Band engineering of two-dimensional material systems for tuneability of electronic and optoelectronic properties

Modern day electronics pursue the integration of components with different capabilities within a single electronic device, such as smart phones and smart watches. To incorporate more functionalities in a similar or smaller footprint, miniaturisation of each component into the nanoscale regime is required. However, reducing the footprint of current silicon technology is plateauing, as silicon, which is the active material used in these components, is reaching its grain boundary limits for further miniaturisation. Researchers have instead turned to alternative materials to replace silicon, such as two dimensional (2D) materials. Unlike conventional bulk semiconductors, 2D materials are only a single or few atomic layers thick, and have optical, mechanical, chemical and electronic properties that are superior to silicon due to quantum confinement of charge carriers within a single layer. This expands the applications of 2D materials in optoelectronics, biosensing, and piezoelectronics to name a few. This research focuses on the use of band engineering to tailor the optoelectronic properties of 2D materials to tune them for the application required, particularly in optoelectronics. Although 2D materials have superior performances compared to their bulk counterpart, their applications are still limited by their intrinsic properties. Black phosphorus (BP) which is a 2D semiconducting material, is widely sought after for its broadband optical absorption. However, its absorbance in the visible to near infrared (IR) is fairly low, and typically requires an electric field in the form of external gating to modulate its band energy level to improve its photodetection capabilities in this region. In this study, the optoelectronic property of BP is altered through surface transfer doping and hybridisation instead. By combining BP with an organic semiconducting polymer such as diketopyrrolopyrrole (DPP), its photoresponsivity to visible and near IR light excitation enhanced by 35 folds, without the need for external gate application. As DPP readily donates charge carriers to BP, photoexcited charge carriers in DPP flows into BP which has a lower band energy level and higher carrier mobility, thus increasing its photodetection capabilities. Furthermore, the polymer also acts as an encapsulation layer to BP, slowing down its rapid oxidation in ambient conditions. This hybridisation method provides a simple process of tuning BP’s optoelectronic properties, whilst also making it more energy efficient. Other than through hybridisation, this thesis also studies the tuneability of BP’s band gap through strain engineering. Fabrication complexities arising from weak van der Waals interaction induced slippage, coupled with mechanical breakdown of metal electrodes have prevented fundamental investigations into strain effects on electrical and optoelectronic characteristics of these material systems. To overcome this limitation, a simple pre-stretch fabrication technique is employed that allowed the demonstration of a functional multilayer BP based device on a stretchable elastomeric platform. By applying a uniaxial compressive strain of up to -10%, the electronic and optical properties of BP can be effectively modulated with mechanical strain. This simple strategy can be extended well beyond BP to other 2D materials creating opportunities for fundamental investigations into strain effects in 2D material systems and potential applications in strain engineered sensors for optical synapses applications. Lastly, broadband photovoltaic devices which are fundamental components in optoelectronics for applications such as self driven photodetectors and solar cells was explored in this thesis using 2D materials to form ultrathin p n junctions. As 2D materials lack dangling bonds in the third dimension, heterostructuring of 2D materials is possible without crystal lattice or grain boundary constraints, which are common issues in bulk semiconductors. In this thesis, 2D p n junctions were formed with iron phosphate trisulfide (FePS3) and molybdenum disulfide (MoS2) as the p  and n type material respectively. The resulting heterojunction exhibited 60 – 99% faster charge transfer with light illumination compared to the individual materials and is further validated through experimental means such as photoluminescence and excitonic lifetime measurements. Furthermore, the heterojunction also exhibited broadband photovoltaic response to visible light excitation and is also translatable onto flexible platforms such as polyimide. This opens up opportunities in creating flexible and self driven photodetectors or solar cells, which are beneficial for wearable and medical applications.