Showing papers on "Photoluminescence published in 2000"

Optical gain and stimulated emission in nanocrystal quantum dots.

Abstract: The development of optical gain in chemically synthesized semiconductor nanoparticles (nanocrystal quantum dots) has been intensely studied as the first step toward nanocrystal quantum dot lasers. We examined the competing dynamical processes involved in optical amplification and lasing in nanocrystal quantum dots and found that, despite a highly efficient intrinsic nonradiative Auger recombination, large optical gain can be developed at the wavelength of the emitting transition for close-packed solids of these dots. Narrowband stimulated emission with a pronounced gain threshold at wavelengths tunable with the size of the nanocrystal was observed, as expected from quantum confinement effects. These results unambiguously demonstrate the feasibility of nanocrystal quantum dot lasers.

Ordered semiconductor ZnO nanowire arrays and their photoluminescence properties

Abstract: Ordered semiconductor ZnO nanowire arrays embedded in anodic alumina membranes (AAM) were fabricated by generating alumina templates with nanochannels, electrodepositing Zn in them, and then oxidizing the Zn nanowire arrays. The polycrystalline ZnO nanowires with the diameters ranging from 15 to 90 nm were uniformly assembled into the hexagonally ordered nanochannels of the AAM. Photoluminescence (PL) measurements show a blue PL band in the wavelength range of 450–650 nm caused by the singly ionized oxygen vacancy in ZnO nanowires.

The Electronic Structure of Semiconductor Nanocrystals1

Abstract: ▪ Abstract We review the rapid progress made in our understanding of the crystal properties of semiconductors and nanocrystals focussing on theoretical results obtained within the multiband effective mass approximation. A comparison with experiment shows these results are valid for nanocrystals down 22–26 A in diameter. The effect of the electron-hole Coulomb interaction on the optical spectra is analyzed. A theory of the quantum–size levels in wide gap (CdSe) and narrow gap semiconductors (InAs) is presented that describes the absorption spectra of these semiconductors well. A great enhancement of the electron-hole exchange interaction leads to the formation of the optically forbidden Dark Exciton in nanocrystals, which strongly affects their photoluminescence. A theory of the band-edge exciton fine structure is presented and applied to the study of the PL in CdSe nanocrystals. The effect of doping on nanocrystal spectra is considered. The enhancement of the short–range spin-spin interaction in Mn-doped ...

Controlling Interchain Interactions in Conjugated Polymers: The Effects of Chain Morphology on Exciton-Exciton Annihilation and Aggregation in MEH-PPV Films

Abstract: The presence of interchain species in the photophysics of conjugated polymer films has been the subject of a great deal of controversy. In this paper, we present strong evidence that interchain species do form in conjugated polymer films, and that the degree of interchain interactions can be controlled by varying the solvent and polymer concentration of the solution from which the films are cast. Thus, much of the controversy in the literature can be resolved by noting that the polymer samples in different studies had different side groups or were prepared in different ways and thus have different degrees of interchain interaction. The photoluminescence (PL) of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene), MEH-PPV, changes both its spectral shape and quantum yield when the films are prepared from different solutions or when the morphology is varied by annealing. Increasing the amount of interchain interactions enhances the red portion of the film’s PL, a result assigned to a combination of changes in the vibronic structure of the PL of the exciton and increased numbers of weakly emissive interchain species. Photoluminescence excitation spectroscopy shows that excitation to the red edge of the absorption band preferentially enhances the red emission, suggesting that the interchain species are aggregates with a distinct ground state absorption. Scanning force microscopy shows topographic features that correlate with the degree of interchain interactions, verifying that the morphology of conjugated polymer films changes with polymer concentration, choice of solvent, and spin-casting speed. Even at low excitation intensities, photooxidative damage occurs quickly in MEH-PPV films excited in air, and the rate at which damage occurs is sensitive to the packing of the polymer chains. For samples under vacuum at low excitation intensity, a long-lived emissive tail, in combination with excitedstate absorption dynamics that do not match those of the emissive species, provide direct evidence for the production of interchain aggregates. Annealing an MEH-PPV film produces a photophysical signature similar to photooxidation, implying that defects in conjugated polymer films are intrinsic and depend on the details of how the chains are packed. At higher excitation intensities, we find that exciton -exciton annihilation occurs, and that the probability for annihilation can vary by an order of magnitude depending on the degree of interchain contact in the film. Finally, we show that changing the film morphology has a direct effect on the performance of MEH-PPV-based light-emitting diodes. Higher degrees of interchain interaction enhance the mobility of carriers at the expense of lower quantum efficiencies for electroluminescence. Taken together, the results reconcile much of the contradictory literature and provide a prescription for the optimization of conjugated polymer films for particular device applications.

