- Email: [email protected]
The phenomenon of pyroelectric luminescence
311
Chemical Physics 116 (1987) 311-318 North-Holland, Amsterdam
THE PHENOMENON MC. NELSON, Department
OF PYROELECTRIC
LUMINESCENCE
D.M. HANSON
of Chemistty,
State University of New York, Stony Brook NY II 794, USA
and J.S. PATEL Bell Communications
Research, Navesink Research and Engineering Center, Red Bank, NJ 07701, USA
Received 18 May 1987; in final form 3 August 1987
Characteristics of spontaneous light emission that occurs from pyroelectric crystals when the temperature is varied are reported. Dam pertaining to the spectral, spatial, and temporal resolution of this luminescence are given, and a theory accounting for some of the characteristics is described. It is concluded that the luminescence results from charge carrier recombination on the surface and from dielectric breakdown of the ambient atmosphere, It is important to identify the source and mechanism of this luminescence because these materials have a potential for use as heat sensors.
1. Introduction
A new luminescent phenomenon, observed when certain crystals are cooled, recently was reported [l-3]. In an effort to deionize liquid helium for spectroscopic experiments in high electric fields, pyroelectric crystals were dropped into the 4 K liquid. It then was observed that these crystals began to glow. While triboluminescence can be brought about by such thermal shock, it later was demonstrated that only small, gradual temperature changes are necessary to cause luminescence, and that some triboluminescent compounds, those that are not pyroelectric, do not exhibit luminescence under these conditions. Although these effect could be called “cryoluminescence”, it is called “pyroelectric luminescence” (PEL) because it is associated with the pyroelectric properties of materials. Such luminescence appears to have been observed first in beryllium oxide [4], as a source of noise when this materials was used for radiation detection. Studies of PEL in N-isopropylcarbazole also have been reported [5-71. In this article we review the characteristics of PEL, report new results pertaining to the spectral, spatial, and tem-
poral resolution of this luminescence, and describe a theory that explains some of the characteristics. Pyroelectric luminescence has been separated into three classes [l]. Class I luminescence is characterized by a series of intense light pulses, accompanied by current pulses in an external circuit connecting opposite faces of the crystal. This behavior, which generally occurs when the pressure of the ambient atmosphere is above 1 Torr, is shown in fig. 1. Class II luminescence is characterized by each intense pulse being preceded by a series of less intense pulses as shown in fig. 2. The decay of these pulses often consists of a fast component and a slow component, and the electrical pulse is synchronous only with the fast component. Class II PEL generally is found when the pressure of the ambient atmosphere is between 1 torr and 1 mTorr. Class III luminescence is identified by a steady glow evolving slowly over periods of seconds or minutes as the temperature is changing, as shown in fig. 3. Pyroelectric crystals are characterized by the existence of a unique polar axis. At equilibrium the electric polarization is compensated by free charges. Heating or cooling induces a polarization
0301-0104/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
M. C. Nelson et al. / Pyroeleciric
312
,CURRENT
LIGHT
. TIME
-
Fig. 1. Simultaneous current and luminescence pulses for class I PEL from cow-win. The traces are offset for clarity.
until the charges again reach equilibrium. Charge carriers are mobile in the electric fields produced by this polarization, and these fields can be large enough to cause dielectric breakdown. It was proposed [l-3] that class I PEL is caused by breakdown in the ambient atmosphere, class II PEL seems to involve breakdown in the crystal or on the crystal surface, and class III PEL appears to result from charge-carrier recombination because of the slowly evolving time structure. In class II PEL the slow component then results from luminescence of the molecules comprising the crystal. The molecular luminescence could be excited by charge-carrier recombination and by radiation from the breakdown. Photoluminescence is implicated by the absence of an electrical current
Fig. 2. Luminescence pulse train of class II PEL from coumarin.
luminescence
2 a 3
290 185 (K) TEMPERATURE Fig. 3. Class III PEL from m-bromonitrobenzene being heated at the rate of 8 “/min.
accompanying the slow decay. The series of less intense pulses could result from breakdown or charge redistribution involving microdomains leading to the formation of larger domains that produce the more intense pulses. On the basis of the observations that have been made [l-3] PEL can be distinguished from other forms of luminescence. For example, thermoluminescence is temperature driven but only occurs with increases in temperature and usually is the result of thermal detrapping of charge carriers, electroluminescence coincides with an electrical current but requires the introduction of charge carriers from an external circuit. Although the mechanical stress required for tribohnninescence can be generated by thermal shock, PEL can be observed with only gradual changes in temperature and without signs of cracking, even after repeated temperature cycles. Also, the slowly evolving class III PEL is not similar to triboluminescence.
2. Experimental To obtain information about the spectral, spatial, and temporal distribution of PEL, single crystals of various compounds were mounted on the cold finger of a variable temperature cryorefrigerator with a small amount of high vacuum silicon grease. The ambient atmosphere was a 1: 3 mixture of helium and neon at various pressures. Luminescence of the atmosphere thus was the characteristic red of neon; whereas, the luminescence of the selected samples was blue. The blue and red components of PEL were detected with different photomultiplier tubes fitted
313
iU.C. Nelson et al. / Pyroelecttic luminescence N-AAA
:oumarin PEL
PEL
2 torr
I
0.25
TIME
& (3
(ms)
I 0.50
200 mtorr
?
