Effects of cross-sectional area on the tunneling-junction array in octahedral PbSe colloidal-nanocrystal solids.

Octahedral PbSe colloidal nanocrystals (NCs) are used to assemble a solid. Because of the special feature of the apexes of the octahedrons, the cross-sectional area of the inter-dot tunneling junctions is much smaller than that formed between spherical NCs. The inter-dot separation between NCs is easily adjusted by mild thermal treatment. Like a spherical NC-solid, the resistance of the octahedral NC-solid is exponentially dependent on the inter-dot separation. On the contrary, due to the difference in the cross-sectional area between the NCs, electron transport in the octahedral NC-solid does not follow the same model used for the explanation of electron transport in a spherical NC-solid. Through analyses of current-voltage and resistance-temperature behaviors, we have confirmed that the model of fluctuation-induced tunneling conduction fits very well with all of the data and explains the variation in the electrical properties of octahedral PbSe colloidal NC-solids after thermal annealing.

Dielectrophoretic placement of quasi-zero-, one-, and two-dimensional nanomaterials into nanogap for electrical characterizations.

DEP is one of promising techniques for positioning nanomaterials into the desirable location for nanoelectronic applications. In contrast, the lithography technique is commonly used to make ultra-thin conducting wires and narrow gaps but, due to the limit of patterning resolution, it is not feasible to make electrical contacts on ultra-small nanomaterials for a bottom-up device fabrication. Thus, integrating the lithography and dielectrophoresis, a real bottom-up fabrication can be achieved. In this work, the device with the nanogap in between two nanofinger-electrodes is made using electron-beam lithography from top down and the ultra-small nanomaterials, such as colloidal PbSe quantum dots, polyaniline nanofibers, and reduced-graphene-oxide flakes, are placed in the nanogap by DEP from bottom up. The threshold electric field for the DEP placement of PbSe nanocrystals was roughly estimated to be about 8.3 × 10(4) V/cm under our experimental configuration. After the DEP process, several procedures for reducing contact resistances are attempted and measurements of intrinsic electron transport in versatile nanomaterials are performed. It is experimentally confirmed that electron transport in both PbSe nanocrystal arrays and polyaniline nanofibers agrees well with Prof. Ping Sheng's model of granular metallic conduction. In addition, electron transport in reduced-graphene-oxide flakes follows Mott's 2D variable-range-hopping model. This study illustrates an integration of the electron-beam lithography and the DEP techniques for a precise manipulation of nanomaterials into electronic circuits for characterization of intrinsic properties.

Study of electrical characterization of 2-methyl-9, 10-di(2-naphthyl)anthracene doped with tungsten oxide as hole-transport layer.

An efficient p-doped transport layer composed of an ambipolar material, 2-methyl-9,10-di(2-naphthyl)anthracene (MADN) and tungsten oxide (WO(3)) has been developed. The admittance spectroscopy studies show that the incorporation of WO(3) into MADN can greatly improve the hole injection and the conductivity of the device. Moreover, when this p-doped layer was incorporated in the tris(8-quinolinolato)aluminum-based device, it achieved a current efficiency of 4.0 cdA and a power efficiency of 2.4 lmW at 20 mAcm(2). This work paves the way to simplify the fabrication of future p-i-n organic light-emitting devices with a single common ambipolar MADN material.

Analysis of InAsN quantum dots by transmission electron microscopy and photoluminescence.

Quantum dots (QDs) have great potential in optical fiber communication applications were widely recognized. The structure of molecular beam epitaxy (MBE) grew InAsN QDs were investigated by transmission electron microscopy (TEM) and measured their optical properties by photoluminescence (PL). TEM images show that the InAsN QDs are irregular or oval shaped. Some of the InAsN QDs are observed to have defects, such as dislocations at or near the surface in contrast to InAs QDs, which appear to be defect free. PL results for InAsN QDs showed a red-shifted emission peak. In addition, the InAsN emission peak is broader than InAs QDs, which supports the TEM observation that the size distribution of the InAsN QDs is more random than InAs QDs. The results show that the addition of nitrogen to InAs QDs leads to a decrease in the average size of the QDs, bring changes in the QD's shape, compositional distribution, and optical properties.