Optical emission from a charge-tunable quantum ring

Abstract: Quantum dots or rings are artificial nanometre-sized clusters that confine electrons in all three directions. They can be fabricated in a semiconductor system by embedding an island of low-bandgap material in a sea of material with a higher bandgap. Quantum dots are often referred to as artificial atoms because, when filled sequentially with electrons, the charging energies are pronounced for particular electron numbers; this is analogous to Hund's rules in atomic physics. But semiconductors also have a valence band with strong optical transitions to the conduction band. These transitions are the basis for the application of quantum dots as laser emitters, storage devices and fluorescence markers. Here we report how the optical emission (photoluminescence) of a single quantum ring changes as electrons are added one-by-one. We find that the emission energy changes abruptly whenever an electron is added to the artificial atom, and that the sizes of the jumps reveal a shell structure.

Correlation between luminescence and structural properties of Si nanocrystals

Abstract: Strong room-temperature photoluminescence (PL) in the wavelength range 650–950 nm has been observed in high temperature annealed (1000–1300 °C) substoichiometric silicon oxide (SiOx) thin films prepared by plasma enhanced chemical vapor deposition. A marked redshift of the luminescence peak has been detected by increasing the Si concentration of the SiOx films, as well as the annealing temperature. The integrated intensity of the PL peaks spans along two orders of magnitude, and, as a general trend, increases with the annealing temperature up to 1250 °C. Transmission electron microscopy analyses have demonstrated that Si nanocrystals (nc), having a mean radius ranging between 0.7 and 2.1 nm, are present in the annealed samples. Each sample is characterized by a peculiar Si nc size distribution that can be fitted with a Gaussian curve; by increasing the Si content and/or the annealing temperature of the SiOx samples, the distributions become wider and their mean value increases. The strong correlation betw...

Highly monodisperse polymer-capped ZnO nanoparticles: Preparation and optical properties

Abstract: We report the preparation of highly monodisperse ZnO nanoparticles using poly(vinyl pyrrolidone) (PVP) as the capping molecules. The surface-modified ZnO nanoparticles were found to be remarkably stable. The optical absorption shows distinct excitonic features. Markedly enhanced near-band-edge ultraviolet photoluminescence and significantly reduced defect-related green emission were also observed. We attribute this observation to the nearly perfect surface passivation of the ZnO nanoparticles by the PVP molecules. The third-order nonlinear optical response of these PVP-capped ZnO nanoparticles in a dilute solution was found to be significantly larger (by at least two orders of magnitude) than that of the bulk ZnO.

Photoluminescence properties of silicon nanocrystals as a function of their size

Abstract: We present results on the photoluminescence (PL) properties of silicon nanocrystals as a function of their size. The nanocrystals are synthesized by laser pyrolysis of silane in a gas flow reactor and deposited at low energy on a substrate after a mechanical velocity and size selection. Both the photoluminescence spectroscopy and yield have been studied as well as the effect of aging of the samples in air. The measurements show that the PL of the silicon nanocrystallites follows the quantum confinement model very closely. The apparent PL yields are rather high (up to 18%). From evaluation of the size distribution obtained by atomic force microscopy it is concluded that the intrinsic PL yield of the nanocrystals can reach almost 100%. These results enabled us to develop a simple theoretical model to describe the PL of silicon nanocrystals. This model can also explain the changes of PL with aging of the sample, just by invoking a decrease of the size of the crystalline core as a result of oxidation.

Luminescence properties and defects in GaN nanocolumns grown by molecular beam epitaxy

Abstract: Wurtzite GaN nanocolumns are reproducibly grown by plasma-assisted molecular beam epitaxy on Si(111) and c-sapphire substrates. The nanocolumns density and diameter (600\char21{}1500 \AA{}) are effectively controlled by means of the III/V ratio. The nanocolumns are fully relaxed from lattice and thermal strain, having a very good crystal quality characterized by strong and narrow (2 meV) low-temperature photoluminescence excitonic lines at 3.472\char21{}3.478 eV. In addition, the spectra reveal a doublet at 3.452\char21{}3.458 eV and a broad line centered at 3.41 eV. This broad emission shows a sample-dependent spectral energy dispersion, from 3.40 to 3.42 eV, explained as due to the effect of strain and/or electric fields associated with extended structural defects located at the nanocolumns bottom interface. From cathodoluminescence data, it is concluded that the doublet emission lines originate at the nanocolumns volume, most probably related to ${\mathrm{Ga}}_{\mathrm{I}}$ defects, given the column growth mode (Ga balling).

