A current imaging tunneling spectroscopy study of localized states at the Fermi level on circular pit edges in graphite

Abstract

Thermal oxidation of the (0001) basal plane of highly oriented pyrolitic graphite leads to the removal of monoatomic carbon layers from the surface and to the formation of circular pits and steps on the exposed plane. Scanning tunneling microscopy and current imaging tunneling spectroscopy are used to study a localized surface state at the Fermi level on the circular edges of the pit. The surface state appears as the maximum of the local density of states at 10–50 meV below the Fermi level. The appearance of the state does not depend on the orientation of the step. The explanation for this effect includes the superposition of the two different surface states. The first one is due to the existence of an edge in the graphite sheet. The second one is due to fused disclination centers caused by mechanical deformations on the terrace. The first-principles band calculations performed on the stepped surface of graphite show an existence of a localized state at the Fermi level [1]. This state is due to the cut of the graphite sheet and is localized near the step. However, to the best of our knowledge, the existence of this state has not yet been confirmed experimentally. The experimental observation of this state might be difficult since its appearance depends on the orientation of the step. As has been proved by tight-binding calculations, if the orientation of the step is rotated by 30◦, the localized state does not appear [1] and the electronic structure near the step is similar to that of a bulk sample. Since the detection of the localized state near the step edges can cause some problems, specific surface preparations and techniques are required to give us local spectroscopic information at atomic level. The thermal oxidation of the (0001) basal plane of highly oriented pyrolitic graphite (HOPG) leads to the removal of monoatomic carbon layers from the surface and to the formation of circular pits and steps on the exposed plane. The circular geometry of the pit makes it possible to find the ∗ To whom all correspondence should be addressed. (Fax: +48-42/790030, E-mail: zbklusek@mvii.uni.lodz.pl) proper orientation of the step edges (running around the pit) and to detect the localized state at the Fermi level. The invention of current imaging tunneling spectroscopy techniques (CITS) enables us to obtain spectroscopic information from the tunneling current characteristics showing spectral features in agreement with other surface-sensitive techniques [2]. The purpose of this study is to obtain a detailed understanding of the graphite tunneling spectra recorded near the pit step edges, which can be used further to determine whether the localized state at the Fermi level exists. The results obtained will make it possible to understand better the thermal oxidation process and can be useful in interpreting the spectra obtained from thin metal layers deposited on thermally oxidized graphite, the deposited clusters on thermally oxidized graphite, or graphite in general. 1 Sample preparation and experimental setup In order to create surface pits and steps the thermal oxidation of graphite is applied [3–5]. The oxidation process is carried out in an externally heated quartz tube in ambient air conditions. The heating is started by the direct inserting of the sample into the tube preheated to the desired temperature and is stopped by withdrawing it from the tube. The experiments are performed at the temperature of 800 ◦C. The heating time is 30 min. After the treatment the samples left to cool in ambient air. STM/CITS investigations are performed in the air in the constant-current mode with the use of a digital feedback loop. The typical tunneling current is 1 nA. Tips are obtained by electrochemical etching from the Pt90-Ir10 alloy wires. The method proposed by Feenstra et al. [6] is applied to normalize all the measurements. The divergence problem in the case of (dI/dV)/(I/V) is overcome by applying some amount of broadening (∆V ) to the I/V values [7]. The positions of spectral features are insensitive to the value chosen for ∆V when the normalization of dI/dV is carried out. However, the half width of the peaks decreases when the ∆V tends to larger values. That is why we observe the narrower peaks on the (dI/dV)/(I/V) curves in comparison to the (dI/dV).