Unlocking the Nanoscale: The Power of AFM-IR in 2D Materials Research
2D materials, characterized by their atomically flat plane, exhibit exceptional properties such as strength and conductivity. These properties open new avenues for electronic device development and enhance silicon-based chips. These materials can be stacked to form heterostructures, allowing for functional customization. Their high surface area also promotes chemical reactivity, useful in catalysis, sensing, energy storage, and drug delivery.
Atomic Force Microscopy (AFM) plays a critical role in characterizing 2D materials by providing high-resolution mapping of topographical, mechanical, and electrical properties at the nanoscale. The integration of infrared spectroscopy with AFM, known as photothermal AFM-IR (AFM-IR), enables localized chemical identification with nanometer-scale spatial resolution.
This technique probes both material structure and chemistry, facilitating in-depth studies of complex 2D material systems.
AFM-IR vs. s-SNOM: A Comparative Overview
The article discusses the compatibility and complementary use of AFM-IR with scattering-type scanning near-field optical microscopy (s-SNOM). s-SNOM is an established AFM-based method for acquiring nanoscale chemical and optical properties. It utilizes the AFM tip as a local antenna to focus incident light, with an interferometer identifying scattered light transmitted over the local complex refractive index. This allows for spatial resolution under 10 nm and is widely used in surface polariton research.
Photothermal AFM-IR, while newer, offers distinct operational differences. In AFM-IR, tunable pulsed IR light is focused onto the sample at the AFM tip. When the IR wavelength matches the material's absorption band, rapid local thermal expansion occurs, stimulating cantilever oscillations measurable by the AFM. The signal directly measures sample absorption, producing spectra consistent with traditional Fourier-transform infrared (FTIR) in transmission mode.
Advancements and Advantages of AFM-IR
Early studies of AFM-IR for 2D materials faced challenges due to weak signals. However, breakthroughs in detection sensitivity (down to monolayers/single molecules) and spatial resolution (<5 nm) have made AFM-IR an effective tool.
AFM-IR presents several advantages over s-SNOM, including faster measurements, simpler interpretation, and displacement-based detection rather than purely optical methods, positioning it strongly in 2D materials research.
Applications of AFM-IR in 2D Materials Research
AFM-IR has been widely used to support s-SNOM in analyzing 2D materials, including plasmon polaritons in graphene monolayers and phonon polaritons in hBN and MoO3.
Beyond complementing s-SNOM, AFM-IR is capable of measuring properties difficult for purely optical near-field imaging techniques, such as:
- Heat dissipation mechanisms in graphene
- Non-radiative states in hBN
- Photothermal effects in thicker hBN
- Functionalized graphene
- MXene/graphene oxide hybrids
It has been used to identify distinct chemical patterns on functionalized graphene and study hydrogen bonds in MXene/graphene oxide nanosheets.
Case Studies: AFM-IR in Action
Phonon Polaritons in hBN
AFM-IR has been employed to study phonon polaritons (PhPs) in hBN flakes. Measurements on a 75 nm-thick hBN flake on a Si/SiO2 substrate confirmed that phonon polaritons correlate with stimulated wavelengths. Results aligned with theoretical models and prior s-SNOM studies. High-resolution spatio-spectral imaging demonstrated polaritonic features consistent with s-SNOM, validating AFM-IR's suitability for quantitative PhP studies.
Stacking Order in Multilayer Graphene
AFM-IR, often combined with techniques like scanning microwave impedance microscopy (sMIM), has proven effective in identifying stacking orders (e.g., Bernal, rhombohedral) in multilayer graphene. Distinct electrical properties of different domains were observed and validated. High-resolution AFM-IR imaging also revealed narrow domain walls (approximately 10 nm wide) separating various regions, corresponding to shear strain during domain transitions. Furthermore, AFM-IR can visualize graphene layers within van der Waals heterostructures, even when encapsulated by other materials like hBN.
Conclusion: The Power of Photothermal AFM-IR
Photothermal AFM-IR is a robust technique for characterizing 2D materials, including their plasmon-polariton and phonon-polariton modes. While it complements s-SNOM, AFM-IR offers unique benefits such as direct probing of non-radiative “dark” states, heat transfer functionality, and the chemistry of functionalized graphene.
Its advanced sensitivity, spatial resolution, speed, and user-friendliness make it a valuable tool for advancing the understanding and development of 2D materials.