High Aspect Ratio Nanostructures Research Articles

A route to engineered high aspect-ratio silicon nanostructures through regenerative secondary mask lithography

Silicon nanostructuring imparts unique material properties including antireflectivity, antifogging, anti-icing, self-cleaning, and/or antimicrobial activity. To tune these properties however, a good control over features’ size and shape is essential. Here, a versatile fabrication process is presented to achieve tailored silicon nanostructures (thin/thick pillars, sharp/truncated/re-entrant cones), of pitch down to ∼50 nm, and high-aspect ratio (>10). The approach relies on pre-assembled block copolymer (BCP) micelles and their direct transfer into a glass hard mask of an arbitrary thickness, now enabled by our recently reported regenerative secondary mask lithography. During this pattern transfer, not only can the mask diameter be decreased but also uniquely increased, constituting the first method to achieve such tunability without necessitating a different molecular weight BCP. Consequently, the hard mask modulation (height, diameter) advances the flexibility in attainable inter-pillar spacing, aspect ratios, and re-entrant profiles (= glass on silicon). Combined with adjusted silicon etch conditions, the morphology of nanopatterns can be highly customized. The process control and scalability enable uniform patterning of a 6-inch wafer which is verified through cross-wafer excellent antireflectivity (<5%) and water-repellency (advancing contact angle 158°; hysteresis 1°). The implementation of this approach to silicon nanostructuring is envisioned to be far-reaching, facilitating fundamental studies and targeting applications spanning solar panels, antifogging/antibacterial surfaces, sensing, amongst many others.

Approaches for the RCWA-based non-destructive characterization of subwavelength-structured gratings

Nano-structuring enables us to add additional degrees of freedom to the design of optical elements. Especially the possibility of controlling the polarization is of great interest in the field of nano-structured optics. For being able to exploit the whole range of form-birefringent phase shifts, the aspect ratios of the resulting element are typically much higher than the aspect ratios of conventional diffractive optical elements (DOEs), which does not only pose a challenge on fabrication but also on characterization. We evaluate several well-established approaches for the nondestructive characterization, including Müller-Matrix-Ellipsometry, measurement of the diffraction efficiencies, scattering measurements and calibration with rigorous coupled-wave modelling. The goal is to understand the challenges with all these techniques and combine them to a reliable method for structural reconnaisance of high aspect ratio nanostructures.

High-efficiency diffraction gratings for EUV and soft x-rays using spin-on-carbon underlayers

We report on the fabrication and characterization of high-resolution gratings with high efficiency in the extreme ultraviolet (EUV) and soft x-ray ranges using spin-on-carbon (SOC) underlayers. We demonstrate the fabrication of diffraction gratings down to 20 nm half-pitch (HP) on Si3N4 membranes with a bilayer of hydrogen silsesquioxane (HSQ) and spin-on-carbon and show their performance as a grating mask for extreme ultraviolet interference lithography (EUV-IL). High-resolution patterning of HSQ is possible only for thin films due to pattern collapse. The combination of this high-resolution resist with SOC circumvents this problem and enables the fabrication of high aspect ratio nanostructures. Rigorous coupled-wave analysis shows that the bilayer gratings exhibit higher diffraction efficiency than what is feasible with a grating made of HSQ. We also demonstrate a simple and accurate method to experimentally measure the diffraction efficiency of high-resolution gratings by measuring the relative ratio of the dose-to-clear curves of the photoresist. The measured diffraction efficiencies are in good agreement with the theoretically predicted values. Furthermore, we verify our calculations and measurements by printing line/space patterns in chemically amplified resists down to 10 nm HP with both HSQ and bilayer grating masks using EUV-IL. The improved diffraction efficiency of the bilayers is expected to have applications not only in gratings for interference lithography, but also in Fresnel zone plates and gratings for spectroscopy in the EUV and soft x-ray ranges.

Mechanical Stimulation after Centrifuge‐Free Nano‐Electroporative Transfection Is Efficient and Maintains Long‐Term T Cell Functionalities (Small 38/2021)

Transfection In article number 2103198, Andy Tay and Nicholas Melosh illustrate the use of high aspect-ratio hollow nanostructures coupled with transient electroporation for electrophoretic delivery of DNA into T cell. Magnetic stimulation was added subsequently to enhance intracellular DNA transport towards the nucleus to achieve high transfection efficiency. Facilitated by magnetic manipulation, the whole gene delivery process is centrifuge-free, making it compatible with end-to-end cell manufacturing.

Mechanical Stimulation after Centrifuge-Free Nano-Electroporative Transfection Is Efficient and Maintains Long-Term T Cell Functionalities.

