One-Step Mask-Based Diffraction Lithography for the Fabrication of 3D Suspended Structures

We propose a novel one-step exposure method for fabricating three-dimensional (3D) suspended structures, utilizing the diffraction of mask patterns with small line width. An optical model of the exposure process is built, and the 3D light intensity distribution in the photoresist is calculated based on Fresnel-Kirchhoff diffraction formulation. Several 3D suspended photoresist structures have been achieved, such as beams, meshes, word patterns, and multilayer structures. After the pyrolysis of SU-8 structures, suspended and free-standing 3D carbon structures are further obtained, which show great potential in the application of transparent electrode, semitransparent solar cells, and energy storage devices.     

Investigation on the solid electrolyte interface formed on pyrolyzed photoresist carbon anodes for C-MEMS lithium-ion batteries

Carbon-microelectromechanical systems (C-MEMS) obtained from the pyrolysis of patterned photoresists are a powerful solution for the miniaturization of energy storage/conversion devices such as fuel-cells and microbatteries. The 3D shapes and the high aspect ratio of the resulting carbon structures are critical factors in applications where specific surface area plays a key role. Furthermore, lithographic techniques used in the C-MEMS technology may solve the downscaling problems of conventional carbon manufacturing techniques. The application of C-MEMS as a lithium-ion battery anode has already been demonstrated in our previous work. It is well known that a protective film, referred to as the solid electrolyte interface (SEI), forms on carbonaceous materials used as negative electrodes in commercial lithium-ion batteries. The passivating film forms during the first battery charging cycle. Detailed SEI investigations of this film, made up of electrolyte decomposition products, have not been explored yet. In this paper the evolution of the pyrolyzed carbon/electrolyte interface functioning as a lithium-ion battery anode is studied and discussed in detail.     

Fabrication and electrical conductivity of suspended carbon nanofiber arrays

We demonstrate a simple, efficient and novel self-assembly based method to fabricate arrays of suspended polymeric nanofibers of polyacrylonitrile and SU-8 negative photoresist by electrospinning on micro-fabricated posts of resorcinol–formaldehyde (RF) gel. The suspended electrospun nanofibers together with the RF gel posts were subsequently pyrolyzed in an inert atmosphere to yield large area monolithic structures of suspended glassy carbon nanofibers (CNF) integrated on RF gel derived carbon posts. The electrospun nanofibers self-assemble to connect the posts owing to a stronger electric field on their tips, obviating the need for positioning and integration of carbon nanowires with the underlying microstructures and paving the way for fabricating novel carbon based micro and nanoscale devices. The fabrication technique also allowed measurements of electrical conductivity of a single suspended CNF between carbon electrodes using I–V characteristics and comparison of the carbon nanowire conductivities for the CNF derived from different polymer precursors.     

Local chemical vapor deposition of carbon nanofibers from photoresist

The addition of nanofeatures to carbon microelectromechanical system (C-MEMS) structures would greatly increase surface area and enhance their performance in miniature batteries, super-capacitors, electrochemical and biological sensors. Negative photoresist posts were patterned on a Au/Ti contact layer by photolithography. After pyrolyzing the photoresist patterns to carbon patterns, graphitic nanofibers were observed near the contact layer. The incorporation of carbon nanofibers in C-MEMS structures via a simple pyrolysis of modified photoresist was investigated. Both experimental results considered to consist of a local chemical vapor deposition mechanism. The method represents a novel, elegant and inexpensive way to equip carbon microfeatures with nanostructures, in a process that could possibly be scaled up to the mass production of many electronic and biological devices.     

Improved conductivity of suspended carbon fibers through integration of C-MEMS and Electro-Mechanical Spinning technologies

Carbon microfibers suspended across carbon walls were fabricated by Electro-Mechanical Spinning and subsequent pyrolysis of a SU-8 based carbon precursor. The shrinkage and elongation of these polymer fibers during the pyrolysis process was observed to depend on the height of the supporting walls. We demonstrate that this shrinkage and elongation during pyrolysis strongly influences the resulting carbon electrical properties. Compared to fibers that retained their length during pyrolysis, conductivity was enhanced by a factor of seven after fibers were elongated four times their initial pre-pyrolysis length with a concurrent shrinkage of their diameter by half.     

Fabrication of porous carbon micropillars using a block copolymer as porogen

A technique to fabricate porous carbon micropillars using a block copolymer, F127, as porogen is described. In this process, negative tone photoresist (i.e. SU-8) mixed with F127 was photopatterned and carbonized under inert atmosphere. The thermal behavior of the photoresist precursor (F127 + SU-8) during carbonization process was characterized using differential scanning calorimetry and thermogravimetric analysis. Texture analysis on the carbon surface showed a mesoporous feature distribution. Electrochemical characterization based on the reaction of redox couple was utilized to study the change in the effective surface area (A_eff) of the porous carbon electrodes with different weight percentages of F127 in SU-8. These results indicated that porous carbon thin film electrodes derived from 10% F127 mixed in SU-8 had an A_eff 185% compared to the conventional photoresist derived carbon electrode. This fabrication approach can be employed to produce reproducible high aspect ratio carbon microelectrodes with different shapes for various electrochemical devices.     

