S. Samanta

Bias and temperature dependent charge transport in high mobility cobalt-phthalocyanine thin films

The temperature dependent current-voltage (J-V) characteristics of highly-oriented cobalt phthalocyanine films (rocking-curve width=0.11°) deposited on (001) LaAlO3 substrates are investigated. In the temperature range 300–100 K, charge transport is governed by bulk-limited processes with a bias dependent crossover from Ohmic (J∼V) to trap-free space-charge-limited conduction (J∼V2). The mobility (μ) at 300 K has a value of ∼7 cm2 V−1 s−1 and obeys Arrhenius-type (ln μ∼1/T) behavior. However, at temperatures <100 K, the charge transport is electrode-limited, which undergoes a bias dependent transition from Schottky (ln J∼V1/2) to multistep-tunneling (conductivity varying exponentially on the inverse of the square-root of electric field).

Room temperature ppb level Cl2 sensing using sulphonated copper phthalocyanine films

We present room temperature chemiresistive gas sensing characteristics of drop casted sulphonated copper phthalocyanine (CuTsPc) films. It has been demonstrated that these films are highly selective to Cl(2) and the sensitivity in the 5-2000 ppb range varies linearly between 65 and 625%. However, for concentrations >or=2000 ppb, the response becomes irreversible, which is found to be due to the chemical bond formation between Cl(2) and SO(3)Na group of CuTsPc films. The X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) data confirms the oxidation of SO(3)Na group by Cl(2) gas.

Bending stress induced improved chemiresistive gas sensing characteristics of flexible cobalt-phthalocyanine thin films

We report chemiresistive gas sensing characteristics of cobalt phthalocyanine films deposited on flexible bi-axially oriented polyethylene terephthalate substrates. Charge carrier mobility in these films systematically decreases upon bending, which implies that bending reduces π-π interactions among molecules. At room temperature, these films exhibited a reversible change in resistance on exposure to ammonia (5–50 ppm) along with high sensitivity and selectivity as well as fast response and recovery. The chemiresistive sensing properties were found to improve significantly under bend conditions owing to creation of more numbers of interaction sites.

Charge transport in polypyrrole:ZnO-nanowires composite films

Low temperature electrical transport properties of composite polypyrrole (PPy) films having ZnO-nanowires (ZnONWs) in the range of 0–50 wt. % have been investigated. It has been found that pure PPy film is in the critical regime of the metal-to-insulator transition, and adding ZnONWs drive composite films into the insulating regime. For PPy films, the transport is governed by Motts three-dimensional-variable range hopping (VRH) (ln ρ∼T−1/4) conduction and with increasing ZnONWs content above 10 wt. % results in a crossover to one-dimensional-VRH conduction (ln ρ∼T−1/2). It has been observed that films having ZnONWs content ≥10 wt. % exhibit reproducible hysteresis in the current-voltage characteristics.

Metallic-like conduction in Co-phthalocyanine/Fe-phthalocyanine composite films grown on sapphire substrates

Charge transport properties of pure and composite thin films of cobalt phthalocyanine (CoPc) and iron phthalocyanine (FePc) deposited on (0001) sapphire substrate has been investigated. Temperature dependence of resistivity showed that composite films are in metallic regime, while pure films are in the critical regime of metal–insulator transition. Composite films showed trap free space charge limited conduction (SCLC) along with two order of magnitude higher mobility (∼110 cm2 V−1 s−1) compared to pure films. Pure films showed SCLC with exponential distribution of traps. High mobility and better structural ordering in composite films is attributed to the formation of CoPc-FePc dimers.

Improved charge conduction in cobalt-phthalocyanine thin films grown along 36.8° boundary of SrTiO3 bicrystals

We have investigated temperature dependence of resistivity (ρ) and current-voltage characteristics of cobalt-phthalocyanine (CoPc) films deposited (i) along the 36.8° boundary (ATB) and (ii) in the single crystal region (SCR) of SrTiO3 bicrystal substrates. The analyses of ρ-T data show that while ATB films exhibit critical behavior, SCR films follow insulating three-dimensional variable-range-hopping mechanism. The mobility (μ) at 300 K has been estimated to be ∼148 and 3.8×10−3 cm2 V−1 s−1, respectively for ATB and SCR films. A very high μ for ATB films indicates that the bicrystal boundary acts as a template for the ordering of CoPc molecules.

