“Magnetic dead layers” on chemisorption at ferromagnetic surfaces

Chemisorption induced “magnetic dead layers” have been determined from Ferromagnetic Resonance (FMR) absorption of Ni and Fe thin films. Within experimental error “magnetic dead” or “live layers” can be excluded at free Ni surfaces. Approximately one hydrogen atom diminishes the contribution of one nickel atom to the thin film ferromagnetism in the β₂-state. Twice the effect is found for CO adsorption on nickel. Possible models to explain chemisorption effects in surface magnetism are discussed briefly.     

Benzene adsorption on the Rh(111) metal surface: A theoretical study

The chemisorption of benzene on the Rh(111) surface is studied in the general framework of the extended Hückel theory, with optimized Rh parameters. MO arguments are used to discuss the main features of the electronic interaction of the aromatic system with the surface. Binding energy curves for adsorption on the most likely surface sites are computed, and the effect of the CH bond back-bending and of tilting of the benzene plane are examined. The most favorable chemisorption geometry is found when the C-atom ring is parallel to the surface and the center of the ring is above a threefold hollow site, but H atoms are farther away from the surface than C atoms. The optimum CRh distance compares well with those found in metal complex molecules. The on-top site is unfavorable for benzene chemisorption. The results of tight-binding calculations on infinite slabs agree with, and support, those of simple cluster calculations. A slight “Kekulé distortion” and a small activation barrier to chemisorption are predicted.     

Chemisorption of nitric oxide by nickel

The adsorption of nitric oxide on clean and pre-oxidized nickel has been investigated by X-ray photoelectron spectroscopy. Three distinct states of chemisorption have been recognised at room temperature; one is dissociative while two involve molecularly adsorbed NO. Pre-exposing the nickel surface to oxygen enabled the activity of the surface to be controlled such that adsorption was confined to only one of the molecular states. The two molecular states are suggested to arise from “bent” and “linear” forms of NO.     

Interaction of ZnO surfaces with oxygen after illumination in the presence of CO and O2

It was found that oxygen chemisorption on ZnO surfaces is accelerated by as much as three orders of magnitude after illumination in the presence of CO + O₂. This accelerated chemisorption is characterized by electron transfer and thermal activation as is oxygen chemisorption on “real” ZnO surfaces. The observed behavior is attributed to the formation of surface complexes (involving carbon, oxygen and Zn ions) which enhance oxygen chemisorption. These complexes are stable at room temperature and render the surface inert to further CO-interaction.     

Embedded cluster calculations for hydrogen chemisorption on the (111) surfaces of fcc Co,Ni, Cu,Rh, Pd and Ag

We present a muffin tin based calculation on (TM)₃H, (TM)₇H and (TM)₁₉H clusters embedded at the surface of an effective jellium-like medium whose potential is treated in scattering length approximation. We consider the changes occurring when the d-like perturbation of the TM muffin tins is switched on. The broad chemisorption-induced resonance seen for H on the effective jellium surface is narrowed and shifted down in energy. Furthermore the occupation of this resonance is increased from about 1.1 electrons to about 1.4 (on 3d metals) or 1.8 (on 4d metals), due to d-like states dropping down from the d band to form a relatively welldefined “bonding state”. An antibonding state containing about 0.4 electrons is formed at the top of the d band. The results are compared with other calculations and with photoemission data. Implications for the metal-hydrogen distance and (for Ni) the demagnetizing effect of hydrogen chemisorption are discussed. We use the change in total single particle energy when the d-like perturbation is switched on to estimate trends in chemisorption energy along the 3d and 4d series. In the 3d case experimental data is available on the difference in chemisorption energy between Ni and Cu which is in reasonable agreement with our estimate.     

