Near-infrared luminescence spectroscopy of Al2O3 : V3+ and YP3O9: V3+

Volume 154. number 5

CHEMICAL

NEAR-INFRARED LUMINESCENCE Christian

REBER

PHYSICS LETTERS

3 February

1989

SPECTROSCOPY OF A1203 : V3+ AND YI’,O, : V3+

and Hans U. GODEL

Institut ftir Anorganische Chemie, Vniversitiif Bern, CH-3000 Bern 9. Switzerland Received 2 November

1988

Near-infrared luminescence and excitation spectra of A1203 : V3+ and YP,Os : V’+ are presented. In Al,OJ : V”+ the trigonal and spin-orbit splitting of the ‘T,, (0, notation) ground state is determined: spinor levels {with D, symmetry designations) are at 0 (A, ), 8.1 (E), 992 (E) and 1134 cm-’ (E, A, and Ai overlapping). Errors in the literature concerning these splittings are corrected. A full crystal field calculation is used to rationalize all the information from absorption and luminescence spectra. There is spectroscopic evidence for inequivalent V )+ sites in YP,09 : V’+. The quantum efficiency in both systems is low, in contrast to V3+ doped in chloride and bromide host lattices. A simple model is presented to account for this difference.

1. Introduction The luminescence properties of the V3+ ion have not received much attention in the past. The reason for this is most likely the difficult detection in the near-infrared (NIR, 1000-1200 nm) region. In addition AllO : V3+, the first system investigated, shows extremely weak features with unusually short luminescence decay times around 9750 cm-i, which can barely be detected with an Sl photomultiplier [l-3]. Measurements to lower energy, i.e. beyond the Sl PM range, have not been reported. Very similar observations to AlzOs : V3+ have been made for V3+ doped in Y3A150i2 (YAG) [ 41. The contrasting behavior of these two systems to ruby, A1203 : Cr3+, was then taken as evidence for intrinsically more efficient nonradiative relaxation processes in d2 versus d’ ions. This was backed by a theoretical model, in which the electronic factor in the relevant nonradiative transition rate was predicted to be much larger for a d2 ion [ 5,6]. It came as a surprise, therefore, that recent measurements on V3+ and Ti2+ in various halide host crystals showed very intense and long-lived luminescences in the NIR [ 7,8]. On the basis of our experience with these halide lattices we decided to reinvestigate A1203 : V3+ and possibly extend the energy range further into the NlR by using highly sensitive detectors. One of the main aims was the location of the upper trigonal compo0 009-2614/89/s ( North-Holland

03.50 0 Elsevier Science Publishers Physics Publishing Division )

nent of the ‘T,, (0, notation) ground state. This splitting is critical for a reliable evaluation of the trigonal crystal field parameters, and it had not been directly determined before. A broad feature between 800 and 1000 cm-’ in the Raman spectrum was ascribed to an electronic transition between the two trigonal components, but this assignment was not fully convincing [ 91. In addition to this question of the trigonal splitting we were interested in a determination of further ground state splittings due to spin-orbit coupling. And then, of course, there was the interesting question as to why nonradiative relaxation processes are much more efficient in A1203 : V3+ than in Al,Os : Cr3+ or in various V’+doped halide lattices. We chose another lattice with octahedral oxygen coordination, YP309, which could be doped with V’+, to test whether the properties of Alz03 : V’+ or YAG : V3+ are unique or whether there is a typical luminescence and relaxation behavior of oxygen coordinated to V3+.

2. Experimental A crystal boule of A1203 : V’+ (doping level 0.8%) grown with the Vemeuil technique was obtained from Djevahirdjian SA. YP309 : V3+ was prepared according to ref. [ lo] with the trivalent vanadium B.V.

