Doped Nanocrystalline Powders Research Articles

Extraction–Pyrolytic Method for TiO2 Polymorphs Production

The unique properties and numerous applications of nanocrystalline titanium dioxide (TiO2) are stimulating research on improving the existing and developing new titanium dioxide synthesis methods. In this work, we demonstrate for the first time the possibilities of the extraction–pyrolytic method (EPM) for the production of nanocrystalline TiO2 powders. A titanium-containing precursor (extract) was prepared by liquid–liquid extraction using valeric acid C4H9COOH without diluent as an extractant. Simultaneous thermogravimetric analysis and differential scanning calorimetry (TGA–DSC), as well as the Fourier-transform infrared (FTIR) spectroscopy were used to determine the temperature conditions to fabricate TiO2 powders free of organic impurities. The produced materials were also characterized by X-ray diffraction (XRD) analysis and transmission electron microscopy (TEM). The results showed the possibility of the fabrication of storage-stable liquid titanium (IV)-containing precursor, which provided nanocrystalline TiO2 powders. It was established that the EPM permits the production of both monophase (anatase polymorph or rutile polymorph) and biphase (mixed anatase–rutile polymorphs), impurity-free nanocrystalline TiO2 powders. For comparison, TiO2 powders were also produced by the precipitation method. The results presented in this study could serve as a solid basis for further developing the EPM for the cheap and simple production of nanocrystalline TiO2-based materials in the form of doped nanocrystalline powders, thin films, and composite materials.

Photoluminescence and photometric studies of low temperature prepared red emitting MgAl2O4:Cr3+ nanophosphors for solid state displays

Abstract MgAl2O4:Cr3+ (0.01–0.11 mol) doped nanocrystalline powders were prepared by a combustion method at low temperature. The Scherer's method was used to estimate crystallite size and values were found to be in the range 20–30 nm and the same was confirmed by transmission electron microscopy (TEM) images. The energy band gap of the prepared samples was estimated from diffusion reflection spectra (DRS) and found to be 3.7–4.5 eV. The characteristic emission peaks recorded at 17123.3 cm−1 and 14727.5 cm−1 were attributed to the transitions 4T2→4A2 and 2E→4A2 respectively upon 357 nm excitation. The emission intensity increases up to 5 mol % Cr3+ particles and then decreases, which may be due to concentration quenching. The CIE (Commission International de I'E' clairage) chromaticity co-ordinates were calculated from emission spectra. They were close to the NTSC (National television standard committee) standard value of red emission. The average correlated color temperature (CCT) was found to be 2109 K. Therefore, the MgAl2O4:Cr3+ phosphor is useful for laser and solid state display applications.

Thickness dependence of properties Ga-doped ZnO thin films deposited by magnetron sputtering

In this work, we studied gallium doped zinc oxide (ZnO:Ga) thin films deposited by rf-magnetron sputtering at room temperature using gallium doped nanocrystalline powder synthesized by sol–gel method. The effect of the thickness, on the physical properties of the GZO thin films was analyzed. The influence of the thickness, on structure, surface morphologies, chemical atomic composition, electrical and optical properties was investigated by XRD, SEM, TEM, AFM, Hall measurement and UV Vis–NIR spectrophotometer, respectively. X-ray diffraction (XRD) results revealed the polycrystalline nature of the films with hexagonal wurtzite structure having preferential orientation along [002] direction normal to the substrate. The lowest resistivity obtained from electrical studies was 10−4 Ω cm. Optical transmittance measurement results show a good transparency within the visible wavelength range for all the films.

Two-color random laser based on a Nd3+ doped crystalline powder

Two-color random laser (RL) emission is demonstrated for the first time in a neodymium (Nd3+) doped nanocrystalline powder. Emissions at 1063.5 and 1065.1nm, due to the Nd3+ transition 4F3/2 →4I11/2 in NdAl3(BO3)4 nanocrystals, were observed by exciting the powder in resonance with the Nd3+ transition 4I9/2→4F5/2, at 811nm. The dual-wavelength emission is attributed to the radiative decay from two nonequivalent site-symmetries occupied by the Nd3+ in the NdAl3(BO3)4 crystalline structure. The behavior of the Nd3+ emissions versus the samples׳ temperature indicates the possibility of the RL application as a near infrared optical sensor with a large thermal sensitivity of 0.020°C−1.

