PHOTO-INDUCED E.S.R. IN GLASSY SULPHUR

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PHOTO-INDUCED E.S.R. IN GLASSY SULPHUR

S. Elliott

To cite this version:

S. Elliott. PHOTO-INDUCED E.S.R. IN GLASSY SULPHUR. Journal de Physique Colloques, 1981,

42 (C4), pp.C4-387-C4-390. �10.1051/jphyscol:1981483�. �jpa-00220940�

CoZZoque C4, suppZ6ment azi Tome 42, octobre 1981

PHOTO-INDUCED EBSeRa IN GLASSY SULPHUR

S.R. Elliott

Dept. of PhysicaZ Chemistry, University of Cambridge, Lensfield Road, Cambridge, U. K.

Abstract.

-

Photo-induced E.S.R. h a s been observed f o r the f i r s t time i n glassy sulphur. 111un;ination a t 77K by l i g h t of energy 3.1 eV (corresponding t o a- 103an-' f o r c r y s t a l l i n e orthorhombic sulphur) produces an E. S. R. signal which contains resolved s t r u c t u r e c h a r a c t e r i s t i c of a completely anisotropic g tensor; t h i s i s the f i r s t observation of s t r u c t u r e i n the E.S.R. spectrum of a chalcogen centre i n chalcogenide glasses. blolecular o r b i t a l calculations on models f o r possible centres indicate t h a t a simple dangling bond (C:) i s more l i k e l y t o be responsible f o r the observed line-shape than a hole trapped i n a valence band lone-pair o r b i t a l . However, power-dependence s t u d i e s indi- cate t h a t more than one centre may be present.

Introduction.

-

Chalcogenide glasses a r e distinguished by t h e i r ground-state dia- magnetism, although it i s hown from photoluminescence, a.c. conductivity e t c . t h a t defect s t a t e s do e x i s t . Paramagnetism can b'e induced, however, by the application of near-bandgap l i g h t an o p t i c a l l y induced E.S.R. signal i s observed; t h i s has been shown f o r glassy Se, As,Se,, As,S, and GeSe, ( I ) , amongst others. I t was found t h a t l i g h t of energy corresponding t o an absorption coefficient a=100 an-' was most e f f i c i e n t a t exciting the E.S.R.. The o p t i c a l l y induced signal i s metastable a t l a u temperatures, and can be annealed away by r a i s i n g the temperature (say above 150K), o r can be bleached by the application of approximately mid-bandgap l i g h t . The

E.s.~.

signal i n the case of As,Se, o r AsLS3, has been ascribed t o two d j s t i n c t centres, one A s r e l a t e d and the other chalcogen r e l a t e d (1); the A s centre was concluded t o be an electron centre with the unpaired spin located mainly on one atom i n a s i n g l e p-orbital, whereas the chalcogen centre was deduced t o be a hole centre, but it was not possible t o decide whether t h i s was due t o a dangling bond o r a hole i n a lone- p a i r o r b i t a l comprising the top of the valence band. The line-shapes of the deriva- t i v e E.S.R. spectra a r e typically dominated by the chalcogen resonance which i s s t r u c t u r e l e s s and asymmetric (of width= 250G f o r Se andr75G f o r S centres, respec- t i v e l y ) , with a weaker, broader (-1400 G width) resonance due t o the A s centre which appears a s shoul6ers t o the main resonance.

The model fir s t r u c t u r a l defects i n chalcogenide glasses proposed by Mott e t a l . (2) and Kastner e t a l . (31, i n which diamagnetic, spin-paired over- (c: o r D+) and under-coordinated C; o r D ) s t r u c t u r a l defects a r e present i n equilibrium, accounts f o r the observation of o p t i c a l l y induced E.S.R. i n a natural way by assum- ing t h a t neutral, paramagnetic centres a r e produced by the capture of photo-generat- ed electrons o r holes by positively o r negatively charged defects, respectively.

Emin (4), on the other hand, ascribes the E.S.R. t o the presence of o p t i c a l l y in- duced small polarons i n the chalcogen lone-pair-like top t o the valence band. Thus, the nature of the chalcogen centre responsible f o r o p t i c a l l y induced E.S.R. i s a matter of some debate, p a r t i c u l a r l y i n view of recent r e a l i s t i c calculations (4) which indicate t h a t the lowest energy neutral defect is the Cy, rather than t h e C!

previously believed t o be of lowest energy (3).

