Photoluminescent properties of MCM-41 molecular sieves

ELSEVIER

P~otoluminescent properties of ~C~-41

I. Introduction

The most widely studied MCM-41 (11 materials are the silica and aluminosilicnte hexagonal forms [2]. These high surface area. monodispersed mesoporous materials may find utility in areas ranging from catalysis to chemicaf sensors.Many of thcx ~i~pl~~~tiollscould benefit from thin lihn configuration5 of the mcsopot-ous materials. There has been some recent progress in this direction 1.3 71. Ozin and coworkers [3] have synthesized oricntcd mesoporous silicate films at the air water interfax. The as-synthesized films can be lifted from the water and show a resilience to bending. Ash:)! et al. 151have grown mewporous silicate films on if variety of interfaces including ~;tter niicu. water graphite. and water;silica. The iiiino.‘itruc-

molecular sieves

It is well recognized [ 13f that the cut:ifytic :tnci other physical properties ofmicroporous materials, such as zeolites, can be affected by the presence of framework defects. While much research has tixused on the synthetic routes to mesoporous phases, there have been few investigations of the structural defects (ordering of the walls [ 14.-171) of’ the MCM-41 ~~~~~7le~ork, The&ore. WC have started to evaluate the electronic energy states induced by the ~b~orptioll of Liltr~ivi(~l~t photons by these materials. Light can either dire+ or indirectly via multiphoton absorption processes generate electronic excitations which may rclas b> emitting photons cll~fr~cterist~c of’thc d&ct site(s). ~~~?ders~~ndjn~ &he optical propcrliL*s of the MC?&-41 materials might contribute to ~Llrttl~r characterization of the channel walls a \vell as lcad to applications that could exploit the optical properties. For example. luminescent MC‘M-41 mater&s (either pure or modif&ci s~~t~letic~lil~~ have the potential to serve ;IS optical ~enstfrs i’or small organics. In this paper we rcpor’t photoluminescence measurements on siliceous and 31~ mi~~~3~ili~tc MCM-41 molccul:tr siew powders which have not been previousty reported. .~dditi(~n~lf~. front these results it would appear that the luminescence spectra msy be uscf‘ui in identifying the presence of’ l’ramework aluminum in MCM-41.

2. ~~perim~fft~l

The siliceous and aluminosificate MCM-41 materials were synthesized according to the published procedures [IS]. A typical pr~p~~r~~ti[3[~ for the siliceous synthesis gel involved the mixing of’ reagents in the following molar ratios. I SQ:O.l7 (CTMA)20:70 H,O where CTMA is cet~ltrimethylammonjum ion. The source of silicon was a sodium silicate solution (27% silica, Aldrich). The resulting gel was heated under static conditions at 150 C i’or 24 h. The solid products were isolated by suction filtr~~t~o~l~sashed with deionized water and dried in air at room tel~~~3er~lture.

The two ~~fumin~3siiic~~te MCM-41 powders were prepared by mixing the reagents in the following molar ratios: 1 Si02:0.013 A1,0,:0.16 (TMA),O: (f.WCTMA f70: ift Hz0 and 1 Si02:0. 1 Al, rcspec(SO,),:O.?A (TMA f20:0.3? (CTMA)zO, tively. where TMA is the trimethylammonium ion. The sources of silicon and aluminum were fumed silica (Aldrich~. Catapal B alumina (73.3% A120,, VISTA) and Cab-0---Sil MS fumed silica (Cabot) and ~illlrninun~ sulfate (~~ldr~ch~,respectively. The resulting gets were heated under static conditions at 1511C for 24 and 48 h, respectively. The solid products were isolated by suction filtration, washed with deionized water and dried in air at room temperature. Portions of the siliceousand atuminosilicate MCM-41 were calcined in air at 540 C for 14 h to remove the organic template.

The silicon to aluminum ratios (SijAI ) of‘ the aluminosilicate MCM-41 sampleswere determined to be SijAl-34 (Catapal B alumina as aluminum source) and - 13 (~~lumjnurnsulfate as ~iluininum source) by Galbraith Laboratories, Knoxville. TN. The sampleswere l~orni~~~~lf~ pure hexagonal phase material 3s determined by powder X-ray diffraction (XRD) patterns collected on a Scintag XDS 7000 diffrwtometer using c’u Kr monochromatic radiation. CaF, was used as an internal standard. Front-face emission and the corresponding cucitation spectra were recorded from s~lf-s~lp~3ortin~ pressed pellets f t in diamcterf using a SPEX FLUOROLOG spectrophotometer. No provision was available for filtering out second and,‘or third order difliwtion ofultra\iolet light from the rough samplesurface. For each spectrum. fO scansof the \~~~vcleIl~tll range wet-c averaged in order to improve signal-to-noise. The curve fitting feature ot‘GRAMS31 version 4.1)(Galactic, Inc.) was used to determine the centroid of’ overlapping emission bands in the phototuminewcnce spectra.

