Nanopatterning Technique Research Articles

Proximity-field nanopatterning for high-performance chemical and mechanical sensor applications based on 3D nanostructures

In this era of the Internet of Things, the development of innovative sensors has rapidly accelerated with that of nanotechnology to accommodate various demands for smart applications. The practical use of three-dimensional (3D) nanostructured materials breaks several limitations of conventional sensors, including the large surface-to-volume ratio, precisely tunable pore size and porosity, and efficient signal transduction of 3D geometries. This review provides an in-depth discussion on recent advances in chemical and mechanical sensors based on 3D nanostructures, which are rationally designed and manufactured by advanced 3D nanofabrication techniques that consider structural factors (e.g., porosity, periodicity, and connectivity). In particular, we focus on a proximity-field nanopatterning technique that specializes in the production of periodic porous 3D nanostructures that satisfy the structural properties universally required to improve the performance of various sensor systems. State-of-the-art demonstrations of high-performance sensor devices such as supersensitive gas sensors and wearable strain sensors realized through designed 3D nanostructures are summarized. Finally, challenges and outlooks related to nanostructures and nanofabrication for the practical application of 3D nanostructure-based sensor systems are proposed.

Self-packaged high-resolution liquid metal nano-patterns

•Laser interference lithography is invented for nano-patterning liquid metal (LM) •The resolution in LM patterns breaks the optical limit of laser beams •Pulsed-laser-induced compression enables uniform ∼500-nm LM nanolayers •The robust oxide shell on LM boosts the mechanical properties and reliability High-resolution self-packaged conductive patterns are important in integrated electronics used in harsh environments. One of the most promising candidates is gallium-based liquid due to its unique properties. Here, we introduce an advanced liquid metal nano-patterning technique based on pulsed laser lithography (PLL) to create self-packaged, high-resolution liquid metal patterns. The method described here, for the first time, can directly generate liquid metal nano-patterns with ∼500-nm line width without being limited by laser beam size. Line-scanning pulsed-laser-induced shock and thermal effects could generate compression on the liquid metal to extrude ∼200-nm particles to an ∼30-nm layer covered by an ∼20-nm oxide shell with boosted mechanical properties. When subjected to external damage, the electrical functionality of the nano-patterns is well maintained due to the protective self-packaged shell and its 3D structure. The electrically self-packaged material with high resolution is a promising candidate to serve in demanding applications with high integration densities. High-resolution self-packaged conductive patterns are important in integrated electronics used in harsh environments. One of the most promising candidates is gallium-based liquid due to its unique properties. Here, we introduce an advanced liquid metal nano-patterning technique based on pulsed laser lithography (PLL) to create self-packaged, high-resolution liquid metal patterns. The method described here, for the first time, can directly generate liquid metal nano-patterns with ∼500-nm line width without being limited by laser beam size. Line-scanning pulsed-laser-induced shock and thermal effects could generate compression on the liquid metal to extrude ∼200-nm particles to an ∼30-nm layer covered by an ∼20-nm oxide shell with boosted mechanical properties. When subjected to external damage, the electrical functionality of the nano-patterns is well maintained due to the protective self-packaged shell and its 3D structure. The electrically self-packaged material with high resolution is a promising candidate to serve in demanding applications with high integration densities.

Nanoimprinting for high-throughput replication of geometrically precise pillars in fused silica to regulate cell behavior

