Journal of Luminescence 102–103 (2003) 608–613

Luminescence of closed shell molecular complex centers innanoporous sol–gel SiO2 glasses

M. Morita*, S. Kajiyama, D. Rau, T. Sakurai, M. Iwamura

Department of Industrial Chemistry, Faculty of Engineering, Seikei University, Kichijoji, Musashino-shi, Tokyo 180-8633, Japan

Abstract

Sol–gel silica glasses doped with transition metal ions (M=Ti4+, V5+, Cr6+, Zr4+, Nb5+, Mo6+, Hf4+, Ta5+, W6+)

were prepared to investigate luminescence properties of nanoporous phosphors by measurements of luminescence

spectra and lifetimes in the temperature range between 10 and 300K. The glasses show broad emission bands at around

18,000 cm�1 under a N2 laser excitation. Luminescence is due to the ligand-to-metal charge transfer (3LMCT) transition

of ions with closed-shell electronic structures, based on tetrahedral molecular complex [MO4] centers. These ions reveal

vibronic structures depending on surface conditions. The presence of oxygen-coordinated octahedral [MO6] centers

must also be considered to understand the spectral band at higher energy observed in Ti, Nb, Ta, and W doped glasses.

Enhancement of quantum efficiency up to 10% is found in hybrid glasses doped with Al and V ions.

r 2003 Elsevier Science B.V. All rights reserved.

Keywords: Sol–gel; SiO2 glass; Transition metal ion

1. Introduction

Current interests are focused on the opticalproperties of nano-porous sol–gel silica glasses,because transparent and homogeneous glasses areobtained easily at low temperatures as compactbricks, thin films and fibers for applications inelectro-optical (EO) devices, displays, screens, LSIlogic and memory, compact lasers and so on [1].We have been interested in exploring the lumines-cence properties of nano-structured sol–gel SiO2glasses doped with transition metal ions, rare earthions and semiconductor nanocrystals [2–4]. In thecase of 3d-transition metal ions, we have observed

broad luminescence bands in the visible spectralregion, which are due to the ligand-to-metalcharge transfer (3LMCT) transition from tetrahe-dral [MO4]

n� complex centers of highly oxidizedions (M=Ti4+, V5+, Cr6+ and Mn7+). Lumines-cence properties are comparable with those ofScheelite compounds (e.g. YVO4, CaWO4, etc.)[5,6] which are considered to be possible lasermedia [7]. Recently, we reported luminescence ofsol–gel silica glasses doped with chromium (III)and vanadium (III) complexes to examine changesof oxidation states of ions in sol–gel and xerogelglasses, using materials annealed at differenttemperatures [8]. The [CrO4]

2� (d0, Cr6+) centerin sol–gel silica glass was confirmed by comparisonof emission from d1 (Cr5+), d2 (Cr4+) and d3

(Cr3+) electronic configurations. If we extend ourstudy of early transition metal ions with 3d

*Corresponding author. Tel.: +81 422 37 3749; fax: +81 422

37 3871.

E-mail address: morita@ch.seikei.ac.jp (M. Morita).

0022-2313/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0022-2313(02)00609-9

electronic configurations to those of later transi-tion metal ions (M=Zr, Nb, Mo, Hf, Ta, W),strong emission spectra are expected from 4d and5d elements due to increase of spin–orbit interac-tion and changes of chemical properties. In thispaper, spectral profiles and decay times at lowtemperatures are discussed by taking into accounttwo complex centers in sol–gel glasses.

2. Experimental

Sol–gel silica glasses doped with transition metalions were prepared by dissolving transition metalions (nominally 0.1mol% of M=Ti, V, Cr, Zr,Nb, Mo, Hf, Ta, W) into tetramethoxyorthosili-cate (TMOS), according to the sol–gel technique[8]. After the redox reaction process was completedin a few months, transparent and homogeneousxerogel (wet glass) was obtained at room tempera-ture. Sol–gel silica glasses were then prepared astransparent and flat plates of pale yellow colors byannealing the xerogel for 24 h in an electric furnaceat 5001C. We can avoid darkening and cracking ofglasses by raising temperatures very slowly in the

annealing process. Luminescence, time-resolvedluminescence, and decay times were measured attemperatures between 10 and 300K under a N2laser (337.1 nm) excitation using a computercontrolled spectroscopic system based on a Spex1401 double monochromator, HR320 photoncounter and DG420 digital oscilloscope, developedin our laboratory [9]. Luminescence excitationspectra were measured by a similar system coupledwith an AMKO monochromator and a 100W Xe-arc lamp.

