Pressure tuning of light-induced superconductivity in K3C60

Optical excitation at terahertz frequencies has emerged as an effective means to manipulate complex solids dynamically. In the molecular solid K3C60, coherent excitation of intramolecular vibrations was shown to transform the high temperature metal into a non-equilibrium state with the optical conductivity of a superconductor. Here we tune this effect with hydrostatic pressure, and we find it to disappear around 0.3 GPa. Reduction with pressure underscores the similarity with the equilibrium superconducting phase of K3C60, in which a larger electronic bandwidth is detrimental for pairing. Crucially, our observation excludes alternative interpretations based on a high-mobility metallic phase. The pressure dependence also suggests that transient, incipient superconductivity occurs far above the 150 K hypothesised previously, and rather extends all the way to room temperature.Comment: 33 pages, 17 figures, 2 table

Synthesis and fundamental property studies of energy material under high pressure.

Recently, high-pressure science and technology has flourished and rapidly advanced to impact a wide domain of materials and physical sciences. One of the most substantial technological developments is the integration of samples at ultrahigh pressure with a wide range of in-situ probing techniques. Applications of extreme pressure have significantly enriched our understanding of the electronic, phonon, and doping effects on the newly emerged two-dimensional (2D) materials. Under high pressure, materials’ atomic volume radically decreases, and electronic density rises, which will lead to extraordinary chemical reaction kinetic and mechanisms. The promising capability of high pressure combine with the significance of novel emerging 2D materials in energy-related research was the main motivation of this dissertation. Firstly, the application of high pressure to enable the direct synthesis of α-AgGaO2 through a reaction of Ag2O and Ga2O3 is demonstrated. The synthesized samples were extensively characterized, and their crystal phase and chemical composition were confirmed. Especially, the rhombohedral delafossite crystal phase of the prepared sample was verified by electron diffraction. The vibrational phonon modes were investigated using a combination of ab initio density functional theory (DFT) and experimental Raman measurement. In addition, using a modified DFT to calculate the electronic band structure of α-AgGaO2 reported a more accurate valu of theoretical[1] band gap than those have been reported previously. Two-dimensional (2D) materials with efficient ion transport between the layers and the large surface areas have demonstrated promise for various energy-related applications. Few-layer black phosphorus (phosphorene), as a novel two-dimensional (2D) material, is gaining researchers’ attention due to the exceptional properties, including puckered layer structure, widely tunable band gap, strong in-plane anisotropy, and high carrier mobility. Phosphorene application expanded from energy storage and conversion devices to thermoelectrics, optoelectronic and spintronic to sensors and actuators. Several recent theoretical studies have indicated that strain engineering can be a viable strategy to tune the electronic structure of phosphorene. Although several theoretical studies have predicted an electronic phase transition such as direct-indirect bandgap and semiconductor-metal transitions, there is not experimental study to indicate the transition. Next, in this dissertation, a systematic experimental study of in situ high-pressure Raman and PL spectroscopy of phosphorene was reported. Furthermore, short transport growth of bulk black phosphorus and also, liquid-phase exfoliation technique to preparing few-layer black phosphorus was described. The study motivated by a better understanding of high-pressure effects on optical properties and band structure of this material system. This study help to verify theoretical predictions and to enhance fundamental understanding of relationships between strain and electronic band structure, enabling rational strain engineering towards additional functionalities and device applications of phosphorene and few-layer BP. In situ characterization techniques are invaluable for a fundamental understanding of materials, their processing, and functionalities. Three-dimensional architecture of graphene has also attracted considerable attentions as an effective way to utilize the unique inherent properties of graphene sheets in practical applications. Three-dimensional graphene-based materials offer an easy and versatile platform for functionalization and integration into devices. Furthermore, the interlocking of graphene sheets into 3D structures solve the restacking issue and make 3D graphene-based materials more compatible with conventional material processing. Finally, in the dissertation, we report a novel, inexpensive, and highly scalable, approach of fabricating a three-dimensional graphene network (foam) via pyrolysis of organic materials as the source of carbon. A template-assisted method to prepare and tune the properties of a high-quality 3D graphene network was described. In this simple method, the 3D graphene foam is synthesized in a controlled environment by thermal decomposition of the organic materials in the presence of Ni foam which plays a dual role of catalyst and 3D template. This technique can efficiently facilitate and control the in situ nitrogen doping of 3D graphene structure by adjusting the growth parameters and choosing the right organic materials (i.e. nitrogen-containing organic acids). In this work, inexpensive organic materials including caffeine (C8H10N4O2), urea (CH4N2O) and acetaminophen (C8H9NO2) were used with a citric acid solution as the source of carbon and nitrogen. Nitrogenation of 3D graphene foam create an effective improvement of properties which is suitable for an extensive range of new energy and environmental applications. Our Raman analysis indicated an improvement of graphene network quality with an increase of synthesis temperature between 650 °C and 1000 °C. Both Raman and TEM study (HRTEM, SAED, and EELS) showed uniform coverage and high crystallinity of multilayered graphitic shells formed in samples synthesis at 1000 °C. The motivation for this 3D graphene research is to use in-situ high-pressure measurements to study fundamental properties of these materials including its vibrational structures, doping and functionalization. With its distinct Raman signatures dependent on the quality and structure, defect distribution, as well types of dopants and their concertation, 3D graphene seems well-suited for high-pressure in-situ Raman studies. These type of measurements are proposed as part of the future, follow-up research. [1] Performed by Dr. Madhu Menon (Center for Computational Sciences at the University of Kentucky)

