First-principles study of Sr adsorption on InN (0001)

Structures of Sr adsorbed on InN (0001) surfaces are theoretically investigated by first-principles calculations. The adsorption energies of Sr on InN (0001) decrease with decreasing Sr coverage. An InN (0001)-(2 ×2) surface structure covered by a 1/4 monolayer of Sr at the T₄ sites may be the most energetically favourable. Sr atoms may substitute indium atoms, or accumulate at the voids inside InN films. The interstitial Sr defects may act as a potential source of compensation for the p-type behaviour of Sr-doped InN at the surface.     

Epitaxial growth of InN on nearly lattice-matched (Mn,Zn)Fe2O4

We have grown InN films on nearly lattice-matched (Mn,Zn)Fe₂O₄ (111) substrates at low temperatures by pulsed laser deposition (PLD) and investigated their structural properties. InN films grown at substrate temperatures above 400 °C show poor crystallinity, and their in-plane epitaxial relationship is [10-10]InN//[11-2](Mn,Zn)Fe₂O₄, which means that their lattice mismatch is quite large (11%). By contrast, high quality InN films with flat surfaces can be grown at growth temperatures lower than 150 °C with the ideal in-plane epitaxial relationship of [11-20]InN//[11-2](Mn,Zn)Fe₂O₄, which produces lattice mismatches of as low as 2.0%. X-ray reflectivity measurements have revealed that the thickness of the interfacial layer between the InN and the substrates is reduced from 14 to 8.4 nm when the growth temperature is decreased from 400 °C to room temperature. This suppression of the interface reactions by reducing the growth temperature is probably responsible for the improvement in crystalline quality. These results indicate that the use of (Mn,Zn)Fe₂O₄ (111) substrates at low growth temperatures allows us to achieve nearly lattice matched epitaxial growth of InN.     

Growth of InN films on spinel substrates by pulsed laser deposition

We have grown InN films on MgAl₂O₄(111) substrates with atomically flat surfaces using pulsed laser deposition (PLD) and compared their structural properties with those grown on (Mn,Zn)Fe₂O₄(111) substrates. It has been revealed that InN(0001) films grow on MgAl₂O₄(111) with an in‐plane epitaxial relationship of InN[1 00] // MgAl₂O₄[1 0], achieving a lattice mismatch minimum. The InN films exhibited a clear sixfold rotational symmetry, without 30° rotational domains and with a full width at half maximum value of the InN 0002 rocking curve being 17.5 arcmin. Comparison between InN films grown on MgAl₂O₄ and those on (Mn,Zn)Fe₂O₄ led us to conclude that suppression of the interfacial reactions between the InN films and the substrate is inherently important to obtain high quality InN on substrates with a spinel structure. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural properties of oxygen on InN(0 0 0 1) surface

First-principles calculations are performed to study the various structures of oxygen (O) adsorbed on InN(0 0 0 1) surfaces. It is found that the formation energy of O on InN(0 0 0 1) decreases with decreasing oxygen coverage. Of all the adsorbate induced surface structures examined, the structure of InN(0 0 0 1)-(2 × 2) as caused by O adsorption at the H₃ sites with 0.25 monolayers coverage is most energetically favorable. Meanwhile, nitrogen (N) vacancy can form spontaneously. Oxygen atoms may also substitute N atoms, or accumulate at the voids inside InN film or simply stay on the surface during growth. The oxygen impurity then acts as a potential source for the n-type conductivity of InN as well as the large energy band gap measured.     

Atomic structure for Mg on InN(0001) surface

Structures of Mg adsorbed on InN(0001) surfaces are theoretically investigated by first-principle calculations. Of all the structures examined, the structure of R30° as caused by Mg adsorption at the Top sites with 1/3 monolayers coverage is most energetically favorable. Mg atoms may also substitute indium atoms, or accumulate at the voids inside InN film. The interstitial Mg defects may act as a potential source of compensation for the p-type behavior of Mg-doped InN at the surface.     

