Achintya Singha

Optical properties of nanocrystalline Y2O3:Eu3+

Optical properties of nanocrystalline red-emitting phosphor, europium-doped yttria (Y2O3:Eu3+), of average particle size of 15nm are investigated. The intensity of the strongest emission line at 612nm is found to be highest in the nanocrystalline sample with 4at.wt% of europium. The narrow electronic emission spectrum suggests a crystalline surrounding in this nanomaterial. We have estimated the strength of the crystal-field parameter at the dopant site, which plays a crucial role in determining the appearance of the intense emission line. The equilibrium temperature of this system has also been calculated from the intensity ratio of Stokes and anti-Stokes Raman scattering. Though known for the bulk samples, our approach and consequent results on the crystalline nanomaterial of Y2O3:Eu3+ provide a unique report, which, we believe, can be of considerable significance in nanotechnology.

Engineering artificial graphene in a two-dimensional electron gas

At low energy, electrons in doped graphene sheets behave like massless Dirac fermions with a Fermi velocity, which does not depend on carrier density. Here we show that modulating a two-dimensional electron gas with a long-wavelength periodic potential with honeycomb symmetry can lead to the creation of isolated massless Dirac points with tunable Fermi velocity. We provide detailed theoretical estimates to realize such artificial graphenelike system and discuss an experimental realization in a modulation-doped GaAs quantum well. Ultrahigh-mobility electrons with linearly dispersing bands might open new venues for the studies of Dirac-fermion physics in semiconductors.

Quantitative analysis of hydrogenated diamondlike carbon films by visible Raman spectroscopy

The correlations between properties of hydrogenated diamondlike carbon films and their Raman spectra have been investigated. The films are prepared by plasma deposition technique, keeping different hydrogen to methane ratios during the growth process. The hydrogen concentration, sp3 content, hardness, and optical Tauc gap of the materials have been estimated from a detailed analysis of their Raman spectra. We have also measured the same parameters of the films by using other commonly used techniques, such as sp3 content in films by x-ray photoelectron spectroscopy, their Tauc gap by ellipsometric measurements, and hardness by microhardness testing. The reasons for the mismatch between the characteristics of the films, as obtained by Raman measurements and by the above mentioned techniques, have been discussed. We emphasize on the importance of the visible Raman spectroscopy in reliably predicting the above key properties of diamondlike carbon films.

DNA-Mediated Wirelike Clusters of Silver Nanoparticles: An Ultrasensitive SERS Substrate

Stable metal nanoclusters (NCs) with uniform interior nanogaps reproducibly offer a highly robust substrate for surface-enhanced Raman scattering (SERS) because of the presence of abundant hot spots on their surface. The synthesis of such an SERS substrate by a simple route is a challenging task. Here, we have synthesized a highly stable wirelike cluster of silver nanoparticles (Ag-NPs) with an interparticle gap of ~1.7 ± 0.2 nm using deoxyribonucleic acid (DNA) as the template by exploiting an easy and inexpensive chemical route. The red shift in the surface plasmon resonance (SPR) band of Ag-NCs compared to SPR of a single Ag-NP confirms the strong interplasmonic interaction. Methylene Blue (MB) is used as a representative Raman probe to study the SERS effect of the NCs. The SERS measurements reveal that uniform, reproducible, and strong Raman signals were observed up to the single-molecule level. The intensity of the Raman signal is not highly dependent on the polarization of the excitation laser. The DNA-based Ag-NCs as a substrate show better isotropic behavior for their SERS intensity compared to the dimer, as confirmed from both the experimental and theoretical simulation results. We believe that in the future the DNA-based Ag-NCs might be useful as a potential SERS substrate for ultrasensitive trace detection, biomolecular assays, NP-based photothermal therapeutics, and a few other technologically important fields.

Hematoporphyrin-ZnO nanohybrids : Twin applications in efficient visible-light photocatalysis and dye-sensitized solar cells

Light-harvesting nanohybrids (LHNs) are systems composed of an inorganic nanostructure associated with an organic pigment that have been exploited to improve the light-harvesting performance over individual components. The present study is focused on developing a potential LHN, attained by the functionalization of dense arrays of ZnO nanorods (NRs) with a biologically important organic pigment hematoporphyrin (HP), which is an integral part of red blood cells (hemoglobin). Application of spectroscopic techniques, namely, Fourier transform infrared spectroscopy (FTIR) and Raman scattering, confirm successful monodentate binding of HP carboxylic groups to Zn(2+) located at the surface of ZnO NRs. Picosecond-resolved fluorescence studies on the resulting HP-ZnO nanohybrid show efficient electron migration from photoexcited HP to the host ZnO NRs. This essential photoinduced event activates the LHN under sunlight, which ultimately leads to the realization of visible-light photocatalysis (VLP) of a model contaminant Methylene Blue (MB) in aqueous solution. A control experiment in an inert gas atmosphere clearly reveals that the photocatalytic activity is influenced by the formation of reactive oxygen species (ROS) in the media. Furthermore, the stable LHNs prepared by optimized dye loading have also been used as an active layer in dye-sensitized solar cells (DSSCs). We believe these promising LHNs to find their dual applications in organic electronics and for the treatment of contaminant wastewater.

