- Email: [email protected]
Long lived red luminescence in diamond
JournalofLuminescence38 (1987) 181—183 North-Holland, Amsterdam
181
LONG LIVED RED LUMINESCENCE IN DIAMOND Estela PEREIRA and LucIlia SANTOS Departarnento e Centro de Fisica (INIC), Universidade de .4veiro, 3800 A veiro, Portugal Naturally occurring brown diamonds present a widely variety of luminescing centres, several ofthem with lifetimes ofthe order ofa millisecond. Two ofsuch centres, with zero phonon lines (ZPL) at 2.082 and 2.052 eV. are analysed. The width of the ZPLs is typical of centres in diamond, indicating symmetry-allowed transitions. Although ZPLs are close in energy, the centres are not related, nor are they related with centres with fast decays (ns) whose luminescence occurs in the same spectral region: they have different luminescence excitation spectra and different relative intensities in different samples. The complex. sample-independent, but temperature-dependent. decay processes show the presence of non-radiative states in thermal equilibrium with the emitting states. Non-radiative transitions to the ground state become important above 90 K. The analogies with other slow emissions in brown diamonds are discussed.
1. Introduction The optical properties of brown diamonds have been studied only in recent years [11. They show a wide variety of luminescing centres in the visible region of the spectrum. Most of the centres found in other types of diamond have lifetimes of the order of nanoseconds and show a ~mirror image’ relationship between absorption and emission spectra, an exeption being the Sl centre [2] found usually in yellow diamonds. However, in brown diamonds, besides the S 1 centre that is usually found in this type of diamonds [31,there are several luminescing centres with lifetimes of the order of a millisecond. They have in common well defined zero phonon lines (ZPL) with widths typical of centres in diamond pointing to symmetry-allowed transitions, and have usually rather strong vibronic bands. No mirror image relationship with the emission band can be found in the absorption of luminescence excitation spectra: they all are excited through higher energy states. Brown diamonds usually show in the red region of the spectrum a strong band with ZPL at 2.145 eV and a Huang—Rhys factor of 9.9, with a dominant phonon of 30 meV [4], and a lifetime is of 6 ns [5]. However in an interval between 2.166 and 2.052 eV eleven independent ZPLs can be detected [4]. Of these the only slow transitions are those at 2.082 and 2.052 eV. Their decay processes are analysed in the present work.
tra by taking the ratio of the monitored luminescence sig-
nal and a reference signal of the light emerging from the excitation monochromator viewed by rhodamine-B.
3. The vibronic band shape In figure 1 we show for comparison a steady state spectrum ofa typical brown diamond taken with excitation at 520 nm and a spectrum viewed from 0.1 to 5 ms after a 10 jts light pulse of the same wavelength. This clearly shows that only the 2.082 and 2.052 eV ZPLs and associated vibronic bands have long lifetimes. The intensities are not correlated from sample to sample and also they have different excitation spectra, that are identical to the steady state ones [4]. Therefore they come from different centres. From a comparison of spectra taken at different cxcitation energies. different time windows and in different samples, the vibronic band shapes of each centre can be obtained. Figure 2 shows two such spectra. The arrows indicate that the dominant phonons associated with each
A
2. Experimental details Time-resolved spectra and lifetime measurements were carried out with a Spex 1934C phosphorimeter that gives light pulses ofca. 10 its from a Xe arc. The delay and window of detection can be varied by l0~ssteps. Luminescence spectra were corrected by comparison with a W lamp ofknown temperatue. and luminescence excitation spec0022-2313/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
.
2.0
2.1
~V)
Fig. 1. Luminescence spectra for excitation at 520 nm: (A) steady state: (B) observed between 0.1 and 5 ms.
L. Pi’ri’iru, L Santo /Loiiy’ /01(1 red /iIl?11/U’.Oi’IilC in diamond
I
time vs temperature for the 2.054 eV centre. Above 40 K there is a single exponential decay, while for lower temperatures two exponential decays can be obtained from 26meV
I
the decay curve. This behaviour is ver~similar to the behavtour found in other centres of diamond. namely of the SI centre and centres at 2.974. 2.818 and 2.490 eV [61.
