Fabricating complex three-dimensional nanostructures with high-resolution conformable phase masks

Layered nano-gratings by electron beam writing to form 3-level diffractive optical elements for 3D phase-offset holographic lithography

A multi-level nanophotonic structure is a major goal in providing advanced optical functionalities as found in photonic crystals and metamaterials. A three-level nano-grating phase mask has been fabricated in an electron-beam resist (ma-N) to meet the requirement of holographic generation of a diamond-like 3D nanostructure in photoresist by a single exposure step. A 2D mask with 600 nm periodicity is presented for generating first order diffracted beams with a preferred π/2 phase shift on the X- and Y-axes and with sufficient 1(st) order diffraction efficiency of 3.5% at 800 nm wavelength for creating a 3D periodic nanostructure in SU-8 photoresist. The resulting 3D structure is anticipated to provide an 8% complete photonic band gap (PBG) upon silicon inversion. A thin SiO2 layer was used to isolate the grating layers and multiple spin-coating steps served to planarize the final resist layer. A reversible soft coating (aquaSAVE) was introduced to enable SEM inspection and verification of each insulating grating layer. This e-beam lithographic method is extensible to assembling multiple layers of a nanophotonic structure.

Three-dimensional micro/nanoscale architectures: fabrication and applications

Three-dimensional (3D) functional solids with programmable hierarchical micro/nanoarchitectures are critical for several fundamental applications, including structural composites, microfluidics, photonics, and tissue engineering. Due to the broad range of application possibilities, a large amount of effort has been devoted to the in-depth exploration of various top-down and bottom-up strategies to construct these complex multi-dimensional structures. In this review, we introduce and discuss selected examples of fabrication techniques which have successfully developed large area, novel 3D functional architectures with exquisite control over their morphology at the nano/subnanolevel. Emphasis is placed on the nanofabrication techniques, their salient features as well as advantages. A summary of the emerging application possibilities of such structures, especially in biomedicine, energy, and device construction, is also discussed.

Photonic-crystal waveguide structure by pattern-integrated interference lithography

The pattern-integrated interference lithography (PIIL) technique combines multi-beam interference lithography (MBIL) and imaging to produce functional periodic-lattice-based microstructures in a rapid single-exposure step. A photonic-crystal waveguide structure with sub-micron resolution is designed, fabricated by PIIL, and characterized. Scanning electron and atomic force microscope images are found to be in good qualitative agreement with three-dimensional simulations of the developed photoresist.

Holographic patterning of high-performance on-chip 3D lithium-ion microbatteries

As sensors, wireless communication devices, personal health monitoring systems, and autonomous microelectromechanical systems (MEMS) become distributed and smaller, there is an increasing demand for miniaturized integrated power sources. Although thin-film batteries are well-suited for on-chip integration, their energy and power per unit area are limited. Three-dimensional electrode designs have potential to offer much greater power and energy per unit area; however, efforts to date to realize 3D microbatteries have led to prototypes with solid electrodes (and therefore low power) or mesostructured electrodes not compatible with manufacturing or on-chip integration. Here, we demonstrate an on-chip compatible method to fabricate high energy density (6.5 μWh cm(-2)⋅μm(-1)) 3D mesostructured Li-ion microbatteries based on LiMnO2 cathodes, and NiSn anodes that possess supercapacitor-like power (3,600 μW cm(-2)⋅μm(-1) peak). The mesostructured electrodes are fabricated by combining 3D holographic lithography with conventional photolithography, enabling deterministic control of both the internal electrode mesostructure and the spatial distribution of the electrodes on the substrate. The resultant full cells exhibit impressive performances, for example a conventional light-emitting diode (LED) is driven with a 500-μA peak current (600-C discharge) from a 10-μm-thick microbattery with an area of 4 mm(2) for 200 cycles with only 12% capacity fade. A combined experimental and modeling study where the structural parameters of the battery are modulated illustrates the unique design flexibility enabled by 3D holographic lithography and provides guidance for optimization for a given application.

