Erik K. Hobbie

Photoelectrochemical complexes for solar energy conversion that chemically and autonomously regenerate

Naturally occurring photosynthetic systems use elaborate pathways of self-repair to limit the impact of photo-damage. Herein, we demonstrate a complex that mimics this process consisting of two recombinant proteins, phospholipids and a carbon nanotube. The components self-assemble into a configuration in which an array of lipid bilayers aggregate on the surface of the carbon nanotube, creating a platform for the attachment of light-converting proteins. The system can disassemble upon the addition of a surfactant and reassemble on its removal over an indefinite number of cycles. The assembly is thermodynamically meta-stable and can only transition reversibly if the rate of surfactant removal exceeds about 10−5 sec−1. Only in the assembled state do the complexes exhibit photoelectrochemical activity. We demonstrate a regeneration cycle that uses surfactant to switch between assembled and disassembled states, resulting in increased photo-conversion efficiency of more than 300% over 168 hours and an indefinite extension of the systems lifetime.

Size Separation of Single-Wall Carbon Nanotubes by Flow-Field Flow Fractionation

Flow-field flow fractionation (flow-FFF) is used to separate single wall carbon nanotubes (SWNTs) dispersed in aqueous medium by the use of DNA. Online measurements are made of SWNT concentration, molar mass, and size by using UV-vis absorption and multiangle light scattering (MALS). Separations are made of both unfractionated SWNTs and SWNT fractions made by use of size exclusion chromatography (SEC). The SEC fractions are well resolved by flow-FFF. SWNT hydrodynamic volume from calibrations with polymer latex particles in flow-FFF are compared to calibrations of hydrodynamic volume from the SEC fractions derived from dissolved polymers. Rod lengths of the SWNTs are calculated from online measurements of MALS and those are compared to rod lengths from hydrodynamic models based on latex sphere calibrations. Samples with varied sizes were prepared by fracturing SWNTs through extended sonication. Flow-FFF of these fractured samples shows very broad size distributions compared to the original SEC and flow-FFF fractions.

Orientation of carbon nanotubes in a sheared polymer melt

Optical measurements of the shear response of semidilute dispersions of polymer-dispersed multiwalled carbon nanotubes are presented. For a weakly elastic polymer melt, the data suggest that the semiflexible tubes orient along the direction of flow at low shear stress, with a transition to vorticity alignment above a critical shear stress, σc, corresponding to a critical Deborah number of approximately 0.15. In contrast, data for a highly elastic polymer solution suggest that the tubes orient with the flow field at high shear rates, in the limit of large Deborah number. The measurements are in qualitative agreement with previous experimental and theoretical studies of fiber orientation in elastic fluids under simple shear flow.

Competing growth kinetics in simultaneously crystallizing and phase-separating polymer blends

The kinetic interplay between crystal superstructure growth and late-stage liquid phase coarsening in a polymer blend has been examined. By controlling the relative quench depths for liquid–liquid phase separation and crystallization, the growth kinetics of the characteristic length scales of the simultaneous ordering processes show a crossover from crystallization dominated to phase-separation dominated behavior. Based on a scaling argument for late-stage coarsening during spinodal decomposition, we argue that this kinetic crossover is inevitable in a blend for which the critical temperature of liquid–liquid phase separation is well above the equilibrium melting temperature of the blend.

Rheo-optical studies of carbon nanotube suspensions.

We use a polarization-modulation technique to investigate the optical anisotropy of multi- and single-wall carbon nanotubes suspended in a variety of solvents under simple shear flow. Measurements of birefringence and dichroism are performed as a function of shear rate, tube concentration, and solvent viscosity. At fixed volume fraction, the anisotropy increases with increasing shear stress due to enhanced flow alignment. At fixed shear stress, the anisotropy increases with volume fraction due to rotational excluded-volume interactions. By considering the rotational diffusivity as a function of nanotube length, diameter, concentration, and solvent viscosity, we demonstrate a leading-order scaling relation for the optical anisotropy in terms of rotary Peclet number Pe. At low Pe, our results are in qualitative agreement with the theoretical predictions of Doi and Edwards. At high Pe, our data suggest that the degree of nanotube alignment scales as Pe16.