Micro-supercapacitors based on three dimensional interdigital polypyrrole/C-MEMS electrodes

Symmetric micro-supercapacitors with three dimensional (3D) interdigital electrode structures have been designed and fabricated through Carbon-microelectrochemical system (C-MEMS) technology. The micro-supercapacitor consists of a 3D C-MEMS structure which serves as a high effective surface area current collector and conformal polypyrrole (PPy) films deposited on the carbon structures as electroactive materials. The electrochemical performance of single electrodes and symmetric micro-supercapacitor cells were evaluated by cyclic voltammetry (CV) at different scan rates and galvanostatic charge/discharge tests. The effect of the 3D electrode structure on the performance of the micro-supercapacitor was studied. Single PPy/C-MEMS electrodes presented a specific capacitance of 162.07 ± 12.40 mF cm⁻² and a specific power of 1.62 ± 0.12 mW cm⁻² at 20 mV s⁻¹ scan rate. The symmetric micro-supercapacitor cells exhibited an average specific capacitance of 78.35 ± 5.67 mF cm⁻² and a specific power of 0.63 ± 0.04 mW cm⁻² at 20 mV s⁻¹ scan rate, demonstrating that 3D micro-supercapacitors are promising for applications that require high power in a limited footprint area of the device.     

Synthesis of carbon xerogel particles and fractal-like structures

A variety of dense and open-architecture amorphous carbon xerogel microspheres and folded fractal-like structures were synthesized by sol-gel polycondensation of resorcinol with formaldehyde in a slightly alkaline aqueous solution. Carbon structures were obtained by inverse emulsification of resorcinol-formaldehyde (RF) sol in cyclohexane containing a non-ionic surfactant Span-80, followed by its pyrolysis at 1173 K in nitrogen. We have investigated the effects of synthesis parameters including stirring time, resorcinol/catalyst (R/C) ratio and surfactant concentration on the structures. The average particle size of the carbon microspheres could be modulated from 5 to 46 μm by increasing the stirring time from 2 to 7 h and by varying the R/C ratio from 0.2 to 500. Particles agglomerated as the R/C ratio increases above 100. Increase in the surfactant concentration from 1% to 4% (v/v) produced smaller spherical particles with narrower size distribution. Further increase in the surfactant concentration from 10% to 50% (v/v) produced branched and folded fractal-like structures of large external area. Thus, RF sol-based precursor chemistry can be easily tuned to produce a spectrum of desired carbon particle morphologies with potential applications in printing technology, adsorbents, resonance-based solar cells, thermal detectors and carbon-based micro-electromechanical devices (C-MEMS).     

Effect of temperature on the SU-8 photoresist filling behavior during thermal nanoimprinting

SU-8 photoresist is an ideal thermal-imprinting polymer, which has been widely used in micro-nano devices in recent years. However, the research on the filling behavior of SU-8 photoresist during thermal nanoimprinting is not complete. In this paper, the stress relaxation curves and shear stress/rate flow curves were measured to analyze the temperature dependent rheological properties of SU-8 photoresist. The filling behavior of SU-8 photoresist was discussed based on thermal imprinting results at various temperatures. High-precision nanochannels were fabricated with replication error of 2.8% at optimized thermal imprinting temperature of 85°C and duration of 10 min, which paves the way for high-precision fabrication of SU-8-based micro-nanofluidic chips.     

Monolithic carbon structures including suspended single nanowires and nanomeshes as a sensor platform

With the development of nanomaterial-based nanodevices, it became inevitable to develop cost-effective and simple nanofabrication technologies enabling the formation of nanomaterial assembly in a controllable manner. Herein, we present suspended monolithic carbon single nanowires and nanomeshes bridging two bulk carbon posts, fabricated in a designed manner using two successive UV exposure steps and a single pyrolysis step. The pyrolysis step is accompanied with a significant volume reduction, resulting in the shrinkage of micro-sized photoresist structures into nanoscale carbon structures. Even with the significant elongation of the suspended carbon nanowire induced by the volume reduction of the bulk carbon posts, the resultant tensional stress along the nanowire is not significant but grows along the wire thickness; this tensional stress gradient and the bent supports of the bridge-like carbon nanowire enhance structural robustness and alleviate the stiction problem that suspended nanostructures frequently experience. The feasibility of the suspended carbon nanostructures as a sensor platform was demonstrated by testing its electrochemical behavior, conductivity-temperature relationship, and hydrogen gas sensing capability.     

