Fabrication Of Nanogap Electrodes Research Articles

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.

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.

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.

Molecular-scale electronics: From device fabrication to functionality

By wiring molecules into circuits, “molecular electronics” aims at studying electronic properties of single molecules and their ensembles, on this basis exploiting their intrinsic functionalities, and eventually applying them as building blocks of electronic components for future electronic devices. Herein, fabricating reliable solid-state molecular devices and developing synthetic molecules endowed with desirable electronic properties, have been two major tasks since the dawn of molecular electronics. This review focuses on recent advances and efforts regarding the main challenges in this field, highlighting fabrication of nanogap electrodes for single-molecule junctions, and self-assembled-monolayers (SAMs) for functional devices. The prospect of molecular-scale electronics is also discussed.

Nanoscale Electrodes: Nanoscale Electrodes for Flexible Electronics by Swelling Controlled Cracking (Adv. Mater. 30/2016).

The fabrication of nanospaced electrodes on a flexible substrate is a great challenge. W. Wang, J. Shao, and co-workers propose a novel approach to fabricate nanogap electrodes for flexible electronics using a swelling-controlled cracking method, which is described on page 6337. This method has the advantages of high applicability, parallel manufacturing capacity, and compatibility with flexible substrates. It provides a new way to create high-performance flexible electronics in a cost-efficient fashion.

Nanoscale Electrodes for Flexible Electronics by Swelling Controlled Cracking.

Nanogap electrodes are realized using pre-patterned electrodes and a swelling controlled cracking method. Parallel fabrication of nanogap electrodes on flexible substrates can be achieved using this method. This swelling-controlled cracking method is promising for fabricating high-performance flexible electronics. UV photodetectors with ZnO nanoparticle-bridged nanogap electrodes exhibit high responsivity and external quantum efficiency.

Thermal induced single grain boundary break junction for suspended nanogap electrodes

Construction of molecular devices is one of the most promising approaches for the ultimate miniaturization of electronic devices, the groundwork of which is the fabrication of nanogap electrodes. Here we report a method to fabricate nanogap electrodes through thermal annealing based on single grain boundary junction. By performing low temperature thermal process, single grain boundary junction can be broken and change into a suspended gap with gap width down to sub-5 nanometers, which is beyond the fabrication precision of traditional lithography technologies. With the advantage of shape stability, no debris and high time efficiency, such nanogap electrodes is promising in constructing molecular devices with two or three-terminals.

