Publications

Chirality is an important molecular property for structural analysis. Similarly, it has been shown that plasmonic chiral systems exhibit strong circular dichroism (CD) responses that can be used to determine the relative positions of their constituent plasmonic elements. Here we show that the sign of the circular dichroism spectrum in a plasmonic system can be controllably changed through small geometric perturbations that change the energetic ordering of the hybridized modes. This mechanism is distinct from geometrical changes that explicitly change the handedness of the system. In a simple system composed of two stacked L-shaped resonators we observe a reversal of the optical rotation spectral signature for small relative shifts, and we show through electromagnetic modeling and experiments on lithographically patterned samples that this is due to a rearrangement of the relative energies between modes. The plasmonic system allows for geometric perturbation along controlled directions and therefore offers more control than corresponding molecular examples. Interestingly, this strong sensitivity in the optical response encodes more spatial information into the optical spectrum, emphasizing the importance of chiral plasmonic assemblies for structural investigations on the nanoscale.

We describe a synthesis of nanometer-sized clusters of CdSe using organometallic reagents in inverse micellar solution and chemical modification of the surface of these cluster compounds. In particular we show how the clusters grow in the presence of added reagents and how the surface may be terminated and passivated by the addition of organoselenides. Passivation of the surface allows for the removal of the cluster molecules from the reaction medium and the isolation of organometallic molecules which are zinc blende CdSe clusters terminated by covalently attached organic ligands. Preliminary cluster characterization via resonance Raman, infrared, and NMR spectroscopy, X-ray diffraction, transmission electron microscopy, and size-exclusion chromatography is reported. © 1988, American Chemical Society. All rights reserved.

We have prepared simple photovoltaic devices based on composite materials formed by mixing cadmium selenide or cadmium sulfide nanocrystals with the conjugated polymer poly(2-methoxy, 5-(2′-ethyl)-hexyloxy-p-phenylenevinylene) (MEH-PPV). When the surface of the nanocrystals is treated so as to remove the surface ligand, we find that the polymer photoluminescence is quenched, consistent with rapid charge separation at the polymer/nanocrystal interface. Transmission electron microscopy (TEM) of these quantum dot/conjugated polymer composites shows clear evidence for phase segregation with length scales in the range 10–200 nm, providing a large area of interface for charge separation to occur. Thin-film photovoltaic devices using the composite materials show quantum efficiencies which are significantly improved over those for pure polymer devices, consistent with improved charge separation. At high concentrations of nanocrystals, where both the nanocrystal and polymer components provide continuous pathways to the electrodes, we find quantum efficiencies of up to 12%. The absorption, charge separation and transport properties of the composites can be controlled by changing the size, material and surface ligands of the nanocrystals.

There has been witness to remarkable progress in the ability to (a) grow well-controlled dots; (b) characterize dots on a near-atomic level; and (c) theoretically describe their electronic properties using not only continuum, but also atomistic approaches. This book focuses on the broad scientific and technological interest in semiconductor quantum dots. Papers on growth and self-assembly/self-organization, as well as characterization of semiconductor quantum dots based upon various materials systems, e.g., silicon-germanium, III-V materials, and II-VI materials, are included. Discussions of technological applications, ranging from biomedical technology, microelectronics and photonics, to more esoteric applications including quantum computing, are also featured. Topics include: Si and Ge dots; II-VI and other free-standing (colloidal) dots; near-field spectroscopy of quantum dots, wires and metals; organized dots and dot arrays; transport, coulomb blockade and metallic dots; optical spectroscopy and phonons; light-emitting quantum dots; and structural characterization and growth.

Reliably accessing nanocrystal luminophores with near-unity efficiencies aids in the ability to understand the upper performance limits in optoelectronic applications that require minimal nonradiative losses. Constructing structure-function relationships at the atomic level, while accounting for inevitable defects, allows for the development of robust strategies to achieve near-unity quantum yield luminophores. For CsPbBr₃ perovskite nanocrystals, bromine vacancies leave behind undercoordinated lead atoms that act as traps, limiting the achievable optical performance of the material. We show that selective etching represents a promising path for mitigating the consequences of optical defects in CsPbBr₃ nanocrystals. A mechanistic understanding of the etching reaction is essential for developing strategies to finely control the reaction. We report a study of the selective etching mechanism of CsPbBr₃ nanocrystal cubes by controlling the etchant chemical potential. We observe optical absorption and luminescence trajectories while varying the extent and rate of lead removal, removing in some cases up to 75% of the lead from the original nanocrystal ensemble. At modest etchant chemical potentials, the size and shape uniformity of the nanocrystal ensemble improves in addition to the quantum yield, proceeding through a layer-by-layer etching mechanism. Operating with excessively high etchant chemical potentials is detrimental to the overall optical performance as the etching transitions to nonselective, while too low of a chemical potential results in incomplete etching. Through this general approach, we show how to finely control selective etching to consistently access a steady state or chemical stability zone of near-unity quantum yield CsPbBr₃ nanocrystals postsynthetically, suggesting a practical framework to extend this treatment to other perovskite compositions and sizes.

Durch größenselektive Fällung konnten homodimere CdSe‐Nanokristalle (siehe rechts) aus einem Gemisch isoliert werden, das bei der Kupplung monodisperser CdSe‐Nanokristalle mit einem Bis(acylhydrazid) entstand. Der konstante Abstand zwischen den verknüpften CdSe‐Teilchen wurde transmissionselektronenmikroskopisch zu einem Viertel des Teilchendurchmessers bestimmt und ist damit in Einklang mit den Abmessungen der organischen Brücke. magnified image

The fluorescence decays from submonolayers of pyrene separated from Si(111) by Xe spacer layers are measured as a function of spacer thickness (17–200 Å), pyrene coverage, and emission wavelength. The results are explained in terms of two decay channels: energy transfer and trapping among the molecules in the two-dimensional pyrene overlayer, and excitation of electrons from the valence to the conduction band in the Si(111) by the dipole near field of the electronically excited pyrene molecule. The intralayer energy transfer is modeled using the Kohlrausch equation N(t)=N0 exp(−t/τ)α, in which α is related to the distribution of pyrene molecules in energy. Energy transfer from the molecule to the semiconductor is modeled using the classical image dipole theory. The classical model is used to calculate the energy transfer rates from a dipole to Si and GaAs as a function of dipole–semiconductor separation, and as a function of dipole emission wavelength.