Violet Band Research Articles

Strong violet luminescence from ZnO nanocrystals grown by the low-temperature chemical solution deposition

The luminescence properties of zinc oxide (ZnO) nanocrystals grown from solution are reported. The ZnO nanocrystals were characterized by scanning electron microscopy, X-ray diffraction, cathodo- and photoluminescence (PL) spectroscopy. The ZnO nanocrystals have the same regular cone form with the average sizes of 100–500 nm. Apart from the near-band-edge emission around 381 nm and a weak yellow–orange band around 560–580 nm at 300 K, the PL spectra of the as-prepared ZnO nanocrystals under high-power laser excitation also showed a strong defect-induced violet emission peak in the range of 400 nm. The violet band intensity exhibits superlinear excitation power dependence while the UV emission intensity is saturated at high excitation laser power. With temperature raising the violet peak redshifts and its intensity increases displaying unconventional negative thermal quenching behavior, whereas intensity of the UV and yellow–orange bands decreases. The origin of the observed emission bands is discussed.

Strong violet and green-yellow electroluminescence from silicon nitride thin films multiply implanted with Si ions

Strong visible electroluminescence (EL) has been observed from a 30 nm silicon nitride thin film multiply implanted with Si ions and annealed at 1100 °C. The EL intensity shows a linear relationship with the current transport in the thin film at lower voltages, but a departure from the linear relationship with a quenching in the EL intensity is observed at higher voltages. The EL spectra show two primary EL bands including the predominant violet band at ∼3.0 eV (415 nm) and the strong green-yellow band at ∼2.2 eV (560 nm). Two weak bands including the ultraviolet band at ∼3.8 eV and the near infrared band at ∼1.45 eV emerge at high voltages. The evolution of each EL band with the voltage has been examined. The phenomena observed are explained, and the EL mechanisms are discussed.

Influence of Fe doping on nanostructures and photoluminescence of sol–gel derived ZnO

The doping of Fe (0.5, 2, and 50 mol%) in ZnO was performed using sol–gel process, which led to noteworthy alterations in the microstructure, phase formations and optical properties. Nanoparticles (20–50 nm) with euhedral shape morphology and aggregated nanowires (20–30 nm diameter) are observed at 2 mol% Fe doping, whereas well developed and facetted nanoparticles (50–100 nm), nanowires (∼60 nm diameter) and ultrafine particles (2–5 nm) are seen at 50 mol% Fe doping. The hexagonal-ZnO was present as the dominant phase with the traces of cubic-ZnFe 2O 4 up to 2 mol% Fe, while at highest 50 mol% Fe doping, ZnFe 2O 4 was the observed prominent phase, which is also responsible for the observed violet (412 nm, 3.0 eV) and blue emission bands (468 nm, 2.65 eV and 440 nm, 2.82 eV) together with quenched orange (632 nm, 1.96 eV) and green (523 nm, 2.37 eV) luminescence.

The influence of defects and impurities in polycrystalline AlN films on the violet and blue photoluminescence

The influence of defects and impurities in polycrystalline aluminum nitride films on the violet and blue photoluminescence properties was investigated. The photoluminescence spectra show a broad emission band in the range from 380 nm to 550 nm, which consists of two components of the violet band centered at 400 nm and the blue band centered at 480 nm. When the native defects reduce and the crystal quality is improved by annealing in nitrogen atmosphere, the shoulder band around 480 nm declines. The center of the violet luminescence shifts from 400 nm to about 440 nm as the oxygen content increase from 2.9 at.% to 12.3 at.%. The intensity and the center of the violet emission vary mostly linearly with the oxygen content. Combining the results of X-ray diffraction and Auger Electron Spectroscopy, the violet emission around 400 nm (3.09 eV) can be attributed to the transition from the shallow donor to the deep acceptor related to the oxygen impurity, while the blue emission around 450 nm (2.58 eV) may originate from transition from the shallow donor to the deep acceptor related to the native defect.

