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Deexcitation of rare-gas metastable atoms on semiconductor surfaces: MoS2 and LaCoO3
kurface science
Surface Science 283 (1993) 78-83 North-Holland
.. .,.,. :.::, :::;i:;, ‘:::.:i:::~::~..:.:.~ ....:.:..> .,,.,, ,,,,,,,,,,, ...,,.,,, ..,,:;, .~:.::::.~::i::,::i:i,::~~~~, “~“‘:::?.-:p ‘i.‘.. ,,,,.,,,:,: ,,./ ““‘i’.’ ‘i’ ‘.‘.>.A.. ..\ ...:,:.:_)...:.,., ..,..... ....j,.,.;,,,;, .,/ ,,.,,;: ““.‘.:.:::::::::‘:::~~.~:::::~~:~,.:::~,: :.:, 3:.,.,..,.
Deexcitation of rare-gas metastable atoms on semiconductor MoS, and LaCoO, Shigeru
Masuda
and Yoshiya
Harada
Department of Chemistry, College of Arts and Sciences, The University of Tokyo, Komaba, Meguro, Tobo Received
6 May 1992; accepted
surfaces:
for publication
153, Japan
7 May 1992
The metastable atom electron spectra (MAES) obtained by thermal collisions between He*(ls2s) atoms and semiconductor MoS, and LaCoO, surfaces were compared to the corresponding He I ultraviolet photoemission spectra. The metastable atoms are deexcited on the MoS, surface predominantly via resonance ionization followed by Auger neutralization and on the LaCoO, surface via Penning ionization. The deexcitation channel and the relative band intensity of the MAES are found to depend strongly on the spatial distributions of the occupied and unoccupied wavefunctions at the outermost surface layer.
1. Introduction
The valence electronic structure localized on a solid surface has attracted considerable attention both from scientific and technological points of view, since it plays an important role in the surface phase transition, adsorption, catalytic reaction, etc. So far the two-dimensional band dispersion (binding energy versus wave vector) and the symmetry of wavefunctions have been well studied by use of angle-resolved photoemission and inverse photoemission measurements. However, information on the spatial distribution of wavefunctions is rather limited, although it is a key factor governing the surface electronic properties and the reactivities with incoming atoms and molecules. Our recent studies reveal that such information can be obtained by metastable atom electron spectroscopy, in which the kinetic energy of electrons emitted by thermal collisions of rare-gas metastable atoms with a solid surface is analyzed. When a metastable atom such as He*(ls2s, 23S) collides with a solid surface, it is known to deexcite to the ground state via one of the follow0039-6028/93/$06.00
0 1993 - Elsevier
Science
Publishers
ing two mechanisms [l], schematically shown in fig. 1. On transition- and noble-metal surfaces, in which the level of an unoccupied state (4,) lies opposite to the excited 2s level of He *, the 2s electron tunnels resonantly into the solid (resonance ionization, RI) and then the remaining He+ ion is neutralized by an Auger transition (Auger neutralization, AN). The AN process produces two holes in the valence band and hence the spectrum exhibits a broad feature reflecting the self-convolution of the local density of states. The transition rate r for the RI process is governed by the wavefunction overlap:
r,,
a C I(xdr)A(r))
1’
unoec
where xZs is the 2s orbital of He*. On insulator and alkali-metal surfaces having no empty level opposite the He* 2s level, the RI process is suppressed and Penning ionization (PI) [or Auger deexcitation (AD)] takes place, where an electron in the valence state (4,) of the surface fills the He* 1s hole and its 2s electron is emitted to the continuum state (en) simultaneously. In this case a single hole is produced in the valence band, this
B.V. All rights reserved
S. Masuda, Y. Harada / Deexcitation of rare-gas metastable atoms on MoS, and LaCoO,
E vat
f
T
7%
;” +I! 2s
2s
$<
EFs, \ \
2s
\-
k
15
(a,) RI
IS
(a?) AN
f
1s
(b) AD (PI)
Fig. 1. Deexcitation mechanisms of a metastable atom. (a,), (a,) Resonance ionization (RI) followed by Auger neutralization (AN) on transition- and nobel-metal surfaces. (b) Penning ionization (PI) [or Auger deexcitation (AD)] on insulator and alkali-metal surfaces.
