Surface Al Atoms Research Articles

Stoichiometric reconstruction of the Al2O3(0001) surface.

Macroscopic properties of materials stem from fundamental atomic-scale details, yet for insulators, resolving surface structures remains a challenge. We imaged the basal (0001) plane of α-aluminum oxide (α-Al2O3) using noncontact atomic force microscopy with an atomically defined tip apex. The surface formed a complex ([Formula: see text] × [Formula: see text])R±9° reconstruction. The lateral positions of the individual oxygen and aluminum surface atoms come directly from experiment; we determined with computational modeling how these connect to the underlying crystal bulk. Before the restructuring, the surface Al atoms assume an unfavorable, threefold planar coordination; the reconstruction allows a rehybridization with subsurface O that leads to a substantial energy gain. The reconstructed surface remains stoichiometric, Al2O3.

Investigation on Be, Mg, Ca, B, Ga, C, Si, and Ge atoms doped on Al (111) surface and their effect on oxygen adsorption: A first-principle calculation

Oxygen corrosion of the aluminum alloy has a substantial impact on the durability of engineering materials and equipment. To better understand how various alloy elements affect oxygen corrosion, we used first-principle DFT method to study the role of each given dopant atom (Be, Mg, Ca, B, Ga, C, Si, and Ge) in the formation of surface oxide film on the Al (111) surface, in the case replacing a surface Al atom. Our calculations show that the electron gain or loss abilities of these alloy elements differ from that of Al metal, with Be, B, C, and Si atoms tending to penetrate into the surface, Mg, Ca, and Ga atoms protruding, and Ge atom just embedded into the Al (111) surfaces. Additionally, the type of doping element at the Al (111) surface can greatly affect the O2 absorption, predicting that Ca-doped surface absorb the least amount of oxygen, followed by Si-doped surface, with Be-, Mg-, B-, Ga-, C-, and Ge-doped surfaces showing higher O2 absorption. Based on adsorption energy (Eads) and partial density of states (PDOS) analysis, we conclude that the doped Al (111) surfaces can make O2 adsorption different owing to the difference in electronic properties of the doping atoms. These insights draw a greater knowledge of the role of alloy elements in the oxygen corrosion of Al alloys, enabling guidance for designing more corrosion-resistant materials.

H2 generation from H2S decomposition on Al

Direct decomposition of H2S not only generates H2 but also reduces sulfur (S) emission. Proper catalysts and/or reactant materials are needed to catalyze its dissociation and accommodate the decomposed S in an economical way. Here, by using the density functional theory (DFT) based calculations, we show that Aluminum (Al) can facilitate the dehydrogenation of H2S and the formation of H2. The remaining S adatoms continue to accumulate on Al surface to reach high S coverage (>0.6 monolayer, ML). Then, S atoms react with surface Al atoms to afford a layer of amorphous Al2S3 (a cathode material for Al–S battery) at relatively low temperature (e.g. 773 K), which will transform into crystal phase at higher temperature (e.g. 1073 K). Our research presents a new way to produce H2 and Al2S3 from H2S and Al.

Effect of shear strain on the electronic and optical properties of Al-doped stanane.

