Raman and photoluminescence spectroscopy of zinc tungstate powders

ZnWO₄ powders, synthesized using co-precipitation technique and annealed in air at different temperatures in the range of 80-, were studied by Raman and photoluminescence spectroscopy. ZnWO₄ single crystal was used for comparison. The interpretation of the observed variations of the Raman spectra and intrinsic photoluminescence band upon annealing is suggested.     

Probing disorder and charged impurities in graphene by Raman spectroscopy

Disorder and doping can strongly affect the properties of graphene. Here we analyze these effects on several samples by Raman spectroscopy. In particular, we show that pristine and unprocessed graphene samples deposited on silicon, covered with a thin silicon oxide layer, show strong variations in their Raman spectra, even in absence of disorder. The variation in the Raman parameters is assigned to charged impurities. This shows that as‐deposited graphene is unintentionally doped, reaching charge concentrations up to 10¹³ cm^–2 under ambient conditions. The doping varies from sample to sample and the charges are inhomogeneously distributed on a submicron scale. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characteristics of photoluminescence and Raman spectra of InP doped silica fiber

We fabricated a silica optical fiber doped with InP sandwiched in the core and the inner cladding layers by using the conventional modified chemical vapor deposition process. We presented the experimental X-ray analysis on the optical properties and found that compound InP was contained in the fiber core after annealing process. Broadband photoluminescence observed in InP doped fiber was well coincided with those coming from the recombination of deep levels in InP. The occurrence of temperature-dependent photoluminescence both at the lower and room temperature would be related with the localized defects and their confinement in the micro-network structure of Si-O-Si. The Raman spectra reveal that Si-O-In vibration would disorder the silica ring structures and enervate their vibrations.     

Nano-star formation in Al-doped ZnO thin film deposited by dip-dry method and its characterization using atomic force microscopy, electron probe microscopy, photoluminescence and laser Raman spectroscopy

Zinc oxide doped with Al (AZO) thin films were prepared on borosilicate glass substrates by dip and dry technique using sodium zincate bath. Effects of doping on the structural and optical properties of ZnO film were investigated by XRD, EPMA, AFM, optical transmittance, PL and Raman spectroscopy. The band gap for ZnO:Al (5.0 at. wt.%) film was found to be 3.29 eV compared with 3.25 eV band gap for pure ZnO film. Doping with Al introduces aggregation of crystallites to form micro-size clusters affecting the smoothness of the film surface. Al³⁺ ion was found to promote chemisorption of oxygen into the film, which in turn affects the roughness of the sample. Six photoluminescence bands were observed at 390, 419, 449, 480, 525 and 574 nm in the emission spectra. Excitation spectra of ZnO film showed bands at 200, 217, 232 and 328 nm, whereas bands at 200, 235, 257 and 267 nm were observed for ZnO:Al film. On the basis of transitions from conduction band or deep donors (CB, Zn_i or V_OZn_i) to valence band and/or deep acceptor states (VB, V_Zn or O_i or O_Zn), a tentative model has been proposed to explain the PL spectra. Doping with Al³⁺ ions reduced the polar character of the film. This has been confirmed from laser Raman studies.     

Origin of photoluminescence in nanocrystalline Si:H films

We have studied the origin of photoluminescence (PL) from hydrogenated nanocrystalline silicon (nc-Si:H) films produced by a plasma-enhanced chemical vapor deposition technique using SiF₄/SiH₄/H₂ gas mixtures. The nc-Si:H films were characterized using X-ray diffraction, infrared, Raman spectroscopy, optical absorption and stress, and were examined for PL by varying the deposition temperature (T_d) under two different hydrogen flow rate ([H₂]) conditions. The PL exhibited two peaks at around 1.7-1.75 and 2.2-2.3 eV. The peak energy, E_PL, of the 1.7-1.75-eV PL band was found to shift as T_d or [H₂] changes. It was found that the decrease in T_d acts to decrease the average grain size, 〈δ〉, and to increase both the optical band gap, , and the E_PL values. By contrast, the increase in [H₂] decreased the 〈δ〉 value, while increased the values of and E_PL. Thus, as either T_d decreases or [H₂] increases, it is found that a decrease in 〈δ〉 corresponds well with increases in and E_PL. As a consequence, it was suggested that an increase in E_PL of the 1.7-1.75-eV PL band can be connected with an increase in , through a decrease in 〈δ〉. However, the PL process cannot be connected with the transition between both the bands, related to formation of nanocrystals. Based on these results, it was proposed that the use of both low T_d and high [H₂] conditions would allow to grow nc-Si:H films with small grains.     

