Understanding Interfacial Phenomena at 2D Material Interfaces using Atomistic Simulations: Chemical Vapor Deposition Growth, Wetting, and Liquid-Phase Exfoliation
Ananth Govind Rajan Department of Chemical Engineering MIT, Cambridge,
08 August 2017, Friday, 12.00 p.m, Seminar Hall, Dept of Chemical Engineering,
AbBSTRACT :-
Two-dimensional (2D) materials, including graphene and transition metal dichalcogenides (TMDs) like molybdenum disulfide (MoS2), are layered materials, held together by weak, van der Waals forces, and are capable of being exfoliated up to the monolayer limit. In recent years, these materials have received considerable attention due to their layer-number dependent electronic band gaps, high mechanical strengths, and minimal resistance to fluid flow, which have enabled applications in transistors, composite materials, and membranes, respectively. Large-scale synthesis techniques for 2D materials involve chemical vapor deposition (CVD) growth or exfoliation and dispersion in solvents. Consequently, it is important to understand (i) the CVD growth process, and (ii) how MoS2 interacts with various liquids. ABSTRACT CVD methods, while powerful, are notoriously difficult to extend across different reactor types and conditions, with subtle variations often confounding reproducibility, particularly for 2D TMD growth. In this talk, I discuss the formulation of the first generalized TMD synthetic theory, by constructing a thermodynamic and kinetic growth mechanism linked to CVD reactor parameters, that is predictive of specific geometric shape, size, and aspect ratio from triangular to hexagonal growth, depending on specific CVD reactor conditions. The model is validated using experimental data that demonstrate the systemic evolution of MoS2 morphology down the length of a flow CVD reactor, where variations in gas phase concentrations can be accurately estimated using a transport model, under otherwise isothermal conditions. A kinetic Monte Carlo (KMC) simulation, which utilizes a sitedependent activation energy barrier, based on the intrinsic TMD bond energies and a series of EvansPolanyi relations, leads to remarkable, quantitative agreement with both shape and size evolution along the reactor. The predictive capability of this model and its extension to other TMD systems promise to significantly increase the controlled synthesis of such materials.
The preparation of molybdenum disulfide (MoS2) dispersions using liquid-phase exfoliation, as well as several of its proposed applications, involve liquids coming into intimate contact with MoS2 surfaces. Molecular dynamics (MD) simulations offer a robust methodology to investigate nanomaterial/liquid interactions involving weak van der Waals forces. However, MD force fields for MoS2 previously available in the literature incorrectly described the adhesive interactions of MoS2 with liquids such as water. With this in mind, in this talk, I describe the development of a set of force-field parameters that reproduce the properties of bulk 2-H molybdenite, and can also correctly describe the interactions of MoS2 with liquids, yielding good agreement with the experimental water contact angle. Despite the existence of charge separation between Mo and S atoms in MoS2, using the developed force field, we show that electrostatic interactions play a negligible role in determining the water contact angle on the MoS2 basal plane. We found that the nonpolarity of MoS2 is specific to the 2D basal plane of MoS2, and that other planes (e.g., the zigzag plane) in MoS2 are polar with respect to their interactions with water, thereby illustrating the role of edge effects, which could be important in systems involving vacancies or nanopores in MoS2. Using the developed force field in MD simulations, one can predict the efficacy of various organic solvents for dispersing molybdenum disulfide flakes using liquid-phase exfoliation.
ABOUT THE SPEAKER :-
Ananth Govind Rajan completed his Bachelor of Technology degree in Chemical Engineering from IIT Delhi in 2013, where he worked on computational fluid dynamics of bubble column reactors, with Professor Shantanu Roy, for his B.Tech. project. Ananth was awarded the Institute Silver Medal at IIT Delhi. Since 2013, Ananth has been pursuing his Ph.D. at the Massachusetts Institute of Technology (MIT), working with Professors Daniel Blankschtein and Michael Strano. His thesis research involves the use of ab initio electronic-structure calculations, molecular dynamics simulations, and kinetic Monte Carlo simulations, to understand the synthesis and interfacial behavior of nanomaterials, specifically layered materials such as graphene, molybdenum disulfide, and hexagonal boron nitride. These materials, due to their favourable properties, are poised to greatly improve efficiencies of electronic devices and separation membranes for both water and gas separations. At MIT, Ananth has been a recipient of the Landau Chemical Engineering Practice School fellowship and the Rosemary J. Wojtowicz Award from the David H. Koch School of Chemical Engineering Practice.