Nanogap Electrodes Research Articles

Pt Nanogap Electrode Fabrication by Two-Layer Lift-Off UV-NIL and Nanowire Breakdown

Nanogap electrodes are expected to aid the study of the electrical properties of single molecules and nanoparticles, and also have been applied to nonvolatile memory. In this study, we developed a process to fabricate nanogap electrodes by combining ultraviolet nanoimprint lithography (UV-NIL) and a nanowire-breakdown technique. A pattern of Pt/Ti nanowires with a width of 300 nm was formed on a silicon oxide layer by two-layer lift-off UV-NIL using an alkali-soluble polymer. A Pt nanogap with a distance of couple of nanometers was fabricated by controlling the step voltage applied to the nanowire electrodes. We demonstrated the switching behavior of the Pt nanogap. It was confirmed that switching operations could be performed for 100 cycles and a high resistance ratio of over 102 was obtained.

Simultaneous arrayed formation of single-electron transistors using electromigration in series-connected nanogaps

A field-emission-induced electromigration method (activation) is reported for integrating single-electron transistors operating at T = 298 K. The field emission currents between the two opposite electrodes of each series-connected nanogap are tuned to accumulate Ni atoms within the gaps. For ten series-connected nanogaps, the resistance (VD/ID), obtained using the current-voltage (ID-VD) properties of these nanogaps during the activation procedure, is observed to decrease on activation. As a result, island structures are formed within the gaps, and the nanogap-based single-electron transistors can be integrated, when atom migration occurs at the tip of each nanogap electrode. After activating the ten series-connected nanogaps with a preset current, IS = 1 nA, current suppression (representative of coulomb blockade) is not observed in the fabricated devices. On the other hand, coulomb blockade, which depicts the charging and discharging of the nanoislands, can be observed at room temperature, after activation with a preset current, IS = 150 nA. Furthermore, the modulation properties of the coulomb blockade voltage by the gate voltage are also determined at room temperature. These results experimentally demonstrate the arrayed formation of ten single-electron transistors operating at room temperature, constituting a significant step toward the practical realization of single-electron-transistor-based systems.

Nanogap-Based Electrochemical Measurements at Double-Carbon-Fiber Ultramicroelectrodes.

Electrochemical measurements with unprecedentedly high sensitivity, selectivity, and kinetic resolution have been enabled by a pair of electrodes separated by a nanometer-wide gap. The fabrication of nanogap electrodes, however, requires extensive nanolithography or nanoscale electrode positioning, thereby preventing the full exploration of this powerful method in electrode design and application. Herein, we report the simple fabrication of double-carbon-fiber ultramicroelectrodes (UMEs) with nanometer-wide gaps not only to facilitate nanogap-based electrochemical measurements but also to gain high time resolution, signal-to-background ratio, and kinetic selectivity for dopamine against ascorbic acid. Specifically, ∼7 μm-diameter carbon fibers are inserted into a double-bore glass capillary, heat-pulled, and milled by focused ion-beam technology to yield ∼50 μm-long double-cylinder UMEs. The redox cycling of the Ru(NH3)63+/2+ couple across a nanogap between voltammetric generator and amperometric collector electrodes reaches quasi-steady states at fast scan rates of 100 V/s as demonstrated experimentally and even 1000 V/s as predicted theoretically. The transient background of the amperometric collector response is suppressed ∼100 times in comparison with that of the voltammetric generator response. Nanogap voltammograms based on the collector response against the cycled generator potential are quantitatively analyzed without background subtraction to reproducibly yield nanogap widths of ∼0.18 μm and a standard electron-transfer rate constant of 0.9 cm/s. Moreover, nanogap-mediated redox cycling can be initiated by dopamine oxidation at the generator electrode to largely improve the dopamine selectivity of the collector response against ascorbic acid, which is also oxidized at the generator electrode to immediately and irreversibly produce a redox-inactive species.

