In nanoscience and nanotechnology, nanofabrication is critical. Among the required processes for nanofabrication, lithography is one of core issues. Although conventional photolithography with recent remarkable improvement has contributed to the industry during the past few decades, fabrication of 3-dimensional (3D) nanostructure is still challenging. In this review, we summarize recent advances for the construction of 3D nanostructures by unconventional lithography and the combination of two top-down approaches or top-down and bottom-up approaches. We believe that the 3D hierarchical nanostructures described here will have a broad range of applications having adaptable levels of functional integration of precisely controlled nanoarchitectures that are required by not only academia, but also industry.
1. Introduction
Nanofabrication is a “hot topic” in nanotechnology and nanoscience. It is absolutely indispensable to solve all scientific questions in nanoscale photonics, biotechnology, and electronics. However, with every increasing demand for smaller features on integrated and complicated geometries, its realization in industry relies on photo (or electron-beam) lithography which has limitations based on the physics on light diffraction and poor throughput that are impractical to overcome [1]. Conventional techniques are reaching their resolution limits and have relatively high cost for the fabrication of patterned nanostructures. On the other hand, unconventional lithography techniques were first seen in early 1990’s [2–6]. Interestingly, at that time, new research had been exploring nanoimprint lithography (NIL) [7], capillary force lithography (CFL) [8, 9], dip-pen lithography [10], soft molding [11, 12], microcontact printing (μCP) [2, 13], and various types of self-assembly [14, 15]. They provided various alternative approaches and unique accomplishments: diverse, novel, and reliable processes to reliably generate smaller pattern features (sub-100 nm scale) with high cost effectiveness. Because there are significant potential and substantial developments for academia and small companies, unconventional lithography has been explosively developed. Therefore, the number of scientific publications and citations of unconventional lithography has been dramatically increasing in the last 20 years [16]. Even with the recent remarkable improvements, unconventional lithography still has huge hurdles, for example, multiple aspect-ratio (3D hierarchical) nanostructures, large area, and low-cost manufacturing with sub-10 nm feature sizes [17–19]. In this review article, we summarize previously demonstrated unconventional lithography and introduce the challenging fields of 3D hierarchical nanostructures and their applications. The paper is composed of five separate parts. The first part presents unconventional lithography methods developed in the last 20 years. The second and third parts deal with 3D hierarchical nanostructures and their applications formed by molding or photolithography (combined top-down lithographic techniques). The fourth and fifth parts cover integrated assembly with merged top-down with bottom-up approaches.
2. Unconventional Lithography
In this section, we attempt to summarize representative sampling of unconventional lithography (Figure 1). The processes of making functional structures with patterns are labeled as top-down (NIL, Figure 1(a); CFL, Figure 1(b); nanotransfer printing (NTL), Figure 1(c)) and bottom-up (colloidal lithography (CL), Figure 1(d), and block copolymer (BCP) lithography, Figure 1(e)) approaches.
Various unconventional lithographic processes. Schematic illustration of (a) thermal nanoimprint lithography (T-NIL, also called hot embossing) and UV nanoimprint lithography (UV-NIL, also called photocurable nanoimprint lithography), (b) capillary force lithography (CFL), (c) nanotransfer printing lithography (NTL), (d) colloidal lithography (CL, also called nanosphere lithography), and (e) block copolymer lithography (BCPL).
The process of NIL was first demonstrated by Chou et al. in 1995 [7]. The concept of the technique is mechanical replication where surface reliefs from prepatterned mold are embossed into a thin layer on the substrate [20, 21]. Figure 1(a) gives an overview of the NIL processing steps. In the NIL, resist layer (with thermo- [22, 23] or photocurable polymer [24]) is coated on a substrate and then pressed by a rigid mold with patterns at 1 : 1 scale through mechanical contact. The resist layer is cured with pressure by thermal or UV curing known, respectively, as temperature-based processing and light-initiated polymerization. In principle, NIL does not have limitations of pattern geometry, which means NIL can copy any patterns on a wide range of substrates. Therefore, NIL can bridge the gap between lab level nanobased technique studies and production level manufacturing. However, it requires high pressure (~MPa) and new challenging processes for large area fabrications, that is, roll-to-roll [25–27] or step-and-repeat processes [28].
2.1.2. Capillary Force Lithography (CFL)
CFL is a method that uses nature force, capillarity with soft or flexible nanoscale molds. Lee and Suh [8, 9] presented large-area patterning by capillarity of melted polymer or polymeric precursors. In this method, by using patterned soft mold such as polydimethylsiloxane (PDMS) or rigiflex molds, thin polymer layers on the substrate are contacted and then heated above the polymer’s glass transition temperature (Tg). Laplace pressure induced by a micro/nanoscale mold allows the melted polymer to fill the void space of the channels formed between the mold and the polymer without applying pressure (Figure 1(b)) [29, 30]. Notably, the strength of CFL is that it allows for smaller sizes of feature patterns with higher Laplace pressure and higher throughput [31]. However, CFL is still limited in forming high aspect and multiple geometry nanostructures.
2.1.3. Nanotransfer Printing (NTP)
Another method to achieve aligned nanopatterns over a large area is to transfer the desired pattern to the target substrate [32–40]. This technique, known as NTP, was introduced by Rogers et al. [32–34]. As shown in Figure 1(c), patterned structures deposited on a polymeric stamp are selectively transferred onto different surfaces. First a stamp contacts a donor substrate coated with the target structure and then quickly peels it away. After the micro/nanostructures are transferred from the donor substrate to the stamp, the stamp contacts the receiving substrate and then slowly peels it away. Consequently the structures are transferred from the stamp to the receiver. In previous studies, ultrathin inorganic devices like single crystal silicon nanoribbons [41–43], photodiodes [44–47], and solar cells were printed onto a sheet of plastic by kinetically adhesion-controllable stamp [34]. These results show that NTP has potential to be useful to transfer conventional electronics to deformable substrates for flexible and stretchable devices. Also, NTP might be used for many other applications in biomedicine, that is, optogenetics [48, 49], catheter [50], and optoelectronics. Although recent advances as mentioned above are presented, it is still suffering from low throughout and poor yield for large area fabrication.
Colloidal lithography (CL) uses an array of self-organized and self-assembled colloids as templates for advanced functional materials [51–55]. CL has simple steps to create nanostructures [51]; a close packed monolayer of colloids (nanosphere) arranged into an array through self-assembly on a substrate. And then reactive ion etching (RIE), such as plasma etching, fabricates nanopillar structures (Figure 1(d)). A remarkable characteristic of the process is that the colloids assemble themselves spontaneously in ordered architectures which are easily controlled over a large area without complex equipment. Modification of the self-assembled etch mask improves the versatility of CL in fabricating nanopatterns such as hollow shells [52], nanocylinders [53], and even multiple layer [54, 55] by spontaneous formation of well-ordered colloid. Therefore this fabrication method has some advantages over conventional lithography; it is a simple cost-effective process that provides easy control of feature size.
2.2.2. Block Copolymer Lithography
Another self-assembled material is block copolymers (BCPs). BCPs are macromolecules consisting of more than two components with chemically different polymeric segments [56–58]. Owing to their segmented structures, BCPs have various shapes of phase separation with different processing conditions with well-aligned arrays at equilibrium with lots of complex morphologies, from lamellar to cylinder (or gyroid) nanoscale structures (Figure 1(e)) [15]. Conveniently, BCP lithography is compatible with conventional manufacturing processes used in the semiconductor industry, showing a wide range of potential as an emerging technology for nanoscale device fabrication.
3. Unconventional Lithography for 3D Hierarchical Micro- and Nanostructure by Combined Top-Down Approaches
All approaches we mentioned in Section 2 demonstrate only monoscale micro- or nanostructures with single step process. Although the previous approaches are useful in many fields, more complicated, denser, and 3-dimensional hierarchical patterns with higher aspect ratio are also required. In this section, we describe several approaches to fabricate high aspect ratio hierarchical structures having nanoscale patterns on microscale structures by using combined top-down lithographic techniques (Figure 2). Precisely controlled hierarchical patterning at multilevel and size-control of individual structures is possible by the techniques. As a result, the range of its applications is being widened.
Schematic illustrations of fabrication process for 3D hierarchical micro- and nanostructures via (a) two-step photolithography and soft lithography, (c) sequential t-NIL, and (e) vacuum-assisted CFL. (b) SEM images showing the fabricated 3D micro/nano-hierarchical structures via two-step photolithography, followed by molding. Reproduced with permission from [59]. Copyright 2009 WILEY-VCH Verlag GmbH & Co. KGaA. The bottom left SEM image shows close-up view of the selected area in top SEM image. Right SEM image of the bottom shows single-level pillars. (d) SEM images show the tilted cross section of the primary imprint 2 μm (top left) grating mold and the secondary imprint 250 nm (top right) grating mold. The hierarchical structures were obtained from two steps (bottom left) and three steps (bottom right) of imprinting. Reproduced with permission from [61]. Copyright 2006 IOP Publishing Ltd. (f) SEM images show the several connected bridges of different width and density for given base structures via vacuum-assisted CFL. Reproduced with permission from [71]. Copyright 2009 Wiley-VCH Verlag GmbH & Co. KGaA.
