Probing Nanoparticle Optical Properties and Responses with In Situ Electron Microscopy

The interaction of materials with photons is critical to many technologies including opto-electronic devices, photovoltaics and photocatalysis. The optical response is determined by the real and imaginary parts of the dielectric function which can in turn be impacted by the ambient conditions such as temperature, gas/liquid environment as well as structural factors such as point defects and interfaces. To explore the optical response of materials at the nanometer level, we have developed <i>in situ</i> optical illumination systems for transmission electron microscopy (TEM) [1-3]. The system is installed in an FEI Titan environmental TEM and is compatible with both heating, cooling and gas ambients. Materials can be thermally processed in a variety of different gases and illuminated with light. Optical properties and responses can be probed <i>in situ</i> with monochromated electron energy-loss spectroscopy (EELS) and electron holography.<br/>In semiconducting devices, separation of electron-hole pairs generated via photon absorption is often facilitated by selective charge transport across heterostructure interfaces. The efficiency of the charge transfer process will depend on materials’ thermodynamics and kinetic factors regulated by interface characteristics (such as Schottky barriers). We have been exploring the variation in the charge state of supported metal nanoparticle photocatalysts using <i>in situ</i> electron microscopy. Specifically, we have investigated the effect of visible light absorption in Ni supported on Rh-doped strontium titanate particles [4]. During illumination, electrons generated from photon absorption may transfer to the metal nanoparticle changing its charge state. Electron holography allows us to detect phase shifts in the fast electron wavefunction associated with photon illumination. <br/>The optical properties of oxides are strongly affected by the concentration of point defects such as oxygen vacancies and dopants. In dielectric materials, photonic or guided light modes may be present which depend on both the nanoparticles shape, size, and refractive index [5]. For many non-stoichiometric oxides such as CeO₂, the oxygen vacancy concentration can be affected by the temperature and oxygen partial pressure. We are investigating the variation in photonic modes with changes in temperature and gas environment. The change in the photonic modes might be associated with changes in density and oxygen vacancy concentration leading to corresponding variations in the refractive index [6]. The sensitivity of photonic modes to small changes in the optical properties of materials is being explored. <br/>1. B. K. Miller and P. A. Crozier, Microscopy and Microanalysis 2013. <b>19</b> Issue 2 Pages 461-469<br/>2. Q. Liu et al, Microscopy and Microanalysis 2016 Vol. 22 Issue S3 Pages 730-731<br/>3. Q. Liu et al, arXiv preprint arXiv:2104.02006 2021<br/>4. P. Haluai and P.A. Crozier Microscopy and Microanalysis 2022 28 (S1), 1880-1882.<br/>5. Q. Liu et al, Physical Review B 2019 Vol. 99 Issue 16 Pages 165102, DOI: 10.1103/PhysRevB.99.165102<br/>6. Y Wang et al, Microscopy and Microanalysis 28 (S1), 1972-1974<br/>7. We gratefully acknowledge support of DOE grant BES DE-SC0004954, the use of ASU’s John M. Cowley Center for High Resolution Electron Microscopy.