Detecting, Distinguishing, and Spatiotemporally Tracking Photogenerated Charge and Heat at the Nanoscale

Abstract

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 lowdimensional 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, we employ stroboscopic optical scattering microscopy (stroboSCAT) to simultaneously map both heat and exciton populations in few-layer MoS 2 on relevant nanometer and picosecond 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 the 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 it will enable direct and quantitative discernment of different types of coexisting energy without recourse to complex models or underlying assumptions.

Dual-Hyperspectral Optical Pump–Probe Microscopy with Single-Nanosecond Time Resolution

“…While our work employed a wide-field pump configuration, where both above- and below-gap pump pulses were focused with lenses (with spot sizes of hundreds of μm), it is conceivable to steer the pump into a second objective placed on the backside of the sample for tighter pump focusing. This focused-pump setup can enable more delicate studies of charge and heat flow, complementing previous research focusing on the ps-to-ns time range . We anticipate that the dual-hyperspectral PPM setup introduced in this study will open new avenues for investigating charge, heat, and spin transport in various materials including inorganic and hybrid semiconductors, organic and soft materials, magnetic materials, and 2D heterostructures.…”

Spatiotemporal Microscopy: Shining Light on Transport Phenomena

“…These diffusing species -charge versus heat -give rise to a different sign of the transient signal, which makes it easy to distinguish them (Figure 6a). With the same technique, the thermal diffusivity of hBN-encapsulated MoS 2 [59] and disordered films of gold nanocrystals [60] were measured recently.…”