Calculated Work Functions Research Articles

Improving the precision of work-function calculations within plane-wave density functional theory

Abstract Work function is a fundamental property of metals and is related to many surface-related phenomena of metals. Theoretically, it can be calculated with a metal slab supercell in density functional theory (DFT) calculations. In this paper, we discuss how the commensurability of atomic structure with the underlying fast Fourier transform (FFT) grid affects the accuracy of work function obtained from plane-wave pseudopotential DFT calculations. We show that the macroscopic average potential, which is an important property in work function calculations under the ‘bulk reference’ method, is more numerically stable when it is calculated with commensurate FFT grids than with incommensurate FFT grids. Due to the stability of the macroscopic average potential, work function calculated with commensurate FFT grids shows better convergence with respect to basis set size, vacuum length and slab thickness of a slab supercell. After we control the FFT grid commensurability issue in our work function calculations, we obtain well-converged work functions for Al, Pd, Au and Pt of (100), (110) and (111) surface orientations. For all the metals considered, the ordering of our calculated work functions of the three surface orientations agrees with experiment. Our findings reveal the importance of the FFT grid commensurability issue, which is usually neglected in practice, in obtaining accurate metal work functions, and are also meaningful to other DFT calculations which can be affected by the FFT grid commensurability issue.

Artificial intelligence combined with high-throughput calculations to improve the corrosion resistance of AlMgZn alloy

Efficiently designing lightweight alloys with combined high corrosion resistance and mechanical properties remains an enduring topic in materials engineering. Due to the inadequate accuracy of conventional stress-strain machine learning (ML) models caused by corrosion factors, a novel reinforcement self-learning ML algorithm combined with calculated features (accuracy R2 >0.92) is developed. Based on the ML models, calculated work functions and mechanical moduli, a Computation Designed Corrosion-Resistant Al alloy is fabricated and verified. The performance (elongation reaches ∼30%) is attributed to the H trapping Al-Sc-Cu phases (-1.44 eV H−1) and Cu-modified η/η' precipitates inside the grain boundaries (GBs).

Pressure-induced novel ZrN4 semiconductor materials with high dielectric constants: a first-principles study.

In addition to Zr3N4 and ZrN2 compounds, zirconium nitrides with a rich family of phases always exhibit metal phases. By employing an evolutionary algorithm approach and first-principles calculations, we predicted seven novel semiconductor phases for the ZrN4 system at 0-150 GPa. Through calculating phonon dispersions, we identified four dynamically stable semiconductor structures under ambient pressure, namely, α-P1̄, β-P1̄, γ-P1̄, and β-P1 (with bandgaps of 1.03 eV, 1.10 eV, 2.33 eV, and 1.49 eV calculated using the HSE06 hybrid density functional, respectively). The calculated work functions and dielectric functions show that the four dynamically stable semiconductor structures are all high dielectric constant (high-k) materials, among which the β-P1̄ phase has the largest static dielectric constant (3.9 times that of SiO2). Furthermore, we explored band structures using the HSE06 functional and density of states (DOS) and the response of bandgaps to pressure using the PBE functional for the four new semiconductor configurations. The results show that the bandgap responses of the four structures exhibit significant differences when hydrostatic pressure is applied from 0 to 150 GPa.

First-Principles Study on the Origin of Tribo-Negative Nature of Polytetrafluoroethylene

Triboelectric nanogenerator (TENG) is an energy harvesting device that converts mechanical energy into electricity based on the coupling of contact electrification and electrostatic induction. Fluorocarbon polymers, such as polytetrafluoroethylene (PTFE) and perfluoroalkoxy alkane (PFA) are the widely used polymeric triboelectric materials, however, their tribo-negative nature has been poorly understood. Here, using first-principles calculations, we have systematically investigated the electronic structure of PTFE and PFA. It is revealed that the defects associated with carbon dangling bonds act as charge trapping centers during contact electrification. Furthermore, the simulation on the contact model of PTFE and nylon confirms that the charge transfer occurs when the distance between them is as near as the atomic orbitals overlap. In addition, the calculated work functions of polymeric materials are consistent with the triboelectric series, giving a powerful method to predict the triboelectric property of materials. We expect our study provides fundamental understandings on the origin of the triboelectric property of materials and the atomistic phenomena during contact electrification. Our results also give insight into triboelectric material designs with a strong electron-withdrawing property.

