Dual-gated single-molecule field-effect transistors beyond Moore's law

In light of the semiconductor industry’s acknowledgment that Moore’s law is nearing its end and the shift toward a “More than Moore” strategy emphasizing both miniaturization and increased functionality, single-molecule electronics emerges as a promising path to ultimate device scaling. A key milestone is a reliable unimolecular field-effect transistor (FET), since FETs are foundational to integrated circuits. This study aims to realize a solid-state single-molecule FET exhibiting a high on/off ratio suitable for practical applications and to enable remote control of performance via optical stimuli. Prior single-molecule transistors often suffered from low on/off ratios (generally <10) or required solution operation. The authors demonstrate dual-gated single-molecule transistors—combining electrical gating and molecular photoisomerization—based on a dinuclear ruthenium–diarylethene (Ru-DAE) complex covalently bridged between nanogapped graphene electrodes over an ultrathin high-k HfO2/Al2O3 dielectric on an Al back gate, achieving robust FET behavior with large modulation.

The work builds on prior demonstrations of molecular-scale devices and single-molecule transistors, noting limitations such as low on/off ratios and solution-based operation. Strategies to enhance on/off include exploiting destructive quantum interference (anti-resonance) and engineering current blockade through small molecular cores with weak electrode coupling. Graphene electrodes have been used to mitigate gate screening and enable stable covalent contacts, offering atomic thickness and robust chemistry compared to metal–thiol junctions. A central challenge in scaling to molecular channels is the short-channel effect, which can be addressed by using ultrathin high-k dielectrics to improve gate coupling. Prior reports have shown molecular orbital gating and Dirac-cone induced gating enhancement in graphene-based systems, motivating the use of HfO2/Al2O3 dielectrics and graphene contacts here. Diarylethene (DAE) molecular photoswitches are known to isomerize between ring-open and ring-closed forms with distinct conjugation and conductance, providing an optical control modality to complement electrical gating.

Device architecture: A single Ru-DAE molecule forms the transport channel, covalently linking nanogapped graphene source/drain electrodes via amide bonds. The back-gate stack comprises an Al gate with native Al2O3 plus a sol–gel-deposited HfO2 layer (each ~5 nm; total dielectric ~10 nm). Graphene serves as atomically thin electrodes to reduce gate screening and provide robust covalent contacts. Fabrication: Gate electrode arrays (8 nm Cr/60 nm Au) were patterned on Si/300 nm SiO2 by photolithography and thermal evaporation; Al was deposited and oxidized in air to Al2O3, followed by sol–gel HfO2 deposition to complete the dielectric. CVD-grown monolayer graphene was transferred onto HfO2. Source/drain metal arrays (8 nm Cr/60 nm Au) were defined by photolithography and evaporation. Nanogapped graphene point contacts terminated with carboxylic acids were formed using dash-line lithography. Ru-DAE molecules were covalently connected through carbodiimide coupling: molecules (≈10⁻⁴ M in anhydrous pyridine) with EDCI were reacted with freshly prepared devices under argon, in the dark, for two days; devices were then rinsed and dried. Characterization and measurements: Electrical measurements were performed under vacuum at 80 K using a Keysight B1500A and a Janis ST-500 probe station (liquid nitrogen cooling). For I–V, gate voltage steps were 10 mV; for I–t, integration time was 50 ms. Optical switching employed UV (380 nm) and visible (650 nm) irradiation to toggle DAE between ring-closed and ring-open isomers. Cross-sectional STEM and EDX validated the dielectric bilayer thickness and composition. Theory: DFT structural relaxations used VASP with PAW and PBE-GGA (350 eV cutoff; forces <0.05 eV/Å). Charge transport employed NEGF-DFT (Nanodcal) with double-zeta polarized basis, PBE-GGA, 1360 eV real-space grid cutoff; convergence threshold 1e-5 a.u. for Hamiltonian and density matrices. Transmission spectra were computed via Green’s functions. Gate voltage effects were included by self-consistently solving Poisson’s equation with appropriate boundary conditions. A single-level model was used to analyze peak bias evolution with gate.