原子氢在Be(1010)薄膜上吸附的第一性原理计算

对原子氢在Be(1010)薄膜表面的吸附性质做了第一性原理计算研究.根据原子面间距的不同，可把Be(1010)表面分为两种.计算结果表明，原子氢在这两种表面上的吸附性质显著不同.为阐明和分析这些不同，系统计算和分析了Be(1010)薄膜的表面电子结构、电子功函数、平均静电势和局域电荷密度.这些物理量都自洽地表明，吸附过程中原子氢和表面铍原子间的电荷转移过程对于两种表面是完全不同的.对于L型表面来说，电荷由吸附原子氢向表面Be原子层转移，而对于S型表面而言，电荷转移过程恰恰相反. 关键词： 表面能 功函数 量子尺度效应     

原子氢在Be(100)薄膜上吸附的第一性原理计算

对原子氢在Be(1010)薄膜表面的吸附性质做了第一性原理计算研究.根据原子面间距的不同,可把Be(1010)表面分为两种.计算结果表明,原子氢在这两种表面上的吸附性质显著不同.为阐明和分析这些不同,系统计算和分析了Be(1010)薄膜的表面电子结构、电子功函数、平均静电势和局域电荷密度.这些物理量都自洽地表明,吸附过程中原子氢和表面铍原子间的电荷转移过程对于两种表面是完全不同的.对于L型表面来说,电荷由吸附原子氢向表面Be原子层转移,而对于S型表面而言,电荷转移过程恰恰相反.     

LCAO studies of hydrogen chemisorption on nickel: II. Cluster models for adsorption sites

The adsorption of single hydrogen atoms, investigated by means of cluster calculations, has been compared with the adsorption of hydrogen monolayers on periodic crystals (paper I). From the similarity of the adsorption energy curves we conclude that the (direct and indirect) interactions between adsorbed hydrogen atoms are relatively small up to monolayer coverage. For adsorption on different sites of ideal low index surfaces the stability decreases in the order Atop > Bridge > Centred. For Atop adsorption it increases with a decreasing number of nearest neighbours to the nickel atom in the NiH “surface molecule”, thus leading to especially strong adsorption sites at the edges of a stepped surface and to low stability in the notches. In general, we find that the Ni_nH “surface molecule” with n = 1, 2, 3 or 4 determines the equilibrium positions for H adsorption; the inclusion of one shell of neighbours to the nickel atoms is sufficient to explain the differences in adsorption energy. The Extended Hückel method is not well suited to study dissociative chemisorption of H₂, although some qualitative trends are correct.     

Resonant surface enhanced Raman scattering by ultra thin Langmuir-Blodgett polydiacetylene polymer films

Using resonant Raman scattering and surface enhanced Raman scattering techniques, changes in the structural and electronic properties of Langmuir-Blodgett (L-B) polydiacetylene films were observed as film thicknesses were increased from one monolayer (or one bilayer) to several bilayers. The L-B films, starting with a single monolayer (or one bilayer) in a disordered “red” phase, were found to change into a mixed phase (ordered “blue” and disordered “red”) as one or more additional bilayers were deposited. This is the first observation of a “disorder to order” transformation in a L-B film. The observed effect is attributed to the ordering brought about by interactions between the initial and subsequent L-B PDA bilayers.     

On the nature of “sers active sites”

Active sites of short range enhancement in surface enhanced Raman scattering (SERS) of adsorbates on cold deposited, porous silver films have been envisioned either as “cavity sites with local electromagnetic resonances” or as “sites of atomic scale roughness with photon driven charge transfer between metal and adsorbate”. Whereas SERS of CO and N₂ is observed at low coverages, the Raman signal of physisorbed O₂ is below the noise level, even at monolayer coverage of the “internal surface” or “filling of the pores”. On the other hand, SERS of ethylene and pyridine is quenched by some percent of a monolayer of oxygen. The Raman signal of multilayer condensed O₂ is stronger for a “compact” film than for a porous film. These observations cannot be reconciled with the first hypothesis, but they are consistent with the second.     

Co chemisorption on PtCu surfaces II. (1 × 1)-CO/Pt0.98Cu0.02(110)

The chemisorption of CO on the Pt atoms of an initially (1 × 3) reconstructed Pt_0.98Cu_0.02(110) surface at ~ 373 K can lead to the formation of a (1 × 1) surface. Comparisons are made with (1 × 3)-CO surfaces formed by CO exposures at 293 or 155 K. Thermal desorption shows that the (1 × 1)-CO surface has an enhanced population of high temperature CO peak ( ~ 543 K) from Pt sites. The CO-induced structural conversion also leads to a decrease in the subsequent CO uptake on the low temperature Pt sites and on the Pt-Cu “mixed” sites, with a concomitant increase in adsorption on the Cu-like sites. Such a reduction in the number of the Pt-Cu “ mixed” sites is also reflected in the CO-induced changes of the Cu 3d-derived states and the $\text{Cu 2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ core levels. A dynamic interplay between chemisorption and surface structure is thus demonstrated.     

