Michael D. Fischbein

DNA Translocation through Graphene Nanopores

We report on DNA translocations through nanopores created in graphene membranes. Devices consist of 1-5 nm thick graphene membranes with electron-beam sculpted nanopores from 5 to 10 nm in diameter. Due to the thin nature of the graphene membranes, we observe larger blocked currents than for traditional solid-state nanopores. However, ionic current noise levels are several orders of magnitude larger than those for silicon nitride nanopores. These fluctuations are reduced with the atomic-layer deposition of 5 nm of titanium dioxide over the device. Unlike traditional solid-state nanopore materials that are insulating, graphene is an excellent electrical conductor. Use of graphene as a membrane material opens the door to a new class of nanopore devices in which electronic sensing and control are performed directly at the pore.

Electron beam nanosculpting of suspended graphene sheets

We demonstrate high-resolution modification of suspended multilayer graphene sheets by controlled exposure to the focused electron beam of a transmission electron microscope. We show that this technique can be used to realize, on time scales of a few seconds, a variety of features, including nanometer-scale pores, slits, and gaps that are stable and do not evolve over time. Despite the extreme thinness of the suspended graphene sheets, extensive removal of material to produce the desired feature geometries is found not to introduce long-range distortion of the suspended sheet structure.

Efficient polymer-nanocrystal quantum-dot photodetectors

We have realized highly efficient photodetectors based on composites of the semiconducting polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] and PbSe nanocrystal quantum dots. The external quantum efficiency in these devices is greater than 1 for electric fields E∼7×105V∕cm. The observed photocurrent gain could be attributed to the carrier multiplication in PbSe nanocrystal quantum dots via multiple exciton generation, and the efficient charge conduction through the host polymer material. This photocurrent gain is observed only when the PbSe nanocrystal band gap is at least three times smaller than the optical energy gap of the active polymer material.

CdSe nanocrystal quantum-dot memory

Memory effects in the electronic transport in CdSe nanocrystal (NC) quantum-dot arrays have been observed and characterized. Conduction through a NC array can be reduced with a negative voltage and then restored with a positive voltage. Light can also be used to restore or even increase the NC array conduction. We have studied the switching of the conduction in CdSe NC arrays and found the behavior to be highly sensitive to the value and duration of the laser and voltage pulses.

Nanogaps by direct lithography for high-resolution imaging and electronic characterization of nanostructures

We report a method for fabricating nanogaps directly with electron beam lithography (EBL). The primary resolution-limit of EBL, electron back-scattering, is reduced dramatically by using a thin-film as a substrate. We show that this resolution enhancement allows one to fabricate metal electrodes with separation from arbitrarily large to under one nanometer. Furthermore, because these nanogaps are on a thin film, they can be imaged with high-resolution transmission electron microscopy (HRTEM). Using these nanogaps we measured the charge transport through several coupled PbSe nanocrystals and correlated the data with detailed structural information obtained by performing HRTEM on the same device.

Controlling Nanogap Quantum Dot Photoconductivity through Optoelectronic Trap Manipulation

Nanoscale devices are being extensively studied for their tunable electronic and optical properties, but the influence of impurities and defects is amplified at these length scales and can lead to poorly understood variations in characteristics of semiconducting materials. By performing a large ensemble of photoconductivity measurements in nanogaps bridged by core-shell CdSe/ZnS semiconductor nanocrystals, we discover optoelectronic methods for affecting solid-state charge trap populations. We introduce a model that unifies previous work and transforms the problem of irreproducibility in nanocrystal electronic properties into a reproducible and robust photocurrent response due to trap state manipulation. Because traps dominate many physical processes, these findings may lead to improved performance and device tunability for various nanoscale applications through the control and optimization of impurities and defects.

Monolayer Suppression of Transport Imaged in Annealed PbSe Nanocrystal Arrays

We use correlated electrostatic force, transmission electron, and atomic force microscopy (EFM, TEM, and AFM) to visualize charge transport in monolayers and up to five layers of PbSe nanocrystal arrays drop-cast on electrode devices. Charge imaging reveals that current paths are dependent on the locally varying thickness and continuity of an array. Nanocrystal monolayers show suppressed conduction compared to bilayers and other multilayers, suggesting a departure from linear scaling of conductivity with array thickness. Moreover, multilayer regions appear electrically isolated if connected solely by a monolayer. Partial suppression is also observed within multilayer regions that contain narrow junctions only several nanocrystals wide. High-resolution TEM structural imaging of the measured devices reveals a larger reduction of inter-nanocrystal spacing in multilayers compared to monolayers upon vacuum-annealing, offering a likely explanation for the difference in conductivity between these two cases. This restriction of transport by monolayers and narrow junctions is an important factor that must be addressed in future designs of optoelectronic devices based on nanocrystals.