Emerging low-cost,large-scale photonic platforms with soft lithography and self-assembly

Advancements in micro/nanofabrication have enabled the realization of practical micro/nanoscale photonic devices such as absorbers,solar cells,metalenses,and metaholograms.Although the performance of these photonic devices has been improved by enhancing the design flexibility of structural materials through advanced fabrication methods,achieving large-area and high-throughput fabrication of tiny structural materials remains a challenge.In this aspect,various technologies have been investigated for realizing the mass production of practical devices consisting of micro/nanostructural materials.This review describes the recent advancements in soft lithography,colloidal self-assembly,and block copolymer self-assembly,which are promising methods suitable for commercialization of photonic applications.In addition,we introduce low-cost and large-scale techniques realizing micro/nano devices with specific examples such as display technology and sensors.The inferences presented in this review are expected to function as a guide for promising methods of accelerating the mass production of various sub-wavelength-scale photonic devices.

Confinement-induced nanocrystal alignment of conjugated polymer by the soft-stamped nanoimprint lithography

Soft-stamped nanoimprint lithography（NIL） is considered as one of the most effective processes of nanoscale patterning because of its low cost and high throughput. In this work, this method is used to emboss the poly（9, 9-dioctylfluorene）film. By reducing the linewidth of the nanogratings on the stamp, the orientations of nanocrystals are confined along the grating vector in the nanoimprint process, where the confinement linewidth is comparable to the geometrical size of the nanocrystal. When the linewidth is about 400 nm, the poly（9, 9-dioctylfluorene）（PFO） nanocrystals could be orderly arranged in the nanogratings, so that both pattern transfer and well-aligned nanocrystal arrangement could be achieved in a single step by the soft-stamped NIL. The relevant mechanism of the nanocrystalline alignment in these nanogratings is fully discussed. The modulation of nanocrystal alignment is of benefit to the charge mobilities and other performances of PFO-based devices for the future applications.

Sol-Gel Based Soft Lithography and Piezoresponse Force Microscopy of Patterned Pb(Zr_(0.52)Ti _(0.48))O_3 Microstructures

Sol-gel based soft lithography technique has been developed to pattern a variety of ferroelectric Pb（Zr0.52Ti0.48）O3（PZT） microstructures,with feature size approaching 180 nm and good pattern transfer between the master mold and patterned films.X-ray diffraction and high-resolution transmission electron microscopy confirm the perovskite structure of the patterned PZT.Piezoresponse force microscopy（PFM） and switching spectroscopy piezoresponse force microscopy（SSPFM） confirm their piezoelectricity and ferroelectricity.Piezoresponse as high as 2.75 nm has been observed,comparable to typical PZT films.The patterned PZT microstructures are promising for a wide range of device applications.

Fabrication of a 256-bits organic memory by soft x-ray lithography

This paper reports a procedure of soft x-ray lithography for the fabrication of an organic crossbar structure. Electron beam lithography is employed to fabricate the mask for soft x-ray lithography, with direct writing technology to the lithograph positive resist and polymethyl methacrylate on the polyimide film. Then Au is electroplated on the polyimide film. Hard contact mode exposure is used in x-ray lithography to transfer the graph from the mask to the wafer. The 256-bits organic memory is achieved with the critical dimension of 250 nm.

Soft thermal nanoimprint lithography using a nanocomposite mold

Soft nanoimprint lithography has been limited to ultraviolet （UV） curable resists. Here, we introduce a novel approach for soft thermal nanoimprinting. This unprecedented combination of the terms ＂soft＂ and ＂thermal＂ for nanoimprinting became possible thanks to an innovative nanocomposite mold consisting of a flexible polydimethylsiloxane （PDMS） substrate with chemically attached rigid relief features. We used soft thermal nanoimprinting to produce high-resolution nanopatterns with a sub-100 nm feature size. Furthermore, we demonstrate the applicability of our nanoimprint approach for the nanofabrication of thermally imprinted nanopattems on non-planar surfaces such as lenses. Our new nanofabrication strategy paves the way to numerous applications that require the direct fabrication of functional nanostructures on unconventional substrates.

