Ultrafast Electron Tunneling Devices-From Electric-Field Driven to Optical-Field Driven

Light Emission and Conductance Fluctuations in Electrically Driven and Plasmonically Enhanced Molecular Junctions

Electrically connected and plasmonically enhanced molecular junctions combine the optical functionalities of high field confinement and enhancement (cavity function), and of high radiative efficiency (antenna function) with the electrical functionalities of molecular transport. Such combined optical and electrical probes have proven useful for the fundamental understanding of metal-molecule contacts and contribute to the development of nanoscale optoelectronic devices including ultrafast electronics and nanosensors. Here, we employ a self-assembled metal-molecule-metal junction with a nanoparticle bridge to investigate correlated fluctuations in conductance and tunneling-induced light emission at room temperature. Despite the presence of hundreds of molecules in the junction, the electrical conductance and light emission are both highly sensitive to atomic-scale fluctuations-a phenomenology reminiscent of picocavities observed in Raman scattering and of luminescence blinking from photoexcited plasmonic junctions. Discrete steps in conductance associated with fluctuating emission intensities through the multiple plasmonic modes of the junction are consistent with a finite number of randomly localized, point-like sources dominating the optoelectronic response. Contrasting with these microscopic fluctuations, the overall plasmonic and electronic functionalities of our devices feature long-term survival at room temperature and under an electrical bias of a few volts, allowing for measurements over several months.

Analytical Model of Optical-Field-Driven Subcycle Electron Tunneling Pulses from Two-Dimensional Materials

We develop analytical models of optical-field-driven electron tunneling from the edge and surface of free-standing two-dimensional (2D) materials. We discover a universal scaling between the tunneling current density (J) and the electric field near the barrier (F): In(J/|F|β) ∝ 1/|F| with β values of 3/2 and 1 for edge emission and vertical surface emission, respectively. At ultrahigh values of F, the current density exhibits an unexpected high-field saturation effect due to the reduced dimensionality of the 2D material, which is absent in the traditional bulk material. Our calculation reveals the dc bias as an efficient method for modulating the optical-field tunneling subcycle emission characteristics. Importantly, our model is in excellent agreement with a recent experiment on graphene. Our results offer a useful framework for understanding optical-field tunneling emission from 2D materials, which are helpful for the development of optoelectronics and emerging petahertz vacuum nanoelectronics.

Light-Emitting Plasmonic Tunneling Junctions: Current Status and Perspectives

Quantum tunneling, in which electrons can tunnel through a finite potential barrier while simultaneously interacting with other matter excitation, is one of the most fascinating phenomena without classical correspondence. In an extremely thin metallic nanogap, the deep-subwavelength-confined plasmon modes can be directly excited by the inelastically tunneling electrons driven by an externally applied voltage. Light emission via inelastic tunneling possesses a great potential application for next-generation light sources, with great superiority of ultracompact integration, large bandwidth, and ultrafast response. In this Perspective, we first briefly introduce the mechanism of plasmon generation in the inelastic electron tunneling process. Then the state of the art in plasmonic tunneling junctions will be reviewed, particularly emphasizing efficiency improvement, precise construction, active control, and electrically driven optical antenna integration. Ultimately, we forecast some promising and critical prospects that require further investigation.

Engineering the Outcoupling Pathways in Plasmonic Tunnel Junctions via Photonic Mode Dispersion for Low-Loss Waveguiding

Outcoupling of plasmonic modes excited by inelastic electron tunneling (IET) across plasmonic tunnel junctions (TJs) has attracted significant attention due to low operating voltages and fast excitation rates. Achieving selectivity among various outcoupling channels, however, remains a challenging task. Employing nanoscale antennas to enhance the local density of optical states (LDOS) associated with specific outcoupling channels partially addressed the problem, along with the integration of conducting 2D materials into TJs, improving the outcoupling to guided modes with particular momentum. The disadvantage of such methods is that they often involve complex fabrication steps and lack fine-tuning options. Here, we propose an alternative approach by modifying the dielectric medium surrounding TJs. By employing a simple multilayer substrate with a specific permittivity combination for the TJs under study, we show that it is possible to optimize mode selectivity in outcoupling to a plasmonic or a photonic-like mode characterized by distinct cutoff behaviors and propagation length. Theoretical and experimental results obtained with a SiO2-SiN-glass multilayer substrate demonstrate high relative coupling efficiencies of (62.77 ± 1.74)% and (29.07 ± 0.72)% for plasmonic and photonic-like modes, respectively. The figure-of-merit, which quantifies the tradeoff between mode outcoupling and propagation lengths (tens of μm) for both modes, can reach values as high as 180 and 140. The demonstrated approach allows LDOS engineering and customized TJ device performance, which are seamlessly integrated with standard thin film fabrication protocols. Our experimental device is well-suited for integration with silicon nitride photonics platforms.

Vacuum Tunneling Transistor with Nano Vacuum Chamber for Harsh Environments

A nano vacuum tube which consists of a vacuum transistor and a nano vacuum chamber was demonstrated. For the device, a vacuum region is an electron transport channel, and a vacuum is a tunneling barrier. Tilted angle evaporation was studied for the formation of the nano level vacuum chamber structure. This vacuum tube was ultraminiaturized with several tens of 10-18 L scale volume and 10-6 Torr of pressure. The device structure made it possible to achieve a high integration density and to sustain the vacuum state in various real operations. In particular, the vacuum transistor performed stably in extreme external environments because the tunneling mechanism showed a wide range of working stability. The vacuum was sustained well by the sealing layer and provided a defect-free tunneling junction. In tests, the high vacuum level was maintained for more than 15 months with high reliability. The Al sealing layer and tube structure can effectively block exposed light such as visible light and UV, enabling the stable operation of the tunneling transistor. In addition, it is estimated that the structure blocks approximately 5 keV of X-ray. The device showed stable operating characteristics in a wide temperature range of 100-390 K. Therefore, the vacuum tube can be used in a wide range of applications involving integrated circuits while resolving the disadvantages of a large volume in old vacuum tubes. Additionally, it can be an important solution for next-generation devices in various fields such as aerospace, artificial intelligence, and THz applications.

High Responsivity Vacuum Nano-Photodiode Using Single-Crystal CsPbBr3 Micro-Sheet

Field electron emission vacuum photodiode is promising for converting free-space electromagnetic radiation into electronic signal within an ultrafast timescale due to the ballistic electron transport in its vacuum channel. However, the low photoelectric conversion efficiency still hinders the popularity of vacuum photodiode. Here, we report an on-chip integrated vacuum nano-photodiode constructed from a Si-tip anode and a single-crystal CsPbBr₃ cathode with a nano-separation of ~30 nm. Benefiting from the nanoscale vacuum channel and the high surface work function of the CsPbBr₃ (4.55 eV), the vacuum nano-photodiode exhibits a low driving voltage of 15 V with an ultra-low dark current (50 pA). The vacuum nano-photodiode demonstrates a high photo responsivity (1.75 AW^-1@15 V) under the illumination of a 532-nm laser light. The estimated external quantum efficiency is up to 400%. The electrostatic field simulation indicates that the CsPbBr₃ cathode can be totally depleted at an optimal thickness. The large built-in electric field in the depletion region facilitates the dissociation of photoexcited electron-hole pairs, leading to an enhanced photoelectric conversion efficiency. Moreover, the voltage drop in the vacuum channel increases due to the photoconductive effect, which is beneficial to the narrowing of the vacuum barrier for more efficient electron tunneling. This device shows great promise for the development of highly sensitive perovskite-based vacuum opto-electronics.