Robert R. Mallik

Easily realized inelastic electron tunneling spectrometer

An easily realized inelastic electron tunneling spectrometer (IETS) controlled by computer through an IEEE‐488 interface bus is described. Components and circuits of the system are described in detail in order to help newcomers to IETS build a research quality spectrometer on a relatively low budget. The system design is much simpler and easier to implement than others reported in the literature, and experimental results indicate that the spectrometer has comparable resolution and signal‐to‐noise ratio. Additionally, in‐house software routines offer the system considerable flexibility in spectral data manipulation, for example, background correction, numerical differentiation, and subtraction of one spectrum from another may be performed.

Comparison of the vibrational spectra of pyruvic acid on alumina using inelastic electron tunneling spectroscopy (IETS) and Fourier transform multiple reflection absorption infrared spectroscopy (MRAIRS)

Abstract Vibrational spectra obtained using IETS and MRAIRS are presented for monolayers of pyruvic acid adsorbed on alumina, incorporated in thin film Al/alumina/pyruvic acid/Pb samples. Spectra obtained using both techniques do not exhibit a carbonyl peak, however MRAIR spectra for similar samples without a Pb cover film do. This behaviour may be attributed to interactions between the adsorbed molecules and the cover film. Possible reasons for this, and the disappearance of certain peaks in the IET spectra when Sn and Al are employed as cover films instead of Pb, are discussed.

Inelastic electron tunneling spectroscopic studies of alkoxysilanes adsorbed on alumina

Inelastic electron tunneling spectroscopy is used to investigate the adsorption of dimethyldimethoxysilane, dimethyldiethoxysilane, and dimethylvinylethoxy silane on alumina at the monolayer level. Data obtained indicate that different adsorbed layers are produced when the silanes are introduced onto the oxide surface from solution or as a vapor. Silanes introduced in the same way onto different types of oxides suggest that alumina morphology also affects the adsorbed configuration.

Inelastic electron tunnelling spectra of some plasma polymers on aluminium oxide

Abstract Inelastic electron tunnelling spectra have been obtained for plasma-polymerized polyvinylacetate, polyethylacrylate, polyacrylic acid, polymethylmethacrylate and polyacrylonitrile on hydrated aluminium oxide. Spectra of plasma-polymerized polyvinyl-acetate and polymethylmethacrylate are very similar to those of conventional free radical polymers. All polymers exhibit spectral peaks at about 1440 cm−1 and 1600 cm−1 which are due to the carboxylate anion, and this indicates that ester groups are cleaved on the oxide to produce ionic attractions at the interface which are advantageous for adhesive bonding.

Spectroscopic, topological, and electronic characterization of ultrathin a-CdTe:O tunnel barriers

Ultrathin oxygenated amorphous CdTe (a-CdTe:O) films are prepared by rf sputtering of CdTe in a background of argon or argon/nitrogen/oxygen mixtures. Atomic force microscopy (AFM) is used to characterize the films and shows that they have an island structure typical of most sputtered thin films. However, when sufficiently low powers and deposition rates are employed during sputtering, the resulting films are remarkably smooth and sufficiently thin for use as barrier layers in inelastic electron tunneling (IET) junctions. Four terminal current–voltage data are recorded for Al/a-CdTe:O/Pb tunnel junctions and conductance–voltage curves are derived numerically. WKB fits to the conductance–voltage curves are obtained using a two-component trapezoidal plus square (TRAPSQR) model barrier potential to determine values for the tunnel barrier parameters (height, shape, and width); these parameters are consistent with AFM topological measurements and values from similar devices reported in the literature. IET spec...

Variations in carbonyl mode intensities in inelastic electron tunneling, and multiple reflection absorption infrared spectroscopy Part I: effects due to the lead cover films for selected mono-carboxylic acids on alumina

Abstract In the first of a series of three articles, evidence for attenuation of the carbonyl vibrational mode intensity in inelastic electron tunneling spectroscopy (IETS), and multiple reflection absorption infrared spectroscopy (MRAIRS) is presented. The work may have considerable impact on the interpretation of IET and MRAIR spectra of certain polymers adsorbed on alumina of interest in the field of adhesion. For a better comparison between the two techniques, MRAIR spectra are recorded for compounds adsorbed on conventional oxidized aluminum mirror samples and on samples prepared with thin-film lead cover films to form planar aluminum/alumina/compound/lead structures similar to the tunnel junctions used for IETS. The compounds chosen for the study are all carboxylic acids, known to adsorb on alumina by coordinate bonding of the carboxylate anion in acid–base reactions. The acids contain non-bonding carbonyl groups on the carbon backbone of the molecules whose intensities are investigated. When the compounds are constrained by the above sample geometry, similar carbonyl attenuation is observed in both IETS and MRAIRS, but to varying degrees depending on the geometry of the adsorbed molecules. The effect, which disappears in MRAIR spectra for samples without the lead cover films, can be explained by consideration of the deformation of the molecules by the lead cover film, which results in steric hindrance of the carbonyl groups and a corresponding reduction in their activity in accordance with the orientational selection rules for both IETS and MRAIRS. This and further spectroscopic evidence for deformation of the molecules by the lead cover film is discussed.