3.1. Two-Step Photolithography for 3D Hierarchical Structure
Greiner et al. [59] reported production of multiscale micropillar structured PDMS using two-step photolithography. The process to fabricate the hierarchical structure involves double layer-by-layer (LbL) coating and exposure steps using SU-8 resist (Figure 2(a)). A single development step after the second exposure was sufficient to remove the unexposed regions from the double layers. Subsequently, the 3D hierarchical PDMS replica was fabricated by soft molding on hierarchical SU-8 hole patterns. Fabricated PDMS replicas were characterized by scanning electron microscopy (SEM) (Figure 2(b)). The bottom pillars had a diameter of 50 μm and a height of 200 μm (aspect ratio λ=4), while the top pillars had a diameter of 5 μm and heights ranging from 2.5 to 10 μm (aspect ratio λ from 0.2 to 2). However, in the aspect of practical application, this approach has lack of processibility due to highly viscous photoresist onto pre-patterned structures.
Although NIL has been recognized as a simple, low-cost, high-throughput, and scalable nanofabrication technique [7, 22, 23], it is difficult to form high aspect ratio patterns with thermal NIL [60]. The large contact areas between the mold and melted polymer and limited mold materials that cannot be controlled in surface energy allow multiple (~2) aspect ratio patterns to be possible. Recently, Zhang and Low [61] demonstrated a new fabrication method for 3D hierarchical structures by NIL by exploiting the properties of thermoplastic polymers. The sequential step process for fabrication of the 3D hierarchical structure is shown in Figure 2(c). First, a mold is pressed on a polymeric film above its glass transition temperature Tg. After demolding, a different mold is aligned with a specific orientation and pressed on the prepatterned polymer film below its glass transition temperature Tg in order to fabricate a secondary pattern. In the same manner, a tertiary or higher-order pattern can also be created. Figure 2(d) shows scanning electron microscopy (SEM) images for the imprint of 2 μm and 250 nm Si grating molds (top images). SEM images show polycarbonate (PC) film imprinted first with 2 μm grating and then with 250 nm grating in parallel orientation and tertiary 250 nm grating imprinted perpendicular to other gratings (bottom images). They demonstrated that this approach offers a fast low-cost process to fabricate 3D hierarchical structures. However, the rigid and thick Si mold could lead to poor yield of demolding and it is difficult to control the temperature to prevent collapse or reflow of prepatterned structures [62]. Therefore, to extend this method, a highly precise temperature-controllable and rigiflex mold [39] could enhance the yield and reduce mechanical damage of both stamp and imprinted hierarchical structures as the alternatives to rigid Si molds.
3.3. Vacuum-Assisted CFL for Fabricating 3D Hierarchical Bridge Structure
Hierarchically suspended polymeric-bridge structures have a wide range of applications as smart electronics, photonic devices, and microfluidic systems. Earlier available fabrication techniques including reverse imprint lithography [63, 64], microtransfer molding [65], edge lithography [66, 67], direct drawing [68], and electrochemical growth [69, 70] have been used to create bridge-like inverted 3D nanostructures. Although these approaches are useful, such suspended structures might contain a heterogeneous interface between their top and bottom structures, which is associated with structural defects and increases contact failure in electronic devices as well as partial current leakage in multichannel devices [71].
Recently, a novel approach to generate 3D hierarchical bridge structures was proposed by Kwak et al. in 2009 [71]. They combined partially curing UV-NIL and vacuum-assisted CFL. As shown in Figure 2(e), vacuum-assisted CFL consists of three steps: (i) the first molding (the UV curable polymer surface is partially cured), (ii) the second molding, and (iii) vacuum-assisted molding. Fundamentally, oxygen inhibits UV-crosslinking since it reacts with scavenging initiator radicals in free-radical polymerization. These seemingly undesirable inhibitory effects form partially uncured regions of pregenerated microstructures. After fabricating a partially cured base microstructure (the first molding step), capillarity action of the partially cured resin simultaneously took place (the second molding step). Finally in the vacuum-assisted molding step, the migration of a partially cured polymer was used to create 3D monolithic bridge like structures. The polymer then spontaneously moved into the second mold’s cavity by capillary action and then fully cured after UV exposure. Figure 2(f) shows SEM images of various monolithic nanowire bridges of different width and density for a given base structure formed by the vacuum-assisted CFL method. The method could serve as a novel tool for fabricating 3D hierarchical structures in a wide range of applications.
4. Diverse Applications with 3D Hierarchical Structure
In general, 3D hierarchical structure fabricated by combined top-down approaches leads to physically and mechanically unique properties. In this section, we introduce several applications using the 3D hierarchical structure and how to realize important properties including hydrophobicity, adhesion force, and robustness compared with conventional methods (Figure 3).
Applications of 3D hierarchical structures fabricated by combined top-down lithographical techniques. (a-b) (a) SEM image of the rice leaf-like hierarchical Pd structures (after heat treatment) with primary lines of 1.7 mm and secondary pillars of 90 nm and corresponding height of 170 nm and 60 nm, respectively, from AFM profiles. (b) The different contact angles in parallel and perpendicular direction of the metallic rice leaf structure due to anisotropic wetting property (left). After hexadecanethiol absorption, the contact angle is increased in both directions (right). Reproduced with permission from [82]. Copyright 2013 Macmillan Publishers Ltd. (c–f) (c) Conceptual illustration (left), SEM image (top right), and photograph (bottom right) of bioinspired dry adhesive as medical skin patch. (d) Photographs show the use of dry adhesive patch to monitor electrocardiograms (ECGs) on the volunteer’s chest (top) and wrist (bottom). The inset images show the corresponding ECG signals from the volunteer’s heart. (e) Photographs show the skin condition after use of acrylic adhesive (left) and bioinspired dry adhesive during 48 hours. (f) Variations in normal adhesion strength of acrylic adhesive and bioinspired dry adhesive on the skin with 30 repeating cycles. Both adhesives were cleaned every 5 times. Reproduced with permission from [90, 91]. Copyright 2011 and 2013 WILEY-VCH Verlag GmbH & Co. KGaA. (g–j) Large-area photograph (g) and SEM images (h) of the three-level hierarchical apertures (800 nm/20 μm/500 μm apertures) at different magnifications. (i) Photographs of the three-level polymeric membrane with hierarchical apertures onto a tip of a syringe. The inset illustration depicts the reorganization process near the apertures. (j) Size distributions of emulsions before and after the filtration of apertures having two different diameters (800 nm apertures and 350 nm apertures). Reproduced with permission from [107]. Copyright 2014 Macmillan Publishers Ltd.
4.1. Directional Wetting and Spreading on Hierarchical Patterns
Anisotropic wetting and spreading of water droplet have attracted much attention [72–75] due to an ability to control liquid flow in a desired direction, which can be applied to microfluidic devices and a unidirectional wetting system, similar to the properties found on butterfly wings [76, 77], shark skin [78], and rice leaves [79]. Natural rice leaf possesses a hierarchical structure where the micropillars, with average diameter of about 5–8 μm, are arranged in parallel with nanoprotrusions on the top of the surface [80]. As a result, the surfaces of rice leaves present anisotropic wettability. Also, beetles that live in the Nambia desert have remarkable water collecting ability using their shell’s nanoarchitectures that induce a gradient of hydrophobicity [81].
Recently, Radha et al. [82] applied sequential imprinting to mimic the hierarchical structures of rice leaves. They created a metallic rice leaf structure not the hierarchical patterning of the polymers. The advantage of this hierarchical metal surface allows it to survive in harsher environments, where polymers are infeasible, and to obtain highly desirable abilities, including anticorrosion, anti-icing, and self-cleaning [82, 83]. The metallic patterns inspired by the rice leaf were generated by palladium (Pd) benzylthiolate (metal-organic ink). The ink was first imprinted using a primary grating with a 2 μm mold, and then fine pillars (200 nm) were patterned. Figure 3(a) shows SEM image of the rice leaf-like Pd structure with primary lines of 1.7 μm and secondary pillars of 90 nm and corresponding height of 170 nm and 60 nm, respectively, after heat treatment to remove organic materials. The structure has 130° and 92° contact angles, respectively, in parallel and perpendicular directions. To make a more hydrophobic surface, hexadecanethiol molecules were used on the metallic rice leaf surface (Figure 3(b)). In this case, this approach was a versatile method for fabricating the hierarchically hydrophobic metal surface.
