Photoinduced force microscopy resolves nanometer-scale topology with chemical recognition based on material absorption.

A theoretical and experimental analysis of the dominant forces measured in photoinduced force microscopy is presented. It is shown that when operated in the noncontact and soft-contact modes, the microscope is sensitive to the optically induced gradient force (F g ) and the scattering force (F sc ). The reconstructed force-distance curve reveals a tip-dependent scattering force in the 30-60 pN range. Whereas the scattering force is virtually insensitive to the nanoscopic tip-sample distance, the gradient force shows a z −4 dependence and is manifest only for tip-sample distances of a few nm. Measurements on glass, gold nanowires, and molecular clusters of silicon naphtalocyanine confirm that the gradient force is strongly dependent on the polarizability of the sample, enabling spectroscopic imaging through force detection. The nearly constant F sc and the spatially dependent F g give rise to a complex force-distance curve, which varies from point to point in the specimen and dictates the image contrast observed for a given set point of the cantilevered tip.

The enormous advances made in nanotechnology have also intensified the need for tools that can characterize newly synthesized nanoaterials with high sensitivity and with high spatial resolution. Many existing tools with nanoscopic resolution or better, including scanning electron microscopy (SEM), atomic force microscopy (AFM), and scanning tunneling microscopy (STM) methods, can generate highly detailed maps of nanoscopic structures. However, while these approaches provide great views of the morphological properties of nanomaterials, it has proven more challenging to derive chemical information from the corresponding images. To address this issue, attempts have been made to dress existing nanoscopy methods with spectroscopic sensitivity. A powerful approach in this direction is the combination of scan probe techniques with optical illumination, which aims to marry the nanoscopic resolution provided by a sharp tip with the chemical selectivity provided by optical spectroscopy. Examples of this approach include existing techniques such as scattering-type scanning near-field optical microscopy and tip-enhanced Raman spectroscopy. A new and emerging technique in this direction is photoinduced force microscopy (PiFM), which enables spectroscopic probing of materials with a spatial resolution well under 10 nm. In PiFM, the sample is optically excited and the response of the material is probed directly in the near-field by reading out the time-integrated force between the tip and the sample. Because the magnitude of the force is dependent on the photoinduced polarization in the sample, PiFM exhibits spectroscopic sensitivity. The photoinduced forces measured in PiFM are spatially confined on the nanometer scale, which translates into a very high spatial resolution even under ambient conditions. The PiFM approach is compatible with a wide range optical excitation frequencies, from the visible to the mid-infrared, enabling nanoscale imaging contrast based on either electronic or vibrational transitions in the sample. These properties make PiFM an attractive method for the visualization and spectroscopic characterization of a vast variety of nano materials, from semiconducting nanoparticles to polymer thin films to sensitive measurements of single molecules. In this Account, we review the principles of the PiFM technique and discuss the basic components of the photoinduced force microscope. We highlight the imaging properties of the PiFM instrument and demonstrate the inherent spectroscopic sensitivity of the technique. Furthermore, we show that the PiFM approach can be used to probe both the linear and nonlinear optical properties of nano materials. In addition, we provide several examples of PiFM imaging applications.

We investigate the tip-enhanced thermal expansion force for nanoscale chemical imaging and spectroscopy in the tip-sample junction. It is found, both theoretically and experimentally, that the tip-enhanced absorption of the near-field at the tip followed by sample expansion shows characteristic behaviors with respect to the sample thickness and the incident laser pulse width. The van der Waals interaction plays a major role in exerting a force on the tip from the thermally expanded sample. The force behavior of the photoinduced force microscope (PiFM) is compared with that of the existing photothermal-induced resonance technique (PTIR) to unravel the ambiguous thermal expansion force mechanism. The present study opens up new opportunities for enhancing the performance of optical nanoscopy and spectroscopy such as chemical imaging of nanobiomaterials and the local field mapping of photonic devices, including surface polaritons on van der Waals materials with the assistance of the thermal expansion of a functionalized tip.

We perform time-resolved pump-probe microscopy measurements by recording the local force between a sharp tip and the photo-excited sample as a readout mechanism for the material's nonlinear polarization. We show that the photo-induced force is sensitive to the same excited state dynamics as measured in an optical pump-probe experiment. Ultrafast pump-probe force microscopy constitutes a non-optical detection technique with nanoscale resolution that pushes pump-probe sensitivities close to the realm of single molecule studies.

Advances in nanotechnology have intensified the need for tools that can characterize newly synthesized nanomaterials. A variety of techniques has recently been shown which combines atomic force microscopy (AFM) with optical illumination including tip-enhanced Raman spectroscopy (TERS), scattering-type scanning near-field optical microscopy (sSNOM), and photothermal induced resonance microscopy (PTIR). To varying degrees, these existing techniques enable optical spectroscopy with the nanoscale spatial resolution inherent to AFM, thereby providing nanochemical interrogation of a specimen. Here we discuss photoinduced force microscopy (PiFM), a recently developed technique for nanoscale optical spectroscopy that exploits image forces acting between an AFM tip and sample to detect wavelength-dependent polarization within the sample to generate absorption spectra. This approach enables ∼10 nm spatial resolution with spectra that show correlation with macroscopic optical absorption spectra. Unlike other techniques, PiFM achieves this high resolution with virtually no constraints on sample or substrate properties. The applicability of PiFM to a variety of archetypal systems is reported here, highlighting the potential of PiFM as a useful tool for a wide variety of industrial and academic investigations, including semiconducting nanoparticles, nanocellulose, block copolymers, and low dimensional systems, as well as chemical and morphological mixing at interfaces.

We present a theoretical and experimental analysis of the cantilever motions detected in photoinduced force microscopy (PiFM) using the sideband coupling detection scheme. In sideband coupling, the cantilever dynamics are probed at a combination frequency of a fundamental mechanical eigenmode and the modulation frequency of the laser beam. Using this detection mode, we develop a method for reconstructing the modulated photo-induced force gradient from experimental parameters in a quantitative manner. We show evidence, both theoretically and experimentally, that the sideband coupling detection mode provides PiFM images with superior contrast compared to images obtained when detecting the cantilever motions directly at the laser modulation frequency.

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