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A surface state on VC(100) identified by photoelectron spectroscopy and gas adsorption studies
366
Surface Science 192 (1987) 366-372 North-Holland, Amsterdam
A SURFACE STATE ON VC(IOO) IDENTIFIED BY PHOTOELECTRON SPECTROSCOPY AND GAS ADSORPTION STUDIES P.A.P. LINDBERG and
r.r. JOHANSSON
Department of Physics and Measurement Technology, Linkbping University, 8-581 83 Linkiiping, Sweden
Received 2 March 1987; accepted for publication 6 August 1987
A surface state has been identified at about 2.3 eV binding energy in the normal emission spectra from veo.so(lDD) using the angle-resolved photoemission technique. The surface induced structure observed in the spectra is found to be strongly attenuated by exposing the clean surface to carbon monoxide. For oxygen exposures, the surface induced structure is found to shift towards higher initial energies. Work function changes upon contaminating the surface to oxygen and carbon monoxide are also reported and used in the discussion of the surface induced state.
I. Introduction Previous angle-resolved photoemission studies [1-4] of the electronic structure of the transition metal nitrides and carbides have revealed a surface induced state on the (100) surface of TiN and ZrN. These states were attributed to Tamrn states, which were pulled off the nitrogen-localized ~5 bulk energy bands of TiN(100) and ZrN(100) by the change in the electrostatic potential at the surface. On the transition metal carbides a surface induced state has been identified on the homopolar TiC(l11) surface [5], but on the (100) surface of the refractory carbides there is, so far, no experimental evidence of surface related states. Clearly, the surface electronic structure of the refractory carbides is of great scientific interest because of their unusual combination of properties and binding mechanisms [6]. In an earlier angle-resolved photoemission study [7] of the VC o.8o(100) crystal the experimental energy distribution curves were compared with theoretical photoemission spectra calculated for the stoichiometric composition, VCl.O' The most significant difference between the experimental and theoretical spectra was the appearance of two structures exhibiting ~ 5 symmetry in the experimental normal emission spectra that both could not be accounted for by the bulk energy band structure of VCl.o(lOO). 0039-6028/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
P.A.P. Lindberg, L.I. Johansson / A surface stale on
vcr100)
367
In the present paper we show that the uppermost of these peaks can be associated with a surface induced state. Angle-resolved photoemission spectra recorded at normal emission from VC o.8o(100), clean and exposed to various amounts of CO and O2 are presented. The emission intensity from the surface induced state is found to be rapidly attenuated by contaminating the clean surface, and for the oxygen adsorbate clear energy shifts towards higher initial energies are observed. Changes in the electronic work function upon gas adsorption are also observed and used in the discussion of the surface induced state. 2. Experimental details Angle-resolved photoemission measurements were carried out in a VO ADES 400 photoemission system equipped with a hemispherical electrostatic analyser. The analyser had an angular resolution of ± 2 0 and was operating at an energy resolution of 0.2 eV. A resonance lamp producing unpolarized radiation was used for excitation. The experiments were performed on the (100) face of a single crystal with composition VCO. BO' The preparation of the crystal is given elsewhere [7]. The crystal was heated in situ by high-temperature flashings to about 1100 0 C. The order and cleanliness of the surface were examined using low energy electron diffraction (LEED) and UPS. A clear, 1 X 1 ordered, LEED pattern was observed and no sign of oxygen or other contaminants could be detected in the UPS spectra. During the gas adsorption studies the surface was exposed to different amounts of CO and O2 using pressures in the 10- 9_10- B Torr range. The total pressure read off the ion gauge was used for determining the exposure. The base pressure of the system was less than 1 X 10 -10 Torr. In the spectra shown below, the energies are given with respect to the midpoint of the Fermi energy, and the incidence angle of the radiation, ()j, is given relative to the surface normal. 3. Results and discussion Angle-resolved photoemission spectra measured in the normal errussion direction from the VC o.Bo(100) surface, before and after exposing the clean surface to various amounts of CO, are shown in fig. 1. Unpolarized Ne I radiation was incident along the (011 > azimuth at an angle of ()j = 17 o. Three emission peaks are seen in the clean spectrum. The lowest lying peak was previously shown [7] to be due to direct transitions from initial states of Ll 1 symmetry. The other two peaks are more intense at smaller incidence angles and are therefore associated with initial states of Ll s symmetry. Upon exposing the clean surface to CO the peak located at about -2.3 eV is dramatically
368
P.A.P. Lindberg, LI. Johansson / A surface stal e on VC(lOOj
eV
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81= 17
~:
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-6 -4 -2 Q=E F INITIAL ENERGY (eVl Fig. 1. Photoelectron energy spectra recorded at normal emission from the VC(lOO) surface, clean and exposed to various amounts of CO. Unpolarized Ne I rad iation incident at 8; = 17 0 was used for excitation.
