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The effect of cadmium doping on the photoluminescence in CaS at 77 and 295 K
Journal of Crystal Growth 214/215 (2000) 939}943
The e!ect of cadmium doping on the photoluminescence in CaS at 77 and 295 K B. Ray *, J.C. Bickerton, J.W. Brightwell, I.V.F. Viney Research Division, Newman College of Higher Education, Bartley Green, Birmingham B32 3NT, UK School of Natural and Environmental Sciences, Coventry University, Priory Street, Coventry CV1 5FB, UK
Abstract Ca Cd S microcrystalline samples, prepared by sintering of the constituent compounds at 1325 K, yielded intense \V V broad band #uorescent materials at 77 and 295 K. At room temperature, excitation at 298 nm gave peak emission wavelengths which shifted from 370 to 410 nm between x"0.000365 and 0.0164 with FWHM rising from 80 to 125 nm. Excitation at 77 K in the wavelength range 250}310 nm permitted optimisation of the brightness with emission peaks at 345, 385 and 370 nm depending on the exciting wavelength and x. The role of cadmium as an emitting centre is discussed. 2000 Elsevier Science B.V. All rights reserved. PACS: 78.55.!m; 78.55.Et Keywords: Calcium cadmium sulphide; Photoluminescence; Fluorescence characteristics; Synthesis of microcrystallites; Ca Cd S \V V
1. Introduction The studies reported extend the "ndings of Lehmann [1] on the enhancement of #uorescence in microcrystalline calcium sulphide with the incorporation of cadmium. Lehmann's investigations were focused on cathodoluminescence whereas more recent studies [2}6] have been centred on photoluminescence and electroluminescence in microcrystalline Ca Cd S. These studies have \V V indicated that introduction of cadmium with x up to 0.05 resulted in immensely bright, near-white #uorescent materials at room temperature; for electroluminescence, it was necessary to introduce copper into the microcrystalline particulate layers with * Corresponding author. Tel: #44-121-476-1181 extn. 287; fax: #44-121-475-6917. E-mail address: [email protected] (B. Ray).
a consequential shift to longer wavelengths of the broad spectral emissions [5]. The investigations described here are focused on photoluminescence at 77 and 295 K in microcrystalline particulate samples of Ca Cd S with in\V V itial x-values from 0.00003 up to 0.03. Systematic analysis of the "nal cadmium content after heat treatment has con"rmed that cadmium loss can be considerable. X-ray powder di!ractometry has been used to assess phase purity.
2. Preparation and characterisation of samples 2.1. Sample preparation Ca Cd S samples, with initial x-values of 0, \V V 0.00003, 0.0001, 0.001, 0.003, 0.01 and 0.03, were prepared by accurately weighing out 0.05 mol of
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 2 5 6 - 6
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B. Ray et al. / Journal of Crystal Growth 214/215 (2000) 939}943
calcium sulphide (3N-Aldrich Chemicals) and mixing with the requisite quantity of cadmium sulphide (5N-Aldrich Chemicals) with the addition of 5 mg of sulphur. The mixtures were ground to give uniform colour and texture and then roll-milled using agate cylinders for 1 h. Each sample was transferred to a silica tube (10 mm internal diameter, 50 mm long) sealed at one end. The powder was lightly compacted and the tube capped by a close "tting sealed tube over the open end. The sample was heated from 300 to 1325 K at 50 K per minute under argon. On reaching the maximum temperature, a stream of hydrogen with added hydrogen sulphide was introduced and maintained for 2 h. Quench cooling followed under argon and the "nal samples were lightly ground and stored under argon. 2.2. Sample characterisation All samples were subject to routine analysis to determine the "nal cadmium content using a Perkin Elmer Plasma 400 ICP spectrometer. The samples, in weighed amounts, were fully dissolved in acid and compared with a set of standard solutions. Scanning electron micrographs (SEMs) were recorded for each sample to determine the distribution of particle sizes. X-ray powder di!raction traces were produced for each sample using a Philips PW 1740/1710 di!ractometer, with monochromated Cu K radiation. Samples with illa de"ned K -K resolution or traces of other phases ? ? were rejected. The photoluminescence (PL) measurements, at 77 and 295 K, were undertaken using a Perkin Elmer LS50 #uorescence spectrophotometer. Fluorescence emission spectra within the range 300}550 nm were determined by excitation between 250 and 310 nm.
