Imaging Non-equilibrium Hot Carrier Dynamics in 2D Materials and Their Heterostructures with Scanning Ultrafast Electron Microscopy - Novel Functional Materials
Abstract
This proposed research is aimed at directly visualizing and quantitatively analyzing the non-equilibrium dynamics of photo-excited hot carriers in two-dimensional (2D) materials and their heterostructures using scanning ultrafast electron microscopy (SUEM) and accompanying multiscale ab initio transport simulations. 2D materials and their heterostructures hold great promise as the core units of next-generation optoelectronic devices. In particular, the reduced electron-phonon scattering phase space due to the reduced dimensionality and the atomic thickness of 2D materials indicate a great potential of extracting and utilizing the photo-excited hot carriers for energy harvesting and photo-sensing applications. To fully capture the dynamics of hot carriers in 2D materials immediately after photo-excitation, extreme spatial (nanometer) and temporal (femtosecond) resolutions are required at the same time, beyond the capability of conventional ultrafast optical spectroscopic methods due to the optical diffraction limit. SUEM is a newly developed photon-pump-electron-probe technique that combines nanometer spatial resolution and femtosecond temporal resolution for in situ and real-time imaging of photo-induced carrier dynamics, utilizing ultrafast-laser-induced photo-emission of pulsed electron beams. We propose to use SUEM to directly visualize the dynamics of hot carriers photo-excited in uniform 2D materials or right at the heterojunctions formed by 2D materials, with the following scientific objectives: l) understand the superdiffusive behavior of hot carriers immediately after photo-excitation caused by the initial high electronic temperature and reduced charge-lattice coupling in uniform 2D materials, which is expected to be fundamentally different from near-equilibrium carrier dynamics; 2) understand the charge separation, oscillation and recombination behaviors of hot carriers excited right at heterojunctions formed by 2D materials with different thicknesses, defined by electrostatic gates or fabricated by "stitching" different 2D materials, which will form the knowledge base for designing 2D-material-based photovoltaic cells and photo-sensors; 3) understand how hot carrier dynamics in 2D materials is modified by the interaction with various substrates, especially the roles played by polar optical phonons in polar substrates and the Schottkey junction formed with metallic substrates, which will provide both practical guidelines for device fabrication and new means to tailor hot carrier dynamics for specific applications; 4) as an exploratory direction, we will develop a time-resolved electron backscattering diffraction (tr-EBSD) mode in SUEM to directly probe the interaction between photo-excited hot carriers and specific phonon modes in 2D materials, in particular the out-of-plane flexural phonon modes; 5) to obtain quantitative understandings of these scientific objectives, a nonequilibrium-Boltzmann-equation-based simulation framework will be developed in parallel to quantitatively interpret experimental measurements, with inputs of electron-electron, electron-phonon and phonon-phonon scattering rates from state-of-the-art ab initio calculations. The proposed research will significantly advance our knowledge of the spatial-temporal dynamics of photo-excited hot carriers, which has been extremely challenging to study due to the required spatial-temporal resolution and the lack of simulation tools based on first-principles. An accurate and detailed understanding of photo-excited hot carrier dynamics, which can be drastically different from near-equilibrium dynamics, is essential for developing next-generation photovoltaic and optoelectronic devices based on 2D materials and their heterostructures.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Feb 14, 2019
- Source ID
- W911NF1910060
Entities
People
- Bolin Liao
Organizations
- Army Contracting Command
- United States Army
- University of California, Santa Barbara