Nanogap Electrodes Research Articles

Electrochemistry in Micro- and Nanochannels Controlled by Streaming Potentials.

Fluid and charge transport in micro- and nanoscale fluidic systems are intrinsically coupled via electrokinetic phenomena. While electroosmotic flows and streaming potentials are well understood for externally imposed stimuli, charge injection at electrodes localized inside fluidic systems via electrochemical processes remains to a large degree unexplored. Here, we employ ultramicroelectrodes and nanogap electrodes to study the subtle interplay between ohmic drops, streaming currents, and faradaic processes in miniaturized channels at low concentrations of supporting electrolyte. We show that electroosmosis can, under favorable circumstances, counteract the effect of ohmic losses and shift the apparent formal potential of redox reactions. This interplay can be described by simple circuit models, such that the results described here can be adapted to other micro- and nanofluidic electrochemical systems.

Nanogap Electrodes: Direct Deposited Angstrom‐Scale Nanogap Electrodes with Macroscopically Measurable and Material‐Independent Capabilities for Various Applications (Adv. Mater. Technol. 12/2019)

In article number 1900641, Yi Luo and co-workers successfully fabricate angstrom-scale nanogap electrodes (NGEs) with micrometer-scale pads. NGEs with conducting, semiconducting and insulating materials are fabricated with low-cost direct-deposition technology which is totally lithography-free and CMOS-compatible. The macroscopically measurable and material-independent NGEs could bring scientists from various research areas to explore the molecular and even atomic worlds.

Single-molecule single-electron transistor (SM-SET) based on π-conjugated quinoidal-fused oligosilole and heteroepitaxial spherical Au/Pt nanogap electrodes

A single-molecule single-electron transistor (SM-SET) consisting of a π-conjugated quinoidal-fused oligosilole derivative, Si-2, of radius 0.75 nm and heteroepitaxial spherical (HS) Au/Pt nanogap electrodes is demonstrated. Si-2 with two inner silicon atoms and two ethylenethiol groups at both ends is chemisorbed by an S–Au bond between HS-Au/Pt nanogap electrodes fabricated by combining electron-beam lithography and electroless Au plating. An ideal Coulomb staircase, Coulomb oscillation, and Coulomb diamond were observed using the 1.5 nm Si-2 SM-SET, which were supported by theoretical results based on the orthodox model.

Heteroepitaxial spherical electroless Au-plated Pt-based nanogap electrodes of radius 5 nm and gap separation 0.7 nm

Heteroepitaxial growth is demonstrated by electroless Au plating (ELGP) on polycrystalline Pt surface and initial ultrafine-linewidth Pt nanogap electrodes, confirmed by scanning transmission electron microscope cross-sectional images, atomic-resolution scanning electron microscope images, and energy-dispersive X-ray spectroscopy elemental maps. The gap between a pair of spherical multidomain heteroepitaxial Au on Pt electrodes of radius 5 nm is controlled to a value as small as 0.7 nm by the self-termination mechanism of ELGP without short circuit. The heteroepitaxial spherical Au/Pt nanogap electrodes are found to be robust against annealing at temperatures up to 573 K.

Direct Deposited Angstrom‐Scale Nanogap Electrodes with Macroscopically Measurable and Material‐Independent Capabilities for Various Applications

AbstractNanogap electrodes (NGEs) with decreasing separations have received growing attention due to their potential in stimulating extensive applications with high scientific impact, including molecular electronics, quantum tunneling, plasmonic optics, and highly sensitive sequencing. Although various NGE‐manufacturing methods have been developed and offer promising results, angstrom‐scale NGEs have not been achieved without requiring sophisticated equipment, restricting materials or substrates, compromising mass‐fabrication compatibility, or/and failing to connect with macroscopic contacts. Here, macroscopically measurable and material‐independent NGEs with separations as low as to 6 angstroms have been achieved through a direct‐deposition technology that simultaneously addresses almost all of the issues mentioned above, highlighting the versatility of the formed NGEs, thus unlocking their full potential for application. The potential application in molecular electronics has been demonstrated experimentally. Importantly, the simple, low‐cost, lithography‐free and CMOS‐compatible fabrication technology will enable a breakthrough in NGE fabrication, enhancing the development of NGE‐related applications in various research fields.

