Room-Temperature Epitaxy of Metal Thin Films on Tungsten Diselenide

Author Kayla A. Cooley, Rajeh Alsaadi, Ramya L. Gurunathan, Anna C. Domask, Lauren Kerstetter, Wissam A. Saidi, and Suzanne E. Mohney
The orientation of selected metals (Pd, Ni, Al, and Co) deposited on WSe2 by physical vapor deposition was examined using transmission electron microscopy and selected area electron diffraction. We discovered that Ni demonstrates room-temperature epitaxy, similarly to other face centered cubic (FCC) metals Au, Ag, and Cu. These epitaxial metals exhibit the following orientation relationship, where M stands for metal: M() || WSe2 (); M[] || WSe2 []. Hexagonally close-packed Co, and FCC Pd and Al, were not epitaxial on deposition; however, Pd became epitaxial after annealing at 673 K for 5 h. To uncover critical variables for epitaxial growth, we correlated our experimental work and reports from the literature on Cu, Ag, and Au with density functional theory calculations of the energetics of metal atoms on the surface of WSe2 and thermodynamic calculations of metal-W-Se phase equilibria. Furthermore, we compared the findings to our previous work on metal/MoS2 systems to draw conclusions more generally applicable to epitaxial growth of metals on transition metal dichalcogenides (TMDs). We observed that epitaxy of metals on TMDs can occur when there is a match in crystallographic symmetry, even with a large lattice mismatch, and it is favored by metals exhibiting a low diffusion barrier on the TMD surface. However, reaction processes between the metal and WSe2 can prevent epitaxy even when the other factors are favorable, as occurred for Al/WSe2 with the formation of aluminum selenide, tungsten aluminide, and elemental tungsten. Consideration of crystallographic symmetry, surface diffusion barriers, and reactivity can be used to predict room-temperature epitaxy in other metal/TMD systems.
Temperature can have a dramatic effect on the solar efficiency of methylammonium lead iodide (CH3NH3PbI3) absorbers due to changes in the electronic structure of the system even within the range of stability of a single phase. Herein, using first-principles density functional theory, we investigate the electron band structure of the tetragonal and orthorhombic phases of CH3NH3PbI3 as a function of temperature. The electron–phonon interactions are computed to all orders using a Monte Carlo approach, which is needed considering that the second-order Allen–Heine–Cardona theory in electron–phonon coupling is not adequate. Our results show that the band gap increases with temperature, in excellent agreement with experimental results. We verified that anharmonic effects are only important near the tetragonal–cubic phase transition temperature. We also found that temperature has a significant effect on the effective masses and Rashba coupling. At room temperature, electron–phonon coupling is found to enhance the band effective mass by a factor of 2 and to diminish the Rashba coupling by the same factor compared to T = 0 K values. Our results underscore the significant impact of electron–phonon coupling on electronic properties of the hybrid perovskites.

Proton Migration in Hybrid Lead Iodide Perovskites: From Classical Hopping to Deep Quantum Tunneling

Author Yexin Feng, Yicheng Zhao, Wen-Ke Zhou, Qi Li, Wissam A. Saidi, Qing Zhao, and Xin-Zheng Li
The organic–inorganic halide perovskites (OIHPs) have shown enormous potential for solar cells, while problems like the current–voltage hysteresis and the long-term instability have seriously hindered their applications. Ion migrations are believed to be relevant. But the atomistic details still remain unclear. Here we study the migrations of ions in CH3NH3PbI3 (MAPbI3) at varying temperatures (T’s), using combined experimental and first-principle theoretical methods. Classical hopping of the iodide ions is the main migration mechanism at moderate T’s. Below ∼270 K, the kinetic constant for ionic migration still shows an Arrenhius dependency, but the much lower activation energy is attributed to the migration of H+. A gradual classical-to-quantum transition takes place between ∼140 and ∼80 K. Below ∼80 K, the kinetic constant becomes T-independent, suggesting that deep quantum tunneling of H+ takes over. This study gives direct experimental evidence for the migrations of H+s in MAPbI3 and confirms their quantum nature.

First-principles Investigations of the Temperature Dependence of Electronic Structure and Optical Properties of Rutile TiO2

Author Yu-Ning Wu, Wissam A. Saidi, Paul Ohodnicki, Benjamin Chorpening, and Yuhua Duan
To gain additional insight into high-temperature functional material properties for applications in optical gas sensing, the temperature effects on the band gap and optical properties of rutile TiO2 are investigated using ab initio methods. By analyzing the contributions from electron-phonon interaction and lattice thermal expansion, we show that the electron-phonon interaction is the dominant factor for temperature band gap renormalization. As the temperature increases, the band gap increases until 300 K and then narrows above 300 K. This behavior results from the acoustic phonons, which widen the band gap, dominating below 300 K, while the optical phonons, which narrows the band gap, dominate above 300 K. Our study suggests that the band gap is narrowed by about 138 meV at 1000 K. We also investigated the temperature effects on the dielectric constants, the refractive index as well as the extinction coefficient. Both the rate of decrease of the refractive index at 650nm and 800nm as well as the experimentally derived bandgap agree with experimentally measured data as temperature increases. Our results and computational methods are of interest for developing high-temperature functional materials with applications towards gas sensing.

In Situ Study of Nucleation and Growth Dynamics of Au Nanoparticles on MoS2 Nanoflakes

Author Boao Song, Kun He, Yifei Yuan, Seyyed Sharifi-Asl, Meng Cheng, Jun Lu, Wissam A Saidi and Reza Shahbazian Yassar
Two-dimensional (2D) substrates decorated with metal nanoparticles offer new opportunities to achieve high-performance catalytic behavior. However, little is known on how the substrates control the nucleation and growth processes of the nanoparticles. This paper presents the visualization of dynamic nucleation and growth processes of gold nanoparticles on ultrathin MoS2 nanoflakes by in situ liquid-cell transmission electron microscopy (TEM). The galvanic displacement resulting in Au nuclei formation on MoS2 was observed in real time inside the liquid cell. We found that the growth mechanism of Au particles on pristine MoS2 is in between diffusion-limited and reaction-limited, possibly due to presence of electrochemical Ostwald ripening. A larger size distribution and more orientation variation is observed for the Au particles along MoS2 edge than on interior. Differ from pristine MoS2, sulfur vacancies on MoS2 induce Au particle diffusion and coalescence during growth process. Density functional theory (DFT) calculations show that the size difference is because the exposed molybdenum atoms at the edge with dangling bonds can strongly interact with Au atoms, whereas sulfur atoms on MoS2 interior have no dangling bonds and weakly interact with gold atoms. In addition, S vacancies on MoS2 generate strong nucleation centers that can promote diffusion and coalescence of Au nanoparticles. The present work provides key insights on the role of 2D materials in controlling the size and orientation of noble metal nanoparticles vital to the design of next generation catalysts.

Superatom Molecular Orbital as an Interfacial Charge Separation State

Author Hongli Guo, Chuanyu Zhao, Qijing Zheng, Zhenggang Lan, Oleg V Prezhdo, Wissam A Saidi, and Jin Zhao
Hot electron cooling by energy loss to heat through electron–phonon (e–ph) interaction is an important mechanism that can limit the efficiency of solar energy conversion. To avoid such energy loss, sufficient charge separation needs to be realized by extracting hot carriers from the photoconverter before they cool, which requires fast interfacial charge transfer and slow internal hot carrier relaxation. Using ab initio time-dependent nonadiabatic molecular dynamics and taking C60/MoS2 as a prototype system, we show that the superatom molecular orbitals (SAMOs) of fullerenes, which are bound by the central potential of the whole molecule induced by the charge screening, are ideal media for charge separation. The diffuse character of SAMOs results in extremely weak e–ph interaction and therefore acts as a “phonon bottleneck” for hot electron cooling. Furthermore, it also leads to significant hybridization with other atoms at the interface that induces fast charge transfer. The interfacial charge-transfer rate at the C60/MoS2 interface is found to be 2 orders of magnitude faster than the hot electron cooling from s-SAMO in C60. This conclusion is generally applicable for different carbon nanostructures that have SAMOs. The proposed SAMO-induced charge separation provides unique and essential insights into the material design and function for solar energy conversion.

Segregation induced order-disorder transition in Cu(Au) surface alloys

Author Lianfeng Zou, Wissam A Saidi, Yinkai Lei, Zhenyu Liu, Jonathan Li, Liang Li, Qing Zhu, Dmitri Zakharov, Eric A Stach, Judith C Yang, Guofeng Wang, and Guangwen Zhou
Using in-situ transmission electron microscopy and atomistic simulations, we report atomic-scale observations of segregation-induced structure changes in the surface and subsurface region of a Cu(Au) solid solution in both reductive and oxidative environments. In a H2 atmosphere, Au segregation induces the formation of a two-atomic-layer thick ordered surface alloy with an L10 terminated surface layer. By switching to an O2 atmosphere, the outermost surface develops into an Au-missing row reconstruction and simultaneously the second layer experiences an order-disorder transition via intralayer atomic exchanges. The chemical disordering then propagates to the outermost surface, driven by oxygen-adsorption induced Cu surface segregation. This transforms the L10 missing-row reconstruction into a non-reconstructed, oxygenated surface. These observations provide a mechanistic detail regarding the evolution of the surface and subsurface of this alloy in response to environmental stimuli, and are relevant to a wide range of technologically relevant processes.

Tuning Solvated Electrons by Polar-Nonpolar Oxide Heterostructure

Author Yanan Wang, Hongli Guo, Qijing Zheng, Wissam A. Saidi and Jin Zhao
Solvated electron states at oxide/aqueous interface represent the lowest energy charge transfer pathways, thereby playing an important role in photocatalysis and electronic device applications. However, their energies are usually higher than the conduction band minimum (CBM), which makes the solvated electrons difficult to utilize in charge transfer processes. Thus, it is essential to stabilize the energy of the solvated electron states. In this report, taking LaAlO3/SrTiO3 (LAO/STO) oxide heterostructure with H2O adsorbed monolayer as a prototypical system, we show using DFT and ab initio time dependent nonadiabatic molecular dynamics simulation that the energy and dynamics of solvated electrons can be tuned by the electric field in the polar-nonpolar oxide heterostructure. Particularly, for LAO/STO with p-type interface, the CBM is contributed by the solvated electron state when LAO is thicker than 4 unit cells. Furthermore, the solvated electron band minimum can be partially occupied when LAO is thicker than 8 unit cells. We propose that the tunability of solvated electron states can be achieved on polar-nonpolar oxide heterostructure surfaces as well as on ferroelectric oxides, which is important for charge and proton transfer at oxide/aqueous interfaces.

Experimentally Validated Structures of Supported Metal Nanoclusters on MoS2

Author Yongliang Shi, Boao Song, Reza Shahbazian-Yassar, Jin Zhao and Wissam A. Saidi
In nanometer clusters (NCs), each atom counts. It is the specific arrangement of these atoms that determines the unique size-dependent functionalities of the NCs and hence their applications. Here, we employ a self-consistent, combined theoretical and experimental approach to determine atom-by-atom the structures of supported Pt NCs on MoS2. The atomic structures are predicted using a genetic algorithm utilizing atomistic force fields and density functional theory, which are then validated using aberration-corrected scanning transmission electron microscopy. We find that relatively small clusters grow with (111) orientation such that Pt[11̅0] is parallel to MoS2[100], which is different from predictions based on lattice-match for thin-film epitaxy. Other 4d and 5d transition metals show similar behavior. The underpinning of this growth mode is the tendency of the NCs to maximize the metal–sulfur interactions rather than to minimize lattice strain.

Phonon-coupled ultrafast interlayer charge oscillation at van der Waals heterostructure interfaces

Author Qijing Zheng, Yu Xie, Zhenggang Lan, Oleg V Prezhdo, Wissam A Saidi, and Jin Zhao
Van der Waals (vdW) heterostructures of transition-metal dichalcogenide (TMD) semiconductors are central not only for fundamental science, but also for electro- and optical-device technologies where the interfacial charge transfer is a key factor. Ultrafast interfacial charge dynamics has been intensively studied, however, the atomic scale insights into the effects of the electron-phonon (e-p) coupling are still lacking. In this paper, using time dependent ab initio nonadiabatic molecular dynamics, we study the ultrafast interfacial charge transfer dynamics of two different TMD heterostructures MoS2/WS2 and MoSe2/WSe2, which have similar band structures but different phonon frequencies. We found that MoSe2/WSe2 has softer phonon modes compared to MoS2/WS2, and thus phonon-coupled charge oscillation can be excited with sufficient phonon excitations at room temperature. In contrast, for MoS2/WS2, phonon-coupled interlayer charge oscillations are not easily excitable. Our study provides an atomic level understanding on how the phonon excitation and e-p coupling affect the interlayer charge transfer dynamics, which is valuable for both the fundamental understanding of ultrafast dynamics at vdW hetero-interfaces and the design of novel quasi-two-dimensional devices for optoelectronic and photovoltaic applications.

