US Researchers Successfully Combine Infrared Spectroscopy with STM

US Researchers Successfully Combine Infrared Spectroscopy with STM

26 September 2013

Researchers from the University of California, Berkeley (UCB) have devised a new technology that can help better understand molecules' behavior on surfaces by integrating infrared spectroscopy with scanning tunneling microscopy (STM). The physicists believe that the technology could break ground for the development of more efficient catalysts for industrial processes and of tools that can identify the locations of surface molecules on a nanometer scale.

In the past, a number of scientists have tried to combine infrared spectroscopy with STM, but their experiments have not been very successful since the infrared light warms up the atomically sharp tip which is scanned very close to the surface monitored, making it expand and crash into the surface. In order to overcome these limitations, students in Professor Michael Crommie's research group at UCB adopted another approach to infrared spectroscopy and STM combination. The researchers applied a tunable infrared laser created by UCB Professor Feng Wang to illuminate a gold surface that is covered in part with two single molecules of tetramantane. The STM's tip was then placed above the gold surface, at a distance of around a millimeter from the area irradiated by the laser, or far enough away to prevent the tip from heating up.

The experiment showed that when the laser's frequency matches one of the absorption frequencies of the adsorbed hydrocarbon, there is an increase in the tunneling current between the tip and the surface. According to the researchers, this was a result of the infrared light absorbed by the surface molecules, causing the energy to quickly scatter into the gold substrate as heat. The heat makes the gold expand to bring the surface closer to the tip and extend the tunneling current.

Thanks to the identification of the exact frequencies at which the tunneling current increases, the team could determine the spectral fingerprints of the tetramantane, which helped them say which of the two molecules was adsorbed on the gold substrate. Besides, the spectral resolution of the technology was much higher than that of previous techniques based on STM.