Surface Science

Surface origin of high conductivities in undoped In2O3 thin-films

by Andriy Zakutayev

Phys. Rev. Lett. 108, 016802 (2012)

Bulk and surface plane electromagnetic waves in anisotropic media

by Vladimir K. Ignatovich

Coauthored with Filipp V. Ignatovich

A new analytical approach to description of electromagnetic waves in nonmagnetic anisotropic media is presented.... more A new analytical approach to description of electromagnetic waves in nonmagnetic anisotropic media is presented. Amplitudes of their reflection and refraction at interfaces and also reflection and transmission of plane parallel plates are derived. Beam splitting at reflection, and creation of surface waves at the interfaces are studied. A simple laboratory demonstration of the beam splitting is proposed. D'yakonov surface waves, their description and observation are discussed.

Inversion of Pattern Anisotropy During CO Oxidation on Pt(110) Correlated with Appearance of Subsurface Oxygen

by Christian Punckt

P. Sadeghi, K. Dunphy, C. Punckt. H. H. Rotermund
J. Phys. Chem. C 116 (2012) 4686

Spatiotemporal patterns occurring during the catalytic oxidation of carbon monoxide on Pt(110) show a strong... more Spatiotemporal patterns occurring during the catalytic oxidation of carbon monoxide on Pt(110) show a strong anisotropy due to differences in the diffusion coefficients along the two major crystallographic axes of the catalyst: Reaction waves propagating parallel to the missing rows of the reconstructed Pt surface are much faster than reaction waves propagating in the perpendicular direction. In this Article, we report on the observation that, as a function of the exposure to reactant gases, the propagation velocities on the Pt surface change drastically, resulting in a complete reversal of the anisotropy. This observation is correlated with the appearance of subsurface oxygen in photoemission electron microscopic images of the reaction patterns.

Effect of chemical environments on palladium phthalocyanine thin film sensor for humidity analysis

by Mohammad Javad Jafari

Journal of Materials Science

Chemiresistors based on palladium phthalocyanine (PdPc) thin films were investigated as humidity sensors. The samples... more Chemiresistors based on palladium phthalocyanine (PdPc) thin films were investigated as humidity sensors. The samples were thermally evaporated onto gold electrodes with a thickness about 100 nm. Optical and electrical characteristics of PdPc thin films were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and electrical measurements. The SEM image demonstrates PdPc (30–60 nm) nanosized particles, and XRD pattern shows that thin films are in α-phase at room temperature. Electrical measurements also confirm that PdPc exhibit semiconducting and photoconducting behaviors, and thermal activation energies of thin films were calculated. After that, the sensitivity and reversibility of devices were investigated on exposure to 20–90% RH in various chemical environments at 293 and 323 K. The response time (35–45 s) and recovery time (75–105 s) of sensors were measured at 293 K with respect to different chemical environments. At last, the stability of devices versus different RH% and chemical environments were tested. The sensors show very good stability on exposure to RH for a period of 2 months but their stability has been reduced in ethanol, acetone, and ammonia environments.

Silicene Nanoribbons - Molecular-Dynamics Simulations

by Alper İnce

Effect of post-deposition annealing on surface morphology and gas sensing properties of palladium phthalocyanine thin films

by Mohammad Javad Jafari

Surface and Interface Analysis

The effect of post-deposition annealing on surface morphology and gas sensing properties of palladium phthalocyanine... more The effect of post-deposition annealing on surface morphology and gas sensing properties of palladium phthalocyanine (PdPc) nanostructured thin films has been studied. PdPc thin films were deposited on polyborosilicate substrate by thermal evaporation technique at room temperature. The surface morphology of thin films was investigated by SEM, X-ray diffraction, and optical absorption. X-ray diffraction patterns showed a phase transition from α to β based on post-deposition annealing at temperatures above 200 °C. The SEM and optical absorption confirmed that annealing strongly influenced the surface morphology of nanostructured thin films. Sandwich devices (Au|PdPc|Al) were fabricated and exposed to different concentrations of NO2 and NH3 as oxidizing and reducing gases at different temperatures, and the sensitivity of devices were obtained versus gases. Obtained results showed α-PdPc thin film devices had higher sensitivity in comparison with devices in β-phase. In particular, it was found that the sensitivity of devices is temperature dependent and the best operating temperature range of devices was measured at about 90–100 °C. Devices showed good reversibility, response, and recovery time at room temperature. Finally, the stability of sensors was investigated for a period of about 1 year; results showed that the sensors were stable for 2 months and lost about 30% of their sensitivity after 1 year.

