Resonance enhanced AFM-IR

How it works

Resonance Enhanced AFM-IR schematic (left) and cantilever deflection measurements for conventional AFM-IR (top right) and Resonance Enhanced (bottom right).


Resonance Enhanced AFM-IR schematic (left) and cantilever deflection measurements for conventional AFM-IR (top right) and Resonance Enhanced (bottom right).

The Resonance Enhanced AFM-IR technique is a new method to collect nanoscale IR spectroscopy. In traditional AFM-IR, the IR source pulses at a fixed slower rate. With the Resonance Enhanced technique an alternate IR source is used which has a much faster variable pulse rate. This source is a semiconductor laser that emits in the mid IR called a Quantum Cascade Laser (QCL). Since the QCL laser has a variable pulse rate, the pulse rate can be tuned to a contact resonance of the AFM cantilever using an automatic software algorithm. By matching the frequences, cantilever oscillation is driven by the expansion of the sample, caused by IR absorption, at each oscillation. Larger continuous oscillations result, and can be rapidly measured using a lock-in amplifier.

Enabling monolayer sensitivity

When operating at a contact resonance of the AFM cantilever, the Resonance Enhanced mode provides higher sensitivity allowing faster spectra acquisition and measurements of thinner samples. If a sample is prepared on an enhancing substrate, such as Au, measurements of samples as thin as a monolayer have been demonstrated. An example of a monolayer measurement is shown, in which the absorption bands of a monolayer of Polyethylene glycol (PEG) could be clearly resolved and the sub-monolayer distribution of the PEG on the Au substrate could be imaged.

Resonance Enhanced AFM-IR spectrum (top), AFM topography image (bottom left) and IR image collected at 1340 cm-1 showing the sub monolayer coverage of a PEG layer on Au.

Resonance Enhanced AFM-IR spectrum (top), AFM topography image (bottom left) and IR image collected at 1340 cm-1 showing the sub monolayer coverage of a PEG layer on Au.