Developing high-throughput nanopatterning techniques that also allow for precise control over the dimensions of the fabricated features is essential for the study of cell-nanopattern interactions. Here, we developed a process that fulfills both of these criteria. Firstly, we used electron-beam lithography (EBL) to fabricate precisely controlled arrays of submicron pillars with varying values of interspacing on a large area of fused silica. Two types of etching procedures with two different systems were developed to etch the fused silica and create the final desired height. We then studied the interactions of preosteoblasts (MC3T3-E1) with these pillars. Varying interspacing was observed to significantly affect the morphological characteristics of the cell, the organization of actin fibers, and the formation of focal adhesions. The expression of osteopontin (OPN) significantly increased on the patterns, indicating the potential of the pillars for inducing osteogenic differentiation. The EBL pillars were thereafter used as master molds in two subsequent processing steps, namely soft lithography and thermal nanoimprint lithography for high-fidelity replication of the pillars on the substrates of interest. The molding parameters were optimized to maximize the fidelity of the generated patterns and minimize the wear and tear of the master mold. Comparing the replicated feature with those present on the original mold confirmed that the geometry and dimensions of the replicated pillars closely resemble those of the original ones. The method proposed in this study, therefore, enables the precise fabrication of submicron- and nanopatterns on a wide variety of materials that are relevant for systematic cell studies. Statement of significanceSubmicron pillars with specific dimensions on the bone implants have been proven to be effective in controlling cell behaviors. Nowadays, numerous methods have been proposed to produce bio-instructive submicron-topographies. However, most of these techniques are suffering from being low-throughput, low-precision, and expensive. Here, we developed a high-throughput nanopatterning technique that allows for control over the dimensions of the features for the study of cell-nanotopography interactions. Assessing the adaptation of preosteoblast cells showed the potential of the pillars for inducing osteogenic differentiation. Afterward, the pillars were used for high-fidelity replication of the bio-instructive features on the substrates of interest. The results show the advantages of nanoimprint lithography as a unique technique for the patterning of large areas of bio-instructive surfaces.

Cryo-Focused Ion Beam-Induced Deposition of Tungsten–Carbon Nanostructures Using a Thermoelectric Plate

Focused Ion Beam-Induced Deposition (FIBID) is a single-step nanopatterning technique that applies a focused beam of ions to induce the decomposition of a gaseous precursor. The processing rate of FIBID increases by two orders of magnitude when the process is performed at cryogenic temperatures (Cryo-FIBID): the precursor forms a condensed layer on the surface of the cooled substrate, greatly enhancing the amount of material available for decomposition. Cryo-FIBID has been achieved so far by making use of liquid nitrogen-based cooling circuits, which require the passage of a flowing gas as a cooling agent. Here, the Cryo-FIBID of the W(CO)6 precursor is performed using a coolant-free thermoelectric plate utilizing the Peltier effect. Performed at −60 ∘C, the procedure yields a W–C-based material with structural and electrical properties comparable to those of its counterpart grown in coolant-based Cryo-FIBID. The use of the thermoelectric plate significantly reduces the vibrations and sample drift induced by the flow of passing coolant gas and allows for the fabrication of similar nanostructures. In summary, the reported process represents a further step towards the practical implementation of the Cryo-FIBID technique, and it will facilitate its use by a broader research community.

Atomic Layer Deposition of Nanolayered Carbon Films

In this paper, carbon thin films were grown using the plasma-enhanced atomic layer deposition (PE-ALD). Methane (CH4) was used as the carbon precursor to grow the carbon thin film. The grown film was analyzed by the high-resolution transmission electron micrograph (TEM), X-ray photoelectron spectroscopy (XPS) analysis, and Raman spectrum analysis. The analyses show that the PE-ALD-grown carbon film has an amorphous structure. It was found that the existence of defective sites (nanoscale holes or cracks) on the substrate of copper foil could facilitate the formation of nanolayered carbon films. The mechanism for the formation of nanolayered carbon film in the nanoscale holes was discussed. This finding could be used for the controlled growth of nanolayered carbon films or other two-dimensional nanomaterials while combining with modern nanopatterning techniques.

