Fangfang Xu

Ultrahigh Thermoelectric Performance by Electron and Phonon Critical Scattering in Cu2Se1-xIx

Iodine-doped Cu2 Se shows a significantly improved thermoelectric performance during phase transitions by electron and phonon critical scattering, leading to a dramatic increase in zT by a factor of 3-7 times culminating in zT values of 2.3 at 400 K.

A general preparation strategy for hybrid TiO2 hierarchical spheres and their enhanced solar energy utilization efficiency.

Titanium (IV) dioxide (TiO 2 ) is one of the most attractive d-block transition metal functional oxides due to its unique physical and chemical properties. Many applications of TiO 2 , such as photocatalyst and dye-sensitized solar cells (DSSCs), have been widely investigated. [ 1 , 2 ] To utilize solar energy effi ciently, TiO 2 should be well-crystallized with a high surface area and promote charge separation as well as electron transport. To improve charge separation, TiO 2 /semiconductor composites have proved successful, [ 3 , 4 ] but efforts to enhance electron transport are less so due to the ineffi cient interface between TiO 2 and the semiconductor. A new composite structure of TiO 2 | semimetal | semiconductor is investigated here for solar energy applications. In general, such heterojunction structure requires (i) an alignment of the conduction band of the semiconductor with that of TiO 2 , (ii) little solubility of the semiconductor in TiO 2 , (iii) a highly conductive semimetal interface such as transparent conducting oxide (TCO), and (vi) a high electron mobility in the semiconductor. One such construct is TiO 2 | ZnO:Ti | ZnO, in which ZnO has a similar band structure but much higher electron mobility (205–300 cm 2 V s − 1 ) than TiO 2 (0.1–4 cm 2 V s − 1 ), [ 5 ] Zn 2 + has very low solubility in TiO 2 , [ 6 ] and the Ti-doped ZnO (ZnO:Ti) is a TCO with a high conductivity (up to 1.5 × 10 3 S cm − 1 ) that depends on the doping level and microstructure. [ 7 ]

Enhancing superplasticity of engineering ceramics by introducing BN nanotubes

Introducing carbon nanotubes (CNTs) into polymer or ceramic matrices has been a promising approach to obtain ultra-strong, extra-toughened materials as well as multifunctional composites. Most of the previous work on CNT composites has focused on strengthening and toughening of matrix materials at ambient conditions. However, so far there is a lack of information on the mechanical behavior of these composites at elevated temperature. Recently, single-walled CNTs were found to undergo a superplastic deformation with an appealing 280% elongation at a high temperature (Huang et al 2006 Nature 439 281). This discovery implies the high probability for the potential usage of CNTs as reinforcing agents in engineering high-temperature ceramics with improved ductility. Here, for the first time, we demonstrate that a small addition of boron nitride nanotubes (BNNTs) can dramatically enhance the high-temperature superplastic deformation (SPD) of engineering ceramics. More specifically, 0.5 wt% addition of BNNTs leads to an inspiring brittle-to-ductile transition in Al2O3 ceramics even at a moderate temperature (1300 °C). For Si3N4 ceramics, 0.5 wt% addition of BNNTs could also decrease the true stress by 75% under the same deformation conditions. In contrast, addition of micro-sized or nano-sized BN powders has no or a negative effect on the superplasticity of these ceramics. The underlying SPD-enhancement mechanism is discussed in terms of the inhibition of static and dynamic grain growth of the matrix and the energy-absorption mechanism of BNNTs. The unraveled capability of BNNTs to enhance the SPD behavior will make BNNTs promising components in cost-effective complex ceramics with good comprehensive mechanical properties.

Ultrahigh Thermoelectric Performance in Mosaic Crystals

Successful research strategies to enhance the dimensionless figure of merit zT above 2 rely on either bulk nanomaterials or on single crystals. A new physical mechanism of nanoscale mosaicity is shown that goes beyond the approaches in single crystals or conventional nanomaterials. A zT value of 2.1 at 1000 K in bulk nanomaterials is achieved.

Quasi-aligned single-crystalline GaN nanowire arrays

Quasi-aligned GaN nanowire arrays have been fabricated via a thermal evaporation of the starting reactants Ga2O3∕GaN. The GaN nanowires have uniform diameters of ∼300nm, lengths up to tens of micrometers and possess a sharp six-fold symmetrical pyramidlike tip. High-resolution transmission electron microscopy (TEM) analysis indicated that majority of GaN nanowires have a preferential growth direction along the [0001] direction. Room-temperature field-emission measurement showed that the as-synthesized GaN nanowire arrays have a lower turn-on field of 7.0V/μm. It is believed that both the sharp tips and rough surface of GaN nanowires contribute to the excellent electron emission behavior.