A specific chromogenic reagent for detection of diazepam among other benzodiazepines from biological and nonbiological samples after HPTLC

Anew sensitive chromogenic TLC spray reagent is described for the detection of the benzodiazepine drug diazepam in biological tissues, blood, urine, vomit, and tablets. Diazepam standard and extracts of these samples in alkaline chloroform-isopropyl alcohol 9:1 were applied to TLC plates coated with 0.25 mm layers of silica gel G. Plates were developed with chloroform-methanol 9:1 as mobile phase, dried, and then sprayed with 5% sodium hydroxide solution followed by 1% m -dinitrobenzene in dimethyl sulfoxide (DMSO). Violet bands were obtained for diazepam but other benzodiazepines, for example oxazepam, nitrazepan, lorazepam, chlordiazepoxide, and flurazepam, did not react. The sensitivity of this reagent for diazepam is approximately 5 μg.

Studies of Chemi-Ionization and Chemiluminescence in Supersonic Flows of Combustion Products

A stable ethylene/oxygen/argon flame is sustained and nearly complete combustion is achieved in the combustion chamber of an M = 3 supersonic nozzle, at a stagnation pressure of P 0 =1 atm. Ultraviolet and visible emission is detected both from the combustion chamber and from the M = 3 flow of combustion products. Temperature in the combustor, inferred from the visible emission spectra, is To = 2000 ± 200 K. Electron density in M = 3 flow of combustion products has been measured using Thomson discharge n, = 1.4 ± 0.2·10 8 cm -3 , at an ionization fraction of n e /N = (0.65 ± 0.15) · 10 -9 . This corresponds to an electron density of n e0 = 2.2 ·10 9 cm -3 in the combustor. The chemi-ionization current measured in the M = 3 flow is found to be proportional to the equivalence ratio in the combustor. The time-resolved chemi-ionization current is in very good correlation with the visible emission from ethylene-air and propane-oxygen-argon flames in the combustor at unstable combustion conditions. The results show that nearly all electrons can be removed from the supersonic flow of combustion products by applying a moderate transverse electric field. No effect of electron removal on visible emission has been detected. A similar result was obtained for nitric oxide β bands and cyanogen violet band emission, when nitric oxide was injected into the combustion product flow.

The nature of unusual luminescence in natural calcite CaCO3

The unusual luminescence of particular varieties of natural pink calcite (CaCO{sub 3}) samples was studied by laser-induced time-resolved luminescence spectroscopy at different temperatures. The luminescence is characterized by intense blue emission under short-wave UV lamp excitation with an extremely long decay time, accompanied by pink-orange luminescence under long wave UV excitation. Our investigation included optical absorption, natural thermostimulated luminescence (NTL) and Laser-Induced Breakdown Spectroscopy (LIBS) studies. Two luminescence centers were detected: a narrow violet band, with {lambda}{sub max} = 412 nm, {Delta} = 45 nm, two decay components of {tau}{sub 1} = 5 ns and {tau}{sub 2} = 7.2 ms, accompanied by very long afterglow, and an orange emission band with {lambda}{sub max} = 595 nm, {Delta} = 90 nm and {tau} = 5 ns. Both luminescence centers are thermally unstable with the blue emission disappearing after heating at 500 C, and the orange emission disappearing after heating at different temperatures starting from 230 C, although sometimes it is stable up to 500 C in different samples. Both centers have spectral-kinetic properties very unusual for mineral luminescence, which in combination with extremely low impurity concentrations, prevent their identification with specific impurity related emission. The most likely explanation of these observationsmore » may be the presence of radiation-induced luminescence centers. The long violet afterglow is evidently connected with trapped charge carrier liberation, with their subsequent migration through the valence band and ultimate recombination with a radiation-induced center responsible for the unusual violet luminescence.« less

Characterization of defects in ZnO nanocrystals: Photoluminescence and positron annihilation spectroscopic studies

Defects present in ZnO nanocrystals prepared by a wet chemical method have been characterized by photoluminescence (PL) and positron annihilation spectroscopy (PAS) techniques. The as-prepared sample was heat treated at different temperatures to obtain nanocrystals in the size range of 19–39nm. X ray diffractograms confirmed the single-phase wurtzite structure formation. Photoluminescence measurements showed a strong violet band at 434nm, which has been identified as due to electronic transitions from the zinc interstitial defect level to the top of the valence band. A marked decrease in the intensity of the violet emission with increasing heat-treatment temperature has been observed, which is attributed to recombination of zinc interstitials with zinc vacancies. Positron annihilation spectroscopy has been employed to understand the dynamics of the vacancy-type defects and their annealing behavior. The observed variation of the defect related lifetime components with heat-treatment temperature has been successfully explained by using a three-state trapping model. The results of PL and PAS studies in the present case are found to be complementary to each other.

