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Influence of trapped impurities on luminescence from MgO:Cr
Nuclear Instruments and Methods in Physics Research B 191 (2002) 181–185 www.elsevier.com/locate/nimb
Influence of trapped impurities on luminescence from MgO:Cr M. Maghrabi b
a,b
, F. Th€ orne b, P.D. Townsend
c,*
a Department of Physics, Hashemite University, P.O. Box 150459, Zarqa 13115, Jordan Department of Chemistry, Physics and Environmental Science, University of Sussex, Brighton BN1 9QH, UK c Department of Engineering and Information Technology, University of Sussex, Brighton BN1 9QH, UK
Abstract Luminescence measurements during heating of MgO and MgO:Cr reveal a consistent set of temperature related discontinuities in intensity, accompanied by changes in emission wavelengths. Whilst the luminescence signals are derived from the host material the intensity and wavelength discontinuities are caused by impurities. It is proposed they are caused by phase changes of nanoparticle size impurity inclusions incorporated during growth, and/or from later surface treatments. The evidence suggests there are inclusions of impurities such as neon, oxygen, nitrogen and possibly chlorine. Additionally the influence of solvents, cleaning materials and water vapour alters the luminescence from the surface. Surface and bulk signals are separable by comparisons of cathodoluminescence and radioluminescence. In order to show impurity phase transition effects the nanoparticle inclusions must contain several hundred atoms. Ó 2002 Published by Elsevier Science B.V. Keywords: Luminescence; Absorbates; Nanoparticles; MgO
1. Introduction Changes in luminescence properties of insulators, such as intensity, spectra and lifetimes can occur when a material undergoes a phase transition [1]. Dramatic first order luminescence effects exist in some cases (ammonium bromide [2] and potassium niobate [3]) and more subtle changes in others (e.g. fullerenes, sodium sulphate, KTN). In addition to the phase changes of the bulk material it is possible to observe indirect effects where inclusions, precipitates or absorbates influence the spectra and/or the intensity. For example in
*
Corresponding author. Tel./fax: +44-1273-678073. E-mail address: [email protected] (P.D. Townsend).
Nd:YAG the line positions of Nd are modified by the crystal field, and so clearly respond to lattice expansion, dislocations and surface impurities. For example, a major discontinuity in intensity and wavelength was noted near 200 K which was attributed to trapped CO2 . On heating, the solid CO2 sublimes at this temperature and thus causes a significant pressure change [4]. The impurity phase transition was reflected both by changes in the lattice spacing and the Nd luminescence. Similarly, precipitate phase effects have been claimed for features in CaSO4 :Dy [5]. Radioluminescence during heating or cooling (RLTL) and subsequent thermoluminescence (TL), respond to bulk properties, whereas cathodoluminescence (CL) is sensitive to surface effects since the electrons only penetrate a micron or so into the surface. Thus CL responds to the trapped
0168-583X/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 5 5 5 - 4
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M. Maghrabi et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 181–185
gases and solvents, for example absorbed within the dislocations of the surface. In general the impurity phase changes influence the luminescence efficiency and introduce wavelength shifts of signals characteristic of the host. However, the inverse process was noted for 60 C luminescence where the luminescence signal is derived from a solvent (toluene) and the luminescence intensity was modified by phase transitions of the fullerene host [6]. One characteristic signature feature of phase driven events, as opposed to normal TL, RL or CL emission, is a discontinuous intensity change across the spectrum. This can be seen on isometric plots of wavelength, temperature and intensity and, more obviously, as a line on a contour map of the data. The intensity steps occur within a narrow temperature range of just a few degrees. The steps in luminescence efficiency may either increase, or decrease, in intensity. In some examples (e.g. [3]) different emission bands show opposite effects at the same phase transition. In the current studies at Sussex luminescence discontinuities have been noted in a very wide range of optical materials and minerals. Despite this widespread behaviour their presence has essentially been overlooked, probably for experimental reasons. Current successes in recording such changes have been possible because of the use of a sensitive wavelength multiplexed spectrometer. The present data were taken using crystals of MgO and MgO:Cr and the objective was to seek any evidence for bulk inclusion of gas during crystal growth, and/or to note absorbate effects. The advantage of the chromium doped material is that it offers well defined red emission from lattice sites which are sensitive to the crystal field.
