SAM Technique for Molecular Electronic Devices: Nanoscale Patterning and its Electrical Conduction
Takao Ishidaa, b, Wataru Mizutania, Nami Choic and Hiroshi Tokumoto*, a
aJoint Research Center for Atom Technology, National Institute for Advanced Interdisciplinary Research 1-1-4 Higashi, Tsukuba, Ibaraki, 305-8562, Japan. bPRESTO-Japan Science and Technology Corporation. cJoint Research Center for Atom Technology, Angstrom Technology Partnership 1-1-4 Higashi, Tsukuba, Ibaraki, 305-8562, Japan.
Development of technology frequently requires revision of previous attempts. When an idea of ýMolecular Devicesţ was first proposed, it was very difficult to characterize the structures and functions in the molecular level. An invention of scanning probe (SPM) techniques such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM) has changed the situation and has enabled us to observe and even to manipulate single molecules. Now the idea of the molecular devices attracts many researchers again. The important factor for fabricating molecular devices is to control both the position and orientation of molecules precisely. These requirements may be satisfied by patterning self-assembled monolayers (SAMs). There are various patterning techniques of SAMs such as the direct molecule-removal by an SPM-tip, the SAM formation on a pre-patterned substrate and the control of the phase separation. In this talk, we shall take the phase separation technique paying attention to patterning molecules and to characterizing its electrical conduction.
We have chosen mixed-SAMs on Au(111) substrates, consisting of conjugated conductive molecules, [1,1':4',1''-Terphenyl]-4-methanethiol (TP), and insulative n-alkanethiols (Cn). First an uniform SAM of C9 was formed on a Au substrate in the solution containing C9 molecules, whose STM images exhibited molecular resolution and local defects. Then this sample was immersed into the solution of TP molecules. STM images of this surface revealed that the TP molecules adsorbed mainly on the uncovered areas such as defects and domain boundaries of the pre-assembled C9-SAM and formed small TP domains there. It is known that the alkanethiol SAMs were well-ordered and partially desorbed by annealing. Indeed we have found that C9 and TP are phase-separated and TP domains form nanoscale structures after annealing at 85 C for 3 hours in vacuum. It should be noted here that the apparent height difference of the TP domains in the STM image is not uniform, but changes depending on the size of domains: the large TP domains appear higher than the small domains. By analyzing the dependence of the measured height of the TP domains on their lateral sizes, we could deduce the electronic conductivity of the molecular domains. The vertical and lateral conductions of the TP-SAMs are estimated to be about 0.02 S/cm and 0.01 S/cm, respectively.  These values seem to be very large compared with values of 10-10 - 10-13 S/cm reported for the most molecular materials such as TPs. We could expect a carrier doping from the Au substrate to the molecules so as to enhance the electrical conduction.
We are grateful to Dr. Uichi Akiba and Prof. Masamichi Fujihira for providing us valuable molecules and discussions. This work was supported by the New Energy and Industrial Development Organization (NEDO).
T. Ishida, W. Mizutani, U. Akiba, K. Umemura, A. Inoue, N. Choi, M. Fujihira and H. Tokumoto,J. Phys. Chem. B103(1999) 1686.