Spatio-temporal-spectral imaging of photo-carrier dynamics in perovskites and two-dimensional semiconductors

Laser-illuminated microwave impedance microscopy (iMIM) is a scanning probe microscopy technique that measures the spatiotemporal and spectral responses of photo-carriers in materials under laser illumination. In this dissertation, we use iMIM to study the photo-carrier dynamics in perovskites and two-dimensional (2D) semiconductors, which are promising materials for optoelectronic applications. We investigate the effects of laser power, wavelength, and sample temperature on the iMIM signals, and we reveal the underlying mechanisms of photo-carrier generation, transport, and recombination in these materials. We also demonstrate the potential of iMIM for imaging the spatial distribution of defects, grain boundaries, and heterostructures in perovskites and 2D semiconductors. Our results provide new insights into the photophysical properties of these materials and pave the way for their optimization and integration in future devices. In this dissertation, I introduce the background and motivation of studying perovskites and 2D semiconductors using iMIM in Chapter 1. In Chapter 2, I explain the basic principles and finite-element analysis of microwave impedance microscopy (MIM). In Chapter 3, I describe the optical MIM systems that I have contributed to or led their development, including systems for photoconductivity mapping, carrier diffusion measurement, time-resolved iMIM, and iMIM spectroscopy. In Chapter 4, I present the paper that utilized the illuminated MIM systems to reveal the dynamics of photo-carriers in organic-inorganic perovskites. I show how defects and varying types of carriers influence the optoelectronic properties of perovskite solar cells on a microscopic scale. In Chapter 5, I illustrate how our innovatively designed iMIM spectroscopy equipment can effectively reveal the exciton properties inherent in monolayer transition metal dichalcogenides (TMDs), with a special focus on WS₂ and WSe₂. I discuss how exciton binding energy, lifetime, and density depend on laser wavelength. In Chapter 6, I summarize the main findings and contributions of this dissertation and provide some outlooks for future research directions.Physic

Spatial scale effect of surface routing and its parameter upscaling for urban flood simulation using a grid‐based model

Urban catchments are characterized by a wide variety of complex juxtapositions and surface compositions that are linked to multiple overland flow paths. Their extremely high spatial heterogeneity leads to great sensitivity of hydrologic simulation to the scale variation of calculation units. Although extensive efforts have been made for investigating the scale effects and indicate its significance, less is understood of how routing features vary with spatial scales and further how the variation of routing features influences the hydrological response. In this paper, a grid-based distributed urban hydrological model is applied to study spatial scale effects ranging from 10 to 250 m. Two parameters are proposed to quantitatively depict the routing features of overland flow specified for impervious and pervious areas. The results show that routing features are quite sensitive to spatial resolution. Large differences among simulations exist in the infiltration amounts attributed to the combined effects of the two routing parameters, which leads to opposite effects for both total flow volume and peak flow for various rainfall events. The relationship of the key model parameters at different spatial resolutions can be explicitly expressed by corresponding routing features. With this relationship, parameters transfer among different spatial scales can be realized to obtain consistent simulation results. This study further revealed the quantitative relationship between spatial scales, routing features and the hydrologic processes, and enabled accurate and efficient simulations required by real time flooding forecasting and land-atmosphere coupling, while fully taking the advantages of detailed surface information. Plain Language Summary Given the inherent complex underlying surface compositions and overland flow paths in urban areas, underlying high spatial resolution surface data eventually become necessary. Unfortunately, high resolution modelling in urban catchment is still challenging in terms of computational restricts, proper setting up of parameters etc., due to the high spatial heterogeneity. Practical simulation requirements often limit the use of high resolution models, as in the case of real time prediction of urban flooding, the coupling of land-atmosphere processes. Therefore, it is necessary to investigate the scale effects and its mechanism, and then to explore an accommodation approach to enable precise flooding prediction with a coarse model. For grid-based and distributed hydrologic models, the mosaic method can basically eliminate the scale effects on the runoff generation process. However, the scale effects on overland flow routing remain insufficiently understood, and to help understand the scale effects, simulations were performed under five different resolutions, ranging from 10 m to 250 m, for various rainfall events. Two physical parameters are introduced to quantify the scale effects on routing features. Three variables are concurrently calculated to assess the effects on modeling outputs. The results indicate that routing features are sensitive to changes in spatial resolution, which results in opposite effects on simulation results under different rainfall conditions. In conclusion, an accommodation approach is proposed based on the affecting mechanism

