Switching-Modulated Phase Change Memory Realized by Si-Containing Block Copolymers.

The phase change memory (PCM) is one of the key enabling memory technologies for next-generation non-volatile memory device applications due to its high writing speed, excellent endurance, long retention time, and good scalability. However, the high power consumption of PCM devices caused by the high switching current from a high resistive state to a low resistive state is a critical obstacle to be resolved before widespread commercialization can be realized. Here, a useful approach to reduce the writing current of PCM, which depends strongly on the contact area between the heater electrode and active layer, by employing self-assembly process of Si-containing block copolymers (BCPs) is presented. Self-assembled insulative BCP pattern geometries can locally block the current path of the contact between a high resistive film (TiN) and a phase-change material (Ge2 Sb2 Te5 ), resulting in a significant reduction of the writing current. Compared to a conventional PCM cell, the BCP-modified PCM shows excellent switching power reduction up to 1/20 given its use of self-assembled hybrid SiFex Oy /SiOx dot-in-hole nanostructures. This BCP-based bottom-up process can be extended to various applications of other non-volatile memory devices, such as resistive switching memory and magnetic storage devices.

Rotating Cylinder-Assisted Nanoimprint Lithography for Enhanced Chemisorbable Filtration Complemented by Molecularly Imprinted Polymers.

Rotating cylindrical stamp-based nanoimprint technique has many advantages, including the continuous fabrication of intriguing micro/nanostructures and rapid pattern transfer on a large scale. Despite these advantages, the previous nanoimprint lithography has rarely been used for producing sophisticated nanoscale patterns on a non-planar substrate that has many extended applications. Here, the simple integration of nanoimprinting process with a help of a transparent stamp wrapped on the cylindrical roll and UV optical source in the core to enable high-throughput pattern transfer, particularly on a fabric substrate is demonstrated. Moreover, as a functional resin material, this innovative strategy involves a synergistic approach on the synthesis of molecularly imprinted polymer, which are spatially organized free-standing perforated nanostructures such as nano/microscale lines, posts, and holes patterns on various woven or nonwoven blank substrates. The proposed materials can serve as a self-encoded filtration medium for selective separation of formaldehyde molecules. It is envisioned that the combinatorial fabrication process and attractive material paves the way for designing next-generation separation systems in use to capture industrial or household toxic substances.

Enabling the Selective Detection of Endocrine-Disrupting Chemicals via Molecularly Surface-Imprinted "Coffee Rings".

Molecularly imprinted polymers (MIPs) represent an intriguing class of synthetic materials that can selectively recognize and bind chemical or biological molecules in a variety of value-added applications in sensors, catalysis, drug delivery, antibodies, and receptors. In this context, many advanced methods of implementing functional MIP materials have been actively studied. Herein, we report a robust strategy to produce highly ordered arrays of surface-imprinted polymer patterns with unprecedented regularity for MIP-based sensor platform, which involves the controlled evaporative self-assembly process of MIP precursor solution in a confined geometry consisting of a spherical lens on a flat Si substrate (i.e., sphere-on-flat geometry) to synergistically utilize the "coffee-ring" effect and repetitive stick-slip motions of the three-phase contact line simply by solvent evaporation. Highly ordered arrays of the ring-patterned MIP films are then polymerized under UV irradiation to achieve semi-interpenetrating polymer networks. The extraction of templated target molecules from the surface-imprinted ring-patterned MIP films leaves behind copious cavities for the recognizable specific "memory sites" to efficiently detect small molecules. As a result, the elaborated surface structuring effect, sensitivity, and specific selectivity of the coffee-ring-based MIP sensors are scrutinized by capitalizing on an endocrine-disrupting chemical, 2,4-dichlorophenoxyacetic acid (2,4-D), as an example. Clearly, the evaporative self-assembly of nonvolatile solutes in a confined geometry renders the creation of familiar yet ordered coffee-ring-like patterns for a wide range of applications, including sensors, scaffolds for cell motility, templates, substrates for neuron guidance, etc., thereby dispensing with the need of multistep lithography techniques and external fields.

