R30 Degrees Research Articles

A structural study of the Al(111)( square root 3* square root 3)R30 degrees -Rb phase at different temperatures

Normal-incidence standing x-ray wavefield (NISXW) measurements have been made of the local adsorption site of Rb on Al(111) surfaces, particularly in an ordered ( square root 3* square root 3)R30 degrees phase, as a function of the sample temperature during adsorption or subsequent annealing. The results confirm the a top-site occupation for low-temperature (around 150 K) preparation, but show that room-temperature preparation leads to a structure having Rb atoms in surface substitutional sites. The overall structural situation is therefore essentially the same as that found previously by low-energy electron diffraction LEED for the Al(111)( square root 3* square root 3)R30 degrees -K phases. However, experiments involving annealing of the low-temperature prepared surface to room temperature indicate that only a small part of the surface easily transforms to the higher-temperature form, and indeed there is evidence that even in room-temperature preparations some fraction of the adsorbed atoms may remain in atop sites. The apparent conflict of this result with that from recent photoemission core-level shift and LEED data is discussed.

The relativistic layer-resolved electronic structure and photoemission of the adsorbate system Xe/Pt(111)

For the ( square root 3* square root 3)R30 degrees commensurate physisorption system Xe/Pt(111), the K/sub //-/, layer- and double-group-symmetry-resolved density of states (LDOS) as well as spin-resolved normal photoemission spectra for circularly and linearly polarized light have been calculated by means of a fully relativistic Green function formalism. Comparison with the LDOS obtained for an isolated Xe monolayer shows that the mj splitting of the 5ps2/-derived states of the Xe adsorbate is almost exclusively due to the lateral interaction within the Xe layer. In contrast. Pt derived features are strongly determined by the Xe overlayer via 'surface umklapp': firstly, electronic states localized in the two topmost Pt monolayers; and secondly, photoemission features originating from umklapped Pt bulk initial states. For circularly polarized light, we obtain very good agreement with experiment with regard to peak positions and sign of the spin polarization. Substantial discrepancies in absolute polarization values are ascribed to Auger-like electron-hole processes and call for an extension of our photoemission theory to incorporate these additional excitation channels. For linearly s-polarized light, the calculated spin polarization spectra depend very strongly on the lateral position of the Xe layer relative to the substrate.

A synchrotron X-ray diffraction study of the structural phase behaviour of multilayer xenon on single-crystal graphite

We present a high-resolution synchrotron X-ray diffraction study of xenon adsorbed on single-crystal graphite for coverages of two, three and six monolayers. We observe the evolution of the lattice mismatch between the xenon overlayer and a square root 3* square root 3R30 degrees structure commensurate with the underlying substrate. In the case of the bilayer we see a first-order incommensurate-commensurate (IC-C) phase transition at 62.0 K+or-1.2 K, which we demonstrate is consistent with the layer closest to the substrate having locked into the commensurate sites. The role of substrate quality is discussed and we conclude that previous studies on relatively poorer substrates may have favoured equilibrium structures strongly influenced by the surface imperfections. The risk of an insufficiently equilibrated adsorbate is discussed in the context of the very slow kinetics we observe for multilayer xenon on high-quality substrates. For the three- and six-layer adsorbates we observe a systematic increase in lattice mismatch with film thickness and no indication of an incommensurate-commensurate transition. We conclude that the trilayer system consists of many distinct domains approximately 500 AA across, each of which has a random close packed structure.

Sodium-promoted oxidation of Al(111) studied by core-level photoemission spectroscopy

The influence of preadsorbed sodium on the early stages of oxidation of Al(111) has been investigated using synchrotron radiation photoemission measurements of the Al and Ha 2p states. There is a strong promotion (by a factor of up to 100 or more) of the rate of oxygen dissociation and formation of the three-dimensional oxide induced by the presence of the sodium. At the highest Na coverage of 0.33 ML (associated with the Al(111) ( square root 3* square root 3)R30 degrees -Na phase), the Al 2p chemically shifted states characteristic of the chemisorption precursor to oxidation seen on the N-free surface are no longer observed. However, a state of intermediate chemical shift is seen in the Na 2p photoemission peak, at much lower oxygen exposures and appears to be a characteristic of a new oxidation precursor.

