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The band structure of molybdenum disulphide from the angular variation of UV stimulated photoelectric emission
Volume 17, number
1 November
CKIZhiICAL PHYSICS LETTERS
1
1972
THE BAND STRUCTURE OF MOLYBDENUM DISULPHPDE FRdM THE ANGULAR VARIATION OF UV STIMULATED PHOTOELECTRIC EMISSLON
J.M. THOMAS Edxvard DcGes Chenlical
Laboratories,
University
College of Wa?es, A
berysmyrh,
UK
and
M. BARBER and N. ALFORD A.EL
Lrd., BattorI Dock Rood, Marxhester,
UK
Received 4 August 1972 The photoeiectron spectrum produced by He1 radiation (21.2 eV) has been recorded for cleaved molybdenire at ;I range of cmissicn ;mgics (from E I c to E il c). The esistencc of the non-bonding MO 4d band (ca. 2 eV wide) is identified esperimentalfy for the first time.
Detailed knowledge of the eIecttonic band structure of MoS, and related layered metaI dichalcogenides is required in order to understand the wide range of optical, electrical and chemical properties displayed by these materials [I] . Thus the refinements of the band structure should be able to explain the semi- and superconducting behaviour of these solids, the reason why, in certain csses, guest specie5 are readily accommodated ro form stable intercalates [I--3] , and, ultimately, the variation in behaviour of a particular compound amongst its various possible polytypic structures. Theoretical approaches [4,5] to the band structures have been limited by the severe intrinsic difficulties involved, though some progress has been made recently, and future cafcu13tions based on tight-binding and group-theoretical models are likely to prove helpful. Ex~riment~ly determined information relating to the band structure ofMoS2 has previously been obtained from electron-energy-loss analysis [6,7], optical reflectivity [S] , low-energy (< 10 eV photons) photo&ctric emission f9] and high-energy (X-ray induced) photoelectron emission studies [ LO]. Also, 142
Yoffe and his co-workers 11 I] from consideration of the exciton and electrical behaviour were able to suggest a qualitative picture (see below) of the 2H polytype of MoS*. Although these methods indicate the presence of two welt-separated maxima in the valcnceband densities of states, they are inherently incapable of yielding the exact widths and form of the energy bands, required for, amongst other things, comparison with theoretical calcuhitions. We report here the finer details of the band structure obtained by inducing electron emission from the (~~0~) cleaved basal faces of a moiybdenite crystal, the surface cleanliness of which was independently proven to be of the highest quality (see Williams and McEvoy [12] who showed that, inter alia, the sticking coefficients for common contaminants on (0001) faces are well below 10-13). The cleaved crystals (typically 1 X 1 X 0.05 cm) were mounted inside an AEI ES200 photoelectron spectrometer modified so as to permit the collection angle f3 of electrons photoreleased by He1 (or HeII) radiation to be varied in steps from 2 to 90”. Light was always incident on the basal surface; there was an approximately constant
Volume 17, number 1
CHEWCAL PHYSICSLETTERS
instrumental response as a function of energy, and the contribution of the satellite HeI, line to the final band spectrum of MoS2 was shown to be negligible. There is remarkable detail (see fig. 1) in the energy distribution of photoelectrons for a variety of emission angles, there being, in general, a striking variation in peak amplitudes and positions as a function of 0. The top part of the valence band is 9 to 10 eV wide being almost 2 eV wider for high-emission (radiation electric vector E 1 c) than for low-emission angles (E IIc). In view of the weak inter-layer interaction in this solid, this is to be expected. All the curves possess a pronounced minimum at roughly 4.5 eV below the Fermi level E, (located precisely along with the valence-band edge in previous photoemission experiments on the same samples), a result which like electron-loss measurements, confirms that there are two groups of valence bands