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Tip-enhanced thermal expansion force for nanoscale chemical imaging and spectroscopy in photo-induced force microscopy Junghoon Jahng, Eric Olaf Potma, and Eun Seong Lee Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b02871 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 24, 2018
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Analytical Chemistry
Tip-enhanced thermal expansion force for nanoscale chemical imaging and spectroscopy in photo-induced force microscopy Junghoon Jahng†, Eric O. Potma∥, and Eun Seong Lee†* †
Center for Nanocharacterization, Korea Research Institute of Standards and Science, Daejeon 34113, South Korea
∥
Department of Chemistry, University of California, Irvine, CA 92697, USA
*
Corresponding authors. Email:
[email protected] (E. S. L.);
KEYWORDS: tip-enhanced thermal expansion, photo-induced thermal expansion force, vibrational nanoscopy
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 photo-induced 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 nano-biomaterials 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.
A nondestructive and label-free characterization platform for nanoscale chemical imaging and spectroscopy meets the growing need of compositional analysis in nano devices and materials researches. Because traditional optical microscopic methods have insufficient spatial resolution to meet this need, recent efforts have focused on technologies that combine the high-resolution capabilities of an atomic force microscopy (AFM)-based platform with the chemical selectivity of optical spectroscopy. The scattering type scanning near-field optical microscopy (sSNOM) is a prime example, which has made it possible to characterize physical and chemical properties of nanoscale materials with great success1. Nonetheless, because the s-SNOM signal is obtained from the perturbation of the near-field in the tip-sample junction by using a far-field optical detector, it is still challenging to isolate the actual near-field response from the background. It often requires complex methods for background suppression such as the pseudo heterodyne technique2 or higher order demodulation techniques3, which may cause signal degradation. An alternative near-field method probes the optomechanical force as the read-out mechanism, which uses exclusively the near-field to register the motion of a probe. These techniques include photo-induced force microscopy (PiFM)4, 5, photothermal-induced resonance
microscopy (PTIR)6-8 and peak force infrared microscopy (PFIR)9. The PiFM operates in the non-contact/tapping mode while monitoring the interaction force between photo-induced dipoles in the tip and the sample. On the other hand, the PTIR and the PFIR probe the thermal expansion of the material due to the light absorption while in mechanical contact with the tip at the contact resonance. Theoretically, the spectral response of induced-dipole forces follows a dispersive lineshape, implying image contrast that differs from signals based on an absorptive lineshape. Indeed, in some reported spectral PiFM measurements the dispersive lineshape is observed5. However, recent PiFM experiments in the mid-IR have reported dissipative line shapes10, similar to those observed in PTIR7 and PFIR measurements9. The latter observations suggest that optical absorption followed by a thermal process is somehow involved. Thus the question is raised how the thermal contributions affect the PiFM signal and whether they may overwhelm the instantaneous induced-dipole forces. In this work we reveal how the thermal process constitutes another force generation mechanism in the PiFM measurement. A rigorous theoretical description of the field enhancement and the thermodynamics, followed by an experimental demonstration in the illuminated tipsample geometry, will be presented.
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RESULTS AND DISCUSSION The light absorption in the sample gives rise to a temperature increase of the sample, which results in the strain deformation that eventually produces the thermal expansion11. The thermal expansion in the tip-sample junction under light illumination may be considered as two different cases: one is the tip-enhanced thermal expansion due to the absorption of distance dependent near-field from the tip. The other one is the direct thermal expansion irrelevant to the tip, which is based on the far-field transmission and absorption. The schematic diagrams of both processes are sketched in Figure 1a and 1b, respectively. The total thermal expansion is the sum of both cases. Generally, the temperature rise inside the sample with a heating source Q(t) is described by the heat equation12:
&
* +,-./+,-
) 0* +,-1 2./+,-1
which is the absorption coefficient
of the sample at the molecular vibration12, 13, λ, c and E are the wavelength, the speed of light and the electric field inside the sample respectively. The complete time dependent temperature change of Eq. (1) is given as7, 14: 3, " 5
Δ/7 8 229: ⁄9;