excitation and TL is a standard option. The equipment was designed for high sensitivity, rather than high resolution, hence only major spectral shifts of the Cr emission will be noted in this experiment. Note also that temperature gradients within the samples will introduce an inherent temperature spread in response and the temperatures are quoted in terms of those recorded at the cryostat head. These values may thus systematically differ slightly from the sample temperatures. MgO crystals of only 99.99% purity were deliberately used in the expectation that these samples might provide favourable impurity signals. In particular, they are likely to have trapped impurities and gases from the original growth procedure.
3. Results and comments In order to emphasise differences between bulk and surface effects data are presented for luminescence from RLTL (bulk) and CLTL (which for 10 keV electrons is from the near surface zone).
2. Experimental The data were recorded on a high sensitivity wavelength multiplexed spectrometer described in numerous earlier papers [2–6] which includes facility for in situ X-ray (RL) and electron (CL) irradiation. It has options to cover the range from 20 to 673 K whilst recording RL, RLTL, TL, CL or CLTL. Heating (and cooling) can be used during
Fig. 1. The red RL spectra from MgO with 1300 ppm Cr recorded during heating. Note the intensity and wavelength discontinuities reflect bulk responses.
M. Maghrabi et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 181–185
Figs. 1 and 2, for a MgO sample doped with 1300 ppm Cr, show similar temperature trends of intensity declining with increasing temperature. Superposed on the smooth background trend are intensity steps. Associated with them are sideways steps in the Cr emission lines. A key observation is that the wavelength directions of the steps differ between bulk and surface emission, but occur at the same temperatures. Similar data were noted for a range of samples including nominally pure MgO or MgO crystals doped with 800, 1300 or 4400 ppm Cr. As expected, there are more longer wavelength red emission lines for the higher Cr concentrations and as for ruby, such features are thought to arise from regions distorted by high concentrations of Cr pairs or higher order impurity effects. The relative scale of the intensity and wavelength discontinuities differed slightly between samples. For CLTL measurements the initial set of data showed greater variability, particularly with re-
Fig. 2. The red CL spectra from MgO with 1300 ppm Cr recorded during heating. The CL responds to surface effects and these data are typical of repeat measurements. The wavelength steps are in the opposite sense from those of RL for the same sample, shown in Fig. 1.
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spect to an intensity step near 170 K. This feature was greatly reduced in subsequent runs. It could be emphasised by pre-treatments such as cleaning in ethanol or acetone. Related behaviour has been noted with other insulating crystals. Therefore it is assumed the electron beam removes surface absorbates during the first few measurements. A particularly strong initial response anomaly was induced in the 1300 ppm Cr sample and this is shown in Fig. 3. Note later measurements are more typical of Fig. 2. The various chromium transitions do not behave identically and intensity versus temperature plots are shown in Fig. 4 for several wavelength regions for the examples presented in preceding figures. In addition, an intensity plot of CLTL for the first measurements of the 4400 ppm Cr sample are shown, since these also show a quenching of the luminescence prior to signal recovery near 170 K.
Fig. 3. The initial CL response for this sample was different from the majority of the MgO samples, at any doping level, although less dramatic features are sometimes apparent at the same temperatures.
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M. Maghrabi et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 181–185
Fig. 4. The intensity plots show signal integration in the bands of (i) 695–710, (ii) 720–735 and (iii) 740–760 nm. (a), (b) and (c) are RL, and CL data as shown in Figs. 1–3 for MgO with 1300 ppm Cr. (d) is for CL data with a higher dopant concentration of 4400 ppm Cr.
