Plasmonic Junction Research Articles

Efficient TE-polarized mode coupling between a plasmonic tunnel junction and a photonic waveguide

Efficient TE-polarized mode coupling between a plasmonic tunnel junction and a photonic waveguide

Optical Extinction-Based 3D Nano-Imaging of WS2 on Gold.

Broadband nanoextinction images recorded in tip-enhanced optical spectroscopy geometry track the 3D topography of a single layer of WS2 on Au substrate. The described nano-optical method is complementary to conventional atomic force microscopy and offers additional information about the buried material-metal interface that is not accessible using conventional topographic imaging. Beyond 3D optical imaging, we observe large variations in the junction plasmon resonance on the nanoscale. The latter is important to understand and account for in tip-enhanced Raman and photoluminescence studies that target low-dimensional materials specifically. Our observations and (coherent) optical scattering-based method are also relevant to emerging efforts aimed at exploring strong coupling and Fano interferences in hybrid plasmonic-low dimensional quantum material systems.

Plasmonic tandem heterojunctions enable high-efficiency charge transfer for broad spectrum photocatalytic hydrogen production

Rational engineering of semiconductor photocatalysts for efficient hydrogen production is of great significance but still challenging, primarily due to the limitations in charge transfer kinetics. Herein, a fascinating plasmonic tandem heterojunction with the hc-CdS/Mo2C@C heterostructure is aimfully prepared for effectively promoting the charge separation kinetics of the CdS photocatalyst via the synergistic strategy of phase junction, Schottky junction, and photothermal effect. The difference in atomic configuration between cubic-CdS (c-CdS) and hexagonal-CdS (h-CdS) leads to effective charge separation through a typical II charge transfer mechanism, and plasmonic Schottky junction further extracts the electrons in the hc-CdS phase junction to realize gradient charge transfer. Besides, the photothermal effect of Mo2C@C helps to expand the light absorption, accelerate charge transfer kinetics, and reduce the hydrogen evolution energy barrier. The carbon layer provides a fast channel for charge transfer and protects the photocatalyst from photocorrosion. As a result, the optimized hc-CMC photocatalyst exhibits a significantly high photocatalytic H2 production activity of 28.63 mmol/g/h and apparent quantum efficiency of 61.8%, surpassing most of the reported photocatalysts. This study provides a feasible strategy to enhance the charge transfer kinetics and photocatalytic activity of CdS by constructing plasmonic tandem heterogeneous junctions.

Rapidly photocatalytic degradation of toluene boosted by plasmonic effect and Schottky junction on Pt nanoparticles engineered self-supporting Cu2O nanowires

Photocatalytic degradation indoor toluene is one of the most environment-compatible strategies for manufacturing advanced air purifiers. A novel self-supporting photocatalysts, Pt/Cu2O nanowires growing from Cu mesh were constructed using energy-saving methods. The 5 nm Pt nanoparticles are well dispersed on Cu2O, establishing intimate Pt/Cu2O Schottky junction that curtails electron-hole recombination. Additionally, Pt nanoparticles contribute a plasmonic heating effect and an oscillating electric field for accelerating the photocatalytic rates and enhancing overall catalytic activity. By virtue of the Pt/Cu2O Schottky junction and plasmonic effect of the Pt nanoparticles, toluene is efficiently mineralized into CO2 and H2O. The optimal Pt/Cu2O catalyst achieve a toluene elimination rate of 93.65%, with the mineralization degree high up to 92.23%. This study confirms the practicability of the self-supporting Pt/Cu2O catalyst, which figures out new ideas for designing high-performance photocatalyst for removing typical and toxic volatile organic compounds.

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.

Fabrication of plasmonic junction diodes based on Ag@ZnO core-shell nanostructures

In this study, Ag nanoparticles and Ag@ZnO core–shell nanostructures were prepared using the wet chemical method and these nanostructures were used for Ag@ZnO/p-Si diode fabrication. Structural, morphological, and optical characterization techniques were applied for Ag@ZnO core–shell NPs prepared by using different molarity of precursor ZnCl2 (10 mM, 20 mM, 30 mM) and showed that the effect of increasing precursor amount on these physical properties of nanoparticles is important. For Ag@ZnO, transmission electron microscopy shows an average diameter of Ag nanoparticles was 51.32 nm and Ag@ZnO core–shell nanostructures were found to be between 31 and 92 nm. The UV-visible absorbance also shows significant plasmonic resonance for NPs, with a slight red shift increasing precursor molarity. The peaks are found to be from 412 nm to 432 nm. This redshift in surface plasmon absorption of Ag@ZnO core–shell structures are consistent with XPS survey. The current–voltage (I-V) characteristic curves of heterojunction diodes were taken in the dark and at room temperature, and it was observed that they showed a rectifying feature. Ideality factor and barrier height values have been found between 2.14 and 3.87, and 0.56 and 0.78, respectively. The results revealed that Ag@ZnO was successfully synthesized and can be used in rectification applications.

