Intensity Of Exciton Emission Research Articles

On the applicability of a few level rate equation model to the determination of exciton versus biexciton kinetics in quasi-zero-dimensional structures

Hereby, we present a few level rate equation model in a context of the interpretation of excitation power dependent exciton and biexciton emission intensity from single quantum-dot-like structures. We emphasize that it not only allows identifying the excitonic and biexcitonic emission from one quasi-zero-dimensional object, but gives also an insight into the kinetics of the carriers confined in the system (both the internal dynamics of the exciton within its fine structure and the relative exciton to biexciton lifetimes ratio), the regime of the confinement itself and the importance of the higher energy levels occupation. Eventually, there are presented and discussed examples of the rate equation model application for an analysis of the experimental data for several kinds of epitaxial nanostructures.

Impact of noble metal nanostructures on surface trapping state of semiconductor quantum dots

The impact of noble metal nanostructures on the surface trapping state of semiconductor quantum dots was investigated by taking nanosecond time resolved photoluminescence measurements. It was observed that nonradiative resonant energy transfer from the excitonic state of quantum dots to Au nanofilm reduced the lifetime and intensity of excitonic state emission. No nonradiative resonant energy transfer from surface trapping state to Au nanofilm was observed. It is possibly implied that noble metal nanostructures have no impact on the surface trapping state of semiconductor quantum dots.

Electroplex emission of the blend film of PVK and DPVBi

Influences of electric fields on the emission from organic light-emitting diodes (OLEDs) based on poly(N-vinylcarbazole) (PVK) and 4′-bis(2-2diphenylvinyl)-1,1′-biphenyl (DPVBi) as the active emission layer are studied. Electroluminescence (EL) spectra of PVK:DPVBi (1:1 w/w) films show one new emission peak locating at 640nm compared with its photoluminescence (PL) spectra. There may be exists an electroplex emission between the PVK and DPVBi under high electric field strength. The emission intensity of peaking at 640nm strongly depends on the driving voltage, and the ratio of electroplex emission intensity to exciton emission intensity (Ielectroplex/Iexciton) increases with the increase of driving voltage.

Synthesis and Characterization of Copper (I) Chloride (CuCl) Nanocrystals in Conductive Polymer for UV Light Emitters

AbstractIntrinsic γ-Copper (I) Chloride is an ionic I-VII compound semiconductor material with relatively low conductivity. To fabricate an efficient electroluminescent device based on CuCl nanocrystals (NC) the conductivity of the CuCl NC film should be relatively high. In order to improve the conductivity of CuCl films, nanocrystals were embedded in a highly conductive polymer (Polyaniline) and deposited on glass substrates via the spin-coating method. The deposited films were heated at 140°C for durations between 1 and 12 hours in vacuo. The room temperature UV-Vis absorption spectra for all CuCl films showed both Z1,2 and Z3 excitonic absorption features and the absorption intensity increased as the anneal time increased. Room temperature photoluminescence (PL) measurements of the hybrid films reveal very intense Z3 excitonic emission. Room temperature X-ray diffraction (XRD) confirmed the preferential growth of CuCl nanocrystals whose average size is ≈40 nm in the <111> orientation. Resistivity measurements were carried out using a four-point probe system, which confirmed that the resistivity of the composite film was ≈500 Ω/cm. This is an improvement when compared to the vacuum evaporated CuCl thin films.

Hybrid organic–inorganic spin-on-glass CuCl films for optoelectronic applications

Cuprous halides are among the most studied inorganic materials for excitonic related linear/non-linear optical processes due to their large excitonic binding energies (∼190 and 108 meV for CuCl and CuBr, respectively). In recent years, we have studied CuCl thin films deposited by vacuum evaporation and sputtering techniques on a variety of substrates. We now report on the extension of this research to the deposition of CuCl nanocrystals on flexible substrates via a spin-on technology. In this study, we present the synthesis, deposition and characterization of CuCl nanocrystals embedded in organic polysilsesquioxane (PSSQ) films on a variety of substrates via the spin coating method. The nanocrystals were synthesized by a complexation–reduction–precipitation mechanism reaction of CuCl2 · 2H2O, alpha D-glucose and de-ionized (DI) water with a PSSQ based solution as the host matrix material. The deposited films were heated at 120 °C for durations between 1 and 24 h in vacuo. The room temperature UV–Vis absorption spectra for all hybrid films, except the as-deposited film, showed both Z1,2 and Z3 excitonic absorption features. Room temperature photoluminescence measurements of all heated films reveal very intense Z3 excitonic emission at 3.221 eV. Room temperature x-ray diffraction (XRD) of the as-deposited films gave no evidence of the crystallization of CuCl. However, after heating the films, XRD confirmed the preferential growth of CuCl nanocrystals whose average size is ≈25–45 nm in the ⟨1 1 1⟩ orientation. The CuCl hybrid films showed bright electroluminescent emission at 384 nm when subjected to an ac voltage of about 100 V peak to peak.

