Photo-induced force microscopy (PiFM) detects photo-induced molecular polarizability of samples down to the molecular level by mechanical detection of the force gradient of the interaction between the optically driven dipoles in the sample and a metal coated AFM tip. PiFM is unique among AFM IR techniques in that it not only excites the sample with near-field but also detects the response in near-field, making it significantly easier and more robust to operate than other nano-IR techniques. Compared to scattering SNOM (s-SNOM), it enjoys a major advantage due to the absence of far-field background signal even on samples with significant topography, eliminating the problem of convolution with topography. Compared to photothermal techniques, it enjoys excellent signal-to-noise ratio even with low excitation power and from extremely thin samples (as thin as ~ 1 nm thick) due to its patented Sideband-BimodalTM detection scheme. Operation in non-contact or light tapping Sideband-BimodalTM AFM mode prevents even the softest samples from damage and achieves higher spatial resolution than AFM topography due to the steeper functional dependence of dipole-dipole force gradient on the tip-sample distance. Download Apps Note to read further.
Sideband-BimodalTM Explained: When an external laser, modulated fm, is focused at the tip/sample area of a non-contact mode (NC) AFM (operating on the second mechanical resonance of cantilever f1), the interaction between the dipoles in the sample and the metallic will experience two types of modulation: one due to the laser turning on and off at fm and another due to the tip-sample gap changing at f1. These two periodic signals will produce sidebands at f1 + fm and f1 – fm. To take advantage of the quality factor Q of the cantilever to enhance the sensitivity, we choose fm such that f1 – fm equals f0, the first mechanical resonance of the cantilever.
A schematic diagram of PiFM is shown below: AFM topography and PiFM signal are collected simultaneously by demodulating the signal at f1 and f0 respectively.
When the wavelength of the external laser, l, matches an excitation energy of the sample, the optically induced force between the sample and tip dipoles will be the strongest and exhibit a strong attractive force that can be measured by the AFM detection at f0. The resonances can be due to excitons, plasmons, or phonons ranging from UV to infrared.
Vista-IR couples VistaScope with a widely tunable infrared laser source to excite the vibrational modes of molecules. Vista-IR produces stunning nanoscale images with chemical specificity. Check out the videos and images at our Media Gallery and application-specific PiFM @ Work blogs to learn more about PiFM and exciting hyPIR imaging capability.