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added as V,03 (doping level 2%). The resulting green-grey powder was checked for purity using powder X-ray diffraction (Cu Ka, radiation). The diffraction pattern agrees well with a literature diagram [ 111. All the luminescence and excitation spectra were performed on powder samples of YP,09 : V3+. A crystal of AlJO : V 3+ suitable for spectroscopic experiments was cut from the boule with a diamond wavering blade in such a way that all the faces are either perpendicular or parallel to the optical axis. The faces were polished with corundum powder. Polarized absorption spectra of A1203 : V3+ were measured with a Gary 17 spectrometer and agreed perfectly with those in ref. [ 121. Trivalent vanadium therefore determines the absorption properties and is the most abundant chromophore in our sample. High-sensitivity luminescence measurements in the visible spectral range indicated the presence of minor amounts of Ti3+ and Cr3+. NIR luminescence spectra were recorded as follows (a Xe lamp was used as an efficient broad-band excitation source): The luminescence was dispersed with a 3/4 m monochromator (Spex 1702) and detected with a cooled Ge detector/preamplifier system (ADC 403L). The signal was further amplified with a lock-in amplifier (PAR 186A). Excitation spectra of YP309 : V3+ were recorded with a tungsten halogen lamp and a l/4 m monochromator (Spex minimate) substituting for the Xe lamp. The excitation spectra were corrected for system response. The second and their harmonic lines of a Nd : YAG laser (Quanta-Ray DCR 3D) were used as a pulsed excitation source for luminescence decay measurements. The luminescence was dispersed with the 3/4 m monochromator and detected with a fast response Ge detector ( ADC 403H.S). Decay curves were recorded with a digital oscilloscope (Tektronix 2430A). Time-resolved spectra of AlLO : V3+ were recorded with a boxcar averager (PAR 162/164). The samples were cooled using the helium gas flowtube technique [ 13 ] in all experiments.

3. Spectroscopic results The luminescence spectrum of A&O3 : V3+ is presented in fig. 1. The width and shape of the observed 426

PHYSICS LETTERS

3 February

1989

rel.

0

50

100 T[KI

-

cm-1

7K& 8500 Fig. 1. Near-infrared luminescence signments of the electronic origins insert at right shows the highest resolution, the insert at left shows cles) and integrated intensities temperature.

:A, 9500

cm--1

spectrum of AlzO, : Vs* Asin D, symmetry are given. The energy transitions under high luminescence decay times (cir(triangles) as a function of

transitions is indicative of a spin-forbidden intraconfigurational transition, similar to the ruby R lines. The two highest energy bands are shown in detail in the insert and are identical to the published luminescence spectra [ 1,2 1. From fig. 1 it is evident that at lower energy there are no other spectroscopic features that are as narrow as the high energy lines. A rationalization of this behavior is given in section 4.1. The broad rising background with increasing energy is most likely due to emission from Ti3+. Excitation at 355 nm, where the absorption is far more intense for V3+ than for Ti3+, leads to spectra with much less background but otherwise identical to the spectrum presented in fig. 1. Both the luminescence intensity and decay time drop significantly with increasing temperature, as shown in fig. 1. The luminescence decay time is 2.5 ps at 8 K, in agreement with refs. [2,3]. These experimental results are indicative of efficient nonradiative relaxation processes down to the lowest temperatures, as discussed in section 4.3. The wavelength range of the metaphosphate luminescence is similar to AlTO : V3+, but there are clear spectroscopic differences: the prominent high energy transition has a width of 85 cm-’ at IO K, i.e.

CHEMICALPHYSICSLETTERS

Volume 154, number 5

I

4 5.0

n rel. ht.

f

l f *

l

i

2.5

.

*A

50

100

Luminescence

8500

TKI

15000

3T 29’

25000 cm-l

3T x3

9500

cm-

Fig. 2. Near-infrared luminescence spectrum of YP,O, : V7+. The insert at right shows excitation spectra of the luminescence at wavelengths a and b, marked in the main figure. Band assignments are given in 0, notation. The insert at left shows luminescence decay times (circles) and integrated intensities (triangles) as a function of temperature.

an order of magnitude broader than in corundum. In addition, there appears a broad-band transition at lower energy. The intensity ratio of sharp to broad luminescence is temperature independent within experimental accuracy. Excitation spectra of the luminescence observed at wavelengths a and b, respectively, are given in fig. 2. They identify both the sharp-line and the broad-band luminescence as originating from V3+ centers. The luminescence decay times at the two wavelengths are not distinguishable. The 8 K value is 5 us. Both the luminescence intensity and the decay time exhibit a similar drop with increasing temperature as in A1203 : V3+. This is shown in the insert of fig. 2.

