Graphite Surface In Air Research Articles

Convex–concave nanostructure transition on highly oriented pyrolitic graphite surface induced by atomic force microscope tip under bias voltage

Convex and concave nanoscale dots were formed on a highly oriented pyrolitic graphite surface in air by an atomic force microscope tip under positively applied bias voltage. It was found that the increasing amplitude or duration of the bias voltage over threshold values can change the convex profiles into concave ones. The threshold voltage duration for the profile transition increases sharply with decreasing amplitude of the voltage and a nonlinear relationship between them was obtained. For enough long duration, there exists a threshold voltage amplitude, about 4.1–5 V in this study, for the profile transition as well.

Wavelet Transforms to Probe the Torsional Modes of a Thermally Excited Cantilever across the Jump-to-Contact Transition: Preliminary Results

The response of the torsional modes of a thermally excited cantilever across the jump-to-contact transition shows a modification of the oscillation amplitude, frequency, and damping. The measurement of these parameters is important because their analysis provides nanoscale information on the physical, chemical, and topographic properties of the sample. The tip-surface interaction potential is usually reconstructed by Fourier analysis of the cantilever oscillations around its equilibrium position. However, Fourier analysis can be correctly interpreted only in the case of stationary systems. The wavelet transform analysis overcomes these limitations, revealing the temporal evolution of the spectral content of a temporal trace. The one-dimensional time signal from the photodiode is converted into a two-dimensional time-frequency topography, which simultaneously exhibits the time and frequency behavior of the cantilever thermal fluctuations. In the present study, we show preliminary data obtained using wavelet transforms to analyze thermally excited torsional cantilever modes during jump-to-contact transition on a highly oriented pyrolitic graphite surface in air. [DOI: 10.1380/ejssnt.2011.228]

Direct observation of transition between stick-slip and continuous sliding in atomic friction

The distortion and gradual recovery processes after a continuous tunable laser illuminating the graphite surface in air have been observed and analyzed in detail with a scanning tunneling microscope (STM). The slow response of the tip to the laser-induced mechanical elongation is responsible for the observed phenomena, which indicate that the transition between stick-slip and continuous sliding has occurred due to the atomically modulated friction between the STM tip and the graphite substrate.

Nanometer-scale fabrication on graphite surfaces by scanning tunneling microscopy

Holes and carbon dots on a nanometer scale are formed on highly oriented pyrolytic graphite (HOPG) surfaces by scanning tunneling microscopy (STM). A hole of 25 Å in diameter and 7 Å in depth at the center is made by applying a voltage pulse (+10 V, 0.1 ms) on the graphite surface in air. A carbon dot of 8 Å in diameter and 7 Å in height is deposited by applying a short voltage pulse (+4 V, 50 μs) in a mixed gas of acetone and hydrogen at 20 Torr. The results indicate the possibility of nanostructure fabrication by an ångström-size electron beam emitted from a STM tip presumably through reactions with ambient gas molecules.

Atomic resolved material displacement on graphite surfaces by scanning tunnelling microscopy

Atomic scale modifications and subsequent atomic resolution imaging has been achieved on the highly oriented pyrolytic graphite surface in air. Application of short pulse voltages, above a minimum threshold voltage of 3.5 V, across the tunneling gap results in the displacement of a layer or more of atoms to form a hole and create a neighboring mound or ‘‘nanodot’’ from the displaced atoms. We have found a correlation between the hole and ‘‘nanodot’’ volume at the atomic level and observe an asymmetric displacement of material in all cases of feature creation. Nanofeatures as small as four carbon atoms at beta sites have been created. Our experimental observations are consistent with the modification process depending on the gradient in the electric field induced by the rise time of the bias pulse voltage and not the pulse duration. Interesting faceting behavior has also been observed around some hole edges. Tip bias pulsing sometimes induced a tip, and not a surface modification, resulting in a change in the observed tunneling image.

Scanning force/tunneling microscopy of a graphite surface in air

We discuss atomic resolution imaging of a graphite surface in air by simultaneously measuring repulsive contact forces and surface conductance with the scanning force/tunneling microscope (AFM/STM) under two types of operating modes: the constant height mode and the constant current mode. We also discuss the coherence of the images during simultaneous imaging with the AFM/STM, and further clarify the measurement condition for simultaneous imaging.

Simultaneous Imaging of a Graphite Surface with Atomic Force/Scanning Tunneling Microscope (AFM/STM)

We observed the graphite surface in air with an atomic force/scanning tunneling microscope (AFM/STM) system. We obtained atomically resolved AFM and STM images simultaneously under two types of operating modes. In the case of a constant height mode, the AFM (variable force) image showed the hexagonal pattern, while the STM (variable current) image showed the trigonal pattern. On the other hand, in the case of a constant current mode, the AFM (variable force) and STM (constant current) images showed the same trigonal pattern, but those patterns were out of phase with each other. We further investigated the effect of an oxide layer of a lever surface on simultaneous observation of AFM and STM images under the constant height mode.

Direct measurement of forces during scanning tunneling microscope imaging of graphite

The normal force acting on a scanning tunneling microscope tip while imaging a graphite surface in air has been measured directly. Forces in the range of 10 −7 to 10 −6 N are required to achieve tunneling. Further, the force needed to maintain a constant current varies considerably as the tip scans from one part of the graphite unit cell to another. Our results are consistent with a model, originally suggested by Mamin et al., in which the force between the tip and the surface is mediated by a contamination layer, and tunneling occurs at the end of an asperity which pierces this layer. However, we cannot rule out a model where a graphite flake is dragged across the graphite surface to generate an STM image.

Tunneling microscopy study of the graphite surface in air and water.

The recent work of Park and Quate has demonstrated that atomic resolution images of graphite can be obtained in air. Here, we compare our experimental results on graphite in air and water to theoretical calculations. We present an image in close qualitative agreement with the theory and other images which can be understood by considering how images can be degraded under real experimental conditions. In addition, we present images showing graphite to be atomically flat over ${10}^{6}$ A${\r{}}^{2}$ areas when imaged under water.

Effect of n-alkanes on wettability of graphite

Measurements of contact angle were made in the system graphite- n-alkane film-air bubble-water for 11 n-alkanes in the homologous series from hexane to hexadecane. From the results and on the basis of theoretical calculations it was found that stable films of n-alkanes at the graphite-water and graphite-air interfaces can be of such a thickness that their pressure corresponds to the work of adhesional wetting of the alkane on a graphite surface in air. The film stability of n-alkanes increases with carbon chain length. It was also found that a difference exists between the film structures, at the graphite-water and graphite-air interfaces, of an n-alkane with an even number of carbon atoms and the neighbouring odd-numbered alkane.