Broadband white radiation in Yb3+- and Er3+-doped nanocrystalline powders of yttrium orthophosphates irradiated by 972-nm laser radiation

The up-conversion luminescence of Er3+ from the 2 H 11/2, 4 S 3/2, and 4 F 9/2 levels in nanocrystals of Y0.95(1‒x)Yb0.95x Er0.05PO4 (x = 0, 0.3, 0.5, 0.7, 1) orthophosphates activated with Er3+ ions has been studied under the excitation of Yb3+ ions to the 2 F 5/2 level by 972-nm cw laser radiation. Broadband radiation in the wavelength range of 370–900 nm has been observed at certain power densities of exciting laser radiation; this broadband radiation is absent in the case of excitation of the powders under study by pulsed laser radiation with a wavelength of 972 nm at a pulse repetition frequency of 10 Hz and a duration of a pulse of 15 ns. Experimental data indicating that this radiation is thermal in nature have been presented.

Electrical Study of Si/PS/ZnO:In Solar Cell Structure

Indium doped zinc oxide (IZO) thin films have been grown on porous silicon, in order to develop solar cell structure, by rf-sputtering at room temperature using indium (4 at %) doped nanocrystalline powder previously synthesized by the sol-gel method. Such layers are polycrystalline with a hexagonal wurtzite structure with a thickness of about 400nm and preferential orientation in (002) crystallographic direction. Electrical study under illumination reveals that this type of nanostructure is promising for solar cell application. By comparing Si/PS/IZO and Si/PS/ZnO/IZO structures, we illustrate a satisfactory photovoltaic conversion with these structures. Also we reveal that interposing an undoped ZnO layer leads to a degradation of PV parameters, specially the short-circuit current (Isc) was affected. In fact, Isc decreases from 3mA to 0.8mA by adding the undoped ZnO thin layer. The open circuit voltage is less affected. A decay of 30mV was recorded. C-V analysis gave a carrier density of about 1016 cm-3 for both cells.

Cooling of Er(3+) with Tm(3+) for accurate temperature sensing using yttrium silicate compact powders.

Er(3+) doped nanocrystalline powders are extensively used for thermometry based on luminescence spectral analysis. The luminescence from Er(3+) is produced by a nonlinear (two-photon) absorption process which may generate strong internal heat by activation of nonradiative relaxation channels. If the heat dissipation is not efficient, as is the case for compact powders, there will be inaccurate readings of the temperature. Our proposed solution is to cool down Er(3+) by transferring part of its accumulated energy to another rare-earth element in the lattice. Here, we show our results for Er(3+)-Tm(3+) co-doped yttrium silicate powders prepared by combustion synthesis.

Sputtered Al-doped ZnO transparent conducting thin films suitable for silicon solar cells

Highly transparent conducting Al-doped zinc oxide (AZO) thin films have been grown onto p-type porous silicon substrates by RF-magnetron sputtering at room temperature using aluminum doped nanocrystalline powder. The obtained AZO films were polycrystalline with a hexagonal wurtzite structure and preferentially oriented in the (002) crystallographic direction. The films are highly transparent in the visible wavelength region with a transmittance higher than 85% and an electrical resistivity of 1.56×10−4Ω·cm was obtained at room temperature. On the other hand, we have studied the position of the p–n junction involved in the In2O3:SnO2/(n)AZO/Si(p) structure, by electron-beam induced current technique. Current density–voltage characterizations in dark and under illumination were also investigated. The cell exhibits an efficiency of 5%.

Effect of substrate temperature on the properties of Al-doped ZnO films sputtered from aerogel nanopowders for solar cells applications

Aluminum-doped zinc oxide (AZO) thin films have been deposited onto glass substrates by rf-magnetron sputtering using aluminum doped nanocrystalline powder synthesized by sol–gel method. Structural, electrical and optical properties of the films as a function of the substrate temperature were investigated. It has been found that all of the films deposited were polycrystalline with a hexagonal wurtzite structure and preferentially oriented in the (002) crystallographic direction. The films are highly transparent in the visible wavelength region with a transmittance higher than 85%, and have an optical band-gap ranging from 3.41 to 3.51 eV depending on the substrate temperature. The lowest electrical resistivity of 1.6 × 10 − 4 Ω cm was obtained for films deposited at a substrate temperature of 120 °C.

