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Open Access Publications from the University of California

Scholarly Works (4 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2023)

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.

    Cover page: On Light Scattering from Photoexcitations: Making Movies of Nanoscale Energy Transport in Emerging Semiconductors
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2020)
    The ability of energy carriers to move between atoms and molecules underlies biochemical and material function. Understanding and controlling energy flow, however, requires observing it on ultrasmall and ultrafast spatio-temporal scales, where energetic and structural roadblocks dictate the fate of energy carriers. Here, we developed a non-invasive optical scheme that leverages non-resonant interferometric scattering to track tiny changes in material polarizability created by energy carriers. We thus map evolving energy carrier distributions in four dimensions of spacetime with few-nanometre lateral precision and directly correlate them with material morphology. We visualize exciton, charge and heat transport in polyacene, silicon and perovskite semiconductors and elucidate how disorder affects energy flow in three dimensions. For example, we show that morphological boundaries in polycrystalline metal halide perovskites possess lateral- and depth-dependent resistivities, blocking lateral transport for surface but not bulk carriers. We also reveal strategies for interpreting energy transport in disordered environments that will direct the design of defect-tolerant materials for the semiconductor industry of tomorrow.
      Cover page: Imaging material functionality through three-dimensional nanoscale tracking of energy flow
      • Article
      • Peer Reviewed
      UC Davis Previously Published Works (2023)
      The minimization of pain in research animals is a scientific and ethical necessity. Carprofen is commonly used for pain management in mice; however, some data suggest that the standard dosage of 5 mg/kg may not provide adequate analgesia after surgery. We hypothesized that a higher dose of carprofen in mice would reduce pain-associated behaviors and improve analgesia without toxic effects. A pharmacokinetic study was performed in mice given carprofen subcutaneously at 10 or 20 mg/kg. Plasma concentrations were measured at 0.25, 0.5, 1, 2, 4, 8, 12, 24, and 48 h after dosing (n = 3 per time point and treatment). At these doses, plasma levels were above the purported therapeutic level for at least 12 and 24 h, respectively, with respective half-lives of 14.9 and 10.2 h. For the efficacy study, 10 mice per group received anesthesia with or without an ovariectomy. Mice were then given 5 or 10 mg/kg of carprofen, or saline subcutaneously every 12 h. Orbital tightening, arched posture, wound licking, rearing, grooming, nesting behavior, and activity were assessed before surgery and at 4, 8, 12, 24, and 48 h after surgery. The von Frey responses were assessed before and at 4, 12, 24, and 48 h after surgery. The efficacy study showed that all surgery groups had significantly higher scores for orbital tightening, arched posture, and wound licking than did the anesthesia-only groups at 4, 8, 12, and 24-h time points. At the 8 h time point, the surgery groups treated with carprofen had significantly lower arched posture scores than did the surgery group treated with saline only. No significant differences were found between carprofen treatment groups for rearing, grooming, von Frey, activity, or nesting behavior at any time point. These results indicate that subcutaneous carprofen administered at these doses does not provide sufficient analgesia to alleviate postoperative pain in female CD1 mice.
        Cover page: A Pharmacokinetic and Analgesic Efficacy Study of Carprofen in Female CD1 Mice.
        • Article
        • Peer Reviewed
        LBL Publications (2023)
        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, we employ stroboscopic optical scattering microscopy (stroboSCAT) to simultaneously map both heat and exciton populations in few-layer MoS2 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.
          Cover page: Detecting, Distinguishing, and Spatiotemporally Tracking Photogenerated Charge and Heat at the Nanoscale.
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