Efficient Prevention of Aspergillus flavus Spores Spread in Air Using Plasmonic Ag-AgCl/α-Fe2O3 under Visible Light Irradiation.

Aspergillus flavus is a kind of widespread fungi that can produce carcinogenic, teratogenic, and mutagenic secondary metabolites known as aflatoxins. Aspergillus flavus mainly spread through the means of fungal spores in air, thus preventing the spores spread is an effective strategy to control aflatoxins contamination from source. Herein, a rapid and efficient control way to prevent the spread of Aspergillus flavus spores in air was demonstrated. Ag-AgCl nanoparticles were combined with tetrahedral α-Fe2O3 to form plasmonic composites that presented 93.65 ± 1.53% prevention rate of Aspergillus flavus spores under 50 min visible light irradiation. The efficient activity was attributed to the synergy effect of Ag including intrinsic disinfection, electron sink, and localized surface plasmon resonance effect, which were proven by photoelectric characterization, density functional theory, and finite difference time domain methods. The calculated work functions of α-Fe2O3, Ag, and AgCl were 3.71, 4.52, and 5.38 eV, respectively, which could accelerate photoinduced carrier transfer through Ag during photoreaction. Moreover, it was found that the intrinsic disinfection of Ag and hydroxyl radical from photocatalytic reaction were the main factors to the prevention of Aspergillus flavus spores, which resulted in the destruction of spore structure and the leakage of intracellular protein with 62.15 ± 2.63 μg mL-1. Most important, it was proven that the composites also showed high activity (90.52 ± 1.26%) to prevent Aspergillus flavus spore spread in the storage process of peanuts. These findings not only provided useful information for an efficient and potential strategy to prevent Aspergillus flavus contamination but also could be as a reference in toxic fungi control.

Cubine Monolayer as a Super Sensor for NO2 Molecule Detection and Capture

AbstractIn this work, the potential application of cubine monolayer as a highly sensitive sensor for the detection and capture of toxic gas molecule NO2 is investigated by employing density functional theory (DFT) calculations. Eight gas molecules, namely CO, CO2, CH4, H2S, SO2, NH3, NO, and NO2 are considered. After examining the adsorption energy, adsorption distance, variation of bond length and Mulliken charge, the results show that only NO2 molecule present chemical adsorption on cubine monolayer. The calculated work functions and I–V curves indicate that cubine has the highest sensitivity to NO2 molecules. The adsorption energies decrease following the increase of strain in both A and B directions, when the strain along the A direction reaches 12%, the adsorption energy decreases to −0.686 eV, which shows physical adsorption. This provides a desorption strategy for recycling. The calculations employed in this work indicate that the cubine monolayer can be a prominent candidate for the application of a super sensor to detect and capture NO2 toxic gas molecules.

Exploring structural, electronic, and mechanical properties of 2D hexagonal MBenes

A family of two-dimensional (2D) transition metal borides, referred to as MBenes, is recently emerging as novel materials with great potentials in electronic and energy harvesting applications to the field of materials science and technology. Transition metal borides can be synthesized from chemical exfoliation of ternary-layered transition metal borides, known as MAB phases. Previously it has been predicted that thin pristine 2D Sc-, Ti-, Zr-, Hf-, V-, Nb-, Ta-, Mo-, and W-based transition metal borides with hexagonal phase are more stable than their corresponding orthorhombic phase. Here, using a set of first-principles calculations (at absolute zero temperature), we have examined the geometric, dynamic stability, electronic structures, work function, bond strength, and mechanical properties of the hexagonal monolayer of transition metal borides (M = Sc, Ti, Zr, Hf, V, Nb, Ta, Mo, and W) chemically terminated with F, O, and OH. The results of the formation energies of terminated structures imply that the surface terminations could make a strong bond to the surface transition metals and provide the possibility of the development of transition metal borides with those surface terminations. Except for ScBO, which is an indirect bandgap semiconductor, the other transition metal borides are metallic or semimetal. Particularly, TiBF, ZrBF, and HfBF are metallic systems whose band dispersions close to the Fermi level indicate the coexistence of type-I and type-II nodal lines. Our calculated work functions indicate that 2D transition metal borides with OH (O) functionalization obtain the lowest (highest) work functions. The results of the mechanical properties of the considered structures imply that oxygen functionalized transition metal borides exhibit the stiffest mechanical strength with 248 < E (N m−1) < 348 while non-terminated transition metal borides are generally the weakest systems with 206 < E (N m−1) < 283.