High-Transconductance,Highly Elastic,Durable and Recyclable All-Polymer Electrochemical Transistors with 3D Micro-Engineered Interfaces

Organic electrochemical transistors(OECTs)have emerged as versatile platforms for broad applications spanning from flexible and wearable integrated circuits to biomedical monitoring to neuromorphic computing.A variety of materials and tailored micro/nanostructures have recently been developed to realized stretchable OECTs,however,a solid-state OECT with high elasticity has not been demonstrated to date.Herein,we present a general platform developed for the facile generation of highly elastic all-polymer OECTs with high transconductance(up to 12.7 mS),long-term mechanical and environmental durability,and sustainability.Rapid prototyping of these devices was achieved simply by transfer printing lithium bis(trifluoromethane)sulfonimide doped poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)(PEDOT:PSS/LiTFSI)microstructures onto a resilient gelatin-based gel electrolyte,in which both depletion-mode and enhancement-mode OECTs were produced using various active channels.Remarkably,the elaborate 3D architectures of the PEDOT:PSS were engineered,and an imprinted 3D-microstructured channel/electrolyte interface combined with wrinkled electrodes provided performance that was retained(>70%)through biaxial stretching of 100%strain and after 1000 repeated cycles of 80%strain.Furthermore,the anti-drying and degradable gelatin and the self-crosslinked PEDOT:PSS/LiTFSI jointly enabled stability during>4 months of storage and on-demand disposal and recycling.This work thus represents a straightforward approach towards high-performance stretchable organic electronics for wearable/implantable/neuromorphic/sustainable applications.

MICROMOLDING ON CURVED SURFACES WITH THERMOPLASTICS

Microstructures were produced on curved surfaces and micro-protrusions by using direct micromolding with fourthermoplastic polymers. This method is simpler and more convenient than micromolding with liquid prepolymer or using theμTM method. By repeated molding, crossed structures were produced with a stamp prepared only with lines. The processingvariables including the softening temperature of the polymers and heating time were discussed. The result shows that theoptimal molding temperature is preferably slightly higher than the melting temperature of the thermoplastic polymers, atwhich polymers are in the critical states of being melted. This method can be applied to many polymers except those with high softening temperatures or high rate of shrinkage upon temperature change.

Polyurethane Molecular Stamps for the in situ Synthesis of DNA Microarray

Fabrication of polyurethane molecular stamps (PU stamps) based on polypropylene glycol (PPG) and toluene diisocyanate (TDI), using 3, 3-dichloro-4, 4-methylenedianiline (MOCA) as the crosslinker, is reported. It was shown from the contact angle measurement that PU stamps surface has good affinity with acetonitrile, guaranteeing the well distribution of DNA monomers on patterned stamps. Laser confocal fluorescence microscopy images of oligonucleotide arrays after hybridization confirmed polyurethane is an excellent material for molecular stamps when transferring polar chemicals and conducting reactions on interfaces by stamping.

A novel method to mass-produce PDMS stamps based on nanoimprint

Soft lithography is a low-cost and convenient method for the forming and manufacturing of micro/ nanostructures compared to the traditional optical lithography. In soft lithography, poly（dimethylsiloxane） （PDMS） stamps with relief structures have been widely used to transfer patterns. The traditional fabrication approach of PDMS stamps is time-consuming since the master has been occupied during the curing process. By adding and repeating fast nanoimprint step, many intermediate polymeric molds can be produced from the master and these molds can then be employed to replicate more PDMS stamps while the time used is close to that of the common way. We demonstrated this idea by three masters which were made by the DEM （Deepetching, Electroforming and Microreplicating） and FIB （Focused Ion Beam） techniques. The photos show that the patterns on the PMDS stamps successfully duplicated patterns on the origin masters.

Photolithography-free fabrication of photoresist-mold for rapid prototyping of microfluidic PDMS devices

Traditional soft lithography based PDMS device fabrication requires complex procedures carried out in a clean room. Herein, we report a photolithography-free method that rapidly produces PDMS devices in 30 min. By using a laser cutter to ablate a tape, a male photoresist mold can be obtained within 5 min by a simple heating-step, which offers significant superiority over currently used photolithographybased method. Since it requires minimal energy to cut the tape, our fabrication strategy shows good resolution(~ 100 μm) and high throughput. Furthermore, the micro-mold height can be easily controlled by changing the tape types and layers. As a proof-of-concept, we demonstrated that the fabricated PDMS devices are compatible with biochemical reactions such as quenching reaction of KI to fluorescein and cell culture/staining. Collectively, our strategy shows advantages of low input, simple operation procedure and short fabrication time, therefore we believe this photolithography-free method could serve as a promising way for rapid prototyping of PDMS devices and be widely used in general biochemical laboratories.

Plasmonic nanostructure characterized by deep-neuralnetwork-assisted spectroscopy[Invited]

The lateral geometry and material property of plasmonic nanostructures are critical parameters for tailoring their optical resonance for sensing applications.While lateral geometry can be easily observed by a scanning electron microscope or an atomic force microscope,characterizing materials properties of plasmonic devices is not straightforward and requires delicate examination of material composition,cross-sectional thickness,and refractive index.In this study,a deep neural network is adopted to characterize these parameters of unknown plasmonic nanostructures through simple transmission spectra.The network architecture is established based on simulated data to achieve accurate identification of both geometric and material parameters.We then demonstrate that the network training by a mixture of simulated and experimental data can result in correct material property recognition.Our work may indicate a simple and intelligent characterization approach to plasmonic nanostructures by spectroscopic techniques.

