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Raman spectra of SnS2 and SnSe2
Solid State Communications, Vol. 20, pp. 885—887, 1976.
Pergamon Press.
Printed in Great Britain
RAMAN SPECTRA OF SnS2 AND SnSe2 D.G. Mead and J.C. Irwin Physics Department, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6 (Received 11 August 1976 by R. Barrie) The first order Raman spectra of the Group Na layer compounds SnS2 and SnSe2 are presented. The two Raman active modes of each compound have been identified and on the basis of their observed symmetry assigned in accord with group theoretical predictions. The SnSe2 spectrum is compared to the results of a previous investigation and poor agreement is obtained. The present results are also compared to spectra previously obtained from Group Nb dichalcogemdes. INTRODUCTION
Spex 1401 spectrometer with a third monochromator,
SnS2 AND SnSe2 are layer structure compounds cornposed of weakly interacting layers each of which is three atoms thick. compounds Cd12 1 orBoth iT polytype, andcrystallize both haveina the symmetry structure, described by the D~space group. The unit cell spans only one layer and contains three atoms. There are thus
an Argon ion laser, and standard photon counting techniques. With the laser beam vertical and the sample face approximately 5°off vertical a near right angle scattering geometry was obtained. The crystals studied were grown by vapour phase transport techniques.6’7 SnS 2 is a semiconductor with a 6 and the crystal room temperature band edge at 2.2 eV appearance varied from yellow to reddish—brown depending on the thickness. SnSe 8 and the 2 is also a semiconductor with acrystals room temperature band edge sheen. at about had a definite metallic The1 eV spectra were
twelve vibrational at the centre the Brfflouin zone and these canmodes be described by theof irreducible representations of the D~point group :2 I’
Aig + Eg + 3A~+ 3Ev.
There are two non-degenerate Raman modes, namely A lg and Eg. In this paper a Raman study of SnS 2 and SnSe2 crystals over the temperature range 80—300 K is presented. The Raman spectrum of SnSe2 hasthe previously 3 and present been investigated bybeAgnihotri et al. with this work. results are found to in disagreement To the best of the authors’ knowledge the Raman spectrum of SnS 2 has not been previously studied. The resuits presented here can however be compared to results obtained onout a number of investigation similar compounds. Smith et 4 carried a Raman of the disulphides al. diselenides of the group Nb elements Hf, Zr, and and Ti. They found that the observed Raman frequencies were essentially independent of the metal atom. Lucovsky et al.5 have also carried out infrared reflectivity measurements on the Zr, Hf and Ti dichalcogenides. These studies have shown that there is little variation between the force constants in these IVb cornpounds. The present results indicate that the force constants in the Sn dichalcogenides are somewhat smaller than in the IVb compounds. 2. EXPERIMENTAL The Raman measurements were carried out using a
obtained from freshly cleaved faces in all cases. 3. RESULTS AND DISCUSSION The room temperature Raman spectra of SnS2 and SnSe2 are shown in Figs. to I and 2 respectively. Only two orientations are required determine the symmetry character of the two modes for each compound. The solid line in each figure was obtained with the electric vector of the incident light in the plane of the crystal layers (s-spectrum). If we take the z axis of the crystal perpendicular to xy the components layers the s spectrum arises from the diagonal and of the polarizability tensor.9 In this orientation both the E~and Aig modes should be observed. The dashed line in each figure was obtained with the electric vector of the incident light perpendicular to the layers (p-spectrum) and thus gives the spectrum arising from the zx and zy components of the polarizabiity tensor. Hence in this orientation only the Eg mode should appear. As can be seen from the figures however the strong Aig mode appears in both the s and p spectra, because of depolarization and the deviation from a strict 90°geometry. The relative strength, however, of the Eg mode is greatest in the s spectra and enables an unambiguous assignment to be made. A similar degree of “leakage” or “spillage” was observed by Smith et aL4 in similar compounds. The measured 885
886
RAMAN SPECTRA OF SnS2 AND SnSe2 I
I
I
SflS2
Vol. 20, No. 9
I
I
I
A19
300K
317
—**4-.
Eg
\X20
202
—
0
100
200
300
400
500
CM’ Fig. 1. Raman spectra of SnS2 at room temperature using 488.0 nm wavelength excitation. The solid line is the s-spectrum (incident radiation polarized perpendicular to the plane of incidence) and the dashed line is the p-spectrum (incident radiation polarized in the plane of incidence). 11
I
I
I
I
A1g
I
300K
SnSe
—
185 5
0
I
I
I
100
200
300
.
