Resonance-enhanced atomic force microscopy (AFM)–infrared (IR) is a new technique that couples an atomic force microscope with a pulsed tunable IR laser source to provide high spatial resolution chemical analysis of samples as thin as a monolayer. The AFM probe tip acts as a small local detector of the thermal expansion of the sample caused by the absorption of the monochromatic IR radiation. Examples are presented of the use of this technique to obtain highly spatially resolved IR spectra (down to 25 nm) of monolayer levels of material deposited onto gold substrates, including self-assembled monolayers of a hydroxyl-terminated hexa(ethylene glycol) undecanethiol, 4-nitrothiophenol, a monolayer island sample of poly(ethylene glycol) methyl ether thiol, and a 5-nm-thick film of purple membrane from Halobacterium salinarum.
Infrared (IR) microspectroscopy provides a powerful capability for chemically characterizing materials at spatial resolutions down to 5–10 μm. Commercial Fourier transform infrared (FT-IR) spectrometers equipped with microscopes or other microsampling accessories have been an important fixture in most analytical laboratories since the 1980s. In industrial and forensic laboratories, for example, FT-IR microspectroscopy has proven to be one of the most important industrial problem-solving techniques for identifying small amounts of unknown material, including contaminants that occasionally arise during the development of new products or during the actual production processes. In today's world, where nanomaterials are becoming more prevalent, there is an ever-increasing need to chemically characterize smaller and smaller particles and domains. The diffraction-limited spatial resolution of conventional FT-IR microscopes is no longer sufficient to solve many of these important nanoscale problems.
The recent coupling of atomic force microscopy (AFM) with pulsed tunable infrared laser sources has enabled the collection of IR spectra at spatial resolutions below 100 nm × 100 nm (1,2). The sharp AFM tip acts as a local detector of IR absorbance at the surface of a sample it is in contact with. When the wavenumber of the laser source is in resonance with a molecular vibrational frequency, the IR radiation can be absorbed and the sample expands when the molecules return to their ground vibrational state after exchanging energy with the sample matrix. This causes the sample to thermally expand over an area corresponding to the focused IR laser spot. The AFM cantilever will deflect because of the local thermal expansion of the material in proximity to the apex of the AFM probe, providing significantly higher spatial resolution that is not limited by the diffraction limit of the IR wavelength. In the initial configuration of this technique, the optical parametric oscillator (OPO) tunable laser source had a repetition rate of 1 kHz and a pulse length of ~10 ns, which would cause a rapid expansion of the sample inducing an impulse in the cantilever. This would cause the cantilever oscillation to ring down at its natural resonance frequencies after each laser pulse. In this article, we describe how replacing the OPO tunable laser source with a variable repetition rate quantum cascade laser (QCL) produces a signal enhancement of the AFM-IR signal of two orders of magnitude. This enhancement is accomplished by tuning the QCL repetition rate to match the contact resonant frequency mode of the AFM cantilever (3,4). At the contact resonance, the oscillation amplitude of the cantilever is significantly increased relative to off-resonance frequencies. An additional enhancement of the AFM-IR signal results when a gold-coated AFM tip is used, producing a "lightning rod" effect that enhances or localizes the electric field at the tip apex. The combination of matching the repetition rate of the laser to the contact resonance of the AFM cantilever and using a gold-coated probe allows for the collection of IR spectra of samples on arbitrary substrates down to thicknesses of ~10 nm. If the thin film sample is deposited onto a gold substrate, a further increase in the local enhancement of the electric field allows measurements down to less than 1 nm. This enables the AFM-IR technique to detect monolayer coverages of material on metal surfaces at lateral spatial resolutions down to 25 nm × 25 nm.