Green Luminescence Band Research Articles

Structural and optical properties of zinc oxide films deposited by wire explosion technique

AbstractNanostructured ZnO films were deposited on glass, quartz and Al on silicon mono‐crystal Si (100) substrates by using the wire explosion technique. The growth process, structure and photoluminescence (PL) properties were studied by X‐ray diffraction (XRD), UV–VIS spectroscopy, scanning electron (SEM) and atomic force microscopy (AFM) and photoluminescence measurements. X‐ray diffraction measurements have shown that ZnO films are composed of (100), (002), (101) orientation crystallites. The post‐deposition thermal treatment at 600 °C temperature in air has shown that the composite of Zn/ZnO film was fully oxidized to ZnO film. XRD spectra of the film deposited in oxygen atmosphere at room temperature presents high intensity dominating peak at 2θ = 36.32° corresponding to the (101) ZnO diffraction peak. The small fraction of the film (7%) correspond to (002) peak intensity at 2θ = 34.42°. This result indicates the good crystal quality of the film and hexagonal wurtzite‐type structure deposited by zinc wire explosion. The SEM analysis shows that ZnO films presented different morphologies from fractal network to porous films depending on deposition conditions. AFM analysis revealed the grain size range from 50 to 500 nm. The deposited in air films have shown the weak PL peak at ∼373 nm and a strong green luminescence band. The films deposited in oxygen atmosphere demonstrate increased UV band PL intensity and decreased intensity of green PL band. The obtained PL spectra are similar to powder ZnO and it correlates with the size of ZnO nanoparticles (10–30 nm) measured by AFM. These results demonstrated that the wire explosion technique is a feasible method to produce high‐quality ZnO nanocrystalline thin films and quantum dots. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

First-principles study on the effect of the interstitial oxygen ion on the spectral properties of PWO crystal and the origin of the green luminescence band

The electronic structures and absorption spectra of PWO crystals containing interstitial oxygen ions have been calculated using density functional theory code CASTEP with lattice structure optimized. It is shown by calculation that: (1) the interstitial oxygen ion in the perfect PWO crystal doesn’t bring any obvious absorption in the visible region; (2) the green emission of lead tungstate origin is closely related to the interstitial oxygen ion, and probably originates from the center of “ WO 4+O i ”.

Synthesis, morphology and spectroscopy of cubic Y 3NbO 7:Er

Synthesis of Y 3NbO 7:Er powders with the aid of Li 2SO 4 flux is reported and spectroscopic properties of the resultant powders are presented. The dopant content varied in the range of 0.1–15 at%. The materials crystallized in the fluorite-type cubic structure in which all the metal ions—Y, Nb, and Er—randomly occupy the same site offered by the host lattice and the O-vacancy is also randomly distributed within the metal surrounding. Transmission electron microscopy images revealed that the agglomeration of particles is very low and the sizes of the grains are around 500 nm. Selected area electron diffraction patterns proved that each grain is monocrystalline. Absorption, excitation, and emission spectra are characterized by relatively broad structures related to the Er 3+ ion. The broadening results from some inhomogeneity of the activator ion surroundings related to the specific structure of the host lattice. When the Er content is only 0.1% both photoluminescence and up-converted emission are dominated by a green luminescent band around 550 nm. However, the efficiency of up-conversion is very low . With increasing concentration of the dopant, a red band located around 665 nm appears and becomes systematically stronger. In up-converted emission, the intensity of the red band surpasses the green one when the Er concentration exceeds 5%. For low concentrations, the up-conversion occurs through a sequential absorption of two infrared (IR) (980 nm) photons from the excitation beam by Er 3+ ion through excited-state absorption mechanism. For higher concentrations, the energy transfer between two neighboring excited Er ions plays dominant role. Surprisingly, the mechanism of up-converted low-intensity luminescence from 2H 11/2 state seems to diverge from the mechanism characteristic for the 4S 3/2 level, which conclusion comes from different slopes of the double-log relationships.

Defects in bulk ZnO studied by steady-state and time-resolved photoluminescence

AbstractUnintentionally doped bulk ZnO samples were grown by hydrothermal method in Tokyo Denpa Co. Ltd. (Japan) and MTI Corporation. At low temperatures the PL spectrum contained a very broad band with the peak position (between 2.0 and 2.4 eV) depending on the excitation intensity. Evolution of the PL spectrum after a pulse excitation revealed that the broad band is composed of an orange (OL) and green (GL) luminescence bands having maxima at 1.96 and ∼2.35 eV, respectively. The GL band dominated at times up to 1 ms and then disappeared. The OL band decayed as approximately t−1 over a wide time interval, and its spectrum could be recorded even 24 hours after the excitation source was switched off. The slow nonexponential decay of the OL band is attributed to transitions from shallow donors to a deep acceptor (donor-acceptor pair transitions).

