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Modern electronic and optoelectronic devices rely on the development of the complementary pair of n-type and p-type semiconductors. However, it is often seen that n-type semiconductors are easier to realize and offer superior performances than their p-type counterparts, with p-type semiconductors showing much lower hole mobility and inefficient carrier doping. Here, by using first-principles studies, we demonstrate that lattice strain engineering can be a universal approach to enhance the hole mobility and dopability in p-type semiconductors. A broad class of p-type semiconductors, including anion p orbital derived valence band compounds (nitrides, oxides, halides, and chalcogenides), s orbital based post-transition metal oxides (e.g., SnO), and d-orbital based transition metal oxides (e.g., NiO), have been applied on strain to demonstrate their valence band modulation ability for the purpose of increasing the hole mobility and p-type dopability. We show that compressive lattice strain generally results in an upshifted valence band edge and reduced effective hole mass, leading to enhanced p-type dopability and increased hole mobility. Our work highlights strain engineering as a universal and effective approach for achieving better performed p-type compound semiconductors. [ABSTRACT FROM AUTHOR]

The electronic properties of nitrogen doped Mo(110) surfaces were investigated using the first-principles pseudopotential method within the local density approximation in order to determine the effect of doping on the work function. Nitrogen doping was modeled by adsorbing N in both surface and subsurface positions. Surface adsorption of nitrogen was found to increase the work function by as much as 2 eV due to the negative surface dipole induced by the electronegativity of nitrogen. Subsurface doping of nitrogen is energetically similar to surface adsorption, but has a small effect on the work function and only when within the first two to three surface Mo layers. [ABSTRACT FROM AUTHOR]

We present the results of a systematic study using the density functional theory (within the local density approximation) of the effect of composition on the self-diffusion of silicon and germanium in silicon-germanium alloys diffusing by a vacancy mechanism. The composition dependence of the vacancy formation energy was calculated. A database of ab initio migration energy barriers for vacancy migration in different local environments was systematically developed by considering the effect of the first nearest neighbor sites explicitly and the effect of the other sites by a mean field approximation. A kinetic Monte Carlo simulation based on the migration energy barrier database was performed to determine the dependence (on the composition) of the activation energy for the diffusion of Si and Ge in Si[sub 1-x]Ge[sub x]. A detailed study of the variation of the correlation factor with composition and temperature in Si[sub l-x]Ge[sub x] was performed using the results of the kinetic Monte Carlo simulation. These analyses constitute essential building blocks to understand the mechanism of vacancy mediated diffusion processes at the microscopic level. [ABSTRACT FROM AUTHOR]

Monolayer doping (MLD) is a promising technique for creating ultra shallow junctions (USJs). Here, a novel self assembled monolayer (SAM) grafting technique is proposed through a single oxygen atom capable of MLD. Consequently, this approach can use simple forms of alkylphosphonic acids and avoid carbon contamination altogether during the doping process. In this paper, density functional theory (DFT) is used to explore the way how alkylphosphonic acid molecules can in just one chemical step be grafted on H-terminated Si(111). A maximum coverage of alkylphosphonic acids is found at 2/3 due to steric constrain forces. It is further demonstrated by means of in situ infrared spectroscopy and DFT calculations that the weak link of an alkylphosphonic acid, such as octadecylphosphonic acid (ODPA), is the P-C bond, with typical release of the carbon ligand around 500 °C. Finally, after release of the carbon ligand, an unsaturated electron configuration is driving force for the phosphorous to start the MLD process. [ABSTRACT FROM AUTHOR]

We employ molecular dynamic simulations to study thereductionprocess of graphene oxide (GO) in a chemically active environmentenriched with hydrogen. We examine the concentration and pressureof hydrogen gas as a function of temperature in which abstractionof oxygen is possible with minimum damage to C-sp2bonds,hence preserving the integrity of the graphene sheet. Through thesestudies we find chemical pathways that demonstrate beneficiary mechanismsfor the quality of graphene including formation of water as well assuppression of carbonyl pair holes in favor of hydroxyl and epoxideformation facilitated by hydrogen gas in the environment. [ABSTRACT FROM AUTHOR]

Abstract: Nitrogen-doped (N-doped) single wall nanotubes (SWNTs) have recently been shown to have good catalytic reactivity towards oxygen reduction reaction (ORR). We used density functional theory calculations to explore reaction paths for facile oxygen dissociation on modified SWNTs, including nitrogen doping, Stone–Wales (SW) defects, and a combination of these two. It was found that oxygen dissociation is facilitated on carbon atoms neighboring a nitrogen dopant, with the dissociation barrier reduced from 2eV to 0.68eV. The activation barrier can be further reduced to 0.03eV in the vicinity of a N-doped SW defect. The reduction in barrier height is explained through the local density of states analysis. [Copyright &y& Elsevier]

Abstract: First principles calculations are performed to study the electronic structure and related properties of the Nb–W interface within NbW multilayer metal gate electrode. It is found that the surface work functions of NbW multilayers are located between those of pure Nb and W surfaces due to the strain effect from the lattice dismatch. The interface energy of Nb(110)/W(110) is calculated to be −0.17J/m2, implying that the Nb–W interface is energetically favorable. Additionally, it is found that the interface dipoles are formed in the interface region to balance the work function difference of W and Nb, and the dipole layers are mainly due to the contributions of interface atoms with a very small contribution from the second atomic layers from the interface. It is of interest to note that the interface W atoms give out a little bit more electrons than the interface Nb atoms during the formation of the interface dipole and that these changes are closely related to the DOS changes. [Copyright &y& Elsevier]