Ga-Doped ZnO Films Grown on GaN Templates by Plasma-Assisted Molecular-Beam Epitaxy

Abstract: We have investigated the structural and optical properties of Ga-doped ZnO films grown on GaN templates by plasma-assisted molecular-beam epitaxy. The carrier concentration in Ga-doped ZnO films can be controlled from 1.33×1018/cm3 to 1.13×1020/cm3. Despite high Ga incorporation, the linewidth of (0002) ω-rocking curves of Ga-doped ZnO films still lies in the range from 5 to 15 arc min. Photoluminescence (PL) spectra of Ga-doped ZnO films show dominant near-bandedge emission with negligibly weak deep-level emission, independent of carrier concentration. The PL spectrum exhibits a new emission line at 3.358 eV, which corresponds to exciton emission bound to a Ga donor. To avoid degradation of the PL intensity, the maximum dopability of Ga in ZnO is determined to be around 2.6×1019/cm3.

Device performance and polymer morphology in polymer light emitting diodes: The control of thin film morphology and device quantum efficiency

Abstract: We present the results of a systematic study on how the processing conditions of spin casting affect the morphology of polymer thin films, and how the morphology affects polymer light-emitting diode (LED) performance. The absorption peaks of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1, 4-phenylene vinylene) (MEH-PPV) thin films, which reflects the conjugation of π electrons, are strongly correlated to the spin-casting conditions. At high spin speed, better conjugation is observed. In addition, the photoluminescence emission peak of MEH-PPV films at ∼630 nm has a strong correlation to polymer aggregation. By proper selection of organic solvents, polymer solution concentrations, and spin speeds, we are able to control the aggregation of the polymer chains. Subsequently, we are able to control the emission color and the quantum efficiency of the MEH-PPV LEDs by simply adjusting the spin-casting conditions. Although spin casting is the most commonly used technique for the preparation of polymer thin films, our fin...

Room Temperature Ferromagnetism in Novel Diluted Magnetic Semiconductor Cd1¡xMnxGeP2

Abstract: High concentration of Mn atoms has been incorporated in the surface region of II-IV-V2 type chalcopyrite semiconductor CdGeP2 Photoluminescence spectrum at 20 K shows a peak around 32 eV, suggesting that the incorporation of Mn introduces an energy gap much higher than that of the host semiconductor (Eg=183 eV) Prominent magnetic hysteresis loops with coercivity of 05 kOe has been observed at room temperature Magnetic force microscope (MFM) measurements reveal a stripe-shaped domain pattern on the top surface Magneto-optical Kerr ellipticity spectrum measured at room temperature show a prominent peak at 17 eV and a broad tail up to 35 eV We tentatively attribute the ferromagnetism to the double exchange interaction between Mn2+ and Mn3+ states due to the structural feature of II-IV-V2 type chalcopyrite compounds

Size-dependent photoluminescence from surface-oxidized Si nanocrystals in a weak confinement regime

Abstract: Photoluminescence (PL) from surface-oxidized Si nanocrystals (nc-Si) was studied as a function of the size. The size of nc-Si was comparable with or larger than the Bohr radius of free excitons in bulk Si crystal (5 nm). In contrast to smaller surface-oxidized nc-Si (typically as small as a few nanometers in diameter), these relatively large nc-Si exhibited PL properties with strong size dependence. A high-energy shift of the PL peak from the vicinity of the bulk band gap to the visible region was observed. This PL shift was accompanied by a shortening of the PL lifetime and an increase in the exchange splitting energy of excitons. These size dependences indicate that the PL originates from the recombination of excitons confined in nc-Si. The differences in the PL properties between H-terminated and surface-oxidized nc-Si are also discussed.

Transparent p-type semiconductor: LaCuOS layered oxysulfide

Abstract: La1−xSrxCuOS (x=0, 0.05) thin films prepared by radio-frequency sputtering were found to have high optical transmission (⩾70%) at the visible and near-infrared wavelengths and an energy gap of about 3.1 eV. The dc electrical conductivities of x=0 and 0.05 thin films at room temperature were 1.2×10−2 and 2.6×10−1 S cm−1, respectively. The Seebeck coefficients of these samples were positive, indicating that p-type electrical conduction is dominant in these materials. A sharp photoluminescence peak, probably originating from an interband transition, was observed at the optical absorption edge. The present study demonstrates that LaCuOS is a promising transparent p-type semiconductor for optoelectronic applications. Moreover, our material design, based on chemical modulation of the valence band, was successfully extended to oxysulfide systems.