3. Experimental results 3.1. Sample structure and composition X-ray powder di!raction analyses showed no evidence of any impurity phases in the samples
Table 1 Initial and "nal measured cadmium concentrations in Ca Cd S samples prepared and annealed for 2 h at 1325 K \V V and rapidly quenched to 295 K Initial Cd, x
Final Cd, x
0 0.00003 0.00010 0.00100 0.00300 0.01000 0.03000
0 0.000024 0.000052 0.000365 0.002070 0.006500 0.016400
investigated which all had the rock salt structure of CaS. K -K resolution was sharp in all samples ? ? and the only measurable shift (0.5 pm) in lattice parameter occurred for x"0.014. SEMs indicated that particle sizes were between 5 and 20 lm with a median value of 15 lm; EDAX scans indicated no aggregation of cadmium on the surface of particles. ICP spectrometric analyses con"rmed signi"cant cadmium loss during synthesis. Table 1 compares the initial cadmium concentrations with those measured after "nal processing. 3.2. Fluorescence emission spectra 3.2.1. At 295 K Fig. 1 compares the PL #uorescence emission spectra for samples with x"0, 0.000024, 0.000052, 0.000365, 0.00207, 0.0065 and 0.0164 excited at 298 nm (4.18 eV), which is about 40 nm above the band edge. At x"0.000365, the "rst signi"cant and broadish #uorescence peak at 375 nm is observed, which is shifted marginally (5}10 nm) to longer wavelengths with intensities enhanced by factors of 3 and 4 for x"0.00207 and 0.0065, respectively. The sample containing most cadmium, x"0.0164, exhibited a shift in #uorescence emission peak to 410 nm, with asymmetric broadening to longer wavelengths and FWHM of 125 nm. Table 2(a) summarises the emission peak wavelengths and their relative intensities for the samples studied at 295 K. The #uorescence emission intensity integrated over wavelength increased progressively with
B. Ray et al. / Journal of Crystal Growth 214/215 (2000) 939}943
Fig. 1. Photoluminescence emission intensity versus wavelength plots of Ca Cd S excited by 298 nm wavelength radiation at \V V 295 K. Table 2 Peak emission Ca Cd S \V V
wavelengths
and
relative
intensities
941
3.2.2. At 77 K Intensities of the brightest samples with x"0.00207, 0.0065 and 0.0164 were factors of 2.5}3 greater than observed at 295 K with identical excitation of 298 nm. There was signi"cant broadening of the emission peaks with increasing x, with the FWHM for x"0.0164 being 125 nm. A much more detailed analysis of the #uorescence data was undertaken at exciting wavelengths between 250 and 310 nm at 2 nm intervals. The results of this study are summarised in Table 2(b) whilst Fig. 2 provides contour plots of two of the brightest Ca Cd S samples at 77 K with \V V x"0.00207 and 0.0065; this contouring indicates in detail the dependence of the brightness on
in
(a) Excited by 298 nm wavelength radiation at 295 K x
j /nm
I /relative units
0 0.000024 0.000052 0.000365 0.002070 0.006500 0.016400
355 355, 355, 360, 370, 380, 410
70 85, 80 70, 60 190, 200 590, 600 660, 680 730
370 370 375 380 385
(b) Excited by di!erent wavelength radiation at 77 K x
k /nm
j /nm
I/relative units
0 0.000024
* 258 266 264 262 262 278 298 262 278 298 264 278 298
* 345 345 345 345 345 385 370 345 385 370 345 385 370
90 90 150 600 1300 1050 600 600 1100 650 650 1050 600
0.000052 0.000365 0.00207
0.0065
0.0164
cadmium content and exhibited approximately an x relationship between x"0.000365 and 0.014.