Functional Modifications Induced via X‐ray Nanopatterning in TiO2 Rutile Single Crystals

The possibility to directly write electrically conducting channels in a desired position in rutile TiO2 devices equipped with asymmetric electrodes—like in memristive devices—by means of the X‐ray nanopatterning (XNP) technique (i.e., intense, localized irradiation exploiting an X‐ray nanobeam) is investigated. Device characterization is carried out by means of a multitechnique approach involving X‐ray fluorescence (XRF), X‐ray excited optical luminescence (XEOL), electrical transport, and atomic force microscopy (AFM) techniques. It is shown that the device conductivity increases and the rectifying effect of the Pt/TiO2 Schottky barrier decreases after irradiation with doses of the order of 1011 Gy and fluences of the order of 1012 J m−2. Irradiated regions also show the ability to pin and guide the electroforming process between the electrodes. Indications are that XNP should be able to promote the local formation of oxygen vacancies. This effect could lead to a more deterministic implementation of electroforming, being of interest for production of memristive devices.

Modification of the surface nanotopography of implant devices: A translational perspective.

There is an increasing need for the development of superior, safe, and more sophisticated implants, especially as our society historically has been moving towards an increasingly aging population. Currently, most research is being focused on the next generation of advanced medical implants, that are not only biocompatible but have modified surfaces that direct specific immunomodulation at cellular level. While there is a plethora of information on cell-surface interaction and how surfaces can be nanofabricated at research level, less is known about how the academic knowledge has been translated into clinical trials and commercial technologies. In this review, we provide a clinical translational perspective on the use of controlled physical surface modification of medical implants, presenting an analysis of data acquired from clinical trials and commercial products. We also evaluate the state-of-the-art of nanofabrication techniques that are being applied for implant surface modification at a clinical level. Finally, we identify some current challenges in the field, including the need of more advanced nanopatterning techniques, the comparatively small number of clinical trials and comment on future avenues to be explored for a successful clinical translation.

Frontispiece: Scaling Up DNA Origami Lattice Assembly

Surface-assisted hierarchical DNA origami assembly can be scaled up to produce highly homogeneous polycrystalline DNA origami lattices at the mica-electrolyte interface over cm2 areas with only minor boundary effects at the substrate edges. At total DNA costs of € 0.12 per cm2, this large-scale DNA origami nanopatterning technique holds great promise for the development and fabrication of versatile functional surfaces. For more details see the Full Paper by A. Keller, V. Linko et al. on page 8564 ff.

Mapping of Structural Changes Induced by X-ray Nanopatterning via Nano-X-ray Diffraction and Corresponding Electrical Effects

We have investigated the modifications induced in high-Tc superconductor Bi2Sr2CaCu2O8+δ (Bi-2212) single crystals by irradiation with an X-ray nanobeam 250 × 250 nm2 in size with E = 14.85 keV and a time-averaged photon flux Φ0 = 7.53 × 108 ph s–1. Crystals were mounted onto amorphous substrates with electrical contacts to allow for both X-ray diffraction (XRD) and electrical measurements on the same sample. Changes in the domain structure were monitored via space-resolved nano-XRD mapping, whereas electrical properties were measured off-line at different levels of irradiation dose. The results show that irradiation induces both an increase of the mosaicity spread and a localized release of the mechanical stress of the crystals. This implies a transient local change of mechanical properties that could be due to thermal fatigue or nonthermal melting. Analysis of the electrical properties of the irradiated regions indicates a strong underdoping effect by the nanobeam and an exponential increase of the normal-state resistivity with the irradiation dose, according to the average rate α = (1.48 ± 0.12) × 10–11 Gy–1. These results represent an important step toward the full development of the X-ray nanopatterning technique.

