Patterning at the Resolution Limit of Commercial Electron Beam Lithography.

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.

Angstrom-Scale Transparent Overcoats: Interfacial Nitrogen-Driven Atomic Intermingling Promotes Lubricity and Surface Protection of Ultrathin Carbon.

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 Å) 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 Å carbon is further improved via interfacial nitrogen.

Thermoelectric Properties of Substoichiometric Electron Beam Patterned Bismuth Sulfide.

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.

Room-Temperature Patterning of Nanoscale MoS2 under an Electron Beam.

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 shape and size of MoS2 crystals that can be patterned precisely at required positions. Here, we describe a method of direct nanoscale patterning of MoS2 at room temperature by exposing a molybdenum thiocubane single-source precursor to a beam of electrons. Molybdenum thiocubanes with various alkylxanthate moieties [Mo4S4(ROCS2)6, where R = alkyl] were prepared using a "self-assembly" approach. Micro-Raman and micro-FTIR spectroscopic studies suggest that exposure to a relatively smaller dose of electrons results in the breakdown of xanthate moieties, leading to the formation of MoS2. High-resolution transmission electron micrographs suggest that the growth of MoS2 most likely happens along (100) planes. An electron-beam-induced chemical transformation of a molybdenum thiocubane resist was exploited to fabricate sub-10 nm MoS2 lines and dense dots as small as 13 nm with a pitch of 33 nm. Since this technique does not require the liftoff and etching steps, patterning of MoS2 with interesting shapes, sizes, and thicknesses potentially leading to tunable band gap is possible.

Slippery and Wear-Resistant Surfaces Enabled by Interface Engineered Graphene.

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 (<4 nm in total thickness) shows the highest macroscale wear durability on tape-head (topologically variant surface) that exceeds most previous thicker (∼7-100 nm) overcoats. Detailed nanoscale characterization and atomistic simulations explain the origin of the reduced friction and wear arising from these nanoscale coatings. Overall, this study demonstrates that engineered graphene-based coatings can outperform conventional coatings in a number of technologies.

New gold standard: weakly capped infant Au nanoclusters with record high catalytic activity for 4-nitrophenol reduction and hydrogen generation from an ammonia borane-sodium borohydride mixture.

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.

Ultrasmall Designed Plasmon Resonators by Fused Colloidal Nanopatterning.

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.

Boosting contact sliding and wear protection via atomic intermixing and tailoring of nanoscale interfaces.

Friction and wear cause energy wastage and system failure. Usually, thicker overcoats serve to combat such tribological concerns, but in many contact sliding systems, their large thickness hinders active components of the systems, degrades functionality, and constitutes a major barrier for technological developments. While sub-10-nm overcoats are of key interest, traditional overcoats suffer from rapid wear and degradation at this thickness regime. Using an enhanced atomic intermixing approach, we develop a ~7- to 8-nm-thick carbon/silicon nitride (C/SiN x ) multilayer overcoat demonstrating extremely high wear resistance and low friction at all tribological length scales, yielding ~2 to 10 times better macroscale wear durability than previously reported thicker (~20 to 100 nm) overcoats on tape drive heads. We report the discovery of many fundamental parameters that govern contact sliding and reveal how tuning atomic intermixing at interfaces and varying carbon and SiN x thicknesses strongly affect friction and wear, which are crucial for advancing numerous technologies.

Oxygen insensitive thiol-ene photo-click chemistry for direct imprint lithography of oxides.

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.

Direct Patterning of Zinc Sulfide on a Sub-10 Nanometer Scale via Electron Beam Lithography.

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.

Second order directed positioning of nanoparticles induced by the main terminal meniscus shape in irregular template cavities.

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.

A hybrid dielectrophoretic system for trapping of microorganisms from water.

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.

Fabrication of Self-Cleaning, Reusable Titania Templates for Nanometer and Micrometer Scale Protein Patterning.

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.

Multiscale ommatidial arrays with broadband and omnidirectional antireflection and antifogging properties by sacrificial layer mediated nanoimprinting.

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.

Large area, facile oxide nanofabrication via step-and-flash imprint lithography of metal-organic hybrid resins.

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 sensor with fast response and low leakage detection by nanoimprint lithography.

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.

Metal hierarchical patterning by direct nanoimprint lithography.

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.

Effect of angstrom-scale surface roughness on the self-assembly of polystyrene-polydimethylsiloxane block copolymer.

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.

Direct patterning of TiO2 using step-and-flash imprint lithography.

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.

Uniaxial alignment of liquid-crystalline conjugated polymers by nanoconfinement.

We demonstrate the uniaxial alignment of a liquid-crystalline conjugated polymer, poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) by means of nanoconfinement during nanoimprinting. The orientation of the conjugated backbones was parallel to the nanolines imprinted into the polymer film. Polarized UV-vis absorption and photoluminescence spectra were measured to quantify the degree of alignment, showing that the polarization ratio and uniaxial molecular order parameter were as high as 66 and 0.97, respectively. The aligned F8BT film was used as the active layer in a PLED, which resulted in polarized electroluminescence with a polarization ratio of 11. Ambipolar PFET in a top-gate configuration with aligned F8BT as the active semiconducting layer showed mobility enhancement when the chains were aligned parallel to the transport direction. Mobility anisotropies for hole and electron transport were 10-15 and 5-7, respectively, for current flow parallel and perpendicular to the alignment direction.