I
meV -
Also a similar behavior has been found in .AgNa(NO~),.
— -
The decrease in lifetime above 90 K is accompanied by a decrease in quantum yield indicating that non-radiative 1.95 2.00 2.05 (eV) Luminescence spectra taken between 0.1 and 5 ms. Full line, excitation at 500 nm: broken line, excitation at 530 nm.
decay processes to the ground state become important
of the ZPLs are 26 meV for the 2.082 eV centre and 33 me\’ for the 2.052 eV centre. From the ratio of band to ZPL intensity the Huang—Rhys factors . are estimated to be ofthe order of 3 for the 2.052 eV band and 3.5 for the 2.082 eV band, using the linear electron—phonon coupling model [I,.~~/Ii~r=exp 5’). II is interesting to note that the coupling in these bands
the two lifetimes if there is a double dgenerate level 0.5 meV above the emmiting state; the radiatixe transition probability. k,. is taken as 101 s ‘. and the transition probability between trap and radiative level ~ = l0~s yielding a fast and a slow decay rate given by:
is to phonons of ca. 30 meV. which is a rather low salue
where k~is the transition probability between the emit-
for phonons in diamond, but common in bands found in brown diamonds. namely the 2.145 eV band (30 mex [4] and the 2.721 eV band (34 meV) [1]. This may indicate that these bands are due to similar impurities, that have a larger mass than carbon.
ting level and the trap. given h\
Fig.
~.
above this ternperature.The lower temperature data are suggestive of non-radiative levels close to the emitting one. The dotted lines in fig. 3 show the expected behaviour of
k]
Ikr +2k,
1
~ (k. +2k, +k~)
4krki
I
k~=/cvi exp) —1/a/U). with 1/a=0.5 nieV. the energy difference between levels. It can he seen that the model gives a fair interpretation of the experimental data.
Decay processes
4.
The decay processes in both bands are sample-independent indicating that all processes take place within the centres. The decays arc independent of the energy of excitation indicating that the relaxation from the initially excited state is much faster than the processes that occur at the emitting levels. The dots in fig. 3 show the decay
The 2.082 eV decay time is shown by the crosses in fig. 3. The behaviour is similar, although the data are less re-
liable as the band cannot be well isolated from the 2.052 eV band. The decays were measured from the ZPL intensities, taken from time-resolved spectra. The general behaviour is similar to the other centre. Below 40 K again two decay times are observed: the non-radiative decay processess to the ground state start at a lower temperature (~75K).
X
.5
Due to the presence of a ZPL with a width similar to the ZPI.s of other centres in diamond the bands must originate in symmetry-allowed transitions. The rather long lifetime may be explained by a spin multiplicity change in the transition. The emitting and trap levels may be the sublevels ofa triplet state ofwhich only one may emit In the ground sates for symmetr\ reasons.
A
A
A
x .
—
~.
x
-~
A
•
E
I
,
Yemp.
Fig. 3. Lifetime behaviour. S. 2.052eV centre: X, 7.082eV centre: broken lines. predicted behavior for a iwo-level irap in the 2.052 cV cenire.
Conclusions
.
The vibronic hand shapes with ZPL5 typical of centres in diamond and the presence of non-radiative levels close to the emitting ones suggest a symmetry-allowed, spinforbidden transition. In the centres whose structure has been positivel~ identified in diamond (associated with nitrogen and vacancies) no spin-forbidden optical transitions have so far been observed. For instance in the 1.945 eV centre (nitrogen—vacancy pair) a non-radiative triplet state has been identified by EPR following optical cxci-
F. Pereira, L. Santos/Long I/red red luminescence in diamond tation [8], but it decays non-radiatively to the ground
183
Acknowledgments
state. Brown diamonds however are known to have several
Dialap is thanked for the loan of samples. INIC and
impurities, namely transition metals, like Fe, Mn and Ni.