process being similar to the photoemission one. The transition probability of the PI process is given by r,r a c I%(5)Xz,(%) occ
XXls(Qw2))
WQ,) I **
1 (2)
This equation means that the electron transfer from the surface largely depends on the differential overlap between the 4, and xls. Since the metastable atom having a thermal translational energy (E,, < 0.1 eV> cannot penetrate into the bulk, an orbital extended outside the surface gives more effective overlap with xrs than an orbital localized on the surface, yielding a stronger band in the spectrum. The PI process occurs also on the transition-metal surface as a competing process of the RI, but its contribution is very low, i.e., r,, -ZKr,, [*I, beccuse the xzs orbital (effective radius reff = 2.5 A) 131 is much further extended outside tht atomic surface than the xls orbital (reff = 0.3 A) [4]. Furthermore, the local density of unoccupied states is rather high at the transition-metal surface, making the RI process more favorable. Recently we have measured metastable atom electron spectra (MAES) of some semiconductor and semi-metal surfaces, and found that the deexcitation path (RI + AN or PI) is governed not
79
only by the position of the 4, level relative to the excited level of the metastable atom, but also by the spatial distribution of 4,. Graphite is a typical example [5]. In the present study, we have taken up semiconducting transition-metal compounds, MoS, and LaCoO,, as samples. For MoS, the level ordering of the valence states and their characters are now understood fairly well by photoemission measurements [6,71 and theoretical band calculations [8,9]. For perovskite-type LaCoO, the photoemission spectra have been measured by two groups [lO,ll], but the band assignment (especially for the Co 3d-derived states) is controversial at present. The purpose of the present study is twofold; one is to clarify the deexcitation mechanism of the metastable atom at semiconductor surfaces, and the other is to probe the spatial distribution of wavefunctions at the outermost surface layer.
2. Experimental The details of the experimental apparatus have been described elsewhere [12], except for a newly constructed sample preparation chamber. It holds a four-grid optics for low energy electron diffraction (LEED) and Auger electron spectroscopy (AES), an Ar + ion sputtering gun, evaporation sources, etc. and is evacuated to ultrahigh vacuum (N 2 X lo- lo Torr) by a 6” diffusion pump (Edwards, Diffstak) and a Ti sublimation pump. The MAES and ultraviolet photoemission spectrum (UPS) were measured by a 180” hemispherical-type analyzer with an energy resolution of _ 0.2 eV. The He metastable atoms (2lS, 20.62 eV and 23S, 19.82 eV) were produced by impact of 70-120 eV electrons. The 2% atoms were quenched by a He discharge lamp (using radiation of A = 2.06 pm) and the pure 23S spectrum was obtained. The He* 2lS spectrum was derived by the difference between the two spectra obtained with the quench lamp on and off. The He I resonance line (21.2 eV) was used for measurement of the UPS. The MoS, sample is a natural 2H-MoS, crystal. It was cleaved in air just prior to its fixing in the sample chamber, and then heated at 400-
80
S. Masuda, Y. Harada
/ Deexcitation
of rare-gas metastable atoms on MoS, and LaCoO,
450°C under ultrahigh vacuum for 10 h. The sample thus obtained showed a sharp hexagonal LEED pattern and was free from impurities within the detection limit of AES (I 0.01 monolayer). The powdered LaCoO, sample was pressed into a disk and calcined in the air, and then cleaned by heating in the 0, atmosphere of N 5 x 10e5 Torr for 10 h. The surface contamination (mainly carbon) of the sample was not detected by AES.
3. Results and discussion 3.1. MoS, surface Fig. 2 shows the He*(2iS) and He*(23S) MAES of the MoS, surface with its geometry illustrated in the inset. For comparison the corresponding UPS using He1 radiation is also shown in the figure. The photoemission threshold indicated by a vertical arrow corresponds to the Fermi level, E,. Five sharp peaks A-E in the UPS have
MoSn
been attributed to direct emissions from valence bands: peak A is due to the atomic-like Mo4d,z state [7], peaks B-D are due to dispersive S3pderived bands, and peak E is due to S3s-derived bands 161.A very weak structure observed at the higher E, side is probably due to electrons localized in defect levels. The He* spectra, on the other hand, are characterized by a weak tail in the high E, region and two broad bands X and Y. The band positions and the threshold indicated by a vertical arrow in fig. 2 are considerably different from those in the UPS, even if we take the difference in the excitation energy between the MAES and UPS into account. This indicates that He* metastable atoms are deexcited on the MoS, surface predominantly via resonance ionization (RI) followed by Auger neutralization (AN). In fact the He*(2lS) and He*(23S) spectra are almost identical, reflecting the formation of the He+ ion at the surface. Now, we discuss the occurrence of the RI process. As mentioned in section 1, the RI process corresponds to the electron transfer from the He* 2s state to the unoccupied state of the surface. For the MoS, surface, the ionization energy of the 2s electron of He* near the surface is approximated by IP,, = E’(He+)
Kinetic Fig. 2. He*(2’S) and MoS, measured
energy
(eV)
He*(23S) MAES, and He1 UPS with the geometry in the inset.
