Search

It has been long known that low molecular weight resists can achieve a very high resolution, theoretically close to the probe diameter of the electron beam lithography (EBL) system. Despite technological improvements in EBL systems, the advances in resists have lagged behind. Here we demonstrate that a low-molecular-mass single-source precursor resist (based on cadmium(II) ethylxanthate complexed with pyridine) is capable of a achieving resolution (4 nm) that closely matches the measured probe diameter (∼3.8 nm). Energetic electrons enable the top-down radiolysis of the resist, while they provide the energy to construct the functional material from the bottom-up─unit cell by unit cell. Since this occurs only within the volume of resist exposed to primary electrons, the minimum size of the patterned features is close to the beam diameter. We speculate that angstrom-scale patterning of functional materials is possible with single-source precursor resists using an aberration-corrected electron beam writer with a spot size of ∼1 Å.

Carbon-based materials exhibit distinct structures and dimensionality which allow modification of their electrical properties and enable them to be integrated in various commercial systems. One of the interesting characteristics of carbon-based materials is efficient electron field emission (FE), which makes them good candidates for displays, in electron microscopy, lithography, sensing, micro- and nanoelectronics, X-ray sources and medical applications. While nano carbon materials have been extensively studied for FE applications, their usefulness, electron emission concerns, and fundamental mechanisms for FE technologies are buried in the reported literature, and cross comparison of all nano carbon materials together is rarely explored. Here we present a comprehensive overview of fundamental and FE properties of all carbon-based materials including diamond, nanocrystalline diamond, graphite, carbon nanotubes, carbon nanofibers, carbon nanowires, carbon nanorods, graphene, and amorphous and nanostructured carbon. Some of these carbon materials, such as amorphous and nanostructured carbon, possess the added benefit of room temperature production over large areas on a variety of substrates. We have compiled an up to date summary which critically discusses the material factors, and the factors that control electron emission of these materials. We also propose unique ideas to further improve electron emission for the design of energy efficient carbon-based cold cathode materials for next generation large area electronic devices.

Molybdenum disulfide (MoS2) is traditionally grown at a high temperature and subsequently patterned to study its electronic properties or make devices. This method imposes severe limitations on the...

Increasing the surface area-to-volume ratio of materials through size reduction is a desired approach to access maximum possible surface sites in applications such as catalysis. However, increase in the surface energy with the decrease in dimension warrants strong ligands to stabilize nanosystems, which mask the accessibility of the active surface sites. Owing to this, the realization of the true potential of a catalyst's surface remains challenging. Here, we employed a rationally designed strategy to synthesize infant Au nanoclusters—that alleviates the requirement of any separate ligand removal step—to unleash their actual potential to register a record high maximum turn-over frequency (TOFmax) of 72 900 h−1 and 65 500 h−1 in the benchmark catalytic reduction of 4-nitrophenol and catalytic H2 generation from an ammonia borane–sodium borohydride mixture, respectively. Such a phenomenal catalytic activity has been realized via the synthesis and stabilization of Au nanoclusters using solid citric acid and a super-concentrated aqueous AuCl3 solution, a pathway entirely different from the conventional modifications of the Turkevich and Brust methods. The crux of the synthetic strategy lies in precise control of the water content and thereby ensuring that the final Au nanoclusters remain in the solid state. During the synthesis, citric acid not only acts as a reducing agent to yield ‘infant’ Au nanoclusters but also provides a barrier matrix to arrest their growth. In solution, its weak capping ability and rapid dissolution allows the reactants to easily access the active sites of Au nanoclusters, thus leading to faster catalysis. Our study reveals that the true potential of metal nanoclusters as catalysts is actually far higher than what has been reported in the literature.

Lubricity, a phenomenon which enables the ease of motion of objects, and wear resistance, which minimizes material damage or degradation, are important fundamental characteristics for sustainable technology developments. Ultrathin coatings that promote lubricity and wear resistance are of huge importance for a number of applications, including magnetic storage and micro-/nanoelectromechanical systems. Conventional ultrathin coatings have, however, reached their limit. Graphene-based materials that have shown promise to reduce friction and wear have many intrinsic limitations such as high temperature and substrate-specific growth. To address these concerns, a great deal of research is currently ongoing to optimize graphene-based materials. Here we discover that angstrom-thick carbon (8 A) significantly reduces interfacial friction and wear. This lubricant shows ultrahigh optical transparency and can be directly deposited on a wide range of surfaces at room temperature. Experiments combined with molecular dynamics simulations reveal that the lubricating efficacy of 8 A carbon is further improved via interfacial nitrogen.

Direct patterning of thermoelectric metal chalcogenides can be challenging and is normally constrained to certain geometries and sizes. Here we report the synthesis, characterization, and direct writing of sub-10 nm wide bismuth sulfide (Bi2S3) using a single-source, spin-coatable, and electron-beam-sensitive bismuth(III) ethylxanthate precursor. In order to increase the intrinsically low carrier concentration of pristine Bi2S3, we developed a self-doping methodology in which sulfur vacancies are manipulated by tuning the temperature during vacuum annealing, to produce an electron-rich thermoelectric material. We report a room-temperature electrical conductivity of 6 S m–1 and a Seebeck coefficient of −21.41 μV K–1 for a directly patterned, substoichiometric Bi2S3 thin film. We expect that our demonstration of directly writable thermoelectric films, with further optimization of structure and morphology, can be useful for on-chip applications.

