Scanning Tunnelling Microscopy and Infrared Spectroscopy Gives Rise to a New Technique


A new technique combining two well known and most used technologies has been developed for measuring changes occurring to molecules when they get attached to substrates. Scanning Tunnelling Microscopy (STM) and Infrared Spectroscopy are widely used in the laboratories. Scanning Tunnelling Microscopes are ideal for imaging surfaces at molecular and atomic levels, but they are not capable of detecting different chemical species. Whereas, infrared spectroscopy can be easily used to identify the chemical nature of the molecules but it won’t be able to pinpoint the location of these molecules due to limitations posed by the longer wavelength of infrared light. Physicists from the University of California, Berkeley have devised a technique to get the best of both worlds.

Until now researchers were not able to combine both the technologies efficiently. The STM tips were prone to damage due to the heat produced by infrared source. But, the UCB researchers have used a novel approach to integrate both the technologies by using a custom made tuneable infrared laser and placing the STM tip about a millimetre behind it. In the experiment, the laser is used to irradiate a gold surface partially coated with a unimolecular layer of either [121]tetramantane or [123]tetramantane, and an increase in the tunnelling current between the STM tip and sample surface is found. Increase in the tunnelling current is attributed to dissipation of heat absorbed by the surface molecules to the gold substrate, resulting in its expansion as the laser frequency equals one of the absorption frequencies of adsorbed material. This reduces the distance of separation between the tip and surface to increase the current.

Researchers were able to measure the precise frequency at which the tunnelling current increased, and matched it with the spectral fingerprints of [121]tetramantane and [123]tetramantane to identify the molecules adsorbed onto gold substrate. The absorption spectra obtained by this method for molecules on surface can also be compared with that of the same molecules in bulk sample to gather information about changes occurring due to interaction with substrates. Currently by using this technique only an averaged signal can be detected from all irradiated molecules and the team is working on overcoming it in future. The research has been published in APS-Physics: Physical Review Letters.

Image: STM image, originally created by IBM Corp

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