Nanometer-sized Gap Research Articles

Fabrications of insulator-protected nanometer-sized electrode gaps

We developed SiO2-coated mechanically controllable break junctions for accurate tunneling current measurements in an ionic solution. By breaking the junction, we created dielectric-protected Au nanoprobes with nanometer separation. We demonstrated that the insulator protection was capable to suppress the ionic contribution to the charge transport through the electrode gap, thereby enabled reliable characterizations of liquid-mediated exponential decay of the tunneling conductance in an electrolyte solution. From this, we found distinct roles of charge points such as molecular dipoles and ion species on the tunneling decay constant, which was attributed to local structures of molecules and ions in the confined space between the sensing electrodes. The device described here would provide improved biomolecular sensing capability of tunneling current sensors.

Fabrication of carbon nanotube nanogap electrodes by helium ion sputtering for molecular contacts

Carbon nanotube nanogaps have been used to contact individual organic molecules. However, the reliable fabrication of a truly nanometer-sized gap remains a challenge. We use helium ion beam lithography to sputter nanogaps of only (2.8 ± 0.6) nm size into single metallic carbon nanotubes embedded in a device geometry. The high reproducibility of the gap size formation provides a reliable nanogap electrode testbed for contacting small organic molecules. To demonstrate the functionality of these nanogap electrodes, we integrate oligo(phenylene ethynylene) molecular rods, and measure resistance before and after gap formation and with and without contacted molecules.

Selective Excitation of Single Molecules Coupled to the Bright Mode of a Plasmonic Cavity

Plasmon-based optical antennas featuring a nanometer-sized gap can enhance the photophysical properties of solid-state quantum emitters by several orders of magnitude at room temperature. However, controlling the position and orientation of an isolated emitter in a metallic resonator, at the nanometer scale, has only been achieved in scanning probe geometries. Using radially polarized cylindrical vector beams and DNA-assembled gold nanoparticle dimers, we demonstrate the reproducible interaction of single dye molecules with the bright longitudinal mode of a plasmonic cavity, achieving decay rate enhancements of 2 orders of magnitude. These results demonstrate that interfacing efficiently isolated quantum emitters and optical nanoantennas is possible on a large scale.

Strong Plasmon Reflection at Nanometer-Size Gaps in Monolayer Graphene on SiC

We employ tip-enhanced infrared near-field microscopy to study the plasmonic properties of epitaxial quasi-free-standing monolayer graphene on silicon carbide. The near-field images reveal propagating graphene plasmons, as well as a strong plasmon reflection at gaps in the graphene layer, which appear at the steps between the SiC terraces. When the step height is around 1.5 nm, which is two orders of magnitude smaller than the plasmon wavelength, the reflection signal reaches 20% of its value at graphene edges, and it approaches 50% for step heights as small as 5 nm. This intriguing observation is corroborated by numerical simulations and explained by the accumulation of a line charge at the graphene termination. The associated electromagnetic fields at the graphene termination decay within a few nanometers, thus preventing efficient plasmon transmission across nanoscale gaps. Our work suggests that plasmon propagation in graphene-based circuits can be tailored using extremely compact nanostructures, such as ultranarrow gaps. It also demonstrates that tip-enhanced near-field microscopy is a powerful contactless tool to examine nanoscale defects in graphene.

Simple Preparation of Microstructure by Using an Organic-Inorganic Raspberry-like Hybrid as a Building Block

A single-step strategy has been developed for synthesizing a micrometer-sized structure by using the raspberry-like hybrid as a building block comprising a repeated sequence of a three-dimensional gold nanoparticle (AuNP)-aniline oligomer-AuNP arrangement. The hybrid holds a high density of highly dispersible AuNPs without any contact with each other, and therefore the microstructure has many nanometer-sized gaps between the adjacent AuNPs for both electrical and optical sensing. We have discussed how the formation mechanism of the microstructure is based on using the hybrid.