Elaboration and characterisation of Si(p)/PS/ZnO(i)/ZnO(n) structure obtained by rf-magnetron sputtering from aerogel nanopowder target material

Al-doped zinc oxide (AZO) thin films has been grown onto p type porous silicon (PS) substrate by rf-magnetron sputtering at room temperature using aluminium (3 at%) doped nanocrystalline powder synthesised by the sol-gel method. XRD measurements show that the obtained AZO film is polycrystalline with a hexagonal wurtzite structure, preferentially orientated in the (002) crystallographic direction. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to study the film morphology. The obtained AZO film has a typical columnar structure and very smooth surface.

Elaboration and characterization of Si(n)/PS/ZnO(n) structure obtained by rf-magnetron sputtering from aerogel nanopowder target material

Aluminum doped zinc oxide (AZO) thin film, generally used as TCO in solar cells, has been grown onto n-type porous silicon (PS) substrate by rf-magnetron sputtering at room temperature using aluminum (3 at%) doped nanocrystalline powder synthesized by the sol–gel method. The obtained AZO film with a thickness of about 0.4 μm was polycrystalline with an hexagonal wurtzite structure and preferentially orientated in the (0 0 2) crystallographic direction. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to study the film morphology. The obtained AZO film has a typical columnar structure and very smooth surface. From electric characterizations: current–voltage and capacitance–voltage ( C– V) at different frequency measurements, we can conclude that we have a Schottky electronic behaviors where the depletion-layer is developed principally in the PS region of the PS/ZnO(n) heterojunction. The I(V) characterizations in dark and under illumination were also investigated.

Red up-conversion emission from nanocrystalline GaN powders co-doped with Er 3+ and Yb 3+

The synthesis, structure, and spectroscopy of frequency upconverting lanthanide doped GaN nanocrystals (NCs) are presented. The crystalline wurtzite GaN NCs were synthesized via a nitridation reaction of β-Ga 2O 3 in a flow of anhydrous ammonia. The average sizes of GaN nanocrystallites as determined from XRD and TEM measurements was 30 nm. The photoluminescence (PL) spectra under 980 nm laser diode excitation was measured and analyzed. It was found that GaN Er 3+ and Yb 3+ doped nanocrystalline powders exhibit strong red up-conversion (UC) emission. The mechanisms and pathways of UC are also briefly discussed.

The properties of aluminum-doped zinc oxide thin films prepared by rf-magnetron sputtering from nanopowder targets

Aluminum-doped zinc oxide (ZnO:Al) films were deposited onto glass substrates by rf-magnetron sputtering at ambient temperature using, for the first time, doped nanocrystalline powder synthesized by the sol–gel method. The effects of aluminum on structural, electrical, morphological and optical properties were investigated. The films showed a hexagonal wurtzite structure and high preferential orientation in the (002) crystallographic direction. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to study the films morphology. The obtained samples have a typical columnar structure and a very smooth surface. The optical transmittance spectra showed transmittance higher than 90% within the visible wavelength region. A minimum resistivity of 5.436 · 10 − 5 Ω cm at room temperature was obtained for the 3.0 at.% Al-doped film.

Enhanced nonradiative relaxation and photoluminescence quenching in random, doped nanocrystalline powders

Nonradiative relaxation and photoluminescence quenching in nanocrystalline powders doped with rare-earth elements are of interest in optical bistability, random laser, and other optoelectronic applications. Here, the luminescence quenching of a one-dimensional random medium made of multilayer nanoparticles (Y2O3) doped with rare-earth elements (Yb3+) is analyzed by considering the transport, transition, and interaction of the fundamental energy carriers. The nonradiative decay and luminescence quenching in random media are enhanced compared to single crystals, due to multiple scattering, enhanced absorption, and low thermal conductivity. The coherent wave treatment is used to calculate the photon absorption, allowing for field enhancement and photon localization. The luminescent and thermal emission is considered as incoherent. The size-dependent absorption coefficient and penetration depth are observed. The nonradiative decay is identified as a multiphonon relaxation process, and is found to be enhanced compared to bulk materials. The luminescence quenching and nonlinear thermal emission, occurring with increasing irradiation intensity, are predicted.