Abstract:
Optical microscopy has played an instrumental role in 2-dimensional (2D) materials research. In particular, the phenomenon of thin-film interference of light has been leveraged to improve contrast and vertical resolution of 2D materials down to the sub-nanometer scale, often via Fabry-Pérot (FP) thin-film resonators.
In this thesis, interference reflection microscopy (IRM) and backside absorbing layer microscopy (BALM), both of which harbor FP effects, are developed and utilized to study visibility and topographic inhomogeneities of the 2D semiconductor MoS2. Experimental contrast data are compared against Fresnel-based simulations of contrast. For IRM, an optimal configuration was found by tuning of incident wavelength and top medium refractive index, yielding ≈ 80% contrast. For BALM, the optical properties were measured for both the anti-reflective absorbing layer of nanometric Cr/Au, and an additional insulating AlOx layer, where for the first time the contrast spectrum for this system was acquired and simulated, yielding a maximum experimental contrast of ≈ 79% for 2D MoS2 .
Simulations of the optical stack across a variable range of aperture stop diameters and FP layer thicknesses predict further improvement of BALM conditions for high-contrast MoS2 visibility. Additional aspects including z-focus, optical noise, image post-processing, and others were also considered. Building on the visibility aspects, a charge density imaging capability for 2D MoS2 and other transition metal dichalcogenide crystals was developed by leveraging the charge-dependent complex refractive index near the wavelengths of the excitons.
Capacitors and field-effect transistors (FET) of MoS2 were realized, with multiple in operando experiments performed in widefield at throughputs up to 4 fps. In IRM mode, a liquid electrolyte gate was used, where charging delays and inhomogeneities due to intra- and inter-flakes resistances in polycrystalline MoS2 are presented. For Schottky barrier MoS2 FETs, the drain versus gate voltage competition for control of the local charge density in the channel was studied for the first time by optical microscopy. Solid-state MoS2 capacitor devices integrated in a BALM optical stack are also presented for the first time, both by experiments and simulations. A preliminary solid-state FET device was realized, exemplifying the powerful idea of combining optical charge imaging with electrical characterization in tandem.
This work on visibility and charge imaging aspects aims to widen the role and impact of optical microscopy techniques in the space of 2D materials research.
NIMBE/LICSEN