of
- E’(He*),
(3)
where E’(He+) and E’(He*) are the effective ionization energy of the ground-state He atom and the effective excitation energy of the He* metastable atom near the surface, respectively. When the AN process involves two electrons having the lowest binding energy (which corresponds to the defect level for the present case), the emitted electron has its maximum kinetic energy, E,(max.) = E’(He+) - 24, where 4 is the work function. Using the observed values [+ = 5.0 eV determined from the UPS and E,(max.) = 13.0 eV from the MAES], the E’(He+) is estimated to be 23.0 eV. This value is 1.6 eV lower than the gas-phase value (24.6 eV), mainly owing to the image potential between the surface and the He+ ion [2,13]. Since the E’(He*) value does not change much from the gas-phase one [14], we obtain IP,,, = 23.0 - 19.8 = 3.2 eV for the He*(23S) atom and IP,, = 23.0 - 20.6 = 2.4 eV
S. Masuda, Y. Harada / Deexcitation of rare-gas metastable atoms on MoS, and LaCoO,
for the He*(2’S) atom. The energy diagram of MoS, and He* is summarized in fig. 4. In this figure the valence band width is derived from the experimental band structure [6] and the band gap is derived from the absorption edge of 1.35 eV measured by Huisman et al. [15]. As is seen from the figure, the 2s levels of the 2lS and 23S atoms are located opposite empty state composed of Mo4d and S3p orbitals [8,9]. Thus, the PI process is energetically possible. Further, the band calculations predict [8,93 that the density of the corresponding empty states is very high. Owing to these features, the RI process occurs effectively and the RI + AN mechanism becomes dominant on the MoS, surface. The AN process produces two holes in the valence states of the surface. If we neglect the interaction between two holes in the final state, the kinetic energy of emitted electrons in the AN process is given by E, = E’(He+)
- li - lj - 24
(4)
where li and lj are the binding energies (referred to EF) of the valence electrons involving the transition. Using the one-electron binding energy obtained by the UPS (fig. 2), a weak tail in the high E, region is related to two-holes in the defect levels and the Mo4d,z states, (def)-* and (MO 4d,z)-*, respectively, and broad bands X and Y are assigned to the two-hole states of the S3p-derived valence bands, (S3p)-*. The latter
LaCo03
I! I
*! I so0 e-
A*. hv C
&
UPS L
D
lk I
I
I
C
Binding
MAES
\ I
I
5
10
energy
I
II
0 (eV)
Fig. 4. He*(2%) MAESand He1 UPS of LaCoO, measured withthe geometryin the inset.
structure corresponds reasonably to a broad structure observed in the core-valence-valence (LMM) Auger spectrum [16]. It should be noted that the band intensity of the (Mo4d,z)-* states is extremely weak compared to that of the (S 3p)-* states. Since MO atoms in MoS, are sandwiched by trigonal-prismatic layers of S atoms with the interlayer distance dMo_S of 1.59 A [17], Mo4d,z electrons cannot interact with incident He+ ions effectively. Further, the band intensity of the (def)-* states is comparable to that of the (Mo4d,*)-* states, although the density of the defect states is very low, as observed in the UPS. This indicates that the electron distribution of the defect states is considerably exposed outside the surface to interact effectively with the He+ ion. 3.2. LaCoO,
10
81
surface
S3P
t---
He* MoS2 Fig.3. Energydiagramof MoS, and He* interactingnear the surface.
