Three-Dimensional Au Octahedral Nanoheptamers: Single-Particle and Bulk Near-Field Focusing for Surface-Enhanced Raman Scattering.

Herein, we present a synthetic approach to fabricate Au nanoheptamers composed of six individual Au nanospheres interconnected through thin metal bridges arranged in an octahedral configuration. The resulting structures envelop central Au nanospheres, producing Au nanosphere heptamers with an open architectural arrangement. Importantly, the initial Pt coating of the Au nanospheres is a crucial step for protecting the inner Au nanospheres during multiple reactions. As-synthesized Au nanoheptamers exhibit multiple hot spots formed by nanogaps between nanospheres, resulting in strong electromagnetic near-fields. Additionally, we conducted surface-enhanced Raman-scattering-based detection of a chemical warfare agent simulant in the gas phase and achieved a limit of detection of 100 ppb, which is 3 orders lower than that achieved using Au nanospheres and Au nanohexamers. This pseudocore-shell nanostructure represents a significant advancement in the realm of complex nanoparticle synthesis, moving the field one step closer to sophisticated nanoparticle engineering.

Adaptive Gap-Tunable Surface-Enhanced Raman Spectroscopy.

Gap plasmon (GP) resonance in static surface-enhanced Raman spectroscopy (SERS) structures is generally too narrow and not tunable. Here, we present an adaptive gap-tunable SERS device to selectively enhance and modulate different vibrational modes via active flexible Au nanogaps, with adaptive optical control. The tunability of GP resonance is up to ∼1200 cm-1 by engineering gap width, facilitated by mechanical bending of a polyethylene terephthalate substrate. We confirm that the tuned GP resonance selectively enhances different Raman spectral regions of the molecules. Additionally, we dynamically control the SERS intensity through the wavefront shaping of excitation beams. Furthermore, we demonstrate simulation results, exhibiting the mechanical and optical properties of a one-dimensional flexible nanogap and their advantage in high-speed biomedical sensing. Our work provides a unique approach for observing and controlling the enhanced chemical responses with dynamic tunability.

Plasmonic Annular Nanotrenches with 1 nm Nanogaps for Detection of SARS-CoV-2 Using SERS-Based Immunoassay.

This study represents the synthesis of a novel class of nanoparticles denoted as annular Au nanotrenches (AANTs). AANTs are engineered to possess embedded, narrow circular nanogaps with dimensions of approximately 1 nm, facilitating near-field focusing for detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) via a surface-enhanced Raman scattering (SERS)-based immunoassay. Notably, AANTs exhibited an exceedingly low limit of detection (LOD) of 1 fg/mL for SARS-CoV-2 spike glycoproteins, surpassing the commercially available enzyme-linked immunosorbent assay (ELISA) by 6 orders of magnitude (1 ng/mL from ELISA). To assess the real-world applicability, a study was conducted on 50 clinical samples using an SERS-based immunoassay with AANTs. The results revealed a sensitivity of 96% and a selectivity of 100%, demonstrating the significantly enhanced sensing capabilities of the proposed approach in comparison to ELISA and commercial lateral flow assay kits.

Electric-Field-Driven Localization of Molecular Nanowires in Wafer-Scale Nanogap Electrodes.

As integrated circuits continue to scale toward the atomic limit, bottom-up processes, such as epitaxial growth, have come to feature prominently in their fabrication. At the same time, chemistry has developed highly tunable molecular semiconductors that can perform the functions of ultimately scaled circuit components. Hybrid techniques that integrate programmable structures comprising molecular components into devices however are sorely lacking. Here we demonstrate a wafer-scale process that directs the localization of a conductive polymer, Mw = 20 kg mol-1 polyaniline, from dilute solutions into 50 nm vertical nanogap device architectures using electric-field-driven self-assembly. The resulting metal-polymer-metal junctions were characterized by electron microscopy, Raman spectroscopy and transport measurements demonstrating that our technique is highly selective, assembling conductive polymers only in electrically activated nanogaps. Our results represent a step toward scalable hybrid nanoelectronics that seamlessly integrate established lithographic top-down fabrication with bottom-up synthesized molecular functional circuit components.

Organic Nano-Junctions: Linking Nanomorphology and Charge Transport in Organic Semiconductor Nanoparticles for Organic Photovoltaic Devices.

