The Patterning of Sub-500 nm Inorganic Oxide Structures

Low-cost, durable master molds for thermal-NIL, UV-NIL, and injection molding

Mold cost and mold lifetime are essential concerns for mass production of micro/nano-patterned surfaces by nanoimprint lithography or micro/nanoinjection molding. Master molds are typically produced by subtractive processing using wafer-based clean room techniques. For imprint lithography, polymer copies of such molds can often be employed, but the durability of such molds is quite limited. The conditions of high temperature and pressure for injection molding require use of the durable masters created in stainless steel, nickel or other robust materials, but such approaches are challenged by the high cost of patterning these substrates and limited lifetime. Here, we report the fabrication of durable crystalline zirconium dioxide (ZrO₂) masters via a simple direct imprint technique. ZrO₂ nanoparticles (NPs) were formulated into an ink and imprinted on a variety of substrates using a solvent-assisted patterning technique and subsequently annealed to increase the mechanical durability of the mold. The hardness and modulus values of the ZrO₂ coatings reached 11 ± 2 GPa and 120 ± 10 GPa, respectively after annealing. The hard ZrO₂ mold was then employed for precision patterning of polymer surfaces by thermal and UV nanoimprinting lithography (NIL) techniques, and by injection molding. High fidelity pattern transfer continued throughout 115 000 injection molding cycles, there was no evidence of delamination, breakage or wear in the ZrO₂ mold. Our simple imprint patterning technique using ZrO₂ NPs inks enable us to fabricate robust molds with excellent thermal and mechanical properties as easily as imprinting simple polymer replicas. This simple and low-cost approach to mold preparation can enable a large variety of high throughput or large area nano-replication technologies.

Solution-Based Micro- and Nanoscale Metal Oxide Structures Formed by Direct Patterning for Electro-Optical Applications

Due to their transparency and tunable electrical, optical, and magnetic properties, metal oxide thin films and structures have many applications in electro-optical devices. In recent years, solution processing combined with direct-patterning techniques such as micro-/nanomolding, inkjet printing, e-jet printing, e-beam writing, and photopatterning has drawn much attention because of the inexpensive and simple fabrication process that avoids using capital-intensive vacuum deposition systems and chemical etching. Furthermore, practical applications of solution direct-patterning techniques with metal oxide structures are demonstrated in thin-film transistors and biochemical sensors on a wide range of substrates. Since direct-patterning techniques enable low-cost fabrication of nanoscale metal oxide structures, these methods are expected to accelerate the development of nanoscale devices and systems based on metal oxide components in important application fields such as flexible electronics, the Internet of Things (IoT), and human health monitoring. Here, a review of the fabrication procedures, advantages, limitations, and applications of the main direct-patterning methods for making metal oxide structures is presented. The goal is to highlight the examples with the most promising perspective from the recent literature.

Wetting Regimes for Residual-Layer-Free Transfer Molding at Micro- and Nanoscales

Transfer molding offers a low-cost approach to large-area fabrication of isolated structures in a variety of materials when recessed features of the open-faced mold are filled without leaving a residual layer on the plateaus of the mold. Considering both macroscale dewetting and microscale capillary flow, a proposed map of wetting regimes for blade meniscus coating provides a guide for achieving discontinuous dewetting at maximum throughput. Dependence of meniscus morphology on the azimuthal orientation of the stamp provides insight into the dominant mechanisms for discontinuous dewetting of one-dimensional (1-D) patterns. Critical meniscus velocity is measured and residual-layer-free filling is demonstrated for 1-D patterned soft molds (stamps) with periods ranging from 140 nm to 6 μm. Transfer of isolated lines, and multilayer woodpile structures were achieved through plasma bonding. These results are relevant to other roll-to-roll compatible processes for scalable production of high-resolution structures across large areas.

A review of metal organic resins for environmental applications

In recent years, research on metal organic frameworks (MOFs) has been extended to explore various issues regarding structural flexibility, toxicity, aqueous synthesis, biodegradability, regeneration, reuse, and easy disposal. Based on such efforts, highly-ordered porous MOF structures bound to organic resins (metal organic resins or MORs) have emerged as a new generation of materials with excellent properties feasible for diverse applications. Here, we describe the excellent features of MORs and demonstrate their potential applicability in environmental as well as other relevant fields.