Fig. 4 shows the He*(23S) MAES and He1 UPS of the LaCoO, surface. To facilitate the comparison between MAES and UPS, the kinetic energy scale for the MAES is shifted relative to that of the UPS by difference in excitation en-
82
S. Masuda, Y. Harada / Deexcitation of rare-gas metastable atoms on MoS, and LaCoO,
ergy, 21.2 - 19.8 = 1.4 eV. The UPS has five prominent peaks labelled by A-E, whose corresponding peaks appear also in the MAES. This indicates that He* metastable atoms decay on the LaCoO, surface via the Penning ionization (PI) mechanism. The photoemission spectra of LaCoO, have been measured by some groups, but the assignment is controversial at present. Orchard and Thornton measured the temperature dependence of the spectra and assigned peaks in the high E, region (which correspond to peaks A and B in our spectrum) to the final state structures derived mainly from the low-spin Co(II1) (t2,J6 and highspin Co(IIIXt,,)4(e,)2 initial ions, and lower-lying peaks C-E to direct emissions from the 02p-derived bands [lo]. This ligand-field (localized model) approach for the transition-metal 3d states was also applied successfully to cases of simple transition-metal oxides such as Co0 [18]. In contrast, Kemp et al. [ll] compared the UPS and XPS of LaCoO,, and indicated that the Co3d states are strongly hybridized with the 0 2p states and delocalized over the valence band: namely peaks A-B, C, D-E in our spectrum relate possibly to direct emissions from antibondingly coupled Co3d and 02p orbitals, non-bonding 02p orbitals, and the bonding combination of Co 3d and 02p orbitals, respectively. In the present study we cannot conclude whether the localized or itinerant approach is appropriate. However, it should be noted that peaks A and B are strongly suppressed in intensity compared to peaks C-E in the MAES. Since the Co atom in LaCoO, is coordinated by six 0 atoms in the 0, symmetry, the incoming He* atom interacts preferentially with outer-distributed 02p orbitals, but does scarcely with inner-distributed Co 3d orbitals. Therefore, the strong suppression suggests that the initial-state wavefunctions relating to peaks A and B are composed predominantly of Co3d components and only slightly of 02p components. In this respect the localized model seems to be favorable. The details will be discussed elsewhere [191. Finally we discuss the suppression of the RI process for He*(23S) atoms on the LaCoO, surface , which is in contrast to the cases of usual
semiconductor surfaces such as Si(ll1) [201 and MoS, described in section 3.1. According to the band calculation [ll], the empty states are located from just above E, to N 5 eV and composed of Co 3d and 0 2p orbitals, indicating that the RI process is energetically possible. However, this is not the case. This finding indicates that the unoccupied wavefunctions having its energy corresponding to the 2s level of He* is scarcely exposed outside the surface, as in the case of occupied Co3d states mentioned above. Similar phenomena have been observed in collisions of He* atoms with La,_,Sr,CoO, [191 and polyacetylene [21] surfaces (which are metallic in character). In the latter case, the peaks due to the r valence bands extending below E, are almost missing in the MAES, which is in close resemblance with the present case of LaCoO,. References [l] H.D. Hagstrum, Electron and Ion Spectroscopy of Solids, Eds. L. Fiermans, J. Vennik and W. Dekeyser (Plenum, New York, 1978) p. 273. [2] W. Sesselmann, B. Woratschek, J. Kiippers, G. Ertl and H. Haberland, Phys. Rev. B 35 (19871 1547. [3] P.E. Siska, Chem. Phys. Lett. 63 (1979) 25. [4] S. Fraga, J. Karwowski and K.M.S. Saxena, Handbook of Atomic Data (Elsevier, Amsterdam, 19761. [5] S. Masuda, H. Hayashi and Y. Harada, Phys. Rev. B 42 (1990) 3582. i61 I.T. McGovern, R.H. Williams and A.W. Parke, J. Phys. C (Solid State Phys.) 12 (1979) 2689, and references therein. [71 I. Abbati, L. Braicovich, C. Carbone, J. Nogami, J.J. Yeh, I. Lindau and U. del Pennino, Phys. Rev. B 32 (19851 5459. Bl L.F. Mattheiss, Phys. Rev. B 8 (1973) 3719. [91 D.W. Bullett, J. Phys. C (Solid State Phys.) 11 (1978) 4501. and G. Thornton, J. Electron Spectrosc. DOI A.F. Orchard Relat. Phenom. 22 (1981) 271. [ill J.P. Kemp, D.L. Beal and P.A. Cox, J. Solid State Chem. 86 (1990) 50. 1121 Y. Harada and H. Ozaki, Jpn. J. Appl. Phys. 26 (1987) 1201. Phys. Rev. 122 (1961) 83. [131 H.D. Hagstrum, [141 Y. Harada, H. Ozaki and K. Ohno, Solid State Commun. 49 (1984) 71. R. de Jonge, C. Haas and F. Jellinek, J. [151 R. Huisman, Solid State Chem. 3 (1971) 56. Ml D. Lichtman, J.H. Craig, Jr., V. Sailer and M. Drinkwine, Appl. Surf. Sci. 7 (1981) 325.
S. Masuda, Y Harada
/ Deexcitation
of rare-gas metastable atoms on MoS, and LaCoO,
[17] J.C. Wildetwanck and F. Jellinek, Z. Anorg. AIIgem. Chem. 328 (1964) 309. [18] D.E. Eastman and J.L. Freeouf, Phys. Rev. Lett. 34 (1975) 395. [19] S. Masuda, M. Aoki, Y. Harada, M. Hirohashi and Y. Watanabe, to be published.
83
[20] H. Ishii, S. Masuda and Y. Harada, Surf. Sci. 239 (1990) 222; Solid State Commun. 82 (1992) 587. [21] J. Lee, C. Hanrahan, J. Arias, R.M. Martin and H. Metiu, Phys. Rev. B 32 (1985) 8216.
