The emission characteristics of low-pressure water vapor discharge UV emitters

We have studied the characteristics of UV emission sources operating on low-pressure normal (H₂O) and heavy (D₂O) water vapor excited by periodic-pulsed and glow discharges. The emission in a 300–330 nm wavelength interval has been studied in detail for water vapor pressures ranging from 50 to 2500 Pa. A comparison of the characteristics of emission from discharge plasma at low (50–150 Pa) and elevated (2.0–2.5 kPa) water vapor pressures reveals significant differences in the character of emission spectra, which can be related to the different types of emitting species (hydroxy radicals versus small clusters of such radicals and water molecules). Discharge current and emission intensity pulses in the periodic-pulsed discharge regime have been measured.     

Judd–Ofelt theory: optical absorption and NIR emission spectral studies of Nd3+:CdO–Bi2O3–B2O3 glasses for laser applications

This article reports on the optical absorption and NIR emission properties of Nd³⁺ ions doped CdO–Bi₂O₃–B₂O₃ (CdBiB) glasses. Judd–Ofelt theory has been applied to calculate the oscillatory strengths of the transitions in the absorption spectra and also the intensity parameters Ω_λ (λ = 2, 4, and 6). These results were then used to compute the radiative properties of emission transitions of Nd³⁺:CdBiB glasses. From the NIR emission spectra of Nd³⁺:CdBiB glasses, three NIR emission bands are observed at 900, 1069, and 1338 nm and assigned to the transitions (⁴F_3/2 → ⁴I_9/2), (⁴F_3/2 → ⁴I_11/2), and (⁴F_3/2 → ⁴I_13/2), respectively. The concentration quenching phenomenon has also been explained. The stimulated emission cross-section and FWHM values were calculated for all the Nd³⁺:CdBiB glasses.     

Poly(p-xylene tetrahydrothiophenium chloride) doped photoluminescent sol-gel composite

Poly(p-xylene tetrahydrothiophenium chloride), a polyphenylene vinylene (PPV) precursor in water and sol-gel solution has been used to prepare photoluminescent glass and coatings. Lactic acid-tetramethoxysilane and lactic acid-aminopropyltriethoxysilane sol solutions were doped with 0.02–0.47 wt% PPV precursor solution (2.5 g/dm³). The visible light emission from vacuum dried doped sol coating, at 365 nm excitation is about four times brighter than pure uncured PPV precursor film. A 23 mg of lactic acid-aminopropyltriethoxysilane sol containing 31 μ g PPV precursor, coated on glass slide gave an emission of 286 Cd/m². The brightness increases with dopant concentration and is stable on heating to 130^∘C. The brightness decreases to 27 Cd/m² at 190^∘C due to coversion of poly(p-xylene tetrahydrothiophenium chloride) to PPV. The bright emission from doped composite is due to wave guiding of sol, transparency of the polymer in the matrix, and the morphology of the composite. The nonlinear optical properties of this material may be useful in display technology and optoelectronics and when coated on glass convert UV radiation to visible light.     

Effect of very high temperature short exposures on the dissolution of the γ′ phase in single crystal MC2 superalloy

Time-temperature dependence of the γ′ phase volume fraction was investigated for a second generation single crystal nickel-based superalloy exposed to very short high temperature regimes (1,100–1,200 °C). In this temperature range, the dissolution of the strengthening γ′ phase occurs. Evolution of the γ′ volume fraction in transient regimes has been established for each temperature and activation energy of the dissolution phenomenon were determined. They directly attest from the activity of the diffusing species involved during this phenomenon. From these analyses, the volume fraction at equilibrium was established for the entire temperature range where dissolution occurs. A model, based on a time/temperature equivalence, is proposed to quantify the γ′ volume fraction dissolved during short high temperature exposure.     

