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9Химия твердых веществ и нанотехнология
УДК 666.22 M. Abdelghany, Е.В. Колобкова, М.М. Сычёв
ВЛИЯНИЕ
ТЕРМООБРАБОТКИ НА ОПТИЧЕСКИЕ СВОЙСТВА ФТОРФОСФАТНЫХ СТЕКОЛ С НАНОКРИСТАЛЛАМИ CuCl
Санкт-Петербургский государственный технологический институт (технический университет), Московский пр., 26, Санкт-Петербург, 190013, Россия e-mail: [email protected]
Изучено формирование нанокристаллов CuCl в матрице фторфосфатного стекла в процессе термической обработки при трех различных температурах. Радиус нанокристаллов CuCl рассчитывали из спектров поглощения в соответствии с эффектом квантового ограничения. Установлено, что радиус нанокристаллов CuCl линейно возрастает от 1 до 4 нм с увеличением времени и температуры термообработки. Изучены спектры люминесценции легированных CuCl фторфосфатных стекол до и после термообработки. Показано, что люминесценция обусловлена ионами Cu+ и молекулярными кластерами (CuCl)n.
Ключевые слова: нанокристаллы CuCl, фторфосфат-ные стекла, эффект квантового ограничения, молекулярные кластеры
M. Abdelghany1, E.V. Kolobkova2, M. M. Sychov3
EFFECT OF HEAT TREATMENT ON THE OPTICAL PROPERTIES OF FLUOROPHOSPHATE GLASS WITH CuCl NANOCRYSTALS
St Petersburg State Institute of Technology (Technical University), Moskovsky Pr., 26, St Petersburg, 190013, Russia e-mail: [email protected]
Formation of CuCl nanocrystals in the fluorophosphate glass matrix during the heat treatment (H.T.) process at three different temperatures was studied. The radius of CuCl nanocrystals was calculated from the absorption spectra according to the quantum confinement effect. CuCl nanocrystals radius is linearly increase from 1 to 4 nm with increases of the time and temperature of heat treatment. The luminescence spectra of the fluorophosphate glass doped with CuCl before and after heat treatment have been investigated. It is shown that the luminescence is due to ions Cu+ and molecular clusters (CuCl)n.
Keywords: CuCl nanocrystals, fluorophosphates glass, quantum confinement effect, molecular clusters.
DOI 10.15217/issn1998984-9.2016.33.19
Introduction
In recent years, the interest in inorganic glasses and polymers, containing copper (I) or sliver ions and their chemical compounds has been constantly increasing due to their unique optical properties [1]. Glass consisted from glass matrix and a semiconductor nanocrystal promises the material for producing optical switches, limiters and optical information processing. CuCl is a semiconductor which as a zinc-blende crystal structure at room temperature. It is a direct transition material and its energy gap is about 3.395 eV at 4 K. Since the Bohr radius of an exciton is 0,7 nm and the binding energy is about 190 meV, the exciton structure is very stable in CuCl [2] and observed at room temperature. When the grain-size of a semiconductor crystal diminished to its Bohr radius, it will exhibit quantum confinement effect [3]. CuCl crystals have not optical absorption bands in the visible spectral range, however, they exhibit pronounced two exciton absorption bands separated by spin-orbit splitting appear in the near UV region, one band is a doublet and the other is a single and are called Zi,2 and Z3, respectively [4]. Currently, CuCl nanocrystals segregate in glasses of borate and silicate matrices [5-8]. CuCl nanocrystals are formed in the fluorophosphate glass by addition of HCl to play the role of external chlorine ions
[9, 10]. Cu+ ions in oxide glass are well known to produce blue emission under UV excitation (250-260 nm) [11]. Cu+-Cu+ dimmers have a wide luminescent band in spectral region 530-590 nm when excited by 350-370 nm. However, copper halide nanocrystals have exciton and bi-exciton luminescence in UV region only at cryogenic temperatures [12, 13], the molecular clusters of copper (Cu2O)n and (CuCl)n have luminescence band at 580 nm, Such glasses, borate and phosphate glasses containing these clusters can be used in temperature sensors [14].