In a separate experiment, a pressed pellet of calcined siliceousMCM-41 was placed in 21vacuum chamber and ir~~~ii~~tc~~using a Lunl(3nics

HyperEXexcimer laser [I48 nm ( KrF*), pulse length 14 ns. repetition rate 10 Hz]. Laser energy was measured using a Scientech pyroelectric head (Model 380402) and found to be 136 mJ ,pulse. A computer controlled rastering mirror (Oriel ) ws used to turn the laser beam 90 and move the beam across the pellet surface. A focusing lens was cnlployed to decrease the laser beam to ;L spot size of ca. 0.001 cm2.

The spectral distribution of the siliceous MCM-41 photoluminescence is shown in Fig. I as ;I function of excitation wavelengths 350 nm and 350 mn (5.0 eV and 3.5 eV. respectively). .4t 250 nm excitation of the as-svnthesized siliceous MCM-41 [Fig. 1(a)]. two kcry broad emission bands appear with maxima around 41 1 nm and 620 nm. Similar spectral features, but with intensities reduced by ca. 84%. are recorded for the calcined material. The intensity of the photoluminescence increases as the excitation is changed from 350 nm to 350 nm [Fig. I (b)]. Both blue (410 mn) and green (473 nm) emission bands are recorded. Reduced intensity photoluminescence (ca. 16% ot signal observed for as-synthesized material ) is recorded for the calcincd siliceous MCM-41, The excitation spectra of the luminescence displays broad peaks with maxima around 210 nm und 306 m-u. These absorptions correspond to emission band peaking about 410 nm. The excitation spectrum corresponding to emission band peaking around 473 nm contains broad absorptions with maxima about 225 nm, 314 nm. and 375 nm. The emission band at 620 nm possesses excitation bands which peak around 326 nm and 485 nm. Fig. 2 shows the spectral distribution of the photoluminescence observed for amorphous silica as a function of the same wavelengths used to examine the siliceous MCM-41. Excitation at

3. Results In the following section we describe the characteristic features of the siliceous and aluminosilicatc MCM-4 I (pore size 40 A) photoluminescence spectra. We have examined the as-synthesized and calcined (template-free) MCM-41 powders using ultraviolet photons having energies 2 3.5 cV ( i.c. i~:;350 nm). Photoluminescence of the MCM-41 materials was measured at wavelengths of 200 nm. 250 nm. 300 nm and 350 nm. In addition, photoluminescence spectra of amorphous silica were recorded for comparison. In all the materials examined. very low intensity emission was observed at 101) nm while the emission bands excited by 100 IIIX were not significantly different from that observed at 3SO mn (results not shown). Table I summarizes the results of all emission and excitation experiments on the siliceous and aluminosilicate MCM-41 samples.

Emwion(s)

M~lklUl Silizwu\

Aluminosilicate

MCM-II

MCM-11

(Catapal

B alumina)

(mm)

[nm (eV)]

Excitation(s)

120

211) ( 5.0)

373 610 310

22s

(5.5)

.306 (1.0) (3.9)

21-l

(5.X)

4Yh 54-l 570

214

(5 8)

320 4.3 3

238

(5.2

)

330

(3.X)

3x0 514 570

2-10 (5.2

)

333

(3.7)

314

375

(3.3)

366

1.34)

320

(2.9)

485

(3,

41’)

I 2.0 )

WI

(2 7I

426 177

(2.9) (1.9)

156 168

(7 7) (2.6)

433

Aluminosilicate

MCM-41

(aluminum

sult’atr)

712

(-5.X)

70

second order of crcltalion

250 urn [Fig. 2(a)] generates ;t photolumil7escence spectrum in which narrow bands appear al 433 and 543 nm while broad bands appear at 410 and 570 nm. The intensity of the emission increases upon changing the excitation from 250 nm to 350 nm [Fig. 2(b)]. Like in the case of siliceous MCM-41. the blue emission is composed of a 420 nm band and ;I 475 nm emission band comprises the green emission. Amorphous silica that had been heated at 540 C for 4 h generated an overall lower intensity photolliminesccncc~ signal at each excitation wavelength [SW Fig. 2(a) and (b)].