Abstract: We present a new technique to extend the embedded-atom method (EAM) for the simulations of non-bulk systems down to the atomic cluster level. To overcome the limitation of the traditional bulk-fit EAM interatomic potentials, bond characteristics from first-principles calculations are systematically included by introducing a local structure dependent prefactor with three additional parameters to the conventional EAM many-body term. The additional parameters improve the local potential landscape virtually for the entire range of atomic configuration space in a quantitative sense. The proposed scheme is applied to two different EAM function sets and validated for both bulk and non-bulk environments in elemental platinum. The obtained material properties, including the binding energies of Pt particles and the Pt adatom diffusion barrier on the Pt(111) surface, show a significant improvement over the conventional EAM formalism. [Copyright &y& Elsevier]

NO2 gas adsorption, diffusion, and reaction on a single walled carbon nanotube (SWNT) surface are studied using ab initio simulations. The small diffusion barriers of NO2 on SWNT surface suggest that NO2 molecules can produce NO and NO3 through chemical reactions. From the estimation of diffusion barriers and binding energies of NO2, NO, and NO3 on a SWNT surface, we show that NO3 is the most likely long-lived species on SWNT. This finding enables us to explain why the experimental recovery times of NO2 gas sensors have been measured to be as long as 12 h. [Copyright &y& Elsevier]

Materials problems of alternative high-k dielectric oxides for future metal–oxide–semiconductor field effect transistor (MOSFET) gate oxide application are examined from first-principles modeling perspective. We have analyzed the relationship between local atomic structures and the corresponding ionic contributions to static dielectric constant for metal oxides, and the analysis shows the importance of stabilizing high oxygen coordination structures for metal atoms. Based on the local atomic structure analysis, metal silicates and aluminate are expected to provide a possible solution of continuously scaling dielectric constant well above the values of amorphous silica (ϵ0=3.9) and alumina (ϵ0=9). [Copyright &y& Elsevier]

Thermoelectric materials play a crucial role in converting heat into electricity, offering significant potential for applications in waste heat recovery and cooling. Herein, an innovative approach that combines an organic–inorganic hybrid superlattice structure with nanocrystal‐amorphous composite nanolayers is introduced. The nanocrystal‐amorphous composite enhances the Seebeck coefficient resulting in a notable twofold improvement in the power factor. The superlattice, alternating self‐assembled organic monolayers and inorganic nanolayers, effectively reduces lattice thermal conductivity by creating multiple interfaces that scatter phonons effectively. The integration of the nanocrystal‐amorphous composite nanolayers into the superlattice provides a dual advantage, simultaneously boosting the power factor and suppressing thermal conductivity. This synergistic effect leads to exceptional thermoelectric performance in the 4‐mercaptophenol/Sb2Te3 superlattice, with achieved figure of merit (ZT) values of 3.48 at 300 K and reaching a peak ZT value exceeding 4.0 at 400 K while surpassing 2.5 over the temperature range from 300 to 500 K. These results suggest that this innovative approach paves the way for the development of highly efficient thermoelectric materials, propelling efforts toward more energy‐efficient and environmentally friendly solutions. [ABSTRACT FROM AUTHOR]

First principles calculation reveals that the Mo–W interface dipole is formed due to an equal loss of electrons of interface atoms, and the interface formation is energetically unfavorable with positive interface energy, signifying a tendency of phase separation of Mo and W atoms. Calculation also shows that Mo surface segregation has an important effect in reducing the work function of Mo–W alloys, and that after Mo surface segregation, the Mo–W work function becomes relatively stable when W is between 0–70 at. %. Interestingly, such a composition range seems good for work function modulation of metal gate Mo–W system. [ABSTRACT FROM AUTHOR]

The tensile yielding of multiwall carbon nanotubes (MWCNTs) has been studied using molecular-dynamics simulations and a transition state theory based model. We find a strong dependence of the yielding on the strain rate. A critical strain rate has been predicted above/below which yielding strain of a MWCNT is larger/smaller than that of the corresponding single-wall carbon nanotubes (CNTs). At an experimentally feasible strain rate of l%/h and T = 300 K, the yield strain of a MWCNT is estimated to be about 3%-4% higher than that of an equivalent single-wall CNT. This is in good agreement with recent experimental observations. [ABSTRACT FROM AUTHOR]

A high-infrared (IR) emissivity is required to dissipate heat effectively from high-temperature surfaces even in a near-vacuum environment. This radiative cooling capability is essential for the emissive layer of EUV pellicles operating under high vacuum conditions with increasing EUV source powers to ensure thermomechanical stability. We compute the IR emissivity of metal films by combining the classical oscillator model and DFT calculations. Based on the calculations, we predict a positive correlation between the electrical resistivity and IR emissivity of metal films, which is consistent with a recent experiment on Ru thin films. Our findings can provide a practical indicator for achieving high IR emissivity of metallic thin films based on electrical measurements. This emissivity–resistivity correlation can provide an effective way to search a large material space, and choose and synthesize a highly emissive layer of EUV pellicles. In this study, we computed the infrared absorbance of Ru thin films through the integration of the classical Drude-Lorentz oscillator model and DFT calculations. According to the Kirchhoff's law of thermal radiation, the IR absorbance is equal to the IR emittance. Based on the calculated results, we propose a positive correlation between IR emissivity and electrical resistivity. [ABSTRACT FROM AUTHOR]