Electronic durability of flexible transparent films from type-specific single-wall carbon nanotubes.

The coupling between mechanical flexibility and electronic performance is evaluated for thin films of metallic and semiconducting single-wall carbon nanotubes (SWCNTs) deposited on compliant supports. Percolated networks of type-purified SWCNTs are assembled as thin conducting coatings on elastic polymer substrates, and the sheet resistance is measured as a function of compression and cyclic strain through impedance spectroscopy. The wrinkling topography, microstructure and transparency of the films are independently characterized using optical microscopy, electron microscopy, and optical absorption spectroscopy. Thin films made from metallic SWCNTs show better durability as flexible transparent conductive coatings, which we attribute to a combination of superior mechanical performance and higher interfacial conductivity.

Carbon Nanotubes: Measuring Dispersion and Length

Advanced technological uses of single-walled carbon nanotubes (SWCNTs) rely on the production of single length and chirality populations that are currently only available through liquid-phase post processing. The foundation of all of these processing steps is the attainment of individualized nanotube dispersions in solution. An understanding of the colloidal properties of the dispersed SWCNTs can then be used to design appropriate conditions for separations. In many instances nanotube size, particularly length, is especially active in determining the properties achievable in a given population, and, thus, there is a critical need for measurement technologies for both length distribution and effective separation techniques. In this Progress Report, the current state of the art for measuring dispersion and length populations, including separations, is documented, and examples are used to demonstrate the desirability of addressing these parameters.

Optical measurements of structure and orientation in sheared carbon-nanotube suspensions

We describe an optical metrology for measuring shear-induced structure and orientation in dilute dispersions of multiwalled carbon nanotubes. Small-angle polarized light scattering and optical microscopy are combined in situ to quantify the structural anisotropy of multiwalled carbon nanotubes in semidilute, surfactant-stabilized aqueous suspensions under simple shear flow. Measurements performed as a function of the applied shear rate are used to demonstrate the capabilities and limitations of the experimental technique, which should be suitable for probing the shear response of polymer-nanotube melts and solutions.

Purifying Colloidal Nanoparticles through Ultracentrifugation with Implications for Interfaces and Materials

Liquid-phase processing and colloidal self-assembly will be critical to the successful implementation of nanotechnology in the next generation of materials and devices. A key hurdle to realizing this will be the development of efficient methods to purify nanomaterials composed of a variety of shapes, including nanocrystals, nanotubes, and nanoplates. Although density-gradient ultracentrifugation (DGU) has long been appreciated as a valuable tool for separating biological macromolecules and components, the method has recently emerged as an effective way to purify colloidal nanoparticles by size and optical and electronic properties. In this feature article, we review our recent contributions to this growing field, with an emphasis on some of the implications that our results have for interfaces and materials. Through transient or isopycnic DGU performed in both aqueous and organic environments, we demonstrate some explicit examples of how the mechanical, electronic, and optical properties of thin films assembled from two specific colloidal nanomaterials--single-walled carbon nanotubes and silicon nanocrystals--can be modified in response to fractionation.

Temperature Dependent Photoluminescence of Size-Purified Silicon Nanocrystals

The photoluminescence (PL) of size-purified silicon nanocrystals is measured as a function of temperature and nanoparticle size for pure nanocrystal films and polydimethylsiloxane (PDMS) nanocomposites. The temperature dependence of the bandgap is the same for both sample types, being measurably different from that of bulk silicon because of quantum confinement. Our results also suggest weaker interparticle and environmental coupling in the nanocomposites, with enhanced PL and an unexpected dependence of lifetime on size for the pure nanocrystal films at low temperatures. We interpret these results through differences in the low-temperature size dependence of the ensemble nonradiative equilibrium constants. The response of the PDMS nanocomposites provides a consistent measure of local temperature through intensity, lifetime, and wavelength in a polymer-dispersed morphology suitable for biomedical applications, and we exploit this to fabricate a small-footprint fiber-optic cryothermometer. A comparison of the two sample types offers fundamental insight into the photoluminescent behavior of silicon nanocrystal ensembles.