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.     

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 of Ag and Au atoms on wurtzite ZnO (0001) surface

We perform first-principles calculations to investigate various surface structures in the absorption of Ag and Au atoms on wurtzite ZnO (0001) surface. The results show that both Ag and Au atoms prefer to be absorbed on the H₃ sites (the center of Zn–O ring) of the surface, and the most favorable monolayer (ML) coverage is 1. The calculated electron structure shows that the Ag- and Au-adsorbed ZnO (0001) surfaces exhibit metallic characteristics even the ML coverage of the adatoms is very low. Finally, the work functions of Ag- and Au-adsorbed ZnO (0001) surfaces are calculated and discussed for the first time in the present work.     

H2O分子在Fe（100）, Fe（110）, Fe（111）表面吸附的第一性原理研究

采用第一性原理研究了H₂O分子在Fe（100）,Fe（110）,Fe（111）三个高对称晶面上的表面吸附.结果表明,H₂O分子在三个晶面上的最稳定结构皆为平行于基底表面的顶位吸附结构.H₂O分子与三个晶面相互作用的吸附能及几何结构计算结果表明H₂O分子与三个晶面的相互作用程度不同,H₂O分子与Fe（111）晶面的相互作用最强,其次是Fe（100）,相互作用最弱的是Fe（110）表面,而这与晶面原子 关键词： 第一性原理 Fe单晶表面 2O分子')" href="#">H₂O分子 分子吸附     

Photoemission studies of oxygen adsorbed on a LiAl(1 1 0) alloy surface: Role of Li segregation

We investigated that the effect of the number of segregated Li atoms on the rate of oxidation on a LiAl alloy surface. Oxygen molecules adsorbed on the LiAl alloy react with the surface atoms to form stable oxides. The segregated Li atoms at reconstructed surfaces (c(2×2) and (2×1)) enhance the oxidation rate and form stable LiAlO_x and Li₂O. The degree of enhancement of oxidation by segregated Li atoms varies as a function of O₂ exposure and annealing temperature, where the latter is directly related to the mode of surface reconstruction by Li segregation.     

Structure analysis of stoichiometric LiNbO3(0 0 0 1) surfaces using low-energy neutral scattering spectroscopy

The atomic structure of LiNbO₃(0 0 0 1) surface was investigated by low-energy neutral scattering spectroscopy (LENS). Poled stoichiometric LiNbO₃ (SLN) samples were prepared for the measurements. The LENS was developed for surface structure and composition analysis particularly of highly insulating materials and was successfully applied to the structure analysis of the SLN(0 0 0 1) surface. The polar angle dependences of intensity of scattered He⁰ from the poled SLN surfaces indicate obvious differences between the negatively and the positively charged surfaces. It is suggested that O atoms cover the surfaces, and the first metal layers underneath the O layer consist of Li and Nb for negatively and positively charged surfaces, respectively, parallel to the applied electric field.     

Geometric and electronic structure of positively and negatively poled LiNbO3 (0 0 0 1) surfaces

The effect of ferroelectric poling direction on the structure and electronic properties of the LiNbO₃ (0 0 0 1) surface was characterized. Low energy and reflection high energy electron diffraction indicated that both the positively and negatively poled surfaces were (1 × 1) with no evidence of longer range periodic reconstructions. Low energy ion scattering spectra from both surfaces were dominated by scattering from oxygen atoms. X-ray and ultraviolet photoelectron spectra also showed little difference between the positively and negatively poled surfaces, with the exception of a high binding energy shoulder on the O 1s core level of the negative surface. Exposure of the surfaces to atomic hydrogen caused reduction of the surface Nb rather than an increase in intensity on the high binding energy side of the O 1s peak, indicating that the shoulder on the O 1s peak on the negative surface was not due to surface hydroxyl groups. Temperature programmed desorption measurements indicated that the nearly stoichiometric LiNbO₃ samples were susceptible to loss of Li₂O starting at temperatures as low as 500 K, independent of the poling direction. An adatom/vacancy model is proposed in which oxygen ad-anions accumulate on one side of the crystal while oxygen anion vacancies are created on the opposite surface. This model can explain the apparent oxygen termination of both surfaces and the observed (1 × 1) periodicity of the surfaces, and also effectively screens the thickness dependent electric field associated with the polar orientation of the crystal.     

