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Defect studies in quartz: Composite nature of the blue and UV emissions
Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
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Defect studies in quartz: Composite nature of the blue and UV emissions Marco Martini a,b,⇑, Mauro Fasoli a, Irene Villa a a b
Dipartimento di Scienza dei Materiali, Università degli Studi di Milano Bicocca, via R. Cozzi 55, I-20125 Milano, Italy INFN, Sezione di Milano Bicocca, Piazza della Scienza 1, I-20126 Milano, Italy
a r t i c l e
i n f o
Article history: Received 1 July 2013 Received in revised form 24 September 2013 Available online xxxx Keywords: Quartz Radioluminescence Dating
a b s t r a c t Quartz is an extremely diffused natural luminescence dosimeter. Thanks to the presence of traps and luminescence centres, its TSL and OSL (Thermally and Optically Stimulated Luminescence) properties have been extensively exploited. Quartz is then used for archaeological and geological dating and is one of the most useful materials for accident dosimetry. Many luminescence emissions are known to be present in the OSL and TSL of quartz. Three main emission bands are always reported, as the red, blue and UV bands, centred at around 650, 470, and 360–380 nm, respectively. Although the assignment of the luminescence emissions to specific defect centres in quartz is still undefined, a thorough analysis of the radioluminescence emissions and their response to irradiation and thermal treatments turned out to be very useful in understanding many features. Specifically, the presence of the same emission bands in natural and synthetic quartz and their dependence on the presence of extrinsic impurities is a common characteristic. The main impurities involve Al ions substituting Si ones and charge compensated by nearby either alkali ions, H+, or a hole. The emission spectra dynamics evidenced in our experiment confirm the role of Al-related centres in the luminescence properties of quartz. From the measurements presented in this paper the composite nature of the ‘‘blue’’ emission is confirmed. Two bands labelled as A at 2.5 eV and B at 2.8 eV contribute to the emission in this region, their behaviour being different as a function of irradiation. More complex is the picture in the UV region, where, besides the already detected C and D bands at 3.4 eV and 3.9 eV, respectively, two further emissions have been detected at 3.1 eV and 3.7 eV. Despite both the 3.4 eV and the 3.7 eV bands are shown to be affected by thermal treatments, the annealing temperature dependence of their intensity is markedly different. In fact, whereas the C band intensity, at 3.4 eV, increases after annealing at 500 °C followed by a decrease at higher temperatures, the 3.7 eV intensity is strongly enhanced by annealing at temperature above 700 °C and reaches its highest value after annealing at around 1000 °C. In the light of these results a number of already known features of quartz emissions should be reconsidered. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The defect structure of an insulating material is an essential requirement to understand its dosimetric properties. In fact, intrinsic and extrinsic defects act as trapping and recombination centres, giving origin to the so-called delayed luminescence, a property that characterizes luminescence dosimetry. Quartz is an efficient natural dosimeter [1] and is extensively exploited in dosimetric ⇑ Corresponding author at: Dipartimento di Scienza dei Materiali, Università degli Studi di Milano Bicocca, via R. Cozzi 55, I-20125 Milano, Italy. Tel.: +39 02 64485 166; fax: +39 02 64485 400. E-mail address: [email protected] (M. Martini).
applications. Quartz is one of the most abundant minerals in the continental crust of the Earth and can be used to detect the amount of absorbed energy due to natural or man-made exposure to ionizing radiation, i.e. the absorbed radiation dose. On this measurement are based some luminescence dating techniques, the best known being thermoluminescence dating [2]. Furthermore quartz is applied in accident dosimetry, in that a composite material exposed to unwanted irradiation can record the amount of absorbed dose if it contains enough quantity of some natural dosimeter, typically quartz [3]. It has been possible to determine the dose distribution around some accident zones, like Hiroshima [4] and Chernobyl [5], through the measurement of the thermoluminescence of quartz inclusions in bricks and other artifacts.
http://dx.doi.org/10.1016/j.nimb.2013.09.048 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Martini et al., Defect studies in quartz: Composite nature of the blue and UV emissions, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2013.09.048
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Although a large number of studies has been published on the luminescence properties of crystalline quartz [6,1 and references therein], a comprehensive description of the light emissions under different stimulation is still far from being clear. The relationships between each luminescence emission and the corresponding responsible defects are generally rather tentative and the assignments given by different studies are often in contrast with one another. Even the reported energies of the emission peaks can differ also because it is emerging that many emission bands are composite [7,8], possibly by components uncorrelated with each other, which implies that, in the same energy region, one or the other component can alternatively be detected (or an overlapping of the two components), giving as a result a band at a slightly different energy. Many defects are present both in the crystalline and in the amorphous forms of SiO2, quartz and silica respectively, and are often reviewed together [9] even if their production and response to irradiation and thermal treatments are often quite different. Besides, there are defects occurring in glassy silica that are not found in quartz and vice versa [10]. As a consequence, the luminescence emissions of quartz and silica are not so similar. In this paper, firstly, a short review of recent results from radioluminescence measurements of natural and synthetic quartz will be given, showing the effect of Al-related centres on the luminescence properties of quartz. Then, new results on thermal treatments in different atmospheres will be shown, evidencing a role of Oxygen in inducing emission bands. The dependence of luminescence emissions on the temperature of annealing has also been studied and two distinct UV bands turned out to have different behaviour as a function of annealing temperature.
2. Defects in quartz and luminescence emissions In order to understand the role of the various defects in producing the different luminescence emissions in quartz, it would be necessary to correlate the defects detected by specific spectroscopic techniques to the corresponding luminescence emissions, following their behaviour as a function of irradiation and/or other treatments, typically heat treatments. A number of defects have been definitely identified in crystalline quartz by Electron Spin Resonance (ESR) [11] and it is widely agreed that two types of centres are certainly essential in the dynamics of radiative recombination: Oxygen vacancies and Al centres. The simplest type of O vacancy is the diamagnetic centre resulting from the removal of an O atom from the otherwise perfect a-quartz, leaving a direct bond between two Si atoms. This ‘‘neutral O vacancy’’ is often considered as the precursor of the paramagnetic E01 centre, which is the most recurrent intrinsic defect in quartz and can be described as a hole trapped at a neutral O vacancy. The resulting defect features an unpaired electron on one of the two Si atoms, while the other Si ion, being positively charged, moves away from the vacancy into a nearby plane configuration [12]. Al3+ ion is the main ubiquitous impurity in quartz whom many defects are associated to. It is present as a substitutional ion for Si4+ and is charge compensated by an alkali ion (M+) or by an H+ ion, giving rise to [AlO4/M+]0, [AlO4/H+]0, respectively. When neither M+ nor H+ is present near a substitutional Al3+ ion, this centre can be charge compensated by a hole, giving rise to the [AlO4]0 centre [13]. Such a defect has been reported as responsible for a recurrent luminescence emission at 365–380 nm [14], detected in many types of luminescence, in thermoluminescence (TL) below [15] and above room temperature (RT) [16], in Optically Stimulated Luminescence (OSL) [8], and in radio-luminescence (RL) [17,18].
Fig. 1. Deconvolution into Gaussian components of the radioluminescence emission spectra of (a) pegmatitic quartz (K-2-00) and (b) unswept synthetic one (G0). From Ref. [7].
Some results have been recently obtained through the analysis of RL. In Fig. 1 the RL curves of a natural and a synthetic quartz sample are reported, from [7]. It is noteworthy that the curves can be de-convoluted in the same four components: Martini et al. [7] concluded that two components participate in the blue emission, at 440 nm (2.8 eV) and at 490 nm (2.5 eV) and two emissions are present in the UV, at 360 nm (3.4 eV) and at 310 nm (3.9 eV). A contribution by a further band at 395 nm (3.1 eV) can be hypothesized to account for some discrepancies in the curve fitting. With the aim of investigating how Al centres are involved in luminescence dynamics, it is possible to compare the luminescence of quartzes containing highly different concentrations of alkali ions. By a high temperature electro-diffusion, the so called ‘‘sweeping’’ procedure, the concentration of alkali ions, that are the main charges present et al. sites in untreated quartz, can be strongly reduced and [AlO4]0, [AlO4/H+]0 centres are created. This technique allows the removal (sweep out), or the introduction (sweep in), of the ions compensating the Al centre (H+ or alkali ions) by applying a high electric field (typically 1500–2000 V/cm at 500–600 °C) along the channels running parallel to the c axis of quartz crystals [Ref. 19]. Correspondingly, evident modifications in the luminescence properties are detected. In Fig. 2 the radio-luminescence of two untreated and a swept out quartz samples are compared (from Ref. [18]). There is evidence of luminescence emissions in the blue
Please cite this article in press as: M. Martini et al., Defect studies in quartz: Composite nature of the blue and UV emissions, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2013.09.048
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Fig. 2. Radioluminescence emission spectra of synthetic quartz samples either untreated (G0 and G12) or ‘‘swept out’’ (GS12). From Ref. [18].
