Polaron Dynamics in Conjugated Polymer Nanoparticles and Applications in Superresolution Imaging

Charge carrier dynamics in conjugated polymers are of fundamental interest as they directly affect the performance of conjugated polymer based devices. Through efficient fluorescence quenching by hole polarons, dynamics of individual charge carriers, including generation, recombination, and transport, can be observed through single particle fluorescence study of conjugated polymers nanoparticles (CPNs). In this dissertation, the hopping dynamics of hole polarons in CPNs were studied using superresolution microscopy. Due to quenching of the local fluorescence, a hole polaron can form a “dark spot” that moves with the polaron and results in displacements in the fluorescence centroid. These position fluctuations were used to track the nanoscale motion of hole polarons. In long trajectories, we observed random walk-like behavior consistent with multiple trap sites, whereas in some short segments, repeated hopping between two traps was observed. The hopping times range from a few ms to seconds, following a power law distribution, while the hopping distances range from 2-5 nm, following an exponential distribution. From the hopping time distribution, we estimated the energy barrier height for polaron hopping in CPNs to be from 430 to 570 meV, indicating the presence of deep traps with nearest-neighbor distances of 2-5 nm, consistent with a low or moderate density of structural or chemical defects dominating charge transport at low carrier densities. Based on polaron generation and recombination dynamics, we developed a new class of photoswitchable nanoparticles for superresolution imaging. By doping nanoparticles of conjugated polymer PFBT with fullerene derivative PCBM, a large population of polarons can be efficiently generated in CPNs, sufficient to nearly completely quench the nanoparticle fluorescence. However, fluctuations in the number of quenchers lead to occasional bursts of fluorescence. 3-5×104 photons were detected during each burst event (1-2 orders of magnitude brighter than photoswitchable dyes), resulting in a localization precision of 0.6 nm, ~4 times better than the typical resolution obtained by localization of dye molecules. In addition, since polaron generation is a photo-driven process, we demonstrated that the blinking duty cycle of PCBM doped CPNs can be controlled by excitation intensity as well as by PCBM fraction. The unprecedented brightness and tunable spontaneous photoswitching properties of PCBM doped PFBT CPNs make them a promising class of superresolution probes, which provide clear advantages for imaging of various biological systems

Superresolution mapping of energy landscape for single charge carriers in plastic semiconductors

The performance of conjugated polymer devices is largely dictated by charge transport processes. However, it is difficult to obtain a clear relationship between conjugated polymer structures and charge transport properties, due to the complexity of the structure and the dispersive nature of charge transport in conjugated polymers. Here, we develop a method to map the energy landscape for charge transport in conjugated polymers based on simultaneous, correlated charge carrier tracking and single-particle fluorescence spectroscopy. In nanoparticles of the conjugated polymer poly[9,9-dioctylfluorenyl-2,7-diyl)-co-1,4-benzo-{2,1′-3}-thiadiazole)], two dominant chain conformations were observed, a blue-emitting phase (λmax = 550 nm) and a red-emitting phase (λmax = 595 nm). Hole polarons were trapped within the red phase, only occasionally escaping into the blue phase. Polaron hopping between the red-emitting traps was observed, with transition time ranging from tens of milliseconds to several seconds. These results provide unprecedented nanoscale detail about charge transport at the single carrier level

Crowdsourcing based Room Localization on Smartphones

People spend approximately 90% of their time indoors, and human indoor activities are strongly correlated with the rooms they are in. Room localization, which identifies the room a person or smartphone is in, provides a powerful tool for characterizing human indoor activities and helping address challenges in public health, productivity, building management, etc. Designing a room localization system that is practically useful in real-world environments is challenging. First, due to the complex multi-path propagation problem, Wi-Fi signals obtained by smartphones are dynamic and noisy. Such noise obscures the unique relationship between Wi-Fi signals and individual rooms. Second, existing room localization methods require labor-intensive manual annotation of individual rooms. The process is time-consuming and expensive, which is a key limitation of existing room localization applications. Third, knowledge of indoor floorplans is often required by room localization applications. However, indoor floorplans are either unavailable or obtaining them requires slow, tedious, and error-prone manual labor. In addition, the overhead of room localization, e.g., energy consumption, imposed on personal smartphones must be low. To tackle those challenges, this thesis proposed a set of techniques: (1) an accurate temporal n-gram augmented Bayesian room positioning method that leverages the ordered sequence information of access points and users' daily motion pattern among rooms; (2) an automatic room fingerprinting approach that identifies in-room occupancy ``hotspot(s)" using density of Wi-Fi signals, and then learns the inter-zone correlation -- thereby distinguishing different rooms; (3) an automatic floorplan construction method that determines the geometries of individual rooms, as well as room adjacency information, and then constructs an indoor floorplan through hallway layout learning and force directed dilation; and (4) an energy-efficient trip detection framework that consists of two modes: the deliberation mode learns cell-id patterns using GPS/Wi-Fi based localization methods, and the intuition mode only uses cell-ids and learned patterns for trip detection

Block iliac bone grafting enhances osseous healing of alveolar reconstruction in older cleft patients : a radiological and histological evaluation

Older alveolar cleft patients (&12 years old) often have wide bone defect as well as teeth loss, resulting in poor osseous healing with conventional alveolar bone grafting (ABG). In this study, we investigated a surgical technique of block iliac bone grafting for the alveolar cleft reconstruction and evaluated the clinical and radiological outcomes of these cleft patients. Fifteen patients were included in this study. All cases received preoperative cone bean computed tomography (CBCT) scans for the alveolar cleft evaluation. Osseous outcomes of block iliac bone grafting were assessed at 1 week, 3- and 6-month postoperatively. Volume changes and bone resorption rates were calculated using the measurement modules of Simplant software. Bone samples from one patient undergoing dental implantation were assessed by micro-CT and histological examination. The morbidities of donor-site were analyzed by clinical examination and questionnaire survey. The average age of the case series was 18.53±2.50 years. The intraoral incision of thirteen cases healed well. However, two cases had oronasal fistula and graft exposure at 1-week postoperatively. The results of follow-up CBCT scans showed significant resistance to radiation on both sides of the bone graft, suggesting a good osseous healing and new bone formation. The mean residual bone volume was 1.68±0.26 cm3, 1.29±0.23 cm3 and 1.15±0.23 cm3 at 1-week, 3- and 6-month postoperatively. Correspondingly, the mean bone resorption rates in 3- and 6-month postoperative were 21.78±6.88% and 30.66±8.97%, respectively. From micro-CT and HE examinations, the block bone samples exhibited a cancellous structure in which mature bone trabecula and functional blood vessels appeared. The average scores of donor-site morbidities were drastically decreased at 3- and 6-month postoperatively compared with those at 1-week postoperatively. Our results demonstrated that block iliac bone grafting could achieve satisfying osseous outcomes in older alveolar cleft patients, and this technique provided favorable bony condition for further treatments, especially dental implantation