J. Phys. Chem. 1992,96, 85-89
85
High-pressure Studies of Photoluminescence and Thermoluminescence of 2nS:Cu:CI Phosphors Using Laser Selection Excitation J. M. Lang, Z. A. Dreger; and H. G . Drickamer* School of Chemical Sciences, Department of Physics and Materials Research Laboratory, University of Illinois at Champaign-Urbana, Urbana, Illinois 61801 (Received: July 10, 1991)
We have measured the effect of pressure on the luminescence and thermoluminescence characteristics of three ZnS phosphors doped with Cu and C1. The total dopant concentration varied in the ratio 1/1.5/2.5, but the CI/Cu ratio was constant at -8/1. The excitation was via the 325-nm (3.82-eV) line of a He-Cd laser. The absorption edge of ZnS is at 3.67 eV at 1 atm and increases by 6.35 meV/kbar, so that at high pressure the excitation is to ‘deep levels” provided by the C1. The emission consists of two peaks at 20 000 and 22 000 cm-’ assigned to emission from a C1- center to a Zn vacancy and a Cu+ center, respectively. Both emission peaks increased in energy with pressure. The shifts could be explained in terms of the degree of pinning of donor and acceptor levels to the conduction and valence bands. The thermoluminescence data indicated an initial trap depth of 0.3 eV for all three samples. This trap depth increased with pressure at low pressures but leveled at 0.39,0.46, and 0.54 eV for the high-, intermediate-, and low-concentration samples, respectively. The thermoluminescence intensity decreased by 2 orders of magnitude in 40 kbar. The behavior of the thermoluminescence could be explained largely by differences in the nature and behavior of the “deep levels” initially in the conduction band.
Introduction ZnS phosphors, especially those doped with Cu and C1, are widely used in various luminescent displays. The emission is characterized as “green“ or “blue” depending on the relative intensity of two peaks at 20000 and at 22000 cm-I. In order to characterize better the states involved and the possible modes of excitation and emission, we have studied the effect of pressure on the photoluminescent and thermoluminescent properties of three ZnS phosphors with accurately established composition. In addition to the practical aspects of ZnS phosphors, they have formed the basis of much of our understanding of donoracceptor in phosphors and the nature of “shallow levels”? Le. levels which are, to a significant degree, pinned to the conduction or valence band.4 In this paper we present the effect of pressure on the photoluminescent and thermoluminescent behavior of three wellcharacterized phosphors using excitation from a He-Cd laser at 352 nm (3.82 eV). Since the absorption edge at 1 atm is 3.67 eV and shifts to higher energy with pressure, we excite at low pressures from the valence band to the conduction band and at higher pressures to the “deep levels” initially buried in the conduction band. Our study permits one to understand better the nature of the “deep levelsn5and of the trapping states in the gap.
Experimental Procedure Both photoluminescence and thermoluminescence experiments were performed in a triangular (Merrill-Bassett) diamond anvil cell (DAC). The ruby fluorescence method for pressure calibration was used.6 The Cu-doped materials were used as obtained. The sources of the materials and their compositions are presented in Table I. For both the photoluminescence and thermoluminescence experiments the excitation was performed by an Omnichrome Model 4056-4 laser emitting a line a t 325 nm (3.82 eV). Thermoluminescence was obtained in the range from 77 to 300 K. The samples were excited by laser for 2 min at 77 K. The details of the thermoluminescence apparatus have been described elsewhere.’ For photoluminescence the front-side emission was focused on the slit of a Kratos Model 252 monochromator (0.25 m) and then passed via a light pipe to a cooled EM1 9558QA photomultiplier tube and an Ortec photon counting system. The spectra were collected point by point using an online computer and corrected for the efficiency of the monochromator and photomultiplier. On leave from Department of Molecular Physics, Tech. University of Gdansk, Gdansk, Poland.
TABLE I: ZnS:Cu:CI Samples Studiedo ZnS:Cu:Clb CI cu blue 860 (0.23 mol %) 205 (0.03 mol %) blue-green 1330 (0.36 mol %) 323 (0.05 mol %) 516 (0.08 mol %) green 2000 (0.55 mol %) ’In ppm by weight. *The samples were synthesized by the Sylvania Division of GTE and analyzed by the Micro-analytical Laboratory of the School of Chemical Science at the University of Illinois. All runs were made at least in duplicate. The scatter of the points gives an indication of the precision and reproducibility of data.
