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.     

A multitechnique surface science examination of Sn deposition on Pt(100)

Interfaces prepared by vapor deposition of Sn onto Pt(100) surfaces have been examined using the following techniques: Auger electron and X-ray photoelectron spectroscopy (AES and XPS), low-energy electron diffraction (LEED), and low-energy ion surface scattering (LEISS) with Ne⁺ ions. Tin deposition was conducted at 320 and 600 K, and the surface composition and order was examined as a function of further annealing to 1200 K. The AES uptake plots (signal versus deposition time) indicate that the Sn growth mode can be described by a layer-by-layer process only up to one adayer at 320 K. Some evidence of 3D growth is inferred from LEED and LEISS data for higher Sn coverages. For deposition at 600 K, AES data indicate significant interdiffusion and surface alloy formation. LEED observations (recorded at a substrate temperature of 320 K) show that the characteristic hexagonal Pt(100) reconstruction disappears with Sn exposures of 4.6 × 10¹⁴ atoms cm² (θ_Sn = 0.35 monolayer (ML)). Further Sn deposition results in a c(2 × 2) LEED pattern starting at a coverage of slightly above 0.5 ML. The c(2 × 2) LEED pattern becomes progressively more diffuse with increasing Sn exposure with eventual loss of all LEED features above 2.2 ML. Annealing experiments with various precoverages of Sn on Pt(100) are also described by AES, LEED, and LEISS results. For specific Sn precoverages and annealing conditions, c(2 × 2), p(3√2 × √2)R45°, and a combination of the two LEED patterns are observed. These ordered LEED patterns are suggested to arise from ordered PtSn surface alloys. In addition, the chemisorption of CO and O₂ at the ordered annealed Sn/Pt(100) surfaces was also examined using thermal desorption mass spectroscopy (TDMS), AES, and LEED.     

Electron spectroscopic studies of tin surface segregation on iron single crystal surfaces

AES, ELS, LEED and XPS investigations of the surface segregation of tin dissolved in a Fe-4wt%Sn alloy were performed in ultra-high vacuum at elevated temperatures. The three low indexed surface orientations (100), (110) and (111) were studied. In all cases, no dependence of the maximum tin surface coverage on temperature was detected within the temperature range from 450 to 650°C. An order-disorder transition was observed by LEED, AES and XPS for the (100) oriented surface during tin segregation. The binding state for the segregated tin atoms abruptly changes at the order-disorder transition as determined by XPS. Similar results were obtained for the (111) surface. A deviating behaviour was observed for the (110) surface orientation, where two different ordered hexagonal surface structures were detected by LEED during tin surface enrichment. The first structure is similar to the diamond structure of pure tin, and the second one corresponds to the formation of a thin layer of the intermetallic compound FeSn on the (110) surface. The electron binding energies of the segregated tin atoms determined by XPS increase with increasing tin coverage on the (110) oriented surface. ELS studies on (100) and (111) oriented surfaces saturated with segregated tin show in comparison with literature data of pure tin a surface plasmon loss peak but no signal for the corresponding bulk loss. An energy loss signal found only for the (110) surface at Sn saturation coverage seems to be characteristic of an intermetallic FeSn surface phase.     

Surface segregation of aluminum atoms on Cu-9 at.% Al(1 1 1) studied by Auger electron spectroscopy and low energy electron diffraction

Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) were applied to investigate the segregation of aluminum atoms on a Cu-9 at.% Al(1 1 1) surface. We observed that the Al concentration in the top layer ranged between about 9 and 36 at.% after the sample we used was annealed at different temperatures. The phenomenon of Al atoms segregating on the surfaces was explained well by considering the diffusion length of Al atoms in bulk Cu. LEED measurements showed that R30° structures grew as the concentration of Al atoms increased. The segregation phenomena on surfaces resulted in a stable two-dimensional Cu₆₇Al₃₃ alloy phase in the top layer.     

Substitutional adsorption of K on Fe(100)

The adsorption of potassium on Fe(100) was studied by time-of-flight forward scattering and recoiling spectroscopy (TOF-SARS), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). After heating to 650 K of the potassium saturated surface the formation of a p(3 × 3) potassium superstructure was observed by LEED. TOF-SARS experiments ruled out the adsorption of potassium in the on-top, bridge and four-fold hollow site. The only site which is in agreement with all experimental results is the substitutional site where K replaces an Fe atom of the topmost layer of the crystal. This is the first time a substitutional adsorption site has been found on a bcc surface. On an fcc surface such an adsorption site has been found recently for adsorption of sodium and potassium on Al(111).     