• Robust solid-state single-molecule FETs were realized using a single Ru-DAE molecule between graphene electrodes over an ultrathin HfO2/Al2O3 dielectric on an Al back gate.
• Dual-gated functionality: Ru-DAE retains reversible photoisomerization (UV 380 nm to ring-closed, Vis 650 nm to ring-open), enabling optical switching of conductance; electrical gating modulates transport strongly, especially for the ring-open Ru-oDAE isomer.
• High performance: Transfer characteristics at V_D = 0.3 V show current modulation exceeding three orders of magnitude with negligible gate leakage (<10 pA). Junction leakage at zero gate is also ~10 pA. Effective molecular channel length ~3.1 nm enables ultralow power operation potential.
• Mechanism: For Ru-oDAE, a negative gate shifts the dominant p-HOMO toward the graphene Fermi level, transitioning from off-resonant tunneling (low current) to resonant transport (high current). Calculated zero-bias transmission spectra move upward in energy with increasingly negative V_G, reducing the energy gap in a nonlinear fashion at low |V_G|, consistent with measured I–V and transfer curves.
• Quantitative gate response: I_D–V_D maps acquired for V_G from 0 to −3.8 V (step 0.02 V) exhibit clear FET-like behavior. In single-level analysis, the conductance peak bias V_c decreases nonlinearly with V_G and disappears around V_G ≈ −3.5 V, indicating alignment of p-HOMO with the graphene Fermi level.
• Control comparison: Ru-cDAE shows an essentially different, nearly linear gate dependence of transmission, underscoring that intrinsic properties of Ru-oDAE drive the pronounced FET action.
• Reproducibility: FET behavior was observed across 9 working devices, though quantitative parameters vary due to atomic-scale gate interface variations.
• Materials choices matter: Graphene electrodes’ electronic structure (Dirac cone) and the ultrathin high-k dielectric bilayer enhance gate coupling and contribute to strong modulation with minimal leakage.

The study addresses the challenge of realizing practical unimolecular FETs by integrating a covalently tethered Ru-DAE molecular channel with graphene electrodes and an ultrathin high-k dielectric back-gate, achieving strong gate coupling and mitigating short-channel effects. The high on/off ratio (>10³) directly responds to prior limitations of single-molecule transistors. Dual-gated functionality—electrical gating and reversible photoisomerization—adds reconfigurability and control complexity for advanced circuit functions. Mechanistically, theory and experiment concur that for Ru-oDAE, gate-induced upward shifts of the p-HOMO toward the graphene Fermi level drive a transition from off-resonant to resonant transport, producing large conductance changes at relatively low gate voltages. The graphene electrodes further facilitate gating through their density-of-states characteristics, increasing conducting modes under gate bias. These results demonstrate that judicious molecular design (controlling conjugation and orbital alignment) combined with dielectric engineering can produce high-performance, solid-state single-molecule FETs compatible with integrated architectures.

This work demonstrates rare, high-performance FET behavior at the single-molecule level in a solid-state platform, integrating both electrical gating and reversible photoswitching within a single Ru-DAE molecular device. The combination of graphene electrodes and ultrathin high-k HfO2/Al2O3 dielectrics enables strong gate coupling, low leakage, and on/off ratios exceeding three orders of magnitude. The mechanistic insights into gate-tuned orbital alignment provide design rules for future molecular FETs. These advances suggest a pathway from laboratory demonstrations to scalable, ultraminiaturized, multifunctional molecular electronic circuits beyond Moore’s law. Future research may target room-temperature operation, further reduction of device-to-device variability, alternative molecular kernels with tailored orbital energetics, and integration into larger circuit architectures.

• Device-to-device variability: Atomic-scale variations at the solid gate–dielectric–electrode interfaces lead to quantitative differences in on/off ratios and FET parameters among devices.
• Operating conditions: Measurements were conducted under vacuum at 80 K; room-temperature performance and environmental stability are not established here.
• Sample size: While FET behavior was demonstrated in 9 working devices, broader statistical validation and yield metrics are not provided.
• Specificity of molecular state: The strongest FET behavior hinges on the Ru-oDAE isomer; performance depends on achieving and maintaining the targeted isomeric state.