attenuated, while the other peaks are much less affected. Furthermore, the uppermost peak is seen to shift towards slightly higher initial energies even at an exposure of only 0.1 L CO. This sensitivity to surface contamination is even mor e exaggerated when exposing the surface to oxygen, as can be seen in fig. 2, where normal emission spectra recorded at hv = 16.8 eV are shown for various O 2 exposures. The spectra in fig. 2 demonstrate clearly that upon contaminating the surface with 2 , the peak at - 2.3 eV in the dean spectrum shift s towards higher initial energies, and more over, it is also attenuated. In the right-hand panel of fig.2, the energy shifts are illustrated in more detail. These spectra show that, upon exposing the surface to 1.2 L of 02' the uppermost peak shifts upwards by almost 0.5 eV. Since surface contamination is expected to affect the electronic structure of the surface more severely than that of the bulk , the peak observed at about 2.3 eV binding energy in the clean spectrum may very well be localized to the sur face region and can therefore be associated with a surface induced state. Surface states [8] are often shifted in energy with respect to their parent bulk energy states, due to the different electrostatic potential experienced by the electrons at the surface. In the present case , the surface state is loc ated about 1 eV closer to the Fermi energy than the ~ 5 bulk energy band. This shift is comparable to those obtained for the previously in vestigated surface states on TiN (100) [1,3] and ZrN(lOG) [2,4], for which the surface states were shifted
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P,A.P. Lindberg, L.I. Johansson / A surface state on VC(IOO)
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Fig. 2. Photoelectron energy spectra measured in the normal emission direction from the VC(lOO) surface before and after 02 exposures. The right-hand panel shows the surface state energy shift in more detaiL The spectra were induced by un polarized Ne I radiation incident at OJ = 17 o.
upwards by respectively, about 1 and 0.5 eV with respect to the As bulk energy band. When exposing a clean surface to adsorbates, a charge transfer between the substrate and the absorbate will in general occur. A change in the surface potential will naturally also affect the electronic work function. Hence, it is interesting to compare the observed energy shifts of the surface induced state with the changes in the work function upon contaminating the surface to Oz. In fig. 3, the energy position of the surface state is shown as a function of the 02 exposure, together with the change in the work function. The vertical bars shown in fig. 3 indicate the estimated uncertainties in determining the energy values. The work function was determined by the energy difference between the Fermi energy and the low energy cut-off in the photoemission spectra. For the clean surface a work function of about 4.3 eV was obtained. Both the work function and the position of the surface state are found to increase systematically with increasing Oz exposure. Furthermore, the trends in the curves are fairly similar, indicating a close relationship between the energy shift of the surface state and the charge transfer between substrate and absorbate. This observation is consistent with a theoretical model [9] for atomic adsorbates which shows that a charge transfer from substrate to absorbate, causing a work function increase, is expected to shift a surface state towards higher energies. Oxygen induced energy shifts have been observed earlier on Cs covered eu(111) [10]. The upward energy shift was attributed to donation of
370
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electrons from the surface state to the absorb ate. In the present case, a charge transfer from the substrate to the adsorbate is expected to occur for the 0 2 adsorbate system, due to the difference in electronegativity [11]. A similar investigation was also performed for CO, but only small changes in the work function, less than 0.15 eV, could be observed even at an exposure of 100 L of CO. So far, the origin of the surface state has not been discuss ed. Surface states [12-14] can either be of the Tamm or the Shockley type. The former arises from a shift in the potential at the surface, while the latter requires a hybridization bulk band gap, caused by overlapping bands. For the previously [1-4] observed surface states on TiN(100) and ZrN(lOO), the possibility of Shockley states could be excluded, because of the absence of the necessary band crossing. These states were instead attributed to Tamm states pulled off the tight-bound ~5 bulk bands, which are predominantly localized on the nitrogen atoms. On the transition metal carbides, Shockley surface states may, however, arise due to the existence of hybridization band gaps. On NbC [15], with a similar band structure to VC, a band gap occurs close to the r point of the projected energy band structure of the surface Brillouin zone. Hence, from the present results, we canno t determine the nature of the surface state; it can either be of the Tamm or the Shockley type. We would finally like to compare the surface state on VC(lOO) with those previously observed on TiN(100) and ZrN(lOO). For the latter two surfaces, the main effect upon exposing the surfaces to CO and 2 , was clear shifts of the surface states towards higher energies. Ho wever, no rapid attenuation of the surface induced peak s could be observed. For VC(100) , the surface stat e
