Surface Work Function Research Articles

The effect of O impurity particle adsorption on the Cs/Mo (110) surface

In this work, the O impurity particle adsorption on the Cs/Mo(110) surface, to determine the influence of O on Cs stability on Mo(110) surface and also the work function of Cs/O/Mo(110) surface, has been investigated by performing density functional theory (DFT). There are four independent adsorption sites on Cs/Mo(110) surface for O atom. No matter which adsorption site O was adsorbed, the surface adsorption energy of Cs increased. O atoms attract more charges due to the difference in electronegativity. Although this weakens the interaction between the Cs and Mo atoms, the O atom does connect them like a bridge, resulting in enhanced adsorption of Cs atom. In our model, the surface work function of Cs/Mo(110) is calculated as 1.835 eV. In some cases, the adsorption of O can dramatically reduce the surface work function to 1.633 eV. Conversely, O may also increase it, but this increase is not pronounced. This result suggests that the appropriate addition of O atoms may be beneficial to the development of high-efficiency negative ion sources.

The Neural Correlates of Life Satisfaction: A Systematic Review of Magnetic Resonance Imaging

In this work, the O impurity particle adsorption on the Cs/Mo(110) surface, to determine the influence of O on Cs stability on Mo(110) surface and also the work function of Cs/O/Mo(110) surface, has been investigated by performing density functional theory (DFT). There are four independent adsorption sites on Cs/Mo(110) surface for O atom. No matter which adsorption site O was adsorbed, the surface adsorption energy of Cs increased. O atoms attract more charges due to the difference in electronegativity. Although this weakens the interaction between the Cs and Mo atoms, the O atom does connect them like a bridge, resulting in enhanced adsorption of Cs atom. In our model, the surface work function of Cs/Mo(110) is calculated as 1.835 eV. In some cases, the adsorption of O can dramatically reduce the surface work function to 1.633 eV. Conversely, O may also increase it, but this increase is not pronounced. This result suggests that the appropriate addition of O atoms may be beneficial to the development of high-efficiency negative ion sources.

Efficient reduction of CO2 to CO over grain boundary rich gold film reconstructed by O2 plasma treatment

Electrochemical carbon dioxide reduction offers a potential way to simultaneously mitigate carbon emission and produce value-added carbonaceous fuels and feedstock. This work proposes a facile method of O2 plasma bombardment to obtain efficient Au electrodes for selectively CO2 reduction. A sustainable setup assisted by solar energy is used to realize green utilization of CO2, with Fe2O3 photoanode for voltage compensation and Si solar cell for external bias. The O2 plasma treatment of Au results in formation of grain boundaries via fast surface reconstruction. The Au film with higher density of grain boundary possesses higher electrochemical active surface area, lower surface work function, and faster charge transfer through Au/electrolyte interface, which can facilitate the CO2 reduction. Compared with the as-deposited Au film, the O2 plasma treated Au film displays a lower overpotential and enhanced CO2-to-CO conversion performance. Faradic efficiency of 88.7% towards CO is achieved over plasma treated Au film.

Explicit solvent effects on (1 1 0) ruthenium oxide surface wettability: Structural, electronic and mechanical properties of rutile RuO2 by means of spin-polarized DFT-MD

The structural, electronic and mechanical properties of crystalline ruthenium dioxide RuO2 in its rutile structure have been calculated via forefront spin-polarized Density Functional Theory (DFT) and DFT-Molecular Dynamics (DFT-MD) simulations. Notwithstanding RuO2 is known as a highly active catalyst for a wide number of (photo) electrochemical reactions in aqueous/humid environments, the study of the interaction of RuO2 surfaces with water has been confined largely to (static) surface science and water adsorption calculations. Herein, an atomistic understanding of bulk rutile RuO2 and explicit solvent effects on (110)-RuO2 facet are provided. We especially focus on the comprehension of the mechanistic interplay between surface wettability, interfacial water dynamics and surface chemical activity.Analysis and characterization of the interfacial water and H-bond environment reveal how explicit liquid water and its dynamics play a role in the surface reconstruction and in the hydrophobic nature of the (110)-RuO2 facet with a prevailing H-bond acceptor character. Moreover, we provide a dependence of physical and chemical properties, such as surface electric field and work function, from different degrees of surface wettability of the (110)-RuO2 facet. Results on bulk properties of crystalline ruthenium dioxide RuO2 are in good agreement with the nowadays available experimental evidences and previous DFT studies.

