Abstract
Understanding the microscopic spatio-temporal dynamics of nonequilibrium charge carriers in heterosystems promises optimization of process and device design towards desired energy transfer. Hot electron transport is governed by scattering with other electrons, defects, and bosonic excitations. Analysis of the energy dependence of scattering pathways and identification of diffusive, superdiffusive, and ballistic transport regimes are current challenges. Beyond our previous studies on the /(001) heterostructure, in this work, we determine the energy-dependent change of the electron propagation time through epitaxial /(001) heterostructures as a function of layer thickness. We do so by employing femtosecond time-resolved two-photon photoelectron emission spectroscopy for energies of 0.5–2.0 eV above the Fermi energy. We describe the laser-induced nonequilibrium electron excitation and injection across the / interface using real-time time-dependent density functional theory and analyze electron propagation through the layer by microscopic electron transport simulations. We identify ballistic transport of minority electrons at energies with a nascent optically excited electron population, which is determined by the combination of photon energy and the specific electronic structure of the material. At lower energy, superdiffusive transport with 1–4 scattering events dominates. The effective electron velocity accelerates from 0.3 to 1 nm/fs with an increase in the layer thickness from 10 to 100 nm. This phenomenon is explained by electron transport that becomes preferentially aligned with the interface normal for thicker layers, which facilitates electron momentum or energy selection by choice of the propagation layer thickness.
- Received 7 June 2023
- Revised 19 September 2023
- Accepted 29 September 2023
DOI:https://doi.org/10.1103/PRXEnergy.2.043009
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
The increasing need for sustainable energy is one society's greatest challenges. Improving the efficiency of future energy conversion technologies is therefore of great importance. To boost the efficiency of light-energy conversion in solar cells or photocatalysts, optically excited electrons should transfer their energy only after delivering it to the desired location. Along the way, energy losses by scattering play a crucial role, and the details of such processes are of both fundamental and technological interest. Here, the authors use an iron-gold heterostructure as a model system and combine experimental and theoretical approaches to determine the photoexcited electrons' propagation times, analyze their transmission across an interface, and unravel the relevant scattering mechanisms. One important result is that the injection angle across an interface determines the transport regime of the electrons. These findings may be used to develop energy filters that enable processes with tailored electron energies. Tailoring the allowed electron energies would limit scattering losses and thus improve the efficiency of electron transport in energy-conversion technologies.
