High-Pressure Study of Molecular Solids and 1D Nanostructures by Vibrational Spectroscopy and Synchrotron X-ray Diffraction

Pressure plays a critical role in regulating the structures and properties of materials. Since Percy Bridgeman was recognized by the 1946 Nobel Prize for his contribution to high-pressure physics, high-pressure research as an interdisciplinary area has attracted extensive attentions. Nowadays, the high-pressure research involves broad frontier areas, such as chemistry, physics, biology, material and earth science. For instance, brand new classes of inorganic materials of unusual stoichiometries and crystal structures, with wide range of optical, mechanical, electronic and magnetic properties, have been produced at high pressure. Pressure-induced structural transformations between crystalline and amorphous materials, as well as among insulators, conductors and even superconductors, have been extensive documented. This Ph.D. work focused on investigating pressure-induced structural transformations in materials and understanding the involved chemistry, which was assembled into two parts. In part I, two molecular systems, chlorocyclohexane (CCH) and azobenzene (AB), were investigated to examine the pressure effect on chemical conformations, stability of ring structures and reactivities using Raman spectroscopy. For CCH, pressure-induced conformational change as well as rich phase transformations were observed upon compression. Both AB and hydrazobenzene (HAB) underwent a phase transition at a similar pressure. However, origins of these phase transformations were drastically different. High-pressure structures of both molecules were examined based on spectroscopic data. Their distinctive high-pressure behaviors were analyzed and interpreted with the aid of ab initio molecular orbital calculations. Part II focuses on pressure-induced structural transformations of one dimensional (1D) nanomaterials using vibrational spectroscopy and synchrotron X-ray diffraction. Individual studies were first carried out on BN nanotubes and GaN nanowires aiming at investigating their high-pressure behaviors. Then, systematic studies were conducted on TiO2 and ZnO nanowires focusing on the size- and morphology- effect on their high-pressure behaviors. These 1D nanomaterials behaved dramatically different from bulk counterparts and nanoparticles, in terms of phase transition pressure, phase transition sequence, and compressibility. In addition, morphology of each material before and after compression was examined by scanning electron microscope. Our studies provide more insight into the understanding of unique high-pressure behaviors of nanomaterials and show profound implications for producing controlled structures with new applications achieved by combined pressure-morphology tuning

HIGH-PRESSURE STUDY OF NANOSTRUCTURED SnO2 BY RAMAN SPECTROSCOPY AND X-RAY DIFFRACTION WITH SYNCHROTRON RADIATION

Tin dioxide (SnO2) nanowires and nanobelts were compressed in a diamond anvil cell to around 40 gigapascal (GPa) at room temperature followed by decompression. Raman spectroscopy was employed to monitor the pressure effect on the SnO2 nanobelts. Synchrotron X-ray powder diffraction measurements were also carried out on both nanomaterials during both compression and decompression. The diffraction patterns were analyzed quantitatively by the Rietveld refinement. Pressure-induced phase transformations were observed in both nanomaterials and compared with those observed previously in the bulk materials. However, transition pressures, phase abundances, reversibilities, as well as compressiblities were found to be different from those of the bulk materials in both nanomorphologies. The mechanisms attributed to these differences were discussed. Finally, scanning electron microscopy (SEM) images of nanobelts and nanowires were collected before compression and after decompression. Morphologies were found to be significantly altered by high pressures, providing insight into these pressure-induced transformation

Multicell Edge Coverage Enhancement Using Mobile UAV-Relay

Unmanned aerial vehicle (UAV)-assisted communication is a promising technology in future wireless communication networks. UAVs can not only help offload data traffic from ground base stations (GBSs) but also improve the Quality of Service (QoS) of cell-edge users (CEUs). In this article, we consider the enhancement of cell-edge communications through a mobile relay, i.e., UAV, in multicell networks. During each transmission period, GBSs first send data to the UAV, and then the UAV forwards its received data to CEUs according to a certain association strategy. In order to maximize the sum rate of all CEUs, we jointly optimize the UAV mobility management, including trajectory, velocity, and acceleration, and association strategy of CEUs to the UAV, subject to minimum rate requirements of CEUs, mobility constraints of the UAV, and causal buffer constraints in practice. To address the mixed-integer nonconvex problem, we transform it into two convex subproblems by applying tight bounds and relaxations. An iterative algorithm is proposed to solve the two subproblems in an alternating manner. Numerical results show that the proposed algorithm achieves higher rates of CEUs as compared with the existing benchmark schemes