Photoluminescence. ( I ) Peak Location and Half Width. At atmospheric pressure, two luminescence emission peaks at 22 000 and 20 000 cm-I are observed for all three ZnS:Cu:Cl samples. Figure l a - c illustrates typical spectra. The luminescence peaks shift to higher energy with increasing pressure. The results are illustrated in Figure 2. The higher energy (HE) peaks for all three samples shift in the same way as the absorption edge (see Discussion below) in the low-pressure range, but the rate of shift decreases at higher pressures. This effect is most noticeable for the high-concentration (green) sample. The lower energy (LE) peaks for all three samples shift linearly with increasing pressure (approximately 6.5 meV/kbar, 1 kbar = lo-’ GPa). The halfwidths (full width at half-maximum, fwhm) of both peaks for all three samples are almost the same. The half-widths of H E peaks and LE peaks are 2250 and 3550 cm-I, respectively, and are nearly constant with pressure. ( 2 ) Intensity. The intensities of the photoluminescence emission peaks for three ZnS:Cu:Cl samples decrease with increasing pressure, by almost half an order of magnitude in 40 kbar. The relative intensities between LE peaks and H E peaks for all three samples are almost independent of pressure. The results are shown (1) Apple, E. F.; Williams, F. E. J . Electrochem. Sot. 1959, 106, 224. (2) Thomas, D. G.; Hopfield, J. J.; Augustyniak, W. M. Phys. Rev. A 1965, 140, 202. (3) Mandel, G. Phys. Reu. 1964, 134, A1073. (4) Drickamer, H. G. J . Lumin. 1990, 47, 1. (5) Hjalmarson, H. P.; Vogl, P.; Wolford, D.J.; Dow, J. D. Phys. Reu. Lett. 1980, 44, 810.
(6) Mao, H. K.; Bell, P. M.; Shaner, J. W.; Steinberg, D. J. J . Appl. Phys.
1978, 49, 3276. (7) Dreger, Z.
A.; Lang, J. M.; Drickamer, H. G. J . Lumin., in press.
0022-3654/92/2096-85%03.00/0 0 1992 American Chemical Society
86 The Journal of Physical Chemistry, Vol. 96, No, 1, 1992
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in Figures 3 and 4,respectively. Thermoluminescence. ( 1 ) Glow Peak Location. The typical thermoluminescence glow curves for three ZnS:Cu:Cl samples for a heating rate of 0.74K/s are shown in Figures 5-7. Smoothed experimental data for the glow maxima T , at various pressures are listed in Table 11, for two different heating rates of 0.74and 0.29 K/s. The glow maxima T , for three samples, at low pressure, are almost the same. With increasing pressure, the thermoluminescence peaks shift rapidly to higher temperature in the pressure range 0-15 kbar. But with further increase of pressure, the behavior of the glow maxima T , with pressure for three samples is different. The thermoluminescence glow peaks of the green, blue-green, and blue samples do not shift above -225, -250, and -270 K, respectively, for a 0.74 K/s heating rate. (2) Intensity. The intensities of the thermoluminescence for three ZnS:Cu:Cl samples decrease drastically with increasing pressure. Figure 8 illustrates these results. As the pressure increases from 0 to 50 kbar, the emission peaks of three ZnS:Cu:Cl
Figure 5. Thermoluminescence glow curves for the ZnS:Cu:Cl (blue) sample at different pressures: (a) 0, (b) 8, (c) 17, (d) 26, (e) 36, and (f) 43 kbar. (The 0 kbar intensity has been divided by 4.)
samples decrease in intensity by almost a factor of 100. The intensity changes of the three ZnS:Cu:Cl samples with pressure are similar.