Plasmon-Assisted Delivery of Single Nano-Objects in an Optical Hot Spot

Fully exploiting the capability of nano-optics to enhance light-matter interaction on the nanoscale is conditioned by bringing the nano-object to interrogate within the minuscule volume where the field is concentrated. There currently exists several approaches to control the immobilization of nano-objects but they all involve a cumbersome delivery step and require prior knowledge of the "hot spot" location. Herein, we present a novel technique in which the enhanced local field in the hot spot is the driving mechanism that triggers the binding of proteins via three-photon absorption. This way, we demonstrate exclusive immobilization of nanoscale amounts of bovine serum albumin molecules into the nanometer-sized gap of plasmonic dimers. The immobilized proteins can then act as a scaffold to subsequently attach an additional nanoscale object such as a molecule or a nanocrystal. This universal technique is envisioned to benefit a wide range of nano-optical functionalities including biosensing, enhanced spectroscopy like surface-enhanced Raman spectroscopy or surface-enhanced infrared absorption spectroscopy, as well as quantum optics.

Batch fabrication of gold–gold nanogaps by E-beam lithography and electrochemical deposition

We report on the successful development of a well-controlled two-step batch nano-fabrication process to achieve nanometer-size gaps at the wafer scale. The technique is based on an optimized electron-beam lithography process, which enables the fabrication of nanogaps in the range (15 ± 4) nm. Following this first step, the feedback-controlled electrochemical deposition of gold from an aqueous HAuCl4-based electrolyte is applied to further reduce the size of the gap down to about 0.3–1.0 nm. This protocol was successfully demonstrated by fabricating more than 385 nanogaps on a 4 inch wafer. The reproducible fabrication of nanogaps in the range between 0.3 and 1.0 nm opens up new perspectives for addressing the electrical and reactivity properties of single molecules and clusters in confined space under well-controlled conditions.

Detection of Asperity Contact for Precise Gap Determination in Thin-Film Nanorheometry

Understanding the mechanical properties of a nanometer-thick liquid lubricant film coated on a magnetic disk surface is essential for effective lubrication design of head-disk interface in hard disk drives. We have developed a highly sensitive shear force measurement method, named the fiber wobbling method (FWM), that enables us to measure the viscoelastic properties of such films. For these measurements, the shearing gap needs to be determined precisely since the thickness of the film is around 1–2 nm, and the accuracy of the gap determination depends on the sensitivity of the solid contact point detection, which is considered to correspond to a gap width of zero. In this study, we present a sensitive method for solid contact detection in the FWM that relies on the monitoring of the resonant oscillation of a shearing probe excited by asperity contact. The feasibility of the method is verified by both numerical calculations and experiments. The applicability to a magnetic disk with a lubricant film is also examined and we confirmed the method had an enough sensitivity to determine the nanometer-sized gap width.

Current-induced nanogap formation and graphitization in boron-doped diamond films

A high-current annealing technique is used to fabricate nanogaps and hybrid diamond/graphite structures in boron-doped nanocrystalline diamond films. Nanometer-sized gaps down to ∼1 nm are produced using a feedback-controlled current annealing procedure. The nanogaps are characterized using scanning electron microscopy and electronic transport measurements. The structural changes produced by the elevated temperature, achieved by Joule heating during current annealing, are characterized using Raman spectroscopy. The formation of hybridized diamond/graphite structure is observed at the point of maximum heat accumulation.

Finite Element Modeling of the Cyclic Wetting Mechanism in the Active Part of Wheat Awns

Many plant tissues and organs are capable of moving due to changes in the humidity of the environment, such as the opening of the seed capsule of the ice plant and the opening of the pine cone. These are fascinating examples for the materials engineer, as these tissues are non-living and move solely through the differential swelling of anisotropic tissues and in principle may serve as examples for the bio-inspired design of artificial actuators. In this paper, we model the microstructure of the wild wheat awn (Triticum turgidum ssp. dicoccoides) by finite elements, especially focusing on the specific microscopic features of the active part of the awn. Based on earlier experimental findings, cell walls are modeled as multilayered cylindrical tubes with alternating cellulose fiber orientation in successive layers. It is shown that swelling upon hydration of this system leads to the formation of gaps between the layers, which could act as valves, thus enabling the entry of water into the cell wall. This supports the hypothesis that this plywood-like arrangement of cellulose fibrils enhances the effect of ambient humidity by accelerated water or vapor diffusion along the gaps. The finite element model shows that a certain distribution of axially and tangentially oriented fibers is necessary to generate sufficient tensile stresses within the cell wall to open nanometer-sized gaps between cell wall layers.

