Fourier-Transform Atomic Force Microscope-Based Photothermal Infrared Spectroscopy with Broadband Source.

Area of Science:

• Nanoscale chemical imaging
• Vibrational spectroscopy
• Surface science

Background:

• Atomic force microscope infrared (AFM-IR) spectroscopy enables chemical imaging beyond the diffraction limit.
• Existing AFM-IR methods rely on continuous-wave or pulsed infrared (IR) lasers.
• Nanoscale vibrational spectroscopy is crucial for material characterization.

Purpose of the Study:

• To develop a Fourier transform AFM-IR technique for enhanced nanoscale chemical imaging.
• To investigate multiphoton absorption processes at the nanoscale.
• To demonstrate time-domain detection of AFM-IR signals in the mid-IR regime.

Main Methods:

• Utilized peak force infrared microscopy combined with broadband femtosecond IR pulses.
• Employed a Michelson interferometer to generate time-delayed IR pulses for photothermal signal generation.
• Transduced photothermal signals via AFM and performed Fourier transform to obtain IR absorption spectra.

Main Results:

• Successfully demonstrated Fourier transform AFM-IR microscopy on a polymer blend and hexagonal boron nitride.
• Observed vertical asymmetry in the interferogram, indicating multiphoton absorption under tip-enhancement.
• Achieved time-domain detection of AFM-IR signals in the mid-IR range.

Conclusions:

• The developed Fourier transform AFM-IR technique enables high-resolution nanoscale chemical imaging.
• The observation of multiphoton absorption opens new avenues for nanoscale spectroscopy.
• This method paves the way for multiphoton vibrational spectroscopy below the diffraction limit.