Probing Nonequilibrium Dynamics of Photoexcited Polarons on a Metal-Oxide Surface with Atomic Precision

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Probing Nonequilibrium Dynamics of Photoexcited Polarons on a Metal-Oxide Surface with Atomic Precision

Chaoyu Guo (郭钞宇), Xiangzhi Meng, Huixia Fu, Qin Wang, Huimin Wang, Ye Tian, Jinbo Peng, Runze Ma, Yuxiang Weng, Sheng Meng, Enge Wang, and Ying Jiang

Phys. Rev. Lett. 124, 206801 – Published 19 May 2020

Abstract

Understanding the nonequilibrium dynamics of photoexcited polarons at the atomic scale is of great importance for improving the performance of photocatalytic and solar-energy materials. Using a pulsed-laser-combined scanning tunneling microscopy and spectroscopy, here we succeeded in resolving the relaxation dynamics of single polarons bound to oxygen vacancies on the surface of a prototypical photocatalyst, rutile ${TiO}_{2} (110)$ . The visible-light excitation of the defect-derived polarons depletes the polaron states and leads to delocalized free electrons in the conduction band, which is further corroborated by ab initio calculations. We found that the trapping time of polarons becomes considerably shorter when the polaron is bound to two surface oxygen vacancies than that to one. In contrast, the lifetime of photogenerated free electrons is insensitive to the atomic-scale distribution of the defects but correlated with the averaged defect density within a nanometer-sized area. Those results shed new light on the photocatalytically active sites at the metal-oxide surface.

Received 7 September 2019
Revised 26 January 2020
Accepted 9 April 2020

DOI:https://doi.org/10.1103/PhysRevLett.124.206801

Physics Subject Headings (PhySH)

Polarons

Oxides Surfaces

Density functional theory Photoexcitation Pump-probe spectroscopy Scanning tunneling microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Chaoyu Guo (郭钞宇)^1,2,*, Xiangzhi Meng ^1,*,†, Huixia Fu ^3,*, Qin Wang ^1,*, Huimin Wang³, Ye Tian¹, Jinbo Peng ¹, Runze Ma ¹, Yuxiang Weng ³, Sheng Meng^3,4,‡, Enge Wang^1,3,4,5,§, and Ying Jiang^1,4,5,¶

¹International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871, People’s Republic of China
²Physical Science Laboratory, Huairou National Comprehensive Science Centre, Beijing 101400, People’s Republic of China
³Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China
⁴Collaborative Innovation Center of Quantum Matter, Beijing 100871, People’s Republic of China
⁵CAS Center for Excellence in Topological Quantum Computation, University of Chinese Academy of Sciences, Beijing 100190, People’s Republic of China

^*These authors contributed equally to this work.
^†Present address: Physikalisches Institut, Westfälische Wilhelms-Universität, Wilhelm-Klemm-straße10, 48149 Münster, Germany.
^‡smeng@iphy.ac.cn
^§egwang@pku.edu.cn
^¶yjiang@pku.edu.cn

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Vol. 124, Iss. 20 — 22 May 2020