Aligned Growth of Gold Nanorods in PMMA Channels: Parallel Preparation of Nanogaps

We demonstrate alignment and positional control of gold nanorods grown in situ on substrates using a seed-mediated synthetic approach. Alignment control is obtained by directing the growth of spherical nanoparticle seeds into nanorods in well-defined poly(methyl methacrylate) nanochannels. Substrates with prepatterned metallic electrodes provide an additional handle for the position of the gold nanorods and yield nanometer-sized gaps between the electrode and nanorod. The presented approach is a novel demonstration of bottom-up device fabrication of multiple nanogap junctions on a single chip mediated viain situ growth of gold nanorods acting as nanoelectrodes.

Tunable nanometer electrode gaps by MeV ion irradiation

We report the use of MeV ion-irradiation-induced plastic deformation of amorphous materials to fabricate electrodes with nanometer-sized gaps. Plastic deformation of the amorphous metal Pd(80)Si(20) is induced by 4.64 MeV O(2+) ion irradiation, allowing the complete closing of a sub-micrometer gap. We measure the evolving gap size in situ by monitoring the field emission current-voltage (I-V) characteristics between electrodes. The I-V behavior is consistent with Fowler-Nordheim tunneling. We show that using feedback control on this signal permits gap size fabrication with atomic-scale precision. We expect this approach to nanogap fabrication will enable the practical realization of single molecule controlled devices and sensors.

Thermal Impacts on the Performance of Nanoscale-Gap Thermophotovoltaic Power Generators

The thermal impacts on the performance of nanoscale-gap thermophotovoltaic (nano-TPV) power generators are investigated using a coupled near-field thermal radiation, charge, and heat transport formulation. A nano-TPV device consisting of a tungsten radiator, maintained at 2000 K, and cells made of indium gallium antimonide (In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.18</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.82</sub> Sb) are considered; the thermal management system is modeled assuming a convective boundary with a fluid temperature fixed at 293 K. Results reveal that nano-TPV performance characteristics are closely related to the temperature of the cell. When the radiator and the junction are separated by a 20 nm vacuum gap, the power output and the conversion efficiency of the system are respectively 5.83 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> Wm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> and 24.8% at 300 K, whereas these values drop to 8.09 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> Wm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> and 3.2% at 500 K. In order to maintain the cell at room temperature, a heat transfer coefficient as high as 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> Wm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> is required for nanometer-size vacuum gaps. The reason for this is that thermal radiation since thermal radiation enhancement beyond the blackbody from a bulk radiator of tungsten is broadband in nature, while only a certain part of the spectrum is useful for maximizing nano-TPV performance. In future studies, near-field radiation spectral conditions leading to optimal performance characteristics of the device will be investigated.

Viscoelastic Properties of Monolayer Lubricant Films during Touch-Down and Take-Off Behavior Measured by Fiber Wobbling Method

It is essential to clarify the interaction between the magnetic head and the monolayer lubricant film coated on the magnetic disk in the design of next-generation hard disk drives with ultra-low flying height. However, with previous measuring methods such as AFM, it was difficult to measure the mechanical properties of molecularly thin lubricant films with a precisely controlled nanometer-sized gaps. In this study, we applied the fiber wobbling method to clarify the viscoelastic properties of monolayer lubricant films during touch-down and take-off behavior. The fiber wobbling method is the highly-sensitive measurement of shear force we developed and can control the shearing gap with sub-nanometer resolution. The gap dependence of viscoelastic properties was evaluated both in the approaching and the separating processes and their differences and dependencies on the types of lubricant films were discussed. As sample lubricants, we used non-polar and polar perfluoropolyether lubricants: Fomblin Z03 and Fomblin Zdol4000. Three different types of monolayer lubricant films were prepared. The first one consisted of mobile molecules of Z03 only. The second one contained both mobile and bonded molecules of Zdol4000. The third one was only made of bonded molecules of Zdol4000. Our experimental results indicated that the mobile molecules caused the formation of a liquid bridge between solid surfaces and it leads to the hysteresis of viscoelastic properties between approaching and separating processes. On the other hand, the sample only consisted of bonded molecules did not show such a hysteresis.

