Ultraviolet Excitation Source Research Articles

Al, Mg Co-doped ZnO thin films: Effect of the annealing temperature on the resistivity and ultraviolet photoconductivity

Interesting electrical and optical properties can be obtained from sol-gel doped Zinc Oxide (ZnO) materials by controlled introduction of dopants in the precursor solution. In this work we investigate the effects of annealing temperature on Al and Mg co-doped ZnO thin films. All samples were dip coated from the same solution, using identical techniques and pre-heating temperature, but were annealed in air at various temperatures ranging from 500°C to 650°C. A three-order magnitude reduction in electrical resistivity was achieved for an increase of 100°C annealing temperature. The structural properties and the energy band-gap remained relatively unchanged during this process. Photocurrent measurements carried out using an ultraviolet excitation source indicate a 5x increase in magnitude, along with increase in persistence behavior. The mechanism behind these effects has been linked to temperature-induced variation in Mg to Al ratio in the films, measured using energy dispersive spectroscopy. Our results indicate that strong in-plane variation of resistivity in ZnO thin films can be carried out using localized heating, opening the way to simpler fabrication processes.

Detection of hazardous chemical using dual-wavelength Raman spectroscopy in the ultraviolet region

This study proposes a stand-off Raman spectroscopy system using dual-wavelength in the ultraviolet (UV) region to detect hazardous chemicals. The Raman spectrum generated by the UV excitation source avoids solar background noise during daytime for chemical detection as the spectrum is in the solar blind range. Wavelengths of 213 and 266 nm by 5th and 4th harmonics are generated from Nd:YAG laser. However, Raman spectra of chemicals exhibit different signal-to-noise ratios for both the excitation wavelengths; therefore, to detect such chemicals, Raman spectra by two sources are required. Raman spectra were acquired using a dual-wavelength laser and spectrometer with a single grating and detector at the wavelengths of 213 and 266 nm simultaneously. The Raman spectra of sulfuric acid, 2-chloroethyl ethyl sulfide, and dimethyl methylphosphonate were acquired and analyzed, thus highlighting the application of dual-wavelength Raman spectroscopy. For efficient chemical detection in the field, we have ensured that the system developed in this study is robust.

Large-Area, Flexible, Transparent, and Long-Lived Polymer-Based Phosphorescence Films.

Polymer-based room-temperature phosphorescence (RTP) materials with high flexibility and large-area producibility are highly promising for applications in organic electronics. However, achieving such photophysical materials is challenging because of difficulties in populating and stabilizing susceptible triplet excited states at room temperature. Herein large-area, flexible, transparent, and long-lived RTP systems prepared by doping rationally selected organic chromophores in a poly(vinyl alcohol) (PVA) matrix were realized through a hydrogen-bonding and coassembly strategy. In particular, the 3,6-diphenyl-9H-carbazole (DPCz)-doped PVA film shows long-lived phosphorescence emission (up to 2044.86 ms) and a remarkable duration of afterglow (over 20 s) under ambient conditions. Meanwhile, the 7H-dibenzo[c,g]carbazole (DBCz)-doped PVA film exhibits high absolute luminance of 158.4 mcd m2 after the ultraviolet excitation source is removed. The RTP results not only from suppressing the nonradiative decay by abundant hydrogen-bonding interactions in the PVA matrix but also from minimizing the energy gap (ΔEST) between the singlet state and the triplet state through the coassembly effect. On account of the outstanding mechanical properties and the afterglow performance of these RTP materials, they were applied in the fabrication of flexible 3D objects with repeatable folding and curling properties. Importantly, the multichannel afterglow light-emitting diode arrays were established under ambient conditions. The present long-lived phosphorescent systems demonstrate a bright opportunity for the production of large-area, flexible, and transparent emitting materials.

Development of Illuminant Glow-in-the-Dark Cotton Fabric Coated by Luminescent Composite with Antimicrobial Activity and Ultraviolet Protection.

