Breathing space

Thin semiconducting nanorods have 'breathing' vibrational modes that can be excited via Raman spectroscopy in order to measure their size

Credit: © 2008 ACS

Nanostructures made from the semiconductor cadmium selenide (CdSe) exhibit strong luminescence that could prove useful in electronics or biology. However it can be difficult to characterize these structures because they are near the limit of detection for electron microscopes. Now Holger Lange at Technische Universität Berlin and co-workers¹ have shown that extremely thin CdSe 'nanorods' can be measured using Raman spectroscopy.

Raman spectroscopy is frequently used to measure the size of carbon nanotubes. In the technique, laser light is used to excite a vibration called the radial breathing mode, in which the carbon atoms move in and out as if the nanotube was breathing.

Lange and co-workers observed a similar radial breathing mode in solid CdSe nanorods less than six nanometres in diameter. They could then accurately calculate the sizes of their nanorods, because the vibration frequency depends strongly on the nanorod diameter.

The researchers prepared other CdSe nanorods coated in zinc sulphide, which improves the luminescence of the nanorods and makes them more compatible with biological organisms. These coated nanorods also show a radial breathing mode, but the vibration is at higher frequencies because the zinc sulphide shell compresses the CdSe core.