4-1 ナノ隙間に閉じ込められた液体の粘弾性計測(セッション4:OS1-1 マルチスケール・シミュレーションとナノ計測)

We developed highly sensitive shear force measuring method that can be applied to evaluate viscoelasticity of liquid lubricant confined in nanometer-sized gap width. The measuring method used an optical fiber whose end was fabricated in a sphere shape to detect the shear force. The lubricant was confined between the sphere-shaped fiber tip and the solid substrate and sheared by driving the fiber in the in-plane direction using a piezo actuator. The shear force required to make the lubricant flow or deform was measured by detecting the deflection of the fiber. This method enabled us to detect the shear force with a sensitivity of around 0.1nN and control the shearing gap width with 0.1nm resolution. Our experimental results revealed that the liquid lubricant showed enhanced viscosity under the confinement. Furthermore, elasticity, which is negligibly small in the bulk state, drastically increased at the gap width less than 10nm.

G-1-1 ナノ隙間における液体の粘弾性と剪断配向特性の同時計測法に関する研究(G-1 マイクロナノ理工学,口頭発表:マイクロナノ理工学)

Liquids confined in nanometer-sized gap width have characteristic viscoelastic properties. In our previous study, we have developed highly-sensitive shear force measuring method, which we called the "fiber wobbling method (FWM)", and revealed that liquid polymers exhibit enhanced viscoelasticity when they are sheared. Although mechanisms to explain these phenomena must involve molecular orientation confined in the gap, details are not clarified. In this study, we succeeded in simultaneously measuring viscoelasticity and molecular orientation of confined liquid crystal using FWM combined with birefringence measurement. Our experimental results showed that the relationship between nanometer-sized gap widths and viscoelasticity strongly depends on the molecular orientation.

Nonlinear Transport in Hybrid Polypyrrole–Gold Nanostructures

We have studied charge transport in nanometer scale films of polypyrrole (PPy) that were grown electrochemically onto discontinuous ultrathin films of gold. The gold films consisted of 100 nm size islands, separated from each other by nanometer-size gaps. The thickness of PPy can be varied from 30 to 200 nm. The I-V characteristics of these hybrid PPy-Au nanostructures show strong non-linearity at low temperatures, and in particular for the more insulating samples. The hopping transport is further verified from the log I versus V(1/4) plots. Furthermore, the I-V data follow an empirical relation dlogI/dV(1/4) - T(-1/2).

Lab-in-a-Tube: Detection of Individual Mouse Cells for Analysis in Flexible Split-Wall Microtube Resonator Sensors

We report a method for the precise capturing of embryonic fibroblast mouse cells into rolled-up microtube resonators. The microtubes contain a nanometer-sized gap in their wall which defines a new type of optofluidic sensor, i.e., a flexible split-wall microtube resonator sensor, employed as a label-free fully integrative detection tool for individual cells. The sensor action works through peak sharpening and spectral shifts of whispering gallery modes within the microresonators under light illumination.

Temperature Dependence of the Viscoelastic Properties of a Confined Liquid Polymer Measured by Using an Oscillating Optical Fiber Probe

We measured the temperature dependence of the viscoelastic properties of a liquid polymer confined and sheared within a nanometer-sized gap. In the viscoelastic measurements, we used the fiber wobbling method, a highly sensitive method that we have developed for measuring shear forces. As a liquid sample, we used the fluoropolyether lubricant Fomblin Zdol4000. Our experimental results showed that the temperature dependence of the viscosity was well expressed by the well-known Andrade equation, even in the confined state. The activation enthalpy was calculated by assuming that Eyring's theory of viscosity holds for gaps of a width ranging from 100 nm down to a few nanometers. We observed a significant decrease in the activation enthalpy for gaps smaller than 10 nm. Elasticity, which only appeared for confinement in gaps smaller than 10 nm, roughly decreased with increasing temperature.

MNM-P6-5 レーザー加熱下においてせん断されるナノ厚さ液体潤滑剤の粘弾性特性(P6 情報・精密機器におけるマイクロ・ナノテクノロジー)

In this study, we measured temperature dependence of viscoelastic properties of PFPE lubricant that was confined in nanometer-sized gap widths. In the viscoelastic measurement, we used the fiber wobbling method, which is the highly sensitive shear force measuring method that we have originally developed. Our experimental results showed that the confinement enhanced both viscosity and elasticity. However, they decreased with increasing heating temperature. In addition the temperature dependence of the lubricant's viscosity was well expressed by Andrade's equation, which is the empirical law to explain the temperature dependence of viscosity of bulk liquid, even under the confined state.