The main objective of technical protective clothing is to enhance people safety at work, which may save their life or keep them healthy away against some hazards. We developed a warning cotton fabric with a traffic safety warning photoluminescence character that continues emitting light for a long period of time after the removal of the illuminant source. Rare earth-doped strontium aluminate was dispersed in an aqueous medium of a polyacrylic-based binder to give a cross-linkable photoluminescent formula to be applied onto cotton substrate employing spray-coat approach. To introduce a transparent photoluminescent film, the Rare earth pigment must be fully dispersed to prevent aggregation. The long-persistent photoluminescent layer was deposited on cotton surface employing different concentrations of the rare earth pigment phosphor. The excitation wavelength maximum band of the spray-coated film on cotton fabric was found to occur at 365nm, while the emission was recorded at 515nm. Yellowish-green emissive color was monitored by CIE color data under the ultraviolet excitation source. The spray-coated fabric was characterized by wavelength dispersive X-ray fluorescence (WD-XRF), phosphorescence and excitation spectra, elements mapping, scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX). The comfort measurements were studied by exploring both of fabric stiffness and air-permeability. Furthermore, the spray-coated textile substrates displayed good fastness properties and a reversible luminescent glow in the dark.

Blue-Light-Absorbing Thin Films Showing Ultralong Room-Temperature Phosphorescence.

The development of organic materials displaying ultralong room-temperature phosphorescence (URTP) is a material design-rich research field with growing interest recently, as the luminescence characteristics have started to become interesting for applications. However, the development of systems performing under aerated conditions remains a formidable challenge. Furthermore, in the vast majority of molecular examples, the respective absorption bands of the compounds are in the near ultraviolet (UV) range, which makes UV excitation sources necessary. Herein, the synthesis and detailed analysis of new luminescent organic metal-free materials displaying, in addition to conventional fluorescence, phosphorescence with lifetimes up to 700 ms and tailored redshifted absorption bands, allowing for deep blue excitation, are reported. For the most promising targets, their application is demonstrated in the form of organic programmable tags that have been recently developed. These tags make use of reversible activation and deactivation of the URTP by toggling between the presence and absence of molecular oxygen. In this case, the activation can be achieved with visible light excitation, which greatly increases the use case scenarios by making UV sources obsolete.

Optoelectronic System for Mycotoxin Detection in Food Quality Control

Here, we report on the design, fabrication, and characterization of a prototype of a portable detection system, able to determine if the contamination level of Ochratoxin A (OTA) is below or above the limit imposed by the EU Commission for a specific food. The operating principle of the system is the real-time monitoring of a chromatographic run when an extract of the sample under analysis is spotted on a high-performance thin-layer chromatographic plate. The idea is to induce naturally the fluorescence of the OTA molecules by using an ultraviolet excitation source and to detect the re-emitted light with an array of amorphous silicon photosensors, whose photocurrents will be proportional to the amount of OTA molecules. The system performances have been verified on sample containing pure OTA molecule and OTA extracted from fortified real matrices of wine and beer. The results showed a minimum detectable quantity of OTA of 0.2 ng and repeatability better than 5%. The analysis of the sample is made simultaneously with a reference, which, in the proposed system, corresponds to the amount of OTA equal to the limit for the specific food commodities. Taking into account the extraction efficiency and the minimum acceptable presence of OTA in such food commodities, we infer that the minimum amount of real matrix to be investigated is as low as 5.5 mL.

Synthesis and spectroscopic properties of Yb3+ and Tb3+ co-doped GdBO3 materials showing down- and up-conversion luminescence.

Gadolinium orthoborates doped with Yb(3+) and Tb(3+) ions were synthesised by the sol-gel Pechini method. Materials annealed at 900 °C were composed of the monoclinic GdBO3 phase with micrometre-sized crystals. The structural properties of the products were analysed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The composition of the prepared materials was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). The materials showed intense ultraviolet (UV) or near infrared (NIR) excited green emission, which resulted from down- or up-conversion processes taking place in their structure. Spectroscopic properties were investigated on the basis of the measured excitation and emission spectra. Also, luminescence decays showing a short rise of emission with time after NIR excitation were measured. The dependence of the integral up-conversion intensity on the energy of the pumping laser was measured. The results indicated a two-photon process based on cooperative energy transfer (CET). The analysis of the synthesised series of samples allowed us to identify those with the best emission under a UV or NIR excitation source.