Dependence of H2 and CO2 Selectivity on Cu Oxidation State during Partial Oxidation of Methanol on Cu/ZnO

Author Hao Chi, Christopher M. Andolina, Jonathan Li, Matthew T. Curnan, Wissam A. Saidi, Guangwen Zhou, Judith C.Yang, and Götz Veser,
Partial oxidation of methanol is a promising reaction for on-board production of high purity H2 streams for fuel cell applications. In the present work, the influence of Cu oxidation state on the selectivity of POM catalyzed by Cu/ZnO was investigated via the use of a microreactor and X-ray photoelectron spectroscopy. A strong correlation between H2 selectivity and the metallic copper (Cu° ) content of the catalyst was observed, while, surprisingly, the CO2 selectivity was not significantly affected by the catalyst oxidation state. Instead, CO2 selectivity showed a strong correlation with O2 partial pressure, which could be explained by differences in the energy barriers between CO desorption and CO2 formation from CO* on Cu2O surfaces calculated via first-principles calculations. Our results indicate that maintaining metallic Cu catalyst during methanol oxidation could maximize H2 production for use in fuel cells or other clean energy applications.

Delocalized Impurity Phonon Induced Electron–Hole Recombination in Doped Semiconductors

Author Lili Zhang, Qijing Zheng, Yu Xie, Zhenggang Lan, Oleg Prezhdo, Wissam A. Saidi, and Jin Zhao
Semiconductor doping is often proposed as an effective route to improving the solar energy conversion efficiency by engineering the band gap; however, it may also introduce electron–hole (e–h) recombination centers, where the determining element for e–h recombination is still unclear. Taking doped TiO2 as a prototype system and by using time domain ab initio nonadiabatic molecular dynamics, we find that the localization of impurity-phonon modes (IPMs) is the key parameter to determine the e–h recombination time scale. Noncompensated charge doping introduces delocalized impurity-phonon modes that induce ultrafast e–h recombination within several picoseconds. However, the recombination can be largely suppressed using charge-compensated light-mass dopants due to the localization of their IPMs. For different doping systems, the e–h recombination time is shown to depend exponentially on the IPM localization. We propose that the observation that delocalized IPMs can induce fast e–h recombination is broadly applicable and can be used in the design and synthesis of functional semiconductors with optimal dopant control.

Atomically Visualizing Elemental Segregation Induced Surface Alloying and Restructuring

Author Lianfeng Zou, Jonathan Li, Dmitri N Zakharov, Wissam A. Saidi, Eric A. Stach, and Guangwen Zhou
Using in-situ transmission electron microscopy that spatially and temporally resolves the evolution of the atomic structure in the surface and subsurface regions, we find that the surface segregation of Au atoms in a Cu(Au) solid solution results in the nucleation and growth of a (2×1) missing-row reconstructed, half-unit-cell thick L12 Cu3Au(110) surface alloy. Our in-situ electron microscopy observations and atomistic simulations demonstrate that the (2×1) reconstruction of the Cu3Au(110) surface alloy stays as a stable surface structure as a result of the favored Cu-Au diatom configuration.
Native point and grain boundary (GB) defects are ubiquitous in methylammonium lead iodide (MAPbI3) sensitizers employed in solar cells that are polycrystalline in nature. Here we use density functional theory (DFT) in conjunction with a thermodynamic approach to determine the stability and electronic properties of all native point defects and their interplays with Σ5-(210) GB in MAPbI3. The transition levels of charged defects are investigated with inclusion of electrostatic charge corrections and spin-orbit coupling. We find that the GB region is a sink for most of the native point defects under different synthesis conditions. For the crystalline and bi-crystalline MAPbI3 with Σ5-(210) GB, we find respectively that the p-type antisite defects MAI and PbI, where I substitutes for MA or Pb, introduce deep levels and both are relatively stable under I-rich conditions. Hence, I-poor conditions are more preferable for synthesis of MAPbI3 to have defects with electronically benign character.

Phonon-Assisted Ultrafast Charge Transfer at van der Waals Heterostructure Interface

Author Qijing Zheng, Wissam A. Saidi, Yu Xie, Zhenggang Lan, Oleg V. Prezhdo, Hrvoje Petek, and Jin Zhao
Nano Lett., 2017, 17 (10), pp 6435–6442
The van der Waals (vdW) interfaces of two-dimensional (2D) semiconductor are central to new device concepts and emerging technologies in light-electricity transduction where the efficient charge separation is a key factor. Contrary to general expectation, efficient electron–hole separation can occur in vertically stacked transition-metal dichalcogenide heterostructure bilayers through ultrafast charge transfer between the neighboring layers despite their weak vdW bonding. In this report, we show by ab initio nonadiabatic molecular dynamics calculations, that instead of direct tunneling, the ultrafast interlayer hole transfer is strongly promoted by an adiabatic mechanism through phonon excitation occurring on 20 fs, which is in good agreement with the experiment. The atomic level picture of the phonon-assisted ultrafast mechanism revealed in our study is valuable both for the fundamental understanding of ultrafast charge carrier dynamics at vdW heterointerfaces as well as for the design of novel quasi-2D devices for optoelectronic and photovoltaic applications.

Structure of Defects on Anatase TiO2(001) Surface

Author Yongliang shi, Huijuan Sun, Manh Cuong Nguyen, Cai Zhuang Wang, Kai Ming Ho, Wissam A Saidi and Jin Zhao
Defects on oxide surfaces play a crucial role on the surface reactivity and thus it is crucial to understand their atomic and electronic structures. The defects on anatase TiO2(001)-(1×4) surface are found to be highly reactive, however, due to the surface reconstruction, the defects exhibit complicated characters in different experiments which make it very challenging to determine their atomic structures. Here we present a systematic first-principles investigation of the defects on anatase TiO2(001)-(1×4) surface based on a global-search adaptive genetic algorithm (AGA) and density functional theory (DFT). For different Ti-O ratios, we identify the low energy defect structures, investigate their electronic structure using hybrid functional, and map their regions of stability under realistic conditions. We successfully find novel oxygen vacancy (OV) and Ti interstitial (Tiini) structures that are different from the conventional ones in terms of their charge localization, magnetic state, and their scanning-tunneling-microscopy bright-dark image signature. This provides insight into the complex geometric and electronic structure of the surface defects, and resolves several experimental discrepancies.
Halide perovskites and van der Waals (vdW) heterostructures are both of current interest owing to their novel properties and potential applications in nano-devices. Here, we show the great potential of 2D halide perovskite sheets (C4H9NH3)2PbX4 (X  =  Cl, Br and I) that were synthesized recently (Dou et al 2015 Science 349 1518–21) as the channel materials contacting with graphene and other 2D metallic sheets to form van der Waals heterostructures for field effect transistor (FET). Based on state-of-the-art theoretical simulations, we show that the intrinsic properties of the 2D halide perovskites are preserved in the heterojunction, which is different from the conventional contact with metal surfaces. The 2D halide perovskites form a p-type Schottky barrier (Φh) contact with graphene, where tunneling barrier exists, and a negative band bending occurs at the lateral interface. We demonstrate that the Schottky barrier can be turned from p-type to n-type by doping graphene with nitrogen atoms, and a low-Φh or an Ohmic contact can be realized by doping graphene with boron atoms or replacing graphene with other high-work-function 2D metallic sheets such as ZT-MoS2, ZT-MoSe2 and H-NbS2. This study not only predicts a 2D halide perovskite-based FETs, but also enhances the understanding of tuning Schottky barrier height in device applications.

Enhanced Mass Transfer in the Step Edge Induced Oxidation on Cu(100) Surface

Author Qing Zhu, Wissam A. Saidi, and Judy Yang
In situ TEM experiments have shown that the oxidation of stepped Cu(100) surface results in a flat Cu2O film, which is different from the 3D oxide island structure that usually forms on a flat Cu surface. The mass transport process originating from Cu adatoms that detach from the step edge is argued to be responsible for the different oxide growth behavior. Using molecular dynamics in conjunction with a reactive force field (ReaxFF), we show that the mass transport from the step edge to the flat terrace is enhanced by the unevenly distributed oxygen adatoms on the step top compared to the flat terrace. The ReaxFF force field is optimized using density functional theory calculated energetics and kinetic barriers on various Cu surface models. We investigate two possible mechanisms that can trigger Cu transport: (1) strain due to lattice mismatch between Cu and Cu2O and (2) electrostatic interactions. We show that the formation and diffusion of Cu–O clusters can accelerate the Cu transport process, especially in the presence of surface vacancy defects. Our atomistic simulations demonstrate that the Cu atom detachment progresses from the top of the step edge into deeper layers, and the detachment rate is enhanced with elevated temperatures.

Facile Anhydrous Proton Transport on Hydroxyl Functionalized Graphane

Author Abhishek Bagusetty, Pabitra Choudhury, Wissam A. Saidi, Bridget Derksen, Elizabeth Gatto, and J. Karl Johnson
Graphane functionalized with hydroxyl groups is shown to rapidly conduct protons under anhydrous conditions through a contiguous network of hydrogen bonds. Density functional theory calculations predict remarkably low barriers to diffusion of protons along a 1D chain of surface hydroxyls. Diffusion is controlled by the local rotation of hydroxyl groups, a mechanism that is very different from that found in 1D water wires in confined nanopores or in bulk water. The proton mean square displacement in the 1D chain was observed to follow Fickian diffusion rather than the expected single-file mobility. A charge analysis reveals that the charge on the proton is essentially equally shared by all hydrogens bound to oxygens, effectively delocalizing the proton.
Nickel-based alloys are widely applied materials in high-temperature applications because they exhibit superior corrosion resistance and mechanical properties. The effects of sulfur, which is invariably present in industrial atmospheres, on the early stages of oxidation of Ni-based surfaces are not well understood. Here we use density functional theory to investigate the interactions of sulfur, SO, and SO2 with the Ni(111) and Cr-doped Ni(111) surface and elucidate their electronic interactions and potential energy surfaces. The results show that Cr doping of the Ni(111) surface increases the adsorption energies of sulfur, oxygen on the sulfur pre-adsorbed condition, SO and SO2. Further, this increase positively correlates with Cr concentration on top of the Ni(111) surface, although sulfur does not have any preferential interaction with Cr. This explains why Cr doping has little effect on the activation energy of sulfur for the most preferable diffusion path. Nevertheless, the increase in adsorption energies indicates a strong interaction with Cr-doped surfaces, which is due to the Cr-enhanced charge transfer to sulfur adsorbates. The existence of pre-adsorbed sulfur is shown to have a destabilizing effect on the oxygen interactions with the surfaces. Our results show that Cr doping helps to stabilize the protective oxide scale on Ni(111) surfaces and enhances its corrosion resistance.

Role of Surface Stress on the Reactivity of Anatase TiO2(001)

Author Yongliang Shi, Huijuan Sun, Wissam A. Saidi , Manh Cuong Nguyen, Cai Zhuang Wang, Kaiming Ho, Jinlong Yang, and Jin Zhao
In contrast with theoretical predictions in which anatase TiO2(001) and its (1 × 4) reconstructed surfaces are highly reactive, recent experimental results show this surface to be inert except for the defect sites. In this report, based on a systematic study of anatase TiO2(001)-(1 × 4) surface using first-principles calculations, the tensile stress is shown to play a crucial role on the surface reactivity. The predicted high reactivity based on add-molecule model is due to the large surface tensile stress, which can be easily suppressed by a stress-release mechanism. We show that various surface defects can induce stress release concomitantly with surface passivation. Thus the synthesis of anatase(001) surface with few defects is essential to improve the reactivity, which can be achieved, for example, via H2O adsorption. Our study provides a uniform interpretation of controversial experimental observations and theoretical predictions on anatase TiO2(001) surface and further proposes new insights into the origin of surface reactivity.

Defect-Induced Near-Infrared Photoluminescence of Single-Walled Carbon Nanotubes Treated with Polyunsaturated Fatty Acids

Author Cheuk Fai Chiu, Wissam A. Saidi, Valerian E. Kagan, and Alexander Star
Single-walled carbon nanotubes (SWCNTs) have been incorporated in many emerging applications in the biomedical field including chemical sensing, biological imaging, drug delivery, and photothermal therapy. To overcome inherent hydrophobicity and improve their biocompatibility, pristine SWCNTs are often coated with surfactants, polymers, DNA, proteins, or lipids. In this paper, we report the effect of polyunsaturated fatty acids (PUFAs) on SWCNT photoluminescence. A decrease in the SWCNT bandgap emission (E11) and a new red-shifted emission (E11-) were observed in the presence of PUFAs. We attribute the change in SWCNT photoluminescence to the formation of oxygen-containing defects by lipid hydroperoxides through photo-oxidation. The observed changes in near-infrared emission of SWCNTs are important for understanding the interaction between SWCNTs and lipid biocorona. Our results also indicate that photo-excited SWCNTs can catalyze lipid peroxidation similarly to lipoxygenases.
The use of water electrocatalysis for hydrogen production is a promising, sustainable and greenhouse-gas-free process to develop disruptive renewable energy technologies. Transition metal carbides, in particular β-phase Mo2C, are garnering increased attention as hydrogen evolution reaction (HER) catalysts due to their favourable synthesis conditions, stability and high catalytic efficiency. We use a thermodynamic approach in conjunction with density functional theory and a kinetic model of exchange current density to systematically study the HER activity of β-Mo2C under different experimental conditions. We show that the (011) surface has the highest HER activity, which is rationalized by its lack of strong Mo-based hydrogen adsorption sites. Thus, the HER efficiency of β-Mo2C can be tuned using nanoparticles (NPs) that expose larger fractions of this termination. We give definite maps between NP morphologies and experimental synthesis conditions, and show that the control of carbon chemical potential during synthesis can expose up to 90% of (011) surface, while as H2 ambient has little effect on NPs morphology. The “volcano” plot shows that under these optimum conditions, the NP exchange current density is ~10-5 A/cm2, that is only slightly smaller than that of Pt (111).
Organometal trihalide perovskites are emerging as very promising photovoltaic materials, which is rivaling that of single crystal silicon solar cells despite their polycrystalline nature with relatively high density of grain boundaries (GBs). There is a lack of understanding of the effects of GBs on halide perovskites as their presence in silicon and other photovoltaic materials is generally detrimental to their photovoltaic properties. Using first-principles calculations, we systematically investigate the geometric structures of high-angle tilt GBs in halide perovskites CsPbX3 (X = Cl, Br, and I) starting from the coincidence site lattice model and refining using crystal shifts and lattice expansion. Electronic density of states calculations reveal that GBs in halides perovskites do not generate midgap states because of the large distance between the unsaturated atoms and the atomic reconstructions in the GB region. However, we show that the GBs can induce different very shallow states near the valence band edge that can hinder hole diffusion. We further extend the results to MAPbI3 GBs and also show their benign effect on optoelectronic properties.

