(Invited) Covalent Functionalization of Graphene and MoS2: Chemical Versus Electrochemical Routes

Abstract

Atomically thin sheets of materials, the so-called two-dimensional (2D) materials arrived on the scene in early 2000s with the successful isolation of graphene as freestanding monolayer films. Graphene, a single atom thick sheet of sp2-hybridized carbon bonded in a honeycomb lattice, has exceptional electronic, optical, mechanical and thermal properties that outperform those of most of the existing materials. The research on graphene further fuelled emergence of related 2D materials such as black phosphorous, silicene, germanene and transition metal dichalcogenides (TMDs) such as MoS2. Due to their exotic properties, these materials offer virtually endless opportunities for fundamental research as well as cutting edge applications. Most of these 2D materials however exist as single layers only when supported by another solid surface or when stabilized by physisorbed or chemisorbed organic molecules. Given their layered nature, most 2D materials are extremely difficult to disperse in typical solvents. Chemisorption of organic molecules onto their basal plane allows dispersion of these materials in (organic) solvents thereby improving their solution processability. Such dispersions can be used in composite materials and as functional inks. Moreover, covalent functionalization allows modification of the intrinsic electrical, electronic and optical properties of these 2D materials. In this talk, I will discuss covalent modification graphene, graphite and MoS2 using diazonium chemistry. Two different routes for reductive decomposition of diazonium salts namely, chemical and electrochemical, will be discussed in detail. Special focus is on sub-nanometer characterization of modified materials using scanning tunneling and atomic force microscopy (STM and AFM). Initial results towards patterned covalent functionalization using physisorbed self-assembled templates will also be presented besides the discussion on the use of such surfaces as seed layers for atomic layer deposition (ALD).