Selective Oxidation of Graphite on Diamond Films Induced by UV Exposure

A selective oxidation technique for diamond films based on the extended exposure to UV light in air at room temperature was developed and studied by Raman Spectroscopy and Scanning Electron Microscopy. The diamond films were synthesized by the combustion flame technique in open atmosphere by using an oxy-acetylene gas mixture as the carbon source. A 125 W UV lamp was used to irradiate the films in the wavelength range of 180–250 nm, from 2 to 10 days. The Raman spectrum from the as-deposited diamond films shows the typical band at 1333 cm^–1, characteristic of the sp ³diamond structure, with a certain nondiamond or graphite content. After UV irradiation, graphite was selectively oxidized and partially removed without oxidation of diamond, indicating that the strong reaction of ozone (O₃) and atomic oxygen (O·) produced by the UV irradiation oxidized the graphite, even at room temperature, without the need of an additional heating source. The oxidation of graphite was best observed after 2 days of UV treatment. A sensible improvement in the diamond film quality was obtained after 2 days of irradiation, as revealed by the sharpening of the corresponding Raman band.     

Studies on the TL and decay of BaS:Cu phosphors

Thermoluminescence (TL) and phosphorescence of BaS:Cu phosphors have been studied in this investigation of exciting at 28°C by low energy electrons, X-rays and UV light. While by UV light only one TL glow peak around 80°C can be observed, by X-ray and electron irradiation additional glow peaks at 187°C, 287°C and 427°C have been produced. The TL output diminishes with the time of storage in darkness. Considering the color of fluorescence, TL emission and the nature of decay (afterglow and TL), the traps involved in this system may be associated with Cu centers located either near S^-- ion vacancy or with halogen ions in their neighborhood.     

Thermoluminescence properties of Ce-doped LiSr4(BO3)3 phosphor

Borates LiSr₄(BO₃)₃ were synthesized by high-temperature solid-state reaction. The thermoluminescence (TL) and some of the dosimetric characteristics of Ce³⁺-activated LiSr₄(BO₃)₃ were reported. The TL glow curve is composed of only one peak located at about 209 °C between room temperature and 500 °C. The optimum Ce³⁺ concentration is 1 mol% to obtain the highest TL intensity. The TL kinetic parameters of LiSr₄(BO₃)₃:0.01Ce³⁺ were studied by the peak shape method. The TL dose response is linear in the protection dose ranging from 1 mGy to 1 Gy. The three-dimensional thermoluminescence emission spectra were also studied, peaking at 441 and 474 nm due to the characteristic transition of Ce³⁺.     

Preparation and characterization of an Hf-doped zinc silicate long-lasting phosphorescent material

Hf⁴⁺-doped Zn₂SiO₄ phosphor emitting long-lasting cyan light was prepared by the conventional high temperature solid-state technique. The emission spectrum of the Hf⁴⁺-doped phosphor exhibits one broad band peaking at 471 nm in the visible region excited by 254 nm. The cyan-light afterglow can last about 40 min in darkness after being irradiated with 254 nm UV lamp for 10 min (250 mW/cm²). The afterglow decay curve can be fitted into a second-order exponential curve. The thermoluminescence (TL) curve shows two glow bands centered at about 386 K (0.61 eV) and 440 K (1.56 eV), respectively, with the lower trap energy level being responsible for the long-lasting afterglow emission. A possible mechanism of the long-lasting phosphorescence based on the experimental results is proposed.     

Thermally stimulated luminescence studies of BaB4O7 compound

In this paper thermally stimulated luminescence (TSL) studies of BaB₄O₇ compound are reported for the first time. The polycrystalline sample of BaB₄O₇ was prepared by a melting method and the formation of the BaB₄O₇ compound was confirmed by an X-ray diffraction study. The compound has orthorhombic structure at room temperature. The TSL glow curves of BaB₄O₇ compound when heated at a constant heating rate of 4°C/s exhibit two thermoluminescence (TL) glow peaks at 110 and 150°C followed by a shoulder around 210°C. The trapping parameters associated with the prominent glow peak of BaB₄O₇ are reported using isothermal luminescence decay method (ILDM). As a check the trap parameters are also calculated by glow curve shape (Chen's) method after isolating the prominent glow peak by thermal cleaning technique. Our results show a very good agreement between the trapping parameters calculated by the two methods.     