Surface Science 283 (1993) 78-83 North-Holland
.. .,.,. :.::, :::;i:;, ‘:::.:i:::~::~..:.:.~ ....:.:..> .,,.,, ,,,,,,,,,,, ...,,.,,, ..,,:;, .~:.::::.~::i::,::i:i,::~~~~, “~“‘:::?.-:p ‘i.‘.. ,,,,.,,,:,: ,,./ ““‘i’.’ ‘i’ ‘.‘.>.A.. ..\ ...:,:.:_)...:.,., ..,..... ....j,.,.;,,,;, .,/ ,,.,,;: ““.‘.:.:::::::::‘:::~~.~:::::~~:~,.:::~,: :.:, 3:.,.,..,.
Deexcitation of rare-gas metastable atoms on semiconductor MoS, and LaCoO, Shigeru
Masuda
and Yoshiya
Harada
Department of Chemistry, College of Arts and Sciences, The University of Tokyo, Komaba, Meguro, Tobo Received
6 May 1992; accepted
surfaces:
for publication
153, Japan
7 May 1992
The metastable atom electron spectra (MAES) obtained by thermal collisions between He*(ls2s) atoms and semiconductor MoS, and LaCoO, surfaces were compared to the corresponding He I ultraviolet photoemission spectra. The metastable atoms are deexcited on the MoS, surface predominantly via resonance ionization followed by Auger neutralization and on the LaCoO, surface via Penning ionization. The deexcitation channel and the relative band intensity of the MAES are found to depend strongly on the spatial distributions of the occupied and unoccupied wavefunctions at the outermost surface layer.
1. Introduction
The valence electronic structure localized on a solid surface has attracted considerable attention both from scientific and technological points of view, since it plays an important role in the surface phase transition, adsorption, catalytic reaction, etc. So far the two-dimensional band dispersion (binding energy versus wave vector) and the symmetry of wavefunctions have been well studied by use of angle-resolved photoemission and inverse photoemission measurements. However, information on the spatial distribution of wavefunctions is rather limited, although it is a key factor governing the surface electronic properties and the reactivities with incoming atoms and molecules. Our recent studies reveal that such information can be obtained by metastable atom electron spectroscopy, in which the kinetic energy of electrons emitted by thermal collisions of rare-gas metastable atoms with a solid surface is analyzed. When a metastable atom such as He*(ls2s, 23S) collides with a solid surface, it is known to deexcite to the ground state via one of the follow0039-6028/93/$06.00
0 1993 - Elsevier
Science
Publishers
ing two mechanisms [l], schematically shown in fig. 1. On transition- and noble-metal surfaces, in which the level of an unoccupied state (4,) lies opposite to the excited 2s level of He *, the 2s electron tunnels resonantly into the solid (resonance ionization, RI) and then the remaining He+ ion is neutralized by an Auger transition (Auger neutralization, AN). The AN process produces two holes in the valence band and hence the spectrum exhibits a broad feature reflecting the self-convolution of the local density of states. The transition rate r for the RI process is governed by the wavefunction overlap:
r,,
a C I(xdr)A(r))
1’
unoec
where xZs is the 2s orbital of He*. On insulator and alkali-metal surfaces having no empty level opposite the He* 2s level, the RI process is suppressed and Penning ionization (PI) [or Auger deexcitation (AD)] takes place, where an electron in the valence state (4,) of the surface fills the He* 1s hole and its 2s electron is emitted to the continuum state (en) simultaneously. In this case a single hole is produced in the valence band, this
B.V. All rights reserved
S. Masuda, Y. Harada / Deexcitation of rare-gas metastable atoms on MoS, and LaCoO,
E vat
f
T
7%
;” +I! 2s
2s
$<
EFs, \ \
2s
\-
k
15
(a,) RI
IS
(a?) AN
f
1s
(b) AD (PI)
Fig. 1. Deexcitation mechanisms of a metastable atom. (a,), (a,) Resonance ionization (RI) followed by Auger neutralization (AN) on transition- and nobel-metal surfaces. (b) Penning ionization (PI) [or Auger deexcitation (AD)] on insulator and alkali-metal surfaces.
