Modified Nanosphere Lithography Research Articles

Modified nanosphere lithography: application to the fabrication of ordered arrays of gold nanopyramids

In this work a modified approach to improve the capabilities of nanosphere lithography was performed to obtain ordered arrays of gold nanopyramids that can be used as Surface-Enhanced Raman Scattering (SERS) substrates. Specifically, shadow sphere lithography in combination with nanosphere lithography was used to fabricate metallic nanostructures of different sizes starting from the same mask of silica microspheres. An extensive study of the obtained samples with scanning electron microscopy, atomic force microscopy and Raman characterizations confirms that indeed this modification of the standard nanosphere lithography is an excellent candidate to produce ordered arrays of Au nanopyramids with good control on their geometric properties. Rhodamine 6G dye was used as a probe molecule to test our SERS substrates. These features can be used in turn to optimize the SERS substrates for several potential applications. Remarkably, it is shown that by this simple technique one can achieve enhancement factors and detection capabilities of dyes that are comparable with those obtained by using more sophisticated techniques as proposed recently in the literature.

Novel Nanofabricated Mo Field-Emitter Array for Low-Cost and Large-Area Application

This paper demonstrates a novel nanofabrication technology for producing high-density gated molybdenum (Mo) field-emitter array (FEA) based on modified nanosphere lithography (NSL). The arc-shaped side wall of gate apertures introduced by the conventional NSL process put obstacles in the formation of sharp cone-type emitters. In this paper, a two-method solution was employed to facilitate the realization of gate apertures with sharper cutting side wall which are beneficial for the formation of sharper gated Mo emitters. One method was softening nanosphere to enlarge the contact area between nanosphere and substrate. The other was optimizing gate film deposition conditions to dramatically reduce the diffraction effects. Due to the improvements in the geometrical shape of gated Mo emitters, a low-voltage working high-density gated Mo FEA was demonstrated in this paper. This is the first observation of remarkable electron emission from high-density gated Mo FEA fabricated using such as low-cost, high-throughput, and high-efficiency technique which is expected to substantially overcome the challenges in the traditional fabrication process and promotes the low-cost and large-area application of high-density gated field-emitter devices. In addition, the novel nanofabricating method is expected to offer a simple and scalable route which allows fabrication of many gated field-emission devices.

Optimizing a Long-Range Highly Ordered Plasmonic Nanoarray Pattern for Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) is one of the most promising sensing techniques as it is non-invasive, non-destructive, and can reveal the spectral “fingerprints” of molecules. However, it remains a great challenge to fabricate large-scale, highly reproducible SERS substrates. In this work, we have developed a novel method to fabricate a long-range, highly ordered, plasmonic gold nano-ring array pattern based on a modified nanosphere lithography technique. The optical properties of gold nanoring array have been comprehensively studied and optimized for SERS by tuning the geometric parameters of the nanoring arrays using a combination of finite-difference time-domain (FDTD) simulations and cleanroom fabrication techniques. The optimized gold nanoring array has been further demonstrated for SERS biosensing.

Tunable Plasmonic SERS “Hotspots” on Au-Film Over Nanosphere by Rapid Thermal Annealing

In this work, a novel method combining fabrication of nonlithographic Au-film over nanosphere (AuFON) with gradual rapid thermal annealing (RTA) treatment was developed to produce a high performance of surface-enhanced Raman scattering (SERS) substrate. The highly uniform and periodic AuFON was achieved by the downscaling processs in the modified nanosphere lithography technique using 100-nm polystyrene (PS) nanospheres, which target significant SERS enhancement factor ${\rm{(EF}}_{{\rm{SERS}}})$ due to effectively generated “hotspots.” Two essential approaches in the nanostructure downscaling included the application of oxygen plasma or solvent ratio adjustment during PS nanospheres self-assembly drop-casting procedures. After the production of AuFON substrates, short RTA process on the AuFON was conducted at 250 °C, 500 °C, and 750 °C, respectively, to increase surface roughness that contribute to the improved SERS activity. Results show that the higher temperature of RTA linearly resulted in structural alteration of Au-film confined on PS spherical lining, which led to an eventual enhancement of SERS response on the detection of 1 μM Rhodamine 6G Raman probe. The most effective SERS reinforcement was demonstrated by ${\rm{(EF}}_{{\rm{SERS}}})$ value of 1.08 × 108 by 750 °C of RTA. Overall, a downscaled nonlithographic SERS substrate with tunable plasmonic response by RTA process was successfully fabricated.

