F.H. Jones

Electronic states at oxygen deficient WO3(001) surfaces: a study by resonant photoemission

Abstract Electronic states associated with oxygen deficiency at WO 3 (001) surfaces have been studied by photoemission with the photon energy tuned to a resonance maximum in the W 5d ionization cross-section profile. Mild ion bombardment followed by UHV annealing leads to a reduced surface where the predominant W 5d states lie “deep” within the bulk bandgap. After more energetic and prolonged bombardment and annealing, a quasi metallic surface is obtained with a significant density of states at the Fermi energy. The valence level photoemission measurements are compared with results from scanning tunnelling microscopy and shallow W 4f core level photoemission. The “deep” states are associated with pairs of W ions σ-bonded together in defect troughs that can be imaged in real space by STM. By contrast the metallic states are assigned to reduced (1 × 1) terrace areas where all the W ions lose on-top oxygen.

An STM study of surface structures on WO3(001)

Structure on the (001) surface of monoclinic WO3 has been studied by scanning tunnelling microscopy. Electrostatic instability at the polar surface of WO3(001) is avoided by termination of the outer WO2 layer with half a monolayer of oxygen ions to produce a (2 × 2)R45° arrangement of on-top oxygen. An additional periodicity is introduced in some terrace areas by tilting of WO6 octahedra to produce alternately long and short on-top OO separations along the [110] direction. This produces a (2 × 2) superstructure approximating to the space group c2mm. Prominent defect troughs on oxygen annealed surfaces are attributed to missing oxygen ions. Topographically resolved I-V curves show that significant filled state electron density resides in the trough bottoms. Argon ion bombardment and subsequent annealing leads to the appearance of defects in which on-top oxygen is removed from adjacent W ions to generate linear features three unit cells wide running along the [100] direction. These new defects aggregate with the troughs to produce characteristic branched structures. Prolonged bombardment and annealing leads eventually to large terrace areas displaying a new (2 × 2) superstructure in which only a 14 the W ions carry on-top oxygen.

Electrochemistry at boron doped diamond films grown on graphite substrates: redox-, adsorption and deposition processes

Highly boron-doped (atomic concentration ∼1021 cm−3) conducting diamond films were grown on graphite substrates by microwave assisted vapor deposition from a gaseous feed of hydrogen and methane and solid boron. These diamond films of ca. 5 to 10 μm thickness composed of crystals of up to 10 μm size were characterized by both surface analytical techniques such as Raman spectroscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM), and electrochemical techniques. The reduction of Ru(NH3)63+, the oxidation of chlorpromazine, and the reduction of Pb2+ in aqueous media were studied in order to investigate the processes involving oxidation and reduction as well as adsorption and deposition. The one-electron oxidation of chlorpromazine in aqueous 0.1 M KCl and the one electron reduction of Ru(NH3)63+ in aqueous 0.1 M KCl gave well defined cyclic voltammetric responses at diamond electrodes with peak currents proportional to the square root of the scan rate consistent with diffusion control at a macroscopically uniformly active electrode. For the oxidation of chlorpromazine at concentrations less than 4 mM adsorption of the neutral chlorpromazine at the diamond electrode was additionally detected. At very low scan rates a transition to sigmoidally shaped responses occurred which may be attributed partly to the presence of areas of low or no conductivity on a microscopic level. The reduction of Pb2+ in aqueous 0.1 M HClO4 allowed the deposition of metallic lead on the diamond surface. Studies by ex-situ SEM and in-situ AFM show that Pb nuclei are formed and distributed inhomogeneously over the polycrystalline diamond film in the active surface areas, irrespective of grain boundaries. However, little or no Pb deposition occurred in some less active areas with dimensions of 2 to 100 μm. This effect may be attributed to regions of poor conductivity between the graphite substrate and the diamond film coupled to low lateral conductivity of the film itself. The size of the regions of lower activity decreases with higher applied over-potential. A characteristic delay in the re-dissolution process of the Pb deposit was observed and attributed to both poor adhesion and a resistive contact between deposit and electrode surface.

AN INSIGHT INTO THE MECHANISM OF SURFACE CONDUCTIVITY IN THIN FILM DIAMOND

Abstract Diamond containing hydrogen at or near the surface displays p-type conductivity. The origin of this effect has been controversial. We have used I–V, Hall effect, SIMS, Raman, UPS and XPS to study hydrogenated polycrystalline CVD diamond films. The direct formation of acceptor states by hydrogen, which resides within the top 20 nm of the film, is the origin of the carriers present rather than surface band bending. Up to 1019 holes cm−3 can be measured and mobilities as high as 70 cm2 Vs−1 recorded. H-termination of the surface is important for the formation of high quality metal-diamond interfaces.

