Temperature dependence of photoluminescence from CdSe nanocrystals embedded in silica matrix

In this work, CdSe nanocrystals (NCs) embedded in SiO₂ matrix were grown by radio frequency (RF)-sputtering technique. X-ray technique was used to characterise the structural properties of the system. The NC's size was estimated to be around 4±1 nm in diameter. The temperature dependence of the photoluminescence from the CdSe/SiO₂ system showed carriers thermal exchange between the NCs and deep defects in the matrix. The evolution of the excitonic energy emission with temperature is about 10 meV in the temperature range 15-295 K. This weak shift was explained by thermal mismatch between the matrix and the NCs.     

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.     

Temperature dependence of photoluminescence spectra in n-type modulation-doped quantum dot arrays

Temperature dependence of photoluminescence (PL) spectra of an electrostatically prepared modulation-doped quantum dot array is investigated. We report a strong temperature dependence of the enhanced PL near the Fermi energy between 0.4 and 4.2 K under a negative bias condition, where an interconnected quantum dot array is formed. This strong temperature dependence suggests that the Fermi-surface-mediated many-body interaction between a photo-created hole and the electron-gas plays an important role in the observed enhanced PL near the Fermi energy.     

Temperature dependent photoluminescence study on ZnO/Graphene nanocomposite films

We report observation of both free and defect-mediated excitonic emissions from temperature-dependent PL study on ZnO/graphene oxide (G-O) nanocomposite grown by ultrasonic assisted spray pyrolysis (UASP). From the temperature-dependent photoluminescence (PL) spectra of the ZnO/G-O nanocomposite, new graphene-related peak was observed at 372 nm along with the exciton transition bound to neutral acceptors or deep donors. The PL intensity of new graphene-related peaks (3.33 eV) become more prominent with increasing G-O concentration, and it was saturated or decreased with the addition of >7.0 wt% of G-O. This feature indicates that new graphene-related states were created below conduction band of ZnO, which supports the excitonic PL enhancement by graphene-embedding is contributed not by charge transfer, but by vacancy filling effect of G-O.     

Excitation-power dependence of the near-band-edge photoluminescence of ZnO inverse opals and nanocrystal films

The excitation-power dependence of the near-band-edge emission in ZnO inverse opals and nanocrystal films has been studied. The dependence of the photoluminescence intensity I on the excitation power L can be described by a power law, i.e., I∼L^α, where α is an exponent that is often used to identify the origin of the near-band-edge emission from semiconductors in previous models. However, in this work, it was found that the values of α show a strong variation between ZnO inverse opals and nanocrystal films. And our results show that the change of α is mainly caused by the laser heating effects. Therefore, the value of α could not be simply employed to unequivocally evaluate the origin of the near-band-edge emission in complex nanostructures.     

Temperature and concentration dependences of the photoluminescence of MEH-PPV polymer composite films with ZnO nanoparticles

The absorption and photoluminescence (PL) spectra of MEH-PPV: ZnO composite films have been investigated at different concentrations of ZnO nanoparticles and at different temperatures (in the case of PL). It has been shown that, at 297 K, with increasing concentration of ZnO nanoparticles in the composite, the intensity of the PL lines of MEH-PPV decreases, whereas the intensity of the PL lines of ZnO increases. At a relatively low concentration of ZnO nanoparticles, a decrease in the temperature leads to an increase in the intensity of PL lines associated with MEH-PPV and ZnO, whereas at higher concentrations of ZnO nanoparticles, the intensity of these lines decreases. This is accompanied by a slight shift in the maximum of the PL toward the infrared (IR) region and a narrowing of the PL line of MEH-PPV with a decrease in the temperature and with an increase in the ZnO concentration. The mechanism of energy transfer in composite systems consisting of a polymer and inorganic nanoparticles that can be responsible for the observed effects has been discussed.     

Temperature dependence of photoluminescence in amorphous Si1-xCx:H films

We have investigated the temperature dependence of photoluminescence in hydrogenated amorphous silicon-carbon alloys a-Si_1-xC_x:H prepared by glow discharge in the low-power regime. The radiative recombination process, due to photocarriers trapped on band-edge states, is in competition with the thermal escape of the photocarriers into the mobility bands. The model gives a quantitative fit with experiment, without any adjustable parameter, provided the width of the band-edge distribution of states is taken as the width of the conduction band only (measured by “photo-induced infra-red spectroscopy”) and not as the Urbach energy, as it is usually assumed.     

Morphology and photoluminescence study of electrodeposited ZnO films

ZnO films on ITO substrates and Au coated ITO substrates were fabricated by using electrodeposition technique. We carried out the experiments by adjusting the concentration of solution, potential, substrate, and temperature. The effect of temperature on the growth of the film has been examined. SEM images have shown that there are several kinds of grown competitions for the deposition of ZnO films, but three kinds of them are dominant. One is the discrete hexagonal column structure, the other is the pentagonal structure, and the third one is of well-oriented hexagonal columns with well-aligned structure. The explanation on the grown competition is discussed. ZnO hexagonal column structures with well-aligned and well-perpendicular to the surface were successfully obtained on Au/ITO substrate in aqueous solvent of electrolyte. Clearly the main columns in the film were obtained by increasing the temperature. Its photoluminescence (PL) study at low temperature exhibited the optical properties as wurtzite ZnO and indicated the existence of macrocrystalline ZnO. A better quality of ZnO columnar structures after annealing was demonstrated from PL analysis and discussion on the existence of 370 nm, 384 nm and 639 nm in the emission bands before and after annealing.     