Fabrication of three-dimensional hierarchical nanostructures using template-directed colloidal assembly

Optical effects in template-directed colloidal assembly are explored to fabricate microscale patterns with integrated three-dimensional (3D) nanostructures. This method allows the patterning of periodic nanostructures in arbitrarily designed regions by controlling particle assembly and light illumination. Using both "bottom-up" and "top-down" methods, this approach enables low-cost fabrication of hierarchical devices.

A hydrogel pen for electrochemical reaction and its applications for 3D printing

A hydrogel pen consisting of a microscopic pyramid containing an electrolyte offers a localized electroactive area on the nanometer scale via controlled contact of the apex with a working electrode. The hydrogel pen merges the fine control of atomic force microscopy with non-linear diffusion of an ultramicroelectrode, producing a faradaic current that depends on the small electroactive area. The theoretical and experimental investigations of the mass transport behavior within the hydrogel reveal that the steady-state current from the faradaic reaction is linearly proportional to the deformed length of the hydrogel pen by contact, i.e. signal transduction of deformation to an electrochemical signal, which enables the fine control of the electroactive area in the nanometer-scale regime. Combined with electrodeposition, localized electrochemistry of the hydrogel pen results in the ability to fabricate small sizes (110 nm in diameter), tall heights (up to 30 μm), and arbitrary structures, thereby indicating an additive process in 3 dimensions by localized electrodeposition.

Three-dimensional nanofabrication by block copolymer self-assembly

Thin films of block copolymers are widely seen as enablers for nanoscale fabrication of semiconductor devices, membranes, and other structures, taking advantage of microphase separation to produce well-organized nanostructures with periods of a few nm and above. However, the inherently three-dimensional structure of block copolymer microdomains could enable them to make 3D devices and structures directly, which could lead to efficient fabrication of complex heterogeneous structures. This article reviews recent progress in developing 3D nanofabrication processes based on block copolymers.

Three-dimensional self-assembled photonic crystals with high temperature stability for thermal emission modification

Selective thermal emission in a useful range of energies from a material operating at high temperatures is required for effective solar thermophotovoltaic energy conversion. Three-dimensional metallic photonic crystals can exhibit spectral emissivity that is modified compared with the emissivity of unstructured metals, resulting in an emission spectrum useful for solar thermophotovoltaics. However, retention of the three-dimensional mesostructure at high temperatures remains a significant challenge. Here we utilize self-assembled templates to fabricate high-quality tungsten photonic crystals that demonstrate unprecedented thermal stability up to at least 1,400 °C and modified thermal emission at solar thermophotovoltaic operating temperatures. We also obtain comparable thermal and optical results using a photonic crystal comprising a previously unstudied material, hafnium diboride, suggesting that refractory metallic ceramic materials are viable candidates for photonic crystal-based solar thermophotovoltaic devices and should be more extensively studied.

Holographic fabrication of functionally graded photonic lattices through spatially specified phase patterns

In this paper, we present a method for the mathematically formulated phase engineering of interfering laser beams through a spatial light modulator for a holographic fabrication of graded photonic lattices. The desired phases can be programmed at specific locations by assigning gray levels in cellular structures. The method is demonstrated by embedding single-lattice structures or missing lattices in dual-lattice periodic photonic structures. The demonstrated method can be potentially combined with the coordinate transformation technique in transformation optics for the fabrication of graded photonic devices.

Arithmetic of focused vortex beams in three-dimensional optical lattice arrays

In this work, we present a method to generate a 3D lattice of vortex beams. We apply phase look-up tables (LUTs) designed to generate gratings having an arbitrary content of diffraction orders. This phase LUT can be applied to a variety of diffraction optical elements, such as linear phase gratings, blazed diffractive lenses, and spiral phase patterns. We concentrate on combinations of all of these to create 3D structures of vortex beams. In particular, we generate all of these elements in the first output quadrant and eliminate the zero-order diffraction that often unavoidably accompanies these patterns. We discuss different ways of producing these 3D vortex gratings, and how the various output beams are related to the arithmetic of the 3D distribution of topological charges. Experimental results are provided by means of a liquid crystal spatial light modulator.