Synthesis and optical properties of gallium arsenide nanowires

Abstract: Gallium arsenide (GaAs) nanowires have been synthesized in bulk quantities and high purity by laser-assisted catalytic growth. Field-emission scanning electron microscopy and transmission electron microscopy investigations show that the GaAs nanowires are produced in >90% yield, are single crystals with 〈111〉 growth axes, and have diameters varying from three to tens of nanometers, and lengths extending to tens of micrometers. Photoluminescence (PL) measurements made on individual GaAs nanowires show large blueshifts in the PL peak position compared to bulk GaAs, and are consistent with strong quantum confinement. The implications of these results are discussed.

Photoluminescence in anatase titanium dioxide nanocrystals

Abstract: Titanium dioxide (TiO2) nanocrystals were prepared by a hydrolysis process of tetrabutyl titanate. X-ray diffraction and Raman scattering showed that the as-prepared TiO2 nanocrystals have anatase structure of TiO2, and that the monophase anatase nanocrystals can be achieved through a series of annealing treatments below 650 °C. We measured photoluminescence (PL) spectra of the TiO2 nanocrystals. Under 2.41–2.71 eV laser irradiation, the TiO2 nanocrystals displayed strong visible light emission with maxima of 2.15–2.29 eV even at excitation power as low as 0.06 W/cm2. To identify the PL mechanism in the TiO2 nanocrystals, the dependences of the PL intensity on excitation power and irradiation time were investigated. The experimental results indicated that the radiative recombination is mediated by localized levels related to surface defects residing in TiO2 nanocrystallites.

Effects of native defects on optical and electrical properties of ZnO prepared by pulsed laser deposition

Abstract: ZnO thin film has been deposited on a sapphire (001) at a temperature of 400°C using a pulsed laser deposition (PLD) with oxygen pressures of 50, 200, 300 and 500 mTorr. As the oxygen pressure for the thin film deposition increases, the crystallinity of the samples degrades as measured by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). In contrast, the photoluminescence (PL) intensity of ultra-violet (UV) luminescence increases as the oxygen pressure increases up to 300 mTorr. This is probably because the stoichiometry of oxygen-deficient ZnO film is improved by increasing oxygen pressure. According to the results from Hall measurements, the oxygen vacancy as a native donor defect in the ZnO decreases in concentration as the pressure increases. It is concluded that the UV luminescence intensity strongly depends on the stoichiometry in the ZnO film rather than the micro-structural quality of the crystal.

Surface plasmon enhanced light-emitting diode

Abstract: A method for enhancing the emission properties of light-emitting diodes, by coupling to surface plasmons, is analyzed both theoretically and experimentally. The analyzed structure consists of a semiconductor emitter layer thinner than /spl lambda//2 sandwiched between two metal films. If a periodic pattern is defined in the top semitransparent metal layer by lithography, it is possible to efficiently couple out the light emitted from the semiconductor and to simultaneously enhance the spontaneous emission rate. For the analyzed designs, we theoretically estimate extraction efficiencies as high as 37% and Purcell factors of up to 4.5. We have experimentally measured photoluminescence intensities of up to 46 times higher in fabricated structures compared to unprocessed wafers. The increased light emission is due to an increase in the efficiency and an increase in the pumping intensity resulting from trapping of pump photons within the microcavity.

Near-Infrared Luminescence from Small Gold Nanocrystals

Abstract: A novel photoluminescence is reported for small metal nanocrystals in the near-infrared region (1.1−1.6 μm). Near-infrared photoluminescence spectra were measured at room temperature for 1.1 and 1.7 nm Au nanocrystals using a 1.06 μm excitation source. This photoluminescence is attributed to sp to sp-like transitions, analogous to intraband transitions in bulk gold; however, the exact mechanism is unknown. A conservative estimate for the quantum yield for the 1.7 nm gold nanocrystals is (4.4 ± 1.5) × 10-5 at room temperature, more than 5 orders of magnitude greater than that of bulk gold.