400
Fig. 2. Rarnan spectra of SnSe2 at room temperature using 488.0 nm excitation. The solid line is the s-spectrum and the dashed line is the p-spectrum as in Fig. 1. frequencies are listed in Table 1 for two different ternperatures. 3 in a recent study obtained quite Agnihotri al. the E~and A ig modes of SnSe different valuesetfor 1 for Eg and 399 cm1 for A ig~No2,feanamely 320 cm were found in this frequency range in tures whatsoever the present work despite a careful search. Any such features would have to be an order of i0~less than the intensity of the mode observed at 185.5 cm~.This discrepancy may be due to different methods of crystal
preparation that would result in very different impurity contents Smithand/or eta!.4polytypes. have pointed out that the metal atom is stationary for force the even vibrational modes of the were iT polytype. If the constants in such materials approximately the same one should then find that the frequencies of the A ig and Eg modes were essentially independent of the metal atom. This indeed turned out to be the case for the Zr, Hf, and Ti dichalcogenides, where Smith et al.4 measured values of approximately 335 and
Vol. 20, No.9
RAMAN SPECTRA OF SnS2 AND SnSe2
Tableofvibration cies 1. Assignments (cm’) and forphonon SnS frequen2/Se2. All frequencies are ± 1.0 cm~
887 1°and employed by account as suggested 5 the modes by Keyes should scale approximately Lucovsky et al. as /~ii
__________________________________________________
I
\3
SpiS 2 80K 300K SnSe2 80K 300K
A1~
E~
“SR’~SeaI~M5(r0)~
320.0 317.0
204.7 202.0
188.0 185.5
117.4 116.0
—
1.68, where
r0 is taken as the nearest neighbour separation. Values were 8’~12 obtained from data contained in the literature.From Table 1 one finds that the measured results
1.71 and = 1.74, 235 cm’ for the A lg &id Eg modes of the suiphides and \“se)Eg 195 and 150 cm~for the A ig and Eg modes of the selen- are very close to the predicted result and to the values ides. The smaller values for the A lg and Eg modes of observed4 for the Group IVb dichalcogenides. Thus in SnS 2 and SnSe2 obtained here (Table 1) indicate that summary it is found that the force constants are very the force constants are smaller in these compounds than nearly equal for SnS2 and SnSe2 and somewhat smaller in the group Nb compounds. than those observed for the Zr, Ti, and Hf dichalcogenIf the frequencies of the selenides and sulphides are ides. compared one expects them to scale approximately as ~
~
IM
J
\1~~2 =
1.57.
Acknowledgements We are grateful to Dr. J.L. Brebner for supplying us with the samples, and to the National Research Council of Canada for financial support. —
/
If the separation between atoms is also taken into
REFERENCES 1.
WYCKOFF R.W.G., Crystal Structures, Vol. 1. Wiley, New York (1965).
2.
LOCKWOOD D.J., Light Scattering Spectra ofSolids (Edited by WRIGHT G.B.), p.75. Springer Verlag, New York (1969). AGNIHOTRI O.P., GARG A.K. & SEHGAL H.K., Solid State Commun. 17, 1537 (1975).
3. 4. 5. 6.
SMITH J.E., NATHAN M.I., SHAFER M.W. & TORRANCE J.B., Proc. Eleventh mt. Conf on the Physics of Semicond., Polish Scientific Publishers, Warsaw, p. 1306 (1972). LUCOVSKY G., WHITE R.M., BENDA J.A. & REVELLI J.F.,.Phys. Rev. 7,3859. (1973).
7.
GREENAWAY D.L. & NITSCHE R.,J. Phys. Chem. Solids 26, 1445 (1965); a review is given in WILSON J.A. YOFFE A.A.,Adv. Phys. 18, 193 (1969). SHAEFER H., Chemical Transport Reactions. Academic Press, New York (1964).
8.
BUSCH G., FROHLJCH C., HULLIGER F. & STEIGMEIER E., Helv. Phys. Acta 34, 359 (1961).
9.
LOUDON R., Adv. Phys. 13, 423 (1964).
10.
KEYESR.W.,J.AppLPhys. 33, 3371 (1962).
11. 12.
GAMBLE F.R.,J. Solid State Chem. 9,358(1974). WILSON J.A. & YOFFE A.D.,Adv. Phys. 18, 193 (1969). For (Sn—S) bond length, the ideal z value 1/4, was used.
