Weak antilocalization (WAL) effects in Bi2Te3 single crystals have been investigated at high and low bulk charge carrier concentrations. At low charge carrier density the WAL curves scale with the normal component of the magnetic field, demonstrating the dominance of topological surface states in magnetoconductivity. At high charge carrier density the WAL curves scale with neither the applied field nor its normal component, implying a mixture of bulk and surface conduction. WAL due to topological surface states shows no dependence on the nature (electrons or holes) of the bulk charge carriers. The observations of an extremely large, non-saturating magnetoresistance, and ultrahigh mobility in the samples with lower carrier density further support the presence of surface states. The physical parameters characterizing the WAL effects are calculated using the Hikami-Larkin-Nagaoka formula. At high charge carrier concentrations, there is a greater number of conduction channels and a decrease in the phase coherence length compared to low charge carrier concentrations. The extremely large magnetoresistance and high mobility of topological insulators have great technological value and can be exploited in magneto-electric sensors and memory devices.

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In this study, the growth and properties of LiAlO2 material and a nonpolar GaN-based light-emitting-diode (LED) structure on LiAlO2 have been investigated. The LiAlO2 material is grown by the Czochralski pulling technique and is used as a substrate for nonpolar nitride growth. An improved surface roughness can be obtained by a four-step polishing process. With subsequent nitridation treatment, a pure M-plane (101̱0) GaN can be obtained. An electron microscope shows an abundance of cracks that are oriented parallel to the (001) and (100) planes of the LiAlO2 substrate on the rear surface of GaN. The absence of the polarization-induced electric field of a GaN-based LED structure on LiAlO2 was shown by using photoluminescence measurements. Therefore, this approach is promising to further increase the luminescence performance of GaN-based LEDs.

Au nanoparticle (NP)-enhanced activity of a semiconductor in ultraviolet (UV) photocatalysis is generally observed. However, the photoinduced charge transfer behavior and the beneficial role of Au NPs in promoting photocatalytic reactions remain controversial. In the present work, the surface potentials (SPs) of Au NP-nonpolar ZnO composites in the dark and under UV irradiation were measured using Kelvin probe force microscopy (KPFM). On the basis of the KPFM results, the surface photovoltages (SPVs) of Au NP-ZnO composites were obtained by calculating the difference between the SP values acquired under UV irradiation and in the dark. Three-dimensional band diagrams of the Au NP-ZnO photocatalysts after equilibrium in the dark and under UV irradiation were thus constructed. Accordingly, charge transfer between three fundamental interfaces, namely Au NP/ZnO, ZnO/solution, and Au NP/solution, in the tested photocatalytic system is clearly described. With the positive SPV values of photocatalysts, the excess holes in the photocatalyst under steady-state UV irradiation are likely the major contributor in the present work. Furthermore, the SPV values of the Au NP-ZnO photocatalysts, as an indication of average excess carrier concentration, show a systematic correlation with photocatalytic activity. This result suggests that the SPV value of a photocatalyst could be a reasonable index for the evaluation of photocatalytic activity.

In this work, a high thermoelectric figure of merit, zT of 1.9 at 740 K is achieved in Ge 1−x Bi x Te crystals through the concurrent of Seebeck coefficient enhancement and thermal conductivity reduction with Bi dopants. The substitution of Bi for Ge not only compensates the superfluous hole carriers in pristine GeTe but also shifts the Fermi level ( E F ) to an eligible region. Experimentally, with moderate 6–10% Bi dopants, the carrier concentration is drastically decreased from 8.7 × 10 20 cm −3 to 3–5 × 10 20 cm −3 and the Seebeck coefficient is boosted three times to 75 μVK −1 . In the meantime, based on the density functional theory (DFT) calculation, the Fermi level E F starts to intersect with the pudding mold band at L point, where the band effective mass is enhanced. The enhanced Seebeck coefficient effectively compensates the decrease of electrical conductivity and thus successfully maintain the power factor as large as or even superior than that of the pristine GeTe. In addition, the Bi doping significantly reduces both thermal conductivities of carriers and lattices to an extremely low limit of 1.57 W m −1 K −1 at 740 K with 10% Bi dopants, which is an about 63% reduction as compared with that of pristine GeTe. The elevated figure of merit observed in Ge 1−x Bi x Te specimens is therefore realized by synergistically optimizing the power factor and downgrading the thermal conductivity of alloying effect and lattice anharmonicity caused by Bi doping.

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