<
I
I mtorr
A
Blur
I I
0 I
0
I
I
I
2
I
TIME
(ms)
I
I
3
Fig. 4. Temporal and spectral resolution of PEL from coumarin at two pressures of He/Ne for the ainbient atmosphere. Lowering the pressure causes the intensity of the red component to decrease. The insets correspond to the placement of a second red filter in the optical path of the red channel.
to adjacent windows of the sample chamber with essentially equal views of the sample. The blue channel was filtered with a Schott BGl glass filter and utilized a RCA lP28 photomultiplier tube. The red channel was filtered with a Schott RG610 glass filter and utilized an EM1 9558 tube. The voltages applied to the tubes and the gain in the external amplifiers were adjusted so the sensitivities of the two channels were equivalent. Each channel was monitored with a Biomation model 805 transient digitizer with a 10 kSt termination. Simultaneous triggering of both digitizers was accomplished by monitoring one channel with a Tektronix 5107N oscilloscope in the single sweep mode and using the attenuated ramp as the trigger source. With the oscilloscope time base set at 1 ns/div, reproducible and simultaneous triggering could be obtained within 0.2 ~.ls of the event. Single pulses were observed with durations be-
BIUS
Red
I
I
10.0
TI$c (ms) Fig. 5. Temporal and spectral resolution of PEL from Nacetylanthranilic acid at two pressures of He/Ne for the ambient atmosphere. Decreasing the pressure from 2 to 0.2 Torr causes the intensity of the red component to decreases.
tween 20 and 130 ps. The subsequent discussion centers on these single-pulse events. A test for the transmission of blue light to the red channel detector was accomplished by inserting an additional glass filter (Schott RG590) in this channel. Since only slight attenuation resulted, it was concluded that this channel detects predominantly the red light from PEL. The corresponding question of red sensitivity in the blue channel is resolved by noting that the sensitivity of the lP28 photomultiplier tube falls off sharply beyond 510 nm. With the BGl filter in place, red light cannot produce the same response in both channels, as was observed under some conditions. It therefore is concluded that the blue channel responds predominantly to blue light. Results obtained with coumarin and Nacetylanthranilic acid (AAA) samples are reported here. These compounds were purified by recrystallization and zone refining prior to use. For AAA, PEL was monitored using an oscilloscope, and the single traces were photographed. For a record of spatial distribution of PEL, the
314
M. C. Nelson et al. / Pyroelectric
luminescence
3. Results and discussion
Fig. 6. Photograph of PEL from N-acetylanthranilic acid with a He/Ne pressure of 0.2 Torr, with the accompanying strip chart record of the photomultiplier tube current during the photographic exposure. For clarity, the edges of the crystal have been outlined.
cryorefrigerator was fitted with a 35 mm SLR camera equipped with a close-up lense. The cold finger was positioned to present a full face view to the camera. A photomultiplier tube in an adjacent window of the cryorefrigerator provided a temporal record of the light intensity during the photographic exposure. Photoluminescence lifetimes of the samples were obtained by excitation with a pulsed nitrogen laser and monitored using a Tracer Northern signal analyzer through a double monochromator with an EM1 9558 photomultiplier tube. Luminescence decay of the helium-neon atmosphere excited by an electric discharge was monitored similarly.
The time and spectrally resolved PEL pulses obtained at different pressures of the He/Ne gas are shown in figs. 4 and 5 for coumarin and AAA. Over a pressure range of 5 mTorr to 2 Torr, it was found that increasing the pressure of the gaseous environment produces an increase in the ratio of red to blue luminescence and a decrease in the pulse width. In all cases, the red and blue pulses occur simultaneously and with identical widths. Several observations prove that this coincidence in the red and blue luminescence is not an artifact. The pulse widths vary with gas pressure. Since the ratio of red to blue intensity varies strongly with pressure and since placement of an additional red filter in the red channel did not significantly alter the intensity ratio, see fig. 4, the coincidence of the light pulses is not due to a broad transmission by either channel. A very important feature of the time domain observations is that in all cases where a distinctly single event is observed, the pulse shape is symmetrical. This behavior is quite unlike the fast rise and exponential decay of photoluminescence excited by a pulse laser. Observations of luminescence resulting from an electric discharge in the He/Ne atmosphere and from excitation of coumarin with a 5 ns laser pulse rule out either of these phenomena as the rate determinin g steps in these observations. In both cases, the luminescence lifetimes were less than 5 ps, which was the limit of the apparatus used in the measurements. A photographic record of PEL is shown in fig. 6. The accompanying strip chart record of the photomultiplier tube current during the photographic exposure shows that a single event of significant luminosity was observed, along with several much less intense pulses. Although it is not obvious in all the photographs that the PEL events are on the crystal surface, it appears that in many examples, as illustrated by the figure, the luminescence originates from the edges. The inference that PEL is a surface phenomenon is corroborated by the susceptibility to gas pressure and by the observations of the spatial distribution, which show the luminescence to originate from the edges.