Luminescence of isoelectronically ion‐implanted SiO2 layers

AbstractScanning electron microscopy (SEM) and cathodoluminescence (CL) in combination with scanning transmission electron microscopy (STEM) have been used to investigate thermally grown amorphous silicon dioxide layers implanted isoelectronically with group IV ions (C+, Si+, Ge+, Sn+, Pb+) as well as with group VI ions (O+, S+, Se+). Besides the main luminescent centers in a‐SiO2 layers: the red R luminescence (650 nm; 1.9 eV) of the non‐bridging oxygen hole centers (NBOHC), the blue B (460 nm; 2.7 eV) and the UV (290 nm; 4.3 eV) band of the oxygen deficient centers (ODC), in ion implanted silica additional emission bands are observed. E.g. in Ge+ implanted layers a huge violet band appears at 410 nm (3.1 eV) increasing with the thermal annealing process due to formation of Ge dimers, trimers or higher aggregates, finally leading to destruction of the luminescence centers by further growing to Ge nanoclusters. The Ge cluster size is shown by STEM cross section micrographs.Generally, group IV element implantation and partial substitution of silicon increases the luminescence in the blue/violet region whereas group VI elements and additional oxygen increase the intensity in the red region, confirming the association of the blue and the red luminescence with oxygen deficient centers and oxygen excess centers, respectively. Thus, the cathodoluminescence spectra of sulfur and oxygen implanted SiO2 layers under special conditions show besides the characteristic luminescence bands a multimodal structure beginning in the green region at 500 nm and extending up to the near infrared region at 820 nm. The energy step differences of the sublevels amount in the average 120 meV and indicate vibronic‐electronic transitions, probably of O2– interstitial molecules. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical investigations of InN nanodots capped by GaN at different temperatures

InN nanodots capped with GaN for temperatures from 600to730°C were investigated. While the dot emission intensity at 0.77eV decreased with increasing capping temperature, two extra visible emission bands appeared around 2.37eV (green band) and 2.96eV (violet band). Furthermore, x ray diffraction shows that the 71.7° and 70.2° peaks were tentatively attributed to InGaN alloy with In fractions of 14.8% and 34.2%, respectively. Moreover, the near-field measurements helped reveal the regions of different emissions. The violet-band mapping showed a spatial distribution in contrast to nanodot distribution but the green band showed a uniform distribution that apparently reflects the capping induced InGaN alloy.

Zinc Oxide Nanorods Grown by Arc Discharge

The arc discharge method was employed to fabricate zinc oxide (ZnO) nanorods with wurtzite structure. The microstructure analysis by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) demonstrated that the ZnO nanorods grew along the [0001] direction. On average, the diameter and length of the nanorods were about 40 nm (some are as thin as 5 nm) and several hundred nanometers, respectively. The photoluminescence (PL) of the nanorods showed an ultraviolet band, a violet band, and a green band. The PL mechanism was discussed with the growth process and Raman spectroscopy.

Visible and near-infrared spectrophotometry of the Deep Impact ejecta of Comet 9P/Tempel 1

We have obtained optical spectrophotometry of the evolution of Comet 9P/Tempel 1 after the impact of the Deep Impact probe, using the Supernova Integral Field Spectrograph (SNIFS) at the UH 2.2-m telescope, as well as simultaneous optical and infrared spectra using the Lick Visible-to-Near-Infrared Imaging Spectrograph (VNIRIS). The spatial distribution and temporal evolution of the “violet band” CN (0–0) emission and of the 630 nm [OI] emission was studied. We found that CN emission centered on the nucleus increased in the 2 h after impact, but that this CN emission was delayed compared to the light curve of dust-scattered sunlight. The CN emission also expanded faster than the cloud of scattering dust. The emission of [OI] at 630 nm rose similarly to the scattered light, but then remained nearly constant for several hours after impact. On the day following the impact, both CN and [OI] emission concentrated on the comet nucleus had returned nearly to pre-impact levels. We have also searched for differences in the scattering properties of the dust ejected by the impact compared to the dust released under normal conditions. Compared to the pre-impact state of the comet, we find evidence that the color of the comet was slightly bluer during the post-impact rise in brightness. Long after the impact, in the following nights, the comet colors returned to their pre-impact values. This can be explained by postulating a change to a smaller particle size distribution in the ejecta cloud, in agreement with the findings from mid-infrared observations, or by postulating a large fraction of clean ice particles, or by a combination of these two.