process being similar to the photoemission one. The transition probability of the PI process is given by r,r a c I%(5)Xz,(%) occ
XXls(Qw2))
WQ,) I **
1 (2)
This equation means that the electron transfer from the surface largely depends on the differential overlap between the 4, and xls. Since the metastable atom having a thermal translational energy (E,, < 0.1 eV> cannot penetrate into the bulk, an orbital extended outside the surface gives more effective overlap with xrs than an orbital localized on the surface, yielding a stronger band in the spectrum. The PI process occurs also on the transition-metal surface as a competing process of the RI, but its contribution is very low, i.e., r,, -ZKr,, [*I, beccuse the xzs orbital (effective radius reff = 2.5 A) 131 is much further extended outside tht atomic surface than the xls orbital (reff = 0.3 A) [4]. Furthermore, the local density of unoccupied states is rather high at the transition-metal surface, making the RI process more favorable. Recently we have measured metastable atom electron spectra (MAES) of some semiconductor and semi-metal surfaces, and found that the deexcitation path (RI + AN or PI) is governed not
79
only by the position of the 4, level relative to the excited level of the metastable atom, but also by the spatial distribution of 4,. Graphite is a typical example [5]. In the present study, we have taken up semiconducting transition-metal compounds, MoS, and LaCoO,, as samples. For MoS, the level ordering of the valence states and their characters are now understood fairly well by photoemission measurements [6,71 and theoretical band calculations [8,9]. For perovskite-type LaCoO, the photoemission spectra have been measured by two groups [lO,ll], but the band assignment (especially for the Co 3d-derived states) is controversial at present. The purpose of the present study is twofold; one is to clarify the deexcitation mechanism of the metastable atom at semiconductor surfaces, and the other is to probe the spatial distribution of wavefunctions at the outermost surface layer.
2. Experimental The details of the experimental apparatus have been described elsewhere [12], except for a newly constructed sample preparation chamber. It holds a four-grid optics for low energy electron diffraction (LEED) and Auger electron spectroscopy (AES), an Ar + ion sputtering gun, evaporation sources, etc. and is evacuated to ultrahigh vacuum (N 2 X lo- lo Torr) by a 6” diffusion pump (Edwards, Diffstak) and a Ti sublimation pump. The MAES and ultraviolet photoemission spectrum (UPS) were measured by a 180” hemispherical-type analyzer with an energy resolution of _ 0.2 eV. The He metastable atoms (2lS, 20.62 eV and 23S, 19.82 eV) were produced by impact of 70-120 eV electrons. The 2% atoms were quenched by a He discharge lamp (using radiation of A = 2.06 pm) and the pure 23S spectrum was obtained. The He* 2lS spectrum was derived by the difference between the two spectra obtained with the quench lamp on and off. The He I resonance line (21.2 eV) was used for measurement of the UPS. The MoS, sample is a natural 2H-MoS, crystal. It was cleaved in air just prior to its fixing in the sample chamber, and then heated at 400-
80
S. Masuda, Y. Harada
/ Deexcitation
of rare-gas metastable atoms on MoS, and LaCoO,
450°C under ultrahigh vacuum for 10 h. The sample thus obtained showed a sharp hexagonal LEED pattern and was free from impurities within the detection limit of AES (I 0.01 monolayer). The powdered LaCoO, sample was pressed into a disk and calcined in the air, and then cleaned by heating in the 0, atmosphere of N 5 x 10e5 Torr for 10 h. The surface contamination (mainly carbon) of the sample was not detected by AES.
3. Results and discussion 3.1. MoS, surface Fig. 2 shows the He*(2iS) and He*(23S) MAES of the MoS, surface with its geometry illustrated in the inset. For comparison the corresponding UPS using He1 radiation is also shown in the figure. The photoemission threshold indicated by a vertical arrow corresponds to the Fermi level, E,. Five sharp peaks A-E in the UPS have
MoSn
been attributed to direct emissions from valence bands: peak A is due to the atomic-like Mo4d,z state [7], peaks B-D are due to dispersive S3pderived bands, and peak E is due to S3s-derived bands 161.A very weak structure observed at the higher E, side is probably due to electrons localized in defect levels. The He* spectra, on the other hand, are characterized by a weak tail in the high E, region and two broad bands X and Y. The band positions and the threshold indicated by a vertical arrow in fig. 2 are considerably different from those in the UPS, even if we take the difference in the excitation energy between the MAES and UPS into account. This indicates that He* metastable atoms are deexcited on the MoS, surface predominantly via resonance ionization (RI) followed by Auger neutralization (AN). In fact the He*(2lS) and He*(23S) spectra are almost identical, reflecting the formation of the He+ ion at the surface. Now, we discuss the occurrence of the RI process. As mentioned in section 1, the RI process corresponds to the electron transfer from the He* 2s state to the unoccupied state of the surface. For the MoS, surface, the ionization energy of the 2s electron of He* near the surface is approximated by IP,, = E’(He+)
Kinetic Fig. 2. He*(2’S) and MoS, measured
energy
(eV)
He*(23S) MAES, and He1 UPS with the geometry in the inset.