Flexible nanogap polymer light-emitting diodes fabricated via adhesion lithography (a-Lith)

We report the development of coplanar green colour organic light-emitting diodes (OLEDs) based on asymmetric nanogap electrodes fabricated on different substrates including glass and plastic. Using adhesion lithography (a-Lith) we pattern Al and Au layers acting as the cathode and anode electrodes, respectively, separated by an inter-electrode distance of <15 nm with an aspect ratio of up to 106. Spin-coating the organic light-emitting polymer poly(9,9-dioctylfluorene-alt-bithiophene) (F8T2) on top of the asymmetric Al–Au nanogap electrodes results in green light-emitting nanogap OLEDs with promising operating characteristics. We show that the scaling of the OLED’s width from 4 to 200 mm can substantially improve the light output of the device without any adverse effects on the manufacturing yield. Furthermore, it is found that the light-emitting properties in the nanogap area differ from the bulk organic film, an effect attributed to confinement of the conjugated polymer chains in the nanogap channel. These results render a-Lith particularly attractive for low cost facile fabrication of nanoscale light-emitting sources and arrays on different substrates of arbitrary size.

A wide-bandwidth tri-axial pendulum accelerometer with fully-differential nano-gap electrodes

This paper presents the implementation and characterization of a single proof-mass wide-bandwidth pendulum accelerometer with fully-differential sensitivity in all three axes. The utilization of nano-gap electrodes (300 nm) boosts the electromechanical coupling significantly so that both increased sensitivity and high resonant frequency (fres > 15 kHz) can be attained, which extends the operational bandwidth as well as the dynamic range of the device using a miniaturized proof-mass of 450 × 450 µm2. Furthermore, increased air-damping from the nano-gap structure enables the stable operation of the device at moderate vacuum environment (1–10 Torr), which paves the way toward the single-chip inertial measurement unit by integrating a quasi-static accelerometer with resonant gyroscopes on a common substrate. The presented design is fabricated on a 40 µm thick silicon-on-insulator substrate and wafer-level vacuum-packaged with a TSV substrate at a moderate vacuum level (1–10 Torr). The device, interfaced with a switched capacitor ASIC exhibits sub-milli-g noise levels in a wide bandwidth exceeding 10 kHz and bias instability of 560 µg, 428.16 µg, 145 µg for the X, Y and Z axes, respectively.

Scalable Manufacturing of Nanogaps.

The ability to manufacture a nanogap in between two electrodes has proven a powerful catalyst for scientific discoveries in nanoscience and molecular electronics. A wide range of bottom-up and top-down methodologies are now available to fabricate nanogaps that are less than 10 nm wide. However, most available techniques involve time-consuming serial processes that are not compatible with large-scale manufacturing of nanogap devices. The scalable manufacturing of sub-10 nm gaps remains a great technological challenge that currently hinders both experimental nanoscience and the prospects for commercial exploitation of nanogap devices. Here, available nanogap fabrication methodologies are reviewed and a detailed comparison of their merits is provided, with special focus on large-scale and reproducible manufacturing of nanogaps. The most promising approaches that could achieve a breakthrough in research and commercial applications are identified. Emerging scalable nanogap manufacturing methodologies will ultimately enable applications with high scientific and societal impact, including high-speed whole genome sequencing, electromechanical computing, and molecular electronics using nanogap electrodes.

Scalable Nanogap Sensors for Non-Redox Enzyme Assays.

