K.K. Chattopadhyay

Three Dimensional Ag2O/TiO2 Type-II (p–n) Nanoheterojunctions for Superior Photocatalytic Activity

Type-II p-n junction three-dimensional Ag(2)O/TiO(2) microspheres have been fabricated by assembling p-type Ag(2)O nanoparticle on n-type TiO(2) 3D microsphere. Ag(2)O/TiO(2) microsphere nanoheterojunctions were obtained by hydrothermal synthesis of TiO(2) microspheres at 180 °C followed by photoreduction of AgNO(3). The samples were carefully characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), field-emission scanning electron microscopy (FESEM), and energy dispersive X-ray analysis (EDX). The photocatalytic activity toward degradation of methyl orange (MO) aqueous solution under UV light was investigated. The result showed that type-II p-n nanoheterojunctions Ag(2)O/TiO(2) significantly enhanced the photocatalytic degradation compared to n-type TiO(2) microsphere. It was found that the photocatalytic degradation followed the pseudo first-order reaction model. In particular, heterostructure with molar ratio of TiO(2) and AgNO(3) of 4:1 exhibited best photocatalytic activity and the corresponding apparent first-order rate constant of 0.138 min(-1) which is 4 times than that of pure n-type microsphere.

Amino-functionalized graphene quantum dots: origin of tunable heterogeneous photoluminescence

Graphene quantum dots are known to exhibit tunable photoluminescence (PL) through manipulation of edge functionality under various synthesis conditions. Here, we report observation of excitation dependent anomalous m-n type fingerprint PL transition in synthesized amino functionalized graphene quantum dots (5-7 nm). The effect of band-to-band π*-π and interstate to band n-π induced transitions led to effective multicolor emission under changeable excitation wavelength in the functionalized system. A reasonable assertion that equi-coupling of π*-π and n-π transitions activated the heterogeneous dual mode cyan emission was made upon observation of the PL spectra. Furthermore, investigation of incremented dimensional scaling through facile synthesis of amino functionalized quantum graphene flakes (20-30 nm) revealed it had negligible effect on the modulated PL pattern. Moreover, an effort was made to trace the origin of excitation dependent tunable heterogeneous photoluminescence through the framework of energy band diagram hypothesis and first principles analysis. Ab initio results suggested formation of an interband state as a manifestation of p orbital hybridization between C-N atoms at the edge sites. Therefore comprehensive theoretical and experimental analysis revealed that newly created energy levels can exist as an interband within the energy gap in functionalized graphene quantum structures yielding excitation dependent tunable PL for optoelectronic applications.

Size-dependent optical properties of sputter-deposited nanocrystalline p-type transparent CuAlO2 thin films

Nanocrystalline, p-type semiconducting, transparent CuAlO2 thin films were deposited by direct current sputtering of a prefabricated polycrystalline CuAlO2 target, with deposition time as a variable parameter. Transmission electron micrographs reveal the formation of CuAlO2 nanoparticles. For the films deposited in 3, 9, and 15min, the average particle sizes are determined to be around 10, 20, and 30nm, respectively. The interplaner spacings calculated from selected area electron-diffraction patterns obtained from transmission electron microscopy confirmed the proper phase formation of the material. X-ray diffraction measurements of the films deposited for 15 and 45min show some diffraction peaks, which depict the rhombohedral crystal structure of the material. The band-gap values obtained from the optical transmission and reflection data, for the films deposited in 3 and 9min, are 3.94 and 3.84eV, respectively, whereas for those films deposited in 15 and 45min, the band-gap values lie in the range of 3.7...

Preparation of p-type transparent conducting CuAlO2 thin films by reactive DC sputtering

Abstract P-type transparent conducting thin films of copper aluminate were prepared by reactive DC sputtering of a prefabricated target having 1:1 atomic ratio of Cu and Al. Films of CuAlO2 were deposited on Si (400) and glass substrates. The sputtering was performed in Ar+O2 (40 vol.%) atmosphere and the substrate temperature was 475 K. X-ray diffraction (XRD) spectra of the films showed the peaks which could be assigned with those of the crystalline CuAlO2. UV–Visible spectrophotometric measurement showed high transparency of the films in the visible region. Both direct and indirect band gaps were found to exist and their corresponding estimated values were 3.75 and 1.85 eV, respectively. The room temperature conductivity of the film was fairly high and was of the order of 0.22 S cm−1 while the activation energy was ∼0.25 eV. Seebeck coefficient at room temperature gave a value of +115 μV/K confirming the p-type conductivity. Room temperature Hall effect measurement also indicated positive value of Hall coefficient with a value RH=14.1 cm3/C.

Synthesis of crystalline carbon nitride thin films by electrolysis of methanol–urea solution

Abstract Polycrystalline carbon nitride films were deposited on Si (400) substrates by electrolysis of methanol–urea solution under high voltage, at atmospheric pressure and at temperature below 350 K. Fourier transform infrared spectroscopy (FTIR) measurements suggested the existence of both single and double carbon–nitrogen bonds in the film. X-ray diffraction (XRD) spectrum showed various peaks for different d values which could be assigned to different crystalline carbon nitride phases. Film morphology was studied by scanning electron microscopy (SEM) which indicated the existence of grains with average grain size of ∼2.5 μm.