The purpose of this study was to confirm the formation and growth of CuCl nanocrystals in the fluorophosphate during heat treatment and determine its radius and investigate the nature of the nucleating centers and its influence on the luminescence spectra.
Experimental
Glass of the composition NaPO3-Ba(PO3)2-AlF3 was selected to be a glass matrix in which the CuCl nanocrystals formed. Three glass samples were studied: glass 1 without additives, glass 2 with 1,72 mol. % CuCl and glass 3 with 1,72 mol. % CuCl and 0,688 mol. % NaCl. The high temperature melt-quenching method for glass formation was used. Synthesis was carried out in air atmosphere for 30 min. at a temperature 900 °C in glass carbon crucible. After synthesis, glass was annealed at Tg during 30 min
1 M. Abdelghany, post-graduate student, Theory of Materials Science Department e-mail: [email protected] M. Abdelghany аспирант, каф. теоретических основ материаловедения
2 Elena V. Kolobkova, Dr. Sci. (Chem.), Professor, Theory of Materials Science Department, e-mail: [email protected] Колобкова Елена Вячеславовна - д-р хим. наук, профессор каф. теоретических основ материаловедения
3 Maxim M. Sychov Dr. Sci. (Eng.), Professor, Theory of Materials Science Department, e-mail: [email protected] Сычёв Максим Максимович, д-р техн. наук, профессор каф. теоретических основ материаловедения
Received February, 04 2016
and quenched to room temperature. Colorless glass samples in form of plates were polished with thickness in range 0,3-0,6 mm. The colorless samples indicates the absence (or small amount) of copper divalent (Cu2+ ions). Heat treatment process of glass samples at a temperature 330, 350, 370 and 390 °C was carried out from 10 to 60 min. step 10 min in muffle furnace. The glass absorption spectra were recorded at room temperature using a spectrophotometer SF-56, LOMO, St. Peterburg in wavelength range of 1901100 with step 0,5 nm. The luminescence spectra at different excitation wavelengths were recorded using MPF-44A spectrofluorimeter (Perkin Elmer).
Results and Discussion
Due to the quantum confinement effect in exciton energy states, the energy of these states is expressed in the following equation [15]:
where Eg is the band gap energy of bulk crystal, EB is the binding energy of exciton, M is the effective mass of exciton and R is the average radius of the CuCl nanocrystals. We know that, the energy of the exciton in CuCl nanocrystals becomes larger than that in the bulk crystal, this difference is equal to AE = E - (Eg + EB). Due to this difference, the absorption band of the exciton in CuCl nanocrystals shifted to the higher wavelength (red shift). After estimation this difference, the CuCl nanocrystals can be calculate using equation (1).
Figure 1 shows the room temperature absorption spectra of the glass without doping and initial glasses 1and 2. It can be seen that, the absorption edge of the glass with CuCl (curve 2, 3) is shifted to higher wavelength about 300 nm in compared to absorption edge of the glass without doping, which is associated with Cu+ ions distributed in the glass [16]. The addition of SnO gives the glass colorless, because the most of Cu2+ ions goes to the monovalent state. Addition NaCl to the glass due to additional Cl- ions also decreases Cu2+ ions concentration [14].