°
P.A.P. Lindberg, L.I. Johansson / A surface state
0/1
VC(JOO)
371
behaves quite differently. While clear energy shifts upwards of the surface state was observed for the O2 adsorbate, the energy position of the surface state was only slightly increased by exposing the surface to CO. Furthermore, in contrast to the surface states observed on TiN(100) and ZrN(lOO), the surface induced peak on VC O,80 (100) is rapidly attenuated upon contaminating the clean surface with CO. 4. Comments I t has recently been discussed [16,17] that surface states may occur on sub stoichiometric compounds of the transition metal nitrides and' carbides due to a gradient of the vacancy concentration in the surface region. It was argued that a stoichiometric top layer might be created by annealing the samples at temperatures around 1600 0 C, and thus, a concentration gradient of the vacancies would be obtained. Since UPS is a very surface sensitive probe, such an effect may totally quench the possibility of observing vacancy induced states, which theoretical results for TiN(lOO) indicate [16]. We have, however, no experience of such effects, and recently, it has been shown [18] that a vacancy induced structure and a surface state can be present simultaneously on the (100) face of a ZrNO.93 crystal, which was cleaned by annealing it to about 1300 0 C. The crystal used in the present work, VCo.8o (l OO), was cleaned by repeated high temperature annealings to about 1100 0 C, i.e. a low temperature in this context, and furthermore, a non-dispersive structure that is associated with a vacancy induced state appears at about 1.8 eV binding energy in the ArI induced spectra [19]. Thus, our results indicate that no significant vacancy concentration gradient occurs at the surface. 5. Summary
A surface induced state has been identified on VC o.8o(100) using the angle-resolved photoemission technique and gas adsorption. An upward shift of the surface state was observed when exposing the surface to oxygen. In contrast to the earlier reported surfaced states on TiN(lOO) and ZrN(100), a rapid attenuation of the surface induced structure was observed upon contaminating the clean VC o.8o(10D) surface to carbon monoxid. The present results do not allow an identification of the origin of the surface state, since Shockley surface states may exist on the transition metal carbides. Large changes in the electronic work function were observed, when exposing the surface to oxygen, while only small changes occurred for the carbon monoxide adsorbate. These results, which are in accordance with the energy shifts of the surface induced state, indicate that the surface state is strongly affected by changes in the charge distribution at the surface.
372
P.A.P. Lindberg, L.I. Johansson / A surface state on VC(IOO)
Acknowledgement
This work has got financial support from the Swedish Natural Science Research Council.
References [1] LJ. Johansson, A. Callenas, P.M. Stefan, A.N. Christensen and K. Schwarz, Phys. Rev. B24 (1981) 1883. [2J A. CaIIenas, 1.1. Johansson, A.N. Christensen, K. Schwarz, P. Blaha and 1. Redinger, Phys. Rev. B30 (1984) 635. [3] 1.1. Johansson and A. Callen~s, Solid Slate Cornmun. 42 (1982) 299. [4] A. Callenas and L.I. Johansson, Solid State Commun. 52 (1984) 143. [5] A.M. Bradshaw, J.F. van der Veen, F.J. Himpsel and D.E. Eastman, Solid State Commun. 37 (1980) 37. [6] L.E. Toth, Transition Metal Carbides and Nitrides (Academic Press, New York, 1971). [7] PAP. Lindberg, L.I. Johansson and A.N. Christensen, Surface Sci. 192 (1987) 353. [8] B. Feuerbacher, B. Fitton and R.F. Willis, Eds., Photoemission and the Electronic Structure of Surfaces (Wiley, New York, 1978). [9] P. Ahlqvist, Solid State Commun, 31 (1979) 1029. [10] S.A. Lindgren and 1. Wallden, Chern. Phys. Letters 64 (1979) 239. [11] P.W. Atkins, Physical Chemistry, 2nd ed, (Oxford University Press, Oxford, 1982). [12] I. Tamm, Phys. Z. 1 (1932) 733. [13] W. SchockJey, Phys. Rev. 56 (1939) 317. [14] E.T. Goodwin, Proc, Cambridge Phil. Soc. 35 (1939) 221. [15] M. Tomasek and S. Pick, Intern. J. Quantum Chem. 17 (1980) 1167. [16] J. Redinger and P. Weinberger, Phys. Rev. B35 (1987) 5652. [17] J. Redinger, P. Weinberger and A. Neckel, Phys. Rev. B35 (1987) 5647. [18] J.B. Lindstrom, L.I. Johansson, A. Callenas, D.S.L. Law and A.N. Christensen, Phys. Rev. B35 (1987) 7891. [19] PAP. Lindberg and L.I. Johansson, Z. Phys. B68 (1987) 83.