4.2. Dry Adhesive Inspired by Gecko Using Hierarchical Micro/Nanostructures
Hierarchical structures can be used as bioinspired dry adhesives. Owing to an increase in the contact area by the high aspect ratio hierarchical structure, the adhesion induced by van der Waals force can be maintained even on a rough surface (roughness < 20 μm). These dry adhesives, for example, gecko-like structures, are useful for a wide range of applications including transportation devices [84], NTP [85], and wall climbing robots [86]. In particular, a dry adhesive patch with hierarchy has several advantages, including restoration of adhesion force, self-cleaning capability, and biocompatibility, in medical bandages, compared to commonly used bandages [87–91].
Kwak et al. [90] and Bae et al. [91] reported interesting mushroom like micropillars. They were used with a commercially available unit of skin patch electrode which can monitor biosignals (Figures 3(c) and 3(d)). As shown in Figure 3(e), a widely used acrylic medical patch has several side effects such as redness, allergic response, and adhesion degradation by repeated attachment/detachment. On the other hand, the dry adhesive skin patch shows reduced pain during peel-off and undesirable side effects even after 48 h of use. Also, the dry adhesive patch has more restorable ability after every five cycles with the help of self-cleaning with water compared with widely used acrylic medical patches (Figure 3(f)). Hence, with increasing demands on long-term uses for the ubiquitous healthcare (U-healthcare) industry, hierarchically bioinspired dry adhesive patches will potentially be of great benefit in longitudinal biosignal acquisition.
4.3. Free Standing Polymeric Membrane with Three-Level Hierarchical Apertures
Free standing micro/nanomembranes have a wide range of applications including molecular separation [92–94], shadow masking [95], plasmonics [96–98], energy devices [99–101], and bioinspired microfluidic device [102, 103]. Free standing silicon nitride (SiNx) membranes are commonly used due to the high mechanical rigidity (Young’s modulus, E>130 GPa), which can withstand external forces during the handing process [94, 95, 104–106]. However, a SiNx membrane is fragile under mechanical contact and requires expensive and complicated fabrication processes via e-beam lithography or focused ion-beam milling.
Cho et al. [107] created a new type of flexible, mechanically stable, and free standing polymeric membrane by using a hierarchical mold-based CFL. As shown in Figures 3(g) and 3(h), the robust membrane with three-level hierarchical apertures (800 μm/20 μm/500 nm) was well defined without structure collapses or defects over a large area. Also, because the hierarchical membrane is easy to handle, it is easy to attach to a tip of the syringe to reorganize emulsions (Figure 3(i)). As a result, highly uniform and skewed distributions of emulsions depending on the aperture size were obtained; for example, 800 nm and 350 nm pores can distribute the mean sizes of 927.1 nm and 414.4 nm, respectively (Figure 3(j)). The results suggest that the mechanically stable membrane with hierarchical apertures is entirely possible to reorganize and generate highly uniform emulsions and is useful also in many fields.
5. Integrating Assembly: Merging Top-Down and Bottom-Up Approach in Unconventional Lithography
Despite extensive efforts to develop nanofabrication, a superior technique enabling sub-10 nm scale control as well as large-area patterning is still required. Here, change toward integrated top-down and bottom-up lithographic technique might be a breakthrough strategy. In this section, we present some approaches for integrating assembly combined top-down and bottom-up unconventional lithography (Figure 4).
Integrated assembly with merged top-down with bottom-up approaches. (a) Schematic illustration of procedure for template-assisted self-assembly of spherical colloids (top left). Optical images show the assembly of various PS beads in an array of cylindrical holes (3.1 μm PS beads in cylindrical holes 5 μm in diameter, top right; 2.5 μm PS beads in cylindrical holes 5 μm in diameter, top right; and 2.5 μm PS beads in cylindrical holes 6 μm in diameter, bottom right). The inset illustration indicates assembled structures (the diameter (D) of holes and the diameter (d) of spherical colloids). Reproduced with permission from [109]. Copyright 2003 WILEY-VCH Verlag GmbH & Co. KGaA. (b) Schematic illustration of procedure for fabricating free-standing three-dimensional inverse opal (3D-IO) structure of polyurethane acrylate (PUA). (c) Cross-sectional (left) and top-view (right) SEM images of well-ordered 3D-IO structure of PUA. Reproduced with permission from [112]. Copyright 2012 American Chemical Society. (d) Schematic illustration of process for Sub-10 nm pattern transfer from self-assembled PS-b-PDMS with cylindrical patterns. Thermal nanoimprint induces the alignment of cylindrical BCP domains along the length direction of mold patterns. (e) SEM images showing the PDMS cylinders show no evidence of residual layer after plasma etching. The magnified SEM image shows the PDMS cylinders aligned along the long axis of the imprint grating. Reproduced with permission from [122]. Copyright 2011 American Chemical Society. (f) Schematic illustration of procedure for the sub-10 nm transfer printing of block copolymer self-assembly patterns. The inset SEM image shows the patterns after transfer printing on a bare Si substrate and treatment with oxidative plasma. Reproduced with permission from [123]. Copyright 2012 WILEY-VCH Verlag GmbH & Co. KGaA. (g) SEM (left) image shows the tip of a glass capillary nozzle coated with Au/Pd. Thermal annealing (220°C for 5 min) results in the self-assembled PS-b-PMMA BCPs on a substrate (right). (h) SEM image shows the self-assembled PS-b-PMMA BCPs with two different morphologies (lamellae form with MWs of 37-37 K, left; cylinder form with 46–21 K, right) printed as lines. (i) Schematic illustration of process for directed self-assembly of PS-b-PMMA BCPs printed onto topographically patterned substrate. (j) SEM images show the directed self-assembly of PS-b-PMMA BCPs with MWs of 37-37 K (left) and 25-26 K (right) in adjacent trenches (depth ~70 nm, width ~260 nm). Reproduced with permission from [124]. Copyright 2013 Macmillan Publishers Ltd.
5.1. Template-Assisted Self-Assembly of Spherical Colloids
An interesting approach to control assembly of spherical colloids was proposed by Xia et al. [108, 109]. This method is based on combining physical templating and self-assembly monodispersed spherical colloids. The template can be fabricated by photolithography and either polystyrene (PS) beads or silica spheres were used as the spherical colloids. As illustrated in Figure 4(a), the template-assisted self-assembly process is influenced by three major forces: (i) the capillary force associated with the meniscus of the liquid slug, (ii) the gravitational force associated with the density difference between the particle and the dispersion solvent, and (iii) the electrostatic force caused by charges on the surfaces of the particles and the template. In particular, the capillary force plays a significant role in pushing particles into the template holes, taking excess particles along in the direction of the liquid slug. The yield of this assembly process depends on the balance of these three forces. By controlling the forces, the colloidal particles can be trapped in the template holes. The three optical micrographs in Figure 4(a) show a few examples of the single-layered and well-defined aggregates of colloidal particles during the in situ observation. Here, this method makes it possible to control the structural assemblies by the ratio between the diameter (D) of holes and the diameter (d) of spherical colloids, ranging from D/d=1.00 to 3.30. Furthermore, it might be useful to other systems such as cells [110, 111].
5.2. Colloidal Template Assisted for Fabrication 3D Porous Inverse Opal Structure
Yeo et al. [112] demonstrated an advanced tool of LbL coating of polyelectrolyte multilayers inside 3-dimensional porous inverse opal (3D-IO) templates. Uniformly deposited LbL films could be grown inside the 3D-IO structures since the structures were free-standing with a double-sided open porosity formed by colloidal assembly and selective removal of the colloids. As shown in Figure 4(b), UV-curable polyurethane acrylate (PUA) prepolymer filled the voids between particles that were self-assembled into 3D opal structure in order to prepare the free-standing 3D-IO structures. After curing PUA by flood UV exposure, the colloidal particles were removed by solvent, and freestanding 3D-IO structures were mechanically peeled from the sacrificial substrate. However, the bottom side spontaneously offered surface porosity because of contact between the substrate and the colloidal particle, whereas the topside pores were blocked by overcoated excess PUA. An ethanol solution was spin-coated on the overcoated layer several times because the shear-slip action of ethanol during spin-coating can physically remove the residual PUA. Figure 4(c) shows the successfully fabricated 3D-IO PUA structure, wherein the top surface pores are completely opened. As a result, the LbL treated 3D-IO structures are used as nanofiltration membranes for removal of diluted copper ions and they show excellent membrane performance because of the nanoscale pores integrated in microscale inverse opal structures.