High‐precision and Scalable Fabrication of Biomimetic Culture Environments for Replicating the Extracellular Matrix In Vitro

In vivo, cell and tissue organization is driven by a complex interplay of cells and their environment, including the extracellular matrix (ECM). The ECM can provide varied cues such as structure, protein content, mechanical stiffness, stretch, and electrical stimulation to influence cell and tissue development and function. Traditional in vitro cell culture environments—typically composed of non-dynamic hard and unstructured glass or plastic—fail to fulfill the essential roles of the ECM in development. Consequently, many in vitro cell models fall short in correctly reproducing critical in vivo phenotypes as evidenced when cultured cells lose type-specific characteristics or express phenotypes indicative of an immature developmental stage. Considerable effort is directed at fabricating biomimetic culture environments that maintain or promote mature phenotypes. However, making biomimetic substrates typically involves costly or hard-to-reproduce techniques that are often incompatible with many standard assays. These challenges are further compounded when using high-throughput assays. In this study, our objective is to develop a host of techniques for recapitulating certain aspects of the ECM in vitro. To replicate the shape and stiffness of the extracellular niche, we first developed novel nanopatterning techniques based on high-precision photolithography methods that are highly reproducible, scalable, and amenable to integration with most industry-standard endpoint assays, including high-NA microscopy. Our data demonstrate that a variety of cell types—including cardiac, skeletal, endothelial, neuronal, and cancer—can cue off of patterns that mimic the fibrous structure of the native ECM. We then extend this biomimetic patterning technique to deformable elastomeres which permit the addition of mechanical strain to recapitulate mechanical signals present in the ECM of specific tissues. Our data show that mechanical strain can orient skeletal muscle cells and tissues along the axis of stretch, resulting in a polarized orientation that is similar to what is seen in vivo but is not recapitulated in traditional culture environments. We conclude that our approach is a viable method for recreating specific aspects of the ECM that are critical for driving the development and maturation of cells in culture in a cell, instrument, and assay-agnostic manner.

Cool, Dry, Nano-scale DIC Patterning of Delicate, Heterogeneous, Non-planar Specimens by Micro-mist Nebulization

Background: Application of patterns to enable high-resolution Digital Image Correlation (DIC) at the small scale (μm/nm) is known to be very challenging as techniques developed for the macro- and mesoscale, such as spray painting, cannot be scaled down directly. Moreover, existing nano-patterning techniques all rely on harsh processing steps, based on high temperature, chemicals, physical contact, liquids, and/or high vacuum, that can easily damage fragile, small-scale, free-standing and/or hygro-sensitive specimens, such as MEMS or biological samples. Objective: To present a straightforward, inexpensive technique specially designed for nano-patterning highly delicate specimens for high-resolution DIC. Methods: The technique consists in a well-controlled nebulized micro-mist, containing predominantly no more than one nanoparticle per mist droplet. The micro-mist is subsequently dried, resulting in a flow of individual nanoparticles that are deposited on the specimen surface at near-room temperature. By having single nanoparticles falling on the specimen surface, the notoriously challenging task of controlling nanoparticle-nanoparticle and nanoparticle-surface interactions as a result of the complex droplet drying dynamics, e.g., in drop-casting, is circumvented. Results: High-quality patterns are demonstrated for a number of challenging cases of physically and chemically sensitive specimens with nanoparticles from 1 μm down to 50 nm in diameter. It is shown that the pattern can easily be scaled within (and probably beyond) this range, which is of special interest for micromechanical testing using in-situ microscopic imaging techniques, such as high-magnification optical microscopy, optical profilometry, atomic force microscopy, and scanning electron microscopy, etc. Conclusions: Delicate specimens can conveniently be patterned at near-room temperature (sim 37 ∘C), without exposure to chemicals, physical contact or vacuum, while the pattern density and speckle size can be easily tuned.

