Single Electron Transport in Metal Nanoparticle Decorated Biopolymers
M. N. Wybourne*, a, C.A. Bervena, L. Clarkea, c, J.E. Hutchisonb, J.L. Moosterb
aDepartment of Physics and Astronomy, Dartmouth College,
Hanover, NH 03755, U.S.A. bDepartment of Chemistry, University of Oregon
Eugene, OR 97403, U.S.A. cPresent address: Department of Physics, University of Colorado, Boulder, USA
Molecular based systems are receiving considerable attention as a potential technology to overcome the challenges of continued miniaturization faced by conventional semiconductor devices. For molecular systems to become a viable alternative, many questions need to be addressed, including their defect tolerance. Toward this goal, we present room temperature electrical transport measurements of metal nanoparticle decorated biopolymers arranged randomly on a surface. The conductance has well-resolved features that we discuss in terms of nanoparticle size dispersion, background charge and chain orientation disorder. The samples we have studied are networks of gold nanoparticles between the fingers of interdigitated array electrodes. The networks were fabricated by a straightforward wet chemical procedure that involves the electrostatic assembly of carboxylic acid modified gold nanoparticles onto the amino side chains of the biopolymer poly-L-lysine (PLL). The metal-core radius of the nanoparticles was determined to be 0.7 ± 0.2 nm (±30%) by TEM, and the diameter of the core and ligand shell together is estimated to be 4.2 nm. AFM images of similar samples on mica showed that the fabrication procedure produced extended, chain-like assemblies with a surface coverage below that required to form a continuous path between the electrodes. The room temperature current-voltage (I-V) characteristic of these samples has pronounced non-linear behavior. After the subtraction of a linear I-V background obtained from control measurements of the PLL before nanoparticle decoration, all samples had a region of low-conductance (zero to within experimental accuracy) at low voltages. The onset of current is characterized by a threshold voltage, VT, above which the increase in current is described by the scaling relationship I ~ (V/VT - 1)g , where the exponent is found to be g = 1.2 ± 0.2. Structure of period Vp was observed in the conductance of the samples, with the ratio Vp/VT ~ 2. This behavior provides strong evidence for single electron charging in one-dimensional arrays of nanoparticles at room temperature. Further evidence is obtained from observed changes in the position, but not period, of the conductance features. These are likely caused by differences in the background charge distribution from measurement to measurement. One unusual aspect of the data is the fact that the voltage scale of Vp and VT are at least an order of magnitude larger than expected for single electron effects. We will show that this is related to the nature of the conduction path, which includes potential drops due to the substrate surface-conduction. Finally, the electrode arrangement is expected to provide substantial averaging over the random orientation of the chain-like structures and over disorder, which arises from dispersion of the metal core size and particle-particle spacing. We will discuss simulations that show that the orientation and size disorder are insufficient to remove the conductance features.