Nanoscale Pillar-Enhanced Tribological Surfaces as Antifouling Membranes

We present a nonconventional membrane surface modification approach that utilizes surface topography to manipulate the tribology of foulant accumulation on water desalination membranes via imprinting of submicron titanium dioxide (TiO₂) pillar patterns onto the molecularly structured, flat membrane surface. This versatile approach overcomes the constraint of the conventional approach relying on interfacial polymerization that inevitably leads to the formation of ill-defined surface topography. Compared to the nonpatterned membranes, the patterned membranes showed significantly improved fouling resistance for both organic protein and bacterial foulants. The use of hydrophilic TiO₂ as a pattern material increases the membrane hydrophilicity, imparting improved chemical antifouling resistance to the membrane. Fouling behavior was also interpreted in terms of the topographical effect depending on the relative size of foulants to the pattern dimension. In addition, computational fluid dynamics simulation suggests that the enhanced antifouling of the patterned membrane is attributed to the enhancement in overall and local shear stress at the fluid-TiO₂ pattern interface.

Athermal Azobenzene-Based Nanoimprint Lithography

A novel nanoimprint lithography technique based on the photofluidization effect of azobenzene materials is presented. The tunable process allows for imprinting under ambient conditions without crosslinking reactions, so that shrinkage of the resist is avoided. Patterning of surfaces in the regime from micrometers down to 100 nm is demonstrated.

Direct and accurate measurement of size dependent wetting behaviors for sessile water droplets

The size-dependent wettability of sessile water droplets is an important matter in wetting science. Although extensive studies have explored this problem, it has been difficult to obtain empirical data for microscale sessile droplets at a wide range of diameters because of the flaws resulting from evaporation and insufficient imaging resolution. Herein, we present the size-dependent quantitative change of wettability by directly visualizing the three phase interfaces of droplets using a cryogenic-focused ion beam milling and SEM-imaging technique. With the fundamental understanding of the formation pathway, evaporation, freezing, and contact angle hysteresis for sessile droplets, microdroplets with diameters spanning more than three orders of magnitude on various metal substrates were examined. Wetting nature can gradually change from hydrophobic at the hundreds-of-microns scale to super-hydrophobic at the sub-μm scale, and a nonlinear relationship between the cosine of the contact angle and contact line curvature in microscale water droplets was demonstrated. We also showed that the wettability could be further tuned in a size-dependent manner by introducing regular heterogeneities to the substrate.

Room temperature direct imprinting of porous glass prepared from phase-separated glass

This work describes a room-temperature imprinting of nanoporous glass prepared by selective chemical etching of phase-separated glass. A highly porous (58%) and highly transparent (>90%) porous glass layer can be formed on a transparent phase-separated glass substrate. It is shown that the lateral resolution of the imprinting is a few tens of nanometers. As the porosity increases, the imprint depth increases and reaches up to 90% of the height of the mold pattern. The porous glass has a wider transmittance window (300-2700 nm) and a higher thermal durability (~500 °C) than other materials used for imprinting. The technique has various potential applications such as diffraction optical elements, waveguides, biosensors, and microfluidic devices.

In situ experiments to reveal the role of surface feature sidewalls in the Cassie-Wenzel transition

Waterproof and self-cleaning surfaces continue to attract much attention as they can be instrumental in various different technologies. Such surfaces are typically rough, allowing liquids to contact only the outermost tops of their asperities, with air being entrapped underneath. The formed solid-liquid-air interface is metastable and, hence, can be forced into a completely wetted solid surface. A detailed understanding of the wetting barrier and the dynamics of this transition is critically important for the practical use of the related surfaces. Toward this aim, wetting transitions were studied in situ at a set of patterned perfluoropolyether dimethacrylate (PFPEdma) polymer surfaces exhibiting surface features with different types of sidewall profiles. PFPEdma is intrinsically hydrophobic and exhibits a refractive index very similar to water. Upon immersion of the patterned surfaces into water, incident light was differently scattered at the solid-liquid-air and solid-liquid interface, which allows for distinguishing between both wetting states by dark-field microscopy. The wetting transition observed with this methodology was found to be determined by the sidewall profiles of the patterned structures. Partial recovery of the wetting was demonstrated to be induced by abrupt and continuous pressure reductions. A theoretical model based on Laplace's law was developed and applied, allowing for the analytical calculation of the transition barrier and the potential to revert the wetting upon pressure reduction.

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.

