Mainak Roy

Improved thermoelectric performance of hot pressed nanostructured n-type SiGe bulk alloys

Silicon germanium alloys (Si80Ge20) have been used in thermoelectric generators for deep space missions to convert radioisotope heat into electricity. This work demonstrates the highest value of thermoelectric figure-of-merit (ZT) ∼1.84 at 1073 K for n-type SiGe nanostructured bulk alloys, which is 34% higher than the reported record value for n-type SiGe alloys. The optimized samples exhibit a Seebeck coefficient of ∼284 μV K−1, resistivity of ∼45 μΩ m and thermal conductivity of ∼0.93 W m−1 K−1 at 1073 K. The main contributing factor for the enhanced ZT is very low and almost temperature independent thermal conductivity, which overcomes the low power factor of the material. Significant reduction of the thermal conductivity is caused by the scattering of low, medium and high wavelength phonons by atomic size defects, dislocations, and grain boundaries that are present due to the formation of nanocrystalline grains in the bulk material.

Ag incorporated nano BiPO4: sonochemical synthesis, characterization and improved visible light photocatalytic properties

We report an efficient route for the sonochemical synthesis of undoped BiPO4, Ag3PO4 and silver doped BiPO4:Ag(x%) (x = 2, 5, 10 and 20) nanostructures using bismuth/silver nitrate and ammonium dihydrogen phosphate as precursors. The products obtained have been characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (UV-vis DRS) and X-ray photoelectron spectroscopy (XPS). The size and morphology of BiPO4 exhibited drastic changes on Ag doping. The surface areas of the samples have been estimated using the Brunauer–Emmett–Teller (BET) method. The catalytic activities of all the samples for the rhodamine-B degradation were investigated systematically under UV and visible-light irradiation. Undoped BiPO4 exhibited excellent photocatalytic activity under UV light but the degradation of RhB was only ∼60% under visible light, while Ag doped BiPO4 samples showed almost complete degradation of the dye under visible light. Amongst all of them, BiPO4:Ag(10%) exhibited the best photocatalytic activity. Furthermore, after photocatalysis, the nanoparticles could be readily separated from the reaction system by low-speed centrifugation and reused. Stability of the photocatalysts was ascertained using FT-IR and Raman spectroscopy. After five recycles, the nanoparticles did not exhibit any apparent loss in activity, confirming its stability despite recycling. By tuning the band gap and measuring the surface area of the nanoparticles using BET tests, we found that the combined effect of these two factors resulted in good performance of the BiPO4:Ag(10%) photocatalyst under visible light irradiation.

X-ray diffraction, μ-Raman spectroscopic studies on CeO2−RE2O3 (RE=Ho, Er) systems: Observation of parasitic phases

The phase relations in CeO2−Ho2O3 and CeO2−Er2O3 systems have been established under the slow-cooled conditions. As per x-ray diffraction (XRD), in both the series a single-phasic solid solution forms up to the nominal composition Ce0.6RE0.4O1.8 (RE=Ho, Er) retaining the F-type structure of parent ceria. In Ce1−xErxO2−x/2 system the presence of microdomains of C-type phase have been revealed by Raman spectroscopy for composition x=0.4, which has been identified as single phasic by XRD. Photoluminescence studies also show that biphasic region commences from x=0.4 for Ce1−xHoxO2−x/2 series. The biphasicity continues until x=0.7 for both the series. From x=0.8 the solid solutions exist as C-type single phasic, which is isotypic to another end member RE2O3 (RE=Ho, Er) and as revealed by both XRD and Raman spectroscopy. High temperature XRD studies show that no temperature induced phase change has been observed in either of the series until 1273 K. In this work photoluminescence data was used to delineate the ...

Subliming the Unsublimable: How to Deposit Nanographenes

Quite sublime: Thin-layer fabrication of unsublimable large polycyclic aromatic hydrocarbons (PAHs) by pulsed laser deposition was used to prepare samples for scanning tunneling microscopy. Giant PAHs with up to 222 carbon atoms can be visualized-a size that was previously not possible because of the lack of suitable deposition methods.

