J.F.D. Chubaci

Properties of carbon nitride films with composition ratio C/N=0.5–3.0 prepared by the ion and vapor deposition method

Carbon nitride films with the composition ratio CR(C/N)=0.5–3.0 were prepared by the ion and vapor deposition method, where carbon was evaporated on various substrates while being simultaneously bombarded with 0.5–10.0 keV nitrogen ions. The properties of the films were studied by x‐ray photoelectron spectroscopy (XPS), Fourier transform infrared spectrometry (FT‐IR), ultraviolet transmission spectroscopy, x‐ray diffraction, and hardness measurements. The films formed at energies lower than 0.8 keV and CR(C/N)=0.6–0.7 on tungsten carbide showed the highest Knoop harness of 6400 kgf/mm2 on films with 1 μm thickness and a maximum optical band gap of 2.7 eV. X‐ray diffraction measurements demonstrated that all films have an amorphous structure. The XPS and FT‐IR studies indicated that the peak newly observed at 286.3 eV in the C‐1s1/2 XPS spectra arises from triple bonding C≡N.

NIR to visible up-conversion, infrared luminescence, thermoluminescence and defect centres in Y2O3?:?Er phosphor

Er3+ doped Y2O3 phosphor was prepared by the solution combustion method and characterized using powder x-ray diffraction and energy-dispersive analysis of x-ray mapping studies. Room temperature near infrared (NIR) to green up-conversion (UC) emissions in the region 520–580 nm {(2H11/2, 4S3/2) →4I15/2} and red UC emissions in the region 650–700 nm (4F9/2 →4I15/2) of Er3+ ions have been observed upon direct excitation to the 4I11/2 level using ~972 nm laser radiation of nanosecond pulses. The possible mechanisms for the UC processes have been discussed on the basis of the energy level scheme, the pump power dependence as well as based on the temporal evolution. The excited state absorption is observed to be the dominant mechanism for the UC process. Y2O3 : Er exhibits one thermally stimulated luminescence (TSL) peak around 367 °C. Electron spin resonance (ESR) studies were carried out to study the defect centres induced in the phosphor by gamma irradiation and also to identify the centres responsible for the TSL peak. Room temperature ESR spectrum of irradiated phosphor appears to be a superposition of at least three distinct centres. One of them (centre I) with principal g-values g|| = 2.0415 and g⊥ = 2.0056 is identified as centre while centre II with an isotropic g-factor 2.0096 is assigned to an F+-centre (singly ionized oxygen vacancy). Centre III is also assigned to an F+-centre with a small g-factor anisotropy (g|| = 1.974 and g⊥ = 1.967). Additional defect centres are observed during thermal annealing experiments and one of them appearing around 330 °C grows with the annealing temperature. This centre (assigned to an F+-centre) seems to originate from an F-centre (oxygen vacancy with two electrons) and the F-centre appears to correlate with the observed TSL peak in Y2O3 : Er phosphor. The trap depth for this peak has been determined to be 0.97 eV from TSL data.

Influence of ion energy and arrival rate on x-ray crystallographic properties of thin ZrOx films prepared on Si(111) substrate by ion-beam assisted deposition

Thin zirconium oxide films, formed on Si(111) substrate by ion-beam assisted deposition, have been investigated by x-ray diffractometry with respect to the microstructure of the films, such as preferred orientation, interplanar spacing, crystallite size. The results of the interplanar spacing and diffraction intensity analysis could be interpreted in terms of relative amount of Zr4+ ions estimated by analyses of Zr 3d x-ray photoelectron spectroscopy spectra for the films.

General order and mixed order fits of thermoluminescence glow curves—a comparison

Abstract General order (GO) and mixed order (MO) kinetics expressions are applied to a series of synthetic thermoluminescence (TL) glow peaks derived from different physical models. The correlation between kinetic order (KO) parameter b of the GO kinetics expression and the parameter α (=n 0 /(n 0 +M) where n0 is the initial filled concentration of the active traps and M that of the thermally disconnected deep traps) of the MO expression is checked. It is found that the correlation is not universal except at the limiting values of α and b, when α=0 and 1 always correspond to b=1 and 2 as expected. In the intermediate region the graphical plot does not produce a smooth line. An examination of all the results along with those from the published literature shows that the overall scatter in the values found for b corresponding to any value found of α is within about ±10%. This scatter is attributed to the influence of parameters other than α and b on the symmetry of the glow peak. A conclusion of greater significance reached from this study is that MO expression is a superior alternative to GO for the glow peak characterization. This is because the parameter α remains constant at all temperatures T in the glow curve whereas b, though assumed constant in the GO expression, changes with T when GO is applied to any physically plausible model of TL emission, except when b turns out to be 1 or 2. Due to this variation of b, the value found for E always has some error which increases systematically as the fitted b deviates farther away from 1 and 2. Parallel to the error in E, the figure of merit (FOM) of the fit also deteriorates. The paper also discusses the relevance of other fitted parameters, both in GO and MO fits.

