Robert J. Turner

Chemisorption on plasma-grown aluminium oxide: a study using inelastic electron tunnelling spectroscopy

Abstract Inelastic electron tunnelling (IET) spectra are reported for 2-hydroxycycloheptatrienone (tropolone), 8-hydroxyquinoline (oxine), 1,2-diaminoethane, 1,2-dihydroxyethane and 1-amino-2-hydroxyethane adsorbed from the vapour on to plasma-grown aluminium oxide. The mode of chemisorption of these adsorbates is discussed as are the oxide surface sites involved in the sorption process.

Temporal behaviour of nanofeatures on Au

Abstract Tip-crash techniques allow dents of 5–10 nm dimension to be written with a W tip on bulk Au. These features subsequently decay as a result of surface diffusion of Au atoms. Voltage pulses 5 V high and 5 ms wide applied to a Au-coated W tip enable mounds of 10–20 nm lateral dimension to be formed. The height of these features may be built up by sequential pulsing at one location with an approximate growth rate of 4 nm per pulse. Interestingly, the mounds do not, in general, suffer subsequent decay. They can, however, change shape and be affected somewhat by further mound deposition in their vicinity. An explanation of the persistence of the mounds, as compared with the dents, is offered in terms of inhibition of surface diffusion processes at the base of the mounds where there are atomic sites of increased co-ordination number.

Spectral dependence of light output from LEIT devices on electrode morphology

The light output from Al-I-Au tunnel junctions is observed to depend on the morphology of the Au film electrode. Films deposited quickly (2 nm s-1) give out less light, especially towards the blue end of the spectrum, than those deposited slowly (0.03 nm s-1). An explanations is offered in terms of elastic scattering of surface plasmon polaritons in the Au film. The scattering increases the chance of plasmon decay by internal electromagnetic absorption at the expanse of photon emission. A model, of more general applicability, has been developed which can explain the results. SEM has been used to determine the grain size in the Au films and STM to measure the surface roughness.

Inelastic electron tunnelling spectroscopy of 1,3-diketones and related molecules on plasma-grown aluminium oxide

Inelastic electron tunnelling (IET) spectra are reported for 2,4-pentanedione and other 1,3-diketones, β-ketoester, methyl-2-hydroxybenzoate and 2-hydroxybenzaldehyde adsorbed from the vapour on to plasma-grown aluminium oxide. These adsorbates are shown to exist as the corresponding anions chelated to surface-exposed aluminium cations with the chelate ring held approximately perpendicular to the local surface. The IET spectrum of the corresponding 2,4-pentanedione: magnesium-oxide surface complex is also discussed as is the surface protonation–dehydration reaction which leads to the formation of the chemisorbed chelates.

Imaging tip formation in single-mode optical fibres

The formation of probe tips is a crucial step in all forms of scanning probe microscopy (SPM). In this work single-mode optical fibres are chemically etched in a variable temperature bath of etchant solution (HF acid buffered with ammonium fluoride) to produce tips for optical SPM. Tip evolution is monitored by prematurely truncating the etching process and imaging the tip end-structure using atomic force microscopy (AFM). In the case of a visible regime single-mode fibre the AFM images show a remarkable ring structure in the central cladding region and a tip structure in the core with a central depression; this serves to demonstrate the efficacy of chemical etching for converting compositional variation to three-dimensional topography. In the case of a standard, single-mode optical communications fibre the (projected) tip cone angle is assessed from AFM images in the early stages of tip formation. Values of the cone angle thus determined, for different etch conditions, are compared to those predicted by a model in which the independently determined core and cladding etch rates, and core diameter are the sole determinants of the final tip geometry. The model was devised in the context of etching multi-mode fibres and is shown to be valid here for single-mode fibres within the range of experimental accuracy and etch conditions examined.

Inelastic electron tunnelling spectroscopy of some 1- and 2-substituted adamantanes on plasma-grown aluminium oxide

Inelastic electron tunnelling spectroscopy has been used to explore the adsorption and possible reactions of 1-substituted adamantanes with plasma-grown aluminium oxide. Spectra are reported for 1- and 2-adamantanols, 1-adamantanamine and 1-adamantane carboxylic acid chloride. The formation of carbocations from the alcohols and amine is discussed as is the formation of 1-adamantyl carboxylate from the acid chloride. The predicted adsorbate-orientation–band-intensity relationship is explored.

Cryogenic Magnetic Force Microscope

A magnetic force microscope has been constructed to image fluxlines in superconductors. In order to facilitate operation at low temperatures, cantilever deflections are detected with an interferometer which requires only an optical fibre in the cryostat. The design and modes of operation of the microscope are described. Room temperature AFM and MFM images are presented. The MFM results, combined with planned experimental improvements, point towards future success in imaging fluxlines.

Influence of surface topography on light emission from tunnel junctions

Visible light is emitted from roughened metal-oxide-metal tunnel junctions subjected to a small (2 - 4 V) dc bias. The color of the emitted light is voltage tunable but device quantum efficiency is generally very low (10-5 - 10-7). The optical emission is due to the roughness induced scattering of electronically excited surface plasmon polaritons. The characteristics of the surface roughness are clearly important in determining the overall device efficiency and spectral output. With a view to better understanding and improving the efficiency we have examined the surface topography of variously roughened devices and relate this to the optical output. For example, we have roughened devices by means of holographic crossed diffraction grating substrates which possess surface topography of greater rms roughness height and larger transverse correlation length than that of devices roughened by a pre-deposited CaF2 layer. Devices roughened by means of a particulate aluminum substrate are also discussed.