4. Discussion The clearly evident discontinuities in luminescence intensity, and wavelength movements of the chromium emission, are consistent with a model of lattice changes arising from phase transitions of nanoparticle impurity inclusions in the samples. The fact that surface and bulk features have similar temperatures, but differ in the sense of the wavelength shifts, is equally explicable in terms of pressure changes. For example, within the bulk a liquid to gas transition will greatly increase the pressure and cause lattice compaction, but very close to the surface it will force an outward expansion of the structure, and possibly some anisotropy. Higher wavelength resolution, and measurement of polarisation, might reveal surface anisotropy induced in the normally cubic lattice of the MgO host. For the radioluminescence (i.e. bulk signals) less detailed measurements might have overlooked these effects, except for the intensity drop near 230 K. The variability of the initial CLTL signals is consistent with partial desorption of absorbates. For there to be signals associated with impurity phase transitions the impurities must exist in at least nanoparticle dimensions, and thus involve a few hundred atoms. In principle the transition
temperatures could identify the impurities and so comparisons were made relative to tabulated phase transition temperatures [7] for common bulk materials. However, it should be recognised that these temperatures may differ from those appropriate for phase transitions of nanoparticles (e.g. if the transitions are pressure sensitive). In the case of metals, the melting points are a strong function of particle size and similar changes may exist for the insulator inclusions. There may be additional effects for nanoparticles which preclude the coexistence of mixed phases (e.g. solid/liquid) and these factors may introduce temperature shifts and/or hysteresis in the transition events during heating and cooling [3,8]. With these caveats, discontinuities in luminescence response can tentatively be matched to solid/liquid and liquid/gas transitions for impurities such as Ne, O2 , CO, N2 and Ar. Many other transitions are less simply identified. The most interesting event is the major intensity change which is frequently found near 170 K, and it has been noted in many materials, not just MgO. Potential candidates include a wide range of halide compounds formed with C, O and N as not only do they appropriate phase transitions but are also potential contaminants during the crystal growth. There are tabulated transitions for melting of COCl2 and Cl2 , or boiling of ClF with a good temperature match for the 170 K event. However, these seem unlikely candidates for an absorbate effect which is also commonly observed in other materials in the present survey. A more probable impurity is surface absorbed water since ice undergoes a cubic to hexagonal phase transition in the range from 170 to 175 K [9] with additional suggestions of a liquid/ice modification [10]. Ice nanoparticles within surface dislocations, droplets, or thin surface films are certainly feasible and represent a common contaminant of the surface. Despite the uncertainties as to the origin, the significant point to note is that such impurity transitions strongly influence the host luminescence signals. In the case of CL they introduce unexpectedly large distortions to the recorded intensities and spectra. Such features are therefore worth further examination, particularly since the present survey has indicated that similar effects can be seen in a wide range of insulators.
M. Maghrabi et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 181–185
Acknowledgements We wish to thank the EPSRC and the Hashemite University in Jordan for financial support. References [1] P.D. Townsend, M. Maghrabi, B. Yang, Nucl. Instr. and Meth. B 191 (2002) 767. [2] P.D. Townsend, A.P. Rowlands, G. Corradi, Radiat. Meas. 27 (1997) 31. [3] B. Yang, P.D. Townsend, M. Maghrabi, J. Modern Opt. 48 (2001) 319.
185
[4] M. Maghrabi, P.D. Townsend, G. Vazquez, J. Phys.: Cond. Matter 13 (2001) 2497. [5] A.P. Rowlands, T. Karali, M. Terrones, N. Grobert, P.D. Townsend, K. Kordatos, J. Phys.: Cond. Matter 12 (2000) 7869. [6] T. Karali, A.P. Rowlands, P.D. Townsend, M. Prokic, J. Olivares, J. Phys. D 31 (1998) 754. [7] Handbook of Chemistry and Physics, CRC, Boca Raton, FL. [8] C.N.R. Rao, K.J. Rao, Phase Transitions in Solids, McGraw-Hill, New York, 1978. [9] F. Franks, Water: A Matrix of Life, Royal Society of Chemistry, Cambridge, 2000. [10] P. Jenniskens, S.F. Banham, D.F. Blake, M.R.S. McCoustra, J. Chem. Phys. 107 (1997) 1232.