Metallic Nanomesh with Disordered Dual-Size Apertures As Wide-Viewing-Angle Transparent Conductive Electrode

With the rapid development of display-related markets, transparent conductive films (TCFs) with wide viewing angles, high transmittance and low sheet resistance are in high demand. However, as a promising TCF material, metallic membranes with a submicrometer-sized periodicity pattern fabricated by currently available techniques always reveal the angle-dependent structure color which can be a major issue in the development of wide-angle viewing display-related applications. In this work, we demonstrate an Au nanomesh with disordered dual-size apertures as a novel TCF with wide viewing angles which is made via a modified nanosphere lithography technique. The as-prepared Au nanomesh film shows good optoelectronic properties (Rs = 160 Ω sq(-1), T = 80%; Rs = 8 Ω sq(-1), T = 57%) that are similar to the Au nanomesh with single size apertures, while the former exhibits excellent wide-angle viewing performance. There is no obvious change in the film when the viewing angle, the light incidence angle or the orientation of substrate vary in the range of 0-90°. In contrast, a rainbow color is observed with the film with ordered single-size apertures. Electrochromic devices based on the novel metallic film show more uniform color distribution than the devices based on metallic film with ordered single-size apertures under indoor natural light irradiation. These findings demonstrate the applicability of the Au nanomesh film with dual-size apertures in enhancing display quality of high-performance optoelectronic devices.

Ultrathin plasmonic frequency selective surface with subwavelength hole arrays

ABSTRACTManipulation of optical transmission using plasmonic nanostructures is a significant issue in applications related to ultra‐compact optoelectronic devices. In this paper, we present a type of plasmonic frequency selective surface constructed with an ultrathin gold film (∼20 nm) with arrays of subwavelength holes. By means of modified nanosphere lithography, we fabricate the nanostructured gold films with scalable sizes and tunable transmission in the visible and near‐infrared range. Moreover, electromagnetic simulations are performed and demonstrate good agreement with experimental results and further reveal the evolution of optical properties from the optically thick film to the ultrathin film. The research achievements will provide significant design guidance of ultrathin plasmonic frequency selective surface for future nanophotonic devices. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:2171–2176, 2016

Large-area, size-tunable Si nanopillar arrays with enhanced antireflective and plasmonic properties

In this paper, a novel method using the modified Langmuir–Blodgett and float-transfer techniques was introduced to construct the perfect PS monolayer nanosphere template with large area up to cm2. Based on such templates, the diameter, length, packing density, and the shape of Si nanopillar arrays (Si NPAs) could be precisely controlled and tuned through the modified nanosphere lithography combined with a metal-assisted chemical etching (NSL-MACE) method. Manipulation of the etching time can effectively avoid permanent deformation/clumping to generate size-tunable Si NPAs. The optical properties of the Si NPAs can be controlled by the Si NPA morphologies resulting from the different reactive ion etching (RIE) time and chemical etching time. The enhanced antireflective property and electromagnetic field effect of Au/Si NPAs were proved by the results. The new modified NSL-MACE technique with the capability of scale-up fabrication of Si NPAs would be helpful for potential applications in optoelectronic devices.

Magnetic switching of nanoscale antidot lattices.

We investigate the rich magnetic switching properties of nanoscale antidot lattices in the 200 nm regime. In-plane magnetized Fe, Co, and Permalloy (Py) as well as out-of-plane magnetized GdFe antidot films are prepared by a modified nanosphere lithography allowing for non-close packed voids in a magnetic film. We present a magnetometry protocol based on magneto-optical Kerr microscopy elucidating the switching modes using first-order reversal curves. The combination of various magnetometry and magnetic microscopy techniques as well as micromagnetic simulations delivers a thorough understanding of the switching modes. While part of the investigations has been published before, we summarize these results and add significant new insights in the magnetism of exchange-coupled antidot lattices.