The adsorption of oxygen and carbon monoxide on cleaved polar and nonpolar ZnO surfaces studied by electron energy loss spectroscopy

Electron energy loss spectroscopy (ELS) with primary energies e₀ ? 80 eV has been performed on ultrahigh vacuum (UHV) cleaved nonpolar (11?00) and polar zinc (0001) and oxygen (0001?) surfaces of ZnO to study the adsorption of oxygen and carbon monoxide. Except for CO on the nonpolar surface where no spectral changes in ELS are observed a surface transition near 11.5 eV is strongly affected at 300 K on all surfaces by CO and O₂. At 300 K clear evidence for new adsorbate characteristic transitions is found for oxygen adsorbed on the Zn polar surface near 7 and 11 eV. At 100 K on all three surfaces both CO and O₂ adsorb in thick layers and produce loss spectra very similar to the gas phase, thus indicating a physisorbed state.     

Optical properties of In2O3 oxidized from InN deposited by reactive magnetron sputtering

Unintentionally doped and zinc-doped indium nitride (U-InN and InN:Zn) films were deposited on (0 0 0 1) sapphire substrates by radio-frequency reactive magnetron sputtering, and all samples were then treated by annealing to form In₂O₃ films. U-InN and InN:Zn films have similar photon absorption characteristics. The as-deposited U-InN and InN:Zn film show the absorption edge, ∼1.8-1.9 eV. After the annealing process at 500 °C for 20 min, the absorption coefficient at the visible range apparently decreases, and the absorption edge is about 3.5 eV. Two emission peaks at 3.342 eV (371 nm) and 3.238 eV (383 nm) in the 20 K photoluminescence (PL) spectrum of In₂O₃:Zn films were identified as the free-exciton (FE) or the near band-to-band (B-B) and conduction-band-to-acceptor (C-A) recombination, respectively.     

A first principle study on the magnetic properties of Cu2O surfaces

Spin-polarized density functional theory calculations were performed to investigate the magnetism of bulk and Cu₂O surfaces. It is found that bulk Cu₂O, Cu/O-terminated Cu₂O(111) and (110) surfaces have no magnetic moment, while, the O-terminated Cu₂O(100) and polar O-terminated Cu₂O(111) surfaces have magnetism. For low index surfaces with cation and anion vacancy, we only found that the Cu vacancy on the Cu₂O(110) Cu/O-terminated surface can induce magnetism. For atomic and molecular oxygen adsorption on the low index surfaces, we found that atomic and molecular oxygen adsorption on the Cu-terminated Cu₂O(110) surface is much stronger than on the Cu/O-terminated Cu₂O(111) and Cu-terminated Cu₂O(100) surfaces. More interesting, O and O₂ adsorption on the surface of Cu/O terminated Cu₂O(111) and O₂ adsorption on the Cu-terminated Cu₂O(110) surface can induce weak ferromagnetism. In addition, we analysis origin of Cu₂O surfaces with magnetism from density of state, the surface ferromagnetism possibly due to the increased 2p–3d hybridization of surface Cu and O atoms. This is radically different from other systems previously known to exhibit point defect ferromagnetism, warranting a closer look at the phenomenon.     

Density functional theory study of Ir atom deposited on γ-Al2O3 (001) surface

Iridium adsorption on γ-Al₂O₃ (001) surface has been studied using the ab initio calculation method and the electronic structures of the bare and the Ir adsorbed γ-Al₂O₃ (001) surfaces have been analyzed. By modeling different adsorption sites, one can conclude that the energetically most favorable sites for the Ir are the top sites of the O atoms at the γ-Al₂O₃ (001) surface terminated with octahedral Al. Charge redistribution around the Ir atom adsorbed on the surface improves the activity of the Ir atom as a catalyst.     

