Near-ultraviolet Range Research Articles

High detectivity ZnSe-based Schottky barrier photodetectorsfor blue and near-ultraviolet spectral range

A characterisation of ZnSe-based Schottky barrier photodetectors grown by molecular beam epitaxy is presented. High quality diodes exhibiting a high responsivity of 0.10 A/W at 455 nm and a sharp cut-off of three to four orders of magnitude have been fabricated. A detectivity of 1.4 × 1010 mHz1/2W-1 has been obtained for a structure with a 5 mm2 area operating at –3.5 V bias. These results highlight the potential of ZnSe in blue and near-ultraviolet light detection.

Effect of ultraviolet irradiation on the charge state of ion-implanted silicon-silicon dioxide structures

The effect of irradiation with the light from a near-ultraviolet range (NUV irradiation) on the charge state of Si-SiO2 structures with an argon-implanted silicon dioxide layer was studied. The method of depth profiling based on the measurement of high-frequency capacitance-voltage characteristics of an electrolyte-dielectric-semiconductor system in combination with etching of the dielectric layer was used. It was established that the NUV irradiation resulted in the recharge of positively charged amphoteric centers (formed due to ion implantation in the silicon dioxide layer of the Si-SiO2 structures) into negatively charged ones.

Zn(Mg)BeSe-based p-i-n photodiodes operating in the blue-violet and near-ultraviolet spectral range

We present the growth and characterization of p-i-n photodiodes based on ZnBeSe and ZnMgBeSe compounds. High-quality diodes exhibiting dark current as low as 12 nA/cm2 at −2 V bias have been fabricated. The spectral response shows a high responsivity of 0.17 A/W at 450 nm, with a rejection of ∼104 at longer wavelengths. Our results thus demonstrate the potential of ZnSe-based heterostructures for efficient detection in the visible-ultraviolet region.

Diode structures based on p-GaN for optoelectronic applications in the near-ultraviolet range of the spectrum

It was previously reported that spectrally matched n-SiC and n-GaN-based MIS Schottky barrier diode structures exhibited optical emission and photosensitivity in the near-ultraviolet (UV) range of the spectrum and rectification at elevated temperatures. However, such structures were not practical due to the low mechanical and thermal stability of the semitransparent Au contacts. In addition, we experienced difficulties in achieving stable optical emission from the n-GaN-based structures. In this work various Schottky barrier diode structures based on p-type GaN layers grown on sapphire with silicon (Si), boron nitride (BN), and silicon dioxide (SiO2) interfacial layers were investigated. Blue and wide-spectrum optical emissions at forward and reverse bias, respectively, and photosensitivity were observed from these structures. A spectral match in the range of 365–400 nm between the light emitting diode (LED) and photodetector structures fabricated on the same substrate was achieved. A total Lambertian radiant UV power of ∼466 μW was measured from a blue/UV LED at 22 V. UV-transparent and electrically conductive SnOx layers were fabricated, characterized, and employed for fabrication of p-GaN-based photodiode structures.

Electro-optical studies of a soluble conjugated polymer with particularly low intrachain disorder

In contrast with many soluble conjugated polymers, the absorption and electroabsorption spectra of the methyl-substituted ladder-type poly(para-phenylene) show particularly well-resolved excitonic transitions, whose linewidth approaches the narrow values previously only associated with single crystals or isolated chains of polydiacetylenes. At least two distinct vibrational modes couple strongly to the electronic transitions. The electroabsorption spectrum increases quadratically with the electric-field strength, and closely follows the first derivative of absorption throughout most of the visible and near-ultraviolet range, indicative of a quadratic Stark shift of the ${1B}_{u}$ singlet exciton. In the region 3.2--3.6 eV, the agreement between the electroabsorption spectrum and the first derivative is offset by a broader spectral feature, which does not correspond to any feature in the linear absorption spectrum. Two-photon fluorescence excitation spectroscopy identifies this state as a forbidden exciton ${(mA}_{g}),$ located 0.7 eV above the lowest singlet exciton ${(1B}_{u}).$ A polarizability of 2060 ${\mathrm{\AA{}}}^{3}$ and a dipole moment of $7.17e\mathrm{\AA{}}(34\mathrm{D})$ is calculated for the ${1B}_{u}$ singlet exciton. The spectra show evidence of a small increase of the vibrational frequencies as the polarization of the \ensuremath{\pi} system increases.

