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Adsorbate enhanced Ti-3d photoemission from layered TiSe2 surfaces
46
Surface Science 155 (1985) 46652 North-Holland, Amsterdam
ADSORBATE ENHANCED TiSe, SURFACES G. KARSCHNICK,
Ti3d
0. ANDERSON,
PHOTOEMISSION
W. DRUBE
FROM LAYERED
and M. SKIBOWSKI
lnsrrtutfir Experimentalphysik der Unwersrtiit Kiel. 0l.rhausen.w 40 60, D - 2300 Kiel 1. Fed. Rep. of German,: Received
19 July 1984; accepted
for publication
29 January
1985
Angle resolved photoelectron spectra of IT-TiSe, single crystals (hw = 21.2 eV) have been investigated as a function of exposure to H,O, CO, N, and 0, at room temperature. A significant enhancement of the Ti-3d emission near the Fermi energy E, is observed for Hz0 and CO exposures of several kL. As compared to the clean surface this emission extends over the entire Brillouin zone. The results indicate that by H,O and CO adsorption the lowest d-subband is pulled below E, possibly by local band bending. Additional relaxation effects at the surface associated with changes in d-band matrix elements may occur. The intensity of the Se-4p derived valence band emission down to 7 eV below E,. remains essentially unchanged.
1. Introduction The layered transition metal dichalcogenides (LTMDs) attract considerable interest because of their peculiar anisotropic macroscopic properties associated with their two-dimensional crystal structure and their characteristic d-electron configuration [l]. These properties are closely related to the electronic band structure which is very effectively studied by angle resolved photoemission. We have examined photoelectrons from single crystal surfaces of lT-TiSe,, a material which is known to show a phase transition around 200 K leading to a 2u, x 2c,] superlattice [2,3]. An important point for understanding the interesting properties of TiSe, [4-61 is the behaviour of the Ti-3d like conduction electrons at the Fermi energy. Because the crystal surface is of the Van der Waals type, reminiscent of the weak interlayer interaction in the bulk, previous photoemission studies [7-111 did not discuss any surface effects. However, we found the Ti-3d emission to be very sensitive to specific experimental conditions, in particular to the exposure of the surfaces to distinct adsorbates. We therefore studied these effects in a more systematic way. For the first time we report on changes in the photoemission spectra due to interaction between adsorbates and the Van der Waals surface of a LTMD resulting in an enhanced d-emission in the case of H,O and CO on TiSe,. 0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
G. Karschnick et al. / Adsorbate enhanced Ti-3dphotoemlssion
4-l
2. Experimental In our studies we have measured angle resolved energy distribution curves (EDCs) with a fixed cylindrical mirror analyser equipped with apertures (FWHM resolution: energy AE = 200 meV, azimuth Arp = 5”, polar angle A8 = 3”). The angles were varied by rotating the sample. Photoelectrons were excited by He1 21.2 eV radiation. Further details of the experimental set-up are described elsewhere [7,12]. Single crystals of IT-TiSe, (space group D$) were grown by an iodine gas transport reaction [13]. The sample was of the best attainable stoichiometry concerning Ti excess. This was checked by temperature dependent resistivity measurements [3] which yielded a ratio of ~(165 K) and ~(300 K) of 4.28. The samples were cleaved in situ at a base pressure in the low lo- ” Torr range. They were characterized by Laue, LEED and AES techniques.