The quasi-metallic properties of stanene limit its prospects in optoelectronic devices. Based on first-principles calculations, a systematic study is conducted on the tuning effects of surface hydrogenation and Al atom doping on the electronic and optical properties of stanene. Surface hydrogenation serves as an ideal way to open the forbidden band of stanene, and Al atom doping further increases hydrogenated stanene (stanane) band gap to 0.460eV. Deformation has a minor impact on the stability of the stanane-Al structure, while shear strain can effectively modulate the band gap engineering of the doped system, reducing the band gap value from 0.460 to 0.170eV. Deformation induces a redshift in the absorption peak and reflectance, also slowing down the rate of decrease in the absorption coefficient, and enhancing the peak value of light reflectance, which is positively correlated with the degree of shear strain. These findings hold promise for expanding the potential application of monolayer stanane in semiconductor devices. All calculations are performed using CASTEP module in Materials Studio based on the density functional theory (DFT). The Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) is employed to describe the exchange-correlation energy (Perdew et al., Phys Rev Lett 77(18), 1996). We construct models for both stanene and stanane. The original unit cell of stanene has two Sn atoms, while stanane consists of two Sn atoms and two H atoms, and expand them to a 3 × 3 × 1 supercell with a vacuum layer of 20Å in height to prevent interlayer coupling. After convergence testing, the plane-wave cutoff energy is set to 450eV, and the energy convergence threshold is set to 1 × 10-5eV. The maximum residual stress for each atom is set to 0.01eV/Å. Brillouin zone sampling is performed using a 6 × 6 × 1k-point mesh based on the Monkhorst-Pack method (Monkhorst and Pack, Phys Rev B 13(12), 1976). The k-point accuracy of the density of states and optical properties is 9 × 9 × 1. All calculations are performed using the more advanced OTFG ultrasoft pseudopotential, and structural relaxations are performed using supercells to ensure that the model is fully relaxed. We use the HSE06 functional to calculate the energy band structures of stanane-Al deformed to 0%, 4%, and 8%, resulting in band gap values of 1.465eV, 1.368eV, and 1.016eV, respectively. These values are significantly higher than those obtained using the PBE functional (0.460eV, 0.397eV, and 0.170eV). However, the shapes and trends of the band structures obtained from both PBE and HSE06 calculations are similar. Additionally, the calculation time needed by HSE06 is greatly longer than PBE, which exceeds the capabilities of our computer hardware, and cannot support all subsequent calculations. To investigate the influence of deformations on the variation of band gap values and to conserve computational resources, the subsequent calculations in this study use the PBE functional.

Surface evolution of AlGaN nanowire decorated by cesium atoms: A first principle study

Surface decoration with cesium on the photocathode is an effective method to reduce surface barriers and promote electron emission. In this study, we investigated the changes in surface properties of AlGaN nanowire coated with Cs atoms using first-principle calculations. The N bridge site is the most stable site for a single atom deposition. As more Cs atoms are deposited on the surface, the average adsorption energy increases along with the weakened ionization of Cs adatoms. The Bader charge of surface Al and Ga atoms undertake significant changes in comparison with tiny alteration of N atoms, especially for Al atom with a decrease of 0.41 |e|/atom. The variation in surface work function reaches a maximum of 2.3 ∼ 2.5 eV at Cs coverage of 0.5 ∼ 0.75 ML, which can be attributed to the synergistic effect of dipole length and charge redistribution. The donor states underneath CBM induced by the orbital overlap of Al-p, Ga-p and Cs-d electrons accompanied by the downward band bending in the band structures. This work provides theoretical insight into the surface evolution of Cs-decorated AlGaN nanowires and offers guidance for the development of state-of-the-art nanostructured vacuum devices.

Atomic-Scale Etching Mechanism of Aluminum with Fluorine-Based Plasma

A first-principles density functional theory study has been carried out to investigate the etching and corrosion of aluminum with fluorine-based plasma. Aluminum is modeled with the (111) surface of aluminum metal and (0001) surface of alumina, which forms as a thin layer on aluminum. The adsorption of plasma gases, for example, CF4, CHF3, and F2, on both surfaces is investigated. CF4 and CHF3 are very weakly adsorbed on both surfaces. On the other hand, a F2 molecule dissociates on both surfaces, which leads to the interaction of more F atoms to the same Al atom. Maximum three F atoms are allowed to adsorb on a single surface Al atom, which paves the way to form an AlF3 molecule. For all F adsorption, the interaction between F and surface Al is much stronger with aluminum surface compared to alumina surface, which explains the relative difficulty of etching alumina. The isopropyl alcohol, which is used to clean the plasma chamber and removed nonvolatile AlF3, itself adsorb on Al(111) and Al2O3(0001) surfaces. This preadsorbed isopropyl alcohol further activates the neighboring surface Al atoms and thus induces stronger interaction between F and surfaces.

