Nanotip Arrays Research Articles

Plasma-Etched Black GaAs Nanoarrays with Gradient Refractive Index Profile for Broadband, Omnidirectional, and Polarization-Independent Antireflection.

Black GaAs nanotip arrays (NTs) with 3300 nm lengths were fabricated via self-masked plasma etching. We show, both experimentally and numerically, that these NTs, with three gradient refractive index layers, effectively suppress Fresnel reflections at the air-GaAs interface over a broad range of wavelengths. These NTs exhibit exceptional UV-Vis light absorption (up to 99%) and maintain high NIR absorption (33-60%) compared to bare GaAs. Moreover, possessing a graded layer with a low refractive index (n = 1.01 to 1.12), they achieve angular and polarization-independent antireflection properties exceeding 80° at 632.8 nm, aligning with perfect antireflective coating theory predictions. This approach is anticipated to enhance the performance of optoelectronic devices across a wide range of applications.

Insight into TaSi2 Nanostructures with Different Morphologies and Their Field Emission Properties

AbstractRecognizing the strong potential of cold cathodes for important commercial applications in fields such as electronics, there is a growing interest in the exploration of novel 1D nanomaterials. Among various cold cathode materials, TaSi2 is of great interest for its outstanding field emission performance. In this work, the Si‐TaSi2 eutectic composite with nano‐sized highly oriented TaSi2 fibers and semi‐coherent phase interfaces is prepared by the laser floating zone melting technique with a very high‐temperature gradient of 6000 K cm−1 at a solidification rate of 200 µm s−1. On the basis of directionally solidified Si‐TaSi2 eutectic composite, well‐aligned TaSi2 nanorod and nanotip arrays are fabricated by inductively coupling plasma (ICP) etching process and HNO3/HF wet etching process, respectively. The field emission measurements show that the field enhancement factor, turn‐on electric field, and effective work function are strongly affected by tip morphologies. The TaSi2 array with regular nanotip structure possesses the best field emission characteristic among all TaSi2 nanostructures, with a relatively low turn‐on field of 4.8 V µm−1 and a high current density of 733 µA cm−2. These findings preliminarily establish a clear relationship between the performance and structure of the array, providing technical guidance for the application of this material in electronic devices.

Shape tuning of large area silicon nanotip arrays through reactive ion etching

Nanostructures formed in silicon form an important class of structures that span a broad spectrum of application areas. Of these, columnar structures of silicon featuring tiplike apexes have their own niche applications. The ability to afford shape tunability for these structures further enhances their application potential. In this paper, we present our findings on the large area fabrication of silicon nanotips defined through microsphere lithography and shape tuned through a combination of different reactive ion etching (RIE) techniques. The self-sharpening mechanism of the tips when using nonplanar etch masks (microspheres) under anisotropic etching conditions is elucidated. We further show that depending on the manner of etching (continuous versus discrete multistep etch), identical anisotropic etching recipes produce vastly different tip morphologies. Hourglass-shaped silicon tips were obtained when silicon was subjected to anisotropic followed by isotropic etching conditions. Sharp silicon tips with tip apex radii on the order of 2 nm have been successfully realized when the RIE shape tuned tips were subjected to a series of oxidative sharpening steps.

Application of a nanotip array-based electrochemical sensing platform for detection of indole derivatives as key indicators of gut microbiota health

The human gut microbiota plays a vital role in health and disease. Indole derivatives like indole, tryptamine and indoxyl sulfate represent important microbiota-associated metabolites and biomarkers of gut function. However, current techniques for analysis of these metabolites like mass spectrometry and HPLC suffer from limitations including high cost, complex sample preparation procedures, and long analysis times. In contrast, electrochemical sensing offers a rapid, simple and low-cost alternative with potential for point-of-care applications. We report a nanotip array-based electrochemical sensing platform for rapid, simple and sensitive quantification of indole derivatives. Silicon nanotip arrays were fabricated with controlled sub-micron morphology and coated with gold to form nanotip electrodes. Further modification with silver nanoparticles (AgNPs) led to significantly enhanced electrochemical signal. Using differential pulse voltammetry (DPV), distinct oxidation peaks were observed for indole, tryptamine and indoxyl sulfate, enabling sensitive detection down to nM levels. A broad linear dynamic range of 7 orders of magnitude demonstrated excellent quantitative capabilities. The sensor showed high reproducibility (<5 % RSD) and selectivity for the metabolites in the presence of potential interferents. Importantly, accurate detection of spiked indole derivatives was achieved in complex biological samples like serum and fecal extracts. The capacity to rapidly quantify indole derivatives in minimally processed samples highlights the potential of this nanotip electrochemical sensing strategy for point-of-care analysis of microbiota function. Overall, this work provides a robust nanotechnology-enabled platform for metabolite profiling to elucidate microbiota health and disease.