Preparation of ZnO nanoparticles for bio‐imaging applications

AbstractZnO nanoparticles were prepared by using a gas evaporation method. As ZnO is a luminescent and less toxic material, it can be useful in imaging applications. The nanoparticles prepared in our laboratory showed intense exciton emission in the UV‐blue region. They were processed and surface treated to be compatible for bio‐conjugation. We were able to prepare good dispersions and water‐soluble ZnO nanoparticles from our experiments. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface excitonic recombination dynamics in ZnO nanowires

AbstractThe optical properties of ZnO nanowires are strongly influenced by surface effects. For diameters smaller than 200 nm, a surface exciton recombination band is observed. We report on the recombination dynamics of this emission band using low temperature and time resolved photoluminescence spectroscopy. We find a biexponential decay with different decay constants for different emission wavelengths on the band. We present a model to explain the biexponential decay characteristics and report on possibilities to experimentally enhance the relative intensity of the surface exciton emission by embedding the ZnO nanowires into polymer matrices. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Low temperature diffusion length of excitons in gallium nitride measured by cathodoluminescence technique

Monochromatic cathodoluminescence (CL) images of threading dislocations in GaN epitaxial layers were observed using a transmission electron microscopy combined with CL system. The carrier diffusion lengths were derived from the free exciton emission intensity profile of the dislocation contrast in the CL images. The carrier diffusion lengths in Si-doped and Mg-doped GaN were nearly the same and shorter than that in undoped GaN in the temperature range from 20 to 140 K, respectively. Moreover, the temperature dependence of the diffusion length shows that the acoustic phonon scattering mainly affects the exciton diffusion process at low temperatures from 40 to 120 K.

Electrical and light‐emitting properties of homoepitaxial diamond p–i–n junction

AbstractWe have investigated the electrical and light‐emitting characteristics of (001)‐oriented homoepitaxial diamond p–i–n junction diodes with the boron‐doped p‐type, non‐doped intrinsic, and phosphorus‐doped n‐type layers formed by applying an optimized homoepitaxial growth technique based on microwave plasma‐enhanced chemical vapor deposition. High‐performance p–i–n junction characteristics were confirmed from current–voltage and capacitance–voltage properties. A strong ultraviolet light emission at around 240 nm due to free exciton recombination was observed at a forward current of over 6 mA, while the broad visible light emission from deep levels was significantly suppressed compared to that of reported electroluminescence in diamond p–n junctions. It was elucidated that the excitonic emission intensity increases superlinearly for the current dependence and is stable even at the high temperature of 200 °C. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Excitonic electroluminescence from ZnO-based heterojunction light emitting diodes

This work reports on the fabrication and characteristics of n-ZnO/p-GaN and n-ZnO/n-MgZnO/n-ZnO/p-GaN heterojunction light emitting diodes (LEDs). Both devices exhibited diode-like rectifying current–voltage characteristics. Room temperature electroluminescence (EL) spectra for both LEDs consisted of dominant emission at 375 nm and two weaker bands centred at 415 nm and 525 nm, which were attributed to ZnO excitonic transition and defect-related emissions from GaN and ZnO, respectively. Moreover, it was demonstrated that the single heterojunction required a higher injection current to obtain an excitonic EL than that for the n-ZnO/n-MgZnO/n-ZnO/p-GaN LEDs. This indicated that the insertion of the MgZnO layer confined the injection carriers and thus increased the intensity of excitonic emission in the ZnO active region.

Photoluminescence in heavily doped ZnO:N:In films

Temperature-dependent photoluminescence is used to investigate ZnO films codoped with In and N at different doping levels. Conversion from exciton recombination to band-to-band transition with increasing both doping level and temperature is observed. We suggest that ionization of the N acceptors and dissociation of excitons by impurity-induced local field are responsible for such conversion. For the film with N concentration of 4×1020cm−3, the excitonic emission intensity shows anomalous temperature dependence due to localized carriers. The localization energy and the N acceptor level is determined to be about 5 and 164meV, respectively.

Structure and Physical Properties of Undoped ZnO and Vanadium Doped ZnO Films Deposited by Pulsed Laser Deposition

Undoped ZnO films were deposited using pulsed laser deposition technique on Si and glass substrates in different O2 partial pressures (ranging from 10(-5) mbar to 3 mbar) and substrate temperatures. When the substrate temperature is 500 degrees C and O2 partial pressure (pp) approximately 3 mbar, randomly oriented ZnO hexagons were observed on glass substrate, whereas, dense ZnO hexagonal rod like structures (diameter ranging from 200-500 nm) were observed on Si substrate. The photoluminescence (PL) characterization of ZnO film grown on Si exhibited an intense defect free narrow excitonic emission in the UV region (Full width half maximum (FWHM) approximately 11.26 nm) as compared to broad emission (FWHM approximately 57.06 nm) from that grown on glass. The parent film emission was found to shift from UV to blue region on doping ZnO with Vanadium.