Pergamon Press.
Printed in Great Britain
RAMAN SPECTRA OF SnS2 AND SnSe2 D.G. Mead and J.C. Irwin Physics Department, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6 (Received 11 August 1976 by R. Barrie) The first order Raman spectra of the Group Na layer compounds SnS2 and SnSe2 are presented. The two Raman active modes of each compound have been identified and on the basis of their observed symmetry assigned in accord with group theoretical predictions. The SnSe2 spectrum is compared to the results of a previous investigation and poor agreement is obtained. The present results are also compared to spectra previously obtained from Group Nb dichalcogemdes. INTRODUCTION
Spex 1401 spectrometer with a third monochromator,
SnS2 AND SnSe2 are layer structure compounds cornposed of weakly interacting layers each of which is three atoms thick. compounds Cd12 1 orBoth iT polytype, andcrystallize both haveina the symmetry structure, described by the D~space group. The unit cell spans only one layer and contains three atoms. There are thus
an Argon ion laser, and standard photon counting techniques. With the laser beam vertical and the sample face approximately 5°off vertical a near right angle scattering geometry was obtained. The crystals studied were grown by vapour phase transport techniques.6’7 SnS 2 is a semiconductor with a 6 and the crystal room temperature band edge at 2.2 eV appearance varied from yellow to reddish—brown depending on the thickness. SnSe 8 and the 2 is also a semiconductor with acrystals room temperature band edge sheen. at about had a definite metallic The1 eV spectra were
twelve vibrational at the centre the Brfflouin zone and these canmodes be described by theof irreducible representations of the D~point group :2 I’
Aig + Eg + 3A~+ 3Ev.
There are two non-degenerate Raman modes, namely A lg and Eg. In this paper a Raman study of SnS 2 and SnSe2 crystals over the temperature range 80—300 K is presented. The Raman spectrum of SnSe2 hasthe previously 3 and present been investigated bybeAgnihotri et al. with this work. results are found to in disagreement To the best of the authors’ knowledge the Raman spectrum of SnS 2 has not been previously studied. The resuits presented here can however be compared to results obtained onout a number of investigation similar compounds. Smith et 4 carried a Raman of the disulphides al. diselenides of the group Nb elements Hf, Zr, and and Ti. They found that the observed Raman frequencies were essentially independent of the metal atom. Lucovsky et al.5 have also carried out infrared reflectivity measurements on the Zr, Hf and Ti dichalcogenides. These studies have shown that there is little variation between the force constants in these IVb cornpounds. The present results indicate that the force constants in the Sn dichalcogenides are somewhat smaller than in the IVb compounds. 2. EXPERIMENTAL The Raman measurements were carried out using a
obtained from freshly cleaved faces in all cases. 3. RESULTS AND DISCUSSION The room temperature Raman spectra of SnS2 and SnSe2 are shown in Figs. to I and 2 respectively. Only two orientations are required determine the symmetry character of the two modes for each compound. The solid line in each figure was obtained with the electric vector of the incident light in the plane of the crystal layers (s-spectrum). If we take the z axis of the crystal perpendicular to xy the components layers the s spectrum arises from the diagonal and of the polarizability tensor.9 In this orientation both the E~and Aig modes should be observed. The dashed line in each figure was obtained with the electric vector of the incident light perpendicular to the layers (p-spectrum) and thus gives the spectrum arising from the zx and zy components of the polarizabiity tensor. Hence in this orientation only the Eg mode should appear. As can be seen from the figures however the strong Aig mode appears in both the s and p spectra, because of depolarization and the deviation from a strict 90°geometry. The relative strength, however, of the Eg mode is greatest in the s spectra and enables an unambiguous assignment to be made. A similar degree of “leakage” or “spillage” was observed by Smith et aL4 in similar compounds. The measured 885
886
RAMAN SPECTRA OF SnS2 AND SnSe2 I
I
I
SflS2
Vol. 20, No. 9
I
I
I
A19
300K
317
—**4-.
Eg
\X20
202
—
0
100
200
300
400
500
CM’ Fig. 1. Raman spectra of SnS2 at room temperature using 488.0 nm wavelength excitation. The solid line is the s-spectrum (incident radiation polarized perpendicular to the plane of incidence) and the dashed line is the p-spectrum (incident radiation polarized in the plane of incidence). 11
I
I
I
I
A1g
I
300K
SnSe
—
185 5
0
I
I
I
100
200
300
.
