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.     

The effect of the crystal structure of the field emitter on field ion energy distributions

The field ionization probability of an atom as a function of distance from the field emitter is discussed in terms of the atomic arrangement and the electron scattering properties of the ion cores of the emitter in the immediate neighborhood of the atom to be ionized, and the electron transmission properties of the potential barrier between the emitter and that atom. This approach to field ionization calculations is somewhat similar to field ionization calculations based on low energy electron diffraction (LEED) procedure in that it takes into account electron scattering from the first few atomic layers of the emitter. It differs from LEED type calculations, because it considers the highly localized nature of the ionization near a surface atom. This localization makes the ionization probability relatively insensitive to the two-dimensional periodicity of the emitter surface. A one-dimensional calculation, in which only the potential barrier and three ion core scatterers in line with the field are considered, shows secondary structure in the predicted field ion energy distributions near the critical energy deficit, as well as the well known, primary field induced resonance peaks. The surface orientation dependence of these distributions arises naturally from this model because the secondary structure depends strongly upon the crystal parameter along a line parallel to the field. This one-dimensional calculation can be no more than an approximation to a complete calculation. It is interesting, however, that such a simple physical model, in which scattering from the image potential and only two or three ion cores is considered, rather than scattering from a complete crystal, can give prodicted field ion onergy distributions which are similar to those experimentally observed.     

Calculations on the effect of the surface potential barrier in LEED

The main effects of the surface potential barrier in LEED arise from strong internal reflection of diffraction beams for incidence conditions close to those for their grazing emergence. Near each grazing emergence condition, the details of LEED intensity profiles depend on the amplitude coefficient of internal reflection and hence on the shape of the surface potential barrier. Calculations of the amplitude coefficient for real one-dimensional barrier potentials with shapes specified by three parameters are reported. The calculations are done as a function of the energy associated with the surface-normal momentum component of the internally incident beam. The potential has the image form at large distances from the surface and joins smoothly to a quadratic at an intermediate distance. The results are tabulated and an interpolation formula is given so that the amplitude coefficient of reflection can be evaluated readily for potentials of the assumed form. Applications to the calculation of LEED intensity lineshapes are summarized.     

The growth mode of aluminium on silver (111)

The geometric and electronic structures occuring during the growth of Al on a single crystal Ag(111) surface have been studied using a combination of low energy electron diffraction (LEED), Auger electron spectroscopy (AES), energy loss spectroscopy (ELS) and work function measurements. The Auger signal versus deposition time plots, which were used to monitor the growth mode, are shown to behave in an identical fashion to that expected for layer-by-layer (Frank-van der Merwe) growth. LEED was used to determine the lateral periodicity of thin Al films and shows that Al forms, at very small coverages, 2D islands which have the same structure as the Ag(111) substrate and which grow together to form the first monolayer. At substrate temperatures of 150 K a well defined (1 × 1) structure with the same orientation as the underlying Ag(111) can be seen up to at least 12 ML. After completion of the third monolayer the ELS spectrum approached that observed for bulk aluminium. At a coverage of 3 ML the work function decreases by 0.4 eV from the clean silver value.     

On the choice of surface barrier models for LEED intensity calculations

The significance of surface barrier scattering in LEED is assessed. The scattering properties of various model barriers are compared. Modifications and improvements are suggested for the simpler barrier models. A computational test for Cu(001) shows that a step barrier is satisfactory for LEED intensity calculations except where details of the surface barrier resonance are desired.     

Chemisorption and surface barrier structure

The effects of chemisorption on the potential barrier at a metal surface are discussed. Simple physical considerations predict an outward shift in the barrier origin, an increased saturation of the barrier, and changes in the inner potential. However, an analysis of low-energy electron diffraction (LEED) data for p(2 × 1) O/W(110) and p(2 × 1) H/W(110) indicates that these changes are small, and that the observed modifications are due primarily to changes in the surface scattering properties accompanying chemisorption. LEED fine structure analysis should then be a useful technique for identifying adsorption sites on metal surfaces.     