Thermoluminescent behaviour of diamond films subjected to UV irradiation

The thermoluminescent behaviour of diamond films subjected to UV irradiation was studied by using an UV lamp of 254 nm wavelength. The UV irradiation was achieved by placing the samples 15 cm away from an UV source for different periods. The thermoluminescent signal was integrated from 0 to 350°C at a linear heating rate of 10°C/s in a N₂ atmosphere. The corresponding luminescence spectra show an excitation band centered at 450 nm while the emission band is centered around 500 nm at room temperature. The diamond films were synthesized on molybdenum substrates by the combustion flame technique and characterized by Raman spectroscopy and scanning electron microscopy. Received: 31 May 1999 / Reviewed and accepted: 22 July 1999     

Thermoluminescence in CVD diamond films: application to actinometric dosimetry

Diamond is considered a tissue-equivalent material since its atomic number (Z =6) is close to the effective atomic number of biological tissue (Z =7.42). Such a situation makes it suitable for radiation detection purposes in medical applications. In the present work the analysis is reported of the thermoluminescence (TL) and dosimetric features of chemically vapour deposited (CVD) diamond film samples subjected to ultraviolet (UV) irradiation in the actinometric region. The TL glow curve shows peaks at 120, 220), 320 and 370 degrees C. The 120 and 370 degrees C peaks are too weak and the first one fades away in a few seconds after exposure. The overall room temperature fading shows a 50% TL decay 30 min after exposure. The 320 degrees C glow peak is considered to be the most adequate for dosimetric applications due to its low fading and linear TL behaviour as a function of UV dose in the 180-260 nm range. The TL excitation spectrum presents a broad band with at least two overlapped components around 205 and 220 nm. The results indicate that the TL behaviour of CVD diamond film can be a good alternative to the currently available dosemeter and detector in the actinometric region as well as in clinical and medical applications.     

Performance of CVD diamond as an optically and thermally stimulated luminescence dosemeter

Diamond is a material with extreme physical properties. Its radiation hardness, chemical inertness and tissue equivalence qualify it as an ideal material for radiation dosimetry. In the present work, the optically stimulated luminescence (OSL) and thermoluminescence (TL) characteristics of a 10 microm thick CVD diamond (polycrystalline diamond films prepared by chemical vapor deposition) film were studied in order to test its performance as a beta radiation dosemeter. The TL response is composed of four main TL glow peaks; two of these are in the range of 150-200 degrees C and two additional peaks in the 250-400 degrees C temperature range. The integrated TL as a function of radiation dose is linear up to 100 Gy and increases with increasing dose exposure. The dose dependence of the integrated OSL exhibits a similar behavior. The observed OSL/TL behavior for the CVD diamond film clearly demonstrate its capability for applications in radiation dosimetry with special relevance in medical dosimetry owing to the diamond's intrinsic material properties.     

Temperature effect on structure and surface morphology of indium tin oxide films deposited by reactive ion-beam sputtering

Indium tin oxide (ITO) films were deposited by reactive ion-beam sputtering. The relationship among the surface morphology, the resistivity ρ of the films, the substrate temperature T_S and the film thickness t_F was investigated. The heat power from the ion source during the sputtering was 265 W. T_S increased from 30 to 145 °C with an increase of t_F. The films thinner than 187 nm at T_S lower than 120 °C were amorphous, the film surface was as smooth as the substrate. The films deposited at T_S in the range between 135 and 145 °C were polycrystalline. So, the films thicker than 375 nm were in a multilayer structure of a polycrystalline layer on an amorphous layer. The surface of the polycrystalline films became rough. ρ of the films suddenly decreased at t_F of 375 nm, where the structure of the films changed. Next, the amorphous films with t_F of 39 nm were annealed in the atmosphere. The film structure changed to a polycrystalline structure at annealing temperature T_A of 350 °C. However, the surface roughness of all the films was almost same. As a result, the substrate temperature during the sputtering was important for the deposition of the films with a very smooth surface.     

On the stability of TL traps of alumina

Thermoluminescence (TL) of heat-treated (in the temperature range 450 to 1400° C) -Al₂O₃ powders have been studied by X-irradiation at room temperature (27° C). The TL patterns have indicated essentially two groups of traps; one in the range 50 to 250° C and the other in the range 200 to 450° C. Apart from other thermal stability characteristics, preferential bleaching by light in the wavelength region of 540 to 630 nm has been observed only in the case of the first group of TL traps. It is concluded that the TL phenomena in -Al₂O₃ are in general controlled by trace impurities rather than intrinsic lattice defects and the TL traps of the system may be associated with a distribution of trapped holes (O^–, O°) stabilized by trace impurities.     

