Browsing by Author "Lv, Bing"
Now showing 1 - 20 of 20
- Results Per Page
- Sort Options
Item Carrier Recombination in Perovskites 3D Through 0D(2022-05-01T05:00:00.000Z) Zheng, Yangzi; Campbell, Zachary; Malko, Anton V.; Gartstein, Yuri; Lv, Bing; Shi, Xiaoyan; Zheng, JieLead-halide perovskites have long been demonstrated as materials with exceptional structural and optoelectronic properties. As an important crystalline material, lead-halide perovskites have potential applications in lasers, light-emitting diodes (LEDs), photovoltaic solar cells, photon detectors and biosensors, etc. By tailoring the morphological dimensionality, low-dimensional possessed distinct properties from their bulk (3D) counterparts. Due to the strong quantum confinement and octahedral site isolation, these low dimensional metal halide hybrids at the molecular level exhibit remarkable and unique properties that are significantly different from those of ABX3 perovskites. Considering the rapid development of low dimensional metal halide perovskites, we will discuss the synthesis, characterization, application, computational studies and compare various metal halide perovskites ranging from 3D through 0D. Finally, we show that a modified atomic layer deposition technique may be successfully used to protect 0D perovskite against external environment.Item Cell Nuclei Segmentation Using Deep Learning Techniques(2021-08-01T05:00:00.000Z) K. C. Khatri, Rajendra; Cao, Yan; Lv, Bing; Dabkowski, Mieczyslaw K.; Ramakrishna, Viswanath; Lou, YifeiPathological examination usually involves manual inspection of hematoxylin and eosin (H&E)- stained images, which is labor-intensive, prone to significant variations, and lacking reproducibility. One of the fundamental tasks to automate this process is to find all the cell nuclei in the H&E-stained images for further analysis. We attempt this problem using deep learning techniques. First, we introduce a semantic pixel-wise segmentation technique using dilated convolutions. We show that dilated convolutions are superior in extracting information from textured images. H&E-stained images are highly textured, which makes dilated convolutions an ideal technique to apply. Our dilated convolutional network (DCN) is constructed based on SegNet, a deep convolutional encoder-decoder architecture. Dilated convolution layers with increased dilation factors are used in the encoder to preserve image resolution. Dilated convolution layers with decreased dilation factors are used in the decoder to reduce gridding artifacts. Our DCN network was tested on synthetic data sets and a publicly available data set of H&E-stained images. We achieve better segmentation results than state-of-the-art. To further separate the instance of each cell nuclei, we adapt our DCN with a single shot multibox detector (SSD) and achieve promising results. Our methods are computationally efficient and can be run on a personal laptop computer. This work is the first step to wards using mathematical models to generate diagnostic inferences and providing clinically actionable knowledge to physicians and patients.Item Chemical Doping and High-Pressure Studies of Layered β-PdBi₂ Single Crystals(American Physical Society, 2015-11-04) Zhao, K.; Lv, Bing; Xue, Y. -Y; Zhu, X. -Y; Deng, L. Z.; Wu, Z.; Chu, C. W.; Lv, BingWe have systematically grown large single crystals of the layered compounds β-PdBi₂, and both the hole-doped PdBi₂₋ₓPb₂ and the electron-doped NaₓPdBi₂, and studied their magnetic and transport properties. Hall effect measurements on PdBi₂, PdBi₁₈Pb₀₂, and Na₀.₀₅₇PdBi₂ show that the charge transport is dominated by electrons in all of the samples. The electron concentration is substantially reduced upon Pb doping in PdBi₂₋ₓPbₓ and increased upon Na intercalation in NaₓPdBi₂, indicating effective hole doping by Pb and electron doping by Na. We observed a monotonic decrease of the superconducting transition temperature (T_c) from 5.4 K in undoped PdBi₂ to less than 2 K for x > 0.35 in hole-doped PdBi₂₋ₓPbₓ. Meanwhile, a rapid decrease of T_c with Na intercalation is also observed in the electron-doped NaₓPdBi₂, which is in disagreement with the theoretical expectation. In addition, both the magnetoresistance and Hall resistance further reveal evidence for a possible spin excitation associated with Fermi surface reconstruction at ∼50 K in the Na-intercalated PdBi₂ sample. The complete phase diagram is thus established from hole doping to electron doping. Meanwhile, a high-pressure study of the undoped PdBi₂ shows that the T_c is linearly suppressed under pressure with a dT_c/dP coefficient of -0.28 K/GPa.Item Evidence for Defect-Induced Superconductivity up to 49 K in (Ca₁₋ₓ)Fe₂As₂(Amer Physical Soc, 2016-02-12) Deng, L. Z.; Lv, Bing; Zhao, K.; Wei, F. Y.; Xue, Y. Y.; Wu, Z.; Chu, C. W.; Lv, BingTo explore the origin of the unusual nonbulk superconductivity with a T_c up to 49 K reported in the rare-earth-doped CaFe₂As₂, the chemical composition, magnetization, specific heat, resistivity, and annealing effect are systematically investigated on nominal Ca₁₋ₓ Rₓ)Fe₂As₂ single crystals with different x and R = La, Ce, Pr, and Nd. All display a doping-independent T_c once superconductivity is induced, a doping-dependent low field superconducting volume fraction ʄ, and a large magnetic anisotropy η the superconducting state, suggesting a rather inhomogeneous superconducting state in an otherwise microscale homogenous superconductor. The wavelength dispersive spectroscopy and specific heat show the presence of defects that are closely related to ʄ, regardless of the R involved. The magnetism further reveals that the defects are mainly superparamagnetic clusters for R = Ce, Pr, and