Anamaris Meléndez

Disordered grain growth in polycrystalline GaN obtained by the polymer-derived-ceramic route

Polycrystalline GaN fibers have been produced by the polymer-derived-ceramic (PDC) technique. The wurtzite-polymorphic fibers appear to emerge from complex nucleation and grain growth mechanisms, being mostly unconstrained during initial polymer to ceramic conversion. The importance of carrier polymer architecture and alignment is highlighted towards controlled microstructure.

Electrical Properties of Electrospun Sb-Doped Tin Oxide Nanofibers

Transparent and conducting tin oxide fibers are of considerable interest for solar energy conversion, sensors and in various electrode applications. Appropriate doping can further enhance the conductivity of the fibers without loosing optical transparency. Undoped and antimony-doped tin oxide fibers have been synthesized by our group in previous work using electrospinning and metallorganic decomposition techniques. The undoped tin oxide fibers were obtained using a mixture of pure tin oxide sol made from tin (IV) chloride : water : propanol : isopropanol at a molar ratio of 1:9:9:6, and a viscous solution made from poly(ethylene oxide) (PEO) and chloroform at a ratio of 200 mg PEO/10 mL chloroform. In this work, antimony doped fibers were obtained by adding a dopant solution of antimony trichloride and isopropanol at a ratio of 2.2812 g antimony trichloride/10 ml isopropanol to the original tin oxide precursor solution. The Sb concentration in the precursor solution is 1.5%. After deposition, the fibers were sintered 600°C in air for two hours. The electrical conductivity of single fibers measured at room temperature increases by up to three orders of magnitude when compared to undoped fibers prepared using the same method. The resistivity change as a function of the annealing temperature can be attributed to the thermally activated formation of a nearly stoichoimetric solid. The resistivity of the fibers changes monotonically with temperature from 714Ω-cm at 2 K to 0.1Ω-cm at 300 K. In the temperature range from 2 to 8 K the fibers have a positive magnetoresistance (MR) with the highest value of 155 % at 2 K and ±9 T. At temperatures of 10 and 12 K the sign of MR changes to negative values for low magnetic fields and positive for high magnetic fields. For higher temperatures (15 K and above) the MR becomes negative and its magnitude decreases with temperature.

CNT Dispersion and Precursor Synthesis for Electrospinning of Polymer-CNT Composites

Multiwall Carbon Nanotubes (MWCNTs) composites fabricated in the form of fibers with large surface areas were used in the development of important technological applications such as photoactuators. MWCNT-polymer fibers can be prepared with the simple and fast technique of electrospinning. The precursor for electrospinning was prepared by adding dispersed MWCNTs to a polymeric solution of Poly(dimethylsiloxane) and Poly(methylmethacrylate) dissolved in Tetrahydrofuran (THF) and Dimethylformamide (DMF). The dispersion of the carbon nanotubes in Sodium Dodecyl Sulfate (SDS)/water is expected to enhance the photoactuation properties of the Polymer CNT Composites. The dispersion of the MWCNTS in SDS and the properties of the precursor solution were analyzed using Scanning Electron Microscopy (SEM), Ultraviolet-Visible Spectroscopy (UV-Vis), and X-Ray Diffraction (XRD) techniques.

Sensor response of electrospun poly(lactic acid)/polyaniline nanofibers to aliphatic alcohol vapors of varying sizes

Poly(lactic acid)PLA is a biocompatible and biodegradable polyester with lactic acid being a byproduct of decomposition. PLA can be produced via processing starch, it is mechanically robust and environmentally stable and has therefore attracted interest in applications such as biomedical implants, controlled drug delivery and other kitchen variety commodity products. Making PLA electrically conductive via blending it with conducting polymers will extend its range of applications to include electronic devices and sensors. Commercial PLA has a hard grainy morphology, but is readily soluble in organic solvents and can be cast into thin films, fibers, foams or other forms. A fiber typically has a larger surface area to volume ratio compared to films and is thus technologically advantageous for sensor applications. We have successfully prepared conducting PLA/PANi nanofibers at low PLA concentrations in CHCl3 and used it to sense alcohol vapors of increasing molecular size. The larger the size of the alcohol molecule, the longer it took for the sensor to reach saturation and the sensitivity was smaller. The sensor response times were found to be slower than the recovery times by more than two orders of magnitude for the larger alcohol molecules. Since larger molecules were not able to penetrate the fibers, they were easier to remove. The fiber sensors could be tested on various alcohols without damage and hence were reusable. Conducting PLA based nanofibers therefore present yet another means of fabricating gas sensors and that are biocompatible.