On the mechanism of the interaction between oxygen and close-packed single-crystal aluminum surfaces

Using periodic first principles simulations we investigate the interaction of oxygen molecules with both regular Al(111) and Al(001) surfaces as well as a stepped Al(111) substrate. The limitation of this approach is the use of thin metallic slabs with a limited range for their coverage by adsorbed oxygen. The advantage is the detailed modeling that is possible at an atomic level. On the regular Al(111) surface, we have been able to follow the oxidation process from the approach of O₂ molecules to the surface, through the chemisorption and absorption of O atoms, up to the formation of first Al₂O₃ formula units. An energetically feasible mechanism for the formation of these Al₂O₃ ‘molecules’ is proposed but their aggregation to Al₂O₃ growth nuclei can only be surmised. On the Al(001) surface, absorption of oxygen atoms occurs more readily without any restrictions on the density of their surface overlayer, in agreement with the failure to observe a distinct chemisorption stage for O on Al(001) experimentally. The stepped Al(111) surface contains both {111} and {001} microfacets: the latter are obviously preferred for penetration of the oxygen adatoms into the subsurface space of the substrate. Before considering the O/Al interfaces the computational method is tested thoroughly by simulations on bulk Al and close-packed aluminum surfaces.     

The structure of the V2O3(0 0 0 1) surface: A scanned-energy mode photoelectron diffraction study

Scanned-energy mode photoelectron diffraction (PhD), using the O 1s and V 2p photoemission signals, together with multiple-scattering simulations, have been used to investigate the structure of the V₂O₃(0 0 0 1) surface. The results support a strongly-relaxed half-metal termination of the bulk, similar to that found in earlier studies of Al₂O₃(0 0 0 1) and Cr₂O₃(0 0 0 1) surfaces based on low energy electron and surface X-ray diffraction methods. However, the PhD investigation fails to provide definitive evidence for the presence or absence of surface vanadyl (VO) species associated with atop O atoms on the surface layer of V atoms. Specifically, the best-fit structure does not include these vanadyl species, although an alternative model with similar relaxations but including vanadyl O atoms yields a reliability-factor within the variance of that of the best-fit structure.     

First-principles calculations on electronic structures of Fe-vacancy-codoped TiO2 anatase (1 0 1) surface

The electronic structures of Fe-doped TiO₂ anatase (1 0 1) surfaces have been investigated by all spin-polarized density functional theory (DFT) plane-wave pseudopotential method. The general gradient approximation (GGA)+U (Hubbard coefficient) method has been adopted to describe the exchange-correlation effects. Through the density functional calculations for the formation energies of various configurations, the complex of a substitutional Fe plus an O vacancy was found to form easily in the most range of O chemical potential. The calculated density of the states of the system of Fe-doped surface with a surface oxygen vacancy shows a band gap narrowing from 2.8 to 1.9 eV comparing with the pure surface due to the synergistic effects of surface Fe impurities with O vacancies. The system processes high visible light sensitivity and photocatalytic ability by decreasing extrinsic absorption energy. By comparing the partial DOS of some O and Ti atoms lying in the outermost and bottom layers of Fe-doped surfaces, it was found that the influence of Fe impurities on the electronic structure of the system is localized.     

Origin of HfO2/GaAs interface states and interface passivation: A first principles study

First principles calculations of HfO₂/GaAs interfaces indicate that the interface states originate from the charge mismatch between HfO₂ and GaAs surfaces. We find that a model neutral interface (HfO₂ and GaAs surfaces terminated with two O and one Ga atoms per surface unit cell) removes gap states due to the balance of the interface charge. F and H can neutralize the HfO₂/GaAs interface resulting in useful band offsets, thus becoming possible candidates to passivate the interface states.     

Interaction of water with stepped Co(0001): how is it different from flat Co(0001)?

The adsorption and dissociation of water monomer and dimer on stepped Co(0001) surface were studied by means of first-principles calculations. Present results indicate that the adsorption strength of water is greatly enhanced by the presence of step, while the activity of water monomer dissociation does not exhibit a noticeable improvement. Nevertheless, water dimer partial dissociation on stepped Co(0001) is more active than on flat Co(0001), and the promotion of oxygen atom on O–H bond cleavage of H₂O is more prominent on stepped surface than on flat Co(0001). The findings reveal the importance of low coordinated surface atoms on metallic catalysts and the vital role of surface rippling on water dissociation. Together with previous reports, the activity of water dissociation on cobalt-based catalytic surfaces depends dominantly on O-containing species like oxygen atom, H₂O or hydroxyl.     