Nd with strong intercluster interactions, implying that defects are locally self-organized. Annealing at 500 ⁰C, without varying the doping level x, suppresses ʄ profoundly but not the T_c. The above observations provide evidence for the crucial role of defects in the occurrence of the unusually high T_c ~ to 49K in Ca₁₋ₓ Rₓ Fe₂As₂ and are consistent with the interface-enhanced superconductivity recently proposed.Item High Thermal Conductivity in Cubic Boron Arsenide Crystals(American Association for the Advancement of Science) Li, Sheng; Zheng, Q.; Lv, Y.; Liu, Xiaoyuan; Wang, X.; Huang, P. Y.; Cahill, D. G.; Lv, Bing; Li, Sheng; Liu, Xiaoyuan; Lv, BingThe high density of heat generated in power electronics and optoelectronic devices is a critical bottleneck in their application. New, high thermally-conducting materials are needed to effectively dissipate heat and thereby enable enhanced performance of power controls, solid-state lighting, communication, and security systems. We report our experimental discovery of high thermal conductivity of 1000 ± 90 W/m/K at room temperature in cubic boron arsenide (BAs) grown through modified chemical vapor transport technique. Such thermal conductivity is a factor of 3 higher than that of silicon carbide and surpassed only by diamond and the basal plane value of graphite. This work establishes BAs as the first realization of a new class of ultrahigh thermal conductivity materials predicted by a recent theory, and a potential revolutionary thermal management material.Item High Thermal Conductivity in Isotopically Enriched Cubic Boron Phosphide(WILEY-VCH Verlag GmbH & Co.) Zheng, Q.; Li, S.; Li, C.; Lv, Y.; Liu, X.; Huang, P. Y.; Broido, D. A.; Lv, Bing; Cahill, D. G.; Li, S.; Liu, X.; Lv, BingZinc blende boron arsenide (BAs), boron phosphide (BP), and boron nitride (BN) have attracted significant interest in recent years due to their high thermal conductivity (Λ) predicted by first-principles calculations. This research reports the study of the temperature dependence of Λ (120 K < T < 600 K) for natural isotope-abundance BP and isotopically enriched 11BP crystals grown from modified flux reactions. Time-domain thermoreflectance is used to measure Λ of sub-millimeter-sized crystals. At room temperature, Λ for BP and 11BP is 490 and 540 W m−1 K−1, respectively, surpassing the values of conventional high Λ materials such as Ag, Cu, BeO, and SiC. The Λ of BP is smaller than only cubic BN, diamond, graphite, and BAs among single-phase materials. The measured Λ for BP and 11BP is in good agreement with the first-principles calculations above 250 K. The quality of the crystals is verified by Raman spectroscopy, X-ray diffraction, and scanning transmission electron microscopy. By combining the first-principles calculations and Raman measurements, a previously misinterpreted Raman mode is reassigned. Thus, BP is a promising material not only for heat spreader applications in high-power microelectronic devices but also as an electronic material for use in harsh environments. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimItem Light-emitting Electrochemical Cells: Temperature Dependence and Host-guest-systems(2022-05-01T05:00:00.000Z) Bowler, Melanie; Slinker, Jason D; Chiou, Sy Han Steven; Lv, Bing; Goeckner, Matthew J; Smaldone, Ronald A; Kolodrubetz, MichaelLight-emitting electrochemical cells (LECs) yield high efficiency and long-lasting performance in a simple device architecture. Due to the ionic nature of these devices, electric double layer formation occurs at the electrodes in response to an applied field, producing efficient charge injection and recombination for light emission. LECs from perovskites, nanoparticles, or organic small molecules have potential as thin, conformable, lost-cost solutions for seamless integration in light-emitting applications. Understanding the interplay of electronic and ionic processes of LECs is necessary to improve the luminance, efficiency, stability, and lifetimes for this potential to be actualized. This dissertation focuses on temperature-dependent studies of ionic, electronic, and optical properties of LECs and host-guest systems to improve stability, increase efficiency, and control color. We established that iridium LECs show superior temperature stability to their ubiquitous ruthenium counterparts, resisting radiant flux loss until 67 °C (152 °F). To understand the mechanistic origin of this superior stability, the temperature-dependence of the photoluminescence of iridium and ruthenium complexes was measured. Although textbooks have asserted that iridium complex stability superiority is due to energetic suppression of non-luminescent and antibonding states, it was alternatively found that this superiority was due to favorable radiative recombination rates relative to nonradiative transitions. We implemented novel small-molecule ionic hosts based on carbazole derivatives with ionic transitional metal complex (iTMC) guestsfor use in LECs. These hosts demonstrated wide, tunable bandgaps and accessible oxidation and reduction features, consistent with our design parameters. LECs with a PBI-CzH host demonstrated superior performance, where PBI is 4- bromophenylbenzimidazole, and CzH is an unsubstituted carbazole unit. These LECs achieved 624 cd/m2 luminance at 3.80% external quantum efficiency, competitive in the field of iTMCs. Xray diffraction suggested that host packing caused the superior performance of the PBI-CzH host and offered a key design insight for future LEC hosts. A novel ionic iridium complex guest was prepared to be used in conjunction with a CsPbBr3 perovskite host and a polyelectrolyte to achieve high performance in a perovskite light-emitting