Transfection is an essential step in genetic engineering and cell therapies. While a number of non-viral micro- and nano-technologies have been developed to deliver DNA plasmids into the cell cytoplasm, one of the most challenging and least efficient steps is DNA transport to and expression in the nucleus. Here, the magnetic nano-electro-injection (MagNEI) platform is described which makes use of oscillatory mechanical stimulation after cytoplasmic delivery with high aspect-ratio nano-structures to achieve stable (>2 weeks) net transfection efficiency (efficiency×viability) of 50% in primary human T cells. This is, to the best of the authors' knowledge, the highest net efficiency reported for primary T cells using a centrifuge-free, non-viral transfection method, in the absence of cell selection, and with a clinically relevant cargo size (>12kbp). Wireless mechanical stimulation downregulates the expression of microtubule motor protein gene, KIF2A, which increases local DNA concentration near the nuclei, resulting in enhanced DNA transfection. Magnetic forces also accelerate membrane repair by promoting actin cytoskeletal remodeling which preserves key biological attributes including cell proliferation and gene expressions. These results demonstrate MagNEI as a powerful non-viral transfection technique for progress toward fully closed, end-to-end T cell manufacturing with less human labor, lower production cost, and shorter delay.

Facile synthesis of dual-phase lithium titanate nanowires as anode materials for lithium-ion battery

A facile method to synthesize dual phase Li4Ti5O12-TiO2 nanowires and their performance as anode material for lithium-ion battery are investigated. Li4Ti5O12-TiO2 nanowires are obtained from titanium nanoparticles by applying the wet corrosion process for the nanostructure formation and subsequent ion exchange processes. Post-heat treatments are adopted to tune the mixed phase composition of the nanowires, which significantly influences the electrochemical performance. The dual phase nanowires-based anode materials with a TiO2: Li4Ti5O12 phase ratio of 7:3 exhibit the highest delithiation capacity of 180 mAh/g and 105 mAh/g at 0.5 C and 5 C, respectively. In addition, it shows a remarkable capacity retention of ~95% after 150 cycles at 5 C, which is a significant improvement compared to previous reports on Li4Ti5O12-TiO2 based anode materials synthesized at low temperatures. This dual phase anode material based on Li4Ti5O12-TiO2 nanowires combines two advantages: i) High aspect ratio nanostructures increase the surface area and shorten the diffusion path of lithium ions and ii) the dual phase accelerates ion/electron migration owing to abundant interfaces/grain boundaries. Consequently, the proposed synthesis route for Li4Ti5O12-TiO2 nanowires provides an effective and highly reproducible approach to fabricate dual phase Li4Ti5O12-TiO2 nanowires as anode materials for lithium ion batteries.

Optical, electrical and structural properties of Fe doped sodium titanate nanostructures

In this work we present the synthesis, optical and electrical characterization of nanoparticles of iron doped anatase and iron doped sodium titanate high aspect ratio nanostructures (Na2Ti2O5·H2O). Our study focused mainly in the study of the electronical structure and the derived properties of the dielectric function by DFT (Density Functional Theory) means and their relationship with experimental measurements of optical band gap and electric impedance spectroscopy. Also, structural and morphological characterizations of the systems were carried on by X-ray powder diffraction, high resolution transmission electron microscopy and Raman spectroscopy. It was possible for the doped nanostructures to observe a slight red shift in the band gap of about 0.2 eV and an order of magnitude decrease in the global resistance in the doped system.

Branched High Aspect Ratio Nanostructures Fabricated by Focused Helium Ion Beam Induced Deposition of an Insulator.

Helium ion beam induced deposition using the gaseous precursor pentamethylcyclopentasiloxane is employed to fabricate high aspect ratio insulator nanostructures (nanopillars and nanocylinders) that exhibit charge induced branching. The branched nanostructures are analyzed by transmission electron microscopy. It is found that the side branches form above a certain threshold height and that by increasing the flow rate of the precursor, the vertical growth rate and branching phenomenon can be significantly enhanced, with fractalesque branching patterns observed. The direct-write ion beam nanofabrication technique described herein offers a fast single-step method for the growth of high aspect ratio branched nanostructures with site-selective placement on the nanometer scale.

Replacing Metals with Oxides in Metal-Assisted Chemical Etching Enables Direct Fabrication of Silicon Nanowires by Solution Processing.

