| dc.description.abstract | Molecular electronics explores electronic functionality at the sub-nanometer scale by utilizing individual molecules as active components. The molecules provide unique advantages such as extreme miniaturization, chemical tunability, and access to quantum effects, considering them as potential alternatives to conventional silicon-based electrical components like resistors, diodes, capacitors, memory elements, and sensors. At the heart of molecular electronics lies the ‘molecular junction,’ in which a single molecule or an ensemble of molecules connects two electrodes, and a thorough understanding of its physicochemical properties is essential to fully harness the capabilities of molecular systems.
The thesis begins by discussing the emerging challenges in the modern electronics industry in the context of continued miniaturization trend, followed by a brief exploration of alternative paradigms. Among the alternate approaches, molecular electronics stands out as molecules directly operate at the nanoscale, allowing access to quantum phenomena such as electron tunneling, quantum interference, and discrete energy levels for designing next-generation electronic devices. With this motivation, this thesis adopts molecular electronics as its central theme, starting with the concepts of the molecular junction and progressing toward experimental testbeds that probe charge transport in both single-molecule and large-area junctions. These systems enable detection of subtle changes at the molecule–electrode interface, revealed through changes in electrical and thermoelectric properties upon external stimuli.
Subsequent chapter delve into the fundamentals of nanoscale charge transport and the mechanisms that govern conduction in molecular junctions, including direct tunneling and thermally activated hopping. The discussion then extends to the Seebeck effect, which provides critical insights into carrier type and the electronic structure of junctions. Since electrical conductance and thermopower are complementary yet differently correlated under tunneling and hopping regimes, their combined study offers a deeper understanding of transport processes. In the final part, the thesis introduces quantum light as an external stimulus, demonstrating how vibrational strong coupling within Fabry–Pérot cavities modifies charge transport by hybridizing cavity photons with molecular vibrations to form vibropolaritonic states. By integrating these aspects—charge transport mechanisms,
Abstract
9
thermoelectric effects, and quantum light–matter interactions—the thesis establishes a comprehensive theoretical foundation that underpins the experimental investigations presented in the later chapters.
A key part of this thesis has focused on the design and optimization of an experimental setup for precise electrical and thermoelectric measurements. Using the liquid metal alloy EGaIn as a soft top contact, allowed reliable probing of self-assembled monolayers (SAMs) and thin films without damaging them. Experimental protocols were standardized by carefully controlling environmental parameters such as temperature, humidity, and mechanical stability of the junction to ensure accuracy and reproducibility. The n-alkanethiolate series, known for its ordered self-assembly and well-defined molecule–electrode interfaces, served as a benchmark for calibration and validation, providing a robust platform for subsequent studies.
Building on this foundation, a low-symmetry unimolecular barrel, UNMB (M₈Lun₄), was investigated as an active host system to probe chemical stimulus effects. Guest encapsulation of fullerenes (C₆₀⊂UNMB and C₇₀⊂UNMB) enhanced conductance by ~1.5 orders of magnitude, while the positive Seebeck coefficient dropped to nearly half its initial value, reflecting significant modulation of the UNMB framework. The temperature-dependent current–voltage studies confirmed a thermally activated hopping mechanism in all three supramolecular assemblies, with distinct activation energies. The observed increase in conductance and reduction in thermopower were explained by a reduced HOMO energy offset upon guest encapsulation by using Ultraviolet photoelectron spectroscopy (UPS) measurements. These results highlight host–guest chemistry as a powerful strategy for nanoscale thermoelectric switching, capable of sensing and transducing structural and energetic variations in molecular devices.
The host–guest interactions observed in discrete molecular cages demonstrated pronounced local effects upon guest encapsulation. To extend these interactions across multiple molecular units, the next chapter explores a Pt(II)-based supramolecular system that forms long interconnected networks, or fuel-driven assemblies, upon ATP modulation through Pt···Pt interactions. These ATP-induced structural transitions are confirmed by photoluminescence (PL) and UV–visible spectroscopy in solution, while atomic force microscopy (AFM) reveals a clear transformation from nanosheet-like structures to interconnected nanowires. Electrical measurements further showed a remarkable ~4 orders of magnitude enhancement in conductance compared to the non-templated Pt-1 complex, firmly establishing the role of Pt···Pt interactions in enabling network formation. Interestingly, this effect was not significantly evident from spectroscopic methods, but it was robustly captured in charge transport studies using our measurement setup. Control experiments reinforces these findings: replacing the labile chloro group (–Cl) did not enhance conductance, whereas increasing the denticity of the phosphate group produced a gradual increase in conductance (n = 1, 2, 3). Temperature-dependent current–voltage (I–V) measurements revealed a transition from temperature-independent tunneling to thermally activated hopping upon ATP modulation. Thermopower measurements further showed a negative Seebeck coefficient, consistent with LUMO-dominated charge transport, which
Abstract
10
was corroborated by UPS measurements indicating that the LUMO is the nearest orbital to the Fermi level. The dynamic character of these assemblies have further been demonstrated through transient states regulated by a potato apyrase (PA) enzyme–mediated negative feedback loop, allowing reversible control over the assembly and disassembly of interconnected nanowire network. Collectively, these results highlight ATP/GTP-templated Pt(II) supramolecular assemblies as recyclable, bioinspired electronic materials that combine molecular precision with adaptive functionality.
