New Materials under Extreme Environments
Hybrid halide perovskites: 3D halide perovskites are crystalline materials where an extended anionic network composed of corner-sharing metal-halide octahedra is charge balanced by small organic or inorganic cations. The related 2D halide perovskites can be structurally derived from 3D perovskites by slicing along certain crystallographic directions. The compositional, structural, and electronic flexibility of the 2D and 3D halide perovskites has shown great promise and enabled diverse applications including energy and optoelectronics (Saparov & Mitzi, 2016).
Our recent collaborative work with the Karunadasa group at Stanford on a series of 2D and 3D hybrid halide perovskites demonstrate that the pressure tuning of these materials’ structural, optical, electronic properties is of both fundamental and technological interest. By compressing a 2D Cu-Cl hybrid perovskite we showed the first instance of appreciable conductivity in such systems, and the material underwent an insulating-to-semiconducting transition (Jaffe et al., 2015). Work on 3D Pb-X (X = Br, I) hybrid perovskites demonstrated that pressure induces drastic structural, electronic, and optical modifications to the materials and could improve the performance of solar cells made of perovskites (Jaffe et al., 2016). Another study on a 2D Pb-SCN hybrid perovskite, a proposed analog to 3D Pb-I perovskite, showed a striking piezochromism from red to black to yellow from 0 to 3.8 GPa, and the material’s unusually low bandgap among 2D hybrid halide perovskites can be further modulated by very moderate pressures (Umeyama et al., 2016).
Intercalation complexes of Lewis bases and transition metal chalcogenides (TMCs): TMCs are a class of 2D materials that have gained intense interest in recent years due to their potential application in nano-electromechanical and optoelectronic devices (Wang, Kalantar-Zadeh, et al., 2012, Yuan et al., 2015). Tuning their crystal and electronic structures away from the pristine states through compression allows us to access exotic physical states not available otherwise. We recently reported that upon compression MoSe2, in contrast to MoS2, underwent a continuous structural evolution from an anisotropic 2D layered network to a 3D structure resulting in a continuous semiconductor-to-metal transition through a combined experimental and theoretical effort (Zhao et al., 2015). We plan to complement this static compression work by looking at these materials under dynamic compression at MEC.
Carbon-based nanomaterials: Diamondoids are the newest members of carbon-based
nanomaterials. They represent the ultimate limit for molecular fragments of bulk diamond and a unique molecular hierarchy. Our high-pressure experiments on a series of diamondoids with systematically varying molecular geometries and dimensionalities, ranging from zero-dimensional (0D) adamantane to 3D [1(2,3)4]pentamantane show a clear dependence of the diamondoids’ bulk moduli to their molecular geometries with 3D [1(2,3)4]pentamantane being the least compressible compared to 0D adamantane which is the most compressible, while 1D and 2D diamondoids fall in between (Fig. 1). The connection between nanomaterials’ shape and functionality revealed by our systematic high-pressure study provides guidance in designing nanoscale structures with tunable mechanical strength that implement diamondoids as building blocks. In addition, higher diamondoids were found to display exotic optical and electronic properties (Yang et al., 2007) and rich chemistry (Schwertfeger et al., 2008), but are only present in extremely minute concentrations in nature, and have in the past been largely inaccessible to synthetic methods (Dahl et al., 2003). We propose to combine pressure, temperature, and ultraviolet radiation to explore the formation of higher diamondoids and nanodiamonds using lower diamondoids as precursors and explore whether pressure can encourage the addition of foreign dopants into diamondoids and induce emergent properties such as ferroelectricity and superconductivity in these materials.
Disordered systems: Glasses are thermodynamically metastable and thus prefer to devitrify when provided with enough energy to overcome the kinetic barrier to crystallization. In addition to stability concerns for material applications, research on devitrification can elucidate underlying correlations between disordered glasses and ordered crystals. Due to the absence of the well-defined dislocation defects in crystalline alloys, metallic glasses exhibit very high compressive strength, good corrosion resistance, and large elasticity. With a close-packed Bernal structure, they also represent a model system for understanding structure-property relationships in glasses. For example high pressure X-ray measurements have proven critical in demonstrating that metallic glasses display fractal packing relationship where the nearest neighbor distance (measured by XRD) scales with volume (measured by nanoTXM) via power law of 2.5 (Fig. 2) (Zeng et al., 2014, Zeng et al., 2016). Probing different regions in momentum space may lead to changes in the power law exponent, resulting in a crossover in the power law exponent from fractal values to homogeneous values past the correlation length (Chen et al., 2015). Coupling high pressure nanoTXM with other X-ray techniques could also shed light onto the structure of other glass systems which would provide key information for understanding the nature of glasses and the glass transition.