Nano-and Micro-patterned Biomaterials

1 Results Our objective is two fold: we (ⅰ) aim at the development of novel patterning methodologies in order to (ⅱ) achieve control over the positioning and alignment of living cells.The patterning of the biointerfaces is carried out both at the micro-and nanometer scale and involve (bio)chemical as well as topographic patterns.The former are relatively easily obtained by patterning techniques adapted from (conventional) soft lithography,e.g.by means of micro-contact printing (μ-CP).The topographic pat...

Micro- and Nanostripes of Self-Assembled Au Nanocrystal Superlattices by Direct Micromolding

A simple,inexpensive direct micromolding method for patterning Au nanocrystal superlattices using a polydimethylsiloxane(PDMS)stamp has been developed.The method involves in situ synthesis of Au(I)dodecanethiolate and its decomposition leading to Au nanocrystals in the microchannels of the stamp which order themselves to form patterned superlattice stripes,in conformity with the stamp geometry.Owing to its insolubility in common solvents,the dodecanethiolate was made by reacting Au(PPh3)Cl and dodecanethiol in situ inside the microchannels,by injecting first the former solution in toluene at room temperature followed by the thiol solution at 120°C.Annealing the reaction mixture at 250°C,resulted in formation of nanocrystals(with a mean diameter of 7.5 nm)and hexagonal ordering.By using an external pressure while molding,parallel stripes with sub-100 nm widths were obtained.The choice of parameters such as injection temperature of the thiol and concentrations is shown to be important if an ordered superlattice is to be obtained.In addition,these parameters can be varied as a means to control the nanocrystal size.

Noncovalent reversible binding-enabled facile fabrication of leak-free PDMS microfluidic devices without plasma treatment for convenient cell loading and retrieval

The conventional approach for fabricating polydimethylsiloxane(PDMS)microfluidic devices is a lengthy and inconvenient procedure and may require a clean-room microfabrication facility often not readily available.Furthermore,living cells can’t survive the oxygen-plasma and high-temperature-baking treatments required for covalent bonding to assemble multiple PDMS parts into a leak-free device,and it is difficult to disassemble the devices because of the irreversible covalent bonding.As a result,seeding/loading cells into and retrieving cells from the devices are challenging.Here,we discovered that decreasing the curing agent for crosslinking the PDMS prepolymer increases the noncovalent binding energy of the resultant PDMS surfaces without plasma or any other treatment.This enables convenient fabrication of leak-free microfluidic devices by noncovalent binding for various biomedical applications that require high pressure/flow rates and/or long-term cell culture,by simply hand-pressing the PDMS parts without plasma or any other treatment to bind/assemble.With this method,multiple types of cells can be conveniently loaded into specific areas of the PDMS parts before assembly and due to the reversible nature of the noncovalent bonding,the assembled device can be easily disassembled by hand peeling for retrieving cells.Combining with 3D printers that are widely available for making masters to eliminate the need of photolithography,this facile yet rigorous fabrication approach is much faster and more convenient for making PDMS microfluidic devices than the conventional oxygen plasma-baking-based irreversible covalent bonding method.

Control of visible‑range transmission and refection haze by varying pattern size,shape and depth in fexible metasurfaces

Cost-efective soft imprint lithography technique is used to prepare fexible thin polymeric surfaces containing a periodic arrangement of nanodimples and nanobumps of sub-micron size.Using a single master mold of self-assembled colloidal crystal,metasurfaces with diferent depths and heights of patterns with a fxed pitch are possible,which makes the process inexpensive and simple.These metasurfaces are studied for their difuse and total transmission and refection spectra in the visible range.The transmission haze and refection haze are calculated from the measurements.The surface containing nanobumps of lesser pattern height result in higher values of refection and transmission haze than from surfaces containing nanodimples of much higher depth for the same pitch.The haze is more dependent on the pattern depth or height and less dependent on the pitch of the pattern.Far-feld transmission profles measured in the same wavelength range from the patterned surfaces show that the scattering increases with the increase of the ratio of pattern depth/height to pitch,similar to the haze measurements conducted with a closed integrating sphere.These profles show that the angular spread of scattered light in transmission is within 10°,explaining the reason for the relatively low transmission haze in all the patterned surfaces.Simulation results confrm that the nanobump pattern gives higher transmission haze compared to nanodimple pattern.By controlling the ratio of pattern depth/height to pitch of the features on these surfaces,both an increase in optical haze and a balance between total refection intensity and total transmission intensity can be achieved.