3D hierarchical architectures prepared by single exposure through a highly durable colloidal phase mask

Three-dimensional hierarchical architectures are fabricated using a simple, cost-effective, durable colloidal phase mask containing a colloidal monolayer embedded in a flexible polydimethylsiloxane (PDMS) membrane. These structures give rise to a photonic bandgap that can be tuned over a wide spectral range from the visible to the near-infrared regions.

25th anniversary article: ordered polymer structures for the engineering of photons and phonons

The engineering of optical and acoustic material functionalities via construction of ordered local and global architectures on various length scales commensurate with and well below the characteristic length scales of photons and phonons in the material is an indispensable and powerful means to develop novel materials. In the current mature status of photonics, polymers hold a pivotal role in various application areas such as light-emission, sensing, energy, and displays, with exclusive advantages despite their relatively low dielectric constants. Moreover, in the nascent field of phononics, polymers are expected to be a superior material platform due to the ability for readily fabricated complex polymer structures possessing a wide range of mechanical behaviors, complete phononic bandgaps, and resonant architectures. In this review, polymer-centric photonic and phononic crystals and metamaterials are highlighted, and basic concepts, fabrication techniques, selected functional polymers, applications, and emerging ideas are introduced.

Phototropic growth control of nanoscale pattern formation in photoelectrodeposited Se-Te films

Photoresponsive materials that adapt their morphologies, growth directions, and growth rates dynamically in response to the local incident electromagnetic field would provide a remarkable route to the synthesis of complex 3D mesostructures via feedback between illumination and the structure that develops under optical excitation. We report the spontaneous development of ordered, nanoscale lamellar patterns in electrodeposited selenium-tellurium (Se-Te) alloy films grown under noncoherent, uniform illumination on unpatterned substrates in an isotropic electrolyte solution. These inorganic nanostructures exhibited phototropic growth in which lamellar stripes grew toward the incident light source, adopted an orientation parallel to the light polarization direction with a period controlled by the illumination wavelength, and showed an increased growth rate with increasing light intensity. Furthermore, the patterns responded dynamically to changes during growth in the polarization, wavelength, and angle of the incident light, enabling the template-free and pattern-free synthesis, on a variety of substrates, of woodpile, spiral, branched, or zigzag structures, along with dynamically directed growth toward a noncoherent, uniform intensity light source. Full-wave electromagnetic simulations in combination with Monte Carlo growth simulations were used to model light-matter interactions in the Se-Te films and produced a model for the morphological evolution of the lamellar structures under phototropic growth conditions. The experiments and simulations are consistent with a phototropic growth mechanism in which the optical near-field intensity profile selects and reinforces the dominant morphological mode in the emergent nanoscale patterns.

Monolithic 3D titania with ultrathin nanoshell structures for enhanced photocatalytic activity and recyclability

Titania has attracted considerable interest for use in water purification applications due to its excellent photocatalytic activity. To further improve the efficiency of photocatalysis, numerous nanostructures (i.e. nanoparticles, nanotubes, and nanowires) have been proposed to increase the surface area of titania. Despite the high photocatalytic performance of the nanostructured titania, subsequent difficulties encountered in recollection and reuse of titania inhibit the practical application for water purification systems. Here we successfully fabricate monolithic, three dimensional (3D) nanoshell titania with high uniformity over large areas (~1 × 1 inch(2)) through proximity field nanopatterning (PnP) and low-temperature atomic layer deposition (ALD) techniques. The higher surface area of 3D nanoshell titania increases the photocatalytic performance more than three-fold relative to that of a thin film of equivalent sample size. Also, the monolithic form of titania enables it to be reused without any degradation of photocatalytic activity. The newly developed nanomaterials in this study can serve as an efficient and reusable photocatalyst for water purification systems.

Nanoplasmonic surfaces enabling strong surface-normal electric field enhancement

Conventional two-dimensional (2D) plasmonic arrays provide electric field intensity enhancement in the plane, typically with a surface coverage around 50% in the plan-view. Here, we show nanoplasmonic three-dimensional (3D) surfaces with 100% surface coverage enabling strong surface-normal field enhancement. Experimental measurements are found to agree well with the full electromagnetic solution. Along with the surface-normal localization when using the plasmonic 3D-surface, observed maximum field enhancement is 7.2-fold stronger in the 3D-surface than that of the 2D counterpart structure. 3D-plasmonic nonplanar surfaces provide the ability to generate volumetric field enhancement, possibly useful for enhanced plasmonic coupling and interactions.