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Fig. 4. Sequence of 40 RL emission spectra of a pegmatitic quartz (K-2-00) collected under continuous X-ray irradiation. For better readability only some of the spectra are shown. From Ref. [7].
350–450 °C. There is agreement in the interpretation of the effect as being due to an increase in the concentration of the hole recombination centres, which should emit at 3.4 eV, as seen in Fig. 5. These have been proposed to be [H3O4]0 [21] or [AlO4]0 [14]. It is interesting to note that also in this two-step process there should be the involvement of M+ ions, because the effect is seen only in unswept quartz [22]. 3. Materials and methods Our investigation was carried out on several crystals of different origin. For the annealing in controlled atmosphere, a crystal of natural quartz, named in the text as K-2-00, was considered. The quartz comes from the Black Hills, South Dakota and formed in the last pegmatitic stage (T < 300 °C). The crystal was crushed and sieved to select grains of 100–200 lm dimension. The powder was divided into four parts, put in quartz crucibles and heated in the oven at 600 °C for 5 h in the following atmospheres: Fig. 3. Optically Stimulated Luminescence (OSL) shine down curves, at room temperature, of an untreated and a ‘‘swept out’’ synthetic quartz sample. Stimulation wavelength: 470 nm, preheat 10 s at 125 °C. Curves are taken from Ref. [19].
and in the UV regions. The most relevant effect of the ‘‘sweeping out’’ procedure is that the main UV emission only is present after eliminating the alkali ions, which is interpreted as a fundamental role of Al-related centres in the radiative recombination. Similarly, Optically Stimulated Luminescence (OSL) emission is heavily affected by the ‘‘sweeping out’’ procedure: in Fig. 3, taken from Ref. [20], a comparison of the room temperature OSL shine down curves, under 470 nm stimulation, from an untreated and a ‘‘swept out’’ quartz, is shown, evidently highlighting a difference of three orders of magnitude in the OSL intensity. While in Fig. 4 an increase of the 2.5 eV emission is detected, another band is affected by irradiation followed by heat treatments: in Fig. 5, also taken from Ref. [7], a sequence similar to the one reported in Fig. 4 is shown. The fundamental difference is that now a heating up to 500 °C follows each irradiation, in a sequence similar to what is known as ‘‘pre-dose’’. The result is a strong increase of the UV 3.4 eV band rather than the blue 2.5 eV one. Still unknown electron and hole traps are involved in the so called pre-dose effect. The effect consists in a strong increase in the efficiency of the 110 °C TSL peak, as a consequence of repeated RT irradiation followed by heating to a temperature in the range
Air. Hydrogen: fluxing a gas mixture of N2 (98%) and H2 (2%). Oxygen: in a 50 ml/min O2 flux. Vacuum: under 105 mbar pressure.
Fig. 5. Sequence of RL emission spectra of natural pegmatitic quartz (K-2-00) obtained during X-ray irradiation followed by a heating up to 500 °C at 1 °C/s. From Ref. [7].
Please cite this article in press as: M. Martini et al., Defect studies in quartz: Composite nature of the blue and UV emissions, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2013.09.048
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Fig. 6. Radioluminescence of a natural pegmatitic quartz (K-2-00) powders annealed in controlled atmosphere at 600 °C for 5 h. The reported spectra are the first one out of a sequence of 40 consecutive measurements.
In a second experiment, three different types of quartz were annealed at different temperatures. Single crystals of a natural colourless quartz, a natural smoky one, and a bar of synthetic (Sawyer Premium Quality) one, were crushed and grinded. For each quartz, the fraction of 100–200 lm grain size was selected by sieving and was divided into ten different batches. Each batch was put in a quartz crucible and annealed, in air, for 10 min in a preheated oven. The crucible was then removed from the oven and let cool to room temperature. A set of nine annealing temperatures, from 300 °C to 1100 °C, in 100 °C step, was considered. The radio-luminescence (RL) measurements were carried out at room temperature (RT) using a home-made apparatus featuring, as detection system, a charge coupled device (CCD) (Jobin–Yvon spectrum one 3000) coupled to a spectrograph operating in the 200– 1100 nm range (Jobin–Yvon Triax 180). The data were corrected for the spectral response of the detection system. RL excitation was obtained by X-rays irradiation, through a Be window, using a Philips 2274 X-ray tube with Tungsten target operated at 20 kV. During each measurement the sample was given a dose of 10 ± 2 Gy, where the uncertainty is related to the dose calibration rather than to its repeatability that was quite good (1%). RL spectra were deconvoluted into Gaussian components using the least squares method with the Levenberg–Marquardt algorithm (origin 8.0).
spectra. A similar effect is observed for a high energy emission, close to 3.9 eV, whose intensity is sensibly higher in the annealed samples compared to the untreated one. The enhancement of these two RL emissions is more evidently induced by the annealing in oxidizing atmosphere (air and Oxygen). The evolution of the quartz luminescence, under continuous X-ray irradiation, was monitored on all of the samples through a sequence of 40 successive RL measurements. The effect of the irradiation on the RL spectra is similar regardless the annealing atmosphere. As an example, the results obtained on the quartz annealed in Oxygen atmosphere are shown in Fig. 7. The blue emission around 2.7 eV is progressively reduced, at the same time, the maximum of the emission shifts towards lower energy evidencing a composite nature of the blue emission. In fact, a second band, around 2.5 eV, is enhanced by irradiation becoming quickly the dominant emission. The high energy range of the RL spectrum is not strongly modified by the X-ray irradiation. In particular, the emission close to 3.9 eV seems to be stable, whereas in the 3.0–3.5 eV region the RL emission is slightly increased by irradiation but early reaches a saturation level. Finally, an emission band below 2.0 eV, detected only in annealed samples, is enhanced by X-ray irradiation. Fig. 8 reports the RL spectra recorded at the end of the sequence. The most intense emission is peaked around 2.5 eV both in the untreated quartz and in the annealed ones and the absolute intensity of the band is similar in all of the samples, regardless the annealing atmosphere. A numerical analysis of the emission spectra was carried out following the same approach used in [7,18,24]. The RL spectra were resolved, using the Levenberg–Marquardt algorithm, into the Gaussian components reported in Table 1. All the parameters were allowed to vary within 0.01 eV from the reported value. With respect to our previous work, two more bands were introduced in the fitting process: one at 3.1 eV (FWHM 0.89 eV) and one at 3.7 eV (FWHM 0.45 eV) which were labelled as X and M band, respectively. The presence of the X band was required in order to obtain a fully satisfactory numerical analysis in the range between 3.0 eV and 3.5 eV. Indeed, low temperature RL measurements evidenced the presence of a band close to 3.1 eV in both natural and synthetic quartzes. Assuming that the contribution of this band could account for the discrepancy between the Gaussian fits and the experimental curve, we tentatively added it to the deconvolution analysis. The M band was necessary only to deconvolve the spectra of the samples annealed at temperature higher than 600 °C (see the next section).
4. Results High temperature annealing is known to strongly affect the luminescence characteristics of quartz [23]. We used RL measurements to systematically investigate the effect of both annealing atmosphere and temperature on the emission bands of quartz. 4.1. Effect of annealing atmosphere In a first experiment a set of K-2-00 powders (100–200 lm) were annealed at 600 °C for 5 h in different controlled atmosphere as described in the previous section. In Fig. 6 the RL spectra of the annealed samples are reported as a function of energy. The spectrum of the unannealed sample is also shown as a reference. Regardless the annealing atmosphere, the RL spectra are characterized by a composite emission. Two main components are found to be enhanced by the thermal treatments: an emission with an apparent maximum around 2.7 eV, and not detected in the unannealed sample, dominates the blue region of the annealed quartzes
Fig. 7. Sequence of 40 consecutive radioluminescence measurements on a natural pegmatitic quartz (K-2-00) powders annealed in Oxygen atmosphere at 600 °C for 5 h. For readability sake only three spectra are reported.