New boron nitride whiskers: showing strong ultraviolet and visible light luminescence.

Boron nitride whiskers with a special structure have been synthesized by a thermal reaction process. The as-prepared BN whiskers have a length of tens of micrometers and a mean diameter of 500 nm. High-resolution TEM analysis shows that the as-prepared BN whiskers can be described as a nanofiber-interweaved network. Infrared and electron energy loss spectra reveal that the BN whiskers are composed of both sigma-sp2 and sigma-sp3 chemical bonds. The UV-vis absorption spectrum displays the energy band gap of the BN whiskers and multiple fine absorption peaks of the phonon-electron coupling. Both photoluminescence (PL) and cathodoluminescence (CL) measurements show the specially structured BN emits strong UV and visible luminescences, which is a promising material for deep-blue and UV applications.

Wurtzite-type faceted single-crystalline GaN nanotubes

We report on the direct fabrication of single-crystalline wurtzite-type hexagonal GaN nanotubes via a newly designed, controllable, and reproducible chemical thermal-evaporation process. The nanotubes are single crystalline, have one end closed, an average outer diameter of ∼300 nm, an inner diameter of ∼100 nm, and a wall thickness of ∼100 nm. The structure and morphology of the tubes are characterized using a scanning electron microscope and a transmission electron microscope. The cathodoluminescence of individual nanotubes is also investigated. The growth mechanism, formation kinetics, and crystallography of GaN nanotubes are finally discussed.

Ultrasonic-irradiation-assisted oriented assembly of ordered monetite nanosheets stacking.

Bioactive monetite (anhydrous calcium hydrogen phosphate, CaHPO(4)) with orderly layered structure assembled by nanosheets has been successfully synthesized by a sonochemical-assisted method in the presence of cetyltrimethylammonium bromide (CTAB). The thicknesses of the nanosheets are 100-200 nm, and the lateral sizes are about 2 microm. Because of the strong affinity with the phosphate ions as well as the (200) faces of the crystals, CTAB molecules can make the formation and stabilization of monetite nanosheets with (200) exposed face. Ultrasonic irradiation makes the transition from disordered state to oriented state before the oriented assembly of monetite nanosheets. The ultrasonic irradiation provides enough external work to make the assemble process possible in thermodynamics. The drastic flow stirred by the supersonic jet in the solution accomplishes the transition and successive oriented assembly of nanosheets in dynamics. This study would offer a simple method to design and synthesize oriented-assembled materials.

Visible emission from N-rich turbostratic boron nitride thin films doped with Eu, Tb, and Tm

Red, green, and blue light emissions have been obtained at 80 and 300 K by cathodoluminescence (CL) from N-rich turbostratic BN thin films doped with Eu, Tb, and Tm, respectively. The films were grown by rf magnetron sputtering in an atmosphere of Ar and NH3 gas mixture using EuB6, TbB6, and TmB12 targets, respectively. Infrared absorption measurements, transmission electron microscopy, and electron probe microanalysis show that the matrix of the films is N-rich turbostratic BN. The sharp characteristic emission lines corresponding to Eu3+, Tb3+, and Tm3+ intra-4fn shell transitions are resolved in the CL spectral range from 350 to 800 nm at 80 and 300 K.

Multiformity and fluctuation of Cu ordering in Cu2Se thermoelectric materials

Cuprous selenide (Cu2Se) has recently shown a very high dimensionless thermoelectric figure of merit zT as well as a dramatic increase in thermoelectric performance during the critical second-order phase transition. The present study indicates that the ultrahigh thermoelectric performance arises from its specific structural features involving multiformity of Cu ordering and drastic structural fluctuation during phase transition. The Cu2Se sample consists of domains of different ordered lamellar structures of Cu atoms which are coherently immersed in the long-range ordered Se pseudo-fcc framework. The specific self-independent binary-sublattice structures have been found to enhance phonon scattering while still guaranteeing good carrier mobilities. Upon increasing the temperature to near phase transition, the multiple structures undergo intense and stepwise changes including appearance of new ordered structures for copper, disordering and diffusion of Cu atoms across the interlayers, and finally random distribution of Cu in the Se cubic sublattice which alters a little during the phase transition. Such extreme structural fluctuation results in critical electron and phonon scatterings that expedite an exceptional enhancement of thermoelectric performance.