Expansion microscopy for large-scale super-resolution mapping of atomic defects in 2D materials

Pin-Jen Wang^1*, Wei-Kun Chang², Shun-Min Yang³, Han-Chun Wu⁴, Yeu-Kuang Hwu^2,3, Chun-Wei Chen⁵, T. Tony Yang⁶, Ann-Shyn Chiang², Shi-Wei Chu^1,2

¹Department of Physics, National Taiwan University, Taipei, Taiwan
²Brain Research Center, National Tsing Hua University, Hsinchu, Taiwan
³Institute of Physics, Academia Sinica, Taipei, Taiwan
⁴School of Physics, Beijing Institute of Technology, Beijing, China
⁵Department of Material Science and Engineering, National Taiwan University, Taipei, Taiwan
⁶Department of Electrical Engineering, National Taiwan University, Taipei, Taiwan

* Presenter:Pin-Jen Wang, email:f11222041@ntu.edu.tw

　　Two-dimensional materials have emerged as promising materials for next-generation electronic and optoelectronic devices, in which atomic defects are both prevalent and critical for modulating electronic, optical, and chemical properties. Thus, it is critical to characterize defects at multiple scales, in order to study both single defects in the nanometer-regime and defect distributions in the micrometer-regime. Scanning tunneling microscopy (STM) and transmission electron microscopy (TEM) have been used to extensively study defects at the atomic scale, but lack large field-of-view. Optical methods such as Raman spectroscopy and photoluminescence mapping can characterize large areas but are ultimately diffraction-limited. A metrology gap is present, preventing simultaneous atomic-resolution and high spatiotemporal-throughput, i.e. large field-of-view (FOV) and fast imaging speed. Currently, the area ratio between FOV and point spread function (PSF) of conventional methods is on the scale of 2-4 orders of magnitude.

　　In this work, we utilize expansion microscopy (ExM) to boost the FOV/resolution ratio to 7 orders of magnitude, which embeds fluorescent labels that specifically bind to sulfur vacancies in MoS₂ onto a swellable hydrogel. The spatial distribution of defects can be directly encoded onto the hydrogel without the presence of the target material, which allows for <10 nm-sized labels to be used without fluorescence quenching near the MoS₂ surface, increasing the precision beyond previous works. The hydrogel can be expanded by 4~10X (depending on the gel recipe) laterally in water in a single round of expansion, and iterative expansions boost the factor up to 100x, allowing for unprecedented super-resolution defect mapping using a conventional optical microscope with FOV a thousand times larger than conventional metrology, retaining sub-10-nm resolution.

Keywords: 2D materials, MoS2, defects, hydrogel, optical microscopy