MD study on high-energy reactive cluster impact on diamond (111) and (100) surfaces

Molecular dynamics (MD) simulations of single argon, CO₂ and O₂ cluster impacts on diamond (100) and (111) surfaces are performed in order to investigate the surface erosion process. The transient crater on the (100) surface seems rather unpherical and skew compared to the typical hemispherical crater appeared on the (111) surface due to the orientation-dependent hardness. Argon cluster impacts on the diamond (100) surface resulted in a slightly higher erosion rate than on the (111) surface while it is lowered on the (111) surface for CO₂ cluster impacts. The difference in the susceptibility to the physical erosion appears in the rim or the crater.     

Observation of sputtering damage on Au(111)

The morphology of a Au(111) surface has been observed with the STM (scanning tunneling microscope) after ion bombardment with 2.5 keV Ne⁺ ions at about 400 K. Mostly triangular and hexagonal shaped vacancy islands are seen in the STM topographs. They are bounded by monatomic steps, oriented along the closed packed 110 directions. The general morphology confirms the conclusions inferred from TEAS (thermal energy atom scattering) measurements on ion bombarded Pt(111) surfaces. The observation of a propensity for the formation of {100} microfacetted 110 ledges is discussed.     

A static SIMS/TPD study of the kinetics of methoxy formation and decomposition on O/Pt(111)

The kinetics and mechanism of the decomposition of methanol (CH₃OD) on oxygen-covered Pt(111) were studied using static secondary ion mass spectrometry (SIMS) and temperature programmed desorption (TPD). The initial sticking coefficient and the saturation first layer coverage of CH₃OD are unity and 0.36 ML, respectively. The maximum amounts decomposed in TPD on O/Pt(111) and clean Pt(111) are 0.19 and 0.047 ML, respectively. At low methanol coverages (< 0.05 ML) on O/Pt(111) the only reaction products were CO₂, H₂O and D₂O, whereas at saturation CO, H₂O, D₂O and H₂ were observed. The decomposed amount did not saturate at or before the onset of molecular methanol desorption, but increeased monotonically until saturation of the first layer. No oxygen exchange was observed between CH₃OD and preadsorbed ¹⁸O. A decomposition mechanism involving methoxy and hydroxyl type species is proposed. Methanol coverages as low as 0.002 ML could be detected with SIMS. The CH₃⁺ ion was the most intense ion in the positive SIMS spectrum of both methanol and methoxy. Larger ion clusters such as (CH₃OD)_n⁺ (n = 2, 3) developed successively at specific multilayer coverages. At low coverages on O/Pt(111), methoxy formation occurs above 125 K and its decomposition becomes detectable above 150 K during temperature programming. In the isothermal mode, the SIMS CH₃⁺ ion was used to follow the kinetics. Over the temperature range 120–145 K, the second order Arrhenius rate parameters for methoxy formation are E = 5.5±0.7 kcal/mol and A = 1.5×10^−7±0.6 cm²/s·molecule for an initial methanol coverage of 0.05 ML. Methoxy decomposition was studied in the temperature range 150–165 K and at an initial coverage of 0.015 ML. The first order kinetic parameters, E = 11.4±0.5 kcal/mol and A = 5.3×10^13±1 s⁻¹ were derived. Advantages and limitations of using SIMS as a tool for kinetic studies are discussed.     

Analyses of HF/NH4F buffer-treated Si(111) surfaces using XPS,REM and SIMS

The effect of different cleaning procedures on Si(111) wafers has been studied. A three-step cleaning process was used. The first two steps (thermal oxidation followed by RCA cleaning) were common to all samples. The final step involved rinsing in one of a set of HF/NH4F buffer solutions with a wide range ofpH values. Three different surface techniques were used for characterizing the chemical condition and morphology of the treated surfaces: XPS (X-ray Photoemission Spectroscopy), REM (Reflection Electron Microscopy) and SIMS (Secondary-Ion Mass Spectroscopy). It has been found that thepH value of an HF solution does significantly affect the etching rate and morphology of the Si(111) surface: For the same type of solution, the smaller thepH value, the higher the etching rate. Basic solutions withpH values larger than eight have a much weaker etching effect on the surface, which is contradictory to some previous reports. The most effective solutions for the etching of the Si(111) surface are the solutions of HF buffered by NH₄F, with thepH in the range of 2–6. REM images indicate that the surface morphology after etching in the HF solution is strongly affected by the length of the etching time: Overetching will roughen the surface. The SIMS data show that water rinsing in air during the cleaning process does speed up oxidation, but it is necessary to use water to clean off the residuals from the HF solutions.     