Spatiotemporal coupled-mode equations for arbitrary pulse transformation

Spatiotemporal modulation offers a variety of opportunities for light manipulations. In this paper, we propose a way towards arbitrary transformation for pulses sequentially propagating within one waveguide in space via temporal waveguide coupling. The temporal waveguide coupling operation is achieved by spatiotemporally modulating the refractive index of the spatial waveguide with a traveling wave through segmented electrodes. We derive the temporal coupled-mode equations and discuss how systematic parameters affect the temporal coupling coefficients. We further demonstrated a temporal Mach-Zehnder interferometer and universal multiport interferometer, which enables arbitrary unitary transformation for pulses. We showcase a universal approach for transforming pulses among coupled temporal waveguides, which requires only one spatial waveguide under spatiotemporal modulation, and hence provide a flexible, compact, and highly compatible method for optical signal processing in time domain

Topological Dissipative Photonics and Topological Insulator Lasers in Synthetic Time-Frequency Dimensions

The study of dissipative systems has attracted great attention, as dissipation engineering has become an important candidate towards manipulating light in classical and quantum ways. Here,we investigate the behavior of a topological system with purely dissipative couplings in a synthetic time-frequency space. An imaginary bandstructure is shown, where eigen-modes experience different eigen-dissipation rates during the evolution of the system, resulting in mode competition between edge states and bulk modes. We show that distributions associated with edge states can dominate over bulk modes with stable amplification once the pump and saturation mechanisms are taken into consideration, which therefore points to a laser-like behavior for edge states robust against disorders. This work provides a scheme for manipulating multiple degrees of freedom of light by dissipation engineering, and also proposes a great candidate for topological lasers with dissipative photonics

Author's personal copy Pressure-induced morphology-dependent phase transformations of nanostructured tin dioxide

a b s t r a c t Two morphologies of nanostructured tin dioxide (SnO 2 ) (i.e., nanobelts and nanowires) were compressed in diamond anvil cells up to 38 GPa followed by decompression. In situ Raman spectroscopy and synchrotron X-ray diffraction were employed to monitor the structural transformations. It was found that nanostructured SnO 2 behaved drastically differently than bulk material in terms of transformation pressures, phase stability regions and compressibility. These findings provide new insight into the unique pressure behaviours of nanostructured materials and have profound implications for producing controlled structures with new applications achieved by combined pressure-morphology tuning

Influence of dust on temperature measurement using infrared thermal imager

Temperature measurement by infrared thermal imager is an attractive technique in many fields, and it is of great importance to ensure the measurement accuracy of the infrared thermal imager. Aiming at the influence of dust on the temperature measurement of infrared thermal imager, this paper summarized the dust influence into three categories: dust on the surface of the measured object, dust on the infrared thermal imager’s lens and dust in the optical path between the measured object and the infrared thermal imager, and conducted three dust experiments. To quantify the measurement errors caused by dust, the infrared thermal image features that are affected by dust are extracted and a compensation model is established based on polynomial regression. The results indicate that dust can introduce measurement errors of infrared thermal imager and the proposed compensation method can compensate for the measurement errors caused by dust and improve the accuracy of infrared thermal imager

A Generalized Look at Federated Learning: Survey and Perspectives

Federated learning (FL) refers to a distributed machine learning framework involving learning from several decentralized edge clients without sharing local dataset. This distributed strategy prevents data leakage and enables on-device training as it updates the global model based on the local model updates. Despite offering several advantages, including data privacy and scalability, FL poses challenges such as statistical and system heterogeneity of data in federated networks, communication bottlenecks, privacy and security issues. This survey contains a systematic summarization of previous work, studies, and experiments on FL and presents a list of possibilities for FL across a range of applications and use cases. Other than that, various challenges of implementing FL and promising directions revolving around the corresponding challenges are provided.Comment: 9 pages, 2 figure

Albiflorin attenuates inflammation and apoptosis by upregulating AMPK-mediated expression of CDX2 in a mouse model of ulcerative colitis

Purpose: To investigate the mechanism underlying the ameliorative effect of albiflorin (AF) on ulcerative colitis (UC) in dextran sulphate sodium (DSS)-induced mice model. Method: Female C57BL/6 mice were administered DSS to establish a mice model of UC. After one week, the mice received AF, and the body weight and length of colon were measured. The histopathological features of colon tissues treated with hematoxylin-eosin (H & E) stain were examinedby microscopy. Expression of inflammatory cytokines and apoptosis-related proteins were determined using enzyme-linked immunosorbent assay (ELISA) and western blotting. Results: The relative abundance of goblet cells and crypts of mice were significantly reduced in DSSinduced UC mice model; furthermore, focal ulcers and mucosal damage were apparent. Moreover, treatment with DSS decreased body weight and colon length, downregulated Bcl-2 and AMPK pathwayrelated proteins, increased inflammatory cytokines levels, and upregulated Bax and cleaved caspase-3. In contrast, treatment with AF completely ameliorated DSS-induced effects. Conclusion: AF treatment attenuated DSS-induced inflammation response and apoptosis via AMPK pathway and modulation of CDX2 expression in UC mice model. Keyword: Albiflorin, Ulcerative colitis, AMPK, CDX2, Apoptosi