Deposition and characterisation of MoSi2 films

Deposition of MoSi₂ films on silicon and tantalum substrates applying pulsed laser deposition technique has been performed. Crystalline, hexagonal symmetry, MoSi₂ films were prepared directly from stoichiometric MoSi₂ tetragonal target on room temperature and heated substrates (500 °C). Textured MoSi₂ films having privileged (110) and (115) orientations and average crystallite size of about 105 nm were grown on Si(111) substrates with a good degree of axial texture (rocking curve full width half maximum of 1.5°). MoSi₂ films grown on Ta(211) substrates, instead, turned out to be polycrystalline, with an average crystallite size of about 100 nm and 50 nm on substrates kept at room temperature and at 500 °C, respectively. Vickers hardness for 1.2 μm thick MoSi₂ films on Si(111) substrates resulted to be 15 GPa both at room temperature and 500 °C, while for 0.4 μm thick MoSi₂ films on Ta(211) substrates — 26 GPa at room temperature and 30 GPa at 500 °C.     

Growth of crystalline HgCr2S4 thin films at mild reaction conditions

Thin films of crystalline HgCr₂S₄ have been deposited on glass substrates at low temperature as low as 65 °C using a chemical bath deposition method. Typical thickness of the deposited HgCr₂S₄ thin films was 264 nm.The films were composed of closely packed irregular grains of 165-175 nm in diameter. The X-ray diffraction analysis and the selected area electron diffraction analysis revealed the deposited thin films were polycrystalline with highly (2 2 0) preferential orientation. The films exhibit a pure faint black. Their direct band gap energy was 2.39 eV with room temperature electrical resistivity of the order of 10⁻³ Ω cm.     

Effects of RE3+ as a co-dopant in blue-emitting long-lasting phosphors, Sr3Al10SiO20:Eu2+

A series of novel long-lasting phosphors, Sr₃Al₁₀SiO₂₀:Eu²⁺,RE³⁺, were prepared and studied. Under UV irradiation, broad-band emission long-lasting phosphorescence located at 466 nm was observed in all of these phosphors at room temperature. The effects of RE³⁺ as a co-dopant in Sr₃Al₁₀SiO₂₀:Eu²⁺ were discussed in conjunction with the afterglow decay curves and thermoluminescence (TL) spectra. Quantitative TL spectra revealed that the introduction of RE³⁺ ions into Sr₃Al₁₀SiO₂₀:Eu²⁺ host produces a highly dense trapping level at appropriate depth (335 K), which is considered to be responsible for the long-lasting phosphorescence at room temperature.     

Growth of nanocrystalline diamond films in CCl4/H2 ambient

We investigated the growth characteristics of the nanocrystalline diamond films using CCl₄/H₂ as gas sources in a hot-filament chemical vapor deposition (CVD) reactor. Successful growth of nanocrystalline diamond at typical growth condition of 1.5-2.5% CCl₄ and 550-730 °C substrate temperature has been demonstrated. Glancing angle X-ray diffraction (XRD) clearly indicated the formation of diamond in the films. Typical root-mean-square surface roughness of 10-15 nm and an optimal root-mean-square surface roughness of 6 nm have been achieved. Transmission electron microscopy (TEM) analyses indicated that nanocrystalline diamond film with an average grain size in the range of 10-20 nm was deposited from 2.5% CCl₄/H₂ at 610 °C. Effects of different source gas composition and substrate temperature on the grain nucleation and grain growth processes, whereby the grain size of the nanocrystalline film could be controlled, were discussed.     

Investigation of thermoluminescence response and kinetic parameters of CaMgB2O5: Tb3+ phosphor against UV-C radiation for dosimetric application

The thermoluminescence (TL) response and kinetic parameters of CaMgB₂O₅:Tb³⁺ phosphor against UV-C radiations (λ?=?254 nm) had been investigated. The powder X-ray diffraction results confirm the formation of the monoclinic phase. TL results depict that the glow curve exhibited a broad peak centered at 430 K. The position and the shape of the curve were not influenced by the increase in dose, which is one of the requirements for dosimetric application. TL response curve was studied and showed a linear behavior against the studied dose (10–180 min). The effect of different heating rates on the TL intensity and the position of the glow peak were discussed in detail. In addition to this, the detailed examination of the glow peaks using variable heating rate and glow curve deconvolution methods was done to reveal the trapping parameters and to check the suitability of the present nanophosphor for UV-C dosimetry application.