Surface Science 192 (1987) 366-372 North-Holland, Amsterdam
A SURFACE STATE ON VC(IOO) IDENTIFIED BY PHOTOELECTRON SPECTROSCOPY AND GAS ADSORPTION STUDIES P.A.P. LINDBERG and
r.r. JOHANSSON
Department of Physics and Measurement Technology, Linkbping University, 8-581 83 Linkiiping, Sweden
Received 2 March 1987; accepted for publication 6 August 1987
A surface state has been identified at about 2.3 eV binding energy in the normal emission spectra from veo.so(lDD) using the angle-resolved photoemission technique. The surface induced structure observed in the spectra is found to be strongly attenuated by exposing the clean surface to carbon monoxide. For oxygen exposures, the surface induced structure is found to shift towards higher initial energies. Work function changes upon contaminating the surface to oxygen and carbon monoxide are also reported and used in the discussion of the surface induced state.
I. Introduction Previous angle-resolved photoemission studies [1-4] of the electronic structure of the transition metal nitrides and carbides have revealed a surface induced state on the (100) surface of TiN and ZrN. These states were attributed to Tamrn states, which were pulled off the nitrogen-localized ~5 bulk energy bands of TiN(100) and ZrN(100) by the change in the electrostatic potential at the surface. On the transition metal carbides a surface induced state has been identified on the homopolar TiC(l11) surface [5], but on the (100) surface of the refractory carbides there is, so far, no experimental evidence of surface related states. Clearly, the surface electronic structure of the refractory carbides is of great scientific interest because of their unusual combination of properties and binding mechanisms [6]. In an earlier angle-resolved photoemission study [7] of the VC o.8o(100) crystal the experimental energy distribution curves were compared with theoretical photoemission spectra calculated for the stoichiometric composition, VCl.O' The most significant difference between the experimental and theoretical spectra was the appearance of two structures exhibiting ~ 5 symmetry in the experimental normal emission spectra that both could not be accounted for by the bulk energy band structure of VCl.o(lOO). 0039-6028/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
P.A.P. Lindberg, L.I. Johansson / A surface stale on
vcr100)
367
In the present paper we show that the uppermost of these peaks can be associated with a surface induced state. Angle-resolved photoemission spectra recorded at normal emission from VC o.8o(100), clean and exposed to various amounts of CO and O2 are presented. The emission intensity from the surface induced state is found to be rapidly attenuated by contaminating the clean surface, and for the oxygen adsorbate clear energy shifts towards higher initial energies are observed. Changes in the electronic work function upon gas adsorption are also observed and used in the discussion of the surface induced state. 2. Experimental details Angle-resolved photoemission measurements were carried out in a VO ADES 400 photoemission system equipped with a hemispherical electrostatic analyser. The analyser had an angular resolution of ± 2 0 and was operating at an energy resolution of 0.2 eV. A resonance lamp producing unpolarized radiation was used for excitation. The experiments were performed on the (100) face of a single crystal with composition VCO. BO' The preparation of the crystal is given elsewhere [7]. The crystal was heated in situ by high-temperature flashings to about 1100 0 C. The order and cleanliness of the surface were examined using low energy electron diffraction (LEED) and UPS. A clear, 1 X 1 ordered, LEED pattern was observed and no sign of oxygen or other contaminants could be detected in the UPS spectra. During the gas adsorption studies the surface was exposed to different amounts of CO and O2 using pressures in the 10- 9_10- B Torr range. The total pressure read off the ion gauge was used for determining the exposure. The base pressure of the system was less than 1 X 10 -10 Torr. In the spectra shown below, the energies are given with respect to the midpoint of the Fermi energy, and the incidence angle of the radiation, ()j, is given relative to the surface normal. 3. Results and discussion Angle-resolved photoemission spectra measured in the normal errussion direction from the VC o.Bo(100) surface, before and after exposing the clean surface to various amounts of CO, are shown in fig. 1. Unpolarized Ne I radiation was incident along the (011 > azimuth at an angle of ()j = 17 o. Three emission peaks are seen in the clean spectrum. The lowest lying peak was previously shown [7] to be due to direct transitions from initial states of Ll 1 symmetry. The other two peaks are more intense at smaller incidence angles and are therefore associated with initial states of Ll s symmetry. Upon exposing the clean surface to CO the peak located at about -2.3 eV is dramatically
368
P.A.P. Lindberg, LI. Johansson / A surface stal e on VC(lOOj
eV
hV='16;8
81= 17
~:
H
• . .· : ·. .