DFT study of small Re-Pt clusters supported on γ-Al2O3

The scarcity and high cost of raw materials such as platinum to synthesize catalysts, makes it desirable to reduce the amount of precious metal used preserving their overall catalytic performance. A promising solution to this is for example, using highly dispersed and catalytically active transition metal clusters supported on metal oxides, such as TiO2, CeO2, and γ-Al2O3. Thus, bimetallic clusters can potentially offer additional catalytic enhancement due to a synergy between the involved metal atoms. We focus our attention on a computational, DFT study of small bimetallic Re-Pt clusters supported on γ-Al2O3. We aim to determine global minima configurations of Re-Pt clusters (up to 5 atoms) on the γ-Al2O3 (100) surface by using a Basin Hopping global optimization scheme. The adsorption and mixing energies are calculated as a function of clusters size and composition, thus providing a quantifiable picture of the stability of such bimetallic clusters. Our results show that bimetallic Re-Pt supported clusters exhibit higher stability compared to monometallic Re or Pt clusters. Regarding the bonding mechanism between metal clusters and the oxide substrate, our calculations indicate that the contribution to the PDOS of the 6-s states of both Re and Pt atoms is negligible, compared to that of the 5s-states. Furthermore, projected density of states (PDOS) analysis indicate that no bond is formed between the surface Al atoms and clusters. A Bader charge transfer analysis reveals that Re-rich clusters transfer electrons to the γ-Al2O3 substrate, while the opposite is true for their Pt-rich counterparts. Finally, various cluster nucleation pathways are investigated finding that the most favorable nucleation routes lead to the Re3Pt2 and Re5 clusters, while the coalescence of Pt atoms into Pt clusters is energetically unfavorable.

Congruent dissolution of Al2O3 in a K-disilicate melt, with constraints on the reaction mechanism

High resolution O 1s X-ray Photoelectron Spectroscopy (XPS) was employed to determine oxygen speciation in 31 mol% K2O silicate glass containing 0.0, 1.0 and 3.0 mol% dissolved Al2O3. Highly resolved bridging oxygen (BO) and non-bridging oxygen (NBO) peaks were observed. The NBO peak intensity decreased and the BO increased with dissolution of Al2O3. The enhanced BO intensity is localized at two binding energies (BE), one characteristic of a Si-O-Si moiety (e.g., quartz, or v-SiO2), the other of a Si-O-Al moiety (e.g., kyanite, Al2SiO5). The dissolution reaction proceeds in two steps; NBO− of a Q3 species attacks and bonds to an under-coordinated surface Al atom of crystalline Al2O3. The reaction produces a Si-O-Al surface species thus converting the attacking NBO to a BO and altering the reactant Q3 species to a Q4 product species:[Q3]−melt + [AlO1.5]surface → [Q4]0melt + [AlO2]−meltThe second step involves reaction of [AlO2]−melt with K+converting it from a modifier cation to a charge compensator, resulting in the polymerization reaction:[K-Q3]0melt + [AlO2]−melt → [Q4]0 + [KAlO2]0melt + 1/2O2−The stoichiometric reaction is the sum of the two:[Q3]−melt + [K-Q3]0melt + [AlO1.5]surface → 2[Q4]0 + [KAlO2]0melt + 1/2O2−Al species detached from the solid surface initially must be bonded to some NBO. Within the melt, nucleophilic substitution attack of the Al-bearing species by Q3 species converts NBO to BO, resulting in [KAlO2]melt being bonded four BOs. This reaction may not go to completion in all melts, depending on dissolved Al content and melt composition.

