We compare the spectroscopy of two different morphologies of CdTe/CdSe type-II nanocrystals. Core/ tetrapod and spherical core/shell particles are grown from identical CdTe cores, and both morphologies exhibit type-II spectroscopic behavior. The two morphologies show very different oscillator strengths for the lowest (luminescent) transition; the core/tetrapod particles exhibit larger oscillator strengths for the same amount of spectral shift. A model is presented that explains this difference and accurately predicts emission wavelengths and relative oscillator strengths for the spherical particles. This model uses an elastic continuum treatment to consider strain induced by lattice mismatch at the core/shell interface. The CdTe−CdSe lattice mismatch results in a calculated core pressure of about 2.9 GPa for the particles with the thickest shells. This pressure causes a change in the CdTe conduction band energy of about 0.24 eV and a change in the CdTe lattice parameter of 2.3%. The change in the lattice parameter is also seen in XRD spectra and HRTEM lattice fringe images. Because of the different morphology, core compression is essentially absent in the core/tetrapod particles. The model also considers radial interdiffusion of the selenium and tellurium. Particle annealing results in an alloyed region at the core−shell interface, and the radial composition profile can be calculated from a diffusion treatment. Partial alloying causes the luminescence to shift further to the red, which may be quantitatively understood in terms of calculated radial composition profiles and the known optical band-bowing parameters of CdTe−CdSe alloys.

Precise spatial organization and electronic coupling between quantum dots are pivotal for many potential applications. Typical spherical quantum dots in assemblies are separated by organic ligands and hence weakly coupled. GaSe nanoparticles are disk-like particles that are four atoms thick with tunable lateral dimensions. Previous spectroscopic investigations indicate the formation of nanoscale aggregates in which the quantum dots are strongly coupled. In this report, we show that the anisotropic properties of these particles may be exploited to assemble surface-stabilized superstructures with well-defined distances between the quantum dots. By changing the ligands adsorbed on the nanoparticle edges, three distinct aggregate morphologies can be produced. The surface chemistry of GaSe orients the nanoparticles on a gold surface and induces stacking in the surface normal direction. The discrete heights of such stacked aggregates suggest that the layers are held together by van der Waals interactions with a regular spacing. Such structures, with their well-defined electronic coupling, have potential implications in fundamental studies of photoinduced charge transfer and transport, as well as device fabrications.

We demonstrate the formation and spatial modulation of strongly coupled gallium selenide quantum dot ͑QD͒ nanoassemblies suspended in a nematic liquid-crystal ͑NLC͒ matrix at room temperature. Using static and dynamic optical techniques we show that the coupled QDs aggregate with a well-defined directionality commensurate with the NLC director axis. This results in highly anisotropic spectral properties of the QD assembly. The spatial orientation of the aggregates is selectively controlled in situ by the application of in-plane electric fields. The strong interdot coupling further increases the excitonic recombination rate which is both direction and electric field dependent. This electrical modulation, a noninvasive process, could potentially be an important functionality for the design and creation of building blocks for novel optoelectronic devices.

GaSe nanoparticles have been synthesized in the absence of tightly binding edge ligands. These particles are ligated primarily with TOP and TOPO ligands that are easily displaced by far less bulky alkyl aldehydes. Ligand replacement results in nanoparticle aggregates in which the lowest energy optical transitions are strongly coupled, resulting in a large red shift of the absorption spectrum (about 2600 cm−1) and a reversal of the singlet and triplet states. The reversal of the spin states results in changes in the polarization spectroscopy and a dramatic decrease in the radiative lifetime. Specifically, the exciton singlet states are linear oscillators, and time-resolved fluorescence polarization spectroscopy gives an initial anisotropy very close to the linear oscillator limit of 0.4. The radiative lifetime of this fluorescence is about 12 ns, compared to about 82 ns for the monomers. Upon aggregation, the fluorescence quantum yield increases from 4.7 to 61%.