Dependence of surface free energy on molecular orientation in polymer films

by Urszula Stachewicz

Surface free energy of mechanically drawn polycarbonate films was determined using contact angle measurements and... more Surface free energy of mechanically drawn polycarbonate films was determined using contact angle measurements and shown to increase with orientation. The increase in polymer film surface free energy was attributed to increased polymer chain packing during orientation, supported by film density measurements, which provides enhanced intermolecular interactions. Surface free energy can therefore be increased by, or used to predict, polymer orientation.

Surface characterization of biological nanodomains using NP-ToF-SIMS

by Francisco A. Fernandez-Lima

Surf. Interface Anal. (in press) SIA ID 4901

Experimental analysis of the surface roughness evolution of etched glass for micro/nanofluidic devices

by Jing Ren

Comparison and Reproducibility of H-Passivation of Si(100) with HF in Methanol, Ethanol, Isopropanol and Water by IBA, TMAFM, and FTIR'

by Nicole Herbots

V. Atluri*"#, N. Herbots*, D. Dagel" H. Jacobsson*,##, M. Johnson%, R. Carpio**, B. Fowler**
Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287-1504. *SEMATECH, Austin, TX 78741-6499.
#Presently at Intel Corporation., Chandler, AZ 85226. "•Presently at Ericsson Research Center, Molndal, Sweden.

V. Atluri*"#, N. Herbots*, D. Dagel" H. Jacobsson*,##, M. Johnson%, R. Carpio**, B. Fowler**
Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287-1504. *SEMATECH, Austin, TX 78741-6499.
#Presently at Intel Corporation., Chandler, AZ 85226. "•Presently at Ericsson Research Center, Molndal, Sweden.

Mat. Res. Soc. Symp. Proc. Vol. 477, pp. 281-292 (1997)

Three different HF:alcohol solutions are investigated to etch native Si0 2 and passivate Si(100) surfaces with H which... more Three different HF:alcohol solutions are investigated to etch native Si0 2 and passivate Si(100) surfaces with H which can the be desorbed at low temperature (T < 600'C). The resulting passivated Si(100) surfaces are compared using as a reference Si(100) passivated by a standard aqueous HF: solution (1:98 parts of HF: H2 0). After a modified RCA cleaning, Si(100) etched by HF:Methanol, HF:IPA, or HF:Ethanol, is characterized by Ion Beam Analysis (IBA), Tapping Mode Atomic Force Microscope (TMAFM), and Fourier Transform Infrared Spectroscopy (FTIR). The absolute coverage of 0 and C is measured by nuclear reaction analysis (NRA) combined with ion channeling at 3.05 MeV for 0 and 4.265 MeV for C. Hydrogen is measured via the elastic recoil detection (ERD) of 4He2+at 2.8 MeV.
Compared to aqueous HF, HF:alcohol passivates Si(100) leaving a lower 0 residue by an average factor of 0.62 and a similar C residue. H coverage is higher by an average factor of 1.43. Surface coverages are found to be reproducible in average by 1.4 x 1014 atoms/cm2 for C, and by 1.25 x 1014 atoms/cm2 for 0 when measured by IBA on samples identically processed. H coverage is reproducible within 5.5% when measured by ERD.
Selective area analysis by TMAFM shows that an increasing number of particulates is responsible for the apparent increase in root-mean -square (nns) surface roughness when the rms is measured over a whole image. Taking this effect into account, all passivated surfaces exhibit similar roughness when compared to the original Si(100) surface with little difference between
alcohols and with the reference aqueous HF solution. FTIR in the attenuation total reflection (ATR) mode detected SiHx species mostly as a
dihydride. Both IBA and FTIR detected significant levels of oxygen on surfaces passivated HF in alcohol and aqueous HF. This indicates that while Si(100) exhibits more H when passivated with HF in alcohol and can be desorbed at lower temperature than when treated with aqueous HF, H is
not bonded to Si only but likely bonds into a more complex surface termination, such as SiOH.

Epitaxy and chemical reactions during thin-film formation from low-energy ions: new kinetic pathways, new phases, and new properties

by Nicole Herbots

Nicole Herbots, O.C. Hellman
ARIZONA STATE UNIVERSITY, Departmentof PhysicsandAstronomy, Tempe AZ 85287
0. Vancauwenberghe* MASSACHUSETTS INSTITUTE OFTECHNOLOGY, DepartmentofMaterialsScience & Engineering,Cambridge, MA 02139

Ref: Mat. Res. Soc. Symp. Proc. Vol. 235. pp. 749-762 (1992)