device with a single-layer structure. Maximum luminance (10600 cd/m2 ), current efficiency (11.6 cd/A), and power efficiency (9.04 Lm/W) were achieved at a 14% weight fraction of the guest, and voltage-tunable color was demonstrated. These results show improvement for all reported metrics for perovskite host devices, demonstrating the potential for a host-guest approach with radiationally designed ionic emitters in perovskite LECs. Finally, we followed the low-temperature performance of current, electroluminescence, and photoluminescence of perovskite LECs to reveal the effects on ionic transport, electronic transport, and optical properties. Initially, lowering the temperature increased device efficiency. However, the efficiency was found to decrease as the temperature decreased from 240 K and below. Differential scanning calorimetry was used to assess morphological changes induced by the polyelectrolyte. Ultimately, it was revealed that the interplay of these factors greatly depends on the mechanical properties of the polyethylene oxide electrolyte, and the suppression of the glass transition could substantially improve low-temperature device performance.Item Low-Temperature Microstructural Studies on Superconducting CaFe₂As₂(Nature Publishing Group, 2019-04-23) Huyan, S.; Deng, L. Z.; Wu, Z.; Zhao, K.; Sun, J. Y.; Wu, L. J.; Zhao, Y. Y.; Yuan, H. M.; Gooch, M.; Lv, Bing; Zhu, Y.; Chen, S.; Chu, C. W.; 0000-0002-9491-5177 (Lv, B); Lv, BingUndoped CaFe₂As₂ (Ca122) can be stabilized in two slightly different non-superconducting tetragonal phases, PI and PII, through thermal treatments. Upon proper annealing, superconductivity with a T_{c} up to 25 K emerges in the samples with an admixture of PI and PII phases. Systematic low-temperature X-ray diffraction studies were conducted on undoped Ca122 samples annealed at 350 °C over different time periods. In addition to the diffraction peaks associated with the single-phase aggregation of PI and PII, a broad intermediate peak that shifts with annealing time was observed in the superconducting samples only. Our simulation of phase distribution suggests that the extra peak is associated with the admixture of PI and PII on the nanometer scale. High-resolution transmission electron microscopy confirms the existence of these nano-scale phase admixtures in the superconducting samples. These experimental results and simulation analyses lend further support for our conclusion that interfacial inducement is the most reasonable explanation for the emergence of superconductivity in undoped Ca122 single crystals. ©2019, The Author(s).Item Mechanism of Fermi Level Pinning for Metal Contacts on Transition Metal Dichalcogenides and Their Interface Thermal Stability(December 2022) Wang, Xinglu; Wallace, Robert; Frensley , William; Fischetti, Massimo V.; Young, Chadwin D.; Kim, Jiyoung; Lv, BingTransition metal dichalcogenides (TMDs) have demonstrated immense potential for application in state-of-the-art electronic, optoelectronic, and spintronic devices because of their outstanding electronic, optical, mechanical, and magnetic properties. However, the failure of tuning the Schottky barrier height by the work function of metal contacts strongly limits the efficiency of carrier injection and hence the electronic performance of TMD-based devices. This dissertation focuses on the interface between covalent and van der Waals metal contacts and TMDs to study the origin and mechanism of Fermi level pinning. Firstly, the interface chemistry and band alignment of Ni and Ag contacts on MoS2 is studied. Then the mechanism of Fermi level pinning of metal contacts on other Mo- and W-based TMDs are uncovered by considering interface chemistry, band alignment, defects and impurities of W-TMDs, contact metal adsorption mechanism and the resultant electronic structure. Also, the thermal stability of Ni/MoS2 systems is investigated in the aspects of interface chemistry, elemental diffusion, and band alignment.Item Molecular Beam Epitaxy of La2-xSrxCuO4 Films and Heterostructures(2022-08-01T05:00:00.000Z) Xu, Xiaotao; Shi, Xiaoyan; Morcos, Faruck; Lv, Bing; Lumata, Lloyd; Zakhidov, Anvar A.; Zhang, FanSince 1986, the study of high-temperature superconductivity (HTS) in cuprates has revealed a massive amount of discoveries, such as pseudogap, charge density wave, d-wave superconductivity, etc. These novel states of matter trigger even more unknowns in fundamental science and inspire enormous emergent applications. This dissertation presents our research on the archetypical La2-xSrxCuO4 (LSCO) thin films and heterostructures. Specifically, the research has been driven by several fundamental questions. For example, can we create c-axis Josephson junctions for scientific research and superconductor-based quantum computation? What controls the fundamental behaviors of interface superconductivity? To answer those questions, high-quality crystals are required. Here we utilized and improved the oxide atomic-layer-by-layer molecular beam epitaxy (ALL-MBE) technique to grow atomically smooth cuprate films and heterostructures to answer the proposed research questions. The main results are presented as follows. First, we improved the ALL-MBE growth in several ways to enhance the film quality significantly. Specifically, we studied the thermal annealing of oxide substrates and developed treatment methods for LaSrAlO4(LSAO) and SrTiO3(STO) substrates. Ramp-up rate and annealing temperature are found to be the most critical parameters. We then studied the synthesis of LSCO thin films via the ALL-MBE system. A detailed recipe for the growth of LSCO thin films on LSAO substrates is presented. Unique reflection high energy electron diffraction (RHEED) pattern features are observed in LSCO films. A strategy to monitor the film growth and maintain the correct stoichiometry is developed based on the real-time RHEED feedback. We also investigated the power and stability of ozone oxidation and compiled empirical post-annealing procedures suitable for various doping levels. Substrates and LSCO films were evaluated using atomic force microscopy (AFM) and RHEED. The results indicate that they are atomically perfect with high crystallinity. Mutual inductance (MI) tests reveal that the LSCO films are uniform over the whole sample area with a sharp superconducting transition. Second, LSCO heterostructures and superlattices have been synthesized to study the HTS c-axis Josephson junction and interfacial superconductivity. The method to probe the superconducting dead layer number near the interface is introduced using a series of superlattices. At the LSCO-LSAO interface, MI and transport measurements imply that the first two LSCO layers that are near the LSAO exhibit a substantial suppression of superconductivity, resulting in a barrier that is five layers thick in total. And an overdoped LSCO protective layer is found to be effective against carrier depletion in superconducting layers. Within LSAO barriers, a thickness of 2 unit-cells of LSCO interface superconductor is synthesized. The superconducting transition of the sample is tunable with doping and demonstrates the highest transition temperature of 34 K.Item Paths to Non-ergodic Quantum Dynamics: From Cavity QED to Strong Zero Modes(December 2022) Rahmanian Koshkaki, Saeed; Pereira, Luis Felipe; Kolodrubetz, Michael; Lv, Bing; Vandenberghe, William; Zhang, Fan; Zhang, ChuanweiRecent advances in cold atoms experiments and the development of superconducting circuits have revolutionized the way we can examine, observe and implement new physical phenomena. In such systems, we can realize new classes of quantum systems which exhibit non-equilibrium quantum phenomena. These systems have attracted mcuh attention in the past two decades as they possess new physics absent in equilibrium. Beside interesting rich physics to learn more about quantum systems, understanding non-equilibrium systems are crucial in developing future technologies such as quantum computation and communication. Given that many open questions needed to be answered in the study of non-equilibrium quantum systems, in this dissertation we will present our theoretical and numerical attempts in providing answers to some of these questions. One of the key features of the non-equilibrium system is how the dynamical properties of quantum systems cane be characterized in different conditions. Here we will present our result on two different mechanism a system can avoid ergodicity. Many-body localization (MBL) is an extension of Anderson localization to interacting systems, where adding strong enough disorder (breaking translational symmetry by adding random potential such as impurity in crystals) can impede the conductivity (system becomes insulator) in the quantum system. Most of the known MBL systems are short- range interacting particles, but in this dissertation, we will discuss MBL in the presence of coupling of the matter to cavity/circuit QED mode where the combined system becomes long-range interacting. We will study the two cases of weak coupling and strong coupling regimes and will derive the effective Hamiltonian using the high-frequency expansion for each case of coupling strength. We predict that the cavity QED has new localization behaviors such as an inversion of the mobility edge where the high-energy states are localized and low-energy states are delocalized. Also in the strong coupling limit, we observed that using the idea from coherent destruction of coupling the system can show signs of localization for photon number as low as n ∼ 2. The rest of this dissertation is devoted to understanding how a clean system (no disorder) can possess symmetry-breaking edge modes indefinitely, or for a long enough but finite time. The case with infinite lifetime edge mode is called strong mode (SM) and the case with finite lifetime edge mode is known as almost strong mode (ASM). Our system of interest is a clock Z3 model which is an extension of the Ising Z2 models. In the clock model (Baxter and modified Baxter) we found that the chirality of the interaction is essential in deriving the exact edge mode in the Hermitian model but removing the hermiticity (controlled by a parameter β), the effect of chirality on the stability of the edge mode becomes less important. We attempt to use different numerical and approximation techniques such as Krylov Hamiltonian and dynamical signature to characterize the edge mode in a Z3 model.Item Pyrrole Based Donor-acceptor Building Blocks for Organic Field-effect Transistors(2020-12-01T06:00:00.000Z) Gamage, Prabhath Lakmal; Biewer, Michael C.; Stefan, Mihaela C.; Lv, Bing; Ferraris, John P.; Smaldone, Ronald A.The class of organic semiconductors is a definite contender for replacing high-cost silicon semiconductors owing to unique advantages such as solution processability, flexibility, lightweight, low cost, and the ability to do multiple structural modifications. Hence, a remarkable amount of scientific research has been carried out to improve the electronic properties of these materials. Throughout the past two decades, many improvements in the field have achieved by designing novel building blocks. There remains the possibility, however, for performance improvement through areas that has paid less attention in both conventional and non-conventional building blocks. Because of the appealing performance of organic semiconductors, it is