Clinical diagnostic assays that monitor redox enzyme activity are widely used in small, low-cost readout devices for point-of-care monitoring (e.g., a glucometer); however, monitoring non-redox enzymes in real-time using compact electronic devices remains a challenge. We address this problem by using a highly scalable nanogap sensor array to observe electrochemical signals generated by a model non-redox enzyme system, the DNA polymerase-catalyzed incorporation of four modified, redox-tagged nucleotides. Using deoxynucleoside triphosphates (dNTPs) tagged with para-aminophenyl monophosphate (pAPP) to form pAP-deoxyribonucleoside tetra-phosphates (AP-dN4Ps), incorporation of the nucleotide analogs by DNA polymerase results in the release of redox inactive pAP-triphosphates (pAPP3) that are converted to redox active small molecules para-aminophenol (pAP) in the presence of phosphatase. In this work, cyclic enzymatic reactions that generated many copies of pAP at each base incorporation site of a DNA template in combination with the highly confined nature of the planar nanogap transducers ( z = 50 nm) produced electrochemical signals that were amplified up to 100,000×. We observed that the maximum signal level and amplification level were dependent on a combination of factors including the base structure of the incorporated nucleotide analogs, nanogap electrode materials, and electrode surface coating. In addition, electrochemical signal amplification by redox cycling in the nanogap is independent of the in-plane geometry of the transducer, thus allowing the nanogap sensors to be highly scalable. Finally, when the DNA template concentration was constrained, the DNA polymerase assay exhibited different zero-order reaction kinetics for each type of base incorporation reaction, resolving the closely related nucleotide analogs.

Thermal robustness evaluation of nonvolatile memory using Pt nanogaps

We investigated the thermal robustness of a nonvolatile memory using polycrystalline platinum (Pt) nanogap electrodes. The temperature dependences of resistance states were evaluated from room temperature to 773 K. At high temperatures, the resistance of the high-resistance state (HRS) was slightly altered as the temperature changed. This slight alteration could be neglected, and the thermal robustness was improved by etching a SiO2 layer just under the nanogap. This indicated that the thermal alteration was caused by current leakage through the SiO2 layer. The nonvolatile memory employing Pt nanogaps is expected to be potentially useful as a thermally robust memory up to 773 K.

Tailoring Single-Cycle Near Field in a Tunnel Junction with Carrier-Envelope Phase-Controlled Terahertz Electric Fields.

Light-field-driven processes occurring under conditions far beyond the diffraction limit of the light can be manipulated by harnessing spatiotemporally tunable near fields. A tailor-made carrier envelope phase in a tunnel junction formed between nanogap electrodes allows precisely controlled manipulation of these processes. In particular, the characterization and active control of near fields in a tunnel junction are essential for advancing elaborate manipulation of light-field-driven processes at the atomic-scale. Here, we demonstrate that desirable phase-controlled near fields can be produced in a tunnel junction via terahertz scanning tunneling microscopy (THz-STM) with a phase shifter. Measurements of the phase-resolved subcycle electron tunneling dynamics revealed an unexpected large carrier-envelope phase shift between far-field and near-field single-cycle THz waveforms. The phase shift stems from the wavelength-scale feature of the tip-sample configuration. By using a dual-phase double-pulse scheme, the electron tunneling was coherently manipulated over the femtosecond time scale. Our new prescription-in situ tailoring of single-cycle THz near fields in a tunnel junction-will offer unprecedented control of electrons for ultrafast atomic-scale electronics and metrology.

Large-area plastic nanogap electronics enabled by adhesion lithography

Large-area manufacturing of flexible nanoscale electronics has long been sought by the printed electronics industry. However, the lack of a robust, reliable, high throughput and low-cost technique that is capable of delivering high-performance functional devices has hitherto hindered commercial exploitation. Herein we report on the extensive range of capabilities presented by adhesion lithography (a-Lith), an innovative patterning technique for the fabrication of coplanar nanogap electrodes with arbitrarily large aspect ratio. We use this technique to fabricate a plethora of nanoscale electronic devices based on symmetric and asymmetric coplanar electrodes separated by a nanogap < 15 nm. We show that functional devices including self-aligned-gate transistors, radio frequency diodes and rectifying circuits, multi-colour organic light-emitting nanodiodes and multilevel non-volatile memory devices, can be fabricated in a facile manner with minimum process complexity on a range of substrates. The compatibility of the formed nanogap electrodes with a wide range of solution processable semiconductors and substrate materials renders a-Lith highly attractive for the manufacturing of large-area nanoscale opto/electronics on arbitrary size and shape substrates.