Aluminosilicate MCM-41 described by Beck et al. [ 181 was made using Catapal B ~tlumina 21s the source of aluminum which results in virtually all the aluminum being extra-fl-aiiie~~ork. A second

aluminosilicate MCM-41 was made using aluminum sulfate as the source of aluminum [ 151 and in this case the aluminum is incorporated into the silicate lattice as evidenced by “Al MAS-NMR. The photoluminescencc observed for the as-synthesizedand calcined samplesprepared using Catapal B alumina is shown in Fig. 3. Broad blue,‘green photoluminescencc bands at 420 nm and 570 nm appear when the as-synthesizedaluminosilicate MCM-41 is excited using 250 nm [Fig. 3(a)]. Narrow emission bands at 433 nm and 544 nm are also observed. These same bands, though ca. 52% greater than the original intensity, arc observed from the calcined material. The ditl’erence in behavior observed between the as-synthesized and calcined aluminosilicate MCM-41 is most dramatic at an excitation of 350 nm [Fig. 3(b)]. The preen emission band (4% nm) is more intense than the blue emission band (320 nrn) for the as-synthesized material;

however, the blue emission band (422 nm) is more intense than the green emission band (476 nm) fol the calcined sample that contains the extra-tramcwork aluminum. The excitation spectra of the luminescence displays broad peaks with maxim;t around 214 nm and 366 nm. These absorptions correspond to emission bzmds pc;tking about 420 nm and 433 nm. The excitation spectra corresponding to emission bands peaking around 4961nm. 544 nm XIC~ 570 nm contain broad absorptions with maxim:t ;tbout -114nm, 479nm. met 469 nm. The overall intensity of the photolumincsccnce signal is greater for the ~tlunlinLlrn sulfktte derived mttterial compared with the Cutapal B :Amiina based material (Fig. 4). Like the ‘Catnpal’ sample. broad blue.~green photoluminescence band5 ;~t roughly 420 nm and 570 nm appear \4:hen the as-synthesized ‘aluminum sulfate’ sample is excited using 250 nm (results not shown). In addition, nitrrow emission bands appear at 433 nm Ed 544 nm. The photolliminescence spectrum from

the calcined material has CI band having a maximum around 420 nm. Upon excitation using 350 nm (see Fig. 41, the green emission band (480 nm) is more intense than the blue emission band (420 nm) for the iIs-synthesized material. Again. narrow emission bands at 433 nm and 544 nm are observed. Only the blue emission band (422 nm) and the green emission band (480 nm) ;Lre observed for the calcined ‘aluminum sulfate’ sample. The excitation spectra of the 1L~rniiiesc~ncc displays broad peaks with maxima around 238 nm and 330 nm. These absorptions correspond to emission bands peaking itbout 420 mn and 433 nm. The excitation specfruin c~~rrespondin~ to emission band peaking around 4X0 nm contains broad absorptions with maxima about 240 nm. 333 nm, and 426 nm. The emission band with maximum peaking around 544 nm has absorption bands around 427 mn and 456 nm. Finally. the excitation spectrum for the emission band at 570 nm contains peaks with maxima around 2 12 nm and 46X nm.

77 as-synthesized calcined 540°C

Wavelength

(nm)