for this solid. The maximum at ca. 7 eV below the Fermi level, E,, remains relatively constant in amplitude as the emission angle is varied, and it would seem reasonable to ascribe it to the deepest lying valence band arising from “free” sulphur spherically symmetrical 3s orbit&. But all semi-quantitative treatments of the band structure indicate that the 3s orbitals are at much lower ener,v. (This is indeed borne out by He11 measurements - see later.) The strong angular dependence of peaks located in the range down to 7 eV or so below E, obviously indicates photoelectron emission from bands derived from S 3p, MO 5p, 4d and 5s orbitals. Possibly the most significant feature of fig. 1 is the weak shoulder that extends from the Fermi level to the region of strong emission at ca. 1.5 eV. This is situated at exactly where the non-bonding MO dZz band should occur, with a low-density of states [3]. Notwithstanding the strongly suggestive results of Williams and McEvoy ( 121 this is the first conclusive experimental evidence for the occurrence of this crucially non-bonding d band in MoS2, or in any other layered solid. The rich structure present from 1.5 to 7 eV below EF arises from the valence band formed from the bonding S 3p and MO hybridized valence orbitals: we are unable, as yet, to identify the various maxima. The question does arise, however, as to what extent the observed curves reflect the true density of states of the MoSz valence band. In general, as emphasized
ANGULAR
1 November
VARL4TIQN
1972
Hd
n
I
Fig. 1. Angular variation of intensity of pho;oelectric emission (MCI radiation) from clcavas face of hloS2. The shoulder just below Ey: is identified as the MO 4d band_
by Eastman and Cashion [ 131, the initial and final states matrix elements should be taken into account. 143
Volume
17, number
Fig. 2. Comparison
1
CHEhlfCAL
of HeI and HclI stimulated photoelectric emission curves (at a r%ed angle of incidence) broad peak ca.. I1 eV below GF is tentatively assigned to the S 3s hand.
We have demonstrated, however (fig. 2), thar the relevant regions of the photoelectron spectrum taken using He(U) (40.8 eV), for an emission angle of 20°, showed major features nearly identical with the equivalent &(I) spectrum, so that the coupling between the initiaf and fina! states is, in this case, small*. Nevertheless until such time as more data, both experimental and theoretical, become available, it is reasonable to regard the spectra reported here as a reflection of the optical density of states of MoSz. We thank the Science Research Council for support. * Fig_
2 yields two other significant items of information. Firstly, because of the essential independence of the positic% of the four major (hig&ener_~) peaks UPOR the ent~gy of the stimulating radiation (and therefore upon the wavefen$h of the emitted electroni, diffradtion effec:, do not contribute significantly to the experimentally derived bandstructure. Scconcliy, there is unmistakable evidence of a strong, rather broad band centred some 11 CV or SO below EP: this is likely, on the basis of Yoffe’s work [ 111, to be due to the “free” sutphur 3s orbit&s, mentioned earlier.
144 .(.
PHYSICS LETTERS
1 November
from bfoS,.
1972
The
References r11 J.A. Wilson and AD. Yoffe, Advan. Phys. 18 (1969) 193.
121B. Bach and J.&f. Thomas, J.C.S. Chem. Commun. (1972) 301. 131 F.R. Gamble, J.H. Osiecki, $1, Cais, R. Pisharody, F.J. DiSalvo and T.H. Gcba!lc, Science 174 (1971) 493. (41 P.G. Harper and D.R. Edmondson, Phys. Stat. Sol. 44 0971) 59. [51 R.B. Murray, R.k Bromley and A.D. Yoffe, J. Phys, C 5 (1972) 738. 161 W.Y. Liang and S-L. Cundy, Phi. hfsg. 19 (1969) 103J. L71 K. Zeppcnfuld, Opt. Commun. l(1970) 377. fS1 W.Y. Lian,o,f. Phys. C 5 (I9721 L378. [91 R.H. \Vi!liarns and A.J. McEvoy, Phys. Stat. Sol. (b) 47 (1371) 517. IlO1 J.&f. Thomas, R.H. Williams and I. Adams, in pneparation. [Ill G.A.N. Connell, J.A. Wilson and A.D. Yoffe, J. Phys. Chem. Solids 30 (1963) 287. II?! RX Williams and A.J. XicEvoy, J. Phys. D 4 (1971) 456. f131 D.E. Eastman and.J.K. Cashion; Phys. Rev. Letters 24. <1970? 310.