Exciton photoluminescence from ZnO layers produced by laser-induced gas breakdown processing

The plasma of optically-excited gas breakdown has been used to treat a Zn target in atmospheric pressure gases (air, O2, N2, Ar). The breakdown is produced near the target by a pulsed CO2 laser radiation, yielding to a local erosion of the target under the irradiation spot and the formation of a porous nanostructured layer, consisting of ZnO nanoscale spheres. We show that the produced nanostructured layers exhibit an intense exciton emission band in the ultraviolet range (380–385 nm), while defect-related photoluminescent bands were weak and could be completely removed by varying the fabrication parameters. Properties of the produced layers were found to be very promising for the development of optoelectronic devices.

Photoluminescence of ZnO Nanocrystals Embedded in BaF2 Matrices by Magnetron Sputtering

This paper describes ZnO nanocrystals embedded in BaF2 matrices by the magnetron sputtering method in an attempt to use fluoride as a shell layer to embed ZnO nanocrystals core. BaF2 is a wide-band gap material, and can confine carriers in the ZnO films. As a result, the exciton emission intensity should be enhanced. The sample was annealed at 773 K, and X-ray diffraction (XRD) results showed that ZnO nanocrystals with wurtzite structure were embedded in BaF2 matrices. Raman-scattering spectra also confirmed the formation of ZnO nanoparticles. Abnormal longitudinal-optical (LO) phonon-dominant multiphonon Raman scattering was observed in the sample. Room-temperature photoluminescence (PL) spectra showed an ultraviolet emission peak at 374 nm. The origin of the ultraviolet emission is discussed here with the help of temperature-dependent PL spectra. The ultraviolet emission band was a mixture of free exciton and bound exciton recombination observed in the low temperature PL spectra (at 77 K). Abnormal temperature dependence of ultraviolet near-band-edge emission-integrated intensity of the sample was observed. The band tail state was observed in the absorption spectra, illustrating that the impurity-related defects were caused by the shell of the BaF2 grain layer. For comparison, ZnO films on BaF2 substrates were also fabricated by the magnetron sputtering method, and the same measurement methods were used.

Optical characterization of Zn0.96Be0.04Se and Zn0.93Mg0.07Se mixed crystals

A detailed optical characterization of Zn0.96Be0.04Se and Zn0.93Mg0.07Se mixed crystal samples grown by the modified high-pressure Bridgman method has been carried out via photoluminescence (PL), contactless electroreflectance (CER), and photoreflectance (PR) in the temperature range of 15–400 K. With these optical diagnostic tools we observed features originating from exciton emission, an edge emission due to recombination of shallow donor-acceptor pairs (DAPs), and a broad band related to recombination through deep-level defects, as well as band-edge and spin-orbit splitting critical points interband transitions in CER and PR experiments. The optical properties of these two mixed crystals are compared. Our comparative study revealed that the beneficial effect of beryllium (Be) incorporation is superior to that of magnesium (Mg), in the sense that a Be-containing ternary alloy resulted in better crystalline quality with more intense excitonic emission line and weaker DAPs and deep-level defect-related bands. The peak positions of the excitonic emission lines in the PL spectra correspond quite well to the energies of the fundamental transitions obtained by means of electromodulation spectra. The parameters that describe the temperature dependence of the fundamental and spin split-off band gaps, and the broadening function of the band-edge excitons, are evaluated and discussed.

