Photoluminescence in anatase titanium dioxide nanocrystals

Titanium dioxide (TiO₂) nanocrystals were prepared by a hydrolysis process of tetrabutyl titanate. X-ray diffraction and Raman scattering showed that the as-prepared TiO₂ nanocrystals have anatase structure of TiO₂, and that the monophase anatase nanocrystals can be achieved through a series of annealing treatments below 650 °C. We measured photoluminescence (PL) spectra of the TiO₂ nanocrystals. Under 2.41–2.71 eV laser irradiation, the TiO₂ nanocrystals displayed strong visible light emission with maxima of 2.15–2.29 eV even at excitation power as low as 0.06 W/cm². To identify the PL mechanism in the TiO₂ nanocrystals, the dependences of the PL intensity on excitation power and irradiation time were investigated. The experimental results indicated that the radiative recombination is mediated by localized levels related to surface defects residing in TiO₂ nanocrystallites. Received: 7 April 1999 / Revised version: 23 August 1999 / Published online: 30 November 1999     

Interplay between the local structural disorder and the length of structural coherence in stabilizing the cubic phase in nanocrystalline ZrO2

The atomic-scale structure of three nanocrystalline ZrO₂ samples obtained by different techniques and possessing a different length of structural coherence has been studied using high-energy X-ray diffraction and the atomic pair distribution function technique. The studies reveal that all samples show a monoclinic-like local atomic ordering. Only when the length of structural coherence exceeds 1 nm the atomic arrangement evolves into a cubic-type one. The result underlines the importance of both the local structural disorder and the length of structural coherence, i.e. the spatial extent of longer-range atomic order, in stabilizing the technologically important cubic zirconia at room temperature.     

Molybdophosphoric acid in sol-gel titania: Physico-chemical properties

Titania mixed with molybdophosphoric acid, HMoP, in various proportions (1, 15, 25 and 50 wt.%) was obtained by the sol-gel method. The gels were dried and calcined in air at 400, 600 and 800 °C. The crystalline phases were identified by X-ray diffraction (XRD), whereas the physico-chemical properties were characterized by Raman spectroscopy and ³¹P MAS-NMR spectroscopy. FTIR was a technique used to determine acidic properties by pyridine adsorption. XRD showed that HMoP does not decompose until 700 °C. The stabilization of the anatase phase and the Keggin HMoP crystalline structure were evidenced by Raman spectroscopy; whereas FTIR-pyridine adsorption spectra showed that titania-HMoP present Brönsted and Lewis acid sites. A correlation between the acid sites and the amount of HMoP was observed by ³¹P MAS-NMR.     

Low frequency Raman scattering of anatase titanium dioxide nanocrystals

TiO₂ nanoparticle of size 7.8 nm are synthesized by wet chemical route and characterized by low-frequency Raman scattering (LFRS), transmission electron microscopy (TEM) and X-ray diffraction. The low frequency peaks in the Raman spectra have been explained using the Lamb's theory that predicts the vibrational frequencies of a homogeneous elastic body of spherical shape. Our results show that the observed low-frequency Raman scattering originates from the spherical (l=0) and quadrupolar vibrations (l=2) of the spheriodal mode due to the confinement of acoustic vibrations in TiO₂ nanoparticles. In addition to the low-frequency peak due to the vibrational quadrupolar and spheriodal modes, a band is also observed, which is assigned to the Raman forbidden torsional l=2 mode originating from the near spherical shape of the TiO₂ nanoparticles. The size distribution is also obtained from LFRS, which is in good agreement with TEM.     