Transmission electron microscopy of electrospun GaN nanofibers

We have reported earlier progress in producing polycrystalline wurtzite-polymorph and photo-conductive GaN nanofibers by electrospinning. This paper shows grain stacking during heat treatment and suggests the need to understand nucleation and grain growth following electrospinning. Transmission Electron Microscopy (TEM) analysis of GaN shows brittle fibers, grain stacking, and unfinished grain nucleation. X-Ray Diffraction analysis confirmed dominant hexagonal 101-wurtzite preferential overall orientation and the incipient grains are of high crystalline quality as seen by high resolution TEM.

Electrospun polymer-CNT actuators

Electrospun polymer-MWCNTs fibers were prepared using a precursor solution that consists of multiwall carbon nanotubes (MWCNTs), Poly(dymethylsiloxane) and Poly(methylmethacrylate) in Tetrahydrofuran (THF) and Dimethylformamide (DMF). Before adding them into the precursor, the MWCNTs were dispersed in Sodium Dodecyl Sulfate (SDS) and water. We report evidence of UV photo-conduction and photo-actuation in electrospun PDMS/PMMA-CNT composite fibers.

Electron microscopy and cathodoluminescence in electrospun nanodimensional structures: challenges and opportunities

GaN nanofibers were sintered by electrospinning and analyzed by electron microscopy techniques to study morphology and grain size. After heat treatment, the fibers showed thinner mats with polycrystalline grains with size on the order of 10 nm. For the first time in electospun GaN, optical properties were investigated by room temperature cathodoluminescence. Despite polycrystallinity, the fibers produced a luminescence signal. The NBE might be blueshitfted (by 1.1 eV) by the electron-confinement effect of excitons in the nm-sized grains. The origin of the other two emissions is also compared to GaN fibres sintered by alternative techniques. The existence of a NBE signal from GaN nanofibres could open the door to applications in nanophotonics.

Synthesis and characterization of electrospun gallium nitride nanofibers

The simple and inexpensive technique of electrospinning was used for the production of long GaN nanofibers. The fibers were made using a precursor solution composed of pure Gallium Nitrate dissolved in dimethylacetamide (DMA) and a viscous solution of Cellulose acetate dissolved in a mixture of DMA and acetone. Using a tube furnace, they were sintered under a Nitrogen atmosphere to decompose the polymer and to reduce Oxygen contamination. This process was followed by sintering under a NH3 flow to complete the synthesis of wurtzite GaN. XRD, ESEM, and FTIR analysis were used to verify the chemical and structural composition of the samples. The I-V characteristics of a device constructed using a single GaN nanofiber showed the formation of ohmic contacts.

Electrospun tin oxide nanofibers for gas sensing applications

Tin oxide is a binary semiconductor with a wide band gap (Eg = 3.6 eV at 300 K) and has been used, mostly in the form of thin films, as the active element in gas sensing applications. As a fiber it is expected to have improved sensitivity as the surface-to-volume ratio increases. The authors fabricated undoped tin oxide and antimony-doped tin oxide nanofibers using electrospinning and metallorganic decomposition techniques. The precursor solution for the undoped fibers was based on a tin (IV) chloride and a viscous solution based on poly(ethylene oxide) (PEO). The antimonydoped precursor solution had an additional antimony trichloride solution made from isopropanol to obtain a Sb concentration of 1.5 %. To study the sensitivity of the fibers to gas exposure, both single nanofibers and nanofiber mats were electrospun onto Si/SiO2 wafers. The changes in the nanofiber resistance with exposure and removal of methanol were measured as a function of time and gas concentration. In both configurations, the undoped nanofibers show higher sensitiviy to the presence and removal of methanol. Both the undoped and antimony-doped tin oxide single nanofibers show faster response times than the nanofiber mats. Of all the configurations tested, the antimony-doped single fiber gives more stable and faster response.

Temperature-dependent charge transport mechanisms in carbon sphere/polyaniline composite

Charge transport in the temperature range 80 K < T < 300 K was studied in a composite of carbon spheres (CS), prepared via hydrothermal carbonization of sucrose, and the conducting polymer polyaniline (PANi). PANi was synthesized via the oxidative polymerization of aniline with ammonium peroxydisulfate (APS) in acidic media. The CS/PANi composite was prepared by coating the spheres with a thin polyaniline (PANi) film doped with hydrochloric acid (HCl) in situ during the polymerization process. Temperature dependent conductivity measurements show that three dimensional variable range hopping of electrons between polymeric chains in PANi-filled gaps between CS is the predominant transport mechanism through CS/PANi composites. The high conductivity of the CS/PANi composite makes the material attractive for the fabrication of devices and sensors.