So Young Jang

Size-dependent thermal conductivity of individual single-crystalline PbTe nanowires

We investigated the thermal conductivity of individual single-crystalline PbTe nanowires grown by a chemical vapor transport method. Thermal conductivities of PbTe nanowires 182–436 nm in diameter were measured using suspended microdevices. The thermal conductivity of a PbTe nanowire appeared to decrease with decreasing nanowire diameter and was measured to be 1.29 W/mK for a 182 nm nanowire at 300 K, which is about half of that of bulk PbTe. Our results indicate that phonon transport through a PbTe nanowire is effectively suppressed by the enhanced phonon boundary scattering due to size effects.

Three Synthetic Routes to Single- Crystalline PbS Nanowires with Controlled Growth Direction and Their Electrical Transport Properties

Single-crystalline rock-salt PbS nanowires (NWs) were synthesized using three different routes; the solvothermal, chemical vapor transport, and gas-phase substitution reaction of pregrown CdS NWs. They were uniformly grown with the [100] or [110], [112] direction in a controlled manner. In the solvothermal growth, the oriented attachment of the octylamine (OA) ligands enables the NWs to be produced with a controlled morphology and growth direction. As the concentration of OA increases, the growth direction evolves from the [100] to the higher surface-energy [110] and [112] directions under the more thermodynamically controlled growth conditions. In the synthesis involving chemical vapor transport and the substitution reaction, the use of a lower growth temperature causes the higher surface-energy growth direction to change from [100] to [110]. The high-resolution X-ray diffraction pattern and X-ray photoelectron spectroscopy results revealed that a thinner oxide-layer was produced on the surface of the PbS NWs by the substitution reaction. We fabricated field effect transistors using single PbS NW, which showed intrinsic p-type semiconductor characteristics for all three routes. For the PbS NW with a thinner oxide layer, the carrier mobility was measured to be as high as 10 cm(2) V(-1) s(-1).

Thermoelectric properties of individual single-crystalline PbTe nanowires grown by a vapor transport method

We present the thermoelectric properties of individual PbTe nanowires grown by a vapor transport method. Temperature-dependent thermopower and electrical conductivity in PbTe nanowires were investigated. A PbTe nanowire {d = 136 nm) was found to have a Seebeck coefficient of −72 µV/K and electrical conductivity of 45 S/cm at 300 K. Thermal conductivity of a PbTe nanowire {d = 290 nm) was also found to be 1.36 W/mK at 300 K. Our results demonstrate the enhanced thermoelectric properties of individual single-crystalline PbTe nanowires, which are higher than that in PbTe bulk.

Transport properties of single-crystalline n-type semiconducting PbTe nanowires

Single-crystalline PbTe nanowires were synthesized using the chemical vapor transport method. They consisted of rock-salt structure PbTe nanocrystals uniformly grown in the [100] direction. We fabricated field-effect transistors using a single PbTe nanowire, providing evidence for its intrinsic n-type semiconductor characteristics. The values of the carrier mobility and concentration were estimated to be 0.83 cm(2) V(-1) s(-1) and 8.8 x 10(17) cm(-3), respectively. The Seebeck coefficients (-72 muV K(-1)) of individual nanowires were measured to show their n-type carrier-dominated thermoelectric transport properties.

Transformation of ZnTe nanowires to CdTe nanowires through the formation of ZnCdTe–CdTe core–shell structure by vapor transport

Single-crystalline ZnTe nanowires were transformed into single-crystalline CdTe nanowires by Cd vapor transport. The composition was controlled by adjusting the reaction time, and the formation of the ZnCdTe–CdTe core–shell nanocable structure as an intermediate was observed. Their morphology and crystal structure were retained during the transformation. The progressive composition change from ZnTe to CdTe tunes the emission wavelength from the green (528 nm) to red (855 nm) color regions . We fabricated field effect transistors using the ZnTe and CdTe nanowires, providing evidence that their p-type semiconductor characteristics remain the same after the transformation.

Gas-phase substitution synthesis of Cu1.8S and Cu2S superlattice nanowires from CdS nanowires

Single-crystalline wurtzite CdS nanowires underwent gas-phase substitution to form unique superlattice cubic Cu1.8S (digenite) and hexagonal Cu2S (chalcocite) structures, using thermal evaporation of CuCl2 at 500∼600 °C. The Cu1.8S nanowires consisted of superlattices along the 〈111〉 direction, with controlled growth direction ([110], [211], [100], [111]) and superlattice periods of 1.6 and 1.9 nm. As the temperature decreased, the growth direction evolved from [100] to the higher surface-energy [110] or [211] directions, with longer superlattice periods. The Cu2S nanowires also exhibited a superlattice along the [0001] growth direction, with a periodicity of 2.7 nm and could be irreversibly converted into superlattice Cu1.8S by electron-beam irradiation.

Thermal Conductivity in Individual Single-Crystalline PbTe Nanowires

We investigated the thermal conductivity of individual single-crystalline PbTe nanowires grown by chemical vapor transport method. Suspended MEMS was utilized to precisely measure the thermal conductivity of an individual nanowire. The thermal conductivity of a PbTe nanowire with diameter of 292 nm was measured to be 1.8 W/m·K at 300 K, which is about two thirds of that of bulk PbTe. This result indicates that the thermal conduction through a PbTe nanowire is effectively suppressed by the enhanced phonon boundary scattering. As the diameter of a PbTe nanowire decreases, the corresponding thermal conductivity linearly decreases. (Received November 16, 2009)

The Optoelectronic Properties of PbS Nanowire Field-Effect Transistors

We report on the optoelectronic properties of individual PbS nanowires prepared by gas-phase substitution reaction of pregrown CdS nanowires. The PbS nanowires synthesized by this method were found to be single crystals with high quality. A combination of electron-beam lithography and a lift-off process was utilized to fabricate individual 62-nm-thick PbS nanowire field-effect transistors (FETs). The nanowire FETs showed pronounced photoconductivity under light illumination while they were highly resistive in the dark environment. The conductivity increased by more than 40 folds in the presence of light. Our results are the first demonstration of the highly efficient photoresponse of individual single-crystalline PbS nanowires and provide insights into future works on nanostructured PbS optoelectronics.

Thermoelectric properties of individual single-crystalline PbTe nanowires

We present the thermoelectric properties of individual PbTe nanowires grown by a vapor transport method. Temperature-dependent thermopower and electrical conductivity in PbTe nanowires were investigated. A PbTe nanowire {d = 136 nm) was found to have a Seebeck coefficient of −72 µV/K and electrical conductivity of 45 S/cm at 300 K. Thermal conductivity of a PbTe nanowire {d = 290 nm) was also found to be 1.36 W/mK at 300 K. Our results demonstrate the enhanced thermoelectric properties of individual single-crystalline PbTe nanowires, which are higher than that in PbTe bulk.