Scanning tunneling microscopy

Archives of Materials Science and Engineering

Volume 28 Issue 8 August 2007 Pages 489-494

International Scientific Journal published monthly as the organ of the Committee of Materials Science of the Polish Academy of Sciences

STM probing of the electronic state of highly oriented pyrolytic graphite (HOPG)
H. Sarraf *, L. Škarpová, P. Louda
Faculty of Mechanical Engineering, Departmentof Material Science, Technical University of Liberec, Halkova 6, 46117, Liberec 1, Liberec, Czech Republic * Corresponding author: E-mail address: sarraf_20002000@yahoo.com
Received 26.04.2007; published in revised form 01.08.2007

ABSTRACT Purpose: The purpose of this research paper is focused on probing of the electronic state of condutive highly oriented pyrolytic graphite (HOPG).Design/methodology/approach: Scanning tunneling microscope (STM) and its spectroscopic mode, scanning tunneling spectroscopy (STS) using mechanically sharpened Pt/Ir tip has been carried out for probing of the electronic state of graphite (HOPG) at the nanometer scale in air at room temperature. Findings: STS spectra revealed obvious dependence of the separation between tip and HOPG surface. The change insurface electronic state induced by the approach of the tip to the HOPG surface was resulted as caused by the deformation of HOPG surface. The attractive force presents between the tip and HOPG surface is concluded as the major factor that causes the separation of the surface layer of HOPG and induces a difference in the local density of states (LDOS). The relationship between tip and HOPG surfaceis illustrated along with the interpretation of the results. Research limitations/implications: Due to the operation of the STS measurements in air, it is difficult to obtain qualitatively good STS spectra with high spatial resolution and high energy resolution. In order to overcome these difficulties, in this study we suggest that image data under reference conditions should be taken many timesintermittently during of the process to ensure that no serious deterioration occured in the surfaces of either the sample or the tip. Practical implications: Previously, the electronic state and its relationship with surface deformation of conductive HOPG were not investigated at room temperature in aerial atmosphere by STM and its spectroscopic mode STS together in more details. Therefore, thisspeculation motivated the present study. Originality/value: Results of the STM measurements present that tip-sample relationship, i.e. the current distance between the tip and the surface plays an important role in the formation of the electronic state and also deformation of HOPG surface. This paper could be of an interest to scientists and people dealing with STM to examine surface properties ofdifferent materials and for different industrial purposes. Keywords: Electrical properties; Surface treatment; Scanning Tunneling Microscopy (STM); Highly Oriented Pyrolytic Graphite (HOPG) PROPERTIES

1. Introduction Introduction
Since the appearance of the scanning tunneling microscope (STM), its power has been applied to the observation of real space images of a wide variety of solid surfaces[1, 2]. One of the most

attractive features of STM compared with other microscopic observation techniques is its capability to observe surfaces not only in vacuum but also in liquid or in gas [3]. Many observations have been performed in air for specimens such as solid surfaces [4-7], adsorbed molecules [8, 9] and biomolecules [10-12]. It is very useful to operate STM in air for many purposes.Samples

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H. Sarraf, L. Škarpová, P. Louda

such as biomolecules, for instance, can not maintain their surface features in vacuum as they are in air. Although STM is widely applied to a variety of samples in air, the image of the surface might be affected by molecular adsorption, oxidation or other surface...