Chemical vapor deposition growth and covalent functionalization/interfacing of 2D nanomaterials for electronic applications

Abstract

The evolution of unique electrical, optical, thermal, mechanical, and chemical properties in two-dimensional (2D) nanomaterials due to the atomic confinement in the z-direction has ignited tremendous technology promises. With that promise comes a challenge of incorporating 2D nanomaterials into practical applications, enabling their large-area growth and using covalent or van der Waal bonding to extent and control their properties in electronic applications. This PhD thesis establishes the following results: (a) successfully developing of scalable processes for direct growth of large-area graphene, h-BN, and MoS₂-on-h-BN on SiO₂/Si substrate, (b) demonstrating an electronic sensor for the defection of molecular motion by covalently interfacing 2D nanomaterials with photo-mechanical molecules, and (c) establishing the modulation of structural, electrical, thermal properties of 2D nanomaterials by covalently interfacing metal nanoparticles with 2D nanomaterials. A promising scalable route for large-area growth of 2D nanomaterial on a dielectric substrate is to perform chemical vapor deposition (CVD). Via two patented processes, we have synthesized graphene films directly on a SiO₂ substrate via carbon-diffusion through copper grains, and h-BN film on SiO₂ substrate via surface oxide assisted mechanism. The continuous graphene film grown with large coverage on SiO₂ substrate possessed a crystalline sp² domain size of 140 nm with low defect density (as indicated by low Raman I[subscript]D/I[subscript]G~0.1). The sheet resistance of this turbostratic stacking graphene was ~4 kOhm/sq, with a charge carrier mobility of ~250 cm²V⁻¹s⁻¹. Unprecedented, large coverage of directly grown h-BN film on SiO₂ substrate was demonstrated. This h-BN film showed a 6-fold smoothness enhancement compared to that of SiO₂ substrate. Such smoothness and the nature of free dangling bond of h-BN film reduced Coulombic long range scattering, leading to the 5-fold enhancement in the conductivity of the MoS₂, which is directly grown on the underlying h-BN platform. The next-generation molecular electromechanical systems require controlled manipulation and detection of molecular motion to build systems which respond to molecular mechanics. To achieve this, we covalently interfaced photo-mechanical molecules (azobenzene) (density = 2.5 nm⁻²) onto trilayer graphene (37.5% sp² coverage), where high sensitivity of this trilayer graphene due to high quantum capacitance (6.3 microF/cm²) and carrier confinement was leveraged. This enabled graphene to sensitively detect azobenzene isomerization, where one hundred molecules generated one charged carriers in the graphenic platform (2.44 x 10¹² holes/cm²). As mentioned before, surface modification of 2D nanomaterials opens an avenue to incorporate them into rational applications. We demonstrated the ability to interface noble metal nanoparticles (gold, silver) selectively onto a MoS₂ lattice (60° angular displacement) via both diffusion limited aggregation and instantaneous reaction arresting (using microwaves). Such gold nanoparticle interfaces allowed the modulation of electrical, and thermal properties, confirmed by Raman, electrical, and thermal studies. Consequently, a remarkably capacitive interaction between gold and thin MoS₂ sheet showed a 9-fold increase of effective gate capacitance with low Schottky barrier (14.52 meV), and a 1.5-fold increase in thermal conductivity with a low carrier-transport thermal-barrier (44.18 meV). This long-term work has established the following points: 1) Scalable routes for the growth of 2D nanomaterials, which can be extended to synthesize complex hetero/lateral architectures for integrated thin film circuitries. Furthermore, 2) covalent functionalization of 2D nanomaterials with nanoparticles and molecular systems can futuristically develop rational interfaces with other 2D heterostructures, and molecular machines.