LEED fine structure: Origins and applications

For incident energies below 50 eV, high resolution low energy electron diffraction (LEED) spectra exhibit fine structure arising primarily from the scattering of electrons by the surface potential barrier. We review the experimental techniques used for measuring such spectra, and derive a model for the process involved. The comparison between measured spectra and calculations using this model provides details about the surface barrier structure and adsorption sites on metal surfaces. We discuss current applications of this technique and some promising directions for future work.     

Correlation of electron self-energy with geometric structure in low-energy electron diffraction

In low-energy electron diffraction (LEED) studies of surface geometries where the energy dependence of the intensities is analyzed, the in-plane lattice parameter of the surface is usually set to a value determined by x-ray diffraction for the bulk crystal. In cases where it is not known, for instance in films that are incommensurate with the substrate, it is desirable to fit the in-plane lattice parameters in the same analysis as the perpendicular interlayer spacings. We show that this is not possible in a conventional LEED I(E) analysis because the inner potential, which is typically treated as an adjustable parameter, is correlated with the geometrical structure. Therefore, without having prior knowledge of the inner potential, it is not possible to determine the complete surface structure simply from LEED I(E) spectra, and the in-plane lattice parameter must be determined independently before the I(E) analysis is performed. This can be accomplished by establishing a more precise experimental geometry. Further, it is shown that the convention of omitting the energy dependency of the real part of the inner potential means geometrical LEED results cannot be trusted beyond a precision of approximately 0.01 ?.     

Multilayer relaxations of Ni(110): New medium energy ion scattering results

Recent ion scattering and LEED measurements of the clean Ni(110) surface structure show a significant difference in the magnitude of the first layer relaxation. Ion scattering has measured the first layer contraction to be 4.5% while LEED I–V analyses have resulted in measurement of an 8.5% contraction. In an attempt to resolve this apparent discrepancy we have undertaken a careful reexamination of the clean Ni(110) surface with medium energy ion scattering using the technique of channeling and blocking. The experiment was conducted in a UHV chamber with a toroidal electrostatic analyzer using 110 keV protons. Our analysis was based on comparisons of measured surface blocking curves in two scattering geometries with full crystal Monte Carlo simulations. These simulations include variations of the interlayer spacing of the outer three surface layers and of the surface Debye temperature. Our measurement shows a first layer contraction, D₁₂ = −9.0% ± 1.0% and a second layer expansion, D₂₃ = +3.5% ± 1.5% (measured as percent of the bulk interlayer spacing) with a surface Debye temperature of 395 K. The present result is in excellent agreement with two recent LEED measurements.     

助催化剂Co与单晶MoS2边缘面区域及离子溅射解理面相互作用

为了阐明Co原子在钼硫化态催化剂表面上的助催化作用,本文利用UPS,XPS和LEED,研究了Co-Mo催化剂活性相的层状硫化物半导体MoS₂单晶边缘面区域的成分较高的表面以及离子溅射解理面上过渡金属Co的亚原子单层淀积过程。在覆盖度为某个亚原子单层时,表面上存在的与淀积上去的Co有关的界面态,改变了E_F能级的钉扎位置(提高了0.30—0.35eV),使表面势垒下降,表面功函数减小。在E_F附近电子结构的明显变化和LEED研究的结果表明,在MoS₂表面的无序缺陷位置上,可能形成了有利于催化过程的Co-Mo-S类合金键型的活性相。 关键词：     

The adsorption of Xe and CO on a Cu (311) single crystal surface

LEED, electron energy loss spectroscopy and surface potential measurements have been used to study the adsorption of Xe and CO on Cu (311). Xe is adsorbed with a heat of 19 ± 2 kJ mol^/t-1. The complete monolayer has a surface potential of 0.58 V and a hexagonal close-packed structure with an interatomic distance of 4.45 ± 0.05 Å. CO gives a positive surface potential increasing with coverage to a maximum of 0.34 V and then falling to 0.22 V at saturation. The heat of adsorption is initially 61 ± 2 kJ mol^?1, falling as the surface potential maximum is approached to about 45 kJ mol^?1. At this coverage streaks appear in the LEED pattern corresponding to an overlayer which is one-dimensionally ordered in the [011̄] direction. Additional CO adsorption causes the heat of adsorption to decrease further and the overlayer structure to be compressed in the [011̄] direction. At saturation the LEED pattern shows extra spots which are tentatively attributed to domains of a new overlayer structure coexisting with the first. Electron energy loss spectra (EELS) of adsorbed CO show two characteristic peaks at 4.5 and 13.5 eV probably arising from transitions between the electronic levels of chemisorbed CO.     

