Nanoscale Graphene and Carbon Nanotube Lithography Using an Atomic Force Microscope

In this work, we demonstrate the use of a voltage-applied Atomic Force Microscopy (VAFM) local anodic oxidation nanolithography process to precisely fabricate small (<20 nm) structures from graphene and carbon nanotube material. These graphitic materials have exceptional electrical properties which give them a niche in emerging nanoelectronics applications requiring quantum structures. While several methods for nanoscale patterning of these materials exist, the VAFM nanolithography technique has lately been shown to address the fabrication issues of graphitic nanodevices on the order of tens of nanometers [1]. If the tip is raised sufficiently from the substrate, in high atmospheric humidity, a water meniscus forms between the two (Fig 1). Application of an appropriate electric field between the tip and substrate dissociates the H2O molecules into H+ and OH-. The H+ ions rush towards the negatively charged tip and the OH-ions gather near the positively substrate. The oxygen reacts with the carbon in the graphitic material to form volatile or nonvolatile carbon oxides depending on the voltage applied. This oxidation, coupled with the x-y scanning capability of the AFM allows for thin structure patterning ability. Depending on such process parameters as applied voltage, pulse width, tip dimensions, contact force, and humidity, the oxidation of the graphitic material into carbon oxides enables the formation of insulating trenches or bumps to make any structure or morphology conceivable [2]. This technique can also be performed in the ambient environment, eliminating several fabrication steps, such as the poly(methyl methacrylate) (PMMA) processing required in conventional electron-beam lithography process. We have used the VAFM technique in preliminary studies to cut few layer graphene and “draw” insulating patterns on highly ordered pyrolyzed graphite (HOPG). A negative bias of 10V applied to the AFM tip with no feedback in a high humidity atmosphere created 0.5 nm deep trenches spaced 27 nm apart. Preliminary experiments have also been conducted on 50 nm diameter multi-walled carbon nanotubes. A negative bias of 5V to the AFM tip pulsed for 100 ms segmented the multi-walled nanotube at selected points. Single wall carbon nanotubes were grown using chemical vapor deposition. Graphene was mechanically exfoliated and prepared using methods described elsewhere [3] on 300 nm SiO₂ on Si substrate. The samples were connected electrically to ground and placed in an AFM system (Pacific Nanotechnology NANO-I) with environmental control. The samples were imaged in contact mode with an electrically conductive sharp AFM tip after which humidity was raised to 40–45%. Once the humidity was sufficiently raised, the tip was raised from the desired location on either the Carbon nanotubes or graphene/graphite and feedback was turned off. Patterns were drawn by the tip in this configuration with applied tip voltage running anywhere from −5V to −10V. See Figs. 2 and 3 for results on graphene and carbon nanotubes. Currently, a parametric study on AFM lithography on graphene and carbon nanotubes is underway. By varying voltage, humidity, tip speed, dwell time, and tip-substrate distance, we will determine the optimal conditions required to accomplish precise patterning of graphene and controlled segmentation of carbon nanotubes. In conclusion, we have demonstrated a voltage-applied technique utilizing an atomic force microscope tip to pattern nanoscale features on graphitic materials. A systematic study on oxidation parameters is forthcoming.