Fig. 2. Photoluminescence excitation, emission and intensity pro"les in Ca Cd S at 77 K for (a) x"0.00207, (b) x"0.0065. \V V
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B. Ray et al. / Journal of Crystal Growth 214/215 (2000) 939}943
exciting and emitting wavelengths thereby permitting optimal tuning for particular excitation/emission characteristics.
4. Discussion At room temperature, for x"0, 0.000024 and 0.000052, there is no signi"cant #uorescence under 298 nm excitation. For x"0.000365, 0.00207 and 0.0065, the #uorescence intensity grows substantially with a peak emission at around 365 nm (3.4 eV) in the ultraviolet; however, there is signi"cant broadening of the emission peak in its longwavelength tail into the blue/green for both x"0.00207 and 0.0065. Finally, for x"0.0164, the #uorescence emission peak is shifted to longer wavelengths (410 nm) and lower energies (3.02 eV) with the peak, marginally more intense but signi"cantly broader, running into the yellow at longer wavelengths. The integrated area beneath the spectral emission curve increases signi"cantly over the composition range x"0.000365}0.0164, following approximately an x relationship and pointing to the progressive involvement of cadmium in the #uorescence, albeit in a complex role; however, a signi"cant proportion of the exciting energy must be being dissipated either in delayed emission or non-radiative processes. Previous work [2}6] on the Ca Cd S system \V V suggested that the maximum intensity of emission at room temperature occurs for x between 0.025 and 0.035 whereas in this study the optimum x lies between 0.0065 and 0.0164; this could be related to di!erences in preparation techniques leading to different distribution pro"les of cadmium within the CaS. The investigations at 77 K provided both comparative information, by use of the same exciting wavelength (298 nm) as at room temperature, and some more fundamental information, from stepped excitation between 250 and 310 nm. Excitation at 298 nm gives a signi"cantly brighter #uorescence than that observed at room temperature. However, the stepped excitation between 250 and 310 nm indicates a pattern of near band edge emission growing signi"cantly with increasing x to a maximum at x"0.00207 and then declining thereafter.
At x"0.00207 signi"cant levels of emission occur for the "rst time at 385 and 370 nm [see Fig. 2 and Table 2(b)], which are optimally excited by 278 and 298 nm radiation, respectively and sustained for x"0.0065 and 0.0164. The ratio of these 385 to 370 nm #uorescence peaks remains near constant for x"0.00207, 0.0065 and 0.0164. The solubility of CdS in CaS [1] is extensive and increases sharply with increasing temperature. The lattice parameters of these solid solutions are linearly dependent on Cd concentration, decreasing with increasing Cd content in accord with the Cd substituting for Ca on the cation sites. There is no structural evidence for the Cd ions entering interstitial (tetrahedral) sites and indeed such an incorporation on a signi"cant scale should lead to an increase in lattice constant with increasing Cd content since the Cd> (tetrahedral) ion is marginally larger than the tetrahedral sites in CaS. However, there is the possibility that a small amount of Cd may enter these tetrahedral sites in association with cation vacancies stemming from the Schottky defects inherent in substances with rock salt structures. The origins of the emissions from the d Cd> ion are not clear but there is evidence of two types of behaviour, one at the very low Cd levels stemming from isolated Cd> centres, which may be related to the incorporation of an atom of di!erent electronegativity from that of Ca [7], and the other at higher Cd concentration where association of the Cd occurs. This association could be of two types (a) two or more Cd atoms in octahedral sites and (b) of Cd atoms in both octahedral and tetrahedral sites in conjunction with lattice vacancies. The concentrations of these associations would be dependent on the cation vacancy concentration which, in turn, would be dependent on thermal history and hence be modi"ed by thermal annealing/cooling processes. Such associations would exhibit their own broadband emission and absorption characteristics but would not be large enough to yield entities detectable by X-ray techniques.
