Green PL Research Articles

Luminescence properties of terbium-doped SiCN thin films by rf magnetron reactive sputtering

Terbium-doped SiCN (SiCN:Tb) thin films were deposited by rf magnetron reactive sputtering at 800 °C. The as-prepared samples were characterized by XRD, FTIR, and XPS. The results showed that SiCN:Tb films mainly contained both SiC and Si 3N 4 nano-compositions with complicated chemical bond networks. Photoluminescence measurements indicated that the undoped SiCN films exhibited a blue-green light emission, while SiCN:Tb films emitted a strong green one. The SiC nanocrystallites formed in the undoped SiCN films might be responsible for the blue-green light emission, while the formed quaternary Si–C–Tb–O compositions in the doped samples could account for the strong green PL behaviors.

Role of excitons in the excitation of deep‐level emission in ZnO crystals

AbstractIn undoped ZnO crystals, photoluminescence excitation (PLE) spectra of deep‐level green and orange PL bands as well as photocurrent (PC) spectra were measured at 77 and 290 K. Some PLE and PC peaks whose energy positions coincided with that of free exciton and its excited states were observed, the intensity of PL and PC excited in these peaks being higher than under excitation by band‐to‐band light. At the same time, a sharp minimum was found in PLE and PC spectra at the position of donor‐bound exciton. Obtained results were accounted for by the interaction of excitons with lattice defects. A number of longer wavelength PLE and PC peaks whose energy positions coincided in fact with that of the first, second and third phonon replicas of free exciton were also observed. The origin of these peaks is discussed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of cadmium oxide incorporation on the microstructural and optical properties of pulsed laser deposited nanostructured zinc oxide thin films

CdO doped (doping concentration 0, 1, 3 and 16 wt%) ZnO nanostructured thin films are grown on quartz substrate by pulsed laser deposition and the films are annealed at temperature 500 °C. The structural, morphological and optical properties of the annealed films are systematically studied using grazing incidence X-ray diffraction (GIXRD), energy dispersive X-ray analysis (EDX), scanning electron microscopy (SEM), atomic force microscopy (AFM), Micro-Raman spectra, UV–vis spectroscopy, photoluminescence spectra and open aperture z-scan. 1 wt% CdO doped ZnO films are annealed at different temperatures viz., 300, 400, 500, 600, 700 and 800 °C and the structural and optical properties of these films are also investigated. The XRD patterns suggest a hexagonal wurtzite structure for the films. The crystallite size, lattice constants, stress and lattice strain in the films are calculated. The presence of high-frequency E 2 mode and the longitudinal optical A 1 (LO) modes in the Raman spectra confirms the hexagonal wurtzite structure for the films. The presence of CdO in the doped films is confirmed from the EDX spectrum. SEM and AFM micrographs show that the films are uniform and the crystallites are in the nano-dimension. AFM picture suggests a porous network structure for 3% CdO doped film. The porosity and refractive indices of the films are calculated from the transmittance and reflectance spectra. Optical band gap energy is found to decrease in the CdO doped films as the CdO doping concentration increases. The PL spectra show emissions corresponding to the near band edge (NBE) ultra violet emission and deep level emission in the visible region. The 16CdZnO film shows an intense deep green PL emission. Non-linear optical measurements using the z-scan technique indicate that the saturable absorption (SA) behavior exhibited by undoped ZnO under green light excitation (532 nm) can be changed to reverse saturable absorption (RSA) with CdO doping. From numerical simulations the saturation intensity ( I s ) and the effective two-photon absorption coefficient ( β) are calculated for the undoped and CdO doped ZnO films.

Deep-blue Electroluminescence of 9,9-Dialkyl-substituted Polyfluorene Copolymers Incorporating Diphenylsilane Units

Bis(7-bromo-9,9-dialkylfluorenyl)diphenylsilanes with different alkyl chain lengths ( Cn = 4 and 8) were synthesized in this work. Conventional 9,9-dialkyl-substituted polyfluorenes (PFs) were modified through copolymerization of the diphenylsilane-containing comonomers by the Suzuki coupling method to improve electroluminescence (EL) color purity. The PF copolymers possessed high molecular weights that were enough for film-forming ability and good solubility in various common organic solvents such as toluene. The incorporation of the diphenylsilane units suppressed effectively the PF-inherent green PL, which is undesirable for the present purpose, with a slight blue-shift of the interesting blue emission band. The results are closely related to a bent and bulky structure at the diphenylsilane units which disturb interchain stacking and a controlled conjugation length along the PF main chains. This effect was observed much more prominently in the EL spectra. Even in the di- n-butyl-substituted PF copolymer which tends to show a strong green excimer emission, the incorporation of the diphenylsilane units completely erased the green EL band. The results suggest that the PF copolymers can be a good candidate as a novel deep-blue emission layer material in EL devices.