_
· ·..··
>~
z
:=! ::z: H
" .
CO
...t
0.5 L
::~Q;:~
·..1·· .'.
.
.
+CO
!.. .. exposure:
..
-
ft;
(100)
ee =0·
I
........
~ :
..H....
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It)
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L
.
......"
..
...•........
~:.~ L
oL
-6 -4 -2 Q=E F INITIAL ENERGY (eVl Fig. 1. Photoelectron energy spectra recorded at normal emission from the VC(lOO) surface, clean and exposed to various amounts of CO. Unpolarized Ne I rad iation incident at 8; = 17 0 was used for excitation.
attenuated, while the other peaks are much less affected. Furthermore, the uppermost peak is seen to shift towards slightly higher initial energies even at an exposure of only 0.1 L CO. This sensitivity to surface contamination is even mor e exaggerated when exposing the surface to oxygen, as can be seen in fig. 2, where normal emission spectra recorded at hv = 16.8 eV are shown for various O 2 exposures. The spectra in fig. 2 demonstrate clearly that upon contaminating the surface with 2 , the peak at - 2.3 eV in the dean spectrum shift s towards higher initial energies, and more over, it is also attenuated. In the right-hand panel of fig.2, the energy shifts are illustrated in more detail. These spectra show that, upon exposing the surface to 1.2 L of 02' the uppermost peak shifts upwards by almost 0.5 eV. Since surface contamination is expected to affect the electronic structure of the surface more severely than that of the bulk , the peak observed at about 2.3 eV binding energy in the clean spectrum may very well be localized to the sur face region and can therefore be associated with a surface induced state. Surface states [8] are often shifted in energy with respect to their parent bulk energy states, due to the different electrostatic potential experienced by the electrons at the surface. In the present case , the surface state is loc ated about 1 eV closer to the Fermi energy than the ~ 5 bulk energy band. This shift is comparable to those obtained for the previously in vestigated surface states on TiN (100) [1,3] and ZrN(lOG) [2,4], for which the surface states were shifted
°
P,A.P. Lindberg, L.I. Johansson / A surface state on VC(IOO)
hV= 16,8 eV............ ..... 8 i= 17'
......,." ...... ..
..
..
'
,.
........',
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..
I 8e=O'
.
...............,...I
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Fig. 2. Photoelectron energy spectra measured in the normal emission direction from the VC(lOO) surface before and after 02 exposures. The right-hand panel shows the surface state energy shift in more detaiL The spectra were induced by un polarized Ne I radiation incident at OJ = 17 o.