Nanogaps Fabricated from Electrodeposited Fe-Ni-Co Nanowires

Nanogap electrodes are elements of nanoscale microfluidic devices and sensors, and have been prepared by different techniques, as recently reviewed by Li et al.1 Their use in biomedical microelectromechanical systems (Bio-MEMS) for molecular detection requires the gap dimensions to be of the same order as the target molecule ~10 nm. Several effective strategies to fabricate nanogap electrodes include: on-wire lithography with segmented nanowires that are etched, 2 -3 using a mechanically controllable break junction,4 focused ion beam lithography,5 and by electrodepositing a metal directly onto separated lithographic wires to fill in the separated space to reduce the gap size to the nanometer range.6 The approach studied here is to create electrodeposited nanowires with a sacrificial layer to create the gap. Examples of chemically etched inner regions of electrodeposited nanowires have been demonstrated in trilayers of Au-Ni-Au,2,7 Au-Ag-Au,2-3 Au-Cu-Au,8 and the quinary FeNiCo-Au-Cu-Au-FeNiCo.8 In this work, tri-layered nanowires with FeNiCo and either Cu or oxide regions, FeNiCo-Cu-FeNiCo, and FeNiCo-oxide-FeNiCo, are electrodeposited using the template method. Nanogaps are prepared by chemically etching the sacrifice layer, i.e., Cu, or the oxide layer, prepared in different ways. Conditions to ensure that the wires are robust before etching and that they form reproducible nanogaps are presented. Experimental Conditions used to deposit and etch the nanowires were determined from rotating cylinder studies. Fe-rich, FeNiCo conditions were selected. The electrodeposition of the tri-layered nanowires was fabricated in a standard three electrode cell with a polycarbonate membrane as a template. FeNiCo was pulsed deposited from a sulfamate-boric acid electrolyte at a pH of 2 and at a temperature of 40 °C. Cu was either galvanostatically deposited or deposited by displacement from a citrate-boric acid electrolyte at a pH of 2 and at room temperature. All nanowires were removed from the template and placed on a substrate to chemically etch the sacrificial, middle layer in an etching solution containing: 0.01 M sodium sulfate, 0.45 M sodium citrate, and 0.40 M boric acid at a pH of 5. The nanowires and their resulting gaps were inspected by FE-SEM (Hitachi S4800). Results and Discussion Figures 1 and 2 show gap regions between two FeNiCo segments that are generated after the etching step. Figure 1 shows examples of gaps created from a sacrificial Cu layer, and Figure 2 shows gaps generated from a sacrificial oxide layer. The Cu region was created by either electrodeposition, Figure 1 (a), or by displacement, Figure 1 (b). The smallest gaps were achieved using the Cu displacement technique. The gaps were not reflective of the charge passed during Cu deposition and was attributed to the generation of an Fe-rich oxide. Replacing the Cu sacrificial layer completely with an Fe-rich oxide layer, Figure 2, also resulted in reproducible gaps < 10 nm. Figure 2 (a) shows the gap region when the applied potential, after the first FeNiCo layer, was maintained at the open circuit potential (OCP), and Figure 2 (b) shows the gap created at an anodic potential value of 0.4 V vs OCP. When the potential was anodic, the second FeNiCo layer created a “tip” at the gap surface. Conclusion Sub-10 nm nanogaps were developed through template electrodeposition via etching of a Cu or oxide layer. The results suggest that a sacrificial Cu layer is effective, but unnecessary, and that a nano-gap can be created by the generation of an oxide from the FeNiCo layer at OCP. The oxide can also impart a tip shaping effect when it is created at an applied potential that is in the anodic region. Acknowledgement The authors thank Roche Diagnostics for generous support of this project. References T. Li, W. Hu and Z. Zhu, Adv. Mater. 22, 286 (2010).L. Qin, S. Park, L. Huang, and C. A. Mirkin, Science 309, 113 (2005).S. H. Liu, J. B. H. Tok, Z. N. Bao, Nano Lett. 5, 1071 (2005).M. A. Reed, C. Zhou, C.J. Muller, T. P. Burgin, J. Tour, Science, 278, 252 (1997).T. Nagase, T. Kubota, S. Mashiko, Thin Solid Films 438, 374 (2003).Y. Kashimura, H. Nakashima, K. Furukawa, K.Torimitsu, Thin Sold Films 438, 317 (2003).B. Widt, P. Mali, and P. C. Searson, Langmuir, 22, 10528 (2006).H. Kim, S. A. Soper, E. J. Podlaha, ECS Trans. 53, 9 (2013). Figure 1

Simultaneous fabrication of nanogap electrodes using field-emission-induced electromigration

We present a simple technique for simultaneous control of the electrical properties of multiple Ni nanogaps. This technique is based on electromigration induced by a field emission current and is called “activation.” Simultaneous tuning of the tunnel resistance of multiple nanogaps was achieved by passing a Fowler–Nordheim (F-N) field emission current through an initial group of three Ni nanogaps connected in series. The Ni nanogaps, which had asymmetrical shapes with initial gap separations in the 80–110-nm range, were fabricated by electron-beam lithography and a lift-off process. By performing the activation procedure, the current–voltage properties of the series-connected nanogaps were varied simultaneously from “insulating” to “metallic” via “tunneling” properties by increasing the preset current of the activation procedure. We can also simultaneously control the tunnel resistances of the series-connected nanogaps, which range from a resistance of the order of 100 TΩ–100 kΩ, by increasing the preset current from 1 nA to 30 μA. This tendency is quite similar to that of individually activated nanogaps, and the tunnel resistance values of the simultaneously activated nanogaps were almost the same at each preset current. These results clearly imply that the electrical properties of the series-connected nanogaps can be controlled simultaneously via the activation procedure.

