Low-Voltage Electron Transport in Self-Assembled Nanocrystal Arrays
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Low-Voltage Electron Transport in Self-Assembled Nanocrystal Arrays C. T. Black, C. B. Murray, R. L. Sandstrom, Shouheng Sun IBM T. J. Watson Research Center, Yorktown Heights, NY 10598 ABSTRACT Electrons traverse two-dimensional nanocrystal arrays by sequential tunneling between neighboring nanocrystals. Analysis of array conductance at zero bias-voltage gives information about underlying nanocrystal uniformity, as well as the relevant single-electron charging energy. We discuss low-temperature measurements of two-dimensional self-assembled superlattices composed of 10 nanometer diameter cobalt nanocrystals, with ~2 nm inter-nanocrystal spacing. INTRODUCTION Electron transport through two-dimensional (2D) arrays has long been a subject of experimental interest. The body of experimental work in this area can be broadly separated into two categories, based on the method of sample fabrication. In one case it is possible to fabricate 2D arrays using lithographic means (for example, [1, 2]). In these experiments, array properties are controlled precisely, however dimensions are restricted to larger than ~100 nm. In another experimental realization, electronic conduction in metallic granular thin-films can be understood in terms of a highly non-uniform array [3, 4]. In this system the characteristic length scale can be on ~1-10 nm (the average grain size in the film), however array properties such as number of nearest neighbors and intergrain spacing are not well-controlled. By using self-assembly of chemically-synthesized nanocrystals, we combine the precision of lithographically-defined arrays with the nm size-scale of granular thin films. We discuss the low-voltage conductance of our devices, and its relationship to the uniformity of our nanocrystal building blocks. EXPERIMENT A full discussion of fabrication and electrical measurements of these magnetic nanocrystal superlattices has already been detailed [5]. We fabricate our tunneling devices by first using electron beam lithography and metal liftoff of a 15 nm thick AuPd film. The resulting electrodes are roughly 100 nm wide, and are spaced ~100 nm apart (Fig. 1a). Subsequent deposition of a nanocrystal monolayer on top of the electrodes (Fig. 1b) results in a hexagonal array situated between the electrodes (as well as everywhere on the wafer). For this particular device, the shortest path between the two electrodes contains ~15 nanocrystals in series. Nanocrystal diameters are 10 nm (±5%) in this experiment, and analysis by transmission electron microscopy (TEM) shows a 2 nm inter-nanocrystal spacing. Details of the cobalt nanocrystal synthesis can be found elsewhere [6]. We form nanocrystal monolayers using a Langmuir-Blodgett technique [7]; we deposit a drop of solvent (hexane) containing the nanocrystals onto a water surface. After the solvent evaporates, the nanocrystals remain confined to a single layer due to surface tension. We can then compress the 2D film to form large areas of D10.7.1
Figure 1. (a) SEM image of nanocrystal superlattice device. 100 nm A
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