Wet-Process Molecular Planting in A Specific Site on Silicon with Si-C Covalent Bonds
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Wet-Process Molecular Planting in A Specific Site on Silicon with Si-C Covalent Bonds Hirokazu Tada1, 2, Masato Ara2 and Shoji Tanaka1,2 Institute for Molecular Science, Okazaki National Research Institutes Myodaiji, Oakzaki 444-8585, Japan 2 Department of Structural Molecular Science, The Graduate University for Advanced Studies Myodaiji, Okazaki 444-8585, Japan 1
ABSTRACT Alkyl monolayers anchored covalently on silicon were prepared through the reaction between 1-alkenes and hydrogen-terminated silicon (111) surfaces. The 2D-patterning of the surface was done by local oxidation with an atomic force microscope (AFM) and 3D-molecular assemblies were fabricated by immobilization of molecules in a specific site of the patterned surface. The surfaces were anodized with a contact-mode AFM by applying a positive bias voltage to the surface with respect to the platinum-coated cantilever under ambient conditions, which resulted in nanometer-scale oxidation of surfaces. The anodized areas were etched and terminated with hydrogen atoms by NH4F solution, in which we could immobilize various molecules having C=C bonds. We put arylamine molecules to which organic dyes such as fluoroscein were anchored. The intensity of luminescence varied depending on dopant concentration of substrates. Luminescence was very weak on highly-doped silicon possibly due to effective energy transfer from dyes to substrates. The method demonstrated is one of the promising ways to fabricate 3D-assemblies of molecular-scale electronic devices with a stable interface on silicon. INTRODUCTION Much attention has recently been paid to preparation and characterization of molecular-scale electronic devices [1]. Molecular assemblies based on thiolate / gold systems have been intensively studied as one of the most promising ways to fabricate nanometer scale devices. But the interface between Au and S atoms has not yet been well characterized. In addition, the method to put the molecule in the specific sites of metal electrodes has not been established. One promising way to overcome these difficulties is to assemble organic molecules on inorganic semiconductors including silicon through covalent bonds such as Si-C. A number of dry [2-5] and wet [2, 6-11] processes have been examined to provide organic/inorganic systems based on Si-C covalent bonds. The advantage of Si-C systems comparing to Au-S systems is as follows: (a) the interface is thermally and chemically stable, (b) the substrates with various dopant levels are available, and (c) micro- and nano-fabrication techniques established in the field of semiconductor devices are applicable. In the previous papers, we demonstrated the preparation of highly ordered organic monolayers anchored covalently to Si(111) and 2-dimensional nano-patterning using an atomic force microscope (AFM) [12, 13]. Figure 1 shows nano-patterning and chemical modification of a silicon surface which is covered with alkyl monolayers. Nanometer-scale anodization of the surface was done using contact-mode AFM (Jeol, JSPM-1300)
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