M.C. Nelson et al. / Pyroelectric
(population) of such microstates in the sample. The surface polarization charge density is given by
4. A model for pyroelectric luminescence On the basis of the above observations, it is possible to construct a model for a single pulse PEL event to account for the symmetrical profile in time, the simultaneous production of red and blue light pulses, the dependence of the luminescence intensity and lifetime on pressure, the origin of the pulse trains shown in fig. 2, and possibly the slowly evolving luminescence under class III conditions. F’yroelectricity commonly is defined as a temperature-dependent spontaneous electric polarization of a crystal. The usual description of this phenomenon is given in terms of an observed change in polarization accompanying a small change in temperature dP=ydT,
0)
where y, the pyroelectric coefficient, also varies with temperature. Though y cannot yet be predicted from knowledge of a given crystal structure, it can be determined from experimental measurements and analysis. Typically this approach has been concerned with changes in the equilibrium polarization corresponding to a change in temperature. A pyroelectric crystal is heated rapidly to a few degrees above the temperature of a thermostated bath, and the charge accumulated at opposite faces of the crystal in response to this small temperature jump is determined Y=A,
J0
mQ( t) dt/AT.
(2)
For PEL, it is more informative to consider the kinetic processes associated with changes in the polarization of a crystal resulting from small time-dependent changes in temperature. Here we a present a model derived from empirical considerations of polarization fields in solids and localized transitions between polarization states. From this viewpoint, a central equation for dynamic pyroelectric phenomena would be p=
C4’CPi)
315
luminescence
(3)
where (pi) is the polarization of microstate i, which is an average over a some small volume (microdomain) of the sample. Ni is the number
(4)
a=n=P,
where n is the surface normal. It is the accumulation of charge in response to this polarization and the migration of that charge accompanying thermally induced changes in P that produce observed pyroelectric phenomena. The requirements for observing pyroelectricity are that the arrangements of dipoles in a crystal do not lead to a net cancellation in P. If over a small temperature range, the microstate polarizations are assumed to be constant, then a change in the macroscopic polarization can be ascribed entirely to changes in the populations of the various microstates AP=
cANi(
(5)
From this viewpoint, the pyroelectric constant can be written as y = dP/dT= and dynamic described by dP/dt
z(dNi/dT)(pi), pyroelectric
(6) phenomena
= x(dN,/dt)(p,).
will be
(7)
The problem then is the description of the factors affecting the various d N/d t in response to a given dT/dt. Since it is a difficult problem to incorporate the distribution of d Ni/dt in space and time and account for the non-linearities due to the influence of the local field on the polarization of a microdomain, we consider from this point the simple case of two polarization states. A microdomain can have either polarization ( pl) or ( pz), and Nl and Nz are the numbers of microdomains with these polarizations in the sample. This simplification is introduced to provide a tractable model containing some of the essential features of the problem. In particular, we will see that the symmetrical pulse shapes can arise from a cooperative effect. For this model the time-dependent polarization is described by dP/dt=
(dN,/dt)((P,)
- (PI>),
(8)
ikf.C. Nelson et al. / Pyroelectric
316
with the stipulation that the total number of microdomains is held constant, This requirement is expected to be valid over macroscopic but small regions of the sample and for temperature changes that are not too large. The transition rate for changes in the number of microdomains with a specific polarization can be written as dN,/dt = k2N2 - k,N,.
(9
In order to include cooperative effects, the rate parameters k, and k, are taken to depend upon the electric field and therefore will vary with the polarization and population of the microdomains. This dependence is represented by k, = k,‘N, and k, = k;N,. The rate for producing a particular microdomain therefore is enhanced by the presence of that microdomain. The transition rate then becomes dN,/dt = N,N,(k;
- k;)
(10)
and the corresponding change in the polarization is given by dP/dt=
NJ&; = N,N,K,,
- k;)((p,)
luminescence
tion charge density, ur,. The time evolution of the red luminescence, In, also is related to the gas pressure, G, and a rate constant Kg IR = apGKg .
(12)
The blue light originates from the sample and is a more complex issue. One mechanism that is consistent with the observations is excitation by electron-hole recombination. Our observations indicate that this luminescence also originates from regions near the surface. We therefore assume that the time evolution of the blue luminescence depends upon the surface charge density, (I, and a rate constant K,. The surface charge density, (I, and the surface polarization charge density, up, are taken to be equal I,=uK,.
(13)
The surface charge density is increased by a change in polarization and decreased by the mechanisms leading to luminescence du/dt = A, dN,/dt -A,&
- ABIB,
- (~2)) (11)
which defines K,. In examining eq. (ll), is clear that if one assumes the initial conditions Ni = 1 and N, = 0, then the transition rate is zero and no dynamical behavior will follow. What is observed, however, is that as the temperature is varied gradually, sudden bursts of light and electrical pulses are produced. Eq. (11) is to be used to describe the time evolution of these bursts and pulses. As the temperature approaches the point where these events occur, both microdomains should be present. This situation could be taken into account by including a temperature-dependent term in eq. (11) to account for the temperature-dependent changes in the polarization and population of the microdomains. For the present purposes, this requirement can be met in the calculations by starting the system with a small value for N,. We now consider a model for the production of the luminescence that is observed. The red light originates from the ambient atmosphere and is taken to be a consequence of the surface polariza-
GK,= 0.00
1
I .o
0.01
lj~:o; ’
IO
0
20
Time (scaled)
0.10
1,
IA_ GK,*2.0
5.0
IO.0
0.00
0
5
IO
Time (scaled)
Fig. 7. Calculated time evolution of blue luminescence for single PEL events at various pressures of the ambient gas using the model described in the text.