Please cite this article in press as: M. Martini et al., Defect studies in quartz: Composite nature of the blue and UV emissions, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2013.09.048
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Fig. 8. Radioluminescence of a natural pegmatitic quartz (K-2-00) powders annealed in controlled atmosphere at 600 °C for 5 h. The reported spectra are the last one out of a sequence of 40 consecutive measurements.
A detailed investigation on the irradiation effect on the intensity of the bands was carried out on the quartz annealed in Oxygen. Fig. 9 reports the area of three of the Gaussian components obtained from the numerical analysis as a function of the accumulated dose. As expected from the RL spectra, the B band progressively decreases under continuous X-ray irradiation. However, the numerical deconvolution of the spectra evidences a complex dose dependence of the A band intensity. In fact, in the first irradiation stage (up to 70 Gy approximately), the band intensity is sensibly reduced. At higher dose the band increases and, above 100 Gy, the growth becomes linear. The 3.1 eV emission (X band) is enhanced by X-ray irradiation reaching a saturation level at 100 Gy. It is interesting to note that the increase of the A band starts as soon as the X band has reached the saturation level. A similar correlation between the A and X bands is observed also in the untreated sample (Fig. 10) where, again, the growth of the A band becomes linear as soon as the X band intensity has stabilized after an early increase under irradiation. Finally, the remaining components, in particular the D band, turned out to be not significantly affected by irradiation. 4.2. Effect of annealing temperature
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Fig. 9. Dose dependence of the A (2.5 eV), B (2.8 eV) and X (3.1 eV) radioluminescence bands intensity in a natural pegmatitic quartz (K-2-00) powder annealed in Oxygen at 600 °C for 5 h.
The effect of the annealing on the RL spectra of the smoky and colourless natural quartz turned out to be qualitatively similar (Fig. 11 reports some of the spectra collected for the annealed batch of smoky quartz). In particular, the samples annealed at temperatures between 400 °C and 600 °C showed a dramatic enhancement of the C band (3.4 eV) which becomes the dominant emission component. The maximum sensitization was observed for samples annealed at 500 °C since a treatment at higher temperature caused a reduction of such emission band intensity. The sensitization of the C band was found to be most effective in the smoky quartz whereas no effect at all was observed in the synthetic one. For annealing temperature higher than 700 °C the RL spectra of both natural samples showed an increase of the emission in the 3.5– 4.0 eV range. The numerical analyses evidenced that this is due to a Gaussian component at 3.7 eV (labelled as M band) whose intensity reaches its maximum in samples annealed at 1000 °C and is reduced when the annealing temperature is further increased. The effect of annealing on the RL spectrum of synthetic quartz is significantly different from the one observed in natural samples. In particular, no evidence of either C or M band was found. Moreover no relevant modification of the RL spectrum was detected for annealing temperature below 900 °C whereas, at higher temperature, an increase of the A, B and D bands was
A second experiment was carried out in order to investigate the effect of the annealing temperature on the RL spectrum of different types of quartz. Powders obtained from single crystals of a natural colourless quartz, a natural smoky one, and a synthetic (Sawyer Premium Quality) one were considered. For each quartz type, different batches were annealed at different temperatures as described in the Materials and Methods section. A RL spectrum was then collected for each annealed sample and deconvolved into Gaussian components.
Table 1 Parameter of the Gaussian components used in the numerical deconvolution of the radioluminescence spectra of quartz samples. Band
Wavelength (nm)
Energy (eV)
FWHM (eV)
O A B X C M D
635 490 440 395 360 330 315
1.92 2.51 2.79 3.06 3.42 3.73 3.93
0.39 0.46 0.46 0.89 0.58 0.45 0.49
Fig. 10. Dose dependence of the A (2.5 eV) and X (3.1 eV) radioluminescence bands intensity in an untreated natural pegmatitic quartz (K-2-00) powder.
Please cite this article in press as: M. Martini et al., Defect studies in quartz: Composite nature of the blue and UV emissions, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2013.09.048
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Fig. 11. Radioluminescence spectra of a natural smoky quartz powders annealed in air at different temperatures for 10 min.
observed. The intensity of the 3.4 eV (C band) and 3.7 eV (M band) bands obtained from the Gaussian deconvolution are reported in Figs. 12 and 13, respectively, as a function of the annealing temperature. 5. Discussion The assignment of a luminescence emission band to a specific quartz defect has long been the weakest point in the discussion of many papers found in the literature. The unambiguous identification of a defect to a recombination centre (or to a trap) requires the combined use of different experimental techniques (for example thermoluminescence, optical absorption, photoluminescence, ESR, etc.). Unfortunately, however, matching the experimental data and obtaining their coherent interpretation is often not as straightforward. This often leaves the field to speculations that, in the long run, can be misleading in further investigations. In the case of quartz, the identification of defects involved in the luminescence dynamics is particularly tricky because of the different varieties of crystals, either natural or synthetic. Quartzes of different origin, in fact, contain different amounts of intrinsic and extrinsic ions (Al, alkali ions, and hydrogen in particular). Moreover, the relative concentration of defects can be altered by radiation and/or thermal treatment also because of high ion mobility.
Fig. 12. Annealing temperature dependence of the C (3.4 eV) radioluminecence band for three different types of quartz powders.
Fig. 13. Annealing temperature dependence of the M (3.7 eV) radioluminecence band for three different types of quartz powders.