Designed formation of InVO4/CeVO4 hollow nanobelts with Z-scheme charge transfer: Synergistically boosting visible-light-driven photocatalytic degradation of tetracycline

As the potential candidates for photodegradating pharmaceutical pollutants, the highly efficient, stable and recyclable Z-scheme photocatalysts, that are superior to the widely studied heterojunction photocatalysts, are very fascinating yet challenging. Herein, hollow structured InVO4/CeVO4 nanobelts with large heterogeneous interface and huge internal cavity are well-designed and prepared through an efficient electrospinning-calcination route. Owing to a large surface-to-bulk ratio, when used as visible-light photocatalytic materials for photochemical decomposition of the residual tetracycline (TC) aqueous solution, these InVO4/CeVO4 hollow nanobelts with porous structure and nanocrystal subunits exhibit excellent photocatalytic activity and very high stability, compared with pristine CeVO4 and InVO4 nanobelts. A possible Z-scheme reaction mechanism for the enhanced photocatalytic performance is tentatively proposed by PL, active species trapping experiments, XPS and EPR analysis, as well as calculated work functions of different crystal faces. This work underlines the importance of heterojunction construction, and presents a feasible protocol for the rational design of active photocatalysts under solar light for the photodegradation of environmental pollutants or for environmental purification.

A Study of SiC Nanostructures Grown on Si Substrate as Thermionic Cathode and its Work Function Lowering Effect

Silicon carbide (SiC) nanostructures were synthesized on a silicon (Si) substrate that served as a thermionic cathode. Nanosheets, nanograins, and high-aspect-ratio nanostructures formed on the thermionic cathodes, depending on the synthesis parameters. The calculated work functions of thermionic cathodes ranged from 4.48 eV to 4.83 eV. It can be observed that the work function is attributed to the geometrical characteristics, such as the aspect ratios and the surface-area-to-volume ratios of SiC nanostructures. Among the nanostructures, using SiC nanowire structures (M200SC3) resulted in the greatest reduction in the thermionic cathode’s work function, which was 0.36 eV. This reduction is attributed to the sharp-tip structures having high aspect ratios, which is a strong field enhancement factor to the electron emission. This study suggests that the presence of SiC nanostructures on a thermionic cathode can potentially enhance electron emission in a thermionic energy converter (TEC) system.

Anisotropic work function of elemental crystals

The work function is a fundamental electronic property of a solid that varies with the facets of a crystalline surface. It is a crucial parameter in spectroscopy as well as materials design, especially for technologies such as thermionic electron guns and Schottky barriers. In this work, we present the largest database of calculated work functions for elemental crystals to date. This database contains the anisotropic work functions of more than 100 polymorphs of about 72 elements and up to a maximum Miller index of two and three for non-cubic and cubic crystals, respectively. The database has been rigorously validated against previous experimental and computational data where available. We also propose a weighted work function based on the Wulff shape that can be compared to measurements from polycrystalline specimens, and show that this weighted work function can be modeled empirically using simple atomic parameters. Furthermore, for the first time, we were able to analyze simple bond breaking rules for metallic systems beyond a maximum Miller index of one, allowing for a more generalized investigation of work function anisotropy.

Metal Hexaboride Work Functions: Surface Configurations and the Electrical Double Layer from First‐Principles

AbstractMetal hexaborides are ionic crystals having exceptional thermochemical stability and unique electrochemical behavior, including low work functions, making these materials ideal for electron emission applications. Here, the importance of specific interfacial properties for the development of efficient cathode devices is examined. In particular, the electrostatic behavior of BaB6, CaB6, LaB6, and SrB6 surfaces having various terminal configurations and surface coverage is studied using a repeated symmetric slab model with self‐consistent field calculations. The calculated work functions for each system are found minimal for a monolayer cation coverage and monotonically approach the experimental value for pure boron as cations are subsequently removed. Using three patterns to sample the surface space on stoichiometric slabs with identical surface composition, it is found that configurational changes can substantially alter the calculated work function. Further examination of the bonding charge densities suggest the low work functions typically observed in these materials are mainly due to the electrical double layer formed at the surface, rather than an inherent bulk property. This provides evidence suggesting that a variety of experimental observations may be due to surface features resulting from different preparatory methods. Significant electron density shifts are observed only few atomic layers deep leaving most of the bulk material unaltered.