Biomimetic synthesis of nanocrystalline silver sol using cysteine: stability aspects and antibacterial activities

The study reports the development of a simple, environmentally benign green chemical route to produce stable silver nanoparticle (Ag-np) sols with excellent antibacterial properties under ambient conditions. The method involves the room temperature reduction of AgNO3 by cysteine (aq) and requires no additional capping/stabilizing agent. It essentially mimics the redox reaction that takes place during incubation of the cell-free extract from Trichoderma asperellum in the presence of AgNO3 (aq) (P. Mukherjee, M. Roy, B. P. Mandal, G. K. Dey, P. K. Mukherjee, J. Ghatak, A. K. Tyagi and S. P. Kale, Nanotechnology, 2008, 19, 075103), wherein cysteine, a biomolecule present in the fungal extract, acts as a potential reducing agent. Additionally, cysteine acts as a capping molecule in the present case. Formation of Ag-nps was evidenced from UV-Vis, TEM, XRD and EDS studies. The stability of Ag sols has been shown to depend strongly on the concentration of cysteine relative to that of AgNO3. Sols obtained by reacting 0.1 mM of cysteine with 1 mM of AgNO3 remained stable for more than one month at 24 °C. The role of cysteine as capping molecule and the possible modes of its linkages with Ag-nps was studied by FT-IR, XPS and Raman spectroscopy. Bonding of Ag with either or all the three, thiolate, amino and carboxylate groups of the cysteine molecule via stable PH configuration is believed to have resulted in the stabilization of the Ag-nps. Antibacterial activity of the cysteine capped Ag sol was studied along with that of the Ag sol obtained by fungal route. Both the sols exhibited excellent and comparable efficacies as bactericidal agents against gram negative bacteria E. coli BW (25113), with one of the lowest minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values published so far.

Bias enhanced deposition of highly oriented β-SiC thin films using low pressure hot filament chemical vapour deposition technique

Abstract Highly oriented cubic silicon carbide (β-SiC) thin films are deposited on Si(111) substrates using bias assisted low-pressure hot filament chemical vapour deposition technique. Methane (CH 4 ) is used as the source for carbon, while the substrate itself acts as the source for silicon. The technique is quite simple, cheap, has one step, and requires no stringent reaction conditions; the substrate temperature used being in the range of ∼750 °C and the chamber pressure ∼1 torr. The films have been characterised by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and Raman spectroscopy. Bombardment of negatively biased substrate by high-energy positive ions under relatively low chamber pressure is believed to facilitate the growth of highly oriented SiC films.

Chlorine gas sensors using one-dimensional tellurium nanostructures

Tellurium nanotubes have been grown by physical vapor deposition under inert environment at atmospheric pressure as well as under vacuum conditions. Different techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and optical absorption have been utilized for characterization of grown structures. Films prepared using both types of tellurium nanotubes were characterized for sensitivity to oxidizing and reducing gases and it was found that the relative response to gases depends on the microstructure. Nanotubes prepared at atmospheric pressure (of argon) showed high sensitivity and better selectivity to chlorine gas. Impedance spectroscopy studies showed that the response to chlorine is mainly contributed by grain boundaries and is therefore enhanced for nanotubes prepared under argon atmosphere.

Synthesis of uniform gold nanoparticles using non-pathogenic bio-control agent: Evolution of morphology from nano-spheres to triangular nanoprisms

Green synthesis of gold nanospheres with uniform diameter and triangular nanoprisms with optically flat surface was carried out using a non-pathogenic bio-control agent Trichoderma asperellum for reduction of HAuCl(4). Kinetics of the reaction was monitored by UV-Vis absorption spectroscopy. No additional capping/complexing agent was used for stabilizing the gold nanoparticles. Evolution of morphology from pseudospherical nanoparticles to triangular nanoprisms was studied by transmission electron microscopy (TEM). It revealed that three or more pseudospheres fused to form nanoprisms of different shapes and sizes. Slow rate of reduction of HAuCl(4) by constituents of cell-free fungal extract was instrumental in producing such exotic morphologies. Isolation of gold nanotriangles from the reacting masses was achieved by differential centrifugation.

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.

Detection of nanophase at the surface of HFCVD grown diamond films using surface enhanced Raman spectroscopic technique

Abstract Diamond films prepared by hot filament chemical vapour deposition technique were characterised using X-ray diffraction, scanning electron microscopy and surface enhanced Raman spectroscopic technique. Silver was deposited on to diamond thin film under ultra high vacuum conditions and macro-Raman spectra were recorded during the deposition. An asymmetric broadening of the Raman peak at 1332 cm−1 and gradual emergence of a new peak at 1240 cm−1 with increasing thickness of the silver layer were observed. These observations were explained on the basis of phonon confinement in nanometer-sized crystals. It is proposed that the detected nanophase is present only at the surface and is not a bulk property. The average particle size of the film was estimated from X-ray analysis and also from the symmetric broadening of the 1332 cm−1 line in the Raman spectra of the bare sample. The presence of the nanophase was further evidenced by high resolution scanning electron microscopy. The probable mechanism of formation was briefly explored.