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The growth of Cd x Mg 0.125- x Zn 0.875 O nanoparticles with yellow-orange luminescence is achieved up to 2.5 at. % Cd via a modified sol–gel process. X-ray diffraction analysis confirmed that all the nanoparticles have the hexagonal wurtzite structure. It is found that Cd doping has a considerable effect on the crystal size, microstrain, band gap, and photoluminescence of the Mg 0 · 125 Zn 0 · 875 O structure, originating from a preferred crystallographic orientation along the (101) plane of the wurtzite structure. The shift and broadening of the E 2 (high) mode observed in the Raman spectra due to growth-induced strain corroborates the small distortion observed in the X-ray diffraction data. The optical band gap varies from 3.21 eV to 2.74 eV, being redshifted with increasing Cd concentration (from 0 at. % to 2.5 at. %). The photoluminescence obtained with an excitation wavelength of 325 nm has a broad yellow-orange emission peak at around 640 nm due to transitions related to oxygen vacancies and interstitial oxygen atoms. We located the yellow-orange emission in the chromaticity coordinate diagram in the 2683–2777 K colour temperature region, demonstrating that Cd x Mg 0.125- x ZnO 0.875 nanoparticles have potential applications in white light-emitting diodes. • Enhanced optical properties induced by oxygen defects along the (101) plane of the wurtzite structure are discussed. • Rietveld analysis and the E 2- (high) phonon mode revealed an increase in the tensile stress component with Cd concentration. • By exchange interaction of the Cd 4 d and O 2 p orbitals, the band gap is modified and sublevels are created. • Photoluminescence spectra show transitions associated with oxygen defects, which increase with the amount of Cd. [ABSTRACT FROM AUTHOR]

Raman spectroscopy is widely used to study bulk and nanomaterials, where information is frequently obtained from spectral line positions and intensities. In this study, we monitored the Raman spectrum of ensembles of semiconductor nanocrystals (NCs) as a function of optical excitation intensity (optical excitation experiments). We observe that in NCs the red-shift of the Raman peak position with increasing light power density is much steeper than that recorded for the corresponding bulk material. The increase in optical excitation intensity results also in an increasingly higher temperature of the NCs as obtained with Raman thermometry through the commonly used Stokes/anti-Stokes intensity ratio. More significantly, the obtained dependence of the Raman peak position on temperature in optical excitation experiments is markedly different from that observed when the same NCs are excited only thermally (thermal excitation experiments). This difference is not observed for the control bulk material. The inefficient diffusion of photogenerated charges in nanoparticulate systems, due to their inherently low electrical conductivity, results in a higher steady-state density of photoexcited charges and, consequently, also in a stronger excitation of optical phonons that cannot decay quickly enough into acoustic phonons. This results in a nonthermal population of optical phonons and thus the Raman spectrum deviates from that expected for the temperature of the system. Our study has major consequences to the general application of Raman spectroscopy to nanomaterials. [ABSTRACT FROM AUTHOR]

Surface acoustic wave (SAW) devices are key components in modern wireless communication systems. Besides being used as reflective delay lines, filters or resonators, SAW devices can be used as sensors and fixed-code identification tags. Due to their high-acoustic velo-city, diamond films can be combined with piezoelectric materials to fabricate diamond-based SAW devices with potential for high frequency applications. This paper presents results of coating commercial SAW delay lines with diamond films. The films were deposited on lithium niobate under different conditions in a hot filament chemical vapour deposition system. Seeding was performed using 6-12 μm diamond grit and 20-30 nm nano-diamond particles based suspensions. After tuning the deposition conditions, dies of 74 MHz delay lines were coated with diamond. The quality, morphology and conformability of the coatings were evaluated by scanning electron microscopy (SEM) and Raman spectroscopy. The integrity of the interdigitated transducers (IDTs) after deposition was evaluated by SEM. S-parameters of as-bought and diamond-coated dies were measured using a network analyser. Although the frequency response is attenuated after diamond deposition, integrity of the IDTs and the operating principle of the devices are kept intact. These preliminary results open the door to a simplified fabrication procedure of diamond-based SAW devices. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

