Detecting, distinguishing, and spatiotemporally tracking photogenerated charge and heat at the nanoscale
Since dissipative processes are ubiquitous in semiconductors, characterizing how electronic and thermal energy transduce and transport at the nanoscale is vital for understanding and leveraging their fundamental properties. For example, in low-dimensional transition metal dichalcogenides (TMDCs), excess heat generation upon photoexcitation is difficult to avoid since even with modest injected exciton densities, exciton-exciton annihilation still occurs. Both heat and photoexcited electronic species imprint transient changes in the optical response of a semiconductor, yet the distinct signatures of each are difficult to disentangle in typical spectra due to overlapping resonances. In response, it is necessary to simultaneously map both heat and charge populations in materials on relevant nanometer and picosecond length- and time scales.
By further honing stroboSCAT, a time-resolved optical scattering microscopy capable of capturing spatiotemporal energy flow in a wide range of materials, a STROBE team from UC Berkeley collaborated with Caltech to map both heat and exciton populations in few-layer TMDC MoS2 on the relevant length- and time scales and with 100-mK temperature sensitivity. We discern excitonic contributions to the signal from heat by combining observations close to and far from exciton resonances, characterizing photoinduced dynamics for each. Our approach is general and can be applied to any electronic material, including thermoelectrics, where heat and electronic observables spatially interplay, and lays the groundwork for direct and quantitative discernment of different types of coexisting energy without recourse to complex models or underlying assumptions. This work illustrates the ability to, finally, simultaneously observe and distinguish photogenerated heat from charge in a broad range of systems critical to the performance of next-generation energy conversion modules.
MRS PREM Research Scholars Symposium
Ab initio structures from nanocrystal molecular lattices
Electron diffraction has dramatically increased in popularity amongst chemists given its renewed application for ab initio structure determination from molecular nanocrystals. In one implementation, popularly referred to as 3D ED or MicroED, crystals nanocrystals orders of magnitude too small for conventional X-ray analysis are interrogated by an electron beam to determine atomic structures. However, these approaches are thwarted by disordered, overlapping, or otherwise poorly diffracting domains.
Spatially resolved diffraction mapping techniques can overcome some of these limitations, and have seen limited application in X-ray diffraction. In electron microscopy, such approaches, including 4D scanning electron microscopy (4D-STEM), have grown popular. We demonstrated that 4D-STEM can be used to determine ab initio structures of molecules by direct methods, from small ordered nanodomains of single microcrystals. In our approach 4D-STEM is used to generate diffraction scans that enable ex post facto reconstruction of digitally defined virtual apertures. The synthetic patterns derived from these scans are suitable for direct methods phasing of molecular structures.
In addition, this approach unveils that coherently diffracting zones (CDZs) in molecular crystals form unpredictably distributed striations. The observation of these zones and our ability to determine structures from these regions of nanocrystals empowers us to explore their atomic substructure and their response to radiolytic damage.
Operando Spectral Imaging of the Li-ion Battery’s Solid-Electrolyte Interphase
Considering the scale of the lithium ion battery (LIB) industry, it is surprising how poorly the function of LIBs is understood at the molecular level. While much is certainly known, this knowledge has been gained via inference and expensive trial-and-error because it is difficult to look inside a functioning LIB to “see” what is going on. The battery is a bulk device with a liquid, air-sensitive organic electrolyte. With use, there forms on the LIB electrodes an almost magical solid-electrolyte interphase (SEI) that is an insulator for electrons but a conductor for Li+ ions. The main mysteries of LIB function involve the chemical composition and structure of this layer. We present the first images of the LIB SEI acquired under room-temperature operando conditions with high spatial and spectroscopic resolution. This combination gives us an unprecedented view of the SEI’s development, where we can make chemical identifications localized to nanometer precision while the electrode is in the very act of intercalating. We image the bulk SEI, not just its surface, by contriving electrochemical fluid cells that are only 50 nm thick. With these thin cells we can map the Li itself by its unique spectroscopic fingerprint, an achievement described as “practically impossible” just a few years ago.
Two-color high-harmonic generation from relativistic plasma mirrors
Predicting heat flow in 3D semiconductor nanosystems
Relationships between Compositional Heterogeneity and Electronic Spectra of (Ga1−xZnx)(N1−xOx) Nanocrystals Revealed by Valence Electron Energy Loss Spectroscopy
Many ternary and quaternary semiconductors have been made in nanocrystalline forms for a variety of applications, but we have little understanding of how well their ensemble properties reflect the properties of individual nanocrystals. STROBE researchers at CU Boulder examined electronic structure heterogeneities in nanocrystals of (Ga1−xZnx)(N1−xOx), a semiconductor that splits water under visible illumination. They used valence electron energy loss spectroscopy (VEELS) in a scanning transmission electron microscope to map out electronic spectra of (Ga1−xZnx)(N1−xOx) nanocrystals with a spatial resolution of 8 nm. They examine three samples with varying degrees of intraparticle and interparticle compositional heterogeneity and ensemble optical spectra that range from a single band gap in the visible to two band gaps, one in the visible and one in the UV. The VEELS spectra resemble the ensemble absorption spectra for a sample with a homogeneous elemental distribution and a single band gap and, more interestingly, one with intraparticle compositional heterogeneity and two band gaps. They observe spatial variation in VEELS spectra only with significant interparticle compositional heterogeneity. Hence, they reveal the conditions under which the ensemble spectra reveal the optical properties of individual (Ga1−xZnx)(N1−xOx) particles. More broadly, they illustrate how VEELS can be used to probe electronic heterogeneities in compositionally complex nanoscale semiconductors.