Angle-Resolved Photoelectron Spectroscopy Studies of Two … · 2019. 11. 5. · 24 HIGHLIGHTS...
1
HIGHLIGHTS 25 HIGHLIGHTS 24 1 - Materials Science 12 In situ Angle-Resolved Photoelectron Spectroscopy Studies of Two-Dimensional Electronic States at Anatase TiO 2 (001) Surface Controlled by K Adsorption The nature of the intriguing metallic electronic structures appearing at the surface of anatase titanium dioxide (a-TiO 2 ) remains to be elucidated mainly owing to the difficulty of controlling the depth distribution of the oxygen vacancies. In this study, K atoms were adsorbed onto the (001) surface of a-TiO 2 to dope electrons into the a-TiO 2 and to confine the electrons in the surface region. The success of the electron doping and its controllability were confirmed by performing in situ angle-resolved photoelectron spectroscopy as well as core-level measurements. Clear subband structures were observed in the surface metallic states, indicating the creation of quasi-two-dimensional electron liquid states in a con- trollable fashion. Techniques to control the low-dimensional electronic states appearing at the surface and interface of oxide semiconductors have attracted much attention for de- signing future quantum electronic devices. Recently, the formation of quasi-two-dimensional electron liquid (q2DEL) states induced by photoirradiation has been re- ported at the surface of anatase titanium dioxide (a-TiO 2 ) [1]. However, owing to the difficulty of controlling the distribution of oxygen vacancies in the depth direction, the dimensionality of the electronic states generated by photoirradiation remains a controversial issue [1, 2]. Therefore, in this study, we proposed an alternative route to address the q2DEL states: we adsorbed K atoms on the (001) surface of a-TiO 2 for doping elec- trons in the surface region and investigated the formed q2DEL states by in situ angle-resolved photoemission spectroscopy (ARPES) [3]. Figure 1(a) shows a series of X-ray photoemission spectroscopy (XPS) spectra taken as a function of K adsorption. The K deposition onto the (001) surface of a-TiO 2 causes a weak but distinct structure corre- The possible structure model explaining charge transfer from adsorbed K atoms to an a-TiO 2 surface and the resulting accumulation-layer formation is drawn in Fig. 2(c). Some of the transferred electrons may be preferentially accumulated in the surface regions ini- tially. Analysis of the observed quantized states with a triangular wedge potential suggests that the activated carriers contributing to the formation of q2DEL states are confined in the surface (interface) region extending to a depth of a few nanometers, although the excess electrons could be trapped around K ions and/or oxy- gen vacancies at the interface between the K and a- TiO 2 . Consequently, these results indicate that doping of electrons onto the a-TiO 2 is achieved by chemical adsorption of K atoms onto the a-TiO 2 . Clear subband structures are observed in the ARPES images for the surface metallic states, indicating the creation of q2DEL states in a controllable fashion. Figure 1: (a) Ti-2p core-level spectra of a-TiO 2 obtained by varying the coverage of adsorbed K atoms. The curve-fitting results for the Ti-2p 3/2 states are presented, where the components of the Ti 4+ and Ti 3+ states are indicated by different hatching. The dashed line indicates the peak position of the Ti-2p 3/2 states for a bare a-TiO 2 (001) surface. (b) The plot of relative intensities of the Ti 3+ states as a function of adsorbed K. sponding to Ti 3+ states to appear at a binding energy of 457.5 eV, indicating that electrons are transferred from the K atoms to the a-TiO 2 film and that the doped elec- trons accumulate near the surface region accessible by the present XPS measurements [~1 nm away from the a-TiO 2 surface (the interface between K and a-TiO 2 )]. As the amount of K deposition increases further, the Ti 3+ states exhibit systematic evolution. These results imply that a certain amount of electron doping is induced by K deposition. The evolution of the Ti 3+ states as a function of K deposition is