Application of Neutron Activation Techniques for the Measurement of 238 U (n, γ) and 238 U (n, 2n) Cross Section at Neutron Energies of 13.5 and 17.28 MeV

The (n, γ) and (n, 2n) reaction cross-section of U at average neutron energies of 13.5 and 17.28 MeV from the Li(p, n)Be reaction has been determined using activation and off-line γ–ray spectrometric technique. The experimentally determined U(n, γ)U and U(n, 2n)U reaction cross-sections from present work were compared with the evaluated data of ENDF/BVII.0 and JENDL-4.0, JEFF-3.1/A and CENDL-3.1 (referenced in text). The experimental values were found to be in general agreement with the evaluated values obtained using ENDF/BVII.0, JENDL-4.0 and JEFF-3.1/A but it differs with the values obtained using CENDL-3.1. The present data along with literature data in a wide range of neutron energies were interpreted in terms of competition between U(n, γ), (n, f), (n, nf) and (n, xn) reactions channels. The U(n, γ) and U(n, 2n) reaction cross-sections were also calculated theoretically using the TALYS 1.4 computer code and were found to be in general agreement with the experimental data. KeywordsU (n, γ)U and U(n, 2n)U Reaction Cross-Sections; Li(p, n)Be Reaction; Average Neutron Energy; En = 13.5 and 17.28 MeV; Off-Line γ-Ray Spectrometric Technique; TALYS Calculation

Chemiresistive gas sensing characteristics of cobalt oxide thin films

We report synthesis of 100 nm thick cobalt oxide films – prepared by electron-beam evaporation of Co onto quartz (Q- films) and c-plane sapphire substrate (S- films) followed by oxygen annealing. Films have been characterized for their structure and morphology by using X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The prepared films have been investigated for their chemiresistive gas sensing characteristics for a host of test gases (CH4, CO, NO2, Cl2, NH3 and H2S) as a function of operating temperature (between 50 and 350°C) and gas concentration (3-30 ppm). We demonstrate that both Q and S-films are highly selective to H2S at an operating temperature of 250°C. However it has been observed that films prepared on sapphire show much less base resistance drift as compared to the films on quartz substrate.

Metal–semiconductor transition in ultrathin cobalt-phthalocyanine films grown on SrTiO3 single crystal substrates

We have investigated the low temperature charge transport properties of ultrathin cobalt-phthalocyanine films grown on (100) SrTiO3 single crystal substrates. The temperature dependence of resistivity shows an anomalous behavior, i.e., a transition from semiconducting to metallic behavior at around 110 K. We demonstrated that metallic behavior in these films is triggered by compressive strains of the SrTiO3 substrate due to its cubic to tetragonal structural phase transition at 110 K.

Implication of Structural Disorder in The Charge Transport Properties of Cobalt-phthalocyanine Thin Films

The charge transport properties of 100 nm thick cobalt phthalocyanine (CoPc) films grown on single crystal Al2O3 (0001 oriented) and quartz substrates using molecular beam epitaxy, have been investigated as a function of applied bias (± 50 V) at room temperature. Films grown on Al2O3 are highly ordered and exhibited non‐hysteretic current‐voltage (J‐V) characteristics. On the other hand, films grown on quartz substrates are highly disordered and exhibited hysteretic J‐V characteristics due to charge trapping. The analysis of J‐V characteristics of films on Al2O3 substrates show that the transport is governed by shallow trap mediated space charge limited conduction (SCLC), while for the films grown on the quartz substrate transport is through the exponentially distributed traps mediated SCLC. X‐ray photoelectron spectroscopy data show that charge trapping centers in the films grown on quartz substrates are created by chemisorbed oxygen.