ABSTRACT
Three important effects of low energy direct Ion Beam Deposition (IBD) are the athermal incorporation of... more ABSTRACT
Three important effects of low energy direct Ion Beam Deposition (IBD) are the athermal incorporation of material into a substrate, the enhancement of atomic mobility in the subsurface, and the modification of growth kinetics it creates. All lead to a significant lowering of the temperature necessary to induce epitaxial growth and chemical reactions. The fundamental understanding and new applications of low temperature kinetics induced by low energy ions in thin film growth and surface processing of semiconductors are reviewed. It is shown that the mechanism of IBD growth can be understood and computed quantitatively using a simple model including ion induced defect generation and sputtering, elastic recombination, thermal diffusion, chemical reactivity, and desorption. The energy, temperature and dose dependence of growth rate, epitaxy, and chemical reaction during IBD is found to be controlled by the net recombination rate of interstitials at the surface in the case of epitaxy and unreacted films, and by the balance between ion beam decomposition and phase formation induced by ion beam generated defects in the case of compound thin films. Recent systematic experiments on the formation of oxides and nitrides on Si, Ge/Si(100), heteroepitaxial SixGe1−x/Si(100) and GaAs(lOO) illustrate applications of this mechanism using IBD in the form of Ion Beam Nitridation (IBN), Ion Beam Oxidation (IBO) and Combined Ion and Molecular beam Deposition (CIMD). It is shown that these techniques enable (1) the formation of conventional phases in conditions never used before, (2) the control and creation of properties via new degrees of freedom such as ion energy and lowered substrate temperatures, and (3) the formation of new metastable heterostructures that cannot be grown by pure thermal means.

Voltammetry of surface-bound species: Proton-coupled electrochemical reduction

by Denis Menshykau

Denis menshykau, Christopher Batchelor-Mcauley, Richard G Compton

We consider the one electron, one proton and also the two electron, two proton reductions of surface-bound species.... more We consider the one electron, one proton and also the two electron, two proton reductions of surface-bound species. Two mechanisms of reaction are considered: stepwise and concerted. The voltammetry is modelled under three regimes of proton transport: infinitely fast (fully buffered solution), infinitely slow (infinitely high surface coverage of electrode) and the intermediate case of a finite rate of diffusional mass transport to electrode surface. The types of voltammograms observed in each case are presented and discussed.

Voltammetric Responses of Surface-Bound and Solution-Phase Anthraquinone Moieties in the Presence of Unbuffered Aqueous Media

by Denis Menshykau

Christopher Batchelor-McAuley, Barbara R. Kozub, Denis Menshykau, and Richard G. Compton

The voltammetry of solution-phase redox species involving proton transfer are known to be qualitatively altered under... more The voltammetry of solution-phase redox species involving proton transfer are known to be qualitatively altered under conditions of nonbuffered media (Quan, M. et al. J. Am. Chem. Soc. 2007, 129, 12847−12856). We now report first the voltammetric response of solution-phase anthraquinone monosulphonate on a gold macroelectrode in the presence of a limited concentrations of protons; further we provide quantitative analysis of the voltammetry under unbuffered conditions whereby it is possible to demonstrate through simulation that in some conditions the pH at the electrode may alter by up to 5−6 pH units as compared to that of the bulk solution. This change in local environment adjacent to the electrode is caused by the consumption of protons during the electrochemical process. As a result, in conditions of low buffering the electrochemical reduction is limited by the availability of protons, leading to a measured voltammetric signal with two voltammetric waves. Second the work is developed through the study of an anthraquinone modified pyrolytic graphite electrode under conditions of finite proton concentration. We demonstrate experimentally how analogous split wave results occur for surface confined species. These results provide physical insight into the consumption of protons during the electrochemical process and highlight how methods of pH measurement based upon the use of redox modified electrode surfaces are nonpassive.

Investigating the Concept of Diffusional Independence. Potential Step Transients at Nano-and Micro-Electrode Arrays: Theory and Experiment

by Denis Menshykau

Chronoamperometry on Ring, Ring-Recessed and Disc Electrodes, and Their Arrays. The Sensitive Measurement of Diffusion Coefficients Independent of a Knowledge of …

by Denis Menshykau

Denis Menshykaua, Aoife M. O’Mahonya, Montserrat Cortina-Puigb, F. Javier del Campob, Francesc Xavier Muñozb and Richard G. Compto

Theory of chronoamperometry on ring-recessed microelectrodes and their arrays is presented. Analysis of the current... more Theory of chronoamperometry on ring-recessed microelectrodes and their arrays is presented. Analysis of the current transients measured at thin rings and their arrays shows that chronoamperometry at such devices is an excellent method of simultaneous measurement of the diffusion coefficient D and the product n[A], where n is the number of electrons transferred and [A] is the concentration of electroactive analyte. A generic, accurate and easy to use method of experimental chronoamperometric data analysis is proposed. It is shown that the method can be applied to the simultaneous measurement of D and n[A] in solution. The method is shown to be applicable to an analysis of data recorded at single disk and ring electrodes as well as arrays of these electrodes even in the case of diffusional non-independence of electrodes in the array.