summarized in Fig. 1(b). The occur- rence of electron doping is also confirmed by the shift of the Ti 4+ peak by ~190 meV due to band bending. The increment of the sheet carrier density (n 2D ) as- sociated with K deposition has been clearly observed in in situ ARPES measurements. Figure 2(a) shows a series of ARPES images taken along the [100] direc- tion as a function of K adsorption. Clear subband struc- tures are observed in all of the ARPES images for the K-adsorbed surfaces, indicating the 2D nature of the observed conduction bands. By deposition of K atoms onto the a-TiO 2 surface, the Fermi momentum (k F ) de- termined from momentum distribution curves (MDCs) increases from 0.11 Å −1 for bare a-TiO 2 film to 0.19 Å −1 for a-TiO 2 film with 0.5 Å K adsorption. Because a- TiO 2 forms a Ti-3d-t 2g d xy -derived circular Fermi surface centered at the Γ point, the increase in k F may reflect the increase in n 2D by K deposition [4]. The n 2D values obtained from the Luttinger volumes based on the as- sumption of spin degeneracy of the subbands are plot- ted in Fig. 2(b) as a function of K deposition. The n 2D value steeply increases with increasing K deposition and seems to saturate at 0.3 Å. In contrast to the satu- ration of n 2D at 0.3 Å K deposition, in the Ti 2p core level shown in Fig. 1 the intensity of Ti 3+ states continues to increase after 0.3 Å K deposition and seems to saturate at about 0.8 Å. Assuming that the Ti 3+ states observed in the Ti-2p core levels reflect the contributions of both localized and mobile electrons (namely, activated carri- ers whose density corresponds to n 2D ) [5], these results suggest the charges transferred from K atoms to a-TiO 2 are partially trapped in the surface region. Figure 2: (a) ARPES images taken at h ν = 47 eV for K-adsorbed surfaces of a-TiO 2 with varying K coverage. The ARPES spectra were measured along the k x direction across the Γ 10 point. The MDCs at the Fermi level (E F ) with an energy window of 10 meV are also shown in the respective images. (b) The plot of n 2D estimated from ARPES measurements as a function of K coverage. (c) Schematic illustration of the possible structure model explaining the charge transfer from adsorbed K atoms to an a-TiO 2 surface and the resulting accumulation-layer formation. The schematics of the potential structure and the eigenfunctions of the first (n = 1) and second (n = 2) subbands are shown on the right side. REFERENCES [1] T. C. Rödel, F. Fortuna, F. Bertran, M. Gabay, M. J. Rozenberg, A. F. Santander-Syro and P. Le Fèvre, Phys. Rev. B 92, 041106 (2015). [2] S. Moser, L. Moreschini, J. Jaćimović, O. S. Barišić, H. Berger, A. Magrez, Y. J. Chang, K. S. Kim, A. Bostwick, E. Rotenberg, L. Forró and M. Grioni, Phys. Rev. Lett. 110, 196403 (2013). [3] R. Yukawa, M. Minohara, D. Shiga, M. Kitamura, T. Mitsuhashi, M. Kobayashi, K. Horiba and H. Kumigashira, Phys. Rev. B 97, 165428 (2018). [4] Z. Wang, Z. Zhong, S. McKeown Walker, Z. Ristic, J. Z. Ma, F. Y. Bruno, S. Riccò, G. Sangiovanni, G. Eres, N. C. Plumb, L. Patthey, M. Shi, J. Mesot, F. Baumberger and M. Radovic, Nano Lett. 17, 2561 (2017). [5] A. Koitzsch, J. Ocker, M. Knupfer, M. Dekker, K. Dörr, B. Büchner and P. Hoffmann, Phys. Rev. B 84, 245121 (2011). BEAMLINE BL-2A R. Yukawa 1 , M. Minohara 1 , D. Shiga 1, 2 , M. Kitamura 1 , T. Mitsuhashi 1,2 , M. Kobayashi 1 , K. Horiba 1 and H. Kumigashira 1, 2 ( 1 KEK-IMSS-PF, 2 Tohoku Univ.) Intensity (arb. units) 465 460 455 Ti 2p hν = 800 eV Bare K 0.1 Å K 0.3 Å K 0.5 Å K 0.8 Å K 1.2 Å Ti 3+ Ti 4+ Ti 3+ / (Ti 4+ + Ti 3+ ) Binding energy (eV) K adsorption (Å) (a) (b) E F k x (Å -1 ) E F k x (Å -1 ) E F k x (Å -1 ) Binding energy (eV) (b) Localized (c) (a) -0.2 0 0.2 0.6 0.4 0.2 -0.2 0 0.2 0.6 0.4 0.2 -0.2 0 0.2 0.6 0.4 0.2 Bare hν = 47 eV K 0.1 Å K 0.5 Å E F k x (Å -1 ) -0.2 0 0.2 0.6 0.4 0.2 K 0.3 Å 0 0.5 1.0 1.5 2 0 8 6 4 K adsorption (Å) n 2D (10 13 cm -2 ) K + K + K + K + K + K + Conducting electrons Localized electrons e - e - e - e - e - e - E n = 1 E F Depth 2 (b) (c)
Transcript of Angle-Resolved Photoelectron Spectroscopy Studies of Two … · 2019. 11. 5. · 24 HIGHLIGHTS...
