Assaf Aharoni

Particle Size, Surface Coating, and PEGylation Influence the Biodistribution of Quantum Dots in Living Mice

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.

Tuning energetic levels in nanocrystal quantum dots through surface manipulations.

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.

Fabrication and optical properties of polymeric waveguides containing nanocrystalline quantum dots

We report on the fabrication of polymer waveguides containing infrared-emitting nanocrystal quantum dots. Both PbSe and InAs nanocrystal quantum dots are incorporated into a fluorinated polymer by a surface functionalization method. The optical properties of the nanocrystal quantum dots are shown to be unaffected by the entire fabrication process. This method may provide a versatile platform for integration of nanocrystal quantum dots into planar photonic circuits.

Shape control of III–V semiconductor nanocrystals: Synthesis and properties of InAs quantum rods

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.

Hybrid nanocrystals-organic-semiconductor light sensor

We present a light sensing device based on nearly spherical, defect free colloidal nanocrystals (NCs) of InAs acting as a light activated gate for a GaAs∕AlGaAs field effect semiconductor transistor. We use self-assembled organic monolayer as linkers that attach the InAs NCs to the surface of the semiconductor device, instead of the gate that exists in common transistors. When the NCs absorb light, at a frequency corresponding to their resonance, a change in the current through the transistor takes place while no current flows through the NCs themselves.

Optoelectronic properties of polymer-nanocrystal composites active at near-infrared wavelengths

We report a systematic study of the optoelectronic processes occurring in composites made of near-infrared IR emitting nanocrystals and conjugated polymers. We focus on PbSe and InAs/ZnSe blended with polyphenylenevinylene-type polymers. We find that the process responsible for quenching the visible luminescence of the polymer by the nanocrystal varies depending on the nanocrystal composite. Moreover, the high 66% energy-transfer efficiency from the polymer to the PbSe nanocrystal does result in significant emission at the near IR. Our measurements suggest that the host may be doping the PbSe nanocrystal, thus making the nonradiative Auger process favorable. For InAs we find the energy levels well aligned inside the polymer band gap, making it an efficient charge trap which acts as a luminescence center. Through two-dimensional numerical modeling of the charge transport in such composite films we highlight the importance of morphology nanocrystal distribution control.

Optical gain from InAs nanocrystal quantum dots in a polymer matrix

We report on the observation of optical gain from InAs nanocrystal quantum dots which emit at 1.55microns and are imbedded in a novel polymer platform. The measurements are based on a three-beam time resolved pump-probe technique, which enables extracting the intrinsic gain cross section, lifetime, and recovery time. These experiments are another step toward the realization of active optical devices based on InAs nanocrystals.

Self-assembling of InAs nanocrystals on GaAs: The effect of electronic coupling and embedded gold nanoparticles on the photoluminescence

InAs∕ZnSe core/shell nanoparticles (NP) were self-assembled on GaAs substrates using different organic molecules of varying length and properties as linkers. The molecules provide control over the coupling and tunneling properties between the substrate and the nanocrystals. By coadsorbing of gold NP on the GaAs substrate, enhancement of the photoluminescence from the InAs NP was achieved. The enhancement factor was found to depend on the properties of the organic linkers.

Electronic structure and self-assembly of cross-linked semiconductor nanocrystal arrays

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.

Optical spectroscopy of cadmium-chalcogenide clusters of the type [Cd10E4(E′Ph)12(PR3)4], (E=Te, Se; E′=Se, S)

Abstract The optical properties of a group of cadmium-chalcogenide cluster-molecules with the general formula [Cd 10 E 4 (E ′ Ph) 12 (PR 3 ) 4 ], (E=Te, Se; E ′ =Se, S) were studied to investigate the effect of substitution of the chalcogen atoms in positions E and E ′ . The cluster-molecules were prepared by an organometallic synthesis route and their structures were determined by single crystal X-ray diffraction. The clusters display visible emission only at low temperatures, and the peak position and lifetime of the photoluminescence (PL) depends significantly on the E ′ chalcogen atom type. The PL of clusters with outer selenophenyl ligands is centered at 500 nm while for clusters with thiophenyl ligands the emission is shifted to 420 nm, and the emission lifetime is significantly longer. This provides strong support for assigning the emission to forbidden transitions related with the cluster capping ligands. The onset of the absorption and the PL excitation spectra display a systematic blue-shift upon variation of the E and E ′ atoms from Te to Se to S. This behavior resembles that of the band gap shift in the respective bulk semiconductors demonstrating that such cluster-molecules manifest a molecular limit of the solid state.