Influence of trapped impurities on luminescence from MgO:Cr M. Maghrabi b
a,b
, F. Th€ orne b, P.D. Townsend
c,*
a Department of Physics, Hashemite University, P.O. Box 150459, Zarqa 13115, Jordan Department of Chemistry, Physics and Environmental Science, University of Sussex, Brighton BN1 9QH, UK c Department of Engineering and Information Technology, University of Sussex, Brighton BN1 9QH, UK
Abstract Luminescence measurements during heating of MgO and MgO:Cr reveal a consistent set of temperature related discontinuities in intensity, accompanied by changes in emission wavelengths. Whilst the luminescence signals are derived from the host material the intensity and wavelength discontinuities are caused by impurities. It is proposed they are caused by phase changes of nanoparticle size impurity inclusions incorporated during growth, and/or from later surface treatments. The evidence suggests there are inclusions of impurities such as neon, oxygen, nitrogen and possibly chlorine. Additionally the influence of solvents, cleaning materials and water vapour alters the luminescence from the surface. Surface and bulk signals are separable by comparisons of cathodoluminescence and radioluminescence. In order to show impurity phase transition effects the nanoparticle inclusions must contain several hundred atoms. Ó 2002 Published by Elsevier Science B.V. Keywords: Luminescence; Absorbates; Nanoparticles; MgO
1. Introduction Changes in luminescence properties of insulators, such as intensity, spectra and lifetimes can occur when a material undergoes a phase transition [1]. Dramatic first order luminescence effects exist in some cases (ammonium bromide [2] and potassium niobate [3]) and more subtle changes in others (e.g. fullerenes, sodium sulphate, KTN). In addition to the phase changes of the bulk material it is possible to observe indirect effects where inclusions, precipitates or absorbates influence the spectra and/or the intensity. For example in
*
Corresponding author. Tel./fax: +44-1273-678073. E-mail address: [email protected] (P.D. Townsend).
Nd:YAG the line positions of Nd are modified by the crystal field, and so clearly respond to lattice expansion, dislocations and surface impurities. For example, a major discontinuity in intensity and wavelength was noted near 200 K which was attributed to trapped CO2 . On heating, the solid CO2 sublimes at this temperature and thus causes a significant pressure change [4]. The impurity phase transition was reflected both by changes in the lattice spacing and the Nd luminescence. Similarly, precipitate phase effects have been claimed for features in CaSO4 :Dy [5]. Radioluminescence during heating or cooling (RLTL) and subsequent thermoluminescence (TL), respond to bulk properties, whereas cathodoluminescence (CL) is sensitive to surface effects since the electrons only penetrate a micron or so into the surface. Thus CL responds to the trapped
0168-583X/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 5 5 5 - 4
182
M. Maghrabi et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 181–185
gases and solvents, for example absorbed within the dislocations of the surface. In general the impurity phase changes influence the luminescence efficiency and introduce wavelength shifts of signals characteristic of the host. However, the inverse process was noted for 60 C luminescence where the luminescence signal is derived from a solvent (toluene) and the luminescence intensity was modified by phase transitions of the fullerene host [6]. One characteristic signature feature of phase driven events, as opposed to normal TL, RL or CL emission, is a discontinuous intensity change across the spectrum. This can be seen on isometric plots of wavelength, temperature and intensity and, more obviously, as a line on a contour map of the data. The intensity steps occur within a narrow temperature range of just a few degrees. The steps in luminescence efficiency may either increase, or decrease, in intensity. In some examples (e.g. [3]) different emission bands show opposite effects at the same phase transition. In the current studies at Sussex luminescence discontinuities have been noted in a very wide range of optical materials and minerals. Despite this widespread behaviour their presence has essentially been overlooked, probably for experimental reasons. Current successes in recording such changes have been possible because of the use of a sensitive wavelength multiplexed spectrometer. The present data were taken using crystals of MgO and MgO:Cr and the objective was to seek any evidence for bulk inclusion of gas during crystal growth, and/or to note absorbate effects. The advantage of the chromium doped material is that it offers well defined red emission from lattice sites which are sensitive to the crystal field.