Synergy of cold development and electron-beam exposure for Si nanotip patterning in quantum-electronic devices

We demonstrated an effective method for lithographic patterning of a fanout-shaped Si nanotip array with nanometer-scale feature size/pitch by using a combination of overdose exposure, cold development, and layout design. We optimized process conditions of electron-beam exposure onto poly(methyl methacrylate) resist and methyl isobutyl ketone development temperature (10 to −5 °C) to produce dense Si nanotips with large gradient fanout patterns. A circular array of Si nanotips with a minimum feature size/pitch of 15/30 nm with a high degree of pattern fidelity, uniformity, and reproducibility were demonstrated. Based on the platform of the Si fanout-shaped nanotip array, we have successfully produced an array of self-organized germanium quantum-dots/Si3N4 barriers, which are self-aligned with Si nanotips as coupling barriers or electrodes for implementing advanced quantum-electronic devices.

Multiple-Route Exciton Recombination Dynamics and Improved Stability of Perovskite Quantum Dots by Plasmonic Photonic Crystal.

We have studied the excited-state exciton recombination dynamics of perovskite quantum dots (QDs) through time-resolved photoluminescence (PL), PL blinking, PL intensity-dependent lifetime modulation, and long-term photostability tests. The various spectroscopic characterizations elucidate that the perovskite QDs have multiple intrinsic exciton recombination routes even in a single QD, i.e., exciton, biexciton, and positive/negative trions, which are dissimilarly contributed to ON and OFF state emissions. We also find that the enhanced radiative recombination from placing green QDs on a photonic Ag nanotip array induces notably improved long-term PL stability. We consider that the accelerated radiative recombination of QDs by strong coupling with the plasmonics of the photonic Ag nanotip array, while eliminating nonradiative pathways, is proven to be a critical factor for improved long-term stability.

Stable Field Emission from Vertically Oriented SiC Nanoarrays.

Silicon carbide (SiC) nanostructure is a type of promising field emitter due to high breakdown field strength, high thermal conductivity, low electron affinity, and high electron mobility. However, the fabrication of the SiC nanotips array is difficult due to its chemical inertness. Here we report a simple, industry-familiar reactive ion etching to fabricate well-aligned, vertically orientated SiC nanoarrays on 4H-SiC wafers. The as-synthesized nanoarrays had tapered base angles >60°, and were vertically oriented with a high packing density >107 mm−2 and high-aspect ratios of approximately 35. As a result of its high geometry uniformity—5% length variation and 10% diameter variation, the field emitter array showed typical turn-on fields of 4.3 V μm−1 and a high field-enhancement factor of ~1260. The 8 h current emission stability displayed a mean current fluctuation of 1.9 ± 1%, revealing excellent current emission stability. The as-synthesized emitters demonstrate competitive emission performance that highlights their potential in a variety of vacuum electronics applications. This study provides a new route to realizing scalable field electron emitter production.

NiMoO4-Coated NiCo2O4 Nanotip Arrays on Carbon Cloth as Flexible and Effective Electrodes for Glu Detection

NiMoO₄-Coated NiCo₂O₄ Nanotip Arrays on Carbon Cloth as Flexible and Effective Electrodes for Glu Detection

Plasmonic gold nanojets fabricated by a femtosecond laser irradiation.

