Badding Research Group

Research

Germanium sulfide tube fabricated inside an MOF.

Empty "honeycomb" large air fraction MOF fabricated at the University of Southampton Optoelectronics Research Centre.

Annular filling of a single MOF capillary with germanium.

Long radial Si-Ge heterojunction incorporated within a MOF capillary 6 microns in diameter.

Array of long germanium wires etched out of a honeycomb fiber template

Example of a jet blown PTFE fiber mat.

Pressure tuning of the thermoelectric power of Sb0.51.50.5BiTe3. The thermoelectric power increases substantially, doubling the figure of merit that determines device efficiency.

UV Raman Spectrum of a diamond-like carbon film.

Research Projects

Microstructured Optical Fibers as High Pressure Chemical Reactors
Jet Blowing of Polymer Nanofibers and Nanocomposites
Thermoelectric Materials and Inorganic/Organic Nanostructures and Composites
Carbon-based Hydrogen Storage Materials

Microstructured Optical Fibers as High Pressure Chemical Reactors

This very collaborative (Pier Sazio; University of Southampton, Venkat Gopalan, and Vin Crespi) and interdisciplinary work is oriented towards incorporating a broad range of materials into the capillary holes within "microstructured optical fibers" (MOFs) to enable behaviors and devices within such fibers that have not previously been possible. Fabrication of advanced photonic, electronic, and plasmonic devices requires the capability to spatially organize materials with great complexity at dimensions down to the nanoscale.  Traditionally this has been done in a planar geometry by fabrication processes that selectively combine photolithography, etching, and deposition.   We have developed an approach to fabrication of fiber structures that combines select aspects of both these approaches: flowing precursors at high pressure are used to deposit extremely smooth annular films that can be layered with different materials within the confines of arrays of micro/nanoscale capillaries in MOFs, see figure left for an example of an unfilled MOF. Extreme aspect ratio (105-106) semiconductor and metal tubes, wires, and heterojunctions can be formed within holes less than 100nm in diameter and up to centimetre to metre length scales, long enough for a wide range of devices. See the figure below for an example of a simple in-fiber field effect transistor (FET).   Because suitability for drawing is not a materials constraint, the fiber filling approach allows for a much wider range of materials to be incorporated into nanostructured fibers than traditional drawing approaches.  Longitudinal structuring of the deposition by means of light impinging from the side of the MOFs can also be realized.

Formation of wires in hosts such as mesoporous silica or nanoporous anodic alumina membranes is known, but these materials do not have the functionality or transparency of MOFs. The holes and thus the wires formed within them are relatively short and their spatial configuration relative to each other cannot be controlled with the flexibility of the holes in MOF metamaterials. Holes with diameters from tens of microns down to less than 20 nm can be incorporated into a single MOF in virtually any desired periodic or aperiodic spatial configuration.   The air fraction of the MOF can be varied from nearly zero to in excess of 80 % and the shapes of the holes can be varied considerably. MOFs are now versatile photonic devices and numerous properties can be controlled, including the chromatic dispersion, modal field confinement , birefringence and non-linearity with applications in metrology, sensors, amplifiers and fiber lasers.

There are significant opportunities for increasing the functionality of silica MOF’s yet much more by incorporating metals or semiconductors within them, an ongoing focus of our research. This MOF/CVD platform technology enables the fabrication of materials, structures, and devices that have great potential for impact in diverse areas ranging from biomedicine to nanophotonics and electronics to telecommunications. From Telephony Magazine: "The ultimate in all-optical (telecommunications) networks may be all-fiber networks, in which the functions performed today by big hunks of hardware (switching, monitoring, etc.) are in the future performed beneath the glass of the fiber itself." Our realization of in fiber structures composed of crystalline semicondcutors thus respresents a step forward towards a vision of "All Fiber Optoelectronics."

Support for this research comes from the National Science Foundation via grant DMR-0502906 and also from the Penn State Center for Nanoscale Science, which is supported by grant DMR-0213623. The Penn State-Lehigh Center for Optical Technologies also provides support. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF).


