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.
We report the synthesis of luminescent, free standing silicon quantum dots by dry and wet etching of silicon and silicon oxide core/shell nanostructures, which are synthesized by controlled oxidation of mechanically milled silicon. Dry and wet etching performed with CF(4) plasma and aqueous HF, respectively, result in the removal of the thick oxide shell of the core/shell nanostructures and affect an additional step of size reduction. HF etch is capable of producing isolated, spherical quantum dots of silicon with dimensions ∼ 2 nm. However, the etching processes introduce unsaturated bonds at the surface of the nanocrystals which are subsequently passivated by oxygen on exposure to ambient atmosphere. The photoluminescence spectra of the colloidal suspensions of these nanocrystals are characterized by double peaks and excitation dependent shift of emission energy. Comparison of the structural, absorption and luminescence characteristics of the samples provides evidence for two competing transition processes--quantum confinement induced widened band gap related transitions and oxide associated interface state mediated transitions. The results enable us to experimentally distinguish between the contributions of the two different transition mechanisms, which has hitherto been a challenging problem.
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 transitions involving surface defect states and quantum confinement induced widened band states.
Synthesis of hybrid core-shell nanostructures requires moderate lattice mismatch (<5%) between the materials of the core and the shell and usually results in the formation of structures with an atomically larger entity comprising the core. A reverse situation, where an atomically larger entity encapsulates a smaller atomic radius component having substantial lattice mismatch is unachievable by conventional growth techniques. Here, we report successful synthesis of ultra-small, light-emitting Si quantum dots (QDs) encapsulated by Au nanoparticles (NPs) forming a hybrid nanocomposite that exhibits intense room temperature photoluminescence (PL) and intriguing plasmon-exciton coupling. A facile strategy was adopted to utilize the active surface of oxide etched Si QDs as preferential sites for Au NP nucleation and growth which resulted in the formation of core-shell nanostructures consisting of an atomically smaller Si QD core surrounded by a substantially lattice-mismatched Au NP shell. The PL characteristics of the luminescent Si QDs (quantum yield ∼28%) are dramatically altered following Au NP encapsulation. Au coverage of the bare Si QDs effectively stabilizes the emission spectrum and leads to a red-shift of the PL maxima by ∼37 nm. The oxide related PL peaks observed in Si QDs are absent in the Au treated sample suggesting the disappearance of oxide states and the appearance of Au NP associated Stark shifted interface states within the widened band-gap of the Si QDs. Emission kinetics of the hybrid system show accelerated decay due to non-radiative energy transfer between the Si QDs and the Au NPs and associated quenching in PL efficiency. Nevertheless, the quantum yield of the hybrid remains high (∼20%) which renders these hetero-nanostructures exciting candidates for multifarious applications.
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