NANO LETTERS
Conductive Atomic Force Microscope Nanopatterning of Hydrogen-Passivated Silicon in Inert Organic Solvents
2005 Vol. 5, No. 1 91-95
C. Reagan Kinser, Matthew J. Schmitz, and Mark C. Hersam* Department of Materials Science and Engineering, Northwestern UniVersity, EVanston, Illinois 60208 Received October 20, 2004; Revised Manuscript Received December 9, 2004
ABSTRACT Ambient liquid phase atomic force microscope (AFM) techniques for nanopatterning organic molecules on silicon through direct Si−C bonds rely on reactions that are in direct competition with spurious oxidation. We study the effectiveness of an inert hydrophobic organic solvent at suppressing oxidation of hydrogen-passivated silicon under ambient conditions. Nanometer scale features were fabricated on an H:Si(111) substrate using a conductive AFM in hexadecane. The patterned features show chemical and kinetic behavior consistent with field induced oxidation (FIO) in air. The mechanism for FIO in hexadecane is discussed.
Scanning probe techniques for generating nanometer scale patterns of organic molecules on silicon substrates have attracted much interest for their potential to fabricate chemical and biological sensors and molecular electronic device structures.1-13 Recent studies of molecule-on-silicon nanostructures have demonstrated current rectification4 and negative differential resistance5 charge transport phenomena which result from the unique molecule-silicon contact.6 Strategies for fabricating molecular nanostructures with direct silicon-carbon bonds have been demonstrated on hydrogenterminated silicon surfaces using ultrahigh vacuum scanning tunneling microscope lithography7-9 and ambient atomic force microscope (AFM) techniques including field induced oxidation,10 mechanical scribing,11 and electrochemical dip pen nanolithography.12 In ambient AFM nanolithography, organic molecules are typically delivered to the substrate in the liquid phase, either by diffusion through a water meniscus12,14 or by immersing the substrate and AFM tip in a liquid carrying the desired molecules.11,15 In particular, recent reports demonstrated Si-C bond formation between unsaturated hydrocarbons and a hydrogen-terminated silicon surface patterned directly from the liquid phase in an ambient environment. In chemomechanical surface patterning, an AFM tip was used to mechanically scribe an H:Si(111) surface in liquid containing 1-alkenes or 1-alcohols.11 Upon breaking Si-Si bonds during scribing, the 1-alkene or 1-alcohol molecules in the liquid phase bind to the chemically activated silicon sites. More * Corresponding author, address: Northwestern University, Materials Science Department, 2220 Campus Drive, Evanston, IL 60208. Tel.: (847) 491-2696, FAX: (847) 491-7820, E-mail:
[email protected]. 10.1021/nl048275q CCC: $30.25 Published on Web 12/23/2004
© 2005 American Chemical Society
recently, conductive AFM tip-induced cathodic electrografting was used to pattern 1-alkynes on an H:Si(111) surface by application of a negative sample bias between the H:Si(111) sample and a conductive AFM tip coated with the neat 1-alkyne.12 In each of these strategies, reaction of organic molecules with the chemically activated silicon surface is in direct competition with water and oxygen present under the ambient processing conditions.16 In particular, it has been previously demonstrated that mechanical scribing of an H:Si(111) substrate with an AFM tip in air produces oxidized silicon patterns.17 Furthermore, AFM tip field induced oxidation (FIO) of an H:Si substrate in air is a well-known phenomenon and has been observed at both positive and negative tipsample biases.18-22 Surface oxidation by water and oxygen in organic solvents has been observed in studies of selfassembled monolayers on hydrogen terminated silicon surfaces.23,24 To minimize oxidation during these reactions, solvents and reactants are carefully dried, distilled, and deoxygenated, and the reaction is typically carried out under an inert atmosphere. While the nonpolar organic liquid environments used to pattern organic molecules on silicon surfaces are generally quite hydrophobic, there have been few reports studying the effectiveness of a hydrophobic organic liquid environment as an inhibitor to oxidation under ambient conditions. An early report of scanning-probe-based lithography on graphite showed that under ambient conditions the water content of n-octane and mineral oil was sufficient to generate features that were otherwise suppressed in a dry N2 environment.25 Herein we report that nanoscale features were patterned on an H:Si(111) surface using conductive
AFM in an inert hydrophobic organic liquid in an ambient laboratory environment. The chemical nature of these features and their dependence on patterning parameters were characterized and are shown to be consistent with AFM FIO. Finally, the mechanism for oxidation in the hydrophobic organic solvent is discussed. Si(111) substrates used in this study (Virginia Semiconductor, n-type, As-doped, resistivity < 0.01 Ω cm,