Metal-assisted chemical etching (MACE) has emerged as an effective method to fabricate high aspect ratio nanostructures. This method requires a catalytic mask that is generally composed of a metal. Here, we challenge the general view that the catalyst needs to be a metal by introducing oxide-assisted chemical etching (OACE). We perform etching with metal oxides such as RuO2 and IrO2 by transposing materials used in electrocatalysis to nanofabrication. These oxides can be solution-processed as polymers exhibiting similar capabilities of metals for MACE. Nanopatterned oxides can be obtained by direct nanoimprint lithography or block-copolymer lithography from chemical solution on a large scale. High aspect ratio silicon nanostructures were obtained at the sub-20 nm scale exclusively by cost-effective solution processing by halving the number of fabrication steps compared to MACE. In general, OACE is expected to stimulate new fundamental research on chemical etching assisted by other materials, providing new possibilities for device fabrication.

Thermal conductivity of individual Si and SiGe epitaxially integrated NWs by scanning thermal microscopy.

Semiconductor nanowires have demonstrated fascinating properties with application in a wide range of fields including energy and information technologies. In particular, increasing attention has been focused on Si and SiGe nanowires for application in thermoelectric generation after recent successful implementation in miniaturized devices. Despite this interest, an appropriate evaluation of thermal conductivity in such nanostructures still poses a great challenge, especially if the characterization of the device-integrated nanowire is desired. In this work, a spatially resolved technique based on scanning thermal microscopy has been demonstrated for the assessment of the thermal conductivity of Si and SiGe nanowires integrated in thermoelectrical microgenerators. Thermal conductivity values of 15.8 ± 0.8 W m-1 K-1 and 4.2 ± 0.3 W m-1 K-1 were measured for Si and SiGe nanowires, respectively, epitaxially grown on silicon microstructures. Moreover, the range of applicability according to the sample thermal conductance and associated errors are discussed to establish the potential of the novel technique. The results presented here show the remarkable utility of scanning thermal microscopy for the challenging thermal characterization of integrated nanostructures and the development of multiple devices such as thermoelectric generators or photovoltaic cells.

Ruthenium-Assisted Chemical Etching of Silicon: Enabling CMOS-Compatible 3D Semiconductor Device Nanofabrication.

The semiconductor industry's transition to three-dimensional (3D) logic and memory devices has revealed the limitations of plasma etching in reliable creation of vertical high aspect ratio (HAR) nanostructures. Metal-assisted chemical etch (MacEtch) can create ultra-HAR, taper-free nanostructures in silicon, but the catalyst used for reliable MacEtch-gold-is not CMOS (complementary metal-oxide-semiconductor)-compatible and therefore cannot be used in the semiconductor industry. Here, for the first time, we report a ruthenium MacEtch process that is comparable in quality to gold MacEtch. We introduce new process variables-catalyst plasma pretreatment and surface area-to achieve this result. Ruthenium is particularly desirable as it is not only CMOS-compatible but has also been introduced in semiconductor fabrication as an interconnect material. The results presented here remove a significant barrier to adoption of MacEtch for scalable fabrication of 3D semiconductor devices, sensors, and biodevices that can benefit from production in CMOS foundries.

(Invited) SiC and GaN High Aspect Ratio Nanostructures By Plasma-Free Photo-Assisted Inverse-Macetch

Etching wide bandgap semiconductors including SiC and GaN, has been challenging. Achieving damage-free device quality high aspect ratio nanostructures is even more difficult. Currently, to produce SiC or high aspect ratio structures, ICP-RIE is generally used, which often results in high energy ion induced damages.Metal-assisted chemical etching (MacEtch) defies the isotropic nature of chemical etch by utilizing a patterned metal catalyst for spatially defined etching of semiconductors. MacEtch has been successfully demonstrated for various types of semiconductors, including Si, GaAs, InGaAs, AlGaAs, InP, InGaP, GaN, b-Ga2O3, SiC. The metal catalyst type that has been proven to be effective include Au, Pt, Ir, Ru, TiN, CNTs, and graphene. Aspect ratios produced using MacEtch can readily exceed 100:1 in some cases, while limited in other cases. The resulted tapering and porosity are strong function of the etch kinetics for the particular semiconductor material, metal catalyst type and pattern, and etchant condition. The loading effect is determined by the local carrier generation rate and mass transport rate. Depending on how the etching evolves relative to the catalyst pattern and additional assistance, we classify MacEtch into forward, inverse (i-MacEtch), magnetic-fielded guided (h-MacEtch), self-anchored-catalyst (SAC-MacEtch), vapor phase (VP-MacEtch), and UV assisted (UV-MacEtch).In this work, we report the successful demonstration of SiC and GaN nanostructures with various sidewall morphologies using UV-assisted i-MacEtch. Produced structures are characterized as a function of etch conditions by SEM, 3D profilometer, AFM, reflection and Raman spectroscopy, as well as the interface properties using Schottky barrier diodes and MOSCAP structures. This work lays the foundation for producing 3D wide bandgap device structures with damage-free pristine surfaces and interfaces, as well as controlled defect engineering for quantum states.