Controllable vertical growth onto anatase TiO 2 nanoparticle films of ZnO nanorod arrays and their photoluminescence and superhydrophilic characteristics

Well-aligned ZnO nanorod arrays have been successfully fabricated directly on anatase TiO 2 nanoparticle films via low-temperature hydrothermal processes. The effects of the reactive time, temperature and reactant concentration on the growth of the as-prepared ZnO crystals are investigated in detail, and the possible mechanisms of crystal ZnO nanorod growth are also suggested. The results show that the low reactant concentration is in favor of the increase in the aspect ratio of crystal ZnO nanorods with weak orientation, while the long reactive time and high reactant concentration are useful to prepare well-aligned crystal ZnO nanorod arrays. Interestingly, the typically constructed composite films exhibit superhydrophilic characteristic without UV irradiation. Moreover, a strong near-ultraviolet PL band centering at about 385 nm and a weak green PL band centering at about 525 nm can appear at the room temperature.

Effect of proton irradiation on photoluminescent properties of PDMS–nanodiamondcomposites

Pure poly(dimethylsiloxane) (PDMS) films, PDMS–nanodiamond (ND) and purenanodiamond powder were irradiated with 2 MeV protons under a variety of fluence andcurrent conditions. Upon proton irradiation, these samples acquire a fluence-dependentphotoluminescence (PL). The emission and excitation spectra, photostability and emissionlifetime of the induced photoluminescence of PDMS and PDMS–ND samples are reported.Pure PDMS exhibits a noticeable stable blue PL, while the PDMS–ND composites exhibita pronounced stable green PL under 425 nm excitation. The PL of PDMS–ND composites ismuch more prominent than that of pure PDMS or pure ND powder even when irradiatedat higher doses. The origin of the significantly enhanced PL intensity for theproton-irradiated PDMS–ND composite is explained by the combination of enhancedintrinsic PL within ND particles due to ion-implantation-generated defects andby PL originating from structural transformations produced by protons at thenanodiamond/matrix interface.

Microstructure and photoluminescence properties of ZnO thin films grown by PLD on Si(1 1 1) substrates

ZnO thin films on Si(1 1 1) substrate were deposited by laser ablation of Zn target in oxygen reactive atmosphere; Nd-YAG laser with wavelength of 1064 nm was used as laser source. X-ray diffraction and atom-force microscopy were applied to characterize the structure and surface morphology of the deposited ZnO films. The optical properties of the ZnO thin films were characterized by photoluminescence with an Ar ion laser as a light source. It was found that ZnO film with a majority of c-axis growth grains can be obtained under the condition of substrate temperature 450∼550 °C. Corresponding to the c-axis growth structure, intense UV emission with narrow FWHM was obtained from the ZnO films grown at substrate temperature 500 °C. The green deep level PL emission centering about 518 nm can be attributed to the electron transitions from the bottom of the conduction band to the antisite oxygen O Zn defect levels.

Radiative defects in CuGaS2 thin films

The radiative defects in vacuum-deposited CuGaS2 is studied by photoluminescence spectroscopy (PL). The PL of Cu-rich layers is dominated by donor–acceptor lines at 2.3 and 2.26eV attributed to a common shallow donor (the S vacancy) and two acceptors (possibly Ga vacancies and CuGa antisites). The PL of near-stoichiometric to Ga-rich layers is dominated by green PL in the 2.3–2.45eV range and broad yellow/red luminescence below 2.2eV. The green PL is interpreted in terms of the (0/+) and (+/2+) levels associated with the S vacancy (a double donor), and the Cu vacancy (an acceptor). The yellow/red PL is typical for highly compensated semiconductors and is ascribed to donor–acceptor transitions involving Cu vacancies and deep donor levels.

Optical transitions in Ge nanocrystals formed by high-pressure annealing of Ge + ion implanted SiO 2 films

Optical transitions in Ge nanocrystals formed by high-pressure annealing of the Ge + ion implanted SiO 2 films have been studied by Raman and photoluminescence spectroscopy. It has been found that the E 1, E 1+ Δ 1 Raman resonance shift observed from the unstrained and hydrostatically compressed nanocrystals corresponds to the quantization of the electron–hole state spectrum of the Ge band. It has also been established that the appearance of a green photoluminescence band centered at 420–520 nm correlates with the formation of strained nanocrystals. Comparisons of the PL data with HRTEM results have been made, which suggest that the green PL arises from strained Ge nanocrystals of a radius of less than 5 nm. The direct electron–hole recombination at Γ is discussed as a possible origin of the observed photoluminescence band.