excitation and TL is a standard option. The equipment was designed for high sensitivity, rather than high resolution, hence only major spectral shifts of the Cr emission will be noted in this experiment. Note also that temperature gradients within the samples will introduce an inherent temperature spread in response and the temperatures are quoted in terms of those recorded at the cryostat head. These values may thus systematically differ slightly from the sample temperatures. MgO crystals of only 99.99% purity were deliberately used in the expectation that these samples might provide favourable impurity signals. In particular, they are likely to have trapped impurities and gases from the original growth procedure.
3. Results and comments In order to emphasise differences between bulk and surface effects data are presented for luminescence from RLTL (bulk) and CLTL (which for 10 keV electrons is from the near surface zone).
2. Experimental The data were recorded on a high sensitivity wavelength multiplexed spectrometer described in numerous earlier papers [2–6] which includes facility for in situ X-ray (RL) and electron (CL) irradiation. It has options to cover the range from 20 to 673 K whilst recording RL, RLTL, TL, CL or CLTL. Heating (and cooling) can be used during
Fig. 1. The red RL spectra from MgO with 1300 ppm Cr recorded during heating. Note the intensity and wavelength discontinuities reflect bulk responses.
M. Maghrabi et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 181–185
Figs. 1 and 2, for a MgO sample doped with 1300 ppm Cr, show similar temperature trends of intensity declining with increasing temperature. Superposed on the smooth background trend are intensity steps. Associated with them are sideways steps in the Cr emission lines. A key observation is that the wavelength directions of the steps differ between bulk and surface emission, but occur at the same temperatures. Similar data were noted for a range of samples including nominally pure MgO or MgO crystals doped with 800, 1300 or 4400 ppm Cr. As expected, there are more longer wavelength red emission lines for the higher Cr concentrations and as for ruby, such features are thought to arise from regions distorted by high concentrations of Cr pairs or higher order impurity effects. The relative scale of the intensity and wavelength discontinuities differed slightly between samples. For CLTL measurements the initial set of data showed greater variability, particularly with re-
Fig. 2. The red CL spectra from MgO with 1300 ppm Cr recorded during heating. The CL responds to surface effects and these data are typical of repeat measurements. The wavelength steps are in the opposite sense from those of RL for the same sample, shown in Fig. 1.
183
spect to an intensity step near 170 K. This feature was greatly reduced in subsequent runs. It could be emphasised by pre-treatments such as cleaning in ethanol or acetone. Related behaviour has been noted with other insulating crystals. Therefore it is assumed the electron beam removes surface absorbates during the first few measurements. A particularly strong initial response anomaly was induced in the 1300 ppm Cr sample and this is shown in Fig. 3. Note later measurements are more typical of Fig. 2. The various chromium transitions do not behave identically and intensity versus temperature plots are shown in Fig. 4 for several wavelength regions for the examples presented in preceding figures. In addition, an intensity plot of CLTL for the first measurements of the 4400 ppm Cr sample are shown, since these also show a quenching of the luminescence prior to signal recovery near 170 K.
Fig. 3. The initial CL response for this sample was different from the majority of the MgO samples, at any doping level, although less dramatic features are sometimes apparent at the same temperatures.
184
M. Maghrabi et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 181–185
Fig. 4. The intensity plots show signal integration in the bands of (i) 695–710, (ii) 720–735 and (iii) 740–760 nm. (a), (b) and (c) are RL, and CL data as shown in Figs. 1–3 for MgO with 1300 ppm Cr. (d) is for CL data with a higher dopant concentration of 4400 ppm Cr.