COMPUTATIONAL STUDY OF OPTICAL, MECHANICAL, AND PHASE TRANSITION PROPERTIES OF SILICON TELLURIDE (Si2Te3)

COMPUTATIONAL STUDY OF OPTICAL, MECHANICAL, AND PHASE TRANSITION PROPERTIES OF SILICON TELLURIDE (Si2Te3

High-pressure behaviors of carbon nanotubes

In this paper, we have reviewed the experimental and theoretical studies on pressure-induced polygonization, ovalization, racetrack–shape deformation, and polymerization of carbon nanotubes (CNTs). The corresponding electronic, optical, and mechanical changes accompanying these behaviors have been discussed. The transformations of armchair (n, n) CNT bundles (n = 2, 3, 4, 6, and 8) under hydrostatic or nonhydrostatic pressure into new carbons, including recently proposed superhard bct-C₄, Cco-C₈, and B-B1AL2R2 carbon phases have also been demonstrated. Given the diversity of CNTs from various chiralities, diameters, and arrangements, pressure-induced CNT polymerization provides a promising approach to produce numerous novel metastable carbons exhibiting unique electronic, optical, and mechanical characteristics.Розглянуто експериментальні та теоретичні дослідження з індукованою тиском полігонізації, овалізації, деформації у формі бігової доріжки і полімеризації вуглецевих нанотрубок (ВНТ). Обговорено відповідні електронні, оптичні і механічні зміни, що супроводжують ці процеси. Також продемонстровано перетворення в ВНТ у формі крісла (n, n), зібраних в пучок (n = 2, 3, 4, 6 і 8) під гідростатичним або негідростатичним тиском в нові вуглецеві алотропи, в тому числі недавно запропоновані надтверді bct-C₄, Cco-C₈ і B-B1AL2R2-вуглецеві фази. Різноманітність ВНТ з різними хіральністю, діаметрами та упаковками, а також полімеризація ВНТ, викликана тиском, забезпечує перспективний підхід для отримання численних нових метастабільних вуглецевих фаз, що демонструють унікальні електронні, оптичні і механічні характеристики.Рассмотрены экспериментальные и теоретические исследования по индуцированной давлением полигонизации, овализации, деформации в форме беговой дорожки и полимеризации углеродных нанотрубок (УНТ). Обсуждены соответствующие электронные, оптические и механические изменения, сопровождающие эти процессы. Также продемонстрированы преобразования в УНТ в форме кресла (n, n), собранных в пучок (n = 2, 3, 4, 6 и 8) под гидростатическим или негидростатическим давлением в новые углеродные аллотропы, в том числе недавно предложенные сверхтвердые bct-C₄, Cco-C₈ и B-B1AL2R2-углеродные фазы. Разнообразие УНТ с различными хиральностью, диаметрами и упаковками, а также полимеризация УНТ, вызванная давлением, обеспечивает перспективный подход для получения многочисленных новых метастабильных углеродных фаз, демонстрирующих уникальные электронные, оптические и механические характеристики

Intercalation and high pressure studies of black phosphorous - pathways to novel materials and physics.