Future of the particle replication in nonwetting templates (PRINT) technology

Particle replication in nonwetting templates (PRINT) is a continuous, roll-to-roll, high-resolution molding technology which allows the design and synthesis of precisely defined micro- and nanoparticles. This technology adapts the lithographic techniques from the microelectronics industry and marries these with the roll-to-roll processes from the photographic film industry to enable researchers to have unprecedented control over particle size, shape, chemical composition, cargo, modulus, and surface properties. In addition, PRINT is a GMP-compliant (GMP=good manufacturing practice) platform amenable for particle fabrication on a large scale. Herein, we describe some of our most recent work involving the PRINT technology for application in the biomedical and material sciences.

Lead sulfide nanocrystal quantum dot solar cells with trenched ZnO fabricated via nanoimprinting

The improvement of power conversion efficiency, especially current density (Jsc), for nanocrystal quantum dot based heterojunction solar cells was realized by employing a trenched ZnO film fabricated using nanoimprint techniques. For an optimization of ZnO patterns, various patterned ZnO films were investigated using electrical and optical analysis methods by varying the line width, interpattern distance, pattern height, and residual layer. Analyzing the features of patterned ZnO films allowed us to simultaneously optimize both the pronounced electrical effects as well as optical properties. Consequently, we achieved an enhancement in Jsc from 7.82 to 12.5 mA cm(-2) by adopting the patterned ZnO with optimized trenched shape.

Surface patterning of mesoporous niobium oxide films for solar energy conversion

An array of periodic surface features were patterned on mesoporous niobium oxide films by a soft-lithographic technique with the goal of constructing a photonic crystal (PC) structure on the back side of the oxide. The oxide films, fabricated by mixing sol-gel derived niobium oxide nanoparticles and hydroxypropyl cellulose, were employed as photoelectrodes in dye-sensitized solar cells (DSSCs), and their performance evaluated against their flat counterparts. The surface patterns were imprinted using a photocurable perfluoropolyether (PFPE) soft-replica of a silicon master with a two-dimensional array of cylindrical posts (200 nm (D) × 200 nm (H)) in hexagonal geometry. The PC on the niobium oxide surface caused large changes in optical measurements, particularly in the blue wavelengths. To evaluate the optical effect on solar energy conversion, the incident photon-to-current conversion efficiency (IPCE) was measured in the patterned devices and the control group. The IPCE of patterned niobium oxide anodes exhibited a relative enhancement in photocurrent generation over the wavelength range corresponding to the higher absorption in optical measurements.

A route for fabricating printable photonic devices with sub-10 nm resolution

A novel and robust route for high-throughput, high-performance nanophotonics-based direct imprint of high refractive index and low visible wavelength absorption materials is presented. Sub-10 nm TiO2 nanostructures are fabricated by low-pressure UV-imprinting of an organic-inorganic resist material. Post-imprint thermal annealing allows optical property tuning over a wide range of values. For instance, a refractive index higher than 2.0 and an extinction coefficient close to zero can be achieved in the visible wavelength range. Furthermore, the imprint resist material permits fabrication of crack-free nanopatterned films over large areas and is compatible for fabricating printable photonic structures.

Nanotransfer molding of free-standing nanowire and porous nanomembranes suspended on microtrenches

Direct transfer printing of functional materials has been employed in the development of sensors, displays, and energy-harvesting devices. The transfer process can be applied advantageously to depositions onto nonplanar and flexible surfaces at low temperatures. In this work, we fabricated free-standing nanowire arrays and nanomembranes on micrometer-scale trenches by nanotransfer molding. We also investigated how deposition pattern types vary with trench dimensions as well as processing pressure and temperature. Finally, a free-standing polymer membrane fabricated by nanotransfer molding was employed as a novel mask in the preparation of three-dimensional nanodot arrays.

Nanomoulding of functional materials, a versatile complementary pattern replication method to nanoimprinting

We describe a nanomoulding technique which allows low-cost nanoscale patterning of functional materials, materials stacks and full devices. Nanomoulding combined with layer transfer enables the replication of arbitrary surface patterns from a master structure onto the functional material. Nanomoulding can be performed on any nanoimprinting setup and can be applied to a wide range of materials and deposition processes. In particular we demonstrate the fabrication of patterned transparent zinc oxide electrodes for light trapping applications in solar cells.

