Controlled placement of colloidal quantum dots in sub-15 nm clusters.

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.

Nanopatterned electrically conductive films of semiconductor nanocrystals.

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.

Contact-independent measurement of electrical conductance of a thin film with a nanoscale sensor.

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.

The effect of electrostatic screening on a nanometer scale electrometer.

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.

Measuring charge transport in a thin solid film using charge sensing.

We measure charge transport in a hydrogenated amorphous silicon (a-Si:H) thin film using a nanometer scale silicon MOSFET as a charge sensor. This charge detection technique makes possible the measurement of extremely large resistances even in the presence of blocking contacts. At high temperatures, where the resistance of the a-Si:H 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.