Badding Group PSU

Research Overview

General Theme

The synthesis and probing of these materials is often done in confined geometries that allow for unique nanostructures and/or a desired control over chemical and physical behavior.

A unifying theme in the Badding group's research is the use of pressure to synthesize or probe solid state materials. We are interested in materials that have unusual micro or nano structure or chemical/physical behavior and often apply them to problems of significant technological interest. Photonic materials, energy materials for photovoltaics and hydrogen storage, and high strength materials have recently been of particular interest. The synthesis and probing of these materials is often done in confined geometries that allow for unique nanostructures and/or a desired control over chemical and physical behavior.

Pressure is a thermodynamic variable that is as fundamental as temperature, but is relatively underutilized in materials chemistry research.

Pressure is a thermodynamic variable that is as fundamental as temperature, but is relatively underutilized in materials chemistry research. It can, for example, control interatomic distance (without much variation in other quantities such as the entropy) (1), tune reaction chemical kinetics and thermodynamics (often over a much wider range than is possible with temperature) (1), allow for solvents with hybrid liquid-like and gas-like properties, and infiltrate molecules and materials into near atomic scale voids. Superior materials properties or interesting behavior not possible without the use of pressure for chemical synthesis or tuning can thus be obtained. At the micro and nano scales, high pressure chemistry becomes much more straightforward and practical because pressure is force per unit area and the forces involved become very small as the area decreases. We use a wide range of pressure from just above atmospheric (0.1 megapascals) to tens of gigapascals.

Past
Chemistry in Confined Geometries
Carbon Materials
Polymer Nanofibers

Brief Summary of Past Research

"New" Elements

We have shown that "new" elements (2) such as potassium, although alkali elements at ambient pressure, can react chemically like a transition element with a single electron in a d-orbital upon compression to gigapascal pressures. Unusual graphite-like sheets (see AgK2 figure) of potassium atoms (red) intercalated with silver atoms (black) in AgK2 can form, for example (3).

Carbon Materials

We have also investigated carbon materials such as the sp3-bonded "transparent phase", a new form of carbon (4) that forms upon compression of (sp2-bonded) graphite (5), networks formed from benzene molecules (6), diamond formed directly from C60 at ambient temperature (7), and carbon nitrides (8). Multiwavelength Raman spectroscopy (9) with excitation wavelengths from the deep UV to the near IR has proved to be a powerful tool for characterizing these materials. The insights from these curiosity driven studies of carbons reversible rehybridization behavior may help in the search for improved hydrogen storage materials, new phases of carbon and high strength carbon materials.

Materials Discovery by Pressure Tuning

We have used pressure for a "combinatorial" approach to complex materials discovery because fundamental parameters such as orbital overlap can be tuned very precisely and over a wide range (1). For example, our pressure tuning experiments have shown that the product of the temperature and dimensionless figure of merit (ZT) of the most technologically important thermoelectric material, antimony bismuth telluride, can be doubled from ~1 to ~2. (10). These studies may provide insight into how to reproduce this record ZT at ambient pressure.

AgK2 graphite-like potassium (3) formed by high pressure reaction between potassium and silver.

Thermoelectric power versus pressure for Sb1.5Bi 0.5Te 3. The pressure was initially increased to 1.5 GPa and then released. Upon release of pressure, the thermoelectric power returned to the same ambient pressure value. The data points from this first run in the increasing and decreasing pressure directions are coincident with the low-pressure data in the increasing pressure direction from the second run in which the sample was compressed to 9 GPa (10).
Current Research

Confined Chemistry in Extreme Aspect Ratio Nanotemplates for Photonics and More

We use chemistry to develop new ways to organize inorganic semiconductor, metallic, and molecular materials in 1-d templates at dimensions down to the nanoscale.

The controlled hierarchical assembly of individual nanoscale elements such as quantum dots and nanowires into high-quality, precisely designed functional materials and devices provides a compelling challenge in nanoscience. We use chemistry to develop new ways to organize inorganic semiconductor, metallic, and molecular materials in 1-d templates at dimensions down to the nanoscale. The resulting 1-d structures are natural conduits for light, electrons, and flowing gases or fluids, which can be manipulated and coupled together inside them in many useful ways over centimeters to meters long length scales.

Major research focuses are developing high pressure chemistry to make atomically smooth, geometrically perfect structures with high materials quality that have excellent optical and electronic properties and organizing these structures with increasing sophistication in templates.