Molybdenum disulfide (MoS

Friction and wear remain the primary cause of mechanical energy dissipation and system failure. Recent studies reveal graphene as a powerful solid lubricant to combat friction and wear. Most of these studies have focused on nanoscale tribology and have been limited to a few specific surfaces. Here, we uncover many unknown aspects of graphene's contact-sliding at micro- and macroscopic tribo-scales over a broader range of surfaces. We discover that graphene's performance reduces for surfaces with increasing roughness. To overcome this, we introduce a new type of graphene/silicon nitride (SiNx, 3 nm) bilayer overcoats that exhibit superior performance compared to native graphene sheets (mono and bilayer), that is, display the lowest microscale friction and wear on a range of tribologically poor flat surfaces. More importantly, two-layer graphene/SiNx bilayer lubricant (

Tailoring writability and obtaining better signal-to-noise ratio performance by tuning the magnetic and microstructural properties of FePt media is of great interest in the bid to achieve high areal densities for next generation hard disk drives (HDDs). Conventional ways to tune FePt media are primarily by either the insertion of additional layers, or by inclusion of segregants such as B, C, Ni, SiO2 etc. Here we describe an approach that involves modifying growth kinetics by N inclusion, via plasma treatment in FePt bilayers (consisting of a hard and a soft FePt layer). The soft FePt layer aids in obtaining easy writability, while nitrogen plasma treatment of the interface of the two FePt layers facilitates suitable tailoring of the microstructure. This approach contributed to lowering of grain dimension as well as reduction in domain sizes in the FePt bilayers, but with the unwanted side-effects of reduction in squareness and ordering. However, we further propose a method to alleviate these concerns by the deposition of the soft FePt layer in partial nitrogen atmosphere, that restored the squareness and ordering while retaining the smaller domains obtained by nitrogen plasma treatment. Thus, the proposed approach provides a potential direction towards meeting the mutually conflicting requirements of easy writability and better signal-to-noise ratio performance in FePt media. ASTAR (Agency for Sci., Tech. and Research, S’pore) Accepted version

This work presents a procedure for large-area patterning of designed plasmon resonators that are much smaller than possible with conventional lithography techniques. Fused Colloidal Nanopatterning combines directed self-assembly and controlled fusing of spherical colloidal nanoparticles. The two-step approach first patterns a surface covered with hydrogen silsesquioxane, an electron beam resist, forming traps into which the colloidal gold nanoparticles self-assemble. Second, the patterned nanoparticles are controllably fused to form plasmon resonators of any 2D designed shape. The heights and widths of the plasmon resonators are determined by the diameter of the nanoparticle building blocks, which can be well below 10 nm. By performing the fusing step with UV ozone and heat exposure, we demonstrate that the process is easily scalable to cover large areas on silicon wafers with designed gold nanostructures. The procedure neither requires adhesion layers nor a lift-off process, making it ideally suited for plasmonics, in comparison with regular electron beam lithography. We use monochromated electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy and boundary element method simulations to demonstrate that the designed plasmon resonators are directly tunable via the pattern design. We foresee future expansion of this approach for applications such as plasmon-enhanced photocatalysis and for large-scale patterning where chemical, optical, or confinement properties require sub-10 nm metal lines.

UV-nanoimprint lithography (UV-NIL) is a promising technique for direct fabrication of functional oxide nanostructures. Since it is mostly carried out in aerobic conditions, the free radical polymerization during imprinting is retarded due to the radical scavenging ability of oxygen. Therefore, it is highly desirable to have an oxygen-insensitive photo-curable resin that not only alleviates the requirement of inert conditions but also enables patterning without making substantial changes in the process. Here we demonstrate the formulation of metal-containing resins that employ oxygen-insensitive thiol–ene photo-click chemistry. Allyl acetoacetate (AAAc) has been used as a bifunctional monomer that, on one hand, chelates with the metal ion, and on the other hand, offers a reactive alkene group for polymerization. Pentaerythritol tetrakis(3-mercaptopropionate) (PETMP), a four-arm thiol derivative, is used as a crosslinker as well as an active component in the thiol–ene photo-click chemistry. The FT-IR analyses on the metal-free and metal-containing resin formulations revealed that the optimum ratio of alkene to thiol is 1 : 0.5 for an efficient photo-click chemistry. The thiol–ene photo-click chemistry has been successfully demonstrated for direct imprinting of oxides by employing TiO2 and Ta2O5 as candidate systems. The imprinted films of metal-containing resins were subjected to calcination to obtain the corresponding patterned metal oxides. This technique can potentially be expanded to other oxide systems as well.

Directed self-assembly of nanoparticles using topographical templates has demonstrated great capabilities of ordering particles at their maximum packing fraction resulting from template confinement effects and free energy minimization. However, to self-assemble nanostructures at a lower packing fraction with a precise control over particle's positioning is challenging due to the high entropy of such a system. Here, by fabricating templates of irregular cavities together with appropriate choice of solvent, we demonstrate the positioning of 8 nm Au nanoparticles within individual cavities at a low filling factor. In addition to the first-order of ordering dictated by the template topography, there is a second-order of ordering induced by the interplay between the evaporation of the residual solvent trapped within the cavities and their intrinsic geometries. The experimental results show that the cavities shaped as equilateral triangles exhibit a random positioning of the particles at the corners; in comparison, right-angled scalene (or irregular) triangles show a more controllable positioning of the particles within the corners of the smallest angle. Finally, this technique has been successfully used to fabricate arrays of dimers with a controllable center-to-center distances at sub-5 nm length scales.