Three-dimensional nanolithography using light scattering from colloidal particles

The interaction between light and colloidal elements can result in a wealth of interesting near-field optical patterns. By examining the optical and colloidal properties, the intensity distribution can be tailored and harnessed for three-dimensional nanolithography. Here, we examine the use of light scattering from colloidal particles to fabricate complex hollow nanostructures. In this approach, a single colloidal sphere is illuminated to create a scattering pattern, which is captured by a photoresist in close proximity. No external optical elements are required, and the colloidal elements alone provide the modulation of the optical intensity pattern. The fabricated nanostructures can be designed to have multiple shells, confined volumes, and single top openings, resembling "nano-volcanoes." The geometry of such structures is dependent on the scattered light distribution and can be accurately modeled by examining the light-particle interaction. The hollow nanostructures can be used to trap nanomaterial, and we demonstrate their ability to trap 50 nm silica nanoparticles. These well-defined surface hollow structures can be further functionalized for applications in controlled drug delivery and biotrapping. Colloidal elements with different geometries and material compositions can also be incorporated to examine other light-colloid interactions.

Soft elastomeric nanopillar stamps for enhancing absorption in organic thin-film solar cells

An elastomeric poly(dimethylsiloxane) (PDMS) block engraved with periodically arrayed nanopillars serves as a transferable light-trapping stamp for encapsulated organic thin-film solar cells. Diffracted light rays from the stamp interfere with one another and self-focus onto the active layer of the solar cell, generating enhanced absorption, as indicated in the current density-voltage measurements.

The effect of high vacuum on the mechanical properties and bioactivity of collagen fibril matrices

The extracellular matrix (ECM) environment plays a critical role in organism development and disease. Surface sensitive microscopy techniques for studying the structural and chemical properties of ECMs are often performed in high vacuum (HV) environments. In this report, we examine the affect HV conditions have on the bioactivity and mechanical properties of type I collagen fibrillar matrices. We find that HV exposure has an unappreciable affect on the cell spreading response and mechanical properties of these collagen fibril matrices. Conversely, low vacuum environments cause fibrils to become mechanically rigid as indicated by force microscopy, resulting in greater cell spreading. Time-of-flight secondary ion mass spectrometry results show no noticeable spectral differences between HV-treated and dehydrated matrices. While previous reports have shown that HV can denature proteins in monolayers, these observations indicate that HV-exposure does not mechanically or biochemically alter collagen in its supramolecular configuration. These results may have implication for complex ECM matrices such as decellularized scaffolds.

Rapid self-assembly of brush block copolymers to photonic crystals

The reduced chain entanglement of brush polymers over their linear analogs drastically lowers the energetic barriers to reorganization. In this report, we demonstrate the rapid self-assembly of brush block copolymers to nanostructures with photonic bandgaps spanning the entire visible spectrum, from ultraviolet (UV) to near infrared (NIR). Linear relationships were observed between the peak wavelengths of reflection and polymer molecular weights. This work enables "bottom-up" fabrication of photonic crystals with application-tailored bandgaps, through synthetic control of the polymer molecular weight and the method of self-assembly. These polymers could be developed into NIR-reflective paints, to combat the "urban heat island effect" due to NIR photon thermalization.

Wavelength-scale lens microscopy via thermal reshaping of colloidal particles

Lenses are by far the most simple tools for visualization. Although they are intrinsically limited in resolution, recent efforts have aimed at focusing visible light in micro-scale lenses with subwavelength resolution, triggering an intense interest in further improving and understanding their performances. Herein, we report on a distinctive library of wavelength-scale solid immersion lenses facilitated the self-assembly of polystyrene colloidal particles. The thermally activated structural changes in polystyrene colloidal spheres directly impact the optical performance of the obtained lenses. Similar melting dynamics is observed for spheres of various size spheres at different temperatures. This allows precise control of the contact angle spanning a broad range from 180° to <20°. The fabricated lenses display deviations from the ray optics, allowing us to resolve features as small as 180 nm using a simple microscopy setup. We succeed in proper self-assembly of the colloidal lenses that enables large-area optical nanoscopy through simple and reliable experimental protocols. The limitations and the artifacts of the present technique are discussed.