Sub-15-nm patterning of asymmetric metal electrodes and devices by adhesion lithography.

Coplanar electrodes formed from asymmetric metals separated on the nanometre length scale are essential elements of nanoscale photonic and electronic devices. Existing fabrication methods typically involve electron-beam lithography—a technique that enables high fidelity patterning but suffers from significant limitations in terms of low throughput, poor scalability to large areas and restrictive choice of substrate and electrode materials. Here, we describe a versatile method for the rapid fabrication of asymmetric nanogap electrodes that exploits the ability of selected self-assembled monolayers to attach conformally to a prepatterned metal layer and thereby weaken adhesion to a subsequently deposited metal film. The method may be carried out under ambient conditions using simple equipment and a minimum of processing steps, enabling the rapid fabrication of nanogap electrodes and optoelectronic devices with aspect ratios in excess of 100,000.

Nanocarbon-Scanning Probe Microscopy Synergy: Fundamental Aspects to Nanoscale Devices

Scanning probe techniques scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have emerged as unique local probes for imaging, manipulation, and modification of surfaces at the nanoscale. Exercising the fabrication of atomic and nansocale devices with desired properties have demanded rapid development of scanning probe based nanolithographies. Dip pen nanolithography (DPN) and local anodic oxidation (LAO) have been widely employed for fabricating functional patterns and prototype devices at nanoscale. This review discusses the progress in AFM bias lithography with focus on nanocarbon species on which many functional quantum device structures have been realized using local electrochemical and electrostatic processes. As water meniscus is central to AFM bias lithography, the meniscus formation, estimation and visualization is discussed briefly. A number of graphene-based nanodevices have been realized on the basis AFM bias lithography in the form of nanoribbons, nanorings and quantum dots with sufficiently small dimensions to show quantum phenomena such as conductance fluctuations. Several studies involving graphitic surfaces and carbon nanotubes are also covered. AFM based scratching technique is another promising approach for the fabrication of nanogap electrodes, important in molecular electronics.

Fabrication of Nanogap Electrodes for Electrical Characterization of Single Molecules

Fabrication of Nanogap Electrodes for Electrical Characterization of Single Molecules

Molecular Crystal Lithography: A Facile and Low‐Cost Approach to Fabricate Nanogap Electrodes

A novel cost-efficient and facile technique, molecular crystal lithography, to fabricate nanogap electrodes efficiently is reported. The gap width of the electrodes can be tuned from ∼9 nm to several micrometers. Organic field-effect transistors based on the nanogap electrodes all exhibit a high performance, indicating the effectiveness and practicability of molecular crystal lithography for mass production of nanogap electrodes.

Fabrication of nanogap electrodes via nano-oxidation mask by scanning probe microscopy nanolithography

In this study, a simple technique was introduced for the fabrication of nanogap electrodes by using nano-oxidation scanning probe microscopy lithography with a Cr/Pt coated silicon tip. Silicon electrodes with a gap of sub-31 nm were fabricated successfully by this technique. The current-voltage measurements (I-V) of the electrodes demonstrated excellent insulating characteristics. This technique is simple, controllable, inexpensive, and faster than common methods. The results showed that silicon electrodes have a great potential for the fabrication of single molecule transistors, single electron transistors, and other nanoelectronic devices.

Fabrication of Nanogap Electrodes by the Molecular Lithography Technique

We have developed fabrication processes of nanogap electrodes with various metals, by combining the molecular lithography technique, in which self-assembled molecular (SAM) layers with an accurately defined thickness are used for resist, and the electron beam lithography. Until now, the molecular lithography technique has been applied to Au and Pt to fabricate the nanogap electrodes, because a thiol-group of the termini molecule that the SAM resist contains is easily stuck to these metals. In this work, we have extended the technique to various metals on which the SAM layers is hard to be formed, with the help of the X-ray photoelectron spectroscopy (XPS) to obtain information on adhesion conditions of the SAM layers on the metals. Finally, we demonstrate fabrication of the Nb nanogap electrodes with the developed technique. An Au nanoparticle was embedded in between the nanogap, which showed the Coulomb blockade effect and the superconducting gap in the current–voltage curves.