Dual spectra band emissive Eu2+/Mn2+ co-activated alkaline earth phosphates for indoor plant growth novel phosphor converted-LEDs.

This paper reports designing a novel single composition blue/red color illuminating phosphor followed by fabricating "smart" agricultural/horticultural LED lighting. Color-tunable Eu2+/Mn2+ co-activated alkaline earth phosphates, Na(Sr,Ba)PO4 and Ca3Mg3(PO4)4, are considered, and the stable doping sites for the corresponding activators are identified by using first-principle DFT calculations. We can realize the designated color purity with stable thermal quenching preserved luminescence behavior is induced by the Eu2+ center positioned at different coordination states with intermixed Sr2+/Ba2+ sites in Na(Sr,Ba)PO4 hosts. Moreover, we demonstrate that the resultant LED lighting adopting the proposed novel phosphor composition stimulates the enhanced photosynthesis reaction for indoor hydroponics plants, such as oats and onions, which is superior to the narrow line emission band induced by the mixture of conventional red/green/blue LEDs. Thus, using the color-tunable single composition luminescent material may produce an innovative energy-efficient artificial lighting for indoor plant growth.

Charge gradient microscopy.

Here we present a simple and fast method to reliably image polarization charges using charge gradient microscopy (CGM). We collected the current from the grounded CGM probe while scanning a periodically poled lithium niobate single crystal and single-crystal LiTaO3 thin film on the Cr electrode. We observed current signals at the domains and domain walls originating from the displacement current and the relocation or removal of surface charges, which enabled us to visualize the ferroelectric domains at a scan frequency above 78 Hz over 10 µm. We envision that CGM can be used in high-speed ferroelectric domain imaging and piezoelectric energy-harvesting devices.

Eu(2+)-Activated Phase-Pure Oxonitridosilicate Phosphor in a Ba-Si-O-N System via Facile Silicate-Assisted Routes Designed by First-Principles Thermodynamic Simulation.

Eu(2+)-activated single phase Ba(2+)-oxonitridosilicate phosphors were prepared under a mild synthetic condition via silicate precursors, and their luminescent properties were investigated. Both the preferred oxonitridosilicate formation as for the available host compounds and thermodynamic stability within the Ba-Si-O-N system were elucidated in detail by the theoretical simulation based on the first-principles density functional theory. Those results can visualize the optimum synthetic conditions for Eu(2+)-activated highly luminescent Ba(2+)-oxonitridosilicates, especially Ba3Si6O12N2, as promising conversion phosphors for white LEDs, including Ba3Si6O9N4 and BaSi2O2N2 phases. To prove the simulated design rule, we synthesized the Ba3Si6O12N2:Eu(2+) phosphor using various silicate precursors, Ba2Si4O10, Ba2Si3O8, and BaSiO3, in a carbothermal reduction ambient and finally succeeded in obtaining a phase of pure highly luminescent oxonitridosilicate phosphor without using any solid-state nitride addition and/or high pressure synthetic procedures. Our study provides useful guidelines for robust synthetic procedures for developing thermally stable rare-earth-ion activated oxonitridosilicate phosphors and an established simulation method that can be effectively applied to other multigas systems.

Self-assembly-induced formation of high-density silicon oxide memristor nanostructures on graphene and metal electrodes.

We report the direct formation of ordered memristor nanostructures on metal and graphene electrodes by a block copolymer self-assembly process. Optimized surface functionalization provides stacking structures of Si-containing block copolymer thin films to generate uniform memristor device structures. Both the silicon oxide film and nanodot memristors, which were formed by the plasma oxidation of the self-assembled block copolymer thin films, presented unipolar switching behaviors with appropriate set and reset voltages for resistive memory applications. This approach offers a very convenient pathway to fabricate ultrahigh-density resistive memory devices without relying on high-cost lithography and pattern-transfer processes.

