A K Pramanick

Silicon and silicon oxide core-shell nanoparticles: Structural and photoluminescence characteristics

We report the synthesis of spherical core-shell structures of silicon and silicon oxide by a novel route of forced external oxidation of ball milled silicon. Structural investigations reveal the formation of a crystalline silicon core surrounded by an amorphous oxide shell, with core and shell dimensions varying approximately between 4–10 and 55–170 nm, respectively. The observations suggest partial amorphization of crystalline silicon, invasive oxygen induced passivation of dangling bonds, and formation of different types of defects in the nanocrystalline silicon/silicon oxide core-shell structure, particularly at the interfaces. No detectable photoluminescence (PL) is obtained from the as-milled silicon, but the oxidized core-shell structures exhibit strong room temperature PL, detectable with unaided eye. The peak energy of the PL spectra blueshifts with an increase in excitation energy, with the peak positions varying from 2.24 to 2.48 eV under external excitation ranging from 2.41 to 3.5 eV. The obse...

Luminescent core-shell nanostructures of silicon and silicon oxide: Nanodots and nanorods

We report synthesis and luminescent characteristics of core-shell nanostructures of silicon and silicon oxide having two different morphologies—spherical (nanodot) and rodlike (nanorod), prepared by controlled oxidation of mechanically milled crystalline silicon and by exfoliation of the affected layer of porous silicon. Colloidal suspensions of these nanostructures exhibit intense room temperature photoluminescence (PL), detectable with the unaided eye. PL band peak energies of the colloidal suspensions formed from porous silicon are blue shifted by ∼1 eV compared to the as-prepared films on silicon substrate. In addition, PL spectra of all the colloidal suspensions blueshift with increase in excitation energy but the PL peaks of as-prepared porous silicon are independent of excitation. However, shape of the nanocrystals (spherical or rodlike) is found to have little effect on the emission spectra. These observations are explained in terms discretization of phonon density of states and electronic transit...

Temperature dependent photoluminescence from porous silicon nanostructures: Quantum confinement and oxide related transitions

Temperature dependent photoluminescence (PL) spectroscopy along with structural investigations of luminescent porous Si enable us to experimentally distinguish between the relative contributions of band-to-band and oxide interface mediated electronic transitions responsible for light emission from these nanostructures. Porous Si samples formed using high current densities (J ≥ 80 mA/cm2) have large porosities (P ≥ 85%) and consequently smaller (∼1-6 nm) average crystallite sizes. The PL spectra of these high porosity samples are characterized by multiple peaks. Two dominant peaks—one in the blue regime and one in the yellow/orange regime, along with a very low intensity red/NIR peak, are observed for these samples. The high energy peak position is nearly independent of temperature, whereas the yellow/orange peak red-shifts with increasing temperature. Both the peaks blue shift with ageing and with increasing porosity. The intensity of the blue peak increases whereas the yellow/orange peak decreases with i...

Liquid phase collagen modified graphene that induces apoptosis

Direct exfoliation of graphite (GR) to collagen modified graphene (G) flakes using acylated collagen has been studied. The chemical structure of collagen (CL) and all liquid exfoliants studied so far have striking similarity, namely, the hydrocarbon chain length, aromatic residues and polarity. Here, CL dispersed in acetic (AA), succinic (SA) and propionic (PA) acid behaves as three different surfactants which have been used to simultaneously exfoliate and disperse nano G platelets to a colloidal form. Transmission electron microscopy micrographs show average dimensions of ∼500 × 200 nm; Moire patterns observed at several places in all the three samples indicate periodic perturbations in the graphitic stacking and selected area electron diffraction confirms graphene formation. AFM images confirmed the same lateral dimensions with an average thickness of 1.2 nm. The Raman spectra revealed strain dependent splitting and a red shift of 2D bands, a maximum in G-PA and minimum in G-AA. X-ray photoelectron spectroscopy showed a decrease of the sp2/sp3 ratio GR post CL interaction indicating an interaction. The zeta potential, fluorescence and luminescence values changed in G, with maximum variation in G-PA; suggesting that CL dispersed in PA is the best. The bioactivity of colloidal G-PA was studied in solid, hematological and neuronal cancer cell lines. It induced reactive oxygen species and cell death in cancer cell lines and altered membrane integrity while sparing normal cells, underscoring its possible utility in cancer therapy.

Performance Enhancement of Crystalline Silicon Solar Cells by Coating with Luminescent Silicon Nanostructures

In this work we report a technique that is potentially capable of increasing the efficiency of crystalline silicon solar cells, which dominate the present-day market of photovoltaic devices. The simple and cost-effective method involves coating the surface of a commercially procured silicon solar cell with luminescent silicon nanocrystals. Core/shell silicon/silicon-oxide nanostructures are fabricated by an inexpensive and reproducible technique, where coarse silicon powders are repeatedly milled, oxidized, and etched until their sizes are reduced so as to exhibit room-temperature photoluminescence under ultraviolet excitation. A thin coating of these nanostructures on a standard solar cell, obtained by a simple dip-coating method, increases the open-circuit voltage and short-circuit current, which consequently increases the maximum power delivered by ~16.3% and efficiency by almost ∼39%. We propose that the core/shell nanostructures act as luminescent convertors that convert higher-energy photons to lower-energy photons, thereby leading to less thermal relaxation loss of photoexcited carriers.

Near-grain-boundary characterization by atomic force microscopy

Characterization of near-grain boundary is carried out by atomic force microscopy (AFM). It has been observed to be the most suitable technique owing to its capability to investigate the surface at high resolution. Commercial purity-grade nickel processed under different conditions, viz., (i) cold-rolled and annealed and (ii) thermally etched condition without cold rolling, is considered in the present study. AFM crystallographic data match well with the standard data. Hence, it establishes two grain-boundary relations viz., plane matching and coincidence site lattice (CSL Sigma=9) relation for the two different sample conditions.

Amorphous Carbon Dots and their Remarkable Ability to Detect 2,4,6-Trinitrophenol

Apparently mundane, amorphous nanostructures of carbon have optical properties which are as exotic as their crystalline counterparts. In this work we demonstrate a simple and inexpensive mechano-chemical method to prepare bulk quantities of self-passivated, amorphous carbon dots. Like the graphene quantum dots, the water soluble, amorphous carbon dots too, exhibit excitation-dependent photoluminescence with very high quantum yield (~40%). The origin and nature of luminescence in these high entropy nanostructures are well understood in terms of the abundant surface traps. The photoluminescence property of these carbon dots is exploited to detect trace amounts of the nitro-aromatic explosive — 2,4,6-trinitrophenol (TNP). The benign nanostructures can selectively detect TNP over a wide range of concentrations (0.5 to 200 µM) simply by visual inspection, with a detection limit of 0.2 µM, and consequently outperform nearly all reported TNP sensor materials.