Ido Hadar

Effect of Nanoparticle Dimensionality on Fluorescence Resonance Energy Transfer in Nanoparticle–Dye Conjugated Systems

Fluorescence resonance energy transfer (FRET) involving a semiconductor nanoparticle (NP) acting as a donor, attached to multiple acceptors, is becoming a common tool for sensing, biolabeling, and energy transfer applications. Such nanosystems, with dimensions that are in the range of FRET interactions, exhibit unique characteristics that are related to the shape and dimensionality of the particles and to the spatial distribution of the acceptors. Understanding the effect of these parameters is of high importance for describing the FRET process in such systems and for utilizing them for different applications. In order to demonstrate these dimensionality effects, the FRET between CdSe/CdS core/shell NPs with different geometries and dimensionalities and Atto 590 dye molecules acting as multiple acceptors covalently linked to the NP surface is examined. Steady-state emission and temporal decay measurements were performed on the NPs, ranging from spherical to rod-like shaped systems, as a function of acceptor concentration. Changes in the NP geometry, and consequently in the distributions of acceptors, lead to distinctively different FRET behaviors. The results are analyzed using a modified restricted geometries model, which captures the dimensionality of the acceptor distribution and allows extracting the concentration of dye molecules on the surface of the NP for both spherical and elongated NPs. The results obtained from the model are in good agreement with the experimental results. The approach may be useful for following the spatial dynamics of self-assembly and for a wide variety of sensing applications.

Polarization Properties of Semiconductor Nanorod Heterostructures: From Single Particles to the Ensemble.

Semiconductor heterostructured seeded nanorods exhibit intense polarized emission, and the degree of polarization is determined by their morphology and dimensions. Combined optical and atomic force microscopy were utilized to directly correlate the emission polarization and the orientation of single seeded nanorods. For both the CdSe/CdS sphere-in-rod (S@R) and rod-in-rod (R@R), the emission was found to be polarized along the nanorods main axis. Statistical analysis for hundreds of single nanorods shows higher degree of polarization, p, for R@R (p = 0.83), in comparison to S@R (p = 0.75). These results are in good agreement with the values inferred by ensemble photoselection anisotropy measurements in solution, establishing its validity for nanorod samples. On this basis, photoselection photoluminescence excitation anisotropy measurements were carried out providing unique information concerning the symmetry of higher excitonic transitions and allowing for a better distinction between the dielectric and the quantum-mechanical contributions to polarization in nanorods.

Thermal Doping by Vacancy Formation in Copper Sulfide Nanocrystal Arrays

A new approach for doping of Cu2S nanocrystal arrays using thermal treatment at moderate temperatures (T < 400 K) is presented. This thermal doping process yields conductance enhancement by 6 orders of magnitude. Local probe measurements prove this doping is an intraparticle effect and, moreover, tunneling spectroscopy data signify p-type doping. The doping mechanism is attributed to Cu vacancy formation, resulting in free holes. Thermal-doping temperature dependence exhibits an Arrhenius-like behavior, providing the vacancy formation energy of 1.6 eV. The moderate temperature conditions for thermal doping unique to these nanocrystals allow patterned doping of nanocrystal films through local heating by a focused laser beam, toward fabrication of nanocrystal-based electronic devices.

Size-Dependent Ligand Layer Dynamics in Semiconductor Nanocrystals Probed by Anisotropy Measurements†

Colloidal semiconductor nanocrystals (NC) have reached a high level of synthetic control allowing the tuning of their properties, and their use in various applications. However, the surface of NCs and in particular their size-dependent capping organic ligand behavior, which play an important role in the NC synthesis, dispersibility, and optoelectronic properties, is still not well understood. We study the size-dependent properties of the ligand shell on the surface of NCs, by embedding surface bound dyes as a probe within the ligand shell. The reorientation times for these dyes show a linear dependence on the NC surface curvature indicating size-dependent change in viscosity, which is related to a change in the density of the ligand layer because of the geometry of the surface, a unique feature of NCs. Understanding the properties of the ligand shell will allow rational design of the surface to achieve the desired properties, providing an additional important knob for tuning their functionality.

Mesophase Formation Stabilizes High-Purity Magic-Sized Clusters

Magic-sized clusters (MSCs) are renowned for their identical size and closed-shell stability that inhibit conventional nanoparticle (NP) growth processes. Though MSCs have been of increasing interest, understanding the reaction pathways toward their nucleation and stabilization is an outstanding issue. In this work, we demonstrate that high concentration synthesis (1000 mM) promotes a well-defined reaction pathway to form high-purity MSCs (>99.9%). The MSCs are resistant to typical growth and dissolution processes. On the basis of insights from in situ X-ray scattering analysis, we attribute this stability to the accompanying production of a large (>100 nm grain size), hexagonal organic-inorganic mesophase that arrests growth of the MSCs and prevents NP growth. At intermediate concentrations (500 mM), the MSC mesophase forms, but is unstable, resulting in NP growth at the expense of the assemblies. These results provide an alternate explanation for the high stability of MSCs. Whereas the conventional mantra has been that the stability of MSCs derives from the precise arrangement of the inorganic structures (i.e., closed-shell atomic packing), we demonstrate that anisotropic clusters can also be stabilized by self-forming fibrous mesophase assemblies. At lower concentration (<200 mM or >16 acid-to-metal), MSCs are further destabilized and NPs formation dominates that of MSCs. Overall, the high concentration approach intensifies and showcases inherent concentration-dependent surfactant phase behavior that is not accessible in conventional (i.e., dilute) conditions. This work provides not only a robust method to synthesize, stabilize, and study identical MSC products but also uncovers an underappreciated stabilizing interaction between surfactants and clusters.