Wet-etching mechanism of a semi-sphere pattern on sapphire substrate

A semi-spherical pattern can be converted to a triangular pyramid comprising three r-planes using chemical wet etching with a mixture of sulfuric and phosphoric acids. The apex Al atom on the triangular pyramid with numerous dangling bonds can be subject to etching with prolonged exposure. After removing the apex Al atoms, Al atoms underneath the r-plane can be readily etched and low incline-angle, side-facet planes are developed. The concentrations of SO42- and PO43- ions in the mixed etching solution greatly affect the etching rate on the patterns on the sapphire surface. The SO42- and PO43- etchant ions in the etching solution will react with the etching site (surface Al atoms) and form Al2(SO4)3 and AlPO4. The molar ratios of the reactants (surface Al etching sites and acid molecules) are different. Given the size of the SO42- radicals (the average bond length of the sulfate ion is 1.49 Å), the other surface Al atoms next to the Al atoms engaging with the SO42- radicals cannot be close to any free SO42- radicals in the etching solution. This effect is called the “screening effect” or “spatial barrier effect.” For forming AlPO4, one PO43- ion can bond with one etching Al site without being limited by the spatial screening effect. There is no spatial screening effect for the phosphoric acid etching of a sapphire surface. Thus, phosphoric acid would have a higher etching rate of a sapphire surface than sulfuric acid. In this study, the bottom of the triangular pyramid is not an equilateral triangle. Instead, the sides of the bottom triangular of the triangular pyramid are curved. The availability of etching Al sites on the etching side-facet surface and ridges of the triangular pyramid can explain the curved sides of the bottom triangle of the triangular pyramid.

Spontaneous dissociation of H2 and high energy barrier of H invasion on W doped Al2O3(0001) surface

In view of the wide use of tungsten in fusion experimental devices and the importance of hydrogen isotopes permeation, here we studied the adsorption, dissociation, diffusion and invasion behavior of hydrogen on W doped α-Al2O3 (0001) surface. Based on the first-principle approaches, we found the W substitution for a top surface Al atom is the most energetically favorable. H2 molecule prefers to be adsorbed on the surface W and spontaneously dissociates into two H anions. Near the W defects, H atoms favor to be adsorbed at the W and Al sites rather than O sites on the surface, and within the subsurface layer H can only bond to W stably. As a result, H migration to subsurface should occur around W with an energy barrier as large as 4.22 eV which is much larger than the 1.91 eV around the O atom on undoped α-Al2O3 (0001) surface. These findings suggest that W surface doping is beneficial to α-Al2O3 as tritium permeation barrier.

Atomistic insights into heterogeneous reaction of hydrogen peroxide on alumina particles: Combining DFT calculation and ReaxFF molecular dynamics simulations

Recently, the heterogeneous reaction between hydrogen peroxide and mineral dust particles has a significant effect in the tropospheric chemistry. In this research, a comprehensive and multiscale theoretical study for the heterogeneous interaction between alumina particles and H2O2 has been performed. It is found that H2O2 and H2O could approach to alumina particle via molecular adsorption and decompose adsorption. The H2O2/H2O molecule adsorbs on the alumina by the weaker interactions between its O atom and surface Al atom. The electrostatic effect dominates the attractive part of the adsorption energy and the induction effect is another important attractive part of the binding energy. Besides, H2O2 and H2O decompose on the surface to form Al-OH bond and the surface Al atom is oxide after this interaction. The calculation also showed that the uptake of H2O2 on alumina is suppressed with the increase of humidity, and it may because water molecules could compete for the surface reaction sites. Based on these findings here, we hope it can be useful for further understanding the effect of the heterogeneous reaction between hydrogen peroxide and particulate matter on human health.

Atomic-level mechanism of the effects of NOx species on Pb adsorption over the Al2O3 sorbent surface

NOx is widely considered to be one of the most important components in the flue gas. A systematically theoretical study based on density functional theory was conducted to provide an atomic-level understanding of the effects of NOx on Pb adsorption over the Al2O3 surface. The results suggest that the adsorption energies of Pb over Al2O3 (110) surface are larger than those of Pb over Al2O3 (001) surface. Al2O3 (110) surface is more favorable for Pb adsorption than Al2O3 (001) surface. NO and NO2 are adsorbed at the same active adsorption site (surface Al atom). NOx competes with Pb for the active sites on the Al2O3 surface. The notable charge accumulation and depletion are accountable for the intense interaction of Pb and NOx-covered Al2O3 surface. At low concentration of NOx, the adsorption of NO and NO2 over Al2O3 surface increases the Pb adsorption capacity of Al2O3 sorbent through strengthening the activity of its neighbor Al and O sites. The formation of Pb-Al and Pb-N bonds can greatly strengthen Pb adsorption over Al2O3 (110) surface in the presence of NO/NO2. However, the high-concentration NO and NO2 inhibit Pb adsorption over Al2O3 surface due to the competitive adsorption between Pb and NO/NO2.

