Conventional silicon-based transistors are approaching their physical limits, necessitating the search for alternative materials and architectures. Single-molecule electronics offers a promising path towards miniaturization and enhanced functionality, aligning with the "More than Moore" strategy adopted by the semiconductor industry. A crucial step in realizing this potential is the development of reliable single-molecule FETs, as these form the fundamental building blocks of modern electronics. While previous single-molecule transistors have demonstrated limited on/off ratios and relied on solution-based operating conditions, this research aims to overcome these limitations by constructing a stable solid-state single-molecule FET with a significantly improved on/off ratio and remote controllability. This dual challenge involves achieving an on/off ratio suitable for practical applications and enabling remote control of this performance for advanced functionalities. The successful fabrication of such a device would represent a significant advance in the field, demonstrating the viability of single-molecule electronics as a complementary technology to current microelectronics.

The literature extensively discusses the challenges and opportunities presented by single-molecule electronics. Significant progress has been made in fabricating electronic devices based on individual molecules, with various approaches explored for creating molecular junctions. However, the development of high-performance single-molecule FETs has been hampered by low on/off ratios (generally <10) and solution-based operating conditions. Previous studies have demonstrated photo-controlled conductance switching using molecules, and strategies to improve on/off ratios include exploiting quantum interference effects and designing molecular structures with current blockade. These approaches have shown some success but haven't yet yielded the high on/off ratios needed for practical applications in integrated circuits. The use of graphene as electrodes has shown promise in reducing gate screening and creating more stable and reproducible junctions. Similarly, the use of high-k dielectric materials is crucial in mitigating the short channel effect which hampers the performance of miniaturized FETs.

This study utilizes a novel solid-state single-molecule FET architecture. Individual dinuclear Ru-DAE molecules, synthesized as described in the supplementary information, were covalently bonded to nanogapped graphene electrodes using a dehydration reaction involving 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDCI). Graphene electrodes were chosen for their atomic thickness, good compatibility with molecules, and robust covalent bonding, leading to more stable junctions compared to metal-sulfur bonds commonly used in other molecular junctions. High-k HfO2/Al2O3 dielectric layers were fabricated using thermal evaporation of Al followed by natural oxidation to form Al2O3 and sol-gel deposition of HfO2. This bilayer structure, approximately 10nm thick, ensured efficient gate modulation and negligible leakage currents. The fabrication process involved photolithography and thermal evaporation for patterning the gate and source/drain electrodes, followed by the transfer of CVD-grown single-layer graphene onto the dielectric layer. Nanogapped graphene point contacts were created using a dash-line lithographic method before molecular integration. Device characterization was performed under vacuum at 80 K using a Keysight B1500A semiconductor characterization system and a ST-500-Probe station. Current-voltage (ID-VD) curves were measured at varying gate voltages (VG), and transfer characteristics were obtained by plotting current at a fixed VD as a function of VG. Theoretical calculations using density functional theory (DFT) within the non-equilibrium Green's function (NEGF) formalism were conducted to understand the charge transport mechanisms and the effect of gate voltage on molecular orbitals. These calculations used the Vienna Ab initio Simulation Package (VASP) and the quantum transport package Nanodcal.

The fabricated single-molecule FETs demonstrated a remarkable on/off ratio exceeding three orders of magnitude, significantly surpassing the performance of previously reported single-molecule transistors. The Ru-DAE molecules exhibited reversible photoswitching behavior, with conductance switching observed upon exposure to ultraviolet and visible light. This photoswitching was confirmed both in the solid-state device and in solution measurements. Gate-dependent charge transport was observed in the Ru-oDAE (ring-open) form, with current modulation exceeding three orders of magnitude via a gate voltage variation. Negligible gate and junction leakage currents were observed, suggesting potential for ultralow power consumption. Theoretical calculations confirmed the experimental findings and provided insights into the underlying mechanisms. The gate voltage effectively shifts the molecular orbital energy levels relative to the graphene Fermi level, resulting in the observed conductance modulation. Notably, the ring-open isomer (Ru-oDAE) demonstrated superior FET characteristics compared to the ring-closed isomer (Ru-cDAE). The on/off ratio observed was consistent across multiple devices, suggesting reproducibility and reliability of the fabrication method. The combination of photoswitching and high-performance FET behavior in a single device showcases the potential of single-molecule electronics for creating multifunctional nanocircuits.

The findings address the long-standing challenge of achieving high-performance single-molecule FETs suitable for practical applications. The remarkable on/off ratio observed, exceeding three orders of magnitude, clearly demonstrates the potential for single-molecule devices to compete with conventional silicon-based transistors. The integration of photoswitching functionality further enhances the versatility and potential applications of these devices. The significant gate-dependent current modulation and negligible leakage currents highlight the energy efficiency of this technology. The theoretical calculations provide a firm foundation for understanding the observed behavior, validating the design principles and paving the way for further optimization. This work demonstrates a significant step towards the realization of molecular-scale integrated circuits, potentially offering a path to circumvent the limitations of Moore's law and enabling the creation of advanced multifunctional electronic devices.

This research successfully demonstrated a high-performance, dual-gated single-molecule FET with a remarkable on/off ratio exceeding three orders of magnitude. The device also exhibits reversible photoswitching, combining two crucial active electronic elements into a single molecular architecture. The robust fabrication method, combined with the exceptional device performance, highlights the potential of single-molecule electronics for developing future generation electronics beyond Moore's law. Future research could explore different molecular structures and dielectric materials to further optimize device performance and expand functional capabilities. Integration of these single-molecule FETs into larger circuits and exploring their applications in various fields, such as sensing and memory, are also promising areas for future investigation.

The study was conducted under vacuum at 80 K, which may not fully reflect the performance under ambient conditions. Although the fabrication method demonstrated reproducibility, further optimization could be explored to improve yield and reduce device-to-device variations. While the theoretical calculations support the experimental findings, they involve approximations and assumptions that may affect the accuracy of the predictions. The long-term stability of the molecular junctions under operating conditions also requires further investigation.