Struan M. Gray

The Pd(100)-(root 5 x root 5)R27 degrees-O surface oxide revisited

Combining high-resolution core-level spectroscopy, scanning tunneling microscopy and density-functional theory calculations we reanalyze the Pd(100)-(root5 x root5)R27degrees-O surface oxide phase. We find that the prevalent structural model, a rumpled PdO(001) film suggested by previous low energy electron diffraction (LEED) work [Surf. Sci. 494 (2001) L799], is incompatible with all three employed methods. Instead, we suggest the two-dimensional film to consist of a strained PdO(101) layer on top of Pd(100). LEED intensity calculations show that this model is compatible with the experimental data of Saidy et al

Structural study of adsorption of isonicotinic acid and related molecules on rutile TiO2(110) I: XAS and STM

The adsorption of monolayers of the pyridine-carboxylic acid monomers (isonicotinic acid, nicotinic acid, and picolinic acid) on rutile TiO2(1 1 0) has been studied by means of X-ray photoemission spectroscopy. An investigation of the O 1s spectra shows that the molecular carboxylic groups are deprotonated and, hence, that the molecules bind to the surface in a bidentate mode. Moreover, the binding energy of those core levels that are related to the pyridine ring atoms shift as a function of molecule relative to the substrate O 1s and Ti 3p levels, while the position of the core levels related to emission from the carboxylic group are constant relative to the substrate levels. The molecule-dependent shifts are attributed to local intermolecular interactions that determine the proximity of adjacent molecular rings and thus the core-hole screening response of the neighbouring molecules. We propose a simple molecular arrangement for each case which satisfies the known constraints

Probing Strain in Bent Semiconductor Nanowires with Raman Spectroscopy.

We present a noninvasive optical method to determine the local strain in individual semiconductor nanowires. InP nanowires were intentionally bent with an atomic force microscope and variations in the optical phonon frequency along the wires were mapped using Raman spectroscopy. Sections of the nanowires with a high curvature showed significantly broadened phonon lines. These observations together with deformation potential theory show that compressive and tensile strain inside the nanowires is the physical origin of the observed phonon energy variations.

Adsorption of C60 on Al(111) studied with scanning tunnelling microscopy

Scanning tunnelling microscopy has been used to study the adsorption and interactions of C60 on Al(111) at room temperature. Initially, C60 forms chains of molecules on the lower terraces of step edges, which indicates that the molecules have a high mobility even at room temperature. When the coverage is increased, the C60 molecules form a close-packed overlayer of (23 × 23)R30° symmetry with respect to the substrate surface which is compressed by 1% in all directions compared with the (111) plane of fullerite. Voltage-dependent intramolecular structure is observed, indicating that the adsorbate-substrate bonding is strong enough to prevent the molecules from rotating, and that C60 prefers to have a six-membered ring facing the surface. The (23 × 23)R30° overlayer structure is metastable, irreversibly transforming to (6 × 6) symmetry when heated above 490 K.

Photoemission with the STM

Abstract An STM tip has been used to detect photoelectrons emitted from alkali metal-coated aluminium and silicon surfaces whilst within tunnelling range. I–V spectra clearly show a pair of sharp thresholds which are simply related to the tip workfunction, the sample workfunction, and the incident photon energy. The voltages at which these thresholds occur yield a direct, absolute measurement of the surface workfunction. The photoemitted electrons can be detected independently of other laser-induced excitations of the surface, and maps of the photocurrent show features on the nanometer length scale which are obviously related to the structure of the surface. Both the spectral and spatial resolution of this technique represent significant advances on previous forms of photoemission or workfunction microscopy.

