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Surface metallic nature caused by an in-gap state of reduced NiO: a photoemission study
Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 873–875
Surface metallic nature caused by an in-gap state of reduced NiO: a photoemission study N. Nakajimaa,∗ , H. Katob , Y. Sakisakab a
Department of Physical Science, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan b Faculty of Science and Technology, Hirosaki University, Hirosaki 036-8561, Japan Available online 26 February 2005
Abstract The electronic state of oxygen-reduced NiO(1 0 0) surface has been investigated using angle-resolved and resonant photoemission spectroscopy with synchrotron radiation. We focus on the behavior of an in-gap state induced by oxygen-defects, which is observed at ∼0.5 eV below the Fermi level (EF ). The in-gap state shows slight energy dispersion; however, with a closer look around EF , it seems to create metallic Fermi cut-off both in hν-dependent normal-emission spectra and in off-normal emission ones. The rest of the peaks due to bulk bands are identical to those of stoichiometric bulk NiO; therefore, the observed feature of the in-gap state reveals the fact that the surface of reduced NiO is a metal while the vast underlying bulk is still an insulator. © 2005 Elsevier B.V. All rights reserved. Keywords: Reduced NiO; In-gap state; Surface metallic
1. Introduction
2. Experiment
Wide-gap transition-metal oxides have reawakened their interest, because of their potential applicability for electronic devices and photocatalyst. NiO is one of the most famous typical Mott insulators with an energy gap of 4.3 eV and intensively studied so far. Recently, as for a candidate of a stable substrate for metallic nanofilms, its stable insulating property is in spotlight. By metal adsorption, a spectral feature known as an in-gap state is induced within the band-gap of substrate NiO and, at the initial stage of adsorption, it is of Ni 3d character reflecting the nature of highly correlated carriers. The in-gap state is also created by oxygenreduction. In this case, the in-gap state is not affected by absorbates so that a pure NiO-derived state can be observed. In this paper, we present the results of angle-resolved and resonant photoemission of reduced NiO(1 0 0) using synchrotron radiation, focusing on the behavior of an in-gap state.
The experiments using synchrotron radiation were performed at the beamline 3B of the Photon Factory (KEK-PF). Angle-resolved photoemission spectra were measured using a Vacuum Science Workshop (VSW) analyzer with an acceptance of ±1◦ . The photon incident angle was fixed to 45◦ . Total energy resolution was 0.1 eV or better as determined by a Pt Fermi edge, depending on the photon energy (hν) in the range of 40–80 eV. A Pt foil and a Ta-plate sample holder in an electrical contact with the sample provided the Fermi level (EF ) reference. The base pressure of the chamber was ∼1×10−10 Torr. A single-crystal rocksalt NiO(1 0 0) of 10 mm × 5 mm × 1 mm was prepared with single-side mirror-finished. Ionic bonding between Ni2+ and O2− ions is strong and only heating in ultra-high vacuum has little effect on oxygen reduction. Therefore, a reduced NiO(1 0 0) surface was prepared by repeated cycles of the following procedure: (1) remove surface contamination and oxygen by Ar+ -ion bombardment (0.6–1.2 kV, 3 × 10−7 Torr), (2) anneal at 700 ◦ C for 30 min, (3) flash at 900 ◦ C for 1 min, and (4) examine the surface
∗
Corresponding author. Tel.: +81 82 424 7361; fax: +81 82 424 0717. E-mail address: [email protected] (N. Nakajima).
0368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2005.01.238
874
N. Nakajima et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 873–875
cleanliness and saturation of oxygen-defect by Auger electron spectroscopy. The (1 × 1) low-energy electron diffraction (LEED) pattern was observed with somewhat diffused spots owing to oxygen vacancies. Detailed examination on the defects created by Ar+ -ion bombardment is discussed in Ref. [1]. A stoichiometric NiO(1 0 0) surface was restored by 20 min annealing (∼500 ◦ C) in oxygen at a partial pressure of ∼2×10−7 Torr, and showed good (1 × 1) LEED pattern. Contrary to our apprehension, no charge-up effect was observed. This could be direct evidence for surface metallic nature, we would discuss in the following section. 3. Results and discussion Fig. 1(a) shows the valence-band normal-emission spectra of a clean reduced NiO(1 0 0) surface for binding energies
Fig. 1. (a) The valence-band normal-emission spectra for EB = 0 (=EF ) to 18 eV with hν ranging from 40 to 80 eV. Symmetry points of the nonmagnetic-phase BZ to EB = 0.5 eV are indicated on the right. (b) The hν-dependence of the intensity of peak A. (c) Details of the spectra around the in-gap region for hν = 59–63 eV.