3. Results and discussion

3.1. Luminescence and lifetimes of transition metal

ions in sol–gel SiO2 glasses at room temperature

Fig. 1 shows luminescence spectra at 300K offour transition metal ions (0.1mol%) doped inSiO2 glasses. All these spectra are broad in shapeand centered at around 18,000–16,000 cm�1. Theemission spectra are not disturbed by emission ofthe host glass which is found at 10K as a ratherstrong band centered at 20,000 cm�1 due to

5000

4000

3000

2000

1000

Em

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24x103 22 20 18 16 14 12

Wavenumber/cm-1

2000

1500

1000

500

(1) Ti(right ax.) (2)Cr(left. ax.)

(3)Ta (left ax.)

(4) W(right. ax.)

Fig. 1. Luminescence spectra of transition metal ions (0.1mol%) doped in SiO2 glasses at 300K, displayed in units of number of

photons per second: (1) broken line (emission intensity refers to right ordinate) SiO2:Ti, (2) solid line (left) SiO2:Cr, (3) open triangle

(left) SiO2:Ta, and (4) open circle (right) SiO2:W.

M. Morita et al. / Journal of Luminescence 102–103 (2003) 608–613 609

Si(OH)4 centers or to electron–hole recombinationprocesses [8]. In Fig. 1, a strong band of SiO2:Cr iscentered at 16,000 cm�1. It is different in shapeand position from the luminescence spectra due toCr3+, Cr4+, and Cr5+ ions and is ascribed toluminescence of 3LMCT transition of a complexcenter of Cr6+. Similarly, the relative intensityluminescence spectra of Ti, W and Ta ions,observed at around 17,000 cm�1, are displayed.Table 1 summarizes luminescence properties

and decay times of 9 transition metal ions dopedin sol–gel SiO2 glasses at room temperature.Luminescence from sol–gel glasses doped withZr, Nb, Mo, Hf, Ta, and W are reported for thefirst time in this paper, as far as we know. In orderto emphasize the importance for applications, alldata are accumulated for comparison at roomtemperature. The appearance of strong lumines-cence and vibronic structures is dependent onsample conditions: starting materials, annealingtemperature, heating rate and condition, moisture,measurement temperature, surface conditions, etc.Typical spectroscopic data are displayed in thetable. In some cases of Ti and W ions, vibronicstructures are not clear (Fig. 1).From Table 1, common spectroscopic data of

nine elements are found as follows: the lumines-cence band at room temperature is located ataround 18,000 cm�1 with a band width (FWHM)of 5000 cm�1 and lifetime less than 3ms. The bandshape is usually simple in Zr, Nb, Mo, and Ta, but

other ions manifest vibronic progressions as foundin Fig. 2. By taking into account analogousluminescence properties in Fig. 1 and Table 1,luminescence of nine metal ions should beconsidered to be due to the 3LMCT transition ofions with closed-shell electronic structures. Weassume the presence of a single center of tetra-hedral molecular complex [MO4]

n� of Ti4+, V5+,Cr6+, Zr4+, Nb5+, Mo6+, Hf4+, Ta5+, W6+ ions,but we have to consider the presence of othercenters in luminescence at 10K.

3.2. Luminescence properties of closed-shell

molecular complex centers of V (V), Nb (V),

Ta (V) and W (VI) ions in sol–gel SIO2 glasses

at 10 K

Luminescence from closed shell molecular cen-ters is very sensitive to surface conditions of sol–gel silica glasses. In order to improve luminescenceproperties, we doped metal ions into a mixture ofglass and ceramics using sol–gel methods. Fig. 2shows luminescence of V ion (0.1mol%) dopedsol–gel composite, a mixture of SiO2 (99mol%)and Al2O3 (1mol%), annealed at 5001C. Measure-ments were done at temperatures between 10 and250K. With increase of temperature from 10K to300K, the vibronic structure gradually disappearswith decrease of intensity by thermal quenching.The luminescence lifetimes at 10 and 250K are 9.0and 1.7ms, respectively. In comparison with

Table 1

Luminescence properties of transition metal ions (0.1mol %) doped in sol-gel SiO2 glasses at room temperature. Luminescence is due

to the 3LMCT transition of closed-shell molecular complex centers. See Fig. 1 and text for details