highly desirable to seek and develop new building blocks for the next generation of organic electronics. In this dissertation, the fundamentals, history, and recent developments of conventional and nonconventional materials are covered briefly in the first chapter. Operation principles, charge transport of organic field effect transistors is introduced. Compared to conventional thiophenebased -electron donor materials, promising non-conventional pyrrole-based donor materials employed in organic field effect transistors are discussed and summarized. Chapter 2 describes the effect on organic field effect transistor (OFET) properties of a donor-acceptor polymer consist of a branched ester functionalized bithiophene copolymerized with thiophene vinyl thiophene donor. The influence on frontier molecular orbital energy levels, UV-vis absorption, electrochemical properties, OFET parameters and morphological effects were investigated. In chapter 3, the effect of furan spacer group on a thieno[3,2-b]pyrrole and diketopyrrolopyrrole based copolymer is discussed. Upon changing similar flanking groups, the polymer showed a high hole mobility of 0.42 cm2 /V s while the on-to-off ratio exhibited a drastic improvement 105 . Chapter 4 describes the incorporation of selenium hetero atom in the pyrrole fused rings to yield seleno[3,2-b]pyrrole based small molecules replacing thieno[3,2-b]pyrrole to extend the knowledge of OFETs activity of seleno[3,2-b]pyrrole in banana shaped small molecules. They exhibited moderate charge carrier properties 10-2 cm2 /V s hole mobility. In the Chapter 5 (attached as an appendix), a study on oxidative degradation of polypropylene mesh in Escherichia coli (E. coli.) environment is disscussed. Medical implants of polypropylene (PP) mesh have demonstrated biodegradation inside the body. Among the many possible factors, bacterial colonization is believed to be one of the causes for biodegradation. To gain insights on this hypothesis PP mesh samples were tested in Luria-Bertani broth containing E. coli and the experimental results demonstrated qualitative and quantitative bioerosion, confirming the oxidative degradation in vitro.Item Seeded Growth of Boron Arsenide Single Crystals with High Thermal Conductivity(Amer Inst Physics, 2018-11-05) Tian, Fei; Song, Bai; Lv, Bing; Sun, Jingying; Huyan, Shuyuan; Wu, Qi; Mao, Jun; Ni, Yizhou; Ding, Zhiwei; Huberman, Samuel; Liu, Te-Huan; Chen, Gang; Chen, Shuo; Chu, Ching-Wu; Ren, Zhifeng; Lv, BingMaterials with high thermal conductivities are crucial to effectively cooling high-power-density electronic and optoelectronic devices. Recently, zinc-blende boron arsenide (BAs) has been predicted to have a very high thermal conductivity of over 2000W m⁻¹ K⁻¹ at room temperature by first-principles calculations, rendering it a close competitor for diamond which holds the highest thermal conductivity among bulk materials. Experimental demonstration, however, has proved extremely challenging, especially in the preparation of large high quality single crystals. Although BAs crystals have been previously grown by chemical vapor transport (CVT), the growth process relies on spontaneous nucleation and results in small crystals with multiple grains and various defects. Here, we report a controllable CVT synthesis of large single BAs crystals (400-600 μm) by using carefully selected tiny BAs single crystals as seeds. We have obtained BAs single crystals with a thermal conductivity of 351 ± 21 W m⁻¹ K⁻¹ at room temperature, which is almost twice as conductive as previously reported BAs crystals. Further improvement along this direction is very likely.Item Silicon-based Thermoelectrics for Microelectronic Applications(2022-05-01T05:00:00.000Z) Dhawan, Ruchika; Lee, Mark; Dabkowski, Mieczyslaw; King, Lindsay J.; Slinker, Jason D.; Lv, Bing; Kolodrubetz, MichaelThe large advancement of miniature (∼0.1 cm2 area) silicon integrated circuit (IC) sensors and networking devices for internet-of-things (IoT) and biomedical electronics has prompted the problem of making such devices energy-autonomous, i.e., how to energize such devices reliably and sustainably when they are embedded in isolated environments that have no sunlight for photovoltaics, have no access to wall plug power, and cannot routinely be accessed for regular maintenance or battery replacement. Recently, significant interest has developed in small microelectronic thermoelectric generators (µTEGs) as an autonomous energy source for IoT and biomedical devices wherever a reliable thermal gradient exists. Miniaturized solid-state thermoelectric (TE) devices can interconvert thermal gradients and electric fields for power generation or refrigeration. Most current research on TE technology concentrates on developing new materials with high TE figure-of-merit ZT = α 2σ κ T, where α, κ, σ and T are the material’s Seebeck coefficient, thermal conductivity, electrical conductivity, and the mean operating temperature, because the ideal thermodynamic maximum efficiency of a TEG increases with the ZT of the materials used to form the thermopile. However, high ZT materials are usually toxic or non-earth abundant, expensive to manufacture, and are generally incompatible with the Si IC fabrication process, all of which can substantially increase the cost of integration with standard IC devices, making these materials inappropriate for large-scale integration. Bulk Si is neglected for TE applications because of its poor ZT ∼ 10−2 − 10−3 near room temperature due to its large thermal conductivity. Recently, new opportunities for TE materials have been created because of significant advances in the scientific understanding of nanostructure effects on TE properties. It