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

Abstract

IR spectroscopy has been widely used for chemical identification and quantitative analysis of reactions occurring in a specific time and space domains by measuring an average signal of the entire system 1 . Achieving IR measurements with nanometer-scale spatial resolution is highly desirable to obtain a detailed understanding of the composition, structure and function of interfaces 2-5 . The challenges in IR nanoscopy yet exist owing to the small molecular cross section and pristine optical diffraction limit. Although atomic force microscopy (AFM) based techniques, such as scattering-type scanning near-field optical microscopy and photothermal-induced resonance microscopy (PTIR), can acquire IR spectroscopy in a few tens of nanometer scale resolution 6-9 , IR measurements with monolayer level sensitivity remains elusive and can only be realized under critical conditions 10,11 . Herein, we demonstrate sub-10 nm spatial resolution sampling a volume of ~360 molecules with a strong field enhancement at the sample-tip junction by implementing noble metal substrates (Au, Ag, Pt) in photo-induced force microscopy (PiFM). This technique shows versatility and robustness of PiFM, and is promising for application in interfacial studies with hypersensitivity and super spatial resolution.

Nanoscale Chemical Imaging by Photo‐Induced Force Microscopy: Technical Aspects and Application to the Geosciences

“…2016, Xiao and Schultz 2018, Li et al . 2020). All tunable PiFM lasers have uneven energy output profiles with respect to wavenumber, which need to be corrected for in order to produce accurate sample spectra.…”

“…Li et al . (2020) further improved the resolution capabilities of PiFM imaging to analyse the amide I and II peaks of bovine serum albumin (BSA) proteinaceous monolayer (Figure 7). This work was particularly challenging due to the small size (< 5 nm thickness), low number and heterogeneous distribution of BSA molecules.…”

“…The critical field enhancement of the PiFM signal of BSA at the sample − tip junction was achieved via gold substrates that increased the PiFM signal by more than 15000 times (Li et al . 2020). The AFM topographic image (Figure 7a) shows the BSA molecules as topographically higher features, which correlate spatially with brighter features in the amide I PiFM map (Figure 7b).…”

In situ Nanoscale Infrared Spectroscopy of Water Adsorption on Nanoislands of Surface‐Anchored Metal‐Organic Frameworks

“…[8,10,11] Our manuscript introduces in situ capability to the emerging field of Photoinduced Force Microscopy (PiFM), a highly sensitive and spatially resolved near-field infrared (IR) technique, to gain new insights into guest-host interactions at working conditions and in real-time. [12,13] We showcase that PiFM can be used to study structure-dependent adsorption on defective versus non-defective sites in a MOF by simultaneously uncovering topological and chemical information at the nanoscale with a spatial resolution down to 10 nm. [13] We selected water (D 2 O) as a probe molecule for this proof-of-concept because of its omnipresence in industrial gas streams, but highlight that our approach can be extended to many gases/functional materials.…”

“…[12,13] We showcase that PiFM can be used to study structure-dependent adsorption on defective versus non-defective sites in a MOF by simultaneously uncovering topological and chemical information at the nanoscale with a spatial resolution down to 10 nm. [13] We selected water (D 2 O) as a probe molecule for this proof-of-concept because of its omnipresence in industrial gas streams, but highlight that our approach can be extended to many gases/functional materials. [14,15] We have applied in situ PiFM to a range of surfaceanchored metal-organic frameworks (SURMOFs) as is outlined in Figure 1.…”