Silica microstructured optical fiber nanotemplates can have arrays of pores designed in virtually any desired pattern (see MOF figure). These meters long pores can have diameters from microns to nanometers in one template and can be precisely spatially arranged relative to each within nanometers. Our research has demonstrated that by treating these ultra-high aspect ratio pores as high pressure chemical reactors we can fill them void free (11) with unary (12) and compound semiconductors (13) (26) (see cross-sectional SEM figure). The resulting near atomically smooth (14) nano or microscale diameter semiconductor wires and tubes are much longer and more geometrically perfect than structures typically made by conventional nanofabrication methods. They have the further advantage that they are embedded in a macroscale size, rugged, functional template that makes them easy to handle and allows light, electricity and/or flowing fluids to be readily coupled into them. Major research focuses are developing high pressure chemistry to make atomically smooth, geometrically perfect structures with high materials quality that have excellent optical and electronic properties and organizing these structures with increasing sophistication in templates.

Layer by layer deposition in the pores can also form very uniform doped semiconductor homo and heterojunctions (see junction figures) (15, 27). The "outside-in" nature of this template-based approach to annular junctions contrasts with the usual approach of depositing on the exterior of nanowires. Patterning a junction adjacent to a central silica core allows for a unique light coupling scheme that overcomes the difficulties in transferring light from low refractive index silica to high index silicon without a large impedance mismatch (15).

The extreme aspect ratios, spatial organization, and geometric perfection of these wires and junctions make them of interest for a wide range of applications

The extreme aspect ratios, spatial organization, and geometric perfection of these wires and junctions make them of interest for a wide range of applications (30), including high speed optoelectronic fiber devices (15), nanofluidics, all optical signal processing (29), chemical sensors (16), optical waveguides (17), high quality factor resonators (18), subwavelength high resolution infrared imaging (19), non-linear optics (20), solar cells (27), confined 1-d physics, and fiber lasers. To pursue these applications, group members often work with our collaborators, including the Southampton University ORC in the UK, and the Gopalan group at Penn State. Templated high pressure deposition is practical and scalable in view of the small volumes of reactants employed. See the in the news for some outside perspectives on applications.

In addition to inorganic solids, self-assembled molecules (16, 21) can be layered, patterned, and characterized via Raman spectroscopy within the arrays of pores in fiber nanotemplates to provide further flexibility in hierarchically organizing complex materials.

Nearly every aspect of the pathway from molecular precursor to reaction product, including reactant flow, surface chemistry, chemical kinetics and thermodynamics, and nucleation and growth, differs from that under conventional conditions.

The behavior of molecules compressed to high pressures and constrained to the small dimensions of the nano/microreactors is dramatically altered. Nearly every aspect of the pathway from molecular precursor to reaction product, including reactant flow, surface chemistry, chemical kinetics and thermodynamics, and nucleation and growth, differs from that under conventional conditions (11), giving a rich variety of chemical phenomena to investigate. There are kinetic/flow effects that increase reactant concentration (22). Supersonic flow can form nanonozzles (22). The width of the "stagnant" layer of precursor present near the reaction interface at low reaction pressures in large volume reactors is greatly reduced. Competition between single crystal growth and deposition on pore walls must be controlled (23). The nm long mean free paths and associated high molecular collision frequency in silane precursor allows for reaction at low enough temperatures for templated growth of nanowires of hydrogenated amorphous silicon (11), a material that is useful for non-linear optics and solar cells. Dopant molecule reaction kinetics and thermodynamics has to be understood to allow for carefully regulated impurity doping and formation of junctions with a built-in potential (15). We have also shown that the remarkable transport properties of pressure driven precursors allows for the formation of nanopores in semiconductor wires many centimeters long (13). We use a combination of experiment and theory/modeling to determine how chemistry is altered under the unusual reaction conditions employed (11, 12, 22, 23). Exploiting these differences to make materials and structures is central to our research.

Silica microstructured optical fiber templates drawn at the ORC.

Cross-sectional SEM of precise spatial organization of silicon tubes in a silica MOF template. The pores in these silica templates can be as small as 10 nm and meters long.

Schematic of an annular Pt-Si Schottky junction constructed in a pore using high pressure chemistry. Light (red arrows) is guided in the undoped i-Si layer. This device detects light with 3 GHz bandwidth (15).