In this study, innovative nanoscale devices are developed to investigate the charge transport in organic semiconductor nanoparticles. Using different steps of lithography techniques and dielectrophoresis, planar organic nano-junctions are fabricated from which hole mobilities are extracted in a space charge-limited current regime. Subsequently, these devices are used to investigate the impact of the composition and morphology of organic semiconductor nanoparticles on the charge mobilities. Pure donor nanoparticles and composite donor:acceptor nanoparticles with different donor compositions in their shell are inserted in the nanogap electrode to form the nano-junctions. The results highlight that the hole mobilities in the composite nanoparticles decrease by two-fold compared to pure donor nanoparticles. However, no significant change between the two kinds of composite nanoparticle morphologies is observed, indicating that conduction pathways for the holes are as efficient for donor proportion in the shell from 40% to 60%. Organic photovoltaic (OPV) devices are fabricated from water-based colloidal inks containing the two composite nanoparticles (P3HT:eh-IDTBR and P3HT:o-IDTBR) and no significant change in the performances is observed in accordance with the mobility results. Through this study, the performance of OPV devices have been succesfully correlated to the transport properties of nanoparticles having different morphology via innovative nanoscale devices.

Optimization of e-beam lithography parameters for nanofabrication of sub-50 nm gold nanowires and nanogaps based on a bilayer lift-off process.

Electron beam lithography (EBL) stands out as a powerful direct-write tool offering nanometer-scale patterning capability and is especially useful in low-volume R&D prototyping when coupled with pattern transfer approaches like etching or lift-off. Among pattern transfer approaches, lift-off is preferred particularly in research settings, as it is cost-effective and safe and does not require tailored wet/dry etch chemistries, fume hoods, and/or complex dry etch tools; all-in-all offering convenient, 'undercut-free' pattern transfer rendering it useful, especially for metallic layers and unique alloys with unknown etchant compatibility or low etch selectivity. Despite the widespread use of the lift-off technique and optical/EBL for micron to even sub-micron scales, existing reports in the literature on nanofabrication of metallic structures with critical dimension in the 10-20 nm regime with lift-off-based EBL patterning are either scattered, incomplete, or vary significantly in terms of experimental conditions, which calls for systematic process optimization. To address this issue, beyond what can be found in a typical photoresist datasheet, this paper reports a comprehensive study to calibrate EBL patterning of sub-50 nm metallic nanostructures including gold nanowires and nanogaps based on a lift-off process using bilayer polymethyl-methacrylate as the resist stack. The governing parameters in EBL, including exposure dose, soft-bake temperature, development time, developer solution, substrate type, and proximity effect are experimentally studied through more than 200 EBL runs, and optimal process conditions are determined by field emission scanning electron microscope imaging of the fabricated nanostructures reaching as small as 11 nm feature size.

Suppression of Resistive Coupling in Nanogap Electrochemical Cell: Resolution of Dual Pathways for Dopamine Oxidation.

A nanogap cell involves two working electrodes separated by a nanometer-wide solution to enable unprecedented electrochemical measurements. The powerful nanogap measurements, however, can be seriously interfered with by resistive coupling between the two electrodes to yield erroneous current responses. Herein, we employ the nanogap cell based on double carbon-fiber microelectrodes to suppress resistive coupling for the assessment of intrinsic current responses. Specifically, we modify a commercial bipotentiostat to compensate the Ohmic potential drop shared by the two electrodes through the common current pathway with a fixed resistance in the solution. Resistive coupling through both non-Faradaic and Faradaic processes is suppressed to eliminate erroneous current responses. Our approach is applied to investigate the mechanism of dopamine oxidation at carbon-fiber microelectrodes as important electrochemical sensors for the crucial neurotransmitter. Resistive coupling is suppressed to manifest the intrinsic current responses based on the oxidation of both adsorbed and non-adsorbed forms of dopamine to the respective forms of dopamine-o-quinone. The simultaneous dual oxidation pathways are observed for the first time and can be mediated through either non-concerted or concerted mechanisms of adsorption-coupled electron transfer. The two mechanisms are not discriminated for the two-electron oxidation of dopamine because it can not be determined whether the intermediate, dopamine semi-quinone, is adsorbed on the electrode surface. Significantly, our approach will be useful to manifest intrinsic current responses without resistive coupling for nanogaps and microgaps, which are too narrow to eliminate the common solution resistance by optimizing the position of a reference electrode.