3. Results and discussion A typical photoelectron spectrum of a clean sample taken in the FAML plane of the hexagonal Brillouin zone (lowest spectrum in fig. 1) shows strong peaks attributed to Se-4p derived states. Some of them show a significant energy shift with varying polar angle [8]. The small peak close to the Fermi energy which appears at a k ,, near the Brillouin zone boundary is ascribed to emission from the lowest Ti-3d derived conduction band. The full experimental two-dimensional band structure E(k ,,) is in good agreement with a local density LCAO calculation [14]. Both theory and experiment locate the occupied d-states around the Brillouin zone boundary (fig. 2). A number of careful experiments on various samples revealed that the d-emission was dependent on sample stoichiometry [7] which has also been reported in the case of TiS, [15], but also on the condition of the surface. In order to test a possible influence of adsorbates we exposed our samples to various gases at room temperature. The adsorption of water on the clean surface caused a significant change in the energy distribution near the Fermi energy (fig. 1). These spectra were taken at a fixed polar angle 19= 34” corresponding to a k ,, near the Brillouin zone boundary for the d-peak energy. At exposures of several kL the emission from the Ti-d band increases considerably. Even at exposures of more than 200 kL the spectral features associated with Se-p states remain essentially unchanged. The d-peak intensity (peak area) measured as a function of exposure shows a saturation-like behaviour at exposures of more than 200 kL (fig. 3). This result together with the good detectability of the TiSe, structure at high exposures led to the assumption that when saturation is reached probably only one monolayer of water molecules is adsorbed on the TiSe, surface which would result in a sticking coefficient of
G. Karschnick
et al. / Adsorbate enhanced Tl-3d photoemrssion
0
5 Energy
Fig. 1. EDCs i%w= 21.2 ev.
below
of lT-TiSe,
EF
exposed
(eV)
to H,O
taken
in the anmuthal
direction
I‘M:
8 = 34”:
10P4 to 10P5. By heating the sample up to 150°C the original state of the surface was recovered: The EDC showed the old features, a weaker d-emission and less background (top spectrum in fig. 1). Water desorption was monitored by an increase in pressure. The behaviour indicates that water is only physisorbed on the TiSe, surface. The EDCs of TiSe, nearly “saturated” with water have been measured for various polar angles (fig. 4). The spectra were normalized to the photon flux. As compared to the clean sample the intensity of the TiSe, valence band emission did not change but the emission from Ti-3d-derived states was now
G. Karschnick et al. / Adsorbate enhanced Ti-3dphotoemission
49
M CL)
JI-(A)
Fig. 2. Electronic band structure of TiSe, in the rALM reproduced from [14]; full lines: TM; dashed lines: AL.
plane in the vicinity
of the Fermi level,
clearly visible for all measured polar angles the spectra shifting by less than 0.1 eV to higher binding energies. The Se-4p bands deduced from these spectra remain the same whereas the d-band close to E, is now observed over the whole Brillouin zone. This allows an estimate of the total width of this measuresubband of - 0.2 eV also supported by recent inverse photoemission ments [16]. This value is smaller than the averaged calculated bandwidth. 0-2~ emission was observed at approximately 8.5 eV below E, out of the energy range in fig. 1. It causes the increase of intensity at binding energies above 7 eV. A similar effect was observed for adsorption of CO. We found again an increase in d-band emission. The adsorbate was removable; electron-bombard-
I
I
0
23( H20
exposure
Fig. 3. Ti-3d peak intensity
(kL)
(peak area) as a function
of the exposure
to H,O.
9
Fig, 4. Polar angle dependence of EDCs from TiSe, exposed to 160 kL Hz0 direction TM: h*r = 21.2 eV.
taken in the uimuthd
ment and laser heating recovered the original spectra. By exposing the samples to N, or Oz no change in d-emission was observed. The only effects were enhancement of inelasticaIly scattered electrons and disappearance of the TiSq structure at very high exposures above 200 kL. In a few cases we also observed a “super-d”-emission (fig. 5) appearing after an air leakage in the cooling system. The measured polar angle dependence reveaIed that this peak atso extends over the whole Brillouin zone similar to that of TiSe, exposed to Hz0 (see fig. 4). Systematic studies of the conditions
G. Karschnick et al. / Adsorbate enhanced Ti-3dphotoemission
51
5 Energy
below
Fig. 5. EDC from ‘Me, Ao = 21.2 eV.