JNICT for financial support and Mr. J.M. Januário for
The presence of these atoms in a defect will favour spinforbidden transitions, due to the larger spin—orbit coupling. Also the fact that in these bands the coupling is to low-energy phonons may indicate a resonance due to the
his help with the experiments.
References
presence of a heavier atom.
The non-radiative transitions to the ground state in these two centres occur at lower temperatures than in other slow luminescing centres in diamond. Sl only shows a decrease in quantum yield above 150 K [3];the 2,974 and 2,490 eV centres above 200 K and the 2.818 eV centre show no reduction in lifetime or quantum yield up to room temperature [6]. This may be due to the lower energy gap between excited and ground state in the new centres. In order to give a full interpretation of these centres further studies are under way, namely determination of the symmetry ofthese centres (e.g. by unaxial stress) and
magneto-optical studies (Zeeman, ODMR).
[I[ AT. Collins and K. Mohammed, J. Ph~s.C: Sol. St. Ph~s.IS (1982) 147. [2] D.S. Nedzveskii and V.A. Gaisin, Sos. Phys. Sol. St. 15 (1973) 427. [3] ME. Pereira. M.I.B. Jorgeand M.F. Thomaz. J. PhYs. C: Sol. St. Phys.. in print. [4]ME. Pereira. M.I.B. Jorge and M.F. Thomaz, 3. Ph~s.C: Sal. St. Phys. 19(1986)1009. [5] M.I.B. Jorge. ME. Pereira, M.F. Thomaz. G. Davies and AT. Collins. Portugaliae Physica 14 (1983) 195.
[6] ME. Pereira and L. Santos, presented at ICL’87. Beijing (Aug. 1987) J. Lumin. 40&41 (1988) 139. [7] C. Thomasand H. Happ. J. Phys. C: Sot. St. Ph~s.19 (1986) 6087. [8] J.H.N. Loubser and Van W\k, Diamond Res. (1977) Ii.
181
LONG LIVED RED LUMINESCENCE IN DIAMOND Estela PEREIRA and LucIlia SANTOS Departarnento e Centro de Fisica (INIC), Universidade de .4veiro, 3800 A veiro, Portugal Naturally occurring brown diamonds present a widely variety of luminescing centres, several ofthem with lifetimes ofthe order ofa millisecond. Two ofsuch centres, with zero phonon lines (ZPL) at 2.082 and 2.052 eV. are analysed. The width of the ZPLs is typical of centres in diamond, indicating symmetry-allowed transitions. Although ZPLs are close in energy, the centres are not related, nor are they related with centres with fast decays (ns) whose luminescence occurs in the same spectral region: they have different luminescence excitation spectra and different relative intensities in different samples. The complex. sample-independent, but temperature-dependent. decay processes show the presence of non-radiative states in thermal equilibrium with the emitting states. Non-radiative transitions to the ground state become important above 90 K. The analogies with other slow emissions in brown diamonds are discussed.
1. Introduction The optical properties of brown diamonds have been studied only in recent years [11. They show a wide variety of luminescing centres in the visible region of the spectrum. Most of the centres found in other types of diamond have lifetimes of the order of nanoseconds and show a ~mirror image’ relationship between absorption and emission spectra, an exeption being the Sl centre [2] found usually in yellow diamonds. However, in brown diamonds, besides the S 1 centre that is usually found in this type of diamonds [31,there are several luminescing centres with lifetimes of the order of a millisecond. They have in common well defined zero phonon lines (ZPL) with widths typical of centres in diamond pointing to symmetry-allowed transitions, and have usually rather strong vibronic bands. No mirror image relationship with the emission band can be found in the absorption of luminescence excitation spectra: they all are excited through higher energy states. Brown diamonds usually show in the red region of the spectrum a strong band with ZPL at 2.145 eV and a Huang—Rhys factor of 9.9, with a dominant phonon of 30 meV [4], and a lifetime is of 6 ns [5]. However in an interval between 2.166 and 2.052 eV eleven independent ZPLs can be detected [4]. Of these the only slow transitions are those at 2.082 and 2.052 eV. Their decay processes are analysed in the present work.
tra by taking the ratio of the monitored luminescence sig-
nal and a reference signal of the light emerging from the excitation monochromator viewed by rhodamine-B.
