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

Abstract

As conventional silicon-based transistors are fast approaching the physical limit, it is essential to seek alternative candidates, which should be compatible with or even replace microelectronics in the future. Here, we report a robust solid-state single-molecule field-effect transistor architecture using graphene source/drain electrodes and a metal back-gate electrode. The transistor is constructed by a single dinuclear ruthenium-diarylethene (Ru-DAE) complex, acting as the conducting channel, connecting covalently with nanogapped graphene electrodes, providing field-effect behaviors with a maximum on/off ratio exceeding three orders of magnitude. Use of ultrathin high-k metal oxides as the dielectric layers is key in successfully achieving such a high performance. Additionally, Ru-DAE preserves its intrinsic photoisomerisation property, which enables a reversible photoswitching function. Both experimental and theoretical results demonstrate these distinct dual-gated behaviors consistently at the single-molecule level, which helps to develop the different technology for creation of practical ultraminiaturised functional electrical circuits beyond Moore's law.

Fig. 1. Device diagrams of a graphene-Ru-DAE-graphene single-molecule FET.

a Schematic representation of the device structure. Bottom: Atomic force microscopic image of nanogapped graphene point contacts with the bottom gate. Top: Schematic of the device center that highlights the reversible isomerisation of the DAE unit between ring-open and ring-closed forms that are triggered by optical stimuli. b Optical images of a graphene-Ru-DAE-graphene single-molecule FET array with a common bottom gate based on a HfO₂/Al₂O₃/Al multilayer. The inset shows the complete pattern, where the central region marked by a red circle is enlarged for clarity. c Left: Cross-sectional scanning transmission electron microscope (STEM) image of the HfO₂/Al₂O₃/Al multilayer structure. The sample was prepared using a focused ion beam and imaged using the STEM (200 kV). Right: Analyses of the elemental compositions of the dielectric layer, which includes hafnium, oxygen, and aluminum, performed using an energy-dispersive X-ray spectroscopy system. These characterizations show that the thickness of both the Al₂O₃ and HfO₂ layers is ~5 nm.

Fig. 2. Reversible photoswitching of graphene-Ru-DAE-graphene single-molecule junctions.

a Real-time measurement of the current passing through a diarylethene molecule that switches reversibly between its ring-closed and ring-open forms upon exposure to alternate ultraviolet (UV: 380 nm) and visible (Vis: 650 nm) irradiations. Drain voltage V_D = 300 mV and gate voltage V_G = 0 V. The region with the purple background is under UV irradiation. b Transmission spectra of graphene-Ru-DAE-graphene single-molecule junctions with ring-open (dark) and ring-closed (red) isomers. Reprinted with permission from ref. . Copyright 2021 American Chemical Society.

Fig. 3. Gate-controllable charge transport in Ru-oDAE single-molecule transistors.

a Two-dimensional visualization of I_D vs. V_G and V_D. b Representative I_D–V_D curves for different values of V_G. c Transfer characteristics for the Ru-oDAE single-molecule FET at V_D = 0.3 V.

Fig. 4. Working mechanism of the Ru-oDAE single-molecule FETs.

a Gate-dependent zero-bias transmission spectra at −2.0 V ≤ V_G ≤ 0 V with steps of 0.5 V. The downward triangles mark the p-HOMO for each case. b Energy gaps between the p-HOMO and the graphene Fermi level at various gate voltages. c Schematic energetic diagram showing the alignments between the molecular orbitals (red and blue lines) and the density of graphene electrodes in Ru-oDAE single-molecule transistors under application of different gate voltages.