is now thought to be possible to build nanostructured silicon TEGs with a large reduction in κ due to significant phonon scattering, thus enhancing the ZT of material. In this research, µTEGs with a small area (<< 1 mm2), using doped Si and Si0.97Ge0.03 as the TE material, are fabricated on a standard industrial Si IC process line following the same protocols and process flows used to make commercial IC devices. These µTEGs can generate very large specific power densities (power per unit area for heat flow per square of temperature difference, ∆T ) of 84 µWcm−2K−2, which is comparable to the best existing high ZT bulk TEGs. Moreover, these miniature Si µTEGs can generate voltages exceeding 1.5 V with several µA of current using commonly encountered ∆Ts ∼ 20 to 25 K, which are sufficient to properly energize some existing commercially available low-power IoT ICs. These µTEGs are compatible with industrial fabrication techniques so can be directly integrated on-chip in the same process flow with the circuits they support, providing one solution to energy autonomy at an extremely low marginal cost. These Si-based µTEGs build on an unconventional approach to µTEG device design that emphasizes the application of device physics and circuit engineering principles to optimize a µTEG’s generated power per unit area at any given ∆T, rather than focusing on the thermodynamic efficiency, for applications with a high specific power density as the primary requirement. Using the ability of CMOS processing to fabricate ultrahigh density devices with low packing fraction, we can integrate ∼ 104 thermocouples cm−2 while maintaining a reasonable temperature gradient across the device, thereby producing high total power and voltage density despite relatively low efficiency per TE element. Modern Si processing is also very good at controlling parasitic thermal and electrical resistances, thus minimizing extrinsic degradation of overall TEG performance. Experimental results on µTEG fundamental performance characteristics (i.e. power and voltage generation) as well as TE Peltier cooling are presented. For optimizing the performance of TEGs, Physics-based models at both the material level including the effects of dopant concentration and small percentage Ge alloying, and the device design level including effects of parasitic electrical and thermal resistances and proper design of thermopile packing fraction have been developed. These models help optimize the TEGs to provide maximum power production in future designs. The wide acceptance of TE technology in a broad range of IC applications demands not only the research on suitable TE materials but also understanding the device physics along with the ability to determine basic TE properties such as Seebeck (α) and Peltier (π) coefficients at the device level. A wide range of literature exists on measurement of α but π is rarely measured and is usually derived from α using the first Kelvin relation, π = αT. We have developed a new method for measuring π in any TE device using only standard measured device parameters (i.e thermal impedance and short circuit current) without the need to correct for Ohmic heating (I 2R) that has historically made reliable measurements of π difficult. The experimental verification of the first Kelvin relation is illustrated using the independently measured value of α and π on commercial TE materials.Item Superconductivity from Site-Selective Ru Doping Studies in Zr₅Ge₃ Compound(IOP Publishing Ltd, 2018-10-22) Li, Sheng; Liu, Xiaoyuan; Anand, Varun; Lv, Bing; Li, Sheng; Liu, Xiaoyuan; Anand, Varun; Lv, BingSystematical doping studies have been carried out to search for the possible superconductivity in the transition metal doped Zr₅Ge₃ system. Superconductivity up to 5.7 K is discovered in the Ru doped Zr₅Ge₂․₅Ru₀․₅ sample. Interestingly, with the same Ru doping, superconductivity is only induced with doping at the Ge site, but remains absent down to 1.8 K with doping at the Zr site or interstitial site. Both magnetic and transport studies have revealed the bulk superconductivity nature for Ru doped Zr₅Ge₂․₅Ru₀․₅ sample. The high upper critical field, enhanced electron correlation, and extremely small electron-phonon coupling, have indicated possible unconventional superconductivity in this system, which warrants further detailed theoretical and experimental studies.Item Superconductivity, Magnetism and Topological Properties of Several Quasi-1D Materials(2020-04-16) Liu, Xiaoyuan; Lv, BingThis dissertation focuses on the investigation of several material systems with distinct quasi-onedimensional (quasi-1D) or pseudo-quasi-1D structural features and hosting rather interesting magnetic, superconducting, and topological properties. High quality of powders or single crystals have been synthesized through various methods and carefully characterized through X-ray diffraction, magnetic, electrical transport and thermal transport studies at low temperature and under high magnetic fields. Their unique magnetic, superconducting, and possible topological properties are presented with their future implications further discussed. Firstly, we investigated the magnetic property and superconductivity in two iron-based chalcogenides with quasi-1D structure: BaFe2Se4 and K3Fe2Se4. Both compounds consist of 1D chains of iron chalcogenide with FeSe4 tetrahedra, similar to the fundamental building block FeAs4 in the iron-based superconductors. However, these FeSe4 tetrahedra is edge-shared along one specific crystallographic axis and forms 1D chain structure, rather than the two-dimensional layered Fe2As2 structure seen in the common iron-based superconductors. X-ray powder pure phases of both compounds