Rapid fabrication of 3D terahertz split ring resonator arrays by novel single-shot direct write focused proximity field nanopatterning

For the next generation of phoXonic, plasmonic, opto-mechanical and microfluidic devices, the capability to create 3D microstructures is highly desirable. Fabrication of such structures by conventional top-down techniques generally requires multiple time-consuming steps and is limited in the ability to define features spanning multiple layers at prescribed angles. 3D direct write lithography (3DDW) has the capability to draw nearly arbitrary structures, but is an inherently slow serial writing process. Here we present a method, denoted focused proximity field nanopatterning (FPnP), that combines 3DDW with single or multiphoton interference lithography (IL). By exposing a thick photoresist layer having a phase mask pattern imprinted on its surface with a tightly focused laser beam, we produce locally unique complex structures. The morphology can be varied based on beam and mask parameters. Patterns may be written rapidly in a single shot mode with arbitrary positions defined by the direct write, thus exploiting the control of 3DDW with the enhanced speed of phase mask IL. Here we show the ability for this technique to rapidly produce arrays of "stand-up" far IR resonators.

Integrated three-dimensional filter separates nanoscale from microscale elements in a microfluidic chip

We report on the integration of a size-based three-dimensional filter, with micrometre-sized pores, in a commercial microfluidic chip. The filter is fabricated inside an already sealed microfluidic channel using the unique capabilities of two-photon polymerization. This direct-write technique enables integration of the filter by post-processing in a chip that has been fabricated by standard technologies. The filter is located at the intersection of two channels in order to control the amount of flow passing through the filter. Tests with a suspension of 3 μm polystyrene spheres in a Rhodamine 6G solution show that 100% of the spheres are stopped, while the fluorescent molecules are transmitted through the filter. We demonstrate operation up to a period of 25 minutes without any evidence of clogging. Preliminary validation of the device for plasma separation from whole blood is shown. Moreover, the filter can be cleaned and reused by reversing the flow.

Two-photon continuous flow lithography

A new approach for microfluidics-based production of polymeric particles, namely two-photon continuous flow lithography, is reported. This technique takes advantage of two-photon lithography to create objects with sub-micrometer and 3D features, and overcomes the traditional process limitations of two-photon lithography by using multiple beam production under continuous flow. Polymeric fibers, helical and bow-tie particles with sub-diffraction resolution and surface roughness as low as 10 nm are demonstrated.

Patterning organic/inorganic hybrid Bragg stacks by integrating one-dimensional photonic crystals and macrocavities through photolithography: toward tunable colorful patterns as highly selective sensors

Herein, we report a simple method to fabricate patterned organic/inorganic hybrid 1DPCs by top-down assisted photolithography. Versatile colorful pattern with different size and shape can be produced by selectively exposing the 1DPCs under UV light with predesigned photomask directly. The period change, especially the thickness variation of the top polymer layer, is the main reason for the colorful pattern generation. Because of the swelling property of the polymer layers, the pattern color can be modulated by introducing or taking off organic solvents, leading the as-prepared patterned 1DPCs to be effective sensors with high selectivity.

Mapping of near field light and fabrication of complex nanopatterns by diffraction lithography

We report a single-step lithographic approach for precisely mapping near field light diffraction in photoresist and fabricating complex subwavelength structures. This method relies on the diffraction of UV light from opaque patterns on a photomask, and utilizes the central diffraction maximum (known as the 'Poisson spot' for an opaque disk) and its higher orders. By correlating pattern geometries with the resulting diffraction, we demonstrate that the near field light intensity can be quantified to high precision and is in good agreement with theory. The method is further extended to capture higher order diffraction, which is utilized to fabricate unconventional subwavelength nanostructures with three-dimensional topographies. The simplicity of this process and its capability for light mapping suggest it to be an important tool for near field optical lithography.