The electronic structures and work functions of (100) surface of typical binary and doped REB6 single crystals

The density function theory been used to calculate the electronic structures of binary and doped rare earth hexaborides (REB6), which exhibits the large density of states (DOS) near Fermi level. The d orbital elections of RE element contribute the electronic states of election emission near the Fermi level, which imply that the REB6 (RE=La, Ce, Gd) with wide distribution of high density d orbital electrons could provide a lower work function and excellent emission properties. Doping RE elements into binary REB6 can adjust DOS and the position of the Fermi energy level. The calculated work functions of considered REB6 (100) surface show that the REB6 (RE=La, Ce, Gd) have lower work function and doping RE elements with active d orbital electrons can significantly reduce work function of binary REB6. The thermionic emission test results are basically accordant with the calculated value, proving the first principles calculation could provide a good theoretical guidance for the study of electron emission properties of REB6.

Novel structures of oxygen adsorbed on a Zr(0001) surface predicted from first principles

The structures of O atoms adsorbed on a metal surface influence the metal properties significantly. Thus, studying O chemisorption on a Zr surface is of great interest. We investigated O adsorption on a Zr(0001) surface using our newly developed structure-searching method combined with first-principles calculations. A novel structural prototype with a unique combination of surface face-centered cubic (SFCC) and surface hexagonal close-packed (SHCP) O adsorption sites was predicted using a single-layer adsorption model (SLAM) for a 0.5 and 1.0 monolayer (ML) O coverage. First-principles calculations based on the SLAM revealed that the new predicted structures are energetically favorable compared with the well-known SFCC structures for a low O coverage (0.5 and 1.0ML). Furthermore, on basis of our predicted SFCC+SHCP structures, a new structure within multi-layer adsorption model (MLAM) was proposed to be more stable at the O coverage of 1.0ML, in which adsorbed O atoms occupy the SFCC+SHCP sites and the substitutional octahedral sites. The calculated work functions indicate that the SFCC+SHCP configuration has the lowest work function of all known structures at an O coverage of 0.5ML within the SLAM, which agrees with the experimental trend of work function with variation in O coverage.

Stable, ruggedized, and nanometer-order size transfer mold field emitter array in harsh oxygen radical environment

Extremely sharp, uniform, stable, rugged, low operating voltage, and nanometer size molybdenum field emitter arrays (Mo FEAs) were developed by transfer mold fabrication. These can be used in stable and reliable vacuum nanoelectronic devices such as those used in electric propulsion engines and in environmentally demanding applications. Nanometer size transfer mold Mo FEAs were fabricated with base lengths of 36–370 nm. The 36 nm base length transfer mold Mo FEAs are the smallest FEA's ever reported. The tip radii were as sharp as 2.8–3.5 nm. The turn-on fields of the transfer mold Mo FEAs with a base length of 36–370 nm were found to be as low as 9.0–23.6 V/μm for the short anode-to-apex distance of less than 10 μm. These are lower than those of conventional FEA's such as carbon nanotube FEAs, Spindt-type FEAs, and Gray-type FEAs with approximate turn-on fields of 50–600 V/μm. They can thus be used in reliable vacuum nanoelectronic devices. The emission fluctuation of the 36 nm base length transfer mold Mo FEAs without a resistive layer were found to be as low as ±1.6% and this value is the lowest ever reported. Conventional FEAs with and without resistive layers have values of 5%–100% and higher than 100%, respectively. The field emission characteristics of the FEAs have been evaluated by in-situ oxygen radical treatment with a flux of 1015 atoms cm−2 s−1. This value is 107–108 times higher than that of typical low earth orbit fluxes. The environmental characteristics of the transfer mold Mo FEAs were compared with those of transfer mold Ni FEAs. The turn-on fields of the transfer mold Mo FEAs changed slightly from 10.8 to 22.5 V/μm at 1 min from the initiation time. It was almost saturated at 29.4 V/μm upon 5 min of in-situ oxygen radical treatment. A rapid increase in the turn-on field from 14.9 to 59.4 V/μm upon in-situ radical treatment for 3 min was observed for the Ni FEAs. The experimental work functions measured by ultraviolet photoelectron spectroscopy and the work functions calculated by the slope of Fowler–Nordheim plots for the transfer mold Mo FEAs changed slightly from 4.3 to 5.2 eV and from 4.3 to 5.3 eV, respectively, after oxygen radical treatment for 5 min. The calculated work functions are similar to the experimental work functions because the transfer mold Mo FEAs have extremely uniform geometric factors and stable emission characteristics. The emission fluctuation rates determined without the radical treatment of the transfer mold Mo FEAs without a resistive layer were found to be as low as ±1.7%. Even with in-situ radical treatment for 30 min the transfer mold Mo FEAs exhibited remarkably stable emission without spike noise and with fluctuations as low as ±4.5%. Therefore, the extremely stable and nanometer size transfer mold Mo FEAs are expected to be useful in the production of vacuum nanoelectronic devices.