Discussion 1. 2ns Optical Absorption Edge llnder P " . The absorption edge of ZnS has been measured by a number of investigators over
The Journal of Physical Chemistry, Vol. 96, No. I, 1992 81
Luminescence Studies of ZnS:Cu:Cl TABLE 11: Tbermdumhsceace Data Summa@ p , kbar 0 2 8 11 13 17 18 25 26 32 36 43
ZnS:Cu:Cl (blue) Tml9 K
Tm2. K
156.4 164.2 179.5 191.0 199.0 213.6 213.6 239.5 244.8 266.6 268.7 269.0
150.5 158.0 173.2 184.1 191.9 206.0 206.3 23 1.O 236.0 257.0 259.0 259.4
ZnS:Cu:Cl (blue-green) P, kbar Tmli K Tmz, K 0 2 7 12 18 25 26 30 35 42 44
157.0 164.1 179.4 200.0 222.5 241.0 240.0 246.0 248.0 247.6 248.0
ZnS:Cu:CI (green) p , kbar Tmly K Tm29 K 0 2 6 9 13 18 25 28 34 43 50
151.0 158.0 173.0 193.0 214.5 232.0 231.5 236.6 238.6 238.0 238.5
157.1 165.4 178.0 189.0 206.5 215.0 225.1 224.3 225.1 225.0 224.3
151.0 159.1 171.3 181.7 198.7 206.3 216.0 215.1 215.9 215.7 215.0
T,, and Tm2are the glow maxima at El (0.74 K/s) and B2 (0.29 K/s) heating rates, respectively.
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Figure 8. Thermoluminescence intensity versus pressure for three ZnS:Cu:CI samples: ( 0 ) the blue sample, (0)the blue-green sample, (*) the green sample.
We use the latter vlaues in this paper. I The excitation energy of the He-Cd laser 325 nm (3.82 eV)
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the past 30 years.8-’0 The most recent and probably the most reliable vlaues are those of Beliveau and Carlone” Egap= 3.641 + 6.7 X lO-’p - 20 X 104p2 (1) where E is in electronvolts and p in kilobar, and of Syassen et al.,I2 who obtained Egap= 3.666 + 6.35 X 1 0 - j ~- 13.1 X 10-6p2 (2) (8) Cerdeira, F.; De Witt, J. S.; Rossler, U.; Cardona, M. Phys. Status Solidi 1970, 41, 735. (9) Camphausen, D. L.; Connell, G. A. N.; Paul, W. Phys. Reo. Lett. 1971, 26, 184. (IO) Piper, W. W.; Marple, D. T. F.; Johnson, P. D. Phys. Rev. 1958,110, 323. (11) Beliveau, A.; Carlone, C. Phys. Reo. B 1990, 41, 9860.
is sufficient to excite electrons into the conduction band a t low pressures, but beyond 25-30 kbar the excitation is to the “deep levels” as discussed below. Thus, using constant excitation with varying band gap, we could study the energy-transfer processes via the conduction band or via the “deep levels” not pinned to the conduction band. 2. The Hjalmslson Theory of Deep Impurity Levels. A central point of Hjalmarson theory5J3is that every heterovalent substitutional impurity produces both “deep levels” and shallow levels and that the deep levels do not necessarily lie in the fundamental band gap, but may be resonant with the host bands. The deep and shallow states are two qualitatively different types of impurity states that coexist but are rarely observed simultaneously. The deep levels are controlled by the central-cell potential, are often antibonding in character, and are largely hostlike. The deep level energies are often unattached to nearby band edges and do not follow them when they move as a result of pressure or alloying. Deep impurity states are localized in real space and delocalized in k space. In addition to the central-cell potential, several other physical effects influence the deep levels on a scale of a few tenths of an electronvolt. These include lattice relaxation around the defectI4 and charge state splittingls of the defect levels. In the case of the ZnS:Cu:Cl sample, the chlorine may produce the “deep levels” band. According to the above Hjalmarson theory, (12) Ves, S.;Schwarz, U.;Christensen, N. E.; Syassen, K.; Cardona, M. Phys. Reo. B 1990,42, 9113. (13) Dow, J. D. Localized Perturbation in Semiconductors. In Proceedings of fhe International School of Physics; Papali, P., Ed.; North-Holland Amsterdam, 1985; Vol. 89, p 465. (14) Scheffler, M.; Vigneron, J. P.; Bachelet, G. P. Phys. Reo. Lett. 1982, 49, 1765. (15) Bemholc, J.; Lipari, N. 0.;Pantelides, S. T.; Scheffler, M. Phys. Reo. B 1982, 26, 5706.
