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Li S.,Tsinghua University | Li S.,Beijing National Center for Electronic Microscopy | Zhang X.,Tsinghua University | Zhang X.,Beijing National Center for Electronic Microscopy | And 2 more authors.
Nanotechnology | Year: 2010

Sb-doped ZnO nanowires with kinking structures have been synthesized by a catalyst-free thermal evaporation method with the addition of Sb2O3. Transmission electron microscopy (TEM) observations revealed that the kinks of the nanowires are induced by twinning structures. {0111}, {0113} twins and heavy stacking faults in the (0001) plane were observed in these kinked nanowires. High-resolution TEM and energy dispersive x-ray spectroscopy showed that there exists an Sb-rich segregation layer in the twin boundaries of some nanowires. A formation mechanism of the kinked nanowires was proposed. The optical property of the synthesized nanowires was investigated by room-temperature photoluminescence. © 2010 IOP Publishing Ltd.


Li S.,Tsinghua University | Li S.,Beijing National Center for Electronic Microscopy | Zhang X.,Tsinghua University | Zhang X.,Beijing National Center for Electronic Microscopy | Zhang L.,Peking University
Journal of Physical Chemistry C | Year: 2010

Controlling the morphology and understanding the underlying growth mechanism are essential for the synthesis of designed one-dimensional nanostructures. Here, a tapered ZnO nanowire array has been synthesized by catalyst-free thermal evaporation with a small amount of Sb2O 3 additive in the precursors. The radial vapor-phase epitaxy growth is found to be responsible for the tapered structures. The kinetics of the radial growth is proposed to be modified by the adsorption of SbOx species on the nanocone surfaces, which may have a promotion effect on the two-dimensional nucleations and an inhibition effect on the advances of steps. As predicted, increasing the concentration of the Sb2O3 additive in the precursors has resulted in a morphology evolution from ZnO nanocones to more-tapered nanopillars, while the Sb doping concentration has not increased notably. The photoluminescence spectrum of the Sb-doped nanocones showed a red shifted and broadened near-band-edge emission. Our work demonstrates that Sb2O3 can be used as an effective additive to control the morphology of ZnO nanowires. © 2010 American Chemical Society.


Butt S.,Tsinghua University | Butt S.,Institute of Space Technology | Ren Y.,Tsinghua University | Farooq M.U.,University of Science and Technology Beijing | And 4 more authors.
Energy Conversion and Management | Year: 2014

We present the temperature dependent thermoelectric properties of polycrystalline misfit-layered ceramics; (Ca2- xMxCoO3)0.62 (CoO2) , (M: Ba and Pb, x = 0, 0.1, 0.2), fabricated by sol-gel method followed by spark plasma sintering technique. The X-ray diffraction results confirmed the substitution of Ba and Pb in the lattice of (Ca 2CoO3)0.62 (CoO2) at Ca-site. An improved grain-alignment was observed at small scale with Ba-doping, which helped increasing the electrical conductivity but for the Pb-doped specimens, the electrical conductivity was suppressed by catastrophic grain-alignment. The bivalent metallic-doping induced a spin-entropy enhancement which resulted in an enhanced thermopower. For Ba-doped specimens, the simultaneous increase in the electrical conductivity and the thermopower resulted in an increased power factor exhibiting the highest value of 527 μW/mK2. On the other hand, Pb-doping increased the thermopower but on the expense of electrical conductivity. Although, Pb-doping decreased the electrical conductivity but on the other hand, the enhanced thermopower and the suppressed thermal conductivity were sufficient for achieving ZT value higher than that of pure (Ca 2CoO3)0.62 (CoO2). Among all the pure and doped samples, the highest ZT value of ∼0.33 at 1000 K was achieved by Ba-doping which is about 100% higher than that of the pure (Ca 2CoO3)0.62 (CoO2). © 2014 Elsevier Ltd. All rights reserved.


Ur Rehman Sagar R.,Tsinghua University | Ur Rehman Sagar R.,Beijing National Center for Electronic Microscopy | Zhang X.,Tsinghua University | Zhang X.,Beijing National Center for Electronic Microscopy | And 4 more authors.
Journal of Applied Physics | Year: 2014

Amorphous carbon (a-C) films were fabricated by chemical vapor deposition on SiO2 substrate. The a-C films have nano-crystalline sp2 structure with the grain size of ∼5nm and an optical band gap of ∼1.8eV. The a-C films show negative magnetoresistance (MR) from 300 to 2K and an anomalous shape change of MR-magnetic field curves at 10K. Grain boundary scattering theory and weak localization theory were used to explain the MR mechanism and shape change of MR-magnetic field curves. © 2014 AIP Publishing LLC.