(EB ) from EF = 0 to 18 eV with hν ranging from 40 to 80 eV. We view the excitation process in the direct transition model and assume a parabolic free-electron like final band with a constant inner potential V0 and a constant effective mass m∗ . Therefore, as hν is swept, in the normal-emission geometry, we can measure the electronic states along the [1 0 0] direction (Γ –X line) in the bulk Brillouin zone (BZ). Symmetry points in the nonmagnetic bulk BZ determined for a state of EB = 0.5 eV using V0 = 0.8 eV and m∗ = 0.95 [2] are given on the right side of the figure. Several features labeled A–F are observed. The overall features are identical to those of stoichiometric NiO [2] except peak A, which is an in-gap state we concern. Firstly, peaks B–F are discussed. Peak B is due to emission from the Ni 3d-derived bands. This is confirmed by the Ni 3p → 3d resonance behavior which is characterized by the resonance minimum at hν = 65–68 eV. The features C and D show dispersion below 60 and 65 eV, respectively, as previously observed by Shen et al., and are assigned to oxygen p-derived 5 and 1 bands, respectively. Despite the oxygen-reduction, no significant change was observed in both band dispersion and hν dependence of the intensity. The broad feature E is also previously observed in stoichiometric NiO by Thuler et al. [3],
Fig. 2. (a) The off-normal emission spectra of reduced NiO(1 0 0) at hν = 40 ¯ direction. Symmetry points of the nonmagnetic-phase eV along the Γ¯ –M surface BZ to EB = 0.5 eV are indicated on the right. (b) Details of the spectra around the in-gap region.
N. Nakajima et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 873–875
which is interpreted as a valence-band satellite of unscreened d7 final-state configuration pushed up to higher binding energy due to the strong on-site Coulomb interaction [4]. Its intensity increases at hν ∼ 68 eV, which is consistent with resonant behavior of Ni 3d. The existence of such a satellite indicates that the d-electrons in the oxide are highly correlated. The feature F shows a strong dispersion and rapidly fades away at hν = 60 eV. It is ascribed to Ni 3p state excited by the second-order light, since its kinetic energy is given by 2hν − EB (Ni 3p ∼ 68 eV). Now we look back to the in-gap state, peak A. The hνdependence of this peak is plotted in Fig. 1(b). Just like the case of peaks B and E, peak A also shows the Ni 3p–3d resonance indicating the highly correlated electronic state at EF . With a closer look at EF , it seems to create metallic Fermi cut-off around hν = 61 eV (Fig. 1(c)). It can be concluded that due to the oxygen-reduction the metallic nature is brought to NiO via an in-gap state. Similar behavior was observed for off-normal emission spectra. Fig. 2(a) shows angle-resolved photoemission spectra of reduced NiO(1 0 0) taken at hν = 40 eV and at different ¯ of the emission angle (θe ) along the symmetry direction Γ¯ –M NiO (1 0 0) surface BZ. Corresponding symmetry points of the nonmagnetic-phase surface BZ to the initial energy of EB = 0.5 eV are indicated on the right side of the figure. As was already pointed out by Shen et al. in the case of stoichiometric NiO, the spectra seen in Figs. 1(a) and 2(a) bear striking similarities to each other, because the Γ –X symmetry ¯ symmetry line of the surface line of the bulk BZ and Γ¯ –M BZ have a close relationship. The peaks A–D in Fig. 2(a) correspond to the counterparts in Fig. 1(a) and the behav-
875
ior of them as a whole is comprehended with no difficulty. This may reflect that the reduced surface was rather undisturbed than expected, which was also suggested by McKay et al. [1]. Fig. 2(b) gives the details of peak A. A similar metallic Fermi cut-off is observed for emission angles around 40–50◦ , again confirming that the metallic nature of reduced NiO. Both in hν-dependent normal-emission spectra and in offnormal emission ones, the bulk peaks are identical to those of stoichiometric NiO; therefore, the observed feature of the in-gap state reveals the fact that the surface of reduced NiO(1 0 0) is a metal while the vast underlying bulk is still an insulator. 4. Conclusions An in-gap state created by oxygen vacancies of reduced NiO(1 0 0) surface was investigated by photoemission spectroscopy. This state presents Ni 3d character and has metallic Fermi cut-off both in normal and off-normal spectra. Because the bulk peaks do not differ from those of stoichiometric ones, rather undisturbed surface together with the underlying insulator bulk was expected. References [1] [2] [3] [4]
J.M. McKay, et al., Phys. Rev. B32 (1985) 6764. Z.-X. Shen, et al., Phys. Rev. B44 (1991) 3604. M.R. Thuler, et al., Phys. Rev. B27 (1983) 2082. A. Fujimori, et al., Phys. Rev. B30 (1984) 957.
