Metal nanowire grating patterns

Design of a binary metal micron grating and its application in near-infrared hot-electron photodetectors

Metal plasmonic nano-gratings possess a high absorption ability and exhibit potential applications in sensing, hot-electron photodetection, metasurfaces, etc. However, the fabrication techniques of high-quality nano-gratings are challenging. In this article, a binary metal micron grating for near-infrared hot-electron photodetectors (HEPDs) is designed in which the surface plasmons are excited by high-diffraction-order modes. The high-diffraction-order micron grating can be fabricated by conventional lithography and has a significantly higher tolerance in the grating parameters than a nano-grating. The range of absorption greater than 70% is ∼3 times that of a nano-grating. Moreover, an interesting relationship between the resonant wavelength and the grating duty cycle is found. When the high-diffraction-order micron grating is applied in metal-insulator-metal HEPDs, a high zero-biased responsivity of 0.533 mA/W is achieved.

High-Throughput Direct Writing of Metallic Micro- and Nano-Structures by Focused Ga+ Beam Irradiation of Palladium Acetate Films

Metallic nanopatterns are ubiquitous in applications that exploit the electrical conduction at the nanoscale, including interconnects, electrical nanocontacts, and small gaps between metallic pads. These metallic nanopatterns can be designed to show additional physical properties (optical transparency, plasmonic effects, ferromagnetism, superconductivity, heat evacuation, etc.). For these reasons, an intense search for novel lithography methods using uncomplicated processes represents a key on-going issue in the achievement of metallic nanopatterns with high resolution and high throughput. In this contribution, we introduce a simple methodology for the efficient decomposition of Pd₃(OAc)₆ spin-coated thin films by means of a focused Ga⁺ beam, which results in metallic-enriched Pd nanostructures. Remarkably, the usage of a charge dose as low as 30 μC/cm² is sufficient to fabricate structures with a metallic Pd content above 50% (at.) exhibiting low electrical resistivity (70 μΩ·cm). Binary-collision-approximation simulations provide theoretical support to this experimental finding. Such notable behavior is used to provide three proof-of-concept applications: (i) creation of electrical contacts to nanowires, (ii) fabrication of small (40 nm) gaps between large metallic contact pads, and (iii) fabrication of large-area metallic meshes. The impact across several fields of the direct decomposition of spin-coated organometallic films by focused ion beams is discussed.

Nanowire-based sensor electronics for chemical and biological applications

Detection and recognition of chemical and biological species via sensor electronics are important not only for various sensing applications but also for fundamental scientific understanding. In the past two decades, sensor devices using one-dimensional (1D) nanowires have emerged as promising and powerful platforms for electrical detection of chemical species and biologically relevant molecules due to their superior sensing performance, long-term stability, and ultra-low power consumption. This paper presents a comprehensive overview of the recent progress and achievements in 1D nanowire synthesis, working principles of nanowire-based sensors, and the applications of nanowire-based sensor electronics in chemical and biological analytes detection and recognition. In addition, some critical issues that hinder the practical applications of 1D nanowire-based sensor electronics, including device reproducibility and selectivity, stability, and power consumption, will be highlighted. Finally, challenges, perspectives, and opportunities for developing advanced and innovative nanowire-based sensor electronics in chemical and biological applications are featured.

Stirring revealed new functions of ethylenediamine and hydrazine in the morphology control of copper nanowires