The surface structure of SnO2(110)(4 × 1) revealed by scanning tunneling microscopy

Abstract Conditions required to produce a (4 × 1) reconstruction on SnO 2 (110) have been investigated and atomically resolved STM images of this surface have been obtained for the first time. Stable tunneling is most easily achieved at + 1.5 V sample bias. The (4 × 1) periodicity is well defined in these images. The greyscale contrast changes dramatically in images taken at + 1.0 V sample bias and structure within the unit cell is much better defined at the lower sample bias. The STM images are inconsistent with a model that has been proposed for the (4 × 1) surface which involves a coincident SnO overlayer, and an alternative model involving an ordered array of in-plane oxygen vacancies is proposed.

Observation of reduced (1 × 1) terraces on WO3(001) surfaces using scanning tunnelling microscopy

Abstract Ordered (1 × 1) terraces on a WO 3 (001) surface have been resolved using scanning tunnelling microscopy. (1 × 1) surfaces were observed after sample treatments involving both simple vacuum annealing and argon ion bombardment/anneal cycles. The (1 × 1) surface was observed to coexist with (2 × 2) and (√2 × √2)R45° reconstructed areas of the surface. The (√2 × √2)R45° reconstruction was only amenable to imaging at positive sample biases (i.e. under tunnelling into empty sample states), as is commonly expected for n-type semiconducting transition metal oxides. In contrast, both the (1 × 1) and (2 × 2) reconstructions have been imaged at negative sample biases (tunnelling from filled sample states) as well as at positive bias. These observations suggest that the (1 × 1) reconstruction arises from termination of the bulk structure in a bare WO 2 plane in which all the W ions are reduced from W(VI) to W(V).

STM characterisation of a c(2 × 2) reconstruction on Na0.65WO3(100)

A c(2 × 2) reconstruction on the (100) surface of the perovskite sodium-tungsten-bronze Na0.65WO3 has been characterised by atomically resolved scanning tunnelling microscopy. The reconstruction is formed by annealing crystals in UHV at temperatures above 650°C and probably involves termination of a WO2 surface layer with an O0.5 adlayer that completes the six-fold coordination of half the surface W ions. Surfaces can be imaged in STM under a wide range of bias conditions probing both filled and empty electronic states.

Structure and reactivity of oxide surfaces: new perspectives from scanning tunnelling microscopy

Scanning tunnelling microscopy (STM) has emerged in the past few years as a uniquely powerful tool for the investigation of oxide surfaces. STM is capable of real-space imaging of periodic structures with atomic resolution and of characterising local atomic arrangements associated with defect sites. The power of the technique is further enhanced by the ability to probe filled and empty density of states profiles at specific atomic sites by monitoring variations in tunnelling currents with applied voltage. The interpretation of STM images is not always unambiguous, a question of central importance being whether oxygen or metal ions appear as maxima in the images. The necessity for adequate sample conductivity imposes some constraints on the applicability of the technique, although developments in instrumentation and in techniques for sample preparation are helping to overcome these limitations. The range and limitations of the technique are illustrated by reference to work on tungsten oxides, titanium dioxide and iron oxides. The review concludes with a discussion of recent developments in the study of molecular adsorbates.

Photoelectron spectroscopy studies of barium films on diamond with respect to the modification of negative electron affinity characteristics

Abstract Efficient low-field emission of electrons from diamond requires surface treatments that lead to the occurrence of a negative or low positive electron affinity. The deposition of caesium/caesium oxide films has produced some of the best field emitters to date, but such films have been shown to be thermally unstable. In order to investigate further the nature and properties of deposited films, treatment of a single crystal diamond (111) surface with barium has been carried out. The film thickness and chemical composition have been determined and the electronic properties ascertained, with particular focus on negative electron affinity behaviour. The thermal stability has also been probed. The properties of the Ba adlayers have been found to differ significantly from those of deposited Cs films. In particular, the nature of the reactive interface between the metal and the diamond substrate is quite dissimilar in both cases. Further, the barium overlayers are notably more stable with respect to heat treatment, with treated surfaces retaining negative electron affinity character, even after heating to beyond 1000 °C.

The surface structure of the metallic sodium tungsten bronze Na0.667WO3(001)

Scanning tunnelling microscopy has been used to identify a number of surface reconstructions on the (001) surface of the cubic metallic sodium tungsten bronze, Na0.667WO3. Which is dominant has been found to depend critically on sample preparation. As well as a (2×2)R45° reconstruction that bears a striking similarity to that of the parent material, tungsten trioxide, regions of (2×1) periodicity are observed that can only be explained in terms of an NayO surface layer. In the current work, we relate the effect of sample preparation on the surface electronic structure of Na0.667WO3(001) with that on the atomic structure by comparing photoemission spectra with STM images. Particular interest is focused on band gap defect states in photoemission spectra which, in contrast to similar states in spectra from WO3, do not appear to correlate with the appearance of localised defects or highly reduced terraces in STM images. The existence of peroxide-like oxygen dimers at the (2×2) reconstructed surface, on the other hand, is characterised by the appearance of identifiable states in the valence band spectrum.