Fabrication of Nanogap Electrodes by the Molecular Lithography Technique

Fabrication of Nanogap Electrodes by the Molecular Lithography Technique

Graphene and graphene oxide nanogap electrodes fabricated by atomic force microscopy nanolithography

Nanogap electrodes have been studied intensively both as ideal research tools for molecular electronics and fundamental building blocks for nanodevices. Here, graphene and graphene oxide nanogap electrodes with gap width below 10 nm were fabricated by atomic force microscopy (AFM) nanolithography. It provides a simple and convenient way to fabricate nanogap electrodes and transfer the nanogap electrodes onto any required substrate. As a preliminary demonstration, organic field-effect transistors and organic photoswitchers based on the nanogap electrodes were fabricated, which all exhibited high performance, indicating the great prospect of the nanogap electrodes and the AFM nanolithography technique for molecular electronics.

Self-Aligned Nanogaps on Multilayer Electrodes for Fluidic and Magnetic Assembly of Carbon Nanotubes

A self-aligned nanogap between multiple metal layers has been developed using a new controlled undercut and metallization technique (CUMT), and practically applied for self-assembly of individual carbon nanotubes (CNTs) over the developed nanogap. This new method allows conventional optical lithography to fabricate nanogap electrodes and self-aligned patterns with nanoscale precision. The self-aligned nickel (Ni) pattern on the nanogap electrode works as an assembly spot where the residual iron (Fe) catalyst at the end of the CNT is magnetically captured. The captured CNT is forced to be aligned parallel to the flow direction by fluidic shear force. The combined forces of magnetic attraction and fluidic alignment provide massive self-assembly of CNTs at target positions. Both multiwalled nanotubes (MWNTs) and single walled nanotubes (SWNTs) were successfully assembled over the nanogap electrodes, and their electrical characteristics were fully characterized. The CNTs self-assembled on the developed electrodes with a nanogap and showed a very reliable and reproducible current-voltage (I-V) characteristic. The method developed in this work can envisage the mass fabrication of individual CNT-assembled devices which can be applied to nanoelectronic devices or nanobiosensors.

Fully lithography independent fabrication of nanogap electrodes for lateral phase-change random access memory application

A nanogap electrode fabrication method was developed and nanogap electrode as small as 17 nm was achieved based on sacrificial spacer process and conventional lithography. We have transferred this method to lateral phase-change random access memory (PCRAM) device fabrication. The electrical characterizations of 4.6 μm gap width using conventional lithography and 88 nm width based on this technology are shown. It is found that the threshold voltage and the dc power consumption are remarkably decreased due to nanogap electrode process. Our method cannot only improve the fabrication efficiency of PCRAM but also be easily transferred to other nanoelectronics applications.

Fabrication of Nanogap Electrodes using Electrodeposition Process

Laterally-enhanced growth of Au using electrochemical deposition from an electrode onto SiO2 surface modified with organic molecules for fabrication of nanogap electrodes was investigated. After modifying the SiO2 surface with self-assembled monolayers (SAMs) or dendrimer, electrodeposition was carried out and the enhanced lateral deposition was observed from the edge of Au electrode onto the SiO2 surface. In order to analyze the surface states modified with SAMs or dendrimer, X-ray photoelectron spectroscopy (XPS) were conducted and the existence of the organic molecules and Au ions on the SiO2 surface was confirmed. It was suggested that the enhanced lateral deposition was induced by the adsorbed Au ions. Furthermore, we tried to measure the electrical property of phthalocyanine molecular wire using the gap electrodes. In order to improve the contact state between the wire and the electrodes, electrodeposition was applied. As a result, improvement of contact state was observed from Current-Voltage characterization.