Heterostructure-based devices with enhanced humidity stability for H2 gas sensing applications in breath tests and portable batteries

Semiconducting metal oxide - based gas sensors exhibit outstanding sensitivity, although humidity in the analyte typically hampers precise measurements. In this work it was shown that a 5−6 nm thin Al2O3 nano-layer is particularly beneficial in reducing the interference due to humidity of p-type conductivity copper oxide-based gas sensors. An effective approach from chemical solutions at 75 °C and thermal annealing at 600 °C was used to grow copper oxide nano-crystallite layers. The Al2O3 nano-layers were subsequently deposited on top of copper oxide by atomic layer deposition in a high-aspect-ratio regime at 75 °C. The morphological, structural, chemical, vibrational, electronical and sensor characteristics of the heterostructured nano-crystallite layers have been studied. The final nano-Al2O3/CuO heterostructure showed an increase in the response to H2 gas by 140 %, while long-term stability at low and high relative humidity was observed. The initial sensing response varied by only 10 % for an Al2O3 layer of 5−6 nm on top of CuO with a post-thermal annealing at 600 °C acting as an effective barrier for water vapor and oxygen. A comparison with CuO nanocrystallite layers covered by ALD with 6 nm and 15 nm of Al2O3 ultra-thin films on top demonstrates an exceptional stability of the hydrogen gas response at high relative humidity (84 % RH). Density functional theory-based calculations showed that the H2 molecule spontaneously dissociates over the formed Al2O3/CuO heterostructure, interacting strongly with the surface Al atoms, showing different behavior compared to the pristine CuO (111) surface, where H2 gas molecules are known to form water over the surface. The present study demonstrates that a thorough optimization of technology and surface properties due to coverage and formation of heterostructured nano-materials improves the humidity stability during H2 gas sensing applications which is important for real-world applications, e.g. portable battery analysis, H2 breath tests, along with environmental, medicine, security, and food safety diagnostic tests.

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.

Anomalous behavior of In adatoms during droplet epitaxy on the AlGaAs surfaces

Semiconductor quantum dots (QDs) in the InAs/AlGaAs system are of great importance due to their promising optoelectronic and nanophotonic applications. However, control over emission wavelength governed by Al content in the matrix is still limited because of an influence of surface Al content on QD size and density. In this paper, we study the growth of In nanostructures by droplet epitaxy on various AlGaAs surfaces. We demonstrate that an increase in the Al content leads to a decrease in the droplet density and an increase in their size, which contradicts the Stranski–Krastanov QD growth. Using a hybrid analytical–Monte Carlo model, we explain this phenomenon by the fact that In adatoms acquire higher mobility on a first indium monolayer which is bound to surface Al atoms. This assumption is confirmed by the fact that a temperature decrease does not lead to a great increase in the critical thickness of droplet formation on the Al-containing surfaces whereas it changes considerably on the GaAs surface. Furthermore, the Al content influence on the formation of In droplets is much less significant than on the growth of InAs QDs by the Stranski–Krastanov mode. This gives an opportunity to use droplet epitaxy to control the matrix bandgap without considerable influence on the QD characteristics.

Surface property of the Cu doped γ-Al2O3: A density functional theory study

Cu/γ-Al2O3 catalysts are widely used in many catalytic processes. Investigation into the catalysts structure at molecular level is the basis for the elucidation of the reaction mechanisms and favors the developments of the catalysts. In the present work, periodic density functional theory calculations were performed to investigate the interface of alumina with copper oxides. The interface model is chosen as the substitution of the surface Al atoms of γ-Al2O3 with Cu, and H is used as the ion for charge balance. It is found that the substitution of surface Al3+ by Cu2+ is thermodynamically accessible. Gibbs free energy calculations show that the dehydration temperature for the γ-Al2O3 (110) surface after substitution is higher than that of on the original γ-Al2O3 (110) and CuAl2O4 surface. The oxygen vacancy formation energies for the (100)-5Cu-dehy-2w and (110)-4Cu-dehy-2w are 213 and 367 kJ/mol, respectively. In addition, the Cu doped γ-Al2O3 interface could strengthen the binding of Cu with the alumina surface. The results provide molecular level insights for the understanding of the interface structures and physical chemistry properties of alumina with copper oxides.