Effects of transition metal doping on surface properties and resistance to Cl− adsorption of α-Al2O3

Under the aqueous environment simulated by VASPsol, structures and work functions of surfaces, adsorption behavior of Cl− on the pure α-Al2O3 (0001) and fourth period transition metal (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) doped α-Al2O3 (0001) slabs have been calculated by Vienna Ab-initio Simulation Package (VASP) based on density functional theory (DFT), which ultimately affect the pitting resistance of the target surface. The results evidence that the interlayer spacing of the first and the second atomic layers and characteristic Al-O bond lengths of doped surfaces showed a trend of increasing first and then decreasing with the incremental nuclear charges of doped atoms, which is contrary to the change of work functions and the adsorption energy of Cl− on the corresponding surfaces. These changes are correlative and essentially depend on the variations of the surface charge distribution and surface reactivity. The doping of Sc, Ti, V, Cr, Mn, Fe, Co and Ni is proved to reduce the stability of the Al2O3 surface and makes it easier for Cl− to adsorb on the surface. The doping of Cu and Zn increases the minimum adsorption energy of Cl− on the surface by 0.07 and 0.09 eV, while the surface reactivity was not reduced. The loosening of the upper structure and the decrease of work function are the precursors of surface deterioration.

Construction photothermal/pyroelectric property of hollow FeS2/Bi2S3 nanostructure with enhanced full spectrum photocatalytic activity

In order to take full advantage of solar energy, the development of full spectrum photocatalysts have become focus of attention. Herein, the hollow FeS2/Bi2S3 is constructed to enhance photocatalytic hydrogen evolution by the synergistic effect of photothermal property and thermoelectric effect. This is the first time to exploit the ΔT between the heat generated by photothermal effect and the cold photocatalytic system to provide driving force for pyroelectric effect. The FeS2 can make the most of near infrared (NIR) light to thermogenesis and provide hot end for thermoelectric effect of Bi2S3. Under the environment of circulating condensed water, Bi2S3 will cause spontaneous polarization and generate thermoelectric effect under the influence of temperature difference (ΔT) fluctuation. It releases the charge on the surface, so as to control the migration direction and rate of carriers in FeS2 nanoparticles. The results of surface work function and COMSOL numerical simulation display that the improvement of photocatalytic efficiency is on account of the lower surface work function and high pyroelectric potential. The hydrogen production efficiency of FeS2/Bi2S3-3 reaches 16.8 mmol g−1 h−1 and has favourable cycle stability. This study provides a new insight for the design and construction of novel full spectrum photocatalysts.

Linear Correlation between Water Adsorption Energies and Volta Potential Differences for Metal/water Interfaces

Potential of zero charge (PZC) is an important reference for understanding the interface charge and structure at a given potential, and its difference from the work function of metal surface (ΦM) is defined as the Volta potential difference (ΔΦ). In this work, we model 11 metal/water interfaces with ab initio molecular dynamics. Interestingly, we find ΔΦ is linearly correlated with the adsorption energy of water (Eads) on the metal surface. It is revealed that the size of Eads directly determines the coverage of chemisorbed water on the metal surface and accordingly affects the interface potential change caused by electron redistribution (ΔΦel). Moreover, ΔΦ is dominated by the electronic component ΔΦel with little orientational dipole contributing, which explains the linear correlation between ΔΦ and Eads. Finally, it is expected that this correlation can be helpful for effectively estimating the ΔΦel and PZC of other metal surfaces in the future work.