Adsorption of potassium on the Si(111) sqrt 3 x sqrt 3 R30 degrees:B surface: Observation of an insulating surface at submonolayer coverage.

The Si(111) square-root 3 X square-root 3R 30-degrees:B surface with submonolayer coverages of potassium has been studied with angle-resolved photoelectron spectroscopy. Deposition of potassium leads to the formation of a state 0.7 eV below the Fermi level. The energy position of this state showed only a minor dependence on the actual potassium coverage and the state displayed a small dispersion of 0.1 eV when probed along the GAMMABAR-MBAR' direction in the square-root 3 X square-root 3R 30-degrees-surface Brillouin zone. These results support the idea that a mainly covalent bonding exists between the potassium atoms and the Si surface.

STM studies of solid/liquid interfaces

The underpotential deposition (UPD) of Ag has been discussed on Au(111) in sulfuric acid solutions. The structure of the first adlayer of Ag is ( square root 3* square root 3)R30 degrees in a 'pure solution'. The atomic image of the hydrogen-terminated Si(111) surface has also been reported. The ideal Si(111):H-1*1 surface persists in solution for several hours.

Epitaxial growth of Sn on Si(111): A direct atomic-structure determination of the (2 sqrt 3 x 2 sqrt 3 )R30 degrees reconstructed surface.

Scanning tunneling microscopy (STM) has been used to determine the surface atomic structure of Si(111)(2\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{}2 \ensuremath{\surd}3 )-Sn. The topographic images show four resolved atoms in each (2 \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{}2 \ensuremath{\surd}3 ) unit cell, and the structure is found to be onefold symmetric. Together with coverage measurements, the STM analysis implies that the reconstructed surface is an epitaxial Sn two-layer structure, where the atoms adopt a bonding configuration characteristic of \ensuremath{\alpha}-Sn. A three-dimensional structure model, in accordance with the obtained results, is proposed.

Growth and electronic structure of some monovalent metals on TiS2(001)

The epitaxial growth of Ag on TiS2(001) is characterized using reflection high-energy electron diffraction (RHEED), low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Ag grows in the (111) orientation epitaxially on TiS2(001) with Ag(110)//TiS2(100). The growth mode belongs to the Volmer-Weber type, which means that Ag forms three-dimensional clusters on top of TiS2(001). Ag also diffuses slowly into the substrate. The evolution of the electronic structure during this intercalation process is studied in detail with angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and X-ray photoelectron spectroscopy (XPS). The authors observed a striking difference between surface Ag and subsurface or intercalated Ag. They conclude that Ag atoms on TiS2(001) have little interaction with the substrate, while upon intercalation charge transfer takes place from Ag to TiS2. For comparison, they also studied Na and K at the TiS2(001) surface. At low temperature (-140 degrees C), Na deposition results in an ordered ( square root 3* square root 3)R30 degrees superstructure. Na and K intercalation proceeds very rapidly. The Ti2p core-level spectra provide additional evidence that charge transfer has taken place after deposition and intercalation of Ag, Na and K.

Surface chemistry of silicon-the behaviour of dangling bonds

The role of surface dangling bonds in controlling the surface chemistry of Si(100) and Si(111) has been investigated using surface science methods. The adsorption and decomposition of acetylene on Si(100) has been shown to occur by means of a mobile precursor mechanism, leading to a di- sigma chemisorbed ethylenic species occupying Si dimer sites on Si(100). This species decomposes at elevated temperatures to produce adsorbed carbon and hydrogen (800 K) and then silicon carbide (1000 K). The activity of Si(111) for the adsorption of species such as ammonia and atomic hydrogen can be significantly reduced by doping of the surface region with boron. It has been shown that subsurface boron significantly changes the structure of Si(111)-(7*7) to a square root 3* square root 3 R30 degrees structure. In addition, the local doping of the surface reduces the chemical activity of the dangling bonds so that neither the dissociation of NH3 nor the adsorption of atomic hydrogen will occur.