Evolutionary Stages and Disk Properties of Young Stellar Objects in the Perseus Cloud

We investigated the evolutionary stages and disk properties of 211 Young stellar objects (YSOs) across the Perseus cloud by modeling the broadband optical to mid-infrared (IR) spectral energy distribution (SED). By exploring the relationships among the turnoff wave bands lambda_turnoff (longward of which significant IR excesses above the stellar photosphere are observed), the excess spectral index alpha_excess at lambda <~ 24 microns, and the disk inner radius R_in (from SED modeling) for YSOs of different evolutionary stages, we found that the median and standard deviation of alpha_excess of YSOs with optically thick disks tend to increase with lambda_turnoff, especially at lambda_turnoff >= 5.8 microns, whereas the median fractional dust luminosities L_dust/L_star tend to decrease with lambda_turnoff. This points to an inside-out disk clearing of small dust grains. Moreover, a positive correlation between alpha_excess and R_in was found at alpha_excess > ~0 and R_in > ~10 $\times$ the dust sublimation radius R_sub, irrespective of lambda_turnoff, L_dust/L_star and disk flaring. This suggests that the outer disk flaring either does not evolve synchronously with the inside-out disk clearing or has little influence on alpha_excess shortward of 24 microns. About 23% of our YSO disks are classified as transitional disks, which have lambda_turnoff >= 5.8 microns and L_dust/L_star >10^(-3). The transitional disks and full disks occupy distinctly different regions on the L_dust/L_star vs. alpha_excess diagram. Taking L_dust/L_star as an approximate discriminator of disks with (>0.1) and without (<0.1) considerable accretion activity, we found that 65% and 35% of the transitional disks may be consistent with being dominantly cleared by photoevaporation and dynamical interaction respectively. [abridged]Comment: 31 pages, 13 figures, 2 tables. To appear in a special issue of RAA on LAMOST science

Experimental observation on beach evolution process with presence of artificial submerged sand bar and reef

For observation on the influence mechanism of environmentally and aesthetically friendly artificial submerged sand bars and reefs in a process-based way, a set of experiments was conducted in a 50 m-long flume to reproduce the cross-shore beach morphodynamic process under four irregular wave conditions. The beach behavior is characterized by the scarp (indicating erosion) and the breaker bar (indicating deposition), respectively, and the scarp location can be formulated as a linear equation regarding the natural exponential of the duration time. Overall, main conclusions are: (1) the cross-shore structure of significant wave height and set-up is mainly determined by the artificial reef (AR); (2) the cross-shore distribution of wave skewness, asymmetry, and undertow (indicating shoaling and breaking) is more affected by the artificial submerged sand bar (ASB); (3) the ASB deforms and loses its sand as it attenuates incident waves, which leads to a complex sediment transport pattern; (4) the scarp retreat is related to the beach state, which can be changed by the AR and the ASB; (5) the AR, the ASB, and their combination decrease wave attack on the beach. In conclusion, this study proves positive effects of the AR and the ASB in beach protection through their process-based influences on beach behaviors and beach states for erosive waves

Direct Visualization of Gigahertz Acoustic Wave Propagation in Suspended Phononic Circuits

We report direct visualization of gigahertz-frequency Lamb waves propagation in aluminum nitride phononic circuits by transmission-mode microwave impedance microscopy (TMIM). Consistent with the finite-element modeling, the acoustic eigenmodes in both a horn-shaped coupler and a sub-wavelength waveguide are revealed in the TMIM images. Using fast Fourier transform filtering, we quantitatively analyze the acoustic loss of individual Lamb modes along the waveguide and the power coupling coefficient between the waveguide and the parabolic couplers. Our work provides insightful information on the propagation, mode conversion, and attenuation of acoustic waves in piezoelectric nanostructures, which is highly desirable for designing and optimizing phononic devices for microwave signal processing and quantum information transduction.The TMIM work was supported by NSF Division of Materials Research Award DMR-2004536 and Welch Foundation Grant F-1814. The data analysis was partially supported by the NSF through the Center for Dynamics and Control of Materials, an NSF Materials Research Science and Engineering Center (MRSEC) under Cooperative Agreement DMR-1720595. The phononic waveguide work was supported by NSF Award EFMA-1741656 and EFMA-1641109. Part of this work was conducted at the Washington Nanofabrication Facility / Molecular Analysis Facility, a National Nanotechnology Coordinated Infrastructure (NNCI) site at the University of Washington with partial support from the National Science Foundation via awards NNCI- 1542101 and NNCI-2025489.Center for Dynamics and Control of Material

