NANO LETTERS
Formation of Nanopatterns of a Self-Assembled Monolayer (SAM) within a SAM of Different Molecules Using a Current Sensing Atomic Force Microscope
2002 Vol. 2, No. 2 137-140
Jianwei Zhao and Kohei Uosaki* Physical Chemistry Laboratory, DiVision of Chemistry, Graduate School of Science, Hokkaido UniVersity, Sapporo, Japan 060-0810 Received October 2, 2001; Revised Manuscript Received November 21, 2001
ABSTRACT Nanosized patterns of an alkanethiol self-assembled monolayer (SAM) were prepared within a SAM of a different alkanethiol on a gold substrate using a current sensing atomic force microscope (CSAFM). When the conducting tip with a sufficient bias was scanned over a SAM in toluene solution containing an alkanethiol of a different chain length, the original SAM underneath the tip was removed and a new nanospaceconfined SAM of the alkanethiol in toluene was formed. The formation of the nanopattern was ascertained by the measurements of the conductivity distribution and current-bias relation by the CSAFM.
The fabrication and modification of nanopatterns on a selfassembled monolayer (SAM)1 have attracted much attention due to the wide variety of potential applications such as electronic devices.2-4 Nanopatterns of SAM, e.g., squares and stripes, can be fabricated by a “dig-down” method, i.e., decomposing or scratching the SAM using an electron beam,5 UV light,6 or scanning probe microscope (SPM) tip.7-11 Recently, several methods based on a “built-up” strategy to fabricate the SAM nanopattern such as microcontact printing,12 micrografting,13 and dip-pen nanolithography14,15 have been developed. Among the many “built-up” methods, SPMbased techniques are particularly attractive because of the high spatial resolution and precise control of the tip position.13,16,17 Significant progress has been achieved using an atomic force microscope (AFM)13 and scanning tunneling microscope (STM).11 We recently proposed a novel current sensing atomic force microscope (CSAFM)-based nanolithography that can generate a blank nanospace within a uniform alkanethiol SAM.18 One of the most important advantages of the SPM-based methods over the “dig-down” methods is the possibility to functionalize the nanopattern by using the molecules with specific functions. The functionalized nanopattern can be further used to attach other molecules with different functions, nanoparticles,19 and/or proteins.20 In this paper, we report the formation of nanosized patterns * Corresponding author. E-mail:
[email protected]. 10.1021/nl0100769 CCC: $22.00 Published on Web 12/22/2001
© 2002 American Chemical Society
of SAMs of alkanethiols of various chain lengths (CH3(CH2)nSH, n ) 5 (C6SH), 7 (C8SH), and 9 (C10SH)) within a SAM of CH3(CH2)17SH (C18SH) or of C18SH within a SAM of C10SH on a gold substrate by CSAFM-based lithography. An evaporated gold film on mica with the (111) oriented surface was used as the substrate. It was annealed by a hydrogen flame to increase the morphological order.21 The substrate was dipped in ca. 1 mM C18SH or C10SH ethanol solution for 24 h so that a C18SH SAM or C10SH was formed on the surface. It was then fully rinsed with ethanol and dried under a superpure nitrogen flow. CSAFM (PicoSPM, Molecular Imaging, MI) controlled by a PicoScan control station (MI) was used for both the formation and characterization of the nanopatterns. The C18SH or C10SH SAM-modified gold substrate was mounted on a sample plate fixed to a home-built liquid cell that was placed in a microscope chamber (MI) filled with saturated toluene vapor (temperature 20 ( 2 °C and relative humidity 20-30%). The nanofabrication and characterization were carried out in water-saturated toluene containing ca. 10 mM alkanethiols using a Pt-coated Si3N4 tip with a spring constant of 0.12 N/m (MikroMasch). The tip was scanned with a