A critical look at the kinetic models of thermoluminescence: I. First-order kinetics

Using a generalized scheme of multiple traps, thermoluminescence (TL) glow curves are calculated for different sets of systems parameters. In particular, the conditions under which glow peaks of first-order kinetics are produced are highlighted. The major findings and conclusions are as follows. (1) In the generalized scheme the glow peaks always reduce to first order at low trap occupancies. It is therefore suggested that the peak analysis to determine the parameters should be carried out only at low doses. (2) Glow peaks which follow first-order kinetics can be obtained irrespective of whether the recombination rate is faster, equal to or slower than the retrapping rate (Rret). (3) Quasi-equilibrium (QE) of free carriers in the delocalized band, which is the essential condition for the derivation of the conventional analytical expressions of TL and thermally stimulated conductivity, can be realized irrespective of whether RrecRret. (4) The realization of the QE condition depends on the concentrations of the traps and the recombination centres (RCs) and their cross sections for free carrier capture. It is discussed and shown that, in doped insulating and semiconducting materials, the values of these parameters are sufficiently high for the QE condition to be comfortably held. It is thus concluded that the doubts raised by earlier workers regarding the validity of the QE assumption in the derivation of the analytical expressions are unnecessary as far as these materials are concerned. (5) It is shown that a system in which some of the untrapped charge carriers recombine within the germinate centres and some become delocalized may satisfactorily explain the mechanism of TL emission in most of the phosphors. The properties of first-order, supralinearity and pre-dose sensitization may be easily explained under the framework of this system. (6) Conclusions (2) and (3) above disprove those of earlier workers who had concluded that QE and fast retrapping together do not form a consistent set of conditions and that the apparent dominance of first-order kinetics in nature is due to slow retrapping.

Test for quasi-equilibrium in thermally stimulated luminescence and conductivity

The uncertainty about the existence of quasi-equilibrium (QE) condition during the readout of thermoluminescence and thermally stimulated conductivity glow peaks in real materials causes doubts about the validity of applying the QE-based analytical expressions to analyse these glow peaks. In this paper a simple method is given to verify the QE condition during the readout of these glow peaks in real materials. The method is based on changes in the glow peak shapes caused by changing the heating rate in non-QE cases. It is illustrated by using synthetic glow peaks derived from two different physically meaningful models.

γ-Radiation effects on colourless silicates of beryl

Abstract γ-Ray radiation effects on thermoluminescence (TL), electron spin resonance (ESR) and optical absorption (OA) properties of natural uncoloured beryl (Be3Al2Si6O18) were investigated. All the TL glow curves for annealed and subsequently irradiated samples showed a single peak at 165 °C. This TL peak did not indicate any relation with the H0 center, contrary to the TL process proposed for beryl. Irradiation with 106 Gy plainly enhanced the following in the samples: the so-called “Maxixe” bands at 600–700 nm due to CO3−; an OA band at 315–326 nm which can be assigned to ultraviolet charge transfer (UVCT) from O2− to Fe3+ in the tetrahedral Be2+ site, and an ESR signal of Fe3+ observed at g=4.257. An isochronal thermal anneal study indicated a certain correlation between the OA band at 315–326 nm and the ESR signal of Fe3+. The UVCT process of this OA band is related to Fe3+, and hence the observed correlation is considered to be reasonable.

Study of luminescence, color and paramagnetic centers properties of albite.

A sample of natural albite, NaAlSi3O8, from the state of Minas Gerais, Brazil, has been investigated. The mineral is a solid solution of K-feldspar (4600 ppm--K) and Ca-feldspar (1100 ppm--Ca). The TL spectra of natural and the pre-annealed at high temperature albite presented a very intense band around 275 nm and weaker bands around 400 and 560 nm. Other TL properties have been investigated through monochromatic (275 nm and 400 nm) glow curves. The EPR spectrum measured at low temperature (77K) shows the typical 11 lines signal due to Al-O(-)-Al center superposed on Fe(3+) signal around g=2.0. The EPR spectra above 260 K show only g=2.0 signal due to Fe(3+) ions.

Dependence of quasi-equilibrium on heating rate and its use in the study of thermoluminescent materials

Using the simulated thermoluminescence (TL) glow peaks, it is shown that the level of quasi-equilibrium (QE) during the emission of a glow peak depends on the heating rate (β) used. Any system which may satisfy the QE condition when β is below a certain value during TL stimulation, may turn to the non-QE condition at higher values of β. The level of QE is examined in terms of the two functions called D(T) and q(T). A quantitative limit is suggested on the value of these functions in order to define a system as quasi-equilibrated. At about the turning point of the QE to non-QE condition the glow peaks begin to change their shape. This property is found for the first time and is used as the first ever experimental method to test whether or not the emission of a given glow peak is occurring under the QE condition. Further, at the QE to non-QE turning point the ratio TR/τm appears to be nearly constant with only a weak dependence on the values of τm where TR is the recording time of the glow peak and τm is the recombination lifetime of the free carriers during the peak emission. This near constant value of TR/τm together with the measured value of TR may be used to estimate the τm of the systems which show the non-QE condition above a certain heating rate. The value of τm being a function of the concentration and the cross section of the TL related traps, one is able to correlate the value of τm thus found with the approximate range of these parameters for the given sample. The results are discussed in the perspective of a well known TL phosphor and the plausible range of its concentration and cross section parameters.

Light element analysis using ERDA method with an ionization chamber

Abstract An elastic recoil detection analysis (ERDA) system using ΔE−E technique was assembled for stoichiometric and depth profile studies of materials formed by light elements. An ionization chamber with a surface barrier detector (SBD) was used in this work. From the energy loss (ΔE) information obtained from the gas and the residual energy (E) obtained by the SBD, it was possible to identify atomic number of the arriving particles at the SBD. An incident beam of 35 Cl of 58 MeV was used for the elastic scattering with the sample components. From the spectra obtained for carbon nitride thin films it was possible to clearly identify the elements carbon and nitrogen. The presence of oxygen in the spectra was probably due to the sample exposure to air. The chemical composition of the films was calculated and compared with that obtained from X-ray photoelectron spectroscopy (XPS) spectra for C 1s and N 1s. A LiNiO2 thin film was also analyzed.