On the electronic structure of cleaved GaAs(110) surfaces exposed to formic acid

GaAs(110) surfaces cleaved in UHV and exposed to HCOOH have been studied by work function measurements (Kelvin method), electron energy loss spectroscopy (ELS) and by low energy electron diffraction (LEED). From the different changes of the work function on n- and p-type material information about intrinsic and extrinsic surface states is derived. In the loss spectra the adsorbed formate species causes a loss near 9 eV. The intensity of the loss near 20 eV generally ascribed to an excitonic transition from the Ga 3d core level into surface states is reduced only by a factor of two after saturation with HCOOH. This might be related to the c(2 × 2) superstructure observed in LEED, which suggests a saturation coverage of half a monolayer.     

Iron deposition on the five-fold surface of the icosahedral Al-Pd-Mn quasicrystal

The room temperature adsorption behaviour of Fe on the five-fold surface of i-Al-Pd-Mn has been studied using scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). A complex growth scenario for Fe adsorption on this substrate is observed with STM. At coverages up to about 3 MLE (monolayer equivalent), layer-by-layer growth is observed whereby small clusters and islands are formed which eventually coalesce into almost complete monolayers. No LEED pattern is observed, indicating that the layers are disordered. The AES results rule out intermixing. Above this coverage, there is a transition to a multilayer island growth mode. The islands are rotated by 72° and have the bcc(1 1 0) Fe structure. The results are compared with previous work on Fe adsorption on this substrate and on Al and Fe single crystal substrates.     

On Auger electron spectroscopic measurements of ZnCl2 —coal interaction

By combining electron stimulated desorption (ESD) with low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and work function change (Δφ) measurements the information content of ESD with regard to surface structure and composition is examined, using the surface systems O/W(100) and O/W(110). Although it is not possible to separate the local interaction from the ion escape phase, the comparison of the ESD results with Information derived from LEED, AES and Δφ and the use of simple models of the local interaction gives a rather detailed picture of the location and environment of adsorbed atoms which provides a reasonably reliable basis for the interpretation of UPS spectra of adsorption layers.ESD is extremely sensitive to adsorbed layers. The fact that the ion signal depends not only on coverage but also on the structure and structure-dependent properties of the adsorbate makes on the one hand coverage determination difficult if not impossible, on the other hand opens the door to structure analysis. The potential for obtaining structure information can be easily assessed by comparison with electron probe results.In comparison with other ion probes such as ion scattering spectroscopy and secondary ion mass spectroscopy, ESD is at present the most promising ion probe method for obtaining information on the location of adsorbed atoms from angular and energy distribution measurement (ESDIAD and ESDIED). This is clearly seen by the comparison with the structural data derived from LEED, AES and Δφ measurements for the complex system O/W(100). The consistency of the data obtained with ESD and electron probe techniques lends strong support to the simple models on which the analysis of the ESD results from chemisorbed layers are based. The comparison of ESD results from the system O/W(100) at high coverage and from O/W(110) with 0⁺ ion emission from oxides shows, however, that caution is in place when assigning ESD features to atoms chemisorbed on the metal surface. Without a careful analysis of the ion energy, threshold and/or cross-section such ions cannot be distinguished from ions produced by dissociation of oxides which may be present on the surface only in small quantity. These ions usually are not related to the chemisorbed species which covers most of the surface and therefore dominates the signals seen with (nearly) all other surface probes.If the consistency of LEED, AES, Δφ and ESD data for O/W(100) is not fortuitous, then ESD has already given some important feed-back to the electron probe techniques: the structural models derived from vibrational ELS spectra have to be revised. Increasing accumulation of experimental data and deepening of the theoretical understanding of the physical processes involved in ion emission will have to show how much further information complementary to that from electron probes can be obtained from ion probes.     

Decomposition of carbon monoxide on a (110) nickel surface

New investigations of the (110) nickel/carbon monoxide system have been made using low energy electron diffraction (LEED), Auger electron spectroscopy (AES), mass spectroscopy and work function measurements. Room temperature adsorption of CO on the surface was reversible with the CO easily removable by heating in vacuum to 450°K. The CO formed a double-spaced structure on the surface which, however, was unstable at room temperature for CO pressures less than 1×10^?7 torr. Work function changes greater than + 1.3 eV accompany this reversible CO adsorption. Irreversible processes leading to the build-up of carbon, and under certain circumstances oxygen, on the surface were the primary concern of the measurements reported here. These processes could be stimulated by the electron beams used in LEED and AES, or by heating the clean surface in CO. The results of AES investigations of this carbon (and oxygen) build-up, together with CO desorption results could be explained on the basis of two surface reactions. The primary reaction was the dissociation of chemisorbed CO leaving carbon and oxygen atomically dispersed on the surface. The second reaction was the reduction of the surface oxygen by CO from the gas phase. The significance of the dissociation reaction to COdesorption studies is discussed.     

