Self-Alignment of Colloidal Nanorods
Nanoparticles have intriguing physical properties, which are quite different from those of the bulk material. For example, bulk crystalline CdSe is a semiconductor with an optical band gap at 690 nm, and continuous optical absorption at shorter wavelengths. In CdSe nanocrystallites, on the other hand, the states of electrons and holes are quantised, resulting in a semiconductor band gap which scales with particle size. Smaller particles have wider band gaps, for which fluorescence occurs at shorter wavelengths. Semiconductor nanoparticles have been used in various applications, ranging from light emitting diodes (LEDs) and efficiency enhanced solar cells to quantum dots for biological tagging. Many of these applications require a precise structuring and alignment of the nanoparticles.
We grow CdSe@CdS core-shell nanorods with spherical CdSe cores embedded into rod-shaped CdS shells. Since the CdS shells have larger band gaps than the CdSe cores, the excitation in the core is shielded from the surrounding medium, which leads to a drastic increase of fluorescence yield.
Even though the fluorescence wavelength is determined by the spherical core, the emission shows a strong polarisation. Presumably this is caused by the specific alignment of the electronic bands at the interface between core and shell. While the hole is confined in the CdSe core, the electron is delocalised over the entire nanorod. Apart from a bright and polarized light emission these nanorods show a very narrow size-distribution.
This is a major prerequisite for the formation of aligned structures even in the absence of external fields. These particles show also a high stability against photobleaching. Owing to these properties, CdSe@CdS nanorods are ideal candidates for probing the alignment and spatial transport processes on the nanometer scale. As one example, we use CdSe@CdS nanorods as a model system to study transport phenomena of molecules within the actin network of a cell.