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Two glasses were investigated to study the influence of the doping of the fluorophosphate glasses on the forms of CuCl nanocrystals. The fluorophosphate glass dopes with CuCl (glass 2) was subjected to H.T. process at different temperature 350, 370 and 390 oC for time up to 60 min. There are no characteristic absorption bands of CuCl nanocrystals. By other words, the exciton Z12 & Z3 bands are not observed in the absorption spectra of the glass 2 samples before and after H.T. According to the authors [6, 9, 10], the formation of CuCl nanocrystals were achieved in silicate, borate and phosphate glass matrix by addition of copper with chlorine ions (Cu2O-NaCl) or CuCl-HCl, in addition to the formation of CuCl nanocrystals in NaCl matrix [18]. Samples of glass 3 containing CuCl and NaCl with ratio 1/0.4 were subjected to heat treatment at three different temperatures. The glass sample was subjected to H.T. at 350 oC for 10, 20, 30, 40, 50 and 60 min. The absorption spectra are presented in figure 2. An increase in the time of HT results in an increase in absorption of the Zi,2 exciton band. After 10 and 20 min no noteworthy changes in the absorption spectrum can observe because the energy absorbed during these small times of H.T. is not adequate to form the crystalline phase but may be enough to form molecular clusters (CuCl)n [17]. After HT during 30min the absorption spectrum exhibits an increase in Z12 exciton band. Also, the absorption coefficient increases to value 69 cm-1 after 60 min, the broadness of this absorption band is decrease and the maximum is a shift to high wavelength.
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Figure 1. The absorption spectra of the fluorophosphate glasses at room temperature: 1 - without CuCl and NaCl; 2 - with 1,72 mol % CuCl;
3 - with 1,72 mol % CuCl-0,688 mol % NaCl
There are most of the copper ions in the form of monovalent by reduction due to reaction during the melting process. In potassium-aluminum borate glass [17], which contains Cu2O and NaCl, was formed molecular clusters (Cu2O)n and (CuCl)n, these clusters exhibit strong luminescence in the visible spectra upon excitation by UV.
Figure 2. The room temperature absorption spectra of the fluorophosphate glass before (dashed line) and after the heat treatment at 350 °C 1 - for 10, 2 - 20, 3 - 30, 4 - 40, 5 - 50 and 6 - 60 min.
Figure 3 illustrates the changes that occur in the absorption spectra of the glass 3 during H.T. at 370 °C for different times up to 60 min. The Z12 exciton band starts to grow after 30 min, where this means the CuCl nanocrystals start to form at this time and growth with increase the time of H.T.. However, the absorption coefficient is increase with increase the time of H.T., the broadness of the Z12 exciton band is decrease and its maximum is shifted to high wavelength. The same features of the absorption spectra are observed with H.T. at 390 °C as in figure 4 for the same time interval of 350 and 370 °C.
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Figure 3. The room temperature absorption spectra of the fluorophosphate glass before (dashed line) and after the heat treatment at 370 °C 1 - for 10, 2 - 20, 3 - 30,4 - 40, 5 - 50 and 6 - 60 min.
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From analysis of figures 2, 3 and 4 gives the absorption coefficient at max. Z12 exciton band of the CuCl nanocrystals absorption band in range 350-385 nm, where the results are presented in figure 5. There are shown that, the rate and the amount of CuCl nanocrystals formation in the glass matrix are directly depends on each of the time and the temperature of H.T. As in figure 6, The energy position of Z12 exciton band are shifted to longer wavelength. This shift is a signature of the quantum confinement effect, using the equation (1) with the values of the quantum confinement parameters can determine the variation of the average radius of CuCl nanocrystals from the Z12 exciton band maximum position in the absorption spectra after H.T. at different times and temperatures.
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Figure 4. The room temperature absorption spectra of the fluorophosphate glass before (dashed line) and after the heat treatment at 390 oC 1 - for 10, 2 - 20, 3 - 30,4 - 40, 5 - 50 and 6 - 60 min.
From the comparison between the three temperatures H.T. Absorption spectra, the following point can be listed:
The three temperatures are suitable to form and growth the CuCl nanocrystals in the fluorophosphate glass matrix, however the H.T. at 330 °C for the same times did not achieved any absorption band in the range 300-400 nm.
The molecular clusters (CuCl)n are formed in glass matrix during melting process and initial time of H.T. and these clusters play the role of the crystallization and growth centers for the CuCl nanocrystals.