Synthesis and up-converted luminescence of Y3NbO7:Er

Synthesis of cubic Y3NbO7:Er using Li2SO4 flux is reported and spectroscopic properties of the resultant powders are presented. The dopant content has varied in the range of 0.1–15at.%. Absorption, excitation and emission spectra are characterized by relatively broad structures related to the Er3+ ion. The broadening results from some inhomogeneity of the metal surrounding. For diluted samples both photoluminescence and up-converted emission are dominated by a green luminescent band around 550nm. With an increasing concentration of the dopant a red band located around 665nm gets systematic enhancement and in up-conversion it finally dominates the spectrum. For low concentrations the up-conversion occurs through excited state absorption mechanism while for higher Er contents energy transfer mechanism dominates.

Anti-Stokes photoluminescence in ZnO microcrystal

Low temperature (10K) strong anti-Stokes photoluminescence (ASPL) of ZnO microcrystal excited by low power cw 532nm laser is reported here. Energy upconversion of 1.1eV is obtained in our experiment with no conventional nonlinear effect. Through the study of the normal photoluminescence and temperature dependence of ASPL we conclude that the green band luminescence in ZnO is related to deep donor to valance band transition. Using the two-step two-photon absorption model, we provide a plausible mechanism leading to the ASPL phenomenon in our experiment.

Characterization of free standing GaN grown by HVPE on a LiAlO2 substrate

AbstractA 230 µm thick free standing GaN layer has been grown on a LiAlO2 substrate by hydride vapor phase epitaxy, with the separation of the layer from the substrate occuring spontaneously during cooling down after growth. A strong green luminescence band is observed from the top surface, while from the bottom surface, strong blue and red bands are seen in photoluminescence (PL). The evolution of these luminescence bands with layer thickness was monitored by cross‐sectional cathodoluminescence (CL). PL and CL measurements indicate that the defect structure changes with growing layer thickness. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Anharmonic metastable charge transfer vibronic exciton in potassium tantalate

This study provides an experimental evidence of a non-linear state of charge transfer vibronic excitons (CTVE) and its well-defined metastable behaviors in KTaO 3. An IR source could induce the same green CTVE-recombination luminescence band as that by a UV pumping. This up-conversion luminescence is characterized by a relaxation time τ ( τ ∼ 60 s at 3 K), and theoretically explained as a result of IR-induced transfer of occupation of a metastable anharmonic CTVE state into a harmonic luminescent CTVE state. The observed temperature dependence of τ suggests that the relaxation is controlled by resonant and phonon-induced tunneling processes.

Luminescence of crystals of divalent tungstates

Crystals of divalent tungstates are characterized by two main luminescence spectral ranges: a short-wavelength (blue) luminescence band in the range 390–420 nm and a group (often two groups) of longer wavelength (green) bands in the range 480–520 nm. For crystals of calcium, strontium, barium, cadmium, magnesium, zinc, and lead tungstates, it is shown that the wavelength corresponding to the maximum of the blue luminescence band (λmax) correlates with the melting temperature (T m) of these compounds. The position of the blue luminescence band is the same (in the range 510–530 nm) for crystals with different divalent cations. Annealing in vacuum and electron irradiation decrease the intensity of both blue and green luminescence bands but do not change the ratio of their maximum intensities. This circumstance suggests that vacancies serve as luminescence quenchers to a greater extent rather than facilitate the formation of emission centers responsible for a particular luminescence band.

Theoretical investigation of the luminescence centres in PbWO4 and CdWO4 crystals

Electronic structures of possible centres of luminescence in the perfect lead (PbWO4) and cadmium tungstate (CdWO4) crystals are calculated using cluster approach. Electronic states of Pb2+, and Cd2+ cations, WO42- and WO66- are obtained on the basis of partial densities of electronic states calculated for several clusters containing from 72 to 104 atoms of the crystals. Calculated energies of one-electron transitions in these centers are compared with experimental data on luminescence and luminescence excitation. Obtained results evidence in favor of a tungstate nature of intrinsic luminescence in the PbWO4 and CdWO4 crystals . I ntrinsic luminescence probably originates from the tungstate groups WO42- in scheelite-type, and from the WO66- groups in the raspite-type PbWO4 crystals. Tungstate groups WO66- are assumed to be the centres of excitation of blue–green and yellow luminescence bands in CdWO4 crystals.