EF
[evl
~super-~-omission)
taken in the azimuthal direction TM; 6 = 28O;
under which a “super-d” occurs are under way. As compared to N, and O,, II,0 and CO molecules carry an electric dipole moment which seems to be responsible for the enhanced d-emission. Its visibility through the entire Btillouin zone in the TM direction can be viewed in a simple rigid band model as being due to a shift of the Fermi energy with respect to the energy bands. A small shift is already expected to give a strong change in the occupation of the flat lowest d-band which explains the experimental observations (see fig. 2). Such a shift can be caused by direct electron transfer to empty bulk states, a phenomenon which has been observed in d-electron emissian from Ti, _XV,Se, mixed crystals where vanadium delivers an extra electron to the d-bands [17]. However, because of the weak interaction between the adsorbate and the surface a complete electron transfer seems unlikely and the very critical Fermi energy shift is more probable due to dipole induced band bending localized at the surface. The band bending reveals that the localization observed for clean samples is even stronger than predicted by theory [14]. A contribution of relaxation effects may be possible but cannot be understood by the simple model proposed in ref. [14] where the variation of the Ti-Se distance on the bands is discussed since no significant changes of the Se-p bands especially of those with Se-p, character were observed. If other relaxation effects exist in this class of materials they are probably small and beyond the now available experimental resolution. It is however possible that the adsorption process changes the matrix elements (wavefunctions) associated with the Ti-3d excitation. We believe that a similar mechanism is also responsible for the “super-d” emission possibly triggered by effective co-adsorption of different gas species.
52
G. Karschnick
et al. / Adwrbate
enhanced TI-3dphotoemission
4. Summary We have observed for the first time significant changes of the Ti-3d emission from TiSe, single crystal surfaces by adsorption of H,O and CO although no strong interactions were expected due to their van der Waals character. The effect can be explained by a shift of the Fermi energy caused by potential changes localized in the surface due to weakly bound dipoles in combination with additional changes in the Ti-3d matrix elements. Thereby the lowest d-band becomes visible over the whole Brillouin zone providing an estimate of its total width - 0.2 eV which is smaller than predicted by theory. Relaxation effects due to adsorbed molecules may be present but cannot be unambiguously identified within the framework of existing theoretical calculations. Further careful experiments on clean and adsorbate covered LTMD surfaces seem necessary for a complete understanding of the adsorption mechanism and even of the electronic structure of clean surfaces particularly at low temperatures in connection with phase transitions.
References [l] Physics and Chemistry of Materials with Layered Structures. Vols. 1-6 (Reidel. Dordrecht, 1976-1979). (21 K.C. Woo. F.C. Brown, W.L. McMillan, R.J. Miller. M.J. Schaffman and M.P. Sears. Phys. Rev. B14 (1976) 3242. [3] F.J. DiSalvo, D.E. Moncton and J.V. Waszczak, Phys. Rev. B14 (1976) 4321. [4] I. Taguchi, M. Asai, Y. Watanabe and M. Oka. Physica B+C 105 (1981) 146. [5] F. Levy, J. Phys. Cl3 (1980) 2901. [6] J.A. Wilson, Phys. Status Solidi (b) 86 (1978) 11. [7] G. Karschnick, Thesis, University of Kiel (1982). [X] W. Drube, G. Karschnick, M. Skibowski, R. Thies and K. Volkert, in: Proc. 15th Intern. Conf. on the Physics of Semiconductors, Kyoto, 1980 [J. Phys. Sot. Japan 49, Suppl. A (1980) 1371. [9] R.Z. Bachrach, M. Skibowski and F.C. Brown, Phys. Rev. Letters 37 (1976) 40. [lo] M.M. Traum, G. Margaritondo. N.V. Smith, J.E. Rowe and F.J. DiSalvo. Phys. Rev. B17 (1978) 1836. [ll] C.H. Chen, W. Fabian, F.C. Brown, K.C. Woo, B. Davies, B. DeLong and A.H. Thompson. Phys. Rev. B21 (1980) 615. [12] G. Karschnick, W. Drube and M. Skibowski, to be published. [13] D.L. Greenaway and R. Nitsche, J. Phys. Chem. Solids 26 (1965) 1445. [14] A. Zunger and A.J. Freeman, Phys. Rev. B17 (1978) 1839; Phys. Rev. Letters 40 (1978) 1155. [15] J.J. Barry, H.P. Hughes, P.C. Khpstein and R.H. Friend, J. Phys. Cl6 (1983) 393. [16] W. Drube, I. Schafer. G. Karschnick and M. Skibowski, Phys. Rev. B30 (1984) 6248. [17] 0. Anderson, W. Drube. G. Karschnick and M. Skibowski, in: Proc. 7th Intern. Conf. on Vacuum Ultraviolet Radiation Physics, VUV VII, Jerusalem 1983 [Ann. Israel Phys. Sot. 6 (1983) 3151.
