D.G. Walmsley

A study of the chemisorption of phenol and its derivatives on plasma‐grown aluminium oxide by inelastic electron tunneling spectroscopy

Inelastic electron tunneling spectroscopy has been applied to study the adsorption of a series of phenols on plasma‐grown aluminium oxide. Vapor phase doping was used and spectra recorded for o‐, m‐, p‐chlorophenol, p‐fluorophenol, p‐bromophenol, o‐, m‐, p‐cresol, and phenol. The high resolution spectra obtained at 2 K allow most of the possible modes to be identifield. The relative strengths of these modes are compared in terms of the proposed molecular orientation of the absorbates in an effort to observe the predicted directional sensitivity of the tunneling process. No systematic line intensity variation is evident, and it is postulated that for large molecules this may be because of the complexity of the motions involved in the vibrational modes. It is also suggested that the contouring of the lead electrode round the adsorbate has the important effect of obscuring the directional sensitivity of the tunneling process. An attempt has also been made further to characterize the chemical nature of the ox...

Conductance of clean and doped tunnel junctions

Abstract The conductance of a clean Alue5f8Iue5f8Pb tunnel junction can be understood in terms of an asymmetric oxide barrier but for a doped junction an oxide-organic bilayer must be considered. We report here an organic barrier 8.80 V high and 2.53 A thick.

Inelastic electron tunneling spectroscopy evidence for a surface chemical reaction

Abstract The inelastic electron tunnel effect has been used to show the occurence of a surface chemical reaction between an organic molecule (benzoyl chloride) dopant and the tunnel barrier of a junction (Al-oxide-Pb).

Development of inelastic electron tunnelling spectroscopy: Comparison of adsorbates on aluminum and magnesium oxides

Abstract We report the first detailed comparison of inelastic electron tunnelling spectra obtained from the same dopant molecules adsorbed on two different metal (aluminum and magnesium) oxides. The widths, positions and relative intensities of the spectral lines are remarkably similar on both oxides. Small shifts to lower energy are found in the antisymmetric CO - 2 stretch modes of carboxylate ions when adsorbed on magnesium oxide as compared with the same ions on aluminium oxide. The shifts are thought to arise from stronger electrostatic binding of the ions to magnesium oxide. No significant shift of the CO stretch mode of the phenolate ion is found.

Inelastic electron tunneling spectroscopy (IETS) of simple unsaturated hydrocarbons adsorbed on plasma-grown aluminum oxide

Abstract The paper reports the inelastic electron tunneling spectra (IETS) of 1-hexene, 1-heptene, 1-octene, 1-hexyne, 3-hexyne, 1-heptyne, cyclohexene, and 1,3,5-cycloheptatriene adsorbed from the vapor onto plasma-grown aluminum oxide. Chemisorption, surface reactions, and adsorbate orientation are discussed. The 1-alkenes appear to undergo a Lewis base interaction with surface aluminum cations; the resultant perturbed double bonds lie parallel to the oxide surface. The alkynes are little altered by adsorption but they too may lie parallel to the surface. Many related species failed to adsorb at all.

A sensitive robust circuit for inelastic electron tunnelling spectroscopy

A circuit suited to inelastic electron tunnelling spectroscopy is described in detail. It combines resonant tuning with some high-impedance elements. It has proved to be robust against stray pick-up yet also to give a good signal-to-noise performance. A high-frequency, 100 kHz, version of the circuit is shown to be better than 10 kHz or 1 kHz equivalents. The quality of the spectra is not significantly degraded when the circuit is interfaced to a desktop-computer-controlled data acquisition system.

An exploration of the surface chemistry of thin-film aluminium oxide using inelastic electron tunnelling spectroscopy

Abstract Inelastic electron tunnelling (IET) spectra are reported for 1,3-diamino-, 1,3-dihydroxy- and 1-amino-3-hydroxypropane adsorbed from the vapour on to plasmagrown aluminium oxide. The chemisorption of these adsorbates is discussed as are the oxide surface sites involved in the sorption process.

The chemisorption of pyridine on plasma-grown aluminium oxide: An inelastic electron tunneling spectral study

From a detailed consideration of the inelastic electron tunneling (IET) spectra of pyridinevapor-doped Al/Al oxide/Pb tunnel junctions, it is found that pyridine is adsorbed at both Lewis acid and Bronsted active sites present on the plasma-grown oxide surface. The data presented are compared with that of air-grown oxide, fluorided air-grown oxide, and with the known behavior of bulk aluminas.

Explanation of asymmetry in tunnel spectra

Abstract The molecular vibrational spectrum from an Alue5f8Iue5f8Pb tunnel junction doped with m -cresol has been studied. It is shown that the asymmetry in signal strength with d.c. bias polarity arises from an asymmetry in the junction conductance.

Pre-microscope tunnelling — Inspiration or constraint?

Abstract Before the microscope burst upon the scene, tunnelling had established for itself a substantial niche in the repertoire of the solid state physicist. Over a period of 20 years it has contributed importantly to our understanding of many systems. It elucidated the superconducting state, first by a direct display of the energy gap then by providing detailed information on the phonon spectra and electron-phonon coupling strength in junction electrodes. Its use as a phonon spectrometer was subsequently extended to semiconductors and to the oxides of insulating barriers. Eventually the vibrational spectra of monolayer organic and inorganic adsorbates became amenable with rich scientific rewards. In a few cases electronic transitions have been observed. Plasmon excitation by tunnelling electrons led to insights on the electron loss function in metals at visible frequencies and provided along the way an intriguing light emitting device. With the advent of the microscope it is now appropriate to enquire how much of this experience can profitably be carried over to the new environment. Are we constrained just to repeat the experiments in a new configuration? Happily no. The microscope offers us topographical and spectroscopic information of a new order. One might next ask how great is the contact between the two disciplines? We explore this question and seek to establish where the pre-microscope experience can be helpful in inspiring our use of this marvellous new facility that we know as the scanning tunnelling microscope.