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1.
Phys Rev Lett ; 102(17): 177402, 2009 May 01.
Artigo em Inglês | MEDLINE | ID: mdl-19518829

RESUMO

Multiple energy scales contribute to the radiative properties of colloidal quantum dots, including magnetic interactions, crystal field splitting, Pauli exclusion, and phonons. Identification of the exact physical mechanism which couples first to the dark ground state of colloidal quantum dots, inducing a significant reduction in the radiative lifetime at low temperatures, has thus been under significant debate. Here we present measurements of this phenomenon on a variety of materials as well as on colloidal heterostructures. These show unambiguously that the dominant mechanism is coupling of the ground state to a confined acoustic phonon, and that this mechanism is universal.

2.
Small ; 5(1): 126-34, 2009 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-19051182

RESUMO

This study evaluates the influence of particle size, PEGylation, and surface coating on the quantitative biodistribution of near-infrared-emitting quantum dots (QDs) in mice. Polymer- or peptide-coated 64Cu-labeled QDs 2 or 12 nm in diameter, with or without polyethylene glycol (PEG) of molecular weight 2000, are studied by serial micropositron emission tomography imaging and region-of-interest analysis, as well as transmission electron microscopy and inductively coupled plasma mass spectrometry. PEGylation and peptide coating slow QD uptake into the organs of the reticuloendothelial system (RES), liver and spleen, by a factor of 6-9 and 2-3, respectively. Small particles are in part renally excreted. Peptide-coated particles are cleared from liver faster than physical decay alone would suggest. Renal excretion of small QDs and slowing of RES clearance by PEGylation or peptide surface coating are encouraging steps toward the use of modified QDs for imaging living subjects.


Assuntos
Polietilenoglicóis/química , Pontos Quânticos , Animais , Fígado/metabolismo , Camundongos , Microscopia Eletrônica de Transmissão , Peso Molecular , Tamanho da Partícula , Peptídeos/química , Tomografia por Emissão de Pósitrons , Baço/metabolismo , Propriedades de Superfície
3.
Phys Rev Lett ; 100(5): 057404, 2008 Feb 08.
Artigo em Inglês | MEDLINE | ID: mdl-18352428

RESUMO

A radiationless transition process of long-range, resonance interconversion of electronic-to-vibrational energy transfer (EVET) is discovered between the band-gap excitation of nanocrystal quantum dots to matrix vibrational overtone modes using fluorescence lifetime measurements. A theoretical analysis based on long-range dipole-dipole nonadiabatic couplings, being distinct from the traditional adiabatic or "static-coupling" pictures, is given and is in qualitative agreement with experiments. EVET should be considered in matrix choices for near-infrared optoelectronic applications of nanocrystals.

4.
Nano Lett ; 8(2): 678-84, 2008 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-18179278

RESUMO

We demonstrate tuning of the electronic level positions with respect to the vacuum level in colloidal InAs nanocrystals using surface ligand exchange. Electrochemical as well as scanning tunneling spectroscopy measurements reveal that the tuning is largely dependent on the nanocrystal size and the surface linking group, while the polarity of the ligand molecules has a lesser effect. The implications of affecting the electronic system of nanocrystal through its capping are illustrated through prototype devices.


Assuntos
Cristalização/métodos , Nanoestruturas/química , Nanoestruturas/ultraestrutura , Nanotecnologia/instrumentação , Pontos Quânticos , Transferência de Energia , Desenho de Equipamento , Análise de Falha de Equipamento , Teste de Materiais , Nanotecnologia/métodos , Tamanho da Partícula , Propriedades de Superfície
5.
Nanotechnology ; 19(6): 065201, 2008 Feb 13.
Artigo em Inglês | MEDLINE | ID: mdl-21730694

RESUMO

We studied the electronic level structure of assemblies of InAs quantum dots and CdSe nanorods cross-linked by 1,4-phenylenediamine molecules using scanning tunneling spectroscopy. We found that the bandgap in these arrays is reduced with respect to the corresponding ligand-capped nanocrystal arrays. In addition, a pronounced sub-gap spectral structure commonly appeared which can be attributed to unpassivated nanocrystal surface states or associated with linker-molecule-related levels. The exchange of the ligands by the linker molecules also affected the structural array properties. Most significantly, clusters of close-packed standing CdSe nanorods were formed.

6.
Angew Chem Int Ed Engl ; 45(47): 8001-5, 2006 Dec 04.
Artigo em Inglês | MEDLINE | ID: mdl-17080480
7.
Nano Lett ; 6(10): 2201-5, 2006 Oct.
Artigo em Inglês | MEDLINE | ID: mdl-17034083

RESUMO

The electronic level structure of colloidal InAs quantum dots (QDs) in two-dimensional arrays, forming a QD-solid system, was probed using scanning tunneling spectroscopy. The band gap is found to reduce compared to that of the corresponding isolated QDs. Typically, the electron (conduction-band) ground state red shifts more than the hole (valence-band) ground state. This is assigned to the much smaller effective mass of the electrons, resulting in stronger electron delocalization and larger coupling between electron states of neighboring QDs compared to the holes. This is corroborated by comparing these results with those for InAs and CdSe nanorod assemblies, manifesting the effects of the electron effective mass and arrangement of nearest neighbors on the band gap reduction. In addition, in InAs QD arrays, the levels are broadened, and in some cases their discrete level structure was nearly washed out completely and the tunneling spectra exhibited a signature of two-dimensional density of states.