Revealing Charge‐ and Temperature‐Dependent Movement Dynamics and Mechanism of Individual Molecular Machines

AbstractSingle‐molecule detection is able to reveal rich spatial/temporal information and elucidate intrinsic mechanisms of inter or intramolecular interactions that are not accessible in ensemble experiments, which is of fundamental importance to solve the key issues in physical, chemical, and biological sciences. Here, a robust, label‐free single‐molecule electrical approach capable of directly exploring the detailed dynamic process of stochastic movement of alkyl chains bearing different charges through a macrocycle in a molecular machine at the single‐event level is represented by using stable graphene–molecule–graphene single‐molecule junctions (GMG‐SMJs). These junctions are built by covalently sandwiching a pseudorotaxane featuring a permethylated α‐cyclodextrin between nanogapped graphene electrodes. In situ long‐term single‐molecule electrical measurements unambiguously show reproducible large‐amplitude multiple‐level fluctuations that are highly dependent on charge and temperature. Both theoretical simulations and experimental data prove that this observation originates from random motions of alkyl chains along the axle within a time scale of a few milliseconds. Observation of transient changes in current signals by gating effects allows direct real‐time monitoring of the submolecular translational process. These investigations conceptualize the capability of GMG‐SMJs to probe fast single‐molecule chemical reactions or behaviors.

Nanogap Lithography: Spontaneous Formation of Nanogap Electrodes by Self‐Peeling Adhesion Lithography (Adv. Mater. Interfaces 17/2019)

Nanogap Lithography: Spontaneous Formation of Nanogap Electrodes by Self‐Peeling Adhesion Lithography (Adv. Mater. Interfaces 17/2019)

Nanotransfer-printed stamps allow fabrication of top-contact nanogap electrodes

Molecular beam epitaxy and nanotransfer-printing pave way for depositing electrodes directly on top of nanoscale objects, providing a way to integrate the electrodes with external testing.Molecular beam epitaxy and nanotransfer-printing pave way for depositing electrodes directly on top of nanoscale objects, providing a way to integrate the electrodes with external testing.

One-step transfer printing of patterned nanogap electrodes

Direct printing of nanogap-separated metallic contact pairs is described that enables novel nanoelectronic device architectures. Nanotransfer printing (nTP) stamps are grown by molecular beam epitaxy involving layered III-V semiconductors that are selectively etched. Finished stamps comprise both the nanoscale surface trench that becomes the nanogap on printing and a microscale, predetermined geometry that affords the simultaneous integration of contact pads for external electrical testing. This nTP technique is well suited for top-contacting sensitive thin films for electrical characterization; a typical electrode configuration is illustrated by transfer-printed 13 nm thin metal films that are separated by an electrically insulating gap of ca. 30 nm.

Spontaneous Formation of Nanogap Electrodes by Self‐Peeling Adhesion Lithography

AbstractAdhesion lithography (“a‐lith”) is a simple method for forming nanoscale gaps between dissimilar metals. In its usual form, a metal is patterned on a substrate, and conformally coated with an alkyl‐functionalized self‐assembled monolayer, rendering it nonadhesive to other metals; a second metal is then deposited uniformly over the full area of the substrate; finally, the parts of the second metal that are in contact with the self‐assembled monolayer are stripped away using an adhesive tape or film, leaving the first and second metals side‐by‐side on the substrate, with a nanoscale spacing between them. It is shown here that, by depositing onto the second metal an adhesive film with high internal strain, it is possible to induce spontaneous delamination of the peeling layer without the need for any applied force. The modified procedure simplifies implementation and eliminates external stresses that can cause unwanted widening of the gap. The resultant electrode separations of ≈10 nm are amongst the smallest values achieved to date using adhesion lithography.