LEED,Auger and plasmon studies of negative electron affinity on Si produced by the adsorption of Cs and O

We have made LEED, Auger, and Plasmon measurements to study how Cs and O adsorb onto the (100) surface of p-type degenerate Si to produce negative electron affinity (NEA). A key factor to producing NEA was found to be a highly ordered Si surface as reflected by very high quality 2×2 LEED patterns. When NEA is produced, both the adsorbed Cs and O give the same LEED pattern as the original Si surface, but with a general enhancement of the half-order spot intensity. The adsorption of both Cs and O is strongly self-limiting, apparently controlled by the number of available appropriate sites on the surface. If Cs and O adsorb amorphously, NEA is not achieved. The thermal desorption of Cs occurs over a fairly broad temperature range centered at about 550°C. After Cs desorbs, the remaining O reverts spontaneously from an ordered layer to an amorphous layer, and then desorbs at about 800°C with an activation energy of 3.3 eV.Measurements of backscattered electron energy losses due to plasmons have shown that the Si surface plasmon is reduced in energy from 12.5 V to 7.0 V by the Cs-O layer. From this, an effective dielectric constant ε = 5.3 for this layer can be deduced which, in turn, enables us to characterize completely the Cs-O dipole layer.The geometrical model described by Levine for the NEA surface is consistent with our experimental results.     

The clean thermally induced W {001} (1 × 1) → (√2 × √2) R 45° surface structure transition and its crystallography

A (√2 × √2)R45° surface structure on W {001} produced only by cooling below ~370 K, first reported by Yonehara and Schmidt, has been investigated by LEED, AES, work function change, characteristic loss and low energy Auger fine structure measurements. No significant changes at any energy up to 520 eV occur in the standard Auger spectrum upon cooling to 220 K for as long as 30 min after a flash to >2 500 K. The work function of the (√2 × √2) R45° at 210 K is 20 ± 10 mV below that of the (1 × 1) surface, and a sensitive feature in the fine structure of the N₇VV AES transition shows approximately 60% attenuation. Unlike for H₂ adsorption, the “surface plasmon” loss peak exhibits little if any measurable attenuation and no measurable shift in energy as the crystal cools to form the (√2 × √2)R45°. The rate of intensity buildup in the $\text{1}\text{2}$-order LEED beams is strictly temperature dependent, and significant differences exist between the $\text{1}\text{2}$-order LEED spectra produced by cooling and those produced by H₂ adsorption. Only 2-fold symmetry was observed in the LEED beam intensities at exactly normal incidence, rather than 4-fold as expected for statistically equal numbers of rotationally equivalent domains. The LEED I-V spectra for 24 fractional order beams and 12 integral order beams, taken over large energy ranges at normal incidence, clearly establish that the beam intensities display 2 mm point group symmetry, and hence a preference of one domain orientation over the other. No beam broadening or splitting effects were apparent, implying only incoherent scattering from the various domains. The half-order beam spectra (±h/2, ±h/2) are identical in relative intensity to the (±h/2, ±h/2) spectra but different in absolute intensity by a constant factor, which can be explained only by domains with p2mg space group symmetry rather than just p2mm. Adsorption of H₂ onto the cooled (√2 × √2)R45° structure restores the 4-fold symmetry in the LEED beam intensities at normal incidence, giving a c(2 × 2) hydrogen structure, the same as when adsorbing H₂ onto the above room temperature (1 × 1) crystal. This strongly supports the observed p2mg symmetry as being a true property of the cooled (√2 × √2)R45° surface structure. These results show that the (1 × 1) → (√2 × √2) R45° transition produced by cooling is a transition involving displacement of surface W atoms, and that it apparently can be characterized as an order-order, second degree, homogeneous nucleation process, which is strongly prohibited by the presence of impurities or defects.     