By integrating the findings of the first two experimental chapters, this work demonstrates how chemical stimuli can modulate supramolecular interactions to produce significant changes in conductance and thermopower. Building on this foundation, the thesis then extends to polymeric systems—intrinsically more heterogeneous and chemically inert—where cavity-confined quantum light is employed as an external stimulus to influence charge transport under vibrational strong coupling (VSC).
Vibrational strong coupling (VSC), where the confined cavity photons act as quantum light stimulus, is employed to study the charge transport in amorphous, non-conducting polymer thin films inside Fabry–Pérot cavities. Surprisingly, intrinsically insulating polymers—polystyrene (PS), poly(benzyl methacrylate) (PBMA), poly(4-methylstyrene) (P4MS), and poly(4-tert-butylstyrene) (P4TS)—displayed an extraordinary enhancement in electrical conductance, by six orders of magnitude, when the cavity thickness was tuned to specific vibrational modes, most prominently the aromatic out-of-plane bending mode of the benzene ring. The conductance response is clearly divisible into two regimes: on-resonance and off-resonance, governed by cavity path length. At on-resonance, where the cavity mode matches with the vibrational frequency, polymers exhibited a striking, temperature-independent conductance enhancement, demonstrating the pivotal role of VSC in charge transport. Conversely, off-resonance cavities lacking vibrational alignment and coupled at random pathlength and frequency, showed conventional semiconducting behavior with thermally activated charge transport behavior. The reproducibility of this phenomenon across several polymers and cavity conditions has been confirmed VSC as a universal route to modulate charge transport in otherwise insulating systems.
In-situ heating experiments provided further evidence of these distinct transport regimes. In on-resonance cavities, thermal expansion of polymer increases the cavity pathlength subsequently, detunes the cavity from resonance, producing a sharp decrease in conductance by at least two orders of magnitude. Upon cooling, the cavity returned to its original thickness, and the enhanced conductance was fully restored, highlighting the reversible and dynamic character of VSC. In contrast, off-resonance cavities exhibited only the expected thermally activated increase in conductance which is also reversible upon cooling.
These contrasting results indicates fundamentally different transport mechanisms: off-resonance systems followed a diffusive, thermally activated regime, whereas on-resonance cavities displays
Abstract
11
temperature-independent conductance suggestive of ballistic-like transport or even an insulator-to-metal transition. This extraordinary, path length– and temperature–independent conductance arises from the formation of collective delocalized states under VSC. Importantly, the enhancement was observed without external light excitation, showing that it originates from coherent interactions with the vacuum electromagnetic field.
Overall, these findings establish VSC as a transformative approach for controlling the charge transport properties of amorphous polymers, enabling long-range charge transport and paving the way for quantum light–mediated electronic devices.
In the final chapter, thermopower measurements of polystyrene under vibrational strong coupling (VSC) revealed a remarkable resonance-dependent transformation. When the cavity was tuned to the aromatic out-of-plane bending vibration, the Seebeck coefficient was low and positive (+3.6 μV/K) with temperature-independent conductance, consistent with p-type carriers, metal-like or ballistic transport. By contrast, off-resonance conditions produced large negative thermopower (–0.5 to –5 mV/K) and thermally activated, n-type carriers and diffusive transport typical of semiconducting polymers. These findings establish VSC as a reversible tool to modulate both carrier type and charge transport mechanism, offering a powerful strategy for controlling heat and charge flow in insulating polymer systems.
This thesis opens promising avenues at the intersection of molecular electronics, supramolecular chemistry, and quantum light–matter interactions. The supramolecular studies demonstrate how chemical stimuli can reorganize structures and modulate charge transport, suggesting pathways toward adaptive, bioinspired supramolecular assemblies for electronic applications. The findings on vibrational strong coupling (VSC) in polymers reveal a powerful, quantum light-based strategy to control conductance and thermopower, motivating deeper mechanistic studies and theoretical modelling. Together, these directions lay the groundwork for designing responsive molecular assemblies and quantum light–driven materials for next-generation electronic, sensing, and energy-harvesting devices. | en_US |
