Nanoscale Bandgap Tuning across an Inhomogeneous Ferroelectric Interface

📝 Abstract

We report nanoscale bandgap engineering via a local strain across the inhomogeneous ferroelectric interface, which is controlled by the visible-light-excited probe voltage. Switchable photovolatic effects and the spectral response of the photocurrent were explore to illustrate the reversible bandgap variation (~0.3eV). This local-strain-engineered bandgap has been further revealed by in situ probe-voltage-assisted valence electron energy-loss spectroscopy (EELS). Phase-field simulations and first-principle calculations were also employed for illustration of the large local strain and the bandgap variation in ferroelectric perovskite oxides. This reversible bandgap tuning in complex oxides demonstrates a framework for the understanding of the opticallyrelated behaviors (photovoltaic, photoemission, and photocatalyst effects) affected by order parameters such as charge, orbital, and lattice parameters.

💡 Analysis

We report nanoscale bandgap engineering via a local strain across the inhomogeneous ferroelectric interface, which is controlled by the visible-light-excited probe voltage. Switchable photovolatic effects and the spectral response of the photocurrent were explore to illustrate the reversible bandgap variation (~0.3eV). This local-strain-engineered bandgap has been further revealed by in situ probe-voltage-assisted valence electron energy-loss spectroscopy (EELS). Phase-field simulations and first-principle calculations were also employed for illustration of the large local strain and the bandgap variation in ferroelectric perovskite oxides. This reversible bandgap tuning in complex oxides demonstrates a framework for the understanding of the opticallyrelated behaviors (photovoltaic, photoemission, and photocatalyst effects) affected by order parameters such as charge, orbital, and lattice parameters.

📄 Content

1 Nanoscale Bandgap Tuning across Inhomogeneous Ferroelectric Interface
Jing Wang1,#, Houbing Huang2,#, Wangqiang He2, Qinghua Zhang3, Danni Yang1, Yuelin Zhang1, Renrong Liang5, Chuanshou Wang1, Xingqiao Ma2, Lin Gu4, Longqing Chen6, Ce-Wen Nan3 and Jinxing Zhang1,* 1, Department of Physics, Beijing Normal University, 100875, Beijing, China 2, Department of Physics, University of Science and Technology Beijing, 100083, Beijing, China 3, School of Materials Science and Engineering, Tsinghua University, 100084, Beijing, China 4, Institute of Physics, Chinese Academy of Science, 100190, Beijing, China 5, Tsinghua National Laboratory for Information Science and Technology, Institute of Microelectronics, Tsinghua University, 100084, Beijing, China 6, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA

2 KEYWORDS: nanoscale bandgap tuning, probe/film interface, switchable ferroelectric photovoltaic effects, point-contact geometry, local strain

ABSTRACT: We report the nanoscale bandgap engineering via a local strain across the inhomogeneous ferroelectric interface, which is controlled by the visible light-excited probe voltage. Switchable photovoltaic effects and spectral response of the photocurrent were explored to illustrate the reversible bandgap variation (~0.3 eV). This local-strain-engineered bandgap has been further revealed by in-situ probe-voltage-assisted valence electron energy loss spectroscopy (EELS). Phase-field simulations and first-principle calculations were also employed to illustrate the large local strain and the bandgap variation in ferroelectric perovskite oxides. This reversible bandgap tuning in complex oxides demonstrates a framework to understand the optical-related behaviors (photovoltaic, photo-emission, and photo-catalyst effects) affected by order parameters such as charge, orbital and lattice.

3 INTRODUCTION
andgap engineering of semiconductors has attracted people’s interests for the past decades due to the emerging quantum phenomena 1-2 and the applications in photocatalysis 3, solar cells 4 and other optoelectronic devices (e.g., photoemitters 5, photodetectors 6 and photomodulators 7). The bandgap of semiconductors can be successfully engineered by employing quantum confinement effects 1-2, chemical doping 8, epitaxial strain 9-10, bending effects (free- standing carbon nanotube 11 and MoS2 monolayer 12 etc.) or hetero-structures 13-14. However, a reversible control of the bandgap under the external stimulus in integrated semiconductors may be challenging. Therefore, the exploration of bandgap engineering in new material systems is highly desired. In strong correlated materials, the optical band structures are coupled with other order parameters, where the bandgap could be modulated under external stimuli (stress, optical excitation and electric/magnetic fields etc.).
Multiferroic perovskite oxides exhibit the coexistence of ferro/piezoelectricity, magnetism and ferroelasticity due to the strong correlation between lattice and other degrees of freedom (e.g., charge, orbital and spin), which provide people potential candidates to study the electronic and optical properties under the application of external stimuli. Dong et al. and Wang et al. examined bandgap change in ferroelectric BiFeO3 (BFO) 15 and KNbO3 16 via a giant external compressive strain. Although such a large strain (over several percent) cannot be realized in bulk crystals in practice, these studies theoretically demonstrate that the electronic band structures in ferroelectric materials could be reversibly modulated by a compressive mechanical stimulus. Therefore, it gives us a strong push to find a way to reversibly input a large external controllable strain (compressive) in ferroelectric oxides without breaking the crystals.
The distinct structural deformation and the consequent large local strain in multiferroic materials can be achieved by the application of a mechanical force 17 or electric field 18-19 on the nanoscale probe. This probe-assisted large local strain can be reversibly achieved without breaking the crystal due to the gradual release of internal stress. In this work, we explored the reversible nanoscale bandgap tuning via a local strain across the inhomogeneous ferroelectric interface, which is controlled by the visible light-excited probe voltage. Switchable photovoltaic effects and spectral response of the photocurrent were explored by photoexcitation-assisted atomic force microscopy (P-AFM) technique to illustrate the reversible bandgap variation (~0.3 eV). This local- B

4 strain-engineered bandgap has been further revealed by in-situ probe-voltage-assisted valence electron energy loss spectroscopy (EELS). Phase-field simulations and first-principle calculations were also employed to illustrate the large local strain and the ba

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