Direct Optical Visualization of Graphene and Its Nanoscale Defects on Transparent Substrates

Abstract

The discovery and rise of graphene were historically enabled by its ∼10% optical contrast on specialized substrates like oxide-capped silicon. However, substantially lower contrast is obtained on transparent substrates. Moreover, it remains difficult to visualize nanoscale defects in graphene, including voids, cracks, wrinkles, and multilayers, on most device substrates. We report the use of interference reflection microscopy (IRM), a facile, label-free optical microscopy method originated in cell biology, to directly visualize graphene on transparent inorganic and polymer substrates at 30-40% image contrast per graphene layer. Our noninvasive approach overcomes typical challenges associated with transparent substrates, including insulating and rough surfaces, enables unambiguous identification of local graphene layer numbers and reveals nanoscale structures and defects with outstanding contrast and throughput. We thus demonstrate in situ monitoring of nanoscale defects in graphene, including the generation of nanocracks under uniaxial strain, at up to 4× video rate.

Graphene-Enabled, Spatially Controlled Electroporation of Adherent Cells for Live-Cell Super-resolution Microscopy

“…Two-point measurements showed resistances of a few kΩ across the as-prepared devices. Interference reflection microscopy (IRM) 28 was employed to confirm that graphene in the final devices to be continuous monolayers with minimal defects. Adherent mammalian cells (A549 and PtK2 cells) were cultured on the graphene surface under standard tissue culture conditions.…”

“…4a). IRM 28 (Fig. 4b-d) showed that the scratch was ~20 µm-wide, for which region graphene was fully removed, and conductance measurements indicated that the two halves were electrically isolated.…”

Visualizing Transparent 2D Sheets by Fluorescence Quenching Microscopy

“…Optical imaging of single-layer graphene was first achieved for samples deposited on Si wafers with a dielectric layer (e.g., SiO 2 or Si 3 N 4 ) of several hundreds of nanometers [31] based on an interference mechanism, [32] which was later extended to optically image many other 2D materials, [33] as well as graphene on transparent substrates. [34] However, the thickness of the dielectric layer and/or wavelength of the incident light must be carefully optimized to provide discernable optical contrast for direct visual observation, which is even more challenging for weakly absorbing clay and metal oxide 2D sheets. [35] In earlier works, we have developed FQM for rapid, highcontrast optical imaging of graphene-based [1,5] and transitionmetal-dichalcogenide 2D sheets [6] on arbitrary substrates and even in solution.…”

“…Although its lateral resolution is diffraction‐limited, optical microscopy is practically most useful for quick evaluation of relatively large, micron‐sized 2D sheets, which is also most relevant to current synthetic efforts. Optical imaging of single‐layer graphene was first achieved for samples deposited on Si wafers with a dielectric layer (e.g., SiO 2 or Si 3 N 4 ) of several hundreds of nanometers31 based on an interference mechanism,32 which was later extended to optically image many other 2D materials,33 as well as graphene on transparent substrates 34. However, the thickness of the dielectric layer and/or wavelength of the incident light must be carefully optimized to provide discernable optical contrast for direct visual observation, which is even more challenging for weakly absorbing clay and metal oxide 2D sheets 35…”

Layer-number determination of two-dimensional materials by optical characterization

“…(d) The intensity of the SHG and THG as a function of LNs [47] . as interference reflection [51] and hyperspectral imaging, have been developed. Interference reflection imaging employs interference reflection microscopy (IRM) to characterize the LNs, which is commonly used in cell biology.…”