Molecular vibrations of the layered-perovskite cobalt oxides characterized by infrared and Raman spectroscopies coupled with crystal structure refinement

Molecular vibrations of the polycrystalline Nd_2−xSr_xCoO₄ are systematically characterized by powdered x-ray diffraction, Raman scattering and infrared spectra at different dopings as well as at different temperatures. Structural refinements reveal that all the specimens crystallize in the K₂NiF₄ structure with space group I4/mmm as is also confirmed by the phonon bands of Raman scattering and infrared transmittance. The frequency shifts of the phonon bands are found to be very well consistent with the elongation and/or contraction of the bond lengths. Moreover, for Nd_0.75Sr_1.25CoO₄, the phonon bands of the Raman active modes seem to retrace the evolutions of magnetic and structural temperature.     

Transition of 3D to 2D growth modes of InAs grown on GaAs

We report a method to grow thin strain-released InAs layer on GaAs (1 0 0) substrates by molecular beam epitaxy. We have shown that by controlling the growth parameters, a thin 2D InAs layer can be grown during initial stages, which eventually serves as a buffer layer to trap dislocations and epitaxial regrowth of InAs on this buffer results in high crystal quality. The size dependence of the InAs islands formed during initial stages with growth time has been studied by atomic force microscopy. With continuous short-time epitaxial growth during various stages, the InAs growth mode transfers from 3D to 2D. The introduction of dislocations into InAs epitaxial islands and their behavior during initial growth stage has been theoretically studied. The theoretical results are in remarkable agreement with the experimental results and shows that once the film is formed, the film strain is totally relaxed. The 200 nm thick InAs epilayer grown on this buffer shows a narrow X-ray diffraction peak. Such InAs strain-released buffer layer would be useful for regrowth of high In content based materials on top of it for electronics and optoelectronics device applications.     

Effect of the crystallite size in the structural and textural properties of sulfated and phosphated titania

Sulfated and phosphated titania (TiO₂) were obtained by the sol-gel method. The sulfation was prepared in situ using sulfuric acid as hydrolysis catalyst, or by impregnation with ammonium sulphate. Phosphated sol-gel titania was synthesized with phosphoric acid. Long thermal treatments were carried out from 100 to 1000 °C to study the crystallization process of titania. All present phases were nanocrystalline, and the anatase-rutile polymorphic transformation was dependent on the crystallite size of the materials and on the preparation method. The smallest crystallite values were obtained for the phosphated samples. Titania sulfated by impregnation was found to be an efficient and selective catalyst towards propene, whereas di-isopropylether was obtained only in the catalyst prepared in situ with H₂SO₄.     

Crystallinity effect in the textural properties of titania-TPA catalysts

Catalysts based in titania mixed with tungstophosphoric acid (TPA), H₃PW₁₂O₄₀, in various proportions (1, 15, 25 and 50 wt%) were obtained by the sol-gel method. The gels were prepared by hydrolysis and gellation of titanium n-butoxide with a TPA solution, using HNO₃ as a catalyst to obtain a pH 3. Fresh samples were thermally treated from 100 to 800 °C, in a stepwise increment of 100 °C during 20 h per step. Specific surface areas were calculated by the BET method from the nitrogen adsorption isotherms; it was found that the surface area increased with TPA content. The crystallization behavior was followed by powder X-ray diffraction. Crystallite size measurements showed that anatase remains nanocrystalline in the studied temperature range. From the X-ray data, it was clear that below 700 °C TPA is highly dispersed in an amorphous state.     

Versatile synthesis of rectangular shaped nanobat-like CuO nanostructures by hydrothermal method; structural properties and growth mechanism

This paper describes a simple and versatile method for growing highly anisotropic rectangular shaped nanobat-like CuO nanostructures by simple, low temperature and cost effective hydrothermal method. Field emission scanning electron microscopy illustrated that these CuO nanostructures have diameter of ∼70 nm, thickness of ∼8 nm and length of ∼174 nm. Structural analysis reveals that the CuO nanostructures have a high crystal quality with monoclinic crystal structure. X-ray photoelectron spectroscopy studies demonstrate that the sample is composed of CuO. The Raman study also indicates the single phase property and high crystallinity of as-grown CuO nanostructures. The plausible growth mechanism for the formation of nanobat-like CuO structure is proposed.     