Comparative friction measurements of InAs nanowires on three substrates

We have investigated friction between InAs nanowires and three different substrates: SiO2, fluorosilanized SiO2, and Si3N4. The nanowires were pushed laterally with the tip of an atomic force microscope and the friction force per unit length for both static and sliding friction was deduced from the equilibrium shape of the bent wires. On all three substrates, thick wires showed a difference between sliding and static friction of up to three orders of magnitude. Furthermore, all substrates display a transition to stick-slip motion for nanowires with a diameter of less than about 40 nm. Hydrophobic and hydrophilic substrates display similar friction behavior suggesting that a condensed water layer does not strongly influence our results. The patterns and trends in the friction data are similar for all three substrates, which indicates that they are more fundamental in character and not specific to a single substrate.

The Pd(100)–(5×5)R27°-O surface oxide revisited

Combining high-resolution core-level spectroscopy, scanning tunneling microscopy and density-functional theory calculations we reanalyze the Pd(100)-(root5 x root5)R27degrees-O surface oxide phase. We find that the prevalent structural model, a rumpled PdO(001) film suggested by previous low energy electron diffraction (LEED) work [Surf. Sci. 494 (2001) L799], is incompatible with all three employed methods. Instead, we suggest the two-dimensional film to consist of a strained PdO(101) layer on top of Pd(100). LEED intensity calculations show that this model is compatible with the experimental data of Saidy et al

Mn-induced NO dissociation on Pd(100)

The changes in the NO adsorption properties of Pd(1 0 0) due to the addition of Mn have been investigated using high resolution core level photoelectron spectroscopy. The Pd(1 0 0) surface was modified by forming a c(2 x 2)-PdMn surface alloy at two different Mn coverages, giving surfaces partly and fully alloyed, respectively, as shown by scanning tunneling microscopy. NO adsorption on the alloy films was found to destroy the c(2 x 2) structure. Dissociation of the NO molecules upon heating is observed, in stark contrast to NO on the clean Pd(1 0 0) surface from which all the molecules desorb intact upon heating. The dissociation process on the c(2 x 2)-PdMn-(1 x 1)-Pd mixed surface is completed at a significantly higher temperature than for the homogeneous c(2 x 2) surface. It is suggested that Mn atoms give rise to NO dissociation at lower temperatures. whereas Pd atoms situated at c(2 x 2)-(1 x 1) boundaries are responsible for the NO decomposition at higher temperatures.

High resolution core-level spectroscopy study of low-coverage lithium adsorption on the Si(100)2 × 1 surface

Abstract In this high-resolution core-level photoelectron spectroscopy study, the adsorption of lithium on the Si(100)2 × 1 surface has been studied for low Li coverages. The basic dimer reconstruction of the clean surface was preserved up to a coverage corresponding to a work function change of Δφ=−1.8eV, as indicated by low-energy electron diffraction (LEED) and the Li 1s and Si 2p core-level spectra. The changes in the Si 2p spectra indicate that the surface reconstruction has changed from asymmetric to symmetric dimers at this 2 × 1 saturation coverage, with the dimer atoms giving rise to a single Si 2p core-level binding-energy shift of −0.37 eV. For higher Li coverages the Li atoms react with the substrate under the formation of Li silicides.

Droplet Epitaxy of InAs Nanocrystals on GaN/GaAs

The growth of InAs nanocrystals fabricated on GaAs by means of droplet epitaxy in a metal-organic chemical vapour-deposition chamber was investigated. To prevent deterioration of the droplet/substrate interface a nitride passivation layer was introduced. For this purpose both, nitridation of GaAs and GaN growth at different temperatures have been applied. In all nitrided samples and when GaN was grown at Tg=450°C, deep pit-like defects have been found in the GaAs below the InAs islands, caused by In that diffused from the droplets into the substrate along the {111} planes. Only the growth of a thin cubic GaN layer at Tg=550°C effectively prevented this indiffusion of indium into the substrate. The InAs islands contained stacking faults and twins that arose from defects in the underlying GaN layer. Their sidewalls do not only consist of (001) and {111} facets, but also of the commonly not observed {110} and even {111} facets. The frequent occurrence of holes at the center base of the InAs islands is discussed within a growth model for the transformation of In droplets to InAs nanocrystals.