Metal surface nanopatterning for enhanced interfacial adhesion in fiber metal laminates

A nano-scale electrochemical sculpture (denoted here as NES) method has been developed to create TiO2 nanotube arrays on the surface of titanium sheets for improved bonding between titanium and carbon fiber reinforced epoxy composite laminates. For comparison, other surface treatments including anodization in NaOH, Na2SiO3, Na2C4H4O6 and EDTA electrolyte (denoted as NaTESi) and electrochemical etching in alkali solution (denoted as ALK) were carried out to investigate their effects on the surface morphology, wettability and the shear strength between titanium sheets and epoxy resin. The results from single-lap tests showed that the apparent shear strengths between titanium sheet and epoxy with NaTESi, ALK and optimized NES treatments have been improved by 37.2%, 53.9%, and 70.9%, respectively, in comparison with that of non-treated pristine samples. The samples in single-lap tests showed a mixture mode of cohesive and interfacial failures where the interfacial failure occurred at the epoxy/metal-oxide interface and the oxide films formed during the surface treatments remained intact. Utilizing the treated titanium sheets, fiber metal laminates (FMLs) were fabricated and their interlaminar shear strengths (ILSS) were found to increase by 43.5%, 56.1%, and 75.0%, respectively, compared to pristine samples, showing the promise of the nano-patterning technique.

Microsphere Photolithography Patterned Nanohole Array on an Optical Fiber

Microsphere Photolithography (MPL) is a nanopatterning technique that utilizes a self-assembled monolayer of microspheres as an optical element to focus incident radiation inside a layer of photoresist. The microspheres produces a sub-diffraction limited photonic-jet on the opposite side of each microsphere from the illumination. When combined with pattern transfer techniques such as etching/lift-off, MPL provides a versatile, low-cost fabrication method for producing hexagonal close-packed metasurfaces. This article investigates the MPL process for creating refractive index (RI) sensors on the cleaved tips of optical fiber. The resonant wavelength of metal elements on the surface is dependent on the local dielectric environment and allows the refractive index of an analyte to be resolved spectrally. A numerical study of hole arrays defined in metal films shows that the waveguide mode provides good sensitivity to the analyte refractive index. This can be readily tuned by adjusting the MPL exposure and the simulation results guide the fabrication of a defect tolerant refractive index sensor on the tip of a fiber tip with a sensitivity of 613 nm/RIU. The conformal nature of the microsphere monolayer simplifies the fabrication process and provides a viable alternative to direct-write techniques such as Focused Ion Beam (FIB) milling.

Nanoimprinting for High-Throughput Fabrication of Geometrically Precise Pillars to Regulate Cell Behavior

Developing high-throughput nanopatterning techniques that also allow for precise control over the dimensions of the fabricated features is essential for the study of cell-nanopattern interactions. Here, we developed a process that fulfills both of these criteria. Firstly, we used electron-beam lithography (EBL) to fabricate precisely controlled arrays of high aspect ratio submicron pillars with varying values of interspacing on a large area (3 × 5 mm2) of fused silica substrates. Two types of etching procedures with two different systems were developed to etch the fused silica and create the final desired height. We then studied the interactions of preosteoblasts (MC3T3-E1) with these pillars. Varying interspacing was observed to significantly affect the morphological characteristics of the cell, the organization of actin fibers, and the formation of focal adhesions. The expression of osteopontin (OPN) significantly increased on the patterns, indicating the potential of the pillars for inducing osteogenic differentiation. The EBL arrays of pillars were thereafter used as master molds in two subsequent processing steps, namely soft lithography (using hybrid PDMS) and thermal nanoimprint lithography for high-fidelity replication of the pillars on the substrates of interest. The molding parameters were optimized to maximize the fidelity of the generated patterns and minimize the wear and tear of the master mold. Comparing the replicated feature with those present on the original mold confirmed that the geometry and dimensions of the replicated pillars closely resemble those of the original ones (i.e., 1% deviation in pillar interspacing, < 20% deviation in pillar diameter). The method proposed in this study, therefore, enables the precise fabrication of submicron- and nanopatterns on a wide variety of substrate materials that are relevant for systematic cell studies.