~~-~-~-~

extra-framework aluminum MCM-41 framework aluminum MCM-41

IIi \

I , I

\

i ;i /

\1 ‘i second order /, of excitation \ ‘\ ,fl .. _,l-T’V-‘\._ ,. *- ..__-_” ,’ \-

t 100

L 200

I 300

I 400

I 500

1 600

1 700

-1 800

1 900

6 1000

4. Discussion Luan et al. [IS] report that the ‘“Si magic-angle spinninp nuclear magnetic resonance ( MAS NMR) spectra of the us-synthesizeti and the calcined siliceous MCM-41 arc identical to those from amorphous silica, indicating an irregular arrangemcnt of Si-O--Si bonds and iI wide range of bond angles in the pore walls. Indeed. we observe similar photolluninescenue spectra from the as-synthesized MCM-41 and amorphous silica. In addition. ‘H -“‘Si cross-polarization MAS NMR spectra show that a portion of the Si atoms exist as silanol groups. In fact. Beck et al. [ 181 hnvc estimated that ~~pproxim~tely 14% of the silicon atoms in MCM-41 are in the form of siloxy groups (Si 0 ). which play a charge balancing role fillthe CTMA template used in the synthesis. Calcination of the MCM-41 to remove the organic template causes protonation of these siloxy group?; which then renders the pores lined with silanol groups (Si--OH ). Modeling studies performed by Fcuston and Higgins [ 191 suggest that the silanol content of the siliceous MCM-41 is in the range of 17% to 28%. A notable decrease in the bluegreen photolun~ill~s~en~e is observed upon ~~llcin~l~ion of the siliceous MCM-41 material. This observation is probably a direct result of the loss of Hz0 1~1 condensation of silanol groups during the calcination. At 350 nm excitation. the residual photnluminescence intensity after calcination is roughI!. 16% of that observed for the as-synthesized siliceous sample. This observation suggests that silano1 groups present throughout the material prior to calcination. but only at the material surface after calcination, are involved in the generation of photoluminescen~e. Iliterestingly. pulseci 248 nm irr~~di~~tion of the calcined siliceous MCM-41 material results in 8 regenerittion of the phntoluminescence intensity (see Fig. 5). It has been reported that 460 nm and 650 nm luminescence bands are caused by 248 nm and 258 nm excitations, respcctively, in certain silica core optical fibers [20]. It should be noted that 24X nm irradiation of a highly crystalline, put-c silicu zeolite such as as-synthcsiyed UTD-I does not generate any observable phoro-

luminescence [71]. It may bc that this luminescence is characteristic of non-crystalline silicas. The optical properties of the aluminosilicatc MCM-4 1 samples primarily ( 7 I%) framework alurnil~itlil (*~lilmin~lrn sulfate‘) and all extra-framework ~~l~~~~~~~u~~ (‘Catapal’) is most interesting. Clearly, the absence of virtually all (only 16%) framework aluminum species in the ‘Catapal’ material results in a dramatic decrease in photoluminescence intensity. Our results suggest that the density of defect sites responsible for the alutninosilicate MCM-41 photolumiiiescerrce signal is related to the amount of aluminum substituted into the silicate framework. Ryoo ct al. [22] have shown that a m;tjor effect of aluminum incorporation in MCM-41 is the loss rtf stru~tur~~l order. R~~ks~hl~~~~ and coworkers [23,24] have examined the blue green photo1LlrniIi~s~ell~e of a variety of hydrated metal oxides (where M =Al. Si, Zn. and Pb). They attribute the blue green photoluminescence to the presence of M--OH moieties and have shown that the intensity of this photoluminescent signal scales with the density of defects with optical absorptions in the spectral region /1\1>3.5 eV (i < 354 nm). Similar photolumincscencc. in terms of spectral distribution and dependency on Si-OH content. has been found in various silica glasses and correlated with the density of structural defects [25.X]. In un ideal silica lattice. only Si- 0 bonds (all Si sites being four-coordinate and all 0 sites being two-coordinate) are present and the band gap is approximately 9 eV (see Fig. 6). The presence of structural defects will introduce electronic states into the band gap. Three intrinsic defect sites that are known to introduce electronic states into the hand gap are pcroxyl radicals. I” centers, and nonbridging oxygen hole centers I N BOHC ) [27]. The pcroxyl radicals consist of an unpaired electron localized over two oxygens. one ofwhich is bonded to Si. The E’ center consists of an unpaired electron localized in a single Si sp” orbital, i.e. a Si dangling bond. The NBOHC consists of an unpaired electron residing in it p7-t orbital of a monovalent OXY~UI. Both peroxyl radicals and NBOHC ;Irt’ fbund most prevalently in silica having high OH content [2X]. Each of these defect sites hiis ;I characteristic photolutninesconce when excited

Peroxyl

radical

F center

(%-0,l) f?g.