Initial oxidation of the Mg(0001) surface,an electron spectroscopy study

The initial oxidation of Mg(0001) has been studied using AES (Auger electron spectroscopy), LEED (low energy electron diffraction), and EELS (electron energy loss spectroscopy). The oxidation proceeds through different stages; first oxygen atoms are incorporated to chemisorption sites below the top layer magnesium. This chemisorption phase is followed by the formation of an oxide layer. The oxide layer covers the Mg surface after an oxygen exposure of ～ 10 L O₂. After this exposure the bulk-like MgO formation slowly increases the oxide thickness. The oxide layer formed for exposures up to ≤ 10 L O₂ gives rise to a diffuse LEED pattern of the same symmetry as the original “clean” LEED pattern; the possibility of an epitaxial oxide formation at this stage is discussed.     

Growth of gold nanoparticles on faceted O/Ru(11–20) nanotemplate

The growth of gold nanoparticles on the oxygen-covered faceted Ru(11–20) surface is investigated by scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). By depositing gold onto the faceted surface held at room temperature, gold nanoparticles with regular spacings are fabricated. Gold nanoparticles are found to nucleate preferentially within valleys of the faceted surface. Our work demonstrates that the faceted metal surfaces are promising templates for the growth of metal nanoparticles.     

Surface science studies of cobalt overlayers on clean and sulfur covered Mo(100) single crystal surfaces

The interaction of cobalt with clean and sulfur covered Mo(100) surfaces was investigated with Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and temperature programmed desorption (TPD). On the clean surface, the deposition and subsequent annealing of one monolayer of cobalt resulted in the formation of an ordered overlayer with (1 × 1) surface structure. When cobalt was deposited on sulfur covered Mo(100) surfaces, after annealing the sulfur overlayer migrated on top of the cobalt layer. This topmost sulfur overlayer did not significantly affect the thermal desorption of cobalt from the Mo(100) surface. Various ordered structures of sulfur, cobalt and coadsorbed sulfur and cobalt were observed by LEED. A new surface structure showing (3 × 1) symmetry was observed when at least one monolayer of cobalt was deposited and annealed at 870 K on an ordered monolayer of sulfur on the Mo(100) surface. This surface structure was stable in ultrahigh vacuum up to 940 K.     

Growth of extra-thin ordered aluminum films on Si(110) surfaces

The formation of surface structures upon Al deposition onto a Si(110) surface was studied by LEED and AES. The “4 × 6”, “1 × 9”, 2 × 1, 1 × 1 ordered Si(110)-Al surface phases and epitaxial Al(110) domains were observed depending on Al coverage and substrate temperature. The formation phase diagram was drawn for the Al/Si(110) system.     

The condensation of gold onto tantalum (100) single crystal surfaces: LEED,AES analysis

The condensation of gold onto clean and contaminated, single crystal, tantalum (100) surfaces has been followed by using LEED and AES. On a contaminated surface gold condenses as crystallites in a (211) surface orientation with some degree of preferred, azimuthal orientation. On a clean surface gold condenses in an ordered overlayer. Up to approximately $\text{3}\text{4}$ monolayer the structure conforms to the (1 × 1) tantalum surface. Beyond this, the observed LEED structure may be interpreted initially in terms of a TaAu overlayer made up of 90° rotated domains with (001)TaAu//(100)Ta and 〈 10 〉 TaAu// 〈 11 〉 Ta, and then in terms of a gold overlayer in a “distorted (111)” orientation. Annealing of these gold films always results in the formation of a (1 × 1) TaAu overlayer of small crystallite size.     

Obtaining density of states information from self-deconvolution of Auger band-type spectra

A systematic revision of the techniques to isolate Auger peaks in experimental spectra obtained with LEED/AES analyzers has been carried out: Background subtraction, deconvolution with the elastic peak and inversion of self-convolution. New techniques have been devised and applied to the Auger spectra of graphite, aluminum, magnesium, magnesium oxide and silicon. Self-deconvolution of these spectra leads to a transition density function that has been compared with the density of states of the valence band near the surface, as given by other competitive techniques (XPS, UPS, XES) and by theory. This comparison shows that in those meterials, Auger electron spectroscopy (AES) is a sensitive probe of the electron environment of surface atoms.     