Adsorption of water molecule on (001) and (110) surfaces of MgH2

First principle calculations have been performed to explore the adsorption characteristics of water molecule on (001) and (110) surfaces of magnesium hydride. The stable adsorption configurations of water molecule on the surfaces of MgH₂ were identified by comparing the total energies of different adsorption states. The (110) surface shows a higher reactivity with H₂O molecule owing to the larger adsorption energy than the (001) surface, and the adsorption mechanisms of water molecule on the two surfaces were clarified from electronic structures. For both (001) and (110) surface adsorptions, the O p orbitals overlapped with the Mg s and p orbitals leading to interactions between O and Mg atoms and weakening the O–H bonds in water molecule. Due to the difference of the bonding strength between O and Mg atoms in the (001) and (110) surfaces, the adsorption energies and configurations of water molecule on the two surfaces are significantly different.     

Adsorption mechanisms of fluorocarbon polymers at ultra low-k surfaces

We report ab initio investigations for the adsorption mechanisms of fluorocarbon polymers at ultra low-k (ULK) surfaces. As prototypical example we study a C₂F₄-molecule adsorbed at a passivated SiO₂:CH₃(001) surface. The calculated adsorption energies between 1.37 and 1.95 eV imply a chemisorption bonding mechanism. The detailed analysis of all investigated structures provides a simple rule to make a rough estimate of the stability of adsorbed fluorocarbon polymers. In addition, an adsorption pathway is proposed and used to suggest a possible adsorption mechanism, triggered by an electron transfer between the ULK surface and the C₂F₄ molecule.     

Site-specific interaction of H2O with ZnO single-crystal surfaces studied by thermal desorption and UV photoelectron spectroscopy

The adsorption and condensation of H₂O(D₂O) on ZnO(101̄0), (0001)Zn and (0001̄)O surfaces was investigated by means of thermal desorption (TDS) and UV photoelectron spectroscopy (UPS). The clean ZnO single-crystal surfaces were prepared by Ar-ion sputtering and annealing and characterised by Auger electron spectroscopy, LEED, UPS and work-function measurements. On all three surfaces six different adsorption states were found. In the monolayer regime there is a stronger bonding to Zn sites (desorption temperature 340 K) than to O sites (190 K), The bonding to the Zn sites seems to be accompanied by some clustering. Before the chemisorption layer is completed a first ice state is found whose desorption temperature shifts from 162 to 168 K with increasing exposures. At higher exposures the multilayer ice state is found at 152 K. On the (0001̄)O face defect-induced features were identified. The water lone-pair orbital 1b₁, whose energy falls between the O p and the Zn 3d emission of the substrate and which is known to show bonding shifts, was analysed using angle-resolved UPS. In the monolayer, the main chemisorption states are found at E_B^V(1b₁) = ?9.6 eV for the (0001)Zn face and at ? 10.6 eV for the (0001̄)O face and are compared with the multilayer ice emission at 1̄1.1 eV. The difference in binding energies shows the same trend as the TDS data. For the (101̄0) face the 1b₁ emission is very broad, indicating some overlap between different states.     

Interaction of O2 with low index faces of α-CuZn

The adsorption of O₂ and initial step of oxidation have been investigated, mainly at room temperature, for three different α-CuZn (75%Cu/25%Zn) surfaces ((110), (100) and (111)) by XPS. XAES, LEED, CPD and HREELS. No superstructures were detected on the LEED patterns during O₂ admission for the three faces, and from the beginning of adsorption Zn segregated to the surface. For (110), the interaction of oxygen follows the sequence: (1) dissociative chemisorption (up to ~ 20 L), accompanied by an increase of the work function and a single site occupancy as revealed by HREELS; (2) nucleation of ZnO only, indicated by a decrease of the work function, a shift of the Zn L₃M₄₅M₄₅ Auger transition and an emergence of a vibration at 550 cm^?1. corresponding to the surface phonon of ZnO. The (111) face follows the same scheme, except that the sticking coefficient for oxygen is very low. For (100), it is clear that two states of oxygen exist simultaneously, even at the beginning, as revealed by HREELS and CPD measurements. No copper oxides are ever detected, even after heat treatment. In addition, different bonding properties of OH groups on the three surfaces are reported.     

