Molecular electronics and electrochemistry: the case of ferrocene-tagged Interfaces and of organic molecular junctions studied by time-dependent methods

• Ano de defesa: 2023
• Autor(a) principal: Sánchez, Yuliana Pérez [UNESP]
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Universidade Estadual Paulista (Unesp)
• Palavras-chave em Português: Charge transfer; Impedance spectroscopy; Nanoeletronics; Tunneling; Quantum electrodynamics; Espectroscopia de impedância; Nanoeletrônica; Tunelamento; Eletrodinâmica quântica
• Link de acesso: https://hdl.handle.net/11449/251082

The understanding of electron transmittance, including electron transfer (ET) and transport, is essential not only for comprehending biological systems but also for developing electrical components in the pursuit of fabricating electronic devices. The quantum rate theory proposes a first-principle fundamental quantum rate ν = e2/hCq, where e is the elementary charge, h is the Planck constant and Cq is the quantum capacitance that informs on the density-of-states (DOS) as Cq ∝ e2DOS. This quantum rate concept is utilized to elucidate the ET rate constant k of electrochemical reaction as k = G/Cμ, where G is the quantum of conductance and Cμ is the electrochemical capacitance. This thesis presents a fundamental study, within the framework of the quantum rate theory, of the ET and transport processes in two kinds of molecular junctions (MJ): electroactive self-assembled monolayer (SAM) over electrodes and aromatic monolayers sandwiched between carbon electrodes. Time-dependent methods, such as impedance spectroscopy (IS) and electrochemical impedance-derived capacitive spectroscopy (ECS), were employed for this study. The impedance data were analyzed by equivalent circuit fitting, specifically as quantum resistive capacitive (RC) point contact. Redox SAMs were formed using Ferrocene-tagged peptides and thiol molecules. These redox SAMs feature Cq, which was determined by ECS, enabling the determination of k as = 2(G0/Cq), where G0 represents the universally known constant for quantum conductance, approximately 77.5 μS. The reciprocal of G0 defines the quantum resistance as Rq = 1/G0 ∼ 12,9 kΩ, establishing a quantum limit for ET. This demonstrates that the electrodynamics is governed by Rq and Cq elements. Furthermore, it was determined that Rq comprises two resistive components connected in series: the relaxation resistance (Rqt) and the combined resistance of solution and contact (Rs), leading to Rq ∼ 13 kΩ. Consequently, the interpretation of Rq within the quantum rate model prompted a reassessment of the meaning assigned to the charge transfer resistance Rct, revealing that Rq is equivalent to Rct. Moreover, it was demonstrated that regardless of the molecular backbone, the Cq states of the redox interface mediate the ET between electroactive free species and the electrode, conforming to maximum efficiency of electron transmittance. Specifically, it was proven that an increase in the number of quantum channels N, and consequently of Cq by almost 40%, was achieved through suitable energy level alignment between the level states of the free redox species with that of the DOS of the redox SAM. This exceptional setting allowed for the amplification of the Cq signal for the detection of a biological target. As a proof-of-concept, this amplification mechanism was applied for sensing the nonstructural protein 1 (NS1) biomarker of the dengue virus, resulting in an observed increase in sensitivity of redox capacitive biosensors by at least 1,000. However, it was evidenced that the acidity of the quantum capacitive interface affects the electron transfer mediation as a result of the electrostatic repulsion between the negatively charged surface and redox charged species. Furthermore, the determination of properties G and Cμ using SI measurement of the aromatic MJs demonstrated that G and k are correlated by Cμ, implying that the associated physical concepts studied separately by the electrochemist and physics are inherently related. The results obtained in this thesis demonstrate that the quantum rate model is a suitable theory enable to address phenomena involved with electron transmittance such as electron transfer, quantum conductance, and capacitance in different MJs.