3. The vibronic band shape In figure 1 we show for comparison a steady state spectrum ofa typical brown diamond taken with excitation at 520 nm and a spectrum viewed from 0.1 to 5 ms after a 10 jts light pulse of the same wavelength. This clearly shows that only the 2.082 and 2.052 eV ZPLs and associated vibronic bands have long lifetimes. The intensities are not correlated from sample to sample and also they have different excitation spectra, that are identical to the steady state ones [4]. Therefore they come from different centres. From a comparison of spectra taken at different cxcitation energies. different time windows and in different samples, the vibronic band shapes of each centre can be obtained. Figure 2 shows two such spectra. The arrows indicate that the dominant phonons associated with each
A
2. Experimental details Time-resolved spectra and lifetime measurements were carried out with a Spex 1934C phosphorimeter that gives light pulses ofca. 10 its from a Xe arc. The delay and window of detection can be varied by l0~ssteps. Luminescence spectra were corrected by comparison with a W lamp ofknown temperatue. and luminescence excitation spec0022-2313/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
.
2.0
2.1
~V)
Fig. 1. Luminescence spectra for excitation at 520 nm: (A) steady state: (B) observed between 0.1 and 5 ms.
L. Pi’ri’iru, L Santo /Loiiy’ /01(1 red /iIl?11/U’.Oi’IilC in diamond
I
time vs temperature for the 2.054 eV centre. Above 40 K there is a single exponential decay, while for lower temperatures two exponential decays can be obtained from 26meV
I
the decay curve. This behaviour is ver~similar to the behavtour found in other centres of diamond. namely of the SI centre and centres at 2.974. 2.818 and 2.490 eV [61.
I
meV -
Also a similar behavior has been found in .AgNa(NO~),.
— -
The decrease in lifetime above 90 K is accompanied by a decrease in quantum yield indicating that non-radiative 1.95 2.00 2.05 (eV) Luminescence spectra taken between 0.1 and 5 ms. Full line, excitation at 500 nm: broken line, excitation at 530 nm.
decay processes to the ground state become important
of the ZPLs are 26 meV for the 2.082 eV centre and 33 me\’ for the 2.052 eV centre. From the ratio of band to ZPL intensity the Huang—Rhys factors . are estimated to be ofthe order of 3 for the 2.052 eV band and 3.5 for the 2.082 eV band, using the linear electron—phonon coupling model [I,.~~/Ii~r=exp 5’). II is interesting to note that the coupling in these bands
the two lifetimes if there is a double dgenerate level 0.5 meV above the emmiting state; the radiatixe transition probability. k,. is taken as 101 s ‘. and the transition probability between trap and radiative level ~ = l0~s yielding a fast and a slow decay rate given by:
is to phonons of ca. 30 meV. which is a rather low salue
where k~is the transition probability between the emit-
for phonons in diamond, but common in bands found in brown diamonds. namely the 2.145 eV band (30 mex [4] and the 2.721 eV band (34 meV) [1]. This may indicate that these bands are due to similar impurities, that have a larger mass than carbon.
ting level and the trap. given h\
Fig.
~.
above this ternperature.The lower temperature data are suggestive of non-radiative levels close to the emitting one. The dotted lines in fig. 3 show the expected behaviour of
k]
Ikr +2k,
1
~ (k. +2k, +k~)
4krki
I
k~=/cvi exp) —1/a/U). with 1/a=0.5 nieV. the energy difference between levels. It can he seen that the model gives a fair interpretation of the experimental data.
Decay processes
4.
