Charge transport in PbSe nanocrystal arrays

Abstract

We report electrical transport measurements of arrays of PbSe nanocrystals forming the channels of field effect transistors. We measure the current in these devices as a function of source-drain voltage, gate voltage and temperature. Annealing is necessary to observe measurable current after which a simple model of hopping between intrinsic localized states describes the transport properties of the nanocrystal solid. We find that the majority carriers are holes, which are thermally released from acceptor states. At low source-drain voltages, the activation energy for the conductivity is given by the energy required to generate holes plus the activation over barriers resulting from site disorder. At high source-drain voltages the activation energy is given by the former only. The thermal activation energy of the zero-bias conductance indicates that the Fermi energy is close to the highest-occupied valence level, the 1S h state, and this is confirmed by field-effect measurements, which give a density of states of approximately eight per nanocrystal as expected from the degeneracy of the 1S h state.PACS numbers:Colloidal semiconductor nanocrystals can be made to self assemble into a close-packed array, creating a novel material known as a nanocrystal (NC) solid. 1,2,3,4,5,6 Electrons in an NC solid made from semiconductor NCs, in contrast to metallic ones, have long-range Coulomb interactions; 7 therefore, the motion of electrons is expected to be highly correlated as long as the number of electrons per NC is not too small. The ability to tune both the energy levels of the individual NCs and the electronic coupling between NCs makes these solids a promising test bed for investigating many-body physics. It has been demonstrated that the charge density in semiconductor NC solids can be modulated, 8 which makes the system suitable for observing the predicted characteristics of electronic correlations as a function of charge density. 9We study PbSe NC solids because they have higher conductances than the well-studied solids composed of II-VI NCs. 10,11,12,13,14,15 PbSe NCs display a narrower dispersion in electronic energy levels than the II-VI NCs as well as a higher degeneracy, 16 which increases the density of states available for conduction. 8,17 It is possible that charge carriers in PbSe NC solids, which are holes, are generated by thermal excitation of electrons into midgap states that arise from dangling bonds on the surface. Bulk PbSe has midgap states closer to the band edge than bulk CdSe, and thus we expect a higher density of charge carriers in PbSe NCs. Furthermore, PbSe NCs have attracted much attention because of their interband transitions in the IR and multiple exciton generation, 18 and hence potential for application in novel optoelectronic devices. 19,20,21 Such applications involve electronic transport through NC solids, thus necessitating a fundamental understanding of the conduction properties.Experimental studies of charge transport in PbSe NC solids have revealed varied behavior.It has been rep...

Charge Transport in Cu2S Nanocrystals Arrays: Effects of Crystallite Size and Ligand Length

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