Photochromic coloration of WO3 with visible light

Thin amorphous films of tungsten oxide (WO3) are well known to change their optical absorption behavior upon light exposure, usually referred to as photochromic effect. Since the formation of color centers is closely related to the optical creation of electron-hole pairs the sensitivity of the photochromic effect in WO3 is limited to energies above its band-gap energy of 3.25 eV, corresponding to the near-ultraviolet range. We will demonstrate that in case of a thin cadmium sulfide (CdS) layer underneath the tungsten oxide film the sensitivity of photochromism is shifted into the visible range. This result is interpreted in terms of charge carrier injection from the CdS into the WO3. Apart from a more detailed understanding of the photochromic effect this may have also relevance for technical applications.

A chromatic action spectrum for planktonic predation by juvenile yellow perch, Perca flavescens

The intensity of light required for juvenile yellow perch, Perca flavescens, to strike at planktonic prey was determined at 14 wavelengths, particular attention being paid to the near-ultraviolet region of the spectrum. Juvenile yellow perch are known to possess near-ultraviolet-sensitive photoreceptors, which are absent in adults. The action spectrum obtained shows three peaks: one between 640 and 700 nm, one between 490 and 525 nm, and one in the near-ultraviolet range, between 360 and 400 nm. The relative amplitude of the near-ultraviolet peak is greater than the amplitudes of the other two peaks. These results reveal that juvenile yellow perch can detect and recognize prey using only near-ultraviolet visual cues. This finding is discussed in terms of the visual ecology of juvenile and adult yellow perch.

Evidence of near-ultraviolet emission during electrical-tree initiation in polyethylene

The spectra of light emitted during electrical-tree initiation in low-density polyethylene used in high-voltage cables were investigated. Light in the visible and in the near-ultraviolet range was detected. Low-density polyethylene subjected to highly divergent fields at voltages below the light-inception level did not develop an electrical tree, and it is suggested that the light-inception voltage is the threshold for insulation degradation. The ultraviolet light emitted at voltages above the light-inception level could cause degradation of the polymer. A model is proposed to explain the mechanism of electroluminescence. Ultraviolet stabilizers added to the polymer prolonged the time to electrical-tree initiation by preventing photodegradation of the polymer.

Optical properties of the semiconductor 7, 7′, 8, 8′, tetracyanoquinodimethane (TCNQ)

The reflectivity and absorption spectra of 7, 7′, 8, 8′ tetracyanoquinodimethane thin films, in the visible and near-ultraviolet range, have been measured. Samples have been grown and characterized by means of elemental microanalysis and differential scanning calorimetry (DSC) in order to obtain the purity degree. The experimental optical spectra are interpreted on the basis of a calculation of the electronic molecular energy levels by the complete neglect of differential-overlap method in a new parametrization scheme.

The magneto-optical polar Kerr effect and birefringence spectra in PrFeO3between 2 and 5.6 eV

The complex polar Kerr effect and the effect of birefringence in PrFeO3 are reported between 2 and 5.6 eV. It is shown that the birefringence observed in transmission has its origin in the electronic transitions involving 3d levels of Fe3+ which are situated in the near-ultraviolet spectral range. The low-energy Kerr ellipticity and the associated Faraday rotation in PrFeO3 are of opposite sign compared to the other orthoferrites studied so far. This situation is analogous to that observed in Pr3+ and Nd3+ substituted iron garnets. It is concluded that the mechanism responsible for the anomalous Faraday rotation in Pr3+ and Nd3+ iron garnets does not require the presence of tetrahedral iron ions.

Genetic damage in Escherichia coli K12 AB2480 by broad-spectrum near-ultraviolet radiation.

Irradiation with either broad-spectrum near-ultraviolet (fluorescent BLB (black light blue)) or monochromatic wavelengths in the near-ultraviolet range (320 to 400 nanometers) can cause specific damage to DNA as shown in experiments with Escherichia coli K12 AB2480 at the stationary phase of growth.

Determination of cathode quantum efficiencies using statistical time lag measurements

The measurement of statistical time lags of cold-cathode gas discharge tubes and the application of these measurements to the determination of the quantum efficiencies of metal cathodes in the near-ultraviolet wavelength range are described. It is proposed that a thin layer of contamination on the cathode accounts for the departure at high voltage from the Wijsman theory describing the statistical time lag variation with applied voltage.

Fluorescence and optical characteristics of reduced flavines and flavoproteins.