     

Structural and optical studies of nanocrystalline V2O5 thin films

We report the structural and optical properties of nanocrystalline thin films of vanadium oxide prepared via evaporation technique on amorphous glass substrates. The crystallinity of the films was studied using X-ray diffraction and surface morphology of the films was studied using scanning electron microscopy and atomic force microscopy. Deposition temperature was found to have a great impact on the optical and structural properties of these films. The films deposited at room temperature show homogeneous, uniform and smooth texture but were amorphous in nature. These films remain amorphous even after postannealing at 300 °C. On the other hand the films deposited at substrate temperature T_S > 200 °C were well textured and c-axis oriented with good crystalline properties. Moreover colour of the films changes from pale yellow to light brown to black corresponding to deposition at room temperature, 300 °C and 500 °C respectively. The investigation revealed that nanocrystalline V₂O₅ films with preferred 001 orientation and with crystalline size of 17.67 nm can be grown with a layered structure onto amorphous glass substrates at temperature as low as 300 °C. The photograph of V₂O₅ films deposited at room temperature taken by scanning electron microscopy shows regular dot like features of nm size.     

Growth and characterization of Bi2Se3 thin films by pulsed laser deposition using alloy target

Bi₂Se₃ thin films were deposited on the (100) oriented Si substrates by pulsed laser deposition technique at different substrate temperatures (room temperature −400 °C). The effects of the substrate temperature on the structural and electrical properties of the Bi₂Se₃ films were studied. The film prepared at room temperature showed a very poor polycrystalline structure with the mainly orthorhombic phase. The crystallinity of the films was improved by heating the substrate during the deposition and the crystal phase of the film changed to the rhombohedral phase as the substrate temperature was higher than 200 °C. The stoichiometry of the films and the chemical state of Bi and Se elements in the films were studied by fitting the Se 3d and the Bi 4d5/2 peaks of the X-ray photoelectron spectra. The hexagonal structure was seen clearly for the film prepared at the substrate temperature of 400 °C. The surface roughness of the film increased as the substrate temperature was increased. The electrical resistivity of the film decreased from 1 × 10⁻³ to 3 × 10⁻⁴ Ω cm as the substrate temperature was increased from room temperature to 400 °C.     

Influence of thickness on H2 gas sensor properties in polycrystalline SnO x films prepared by ion-beam sputtering

Amorphous SnO _x films were deposited on sintered alumina substrates by ion-beam sputtering. They were annealed at 500° C for 2 h in air and polycrystalline films with thickness varying from about 1 to 700 nm were prepared. Film-sensor properties against 0.47% H₂ gas were measured as a function of thickness and the operating temperature for 150 to 350° C. The film thickness exhibiting a sensitivity maximum increased gradually with temperature. The optimum thickness shifted from 7 nm at 150° C to 175 nm at 350° C. Highly sensitive films lay in a narrow thickness range of 60 to 180 nm and films thinner or thicker than this were relatively insensitive at 300 and 350°C. A model was proposed to interpret the sensitivity behaviour in terms of thickness and grain-boundary effect.     

Structural, dielectric and ferroelectric properties of multilayer lithium tantalate thin films prepared by sol-gel technique

Multilayer lithium tantalate thin films were deposited on Pt-Si [Si(111)/SiO₂/TiO₂/Pt(111)] substrates by sol-gel process. The films were annealed at different annealing temperatures (300, 450 and 650 °C) for 15 min. The films are polycrystalline at 650 °C and at other annealing conditions below 650 °C the films are in amorphous state. The films were characterized using X-ray diffraction, atomic force microscopy (AFM) and Raman spectroscopy. The AFM of images show the formation of nanograins of uniform size (50 nm) at 650 °C. These polycrystalline films exhibit spontaneous polarization of 1.5 μC/cm² at an application of 100 kV/cm. The dielectric constant of multilayer film is very small (6.4 at 10 kHz) as compared to that of single crystal.     

A novel ozone detection at room temperature through UV-LED-assisted ZnO thick film sensors

In this work a novel ozone detection at room temperature (RT) has been investigated. Two functional materials, ZnO and (W_0.9Sn_0.1)O_3 − x (WS10) oxides, have been synthesized to prepare thick film gas sensors, both used in conventional heated mode as well as at RT assisted by UV irradiation. As a source of light, a light emitting diode (LED) of 400 nm peak wavelength was used. Under typical operating conditions of the UV-LED, the radiation flux density ? over the sensor was of about 5 · 10¹⁷ photons/cm². Powders and films have been characterized by means of TG-DTA, SEM, TEM and XRD. Finally, electrical measurements have been performed on sensing films with the aim to compare conductive properties, surface barrier heights and ozone sensing features with and without UV irradiation. Despite the fact that two types of conventional heated sensors offered quite similar results with respect to ozone sensing, it turned out that, at RT and with the assistance of UV light, ZnO behaved excellently fast detecting ozone at concentrations down to 10 ppb, while for WS10 under the same operating conditions an opposite result was observed, i.e. very low response and long response time.