Dipanwita Majumdar

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.

Graphene Quantum Sheets: A New Material for Spintronic Applications

www.MaterialsViews.com C O M M U Graphene Quantum Sheets: A New Material for Spintronic Applications N IC By Shyamal K. Saha , * Moni Baskey , and Dipanwita Majumdar A IO N Since the discovery of the giant magnetoresistance (GMR) effect, extensive research has been devoted to fi nding new materials for application in spintronic devices. [ 1 – 4 ] GMR devices (spin valves) basically consist of artifi cial thin fi lm materials of alternate ferromagnetic and non-magnetic layers. Resistance of the materials is at a minimum when the magnetic moments in ferromagnetic layers are aligned and maximum when they are anti-aligned. The major problem in spin valve structures is the interface scattering of spins between magnetic and nonmagnetic layers. To fabricate a clean (defect-free) interface, where there is no scattering, is a great technological challenge. Therefore, the ideal and most effective effort would be to search for a suitable material that intrinsically behaves as a spin-valve, with two ferromagnetic edges separated by a non-magnetic core. It has been reported that the zigzag edge states of graphene sheets are ferromagnetic and these opposite ferromagnetic edges are coupled through antiferromagnetic interaction. [ 5 , 6 ]

Microwave-assisted synthesis of porous Mn2O3 nanoballs as bifunctional electrocatalyst for oxygen reduction and evolution reaction

Technological hurdles that still prevent the commercialization of fuel cell technologies necessitate designing low-cost, efficient and non-precious metals. These could serve as alternatives to high-cost Pt-based materials. Herein, a facile and effective microwave-assisted route has been developed to synthesize structurally uniform and electrochemically active pure and transition metal-doped manganese oxide nanoballs (Mn2O3 NBs) for fuel cell applications. The average diameter of pure and doped Mn2O3 NBs was found to be ~610 nm and ~650 nm, respectively, as estimated using transmission electron microscopy (TEM). The nanoparticles possess a good degree of crystallinity as evident from the lattice fringes in high-resolution transmission electron microscopy (HRTEM). The cubic crystal phase was ascertained using X-ray diffraction (XRD). The energy dispersive spectroscopic (EDS) elemental mapping confirms the formation of copper-doped Mn2O3 NBs. The experimental parameter using trioctylphosphine oxide (TOPO) as the chelating agent to control the nanostructure growth has been adequately addressed using scanning electron microscopy (SEM). The solid NBs were formed by the self-assembly of very small Mn2O3 nanoparticles as evident from the SEM image. Moreover, the concentration of TOPO was found to be the key factor whose subtle variation can effectively control the size of the as-prepared Mn2O3 NBs. The cyclic voltammetry and galvanostatic charge/discharge studies demonstrated enhanced electrochemical performance for copper-doped Mn2O3 NBs which is supported by a 5.2 times higher electrochemically active surface area (EASA) in comparison with pure Mn2O3 NBs. Electrochemical investigations indicate that both pure and copper-doped Mn2O3 NBs exhibit a bifunctional catalytic activity toward the four-electron electrochemical reduction as well the evolution of oxygen in alkaline media. Copper doping in Mn2O3 NBs revealed its pronounced impact on the electrocatalytic activity with a high current density for the electrochemical oxygen reduction and evolution reaction. The synthetic approach provides a general platform for fabricating well-defined porous metal oxide nanostructures with prospective applications as low-cost catalysts for alkaline fuel cells.

Epitaxial Growth of Crystalline Polyaniline on Reduced Graphene Oxide

Due to its unique electronic properties, graphene has already been identified as a promising material for future carbon based electronics. To develop graphene technology, the fabrication of a high quality P-N junction is a great challenge. Here, we describe a general technique to grow single crystalline polyaniline (PANI) films on graphene sheets using in situ polymerization via the oxidation-reduction of aniline monomer and graphene oxide, respectively, to fabricate a high quality P-N junction, which shows diode-like behavior with a remarkably low turn-on voltage (60 mV) and high rectification ratio (1880:1) up to a voltage of 0.2 V. The origin of these superior electronic properties is the preferential growth of a highly crystalline PANI film as well as lattice matching between the d-values [∼2.48 Å] of graphene and {120} planes of PANI.

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.

Probing Thermal Flux in Twinned Ge Nanowires through Raman Spectroscopy

We report a noninvasive optical technique based on micro-Raman spectroscopy to study the temperature-dependent phonon behavior of normal (nondefective) and twinned germanium nanowires (Ge-NWs). We studied thermophysical properties of Ge-NWs from Raman spectra, measured by varying excitation laser power at ambient condition. We derived the laser-induced temperature rise during Raman measurements by analyzing the Raman peak position for both the NWs, and for a comparative study we performed the same for bulk Ge. The frequency of the Ge-Ge phonon mode softens for all the samples with the increase in temperature, and the first-order temperature coefficient (χT) for defected NWs is found to be higher than normal NWs and bulk. We demonstrated that apart from the size, the lamellar twinning and polytype phase drastically affect the heat transport properties of NWs.

Observation of ferroelectric response in conjugated polymer nanotubes

Long range charge delocalization usually inhibits ferroelectric response in conjugated polymers. We observe remarkable ferroelectric response (remnant polarization 2.8 μC cm−2) in polyaniline (PANI) nanotubes synthesized by oxidative polymerization technique in methanol medium using anodic alumina (AAO) template. Ferroelectricity in PANI nanotubes arises due to spontaneous polarization caused by hydrogen bonds, created due to charge transfer between methanol molecule and aligned polymer chain. Due to directional growth along the AAO nanochannel, PANI chains are well-aligned and all these dipoles associated with hydrogen bonds are arranged in regular order—exhibiting exceptional ferroelectric response. However, such effect is absent in bulk PANI.

Synthesis of single crystalline micron-sized rectangular silver bar

The synthesis of single crystalline rectangular silver bar using polyacrylamide (PAM) and silver nitrate (AgNO 3 ) by a hydrothermal process is reported. PAM has been used to create a reducing atmosphere as well as nucleation sites to produce silver seeds along the PAM chain. Several silver nanostructures viz. nanoparticles, growth of silver nanowires, and finally a single crystalline silver nanobar with a square cross section and of several microns in length, depending upon maturity and temperature of the hydrosol, are synthesized. At relatively lower temperatures (above 380 K) and higher pressure amide group of PAM is hydrolyzed with the liberation of ammonia (NH 3 ), which produces a reducing atmosphere. As a result, the degraded PAM chain acts as nucleation sites to produce the assembly of silver nanocrystals along the chain. As the hydrosol becomes more and more mature, a directional growth of silver nanocrystals, called a mesocrystal, is formed. This mesocrystal is converted into single crystals due to fusion of higher energy surfaces (100) of nanocrystals to minimize the total surface energy. This growth process is completed with the formation of a single crystalline rectangular silver bar with a square cross section due to the growth of silver along the [110] direction.

Observation of microwave plasmons in one-dimensional conjugated polymer chain

Observation of extremely low frequency plasmons in highly ordered quasi-one-dimensional (quasi-1D) interrupted metallic polymer chain segments is reported. Rice and Bernascony [Phys. Rev. Lett. 29, 113 (1972)] predicted giant permittivity in interrupted 1D metal strands because of quantum confinement. We have used this quasi-1D electron system with giant permittivity to realize 1D plasmons in microwave frequency. Polypyrrole nanorods with ordered and aligned chains have been synthesized. These ordered and perfectly conjugated systems interrupted by defects are ideal systems to achieve giant permittivity and as a result, 1D microwave plasmons, which have potential applications in microwave devices, are observed.