of
- E’(He*),
(3)
where E’(He+) and E’(He*) are the effective ionization energy of the ground-state He atom and the effective excitation energy of the He* metastable atom near the surface, respectively. When the AN process involves two electrons having the lowest binding energy (which corresponds to the defect level for the present case), the emitted electron has its maximum kinetic energy, E,(max.) = E’(He+) - 24, where 4 is the work function. Using the observed values [+ = 5.0 eV determined from the UPS and E,(max.) = 13.0 eV from the MAES], the E’(He+) is estimated to be 23.0 eV. This value is 1.6 eV lower than the gas-phase value (24.6 eV), mainly owing to the image potential between the surface and the He+ ion [2,13]. Since the E’(He*) value does not change much from the gas-phase one [14], we obtain IP,,, = 23.0 - 19.8 = 3.2 eV for the He*(23S) atom and IP,, = 23.0 - 20.6 = 2.4 eV
S. Masuda, Y. Harada / Deexcitation of rare-gas metastable atoms on MoS, and LaCoO,
for the He*(2’S) atom. The energy diagram of MoS, and He* is summarized in fig. 4. In this figure the valence band width is derived from the experimental band structure [6] and the band gap is derived from the absorption edge of 1.35 eV measured by Huisman et al. [15]. As is seen from the figure, the 2s levels of the 2lS and 23S atoms are located opposite empty state composed of Mo4d and S3p orbitals [8,9]. Thus, the PI process is energetically possible. Further, the band calculations predict [8,93 that the density of the corresponding empty states is very high. Owing to these features, the RI process occurs effectively and the RI + AN mechanism becomes dominant on the MoS, surface. The AN process produces two holes in the valence states of the surface. If we neglect the interaction between two holes in the final state, the kinetic energy of emitted electrons in the AN process is given by E, = E’(He+)
- li - lj - 24
(4)
where li and lj are the binding energies (referred to EF) of the valence electrons involving the transition. Using the one-electron binding energy obtained by the UPS (fig. 2), a weak tail in the high E, region is related to two-holes in the defect levels and the Mo4d,z states, (def)-* and (MO 4d,z)-*, respectively, and broad bands X and Y are assigned to the two-hole states of the S3p-derived valence bands, (S3p)-*. The latter
LaCo03
I! I
*! I so0 e-
A*. hv C
&
UPS L
D
lk I
I
I
C
Binding
MAES
\ I
I
5
10
energy
I
II
0 (eV)
Fig. 4. He*(2%) MAESand He1 UPS of LaCoO, measured withthe geometryin the inset.
structure corresponds reasonably to a broad structure observed in the core-valence-valence (LMM) Auger spectrum [16]. It should be noted that the band intensity of the (Mo4d,z)-* states is extremely weak compared to that of the (S 3p)-* states. Since MO atoms in MoS, are sandwiched by trigonal-prismatic layers of S atoms with the interlayer distance dMo_S of 1.59 A [17], Mo4d,z electrons cannot interact with incident He+ ions effectively. Further, the band intensity of the (def)-* states is comparable to that of the (Mo4d,*)-* states, although the density of the defect states is very low, as observed in the UPS. This indicates that the electron distribution of the defect states is considerably exposed outside the surface to interact effectively with the He+ ion. 3.2. LaCoO,
10
81
surface
S3P
t---
He* MoS2 Fig.3. Energydiagramof MoS, and He* interactingnear the surface.