Controlling nucleation, growth, and orientation of metal halide perovskite thin films with rationally selected additives

Author Benjamin J. Foley, Justin Girard, Blaire A. Sorenson, Alexander Z. Chen, J. Scott Niezgoda, Matthew R. Alpert, Angela F. Harper, Detlef-M. Smilgies, Paulette Clancy, Wissam A. Saidi and Joshua J. Choi
Accelerating the progress toward realizing metal halide perovskite solar cells with improved efficiency, stability and reliability requires a deeper understanding of the thin film formation processes. This paper investigates the impact of rationally selected chemical additives in precursor solutions on the nucleation and growth of metal halide perovskite thin films. Computational screening was performed to guide the selection of tetrahydrothiophene oxide as an additive with stronger solvation efficacy than all other commonly used solvents. In situ grazing incidence X-ray diffraction measurements show that the additives suppress the formation of homogeneous nuclei as well as crystalline intermediate structures. Instead, heterogeneous nucleation on the substrate surface and growth of a thin film with a strongly preferential crystallographic orientation occur directly from the precursor solution. Density functional theory calculations show that the crystallographic orientation of the thin films can be tuned by altering the surface energies with the chemical additives. The crystallographic orientation of the thin films is found to have a significant impact on the open circuit voltage of solar cell devices, highlighting the importance of controlling the metal halide perovskite thin film orientation for improved solar cell efficiency.

Hydrogen-induced atomic structure evolution of the oxygen-chemisorbed Cu(110) surface

Author Weitao Shan, Qianqian Liu, Jonathan Li, Na Cai, Wissam A. Saidi, and Guangwen Zhou
Using a combination of scanning tunneling microscopy (STM) and density functional theory (DFT) modeling, we determine the mechanism of the atomic structural evolution of the oxygenated Cu(110) surface induced by the reaction of adsorbed hydrogen with chemisorbed oxygen in the Cu(110)-c(6 × 2)-O structure. Our STM observations show that the reconstructed Cu(110)-c(6 × 2)-O surface undergoes a phase transition to the (2 × 1)-O reconstruction in the course of oxygen loss induced by the reaction with H2 gas. Using DFT modeling, we find that the surface phase transition is initiated via the adsorption of molecular hydrogen on the chemisorbed oxygen, which results in the formation of H2O molecules that desorb spontaneously from the surface. The loss of chemisorbed oxygen induces the c(6 × 2) → (2 × 1) transition that involves the diffusion of Cu―O―Cu chains along the ⟨1¯10⟩ direction.

Temperature Dependence of the Energy Levels of Methylammonium Lead Iodide Perovskite from First-Principles

Author Wissam A. Saidi, Samuel Ponce, and Bartomeu Monserrat
Environmental effects and intrinsic energy-loss processes lead to fluctuations in the operational temperature of solar cells, which can profoundly influence their power conversion efficiency. Here we determine from first-principles the effects of temperature on the band gap and band edges of the hybrid pervoskite CH3NH3PbI3 by accounting for electron–phonon coupling and thermal expansion. From 290 to 380 K, the computed band gap change of 40 meV coincides with the experimental change of 30–40 meV. The calculation of electron–phonon coupling in CH3NH3PbI3 is particularly intricate as the commonly used Allen–Heine–Cardona theory overestimates the band gap change with temperature, and excellent agreement with experiment is only obtained when including high-order terms in the electron–phonon interaction. We also find that spin–orbit coupling enhances the electron–phonon coupling strength but that the inclusion of nonlocal correlations using hybrid functionals has little effect. We reach similar conclusions in the metal–halide perovskite CsPbI3. Our results unambiguously confirm for the first time the importance of high-order terms in the electron–phonon coupling by direct comparison with experiment.
Hybrid organic-inorganic perovskites, as well as the perovskites in general, are known for their phase complexity evidenced by the stabilization of different polymorphs, and thus an understanding of their regions of stability and transitions can be important for their photovoltaic and optoelectronic technologies. Here we use a multiscale approach based on first-principles calculations with van der Waals corrections and classical force-field molecular dynamics to determine the finite-temperature properties of the tetragonal and cubic phases of CH3NH3PbI3. Temperature effects are implicitly included using the quasi-harmonic approximation that can describe anharmonic behavior due to thermal expansion through the dependence of the harmonic frequencies on structural parameters. Our finite-temperature free-energy surfaces predict the lattice and elastic moduli evolution with temperature, and show in particular that the calculated lattice parameters of the cubic and tetragonal phases are to within 1% of experimental values. Further, our results show that the phonons are the major contributing factor for stabilizing the cubic phase at high temperatures mainly due to the low-energy phonon modes that are associated with the inorganic lattice. On the other hand, the configurational entropy due to CH3NH3 + rotational degrees of freedom is slightly more favored in the cubic phase and amounts to less than 0.2% of the T = 0 K free-energy difference between the two phases.
The two-dimensional electron gas (2DEG) formed at the interface between two insulating materials LaAlO3 (LAO) and SrTiO3 (STO) has recently generated a lot of interest. Here, based on first-principles density functional theory calculations, we investigate the existence and stability of the 2DEG under the application of a biaxial strain on the LAO/STO(001) heterostructure. The compressive strain induces ferroelectric (FE) polarization in STO, which allows for the tunability of the 2DEG by reversing the STO polarization orientation. We show that the formation of the 2DEG is unstable when LAO and STO have the same polarization direction. On the other hand, the 2DEG will always form if the two polarizations are in the opposite directions regardless of the LAO thickness, which is in contrast to the unstrained interface that has a critical thickness for stabilizing the 2DEG. We show that the underpinnings of this behavior are due to charge passivation and band gap alignment.

Ultrafast Dynamics of Photongenerated Holes at a CH3OH/TiO2 Rutile Interface

Author Weibin Chu, Wissam A. Saidi, Qijing Zheng, Yu Xie, Zhenggang Lan, Oleg V. Prezhdo, Hrvoje Petek, and Jin Zhao
Photogenerated charge carrier dynamics near molecule/TiO2 interfaces are important for the photocatalytic and photovoltaic processes. To understand this fundamental aspect, we performed a time-domain ab initio nonadiabatic molecular dynamics study of the photogenerated hole dynamics at the CH3OH/rutile TiO2(110) interface. We studied the forward and reverse hole transfer between TiO2 and CH3OH as well as the hole energy relaxation to the valence band maximum. First, we show that the hole-trapping ability of CH3OH depends strongly on the adsorption structure. Only when the CH3OH is deprotonated to form chemisorbed CH3O will ∼15% of the hole be trapped by the molecule. Second, we find that strong fluctuations of the HOMO energies of the adsorbed molecules induced by electron-phonon coupling provide additional channels, which accelerate the hole energy relaxation. Third, we demonstrate that the charge transfer and energy relaxation processes depend significantly on temperature. When the temperature decreases from 100 to 30 K, the forward hole transfer and energy relaxation processes are strongly suppressed because of the reduction of phonon occupation. These results indicate that the molecule/TiO2 energy level alignment, thermal excitation of a phonon, and electron-phonon coupling are the key factors that determine the photogenerated hole dynamics. Our studies provide valuable insights into the photogenerated charge and energy transfer dynamics at molecule/semiconductor interfaces.
The van der Waals C6 coefficients of fullerenes are shown to exhibit an anomalous dependence on the number of carbon atoms N such that C 6 ∝ N 2.2 as predicted using state-of-the-art quantum mechanical calculations based on fullerenes with small sizes, and N 2.75 as predicted using a classical-metallic spherical-shell approximation of the fullerenes. We use an atomistic electrodynamics model where each carbon atom is described by a polarizable object to extend the quantum mechanical calculations to larger fullerenes. The parameters of this model are optimized to describe accurately the static and complex polarizabilities of the fullerenes by fitting against accurate ab initio calculations. This model shows that C 6 ∝ N 2.8, which is supportive of the classical-metallic spherical-shell approximation. Additionally, we show that the anomalous dependence of the polarizability on N is attributed to the electric charge term, while the dipole–dipole term scales almost linearly with the number of carbon atoms.

Step-Edge Directed Metal Oxidation

Author Qing Zhu, Wissam A. Saidi, and Judith Yang
Metal surface oxidation is governed by surface mass transport processes. Realistic surfaces have many defects such as step edges, which often dictate the oxide growth dynamics and result in novel oxide nanostructures. Here we present a comprehensive and systematic study of the oxidation of stepped (100), (110) and (111) Cu surfaces using a multiscale approach employing density functional theory (DFT) and reactive force field molecular dynamics (MD) simulations. We show that the early stages of oxidation of these stepped surfaces can be qualitatively understood from the potential energy surface of single oxygen adatoms, namely, adsorption energies and Ehrlich-Schwöbel barriers. These DFT predictions are then validated using classical MD simulations with a newly optimized ReaxFF force field. In turn, we show that the DFT results can be explained using a simple bond-counting argument that makes our results general and transferable to other metal surfaces.
We have used time-dependent density functional theory in conjunction with the CAM-B3LYP functional and MWB28/aug-cc-pVDZ basis set to determine non-, near-, and on-resonance Raman spectra for a complex formed by 4-mercaptopyridine (4-Mpy) binding with a Ag13 cluster via the thiolate Ag–S bond. Geometry optimizations of the Ag13-4-Mpy complex showed an on-top structure directly bound to one Ag atom with the ring of the molecule almost flat with respect to two Ag atoms of the complex. The corresponding B3LYP/MWB28/aug-cc-pVDZ geometry is also an on-top structure directly bound to one Ag atom, but the molecule is directed away from the surface. The near-resonance Raman calculations were carried out in the infinite lifetime approximation, while the on-resonant Raman excitation profiles were calculated with the complex polarization propagator (CPP) approach, introducing a half width at half-maximum spectral broadening of 0.2 eV. Calculation of the UV–vis spectra of the isolated 4-Mpy and of the Ag13-4-Mpy complex showed that binding shifts the spectra from deep in the UV to the visible region. Calculation of the near-resonance Raman spectra of the two structures of the complex at 410 (3.025 eV) and 425 nm (2.918 eV) showed a strong enhancement. A very large variation across vibrational modes by a factor of at least 103 was found for both the static chemical enhancement and charge-transfer (CT) enhancement mechanisms. This large variation in enhancement factor indicates that B-term Herzberg–Teller scattering is occurring because inactive or very low intensity modes in the static spectra of the molecule are much stronger in both the static and near-resonance spectra of the complex. From the excitation profile using the CPP method, an overall surface enhancement on the order 103 or higher was found for individual modes on excitation into a CT excited state.

Early and transient stages of Cu oxidation: Atomistic insights from theoretical simulations and in situ experiments

Author Qing Zhu, Lianfeng Zou, Guangwen Zhou, Wissam A. Saidi, and Judith C. Yang
Understanding of metal oxidation is critical to corrosion control, catalysis synthesis, and advanced materials engineering. Although, metal oxidation process is rather complicated, different processes, many of them coupled, are involved from the onset of reaction. Since first introduced, there has been great success in applying heteroepitaxial theory to the oxide growth on a metal surface as demonstrated in the Cu oxidation experiments. In this paper, we review the recent progress in experimental findings on Cu oxidation as well as the advances in the theoretical simulations of the Cu oxidation process. We focus on the effects of defects such as step edges, present on realistic metal surfaces, on the oxide growth dynamics. We show that the surface steps can change the mass transport of both Cu and O atoms during the oxide growth, and ultimately lead to the formation of different oxide morphology. We also review the oxidation of Cu alloys and explore the effect of secondary element to the oxide growth on a Cu surface. From the review of the work on Cu oxidation, we demonstrate the correlation of theoretical simulations at multiple scales with various experimental techniques.

Nano-scale Polar-Nonpolar Oxide Heterostructures for Photocatalysis

Author Hongli Guo, Wissam A Saidi, Jinlong Yang and Jin Zhao
We propose that a nano-scale thin film based on polar-nonpolar transition-metal oxide heterostructure can be used as a highly-efficient photocatalyst. This is demonstrated using a SrTiO3/LaAlO3/SrTiO3 sandwich-like heterostructure with a photocatalytic activity in the near-infrared region using first principles calculations. The effect of the polar nature of LaAlO3 is two-fold. First, the induced electrostatic field accelerates the photo-generated electrons and holes into opposite directions and minimizes their recombination rates. Hence, the reduction and oxidation reactions can be instigated at the SrTiO3 surfaces located on opposite sides of the heterostructure. Second, the electric field reduces the band gap of the system making it photoactive in the infrared region. We also show that charge separation can be enhanced in using compressive strain engineering that creates a ferroelectric instability in STO. The proposed setup is ideal for tandem oxide photocatalysts especially when combined with photoactive polar materials.