Oxygen adsorption on the doped TiAl(1 0 0) surface

The oxygen adsorption on the doped TiAl(1 0 0) surface is investigated by the projector augmented-wave method within density functional theory. The changes of the oxygen adsorption energy due to elements of the 4d-period and IIIA and IVA groups being its first neighbors are estimated. It is shown that the 4d-metals substituting the Ti surface atom lead to decrease of the oxygen adsorption energy whereas opposite trend is occurred, if these metals substitute Al surface atom. Elements of IVA group on the Al-sublattice affect oxygen adsorption energy in the preferred site on the surface lesser than on the Ti-sublattice. In order to understand the mechanisms being responsible for the change of the oxygen adsorption energy, some electronic characteristics of the TiAl(1 0 0) surface doped by Zr, Rh, Ga and Si are analyzed. The factors responsible for the increase/decrease of the adsorption energy are discussed.

Full-visible multifunctional aluminium metasurfaces by in situ anisotropic thermoplasmonic laser printing

A new level of anisotropic plasmonic laser printing for aluminum multifunctional metasurfaces.

Adsorption of tetracyanoquinodimethane and tetrathiafulvalene on aluminium (100) surface – a first principle study of structural and electronic properties

ABSTRACTPossible adsorption configurations and electronic properties, such as charge analysis, density of states, work function and Schottky barrier height of tetracyanoquinodimethane (TCNQ) and tetrathiafulvalene (TTF) on Aluminium (100) surface is studied by using density functional theory methods with local density approximation (LDA), generalised gradient approximation (GGA), PBE and PBE-D2 methods. TCNQ is strongly adsorbed on Al(100) with adsorption energy of −3.66 eV. The charge is transferred from Al(100) to TCNQ and charge transfer occurs mainly through cyano group of TCNQ. Adsorption on Al(100) surface leads to downshift in energy difference between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of TCNQ by 2.5 eV as result of hybridisation of p orbital of carbon and nitrogen atoms of TCNQ and p orbital of surface Al atoms. Compared to TCNQ adsorption, TTF adsorption on Al(100) surface is having less adsorption energy and type of interaction is physisorption. The charge transfer from TTF to Al(100) surface leads to decrease in work function of Al by 0.24 eV. The n-type Schottky barrier height of TCNQ/Al(100) and p-type Schottky barrier height of TTF/Al(100) is 0.68eV and 1.97 eV, respectively showing that TCNQ and Al(100) are suitable for organic photovoltaic and electrochemical applications.

Diffusion-Driven Al-Doping of ZnO Nanorods and Stretchable Gas Sensors Made of Doped ZnO Nanorods/Ag Nanowires Bilayers.

A crystal-damage-free nanodoping method, which utilized the vacuum drive-in diffusion of Al into ZnO nanorods, was developed. In this method, vertical ZnO nanorod arrays that were grown by chemical bath deposition beforehand were deposited with Al thin film and subsequently heat-treated under a high vacuum. At an optimum condition, the surface Al atoms were completely diffused into ZnO nanorods, resulting in Al-doped ZnO nanorods. Stretchable gas sensors were fabricated by sequentially drop-casting the Al-doped ZnO nanorods and silver nanowires on polydimethylsiloxane substrate. The resistance and response of the sensor could be optimized through the elaborate control of relative densities of Al-doped ZnO nanorods and silver nanowires. The sensor showed a high response of 32.3% to 10 ppm of NO2 gas at room temperature, even under a large strain of 30%. The NO2-sensing mechanism of Al-doped ZnO nanorod/silver nanowire bilayer sensors is discussed on the basis of a synergistic interplay of Al-doped ZnO nanorods and silver nanowires.