Superconducting properties of in-plane W-C nanowires grown by He+ focused ion beam induced deposition

Focused ion beam induced deposition (FIBID) is a nanopatterning technique that uses a focused beam of charged ions to decompose a gaseous precursor. So far, the flexible patterning capabilities of FIBID have been widely exploited in the fabrication of superconducting nanostructures, using the W(CO)6 precursor mostly in combination with a focused beam of Ga+ ions. Here, the fabrication and characterization of superconducting in-plane tungsten-carbon (W-C) nanostructures by He+ FIBID of the W(CO)6 precursor is reported. A patterning resolution of 10 nm has been achieved, which is virtually unattainable for Ga+ FIBID. When the nanowires are patterned with widths of 20 nm and above, the deposited material is superconducting below 3.5–4 K. In addition, nanowires with widths of 60 and 90 nm have been found to sustain long-range controlled nonlocal superconducting vortex transfer along 3 μm. Overall, these findings strengthen the capabilities of He+ FIBID of W-C in the growth and patterning of in-plane superconducting nanodevices.

Robust quantum point contact operation of narrow graphene constrictions patterned by AFM cleavage lithography

Detecting conductance quantization in graphene nanostructures turned out more challenging than expected. The observation of well-defined conductance plateaus through graphene nanoconstrictions so far has only been accessible in the highest quality suspended or h-BN encapsulated devices. However, reaching low conductance quanta in zero magnetic field, is a delicate task even with such ultra-high mobility devices. Here, we demonstrate a simple AFM-based nanopatterning technique for defining graphene constrictions with high precision (down to 10 nm width) and reduced edge-roughness (+/−1 nm). The patterning process is based on the in-plane mechanical cleavage of graphene by the AFM tip, along its high symmetry crystallographic directions. As-defined, narrow graphene constrictions with improved edge quality enable an unprecedentedly robust QPC operation, allowing the observation of conductance quantization even on standard SiO2/Si substrates, down to low conductance quanta. Conductance plateaus, were observed at n × e2/h, evenly spaced by 2 × e2/h (corresponding to n = 3, 5, 7, 9, 11) in the absence of an external magnetic field, while spaced by e2/h (n = 1, 2, 3, 4, 5, 6) in 8 T magnetic field.

High-resolution combinatorial patterning of functional nanoparticles

Fast, low-cost, reliable, and multi-component nanopatterning techniques for functional colloidal nanoparticles have been dreamed about by scientists and engineers for decades. Although countless efforts have been made, it is still a daunting challenge to organize different nanocomponents into a predefined structure with nanometer precision over the millimeter and even larger scale. To meet the challenge, we report a nanoprinting technique that can print various functional colloidal nanoparticles into arbitrarily defined patterns with a 200 nm (or smaller) pitch (>125,000 DPI), 30 nm (or larger) pixel size/linewidth, 10 nm position accuracy and 50 nm overlay precision. The nanopatterning technique combines dielectrophoretic enrichment and deep surface-energy modulation and therefore features high efficiency and robustness. It can form nanostructures over the millimeter-scale by simply spinning, brushing or dip coating colloidal nanoink onto a substrate with minimum error (error ratio < 2 × 10−6). This technique provides a powerful yet simple construction tool for large-scale positioning and integration of multiple functional nanoparticles toward next-generation optoelectronic and biomedical devices.

Large-Scale Synthesis of Highly Uniform Silicon Nanowire Arrays Using Metal-Assisted Chemical Etching.