Influence of Electrode Roughness on Stripping Voltammetry: Mathematical Modeling and Numerical Simulation

by Denis Menshykau

Denis Menshykau and Richard G. Compton

Electrodes with rough surfaces inevitably have practical importance from both applied and fundamental points of view... more Electrodes with rough surfaces inevitably have practical importance from both applied and fundamental points of view including electroanalysis where stripping voltammetry is a popular technique due to its simplicity and high sensitivity. The diffusional domain approach is used to model stripping voltammetry at rough electrodes: two models of the electrode surface, “rough” and “scratched”, are considered. Electron transfer is described by three models which correspond to cases of stripping of a monolayer, a thin layer, and a bulk layer. The shape of the votammograms strongly depends on the model of the electron transfer but is not always sensitive to the precise model of the electrode surface; the conditions under which this is the case are identified, and generic roughness effects on stripping voltammetry are quantified. We conclude that electrode roughness can have a significant effect on the stripping of the metals from the solid electrode especially in respect of the voltammetric waveshape.

Current Collection Efficiency of Micro-and Nano-Ring-Recessed Disk Electrodes and of Arrays of These Electrodes

by Denis Menshykau

Denis Menshykaua, F. Javier del Campob, Francesc Xavier Muñoz and Richard G. Compton

Diffusional transport theory for micro- and nano-ring-recessed disc microelectrodes has been developed using finite... more Diffusional transport theory for micro- and nano-ring-recessed disc microelectrodes has been developed using finite difference methods. The device is typically operated in generator–collector mode with the disc acting as generator and the ring as the collector. The collection efficiency, defined as the ratio of the detector to generator currents under diffusion controlled conditions, is shown to depend on the depth of the recession of the disc below the ring and the radius of the ring relative to the disc radius. The latter is also the inner radius of the ring. The theory shows that collection efficiencies in excess of 90% are possible with this geometry, which is larger than most conventional hydrodynamic systems. This level of efficiency is in the same range as interdigitated microband electrodes. The diffusion domain approach is used to extend the theory to arrays of such micro- and nano-ring-recessed disc systems.

The Influence of Electrode Porosity on Diffusional Cyclic Voltammetry

by Denis Menshykau

Denis Menshykau, Richard G. Compton

A simple generic model to predict the influence of electrode porosity on the cyclic voltammetric response of an... more A simple generic model to predict the influence of electrode porosity on the cyclic voltammetric response of an electrode is presented. The conditions under which deviation from the behavior of a perfectly flat, planar electrode can be expected are predicted. The scope for misinterpretation when conventional flat electrode theory is applied to porous electrodes is highlighted, especially in respect to the extraction of electrode kinetic parameters and the influence of electrocatalysis.

Electrodes Modified With Electroinactive Layers: Distinguishing Through-Film Transport From Pinhole (Pore) Diffusion

by Denis Menshykau

Denis Menshykau and Richard G. Compton

Electrodes modified with layers, for example, of polymers or self-assembled monolayers, are of great importance from... more Electrodes modified with layers, for example, of polymers or self-assembled monolayers, are of great importance from both the fundamental and applied points of view. Two different models of electrodes covered with electroinactive layers can be proposed. First, the electrode is covered with a uniform layer into which the electroactive species dissolves and then diffuses through, or second, the layer contains pinholes that are exclusively responsible for diffusional transport to the electrode. Both models are simulated and then compared to identify conditions under which they can be distinguished. The models are studied for a broad range of parameters reflecting experimentally viable values. Different types of cyclic voltammograms can be observed in the studied models corresponding to classical Randles-Sevchik, thin layer, and steady-state behaviors. We show that the models can be distinguished experimentally through recording cyclic voltammograms over a sufficiently broad range of voltage scan rates.

Influence of Electrode Roughness on Cyclic Voltammetry

by Denis Menshykau

Denis Menshykau, Ian Streeter and Richard G. Compton

Electrodes with rough surfaces are of great practical importance from both applied and fundamental points of view. The... more Electrodes with rough surfaces are of great practical importance from both applied and fundamental points of view. The diffusion domain approach is used to model cyclic voltammetry at such electrodes. Electrode roughness only has a significant effect on the shape of cyclic voltammograms and peak currents at relatively high values of electrode roughness. To verify the theory two experimental systems were used: TMPD in acetonitrile and Ru(NH3)6Cl3 in aqueous solution. In both cases cyclic voltammograms on the flat and roughened glassy carbon electrodes were in agreement with theory. Even significant surface roughness produced by deliberate polishing or scratching is not sufficient to be distinguished in cyclic voltammetry experiments conducted under the usual conditions.