Light-Emitting Electrochemical Cells Based on Nanogap Electrodes.

In a light-emitting electrochemical cell (LEC), electrochemical doping caused by mobile ions facilitates bipolar charge injection and recombination emissions for a high electroluminescence (EL) intensity at low driving voltages. We present the development of a nanogap LEC (i.e., nano-LEC) comprising a light-emitting polymer (F8BT) and an ionic liquid deposited on a gold nanogap electrode. The device demonstrated a high EL intensity at a wavelength of 540 nm corresponding to the emission peak of F8BT and a threshold voltage of ∼2 V at 300 K. Upon application of a constant voltage, the device demonstrated a gradual increase in current intensity followed by light emission. Notably, the delayed components of the current and EL were strongly suppressed at low temperatures (<285 K). The results clearly indicate that the device functions as an LEC and that the nano-LEC is a promising approach to realizing molecular-scale current-induced light sources.

How to Obtain the Correct Rabi Splitting in a Subwavelength Interacting System.

We unambiguously extract the individual decay channels of a coupled plasmon-exciton system by using correlated single-particle absorption and scattering measurements. A remarkable difference in the two channels is present─clear Rabi splitting in the plasmon channel but no Rabi splitting in the exciton channel. Discordance in the absorption and scattering spectra are mainly originated from the distinct contributions of plasmon and exciton channels in the absorption and scattering process. Our findings provide insights into plasmon-exciton interaction in an open cavity and can impact the design of plexcitonic devices for ultrafast nonlinear nanophotonics.

Bioinspired Plasmo-virus for Point-of-Care SARS-CoV-2 Detection.

Directly identifying the presence of the virus in infected hosts with an appropriate speed and sensitivity permits early epidemic management even during the presymptomatic incubation period of infection. Here, we synthesize a bioinspired plasmo-virus (BPV) particle for rapid and sensitive point-of-care (POC) detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) via a self-assembled plasmonic nanoprobe array on spike proteins. The BPV enables strong near-infrared (NIR) extinction peaks caused by plasmonic nanogaps. We quantify SARS-CoV-2 in viral transport medium (VTM) at low titers within 10 min with a limit of detection (LOD) of 1.4 × 101 pfu/mL, which is 103 times more sensitive than the current gold-standard method. The high-sensitivity and high-speed POC detection may be widely used for the timely, individualized diagnosis of infectious agents in low-resource settings.

More Than 30 000-fold Field Enhancement of Terahertz Nanoresonators Enabled by Rapid Inverse Design.

The rapid development of 6G communications using terahertz (THz) electromagnetic waves has created a demand for highly sensitive THz nanoresonators capable of detecting these waves. Among the potential candidates, THz nanogap loop arrays show promising characteristics but require significant computational resources for accurate simulation. This requirement arises because their unit cells are 10 times smaller than millimeter wavelengths, with nanogap regions that are 1 000 000 times smaller. To address this challenge, we propose a rapid inverse design method using physics-informed machine learning, employing double deep Q-learning with an analytical model of the THz nanogap loop array. In ∼39 h on a middle-level personal computer, our approach identifies the optimal structure through 200 000 iterations, achieving an experimental electric field enhancement of 32 000 at 0.2 THz, 300% stronger than prior results. Our analytical model-based approach significantly reduces the amount of computational resources required, offering a practical alternative to numerical simulation-based inverse design for THz nanodevices.

Nanoscale Channel Gate-Tunable Diodes Obtained by Asymmetric Contact and Adhesion Lithography on Fluoropolymers.

Adhesion lithography offers to fabrication of coplanar asymmetric nanogap electrodes with a low-cost and facile process. In this study, a gate-tunable diode with coplanar asymmetric nanogap is fabricated using adhesion lithography. A fluoropolymer material is introduced to the adhesion lithography process to ensure a manufacturing patterning process yield as high as 96.7%. The asymmetric electrodes formed a built-in potential, leading to rectifying behavior. The coplanar electrode structure allowed the use of a gate electrode in vertical contact with the channel, resulting in gate-tunable diode characteristics. The nanoscale channel induced a high current density (3.38 × 10-7  A∙cm-1 ), providing a high rectification ratio (1.67 × 105  A∙A-1 ). This rectifier diode is confirmed to operate with pulsed input signals and suggests the gate-tunability of nanogap diodes.