Gold nanojets with various morphologies, from nanopillar to nanotip with up to 800 nm height, and finally to nanotip with droplet, are fabricated on gold thin film by a femtosecond laser irradiation. The near-field localized surface plasmon resonance (LSPR) and photothermal effects of gold nanojets are studied through finite element electromagnetic (EM) analysis, supporting in nanojets design for potential applications of high-resolution imaging, nanomanipulation and sensing. For an individual nanotip, the confined electron oscillations in LSPR lead to an intense local EM field up to three orders of magnitude stronger than the incident field strength at the end of gold tip, where the vertical resolution for the field enhancement was improved down to nanoscale due to the small size of the sharp gold tip (5-nm-radius). At specific wavelength, nanopillar can serve as an effective light-to-heat converter and its heating can be fine-tuned by external irradiation, and its dimension. The long-range periodic nanojet arrays (periods from 1.5 µm to 2.5 µm) with different geometry were printed using several pulse energy levels. By confining more light into the tip (two orders of magnitude stronger than single tip), nanotip array shows more pronounced potential to serve as a refractometric sensor due to their high sensitivity and reproducibility. These results promote fs laser printing as a high-precision tool for nanoarchitecture in optical imaging, nanomanipulation and sensing application.

Thermal Oxidation Fabricated Copper Oxide Nanotip Arrays with Tunable Wettability and Robust Stability: Implications for Microfluidic Devices and Oil/Water Separation

Materials with different wettability features have attracted intensive attention due to their outstanding performances and the broad application prospects. In this work, we report an applicable and...

Light absorption and nanofocusing on a tapered magnetic metasurface

A type of metasurface was constructed on a silicon wafer using a nanopatterned magnetic film to achieve ideal light absorption within a wide wavelength range of 3 μm–15 μm. Using the metasurface, the surface electrons could be localized efficiently into an arrayed planar magnetic nanotip and then modulated by configuring the surface architecture to produce remarkable infrared reflectivity variation. A theoretical analysis showed that the excited surface plasmon exhibit stronger electric field components at the common metal-to-air interface. The Tb14Fe68Co18 nanotip array provided more powerful nanofocusing and a lower infrared reflectivity than an array shaped on a traditional aluminum film. By adjusting the structural parameters of the nanorhombus array formed on the TbCo film system, the convergent light spot could be modulated to improve light absorption markedly.

Amplified fluorescence imaging using photonic Ag nanotip array: A comparative study on surface morphology effects

We studied amplified fluorescence and applied to fluorescent cell imaging using plasmonic Ag substrates with the different surface nano-morphologies of tip and porous shapes. For a comparative study, we deliberately fabricated wafer-scale Ag nanotip array and nanoporous Ag substrates by a nanoimprint lithography and a chemical reduction route, respectively. Time- and space-resolved spectroscopy precisely evaluated metal-induced fluorescence characteristics by analyzing fluorescence intensity and lifetime modulations. For the standard molecular probe, the photonic Ag nanotip array showed slightly enhanced fluorescence compared to the cases on the irregular nanoporous Ag substrate and a bare glass. On the contrary, from the green fluorescent protein (GFP) expressed cell imaging study, fluorescence was remarkably amplified on the photonic nanotip array compared to that simply placed on a bare glass, whereas the GFP expressed cells suffered fluorescence quenching on the nanoporous Ag. The observed notable amplification of cell fluorescence on the photonic nanotip array is ascribed to the multiple constructive effects: the nanotip array for improved cell adherence, accumulated optical fields surrounding Ag nanotips, and photonic band-gap effect from the regularly arrayed nanotips.

Ultrafast and On-Demand Oil/Water Separation Membrane System Based on Conducting Polymer Nanotip Arrays.

Ultrafast oil/water separation based on tunable superwettability switch remains a big challenge. Here, inspired by the ultrafast water transport mechanism in sarracenia, we develop a micro/nanostructured porous membrane with conducting polymer nanotip arrays through the surface-initiated polymerizations. By modulating the height (ranging from 49-529 nm) and redox states of nanotips, a smart reversible superwettability switch is facile to obtain with contact angles of water/oil arranging from 161° to about 0°. Besides, liquid transport speed was accelerated more than 1.5 times by increasing the nanotip length. The water flux could reach up to 50326 L m-2 h-1 (1000 times that of a typical industrial ultrafiltration membrane). This is attributed to the stable and continuous water film along the nanotips, which provide a lubrication layer, leading to an increase of permeability. This work provides significant insights into macro/nanostructured membrane design for smart separation, blood lipid filtration, and smart nanoreactors with high permeability.