4. Discussion The clearly evident discontinuities in luminescence intensity, and wavelength movements of the chromium emission, are consistent with a model of lattice changes arising from phase transitions of nanoparticle impurity inclusions in the samples. The fact that surface and bulk features have similar temperatures, but differ in the sense of the wavelength shifts, is equally explicable in terms of pressure changes. For example, within the bulk a liquid to gas transition will greatly increase the pressure and cause lattice compaction, but very close to the surface it will force an outward expansion of the structure, and possibly some anisotropy. Higher wavelength resolution, and measurement of polarisation, might reveal surface anisotropy induced in the normally cubic lattice of the MgO host. For the radioluminescence (i.e. bulk signals) less detailed measurements might have overlooked these effects, except for the intensity drop near 230 K. The variability of the initial CLTL signals is consistent with partial desorption of absorbates. For there to be signals associated with impurity phase transitions the impurities must exist in at least nanoparticle dimensions, and thus involve a few hundred atoms. In principle the transition
temperatures could identify the impurities and so comparisons were made relative to tabulated phase transition temperatures [7] for common bulk materials. However, it should be recognised that these temperatures may differ from those appropriate for phase transitions of nanoparticles (e.g. if the transitions are pressure sensitive). In the case of metals, the melting points are a strong function of particle size and similar changes may exist for the insulator inclusions. There may be additional effects for nanoparticles which preclude the coexistence of mixed phases (e.g. solid/liquid) and these factors may introduce temperature shifts and/or hysteresis in the transition events during heating and cooling [3,8]. With these caveats, discontinuities in luminescence response can tentatively be matched to solid/liquid and liquid/gas transitions for impurities such as Ne, O2 , CO, N2 and Ar. Many other transitions are less simply identified. The most interesting event is the major intensity change which is frequently found near 170 K, and it has been noted in many materials, not just MgO. Potential candidates include a wide range of halide compounds formed with C, O and N as not only do they appropriate phase transitions but are also potential contaminants during the crystal growth. There are tabulated transitions for melting of COCl2 and Cl2 , or boiling of ClF with a good temperature match for the 170 K event. However, these seem unlikely candidates for an absorbate effect which is also commonly observed in other materials in the present survey. A more probable impurity is surface absorbed water since ice undergoes a cubic to hexagonal phase transition in the range from 170 to 175 K [9] with additional suggestions of a liquid/ice modification [10]. Ice nanoparticles within surface dislocations, droplets, or thin surface films are certainly feasible and represent a common contaminant of the surface. Despite the uncertainties as to the origin, the significant point to note is that such impurity transitions strongly influence the host luminescence signals. In the case of CL they introduce unexpectedly large distortions to the recorded intensities and spectra. Such features are therefore worth further examination, particularly since the present survey has indicated that similar effects can be seen in a wide range of insulators.
M. Maghrabi et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 181–185
Acknowledgements We wish to thank the EPSRC and the Hashemite University in Jordan for financial support. References [1] P.D. Townsend, M. Maghrabi, B. Yang, Nucl. Instr. and Meth. B 191 (2002) 767. [2] P.D. Townsend, A.P. Rowlands, G. Corradi, Radiat. Meas. 27 (1997) 31. [3] B. Yang, P.D. Townsend, M. Maghrabi, J. Modern Opt. 48 (2001) 319.
185
[4] M. Maghrabi, P.D. Townsend, G. Vazquez, J. Phys.: Cond. Matter 13 (2001) 2497. [5] A.P. Rowlands, T. Karali, M. Terrones, N. Grobert, P.D. Townsend, K. Kordatos, J. Phys.: Cond. Matter 12 (2000) 7869. [6] T. Karali, A.P. Rowlands, P.D. Townsend, M. Prokic, J. Olivares, J. Phys. D 31 (1998) 754. [7] Handbook of Chemistry and Physics, CRC, Boca Raton, FL. [8] C.N.R. Rao, K.J. Rao, Phase Transitions in Solids, McGraw-Hill, New York, 1978. [9] F. Franks, Water: A Matrix of Life, Royal Society of Chemistry, Cambridge, 2000. [10] P. Jenniskens, S.F. Banham, D.F. Blake, M.R.S. McCoustra, J. Chem. Phys. 107 (1997) 1232.