Assuntos
Arsenicais/química , Cristalização/métodos , Índio/química , Modelos Químicos , Pontos Quânticos , Semicondutores , Arsenicais/análise , Simulação por Computador , Condutividade Elétrica , Transporte de Elétrons , Índio/análise , Teste de Materiais
8.
J Phys Chem A ; 110(27): 8297-303, 2006 Jul 13.
Artigo em Inglês | MEDLINE | ID: mdl-16821813

RESUMO

We investigate the modification of photoluminescence (PL) from single semiconductor nanocrystal quantum dots (NCs) in the proximity of metal and semiconducting atomic force microscope (AFM) tips. The presence of the tip alters the radiative decay rate of an emitter via interference and opens efficient nonradiative decay channels via energy transfer to the tip material. These effects cause quenching (or enhancement) of the emitter's PL intensity as a function of its distance from the interacting tip. We take advantage of this highly distance-dependent effect to realize a contrast mechanism for high-resolution optical imaging. AFM tips are optimized as energy acceptors by chemical functionalization with InAs NCs to achieve optical resolution down to 30 nm. The presented experimental scheme offers high-resolution optical information while maintaining the benefits of traditional AFM imaging. We directly measure the PL intensity of single NCs as a function of the tip distance. Our results are in good agreement with calculations made by a classical theoretical model describing an oscillating dipole interacting with a planar mirror.

9.
J Am Chem Soc ; 128(1): 257-64, 2006 Jan 11.
Artigo em Inglês | MEDLINE | ID: mdl-16390155

RESUMO

A complex InAs/CdSe/ZnSe core/shell1/shell2 (CSS) structure is synthesized, where the intermediate CdSe buffer layer decreases strain between the InAs core and the ZnSe outer shell. This structure leads to significantly improved fluorescence quantum yield as compared to previously prepared core/shell structures and enables growth of much thicker shells. The shell growth is done using a layer-by-layer method in which the shell cation and anion precursors are added sequentially allowing for excellent control, and a good size distribution is maintained throughout the entire growth process. The CSS structure is characterized using transmission electron microscopy, as well as by X-ray diffraction and X-ray photoelectron spectroscopy which provide evidence for shell growth. The quantum yield for CSS with small InAs cores reaches over 70%-exceptional photoluminescence intensity for III-V semiconductor nanocrystals. In larger InAs cores there is a systematic decrease in the quantum yield, with a yield of approximately 40% for intermediate size cores down to a few percent in large cores. The CSS structures also exhibit very good photostability, vastly improved over those of organically coated cores, and transformation into water environment via ligand exchange is performed without significant decrease of the quantum yield. These new InAs/CdSe/ZnSe CSS nanocrystals are therefore promising near-IR chromophores for biological fluorescence tagging and optoelectronic devices.

10.
Faraday Discuss ; 125: 23-38; discussion 99-116, 2004.
Artigo em Inglês | MEDLINE | ID: mdl-14750662

RESUMO

A novel approach for synthesis of soluble semiconductor quantum rods using metal nanoparticles to direct and catalyze one-dimensional growth is developed. The method is useful in particular for III-V semiconductors with cubic lattice, where the utilization of surfactant-controlled rod-growth is not easily realized. The growth takes place via the solution-liquid-solid (SLS) mechanism where proper precursors are injected into a coordinating solvent. Centrifugation is used for separation of rod-fractions with different lengths. The reaction is demonstrated for InAs, InP and GaAs. Focusing on InAs rods as a model system, we examined the effects of the type of metal catalyst, and the tuning of reaction conditions with respect to temperature, concentration, catalyst content and reaction time. Within the three types of metal catalysts used--Au, Ag and In, Au was found to provide the best control for achieving rod-growth even though the melting point of bulk gold is significantly higher then the reaction temperature. The structural properties of the rods were characterized by transmission electron microscopy, X-ray diffraction and energy dispersive X-ray spectroscopy. Rods have a cubic lattice and grow mainly along the [111] direction. The relative gold content decreases in shorter rods suggesting Au depletion as a cause for limiting the growth. Room and low temperature absorption and photoluminescence measurements show that the band-gap shifts to the red upon increasing rod length revealing strong quantum confinement along the long axis in InAs rods, providing spectral coverage of the near-IR range relevant for telecommunication applications. Emission intensity also decreases with increased rod-length. These length dependent properties manifest the transition from 0D to 1D quantum confined systems.

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