Ahmad M.,Beijing National Center for Electronic Microscopy | Pan C.,Beijing National Center for Electronic Microscopy | Zhao J.,Beijing National Center for Electronic Microscopy | Iqbal J.,Tsinghua University | Zhu J.,Beijing National Center for Electronic Microscopy
Materials Chemistry and Physics | Year: 2010

The effect of high-energy (200 keV) electron irradiation on ZnO-tetrapod (ZnO-T) nanostructure has been investigated by employing in situ scanning tunneling microscopy (STM) holder inside TEM. The microscopic results have revealed that the product consists of highly single-crystalline ZnO-T structures. The photoluminescence spectra show the increased amount of defects which lead to shift in the emission peak position in ultraviolet (UV) region and enhance the PL performance in visible luminescence (VL) region of ZnO-T nanocrystals. The in situ measurements show asymmetric Schottky contacts at the both ends interfaces under electron irradiation. The current-voltage (I-V) characteristics have revealed that the increase in electron density (range of ∼0-25 pA cm-2) leads to an increase in the current along with the increase in carrier concentration from 1.1 × 1017 cm-3 to 3.2 × 1017 cm-3. In addition, it has been interestingly found that at high bias voltage, Schottky contacts turn to Ohmic contacts at the both ends with the influence of irradiation-matter interaction. The results strongly suggest that the ZnO-T is considered as a promising candidate for applications in irradiation environments. © 2009 Elsevier B.V. All rights reserved.


Huang R.,Beijing National Center for Electronic Microscopy | Zhu J.,Beijing National Center for Electronic Microscopy
Materials Chemistry and Physics | Year: 2010

Silicon nanowire array films were prepared by metal catalytic etching method and applied as anode materials for rechargeable lithium-ion batteries. The films completely consisted of silicon nanowires that were single crystals. Aluminum films were plated on the backs of the silicon nanowire films and then annealed in an argon atmosphere to improve electronic contact and conduction. In addition to easy preparation and low cost, the silicon nanowire film electrodes exhibited large lithium storage capacity and good cycling performance. The first discharge and charge capacities were 3653 mAh g -1 and 2409 mAh g -1, respectively, at a rate of 150 mA g -1 between 2 and 0.02 V. A stable reversible capacity of about 1000 mAh g -1 was maintained after 30 cycles. The good properties were ascribed to the silicon nanowires which better accommodated the large volume change during lithium-ion intercalation and de-intercalation. © 2010 Elsevier B.V. All rights reserved.


Wu L.H.,Tsinghua University | Wu L.H.,Beijing National Center for Electronic Microscopy | Wu L.H.,Shanghai Aircraft Design and Research Institute | Zhang X.,Tsinghua University | And 6 more authors.
Applied Physics Letters | Year: 2011

Nonsaturating positive magnetoresistance (MR) of intrinsic bulk silicon (i-Si) was observed at forward bias, exhibiting an almost linear behavior at high magnetic fields (5 T


Sagar R.U.R.,Tsinghua University | Zhang X.,Tsinghua University | Zhang X.,Beijing National Center for Electronic Microscopy | Xiong C.,Tsinghua University | Yu Y.,Tsinghua University
Carbon | Year: 2014

Semiconducting amorphous carbon thin films were directly grown on SiO 2 substrate by using chemical vapor deposition. Raman spectra and transmission electron microscopy image showed that the a-C films have a short-range ordered amorphous structure. The electrical and optical properties of the a-C thin films were investigated. The films have sheet resistance of 3.7 kΩ/□ and high transmittance of 82%. They exhibit metal-oxide- semiconductor field effect transistor mobility of 10-12 cm2 V -1 s-1 at room temperature, which is comparable to previous reported mobility of amorphous carbon. The optical band gap was calculated by Tauc's relationship and photoluminescence spectra showed that the films are semiconductor with an optical band gap of 1.8 eV. These good physical properties make the a-C films a candidate for the application of transparent conducting electrodes. © 2014 Elsevier Ltd. All rights reserved.


Ahmad M.,Beijing National Center for Electronic Microscopy | Pan C.,Beijing National Center for Electronic Microscopy | Zhao J.,Beijing National Center for Electronic Microscopy | Zhu J.,Beijing National Center for Electronic Microscopy
Journal of Nanoscience and Nanotechnology | Year: 2011

In this letter, effect of Pb-doping on the electrical and optical properties of the as grown ZnO nanowires (NWs) have been investigated. The microstructural investigations show that the Pb-dopant substituted into wurtzite ZnO nanowires without forming any secondary phase. The amount of contents and valence state of Pb ions has been investigated through energy dispersive spectroscopy and X-ray photospectroscopy The doped nanowires show a remarkable reduction of 15.3 nm (127.4 meV) in the optical band gap, while an increase amount of deep-level defects transition in visible luminescence. Furthermore, the reduction in the band gap and the presence of deep-level defects induces strong effect in the electrical resistivity of doped NWs, which makes their potential for the fabrication of nanodevices. The possible growth mechanism is also briefly discussed. © 2011 American Scientific Publishers.


Li S.,Tsinghua University | Li S.,Beijing National Center for Electronic Microscopy | Zhang X.,Tsinghua University | Zhang X.,Beijing National Center for Electronic Microscopy | And 2 more authors.
Materials Letters | Year: 2011

A one-step thermal evaporation and vapor-phase transport method was utilized to synthesize porous ZnO nanofibers in large scale. The synthesized nanofibers are highly porous, with diameters in the range of 200-700 nm and lengths of several micrometers. The addition of the CuCl2• 2H2O into the Zn precursor powder was proved to be critical for the formation of the porous structures, which were proposed to be resulted from the decomposition of the unstable Cl-containing intermediate products such as zinc hydroxide chloride or zinc oxide chloride hydrate phases. In addition, a demonstration of applying the porous ZnO nanofibers as the photoanode of dye-sensitized solar cells is provided. © 2011 Elsevier B.V. All Rights Reserved.

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