Fig. 4 shows the He*(23S) MAES and He1 UPS of the LaCoO, surface. To facilitate the comparison between MAES and UPS, the kinetic energy scale for the MAES is shifted relative to that of the UPS by difference in excitation en-
82
S. Masuda, Y. Harada / Deexcitation of rare-gas metastable atoms on MoS, and LaCoO,
ergy, 21.2 - 19.8 = 1.4 eV. The UPS has five prominent peaks labelled by A-E, whose corresponding peaks appear also in the MAES. This indicates that He* metastable atoms decay on the LaCoO, surface via the Penning ionization (PI) mechanism. The photoemission spectra of LaCoO, have been measured by some groups, but the assignment is controversial at present. Orchard and Thornton measured the temperature dependence of the spectra and assigned peaks in the high E, region (which correspond to peaks A and B in our spectrum) to the final state structures derived mainly from the low-spin Co(II1) (t2,J6 and highspin Co(IIIXt,,)4(e,)2 initial ions, and lower-lying peaks C-E to direct emissions from the 02p-derived bands [lo]. This ligand-field (localized model) approach for the transition-metal 3d states was also applied successfully to cases of simple transition-metal oxides such as Co0 [18]. In contrast, Kemp et al. [ll] compared the UPS and XPS of LaCoO,, and indicated that the Co3d states are strongly hybridized with the 0 2p states and delocalized over the valence band: namely peaks A-B, C, D-E in our spectrum relate possibly to direct emissions from antibondingly coupled Co3d and 02p orbitals, non-bonding 02p orbitals, and the bonding combination of Co 3d and 02p orbitals, respectively. In the present study we cannot conclude whether the localized or itinerant approach is appropriate. However, it should be noted that peaks A and B are strongly suppressed in intensity compared to peaks C-E in the MAES. Since the Co atom in LaCoO, is coordinated by six 0 atoms in the 0, symmetry, the incoming He* atom interacts preferentially with outer-distributed 02p orbitals, but does scarcely with inner-distributed Co 3d orbitals. Therefore, the strong suppression suggests that the initial-state wavefunctions relating to peaks A and B are composed predominantly of Co3d components and only slightly of 02p components. In this respect the localized model seems to be favorable. The details will be discussed elsewhere [191. Finally we discuss the suppression of the RI process for He*(23S) atoms on the LaCoO, surface , which is in contrast to the cases of usual
semiconductor surfaces such as Si(ll1) [201 and MoS, described in section 3.1. According to the band calculation [ll], the empty states are located from just above E, to N 5 eV and composed of Co 3d and 0 2p orbitals, indicating that the RI process is energetically possible. However, this is not the case. This finding indicates that the unoccupied wavefunctions having its energy corresponding to the 2s level of He* is scarcely exposed outside the surface, as in the case of occupied Co3d states mentioned above. Similar phenomena have been observed in collisions of He* atoms with La,_,Sr,CoO, [191 and polyacetylene [21] surfaces (which are metallic in character). In the latter case, the peaks due to the r valence bands extending below E, are almost missing in the MAES, which is in close resemblance with the present case of LaCoO,. References [l] H.D. Hagstrum, Electron and Ion Spectroscopy of Solids, Eds. L. Fiermans, J. Vennik and W. Dekeyser (Plenum, New York, 1978) p. 273. [2] W. Sesselmann, B. Woratschek, J. Kiippers, G. Ertl and H. Haberland, Phys. Rev. B 35 (19871 1547. [3] P.E. Siska, Chem. Phys. Lett. 63 (1979) 25. [4] S. Fraga, J. Karwowski and K.M.S. Saxena, Handbook of Atomic Data (Elsevier, Amsterdam, 19761. [5] S. Masuda, H. Hayashi and Y. Harada, Phys. Rev. B 42 (1990) 3582. i61 I.T. McGovern, R.H. Williams and A.W. Parke, J. Phys. C (Solid State Phys.) 12 (1979) 2689, and references therein. [71 I. Abbati, L. Braicovich, C. Carbone, J. Nogami, J.J. Yeh, I. Lindau and U. del Pennino, Phys. Rev. B 32 (19851 5459. Bl L.F. Mattheiss, Phys. Rev. B 8 (1973) 3719. [91 D.W. Bullett, J. Phys. C (Solid State Phys.) 11 (1978) 4501. and G. Thornton, J. Electron Spectrosc. DOI A.F. Orchard Relat. Phenom. 22 (1981) 271. [ill J.P. Kemp, D.L. Beal and P.A. Cox, J. Solid State Chem. 86 (1990) 50. 1121 Y. Harada and H. Ozaki, Jpn. J. Appl. Phys. 26 (1987) 1201. Phys. Rev. 122 (1961) 83. [131 H.D. Hagstrum, [141 Y. Harada, H. Ozaki and K. Ohno, Solid State Commun. 49 (1984) 71. R. de Jonge, C. Haas and F. Jellinek, J. [151 R. Huisman, Solid State Chem. 3 (1971) 56. Ml D. Lichtman, J.H. Craig, Jr., V. Sailer and M. Drinkwine, Appl. Surf. Sci. 7 (1981) 325.
S. Masuda, Y Harada
/ Deexcitation
of rare-gas metastable atoms on MoS, and LaCoO,
[17] J.C. Wildetwanck and F. Jellinek, Z. Anorg. AIIgem. Chem. 328 (1964) 309. [18] D.E. Eastman and J.L. Freeouf, Phys. Rev. Lett. 34 (1975) 395. [19] S. Masuda, M. Aoki, Y. Harada, M. Hirohashi and Y. Watanabe, to be published.
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[20] H. Ishii, S. Masuda and Y. Harada, Surf. Sci. 239 (1990) 222; Solid State Commun. 82 (1992) 587. [21] J. Lee, C. Hanrahan, J. Arias, R.M. Martin and H. Metiu, Phys. Rev. B 32 (1985) 8216.