Universal alignment of surface and bulk oxygen levels in semiconductors

Oxygen and hydrogen are the two most important impurities in semiconductors because of their ubiquitous presence in growth and device processing environments, and consequently, their incorporation strongly influences electronic and electrical properties. Therefore, a deeper understanding of the interaction of these species with the semiconductor surface and bulk defects is necessary for enabling the development of devices based on them, such as photovoltaic and photocatalytic systems and fuel cells. It is shown here, through the analysis of the reported surface work function values and substitutional bulk O-defect energies, that the surface Fermi level of semiconductors with physisorbed O2 lies universally at approximately −5.1 eV below the vacuum level. Similarly, the results show that the energy of substitutional bulk O-related amphoteric defects incorporated during the crystal growth also has a universal energy of ∼−5.0 eV with respect to the vacuum level for most semiconductors investigated. It is shown that the process of “surface transfer doping” involving an adsorbed water film on the semiconductor surface is likely responsible for the universal alignment of oxygen levels.

Sensing by Surface Work Function Modulation: High Performance Gas Sensing using van der Waals Stacked Bipolar Junction Transistor

We propose and demonstrate a new principle for sensing. Impact of light, radiation, charge particle and gas molecular etc. can induce the change of the surface work function of a solid. This will result in a profound effect to a device’s output, such as current and voltage. Such an effect is in particular outstanding in a nanodevice using van der Waals stacked two-dimensional (2D) atomic crystals. In this paper, a study of a gas sensing device relying on such a principle is presented. Van der Waals bipolar junction transistors (V2D-BJT) based on vertically stacked MoS2/WSe2/MoS2 and WSe2/MoS2/WSe2 heterostructures are separately fabricated to detect NH3 and NO2 gas. Kelvin probe force microscope measurement shows that the work function change at the overlapped region of MoS2/WSe2/MoS2 heterostructure upon exposure to 50 ppm NH3 gas is 80 meV. In addition, the MoS2/WSe2/MoS2 V2D-BJT manifest superior sensing performances in detecting NH3, while the WSe2/MoS2/WSe2 V2D-BJT exhibited superior NO2 sensing properties in terms of high sensitivity (18), low power dissipation (12 nW), fast response (26 s) and recovery (14 s) time. We demonstrate that the NH3 and NO2 gas can be selectively detected by the V2D-BJT through changing the van der Waals heterostructure stacking sequences.

Covalent Organic Frameworks with Low Surface Work Function Enabled Stable Lithium Anode.

Uniform deposition and distribution of lithium ion (Li+ ) on the surface of lithium metal anode is crucial for long-life and high-safety lithium metal batteries. However, the preparation of stable solid-electrolyte interphase (SEI) is mostly based on trial and error in the absence of guideline. Herein, covalent organic framework (COF) with high Young's modulus and low surface work function is in situ synthesized on Li anode to stabilize Li|electrolyte interface. Notably, Young's modulus, mechanical index for Li dendrite resistance, and surface work function, electrical index for Li+ distribution, can be regarded as macroscopically detectable indicators to evaluate the artificial SEI before battery assembly. The COFTpPa modified Li metal anodes delivered stable cycling over 1000 (2000) h at high current density of 5 (2) mA cm-2 in the ether-based electrolyte, and the full cells with commercial LiFePO4 electrode (mass loading of 16.5mg cm-2 ) demonstrate remarkably enhanced cycling performance with a high reversible capacity of 152.3 mAh g-1 (retention of 96.8%) after 300 cycles.

All solution-processed composite (ZnO+PEG)/Cs2CO3 with robust electron injection tailoring and its application to efficient inverted near ultraviolet organic light-emitting diodes

Electron injection/extraction is always highly emphasized in tailoring optoelectronic properties of organic electronic devices. Solution-processed ZnO, ZnO doped with poly(ethylene glycol) (ZnO+PEG) and composite (ZnO+PEG)/Cs2CO3 have been synthesized and characterized with X-ray diffraction, scanning electron microscope, energy dispersive X-ray spectroscopy, atomic force microscopy, X-ray/ultraviolet photoelectron spectroscopy. It is demonstrated that ZnO, ZnO+PEG and (ZnO+PEG)/Cs2CO3 exhibit superior film morphology and robust electron injection tailoring. With (ZnO+PEG)/Cs2CO3 as electron injection layer and 9-(4′-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)-[1,1′-biphenyl]-4-yl)-9H-carbazole as fluorescent molecule, efficient inverted near ultraviolet organic light-emitting diode is assembled and gives satisfactory radiance of 10.6 mW cm−2 and external quantum efficiency of 1.9% with short-wavelength peaking at 409 nm and full width at half maximum of 52 nm, which are superior to the corresponding counterparts. The electron injection capacity of ZnO, ZnO+PEG and (ZnO+PEG)/Cs2CO3 are further confirmed by current-voltage characteristics, impedance spectroscopy transition curves and surface work function measurements. Such a solution-processed composite (ZnO+PEG)/Cs2CO3 also pave an alternative approach for constructing other multilayer electronic devices.