HIGHLIGHTS 25HIGHLIGHTS24
1 - Materials Science 12
In situ Angle-Resolved Photoelectron Spectroscopy Studies of Two-Dimensional Electronic States at Anatase TiO2 (001) Surface Controlled by K Adsorption
The nature of the intriguing metallic electronic structures appearing at the surface of anatase titanium dioxide (a-TiO2) remains to be elucidated mainly owing to the difficulty of controlling the depth distribution of the oxygen vacancies. In this study, K atoms were adsorbed onto the (001) surface of a-TiO2 to dope electrons into the a-TiO2 and to confine the electrons in the surface region. The success of the electron doping and its controllability were confirmed by performing in situ angle-resolved photoelectron spectroscopy as well as core-level measurements. Clear subband structures were observed in the surface metallic states, indicating the creation of quasi-two-dimensional electron liquid states in a con-trollable fashion.
Techniques to control the low-dimensional electronic states appearing at the surface and interface of oxide semiconductors have attracted much attention for de-signing future quantum electronic devices. Recently, the formation of quasi-two-dimensional electron liquid (q2DEL) states induced by photoirradiation has been re-ported at the surface of anatase titanium dioxide (a-TiO2) [1]. However, owing to the difficulty of controlling the distribution of oxygen vacancies in the depth direction, the dimensionality of the electronic states generated by photoirradiation remains a controversial issue [1, 2]. Therefore, in this study, we proposed an alternative route to address the q2DEL states: we adsorbed K atoms on the (001) surface of a-TiO2 for doping elec-trons in the surface region and investigated the formed q2DEL states by in situ angle-resolved photoemission spectroscopy (ARPES) [3].
Figure 1(a) shows a series of X-ray photoemission spectroscopy (XPS) spectra taken as a function of K adsorption. The K deposition onto the (001) surface of a-TiO2 causes a weak but distinct structure corre-
The possible structure model explaining charge transfer from adsorbed K atoms to an a-TiO2 surface and the resulting accumulation-layer formation is drawn in Fig. 2(c). Some of the transferred electrons may be preferentially accumulated in the surface regions ini-tially. Analysis of the observed quantized states with a triangular wedge potential suggests that the activated carriers contributing to the formation of q2DEL states are confined in the surface (interface) region extending to a depth of a few nanometers, although the excess electrons could be trapped around K ions and/or oxy-gen vacancies at the interface between the K and a-TiO2. Consequently, these results indicate that doping of electrons onto the a-TiO2 is achieved by chemical adsorption of K atoms onto the a-TiO2. Clear subband structures are observed in the ARPES images for the surface metallic states, indicating the creation of q2DEL states in a controllable fashion.
Figure 1: (a) Ti-2p core-level spectra of a-TiO2 obtained by varying the coverage of adsorbed K atoms. The curve-fitting results for the Ti-2p3/2 states are presented, where the components of the Ti4+ and Ti3+ states are indicated by different hatching. The dashed line indicates the peak position of the Ti-2p3/2 states for a bare a-TiO2
(001) surface. (b) The plot of relative intensities of the Ti3+ states as a function of adsorbed K.
sponding to Ti3+ states to appear at a binding energy of 457.5 eV, indicating that electrons are transferred from the K atoms to the a-TiO2 film and that the doped elec-trons accumulate near the surface region accessible by the present XPS measurements [~1 nm away from the a-TiO2 surface (the interface between K and a-TiO2)]. As the amount of K deposition increases further, the Ti3+ states exhibit systematic evolution. These results imply that a certain amount of electron doping is induced by K deposition. The evolution of the Ti3+ states as a function of K deposition is summarized in Fig. 1(b). The occur-rence of electron doping is also confirmed by the shift of the Ti4+ peak by ~190 meV due to band bending.