Spin and surface orientation effects of work function forCr/Ni(111),Cr/Ni(100) andCr/Ni(110) metal gates

Work functions of Cr/Ni (111), Cr/Ni (100) and Cr/Ni (110) surfaces with different magnetic configurations for Cr atoms in the topmost Cr monolayer are investigated using first-principles methods based on density functional theory. The calculated results reveal that work functions vary with crystal orientations and magnetic configurations. The magnitude of the Cr magnetic moments for the three (111), (100) and (110) surfaces follows a change trend with MCr, Cr/Ni(111)&lt; MCr, Cr/Ni(100)&lt; MCr, Cr/Ni(110). Altering the magnetic configurations of the systems from an original ground state to an excited one will have the total energy and the Fermi level increase. Consequently, it will give rise to the reduction of the work function for the system. Moreover, the quite favorable variation range (4.92–4.41 eV) of the calculated work functions for Cr/Ni (100) system modulated by spin effect implies that the Cr/Ni (100) system may be a more promising candidate. Our work suggests that changing magnetic configurations can modulate the work functions of magnetic metal gates well.

Hybrid molecular dynamics and first-principles study on the work function of a Pt(111) electrode immersed in aqueous solution at room temperature

With a combined molecular dynamics simulation and first-principles calculations, we have investigated a metal surface immersed in aqueous solution at room temperature using a Pt(111) electrode as an example. With the inclusion of thermal average effects at room temperature, the calculated averaged work function is found to be in good agreement with the experimental measurements. The electron redistribution at the interface of the topmost Pt(111) slab layer and the first water layer plays an important role in controlling the work function. A broad distribution of calculated work functions caused by the thermal motions of the dipolar solvents is obtained from statistical sampling, which implies that the chemical reactivity of a metal electrode in aqueous solution is a dynamic property at least in the nanoscale. Such a microscopic understanding helps to understand the behavior of complex electrochemical double layers.

On the HSAB based estimate of charge transfer between adsorbates and metal surfaces

The applicability of the HSAB based electron charge transfer parameter, Δ N, is analyzed for molecular and atomic adsorbates on metal surfaces by means of explicit DFT calculations. For molecular adsorbates Δ N gives reasonable trends of charge transfer if work function is used for electronegativity of metal surface. For this reason, calculated work functions of low Miller index surfaces for 11 different metals are reported. As for reactive atomic adsorbates, e.g., N, O, and Cl, the charge transfer is proportional to the adatom valence times the electronegativity difference between the metal surface and the adatom, where the electronegativity of metal is represented by a linear combination of atomic Mulliken electronegativity and the work function of metal surface. It is further shown that the adatom-metal bond strength is linearly proportional to the metal-to-adatom charge transfer thus making the Δ N parameter a useful indicator to anticipate the corresponding adsorption energy trends.

