Effects of Size Dispersion Disorder on the Charge Transport in Self-Assembled 2-D Ag Nanoparticle Arrays K. C. Beverly, J. F. Sampaio,* and J. R. Heath † The California NanoSystems Institute and the UCLA Department of Chemistry and Biochemistry, 603 Charles E. Young DriVe East, Los Angeles, California 90095-1569 ReceiVed: June 13, 2001; In Final Form: October 31, 2001 Temperature-dependent DC transport measurements on self-assembled Ag nanoparticle monolayers have been obtained as a function of particle size distribution induced disorder. A transition temperature between the transport regimes of simple, activated transport to variable range hopping is observed to be in tune with increasing disorder of the monolayer. The overall transport behavior is described in terms of a mobility gap in the Anderson localized regime and an Efros-Shklovskii variable range hopping between localized states in the gap below some temperature characterized by the degree of disorder. From these results, we predict a true insulator-metal transition at narrow but still finite (<3%) size distributions. Introduction Chemically synthesized nanoparticles and quantum dots (QDs) are becoming model systems for investigating the fundamental physics 1,2 and chemistry 3 of low dimensional solids and nanoscopic materials. Scaling behavior, size and shape dependence, and quantum confinement phenomena are all areas of interest in which these systems provide a powerful and often unique environment for manipulation and investigation. A natural extension of these studies has been the exploration of the collective properties of assembled arrays of QDs. 4 The advent of synthetic techniques for preparing narrow and tunable size distributions of metal and semiconductor nanoparticles has provided building blocks for assembling ordered 2D and 3D superlattices with domain sizes on the order of hundreds of unit cells. 5,6 The nanoparticles, often passivated with organic ligands attached to their surfaces, play the role of atoms in the superlattice “artificial” solids. Several groups are now exploring properties of such QD solids, although the field remains relatively open due to the extraordinary difficulty in probing these structures in a consistent and controllable fashion. Nevertheless, reports on the transport characteristics of mag- netic, 7 noble metal, 8-10 and semiconductor nanoparticle 11 arrays as well as three-dimensional metal nanoparticle superlattices 12 have appeared in the recent literature. Over the past few years we have been investigating two- dimensional (2-D) arrays of organically passivated Ag nano- particles that are prepared at the air-water interface of a Langmuir trough. The Langmuir compression technique, coupled with the choice of the passivating ligand, can be utilized to continuously or stepwise tune the interparticle separation distance, over the range from >20 Å to about 5 Å. These QD monolayers have been characterized using a variety of tech- niques including optical, 13 impedance, 14 and tunneling spec- troscopies. 15 The electronic nature of the QD arrays can be tuned quite extensively through the manipulation of several funda- mental variables, including particle size, interparticle separation distance, and lattice order/disorder. Notably, this control has been demonstrated in the reversible metal-insulator transition in a Langmuir monolayer of silver QDs, observed at room temperature and as a function of monolayer compression on the Langmuir trough. We and other groups 7-9,12 have begun to investigate the temperature-dependent DC transport characteristics of 2D QD films. Such measurements are the standard for elucidating mechanistic information about transport in the solid state. In our case, we have reported on the preparation of up to six different devices from a single Langmuir monolayer of al- kanethiol-passivated 7.8 ((0.6) nm Ag QDs. 10 Each device corresponded to a transfer at a different point along the pressure/ area (π/A) isotherm, and thus to a different interparticle separation distance. The temperature-dependent resistance of those devices, measured over the range of ∼50 to 300 K, revealed two distinct transport regimes. For highly compressed monolayers, the films exhibited metallic conductivity (decreas- ing resistance (R) with decreasing temperature (T)) above ∼190 K, and simple, activated transport (ln(R) ∝ E µ /T, where E µ is the activation energy) at lower temperatures. Throughout the activated regime, E µ decreased steadily with lattice compression, and particles that were passivated with shorter ligands exhibited a lower E µ for similar values of compression. In addition, the temperature of minimum resistance (T min )si.e., the temperature at which the transport properties passed from metallic to insulatingsdecreased with decreasing E µ . While these measure- ments revealed a number of trends, they opened up a number of mechanistic questionssespecially as related to the importance of disorder in the transport characteristics. In this paper, we address this issue by a stepwise fine-tuning of the variable of superlattices disorder across a series of devices, while attempting to maintain similar values for the parameters of mean particle size, passivating ligand length, and lattice compression. The critical experimental handle is the width of the particle size distribution. We report on the transport properties, from 300 to ∼10 K, of six different monolayers of ∼70 Å diameter, dodecanethiol-passivated Ag QDs, in which the width of the particle size distribution was varied from 6.6% to 13.8%. Similar * Corresponding author. E-mail: heath@chem.ucla.edu. † On sabbatical leave. Permanent address: Universidade Federal de Minas Gerais, Depto. de Fisica, C. P. 702, CEP. 30123-970, Belo Horizonte, M. G., Brazil. 2131 J. Phys. Chem. B 2002, 106, 2131-2135 10.1021/jp012261g CCC: $22.00 © 2002 American Chemical Society Published on Web 02/12/2002