The joint measurement of chemical composition and mechanical properties at sub 20 nm spatial resolution is highly desirable for characterization of inhomogeneous soft materials. While the mechanical properties can be measured with the nano-indentation technique of the atomic force microscopy (AFM), the measurement of the chemical composition has been a challenge. The AFM-based scattering-type near-field infrared or Raman microscopy is used to obtain the spectroscopic information with sub-diffraction limit spatial resolutions. However, there has been very few examples of joint spectroscopic and mechanical measurement at the nanoscale. The difficulty is the near-field based techniques require near-field enhancement that is easily perturbed or reduced by deformations or contaminations that are likely to occur during the nano-indentation. Moreover, the scattering-type scanning near-field optical microscopy suffers from low signal strength and high non-resonant background for soft matters, such as polymers. The tip-enhanced Raman spectroscopy suffered from low reproducibility of Raman-active probe. These factors severely limit the joint or simultaneous measurement of chemical composition and mechanical properties with AFM at the nanoscale.<br/>To address this need, we have developed an action-based infrared scanning probe microscopy that is capable of simultaneous measurement of the infrared spectroscopic response and mechanical properties. With the use of the frequency-tunable infrared laser combined with the AFM, the method maps the infrared absorption responses of polymers at sub 20 nm spatial resolutions, together with the mechanical properties at similar or better spatial resolutions. Our proof-of-principle results show that the method is more sensitive than the infrared scattering-type scanning near-field optical microscopy for soft, polymeric materials and provide simple access to background free infrared spectra much below the diffraction limit. Besides polymers, we have also used the method to characterize urban PM2.5 aerosols to decipher their compositions and possible origins. The method provides correlations between the mechanical properties and chemical compositions, which provides insight into the organization of inhomogeneous materials at the nanometer scales. The new approach will routinely enable non-invasive, high spatial resolution characterizations of inhomogeneous soft materials in research areas across multiple disciplines.

Joint nanoscale chemical-sensitive infrared imaging and mechanical property mapping of polymers at sub 20 nm spatial resolution

253rd National Meeting (2017)

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