88 The Journal of Physical Chemistry, Vol. 96, No. 1 , 1992 the deep levels band may lie above the conduction band edge with a “negative binding energy”, which can trap excitons and enhance the nonradiative recombination of electron and hole. Here we name this band as the deep levels T band. With increasing pressure, the deep levels which often possess antibonding character do not shift with the energy band edge. Above a certain pressure, the deep levels T energy band will be lower in energy than the conduction band and the laser can excite to it. In addition, the deep levels T energy bands for three ZnS:Cu:Cl samples may be different in energy. This may be due differences in C1- concentration giving different lattice relaxation around the defect produced by doping with chlorine. 3. Photoluminescence of ZnS:Cu:Cl under Pressure. Many workers16-19have reported that the emission color of ZnS:Cu:Cl phosphors depends on the amount of copper and chlorine present in the sample and also on the energy of excitation. Godlewski et al.17 suggested that it was possible to identify three different centers, viz., Cu2+,Cu+, and C1- introduced into the ZnS lattice as a result of the firing process. The relative concentration of Cu2+, Cu+, and C1- centers should depend entirely on the amount of chlorine added in the initial mixture. Pillai and Vallabhan’* have reported the effect of chlorine concentration on the spectral characteristics of electroluminescence in ZnS:Cu:Cl phosphors. The experimental results showed that beyond a certain chlorine concentration the Cu2+center would disappear, and the emission peak for Cu2+was completely suppressed. As chlorine concentration was increased, the number of zinc vacancies, which also could become a radiative recombination center,19 would be increased. There were many different models which have been suggested to explain the occurrence of the emission bands. In the donoracceptor model of zinc sulfide luminescence a zinc vacancy with a chlorine [Zno(Cl)]+ and Cu+ center each could play the role of acceptor,17J9whereas the isolated Cl- are donors17-19which could be stabilized by interaction with a Cu+ or zinc vacancy. Our experimental observations can be satisfactorily explained on the basis of an energy level scheme shown in Figure 9. Three different centers, viz., [Zno(Cl)]+,Cu+, and C1-, are introduced into the band gap of the ZnS lattice as a result of the fuing process. The H E peaks for three ZnS:Cu:Cl samples are associated with the emission from C1- to [Zno(Cl)]+,whereas the LE peaks are from Cl- to Cu+. THe relative concentrations of [Zno(Cl)]+,a+, and C1- centers depend on the amount of chlorine added in the initial mixture, so the relative intensities of the LE and H E peaks for ZnS:Cu:Cl samples are different (see Figure 1). The stronger LE photoluminescence emission in the samples with the highest dopant concentration is apparently due to creation of additional Cu+ and C1- centers. The relative emission intensity of the LE and H E peaks does not change with pressure. This implies that the same donor level is involved in both emissions. The fact that the LE peak shifts the same as the absorption edge implies that the donor level is pinned to the bottom of conduction band and the acceptor (Cu’) center is pinned to the top of the valence band! Then the slightly different shift of the H E peak implies some movement of the zinc vacancy level with respect to the top of the valence band. The loss of intensity with pressure could be related directly to changes in donor energy level. The increase in energy of the donor state decreases the overlap with the acceptor state, thus allowing competing nonradiative pathways to quench t h e luminescence as discussed in earlier work.20,21 As the changes of pressure do not alter the relative concentrations of centers, the relative intensities between H E peaks with LE peaks for the three ZnS:Cu:Cl samples are constant with pressure. 4. Thermoluminescence of ZnS:Cu:Cl under Pressure. One of the objectives of a thermoluminescence experiment is to extract (16) Thornton, W. A. J. Electrochem. SOC.1960, 107, 895. (17) Godlewski, M.; Lamb, W. E.; Cavenett, B. C. J . Phys. C: Solid State
Phys. 1982, 15, 3925. (18) (19) (20) (21)
Pillai, S. M.; Vallabhan, C. P. G. SolidSrare Commun. 1983,47,909. Curie, D. Luminescence in Crystals; Methuen: London, 1963. House, G. L.; Drickamer, H. G. J. Chem. Phys. 1977, 67, 3221. Hook, J. W.; Drickamer, H. G . J . Appl. Phys. 1978, 49, 2503.