5.3. Nanoimprint Directed Self-Assembly of Block Copolymers
Directed self-assembly (DSA) of block copolymers (BCPs) has focused on promising nanolithography to realize high-resolution nanopatterning with a typical feature size in the 3–50 nm region [56, 113, 114]. Generally, there are two approaches for DSA: (i) chemically [115–117] and (ii) topographically [118–121] patterned substrates. These approaches are useful but require high-cost and time-consuming processes for lithographic prepatterning of each substrate. Recently, Park et al. [122] presented an integrated assembly approach for the nanoimprint DSA of BCPs to address these shortcomings. The process for nanoimprint DSA of PS-b-PDMS BCP is shown in Figure 4(d). In particular, the PS brush layer, which is deposited on the substrate, plays a significant role in attracting the majority of PS domains since bare silicon or silicon dioxide attracts a PDMS layer. After forming self-assembled BCPs via thermal imprint and release, the aligned PDMS cylinders are used as a template or etch-mask for sub-10 nm pattern transfer on the underlying substrate. As shown in Figure 4(e), after a plasma etching step, perfectly aligned BCP patterns were obtained along the long axis of the grating pattern without a residual layer.
5.4. NTP with Directed Self-Assembly of BCPs
Another interesting approach for integrating transfer printing and self-assembly BCPs was demonstrated by Jeong et al. [123] The process uses PDMS molds for directing the self-assembly of copolymers. The polydimethyl siloxane (PDMS) mold was prepared from a silicon master patterned with trenches with a width of 250 nm and a depth of 40 nm with a periodicity of 1.25 μm. In the next step, PS-b-PDMS films were spin-coated on the PDMS mold and the mold was placed in a solvent annealing chamber containing acetone and toluene to form self-assembled BCPs. The PDMS mold coated with self-assembled BCPs was placed on the target substrate, and mild pressure of ~20 kPa was applied to transfer the self-assembled BCPs. Finally, after the transfer of the self-assembled BCP pattern, the elastomeric mold was removed from the substrate (Figure 4(f)). The inset SEM image shows that this approach can realize sub-10 nm transfer printing of BCPs on the Si substrate.
5.5. Self-Assembly of BCPs via High-Resolution Patterning Electrohydrodynamic Jet Printing
With increasing demand for nanoscale device applications, self-assembled BCPs have received a great deal of research attention as a means to the nanofabrication. However, rapid and high resolution patterning of multiple scale BCPs with diverse molecular weights (MWs) and composition is still challenging. Onses et al. [124] demonstrated a creative technique for self-assembling multiple BCPs via electrohydrodynamic jet (e-jet) printing on the same surface. E-jet printing that requires applying electric field during the jet printing process is possible to create complex patterns with different conductive inks such as conductive polymers, Si nanoparticles, or rods, and single-walled carbon nanotubes [125]. Here, applied electric fields drive the flow of inks based on PS-b-PMMA BCPs with different MWs through nozzle tips. Thermal annealing then induces phase separation of the BCPs on the same substrate (Figure 4(g)). Figure 4(h) shows successful high-resolution printed BCPs with two different MWs. Moreover, the authors presented that the printing is useful to direct self-assembly of BCPs based on surface topography (Figure 4(i)). As shown in Figure 4(j), by using two BCP inks with different MWs, it is possible to generate the BCP domain with two different periodicities within the same trench area.
6. Applications with Integrating Assembly
The best advantage of integrating assembly lithographic involves the possibility to adjust the desired physical and chemical properties of fabricated samples by changing the feature size, thickness, and applied materials. In this section, we introduce several applications as examples of integrating assembly: photoluminescent microtags, graphene nanoribbon transistors, and multiscale porous nanocolander networks (Figure 5).
Applications of 3D hierarchical structures fabricated by integrated assembly with merged top-down with bottom-up approaches. (a-b) (a) Schematic illustration for the transfer procedure for developed QR code stickers that could be laminated on target products. (b) An example of a 1 cm2 sticker containing one thousand 50 μm sized QR codes made of 28 nm β-NaYF4 nanocrystals, integrated on a private official document (top) and optical microscopy image of the PL mapping at 545 nm of one of the QR codes (middle). After black and white conversion and inversion of the image, the 50 μm NC-QR code, which directs to the internet link, is readable using an appropriate smartphone application (bottom). Reproduced with permission from [129]. Copyright 2014 IOP Publishing Ltd. (c–e) (c) Conceptual illustration for the multichannel transistor with densely aligned sub-10 nm graphene nanoribbons. (d) SEM image showing well-oriented 9 nm half-pitch PDMS cylinders that were revealed from the segregated PS-b-PDMS film by O2 plasma etching. (e) IDS-VG characteristic curves of the three multichannel FETs with ribbon-to-ribbon width variation of 7.4 nm (left), 5.1 nm (middle), and 2.4 nm (right). Reproduced with permission from [145]. Copyright 2012 American Chemical Society. (f–h) (f) SEM images show morphology control of the BCP thin films inside the hollow chamber of the inverse opal (IO) frame according to the BCP film thickness. Inset illustration depicts structures ranging from nanosieves (left) to nanodomes (middle) and corrugated structures containing both perpendicular and parallel cylinders (right). (g) Size-selective separation performance of the 3D nanocolander membrane with varying particulate size of Au nanoparticles (5 to 30 nm). The pore size of the BCP nanosieve was fixed at 18 nm in diameter (corresponding to the volume fraction of PMMA at 28%). The dotted line represents the estimated separated efficiency of filtration with 18 nm-sized pores. (h) Plot of pure water flux through membranes with respect to the operating pressure with varying numbers of BCP solution coatings. Reproduced with permission from [153]. Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA.
6.1. Colloid-Based Photoluminescent Microtags
Recently, Ressier et al. [126] reported a new patterning technique called electrical-NIL (e-NIL) for topographic and electrostatic imprinting of thermoplastic electret films at the nanoscale. The idea stems from electrical microcontact printing (e-μCP) which can selectively obtain chemical patterning having a surface charge gradient [127, 128]. E-NIL consists of three steps: (i) a patterned conductive mold is pressed on the thermoplastic electret film above its glass transition temperature Tg, (ii) after cooling the temperature below its Tg, an electric field is applied between the mold and the substrate, and (iii) the mold is removed. Through e-NIL, it is possible to fabricate topographic and electrostatic patterns on desired spots. These patterns allow opportunities for directed assembly of colloidal nanoparticles on desired surfaces.
Diaz et al. [129] realized micron-sized photoluminescent quick response (QR) codes which applied to directed electrostatic assembly of 28 nm upconverting lanthanide-doped β-NaYF4 nanocrystals inside e-NIL patterns. Figure 5(a) shows the transfer procedure of nanocrystals-based QR codes. First, PMMA was chosen as the thermoplastic electret due to its excellent charge storage properties and the capability to be readily imprinted by T-NIL [130–132]. The glue is cured by UV exposure, allowing nanocrystals to bind to e-NIL patterns, and then immersed in a water bath to remove the polyvinyl acetate (PVA) sacrificial layer. These NaYF4 nanocrystal-based QR codes were transferred to an adhesive polyethylene terephthalate (PET) film and the thin film can be used as a sticker to tag desired products. Also, after black and white light conversion and inversion of optical microscopy image of photoluminescence (PL) mapping at 545 nm, the 50 micron-sized nanocrystal-based QR code can be read with an available smartphone application (Figure 5(b)). Beyond this application, combining localized charge injection through e-NIL with other charged or polarizable colloidal particles has great potentials for colloid-based devices and sensors.
6.2. Fabrication of Sub-10 nm Scale Graphene Nanoribbon Transistor by BCP Lithography
Graphene has remarkable electronic and mechanical properties [133–136]. However, field-effect transistors (FETs) based on graphene show poor on/off current ratios because graphene is a zero-bandgap semiconductor [137]. In order to open up the bandgap, graphene nanostructures, such as graphene nanoribbons (GNRs) [138, 139] and graphene nanomeshes (GNMs) [140–142] have been extensively studied. In particular, to create densely aligned arrays of sub-10 nm wide graphene nanostructures over a large area, block copolymer (BCP) self-assembly in combination with nanoimprint lithography is a promising lithographic technique [122, 143–145].
Recently, Liang and Wi [145] showed FETs consisting of densely arranged GNRs (the total number of GNR channels in a FET is approximately 50) made by a nanofabrication method (sub-10 nm) that merges NIL and directed self-assembly of BCPs (Figures 5(c) and 5(d)). Furthermore, they showed that the on/off current ratio of the FETs bearing such GNRs is significantly affected by ribbon-to-ribbon width variation (RWV) of multiple GNRs depending on the processing conditions of BCP self-assembly. As shown in Figure 5(e), a relatively large RWV among GNRs results in a lower on/off current ratio, which is caused by the nonsynchronization of the off states of multiple nonuniform GNRs. In particular, the 8 nm half-pitch GNRs-based FET with RWV of less than 3 nm exhibits a high on/off current ratio (>10), which is of higher value than reported transistors bearing densely aligned GNRs [146]. This research offers crucial insights for understanding the transport characteristics of the FETs based on multiple GNRs, which may lead to high resolution graphene nanostructures for future electronic applications.
