Hyunsung Jung

Ultra-long bismuth telluride nanoribbons synthesis by lithographically patterned galvanic displacement

We demonstrated the wafer level batch synthesis and fabrication of single semiconducting thermoelectric nanoribbon based devices by Lithographically Patterned Galvanic Displacement (LPGD). The shape, composition, and dimension of nanoribbons were tailored by adjusting deposition conditions. High resolution TEM images with fast Fourier transform (FFT)-converted selected area electron diffraction (SAED) patterns confirmed the formation of polycrystalline Bi2Te3 intermetallic compound with a rhombohedral structure without elemental Te and Bi. The thickness dependent electrical resistivity of BixTey nanoribbons shows a classic size effect due to the increase in surface boundary scattering. The as-synthesized nanoribbons were n-type semiconductors with no clear trend between field effect carrier mobility and composition, which might be attributed to the trapped charges at the interface between the channel and dielectric layer. The preliminary results on thermoelectric properties (i.e. Seebeck coefficient and power factor) show that the Seebeck coefficient of as-synthesized 0.1 µm thick Bi30Te70 nanoribbon is comparable with bulk counterparts, however, the power factor was lower because of poor crystallinity which leads to higher resistivity.

The influence of polarity of electrodeposited Cu2O thin films on the photoelectrochemical performance

Both polar and non-polar Cu2O thin films were electrodeposited in electrolytes of different pH, and their photoelectrochemical activities in water were analyzed. The surface morphology of these tailored-polarity Cu2O films was analyzed by field emission scanning electron microscope (FE-SEM) and X-ray diffraction (XRD), and their photoactivity was characterized through photoluminescence and Mott–Schottky measurement of the defect structure, diffusion length, carrier concentration, and charge separation. Through this, the tetrahedral surface morphology of a polar Cu2O thin film was found to exhibit a higher photocurrent in the visible spectrum than the pyramidal surface morphology of a non-polar Cu2O film. Furthermore, the higher flat-band potential and carrier concentration identified in the polar Cu2O by Mott–Schottky analysis was found to result in a higher charge separation.

Invited paper: Composition-dependent electrical properties of ternary AgxSb1−xTey thin films synthesized by cationic exchange reaction

Ternary silver antimony telluride (AgxSb1−xTey) thin films with tailored compositions were synthesized by a cationic exchange reaction of thermally evaporated antimony telluride thin films, as a simple and costeffective approach. The composition of AgxSb1−xTey thin films was controlled by the reaction time. Temperaturedependent electrical properties of the AgxSb1−xTey thin films demonstrated phase transition behavior from 323 K to 343 K. The composition-dependent thermoelectric properties (i.e., electrical resistivity (ρ), Seebeck coefficient (S) and power factor (S2ρ)) of the as-deposited Sb54Te46, the transformed AgxSb1−xTey and the annealed AgxSb1−xTey thin films were investigated as a function of temperature.