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Images

Figure 1
In-gap state (polaron state) derived from the surface oxygen vacancies on ${TiO}_{2} (110)$ . (a) Experimental setup of the laser-combined STM. (b) Atomic structure of the ${TiO}_{2} (110)$ surface. The hexacoordinated Ti ( ${Ti}_{6 c}$ ), pentacoordinated terminal Ti ( ${Ti}_{5 c}$ ), in-plane oxygen ( $O_{ip}$ ), and bridge oxygen ( $O_{b}$ ) are indicated. (c) STM topography of the ${TiO}_{2} (110)$ surface, where the surface $O_{b}$ vacancy ( $V_{O}^{surf}$ ) and ${Ti}_{5 c}$ row are denoted by the white dashed circle and white arrow, respectively. The set point of the STM image: $V = 1.7 V$ and $I = 80 pA$ . (d) $d I / d V$ spectra taken around $V_{O}^{surf}$ and at the defect-free surface area (bkgd). Set points for $V_{O}^{surf}$ (1.7 V and $I = 200 pA$ ) and bkgd (1.7 V and $I = 30 pA$ ) were chosen differently to highlight the in-gap states. The spectrum of $V_{O}^{surf}$ was offset by $8 pA / V$ for clarity. The peak position of the polaron state is denoted by the blue arrow. The valence band (VB) and conduction band (CB) are represented by filled rectangles. The inset is the statistics of polaron energy distribution. Bin size, 100 mV. The red line represents the Gaussian (normal) curve fitting to the data.
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Figure 2
The photoexcitation of a single polaron. (a),(b) The topography and $d I / d V$ mapping of $V_{O}^{surf}$ . The white dashed lines highlight the ${Ti}_{5 c}$ rows. In (b), the filled ellipse denotes the $V_{O}^{surf}$ site, and the white arrows indicate the distribution of the polaron states. Set points of STM topography and $d I / d V$ mapping: $V = 1.7 V$ and $I = 70 pA$ (a),(b). The $d I / d V$ mapping was acquired with an open feedback loop at the bias: $V = - 1.2 V$ . The red (black) curves in (c) show the $d I / d V$ spectra with (without) the 700-nm laser irradiation. Laser power, 2 mW. The arrows mark the peak positions of polaron states. Set point: $V = 1.7 V$ and $I = 100 pA$ . (d) Optimized structure of one $V_{O}^{surf}$ on rutile (110) surface and the excess electron distribution. The excess electron density is highlighted by dark green, and the value of the isosurface is $10^{- 2} electrons / A^{3}$ . Gray and red spheres are the Ti and O ions, respectively. (e) The strength of optical transition probability between the defect state and the states in the VB and CB. Here α is the transition dipole moment between the defect state and VB or CB states. The zero energy was set at the CBM. The curved arrows denote the direction of the charge transfer. (f) The projected DOS of defect states for $V_{O}^{surf}$ before (filled) and after (dashed) the charge transfer ( $0.2 e$ ). The arrows denote the shift of the defect states. The dashed vertical lines denote the position of defect levels.
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Figure 3
Spatially dependent lifetimes of photoexcited electrons and polaron states. (a),(b) and (c),(d) Photoinduced current $I_{ph}$ versus delay time $t_{d}$ between the two 532-nm laser pulses arising from the photoexcited electrons in the CB and the photodepletion of polarons, respectively. The red lines represent the exponential fitting to the data. Set points: $V = 1.7 V$ and $I = 80 pA$ (a),(b), $V = - 2.2 V$ and $I = 2 pA$ (c), and $V = - 1.7 V$ and $I = 50 pA$ (d). Laser power, 0.1 mW. The result in (c) has the identical relative resolution as that in (d) although the set point is about a factor of 10 lower. (e) The topography of a $V_{O}^{surf}$ dimer. The filled circles denote the position of two ground-state polarons, where polaron 1 is isolated and polaron 2 is bound to two defects. (a),(c) and (b),(d) were taken at defect-free region and defect site [location 1 in (e)], respectively. (f) Location dependence of the lifetimes for photoexcited electrons and polarons, which were extracted from the exponential fitting of $I_{ph}$ ( $t_{d}$ ) curves. The error bars arise from the fitting error.
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Figure 4
The lifetimes of photoexcited free electrons in the CB as a function of the defect density. The lifetimes were extracted from the $I_{ph} (t_{d})$ spectra taken at defect-free regions. Set points: $V = 1.7 V$ and $I = 100 pA$ . Laser power, 0.1 mW. The averaged defect density was calculated within $20 \times 20 {nm}^{2}$ areas. The error bars of the lifetime and the defect density reflect the fitting error and the statistical error, respectively. The insets are the defect density mapping of six $20 \times 20 {nm}^{2}$ areas with the averaged defect density ranging from $0 . {51 / nm}^{2}$ to ${0.70 / nm}^{2}$ . The local density was analyzed in square areas of $1.8 \times 1.8 {nm}^{2}$ , which defines the resolution of the density mapping.
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