Photoluminescence Excitation Spectroscopy Studies of Anodically Etched and Oxidized Porous Zn

Photoluminescence excitation (PLE) spectroscopy studies were performed for anodically etched porous Zn, which exhibited a PL in the blue/violet spectral range peaking at 420nm (2.95eV), and oxidzed porous Zn at 380℃ for 10min and 12 h. A broad absorption band was observed at 4.07eV (305 nm), 3.49 (355 nm) for anodically etched porous Zn. In contrast, both the oxidized porous Zn and sintered ZnO exhibited an almost identical one broad absorption band at 3.85eV (322 nm), when PLE spectra were measured at 378 nm (3.28eV). The oxidized porous Zn and sintered ZnO, which displayed both UV and green luminescence band, showed an additional absorption band at 389 nm (3.19eV) and 467 nm (2.66eV). In contrast, no significant absorption band was detected for a 10-min oxidized porous Zn, which only displayed one UV luminescence void of deep-level luminescence. These absorption bands determined by PLE studies enabled a clear understanding of an emission mechanism for the UV and green luminescence from ZnO.

Aquamarine Luminescence Band in Undoped GaN

ABSTRACTWe report a new defect-related photoluminescence (PL) band peaking at 2.56 eV at 15 K in undoped GaN layers grown by molecular beam epitaxy (MBE) on GaN templates. The maximum of the aquamarine luminescence (AL) band shifts to higher energies by about 100 meV with increasing temperature from 15 to 300 K. In spite of the fact that the shape and the peak position of the AL band are close to the characteristics of the green luminescence band in a freestanding GaN template, we were able to delineate these two bands for their markedly different behavior with temperature and excitation intensity. The origin of defect responsible for this PL band remains unknown.

Unusual Properties of the Red and Green Luminescence Bands in Ga-rich GaN

ABSTRACTWe studied photoluminescence (PL) from deep-level defects in GaN grown under Garich conditions at relatively low temperatures (700–800°C) by molecular-beam epitaxy (MBE). The dominant features of PL spectrum are red and green bands peaking respectively at ∼1.8 and ∼2.35 eV. Both PL bands decay exponentially at low temperatures (15 – 100 K) after pulsed excitation. The characteristic lifetime for the red band decreases by almost two orders of magnitude from 110 to 2 μs with increasing temperature from 15 to 100 K, while its integrated intensity after each pulse remains nearly unchanged in the same temperature range due to an increase in the peak intensity in the time-resolved PL curve. The lifetime of the green band remained unchanged in this temperature range. We suggest that these PL bands are caused by transitions between excited and ground states of some deep defects rather than transitions involving a shallow donor, conduction or valence bands.

The visible luminescent characteristics of ZnO supported on SiO2powder

In situ laser-induced luminescence spectroscopy is used to study the visible luminescent characteristics of ZnO during the preparation process of ZnO supported on SiO2 by the pyrolysis of different Zn precursors in N2 or O2 atmosphere. The excitation source is 325 nm light, which is above the band gap (3.37 eV) of ZnO. In N2 atmosphere, it is shown that green (centered at ca. 520 nm), yellow (centered at ca. 580 nm) and orange (centered at ca. 640 nm) luminescence bands appear for ZnO produced from zinc acetate, zinc hydroxide and zinc nitrate, respectively. After these samples are treated by O2, green band is changed into yellow band and yellow band is changed into orange band. On the other hand, it is also found that the laser irradiation on the sample could alter the luminescent behavior of ZnO produced at the beginning decomposition temperature of the Zn precursors. While this sample is irradiated, the orange band is gradually changed to a yellow band, the luminescent intensity finally increases more than 30 times that at the beginning of irradiation. However, irradiation hardly affects the luminescent properties of ZnO after calcination above 160 °C. The results indicate that the visible luminescence from ZnO is associated with the oxygen vacancies in ZnO, and the electronic state levels responsible for the visible luminescence bands are changing with the density of oxygen vacancies in ZnO. The green, yellow and orange bands are ascribed to the state of ZnO with high density of oxygen vacancies, with moderate density of oxygen vacancies and with less oxygen vacancies, respectively.