Direct nanoimprint lithography of Al₂O₃ using a chelated monomer-based precursor

Nanostructuring of Al₂O₃ is predominantly achieved by the anodization of aluminum film and is limited to obtaining porous anodized aluminum oxide (AAO). One of the main restrictions in developing approaches for direct fabrication of various types of Al₂O₃ patterns, such as lines, pillars, holes, etc, is the lack of a processable aluminum-containing resist. In this paper, we demonstrate a stable precursor prepared by reacting aluminum tri-sec-butoxide with 2-(methacryloyloxy)ethyl acetoacetate, a chelating monomer, which can be used for large area direct nanoimprint lithography of Al₂O₃. Chelation in the precursor makes it stable against hydrolysis whilst the presence of a reactive methacrylate group renders it polymerizable. The precursor was mixed with a cross-linker and their in situ thermal free-radical co-polymerization during nanoimprinting rigidly shaped the patterns, trapped the metal atoms, reduced the surface energy and strengthened the structures, thereby giving a ~100% yield after demolding. The imprinted structures were heat-treated, leading to the loss of organics and their subsequent shrinkage. Amorphous Al₂O₃ patterns with line-widths as small as 17 nm were obtained. Our process utilizes the advantages of sol-gel and methacrylate routes for imprinting and at the same time alleviates the disadvantages associated with both these methods. With these benefits, the chelating monomer route may be the harbinger of the universal scheme for direct nanoimprinting of metal oxides.

Nanotransfer printing with sub-10 nm resolution realized using directed self-assembly

An extraordinarily facile sub-10 nm fabrication method using the synergic combination of nanotransfer printing and the directed self-assembly of block copolymers is introduced. The approach is realized by achieving the uniform self-assembly of polydimethylsiloxane (PDMS)-containing block copolymers on a PDMS mold through the stabilization of the block copolymer thin films. This simple printing method can be applied on oxides, metals, polymers, and non-planar substrates without pretreatments. The fabrication of well-aligned metallic and polymeric functional nanostructures and crossed wire structures is also presented.

Graphoepitaxy of block-copolymer self-assembly integrated with single-step ZnO nanoimprinting

A highly efficient, ultralarge-area nanolithography that integrates block-copolymer lithography with single-step ZnO nanoimprinting is introduced. The UV-assisted imprinting of a photosensitive sol-gel precursor creates large-area ZnO topographic patterns with various pattern shapes in a single-step process. This straightforward approach provides a smooth line edge and high thermal stability of the imprinted ZnO pattern; these properties are greatly advantageous for further graphoepitaxial block-copolymer assembly. According to the ZnO pattern shape and depth, the orientation and lateral ordering of self-assembled cylindrical nanodomains in block-copolymer thin films could be directed in a variety of ways. Significantly, the subtle tunability of ZnO trench depth enabled by nanoimprinting, generated complex hierarchical nanopatterns, where surface-parallel and surface-perpendicular nanocylinder arrays are alternately arranged. The stability of this complex morphology is confirmed by self-consistent field theory (SCFT) calculations. The highly ordered graphoepitaxial nanoscale assembly achieved on transparent semiconducting ZnO substrates offers enormous potential for photonics and optoelectronics.

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.

Thin films of two functional oxides patterned laterally by soft lithography

Thin films of two laterally patterned functional oxides of uniform thickness were obtained in a two-step soft-lithographic micromolding process. CoFe(2)O(4)/ZnO and CoFe(2)O(4)/BaTiO(3) dual-phase patterns were fabricated. The films showed good replication of the pattern that was defined in the first patterning step. X-ray diffraction showed that the films consisted of two distinct phases, and magnetic force microscopy showed that the compounds were laterally separated, the separation pattern being the same as that of the initial soft-lithographic process. The films exhibited slight height variations near the edges of the phases, which were introduced in the first deposition step and were not fully compensated in the second deposition step. The films are sufficiently smooth to allow fabrication of multilayer structures.

Mechanism of titania deposition into cylindrical poly(styrene-block-4 vinyl pyridine) block copolymer templates

A simple and effective way for TiO(2) to be deposited on silicon or indium tin oxide (ITO) substrates has been achieved by using a poly(styrene-block-4-vinyl pyridine) (PS-b-P4VP) block copolymer template. In particular, a mechanism for the formation of TiO(2) within the P4VP phase was developed. Within this model, the TiO(2) deposition occurs by swelling of the protonated P4VP segments followed by transport of Ti precursor, probably protonated Ti(OH)(4) given the low pH conditions used, into the swollen P4VP followed by condensation into TiO(2) during the heating/plasma etch processes. TiO(2) nanostructure morphology is affected by pH and deposition temperatures, because these parameters affect the degree of protonation of P4VP segments and diffusion of the titanium(IV) bis(ammonium lactato)dihydroxide (TALH) precursor into the film. A pH range of 2.1-2.5 for silicon substrates and pH = 2.1 for ITO substrates gave the narrower TiO(2) nanostructures distributions, and deposition at 70 °C gave TiO(2) nanostructures with more regular arrangements and smoother surface than those deposited at room temperature. The use of 1,4-diiodobutane as a P4VP cross-linking compound is demonstrated to be a critical parameter for maintaining good cylindrical surface morphology for both the block copolymer template and the TiO(2) nanostructures.