are synthesized and found to be semiconducting and magnetic. Intuitively doping studies based on the nominal iron valence charges are also carried out, however, no superconductivity is detected yet at this stage in the doped samples. Secondly, we carried out systematical doping studies aiming to induce superconductivity in the Zr5Ge3 system with Mn5Si3-type structure and pseudo-quasi-1D Zr3Ge3 chain along the c axis. Different transition metal doping at different crystallographic sites in this Zr5Ge3 system have been thoroughly investigated. Interestingly, superconductivity was successfully induced through Ru and Pt doping, but only at the Ge site. Superconductivity is found to be absent with same amount of carrier doping at the Zr site or interstitial site. The highest superconducting Tc is found to be at 5.7 K for Ru doping, and 2.8 K for Pt doping. Detailed transport and magnetic studies have suggested bulk superconductivity, high upper critical field, enhanced electron correlation, and extremely small electron-phonon coupling, indicating possible unconventional superconductivity in this system. In addition, we also investigated the superconductivity properties of the A2Mo3As3 (A = K, Rb), which has a more 1D-like Mo3As3 chain structure but structurally very close to the Zr5Ge3 compound. The superconducting Tc is ~ 10.6 K, and the electrical transport studies suggested a much higher upper critical field ~ 27 T, exceeding the Pauli limit based on conventional criteria. Thirdly, we present a study of synthesis and crystal growth of α, β-Bi4X4 (X = I, Br) materials with different stacking of 1D BiX chains and various interesting topological properties. Different types of synthetic methods have been tested for the high quality crystal growth, and crystals as large as 8 mm size of Bi4X4 was successfully synthesized. A hypothetical growth mechanism was proposed for Bi4Br4. The transport measurements have revealed a topological phase transition from β-Bi4I4 as a strong topological insulator to α-Bi4I4 with high ordered hinge states around room temperature at 300 K. The exact origin of several transport anomaly in the α-Bi4I4 and α’-Bi4Br4 remain elusive at this stage. Single crystals of mixed Bi4I4-xBrx (0.1 ≤ x ≤ 3) were also synthesized and a rather interesting structural transformations upon Br doping have been observed, and further studies are needed to fully understand the topological phase transitions in this system. Last but not least, chemical intercalation studies were also carried out, and preliminary data suggests superconductivity is successfully induced in these materials. The last part of this dissertation focus on a two-dimensional material Black phosphorus (Black-P). We carried out thorough experimental studies to investigate the growth mechanism for Black-P aiming to facilitate its future layer-by-layer thin film growth. A new synthetic strategy to grow large size of Black-P crystals through a ternary clathrate Sn24P22-xI8 under lower temperature and pressure was reported. The chemical vapor transport mechanism was found not play a critical role for the growth of Black-P, but rather the vapor-solid-solid (VSS)-like growth mechanism is found to be crucial for the quality growth. The ternary clathrate Sn24P22-xI8 acts as the solid catalyst and the P vacancies in Sn24P22-xI8 was found plays an important role in this mechanism.Item The Dielectric Properties of Two-dimensional Materials and Their Applications in Electronic Devices: a First-principles Study(2022-05-01T05:00:00.000Z) Rostami Osanloo, Mehrdad; Vandenberghe, William; Zhang, Li; Lv, Bing; Cho, Kyeongjae; Kim, Moon J.; Wallace, Robert M.Recent developments in the field of two-dimensional (2D) van der Waals (vdW) materials have captured great interest for their possible applications in the next generation of complementary Metal-Oxide Semiconductor (CMOS) technologies. Easy cleavage along the layer planes, naturally passivated surfaces, and intriguing anisotropic electrical, thermal, and optical properties make vdW materials ideal candidates to reach the ultimate scaling limit of transistors. Moreover, vdW transistors could add great functionality in the backend-of-line or for other novel applications like neuromorphic computing. In this doctoral dissertation, I will identify and study novel 2D vdW dielectric candidates to address some drawbacks of currently available non-van der Waals (non-vdW) dielectrics, e.g., HfO2 and Al2O3 . Although traditional three-dimensional (3D) dielectrics provide high-k solutions for silicon-based semiconductor technologies, they cannot be easily scaled and therefore deteriorate device performance in vdW channel transistors. Moreover, the covalent bonds in non-vdW dielectrics may destroy the naturally passivated bonds in a vdW channel material. To address this concern, we investigate 40 alternative 2D vdW dielectrics and evaluate their performance in conjugation with six Transition Metal Dichalcogenide (TMD) channels. We employ Density Functional Theory (DFT) and Density Functional Perturbation Theory (DFPT) to calculate the electronic and dielectric properties of a wide range of 2DvdW dielectrics. We perform highly accurate calculations to investigate the electronic band structure, thermodynamic properties, structural stability, and dielectric properties of 2D vdW structures. We perform precise calculations at high DFT and DFPT levels to obtain the bandgap, electron affinity, out-of-plane electron effective mass, as well as in-plane and out-of-plane dielectric constant. We calculate the band offset (conduction and valence band edge) of each material to evaluate their insulating properties for the potential application in n-MOS and p-MOS