Direct imprinting of metals is predominantly achieved by using polydimethylsiloxane (PDMS) molds to pattern metal nanoparticles and subsequently melting them to form continuous structures. Although such a combination can successfully imprint metals, the yield and reproducibility are usually low when sub-100 nm features over large areas are desired. In this work, we demonstrate a simple method involving the addition of a cross-linker ethylene glycol dimethacrylate (EDMA) to a palladium metal precursor, and its in situ free radical polymerization during imprinting, which not only dramatically increases the yield to ∼100% but also enables high reproducibility. Palladium mercaptide resist was formed by dissolving acetoxy(benzylthio)palladium, EDMA and azobis-(isobutyronitrile) in an organic solvent mixture. The resist underwent polymerization when imprinted using a silicon mold at 120 °C with pressures as low as 30 bar. Polymerization rigidly shapes the imprinted patterns, traps the metal atoms, reduces the surface energy and strengthens the structures, thereby giving ∼100% yield after demolding. Heat-treatment of the imprinted structures at 330 °C resulted in the loss of organics and their subsequent shrinkage without the loss of integrity or aspect ratio and converted them to palladium nanostructures as small as ∼35 nm wide, over areas >1 cm × 1 cm. With suitable precursors, our technique can potentially be extended to pattern noble metals such as platinum, gold and silver.

The Co-Fe-B/MgO system with perpendicular magnetic anisotropy finds extensive application in modern magnetic memories. Critical issues for the practical usage of magnetic tunnel junctions (MTJs) include the limited thermal stability of bit storage, and low switching efficiency. This study elucidates the deterministic influence of Co-Fe-B composition and the MgO interface on an MTJ's fundamental magnetic properties, which affect both thermal stability and switching current. Furthermore, the authors observe an anomalous trend in saturation magnetization, which is attributed to a change in magnetic anisotropy.

The photocatalytic self-cleaning characteristics of titania facilitate the fabrication of reuseable templates for protein nanopatterning. Titania nanostructures were fabricated over square centimeter areas by interferometric lithography (IL) and nanoimprint lithography (NIL). With the use of a Lloyd's mirror two-beam interferometer, self-assembled monolayers of alkylphosphonates adsorbed on the native oxide of a Ti film were patterned by photocatalytic nanolithography. In regions exposed to a maximum in the interferogram, the monolayer was removed by photocatalytic oxidation. In regions exposed to an intensity minimum, the monolayer remained intact. After exposure, the sample was etched in piranha solution to yield Ti nanostructures with widths as small as 30 nm. NIL was performed by using a silicon stamp to imprint a spin-cast film of titanium dioxide resin; after calcination and reactive ion etching, TiO2 nanopillars were formed. For both fabrication techniques, subsequent adsorption of an oligo(ethylene glycol) functionalized trichlorosilane yielded an entirely passive, protein-resistant surface. Near-UV exposure caused removal of this protein-resistant film from the titania regions by photocatalytic degradation, leaving the passivating silane film intact on the silicon dioxide regions. Proteins labeled with fluorescent dyes were adsorbed to the titanium dioxide regions, yielding nanopatterns with bright fluorescence. Subsequent near-UV irradiation of the samples removed the protein from the titanium dioxide nanostructures by photocatalytic degradation facilitating the adsorption of a different protein. The process was repeated multiple times. These simple methods appear to yield durable, reuseable samples that may be of value to laboratories that require nanostructured biological interfaces but do not have access to the infrastructure required for nanofabrication.

Moth's eye inspired multiscale ommatidial arrays offer multifunctional properties of great significance in optoelectronic devices. However, a major challenge remains in fabricating these arrays on large-area substrates using a simple and scalable technique. Here we present the fabrication of these multiscale ommatidial arrays over large areas by a distinct approach called sacrificial layer mediated nanoimprinting, which involves nanoimprinting aided by a sacrificial layer. The fabricated arrays exhibited excellent pattern uniformity over the entire patterned area. Optimum dimensions of the multiscale ommatidial arrays determined by the finite-difference time domain simulations served as the design parameters for replicating the arrays on glass. A broadband suppression of reflectance to a minimum of ∼1.4% and omnidirectional antireflection for highly oblique angles of incidence up to 70° were achieved. In addition, superhydrophobicity and superior antifogging characteristics enabled the retention of optical properties even in wet and humid conditions, suggesting reliable optical performance in practical outdoor conditions. We anticipate that these properties could potentially enhance the performance of optoelectronic devices and minimize the influence of in-service conditions. Additionally, as our technique is solely nanoimprinting-based, it may enable scalable and high-throughput fabrication of multiscale ommatidial arrays.

Nanostructures of metal sulfides are conventionally prepared via chemical techniques and patterned using self-assembly. This poses a considerable amount of challenge when arbitrary shapes and sizes of nanostructures are desired to be placed at precise locations. Here, we describe an alternative approach of nanoscale patterning of zinc sulfide (ZnS) directly using a spin-coatable and electron beam sensitive zinc butylxanthate resist without the lift-off or etching step. Time-resolved electron beam damage studies using micro-Raman and micro-FTIR spectroscopies suggest that exposure to a beam of electrons leads to quick disappearance of xanthate moieties most likely via the Chugaev elimination, and further increase of electron dose results in the appearance of ZnS, thereby making the exposed resist insoluble in organic solvents. Formation of ZnS nanocrystals was confirmed by high-resolution transmission electron microscopy and selected area electron diffraction. This property was exploited for the fabrication of ZnS lines as small as 6 nm and also enabled patterning of 10 nm dots with pitches as close as 22 nm. The ZnS patterns fabricated by this technique showed defect-induced photoluminescence related to sub-band-gap optical transitions. This method offers an easy way to generate an ensemble of functional ZnS nanostructures that can be arbitrarily patterned and placed in a precise way. Such an approach may enable programmable design of functional chalcogenide nanostructures.

