Douglas Chinn

Multiple resonances in microdisk lasers of π-conjugated polymers

We have fabricated microdisk lasers from π-conjugated polymers that show multiline emission spectrum upon optical excitation. Using Fourier transform analysis, each emission line is assigned integer to an interger Bessel function that helps to estimate the field distribution inside the photoexcited polymer microdisk. We found that the microdisk can sustain two different laser modes with different, though complementary, field distributions and that the index of refraction decreases with increasing excitation intensity.

Exploratory search for improved oxidizing agents used in the reduction of surface leakage currents of CdZnTe detectors

We have studied the ability of different oxidizing agents, other than H2O2 to reduce the surface leakage current of CdZnTe devices. All chemical treatments were performed in aqueous solutions, at room temperature, with weight percent concentrations of 2.5g/25ml. Before and after I-V curves were obtained. It was found that by increasing the basicity of the chemical treatment, greater reduction in surface leakage current occurred. The result show that these alternative chemical treatments reduced the surface leakage current as well as or better than H2O2 chemical treatment.

MEMS-based bubble pressure sensor for prosthetic socket interface pressure measurement

The ability to chronically monitor pressure at the prosthetic socket/residual limb interface could provide important data to the research and clinical communities. With this application in mind, we describe a novel type of sensor which consists of a MEMS pressure sensor and custom electronics packaged in a fluid-filled bubble. The sensor is characterized and compared to two commercially-available technologies. The bubble sensor has excellent drift performance and good sensing resolution. It exhibits hysteresis which may be due to the silicone that the sensor is molded in. To reduce hysteresis, it may be advisable to place the sensor between the liner and the socket rather molding directly into the liner.

Postpolymerization alignment of bulk conjugated polymers

We have demonstrated postpolymerization stretch alignment of substituted conjugated polymers as evidenced by polarized infrared spectroscopy, with a Hermans orientation function value of 0.66 for a stretch ratio of 3.7. The effects of stretch rate and substrate on the material order are examined. We further show that the stretch-induced increase in long-range order leads to an increase in photoconductive response parallel to the stretch direction, and a decrease in the orthogonal direction. The introduction of small amounts of substituted fullerenes does not interfere with the ordering of the polymer on a bulk scale.

Order and charge collection correlations in organic materials for neutron detection

Organic materials, and in particular, poly(p-phenylene vinylene)s, are being investigated for solid state neutron detection. Semiconducting organics can offer direct detection because of high resistivity, high dielectric strength, natural gamma discrimination due to low Z, and room temperature operation. However, the effective charge collection is dependant on several material processing variables, including solvent choice and concentration, substrate, deposition method and conditions, post-deposition processing, and other factors, all of which can influence the local and bulk order of the material. We have investigated the effects of processing variables on the material order through infrared dichroism. The charge collection of the device was measured with visible laser excitation, and related to the order.

Studies of π-conjugated polymer coupled microlasers

A resonance structure formed from two connected microdisks was fabricated on a film of dioctyloxy poly-phenlene vinylene (DOO-PPV). When optically excited above the laser threshold this structure showed multimode laser emission, which at high intensities form a whispering gallery (WG) mode that traverses both microdisks.

Computational and experimental study of nanoporous membranes for water desalination and decontamination.

Fundamentals of ion transport in nanopores were studied through a joint experimental and computational effort. The study evaluated both nanoporous polymer membranes and track-etched nanoporous polycarbonate membranes. The track-etched membranes provide a geometrically well characterized platform, while the polymer membranes are more closely related to ion exchange systems currently deployed in RO and ED applications. The experimental effort explored transport properties of the different membrane materials. Poly(aniline) membranes showed that flux could be controlled by templating with molecules of defined size. Track-etched polycarbonate membranes were modified using oxygen plasma treatments, UV-ozone exposure, and UV-ozone with thermal grafting, providing an avenue to functionalized membranes, increased wettability, and improved surface characteristic lifetimes. The modeling effort resulted in a novel multiphysics multiscale simulation model for field-driven transport in nanopores. This model was applied to a parametric study of the effects of pore charge and field strength on ion transport and charge exclusion in a nanopore representative of a track-etched polycarbonate membrane. The goal of this research was to uncover the factors that control the flux of ions through a nanoporous material and to develop tools and capabilities for further studies. Continuation studies will build toward more specific applications, such as polymers with attached sulfonate groups, and complex modeling methods and geometries.

Transport Properties of Stretch-Oriented PPV Films

Organic semiconductors are under investigation for radiation sensors at Sandia National Laboratories. The wide band gaps, high resistivities, low dielectric constants, and high dielectric strengths of conjugated polymers suggest these materials may be suitable for solid-state particle counting detectors. A range of solution cast materials have been evaluated for this application, including polythiophenes and poly( p -phenylene vinylene)s, or PPVs. Films were prepared by novel solution casting and mechanical stretching methods. Device structures including interdigital metal electrodes on glass and thin film transistors on SiO 2 on silicon were fabricated by drop casting from solution and lamination of solid films. Transient and DC responses were recorded and analyzed. Experiments include laser stimulus for photoconductive pulse response, and field effect transistor testing. Mechanical stretching was shown to dramatically alter electrical properties of polymer films. Future work will analyze the feasibility of single particle detection and analyze various geometries for optimization. The effects of traps and methods for reduction of trapping effects will be analyzed.