Chemisorption of oxygen to gold: AES study of catalytic effect of calcium

Surface-segregating calcium impurity is removed from a polycrystalline gold sample by heating, exposing to oxygen, and ion-bombarding over an extended period of time. The rate of oxygen chemisorption is found to decrease drastically from that at the beginning of the purification period where calcium segregation is substantial to that at the end where segregated calcium is nearly undetectable. The effect of calcium appears to involve dissociative chemisorption of molecular oxygen at the calcium sites followed by migration of oxygen atoms to gold sites, in a manner analogous to “hydrogen spillover” occurring on the surface of supported catalysts. It is also found that oxygen previously chemisorbed to a gold surface can promote the surface segregation of calcium even in the absence of gas-phase oxygen. Equations are presented to account for simultaneous catalysis of oxygen chemisorption to gold by calcium, and promotion of calcium surface-segregation by oxygen chemisorbed to gold.     

The chemisorption of H2O,HCOOH and CH3COOH on thin amorphous films of Al2O3

Inelastic electron tunneling spectroscopy (IETS) has been used to probe the irreversible chemisorption of H₂O, HCOOH, and CH₃COOH on a thin amorphous film of Al₂O₃. The “clean” Al₂O₃ film was also probed in a similar way. The measurement involves examining the second derivative of the I–V curve for electrons tunneling between the metal electrodes in an AlAl₂O₃Pb junction. When chemisorbed species are present on the Al₂O₃, the second derivative measurement is a representation of vibronic excitations of the chemical bonds in the adspecies, i.e. the tunneling spectra are analogous to IR spectra. A “clean” Al₂O₃ surface is found to possess free OH groups essentially equivalent to a surface exposed to large concentrations of H₂O. Saturation coverages of HCOOH and CH₃COOH yield similar spectra to each other, as expected. However, chemical shifts as well as variations in intensity among the equivalent bonds in the adsorbed HCOOH and CH₃COOH are noted. Tentative assignments are given for the observed peaks, and it is concluded that the acids adsorb as formate and acetate ions on the surface. Tunneling spectra for several submonolayer coverages of HCOOH (θ = 0.03, 0.08, 0.3 and 0.4) have been measured in addition to the spectrum corresponding to saturation coverage. Small shifts in certain vibronic transitions as a function of surface coverage have been noted. The shifts in the peak positions are related to variations in adsorbate-adsorbent and adsorbate-adsorbate interactions, and the variations in relative peak intensity are associated with the orientation of the adsorbed molecules. The kinetics of the chemisorption of HCOOH, i.e. the sticking probability as a function of surface coverage, has also been determined for surface coverages between 0.03 and 0.4 monolayer.     

The adsorption of ammonia on an Fe(110) single-crystal surface studied by high-resolution electron energy-loss spectroscopy (eels)

The EELS spectra of ammonia adsorbed on an Fe(110) surface at 110 K reveal four different adsorption states of molecular ammonia with increasing coverage : chemisorption at “on-top” sites, chemisorption at multi-coordinated sites, adsorption in a second layer, and multilayer condensation.Thermal processing of an ammonia-covered sample to 155 K causes desorption of both the condensed phase and the second layer without any fragmentation of ammonia.Further heating of the sample leads to a much weaker desorption of molecular ammonia up to a temperature of 260 K. EELS spectra recorded after heating to 290 K show only small amounts of atomic nitrogen and hydrogen present on the surface, indicating partial decomposition of ammonia.The formation of species such as NH₂ (ads) or NH (ads) during the thermal-processing experiments could not be observed.     

