Low-density InAs QDs with subcritical coverage obtained by conversion of In nanocrystals
We report growth of InAs/GaAs quantum dots (QDs) by molecular beam epitaxy with low density of 2 μm−2 by conversion of... more We report growth of InAs/GaAs quantum dots (QDs) by molecular beam epitaxy with low density of 2 μm−2 by conversion of In nanocrystals deposited at low temperatures. The total amount of InAs used is about one monolayer, which is less than the critical thickness for conventional Stranski–Krastanov QDs. We also demonstrate the importance of the starting surface reconstruction for obtaining uniform QDs. The QD emission wavelength is easily tunable upon post-growth annealing with no wetting layer signal visible for short anneals. Microphotoluminescence measurements reveal well separated and sharp emission lines of individual QDs.
Preparation and photo/chemical-activation of wormlike network micelles of core–shell quantum dots and block copolymer hybrids
by Meng Zhang
Zhang, M.; Rene-Boisneuf, L.; Hu, Y.; Moozeh, K.; Hassan, Y.; Scholes, G.; Winnik, M. A., J. Mater. Chem. 2011, 21 (26), 9692-9701.
Poly(styrene-b-4-vinylpyridine) diblock copolymers PS404-b-P4VP76 and PS317-b-P4VP76 (the subscripts indicate the... more
Poly(styrene-b-4-vinylpyridine) diblock copolymers PS404-b-P4VP76 and PS317-b-P4VP76 (the subscripts indicate the degree of polymerization) self-assemble into spherical ‘‘crew-cut’’ micelles with a PS core and P4VP corona when prepared in a mixture of chloroform and 2-propanol. When the micelles are formed in the presence of quantum dots (QDs), the nature of the structures formed depends upon the polymer and the type of QDs. In our previous report [Macromolecules, 2010, 43, 5066–5074], PS404-b-P4VP76 + CdSe QDs formed stable spherical hybrid micelles, but prolonged vigorous stirring of the solutions led to a rearrangement into wormlike networks and loss of photoluminescence (PL) from the QDs. Here we report that PS317-b-P4VP76 + CdSe/ZnS core–shell QDs behave differently.
Partial loss of PL intensity occurred upon addition of 2-propanol to the chloroform solution of the components, and the rearrangement to a network structure occurred spontaneously. We describe two strategies for recovery of the PL intensity for the QDs within the network, photo-activation and chemical activation with elemental sulfur.
Download (.pdf) In vivo nanotoxicity assessment: the role of size, surface coating, nanostructuration, and dose-metrics
G. Vecchio, A. Galeone, V. Brunetti, G. Maiorano, M. A. Malvindi, S. Sabella, R. Cingolani, and P.P. Pompa
Nanotech 2011, Vol 3, Chapter 7, Environment, Health & Safety, Pages 509–512
ISBN: 978-1-4398-7138-6
Conference Technical Proceedings, TechConnectWorldConference and Expo 2011, June 13-16, Boston (MA)
The growing use of nanomaterials in commercial goods and as novel carriers for drug delivery is generating increasing... more The growing use of nanomaterials in commercial goods and as novel carriers for drug delivery is generating increasing questions about possible risks for human health and environment, due to the lack of an in-depth assessment of their potential toxicity. In this frame, we focused on the study of metrology of nanoparticles (characterization of their size, shape, surface chemistry, aggregation/agglomeration, formation of protein/NP complexes) in order to design standardized in vitro protocols to assess dose-dependence toxicity. Moreover, we investigated the in vivo effects of AuNPs and QDs with different sizes, surface coatings, and nanostructuration, on the model system Drosophila melanogaster upon ingestion. We observed that nanoparticles induce clear adverse effects in treated organisms, such as a strong reduction of their life span and fertility, presence of DNA fragmentation and apoptosis, as well as a significant overexpression of the stress proteins. Interestingly, the toxic effects were found to be dependent on size, surface coatings and nanoscale surface features. These results open up important questions related to the safe use of nanoparticles and suggest some experimental routes for the development of safe nanocarriers.