M.C. Nelson et al. / Pyroelectric luminescence
where the Aj are constants. The change in the charge density is related to the number of photons produced by A, and A,, and to the change in the polarization and microdomain population by A,. Nonradiative processes, which would affect the quantum yields and may be important, are neglected to avoid introducing additional parameters that are not necessary to model the observations. The following characteristics of PEL are predicted by eqs. (12) (13) and (14). The red and blue light always will be emitted simultaneously and with identical time behavior. The overall time evolution, when GKs differs appreciably from K,, is set almost entirely by the larger of these factors and by K,, so the upper limit of the time response is set by l/K, and the lower limit is set by l/K,. These limits are scaled by the coefficients in eq. (14). Plots of the time-dependent blue luminescence obtained for different values of gas pressure, G, are shown in fig. 7. These plots were generated by numerical calculations using a Runya-Kutta routine [8] with values for A,, A,, and A, of lo-“, 1, and 1, respectively, and using K, = 10p4. As shown in fig. 7, this model produces a range of luminescence pulse widths and shapes depending on the value of GK, at a given K,. For small GK, at low gas pressures, the decay is slow because the gas provides a discharge mechanism. As GK, increases, the pulses get shorter and more symmetrical. At large GK, the pulse width reaches a limit because of l/K,, feedback, and cooperativity in the kinetics. The red intensity is given by IB(f)GKg/KB, and the surface charge density is given by I,(t)/GK,. With K, set a lo5 s-l, the time scale is approximately that of the observed PEL. The decay half-life generated by the model is plotted in fig. 8 against the logarithm of the red to blue intensity (log IR/Ia), and in the bottom half the same information is presented from the actual PEL data. Since the effective pressure of the gas surrounding the sample in a dynamic system is not an easily measured quantity, the ratio of the red to blue luminescence is a more viable means of characterizing the observations in terms of the model. By taking the logarithm of this quantity, any difference in the sensitivities of the two chan-
317
Model
log t Experiment
I\ 0
0
-I
0 log *
Fig. 8. A comparison of the decay half-life as a function of the ambient gas pressure, which is characterized by the ratio of red to blue luminescence, for the model described in the text and for the experimental data. The relative translation of the x-axes of the two graphs can be caused by differences in the sensitivities of the red and blue channels and by an arbitrariness in the choice of parameters in the model.
nels or arbitrariness in the choice of parameters is transformed into a simple translation along the x-axis. The parameters have not been scaled to make the x-coordinates of the plots in fig. 8 agree. If the pyroelectric activity is considered in terms of microdomains, then it is readily envisaged that as the process occurs throughout some region of the crystal, there will come a point when the remaining microdomains are surrounded by neighbors already in the new configuration. As a result these remaining microdomains will switch all at once. In this way, a cycle of behavior will be a produced in which a series of small pulses will seem to trigger a larger pulse, as observed in fig. 2 for class II PEL. The model suggests that at very low gas pressures, the series of discrete events could occur with an average spacing less than the
318
M.C. Nekon et al. / Pyroelectric luminescence
duration of an event. The resulting slowly evolving intensity in the luminescence then would be identified as class III PEL.
5. Summary Observations of the temporal, spatial, and spectral resolution of pyroelectric luminescence (PEL) have been presented. These observations revealed that PEL, which appears to be a surface phenomenon, results from luminescence of both the sample and the ambient atmosphere. Light pulses from the sample and the atmosphere generally are symmetrical in time and occur simultaneously. These observations can be reproduced by a model of dynamic pyroelectricity. In the model the sample luminescence results from charge-carrier recombination, and the atmosphere luminescence results from dielectric breakdown induced by surface polarization fields. These two sources of luminescence are coupled by their mutual dependence on the surface charge density. The symmetrical pulse shape is obtained by a cooperative switching of microdomain polarizations. From the observations and the qualitative
success of this simple model, it seems likely that a quantitative description of dynamic pyroelectric phenomena, including PEL, is a tractable though not necessarily simple or straightforward problem. Studies along these lines should reveal some of the more intricate details of pyroelectricity. Acknowledgement Support for this research from the National Science Foundation under Grant DMR-8305050 is gratefully acknowledged. References [l] J.S. Pate1 and D.M. Hanson, Nature 293 (1981) 445. [2] J.S. Pate1 and D.M. Hanson, Ferroelectrics 38 (1981) 923. [3] D.M. Hanson, J.S. Pate1 and M.C. Nelson, Materials Sci. (Wroclaw) 10 (1984) 459. [4] R.B. Gammage and J.S. Cheka, Health Phys. 32 (1977) 189. [5] R. Nowak and R. Poprawski, Ferroelectric Letters 1(1984) 175. [6] Z. Dreger, J. Kalinowski, R. Nowak and J. Sworakowski, Materials Sci. (Wroclaw) 10 (1984) 67. [7] Z. Dreger, J. Kalinowski, R. Nowak and J. Sworakowsld, Chem. Phys. Letters 116 (1985) 192. [8] S. Gill, Proc. Cambridge Phil. Sot. 47 (1951) 96.
