Search query: , Nanocrystals, and Quantum Dots

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abstract: Current research into semiconductor clusters is focused on the properties of quantum dots fragments of semiconductor consisting of hundreds to many thousands of atoms with the bulk bonding geometry and with surface states eliminated by enclosure in a material that has a larger band gap. Quantum dots exhibit strongly size dependent optical and electrical properties. The ability to join the dots into complex assemblies creates many opportunities for scientific discovery.
author_names: A Paul Alivisatos
title: Semiconductor Clusters, Nanocrystals, and Quantum Dots
venue: Science
year: 1996

abstract: Recent advances in strategies for synthesizing nanoparticles such as semiconductor quantum dots, magnets and noble metal clusters have enabled the precise control of composition, size, shape, crystal structure, and surface chemistry. The distinct properties of the resulting nanometre scale building blocks can be harnessed in assemblies with new collective properties, which can be further engineered by controlling interparticle spacing and by material processing. Our study is motivated by the emerging concept of metamaterials materials with properties arising from the controlled interaction of the different nanocrystals in an assembly. Previous multi component nanocrystal assemblies have usually resulted in amorphous or short range ordered materials because of non directional forces or insufficient mobility during assembly. Here we report the self assembly of PbSe semiconductor quantum dots and Fe2O3 magnetic nanocrystals into precisely ordered three dimensional superlattices. The use of specific size ratios directs the assembly of the magnetic and semiconducting nanoparticles into AB13 or AB2 superlattices with potentially tunable optical and magnetic properties. This synthesis concept could ultimately enable the fine tuning of material responses to magnetic, electrical, optical and mechanical stimuli.
author_names: Franz X Redl

K -S Cho

Christopher B Murray

Stephen O'Brien
title: Three dimensional binary superlattices of magnetic nanocrystals and semiconductor quantum dots
venue: Nature
year: 2003

abstract: Understanding the surface of semiconductor nanocrystals (NCs) prepared using colloidal methods is a long standing goal of paramount importance for all their potential optoelectronic applications, which remains unsolved largely because of the lack of site specific physical techniques. Here, we show that multidimensional 113Cd dynamic nuclear polarization (DNP) enhanced NMR spectroscopy allows the resolution of signals originating from different atomic and magnetic surroundings in the NC cores and at the surfaces. This enables the determination of the structural perfection, and differentiation between the surface and core atoms in all major forms of size and shape engineered CdSe NCs: irregularly faceted quantum dots (QDs) and atomically flat nanoplatelets, including both dominant polymorphs (zinc blende and wurtzite) and their epitaxial nanoheterostructures (CdSe/CdS core/shell quantum dots and CdSe/CdS core/crown nanoplatelets) as well as magic sized CdSe clusters. Assignments of the NMR signals to specific crystal facets of oleate terminated ZB structured CdSe NCs are proposed. Significantly, we discover far greater atomistic complexity of the surface structure and the species distribution in wurtzite as compared to zinc blende CdSe QDs, despite an apparently identical optical quality of both QD polymorphs.
author_names: Laura Piveteau

Ta-Chung Ong

Brennan J Walder

Dmitry N Dirin

Daniele Moscheni

Barbara Schneider

Janine Baer

Loredana Protesescu

Norberto Masciocchi

Antonietta Guagliardi

Lyndon Emsley

Christophe Coperet

Maksym V Kovalenko
title: Resolving the Core and the Surface of CdSe Quantum Dots and Nanoplatelets Using Dynamic Nuclear Polarization Enhanced PASS PIETA NMR Spectroscopy
venue: ACS central science
year: 2018