Hydrothermal synthesis of CdTe QDs: Their luminescence quenching in presence of bio-molecules and observation of bistable memory effect in CdTe QD/PEDOT:PSS heterostructure
We report one-pot hydrothermal synthesis of nearly mono-disperse 3-mercaptopropionic acid capped water-soluble cadmium... more We report one-pot hydrothermal synthesis of nearly mono-disperse 3-mercaptopropionic acid capped water-soluble cadmium telluride (CdTe) quantum dots (QDs) using an air stable Te source. The optical and electrical characteristics were also studied here. It was shown that the hydrothermal synthesis could be tuned to synthesize nano structures of uniform size close to nanometers. The emissions of the CdTe QDs thus synthesized were in the range of 500–700 nm by varying the duration of synthesis. The full width at half maximum (FWHM) of the emission peaks is relatively narrow (40–90 nm), which indicates a nearly uniform distribution of QD size. The structural and optical properties of the QDs were characterized by transmission electron microscopy (TEM), photoluminescence (PL) and Ultraviolet–visible (UV–Vis) spectroscopy. The photoluminescence quenching of CdTe QDs in the presence of l-cysteine and DNA confirms its biocompatibility and its utility for biosensing applications. The room temperature current–voltage characteristics of QD film on ITO coated glass substrate show an electrically induced switching between states with high and low conductivities. The phenomenon is explained on the basis of charge confinement in quantum dots.
Synthesis of cationic quantum dots via a two-step ligand exchange process
A new class of quaternary ammonium derivatives has been used
to synthesize cationic CdSe/ZnS quantum dots with... more
A new class of quaternary ammonium derivatives has been used
to synthesize cationic CdSe/ZnS quantum dots with exceptional
stability in water as well as in biological media.
Reverse Stern–Volmer behavior for luminescence quenching in carbon nanoparticles
by Parambath "Anil " Anilkumar
L. Cao, P.Anilkumar et al. Canadian Journal of Chemistry, 2011, 89,104-109
Photoluminescence properties of carbon nanoparticles from different treatments in various suspensions were evaluated,... more Photoluminescence properties of carbon nanoparticles from different treatments in various suspensions were evaluated, and the results were in general agreement with those available in the literature. An interesting observation was that the quenching of luminescence emissions in the aqueous suspended carbon nanoparticles by aliphatic amines deviated from the classical Stern–Volmer behavior in a somewhat unusual fashion, exhibiting reversed quenching (increasing luminescence intensities) at very low quencher concentrations. The implication of the observation to the mechanistic framework that may account for photoluminescence emissions in both functionalized and naked carbon nanoparticles is proposed and discussed.
Toward quantitatively fluorescent carbon-based ‘‘quantum’’ dots
by Parambath "Anil " Anilkumar
P. Anilkumar et al. Nanoscale, 2011, 3, 2023-2027
Carbon-based “quantum” dots (or “carbon dots”) are generally defined as surface-passivated small carbon nanoparticles... more Carbon-based “quantum” dots (or “carbon dots”) are generally defined as surface-passivated small carbon nanoparticles that are brightly fluorescent. Apparently, the carbon particle surface passivation in carbon dots is critical to their fluorescence performance. An effective way to improve the surface passivation is to dope the surface of the precursor carbon nanoparticles with an inorganic salt, followed by the typical functionalization with organic molecules. In this work we passivated small carbon nanoparticles by a combination of the surface-doping with nanoscale semiconductors and the organic functionalization, coupled with gel column fractionation to harvest the most fluorescent carbon dots, which exhibited fluorescence emission quantum yields of up to 78%. Experimental and mechanistic issues relevant to potentially further improve the performance of carbon dots toward their being quantitatively fluorescent are discussed.
Carbon Nanoparticles as Visible-Light Photocatalysts for Efficient CO2 Conversion and Beyond
by Parambath "Anil " Anilkumar
L. Cao, S. Sahu, P. Anilkumar et al. J. Am. Chem. Soc., 2011, 133, 4754–4757
Increasing atmospheric CO2 levels have generated much concern, driving the ongoing carbon sequestration effort. A... more Increasing atmospheric CO2 levels have generated much concern, driving the ongoing carbon sequestration effort. A compelling CO2 sequestration option is its photocatalytic conversion to hydrocarbons, for which the use of solar irradiation represents an ultimate solution. Here we report a new strategy of using surface-functionalized small carbon nanoparticles to harvest visible photons for subsequent charge separation on the particle surface in order to drive the efficient photocatalytic process. The aqueous solubility of the catalysts enables photoreduction under more desirable homogeneous reaction conditions. Beyond CO2 conversion, the nanoscale carbon-based photocatalysts are also useful for the photogeneration of H2 from water under similar conditions.
