Infrared nanospectroscopy (nanoIR) has garnered significant attention from the academic community since 2013. Initially, the nanoIR technique faced challenges in aqueous environments due to the strong absorption of infrared light and the damping of the atomic force microscope (AFM) probe’s oscillation by the aqueous solution. Characterizing samples in liquid environments, particularly at electrochemical interfaces, has been a persistent challenge. 

  Lead by Prof. Zhong-Qun Tian, our team has developed a novel nanoIR system based on Peak-force technology. This system overcomes aqueous environment challenges through a strategically designed bottom excitation module. Additionally, by integrating a customized electrochemical cell, we have achieved in-situ nanoIR spectroscopy and imaging of electrochemical reactions at the solid/liquid interface, with a spatial resolution of approximately 4 nm. 

  The spectroelectrochemical cell comprises a main body, a BaF2 substrate coated with a 10-nm gold film serving as the working electrode, O-rings, and other mechanical components. A reflective objective (N.A. = 0.5) illuminates the cell directly from the bottom. The cell is mounted on a three-axis nanopositioning stage, facilitating both sample and probe scanning. The overall optomechanical structure employs a multi-axis cage system, ensuring high stability and compatibility. 

 We have also developed control software that integrates mid-IR laser control, AFM operation, electrochemical scanning, and data processing. The embedded algorithm automatically enhances the excitation efficiency of IR pulses based on trigger-demodulation frequency and infrared amplitude intensity imaging. The nanoIR signal is transmitted to the AFM controller, enabling simultaneous display of nanoIR imaging with correlated morphology. Furthermore, the electrochemical control module facilitates electrochemical nano-infrared spectroscopy and imaging.

Figure 1. (a) Design schematic diagram of the developed nanoIR system. (b) Photograph of spectroelectrochemical cell loaded in the system. (c) Structural design drawing of the spectroelectrochemical cell. (d) Physical photo of the third generation of instruments and devices, including multi-axis cage structure and spectroelectrochemical cells

Performance

  We use standard polymer samples, specifically PMMA films, to characterize the instrument's performance. 

  The force curves of AFM probes scanning the PMMA surface vary significantly with the environment. In air, the free vibration amplitude of the probes oscillates for an extended period (~ms), whereas in aqueous solutions, it attenuates rapidly. The 'jump to contact' point is prominent in air when the probe approaches the sample film but is less noticeable in solution. 

  The signal-to-noise ratio of nanoIR spectra is high in both air and solution conditions. However, the spectral baseline is elevated in solution. Imaging the C=O bond distribution at a fixed excitation wavelength of 1730 cm⁻¹ reveals that morphology and chemical information are correlated. The C=O bonds exhibit distinct heterogeneous distribution at the nanometer scale, demonstrating that our nanoIR microscopy operates stably and with high performance in aqueous environments.

Figure 2. NanoIR measurement of the polymer film surface of a standard sample in the aqueous environment (a) The force curves of the tip on the surface of the polymer film in aqueous solution and air respectively. The box shows the obvious difference between the two curves. (b) Comparison of nanoIR spectra of PMMA film in air and water solution. (c) Surface topography of PMMA film. (d) C=O bond imaging on the surface of PMMA film.

  To validate the sensitivity of our instrument, we prepared few-layer ultra-thin samples on an ultra-thin gold film substrate using the Langmuir-Blodgett (LB) method. The AFM topography imaging closely correlates with the nanoIR imaging at 1630 cm⁻¹. Comparing the topography profile and nanoIR intensity, the results indicate a spatial resolution of 3.16 nm perpendicular to the edge of the LB film, surpassing the previously reported 6 nm resolution. For further validation, DNA double helix chains were imaged at 1280 cm⁻¹ and 1700 cm⁻¹, revealing helix-like structures at the edge of the DNA strand. These experiments demonstrate the NanoIR system's exceptional sensitivity and spatial resolution on 1.2-nm thick samples.

Figure 3. Ultra-sensitive and ultra-high spatial resolution nano-infrared imaging measurement of ultra-thin sample film.(a) AFM imaging of ultrathin samples prepared by LB film technique. (b) the correlated nano-infrared imaging excited at 1630 cm-1. (c) the contour of white line area in morphology and nano-infrared imaging， and the correlation between nano-infrared intensity and appearance. (d) morphological imaging of DNA strands. (e) nano-infrared imaging excited at 1280 cm-1. (f) nano-infrared imaging excited at 1700 cm-1.

References

1. Hai-Long Wang, Xu-Cheng Li, Chuan-Cheng Guo, Yu-Fan Cheng, Wen-Han Zhang, Zi-Ang Nan, Li-Na Shen, Li-Qiang Xie*, Bing-Wei Mao, Zhong-Qun Tian, and Jun Yi*, Chemical Nanoimaging of Octylphosphonic Acid Molecular Additives on Hybrid Organic-Inorganic Perovskite Films, Journal of Materials Chemistry A(2024)