Synthesis and Characterization of Nano-Crystalline Fluorine-Doped Tin Oxide Thin Films by Sol-Gel Method

Thin films of fluorine-doped tin-oxide (FTO) were prepared by sol-gel dip-coating technique. Stannous chloride (SnCl2ċ2H2O) and hydrogen fluoride (HF) were mixed with isopropyl alcohol to serve as source solution. X-ray diffraction (XRD) spectrum showed all the peaks of the crystalline SnO2. Analysis of XRD spectrum showed the particle size to be nearly 6 nm, which indicated the nanocrystalline structure of the films. Strain calculation by integral breadth (IB) method from XRD data showed a value of 0.010. UV-Visible spectrophotometric measurement showed high transparency of the films in the visible region and the band gap was calculated to be 3.34 eV. The room temperature resistivity of the films were of the order of 1 Ωcm. Fluorine concentration in the films was determined from energy dispersive X-ray (EDX) study. Current-voltage (I-V) characteristics at high temperatures showed the Poole-Frenkel effect of thermionic emission. SEM study indicated the existence of fine grains in the film. FT-IR spectroscopy showed strong Sn—O and Sn—O—Sn bonding.

Grain boundary scattering in CuInSe2 films

Electrical conductivity and Hall mobility of CuInSe2 films were measured in the temperature range of 77–400 K. The films were deposited with different Cu/In ratios ranging from 0.7–0.9 and at substrate temperatures of 620–720 K. Effects of grain boundary scattering on the electron transport properties were studied carefully and it was observed that scattering at the grain boundaries is a predominant factor controlling the electron transport properties at lower temperatures while complex scattering mechanisms become operative at higher temperatures. The energy values of trap levels and the densities of trap states were also obtained.

Nano-diamond films produced from CVD of camphor

Abstract Nano-diamond films were deposited by chemical vapour deposition (CVD) of camphor (C 10 H 16 O) and hydrogen (∼ 75 vol.%) on glass (at 523–623 K) and quartz/Si substrate (at 523–713 K). The films had low values of surface roughness (∼ 17 nm on quartz) along with high values of hardness and band gap. IR studies of the films deposited on Si substrate indicated the films to be transparent in the IR region. Optical properties of the films deposited on quartz (fused silica) and glass (Corning) substrates were studied. The absorption data in the below band gap region were utilized to estimate the strain (2–12 × 10 −3 ), stress (1.9–8.4 GPa) and hardness (47–62 GPa) in the films. The FTIR studies of the films did not show absorption around 2900 cm −1 , while the Raman spectra indicated a sharp peak (with FWHM ∼8 cm −1 ) at 1337 cm −1 corresponding to good quality diamond films.

Low temperature solution processed ZnO/CuO heterojunction photocatalyst for visible light induced photo-degradation of organic pollutants

The possibility of integrating manifold functionalities, coupled with various associated noble interface phenomena in the hierarchical nanoforms, either comprised of geometrical intricacies or achieved via the rational coupling of several components, has made them immensely pertinent from both research and technological aspects. Here, an oxide based nanostructure hybrid has been realized by integrating low bandgap copper oxide nanosheet with high bandgap one dimensional zinc oxide nanowires on a flexible carbon cloth as well as on a flat substrate. These bandgap modulated hybrid nanostructures are generated for the efficient absorption of visible light, targeting their possible use in waste water management. Our work presents a novel ambient condition protocol for morphological tuning in the nanoscale or their organization in a hierarchical structure. Environmental remediation through catalytic activity under the visible light irradiation of the synthesized samples was inspected using both anionic and cationic dyes (methyl orange and Rhodamine B, respectively) as the model contaminants, where the optimized heterostructure exhibits significantly better performance than the mono component oxides. Such enhanced performance could be explained by the formation of favorable staggered gap multiple p–n junctions at ZnO/CuO interface, which in turn retards the photogenerated electron–hole pair recombination within the heterostructure. The signature of successful p–n junction formation at ZnO nanorod/CuO nanosheet interface has been identified via current–voltage measurements with a conducting tip AFM in contact mode. The creative designing of novel heterojunctions adopting this protocol will pave the way for the utilization of the entire visible light range: thus, offering potential in solar energy conversion devices.

Recent advances in low temperature, solution processed morphology tailored ZnO nanoarchitectures for electron emission and photocatalysis applications

Having the benefit of several features such as low cost, straightforward processing, easy fabrication, large area deposition and physical flexibility, low temperature solution processed electronic devices have gained traction in the eyes of the research community as the difficulties associated with conventional higher temperature crystalline semiconductor devices are incrementally rendering them outdated. Over the last decade, amongst metal oxides, ZnO with its rich variety of nanoforms has been documented as the candidate with the highest economic impact by virtue of its diverse use in a plethora of electronic and optoelectronic applications relying on its associated unique functional features such as high mobility, excellent thermal stability and high transparency. To date, most review articles in the literature focused on ZnO nanostructures realized via the vapour phase method whereas a comprehensive study on solution processed nanoforms and their widespread use is still lacking. The current article mainly highlights an overview of recent developments in multi-dimensional ZnO nanoarchitectures processed via low temperature, rational approaches and their functional properties in field emission devices and environmental remediation. In addition to these descriptions of controlled morphology design and the usage perspective, significant issues pertinent to such geometrical evolution and device performance determination and the possible outlook for low temperature research on ZnO are also described.