Enhanced visible light emission from Co doped ZnS nanoparticles

ZnS nanoparticles with Co²⁺ doping have been prepared at room temperature through a soft chemical route, namely the chemical co-precipitation method. The nanostructures of the prepared nanoparticles have been analyzed using X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), selected-area electron diffraction (SAED), and UV-vis spectrophotometer. The sizes of as prepared nanoparticles are found to be in 1–4 nm range. Room-temperature photoluminescence (PL) spectrum of the undoped sample exhibits emission in the blue region with multiple peaks under UV excitation. On the other hand, in the Co²⁺ doped ZnS samples enhanced visible light emissions with emission intensities of ~35 times larger than that of the undoped sample are observed under the same UV excitation wavelength of 280 nm.     

Preparation and luminescence of water soluble poly (N-vinyl-2-pyrrolidone)/LaF3:Eu nanocrystals

Lanthanide-doped luminescent nanocrystals have great potential as biological luminescent labels, but their use has been limited because of most of these nanocrystals are hydrophobic. In this work, water soluble LaF₃:Eu³⁺ down-conversion nanocrystals were prepared by encapsulated individual nanocrystals with polyvinylpyrrolidone (PVP). Their morphology, surface structure and luminescence properties were explored in detail. The results indicate that these nanocrystals can be readily dispersed in water, forming a stable and transparent colloidal solution. The colloidal solution displayed unique red luminescence with high emission intensity under ultraviolet excitation. These results suggest that these nanocrystals have great potential as luminescent labeling materials for biological applications.     

Emission of Cu-related complexes in ZnO:Cu nanocrystals

Photoluminescence (PL), its temperature dependence, scanning electronic microscopy (SEM) and X ray diffraction (XRD) have been applied for the comparative study of varying the emission, morphology and crystal structure of ZnO and ZnO:Cu nanocrystals (NCs) versus technological routines, as well as the dependence of ZnO:Cu NC parameters on the Cu concentration. A set of ZnO and ZnO Cu NCs was prepared by the electrochemical (anodization) method at a permanent voltage and different etching durations with follows thermal annealing at 400 °C for 2 h in ambient air. The size of ZnO NCs decreases from 300 nm×540 nm down to 200 nm×320 nm with etching duration increasing. XRD study has confirmed that thermal annealing stimulates the ZnO oxidation and crystallization with the formation of wurtzite ZnO crystal lattice. XRD method has been used for monitoring the lattice parameters and for confirming the Cu doping of ZnO Cu NCs. In ZnO Cu NCs four defect related PL bands are detected with the PL peaks at 1.95–2.00 eV (A), 2.15-2.23  eV (B), 2.43–2.50 eV (C) and 2.61–2.69 eV (D). Highest PL intensities of orange, yellow and green emissions have been obtained in ZnO Cu NCs with the Cu concentration of 2.28 at%. At Cu concentration increasing (≥2.28 at%) the PL intensities of the bands A, B, C decrease and the new PL band peaked at 2.61–2.69 eV at 10 K appears in the PL spectrum. The variation of PL intensities for all PL bands versus temperature has been studied and the corresponding activation energies of PL thermal decay have been estimated. The type of Cu-related complexes is discussed using the correlation between the PL spectrum transformation and the variation of XRD parameters in ZnO Cu NCs.     

Synthesis and time-resolved photoluminescence spectroscopy of capped CdS nanocrystals

Here, we report the synthesis of colloidal CdS nanoparticles by capping with starch, phenol and pyridine. We also study the photophysical properties of CdS nanoparticles by steady state and time-resolved photoluminescence spectroscopy. The experimental results show that the relaxation of the excited state of CdS nanoparticles is composed of two different components. Our analysis suggests that the fast and slow components decay times of these capped CdS nanocrystals are due to trapping of carriers in surface state and e–h radiative recombination processes, respectively.     