Surface metallic nature caused by an in-gap state of reduced NiO: a photoemission study N. Nakajimaa,∗ , H. Katob , Y. Sakisakab a
Department of Physical Science, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan b Faculty of Science and Technology, Hirosaki University, Hirosaki 036-8561, Japan Available online 26 February 2005
Abstract The electronic state of oxygen-reduced NiO(1 0 0) surface has been investigated using angle-resolved and resonant photoemission spectroscopy with synchrotron radiation. We focus on the behavior of an in-gap state induced by oxygen-defects, which is observed at ∼0.5 eV below the Fermi level (EF ). The in-gap state shows slight energy dispersion; however, with a closer look around EF , it seems to create metallic Fermi cut-off both in hν-dependent normal-emission spectra and in off-normal emission ones. The rest of the peaks due to bulk bands are identical to those of stoichiometric bulk NiO; therefore, the observed feature of the in-gap state reveals the fact that the surface of reduced NiO is a metal while the vast underlying bulk is still an insulator. © 2005 Elsevier B.V. All rights reserved. Keywords: Reduced NiO; In-gap state; Surface metallic
1. Introduction
2. Experiment
Wide-gap transition-metal oxides have reawakened their interest, because of their potential applicability for electronic devices and photocatalyst. NiO is one of the most famous typical Mott insulators with an energy gap of 4.3 eV and intensively studied so far. Recently, as for a candidate of a stable substrate for metallic nanofilms, its stable insulating property is in spotlight. By metal adsorption, a spectral feature known as an in-gap state is induced within the band-gap of substrate NiO and, at the initial stage of adsorption, it is of Ni 3d character reflecting the nature of highly correlated carriers. The in-gap state is also created by oxygenreduction. In this case, the in-gap state is not affected by absorbates so that a pure NiO-derived state can be observed. In this paper, we present the results of angle-resolved and resonant photoemission of reduced NiO(1 0 0) using synchrotron radiation, focusing on the behavior of an in-gap state.
The experiments using synchrotron radiation were performed at the beamline 3B of the Photon Factory (KEK-PF). Angle-resolved photoemission spectra were measured using a Vacuum Science Workshop (VSW) analyzer with an acceptance of ±1◦ . The photon incident angle was fixed to 45◦ . Total energy resolution was 0.1 eV or better as determined by a Pt Fermi edge, depending on the photon energy (hν) in the range of 40–80 eV. A Pt foil and a Ta-plate sample holder in an electrical contact with the sample provided the Fermi level (EF ) reference. The base pressure of the chamber was ∼1×10−10 Torr. A single-crystal rocksalt NiO(1 0 0) of 10 mm × 5 mm × 1 mm was prepared with single-side mirror-finished. Ionic bonding between Ni2+ and O2− ions is strong and only heating in ultra-high vacuum has little effect on oxygen reduction. Therefore, a reduced NiO(1 0 0) surface was prepared by repeated cycles of the following procedure: (1) remove surface contamination and oxygen by Ar+ -ion bombardment (0.6–1.2 kV, 3 × 10−7 Torr), (2) anneal at 700 ◦ C for 30 min, (3) flash at 900 ◦ C for 1 min, and (4) examine the surface
∗
Corresponding author. Tel.: +81 82 424 7361; fax: +81 82 424 0717. E-mail address: [email protected] (N. Nakajima).
0368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2005.01.238
874
N. Nakajima et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 873–875
cleanliness and saturation of oxygen-defect by Auger electron spectroscopy. The (1 × 1) low-energy electron diffraction (LEED) pattern was observed with somewhat diffused spots owing to oxygen vacancies. Detailed examination on the defects created by Ar+ -ion bombardment is discussed in Ref. [1]. A stoichiometric NiO(1 0 0) surface was restored by 20 min annealing (∼500 ◦ C) in oxygen at a partial pressure of ∼2×10−7 Torr, and showed good (1 × 1) LEED pattern. Contrary to our apprehension, no charge-up effect was observed. This could be direct evidence for surface metallic nature, we would discuss in the following section. 3. Results and discussion Fig. 1(a) shows the valence-band normal-emission spectra of a clean reduced NiO(1 0 0) surface for binding energies
Fig. 1. (a) The valence-band normal-emission spectra for EB = 0 (=EF ) to 18 eV with hν ranging from 40 to 80 eV. Symmetry points of the nonmagnetic-phase BZ to EB = 0.5 eV are indicated on the right. (b) The hν-dependence of the intensity of peak A. (c) Details of the spectra around the in-gap region for hν = 59–63 eV.