First principles density functional theory calculations are carried out to investigate the scaling trends of band offsets at model silicon/zirconium silicate interfaces. Owing to the d character of zirconium silicate conduction bands, the band gap and band offset are shown to decrease as the zirconium concentration is increased. Since the valence band character of silicates remains unchanged relative to SiO[sub 2], the conduction band offset alone decreases, leading to increasingly asymmetric band offsets at higher zirconium concentrations. The use of charge transfer dipoles at the interface is investigated as a possible remedy to restore the band offset symmetry by shifting the silicate bands relative to the silicon bands. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

First principles calculation shows that the Nb and W atoms have a tendency to mix instead of phase separate in the Nb–W bulk within the entire composition range, and the interaction between Nb and W atoms has a nonlinear effect on the density of states of the Nb–W bulk at the Fermi level. Calculation also reveals that the surface segregation of Nb atoms is energetically favorable with an energy decrease of 0.4–0.55 eV/atom, and the segregation has important effects on the electronic structures of Nb–W surfaces. Interestingly, the work function of Nb–W phases is insensitive to the composition as well as the Nb surface segregation when W is in the range of 0–60 at. %, and over this composition range, the work function remains relatively stable. [ABSTRACT FROM AUTHOR]

Density functional theory simulations of HfO2/SiO2 interfaces predict the presence of midgap states associated with nonbonding Hf d electrons which result from the reduced oxygen coordination of near-interface Hf ions. These states are expected to be unoccupied in actual device structures, producing a high density of positive fixed charge during the operation of Si field effect devices containing high permittivity HfO2 dielectrics. Our results further demonstrate how the segregation of electronegative species to the HfO2/SiO2 interface can remove gap states by accepting the nonbonding electrons. [ABSTRACT FROM AUTHOR]

Point defect mediated diffusion of impurities in crystalline materials involves a sequence of several processes, which are repeated in varying combinations a multiple number of times. The concept of “activation energy” has been borrowed from simple chemical reactions, where the reactants are postulated to form an activated complex before decomposing into products. While ideas such as the smallest rate (or the rates of a select few “important” processes) being the rate determining step and hence the overall activation energy may be applicable in the case of chemical reactions that are sequential, such ideas are shown to be too simplistic to be applicable to describe diffusion in the crystalline phase. In this paper, we present a systematic scheme to arrive at the macroscopic activation energy in terms of the energy barriers for the constituent microscopic processes. We apply this scheme to the case of vacancy mediated diffusion of impurities in a diamond lattice. We present results of numerical verification of the scheme performed by kinetic Monte Carlo simulations based on the energy barriers obtained using the density functional theory within the local density approximation. We then present observations on the dependence of the macroscopic “migration energy” on the energy barriers for the constituent microscopic processes. As an illustration of how the energy barriers for the microscopic processes can be affected, we present first principles calculation of the effect of biaxial strain on these energy barriers. [ABSTRACT FROM AUTHOR]

Conventional mechanical or chemical exfoliation approach of 2D material synthesis is largely dependent on the inherent structure of the parent material, i.e., whether it is a layered structure or a 3D bulk structure with embedded 2D substructures. A recent experiment demonstrated that unprecedented atomically thin metal oxides without bulk layered structures can be synthesized by using liquid metals. Supported by an experimental realization of atomically thin W layers through the metal atomic layer deposition method, we propose a new type of transition metal (TM)-based 2D materials that can be stabilized at the oxide interfaces with oxide substrates and overlayers. Based on the ab initio density functional theory calculations, we show that most of the TM elements can form unprecedented atomically thin 2D materials by the surface oxygen passivation, which is available from the oxide substrate and the overlayer. The stabilized 2D TM layers show diverse electronic and magnetic properties. Our results suggest a novel way to extend 2D materials study and a possible application of those 2D TM layers embedded in oxides. [ABSTRACT FROM AUTHOR]

Hafnium–zirconium oxide (HZO) thin films are of interest due to their ability to form ferroelectric (FE) and antiferroelectric (AFE) oxide phases. Density functional theory is employed to elucidate the stabilization mechanisms of both FE HZO thin films and AFE ZrO2 films. The FE orthorhombic phase is primarily stabilized by in-plane tensile strain, which spontaneously occurs during the synthesis process, and this is more effective for HZO than HfO2. Layer-by-layer stack models and core-matrix three-dimensional models of the polymorphs reveal that the electrostatic component of interfacial free energy can play a critical role in the formation of the AFE tetragonal phase in ZrO2 and the "wake-up" effect for FE HZO. [ABSTRACT FROM AUTHOR]

The poor reversibility of Zn metal anodes arising from water‐induced parasitic reactions poses a significant challenge to the practical applications of aqueous zinc‐ion batteries (AZIBs). Herein, a novel quasi‐solid‐state "water‐in‐swelling‐clay" electrolyte (WiSCE) containing zinc sulfate and swelling clay, bentonite (BT), is designed to enable highly reversible Zn metal anodes. AZIB full cells based on the WiSCE exhibit excellent cyclic stability at various current densities, long shelf life, low self‐discharge rate, and outstanding high‐temperature adaptability. Particularly, the capacity of WiSCE‐based AZIB full cells retains 90.47% after 200 cycles at 0.1 A g−1, 96.64% after 2000 cycles at 1 A g−1, and 88.29% after 5000 cycles at 3 A g−1. Detailed density functional theory calculations show that strong hydrogen bonds are formed between BT and water molecules in the WiSCE. Thus, water molecules are strongly confined by BT, particularly within the interlayers, which significantly inhibits water‐induced parasitic reactions and greatly improves cyclic stability. Compared to the state‐of‐the‐art "water‐in‐salt" electrolytes, the WiSCE can provide a significantly higher capacity at the full‐cell level with a substantially reduced cost, which is promising for the design of next‐generation high‐performance AZIBs. This work provides a new direction for developing cost‐competitive AZIBs as alternatives to grid‐scale energy storage. [ABSTRACT FROM AUTHOR]

