Synthesis And Characterization Of Functional Materials Using Silica Colloidial Crystals, Their Inverse Replicas, And Layered Double Hydroxides

The design of materials with tunable properties is at the forefront of material-based applications. The key to materials design is understanding their fundamental characteristics and establishing a structure-property correlation. This dissertation explores fundamental aspects of synthesis and characterization of functional materials designed using colloidal crystals, inverse replicas, and layered materials for electronics and energy devices applications. We have combined particle assembly and High Pressure confined Chemical Vapor Deposition (HPcCVD) to create ordered and electrically continuous 3D nanostructures of metals and semiconductors, defined as metalattices. These nanostructures have crystalline arrays of uniform particles in which the period of the crystal is close to the characteristic physical length scale of the material, for example, exciton Bohr radius in semiconductors, making them tunable for electronic, plasmonic, thermoelectric and spintronics applications. Silica nanoparticles in the range of 20-120 nm, assembled as micron thick films using vertical deposition technique, were used as templates for metalattice design. The interstices in the colloidal crystal films were infiltrated with polycrystalline semiconductors (Ge/Si/ZnSe) and metals (Ni/Pt/Ag/Pd/Au) using HPcCVD to obtain corresponding metalattices.We have developed a core-shell chemical passivation strategy for Ge metalattice prepared by infiltration of ~70 nm silica colloidal crystal using HPCVD. The oxide-free Ge core shows quantum confinement which depends on the void size in the silica template. The size of Ge sites dictated by the voids in the template and core-shell interdiffusion of Si and Ge can, in principle, be tuned to modify the electronic properties of the Ge metalattice. We have also investigated the structures of colloidal crystalline films and germanium metalattice in detail by scanning electron microscopy (SEM) and small angle x-ray scattering (SAXS). Particles smaller than ~32 nm diameter assemble into body centered cubic, whereas particles larger than 32 nm assemble into random hexagonal close pack structures with 2D hexatic phase. Polycrystalline films of these materials retain their structure, and long-range order upon infiltration at high temperature and pressure, and the structure is preserved in Ge metalattice. This detailed understanding of particle arrangements in the template can help in establishing structure-property relationships in the metalattices. We also explore material design made from layered materials for application in energy systems. We discuss method for controlled assembly of oppositely charged nanosheets using tri-block co-polymer F127 to tune their interactions and study the synthesis and anion exchange of Mg-Al, Zn-Al and Co-Al layered double hydroxides. We characterize their structural, thermochemical, and ionic conduction properties to understand their fundamental behavior for applications as anionic conductors in electrochemical systems operating between 100-250 °C

Chemical exfoliation of MoS2 leads to semiconducting 1T' phase and not the metallic 1T phase

A trigonal phase existing only as small patches on chemically exfoliated few layer, thermodynamically stable 1H phase of MoS2 is believed to influence critically properties of MoS2 based devices. This phase has been most often attributed to the metallic 1T phase. We investigate the electronic structure of chemically exfoliated MoS2 few layered systems using spatially resolved (lesser than 120 nm resolution) photoemission spectroscopy and Raman spectroscopy in conjunction with state-of-the-art electronic structure calculations. On the basis of these results, we establish that the ground state of this phase is a small gap (~90 meV) semiconductor in contrast to most claims in the literature; we also identify the specific trigonal (1T') structure it has among many suggested ones

Synthesis and Characterization of Functional Materials Using Silica Colloidial Crystals, Their Inverse Replicas, and Layered Double Hydroxides

The design of materials with tunable properties is at the forefront of material-based applications. The key to materials design is understanding their fundamental characteristics and establishing a structure-property correlation. This dissertation explores fundamental aspects of synthesis and characterization of functional materials designed using colloidal crystals, inverse replicas, and layered materials for electronics and energy devices applications. We have combined particle assembly and High Pressure confined Chemical Vapor Deposition (HPcCVD) to create ordered and electrically continuous 3D nanostructures of metals and semiconductors, defined as metalattices. These nanostructures have crystalline arrays of uniform particles in which the period of the crystal is close to the characteristic physical length scale of the material, for example, exciton Bohr radius in semiconductors, making them tunable for electronic, plasmonic, thermoelectric and spintronics applications. Silica nanoparticles in the range of 20-120 nm, assembled as micron thick films using vertical deposition technique, were used as templates for metalattice design. The interstices in the colloidal crystal films were infiltrated with polycrystalline semiconductors (Ge/Si/ZnSe) and metals (Ni/Pt/Ag/Pd/Au) using HPcCVD to obtain corresponding metalattices.We have developed a core-shell chemical passivation strategy for Ge metalattice prepared by infiltration of ~70 nm silica colloidal crystal using HPCVD. The oxide-free Ge core shows quantum confinement which depends on the void size in the silica template. The size of Ge sites dictated by the voids in the template and core-shell interdiffusion of Si and Ge can, in principle, be tuned to modify the electronic properties of the Ge metalattice. We have also investigated the structures of colloidal crystalline films and germanium metalattice in detail by scanning electron microscopy (SEM) and small angle x-ray scattering (SAXS). Particles smaller than ~32 nm diameter assemble into body centered cubic, whereas particles larger than 32 nm assemble into random hexagonal close pack structures with 2D hexatic phase. Polycrystalline films of these materials retain their structure, and long-range order upon infiltration at high temperature and pressure, and the structure is preserved in Ge metalattice. This detailed understanding of particle arrangements in the template can help in establishing structure-property relationships in the metalattices.We also explore material design made from layered materials for application in energy systems. We discuss method for controlled assembly of oppositely charged nanosheets using tri-block co-polymer F127 to tune their interactions and study the synthesis and anion exchange of Mg-Al, Zn-Al and Co-Al layered double hydroxides. We characterize their structural, thermochemical, and ionic conduction properties to understand their fundamental behavior for applications as anionic conductors in electrochemical systems operating between 100-250 °C