Single layer homogeneous model for surface roughness by polarized light scattering

In this paper a homogeneous single layer model for surface roughness by polarized light has been developed. It has been shown that the reflectance change in non-absorbing layer is directly proportional to the refractive index of the ambient and substrate media for s polarization but inversely proportional to the p polarization and it is directly proportional to the square of the thickness of the layer for both the polarization. In an absorbing layer, it has been shown that the thickness of the layer is equal to the twice of surface roughness of the single layer identical system for s polarization but it is ratio of twice of surface roughness to the square of refractive index of thin film for p polarization. The extinction coefficient of the layer is directly proportional to the thickness of that layer for both the polarization. The consequence of the scattered light on the specular reflectance and transmittance for oblique incidence shows that there is reduction in reflectance (in both non-absorbing and absorbing cases) and transmittance (in the absorbing case for p polarization only), due to roughness on the surface under the Drude effective-medium approximation. Thus such an absorbing layer provides a valid model for the effect of scatter on the transmittance for p polarization only.     

Synthesis of NaYF4 nanocrystals doped with Yb/Er and influence of citric acid on the green and red luminescence

In this paper, NaYF₄ nanocrystals doped with Yb³⁺ and Er³⁺ were synthesized in a medium containing polyethylene glycol and citric acid. This nanocrystal presents up-converting green and red emission bands which were simultaneously observed under the excitation of a 980 infrared diode laser. Mainly, the green to red ratio (GRR) of the up-conversion emission of the hexagonal NaYF₄ nanocrystals doped with Yb³⁺ and Er³⁺ can be finely tuned by changing the content of citric acid to be nearly an arithmetic progression, i.e. 6/4, 5/4, 4/4, and 3/4. The further analysis revealed that citric acid plays a key role in improving the surface crystallinity of NaYF₄ nanocrystals doped with Yb³⁺ and Er³⁺, to which the achievement of fine controlling on GRR is ascribed.     

Defect induced paramagnetism in lightly doped ZnS:Fe nanoparticles

Nanoparticles of ZnS:Fe (0, 1, 3, and 5 at%) were synthesized by a refluxing route at 80 °C. All the samples exhibited cubic structure as revealed by X-ray powder diffraction studies. Blue emission of undoped samples was totally quenched by Fe doping. Magnetic measurements showed that the undoped ZnS was diamagnetic whereas all the doped samples were paramagnetic at room temperature. EPR signal characteristic of Fe³⁺ was observed in all the doped samples at room temperature. The paramagnetism of the present samples is attributed to the presence of uncoupled Fe³⁺ ions mediated by cation vacancies.     

Origins of visible-light emissions in hydrogen-coated silicon nanocrystals: Role of passivating coating

We present a theoretical investigation of the electronic and optical properties of hydrogen-coated silicon nanocrystals (Si:H NCs). On one hand, the density-functional theory (DFT) is used to both calculate the total energy and relax the NCs. On a second hand, the tight-binding method, which includes the minimal sp³-basis set within the second-nearest-neighbor interaction scheme, is applied to calculate the electronic structures, oscillator strength (OS) and recombination rate (RR) versus the NC size, coating and atomic relaxation. Three main findings are reported: (i) The quantum confinement in these NCs do follow similar rule to the case of a single-particle in a box, where the confinement energy decays in power-law with the increasing NC's size. (ii) The coating is shown to play the essential role in creation of large band-gap energy lying within the visible-light energy spectrum. (iii) The surface atomic relaxation is found to reduce the band-gap energy by about 150 meV and enhance both OS and RR. Our claims are corroborated by the available experimental data.     

Effect of thermal annealing on r.f. sputtering-deposited nanocrystalline GaN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> thin films

