Nicholas J. Pinto

Electrostatically-generated nanofibers of electronic polymers

Long nanofibers of conducting electronic polymers and their blends and also conventional polymers are conveniently fabricated in air by a non-mechanical electrostatic dispersion method. All fibers in a given preparation of certain polymers have diameters < 100nm. Fibers of 100% doped polyaniline as well as its blends in polymers such as polystyrene and polyethylene oxide have been prepared. Current/voltage in addition to conductivity/temperature relationships of single fibers as small as 419 nm have been obtained. Polyacrylonitrile and polystyrene fibers can be easily and uniformly coated from aqueous solution with conducting polypyrrole or with gold by electroless deposition. Polyacrylonitrile fibers can be thermally converted to conducting carbon nanofibers.

Electrospun polyaniline/polyethylene oxide nanofiber field-effect transistor

We report on the observation of field-effect transistor (FET) behavior in electrospun camphorsulfonic-acid-doped polyaniline/polyethylene oxide (PEO) nanofibers. Saturation channel currents are observed at surprisingly low source–drain voltages. The hole mobility in the depletion regime is 1.4×10−4 cm2/V s, while the one-dimensional (1-D) charge density (at zero gate bias) is calculated to be approximately 1 hole per 50 two-ring repeat units of polyaniline, consistent with the rather high channel conductivity (∼10−3 S/cm). Reducing or eliminating the PEO content in the fiber is expected to enhance device parameters. Electrospinning is thus proposed as a simple method of fabricating one-dimensional polymer FETs.

Fabrication and electrical characterization of polyaniline-based nanofibers with diameter below 30 nm

We fabricate and electrically characterize electrospun nanofibers of doped polyaniline/polyethylene oxide (PAn/PEO) blend with sub-30 nm diameter. Fiber diameters near 5 nm are obtained for optimized process parameters. Scanning conductance microscopy (SCM) shows that fibers with diameter below 15 nm are electrically insulating; the small diameter may allow complete dedoping in air or be smaller than phase-separated grains of PAn and PEO. Electrical contacts to nanofibers are made by shadow mask evaporation with no chemical or thermal damage to the fibers. Single fiber I–V characteristics show that thin fibers conduct more poorly than thick ones, in agreement with SCM data. I–Vs of asymmetric fibers are rectifying, consistent with formation of Schottky barriers at the nanofiber-metal contacts.

Controlled Doping of Graphene Using Ultraviolet Irradiation

The electronic properties of graphene are tunable via doping, making it attractive in low dimensional organic electronics. Common methods of doping graphene, however, adversely affect charge mobility and degrade device performance. We demonstrate a facile shadow mask technique of defining electrodes on graphene grown by chemical vapor deposition (CVD) thereby eliminating the use of detrimental chemicals needed in the corresponding lithographic process. Further, we report on the controlled, effective, and reversible doping of graphene via ultraviolet (UV) irradiation with minimal impact on charge mobility. The change in charge concentration saturates at ∼2 × 1012 cm–2 and the quantum yield is ∼10−5 e/photon upon initial UV exposure. This simple and controlled strategy opens the possibility of doping wafer-size CVD graphene for diverse applications.

Apparent dependence of conductivity of a conducting polymer on an electric field in a field effect transistor configuration

A curious effect is reported whereby an electric field apparently greatly affects the conductivity of an organic polymer, poly-3,4-ethylenedioxythiophene (PEDOT), doped to the “metallic” conducting regime when it is used in an all-organic polymer field effect transistor configuration. The response time for change in current in the source/drain PEDOT polymer with a change of gate voltage is slow (⩽∼2 s), suggesting that ionic diffusion is involved in the phenomenon. It is suggested that the electric field changes only the conductivity of the lowly conducting polymer matrix, which contains highly conducting islands of PEDOT, thus changing the extent of percolation of electric charge between metallic islands, and thereby affecting the bulk conductivity of the PEDOT source/drain material.

An EPR investigation of electrospun polyaniline-polyethylene oxide blends

Abstract The EPR magnetic susceptibility behavior of the camphorsulfonic acid doped polyaniline (PANCSA) blends with polyethylene oxide (PEO) is reported in fibers and films. In particular, EPR investigations on electrospun (PANCSA)0.72(PEO)0.28 nanofibers, cast films of (PANCSA)0.72(PEO)0.28 and cast films of (PANCSA) were performed to investigate differences in the mesoscopic disorder as induced by the process of electrospinning. The changes observed in the Pauli susceptibility, EPR lineshape, EPR linewidth, and dc conductivity are interpreted as due to increased chain alignment in the fibers compared with the cast films.

Electrospun hybrid organic/inorganic semiconductor Schottky nanodiode

We report on a simple method to fabricate, under ambient conditions and within seconds, Schottky nanodiodes using electrospun polyaniline nanofibers and an inorganic n-doped semiconductor. In addition to being a rectifier, the advantage of our design is the complete exposure of the rectifying nanojunction to the surrounding environment, making them attractive candidates in the potential fabrication of low power, supersensitive, and rapid response sensors as well. The diode parameters were calculated assuming the standard thermionic emission model of a Schottky junction, and the use of this junction as a gas sensor was examined.

Electrical conductivity of polyaniline as a function of pressure using a diamond anvil cell

We report our results on the electrical resistance of thin films of polyaniline as a function of pressure which was applied to the samples using a diamond anvil cell up to 22 GPa at room temperature. Samples of polyaniline cast from N-methylpyrrolidinone (NMP) and doped with HCl and also those doped with camphor sulfonic acid (CSA) and cast from m-cresol were studied. Resistance versus pressure measurements recorded during the loading and unloading cycle show asymmetry, suggesting irreversible structural changes. Since the primary effect of applying pressure is sample compression, this leads to reduced interchain separation resulting in enhanced interchain charge transport. Such effects are reflected in reduced resistance with increasing pressure as depicted in our results.

Dielectric permittivity study on weakly doped conducting polymers based on polyaniline and its derivatives

Abstract We report the results on low frequency dielectric study of pressed pellets of polyaniline (PAN) and its derivatives in the emeraldine base and weakly doped emeraldine salt forms. The parameter describing the doping is defined as y=[H+]/[N]. The samples were synthesized with y=0.00, 0.07 and 0.50. It is suggested that the presence of polarons and bipolarons are responsible for the dielectric relaxation mechanism as well as the frequency and temperature dependence of conductivity. A broadening in the spectrum of relaxation times of the dielectric relaxation process due to charge motion via creation/annihilation of polarons and bipolarons increases with increasing doping. The subsequent diffusion of polarons and bipolarons become faster as doping increases.

Effects of confinement on the phase separation in emeraldine base polyaniline cast from 1-methyl-2-pyrrolidinone studied via dielectric spectroscopy

Measurement in the frequency range 3 mHz–10 6 Hz of the dielectric characteristics of emeraldine base polyaniline dissolved in 1-methyl-2pyrrolidinone (NMP) and cast into bulk free-standing polymer films shows features similar to those reported by others and which are a result of microphase separation into reduced and oxidized repeat units. However, upon confinement into the cylindrical pores, of average diameter 20 nm, of a porous membrane such features of microphase separation do not occur. Strong pinning of the charge carriers due to interactions of the polymer with pore walls together with constrained chain packing and a non-uniform rate of evaporation of the NMP solvent from the pores suggests that the microphase separation observed in the bulk polymer is suppressed. This has the potential of being able to enhance th eb ulk conductivity after doping by reducing the internal intrachain disorder introduced by microphase separation.