Fast leed intensity measurements from Ni(100)c(2 × 2)CO

The Ni(100)c(2 × 2)CO surface structure has been investigated by very fast LEED intensity measurements using a computer controlled television method. It turns out that the intensity spectra are strongly influenced by intolerably long measuring times during which the primary electron beam impinges onto the surface. The spectra have been taken within 16 sec at 100 K immediately after termination of the adsorption process for all beams simultaneously. They are compared with other measurements and with Pendrys model calculations for a CO molecule bonded linearly on top of a Ni atom with straight molecular axis normal to the surface. Using the r-factor formalism for theory-experiment comparison the bond length results to be 1.15 ± 0.1 Å for CO and 1.80 ± 0.1 Å for NiC. This is in agreement with the results of other methods and removes some discrepancies with those of earlier LEED experiments.     

On the electronic and geometric structure of the Si(113) surface

The Si(113) surface of a p-type sample was studied by AES, LEED and ARUPS. On the clean surface, a (3 × 2) and a (3 × 1) LEED pattern coexist for a large range of annealing temperatures. Annealing to 900 K results in (3 × 1), while temperatures higher than 1050 K favour the (3 × 2) superstructure. ARUPS reveals two weakly dispersing surface resonances around 0.9 and 2.6 eV below E_F which are connected with the (3 × 2) and (3 × 1) structures, respectively. The work function was determined as φ = 4.81 eV and the photoionization threshold as ξ = 5.36 eV. The bands are bent downwards by 0.43 eV at room temperature.     

Reconstruction of W(110) induced by exposure to NO and by subsequent heat treatment

The (110) face of a tungsten single crystal was found to be partially reconstructed after an exposure, at 300 K, of 300 L of nitric oxide. This surface liberated N₂ when heated to 975 K, after which the reconstruction appeared to have been completed. At this stage a well developed c(11 × 5) LEED pattern was observed and a surface oxide, W₃O₂, is proposed for this reconstructed surface. The above mentioned surface reconstructs again after further heat treatment and is characterised by a weak p(2 × 2) LEED pattern. Work function measurements and the thermal stability of this surface structure indicate that the latter is not the same as that produced by oxygen adsorption on W(110).     

酞菁铜与MoS_2(0001)范德瓦耳斯异质结研究

利用光电子能谱、原子力显微镜以及低能电子衍射等表面研究手段系统研究了真空沉积生长的酞菁铜薄膜与衬底MoS2(0001)之间的范德瓦耳斯异质结界面电子结构和几何结构.角分辨光电子能谱清楚地再现了MoS2(0001)衬底在Γ点附近的能带结构.低能电子衍射结果表明,CuPc薄膜在MoS2(0001)表面沿着衬底表面[11ˉ20],[1ˉ210]和[ˉ2110]三个晶向有序生长,反映了衬底对CuPc的影响.原子力显微镜结果表明,CuPc在MoS2衬底上遵循层状-岛状生长模式:在低生长厚度下(单层薄膜厚度约为0.3 nm),CuPc分子平面平行于MoS2表面上形成均匀连续的薄膜;在较高的沉积厚度下,CuPc沿衬底晶向形成棒状晶粒,表现出明显的各向异性.光电子能谱显示界面偶极层为0.07 eV,而且能谱在膜厚1.2 nm饱和,揭示了酞菁铜与MoS2(0001)范德瓦耳斯异质结的能级结构.     

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 backscattering of low energy ions and surface structure

Low energy ion scattering (< 2 keV) in combination with LEED allows surface structure analysis in the case of gases adsorbed on single crystal surfaces. Strong shadowing and anisotropic effects observed are used to estimate the position of the adsorbed species. These techniques may also be extended to study adsorption on polycrystals. For the study of the faces of polar crystals multiple scattering effects in conjunction with shadowing offer a method of surface structure analysis. Surface relaxation effects may possibly be successfully measured by high energy (100 keV – 2 MeV) ion scattering making use of channelling and blocking effects.