Monolayer Transition Metal Dichalcogenides (TMDs) have emerged into the scientific foreground as a promising group of materials for the development of advanced optoelectronic applications. Their unique crystal structure combined with their electronic band formation and their atomically thin nature, inherit them a number of extraordinary properties, making them ideal for applications like photovoltaics, light-emitting devices, field-effect transistors, quantum computing and flexible electronics.
Being membrane materials, they are highly sensitive to their surrounding environment and by extension to their supporting substrate. Fluctuations in dielectric environment, local strain effects and carrier doping resulting from atmospheric molecules can effectively alter the 2D TMDs optoelectronic response. Substrate engineering can be utilized targeting the control of 2D material properties; therefore, the appropriate substrate material and configuration is of high importance.
In this thesis, we investigate the substrate effect on monolayer WS2 (1L-WS2) with optical spectroscopy means by suspending the atomically thin material over pre-patterned Si/SiO2 with cylindrical wells. We explicitly show that suppressed substrate doping due to its absence in suspended regions leads to a more pronounced Photoluminescence (PL) emission from the neutral exciton (X0). Furthermore, reduced screening stemming from the fact that the monolayer is freestanding, leads to a slight shift in the band gap which impacts the valley polarization response of 1L-WS2.
The same sample configuration was examined under the premise of non-linear imaging. We present enhanced Second Harmonic Generation (SHG) signal from suspended 1L-WS2 with respect to supported and strained regions, owing to light interference effects. In addition, polarization-resolved SHG (P-SHG) provided images of the armchair orientation of suspended, supported and strained regions. In the latter, a characteristic cross-shaped pattern was found, giving the fingerprint of strain for monolayer WS2.
Finally, we study the effect of biaxial strain on the exciton resonance and the degree of valley polarization (VP), of 1L-WS2/Graphite at room temperature. By encapsulating the heterostructure and transferring it on strain-dedicated device we managed to tune the VP degree in a fully reversible manner. Our results can offer significant contributions towards the realization of novel, cutting edge optoelectronic devices.
(EN)
University of Crete
