Spatiotemporal Study of Electronic and Thermal Transport in Low-Dimensional Semiconductors Using Transient Reflection Microscopy

The efficiency of silicon-based optoelectronic devices has greatly improved due to the growing knowledge of the material, yet the increasing miniaturization and performance demand is pushing research towards fundamental physical limits. At nanometer scales, carrier mobility is degraded, recombination rates affect operation speed, and thermal management becomes a challenge. Low-dimensional semiconductors such as colloidal nanocrystal (NC) films and transition metal dichalcogenides (TMDs) are therefore investigated as promising candidates for next-generation optoelectronics. The objective of this thesis is to characterize the energy transport mechanisms in these nanomaterials and identify the factors that impact on their behavior. To simultaneously access electronic and thermal carriers with sub-micrometer spatial and nanosecond temporal resolution, we employ stroboscopic scattering microscopy (stroboSCAT). In this pump–probe technique, a focused laser pulse generates localized excited energy carriers, while a delayed wide-field probe pulse detects refractive index perturbations, enabling direct reconstruction of diffusion profiles, lifetimes, and diffusivities. Experiments on CdSe and PbS NC films reveal that pump–probe signals contain overlapping electronic and thermal components that need to be correctly addressed. Near-gap, low-fluence excitation produces subdiffusive transport consistent with hopping-mediated dynamics, whereas above-gap or high-fluence excitation enhances Auger recombination and hot-carrier cooling, leading to significant transient heating. Thickness-dependent relaxation time, comparison with simulations and timescale analysis confirm the thermal origin of late-time signals, allowing thermal and electronic regimes to be correctly distinguished. The methodology is extended to WSe₂ flakes mechanically exfoliated up to atomically thin films. High-fluence, above-gap excitation produces heat-dominated signals which have been correlated to heat relaxation finite element simulations (COMSOL). While near-gap, low-fluence excitation allows us to selectively measure electronic transport within the first nanoseconds. Probe-wavelength dependent studies near the excitonic resonances further enhance selectivity, yielding diffusivities of 1–3 cm²/s, consistent with literature values. Correlation with AFM profiling and COMSOL simulations shows that stroboSCAT can also access interfacial heat dissipation in layered structures. In conclusion, stroboSCAT emerges as a versatile platform to disentangle electronic and thermal transport in nanomaterials, providing mechanistic insight and quantitative parameters for the design of efficient and stable optoelectronic devices.