5. Conclusion The investigation has indicated the critical nature of the processing and of the excitation
B. Ray et al. / Journal of Crystal Growth 214/215 (2000) 939}943
conditions in optimising and achieving high #uorescent brightness in calcium sulphides with much lower levels of cadmium incorporation than observed in earlier studies [6]. Single-phase samples have been produced by a preparative method that does not involve the use of #uxes, which would introduce impurity atoms into the structure, and allows a signi"cant formation time of several hours at 1325 K prior to quenching, which appears to have reduced the surface concentration of cadmium while increasing the cadmium incorporation onto lattice substitutional sites. There is clear scope for further optimisation of the processing conditions but the evidence from the study is that exceptionally bright UV/visible #uorescence can be obtained for x"0.002 in Ca Cd S which is a signi"cantly \V V lower concentration of cadmium than previously observed. The use of the contour plots to optimise
943
the #uorescence characteristics has enhanced the approach to photoluminescence studies in these and similar materials.
References [1] W. Lehmann, J. Lumin. 5 (1972) 87. [2] B. Ray, J.W. Brightwell, D. Allsop, A.G.J. Green, J. Crystal Growth 86 (1988) 644. [3] I.V.F. Viney, B. Ray, J.W. Brightwell, B.. Arterton, J. Crystal Growth 117 (1992) 806. [4] I.V.F. Viney, B.W. Arterton, B. Ray, J.W. Brightwell, J. Crystal Growth 138 (1994) 1055. [5] B. Ray, J.W. Brightwell, J.C. Bickerton, S. Clough, I.V.F. Viney, Electrochem. Soc. Proc. 97-29 (1998) 67. [6] B. Ray, P. Lenton, B.W. Arterton, I.V.F. Viney, J.W. Brightwell, S. Clough, J. SID 5/2 (1997) 111. [7] G. Blasse, G.J. Dirkson, M.E. Brenchly, M.T. Weller, Chem. Phys. Lett. 234 (1995) 177.
The e!ect of cadmium doping on the photoluminescence in CaS at 77 and 295 K B. Ray *, J.C. Bickerton, J.W. Brightwell, I.V.F. Viney Research Division, Newman College of Higher Education, Bartley Green, Birmingham B32 3NT, UK School of Natural and Environmental Sciences, Coventry University, Priory Street, Coventry CV1 5FB, UK
Abstract Ca Cd S microcrystalline samples, prepared by sintering of the constituent compounds at 1325 K, yielded intense \V V broad band #uorescent materials at 77 and 295 K. At room temperature, excitation at 298 nm gave peak emission wavelengths which shifted from 370 to 410 nm between x"0.000365 and 0.0164 with FWHM rising from 80 to 125 nm. Excitation at 77 K in the wavelength range 250}310 nm permitted optimisation of the brightness with emission peaks at 345, 385 and 370 nm depending on the exciting wavelength and x. The role of cadmium as an emitting centre is discussed. 2000 Elsevier Science B.V. All rights reserved. PACS: 78.55.!m; 78.55.Et Keywords: Calcium cadmium sulphide; Photoluminescence; Fluorescence characteristics; Synthesis of microcrystallites; Ca Cd S \V V
1. Introduction The studies reported extend the "ndings of Lehmann [1] on the enhancement of #uorescence in microcrystalline calcium sulphide with the incorporation of cadmium. Lehmann's investigations were focused on cathodoluminescence whereas more recent studies [2}6] have been centred on photoluminescence and electroluminescence in microcrystalline Ca Cd S. These studies have \V V indicated that introduction of cadmium with x up to 0.05 resulted in immensely bright, near-white #uorescent materials at room temperature; for electroluminescence, it was necessary to introduce copper into the microcrystalline particulate layers with * Corresponding author. Tel: #44-121-476-1181 extn. 287; fax: #44-121-475-6917. E-mail address: [email protected] (B. Ray).
a consequential shift to longer wavelengths of the broad spectral emissions [5]. The investigations described here are focused on photoluminescence at 77 and 295 K in microcrystalline particulate samples of Ca Cd S with in\V V itial x-values from 0.00003 up to 0.03. Systematic analysis of the "nal cadmium content after heat treatment has con"rmed that cadmium loss can be considerable. X-ray powder di!ractometry has been used to assess phase purity.