Non-equilibrium induction of tin in germanium: towards direct bandgap Ge1-xSnx nanowires.

The development of non-equilibrium group IV nanoscale alloys is critical to achieving new functionalities, such as the formation of a direct bandgap in a conventional indirect bandgap elemental semiconductor. Here, we describe the fabrication of uniform diameter, direct bandgap Ge1−xSnx alloy nanowires, with a Sn incorporation up to 9.2 at.%, far in excess of the equilibrium solubility of Sn in bulk Ge, through a conventional catalytic bottom-up growth paradigm using noble metal and metal alloy catalysts. Metal alloy catalysts permitted a greater inclusion of Sn in Ge nanowires compared with conventional Au catalysts, when used during vapour–liquid–solid growth. The addition of an annealing step close to the Ge-Sn eutectic temperature (230 °C) during cool-down, further facilitated the excessive dissolution of Sn in the nanowires. Sn was distributed throughout the Ge nanowire lattice with no metallic Sn segregation or precipitation at the surface or within the bulk of the nanowires. The non-equilibrium incorporation of Sn into the Ge nanowires can be understood in terms of a kinetic trapping model for impurity incorporation at the triple-phase boundary during growth.

Delocalized-localized transition in a semiconductor two-dimensional honeycomb lattice

We report the magnetotransport properties of a two-dimensional electron gas in a modulation-doped AlGaAs/GaAs heterostructure subjected to a lateral potential with honeycomb geometry. Periodic oscillations of the magnetoresistance and a delocalized-localized transition are shown by applying a gate voltage. We argue that electrons in such artificial-graphene lattices offer a promising approach for the simulation of quantum phases dictated by Coulomb interactions.

Inherent Control of Growth, Morphology, and Defect Formation in Germanium Nanowires

The use of bimetallic alloy seeds for growing one-dimensional nanostructures has recently gained momentum among researchers. The compositional flexibility of alloys provides the opportunity to manipulate the chemical environment, reaction kinetics, and thermodynamic behavior of nanowire growth, in both the eutectic and the subeutectic regimes. This Letter describes for the first time the role of Au(x)Ag(1-x) alloy nanoparticles in defining the growth characteristics and crystal quality of solid-seeded Ge nanowires via a supercritical fluid growth process. The enhanced diffusivity of Ge in the alloy seeds, compared to pure Ag seeds, and slow interparticle diffusion of the alloy nanoparticles allows the realization of high-aspect ratio nanowires with diameters below 10 nm, via a seeded bottom-up approach. Also detailed is the influence the alloyed seeds have on the crystalline features of nanowires synthesized from them, that is, planar defects. The distinctive stacking fault energies, formation enthalpies, and diffusion chemistries of the nanocrystals result in different magnitudes of {111} stacking faults in the seed particles and the subsequent growth of <112>-oriented nanowires with radial twins through a defect transfer mechanism, with the highest number twinned Ge nanowires obtained using Ag(0.75)Au(0.25) growth seeds. Employing alloy nanocrystals for intrinsically dictating the growth behavior and crystallinity of nanowires could open up the possibility of engineering nanowires with tunable structural and physical properties.

Correlated Electrons in Optically Tunable Quantum Dots: Building an Electron Dimer Molecule

We observe the low-lying excitations of a molecular dimer formed by two electrons in a GaAs semiconductor quantum dot in which the number of confined electrons is tuned by optical illumination. By employing inelastic light scattering we identify the intershell excitations in the one-electron regime and the distinct spin and charge modes in the interacting few-body configuration. In the case of two electrons, a comparison with configuration-interaction calculations allows us to link the observed excitations with the breathing mode of the molecular dimer and to determine the singlet-triplet energy splitting.

Tuning the photoluminescence and ultrasensitive trace detection properties of few-layer MoS2 by decoration with gold nanoparticles

We report an easy and inexpensive chemical route for the decoration of few-layer MoS2 with Au nanoparticles (NPs). The Au-NPs are formed on the defect sites of the MoS2 and localized by a non-covalent bond. The NPs act as a p-type dopant in the MoS2 layer. An enhancement in the photoluminescence (PL) intensity of the Au–MoS2 composite with respect to bare few-layer MoS2 has been observed. We also systematically observed a blue shift in the excitonic emission as the number and size of the Au-NPs on MoS2 increased. Both phenomena have been understood to result from the switching between charged exciton (trion) recombination and neutral exciton recombination. A potential application for the Au–MoS2 composite has been demonstrated, by using it as a substrate for surface-enhanced Raman scattering (SERS). The SERS measurements show a uniform, reproducible, and strong Raman signal from the adsorbed molecules with concentrations as low as 10−12 M. Our work provides a method to tune the optical and electronic properties of MoS2, and the Au–MoS2 composite might be useful as an efficient SERS substrate for the ultrasensitive detection of biomolecules.