Nanoscale Chemical Characterization with Photo-induced Force Microscopy
Advances in nanotechnology have increased the need for tools to characterize a wide variety of nanomaterials. Photo-induced force microscopy (PiFM) is a powerful approach that addresses this need by combining the capabilities of scanning probe techniques with optical illumination. PiFM has been used to chemically resolve samples as small as 4 nm. Based on the traditional atomic force microscope, a cantilever tip is operated in tapping mode. Laser excitation resonant with a sample of interest, induces a dipole force in the sample that can be measured in its response on the tip.
In this work, we describe PiFM and its unique approach to nanoscale chemical characterization. We compare it with other contemporary techniques in this field, and demonstrate why PiFM stands out. We outline the theoretical description of the photo-induced force using the dipole approximation. Accounting for multiple scattering pathways, the approximation can be improved, and calculated forces agree with experimental results. We discuss the impact of the thermal expansion force to PiFM measurements and how to experimentally maximize or minimize its effect on the total force.
We present PiFM results in our recent work with (6,5) chirality single-walled carbon nanotubes (SWCNTs). We perform time-resolved pump-probe measurements on the SWCNTs with transient absorption spectroscopy and confirm the findings on individual clusters of SWCNTs on the nanoscale with ultrafast pump-probe PiFM. We then explore the utility of PiFM in chemically resolving SWCNTs under a polymer layer, minding thermal expansion effects, the properties of our polymer, and the distance dependence of the measured gradient force. We show that where topographically the SWCNTs are difficult to resolve, PiFM shows surprising resolution through the polymer layer.