upwards by respectively, about 1 and 0.5 eV with respect to the As bulk energy band. When exposing a clean surface to adsorbates, a charge transfer between the substrate and the absorbate will in general occur. A change in the surface potential will naturally also affect the electronic work function. Hence, it is interesting to compare the observed energy shifts of the surface induced state with the changes in the work function upon contaminating the surface to Oz. In fig. 3, the energy position of the surface state is shown as a function of the 02 exposure, together with the change in the work function. The vertical bars shown in fig. 3 indicate the estimated uncertainties in determining the energy values. The work function was determined by the energy difference between the Fermi energy and the low energy cut-off in the photoemission spectra. For the clean surface a work function of about 4.3 eV was obtained. Both the work function and the position of the surface state are found to increase systematically with increasing Oz exposure. Furthermore, the trends in the curves are fairly similar, indicating a close relationship between the energy shift of the surface state and the charge transfer between substrate and absorbate. This observation is consistent with a theoretical model [9] for atomic adsorbates which shows that a charge transfer from substrate to absorbate, causing a work function increase, is expected to shift a surface state towards higher energies. Oxygen induced energy shifts have been observed earlier on Cs covered eu(111) [10]. The upward energy shift was attributed to donation of
370
P.A.P. Lindberg, L.I. Johansso n / A surface slat e on VCr100)
I
:; 1.0~ lJ.l t!l
-
• WORK FUNCHON CHANGE (eV) x SURFACE STATE SHIFT lev)
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electrons from the surface state to the absorb ate. In the present case, a charge transfer from the substrate to the adsorbate is expected to occur for the 0 2 adsorbate system, due to the difference in electronegativity [11]. A similar investigation was also performed for CO, but only small changes in the work function, less than 0.15 eV, could be observed even at an exposure of 100 L of CO. So far, the origin of the surface state has not been discuss ed. Surface states [12-14] can either be of the Tamm or the Shockley type. The former arises from a shift in the potential at the surface, while the latter requires a hybridization bulk band gap, caused by overlapping bands. For the previously [1-4] observed surface states on TiN(100) and ZrN(lOO), the possibility of Shockley states could be excluded, because of the absence of the necessary band crossing. These states were instead attributed to Tamm states pulled off the tight-bound ~5 bulk bands, which are predominantly localized on the nitrogen atoms. On the transition metal carbides, Shockley surface states may, however, arise due to the existence of hybridization band gaps. On NbC [15], with a similar band structure to VC, a band gap occurs close to the r point of the projected energy band structure of the surface Brillouin zone. Hence, from the present results, we canno t determine the nature of the surface state; it can either be of the Tamm or the Shockley type. We would finally like to compare the surface state on VC(lOO) with those previously observed on TiN(100) and ZrN(lOO). For the latter two surfaces, the main effect upon exposing the surfaces to CO and 2 , was clear shifts of the surface states towards higher energies. Ho wever, no rapid attenuation of the surface induced peak s could be observed. For VC(100) , the surface stat e
°
P.A.P. Lindberg, L.I. Johansson / A surface state
0/1
VC(JOO)
371
behaves quite differently. While clear energy shifts upwards of the surface state was observed for the O2 adsorbate, the energy position of the surface state was only slightly increased by exposing the surface to CO. Furthermore, in contrast to the surface states observed on TiN(100) and ZrN(lOO), the surface induced peak on VC O,80 (100) is rapidly attenuated upon contaminating the clean surface with CO. 4. Comments I t has recently been discussed [16,17] that surface states may occur on sub stoichiometric compounds of the transition metal nitrides and' carbides due to a gradient of the vacancy concentration in the surface region. It was argued that a stoichiometric top layer might be created by annealing the samples at temperatures around 1600 0 C, and thus, a concentration gradient of the vacancies would be obtained. Since UPS is a very surface sensitive probe, such an effect may totally quench the possibility of observing vacancy induced states, which theoretical results for TiN(lOO) indicate [16]. We have, however, no experience of such effects, and recently, it has been shown [18] that a vacancy induced structure and a surface state can be present simultaneously on the (100) face of a ZrNO.93 crystal, which was cleaned by annealing it to about 1300 0 C. The crystal used in the present work, VCo.8o (l OO), was cleaned by repeated high temperature annealings to about 1100 0 C, i.e. a low temperature in this context, and furthermore, a non-dispersive structure that is associated with a vacancy induced state appears at about 1.8 eV binding energy in the ArI induced spectra [19]. Thus, our results indicate that no significant vacancy concentration gradient occurs at the surface. 5. Summary
A surface induced state has been identified on VC o.8o(100) using the angle-resolved photoemission technique and gas adsorption. An upward shift of the surface state was observed when exposing the surface to oxygen. In contrast to the earlier reported surfaced states on TiN(lOO) and ZrN(100), a rapid attenuation of the surface induced structure was observed upon contaminating the clean VC o.8o(10D) surface to carbon monoxid. The present results do not allow an identification of the origin of the surface state, since Shockley surface states may exist on the transition metal carbides. Large changes in the electronic work function were observed, when exposing the surface to oxygen, while only small changes occurred for the carbon monoxide adsorbate. These results, which are in accordance with the energy shifts of the surface induced state, indicate that the surface state is strongly affected by changes in the charge distribution at the surface.
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Acknowledgement
This work has got financial support from the Swedish Natural Science Research Council.
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