Kelvin probe force microscopy (KFM) was applied to two-dimensional profiling of silicon pn-structures covered with a 2 nm-thick oxide layer. The surface potential contrast between the p- and n-type regions depended on the hydrophobicity of the oxide surface when KFM imaging was conducted in air with a relative humidity of more than 50%. By decreasing the density of surface hydroxyl groups on the oxide layer through thermal annealing, the potential contrast between the p- and n-type regions increased. While there was no detectable contrast on samples covered with hydrophilic oxide with a water contact angle of almost 0°, contrast increased to greater than 50 mV on the samples covered with hydrophobic oxide with a water contact angle of about 80°. However, when KFM imaging was conducted in a dry nitrogen atmosphere with relative humidity less than 0.6%, a clear potential contrast of about 50 mV could be acquired even on samples covered with the hydrophilic oxide layer. Since samples with less adsorbed water on their surface showed greater potential contrast, contrast degradation is attributed to a shielding effect of the adsorbed water layer.
Coplanar nanostructures consisting of two different types of
organosilane monolayers have been fabricated
using scanning probe microscopes (SPMs) and served as templates for the
area-selective immobilization of various
materials. The first organosilane monolayer, which had been
uniformly prepared on a substrate, was locally degraded
through electrochemistry of adsorbed water at the junction of the
monolayer and the SPM probe. This probe-scanned region chemisorbed molecules of the second organosilane,
resulting in the creation of a self-assembled
monolayer (SAM) confined to the SPM-defined pattern. Latex
nanoparticles or proteins selectively assembled onto
this patterned SAM.
Microstructures composed of two types of organosilane self-assembled monolayers (SAMs) terminated with different functional groups have been constructed on silicon substrates covered with native oxide. Surface potential images of these microstructures were acquired by Kelvin-probe force microscopy (KFM). By chemical vapor deposition (CVD), the SAMs were fabricated from n-octadecyltrimethoxysilaneand n-(6-aminohexyl)aminopropyltrimethoxysilane (AHAPS) [H2N(CH2)6NH(CH2)3Si(OCH3)3]. Through a photolithographic technique, binary microstructures consisting of ODS/FAS and ODS/AHAPS were constructed. A surface potential contrast of the two SAMs in each of the binary microstructures was clearly detected by KFM. The surface potential of the FAS-terminated region was ca. 180 mV lower than that of the ODS-terminated region, while the potential of the AHAPSterminated region was ca. 50 mV higher. These results agree with surface potentials of the SAMs predicted from dipole moments of the corresponding precursor organosilane molecules as estimated by ab initio molecular orbital (MO) calculations. Moreover, by applying the surface potential contrasts experimentally acquired by KFM to surface potential contrast occupied area with single molecule curves theoretically derived by MO calculations, packing densities of the SAMs prepared by CVD were confirmed to be smaller than those of Langmuir-Blodgett monolayers.
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