Cross sectional image of a centimeters long annular doped Pt-Si Schottky junction constructed in a pore using high pressure chemistry (15). The bottom panel shows the Raman spectroscopic q parameter, which reveals changes in doping, across the junction.
Current Research

Rehybridization in Carbon Materials

Unsaturated carbon molecules and materials can rehybridize to sp3-bonded carbon upon compression (4). Hydrogen can affect this rehybridization by bonding to compressed carbon. We are investigating the high pressure behavior of unsaturated carbon molecules as part of the Carnegie Institution of Washington DOE EFREE Energy Frontier Research Center. Applications of interest include hydrogen storage materials, luminescent materials, and high strength materials. Some solids can reversibly rehybridize from sp2 to sp3 bonding and back to the same sp2 bonding upon release of pressure (4). This behavior remains largely not understood and thus probing it is a key focus of our research.

Ambient temperature reversible hydrogenation of carbon is of particular interest because it places strict constraints on the C-H bond energetics: 2 C-H bonds must have an energy close to that of an H-H bond. Most C-H bonds in organic molecules and organic solids have an energy closer to that of H-H.

Our research collaboration with Angela Lueking and Vin Crespi at Penn State has shown that carbons doped with catalytic Pt nanoparticles can be hydrogenated reversibly at room temperature (28). A new mode at ~1180 cm-1 arises in the in Raman spectrum of oxidized and Pt doped activated carbon in the presence of hydrogen and disappears upon removal of hydrogen at ambient temperature. This mode also appears to arise in the Raman spectrum of hydrogen implanted graphene (28), but has not previously been recognized as such. Other carbons, such as graphene, also exhibit the mode, but it does not disappear upon removal of hydrogen. The bond can be convincingly attributed to H because it downshifts by the expected amount upon deuteration. Ambient temperature reversible hydrogenation of carbon is of particular interest because it places strict constraints on the C-H bond energetics: 2 C-H bonds must have an energy close to that of an H-H bond. Most C-H bonds in organic molecules and organic solids have an energy closer to that of H-H. Reversibility is a central issue in hydrogen storage, for example; chemical bonds are often too strong to allow for reversibility while physisorption does not allow for much storage at ambient temperature. Density functional calculations show that certain arrangements of paired H atoms bonded to a graphene sheet allow for bond strengths in the range needed for reversibility. Steric effects associated with the rehybridization of the sp2 graphene carbons to sp3 upon reaction with hydrogen weaken the bond. Unraveling the origin of the reversibility in certain carbons and the lack of it in other carbons is a central goal of this project. The experiments to date show that Raman spectroscopy can give new information at the microscopic level, i.e., the level of bonds, about the reversible and irreversible hydrogenation of carbon by catalytic Pt nanoparticles. Macroscopic measurements and inelastic neutron scattering measurements, for example, do not give such direct microsocopic level information about the C-H bonding.

Characterization of Pt supported on oxidized activated carbon (Pt/ACox) (28). (A) Transmission electron microscopy and (B) high-angle annular dark-field scanning images showing size and distribution of Pt nanoparticles as well as carbon morphology. (C) Raman spectra at 514.4 nm, 0.2 mW showing the carbon D (~1350 cm-1) and G bands (~1590 cm-1) at 298K in a capillary exposed sequentially to He, H2 and He. A new C-H mode appears reversibly at 1184 cm-1 under hydrogen exposure. Relative area and Lorentzian fitting parameters are shown. (D) Raman spectra (enlarged 3x) of a second H2 exposure, a D2 exposure exhibiting a C-D wagging mode at 830 cm-1 in D2, and a CO poisoning that eliminates the reversibility of this feature, with the CH wag remaining after a 72 hour degas at 298K. The sharp features at 817 cm-1 and 1037 cm-1 in the H2 spectra are rotational transitions of free H2. The remaining features at 753 and 859 cm-1 are the free D2 rotational transitions S(5) and S(6).
Current Research

Nanofibers of Non-Melt Processible Polymers

We discovered that pressure driven flow can be used to make nanofibers of polytetrafluoroethylene (Teflon) (24) that cannot be made by other means because of the non-melt processible nature of this polymer. Adhesion of proteins and cells (25) to these materials is now under investigation.

Jet Blown PTFE fibers.