Atomic

number

Dopant

(periodic

group )

Band center/

cm�1Band width /

cm�1Number of

em. Center

Spectral

band shape

Lifetime/ms Comment

22 Ti (4A) 17,000 4000 2 Vibronic NA (a)

23 V (5A) 18,600 4000 1 Vibronic 1.50 (a)

24 Cr (6A) 16,000 5000 >1 Vibronic 0.012 (b)

40 Zr (4A) 17,000 6000 1 Simple NA —

41 Nb (5A) 17,000 4000 2 Simple 2.9 (c)

42 Mo (6A) 18,000 5500 1 Simple NA —

72 Hf (4A) 18,000 5000 1 Vibronic NA —

73 Ta (5A) 17,600 4000 2 Simple 1.22 (c)

74 W (6A) 17,800 5000 2 Simple 0.065 (c)

(a) Surface condition dependence, (b) Coexistence with Cr3+, (c) Annealing condition dependence.

M. Morita et al. / Journal of Luminescence 102–103 (2003) 608–613610

luminescence of SiO2:V, vibronic structures aremore precise and luminescence increases by afactor of 20 (quantum efficiency of 13%) in thehybrid-glass SiO2:Al, V. In Eu

3+-doped sol–gelSiO2 glass, increase of Eu

3+ hydroxylation and itsisolation were observed by codoping of Al2O3 indecay and line narrowing experiments [10]. Chargecompensation and increased structural flexibility

are important factors for isolation of V ion toimprove luminescence efficiency in the new amor-phous network.Table 2 is a summary of luminescence properties

of V, Nb, Ta, and W ions (0.1mol%) doped insol–gel SiO2 glasses. The band position, lifetimes,bandwidth, vibronic progression, Huang-Rhysfunction and activation energy DE at 10K are

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ity

24,000 22,000 20,000 18,000 16,000 14,000 12,000

Wavenumber / cm-1

10K 30K 50K 70K 100K 150K 200K 250K

Fig. 2. Luminescence spectra of V5+ (0.1mol%)-doped sol–gel composite of SiO2 (99%) and Al2O3 (1mol%), annealed at 5001C, as a

function of temperature between 10 and 250K. Enhancement of luminescence efficiency is realized by addition of Al2O3. See text for

details.

Table 2

Luminescence band position, lifetimes, band width (FWHM), vibronic progression, Huang-Rhys function and activation energy in the3LMCT transition of V, Nb, Ta, and W ions (0.1mol%) doped in sol–gel SiO2 glasses at 10K. See text for details

Host glass/oxide:dopant V5+ SiO2:V K2NbOF5.H2O SiO2:Nb YTaO4 SiO2:Ta SiO2:W

Annealing temp. (1C) — 500 — 500 — 500 500

Band center (kcm�1) — 18.3 22.6 17.4 29.8 17.5 18.0 (II) 23.0(I)

Lifetime (ms) — 8.4 0.56 (77K) 5.0 NA 2.5 0.055(II) 0.175(I)

Lifetime (250K) (ms) — 1.5 NA 2.9 NA 1.9 NA(II) 0.065(I)

FWHM (kcm�1) 5.3 4.9 NA 3.4 NA 5.9 5.7(II) 4.2(I)

Vibronic progression (cm�1) 1060 970 290 — NA — 1350(II) 4250(I)

Huang-Rhys function 4.5 5.3 10.1 — NA — 10.5(II)

Act. Energy DEðIÞ (cm�1) — 600 NA 935 NA 295 59(II)/527(I)

Reference DFT theory [13] [8] [12] Present work [5] Present work Present work

M. Morita et al. / Journal of Luminescence 102–103 (2003) 608–613 611

displayed. The spectroscopic data are comparablewith those of sol–gel SiO2 glasses doped withV (III) and Cr (III) complexes [8]. For reference,we need to review previous investigations incrystals [5,11,12]. Recently, theoretical argumentson charge transfer spectra in tetrahedral VO4

3� andrelated transition metal oxo-anions were publishedby Atanasov et al. [13]. Optical properties ofV centers by DFT theory correspond to ourobservation. By assuming the presence of twoexcited-states levels, the activation energy DE wasestimated from the temperature dependence ofdecay profiles/intensity in the last column. Wepresent spectroscopic differences of two W centers,I and II, using different materials. The lumines-cence bands observed in W (at 23,000 and18,000 cm�1) correspond in energy to those inNb (at 21,000 and 17,400 cm�1).