Uniform graphene quantum dots patterned from self-assembled silica nanodots.

Graphene dots precisely controlled in size are interesting in nanoelectronics due to their quantum optical and electrical properties. However, most graphene quantum dot (GQD) research so far has been performed based on flake-type graphene reduced from graphene oxides. Consequently, it is extremely difficult to isolate the size effect of GQDs from the measured optical properties. Here, we report the size-controlled fabrication of uniform GQDs using self-assembled block copolymer (BCP) as an etch mask on graphene films grown by chemical vapor deposition (CVD). Electron microscope images show that as-prepared GQDs are composed of mono- or bilayer graphene with diameters of 10 and 20 nm, corresponding to the size of BCP nanospheres. In the measured photoluminescence (PL) spectra, the emission peak of the GQDs on the SiO(2) substrate is shown to be at ∼395 nm. The fabrication of GQDs was supported by the analysis of the Raman spectra and the observation of PL spectra after each fabrication step. Additionally, oxygen content in the GQDs is rationally controlled by additional air plasma treatment, which reveals the effect of oxygen content to the PL property.

High-Resolution Nanotransfer Printing of Porous Crossbar Array Using Patterned Metal Molds by Extreme-Pressure Imprint Lithography.

High-resolution nanotransfer printing (nTP) technologies have attracted a tremendous amount of attention due to their excellent patternability, high productivity, and cost-effectiveness. However, there is still a need to develop low-cost mold manufacturing methods, because most nTP techniques generally require the use of patterned molds fabricated by high-cost lithography technology. Here, we introduce a novel nTP strategy that uses imprinted metal molds to serve as an alternative to a Si stamp in the transfer printing process. We present a method by which to fabricate rigid surface-patterned metallic molds (Zn, Al, and Ni) based on the process of direct extreme-pressure imprint lithography (EPIL). We also demonstrate the nanoscale pattern formation of functional materials, in this case Au, TiO2, and GST, onto diverse surfaces of SiO2/Si, polished metal, and slippery glass by the versatile nTP method using the imprinted metallic molds with nanopatterns. Furthermore, we show the patterning results of nanoporous crossbar arrays on colorless polyimide (CPI) by a repeated nTP process. We expect that this combined nanopatterning method of EPIL and nTP processes will be extendable to the fabrication of various nanodevices with complex circuits based on micro/nanostructures.

Formation of Multiscale Pattern Structures by Combined Patterning of Nanotransfer Printing and Laser Micromachining.

Various lithography techniques have been widely used for the fabrication of next-generation device applications. Micro/nanoscale pattern structures formed by lithographic methods significantly improve the performance capabilities of the devices. Here, we introduce a novel method that combines the patterning of nanotransfer printing (nTP) and laser micromachining to fabricate multiscale pattern structures on a wide range of scales. Prior to the formation of various nano-in-micro-in-millimeter (NMM) patterns, the nTP process is employed to obtain periodic nanoscale patterns on the target substrates. Then, an optimum laser-based patterning that effectively engraves various nanopatterned surfaces, in this case, spin-cast soft polymer film, rigid polymer film, a stainless still plate, and a Si substrate, is established. We demonstrate the formation of well-defined square and dot-shaped multiscale NMM-patterned structures by the combined patterning method of nTP and laser processes. Furthermore, we present the generation of unusual text-shaped NMM pattern structures on colorless polyimide (CPI) film, showing optically excellent rainbow luminescence based on the configuration of multiscale patterns from nanoscale to milliscale. We expect that this combined patterning strategy will be extendable to other nano-to-micro fabrication processes for application to various nano/microdevices with complex multiscale pattern geometries.

Direct Printing of Ultrathin Block Copolymer Film with Nano-in-Micro Pattern Structures.

