Quantum Transport of Alkane Derivatives in Nanoscale Structures

Alshehab, Abdullah and Lambert, Colin (2022) Quantum Transport of Alkane Derivatives in Nanoscale Structures. PhD thesis, Lancaster University.

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Abstract

As a result of recent developments, alkane derivatives have been used in many applications, including sensors for safety and security. This thesis provides a theoretical contribution to these developments by exploring molecular junctions formed from nearly 40 alkane derivatives including linear chains and rings. This study also employs 4 different anchor groups such as amine, thiol, direct carbon and thiomethyl. Within this thesis, I will provide a simple introduction to the theoretical tools used to explore electron transport through single-moelcule junctions. In Chapter 2, Density Functional Theory (DFT) and its applications within the SIESTA code will be discussed. This allows computation of ground state wave functions for molecules and provides the underlying Hamiltonians for molecular junctions which are used as a starting point for transport calculations. Chapter 3, provides the theoretical framework that is used to calculate electron transport properties such as electrical conductance and thermopower . This is based on Green’s and Dyson’s equation and is embodied in the GOLLUM code, which is the second major tool used during in this thesis. In chapter 3, I present some solutions of Green’s functions for infinite and semi-infinite chains and the transmission coefficient equations that implemented in the GOLLUM code. Chapter 4 is the first results chapter in this thesis, which provides thorough and vigorous theoretical investigations about series of alkane chains using different anchor groups including amine (NH2), thiol (S), direct carbon contact (C), and thiomethyl (SMe). Thus, I demonstrate the impact of using different terminal groups on the electrical conductance of alkane molecules. As expected, the conductance of alkane chains decreases exponentially with length, regardless of the type of anchor groups. However, the precise value of the conductance differs from one anchor to another. The theoretical predictions of the four anchors are checked against the STM measurements and are found to be well supported by the experimental measurements. Chapter 5 is the second results chapter in my thesis, and investigates a series of alkane chains and their corresponding symmetric and asymmetric alkane rings were explored. The electrical conductance and Seebeck coefficient of three linear chains and six rings were investigated using DFT. Remarkably, I found that the conductances of the double-branched alkane rings were smaller than those of the corresponding individual chains and much smaller than the value predicted by Kirchhoff’s law. This result is well supported by previous published work. The Seebeck coefficients of the rings were also higher than those of the corresponding chains, which is consistent with the presence of phase coherent tunnelling in the alkane rings. Further characterizations of asymmetric rings found that their conductances and Seebeck coefficients were between those of their corresponding shorter and longer chains. With the elongation of the longer chain, the conductance of the asymmetric ring became close to that of the shorter chain. This suppression of conductance in symmetric rings agrees with experimental results using the scanning tunneling microscope break junction (STM-BJ) method.