Three-dimensional hexagonal GDC interlayer for area enhancement of low-temperature solid oxide fuel cells

A three-dimensional hexagonal gadolinium-doped ceria (GDC) interlayer is fabricated to enhance the cathodic surface area of low-temperature solid oxide fuel cells (LT-SOFCs). By using modified nanosphere lithography method, 3-D nanostructure is successfully fabricated. As a result of the nanostructuring, the 3-D hexagonal GDC interlayer has a 70% increase in cathodic surface area. Electrochemical impedance spectroscopy reveals a 2-fold decrease in the electrode resistance compared with a plane cell, resulting in a current–voltage behavior with a 60% higher peak power density. The superior electrode reaction is attributed to the large surface area (~1.7 times larger) due to the 3-D hexagonal cathodic interlayer. This novel fabrication method can be applied to almost all kinds of LT-SOFC structures, including large-scale and substrate-supported platforms, and thereby it will contribute to commercializing LT-SOFCs.

Metal Organic Frameworks (MOFs) Cooperated Si Nanorod Arrays Used in Rechargeable Batteries

Given the large surface area and unique channels, metal-organic frameworks (MOFs),1 have served as a “generalist” in the fields of clean energy,2 such as adsorbents for hydrogen,3 electrolytes for fuel cells,4 electrode materials for supercapacitors5 and so on. However, only a few immature applications of MOFs as electrodes for lithium ion batteries (LIBs) or micro-LIBs have been researched.6-9To the contrary, Si anode, with the highest theoretical capacity, low working potential and feasible integration with other micro- or nano-electronic devices, has been intensively investigated experimentally and theoretically to address the volume expansion issue during discharge/charge processes. But, fully addressing the deterioration of the electrode structures caused by the volume expansion, which would generally results in performance fading and safety issues, is still challenging. In this work, a novel class of MOFs, was initially proposed to composite/network with Si nanorod (NR) arrays as anodes for LIBs employing the modified nanosphere lithography (NSL) and inductive coupled plasma (ICP) dry etching followed by a method of solution growth. An enhanced capacity was accomplished in this hybrid material nanocomposite electrode compare with that in the only Si NR anodes. In addition, with the employment of a buffer layer of Ti/TiN to modify the electronic conductivity of the interfaces between Si inner core and outer MOFs, a further enhanced capacity was realized accompanied with the high Coulombic Efficiency (CE).First principles calculations were performed to verify the favorable lithium diffusion in the MOF cooperated Si structure.

Template-Assisted Electrochemical Liquid-Liquid-Solid Deposition of Crystalline Ge Nanowires

Electrodeposition of crystalline Ge nanowires from GeO2 aqueous solutions through an electrochemical liquid-liquid-solid (ec-LLS) process using Ga nano/microdroplets of various sizes has been demonstrated. A silica template featuring regularly spaced opening with defined sizes was prepared on Si substrates by a modified nanosphere lithography patterning process. These platforms were used to electrodeposit Ga droplet arrays with defined sizes and pitch. The Ga nano/microdroplets subsequently enabled growth of crystalline Ge nanowires through ec-LLS in aqueous electrolyte as a function of droplet size. Scanning electron micrographs of the as-produced Ge nanowires showed the base size of the Ge nanowires tracks the Ga droplet diameter. All nanowires featured a consistent taper, with a cone angle of approximately 10°. For the Ga droplets larger than 1 µm in diameter, no nanowires were observed under the employed ec-LLS conditions, but a large isotropic Ge deposit was formed. These cumulative results demonstrate effects that liquid metal size(volume) have on electrodeposition by ec-LLS.