Nanotransfer printing (nTP) is one of the most promising nanopatterning methods given that it can be used to produce nano-to-micro patterns effectively with functionalities for electronic device applications. However, the nTP process is hindered by several critical obstacles, such as sub-20 nm mold technology, reliable large-area replication, and uniform transfer-printing of functional materials. Here, for the first time, a dual nanopatterning process is demonstrated that creates periodic sub-20 nm structures on the eight-inch wafer by the transfer-printing of patterned ultra-thin (<50 nm) block copolymer (BCP) film onto desired substrates. This study shows how to transfer self-assembled BCP patterns from the Si mold onto rigid and/or flexible substrates through a nanopatterning method of thermally assisted nTP (T-nTP) and directed self-assembly (DSA) of Si-containing BCPs. In particular, the successful microscale patternization of well-ordered sub-20 nm SiOx patterns is systematically presented by controlling the self-assembly conditions of BCP and printing temperature. In addition, various complex pattern geometries of nano-in-micro structures are displayed over a large patterning area by T-nTP, such as angular line, wave line, ring, dot-in-hole, and dot-in-honeycomb structures. This advanced BCP-replicated nanopatterning technology is expected to be widely applicable to nanofabrication of nano-to-micro electronic devices with complex circuits.

Highly tunable self-assembled nanostructures from a poly(2-vinylpyridine-b-dimethylsiloxane) block copolymer.

An extraordinarily large degree of tunability in geometry and dimension is demonstrated in films of a self-assembled block copolymer. A poly(2-vinylpyridine-b-dimethylsiloxane) block copolymer with highly incompatible blocks was spun-cast on patterned substrates and treated with various solvent vapors. The degree of selective swelling in the poly(2-vinylpyridine) matrix block could be controlled over an extensive range, leading to the formation of various microdomain morphologies such as spheres, cylinders, hexagonally perforated lamellae, and lamellae from the same block copolymer. The systematic control of swelling ratio and the choice of solvent vapors offer the unusual ability to control the width of very well-ordered linear features within a range between 6 and 31 nm. This methodology is particularly useful for nanolithography based on directed self-assembly in that a single block copolymer film can form microdomains with a broad range of geometries and sizes without the need to change molecular weight or volume fraction.

Superhydrophobic Electrodeposited Copper Surface for Robust Condensation Heat Transfer.

Superhydrophobic surfaces have great potential for various applications owing to their superior dewetting and mobility of water droplets. However, the physical robustness of nano/microscale rough surface structures supporting superhydrophobicity is critical in real applications. In this study, to create a superhydrophobic surface on copper, we employed copper electrodeposition to create a nano/microscale rough surface structure as an alternative to the nanoneedle CuO structure. The rough electrodeposited copper surface with a thin Teflon coating shows superhydrophobicity. The enhancement of dewetting and mobility of water droplets on copper surfaces by electrodeposition and hydrophobization significantly improved the condensation heat transfer by up to approximately 78% compared to that of copper substrates. Moreover, the nano/microscale rough surface structure of the electrodeposited copper surface exhibits better tolerance to physical rubbing, which destroys the nanoneedle-structured CuO surface. Therefore, the condensation heat transfer of the superhydrophobic electrodeposited copper surface decreased by only less than 10%, while that of the nanoneedle-structured CuO surface decreased by approximately 40%. This suggests that an electrodeposited copper surface can lead to the stable performance of superhydrophobicity for real applications.

Controlled self-assembly of block copolymers in printed sub-20 nm cross-bar structures.