From the data reported in this paper and in recent publications [7,18,24] some conclusions can be drawn, particularly thanks to a better defined picture of radioluminescence mechanisms. The first important result is that it has been demonstrated that all RL spectra, both in natural and in synthetic quartz, are composed of the same luminescence emissions, as shown in Fig. 1 (from Ref. [18]). Most papers in the literature report a blue emission as the main recombination centre in the TSL glow curves. We found always a blue luminescence in the RL spectra of unswept synthetic and natural quartz and clearly demonstrated that two bands, labelled as A and B, at 2.5 eV and 2.8 eV respectively, contribute to this emission. A perfect agreement between the growth with irradiation of the A RL emission and of the 110 °C TL peak has been shown, see Fig. 4 (from Ref. [7]). The 110 °C TSL peak is also increased through a procedure similar to the so called pre-dose, a series of irradiation followed by heat treatments [25,26]. This TL growth has been clearly related to the growth of a further RL emission band, the C emission at 3.4 eV, see Fig. 5. It is then clear that the same quartz crystal can alternatively strongly emit, both in RL and TL, in the blue (A band) or in the UV (C band), depending on the treatments it has been submitted to. It is not easy to correlate these changes to specific recombination centres, but an indication of the role of Al-related centres (charge compensated substitutional Al) comes from some experimental results. Figs. 2 and 3 summarize the effects due to the reduction of the concentration of alkali ions (‘‘sweeping’’) on RL and OSL emissions: it is evident that most RL bands disappear after a ‘‘sweeping’’ procedure and similarly a reduction in the intensity of OSL is seen in swept out samples. As already discussed by Halliburton et al. [13], the modifications that occur et al. centres are at the basis of most luminescence emissions. Martini et al. [14], through high temperature treatment, induced substitutional Al3+ ions to be charge compensated by a hole, giving rise to the [AlO4]0 centre. This latter has been proposed as the recombination centre responsible for the main UV emission in TSL, that has been identified as the C band, at 3.4 eV [7]. Jani et al. [27] reported a correlation between the growth of the E01 centre and the decay of the [AlO4]0 centre and observed that sweeping out the alkali ions strongly reduces the intensity of the E01 ESR signal. Poolton et al. [28] suggested that E01 may act as non-radiative centre competing in OSL with the [AlO4]0 recombination centre, while a good correlation between the growth of E01 and of [AlO4]0 was found by Usami et al. [29]. Then E01 centre has not been clearly identified in its role in indirectly contributing to the
Please cite this article in press as: M. Martini et al., Defect studies in quartz: Composite nature of the blue and UV emissions, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2013.09.048
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luminescence processes. Some indications could come from the effect of high temperature annealing in different atmosphere. Looking at Figs. 6–8 the presence of O is seen to enhance both the blue and the UV emissions. Specifically, in the blue region, both A and B bands are induced by the thermal treatments; in the UV region, the D band at 3.9 eV is mainly enhanced. Furthermore, a confirmation of the effect of irradiation came from the growth of the A band at 2.5 eV clearly seen in Fig. 8: whatever the atmosphere of treatment, after a series of irradiation this band increases of the same amount. Despite the assignment of the emission bands detected in RL to a particular defect remains matter of investigation, some interesting considerations, based on our results, can be done suggesting possible correlations that can constitute a clue for further studies. In particular, the evolution of the RL band intensities of both annealed and untreated samples under irradiation show some common features. The fact that both the A and X bands intensities evidence a clear change in the cumulative dose dependence after about 100 Gy suggests some kind of correlation between them. Unfortunately, until the exact nature of the related defects is identified, it is not possible to determine whether this correlation is due to a simple competition in charge capture or if some more complex interaction or spatial correlation is implied. Whatever the nature of the correlation between the A and X bands, however, it is interesting to notice that the B band seems not to be involved in it. The B band intensity in the annealed samples, in fact, is progressively reduced by irradiation with no evidence of a discontinuity at about 100 Gy of cumulative dose. If we now consider the RL spectra as a function of the annealing temperature, our results point out, one more time, the complexity of the mechanisms involved in quartz luminescence. In fact, not only the emission spectrum is due to the contribution of several Gaussian component, but how such bands are affected by the thermal treatments depends on the specific emission. Despite that, the set of RL measurements carried out on different types of quartz allowed to highlight the common and most relevant features. Of particular interest is how the annealing temperature affects the C band emitting at 3.4 eV. Such band was proven to be detected also in TSL and to be activated by a heating following an irradiation like in the pre-dose sequence. The fact that an annealing at 500 °C could induce the C band only in natural samples is a further indication of its extrinsic nature, being the concentration of Al and alkali ions orders of magnitude lower in synthetic quartz. A further important result is related to the presence of a new emission band in the UV region, specifically the M band emitting at 3.7 eV, in samples annealed at temperatures higher than 700 °C. As we have already observed for the so called blue emission which was found to be composite, the same holds for the UV quartz emission. At least three emission components, specifically the C, D and M band contribute to the RL emission in the high energy range. The UV quartz emission has long been reported in the literature, however, referring to the high energy emission of quartz with a not better defined ‘‘UV emission’’, could be misleading since each of the involved components is related to a different luminescent centre which is differently affected by irradiation or thermal treatments. The effect of the annealing temperature on the RL spectrum of quartz was already reported in [23] and an overall good agreement with our results is evidenced. The authors, however, report of a single UV emission centred around 360 nm whose dependence on the annealing temperature is compatible with the sum of C and M bands. It is possible that many results reported insofar on the UV emission in quartz do not always refer to the same luminescence band due to the strong overlapping of the emissions. We remind however that a luminescence band
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detected in RL is not necessarily involved in the TSL mechanism. Further detailed investigations featuring a deconvolution of TSL spectra are thus required in order to determine whether the bands reported in Table 1, beside A and C, are detected in TSL. 6. Conclusions Although the assignment of the luminescence emissions to specific defect centres in quartz is still undefined, a thorough analysis of the RL emissions and their response to irradiation and thermal treatments turned out to be very useful in understanding many features. Specifically, the presence of the same emission bands in natural and synthetic quartz and their dependence on the presence of extrinsic impurities is a common characteristic which confirms the role of Al centres in the luminescence properties of quartz. From the measurements presented in this paper the composite nature of the ‘‘blue’’ emission is confirmed. Two bands labelled as A at 2.5 eV and B at 2.8 eV contribute to the emission in this region, their behaviour being different as a function of irradiation, the A band featuring a growth with irradiation in perfect agreement with the growth of the 110 °C TSL peak. More complex is the picture in the UV region, where besides the already detected C and D bands at 3.4 eV and 3.9 eV, respectively, two further emissions have been detected at 3.1 eV and 3.7 eV. This latter is seen to be strongly enhanced by high temperature treatments, its intensity reaching its highest value after an annealing at around 1000 °C, while it is seen that the C band increases in intensity after annealing at around 500 °C, followed by a decrease. In the light of these results a number of already known features should be reconsidered. References [1] F. Preusser, M.L. Chithambo, T. Götte, M. Martini, K. Ramseyer, E.J. Sendezera, G.J. Susino, A.G. Wintle, Earth–Sci. Rev. 97 (2009) 184. [2] M.J. Aitken, Thermoluminescence Dating, Academic Press, London, 1985. [3] S.J. Fleming, J. Thompson, Health Phys. 18 (1970) 567. [4] T. Higashimura, Y. Ichikawa, T. Sidei, Science 139 (1963) 1284. [5] I.K. Bailiff, Radiat. Meas. 24 (1995) 507. [6] K.P. Lieb, J. Keinonen, Contemp. Phys. 47 (2006) 305. [7] M. Martini, M. Fasoli, I. Villa, P. Guibert, Radiat. Meas. 47 (2012) 846. [8] M. Martini, M. Fasoli, A. Galli, Radiat. Meas. 44 (2009) 458. [9] M.A.S. Kalceff, M.R. Phillips, Phys. Rev. B 52 (1995) 3122. [10] D.L. Griscom, J. Non-Cryst. Solids 357 (2011) 1945. [11] J.A. Weil, In Defects in SiO2 and Related Dielectrics: Science and Technology, in: G. Pacchioni, L. Skuja, D.L. Griscom (Eds.), Kluwer Academic Publishers., Amsterdam, 2000, pp. 197–212. [12] J.K. Rudra, W.B. Fowler, Phys. Rev. B 35 (1987) 8223. [13] L.E. Halliburton, N. Koumvakalis, M.E. Markes, J.J. Martin, J. Appl. Phys. 52 (1981) 3565. [14] M. Martini, A. Paleari, G. Spinolo, A. Vedda, Phys. Rev. B 51 (1995) 138. [15] M. Martini, F. Meinardi, A. Vedda, Radiat. Meas. 32 (2000) 673. [16] W.J. Rink, H.M. Rendell, E.A. Marseglia, B.J. Luff, P.D. Townsend, Phys. Chem. Miner. 20 (1993) 353. [17] A. Halperin, E.W. Sucov, J. Phys. Chem. Solids 54 (1993) 43. [18] M. Martini, M. Fasoli, A. Galli, I. Villa, P. Guibert, J. Lumin. 132 (2012) 1030. [19] J.J. Martin, IEEE T. Ultrason. Ferr. 35 (1988) 288. [20] M. Martini, A. Galli, Phys. Status Solidi C 4 (2007) 1000. [21] X.H. Yang, S.W.S. McKeever, Radiat. Prot. Dosim. 33 (1990) 27. [22] M. Martini, E. Sibilia, G. Spinolo, A. Vedda, Nucl. Tracks 10 (1985) 497. [23] T. Schilles, N.R.J. Poolton, E. Bulur, L. Bøtter-Jensen, A.S. Murray, G.M. Smith, P.C. Riedi, G.A. Wagner, J. Phys. D: Appl. Phys. 34 (2001) 722. [24] V. Pagonis, M. Chithambo, R. Chen, A. Chrus´cin´ska, M. Fasoli, S.H. Li, M. Martini, K. Ramseyer, J. Lumin. 145 (2014) 38. [25] J. Zimmerman, J. Phys. C: Solid Status Phys. 4 (1971) 3265. [26] N. Itoh, D. Stoneham, A.M. Stoneham, J. Appl. Phys. 92 (2002) 5036. [27] M.G. Jani, R.B. Bossoli, L.E. Halliburton, Phys. Rev. B 27 (1983) 2285. [28] N.R.J. Poolton, G.M. Smith, P.C. Riedi, E. Bulur, L. Bøtter-Jensen, A.S. Murray, M. Adrian, J. Phys. D: Appl. Phys. 33 (2000) 72207. [29] T. Usami, S. Toyoda, H. Bahadur, A.K. Srivastava, H. Nishido, Phys. B: Condens. Matter 404 (2009) 3819.