6.3. Multiscale Porous Nanocolander Network with Tunable Transport Properties by Combined CL with LbL Processes
3D inverse opal (IO) structure, which is constructed from the self-assembled colloidal particles, is an attractive structure for various applications such as photonic crystals [147], optical biosensors [148], and energy devices [149–152]. Previous researches take advantage of large interfacial area and highly periodic structures generally have morphological benefits including high open porosity. Nevertheless, systematic control of permeability and selectivity of porous media is challenging because the size of colloids used for each templating must change.
Kim et al. [153] presented an interesting approach for a multiscale porous network with tunable transport properties. The microphase-separated block copolymer creates well-defined mesopores inside the 3D-IO structured template that was obtained from a UV-curable polymer, PUA. According to the increasing numbers of BCP solution coating, the film thickness is increased and BCP morphology is changed from nanosieves to nanodomes and corrugated structures (Figure 5(f)). The multiscale porous networks were tested for membrane separation performance with Au nanoparticle solutions (the pore size of the BCP nanosieve was fixed at 18 nm and Au nanoparticle solution contained particles with diameters of 5, 10, 20, or 30 nm) (Figure 5(g)). The results show complete separation of the nanoparticles, when the number of BCP solution coatings is greater than 8 without forming bulk-scale defects. Also the value of water permeability for testing the transport performance is higher than other presented BCP-based membranes (Figure 5(h)) [154–156]. Therefore, multiscale porous network structures with tunable transport properties can serve as a significant platform for filtration applications.
7. Conclusion
In this review, advanced methods in unconventional lithography for 3D hierarchical nanostructures are described. These methods can produce structures that can be used for many applications that can complement those already available from conventional nanolithography or nanotechnology. The range of the 3D nanostructures and techniques can create an appealing set of circumstances for development of uses for basic science and industry in the near future.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This work is supported by the IBS-R015-D1, Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT and Future Planning (NRF-2013-R1A1A1061403), and the Pioneer Research Center Program (NRF-2014M3C1A3001208).
LevinsonH. J.2005Bellingham, Wash, USASPIEXiaY.WhitesidesG.Soft lithography199837550575XiaY.RogersJ. A.PaulK. E.WhitesidesG. M.Unconventional methods for fabricating and patterning nanostructures19999971823184810.1021/cr980002q2-s2.0-0346704264GatesB. D.XuQ.LoveJ. C.WolfeD. B.WhitesidesG. M.Unconventional nanofabrication20043433937210.1146/annurev.matsci.34.052803.0911002-s2.0-4344679349BucknallD. G.2005Boca Raton, Fla, USACRC PressGatesB. D.XuQ.StewartM.RyanD.WillsonC. G.WhitesidesG. M.New approaches to nanofabrication: molding, printing, and other techniques200510541171119610.1021/cr030076o2-s2.0-18044384992ChouS. Y.KraussP. R.RenstromP. J.Imprint of sub-25 nm vias and trenches in polymers1995673114311610.1063/1.1148512-s2.0-0142037327SuhK. Y.KimY. S.LeeH. H.Capillary force lithography200113181386138910.1002/1521-4095(200109)13:18<1386::aid-adma1386>3.0.co;2-xSuhK. Y.LeeH. H.Capillary force lithography: large-area patterning, self-organization, and anisotropic dewetting2002126-74054132-s2.0-0036612021PinerR. D.ZhuJ.XuF.HongS.MirkinC. A.‘Dip-pen’ nanolithography1999283540266166310.1126/science.283.5402.6612-s2.0-0033614026KimY. S.SuhK. Y.LeeH. H.Fabrication of three-dimensional microstructures by soft molding200179142285228710.1063/1.14078592-s2.0-0035474532KimY. S.ParkJ.LeeH. H.Three-dimensional pattern transfer and nanolithography: modified soft molding20028161011101310.1063/1.14981492-s2.0-79956024799PerlA.ReinhoudtD. N.HuskensJ.Microcontact printing: limitations and achievements200921222257226810.1002/adma.2008018642-s2.0-67649274017YangS.-M.JangS. G.ChoiD.-G.KimS.YuH. K.Nanomachining by colloidal lithography20062445847510.1002/smll.2005003902-s2.0-33644887719ParkM.HarrisonC.ChaikinP. M.RegisterR. A.AdamsonD. H.Block copolymer lithography: periodic arrays of ~1011 holes in 1 square centimeter199727653171401140410.1126/science.276.5317.1401RogersJ. A.LeeH. H.2009Hoboken, NJ, USAJohn Wiley & SonsJeongH. E.KwakR.KhademhosseiniA.SuhK. Y.UV-assisted capillary force lithography for engineering biomimetic multiscale hierarchical structures: from lotus leaf to gecko foot hairs20091333133810.1039/b9nr00106a2-s2.0-77953659861JeongH. E.LeeS. H.KimJ. K.SuhK. Y.Nanoengineered multiscale hierarchical structures with tailored wetting properties20062241640164510.1021/la05264342-s2.0-33644558075BaeW.-G.KimH. N.KimD.ParkS.-H.JeongH. E.SuhK.-Y.25th anniversary article: scalable multiscale patterned structures inspired by nature: the role of hierarchy201426567569910.1002/adma.2013034122-s2.0-84895063766GuoL. J.Recent progress in nanoimprint technology and its applications20043711R123R14110.1088/0022-3727/37/11/r012-s2.0-2942558559GuoL. J.Nanoimprint lithography: methods and material requirements200719449551310.1002/adma.2006008822-s2.0-34250642011ChouS. Y.KraussP. R.RenstromP. J.Imprint lithography with 25-nanometer resolution19962725258858710.1126/science.272.5258.852-s2.0-0030570065ChouS. Y.KraussP. R.RenstromP. J.Nanoimprint lithography19961464129413310.1116/1.5886052-s2.0-5344241941ColburnM.JohnsonS. C.StewartM. D.Step and flash imprint lithography: a new approach to high-resolution patterning3676Emerging Lithographic Technologies III1999379389Proceedings of SPIE10.1117/12.351155SeoS.-M.KimT.-I.LeeH. H.Simple fabrication of nanostructure by continuous rigiflex imprinting200784456757210.1016/j.mee.2006.11.0082-s2.0-33847620278AhnS. H.GuoL. J.High-speed roll-to-roll nanoimprint lithography on flexible plastic substrates200820112044204910.1002/adma.2007026502-s2.0-55149110583AhnS. H.GuoL. J.Large-area roll-to-roll and roll-to-plate Nanoimprint Lithography: a step toward high-throughput application of continuous nanoimprinting2009382304231010.1021/nn90036332-s2.0-69549086726HaatainenT.AhopeltoJ.Pattern transfer using step&stamp imprint lithography200367435736010.1238/physica.regular.067a003572-s2.0-0037397630SuhK. Y.KimP.LeeH. H.Capillary kinetics of thin polymer films in permeable microcavities200485184019402110.1063/1.18102122-s2.0-10044235308SuhD.TakH.ChoiS.-J.KimT.-I.Permeability- and surface-energy-tunable polyurethane acrylate molds for capillary force lithography2015743238242383010.1021/acsami.5b06975YoonH.KimT.-I.ChoiS.SuhK. Y.KimM. J.LeeH. H.Capillary force lithography with impermeable molds20068825410410.1063/1.22062472-s2.0-33745477892ZaumseilJ.MeitlM. A.HsuJ. W. P.AcharyaB. R.BaldwinK. W.LooY.-L.RogersJ. A.Three-dimensional and multilayer nanostructures formed by nanotransfer printing2003391223122710.1021/nl03440072-s2.0-0141461371CarlsonA.BowenA. M.HuangY.NuzzoR. G.RogersJ. A.Transfer printing techniques for materials assembly and micro/nanodevice fabrication201224395284531810.1002/adma.2012013862-s2.0-84867030009MeitlM. A.ZhuZ.