Sulphur can be rendered glassy l i k e Se, but no previous work on E.S.R. of glassy S has been reported. Since S i s also a constituent of many archetypal chalcogenide glasses, it i s of i n t e r e s t t o investigate whether an equilibrium o r

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981483

JOURNAL DE PHYSIQUE

photo-induced E.S.R. signal e x i s t s , and t o compare i t s behaviour with other chalco- genide glasses.

Sulphur e x i s t s i n a great number of a l l o t r o p i c forms (5,6), of which the most common contain Sg rings (cyclooctasulphur) and a r e the a-orthorhombic and 8-mono- c l i n i c c r y s t a l l i n e forms. However, l i k e Se, S can a l s o e x i s t i n a polymeric (cat- ena-) form, and an abrupt transformation from cycloocaa- t o polycatena-sulphur i s responsible f o r the rapid increase i n viscosity a t 432K on heating a melt of origi- nally o r t h o r h o ~ b i c S. I f the melt i s quenched from a temperature above 432K (pref- erably 470-520K) t o a temperature below about 240K, a transparent yellow metastable glassy form i s obtained, consisting of chains of S; c r y s t a l l i z a t i o n t o the a-orthor- hombic form however occurs rapidly a t temperatures above T =243K(7).

a

E erimental Sulphur of p u r i t y 4N (Koch-Light) was sealed under vacuum i n a spec- k : - t u b e , heated t o above 470K and quenched i n l i q u i d nitrogen t o form

the glass. The sample tube was then transferred t o a s p e c t r o s i l cryostat containing l i q u i d nitrogen. The tail-piece of t h i s dewar f i t t i n g i n t o the microwave cavity was unsilvered, thereby allowing illumination of the sample (through the l i q u i d nitrogen surrounding the sample tube) e i t h e r i n place i n the cavity o r externally.

I l l m i n a t i o n was provided by a lOOW Hg lamp i n conjunction with a Wratten 35 f i l t e r , which transmits between 380-450nm f o r which the absorption coefficient of or tho^- hombic S i s 10'-lo3 cm" (8) ,(and beyond 670nm), the sample being r o t a t e m g illumination t o ensure maximum exposure. The E.S.R. spectra were taken using a Varian V-4500 spectrometer operating a t X-band, and DPPH was used a s a c a l i b r a t i o n standard f o r the magnetic f i e l d .

Results.

-

The photo-inducedE.S.R. signal observed i n glassy sulphur a t 77K a f t e r

6

i l l m i n a t i o n i s shown i n fig.1. A strong signal was observed which has s t r u c t u r e i n the resonance l i n e . This is the f i r s t observation of resolved s t r u c t u r e i n the l i n e of a chalcogen centre of a chalcogenide glass (excluding the A s resonance peaks present t o a l e s s e r (1) o r greater (9) extent i n A s chalcogenides)

.

The resonance i s r e l a t i v e l y narrow overall ( ~ 1 0 0 G wide), although of course the resolved features are even narrower. No E.S.R. signal was observed i n the unirradiated glass

- r

_C

3

L

5'

C

13

a

L

-

t A

. -

_w

2

C

.

C

-

* Y

nor i n the orthorhombic c r y s t a l l i n e form, e i t h e r unirradiated o r irradiated.

The magnitude of the signal increased with the period of illumination, show- ing signs of saturation a t the longest times measured; fig.2 shows typical growth curves f o r each of the resolved features (1-4) of the spectrum shavn

-

i n fig.1.

z

_C

2

i?6

12 2

+ %

.-

V1 C

E

E

*

t t + ^-

^W

I

9,

^{g y gz}

, -

3.1 3.2 3.3 H ( ~ O O

SO -100 150 200 250 300 350 t~me(mms) Fig. 1. Optically-induced E. S. R. spectrum Fig. 2. Time evolution of the optically- f o r glassy S, irradiated by 3.leV l i g h t inducsd E.S.R. signal f o r each of the a t 77K. The principal g values a r e indi- features (1-4) i n fig.1. The l i n e s a r e

cated. drawn t o guide the eye.

