Publications

We report the first observation of a large pressure-induced red-shift of the photoluminescence band in single crystals of C60 and the associated colour change of the sample from red to black at 3.2 GPa. Interpreting the red shift of the luminescence band as a reduction of the bandgap Eg of the crystal, we get the pressure derivative ∂E g /∂P = −0.138 eV/GPa. This result taken in conjunction with the known compressibility, gives the deformation potential ∂E g /∂(ln a) = 6.0 eV, where ‘a’ is the lattice constant. Taking the same value for the alkali-doped fullerene, and combining it with the known pressure dependence of the transition temperature in the context of a single BCS expression for Tc we estimate the electron-phonon coupling parameter λ - 0.3.

Graphene analogues of TaS 2 and TiS 2 (3–4 layers), prepared by Li intercalation followed by exfoliation in water, were characterized. Nanocomposites of CdS with few‐layer TiS 2 and TaS 2 were employed for the visible‐light‐induced H 2 evolution reaction (HER). Benzyl alcohol was used as the sacrificial electron donor, which was oxidized to benzaldehyde during the reaction. Few‐layer TiS 2 is a semiconductor with a band gap of 0.7 eV, and its nanocomposite with CdS showed an activity of 1000 μmol h −1 g −1 . The nanocomposite of few‐layer TaS 2 , in contrast, gave rise to higher activity of 2320 μmol h −1 g −1 , which was attributed to the metallic nature of few‐layer TaS 2 . The amount of hydrogen evolved after 20 and 16 h for the CdS/TiS 2 and CdS/TaS 2 nanocomposites was 14833 and 28132 μmol, respectively, with turnover frequencies of 0.24 and 0.57 h −1 , respectively.

The click reaction involving the Huisgen 1,3-cycloaddition reaction between azide and ethynyl groups has been employed to obtain assemblies of nanostructures. The click reaction between gold nanorods capped with azidoalkane- and alkyne-thiols yields chains and complex assemblies, accompanied by a large red shift of the longitudinal surface plasmon band. A similar reaction between CdSe nanocrystals results in a splitting of the photoluminescence band, possibly due to exciton splitting. Carbon nanotubes decorated by covalently linked gold nanoparticles have been obtained through click chemistry.

We report the results of an in situ small-angle x-ray scattering (SAXS) study of the aggregation of gold nanoparticles formed by an interfacial reaction at the toluene-water interface. The SAXS data provide a direct evidence for aggregate formation of nanoparticles having 1.3 nm gold core and an organic shell that gives a core-core separation of about 2.5 nm. Furthermore, the nanoparticles do not occupy all the cites of 13-member cluster. This occupancy decreases with reaction time and indicate reorganization of the clusters that generates planner disklike structures. A gradual increase in fractal dimension from 1.82 to 2.05 also indicate compactification of cluster aggregation with reaction time, the final exponent being close to 2 expected for disklike aggregates.

High resolution electron microscopic studies show that bismuth forms intergrowth tungsten bronzes containing varying widths of the WO3 slab and one-tunnel wide HTB strips.

The n = 1 and n = 3 members of the new (Tl, Pb)Srn+1CunO2n+3 series of cuprates possess primitive tetragonal structures. While the n =3 member is superconducting (Tc ∼ 60K), the n =1 member is metallic immaterial of the Tl Sr ratio.

Complexes of melamine with 4,4′-bipyridyl and AgNO3 having compositions, C10H8N2:0.5(C3H6N6):H2O and [Ag(C3H6N6)]NO3, respectively, have been prepared for the first time. A crystallographic investigation has shown unique hydrogen bonding patterns in the two systems. The melamine–4,4′-bipyridyl complex shows a supramolecular organization wherein hydrogen bonding between melamine and water molecules results in tapes. The tapes are connected by hydrogen bonding to 4,4′-bipyridyl giving rise to a rectangular box. In the melamine–AgNO3 complex, the melamine molecules are bound to Ag+ ions forming N–Ag–N chains, giving rise to a helix. The helices are held together by interchain hydrogen bonds.

Hydrogen production by photochemical and electrochemical means is an important area of research related to renewable energy. 2D nanomaterials such as C3N4 and MoS2 have proven to be active for the hydrogen evolution reaction (HER). Phosphorene, a mono-elemental 2D layer of phosphorus, is known to catalyze the HER, but the activity is marginal. The use of phosphorene is also limited by its ambient instability. We have been able to prepare covalently cross-linked nanocomposites of phosphorene with MoS2 as well as MoSe2. The phosphorene-MoS2 nanocomposite shows excellent photochemical HER activity yielding 26.8 mmol h-1 g-1 of H2, while only a negligible amount is produced by the physical mixture of phosphorene and MoS2. The phosphorene-MoS2 composite also displays high electrochemical HER activity with an onset overpotential of 110 mV, close to that of Pt. The enhanced HER activity of the phosphorene-MoS2 nanocomposite can be attributed to the ordered cross-linking of the 2D sheets, increasing the interfacial area as well as the charge-transfer interaction between phosphorene and MoS2 layers. The phosphorene-MoSe2 nanocomposite also exhibits good photochemical HER activity.

Tunneling spectroscopy measurements have been carried out on a single molecule device formed by two Pd nanocrystals (dia, $\sim$5 nm) electronically coupled by a conducting molecule, dimercaptodiphenylacetylene. The I-V data, obtained by positioning the tip over a nanocrystal electrode, exhibit negative differential resistance (NDR) on a background M-I-M characteristics. The NDR feature occurs at $\sim$0.67 V at 300 K and shifts to a higher bias of 1.93 V at 90 K. When the tip is held in the middle region of the device, a coulomb blockade region is observed ($\pm\sim$0.3 V).