Simultaneous fabrication of line defects-embedded periodic lattice by topographically assisted holographic lithography

We have demonstrated simultaneous fabrication of designed defects within a periodic structure. For rapid fabrication of periodic structures incorporating nanoscale line-defects at large area, topographically assisted holographic lithography (TAHL) technique, combining the strength of hologram lithography and phase-shift interference, was proposed. Hot-embossing method generated the photoresist patterns with vertical side walls which enabled phase-shift mask effect at the edge of patterns. Embossing temperature and relief height were crucial parameters for the successful TAHL process. Periodic holes with a diameter of 600 nm at a 1 μm-pitch incorporating 250 nm wide line-defects were obtained simultaneously.

Focused laser spike (FLaSk) annealing of photoactivated chemically amplified resists for rapid hierarchical patterning

Lithographic alternatives to conventional layer-by-layer processes for the design of 3D structures such as photonic or phononic crystals often present a dichotomy: patterning control versus patterning area. We demonstrate a combined technique of large area interference lithography and local area direct write focused laser spike (FLaSk) annealing that can enable the microscale patterning of hierarchical structures defined in their morphology by the interference and defined in placement and shape by the direct write. This is accomplished by doping a commercial chemically amplified photoresist (SU-8) with an absorbing dye to provide thermal activation at a wavelength shifted from that causing UV crosslinking. In this way, the necessary post-exposure bake to complete the crosslinking of the resist is locally performed by the FLaSk laser, rather than globally on a hot plate. By utilizing the same experimental setup as used by a 3D direct write system, it is possible to integrate another level of patterning by enabling fully dense, arbitrarily written features on multiple length scales. Both experimental and simulated results of this novel processing method are shown.

From two-dimensional colloidal self-assembly to three-dimensional nanolithography

A number of "top-down" lithographic and "bottom-up" self-assembly methods have been developed to fabricate three-dimensional (3D) nanostructures to support the recent advances in nanotechnology. But they are limited by a number of factors such as fabrication cost, pattern resolution, and/or flexibility of geometry. Here we present a 3D nanolithography process that utilizes self-assembled nanospheres to create a periodic array of focal spots, which are then replicated across multiple depth in a transparent medium according to the Talbot effect. The Talbot field then exposes a pattern onto the underlying photoresist, recording the 3D intensity distribution. We have demonstrated designable complex 3D periodic structures with 80 nm minimum feature size, roughly one-fourth of the operating wavelength. This approach combines 2D colloidal self-assembly and 3D phase lithography, is robust, cost-effective, and widely applicable to nanoscale research and manufacturing.

Practical approach for macroporous structure embedded microfluidic system and the catalytic microchemical application

We present a low cost and practical approach to integrate 3D ordered macroporous polyfluoropolyether (PFPE) patterns into a microchannel by a series of porous pattern fabrication processes and subsequent photolithography in a site- and shape-selective manner. The 3D ordered macroporous patterns with high-resolution edges were firstly fabricated by microtransfer molding (μ-TM) of the sacrificial polystyrene (PS) template infiltrated with PFPE as a non-adhesive and solvent-resistant skeletal material. The resulting robust PFPE porous structures with high solvent resistance on a silicon wafer can easily be embedded into the microchannel with the aid of conventional photolithography, leading to a microfluidic system with a built-in microstructure. Moreover, catalytic Pd nanoparticles implanted on the surface of the porous structure were obtained by use of Pd nanoparticle deposited PS spheres, the porous structure embedded channel was utilized to perform a Suzuki coupling reaction.

Single exposure fabrication and manipulation of 3D hydrogel cell microcarriers

We present a simple and high-throughput method for fabricating free-floating hydrogel cell microcarriers using single exposure UV patterning. We also demonstrate magnetic manipulation of the free-floating cell microcarriers using a magnetic nanoparticle-embedded structure for an active agitation and a solution exchange.