Upconversion electroluminescence in 2D semiconductors integrated with plasmonic tunnel junctions.

Plasmonic tunnel junctions are a unique electroluminescent system in which light emission occurs via an interplay between tunnelling electrons and plasmonic fields instead of electron-hole recombination as in conventional light-emitting diodes. It was previously shown that placing luminescent molecules in the tunneling pathway of nanoscopic tunnel junctions results in peculiar upconversion electroluminescence where the energy of emitted photons exceeds that of excitation electrons. Here we report the observation of upconversion electroluminescence in macroscopic van der Waals plasmonic tunnel junctions comprising gold and few-layer graphene electrodes separated by a ~2-nm-thick hexagonal boron nitride tunnel barrier and a monolayer semiconductor. We find that the semiconductor ground exciton emission is triggered at excitation electron energies lower than the semiconductor optical gap. Interestingly, this upconversion is reached in devices operating at a low conductance (<10-6 S) and low power density regime (<102 W cm-2), defying explanation through existing proposed mechanisms. By examining the scaling relationship between plasmonic and excitonic emission intensities, we elucidate the role of inelastic electron tunnelling dipoles that induce optically forbidden transitions in the few-layer graphene electrode and ultrafast hot carrier transfer across the van der Waals interface.

Controlling the Fluctuating Tip-Enhanced Raman Spectra of Chloramben on Silver Nanocubes.

Tip-enhanced Raman (TER) scattering from molecules residing at plasmonic junctions can be used to detect, identify, and image single molecules. This is most evident for flat molecules interrogated under conditions of extreme temperatures and pressure. It is also the case for (bio)molecular systems that feature preferred orientations/conformations under ambient laboratory conditions. More complex molecules that can adopt multiple conformations and/or feature different protonation or charge states give rise to complex TER spectra. We illustrate how the latter can be controlled in the case of chloramben molecules coated onto plasmonic silver nanocubes. We show that characteristic molecular Raman spectra cannot be obtained when tunneling plasmons are operative, i.e., when the tip is in direct contact with the chemically functionalized plasmonic nanoparticles. We rationalize these observations and propose an approach to less invasive and hence more analytical TER spectral imaging.

Determining Transmission Characteristics from Shot-Noise-Driven Electroluminescence in Single-Molecule Junctions.

Biased metal-molecule-metal junctions emit light through electroluminescence, a phenomenon at the intersection of molecular electronics and nanoplasmonics. This can occur when the junction plasmon mode is excited by inelastic electron current fluctuations. Here, we simultaneously measure the conductance and electroluminescence intensity from single-molecule junctions with time resolution in a solution environment at room temperature. We use current versus bias data to determine the molecular junction transport parameters and then relate these to the expected current shot noise. We find that the electroluminescence signal accurately matches the theoretical prediction of shot-noise-driven emission in a large fraction of the molecular junctions studied. This introduces a novel experimental method for qualitatively estimating finite-frequency shot noise in single-molecule junctions under ambient conditions. We further demonstrate that electroluminescence can be used to obtain the level alignment of the frontier orbital dominating transport in the molecular junction.

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.

Electroluminescence as a Probe of Strong Exciton-Plasmon Coupling in Few-Layer WSe2.

The manipulation of coupled quantum excitations is of fundamental importance in realizing novel photonic and optoelectronic devices. We use electroluminescence to probe plasmon-exciton coupling in hybrid structures consisting of a nanoscale plasmonic tunnel junction and few-layer two-dimensional transition-metal dichalcogenide transferred onto the junction. The resulting hybrid states act as a novel dielectric environment that affects the radiative recombination of hot carriers in the plasmonic nanostructure. We determine the plexcitonic spectrum from the electroluminescence and find Rabi splittings exceeding 50 meV in the strong coupling regime. Our experimental findings are supported by electromagnetic simulations that enable us to explore systematically and in detail the emergence of plexciton polaritons as well as the polarization characteristics of their far-field emission. Electroluminescence modulated by plexciton coupling provides potential applications for engineering compact photonic devices with tunable optical and electrical properties.