A new visible photoluminescence in the conducting Ta-Si-N nanocomposite thin films

The conventional visible photoluminescence (PL) behavior at room temperature (RT) is observed in the semiconducting or insulating nanostructured materials with large bandgaps. Here, we have demonstrated the visible-color PL behavior at RT of the conducting Ta-Si-N thin films nanocomposite using simple magnetron sputtering without any post annealing. X-ray diffraction, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) depth profile results evidence that the microstructure of conducting Ta-Si-N with a low resistivity of 220-375 μΩ cm exhibits (Ta_x,Si_y)N nanocrystalline grains embedded in an amorphous matrix, which can emit the distinct PL intensity from 390 to 750 nm wavelengths. No PL is detected in the coarse-grain polycrystalline films. Qin's extended quantum confinement/luminescence center model [G.G. Qin, Mater. Res. Bull. 33 (1998) 1857-1866] was adopted and modified to discuss the RT PL mechanism in the conducting Ta-Si-N films using three types of competitive photoexcitation/photoemission processes.     

Depth dependence of photoluminescence and chemical bonding in porous silicon

Porous silicon (PS) is studied by stepwise peeling of the surface layer to clarify the non-uniformity in the photoluminescence (PL) and correlate it with the in-depth chemical bonding and structure of the 30 μm thick layer. The PL intensity grows by an order of magnitude after the peeling off of the first 10 μm and decreases five times in the next 5 μm while the peak maximum position shifts from 730 to 800 nm. X-ray photoelectron spectroscopy (XPS) measurements show that Si–Si and Si–O bonds are present both on the surface and below, and the preferential oxidation state of silicon changes from 3+ and 4+ on the surface to 1+ and 2+ below 10 μm. Using Raman spectroscopy silicon nanocrystals are shown to exist. Their mean size can be estimated at about 3 nm. These results show that the strongest PL comes from a region in the PS layer where silicon nanocrystallites are surrounded by oxides with a low level of oxidation and not from the strongly oxidized surface layer.     

Relationship between crystal morphology and photoluminescence in polynanocrystalline lead sulfide thin films

Thin films of lead sulfide (PbS) nanoparticles were grown on corning glass and Si(1 0 0) substrates by polyethylene glycol-assisted chemical bath deposition (CBD) method. This paper compares the morphology and the luminescence properties (PL) of the deposited thin films in the presence (or absence) of PEG300 and investigates the effect of deposition temperatures. Surface morphology and photoluminescence properties of samples were analyzed. The PL data show a blue-shift from the normal emission at ∼2900 nm in PbS bulk to ∼360 nm in nanoparticles of PbS thin films. Furthermore, the PL emission of the films obtained without the addition of PEG300 (type 1) was slightly shifted from that of the films obtained in presence of PEG300 (type 2) from ∼360 to ∼470 nm. The blue-shifting of the emission wavelengths from 2900 to ∼360 or 470 nm is attributed to quantum confinement of charge carriers in the restricted volume of nanoparticles, while the shift between the two types of PbS nanoparticles thin films is speculated to be due to an increase in the defect concentration. The blue-shift increased with increase of the deposition temperature, which suggests that there has been a relative depletion in particle sizes during the CBD of the films at higher temperatures. The PbS nanocrystalline thin films obtained in the presence of PEG300 at 60 °C exhibit a high blue luminescence.     

Temperature dependence of GMR and effect of annealing on electrodeposited Co-Ag granular films

The magnetoresistance of Co-Ag granular films composed of superparamagnetic and ferromagnetic particles was studied at different temperatures. The increase in the GMR values while decreasing temperature down to 20 K was quantified. The non-saturating behaviour of the MR(H) curves was retained even at the lowest measurement temperature, which was mainly attributed to the dipolar interaction among the superparamagnetic particles. The influence of the annealing conditions on the magnetoresistance was also studied. In all conditions, a decrease in the GMR values was measured being attributed to an increase in the particle size.     