Surface Science 155 (1985) 46652 North-Holland, Amsterdam
ADSORBATE ENHANCED TiSe, SURFACES G. KARSCHNICK,
Ti3d
0. ANDERSON,
PHOTOEMISSION
W. DRUBE
FROM LAYERED
and M. SKIBOWSKI
lnsrrtutfir Experimentalphysik der Unwersrtiit Kiel. 0l.rhausen.w 40 60, D - 2300 Kiel 1. Fed. Rep. of German,: Received
19 July 1984; accepted
for publication
29 January
1985
Angle resolved photoelectron spectra of IT-TiSe, single crystals (hw = 21.2 eV) have been investigated as a function of exposure to H,O, CO, N, and 0, at room temperature. A significant enhancement of the Ti-3d emission near the Fermi energy E, is observed for Hz0 and CO exposures of several kL. As compared to the clean surface this emission extends over the entire Brillouin zone. The results indicate that by H,O and CO adsorption the lowest d-subband is pulled below E, possibly by local band bending. Additional relaxation effects at the surface associated with changes in d-band matrix elements may occur. The intensity of the Se-4p derived valence band emission down to 7 eV below E,. remains essentially unchanged.
1. Introduction The layered transition metal dichalcogenides (LTMDs) attract considerable interest because of their peculiar anisotropic macroscopic properties associated with their two-dimensional crystal structure and their characteristic d-electron configuration [l]. These properties are closely related to the electronic band structure which is very effectively studied by angle resolved photoemission. We have examined photoelectrons from single crystal surfaces of lT-TiSe,, a material which is known to show a phase transition around 200 K leading to a 2u, x 2c,] superlattice [2,3]. An important point for understanding the interesting properties of TiSe, [4-61 is the behaviour of the Ti-3d like conduction electrons at the Fermi energy. Because the crystal surface is of the Van der Waals type, reminiscent of the weak interlayer interaction in the bulk, previous photoemission studies [7-111 did not discuss any surface effects. However, we found the Ti-3d emission to be very sensitive to specific experimental conditions, in particular to the exposure of the surfaces to distinct adsorbates. We therefore studied these effects in a more systematic way. For the first time we report on changes in the photoemission spectra due to interaction between adsorbates and the Van der Waals surface of a LTMD resulting in an enhanced d-emission in the case of H,O and CO on TiSe,. 0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
G. Karschnick et al. / Adsorbate enhanced Ti-3dphotoemlssion
4-l
2. Experimental In our studies we have measured angle resolved energy distribution curves (EDCs) with a fixed cylindrical mirror analyser equipped with apertures (FWHM resolution: energy AE = 200 meV, azimuth Arp = 5”, polar angle A8 = 3”). The angles were varied by rotating the sample. Photoelectrons were excited by He1 21.2 eV radiation. Further details of the experimental set-up are described elsewhere [7,12]. Single crystals of IT-TiSe, (space group D$) were grown by an iodine gas transport reaction [13]. The sample was of the best attainable stoichiometry concerning Ti excess. This was checked by temperature dependent resistivity measurements [3] which yielded a ratio of ~(165 K) and ~(300 K) of 4.28. The samples were cleaved in situ at a base pressure in the low lo- ” Torr range. They were characterized by Laue, LEED and AES techniques.
3. Results and discussion A typical photoelectron spectrum of a clean sample taken in the FAML plane of the hexagonal Brillouin zone (lowest spectrum in fig. 1) shows strong peaks attributed to Se-4p derived states. Some of them show a significant energy shift with varying polar angle [8]. The small peak close to the Fermi energy which appears at a k ,, near the Brillouin zone boundary is ascribed to emission from the lowest Ti-3d derived conduction band. The full experimental two-dimensional band structure E(k ,,) is in good agreement with a local density LCAO calculation [14]. Both theory and experiment locate the occupied d-states around the Brillouin zone boundary (fig. 2). A number of careful experiments on various samples revealed that the d-emission was dependent on sample stoichiometry [7] which has also been reported in the case of TiS, [15], but also on the condition of the surface. In order to test a possible influence of adsorbates we exposed our samples to various gases at room temperature. The adsorption of water on the clean surface caused a significant change in the energy distribution near the Fermi energy (fig. 1). These spectra were taken at a fixed polar angle 19= 34” corresponding to a k ,, near the Brillouin zone boundary for the d-peak energy. At exposures of several kL the emission from the Ti-d band increases considerably. Even at exposures of more than 200 kL the spectral features associated with Se-p states remain essentially unchanged. The d-peak intensity (peak area) measured as a function of exposure shows a saturation-like behaviour at exposures of more than 200 kL (fig. 3). This result together with the good detectability of the TiSe, structure at high exposures led to the assumption that when saturation is reached probably only one monolayer of water molecules is adsorbed on the TiSe, surface which would result in a sticking coefficient of
G. Karschnick
et al. / Adsorbate enhanced Tl-3d photoemrssion
0
5 Energy
Fig. 1. EDCs i%w= 21.2 ev.