Publisher Summary This chapter discusses the fluorescence and optical characteristics of reduced flavins and flavoproteins. Flavins and flavoproteins in their oxidized state (Flex) are typically yellow, with the absorption spectrum in the visible and near-ultraviolet (UV) range being characterized, by two well-resolved bands centered around 450 and 370 nm. This chromophore, free in solution, exhibits a strong fluorescence that is maximal around 520 nm; when the chromophore is bound to flavoproteins, its emission can be fully quenched. Studies show that the near-UV absorption spectrum of the 4a,5-dihydroflavin chromophore is similar, in its appearance, to that of its 1,5-dihydro isomer, but its shape is even more dependent on temperature and solvent. In particular, substituents that have an effect on the internal mobility of the molecule were found to cause major spectral changes on the chromophore free in solution and also when bound to various apoflavoproteins. At ambient temperature, 4a,5-dihydroflavins show a weak fluorescence only when an appropriate substituent decreases their internal mobility. However, at low temperature in rigid ethanol glasses or when bound to apoflavoproteins, 4a,5-dihydroflavins exhibit a rather strong fluorescence emission maximal in the range 480-530 nm.

The Influence of Lipid on the Conformation of Human Plasma High Density Apolipoproteins

Abstract The interaction of human plasma high density apolipoproteins (apoHDL) with lipids was examined by circular dichroism (CD) of the peptide and aromatic chromophores. By CD criteria native HDL contained about 70% α helical, 5 to 15% β, and 15 to 20% disordered structure. Delipidation decreased the helical content by about 20% with a corresponding increase in disordered structure. The two major apoproteins of HDL (apoA-I and apoA-II), isolated by chromatography in urea, refolded to different extents after removal of urea; apoA-I had more ordered structure than apoA-II (approximately 55% and 35% α helix, respectively). Reconstitution of apoA-I, apoA-II, and apoHDL with both phosphatidylcholine and cholesteryl ester restored, respectively, 119%, 87%, and 100% of the helical structure of the parent HDL. Phosphatidylcholine alone restored 50 to 70% of the increase produced by both phosphatidylcholine and cholesteryl oleate. With each substrate, the increase in helical content on lipid-protein recombination was accompanied by a corresponding decrease in disordered structure. Spectra of HDL in the near-ultraviolet range showed peaks at 258, 264, 283.5, and 290.5 nm. Delipidation reduced and markedly altered the ellipticity bands of the near-ultraviolet CD spectrum. The bands at 283.5 and 290.5 nm, tentatively assigned to one or more of the tryptophan chromophores in apoA-I, were reversed in sign and shifted to 286 and 292 nm, respectively. The near-ultraviolet CD spectrum of the parent HDL was partially restored by recombination of apoHDL with phosphatidylcholine alone and almost completely restored by recombination with both phosphatidylcholine and cholesteryl oleate. Recombination with phosphatidylcholine alone was sufficient to restore the 286 and 292 nm peaks to their original sign and position. These studies indicate that about 20 to 30% of the amino acid residues in the two major HDL apoproteins are involved in a helix-disordered transition on lipid removal or restoration. The near-ultraviolet CD spectrum is especially sensitive to these lipid-induced structural changes. Both phospholipid and cholesteryl esters are required for complete reorganization of the secondary and tertiary structure of HDL.

Circular dichroism of N-thiobenzoyl-L-α-amino-acids. Part III. Their circular dichroism through the near-ultraviolet wavelength range

Solutions in ether and in methanol of a series of N-thiobenzoyl-L-α-amino-acids exhibit circular dichroism associated with their π→π* ultraviolet absorption maximum (log Iµ∼ 3·85) near 290 mµ. In contrast to the circular dichroism behaviour of this series near 395 mµ(n→π* transition), that near 290 mµ is little influenced in sign, and in the magnitude and wavelength of the circular dichroism maximum, by changing the solvent from ether to methanol. Thus, ether solutions of the N-thiobenzoyl derivatives of all L-α-amino-acids in the present series show positive circular dichroism near 290 mµ; the same relationship holds for methanol solutions of most of these compounds and corresponding cyclohexylammonium salts. Values of (Iµl–Iµr) at the circular dichroism maximum near 290 mµ are particularly large for the N-thiobenzoyl derivatives of α-amino-acids which carry an aromatic group in their side-chain. The derivatives of the α-imino-acids, L-proline and hydroxy-L-proline, however, show negative circular dichroism near 290 mµ, probably reflecting a different spatial relationship of chromophore to asymmetric centre from that adopted by the derivatives of the α-amino-acids.The 243 mµ absorption (log Iµ∼ 4·05) maximum of the thiobenzamide chromophore in these compounds is also “optically active”; a circular dichroism maximum can be detected near this wavelength in solutions of the N-thiobenzoyl-L-α-imino-acids and of the derivatives of L-α-amino-acids which exhibit the larger values of (Iµl–Iµr) near 290 mµ, though the circular dichroism of most members of the series is too weak near 243 mµ for detection with the present Dichrographe.