Batch-Fabricated α-Si Assisted Nanogap Tunneling Junctions.

This paper details the design, fabrication, and characterization of highly uniform batch-fabricated sidewall etched vertical nanogap tunneling junctions for bio-sensing applications. The device consists of two vertically stacked gold electrodes separated by a partially etched sacrificial spacer layer of sputtered α-Si and Atomic Layer Deposited (ALD) SiO2. A ~10 nm wide air-gap is formed along the sidewall by a controlled dry etch of the spacer. The thickness of the spacer layer can be tuned by adjusting the number of ALD cycles. The rigorous statistical characterization of the ultra-thin spacer films has also been performed. We fabricated nanogap electrodes under two design layouts with different overlap areas and spacer gaps, from ~4.0 nm to ~9.0 nm. Optical measurements reported an average non-uniformity of 0.46 nm (~8%) and 0.56 nm (~30%) in SiO2 and α-Si film thickness respectively. Direct tunneling and Fowler–Nordheim tunneling measurements were done and the barrier potential of the spacer stack was determined to be ~3.5 eV. I–V measurements showed a maximum resistance of 46 × 103 GΩ and the average dielectric breakdown field of the spacer stack was experimentally determined to be ~11 MV/cm.

High Responsivity and Response Speed Single‐Layer Mixed‐Cation Lead Mixed‐Halide Perovskite Photodetectors Based on Nanogap Electrodes Manufactured on Large‐Area Rigid and Flexible Substrates

AbstractMixed‐cation lead mixed‐halide perovskites are employed as the photoactive material in single‐layer solution‐processed photodetectors fabricated with coplanar asymmetric nanogap Al–Au and indium tin oxide–Al electrodes. The nanogap electrodes, bearing an interelectrode distance of ≈10 nm, are patterned via adhesion lithography, a simple, low‐cost, and high‐throughput technique. Different electrode shapes and sizes are demonstrated on glass and flexible plastic substrates, effectively engineering the device architecture, and, along with perovskite film and material optimization, paving the way toward devices with tunable operational characteristics. The optimized coplanar nanogap junction perovskite photodetectors show responsivities up to 33 A W−1, specific detectivity on the order of 1011 Jones, and response times below 260 ns, while retaining a low dark current (0.3 nA) under −2 V reverse bias. These values outperform the vast majority of perovskite photodetectors reported so far, while avoiding the complicated fabrication steps involved in conventional multilayer device structures. This work highlights the promising potential of the proposed asymmetric nanogap electrode architecture for application in the field of flexible optoelectronics.

(Invited) Fast-Scan Nanogap Voltammetry at Double-Carbon-Fiber Ultramciroelectrodes

In this presentation, we discuss theory [1] and experiment [2] of double-carbon-fiber (CF) ultramicroelectrodes (UMEs) with nanometer-wide gaps to voltammetrically obtain high time resolution, signal-to-background ratio, and kinetic selectivity for dopamine (DA) against ascorbic acid (AA). The novel nanogap-based electrodes are fabricated by using a pair of ~7 µm-diameter CFs, which 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 separated by a nanometer-wide gap without extensive nanolithography or nanoscale electrode positioning required for other nanogap electrodes. Double-CF UMEs are characterized electrochemically by investigating the redox cycling of the Ru(NH3)6 3+/2+ couple across a nanogap between voltammetric generator and amperometric collector electrodes to achieve quasi-steady states at fast scan rates of 100 V/s, which is ~1000 times faster than reported with any nanogap-based electrochemical cell. Importantly, the transient background of the amperometric collector response is suppressed ~100 times in comparison with that of the voltammetric generator response to enable the quantitative analysis of the collector response against the cycled generator potential without background subtraction. Finite element analysis of oxidative amperometric responses to Ru(NH3)6 2+ at the collector electrode at up to 100 V/s yield consistent gap 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 DA oxidation at the generator electrode to largely improve the DA selectivity of the collector response against AA, which is also oxidized at the generator electrode to immediately and irreversibly produce a redox-inactive species, thereby minimizing the corresponding collector response. The probe-type design of double-CF UMEs as well as their high DA selectivity against AA is attractive for in-vivo applications. [1] Pathirathna, P.; Balla, R. J.; Amemiya, S. “Simulation of fast-scan nanogap voltammetry at double-cylinder ultramicroelectrodes.” J. Electrochem. Soc., Focus Issue on the Brain and Electrochemistry Honoring R. Mark Wightman and Christian Amatore, 2018, 165, G3026. [2] Pathirathna, P.; Balla, R. J.; Amemiya, S. “Nanogap-based electrochemical measurements at double-carbon-fiber ultramicroelectrodes.” Anal. Chem. 2018, 90, 11746.

