Mallar Ray

Free standing luminescent silicon quantum dots: evidence of quantum confinement and defect related transitions.

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.

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...

Highly lattice-mismatched semiconductor-metal hybrid nanostructures

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.

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...

Superparamagnetic iron oxide nanoparticle attachment on array of micro test tubes and microbeakers formed on p-type silicon substrate for biosensor applications

A uniformly distributed array of micro test tubes and microbeakers is formed on a p-type silicon substrate with tunable cross-section and distance of separation by anodic etching of the silicon wafer in N, N-dimethylformamide and hydrofluoric acid, which essentially leads to the formation of macroporous silicon templates. A reasonable control over the dimensions of the structures could be achieved by tailoring the formation parameters, primarily the wafer resistivity. For a micro test tube, the cross-section (i.e., the pore size) as well as the distance of separation between two adjacent test tubes (i.e., inter-pore distance) is typically approximately 1 μm, whereas, for a microbeaker the pore size exceeds 1.5 μm and the inter-pore distance could be less than 100 nm. We successfully synthesized superparamagnetic iron oxide nanoparticles (SPIONs), with average particle size approximately 20 nm and attached them on the porous silicon chip surface as well as on the pore walls. Such SPION-coated arrays of micro test tubes and microbeakers are potential candidates for biosensors because of the biocompatibility of both silicon and SPIONs. As acquisition of data via microarray is an essential attribute of high throughput bio-sensing, the proposed nanostructured array may be a promising step in this direction.

Artificial Neural Network (ANN)-Based Model for In Situ Prediction of Porosity of Nanostructured Porous Silicon

Nanostructured porous silicon (PS) is usually formed by anodic etching in HF-based solution, and its porosity is measured by a destructive gravimetric technique. In this article, we report the development of an artificial neural network (ANN)-based model permitting in situ prediction of porosity of PS samples. The sensitive and nonlinear dependence of porosity on the formation parameters demanded a nonclassical treatment, and ANN was found suitable for handling this problem. A series of experiments were performed on p-type Si having resistivity 2–5 Ω-cm in 24% HF solution to generate the data for development of the ANN model. The voltage fluctuations across the electrodes during the formation of PS samples were recorded and used to develop an ANN model for prediction of voltage during the transient state of PS evolution. The predicted voltages were then used to predict porosity for different values of current density (J) at any time instant. Porosity was also measured by the conventional and destructive gravimetric method for different values of J and time. The predicted porosities agreed well with gravimetrically determined values.

Remarkable thermal conductivity reduction in metal-semiconductor nanocomposites

Remarkable reduction in thermal conductivity, by ∼2 orders of magnitude compared to the bulk counterpart, is observed in a metal-semiconductor nanocomposite consisting of silver (Ag) and silicon (Si) nanostructures. The variation of thermal conductivity with temperature and with volume fraction of metallic inclusion exhibits counter-intuitive behavior. Contrary to bulk composites, thermal conductivity decreases with the increase in the volume fraction of Ag nanocrystals (at least till 0.067 experimented) and increases with temperature over the range of 303-473 K. This remarkable reduction in the thermal conductivity of the nanocomposite is due to the interplay of size-dependent reduction in thermal conductivity of the individual nanostructures, increased contribution of phonon scattering at the interfaces between nanoparticles, and electron-phonon coupling inside metallic nanocrystals and across metal-semiconductor interface. Such hybrid metal-semiconductor nanostructures with reduced thermal conductivity...

Unipolar resistive switching and tunneling oscillations in isolated Si–SiO x core–shell nanostructure

Unipolar resistive switching (URS) is observed in isolated Si-SiO x core-shell nanostructures. I-V characteristics recorded by a conductive atomic force microscope tip show SET and RESET processes with self compliance behavior. Hopping of carriers through defect states in the high resistance state (HRS) and space charge limited conduction in the low resistance state (LRS) are found to be the dominant carrier transport mechanisms in Si-SiO x core-shell nanostructures. URS between LRS and HRS may be attributed to the transition between hydrogen bridge (Si-H-Si) and hydrogen doublet (Si-HH-Si) defects. During RESET process, charge carriers tunnel through the nanostructure giving rise to oscillatory conduction.

Collective charge transport in semiconductor-metal hybrid nanocomposite

Collective charge transport through a hybrid nanocomposite made of Ag nanoparticles (NPs) embedded in ultra-small Si quantum dot (QD) matrix exhibits unexpected and fascinating characteristics. Metallic inclusion (10 wt. % of Ag NPs) in the Si QD matrix affects six orders of magnitude increase in current. In the semiconductor-metal hybrid, three different charge transport mechanisms—quantum tunneling through insulating barriers, variable range hoping, and simple thermally activated conduction dominate in three different temperature regimes that are influenced by bias voltage. We show that there is a cross-over from one transport mechanism to the other and determine the voltage dependent cross-over temperatures.