ß-Ga2O3: a potential high-temperature thermoelectric material.

The thermoelectric properties of intrinsic n-type ß-Ga2O3 are evaluated by first-principles calculations combined with Boltzmann transport theory and relaxation time approximation. The electron mobility is predicted by considering polar optical phonon scattering in ß-Ga2O3. A temperature power law of T-0.67 is obtained for the intrinsic electron mobility. Due to the ultra-wide band gap of 4.7-4.9 eV, ß-Ga2O3 has a large Seebeck coefficient. As a result, a maximum power factor of 3.1 × 10-3 W m-1 K-2 is obtained at 1600 K. A clear anisotropy in lattice thermal conductivity is observed, with the highest thermal conductivity of 23.1 W m-1 K-1 at 300 K along the [010] direction, and a lower value of 13.2 and 12.2 W m-1 K-1 along the [001] and [100] directions, respectively. A high ZT value of 1.07 at 1600 K can be obtained at the optimal carrier concentration of 2.4 × 1019 cm-3, which is superior to that of most other oxides such as ZnO. In addition, the lattice thermal conductivity can be reduced by precisely adjusting the grain size, and the lattice thermal conductivity at 300 K (1600 K) can be reduced by 73% (39%) when the grain size is decreased to 10 nm. The excellent thermoelectric properties of ß-Ga2O3 have promoted its potential application in the field of high temperature thermoelectric conversion.

Significant improvement in thermoelectric performance of SnSe/SnS via nano-heterostructures.

In this work, we study theoretically the electronic and phonon transport properties of heterojunction SnSe/SnS, bilayer SnSe and SnS. The energy filtering effect caused by the nano heterostructure in SnSe/SnS induces an increase in the Seebeck coefficient, causing a large power factor. We calculate the phonon relaxation time and lattice thermal conductivity κL for the three structures; the heterogeneous nanostructure could effectively reduce κL due to the enhanced phonon boundary scattering at interfaces. The average κL notably reduces from around 3.3 (3.2) W m-1 K-1 for bilayer SnSe (SnS) to nearly 2.2 W m-1 K-1 for SnSe/SnS at 300 K. As a result, the average ZT (ZTave in b and c directions) reaches 1.63 with temperature range around 300-800 K, which is improved by 63% (25%) compared with that of bilayer SnSe (SnS). Our theoretical results show that the heterogeneous nanostructure is an innovative approach for improving the Seebeck coefficient and significantly reducing κL, effectively enhancing thermoelectric properties.

Microscopic origin of the extremely low thermal conductivity and outstanding thermoelectric performance of BiSbX3 (X = S, Se) revealed by first-principles study.

In this paper, we performed comprehensive investigations of both the thermal and electrical transport properties of BiSbSe3 and BiSbS3 by using first-principles calculations and Boltzmann transport theory. Due to the repulsion between the lone-pair electrons of Sb and the p orbital of Se(S), both BiSbSe3 and BiSbS3 show strong anharmonicity with Grüneisen parameters of 1.90 and 1.79, respectively. As a result, these two materials possess extremely low lattice thermal conductivities. Meanwhile, both BiSbSe3 and BiSbS3 exhibit similar anisotropic thermal transport behaviors, which is due to the smaller phonon group velocities along the a axis. The predicted highest ZT values at 750 K are 2.9 for n-type BiSbSe3 and 1.2 for p-type BiSbS3. Our calculations provide insights into the origin of the extremely low thermal conductivity in BiSbSe3 and BiSbS3, which is meaningful for exploiting high performance thermoelectric materials with low thermal conductivity.

High thermoelectric performance of topological half-Heusler compound LaPtBi achieved by hydrostatic pressure.

The topological phase transition and thermoelectric performance of LaPtBi under hydrostatic pressure up to 34.6 GPa have been systematically investigated using first-principles calculations based on density functional theory. The results indicate that the band structure can be tuned by applying hydrostatic pressure. As the energy band gap is opened under the hydrostatic pressure, a topological phase transition occurs in this material, changing from a topologically nontrivial semimetal to a trivial semiconductor. In addition, the hydrostatic pressure also has a remarkable effect on the thermoelectric properties of the topological half-Heusler compound LaPtBi. Though the lattice thermal conductivity shows a continuous increase with increasing hydrostatic pressure, the power factor is greatly enhanced due to the increase of the Seebeck coefficient. As a result, a maximum ZT value of 1.74 at 1000 K is achieved in n-type LaPtBi under pressure of 21.0 GPa. It is obvious that the thermoelectric figure of merit of LaPtBi is far beyond that of state-of-the-art half-Heusler thermoelectric materials, such as ZrNiSn, FeNbSb and TiCoSb. The realization of high thermoelectric performance in the half-Heusler compound LaPtBi under hydrostatic pressure could provide a new way to further explore other topological thermoelectric materials.

Controllable photoluminescence enhancement of CdTe/CdS quantum dots thin films incorporation with Au nanoparticles.

Au nanoparticles (Au NPs)/CdTe/CdS QDs nanocomposite films were fabricated by deposition of Au NPs and layer-by-layer self-assembly of colloidal CdTe/CdS QDs. Photoluminescence (PL) spectra showed that Au NPs incorporation resulted in an increase of PL intensity about 16-fold compared with that of the samples without Au NPs. PL enhancement of Au NPs/CdTe/CdS QDs nanocomposite films can be controlled by tuning the thickness of spacer layer between the metal nanoparticles (MNPs) and QDs. Optical absorption spectra exhibited the incorporation of Au NPs boosted the absorption of Au NPs/CdTe/CdS QDs nanocomposite films. The results of finite-difference time-domain (FDTD) simulation indicated that the increased sizes of Au NPs resulted in stronger localization of electric field, which boosted the PL intensity of QDs in the vicinity of Au NPs. We thought that these were mainly attributed to localized SP enhancement effects of the Au NPs. Our experiment results demonstrated that Au NPs/QDs nanocomposite films would be a promising candidate for optoelectronic devices application. PACS 78.55.-m; 82.33.Ln; 68.65.Hb.

Segmented gray-code kernels for fast pattern matching.

The gray-code kernels (GCK) family, which has Walsh Hadamard transform on sliding windows as a member, is a family of kernels that can perform image analysis efficiently using a fast algorithm, such as the GCK algorithm. The GCK has been successfully used for pattern matching. In this paper, we propose that the G4-GCK algorithm is more efficient than the previous algorithm in computing GCK. The G4-GCK algorithm requires four additions per pixel for three basis vectors independent of transform size and dimension. Based on the G4-GCK algorithm, we then propose the segmented GCK. By segmenting input data into L(s) parts, the SegGCK requires only four additions per pixel for 3L(s) basis vectors. Experimental results show that the proposed algorithm can significantly accelerate the full-search equivalent pattern matching process and outperforms state-of-the-art methods.