4. Discussion 4.1.

Ground state splitting of V3+ in A1203

The trigonal symmetry of V 3f in A1203 is known to influence the absorption spectrum [ 121. It is observed as a broadening of the ‘T& band but no distinct electronic origins belonging to the trigonal components are identified. The luminescence spec-

3 February 1989

trum is more informative in this respect. The emitting state at low temperatures is a trigonal component of ‘TZg,which derives from the same strong-field configuration as the ground state. The luminescence transitions therefore take place within this (t2,)2 electron configuration and appear as sharp lines in the spectrum. The situation is quite analogous to the well-known R line emission of d3 ions taking place within the (t2,)3 configuration. There is a notable difference, however, between the two cases, in that the ground state of d3 is an orbital singlet, whereas for the d2 ion it is 3T&. This state, which is split by the trigonal crystal field potential and by spin-orbit coupling, is directly accessible by luminescence spectroscopy. In Ti’+-doped MgC12 this ground-state splitting into six spinor states could be determined very accurately and rationalized in terms of a simple crystal field model. In A1203 : V’+ the trigonal splitting is dominant and we will discuss it first, momentarily ignoring the spin-orbit effects. The 3T,g ground state of a V3+ ion splits into 3A2 and ‘E components in D-(. It is known from EPR [ 141, magnetic susceptibility [ 151 and existing luminescence measurements that the 3Az component is lower in energy. The 3E component is not thermally populated at room temperature, therefore EPR and magnetic susceptibility experiments cannot provide information about it, From the luminescence spectrum in fig. 1 the trigonal splitting of the ground state can be directly determined. The 3A2 component corresponding to the prominent sharp band at about 9740 cm- ’ has been identified before. We assign the much broader and more intense band centered at approximately 8610 cm-’ to the 3E component. This is based on its energy displacement of about 1130 cm-’ from the 3A2 origin, which is beyond the vibrational energy range in this lattice. In addition, the 8610 cm-’ band shows some structure as expected for 3E and discussed below, and associated with it we observe a vibrational sideband structure as expected for an origin. The difference in bandwidths between the ‘A> and the 3E origins is approximately an order of magnitude. Part of the 3E broadening is due to the fact that this band is composed of four overlapping spinor transitions separated by some 100 cm-‘, as discussed below. Besides this, there remains a significant difference, which is most likely due to a ho427

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CHEMICAL

mogeneous broadening of the 3E transitions. We ascribe it to a direct phonon emission process from 3E to ‘AZ in the ground state. Such processes are known to be important factors determining luminescence linewidths of Cr3+ in various oxides [ 161. A theoretical overview of such homogeneous broadening effects is given in ref. [ 17 1. In the present case the energy gap of 1130 cm- ’ is much larger than in the Cr3 + systems mentioned above. It can be bridged by two quanta of the high-energy V3+-02- stretching and bending vibrations, which extend up to about 900 cm-’ in the corundum lattice [ 121. In contrast, the energy range of Ti2+-CLvibrations in MgC& : TiZ+ only extends to about 250 cm-‘, so that a larger number of quanta is required to bridge the 3E-3A2 energy gap. As a result the 3E broadening is much less pronounced in MgCl* : Ti’+. We therefore feel confident in ascribing the increased luminescence linewidth of the ‘E transitions in A1203 : V3* to a phonon emission process in the final state of the luminescence transition. The trigonal 3E-3Az splitting, a quantity that was up to now experimentally undetermined and thus only estimated from crystal field calculations, can now be used as input for such a calculation. The position of the highest energy luminescence line (9744 cm-‘) determines the energy of the lowest singlet excited state. The energies of the 3T2, and 3T& excited states are taken from absorption spectra. The crystal field calculation was done in the computational scheme outline in ref. [ 7 ] _ All the observed and calculated energies are collected in table 1. The crystal field parameters differ appreciably from those reported in refs. [ 18-201. The main reason for this discrepancy is that the lowest energy excited state was wrongly assumed to be at 8770 cm-’ [ 181. Later on the position of the first excited state was corrected [21-231 to agree with the highest energy luminescence transition, but the trigonal ground state splitting was still not known. Our parameter set is similar to that reported in refs. [22,23], but more experimental information could be used to back it up. So far in the discussion we have ignored the spinorbit splittings of the 3E and 3A2 states. The 3Az splitting is nicely resolved in the spectrum, as shown in the insert of fig. 1, and its magnitude of 8.1 cm- ’ is well reproduced by the crystal field calculation. In the 3E state the situation is not as clear-cut because 428

PHYSICS LETTERS

3 February

1989

Table 1 Crystal field parameters (in cm -’ ) and calculated levels for VI+ in Al2O3. Spectroscopically determined levels are included for comparison IODq

18328 329

DC? DT B c

590 2830

i

164

Level

0,

-214

Energy

D3

spinor

‘4

AI

0

E

8.2

(cm- ’ )

talc.

obs.

notation

Tp

Ea 3E

990

Eb

1096

AZ

1178

Al

1197

‘T&

IE

E

9753

‘E,

‘E

E

10548

‘A,

AI

10706

)A!