Directed self-assembly (DSA) of block copolymers (BCPs) has garnered much attention due to its excellent pattern resolution, simple process, and good compatibility with many other lithography methods for useful nanodevice applications. Here, we present a BCP-based multiple nanopatterning process to achieve three-dimensional (3D) pattern formation of metal/oxide hybrid nanostructures. We employed a self-assembled sub-20 nm SiO x line pattern as a master mold for nanotransfer printing (nTP) to generate a cross-bar array. By using the transfer-printed cross-bar structures as BCP-guiding templates, we can obtain well-ordered BCP microdomains in the distinct spaces of the nanotemplates through a confined BCP self-assembly process. We also demonstrate the morphological evolution of a cylinder-forming BCP by controlling the BCP film thickness, showing a clear morphological transition from cylinders to spheres in the designated nanospaces. Furthermore, we demonstrate how to control the number of BCP spheres within the cross-bar 3D pattern by adjusting the printing angle of the multiple nTP process to provide a suitable area for spontaneous BCP accommodation. This multiple-patterning-based approach is applicable to useful 3D nanofabrication of various devices with complex hybrid nanostructures.

Formation of Li2CO3 Nanostructures for Lithium-Ion Battery Anode Application by Nanotransfer Printing.

Various high-performance anode and cathode materials, such as lithium carbonate, lithium titanate, cobalt oxides, silicon, graphite, germanium, and tin, have been widely investigated in an effort to enhance the energy density storage properties of lithium-ion batteries (LIBs). However, the structural manipulation of anode materials to improve the battery performance remains a challenging issue. In LIBs, optimization of the anode material is a key technology affecting not only the power density but also the lifetime of the device. Here, we introduce a novel method by which to obtain nanostructures for LIB anode application on various surfaces via nanotransfer printing (nTP) process. We used a spark plasma sintering (SPS) process to fabricate a sputter target made of Li2CO3, which is used as an anode material for LIBs. Using the nTP process, various Li2CO3 nanoscale patterns, such as line, wave, and dot patterns on a SiO2/Si substrate, were successfully obtained. Furthermore, we show highly ordered Li2CO3 nanostructures on a variety of substrates, such as Al, Al2O3, flexible PET, and 2-Hydroxylethyl Methacrylate (HEMA) contact lens substrates. It is expected that the approach demonstrated here can provide new pathway to generate many other designable structures of various LIB anode materials.

Topographically designed hybrid nanostructures via nanotransfer printing and block copolymer self-assembly.

Nanotransfer printing (nTP) has attracted much attention due to its high pattern resolution, simple process, and low processing cost for useful nanofabrication. Here, we introduce a thermally assisted nTP (T-nTP) process for the effective fabrication of various periodic three-dimensional (3D) nanosheets, such as concavo-convex lines, spine lines, square domes, and complex multi-line patterns. The T-nTP method allows continuous nanoscale 3D patterns with functionality to be transferred onto both rigid and flexible substrates by heat without any collapse of uniform convex nanostructures with nanochannels. We also show the pattern formation of multi-layered hybrid structures consisting of two or more materials by T-nTP. Furthermore, the formation of silicon oxide nanodots (0D) within a printed metallic nanowave structure (3D) can be achieved by the combined method of T-nTP and the self-assembly of poly(styrene-b-dimethylsiloxane) (PS-b-PDMS) block copolymers. Moreover, we demonstrate how to obtain well-defined oxide-metal hybrid nanostructures (0D-in-3D) through the spontaneous accommodation of PDMS spheres in the confined spaces of an Au-wave nanotemplate. This approach is applicable during the nanofabrication of various high-resolution devices with complex geometrical nanopatterns.

Extreme-Pressure Imprint Lithography for Heat and Ultraviolet-Free Direct Patterning of Rigid Nanoscale Features.

Nanoimprint lithography (NIL) is typically performed by filling up of molds by heated polymers or UV-curable liquid resists, inevitably requiring subsequent pattern-transfer processes. Although direct NIL techniques have been suggested alternatively, they usually require precursors or ink-type resists containing undesired organic components. Here, we demonstrate extreme-pressure imprint lithography (EPIL) that effectively produces well-defined multiscale structures with a wide range from 10 nm to 10 mm on diverse surfaces even including pure or alloy metals without using any precursors, heating, UV exposure, or pattern transfer. In particular, EPIL is accomplished through precise control of room-temperature plastic deformation in nanoscale volumes, which is elucidated by finite element analyses and molecular dynamics simulations. In addition to scalability to macroscopic areas, we confirm the outstanding versatility of EPIL via its successful applications to Ni, Cu, steel, and organics. We expect that the state-of-the-art EPIL process combined with other emerging nanopatterning technologies will be extendable to the future large-area nanofabrication of various devices.