Previous studies have shown that fundamental cell functions such as adhesion, proliferation, and morphology are regulated by the interaction of cells with basement membrane nano- and micron- scale surface topography. By taking the basement membrane as a guiding principle, the surface can be designed with biophysical cues in the form of surface topographies to modulate the cellular functions of vascular endothelial and smooth muscle cells, which are crucial for vascular diseases. Titanium-based materials whose surfaces are covered by a thin layer of titanium dioxide (TiO2) are utilized in several regenerative medicine applications such as vascular prosthetics. The utilization of TiO2-covered materials makes it essential to understand the interaction of cells with the TiO2 layer to control the cell response. While it has been shown by means of patterned micro- and nano-topography that it is important to regulate cell functions on non-metallic materials, it would be of interest in the field of regenerative medicine to study the cell response on patterned TiO2 surface layers. Previous studies have mostly focused on studying the cell response on random micro- and nano-roughened metallic and metal oxide surfaces as it is challenging to fabricate patterned TiO2 surface layers. Here, the biocompatibility of a method that is capable of the rational patterning of a continuous TiO2 surface layer of sub-100 nm resolution scale using thermally curable resin-based nanoimprinting was studied. The responses of human umbilical vein endothelial cell (HUVEC), such as proliferation, morphology and focal adhesions, and smooth muscle cell (SMC) proliferation and morphology to the nano-patterned TiO2 layer were investigated. Overall, the TiO2 layer with surface nano-gratings showed enhanced proliferation of HUVECs, while it significantly lowered the proliferation of SMCs as compared to the unpatterned control. The HUVECs and SMCs were shown by topography to be sensitive to the 70 nm gratings as evident by the regulation of proliferation and cell shape. A significantly lower focal adhesion density was found of HUVECs on TiO2 nano-gratings, while a significantly higher average focal adhesion size of HUVECs was seen on TiO2 nano-wells and nano-gratings, compared to the unpatterned controls.

Current induced spin orbit torques (SOTs) offer an efficient pathway to manipulate the magnetization of a ferromagnet for future magnetic memories and logic devices. Among the various non-magnets utilized, the topological insulators, such as Bi2Se3, have proven to be one of most efficient spin current generators for SOTs. So far, the preferred growth technique for such materials is the molecular beam epitaxy technique which is not compatible with the magnetic memory industry. In this study, we utilize a sputter deposited Bi2Se3 to demonstrate highly efficient SOT generation and subsequently magnetization switching.

Although step-and-flash imprint lithography, or S-FIL, has brought about tremendous advancement in wafer-scale fabrication of sub-100 nm features of photopolymerizable organic and organo-silicon-based resists, it has not been successful in direct patterning of inorganic materials such as oxides because of the difficulties associated with resist formulation and its dispensing. In this paper, we demonstrate the proof-of-concept S-FIL of titanium dioxide (TiO2) carried by an acrylate-based formulation containing an allyl-functionalized titanium complex. The prepolymer formulation contains 48 wt % metal precursor, but it exhibits low enough viscosity (∼5 mPa·s) to be dispensed by an automatic dispensing system, adheres and spreads well on the substrate, is insensitive to pattern density variations, and rapidly polymerizes when exposed to broadband UV radiation to give a yield close to 95%. Five fields, each measuring 1 cm × 1 cm, consisting of 100 nm gratings were successively imprinted. Heat-treatment of the...

The fabrication of very narrow metal lines by the lift-off technique, especially below sub-10 nm, is challenging due to thinner resist requirements in order to achieve the lithographic resolution. At such small length scales, when the grain size becomes comparable with the line-width, the built-in stress in the metal film can cause a break to occur at a grain boundary. Moreover, the line-width roughness (LWR) from the patterned resist can result in deposited metal lines with a very high LWR, leading to an adverse change in device characteristics. Here a new approach that is not based on the lift-off technique but rather on low temperature hydrogen reduction ofelectron-beam patterned metal naphthenates is demonstrated. This not only enables the fabrication of sub-10 nm metal lines of good integrity, but also of low LWR, below the limit of 3.2 nm discussed in the International Technology Roadmap for Semiconductors. Using this method, sub-10 nm nickel wires are obtained by reducing patterned nickel naphthenate lines in a hydrogen-rich atmosphere at 500°C for 1 h. The LWR (i.e., 3 σ LWR ) of these nickel nanolines was found to be 2.9 nm. The technique is general and is likely to be suitable for fabrication of nanostructures of most commonly used metals (and their alloys), such as iron, cobalt, nickel, copper, tungsten, molybdenum, and so on, from their respective metal―organic compounds.

L10 FePtC granular media are being studied as potential future magnetic recording media and are set to be used in conjunction with heat assisted magnetic recording (HAMR) to enable recording at write fields within the range of current day recording heads. Media structures based on a FePtC storage layer and a capping layer can alleviate the switching field distribution (SFD) requirements of HAMR and reduce the noise originating from the writing process. However, the current designs suffer from SFD issues due to high temperature writing. To overcome this problem, we study a CoPt/FePtC exchange coupled composite structure, where FePtC serves as the storage layer and CoPt (with higher Curie temperature, Tc) as the capping layer. CoPt remains ferromagnetic at near Tc of FePtC. Consequently, the counter exchange energy from CoPt would reduce the noise resulting from the adjacent grain interactions during the writing process. CoPt/FePtC bilayer samples with different thicknesses of CoPt were investigated. Our studies found that CoPt forms a continuous layer at a thickness of 6 nm and leads to considerable reduction in the saturation field and its distribution. ASTAR (Agency for Sci., Tech. and Research, S’pore) Published version

Conventional patterning of HfO2 as a gate dielectric is a multistep complicated process that involves deposition of oxide, photolithography, and hard mask etching. In order to simplify the process of HfO2 patterning, the authors have developed photo- and electron beam-sensitive spin-coatable HfO2 resists for direct writing. They were prepared by reacting hafnium tert-butoxide with benzoylacetone in n-butanol and were found to be highly stable in air. Fourier transform infrared studies suggest that exposure to radiation results in the gradual removal of organic material from the resist and the enrichment of the resist with inorganics. This makes the exposed resist insoluble in organic solvents such as ethanol, thereby providing high-resolution negative patterns as small as ∼10nm wide. A silicon-on-insulator field effect transistor has been fabricated using sol-gel-derived HfO2 resist as a high-k gate dielectric and characterized over a range of temperatures. At room temperature a relatively high gate leaka...