Adsorption and dissociation of molecular oxygen on α-Pu (0 2 0) surface: A density functional study

Molecular and dissociative oxygen adsorptions on the α-Pu (0 2 0) surface have been systematically studied using the full-potential linearized augmented-plane-wave plus local orbitals (FP-LAPW+lo) basis method and the Perdew–Burke–Ernzerhof (PBE) exchange-correlation functional. Chemisorption energies have been optimized for the distance of the admolecule from the Pu surface and the bond length of O–O atoms for four adsorption sites and three approaches of O 2 admolecule to the (0 2 0) surface. Chemisorption energies have been calculated at the scalar relativistic level with no spin–orbit coupling (NSOC) and at the fully relativistic level with spin–orbit coupling (SOC). Dissociative adsorptions are found at the two horizontal approaches (O 2 is parallel to the surface and perpendicular/parallel to a lattice vector). Hor2 (O 2 is parallel to the surface and perpendicular to a lattice vector) approach at the one-fold top site is the most stable adsorption site, with chemisorption energies of 8.048 and 8.415 eV for the NSOC and SOC cases, respectively, and an OO separation of 3.70 Å. Molecular adsorption occurs at the Vert (O 2 is vertical to the surface) approach of each adsorption site. The calculated work functions and net spin magnetic moments, respectively, increase and decrease in all cases upon chemisorption compared to the clean surface. The partial charges inside the muffin-tins, the difference charge density distributions, and the local density of states have been used to investigate the Pu-admolecule electronic structures and bonding mechanisms.

First-principles investigation of the structure and electronic properties of CdS/CdSe/CdS and CdS/CdTe/CdS quantum wells using a slab approximation

The structure and electronic properties of CdS, CdSe, and CdTe thin films and multilayer CdS/CdSe/CdS and CdS/CdTe/CdS heterostructures simulating quantum wells of these compositions are performed using density functional theory and a slab approximation. Various crystal modifications and various crystallographic surfaces of the specified compounds are considered. In the calculations, the GGA approximation for the exchange-correlation functional (PW91) and ultrasoft pseudopotentials are used. For the systems in study, the work functions are calculated for various surfaces. The calculated structural data are in good agreement with experiments. The calculated surface reconstruction can be qualitatively described as an inward displacement of cadmium atoms into the surface crystal layer and an outward displacement of chalcogen atoms. The calculated surface energies indicate that the (110) surface formation energy is lower than the (100) surface formation energy. The calculated work functions decrease in the sequence CdS > CdSe > CdTe, while the calculated work functions for multilayer structures are between the corresponding values for the pure components of the structure.

Diameter dependence of mechanical, electronic, and structural properties of InAs and InP nanowires: A first-principles study

Semiconductor nanowires (NWs) have ideal morphologies to act as active parts and connections in nanodevices since they naturally restrict the conduction channels and periodicity to one dimension. The advantages from the reduced spatial dimension can be greatly enhanced by wisely selecting the materials composing the NWs, through the knowledge of the properties of their bulk counterparts. NW's properties can still be tailored by managing (i) internal or intrinsic characteristics as diameters, growth directions, structural phases, and the faceting or saturation of surfaces, and/or (ii) external or extrinsic influences as applied electric, magnetic, thermal, and mechanical fields. Bulk InAs has one of the lowest electron effective-masses among binary III-V semiconducting materials while bulk InP shows excellent optical properties, which make InAs and InP NWs candidates for optoelectronic materials. In this work, we use first-principles calculations to study the structural, electronic, and mechanical properties of [111] zinc-blende InAs and InP NWs as a function of diameter (ranging from 0.5 to 2.0 nm). The influence of external mechanical stress on the electronic properties is also analyzed. The axial lattice constants of the NWs are seen to decrease with decreasing diameter, as a consequence of a shorter surface lattice constant of the NWs, as compared to their bulk values. The Young's modulus of both InAs and InP NWs is determined to decrease while the Poisson's ratio to increase with decreasing diameters, with deviations from the bulk Young's modulus estimated to occur for NWs with diameters lower than 15 nm. The increase in the band-gaps with decreasing diameters is seen to be slower than the expected from simple quantum-mechanical models ($1/{D}^{2}$, where $D$ is the diameter), mainly for the smallest $(&lt;1.0\text{ }\text{nm})$ diameters. The electron effective-masses are seen to increase with decreasing diameters, due to a $k$-dependent energy shift of the conduction-band minimum. The calculated work functions for both NWs show a decrease with decreasing diameters. The change in the NWs' band-edge eigenvalues with axial strain is calculated and the band-gap deformation potentials are determined and shown to change in signal within the range of studied diameters. The influence of the mechanical strain on the electronic bands is analyzed in terms of electronic charge decompositions in directions parallel and perpendicular to the NWs' axes. Direct to indirect band-gap transitions are observed for compressive strains in very thin NWs. The hole effective-mass is seen to be lower than the corresponding electron effective-mass for the studied NWs.