Pushing the Sample-Size Limit of Infrared Vibrational Nanospectroscopy: From Monolayer toward Single Molecule Sensitivity.
While scattering-scanning near-field optical microscopy (s-SNOM) has demonstrated its potential to extend infrared (IR) spectroscopy into the nanometer scale, it has not yet reached its full potential in terms of spectroscopic sensitivity. We combine broadband femtosecond mid-IR excitation with an optimized spectral irradiance of ∼2 W/cm(2)/ cm(-1) (power/area/bandwidth) and a combination of tip- and substrate enhancement to demonstrate single-monolayer sensitivity with exceptional signal-to-noise ratio. Using interferometric time domain detection, the near-field IR s-SNOM spectral phase directly reflects the molecular vibrational resonances and their intrinsic line shapes. We probe the stretching resonance of ∼1000 carbonyl groups at 1700 cm(-1) in a self-assembled monolayer of 16-mercaptohexadecanoic acid (MHDA) on an evaporated gold substrate with spectroscopic contrast and sensitivity of ≲100 vibrational oscillators. From these results we provide a roadmap for achieving true single-molecule IR vibrational spectroscopy in s-SNOM by implementing optical antenna resonant enhancement, increased spectral pump power, and improved detection schemes.
†Department of Physics, Department of Chemistry, and JILA, University of Colorado, Boulder, Colorado 80309, United States.
Xiaoji G
XG
Xu
‡Forschungsinstitut am Goetheanum, CH-4143 Dornach, Switzerland.
Mathias
M
Rang
§Pacific Northwest National Laboratory, Richland, Washington 99352, United States.
Ian M
IM
Craig
†Department of Physics, Department of Chemistry, and JILA, University of Colorado, Boulder, Colorado 80309, United States.
Markus B
MB
Raschke
femtosecond near-field infrared spectroscopy; near-field microscopy; self-assembled monolayer; vibrational nano-spectroscopy chemical imaging