GaNAs thin films were deposited on Corning glass substrates by radio frequency (r.f.) sputtering in molecular nitrogen ambient. The stoichiometry in the GaNAs alloy was controlled by changing the nitrogen incorporation in the film during the growth process, through the variation of the r.f. power in the range 30–80 watts which produced films with N concentrations in the range: x = 0.85–0.90. The structural and optical properties of the GaNAs thin films were studied by X-ray diffraction (XRD), photoacoustic (PA) and photoluminescence (PL) spectroscopies. XRD measurements show a broad diffraction band with a peak close to the (002) diffraction line of the GaN hexagonal phase, and a slight shoulder at the position corresponding to the (111) GaAs cubic phase. The PA absorption spectra showed a remarkable shift to higher energies of the absorption edge as the r.f. power decreases corresponding to the films with higher N concentrations. Thermal annealing of the GaNAs films at temperatures of 450 °C produced a GaAs nanocrystalline phase with grain sizes in the range 10–13 nm, as confirmed by the XRD measurements that showed a well-defined peak in the (111) GaAs direction, and also by the PA spectra which showed an absorption band at energies around 1.45 eV due to the quantum confinement effects. PL spectra of thermal-annealed GaNAs films showed a very intense emission at 1.5 eV which we have associated to transitions between the first electron excited level and acceptor states in the GaAs nanocrystallites.     

Luminescence enhancement of Mn doped ZnS nanocrystals passivated with zinc hydroxide

Mn-doped ZnS nanocrystals prepared by solvothermal method have been successfully coated with different thicknesses of Zn(OH)₂ shells through precipitation reaction. The impact of Zn(OH)₂ shells on luminescent properties of the ZnS:Mn nanocrystals was investigated. X-ray diffraction (XRD) measurements showed that the ZnS:Mn nanocrystals have cubic zinc blende structure. The morphology of nanocrystals is spherical shape measured by transmission electron microscopy (TEM). ZnS:Mn/Zn(OH)₂ core/shell nanocrystals exhibited much improved luminescent properties than those of unpassivated ZnS:Mn nanocrystals. The luminescence enhancement was observed with the Zn(OH)₂ shell thickening by photoluminescence (PL) spectra at room temperature and the luminescence lifetime of transition from ⁴T₁ to ⁶A₁ of Mn²⁺ ions was also prolonged. This result was led by the effective, robust passivation of ZnS surface states by the Zn(OH)₂ shells, which consequently suppressed nonradiative recombination transitions.     

Spectroscopic ellipsometry of layer by layer deposited colloidal HgTe nanocrystals exhibiting quantum confinement

We have explored the optical properties of bilayers of Mercury telluride (HgTe) nanocrystals (NCs) embedded in polymer which were prepared from a colloidal solution. These NCs show strong luminescence in the near infrared at room temperature, which makes them an interesting material for the telecommunication area. The emission wavelength can efficiently be tuned by controlling the size of the NCs. We report spectroscopic ellipsometry measurements, which clearly show an energy shift of the critical points (CPs) in the dielectric function to higher energies compared to the HgTe bulk properties. This is caused by quantum confinement in the crystals. The exact peak energies of the transitions are fitted with line-shape models for CPs. Surprisingly, concepts coming from semiconductor bulk optics, as CPs, can be applied to NCs with a diameter of less than 5 unit cells.     

Studies of silicon nanocrystals in phosphorus rich SiO2 matrices

In this contribution we present a new type of optoelectronic silicon nanocrystal (Si-nc) based material, namely, Si-nc embedded into solidified pure or doped spin-on-glasses. The resulting self-supporting samples contain thin layers with high Si-nc concentrations. The visible photoluminescence (PL) maximum at room temperature is blue-shifted when the concentration of phosphorus in the spin-on-glass is increased.     

Surface-defect-states photoluminescence in CdS nanocrystals prepared by one-step aqueous synthesis method

CdS nanocrystals were prepared by a simple one-step aqueous synthesis method using thioglycolic acid (TGA) as the capping molecule, and characterized by X-ray powder diffraction (XRD), UV-vis absorption spectra and photoluminescence (PL) emission spectroscopy. The effects of both TGA/Cd and Cd/S molar ratio on the surface-defect-state PL intensity of CdS nanocrystals have been investigated. It was found that all of the as-prepared CdS nanocrystals showed a strong broad emission in the range of 450-700 nm centered at 560 nm, which was attributed to the recombination of an electron trapped in a sulfur vacancy with a hole in the valance band of CdS. The surface-defect-states emission intensity of CdS nanocrystals significantly increased with the increase of Cd/S molar ratio, and showed a maximum when Cd/S molar ratio was 2.0. If Cd/S molar ratio continued to increase, namely more than 2.0, the surface-defect-states emission intensity would decrease. It was found that the surface-defect-states emission intensity increased with the increase the TGA/Cd molar ratio, and showed a maximum when the TGA/Cd molar ratio was equal to 1.8, and a further increase of the TGA/Cd molar ratio would lead to the decrease of the surface-defect-states emission intensity.     