technologies. We compute the Equivalent Oxide Thickness (EOT) for each compound and use direct tunneling and thermionic emission equations to examine the performance of a device made by a 2D dielectric and a TMD channel. We eventually introduce the most promising candidates that can satisfy the low-power limit introduced by the 2020 International Roadmap for Devices and Systems (IRDS) by having an acceptable leakage current. We eventually compare our results with the industry’s most desirable dielectrics (SiO2 , and HfO2) and 2D hexagonal Boron Nitride (h-BN). We explicitly show how some materials in our list outperform the available high-k dielectrics. Given the growing interest in the 2D layered dielectric materials and their possible use in future scaled electronic devices, we believe that 2D vdW dielectric candidates identified in this PhD dissertation could pave the way for the future design of advanced n-MOS and p-MOS transistors.Item The Role of Crystal Growth Conditions on the Magnetic Properties of Ln₂Fe₄₋ₓCoₓSb₅ (Ln = La and Ce)(American Chemical Society, 2019-04-15) Weiland, Ashley; Li, Sheng; Benavides, Katherine A.; Burnett, Joseph V.; Milam-Guerrero, J.; Neer, Abbey J.; McCandless, Gregory T.; Lv, Bing; Chan, Julia Y.; 0000-0001-7198-3559 (Weiland, A); 0000-0002-9491-5177 (Lv, B); 0000-0003-4434-2160 (Chan, JY); Weiland, Ashley; Li, Sheng; Benavides, Katherine A.; Burnett, Joseph V.; Neer, Abbey J.; McCandless, Gregory T.; Lv, Bing; Chan, Julia Y.Single crystals of Ln₂Fe₄₋ₓCoₓSb_{5-y}Bi_y (Ln = La, Ce; 0 ≤ x ≤ 0.5; 0 ≤ y ≤ 0.2) were grown using Bi flux and self-flux methods. The compounds adopt the La₂Fe₄Sb₅ structure type with tetragonal space group I4/mmm. The La₂Fe₄Sb₅ structure type is comprised of rare earth atoms capping square Sb nets in a square antiprismatic fashion and two transition-metal networks forming a PbO-type layer with Sb and transition-metal isosceles triangles. Substituting Co into the transition-metal sublattice results in a decrease in the transition temperature and reduced frustration, indicative of a transition from localized to itinerant behavior. In this manuscript, we demonstrated that Bi can be used as an alternate flux to grow single crystals of antimonides. Even with the incorporation of Bi into the Sb square net, the magnetic properties are not significantly affected. In addition, we have shown that the incorporation of Co into the Fe triangular sublattice leads to an itinerant magnetic system. ©2019 American Chemical Society.Item Tracking the Biochemistry of Cancer Cells and Dynamics of Physical Systems Using Nuclear Magnetic Resonance(2021-08-01T05:00:00.000Z) Khashami, Fatemeh; Krawcewicz, Wieslaw; Lumata, Lloyd; Lee, Mark; Glosser, Robert; Slinker, Jason; Lv, BingNuclear magnetic resonance (NMR) spectroscopy, providing a versatile technique for analyzing cancer metabolism based on 13C NMR analysis, is one of the most important tools for biological and more specifically cancer study purposes. The chemical shifts for 13C nuclei in organic molecules are spread out up to 200 ppm, enabling signal from each carbon in a compound be seen as a distinct peak. The relatively weak signals obtained from the NMR spectroscopy enables probing sensitive physical systems such as living systems without significantly disturbing them. In this dissertation, we have tracked 13C metabolism in different type of cancers, specially Glioblastoma Multiforme (GBM). GBM is an aggressive type of the Central Nervous System (CNS) tumor that grows within the brain tissue. In this study, we have investigated how the individually used fructose and glucose sugars and there combinations as high fructose corn syrup (HFCS) are metabolized in cultured SFxL glioblastoma and Huh-7 hepatocellular carcinoma cells as probed by 13C NMR spectroscopy. To understand more about cancer metabolism we did more study in glycolysis activity and pentose phosphate pathway (PPP). We used [1,2-13C] glucose to investigate the amount of lactate that could be produced from glycolysis versus PPP as the alternative route. Furthermore, we have investigated the metabolism of [1,2-13C] glucose with inhibitors of Lactate dehydrogenase A (LHDA) and sodium oxamate in GBM cells. LDHA is an important enzyme that is active in most of tissues. LDHA catalyzes the reversible conversion of pyruvate to lactate. Moreover, we have investigated the spin-lattice relaxation time (T1) of water-glycerol mixtures at the earth magnetic field. The water 1H T1s at various ratios of water-glycerol contents were measured at different temperatures ranging from 253.15 K to 353.15 K. In summary, this PhD dissertation presents and discusses a unique tool for deciphering cancer metabolism in vitro using NMR spectroscopy.Item Ultrafast Dynamics of Quasiparticles and Coherent Acoustic Phonons in Slightly Underdoped (BaK)Fe₂As₂(Nature Publishing Group, 2016-05-16) Lin, Kung-Hsuan; Wang, Kuan-Jen; Chang, Chung-Chieh; Wen, Yu-Chieh; Lv, Bing; Chu, Ching-Wu; Wu, Maw-Kuen; Lv, BingWe have utilized ultrafast optical spectroscopy to study carrier dynamics in slightly underdoped (BaK)Fe2As2 crystals without magnetic transition. The photoelastic signals due to coherent acoustic phonons have been quantitatively investigated. According to our temperature-dependent results, we found that the relaxation component of superconducting quasiparticles persisted from the superconducting state up to at least 70 K in the normal state. Our findings suggest that the pseudogaplike feature in the normal state is possibly the precursor of superconductivity. We also highlight that the pseudogap feature of K-doped BaFe2As2 is different from that of other iron-based superconductors, including Co-doped or P-doped BaFe2As2.