The formation of CuCl nanocrystals start at small time for high temperature or long time for less temperature, that means, the CuCl nanocrystals formation is a function of the energy absorbed for the crystallization of (CuCl)n molecular clusters.
The spectra exhibit wavelength shift (red shift) of the Zi,2 exciton band maximum to longer wavelength which is associated with the growth of the average radius of CuCl nanocrystals with more time of H.T.
Not only the Z12 exciton band maximum is shifted, but also the absorption coefficient value is increase, this is caused by increase the amount of CuCl nanocrystals formed in glass matrix.
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Figure 6. The max. Zi,2 exciton CuCl nanocrystals position at different temperatures.
The variation in the CuCl nanocrystals radius at different growth temperatures are presented in figure 7. The rate of growth CuCl nanocrystals is different, where this rate increases with the higher temperature. The critical radius of CuCl nanocrystals- at the beginning of the formation- is above 1 nm, where the critical radius of CuCl nanocrystals was achieved before in glass by Valov et.al. [16] with addition of 2 NaCl and CuO at 500 °C of H.T. for 200 min. in the same value. During the H.T. process, the radius of the nanocrystals increases from 1.2 to 1.6 nm, 1.7 to 2.7 nm and 1.95 to 3.55 nm at 350, 370 and 390 °C, respectively. in the same
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Figure 7. The average radius of CuCl nanocrystals in glass dependence on the time of H.T. at different temperatures.
The CuCl nanocrystals in the fluorophosphate glass are formed by the crystallization process of the nucleation centers (CuCl)n molecular clusters. The possibility of the existence of these clusters has been confirmed in borate glass experimentally [17]. To verify this hypothesis, the luminescence spectra of the glass before and after H.T. for 60 min./390 °C which is considered the highest amount of CuCl nanocrystals in the studied samples as discussed above. The measurement results are presented in figure 8 and 9. Before H.T., the irradiation by 250350 nm induced two pronounced luminescence bands with the maximum on 460 and 580 nm, depends on the excitation wavelength, which means, suggested luminescence centers of two types are in the fluorophosphate glass. It can be seen that, the first luminescence band at 460 has maximum intensity at Aex = 250 nm, however, moving towards Aex = 350 nm, the second band become that centered at 580 nm. According to [1-3], the first band is due to the Cu+ ions, while the second band in the long wavelength regions is due to the molecular clusters (CuCl)n.
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Figure 8. The haminescence spectra of the fluorophosphate glass before H.T. (The numbers indicate the excitation wakeleght).
After H.T at 390 °C, the luminescent intensity as figure 9 decreases and the second band which centers at 580 nm disappeared and replaced by band centered at 550 nm. It means there are more than two luminescent centers in the fluorophosphate glass.
Figure 9. The luminescence spectra of the fluorophosphate glass after H.T. for 60 min. at 390 °C(the numbers indicate the excitation wavelength).
As were noted above. The addition of NaCl with CuCl to fluorophosphate glass achieves the formation and growth of molecular clusters (CuCl)n during melting process and initial times of H.T. after reaching the critical size, then transformed to CuCl nanocrystals during H.T.. Therefore, the luminescence of these clusters in the visible region disappears. While CuCl nanocrystals do not contribute to the luminescence band at room temperature.
Conclusion
In this work, we succeeded to form molecular clusters (CuCl)n in the fluorophosphate glass matrix by addition of CuCl and NaCl during the melting and initial heat treatment processes. This glass has a bright luminescence in the visible spectral range due to the Cu+ ions and the (CuCl)n molecular clusters. The heat treatment of fluorophosphate glass produced crystallization of molecular clusters (CuCl)n to CuCl nanocrystals with radius 1-4 nm. The shift of the exciton band of CuCl toward high wavelength is due to the quantum confinement effect. The formation of CuCl nanocrystals decreases the luminescence due to the consuming of one of the luminescent centers, (CuCl)n molecular clusters.
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