High pressure phase transition in metallic LaB6: Raman and X-ray diffraction studies

High pressure Raman and angle dispersive X-ray diffraction (ADXRD) measurements on the metallic hexaboride LaB₆ have been carried out upto the pressures of about 20 GPa. The subtle phase transition around 10 GPa indicated in Raman measurements is confirmed by ADXRD experiments to be a structural change from cubic to orthorhombic phase. Ab-initio electronic band structure calculations using full potential linear augmented plane wave method carried out as a function of pressure show that this transition is driven by the interception of Fermi level by electronic band minimum around the transition pressure.     

Confocal Raman spectroscopic mapping studies on a single CuO nanowire

In this study we are able, using a copper grid substrate, to successfully grow separate nanowires with a high level of crystallinity, for a length of up to 10 μm. They were synthesized under various temperatures. We compare and contrast three types of geometries (micron-, nano-scale, and tip-like single CuO nanowires), to identify their potential for monitoring the size effects of quantum confinements. The confocal Raman spectrometry results confirm the expected outcome, that reducing of the diameter of a cylindrical cross-section of a single nanowire results in Raman frequency downshifts. The results can be explained by the bond polarizability model. The applicability of investigating the size effects of the quantum confinement of the tip-like geometry of a single nanowire without any preparation for different sizes of nanoparticles is possible because the detection is relatively straightforward and the reproduced Raman signals can be observed.     

One-phonon-assisted resonant electron Raman scattering of GaAs quantum dots in an AlAs matrix

We have presented a theoretical calculation of the differential cross section (DCS) for the electron Raman scattering (ERS) process associated with the bulk-like longitudinal optical (LO) and interface optical (IO) phonon modes in semiconductor quantum dots (QDs). Electron states are considered to be confined within the QDs. We consider the Fröhlich electron-phonon interaction in the framework of the dielectric continuum approach. We study selection rules for the processes. Some singularities in the Raman spectra are found and interpreted. A discussion of the phonon behavior for QDs with large and small size is presented. The numerical results are also compared with that of experiments.     

Raman study of photo‐induced degradation of the Ru(II) complex adsorbed on nanocrystalline TiO2 films

Photo‐induced degradation of a monolayer of the Ru(II) complex adsorbed on anatase TiO₂ thin films was studied by using resonant micro‐Raman spectroscopy. We developed two contrastive experiments to analyze the degradation mechanism. An exponential decay law was found when the dye was irradiated in the absence of a reducing agent. While the sensitized TiO₂ thin film electrode was covered by the I^–/I₃^– redox couple, the dye degradation exhibited a slowed linear decay. The experimental result was compared and the degradation mechanism was analyzed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Surface acoustic phonon modes of Ge nanocrystals embedded in SiO2

Ge nanocrystals (NCs) embedded in SiO₂ are synthesized by ion implantation, and the surface vibrational modes of the Ge NCs are investigated using the low-frequency Raman scattering (LFRS) technique. LFRS studies show distinct low-frequency Raman modes in the range 6.5-21.2 cm⁻¹ for the Ge NCs depending on the implant dose and annealing temperature. These low-frequency Raman modes are attributed to the confined surface acoustic phonon modes of Ge NCs with (0,0) spheroidal mode and (0,3) torsional modes. Our results are in excellent agreement with the recent theoretical predictions of surface vibrational modes in Ge NCs.     

Compression behavior of nanocrystalline anatase TiO2

We present a synchrotron X-ray diffraction study of pressure-induced changes in nanocrystalline anatase (with a crystallite size of 30-40 nm) to 35 GPa. The nanoanatase was observed to a pressure above 20 GPa. Direct transformation to the baddeleyite-TiO₂ polymorph was seen at 18 GPa. A fit of the pressure versus volume data to a Birch-Murnaghan equation yielded the following parameters: zero-pressure volume, V₀=136.15 Å³, bulk modulus, K_T=243(3) GPa, and the pressure derivative of bulk modulus, K′=4 (fixed). The bulk modulus value obtained for the nanocrystalline anatase is about 35% larger than that of the macrocrystalline counterpart.     

