Tuning thermal stability and electronic properties of germanium oxide on Ge(001) surface with the incorporation of nitrogen

The application of germanium as a channel material of transistors in near future requires an improved understanding of the interface between germanium and its potential passivation layer. In this study, we study effects of nitrogen incorporation on the thermal stability and electronic properties of GeO_xN_y/Ge interface by using high‐resolution X‐ray photoemission spectroscopy. We find that with the increasing nitrogen concentration in the GeO_xN_y films, the thermal stability can be increased, while the valence band offset with the Ge(001)substrate is decreased. First‐principles calculations further suggest that the unpaired p orbitals of nitrogen atoms induce electronic states near valence band edge, contributing to the reduction of the valence band offset. Our results provide a possibility to tune electronic and thermal properties of GeO_xN_y/Ge interface by controlling nitrogen concentrations during the growth.     

Structural features of zinc hydroxyfluoride

Zinc hydroxyfluoride (ZnOHF), obtained by coprecipitation of a zinc salt in aqueous HF, exhibits a variable stoichiometry Zn(OH)_2−xF_x, where x is tuneable from 0.63 to 0.87 by controlling the pH of the solution. The structure was determined from Rietveld refinements using X-ray powder diffraction data. Crystallizing with the orthorhombic symmetry (SG : Pna2₁), the ZnOHF-type structure exhibits two different anionic sites. A bond valence analysis shows that fluorine exclusively occupies the anionic site that has shorter contacts to zinc. This site is split into two partially occupied sites, one corresponding to the position of a fluoride ion and the other to the position of a hydroxide ion. Bond valence calculations show that the split site model gives a more accurate picture of the local coordination of the anions on this site.     

Octadecyltrichlorosilane adsorption kinetics on Si(100)/SiO2 surface: contact angle,AFM, FTIR and XPS analysis

Octadecyltrichlorosilane (OTS) adsorption kinetics on Si(100)/SiO₂ surface has been studied as a function of concentration by sequential and nonsequential dipping techniques. The contact angle technique is used to evaluate growth kinetics and thermal stability and to determine critical surface tension of the OTS layer. Atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy and X‐ray photoelectron spectroscopy (XPS) are used to confirm OTS adsorption. Langmuir isotherms are employed to analyze the kinetics data to obtain adsorption and desorption rate constants (k_a & k_d) as well as Gibbs free energy, (ΔG_ads). These parameters, k_a, k_d and ΔG_ads(y) are found to depend exponentially (y = y₀ + A.exp(?x/t)) on the OTS concentration (x). The OTS layers are found to be thermally stable up to a temperature of 230 °C and the critical surface tension obtained from the Zisman plot is found to be ～19.8 dynes/cm. OTS monolayer coverage obtained by AFM measurement agrees quite closely with that obtained from contact angle measurements. FTIR and XPS results confirm OTS adsorption. Copyright © 2006 John Wiley & Sons, Ltd.     

Evidence for nickel-(I)-rich mixed oxide with a defect K2NiF4-type structure

Reduced phases obtained from lanthanum mixed nickel oxide, i.e., La_2−xSr_xNiO_4−y, K₂NiF₄ type, have been studied. Reduction under controlled conditions led to the composition LaSrNiO_3.1 containing formally more than 80% of Ni in the valence state (I). Structural calculations and local studies by X-ray adsorption spectroscopy provide evidence for a lowering of the Ni octahedral coordination configuration. V-square-pyramidal and IV-square configurations are obtained, depending on the reduction level. The oxygen vacancies are highly ordered along the b axis of the orthorhombic unit cell, in agreement with recent defect modeling of this structure     

Influence of partial substitution of zinc for copper on electronic structures of YBa2Cu3O y

Summary By use of an approximate band-structure treatment based on the EHMO approach, the energy band structures for the Zn-doped superconductor YBa₂Cu_3–x Zn _x O _y were calculated in the present paper and the influence of partial substitution of zinc for copper on the electronic structures for orthorhombic YBa₂Cu₃O_y was studied. From analysis of the band structures and the densities of states for YBa₂Cu_3–x Zn _x O _y , it was demonstrated that the 2D Cu-O planes in the Y-Ba-Cu-O superconducting system have a direct and dominant influence on superconductivity, whereas the role of the 1D Cu-O ribbons and the O(4) atoms is also of some importance.     

