Photoluminescence Spectral Features Research Articles

Studies on optical absorption and photoluminescence of thioglycerol-stabilized CdS quantum dots

Nanoparticles of CdS were prepared at 303 K by aqueous precipitation method using CdSO 4 and (NH 4) 2S in presence of the stabilizing agent thioglycerol. Adjustment of the thioglycerol (T) to ammonium sulphide (A) ratio (T:A) from 1:25 to 1:3.3 was done during synthesis and nanoparticles of different size were obtained. The prepared colloids were characterized by UV–vis and photoluminescence (PL) spectroscopic studies. Prominent first and second excitonic transitions are observed in the UV–vis spectrum of the colloid prepared with a T:A ratio of 1:3.3. Particle size analysis was done using XRD, high resolution TEM and dynamic light scattering and found to be ∼3 nm. UV–vis and PL spectral features also agree with this particle size in colloid with T:A of 1:3.3. The band gap of CdS quantum dots has increased from the bulk value 2.4–2.9 eV. PL spectra show quantum size effect and the peak is shifted from 628 to 556 nm when the ratio of T:A was changed from 1:25 to 1:3.3. Doping of CdS with Zn 2+ and Cu 2+ is found to enhance the PL intensity. PL band shows blue-shift and red-shift on doping with Zn 2+ and Cu 2+, respectively. UV and PL spectral features of the CdS/Au hybrid nanoparticles obtained by a physical mixing of CdS and Au nanoclusters in various volume ratios is also discussed. Au red-shifts and rapidly quenches the PL of CdS. An additional low energy band ∼650 nm is observed in the UV visible spectrum of the hybrid nanoparticles.

Cooperative Recombination of a Quantized High-Density Electron-Hole Plasma in Semiconductor Quantum Wells

We investigate photoluminescence from a high-density electron-hole plasma in semiconductor quantum wells created via intense femtosecond excitation in a strong perpendicular magnetic field, a fully quantized and tunable system. At a critical magnetic field strength and excitation fluence, we observe a clear transition in the band-edge photoluminescence from omnidirectional output to a randomly directed but highly collimated beam. In addition, changes in the linewidth, carrier density, and magnetic field scaling of the photoluminescence spectral features correlate precisely with the onset of random directionality, indicative of cooperative recombination from a high-density population of free carriers in a semiconductor environment.

Photoluminescence of wurtzite ZnO under hydrostatic pressure

Photoluminescence (PL) spectra of single-crystal ZnO bulk under hydrostatic pressure are studied using the diamond-anvil-cell technique at room temperature. The PL spectrum of ZnO single crystal taken at atmospheric pressure was dominated by a strong near-band-edge exciton emission. The emission line was found to shift towards higher energy with increasing applied pressure. By examining the pressure-dependent PL spectral features, the pressure coefficient of the direct Γ band gap of the wurtzite ZnO was determined. The hydrostatic deformation potential of the band gap has also been deduced from the experimental results.

Photoluminescence study of ZnCdSe/ZnSe quantum dot systems

The optical properties of the populated ZnCdSe/ZnSe quantum dots have been studied by photoluminescence spectra measured with different laser excitation apertures at temperatures from 22 to 300 K. The differences of spectral features between small and large excitation spot suggest the existence of quantum dot size fluctuation in the system. The temperature evolution of photoluminescence spectral features revealed that two types of quantum dots with different densities and sizes coexist in ZnCdSe/ZnSe system. The energy spacings of the two kinds of quantum dot emissions are about 50 meV at various temperatures. The thermally activated lateral transfer processes of carriers populated in the two sorts of quantum dots are investigated by temperature dependences of spectral intensities.

Excitation-Wavelength Dependent and Time-Resolved Photoluminescence Studies of Europium Doped GaN Grown by Interrupted Growth Epitaxy (IGE)

AbstractThe emission properties of Eu doped GaN thin films prepared by interrupted growth epitaxy (IGE) were investigated through excitation-wavelength dependent and time-resolved photoluminescence (PL) studies. Under above-gap excitation (333-363 nm) large differences were observed in the Eu3+ PL intensity and spectral features as a function of Ga shutter cycling time. The overall strongest red Eu3+ PL intensity was obtained from a sample grown with a Gashutter cycling time of 20 minutes. The main Eu3+ emission line originating from 5D0→ 7F2 transition was composed of two peaks located at 620 nm and 622 nm, which varied in relative intensity depending on the growth conditions. The room-temperature emission lifetimes of the samples were non-exponential and varied from ∼50 νs to ∼200 νs (1/e lifetimes). Under resonant excitation at 471 nm (7F0→5D2) all samples exhibited nearly identical PL spectra independent of Ga shutter cycling time. Moreover, the Eu3+ PL intensities and lifetimes varied significantly less compared to above-gap excitation. The excitation wavelengths dependent PL results indicate the existence of different Eu3+ centers in GaN: Eu, which can be controlled by the Ga shutter cycling time.