Tunable light emission and decaying process of photoluminescence from a nanostructured Si-in-SiN x film

Strong photoluminescence (PL) covering the green–violet band was measured at room temperature in an as-deposited amorphous Si-in-SiN x film, which was prepared by plasma-enhanced chemical vapor deposition on cold (below 60 °C) Si(1 0 0) wafer. With an increase in photon energy of excitation, the PL shifts its peak position from 510 to 416 nm at yet-comparable intensities, thus allowing an energy-selected excitation in practical application. Also, a time-resolved analysis was performed for the emissions at various wavelengths, which showed a decay time shorter than 1.0 ns. These results indicate that the nanostructured Si-in-SiN x can be a promising candidate material for the fabrication of silicon-based optical interconnections and switches.

Visible and near-infrared spectrophotometry of the Deep Impact ejecta of Comet 9P/Tempel 1

We have obtained optical spectrophotometry of the evolution of Comet 9P/Tempel 1 after the impact of the Deep Impact probe, using the Supernova Integral Field Spectrograph (SNIFS) at the UH 2.2-m telescope, as well as simultaneous optical and infrared spectra using the Lick Visible-to-Near-Infrared Imaging Spectrograph (VNIRIS). The spatial distribution and temporal evolution of the “violet band” CN (0–0) emission and of the 630 nm [OI] emission was studied. We found that CN emission centered on the nucleus increased in the 2 h after impact, but that this CN emission was delayed compared to the light curve of dust-scattered sunlight. The CN emission also expanded faster than the cloud of scattering dust. The emission of [OI] at 630 nm rose similarly to the scattered light, but then remained nearly constant for several hours after impact. On the day following the impact, both CN and [OI] emission concentrated on the comet nucleus had returned nearly to pre-impact levels. We have also searched for differences in the scattering properties of the dust ejected by the impact compared to the dust released under normal conditions. Compared to the pre-impact state of the comet, we find evidence that the color of the comet was slightly bluer during the post-impact rise in brightness. Long after the impact, in the following nights, the comet colors returned to their pre-impact values. This can be explained by postulating a change to a smaller particle size distribution in the ejecta cloud, in agreement with the findings from mid-infrared observations, or by postulating a large fraction of clean ice particles, or by a combination of these two.

Optical and X-ray Emission Spectroscopy of High-Power Laser-Induced Dielectric Breakdown in Molecular Gases and Their Mixtures

Large-scale plasma was created in molecular gases (CO, CO2, N2, H2O) and their mixtures by high-power laser-induced dielectric breakdown (LIDB). Compositions of the mixtures used are those suggested for the early earth's atmosphere of neutral and/or mildly reducing character. Time-integrated optical spectra emitted from the laser spark have been measured and analyzed. The spectra of the plasma generated in the CO-containing mixtures are dominated by emission of both C2 and CN radicals. A vibrational temperature of approximately 10(4) K was determined according to an intensity distribution in a vibronic structure of the CN (B2Sigma(+)u-X2Sigma(+)g) violet band. For comparison, the NH3-CH4-H2-H2O mixture has been irradiated as a model of the strongly reducing version of the early earth's atmosphere. In this mixture, excited CN seems to be significantly less abundant than C2. The LIDB experiments were in the molecular gases carried out not only in the static cell but also using a large, double stream pulse jet (gas puff target) placed in the vacuum interaction chamber. The obtained soft X-ray emission spectra indicate the presence of highly charged atomic ions in the hot core of high-power laser sparks.