Porous silicon as efficient surface enhanced Raman scattering (SERS) substrate

Silver nanocrystallites are obtained through immersion of porous silicon samples in AgNO₃ solutions and a successive thermal annealing. The efficiency of nanostructures as surface enhanced Raman scattering (SERS) substrates is checked on cyanine-based dyes and horseradish peroxidase, evidencing detectable concentrations as low as 10⁻⁷ to 10⁻⁸ M. The substrate efficiency is strictly related to the Ag particle morphology, which could yield to either local surface plasmons (LSP) coupled to individual particles or to inter-particle short-range interaction.     

Decay dynamics of ultraviolet photoluminescence in ZnO nanocrystals

Time resolved photoluminescence (PL) measurements at low temperature are performed on colloidal ZnO nanocrystals dispersed in t-butanol. Considering the particle size dependence of the decay times we conclude that the luminescence is composed of two trap related emissions one of which undergoes lifetime shortening due to a non-radiative process. Initial fast shift of the spectrum within 30 ps is observed and it is interpreted as a fast hole cooling just after the excitation.     

Phonon confinement effect on nanocrystalline LiCoO2 studied with Raman spectroscopy

A systematic investigation on nanocrystalline LiCoO₂ has been carried out using Raman spectroscopy. We synthesized nanocrystalline LiCoO₂ (ca. 20-50 nm) through a combination of rapid thermal annealing at various annealing temperatures and a sol-gel method assisted with a triblock copolymer surfactant. Powder X-ray diffraction measurements revealed the formation of LiCoO₂. The crystallite size of LiCoO₂ from the Scherrer equation strongly depended on the annealing temperature. The crystallite size was confirmed by SEM and TEM measurements. Raman shifts of the A_1g and E_g modes for nanocrystalline LiCoO₂ exhibited a broadening and a frequency shift according to the crystallite size. While the frequency shift could be ascribed to a structural strain at the surface, the broadening was due to the phonon confinement effect produced by narrow crystal boundaries.     

Photoluminescence scanning on InAs/InGaAs quantum dot structures

The photoluminescence spectra of InAs quantum dots (QDs) embedded into four types of In_xGa_1−xAs/GaAs (x = 0.10, 0.15, 0.20 and 0.25) multi quantum well MBE structures have been investigated at 300 K in dependence on the QD position on the wafer. PL mapping was performed with 325 nm HeCd laser (35 mW) focused down to 200 μm (110 W/cm²) as the excitation source. The structures with x = 0.15 In/Ga composition in the In_xGa_1−xAs capping layer exhibited the maximum photoluminescence intensity. Strong inhomogeneity of the PL intensity is observed by mapping samples with the In/Ga composition of x ≥ 0.20-0.25. The reduction of the PL intensity is accompanied by a gradual “blue” shift of the luminescence maximum at 300 K as follows from the quantum dot PL mapping. The mechanism of this effect has been analyzed. PL peak shifts versus capping layer composition are discussed as well.     

