Ideal optical contrast for 2D material observation using bi-layer antireflection absorbing substrates

Abstract

The capability to observe 2D materials with optical microscopy techniques is of central importance in the development of the field and is a driving force for the assembly and study of 2D material van der Waals heterostructures. Such observation of ultrathin materials usually benefits from antireflection conditions associated with the choice of particular substrate geometry. The most common configuration uses a transparent oxide layer with a thickness minimizing light reflection at the air/substrate interface when light travels from air to substrate. Backside Absorbing Layer Microscopy (BALM) is a newly proposed configuration in which light travels from glass to air (or another medium such as water or a solvent) and the antireflection layer is a light-absorbing material (typically a metal). We recently showed that this technique produces images of 2D materials with unprecedented contrast and can be ideally coupled to chemical and electrochemical experiments. Here, we show that contrast can be optimal using double-layer antireflection coatings. By following in situ and with sub-nm precision the controlled deposition of molecules, we notably establish precisely the ideal observation conditions for graphene oxide monolayers which represents one of the most challenging 2D material cases in terms of transparency and thickness. We also provide guidelines for the selection of antireflection coatings applicable to a large variety of nanomaterials. This work strengthens the potential of BALM as a generic, powerful and versatile technique for the study of molecular-scale materials and phenomena.

Backside absorbing layer microscopy: a universal relationship between physical thickness and reflectivity

“…In the numerical examples presented above, we have only envisaged purely transparent, i.-e. non absorbing sample layers. The case of absorbing layers was addressed in [11]. The high sensitivity of on was demonstrated, which confirms possibilities for molecular layer characterization.…”

“…In the case of BALM, their number exceeds ten. Indeed, BALM is a white light, high aperture technique and there are many benefits to use ARA multi-layers made of two or three superimposed materials [11]. The modeling must take into account the spectral characteristics of the source and of the camera, all incidence angles contributing to the signal on the microscope, and one thickness and one complex refractive index per constitutive layer of the ARA coating.…”

“…However, the reflectivity also depends on the refractive index of the sample, so it must be the same for the sample and the ruler, and this is a serious limitation. An alternative approach is to calibrate the optical measurement with a few AFM measurements on the same sample [11]. It has other drawbacks such as limitations on the nature of the sample or having to extract the sample from its environment in the optical setup, especially inconvenient when in a liquid.…”

Cu(II)-Metallized Three-Layered Cu–8HQ Complex with Hierarchical Crystallite Morphologies Synthesized via Reaction–Diffusion

“…Optical contrast of the SEM images has been used as an effective method for identifying the layered structures and layer numbers of 2D nano-assemblies using the contrast difference ( C D ) determined using eq C normalD = 1 − α 1 + α , goodbreak0em2em⁣ normalw normalh normale normalr normale , α = R normals normall normala normalb R normalb normall normala normaln normalk …”

Single-walled carbon nanotube membranes as non-reflective substrates for nanophotonic applications

“…Apparently, the enhanced contrast is caused by a much lower light reflectance from the SWCNT membranes in comparison with that of the cover glass. Similarly, conventional antireflective layers are used to achieve a better contrast in optical microscopy for the observation of 2D materials [30][31][32]. Hence, non-reflective SWCNT membranes seem to be perfect substrates for an optical detection and characterization of various low-contrast specimens such as live cells, which are usually poorly detectable by BF optical microscopy [33].…”