UC Berkeley

There are many types of excitations that may coexist and interact after light illumination in a semiconductor. In emerging semiconductors like low-dimensional and solution-processed materials, these excitations often encounter nanoscale disorder over their lifetime, including point defects, different crystalline phases, boundaries, interfaces and dielectric disorder. In addition, distinguishing between different types of excitations is vital for teasing out the intrinsic material properties so that they may be tuned and optimized for optoelectronic device applications. Optical scattering is a sensitive probe that, when incorporated into a time-resolved pump-probe microscope, correlates sample morphology to nanoscale energy transport. The unique spectrally-dependent scattering contrast from different photoexcitations and their spatiotemporal evolution together permit the distinction between coexisting excitations. We call this approach stroboscopic scattering microscopy (stroboSCAT) and demonstrate its versatility across a range of organic, organic–inorganic, and inorganic semiconductors, enabling model-free structure–function correlations with few-nanometer precision in all spatial dimensions.

Chapter 1 provides an introduction to the two classes of materials that are explored in this dissertation with an emphasis on the 2D material archetype, molybdenum disulfide (MoS2). Next, we introduce the playing field of energy flow microscopies well-suited to study the energy transport and transduction in emerging semiconductors.

Chapter 2 explores the theoretical underpinnings and practical operation of the stroboSCAT microscope. We provide background on two approaches for understanding the optical properties of materials: scattering and frequency-dependent complex dielectric functions. We describe the photoinduced changes to the optical response and how they may be probed with transient scattering approaches in transmission or reflection. After a crash course in basic microscopy principles, the nuts and bolts of the stroboSCAT setup are described in detail, including its timing control, resolution limits, and stability.

Chapter 3 illustrates how stroboSCAT is a versatile technique that directly correlates microscopic structural motifs and macroscopic function across a broad range of materials. In TIPS-Pentacene, a molecular crystal, exciton transport is hindered by material interfaces. In a thin film halide perovskite, charge carriers find their way around resistive morphological boundaries using 3D trajectories. Finally, in silicon, oppositely signed contrast for charge carriers and heat, along with a separation in time scales, enables isolating the dynamics of each population even though they overlap.

Chapter 4 takes stroboSCAT into the 2D realm in several layered architectures and an in-depth exploration of exciton and heat transport and transduction in four-layer MoS2. We combine near- and far-from resonant stroboSCAT measurements with temperature-dependent reflectance contrast spectroscopy to isolate overlapping exciton and heat distributions. This study demonstrates the importance of access to spatially resolved information and tunable contrast for directly discerning photoinduced heat from charge without complex models or assumptions.

As summarized in Chapter 5, this dissertation illustrates the advantages of using transient light scattering to probe photoinduced changes in the optical response of a wide range of novel semiconducting materials. With high spatiotemporal resolution and sensitivity, tunable imaging contrast and direct structure-function correlation, stroboSCAT is a powerful tool for uncovering the intricacies of energy flow on relevant length- and time scales without averaging over heterogeneous energetic landscapes or misinterpreting the emergent dynamics when different excitations coexist.