1 November
CKIZhiICAL PHYSICS LETTERS
1
1972
THE BAND STRUCTURE OF MOLYBDENUM DISULPHPDE FRdM THE ANGULAR VARIATION OF UV STIMULATED PHOTOELECTRIC EMISSLON
J.M. THOMAS Edxvard DcGes Chenlical
Laboratories,
University
College of Wa?es, A
berysmyrh,
UK
and
M. BARBER and N. ALFORD A.EL
Lrd., BattorI Dock Rood, Marxhester,
UK
Received 4 August 1972 The photoeiectron spectrum produced by He1 radiation (21.2 eV) has been recorded for cleaved molybdenire at ;I range of cmissicn ;mgics (from E I c to E il c). The esistencc of the non-bonding MO 4d band (ca. 2 eV wide) is identified esperimentalfy for the first time.
Detailed knowledge of the eIecttonic band structure of MoS, and related layered metaI dichalcogenides is required in order to understand the wide range of optical, electrical and chemical properties displayed by these materials [I] . Thus the refinements of the band structure should be able to explain the semi- and superconducting behaviour of these solids, the reason why, in certain csses, guest specie5 are readily accommodated ro form stable intercalates [I--3] , and, ultimately, the variation in behaviour of a particular compound amongst its various possible polytypic structures. Theoretical approaches [4,5] to the band structures have been limited by the severe intrinsic difficulties involved, though some progress has been made recently, and future cafcu13tions based on tight-binding and group-theoretical models are likely to prove helpful. Ex~riment~ly determined information relating to the band structure ofMoS2 has previously been obtained from electron-energy-loss analysis [6,7], optical reflectivity [S] , low-energy (< 10 eV photons) photo&ctric emission f9] and high-energy (X-ray induced) photoelectron emission studies [ LO]. Also, 142
Yoffe and his co-workers 11 I] from consideration of the exciton and electrical behaviour were able to suggest a qualitative picture (see below) of the 2H polytype of MoS*. Although these methods indicate the presence of two welt-separated maxima in the valcnceband densities of states, they are inherently incapable of yielding the exact widths and form of the energy bands, required for, amongst other things, comparison with theoretical calcuhitions. We report here the finer details of the band structure obtained by inducing electron emission from the (~~0~) cleaved basal faces of a moiybdenite crystal, the surface cleanliness of which was independently proven to be of the highest quality (see Williams and McEvoy [12] who showed that, inter alia, the sticking coefficients for common contaminants on (0001) faces are well below 10-13). The cleaved crystals (typically 1 X 1 X 0.05 cm) were mounted inside an AEI ES200 photoelectron spectrometer modified so as to permit the collection angle f3 of electrons photoreleased by He1 (or HeII) radiation to be varied in steps from 2 to 90”. Light was always incident on the basal surface; there was an approximately constant
Volume 17, number 1
CHEWCAL PHYSICSLETTERS
instrumental response as a function of energy, and the contribution of the satellite HeI, line to the final band spectrum of MoS2 was shown to be negligible. There is remarkable detail (see fig. 1) in the energy distribution of photoelectrons for a variety of emission angles, there being, in general, a striking variation in peak amplitudes and positions as a function of 0. The top part of the valence band is 9 to 10 eV wide being almost 2 eV wider for high-emission (radiation electric vector E 1 c) than for low-emission angles (E IIc). In view of the weak inter-layer interaction in this solid, this is to be expected. All the curves possess a pronounced minimum at roughly 4.5 eV below the Fermi level E, (located precisely along with the valence-band edge in previous photoemission experiments on the same samples), a result which like electron-loss measurements, confirms that there are two groups of valence bands for this solid. The maximum at ca. 7 eV below the Fermi level, E,, remains relatively constant