Fabrication of size‐tunable hierarchical CrN nanohole arrays for two‐dimensional nanomould using modified nanosphere lithography

This Letter presents a low-cost, high-throughput strategy for fabricating size-tunable hierarchical CrN nanohole arrays for a nanomould using a magnetron sputtering approach with nanosphere lithography (NIL)-based technology. The size of the polystyrene nanospheres has a direct influence on the diameter and period of the CrN nanohole structure. The reactive ion etching and magnetron sputtering process can easily control the nanohole size and depth. The hole depth generally depends on the film thickness. The contact angles were measured by the sessile drop method using distilled water, ethylene glycol and diiodomethane. The surface-free energy of the CrN nanomould was calculated using the Owens–Wendt geometric mean approach. This Letter reports the successful fabrication of a series of nanoholes with diameters decreased from 347 ± 9.6 to approximately 166 ± 11.8 nm with a depth of 100 ± 5.6 nm. The corresponding surface-free energy decreased from 40.83 to approximately 24.58 mN/m. The diameter of nanoholes has an obvious effect on the surface-free energy and there is tendency for the surface-free energy to decrease with the decrease in the diameter of the nanoholes. This new approach of ordered CrN nanohole array structures can be used to create a two-dimensional nanomould for NIL.

Efficiency enhancement of InGaN/GaN solar cells with nanostructures

We demonstrate InGaN/GaN multi-quantum-well solar cells with nanostructures operating at a wavelength of 520 nm. Nanostructures with a periodic nanorod or nanohole array are fabricated by means of modified nanosphere lithography. Under 1 sun air-mass 1.5 global spectrum illumination, a fill factor of 50 and an open circuit voltage of 1.9 V are achieved in spite of very high indium content in InGaN alloys usually causing degradation of crystal quality. Both the nanorod array and the nanohole array significantly improve the performance of solar cells, while a larger enhancement is observed for the nanohole array, where the conversion efficiency is enhanced by 51%.

Fabrication of two-dimensional InGaN/GaN photonic crystal structure using a modified nanosphere lithography technique

By means of combining a very cost-effective lift-off process and a nanosphere lithography technique, we have fabricated two dimensional (2D) photonic crystal (PhC) structures on an InGaN/GaN multiple quantum well structure. Significant enhancement in photoluminescence (PL) intensity has been observed when the emission wavelength is within the photonic bandgap. Time-resolved PL measurements have shown that the spontaneous emission rate is strongly reduced by a factor of ∼4 due to the PhC effect. As a consequence, the emission intensity along 2D PhC slab-plane directions is effectively suppressed and redistributed to the direction normal to the 2D PhC slab-plane simultaneously. Temperature-dependent PL measurements have confirmed that the enhanced PL intensity is due to an increase in extraction efficiency as a result of the PhC effect.

Effect of Fabricating Nanopatterns on GaN-Based Light Emitting Diodes by a New Way of Nanosphere Lithography

Nanosphere lithography is an inexpensive, simple, high-throughput nanofabrication process. NSL can be done in different ways, such as drop coating, spin coating or by means of tilted evaporation. Nitride-based light-emitting diodes (LEDs) are applied in different places, such as liquid crystal displays and traffic signals. The characteristics of gallium nitride (GaN)-based LEDs can be enhanced by fabricating nanopatterns on the top surface of the LEDs. In this work, we created differently sized (420, 320 and 140 nm) nanopatterns on the upper surfaces of GaN-based LEDs using a modified nanosphere lithography technique. This technique is quite different from conventional NSL. The characterization of the patterned GaN-based LEDs revealed a dependence on the size of the holes in the pattern created on the LED surface. The depths of the patterns were 80 nm as confirmed by AFM. Both the photoluminescence and electroluminescence intensities of the patterned LEDs were found to increase with an increase in the size of holes in the pattern. The light output power of the 420-nm hole-patterned LED was 1.16 times higher than that of a conventional LED. Moreover, the current-voltage characteristics were improved with the fabrication of differently sized patterns over the LED surface using the proposed nanosphere lithography method.