The increment of the sheet carrier density (n2D) as-sociated with K deposition has been clearly observed in in situ ARPES measurements. Figure 2(a) shows a series of ARPES images taken along the [100] direc-tion as a function of K adsorption. Clear subband struc-tures are observed in all of the ARPES images for the K-adsorbed surfaces, indicating the 2D nature of the observed conduction bands. By deposition of K atoms onto the a-TiO2 surface, the Fermi momentum (kF) de-termined from momentum distribution curves (MDCs) increases from 0.11 Å−1 for bare a-TiO2 film to 0.19 Å−1 for a-TiO2 film with 0.5 Å K adsorption. Because a-TiO2 forms a Ti-3d-t2g dxy-derived circular Fermi surface centered at the Γ point, the increase in kF may reflect the increase in n2D by K deposition [4]. The n2D values obtained from the Luttinger volumes based on the as-sumption of spin degeneracy of the subbands are plot-ted in Fig. 2(b) as a function of K deposition. The n2D value steeply increases with increasing K deposition and seems to saturate at 0.3 Å. In contrast to the satu-ration of n2D at 0.3 Å K deposition, in the Ti 2p core level shown in Fig. 1 the intensity of Ti3+ states continues to increase after 0.3 Å K deposition and seems to saturate at about 0.8 Å. Assuming that the Ti3+ states observed in the Ti-2p core levels reflect the contributions of both localized and mobile electrons (namely, activated carri-ers whose density corresponds to n2D) [5], these results suggest the charges transferred from K atoms to a-TiO2 are partially trapped in the surface region.
Figure 2: (a) ARPES images taken at h
ν = 47 eV for K-adsorbed surfaces of a-TiO2 with varying K coverage. The ARPES spectra were
measured along the kx direction across the Γ 10 point. The MDCs at the Fermi level (EF) with an energy window of 10 meV are also shown in the respective images. (b) The plot of n2D estimated from ARPES measurements as a function of K coverage. (c) Schematic illustration of the possible structure model explaining the charge transfer from adsorbed K atoms to an a-TiO2 surface and the resulting accumulation-layer formation. The schematics of the potential structure and the eigenfunctions of the first (n = 1) and second (n = 2) subbands are shown on the right side.
REFERENCES[1] T . C. Röde l , F . For tuna, F . Ber t ran , M. Gabay,
M. J. Rozenberg, A. F. Santander-Syro and P. Le Fèvre, Phys. Rev. B 92, 041106 (2015).
[2] S. Moser, L. Moreschini, J. Jaćimović, O. S. Barišić, H. Berger, A. Magrez, Y. J. Chang, K. S. Kim, A. Bostwick, E. Rotenberg, L. Forró and M. Grioni, Phys. Rev. Lett. 110, 196403 (2013).
[3] R. Yukawa, M. Minohara, D. Shiga, M. Kitamura, T. Mitsuhashi, M. Kobayashi, K. Horiba and H. Kumigashira, Phys. Rev. B 97, 165428 (2018).
[4] Z. Wang, Z. Zhong, S. McKeown Walker, Z. Ristic, J. Z. Ma, F. Y. Bruno, S. Riccò, G. Sangiovanni, G. Eres, N. C. Plumb, L. Patthey, M. Shi, J. Mesot, F. Baumberger and M. Radovic, Nano Lett. 17, 2561 (2017).
[5] A. Koitzsch, J. Ocker, M. Knupfer, M. Dekker, K. Dörr, B. Büchner and P. Hoffmann, Phys. Rev. B 84, 245121 (2011).
BEAMLINEBL-2A
R. Yukawa1, M. Minohara1, D. Shiga1, 2, M. Kitamura1, T. Mitsuhashi 1,2, M. Kobayashi 1, K. Horiba 1 and H. Kumigashira1, 2 (1KEK-IMSS-PF, 2Tohoku Univ.)