2. Preparation and characterisation of samples 2.1. Sample preparation Ca Cd S samples, with initial x-values of 0, \V V 0.00003, 0.0001, 0.001, 0.003, 0.01 and 0.03, were prepared by accurately weighing out 0.05 mol of
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 2 5 6 - 6
940
B. Ray et al. / Journal of Crystal Growth 214/215 (2000) 939}943
calcium sulphide (3N-Aldrich Chemicals) and mixing with the requisite quantity of cadmium sulphide (5N-Aldrich Chemicals) with the addition of 5 mg of sulphur. The mixtures were ground to give uniform colour and texture and then roll-milled using agate cylinders for 1 h. Each sample was transferred to a silica tube (10 mm internal diameter, 50 mm long) sealed at one end. The powder was lightly compacted and the tube capped by a close "tting sealed tube over the open end. The sample was heated from 300 to 1325 K at 50 K per minute under argon. On reaching the maximum temperature, a stream of hydrogen with added hydrogen sulphide was introduced and maintained for 2 h. Quench cooling followed under argon and the "nal samples were lightly ground and stored under argon. 2.2. Sample characterisation All samples were subject to routine analysis to determine the "nal cadmium content using a Perkin Elmer Plasma 400 ICP spectrometer. The samples, in weighed amounts, were fully dissolved in acid and compared with a set of standard solutions. Scanning electron micrographs (SEMs) were recorded for each sample to determine the distribution of particle sizes. X-ray powder di!raction traces were produced for each sample using a Philips PW 1740/1710 di!ractometer, with monochromated Cu K radiation. Samples with illa de"ned K -K resolution or traces of other phases ? ? were rejected. The photoluminescence (PL) measurements, at 77 and 295 K, were undertaken using a Perkin Elmer LS50 #uorescence spectrophotometer. Fluorescence emission spectra within the range 300}550 nm were determined by excitation between 250 and 310 nm.
3. Experimental results 3.1. Sample structure and composition X-ray powder di!raction analyses showed no evidence of any impurity phases in the samples
Table 1 Initial and "nal measured cadmium concentrations in Ca Cd S samples prepared and annealed for 2 h at 1325 K \V V and rapidly quenched to 295 K Initial Cd, x
Final Cd, x
0 0.00003 0.00010 0.00100 0.00300 0.01000 0.03000
0 0.000024 0.000052 0.000365 0.002070 0.006500 0.016400
investigated which all had the rock salt structure of CaS. K -K resolution was sharp in all samples ? ? and the only measurable shift (0.5 pm) in lattice parameter occurred for x"0.014. SEMs indicated that particle sizes were between 5 and 20 lm with a median value of 15 lm; EDAX scans indicated no aggregation of cadmium on the surface of particles. ICP spectrometric analyses con"rmed signi"cant cadmium loss during synthesis. Table 1 compares the initial cadmium concentrations with those measured after "nal processing. 3.2. Fluorescence emission spectra 3.2.1. At 295 K Fig. 1 compares the PL #uorescence emission spectra for samples with x"0, 0.000024, 0.000052, 0.000365, 0.00207, 0.0065 and 0.0164 excited at 298 nm (4.18 eV), which is about 40 nm above the band edge. At x"0.000365, the "rst signi"cant and broadish #uorescence peak at 375 nm is observed, which is shifted marginally (5}10 nm) to longer wavelengths with intensities enhanced by factors of 3 and 4 for x"0.00207 and 0.0065, respectively. The sample containing most cadmium, x"0.0164, exhibited a shift in #uorescence emission peak to 410 nm, with asymmetric broadening to longer wavelengths and FWHM of 125 nm. Table 2(a) summarises the emission peak wavelengths and their relative intensities for the samples studied at 295 K. The #uorescence emission intensity integrated over wavelength increased progressively with