6. Enwg\i

(%i band

diap~tt~

showing

with high energy photons (ht*>i.5 eV f: 459 nm [39] ( peroxyl radical ), 288 nm [Xl] (E’ center), and 652.-670 nm [28] (non-bridling oxygen hole Cleariy. ~hotoluminescence is not center). observed at 288 nm: however, the presence of oxygen-related radiative centers cannot bc ruled out based on our results. Brimacombe et al. [?I] also report a pi~ot(?lum~nesce~lt band related to E’

Non-bridging

I)

oxygen

(%i-O

possible

defect

sites

in amorphous

hole center

I) silica.

center concentrations at 45X nm. This particular band disappears upon annealing silica samples to tempcratur~s > 130°C. Insight into the nature of the defect sites responsible for the photoluminescence can be drawn from the luminescence excitation spectra. The siliceous and aluminosilicate MCM-41 samples have at least one emission band that is excited at wa\;elengths

I 250 nm (2 5.0 eV ). Absorptions also 0x111 in the range 300~380 nm (4.1 -3.3 eV) anti 426-490 nm (2.9-2.6 eV ). There also appear\ to be some transfer of energy between excited species. For example, in the siliceous MCM-41. the low energy emission band having a maximum around 620 nm is excited by the broad luminescence bands around 420 and 473 nm. The alumii~osilicatc MCM-41 samples have emission bands with maxima around 496 nm, 544 nm and S70 nm (Y’atapal’) and 4X0 nm and 544 nm (‘aluminum sulfate’) which are excited by a 433 nm emission band. It is clear that the exact nature of the radiative sites in the siliceous and aluminosilicate MCM-41 will require further spectroscopic stud& such as ESR to verify the presence or absence of paramagnetic defects such as peroxyl radicals. I-’ centers andior NBOHCs. We have previously noted a sharp ESR band at s= 2.005 ( 77 K ) associated with siliceous MCM-41 which \ve assigned to an organic radical impurity [37]. Therefore. we need to revisit these materials and evaluate them from this mm perspective.

Acknowledgement

Funds for this research were provided by the Texas Advanced Technology program. We would like to thank Professor John P. Ferraris for the use of the SPEX FLUOROLOG spectrophotometer. We would also like to thank Kevin J. Sutovich and Professor Karl 7’. Mueller from the Department of Chemistry at Penn State University for running the “Al MAS-NMR experiments.

References [I] [?I

and

rclk2nc~~

Vartuli.

F&l.

35 (19%)

I. Honma.

N.

515

tIlereIn

I..4

[h] [7]

[Y] [IO] [I I]

Trau.

S. Mannc‘.

Brinker. It. Sqw. B. Dunn. Mti Natul-c 3% ( lYY7) 363. K.J. Balkua. Jr.. L.J. Sotille. S.J. Riley. Sold Film, 260 ( I YYi ) I I X.3.

Huang, B.E.

Yao.

J.I. Gnadc.

I-.

Link. Thin

K.J. Balku\. Jr., L.J. Sottile. H. Ugycn. S.J. Riley. B.E Gnxic. Mater. Re\. SW. S!,mp. Proc. 375 (lYY4) 33 [...I. Sottdc. K.J. Balkua, JI-.. S J. Rdcy. B.E. Gnade. Mater. Rex. SW. Symp. Proc. 351 ( IYY4) 263. K.J. Symp.

[ 121

M.

J.C.

Zhou. P. I’cntcr. P.M. Eisenhergct-. S.M. Pruner. Scicncc 277 ( 1’196) XY7. \I: Ogawa. (‘hem. C‘ommi1n. I lYO6) 113’). R Ganguli. \‘. Lu, M.T. Anderson. C‘.A. Drewien. C.J

[X]

Aksaq.

cltcd

Roth,

[S]

5. Conclusions

The photoluminescence of as-synthesized and calcined siliceous as well as aluminosilicatc MCM-41 molecular sieves is examined as a function of photon wavelengths 350, 300. 2.50, and 300 nm. The observation of high energy emission bands (/IY> 2.5 eV ) from the as-synthesized siliceous MCM-41 suggeststhe presence of oxygen related defect sites similar to those found in amorphous silica. Upon calcination, the intensity of the photoluminescent signal decreases dramatically but can be regenerated by laser irradiation at 24X nm. Thus. laser irradiation provides ;I COIIWnient method of generating defect sites in the calcined material. The photoluminescencc intensity of the aluminosilicatc MCM-41 material increases dramatically when the material contains virtually all framework aluminum. Thus, this photoluminesccnccbehavior may represent a quick qualitative method for the detection of framework aluminum in the absenceof NMR.

C‘.T. Krebg~~ M.E. Lronowic/. W.J. J.S. Beck. Nature 35’) (IYY?) 710. P. Behren,. 4ngcw. Chen~ (nl Ed.

1d.E.

Balkus.