Si nanoparticle-Er3+ coupling through contact in as-deposited nanostructured films
Published in Applied Physics Letters. Authors: S. Núñez-Sánchez, P. M. Roque, R. Serna, and A. K. Petford-Long
The efficient excitation of Er3+ ions through contact with Si nanoparticles (NPs) is demonstrated. A nanostructured... more The efficient excitation of Er3+ ions through contact with Si nanoparticles (NPs) is demonstrated. A nanostructured doping process has been developed that leads to contact between Si NPs formed in situ and optically-active Er3+ ions embedded in Al2O3. This is achieved by independent and consecutive deposition of the dopants and matrix. The Si NP–Er3+ contact regime enhances the probability of efficient interaction due to the local spatial overlap of the electronic states of the Er3+ and of the Si NP exciton, enabling energy transfer by interband exciton recombination. This leads to up to 53% of the Er3+ ions being excited in as-deposited films.
Förster signatures and qubits in optically driven quantum dot molecules
by Juan E Rolon
Physica E: Low-dimensional Systems and Nanostructures
Volume 40, Issue 5, March 2008, Pages 1481-1483
17th International Conference on Electronic Properties of Two-Dimensional Systems
An interesting approach to achieve quantum gate operations in a solid state device is to implement an optically driven... more An interesting approach to achieve quantum gate operations in a solid state device is to implement an optically driven quantum gate using two vertically coupled self-assembled quantum dots, a quantum dot molecule (QDM). We present a realistic model for exciton dynamics in InGaAs/GaAs QDMs under intense laser excitation and applied electric fields. The dynamics is obtained by solutions of the Lindblad master equation. A map of the dressed ground state as function of laser energy and applied electric field exhibits rich structure that includes excitonic anticrossings that permit the identification of the relevant couplings. The optical signatures of the dipole–dipole Förster energy transfer mechanism show as splittings of several (spatially) indirect excitonic lines. Moreover, we construct a model for exciton qubit rotations by adiabatic electric field cyclic sweeps into a Förster-tunneling regime which induces level anticrossings. The proposed qubit exhibits Rabi oscillations among two well-defined exciton pairs as function of the residence time at the anticrossing.
Forster energy transfer signatures in optically driven quantum dot molecules
by Juan E Rolon
Phys. Rev. B 79, 245309 (2009)
Juan E. Rolon and Sergio E. Ulloa
http://prb.aps.org/abstract/PRB/v79/i24/e245309
The Förster resonant energy-transfer mechanism (FRET) is investigated in optically driven and electrically gated... more The Förster resonant energy-transfer mechanism (FRET) is investigated in optically driven and electrically gated tunnel-coupled quantum dot molecules. Two novel FRET-induced optical signatures are found in the dressed excitonic spectrum. This is constructed from exciton-level occupation as a function of pump-laser energy and applied bias, resembling a level anticrossing spectroscopy measurement. We observe a redistribution of spectral weight and splitting of the exciton spectral lines. FRET among single excitons induces a splitting in the spatially direct exciton lines, away from the anticrossing due to charge tunneling in the molecule. However, near the anticrossing, a novel signature appears as a weak satellite line following an indirect exciton line. FRET signatures may also occur among indirect excitons, appearing as split indirect lines. In that case, the signatures appear also in the direct biexciton states, as the indirect satellite mixes in near the tunneling anticrossing region.