Generation of ground-state structures and electronic properties of ternary AlxTiyNiz clusters (x + y + z = 6) with a two-stage density functional theory global search approach

The structural and electronic properties of ternary Al_xTi_yNi_z clusters, where x, y, and z are integers and x + y + z = 6 , are investigated. Both Slater, Vosko, Wilks, and Nusair and B3LYP exchange-correlation (XC) functionals are employed in a two-stage density functional theory (DFT) calculations to generate these clusters. In the first stage, a minimum energy cluster structure is generated by an unbiased global search algorithm coupled with a DFT code using a light XC functional and small basis sets. In the second stage, the obtained cluster structure is further optimized by another round of global minimization search coupled with a DFT calculator using a heavier XC functional and more costly basis set. Electronic properties of the structures are illustrated in the form of a ternary diagram. Our DFT calculations find that the thermodynamic stability of the clusters increases with the increment in the number of constituent nickel atoms. These results provide a new insight to the structure, stability, chemical order, and electronic properties for the ternary alloy nanoclusters.     

A theoretical study on the pure and doped ZnO nanoclusters as effective nanobiosensors for 5-fluorouracil anticancer drug adsorption

The electronic sensitivity and effectiveness of the pristine, Fe,- Mg-, Al- and Ga-doped ZnO nanoclusters interacted with 5-fluorouracil (5-FU) anticancer drug are theoretically investigated in the gas phase using the B3LYP/wB97XD density functional theory calculations with LANL2DZ basis set. It is concluded that 5-FU adsorption on the doped nanoclusters has relatively higher adsorption energy as compared with the pristine zinc oxide. A number of thermodynamic parameters, such as band gap energy (E_g), adsorption energy (E_ad), molecular electrostatic potential, global hardness (η) and density of electronic states, are attained and compared. Also, calculated geometrical parameters and electronic properties for the studied systems indicate that Mg- and Ga-doped Zn₁₂O₁₂ present higher sensitivity to 5-FU compared with the pristine nanocluster. Theoretical results reveal that adsorption of 5-FU on the doped nanoclusters is influenced by the electronic conductance of the nanocluster. Therefore, Mg- and Ga-doped ZnO can be considered as promising nanobiosensors for detection of 5-FU in medicine.     

P-doped g-C3N4 as an efficient photocatalyst for CO2 conversion into value-added materials: a joint experimental and theoretical study

The photocatalytic yield of g-C₃N₄ for CO₂ reduction was modified by phosphorus doping. Possible reaction pathways for CO₂ reduction on the P-doped g-C₃N₄ (PCN) surface were investigated by density function theory calculations for the first time. The experimental results showed that P doping increases the carriers' lifetime, which improves the production of CH₄ through the increase in the driving force of the electrons. The partial density of states of the PCN showed that the valence band maximum and conduction band minimum are composed of p_x, p_y, and, s orbitals of the N atoms and p_z states of carbon, nitrogen, and phosphorus, respectively. Mechanism studies confirm that formic acid, formaldehyde, methanol, and methane are the most probable products. Methane, having positive adsorption energy, can be easily desorbed from the PCN surface, and the Gibbs activation energy of the final step is 1.98 eV. The formation of H₂COOH is the rate-determining step.     

Electronic structures of superconductors YBa2Cu3−x M x O y (M=Sn,Ni)

Based on the EHMO approach, the energy band structures for superconductors YBa₂Cu_3–x Sn _x O _y (y>7) and YBa₂Cu_3–x Ni _x O_y (y<7) were calulated in the present paper. The influence of the cation doping at the Cu site in the unit cell and the oxygen content on their electronic structures was studied. The results showed that the cation doping at the Cu site resulted in the great decreases in the bandwidths of the broad anisotropic Cu-O bands and the densities of states. In YBa₂Cu_3–x Sn _x O _y , however, these decreases are compensated by the increase in the oxygen content caused by the Sn-doping, which results in a small change in the total densities of states. For YBa₂Cu_3–x Ni _x O _y , the effect of the doping on its electronic structures in dominant. The Ni-doping, therefore, results in a great change in the electronic structures. In addition, the study on the projected densities of states of the Ni-doped system revealed that the 2D Cu-O planes in the Y-Ba-Cu-O system played a dominant role in superconductivity.     

The Adsorption of Oxygen and Coadsorption of CO and Oxygen on Structurally Well‐Defined PdAg Surface Alloys

The dissociative interaction of oxygen with structurally well‐defined monolayer Pd_xAg_1?x/Pd(111) surface alloys of different compositions, with well‐known distributions of the respective surface atoms (A. K. Engstfeld et al., Phys. Chem. Chem. Phys. 2012 , 14, 10754–10761), and the coadsorption of/reaction with CO on oxygen pre‐covered surfaces were studied by high‐resolution electron energy loss spectroscopy (HREELS) and temperature‐programmed desorption/reaction spectroscopy (TPD/TPR). The impact of geometric ensemble effects as well as electronic ligand and strain effects on the adsorption and reaction behaviour of the respective species on the bimetallic surfaces is elucidated and compared with related systems such as CO adsorption on similar surfaces and oxygen adsorption on a Pd₆₇Ag₃₃(111) bulk alloy surface. The data show a clear dominance of ensemble effects on the oxygen adsorption and CO coadsorption behaviour, with oxygen adsorption limited to threefold‐hollow sites on Pd₃ sites, while the combined electronic effects, as evident from modifications in the adsorption and reaction characteristics on the Pd sites, are small.     