Chemical Physics 116 (1987) 311-318 North-Holland, Amsterdam
THE PHENOMENON MC. NELSON, Department
OF PYROELECTRIC
LUMINESCENCE
D.M. HANSON
of Chemistty,
State University of New York, Stony Brook NY II 794, USA
and J.S. PATEL Bell Communications
Research, Navesink Research and Engineering Center, Red Bank, NJ 07701, USA
Received 18 May 1987; in final form 3 August 1987
Characteristics of spontaneous light emission that occurs from pyroelectric crystals when the temperature is varied are reported. Dam pertaining to the spectral, spatial, and temporal resolution of this luminescence are given, and a theory accounting for some of the characteristics is described. It is concluded that the luminescence results from charge carrier recombination on the surface and from dielectric breakdown of the ambient atmosphere, It is important to identify the source and mechanism of this luminescence because these materials have a potential for use as heat sensors.
1. Introduction
A new luminescent phenomenon, observed when certain crystals are cooled, recently was reported [l-3]. In an effort to deionize liquid helium for spectroscopic experiments in high electric fields, pyroelectric crystals were dropped into the 4 K liquid. It then was observed that these crystals began to glow. While triboluminescence can be brought about by such thermal shock, it later was demonstrated that only small, gradual temperature changes are necessary to cause luminescence, and that some triboluminescent compounds, those that are not pyroelectric, do not exhibit luminescence under these conditions. Although these effect could be called “cryoluminescence”, it is called “pyroelectric luminescence” (PEL) because it is associated with the pyroelectric properties of materials. Such luminescence appears to have been observed first in beryllium oxide [4], as a source of noise when this materials was used for radiation detection. Studies of PEL in N-isopropylcarbazole also have been reported [5-71. In this article we review the characteristics of PEL, report new results pertaining to the spectral, spatial, and tem-
poral resolution of this luminescence, and describe a theory that explains some of the characteristics. Pyroelectric luminescence has been separated into three classes [l]. Class I luminescence is characterized by a series of intense light pulses, accompanied by current pulses in an external circuit connecting opposite faces of the crystal. This behavior, which generally occurs when the pressure of the ambient atmosphere is above 1 Torr, is shown in fig. 1. Class II luminescence is characterized by each intense pulse being preceded by a series of less intense pulses as shown in fig. 2. The decay of these pulses often consists of a fast component and a slow component, and the electrical pulse is synchronous only with the fast component. Class II PEL generally is found when the pressure of the ambient atmosphere is between 1 torr and 1 mTorr. Class III luminescence is identified by a steady glow evolving slowly over periods of seconds or minutes as the temperature is changing, as shown in fig. 3. Pyroelectric crystals are characterized by the existence of a unique polar axis. At equilibrium the electric polarization is compensated by free charges. Heating or cooling induces a polarization
0301-0104/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
M. C. Nelson et al. / Pyroeleciric
312
,CURRENT
LIGHT
. TIME
-
Fig. 1. Simultaneous current and luminescence pulses for class I PEL from cow-win. The traces are offset for clarity.
until the charges again reach equilibrium. Charge carriers are mobile in the electric fields produced by this polarization, and these fields can be large enough to cause dielectric breakdown. It was proposed [l-3] that class I PEL is caused by breakdown in the ambient atmosphere, class II PEL seems to involve breakdown in the crystal or on the crystal surface, and class III PEL appears to result from charge-carrier recombination because of the slowly evolving time structure. In class II PEL the slow component then results from luminescence of the molecules comprising the crystal. The molecular luminescence could be excited by charge-carrier recombination and by radiation from the breakdown. Photoluminescence is implicated by the absence of an electrical current
Fig. 2. Luminescence pulse train of class II PEL from coumarin.
luminescence
2 a 3
290 185 (K) TEMPERATURE Fig. 3. Class III PEL from m-bromonitrobenzene being heated at the rate of 8 “/min.
accompanying the slow decay. The series of less intense pulses could result from breakdown or charge redistribution involving microdomains leading to the formation of larger domains that produce the more intense pulses. On the basis of the observations that have been made [l-3] PEL can be distinguished from other forms of luminescence. For example, thermoluminescence is temperature driven but only occurs with increases in temperature and usually is the result of thermal detrapping of charge carriers, electroluminescence coincides with an electrical current but requires the introduction of charge carriers from an external circuit. Although the mechanical stress required for tribohnninescence can be generated by thermal shock, PEL can be observed with only gradual changes in temperature and without signs of cracking, even after repeated temperature cycles. Also, the slowly evolving class III PEL is not similar to triboluminescence.
2. Experimental To obtain information about the spectral, spatial, and temporal distribution of PEL, single crystals of various compounds were mounted on the cold finger of a variable temperature cryorefrigerator with a small amount of high vacuum silicon grease. The ambient atmosphere was a 1: 3 mixture of helium and neon at various pressures. Luminescence of the atmosphere thus was the characteristic red of neon; whereas, the luminescence of the selected samples was blue. The blue and red components of PEL were detected with different photomultiplier tubes fitted
313
iU.C. Nelson et al. / Pyroelecttic luminescence N-AAA
:oumarin PEL
PEL
2 torr
I
0.25
TIME
& (3
(ms)
I 0.50
200 mtorr
?
