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A paper describing these findings is published in the journal National Science Review.
To verify this result, the team compared the spectral response of quartz using photothermal induced resonance (PTIR) and photoinduced force microscopy (PiFM), showing that photoinduced dipole force (PiDF) dominates the photoinduced thermal forces (PiTF) of quartz. As PiDF shows a more pronounced relationship with the tip-quartz distance (~z−4) compared to the PiTF ( ~z−3), Dr. Li proposed a general approach for nano-IR contrast imaging of ultrathin samples loaded on top of quartz.
The ultrathin sample, characterized by a positive real part of the permittivity (weak oscillator), is expected to manifest weak PiTF and PiDF near its infrared (IR) resonance. However, a significant PiDF change is anticipated near the tip-induced nearfield resonance of the quartz substrate.
These spectral distinctions contribute to the contrasts in nano-IR imaging. Notably, the PiDF response on quartz exhibits a more conspicuous signal variation with respect to sample thickness compared to the PiTF of the sample. For ultrathin samples, PiDF imaging on quartz presents an opposite contrast with enhanced sensitivity compared to the nano-IR contrast imaging with the PiTF of the sample.
The team used a polydimethylsiloxane (PDMS) wedge prepared on a quartz substrate to demonstrate the substrate-enhanced nano-IR contrast imaging. The results provide clear evidence that the PiDF can be employed for sensitive nano-IR imaging of ultrathin samples under nanocavity geometry with improved contrast and sensitivity.
The researchers further applied the nano-IR imaging method to visualize thin covalent organic framework layers and subsurface defects under block-copolymer films. They hypothesized that by selecting suitable IR materials that exhibit phonon polaritons/reststrahlen bands, users could achieve high-resolution nanoimaging of specific crystals and polymer molecules, as well as biomolecules with known vibrational mode frequencies.