Optical transitions,XPS, electronic states in NiPS3

We have measured the optical absorption below the fundamental threshold, the normal-incidence reflectivity between 1.5 and 30 eV and the X-ray photoemission spectra of NiPS₃. Shake-up satellites present at the Ni 2p and 3p core levels are strong evidence for the ionicity of the NiS bonds. We have also derived a qualitative molecular orbital model of NiPS₃ in which the trigonal crystal field splits the P and S 3p_xp_y-3p_a states, and strong covalent hybridization between P and S p_xp_y orbitals leads to covalent electronic bonding. Ni is envisaged as a divalent ion which plays little role in the electronic bonding and its 3d levels are localized, lying near the top both of the valence states. This model accounts well for both the valence band XPS data and the low energy optical transitions. Our model should represent, at the center of the Brillouin zone but not at the boundaries, the energy level sequence in NiPS₃ and other related MPX₃ layer-type compounds where M Co²⁺, Mn²⁺, Fe²⁺, Zn²⁺ and X is sulfur or selenium.The XPS spectra and optical properties of NiPS₃ have been obtained and interpreted on a qualitative molecular orbital model in which the Ni is a divalent positive ion which plays little role in the bonding. Evidence for such ionicity appears in the optical properties and XPS satellite structures, as well as in the magnetic properties. The model should represent qualitatively the band structure at the center of the Brillouin zone, but not at the boundaries. It should also be valid for other compounds similar to NiPS₃, i.e. those with other metals in place of Ni and those with Se in place of S.     

First‐principles calculations of oxygen adsorption on the Ti3Al (0001) surface

Adsorption energies and density of states for O atoms adsorption on the Ti₃Al (0001) surface have been calculated using first‐principles calculations based on density functional theory. It is found that the order of O atom adsorption on the Ti₃Al (0001) surface is associated with the adsorption energy as well as the distance of O atoms because of the interaction. The adsorption energy mainly depends on the bond number and bond strength between O and Ti atoms, and the adsorption site with rich‐Ti surface (H_I and HCP_Al) is first priority. The adsorption energy decreases with the increase of the oxygen coverage because of the characteristics of the valence d‐orbitals of transition metals surface. Furthermore, the density of states indicates that the hybridization peak of O and Ti atoms is mainly from the contribution of Ti 3d‐ and O 2p‐orbitals, and the hybridization peak of O and Al atoms from the contribution of Al 2p‐ and O 2p‐orbitals. Copyright © 2016 John Wiley & Sons, Ltd.     

Exfoliation methods for compositional and electronic characterization of interfacial Mo(Sex,Sy) in Cu(In,Ga)(Se,S)2 solar cells by X-ray and UV photoelectron spectroscopy

Mo(Se_xS_y) is a transition metal dichalcogenide typically applied as a back contact interlayer in Cu(In,Ga)(Se,S)₂ (CIGSSe) solar cells. Band alignment at the buried Mo/CIGSSe junction mediated by Mo(Se_xS_y) is important for current transport and enables quasi-ohmic behavior between the CIGSSe absorber and the Mo back electrode. Furthermore, the S/(Se + S) ratio is a crucial parameter that determines the height of the valence band offset at the CIGSSe/Mo(Se_xS_y) interface. Because the interlayer is formed during rapid thermal processing, an MoSe₂ or MoS₂ thin film grown on free substrate surfaces will not be representative for a realistic solar cell device. Thus, for fundamental thin-film material analysis, as well as functional characterization and modeling, appropriate preparation and analytical techniques are required in order to prevent artifacts. In principal, the weak van der Waals forces between two-dimensional stacked Mo(Se_xS_y) sheets allow the implementation of exfoliation procedures to generate free Mo(Se_xS_y) surfaces out of CIGSSe solar cell layer stacks. In this article, two different exfoliation-based Mo(Se_x,S_y) preparation methods are investigated and evaluated with respect to subsequent surface analytical characterization by X-ray and ultraviolet photoelectron spectroscopy. A special focus is laid on an artifact-free characterization of chemical and electronical properties of the exposed layers for a number of samples. In a first instance, the compositional Se/S and (Se + S)/Mo ratios at the surface are quantitatively analyzed on the basis of dedicated peak-fitting routines. Artifacts from carbonaceous contamination due to different exfoliation glues can be prevented through a detailed comparative analysis of carbon 1s and KLL Auger peaks. Furthermore, a significant surface band bending is observed that can be reduced by low-energy Ar ion in situ sputtering. A simple model for the sputter removal of a charged surface layer is presented, which allows to approximately calculate the absolute valence band maximum (VBM) positions required for band alignment and numerical device simulations. The presented exfoliation surface analysis methodology is important for the whole CIGS(Se) solar cell community and may be of general interest for emerging applications of further 2D transition metal dichalcogenides as well.     