Photonic crystal patterning of luminescent sol-gel films for light extraction

Structured luminescent thin films are investigated in the context of improved light extraction of phosphors for solid-state-lighting applications. Thin films composed of a sol-gel titania matrix doped with europium chelates are studied as a model system. These films, patterned with a square photonic lattice by soft nanoimprint lithography, are characterized by angle-resolved fluorescence. Modeling of this simple technique is shown to fit well the experimental data, revealing in great detail the guided modes of the film and their extraction parameters. An eightfold extraction enhancement factor of the film emission is measured. To further improve the extraction efficiency, we investigate the role of an additional low-index mesoporous silica underlayer through its influence on the guided modes of different polarizations and their interactions with the photonic crystal. Results obtained on model systems open the way towards the optimization of light-emitting devices, using a strategy of dielectric microstructure engineering using the sol-gel process.

Transfer molding of nanoscale oxides using water-soluble templates

We report a facile method for creating nanoscopic oxide structures over large areas that is capable of producing high aspect ratio nanoscale structures with feature sizes below 50 nm. A variety of nanostructured oxides including TiO(2), SnO(2) and organosilicates are formed using sol-gel and nanoparticle precursors by way of molding with water-soluble polymeric templates generated from silicon masters. Sequential stacking techniques are developed that generate unique 3-dimensional nanostructures with combinatorially mixed geometries, scales, and materials. Applicable to a variety of substrates, this scalable method allows access to a broad range of new thin film morphologies for applications in devices, catalysts, and functional surface coatings.

Nanoimprint lithography for functional three-dimensional patterns

Nanoimprint lithography (NIL) is viewed as an alternative nanopatterning technique to traditional photolithography, allowing micrometer-scale and sub-hundred-nanometer resolution as well as three-dimensional structure fabrication. In this Research News article we highlight current activities towards the use of NIL in patterning active or functional materials, and the application of NIL in patterning materials that present both chemistry and structure/topography in the patterned structures, which provide scaffolds for subsequent manipulation. We discuss and give examples of the various materials and chemistries that have been used to create functional patterns and their implication in various fields as electronic and magnetic devices, optically relevant structures, biologically important surfaces, and 3D particles.

Direct imprinting of high resolution TiO(2) nanostructures

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.

High-resolution PFPE-based molding techniques for nanofabrication of high-pattern density, sub-20 nm features: a fundamental materials approach

Several perfluoropolyether (PFPE)-based elastomers for high-resolution replica molding applications are explored. The modulus of the elastomeric materials was increased through synthetic and additive approaches while maintaining relatively low surface tension values (<25 mN/m). Using large area (>4 in.(2)) master templates, we experimentally show the relationship between mold resolution and material properties such as modulus and surface tension for materials used in this study. A composite mold approach was used to form flexible molds out of stiff, high modulus materials that allow for replication of sub-20 nm post structures. Sub-100 nm line grating master templates, formed using e-beam lithography, were used to determine the experimental stability of the molding materials. It was observed that as the feature spacing decreased, high modulus PFPE tetramethacrylate (TMA) composite molds were able to effectively replicate the nanograting structures without cracking or tear-out defects that typically occur with high modulus elastomers.

Direct patterning of CdSe quantum dots into sub-100 nm structures

Ordered, two-dimensional cadmium selenide (CdSe) arrays have been fabricated on indium-doped tin oxide (ITO) electrodes using the pattern replication in nonwetting templates (PRINT) process. CdSe quantum dots (QDs) with an average diameter of 2.7 nm and a pyridine surface ligand were used for patterning. The PRINT technique utilizes a perfluoropolyether (PFPE) elastomeric mold that is tolerant of most organic solvents, thus allowing solutions of CdSe QDs in 4-picoline to be used for patterning without significant deformation of the mold. Nanometer-scale diffraction gratings have been successfully replicated with CdSe QDs.

Thin films of conductive ZnO patterned by micromolding resulting in nearly isolated features

Patterned and continuous thin films of conductive Al-doped zinc oxide (ZnO:Al) were prepared on different substrates from a polymeric precursor solution. Their electric conductivity and light transmittance (for visible and UV light) was measured at room temperature. By means of a simple device, conductive ZnO:Al films with high fidelity patterns with features of 2-20 microm width could be obtained by simply micromolding the liquid precursor film prior to heat treatment. The individual features were interconnected by a very thin residual ZnO layer.

