Achieving High Visibility of Monolayer MoS₂ Using Backside-Illuminated Thin Films

Interference reflection microscopy (IRM) and backside absorbing layer microscopy (BALM) have emerged as powerful optical microscopy methods for the study of nanomaterials and biological samples. These techniques consist in using an inverted optical microscope in reflection mode to observe objects deposited either on glass (IRM) or on a nanometric absorbing metallic film (BALM). The thickness of the BALM absorbing layer and of optional additional transparent layers, as well as the choice of incident wavelength and top medium, act as powerful levers for maximizing the resultant contrast of a given sample. However, the use of BALM to study samples with high absorption coefficient has been limited so far in the literature. Furthermore, the complex refractive index (ñ = n + iκ) of layers in a specific BALM optical stack have so far not been measured, and thus experimentally informed simulations are nonexistent. In this work, we use variable angle spectroscopic ellipsometry (VASE) to measure ñ(λ) of each layer in BALM (and IRM) stacks consisting of different combinations of glass, Cr, Au, and AlO, with the two-dimensional (2D) transition metal dichalcogenide (TMD) MoS₂ as the sample under study. Using the measured values, we simulate contrast spectra and compare them against data. We manipulate the film thicknesses and top media to engineer favorable contrast conditions both in the blue and red regimes of the visible spectrum, with peak contrasts of ≈80%. Finally, we demonstrate how the 2-layer BALM stack can double as both an optically sensitive system and at the same time a metal-oxide-semiconductor structure allowing for electrostatic gating. We use this structure to do in situ local charge density imaging of a 2D MoS₂ capacitor, which showcases the versatility of these types of BALM stacks.