B. Ray et al. / Journal of Crystal Growth 214/215 (2000) 939}943
Fig. 1. Photoluminescence emission intensity versus wavelength plots of Ca Cd S excited by 298 nm wavelength radiation at \V V 295 K. Table 2 Peak emission Ca Cd S \V V
wavelengths
and
relative
intensities
941
3.2.2. At 77 K Intensities of the brightest samples with x"0.00207, 0.0065 and 0.0164 were factors of 2.5}3 greater than observed at 295 K with identical excitation of 298 nm. There was signi"cant broadening of the emission peaks with increasing x, with the FWHM for x"0.0164 being 125 nm. A much more detailed analysis of the #uorescence data was undertaken at exciting wavelengths between 250 and 310 nm at 2 nm intervals. The results of this study are summarised in Table 2(b) whilst Fig. 2 provides contour plots of two of the brightest Ca Cd S samples at 77 K with \V V x"0.00207 and 0.0065; this contouring indicates in detail the dependence of the brightness on
in
(a) Excited by 298 nm wavelength radiation at 295 K x
j /nm
I /relative units
0 0.000024 0.000052 0.000365 0.002070 0.006500 0.016400
355 355, 355, 360, 370, 380, 410
70 85, 80 70, 60 190, 200 590, 600 660, 680 730
370 370 375 380 385
(b) Excited by di!erent wavelength radiation at 77 K x
k /nm
j /nm
I/relative units
0 0.000024
* 258 266 264 262 262 278 298 262 278 298 264 278 298
* 345 345 345 345 345 385 370 345 385 370 345 385 370
90 90 150 600 1300 1050 600 600 1100 650 650 1050 600
0.000052 0.000365 0.00207
0.0065
0.0164
cadmium content and exhibited approximately an x relationship between x"0.000365 and 0.014.
Fig. 2. Photoluminescence excitation, emission and intensity pro"les in Ca Cd S at 77 K for (a) x"0.00207, (b) x"0.0065. \V V
942
B. Ray et al. / Journal of Crystal Growth 214/215 (2000) 939}943
exciting and emitting wavelengths thereby permitting optimal tuning for particular excitation/emission characteristics.
4. Discussion At room temperature, for x"0, 0.000024 and 0.000052, there is no signi"cant #uorescence under 298 nm excitation. For x"0.000365, 0.00207 and 0.0065, the #uorescence intensity grows substantially with a peak emission at around 365 nm (3.4 eV) in the ultraviolet; however, there is signi"cant broadening of the emission peak in its longwavelength tail into the blue/green for both x"0.00207 and 0.0065. Finally, for x"0.0164, the #uorescence emission peak is shifted to longer wavelengths (410 nm) and lower energies (3.02 eV) with the peak, marginally more intense but signi"cantly broader, running into the yellow at longer wavelengths. The integrated area beneath the spectral emission curve increases signi"cantly over the composition range x"0.000365}0.0164, following approximately an x relationship and pointing to the progressive involvement of cadmium in the #uorescence, albeit in a complex role; however, a signi"cant proportion of the exciting energy must be being dissipated either in delayed emission or non-radiative processes. Previous work [2}6] on the Ca Cd S system \V V suggested that the maximum intensity of emission at room temperature occurs for x between 0.025 and 0.035 whereas in this study the optimum x lies between 0.0065 and 0.0164; this could be related to di!erences in preparation techniques leading to different distribution pro"les of cadmium within the CaS. The investigations at 77 K provided both comparative information, by use of the same exciting wavelength (298 nm) as at room temperature, and some more fundamental information, from stepped excitation between 250 and 310 nm. Excitation at 298 nm gives a signi"cantly brighter #uorescence than that observed at room temperature. However, the stepped excitation between 250 and 310 nm indicates a pattern of near band edge emission growing signi"cantly with increasing x to a maximum at x"0.00207 and then declining thereafter.