Green luminescence band of zinc oxide films copper-doped by thermal diffusion

High quality ZnO single-crystal films were doped with copper by thermal diffusion, and their luminescent properties were studied by cathodoluminescence spectroscopy. Doping with copper increases the intensity of the green-emission band of the cathodoluminescence spectrum, whose peak, width, and shape at 78 and 300 K remain unchanged. At 4.2 K, a pronounced phonon structure with a phonon energy of 72 meV is detected in the cathodoluminescence green-emission band of the doped samples. In this case, the phonon peaks feature a triplet fine structure instead of the doublet one generally observed. This feature is attributed to radiative recombination of acceptor excitons that are localized at copper atoms and interact with each one of the subbands of the ZnO valence band. An analysis of the experimental data on the film cathodoluminescence and comparative studies of luminescence and electron spin resonance in single crystals allow one to conclude that the uncontrollable copper impurity typically existing in ZnO is responsible for green-emission luminescence in this material.

Unusual properties of the dominant acceptor in freestanding GaN

Photoluminescence (PL) study of high-purity undoped GaN revealed the yellow (YL) and green luminescence (GL) bands, attributed to two charge states of the same defect, presumably a gallium vacancy–oxygen complex. We have used time-resolved PL in a wide temperature range to study this defect. The GL band was visible only for short time (below 10−4s) and then transformed into the YL band, consistent with the attribution of these bands to an acceptor binding two and one holes, respectively. The GL exhibits fast exponential decay (lifetime of 1.5μs) above 30K. In contrast to the YL band, where the lifetime decreases with increasing temperature due to increasing concentration of free electrons, the lifetime of the GL stays unchanged up to 70K and then increases by a factor of 20 at 300K. This anomalous behavior of the GL band is attributed to the existence of an excited state near the conduction band.

Characterization of enhanced emission from excimer laser treated ZnO ceramics using one- and two-photon luminescence spectroscopy and microscopy

The effects of excimer laser irradiation on the surface structure and luminescence of sintered ZnO ceramics were investigated. Photoluminescence spectra of three materials (as-prepared ZnO ceramic, the ceramic blackened by high-energy irradiation and the recovered ceramic treated using lower-energy irradiation) were compared in the visible region at room temperature and 10 K. Each material exhibited a green luminescence band but the blackened ZnO ceramics had the strongest emission in the visible region while the recovered ceramics had significantly shorter decay times.

Properties of annealed anodically etched porous Zn studied by scanning tunneling microscopy

We have studied the annealing behavior of anodically etched porous Zn (p-Zn) as a function of annealing time. It is found that 10 min ambient air annealing of Zn yields efficient UV luminescence with a very weak deep-level defect-related luminescence. The emergence of the green luminescence band is observed with an increase of annealing time. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) have been performed on annealed p-Zn. STM studies reveal a more smooth surface for p-Zn annealed for a short period. STS analysis shows a good agreement with observed PL spectra, especially concerning the existence or absence of defect-related luminescence.

Evolution of visible luminescence in ZnO by thermal oxidation of zinc films

Zinc oxide (ZnO) films were prepared by thermal oxidation of metallic zinc films. The intensities of visible luminescence increase with oxidation time; however the shape and position of the green and yellow luminescence bands are determined by oxidation temperature only and do not vary with oxidation time and excitation laser powers. The green and yellow luminescent bands should originate from different intrinsic defects or defect complexes, which are formed in ZnO at certain temperatures. It suggests that the possibility of donor–acceptor pair recombination as the mechanism responsible for the green and yellow emissions can be excluded.

Optical study of SrAl1.7B0.3O4:Eu, R (R=Nd, Dy) pigments with long-lasting phosphorescence for industrial uses

We have studied and compared the optical properties of SrAl1.7B0.3O4:Eu, R (R=Nd, Dy) pigments that present long-lasting phosphorescence obtained by different synthesis techniques. Samples obtained by ceramic methods, in our laboratories and by an industrial process, present better phosphorescent properties than those obtained by sol–gel technique. Raman spectra show that grinding produces severe damage of the lattice. We have obtained and analyzed the Eu3+ crystal field luminescence indicating that Eu3+ is found in quite different sites comparing ceramic and sol–gel samples. Codoping, with Nd or Dy is necessary in order to reduce the Eu3+ content, in all cases. The green luminescence band, obtained under UV illumination, can be fitted to two and three components in ceramic and sol–gel samples, respectively, due to different Eu2+ sites. Eu–Dy samples present the longest and the most efficient phosphorescence. The time evolution of the afterglow is well described by a t−1 law, up to about 2h, indicating that the recombination process is achieved by electron–hole tunneling.