Temperature dependent energy levels of methylammonium lead iodide perovskite

Author Benjamin J Foley, Daniel L Marlowe, Keye Sun, Wissam A Saidi, Louis Scudiero, Mool C Gupta, and Joshua J Choi
Temperature dependent energy levels of methylammonium lead iodide are investigated using a combination of ultraviolet photoemission spectroscopy and optical spectroscopy. Our results show that the valence band maximum and conduction band minimum shift down in energy by 110 meV and 77 meV as temperature increases from 28 °C to 85 °C. Density functional theory calculations using slab structures show that the decreased orbital splitting due to thermal expansion is a major contribution to the experimentally observed shift in energy levels. Our results have implications for solar cell performance under operating conditions with continued sunlight exposure and increased temperature.
Contacts between metal surfaces and MoS2 are crucial for the utilization of MoS2 in different technologies. Here we systematically investigate using first-principles density functional theory the adsorption and diffusion on MoS2(001) of a wide range of metals from Groups I–IV in addition to all of the 3d transition metals (TMs) and selected 4d and 5d TMs. The binding mechanisms as well as trends in the binding energies are elucidated by examining the electronic structure of the system, and in particular the interplay between Coulomb interactions, Pauli repulsion, and ndm(n + 1)sxndm+1(n + 1)sx–1 (x = 1, 2; n = 3, 4, 5) promotion energies. We show that the metal-induced workfunction reduction is correlated with the ionization potential of the isolated atom and is furthermore linearly dependent on the interfacial dipole moment with an offset term. Additionally, the growth morphologies of the metal nanoparticles on MoS2 are predicted by analyzing the monomer adhesion energy and its mobility on the substrate. Our results are in line with recent experiments showing that Ag and Au follow a Volmer–Weber growth mode on MoS2(001).
We use density-functional theory (DFT) and molecular dynamics (MD) to resolve the role of polyvinylpyrrolidone (PVP) in the shape-selective synthesis of Au nanostructures. Using DFT, we probe the adsorption-induced surface energies and spatially resolved binding of PVP monomer analogs on Au(111), Au(100), and (5 × 1) Au(100)-hex. These calculations suggest that {111} facets should be prevalent in Au nanostructures grown with the help of PVP. We explore the role of solvent and find that, while solvent weakens binding, it does not change the trends we observe in vacuum. We fit an ad hoc interatomic potential to the DFT results so we can describe the binding of PVP to the Au surfaces. Using MD simulations based on this potential, we investigate the PVP-induced surface energies, PVP binding affinities, and oxygen density profile of atactic PVP icosamers on Au(111) and (5 × 1) Au(100)-hex. We conclude that {111}-faceted Au nanocrystals are preferred in PVP-mediated synthesis of Au nanostructures. The reconstruction of Au(100) is important in achieving {111}-facet selectivity.

Modified Schottky emission to explain thickness dependence and slow depolarization in BaTiO3 nanowires

Authors Y. Qi, J. M. P. Martirez, Wissam A. Saidi, J. J. Urban, W. S. Yun, J. E.Spanier, and A. M. Rappe
We investigate the origin of the depolarization rates in ultrathin adsorbate-stabilized ferroelectric wires. By applying density functional theory calculations and analytic modeling, we demonstrate that the depolarization results from the leakage of charges stored at the surface adsorbates, which play an important role in the polarization stabilization. The depolarization speed varies with thickness and temperature, following several complex trends. A comprehensive physical model is presented, in which quantum tunneling, Schottky emission, and temperature-dependent electron mobility are taken into consideration. This model simulates experimental results, validating the physical mechanism. We also expect that this improved tunneling-Schottky emission model could be applied to predict the retention time of polarization and the leakage current for various ferroelectric materials with different thicknesses and temperatures.
The dispersion of Pt metallic nanoparticles on different supports is of high relevance for designing more efficient and less expensive catalysts. In order to understand the nucleation and epitaxial growth of Pt nanoparticles and thin films on MoS2 monolayers, we have systematically analyzed, by first-principles density functional calculations, the evolution of morphology and atomic structure of supported (Pt)n nanoparticles (NPs) on MoS2(001) for n ≤ 12. We find that n = 5 is the cluster size where the growth of the NPs transforms from two- to three-dimensional (2D to 3D). Owing to the topography of MoS2(001), the 2D NPs mostly attach to the support via direct bonding with Mo atoms that sit in the troughs of the surface, while the 3D NPs are bonded to the sulfur atoms that are more extended in the vacuum region. Furthermore, we find that Pt is sufficiently mobile on the surface where the number of hopping events per second is ≈103 s–1 along [101̅] and ≈10 s–1 along [11̅0] at room temperature. The somewhat large mobility suggests that monomer diffusion is not likely to be the rate-limiting step for Oswald ripening and that Pt sputtering on MoS2(001) will result in relatively large particles rather than a fine dispersion. The existence of a fast diffusion channel along [101̅] suggests that the morphology of the NPs is anisotropic.

Step-Induced Oxygen Upward Diffusion on Stepped Cu(100) Surface

Authors Qing Zhu, Wissam A. Saidi, and Judith C. Yang
Surface defects such as step edges play an important role in determining the surface properties, affecting immensely the growth mechanisms and morphologies of the nanostructures in epitaxial film growth processes. Here, we probe the dynamics of the oxidation on stepped Cu(100) using molecular dynamic simulations in conjunction with a reactive force field, and we elucidate the mechanisms and energy barriers affecting the oxidation process. Molecular dynamic simulations show that the adsorbed oxygen adatoms are unevenly distributed on the stepped surface, favoring the top terrace. We show that this behavior is due to Ehrlich–Schwöbel (ES) barrier effect. However, differently from the reduced interlayer self-diffusion in descending a step as in a conventional ES barrier effect, we find instead that the ES barrier reduces the ascending diffusion barrier for oxygen, promoting its transport across the step edge and enhancing oxidation of the upper terrace. Additionally, we find that the ES barrier is step-height dependent, where higher step edges reduce more the oxygen-ascending diffusion barrier and favor more oxidation of the upper terraces of stepped surfaces.

Origins of thermal conductivity changes in strained crystals

Authors Kevin D Parrish, Ankit Jain, Jason M Larkin, Wissam A Saidi, Alan JH McGaughey
The strain-dependent phonon properties and thermal conductivities of a soft system [Lennard-Jones (LJ) argon] and a stiff system (silicon modeled using first-principles calculations) are predicted using lattice dynamics calculations and the Boltzmann transport equation. As is commonly assumed for materials under isotropic strain, the thermal conductivity of LJ argon decreases monotonically as the system moves from compression into tension. The reduction in thermal conductivity is attributed to decreases in both the phonon lifetimes and group velocities. The thermal conductivity of silicon, however, is constant in compression and only begins to decrease once the system is put in tension. The silicon lifetimes show an anomalous behavior, whereby they increase as the system moves from compression into tension, which is explained by examining the potential energy surface felt by an atom. The results emphasize the need to separately consider the harmonic and anharmonic effects of strain on material stiffness, phonon properties, and thermal conductivity.

Strong Reciprocal Interaction between Polarization and Surface Stoichiometry in Oxide Ferroelectrics

Authors Wissam A. Saidi, John Mark P. Martirez, and Andrew M. Rappe
We present a systematic evaluation of the effects of polarization switchability on surface structure and stoichiometry in BaTiO3 and PbTiO3 ferroelectric oxides. We show that charge passivation, mostly by ionic surface reconstructions, is the driving force for the stability of the surfaces, which suggests that varying the substrate polarization offers a new mechanism for controlling surface reconstructions in polar systems and inducing highly nonstoichiometric structures. Conversely, for thin-films the chemical environment can drive polarization switching via induced compositional changes on the surface. We find that the value of the oxygen partial pressure for the positive-to-negative polar transition is in good agreement with the recent experimental value for thin-film PbTiO3. For BaTiO3, we show that it is harder for oxygen control to drive polar transition because it is more difficult to reduce. This study opens up the possibility of real-time control of structure and composition of oxide surfaces.

Role of oxygen in Cu (110) surface restructuring in the vicinity of step edges

Authors Liang Li, Na Cai, Wissam A Saidi, Guangwen Zhou
Surface steps are typically assumed as a source of adatoms for oxygen-chemisorption induced surface reconstruction, but few microscopic observations have been made in the vicinity of steps on reconstructing surfaces. Using in situ scanning tunneling microscopy, we provide direct evidence that surface steps are the source of Cu adatoms for the Cu(1 1 0)single bond(2 × 1)-O restructuring. Using density functional theory, we show that the role of oxygen is to stabilize Cu adatoms detached from step edges via the barrier-less formation of Cusingle bondO dimers on terraces. Incorporating this atomic process of capturing Cu adatoms into kinetic Monte Carlo simulations reproduces the experimentally observed (2 × 1)-O reconstruction.

Oxygen chemisorption-induced surface phase transitions on Cu(110)

Authors Qianqian Liua, Liang Lia, Na Caia, Wissam A. Saidib, Guangwen Zhou
From an interplay between variable temperature scanning tunneling microscopy and density–functional theory calculations, the evolution of oxygen chemisorption-induced surface reconstructions of the Cu(110) surface is determined. The surface reconstructions proceed via a sequential pathway with increasing oxygen surface coverage. The (2 × 1) reconstruction occurs first and then transits to the c(6 × 2) phase with a higher oxygen coverage through a mechanism that consumes the existing (2 × 1) phase with the supply of Cu adatoms from step edges and terraces. The temperature dependence of the (2 × 1) ? c(6 × 2) transition demonstrates that the surface phase transition is an activated process for breaking up added Cu–O–Cu rows in the (2 × 1) structure. Comparison between the experimental observations and the theoretical surface phase diagram obtained from first-principles thermodynamic calculations reveals that the (2 × 1) ? c(6 × 2) transition takes place at the oxygen chemical potentials that are far above the chemical potential for Cu2O bulk oxide formation, reflecting the existence of kinetic limitations to the surface phase transition and the bulk oxide formation.

Surface-Step-Induced Oscillatory Oxide Growth

Authors Liang Li, Langli Luo, Jim Ciston, Wissam A. Saidi, Eric A. Stach, Judith C. Yang, and Guangwen Zhou
We report in situ atomic-resolution transmission electron microscopy observations of the oxidation of stepped Cu surfaces. We find that the presence of surface steps both inhibits oxide film growth and leads to the oxide decomposition, thereby resulting in oscillatory oxide film growth. Using atomistic simulations, we show that the oscillatory oxide film growth is induced by oxygen adsorption on the lower terrace along the step edge, which destabilizes the oxide film formed on the upper terrace.

Influence of strain and metal thickness on metal-MoS2 contacts

Author Wissam A. Saidi
MoS2 and other transition metal dichalcogenides are considered as potential materials in many applications including future electronics. A prerequisite for these applications is to understand the nature of the MoS2 contact with different metals. We use semi-local density functional theory in conjunction with dispersion corrections to study the heterostructures composed of Pd and Pt monolayers with (111) orientation grown pseudomorphically on MoS2(001). The interface properties are mapped as a function of the number of deposited overlayers, as well as a function of tensile and compressive strains. Although we show that the dependence of the contacts on strain can be fully explained using the d-band model, we find that their evolution with the number of deposited metal layers is markedly different between Pd and Pt, and at variance with the d-band model. Specifically, the Pt/MoS2 heterostructures show an anomalous large stability with the deposition of two metal monolayers for all investigated strains, while Pd/MoS2 exhibits a similar behavior only for compressive strains. It is shown that the results can be rationalized by accounting for second-nearest-neighbor effect that couples MoS2 with the subsurface metal layers. The underpinnings of this behavior are attributed to the larger polarizability and cohesive energy of Pt compared to Pd, that leads to a larger charge-response in the subsurface layers.
The stability and the electronic structure of layered heterostructures MX2 (M = Mo or W and X = S or Se) and graphene (GA) are systematically investigated using first-principles methods. The calculations cover pristine and defected GA systems with up to 12% nitrogen substitutional defects. It is found that the van der Waals (vdW) epitaxy of MX2 on undoped GA substrate, whether pristine or defected, follows a Volmer–Weber growth-mode resulting in thick MX2 films. On the other hand, nitrogen doping of pristine GA (N-GA) and also of GA with Stone–Wales (SW) defects increases the MX2/GA heterostructure adhesion energy favoring the growth of ultrathin MX2 layers. This growth-mode change in MoS2 due to nitrogen doping is in agreement with recent experiments. Furthermore, our study demonstrates that the yield of ultrathin MX2 films can be increased if the N-GA samples have a larger concentration of SW defects or nitrogen. The underpinnings of the extra stability of these N-GA substrates are due to charge-transfer effects that decrease the Pauli repulsion between the two layered systems.

TFOx: A versatile kinetic Monte Carlo program for simulations of island growth in three dimensions

Authors Qing Zhu, Chris Fleck, Wissam A. Saidi, Alan McGaughey, Judith C. Yang
A three dimensional (3D) kinetic Monte Carlo (KMC) code has been developed that simulates the general behavior of the 3D irreversible nucleation and growth of epitaxial islands, as motivated by experimental observations of oxide nuclei formation and growth during the early stages of copper oxidation. This package was originally a versatile two dimensional (2D) KMC code [Thin Film Oxidation (TFOx)] that considered a variety of elementary steps, including deposition, adsorption, surface diffusion, aggregation, desorption, and substrate-mediated indirect interactions between static adatoms. We extended TFOx to describe 3D island growth. This new version of TFOx is composed of a C++ console program and Python graphical user interface (GUI), such that parameterized simulation, parallel execution, and 3D growth capabilities are feasible. We examined the effects of the potential gradient and the Ehrlich–Schwöbel barrier and found that the 3D island morphology is significantly influenced by the incorporation of these two factors.