We report on near band edge and local defects emissions in Cd 0·025 Mg 0·10 Zn 0·875 O (CdMgZnO) nanoparticles (NPs) as a function of temperature, where a strong temperature-dependent near-infrared emission around 1.7 eV (~730 nm) has been observed. The NPs were synthesized by a modified sol-gel method and were annealed at 750 °C after growing. The crystallographic parameters have been determined by 2-dimensional synchrotron x-ray diffraction (XRD) and conventional XRD analysis, confirming their growth within the wurtzite phase with a preferred orientation along the (101) plane and an apparent crystallite size of 52.72 ± 0.18 nm. This apparent crystallite size is consistent with the nearly hexagonal particle shape (50.15 ± 0.38 nm) obtained from high resolution transmission electron microscopy. Valence Electron Energy Loss Spectroscopy (VEELS) has been conducted to gain information on the band structure and interband transitions. From the VEELS spectrum, an onset bandgap of 3.2 ± 0.3 eV was attributed to Cd/Mg co-doped ZnO. The temperature-dependent photoluminescence properties and enhancement of the optically active local defects emission have been analyzed. • Cd 0·025 Mg 0·10 Zn 0·875 O nanoparticles showed near band edge (NBE) and local defects emission at low temperature. • The 2D, 1D XRD pattern, and their Reitveld refinements established a disorder of wurtzite phase. • EELS generated an onset band gap at 3.22 ± 0.02 eV in particles of a nearly hexagonal shape. • Near infrared emission intensity enhancement changes significantly with decrease in temperature. [ABSTRACT FROM AUTHOR]

Abstract: Titano-vanadium silicate with the structure of ETS-4 has been hydrothermally synthesized to produce V-ETS-4. Direct comparison between ETS-4 and V-ETS-4 showed successful isomorphous substitution of titanium by vanadium. The existence of vanadium is confirmed by energy dispersive X-ray spectrometry (EDS). 51V and 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy revealed that the vanadium is present as V4+ and prefers the octahedral site in the ETS-4 structure. Diffuse reflectance ultraviolet–visible (DR UV–Vis) and Raman spectroscopy analyses showed the existence of five- and six-coordinated V4+, which partly substitute TiO5 square-pyramids and interrupts the chains of TiO6 octahedra in the ETS-4 structure. This isomorphous substitution also impacts the thermal stability of V-ETS-4 and it was observed by differential scanning calorimetry (DSC). [Copyright &y& Elsevier]

ZnO microrods were grown by laser assisted flow deposition technique in order to study their luminescence behaviour in the near band edge spectral region. Transmission electron microscopy analysis put in evidence the high crystallinity degree and microrod's compositional homogeneity. Photoluminescence revealed a dominant 3.31 eV emission. The correlation between this emission and the presence of surface states was investigated by performing plasma treatments with hydrogen and nitrogen. The significant modifications in photoluminescence spectra after the plasma treatments suggest a connexion between the 3.31 eV luminescence and the surface related electronic levels. [ABSTRACT FROM AUTHOR]

We studied the optical properties of metalorganic chemical vapour deposited (MOCVD) InGaN/GaN multiple quantum wells (MQW) subjected to nitrogen (N) implantation and post-growth annealing treatments. The optical characterization was carried out by means of temperature and excitation density-dependent steady state photoluminescence (PL) spectroscopy, supplemented by room temperature PL excitation (PLE) and PL lifetime (PLL) measurements. The as-grown and as-implanted samples were found to exhibit a single green emission band attributed to localized excitons in the QW, although the N implantation leads to a strong reduction of the PL intensity. The green band was found to be surprisingly stable on annealing up to 1400°C. A broad blue band dominates the low temperature PL after thermal annealing in both samples. This band is more intense for the implanted sample, suggesting that defects generated by N implantation, likely related to the diffusion/segregation of indium (In), have been optically activated by the thermal treatment. [ABSTRACT FROM AUTHOR]