Controlled Nanoscale Growth of 2D Materials and Their Heterostructures

Unique ranges of properties of two dimensional (2D) materials foster new opportunities in many scientific and technological fields. Beyond pristine 2D materials, their heterostructures provide more possibilities in real application as the stack can be tailored according to desired properties. Besides, exotic combination can give rise to new functionalities. Despite experimental success in many lab-grade devices, large scale commercialization of 2D technology is still being actively worked on. This requires easy and cost effective synthesis technique which can give high quality sample in large scale. In this thesis, we discuss the scalable, application specific growth of 2D materials and their heterostructures. The easiest preparation method (micromechanical cleavage) does not offer a very good route for commercial use and alternative approaches can offer large scale production. A brief survey on the various production methods have been carried out in Chapter 2. It is shown that through bottom up approach a balance between growing large scale and high quality can be achievable and CVD is one such controllable growth platform. However to control materials at such atomic level require detail understanding of nucleation, surface science and defect control. Next in Chapter 3, taking graphene growth as an example, a CVD based growth method is first established which can also be applied for other 2D materials. The grain size of a CVD grown monolayer large area graphene film is key to its performance. Microstructural design for the desired grain size requires a fundamental understanding of graphene nucleation and growth. Hence, first the CVD growth phenomenon of graphene has been exploited. In this course, two ultimate levers for controlling the nucleation density are identified and they are substrate defect density, which are the active nucleation sites and “gas-phase supersaturation”. It is observed that defects on copper surface, namely dislocations, grain boundaries, triple points and rolling marks, initiate the nucleation of graphene. It has been shown that among these defects, dislocations are the most potent nucleation sites, as they get activated at lowest supersaturation. Defects in graphene can be made useful in functionalization or making heterostructures which can be useful in chemical sensing or energy generating fields. Specifically, combination of graphene with plasmonic nanostructures would allow for making surface enhanced Raman spectroscopy (SERS) based sensing platform. Graphene being atomically thin can ideally be placed between plasmonic metal dimers to create precise sub-nm gap. Such hybrid structure of metal dimers with graphene spacer increases the Raman signal by several orders by near-field enhancement resulting from strong electromagnetic coupling between the particles. Besides these hybrids have applications in other different areas such as optical switch, displays, and photodetectors. In Chapter 4, wet-chemistry-based method is applied to fabricate a SERS substrate with 7x10cm Au nanoparticle…