Realizing field-dependent conduction in ZnO nanowires without annealing.

We report on the low-temperature fabrication of field-effect transistors by bridging pre-patterned electrodes using ZnO nanowires grown in situ, which operate without requiring post-growth processing or annealing. The devices show good performance using as-grown nanowires, with on-off ratios of 105 and threshold voltages of 2 V. Electron microscopy shows the field-dependent nanowires hierarchically nucleate from larger ZnO nanorods, and both are oriented along a common c-axis. A high nanowire surface-to-volume ratio allows depleting electron traps on the nanowire surface to compensate intrinsic electron donors present throughout the nanowire bulk. This eliminates the need to reduce the electron concentration through high-temperature annealing, making the nanowires naturally field-dependent in their as-grown state.

Oxygen non-stoichiometry phenomena in Pr1-xZrxO2-y compounds (0.02 < x < 0.5).

New Pr1-xZrxO2-y oxides with x < 0.5 have been prepared by co-precipitation in basic medium and annealed under air at high temperatures (T≤ 1200 °C). Defined compositions with x = 0.02, 0.1, 0.2, 0.35, 0.40 and 0.5 have been characterized by XRD, Zr-K-edge EXAFS for the local structure, magnetic susceptibility measurements, and Pr LIII-edge XANES in order to identify the variation of the cell parameter and Zr local environment versus Zr content and Pr(n+) (4 < n < 3) oxidation states. The higher the Zr content, the lower the Pr valence state. The Zr amount stabilized in the distorted octahedral site is at the origin of the formation of defined compositions as discovered by Leroy Eyring et al. in the PrnO2n-2m series and the generation of oxygen vacancies stabilized in the fluorite-type network. TGA and TPR analyses help to follow the reduction properties under Ar/5% H2 and show high Pr reducible rates at low temperatures (T < 250 °C). The identification of the fluorite-type superstructure (SG: Ia3[combining macron]) of reduced compositions annealed at T = 900 °C under Ar/5% H2 shows the cationic and oxygen vacancy ordering. This feature plays a key role with Zr(4+) cations stabilized in flattened octahedral sites for the generation of oxygen vacancies and the stabilization of Pr(3+) in the reduced states.

Thermochromic phase transition on CuMo0.9W0.1O4@SiO2 core-shell particles.

The thermochromic phase transition of CuMo(1-x)W(x)O(4) oxide was delayed by the deposit of SiO(2) shells. The phase transition temperature was investigated by optical reflectivity versus temperature. The effect of the shell thickness on the transition temperature is established. The cyclability of the phenomenon is also discussed.

Characterization of the piezochromic behavior of some members of the CuMo1-xWxO4 series.

The members of the CuMo(1- x)WxO4 series (0 < or = x < 0.1) undergo a first-order phase transition that can be induced by pressure application; the thermochromic properties of such a series have already been reported. The two polymorphic forms exhibit two distinguishable colors: green for the low pressure form (alpha) and brownish-red for the high pressure one (gamma). These oxides can open up a new market for friendly pressure indicators, particularly for the compositions (0.07 < or = x < or = 0.1) for which the two polymorphs are stable at room temperature, that is, for which the color transition via pressure application is nonreversible. Within the CuMo(1- x)WxO4 solid solution domain, the dependence of the transition pressure versus tungsten content, temperature of measurement, and sample thermal-pressure history was studied. A large control of the transition pressure (from 5 to several 100 MPa) was brought to the fore. The transition was then studied using X-ray diffraction and transmission electron microscopy-energy dispersive X-ray analyses. This first-order transition, occurring by atomic migration inside the cell, seems to be preceded by an atomic disordering; moreover, transition temperatures may be modified by W segregation at the surface of the grains.

Synthesis of lead chalcogenide nanocrystals by sequential ion implantation in silica.

Lead chalcogenide (PbS, PbSe, and PbTe) nanocrystals were synthesized by sequential implantation of Pb and one of the chalcogen species into pure silica. The implantation energy and fluence were chosen so that the implantation profiles practically overlap at a depth approximately 150 nm with a maximum concentration of about 0.3 atom %. Annealing for 1-8 h at 850-900 degrees C triggers nanocrystal growth, which is monitored by high-resolution (HRTEM) and conventional transmission electron microscopy (TEM), secondary-ion mass spectrometry (SIMS), and Rutherford backscattering spectrometry (RBS). Striking differences are found in the depth distributions and microstructures of the resulting nanocrystals. We show that the differing chemical interactions of Pb and chalcogens (between each other and with silica) play a crucial role in chalcogenide nucleation and growth. Using available information on chalcogen redox states in silicate glass, we propose a nonclassical nucleation and growth mechanism consistent with our experimental results. The complex chemistry involved at the microscopic level is shown to impair control over the nanocrystal size distribution. Finally, PbS nanocrystal-doped silica is shown to emit intense photoluminescence (PL) in the 1.5-2 microm wavelength range, an effect that we relate to the above nucleation and growth scheme.