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New technique with scanning tunneling microscope

Multimetallic nanoframes with 3D surfaces image
a) STM image and (b) IRSTM spectrum (black line) of [121]tetramantane molecules on Au(111). The blue line shows an STM-IETS spectrum of [121]tetramantane on Au(111). (c) STM image and (d) IRSTM spectrum of [123]tetramantane on Au(111).

Scientific Achievement
The Characterization of Nanomachines Program developed a new technique that can measure infrared (IR) absorption spectra of molecular adsorbates with a scanning tunneling microscope.

Significance and Impact
This enables a new combination of IR spectroscopy and real-space imaging for nanoscale structures, allowing superior characterization of molecular adsorbates and other scientific and technologically important systems.

Summary
Simultaneously probing the vibrational behavior and local electronic or geometric structure of molecular adsorbates is important for understanding their physical and chemical properties. While scanning tunneling microscopy (STM) can yield subnanometer-resolved topographical information and vibrational properties, current techniques do not allow direct probing of optical processes and have a spectral resolution limited by thermal broadening.

Michael Crommie’s team developed a new technique whereby STM can be used to perform vibrational spectroscopy of molecular adsorbates at the submonolayer level in combination with a tunable infrared laser source. IR vibrational spectroscopy of molecules on surfaces provides valuable insights into molecule-surface and molecule-molecule interactions and self-assembly, as well as chemical and structural properties. Traditionally this information is obtained via ensemble infrared spectroscopic techniques that contain no real-space information. Combining optical vibrational spectroscopy with atom-scale microscopy, as demonstrated here, is extremely useful for better understanding this type of microscopic phenomena. By using the STM as a sensitive detector to measure optically-induced expansion of the sample surface, the new technique takes advantage of the high spectral resolution inherent to infrared measurements while avoiding many difficulties related to pure optical detection. Using this technique, infrared absorption spectra of several diamondoid molecules deposited onto a Au(111) surface at submonolayer levels were obtained. The significant differences seen between the IRSTM spectra of molecular isomers show the power of this new technique to differentiate the chemical structure of adsorbates by their IR spectroscopic fingerprints. This is an important step forward toward the goal of single-molecule IR spectroscopy.

“Infrared spectroscopy of molecular submonolayers on surfaces by infrared scanning tunneling microscopy: Tetramantane on Au(111),” I. Pechenezhskiy, X. Hong, G. Nguyen, J. Dahl, R. Carlson, F. Weng, and M. Crommie, Phys. Rev. Lett. 111, 126101 (2013). DOI: 10.1103/PhysRevLett.111.126101