Early work by Gardner and Fraenkel ( l o ) , and l a t e r by Koningsberger and de Neef ( l l ) , on liquid sulphur showed a single symmetric line. The spin density increased with increasing temperature, and was assumed t o a r i s e from progressive chain breaking and consequent production of dangling bonds. Pinkus and P i e t t e (12) also observed a narrow single asymmetric l i n e i n "Crystex" (Stauffer Chemical Co.), a form of sulphur containing only polymer chains (the S, rings having been dissolved i n CS,)

.

Finally, Radford and Rice (13) and ~ h h t e l a i n and Buttet (14) have studied sulphur condensed from the vapour onto substrates a t 77K (presumably forming amorphous films) which exhibit E.S.R. spectra very similar t o those reported here f o r photo-irradiated glassy sulphur. The table displays values f o r the principal values of the g tensor (gx,gy ,g,) f o r those cases which have been reported o r the average value g = 1/3 x (g,+g,.+g,) otherwise, together with comments on whether a single l i n e , (symmetric o r asymmetric), o r a complex l i n e (revealing resolved anisotropies i n g values) was observed.

E.S.R. data f o r allotropes of sulphur.

Allotrope

ex

^{g y gz}

Glass 2.041 2.026 2.003

Liquid

- ^- -

Liquid

- - -

"Crys tex"

- - -

Vapour deposited 2.039 2.025 2.000 Vapour deposited 2.0405 2.0259 2.0023

- - -

g Line shape Reference

2.023 resolved anisotropic This work 2.024 symmetric

symmetric (1 01

2.017 (11)

2.0044 unresolved ahisotropic (12) 2.021 resolved anisotropic (1 3 ) 2.0229 resolved anisotropic (1 4)

2.025 symmetric (14)

The line-shape of the E.S.R. spectnun shown i n fig.1 i s characteristic of a paramagnetic centre having complete asymmetry i n the spin Hmiltonian and hence com- p l e t e anisotropy i n the g tensor (i.e. the principal components are such t h a t g,>g>

g, ). Principaf features i n the derivative spectrum ( f i g . l ) , such as peaks 1 and 4 and the point of inflection between 2 and 3, a r i s e because of shoulders o r diver- gences i n the absorption p r o f i l e o r "powder pattern" of a random distribution of spin centres; the g values g,, g and g, may be read off from the spectrum a s indicated i n fig.1, and are alsoygiven i n the table. Detailed computer-fitting of the line-shape w i l l be reported elsewhere; here we concern ourselves with an i n i t i a l inquiry i n t o the possible nature of the centre responsible f o r the photo-induced signal.

Since sulphur i s composed of the i ~ o t o ~ e ~ ~ ~ ' 3 ? n u c l e a r spin zero) a t a natural abundance >99%, no hyperfine structure resulting from interactions between the un- paired electron and the nuclear spin i s expected t o be observed. This linlits the amount of information t h a t can be gained from the E.S.R. spectrum, i n p a r t i c u l a r relating t o the localization of the unpaired electron. Nevertheless, we can make some progress by i n i t i a l l y assuming t h a t the line-shape i n f i g . 1 (and resulting g values) a r i s e from a single resonance; we can then model the configuration of var- ious centres, calculate the g values and by comparison with experiment we may dis- tinguish between different alternatives.

We choose two models as a preliminary step, the f i r s t being a dangling bond, the second being a hole i n a lone-pair o r b i t a l a t the top of the valence band. In both cases, we use molecular o r b i t a l theory; f o r the former case it i s assumed t h a t the dangling bond centre occurs as one of two equivalent s-p hybridized bonding o r b i t a l s (having a fraction a of s-character) together with two equivalent non-bond- ing o r b i t a l s a l s o s-p hybridized but with a different amount of s-character. The other centre has a hole i n a lone-pair p-orbital ( i . e . a polaron), together with two equivalent s-p hybrid bonding o r b i t a l s (again having a fraction a s-character) and a deep non-bonding o r b i t a l having a greater degree of s-admixture. The g values were calculated using the formula (IS), r e l a t i n g the unpaired o r b i t a l wavefunction and energy, the wavefunction and energy f o r excited s t a t e s , the angular momentum operator L and il;he spin-orbit coupling constant

c

(3.84x10-' an-' f o r S)

.

