Search query: Core-shell semiconductor nanocrystals

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abstract: Colloidal core/shell nanocrystals contain at least two semiconductor materials in an onionlike structure. The possibility to tune the basic optical properties of the core nanocrystals, for example, their fluorescence wavelength, quantum yield, and lifetime, by growing an epitaxial type shell of another semiconductor has fueled significant progress on the chemical synthesis of these systems. In such core/shell nanocrystals, the shell provides a physical barrier between the optically active core and the surrounding medium, thus making the nanocrystals less sensitive to environmental changes, surface chemistry, and photo oxidation. The shell further provides an efficient passivation of the surface trap states, giving rise to a strongly enhanced fluorescence quantum yield. This effect is a fundamental prerequisite for the use of nanocrystals in applications such as biological labeling and light emitting devices, which rely on their emission properties. Focusing on recent advances, this Review discusses the fundamental properties and synthesis methods of core/shell and core/multiple shell structures of II VI, IV VI, and III V semiconductors.
author_names: Peter Reiss

Myriam Protiere

Liang Li
title: Core/Shell semiconductor nanocrystals.
venue: Small
year: 2009

abstract: A strong electron hole exchange interaction (EI) in semiconductor nanocrystals (NCs) gives rise to a large (up to tens of meV) splitting between optically active 'bright' and optically passive 'dark' excitons. This dark bright splitting has a significant effect on the optical properties of band edge excitons and leads to a pronounced temperature and magnetic field dependence of radiative decay. Here we demonstrate a nanoengineering based approach that provides control over EI while maintaining nearly constant emission energy. We show that the dark bright splitting can be widely tuned by controlling the electron hole spatial overlap in core shell CdSe/CdS NCs with a variable shell width. In thick shell samples, the EI energy reduces to <250 meV, which yields a material that emits with a nearly constant rate over temperatures from 1.5 to 300 K and magnetic fields up to 7 T. The EI manipulation strategies demonstrated here are general and can be applied to other nanostructures with variable electron hole overlap.
author_names: Sergio Brovelli

Richard D Schaller

Scott A Crooker

Florencio Garcia-Santamaria

Yuan Chen

Ranjani Viswanatha

Jennifer Ann Hollingsworth

Han Htoon

Victor I Klimov
title: Nano engineered electron hole exchange interaction controls exciton dynamics in core shell semiconductor nanocrystals
venue: Nature communications
year: 2011

abstract: A theoretical study of the positive and negative trion channels in the nonradiative Auger recombination of band edge biexcitons (BXs) in CdSe/CdS core/shell nanocrystals (NCs) is presented. The theory takes into account the BX fine structure produced by NC asymmetry and hole hole exchange interaction. The calculations show that growth of CdS shell upon CdSe core suppresses the rate of the Auger recombination via negative trion channel, while the more efficient Auger recombination via positive trion channel shows much weaker dependence on the shell thickness. The demonstrated oscillatory dependence of the BX Auger rate on the core and shell sizes is explained qualitatively in terms of overlap of the ground and excited carrier wave functions. The calculations show that raise of temperature accelerates the Auger recombination in CdSe/CdS NCs due to reduction of the bulk energy gaps of CdSe and CdS.
author_names: Roman Vaxenburg

Anna V Rodina

Efrat Lifshitz

Alexander L Efros
title: Biexciton Auger Recombination in CdSe/CdS Core/Shell Semiconductor Nanocrystals.
venue: Nano letters
year: 2016