Spontaneous long-wavelength interlevel emission in quantum-dot laser structures

Spontaneous emission has been observed for the first time as a result of interband transitions of holes and electrons between size-quantization levels in vertically coupled quantum dots and also as a result of transitions from quantum-well states to a quantum-dot level. The spectral range of the emission was in the far-infrared (λ≅10–20 μm). The long-wavelength emission was only recorded simultaneously with short-wavelength interband emission (λ≅0.94 μm) in InGaAs/AlGaAs quantum-dot laser structures at above-threshold currents. Pis’ma Zh. Tekh. Fiz. 24, 20–26 (August 12, 1998)     

Determination of the kinetic curves of drying of blocks of adsorbents-dessicants with different cross sections of channels and of a granulated adsorbent on a change in the flow velocity

The kinetics of convective drying (regeneration) of different types of adsorbents-dessicants with different geometric parameters has been studied. It is shown that adsorbents of a “channel” (cellular) type have a long constant-rate stage of drying, whereas a granulated polydisperse adsorbent is mainly dried at a variable rate. Adsorbents having a maximum water yielding capacity have been revealed. It has been established that the velocity of a drying gas flow substantially influences the process of moisture extraction from all types of adsorbents. In order to describe the kinetic curves of the given phenomenon, a model of a relaxation kinetic equation that quite satisfactorily describes the kinetics of drying of the adsorbents studied was used. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 82, No. 1, pp. 65–68, January–February, 2009.     

Iron and gold related defects in water quenched silicon

Thermally induced defects in heat-treated and then quenched in water n-silicon samples have been studied using deep level transient spectroscopy. Two deep levels at energies E _c – 0.48 eV and E _c – 0.25 eV are observed in high concentration. The emission rate signatures and annealing characteristics showed the DLTS signal due to level at energy position E _c – 0.48 eV is not only due to Au(A) but some other level also contributes to this signal. The energy state at E _c – 0.25 eV is identified to be pinned with E _c – 0.48 eV. The annealing characteristics also revealed the contribution of Au–Fe complex in DLTS signal of E _c – 0.25 eV level. A complementary behavior of these two levels in annealing characteristics has also been observed.     

Formation of the continuous visible emission spectrum of pulsed discharge in cesium vapor

The formation of a continuous emission spectrum of pulsed discharge in high-pressure cesium vapor (P ≤ 1 bar) has been experimentally studied under conditions of increased power deposited in discharge. Experimental data well agree with the results of theoretical calculations. Physical mechanisms underlying the observed phenomenon are discussed. It is shown that, as the pumping current pulse amplitude is increased from 15 to 80 A, a discrete line spectrum transforms into the continuous spectrum. This is accompanied by an increase in the light yield (from 15 to 85 lm/W) and color-rendering index (from 86 to 98).     

Controlling the wavelength of InGaAs/GaAs/InGaP lasers by ion implantation

The possibility of controlling the wavelength of emission from an InGaAs/GaAs/InGaP laser heterostructure with strained quantum wells (QWs) using medium-energy proton implantation followed by thermal annealing has been studied. It is established that the optimum proton energy is related to the arrangement of QWs in the structure (e.g., 150 keV for QWs at a depth of ≈1.3 μm). Proton irradiation to a total dose of 6 × 10¹⁴ cm⁻² followed by annealing at 700°C allows the wavelength of emission from the modified region to be decreased by 8–10 nm at minimum losses in the output intensity. The observed effect can be used to obtain two-band emission from the same chip and has good prospects for use in the development of new optoelectronic schemes.     

Enhanced emission during submillisecond low-energy electron beam generation in a diode with grid-stabilized plasma cathode and open anode plasma boundary

We have experimentally studied the phenomenon of emission enhancement in a gas-filled diode with grid-stabilized plasma cathode and open (mobile) anode plasma boundary at an accelerating voltage of up to 20 kV. As the working gas pressure is increased to p ≥ 10⁻² Pa and the longitudinal magnetic field is increased to B _z ≥ 20 mT, the current in the accelerating gap exhibits significant growth, sometimes by a factor of two or more. Experimental data show that the most probable mechanism responsible for this effect is ion-induced secondary electron emission from the emitting electrode surface bombarded by ions from plasma generated by the electron beam in the drift space. These ions are accelerated in the space charge layer between the emitting electrode surface and the mobile boundary of the beam-generated (anode) plasma.     