Inte
nsity
(arb
. uni
ts)
465 460 455Binding energy (eV)
Ti 2phν = 800 eV
Bare
K 0.1 Å
K 0.3 Å
K 0.5 Å
K 0.8 Å
K 1.2 Å Ti3+
Ti4+
K adsorption (Å)
Ti3+
/ (T
i4+ +
Ti3+
)
(b)(a)
Inte
nsity
(arb
. uni
ts)
465 460 455Binding energy (eV)
Ti 2phν = 800 eV
Bare
K 0.1 Å
K 0.3 Å
K 0.5 Å
K 0.8 Å
K 1.2 Å Ti3+
Ti4+
K adsorption (Å)
Ti3+
/ (T
i4+ +
Ti3+
)
(b)(a)
Inte
nsity
(arb
. uni
ts)
465 460 455Binding energy (eV)
Ti 2phν = 800 eV
Bare
K 0.1 Å
K 0.3 Å
K 0.5 Å
K 0.8 Å
K 1.2 Å Ti3+
Ti4+
K adsorption (Å)
Ti3+
/ (T
i4+ +
Ti3+
)
(b)(a)
Inte
nsity
(arb
. uni
ts)
465 460 455Binding energy (eV)
Ti 2phν = 800 eV
Bare
K 0.1 Å
K 0.3 Å
K 0.5 Å
K 0.8 Å
K 1.2 Å Ti3+
Ti4+
K adsorption (Å)
Ti3+
/ (T
i4+ +
Ti3+
)
(b)(a)
Inte
nsity
(arb
. uni
ts)
465 460 455Binding energy (eV)
Ti 2phν = 800 eV
Bare
K 0.1 Å
K 0.3 Å
K 0.5 Å
K 0.8 Å
K 1.2 Å Ti3+
Ti4+
K adsorption (Å)
Ti3+
/ (T
i4+ +
Ti3+
)
(b)(a)
Inte
nsity
(arb
. uni
ts)
465 460 455Binding energy (eV)
Ti 2phν = 800 eV
Bare
K 0.1 Å
K 0.3 Å
K 0.5 Å
K 0.8 Å
K 1.2 Å Ti3+
Ti4+
K adsorption (Å)
Ti3+
/ (T
i4+ +
Ti3+
)
(b)(a)
(a) (b)
EF
kx (Å-1)
EF
kx (Å-1)
EF
kx (Å-1)
Bind
ing
ener
gy (e
V)
(b)
0 0.5 1.0 1.5
2
0
8
6
4
K adsorption (Å)
n 2D (1
013 c
m-2
) K+K+ K+ K+K+ K+
Conducting electrons
Localized electrons
e-e- e- e- e-
e-
(c)
E
n =
1
EF
Dep
th
(a)
-0.2 0 0.20.6
0.4
0.2
-0.2 0 0.20.6
0.4
0.2
-0.2 0 0.20.6
0.4
0.2
Barehν = 47 eV
K 0.1 Å K 0.5 Å
2
EF
kx (Å-1)-0.2 0 0.2
0.6
0.4
0.2
K 0.3 Å
EF
kx (Å-1)
EF
kx (Å-1)
EF
kx (Å-1)
Bind
ing
ener
gy (e
V)
(b)
0 0.5 1.0 1.5
2
0
8
6
4
K adsorption (Å)
n 2D (1
013 c
m-2
) K+K+ K+ K+K+ K+
Conducting electrons
Localized electrons
e-e- e- e- e-
e-
(c)
E
n =
1
EF
Dep
th
(a)
-0.2 0 0.20.6
0.4
0.2
-0.2 0 0.20.6
0.4
0.2
-0.2 0 0.20.6
0.4
0.2
Barehν = 47 eV
K 0.1 Å K 0.5 Å
2
EF
kx (Å-1)-0.2 0 0.2
0.6
0.4
0.2
K 0.3 Å
EF
kx (Å-1)
EF
kx (Å-1)
EF
kx (Å-1)
Bind
ing
ener
gy (e
V)
(b)
0 0.5 1.0 1.5
2
0
8
6
4
K adsorption (Å)
n 2D (1
013 c
m-2
) K+K+ K+ K+K+ K+
Conducting electrons
Localized electrons
e-e- e- e- e-
e-
(c)
E
n =
1
EF
Dep
th
(a)
-0.2 0 0.20.6
0.4
0.2
-0.2 0 0.20.6
0.4
0.2
-0.2 0 0.20.6
0.4
0.2
Barehν = 47 eV
K 0.1 Å K 0.5 Å
2
EF
kx (Å-1)-0.2 0 0.2
0.6
0.4
0.2
K 0.3 Å
EF
kx (Å-1)
EF
kx (Å-1)
EF
kx (Å-1)
Bind
ing
ener
gy (e
V)
(b)
0 0.5 1.0 1.5
2
0
8
6
4
K adsorption (Å)
n 2D (1
013 c
m-2
) K+K+ K+ K+K+ K+
Conducting electrons
Localized electrons
e-e- e- e- e-
e-
(c)
E
n =
1
EF
Dep
th
(a)
-0.2 0 0.20.6
0.4
0.2
-0.2 0 0.20.6
0.4
0.2
-0.2 0 0.20.6
0.4
0.2
Barehν = 47 eV
K 0.1 Å K 0.5 Å
2
EF
kx (Å-1)-0.2 0 0.2
0.6
0.4
0.2
K 0.3 Å