^{The g}

s h i f t s Agx,Agy (AgZ=O i n both cases) were calculated as a function of a, and are

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shown i n figs. 3(a) and 3(b)

.

2 0 - -70 70-

-60 6 0 - 15-

-50 5.0-

1.0

-

^-70

- 4 0 40- - 3 0 10-

( , @ . 2 0 2 0

aoo 005 oio 01s 020 025 0.3 03s 0.40 045 cc

Fig. 3.- Calculated g values f o r model centres.

a) Dangling bond centre, with two equivalent non-bonding orbitals.

b) Hole i n p-like lone-pair o r b i t a l ( a t top of valence band)

In both cases, the difference between g values and the free electron value (Ag=g-ge) i s plotted as a function of s-admixture (a) of the bonding orbitals a t the centres.

I t can be seenwith reference to the table t h a t the g values predicted by the 'polar- on1 model a r e consistently much smaller than the experimental values f o r a l l a , whereas the values predicted by the dangling bond model qualitatively agree. However

close examination shows that agreement with experiment f o r Ag, i s obtained for a*. 37, whereas f o r Agy it is ~ 0 . 4 4 . A more r e a l i s t i c model f o r the wavefunctions a t the dangling bond (i.e. two inequivalent non-bonding orbitals, one with mainly p-character) would probably improve the f i t so the same value of a would give Ag,

and Ag,.

,

both in agre~ment with experiment. I n a s n , n-delocalization between

the dangling bond o r b i t a l and the one-pair on the neighbouring atom i s expected t o take place, altering the calculated g values.

There is evidence that the line-shape changes with microwave power; indeed a t the highest powers peaks 3 and 4 reverse t h e i r relative heights compared with fig.1.

This implies, contrary t o the assumptions made so f a r , t h a t two centres are present, having different saturation behaviour. I t is of i n t e r e s t t o E t e that the E.S.R.

spectrum obtained from vapour deposited sulphur (14) was deconvoluted into two com- ponents, one having the same g value as observed i n liquid sulphur (10)

-

see table.

Whether the optically induced E.S.R. spectrum of glassy sulphur also is composed of two resonance lines awaits detailed line-shape f i t t i n g . Preliminary experiments using sub-band-gap l i g h t (Wratten 70 filter,)\greater than 66Onm) indicate t h a t some, but not t o t a l , bleaching can be achieved. This could be due t o one of two causes: either the bleaching wavelength used was not e f f i c i e n t or the centres are unbleachable, im- plying that unpaired spins occur as a r e s u l t of bond scission rather than charge trapping.

References

. -

1. BISHOP S.G., STROM U. and TAYLOR P.C., Phys .Rev.= (1977) 2278.

2. MYIT N.F., STREET R.A. and DAVIS E.A., Phil.Mag.

2

(1975) 961.

3. KASTNER M., ADLER D. and FRITZW H., Phys.Rev.Lett. 37 (1976) 1504.

4. VANDERBILT D., and JOANNOPOLLOS J.D., Phys. Rev. B22 n980) 2927.

5. DONOHUE J. and MEYER B., i n "Elemental Sulphur", B.MEYER (Interscience: 1965)

.

6 . MEYER B., Adv. Inorg. Radiochem. 18 (1976) 287.

7. MACKNIGHT W. J

.

and TOBOLSKY A.V. , T i n ref. 5)

.

8. SPEAR W.E. and ADAMS A.R., ( i n ref. 5).

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38

(1981) 23.

10.GARDNER D.M. and FRAENKEL G.K., J. Am. Chem. Soc. 78 (1956) 3279.

11.KONINGSBERGER D.C. and DE NEEF T., Chem. Phys. L e t E 4 (1970) 615.

12.PINKUS A.G. and PIEl'TE L.H., J. Phys. Chem.

63

(19597 2086.

13.RADFORD H.E. and RICE F.O., J. Chem. Phys.

2

(1966) 774.

~~.&TELAIN A. and BUITET J., ( i n ref. 5)

.

1 5 . W I . and DAS T.P., J. Chem. Phys.

45

(1966) 3526.

16.MEYER B. ( i n ref. 5)

.