The combination of metal-assisted chemical etching (MACE) with colloidal lithography has emerged as a simple and cost-effective approach to nanostructure silicon. It is especially efficient at synthesizing Si micro- and nanowire arrays using a catalytic metal mesh, which sinks into the silicon substrate during the etching process. The approach provides a precise control over the array geometry, without requiring expensive nanopatterning techniques. Although MACE is a high-throughput solution-based approach, achieving large-scale homogeneity can be challenging because of the instability of the metal catalyst when the experimental parameters are not set appropriately. Such instabilities can lead to metal film fracture, significantly damaging the substrate and thus compromising the nanowire array quality. Here, we report on the critical parameters that influence the stability of the metal catalyst layer for achieving large-scale homogeneous MACE: etchant composition, metal film thickness, adhesion layer thickness, nanowire diameter and pitch, metal film coverage, Si/Au/etchant interface length, and crystalline quality of the colloidal template (grain size and defects). Our results investigate the origin of the catalyst film fracture and reveal that MACE experiments should be optimized for each Si wire array geometry by keeping the etch rate below a certain threshold. We show that the Si/Au/etchant interface length also affects the etch rate and should thus be considered when optimizing the MACE experimental parameters. Finally, our results demonstrate that colloidal templates with small grain sizes (i.e., <100 μm2) can yield significant problems during the pattern transfer because of a high density of defects at the grain boundaries that negatively affects the metal film stability. As such, this work provides guidelines for the large-scale synthesis of Si micro- and nanowire arrays via MACE, relevant for both new and experienced researchers working with MACE.

High-Resolution Three-Dimensional Sculpting of Two-Dimensional Graphene Oxide by E-Beam Direct Write.

On-demand switchable "additive/subtractive" patterning of two-dimensional (2D) nanomaterials is an essential capability for developing new concepts of functional nanomaterials and their device realizations. Traditionally, this is performed via a multistep process using photoresist coating and patterning by conventional photo or electron beam lithography, which is followed by bulk dry/wet etching or deposition. This limits the range of functionalities and structural topologies that can be achieved as well as increases the complexity, cost, and possibility of contamination, which are significant barriers to device fabrication from highly sensitive 2D materials. Focused electron beam-induced processing (FEBIP) enables a material chemistry/site-specific, high-resolution multimode atomic scale processing and provides unprecedented opportunities for "direct-write", single-step surface patterning of 2D nanomaterials with an in situ imaging capability. It allows for realizing a rapid multiscale/multimode approach, ranging from an atomic scale manipulation (e.g., via targeted defect introduction as an active site) to a large-area surface modification on nano- and microscales, including patterned doping and material removal/deposition with 2D (in-plane)/three-dimensional (3D) (out-of-plane) control. In this work, we report on a new capability of FEBIP for nanoscale patterning of graphene oxide via removal of oxygenated carbon moieties with no use of reactive gas required for etching complemented by carbon atom deposition using a focused electron beam. The mechanism of experimentally observed phenomena is explored using the density functional theory (DFT) calculations, revealing that interactions of e-beam that liberated reactive oxygen radicals with carbon atoms on the graphene basal plane lead to the creation of atomic vacancies in the material. The reaction byproducts are volatile carbon dioxides, which are dissociated and volatilized from the graphene oxide surface functional groups by interactions with an energetic focused electron beam. Along with selective subtractive patterning of graphene oxide, the same electron beam with increased irradiation doses can deposit out-of-plane 3D carbon nanostructures on top of or around the 2D etched pattern, thus forming a hybrid 2D/3D nanocomposite with a feature control down to a few nanometers. This in operando dual nanofabrication capability of FEBIP is unmatched by any other nanopatterning techniques and opens a new design window for forming 2D/3D complex nanostructures and functional nanodevices.

Polyvinyl alcohol: a high-resolution hydrogel resist for humidity-sensitive micro-/nanostructure

A high-resolution nanopatterning technique is desirable with the present rapid development of hydrogel nanodevices. Here, we demonstrate that polyvinyl alcohol (PVA), a popular polymeric hydrogel, can function as the negative-tone resist for electron beam lithography (EBL) with a resolution capability as narrow as 50 nm half-pitch. Furthermore, the hydrophilic groups of PVA are stable after EBL exposure, and thus the pattern still shows rapid responsivity to humidity change. An aqueous nanopatterning process including dissolution, spin-coating and development is setup, which is friendly for organic device fabrication free of organic solvent. This high-resolution nanopatterning technique with PVA is helpful for the design and realization of hydrogel-related nanodevices in the future.