Synthesis of iron-doped TiO2 nanoparticles by ball-milling process: the influence of process parameters on the structural, optical, magnetic, and photocatalytic properties

Titanium dioxide (TiO2) absorbs only a small fraction of incoming sunlight in the visible region thus limiting its photocatalytic efficiency and concomitant photocatalytic ability. The large-scale application of TiO2 nanoparticles has been limited due to the need of using an ultraviolet excitation source to achieve high photocatalytic activity. The inclusion of foreign chemical elements in the TiO2 lattice can tune its band gap resulting in an absorption edge red-shifted to lower energies enhancing the photocatalytic performance in the visible region of the electromagnetic spectrum. In this research work, TiO2 nanoparticles were doped with iron powder in a planetary ball-milling system using stainless steel balls. The correlation between milling rotation speeds with structural and morphologic characteristics, optical and magnetic properties, and photocatalytic abilities of bare and Fe-doped TiO2 powders was studied and discussed.

Ultraviolet photosensitivity of sulfur-doped micro- and nano-crystalline diamond

The room-temperature photosensitivity of sulfur-doped micro-, submicro-, and nano-crystalline diamond films synthesized by hot-filament chemical vapor deposition was studied. The structure and composition of these diamond materials were characterized by Raman spectroscopy, scanning electron microscopy, and x-ray diffraction. The ultraviolet (UV) sensitivity and response time were studied for the three types of diamond materials using a steady-state broad UV excitation source and two pulsed UV laser radiations. It was found that they have high sensitivity in the UV region (as high as 109 s−1 mV−1 range), a linear response in a broad spectral range below 320 nm, photocurrents around ∼10−5 A, and a short response time better than 100 ns, which is independent of fluency intensity. A phenomenological model was applied to help understand the role of defects and dopant concentration on the materials’ photosensitivity.

Indirect interband transition in hexagonal GaN

In this paper, we report on optical investigations with Raman experiment to underline a new ultraviolet (UV) luminescence band in hexagonal gallium nitride (GaN) at 4.56 eV. GaN is a direct band gap semiconductor, the photoluminescence peak corresponding to the energy gap at 3.43 eV dominates the spectrum. Nevertheless, other electronic interband transitions can appear on the spectrum: the electronic indirect interband transitions. We attribute one of them to the observed new photoluminescence band at 4.56 eV. This interpretation is supported by photoluminescence spectra obtained on three different samples at room temperature and at −50 °C with UV excitation source: mbd-266 nm solid laser (4.66 eV) and by the study of three criteria: the partly opposite parities of initial and final wave function, the implication of acoustic phonons and temperature control.

Sp2 Carbon Phases in Nano Diamond.

Hot filament and microwave plasma CVD nanocrystalline diamond films are analysed by visible and ultra-violet excitation source Raman spectroscopy. The analysis of the shape and relative intensity of the different Raman features due to the sp2 phases, i.e. the sp2 1550 cm-1 G band attributed to C=C sp2 bond stretching mode, the D band (collective breathing mode of aromatic rings) and the shoulder observed at about 1150 cm-1 (assigned to transpolyacetylene segments) gives some information on the local structure of the sp2 phases. The surface of the samples is analysed by X-ray Photoelectron Spectroscopy (XPS). The evolution of the sp2 phases is analysed at the surface and in the bulk when the diamond films are submitted to thermal annealing under ultra-high vacuum or acidic surface oxidation. The decomposition of the C1s peak evidences graphitic phases attributed to the high density of grain boundaries.

Sp 3/sp 2 character of the carbon and hydrogen configuration in micro- and nanocrystalline diamond

Hot filament and microwave plasma CVD micro- nanocrystalline diamond films are analysed by visible and ultra-violet excitation source Raman spectroscopy. The sample grain size varies from 20 nm to 2 μm. The hydrogen concentration in samples is measured by SIMS and compared to the grain size, and to the ratio of sp 2 carbon bonds determined by Raman spectroscopy from the 1332 cm − 1 diamond peak and the sp 2 1550 cm − 1 G band. Hydrogen concentration appears to be proportional to the sp 2 bonds ratio. The 3000 cm − 1 CH x stretching mode band intensity observed on the Raman spectra is decreasing with the G band intensity. Thermal annealing modifies the sp 2 phase structure and concentration, as hydrogen outdiffuses.