Discovery of graphene in 2004 initiated a new trend of materials known as two-dimensional (2D) materials which have exciting surface properties and anisotropies than their bulk counterparts. Phosphorene, which is the layered version of black phosphorous (BP) is one of the top 2D materials in terms of research interests and applications of the present day. Moving a step further, our interest is to understand the possibilities for structural modifications of phosphorene, by means of stimuli such as intercalation and high-pressure. It has been predicted by theoretical studies that these stimuli may lead to the formation of new structures and phases which widens the applications of these materials. The initial phase of this work was dedicated to synthesize BP in laboratory conditions. In today’s market, 1g of pure black phosphorous crystals costs as high as $800. Thus, high quality BP crystals were grown in our lab using chemical vapor transport technique, and characterized for its quality using several characterization techniques. In the next phase, A systematic study on electrochemical charge transfer in Li-doped black phosphorus (BP) was carried out by both in-situ and ex-situ Raman scattering. Galvanostatic discharge of dedicated in-situ electrochemical cell for Raman spectroscopy was used to study time evolution of vibrational modes under lithiation. In addition to the peak broadening, which is a result of structural expansion, peaks corresponding to all three Raman-active atomic vibrational modes were found to redshift as a result of lithiation. Peaks corresponding to in-plane modes were red-shifted about 1.6 times faster than the out-of-plane mode. Further characterizations using optical and electron microscopy showed that the intercalation of BP is highly anisotropic, where channels along the zigzag direction were found to be the easy direction for intercalation. X-ray diffraction on intercalated samples confirmed a reduction of thickness as lithiation weakens interlayer bonding, thus resulting in a partial exfoliation of BP flakes. Furthermore, first principle studies using density functional theory were used (performed by DR. Ming Yu and Md. Rajib Khan Musa) to develop a theoretical model for the intercalation mechanism. The discrepancy between the experimental and the theoretical results was also addressed. Next, the focus was on the high-pressure response of pristine and Li intercalated BP. Structural evolution of Li-intercalated and pristine black phosphorous (BP) under high-pressure (up to ∼ 8 GPa) was studied using in-situ Raman spectroscopy. Even though both materials showed a monotonic blueshift of the out-of-plane vibrational mode with pressure, Li-intercalated BP did not show a blueshift until a threshold pressure (2.4 GPa) was reached to compensate the structural expansion caused by intercalation. However, the in-plane modes in each sample responded differently. In the mid-pressure region, they both showed redshifts which in Li-intercalated BP was also followed by abrupt blueshifts. Such behavior indicated pressure-induced structural reorganizations inside the material. Computational modeling revealed the existence of a process of P-P bond breaking and reforming in the system due to the redistribution of intercalated Li atoms under pressure. This work shows the significance of combined effect of pressure and intercalation on structural changes in the search for new phases of BP and other 2D materials. Alloying BP with another group V element like arsenic (As), is another way of tuning the structure as well as improving the stability of BP in ambient conditions. Thus, in addition to the intercalation and high pressure approaches, black arsenic-phosphorous alloys were synthesized as an initial step for future research. The structural properties of AsxP1-x (x = 0, 0.2, 0.5, 0.83, and 1) alloys. It is observed that black phosphorous-related phonon modes in the alloy samples are redshifted with increasing arsenic concentration, while black arsenic-related modes in these samples are blue-shifted with increasing phosphorus concentration. In addition, these materials were tested for their temperature-dependent transport properties which gave much promise on its usage as thermoelectric materials

Novel Growth Routes and Fundamental Understanding of Pseudo-One-Dimensional Materials

abstract: Recently, two-dimensional (2D) materials have emerged as a new class of materials with highly attractive electronic, optical, magnetic, and thermal properties. However, there exists a sub-category of 2D layers wherein constituent metal atoms are arranged in a way that they form weakly coupled chains confined in the 2D landscape. These weakly coupled chains extend along particular lattice directions and host highly attractive properties including high thermal conduction pathways, high-mobility carriers, and polarized excitons. In a sense, these materials offer a bridge between traditional one-dimensional (1D) materials (nanowires and nanotubes) and 2D layered systems. Therefore, they are often referred as pseudo-1D materials, and are anticipated to impact photonics and optoelectronics fields. This dissertation focuses on the novel growth routes and fundamental investigation of the physical properties of pseudo-1D materials. Example systems are based on transition metal chalcogenide such as rhenium disulfide (ReS2), titanium trisulfide (TiS3), tantalum trisulfide (TaS3), and titanium-niobium trisulfide (Nb(1-x)TixS3) ternary alloys. Advanced growth, spectroscopy, and microscopy techniques with density functional theory (DFT) calculations have offered the opportunity to understand the properties of these materials both experimentally and theoretically. The first controllable growth of ReS2 flakes with well-defined domain architectures has been established by a state-of-art chemical vapor deposition (CVD) method. High-resolution electron microscopy has offered the very first investigation into the structural pseudo-1D nature of these materials at an atomic level such as the chain-like features, grain boundaries, and local defects. Pressure-dependent Raman spectroscopy and DFT calculations have investigated the origin of the Raman vibrational modes in TiS3 and TaS3, and discovered the unusual pressure response and its effect on Raman anisotropy. Interestingly, the structural and vibrational anisotropy can be retained in the Nb(1-x)TixS3 alloy system with the presence of phase transition at a nominal Ti alloying limit. Results have offered valuable experimental and theoretical insights into the growth routes as well as the structural, optical, and vibrational properties of typical pseudo-1D layered systems. The overall findings hope to shield lights to the understanding of this entire class of materials and benefit the design of 2D electronics and optoelectronics.Dissertation/ThesisDoctoral Dissertation Materials Science and Engineering 201