The development of high-performance p-type oxides with high hole mobility and a wide bandgap is critical for the applications of metal oxide semiconductors in vertically integrated CMOS devices [Salahuddin et al., Nat. Electron. 1, 442 (2018)]. Sn2+-based oxides such as SnO and K2Sn2O3 have recently been proposed as high-mobility p-type oxides due to their relatively low effective hole masses, which result from delocalized Sn s-orbital character at the valence band edge. Here, we introduce a promising ternary Sn-O-X compound, Ta2SnO6, which exhibits strong valence band dispersion and a large bandgap. In order to evaluate the performance of this oxide as a p-type semiconductor, we perform first-principles calculations of the phonon-limited room-temperature carrier mobilities in SnO, SnO2, and Ta2SnO6. Electron relaxation time is evaluated, accounting for the scatterings from acoustic deformation potentials and polar optical phonons (POP), within the isotropic and dispersionless approximation. At room temperature, the electron/hole mobilities in a given material (SnO, SnO2, and Ta2SnO6) are found to be limited by POP scattering. SnO2 shows high room-temperature electron mobility of 192 cm2/(V s), while SnO and Ta2SnO6 exhibit impressive hole mobilities, with the upper limit at 60 and 33 cm2/(V s), respectively. We find that carrier effective mass largely accounts for the differences in mobility between these oxides with correspondingly different POP scattering rates. The theoretically predicted intrinsic mobilities of each material will provide the upper limit to the real mobilities for their device applications. Our findings also suggest a necessity of further investigation to identify even higher mobility p-type oxides with smaller hole effective masses. [ABSTRACT FROM AUTHOR]

Short-range atomic order in semiconductor alloys is a relatively unexplored topic that may promote design of new materials with unexpected properties. Here, local atomic ordering is investigated in Ge–Sn alloys, a group-IV system that is attractive for its enhanced optoelectronic properties achievable via a direct gap for Sn concentrations exceeding ≈10 at. %. The substantial misfit strain imposed on Ge–Sn thin films during growth on bulk Si or Ge substrates can induce defect formation; however, misfit strain can be accommodated by growing Ge–Sn alloy films on Ge nanowires, which effectively act as elastically compliant substrates. In this work, Ge core/Ge1−xSnx (x ≈ 0.1) shell nanowires were characterized with extended x-ray absorption fine structure (EXAFS) to elucidate their local atomic environment. Simultaneous fitting of high-quality EXAFS data collected at both the Ge K-edge and the Sn K-edge reveals a large (≈ 40%) deficiency of Sn in the first coordination shell around a Sn atom relative to a random alloy, thereby providing the first direct experimental evidence of significant short-range order in this semiconductor alloy system. Comparison of path length data from the EXAFS measurements with density functional theory simulations provides alloy atomic structures consistent with this conclusion. [ABSTRACT FROM AUTHOR]

Amination of surfaces is useful in a variety of fields, ranging from device manufacturing to biological applications. Previous studies of ammonia reaction on silicon surfaces have concentrated on vapor phase rather than wet chemical processes, and mostly on clean Si surfaces. In this work, the interaction of liquid and vapor-phase ammonia is examined on three types of oxide-free surfaces – passivated by hydrogen, fluorine (1/3 monolayer) or chlorine – combining infrared absorption spectroscopy, X-ray photoelectron spectroscopy, and first-principles calculations. The resulting chemical composition highly depends on the starting surface; there is a stronger reaction on both F- and Cl-terminated than on the H-terminated Si surfaces, as evidenced by the formation of Si-NH 2 . Side reactions can also occur, such as solvent reaction with surfaces, formation of ammonium salt by-products (in the case of 0.2 M ammonia in dioxane solution), and nitridation of silicon (in the case of neat and gas-phase ammonia reactions for instance). Unexpectedly, there is formation of Si-H bonds on hydrogen-free Cl-terminated Si(111) surfaces in all cases, whether vapor phase of neat liquid ammonia is used. The first-principles modeling of this complex system suggests that step-edge surface defects may play a key role in enabling the reaction under certain circumstances, despite the endothermic nature for Si-H bond formation. [ABSTRACT FROM AUTHOR]

We have performed first-principle calculations to explore the possibility of synthesizing atomically thin transition metal (TM) layers. Buckled structures as well as planar structures of elemental 2D TM layers result in significantly higher formation energies compared with sp-bonded elemental 2D materials with similar structures, such as silicene and phosphorene. It is shown that the TM layers can be stabilized by surface passivation with HS, C6H5S2, or O, and O passivation is most effective. The surface oxygen passivation can improve stability leading to thermodynamically stable TM monolayers except Au, which is the most non-reactive metal element. Such stabilized TM monolayers also show an electronic structure transition from metallic state of free-standing TM layer to semiconducting O-passivated Mo and W monolayers with band gaps of 0.20-1.38 eV. [ABSTRACT FROM AUTHOR]