-T.KumarV.LeeK. J.FengX.HuangY. Y.AdesidaI.NuzzoR. G.RogersJ. A.Transfer printing by kinetic control of adhesion to an elastomeric stamp200651333810.1038/nmat15322-s2.0-30044447991KimT.-I.KimJ.-H.SonS. J.SeoS.-M.Gold nanocones fabricated by nanotransfer printing and their application for field emission2008192929530210.1088/0957-4484/19/29/2953022-s2.0-47249149202KimT.-I.SeoS.-M.The facile fabrication of a wire-grid polarizer by reversal rigiflex printing2009201414530510.1088/0957-4484/20/14/1453052-s2.0-65149085669KwakM. K.KimT.-I.KimP.LeeH. H.SuhK. Y.Large-area dual-scale metal transfer by adhesive force20095892893210.1002/smll.2008012622-s2.0-65649114365KimY. S.BaekS. J.HammondP. T.Physical and chemical nanostructure transfer in polymer spin-transfer printing200416758158410.1002/adma.2003062312-s2.0-2442585608SuhD.ChoiS.-J.LeeH. H.Rigiflex lithography for nanostructure transfer200517121554156010.1002/adma.2004020102-s2.0-20644436244LeeM. H.LinJ. Y.OdomT. W.Large-area nanocontact printing with metallic nanostencil masks201049173057306010.1002/anie.2009068002-s2.0-77951154342MenardE.LeeK. J.KhangD.-Y.NuzzoR. G.RogersJ. A.A printable form of silicon for high performance thin film transistors on plastic substrates200484265398540010.1063/1.17675912-s2.0-3242707892KimD.-H.AhnJ.-H.WonM. C.KimH.-S.KimT.-H.SongJ.HuangY. Y.LiuZ.LuC.RogersJ. A.Stretchable and foldable silicon integrated circuits2008320587550751110.1126/science.11543672-s2.0-42549116193KimT.-H.CarlsonA.AhnJ.-H.WonS. M.WangS.HuangY.RogersJ. A.Erratum: ‘kinetically controlled, adhesiveless transfer printing using microstructured stamps’ [Appl. Phys. Lett.94, 113502 (2009)]20099418990210.1063/1.31371832-s2.0-65549135737KimT.-I.KimR.-H.RogersJ. A.Microscale inorganic light-emitting diodes on flexible and stretchable substrates20124260761210.1109/JPHOT.2012.21889982-s2.0-84866125758KimT.-I.JungY. H.SongJ.KimD.LiY.KimH.-S.SongI.-S.WiererJ. J.PaoH. A.HuangY.RogersJ. A.High-efficiency, microscale GaN light-emitting diodes and their thermal properties on unusual substrates20128111643164910.1002/smll.2012003822-s2.0-84861885668KimR.-H.TaoH.KimT.-I.ZhangY.KimS.PanilaitisB.YangM.KimD.-H.JungY. H.KimB. H.LiY.HuangY.OmenettoF. G.RogersJ. A.Materials and designs for wirelessly powered implantable light-emitting systems20128182812281810.1002/smll.2012009432-s2.0-84866413107KimR.-H.KimS.SongY. M.JeongH.KimT.-I.LeeJ.LiX.ChoquetteK. D.RogersJ. A.Flexible vertical light emitting diodes20128203123312810.1002/smll.2012011952-s2.0-84867565775KimT.-I.McCallJ. G.JungY. H.HuangX.SiudaE. R.LiY.SongJ.SongY. M.PaoH. A.KimR.-H.LuC.LeeS. D.SongI.-S.ShinG.Al-HasaniR.KimS.TanM. P.HuangY.OmenettoF. G.RogersJ. A.BruchasM. R.Injectable, cellular-scale optoelectronics with applications for wireless optogenetics2013340612921121610.1126/science.12324372-s2.0-84876310253JeongJ.-W.MccallJ. G.ShinG.ZhangY.Al-HasaniR.KimM.LiS.SimJ. Y.JangK.-I.ShiY.HongD. Y.LiuY.SchmitzG. P.XiaL.HeZ.GambleP.RayW. Z.HuangY.BruchasM. R.RogersJ. A.Wireless optofluidic systems for programmable in vivo pharmacology and optogenetics2015162366267410.1016/j.cell.2015.06.042KimD.-H.LuN.GhaffariR.KimY.-S.LeeS. P.XuL.WuJ.KimR.-H.SongJ.LiuZ.ViventiJ.De GraffB.ElolampiB.MansourM.SlepianM. J.HwangS.MossJ. D.WonS.-M.HuangY.LittB.RogersJ. A.Materials for multifunctional balloon catheters with capabilities in cardiac electrophysiological mapping and ablation therapy201110431632310.1038/nmat29712-s2.0-79953038482CheungC. L.NikolićR. J.ReinhardtC. E.WangT. F.Fabrication of nanopillars by nanosphere lithography20061751339134310.1088/0957-4484/17/5/0282-s2.0-33144470243VlasovY. A.BoX.-Z.SturmJ. C.NorrisD. J.On-chip natural assembly of silicon photonic bandgap crystals2001414686128929310.1038/351045292-s2.0-0035891246MoonJ. H.KimS.YiG.-R.LeeY.-H.YangS.-M.Fabrication of ordered macroporous cylinders by colloidal templating in microcapillaries20042052033203510.1021/la03580152-s2.0-1542337011LimJ.-M.YiG.-R.MoonJ. H.HeoC.-J.YangS.-M.Superhydrophobic films of electrospun fibers with multiple-scale surface morphology200723157981798910.1021/la700392w2-s2.0-34547352383LiY.KoshizakiN.WangH.ShimizuY.Untraditional approach to complex hierarchical periodic arrays with trinary stepwise architectures of micro-, submicro-, and nanosized structures based on binary colloidal crystals and their fine structure enhanced properties20115129403941210.1021/nn203239n2-s2.0-84555223737BatesF. S.FredricksonG. H.Block copolymer thermodynamics: theory and experiment199041152555710.1146/annurev.pc.41.100190.0025212-s2.0-4444340443ParkC.YoonJ.ThomasE. L.Enabling nanotechnology with self assembled block copolymer patterns200344226725676010.1016/j.polymer.2003.08.0112-s2.0-0141755407BatesC. M.MaherM. J.JanesD. W.EllisonC. J.WillsonC. G.Block copolymer lithography201447121210.1021/ma401762n2-s2.0-84892597058GreinerC.ArztE.del CampoA.Hierarchical gecko-like adhesives200921447948210.1002/adma.2008015482-s2.0-59249109078HiraiY.YoshidaS.TakagiN.Defect analysis in thermal nanoimprint lithography20032162765277010.1116/1.16292892-s2.0-0942300052ZhangF.LowH. Y.Ordered three-dimensional hierarchical nanostructures by nanoimprint lithography20061781884189010.1088/0957-4484/17/8/0132-s2.0-33645310219SchiftH.Nanoimprint lithography: an old story in modern times? A review200826245848010.1116/1.28909722-s2.0-41549124541BaoL.-R.ChengX.HuangX. D.GuoL. J.PangS. W.YeeA. F.Nanoimprinting over topography and multilayer three-dimensional printing20022062881288610.1116/1.15263552-s2.0-0036873980KehagiasN.ReboudV.ChansinG.ZelsmannM.JeppesenC.ReutherF.SchusterC.KubenzM.GruetznerG.KehagiasN.Sotomayor TorresC. M.Submicron three-dimensional structures fabricated by reverse contact UV nanoimprint lithography20062430023005ZhaoX.-M.XiaY.WhitesidesG. M.Fabrication of three-dimensional micro-structures: microtransfer molding199681083784010.1002/adma.199600810162-s2.0-0030263409LipomiD. J.ChiechiR. C.DickeyM. D.WhitesidesG. M.Fabrication of conjugated polymer nanowires by edge lithography2008872100210510.1021/nl80093182-s2.0-53149142545BruininkC. M.PéterM.MauryP. A.de BoerM.KuipersL.HuskensJ.ReinhoudtD. N.Capillary force lithography: fabrication of functional polymer templates as versatile tools for nanolithography200616121555156510.1002/adfm.2005006292-s2.0-33747610619HarfenistS. A.CambronS. D.NelsonE. W.BerryS. M.IshamA. W.CrainM. M.WalshK. M.KeyntonR. S.CohnR. W.Direct drawing of suspended filamentary micro- and nanostructures from liquid polymers20044101931193710.1021/nl048919u2-s2.0-7544237248YunM.MyungN. V.VasquezR. P.LeeC.MenkeE.PennerR. M.Electrochemically grown wires for individually addressable sensor arrays20044341942210.1021/nl035069u2-s2.0-1642487781KempN. T.McGroutherD.CochraneJ. W.NewburyR.Bridging the gap: polymer nanowire devices200719182634263810.1002/adma.2006027262-s2.0-34748914366KwakR.JeongH. E.SuhK. Y.Fabrication of monolithic bridge structures by vacuum-assisted capillary-force lithography20095779079410.1002/smll.2009002192-s2.0-65449182048NeuhausS.SpencerN. D.PadesteC.Anisotropic wetting of microstructured surfaces as a function of surface chemistry20124112313010.1021/am201104q2-s2.0-84856265251XiaD.BrueckS. R. J.Strongly anisotropic wetting on surfaces 20082008828192824ChungJ. Y.YoungbloodJ. P.StaffordC. M.Anisotropic wetting on tunable micro-wrinkled surfaces2007391163116910.1039/b705112c2-s2.0-34547917960KwakM. K.JeongH.