Variations in carbonyl mode intensities in inelastic electron tunneling and multiple reflection absorption infrared spectroscopy. Part III: effects due to surface coverage of adsorbates

In this paper, the last in the series of three articles, inelastic electron tunneling (IET) spectra are obtained for a series of aliphatic dicarboxylic acids of increasing chain length with one of the methylene groups substituted by a carbonyl group which is not involved in bonding with the alumina surface. The effect of surface coverage on the relative intensity of the carbonyl mode is discussed. For lower surface coverages, the number of possible configurations for the adsorbed molecules increases, and in certain cases, the carbonyl group is located closer to the lead cover film thereby attenuating its intensity. The effect of changing location of the carbonyl group within the molecule (and hence, the tunnel barrier) and increasing chain length on the relative intensity of the carbonyl mode is also discussed.

Electronic signal regulator for constant resolution inelastic electron tunneling spectroscopy

A relatively simple and inexpensive ac signal regulator is described which facilitates constant resolution inelastic electron tunneling spectroscopy (IETS). Constant resolution is achieved by maintaining an approximately constant ac modulation voltage across IET junctions during spectral scans. The regulator circuit is based upon a field-effect transistor optoisolator with appropriate feedback control acting as a voltage comparator. It is modular in design and can easily be added in the signal path of existing IET spectrometers. A complete schematic diagram of the circuit is provided as well as a discussion on the theory of operation. IET spectra obtained from tunnel junctions with various degrees of nonlinear conductance-voltage behavior are presented with, and without, the circuit. Analysis of these spectra shows that the regulator increases the spectrometer’s signal-to-noise ratio, produces no distortion and, in the case of severely nonlinear junctions, reveals spectral features at mid to high bias, wh...

Inelastic electron tunneling spectroscopy and atomic force microscopy investigation of ultrathin sputtered amorphous silica films on gold

Ultrathin amorphous SiO2 films are radio frequency sputter deposited onto thin film gold electrodes and incorporated as the insulating barrier in metal/insulator/metal tunnel junctions of the type Au/SiO2/Pb. Inelastic electron tunneling spectra (IETS) recorded at 4.2 K reveal characteristic zero bias anomalies associated with rapid variations in the Pb density of states on either side of the superconducting gap which shows unequivocally that tunneling is the primary conduction mechanism through the junctions at this temperature. Full IET spectra reported are essentially identical to those previously recorded for similar SiO2 films on aluminum. Atomic force and scanning tunneling microscope images of the SiO2 and underlying gold films are also presented; these data confirm that the SiO2 films are sufficiently uniform and continuous for IETS.

Inelastic electron tunneling spectroscopy of triethoxysilane on germania: a model study for silane-glass adhesion

Abstract IET spectra are presented for the first time for inorganic monolayers adsorbed on artificial tunnel barriers. They are obtained from hydrolyzed triethoxysilane (TES) monolayers introduced onto ultra-thin sputtered germania films incorporated in Al/germania/TES/Pb tunnel junctions. The minimum thickness required for germania film continuity is determined to be ∼ 1.5nm using a reliable method which is described. Three techniques are then used to dope the germania surface with TES; these are by exposure to: (i) TES vapor under a dry nitrogen atmosphere, (ii) TES vapor under a water vapor/nitrogen atmosphere, and (iii) aqueous acidic hydrolyzed TES solution. Vibrational spectra obtained using (i) and (iii) are similar; in both cases strong Si H stretching and bending modes are present at approximately 2000–2200 cm −1 , and 900 cm −1 respectively. However, only very weak features due to residual CH species are present. These results confirm that the TES hydrolyses to a large degree and condenses on the germania surface. Spectra obtained using (ii) are similar to those obtained by the other two methods but also contain vibrational modes due to Si O Si stretching vibrations, which indicates that the TES not only hydrolyzes and condenses on the surface, but also polymerizes to form a hydrogenated polysiloxilane matrix. In contrast, spectra obtained for TES adsorbed on alumina using the same three techniques indicate that the TES neither hydrolyzes, condenses, nor polymerizes unless introduced from an aqueous acidic solution.