3.3. Emission centers in sol–gel SiO2 glasses

We have investigated luminescence of SiO2:Vglasses prepared by annealing at different tem-peratures between 801C and 7501C. With increaseof annealing temperatures, luminescence gainedand the bandwidth increased as the crystallization

processes progressed. However, the broad banddid not show red shift by increase of annealingtemperatures. On the other hand, we foundunusual temperature-dependence of band shift inthe case of Nb, Ta, and W ions. Fig. 3 showsluminescence spectra at 10K of SiO2:Nb(0.1mol%), annealed for 24 h at (I) 1001C, (II)5001C, and (3) 7501C. The centers of band I, II andIII are 21,000, 17,400 and 19,500 cm�1, respec-tively. Although the center of band I shifts to reddue to enhancement of Huang-Rhys parameterwith increase of the annealing temperature, bandII returns back to center III in blue at a highertemperature. Therefore, we assume the presence oftwo centers: the lower energy center (II) due totetrahedral coordination [MO4] and the center I ata higher energy side. The latter is considered tooriginate from an octahedral center [MO6] as inthe case of Zr4+ center in (Pb, La)(Zr, Ti) O3ceramics [9]. The absorption bands of sample IIare found at 27,500 and 37,500 cm�1 at roomtemperature. Using the Huang-Rhys factor S of 5and a progression of 1000 cm�1, a Stokes shift of9000 cm�1 is derived which is comparable to theobserved shift of 10,500 cm�1. Band III is anoverlap of bands I and II. Similarly, we assume the

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24000 22000 20000 18000 16000 14000 12000

Wavenumber / cm-1

19,500cm-1

17,400cm-1

21,000 cm-1 (I)100 0C

(II)500 0C

(III)750 0C

I

II

III

Fig. 3. Luminescence spectra at 10K of sol–gel SiO2:Nb (0.1mol%) glasses, annealed at different temperatures: (I) 1001C, (II) 5001C,

and (3) 7501C.

M. Morita et al. / Journal of Luminescence 102–103 (2003) 608–613612

presence of two centers in Ta and W doped sol–gelglasses.Fig. 4 shows a schematic energy level diagram

for luminescence processes in the 3LMCT transi-tion due to a tetrahedral closed shell molecularcomplex center of [VO4]

3� in sol–gel SiO2 glassesunder a N2 laser excitation. This three-levelscheme is sufficient to understand temperaturedependence of luminescence and decay profiles of[MO4]

n�centers in general. At low temperatures,10 cm�1 splitting of the lowest triplet state must betaken into account. In the case of tetragonal andoctahedral centers of [MO6] type, the term levelsshould be reversed in energy. This model is usefulto interpret luminescence properties of variousmetal ions displayed in Table 1. Luminescence rateconstants at 10K were found to increase in thefollowing order: VoNboTa. This order origi-nates from the enhancement of intersystem cross-ing rates between the triplet and the singlet state inthe model scheme, because spin–orbit interactionHSO increases with increase of atomic numbers.We have observed luminescence from tetrahedraland octahedral centers in Ti, Nb, Ta, and

W-doped SiO2. Recently, in luminescence of V5+

ion species in zeolite the presence of three differentV ion sites was reported [14]. Therefore, in the caseof sol–gel silica glasses, we have to examine centersof various configurations of [MO8] or [M2O9]types.In summary, we investigated photoluminescence

and decay times of transition metal ions (M=Ti,V,Cr, Zr, Nb, Mo, Hf, Ta, W) doped in sol–gelsilica glasses to clarify electronic structures ofdopants at 300 and 10K. Spectroscopic data ofvarious samples are summarized in Tables 1 and 2.Broad luminescence spectra were found newly inall these ions, due to the 3LMCT transitions ofclosed shell molecular [MO4] centers. The presenceof octahedral [MO6] centers and enhancement ofluminescence in the sol–gel hybrid silica glass arealso discussed. In the present meso-porous ma-trices, particles of nano-meter size promote redoxreactions in solution and glasses.

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1T2

1T1

3T2

3T1

35,000cm-1

30,000cm-1

∆E =600cm-1

1A1

Exc. Em.18,600 cm-1

Laser

Fig. 4. Luminescence process in a schematic energy level

diagram for a tetrahedral molecular complex [MO4]n�center

in sol–gel SiO2 glasses.

M. Morita et al. / Journal of Luminescence 102–103 (2003) 608–613 613