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Defect studies in quartz: Composite nature of the blue and UV emissions Marco Martini a,b,⇑, Mauro Fasoli a, Irene Villa a a b
Dipartimento di Scienza dei Materiali, Università degli Studi di Milano Bicocca, via R. Cozzi 55, I-20125 Milano, Italy INFN, Sezione di Milano Bicocca, Piazza della Scienza 1, I-20126 Milano, Italy
a r t i c l e
i n f o
Article history: Received 1 July 2013 Received in revised form 24 September 2013 Available online xxxx Keywords: Quartz Radioluminescence Dating
a b s t r a c t Quartz is an extremely diffused natural luminescence dosimeter. Thanks to the presence of traps and luminescence centres, its TSL and OSL (Thermally and Optically Stimulated Luminescence) properties have been extensively exploited. Quartz is then used for archaeological and geological dating and is one of the most useful materials for accident dosimetry. Many luminescence emissions are known to be present in the OSL and TSL of quartz. Three main emission bands are always reported, as the red, blue and UV bands, centred at around 650, 470, and 360–380 nm, respectively. Although the assignment of the luminescence emissions to specific defect centres in quartz is still undefined, a thorough analysis of the radioluminescence emissions and their response to irradiation and thermal treatments turned out to be very useful in understanding many features. Specifically, the presence of the same emission bands in natural and synthetic quartz and their dependence on the presence of extrinsic impurities is a common characteristic. The main impurities involve Al ions substituting Si ones and charge compensated by nearby either alkali ions, H+, or a hole. The emission spectra dynamics evidenced in our experiment confirm the role of Al-related centres in the luminescence properties of quartz. From the measurements presented in this paper the composite nature of the ‘‘blue’’ emission is confirmed. Two bands labelled as A at 2.5 eV and B at 2.8 eV contribute to the emission in this region, their behaviour being different as a function of irradiation. More complex is the picture in the UV region, where, besides the already detected C and D bands at 3.4 eV and 3.9 eV, respectively, two further emissions have been detected at 3.1 eV and 3.7 eV. Despite both the 3.4 eV and the 3.7 eV bands are shown to be affected by thermal treatments, the annealing temperature dependence of their intensity is markedly different. In fact, whereas the C band intensity, at 3.4 eV, increases after annealing at 500 °C followed by a decrease at higher temperatures, the 3.7 eV intensity is strongly enhanced by annealing at temperature above 700 °C and reaches its highest value after annealing at around 1000 °C. In the light of these results a number of already known features of quartz emissions should be reconsidered. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The defect structure of an insulating material is an essential requirement to understand its dosimetric properties. In fact, intrinsic and extrinsic defects act as trapping and recombination centres, giving origin to the so-called delayed luminescence, a property that characterizes luminescence dosimetry. Quartz is an efficient natural dosimeter [1] and is extensively exploited in dosimetric ⇑ Corresponding author at: Dipartimento di Scienza dei Materiali, Università degli Studi di Milano Bicocca, via R. Cozzi 55, I-20125 Milano, Italy. Tel.: +39 02 64485 166; fax: +39 02 64485 400. E-mail address: [email protected] (M. Martini).
applications. Quartz is one of the most abundant minerals in the continental crust of the Earth and can be used to detect the amount of absorbed energy due to natural or man-made exposure to ionizing radiation, i.e. the absorbed radiation dose. On this measurement are based some luminescence dating techniques, the best known being thermoluminescence dating [2]. Furthermore quartz is applied in accident dosimetry, in that a composite material exposed to unwanted irradiation can record the amount of absorbed dose if it contains enough quantity of some natural dosimeter, typically quartz [3]. It has been possible to determine the dose distribution around some accident zones, like Hiroshima [4] and Chernobyl [5], through the measurement of the thermoluminescence of quartz inclusions in bricks and other artifacts.
http://dx.doi.org/10.1016/j.nimb.2013.09.048 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.
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Although a large number of studies has been published on the luminescence properties of crystalline quartz [6,1 and references therein], a comprehensive description of the light emissions under different stimulation is still far from being clear. The relationships between each luminescence emission and the corresponding responsible defects are generally rather tentative and the assignments given by different studies are often in contrast with one another. Even the reported energies of the emission peaks can differ also because it is emerging that many emission bands are composite [7,8], possibly by components uncorrelated with each other, which implies that, in the same energy region, one or the other component can alternatively be detected (or an overlapping of the two components), giving as a result a band at a slightly different energy. Many defects are present both in the crystalline and in the amorphous forms of SiO2, quartz and silica respectively, and are often reviewed together [9] even if their production and response to irradiation and thermal treatments are often quite different. Besides, there are defects occurring in glassy silica that are not found in quartz and vice versa [10]. As a consequence, the luminescence emissions of quartz and silica are not so similar. In this paper, firstly, a short review of recent results from radioluminescence measurements of natural and synthetic quartz will be given, showing the effect of Al-related centres on the luminescence properties of quartz. Then, new results on thermal treatments in different atmospheres will be shown, evidencing a role of Oxygen in inducing emission bands. The dependence of luminescence emissions on the temperature of annealing has also been studied and two distinct UV bands turned out to have different behaviour as a function of annealing temperature.
2. Defects in quartz and luminescence emissions In order to understand the role of the various defects in producing the different luminescence emissions in quartz, it would be necessary to correlate the defects detected by specific spectroscopic techniques to the corresponding luminescence emissions, following their behaviour as a function of irradiation and/or other treatments, typically heat treatments. A number of defects have been definitely identified in crystalline quartz by Electron Spin Resonance (ESR) [11] and it is widely agreed that two types of centres are certainly essential in the dynamics of radiative recombination: Oxygen vacancies and Al centres. The simplest type of O vacancy is the diamagnetic centre resulting from the removal of an O atom from the otherwise perfect a-quartz, leaving a direct bond between two Si atoms. This ‘‘neutral O vacancy’’ is often considered as the precursor of the paramagnetic E01 centre, which is the most recurrent intrinsic defect in quartz and can be described as a hole trapped at a neutral O vacancy. The resulting defect features an unpaired electron on one of the two Si atoms, while the other Si ion, being positively charged, moves away from the vacancy into a nearby plane configuration [12]. Al3+ ion is the main ubiquitous impurity in quartz whom many defects are associated to. It is present as a substitutional ion for Si4+ and is charge compensated by an alkali ion (M+) or by an H+ ion, giving rise to [AlO4/M+]0, [AlO4/H+]0, respectively. When neither M+ nor H+ is present near a substitutional Al3+ ion, this centre can be charge compensated by a hole, giving rise to the [AlO4]0 centre [13]. Such a defect has been reported as responsible for a recurrent luminescence emission at 365–380 nm [14], detected in many types of luminescence, in thermoluminescence (TL) below [15] and above room temperature (RT) [16], in Optically Stimulated Luminescence (OSL) [8], and in radio-luminescence (RL) [17,18].
Fig. 1. Deconvolution into Gaussian components of the radioluminescence emission spectra of (a) pegmatitic quartz (K-2-00) and (b) unswept synthetic one (G0). From Ref. [7].
Some results have been recently obtained through the analysis of RL. In Fig. 1 the RL curves of a natural and a synthetic quartz sample are reported, from [7]. It is noteworthy that the curves can be de-convoluted in the same four components: Martini et al. [7] concluded that two components participate in the blue emission, at 440 nm (2.8 eV) and at 490 nm (2.5 eV) and two emissions are present in the UV, at 360 nm (3.4 eV) and at 310 nm (3.9 eV). A contribution by a further band at 395 nm (3.1 eV) can be hypothesized to account for some discrepancies in the curve fitting. With the aim of investigating how Al centres are involved in luminescence dynamics, it is possible to compare the luminescence of quartzes containing highly different concentrations of alkali ions. By a high temperature electro-diffusion, the so called ‘‘sweeping’’ procedure, the concentration of alkali ions, that are the main charges present et al. sites in untreated quartz, can be strongly reduced and [AlO4]0, [AlO4/H+]0 centres are created. This technique allows the removal (sweep out), or the introduction (sweep in), of the ions compensating the Al centre (H+ or alkali ions) by applying a high electric field (typically 1500–2000 V/cm at 500–600 °C) along the channels running parallel to the c axis of quartz crystals [Ref. 19]. Correspondingly, evident modifications in the luminescence properties are detected. In Fig. 2 the radio-luminescence of two untreated and a swept out quartz samples are compared (from Ref. [18]). There is evidence of luminescence emissions in the blue
Please cite this article in press as: M. Martini et al., Defect studies in quartz: Composite nature of the blue and UV emissions, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2013.09.048
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Fig. 2. Radioluminescence emission spectra of synthetic quartz samples either untreated (G0 and G12) or ‘‘swept out’’ (GS12). From Ref. [18].