Cu nanowires, as promising candidates in many fields because of their merits, are commonly prepared by the solution phase based synthesis which is a simple and scalable method. However, precise control of the morphology, particularly surface roughness, of Cu nanowires is still challenging; and moreover, detailed formation mechanisms of Cu nanowires, in solution phase based synthesis, are still unclear. We here show the morphology manipulation of Cu nanowires by adjusting the stirring rate and the amounts of ethylenediamine and hydrazine (N2H4), yielding Cu nanowires with either smooth or rough surface. Importantly, according to our experimental results and theoretical investigation, new functions of ethylenediamine and N2H4 are found, and a growth process of Cu nanowires is proposed accordingly. In addition to typically accepted roles of ethylenediamine and N2H4, we find that ethylenediamine can facilitate the growth of Cu nanowires by etching Cu oxides and even Cu on the surface of Cu nanowires. Meanwhile, N2H4 molecules can modulate the growth of Cu nanowires as a capping agent, which can be easily influenced by stirring. Additionally, the as-synthesized Cu nanowires with different morphologies exhibit different optical and catalytic properties. This study provides new fundamental insights into the growth mechanism of Cu nanowires, and thus can facilitate controlled synthesis of Cu nanowires for further applications, including electronics, catalysis, and sensing.

Confined Chemical Etching for Electrochemical Machining with Nanoscale Accuracy

In the past several decades, electrochemical machining (ECM) has enjoyed the reputation of a powerful technique in the manufacturing industry. Conventional ECM methods can be classified as electrolytic machining and electroforming: the former is based on anodic dissolution and the latter is based on cathodic deposition of metallic materials. Strikingly, ECM possesses several advantages over mechanical machining, such as high removal rate, the capability of making complex three-dimensional structures, and the practicability for difficult-to-cut materials. Additionally, ECM avoids tool wear and thermal or mechanical stress on machining surfaces. Thus, ECM is widely used for various industrial applications in the fields of aerospace, automobiles, electronics, etc. Nowadays, miniaturization and integration of functional components are becoming significant in ultralarge scale integration (ULSI) circuits, microelectromechanical systems (MEMS), and miniaturized total analysis systems (μ-TAS). As predicted by Moore's law, the feature size of interconnectors in ULSI circuits are down to several nanometers. In this Account, we present our perseverant research in the last two decades on how to "confine" the ECM processes to occur at micrometer or even nanometer scale, that is, to ensure ECM with nanoscale accuracy. We have been developing the confined etchant layer technique (CELT) to fabricate three-dimensional micro- and nanostructures (3D-MNS) on different metals and semiconductor materials since 1992. In general, there are three procedures in CELT: (1) generating the etchant on the surface of the tool electrode by electrochemical or photoelectrochemical reactions; (2) confining the etchant in a depleted layer with a thickness of micro- or nanometer scale; (3) feeding the tool electrode to etch the workpiece. Scavengers, which can react with the etchant, are usually adopted to form a confined etchant layer. Through the subsequent homogeneous reaction between the scavenger and the photo- or electrogenerated etchant in the electrolyte solution, the diffusion distance of the etchant is confined to micro- or nanometer scale, which ensures the nanoscale accuracy of electrochemical machining. To focus on the "confinement" of chemical etching reactions, external physical-field modulations have recently been introduced into CELT by introducing various factors such as light field, force field, hydrodynamics, and so on. Meanwhile, kinetic investigations of the confined chemical etching (CCE) systems are established based on the finite element analysis and simulations. Based on the obtained kinetic parameters, the machining accuracy is tunable and well controlled. CELT is now applicable for 1D milling, 2D polishing, and 3D microfabrication with an accuracy at nanometer scale. CELT not only inherits all the advantages of electrochemical machining but also provides advantages over photolithography and nanoimprint for its applicability to different functional materials without involving any photocuring and thermoplastic resists. Although there are some technical problems, for example, mass transfer and balance, which need to be solved, CELT has shown its prospective competitiveness in electrochemical micromachining, especially in the semiconductor industry.

Large area sub-100 nm direct nanoimprinting of palladium nanostructures

We demonstrate a simple direct nanoimprinting method for fabricating palladium nanostructures that involves in situ free radical polymerization of a resin consisting of an acrylate-based crosslinker and a palladium metal precursor.

