Phys. Rev. Lett. 111, 126101 (2013)

Scanning tunnelling microscopy (STM) is a powerful technique for imaging single molecules on surfaces, and when combined with inelastic electron tunnelling spectroscopy it can provide simultaneous topographical and vibrational information. However, it does not permit optical processes to be directly probed and its spectral resolution is limited by thermal broadening. Now, researchers in California, USA, have devised a new technique called infrared scanning tunnelling microscopy that overcomes these limitations. It essentially involves using an STM tip in tunnelling mode to remotely (at a distance of ∼1 mm) probe the response of a submonolayer of adsorbed molecules to infrared excitation. The wavelength of a tunable infrared laser is scanned, and the STM tip is used to monitor the thermal expansion of the substrate induced by absorption of the laser light. When the laser wavelength is resonant with a vibration of the absorbed molecules, this expansion is enhanced, producing a peak in the spectrum. The technique can probe a wide range of molecular species with a subthermal-energy resolution that is superior to that of STM-based inelastic electron tunnelling spectroscopy. The researchers demonstrated the effectiveness of this technique in differentiating chemical structures by obtaining clearly distinct spectra from two closely related isomers.