Peak force visible microscopy

The optical responses of molecules and materials provide a basis for chemical measurement and imaging. The optical diffraction limit in conventional light microscopy is exceeded by mechanically probing optical absorption through the photothermal effect with atomic force microscopy (AFM). However, the spatial resolution of AFM-based photothermal optical microscopy is still limited, and the sample surface is prone to damage from scratching due to tip contact, particularly for measurements on soft matter. In this article, we develop peak force visible (PF-vis) microscopy for the measurement of visible optical absorption of soft matter. The spatial resolution of PF-vis microscopy is demonstrated to be 3 nm on green fluorescent protein-labeled virus-like particles, and the imaging sensitivity may approach a single protein molecule. On organic photovoltaic polymers, the spatial distribution of the optical absorption probed by PF-vis microscopy is found to be dependent on the diffusion ranges of excitons in the donor domain. Through finite element modeling and data analysis, the exciton diffusion range of organic photovoltaics can be directly extracted from PF-vis images, saving the need for complex and delicate sample preparations. PF-vis microscopy will enable high-resolution nano-imaging based on light absorption of fluorophores and chromophores, as well as deciphering the correlation between the spatial distribution of photothermal signals and underlying photophysical parameters at the tens of nanometer scale.