Nanoscopy through a plasmonic nanolens.

Plasmonics now delivers sensors capable of detecting single molecules. The emission enhancements and nanometer-scale optical confinement achieved by these metallic nanostructures vastly increase spectroscopic sensitivity, enabling real-time tracking. However, the interaction of light with such nanostructures typically loses all information about the spatial location of molecules within a plasmonic hot spot. Here, we show that ultrathin plasmonic nanogaps support complete mode sets which strongly influence the far-field emission patterns of embedded emitters and allow the reconstruction of dipole positions with 1-nm precision. Emitters in different locations radiate spots, rings, and askew halo images, arising from interference of 2 radiating antenna modes differently coupling light out of the nanogap, highlighting the imaging potential of these plasmonic "crystal balls." Emitters at the center are now found to live indefinitely, because they radiate so rapidly.

Ultrafast Lifetime and Bright Emission from Graphene Quantum Dots Using Plasmonic Nanogap Cavities.

Graphene quantum dots (GQDs) are quasi-zero-dimensional, carbon-based luminescent nanomaterials that possess desirable physical properties, such as high photostability, low cytotoxicity, good biocompatibility, and excellent water solubility; however, their long radiative lifetimes significantly limit their use in, e.g., light emitting devices where a fast spontaneous emission rate is essential. Despite a few reports on GQD fluorescence enhancements using metal nanostructures, studies of enhanced spontaneous emission rate remain outstanding. Here, we report fast and bright luminescence by coupling gap plasmon modes to nanoparticle emitters. Through precise control over the nanoparticle's local density of states (LDOS), we achieved a 220-fold increase in the PL intensity. The shortest radiative lifetime obtained was below 8.0 ps and limited by the instrument response, which is over 288-fold shorter than the lifetime of uncoupled GQDs. These findings may benefit the future development of rapid displays and open the possibility of constructing high-frequency classical or quantum telecommunication systems.

Review of the use of nanodevices to detect single molecules.

The development of methodologies to identify single molecules and/or to detect/monitor molecular behavior at the single-molecule level is one of the important research topics in chemistry and biology. In this review, we summarized the state-of-the-art of single molecule measurement methods and its latest applications using nanodevices integrated with molecular-size functional nanostructures, nanopores, nanogaps, and nanofluidic channels, which detect differences in chemical species, presence or absence of translational modifications, changes in steric structure, and changes in interactions between molecules. Besides these fundamental analytical achievements of molecular identification abilities, the latest applications include the single-molecule electrical sequencing, disease diagnosis, viral testing, single-molecule drug screening, and environmental monitoring. Finally, we added some discussion on the current status of single-molecule measurement as a method and technology to solve the problems to expand the future application needs of single-molecule measurement.

Recent advances in techniques for fabrication and characterization of nanogap biosensors: A review.

Nanogap biosensors have fascinated researchers due to their excellent electrical properties. Nanogap biosensors comprise three arrays of electrodes that form nanometer-size gaps. The sensing gaps have become the major building blocks of several sensing applications, including bio- and chemosensors. One of the advantages of nanogap biosensors is that they can be fabricated in nanoscale size for various downstream applications. Several studies have been conducted on nanogap biosensors, and nanogap biosensors exhibit potential material properties. The possibilities of combining these unique properties with a nanoscale-gapped device and electrical detection systems allow excellent and potential prospects in biomolecular detection. However, their fabrication is challenging as the gap is becoming smaller. It includes high-cost, low-yield, and surface phenomena to move a step closer to the routine fabrications. This review summarizes different feasible techniques in the fabrication of nanogap electrodes, such as preparation by self-assembly with both conventional and nonconventional approaches. This review also presents a comprehensive analysis of the fabrication, potential applications, history, and the current status of nanogap biosensors with a special focus on nanogap-mediated bio- and chemical sonsors.

Highly sensitive near-infrared SERS nanoprobes for in vivo imaging using gold-assembled silica nanoparticles with controllable nanogaps.

