Jianglong Wang

Epitaxial growth and thermoelectric properties of c-axis oriented Bi1−xPbxCuSeO single crystalline thin films

c-axis oriented Bi1−xPbxCuSeO (0 ≤ x ≤ 0.8) single crystalline thin films have been successfully epitaxially grown on SrTiO3 (001) substrates via the pulsed laser deposition technique. Detailed X-ray diffraction and transmission electron microscopy measurements reveal the excellent c-axis and ab-plane texture of the films. As the Pb doping concentration increases, both the resistivity and the Seebeck coefficient decrease due to the increase in hole carrier concentration. The highest power factor of about 1.2 mW m−1 K−2 has been achieved in Bi0.96Pb0.06CuSeO single crystalline thin films at 673 K, which is much higher than those of polycrystalline ceramics with random orientation, suggesting that c-axis oriented BiCuSeO-based single crystalline thin films might have great potential for application in thermoelectric thin film devices.

Enhanced thermoelectric performance in CdO by nano-SiO2 inclusions

We report enhanced thermoelectric (TE) performance in CdO by using thermally insulating nano-particles to mimic nano pores in the composite. Through simply mechanical alloying, we fabricated CdO-SiO2 composites with varying nano-SiO2 concentration from 0.1 to 3 at%. Due to the very low thermal conductivity of nano-SiO2 distributed in the CdO matrix, the thermal conductivity of the composite was substantially reduced by about 80%, which lead to the dimensionless figure of merit (ZT) value increment about 28% (from 0.32 to 0.41) at 1000 K. TEM shows the nano inclusions formed within the CdO matrix and grain boundaries as well, which is thought to contribute to the reduction of thermal conductivity of the composite by additional scattering mechanism for the mid- to long-wavelength phonons. This facile and low-cost approach might be widely adopted and synergized to other TE materials systems to further improve their performance.

The enhancement of photo-thermo-electric conversion in tilted Bi 2 Sr 2 Co 2 Oy thin films through coating a layer of single-wall carbon nanotubes light absorber

Light-induced transverse thermoelectric effect has been investigated in c-axis tilted Bi(2)Sr(2)Co(2)O(y) thin films coated with a single-wall carbon nanotubes light absorption layer. Open-circuit voltage signals were detected when the sample surface was irradiated by different lasers with wavelengths ranging from ultraviolet to near-infrared and the voltage sensitivity was enhanced as a result of the increased light absorption at the carbon nanotubes layer. Moreover, the enhancement degree was found to be dependent on the laser wavelength as well as the absorption coating size. This work opens up new strategy toward the practical applications of layered cobaltites in photo-thermo-electric conversion devices.

Enhancement of thermoelectric power factor in NaxCoO2/Au multilayers

NaxCoO2/Au thin film multilayers, with a thickness of the Au layer of 0.5–12 nm, have been fabricated on c-Al2O3 by post annealing of the CoO/Au thin film multilayers in Na vapor at high temperature in air. X-ray diffraction measurements revealed that the resulting NaxCoO2 layer was c-axis oriented with perfect ab-plane alignment and there was no reaction between Au and NaxCoO2. The resistivity of the NaxCoO2/Au multilayers decreased significantly with increasing the volume fraction of the Au layer without obvious deterioration of the Seebeck coefficient. Consequently, a considerable enhancement of the power factor was achieved in NaxCoO2/Au multilayers with the right Au volume fraction, which was about 2.2 times larger than that of the single NaxCoO2 layer with the same thickness.

Dependence of oxygen content on transverse thermoelectric effect in tilted Bi 2 Sr 2 Co 2 O y thin films

The transverse thermoelectric (TE) effect has been investigated in c axis tilted Bi2Sr2Co2Oy thin films with different oxygen content. The film samples were fabricated by a chemical solution deposition method annealed under different atmospheres of O2, air, and N2, respectively. Open-circuit transverse voltage signals were observed when the surface of the films was heated by a pulsed laser as well as a continuous thermal source. With the increase of oxygen content in the films, the amplitude of the observed voltage signals increased while the response time decreased. The experimental results can be explained by a mechanism involving the transverse TE effect as well as the transport theory of TE materials.