Raman spectroscopic and photoluminescence study of single-crystalline SnO2 nanowires

Single-crystalline SnO₂ nanowires with sizes of 4-14 nm in diameter and 100-500 nm in length were produced in a molten salt approach by using hydrothermal synthesized precursor. Structural characters of the nanowires were examined by X-ray diffraction and high-resolution electron transmission microscopy. Raman, photoluminescence and X-ray photoelectron spectra of the samples were examined under heat treatments. Three new Raman modes at 691, 514 and 358 cm⁻¹ were recorded and assigned. The former two are attributed to activation of original Raman-forbidden A_2uLO mode and the third is attributed to defects in small-sized nanowires. A strong photoluminescence is observed at about 600 nm, the temperature effects is examined and the origin of the PL process is discussed via X-ray photoelectron spectra.     

Optical property investigation of SiGe nanocrystals formed by electrochemical anodization

We report on the optical property investigation of SiGe nanocrystals (NCs) prepared by electrochemical anodization (ECA) of SiGe layer grown by ultrahigh vacuum chemical vapor deposition (UHVCVD). At room temperature, SiGe NCs with higher Ge content demonstrate a redshift of the photoluminescence (PL) peak compared to Si NCs. It was found that the surface chemical composition, density, and the size of the SiGe NCs were very sensitive to the annealing conditions. Various spectroscopy measurements such as PL, FTIR, and XPS have been carried out to reveal the mechanism of the PL peak transition. The results indicated that the PL peak position was determined by two major factors, namely, interface state density and the size of SiGe NCs. It was shown that the higher the interface state density, the more significant the redshift of the peak position. While the smaller the size of the SiGe NCs, the more significant the quantum size effects become, resulting in the blueshift of the PL peak position.     

Influence of the SiO thickness on the photoluminescence properties of Er-doped SiO/SiO2 multilayers

Er-doped SiO single layer and Er-doped SiO/SiO₂ multilayers with different SiO thicknesses were prepared by evaporation. In the as-deposited samples, the erbium ions exhibit a very weak photoluminescence emission at 1.54 μm. This luminescence is strongly enhanced after annealing treatments between 500 and 1050 °C, with an optimal annealing temperature which is dependent from the SiO thickness. For the SiO single layer, this optimal temperature is around 700 °C while it is shifted at highest temperature for the multilayers. The origin of the higher luminescence intensity in the SiO layer is also discussed.     

Ar irradiation of Si nanocrystal-doped SiO2: Evolution of photoluminescence

We report the evolution of photoluminescence (PL) of Si nanocrystals (nc-Si) embedded in a matrix of SiO₂ during Ar⁺ ion bombardment. The integrated intensity of nc-Si PL falls down drastically before the Ar⁺ ion fluence of 10¹⁵ ions cm⁻², and then decreases slowly with the increasing ion fluence. At the meantime, the PL peak position blueshifts steadily before the fluence of 10¹⁵ ions cm⁻², and then changes in an oscillatory manner. Also it is found that the nc-Si PL of the Ar⁺-irradiated sample can be partly recovered after annealing at 800 °C in nitrogen, but can be almost totally recovered after annealing in oxygen. The results confirm that the ion irradiation-induced defects are made up of oxygen vacancies, which absorb light strongly. The oscillatory peak shift of nc-Si can be related to a size-distance distribution of nc-Si in SiO₂.     

Effects of citric acid additive on photoluminescence properties of YAG:Ce nanoparticles synthesized by glycothermal reaction

We synthesize Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) nanoparticles in the presence of citric acid by glycothermal method. Fourier transform infrared absorption spectroscopy measurement indicates that the intensity of the peak corresponding to carboxyl groups coordinating to the nanoparticles increases with increasing amount of citric acid. At the same time, the primary particle diameter decreases from 10.2 to 4.0 nm. In addition, the internal quantum efficiency of the photoluminescence (PL) due to the 4f-5d transition of Ce³⁺ increases from 22.0% to 40.1% with increasing amount of citric acid. Two kinds of PL decay lifetimes, 16-26 and 72-112 ns, are detected for YAG:Ce³⁺ nanoparticles, whereas the micron sized YAG:Ce³⁺ bulk shows the lifetime of 57 ns. We discuss these phenomena from the aspects of the coordination of citric acid and the incorporation of Ce³⁺ ions into the nanoparticles.     