At x"0.00207 signi"cant levels of emission occur for the "rst time at 385 and 370 nm [see Fig. 2 and Table 2(b)], which are optimally excited by 278 and 298 nm radiation, respectively and sustained for x"0.0065 and 0.0164. The ratio of these 385 to 370 nm #uorescence peaks remains near constant for x"0.00207, 0.0065 and 0.0164. The solubility of CdS in CaS [1] is extensive and increases sharply with increasing temperature. The lattice parameters of these solid solutions are linearly dependent on Cd concentration, decreasing with increasing Cd content in accord with the Cd substituting for Ca on the cation sites. There is no structural evidence for the Cd ions entering interstitial (tetrahedral) sites and indeed such an incorporation on a signi"cant scale should lead to an increase in lattice constant with increasing Cd content since the Cd> (tetrahedral) ion is marginally larger than the tetrahedral sites in CaS. However, there is the possibility that a small amount of Cd may enter these tetrahedral sites in association with cation vacancies stemming from the Schottky defects inherent in substances with rock salt structures. The origins of the emissions from the d Cd> ion are not clear but there is evidence of two types of behaviour, one at the very low Cd levels stemming from isolated Cd> centres, which may be related to the incorporation of an atom of di!erent electronegativity from that of Ca [7], and the other at higher Cd concentration where association of the Cd occurs. This association could be of two types (a) two or more Cd atoms in octahedral sites and (b) of Cd atoms in both octahedral and tetrahedral sites in conjunction with lattice vacancies. The concentrations of these associations would be dependent on the cation vacancy concentration which, in turn, would be dependent on thermal history and hence be modi"ed by thermal annealing/cooling processes. Such associations would exhibit their own broadband emission and absorption characteristics but would not be large enough to yield entities detectable by X-ray techniques.
5. Conclusion The investigation has indicated the critical nature of the processing and of the excitation
B. Ray et al. / Journal of Crystal Growth 214/215 (2000) 939}943
conditions in optimising and achieving high #uorescent brightness in calcium sulphides with much lower levels of cadmium incorporation than observed in earlier studies [6]. Single-phase samples have been produced by a preparative method that does not involve the use of #uxes, which would introduce impurity atoms into the structure, and allows a signi"cant formation time of several hours at 1325 K prior to quenching, which appears to have reduced the surface concentration of cadmium while increasing the cadmium incorporation onto lattice substitutional sites. There is clear scope for further optimisation of the processing conditions but the evidence from the study is that exceptionally bright UV/visible #uorescence can be obtained for x"0.002 in Ca Cd S which is a signi"cantly \V V lower concentration of cadmium than previously observed. The use of the contour plots to optimise
943
the #uorescence characteristics has enhanced the approach to photoluminescence studies in these and similar materials.
References [1] W. Lehmann, J. Lumin. 5 (1972) 87. [2] B. Ray, J.W. Brightwell, D. Allsop, A.G.J. Green, J. Crystal Growth 86 (1988) 644. [3] I.V.F. Viney, B. Ray, J.W. Brightwell, B.. Arterton, J. Crystal Growth 117 (1992) 806. [4] I.V.F. Viney, B.W. Arterton, B. Ray, J.W. Brightwell, J. Crystal Growth 138 (1994) 1055. [5] B. Ray, J.W. Brightwell, J.C. Bickerton, S. Clough, I.V.F. Viney, Electrochem. Soc. Proc. 97-29 (1998) 67. [6] B. Ray, P. Lenton, B.W. Arterton, I.V.F. Viney, J.W. Brightwell, S. Clough, J. SID 5/2 (1997) 111. [7] G. Blasse, G.J. Dirkson, M.E. Brenchly, M.T. Weller, Chem. Phys. Lett. 234 (1995) 177.