Bilayer graphene (BLG) shows great potential as a new material for opto-electronic devices because its bandgap can be controlled by varying the stacking orders, as well as by applying an external electric field. An imaging technique that can visualize and characterize various stacking domains in BLG may greatly help in fully utilizing such properties of BLG. Here we demonstrate that infrared (IR) scattering-type scanning near-field optical microscopy (sSNOM) can visualize Bernal and non-Bernal stacking domains of BLG, based on the stacking-specific inter- and intra-band optical conductivities. The method enables nanometric mapping of stacking domains in BLG on dielectric substrates, augmenting current limitations of Raman spectroscopy and electron microscopy techniques for the structural characterization of BLG.

The demand to visualize the spatial distribution of chemical species based on vibrational spectra is rapidly increasing. Driven by such need, various Raman and infrared spectro-microscopies with a nanometric spatial resolution have been developed over the last two decades. Despite rapid progress, a large gap still exists between the general needs and what these techniques can achieve. This perspective highlights the key challenges and recent breakthroughs of the two vibrational nano-imaging techniques, the scattering-type scanning near-field optical microscopy and tip-enhanced Raman scattering.

We report, for the first time, that the oxidation of bilayer graphene (BLG) can be reversibly and stacking-specifically controlled. The infrared (IR) absorption, IR nanoscopy, and Raman spectroscopy measurements on BLG consistently show reversible changes in the spectra and images upon exposure to O 2 and H 2 at elevated temperatures. We also obtain spectroscopic and theoretical evidence that stacking orders of graphene layers have a profound influence on the oxide structures: AB-BLG reacting with singlet and triplet oxygen results in endoperoxides (−C−O−O− C−), whereas AA′-BLG reacting with oxygen generates both the epoxides (singlet, −C−O−C−) and endoperoxides (triplet). We believe that our result provides deeper understanding on the layer-dependent catalytic activities of graphene, which is crucial for the design of high-performance graphene-based catalysts needed for various electrochemical, biological, and environmental applications.

Infrared (IR) vibrational spectroscopy is one of the oldest and the most widely used analytical tool for the characterization of chemical species in samples. Spatially resolved IR spectroscopy, the IR spectro‐microscopy, may offer information on the spatial arrangement of chemical species on sample surfaces as well as the chemical identity of the species. However, conventional far‐field IR microscopy offers spatial resolution of 5 μm, which is not nearly enough for fully characterizing the sample structures in many cases. In this article, we review the recently developed spectro‐microscopy methods that can overcome the diffraction limit of light and offer chemical maps of samples at nanometric scales. We mainly focus on IR near‐field microscopy, IR photothermal microscopy, IR photo‐induced force microscopy, and finally the IR‐Visible photothermal lensing microscopy. The key principles of each technique, practical merits, and limitations are described.