Applications of Surface Science最新文献

The chemisorptive interaction of the CO/Cu(100) system is studied with a Cu₅(1,4)CO cluster model using the constrained space orbital variation (CSOV) analysis. This analysis has been used previously to examine the origin of the CO-metal bond and to show that the energetic importance of the metal to CO π donation is larger than that of the CO to metal σ donation. In the present work, we use it to determine the origin of the differential CO 5σ ionization potential shift observed in the photoemission spectra of CO/Cu(100). This shift is shown to be largely due to electrostatic, environmental potential effects. It does not indicate the nature of the CO-metal bond but does indicate the CO adsorption geometry.

The chemisorption of oxygen at room temperature on polycrystalline zirconium has been studied using AES and ESCA as a function of sample pretreatment. It is shown that annealing at high temperature greatly reduces the ability of zirconium to chemisorb oxygen but that its activity can be restored by argon-ion bombardment. This suggests that chemisorption may be site specific and that the concentration of these sites can be manipulated by varying sample pretreatment.

The two-hole and the two-electron spectra, which are related to the XVV Auger and to the APS spectra, respectively, have been computed for an Fe-Ni alloy of different compositions. A recently proposed theory has been used which, by means of some approximations, is capable of treating the two-particle spectra of a Hubbard-type Hamiltonian in the important case of a not completely filled valence band. In that situation, the role of the Coulomb repulsion between two particles is quite relevant in determining shifts and broadenings of the spectra. The Coulomb correlation results also in a renormalization of the one-particle spectrum. The results show that, while the Auger spectra are contributed to by both spin up and spin down bands, the APS spectra are contributed to only by spin down bands.

This paper reviews a theory that justifies “lattice match” assumptions in epitaxy and the application of the theory to epitaxy at {111}fcc/{110}bcc metal interfaces. The basic assumptions of the model are (i) that the adatom-substrate interaction has the periodicity and symmetry of the substrate surface, (ii) that the overlayer is crystalline, (iii) that the adatom-adatom interaction is harmonic and (iv) that the ideal epitaxial configurations are the same as for rigid half crystals. The theory correlates epitaxy with lattice match and predicts the occurrence of the Nishiyama-Wassermann and Kurdjumov-Sachs orientations in thick overlayers with astonishing accuracy. In thin overlayers there is good qualitative agreement with empirical data. The theory stresses the need for quantifying the Fourier coefficients.

Low energy ion scattering can provide atom location and structural information from a number of different measurements. Low angle scattering can yield information on atom location and surface crystallography from the measurement of the azimuthal angular distribution of scattered particles. Large angle backscattering provides a direct measurement of shadow cone geometry and combined with the angular dependence of the backscattered yield, this large angle scattering event can provide accurate information on the surface structure. Studies of the angular distribution of recoil ions from the surface can also provide information on the site from which the recoil came. These three techniques have been applied to the study of the Ni(110) surface and to oxygen adsorption on that surface. A comparison of the information available from each technique is presented.

The excess-carrier charge transfer velocity, υ_cp, is an important parameter describing the efficiency of charge transfer across a solid-liquid interface. By using recent theory to analyze current-voltage curves of photoelectrochemical cells, we have been able to derive values of υ_cp for CdSe films in aqueous polysulfide electrolytes and measure the effects of varying the sulfur concentration. The parameter, υ_cp, is found to vary at short circuit conditions, from 1.2 × 10⁶ to 1.36 × 10⁶ cm s⁻¹ on increasing the sulfur concentration from 0 to 3.0M. In addition it has been found that υ_vp has only a weak voltage dependence described by an equation linear in voltage.

The effects of processes used in making thin film ITO/CdS/CdTe/Au heterojunction solar cells have been investigated. In particular, air annealing the ITO/CdS/CdTe layer is shown, from spectral evidence, to produce a heterojunction. XPS evidence shows that this heating forms CdO and TeO₂ on the CdTe surface and that these are removed by a KOH etch. A further bromine/methanol etch leaves a Te-rich surface which forms an injecting contact with the Au top electrode into which the Te mixes.