Behavior of Methylammonium Dipoles in MAPbX(3) (X = Br and I)

Dielectric constants of MAPbX(3) (X = Br, I) in the 1 kHz-1 MHz range show strong temperature dependence near room temperature, in contrast to the nearly temperature -independent dielectric constant of CsPbBr3. This strong temperature dependence for MAPbX(3) in the tetragonal phase is attributed to the MA+ dipoles rotating freely within the probing time scale. This interpretation is supported by ab initio molecular dynamics simulations on MAPbI(3) that establish these dipoles as randomly oriented with a rotational relaxation time scale of similar to 7 ps at 300 K. Further, we probe the intriguing possibility of transient polarization of these dipoles following a photo excitation process with important consequences on the photovoltaic efficiency, using a photoexcitation pump and second harmonic generation efficiency as a probe with delay times spanning 100 fs-1.8 ns. The absence of a second harmonic signal at any delay time rules out the possibility of any transient ferroelectric state under photoexcitation

Is CH3NH3PbI3 Polar?

In view of the continued controversy concerning the polar/nonpolar nature of the hybrid perovskite system, CH3NH3PbI3, we report the first investigation of a time resolved pump probe measurement of the second harmonic generation efficiency as well as using its more traditional form as a sensitive probe of the absence/presence of the center of inversion in the system both in its excited and ground states, respectively. Our results clearly show that SHG efficiency, if nonzero, is below the limit of detection, strongly indicative of a nonpolar or centrosymmetric structure. Our results on the same samples, based on temperature dependent single crystal X-ray diffraction and P-E loop measurements, are entirely consistent with the above conclusion of a centrosymmetric structure for this compound in all three phases, namely the high temperature cubic phase, the intermediate temperature tetragonal phase and the low temperature orthorhombic phase. It is important to note that all our experimental probes are volume averaging and performed on bulk materials, suggesting that basic material properties of CH3NH3PbI3 are consistent with a centrosymmetric, nonpolar structure

Ultra-low thermal conductivity and acoustic dynamics of Si nanostructured metalattices probed using ultrafast high harmonic beams

We extend optical nanometrology capabilities to smaller dimensions by using tabletop coherent extreme ultraviolet (EUV) beams. Specifically, we characterize thermal transport and acoustic wave propagation in 3D periodic silicon inverse metalattices with <15nm characteristic dimensions. Measurements of the thermal transport demonstrate that metalattices may significantly impede heat flow, making them promising candidates for thermoelectric applications. Extraction of the acoustic wave dispersion down to ~100nm shows good agreement with finite element predictions, confirming that these semiconductor metalattices were fabricated with a very high-quality. These results demonstrate that EUV nanometrology is capable of extracting both dispersion relations, and thermal properties of 3D complex nano-systems, with applications including informed design and process control of nanoscale devices

Ultra-low thermal conductivity and acoustic dynamics of Si nanostructured metalattices probed using ultrafast high harmonic beams

We extend optical nanometrology capabilities to smaller dimensions by using tabletop coherent extreme ultraviolet (EUV) beams. Specifically, we characterize thermal transport and acoustic wave propagation in 3D periodic silicon inverse metalattices with <15nm characteristic dimensions. Measurements of the thermal transport demonstrate that metalattices may significantly impede heat flow, making them promising candidates for thermoelectric applications. Extraction of the acoustic wave dispersion down to ~100nm shows good agreement with finite element predictions, confirming that these semiconductor metalattices were fabricated with a very high-quality. These results demonstrate that EUV nanometrology is capable of extracting both dispersion relations, and thermal properties of 3D complex nano-systems, with applications including informed design and process control of nanoscale devices