Validity of rigid band approximation of PbTe thermoelectric materials

Yoshiki Takagiwa, Yanzhong Pei, Gregory Pomrehn, and G. Jeffrey Snyder

The tuning of carrier concentration through chemical doping is very important for the optimization of thermoelectric materials. Traditionally, a rigid band model is used to understand and guide doping in such semiconductors, but it is not clear whether such an approximation is valid. This letter focuses on the changes in the electronic density of states (DOS) near the valence band maximum for different p-type dopants (Na, K, Tl, or vacancy on Pb site) maintaining the high symmetry of the NaCl structure. Na- and K-doped, and vacancy-introduced PbTe show a clear rigid-band like change in DOS unlike that concluded from supercell based calculations.

APL Mater. 1, 011101 (2013) | HTML | PDF

Transparent, flexible, and solid-state supercapacitors based on graphene electrodes

Y. Gao, Y. S. Zhou, W. Xiong, L. J. Jiang, M. Mahjouri-samani, P. Thirugnanam, X. Huang, M. M. Wang, L. Jiang, and Y. F. Lu

In this study, graphene-based supercapacitors with optical transparency and mechanical flexibility have been achieved using a combination of poly(vinyl alcohol)/phosphoric acid gel electrolyte and graphene electrodes. An optical transmittance of ∼67% in a wavelength range of 500-800 nm and a 92.4% remnant capacitance under a bending angle of 80° have been achieved for the supercapacitors. The decrease in capacitance under bending is ascribed to the buckling of the graphene electrode in compression. The supercapacitors with high optical transparency, electrochemical stability, and mechanical flexibility hold promises for transparent and flexible electronics.

APL Mater. 1, 012101 (2013) | HTML | PDF

High quality transparent TiO₂/Ag/TiO₂ composite electrode films deposited on flexible substrate at room temperature by sputtering

Aritra Dhar and T. L. Alford

Multilayer structures of TiO₂/Ag/TiO₂ have been deposited onto flexible substrates by room temperature sputtering to develop indium-free transparent composite electrodes. The effect of Ag thicknesses on optical and electrical properties and the mechanism of conduction have been discussed. The critical thickness (tc) of Ag mid-layer to form a continuous conducting layer is 9.5 nm and the multilayer has been optimized to obtain a sheet resistance of 5.7 Ω/sq and an average optical transmittance of 90% at 590 nm. The Haacke figure of merit (FOM) for t_c has one of the highest FOMs with 61.4 × 10⁻³ Ω⁻¹/sq.

APL Mater. 1, 012102 (2013) | HTML | PDF

Spatial mapping of exciton lifetimes in single ZnO nanowires

J. S. Reparaz, G. Callsen, M. R. Wagner, F. Güell, J. R. Morante, C. M. Sotomayor Torres, and A. Hoffmann

The authors investigate the spatial dependence of the exciton lifetimes in single ZnO nanowires. We have found that the free exciton and bound exciton lifetimes exhibit a maximum at the center of nanowires, while they decrease by 30% towards the tips. This dependence is explained by considering the cavity-like properties of the nanowires in combination with the Purcell effect. The authors show that the lifetime of the bound-excitons scales with the localization energy to the power of 3/2, which validates the model of Rashba and Gurgenishvili at the nanoscale.

APL Mater. 1, 012103 (2013) | HTML | PDF