Selective area heteroepitaxy of nano-AlGaN ultraviolet excitation sources for biofluorescence application

We report on the selective area heteroepitaxy and facet evolution of AlGaN nanostructures on GaN/sapphire substrate using various mask materials. We also report on the challenges associated with selection of an appropriate mask material for selective area heteroepitaxy of AlGaN with varying Al composition. The shape and the growth rate of the nanostructures are observed to be greatly affected by the mask material. The evolution of the AlGaN nanostructures and Al incorporation were studied exhaustively as a function of growth parameters including temperature, pressure, NH3 flow, total alkyl flow, and TMAl/(TMAl+TMGa) ratio. The growth rate of nanostructures was reduced drastically when higher Al percentage AlGaN nanostructures were grown. The growth rates were increased for higher Al percentage AlGaN using a surfactant, which resulted in a high-quality pyramidal structure. As indicated by high-resolution x-ray diffraction and cathodoluminescence spectroscopy, the composition of Al in the AlGaN nanostructure is significantly different from that of a thin film grown under the same growth conditions.

Atomic and electronic structures of amorphous ZrO 2 and HfO 2 films

Atomic and electronic structures of ZrO 2 and HfO 2 films deposited using sputtering technique were studied by X-ray diffraction (XRD), EXAFS spectroscopy, photoemission (with X-ray and ultraviolet excitation sources) and electron energy loss spectroscopy (EELS). XRD results indicated that the as-deposited films were amorphous and a monoclinic phase was detected after annealing them in ambient at 800 °C. Photoemission results indicated that the ZrO 2 valence band consists of two bands separated by an ionic gap of 6 eV. The lower band of 10 eV width is occupied mainly by O 2s states. EELS results indicated that the bandgap of ZrO 2 and HfO 2 films are 4.7 and 5.7 eV, respectively.

Blue light-emitting diode demonstrated as an ultraviolet excitation source for nanosecond phase-modulation fluorescence lifetime measurements

We have produced amplitude-modulated near-ultraviolet light, centered at 390 nm, using an inexpensive, commercially available blue light-emitting diode (LED). The LED was amplitude modulated with the +13 dBm ac output from a frequency generator while biased through a bias tee with 60 mA of dc current. The LED produced 45 to 54 μW of UV light over the modulation bandwidth of 0.01 to 200 MHz, when measured after optical filters to remove the residual blue output. Since the filter attenuated the UV output about 3 dB, more than 100 μW of UV light was initially produced. Modulated UV light was available to approximately 200 MHz, with a −3 dB point of 31 MHz, allowing the measurement of ns fluorescence lifetimes. The fluorescence lifetimes of standard fluorophores (9-cyanoanthracene and green fluorescent protein) were measured in the frequency domain using the phase-modulation technique, producing lifetimes that closely agree with those reported in the literature, confirming that the UV-emitting blue LED is practical for spectroscopic and sensor applications. When compared to a laser modulated with a Pockels cell, the LED was smaller, less expensive, required less power, generated less heat, and required less alignment. The ability to modulate the LED at high frequencies, along with the UV output, allows its use as an inexpensive UV light source in fluorescence lifetime optical sensors and even frequency-domain fluorometers.

Microcavity optical mode structure measurements via absorption and emission of polymer thin films

Langmuir-Blodgett deposition was used to control the positions of luminescent thin films of the polymer substituted poly( p -phenylene) (SPPP) within metal mirror microcavities. In this way, the position of the absorbing/emitting layers were precisely defined, permitting the optical mode structure of the cavities to be probed by both the emission and absorption of the chromophore. It was demonstrated that the spontaneous emission from a thin film of chromophore varied as a function of the optical electric field intensity within the microcavity and was therefore dependant on its position within the cavity. The absorption of the chromophore film was investigated by coupling the ultraviolet excitation source to the cavity mode and was also found to vary as a function of the optical electric field intensity of the cavity mode.

The photodissociation of D 2O by vacuum ultraviolet excitation sources

The photodissociation of D 2O was carried out at excitation wavelengths of 1216, 1236, and 1302 Å. The distribution of the excess energy to rotational and vibrational degrees of freedom of the product OD was determined by an investigation of the OD 2Σ + emission. The population of the highest allowed rotational levels was observed with results similar to those obtained for H 2O. It has been found that the two spin components of the higher rotational levels were unequally populated. An attempt is made to account for this anomalous population distribution.