Vacuum-UV emitter using low-pressure discharge in helium-water vapor mixture

The spectral characteristics of emission from plasma of periodic-pulsed capacitive discharge in a mixture of water vapor with helium have been studied in a spectral range of 140–315 nm. Dependences of the intensity of the characteristic emission lines of hydroxy groups in He-H₂O plasma on the partial pressure of water vapor at a helium pressure of p(He) = 2.6 kPa are presented. The results provide a basis for the creation of simple sources of vacuum-UV radiation with a cheap working medium based on water vapor.     

Optical emission from metal targets bombarded by 5–20 keV argon ions

The spectral composition of optical emission from high-purity iron (99.99% Fe), zirconium (99.98% Zr), and tungsten (99.96% W) targets bombarded by 5–20 keV Ar⁺ ions has been studied. The ion-bombardment-induced emission spectra of all metals exhibit two broad bands in the visible and IR spectral range. The first band is assigned to the emission from thermal spikes representing nanosized regions heated to 5000–6000 K, which are formed at the target surface in the course of evolution of high-density atomic collision cascades. The presence of an IR emission band is explained by the integral heating of targets to 500–800 K in the course of ion irradiation. This interpretation is confirmed by agreement between the experimentally measured and calculated temperatures in the region of thermalized collision cascades and the relative intensities of emission bands.     

Temperature dependent growth and optical properties of SnO<Subscript>2</Subscript> nanowires and nanobelts

SnO₂ nanowires and nanobelts have been grown by the thermal evaporation of Sn powders. The growth of nanowires and nanobelts has been investigated at different temperatures (750–1000°C). The field emission scanning electron microscopic and transmission electron microscopic studies revealed the growth of nanowires and nano-belts at different growth temperatures. The growth mechanisms of the formation of the nanostructures have also been discussed. X-ray diffraction patterns showed that the nanowires and nanobelts are highly crystalline with tetragonal rutile phase. UV-visible absorption spectrum showed the bulk bandgap value (∼ 3–6 eV) of SnO₂. Photoluminescence spectra demonstrated a Stokes-shifted emission in the wavelength range 558–588 nm. The Raman and Fourier transform infrared spectra revealed the formation of stoichiometric SnO₂ at different growth temperatures.     

Effect of chemical degradation on superconducting properties of the HTSC ceramics

The effect of humid atmosphere (59 and 86%) on phase composition and properties of YBa₂Cu₃O_7−y powder and ceramics prepared by hot pressing has been studied. It has been found that with increase of water content in the powder to 1·6 mass.% the superconducting properties of the corresponding ceramics gradually improve; on further increase of the water content the ceramics degrade resulting in a complete loss of superconductivity. The observed phenomenon can be explained by a plastifying effect of water on the powder by hot pressing on the one hand and by a solid-phase reaction of the products of decomposition (1-2-3) at a high temperature annealing, on the other.     

Depolarization of ferroelectrics by nanosecond electrical pulses

It is shown that the ordinary pulsed action on a ferroelectric cold cathode, which induces electron emission, results in depolarization of the ferroelectric. This phenomenon must be taken into account in the development and use of cold cathodes. Pis’ma Zh. Tekh. Fiz. 23, 71–74 (January 26, 1997)     

Enhancing electron field emission of carbon nanoflakes by hydrogen post-annealing process

In this study, the carbon nanoflakes (CNFs) fabricated by sputtering were chosen as the field emission emitters because of their very sharp and thin edges which are potentially good electron field emission sites. The as-deposited CNFs were annealed in the furnace under hydrogen atmosphere. The results showed that the optimum field emission properties with smaller turn-on field and larger current density were obtained at annealing temperature of 600 °C for 10 min. The hydrogen thermal annealing has chemical etching on the surface of the CNFs and produces appropriate emission site density to increase the emission current density. The turn-on field was reduced from 6.7 to 5.8 V/μm and electric current density was increased from 22 to 187 μA/cm² under 8 V/μm after hydrogen thermal annealing.     