Phonon modes at the reconstructed surface of a cubic lattice

The author considers the ( square root 2* square root 2)R45 degrees -reconstruction on the (001) surface of a simple cubic lattice with interaction forces acting between nearest neighbours according to Hooke's law. Using Feynman's complex-time path integral he finds the energy of the surface, which provides the usual T2-law for specific heat. He comes to the conclusion that the energy of zero point vibrations at the surface, which can be compared to the Casimir energy, could influence the phenomenon of reconstruction at low temperatures.

Spin-resolved photoemission from submonolayer Ag on Si(111): Ag adsorption in the ( sqrt 3 x sqrt 3 )R30 degrees phase with no Ag-Ag interaction and in the 1 x 1 structure with strong Ag-Ag interaction.

Ag/Si(111) has been studied by spin-, angle-, and energy-resolved photoemission with circularly polarized radiation from the Berliner Elektronenspeicherring-Gesellschaft f\"ur Synchrotionstrahlung (BESSY) for the 1\ifmmode\times\else\texttimes\fi{}1 and (\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{} surfaces at a converage of 0.5 monolayer. The spin-resolved photoemission data were obtained for normal incidence of the light and normal emission of the photoelectrons. For both the 1\ifmmode\times\else\texttimes\fi{}1 and (\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{} structures we find no dispersion in the Ag photoelectron peaks depending on ${\mathit{K}}_{\mathrm{\ensuremath{\parallel}}}$ and ${\mathit{K}}_{\mathrm{\ensuremath{\perp}}}$. For the 1\ifmmode\times\else\texttimes\fi{}1 structure and for photon energies between 20 and 24 eV, we observe an additional Ag photoelectron peak at about 1 eV higher binding energy which results from a d-level splitting due to Ag-Ag interaction. This peak is absent for the (\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)R30\ifmmode^\circ\else\textdegree\fi{} phase, which demonstrates that the Ag-Ag neighbor interaction is negligible. This supports the models that do not imply Ag-Ag interaction for the Ag adsorption in the (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{} phase.

Electronic structure of Si(111)-B( sqrt 3 x sqrt 3 )R30 degrees studied by Si 2p and B 1s core-level photoelectron spectroscopy.

An investigation of the Si(111)-B(\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{} system has been performed using high-resolution photoelectron spectroscopy of the Si 2p core level and polarization-dependent studies of the B 1s absorption edge. Least-squares analysis of the Si 2p core-level line shape reveals that it comprises a bulk component and a surface component shifted by 0.40\ifmmode\pm\else\textpm\fi{}0.02 eV to higher binding energy. The exceptionally large surface-to-bulk ratio that is observed in this system suggests that the range of influence of the B atoms extends to more than 1 monolayer of Si atoms. The magnitude of the surface-to-bulk ratio is consistent with a model in which B occupies a subsurface site below a Si adatom. The B 1s edge contains a feature which is excited by the component of the electric field vector perpendicular to the surface. We argue that this arises from an electronic transition from the B 1s level into an empty surface orbital, orientated perpendicularly to the surface. We also study the change of the electronic structure as the surface is covered with Si, thereby producing a buried \ensuremath{\delta}-doping layer.

Chlorine adsorption on Cu(111): low-temperature adsorption, electron-stimulated desorption and chloride formation

Chlorine adsorption on Cu(111) has been studied at low temperatures using LEED and AES. AT 165 K a (6 square root 3*3*6 square root 3)R30 degrees chemisorbed structure was formed at saturation. At 110 K adsorption continued, forming multilayers of copper chloride with the (6 square root 3*6 square root 3)R30 degrees structure still intact at the interface between the chloride and the copper. Electron-stimulated desorption was observed from the (6 square root 3*6 square root 3)R30 degrees structure at 165 K but not at 300 K, which is consistent with a high surface mobility for chemisorbed chlorine.