Luminescence and epitaxy of ion-implanted α-quartz

Ion implantation is a promising route for doping quartz with luminescent impurity atoms and growing photoactive nanoparticles of them. The present work focuses on cathodoluminescence (CL) and epitaxy during or after Na, Rb, Cs, Ge, and Ba ion implantation as well as Rb/Ge double implantation in crystalline α-quartz, under the conditions of dynamic, chemical or laser epitaxy. In addition to the known intrinsic sub-bands in the CL spectra, which have been associated with specific defects in the silica matrix, very intense blue or violet bands were found. They are correlated with the implanted ion species and follow their thermal behaviour. The quantum efficiency and possible origins of the blue/violet CL emission for various ions and processing treatments will be discussed. The case of double Rb/Ge implantation appears to be very promising with respect to a combination of a high CL output and full chemical epitaxy.

Light-emitting defects and epitaxy in alkali-ion-implanted α quartz

Light-emitting centers in alkali-ion-implanted α quartz have been investigated with respect to the solid phase epitaxial growth of the ion irradiation induced amorphous zone. Cathodoluminescence was studied under the conditions of chemical epitaxy in annealing the samples, implanted with 2.5×1016 50keV Na ions/cm2 or 175keV Rb ions/cm2, in O218 atmosphere in the temperature range of 673–1173K. In addition to the known intrinsic subbands at 2.40, 2.79, and 4.30eV, which previously were associated with specific defects in the silica matrix, a strong violet band at 3.65eV and a band at 3.25eV have been identified. Both are intimately correlated with the presence of the implanted alkali atoms and recrystallization process. With respect to the 3.25eV band reported in the literature, they are discussed to be correlated with the presence of nanoclusters in Si-enriched, and Ge- and Sn-implanted SiO2 structures.

InGaN blue cathodoluminescence phosphors synthesized with long reaction time in a new reactor

AbstractInGaN microcrystals were grown by two‐stage growth method for 10 hours in a new reactor without clogging an exhaust tube by fine particles. Obtained InGaN showed double peaked cathodoluminescence spectra, violet peak at 385‐389 nm and blue at 438‐444 nm when excited at 5‐15 kV. The uniformity of the blue emission spectra among the crystals were improved for those grown for the 10‐hour reaction time. The yields of emissive GaN and InGaN crystals were not improved. The compatibility of the position of the violet band of the InGaN crystals to that of the GaN crystals and the resemblance of morphology of the GaN and InGaN crystals imply the growth of InGaN on GaN facets in two‐stage growth. The cause of the deterioration of the CL efficiency of the grown InGaN crystals at a low acceleration voltage was discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cathodoluminescence and epitaxy after group-IV and alkali-ion implantation in α-quartz

Ion implantation is a promising route for doping quartz with luminescent impurity atoms and nanoparticles at moderate fluences. The present work focuses on the comparison of epitaxy and cathodoluminescence (CL) after Ba, Ge, Na and Rb ion implantation as well as Rb/Ge double implantation in α-quartz, under the conditions of either dynamic or chemical epitaxy. In addition to the known intrinsic sub-bands in the CL spectra, due to the silica matrix or electron irradiation, we were able to line up several new blue or violet bands with the implanted ion species and follow their thermal behaviour. In the case of Na and Rb ions, complete chemical epitaxy has been achieved, but only rather weak CL output. For Ba ions, almost complete dynamic epitaxy and fairly high CL efficiency have been realized, but no epitaxy for Ge implantation. The case of dual Rb/Ge ion implantation appears as the most promising route to gain a high CL light output and full epitaxy.

Hydrothermal synthesis and optical properties of Pb 2+ doped ZnO nanorods

Single-crystalline Pb 2+ doped ZnO nanorods were synthesized by a hydrothermal method in the presence of cetyltrimethyl ammonium bromide (CTAB). The obtained Pb 2+ doped ZnO nanorods were in diameters of ∼150 nm and in lengths of ∼3 μm. The nanorods grow along the [011¯0] direction. The Pb 2+ doped ZnO nanorods show violet and blue emission bands centered at 415, 438 and 467 nm, respectively. The possible growth mechanism of the Pb 2+ doped ZnO nanorods was also discussed.