A new blue-emitting phosphor, SrZnO2:Pb, synthesized by the adipic acid templated sol-gel route

A new blue-emitting phosphor, Sr_1−xPb_xZnO₂, was prepared by a novel adipic acid templated sol-gel route. Photoluminescence and crystalline properties were investigated as functions of calcination temperatures and the Pb²⁺ doping levels. It was found that under UV excitation with a wavelength of 283 or 317 nm, the phosphors gave emission from 374 to 615 nm with a peak centered at 451 nm. This broad-band was composed of UV and the visible range was attributed to an impurity-trapped exciton-type emission. The maximum emission intensity of the Sr_1−xPb_xZnO₂ phosphors occurred at a Pb concentration of x=0.01. The decay time was observed to be ∼33 ms for the compound doped with 1 mol% Pb prepared at 1000 °C. Diffuse reflectance spectra revealed the characteristic absorption peaks and the bandgap energy of SrZnO₂ was found to be 3.4 eV. SEM analysis indicated that phosphor particles have an irregularly rounded morphology and the average particle size was found to be approximately 1 μm.     

The dynamics of excitons and trions in resonant tunneling diodes

The aim of this work is to study the dynamic formation and dissociation of trions and excitons in double barrier resonant tunneling diodes. We propose a system of rate equations that takes into account the formation, dissociation and annihilation of these complexes inside the quantum well. From the solutions of the coupled equations, we are able to study the modulation of excitons and trions formation in the device as a function of the applied bias. The results of our model agree qualitatively with the experiments showing the viability of these rate equations system to study the dynamics of complex systems.     

Superheating of silicon nanocrystals observed by Raman spectroscopy

In the Raman spectra of silicon nanocrystals a new anomalous component was detected. Close to the usual first order Raman peak situated for a bulk crystal at 521 cm⁻¹ at room temperature, two peaks arise shifting towards lower energy and demonstrating a huge temperature increase, as measured by the ratio of the Stokes/anti-Stokes peak intensities. This behavior is dependent on the laser power and on the morphology of the nanocrystals. We can exclude, however, confinement effects, although surface enhanced phonon modes could be responsible of such superheating. Alternative explanations are also suggested and discussed.     

1.54 μm room temperature emission from Er-doped Si nanocrystals deposited by ECR-PECVD

In this work, silicon nanocrystals (Si-nc) embedded in a silicon-rich silicon oxide (SRSO) matrix doped with Er³⁺ ions for different erbium and silicon concentrations have been deposited by electron-cyclotron resonance plasma-enhanced chemical-vapor-deposition (ECR-PECVD) technique. Their optical properties have been investigated by photoluminescence (PL) and reflectance spectroscopy.Room temperature emission bands centered at ∼1.54 and at 0.75 μm have been obtained for all samples. The most intense emission band at ∼1.54 μm was obtained for samples with concentrations of 0.45% and 39% for erbium and silicon, respectively. Moreover, it has been found that the broad emission band centered at ∼0.75 μm for all samples shows a very strong interference pattern related to the a specific sample structure and a high sample quality.     

Preparation and luminescent properties of europium-doped yttria fibers by electrospinning

Sub-micrometer-sized fibers of europium-doped yttria (Y₂O₃:Eu³⁺) were prepared by electrospinning followed by high-temperature calcinations for the first time. The fibers were with diameters of 200-400 nm and lengths of several 10 μm and cubic in phase. The spectral properties of the Y₂O₃:Eu³⁺ fibers were studied, in contrast with those of bulk powders. The results indicated that in the present Y₂O₃:Eu³⁺ fibers the excited charge transfer band had slightly blue shift in comparison with that in the bulk due to weaker covalence of Eu-O bonds. In addition, both of the lifetimes of the ⁵D₁ and ⁵D₀ states in the fibers became shorter than that in the bulk due to improved nonradiative transition rates.