Strain and electric field co-modulation of electronic properties of bilayer boronitrene

The electronic properties of bilayer strained boronitrenes are investigated under an external electric field using density functional methods. Our result is just the same as the previous conclusion: ie, that the electric field will reduce their band gaps. Except for the decrease of their band gaps, the degeneracy of π valence bands at K points will be lifted and the degenerate gap will increase with the electric field increasing. Moreover, the widths of π valence bands are nearly robust and increase a little. In addition, a simple tight-binding model, where different electrostatic potentials are applied to boronitrene layers, can be sufficient to describe the variations of their band gaps. It is found that the interlayer hopping interaction increases while the intralayer hopping parameter changes little with increasing the electric field. Furthermore, a band gap phase diagram is determined within the in-plane strain [-0.2, 0.2] and the interlayer bias [0, 10] V nm(-1). The strain could make the bottom of conduction bands shift from K to M, then to Γ in the Brillouin zone, while the top of valence bands shifts from K to Γ. Thus, a direct-gap semiconductor at K points is changed into an indirect-gap semiconductor, and then a semiconductor with the direct band gap at Γ points. When bilayer boronitrene is a semiconductor with a direct gap at K points, the electric field and strain are inverse proportional relationships. Particularly, when the compressive strain exceeds  -0.194, there is an insulator-metal transition and the system becomes metallic with sizable pocket Fermi surfaces.

Enhanced transparent conducting performance of c-axis oriented Ca3Co4O9 thin films

p-type transparent conductive Ca3Co4O9 thin films were grown on LaAlO3 substrates using a pulsed laser deposition technique. The films showed perfect c-axis orientation with a resistivity of about 7.3 mΩ cm at room temperature and optical transmittance of about 50% in the visible range. A high figure of merit of about 988 MΩ−1 was achieved in these c-axis oriented Ca3Co4O9 films, which is comparable to those reported for the state-of-the-art p-type transparent conducting oxides. This work demonstrated the potential applications of c-axis oriented Ca3Co4O9 films in optoelectronic devices as a new class of transparent conducting oxide coatings.

Epitaxial Bi2Sr2Co2Oy thin films as a promising p-type transparent conducting oxides

National Natural Science foundation of China [51372064]; Nature Science Foundation of Hebei Province, China [A2013201249, A2014201176, QN20131040]

Impact of contact couplings on thermoelectric properties of anti, Fano, and Breit-Wigner resonant junctions

Quantum interference is a well-known phenomenon which results in unique features of the transmission spectra of molecular junctions at the nanoscale. We investigate and compare the thermoelectric properties of three types of junctions like the anti, Breit-Wigner, and Fano resonances. Due to its asymmetric line-shaped transmission function, Fano resonances lead to a larger thermoelectric figure of merit (ZT) than the symmetric anti and Breit-Wigner resonances. The occurrence of quantum interference in molecular and other nanoscale junctions is independent of contact couplings between the sandwiched molecules and left/right electrodes. However, it is found that the contact couplings determine the electric and thermoelectric performances of quantum interference junctions. In anti-resonant junctions, the Seebeck coefficient is enhanced by strong contact couplings. By contrast, for Breit-Wigner resonant junctions, this same property will increase in the weak contact coupling regime. Contrary to what is observe...

Intra- and inter-layer charge redistribution in biased bilayer graphene

We investigate the spatial redistribution of the electron density in bilayer graphene in the presence of an interlayer bias within density functional theory. It is found that the interlayer charge redistribution is inhomogeneous between the upper and bottom layers and the transferred charge from the upper layer to the bottom layer linearly increases with the external voltage which further makes the gap at K point linearly increase. However, the band gap will saturate to 0.29 eV in the strong-field regime, but it displays a linear field dependence at the weak-field limit. Due to the AB-stacked way, two carbon atoms per unit cell in the same layer are different and there is also a charge transfer between them, making the widths of π valence bands reduced. In the bottom layer, the charge transfers from the direct atoms which directly face another carbon atom to the indirect atoms facing the center of the hexagon on the opposite layer, while the charge transfers from the indirect atoms to the direct atoms in ...