<
I
I mtorr
A
Blur
I I
0 I
0
I
I
I
2
I
TIME
(ms)
I
I
3
Fig. 4. Temporal and spectral resolution of PEL from coumarin at two pressures of He/Ne for the ainbient atmosphere. Lowering the pressure causes the intensity of the red component to decrease. The insets correspond to the placement of a second red filter in the optical path of the red channel.
to adjacent windows of the sample chamber with essentially equal views of the sample. The blue channel was filtered with a Schott BGl glass filter and utilized a RCA lP28 photomultiplier tube. The red channel was filtered with a Schott RG610 glass filter and utilized an EM1 9558 tube. The voltages applied to the tubes and the gain in the external amplifiers were adjusted so the sensitivities of the two channels were equivalent. Each channel was monitored with a Biomation model 805 transient digitizer with a 10 kSt termination. Simultaneous triggering of both digitizers was accomplished by monitoring one channel with a Tektronix 5107N oscilloscope in the single sweep mode and using the attenuated ramp as the trigger source. With the oscilloscope time base set at 1 ns/div, reproducible and simultaneous triggering could be obtained within 0.2 ~.ls of the event. Single pulses were observed with durations be-
BIUS
Red
I
I
10.0
TI$c (ms) Fig. 5. Temporal and spectral resolution of PEL from Nacetylanthranilic acid at two pressures of He/Ne for the ambient atmosphere. Decreasing the pressure from 2 to 0.2 Torr causes the intensity of the red component to decreases.
tween 20 and 130 ps. The subsequent discussion centers on these single-pulse events. A test for the transmission of blue light to the red channel detector was accomplished by inserting an additional glass filter (Schott RG590) in this channel. Since only slight attenuation resulted, it was concluded that this channel detects predominantly the red light from PEL. The corresponding question of red sensitivity in the blue channel is resolved by noting that the sensitivity of the lP28 photomultiplier tube falls off sharply beyond 510 nm. With the BGl filter in place, red light cannot produce the same response in both channels, as was observed under some conditions. It therefore is concluded that the blue channel responds predominantly to blue light. Results obtained with coumarin and Nacetylanthranilic acid (AAA) samples are reported here. These compounds were purified by recrystallization and zone refining prior to use. For AAA, PEL was monitored using an oscilloscope, and the single traces were photographed. For a record of spatial distribution of PEL, the
314
M. C. Nelson et al. / Pyroelectric
luminescence
3. Results and discussion
Fig. 6. Photograph of PEL from N-acetylanthranilic acid with a He/Ne pressure of 0.2 Torr, with the accompanying strip chart record of the photomultiplier tube current during the photographic exposure. For clarity, the edges of the crystal have been outlined.
cryorefrigerator was fitted with a 35 mm SLR camera equipped with a close-up lense. The cold finger was positioned to present a full face view to the camera. A photomultiplier tube in an adjacent window of the cryorefrigerator provided a temporal record of the light intensity during the photographic exposure. Photoluminescence lifetimes of the samples were obtained by excitation with a pulsed nitrogen laser and monitored using a Tracer Northern signal analyzer through a double monochromator with an EM1 9558 photomultiplier tube. Luminescence decay of the helium-neon atmosphere excited by an electric discharge was monitored similarly.
The time and spectrally resolved PEL pulses obtained at different pressures of the He/Ne gas are shown in figs. 4 and 5 for coumarin and AAA. Over a pressure range of 5 mTorr to 2 Torr, it was found that increasing the pressure of the gaseous environment produces an increase in the ratio of red to blue luminescence and a decrease in the pulse width. In all cases, the red and blue pulses occur simultaneously and with identical widths. Several observations prove that this coincidence in the red and blue luminescence is not an artifact. The pulse widths vary with gas pressure. Since the ratio of red to blue intensity varies strongly with pressure and since placement of an additional red filter in the red channel did not significantly alter the intensity ratio, see fig. 4, the coincidence of the light pulses is not due to a broad transmission by either channel. A very important feature of the time domain observations is that in all cases where a distinctly single event is observed, the pulse shape is symmetrical. This behavior is quite unlike the fast rise and exponential decay of photoluminescence excited by a pulse laser. Observations of luminescence resulting from an electric discharge in the He/Ne atmosphere and from excitation of coumarin with a 5 ns laser pulse rule out either of these phenomena as the rate determinin g steps in these observations. In both cases, the luminescence lifetimes were less than 5 ps, which was the limit of the apparatus used in the measurements. A photographic record of PEL is shown in fig. 6. The accompanying strip chart record of the photomultiplier tube current during the photographic exposure shows that a single event of significant luminosity was observed, along with several much less intense pulses. Although it is not obvious in all the photographs that the PEL events are on the crystal surface, it appears that in many examples, as illustrated by the figure, the luminescence originates from the edges. The inference that PEL is a surface phenomenon is corroborated by the susceptibility to gas pressure and by the observations of the spatial distribution, which show the luminescence to originate from the edges.