-E.KimT.-I.YoonH.SuhK. Y.Bio-inspired slanted polymer nanohairs for anisotropic wetting and directional dry adhesion2010691849185710.1039/b924056j2-s2.0-77951604541ZhengY.GaoX.JiangL.Directional adhesion of superhydrophobic butterfly wings20073217818210.1039/b612667g2-s2.0-33846453543KimT.-I.SuhK. Y.Unidirectional wetting and spreading on stooped polymer nanohairs20095214131413510.1039/b915079j2-s2.0-70350087374OeffnerJ.LauderG. V.The hydrodynamic function of shark skin and two biomimetic applications2012215578579510.1242/jeb.0630402-s2.0-84857612961FengL.LiS.LiY.LiH.ZhangL.ZhaiJ.SongY.LiuB.JiangL.ZhuD.Super-hydrophobic surfaces: from natural to artificial200214241857186010.1002/adma.200290020GuoZ.LiuW.SuB.-L.Superhydrophobic surfaces: from natural to biomimetic to functional2011353233535510.1016/j.jcis.2010.08.0472-s2.0-78149411640ZhaiL.BergM. C.CebeciF. Ç.KimY.MilwidJ. M.RubnerM. F.CohenR. E.Patterned superhydrophobic surfaces: toward a synthetic mimic of the namib desert beetle2006661213121710.1021/nl060644q2-s2.0-33745783951RadhaB.LimS. H.SaifullahM. S. M.KulkarniG. U.Metal hierarchical patterning by direct nanoimprint lithography20133, article 1078810.1038/srep010782-s2.0-84874618004VorobyevA. Y.GuoC.Multifunctional surfaces produced by femtosecond laser pulses2015117303310310.1063/1.4905616JeongH. E.LeeJ.-K.KimH. N.MoonS. H.SuhK. Y.A nontransferring dry adhesive with hierarchical polymer nanohairs2009106145639564410.1073/pnas.09003231062-s2.0-65249190864KimS.WuJ.CarlsonA.JinS. H.KovalskyA.GlassP.LiuZ.AhmedN.ElganS. L.ChenW.FerreiraP. M.SittiM.HuangY.RogersJ. A.Microstructured elastomeric surfaces with reversible adhesion and examples of their use in deterministic assembly by transfer printing201010740170951710010.1073/pnas.10058281072-s2.0-78049300014KimS.SpenkoM.TrujilloS.HeynemanB.SantosD.CutkosklyM. R.Smooth vertical surface climbing with directional adhesion2008241657410.1109/tro.2007.9097862-s2.0-40949135172HansenW. R.AutumnK.Evidence for self-cleaning in gecko setae2005102238538910.1073/pnas.04083041022-s2.0-12244266092LeeJ.FearingR. S.Contact self-cleaning of synthetic gecko adhesive from polymer microfibers20082419105871059110.1021/la80214852-s2.0-54549102232KimS.CheungE.SittiM.Wet self-cleaning of biologically inspired elastomer mushroom shaped microfibrillar adhesives200925137196719910.1021/la900732h2-s2.0-67650047733KwakM. K.JeongH.-E.SuhK. Y.Rational design and enhanced biocompatibility of a dry adhesive medical skin patch201123343949395310.1002/adma.2011016942-s2.0-80052424584BaeW. G.KimD.KwakM. K.HaL.KangS. M.SuhK. Y.Enhanced skin adhesive patch with modulus-tunable composite micropillars20132110911310.1002/adhm.2012000982-s2.0-84879608549DekkerC.Solid-state nanopores20072420921510.1038/nnano.2007.272-s2.0-34248351114StriemerC. C.GaborskiT. R.McGrathJ. L.FauchetP. M.Charge- and size-based separation of macromolecules using ultrathin silicon membranes2007445712974975310.1038/nature055322-s2.0-33847132253KowalczykS. W.KapinosL.BlosserT. R.MagalhãesT.Van NiesP.LimR. Y. H.DekkerC.Single-molecule transport across an individual biomimetic nuclear pore complex20116743343810.1038/nnano.2011.882-s2.0-79960098185Vazquez-MenaO.VillanuevaG.SavuV.SidlerK.van den BoogaartM. A. F.BruggerJ.Metallic nanowires by full wafer stencil lithography20088113675368210.1021/nl801778t2-s2.0-58149279809HenzieJ.LeeM. H.OdomT. W.Multiscale patterning of plasmonic metamaterials20072954955410.1038/nnano.2007.2522-s2.0-34548399047AksuS.YanikA. A.AdatoR.ArtarA.HuangM.AltugH.High-throughput nanofabrication of infrared plasmonic nanoantenna arrays for vibrational nanospectroscopy20101072511251810.1021/nl101042a2-s2.0-77955331450OdomT. W.GaoH.McMahonJ. M.HenzieJ.SchatzG. C.Plasmonic superlattices: hierarchical subwavelength hole arrays20094834–618719210.1016/j.cplett.2009.10.0842-s2.0-71649115494LeeW.HanH.LotnykA.SchubertM. A.SenzS.AlexeM.HesseD.BaikS.GöseleU.Individually addressable epitaxial ferroelectric nanocapacitor arrays with near Tb inch−2 density20083402407FanZ.RazaviH.DoJ.-W.MoriwakiA.ErgenO.ChuehY.-L.LeuP. W.HoJ. C.TakahashiT.ReichertzL. A.NealeS.YuK.WuM.AgerJ. W.JaveyA.Three-dimensional nanopillar-array photovoltaics on low-cost and flexible substrates20098864865310.1038/nmat24932-s2.0-69249213906MoghaddamS.PengwangE.JiangY.-B.GarciaA. R.BurnettD. J.BrinkerC. J.MaselR. I.ShannonM. A.An inorganic–organic proton exchange membrane for fuel cells with a controlled nanoscale pore structure20105323023610.1038/nnano.2010.132-s2.0-77949262942HuhD.MatthewsB. D.MammotoA.Montoya-ZavalaM.Yuan HsinH.IngberD. E.Reconstituting organ-level lung functions on a chip201032859861662166810.1126/science.11883022-s2.0-77954038080JangK.-J.SuhK.-Y.A multi-layer microfluidic device for efficient culture and analysis of renal tubular cells2010101364210.1039/b907515a2-s2.0-77951884924LeenJ. B.HansenP.ChengY.-T.HesselinkL.Improved focused ion beam fabrication of near-field apertures using a silicon nitride membrane200833232827282910.1364/ol.33.0028272-s2.0-57649208448YanX.-M.ContrerasA. M.KoebelM. M.LiddleJ. A.SomorjaiG. A.Parallel fabrication of sub-50-nm uniformly sized nanoparticles by deposition through a patterned silicon nitride nanostencil2005561129113410.1021/nl05068122-s2.0-21644450668TongH. D.JansenH. V.GadgilV. J.BostanC. G.BerenschotE.Van RijnC. J. M.ElwenspoekM.Silicon nitride nanosieve membrane20044228328710.1021/nl03501752-s2.0-1442324498ChoH.KimJ.ParkH.Won BangJ.Seop HyunM.BaeY.HaL.Yoon KimD.Min KangS.Jung ParkT.SeoS.ChoiM.SuhK.Replication of flexible polymer membranes with geometry-controllable nano-apertures via a hierarchical mould-based dewetting20145, article 313710.1038/ncomms4137YinY.LuY.GatesB.XiaY.Template-assisted self-assembly: a practical route to complex aggregates of monodispersed colloids with well-defined sizes, shapes, and structures2001123368718872910.1021/ja011048v2-s2.0-0035850516XiaY.YinY.LuY.McLellanJ.Template-assisted self-assembly of spherical colloids into complex and controllable structures2003131290791810.1002/adfm.2003000022-s2.0-0346394257KimP.LeeS. E.JungH. S.LeeH. Y.KawaiT.SuhK. Y.Soft lithographic patterning of supported lipid bilayers onto a surface and inside microfluidic channels200661545910.1039/b512593f2-s2.0-33644848017YapF. L.ZhangY.Assembly of polystyrene microspheres and its application in cell micropatterning200728142328233810.1016/j.biomaterials.2007.01.0342-s2.0-33847187058YeoS. J.KangH.KimY. H.HanS.YooP. J.Layer-by-layer assembly of polyelectrolyte multilayers in three-dimensional inverse opal structured templates2012442107211510.1021/am300072p2-s2.0-84860347311FredricksonG. H.BatesF. S.Dynamics of block copolymers: theory and experiment199626150155010.1146/annurev.ms.26.080196.0024412-s2.0-0029713327BatesF. S.FredricksonG. H.Block copolymers-designer soft materials199952232382-s2.0-0348046829KimS. O.SolakH. H.StoykovichM. P.FerrierN. J.De PabloJ. J.NealeyP. F.Epitaxial self-assembly of block copolymers on lithographically defined nanopatterned substrates2003424694741141410.1038/nature017752-s2.0-0042532330RuizR.KangH.DetcheverryF. A.DobiszE.KercherD. S.AlbrechtT. R.de PabloJ. J.NealeyP. F.Density multiplication and improved lithography by directed block copolymer assembly2008321589193693910.1126/science.11576262-s2.0-49649099742ChengJ. Y.RettnerC. T.SandersD. P.KimH.