High-Resolution Infrared Spectroscopy for Nanoscale Material Analysis

Background

Infrared (IR) spectroscopy is a cornerstone analytical technique in materials science, chemistry, and biology, valued for its ability to identify chemical compositions and molecular structures without requiring specialized labeling. As modern research increasingly focuses on complex nanomaterials, polymer thin films, and polaritonic devices, there is a critical need to go beyond simple chemical identification. Researchers must be able to map ultrafast molecular dynamics, vibrational mode couplings, and nanoscale energy transfer pathways at the precise spatial scale of localized material features, such as structural boundaries and scattering sites.

However, existing analytical methods struggle to simultaneously provide high spatial and high spectral dimensionality. Conventional ultrafast multidimensional techniques, such as two-dimensional infrared (2DIR) spectroscopy, can successfully resolve complex vibrational couplings and anharmonicities, but they rely on far-field optics and are restricted by the optical diffraction limit to micrometer-scale spatial resolution. Conversely, while conventional scanning probe techniques like photothermal atomic force microscopy-infrared (AFM-IR) bypass the diffraction limit to achieve nanometer-scale resolution, they are limited to linear, one-dimensional measurements. Consequently, these nanoscale approaches suffer from severe spectral congestion, which obscures underlying excited-state absorptions and prevents the characterization of complex energy transfer mechanisms at the nanoscale.

Technology

Developed at Lehigh University, the photothermal action-based two-dimensional nanoscale infrared (AFM-2DIR) spectroscopy and microscopy system integrates atomic force microscopy with ultrafast multidimensional infrared spectroscopy. The apparatus uses a laser amplifier and stacked Michelson interferometers to generate a sequence of collinear, time-delayed femtosecond mid-infrared pulses. These pulses are focused onto a metallic AFM tip operating in Peak Force Tapping mode, which acts as an optical antenna to confine excitation to a nanometer-scale volume. Synchronized to strike precisely when the tip reaches peak contact force, the absorbed pulses cause localized thermal expansion, triggering high-frequency contact resonance oscillations in the cantilever. High-speed electronics and a photodiode record these mechanical deflections across varied inter-pulse delays, using sequential Fourier transformations to generate a two-dimensional frequency-frequency spectrum.

This technology is differentiated by its ability to overcome the optical diffraction limit of conventional infrared techniques, achieving sub-10 nm spatial resolution under ambient conditions without requiring specialized sample labeling. Unlike traditional far-field optical detection or standard one-dimensional AFM-IR measurements—which often suffer from spectral congestion—this mechanical photothermal approach extracts multidimensional spectra that reveal complex cross-peaks. Consequently, researchers can directly characterize vibrational anharmonicity, mode couplings, excited-state absorption, and nanoscale energy transfer pathways in materials like polymer nanofilms and polaritonic 2D flakes. Furthermore, the system’s architecture can be adapted for visible femtosecond pulses, uniquely enabling two-dimensional electronic nanoscopy to evaluate excitonic dynamics with nanoscale precision.

Benefits

• Achieves sub-10 nm spatial resolution by overcoming the optical diffraction limit of conventional infrared techniques
• Operates under ambient conditions without requiring specialized sample labeling
• Resolves spectral congestion in conventional FTIR and AFM-IR measurements through multidimensional spectral mapping
• Characterizes nanoscale vibrational mode couplings, anharmonicity, excited-state absorption, and energy transfer pathways
• Enables high-sensitivity mechanical detection of localized photothermal expansion rather than relying on far-field optical transmission or reflection
• Adaptable to visible femtosecond pulse sequences to perform two-dimensional electronic nanoscopy for evaluating excitonic dynamics and electronic coupling

Applications

• Nanoscale polymer thin-film characterization
• 2D material polariton mapping
• Semiconductor nanoscale defect analysis
• Label-free biomolecular structure analysis
• Optoelectronic excitonic dynamics evaluation

Additional info

The AFM-2DIR platform couples atomic force microscopy with ultrafast mid-infrared spectroscopy to achieve sub-10 nm spatial resolution. An optical parametric system generates femtosecond pulse sequences focused at a metallic tip during Peak Force Tapping contact. Localized photothermal expansion drives cantilever contact resonances monitored by a photodiode. Sequential Fourier transformations across calibrated inter-pulse delays generate two-dimensional spectra mapping vibrational coupling, anharmonicity, and nanoscale energy transfer.