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.
We examine the modified electronic structure and single-carrier transport of individual hybrid core-shell metal-semiconductor Au-ZnS quantum dots (QDs) by a scanning tunnel microscope. Nearly monodisperse ultra-small QDs are achieved by...
Semiconductor–metal
hybrid nanostructures are extremely promising candidates for exploring
fundamental physics and chemistry of small systems and have multifarious
potential applications. However, most of the synthesis techniques
result in the formation of core–shell nanostructures with the
atomically larger entity comprising the core, and allows moderate
lattice-mismatch between the materials of the core and the shell.
In this work we demonstrate the synthesis of a stable hybrid core–shell
nanocomposite, consisting of Si quantum dots encapsulated by atomically
larger and highly lattice-mismatched Au nanoparticles, induced by
charge transfer between the two species. The semiconductor–metal
hybrid system exhibits strong and stable room temperature photoluminescence
with a substantially high quantum yield of 17.1%. Structural and spectroscopic
investigations confirm the encapsulation of light emitting Si quantum
dots by several Au nanoparticles, which consequently restrict surface
oxidation of the quantum dots and effectively stabilizes the emission
spectrum. These luminescent heteronanostructures have huge potential
applications ranging from energy harvesting to bioimaging.
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