Ultrahigh resolution electron microscopy of the surface structure of (110) Ag2O

An ultrahigh resolution transmission electron microscope and diffraction investigation of the residual oxidized silver on the surface of (111) Au films has shown that it is possible to resolve a monolayer coverage of silver atoms and the atomic structure of crystalline Ag₂O surfaces. Localized patches often under 100 Å in lateral extent show a reoriented (110) oxide which often exhibits a (2 × 1) surface superstructure. The structure of various different layer coverages has been analyzed from the high resolution micrographs via digital image processing and model building to interpret surface morphology.     

Adsorption states and Ga depletion during oxygen adsorption on clean polar GaAs surfaces

Ultraviolet photoelectron spectroscopy (UPS), thermal desorption spectroscopy (TDS) and Auger (AES) measurements were used to study oxygen adsorption on sputtered an annealed GaAs(111)Ga, (1̄1̄1̄)As, and (100) surfaces. Two forms of adsorbed oxygen are seen in UPS. One of them is associatively bound and desorbs at 400–550 K mainly as molecular O₂. It is most probably bound to surface As atoms as indicated by the small amounts of AsO which desorb simultaneously. The second form is atomic oxygen bound in an oxidic environment. It desorbs at 720–850 K in the form of Ga₂O. Electron irradiation of the associatively bound oxygen transforms it into the oxidic form. This explains the mechanism of the known stimulating effect of low energy electrons on the oxidation of these surfaces. During oxygen exposure a Ga depletion occurs at the surface which indicates that oxygen adsorption is a more complex phenomenon then is usually assumed. The following model for oxygen adsorption is proposed: oxygen impinges on the surface, removes Ga atoms and thus creates sites which are capable of adsorbing molecular oxygen on As atoms of the second layer and are surrounded by Ga atoms of the first layer. This molecular oxygen is stable and simultaneously forms the precursor state for the dissociation to the oxidic form.     

The configuration and electronic state of SO3 adsorbed on Au surface

We evaluated the adsorption of SO₃ molecule on Au (1 1 1) surface using first principles calculation by a slab model with a periodic boundary condition. We find that there are six stable adsorption configurations on an Au surface, where the SO₃ molecule is adsorbed above the three-fold fcc and hcp hollow sites and on the atop site. In two of these configurations, S and two O atoms are bound to the Au atoms, the next two configurations have all the three O atoms bound to the Au surface atoms, and the last two configurations have the S atom bound to an Au surface atom on the atop site and O atoms situated above the hollow sites. In these configurations, the electronic structures of SO₃ on the Au surface show that molecular orbitals of SO₃ and those of the Au surface are hybridized in the active metal d-band region, that the localized molecular orbitals in SO₃ are stabilized, and that charge is transferred from Au to S 3p by SO₃ adsorption on the Au surface though there is little other interaction of the S and O (bound to Au) component with Au. Moreover, the bond between the S and O atoms bound to Au is weakened due to SO₃ adsorption on the Au surface due to the charge polarization of the O-Au bond. This interaction is likely to encourage the S-O bond to break.     

SrTiO3(001)表面上Au和N原子相互作用的第一性原理研究

采用基于密度泛函理论的第一性原理平面波赝势方法研究了SrTiO₃(001)表面上Au和N原子间相互作用的微观机理.通过比较分析N置换表面层O原子前后SrTiO₃(001)表面吸附Au原子体系的相关能量和电子结构,发现SrTiO₃(001)表面吸附Au原子和N替代表面层O原子的置换过程二者之间存在明显的"协同效应",即N原子置换SrTiO₃(001)表面层O原子的过程增强了相应表面吸附Au原子的稳定性,而SrTiO关键词： 表面结构 相互作用 第一性原理     

Evaluation for the configurational and electronic state of SO3 adsorbed on Pt surface

We evaluate the adsorption of SO₃ molecule on the Pt (1 1 1) surface using the first-principles calculations by a slab model with a periodic boundary condition. We find that there are four stable adsorption configurations on the Pt surface, where SO₃ molecules are adsorbed above the three-fold fcc and hcp sites. In two of these configurations, S and two O atoms are bound to the Pt atoms, and in two other of them, all the three O atoms are bound to Pt surface atoms. Besides, it is found that molecular orbitals of SO₃ and those of Pt surface are hybridized in the active metal d-bands region, that the localized molecular orbitals in SO₃ are stabilized, and that the charge is transferred from Pt to S 3p by SO₃ adsorption on Pt surface though the other interaction of S and O (bound to Pt) component with Pt is little. In addition, the bond between S and O bound to Pt become weak by SO₃ adsorption on Pt surface because the charge polarization to O-Pt bond weakens the bond between S and O bound to Pt. This interaction is assumed to encourage the breakage of S-O bond.