Fabrication of size-tunable hierarchical porous Cr nanoring arrays by modified nanosphere lithography

This study presents a low-cost and high-throughput strategy for fabricating size-tunable hierarchical porous Cr nanoring arrays using modified nanosphere lithography-based technology. The period and diameter of the Cr nanorings can be easily controlled by the initial diameter of the self-assembled nanospheres and the following Cr coating and reactive ion-etching processes. The wettability can be manipulated by changing the pore size and the height of the nanorings. The optical transmittance of the periodic porous Cr nanoring arrays was enhanced because of a surface plasmon resonance effect. This new approach will surely facilitate further exploration of the structure of hierarchical porous Cr nanoring arrays for potential applications in interesting scientific fields such as optical sensors and electro-optic devices.

Fabrication of size-tunable Cr nanodots and nanorings array by modified nanosphere lithography

This Letter is devoted to developing a novel and low-cost strategy for fabricating Cr triangular-shaped nanodot and nanoring array patterns by a distinct magnetron sputtering approach using nanosphere lithography-based technology. The size of polystyrene nanospheres had a direct influence on the size and period of Cr nanorings. The lateral sizes, inner sizes, wall thickness and wall height are controllable by varying the process conditions of reactive ion etching, magnetron sputtering and the lift-off process. The wettability can be manipulated by changing the size of the nanorings. This new approach will surely facilitate further exploration of metal nanorings for potential applications in electro-optic devices.

Fabrication of size-tunable, periodic Si nanohole arrays by plasma modified nanosphere lithography and anisotropic wet etching

In this study, we propose a new, facile and efficient method to define the desired patterns on Si surfaces and to fabricate periodic arrays of size- and position-controllable Si nanoholes on blank- and pre-patterned (001)Si substrates, which is based on the O2 plasma modified nanosphere lithography with an anisotropic wet etching technique. In contrast to earlier approaches, no additional thin-film deposition equipments and metal film hard masks are needed for this method. The Si nanohole arrays exhibit the same hexagonal arrangement as the initial colloidal nanospheres template. The sizes of the Si nanoholes can be tuned from 150nm to 480nm by varying the etching time. The Si substrates with nanohole-textured surfaces exhibit strong antireflection properties. UV–Vis spectroscopic measurements revealed that by increasing the nanohole size, the optical reflectance of the nanohole-structured (001)Si surfaces was gradually decreased down to less than 8%. Since the nanohole sizes, shapes and spacings can be readily controlled by adjusting the diameters of the colloidal nanospheres and anisotropic wet etching conditions, the combined approach presented here provides the capability to fabricate a variety of nanohole array structures on various Si-based substrates without complex lithography.

High aspect ratio silicon and polyimide nanopillars by combination of nanosphere lithography and intermediate mask pattern transfer

Graphical abstractDisplay Omitted Highlights? Novel multilayer mask stack for high aspect ratio etching using nanosphere lithography. ? Silicon nanopillars with 75nm diameter and an aspect ratio of 17. ? High aspect ratio polyimide nanopillars with sub-100nm diameter. ? Silicon pillar covered surface showed superhydrophilic properties under repeated wetting conditions. Silicon and polymer nanopillar structures with sub-100nm diameter and high aspect ratios were fabricated using a modified nanosphere lithography process. A thin silicon film was sputtered onto spin-coated polyimide films. A self-assembled layer of nanospheres was then formed on the silicon. Reactive ion etching with SF6/C4F8 was used to transfer the nanosphere pattern to the silicon. Oxygen plasma etching then transferred the silicon pattern into the polymer to create nanopillars. The nanopillar diameter could be finely tuned by oxygen plasma thinning of the nanospheres and the conditions for silicon mask etching. In the last step the polyimide pillars served as etch masks to transfer the structures back into the silicon substrate to give well-ordered pillars of 1.3µm height and 75nm diameter.

Nanoribbon Plasmonic Gratings and their Anomalous Interaction with Electromagnetic Waves

An inexpensive method to fabricate arrays of metallic nanoribbons on planar substrates by a modified nanosphere lithography is developed. The transmission characteristics of the synthesized Au and Fe gratings deposited on transparent substrates are studied experimentially and using simulations in the visible and infrared frequency ranges. Plasmonic effects control these characteristics for the Au gratings in the visible range, leading to their anomalous, reverse polarizer action.