Amine termination of surfaces constitutesa core platform for fieldsas diverse as microelectronics and bioengineering, and for nanotechnologyin general. Diamines are particularly attractive for surface aminationbecause unlike ammonia or simple amine molecules, they have a metalchelating capability useful in fabricating heterostructures. Theycan act as a linker molecule between inorganic electronic materialsand biomolecules or photoactive quantum dots for applications in microelectronic,photonics, and biosensing. In contrast to ammonia modification ofsilicon surfaces, the direct grafting of diamine on silicon surfaceshas been less explored. In this work, the attachment of liquid andvapor-phase ethylenediamine (EDA) on three types of oxide-free (H-,1/3 ML F-, and Cl-terminated) Si(111) surfaces is therefore examinedby infrared absorption spectroscopy and X-ray photoelectron spectroscopyin conjunction with first-principle calculations. We find that EDAchemisorption is only possible on 1/3 ML F- and Cl-terminated Si(111)surfaces: EDA only physisorbs on H-terminated Si(111) surfaces. OnCl-terminated Si(111) surfaces, EDA molecules adsorb in a mixtureof monodentate and bridging configurations (chemical reaction of bothEDA end groups), while on 1/3 ML F-terminated Si(111) surfaces theadsorption occurs primarily at one end of the molecule. EDA reactionwith Cl-terminated Si(111) surfaces is also characterized by completeremoval of Cl and partial Si–H (∼25% ML) formation onthe surface. This unexpected Si–H product suggests that a proton–chlorineexchange may take place, with the endothermic barrier possibly reducedvia concerted chemical reactions after an initial attachment of EDAto the surface. [ABSTRACT FROM AUTHOR]

Partially reduced graphite oxide (GOpr) inherits expanded interlayer distance from the parent, graphite oxide (GO), and electronic conductivity from the grandparent, graphite. Indebted to the dual properties, GOpr shows unique behavior so-called electrochemical activation in organic electrolytes; when GOpr is polarized over a certain electrode potential, graphitic layers in GOpr are injected by solvated ions and become ion adsorbing sites permanently. Resultant gravimetric or volumetric capacitance (up to 200 F g −1 or 150 F ml −1 ) exceeds that of commercially available activated carbon electrodes for electric double-layer capacitor (EDLC). Previous literatures have reported that larger interlayer distance ( d ) lowers the on-set potential of electrochemical activation of graphitic carbons. However, the reason of this phenomenon has not yet been studied. In this paper, we combined experimental and theoretical approaches to reveal the effect of interlayer distance and van der Waals energy ( U vdW ) on the electrochemical activation of graphitic carbon. More specifically, we compared the energy for the electrochemical activation ( U EA ) and the van der Waals energy with respect to the interlayer distance. For this purpose, we devised an experimental method to measure U EA from cyclic voltammogram, and this method was verified by in-situ electrochemical dilatometry. The theoretical value of U vdW was calculated by semi-empirical density functional theory (DFT) and those experimental and theoretical values were linearly in accordance with each other. This finding signifies that (1) the electrochemical activation of graphitic carbon occurs by overcoming van der Waals energy to expand the interlayer distance, (2) and the on-set potential for electrochemical activation is varied by the interlayer distance because of the different van der Waals energy. [ABSTRACT FROM AUTHOR]

Ab initio predictions for the stability of different graphene oxide (GO) structures have been shown to conflict with experimental observations. While ab initio studies predict that the most stable GOs are fully oxygen-covered (either with epoxide or hydroxyl), stable as-produced GOs are partially oxygen-covered and predominantly epoxide-covered structures. Although this discrepancy is being examined in terms of calculations of free energies of GOs and large diffusion energy-barriers for oxygen groups on graphene, there is still a lack of understanding on the energetic properties of GOs using classical molecular dynamics, which is able to investigate their structural distortion. Here, using the reactive empirical bond order (REBO) molecular dynamics potential, we compute the free energy and binding energy of GOs at different oxygen concentrations and epoxide to hydroxyl ratios, as well as the distortion energies of graphene lattice. Although epoxide causes more distortion on the carbon hexagonal planar structure, it provides more stability to the GO structure. The difference between free energy and binding energy of GOs is shown to be independent of oxygen coverage. These results allow gaining more insight on the issue of GO stability and show that REBO can capture most of experimental properties of GOs. [ABSTRACT FROM AUTHOR]

A diffusion induced stress (DIS) model based on the finite element method was used to analyze mechanical stress within a multiscale framework for silicon nanowire anodes designed for use in Li-ion batteries. With a prelithiated nanowire, the mechanical moduli of lithium silicide and the migration energy barriers of lithium were calculated by density functional theory method, while the diffusion constants for lithium silicide were obtained using kinetic Monte Carlo simulations. Unlike previous DIS analyses, this multiscale approach reveals a decreasing hoop stress with increasing lithium concentration. Furthermore, an increase in the diffusion coefficient with Li concentration was also found to be a much more significant factor than the reduction in the mechanical moduli, causing a prelithiated silicon anode to experience lower stress levels than pristine silicon. On the basis of this finding, it is concluded that prelithiation reduces the loss in cycling performance that typically occurs due largely to an excess of induced stress. [ABSTRACT FROM AUTHOR]