abstract: The insertion of intentional impurities, commonly referred to as doping, into colloidal semiconductor quantum dots (QDs) is a powerful paradigm for tailoring their electronic, optical, and magnetic behaviors beyond what is obtained with size control and heterostructuring motifs. Advancements in colloidal chemistry have led to nearly atomic precision of the doping level in both lightly and heavily doped QDs. The doping strategies currently available, however, operate at the ensemble level, resulting in a Poisson distribution of impurities across the QD population. To date, the synthesis of monodisperse ensembles of QDs individually doped with an identical number of impurity atoms is still an open challenge, and its achievement would enable the realization of advanced QD devices, such as optically/electrically controlled magnetic memories and intragap state transistors and solar cells, that rely on the precise tuning of the impurity states (i.e. number of unpaired spins, energy and width of impurity levels) within the QD host. The only approach reported to date relies on QD seeding with organometallic precursors that are intrinsically unstable and strongly affected by chemical or environmental degradation, which prevents the concept from reaching its full potential and makes the method unsuitable for aqueous synthesis routes. Here, we overcome these issues by demonstrating a doping strategy that bridges two traditionally orthogonal nanostructured material systems, namely, QDs and metal quantum clusters composed of a "magic number" of atoms held together by stable metal to metal bonds. Specifically, we use clusters composed of four copper atoms (Cu4) capped with d penicillamine to seed the growth of CdS QDs in water at room temperature. The elemental analysis, performed by electrospray ionization mass spectrometry, X ray fluorescence, and inductively coupled plasma mass spectrometry, side by side with optical spectroscopy and transmission electron microscopy measurements, indicates that each Cu:CdS QD in the ensemble incorporates four Cu atoms originating from one Cu4 cluster, which acts as a "quantized" source of dopant impurities.
author_names: Beatriz Santiago-Gonzalez

Angelo Monguzzi

Valerio Pinchetti

Alberto Casu

Mirko Prato

Roberto Lorenzi

Marcello Campione

Norberto Chiodini

Carlo Santambrogio

Francesco Meinardi

Liberato Manna

Sergio Brovelli
title: "Quantized" Doping of Individual Colloidal Nanocrystals Using Size Focused Metal Quantum Clusters.
venue: ACS nano
year: 2017

abstract: Magic sized clusters (MSCs) of semiconductor are typically defined as specific molecular scale arrangements of atoms that exhibit enhanced stability. They often grow in discrete jumps, creating a series of crystallites, without the appearance of intermediate sizes. However, despite their long history, the mechanism behind their special stability and growth remains poorly understood. It is particularly difficult to explain experiments that have shown discrete evolution of MSCs to larger sizes well beyond the "cluster" regime and into the size range of colloidal quantum dots. Here, we study the growth of MSCs, including these larger magic sized CdSe nanocrystals, to unravel the underlying growth mechanism. We first introduce a synthetic protocol that yields a series of nine magic sized nanocrystals of increasing size. By investigating these crystallites, we obtain important clues about the mechanism. We then develop a microscopic model that uses classical nucleation theory to determine kinetic barriers and simulate the growth. We show that magic sized nanocrystals are consistent with a series of zinc blende crystallites that grow layer by layer under surface reaction limited conditions. They have a tetrahedral shape, which is preserved when a monolayer is added to any of its four identical facets, leading to a series of discrete nanocrystals with special stability. Our analysis also identifies strong similarities with the growth of semiconductor nanoplatelets, which we then exploit to further increase the size range of our magic sized nanocrystals. Although we focus here on CdSe, these results reveal a fundamental growth mechanism that can provide a different approach to nearly monodisperse nanocrystals.
author_names: Aniket S Mule

Sergio Mazzotti

Aurelio A Rossinelli

Marianne Aellen

P Tim Prins

Johanna C van der Bok

Simon F Solari

Yannik M Glauser

Priyank V Kumar

Andreas Riedinger

David J Norris
title: Unraveling the Growth Mechanism of Magic Sized Semiconductor Nanocrystals.
venue: Journal of the American Chemical Society
year: 2021

abstract: Understanding the pathways of hot exciton relaxation in photoexcited semiconductor nanocrystals, also called quantum dots (QDs) is of paramount importance in multiple energy, electronics and biological applications. An important nonradiative relaxation channel originates from the nonadiabatic (NA) coupling of electronic degrees of freedom to nuclear vibrations, which in QDs depend on the confinement effects and complicated surface chemistry. To elucidate the role of surface ligands in relaxation processes of nanocrystals, we study the dynamics of the NA exciton relaxation in Cd(33)Se(33) semiconductor quantum dots passivated by either trimethylphosphine oxide or methylamine ligands using explicit time dependent modeling. The large extent of hybridization between electronic states of quantum dot and ligand molecules is found to strongly facilitate exciton relaxation. Our computational results for the ligand contributions to the exciton relaxation and electronic energy loss in small clusters are further extrapolated to larger quantum dots.
author_names: Svetlana V Kilina

