Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

dithiolates to thiol-terminated DNA [101], these gold cluster solids exhibit anonmetallic, Arrhenius-like, dependence of conductivity, r, on temperaturerðTÞ ¼ r 0 exp½ E A = ðk B TÞŠ ð19Þwhere r 0 is a constant, similar to the behavior seen in metal island films [102].This simple Arrhenius form is obeyed, rather than the ln(r) ~ T0.5 of sputteredgranular films [103] only if the cores are relatively monodisperse, and thetunnel junction lengths are not correlated with the particle sizes. Theactivation energies for charge transport correspond well to the capacitativecharging energies of the particles embedded in the array [95,98] which can beapproximated by the electrostatic energy derived by Abeles et al. [103] forcarrier generation in granular metal films (cermets) in the low-field regime:E A ¼ 1 2 E C ¼ 1 e 2 1 1ð20Þ2 4pee 0 r r þ sIn this case, E C is the Coulomb charging energy required to generate twocharged nanoparticles from two originally neutral particles embedded in amaterial of relative permittivity e. The average particle radius is r, and theaverage spacing between particle surfaces is s. It is important to note that thisis the activation energy for carrier generation, as in an intrinsic semiconductor,and not for electron transport (tunneling) through the array.The degree of electronic coupling between the metal cores strongly influencesthe tunneling rate. For alkanethiolate ligands, the conductivitydecreases exponentially with increasing ligand length [97,99,100] asrðsÞ ¼ r 0 expð Þ ð21Þb sThe electronic coupling, b, through the alkanethiolate chains falls in the rangefrom 0.6 to 1.25 A˚ 1 [97,100]. Murray and co-workers illustrated the effects ofboth carrier density and tunneling rate on sample conductivity [100]. Theyfound that for small particles with large charging energies, the carrierconcentration in the array can be dominated by intentional ‘‘doping’’ (mixedvalency) of the nanoparticle cores rather than through thermal activation,and they considered the possibility that Marcus theory activation energies(Sect. II) could describe the activated transport observed in their samples.Gold is not the only metal that can be used to synthesize arrays ofnanocrystals suitable for charge transport studies. Black et al. observed anumber of the above effects, including power-law scaling of current withvoltage and Arrhenius-like activated conductivity in arrays of Co nanocrystals[93]. Because they used a ferromagnetic metal, they were also ableto observe magnetoresistance indicative of spin-dependent tunneling in theirsamples.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.