-C.HinsbergW. D.Dense self-assembly on sparse chemical patterns: rectifying and multiplying lithographic patterns using block copolymers200820163155315810.1002/adma.2008008262-s2.0-52649100977SegalmanR. A.YokoyamaH.KramerE. J.Graphoepitaxy of spherical domain block copolymer films200113151152115510.1002/1521-4095(200108)13:15<1152::AID-ADMA1152>3.0.CO;2-52-s2.0-0035800414BitaI.YangJ. K. W.YeonS. J.RossC. A.ThomasE. L.BerggrenK. K.Graphoepitaxy of self-assembled block copolymers on two-dimensional periodic patterned templates2008321589193994310.1126/science.11593522-s2.0-49649127635ParkS.LeeD. H.XuJ.KimB.SungW. H.JeongU.XuT.RussellT. P.Macroscopic 10-terabit-per-square-inch arrays from block copolymers with lateral order200932359171030103310.1126/science.11681082-s2.0-60749101893YangJ. K. W.JungY. S.ChangJ.-B.MickiewiczR. A.Alexander-KatzA.RossC. A.BerggrenK. K.Complex self-assembled patterns using sparse commensurate templates with locally varying motifs20105425626010.1038/nnano.2010.302-s2.0-77950847801ParkS.-M.LiangX.HarteneckB. D.PickT. E.HiroshibaN.WuY.HelmsB. A.OlynickD. L.Sub-10 nm nanofabrication via nanoimprint directed self-assembly of block copolymers20115118523853110.1021/nn201391d2-s2.0-81855172058JeongJ. W.ParkW. I.DoL.-M.ParkJ.-H.KimT.-H.ChaeG.JungY. S.Nanotransfer printing with sub-10 nm resolution realized using directed self-assembly201224263526353110.1002/adma.2012003562-s2.0-84863712046OnsesM. S.SongC.WilliamsonL.SutantoE.FerreiraP. M.AlleyneA. G.NealeyP. F.AhnH.RogersJ. A.Hierarchical patterns of three-dimensional block-copolymer films formed by electrohydrodynamic jet printing and self-assembly20138966767510.1038/nnano.2013.1602-s2.0-84883742518ParkJ.-U.HardyM.KangS. J.BartonK.AdairK.MukhopadhyayD. K.LeeC. Y.StranoM. S.AlleyneA. G.GeorgiadisJ. G.FerreiraP. M.RogersJ. A.High-resolution electrohydrodynamic jet printing200761078278910.1038/nmat19742-s2.0-34848838673RessierL.PalleauE.BeharS.Electrical nano-imprint lithography2012232525530210.1088/0957-4484/23/25/2553022-s2.0-84861855599JacobsH. O.WhitesidesG. M.Submicrometer patterning of charge in thin-film electrets200129155091763176610.1126/science.10570612-s2.0-0035793890BarryC. R.GuJ.JacobsH. O.Charging process and Coulomb-force-directed printing of nanoparticles with sub-100-nm lateral resolution20055102078208410.1021/nl05119722-s2.0-27544506999DiazR.PalleauE.PoirotD.SangeethaN. M.RessierL.High-throughput fabrication of anti-counterfeiting colloid-based photoluminescent microtags using electrical nanoimprint lithography2014253434530210.1088/0957-4484/25/34/3453022-s2.0-84905739238SchulzH.ScheerH.-C.HoffmannT.TorresC. M. S.PfeifferK.BleidiesselG.GrütznerG.CardinaudC.GaboriauF.PeignonM.-C.AhopeltoJ.HeidariB.New polymer materials for nanoimprinting20001841861186510.1116/1.13053312-s2.0-23044523167RessierL.Le NaderV.Electrostatic nanopatterning of PMMA by AFM charge writing for directed nano-assembly2008191313530110.1088/0957-4484/19/13/1353012-s2.0-40549126638KnorrN.RosselliS.NellesG.Surface-potential decay of biased-probe contact-charged amorphous polymer films201010705410610.1063/1.33097632-s2.0-77949768709NilssonJ.NetoA. H. C.GuineaF.PeresN. M. R.Electronic properties of graphene multilayers2006972626680110.1103/PhysRevLett.97.2668012-s2.0-33846402445StankovichS.DikinD. A.DommettG. H. B.KohlhaasK. M.ZimneyE. J.StachE. A.PinerR. D.NguyenS. T.RuoffR. S.Graphene-based composite materials2006442710028228610.1038/nature049692-s2.0-33746344730NetoA. H. C.PeresN. M. R.NovoselovK. S.GeimA. K.The electronic properties of graphene200981109162YanJ.-A.XianL.ChouM. Y.Structural and electronic properties of oxidized graphene2009103808680210.1103/physrevlett.103.086802ChoiD.KuruC.KimY.KimG.KimT.ChenR.JinS.Uniformly nanopatterned graphene field-effect transistors with enhanced properties201510, article 28910.1186/s11671-015-0976-2HanM. Y.ÖzyilmazB.ZhangY.KimP.Energy band-gap engineering of graphene nanoribbons2007982020680510.1103/physrevlett.98.2068052-s2.0-34547334459LiX.WangX.ZhangL.LeeS.DaiH.Chemically derived, ultrasmooth graphene nanoribbon semiconductors200831958671229123210.1126/science.11508782-s2.0-40049093097LiangX.JungY.-S.WuS.IsmachA.OlynickD. L.CabriniS.BokorJ.Formation of bandgap and subbands in graphene nanomeshes with sub-10 nm ribbon width fabricated via nanoimprint lithography20101072454246010.1021/nl100750v2-s2.0-77955340828BaiJ.ZhongX.JiangS.HuangY.DuanX.Graphene nanomesh20105319019410.1038/nnano.2010.82-s2.0-77749323301AkhavanO.Graphene nanomesh by ZnO nanorod photocatalysts2010474174418010.1021/nn10074292-s2.0-77955547940LiH.-W.HuckW. T. S.Ordered block-copolymer assembly using nanoimprint lithography2004491633163610.1021/nl049209r2-s2.0-4644288208YangE. L.LiuC. C.YangC. Y. P.SteinhausC. A.NealeyP. F.SkinnerJ. L.Nanofabrication of surface-enhanced Raman scattering device by an integrated block-copolymer and nanoimprint lithography method201028C6M9310.1116/1.35013412-s2.0-84905959766LiangX.WiS.Transport characteristics of multichannel transistors made from densely aligned sub-10 nm half-pitch graphene nanoribbons20126119700971010.1021/nn303127y2-s2.0-84870424667PanZ.LiuN.FuL.LiuZ.Wrinkle engineering: a new approach to massive graphene nanoribbon arrays201113344175781758110.1021/ja207517u2-s2.0-80455123827SchrodenR. C.Al-daousM.SteinA.Self-modification of spontaneous emission by inverse opal silica photonic crystals20011392945295010.1021/cm010230s2-s2.0-0035195094CassagneauT.CarusoF.Inverse opals for optical affinity biosensing200214221629163310.1002/1521-4095(20021118)14:22<1629::AID-ADMA1629>3.0.CO;2-22-s2.0-0037132297SakamotoJ. S.DunnB.Hierarchical battery electrodes based on inverted opal structures200212102859286110.1039/b205634h2-s2.0-0036795537KwakE. S.LeeW.ParkN.-C.KimJ.LeeH.Compact inverse-opal electrode using non-aggregated TiO2 nanoparticles for dye-sensitized solar cells20091971093109910.1002/adfm.2008015402-s2.0-64549139312KimJ.-H.KangS. H.ZhuK.KimJ. Y.NealeN. R.FrankA. J.Ni–NiO core-shell inverse opal electrodes for supercapacitors201147185214521610.1039/c0cc05191h2-s2.0-79954596313KimO. H.ChoY. H.KangS. H.ParkH. Y.KimM.LimJ. W.ChungD. Y.LeeM. J.ChoeH.SungY. E.Ordered macroporous platinum electrode and enhanced mass transfer in fuel cells using inverse opal structure201341910.1038/ncomms3473KimY. H.KangH.ParkS.ParkA. R.LeeY. M.RheeK.HanS.ChangH.RyuD. Y.YooP. J.Multiscale porous interconnected Nanocolander network with tunable transport properties201426477998800310.1002/adma.2014024362-s2.0-84919725136PeinemannK.-V.AbetzV.SimonP. F. W.Asymmetric superstructure formed in a block copolymer via phase separation200761299299610.1038/nmat20382-s2.0-36749015118YangS. Y.ParkJ.YoonJ.ReeM.JangS. K.KimJ. K.Virus filtration membranes prepared from nanoporous block copolymers with good dimensional stability under high pressures and excellent solvent resistance20081891371137710.1002/adfm.2007008322-s2.0-44249108375Zavala-RiveraP.ChannonK.NguyenV.SivaniahE.KabraD.FriendR. H.NatarajS. K.Al-MuhtasebS. A.HexemerA.CalvoM. E.MiguezH.Collective osmotic shock in ordered materials2012111535710.1038/nmat31792-s2.0-83655198311