Charge Transport in Molecular Systems

Abstract

Understanding charge transport in molecular systems is of fundamental interest in rapidly growing field of molecular electronics as well as to understand biological signal transfer processes which are accompanied by the spatial transport of charge. In this thesis, I describe the charge transport properties of various molecular systems using thermally activated hopping mechanism as described in the framework of Marcus- Hush formalism. The thesis is organized as follows: In the introduction of the thesis, I present a general overview of the various theoretical descriptions of the charge transport phenomenon in molecular systems. I describe the diffusive transport of charge captured by the classical Drude theory and present the Landauer-B uttiker formalism which describes the transport of charge as the quantum mechanical tunnelling. I also narrate the semi-classical Marcus-Hush kind of hopping mechanism of charge transport which holds good for the systems having disorder causing localization of the charge carrier. 2nd chapter of the thesis presents a brief introduction of the various numerical techniques and force field used in all the works presented in the thesis. In the rest of the thesis, I discuss the charge transport properties of a variety of molecular systems. The charge transport properties are studied either in a single molecule level or in a self-assembled morphology of the molecules. To study the charge transport properties of the system we use multiscale modelling simulation technique combining classical molecular dynamics simulation, quantum mechanical calculation and Kinetic Monte Carlo Simulation. The molecular dynamics simulations are done to predict the equilibrium morphology of the system. Once the equilibrium morphology is generated, the system is partitioned into charge hopping sites with the hopping rates between the neighbouring sites described by Semi-Classical Marcus-Hush formalism. Quantum mechanical calculations are performed to calculate these hopping rates. Using these rates, Kinetic Monte Carlo Simulation is done to simulate the movement of charge and to predict the mobility of charge carriers in the system. In the 3rd chapter of the thesis, I explore the charge transport properties of a recently synthesized discotic liquid crystal[Adv. Mater. 26, 2066 (2014)]. We perform all-atom molecular dynamics simulation to probe the molecular organization in a hexagonal columnar liquid crystalline phase formed by the discotic molecule Hexa- Peri-Hexabenzocoronene/Oligothiophene hybrid. We also report the hole mobility along the column of this liquid crystal phase and found that the mobility is limited by the defects in molecular arrangement in the column. Understanding that the defects in the column limits the mobility, we arrange the discotic coronene molecules inside the single walled carbon nanotube(CNT). Coronene molecules are found to form defect free column inside the nanotube for a particular radius of the CNT which leads to ultrahigh charge carrier (hole) mobility along the coronene stack. The details of these calculations are presented in chapter 4 of the thesis. In the 5th chapter, I report the hole and electron mobility of two different dendrimer melt systems: Dendritic phenylazomethine with a triphenylamine core (Dpa-Tpa) and dendritic carbazole with cyclic phenylazomethine as the core (Cpa-Cz). We present a way to tune the mobility of the system by changing the dendrimer generation. In chapter 6 of the thesis, I report the V-I characteristics of a ds-DNA which is attached between two electrodes. The DNA is further stretched using different protocols and the current is measured in the course of pulling. We found abrupt jump in current as the DNA is stretched beyond a critical stretching length. The value of the critical stretching length was strongly dependent on the pulling protocol. When the DNA is stretched along 3'end1-3'end2 direction, the conductance jump happens at a larger stretching length compared to other pulling protocols. We attribute this observation to the S-form of DNA when pulled along 3'end1-3'end2 direction. In contrast, pulling along 5'end1-5'end2 direction leads to immediate melting of DNA which leads sharp conductance jump at very short stretching length. I believe that the content of the thesis will have significant impact on the field of molecular electronics and will help to understand the biological charge transfer processes.