Fano resonance properties of gold nanocrescent arrays

The Fano resonance induced by symmetry breaking could improve the sensitivity of localized surface plasmon resonance sensors. In this work, the spectra of gold nanocrescent arrays are measured and confirmed by simulation results through the finite element method (FEM). The Fano resonance presented in the spectra could be modulated by the symmetry breaking with different waist widths, which are understood through plasmonic hybridization theory with the help of surface charge distribution. Our results indicate the Fano lineshape is generated by the coherent coupling of the quadrupole plasmon mode Q_H of nanohole and the antibonding plasmon mode D(AB) of nanocrescent. Finally, the high figure of merit (FoM=1.6-3.5) of the Q mode in the visible region illustrates this nanocrescent Fano sensor is of great value in the biological and chemical scientific fields.

Silica nanowire arrays for diffraction-based bioaffinity sensing

Arrays of electrodeposited silica nanowires (SiO2 NWs) have been fabricated over large areas (cm(2)) on fluoropolymer thin films attached to glass substrates by a combination of photolithography and electrochemically triggered sol-gel nanoscale deposition. Optical and scanning electron microscopy (SEM) measurements revealed that the SiO2 NW arrays had an average spacing of ten micrometers and an average width of 700 nm with a significant grain structure that was a result of the sol-gel deposition process. The optical diffraction properties at 633 nm of the SiO2 NW arrays were characterized when placed in contact with solutions by using a prism-coupled total internal reflection geometry; quantification of changes in these diffraction properties was applied in various sensing applications. Bulk refractive index sensing by using the SiO2 NW grating was demonstrated with a sensitivity of 1.30×10(-5) RIU. Toposelectively chemically modified SiO2 NW arrays were used for diffraction biosensing measurements of surface binding events, such as the electrostatic adsorption of gold nanoparticles and the bioaffinity adsorption of streptavidin onto a biotin monolayer. Finally, the application of the SiO2 NW arrays for practical medical-diagnostic applications was demonstrated by monitoring the diffraction of SiO2 NW arrays functionalized with a single-stranded (ss)DNA aptamer to detect human α-thrombin from solutions at sub-pathologic nanomolar concentrations.

Large area metal nanowire arrays with tunable sub-20 nm nanogaps

We report a new top-down nanofabrication technology to realize large area metal nanowire (m-NW) arrays with tunable sub-20 nm separation nanogaps without the use of chemical etching or milling of the metal layer. The m-NW array nanofabrication technology is based on a self-regulating metal deposition process that is facilitated by closely spaced and isolated heterogeneous template surfaces that confine the metal deposition into two dimensions, and therefore, electrically isolated parallel arrays of m-NW can be realized with uniform and controllable nanogaps. Au-NW and Ag-NW arrays are presented with high-density ~10(5) NWs cm(-1), variable NW diameters down to ~50 nm, variable nanogaps down to ~5 nm, and very large nanogap length density ~1 km cm(-2). The m-NW arrays are designed and implemented as interdigitated nanoelectrodes for electrochemical applications and as plasmonic substrates where the coupled-mode localized surface plasmon resonance (LSPR) wavelength in the nanogaps between adjacent m-NW dimers can be precisely tuned to match any excitation source in the range from 500 to 1000 nm, thus providing optimal local electromagnetic field enhancement. A spatially averaged (n = 2500) surface-enhanced Raman scattering (SERS) analytical enhancement factor of (1.2 ± 0.1) × 10(7) is demonstrated from a benzenethiol monolayer chemisorbed on a Au-NW array substrate with LSPR wavelength matched to a He-Ne laser source.

Fabrication of ultrahigh density metal-cell-metal crossbar memory devices with only two cycles of lithography and dry-etch procedures

A novel approach to the fabrication of metal-cell-metal trilayer memory devices was demonstrated by using only two cycles of lithography and dry-etch procedures. The fabricated ultrahigh density crossbar devices can be scaled down to ≤70 nm in half-pitch without alignment issues. Depending on the different dry-etch mechanisms in transferring high and low density nanopatterns, suitable dry-etch angles and methods are studied for the transfer of high density nanopatterns. Some novel process methods have also been developed to eliminate the sidewall and other conversion obstacles for obtaining high density of uniform metallic nanopatterns. With these methods, ultrahigh density trilayer crossbar devices (~2 × 10(10) bit cm(-2)-kilobit electronic memory), which are composed of built-in practical magnetoresistive nanocells, have been achieved. This scalable process that we have developed provides the relevant industries with a cheap means to commercially fabricate three-dimensional high density metal-cell-metal nanodevices.