Optically active vibrational modes in the photoluminescence line shape of BDMO-PPV films

In this work we simulate the photoluminescence (PL) spectra of BDMO-PPV thin films prepared by spin-coating technique on glass and on copper, as a function of temperature (12-300 K). Simulations were done using two theoretical models based on (i) the SSH theory where the line shape of the purely electronic transition is partly generated by localized states and partly by delocalized states and (ii) the semi-empirical model containing the coupling between localized molecular excitons and vibrational modes in Franck-Condon approach. Four active vibrational modes have been considered: C-C stretching coupled to a C-H bending of the phenyl ring at 1111.5 cm⁻¹, inter-rings C-C stretching at 1282.2 cm⁻¹, CC stretching coupled to a C-H bending of the vinyl group at 1309.3 cm⁻¹, C-C stretching of the phenyl ring at 1580.2 cm⁻¹. Additional vibrational mode of 403 cm⁻¹ associated with C-C-C out-of-plane bending allowed leastwise for this material to adjust well with the characteristic asymmetry of the purely electronic transition. Finally, application of theoretical models are strongly dependent on the well-resolved PL spectra, i.e., electronic transition peak presented a relatively thinner HWHM and an asymmetric line shape.     

Effect of Ag doping on the photoluminescence properties of ZnO films

ZnO:Ag films were grown on Si (1 0 0) substrates by ultrasonic spray pyrolysis at various substrate temperatures. The effect of deposition temperature on the structural and the room temperature photoluminescence (RT-PL) properties of ZnO:Ag films was studied. With the deposition temperature rising to 550 °C, the intensity of the near-band edge (NBE) emission at 378 nm decreased and a new emission peak at 399 nm was observed. On the basis of the X-ray diffraction pattern (XRD), the X-ray photoelectron (XPS) spectra of ZnO:Ag films, and the effects of annealing on the PL, we suggest that the 399 nm emission should be attributed to the electron transition from the conduction band to Ag_Zn-related complexes defects radiative centers above the valence band.     

Temperature dependence of photoluminescence from mono-dispersed Si nanoparticles

The temperature dependence of photoluminescence (PL) from mono-dispersed Si nanoparticles was studied from 4 to 300 K. Si nanoparticles produced by pulsed laser ablation in He background gas were sorted into the 6 nm size range by a differential mobility analyzer (DMA). The spread of the size distribution was narrowed to a geometrical standard deviation _g = 1.05. On decreasing the temperature from 300 to 4 K, the intensity of the PL spectra increased gradually, peaked at about 60 K, and then decreased rapidly. The temperature dependences of the intensity and the full width at half maximum (FWHM) on the PL spectra are discussed in terms of radiative and nonradiative decay rates.     

Violet-to-red photoluminescence of spiro-TAD organic films doped with different organic dyes

Photoluminescence (PL) and photoluminescence excitation (PLE) spectra of undoped spiro-TAD films and spiro-TAD films doped either by the organic dyes coumarin 7 or DCM as well as by both of these dyes simultaneously were investigated at different dye concentrations. A widened PL spectrum caused by doping was encountered and violet-to-red emission was obtained. It was established that excitation of the dyes is realized most efficiently through spiro-TAD. The overall integral PL intensity of the coumarin 7 doped films increased with dye concentration due to the suppression of nonradiative recombination in the film caused by a transfer of spiro-TAD excitation energy to the dye molecules. Mainly radiative energy transfer from semiconductor to dye molecules occurs in the case of DCM doping. No mutual influence on the luminescence of both dyes in the spiro-TAD film was observed and as a consequence, the PL band intensity of each dye can be adjusted separately.     

Structure and photoluminescence properties of ZnS films grown on porous Si substrates

ZnS films were deposited on porous silicon (PS) substrates with different porosities. With the increase of PS substrate porosity, the XRD diffraction peak intensity decreases and the surface morphology of the ZnS films becomes rougher. Voids appear in the films, due to the increased roughness of PS structure. The photoluminescence (PL) spectra of the samples before and after deposition of ZnS were measured to study the effect of substrate porosity on the luminescence properties of ZnS/PS composites. As-prepared PS substrates emit strong red light. The red PL peak of PS after deposition of ZnS shows an obvious blueshift. As PS substrate porosity increases, the trend of blueshift increases. A green emission at about 550 nm was also observed when the porosity of PS increased, which is ascribed to the defect-center luminescence of ZnS. The effect of annealing time on the structural and luminescence properties of ZnS/PS composites were also studied. With the increase of annealing time, the XRD diffraction peak intensity and the self-activated luminescence intensity of ZnS increase, and, the surface morphology of the ZnS films becomes smooth and compact. However, the red emission intensity of PS decreases, which was associated with a redshift. White light emission was obtained by combining the luminescence of ZnS with the luminescence of PS.     

Enhanced photoluminescence of conjugated polymer thin films on nanostructured silver

Photoluminescence of a conjugated polymer in the presence of surface plasmons on metallic nanoparticles is studied. A layered device structure was constructed that enabled control over nanoparticle diameter and separation between the polymer and nanoparticles layers. The dependence of the surface plasmon evanescent field and energy transfer has been investigated with the largest enhancement in photoluminescence observed at a 40 nm distance separation between the fluorophore and the surface plasmon. A spectrum of surface plasmon resonances ranging from the emission to the absorption energies of the conjugated polymer revealed largest enhancements when the resonance was tuned to the conjugated polymer emission energy. At peak photoluminescence the maximum photoluminescent enhancement was found to be 5.6 times of the photoluminescence of the control structure and the total integrated enhancement was 5.9 times.