BACKGROUND: To take advantages, such as multiplex capacity, non-photobleaching property, and high sensitivity, of surface-enhanced Raman scattering (SERS)-based in vivo imaging, development of highly enhanced SERS nanoprobes in near-infrared (NIR) region is needed. A well-controlled morphology and biocompatibility are essential features of NIR SERS nanoprobes. Gold (Au)-assembled nanostructures with controllable nanogaps with highly enhanced SERS signals within multiple hotspots could be a breakthrough. RESULTS: Au-assembled silica (SiO2) nanoparticles (NPs) (SiO2@Au@Au NPs) as NIR SERS nanoprobes are synthesized using the seed-mediated growth method. SiO2@Au@Au NPs using six different sizes of Au NPs (SiO2@Au@Au50-SiO2@Au@Au500) were prepared by controlling the concentration of Au precursor in the growth step. The nanogaps between Au NPs on the SiO2 surface could be controlled from 4.16 to 0.98 nm by adjusting the concentration of Au precursor (hence increasing Au NP sizes), which resulted in the formation of effective SERS hotspots. SiO2@Au@Au500 NPs with a 0.98-nm gap showed a high SERS enhancement factor of approximately 3.8 × 106 under 785-nm photoexcitation. SiO2@Au@Au500 nanoprobes showed detectable in vivo SERS signals at a concentration of 16 µg/mL in animal tissue specimen at a depth of 7 mm. SiO2@Au@Au500 NPs with 14 different Raman label compounds exhibited distinct SERS signals upon subcutaneous injection into nude mice. CONCLUSIONS: SiO2@Au@Au NPs showed high potential for in vivo applications as multiplex nanoprobes with high SERS sensitivity in the NIR region.

Ultralow Voltage GaN Vacuum Nanodiodes in Air.

The III-nitride semiconductors have many attractive properties for field-emission vacuum electronics, including high thermal and chemical stability, low electron affinity, and high breakdown fields. Here, we report top-down fabricated gallium nitride (GaN)-based nanoscale vacuum electron diodes operable in air, with record ultralow turn-on voltages down to ∼0.24 V and stable high field-emission currents, tested up to several microamps for single-emitter devices. We leverage a scalable, top-down GaN nanofabrication method leading to damage-free and smooth surfaces. Gap-dependent and pressure-dependent studies provide new insights into the design of future, integrated nanogap vacuum electron devices. The results show promise for a new class of high-performance and robust, on-chip, III-nitride-based vacuum nanoelectronics operable in air or reduced vacuum.

High-Performance Vertical Organic Transistors of Sub-5 nm Channel Length.

Miniaturization of electronic circuits increases their overall performance. So far, electronics based on organic semiconductors has not played an important role in the miniaturization race. Here, we show the fabrication of liquid electrolyte gated vertical organic field effect transistors with channel lengths down to 2.4 nm. These ultrashort channel lengths are enabled by using insulating hexagonal boron nitride with atomically precise thickness and flatness as a spacer separating the vertically aligned source and drain electrodes. The transistors reveal promising electrical characteristics with output current densities of up to 2.95 MA cm-2 at -0.4 V bias, on-off ratios of up to 106, a steep subthreshold swing of down to 65 mV dec-1 and a transconductance of up to 714 S m-1. Realizing channel lengths in the sub-5 nm regime and operation voltages down to 100 µV proves the potential of organic semiconductors for future highly integrated or low power electronics.

Spin-Orbit Angular-Momentum Transfer from a Nanogap Surface Plasmon to a Trapped Nanodiamond.

The ability to control the motion of single nanoparticles or molecules is currently one of the major scientific and technological challenges. Despite tremendous progress in the field of plasmonic nanotweezers, controlled nanoscale manipulation of nanoparticles trapped by a plasmonic nanogap antenna has not been reported yet. Here, we demonstrate the controlled orbital rotation of a single fluorescent nanodiamond trapped by a gold trimer nanoantenna irradiated by a rotating linearly polarized light or circularly polarized light. Remarkably, the rotation direction is opposite to the light's polarization rotation. We numerically show that this inversion comes from sequential excitation of individual nanotriangles in the reverse order when the linear polarization is rotated, whereas using a circular polarization, light-nanoparticle angular momentum transfer occurs via the generation of a Poynting vector vortex of reversed handedness. This work provides a new path for the control of light-matter angular momentum transfer using plasmonic nanogap antennas.