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...

We present the first semiconductor nanocrystal films of nanoscale dimensions that are electrically conductive and crack-free. These films make it possible to study the electrical properties intrinsic to the nanocrystals unimpeded by defects such as cracking and clustering that typically exist in larger-scale films. We find that the electrical conductivity of the nanoscale films is 180 times higher than that of drop-cast, microscopic films made of the same type of nanocrystal. Our technique for forming the nanoscale films is based on electron-beam lithography and a lift-off process. The patterns have dimensions as small as 30 nm and are positioned on a surface with 30 nm precision. The method is flexible in the choice of nanocrystal core-shell materials and ligands. We demonstrate patterns with PbS, PbSe, and CdSe cores and Zn(0.5)Cd(0.5)Se-Zn(0.5)Cd(0.5)S core-shell nanocrystals with a variety of ligands. We achieve unprecedented versatility in integrating semiconductor nanocrystal films into device structures both for studying the intrinsic electrical properties of the nanocrystals and for nanoscale optoelectronic applications.

We report the influence of trap states on charge transport through films of mixed CdTe and CdSe nanocrystals ͑NCs͒ between lateral electrodes, through layered films of CdTe and CdSe NCs in a layered geometry, and through films of CdTe/CdSe nanobarbells in a layered geometry. We find that an electron trapping state on the surface of the CdTe NCs dominates the conduction in all devices studied. X-ray photoelectron spectroscopy and thermal activation studies implicate unpassivated or oxidized Te as the electron-trapping site.

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We measure charge transport in hydrogenated amorphous silicon (a-Si:H) using a nanometer scale silicon MOSFET as a charge sensor. This charge detection technique makes possible the measurement of extremely large resistances. At high temperatures, where the a-Si:H resistance is not too large, the charge detection measurement agrees with a direct measurement of current. The device geometry allows us to probe both the field effect and dispersive transport in the a-Si:H using charge sensing and to extract the density of states near the Fermi energy.

Abstract:We demonstrated a technique to control the placement of 6-nm-diameter CdSe and 5-nm-diameter CdSe/CdZnS colloidal quantum dots (QDs) through electron-beam lithography. This QD-placement technique resulted in an average of three QDs in each cluster, and 87% of the templated sites were occupied by at least one QD. These QD clusters could be in close proximity to one another, with a minimum separation of 12 nm.Photoluminescence measurements of the fabricated QD clusters showed intermittent photoluminescence, which indicates that the QDs were optically active after the fabrication process. This optimized top-down lithographic process is a step towards the integration of individual QDs in optoelectronic and nano-optical systems. Manuscript Preprint 3

Contact effects are a common impediment to electrical measurements throughout the fields of nanoelectronics, organic electronics, and the emerging field of graphene electronics. We demonstrate a novel method of measuring electrical conductance in a thin film of amorphous germanium that is insensitive to contact effects. The measurement is based on the capacitive coupling of a nanoscale metal-oxide-semiconductor field-effect transistor (MOSFET) to the thin film so that the MOSFET senses charge diffusion in the film. We tune the contact resistance between the film and contact electrodes and show that our measurement is unaffected. With the MOSFET, we measure the temperature and field dependence of the conductance of the amorphous germanium, which are fit to a model of variable-range hopping. The device structure enables both a contact-independent and a conventional, contact-dependent measurement, which makes it possible to discern the effect of the contacts in the latter measurement. This measurement method can be used for reliable electrical characterization of new materials and to determine the effect of contacts on conventional electron transport measurements, thus guiding the choice of optimal contact materials.

We investigate the effect of electrostatic screening on a nanoscale silicon MOSFET electrometer. We find that screening by the lightly doped p-type substrate, on which the MOSFET is fabricated, significantly affects the sensitivity of the device. We are able to tune the rate and magnitude of the screening effect by varying the temperature and the voltages applied to the device, respectively. We show that despite this screening effect, the electrometer is still very sensitive to its electrostatic environment, even at room temperature.