Digital image analysis for measuring nanogap distance produced by adhesion lithography

A simple digital image analysis for measuring nanogap distance produced by adhesion lithography is proposed. Adhesion lithography produces metal electrodes with sub-15 nm undulated space and μm to mm scale width without using electron beam lithography. Although the process has been rapidly improved in recent years, there has been no generalized procedure to evaluate the nanogap distance. In this study, we propose a procedure to evaluate a nanogap electrode with large width/gap distance ratios (>1000). The procedure is to determine the average distance of nanogap space from the area and the perimeter of the space by the analysis of the grayscale image. This procedure excludes any arbitrariness of the estimation and gives quantitative comparison of nanogap electrodes produced by different processes.

Single-Molecule Detection of DNA in a Nanochannel by High-Field Strength-Assisted Electrical Impedance Spectroscopy.

Many researchers have fabricated micro and nanofluidic devices incorporating optical, chemical, and electrical detection systems with the aim of achieving on-chip analysis of macromolecules. The present study demonstrates a label-free detection of DNA using a nanofluidic device based on impedance measurements that is both sensitive and simple to operate. Using this device, the electrophoresis and dielectrophoresis effect on DNA conformation and the length dependence were examined. A low alternating voltage was applied to the nanogap electrodes to generate a high intensity field (>0.5 MV/m) under non-faradaic conditions. In addition, a 100 nm thick gold electrode was completely embedded in the substrate to allow direct measurements of a solution containing the sample passing through the gap, without any surface modification required. The high intensity field in this device produced a dielectrophoretic force that stretched the DNA molecule across the electrode gap at a specific frequency, based on back and forth movements between the electrodes with the DNA in a random coil conformation. The characteristics of 100 bp, 500 bp, 1 kbp, 5 kbp, 10 kbp, and 48 kbp λ DNA associated with various conformations were quantitatively analyzed with high resolution (on the femtomolar level). The sensitivity of this system was found to be more than about 10 orders of magnitude higher than that obtained from conventional linear alternating current (AC) impedance for the analysis of bio-polymers. This new high-sensitivity process is expected to be advantageous with regard to the study of complex macromolecules and nanoparticles.

Concepts in the design and engineering of single-molecule electronic devices

Over the past two decades, various techniques for fabricating nano-gapped electrodes have emerged, promoting rapid development in the field of single-molecule electronics, on both the experimental and theoretical sides. To investigate intrinsic quantum phenomena and achieve desired functionalities, it is important to fully understand the charge transport characteristics of single-molecule devices. In this Review, we present the principles that have been developed for fabricating reliable molecular junctions and tuning their intrinsic properties from an engineering perspective. Through holistic consideration of the device structure, we divide single-molecule junctions into three intercorrelated components: the electrode, the contact (spacer–linker) interface and the molecular backbone or functional centre. We systematically discuss the selection of the electrode material and the design of the molecular components from the point of view of the materials, the interface and molecular engineering. The influence of the properties of these elements on the molecule–electrode interface coupling and on the relative energy gap between the Fermi level of the electrode and the orbital energy levels of the molecule, which directly influence the charge transport behaviour of single-molecule devices, is also a focus of our analysis. On the basis of these considerations, we examine various functionalities demonstrated in molecular junctions through molecular design and engineering. In this Review, the principles developed for fabricating reliable molecular junctions and tuning their intrinsic properties are examined from the point of view of the electrode, the interface and molecular engineering. The various functionalities demonstrated in molecular junctions through molecular design are discussed, along with the open challenges in the field.