Jet Blowing of Polymer Nanofibers and Nanocomposites

Polytetrafluoroethylene (PTFE) fluoropolymer exhibits extreme chemical and thermal stability, low friction coefficients and many other exceptional properties. Unfortunately, very high molecular weight PTFE has an extremely high melt viscosity and negligible solubility in common solvents. Under tensile stress, molten PTFE elongates and breaks rather than forming fibers and processing it is generally laborious and expensive . Together with Ayusman Sen, we have developed a simple, environmentally friendly, single step, solvent-free technique to process very high molecular weight PTFE and a broad range of other polymers, including those that are intractable or difficult to process, inside of a high pressure jet of gases such as nitrogen or argon into mats of micro and nanofibers that are up to several mm long. Plasticization of the PTFE by the hot, high pressure gases within the jet and extensional stretching in the jet nozzle appear to facilitate fiber formation even at temperatures below the melting point. Polarized Raman spectra demonstrate that the PTFE polymer chains exhibit substantial alignment along the fiber axis. The fibers are produced at a rapid rate and adhere to many different materials, allowing for facile fabrication of surface modifying coatings and dense fibrous mats to control properties such as surface hydrophobicity, drag, and biocompatibility.

Jet blowing is performed by spraying a two-phase mixture of gas and solid polymer at elevated temperature and pressure through a single capillary orifice.   Thus far we have used orifices ranging in diameter from 50 m m to 150 m m and in lengths from 0.5 to 2 mm long.   Behind the orifice the flowing high pressure gas and polymer are mixed in a tube that is larger in diameter, such that there is an approximately 25:1 axisymmetric contraction. The flow driven through this abrupt contraction at the entrance of the capillary by the large pressure drop has strong extensional and shear components, which result in deformation, extension, and reorientation of the polymer chains to form fibers.   It is not possible to form fibers of PTFE by conventional melt blowing or electrospinning, in which a polymer extruded through an orifice is stretched outside of it by a pair of nearly parallel impinging jets of gas or an electric field, respectively. Thus PTFE is typically considered to be neither melt nor solvent processible. In contrast, during the jet blowing process a two phase mixture of gas and polymer is driven by pressure through a single nozzle, resulting in fiber formation inside the nozzle accompanied by much higher shear rates and extensional stretching than are otherwise possible.   Jet blowing is different from a simple extrusion process because the multiple fibers formed in parallel within the nozzle are much smaller than the nozzle diameter.

PTFE is intrinsically hydrophobic and the complex structure down to the nanoscale of the surfaces coated with jet blown fibers might be expected to further increase the hydrophobicity.   There is considerable interest in the fabrication of superhydrophic surfaces from both scientific and applied perspectives. Water droplets placed upon the surface of jet blown PTFE 601A fibers exhibit a contact angle of 147°, much higher than the contact angle of a flat PTFE surface, which is 113° . Thus jet blowing allows for single step fabrication of very hydrophobic surfaces that exhibit extreme chemical inertness. We are currently collaborating with Mike Pishko and Tim Ritty (Hershey Medical Center) to characterize the surface biological properties of these novel PTFE materials.

Thermoelectric Materials and Inorganic/Organic Nanostructures and Composites

Our group has had a long-standing interest in thermolectric materials, which allow for the fabrication of solid state devices that can directly convert heat into electricity and use electricity to cool. In the past , we have shown that dramatic improvments, to unprecented thermoelectric figures of merit, are possible by means of pressure-tuning these materials. It is thus now clear that pressure tuning can be used to rapidly optimize and rapidly demonstrate the potential of novel thermoelectric materials.

Our more recent efforts in this area now involve nanostructures and composites. It is currently thought that nanostructured thermoelectric materials may have superior thermoelectric figure of merit, ZT. In collaboration with Aysuman Sen, David Allara, and Gerald Mahan, we are synthesizing new examples of inorganic/organic nanostructures and composites that may exhibit improved thermoelectric properties or other types of improved behavior in view of their carefully designed structural and interfacial properties.

Carbon-based Hydrogen Storage Materials


Our group has had a long-standing interest in the synthesis and characterization under pressure of carbon materials. We are now collaborating with Angela Lueking to investigate nanostructured carbon materials for hydrogen storage applications.

To characterize these materials, we are using high pressure techniques, x-ray diffraction, electron microsocopy, infrared spectroscopy, visible Raman spectroscopy and, recently, continuous wave (cw) ultraviolet (257 nm) Raman spectroscopy, a technique which has emerged within the past few years. There are numerous advantages to cw UV Raman, including reduced interference from flourescence, the ability to observe Raman spectra of sp3 bonded carbon in the presence of substantial amounts of sp2 bonded carbon, and the ability to collect Raman spectra at very high temperatures. This type of spectroscopy opens a multitude of opportunities for novel science in carbon-based materials.