Experimental Studies of Electron Affinity and Work Function from Aluminium on Oxidized Diamond (100) and (111) Surfaces

Three different procedures are used to deposit aluminium onto O‐terminated (100) and (111) boron‐doped diamond, with the aim of producing a thermally stable surface with low work function and negative electron affinity. The methods are 1) deposition of a > 20 nm film of Al by high‐vacuum evaporation followed by HCl acid wash to remove excess metallic Al, 2) deposition of <3 Å of Al by atomic layer deposition, and 3) thin‐film deposition of Al by electron beam evaporation. The surface structure, work function, and electron affinity are investigated after annealing at temperatures of 300, 600, and 800 °C. Except for loss of excess O upon first heating, the Al + O surfaces remain stable up to 800 °C. The electron affinity values are generally between 0.0 and −1.0 eV, and the work function is generally 4.5 ± 0.5 eV, depending upon the deposition method, coverage, and annealing temperature. The values are in broad agreement with those predicted by computer simulations of Al + O (sub)monolayers on a diamond surface.

Ultraviolet light stimulated water desorption effect on emission performance of gated field emitter array

The performance of silicon gated field emission arrays (GFEAs) was characterized before and after ultraviolet (UV) light exposure. Emission and gate leakage currents were measured on 1000 × 1000 tip arrays by sweeping the gate voltage to 40 V DC with a fixed DC collector voltage of 100 V DC. UV light exposure was used to desorb water molecules from the GFEA surfaces. It was found that, before UV exposure, the gate current was 6 mA at 40 V, whereas after 70 min of UV exposure, the gate current decreased to 0.46 mA, indicating a more than ten times reduction in leakage current between the gate and the emitter. Similarly, the observed collector current was 94 μA at 40 V before exposure, and after UV exposure, the collector current increased to 1.33 mA, indicating an improvement of more than 14 times. During the experiments with UV light, residual gas analyzer measurements showed that the partial pressure for water increased by greater than ten times after 60 min of exposure and then decreased by 1 order of magnitude after 100 min of exposure. The emission and leakage current changes remained even after turning off the UV lamps for several tens of minutes; however, upon the exposure to the atmosphere for a few days, those changes reversed. The enhancement could again be observed after additional UV exposure indicating that the adsorbates (mainly water along with others) on the surface affected the leakage between gate and emitter and field emission. Based on analysis of the IV characteristics before and after UV exposure, the work function of the emitter surfaces increases while the portion of the array tips that emits expands resulting in a decrease in the calculated array tip sharpness as duller tips now emit.

Boosted photoinduced cathodic protection performance of ZnIn2S4/TiO2 nanoflowerbush with efficient photoelectric conversion in NaCl solution