Penetration mechanism of cells by vertical nanostructures.

Cell penetration by high aspect-ratio vertical nanostructures such as nanowires and nanopillars provides a powerful method for accessing the cell interior for delivery and sensing. However, there is a lack of studies on the understanding of the mechanism of cell membrane penetration and how design nanostructures to optimize the efficiency of penetration remains unclear. Here, we propose an analytical model to elucidate the mechanism of cells penetration by analyzing the free-energy change of cells adhered to the nanostructures surface. Furthermore, we provide a simple method to evaluate the crossover radius or density for cell membrane penetration. By introducing a dimensionless parameter, i.e., adhesion area factor, we investigated the effects of the radius and distribution densities of nanostructures on cell membrane penetration which is determined by the competition between adhesion energy and deformation energy. Besides, a diagram of the distribution of cell penetration and no penetration is obtained. From the cell penetration diagram, one can determine easily and intuitively the relations of cell penetration state with the radius and distribution densities of nanostructures. Our theoretical results seem to show broad agreement with experimental observations, which implies that these studies would provide useful guidance to the design of nanopatterned surfaces for biomedical applications.

Bacterial Envelope Damage Inflicted by Bioinspired Nanostructures Grown in a Hydrogel.

Surface-associated bacterial communities, known as biofilms, are responsible for a broad spectrum of infections in humans. Recent studies have indicated that surfaces containing nanoscale protrusions, like those in dragonfly wings, create a hostile niche for bacterial colonization and biofilm growth. This functionality has been mimicked on metals and semiconductors by creating nanopillars and other high aspect ratio nanostructures at the interface of these materials. However, bactericidal topographies have not been reported on clinically relevant hydrogels and highly compliant polymers, mostly because of the complexity of fabricating nanopatterns in hydrogels with precise control of the size that can also resist aqueous immersion. Here, we report the fabrication of bioinspired bactericidal nanostructures in bacterial cellulose (BC) hydrogels using low-energy ion beam irradiation. By challenging the currently accepted view, we show that the nanostructures grown in BC affect preferentially stiff membranes like those of the Gram-positive bacteria Bacillus subtilis in a time-dependent manner and, to a lesser extent, the more deformable and softer membrane of Escherichia coli. Moreover, the nanostructures in BC did not affect the viability of murine preosteoblasts. Using single-cell analysis, we demonstrate that indeed B. subtilis requires less force than E. coli to be penetrated by nanoprobes with dimensions comparable to those of the nanostructured BC, providing the first direct experimental evidence validating a mechanical model of membrane rupture via a tension-induced mechanism within the activation energy theory. Our findings bridge the gap between mechano-bactericidal surfaces and low-dimensional materials, including single-walled carbon nanotubes and graphene nanosheets, in which a higher bactericidal activity toward Gram-positive bacteria has been extensively reported. Our results also demonstrate the ability to confer bactericidal properties to a hydrogel by only altering its topography at the nanoscale and contribute to a better understanding of the bacterial mechanobiology, which is fundamental for the rational design bactericidal topographies.

Omnidirectional, Broadband Light Absorption in a Hierarchical Nanoturf Membrane for an Advanced Solar‐Vapor Generator

AbstractSolar‐thermal materials have been intensively studied in the context of production and localization of thermal energy, targeting an industry level application. Although photonic and optical strategies for enhancing light absorption have increased the efficiency of photo excitation/conversion into thermal energy, most of them have several limitations such as large area fabrication, thermal stability and broadband/omnidirectional light absorption. In this study, a gold‐coated hierarchical nanoturf membrane (Au/h‐Nanoturf membrane) incorporated with randomly distributed high aspect ratio (AR) nanostructures and micro‐through holes is proposed. The Au/h‐Nanoturf has peculiar black structures that provide advantages in forming a membrane with a large area and in absorbing broadband solar light spectrum. Furthermore, the membrane is combined with micro‐cone array which makes it exhibit exceptionally omnidirectional light absorption properties. Using computational analysis, it is confirmed that the micro‐cone array substantially contributes to the omnidirectional solar absorption irrespective of the wavelength. The optimized structural parameters for the maximum efficiency of the solar‐thermal materials are also found. The prepared solar‐vapor generator with the optimized structural features exhibits 91% average conversion efficiency under one sun condition. The efficiency is sustainable for up to four weeks. The highly efficient and omnidirectional broadband‐absorbing solar‐thermal membrane can be readily employed, targeting an industry level application.