Enhanced energy dissipation in periodic epoxy nanoframes

Periodic nanostructures fabricated by interference lithography can be precisely designed to have a specific cell geometry, topology, and porosity in contrast to typical stochastic cellular materials. We use nanoindentation to elucidate the mechanical characteristics of the nanoframe as a function of its relative density and model the deformation behavior via numerical simulations. The nanoframe exhibits a scaling exponent of relative modulus versus relative density of 1.26, which is less sensitive than for conventional foams. Moreover, the nanoframe shows large mechanical energy dissipation/volume (up to 4.5 MJ/m(3)), comparable to the highest values achieved in the conventional polymer foams but at a far smaller strain. Counterintuitively, a nanoframe of smaller relative density can dissipate more energy per volume because the geometry of the nanoframe evolves during deformation to engage more of the material in plastic deformation. The results demonstrate how geometrical control at the nano- and microstructural scale can tailor modulus and energy dissipation and suggest means for engineering of mechanically superior materials in the future.

Proximity field nanopatterning of azopolymer thin films

A method for inscribing surface relief gratings in azopolymer thin films via proximity field nanopatterning is reported. Azopolymers prepared by ring opening metathesis polymerization were cast as thin films and brought into conformal contact with transparent polydimethylsiloxane phase masks. Irradiation of the film surface through the phase masks induces mass transport of azopolymer that generates surface relief structures on the basis of the intensity modulation of the light by structures on the phase mask. The experimental images obtained matched well with those produced by optical simulation. A wide variety of structures could be inscribed in the film surface which depended on the molecular weight of the azopolymer and irradiation time. Control experiments conducted suggest that the process is entirely photonic and that the presence of the phase mask on the film surface did not affect the inscription process.

Three-dimensional fabrication at small size scales

Despite the fact that we live in a 3D world and macroscale engineering is 3D, conventional submillimeter-scale engineering is inherently 2D. New fabrication and patterning strategies are needed to enable truly 3D-engineered structures at small size scales. Here, strategies that have been developed over the past two decades that seek to enable such millimeter to nanoscale 3D fabrication and patterning are reviewed. A focus is the strategy of self-assembly, specifically in a biologically inspired, more deterministic form, known as self-folding. Self-folding methods can leverage the strengths of lithography to enable the construction of precisely patterned 3D structures and "smart" components. This self-assembly approach is compared with other 3D fabrication paradigms, and its advantages and disadvantages are discussed.

Controlled assembly for well-defined 3D bioarchitecture using two active enzymes

This paper reports that a bioarchitecture with two different active enzymes can be fabricated conveniently on a prepatterned surface by highly selective surface-templated layer-by-layer (LBL) assembly by coupling a bilayer of avidin/biotin-lactate oxidase (biotin-LOD) with a bilayer of avidin/biotin-horseradish peroxidase (biotin-HRP). The two different active enzymes can be utilized as excellent building blocks for the fabrication of well-defined 3D nanostructures with precise control of the position and height on the surface. In addition, the LBL assembled bienzyme structures are highly functional, and bioarchitectures based on LOD and HRP-mediated coupling reaction can be employed in a number of viable biosensing applications.

Multidimensional architectures for functional optical devices

Materials exhibiting multidimensional structure with characteristic lengths ranging from the nanometer to the micrometer scale have extraordinary potential for emerging optical applications based on the regulation of light-matter interactions via the mesoscale organization of matter. As the structural dimensionality increases, the opportunities for controlling light-matter interactions become increasingly diverse and powerful. Recent advances in multidimensional structures have been demonstrated that serve as the basis for three-dimensional photonic-bandgap materials, metamaterials, optical cloaks, highly efficient low-cost solar cells, and chemical and biological sensors. In this Review, the state-of-the-art design and fabrication of multidimensional architectures for functional optical devices are covered and the next steps for this important field are described.