Transparent conductive indium tin oxide (ITO) thin films, electrochemically intercalated with sodium or other cations, show tunable superconducting transitions with a maximum Tc at 5 K. The transition temperature and the density of states, D(EF) (extracted from the measured Pauli susceptibility χp) exhibit the same dome shaped behavior as a function of electron density. Optimally intercalated samples have an upper critical field ≈ 4 T and Δ/kBTc ≈ 2.0. Accompanying the development of superconductivity, the films show a reversible electrochromic change from transparent to colored and are partially transparent (orange) at the peak of the superconducting dome. This reversible intercalation of alkali and alkali earth ions into thin ITO films opens diverse opportunities for tunable, optically transparent superconductors. [ABSTRACT FROM AUTHOR]

We have performed the density functional theory (DFT) method to study the electronic structure of ethylene carbonate (EC) in the gas phase and ab initio molecular dynamics (AIMD) simulations for its liquid phase at T = 450K to avoid freezing. The calculated characteristics of EC in both gas and liquid phases are found to be consistent with the experimental results. The coordination of one Li+ with 1-5 EC molecules is studied using the DFT method in the gas phase. At the same time, the formation of the first solvation shell of one Li+ within the EC solution at T = 450K is investigated using the AIMD simulation technique. The calculated results reveal the stability of the [Li+(EC)4] solvation shell, which contains four strongly bound EC molecules in a tetrahedral arrangement both in gas and liquid phases. The diffusion coefficient of the EC crystal at T = 450K is found to be 2.42×10-9 m2 s-1 and that of Li+ in liquid EC at T = 450K is found to be 1.27 × 10-9 m2 s-1. The conductivity of Li+ is found to be 5.46mScm-1. Such diffusion and conduction of Li+ in liquid EC may be helpful in understanding the formation of the primary solvation shell of Li+ and the solid electrolyte interphase formation mechanism, and hence may be useful in applications of lithium ion batteries. [ABSTRACT FROM AUTHOR]

Density functional theory is used to study the interaction of Li cation with ethylene carbonate (EC) and propylene carbonate (PC) comparatively, which are the most popular solvents used in lithium-ion battery composite. In our theoretical calculations, we use DFT hybrid parameter B3LYP5 with a basis set 6-31G** by means of PCGAMESS/Firefly software package. We analyze the optimized structures of EC, PC, and their clusters including lithium-ion. We then calculate solvation energy, desolvation energy, electron affinity, Gibbs free energy, heats of formation of Li solvated by EC and PC, and the charge on Li. From the above analysis, we observe EC as a better solvent than PC in applications of lithium-ion batteries. [ABSTRACT FROM AUTHOR]

The interaction of lithium (Li+) cation and hexafluorophosphate (PF6-) anion with nonaqueous electrolytes is studied by using density functional theory at the B3LYP/6-311++G(d,p) level in the gas phase in terms of the coordination of Li+ and PF6- with these solvents. Ethylene carbonate (EC) coordinates with Li+ and PF6- most strongly and reaches the anode and cathode most easily because of its highest dielectric constant among all the solvent molecules, resulting in its preferential reduction on the anode and oxidation on the cathode. For cyclic carbonates EC and propylene carbonate (PC), the structure Li+(S)4 is found to be the most stable. However, for linear carbonates dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC), the formation of PF6-(S) n=1-3 is not favorable. Such analysis may be useful in applications for lithium ion batteries. [ABSTRACT FROM AUTHOR]

Abstract: Electronic structures of Li+ ion–ethylene carbonate (EC) complexes were studied by density functional theory. The structural, electronic and dynamical properties of Li+–EC complexes were studied for both an isolated EC molecule and clusters including Li+ ion. Our structural analysis showed only one type of Li+ coordination with EC through Li+⋯Oas supported by the vibration spectral analysis for interaction between Li+ ion and a solvent (EC) molecule. It was analyzed that the solvation energy and Mulliken charge of Li+ ion solvated by EC molecule decrease with increase in number of EC molecule. However, electron affinity shows the opposite change. This analysis with solvation energy, electron affinity and Mulliken charge supported the stabilization of 4-coordinated solvation shell among [Li+(EC) n ] n=1–5 complexes. [ABSTRACT FROM AUTHOR]

Abstract: Carbon, Ni, and C–Ni alloy modified embedded atom method (MEAM) potentials were developed to study the initial process of carbon nanotube growth on Ni catalyst particles. The MEAM potentials were used to study the atomistic interaction between a carbon atom and a fcc Ni nano particle, both on the particle surfaces and inside the Ni nano particles. The result shows that surface carbon atom is more stable than those in the bulk and sub-surface interstitial positions. Carbon atoms are expected to diffuse from the bulk to the surface, and the single walled and double-walled carbon nanotubes would be more favorable to form on Ni nano particle catalyst. The carbon and Ni nano particle interaction calculation shows that the corner and the edge of the particle are the energetically more favorable sites for the carbon adatom. The carbon nanotube may grow from the corner and edge of the particle. [Copyright &y& Elsevier]

In this work, indium-tin-oxide (ITO) transistors with atomically thin channel thickness (${T}_{ch}$) of 2.1 nm realized by atomic layer deposition (ALD) are demonstrated. A maximum ON-state current of 970 mA/mm at ${V}_{DS}$ of 0.8 V and an ON/OFF ratio up to 107 are achieved in ITO transistor with In:Sn ratio of 9:1, channel length (${L}_{ch}$) of 60 nm, and dielectric equivalent oxide thickness (EOT) of 2.1 nm. Comparison between devices with different In:Sn ratios indicates a significant reduction of electron transport resulting from more Sn concentrations in ITO. The impact of back-end-of-line (BEOL) compatible low-temperature annealing is also investigated. An enhancement-mode operation with minimum subthreshold slope (SS) of 80 mV/dec and maximum field-effect mobility ($\mu _{FE}$) of 28 cm2/ $\text{V}\cdot \text{s}$ is achieved after O2 annealing. Besides, bias instability measurement shows the negative threshold voltage (${V}_{T}$) shift under both positive and negative gate bias stress due to donor-like interface states below the trap neutral level (TNL). The realization of large-area synthesis of atomically thin ITO films by ALD and decent electrical performance provide opportunities in future monolithic 3-D device integration with BEOL compatibility. [ABSTRACT FROM AUTHOR]