Kinetic Barriers of the Phase Transition in the Oxygen Chemisorbed Cu(110)-(2 × 1)-O as a Function of Oxygen Coverage

Authors Liang Li, Qianqian Liu, Jonathan Li, Wissam A. Saidi, and Guangwen Zhou
Oxygen chemisorption induced surface reconstructions are widely observed, but the atomic processes leading to transitions among oxygen chemisorbed phases are largely unknown. Using ab initio molecular dynamics and density-functional theory, we study the kinetic process of the Cu(110)-(2 × 1) ? c(6 × 2) phase transition upon increasing oxygen surface coverage. We show that the phase transition involves initially Cu–O dimer and Cu–O–Cu trimer formation with a kinetic barrier of ?0.13 eV, followed by a barrierless process of forming a four Cu–O–Cu–O chains configuration that transitions to the c(6 × 2) reconstruction via concerted movement of three Cu atoms with an associated energy barrier of ?1.41 eV. The larger kinetic barrier is suggested as the origin of the kinetic hindrance that is inferred from the significant discrepancy between the experimentally observed temperature and pressure dependent (2 × 1) ? c(6 × 2) phase transition and the equilibrium thermodynamics prediction.

Insight into the Mechanism of Graphene Oxide Degradation via the Photo-Fenton Reaction

Authors Hao Bai, Wentao Jiang, Gregg P. Kotchey, Wissam A. Saidi, Benjamin J. Bythell, Jacqueline M. Jarvis, Alan G. Marshall, Renã A. S. Robinson, and Alexander Star
Graphene represents an attractive two-dimensional carbon-based nanomaterial that holds great promise for applications such as electronics, batteries, sensors, and composite materials. Recent work has demonstrated that carbon-based nanomaterials are degradable/biodegradable, but little work has been expended to identify products formed during the degradation process. As these products may have toxicological implications that could leach into the environment or the human body, insight into the mechanism and structural elucidation remain important as carbon-based nanomaterials become commercialized. We provide insight into a potential mechanism of graphene oxide degradation via the photo-Fenton reaction. We have determined that after 1 day of treatment intermediate oxidation products (with MW 150–1000 Da) were generated. Upon longer reaction times (i.e., days 2 and 3), these products were no longer present in high abundance, and the system was dominated by graphene quantum dots (GQDs). On the basis of FTIR, MS, and NMR data, potential structures for these oxidation products, which consist of oxidized polycyclic aromatic hydrocarbons, are proposed.

Coexisting Surface Phases and Coherent One-Dimensional Interfaces on BaTiO3(001)

Authors Erie H. Morales, John Mark P. Martirez, Wissam A. Saidi, Andrew M. Rappe, and Dawn A. Bonnell
Coexistence of surface reconstructions is important due to the diversity in kinetic and thermodynamic processes involved. We identify the coexistence of kinetically accessible phases that are chemically identical and form coherent interfaces. Here, we establish the coexistence of two phases, c(2 × 2) and c(4 × 4), in BaTiO3(001) with atomically resolved Scanning Tunneling Microscopy (STM). First-principles thermodynamic calculations determine that TiO adunits and clusters compose the surfaces. We show that TiO diffusion results in a kinetically accessible c(2 × 2) phase, while TiO clustering results in a kinetically and thermodynamically stable c(4 × 4) phase. We explain the formation of domains based on the diffusion of TiO units. The diffusion direction determines the observed 1D coherent interfaces between c(2 × 2) and c(4 × 4) reconstructions. We propose atomic models for the c(2 × 2), c(4 × 4), and 1D interfaces.

Understanding the Adsorption of CuPc and ZnPc on Noble Metal Surfaces by Combining Quantum-Mechanical Modelling and Photoelectron Spectroscopy

Authors Yu Li Huang, Elisabeth Wruss, David A. Egger, Satoshi Kera, Nobuo Ueno, Wissam A. Saidi, Tomas Bucko, Andrew T.S. Wee and Egbert Zojer
Phthalocyanines are an important class of organic semiconductors and, thus, their interfaces with metals are both of fundamental and practical relevance. In the present contribution we provide a combined theoretical and experimental study, in which we show that state-of-the-art quantum-mechanical simulations are nowadays capable of treating most properties of such interfaces in a quantitatively reliable manner. This is shown for Cu-phthalocyanine (CuPc) and Zn-phthalocyanine (ZnPc) on Au(111) and Ag(111) surfaces. Using a recently developed approach for efficiently treating van der Waals (vdW) interactions at metal/organic interfaces, we calculate adsorption geometries in excellent agreement with experiments. With these geometries available, we are then able to accurately describe the interfacial electronic structure arising from molecular adsorption. We find that bonding is dominated by vdW forces for all studied interfaces. Concomitantly, charge rearrangements on Au(111) are exclusively due to Pauli pushback. On Ag(111), we additionally observe charge transfer from the metal to one of the spin-channels associated with the lowest unoccupied ?-states of the molecules. Comparing the interfacial density of states with our ultraviolet photoelectron spectroscopy (UPS) experiments, we find that the use of a hybrid functionals is necessary to obtain the correct order of the electronic states. - See more at: http://www.mdpi.com/1420-3049/19/3/2969/htm#sthash.dOxH6z7Z.dpuf

Probing single-walled carbon nanotube defect chemistry using resonance Raman spectroscopy

Authors Wissam A. Saidi and Patrick Norman
Polyvinylpyrrolidone (PVP), ethylene glycol (EG), and polyethylene oxide (PEO) are key molecules in the solution-phase synthesis of Ag nanostructures. To resolve various aspects of this synthesis, we develop a classical force field to describe the interactions of these molecules with Ag surfaces. We parametrize the force field through force and energy matching to results from first-principles density-functional theory (DFT). Our force field reproduces the DFT binding energies and configurations of these molecules on Ag(100) and Ag(111). Our force field also yields a binding energy for EG on Ag(110) that is in agreement with experiment. Molecular-dynamics simulations based on this force field indicate that the preferential binding affinity of the chains for Ag(100) increases significantly beyond the segment binding energy for PVP decamers, but not for PEO. This agrees with experimental observations that PVP is a more successful structure-directing agent than is PEO.

Correcting density functionals for dispersion interactions using pseudopotentials

Authors Ozan Karalti, Xiaoge Su, Wissam A. Al-Saidi, Kenneth D. Jordan
We present a two-channel dispersion-corrected atom-centered potential (DCACP) method for correcting BLYP and PBE density functionals for long-range dispersion. The approach, designated DCACP2, is tested on the S22X5 test set and on isomers of the water hexamer. The DCACP2 method provides a significantly improved description of the interaction energies at distances beyond Req than does the single-channel DCACP procedure.
The optical properties, including UV-vis spectra and resonance Raman profiles, of pristine and defected single-walled carbon nanotubes (SWCNTs) are computed using state-of-the-art time-dependent density functional theory (TDDFT) as implemented using the Liouville–Lanczos approach to linear-response TDDFT. The CNT defects were of the form of Stone–Wales and diatom-vacancies. Our results are in very good agreement with experimental results where defects were introduced into a part of defect-free CNTs. In particular, we show that the first and second ?–?* excitation energies are barely shifted due to the defects and associated with a relatively small reduction in the maxima of the absorption bands. In contrast, the resonance Raman spectra show close to an order of magnitude reduction in intensities, offering a means to distinguish between pristine and defected SWCNTs even at low defect concentrations.
Defects are ubiquitous in carbon nanotubes (CNTs), despite their large formation energies, and have astounding effects on their physicochemical properties. In this study, we employ density-functional theory (DFT) calculations to study systematically the atomic structure, stability, and characteristic vibrations of pristine and defected zigzag CNTs, where the defects are of the form of Stone–Wales (SW) and diatom vacancies (DV). The DFT optimized structures and the phonon modes are subsequently used in conjunction with a semiempirical bond-polarization model to study the nonresonant Raman spectra. For each defect type, we find two CNT structures with defects parallel or oblique to the tube axis. For the SW defects, the two structures have similar formation energies, whereas for the DV defect, only defects parallel to the tube axis are likely to exist. The results show that the defects induce a blue shift in the radial breathing mode (RBM) of metallic CNTs, whereas this mode is not shifted for semiconducting CNTs. However, the RBM shift or its Raman profile is not sensitive to the defect type. The G-band showed more sensitivity to the defects in the form of a red/blue shift in the frequency, or a partial/complete defragmentation of the G bands.
First-principles investigations of the electrocatalytic activity toward the four-electron oxygen reduction-reaction in N-doped graphene quantum dots reveal that pyridinic and graphitic nitrogen are the most active sites with overpotentials of 0.55 and 0.79–0.90 V, respectively. This agrees with experimental findings. Our calculations account for van der Waals interactions, solvent effects, and describe the electrochemistry using standard hydrogen electrode model. The results show correlations between OH*, OOH*, and O* binding energies that impose a lower limit on the oxygen reduction overpotential.

The Effect of Metal Catalyst on the Electrocatalytic Activity of Nitrogen-Doped Carbon Nanotubes

Authors Yifan Tang, Seth C. Burkert, Yong Zhao, Wissam A. Saidi, and Alexander Star
Nitrogen-doped and undoped carbon nanotubes (CNTs) were synthesized from ferrocene, nickelocene, and cobaltocene metal catalysts. Electrochemical testing for an oxygen reduction reaction (ORR) showed that nitrogen-doped CNTs synthesized from ferrocene had improved catalytic activity while nanotubes synthesized from nickelocene and cobaltocene, doped with a comparable amount of nitrogen and having similar stacked-cups structure as nitrogen doped CNTs from ferrocene, had a performance only slightly better than that of undoped CNTs. Ferrocene-based nitrogen-doped CNTs also demonstrated similar long-term stability and higher CO tolerance compared to Pt/C catalyst. Detailed ORR mechanisms were also studied and carbon nanomaterials showed different ORR processes as a result of the metal catalyst utilized in the chemical synthesis. Nitrogen-doped and undoped CNTs synthesized from nickelocene show a preferential 4-electron process as compared to materials synthesized from ferrocene and cobaltocene. We believe that the metal used in the growth process regulates the mechanism of oxygen reduction and can be used to develop improved nitrogen-doped carbon nanomaterials as nonprecious-metal catalysts for fuel cells.

Deliquescence of NaBH4 from Density Functional Theory and Experiments

Authors Ping Li, Lin Yu, Michael A. Matthews, Wissam A. Saidi, and J. Karl Johnson
We report a theoretical investigation of H2O adsorption on the NaBH4(100) surface based on first principles density functional theory with inclusion of dispersion corrections in order to explore the initial stages of deliquescence at the molecular level. In the zero coverage limit, H2O is found to bind strongly to sodium sites on NaBH4(100) through O··· Na and O–H···H–B attractions. As the coverage increases H2O molecules adsorb on boron sites. H atoms in the adsorbed H2O monomer adopt tilted down (15°â€“20°) configurations with respect to the NaBH4(100) surface, which undergoes reconstruction in response to adsorbed H2O by rotations of BH4– groups of up to 90° and slight distortions of the positions of Na+ and BH4–. The adsorption energy per H2O is roughly independent of water coverage up to at least a coverage of four monolayers, suggesting that it is energetically feasible for water to condense on the surface, in agreement with experiments. We have experimentally studied the deliquescence of a mixture of NaBH4 with 10 wt % CoCl2. We found that CoCl2 lowers the deliquescence temperature compared to that for pure NaBH4 at a given vapor phase mole fraction of water; i.e., the deliquescence relative humidity is increased because of addition of CoCl2. Thus, while CoCl2 is a catalyst for aqueous phase hydrolysis of NaBH4, it actually inhibits deliquescence and hence delays the onset of steam hydrolysis.

Enzyme-Catalyzed Oxidation Facilitates the Return of Fluorescence for Single-Walled Carbon Nanotubes

Authors Cheuk Fai Chiu, Brian A. Barth, Gregg P. Kotchey, Yong Zhao, Kristy A. Gogick, Wissam A. Saidi, Stéphane Petoud, and Alexander Star
In this work, we studied enzyme-catalyzed oxidation of single-walled carbon nanotubes (SWCNTs) produced by the high-pressure carbon monoxide (HiPco) method. While oxidation via strong acids introduced defect sites on SWCNTs and suppressed their near-infrared (NIR) fluorescence, our results indicated that the fluorescence of SWCNTs was restored upon enzymatic oxidation, providing new evidence that the reaction catalyzed by horseradish peroxidase (HRP) in the presence of H2O2 is mainly a defect-consuming step. These results were further supported by both UV–vis–NIR and Raman spectroscopy. Therefore, when acid oxidation followed by HRP-catalyzed enzyme oxidation was employed, shortened (<300 nm in length) and NIR-fluorescent SWCNTs were produced. In contrast, upon treatment with myeloperoxidase, H2O2, and NaCl, the oxidized HiPco SWCNTs underwent complete oxidation (i.e., degradation). The shortened, NIR-fluorescent SWCNTs resulting from HRP-catalyzed oxidation of acid-cut HiPco SWCNTs may find applications in cellular NIR imaging and drug delivery systems.