Energy transfer dynamics between Alq3 and CdTeS/ZnS core shell nanocrystals

The photoluminescence (PL) properties of the guest-host films, using CdTeS/ZnS core shell quantum dots (QDs) as the guest and organic small-molecule material Alq₃ as the host, are studied by steady-state and time-resolved PL spectroscopy. Both the relative intensity and the PL lifetime are intensively dependent on the weight ratio of Alq₃ and CdTeS/ZnS QDs. The detailed analysis provides clear evidence for a Förster energy transfer from Alq₃ host to QDs guest, based on the nonradiative resonant transfer mechanism. The results are relevant to the application of hybrid organic/inorganic systems to OLEDs.     

Superlinear photoluminescence in silicon nanocrystals: The role of excitation wavelength

We study the influence of the wavelength of picosecond excitation pulses on the properties of photoluminescence (PL) in a series of samples of silicon nanocrystals prepared by ion implantation into silica matrix. We observed a gradual change in the behaviour of the PL fast component (spectral shape, decay times, pump-intensity dependence) when tuning the excitation wavelength from 355 to 532 nm. We interpret the results in terms of an interplay between the PL originating from volume states of nanocrystals containing two photoexcited carrier pairs, and the PL due to the silicon oxide states. We discuss also the role of the implant fluence on the PL properties of samples.     

Surface control of optical properties in silicon nanocrystals produced by laser pyrolysis

Macroscopic quantities (g/h) of Si nanoparticles were prepared by laser pyrolysis of silane and showed photoluminescence (PL) emission in the range 700-1050 nm after oxidation in air at a temperature T ≥ 700 °C. Two different strategies were followed to reduce as-produced particle agglomeration which hinders most of the applications, namely etching with either acid or alkaline solutions. Well isolated single particles were detected after acid etching in HF. Disaggregation was also achieved by the combined effect of the high power sonication and alkaline etching by tetra-methyl ammonium hydroxide (TMAH), which leaves OH terminated surfaces. However, in both cases re-aggregation was observed within a few hours after oxide removal. Stable dispersions of Si nanoparticles in different solvents were obtained by treatments of H-terminated surfaces with the surfactant TOPO (C₂₄H₅₁PO, trioctylphospine oxide) and by treatment of OH-terminated surfaces with Na₃PO₄.     

Temperature dependence of photoluminescence and magnetic properties in Yb-doped ZnS nanocrystals

Zinc sul?de semiconductor nanocrystals doped with Yb³⁺ ions have been prepared through a chemical precipitation method using PVP (polyvinylpyrrolidone) as a capping agent. The structure of the nanoparticles has been analyzed by X-ray diffraction (XRD). The average size of the nanoparticles is found to be 3.7±0.4 nm. Photoluminescence spectra were recorded for doped ZnS nanoparticles as a function of temperature between 9 and 300 K. The results suggest that two emission bands have been observed at different temperatures. The width of 1.269 eV peak increases as temperature is raised. A shift of lower energy emission band has been observed with the change of temperature. Moreover, the magnetic measurement showed that the sample exhibits paramagnetic behavior.     

弱场中光抽运极化Eu原子束的制备

自五十年代首次实现光抽运技术以来,人们用它已对多种原子进行了极化原子束制备的研究[1～5],但所有这些工作只是以相对简单的碱金属原子为对象,而对价电子数大于1的复杂原子的极化原子束的制备目前尚未见报道.描述了用一台连续波染料激光器对复杂原子Eu进行了弱场制备极化原子束的实验研究,得到了较满意的结果.