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Fig. 4. Sequence of 40 RL emission spectra of a pegmatitic quartz (K-2-00) collected under continuous X-ray irradiation. For better readability only some of the spectra are shown. From Ref. [7].
350–450 °C. There is agreement in the interpretation of the effect as being due to an increase in the concentration of the hole recombination centres, which should emit at 3.4 eV, as seen in Fig. 5. These have been proposed to be [H3O4]0 [21] or [AlO4]0 [14]. It is interesting to note that also in this two-step process there should be the involvement of M+ ions, because the effect is seen only in unswept quartz [22]. 3. Materials and methods Our investigation was carried out on several crystals of different origin. For the annealing in controlled atmosphere, a crystal of natural quartz, named in the text as K-2-00, was considered. The quartz comes from the Black Hills, South Dakota and formed in the last pegmatitic stage (T < 300 °C). The crystal was crushed and sieved to select grains of 100–200 lm dimension. The powder was divided into four parts, put in quartz crucibles and heated in the oven at 600 °C for 5 h in the following atmospheres: Fig. 3. Optically Stimulated Luminescence (OSL) shine down curves, at room temperature, of an untreated and a ‘‘swept out’’ synthetic quartz sample. Stimulation wavelength: 470 nm, preheat 10 s at 125 °C. Curves are taken from Ref. [19].
and in the UV regions. The most relevant effect of the ‘‘sweeping out’’ procedure is that the main UV emission only is present after eliminating the alkali ions, which is interpreted as a fundamental role of Al-related centres in the radiative recombination. Similarly, Optically Stimulated Luminescence (OSL) emission is heavily affected by the ‘‘sweeping out’’ procedure: in Fig. 3, taken from Ref. [20], a comparison of the room temperature OSL shine down curves, under 470 nm stimulation, from an untreated and a ‘‘swept out’’ quartz, is shown, evidently highlighting a difference of three orders of magnitude in the OSL intensity. While in Fig. 4 an increase of the 2.5 eV emission is detected, another band is affected by irradiation followed by heat treatments: in Fig. 5, also taken from Ref. [7], a sequence similar to the one reported in Fig. 4 is shown. The fundamental difference is that now a heating up to 500 °C follows each irradiation, in a sequence similar to what is known as ‘‘pre-dose’’. The result is a strong increase of the UV 3.4 eV band rather than the blue 2.5 eV one. Still unknown electron and hole traps are involved in the so called pre-dose effect. The effect consists in a strong increase in the efficiency of the 110 °C TSL peak, as a consequence of repeated RT irradiation followed by heating to a temperature in the range
Air. Hydrogen: fluxing a gas mixture of N2 (98%) and H2 (2%). Oxygen: in a 50 ml/min O2 flux. Vacuum: under 105 mbar pressure.
Fig. 5. Sequence of RL emission spectra of natural pegmatitic quartz (K-2-00) obtained during X-ray irradiation followed by a heating up to 500 °C at 1 °C/s. From Ref. [7].
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Fig. 6. Radioluminescence of a natural pegmatitic quartz (K-2-00) powders annealed in controlled atmosphere at 600 °C for 5 h. The reported spectra are the first one out of a sequence of 40 consecutive measurements.
In a second experiment, three different types of quartz were annealed at different temperatures. Single crystals of a natural colourless quartz, a natural smoky one, and a bar of synthetic (Sawyer Premium Quality) one, were crushed and grinded. For each quartz, the fraction of 100–200 lm grain size was selected by sieving and was divided into ten different batches. Each batch was put in a quartz crucible and annealed, in air, for 10 min in a preheated oven. The crucible was then removed from the oven and let cool to room temperature. A set of nine annealing temperatures, from 300 °C to 1100 °C, in 100 °C step, was considered. The radio-luminescence (RL) measurements were carried out at room temperature (RT) using a home-made apparatus featuring, as detection system, a charge coupled device (CCD) (Jobin–Yvon spectrum one 3000) coupled to a spectrograph operating in the 200– 1100 nm range (Jobin–Yvon Triax 180). The data were corrected for the spectral response of the detection system. RL excitation was obtained by X-rays irradiation, through a Be window, using a Philips 2274 X-ray tube with Tungsten target operated at 20 kV. During each measurement the sample was given a dose of 10 ± 2 Gy, where the uncertainty is related to the dose calibration rather than to its repeatability that was quite good (1%). RL spectra were deconvoluted into Gaussian components using the least squares method with the Levenberg–Marquardt algorithm (origin 8.0).
spectra. A similar effect is observed for a high energy emission, close to 3.9 eV, whose intensity is sensibly higher in the annealed samples compared to the untreated one. The enhancement of these two RL emissions is more evidently induced by the annealing in oxidizing atmosphere (air and Oxygen). The evolution of the quartz luminescence, under continuous X-ray irradiation, was monitored on all of the samples through a sequence of 40 successive RL measurements. The effect of the irradiation on the RL spectra is similar regardless the annealing atmosphere. As an example, the results obtained on the quartz annealed in Oxygen atmosphere are shown in Fig. 7. The blue emission around 2.7 eV is progressively reduced, at the same time, the maximum of the emission shifts towards lower energy evidencing a composite nature of the blue emission. In fact, a second band, around 2.5 eV, is enhanced by irradiation becoming quickly the dominant emission. The high energy range of the RL spectrum is not strongly modified by the X-ray irradiation. In particular, the emission close to 3.9 eV seems to be stable, whereas in the 3.0–3.5 eV region the RL emission is slightly increased by irradiation but early reaches a saturation level. Finally, an emission band below 2.0 eV, detected only in annealed samples, is enhanced by X-ray irradiation. Fig. 8 reports the RL spectra recorded at the end of the sequence. The most intense emission is peaked around 2.5 eV both in the untreated quartz and in the annealed ones and the absolute intensity of the band is similar in all of the samples, regardless the annealing atmosphere. A numerical analysis of the emission spectra was carried out following the same approach used in [7,18,24]. The RL spectra were resolved, using the Levenberg–Marquardt algorithm, into the Gaussian components reported in Table 1. All the parameters were allowed to vary within 0.01 eV from the reported value. With respect to our previous work, two more bands were introduced in the fitting process: one at 3.1 eV (FWHM 0.89 eV) and one at 3.7 eV (FWHM 0.45 eV) which were labelled as X and M band, respectively. The presence of the X band was required in order to obtain a fully satisfactory numerical analysis in the range between 3.0 eV and 3.5 eV. Indeed, low temperature RL measurements evidenced the presence of a band close to 3.1 eV in both natural and synthetic quartzes. Assuming that the contribution of this band could account for the discrepancy between the Gaussian fits and the experimental curve, we tentatively added it to the deconvolution analysis. The M band was necessary only to deconvolve the spectra of the samples annealed at temperature higher than 600 °C (see the next section).
4. Results High temperature annealing is known to strongly affect the luminescence characteristics of quartz [23]. We used RL measurements to systematically investigate the effect of both annealing atmosphere and temperature on the emission bands of quartz. 4.1. Effect of annealing atmosphere In a first experiment a set of K-2-00 powders (100–200 lm) were annealed at 600 °C for 5 h in different controlled atmosphere as described in the previous section. In Fig. 6 the RL spectra of the annealed samples are reported as a function of energy. The spectrum of the unannealed sample is also shown as a reference. Regardless the annealing atmosphere, the RL spectra are characterized by a composite emission. Two main components are found to be enhanced by the thermal treatments: an emission with an apparent maximum around 2.7 eV, and not detected in the unannealed sample, dominates the blue region of the annealed quartzes
Fig. 7. Sequence of 40 consecutive radioluminescence measurements on a natural pegmatitic quartz (K-2-00) powders annealed in Oxygen atmosphere at 600 °C for 5 h. For readability sake only three spectra are reported.