Evaluation of Nanoimprinting Multilayer Lift-off Process using Spin-on-glass for Nanogap Electrode Array

Evaluation of Nanoimprinting Multilayer Lift-off Process using Spin-on-glass for Nanogap Electrode Array

Novel fabrication of extremely high aspect ratio and straight nanogap and array nanogap electrodes

We reported a reproducible, simple and novel method for fabricating electrodes with high aspect ratio and highly straight nanometer size gap. The gap size could be controlled in a wide range between several nm up to several hundreds of nm, although in this work our individual gap size was 172 nm. In this method a single nanofiber was aligned as a sacrificial structure using a secondary electrical field and its diameter lowered through thermal treatment to obtain smallest gap size. Then, after deposition of blanket metal, nanofiber was removed obtaining nano scale gap. We predict the application of these simple fabricated gaps for diverse fields. The expensive, time consuming, none-reproducible and low accuracy fabrication methods can supersede by this cost effective, facile and fast method. It can provide mass production for precisely positioned nanoscale gaps, ignoring electrode and substrate materials. The electrodes hold the advantage of maximum straightness comparing to other similar nano fabrication methods. Fabricated gaps have large aspect ratio (in the order of 106). Also array gaps with average gap size of 400 nm were fabricated. Same size gaps could be fabricated in a row by this method. Different parameters in alignment were taken into account and studied. The process shows great potential for facile and batch fabrication of parallel submicron gaps for diverse applications including gas sensing, bio-sensing and nanofluidic systems.

Label-Free Dynamic Detection of Single-Molecule Nucleophilic-Substitution Reactions.

The mechanisms of chemical reactions, including the transformation pathways of the electronic and geometric structures of molecules, are crucial for comprehending the essence and developing new chemistry. However, it is extremely difficult to realize at the single-molecule level. Here, we report a single-molecule approach capable of electrically probing stochastic fluctuations under equilibrium conditions and elucidating time trajectories of single species in non-equilibrated systems. Through molecular engineering, a single molecular wire containing a functional center of 9-phenyl-9-fluorenol was covalently wired into nanogapped graphene electrodes to form stable single-molecule junctions. Both experimental and theoretical studies consistently demonstrate and interpret the direct measurement of the formation dynamics of individual carbocation intermediates with a strong solvent dependence in a nucleophilic-substitution reaction. We also show the kinetic process of competitive transitions between acetate and bromide species, which is inevitable through a carbocation intermediate, confirming the classical mechanism. This unique method creates plenty of opportunities for carrying out single-molecule dynamics or biophysics investigations in broad fields beyond reaction chemistry through molecular design and engineering.

Batch fabrication of nanogap electrodes arrays with controllable cracking for hydrogen sensing

Hydrogen sensor based on Pd nanogap arrays was fabricated using the swelling induced cracking method. The square hole Pd array is patterned on the polymer to define the position of the nanogaps. We discuss the influence of the size and the shape of the holes on the formed nanogaps. The structural and morphological characteristics of the metal array and the nanogaps were investigated using laser scanning confocal microscopy (LSCM) and scanning electron microscopy (SEM). To illustrate an application of the nanogap structure, the Pd nanogap array was deposited on epoxy and polyimide (PI) substrate and used as H2 sensor, showing a good sensitivity and stability at room temperature. Moreover, the performance of the nanogap array on PI substrate remains stable after annealing at 80–180 °C. This effect is attributed to the high thermal stability of the PI substrate. The effective fabrication method, good sensitivity and the high stability of the Pd nanogap arrays show promising application for H2 sensing.

Hot-electron photodetection based on embedded asymmetric nano-gap electrodes

Hot electrons generated from the nonradiative decay of surface plasmons can be employed in photodetection. However, only a small percentage of these hot electrons can be collected. In this paper, a type of hot-electron photodetector based on embedded asymmetric nano-gap electrodes is proposed which can enhance hot-electron collection efficiency. Due to this structure, the device can achieve responsivities as high as 3.75 mA/W and 1.58 mA/W for wavelengths of 1310 nm and 1550 nm, respectively. These insights can enhance efficiencies and lower cost in optical communication systems.

