Publication
This thesis studies the optical and electro-mechanical properties of novel atomically thin crystals. It deals with the carbon based material, graphene, and with layered transition metal dichalcogenides with the common formula MX2. The first part describes a new optical model for detecting ultra-thin, two dimensional nanocrystals under optical microscope. A simple optical model is used to calculate the contrast of nanolayers on SiO2. The model is extended for imaging using the green channel of a video camera. Atomic force microscopy combined with optical imaging confirms that single layers of graphene, MoS2, WSe2 and NbSe2 are detectable on 90 nm and 270 nm SiO2. The optical contrast of multi-layer nanocrystals is also reported and allows easy differentiation between single, double and triple layers. Optical imaging is proposed as a rapid, non invasive and low cost method for the detection of ultra-thin nanocrystals. The second part is devoted to studying the electromechanical properties of mono-layer and bilayer graphene. The interplay between the mechanical and the electrical properties of graphene nanoribbons is probed by nanoindentation techniques based on AFM. Deflection of monolayer graphene nanoribbons with widths ranging between 60 nm and 300 nm results in a linear increase in their electrical resistance. The sensitivity of the response increases for nar- rower ribbons and variations of the resistance up to 15% are measured. The electromechanical response of bilayer graphene shows a superposition of a linear response with oscillations of the electrical resistance. We present possible scenarios to explain the origin of these oscillations.
Kumar Varoon Agrawal, Shaoxian Li