in amplitude as the emission angle is varied, and it would seem reasonable to ascribe it to the deepest lying valence band arising from “free” sulphur spherically symmetrical 3s orbit&. But all semi-quantitative treatments of the band structure indicate that the 3s orbitals are at much lower ener,v. (This is indeed borne out by He11 measurements - see later.) The strong angular dependence of peaks located in the range down to 7 eV or so below E, obviously indicates photoelectron emission from bands derived from S 3p, MO 5p, 4d and 5s orbitals. Possibly the most significant feature of fig. 1 is the weak shoulder that extends from the Fermi level to the region of strong emission at ca. 1.5 eV. This is situated at exactly where the non-bonding MO dZz band should occur, with a low-density of states [3]. Notwithstanding the strongly suggestive results of Williams and McEvoy ( 121 this is the first conclusive experimental evidence for the occurrence of this crucially non-bonding d band in MoS2, or in any other layered solid. The rich structure present from 1.5 to 7 eV below EF arises from the valence band formed from the bonding S 3p and MO hybridized valence orbitals: we are unable, as yet, to identify the various maxima. The question does arise, however, as to what extent the observed curves reflect the true density of states of the MoSz valence band. In general, as emphasized
ANGULAR
1 November
VARL4TIQN
1972
Hd
n
I
Fig. 1. Angular variation of intensity of pho;oelectric emission (MCI radiation) from clcavas face of hloS2. The shoulder just below Ey: is identified as the MO 4d band_
by Eastman and Cashion [ 131, the initial and final states matrix elements should be taken into account. 143
Volume
17, number
Fig. 2. Comparison
1
CHEhlfCAL
of HeI and HclI stimulated photoelectric emission curves (at a r%ed angle of incidence) broad peak ca.. I1 eV below GF is tentatively assigned to the S 3s hand.
We have demonstrated, however (fig. 2), thar the relevant regions of the photoelectron spectrum taken using He(U) (40.8 eV), for an emission angle of 20°, showed major features nearly identical with the equivalent &(I) spectrum, so that the coupling between the initiaf and fina! states is, in this case, small*. Nevertheless until such time as more data, both experimental and theoretical, become available, it is reasonable to regard the spectra reported here as a reflection of the optical density of states of MoSz. We thank the Science Research Council for support. * Fig_
2 yields two other significant items of information. Firstly, because of the essential independence of the positic% of the four major (hig&ener_~) peaks UPOR the ent~gy of the stimulating radiation (and therefore upon the wavefen$h of the emitted electroni, diffradtion effec:, do not contribute significantly to the experimentally derived bandstructure. Scconcliy, there is unmistakable evidence of a strong, rather broad band centred some 11 CV or SO below EP: this is likely, on the basis of Yoffe’s work [ 111, to be due to the “free” sutphur 3s orbit&s, mentioned earlier.
144 .(.
PHYSICS LETTERS
1 November
from bfoS,.
1972
The
References r11 J.A. Wilson and AD. Yoffe, Advan. Phys. 18 (1969) 193.
121B. Bach and J.&f. Thomas, J.C.S. Chem. Commun. (1972) 301. 131 F.R. Gamble, J.H. Osiecki, $1, Cais, R. Pisharody, F.J. DiSalvo and T.H. Gcba!lc, Science 174 (1971) 493. (41 P.G. Harper and D.R. Edmondson, Phys. Stat. Sol. 44 0971) 59. [51 R.B. Murray, R.k Bromley and A.D. Yoffe, J. Phys, C 5 (1972) 738. 161 W.Y. Liang and S-L. Cundy, Phi. hfsg. 19 (1969) 103J. L71 K. Zeppcnfuld, Opt. Commun. l(1970) 377. fS1 W.Y. Lian,o,f. Phys. C 5 (I9721 L378. [91 R.H. \Vi!liarns and A.J. McEvoy, Phys. Stat. Sol. (b) 47 (1371) 517. IlO1 J.&f. Thomas, R.H. Williams and I. Adams, in pneparation. [Ill G.A.N. Connell, J.A. Wilson and A.D. Yoffe, J. Phys. Chem. Solids 30 (1963) 287. II?! RX Williams and A.J. XicEvoy, J. Phys. D 4 (1971) 456. f131 D.E. Eastman and.J.K. Cashion; Phys. Rev. Letters 24. <1970? 310.