How it worksThe 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.