Molecular Dynamics Simulations of Carbon Dioxide Intercalation in Hydrated Na-Montmorillonite

Authors Evgeniy M. Myshakin, Wissam A. Saidi , Vyacheslav N. Romanov , Randall T. Cygan, and Kenneth D. Jordan
Molecular dynamics simulations using classical force fields were carried out to study the structural and transport properties of clay mineral–water–CO2 systems at pressure and temperature relevant to geological carbon storage. The simulations show that the degree of swelling caused by intercalation of CO2 strongly depends on the initial water content in the interlayer space and that CO2 intercalation stimulates inner-sphere adsorption of the positively charged interlayer ions on the internal clay surfaces, which modifies the wetting properties of the surfaces. DFT-based molecular dynamics simulations were used to interpret the origin of the observed shift in the asymmetric stretch vibration of CO2 trapped in montmorillonite. The origin of the shift is attributed to the electric field effects on the CO2 molecules induced by the water molecules.
We investigate the origins of experimentally observed differences in the structure-directing capabilities of polyvinylpyrrolidone (PVP) and polyethylene oxide (PEO) for the shape-selective, colloidal synthesis of {100}-faceted Ag nanostructures. Using dispersion-corrected density-functional theory, we calculate the binding energies of polymer repeat units to Ag(100) and Ag(111). At the level of the repeat unit, the energetic preference for PEO to bind to {100} facets (?40 meV) is half that of PVP. At the chain level, we use the Kuhn length to define the polymer binding unit and this is 9 (3) repeat units for PVP (PEO). A thermodynamic model predicts that the {100} binding selectivity of PVP is 107 times higher than that of PEO at synthesis temperatures. These results are consistent with experiment and demonstrate that a distinguishing characteristic of a successful polymer structure-directing agent is the pairing of facet-selective binding of the repeat unit with a sufficiently stiff chain.

Experimental and Theoretical Comparison of Gas Desorption Energies on Metallic and Semiconducting Single-Walled Carbon Nanotubes

Authors Lynn Mandeltort, De-Li Chen, Wissam A. Saidi, J. Karl Johnson, Milton W. Cole, and John T. Yates Jr.
Single-walled carbon nanotubes (SWNTs) exhibit high surface areas and precisely defined pores, making them potentially useful materials for gas adsorption and purification. A thorough understanding of the interactions between adsorbates and SWNTs is therefore critical to predicting adsorption isotherms and selectivities. Metallic (M-) and semiconducting (S-) SWNTs have extremely different polarizabilities that might be expected to significantly affect the adsorption energies of molecules. We experimentally and theoretically show that this expectation is contradicted, for both a long chain molecule (n-heptane) and atoms (Ar, Kr, and Xe). Temperature-programmed desorption experiments are combined with van der Waals corrected density functional theory, examining adsorption on interior and exterior sites of the SWNTs. Our calculations show a clear dependence of the adsorption energy on nanotube diameter but not on whether the tubes are conducting or insulating. We find no significant experimental or theoretical difference in adsorption energies for molecules adsorbed on M- and S-SWNTs having the same diameter. Hence, we conclude that the differences in polarizabilities between M- and S-SWNTs have a negligible influence on gas adsorption for spherical molecules as well as for highly anisotropic molecules such as n-heptane. We expect this conclusion to apply to all types of adsorbed molecules where van der Waals interactions govern the molecular interaction with the SWNT.
Carboxylation of carbon nanotubes (CNTs) is a byproduct of acid oxidation treatments that are applied routinely for several purposes including cleaning of CNTs and as a first step of functionalization procedures. In this study, we employ density functional calculations to study the atomic and electronic structures of side-wall —COOH-functionalized zigzag CNTs and elucidate their dependence on the tube diameter and the metallic or semiconducting character. Adsorption of a —COOH group shows a covalent bonding character associated with a small charge transfer from the CNT to the carboxyl groups. The amount of charge transfer, as well as the binding energy, of the carboxyl to the CNT decreases with the tube diameter. We find that it is thermodynamically more favorable for —COOH to adsorb in pairs on top of two neighboring carbon atoms that are bonded along the tube axis. This clustering effect becomes more favorable for larger diameter CNTs, because the difference in adsorption energy between isolated and pair carboxylation increases with tube diameter. Furthermore, we find that pair adsorption is not kinetically hindered and shows similar activation energies to that of the isolated adsorption. The electronic mechanism for the clustering effect is discussed.

Is there a Difference in Van Der Waals Interactions between Rare Gas Atoms Adsorbed on Metallic and Semiconducting Single-Walled Carbon Nanotubes?

Authors De-Li Chen, Lynn Mandeltort, Wissam A. Saidi, John T. Yates, Jr., Milton W. Cole, and J. Karl Johnson
The differences in the polarizabilities of metallic (M) and semiconducting (S) single-walled carbon nanotubes (SWNTs) might give rise to differences in adsorption potentials. We show from experiments and van der Waals—corrected density functional theory that the binding energies of Xe adsorbed on M- and S-SWNTs are nearly identical. Temperature programed desorption experiments of Xe on purified M- and S-SWNTs give similar peak temperatures, indicating that desorption kinetics and binding energies are independent of the type of SWNT. Binding energies computed from vdW-corrected density functional theory are in good agreement with experiments.

Non-additivity of polarizabilities and van der Waals C6 coefficients of fullerenes

Authors Joanna Kauczor, Patrick Norman and Wissam A. Saidi
We present frequency-dependent polarizabilities and C 6 dipole-dipole dispersion coefficients for a wide range of fullerene molecules including C60, C70, C78, C80, C82, and C84. The static and dynamic polarizabilities at imaginary frequencies are computed using time-dependent Hartree-Fock, B3LYP, and CAM-B3LYP ab initio methods by employing the complex linear polarization propagator and are subsequently utilized to determine the C 6 coefficients using the Casimir-Polder relation. Overall, the C60 and C70 average static polarizabilities ???(0) agree to better than 2% with linear-response coupled-cluster single double and experimental benchmark results, and the C 6 coefficient of C60 agrees to better than 1% with the best accepted value. B3LYP provides the best agreement with benchmark results with deviations less than 0.1% in ???(0) and C 6. We find that the static polarizabilities and the C 6 coefficients are non-additive, and scale, respectively, as N 1.2 and N 2.2 with the number of carbon atoms in the fullerene molecule. The exponent for C 6 power-dependence on N is much smaller than the value predicted recently based on a classical-metallic spherical-shell approximation of the fullerenes.

Understanding Structure and Bonding of Multilayered Metal-Organic Nanostructures

Authors David A. Egger, Victor G. Ruiz, Wissam A. Saidi, Tomas Bucko, Alexandre Tkatchenko, and Egbert Zojer
For organic and hybrid electronic devices, the physicochemical properties of the contained interfaces play a dominant role. To disentangle the various interactions occurring at such heterointerfaces, we here model a complex, yet prototypical, three-component system consisting of a Cu–phthalocyanine (CuPc) film on a 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) monolayer adsorbed on Ag(111). The two encountered interfaces are similar, as in both cases there would be no bonding without van der Waals interactions. Still, they are also distinctly different, as only at the Ag(111)–PTCDA interface do massive charge-rearrangements occur. Using recently developed theoretical tools, we show that it has become possible to provide atomistic insight into the physical and chemical processes in this comparatively complex nanostructure distinguishing between interactions involving local rearrangements of the charge density and long-range van der Waals attraction.
We use dispersion-corrected density functional theory (DFT) to resolve the role of polyvinylpyrrolidone (PVP) in the shape-selective synthesis of Ag nanostructures by probing the interaction of its 2-pyrrolidone (2P) ring with Ag(100) and Ag(111). We employ two different semiempirical methods for including van der Waals (vdW) interactions in DFT calculations: DFT+vdWsurf and DFT-D2. We find that DFT-D2, in its original parametrization, overestimates the Ag metal dispersion interaction and causes an unphysical herringbone-like reconstruction of Ag(100). This can be remedied in DFT-D2 by using modified vdW parameters for Ag that account for many-body screening effects. The results obtained using DFT-D2 with the modified parameters agree well with experiment and with DFT+vdWsurf results. We find that 2P binds more strongly to Ag(100) than Ag(111), consistent with experiment. We analyze the origins of the surface-sensitive binding and find that vdW attraction is stronger on Ag(111), but the direct chemical bonding of 2P is stronger on Ag(100). We also study the influence of strain on binding energies and find that tension tends to lower the vdW interaction with the surfaces, while increasing the direct chemical-bonding interaction, consistent with the d-band center model. Overall, our work indicates that strain has little impact on the structure-directing capabilities of PVP, which is consistent with the fact that strained, 5-fold twinned Ag nanowires have extensive {100} facets and relative small {111} facets.

In situ atomic-scale visualization of oxide islanding during oxidation of Cu surfaces

Authors Guangwen Zhou, Langli Luo, Liang Li, Jim Ciston, Eric A. Stach, Wissam A. Saidi and Judith C. Yang
Oxidation of Cu occurs via Cu2O islanding on an oxide wetting layer at a critical thickness of two atomic layers. The transition from 2D wetting-layer growth to 3D oxide islanding is driven energetically arising from the Cu–Cu2O interfacial interaction.

Ab initio atomistic thermodynamics study of the early stages of Cu(100) oxidation

Authors Wissam A. Saidi, Minyoung Lee, Liang Li, Guangwen Zhou, and Alan J. H. McGaughey
Using an ab initio atomistic thermodynamics framework, we identify the stable surface structures during the early stages of Cu(100) oxidation at finite temperature and pressure conditions. We predict the clean surface, the 0.25 monolayer oxygen-covered surface, and the missing-row reconstruction as thermodynamically stable structures in range of 100–1000 K and 10?15–105atm, consistent with previous experimental and theoretical results. We also investigate the thermodynamic stabilities of possible precursors to Cu2O formation including missing-row reconstruction structures that include extra on- or subsurface oxygen atoms as well as boundary phases formed from two missing-row nanodomains. While these structures are not predicted to be thermodynamically stable for oxygen chemical potentials below the nucleation limit of Cu2O, they are likely to exist due to kinetic hindrance.

Atomic and Electronic Structure of the BaTiO3(001) (5?×5?)R26.6° Surface Reconstruction

Authors John Mark P. Martirez, Erie H. Morales, Wissam A. Saidi, Dawn A. Bonnell, and Andrew M. Rappe
This contribution presents a study of the atomic and electronic structure of the (5?×5?)R26.6°surface reconstruction on BaTiO3 (001) formed by annealing in ultrahigh vacuum at 1300 K. Through density functional theory calculations in concert with thermodynamic analysis, we assess the stability of several BaTiO3 surface reconstructions and construct a phase diagram as a function of the chemical potential of the constituent elements. Using both experimental scanning tunneling microscopy (STM) and scanning tunneling spectroscopy measurements, we were able to further narrow down the candidate structures, and conclude that the surface is either TiO2-Ti3/5, TiO2-Ti4/5, or some combination, where Ti adatoms occupy hollow sites of the TiO2 surface. Density functional theory indicates that the defect states close to the valence band are from Ti adatom 3d orbitals (?1.4??eV below the conduction band edge) in agreement with scanning tunneling spectroscopy measurements showing defect states 1.56±0.11??eV below the conduction band minimum (1.03±0.09??eV below the Fermi level). STM measurements show electronic contrast between empty and filled states’ images. The calculated local density of states at the surface shows that Ti 3d states below and above the Fermi level explain the difference in electronic contrast in the experimental STM images by the presence of electronically distinctive arrangements of Ti adatoms. This work provides an interesting contrast with the related oxide SrTiO3, for which the (001) surface (5?×5?)R26.6° reconstruction is reported to be theTiO2 surface with Sr adatoms.

The role of van der Waals interactions in the adsorption of noble gases on metal surfaces

Authors De-Li Chen, W A Al-Saidi and J Karl Johnson
Adsorption of noble gases on metal surfaces is determined by weak interactions. We applied two versions of the nonlocal van der Waals density functional (vdW-DF) to compute adsorption energies of Ar, Kr, and Xe on Pt(111), Pd(111), Cu(111), and Cu(110) metal surfaces. We compared our results with data obtained using other density functional approaches, including the semiempirical vdW-corrected DFT-D2. The vdW-DF results show considerable improvements in the description of adsorption energies and equilibrium distances over other DFT based methods, giving good agreement with experiments. We also calculated perpendicular vibrational energies for noble gases on the metal surfaces using vdW-DF data and found excellent agreement with available experimental results. Our vdW-DF calculations show that adsorption of noble gases on low-coordination sites is energetically favored over high-coordination sites, but only by a few meV. Analysis of the two-dimensional potential energy surface shows that the high-coordination sites are local maxima on the two-dimensional potential energy surface and therefore unlikely to be observed in experiments; this provides an explanation of the experimental observations. The DFT-D2 approach with the standard parameterization was found to overestimate the dispersion interactions, and to give the wrong adsorption site preference for four of the nine systems we studied.