JI-.. S.J. Rile!. B.1’. (;nade. Mater. Res Sot. 151 ( lYY4) 13: Ciimon-Kinael. K. Crr
prc}mltion.

[I?]

D.W. HI-sck. Zwllto MolecularSieves: Structure. (‘hcmistry. and Use. Rohel-1 1: Krxgcr Publishing Co.. Malahar, I-L. I YX1.

[ 141 (‘.-Y. <‘hen. Il.-X. LI. 2 (I’)‘)?) 17. II-51 Z. Luan. C.-F. Chcng. <'l1cn1. 99 (IYYC) 101x. [I h] C. Pophal. Y.S. Llh Materidls. Vol. 105. I’. i(ll.

M.E. W.

Da\~s. Zhou.

Micropol-ous _I Klinowski.

Mater. J. Phys.

R. Schnell. H. Fuc\\. in. H. Chon. S.-K. Ihm. ( Ids.), Progrw. in Zeollte and Micropvrous Studies in Surli~cc Swncc and Culaylsis. tlwvier Scicncc B.V.. The Netherlands, lYY7.

[ 171 S. Viaic. E. Garrone, I’. Di 111. I I. Chon. S.-K. Ihm. Y.S. and Micropol-ous Mawrial~.

Ken/o. B. Chiche. F. Fajula. I111 ( lida.). Progress in Zoolitc Studier in Surface Science

Xld

The [IX]

[IS]

Cal;lylsls.

Netherlands.

Vol.

1997,

105.

ElWVit!l

Scletlcc

B.V

[26]

H. Tamura. Thin Solid

1771

J. Robertbon. in. R.A.B Devtne (Ed.). The Physics and Technology of Amorphous SiOz, Plenum Press. Nev, York. I9XX. p. 91. S. Munekuni. T. Yamanaka. Y. Shimogaichi. K. Nagasawn. Y. Hama. J. Appl. Phys. 6X (1990) 1212.

p. 533

J.S. Beck. J.C. Vartuli. W.J. Roth. M.F. Leono\vic/. C.T. Kresge. K.D. Schmttr, C.T.-W. Chu. D.H. Olwn. E.W. Sheppard. S.B. McCullen. J.B. Iltpgtn~. J.L. Schlenkct-. J. Am. Chem. Sot. I I3 (1993) 10833. B.P. Fueston. J.B. Higgins, .I. Phya. c‘hcm. 9X ( 1994 I 4359

[2X]

[N]

M. Films

RiickschloD. 2.55 ( 1995)

Th. 92.

Wtrschem.

S. Veprck.

S. Kannan. J. L.i. R.L. Lehman, G.H. Sigel Jr.. Appl. Opt. 32 ( 1993) 6681. [?I ] K.J. Balkus. Jt-.. T. Mutioz. Jr.. M E. (Simon-kinwl. (‘hem

[li)]

M. Guzri. M. Marttni. F. Pio, G. Spin&. A. Vcddn. in: R.A.B. Dminc ( Ed 1. I‘he Physics and Technology 01 Amorphous SiO,. Plenum Pras. New York. 198X. 1’. 175.

Mater.. in prw. R. Ryoo. C.II. Ko. R.F. Hwe. (‘hem Mater. 9 I 1907) 1607. FI. Tamura. hl. Ritck~chloD. ‘II. Wtt-achem. S Vept-ck. Appl. Phys. Lctt. 6s 11995) 1577.

[30]

Ii. Imai. K Arai. T. Saitc,. S. Ichimura. H Nonaka. J. P. Vtgouroux. H. Imapawt. I I. lioaono. Y. Abe. in: R.A.B. Dcvtnc ( Ed.). The Physics .md Technology of Amorphous

[22] [I’?] 1241 [2]

M. RiickschloO. Th. Wirschem. Il. l,tmut-a. Oswald. S. Vcprck. J. Lumin 63 ( 1995 I 379). H. Tamura. M RtichschlolS. Th Wirwhctn. Appl. Phy. Lctt. 65 ( 1995) 1171.

G. Rtthl. S. Vcpreh.

J.

Si02. [3 I ] R.K. Phvs. [?2] J.?. Kevan,

Plenum Pruss. hen, \‘ork. 1988, p, 153. Brtmacombe. II S. Taylor. K.E. Leopold. J. Appl. 66 ( 19x9)) ‘Io35. Diar. K.J. Balkua. Jt- 1:. Bediotti. V. Kut-bhev. L. Chem. Mater. 9 ( 19%) 61.