Work function measurements with a high resolution electron energy loss spectrometer: Application to the interaction of oxygen with Ag(110)

The monochromatized electron beam of a high resolution electron energy loss (HREEL) spectrometer is used for accurate (±5 meV) measurement of the work function changes during exposure of a Ag(110) single crystal surface to oxygen. Absolute calibration of the results is made by comparison with Kelvin probe data. The procedure allows the precise determination of the electron impact energy, which is an important parameter for quantitative HREELS analysis. Furthermore, in the case of oxygen adsorbed on Ag(110), the occurrence of several LEED (n×1) superstructures enables a calibration of the HREELS data with respect to surface coverage.     

Elemental composition of β-Sic(001) surface phases studied by medium energy ion scattering

β-SiC surfaces have been investigated in terms of surface composition and reconstruction by medium energy ion scattering (MEIS), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED). A (3 × 2) phase is produced by evaporating Si on a β-SiC surface. Heat treatment at 1065°C causes consecutive transformation into (5 × 2), c(4 × 2), (2 × 1), (1 × 1) and c(2 × 2) phases. Quantitative analysis of MEIS spectra shows that the c(4 × 2) surface has a full silicon topmost layer, whereas the c(2 × 2) surface has a full carbon topmost layer. The (3 × 2) and (5 × 2) phases are believed to originate from additional Si dimer rows on top of a Si terminated crystal.     

Adsorption of glycine on a NiAl(1 1 0) alloy surface

The adsorption and desorption of glycine (NH₂CH₂COOH), vacuum deposited on a NiAl(1 1 0) surface, were investigated by means of Auger electron spectroscopy (AES), low energy electron diffraction (LEED), temperature-programmed desorption, work function (Δφ) measurements, and ultraviolet photoelectron spectroscopy (UPS). At 120 K, glycine adsorbs molecularly forming mono- and multilayers predominantly in the zwitterionic state, as evidenced by the UPS results. In contrast, the adsorption at room temperature (310 K) is mainly dissociative in the early stages of exposure, while molecular adsorption occurs only near saturation coverage. There is evidence that this molecularly adsorbed species is in the anionic form (NH₂CH₂COO⁻). Analysis of AES data reveals that upon adsorption glycine attacks the aluminium sites on the surface. On heating part of the monolayer adsorbed at 120 K is converted to the anionic form and at higher temperatures dissociates further before desorption. The temperature-induced dissociation of glycine (<400 K) leads to a series of similar reaction products irrespective of the initial adsorption step at 120 K or at 310 K, leaving finally oxygen, carbon and nitrogen at the surface. AES and LEED measurements indicate that oxygen interacts strongly with the Al component of the surface forming an “oxide”-like Al-O layer.     

LEED and thermal desorption studies of small molecules (H2, O2, CO,CO2, NO,C2H4, C2H2 and C) chemisorbed on the stepped rhodium (755) and (331) surfaces

The chemisorption of H₂, O₂, CO, CO₂, NO, C₂H₂, C₂H₄ and C has been studied on the clean stepped Rh(755) and (331) surfaces. Low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and thermal desorption spectroscopy (TDS) were used to determine the size and orientation of the unit cells, desorption temperatures and decomposition characteristics for each adsorbate. All of the molecules studied readily chemisorbed on both stepped surfaces and several ordered surface structures were observed. The LEED patterns seen on the (755) surface were due to the formation of surface structures on the (111) terraces, while on the (331) surface the step periodicity played an important role in the determination of the unit cells of the observed structures. When heated in O₂ or C₂H₄ the (331) surface was more stable than the (755) surface which readily formed (111) and (100) facets. In the CO and CO₂ TDS spectra a peak due to dissociated CO was observed on both surfaces. NO adsorption was dissociative at low exposures and associative at high exposures. C₂H₄ and C₂H₂ had similar adsorption and desorption properties and it is likely that the same adsorbed species was formed by both molecules.     

Single crystal RuO2/Ti and RuO2/TiO2 interface: LEED, Auger and XPS study

The interactions at the evolving RuO₂/titanium interface have been studied by LEED, AES and XPS. Titanium films of up to 5 monolayers were evaporated onto well ordered and ion sputtered ruthenium dioxide crystal surfaces of (110) and (100) orientation. Stabilization of the surface oxygen content under thermal treatment in UHV (up to 600°C) with increasing titanium coverage was established. After extended (up to 4 h) annealing in O₂ at 600°C an epitaxial ordering of TiO₂ on RuO₂(110) was observed. The (1 × 1) LEED patterns from the epitaxial layer exhibit a reduced background level when compared to the RuO₂ substrate itself. These findings are correlated with the XPS data and are interpreted in connection with the disappearance of the defect RuO₂ phase in the surface layer of the RuO₂. The appearance of the (1 × 2) surface reconstruction at the RuO₂(100)/Ti interface is discussed in the context of maximum cation coordination by oxygen atoms.