Exact crossover Green function in the two-channel and two-impurity Kondo models
Phys. Rev. Lett. 106, 147202 (2011)
Can also be found at:
http://arxiv.org/abs/1101.3028
Symmetry-breaking perturbations destabilize the critical points of the two-channel and two-impurity Kondo models,... more Symmetry-breaking perturbations destabilize the critical points of the two-channel and two-impurity Kondo models, thereby leading to a crossover from non-Fermi liquid behavior to standard Fermi liquid physics. Here we use an analogy between this crossover and one occurring in the boundary Ising model to calculate the full crossover Green function analytically. In remarkable agreement with our numerical renormalization group calculations, the single exact function applies for an arbitrary mixture of the relevant perturbations in each model. This rich behavior resulting from finite channel asymmetry, interlead charge transfer, and/or magnetic field should be observable in quantum dot or tunneling experiments.
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Synthesis of reversible circuits by different methods and their optimization
by Sumit Raj
In the recent years, reversible logic has emerged as a promising technology having its applications in low power CMOS,... more In the recent years, reversible logic has emerged as a promising technology having its applications in low power CMOS, quantum computing, nanotechnology, and optical computing. A reversible circuit maps each output vector into a unique input vector, and vice versa. There always has been a hurdle in realization and optimization of reversible circuit. One way of realizing reversible logic is quantum computers. Quantum computing has been a field of growing interest in the last decade because of its promises to reduce power consumption. This paper presents realization of reversible circuits such as adder and multiplier using three different methods which are as follows CMOS logic, Quantum cellular automata (QCA), and jQuantum. Although CMOS don’t take full benefit of reversibility criteria but it is used for functional verification of reversible circuit design. In this paper we have implemented few reversible circuits in CMOS and their layout is presented. QCA is a new technology for realization of quantum circuits. Minimum area full adder has been implemented in QCAD and presented in this paper. This paper also proposes a design of a reversible multiplier with minimum complexity in terms of gates. This multiplier design has been verified using jQuantum which is a JAVA simulator which designs reversible circuits based on quantum wires. A novel design of a 4x4 multiplier has also been proposed. Thus, this paper proposes different methods for realising reversible logic and their optimization techniques.
Solvothermal synthesis of Hg1−xCdxTe nanostructures—Their structural and optical properties
Journal of Alloys and Compounds
Volume 509, Issue 13, 31 March 2011, Pages 4632-4635
This article describes a facile, low-cost, solution-phase approach to the large-scale preparation of Hg1−xCdxTe... more
This article describes a facile, low-cost, solution-phase approach to the large-scale preparation of Hg1−xCdxTe nanostructures of different shapes such as nanorods, quantum dots, hexagonal cubes of different sizes and different compositions at a growth temperature of 180 °C using an air stable Te source by solvothermal technique. The XRD spectrum shows that the crystals are cubic in their basic structure and reveals the variation in lattice constant as a function of composition. The size and morphology of the products were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The formation of irregular shaped particles and few nano-rods in the present synthesis is attributed to the cetyl trimethylammonium bromide (CTAB). The room temperature FTIR absorption and PL studies for a compositon of x = 0.8 gives a band gap of 1.1 eV and a broad emission in NIR region (0.5–0.9 eV) with all bands attributed to surface defects.
Microwave-assisted synthesis of monodispersed CdTe nanocrystals
by John Bass
published in 'Chemical Communications', 2010
Microscale Synthesis of Quantum Dots
Microfluidic reactors are emerging as a highly promising technology for quantum dot synthesis due to the unparalleled... more Microfluidic reactors are emerging as a highly promising technology for quantum dot synthesis due to the unparalleled control they provide over particle properties. In this article, we review recent developments in the microfluidic synthesis of quantum dots, and discuss some of the advantages and challenges of preparing nanocrystalline materials in microscale fluidic channels. The relative merits of continuous-flow and segmented-flow reactors are considered, together with a number of outstanding issues that must be successfully addressed for microfluidics to become a truly viable technology for quantum dot synthesis.
Controlled Synthesis of III-V Quantum Dots In Microfluidic Reactors
Microfluidic reactors have recently emerged as near-ideal systems for the growth of high quality colloidal... more Microfluidic reactors have recently emerged as near-ideal systems for the growth of high quality colloidal nanoparticles due to the unprecedented control they offer over reaction conditions. High quality InP quantum dots are synthesised in simple microfluidic devices, and their spectral characteristics are easily tuned.