abstract: Concomitant with the development of polymer nanocomposite (PNC) technologies across numerous industries is an expanding awareness of the uncertainty with which engineered nanoparticles embedded within these materials may be released into the external environment, particularly liquid media. Recently there has been an interest in evaluating potential exposure to nanoscale fillers from PNCs, but existing studies often rely upon uncharacterized, poor quality, or proprietary materials, creating a barrier to making general mechanistic conclusions about release phenomena. In this study we employed semiconductor nanoparticles (quantum dots, QDs) as model nanofillers to quantify potential release into liquid media under specific environmental conditions. QDs of two sizes were incorporated into low density polyethylene by melt compounding and the mixtures were extruded as free standing fluorescent films. These films were subjected to tests under conditions intended to accelerate potential release of embedded particles or dissolved residuals into liquid environments. Using inductively coupled plasma mass spectrometry and laser scanning confocal microscopy, it was found that the acidity of the external medium, exposure time, and small differences in particle size (on the order of a few nm) all play pivotal roles in release kinetics. Particle dissolution was found to play a major if not dominant role in the release process. This paper also presents the first evidence that internally embedded nanoparticles contribute to the mass transfer, an observation made possible via the use of a model system that was deliberately designed to probe the complex relationships between nanoparticle enabled plastics and the environment.
author_names: K Venkatanarasimha Pillai

Patrick Gray

C C Tien

Reiner Bleher

Li Piin Sung

Timothy V Duncan
title: Environmental release of core shell semiconductor nanocrystals from free standing polymer nanocomposite films.
venue: Environmental science. Nano
year: 2016

abstract: The present study investigates the photocatalytic activity of ZnSe/CdS core/shell semiconductor nanocrystals. These nanoparticles exhibit a spatial separation of photoinduced charges between the core and the shell domains, which makes them potentially viable for photocatalytic applications. Unfortunately, one of the excited charges remains inside the core semiconductor and thus cannot efficiently react with the external environment. Here, we explore this issue by investigating the mechanisms of hole extraction from the ZnSe core to the surface of the CdS shell. In particular, the effect of shell thickness in ZnSe/CdS core/shell nanocrystals on the ability of core localized charges to perform oxidative reactions was determined. By using a combination of time resolved spectroscopy and electrochemical techniques, we demonstrate that the use of hole scavenging surfactants facilitates an efficient transfer of core localized holes to the surface even in the case of shells exceeding 7 nm in thickness. These meas.
author_names: Dimuthu Perera

Ryan Lorek

Rony S Khnayzer

Pavel Moroz

Timothy F O'Connor

Dmitry Khon

Geoffrey M Diederich

Erich W Kinder

Scott Lambright

Felix N Castellano

Mikhail Zamkov
title: Photocatalytic Activity Of Core/shell Semiconductor Nanocrystals Featuring Spatial Separation Of Charges
year: 2012

abstract: Quantum dots are semiconducting nanocrystals that exhibit extraordinary optical properties. QD have shown higher photostability compared to standard organic dye type probes. Therefore, they have been heavily explored in the biomedical field. This review will discuss the different approaches to synthesis, solubilise and functionalise QD. Their main biomedical applications in imaging and photodynamic therapy will be highlighted. Finally, QD biodistribution profile and in vivo toxicity will be discussed.
author_names: Wafa' T Al-Jamal
title: Core shell Semiconductor Nanocrystals: Effect of Composition, Size, Surface Coatings on their Optical Properties, Toxicity and Pharmacokinetics.
venue: Current pharmaceutical design
year: 2017

abstract: We report the synthesis of spherical ZnTe nanocrystals and the successive coating with a ZnS shell to afford core/shell quantum dots. These nanocrystals can represent alternatives to cadmium based quantum dots but their preparation and properties are challenging and relatively unexplored. The effect of various synthetic parameters on the reaction outcome was investigated, and the resulting nanocrystals were characterized by TEM, EDX, XPS, and spectroscopic measurements. The optical data indicate that these core/shell quantum dots belong to type I, i.e. both the electron and the hole are confined within the ZnTe core. Both the ZnTe core and ZnTe/ZnS core/shell quantum dot samples absorb in the visible region and are not luminescent. The ZnS shell preserves the optical properties of the core and improves the chemical and photochemical stability of the nanoparticles in air equilibrated solution, whereas they appear to be quite fragile in the solid state. XPS results have evidenced the distinct nature of core and core/shell QDs, confirming the formation of QDs with shells of different thicknesses and their evolution due to oxidation upon air exposure. Anodic photocurrent generation was observed when an ITO electrode functionalized with ZnTe/ZnS nanocrystals was irradiated in the visible region in a photoelectrochemical cell, indicating that the quantum dots perform spectral sensitization of the electron injection into the ITO electrode. Conversely, cathodic photocurrent generation was not observed; hence, the QD modified electrode performs electrical rectification under a photon energy input.
author_names: Christophe Lincheneau