Chemical and geometrical criteria for the release of elastomeric 1D nanoarrays from porous nanotemplates

Releasing 1D nanoarrays from nanotemplates is a significant challenge for the integration of mechanically soft materials in a variety of newly emerging technological areas. To fabricate nanoarrays without defects, the combined effects of the surface energy and the geometric features of the nanotemplate should be considered. A previously reported approach based on the correlation between the adhesion energy and the real contact area was not satisfactory to describe the rupture conditions of the nanofibers while they were being peeled off from the porous template. Here we demonstrate that the aspect ratio rather than the contact area of the nanoporous template is the key factor determining the upper limit of the pore length of the nanotemplate with respect to the rupture of the nanoarray during separation. We propose that the value of alpha(c)*, which is calculated with a simple expression in which the adhesion energy is multiplied by the aspect ratio, can be used as an excellent criterion for the fabrication of 1D nanoarrays without defects with a simple peel-off processes. Our approach opens up new applications for unconventional lithographic techniques, such as soft lithography, imprint lithography, and others.

Transferring vertically aligned carbon nanotubes onto a polymeric substrate using a hot embossing technique for microfluidic applications

We explored the hot embossing method for transferring vertically aligned carbon nanotubes (CNTs) into microfluidic channels, fabricated on poly-methyl-methacrylate (PMMA). Patterned and unpatterned CNTs were synthesized by microwave plasma-enhanced chemical vapour deposition on silicon to work as a stamp. For hot embossing, 115 degrees C and 1 kN force for 2 min were found to be the most suitable parameters for the complete transfer of aligned CNTs on the PMMA microchannel. Raman and SEM studies were used to analyse the microstructure of CNTs before and after hot embossing. The PMMA microparticles with dimensions (approx. 10 microm in diameter) similar to red blood cells were successfully filtered using laminar flow through these microfluidic channels. Finally, a microfluidic-based point-of-care device for blood filtration and detection of bio-molecules is drawn schematically.

Flexible fabrication of three-dimensional optical-domain photonic crystals using a combination of single-laser-exposure diffractive-optics lithography and template inversion

We demonstrate inversion of three-dimensional photonic crystal templates fabricated in a large area with diffractive-optics lithography. A custom-designed two-dimensional diffractive optical element was used to generate highly uniform, bicontinuous, three-dimensional photonic crystal templates in a single-laser exposure. Chemical vapor deposition successfully infiltrated the thick periodic polymer structure to deposit amorphous silica and thereby define an all-silica inverted photonic crystal after polymer removal, as confirmed by focused ion-beam milling and energy-dispersive x-ray spectroscopy. The diffractive-optics lithography permitted a large number of uniform layers to form that manifested in the recording of a strong -28 dB stopband in the telecom band.

Template-assisted three-dimensional nanolithography via geometrically irreversible processing

An innovative and versatile nanofabrication technique based on template assisted three-dimensional (3D) nanolithography is presented that takes advantage of the irreversibility of conformal growth and conformal etching at locations with negative surface curvatures in 3D templates. Using colloidal crystals as templates, nanoring particles are generated with quantity much higher than conventional methods. Relying on the same principle, metallodielectric photonic crystals with discrete metal elements are fabricated that show strong absorption in the near-IR and transmission at longer wavelengths.

Holographic fabrication of three-dimensional nanostructures for microfluidic passive mixing

In this study, we incorporated mixing units of three-dimensional (3D) interconnected pore network inside microfluidic channels by combining single prism holographic lithography and photolithography. 3D pore network structures were generated by the interference of four laser beams generated by a truncated triangular pyramidal prism. The levelling between the 3D porous structures and the channel walls was greatly improved by employing supercritical drying, which induced negligible internal capillary stresses and reduced substantially anisotropic volume shrinkage of 3D structures. Also, complete sealing of the microfluidic chips was achieved by attaching flexible PDMS cover substrates. Overall mixing performance of the systems with completely sealed mixing units was 84% greater than that obtained without such mixers. Splitting and recombination of flows in the 3D interconnected pore structures enhanced the mixing efficiency by decreasing the diffusion path and increasing the surface contact between two liquid streams. Because the flow splitting and recombination was developed through the 3D interconnected pore network, high mixing efficiency (>0.60) was achieved at low Reynolds numbers (Re < 0.05) and Péclet numbers in the regime of Pe < 1.4 x 10(3).