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Langmuir 2001, 17, 7784-7788
A Novel Nanolithography Technique for Self-Assembled Monolayers Using a Current Sensing Atomic Force Microscope Jianwei Zhao and Kohei Uosaki* Physical Chemistry Laboratory, Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan Received April 30, 2001. In Final Form: September 7, 2001 A nanolithography technique for a self-assembled monolayer (SAM) using a current sensing atomic force microscope (CSAFM) is proposed. Using this method, we prepared nanometer-sized patterns on the octadecanethiol SAM by removing octadecanethiol molecules at a force of a few nanonewtons in toluene when an appropriate positive or negative bias was applied. The relation between the pattern formation and the applied bias was studied at both positive and negative polarizations. The minimum pattern size achieved by this method was 15 nm. The formation probability as a function of the applied force and the trace amount of water contained in toluene was also systematically investigated. These results suggested an electrochemical mechanism for the removal of thiol molecules.
Introduction Recently, the formation of a well-defined nanopattern on a self-assembled monolayer (SAM)1 has attracted much attention because the patterned SAM could be used in a wide variety of applications, for example, as a resist for the pattern transfer2,3 and as a template to pattern biomolecules4 or nanoparticles.5,6 Substantial progress has been made to pattern thiol SAM on gold using various procedures.1 Scanning probe lithography (SPL) is one of the most promising methods because of its high spatial resolution and feasible manipulation in a nanometer space.7-9 Most of the SPL techniques are based on either a mechanical or electrical mechanism. In the mechanical mode, the nanopattern is fabricated by mechanically removing the original SAM using an atomic force microscope (AFM) tip when the applied force is large enough (hundred times the one used in normal imaging).9-11 In the electrical mode, an additional bias is applied between a conductive probe and the substrate, and then the SAM underneath the probe is removed by a chemical or physical effect. Scanning tunneling microscope (STM) based lithography12-19 is a typical example of this kind of SPL. * To whom correspondence should be addressed. (1) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 550. (2) Bishop, A.; Nuzzo, R. G. Curr. Opin. Colloid Interface Sci. 1996, 1, 127. (3) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. M.; Burgin, T. P.; Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705. (4) Wadu-Mesthrige, K.; Xu, S.; Amro, N. A.; Liu, G. Y. Langmuir 1999, 15, 8580. (5) Mu¨ller, W. T.; Klein, D. L.; Lee, T.; Clarke, J.; McEuen, P. L.; Schultz, P. G. Science 1995, 268, 272. (6) Zheng, J.; Zhu, Z.; Chen, H.; Liu, Z. F. Langmuir 2000, 16, 4409. (7) Carpick, R. W.; Salmeron, M. Chem. Rev. 1997, 97, 1163. (8) Nyffenegger, R. M.; Penner, R. M. Chem. Rev. 1997, 97, 1195. (9) Liu, G.-Y.; Xu, S.; Qian, Y. Acc. Chem. Res. 2000, 33, 457. (10) Xu, S.; Laibinis, P. E.; Liu, G. Y. J. Am. Chem. Soc. 1998, 120, 9356. (11) Amro, N. A.; Xu, S.; Liu, G. Y. Langmuir 2000, 16, 3006. (12) Schoer, J. K.; Zamborini, F. P.; Crooks, R. M. J. Phys. Chem. 1996, 100, 11086. (13) Gorman, C. B.; Carroll, R. L.; He, Y.; Tian, F.; Fuierer, R. Langmuir 2000, 16, 6312. (14) Kim, Y. T.; Bard, A. J. Langmuir 1992, 8, 1096.
Compared with the AFM-based mechanical lithography, the latter mechanism seems to be much more complicated. First, the trace water in air or solvent seriously affects the patterning performance.12,13 Second, the patterning performance and mechanism are also dominated by the bias. At a small bias, the tip may penetrate into the SAM and apply a large force; hence, the patterning under this condition is more likely to be ascribed as a mechanical removal.14 On the other hand, the large bias is powerful enough to pattern the SAM, but we are still unclear if the mechanical interaction can be neglected, especially when we use the self-assembly molecule with a long chain.15-17 Third, the pattering performance seems to be sensitive to the polarization direction. Up to now, there has been no report on the patterning of SAM at a negative bias by STM even though several groups have attempted to do so.15-18 Despite these complexities, the electrical mode of SPL, which employs bias to remove the molecules so that a large mechanical force can be avoided, seems to be more attractive. Thus, it has been extensively studied in different environments, including ultrahigh vacuum (UHV),17,19 air,12,15 and liquid.13,16 In this paper, we propose a novel technique based on a current-sensing atomic force microscope (CSAFM) to form the nanometer-sized pattern on SAM-coated gold. In this technique, a well-defined nanopattern can be formed by removing the alkanethiol molecules by applying a sufficient bias at a constant force in an inert solvent. By combination of the features of AFM and STM, CSAFM can separately control the force and bias during patterning, thus enabling us to investigate in detail the patterning performance as a function of the applied bias and force. Our results indicate that the CSAFM-based SPL is a very promising method to fabricate and characterize the nanopattern. (15) Schoer, J. K.; Crooks, R. M. Langmuir 1997, 13, 2323. (16) Chen, J.; Reed, M. A.; Asplund, C. L.; Cassell, A. M.; Myrick, M. L.; Rawlett, A. M.; Tour, J. M.; Van Patten, P. G. Appl. Phys. Lett. 1999, 75, 624. (17) Mizutani, W.; Ishida, T.; Tokumoto, H. Langmuir 1998, 14, 7197. (18) Gratratzke, U.; Simon, G. Phys. Rev. B 1995, 52, 8535. (19) Hartwich, J.; Sundermann, M.; Kleineberg, U.; Heinzmann, U. Appl. Surf. Sci. 1999, 144-145, 538.
10.1021/la010635r CCC: $20.00 © 2001 American Chemical Society Published on Web 11/10/2001
A Nanolithography Technique for SAMs
Langmuir, Vol. 17, No. 25, 2001 7785
Experimental Section Octadecanethiol, CH3(CH2)17SH (C18SH) (Tokyo Chemical, >97%), and toluene (Wako Pure Chemicals, >99.0%, water concentration