Nano-patterning on the order of sub-10 nm is integral to achieve high-density nano-scale devices for various data storage and data processing applications. However, the additional requirement of planarization and unwanted side-effects of physical or chemical etching have so far limited the patterning of sub-10 nm devices. In this work, we have demonstrated the creation of an array of ∼10 nm ferromagnetic islands through selective phase transformation of paramagnetic multilayers by low-energy proton irradiation. Paramagnetic Co3O4/Pd multilayers masked with patterned PMMA (polymethyl methacrylate) were reduced to ferromagnetic Co/Pd by proton irradiation. A clear contrast of the nano-islands was observed using magnetic force microscopy, establishing the formation of ferromagnetic nano-islands with perpendicular magnetic anisotropy. This process provides a way to circumvent the side-effects associated with both conventional nano-scale pattering and high-energy ion irradiation. Therefore, phase transformatio...

This article reviews the progress made in the sub-10 nm electron beam patterning of metal oxides over the last thirty years. The patterning of inorganic resists began with metal halides, they were soon taken over by metal oxides due to their excellent environmental stability. However, these inorganic materials, both halides and oxides, suffered from the requirement of very steep dose, thus rendering them useless for practical applications. This gave way to highly electron beam-sensitive stabilized metal alkoxides and metal naphthenates, with sensitively close to conventional electron beam resists such as poly(methylmethacrylate) (PMMA) and calixarene. Furthermore, they show excellent line edge roughness characteristics at sub-10 nm scale, which is currently unmatched by any other electron beam resist. Recent applications of these resists such as an etch mask and their suitability as gate oxides will be highlighted.

A silicon on insulator field effect transistor for cryogenic operation has been fabricated using a sol-gel derived TiO2 electron beam resist as a high-k gate dielectric and characterized over a range of temperatures. The TiO2 dielectric layer allows too large a gate leakage current for good device operation at room temperature, but the leakage current is strongly suppressed at cryogenic temperatures and good transistor characteristics were observed. The temperature dependence of the gate leakage current suggests that Frenkel-Poole and trap-assisted tunneling dominates the conduction in the dielectric layer. The drain current shows peaks at certain frequencies under continuous wave microwave irradiation, which may be caused by the resonance of electrons trapped in defects at the TiO2/SiO2 interface. These resonances offer the possibility to manipulate single electrons for nonclassical information processing.

High temperature nanoimprinting of viscous polymers which are glassy at room temperature is usually performed using brittle and expensive molds made of inorganic materials. As a replacement, soft molds made of plastics or elastomers have been used because of their low cost and ease of fabrication. However, the deformation of polymer molds under pressure remains a major issue which limits their resolution in high temperature nanoimprinting. Moreover, the replicated structures are often broken or lack definition due to sticking of the embossed polymer to the mold. We report a method for imprinting fine, densely packed nanostructures down to 12 nm into a wide range of technologically important polymers using a flexible and robust mold made from ethylene(tetrafluoroethylene) (ETFE). The high resolution achieved is due to the mold's mechanical stability and resistance to distortion at high pressures and high temperatures. The flexibility and low surface energy of ETFE provide a clean mold release without fracture or deformation of the embossed structures. Multiple imprinting and patterning on large areas is also made possible because of the good conformal contact and low-adhesion of the ETFE mold. Finally, this simple and inexpensive method allows reproduction of the stamps from one single master, thus providing an economical alternative to expensive and brittle inorganic materials.

To explore the full capabilities of proton beam writing (PBW) as a lithographic tool it is important to investigate potential new resist materials. In PBW the interactions of the protons with the resist are comparable to the electron interactions with the resist in electron beam writing. In both techniques the induced secondary electrons will modify the molecular structure of the resist, therefore electron beam resists are potential candidates for PBW. Here we discuss resist properties such as contrast and sensitivity of two new negative resists for PBW. The first resist is a spin-coatable TiO2 resist for which sub 10 nm resolution has been reported using electron beam writing. In PBW smooth side walls have been observed for this resist. Despite a relative low sensitivity of this resist for PBW (8000 nC/mm2) it has potential applications in the area of integrated optical components such as waveguides and gratings because of its high refractive index. WL-7154 is a UV-sensitive negative resist and shows high sensitivity for PBW (4 nC/mm2). This resist could function as a mold for Ni electroplating to fabricate Ni stamps for nanoimprint- and soft-lithography.