산소와 수소 플라즈마로 처리한 사파이어 기판 위에 성장된 ZnO 박막의 구조적.광학적 특성

[ <TEX>$Al_2O_3$</TEX> ]기판을 산소 plasma 또는 수소 plasma로 표면 처리한 후 그 위에 plasma-assisted molecular beam epitaxy 방법으로 성장된 ZnO 박막의 구조적 특성과 광학적 발광 특성을 체계적으로 조사하였다. 제작된 ZnO 박막은 high resolution X-ray diffraction 측정과 atomic force microscope를 사용하여 구조적 특성과 표면 특성을 관찰하였으며, photoluminescence (PL) 측정을 통하여 엑시톤과 관련된 광학적 전이특성을 온도에 따라 조사하였다. free exciton, bound exciton, 그리고 이들의 phonon replica들의 특성을 온도에 따라 분석하였으며, 산소 plasma로 표면 처리한 시료의 PL 세기가 수소 plasma 표면 처리한 시료의 PL 세기보다 상당히 커짐을 관찰하였다. 산소 plasma로 처리된 기판 위에 성장된 ZnO 시료가 수소 plasma로 처리된 경우보다 우수한 구조적 특성과 광학적 특성을 보였는데, 이는 산소 plasma로 표면 처리함으로써 산소 공공(oxygen vacancy)과 같은 결함 구조가 적게 생성되고 좋은 격자 상수 일치를 보여주므로 구조적 특징과 발광 특징이 향상되는 것으로 해석되었다. Structure and optical properties of ZnO epilayers grown on oxygen- and hydrogen-plasma treated sapphire substrates by plasma-assisted molecular beam epitaxy (denoted as samples A and B, respectively) have been investigated by various techniques. The crystal quality and structural properties of the surface for the ZnO epilayers were investigated by high-resolution X-ray diffraction and atomic force microscope. For investigating the optical properties of excitonic transition of ZnO, we carried out photoluminescence experiments as a function of temperature. The free exciton, bound exciton emission and their phonon replicas were investigated as a function of temperature from 10 to 300 K, and the intensity of excitonic PL peak emission from the sample A is found to be higher than that of sample B. From the results, we found that sample A has better crystal structure quality and optical properties as compared to sample B. The number of oxygen vacancies may be decreased in sample A, resulting in an enhancement of the crystal quality and a higher intensity of excitonic emission band as compared to sample B.

Free exciton emission and dephasing in individual ZnO nanowires

We report on systematic measurements of the dephasing of the free excitonic transition in individual ZnO nanowires with a diameter of about 70nm. The temperature-dependent intensity of the free exciton (FX) emission is well reproduced by a thermally activated behavior having two constant activation energies of 5.98 and 30.63meV. The FX linewidth broadening with temperature reveals the dephasing of FX scattered by acoustic phonons rather than longitudinal optical phonons. These findings indicate that the dephasing mechanism scattering from acoustic phonons in individual ZnO nanowires result from the compensation of acoustic phonons to the nonconservation of crystal momentum.

Photoluminescence and acceptor level state of p-type nitrogen-doped MgZnO films

A wurtzite nitrogen-doped MgZnO (MgZnO:N) film was grown by plasma-assisted molecular-beam epitaxy (PAMBE) on c-plane sapphire using radical NO as oxygen source and nitrogen dopant. The as-grown film shows n-type conduction at room temperature, but transforms into p-type conduction after annealed. Photoluminescence (PL) spectrum measured at 80 K is dominated by neutral donor-bound exciton emission (D0X) located at 3.522 eV for the n-type MgZnO:N film, but by neutral acceptor-bound exciton emission (A0X) located at 3.515 eV for the p-type MgZnO:N film. By fitting exciton emission intensity of temperature-dependent PL spectra, the binding energies of the D0X and A0X were estimated to be 32 and 43 meV, respectively. Based on the energy shift of exciton emission, the band gap of the MgZnO:N film is estimated to be 3.613 eV, which is 179 meV larger than that of ZnO. Using the Haynes rule, the acceptor energy level of the MgZnO:N film was evaluated to be about 176 meV above the valence band.

Temperature dependent optical properties of UV emitting γ-CuCl thin films

In this work, ultra violet emitting properties of RF sputtered CuCl thin films were studied as a function of temperature in the region of 15–300 K. Deposition parameters were optimised to obtain good optical quality films exhibiting an intense and sharp Z3 free exciton emission (at 383 nm) with an FWHM of ∼74 meV at room temperature. At lower temperatures, there are two other emissions observed at 375 and 390 nm, and they are identified as Z12 and I1 exciton lines, respectively. The free exciton emission is extremely stable at room temperature and it was found to be less influenced by the exciton–phonon interaction compared to the bound exciton emission which dissociates after 40 K. The Z3 free exciton line exhibits a slight blue-shift on increasing the temperature. The temperature dependence of exciton line shift, line broadening and emission intensity are discussed in detail.

Electroplex emission from bi-layer blue emitting organic materials

Electroluminescence (EL) and photoluminescence (PL) spectra of an electron donor, an (poly(N-vinylcarbazole) (PVK))/electron acceptor, and a (2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD)) bi-layer solid film are analysed. The EL emission peak has an apparent red-shift with the increase of driving voltage. There maybe exist an electroplex emission between the PVK and PBD interface under high electric field strength. According to their energy level, the electroplex emission peak should locate at 460 nm. There are great spectra overlapping between PVK emission and electroplex emission, and the ratio of electroplex emission intensity to exciton emission intensity (Ielectroplex/Iexciton) increases from 0.38 at 10 to 0.81 at 16 V. Therefore the measured emission peaks continuously shift from 410 nm at 10 V to 445 nm at 16 V.