M.C. Nelson et al. / Pyroelectric
(population) of such microstates in the sample. The surface polarization charge density is given by
4. A model for pyroelectric luminescence On the basis of the above observations, it is possible to construct a model for a single pulse PEL event to account for the symmetrical profile in time, the simultaneous production of red and blue light pulses, the dependence of the luminescence intensity and lifetime on pressure, the origin of the pulse trains shown in fig. 2, and possibly the slowly evolving luminescence under class III conditions. F’yroelectricity commonly is defined as a temperature-dependent spontaneous electric polarization of a crystal. The usual description of this phenomenon is given in terms of an observed change in polarization accompanying a small change in temperature dP=ydT,
0)
where y, the pyroelectric coefficient, also varies with temperature. Though y cannot yet be predicted from knowledge of a given crystal structure, it can be determined from experimental measurements and analysis. Typically this approach has been concerned with changes in the equilibrium polarization corresponding to a change in temperature. A pyroelectric crystal is heated rapidly to a few degrees above the temperature of a thermostated bath, and the charge accumulated at opposite faces of the crystal in response to this small temperature jump is determined Y=A,
J0
mQ( t) dt/AT.
(2)
For PEL, it is more informative to consider the kinetic processes associated with changes in the polarization of a crystal resulting from small time-dependent changes in temperature. Here we a present a model derived from empirical considerations of polarization fields in solids and localized transitions between polarization states. From this viewpoint, a central equation for dynamic pyroelectric phenomena would be p=
C4’CPi)
315
luminescence
(3)
where (pi) is the polarization of microstate i, which is an average over a some small volume (microdomain) of the sample. Ni is the number
(4)
a=n=P,
where n is the surface normal. It is the accumulation of charge in response to this polarization and the migration of that charge accompanying thermally induced changes in P that produce observed pyroelectric phenomena. The requirements for observing pyroelectricity are that the arrangements of dipoles in a crystal do not lead to a net cancellation in P. If over a small temperature range, the microstate polarizations are assumed to be constant, then a change in the macroscopic polarization can be ascribed entirely to changes in the populations of the various microstates AP=
cANi(
(5)
From this viewpoint, the pyroelectric constant can be written as y = dP/dT= and dynamic described by dP/dt
z(dNi/dT)(pi), pyroelectric
(6) phenomena
= x(dN,/dt)(p,).
will be
(7)
The problem then is the description of the factors affecting the various d N/d t in response to a given dT/dt. Since it is a difficult problem to incorporate the distribution of d Ni/dt in space and time and account for the non-linearities due to the influence of the local field on the polarization of a microdomain, we consider from this point the simple case of two polarization states. A microdomain can have either polarization ( pl) or ( pz), and Nl and Nz are the numbers of microdomains with these polarizations in the sample. This simplification is introduced to provide a tractable model containing some of the essential features of the problem. In particular, we will see that the symmetrical pulse shapes can arise from a cooperative effect. For this model the time-dependent polarization is described by dP/dt=
(dN,/dt)((P,)
- (PI>),
(8)
ikf.C. Nelson et al. / Pyroelectric
316
with the stipulation that the total number of microdomains is held constant, This requirement is expected to be valid over macroscopic but small regions of the sample and for temperature changes that are not too large. The transition rate for changes in the number of microdomains with a specific polarization can be written as dN,/dt = k2N2 - k,N,.
(9
In order to include cooperative effects, the rate parameters k, and k, are taken to depend upon the electric field and therefore will vary with the polarization and population of the microdomains. This dependence is represented by k, = k,‘N, and k, = k;N,. The rate for producing a particular microdomain therefore is enhanced by the presence of that microdomain. The transition rate then becomes dN,/dt = N,N,(k;
- k;)
(10)
and the corresponding change in the polarization is given by dP/dt=
NJ&; = N,N,K,,
- k;)((p,)
luminescence
tion charge density, ur,. The time evolution of the red luminescence, In, also is related to the gas pressure, G, and a rate constant Kg IR = apGKg .
(12)
The blue light originates from the sample and is a more complex issue. One mechanism that is consistent with the observations is excitation by electron-hole recombination. Our observations indicate that this luminescence also originates from regions near the surface. We therefore assume that the time evolution of the blue luminescence depends upon the surface charge density, (I, and a rate constant K,. The surface charge density, (I, and the surface polarization charge density, up, are taken to be equal I,=uK,.
(13)
The surface charge density is increased by a change in polarization and decreased by the mechanisms leading to luminescence du/dt = A, dN,/dt -A,&
- ABIB,
- (~2)) (11)
which defines K,. In examining eq. (ll), is clear that if one assumes the initial conditions Ni = 1 and N, = 0, then the transition rate is zero and no dynamical behavior will follow. What is observed, however, is that as the temperature is varied gradually, sudden bursts of light and electrical pulses are produced. Eq. (11) is to be used to describe the time evolution of these bursts and pulses. As the temperature approaches the point where these events occur, both microdomains should be present. This situation could be taken into account by including a temperature-dependent term in eq. (11) to account for the temperature-dependent changes in the polarization and population of the microdomains. For the present purposes, this requirement can be met in the calculations by starting the system with a small value for N,. We now consider a model for the production of the luminescence that is observed. The red light originates from the ambient atmosphere and is taken to be a consequence of the surface polariza-
GK,= 0.00
1
I .o
0.01
lj~:o; ’
IO
0
20
Time (scaled)
0.10
1,
IA_ GK,*2.0
5.0
IO.0
0.00
0
5
IO
Time (scaled)
Fig. 7. Calculated time evolution of blue luminescence for single PEL events at various pressures of the ambient gas using the model described in the text.
