Single Molecular Break Junctions: Insights into Charge Transport and Chemical Reactivity

Abstract

Molecular electronics leverages the unique quantum transport properties of single molecules or molecular assemblies to create nanoscale components with tailored functionalities. Constructing a detailed knowledge base of the physicochemical properties of single (or ensemble) molecule-based junctions and the mechanisms behind the single (or ensemble) molecule charge transport is the first step towards designing the next generation of hybrid electronic high-performance devices with a myriad of functionalities. The approach of using single molecules as electrical components offers several advantages. These include the scope for ultimate miniaturization beyond the limits posed by the device physics at the nanoscale, utilizing quantum mechanical effects for enhanced device performance and improved functionalities with low energy consumption. This requires an interdisciplinary approach where the fundamental principles of chemistry, physics, and engineering together unravel and harness charge transport phenomena at the molecular scale. This thesis explores the basic theoretical principles, experimental advancements, and applications of molecular electronics, with a particular focus on molecular junctions as pivotal building blocks of nanoscale devices. It provides insights into the evolution of the field, methodologies, and intriguing experimental results, offering a pathway to the realization of reliable molecular-scale technologies. The thesis starts with establishing the theoretical basis of molecular electronics, emphasizing the historical context and conceptual evolution that laid the groundwork for this field. The principles of electron transport at the molecular level are briefly examined, including key mechanisms such as quantum mechanical tunneling, incoherent hopping, and thermionic emissions. These discussions are followed by a brief context of the Landauer formalism, which forms a cornerstone for calculating transport properties in molecular junctions. In addition, the influence of molecular structure, electrode materials, and external environmental factors on charge transport phenomena is explained, paving the way for subsequent experimental studies. This work places much emphasis on the development and application of state-of-the-art experimental techniques to probe charge transport in molecular systems. The thesis demonstrates the development of several experimental methodologies, including the EGaIn-based large-area molecular junction setup, mechanically controlled break junctions (MCBJs), and scanning tunneling microscope break junctions (STM-BJs). These tools enable precise characterization of electrical conductance, thermoelectric properties, and other transport characteristics at the nanoscale, overcoming challenges associated with molecular junction instability and measurement sensitivity. These techniques have successfully enabled the understanding of charge transport in a wide array of molecular systems at the single-molecular level. One key contribution of the research is the exploitation of light-responsive molecular actuators. Utilizing the DTE-based architecture, the PhD thesis shows a photothermal isomerization-based, reversible, and single-molecule linear actuator under ambient conditions. This molecular system exhibits mechanical actuation without altering conductance properties around the Fermi level, a feature critical for functional device applications. This functionality is achieved with a delicate balance in the tunneling width and the barrier height change through photothermal isomerization. This work demonstrates a novel paradigm in the design of functional nanoscale devices with reproducible and reliable performance. This work also explores the non-conventional stimuli to drive isomerization in different DTE derivatives, thereby mapping how mechanical forces and electric fields alter ground-state reactivity landscapes. With the help of the single-molecule break junction technique, the thesis addresses the mechanical and electric field-induced isomerization of DTE derivatives, usually obtained through photochromism or electrochromism. These studies showed that external perturbations can control reaction pathways, open new dimensions of molecular reactivity, and extend ground-state potential energy surfaces. This work gives a roadmap on how to harness mechanical and electrical forces to induce and control chemical transformations selectively at the molecular scale, with significant implications for catalysis and materials science. To probe the interfacial dynamics, molecular-metal interactions are investigated with a particular focus on the gold-thiol (Au-S) interface, which has been a key element in several domains, from chemotherapy to nanoparticle synthesis to molecular electronics. Using a custom-built MCBJ setup, the study investigates the binding strength and dynamic behavior of Au-S junctions by conductance and flicker noise measurements. The experiments show the subtle interplay between chemisorption and physisorption phenomena and their implications for molecular stability and transport properties. Another particularly intriguing aspect of the work presented in this chapter is the demonstration of interfacial electric field-induced chemical transformations at the single-molecule level, which otherwise requires harsh chemical conditions. The results highlight the potential of single molecular conductance and flicker noise as a diagnostic tool for probing molecular junction dynamics and distinguishing between different interaction mechanisms at the molecular-metal interface. Another critical dimension probed in this thesis is the conductance variability in cyclic aliphatic systems and the role of anchoring groups in modulating molecular transport properties. Using mechanically controlled break junction (MCBJ) experiments and in-situ dithiocarbamylation, it demonstrates that, unlike aromatic systems, these σ-conductive aliphatic systems exhibit no meta effect, with conductance trends following para < meta < ortho configurations. It also shows that differential overlap between frontier molecular orbitals and metal electrodes due to a change in junction geometry during junction stretching leads to distinct conductance plateaus in aliphatic systems. Further, anchoring group modifications (secondary amine to dithiocarbamate) significantly alter conductance decay factor (β) and transmission pathways, as validated by single molecular conductance behavior upon junction evolution. These findings reveal that junction configurational changes govern conductance variability in saturated systems, providing a framework for designing robust molecular junctions with controlled electronic properties. The inclusion of metal-organic complexes into molecular electronics represents a new paradigm, as has been proven by this dissertation. The STM-BJ technique was utilized to explore the stability and transport properties of Ag(I) and Au(I) tetragonal prismatic metal-organic cages. In this process, denaturation of the said complexes was illustrated into smaller fragments with distinguished transport properties, ultimately leading to metal-metal filament formation across the interelectrode gaps under high applied fields. The observed fragmentation of metal cages under applied bias break junction conditions highlights a critical trade-off in molecular electronics: while metal-organic cages offer tunable electronic properties, their structural instability in high-field environments can limit their direct utility in robust single molecular devices. However, the transient formation of conductive metal-organic fragments with very low attenuation factors suggests an alternative pathway for achieving time-dependent dynamic junctions. These fragments, with their wire-like conductance behavior, could serve as intermediates in reconfigurable circuits or adaptive components for neuromorphic computing, where controlled breakdown and self-healing processes are advantageous—the findings open avenues for leveraging instability as a functional feature. For instance, metastable cages could act as "sacrificial templates" for transient ion release in nanoelectrochemical systems or self-limiting conductive networks. This work opens new avenues for applications of metal-organic systems toward advanced molecular circuitry and electronics by illustrating instability and fragmentation-related challenges. This work underscores the importance of evaluating not just electronic properties but also operational durability when integrating supramolecular architectures into practical circuits. In conclusion, this thesis explores theoretical principles, experimental methodologies, instrumentation development, and key experimental results on different molecular structures to unravel the charge transport phenomena and molecular reactivity at the single-molecule level. The insights and methodologies developed in this thesis pave the way for applications in catalysis, organic electronics, and nanoscale device engineering. This work demonstrates the transformative potential of studying charge transport in molecular junctions in understanding the chemical transformations at the nanoscale through several key contributions. Future research should expand ground-state reactivity landscapes, integrate machine learning for data-driven design, and explore in-situ spectroscopic methods to decode dynamic junction behavior.