Controlled fabrication of gold nanotip arrays by nanomolding-necking technology

The fabrication of nanotips has been driven by the increasing industrial demands in developing high-performance multifunctional nanodevices. In this work, we proposed a controlled, rapid as well as low cost nanomolding-necking technology to fabricate gold nanotips arrays. The geometries of gold nanotips having cone angle range of ∼28–77° and curvature radii of <5 nm can be prepared by tailoring the diameters of raw nanorods in nanomolding process or modulating the necking temperature. Molecular dynamics simulation reveals that the formation of the nanotip geometry is determined by the interplay between dislocation-based and diffusion-based deformation mechanisms, intrinsically arising from the nonlinear dependence of atom diffusion on temperature and sample size. The good controllability, mass production and low cost of the developed nanomolding-necking technology make it highly promising in developing nanodevices for a wide range of applications, such as probing, sensing, antireflection coating and nanoindentation.

Design of wafer-scale uniform Au nanotip array by ion irradiation for enhanced single conductive filament resistive switching

Here, we report a simple and low-cost method to fabricate wafer-scale uniform and flexible nanotip array substrates via ion irradiation and replica-molding process. The diameter of the nanotip can be accurately modulated, and the minimum tip size can reach approximately 50 nm. Then, we exploit this nanotip array to fabricate high-performance single conductive filament (CF) resistive switching memory. As a result, the tailored Ag/HfO2/Au nanotip resistive random access memory (RRAM) devices on polyethylene terephthalate (PET) substrates show good flexibility, a large switching window over 108, an ultralow off-state current of 10−12 A, and low and uniform programming voltages. It is believed that the randomness of the CFs is depressed by the enhanced local electric field near the Au nanotips, contributing to the reliable performance of the tailored Ag/HfO2/Au nanotip/PET-based RRAM devices. This novel method paves the way for fabrication of large single-filament flexible RRAM device arrays.

Dual microelectrodes decorated with nanotip arrays: Fabrication, characterization and spectroelectrochemical sensing

Gold microelectrodes decorated with nanotips were developed for spectroelectrochemical experiments. These new dual probes were fabricated by combining fabrication processes of scanning near-field optical microscopy with photolithography. A nanotip array was produced at the surface of a coherent optical fiber bundle by a wet chemical etching step. The resulting nanostructured surface was sputter-coated with a thin gold layer. This gold film conferred plasmonic properties to the sharp nanotips and served as well as the electrode material to enable electrochemical reactions. A photolithographic process was used to delimit on the bundle surface nanotips-decorated microelectrodes with tunable dimensions (radii ranging between 20 μm and 3.5 μm) individually or in an array format. The resulting microelectrodes with a regular nanotip pattern were characterized first by cyclic voltammetry. Numerical simulation was used to assess the electrochemical properties of these platforms and the influences of the recessed geometry and of the nanotips. Approach curves were recorded in negative and positive feedback modes of scanning electrochemical microscopy (SECM) on insulating and conducting substrates. Finally, spatially resolved Raman imaging allowed us to detect a mercaptobenzoic acid monolayer adsorbed on the microelectrode surface, demonstrating a surface-enhanced Raman scattering (SERS) effect induced by the gold-coated nanotips with a typical enhancement factor of ∼7 × 104. Such an approach introduces a reproducible method to fabricate promising SERS-active platforms with microelectrode behavior for SECM experiments.

Label-free SERS diagnostics of radiation-induced injury via detecting the biomarker Raman signal in the serum and urine bio-samples based on Au-NPs array substrates.