Patterning lead zirconate titanate nanostructures at sub-200-nm resolution by soft confocal imprint lithography and nanotransfer molding

Patterned sol-gel-derived lead zirconate titanate (PZT) thin films with lateral resolutions down to 100 nm on silicon are reported. Both an imprint and a transfer-molding method were employed. The formed patterns after annealing were characterized with scanning electron microscopy, atomic force microscopy, and X-ray diffraction. Despite the small dimensions and flexibility of the poly(dimethylsiloxane) (PDMS) stamps used for patterning, the quality of replication was found to be good. The influence of the surface energies of the substrate, PDMS mold, and precursor solution on the quality of pattern replication is discussed. The colloidal structure of the PZT sol-gels from which the patterns were made was studied with small-angle X-ray scattering. The sols were found to be chemically homogeneous down to a length scale of approximately 2 nm and higher, which is sufficient for pattern replication on approximately 100 nm scale.

Photonic crystal geometry for organic solar cells

We report organic solar cells with a photonic crystal nanostructure embossed in the photoactive bulk heterojunction layer, a topography that exhibits a 3-fold enhancement of the absorption in specific regions of the solar spectrum in part through multiple excitation resonances. The photonic crystal geometry is fabricated using a materials-agnostic process called PRINT wherein highly ordered arrays of nanoscale features are readily made in a single processing step over wide areas (approximately 4 cm(2)) that is scalable. We show efficiency improvements of approximately 70% that result not only from greater absorption, but also from electrical enhancements. The methodology is generally applicable to organic solar cells and the experimental findings reported in our manuscript corroborate theoretical expectations.

DIRECT IMPRINTING OF SiO2 WAVEGUIDE STRUCTURES ON GaAs AND ITS APPLICATION IN InAs/GaAs QUANTUM DOT INTERMIXING

A novel approach for intermixing of InAs / GaAs quantum dot (QD) structure by direct imprinting of SiO 2 strips using silicafilm is presented. The silicafilm SiO 2 strips were imprinted on a InAs / GaAs QD structure by an Obducat nanoimprinter using a polyethylene terepthalate soft mold. Inductively coupled plasma etching of GaAs waveguide structures using the imprinted SiO 2 strips as hard mask has been demonstrated. By using the silicafilm SiO 2 as a capping layer, the effects of impurity-free vacancy disordering intermixing on optical characteristics of the InAs / GaAs QD structure have been examined. The measured photoluminescence spectra of the InAs / GaAs QD structure suggest that differential wavelength blueshift of up to 105 nm for region with and without the SiO 2 film can be achieved. A cost effective one step SiO 2 hard mask with graded thickness profile for quantum well or QD intermixing by imprinting technique is proposed for application in broadband lasers and superluminescent diodes.

The pursuit of a scalable nanofabrication platform for use in material and life science applications

In this Account, we describe the use of perfluoropolyether (PFPE)-based materials that are able to accurately mold and replicate micro- and nanosized features using traditional techniques such as embossing as well as new techniques that we developed to exploit the exceptional surface characteristics of fluorinated substrates. Because of the unique partial wetting and nonwetting characteristics of PFPEs, we were able to go beyond the usual molding and imprint lithography approaches and have created a technique called PRINT (Particle [or Pattern] Replication In Nonwetting Templates). PRINT is a distinctive "top-down" fabrication technique capable of generating isolated particles, arrays of particles, and arrays of patterned features for a plethora of applications in both nanomedicine and materials science. A particular strength of the PRINT technology is the high-resolution molding of well-defined particles with precise control over size, shape, deformability, and surface chemistry. The level of replication obtained showcases some of the unique characteristics of PFPE molding materials. In particular, these materials arise from very low surface energy precursors with positive spreading coefficients, can be photocured at ambient temperature, and are minimally adhesive, nonswelling, and conformable. These distinctive features enable the molding of materials with unique attributes and nanometer resolution that have unprecedented scientific and technological value. For example, in nanomedicine, the use of PFPE materials with the PRINT technique allows us to design particles in which we can tailor key therapeutic parameters such as bioavailability, biodistribution, target-specific cell penetration, and controlled cargo release. Similarly, in materials science, we can fabricate optical films and lens arrays, replicate complex, naturally occurring objects such as adenovirus particles, and create 2D patterned arrays of inorganic oxides.