Diamond-like C2H nanolayer, diamane: Simulation of the structure and properties

We consider a new C₂H nanostructure based on bilayer graphene transformed under the covalent bond of hydrogen atoms adsorbed on its external surface, as well as compounds of carbon atoms located opposite each other in neighboring layers. They constitute a “film” of the 〈111〉 diamond with a thickness of less than 1 nm, which is called diamane. The energy characteristics and electron spectra of diamane, graphene, and diamond are calculated using the density functional theory and are compared with each other. The effective Young’s moduli and destruction thresholds of diamane and graphene membranes are determined by the molecular dynamics method. It is shown that C₂H diamane is more stable than CH graphane, its dielectric “gap” is narrower than the band gap of bulk diamond (by 0.8 eV) and graphane (by 0.3 eV), and is harder and more brittle than the latter.     

大尺寸化学Ni源金属诱导晶化多晶硅的研究

用无电电镀的化学方法，在VHF-PECVD沉积获得的非晶硅薄膜表面形成镍诱导源，在550℃下退火若干小时，可以诱导产生微米量级的多晶硅晶粒.用此法形成的镍源可以均匀地分布在非晶硅薄膜的表面.非晶硅薄膜上形成晶核的数量取决于镍溶液的浓度、pH值和无电电镀的时间等参量.当成核密度比较低时可以观察到径向晶化现象.用VHF-PECVD非晶硅薄膜作为晶化前驱物，晶化后多晶硅的最大晶粒尺寸可达到90μm.用此多晶硅试制的TFT，获得了良好的器件特性. 关键词： 金属诱导晶化 化学源 多晶硅 薄膜晶体管     

Auger and secondary electrons excited by backscattered electrons; An approach to quantitative analysis

The contribution of backscattered electrons (BE) to Auger electrons (AE) and secondary electrons (SE) was studied by depositing Be onto a polycrystalline or deposited Cu substrate. The effects of backscattering on SE and maximum escape depth of SE were obtained by using the so called δ-η method. This method was also applied to the AE and the effects of BE on AE were experimentally evaluated. The AE yield versus primary energy curve which was corrected for BE was compared with other experiments and theories and considerably good agreement was obtained. From this analysis, the excitation efficiencies of AE by primary electrons and by BE could also be obtained. The absolute AE yield of Be (KVV) was estimated by “area” measurements. The changes of plasma losses, the elastic peaks, the energy distribution of BE, and the true SE were also observed as a function of deposited film thickness, and the results are discussed.     

Infrared vibration spectroscopy of molecular adsorbates on tungsten using reflection inelastic electron scattering

The observation of adsorbate vibrational energies in the range, 30 ?, hv_vib ? 1000 meV, by electron-energy-loss spectroscopy, provides detailed information on the geometry of atomic and molecular complexes. The “surface normal dipole selection rule”, is discussed and illustrated with results obtained for CO and C₂H₂ adsorption on the principal low-index faces of tungsten, viz.: W(100), W(110) and W(111) using a high-resolution electron reflection spectrometer. Specifically, the behaviour of chemisorbedd diatomic carbon monoxide and polyatomic acetylene is compared as a function of coverage and surface crystallography. Comparison is made with the spectral information obtained by reflection infrared spectroscopy and recent ultraviolet photoelectron spectroscopy studies of the chemisorption binding energies. The energy loss spectra are discussed in terms of current adsorbate models and the possible formation of “distorted rehybridized surface molecular complexes” based on molecular orbital theories of organometallic compounds.     

Atomic exchange mean field study of intermixing of Au on Ag(110)

A mean field method to study heteroepitaxial thin film growth is applied to growth, intermixing and surface reconstructions of Au on Ag(110). The results are in accordance with experimentally observed “burrowing” at submonolayer coverages and growth of elongated, (1 × 3) reconstructed, Au clusters at higher coverages. At coverages of a few monolayers the surface between the clusters has a high concentration of Ag, and ordered rows of Au are formed just beneath the surface.     

Results on low pressure oxygen adsorption on a Pt60Re40 alloy sample: A SIMS and XPS study

Low pressure oxygen adsorption on Pt₆₀Re₄₀ was investigated at 300, 570 and 770 K employing XPS and SIMS. At 300 K oxygen chemisorption on both Re and Pt surface atoms was observed. At 570 and 770 K. a rhenium surface oxide covers the alloy surface after oxygen treatment. It is suggested that this surface oxide equals “ReO” as observed by Alnot et al. on pure Re.