Nanoscale Optical Analysis Using AFM-Based Techniques

Recent advances in atomic force microscopy (AFM)-based techniques have significantly expanded the ability to probe optoelectronic and quantum materials at the nanoscale, enabling high-resolution spatial mapping of chemical, optical, and electronic properties. Techniques such as scanning near-field optical microscopy (s-SNOM), AFM-based infrared (AFM-IR), tip-enhanced Raman spectroscopy (TERS), and tip-enhanced photoluminescence (TEPL) are pivotal in addressing the challenges of characterizing quantum dots, 2D semiconductors, perovskites, and other complex materials with unique quantum confinement and light-matter interaction effects. These methods provide unparalleled insights into the local structure-function relationships and defect distributions, which are critical for optimizing device performance and stability.

One of the primary advantages of s-SNOM is its ability to achieve sub-wavelength optical resolution, making it an ideal tool for mapping the optical near-field properties of materials. In 2D materials such as transition metal dichalcogenides (TMDs), s-SNOM has been successfully applied to image plasmonic excitations and phonon polaritons, allowing for a deeper understanding of optical anisotropies at the nanoscale. This technique has also been instrumental in studying local charge distributions in perovskite materials, providing key insights into surface defects and their effects on device performance, particularly in photovoltaic applications.
AFM-IR, which combines the spatial resolution of AFM with the chemical specificity of infrared spectroscopy, has emerged as a powerful technique for studying chemical composition and molecular interactions at the nanoscale. In the field of organic photovoltaics (OPVs), AFM-IR has been used to map the distribution of polymer phases, revealing how phase separation impacts charge carrier mobility. Similarly, in perovskite solar cells, AFM-IR has been utilized to identify degradation pathways and the distribution of halide species, aiding in the development of more stable perovskite formulations.

Tip-enhanced Raman spectroscopy (TERS) extends the capabilities of traditional Raman spectroscopy by combining it with AFM, enabling the detection of vibrational modes with nanometer-scale spatial resolution. This technique has proven particularly useful in studying strain and defects in 2D materials such as graphene and MoS_2, where slight changes in lattice structure can significantly impact electronic properties. For example, TERS has been used to visualize strain-induced bandgap modifications in MoS₂, which are important for designing next-generation flexible electronics and photodetectors.
Tip-enhanced photoluminescence (TEPL) is another powerful technique, particularly for examining exciton dynamics in quantum-confined systems. In quantum dot arrays and TMD monolayers, TEPL has been employed to map exciton localization and quenching effects, which are critical for improving light-emitting devices and photovoltaic efficiencies. This method allows researchers to pinpoint non-radiative recombination centers, which can act as efficiency bottlenecks in devices such as quantum dot solar cells and light-emitting diodes (LEDs).

Together, these AFM-based techniques are revolutionizing the study of emerging optoelectronic and quantum materials by providing unprecedented nanoscale detail on the optical, chemical, and electronic properties. Their applications span a wide range of materials and technologies, from improving the performance of perovskite solar cells by identifying degradation mechanisms, to advancing quantum computing through the precise characterization of defect states in quantum materials. As these multimodal approaches continue to evolve, they will further deepen our understanding of the fundamental properties of materials and accelerate the development of next-generation devices.