Tip-Enhanced Thermal Expansion Force for Nanoscale Chemical Imaging and Spectroscopy in Photoinduced Force Microscopy

Abstract

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.

Tip-Enhanced Infrared Imaging with Sub-10 nm Resolution and Hypersensitivity

“…The results are listed in Table 1 and the detailed calculation process can be found in the supplementary materials (Section 3). Since the noise level of PiFM is roughly ~ 0.1 pN, 12 MBA and MDHA are barely measurable on SiO2 substrates but they are clearly measurable on Au films. Hence, for small molecules, our set-up is better than conventional PiFM detection in principle.…”

“…A diagram of the forces involved in the PiFM measurement is shown in Figure 2a. In the mid-IR region, expansion-modulated van der Waals force has recently been proposed as the dominant term for molecules in PiFM operation 12 . The van der Waals force is proportional to the thermal expansion of sample under illumination, which can be written as, (1) where, ΔL is the tip-enhanced thermal expansion, σ is the linear thermal expansion coefficient and E is the position dependent field strength.…”

“…A more in detailed mechanism of detection can be found in some previous works. 12,13 The first demonstration of PiFM to measure thin films was pioneered by Derek et al 13 in 2016. They distinguished the patterns of 30 nm thick block copolymer (BCP) films and revealed the topography and composition relation within 10 nm spatial resolution.…”

Optical force microscopy: combining light with atomic force microscopy for nanomaterial identification

“…Aside from optical gradient forces and radiation pressures, light-induced thermal forces (or photo-thermal forces) can be used in nanoscale spectroscopy [26,47]. For example, near material absorption resonances, a temperature increment of the sample can give rise to thermal expansion.…”

Revealing Phonon Polaritons in Hexagonal Boron Nitride by Multipulse Peak Force Infrared Microscopy

“…[17,18] However, the signal generation mechanism of PiFM is under debate. [17,19] Recently-developed peak force infrared (PFIR) microscopy is a new type of photothermal AFM-IR microscopy that overcomes the limitation of the contact mode PTIR microscopy. [20] PFIR microscopy operates in the peak force tapping mode [21] that can avoid sample damage.…”