X-ray spectroscopic study of the electronic structure of boron carbonitride films obtained by chemical vapor deposition on Co/Si and CoO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/Si substrates

Boron carbonitride films are synthesized by chemical vapor deposition from a mixture of triethylamine borane and ammonia on a metallic or oxidized cobalt sublayer sprayed over Si(100) substrates. Scanning electron microscopy shows that the surface of a BC _x N _y /Co/Si sample has a homogeneous fine-grained structure; filamentous entities are found on the surface of the BC _x N _y /CoO _x /Si sample. The electronic structure of the films is investigated by X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). An analysis of the spectra shows that BC _x N _y films are composed of graphite and hexagonal boron nitride (h-BN) regions and complex BC _x N _y O _z components with B-C, N-C, B-O, N-O, and C-O bonds. The deposition of the BC _x N _y film on the oxidized Co sublayer results in an increase in the number of C-O, N-O, B-O, and C-N bonds and a decrease of the graphite and h-BN components and in the number of C-B bonds. The XPS data are used to estimate the surface elemental composition of the BC _x N _y /CoO _x /Si sample. It is found that the film consists of 66 at.% graphite component and 3 at.% h-BN; the proportion of complex C_0.46B_0.11N_0.05O_0.38 components is 31 at.%.     

Tuning Binding Strength of Multiple Intermediates towards Efficient pH-universal Electrocatalytic Hydrogen Evolution by Mo8O26-NbNxOy Heterocatalysts

Developing efficient and robust hydrogen evolution reaction (HER) catalysts for scalable and sustainable hydrogen production through electrochemical water splitting is strategic and challenging. Herein, heterogeneous Mo₈O₂₆-NbN_xO_y supported on N-doped graphene (defined as Mo₈O₂₆-NbN_xO_y/NG) is synthesized by controllable hydrothermal reaction and nitridation process. The O-exposed Mo₈O₂₆ clusters covalently confined on NbN_xO_y nanodomains provide a distinctive interface configuration and appropriate electronic structure, where fully exposed multiple active sites give excellent HER performance beyond commercial Pt/C catalyst in pH-universal electrolytes. Theoretical studies reveal that the Mo₈O₂₆-NbN_xO_y interface with electronic reconstruction affords near-optimal hydrogen adsorption energy and enhanced initial H₂O adsorption. Furthermore, the terminal O atoms in Mo₈O₂₆ clusters cooperate with Nb atoms to promote the initial H₂O adsorption, and subsequently reduce the H₂O dissociation energy, accelerating the entire HER kinetics.     

Hybrid Polyoxotungstates as Functional Comonomers in New Cross‐Linked Catalytic Polymers for Sustainable Oxidation with Hydrogen Peroxide

Anchoring terminal octenyl tails on molecular polyoxotungstates yield polymerizable organic–inorganic monomers with formula [{CH₂?CH(CH₂)₆Si}_xO_ySiW_wO_z]^4? [x=2, w=11, y=1, z=39 ( 1 ); x=2, w=10, y=1, z=36 ( 2 ); and x=4, w=9, y=3, z=34 ( 3 )]. These molecular hybrids can use aqueous hydrogen peroxide to catalyze the selective oxidation of organic sulfides in CH₃CN. Copolymerization of 1 – 3 with methyl methacrylate and ethylene glycol dimethacrylate leads to porous materials with a homogeneous distribution of the functional monomers, as indicated by converging evidence from FTIR spectroscopy and electronic microscopy. The catalytic polymers activate hydrogen peroxide for oxygen transfer, as demonstrated by the quantitative and selective oxidation of methyl p‐tolyl sulfide, which was screened as model substrate. The hybrid material containing monomer 2 was also tested in n‐octane to evaluate its potential for the oxidation and removal of dibenzothiophene, a well‐known gasoline contaminant.