Boosting the photoinduced cathodic protection (CP) for metals in NaCl solution is crucial for achieving the application of photoelectrochemical conversion in marine corrosion protection. An environmentally friendly three-dimensional ZnIn2S4/TiO2 nanoflowerbush (NFB) photoelectrode was constructed with sufficient interfacial incorporation based on the ultrafine branched TiO2 nanobush substrate via a facile in situ hydrothermal method. This ZnIn2S4/TiO2 NFB nanoheterojunction composite behaves excellent photoinduced CP performance for metallic materials. This is owing to the optimized construction of photoelectrode with relatively negative band potential, wide solar spectrum response, effective generation and collection of photoinduced electrons. The morphology, distribution and performance of ZnIn2S4 vary with different substrates. The pure ZnIn2S4 powder is nanoflower sphere composed of a lot of thin petals; when ZnIn2S4 grow onto the FTO glass, the ZnIn2S4 petals are distributed vertically on the two-dimensional structure; when ZnIn2S4 grow on the three-dimensional ultrafine branched TiO2 nanobush substrate, a ZnIn2S4/TiO2 NFB architecture with ultrafine TiO2 branchlet cores are formed. This ZnIn2S4/TiO2 NFB photoelectrode can achieve efficient photoinduced CP performance for pure copper and low alloy steels with different negative self-corrosion potentials, such as Cu (−0.18 V), E40 (−0.45 V), Q345 (−0.55 V) and Q235 carbon steel (−0.65 V) under simulated sunlight irradiation in NaCl solution without additional hole scavengers, and the photoinduced CP current densities reach as high as 170, 72, 63 and 44μAcm−2, respectively. The high-efficiency photoinduced CP performance is closely related to the band structure gradient matching between the TiO2 and ZnIn2S4, prompting both the widened absorption of solar spectrum and high-efficiency transfer of the photogenerated electron–hole pairs. Importantly, the negative band potential of ZnIn2S4 endues a lower surface work function which benefits the electron transport to the under-protected metallic materials to achieve photoinduced CP. This photoelectrode exhibits huge photoinduced CP application potential in marine environments.

Exploring the electronic properties of Si-doped AlN (0001) surface with Cs adsorption layer for solar cell anode applications

The first-principles calculations are utilized to systematically investigate the adsorption behaviors of Cs on Si-doped AlN(0001) surfaces and its potential applications as anode material with low work function. Five Si substitution doping models and eight Cs adsorption surface models are established, respectively. Results show that Si atom tends to replace surface Al atom to form an n-type doping structure. Based on the most stable Si-doped surface, different Cs adsorption configurations with different adsorption sites and Cs coverage are analyzed. During Cs adsorption process, Cs acts as the electron donor and AlN surface is the electron acceptor. The large charge transfer at Cs adsorption interface induces a dipole moment pointing from Cs to AlN surface, which can effectively cut down the surface work function. Specifically, the 0.75 ML Cs-adsorbed surface shows the lowest work function of 0.62 eV, which has great potential application as the anode material of PETE devices. The findings in this work are generally applicable, which may shed new light on future exploration and development of low work function anode materials of PETE devices.

Modeling and Simulation of Ultrahigh Sensitive AlGaN/AlN/GaN HEMT-Based Hydrogen Gas Detector With Low Detection Limit

Presented through this work is a steady state analytical model of the GaN HEMT based gas detector. GaN with high chemical and thermal stability provides promises for detectors in hazardous environments. However, HEMT sensor resolution must be improved to develop high precision gas sensors for automotive and space applications. The proposed model aids in systematical study of the sensor performance and prediction of sensitivities. The linear relation of threshold voltage shift at thermal equilibrium is used in predicting the sensor response. Numerical model for the reaction rates and the electrical dipole at the adsorption sites at the surface and metal/semiconductor interface have been developed and the sensor performance is analyzed for various gas concentrations. The validation of the model has been achieved through surface and interfacial charge adsorption-based gate electrode work function, Schottky barrier, 2DEG and threshold voltage deduction using MATLAB and SILVACO ATLAS TCAD. Further the applicability of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{g}_{d}$ </tex-math></inline-formula> (channel conductance) as gas sensing metric is also presented. With high <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{I}_{D}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{g}_{d}$ </tex-math></inline-formula> percentile sensitivities of 118.5% and 92 % for 10 ppm hydrogen concentration. The sensor shows capability for detection in sub-ppm levels by exhibiting a response of 0.043% for 0.01ppm (10 ppb) hydrogen concentration. The detection limit of the sensor (1% sensitivity) presented here is 169 ppb and the device current increases by <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$34.2~\mu \text{A}$ </tex-math></inline-formula> for 1ppb hydrogen concentration.