400
Fig. 2. Rarnan spectra of SnSe2 at room temperature using 488.0 nm excitation. The solid line is the s-spectrum and the dashed line is the p-spectrum as in Fig. 1. frequencies are listed in Table 1 for two different ternperatures. 3 in a recent study obtained quite Agnihotri al. the E~and A ig modes of SnSe different valuesetfor 1 for Eg and 399 cm1 for A ig~No2,feanamely 320 cm were found in this frequency range in tures whatsoever the present work despite a careful search. Any such features would have to be an order of i0~less than the intensity of the mode observed at 185.5 cm~.This discrepancy may be due to different methods of crystal
preparation that would result in very different impurity contents Smithand/or eta!.4polytypes. have pointed out that the metal atom is stationary for force the even vibrational modes of the were iT polytype. If the constants in such materials approximately the same one should then find that the frequencies of the A ig and Eg modes were essentially independent of the metal atom. This indeed turned out to be the case for the Zr, Hf, and Ti dichalcogenides, where Smith et al.4 measured values of approximately 335 and
Vol. 20, No.9
RAMAN SPECTRA OF SnS2 AND SnSe2
Tableofvibration cies 1. Assignments (cm’) and forphonon SnS frequen2/Se2. All frequencies are ± 1.0 cm~
887 1°and employed by account as suggested 5 the modes by Keyes should scale approximately Lucovsky et al. as /~ii
__________________________________________________
I
\3
SpiS 2 80K 300K SnSe2 80K 300K
A1~
E~
“SR’~SeaI~M5(r0)~
320.0 317.0
204.7 202.0
188.0 185.5
117.4 116.0
—
1.68, where
r0 is taken as the nearest neighbour separation. Values were 8’~12 obtained from data contained in the literature.From Table 1 one finds that the measured results
1.71 and = 1.74, 235 cm’ for the A lg &id Eg modes of the suiphides and \“se)Eg 195 and 150 cm~for the A ig and Eg modes of the selen- are very close to the predicted result and to the values ides. The smaller values for the A lg and Eg modes of observed4 for the Group IVb dichalcogenides. Thus in SnS 2 and SnSe2 obtained here (Table 1) indicate that summary it is found that the force constants are very the force constants are smaller in these compounds than nearly equal for SnS2 and SnSe2 and somewhat smaller in the group Nb compounds. than those observed for the Zr, Ti, and Hf dichalcogenIf the frequencies of the selenides and sulphides are ides. compared one expects them to scale approximately as ~
~
IM
J
\1~~2 =
1.57.
Acknowledgements We are grateful to Dr. J.L. Brebner for supplying us with the samples, and to the National Research Council of Canada for financial support. —
/
If the separation between atoms is also taken into
REFERENCES 1.
WYCKOFF R.W.G., Crystal Structures, Vol. 1. Wiley, New York (1965).
2.
LOCKWOOD D.J., Light Scattering Spectra ofSolids (Edited by WRIGHT G.B.), p.75. Springer Verlag, New York (1969). AGNIHOTRI O.P., GARG A.K. & SEHGAL H.K., Solid State Commun. 17, 1537 (1975).
3. 4. 5. 6.
SMITH J.E., NATHAN M.I., SHAFER M.W. & TORRANCE J.B., Proc. Eleventh mt. Conf on the Physics of Semicond., Polish Scientific Publishers, Warsaw, p. 1306 (1972). LUCOVSKY G., WHITE R.M., BENDA J.A. & REVELLI J.F.,.Phys. Rev. 7,3859. (1973).
7.
GREENAWAY D.L. & NITSCHE R.,J. Phys. Chem. Solids 26, 1445 (1965); a review is given in WILSON J.A. YOFFE A.A.,Adv. Phys. 18, 193 (1969). SHAEFER H., Chemical Transport Reactions. Academic Press, New York (1964).
8.
BUSCH G., FROHLJCH C., HULLIGER F. & STEIGMEIER E., Helv. Phys. Acta 34, 359 (1961).
9.
LOUDON R., Adv. Phys. 13, 423 (1964).
10.
KEYESR.W.,J.AppLPhys. 33, 3371 (1962).
11. 12.
GAMBLE F.R.,J. Solid State Chem. 9,358(1974). WILSON J.A. & YOFFE A.D.,Adv. Phys. 18, 193 (1969). For (Sn—S) bond length, the ideal z value 1/4, was used.