Characterization of Defects Generated During Boron Diffusion in SiC

AbstractThe defects associated with the implantation and diffusion of boron in silicon carbide have been studied using secondary ion mass spectrometry (SIMS) and photoluminescence (PL) imaging and spectroscopy. An n-type epitaxial SiC (1000) substrate was implanted with 2×1014 atoms/cm2 B and annealed to 1700°C. PL data was acquired before and after annealing, and following removal of various thicknesses of the sample by mechanical polishing. Thermal annealing generated a B diffusion profile measured by SIMS to extend to about 3 microns depth. After removing the diffused B layer, a PL spectral feature at 415nm disappeared, which is consistent with its previous identification as arising from donor-acceptor pairs (DAP). The D1 spectral features survived polishing, supporting previous suggestions that these features are intrinsic defects due to the di-interstitial (Ic-Ic or Isi-Isi) or di-vacancy (Vc-Vc or Vsi-Vsi) defects.

Modeling of the photophysical properties of α-sexithiophene thin films

Photoluminescence spectral features of α-sexithiophene (α-T 6) thin films have been demonstrated to be directly linked to film morphology and temperature. Ultra-thin films at low temperature show vibronic resolved photoluminescence (PL) spectra typical of intrinsic excitonic emission. As film thickness increases, PL spectra show additional emission at low energy, which is attributed to trap states. The nature of the trap states is to date elusive as well as the non-radiative mechanism involving exciton and trap states. Here, we report a phenomenological model that allows to determine the photophysical rate constants of the excitonic and trap state transitions and the density of traps in α-T 6 thin films. Our model accounts for the thickness dependence of α-T 6 films optical emission and is well in agreement with the observed variation of photoluminescence as a function of temperature in the range of 4–300 K.

Broad and narrow bands in the photoluminescence spectrum of solid-state oligothiophenes: Two marks of an intrinsic emission

The optical properties of quaterthiophene (4T) films have been studied and related to the presence of perfectly crystalline and orientationally disordered domains. The translational order is always retained, however, since 4T molecules lack a reflection symmetry plane perpendicular to the long molecular axis, each molecule can assume two nonequivalent orientations. The electronic state energies are almost configuration independent, but the selection rules for the transition from the first excited state towards the ground state are completely different in well-oriented and orientationally disordered domains. All observed photoluminescence spectral features of high-quality films are interpreted in terms of intrinsic emissions. The model of chiral and achiral pinwheel aggregates provides theoretical support to this picture.

Relationship between mechanical grinding and photoluminescence of zirconia-toughened-alumina ceramics

Zirconia-toughened-alumina (ZTA) ceramics have been investigated by means of photoluminescence (PL) experiments at room temperature. Alumina and zirconia components of ZTA ceramics display characteristic PL emission spectra in the visible region. The spectrum for the former is confirmed to be originated in Cr3+ ion impurities while for the latter it does in FA-type centres (oxygen vacancy associated with an impurity). Samples mechanically polished and grown for one face with sandpaper—of different grades—show significant changes in the main PL spectral features with respect to untreated faces, also in time decay curves. The changes are discussed in terms of effects of microstructural variations by the mechanical grinding on the electron-hole recombination processes in the centres mentioned above. The main structural variation in the samples is related to the transformation of the tetragonal to the monoclinic phase of zirconia.

Luminescence properties of Tb 3+-doped PbO–Bi 2O 3–GeO 2 glasses

This paper reports the luminescence properties of (13− x)PbO–25Bi 2O 3–62GeO 2 glasses doped with Tb 3+ ions ( x=0.2, 0.5, 1.0, 1.5 and 2.0 mol%) that have been investigated upon excitation at λ ex=248, 261, 352, 370 and 380 nm, respectively. At excitation wavelength of 370 nm, 1 mol% Tb 3+ glass has shown an intense green emission and the photoluminescence spectral features have been analyzed.

Luminescence properties of Tb 3+ doped PbO–Bi 2O 3–GeO 2 glasses

This paper reports the luminescence properties of (13− x)(PbO–25Bi 2O 3–62GeO 2) glasses doped with Tb 3+ ions ( x=0.2, 0.5, 1.0, 1.5 and 2.0 mol%) which have been investigated upon excitation at λ ex=248, 261, 352, 370 and 380 nm, respectively. At excitation wavelength 370 nm, 1 mol% of Tb 3+ glass has shown an intense green emission and the photoluminescence spectral features have been analysed.

Influence of annealing on optical properties of cerium doped soda-lime–silicate glasses

Effects of annealing on optical absorption (OA) and photoluminescence (PL), emission and excitation spectra from a cerium doped soda-lime–silicate glass ion-exchanged with silver were investigated. Annealing experiments were performed at temperatures of 350 and 400°C, far below the 569°C transition temperature measured for the doped glass. Evidence of surface-plasmon resonance of metallic silver nanoparticles and its subsequent disappearance in the OA spectra by annealing effects are reported. In this case, the reduction of Ag + to elemental silver and further reoxidation are governed in part by the reaction Ce 3++Ag +↔Ce 4++Ag 0. Results are in agreement with a preferential formation of silver nanoparticles at the surface, where the strain energy is more easily relaxed. On the other hand, PL spectra are also affected by the annealing, principally excitation spectra. Analyses of these spectra by de-convolution into three Gaussian shapes were carried out. The influence of annealing has a special significance for the excitation component centered between 3.94 and 3.84 eV (∼315 and 323 nm), related to Ce 3+ centers. Superficial increase in the number of Ce 4+ partially prevents excitation of remaining Ce 3+ ions and, therefore, enhances self-absorption in the glass. The main changes in the OA and PL spectral features are discussed in terms of the reducing role of Ce 3+ ions and on the basis of thermal relaxation of the surface tensile stress caused by differences in size between Ag + and Na + ions.

