David W. Tomlin

Morphology of dispersed carbon single-walled nanotubes

Using scattering methods, we determine the morphology of carbon nanotube suspensions over length scales from 1 nm to 50 μm. We find no evidence of rod-like character at any length. Rather, a network structure of aggregated tubes, similar to that seen in dry samples, is found. These observations have significant implications regarding the use of single-walled nanotubes as a composite reinforcing filler since the network structure has significantly lower modulus than fully dispersed tubes. We also show that it is possible to isolate a rod-like fraction from the aggregated suspension using intense sonication, providing a potential route to fully dispersed nanotubes.

Voltage Creep in Holographic PDLC Gratings

In holographic Bragg gratings formed by anisotropic photopolymerization of free-radical monomers, an electric field is required to switch the grating between a diffracting and a transmitting state. The voltage necessary to turn the grating completely off is defined as the switching voltage. In this work, we report on the gradual increase of this switching voltage with time after initial fabrication. The switching field increases approximately 60% over a seven-day period for a pentaacrylate/E7 material system. Using dielectric measurements, it is observed that the resistance of the cell does not change over time while the capacitance decreases. FT-IR spectroscopy was also used to explore the continued post-polymerization after fabrication of the gratings. Increases in switching field were observed for samples fully illuminated with light as well as those kept in the dark. We speculate that the rise in the switching voltage is caused by post polymerization of residual reactive moieties located near the polymer/nematic droplet interface. An increase in the amount of interfacial area generated by post-polymerization yields more surface area for the LC molecules to bind; thus a larger field is needed to switch.

Investigation of Transition Metal-Xanthate Complexes for Nonlinear Optical Applications

We have synthesized several metal xanthate complexes from metal salts and xanthate potassium salts. Powders of several of the materials were sorted and tested for their nonlinear optical properties. Cadmium xanthate demonstrated phase matchability for frequency doubling of 1.06 micron light and generated a very strong second harmonic signal.

Holographic photopolymerization for fabrication of electrically switchable inorganic-organic hybrid photonic structures

Holography offers a versatile, rapid and volume scalable approach for making large area, multi-dimensional, organic PBGs; however, the small refractive index contrast of organics prevents formation of a complete band-gap. The introduction of inorganic nanoparticles to the structure provides a possible solution. In contrast to the multiple steps (exposure, development and infiltration) necessitated by lithographic-based holography (e.g. photoresists), holographic photopolymerization of monomer-nanoparticle suspensions enables one-step fabrication of multidimensional organic-inorganic photonic band gap (PBG) structures with high refractive index contrast. The PBGs are formed by segregation of semiconductor nanocrystals during polymerization of the polymer network. Addition of CdSe/ZnS polymerization of the highly cross-linked polymer network. Addition of CdSe/ZnS quantum dots or ZnO nanocrystals to the H-PDLCs formulation results in phase segregation of the nanoparticles into the liquid crystal rich lamellae, producing photonic structures with high diffraction efficiencies that may be modulated by application of an external electric field.

Growth of carbon nanotubes by sublimation of silicon carbide substrates

Aligned carbon nanotubes (CNT’s) have been found to form on both the Si and C faces of silicon carbide (SiC) wafers at high temperature. The CNT’s form when the SiC wafer is exposed to temperatures in the range 1400-1700°C under moderate vacuum. The CNT’s are aligned roughly parallel to the surface. After a half hour at 1700°C under vacuum of 10-4torr, a near continuous CNT layer about 250nm thick is formed. The entire surface of the SiC is covered with CNT’s including both single and multiwalled tubes, and some graphitic carbon. SEM, TEM, AFM, XPS and Raman scattering measurements have been used to analyse the CNT/SiC structures. The metal catalyst free CNT’s on SiC exhibit low density of structural defects and are very straight. The carbon source is believed to be residual carbon from the SiC left on the surface after preferential evaporation of Si. It is speculated that CNTs growth is catalysed by low concentrations of residual oxygen in the chamber during growth. The vacuum conditions can significantly affect CNTs growth. Single wall carbon nanotubes are evident in Raman spectra on the samples grown at 10-3 Torr, not on these grown at 10-5Torr.

Hybrid polymer-based photovoltaics via carbon nanotubes and electrostatic self-assembly

Recently, there has been increased interest in polymer-based photovoltaic devices due to their promise for the creation of lightweight, flexible, and inexpensive electrical power. WE examined the possibility of using nanoparticles and nanoparticles with tailored interfaces for the creation of hybrid polymer-based devices with enhanced photovoltaic response. Initially, we investigated the incorporation of multi-walled carbon nanotubes (MWNT) in the poly(benzimidazo-benzophenanthroline) ladder (BBL) layer of two-layer poly(p-phenylene vinylene)(PPV)-BBL photovoltaic devices. Subsequently, we explored the possibility of tuning polymer-particle interfaces through the creation of core-shell particles fabricated using electrostatic self- assembly. For the PPV/BBL(MWNT) devices, a doubling of the photocurrent and a drastic reduction in photovoltage with MWNT incorporation is observed for a range of BBL layer thickness values. This behavior is consistent with the MWNTs functioning as a three dimensional extension of the top aluminum electrode. Fabrication studies on core-shell particles demonstrate that the interfacial properties of a variety of particles can be manipulated, shells of up to 10 bilayers can be achieved, and TiO2 nanoparticles with PPV polymer shells are possible.

Real Time Study of Reflective H-PDLC Gratings

The formation of holographic reflection gratings in polymer dispersed liquid crystals(H-PDLC) was investigated in real time both in reflection and transmission modes. Real time study is useful for obtaining information on the speed of grating formation, phase separation dynamics, host polymer shrinkage, polarization dependence of the reading beam, bleaching of the photointiator dye and scattering of the writing beam during grating formation. The real time studies showed that the formation of the grating is fast and reaches a maximum in a few seconds. The diffraction efficiency (DE) VS time curve shows an initial increase followed by a sharp decrease and a recovery to higher efficiencies. Variation of monomer functionality in the pre-polymer syrup resulted in a decrease of reflection efficiency as the functionality is lowered. Varying the writing power of the laser beam gave higher efficiencies at powers exceeding 10 mW/cm 2 . Liquid crystal (LC) loadings of >16% are needed to obtain measurable efficiencies. Varying the polarization of the probe beam (S vs P) revealed differences in the temporal evolution of DE. Morphology studies are indicative of the differences in the phase separation due to monomer functionality, LC concentration and also laser power. The growth dynamics of H-PDLC gratings are very different from the well known Du Pont photopolymer films which may be due to the phase separation process accompanying gelation in H-PDLC systems.