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data from an experimental glow curve and to use these data to calculate the value of the trap depths ( E ) . There exist many s~~ m e t h ~ d s which ~ ~ J ~depend on different models. K i ~ i t calculated the influence of several complex models and showed that the values of the trap depths determined after application of most approximate methods are unreliable. The method of HoogenstraatenZ5 gave the most consistent comparisons with model systems. Hoogenstraaten’s peak position heating rate method can be expressed by the formula
where E is the trap depth, k is Boltzmann’s constant, and T,, and Tm2are the glow maxima at B1 and B2 heating rates, respectively. Using this peak position heating rate method, we calculated the trap depth for each of three of ZnS:Cu:Cl samples at various pressures. The results are shown in Figure 10. At atmospheric pressure the trap depths are about 0.30 eV for all three samples. Within the range 0-1 5 kbar, the traps get deeper when the pressure increases. The shifts of trap depth energy are quite similar to the so that means the trapped electrons change of energy gap Egapr thermally release from the trap to the conduction band. With further increase of pressure, the trap depths of the green, bluegreen, and blue samples reach their plateaus of 0.39, 0.46, and (22) Chen, R.; Kirsch, Y. Analysis of Thermally Stimulated Processes; Pergamon: Oxford, 198 1. (23) McKeever, S . W. S. Thermoluminescence of Solids; Cambridge University Press: Cambridge, 1985. (24) Kivits, P. J. Lumin. 1978, 16, 119. ( 2 5 ) Hoogenstraaten, W. Philips Res. Rep. 1958, 13, 515.
J. Phys. Chem. 1992, 96, 89-94 0.54 eV, respectively, and keep at these levels thereafter. This experimental phenomena can be explained by using the Hjalmarson theory of deep impurity levels. As increasing pressure, the “deep levels” bands do not shift like the ZnS energy gap with pressure. Beyond a certain pressure the deep levels T energy band falls below the ZnS conduction band. The 325-nm (3.82-eV) laser energy is not sufficient to excite the electrons to the conduction band but can excite the electrons to the deep levels T energy band. The deep levels T energy band plays a role similar to the ZnS conduction band; thus, the trap depths obtained at higher pressure are attributed to the electrons thermally released from the trap to the deep levels T energy band. As discussed above, there is a range of energies for the deep levels depending on the amount of C1 present. The trapping ‘level” also consists of a distribution or band of states which may be several tenths of an electronvolt in width. The irradiation times were intentionally kept constant at 2 min at all pressures. At low pressure we could saturate the traps in a few seconds as demonstrated by auxiliary experiments using short irradiation times. At high pressures only the lowest trap levels were filled due to the relative inefficiency of the ‘deep levels” as feeders. We demonstrated this by several partial heating experiments which showed a significantly narrower range of filled traps (smaller half-width of the thermoluminescence curves) at high pressure. For example, for the low-concentration (blue) sample at 0 kbar the fwhm is -60 K while at 43 kbar the fwhm is -45 K. The situation is represented in Figure 9 where the dashed cross hatching represents unfilled states in the trapping level at high pressure. ‘Deep levels” have been extensively studied in GaAs and A1,Gal-,As. In GaAs the deep levels are resonant with the conduction band at ambient pressure. The energy gap increases with pressure (by 11 meV/kbar) so that above -25 kbar the deep levels are exposed in the gap.26,27 Adding A1 to GaAs
-
89
increases the gap so that at x = 0.22 the deep levels appear in the gap at ambient pressure. The properties of deep levels in 111-V semiconductors have been reviewed by Mooney.28 The kas of thermoluminescence intensity with prtssure involves several factors:20.21the loss in luminescence intensity discussed above due to thermal quenching as T, increases and relative inefficiency of the localized deep levels as intermediate states.
