D. B. Tanner

Insulator-to-metal transition in polyaniline

Abstract The emeraldine base (EB) form of polyaniline can be varied from insulating ( σ −10 ohm −1 cm −1 ) to conducting ( σ ≈ 10 +1 ohm − cm − ) through protonation. That is, the number of electrons on the polymer backbone is constant while the number of protons is increased. We present here extensive magnetic, optical and transport data that demonstrate that the resulting emeraldine salt (ES) is metallic with a finite density of states at the Fermi energy. The results are consistent with segregation into fully protonated emeraldine salt and unprotonated emeraldine base polymer regions. it is proposed that the observed transition is an isolated bipolaron-to-polaron lattice transition. The correspondence of this concept to the disproportionation between protonated imine plus amine to form two semiquinones is shown.

SQUID-Based Microwave Cavity Search for Dark-Matter Axions

Axions in the microeV mass range are a plausible cold dark-matter candidate and may be detected by their conversion into microwave photons in a resonant cavity immersed in a static magnetic field. We report the first result from such an axion search using a superconducting first-stage amplifier (SQUID) replacing a conventional GaAs field-effect transistor amplifier. This experiment excludes KSVZ dark-matter axions with masses between 3.3 microeV and 3.53 microeV and sets the stage for a definitive axion search utilizing near quantum-limited SQUID amplifiers.

A 410GHz CMOS Push-Push Oscillator with an On-Chip Patch Antenna

The uses of terahertz systems (300 GHz to 3 THz) in radars, remote sensing, advanced imaging, and bio-agent and chemical detection have been extensively studied. A compact and low-cost signal source is a key circuit block of terahertz systems. Traditionally, the circuits have been built using highly optimized III-V technologies. With the advances of CMOS, it has become realistic to consider terahertz circuits in CMOS. This paper reports a signal source operating near 410 GHz that is fabricated using low-leakage transistors in a 6 M 45 nm digital CMOS technology.

Infrared and Raman studies of the Verwey transition in magnetite

We present infrared and Raman measurements of magnetite (Fe3O4). This material is known to undergo a metal-insulator and a structural transition (Verwey transition) at T-V = 120 K. The structural aspect of the Verwey transition is disclosed by the appearance of additional infrared-active and Raman-active phonons. The frequencies of the infrared-active phonons show no significant singularities at the transition whereas their linewidths increase. The frequency and linewidth of the Raman-active phonon at 670 cm(-1) change abruptly at the transition. For T<T-V, we observe fine structure in the infrared and Raman spectra which may indicate strong anharmonicity of the system below the transition. The effective mass of the mobile carriers is estimated to be m*approximate to 100m, where m is the electronic mass.

Insulator-to-metal transition in polyaniline: Effect of protonation in emeraldine

Abstract The emeraldine base form of the polymer can be varied from insulating ( σ ∼ 10 −10 ohm −1 cm −1 ) to conducting ( σ ∼ 10 0 ohm −1 cm −1 ) states through protonation. Based upon extensive magnetic, optical and transport data, we demonstrate that the resulting emeraldine salt is metallic with a finite density of states at the Fermi energy. The roles of a novel bipolaron-to-polaron lattice transition and phase segregation into conducting and non-conducting regions are discussed.

Terahertz study of 1,3,5-trinitro-s-triazine by time-domain and Fourier transform infrared spectroscopy

This letter describes the use of THz time-domain spectroscopy (TDS) applied in transmission to the secondary explosive 1,3,5 trinitro-s-triazine. Samples were also subjected to Fourier transform infrared spectroscopy over the same range for comparison. A detailed spectroscopy study is presented. General agreement between results from both methods confirms the absorption features found. A comparison study with computer molecular simulations shows that THz-TDS is sensitive to collective modes or vibrational modes of material.

Stable hole doping of graphene for low electrical resistance and high optical transparency

We report on the p doping of graphene with the polymer TFSA ((CF(3)SO(2))(2)NH). Modification of graphene with TFSA decreases the graphene sheet resistance by 70%. Through such modification, we report sheet resistance values as low as 129 Ω, thus attaining values comparable to those of indium-tin oxide (ITO), while displaying superior environmental stability and preserving electrical properties over extended time scales. Electrical transport measurements reveal that, after doping, the carrier density of holes increases, consistent with the acceptor nature of TFSA, and the mobility decreases due to enhanced short-range scattering. The Drude formula predicts that competition between these two effects yields an overall increase in conductivity. We confirm changes in the carrier density and Fermi level of graphene through changes in the Raman G and 2D peak positions. Doped graphene samples display high transmittance in the visible and near-infrared spectrum, preserving graphenes optical properties without any significant reduction in transparency, and are therefore superior to ITO films in the near infrared. The presented results allow integration of doped graphene sheets into optoelectronics, solar cells, and thermoelectric solar cells as well as engineering of the electrical characteristics of various devices by tuning the Fermi level of graphene.

High resolution search for dark-matter axions

We have performed a high resolution search for galactic halo axions in cold flows using a microwave cavity detector. The analysis procedure and other details of this search are described. No axion signal was found in the mass range 1.98-2.17 micro-eV. We place upper limits on the density of axions in local discrete flows based on this result.

Low-Band-Gap Platinum Acetylide Polymers as Active Materials for Organic Solar Cells

We report on two pairs of platinum acetylide based polymers and model oligomers utilizing a 2,1,3-benzothiadiazole (BTD) acceptor moiety flanked on either side by either 2,5-thienyl donor units (Pt2BTD-Th and p-PtBTD-Th) or (3,4-ethylenedioxy)-2,5-thienyl donors (Pt2BTD-EDOT and p-PtBTD-EDOT). Both oligomer/polymer pairs absorb strongly throughout the visible region; however, because the (ethylenedioxy)thiophene moiety is a stronger donor than thiophene, the latter oligomer/polymer pair has a correspondingly lower band gap and, therefore, harvests light more efficiently at longer wavelengths. p-PtBTD-Th exhibits a relatively narrow molecular weight distribution with a number-average molecular weight (Mn) of 22 kDa, while p-PtBTD-EDOT exhibits a comparable Mn of 33 kDa but has a high polydispersity index likely due to aggregation. We provide a complete report of the photophysical and electrochemical characterization of the two oligomer/polymer pairs. The photophysical studies reveal that the materials undergo relatively efficient intersystem crossing. In a discussion of the energetics of photoinduced electron transfer from the platinum polymers to [6,6]-phenyl C61 butyric acid methyl ester (PCBM), it is noted that while the singlet state is quenched efficiently, the triplet state is not quenched, indicating that charge generation in the photovoltaic materials must ensue from the singlet manifold. Finally, organic photovoltaic devices based on blends of p-PtBDT-Th or p-PtBDT-EDOT with PCBM were characterized under monochromatic and simulated solar (AM1.5) illumination. Optimized devices exhibit an open-circuit voltage (Voc) of approximately 0.5 V, a short-circuit current density (Isc) of approximately 7.2 mA cm(-2), and a fill factor of approximately 35%, which yields overall power conversion efficiencies of 1.1-1.4%.

An Improved RF Cavity Search for Halo Axions

The axion is a hypothetical elementary particle and cold dark matter candidate. In this RF cavity experiment, halo axions entering a resonant cavity immersed in a static magnetic field convert into microwave photons, with the resulting photons detected by a low-noise receiver. The ADMX Collaboration presents new limits on the axion-to-photon coupling and local axion dark matter halo mass density from a RF cavity axion search in the axion mass range 1.9-2.3 {micro}eV, broadening the search range to 1.9-3.3 {micro}eV. In addition, we report first results from an improved analysis technique.