Nano-IR spectroscopy

Nano-IR spectroscopy using AFM

The ability to identify material under the tip of an atomic force microscope (AFM) is one of the Holy Grails of scanning probe microscopy. While AFM can measure mechanical, electrical, magnetic and thermal properties of materials, it has lacked the robust ability to chemically characterize unknown materials. Infrared spectroscopy is a benchmark technique used in a broad range of sciences and industry to characterize and identify materials via vibrational resonances of chemical bonds. In collaboration with Anasys Instruments (Santa Barbara CA), we have integrated AFM with IR spectroscopy to allow measurement of high quality IR spectra at arbitrary points in an AFM image, thus providing nanoscale chemical characterization. For illumination, we use a compact pulsed IR source, based on a cascaded optical parametric oscillator [1] and is continuously tunable from 2.5 to 10 µm (4000 to 1000 cm-1). This range covers a major portion of important vibrational bands, including CH, NH and CO bands, as well as carbonyl and amide I and II bands. The sample is mounted (Fig. 1) on a ZnSe prism and the IR beam illuminates the sample via total internal reflection. The IR source produces pulses at few kHz repetition rate and a data acquisition system records each ring-down event of the cantilever. Finally, the IR absorption spectrum is recorded through measuring AFM cantilever deflection [2].

Fig.1. The AFM-IR technique combines an atomic force microscope with an IR spectrometer, where an AFM cantilever serves as local detector of absorbed IR radiation.

Fig. 2. High resolution line spectral map across boundary between EAA and Nylon layers in a laminated multilayer film. Across the boundary the transition between the CH and NH absorption peaks is resolved far below the diffraction limit

[1] K.L. Vodopyanov, P.G. Schunemann, Broadly tunable noncritically phase-matched ZGP optical parametric oscillator with a 2-�J pump threshold, Opt. Lett. 28, 441-443 (2003)

[2] C. Prater, K. Kjoller, D. Cook, R. Shetty, G. Meyers, C. Reinhardt, J. Felts, W. King, K. Vodopyanov, A. Dazzi, Nanoscale Infrared Spectroscopy of Materials by Atomic Force Microscopy, Microscopy and Analysis 24, 5-8 (SPM) (2010)