Kirill A Velizhanin

Sergei Ivanov

Oleg V Prezhdo

Sergei Tretiak
title: Surface ligands increase photoexcitation relaxation rates in CdSe quantum dots.
venue: ACS nano
year: 2012

abstract: Surface effects significantly influence the functionality of semiconductor nanocrystals. High quality nanocrystals can be achieved with good control of surface passivation by various hydrophobic ligands. In this work, the chemistry between CdSe quantum dots and common surface capping ligands is investigated using density functional theory (DFT) We discuss the electronic structures and optical properties of small CdSe clusters controlled by their size of particle, self organization, capping ligands, and positive charges. The chosen model ligands reproduce good structural and energetic description of the interactions between the ligands and quantum dots. In order to capture the chemical nature and energetics of the interactions between the capping ligands and CdSe quantum dots, we found that PMe3 is needed to adequately model trioctylphosphine (TOP) NH3 is sufficient for amines, while OPH2Me could be used to model trioctylphosphine oxide. The relative binding interaction strength between ligands was found to decrease in order Cd O Cd N Cd P with average binding energy per ligand being 25 kcal/mol for OPH2Me, 20 kcal/mol for NH3 and 10 kcal/mol for PMe3. Charges on studied stoichiometric clusters were found to have a significant effect on their structures, binding energies, and optical properties.
author_names: Ping Yang

Sergei Tretiak

Sergei Ivanov
title: Influence of Surfactants and Charges on CdSe Quantum Dots
year: 2011

abstract: The International Conference on Semiconductor Quantum Dots (QD2002) was held at Komaba Campus of University of Tokyo, Japan, from Monday, 30 September, through Thursday, 3 October 2002. The purpose of the QD2002 was to bring together scientists from different fields of physics and chemistry to discuss topics of common interest and significance in such growing areas including semiconductor quantum dots, nanocrystals, and clusters. The conference was focused on the optical and electronic properties of three dimensionally confined nanostructures grown both by epitaxial methods and chemical preparation routes. The QD2002 was the second in a series which started in Munich, Germany, in 2000. The scope of the QD2002 covered various research fields including novel fabrication techniques of nanoheterostructures, electronic structures, optical properties, electronic properties/single electron tunneling processes, molecular dots, nanocrystals, device applications such as lasers and memories, coherent processes/quantum computations, and biomedical applications.
author_names: Yasuhiko Arakawa

Seigo Tarucha
title: Proceedings of the 2nd International Conference on Semiconductor Quantum Dots (QD2002)
year: 2003

abstract: physica status solidi (c) conferences and critical reviews publishes conference proceedings, ranging from large international meetings to specialized topical workshops as well as collections of topical reviews on various areas of current solid state physics research. The International Conference on Semiconductor Quantum Dots (QD2002) was held at Komaba Campus of University of Tokyo, Japan, from Monday 30 September through Thursday 3 October 2002. The purpose of the QD2002 was to bring together scientists from different fields of physics and chemistry to discuss topics of common interest and significance in such growing areas including semiconductur quantum dots, nanocrystals, and clusters. The conference was focused on the optical and electronic properties of three dimensionally confined nanostructures grown both by epitaxial methods and chemical preparation routes. The QD2002 was the second in a series which started in Munich, Germany, in 2000. The scope of QD2002 covered various research fields including novel fabrication techniques of nanoheterostructures, electronic structures, optical properties, electronic properties/single electron tunneling processes, molecular dots, nanocrystals, device applications such as lasers and memories, coherent processes/quantum computations, and biomedical applications.
author_names: Huang Chuan Tai Yan

Zun Cha Qing Wu
title: Proceedings of the 2nd international conference on semiconductor quantum dots, Tokyo, Japan 30 September 3 October 2002
year: 2003

abstract: Abstract Sol gel transition is a second order phase transition depending on the creation of the macroscopic solute network, which confines the solvent in its pores. The phase transition from the sol state to the gel state takes place, when small clusters of molecules join together and create one huge cluster, which fills most of the volume of the sample. The aim of this paper is to study the phase transition process using as detectors the semiconductor PbS nanocrystals, called quantum dots, embedded in the solution. As a material we are using the aqueous solution of gelatin, which creates the gel network due to hydrogen bonds between monomers belonging to different chains. The lifetimes of excitons generated in quantum dots by the light pulse appear to be very sensitive to the polymer surrounding. These lifetimes have been measured by use of the transient absorption spectroscopy. In order to show the dynamics of the gelation process the measurements were performed with appropriate time lags in train of the process duration. It appears that dynamics of the exciton lifetime significantly changes during the sol gel transition, what allows the indication of the gel point in the system and the appropriate universal critical exponents.
author_names: Piotr Piatkowski

B Ratajska-Gadomska

Wojciech Gadomski
title: Probing slow dynamics by ultrafast process: Sol gel transition detected by transient absorption spectroscopy of quantum dots
year: 2012