Specific features of plasma etching of helical PbTe structures on BaF2(111) substrates

The morphology of lead telluride surface films with helical formations on (111) barium fluoride substrates in the initial state and after treatment in an argon plasma is studied using atomic force microscopy. It is shown that the presence of large helical stepped structures near the outcrops of threading dislocations results in specific phenomena upon sputtering. The main features of the modification of the surface relief and the relation of the nascent single and grouped microhillocks to the initial surface relief and dislocations are determined.     

Surface coverage measurements by sims for CO adsorption on a number of metals and for CO and H2S CO-adsorption on Ni(110), (100) and (111)

A detailed study of CO adsorption on Ni(100) utilizing static SIMS and a comparison of the data with surface coverage data from the literature shows that there is a linear relationship between CO coverage and the peak intensity ratios (MCO⁺/M⁺ and M₂CO⁺/M⁺₂) of the CO-containing secondary ions, in the region of coverage below which the adlayer becomes compressed. Adsorption isobares were obtained using the intensity ratios and from these, adsorption isosteres were derjved to give heats of adsorption as a function of coverage. These data are in very close agreement with the literature. Confirmatory data were obtained for this relationship for CO adsorption on polycrystalline Ni, Pd, Pt and Cu and Cu(100). The application of this technique of surface coverage measurements to a study of the extent to which H₂S coadsorption reduces the coverage of adsorbed CO on Ni(110), (100) and (111) shows that these faces are poisoned in the order (100) > (111) > (110). Surface coverage measurements on the non-closepacked (110) face are affected by the apparent insensitivity of SIMS to adsorbates located in the “channels”.     

On the structure of the laser irradiated Si(111)-(1 × 1) surface

We have studied the geometric structure of the Si(111)-(1 × 1) surface, prepared from a Si(111)-(7 × 7) surface with a pulsed ruby laser. With the technique of medium energy ion scattering, in combination with channeling and blocking, we find that this surface has a structure very similar to the (7 × 7) surface. Our results are inconsistent with a simple relaxation model.     

Surface reconstruction of Cu (111) upon oxygen adsorption

Low-energy He⁺ ion scattering (IS) in combination with AES, LEED and work function measurements has been applied for the determination of surface reconstruction of Cu(111) upon oxygen adsorption. IS data clearly indicate that oxygen is not significant incorporated into the bulk at room temperature adsorption, however the surface shows reconstruction by displacement of Cu atoms by 0.3 Å. The disappearance of structure of both Cu_IS and O_IS in the “?_in pattern” demonstrate the development of a disordered layer of reconstruction centres. At saturation coverage, a rough and dis-ordered oxygen-copper surface layer is present.     

Interaction of HNCO with Au(111) surfaces

The surface chemistry of isocyanic acid, HNCO, and its dissociation product, NCO, was studied on clean, O-dosed and Ar ion bombarded Au(111) surfaces. The techniques used are high resolution energy loss spectroscopy (HREELS) and temperature-programmed desorption (TPD). The structure of Ar ion etched surface is explored by scanning tunneling microscopy (STM). HNCO adsorbs molecularly on Au(111) surface at 100 K yielding strong losses at 1390, 2270 and 3230 cm^? 1. The weakly adsorbed HNCO desorbs in two peaks characterized by T_p = 130 and 145 K. The dissociation of the chemisorbed HNCO occurs at 150 K to give NCO species characterized by a vibration at 2185 cm^? 1. The dissociation process is facilitated by the presence of preadsorbed O and by defect sites on Au(111) produced by Ar ion bombardment. In the latter case the loss feature of NCO appeared at 2130 cm^? 1. Isocyanate on Au(111) surface was found to be more stable than on the single crystal surfaces of Pt-group metals. Results are compared with those obtained on supported Au catalysts.     

