Remote-Controllable Interfacial Electron Tunneling at Heterogeneous Molecular Junctions via Tip-Induced Optoelectrical Engineering

Abstract

Molecular electronics enables functional electronic behavior via single molecules or molecular self-assembled monolayers, providing versatile opportunities for hybrid molecular-scale electronic devices. Although various molecular junction structures are constructed to investigate charge transfer dynamics, significant challenges remain in terms of interfacial charging effects and far-field background signals, which dominantly block the optoelectrical observation of interfacial charge transfer dynamics. Here, tip-induced optoelectrical engineering is presented that synergistically correlates photo-induced force microscopy and Kelvin probe force microscopy to remotely control and probe the interfacial charge transfer dynamics with sub-10 nm spatial resolution. Based on this approach, the optoelectrical origin of metal-molecule interfaces is clearly revealed by the nanoscale heterogeneity of the tip-sample interaction and optoelectrical reactivity, which theoretically aligned with density functional theory calculations. For a practical device-scale demonstration of tip-induced optoelectrical engineering, interfacial tunneling is remotely controlled at a 4-inch wafer-scale metal-insulator-metal capacitor, facilitating a 5.211-fold current amplification with the tip-induced electrical field. In conclusion, tip-induced optoelectrical engineering provides a novel strategy to comprehensively understand interfacial charge transfer dynamics and a non-destructive tunneling control platform that enables real-time and real-space investigation of ultrathin hybrid molecular systems.

Keywords: DFT calculation; Kelvin probe force microscopy; interfacial charge transfer; molecular tunneling junction; photo-induced force microscopy.

Figure 1

Remote‐controllable interfacial tunneling dynamics at SAMs‐based tunneling junctions via tip‐induced optoelectrical engineering. A) Schematic diagram of experimental design of tip‐induced optoelectrical engineering based on PiFM IR‐excitation and KPFM polarization. B) CAFM current‐voltage curve of sulfur‐anchored (2A5MT, PTT) and nitrogen‐anchored (3ATZ) self‐assembled domain, exhibiting the intrinsic charge transfer properties of molecule. XPS N 1s spectra of C) PTT, D) 3ATZ, E) 2A5MT, and S 2p spectra of F) PTT, G) 2A5MT, which clarifies the evident formation of covalent bonding, activating the hybrid tunneling states at the metal‐molecule interfaces.

Figure 2

DFT‐calculated atomic‐scale characterization of metal‐molecule interfaces. A) Schematic of the self‐assembly process on the Cu (111) surface and the simulated atomic charge distribution. DFT‐calculated electrochemical properties of B) 2A5MT, C) 3ATZ, and D) PTT, including atomic charge distribution, HOMO‐LUMO gap, and dipole moment. The isosurface value for HOMO and LUMO is set at 0.03 e Bohr⁻³. DFT calculated atomic charge distribution theoretically correlates with the heterogeneous tunneling dynamics between Cu─S bonding and Cu─N bonding, which attributed to the interface dipole and electron push/pull effects. The isosurface value of differential charge distribution is 0.015 e Bohr⁻³.

Figure 3

Polarization‐dependent heterogeneity of interfacial electron tunneling dynamics. A) Schematic illustration of KPFM tip‐induced polarization switching and polarization‐dependent charge transfer mechanism. Nanoscale heterogeneity in B) work function shift, C) phase shift, and E) tip oscillation amplitude, which dominantly attributed to polarization‐dependent electrostatic interaction and atomic charge distribution. D) Energy level diagram of metal (Cu)‐molecule‐dielectric barrier (air)‐AFM tip (Pt) junction, describing the mechanism that charge transfer to the AFM tip through the tunneling gap.

Figure 4

Near‐field spatio‐spectroscopic heterogeneity of interfacial tunneling dynamics. A) Schematic illustration of PiFM IR‐excitation switching and photo‐induced charge transfer mechanism. B) Nanoscale spatio‐spectroscopic mapping of morphologies, PiFM IR spectra, and photo‐induced force at the specific wavenumber, which exhibits the highest difference of optoelectrical response. Multidimensional near‐field heterogeneity of C) PiFM signal intensity shift and D) phase shift, E) tip oscillation amplitude, which is accumulatively induced by near‐field electrostatic interaction and photo‐induced charge transfer.

Figure 5

Tip‐induced tunneling control of 4‐inch wafer‐scale vertical MIM capacitor. A) Photography of fabricated 4‐inch wafer‐scale MIM capacitor. B) Schematic diagram of MIM capacitor structure, consist of metal electrode (Cu)‐insulator (2A5MT)‐metal electrode (Cu), enabling tunable capacitance based on 2A5MT monolayers. C) Cross‐sectional HR‐TEM image, D) OM image, and E) corresponding EDS elemental mapping of vertical MIM capacitor (Cu‐2A5MT‐Cu). F) Schematic illustration of remote tunnelling control platform through Cu─S bonding at Cu‐2A5MT‐Cu heterointerfaces. G) CV curve of different voltage sweep range, scanning the −1–+1 V (left) range and −2–+2 V (right) range. Tip‐induced electrical field remarkably enlarges the hysteresis, which is attributed to interfacial electron tunneling to the 2A5MT monolayer.