The Lifshitz theory is applied to calculate the long-range dispersion force between two semi-infinite half planes of dissimilar metals. It is shown that the asymptotic form of the force F, for large separation d between the half planes is F = C₁₂/d³, where an explicit expression is given for C₁₂ in terms of the two plasma frequencies ω_p2 and ω_p1 of the interacting metals. Attention is then given to the contributions to the interfacial energy of the composite system. In an ideal situation, in which the two metals had (a) the same crystal structure, (b) identical lattice parameters and (c) no charge transfer, the interfacial energy is the sum σ₁ + σ₂ of the surface energies of the two pure metals involved. It is argued that the charge transfer contribution (c) takes the form (ΔW)²/ρ²l_c, where ΔW is the difference in work functions of the two pure metals and l_c is a characteristic length. Contributions arising from departures for ideal lattice matching, embodied in point (b) above can be estimated from the isothermal compressibility, κ_T, of the pure metals. Further, invoking for a pure metal the known approximate relation that $\text{σκ}{}_{\mathrm{<mtext>T</mtext>}}\text{∼ 1}\text{A}\text{̊}$. this lattice mismatch contribution is expressed again in terms of surface energies. It will usually alter the contribution σ₁ + σ₂ of ine interfacial energy by a multiplying factor less than, but quite near to unity.

Surface and subsurface compositional changes in Au-Cu alloys and thin films sputtered with 2 keV Ar, Xe, Ne and Kr ions were observed with low energy (60 eV for Cu and 69 eV for Au) and high energy (239 eV for Au and 920 eV for Cu) Auger peaks at temperatures between liquid nitrogen and room temperature. For a Au_0.56Cu_0.44 alloy, Au enrichment at the surface and depletion beneath the surface was observed with high energy as well as low energy Auger peaks. But the compositional change assessed with low energy Auger peaks is greater than that assessed with high energy peaks. This was also calculated with the simulation of sputtering including ion radiation-enhanced segregation and diffusion. This shows clearly that the Gibbsian segregation plays an important role in composition changes at the surface and subsurface of the ion-sputtered alloy. Compositional changes of Au-Cu thin film alloys prepared by the co-evaporation technique were also observed following sputtering with Ar, Xe, Ne and Kr. Except in the case of Xe, the coevaporated composition was preserved during sputtering at low temperatures (<-120°C).

The M_2,3M_4,5V Auger transition obtained from vacuum-cleaved Ge(111) surfaces has been debroadened to remove inelastic scattering, instrumental effects and core level lifetime broadening. The resultant spectrum was further separated into its component - M₂M_4,5V, M₃M_4,5V - transitions to obtain the transition density of states function (TDOS). For the 15.3 eV wide valence band, four definite peaks were found at -4.3, -8.0, -11.9 and -13.7 eV and the main TDOS peak at - 4.3 eV is at variance with the accepted bulk value of approximately - 2.7 eV (analogous to previous silicon results). Detailed AES measurements for excited-oxygen exposed cleaved germanium surfaces are also reported. For oxygen exposures in the range 0.1 to 10³ L a continuous shift was observed in the negative excursion of the M_2,3M_4,5V [N'(E)] transition and the derived TDOS curves showed that a number of oxygen-induced peaks appeared at well-defined energies, -7.7, -8.4, -11.3, -13.5, -14.9, -17.8 and -19.7 eV, below the vacuum level. Three of these peaks, -11.3, -14.9 and -19.7 eV, are consistent with GeO₂ formation. For the range of exposures studied, the valence-band spectroscopic data reported have been interpreted as showing the simultaneous existence of both atomic and molecular chemisorption states and their coexistence with GeO₂ formation during the early oxidation stage.