Fine structure effects and phase transition of Xe nanocrystals in Si

We report on an X-ray diffraction study performed on Xe agglomerates obtained by ion implantation in a Si matrix. At low temperature, Xe nano-crystals were formed in Si with different average sizes according to the preparation procedure. High resolution diffraction spectra were detected as a function of the temperature, in the range 15–300 K, showing evidence of fine structure effects in the growth mode of the Xe nanocrystals. We report the first experimental observation of fcc crystalline agglomerates with a lattice parameter expanded by the epitaxial condensation on the Si cavities, whereas for small agglomerates randomly oriented evidence of a contracted lattice was found. For these nanocrystals, a solid-to-liquid transition temperature, size dependent, was detected; above the transition temperature, a fluid phase was observed. Neither overpressurized clusters were detected at any temperature, nor preferential binary size distribution as reported for a metal matrix.     

Photoluminescence and Raman scattering of Cu-doped ZnO films prepared by magnetron sputtering

The Cu-doped ZnO films were prepared by direct current reactive magnetron sputtering using a zinc target with various Cu-chips attached. The influences of Cu-doping on the microstructure, photoluminescence, and Raman scattering of ZnO films were systematically investigated. The results indicate that ZnO films doped with moderate Cu dopant (2.0-4.4 at.%) can obtain wurtzite structure with strong c-axis orientation. The near band edge (NBE) emission of ZnO film can be enhanced by Cu dopant with a concentration of 2.0 at.% and quench quickly with further increase of doping concentration. Two additional modes at about 230 and 575 cm⁻¹, which could be induced by Cu dopant, can be observed in Raman spectra of the Cu-doped ZnO films.     

Combinatorial ion beam synthesis of CdSxSe1−x nanocrystals

In this presentation we focus on the synthesis of buried multielemental semiconductor nanoparticles by sequential high dose ion implantation and post-implantation annealing. Nanocluster formation and alloying was studied by Raman-, Rutherford Backscattering Spectroscopy (RBS) and X-ray diffraction analysis (XRD) on a materials library of CdS_xSe_1−x nanoclusters buried in thermally grown SiO₂ on silicon. Characteristic peak shifts of the LO-Raman signal and XRD-peaks due to varying S- and Se-fraction indicate that the ion beam synthesized clusters consist of a solid solution of Cd, S and Se. In addition the influence of the implanted dose ratios on the structural evolution of the nanocluster-SiO₂ system will be discussed.     

Microwave plasma synthesis of nano‐crystalline YSZ

Nanocrystalline yttria‐stabilized zirconia (YSZ) powder and compacts have been successfully synthesized by microwave plasma synthesis co‐evaporating Y and Zr precursors from a single source. Both liquid and solid mixtures of chemically homologous precursors were tested. Electron micrographs and X‐ray diffraction studies reveal small crystallites in the range 3–4 nm with a narrow size distribution. Crystallite sizes smaller than 25 nm are preserved even after subsequent annealing at temperatures up to 950 °C. A comparison with other gas‐phase synthesis routes of nanocrystalline YSZ is given. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and characterisation of high quality MBE grown InNx Sb1–x

The growth, structural and optical characterisation of dilute nitride alloys of InSb grown by plasma‐assisted molecular beam epitaxy is presented. The layers were characterised by high‐resolution X‐ray diffraction indicating high crystalline quality and nitrogen incorporations up to 0.68%. Fourier‐transform infrared absorption measurements reveal the position of the absorption edge to be a result of the competing effects of bandgap reduction (due to nitrogen incorporation and bandgap renormalisation) and Moss–Burstein band filling.