Gold nanorods with spatial separation of TiO2 deposition for plasmonic effect and Schottky junction enhanced antioxidant stress and hydrogen therapy of rheumatoid arthritis

Hydrogen (H2) therapy is a new and effective approach for treatment of inflammation-related diseases. Nevertheless, the non-polarity and poor solubility of H2 under physiological status lead to its limited therapeutic effect. Here, we developed spatially separated Au/TiO2 nanodumbbells (NDs) as H2 generators and further conjugated them with Arg-Gly-Asp (RGD) peptides (labeled R-Au/TiO2) to achieve inflammatory joint targeting for precise treatment of rheumatoid arthritis (RA). The Au/TiO2 NDs structure combines the surface plasmon resonance (SPR) excitation of gold nanorods (Au NRs) and Schottky junction to exhibit very efficient photocatalytic H2 production under near-infrared light irradiation. In situ generated H2 gas acts as an active ingredient to scavenge reactive oxygen species (ROS) and clear inflammation. Through active H2 scavenging of ROS and inflammation, R-Au/TiO2 alleviated oxidative stress and reduced levels of inflammatory cytokines, showing considerable reduction in joint destruction and suppression of arthritis severity in a collagen-induced arthritis mouse model. Furthermore, the photothermal properties of R-Au/TiO2 nanoparticles (NPs) endow them with photothermal therapeutic effects, which synergize with the anti-inflammatory and anti-oxidative stress of H2, further enhancing the therapeutic effect. Therefore, R-Au/TiO2 NPs show great promise for treating RA and advancing clinical translation.

Optical properties of plasmonic tunneling junctions.

Over the last century, quantum theories have revolutionized our understanding of material properties. One of the most striking quantum phenomena occurring in heterogeneous media is the quantum tunneling effect, where carriers can tunnel through potential barriers even if the barrier height exceeds the carrier energy. Interestingly, the tunneling process can be accompanied by the absorption or emission of light. In most tunneling junctions made of noble metal electrodes, these optical phenomena are governed by plasmonic modes, i.e., light-driven collective oscillations of surface electrons. In the emission process, plasmon excitation via inelastic tunneling electrons can improve the efficiency of photon generation, resulting in bright nanoscale optical sources. On the other hand, the incident light can affect the tunneling behavior of plasmonic junctions as well, leading to phenomena such as optical rectification and induced photocurrent. Thus, plasmonic tunneling junctions provide a rich platform for investigating light-matter interactions, paving the way for various applications, including nanoscale light sources, sensors, and chemical reactors. In this paper, we will introduce recent research progress and promising applications based on plasmonic tunneling junctions.

Potential controlled redox cycling of 4-aminothiophenol by coupling plasmon mediated chemical reaction with electrochemical reaction

The plasmon-mediated chemical reaction (PMCR) is a promising new direction for photocatalysis. By harvesting light through photon-electron interactions using nanostructures of plasmonic metals, chemical reactions of immobilized molecules on the metal surface can be activated. As demonstrated in recent years, the electrode potential (E) applied to the metal surface also provides a unique means for electrosynthesis by changing the local electrostatic field and controlling the electron transfer and redistribution in the molecules immobilized on the electrode surface. Here, as a case study, we studied the E-modulated PMCR of 4-aminothiophenol (4-ATP) confined in the nano junction formed between the gold nanoparticle (AuNP) and gold nanoelectrode (AuNE) by using the time-resolved electrochemical-surface-enhanced Raman spectroscopy (EC-SERS), a powerful tool to track the chemical reaction in-situ on the plasmonic metal surfaces in solution. We demonstrated that the negative E can effectively suppress the plasmon-mediated photo-oxidation of 4-ATP to 4,4-dimercaptoazobenzene (DMAB) and meanwhile activate the EC reduction of DMAB to 4-ATP. We are able to use E to regulate the competition between photo-oxidation and EC reduction and achieve the redox cycling between 4-ATP and DMAB in continuous E cycles. By analyzing the E-modulated dynamic SERS signal changes, we also found the applied E could affect the SERS changes through two intertwined nonreaction and reaction mechanisms. This approach can improve our understanding of the photon-electron-molecule interactions and the interplay between PMCR and EC reactions in plasmonic molecular junctions.