AZ E

16888

A,

17123

A2

17136

jT*g

3E

3T!s

‘E

‘AZ

0 a) 8.1 .a) 992 *’ 1134 =J

9744 al _

16900

E

17163

E

17188

A,

24483

A2

24502

E

24545

E

24617

E

24822

A,

24830

17200 b,

24800 b,

a’ Position observed from luminescence spectrum. ‘) Position observed from absorption spectrum.

of the line broadening effect. We do observe some structure, however, and we can assign the weak lowenergy component at 990 cm-’ to the lowest energy E” spinor expected at 992 cm-’ and the intense band centered at 1134 cm- ’ to the three close-lying and thus overlapping transitions to Eb, A2 and A,. We note, therefore, that the spin-orbit splittings of both 3A, and 3E are in very good overall agreement with the calculation. There is no evidence for a Ham quenching of the trigonal or spin-orbit splittings and

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thus no evidence of a Jahn-Teller (JT) effect in the ground state. This is confirmed by the data presented in refs. [ 14,22,24], and the postulation of a JT effect in refs. [ 25,261 is suspicious. In contrast, a dominant JT effect is well established in the 3T2, excited state of A1209 : V ‘+, and it has been analyzed in detail [ 24,27 1. The situation is very similar to that found for V3+ and Ti2+ doped into various halide lattices of cubic, trigonal and tetragonal symmetry: no evidence for a JT effect in the ground state, strong evidence for a Ham quenching in 3T2g 4.2. Multiple

sites in YP30g : V3+

The simultaneous occurrence of sharp-line and broad-band luminescence from V j+ in this host lattice can have two physical origins: (i) Ions in identical sites can simultaneously luminesce from a singlet and a triplet excited state which are in thermal equilibrium. (ii) Luminescence occurs from two or more sets of non-equivalent centers in different crystals fields. Situation (i) is encountered in various chloride and bromide lattices doped with V3+ and Ti*+. It can be ruled out in the present case by the observation that the sharp/broad luminescence ratio is temperature independent. YP309 is known to contain four crystallographically nonequivalent Y3+ sites, each with a distorted octahedral oxygen coordination [ lo]. Two of the sites have a center of inversion. The observation that the sharp and broad luminescence bands have the same lifetime and very similar excitation spectra seems, at first sight, to contradict their assignment as multiple emissions. However, as will be shown in section 4.3, nonradiative processes determine the lifetime, and only a small difference in the crystal field is required to change the order of the first excited states. In various chloride and bromide hosts the lowestenergy singlet ( ‘T2,J and triplet (3T29) excited states are separated by a few hundred wavenumbers. Very small distortions of the highly symmetrical elpasolite lattices reported in ref. [ 8 ] can lead to significant changes in the excited state energies or even invert the energy ordering, and thus strongly influence the luminescence behavior. The actual distortions of the various sites in YP309 are not known, but they are likely to be bigger than in the elpasolite lattices, and it is thus conceivable that in some of them V3+ has

3 February

1989

a ‘Tzs component as the emitting state, resulting in broad-band emission. The broad emission dominates the sharp band by about ten to one, and it is tempting to attribute it to a majority of sites. However, this argument ignores the different excitation efficiencies of the sites with and without inversion centers. The relatively large width of 90 cm-’ at 7 K of the sharp emission band is most likely inhomogeneous. The excitation and sharp luminescence band positions can be rationalized with a crystal field calculation assuming Oh symmetry, which is likely to be a rather gross approximation. The resulting parameter values as well as observed and calculated energy levels are summarized in table 2. The experimental data base does not allow a distinction between the inequivalent sites for the crystal field calculation. The electron repulsion parameters obtained for YP309 : V3+ are comparable to those for A&O3 : V3+, as expected for the chemically similar first coordination sphere. The cubic crystal field parameter lODq, on the other hand, is significantly smaller in YP309 : V3+. This difference can be correlated with the different ionic radii of 0.900 and 0.535 A, for Y3+ and Al’+, respectively [ 28 1, causing a much shorter V3+-02- distance and therefore a stronger crystal field in Ai203. The 3Tzs excited state is too high in energy to compete with ‘Tpg as the emitting state in A1203 : V3+. In YP309 : V3+ this is only true for the sharp-band emitting centers. The broad-band luminescence originates in a component of 3T2,. Both the Table 2 Crystal field parameters (in cm- ’ ), observed and calculated els for V’* in YP,O,. O,, symmetry is assumed 1ODq B c Level

‘Tk ‘128

‘T*s ‘T!,

lev-

I4700 560 3020 Energy (cm-‘) talc.

obs.

