Contact resonance force microscopy for viscoelastic property measurements: From fundamentals to state-of-the-art applications
Jason P. Killgore, Frank W. DelRio
Contact resonance force microscopy (CRFM) is an atomic force microscopy (AFM) method that evolved from a curiosity about the detection of ultrasonic vibrations with an AFM cantilever, and an unaddressed need to characterize the mechanical properties of stiffer materials (elastic modulus > 50 GPa). The method has matured to allow near-surface and sub-surface elastic property measurements of single crystals, thin films, nanomaterials, composites, and other advanced materials. More recently, CRFM has been extended to viscoelastic property measurements, where the CR frequency and CR quality factor are utilized to quantitatively assess properties such as storage modulus, loss modulus, and loss tangent. In this perspective, we trace the evolution of CRFM from initial discovery to elastic property measurements to viscoelastic property measurements. The techniques for extending single-point property measurements to two-dimensional property maps are then described in terms of their operational characteristics, demonstrated on calibration materials, and validated via comparisons to other viscoelastic measurement tools. The focus of the discussion then shifts to viscoelastic CRFM in non-ambient conditions to highlight the challenges and developments related to thermomechanical analyses and liquid operation. The current state-of-the-art and best practices in data acquisition and analyses for viscoelastic CRFM are elucidated via a step-by-step demonstration on a wood-polymer composite. Finally, we conclude with a discussion of potential polymer science application areas that are poised to benefit from the recent advances in the ambient and non-ambient CRFM methodologies. Altogether, we feel that the recent addition of CRFM to commercially-available AFMs together with guides that clearly define state-of-the-art and best practices will accelerate its acceptance and adoption in polymer science via viscoelastic property measurements at unprecedented length and time scales.