Coadsorption properties of CO2 and H2O on TiO2 rutile (110): A dispersion-corrected DFT study

Authors Dan C. Sorescu, Junseok Lee, Wissam A. Al-Saidi and Kenneth D. Jordan
Adsorption and reactions of CO2 in the presence of H2O and OH species on the TiO2 rutile (110)-(1×1) surface were investigated using dispersion-corrected density functional theory and scanning tunneling microscopy. The coadsorbed H2O (OH) species slightly increase the CO2adsorption energies, primarily through formation of hydrogen bonds, and create new binding configurations that are not present on the anhydrous surface.Proton transferreactions to CO2 with formation of bicarbonate and carbonic acid species were investigated and found to have barriers in the range 6.1–12.8 kcal/mol, with reactions involving participation of two or more water molecules or OH groups having lower barriers than reactions involving a single adsorbed water molecule or OH group. The reactions to form the most stable adsorbed formate and bicarbonate species are exothermic relative to the unreacted adsorbed CO2 and H2O (OH) species, with formation of the bicarbonate species being favored. These results are consistent with single crystal measurements which have identified formation of bicarbonate-type species following coadsorption of CO2 and water on rutile (110).
BNB is a challenging example of artifactual symmetry-breaking effects due to its susceptibility to a pseudo second-order Jahn–Teller interaction, which results in a structure with unequal BN bondlengths. The fixed-node diffusion Monte Carlo method is employed to calculate the potential curves along the symmetric and asymmetric stretching coordinates. With a multi-determinant wavefunction, the symmetric and asymmetric structures are found degenerate within statistical errors, with the asymmetric configuration lower in energy. The energy difference between the two structures is smaller than the multi-reference coupled cluster result obtained with four determinants, and supports previous conclusions that BNB has a floppy quasi-symmetric ground-state.

Dispersion-Corrected Density Functional Theory and Classical Force Field Calculations of Water Loading on a Pyrophyllite(001) Surface

Authors Guozhen Zhang, W. A. Al-Saidi, Evgeniy M. Myshakin, and Kenneth D. Jordan
Water adsorption on the (001) surface of pyrophyllite [Al(OH)(Si2O5)] was investigated using density functional theory (DFT) with dispersion corrections and force field calculations. The DFT calculations show that a water molecule can bind either to one or to two basal oxygen atoms of the surface, with adsorption energies varying from ?0.10 to ?0.19 eV depending on the binding configuration and binding site. Because the water–water interactions are stronger than the water–surface interactions, the energetically preferred structures with two or more molecules on the surface are clusters reminiscent of their gas-phase counterparts. The trend in water–surface binding energies with the number of water molecules obtained from force field calculations qualitatively agrees with that predicted by the dispersion-corrected DFT calculations. However, the force field calculations give a low-energy structural motif with a water molecule coordinated to a hydroxyl group associated with the octahedral layer of the pyrophyllite surface. This binding motif is found to be unstable in the DFT calculations.
Geometries, UV absorption bands, and resonance Raman (RR) cross sections of TNT and RDX are investigated using density functional theory (DFT) in conjunction with the Coulomb attenuated B3LYP exchange-correlation functional. The absorption and RR spectra are determined with use of vibronic (VB) theory, excited-state gradient, and complex polarizability (CPP) approximations. We examined low-energy isomers (two for TNT and four for RDX) whose energies differ by less than 1 kcal/mol, such that they would appreciably be populated at room temperature. The two TNT isomers differ by an internal rotation of the methyl group, while the four conformers of RDX differ by the arrangements of the nitro group relative to the ring. Our theoretical optical properties of the TNT and RDX isomers are in excellent agreement with experimental and recent CCSD-EOM results, respectively. For the two TNT isomers, the ultraviolet RR (UVRR) spectra are similar and in good agreement with recently measured experimental results. Additionally, the UVRR spectra computed using the excited-state and CPP approaches compare favorably with the VB theory results. On the other hand, the RR spectra of the RDX conformers differ from one another, reflecting the importance of the positioning of the NO2 groups with respect to the ring. In the gas phase or in solution, RDX would give a spectrum associated with a conformationally averaged structure. It is encouraging that the computed spectra of the conformers show similarities to recent measured RDX spectra in acetonitrile solution, and reproduce the 10-fold decrease in the absolute Raman cross sections of RDX compared to TNT for the observed 229 nm excitation. We show that in TNT and RDX vibrational bands that couple to NO2 or the ring are particularly resonance enhanced. Finally, the computed RDX spectra of the conformers present a benchmark for understanding the RR spectra of the solid-phase polymorphs of RDX.

An Assessment of the vdW-TS Method for Extended Systems

Authors W. A. Al-Saidi, Vamsee K. Voora, and Kenneth D. Jordan
The Tkatchenko–Scheffler vdW-TS method [Phys. Rev. Lett.2009,160;102, 073005] has been implemented in a plane-wave DFT code and used to characterize several dispersion-dominated systems, including layered materials, noble-gas solids, and molecular crystals. Full optimizations of the structures, including relaxation of the stresses on the unit cells, were carried out. Internal geometrical parameters, lattice constants, bulk moduli, and cohesive energies are reported and compared to experimental results.

Adsorption of Polyvinylpyrrolidone on Ag Surfaces: Insight into a Structure-Directing Agent

Authors W. A. Al-Saidi, Haijun Feng, and Kristen A. Fichthorn
We use density functional theory to resolve the role of polyvinylpyrrolidone (PVP) in the shape-selective synthesis of Ag nanostructures. At the segment level, PVP binds more strongly to Ag(100) than Ag(111) because of a surface-sensitive balance between direct binding and van der Waals attraction. At the chain level, correlated segment binding leads to a strong preference for PVP bind to Ag(100). Our study underscores differences between small-molecule and polymeric structure-directing agents.

Noble gases on metal surfaces: Insights on adsorption site preference

Authors De-Li Chen, W. A. Al-Saidi, and J. Karl Johnson
We use a nonlocal van der Waals density functional (vdW-DF) approach to reexamine the problem of why noble gases are experimentally observed to adsorb on low-coordination atop sites rather than on high-coordination hollow sites for several different metal surfaces. Previous calculations using density functional theory (DFT) within the local density approximation (LDA) ascribed the site preference to reduced Pauli repulsion at atop sites, largely due to reduced exchange repulsion within LDA-DFT. In contrast, our vdW-DF calculations show that site preference is not due to differences in the exchange repulsion at all, but rather the result of a delicate balance between the electrostatic and kinetic energies; surprisingly, exchange-correlation energies play a negligible role in determining site preference. In contrast to previous calculations, we find that experimental results cannot be explained in terms of binding energy differences between atop and hollow sites. Instead, we show that the hollow sites are transition states rather than minima on the two-dimensional potential energy surface, and therefore not likely to be observed in experiments. This phenomenon is quite general, holding for close-packed and non-close-packed metal surfaces. We show that inclusion of nonlocal vdW interactions is crucial for obtaining results in quantitative agreement with experiments for adsorption energies, equilibrium distances, and vibrational energies.

CO2 adsorption on TiO2(101) anatase: A dispersion-corrected density functional theory study

Authors Dan C. Sorescu, Wissam A. Al-Saidi and Kenneth D. Jordan
Adsorption,diffusion, and dissociation of CO2 on the anatase (101) surface were investigated using dispersion-corrected density functional theory. On the oxidizedsurface several different local minima were identified of which the most stable corresponds to a CO2 molecule adsorbed at a five-fold coordinated Ti site in a tilted configuration. Surfacediffusion is characterized by relatively small activation barriers. Preferential diffusion takes place along Ti rows and involves a cartwheel type of motion. The presence of a bridging oxygen defect or a surfaceinterstitial Ti atom allows creation of several new strong binding configurations the most stable of which have bent CO2 structures with simultaneous bonding to two surface Ti atoms. Subsurface oxygen vacancy or interstitial Ti defects are found to enhance the bonding of CO2 molecules to the surface. CO2dissociation from these defect sites is calculated to be exothermic with barriers less than 21 kcal/mol. The use of such defects for catalytic activation of CO2 on anatase (101) surface would require a mechanism for their regeneration.

Thermal Dehydration and Vibrational Spectra of Hydrated Sodium Metaborates

Authors Amy M. Beaird,Ping Li,Hilary S. Marsh,W. A. Al-Saidi,J. Karl Johnson,Michael A. Matthews, and Christopher T. Williams
Sodium metaborate hydrates are a class of compounds represented by the stoichiometry NaBO2xH2O. Recently, sodium metaborate has received attention as the byproduct of sodium borohydride hydrolysis, a reaction that is under consideration for hydrogen storage applications. The aim of this work was to understand the disposition of water in the crystal structure of hydrated sodium metaborates and to characterize the thermal stability and dehydration of the various hydrated species to optimize hydrogen storage efficiency as well as recyclability of the borate. Observations from a suite of analytical techniques including thermal analyses (thermogravimetric analysis/differential scanning calorimetry), X-ray diffraction, and Raman spectroscopy were correlated to characterize the dehydration mechanism of commercially available sodium metaborates, with an emphasis on the dihydrate (x = 2). A transformation from tetrahedrally coordinated boron to trigonal boron occurs when NaB(OH)4 (x = 2) is heated between 25 and 400 °C. The first dehydration to Na3[B3O5(OH)2] (x = 1/3) releases 5 mol of water between 83 and 155 °C. The final mole of water is released between 249 and 280 °C, and Na3B3O6 (x = 0) is formed. Raman spectra are reported for x = 2 and 1/3 for the first time. First-principles density functional theory was used to compute Raman spectra of the x = 1/3 and 2 material in order to assign the modes. We found reasonably good agreement between the experimentally measured and calculated vibrational frequencies.
The stacking parameters, lattice constants, bond lengths, and bulk moduli of pyrophyllite and montmorillonites (MMTs), with alkali and alkali earth metal ions, are investigated using density functional theory with and without dispersion corrections. For pyrophyllite, it is found that the inclusion of the dispersion corrections significantly improves the agreement of the calculated values of the lattice parameters and bulk modulus with the experimental values. For the MMTs, the calculations predict that the interlayer spacing varies approximately linearly with the cation radius. The inclusion of dispersion corrections leads to sizable shifts of the interlayer spacings to shorter values. In Li-MMT, compaction of the interlayer distance triggers migration of the Li ion into the tetrahedral sheet and close coordination with basal oxygen atoms. Analysis of electron density distributions shows that the isomorphic octahedral Al3+/Mg2+ substitution in MMT causes an increase of electron density on the basal oxygen atoms of the tetrahedral sheets.

Evaluation of Theoretical Approaches for Describing the Interaction of Water with Linear Acenes

Authors Glen R. Jenness, Ozan Karalti, W. A. Al-Saidi, and Kenneth D. Jordan 
The interaction of a water monomer with a series of linear acenes (benzene, anthracene, pentacene, heptacene, and nonacene) is investigated using a wide range of electronic structure methods, including several “dispersion”-corrected density functional theory (DFT) methods, several variants of the random phase approximation (RPA), DFT-based symmetry-adapted perturbation theory with density fitting (DF-DFT-SAPT), MP2, and coupled-cluster methods. The DF-DFT-SAPT calculations are used to monitor the evolution of the electrostatics, exchange-repulsion, induction, and dispersion contributions to the interaction energies with increasing acene size and also provide the benchmark data against which the other methods are assessed.

CO2 adsorption on TiO2(110) rutile: Insight from dispersion-corrected density functional theory calculations and scanning tunneling microscopy experiments

Authors Dan C. Sorescu, Junseok Lee, Wissam A. Al-Saidi and Kenneth D. Jordan
Adsorption of CO2 on the rutile(110) surface was investigated using dispersion-corrected density functional theory and scanning tunneling microscopy(STM). On the oxidizedsurface the CO2 molecules are found to bind most strongly at the five-fold coordinated Ti sites adopting tilted or flat configurations. The presence of bridging oxygen defects introduces two new adsorption structures, the most stable of which involves CO2 molecules bound in tilted configurations at the defect sites. Inclusion of dispersion corrections in the density functional theory calculations leads to large increases in the calculated adsorptionenergies bringing these quantities into good agreement with experimental data. The STM measurements confirm two of the calculated adsorption configurations.

Density functional study of PbTiO3 nanocapacitors with Pt and Au electrodes

Authors W. A. Al-Saidi and Andrew M. Rappe
We present an ab initio density functional study of ferroelectricity in single-domain PbTiO3-based nanocapacitors. We used density functional theory with the recently introduced PBEsol generalized-gradient exchange-correlation functional, which we found to give accurate properties of bulk ferroelectric (FE) materials. Pt and Au electrodes are used in our study to gain a thorough understanding of the electrode-oxide interfaces, and the role of the interfacial chemical bonding and charge transfer in stabilizing the FE polar phase. We found that the FE properties of the thin films depend not only on the electrode and the FE material but also on the electrode-perovskite termination (TiO2 vs PbO), exemplifying the key role of the interface in these systems. The critical thickness was found to be 24–28?Å. In addition, a Löwdin orbital analysis gives a detailed description of the distribution of charges in the system, and shows the importance of charge passivation by the electrodes in stabilizing the FE polar phase.

Assessment of the performance of common density functional methods for describing the interaction energies of (H2O)6 clusters

Authors F.-F. Wang, G. Jenness, W. A. Al-Saidi and K. D. Jordan
Localized molecular orbital energy decomposition analysis and symmetry-adapted perturbation theory (SAPT) calculations are used to analyze the two- and three-body interactionenergies of four low-energy isomers of (H2O)6 in order to gain insight into the performance of several popular density functionals for describing the electrostatic, exchange-repulsion, induction, and short-range dispersion interactions between water molecules. The energy decomposition analyses indicate that all density functionals considered significantly overestimate the contributions of charge transfer to the interactionenergies. Moreover, in contrast to some studies that state that density functional theory(DFT) does not include dispersion interactions, we adopt a broader definition and conclude that for (H2O)6 the short-range dispersion interactions recovered in the DFT calculations account about 75% or more of the net (short-range plus long-range) dispersion energies obtained from the SAPT calculations.
2009 and before

Fixed-node diffusion Monte Carlo study of the structures of m-benzyne

Author W. A. Al-Saidi and C. J. Umrigar
Diffusion Monte Carlo (DMC) calculations are performed on the monocyclic and bicyclic forms ofm-benzyne, which are the equilibrium structures at the CCSD(T) and CCSD levels of coupled clustertheory. We employed multiconfiguration self-consistent field trial wave functions which are constructed from a carefully selected eight-electrons-in-eight-orbitals complete active space [CAS(8,8)], with configuration state function coefficients that are reoptimized in the presence of a Jastrow factor. The DMC calculations show that the monocyclic structure is lower in energy than the bicyclic structure by 1.9(2)kcal?mole, which is in excellent agreement with the best coupled cluster results.