Please cite this article in press as: M. Martini et al., Defect studies in quartz: Composite nature of the blue and UV emissions, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2013.09.048
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Fig. 8. Radioluminescence of a natural pegmatitic quartz (K-2-00) powders annealed in controlled atmosphere at 600 °C for 5 h. The reported spectra are the last one out of a sequence of 40 consecutive measurements.
A detailed investigation on the irradiation effect on the intensity of the bands was carried out on the quartz annealed in Oxygen. Fig. 9 reports the area of three of the Gaussian components obtained from the numerical analysis as a function of the accumulated dose. As expected from the RL spectra, the B band progressively decreases under continuous X-ray irradiation. However, the numerical deconvolution of the spectra evidences a complex dose dependence of the A band intensity. In fact, in the first irradiation stage (up to 70 Gy approximately), the band intensity is sensibly reduced. At higher dose the band increases and, above 100 Gy, the growth becomes linear. The 3.1 eV emission (X band) is enhanced by X-ray irradiation reaching a saturation level at 100 Gy. It is interesting to note that the increase of the A band starts as soon as the X band has reached the saturation level. A similar correlation between the A and X bands is observed also in the untreated sample (Fig. 10) where, again, the growth of the A band becomes linear as soon as the X band intensity has stabilized after an early increase under irradiation. Finally, the remaining components, in particular the D band, turned out to be not significantly affected by irradiation. 4.2. Effect of annealing temperature
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Fig. 9. Dose dependence of the A (2.5 eV), B (2.8 eV) and X (3.1 eV) radioluminescence bands intensity in a natural pegmatitic quartz (K-2-00) powder annealed in Oxygen at 600 °C for 5 h.
The effect of the annealing on the RL spectra of the smoky and colourless natural quartz turned out to be qualitatively similar (Fig. 11 reports some of the spectra collected for the annealed batch of smoky quartz). In particular, the samples annealed at temperatures between 400 °C and 600 °C showed a dramatic enhancement of the C band (3.4 eV) which becomes the dominant emission component. The maximum sensitization was observed for samples annealed at 500 °C since a treatment at higher temperature caused a reduction of such emission band intensity. The sensitization of the C band was found to be most effective in the smoky quartz whereas no effect at all was observed in the synthetic one. For annealing temperature higher than 700 °C the RL spectra of both natural samples showed an increase of the emission in the 3.5– 4.0 eV range. The numerical analyses evidenced that this is due to a Gaussian component at 3.7 eV (labelled as M band) whose intensity reaches its maximum in samples annealed at 1000 °C and is reduced when the annealing temperature is further increased. The effect of annealing on the RL spectrum of synthetic quartz is significantly different from the one observed in natural samples. In particular, no evidence of either C or M band was found. Moreover no relevant modification of the RL spectrum was detected for annealing temperature below 900 °C whereas, at higher temperature, an increase of the A, B and D bands was
A second experiment was carried out in order to investigate the effect of the annealing temperature on the RL spectrum of different types of quartz. Powders obtained from single crystals of a natural colourless quartz, a natural smoky one, and a synthetic (Sawyer Premium Quality) one were considered. For each quartz type, different batches were annealed at different temperatures as described in the Materials and Methods section. A RL spectrum was then collected for each annealed sample and deconvolved into Gaussian components.
Table 1 Parameter of the Gaussian components used in the numerical deconvolution of the radioluminescence spectra of quartz samples. Band
Wavelength (nm)
Energy (eV)
FWHM (eV)
O A B X C M D
635 490 440 395 360 330 315
1.92 2.51 2.79 3.06 3.42 3.73 3.93
0.39 0.46 0.46 0.89 0.58 0.45 0.49
Fig. 10. Dose dependence of the A (2.5 eV) and X (3.1 eV) radioluminescence bands intensity in an untreated natural pegmatitic quartz (K-2-00) powder.
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Fig. 11. Radioluminescence spectra of a natural smoky quartz powders annealed in air at different temperatures for 10 min.
observed. The intensity of the 3.4 eV (C band) and 3.7 eV (M band) bands obtained from the Gaussian deconvolution are reported in Figs. 12 and 13, respectively, as a function of the annealing temperature. 5. Discussion The assignment of a luminescence emission band to a specific quartz defect has long been the weakest point in the discussion of many papers found in the literature. The unambiguous identification of a defect to a recombination centre (or to a trap) requires the combined use of different experimental techniques (for example thermoluminescence, optical absorption, photoluminescence, ESR, etc.). Unfortunately, however, matching the experimental data and obtaining their coherent interpretation is often not as straightforward. This often leaves the field to speculations that, in the long run, can be misleading in further investigations. In the case of quartz, the identification of defects involved in the luminescence dynamics is particularly tricky because of the different varieties of crystals, either natural or synthetic. Quartzes of different origin, in fact, contain different amounts of intrinsic and extrinsic ions (Al, alkali ions, and hydrogen in particular). Moreover, the relative concentration of defects can be altered by radiation and/or thermal treatment also because of high ion mobility.
Fig. 12. Annealing temperature dependence of the C (3.4 eV) radioluminecence band for three different types of quartz powders.
Fig. 13. Annealing temperature dependence of the M (3.7 eV) radioluminecence band for three different types of quartz powders.
From the data reported in this paper and in recent publications [7,18,24] some conclusions can be drawn, particularly thanks to a better defined picture of radioluminescence mechanisms. The first important result is that it has been demonstrated that all RL spectra, both in natural and in synthetic quartz, are composed of the same luminescence emissions, as shown in Fig. 1 (from Ref. [18]). Most papers in the literature report a blue emission as the main recombination centre in the TSL glow curves. We found always a blue luminescence in the RL spectra of unswept synthetic and natural quartz and clearly demonstrated that two bands, labelled as A and B, at 2.5 eV and 2.8 eV respectively, contribute to this emission. A perfect agreement between the growth with irradiation of the A RL emission and of the 110 °C TL peak has been shown, see Fig. 4 (from Ref. [7]). The 110 °C TSL peak is also increased through a procedure similar to the so called pre-dose, a series of irradiation followed by heat treatments [25,26]. This TL growth has been clearly related to the growth of a further RL emission band, the C emission at 3.4 eV, see Fig. 5. It is then clear that the same quartz crystal can alternatively strongly emit, both in RL and TL, in the blue (A band) or in the UV (C band), depending on the treatments it has been submitted to. It is not easy to correlate these changes to specific recombination centres, but an indication of the role of Al-related centres (charge compensated substitutional Al) comes from some experimental results. Figs. 2 and 3 summarize the effects due to the reduction of the concentration of alkali ions (‘‘sweeping’’) on RL and OSL emissions: it is evident that most RL bands disappear after a ‘‘sweeping’’ procedure and similarly a reduction in the intensity of OSL is seen in swept out samples. As already discussed by Halliburton et al. [13], the modifications that occur et al. centres are at the basis of most luminescence emissions. Martini et al. [14], through high temperature treatment, induced substitutional Al3+ ions to be charge compensated by a hole, giving rise to the [AlO4]0 centre. This latter has been proposed as the recombination centre responsible for the main UV emission in TSL, that has been identified as the C band, at 3.4 eV [7]. Jani et al. [27] reported a correlation between the growth of the E01 centre and the decay of the [AlO4]0 centre and observed that sweeping out the alkali ions strongly reduces the intensity of the E01 ESR signal. Poolton et al. [28] suggested that E01 may act as non-radiative centre competing in OSL with the [AlO4]0 recombination centre, while a good correlation between the growth of E01 and of [AlO4]0 was found by Usami et al. [29]. Then E01 centre has not been clearly identified in its role in indirectly contributing to the