M.C. Nelson et al. / Pyroelectric luminescence
where the Aj are constants. The change in the charge density is related to the number of photons produced by A, and A,, and to the change in the polarization and microdomain population by A,. Nonradiative processes, which would affect the quantum yields and may be important, are neglected to avoid introducing additional parameters that are not necessary to model the observations. The following characteristics of PEL are predicted by eqs. (12) (13) and (14). The red and blue light always will be emitted simultaneously and with identical time behavior. The overall time evolution, when GKs differs appreciably from K,, is set almost entirely by the larger of these factors and by K,, so the upper limit of the time response is set by l/K, and the lower limit is set by l/K,. These limits are scaled by the coefficients in eq. (14). Plots of the time-dependent blue luminescence obtained for different values of gas pressure, G, are shown in fig. 7. These plots were generated by numerical calculations using a Runya-Kutta routine [8] with values for A,, A,, and A, of lo-“, 1, and 1, respectively, and using K, = 10p4. As shown in fig. 7, this model produces a range of luminescence pulse widths and shapes depending on the value of GK, at a given K,. For small GK, at low gas pressures, the decay is slow because the gas provides a discharge mechanism. As GK, increases, the pulses get shorter and more symmetrical. At large GK, the pulse width reaches a limit because of l/K,, feedback, and cooperativity in the kinetics. The red intensity is given by IB(f)GKg/KB, and the surface charge density is given by I,(t)/GK,. With K, set a lo5 s-l, the time scale is approximately that of the observed PEL. The decay half-life generated by the model is plotted in fig. 8 against the logarithm of the red to blue intensity (log IR/Ia), and in the bottom half the same information is presented from the actual PEL data. Since the effective pressure of the gas surrounding the sample in a dynamic system is not an easily measured quantity, the ratio of the red to blue luminescence is a more viable means of characterizing the observations in terms of the model. By taking the logarithm of this quantity, any difference in the sensitivities of the two chan-
317
Model
log t Experiment
I\ 0
0
-I
0 log *
Fig. 8. A comparison of the decay half-life as a function of the ambient gas pressure, which is characterized by the ratio of red to blue luminescence, for the model described in the text and for the experimental data. The relative translation of the x-axes of the two graphs can be caused by differences in the sensitivities of the red and blue channels and by an arbitrariness in the choice of parameters in the model.
nels or arbitrariness in the choice of parameters is transformed into a simple translation along the x-axis. The parameters have not been scaled to make the x-coordinates of the plots in fig. 8 agree. If the pyroelectric activity is considered in terms of microdomains, then it is readily envisaged that as the process occurs throughout some region of the crystal, there will come a point when the remaining microdomains are surrounded by neighbors already in the new configuration. As a result these remaining microdomains will switch all at once. In this way, a cycle of behavior will be a produced in which a series of small pulses will seem to trigger a larger pulse, as observed in fig. 2 for class II PEL. The model suggests that at very low gas pressures, the series of discrete events could occur with an average spacing less than the
318
M.C. Nekon et al. / Pyroelectric luminescence
duration of an event. The resulting slowly evolving intensity in the luminescence then would be identified as class III PEL.
5. Summary Observations of the temporal, spatial, and spectral resolution of pyroelectric luminescence (PEL) have been presented. These observations revealed that PEL, which appears to be a surface phenomenon, results from luminescence of both the sample and the ambient atmosphere. Light pulses from the sample and the atmosphere generally are symmetrical in time and occur simultaneously. These observations can be reproduced by a model of dynamic pyroelectricity. In the model the sample luminescence results from charge-carrier recombination, and the atmosphere luminescence results from dielectric breakdown induced by surface polarization fields. These two sources of luminescence are coupled by their mutual dependence on the surface charge density. The symmetrical pulse shape is obtained by a cooperative switching of microdomain polarizations. From the observations and the qualitative
success of this simple model, it seems likely that a quantitative description of dynamic pyroelectric phenomena, including PEL, is a tractable though not necessarily simple or straightforward problem. Studies along these lines should reveal some of the more intricate details of pyroelectricity. Acknowledgement Support for this research from the National Science Foundation under Grant DMR-8305050 is gratefully acknowledged. References [l] J.S. Pate1 and D.M. Hanson, Nature 293 (1981) 445. [2] J.S. Pate1 and D.M. Hanson, Ferroelectrics 38 (1981) 923. [3] D.M. Hanson, J.S. Pate1 and M.C. Nelson, Materials Sci. (Wroclaw) 10 (1984) 459. [4] R.B. Gammage and J.S. Cheka, Health Phys. 32 (1977) 189. [5] R. Nowak and R. Poprawski, Ferroelectric Letters 1(1984) 175. [6] Z. Dreger, J. Kalinowski, R. Nowak and J. Sworakowski, Materials Sci. (Wroclaw) 10 (1984) 67. [7] Z. Dreger, J. Kalinowski, R. Nowak and J. Sworakowsld, Chem. Phys. Letters 116 (1985) 192. [8] S. Gill, Proc. Cambridge Phil. Sot. 47 (1951) 96.