Electronic structure and the attractive interaction between atoms on the (001) surface of W

The band-structure contribution to the force constants of an unrelaxed W(001) system is computed within a tight-binding approach. It is shown that the high density of surface states near the Fermi level gives rise to an enhanced attractive interaction between nearest neighbours on the surface. The attractive nearest-neighbour interaction drives a ( square root 2* square root 2)R45 degrees reconstruction in the surface plane. On the corresponding surface of Ta the surface peak is not occupied and the interaction is less attractive. This may account for the different surface structures observed for the two metals at low temperatures.

Structure factors associated with the continuous melting of two-dimensional lattice gases: Models with ( sqrt 3 x sqrt 3)R30 degrees and p(2 x 2) ordered states on triangular nets.

We study the temperature dependence of the structure factors of two lattice gases which undergo order-disorder phase transitions. Our goal is to determine how much information about the critical behavior of these phase transitions a low-energy electron-diffraction experiment might obtain. We use Monte Carlo simulation to compute the structure factors. Both lattice gases are on triangular nets; one has a (\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)R30\ifmmode^\circ\else\textdegree\fi{} ordered phase; the other has a p(2\ifmmode\times\else\texttimes\fi{}2) ordered phase. The structure factors scale almost halfway from the center of an extra spot to the zone center; for system sizes comparable to those that are physically realizable we see effective critical exponents which are typically within of order 10% of expectations based on universality. Below the transition temperature, nonlinearities in log-log plots are significant, indicating that corrections to scaling cannot be ignored. We consider how asymmetries in the structure factor reflect differences between lattice-gas systems and magnetic analogs in the same universality class and also briefly treat the effects of quenched random vacancies and of a fixed concentration of annealed vacancies.

The reliability of LEED I-V spectra: incidence angle errors and symmetry effects on W (100)(1*1)

A recently published assessment of the reproducibility of experimental LEED intensity data from the tungsten (100)(1*1) surface by Stevens and Russell (1981) is evaluated. I-V curves are presented which suggest that the independently obtained LEED spectra which they compared, including their own, were not at precisely normal incidence, this being the major cause of the differences among the curves. The importance of a valid criterion for establishing normal incidence is emphasised, especially when studying subtle changes in surface-structure symmetry. This is demonstrated by data which suggest the W(100) surface above 370K shows a very subtle loss of mirror-plane symmetry in the direction parallel to the steps existing on the crystal surface studied. Just as below 370K the observed 2 mm point group symmetry of the ( square root 2* square root 2)R45 degrees structure can be attributed to a preferred orientation of p2mg domains, this apparent 1 m point group symmetry of the (1*1) structure is suggested to be a direct step-induced preference for one p1m1 orientation rather than a consequence of deviations from normal incidence.

Long-range inhibition of the W(001) clean surface displacive phase by nitrogen

Half-order LEED beam intensities and half-widths produced by the low-temperature ( square root 2* square root 2)R45 degrees displacive phase on W(001) have been determined as a function of nitrogen adatom coverage. The clean surface structure is found to be strongly inhibited by very low coverages (1-2*1013 atoms/cm2, or 1-2% of a monolayer), and the radius of influence of a N adatom pair is estimated as 20 AA. The results provide evidence for the influence of long-range effects, such as charge-density waves or surface-state coupling, on the formation of the ordered displacive phase.

LEED and angle-resolved AES studies on Si-, P-, and Cl-adsorbed Ni(100) surfaces

A number of adsorbed structures produced by chemisorption of SiH4, PH3 and HCl on Ni(100) surfaces have been investigated by LEED and angle-resolved Auger electron spectroscopy (ARAES). The room-temperature adsorption of SiH4 and PH3 produces the disordered structures, from which the Si(92 eV) and P (120 eV) Auger emissions respectively show a cos theta profile. The Si, P and Cl (180 eV) Auger emissions from the c(2*2)-Si, (2 square root 2* square root 2)R45 degrees -Si, ( square root 5* square root 5)R26.6 degrees -P, c(2*2)-HCl and c(2*2)-Cl structures show the anisotropic angular features as in the case of the c(2*2)-S and -Se on Ni(100) surfaces.