Phosphorylation of NiAl-layered double hydroxide nanosheets as a novel cocatalyst for photocatalytic hydrogen evolution

In previous studies, it has been shown that phosphorus and phosphate can improve the conductivity, change the electronic structure, and accept electrons from catalysts. In this study, we obtained phosphorylated NiAl-layered double hydroxide (P-LDH) nanosheets and used them as a novel cocatalyst in photocatalytic hydrogen evolution. After assembly with g-C3N4 via an in situ process, these noble-metal-free composite photocatalysts showed superior photocatalytic hydrogen evolution activity. It was also found that the efficiency of H2 production on the optimal composite was 1.5 times that of Pt-modified g-C3N4. Characterization of the photocatalysts revealed that the effects of P-LDH were different from those of other bimetallic LDHs, showing a lower overpotential and faster reaction kinetics of H2 evolution. Moreover, it was found that P-LDH has a higher surface work function than that of g-C3N4, leading to the formation of an interfacial electric field from CN toward P-LDH. Therefore, modifying P-LDH can efficiently improve the interfacial charge transfer rate, suppress photogenerated charge recombination, and lower the surface overpotential of g-C3N4. This study serves as guidance on the design of more effective cocatalysts for photocatalytic hydrogen evolution reactions.

Enhanced Weak-Light Detection of Perovskite Photodetectors through Perovskite/Hole-Transport Material Interface Treatment.

Enhancement in weak-light detection and other photodetection properties was observed for organic-inorganic halide perovskite photodetectors as a result of benzylammonium iodide (BzAI) treatment at the methylammonium lead triiodide (MAPbI3) and hole-transport layer (HTL) interface. After treatment, growth of the two-dimensional Ruddlesden-Popper perovskite phase was observed at the MAPbI3 surface, which shifted the overall surface work function upwards and thus effectively facilitated charge transfer across the MAPbI3/HTL interface. As a result, the fully fabricated device with 10 mg/mL (BzAI/isopropanol) treatment exhibited shorter rise time (trise) and decay time (tdecay) of 53 and 38 μs, respectively, compared to trise and tdecay of 214 and 120 μs, respectively, for the pristine MAPbI3 sample. In addition, the BzAI-treated device exhibited larger linearity compared to the pristine MAPbI3 sample, demonstrating a high and stable specific detectivity of 1.49 × 1013 to 2.14 × 1013 Jones under incident light intensity of 10-3 to 100 mW/cm2, respectively.

Prolonged Annealing Improves Hole Transport of Silicon Heterojunction Solar Cells

The Schottky barrier is a fundamental issue when an n‐type transparent conductive oxide (TCO) is contacted with p‐type hydrogenated amorphous silicon (p‐a‐Si:H) in silicon heterojunction (SHJ) solar cells. Herein, it is found that the hydrogen (H) atoms in p‐a‐Si:H diffuse into tungsten‐doped indium oxide (IWO) during annealing, which improves the electric properties of both the IWO films and the p‐a‐Si:H/IWO interface. H diffusion reduces the surface work function of p‐a‐Si:H, and thus reduces the Schottky barrier between the p‐a‐Si:H and the IWO. Consequently, it improves the hole transport of SHJ solar cells; i.e., both the fill factor (FF) and power conversion efficiency (PCE) substantially increase. These findings provide a new strategy to optimize the FF of SHJ solar cells.

Impact of residual gas on the optoelectronic properties of Cs-sensitized In0.53Ga0.47As (0 0 1) surface

Near-infrared InxGa1-xAs photocathode with better optoelectronic properties is a good candidate for low-light-level (LLL) night-vision system. However, the residual gases in the ultra-high vacuum (UHV) system inevitably affects the stability and photo-emission performance of LLL photoelectric devices such as their quantum efficiency and life-time. In this study, the first-principles calculations were used to investigate the adsorption effect of five different residual gas species, including H2, CH4, CO, H2O and CO2 on Cs-sensitized In0.53Ga0.47As (001) β2 (2 × 4) surface. The study results indicate that CO2 gas molecule is the most easily attached to the Cs-sensitized surface. The adsorption of residual gases leads to the formation of a new dipole pointing from inner Cs atoms to gas molecules. It makes the charge center of the adsorbates escape from the surface, which weakens the interaction between the inner Cs atoms and the clean surface. This results in the increase of the surface work function and degradation of the performance of photoelectric devices. Also, the adsorption of residual gas molecules influences the absorption and reflection coefficients of Cs-sensitized In0.53Ga0.47As (001) β2 (2 × 4) surface.