Application Note 151: 2D Materials Characterization Using Nanoscale FTIR Spectroscopy and Near-field Imaging

2D materials are an important emerging field of research due to their unique properties for applications in photovoltaics, semiconductors, battery technology and many other areas. 2D materials have been characterized by multiple nanoscale and microscopy techniques to gain a better understanding of the nature of their properties. Nanoscale FTIR techniques extend this characterization with critical chemical and optical data at the nanoscale.

The Anasys nanoIR3-s system provides two complementary nanoscale FTIR techniques for spectroscopy and near-field imaging, scattering-scanning near-field optical microscopy (s-SNOM) and photothermal AFM-Infrared (AFM-IR), including resonance-enhanced AFM-IR and Tapping AFM-IR. AFM-IR absorption spectra are direct measurements of sample absorption, independent of other complex optical properties of the tip and sample. As such, the spectra correlate very well to conventional bulk transmission IR. Scattering SNOM is a technique that investigates the optical and chemical properties of the material surface, and that providing correlation to reflection-based spectroscopy techniques. Atomic force microscopy (AFM) based techniques provide nanoscale information on the electrical, thermal, and mechanical properties of these materials.

Traditionally, 2D materials have been characterized with s-SNOM imaging techniques for optical and chemical information, but with the development of applications such as functionalization and nanopatterning, the AFM-IR can better provide unique information and valuable insights about materials, accelerating innovative research and capabilities. Together, these complementary techniques provide new insights into the nanoscale chemical and complex optical properties of 2D materials with resolutions of 10 nm, orders of magnitude below the diffraction limit of conventional IR spectroscopy.

This application note describes using these complementary techniques for the characterization of a variety of 2D materials, including graphene, hexagonal boron nitride, nanoantennae and semiconductor materials.

Extending s-SNOM into Nanoscale FTIR Spectroscopy

Tapping AFM-IR is a recent advancement in AFM-IR that provides higher resolution chemical imaging and extends AFM-IR spectroscopy to a broader application range. Recent Bruker developments in s-SNOM technology have enabled the extension of this technique to nanoscale FTIR spectroscopy across the broadest available mid-IR range.

In a single source, the system provides 2 modes. The spectroscopy mode provides the largest spectral range for s-SNOM in a single laser source (670 cm-1 to >4000 cm-1), and the imaging mode provides the largest available imaging range (670 cm-1 to >2000 cm-1), eliminating the need to buy QCL lasers for imaging.

Complementary Nanoscale IR Techniques

The nanoIR3-s has the ability to acquire nanoscale images and IR spectra using two separate near-field spectroscopy techniques: photothermal AFM-IR and s-SNOM. These complementary techniques offer nanoscale chemical analysis, as well as optical, thermal, electrical, and mechanical mapping with spatial resolution down to a few nanometers for both soft and hard matter applications.

Nanoscale IR spectroscopy combines the precise chemical identification of infrared spectroscopy with the nanoscale capabilities of AFM to chemically identify sample components with a chemical spatial resolution down to 10 nm with monolayer sensitivity, breaking the diffraction limit by >100x. AFM-IR absorption spectra are direct measurements of sample absorption, independent of other complex optical properties of the tip and sample. As such, the spectra correlate very well to that of conventional bulk transmission IR.

Imaging of Plasmons and Phonons

Surface plasmon polaritons (SPPs) and surface phonon polaritons (SPhPs) in 2D materials, with their high spatial confinement, can open up new opportunities for enhanced light-matter interaction, super lenses, subwavelength metamaterial, and other novel photonic devices. In-situ characterization of these polaritonic excitations across different applications requires a versatile optical imaging and spectroscopy tool with nanometer spatial resolution. Through a non-invasive near-field light-matter interaction, s-SNOM provides a unique way to selectively excite and locally detect electronic and vibrational resonances in real space. This technique is demonstrated by imaging the SPhPs of hexagonal boron nitride (hBN) as shown in Figure 4. Amplitude and phase near-field optical images provide complementary information for thorough characterization of the polaritonic resonances. Greater than 90° phase shift of SPhPs are observed on hBN, indicating strong light-matter coupling. Similar to the visualization of SPhPs in hBN, the SPPs of graphene can also be investigated using the nanoIR3-s system. Figure 5 illustrates the standing wave of an SPP on a graphene wedge. Generally, the spatial resolution of s-SNOM is limited only by the end radius of the AFM probe, enabling the s-SNOM technique to measure cross sections of the SPP down to ~8 nm.

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