Ion-Induced Nanopatterning of Bacterial Cellulose Hydrogels for Biosensing and Anti-Biofouling Interfaces

Hydrogels provide a solution-mimicking environment for the interaction with living systems that make them desirable for various biomedical and technological applications. Because relevant biological processes in living tissues occur at the biomolecular scale, hydrogel nanopatterning can be leveraged to attain novel material properties and functionalities. However, the fabrication of high aspect ratio (HAR) nanostructures in hydrogels capable of self-standing in aqueous environments, with fine control of the size and shape distribution, remains challenging. Here, we report the synthesis of nanostructures with a HAR in bacterial cellulose (BC) hydrogel via directed plasma nanosynthesis using argon ions. The nanostructures in BC are reproducible, stable to sterilization, and liquid immersion. Using in-situ surface characterization and semi-empirical modeling, we discovered that pattern formation was linked to the formation of graphite-like clusters composed of a mixture of C-C and C=C bonds. Moreover, our model predicts that reactive species at the onset of the argon irradiation accelerate the bond breaking of weak bonds, contributing to the formation of an amorphous carbon layer and nanopattern growth.

Magnetic Nanomaterials as Contrast Agents for MRI.

Magnetic Resonance Imaging (MRI) is a powerful, noninvasive and nondestructive technique, capable of providing three-dimensional (3D) images of living organisms. The use of magnetic contrast agents has allowed clinical researchers and analysts to significantly increase the sensitivity and specificity of MRI, since these agents change the intrinsic properties of the tissues within a living organism, increasing the information present in the images. Advances in nanotechnology and materials science, as well as the research of new magnetic effects, have been the driving forces that are propelling forward the use of magnetic nanostructures as promising alternatives to commercial contrast agents used in MRI. This review discusses the principles associated with the use of contrast agents in MRI, as well as the most recent reports focused on nanostructured contrast agents. The potential applications of gadolinium- (Gd) and manganese- (Mn) based nanomaterials and iron oxide nanoparticles in this imaging technique are discussed as well, from their magnetic behavior to the commonly used materials and nanoarchitectures. Additionally, recent efforts to develop new types of contrast agents based on synthetic antiferromagnetic and high aspect ratio nanostructures are also addressed. Furthermore, the application of these materials in theragnosis, either as contrast agents and controlled drug release systems, contrast agents and thermal therapy materials or contrast agents and radiosensitizers, is also presented.

Nanoscale Elastocapillary Effect Induced by Thin-Liquid-Film Instability.

Dense arrays of high-aspect-ratio (HAR) vertical nanostructures are essential elements of microelectronic components, photovoltaics, nanoelectromechanical, and energy storage devices. One of the critical challenges in manufacturing the HAR nanostructures is to prevent their capillary-induced aggregation during solution-based nanofabrication processes. Despite the importance of controlling capillary effects, the detailed mechanisms of how a solution interacts with nanostructures are not well understood. Using in situ liquid cell transmission electron microscopy (TEM), we track the dynamics of nanoscale drying process of HAR silicon (Si) nanopillars in real-time and identify a new mechanism responsible for pattern collapse and nanostructure aggregation. During drying, deflection and aggregation of nanopillars are driven by thin-liquid-film instability, which results in much stronger capillary interactions between the nanopillars than the commonly proposed lateral meniscus interaction forces. The importance of thin-film instability in dewetting has been overlooked in prevalent theories on elastocapillary aggregation. The new dynamic mechanism revealed by in situ visualization is essential for the development of robust nanofabrication processes.

Fabrication of ordered nanostructures on the Ge surface using electron-beam lithography and ion beam irradiation

Ordered nanostructure fabrication was performed via a two-step process using electron-beam lithography and ion beam irradiation. In the first process, a well-focused electron-beam was used to fabricate ordered holes on an electron-beam resist-coated Ge wafer. In the second process, an ion beam was used to irradiate the entire surface to promote nanostructure growth through the migration of ion beam-induced interstitial atoms. The formation of nanostructures with a 26 nm thickness, 59 nm height, and an aspect ratio of 2.2 was achieved in 60 nm diameter holes with equal center–center distances. High aspect ratio nanostructures were formed via limited migration of interstitial atoms on the walls. An estimated 1.1% point defect was obtained by changing the volume of the nanostructures after irradiation, which contributed to the growth of the structures.

A General and Programmable Synthesis of Graphene-Based Composite Aerogels by a Melamine-Sponge-Templated Hydrothermal Process

Three-dimensional (3D) graphene networks are performance boosters for functional nanostructures in energy-related fields. Although tremendous intriguing nanostructures-decorated 3D graphene network...