Title: Synthesis and investigation of physical properties of extended inorganic solids. Project A.
Description: A Grand Challenge in chemistry is preparing extended solids with designed structures that have desired properties. We recently have shown that it is possible to prepare many compounds that do not exist on equilibrium phase diagrams. These compounds self-assembly from a designed layered reactant. These compounds include structural isomers - extended inorganic solids that consist of two crystallographically aligned constituents with structurally precise layering schemes and turbostratic disorder between the constituents. Our key hypothesis is that the compositional wavelength (λc) and the compositional waveform (what element is where) (ϖc) built into the modulated elemental reactant is sufficient to kinetically trap a desired solid that consists of a pattern of interwoven layers of two or more different structures that self-assembles from the precursor. Figure 2 shows that we are able to use this approach to successfully synthesize structural isomers - compounds with the same number structural units but a different connectivity. The three STEM Z-contrast images shown in Figure 1, for example, are 3 of the 5 isomers of Nb4Pb4Se12.This project seeks to understand how structure and properties evolve as a function of n, m and the ordering of layers in structural isomers. We will prepare these new isomers of [(PbSe)1+δ]m[NbSe2]n compounds, determine their structures and correlate structural distortions, related to the interplay between surface, interface and bulk free energies, with nanostructure. We will also correlate electrical and superconducting properties with nanostructure, measuring electrical properties to derive carrier concentrations and mobility. This is an exciting new area of research to explore

Title: Synthesis and investigation of physical properties of extended inorganic solids. Project B.
Description: Do you want to make compounds that have never been made before? Researchers in my laboratory have discovered that a broad new class of materials can be systematically prepared with turbostratic disorder between crystallographically aligned constituents with a variety of different structure types. Figure 1 shows a STEM image of an example compound, [(PbSe)0.99]2[WSe2]1. We refer to this new state of matter – layered structures with in-plane crystallinity, chemically and structurally abrupt interfaces, layer-to-layer misregistration, and turbostratic disorder ¬– as ferecrystals (from Latin fere, meaning almost). We prepare these compounds using modulated elemental reactants – a synthesis technique developed in my laboratory that provides new parameters that can be used to control the kinetics of solid-state reactions, avoiding thermodynamically stable reaction intermediates. By controlling layer thicknesses, the ratio of layer thicknesses and the order of the layers we have shown that it is possible to prepare compounds that do not exist on equilibrium phase diagrams that form by self-assembly from the layered reactant. Our key hypothesis is that the compositional wavelength (λc) and the compositional waveform (what element is where) (ϖc) built into the modulated elemental reactant is sufficient to kinetically trap a desired solid that consists of a pattern of interwoven layers of two or more different structures. The targeted compounds must be at least local free energy minima, as illustrated in figure 2. Modulated elemental reactants can be designed to contain the correct ratio of atoms, the correct number of atoms to form a specific thickness of each substance, and a long range ordering that approximates that of the desired compound. This project seeks to understand how properties evolve as a function of n, m and the ordering of layers in structural isomers at each composition. We will use the [(PbSe)1+?]m[TiSe2]n compounds as a baseline for structure and property comparisons for the other targeted systems. We will compare the evolution of structural distortions between the [(PbSe)1+?]m[TiSe2]n and [(SnSe)1+?]m[TiSe2]n compounds, since SnSe does not exist with a rock salt structure as a binary compound but has a rock salt structure within ferecrystals, to explore the interplay between surface/interface and bulk free energies. We will seek to determine the m value where the SnSe reverts back to the bulk structure rather than adopting the rock salt structure. Synthesizing LaSe and CeSe containing compounds would permit us to examine the effect of charge transfer between constituents on structure and on physical properties, initially through comparing carrier concentrations for PbSe, SnSe, LaSe and CeSe containing ferecrystals with identical n and m values. We will also test the limit of stability of [MSe]m[TSe2]n compounds where M=Ce or La, as either large values of m or large m to n ratios may be unstable due to charge transfer between constituents.

Title: Experimentally characterize the structure of inorganic clusters (used as thin-film precursors) by applying both solution-phase and solid-state NMR.
Description: We propose to experimentally characterize the structure of inorganic clusters (used as thin-film precursors) by applying both solution-phase and solid-state NMR. The aim is to provide structure and dynamics information about species such as the [Al13(μ3-OH)6(μ-OH)18 (H2O)24]15+ (“Al13”) and [Ga13(μ3-OH)6(μ-OH)18(H2O)24]15+ (“Ga13”) clusters synthesized by teams at UofO and OSU, and other structural variants. NMR is an inherently element-selective, quantitative, non-destructive form of spectroscopy that provides a unique window into reactions. We intend to examine species in the solid-state—which are particularly important for thin film characterization. We also intend to explore solution-phase NMR to locate sites on the cluster for ligand exchange, degradation, and potentially metal- or oxygen-exchange. Skills that will be used include solid-state NMR and solution-phase NMR of less common nuclei (such as 69/71Ga, 27Al), and abundant 1H species. Students can expect to learn techniques for data analysis and sample handling. We have access to a full array of materials characterization facilities at WashU that a student intern may also be able to utilize (i.e., XRD, TEM, SEM).

Title: Increasing the dielectric constant of solution deposited metal oxide thin films
Description: Research in the Page lab mainly deals with metal-oxide thin-films deposited from aqueous solutions. We are currently working on dielectric films with the specific aim of increasing the dielectric constant which is related to the polarizability. Typically, the dielectric constants of metal-oxides are relatively low. Incorporation of alkali-metals can increase the dielectric constants far beyond those of simpler systems while retaining many of the desirable properties of the original metal-oxide. Relatively mobile ions within the metal-oxide framework should introduce new variables, so a range of alkali-metals will be investigated. The goal is to correlate film properties to the identity of the alkali-metal introduced into the films. We will develop a simple procedure for producing such alkali-metal metal-oxide films from aqueous solution (in the absence of organics) and characterize them using a variety of instrumentation and device configurations. Research in the Page lab mainly deals with metal-oxide thin-films deposited from aqueous solutions. We are currently working on dielectric films with the specific aim of increasing the dielectric constant which is related to the polarizability. Typically, the dielectric constants of metal-oxides are relatively low. Incorporation of alkali-metals can increase the dielectric constants far beyond those of simpler systems while retaining many of the desirable properties of the original metal-oxide. Relatively mobile ions within the metal-oxide framework should introduce new variables, so a range of alkali-metals will be investigated. The goal is to correlate film properties to the identity of the alkali-metal introduced into the films. We will develop a simple procedure for producing such alkali-metal metal-oxide films from aqueous solution (in the absence of organics) and characterize them using a variety of instrumentation and device configurations.

This project offers a great introduction to some basic solid-state chemistry, exposure to a variety of techniques commonly used to characterize materials and a chance to fabricate and characterize simple electronic devices. The simplicity of film production and device fabrication process will allow plenty of time for data collection and interpretation.