Van der Waals (vdW) heterostructures (or heterojunctions) are formed by stacking two different 2D materials (e.g., graphene, h‐BN, or transition metal dichalcogenides) across vdW gaps. In a type‐II heterojunction, 2D semiconductors are aligned with staggered bandgaps, which can effectively separate electron and hole carriers, and enable promising high‐performance photovoltaics and photodetectors. Herein, an effective vdW‐homojunction is reported, formed by one 2D material (2H‐WS2) with vdW gap engineering leading to different electronic structures and type‐II junction formation. WS2 films are synthesized by W metal deposition and controlled sulfurization method leading to a nonuniform vdW gap strain in the film. The vdW strain gradients in multilayer WS2 films are confirmed by transmission electron microscopy analysis, and the modeling by density functional theory shows an effective type‐II homojunction formation via modulated bandgaps by the vdW gap strains. The superior performance of a broadband photodetector application is confirmed by photoluminescence and photocurrent experiments. [ABSTRACT FROM AUTHOR]

Low dimensional materials are suitable candidates applying in next-generation high-performance electronic, optoelectronic, and energy storage devices because of their uniquely physical and chemical properties. In particular, one-dimensional (1D) atomic wires (AWs) exfoliating from 1D van der Waals (vdW) bulks are more promising in next generation nanometer (nm) even sub-nm device applications owing to their width of few-atoms scale and free dandling bonds states. Although several 1D AWs have been experimentally prepared, few 1D AW candidates could be practically applied in devices owing to lack of enough suitable 1D AWs. Herein, 367 kinds of 1D AWs have been screened and the corresponding computational database including structures, electronic structures, magnetic states, and stabilities of these 1D AWs has been organized and established. Among these systems, unary and binary 1D AWs with relatively small exfoliation energy are thermodynamically stable and theoretically feasible to be exfoliated. More significantly, rich quantum states emerge, such as 1D semiconductors, 1D metals, 1D semimetals, and 1D magnetism. This database will offer an ideal platform to further explore exotic quantum states and exploit practical device applications using 1D materials. The database are openly available at http://www.dx.doi.org/10.11922/sciencedb.j00113.00004. [ABSTRACT FROM AUTHOR]

We utilize real-time time-dependent density functional theory and Ehrenfest dynamics scheme to investigate excited-state nonadiabatic dynamics of ligand dissociation of cobalt tricarbonyl nitrosyl, Co(CO)3NO, which is a precursor used for cobalt growth in advanced technologies, where the precursor's reaction is enhanced by electronic excitation. Based on the first-principles calculations, we demonstrate two dissociation pathways of the NO ligand on the precursor. Detailed electronic structures are further analyzed to provide an insight into dynamics following the electronic excitations. This study sheds light on computational demonstration and underlying mechanism of the electronic-excitation-induced dissociation, especially in molecules with complex chemical bonds such as the Co(CO)3NO. [ABSTRACT FROM AUTHOR]

Success in making artificial muscles that are faster and more powerful and that provide larger strokes would expand their applications. Electrochemical carbon nanotube yarn muscles are of special interest because of their relatively high energy conversion efficiencies. However, they are bipolar, meaning that they do not monotonically expand or contract over the available potential range. This limits muscle stroke and work capacity. Here, we describe unipolar stroke carbon nanotube yarn muscles in which muscle stroke changes between extreme potentials are additive and muscle stroke substantially increases with increasing potential scan rate. The normal decrease in stroke with increasing scan rate is overwhelmed by a notable increase in effective ion size. Enhanced muscle strokes, contractile work-per-cycle, contractile power densities, and energy conversion efficiencies are obtained for unipolar muscles. [ABSTRACT FROM AUTHOR]

First principles calculation reveals that the compressed (tensile) strain increases (decreases) the work function of Nb and W (110) surfaces, and that such a work function change is due to the combined effects of the surface dipole and bulk electronic structure toward the same direction. Calculation also shows that the interface dipole is formed in the Nb–W interface due to an unequal loss of the electrons from Nb and W interface atoms, and that the formation of the Nb–W interface is energetically favorable with negative interface energy, implying a tendency of interface interdiffusion of Nb and W atoms. [ABSTRACT FROM AUTHOR]

A theoretical study of the oxidation of InP(001)-(2 × 4) surface is performed using density functional theory methods. Our results on surface oxidation show that the oxygen adsorption does not produce any gap states in the bulk InP band gap, due to the saturation of surface In dangling bonds, whereas substitutional oxygen atoms produce gap states. This study also shows that the surface stability increases with the oxygen content, indicating a strong tendency for surface oxidation. Our results help to clarify the origin of surface gap states upon surface oxidation and provide an insight at the atomic level the mechanism of surface oxidation, which will assist in the understanding of the degradation of III-V devices upon oxygen exposure or interfacial oxidation with high dielectric constant oxides. [ABSTRACT FROM AUTHOR]