Chemisorption of oxygen on the silver (111) surface

The interaction of oxygen with the (111) surface of a silver single crystal is studied, mainly in the pressure range from 10^?3 up to 1 torr and at temperatures from room up to 500°C. The experimental techniques employed were LEED, secondary electron spectroscopy, work function variation measurements, and desorption kinetics. Exposure to the high pressures was made with a sample isolation valve. The experimental procedures are examined in detail and critically discussed. The results obtained with the different techniques allow a correlation with many studies of other authors. The LEED technique indicates that in the range of pressures and temperatures examined, a surface superstructure is stable, having a unit mesh with sides four times greater than that of the silver (111) plane. The presence of this surface phase seems to be related to oxygen adsorbed in the dissociated form. On this assumption, an interpretation of the structure is proposed, which is based on a coincidence lattice formed by a (111) plane of Ag₂O on the (111) plane of the metal. This interpretation is also in agreement with the thermodynamic data.     

The adsorption of Xe and CO on Ag(111)

The adsorption of Xe and CO on Ag(111) in the range 66 to 123 K and 10^?7 to 10^?1 Pa has been studied by surface potential, low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and electron energy loss spectroscopic (EELS) measurements. Isotherms derived from both surface potential and AES measurements for submonolayer Xe adsorption reveal successive stages and a two-dimensional phase change. Isosteric heats were 18 ± 1 kJ mol^?1. Temkin isotherms were observed for CO, the heat falling linearly with coverage from an initial value of 27 ± 1.5 kj mol^?1. No ordered CO overlayer structure could be detected. EEL spectra of clean Ag(111) agree with previous studies. Additional loss peaks were recorded for Xe and CO overlayers, and the assignment of the substrate loss features is discussed in relation to the effects of adsorption.     

Vibrational dynamics and surface structure of Bi(111) from helium atom scattering measurements

The Bi(111) surface was studied by elastic scattering of helium atoms at temperatures between 118 and 423 K. The observed diffraction patterns with clear peaks up to third order were used to model the surface corrugation using the eikonal approximation as well as the GR method. Best fit results were obtained with a rather large corrugation height compared to other surfaces with metallic character. The corrugation shows a slight enhancement of the surface electron density in between the positions of the surface atoms. The vibrational dynamics of Bi(111) were investigated by measurements of the Debye-Waller attenuation of the elastic diffraction peaks and a surface Debye temperature of (84 ± 8) K was determined. A decrease of the surface Debye temperature at higher temperatures that was recently observed on Bi nanofilms could not be confirmed in the case of our single-crystal measurements.     

Growth mode and interface structure of Ag on the HF-treated Si(111):H surface

A growth mode and interface structure analysis has been performed for Ag deposited at a high temperature of 300°C on the HF-treated Si(111):H surface by means of medium-energy ion scattering and elastic recoil detection analysis of hydrogen. The measurements show that Ag grows in the Volmer-Weber mode and that the Ag islands on the surface are epitaxial with respect to the substrate. The preferential azimuthal orientation is A-type only when Ag is deposited slowly. The interface does not reconstruct to the √3 × √3-Ag structure, which is normally observed for Ag deposition above 200°C on the Si(111)7 × 7 surface, but retain bulk-like structure. The presence of hydrogen at the interface is demonstrated after deposition of thick (1100 Å) Ag films. However, the amount of hydrogen at the interface is not a full monolayer. This partial desorption of hydrogen from the interface explains why the Schottky barrier heights of Ag/Si(111):H diodes are close to those of Ag/Si(111)7 × 7 and Ag/Si(111)2 × 1.     

Direct observation of an ordered step surface reconstruction on Au(111) by scanning tunneling microscopy

Experiments with a scanning tunneling microscope (STM) are reported which include the first surface topographic images of Au(111), cleaned and annealed in ultrahigh vacuum. The STM system used for this work includes in situ sample manipulation for sample cleaning, annealing and characterization. Topographs with very large atomically flat (111) regions are obtained and no corrugation on the scale of about 0.1 Å is observed. Some single atomic steps are observed. Surprisingly, ordered arrays of single steps are observed over a large region. These represent a type of surface reconstruction whose repeat period agrees with values derived from previous LEED, TEM and ion scattering measurements. These results represent the first critical information about the surface topography of the Au(111) surface.     

