Research Interests
Functional heterostructure fabrication
This project focuses on self-assembling and processing functional materials with precise control of atomic structure, defect level, phase dimension and coupling. The research uses pulsed laser deposition as the main materials synthesis tool, together with atomic layer deposition, sputtering and post-synthetic treatments.
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References:
1. Self-assembled Oxide Nanocomposite Thin Films and Devices. MRS Bull 40, 736-745 (2015).
2. Interfacial Coupling in Heteroepitaxial Vertically Aligned Nanocomposite Thin Films: From Lateral to Vertical Control. Curr Opin Solid State Mater Sci 18, 6-18 (2014).
Nanoscale energy materials
This project studies exotic phenomena of carrier transport and dynamics in top-performing semiconductors and establishes their relation with the photoelectrochemical performance. Representative examples include ZnO (large exciton diffusivity) and BiVO4 (poor electron transport and good hole transport as a n-type semiconductor). The goal aims to accelerate material modeling, design and discovery for solar energy conversion.
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References:
1. Anomalous Conductivity Tailored by Domain-Boundary Transport in Crystalline Bismuth Vanadate Photoanodes. Chem Mater 30, 1677-1685 (2018).
2. Unravelling Photocarrier Dynamics beyond the Space Charge Region for Photoelectrochemical Water Splitting. Chem Mater 29, 4036-4043 (2017).
Quantum materials and device physics
This project explores the interfacial coupling across epitaxial heterointerfaces of correlated materials systems, and studies the effects on physical properties and device functionalities. Current interests are the strain mediation, spin coupling and charge transfer at the heterointerfaces with the technological goals of realizing advanced electric and magnetic devices.
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References:
1. Strong Perpendicular Exchange Bias in Epitaxial La0.7Sr0.3MnO3:BiFeO3 Nanocomposite Films through Vertical Interfacial Coupling. Nanoscale 7, 13808-13815 (2015).
2. Strain Relaxation and Enhanced Perpendicular Magnetic Anisotropy in BiFeO3:CoFe2O4 Vertically Aligned Nanocomposite Thin Films. Appl Phys Lett 104, 062402 (2014).
Research Facility and Collaboration
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Materials Synthesis: Pulsed laser deposition, Atomic layer deposition, Sputtering, E-beam/thermal evaporation
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Structural/Chemical/Optical Characterization: (S)TEM/EDS/EELS/SAED, SEM/EDS, AFM/C-AFM/PFM, XRD/RSM, Raman spectroscopy, XPS, XAS, Spectroscopic ellipsometry, Photoluminescence spectroscopy, Steady-state/transient optical absorption spectroscopy
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Device Fabrication: Focused ion beam, Lithography
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Physical Properties Characterization: PPMS, Electric probe stations, (Photo)electrochemical measurements
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Data Analysis and Visualization: Origin, Igor, Python, Matlab and Microsoft Office Suite.
​DOE Facilities
Research Higlights
Solar Energy Conversion
Work 1: Unconventional Relation between Charge Transport and Photocurrent via Boosting Small Polaron Hopping for Photoelectrochemical Water Splitting
Significance: This work reveals an unconventional carrier transport relation that is tuned by extrinsic molybdenum (Mo) doping in BiVO4 photoanodes. It provides direct evidence that Mo doping improves electron transport by boosting not only the donor density but also the electron mobility in the form of small polaron, an effect not readily recognized by the community.
Next: While the extrinsic doping effectively improves electron doping, it is found to compromise minority carrier (hole) transport. Finding ways to coordinate overall carrier transport, along with light absorption, is critical to enhance the energy conversion efficiency.
Work 2: Anomalous Conductivity Tailored by Domain-Boundary Transport in Crystalline Bismuth Vanadate Photoanodes
Significance: BVO is intrinsically insulating due to the low electron mobility. This work explores vertical domain boundaries as extrinsic electron conduction channels, thus significantly improving carrier transport in this material.
Next: While the extrinsic doping effectively improves electron doping, it is found to compromise minority carrier (hole) transport. Finding ways to coordinate overall carrier transport, along with light absorption, is critical to enhance the energy conversion efficiency.
Work 3: Unravelling Photocarrier Dynamics beyond the Space Charge Region for Photoelectrochemical Water Splitting
Significance: This work reveals non-trivial photocarrier transport at a commonly neglected quasi-neutral region, which plays a significant role in boosting the photoelectrode quantum efficiency. In this case, nanostructing is not necessary, and a planar thin film structure already achieves above 90% quantum efficiency by enabling directional exciton transport towards the space charge region.
Next: Combining the exciton-assisted transport with a material with strong visible light absorption is critical to develop efficient solar water splitting devices.
Solid State Physics
Work 1: Strong perpendicular exchange bias in epitaxial La0.7Sr0.3MnO3:BiFeO3 nanocomposite films through vertical interfacial coupling
Significance: A novel approach for obtaining perpendicular exchange bias (PEB) is achieved by orienting the ferromagnetic/antiferromagnetic coupling in the vertical geometry through a unique vertically aligned nanocomposite (VAN) design. Such PEB effect is much robust than that in conventional layered structures, and does not have a thickness limitation due to the inherently vertical exchange coupling.
Next: The full picture of spin coupling across the interface needs to be established by spectroscopy characterizations. Novel device concepts are expected to integrate the vertical coupling for applications.
Work 2: Perpendicular Exchange-Biased Magnetotransport at the Vertical Heterointerfaces in La0.7Sr0.3MnO3:NiO Nanocomposites
Significance: This work demonstrates the use of magnetic exchange coupling at the interface to control magnetotransport, which represents a fundamentally different mechanism from the conventional spin-polarized tunneling.
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Next: The self-assembled vertical interfaces, with the ultrahigh density, can maximize the interface impact and can be used to manipulate the device performance.
Review: Multifunctional, self-assembled oxide nanocomposite thin films and devices
Summary: This review is a good start to learn epitaxial nanocomposite self-assembly and demonstrated device functionalities.
Complex oxides provide an ideal playground for exploring the interplay among the fundamental degrees of freedom: structural (lattice), electronic (orbital and charge), and magnetic (spin). This work focuses on self-assembled oxide nanocomposite thin films, which provide a novel pathway to create ultrahigh density of vertical interfaces and thus, new functionalities. Representative examples include flux pinning in superconductors, strain-enhanced ferroelectricity, strain- and charge-coupled magnetoelectrics, tunable magnetotransport, novel electrical/ionic transport, memristors, and tunable dielectrics.
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