below
of lT-TiSe,
EF
exposed
(eV)
to H,O
taken
in the anmuthal
direction
I‘M:
8 = 34”:
10P4 to 10P5. By heating the sample up to 150°C the original state of the surface was recovered: The EDC showed the old features, a weaker d-emission and less background (top spectrum in fig. 1). Water desorption was monitored by an increase in pressure. The behaviour indicates that water is only physisorbed on the TiSe, surface. The EDCs of TiSe, nearly “saturated” with water have been measured for various polar angles (fig. 4). The spectra were normalized to the photon flux. As compared to the clean sample the intensity of the TiSe, valence band emission did not change but the emission from Ti-3d-derived states was now
G. Karschnick et al. / Adsorbate enhanced Ti-3dphotoemission
49
M CL)
JI-(A)
Fig. 2. Electronic band structure of TiSe, in the rALM reproduced from [14]; full lines: TM; dashed lines: AL.
plane in the vicinity
of the Fermi level,
clearly visible for all measured polar angles the spectra shifting by less than 0.1 eV to higher binding energies. The Se-4p bands deduced from these spectra remain the same whereas the d-band close to E, is now observed over the whole Brillouin zone. This allows an estimate of the total width of this measuresubband of - 0.2 eV also supported by recent inverse photoemission ments [16]. This value is smaller than the averaged calculated bandwidth. 0-2~ emission was observed at approximately 8.5 eV below E, out of the energy range in fig. 1. It causes the increase of intensity at binding energies above 7 eV. A similar effect was observed for adsorption of CO. We found again an increase in d-band emission. The adsorbate was removable; electron-bombard-
I
I
0
23( H20
exposure
Fig. 3. Ti-3d peak intensity
(kL)
(peak area) as a function
of the exposure
to H,O.
9
Fig, 4. Polar angle dependence of EDCs from TiSe, exposed to 160 kL Hz0 direction TM: h*r = 21.2 eV.
taken in the uimuthd
ment and laser heating recovered the original spectra. By exposing the samples to N, or Oz no change in d-emission was observed. The only effects were enhancement of inelasticaIly scattered electrons and disappearance of the TiSq structure at very high exposures above 200 kL. In a few cases we also observed a “super-d”-emission (fig. 5) appearing after an air leakage in the cooling system. The measured polar angle dependence reveaIed that this peak atso extends over the whole Brillouin zone similar to that of TiSe, exposed to Hz0 (see fig. 4). Systematic studies of the conditions
G. Karschnick et al. / Adsorbate enhanced Ti-3dphotoemission
51
5 Energy
below
Fig. 5. EDC from ‘Me, Ao = 21.2 eV.
