Anil K. Debnath

Fast Response and High Sensitivity of ZnO Nanowires-Cobalt Phthalocyanine Heterojunction Based H2S Sensor.

The room temperature chemiresistive response of n-type ZnO nanowire (ZnO NWs) films modified with different thicknesses of p-type cobalt phthalocyanine (CoPc) has been studied. With increasing thickness of CoPc (>15 nm), heterojunction films exhibit a transition from n- to p-type conduction due to uniform coating of CoPc on ZnO. The heterojunction films prepared with a 25 nm thick CoPc layer exhibit the highest response (268% at 10 ppm of H2S) and the fastest response (26 s) among all samples. The X-ray photoelectron spectroscopy and work function measurements reveal that electron transfer takes place from ZnO to CoPc, resulting in formation of a p-n junction with a barrier height of 0.4 eV and a depletion layer width of ∼8.9 nm. The detailed XPS analysis suggests that these heterojunction films with 25 nm thick CoPc exhibit the least content of chemisorbed oxygen, enabling the direct interaction of H2S with the CoPc molecule, and therefore exhibit the fastest response. The improved response is attributed to the high susceptibility of the p-n junctions to the H2S gas, which manipulates the depletion layer width and controls the charge transport.

Flexible cobalt-phthalocyanine thin films with high charge carrier mobility

The structural and charge transport characteristics of cobalt phthalocyanine (CoPc) films deposited on flexible bi-axially oriented polyethylene terephthalate (BOPET) substrates are investigated. CoPc films exhibited a preferential (200) orientation with charge carrier mobility of ∼118 cm2 V−1 s−1 (at 300 K). These films exhibited a reversible resistance changes upon bending them to different radius of curvature. The charge transport in CoPc films is governed by a bias dependent crossover from ohmic (J–V) to trap-free space-charge limited conduction (J–V2). These results demonstrate that CoPc films on flexible BOPET having high mobility and high mechanical flexibility are a potential candidate for flexible electronic devices.

Change in the Affinity of Ethylene Glycol Methacrylate Phosphate Monomer and Its Polymer Anchored on a Graphene Oxide Platform toward Uranium(VI) and Plutonium(IV) Ions.

The complexation behavior of the carbonyl and phosphoryl ligating groups bearing ethylene glycol methacrylate phosphate (EGMP) monomer and its polymer fixed on a graphene oxide (GO) platform was studied to understand the coordination ability of segregated EGMP units and polymer chains toward UO2(2+) and Pu(4+) ions. The cross-linked poly(EGMP) gel and EGMP dissolved in solution have a similar affinity toward these ions. UV-initiator induced polymerization was used to graft poly(EGMP) on the GO platform utilizing a double bond of EGMP covalently fixed on it. X-ray photoelectron spectroscopy (XPS) of the GO and GO-EGMP was done to confirm covalent attachment of the EGMP via a -C-O-P- link between GO and EGMP. The extent of poly(EGMP) grafting on GO by thermal analyses was found to be 5.88 wt %. The EGMP units fixed on the graphene oxide platform exhibited a remarkable selectivity toward Pu(4+) ions at high HNO3 conc. where coordination is a dominant mode involved in the sorption of ions. The ratio of distribution coefficients of Pu(IV) to U(VI) (DPu(IV)/DU(VI)) followed a trend as cross-linked poly(EGMP) (0.95) < EGMP in solvent methyl isobutyl ketone (1.3) < GO-poly(EGMP) (25) < GO-EGMP (181); the DPu(IV)/DU(VI) values are given in parentheses. The density functional theory computations have been performed for the complexation of UO2(2+) and Pu(4+) ions with the EGMP molecule anchored on GO in the presence of nitrate ions. This computational modeling suggested that Pu(4+) ion formed a strong coordination complex with phosphoryl and carbonyl ligating groups of the GO-EGMP as compared to UO2(2+) ions. Thus, the nonselective EGMP becomes highly selective to Pu(IV) ions when it interacts as a single unit fixed on a GO platform.

Metal nanoparticle catalyzed charge rearrangement in selenourea probed by surface-enhanced Raman scattering

The adsorption behavior of selenourea (SeU) on noble metal silver (Ag) and gold (Au) nanoparticles (NPs) was investigated using the surface-enhanced Raman scattering (SERS) technique in combination with X-ray photoelectron spectroscopy (XPS) and density functional theoretical (DFT) calculations. SeU contains two different anchoring groups, firstly, a selenium atom and secondly, two amino groups. SERS, XPS and DFT studies provided useful insight into the metal–molecule interaction, thus, illustrating the active sites of SeU and the metal substrates being directly involved in binding. The SERS and DFT results provided further insight into the tautomerism of SeU, suggesting that as a solid, the selenone form is exclusively found. On the Ag NP surface, signatures of the selenol form were predominant which indicates charge rearrangement within the molecule while on the Au NP surface, contributions from both selenone and selenol tautomers were observed. In addition, the preferential binding affinity of the anchoring groups in SeU towards Ag and Au was exploited for the synthesis of metal–molecule–metal sandwiched nanoassemblies that further suggested possible charge rearrangement within the molecule.