A sensitive approach based on surface enhanced Raman spectroscopy (SERS) has been developed to evaluate the radiation caused biological injury. To achieve the effective SERS substrate, canonical anodic aluminum oxide (AAO) templates with regular array of nanotips were fabricated, and by plasma sputtering the gold nanoparticles (Au-NPs) were distributed on the nanotips to form the Au-NPs array with plenty of hotspots. The SERS substrates were utilized to examine the serum samples taken from the mice with the treatment of total body irradiation (TBI) of X-ray. The impact of TBI on the mice was analyzed and it was found that the SERS peak intensity at 532 cm-1 increased as a function of duration or dose of TBI. We confirmed that this Raman signature belongs to the myoglobin as a biomarker for the muscle damage due to the radiation caused injury. Furthermore, we also tested several blood and urine specimen of cancer patients who received radiotherapy. The results showed that our approach to some extent could distinguish the bio-samples from normal, X-ray treated and untreated individuals. Therefore, the proposed methodology may have the potential for prompt prognosis of radiation injury at early stage.

Nanofocusing on gold planar nanotip arrays

Nanofocusing of incident light in the visible and infrared regions is achieved using tapered metallic nanostructures (TMNSs). Illumination under 633 nm excitation produces a very small (∼40 nm in the x-direction), very bright spot with a specific geometry, demonstrating near-field nanofocusing of the incident beam to the deep subwavelength scale. The key processes, including a theoretical understanding, numerical calculation, and a near-field optical measurement involving the metallic planar nanotip arrays, are discussed. As in the lightning-rod effect, there is a large number of surface states to accommodate free electrons, resulting in a very high surface density distribution of the free electrons over the nanoapexes. A theoretical model for calculating the free-electron distribution based on the surface energy state of the nanoapex is established. The spread of free electron oscillation, including the guiding of free electrons towards nearby planar nanoapexes to obtain near-field nanofocusing is examined. As the lightning-rod effect is a broadband phenomenon, the TMNSs are also examined in the far-infrared (far-IR) region, at the typical far-IR wavelength of 10.274 μm. It should be noted that the IR radiation can also be focused into a linear light-spot of ∼20 nm, far beyond the IR diffraction limit. A metasurface constructed by the orderly arrangement of TMNSs highlights their potential in applications such as surface enhanced Raman spectroscopy, ray absorbing materials, and low-cost nanolithography.

Structural modification of ZnO nanorod array through Fe-doping: Ramification on UV and humidity sensing properties

Iron (Fe)-doped zinc oxide (ZnO) nanotip arrays (FZO) were prepared using low temperature sol–gel immersion method. The X-ray diffraction analysis reveals that the crystallite size reduces from 30.7 to 28.3 nm after doped with Fe. Besides, it is perceived that the lattice strain metamorphoses from tensile to compressive strain after doping with value of -0.23%. Field emission scanning electron microscopy images show that the Fe doping induced a significant modification to the ZnO structure; transforming the structure from rod-like to tip-like structure. The current–voltage measurement results indicate that the FZO film has good electrical properties with resistance value of 12.34 MΩ. The FZO is also capable of performing well as UV and humidity sensors. This study points out that FZO films are suitable for multifunctional uses particularly for UV and humidity sensing.

Highly parallel remote SPR detection of DNA hybridization by micropillar optical arrays.

Remote detection by surface plasmon resonance (SPR) is demonstrated through microstructured optical arrays of conical nanotips or micropillars. Both geometries were fabricated by controlled wet chemical etching of bundles comprising several thousands of individual optical fibers. Their surface was coated by a thin gold layer in order to confer SPR properties. The sensitivity and resolution of both shapes were evaluated as a function of global optical index changes in remote detection mode performed by imaging through the etched optical fiber bundle itself. With optimized geometry of micropillar arrays, resolution was increased up to 10-4 refractive index units. The gold-coated micropillar arrays were functionalized with DNA and were able to monitor remotely the kinetics of DNA hybridization with complementary strands. We demonstrate for the first time highly parallel remote SPR detection of DNA via microstructured optical arrays. The obtained SPR sensitivity combined with the remote intrinsic properties of the optical fiber bundles should find promising applications in biosensing, remote SPR imaging, a lab-on-fiber platform dedicated to biomolecular analysis, and in vivo endoscopic diagnosis. Graphical abstract We present a single fabrication step to structure simultaneously all the individual cores of an optical fiber bundle composed of thousands of fibers. The resulting sensor is optimized for reflection mode (compatible with in vivo applications) and is used to perform for the first time highly parallel remote SPR detection of DNA via several thousands of individual optical fiber SPR sensors paving the way for multiplexed biological detection.