On the synthesis,characterization and photocatalytic applications of nanostructured TiO<Subscript>2</Subscript>

Nanocrystalline semiconducting materials are attracting much attention due to their potential applications in solar energy conversion, nonlinear optics, and heterogeneous photocatalysis. In the present investigation, we have synthesized nanostructured TiO₂ photocatalysts, which have been used in the photocatalytic degradation of phenol (one of the most common water pollutants). These catalysts have been prepared through sol-gel technique using titanium tetra-isopropoxide as a raw material for synthesis. Characterization techniques such as XRD, SEM and TEM have been employed for structural/microstructural investigations. XRD results show that the as synthesized TiO₂ nanopowder exhibit anatase phase, TiO₂. The average sizes of the TiO₂ nanopowders are ∼ 5–10 nm. The optical properties of the samples were investigated through UV-visible and fluorescence techniques. It has been observed that absorption edge corresponds to ∼ 410 nm (bandgap, ∼ 3.02 eV). The emission peak in the fluorescence spectrum at ∼ 418 nm corresponds to the bandgap energy of ∼ 2.97 eV. Concentration of phenol (initial concentration, ∼ 100 ppm) with illumination time was monitored by measuring the absorbance of pure and illuminated phenol through UV-visible spectrophotometer. Salient feature of this study relates to the fact that the present sol-gel synthesized TiO₂ nanopowders have been found to be better photocatalysts for phenol degradation than the presently employed commercial TiO₂ (P-25, Degussa) photocatalyst. Thus, whereas phenol concentration, with the presently synthesized TiO₂ nanopowders, the concentration of phenol decreases up to ∼ 32% but for commercial TiO₂ nanopowder (P-25, Degussa), it decreased only up to ∼ 25%. The improved surface area is considered as an important factor for the aforesaid decrease in phenol concentration.     

Emission characteristics improvement in structures with InAs/GaAsN/InGaAsN superlattices

The influence of some parameters of nitrogen-containing heterostructures InAs/GaAsN/InGaAsN with strain-compensated superlattices (SCSL) on their emission characteristics has been studied. It is established that the net strain in the structure affects the photoluminescence (PL) linewidth, internal quantum efficiency, intensity, and wavelength. The maximum PL intensity and minimum full width at half maximum (FWHM) of the PL line were achieved with small strains (0–0.2%), whereas the maximum wavelengths (∼1.76 μm) observed for large strain (about +1%). By adding multilayer InAs inserts in the active InGaAsN quantum well in combination with using strain-compensated GaAsN/InGaAsN superlattices, it is possible to control the room-temperature emission wavelength in the range of 1.45–1.76 μm without significantly deteriorating the emissiion characteristics.     

Enhanced and stable electron emission of carbon nanotube emitter arrays by post-growth hydrofluoric acid treatment

Carbon nanotubes (CNT) have been highlighted as possible candidates for field-emission emitters and vacuum nanoelectronic devices. In this article, we studied the effect of acid treatment of CNTs on field emission from carbon nanotube field emitter arrays (FEAs), grown using the resist-assisted patterning process (RAP). The emission current densities of as grown CNT-FEAs and those which were later immersed in hydrofluoric acid (HF) for 20 s, were 19 μA/cm² and 7.0 mA/cm², respectively, when measured at an anode field of 9.2 V/μm. Hence, the emission current densities after HF treatment are 300 times larger than those of as grown CNT-FEAs. Also, it was observed that a very stable electron emission current was obtained after stressing the CNTs with an electric field of 9.2 V/μm for 800 min in dc-mode, where the emission current non-uniformity was 0.13%. The enhancement in electron emission after HF treatment appears to be due to the effect of fluorine bonding. Also, the electron emission characteristics and structural improvement of CNT-FEAs after HF treatment are discussed.     

THz Wave Emission from Intrinsic Josephson Junctions Controlled by Surface Impedance

Recently terahertz wave emission from intrinsic Josephson junctions without external magnetic fields has been intensively studied, and some emission states have been proposed numerically or theoretically. For the surface impedance Z=1, the McCumber-like state with small spatial dependence of the electric field in the junction becomes stable, while for large and complex Z, the π-phase kink state characterized by translational symmetry breaking along the layered structure becomes stable. In the present study, the relations between these two extreme cases are clarified numerically by solving the coupled equations of the Josephson relations and the Maxwell equations for an experimental width of the junction, 86 μm. The dynamical phase diagram in the surface impedance (Z)–current (J) plane and the optimal value of Z for the strongest emission are evaluated.