The decay processes in both bands are sample-independent indicating that all processes take place within the centres. The decays arc independent of the energy of excitation indicating that the relaxation from the initially excited state is much faster than the processes that occur at the emitting levels. The dots in fig. 3 show the decay
The 2.082 eV decay time is shown by the crosses in fig. 3. The behaviour is similar, although the data are less re-
liable as the band cannot be well isolated from the 2.052 eV band. The decays were measured from the ZPL intensities, taken from time-resolved spectra. The general behaviour is similar to the other centre. Below 40 K again two decay times are observed: the non-radiative decay processess to the ground state start at a lower temperature (~75K).
X
.5
Due to the presence of a ZPL with a width similar to the ZPI.s of other centres in diamond the bands must originate in symmetry-allowed transitions. The rather long lifetime may be explained by a spin multiplicity change in the transition. The emitting and trap levels may be the sublevels ofa triplet state ofwhich only one may emit In the ground sates for symmetr\ reasons.
A
A
A
x .
—
~.
x
-~
A
•
E
I
,
Yemp.
Fig. 3. Lifetime behaviour. S. 2.052eV centre: X, 7.082eV centre: broken lines. predicted behavior for a iwo-level irap in the 2.052 cV cenire.
Conclusions
.
The vibronic hand shapes with ZPL5 typical of centres in diamond and the presence of non-radiative levels close to the emitting ones suggest a symmetry-allowed, spinforbidden transition. In the centres whose structure has been positivel~ identified in diamond (associated with nitrogen and vacancies) no spin-forbidden optical transitions have so far been observed. For instance in the 1.945 eV centre (nitrogen—vacancy pair) a non-radiative triplet state has been identified by EPR following optical cxci-
F. Pereira, L. Santos/Long I/red red luminescence in diamond tation [8], but it decays non-radiatively to the ground
183
Acknowledgments
state. Brown diamonds however are known to have several
Dialap is thanked for the loan of samples. INIC and
impurities, namely transition metals, like Fe, Mn and Ni.
JNICT for financial support and Mr. J.M. Januário for
The presence of these atoms in a defect will favour spinforbidden transitions, due to the larger spin—orbit coupling. Also the fact that in these bands the coupling is to low-energy phonons may indicate a resonance due to the
his help with the experiments.
References
presence of a heavier atom.
The non-radiative transitions to the ground state in these two centres occur at lower temperatures than in other slow luminescing centres in diamond. Sl only shows a decrease in quantum yield above 150 K [3];the 2,974 and 2,490 eV centres above 200 K and the 2.818 eV centre show no reduction in lifetime or quantum yield up to room temperature [6]. This may be due to the lower energy gap between excited and ground state in the new centres. In order to give a full interpretation of these centres further studies are under way, namely determination of the symmetry ofthese centres (e.g. by unaxial stress) and
magneto-optical studies (Zeeman, ODMR).
[I[ AT. Collins and K. Mohammed, J. Ph~s.C: Sol. St. Ph~s.IS (1982) 147. [2] D.S. Nedzveskii and V.A. Gaisin, Sos. Phys. Sol. St. 15 (1973) 427. [3] ME. Pereira. M.I.B. Jorgeand M.F. Thomaz. J. PhYs. C: Sol. St. Phys.. in print. [4]ME. Pereira. M.I.B. Jorge and M.F. Thomaz, 3. Ph~s.C: Sal. St. Phys. 19(1986)1009. [5] M.I.B. Jorge. ME. Pereira, M.F. Thomaz. G. Davies and AT. Collins. Portugaliae Physica 14 (1983) 195.
[6] ME. Pereira and L. Santos, presented at ICL’87. Beijing (Aug. 1987) J. Lumin. 40&41 (1988) 139. [7] C. Thomasand H. Happ. J. Phys. C: Sot. St. Ph~s.19 (1986) 6087. [8] J.H.N. Loubser and Van W\k, Diamond Res. (1977) Ii.