Sodium-Directed Photon-Induced Assembly Strategy for Preparing Multisite Catalysts with High Atomic Utilization Efficiency.

Integrating different reaction sites offers new prospects to address the difficulties in single-atom catalysis, but the precise regulation of active sites at the atomic level remains challenging. Here, we demonstrate a sodium-directed photon-induced assembly (SPA) strategy for boosting the atomic utilization efficiency of single-atom catalysts (SACs) by constructing multifarious Au sites on TiO2 substrate. Na+ was employed as the crucial cement to direct Au single atoms onto TiO2, while the light-induced electron transfer from excited TiO2 to Au(Na+) ensembles contributed to the self-assembly formation of Au nanoclusters. The synergism between plasmonic near-field and Schottky junction enabled the cascade electron transfer for charge separation, which was further enhanced by oxygen vacancies in TiO2. Our dual-site photocatalysts exhibited a nearly 2 orders of magnitude improvement in the hydrogen evolution activity under simulated solar light, with a striking turnover frequency (TOF) value of 1533 h-1 that exceeded other Au/TiO2-based photocatalysts reported. Our SPA strategy can be easily extended to prepare a wide range of metal-coupled nanostructures with enhanced performance for diverse catalytic reactions. Thus, this study provides a well-defined platform to extend the boundaries of SACs for multisite catalysis through harnessing metal-support interactions.

A chemically bonded and plasmonic Z-scheme junction for high-performance artificial photosynthesis of hydrogen peroxide

Chemically bonded and plasmonic Z-scheme junction as a model material was produced byin situassembly of nonstoichiometric W18O49(WO) onto two-dimensional carbon nitride nanosheets (CNs) for high-performance artificial photosynthesis of H2O2.

Topological surface plasmon resonance in deep subwavelength structure

We propose a plasmonic topological junction structure that combines topological robustness and subwavelength localization properties. The proposed structure is a junction of two topologically distinguished metal-insulator-metal waveguide gratings. Assuming Au–SiO2–Au waveguide gratings, we theoretically demonstrate a plasmonic Jackiw-Rebbi-state resonance which has substantially small effective mode-field area in the order of 0.1 μm2 at 1560 nm wavelength and requires only 10 periods for fully sustaining the ideal resonance Q-factor of the infinitely large array. Therefore, our proposed approach provides an efficient scheme for creating various plasmonic leaky-mode resonance components that can take advantages of spectral robustness and strong localization properties.

Nanoscale coherent phonon spectroscopy.

Coherent phonon spectroscopy can provide microscopic insight into ultrafast lattice dynamics and its coupling to other degrees of freedom under nonequilibrium conditions. Ultrafast optical spectroscopy is a well-established method to study coherent phonons, but the diffraction limit has hampered observing their local dynamics directly. Here, we demonstrate nanoscale coherent phonon spectroscopy using ultrafast laser–induced scanning tunneling microscopy in a plasmonic junction. Coherent phonons are locally excited in ultrathin zinc oxide films by the tightly confined plasmonic field and are probed via the photoinduced tunneling current through an electronic resonance of the zinc oxide film. Concurrently performed tip-enhanced Raman spectroscopy allows us to identify the involved phonon modes. In contrast to the Raman spectra, the phonon dynamics observed in coherent phonon spectroscopy exhibit strong nanoscale spatial variations that are correlated with the distribution of the electronic local density of states resolved by scanning tunneling spectroscopy.

Tuning Light Emission Crossovers in Atomic-Scale Aluminum Plasmonic Tunnel Junctions.

Atomic-sized plasmonic tunnel junctions are of fundamental interest, with great promise as the smallest on-chip light sources in various optoelectronic applications. Several mechanisms of light emission in electrically driven plasmonic tunnel junctions have been proposed, from single-electron or higher-order multielectron inelastic tunneling to recombination from a steady-state population of hot carriers. By progressively altering the tunneling conductance of an aluminum junction, we tune the dominant light emission mechanism through these possibilities for the first time, finding quantitative agreement with theory in each regime. Improved plasmonic resonances in the energy range of interest increase photon yields by 2 orders of magnitude. These results demonstrate that the dominant emission mechanism is set by a combination of tunneling rate, hot carrier relaxation time scales, and junction plasmonic properties.