Field emission and explosive electron emission process in focused ion beam fabricated platinum and tungsten three-dimensional overhanging nanostructure

Abstract Three-dimensional platinum and tungsten overhanging nanogap (∼70 nm) electrodes are fabricated on a glass substrate using focused ion beam milling and chemical vapour deposition processes. Current-voltage (I-V) characteristics of the devices measured at a pressure of ∼10−6 mbar shows space-charge emission followed by the Fowler-Nordheim (F-N) field emission. After the F-N emission, the system enters into an explosive emission process, at a higher voltage generating a huge current. We observe a sharp and abrupt rise in the emission current which marks the transition from the F-N emission to the explosive emission state. The explosive emission process is destructive in nature and yields micro-/nano-size spherical metal particles. The chemical compositions and the size-distribution of such particles are performed.

Direct single-molecule dynamic detection of chemical reactions.

Single-molecule detection can reveal time trajectories and reaction pathways of individual intermediates/transition states in chemical reactions and biological processes, which is of fundamental importance to elucidate their intrinsic mechanisms. We present a reliable, label-free single-molecule approach that allows us to directly explore the dynamic process of basic chemical reactions at the single-event level by using stable graphene-molecule single-molecule junctions. These junctions are constructed by covalently connecting a single molecule with a 9-fluorenone center to nanogapped graphene electrodes. For the first time, real-time single-molecule electrical measurements unambiguously show reproducible large-amplitude two-level fluctuations that are highly dependent on solvent environments in a nucleophilic addition reaction of hydroxylamine to a carbonyl group. Both theoretical simulations and ensemble experiments prove that this observation originates from the reversible transition between the reactant and a new intermediate state within a time scale of a few microseconds. These investigations open up a new route that is able to be immediately applied to probe fast single-molecule physics or biophysics with high time resolution, making an important contribution to broad fields beyond reaction chemistry.

Electrical Response Characteristics of Electrolyte Solutions Using Micro- and Nanogap Electrodes

Electrical Response Characteristics of Electrolyte Solutions Using Micro- and Nanogap Electrodes

Deep Ultraviolet Copper(I) Thiocyanate (CuSCN) Photodetectors Based on Coplanar Nanogap Electrodes Fabricated via Adhesion Lithography.

Adhesion lithography (a-Lith) is a versatile fabrication technique used to produce asymmetric coplanar electrodes separated by a <15 nm nanogap. Here, we use a-Lith to fabricate deep ultraviolet (DUV) photodetectors by combining coplanar asymmetric nanogap electrode architectures (Au/Al) with solution-processable wide-band-gap (3.5-3.9 eV) p-type semiconductor copper(I) thiocyanate (CuSCN). Because of the device's unique architecture, the detectors exhibit high responsivity (≈79 A W-1) and photosensitivity (≈720) when illuminated with a DUV-range (λpeak = 280 nm) light-emitting diode at 220 μW cm-2. Interestingly, the photosensitivity of the photodetectors remains fairly high (≈7) even at illuminating intensities down to 0.2 μW cm-2. The scalability of the a-Lith process combined with the unique properties of CuSCN paves the way to new forms of inexpensive, yet high-performance, photodetectors that can be manufactured on arbitrary substrate materials including plastic.

Enhancement of discrete changes in resistance in engineered VO2 heterointerface nanowall wire

The nanoconfinement effect in three-dimensional (3D) space was investigated for strongly correlated VO2. Three-dimensional VO2 nanowall wires (nws) with an engineered heterointerface were grown on the side-surface of 3D patterned MgO. A 100-nm-wide VO2 nw 2 µm in length and 100 nm in height showed multistep changes in resistance during the insulator–metal transition owing to confinement of 150 insulator and metal domains in the nw. We showed that further confinement along the longitudinal direction of a nanowall using a nanogap electrode, that is, a reduction of the number of domains to 10, led to much steeper step changes.