Semiconductor heterostructures that have large absorption cross sections, high stability, and quantum yields as well as size-tunable electronic structures are good candidates for light-harvesting and energy conversion applications. Nanoscale CdSe/CdS heterostructures have been reported to exhibit either Type I behavior, in which a photogenerated electron–hole pair remains in one of the materials, or Type II behavior in which the electron and hole separate between the materials. This distinction in carrier migration behavior determines for which applications a structure may be used, light emission (Type I) versus photovoltaics (Type II) for example. A detailed understanding of the origin of the electronic structure of such heterojunctions is not only crucial for engineering particles for the desired application, but can also lead to the ability to fine-tune the material for optimal performance. N.J. Borys and M.J. Walter from the University of Utah, J. Huang and D.V. Talapin from the University of Chicago, and J.M. Lupton from the University of Utah and the Universität Regensburg, Germany, report on the morphological effects of CdSe/CdS nanocrystals on interfacial energy transfer properties as published in the December 3, 2010 issue of Science (DOI: 10.1126/science.1198070; p. 1371).
The researchers used single-particle light-harvesting action spectroscopy on a range of CdSe/CdS heterostructures to probe the electronic structure of the heterojunctions. Specifically they meas-ured photoluminescence excitation (PLE) of absorbing CdS through emission from CdSe. The single-particle approach enables the detection of properties that might be obscured in ensemble measurements. Typically nanoscale CdS exhibits a peak in its PLE spectrum due to the quantum-confined exiton state. Intriguingly, results from single-particle spectroscopy showed this peak only occurred in some fraction of the spectra collected from tetrapods, a CdSe core connecting four arms of CdS.
By studying other particle morphologies, including rods and spheres, the researchers were able to isolate structural features that seemed to control the presence or absence of the peak. A crucial part of this study was developing a method to correlate the single particle PLE with scanning electronic microscope images of the same particle to unambiguously show the connection to particle shape. Notably, spheres and rods with a bulbous coating around the CdSe particles did not show a peak. The researchers said that non-uniform diameter in the arms of the tetrapods or overgrowth of the CdS shell around the central core broadens quantum confinement effects and is responsible for the distinct spectra observed in the tetrapod population.
By spectrally resolving the emission energies observed in tetrapods, a surprising dependence on the excitation energy was observed, primarily in tetrapods exhibiting an overall PLE signature indicating uniform morphology. The researchers said that this phenomenon is due to misalignment of the CdSe and CdS conduction bands, which should be more pronounced in a system with more narrowly defined energy levels. Because both the CdSe core exciton energy and the CdSe/CdS interfacial exciton energy are observed in the emission spectra, the research team asserts that an interfacial barrier preventing complete transfer of the electron to the lowest energy conduction band exists.
The researchers conclude that uniformity of morphology is directly related to the uniformity of quantum confinement in the particles, and affects electronic delocalization across the heterojunction. Band misalignment is more prevalent in morphologically uniform particles, and there appears to be a barrier to electron transfer between CdS and CdSe in these cases.
The researchers suggest that nanostructures with uniform morphologies are desirable for light-emitting devices while those with some structural variation are more suited for light-harvesting applications because the barrier to electron transfer across the junction is less prevalent.