Fabrication of a nickel nanowire mesh electrode suspended on polymer substrate

We report on an efficient strategy for the fabrication of an ultra-long suspended nanowire mesh suitable for nanodevice architectures on a polymer surface. First, nickel nanowires are synthesized directly on a template substrate by magnetron sputtering. Laser interference lithography followed by deep reactive ion etching is used to create the nanograted template substrate constituted of one-dimensional line pattern arrays of 240 nm in periodicity. Ordered alignment of ultra-long nanowires (∼180 nm in diameter) with high fidelity to the template pattern is observed by scanning electron microscopy. The transfer of the pre-defined parallel nanowire array from the template surface to a target polymer substrate for electrical characterization of the system is demonstrated. The electrical behaviour of the nanowire mesh, suspended between two electrodes, was found to be linear, stable, and reproducible. This result suggests that this nanofabrication process will open an efficient way to the design and construction of novel nanodevices.

Electroactive subwavelength gratings (ESWGs) from conjugated polymers for color and intensity modulation

Subwavelength gratings with electroactive polymers such as poly(3-hexylthiophene) (P3HT) and poly(3,4-propylenedioxythiophene-phenylene) (P(ProDOT-Ph)) controlled the color intensity for various visible colors of diffracted light in a single device. Under the illumination of a white light, at a fixed angle of incidence, the color intensity of the diffracted light was reversibly switched from the maximum value down to 15% (85% decrease) by applying -2 to 2 V due to electrochemical (EC) reaction. All spectral colors including red, green, and blue were generated by changing the angle of incidence, and the intensity of each color was modulated electrochemically at a single EC device. With electroactive subwavelength gratings (ESWGs) of P3HT, the maximum modulation of the color intensity was observed in the red-yellow quadrant in the CIE color plot, whereas for the ESWGs of P(ProDOT-Ph), the maximum modulation of the color intensity was observed in the yellow-green and green-blue quadrants. Both ESWGs showed a memory effect, keeping their color and intensity even after power was turned off for longer than 40 hours.

Fabrication of ultra-fine nanostructures using edge transfer printing

The exploration of new methods and techniques for application in diverse fields, such as photonics, microfluidics, biotechnology and flexible electronics is of increasing scientific and technical interest for multiple uses over distance of 10-100 nm. This article discusses edge transfer printing--a series of unconventional methods derived from soft lithography for nanofabrication. It possesses the advantages of easy fabrication, low-cost and great serviceability. In this paper, we show how to produce exposed edges and use various materials for edge transfer printing, while nanoskiving, nanotransfer edge printing and tunable cracking for nanogaps are introduced. Besides this, different functional materials, such as metals, inorganic semiconductors and polymers, as well as localised heating and charge patterning, are described here as unconventional "inks" for printing. Edge transfer printing, which can effectively produce sub-100 nm scale ultra-fine structures, has broad applications, including metallic nanowires as nanoelectrodes, semiconductor nanowires for chemical sensors, heterostructures of organic semiconductors, plasmonic devices and so forth.

Fabrication of periodic metal nanowires with microscale mold by nanoimprint lithography

In this paper, a simple method is demonstrated for fabricating periodic metal nanowires based on the unconventional nanoimprint lithography (NIL) technique. Using this method, sub-100 nm metal nanowires with the rectangular cross-section are fabricated with microscale stamp. Furthermore, the metal nanowires with different widths and heights can be generated by adjusting the imprinting parameters with the same stamp. The metal nanowires prepared with this method can be used for chemical sensing, such as ammonia sensing, and it may have applications in optical signal processing.