Secondary ion emission processes of sputtered alkali ions from alkali/Si(100) and Si(111)

The secondary alkali ion yield vs. the work function change (Δφ) of Na, K and Cs/Si(100) and Si(111) was measured to discuss the details of secondary ion emission processes. In the case of alkali/metal systems, the secondary ion emission is explained by the electron tunneling model. In this model, the ionization of the ejected atom occurs as a result of electron resonant tunneling through the potential barrier separating an atom and a metal, and the secondary ion yield depends on exponentially the work function change of metal surface. For alkali/Si(100) systems, the secondary ion emission processes are explained in terms of the electron tunneling model since the secondary alkali ion yield vs. the work function change (Δφ) follows the exponential manner. However, it is not easy to apply the simple electron tunneling model to our experimental results for alkali/Si(111) systems. There is the essential difference in surface structures between Si(100) and Si(111). Therefore, it is suggested that the local electronic environment around the adsorbates might be taken into consideration for alkali/Si(111) systems.     

Surface states calculation for the (100), (110) and (111) faces of Si

Using essentially the Heine and Jones model of the band structure for diamond-type semiconductors and a 3-dimensional surface state secular equation, we have calculated the bands of surface states for the (100), (110) and (111) orientations of the surface plane of ideal Si. Our bands for the (110) surface are very similar to those obtained by Jones. Based on these bands of surface states, we can explain reasonably both optical and electrical measurements on the (111) surface.     

H原子在Al(111)和Ag(111)面上的吸附

本文用电荷自洽的EHT方法,研究了H原子在Al(111)和Ag(111)面上的吸附,结果指出:在Al(111)面上,H以原子状态吸附在某些对称位置上,它也能渗透到表面层中去,成为填隙原子;H₂分子在表面处发生解离吸附。在Ag(111)表面上,H原子有可能以分子状态吸附,H—H键平行于表面,这与高分辨率电子能量损失谱所得到的实验结果一致;但H₂分子在Ag(111)表面也可能发生解离吸附。 关键词：     

Steady state shapes of periodic profiles on (110), (100), and (111) surfaces of nickel

Periodic surface profiles with amplitudes of ≦0.4 μm and periodicities of 4–20 μm were prepared on Ni(110), (100), and (111) single crystal surfaces. These crystals were annealed in ultra-high vacuum (UHV) at 1073–1327 K after they had been cleaned by Ar ion bombardment and investigated by Auger electron spectroscopy. The geometry of the profiles was studied in UHV by laser diffraction and outside the vacuum by interference microscopy. The profiles have sinusoidal shapes on Ni(110) but trapezoidal shapes on both the (100) and (111) surfaces. This type of faceting can be understood on the basis of the anisotropic surface energy of Ni, with cusps at the (100) and (111) orientations. Model calculations show in the case of anisotropic surface energy that periodic profiles develop facets which correspond to the low surface energy orientations (close-packed surfaces).     

A LEED and AES study of the structure,debye temperature,and oxidation of the (111) crystal face of thorium

The structure, and reactivity towards O₂ and CO, of the (111) crystal face of a single crystal of high purity thorium metal was studied using low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). After the sample was cleaned in vacuum by a combination of ion bombardment and annealing, a (1 × 1) LEED pattern characteristic of a (111) surface was obtained. Extended annealing of the cleaned sample at 1000 K produced a new LEED pattern characteristic of a (9 × 9) surface structure. A model of a reconstructed thorium surface is presented that generates the observed LEED pattern. When monolayer amounts of either O₂ or CO were adsorbed onto the crystal surface at 300 K, no ordered surface structures formed. Upon heating the sample following these exposures the (111) surface structure was restored accompanied by a reduction in the amount of surface carbon and oxygen. With continued exposure to either O₂ or CO and annealing, a new LEED pattern developed which was interpreted as resulting from the formation of thorium dioxide. Debye-Walter factor measurements were made by monitoring the intensity of a specularly reflected electron beam and indicated that the Debye temperature of the surface region is less than it is in bulk thorium. Consequently, the mean displacement of thorium atoms from their equilibrium positions was found to increase at the surface of the crystal. The presence of chemisorbed oxygen on the crystal surface affected the Debye temperature, raising it significantly.     

CO on Pt(111): TDS and ESD study

Electron stimulated desorption of ionic species from CO adsorbed on Pt(111) has been studied and comparison made with EELS results. The “on-top” site which, according to EELS data, fills first is observed to yield O⁺ ion. The bridge adsorption site appears to release CO⁺ during electron bombardment. Coadsorption of H₂ and CO was also examined and compared with the polycrystalline platinum case. Only very weak coadsorption effects are seen on the Pt(111) surface, as evidenced by presence of a weak low energy component associated with the O⁺ ESD energy distribution.