Summary High-pressure luminescence and thermoluminescence studies have been made on well-characterized ZnS samples doped with Cu and Cl. By using a constant excitation energy of 3.82 eV, at low pressures electrons were excited to the conduction band, and at high pressures, to the “deep levels” initially buried in the conduction band. The differences caused by the different types of excitation were especially revealed in the trap depths and efficiency of thermoluminescence. The changes in luminescence energy and efficiency for the two emission peaks could be associated with the degree to which donor and acceptor states were pinned to the conduction and valence bands. Acknowledgment. We thank T. Brumleve for the samples and K. Hess and G. A. Samara for helpful and illuminating discussions of ‘deep levels” in semiconductors. It is a pleasure to acknowledge the continuing support of the Materials Science Division of the Department of Energy under Contract DEFG02-9 1ER 45439. Registry No. ZnS, 1314-98-3; Cu, 17493-86-6; C1, 16887-00-6. (26) Wolford, D. J.; Bradlex, J. A.; Fry, K.; Thompson, J.; King, H. E. Institute of Physics Conference Series; Institute of Physics: London, 1982;
Vol. 65, p 477. (27) Mizuta, M.; Yachikawa, M.; Kukimoto, H.; Minomura, S. J . Appl. Phys. 1985, 24, L143. (28) Mooney, P. M. J . Appl. Phys. 1990, 67, R1.
High-Resolution Spectroscopy of Jet-Cooled Substituted Cyclopentadienyi Radicals David W. Cullin: Lian Yu,*James M. Williamson, and Terry A. Miller* Laser Spectroscopy Facility, Department of Chemistry, The Ohio State University, I20 West 18th Avenue, Columbus, Ohio 4321 0- I I73 (Received: July 12, 1991)
Rotationally resolved electronic spectra have been observed for a number of substituted cyclopentadienyl radicals, C5H4X, with X = F, C1, CN, and CH3. This paper reports the spectral analysis for the radicals with X = F and C1; results for X = CN and CH, have been reported previously. The geometric structural parameters for all the radicals are consistent with a distortion of the five-membered ring to a diene-like structure in the ground state, with C-C bond length alternations in excess of 0.1 A in some cases. The analysis also quite clearly shows a strengthening and shortening of the C-X bond in the radical compared to that of the similarly substituted benzene derivatives. The observed results are rationalized in terms of a simple molecular orbital picture involving conjugation and hyperconjugation.
I. Introduction The implementation of supersonic free jet expansions and high-resolution optical laser induced fluorescence (LIF) techniques have advanced the understanding of the spectroscopy of organic free radicals in the last few years. A great deal of electronic and structural information has been obtained for radicals such as the fluorobenzene ions (c6F6+,C6F3H3+),’cyclopentadienyl (C5H5),233 methylnitrene (CH3N),4 diacetylene cation (C4H2+),5benzyl (C6H5CH2),6methoxy (CH30)? and others because of the ability to rotationally resolve the electronic spectra of these large organic species. We recently reported the spectroscopy of the cyclopentadienyl radical (Cp). We obtained accurate rotational constants along Present Address: Naval Surface Warfare Center, Dahlgren, VA 22448. *Present Address: Lilly Research Laboratories, Lilly Corporate Center, Indianapolis, IN 46285.
with other parameters including those describing the Jahn-Teller effect caused by three electrons occupying the doubly degenerate lel” orbital of Cp. It was found that as a consequence of the Jahn-Teller effect, the five-membered ring in Cp is significantly distorted from a regular pentagon. ( 1 ) Yu. L.; Foster, S.C.; Williamson, J. M.; Miller, T. A. J . Chem. Phys. 199b; 94, 5794. (2) Yu. L.: Foster. S. C.; Williamson, J. M.; Heaven, M. C.; Miller, T. A. J . Phys. Chem. 1988, 92,4263. (3) Yu, L.; Williamson, J. M.; Miller, T. A. Chem. Phys. Let?.1988, 162,
4263. (4) Carrick, P. G.; Brazier, C. R.; Bernath, P. F.; Engleking, P. C. J. Am. Chem. SOC.1987, 109, 5100. (5) Lecoultre, J.; Maier, J. P.; Rijsselein, M. J . Chem. Phys. 1988, 89, 608 1. (6) Cossart-Magos, C.; Goetz, W. J . Mol. Spectrosc. 1986, 115, 366. (7) Liu, X.; Damo, C. P.; Lin, T.-Y.; Foster, S . C.; Misra, P.; Yu, L.; Miller, T. A. J . Phys. Chem. 1989, 93, 2266.
0022-3654/92/2096-89$03.00/0 0 1992 American Chemical Society