Two-photon polymerization of photosensitive materials irradiated by femtosecond laser pulses is now considered as an enabling technology for the fabrication of 3D structures, especially photonic crystals and photonic crystal templates. Depending on the topology and dielectric constant contrast of photonic crystals, their optical properties can be tailored in a desired manner. Since 1987, when the concept of 3D photonic crystal was introduced by Yablonovitch and John, photonic crystals have been a subject of intensive research. In spite of this, the fabrication of photonic crystals with a full 3D (or omni-directional) bandgap in the visible range is still a challenging task. To realize photonic crystals with a full photonic bandgap, 3D structuring of high refractive index materials is required. The high refractive index (ca. 2.4) and high transparency in the visible spectrum makes TiO2 a very promising photonic material. For many applications, it is desirable to have a simple technique for patterning this material in twoand even three dimensions. Conventionally, TiO2 is patterned by sputtering it onto a pre-patterned organic resist followed by lift-off. When thick films of TiO2 or complicated features are desired, the lift-off process does not work well. For 3D fabrication, the most attractive option so far has been to fabricate templates which were later infiltrated with a high refractive index material, followed by the removal of the original template structure. Another approach is to use inorganic–organic photosensitive materials (ormocers) for the fabrication of photonic crystals. In this case, there exists a possibility to skip the replication steps and to fabricate 3D inorganic structures directly. However, the inorganic content in these materials is not very high and the attempts to fabricate in three dimensions have resulted in porous structures. In order to eliminate the issue of making templates and their subsequent infiltration with a high refractive index material, a direct 3D fabrication of structures appears to be an attractive option. As2S3, a chalcogenide glass, was directly patterned in three dimensions by exploiting its photo-induced metastability. On the other hand, examples of direct patterning of oxides are nonexistent. In this paper, we will demonstrate a direct 3D patterning of TiO2 structures using femtosecond laser pulses. To enable such fabrication, a photosensitive sol– gel-based spin-coatable TiO2 resist was developed. The experimental setup used for the patterning is shown in Figure 1 and is similar to that used before. The TiO2 resist is transparent (refractive index = 1.68) to the femtosecond laser radiation of 780 nm wavelength, and therefore, allows focusing of laser pulses tightly into the material volume. In the focal area, where the intensity of femtosecond laser pulses is high enough to initiate multiphoton processes, bond-breaking in the TiO2 resist makes the irradiated regions insoluble in organic solvents such as acetone. This allows the fabrication of any computer-generated 3D structure by direct laser “recording” into the volume of the TiO2 resist. Because of the threshold behavior and nonlinear nature of this bond-breaking proC O M M U N IC A TI O N

L1 0 FePt displays a large magneto-crystalline anisotropy (K u ∼ 7 × 107 ergs/cc), thereby making it a potential candidate for advanced recording technologies such as heat assisted magnetic recording (HAMR) [1, 2]. There have been many studies to tailor the properties of L1 0 FePt for practical application in hard disk drives (HDDs). These include control of grain size by addition of impurities, use of underlayers for epitaxial growth of (001) FePt at reduced temperatures as well as tailoring out-of-plane coercivity [3-6].

Assessment of the microbial safety of water resources is among the most critical issues in global water safety. As the current detection methods have limitations such as high cost and long process time, new detection techniques have transpired among which microfluidics is the most attractive alternative. Here, we show a novel hybrid dielectrophoretic (DEP) system to separate and detect two common waterborne pathogens, Escherichia coli (E. coli), a bacterium, and Cryptosporidium parvum (C. parvum), a protozoan parasite, from water. The hybrid DEP system integrates a chemical surface coating with a microfluidic device containing inter-digitated microelectrodes to impart positive dielectrophoresis for enhanced trapping of the cells. Trimethoxy(3,3,3-trifluoropropyl) silane, (3-aminopropyl)triethoxysilane, and polydiallyl dimethyl ammonium chloride (p-DADMAC) were used as surface coatings. Static cell adhesion tests showed that among these coatings, the p-DADMAC-coated glass surface provided the most effective cell adhesion for both the pathogens. This was attributed to the positively charged p-DADMAC-coated surface interacting electrostatically with the negatively charged cells suspended in water leading to increased cell trapping efficiency. The trapping efficiency of E. coli and C. parvum increased from 29.0% and 61.3% in an uncoated DEP system to 51.9% and 82.2% in the hybrid DEP system, respectively. The hybrid system improved the cell trapping by encouraging the formation of cell pearl-chaining. The increment in trapping efficiency in the hybrid DEP system was achieved at an optimal frequency of 1 MHz and voltage of 2.5 Vpp for C. parvum and 2 Vpp for E. coli, the latter is lower than 2.5 Vpp and 7 Vpp, respectively, utilized for obtaining similar efficiency in an uncoated DEP system.

Tuning the magnetic properties of FePt media is of great interest in the bid to achieve high areal densities. In this work, we demonstrate that properties of FePt media can be tuned using a two-step temperature deposited FePt bilayer. In this bilayer scheme, the first layer of FePt was deposited at a high nominal temperature (600 °C), whilst the second layer was deposited at a lower temperature over the first layer. This resulted in tunable magnetic and improved topographical properties, and was due to the interplay of a number of temperature-dependent factors such as epitaxial growth, new grain nucleation, surface mobility, and difference in the thermal expansion coefficients. Transmission electron microscopy imaging was used to understand the growth dynamics of the second FePt layer in the two-step temperature deposited structures. Accepted version

Electron beam-sensitive spin-coatable Al2O3 resists were prepared by chemically modifying aluminium tri-sec-butoxide with various β-ketoesters (R-acetoacetates, R = methyl, ethyl, isopropyl, isobutyl, isoamyl, heptyl, benzyl and 2-[methacryloyloxy] ethyl) in isopropyl alcohol. The reaction product was a chelated complex. With increasing molecular weight of R, there was an increase in electron beam sensitivity of spin-coatable Al2O3 resists. This appears to be due to decreased stability of the chelated complex formed by higher molecular weight R-acetoacetates. Fourier transform infrared (FTIR) spectroscopy studies indicate that exposure to an electron beam results in the breakdown of chelate rings, making the exposed area insoluble during development. Electron beam nanolithography produced 8 nm wide lines. These are the smallest oxide lines written using a sol-gel-based resist.