Adsorption of hydrogen on palladium single crystal surfaces

The adsorption of hydrogen on clean Pd(110) and Pd(111) surfaces as well as on a Pd(111) surface with regular step arrays was studied by means of LEED, thermal desorption spectroscopy and contact potential measurements. Absorption in the bulk plays an important role but could be separated from the surface processes. With Pd(110) an ordered 1 × 2 structure and with Pd(111) a 1 × 1 structure was formed. Maximum work function increases of 0.36, 0.18 and 0.23 eV were determined with Pd(110), Pd(111) and the stepped surface, respectively, this quantity being influenced only by adsorbed hydrogen under the chosen conditions. The adsorption isotherms derived from contact potential data revealed that at low coverages θ ∞ √p_H2, indicating atomic adsorption. Initial heats of H₂ adsorption of 24.4 kcal/mole for Pd(110) and of 20.8 kcal/mole for Pd(111) were derived, in both cases E_ad being constant up to at least half the saturation coverage. With the stepped surface the adsorption energies coincide with those for Pd(111) at medium coverages, but increase with decreasing coverage by about 3 kcal/mole. D₂ is adsorbed on Pd(110) with an initial adsorption energy of 22.8 kcal/mole.     

Interaction of CO,CO2 and O2 with nonpolar,stepped and polar Zn surfaces of ZnO

The nonpolar (10$\text{1}$0), stepped (40$\text{4}$1) and (50$\text{5}$1), and the polar (0001) surfaces of ZnO were prepared. Stable unreconstructed nonpolar and stepped surfaces were obtained. LEED analyses showed that the step height and the step width of the stepped surfaces were similar to the theoretical values. The polar surface showed a 1 × 1 LEED pattern of six-fold symmetry after annealing at 500°C, and evidence of a more complicated pattern at 300–400°C. Temperature programmed desorption of CO resulted in the desorption of CO from the stepped and the polar surfaces. However, desorption of CO₂ was observed from the stoichiometric nonpolar surface, and no desorption from the reduced nonpolar surface. CO₂ was also observed by interacting CO with all surfaces at elevated temperatures. A total of four temperature programmed desorption peaks of CO₂, α, β, γ, and δ were observed. The α and β peaks were observed on the nonpolar and the stepped surfaces, and the γ peak was observed on the polar surface. The α peak was assigned to adsorption on a surface ZnO pair, and the β peak was assigned to adsorption on an anion vacancy or a step. While adsorbed water enhanced the β, preadsorbed methanol reduced it. O₂ adsorption was similar on the nonpolar and the stepped surfaces, but was weak on the polar surface.     

Interaction of CO with surface PdZn alloys

The adsorption and bonding configuration of CO on clean and Zn-covered Pd(1 1 1) surfaces was studied using low energy electron diffraction (LEED), temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). LEED and TPD results indicate that annealing at 550 K is sufficient to induce reaction between adsorbed Zn atoms and the Pd(1 1 1) surface resulting in the formation of an ordered surface PdZn alloy. Carbon monoxide was found to bond more weakly to the Zn/Pd(1 1 1) alloy surfaces compared to clean Pd(1 1 1). Zn addition was also found to alter the preferred adsorption sites for CO from threefold hollow to atop sites. Similar behavior was observed for supported Pd-Zn/Al₂O₃ catalysts. The results of this study show that both ensemble and electronic effects play a role in how Zn alters the interactions of CO with the surface.     