EF
[evl
~super-~-omission)
taken in the azimuthal direction TM; 6 = 28O;
under which a “super-d” occurs are under way. As compared to N, and O,, II,0 and CO molecules carry an electric dipole moment which seems to be responsible for the enhanced d-emission. Its visibility through the entire Btillouin zone in the TM direction can be viewed in a simple rigid band model as being due to a shift of the Fermi energy with respect to the energy bands. A small shift is already expected to give a strong change in the occupation of the flat lowest d-band which explains the experimental observations (see fig. 2). Such a shift can be caused by direct electron transfer to empty bulk states, a phenomenon which has been observed in d-electron emissian from Ti, _XV,Se, mixed crystals where vanadium delivers an extra electron to the d-bands [17]. However, because of the weak interaction between the adsorbate and the surface a complete electron transfer seems unlikely and the very critical Fermi energy shift is more probable due to dipole induced band bending localized at the surface. The band bending reveals that the localization observed for clean samples is even stronger than predicted by theory [14]. A contribution of relaxation effects may be possible but cannot be understood by the simple model proposed in ref. [14] where the variation of the Ti-Se distance on the bands is discussed since no significant changes of the Se-p bands especially of those with Se-p, character were observed. If other relaxation effects exist in this class of materials they are probably small and beyond the now available experimental resolution. It is however possible that the adsorption process changes the matrix elements (wavefunctions) associated with the Ti-3d excitation. We believe that a similar mechanism is also responsible for the “super-d” emission possibly triggered by effective co-adsorption of different gas species.
52
G. Karschnick
et al. / Adwrbate
enhanced TI-3dphotoemission
4. Summary We have observed for the first time significant changes of the Ti-3d emission from TiSe, single crystal surfaces by adsorption of H,O and CO although no strong interactions were expected due to their van der Waals character. The effect can be explained by a shift of the Fermi energy caused by potential changes localized in the surface due to weakly bound dipoles in combination with additional changes in the Ti-3d matrix elements. Thereby the lowest d-band becomes visible over the whole Brillouin zone providing an estimate of its total width - 0.2 eV which is smaller than predicted by theory. Relaxation effects due to adsorbed molecules may be present but cannot be unambiguously identified within the framework of existing theoretical calculations. Further careful experiments on clean and adsorbate covered LTMD surfaces seem necessary for a complete understanding of the adsorption mechanism and even of the electronic structure of clean surfaces particularly at low temperatures in connection with phase transitions.
References [l] Physics and Chemistry of Materials with Layered Structures. Vols. 1-6 (Reidel. Dordrecht, 1976-1979). (21 K.C. Woo. F.C. Brown, W.L. McMillan, R.J. Miller. M.J. Schaffman and M.P. Sears. Phys. Rev. B14 (1976) 3242. [3] F.J. DiSalvo, D.E. Moncton and J.V. Waszczak, Phys. Rev. B14 (1976) 4321. [4] I. Taguchi, M. Asai, Y. Watanabe and M. Oka. Physica B+C 105 (1981) 146. [5] F. Levy, J. Phys. Cl3 (1980) 2901. [6] J.A. Wilson, Phys. Status Solidi (b) 86 (1978) 11. [7] G. Karschnick, Thesis, University of Kiel (1982). [X] W. Drube, G. Karschnick, M. Skibowski, R. Thies and K. Volkert, in: Proc. 15th Intern. Conf. on the Physics of Semiconductors, Kyoto, 1980 [J. Phys. Sot. Japan 49, Suppl. A (1980) 1371. [9] R.Z. Bachrach, M. Skibowski and F.C. Brown, Phys. Rev. Letters 37 (1976) 40. [lo] M.M. Traum, G. Margaritondo. N.V. Smith, J.E. Rowe and F.J. DiSalvo. Phys. Rev. B17 (1978) 1836. [ll] C.H. Chen, W. Fabian, F.C. Brown, K.C. Woo, B. Davies, B. DeLong and A.H. Thompson. Phys. Rev. B21 (1980) 615. [12] G. Karschnick, W. Drube and M. Skibowski, to be published. [13] D.L. Greenaway and R. Nitsche, J. Phys. Chem. Solids 26 (1965) 1445. [14] A. Zunger and A.J. Freeman, Phys. Rev. B17 (1978) 1839; Phys. Rev. Letters 40 (1978) 1155. [15] J.J. Barry, H.P. Hughes, P.C. Khpstein and R.H. Friend, J. Phys. Cl6 (1983) 393. [16] W. Drube, I. Schafer. G. Karschnick and M. Skibowski, Phys. Rev. B30 (1984) 6248. [17] 0. Anderson, W. Drube. G. Karschnick and M. Skibowski, in: Proc. 7th Intern. Conf. on Vacuum Ultraviolet Radiation Physics, VUV VII, Jerusalem 1983 [Ann. Israel Phys. Sot. 6 (1983) 3151.