Optical and electrical properties of silicon nanocrystals formed by CW laser irradiation of amorphous silicon oxides

Optical and electrical properties of silicon nanocrystals formed by CW laser-treated hydrogenated amorphous silicon–oxygen alloys have been investigated as a function of the irradiation time, for different laser power densities and alloy compositions. Raman scattering measurements indicate that laser irradiation yields a mixed phase structure consisting of silicon nanocrystals (Sinc) embedded in an oxygen-rich amorphous matrix. It is shown that both photoluminescence (PL) and electrical characteristics clearly reflect this phase separation, resulting in a wavelength-dependent PL excitation and spectral features. Largely different conduction paths related to carrier transport within Sinc and intergrain oxide are also detected.

Optical investigation of modified Stranski–Krastanov growth mode in the stacking of self-assembled Ge islands

The modification of the Stranski–Krastanov growth mode produced by the strain-field from buried islands has been non-destructively investigated by photoluminescence (PL) spectroscopy in lattice-mismatched Ge/Si system. The samples consist of double Ge layers with each coverage of 4.5 monolayers separated by a Si spacer layer with various widths. At an intermediate Si spacer thickness of 100–200 Å, the modification of the growth mode of the second Ge layer was clearly evidenced in the PL spectral feature which was characterized by the superposition of two different PL spectra. In addition, a drastic increase of the density of Ge islands in the second layer was observed by atomic force microscopy. These facts show that the strain-field from the buried islands is very important to control the growth mode of lattice-mismatched semiconductor heterostructures.

Influence of impurities (nitrogen, oxygen) on Er-related emission in gallium phosphide

Erbium (Er) was co-implanted with nitrogen (N) or oxygen (O) into gallium phosphide (GaP). Compared with GaP:Er sample, the Er 3+-related 1.54 μm photoluminescence (PL) intensity from GaP:Er doped with N was enhanced approximately six times stronger, while no noticeable enhancement was observed for specimens with O. The PL spectral features of GaP:Er, N or O were shown to be different from these of GaP:Er. We believe that these differences in the Er-related luminescence are due to the formation of optically radiative complexes composed of Er and impurity atoms. As compared with O, it is found that N is better impurity which give rise to more efficient energy-transfer to Er 3+ ion.

Growth of ZnS epitaxial layers by photo-assisted MOCVD method and Na-acceptor impurity related emission properties.

Metalorganic chemical vapor deposition (MOCVD) growth assisted by UV light illumination for nominally pure ZnS epitaxial films has been carried out in a low-pressure system. The effects of UV light (>3 eV) have been investigated using low-temperature photoluminescence (PL) measurement. As a result, an increase in Na-related emission lines and a reduction of deep level emission have been observed in the PL spectra. This suggests that the improvement of the crystallinity and introduction of the Na acceptor-impurity substituted on Zn lattice site occured during the epitaxial growth under UV light illumination. We consider that these effects arise from the migration enhancement of adsorbed Zn atoms and/or Zn vacancies by irradiation during the growth, which will lead to a significant change of Zn and S compositional ratio. Finally, we found it profitable to apply these UV light illumination effect to the Na acceptor doping, and thus carried out the photo-assisted Na doping in the same system. A typical PL spectral feature of p-type ZnS films can be successfully obtained.

Photoluminescence line shape due to arrayed steps at the interfaces of GaAs/AlGaAs single quantum wells grown on vicinal surfaces by molecular beam epitaxy

Systematic variations in excitonic photoluminescence spectral features were observed for growth-interrupted GaAs/AlGaAs SQW's on GaAs(100) vicinal substrates for a variation in the misorientation angle (toward the (111)A) from 0.5° to 4.0°. These were explained by a structural model of hetero-interfaces composed of staircases which have monolayer straight steps and terraces with a fluctuating width characteristic of the angles. An analysis, which includes the effect of the fluctuation in terrace widths relative to the exciton size is given to explain the observed narrowing of the luminescence linewidth for the highest misorientation angle examined.

High-density excitonic state in two-dimensional multiple quantum wells.

A sharp photoluminescence spectral feature has been observed in ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As-GaAs multiple-quantum-well structures under high-intensity, resonant excitation at the ground-state exciton energy. This feature, which appears below the exciton ground state, emerges from a cold dense system of excitons, but prior to the breakup of excitons into an electron-hole plasma. A collective excitonic state is speculated.