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Field effect conductance measurements were made on individual microscopic crystals of the organic semiconductor sexithiophene (6T). These crystals, ranging from 1 to 6 molecules (2-14 nm) in thickness and from 1 to 2 µm in diameter, were deposited by thermal evaporation onto SiO 2 substrates previously patterned with closely spaced (∼400 nm) pairs of Au wires. Atomic force microscopy (AFM) of these substrates demonstrated the growth of individual 6T crystals between the wires. The resulting wire/6T/wire structures were used in a transistor configuration to probe the in-plane conductance of the crystals as a function of transverse electric field (i.e., perpendicular to the substrate). These measurements showed (1) no discernible dependence of the carrier mobility on thicknessseven down to crystals as thin as a monolayerssuggesting that much of the current is carried by the first monolayer next to the SiO 2 , (2) activated transport (E A ) 25 meV) at temperatures above 100 K but nearly temperature-independent transport from 5 to 100 K, and (3) slowly decaying currents associated with reversible charge trapping. In cases where two crystals were isolated between Au electrodes, the resultant grain boundary severely limited conduction. In general, we have developed a reproducible method for electrically contacting two-dimensional molecular assemblies and have demonstrated the utility of transport measurements, in combination with AFM imaging, for elucidating structure-transport relationships in organic materials.

Conducting probe atomic force microscopy (CPAFM) was used to measure the electrical transport characteristics of 2-14 nm thick doped crystallites of the organic semiconductor sexithiophene (6T) grown on Au and SiO 2 substrates by vacuum sublimation. To make the measurements, an AFM was modified to allow in situ switching from tapping mode imaging to point contact electrical characterization with an Au-coated tip. The crystals were characterized structurally by molecular contrast AFM imaging and consist of layers of 6T molecules oriented with their long axes nearly perpendicular to the substrate. For crystals grown on Au substrates, transport is probed through the thickness of the crystals (i.e., the vertical direction) using a CPAFM tip and the substrate as electrical contacts. On SiO 2 substrates, transport is measured parallel to the substrate between the CPAFM tip and a nanofabricated Au electrode in contact with the crystallite. The measurements on Au reveal an unexpected dependence of the conductance on crystallite thickness, namely that conductance is greatest for crystals that are three 6T layers thick, not one layer. Both the vertical and horizontal conductance measurements show nonohmic behavior which may arise from an energy barrier to charge injection at the Au-6T interface. The reproducibility of the CPAFM methodology for probing transport in these extremely thin organic crystals and the observation of nonohmic behavior underscore the importance of nanoscale transport measurements afforded by CPAFM.

The set-up of the holographic grating experiment is sketched in Figure 5. Intensity gratings with a grating constant of L = 5.1 mm were obtained by the interference of two coherent, linearly polarized laser beams in the plane of the sample. We used a continuous wave argon ion laser operating at

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