Nanoscale Conductivity Imaging of Correlated Electronic States in WSe2/WS2 Moir\'e Superlattices

We report the nanoscale conductivity imaging of correlated electronic states in angle-aligned WSe2/WS2 heterostructures using microwave impedance microscopy. The noncontact microwave probe allows us to observe the Mott insulating state with one hole per moir\'e unit cell that persists for temperatures up to 150 K, consistent with other characterization techniques. In addition, we identify for the first time a Mott insulating state at one electron per moir\'e unit cell. Appreciable inhomogeneity of the correlated states is directly visualized in the hetero-bilayer region, indicative of local disorders in the moir\'e superlattice potential or electrostatic doping. Our work provides important insights on 2D moir\'e systems down to the microscopic level.Comment: 12 Pages, 4 figure

Unveiling Defect-Mediated Carrier Dynamics in Monolayer Semiconductors by Spatiotemporal Microwave Imaging

The optoelectronic properties of atomically thin transition-metal dichalcogenides are strongly correlated with the presence of defects in the materials, which are not necessarily detrimental for certain applications. For instance, defects can lead to an enhanced photoconduction, a complicated process involving charge generation and recombination in the time domain and carrier transport in the spatial domain. Here, we report the simultaneous spatial and temporal photoconductivity imaging in two types of WS2 monolayers by laser-illuminated microwave impedance microscopy. The diffusion length and carrier lifetime were directly extracted from the spatial profile and temporal relaxation of microwave signals respectively. Time-resolved experiments indicate that the critical process for photo-excited carriers is the escape of holes from trap states, which prolongs the apparent lifetime of mobile electrons in the conduction band. As a result, counterintuitively, the photoconductivity is stronger in CVD samples than exfoliated monolayers with a lower defect density. Our work reveals the intrinsic time and length scales of electrical response to photo-excitation in van der Waals materials, which is essential for their applications in novel optoelectronic devices.Comment: 21 pages, 4 figure

Superior photo-carrier diffusion dynamics in organic-inorganic hybrid perovskites revealed by spatiotemporal conductivity imaging

The outstanding performance of organic-inorganic metal trihalide solar cells benefits from the exceptional photo-physical properties of both electrons and holes in the material. Here, we directly probe the free-carrier dynamics in Cs-doped FAPbI3 thin films by spatiotemporal photoconductivity imaging. Using charge transport layers to selectively quench one type of carriers, we show that the two relaxation times on the order of 1 μs and 10 μs correspond to the lifetimes of electrons and holes in FACsPbI3, respectively. Strikingly, the diffusion map- ping indicates that the difference in electron/hole lifetimes is largely compensated by their disparate mobility. Consequently, the long diffusion lengths (3~5 μm) of both carriers are comparable to each other, a feature closely related to the unique charge trapping and de- trapping processes in hybrid trihalide perovskites. Our results unveil the origin of superior diffusion dynamics in this material, crucially important for solar-cell applications.The research at UT-Austin was primarily supported by the NSF through the Center for Dynamics and Control of Materials, an NSF Materials Research Science and Engineering Center (MRSEC) under Cooperative Agreement DMR-1720595. The authors also acknowledge the use of facilities and instrumentation supported by the NSF MRSEC. K.L. and X.M. acknowledge the support from Welch Foundation Grant F-1814. X. Li acknowledges the support from Welch Foundation Grant F-1662. The tip-scan iMIM setup was supported by the US Army Research Laboratory and the US Army Research Office under Grants W911NF-16-1-0276 and W911NF-17-1-0190. The work at NREL was supported by the US DOE under Contract No. DE-AC36-08GO28308 with Alliance for Sustainable Energy, Limited Liability Company (LLC), the Manager and Operator of the National Renewable Energy Laboratory. K.Z., J.H., X.C., X.W., and Y.Y. acknowledge the support on charge carrier dynamics study from the Center for Hybrid Organic-Inorganic Semiconductors for Energy (CHOISE), an Energy Frontier Research Center funded by the Office of Basic Energy Sciences, Office of Science within the US DOE. F.Z. acknowledges the support on devices fabrication and characterizations from the De-Risking Halide PSCs program of the National Center for Photovoltaics, funded by the US DOE, Office of Energy Efficiency and Renewable Energy, Solar Energy Technologies Office.Center for Dynamics and Control of Material