Atomic and electronic structure studies of nano-structured systems : Carbon and related materials

Modeling in the framework of density functional theory has been conducted on carbon nanotubes and graphene nano-structures. The results have been extended to non-carbon systems such as boron nanostructures. Computational studies are complemented by experimental methods to refine the structural models and obtain a better understanding of the electronic structure. It is observed that the zigzag edged bilayered graphene nanoribbons are highly unstable as compared to their armchair counterparts. A novel approach has been proposed for the patterning of chirality/diameter controlled single walled carbon nanotubes. Nanotube formation is found to be assisted by edge ripples along with the intrinsic edge reactivity of different types of bilayered GNRs. The effect of bundling on the electronic structure of single walled carbon nanotubes in zigzag single walled carbon nanotubes has been studied. Hydrostatic pressure effects were examined on bundled single walled carbon nanotubes. Nanotubes with chiral indices (3n + 3, 3n + 3) acquire hexagonal cross-sections on application of hydrostatic pressures. The formation of a novel quasi two-dimensional phase of carbon during hydrostatic compression of small and large nanotubes under extreme conditions of pressure is modeled and is understood to be dictated by breaking of symmetry during compression. Nanoscale materials with anisotropic compressibility do not exhibit symmetric compression as in bulk materials. Structural stability of boron nanoribbons derived from \u27α-sheet\u27 and reconstructed {1221} sheets was studied. Antiaromatic instabilities were found to destabilize nanoribbons constructed from reconstructed {1221} sheets when compared to those obtained from the \u27α-sheet\u27. The stability of the nanoribbons was found to increase with increasing width and increase in the hole density (η) of the boron nanoribbons. The study of electronic structure reveals the presence of semiconducting nanostructures. The presence of nanoscale crystalline domains due to random functionalization has made it difficult to resolve the chemical structure of graphene oxide and it remains a much debated topic to date. A combination of analytical, spectroscopic and density functional techniques have been used to determine the structure and properties of such nano materials. Graphene oxide has unusual exotic properties and belongs to this class of materials. Investigations reveal that the chemical structure of graphene oxide can be visualized as puckered graphene sheets linked by oxygen atoms. Density functional theory has been used to calculate the site projected partial density of states for carbon and oxygen atoms involved in different types of bonding. A comparison of these simulations with carbon and oxygen K-edge absorption spectra has led to an understanding of the basic electronic structure of this material

Transport and coupling of phonons, electrons, and magnons in complex materials

In nanoscale systems, in which the relevant length scales can be comparable to the mean free paths and wavelengths of the energy, charge and spin carriers, it is necessary to examine the microscopic transport of heat, spin and charge at the atomic scale and the quantization of the associated quasiparticles. The intricacies of the transport dynamics can be even more complicated in materials with atomic scale complexities, such as incommensurate crystals, magnetic materials, and quasi-one-dimensional systems. Meanwhile, the transport properties and coupling between these quasiparticles is important in determining the strength of various thermoelectric and spincaloritronic phenomena, as well as the reliability of nanoscale electronics. This work seeks to further the understanding of the complicated transport dynamics in complex structured materials at nanometer and micrometer length scales, and to address some of the fundamental questions about the interactions between energy, charge and spin carriers in the conducting polymer poly(3,4- ethylenedioxythiophene) (PEDOT), the incommensurate higher manganese silicide (HMS) thermoelectric material, and the magnetic insulator yttrium iron garnet (YIG). These questions are addressed through a number of combined experimental approaches through the use of thermal conductance and thermoelectric property measurements of suspended nanostructures, inelastic neutron scattering, Brillouin light scattering, and electron microscopy. According to in-plane thermal and thermoelectric transport measurements of PEDOT thin films, the electronic thermal conductivity of this conducting polymer is found to be significant and exceeds that predicted by the Wiedemann-Franz law for metals. Furthermore, thermoelectric transport measurements of suspended HMS nanoribbons show a reduction in the lattice thermal conductivity by approximately a factor of two compared to bulk HMS, which is qualitatively consistent with that predicted from a diffuson model for thermal conductivity derived from the phonon dispersion of HMS. Lastly, pressure dependent Brillouin light scattering spectroscopy is used to determine the influence of hydrostatic stress on the dispersions of magnons and phonons in YIG, in order to determine the magnon and phonon peak frequency shift associated with localized laser heating induced strain.Mechanical Engineerin