Time-resolved photoluminescence of particulate TiO2 photocatalysts suspended in aqueous solutions

Abstract: TiO2 powders showed a broad emission band. The peak position of the emission band for rutile powders was about 450 nm, and that of anatase powders was about 500 nm. The time-resolved photoluminescence of TiO2 powder was investigated with 30 ps laser pulses at 355 and 266 nm. Decay profiles of emission were successfully traced by the extended exponential equation. In the case of rutile powders, the lifetime of the emission lengthened in the relation to the increase of particle size. Their lifetime was also dependent on the wavelength of excitation; i.e., it was longer at 355 nm of excitation than at 266 nm. Based on these results, the decay profiles of rutile particles were concluded to be determined by the migration of carriers from the bulk of the particles to the surface. In contrast, the decay profiles of the emission of small anatase particles were considered to be determined chiefly by the recombination process at the surface. The effects of Fe(III) ions and 2-propanol on the respective decay profiles of the emission from rutile and anatase powders are also discussed.

Strong exciton-erbium coupling in Si nanocrystal-doped SiO2

Abstract: Silicon nanocrystals were formed in SiO2 using Si ion implantation followed by thermal annealing. The nanocrystal-doped SiO2 layer was implanted with Er to a peak concentration of 1.8 at. %. Upon 458 nm excitation the sample shows a broad nanocrystal-related luminescence spectrum centered around 750 nm and two sharp Er luminescence lines at 982 and 1536 nm. By measuring the excitation spectra of these features as well as the temperature-dependent intensities and luminescence dynamics we conclude that (a) the Er is excited by excitons recombining within Si nanocrystals through a strong coupling mechanism, (b) the Er excitation process at room temperature occurs at a submicrosecond time scale, (c) excitons excite Er with an efficiency >55%, and (d) each nanocrystal can have at most ~1 excited Er ion in its vicinity.

Fabrication of GaAs Quantum Dots by Modified Droplet Epitaxy

Abstract: We propose a modified droplet epitaxy method for fabricating self-organized GaAs/AlGaAs quantum dots (QDs) with a high As flux irradiation and a low substrate temperature. By our novel method, GaAs QDs were successfully formed, retaining their pyramidal shape, original base size and density of droplets, and preventing layer-by-layer growth. Quantum size effects of the QDs were distinctly observed by photoluminescence measurements. It was confirmed that this new modified droplet epitaxy method is promising for fabricating a high-quality GaAs/AlGaAs QD system.

Room-temperature stimulated emission of excitons in ZnO/(Mg, Zn)O superlattices

Abstract: We report on the observation of stimulated emission in ZnO/MgxZn1−xO superlattices well above room temperature. Two kinds of superlattices grown by laser molecular-beam epitaxy showed clear systematics on the quantum subband levels in absorption and spontaneous emission spectra. Stimulated emission with excitonic origin could be observed at very low optical pumping levels. The threshold excitation intensity changed from 11 to 40 kW/cm2, and the emission energy could be tuned between 3.2 and 3.4 eV, depending on the well thickness and/or the Mg content in the barrier layers. The excitonic stimulated emission could be observed up to 373 K and the characteristic temperature was as high as 87 K.

Photoluminescence quenching at a polythiophene/C-60 heterojunction

Abstract: Quenching of photoluminescence in a substituted polythiophene in the presence of a deposited C-60 layer is studied by steady-state and time-resolved photoluminescence (PL). The steady-state PL is evaluated by con -sidering the interference of the absorbed and emitted electro-optical field in the thin film coupled to exciton diffusion in the conjugated polymer. PL quenching occurs for excitons generated within 5 nm from the heterojunction. A blueshift of the polymer emission spectrum is observed when C-60 is deposited on top of a polymer thin film. The blueshift is shown to be caused by PL quenching before the excitation is transferred to the lowest-energy sites.

Luminescence of nanocrystalline ZnSe:Mn2+

Abstract: Nanocrystalline ZnS:Pb2+ is synthesised ia a precipitation method. The influence of the size of the nanocrystals and the sulfide concentration used in the synthesis on the luminescence properties is investigated. Nanocrystalline ZnS:Pb2+ shows a white emission under UV excitation with a rather high quantum efficiency (∽5%). At least two luminescence centres are involved. One centre is identified as a Pb2+ ion located on a regular Zn2+ site and gives a red emission under 480 nm excitation. The luminescence properties of this emission are characteristic of 3P0 → 1S0 (A-band) or charge transfer (D-band) transitions on Pb2+ ions. The other centres are not as well defined and give a broad green emission band under 380 nm excitation and also show luminescence properties typically observed for Pb2+. The green emission probably originates from a charge-transfer like D-band emission of Pb2+ in ZnS close to a defect (e.g. an S2− vacancy or an O2− ion on an S2− site). A relation between the temperature quenching of the emissions and the band gap is observed and indicates that photoionisation occurs. The higher quenching temperature for the Pb2+ luminescence in smaller particles can be explained by widening of the band gap as a result of quantum size effects in the ZnS host.