Observation of Gigahertz Topological Valley Hall Effect in Nanoelectromechanical Phononic Crystals

Topological phononics offers numerous opportunities in manipulating elastic waves that can propagate in solids without being backscattered. Due to the lack of nanoscale imaging tools that aid the system design, however, acoustic topological metamaterials have been mostly demonstrated in macroscale systems operating at low (kilohertz to megahertz) frequencies. Here, we report the realization of gigahertz topological valley Hall effect in nanoelectromechanical AlN membranes. Propagation of elastic wave through phononic crystals is directly visualized by microwave microscopy with unprecedented sensitivity and spatial resolution. The valley Hall edge states, protected by band topology, are vividly seen in both real- and momentum-space. The robust valley-polarized transport is evident from the wave transmission across local disorder and around sharp corners, as well as the power distribution into multiple edge channels. Our work paves the way to exploit topological physics in integrated acousto-electronic systems for classical and quantum information processing in the microwave regime.This work was supported by the NSF through the Laboratory for Research on the Structure of Matter, an NSF Materials Research Science & Engineering Center (MRSEC; DMR-1720530). The TMIM work was supported by NSF Division of Materials Research Award DMR-2004536 and Welch Foundation Grant F-1814. The data analysis was partially supported by the NSF through the Center for Dynamics and Control of Materials, an NSF MRSEC under Cooperative Agreement DMR-1720595. This work was carried out in part at the Singh Center for Nanotechnology, which is supported by the NSF National Nanotechnology Coordinated Infrastructure Program under grant NNCI-2025608. The metamaterial design and simulation work was supported by the US Office of Naval Research (ONR) Multidisciplinary University Research Initiative (MURI) grant N00014- 20-1-2325 on Robust Photonic Materials with High-Order Topological Protection and grant N00014-21-1-2703. We would like to express our appreciation for useful discussions with Prof. Troy Olsson and Dr. Qian Niu.Center for Dynamics and Control of Material

CD13 Inhibition Enhances Cytotoxic Effect of Chemotherapy Agents

Multidrug resistance (MDR) of hepatocellular carcinoma is a serious problem. Although CD13 is a biomarker in human liver cancer stem cells, the relationship between CD13 and MDR remains uncertain. This study uses liver cancer cell model to understand the role of CD13 in enhancing the cytotoxic effect of chemotherapy agents. Cytotoxic agents can induce CD13 expression. CD13 inhibitor, bestatin, enhances the antitumor effect of cytotoxic agents. Meanwhile, CD13-targeting siRNA and neutralizing antibody can enhance the cytotoxic effect of 5-fluorouracil (5FU). CD13 overexpression increases cell survival upon cytotoxic agents treatment, while the knockdown of CD13 causes hypersensitivity of cells to cytotoxic agents treatment. Mechanistically, the inhibition of CD13 leads to the increase of cellular reactive oxygen species (ROS). BC-02 is a novel mutual prodrug (hybrid drug) of bestatin and 5FU. Notably, BC-02 can inhibit cellular activity in both parental and drug-resistant cells, accompanied with significantly increased ROS level. Moreover, the survival time of Kunming mice bearing H22 cells under BC-02 treatment is comparable to the capecitabine treatment at maximum dosage. These data implicate a therapeutic method to reverse MDR by targeting CD13, and indicate that BC-02 is a potent antitumor compound