Please cite this article in press as: M. Martini et al., Defect studies in quartz: Composite nature of the blue and UV emissions, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2013.09.048
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luminescence processes. Some indications could come from the effect of high temperature annealing in different atmosphere. Looking at Figs. 6–8 the presence of O is seen to enhance both the blue and the UV emissions. Specifically, in the blue region, both A and B bands are induced by the thermal treatments; in the UV region, the D band at 3.9 eV is mainly enhanced. Furthermore, a confirmation of the effect of irradiation came from the growth of the A band at 2.5 eV clearly seen in Fig. 8: whatever the atmosphere of treatment, after a series of irradiation this band increases of the same amount. Despite the assignment of the emission bands detected in RL to a particular defect remains matter of investigation, some interesting considerations, based on our results, can be done suggesting possible correlations that can constitute a clue for further studies. In particular, the evolution of the RL band intensities of both annealed and untreated samples under irradiation show some common features. The fact that both the A and X bands intensities evidence a clear change in the cumulative dose dependence after about 100 Gy suggests some kind of correlation between them. Unfortunately, until the exact nature of the related defects is identified, it is not possible to determine whether this correlation is due to a simple competition in charge capture or if some more complex interaction or spatial correlation is implied. Whatever the nature of the correlation between the A and X bands, however, it is interesting to notice that the B band seems not to be involved in it. The B band intensity in the annealed samples, in fact, is progressively reduced by irradiation with no evidence of a discontinuity at about 100 Gy of cumulative dose. If we now consider the RL spectra as a function of the annealing temperature, our results point out, one more time, the complexity of the mechanisms involved in quartz luminescence. In fact, not only the emission spectrum is due to the contribution of several Gaussian component, but how such bands are affected by the thermal treatments depends on the specific emission. Despite that, the set of RL measurements carried out on different types of quartz allowed to highlight the common and most relevant features. Of particular interest is how the annealing temperature affects the C band emitting at 3.4 eV. Such band was proven to be detected also in TSL and to be activated by a heating following an irradiation like in the pre-dose sequence. The fact that an annealing at 500 °C could induce the C band only in natural samples is a further indication of its extrinsic nature, being the concentration of Al and alkali ions orders of magnitude lower in synthetic quartz. A further important result is related to the presence of a new emission band in the UV region, specifically the M band emitting at 3.7 eV, in samples annealed at temperatures higher than 700 °C. As we have already observed for the so called blue emission which was found to be composite, the same holds for the UV quartz emission. At least three emission components, specifically the C, D and M band contribute to the RL emission in the high energy range. The UV quartz emission has long been reported in the literature, however, referring to the high energy emission of quartz with a not better defined ‘‘UV emission’’, could be misleading since each of the involved components is related to a different luminescent centre which is differently affected by irradiation or thermal treatments. The effect of the annealing temperature on the RL spectrum of quartz was already reported in [23] and an overall good agreement with our results is evidenced. The authors, however, report of a single UV emission centred around 360 nm whose dependence on the annealing temperature is compatible with the sum of C and M bands. It is possible that many results reported insofar on the UV emission in quartz do not always refer to the same luminescence band due to the strong overlapping of the emissions. We remind however that a luminescence band
7
detected in RL is not necessarily involved in the TSL mechanism. Further detailed investigations featuring a deconvolution of TSL spectra are thus required in order to determine whether the bands reported in Table 1, beside A and C, are detected in TSL. 6. Conclusions Although the assignment of the luminescence emissions to specific defect centres in quartz is still undefined, a thorough analysis of the RL emissions and their response to irradiation and thermal treatments turned out to be very useful in understanding many features. Specifically, the presence of the same emission bands in natural and synthetic quartz and their dependence on the presence of extrinsic impurities is a common characteristic which confirms the role of Al centres in the luminescence properties of quartz. From the measurements presented in this paper the composite nature of the ‘‘blue’’ emission is confirmed. Two bands labelled as A at 2.5 eV and B at 2.8 eV contribute to the emission in this region, their behaviour being different as a function of irradiation, the A band featuring a growth with irradiation in perfect agreement with the growth of the 110 °C TSL peak. More complex is the picture in the UV region, where besides the already detected C and D bands at 3.4 eV and 3.9 eV, respectively, two further emissions have been detected at 3.1 eV and 3.7 eV. This latter is seen to be strongly enhanced by high temperature treatments, its intensity reaching its highest value after an annealing at around 1000 °C, while it is seen that the C band increases in intensity after annealing at around 500 °C, followed by a decrease. In the light of these results a number of already known features should be reconsidered. References [1] F. Preusser, M.L. Chithambo, T. Götte, M. Martini, K. Ramseyer, E.J. Sendezera, G.J. Susino, A.G. Wintle, Earth–Sci. Rev. 97 (2009) 184. [2] M.J. Aitken, Thermoluminescence Dating, Academic Press, London, 1985. [3] S.J. Fleming, J. Thompson, Health Phys. 18 (1970) 567. [4] T. Higashimura, Y. Ichikawa, T. Sidei, Science 139 (1963) 1284. [5] I.K. Bailiff, Radiat. Meas. 24 (1995) 507. [6] K.P. Lieb, J. Keinonen, Contemp. Phys. 47 (2006) 305. [7] M. Martini, M. Fasoli, I. Villa, P. Guibert, Radiat. Meas. 47 (2012) 846. [8] M. Martini, M. Fasoli, A. Galli, Radiat. Meas. 44 (2009) 458. [9] M.A.S. Kalceff, M.R. Phillips, Phys. Rev. B 52 (1995) 3122. [10] D.L. Griscom, J. Non-Cryst. Solids 357 (2011) 1945. [11] J.A. Weil, In Defects in SiO2 and Related Dielectrics: Science and Technology, in: G. Pacchioni, L. Skuja, D.L. Griscom (Eds.), Kluwer Academic Publishers., Amsterdam, 2000, pp. 197–212. [12] J.K. Rudra, W.B. Fowler, Phys. Rev. B 35 (1987) 8223. [13] L.E. Halliburton, N. Koumvakalis, M.E. Markes, J.J. Martin, J. Appl. Phys. 52 (1981) 3565. [14] M. Martini, A. Paleari, G. Spinolo, A. Vedda, Phys. Rev. B 51 (1995) 138. [15] M. Martini, F. Meinardi, A. Vedda, Radiat. Meas. 32 (2000) 673. [16] W.J. Rink, H.M. Rendell, E.A. Marseglia, B.J. Luff, P.D. Townsend, Phys. Chem. Miner. 20 (1993) 353. [17] A. Halperin, E.W. Sucov, J. Phys. Chem. Solids 54 (1993) 43. [18] M. Martini, M. Fasoli, A. Galli, I. Villa, P. Guibert, J. Lumin. 132 (2012) 1030. [19] J.J. Martin, IEEE T. Ultrason. Ferr. 35 (1988) 288. [20] M. Martini, A. Galli, Phys. Status Solidi C 4 (2007) 1000. [21] X.H. Yang, S.W.S. McKeever, Radiat. Prot. Dosim. 33 (1990) 27. [22] M. Martini, E. Sibilia, G. Spinolo, A. Vedda, Nucl. Tracks 10 (1985) 497. [23] T. Schilles, N.R.J. Poolton, E. Bulur, L. Bøtter-Jensen, A.S. Murray, G.M. Smith, P.C. Riedi, G.A. Wagner, J. Phys. D: Appl. Phys. 34 (2001) 722. [24] V. Pagonis, M. Chithambo, R. Chen, A. Chrus´cin´ska, M. Fasoli, S.H. Li, M. Martini, K. Ramseyer, J. Lumin. 145 (2014) 38. [25] J. Zimmerman, J. Phys. C: Solid Status Phys. 4 (1971) 3265. [26] N. Itoh, D. Stoneham, A.M. Stoneham, J. Appl. Phys. 92 (2002) 5036. [27] M.G. Jani, R.B. Bossoli, L.E. Halliburton, Phys. Rev. B 27 (1983) 2285. [28] N.R.J. Poolton, G.M. Smith, P.C. Riedi, E. Bulur, L. Bøtter-Jensen, A.S. Murray, M. Adrian, J. Phys. D: Appl. Phys. 33 (2000) 72207. [29] T. Usami, S. Toyoda, H. Bahadur, A.K. Srivastava, H. Nishido, Phys. B: Condens. Matter 404 (2009) 3819.
Please cite this article in press as: M. Martini et al., Defect studies in quartz: Composite nature of the blue and UV emissions, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2013.09.048