The semiconductor industry is searching for a high-k gate dielectric replacement for silica. We study the electronic properties of Nb, Zr, Y, and Sc aluminates using density functional theory to screen aluminates for further study. The transition metals substituted favorably at the octahedral Al sites in the κ-alumina model system. Zirconium and niobium substitution for Al introduced localized d-electron states 0.6 and 1.1 eV below the conduction band. Yttrium and scandium introduced states 0.1 and 1.4 eV above the delocalized s and p states of the conduction band causing little change in the κ-alumina band gap and are promising replacement candidates that maintain the large alumina band offset. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A method to electronically modulate the energy gap and bandstructure of semiconducting carbon nanotubes is proposed. We investigate this bandstructure modulation mechanism using tight-binding and density functional theory (DFT). Results show that the energy gap of a semiconducting nanotube can be narrowed, when the tube is placed in an electric field perpendicular to the tube axis. In contrast, Metallic tubes were found to exhibit a screening behavior, whereby free charge redistributes about the tube circumference as a result of the external field. In this case, the bandstructure shows little perturbation in response to an applied electric field. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Redox reactions of oxygen have been considered critical in controlling the electrochemical properties of lithium‐excessive layered‐oxide electrodes. However, conventional electrode materials without overlithiation remain the most practical. Typically, cationic redox reactions are believed to dominate the electrochemical processes in conventional electrodes. Herein, we show unambiguous evidence of reversible anionic redox reactions in LiNi1/3Co1/3Mn1/3O2. The typical involvement of oxygen through hybridization with transition metals is discussed, as well as the intrinsic oxygen redox process at high potentials, which is 75 % reversible during initial cycling and 63 % retained after 10 cycles. Our results clarify the reaction mechanism at high potentials in conventional layered electrodes involving both cationic and anionic reactions and indicate the potential of utilizing reversible oxygen redox reactions in conventional layered oxides for high‐capacity lithium‐ion batteries. [ABSTRACT FROM AUTHOR]

Redox reactions of oxygen have been considered critical in controlling the electrochemical properties of lithium‐excessive layered‐oxide electrodes. However, conventional electrode materials without overlithiation remain the most practical. Typically, cationic redox reactions are believed to dominate the electrochemical processes in conventional electrodes. Herein, we show unambiguous evidence of reversible anionic redox reactions in LiNi1/3Co1/3Mn1/3O2. The typical involvement of oxygen through hybridization with transition metals is discussed, as well as the intrinsic oxygen redox process at high potentials, which is 75 % reversible during initial cycling and 63 % retained after 10 cycles. Our results clarify the reaction mechanism at high potentials in conventional layered electrodes involving both cationic and anionic reactions and indicate the potential of utilizing reversible oxygen redox reactions in conventional layered oxides for high‐capacity lithium‐ion batteries. [ABSTRACT FROM AUTHOR]

Substitutional doping in 2D semiconductor MoS2 was investigated by charge transition level (CTL) calculations for Nitrogen group (N, P, As, Sb) and Halogen group (F, Cl, Br, I) dopants at the S site of monolayer MoS2. Both n-type and p-type dopant levels are calculated to be deep mid-gap states (~1 eV from band edges) from DFT total energy-based CTL and separate DFT + GW calculations. The deep dopant levels result from the giant renormalization of hydrogen-like defect states by reduced dielectric screening in ultrathin 2D films. Theoretical analysis based on Keldysh formulation provides a consistent impurity binding energy of ~1 eV for dielectric thin films. These findings of intrinsic deep impurity levels in 2D semiconductors MoS2 may be applicable to diverse novel emerging device applications. [ABSTRACT FROM AUTHOR]

The surface of solid catalysts is one of the most important factors where the interface with reaction products governs the reaction kinetics. Herein, the crystal phase of palladium–copper nanoparticles (PdCu NPs) is controlled to modulate their surface atomic arrangement, which will govern the growth dynamics of discharge products on their surfaces and thus the catalytic performances in non‐aqueous lithium–oxygen (Li‐O2) batteries. First‐principles calculations and experimental validations reveal that homogeneous nucleation and distribution of discharge products are observed on the surface of body‐centered cubic PdCu NPs, promoting the oxygen reduction/evolution reaction (ORR/OER) activities in Li‐O2 batteries. However, the agglomerates formed on the surface of its face‐centered cubic homologue deteriorates ORR/OER activities, which worsen the battery performances. For the first time, this work theoretically and experimentally demonstrates how the crystal phase modulation regulates the nucleation behaviors and growth dynamics of discharge products for ORR/OER. [ABSTRACT FROM AUTHOR]

Recently, spintronics devices using MoS 2 and ferromagnetic electrode have been in the spotlight. However, strong Fermi level pinning (FLP) is known to occur between MoS 2 and ferromagnetic electrode, resulting in a large Schottky barrier height (SBH), which makes it difficult to inject electron from ferromagnetic electrode to semiconductor. To resolve this problem, we study the reduction of FLP occurring between two materials by investigating the effect of atomic passivation at Ni-MoS 2 interfaces on contact behaviors. Such atomic passivation can reduce the FLP and magnetic moments induced at S atoms of MoS 2. Especially, the largest decrease in the FLP occurs in the case of H atom passivation. Besides, the N, O, and F atom passivation confirms the possibility of ohmic contact, indicated from small SBHs (~0.2 eV). As a result, the SBH and thus the efficiency of the device can be controlled by atomic passivation at metal-semiconductor interfaces. [ABSTRACT FROM AUTHOR]