We describe a direct write technique using an electron beam to pattern ZrO2 on a sub-10 nm scale that bypasses the conventional method of sputtering and lift-off. This technique utilizes spin-coatable ZrO2 resist prepared by chemically reacting zirconium n-butoxide with benzoyl acetone in ethanol. The patterned resist has a sensitivity and contrast of and 3, respectively. Baking the resist at 85 °C increases the sensitivity to . The electron-beam-exposed regions become insoluble in acetone, thus yielding negative patterns. This property was exploited to write high-resolution patterns as small as 9 nm wide. Such negative patterns can be written with a pitch as close as 30 nm.

Conventional methods for electron beam patterning of TiO2 are based on sputtering and lift-off. This poses significant problems in producing high aspect ratio and stoichiometric structures, especially in the sub-100 nm size range. We describe an alternative approach of preparing spin-coatable TiO2 resists by chemically reacting titanium n-butoxide with benzoylacetone in methyl alcohol. They have an electron beam sensitivity of ∼35 mC cm-2 and are >107 times more sensitive to an electron beam than sputtered TiOx and crystalline TiO2 films. Fourier transform infrared studies suggest that exposure to an electron beam results in the gradual removal of organic material from the resist. This makes the exposed resist insoluble in organic solvents such as acetone, thereby providing high-resolution negative patterns as small as 8 nm wide. Such negative patterns can be written with a pitch as close as 30 nm.

We demonstrate a high yield production scheme to fabricate sub-5 nm co-planar metal–insulator–metal junctions. This involves determining the relationship between the actual gap between the metallic junctions for a given designed gap, and the use of weak developers with ultrasonic agitation to process the exposed resist. This results in an improved process to achieve narrow inter-electrode gaps. The gaps were imaged using an AFM equipped with a carbon nanotube tip to achieve a high degree of accuracy in measurement. The smallest gap unambiguously measured was ~ 2 nm. Gaps with ≤ 5 nm spacing were produced with a very high yield of about 75% for a designed inter-electrode distance of 0 nm. The leakage resistance of the gaps was found to be of the order of 1012 Ω. The entire junction structure was designed to be co-planar to better than 1 nm over 1 μ m2.

Satisfying the mutually conflicting requirements of easy switchability and high thermal stability still remains a hindrance to achieving ultra-high areal densities in hard disk drives. Exchange coupled composite media used with proper exchange control layers (ECLs) presents a potential solution to circumvent this hindrance. In this work, we have studied the role of MgO and Ta ECLs of different thicknesses in reducing the switching field of FePt media. MgO ECL was found to be more effective than a Ta ECL. For a 2 nm MgO ECL, the switching field could be reduced by 41% and at the cost of only a limited loss in thermal stability. Furthermore, a very high switching efficiency of 1.9 was obtained using 2 nm MgO ECL. So, with a proper choice of ECL material and thickness, the switching field of FePt media can be substantially reduced while ensuring high thermal stability and a better signal-to-noise ratio, thus potentially paving the way for very high areal density media. NRF (Natl Research Foundation, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) Published version

Step-and-flash imprint lithography (S-FIL) is a wafer-scale, high-resolution nanoimprint technique capable of expansion of nanoscale patterns via serial patterning of imprint fields. While S-FIL patterning of organic resins is well known, patterning of metal-organic resins followed by calcination to form structured oxide films remains relatively unexplored. However, with calcination shrinkage, there is tremendous potential utility in easing accessibility of arbitrary nanostructures at 20 nm resolution and below. However, barriers to commercial adoption exist due to difficulties in formulating polymerizable oxide precursors with good dispensability, long shelf life, and resistance to auto-homopolymerization. Here we propose a solution to these issues in the form of a versatile resin formulation scheme that is applicable to a host of functional oxides (Al2O3, HfO2, TiO2, ZrO2, Ta2O5, and Nb2O5). This scheme utilizes a reaction of metal alkoxides with 2-(methacryloyloxy)ethyl acetoacetate (MAEAA), a polymerizable chelating agent. Formation of these inorganic coordination complexes enables remarkable resistance to auto-homopolymerization, greatly improving dispensability and shelf life, thus enabling full scale-up of this facile nanofabrication approach. Results include successively imprinted fields consisting of 100 nm linewidth gratings. Isothermal calcination of these structures resulted in corresponding shrinkage of 75-80% without loss of mechanical integrity or aspect ratio, resulting in 20 nm linewidth oxide nanostructures.

Flexible palladium-based H2 sensors have a great potential in advanced sensing applications, as they offer advantages such as light weight, space conservation, and mechanical durability. Despite these advantages, the paucity of such sensors is due to the fact that they are difficult to fabricate while maintaining excellent sensing performance. Here, we demonstrate, using direct nanoimprint lithography of palladium, the fabrication of a flexible, durable, and fast responsive H2 sensor that is capable of detecting H2 gas concentration as low as 50 ppm. High resolution and high throughput patterning of palladium gratings over a 2 cm × 1 cm area on a rigid substrate was achieved by heat-treating nanoimprinted palladium benzyl mercaptide at 250 °C for 1 h. The flexible and robust H2 sensing device was fabricated by subsequent transfer nanoimprinting of these gratings into a polycarbonate film at its glass transition temperature. This technique produces flexible H2 sensors with improved durability, sensitivity, and response time in comparison to palladium thin films. At ambient pressure and temperature, the device showed a fast response time of 18 s at a H2 concentration of 3500 ppm. At 50 ppm concentration, the response time was found to be 57 s. The flexibility of the sensor does not appear to compromise its performance.