Greatly enhanced H2S sensitivity using defect-rich titanium oxide films

Defect-rich titanium oxide films were prepared using the laser ablation technique. The target pellet used for the ablation was rutile phase pure stoichiometric TiO2. X-ray photoelectron and synchrotron soft X-ray absorption spectroscopy measurements have revealed the chemical composition of the films. Using 200 mJ of laser energy, the stoichiometric transfer of TiO2 from the target to the substrate was achieved. The films prepared using 500 mJ of laser energy were observed to be highly defect-rich with a composition of Ti/TiOx. The gas sensing characteristics of these films were tested. The unique defect-richness of the films favours the formation of excessive overlayers of chemisorbed oxygen on the surface of the films in large proportions as compared to the lattice oxygen, making the films rich in titanium forming Ti–Ti electronic bonds, producing a great enhancement in the sensitivity toward highly toxic H2S gas.

Influence of adsorbed oxygen on charge transport and chlorine gas-sensing characteristics of thin cobalt phthalocyanine films

We have investigated the morphology, charge transport, and gas-sensing characteristics of thin films of cobalt phthalocyanine (CoPc) deposited on glass and sapphire substrates, using molecular beam epitaxy (MBE). CoPc films deposited on glass were found to be highly disordered. The ambient oxygen was found to be chemisorbed and created deep trap states, which led not only to hysteretic current-voltage (J-V) characteristics but also reduce the charge mobility. These properties render them unsuitable for gas-sensing. On the other hand, films deposited on sapphire were polycrystalline, which was attributed to an improved molecule-substrate interaction. The physically sorbed oxygen only created shallow traps, and the J-V characteristics were non-hysteretic, rendering them suitable for gas-sensing applications. It was demonstrated that the ultrathin (20 nm) CoPc films deposited on sapphire acted as highly sensitive and selective sensors for chlorine present in the wCl concentration range of 5 × 10−9−2 × 10−6 (5–2000 ppb).

Interface mediated semiconducting to metallic like transition in ultrathin Bi2Se3 films on (100) SrTiO3 grown by molecular beam epitaxy

Bismuth selenide (Bi2Se3) thin films of nominal thickness ∼10 nm were grown on (100) SrTiO3 and (100) LaAlO3 single crystal substrates using molecular beam epitaxy and their charge transport properties were investigated in the temperature range of 1.5 K–300 K. Bi2Se3 films deposited on (100) LaAlO3 exhibit semiconducting behavior, while films prepared on (100) SrTiO3 exhibited an anomalous semiconductor-to-metal-like transition at 68 K. The low temperature metal like transition is attributed to compressive strains arising due to structural phase transition of SrTiO3 substrate, which modulate the Bi2Se3 grain boundary width and facilitate the electric field assisted tunneling of charge carrier at the grain boundaries. The field assisted tunneling of charge carriers is supported by the inverse square-root field dependence of electrical conductivity.

Photo-Induced Spin State Switching In [Fe(bpp)[sub 2]](NCS)[sub 2]⋅2H[sub 2]O

We present the results of our investigation into the effect of irradiation of green light on the high spin low spin transition behavior of the mononuclear iron(II) compound [Fe(bpp){sub 2}](NCS){sub 2{center_dot}}2H{sub 2}O explored with the help of magnetic as well as Moessbauer spectroscopic studies. It has been found that the compound exhibits molecular bistability under irradiation of light due to LIESST effect.We present the results of our investigation into the effect of irradiation of green light on the high spin ⇆ low spin transition behavior of the mononuclear iron(II) compound [Fe(bpp)2](NCS)2⋅2H2O explored with the help of magnetic as well as Mossbauer spectroscopic studies. It has been found that the compound exhibits molecular bistability under irradiation of light due to LIESST effect.

Study Of Nature Of Structural Defects In A Prussian Blue Type Cu[sub 0.73]Mn[sub 0.77][Fe(CN)[sub 6]]⋅zH[sub 2]O Molecular Magnet

We have determined crystal structure and nature of structural defects in the Cu0.73Mn0.77[Fe(CN)6]⋅zH2O molecular magnetic compound by employing the Rietveld refinement and reverse Monte Carlo (RMC) simulation techniques on the neutron scattering data. It crystallizes in a face centred cubic structure (Space group: Fm3m). The analysis of the pair‐correlation functions for the oxygen atoms of the coordinated and non‐coordinated water molecules reveals a correlated local ordering of water molecules at and around the empty Fe(CN)6 sites in this compound.

Infrared Reflectivity Studies On Composite HfO[sub 2]∕SiO[sub 2] Thin Films

The HfO2‐SiO2 composite films were prepared by reactive electron beam co‐deposition process with various pre‐determined mixing compositions. The films have been characterized by FTIR spectroscopy. IR reflectivity data shows a regular trend with the increase of silica concentration. For low silica concentrations, refractive index shows anomalously hardened value, whereas for low hafnia concentrations, abosorption spectra show the formation of hafnium silicate.