A. K. Debnath

High thermoelectric performance of (AgCrSe2)0.5(CuCrSe2)0.5 nano-composites having all-scale natural hierarchical architectures

Recent studies have shown that thermoelectric materials exhibit a high figure-of-merit if it consists of hierarchically organized microstructures that significantly lower the lattice thermal conductivity without any appreciable change in the power factor. Here, we report a new class of thermoelectric (AgCrSe2)0.5(CuCrSe2)0.5 nano-composites synthesized via the vacuum hot pressing of a mixture of the constituents, which naturally consists of phonon scattering centers in a multiscale hierarchical fashion, i.e. atomic scale disorder, nanoscale amorphous structure, natural grain boundaries due to layered structure and mesoscale grain boundaries/interfaces. The presence of a natural hierarchical architecture of different length scales in the composite samples is confirmed by scanning electron and transmission electron microscopy. Detailed characterization reveals that in the composite samples there is a slight migration of Cu into the Ag site. Composite samples exhibit extremely low thermal conductivity ∼2 mW cm−1 K−1 at 773 K, which is nearly one third of the pure AgCrSe2 and CuCrSe2. The composite samples exhibit a high ZT ∼ 1.4 at 773 K, which is attributed to the scattering of heat carrying phonons of all wavelengths via the natural hierarchical architecture of the material. The ease of synthesis of such high performance (AgCrSe2)0.5(CuCrSe2)0.5 nanocomposites with a natural hierarchical architecture offers a promise for replacing conventional tellurides.

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.

Oxygen induced hysteretic current-voltage characteristics of iron-phthalocyanine thin films

Electrical transport has been investigated in amorphous and polycrystalline (α-phase) iron phthalocyanine (FePc) thin films grown by molecular beam epitaxy. Measurements carried out in the temperature range of 150–300 K showed hysteric current-voltage (I-V) characteristics at temperatures above 200 K. The I-V characteristics measured during the increasing voltage scan showed a transition from the Ohmic conduction to the trap controlled space charge limited conduction followed by a trap free conduction. During the decreasing voltage scan, trap free conduction was observed in full voltage range. The I-V hysteresis is attributed to the filling of deep surface traps created by chemisorbed oxygen. Amorphous films showed higher hysteresis as well as chemisorbed oxygen content.

Phthalocyanine based nanowires and nanoflowers as highly sensitive room temperature Cl2 sensors

Nanowires of Zn(II) 2,3,9,10,16,17,23,24-octakis(octyloxy)-29H,31H-phthalocyanine (ZnPcOC8) and nanoflowers of Cu(II) 2,3,9,10,16,17,23,24-octakis(octyloxy)-29H,31H-phthalocyanine (CuPcOC8) have been uniformly grown onto glass substrate by employing a cost effective solution based self-assembly technique. The nanowires and nanoflowers were characterized by XRD, SEM and UV-Visible absorption spectroscopy. It has been demonstrated that these nanowires and nanoflowers were highly sensitive towards Cl2 at room temperature with a detection limit as low as 5 ppb. The response of nanowires and nanoflowers varied linearly from 93% to 715% and 85% to 550% for 5–1500 ppb of Cl2. Raman spectroscopic and XPS studies revealed that the central metal ions of ZnPcOC8 and CuPcOC8 were the predominant absorption sites for Cl2. The results emphasized the application of these nanostructures as low cost and highly sensitive room temperature Cl2 sensors.

The Effect of Soil Physico-chemical Properties on Rhizome Rot and Wilt Disease Complex Incidence of Ginger Under Hill Agro-climatic Region of West Bengal

A study was conducted to find out the relationship of physico-chemical properties (viz. organic carbon(OC), pH, electrical conductivity, nitrogen, phosphorus and potassium content) of ginger growing soil with incidence percentage of rhizome rot and wilt disease complex of ginger. Organic carbon content and pH of the ginger soil contributed significantly (93%) in the prediction of ginger rhizome rot and wilt disease complex incidence with negative correlation. Soil having weak acidic reaction with OC percent greater than 2.25 was observed to have the lower average incidence of the disease.

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.