Eliminating spin contamination in auxiliary-field quantum Monte Carlo: Realistic potential energy curve of F2

Authors Wirawan Purwanto, W. A. Al-Saidi, Henry Krakauer and Shiwei Zhang
The use of an approximate reference state wave function in electronic many-body methods can break the spin symmetry of Born-Oppenheimer spin-independent Hamiltonians. This can result in significant errors, especially when bonds are stretched or broken. A simple spin-projection method is introduced for auxiliary-field quantum Monte Carlo (AFQMC) calculations, which yields spin-contamination-free results, even with a spin-contaminated ??r?. The method is applied to the difficult F2 molecule, which is unbound within unrestricted Hartree–Fock (UHF). With a UHF ??r?, spin contamination causes large systematic errors and long equilibration times in AFQMC in the intermediate, bond-breaking region. The spin-projection method eliminates these problems and delivers an accurate potential energy curve from equilibrium to the dissociation limit using the UHF ??r?. Realistic potential energy curves are obtained with a cc-pVQZ basis. The calculated spectroscopic constants are in excellent agreement with experiment.

Optimized norm-conserving Hartree-Fock pseudopotentials for plane-wave calculations

Authors W. A. Al-Saidi, E. J. Walter, and A. M. Rappe
We report Hartree-Fock (HF)-based pseudopotentials suitable for plane-wave calculations. Unlike typical effective core potentials, the present pseudopotentials are finite at the origin and exhibit rapid convergence in a plane-wave basis; the optimized pseudopotential method [A. M. Rappe et al., Phys. Rev. B 41, 1227 (1990)] improves plane-wave convergence. Norm-conserving HF pseudopotentials are found to develop long-range non-Coulombic behavior which does not decay faster than 1?r, and is nonlocal. This behavior, which stems from the nonlocality of the exchange potential, is remedied using a recently developed self-consistent procedure [J. R. Trail and R. J. Needs, J. Chem. Phys. 122, 014112 (2005)]. The resulting pseudopotentials slightly violate the norm conservation of the core charge. We calculated several atomic properties using these pseudopotentials, and the results are in good agreement with all-electron HF values. The dissociation energies, equilibrium bond lengths, and frequencies of vibration of several dimers obtained with these HF pseudopotentials and plane waves are also in good agreement with all-electron results.

Bond breaking with auxiliary-field quantum Monte Carlo

Authors W. A. Al-Saidi, Shiwei Zhang and Henry Krakauer
Bond stretching mimics different levels of electron correlation and provides a challenging test bed for approximate many-body computational methods. Using the recently developed phaseless auxiliary-field quantum Monte Carlo (AF QMC) method, we examine bond stretching in the well-studied molecules BH and N2 and in the H50 chain. To control the sign/phase problem, the phaseless AF QMC method constrains the paths in the auxiliary-field path integrals with an approximate phase condition that depends on a trial wave function. With single Slater determinants from unrestricted Hartree-Fock as trial wave function, the phaseless AF QMC method generally gives better overall accuracy and a more uniform behavior than the coupled cluster CCSD(T) method in mapping the potential-energy curve. In both BH and N2, we also study the use of multiple-determinant trial wave functions from multiconfiguration self-consistent-field calculations. The increase in computational cost versus the gain in statistical and systematic accuracy are examined. With such trial wave functions, excellent results are obtained across the entire region between equilibrium and the dissociation limit.
The authors present phaseless auxiliary-field (AF) quantum Monte Carlo (QMC) calculations of the ground states of some hydrogen-bonded systems. These systems were selected to test and benchmark different aspects of the new phaseless AF QMC method. They include the transition state of H+H2 near the equilibrium geometry and in the van der Walls limit, as well as the H2O, OH, and H2O2 molecules. Most of these systems present significant challenges for traditional independent-particle electronic structure approaches, and many also have exact results available. The phaseless AF QMC method is used either with a plane wave basis with pseudopotentials or with all-electron Gaussian basis sets. For some systems, calculations are done with both to compare and characterize the performance of AF QMC under different basis sets and different Hubbard-Stratonovich decompositions. Excellent results are obtained using as input single Slater determinant wave functions taken from independent-particle calculations. Comparisons of the Gaussian based AF QMC results with exact full configuration interaction show that the errors from controlling the phase problem with the phaseless approximation are small. At the large basis-size limit, the AF QMC results using both types of basis sets are in good agreement with each other and with experimental values.

Auxiliary-field quantum Monte Carlo study of first- and second-row post-d elements

Authors W. A. Al-Saidi, Henry Krakauer and Shiwei Zhang
A series of calculations for the first- and second-row post-d elements (Ga--Br and In--I) are presented using the phaseless auxiliary-field quantum Monte Carlo (AF QMC) method. This method is formulated in a Hilbert space defined by any chosen one-particle basis and maps the many-body problem into a linear combination of independent-particle solutions with external auxiliary fields. The phase/sign problem is handled approximately by the phaseless formalism using a trial wave function, which in our calculations was chosen to be the Hartree-Fock solution. We used the consistent correlated basis sets of Peterson et al. [J. Chem. Phys.119, 11099 (2003);119, 11113 (2003)], which employ a small-core relativistic pseudopotential. The AF QMC results are compared with experiment and with those from density functional (generalized gradient approximation and B3LYP) and CCSD(T) calculations. The AF QMC total energies agree with CCSD(T) to within a few millihartrees across the systems and over several basis sets. The calculated atomic electron affinities,ionization energies, and spectroscopic properties of dimers are, at large basis sets, in excellent agreement with experiment.

Auxiliary-field quantum Monte Carlo calculations of molecular systems with a Gaussian basis

Authors W. A. Al-Saidi, Shiwei Zhang and Henry Krakauer
We extend the recently introduced phaseless auxiliary-field quantum Monte Carlo (QMC) approach to any single-particle basis and apply it to molecular systems with Gaussian basis sets. QMC methods in general scale favorably with the system size as a low power. A QMC approach with auxiliary fields, in principle, allows an exact solution of the Schrödinger equation in the chosen basis. However, the well-known sign/phase problem causes the statistical noise to increase exponentially. The phaseless method controls this problem by constraining the paths in the auxiliary-field path integrals with an approximate phase condition that depends on a trial wave function. In the present calculations, the trial wave function is a single Slater determinant from a Hartree-Fock calculation. The calculated all-electron total energies show typical systematic errors of no more than a few millihartrees compared to exact results. At equilibrium geometries in the molecules we studied, this accuracy is roughly comparable to that of coupled cluster with single and double excitations and with noniterative triples [CCSD(T)]. For stretched bonds in H2O, our method exhibits a better overall accuracy and a more uniform behavior than CCSD(T).

Auxiliary-field quantum Monte Carlo study of TiO and MnO molecules

Authors W. A. Al-Saidi, Henry Krakauer, and Shiwei Zhang
Calculations of the binding energy of the transition-metal oxide molecules TiO and MnO are presented, using a recently developed phaseless auxiliary-field quantum Monte Carlo approach. This method maps the interacting many-body problem onto a linear combination of noninteracting problems by a complex Hubbard-Stratonovich transformation, and controls the phase and sign problem with a phaseless approximation relying on a trial wave function. It employs random walks in Slater determinant space to project the ground state of the system, and allows use of much of the same machinery as in standard density functional theory calculations using the plane-wave basis and nonlocal pseudopotentials. The calculations used a single Slater determinant trial wave function obtained from a density functional calculation, with no further optimization. The calculated binding energies are in good agreement with experiment and with recent diffusion Monte Carlo results. Together with previous results for sp-bonded systems, the present study indicates that the phaseless auxiliary-field method is a robust and promising approach for the study of correlation effects in real materials.

Quantum simulations of realistic systems by auxiliary fields

Authors Shiwei Zhang, , Henry Krakauer, Wissam A. Al-Saidi, Malliga Suewattana
To treat interacting quantum systems, it is often crucial to have accurate calculations beyond the mean-field level. Many-body simulations based on field-theoretical approaches are a promising tool for this purpose and are applied in several sub-fields of physics, in closely related forms. An major difficulty is the sign or phase problem, which causes the Monte Carlo variance to increase exponentially with system size. We address this issue in the context of auxiliary-field simulations of realistic electronic systems in condensed matter physics. We show how to use importance sampling of the complex fields to control the phase problem. An approximate approach is formulated with a trial determinant to constrain the paths in field space and completely eliminate the growth of the noise. For ab initio electronic structure calculations, this gives a many-body approach in the form of a “coherent” superposition of mean-field calculations, allowing direct incorporation of state-of-the-art technology from the latter (non-local pseudopotentials; high quality basis sets, etc.). In our test calculations, single Slater determinants from density functional theory or Hartree–Fock calculations were used as trial wave functions, with no additional optimization. The calculated dissociation energies of various molecules and the cohesive energy of bulk Si are in excellent agreement with experiment and are comparable to or better than the best existing theoretical results.

Quantum Monte Carlo study of a disordered 2D Josephson junction array

Authors W. A. Al-Saidi and D. Stroud
We have studied the superconducting–insulating phase transition in a disordered two-dimensional Josephson junction array, using quantum Monte Carlo techniques. We consider disorder in both the capacitive energies and in the values of the offset charges. The calculated phase diagram shows that the lobe structure of the phase diagram disappears for sufficiently strong disorder in the offset charge. Our results agree quite well with the previous calculations carried out using a mean-field approximation.
We have numerically investigated the dynamics of vortices in a clean layered superconductor placed in a perpendicular magnetic field. We describe the energetics using a Ginzburg-Landau free-energy functional in the lowest-Landau-level approximation. The dynamics are determined using the time-dependent Ginzburg-Landau approximation, and thermal fluctuations are incorporated via a Langevin term. The c-axis conductivity at nonzero frequencies, as calculated from the Kubo formalism, shows a strong but not divergent increase as the melting temperature TM is approached from above, followed by an apparently discontinuous drop at the vortex-lattice freezing temperature. The discontinuity is consistent with the occurrence of a first-order freezing. The calculated equilibrium properties agree with previous Monte Carlo studies using the same Hamiltonian. We briefly discuss the possibility of detecting this fluctuation conductivity experimentally.

Phase phonon spectrum and melting in a quantum rotor model with diagonal disorder

Authors W. A. Al-Saidi and D. Stroud
We study the zero temperature (T=0) quantum rotor model with on-site disorder in the charging energy. Such a model may serve as an idealized Hamiltonian for an array of Josephson-coupled small superconducting grains or superfluid 4He in a disordered environment. In the approximation of small-amplitude phase fluctuations, the Hamiltonian maps onto a system of coupled harmonic oscillators with on-site disorder. We study the effects of disorder in this harmonic regime, using the coherent potential approximation, obtaining the density of states and the lifetimes of the spin-wave-like excitations for several choices of the parameters which characterize the disorder. Finally, we estimate the parameters characterizing the T=0 quantum melting of the phase order, using a suitable Lindemann criterion.

Several small Josephson junctions in a resonant cavity: Deviation from the Dicke model

Authors W. A. Al-Saidi and D. Stroud
We have studied quantum mechanically a system of several small identical Josephson junctions in a lossless single-mode cavity for different initial states, under conditions such that the system is at resonance. This system is analogous to a collection of identical atoms in a cavity, which is described under appropriate conditions by the Dicke model. We find that our system can be well approximated by a reduced Hamiltonian consisting of two levels per junction. The reduced Hamiltonian is similar to the Dicke Hamiltonian, but contains an additional term resembling a dipole-dipole interaction between the junctions. This extra term can be understood as a natural consequence of degenerate second-order (Löwdin) perturbation theory. For typical, physically reasonable values of the junction-cavity coupling, we find that this perturbation treatment is an adequate way to include the junction energy levels beyond the lowest two. As in the Dicke model, we find that, when N junctions are present in the cavity, the junction-cavity interaction is enhanced by N???, with a corresponding decrease in the Rabi oscillation period. We find that this enhancement survives even if the junctions differ slightly from one another, as expected in a realistic system. Since coherence effects thus reduce the Rabi period, it may become smaller than the decoherence time due to dissipation, making these oscillations observable.

Eigenstates of a small Josephson junction coupled to a resonant cavity

Authors W. A. Al-Saidi and D. Stroud
We carry out a quantum-mechanical analysis of a small Josephson junction coupled to a single-mode resonant cavity. We find that the eigenstates of the combined junction-cavity system are strongly entangled only when the gate voltage applied at one of the superconducting islands is tuned to certain special values. One such value corresponds to the resonant absorption of a single photon by Cooper pairs in the junction. Another special value corresponds to a two-photon absorption process. Near the single-photon resonant absorption, the system is accurately described by a simplified model in which only the lowest two levels of the Josephson junction are retained in the Hamiltonian matrix. We noticed that this approximation does not work very well as the number of photons in the resonator increases. Our system shows also the phenomenon of “collapse and revival” under suitable initial conditions, and our full numerical solution agrees with the two level approximation result.