Folic Acid Functionalized Carbon Dot/Polypyrrole Nanoparticles for Specific Bioimaging and Photothermal Therapy.

Polypyrrole nanoparticles (PPy-NPs) with excellent near-infrared absorption are commonly used as photothermal therapy (PTT) agents; however, PTT using PPy-NPs has a limitation in that it is difficult to maximize their therapeutic effect because of the lack of specific targeting. In this study, to overcome the difficulty of targeting, folic acid functionalized carbon dots (FA-CDs) with bright green fluorescence properties were combined with carboxylated PPy-NPs via the EDC/NHS coupling reaction to yield a PTT imaging agent. The synthesized FA-CD/PPy-NPs with excellent photostability performed folate receptor (FR) positive HeLa cancer cell imaging by green fluorescence signals of FA-CDs and exhibited high cell viability (above 90%) even at 500 µg/mL. The viability of HeLa cells incubated with 200 µg/mL FA-CD/PPy-NPs was dramatically decreased to 25.02 ± 1.85% by NIR laser irradiation, through photothermal therapeutic effects of FA-CD/PPy-NPs with high photothermal conversion efficiency (η = 40.80 ± 1.54%). The cancer cell death by FA-CD/PPy-NPs was confirmed by fluorescence imaging of FA-CDs as well as live/dead cell staining assay (calcein-AM/PI). These results demonstrate that the FA-CD/PPy-NPs can be utilized as multifunctional theranostic agents for specific bioimaging and treatment of FR-positive cancer cells.

Lithography-Free Route to Hierarchical Structuring of High-χ Block Copolymers on a Gradient Patterned Surface.

A chemically defined patterned surface was created via a combined process of controlled evaporative self-assembly of concentric polymer stripes and the selective surface modification of polymer brush. The former process involved physical adsorption of poly (methyl methacrylate) (PMMA) segments into silicon oxide surface, thus forming ultrathin PMMA stripes, whereas the latter process was based on the brush treatment of silicon native oxide surface using a hydroxyl-terminated polystyrene (PS-OH). The resulting alternating PMMA- and PS-rich stripes provided energetically favorable regions for self-assembly of high χ polystyrene-block-polydimethylsiloxane (PS-b-PDMS) in a simple and facile manner, dispensing the need for conventional lithography techniques. Subsequently, deep reactive ion etching and oxygen plasma treatment enabled the transition of the PDMS blocks into oxidized groove-shaped nanostructures.

Thermally assisted nanotransfer printing with sub-20-nm resolution and 8-inch wafer scalability.

Nanotransfer printing (nTP) has attracted considerable attention due to its good pattern resolution, process simplicity, and cost-effectiveness. However, the development of a large-area nTP process has been hampered by critical reliability issues related to the uniform replication and regular transfer printing of functional nanomaterials. Here, we present a very practical thermally assisted nanotransfer printing (T-nTP) process that can easily produce well-ordered nanostructures on an 8-inch wafer via the use of a heat-rolling press system that provides both uniform pressure and heat. We also demonstrate various complex pattern geometries, such as wave, square, nut, zigzag, and elliptical nanostructures, on diverse substrates via T-nTP. Furthermore, we demonstrate how to obtain a high-density crossbar metal-insulator-metal memristive array using a combined method of T-nTP and directed self-assembly. We expect that the state-of-the-art T-nTP process presented here combined with other emerging patterning techniques will be especially useful for the large-area nanofabrication of various devices.