0 10114 13584 20778

0.’ 10199” 13600” 20800 b’

‘) Position observed from luminescence spectrum. b1 Position observed from excitation spectrum.

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relatively weak overall crystal field and a substantial splitting of 3T2g due to distortions of the octahedral geometry may be responsible for this.

4.3. Nonradiative relaxation In both systems investigated in this study the V3+ luminescence intensity is very weak even at the lowest temperatures and drops even more with increasing temperature as shown in figs. 1 and 2. We estimate that at 10 K the quantum efficiency is about three orders of magnitude smaller than for V3+ in chloride and bromide environments. The luminescence lifetimes, 2.5 and 5 us at 8 K for Al,OJ and YPS09, respectively, are three orders of magnitude shorter than the 10 ms estimated from the 3T,g+‘T2g absorption intensity in A1203 : V3+ for a purely radiative transition [ 2,3]. All these experimental results clearly show the dominance of nonradiative multiphonon relaxation processes down to the lowest temperatures. In the case of the YPS09 lattice this results in decay times for the various emitters which are nondistinguishable, even though the electronic nature of the emitting states is different. This prevents the use of luminescence decay curves and timeresolved luminescence spectra to further resolve the inequivalent sites in YP309 : V3+. In contrast, nonradiative processes in chlorides and bromides doped with V3+ occur only at temperatures higher than 250 K [ 81. These systems are evidence against the suggestion that ions with a dZ configuration are generally more susceptible to nonradiative relaxation processes than d3 ions, as outlined using group theoretical arguments in refs. [ 5,6]. The nature of the chemical bonding of V’+ to the ligands and of the accepting vibrations determine the efficiency of the relaxation pathways. We can make a very rough estimate of the order of magnitude of relative nonradiative relaxation rates with the following comparisons: In A1203 : V3* and AlzO, : Cr3+ low-temperature luminescence quantum efficiencies are approximately 0.0 1% and 1001, respectively. About 10 and 16 quanta of the highestenergy vibrations are required to bridge the energy gap between the emitting state and the ground state in the two systems, respectively. In Cs2NaYC16 : V3+ and Cs2NaYC16 : Cr3+ [ 29 ] with approximately 100% quantum efficiency in both cases, the corre430

3 February1989

sponding numbers are 39 and 42 quanta, respectively. The simplest possible model to account for these effects is an energy gap law [ 301 which, at low temperatures, is given by W$, = const. x cp,

(1)

where p is the number of vibrational quanta required to bridge the energy gap and W$, is the nonradiative relaxation rate. c is a quantity which we assume to have the same value of 0.3 for all four systems compared here. Inserting the experimental numbers of p in eq. ( 1) we then obtain the following ratios, W:;(A1203 : V3+) =1370, W;;(A1203 : Cr3+) W 39( Csz NaYCl, : V3+ ) =37. Wg( Cs2NaYC16 : Cr3+ ) This is only an order of magnitude estimate, but the ratios can nicely account for the observed differences in the quantum efficiencies. We conclude that quenching of NIR luminescence by multiphonon relaxation processes is more strongly dependent on the frequencies of the active vibrations than the quenching of visible luminescence. A more detailed account of the nonradiative relaxation processes in a variety of V3+-doped systems will be published elsewhere 1311. The luminescence quantum efficiency of V3+ doped into oxide lattices is approximately three orders of magnitude lower than in chloride and bromide lattices. We conclude that oxygen coordination is not ideal for technological applications of transition metal ions with luminescence below 10000 cm-‘, in contrast to ions emitting at higher energies; ruby (A1203 : Cr3+) and alexandrite (BeAl,O, : Cr3 + ) are examples of oxides successfully applied as laser media.

Acknowledgement We are indebted to Djevahirdjian SA for providing a sample of A1203 : V 3+, to Claude Daul for making his crystal field programs available to us and to Elmars Krausz for his help with the luminescence decay as well as many discussions. Financial support

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of this work by the Swiss National dation is gratefully acknowledged.

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Science Foun-

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