ANA LAURA ELIAS

Controlled Generation and Understanding of Defects in 2D Nanomaterials.

The understanding of defects in two-dimensional (2D) materials, such as semiconducting transition metal dichalcogenides (TMDs), is key to exploit their properties for applications in electronics, optoelectronics, and catalysis, among others. Moreover, the control of the density and spatial distribution of defects in
semiconductors is also crucial for tuning their properties.
Defects in the form of dopants were introduced in TMD monolayers through the development of a liquid-phase precursor assisted technique. Substitutional dopants in MoS2 and WS2 layers were identified by high-resolution scanning transmission electron microscopy. This synthesis approach can be tuned to control the doping concentration in the samples up to very high levels (more than 10 at%). The consequent changes in the photoluminescence (PL) emission and Raman spectra show that the proposed synthesis route is effective for engineering functionalities in TMD layers. Moreover, the edge termination was found to be a crucial feature in 2D TMD monolayers, influencing the distribution of chemical dopants. For the case of monolayered WS2, a higher density of transition metal dopants is always incorporated in sulfur-terminated domains when compared to tungsten-terminated domains. Two representative examples of this spatial distribution control will be discussed, including iron- and vanadium-doped WS2 monolayers. Density functional theory (DFT) studies of the edge-dependent dopant distribution will also be presented. This research indicates that edge termination in crystalline TMD monolayers can serve as a knob for engineering the spatial distribution of substitutional dopants, leading to in-plane hetero-/multi-junctions that could feature fascinating electronic, optoelectronic, and magnetic properties.
Furthermore, during bottom-up synthesis of TMDs, the atomically thin nanomaterials can be subjected to significant strain. Resonant Raman and PL spectroscopy were used to analyze the anisotropic in-plain strain in MoS2 monolayers grown onto Si/SiO2. The correlation between the second-order Raman bands and PL energy was documented for the first time when the analyzed 2D layers were under tensile strain.

This study presented a novel indirect method to extract the Grüneisen parameter directly from the Raman and PL spectra. Experimental and DFT simulations were compared and a good agreement was found in the Grüneisen parameter calculated values.
In summary, native and induced defects in TMDs were studied through a combination of electron microscopy, Raman, and PL spectroscopies. These results represent a contribution towards the manipulation, quantification and understanding of defects in TMDs, bringing 2D materials one step closer to applications.