Electro-optical properties of water-soluble conjugated polymer

The electro-optical properties of conjugated ionic polymer prepared from 2-ethynyl-N-propargylpyridinium bromide by the cyclopolymerization method were studied. The poly (2-ethynyl-N-propargylpyridinium bromide) [poly(EPPB)] showed characteristic UV-visible absorption band at 300nm and 460nm and green PL spectrum at 510nm corresponding to the photon energy of 2.43eV. The photoluminescence intensity was decreased as the temperature is increased because of the increase of nonradiative decay channels. The electrical conductivities of iodine-doped polymer increased with doping level and reached the value of 4.5 x 10 -2 Ω -1 cm -1 .

Comparison of the Optical Properties of Er3+ Doped Gallium Nitride Prepared by Metalorganic Molecular Beam Epitaxy (Mombe) and Solid Source Molecular Beam Epitaxy (SSMBE)

We report on the luminescence properties of Er doped GaN grown prepared by metalorganic molecular beam epitaxy (MOMBE) and solid-source molecular beam epitaxy (SSMBE) on Si substrates. Both types of samples emitted characteristic 1.54 µm PL resulting from the intra-4f Er3+ transition 4I13/2→4I15/2. Under below-gap excitation the samples exhibited very similar 1.54 µm PL intensities. On the contrary, under above-gap excitation GaN: Er (SSMBE) showed ∼80 times more intense 1.54 µm PL than GaN: Er (MOMBE). In addition, GaN: Er (SSMBE) also emitted intense green luminescence at 537 nm and 558 nm, which was not observed from GaN: Er (MOMBE). The average lifetime of the green PL was determined to be 10.8 µs at 15 K and 5.5 µs at room temperature. A preliminary lifetime analysis suggests that the decrease in lifetime is mainly due to the strong thermalization between the 2H11/2 and 4S3/2 excited states. Nonradiative decay processes are expected to only weakly affect the green luminescence.

Comparison of the Optical Properties of Er3+ Doped Gallium Nitride Prepared by Metalorganic Molecular Beam Epitaxy (MOMBE) and Solid Source Molecular Beam Epitaxy (SSMBE)

AbstractWe report on the luminescence properties of Er doped GaN grown prepared by metalorganic molecular beam epitaxy (MOMBE) and solid-source molecular beam epitaxy (SSMBE) on Si substrates. Both types of samples emitted characteristic 1.54 µm PL resulting from the intra-4f Er3+ transition 4I13/2→4I15/2. Under below-gap excitation the samples exhibited very similar 1.54 µm PL intensities. On the contrary, under above-gap excitation GaN: Er (SSMBE) showed ∼80 times more intense 1.54 µm PL than GaN: Er (MOMBE). In addition, GaN: Er (SSMBE) also emitted intense green luminescence at 537 nm and 558 nm, which was not observed from GaN: Er (MOMBE). The average lifetime of the green PL was determined to be 10.8 µs at 15 K and 5.5 µs at room temperature. A preliminary lifetime analysis suggests that the decrease in lifetime is mainly due to the strong thermalization between the 2H11/2 and 4S3/2 excited states. Nonradiative decay processes are expected to only weakly affect the green luminescence.

Colour-switching effect of photoluminescent silicon after spark-processing in oxygen

We report on a new colour-switching effect of photoluminescing spark-processed silicon which was prepared in pure oxygen. Whereas as-prepared specimens display an orange PL peaking at 655 nm (1.89 eV), the PL changes to a more intense blue emission centred at 475 nm (2.61 eV) when the sample is subjected to a lower pressure. The same blue PL band is observed after heat treatments up to . Moreover, at annealing temperatures above a third radiative transition emerges in the infrared and eventually dominates the spectrum. A model is suggested that links ozone molecules, generated during spark-processing and incorporated into the sp-matrix, to the orange PL band. The results of heat treatments and temperature-dependent PL measurements allow for the relationship of the 2.61 eV band to oxygen deficiency centres, created by out-driven ozone molecules. The IR band is discussed in the light of luminescing properties of porous silicon and silicon oxides. The experiments demonstrate that the UV/blue (3.25 eV) and green (2.36 eV) PL bands for Si spark-processed in air cannot be related to radiative transitions in silicon oxides.

Nature of the Green Luminescent Center in Zinc Oxide

ABSTRACTWe apply a number of complementary characterization techniques including electron paramagnetic resonance, optical absorption, and photoluminescence spectroscopies to characterize a wide range of different ZnO phosphor powders. We generally observe a good correlation between the 510-nm green emission intensity and the density of paramagnetic isolated oxygen vacancies. In addition, both quantities are found to peak at a free-carrier concentration ne, of about 1.4 × 1018 cm-3. We also find that the green emission intensity can be strongly influenced by free-carrier depletion at the particle surface, especially for small particles and/or low doping. Our data suggest that the green PL in ZnO phosphors is due to the recombination of electrons in singly occupied oxygen vacancies with photoexcited holes in the valence band.