Three-dimensional hierarchical patterning of metals is of paramount importance in diverse fields involving photonics, controlling surface wettability and wearable electronics. Conventionally, this type of structuring is tedious and usually involves layer-by-layer lithographic patterning. Here, we describe a simple process of direct nanoimprint lithography using palladium benzylthiolate, a versatile metal-organic ink, which not only leads to the formation of hierarchical patterns but also is amenable to layer-by-layer stacking of the metal over large areas. The key to achieving such multi-faceted patterning is hysteretic melting of ink, enabling its shaping. It undergoes transformation to metallic palladium under gentle thermal conditions without affecting the integrity of the hierarchical patterns on micro- as well as nanoscale. A metallic rice leaf structure showing anisotropic wetting behavior and woodpile-like structures were thus fabricated. Furthermore, this method is extendable for transferring imprinted structures to a flexible substrate to make them robust enough to sustain numerous bending cycles.

UV- and E-beam direct patterning processes using photosensitive precursor films are reviewed in this chapter. For the UV patterning process, discussed in the first part, reaction processes of precursors, such as metal alkoxides modified with β-diketones and metal complexes of carboxylic acids, with UV-irradiation are reported. The discussion of multi-component oxide film, as well as simple oxide film preparation, some applications of direct UV-patterning, and limitation of this process completes this section. The second part covers the progress made in the direct sub-10 nm electron beam patterning of metal oxides over the last 30 years, moving from physical deposition to chemical solution deposition of resists. Patterning of inorganic resists began with thermally evaporated metal halides. They were soon taken over by sputtered metal oxide films due to their excellent environmental stability. However, these inorganic materials, both halides and oxides, suffered from very steep electron dose requirement, thus rendering them useless for practical applications. This gave way to highly electron beam-sensitive chemical solution deposited stabilized metal alkoxides and metal naphthenates, with sensitively close to conventional electron beam resists such as poly(methylmethacrylate), PMMA, and calixarene. Furthermore, they show excellent line edge roughness characteristics at sub-10 nm scale, which is currently unmatched by any other electron beam resist. Recent applications of these resists such as etch mask and their suitability as gate oxides closes the chapter.

Self-assembly of block copolymers has been identified as a potential candidate for high density fabrication of nanostructures. However, the factors affecting its reliability and reproducibility as a patterning technique on various kinds of surfaces are not well-established. Studies pertaining to block copolymer self-assembly have been confined to ultra-flat substrates without taking into consideration the effect of surface roughness. Here, we show that a slight change in the angstrom-scale roughness arising from the surface of a material creates a profound effect on the self-assembly of polystyrene-polydimethylsiloxane block copolymer. Its self-assembly was found to be dependent on both the root mean square roughness (R(rms)) of the surface and the type of solvent annealing system used. It was observed that surface with R(rms)5.0 Å showed self-assembly. Above this value, the kinetic hindrance posed by the surface roughness on the block copolymer leads to its conforming to the surface without observable phase separation.

Although step-and-flash imprint lithography, or S-FIL, has brought about tremendous advancement in wafer-scale fabrication of sub-100 nm features of photopolymerizable organic and organo-silicon-based resists, it has not been successful in direct patterning of inorganic materials such as oxides because of the difficulties associated with resist formulation and its dispensing. In this paper, we demonstrate the proof-of-concept S-FIL of titanium dioxide (TiO(2)) carried by an acrylate-based formulation containing an allyl-functionalized titanium complex. The prepolymer formulation contains 48 wt % metal precursor, but it exhibits low enough viscosity (∼5 mPa·s) to be dispensed by an automatic dispensing system, adheres and spreads well on the substrate, is insensitive to pattern density variations, and rapidly polymerizes when exposed to broadband UV radiation to give a yield close to 95%. Five fields, each measuring 1 cm × 1 cm, consisting of 100 nm gratings were successively imprinted. Heat-treatment of the patterned structures at 450 °C resulted in the loss of organics and their subsequent shrinkage without the loss of integrity or aspect ratio and converted them to TiO(2) anatase nanostructures as small as 30 nm wide. With this approach, wafer-scale direct patterning of functional oxides on a sub-100 nm scale using S-FIL can become a distinct possibility.

We describe measurements of the stiffness and Young’s modulus, Y of single crystals of molybdenum-based compounds. Atomic force microscopy is used first to image, and then to perform stiffness measurements on crystals which are growing up out of a substrate. Y is extracted by comparing the measured stiffness with that calculated from a continuum elasticity model of the crystals, whose experimentally indeterminate parameters are the Young’s modulus and the geometry. We find a value for Y in the range 0.8–1.4 TPa, depending on the assumed geometry of the nanocrystal. As these crystals are essentially perfectly ordered on the micron scale, this opens the possibility of forming composite materials of immense strength.

We demonstrate a different approach to direct nanoimprint lithography of oxides, in particular TiO(2), using the metal methacrylate route which not only gives very high resolution ( approximately 20 nm) but also provides yields of approximately 100% over areas > 1 cm x 1 cm. TiO(2) was imprinted using a polymerizable liquid 'TiO(2) resin' consisting of a mixture of titanium methacrylate, ethylene glycol dimethacrylate, and azobis-(isobutyronitrile). The resin underwent free radical polymerization when imprinted using a silicon mold at 110 degrees C with pressures as low as 10 bar. Polymerization strengthens the imprinted structures, thereby giving approximately 100% yield after demolding. Heat-treatment of the imprinted structures at 400 degrees C resulted in the loss of organics and their subsequent shrinkage ( approximately 75%) without the loss of integrity or aspect ratio, and converted them to TiO(2) nanostructures as small as approximately 20 nm wide. Furthermore, our method demonstrates that large imprinted areas of sub-100-nm features can be achieved by sub-micron molds which translate into huge cost savings with the added flexibility of direct patterning of urinary as well as multi-component oxides.