(EB ) from EF = 0 to 18 eV with hν ranging from 40 to 80 eV. We view the excitation process in the direct transition model and assume a parabolic free-electron like final band with a constant inner potential V0 and a constant effective mass m∗ . Therefore, as hν is swept, in the normal-emission geometry, we can measure the electronic states along the [1 0 0] direction (Γ –X line) in the bulk Brillouin zone (BZ). Symmetry points in the nonmagnetic bulk BZ determined for a state of EB = 0.5 eV using V0 = 0.8 eV and m∗ = 0.95 [2] are given on the right side of the figure. Several features labeled A–F are observed. The overall features are identical to those of stoichiometric NiO [2] except peak A, which is an in-gap state we concern. Firstly, peaks B–F are discussed. Peak B is due to emission from the Ni 3d-derived bands. This is confirmed by the Ni 3p → 3d resonance behavior which is characterized by the resonance minimum at hν = 65–68 eV. The features C and D show dispersion below 60 and 65 eV, respectively, as previously observed by Shen et al., and are assigned to oxygen p-derived 5 and 1 bands, respectively. Despite the oxygen-reduction, no significant change was observed in both band dispersion and hν dependence of the intensity. The broad feature E is also previously observed in stoichiometric NiO by Thuler et al. [3],
Fig. 2. (a) The off-normal emission spectra of reduced NiO(1 0 0) at hν = 40 ¯ direction. Symmetry points of the nonmagnetic-phase eV along the Γ¯ –M surface BZ to EB = 0.5 eV are indicated on the right. (b) Details of the spectra around the in-gap region.
N. Nakajima et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 873–875
which is interpreted as a valence-band satellite of unscreened d7 final-state configuration pushed up to higher binding energy due to the strong on-site Coulomb interaction [4]. Its intensity increases at hν ∼ 68 eV, which is consistent with resonant behavior of Ni 3d. The existence of such a satellite indicates that the d-electrons in the oxide are highly correlated. The feature F shows a strong dispersion and rapidly fades away at hν = 60 eV. It is ascribed to Ni 3p state excited by the second-order light, since its kinetic energy is given by 2hν − EB (Ni 3p ∼ 68 eV). Now we look back to the in-gap state, peak A. The hνdependence of this peak is plotted in Fig. 1(b). Just like the case of peaks B and E, peak A also shows the Ni 3p–3d resonance indicating the highly correlated electronic state at EF . With a closer look at EF , it seems to create metallic Fermi cut-off around hν = 61 eV (Fig. 1(c)). It can be concluded that due to the oxygen-reduction the metallic nature is brought to NiO via an in-gap state. Similar behavior was observed for off-normal emission spectra. Fig. 2(a) shows angle-resolved photoemission spectra of reduced NiO(1 0 0) taken at hν = 40 eV and at different ¯ of the emission angle (θe ) along the symmetry direction Γ¯ –M NiO (1 0 0) surface BZ. Corresponding symmetry points of the nonmagnetic-phase surface BZ to the initial energy of EB = 0.5 eV are indicated on the right side of the figure. As was already pointed out by Shen et al. in the case of stoichiometric NiO, the spectra seen in Figs. 1(a) and 2(a) bear striking similarities to each other, because the Γ –X symmetry ¯ symmetry line of the surface line of the bulk BZ and Γ¯ –M BZ have a close relationship. The peaks A–D in Fig. 2(a) correspond to the counterparts in Fig. 1(a) and the behav-
875
ior of them as a whole is comprehended with no difficulty. This may reflect that the reduced surface was rather undisturbed than expected, which was also suggested by McKay et al. [1]. Fig. 2(b) gives the details of peak A. A similar metallic Fermi cut-off is observed for emission angles around 40–50◦ , again confirming that the metallic nature of reduced NiO. Both in hν-dependent normal-emission spectra and in offnormal emission ones, the bulk peaks are identical to those of stoichiometric NiO; therefore, the observed feature of the in-gap state reveals the fact that the surface of reduced NiO(1 0 0) is a metal while the vast underlying bulk is still an insulator. 4. Conclusions An in-gap state created by oxygen vacancies of reduced NiO(1 0 0) surface was investigated by photoemission spectroscopy. This state presents Ni 3d character and has metallic Fermi cut-off both in normal and off-normal spectra. Because the bulk peaks do not differ from those of stoichiometric ones, rather undisturbed surface together with the underlying insulator bulk was expected. References [1] [2] [3] [4]
J.M. McKay, et al., Phys. Rev. B32 (1985) 6764. Z.-X. Shen, et al., Phys. Rev. B44 (1991) 3604. M.R. Thuler, et al., Phys. Rev. B27 (1983) 2082. A. Fujimori, et al., Phys. Rev. B30 (1984) 957.