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IOP PUBLISHING Nanotechnology 21 (2010) 095306 (4pp)

NANOTECHNOLOGY doi:10.1088/0957-4484/21/9/095306

Scanning probe nanoscale patterning of highly ordered pyrolytic graphite
Norimasa Yoshimizu, Bryan Hicks, Amit Lal and Clifford R Pollock
SonicMEMS Laboratory, School of Electrical and Computer Engineering, Cornell University, Ithaca, NY 14853, USA E-mail: ny22@cornell.edu

Received 13November 2009, in final form 19 January 2010 Published 8 February 2010 Online at stacks.iop.org/Nano/21/095306 Abstract In this work we present precision scanning probe etching of highly ordered pyrolytic graphite. We corroborate that the lithography is due to an electrochemical, polarity-dependent, meniscus-mediated etching of the carbon surface. By changing the etching temperature, we are able toreduce the feature size by 24%. External feedback control and probe tip cleaning enables desired cut patterns with high precision. Using a feedback-controlled atomic force microscope, we demonstrate an array of 105 trenches using 370 etching operations, with 136 ± 6 and 183 ± 5 nm precision over an area of 2.5 µm × 2.5 µm. This results in a precision of 4.4% and 2.7%, respectively. (Some figuresin this article are in colour only in the electronic version)

1. Introduction
The recent growth of interest in carbon-based devices [1] is in part motivated by the nanoscale control of electrical properties with size. For example, this size control in nanoribbons ˚ requires atomic (≈1.4 A) control of their dimensions [2]. Nanofabrication with scanning probe microscopes is an attractive methodfor high resolution nanofabrication beyond photo- or electron-lithographic methods, perhaps down to atom level engineering. There are a number of papers on scanning probe nanofabrication of HOPG, with a variety of empirical observations and proposed etching mechanisms [3–5]. In all cases, a scanning probe approaches the surface of the HOPG and a bias voltage is applied between the sample and thetip. Both carbon oxidation resulting in a convex protrusion and carbon etching resulting in a small concave pit are observed. These processes can be used as a method to fabricate nanoscale carbon-based devices. The origins of graphite etching could be defects induced by an argon plasma [6], atomic oxygen [7], or due to knock-on collisions from energetic electrons [8]. These processes may havesecondary effects in scanning probe etching of graphite, but polarity, temperature, and electric current data suggest that an electrochemical reaction is the fundamental origin. Here, we report on the chemistry of scanning probe electrochemical etching of highly ordered pyrolytic graphite and the demonstration of numerous, precise nanofabrications.
0957-4484/10/095306+04$30.00

2. Scanning probeetching of graphite
The process of scanning probe microscope graphite etching is due to an aqueous electrochemical oxidation and removal of surface carbon atoms. The water is supplied by the meniscus that forms between the carbon surface and the scanning probe tip from the ambient moisture. As a result, HOPG etching does not occur under vacuum. This is best described by the Pourbaix diagram forcarbon, which shows a narrow region of stability at low potentials [9]. The main graphite etching reaction is the generation of carbon dioxide [10–12], C + 2H2 O 4H+ + 4e− + CO2 , (1)

which is essentially an irreversible reaction whose rate is increased at positive carbon, or cathode, voltages. At high bias voltages, the carbon is etched rapidly. At low bias voltages the carbon can be oxidizedrather than etched: while the Pourbaix diagram indicates the possibility of etching at low voltages, the chemical kinetics favor the oxidation reaction [13]. We have been able to etch graphene at voltages as small as +2 V sample bias. At the reverse polarities, the Pourbaix diagram shows the possibility of the generation of methane by combining carbon with four hydrogen ions and four free electrons,...
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