A DFT study the solvent effects of H2 adsorption on Cu(h k l) surface

Density functional theory (DFT) combined with conductor-like solvent model (COSMO) have been performed to study the solvent effects of H₂ adsorption on Cu(h k l) surface. The result shows H₂ can not be parallel adsorbed on Cu(h k l) surface in gas phase and only vertical adsorbed. At this moment, the binding energies are small and H₂ orientation with respect to Cu(h k l) surfaces is not a determining parameter. In liquid paraffin, when H₂ adsorbs vertically on Cu(h k l) surface, solvent effects not only influences the adsorptive stability, but also improves the ability of H₂ activation; When H₂ vertical adsorption on Cu(h k l) surface at 1/4 and 1/2 coverage, H-H bond is broken by solvent effects. However, no stable structures at 3/4 and 1 ML coverage are found, indicating that it is impossible to get H₂ parallel adsorption on Cu(h k l) surfaces at 3/4 and 1 ML coverages due to the repulsion between adsorbed H₂ molecules.     

Oxygen adsorption on the LaB6(100), (110) and (111) surfaces

Oxygen adsorption on the LaB₆(100), (110) and (111) clean surfaces has been studied by means of UPS, XPS and LEED. The results on oxygen adsorption will be discussed on the basis of the structurs and the electronic states on the LaB₆(100), (110) and (111) clean surfaces. The surface states on LaB₆(110) disappear at the oxygen exposure of 0.4 L where a c(2 × 2) LEED pattern disappears and a (1 × 1) LEED pattern appears. The work function on LaB₆(110) is increased to ～3.8 eV by an oxygen exposure of ～2 L. The surface states on LaB₆(111) disappear at an oxygen exposure of ～2 L where the work function has a maximum value of ～4.4 eV. Oxygen is adsorbed on the surface boron atoms of LaB₆(111) until an exposure of ～2 L. Above this exposure, oxygen is adsorbed on another site to lower the work function from ～4.4 to ～3.8 eV until an oxygen exposure of ～100L. The initial sticking coefficient on LaB₆(110) has the highest value of ～1 among the (100), (110) and (111) surfaces. The (100) surface is most stable to oxygen among these surfaces. It is suggested that the dangling bonds of boron atoms play an important role in oxygen adsorption on the LaB₆ surfaces.     

Substrate mediated stabilization of methylphosphonic acid on ZnO non-polar surfaces'

The adsorption of methylphosphonic acid (MPA, formula CH₃–PO₃H₂) on ZnO(10-10) surfaces has been investigated by first-principles density-functional total energy calculations. We show that substrate mediated interactions between co-adsorbates can significantly affect the binding energy of MPA on the ZnO surface, which leads to a preferential molecular dimer assembly along the polar [0001] direction (i.e. along the Zn–O dimer direction). We propose that this is caused by a local charge compensation mechanism due to the relaxation of the ZnO surface and suggest that this concept can be applied to other adsorbates on metal oxide surfaces with metal–oxygen dimers.     

F2+ defect based model for tunable laser activity and interaction of atomic halogens (F,Cl and Br) at the low coordinated surfaces of LiH: ab initio calculations

The twofold potential of F₂⁺ color center at the low coordinated surfaces of LiH in providing tunable laser activity and adsorption properties for atomic halogens is examined using ab initio calculations. Based on Stokes shifts of the examined clusters, the F₂⁺ laser activity was very weak, but increases significantly as the coordination number decreases from 5 (flat) to 4 (edge) to 3 (H⁻ corner). The corner site is the least probable for relaxed excited state orientational destruction of F₂⁺. The exciton (energy) transfer takes place from the corner to the flat or edge sites. The Glasner–Tompkins relation is generalized to include the low coordinated surfaces of LiH. The F⁺₂ color center change the nature of halogen–surface interaction from physical adsorption to chemical adsorption. The halogen–surface interactions were monotonically increasing functions of the electronegativity of the halogen and the amount of charge transferred between the halogen and the surface. The “covalent spin pairing” mechanism play the dominant role in the course of adsorbate–substrate interactions.