Matteo Amelia

Marek F Oszajca

Alice Boccia

Fabio D'Orazi

Mattia Madrigale

Robertino Zanoni

Raffaello Mazzaro

Luca Ortolani

Vittorio Morandi

Serena Silvi

Konrad Szacilowski

Alberto Credi
title: Synthesis and properties of ZnTe and ZnTe/ZnS core/shell semiconductor nanocrystals
year: 2014

abstract: We conducted a comparative synthesis of water soluble CdTe/CdS colloidal nanocrystalline semiconductors of the core/shell type. We prepared the CdS shell using two different methods: a one pot approach and successive ionic layer adsorption and reaction (SILAR) in both cases, we used 3 mercaptopropionic acid (MPA) as the surface ligand. In the one pot approach, thiourea was added over the freshly formed CdTe dispersion, and served as the sulfur source. We achieved thicker CdS layers by altering the Cd:S stoichiometric ratio (1:1, 1:2, 1:4, and 1:8) The Cd:S ratios 1:1 and 1:2 furnished the best optical properties; these ratios also made the formation of surface defects less likely. For CdTe/CdS obtained using SILAR, we coated the surface of three differently sized CdTe cores (2.17, 3.10, and 3.45 nm) with one to five CdS layers using successive injections of the Cd2+ and S2 ions. The results showed that the core size influenced the optical properties of the materials. The deposition of three to five layers over the surface of smaller CdTe colloidal nanocrystals generated strain effects on the core/shell structure.
author_names: Brener R C Vale

Fernanda O Silva

Melissa da Silva Carvalho

Ellen Raphael

Jefferson Luiz Ferrari

Marco Antonio Schiavon
title: Water Soluble CdTe/CdS Core/Shell Semiconductor Nanocrystals: How Their Optical Properties Depend on the Synthesis Methods
year: 2016

abstract: Semiconductor nanocrystals of different formulations have been extensively studied for use in thin film photovoltaics. Materials used in such devices need to satisfy the stringent requirement of having large absorption cross sections. Hence, type II semiconductor nanocrystals that are generally considered to be poor light absorbers have largely been ignored. In this article, we show that type II semiconductor nanocrystals can be tailored to match the light absorption abilities of other types of nanostructures as well as bulk semiconductors. We synthesize type II ZnTe/CdS core/shell nanocrystals. This material is found to exhibit a tunable band gap as well as absorption cross sections that are comparable to CdTe. This result has significant implications for thin film photovoltaics, where the use of type II nanocrystals instead of pure semiconductors can improve charge separation while also providing a much needed handle to regulate device composition.
author_names: Rekha Mahadevu

Aniruddha R Yelameli

Bharati Panigrahy

Anshu Pandey
title: Controlling light absorption in charge separating core/shell semiconductor nanocrystals.
venue: ACS nano
year: 2013

abstract: Summary In this study, we report on CdS/ZnS nanocrystals as a luminescence probe for bioimaging applications. CdS nanocrystals capped with a ZnS shell had enhanced luminescence intensity, stronger stability and exhibited a longer lifetime compared to uncapped CdS. The CdS/ZnS nanocrystals were stabilized in Pluronic F127 block copolymer micelles, offering an optically and colloidally stable contrast agents for in vitro and in vivo imaging. Photostability test exhibited that the ZnS protective shell not only enhances the brightness of the QDs but also improves their stability in a biological environment. An in vivo imaging study showed that F127 CdS/ZnS micelles had strong luminescence. These results suggest that these nanoparticles have significant advantages for bioimaging applications and may offer a new direction for the early detection of cancer in humans.
author_names: Li-wei Liu

Siyi Hu

Ying Pan

Jia-qi Zhang

Yue-shu Feng

Xi-he Zhang
title: Optimizing the synthesis of CdS/ZnS core/shell semiconductor nanocrystals for bioimaging applications
venue: Beilstein journal of nanotechnology
year: 2014