Jan 8, 2009
Phonons confined in nanorods
Colloidal nanocrystals are structures made from particles of a few hundred atoms up to almost µm-sized structures, which are synthesized in solution. They can be grown from many different materials and can be cheaply produced in fairly large amounts and with a high homogeneity in size and shape. Besides potential technological applications, they are a versatile model system to investigate size dependent effects.
In a recent study, which was published in Nanotechnology, the authors have exploited the size-tunability of nanorods made from the direct band-gap compound semiconductor CdSe to study the influence of changing size and shape on the optical phonons in such nanostructures.
The researchers performed low-temperature Raman scattering experiments on several nanorod samples. The most prominent Raman active phonons are longitudinal optical phonons and surface optical phonon modes. Both can be well separated and their behaviour with changing dimensions is quite different.
The longitudinal optical phonons experience a confinement if the diameter of the nanorod is reduced. This results in a changed frequency of the confined phonons and is independent of the nanorod's length and consistent with theory.
The frequency of the surface vibrations depends on the shape of the nanorod. It neither solely depends on the length nor the diameter but the ratio of both, the aspect ratio. This is the reason why the fundamental Raman band, which is the sum of both contributions, has a different shape and position for nanorods of different sizes.
As photonic devices are promising applications for the nanorods, the reasearchers aim to investigate the impact of these results on the luminescence properties of such structures, which are influenced by the behavior of the optical phonons.
About the author
Holger Lange is a researcher in the Thomsen group at the Institut für Festkörperhysik (solid state physics). His interests are the properties of colloidal II-VI nano- and heterostructures. Mikhail Artemyev performs the nanorod synthesis in Minsk and Berlin. Ulrike Woggon holds a chair for optics and photonics and Christian Thomsen holds a chair for solid state physics. The research was performed at the Technische Universität Berlin and supported by the DFG.