Robust Pt-based nanogap electrodes with 10 nm scale ultrafine linewidth

Robust Pt-based nanogap electrodes with a 10 nm scale ultrafine linewidth were fabricated on SiO2/Si substrates by electron-beam lithography. The structures of the fabricated electrodes remained unchanged up to temperatures of 773 K. The robust thermal stability of the fabricated electrodes in comparison with that of Au-based nanogap electrodes was discussed based on Rayleigh instability. A chemically assembled single-electron transistor was prepared by chemisorbing a 6 nm sized Au nanoparticle between the Pt-based nanogap electrodes, and the transistor showed clear and ideal Coulomb diamonds up to 180 K.

Batch Fabrication of Nano-Gap Electrode Array Using Photo-Patterning and Resist UV-Curing

Batch Fabrication of Nano-Gap Electrode Array Using Photo-Patterning and Resist UV-Curing

Graphene nanogap electrodes in electrical biosensing

Graphene nanogap electrodes are reported here for the first time in an electrical biosensor for the detection of biomolecular interactions. Streptavidin-biotin was chosen as a model system for evaluating the sensor’s performance. High-affinity interactions of streptavidin-gold nanoparticles (strep-AuNPs) to the biotin-functionalized nanogap localizes AuNPs, thereby bridging the gap and resulting in changes in device conductance. Biosensing performance was optimized by varying the gap size, AuNP diameter, and streptavidin coverage on AuNPs. The sensitivity and limit of detection (LOD) of streptavidin detection with the optimized parameters were determined to be 0.3 µA/nM and 0.25 pM, respectively. The proposed platform suggests high potential as a portable point-of-use biosensor for the detection of other affinity-based biomolecular interactions, such as antigen-antibody, nucleic acid, or chemo-selective interactions.

A High-Frequency Resonant Framed-Annulus Pitch or Roll Gyroscope for Robust High-Performance Single-Chip Inertial Measurement Units

This paper introduces a novel resonator design for a high-frequency mode-matched pitch (or roll) gyroscope featuring large excitation amplitude and high Coriolis coupling to address the limitations of conventional designs in enabling robust high-performance single-chip inertial measurement units. The resonator consists of a compact framed-annulus structure with substrate-decoupled in-plane and out-of-plane high-frequency gyroscopic modes for high environmental robustness. The out-of-plane mode is carefully engineered to match the Coriolis force generated by the in-plane mode motion under rotation, providing high Coriolis coupling and improving the signal-to-noise ratio. Prototypes are implemented using HARPSS+ process with nano-gap slanted electrodes for quadrature tuning and nano-gap comb-drives for efficient largerange linear actuation. Mode-matched operation of the gyroscope at 657 kHz is demonstrated with a relatively large open-loop bandwidth of 73 Hz, showing a scale-factor of 150 pA/(°/s) with nonlinearity less than 0.08% over a measured 600°/s full-scale range. The gyroscope shows well-behaved Q-factor and scale-factor across temperature from -40°C to 80 °C, giving less than ±0.65% sensitivity variation after applying a quadratic correction curve to compensate for Q variation. The gyroscope demonstrates angle random walk of 0.55°/√h and bias instability of 17.9°/h, limited by package pressure and interface electronics. Resonant operation of the framed-annulus gyroscope with mechanical Q-amplification provides a solution to high-performance single-chip inertial measurement unit with robust large-bandwidth low-power gyroscopes.