Structure Evolution in Polyethylene-Derived Carbon Fiber Using a Combined Electron Beam-Stabilization-Sulphurization Approach

A new approach is described for the production of poly(ethylene) (PE) derived carbon fibers (CFs) that entails the melt spinning of PE fibers from a suitable precursor, their cross-linking by electron beam (EB) treatment, and sulphurization with elemental sulphur (S₈), followed by pyrolysis and carbonization. Instead of focusing on mechanical properties, analysis of CF structure formation during all process steps is carried out by different techniques comprising solid-state nuclear magnetic resonance spectroscopy, thermogravimetric analysis coupled to mass spectrometry/infrared spectroscopy, elemental analysis, energy dispersive X-ray scattering, scanning electron microscopy, Raman spectroscopy, and wide-angle X-ray diffraction. A key step in structure formation is the conversion of PE into poly(thienothiophene)s during sulphurization; these species are stabile under inert gas up to 700 °C as confirmed by Raman analysis. Above this temperature, they condense into poly(napthathienophene)s, which are then converted into graphite-type structures during pyrolysis.     

Improved graphitization and electrical conductivity of suspended carbon nanofibers derived from carbon nanotube/polyacrylonitrile composites by directed electrospinning

Single suspended carbon nanofibers on carbon micro-structures were fabricated by directed electrospinning and subsequent pyrolysis at 900 °C of carbon nanotube/polyacrylonitrile (CNT/PAN) composite material. The electrical conductivity of the nanofibers was measured at different weight fractions of CNTs. It was found that the conductivity increased almost two orders of magnitude upon adding 0.5 wt.% CNTs. The correlation between the extent of graphitization and electrical properties of the composite nanofiber was examined by various structural characterization techniques, and the presence of graphitic regions in pyrolyzed CNT/PAN nanofibers was observed that were not present in pure PAN-derived carbon. The influence of fabrication technique on the ordering of carbon sheets in electrospun nanofibers was examined and a templating effect by CNTs that leads to enhanced graphitization is suggested.     

Patterned growth and differentiation of neural cells on polymer derived carbon substrates with micro/nano structures in vitro

The growth of neuroblastoma (N2a) and Schwann cells has been explored on polymer derived carbon substrates of varying micro and nanoscale geometries: resorcinol–formaldehyde (RF) gel derived carbon films and electrospun nanofibrous (∼200 nm diameter) mat and SU-8 (a negative photoresist) derived carbon micro-patterns. MTT assay and complementary lactate dehydrogenase (LDH) assay established cytocompatibility of RF derived carbon films and fibers over a period of 6 days in culture. The role of length scale of surface patterns in eliciting lineage-specific adaptive response along, across and on the interspacing between adjacent micropatterns (i.e., “on”, “across” and “off”) has been assayed. Textural features were found to affect 3′,5′-cyclic AMP sodium salt-induced neurite outgrowth, over a wide range of length scales: from ∼200 nm (carbon fibers) to ∼60 μm (carbon patterns). Despite their innate randomness, carbon nanofibers promoted preferential differentiation of N2a cells into neuronal lineage, similar to ordered micro-patterns. Our results, for the first time, conclusively demonstrate the potential of RF-gel and SU-8 derived carbon substrates as nerve tissue engineering platforms for guided proliferation and differentiation of neural cells in vitro.     

ESCA characterization of commercial carbon blacks and of carbon blacks from vacuum pyrolysis of used tires

Pyrolytic carbon blacks (CB_p) were obtained by vacuum pyrolysis of used tires in a batch reactor at a total pressure ranging from 0.3 to 20.0 kPa, and temperatures ranging from 420 to 700°C. CB_p differ from commercial carbon blacks used initially in the tire fabrication. A series of commercial carbon blacks with different surface areas and structures and CB_p obtained under different pyrolysis conditions were characterized using ESCA and SEM techniques to investigate the effect of the pyrolysis conditions on the chemical nature of the surface of CB_p.     

Pitch resin addition induced evolution of composition,microstructure and mechanical property of C/C-SiC-ZrC composites

Precursor infiltration and pyrolysis (PIP) has been widely used to fabricate C/C-SiC-ZrC composites. However, the use of organic polymeric precursor of zirconium carbide (PZC) can usually cause the degradation of their mechanical property due to the reaction of ZrO₂ intermediate with pyrocarbon (PyC) and carbon fibers (C_f) during pyrolysis. In this study, pitch resin was directly added into the mixture solution of PZC and polycarbosilane (PCS) to supply extra carbon. The composition, microstructure and mechanical property of the as-prepared composites were investigated systematically. The pure ZrC-SiC with a high ZrC content is obtained at 1500 °C when the PZC/PCS/resin mass ratio is 20:1:5. The resulting C/C-SiC-ZrC composites have the highest flexural strength of 247.4 MPa since the degradation of PyC and C_f is greatly alleviated by the addition of resin. The damage mechanism of PyC and C_f during pyrolysis was revealed under the different fabrication conditions.     

Photolithographically patterned smart hydrogel based bilayer actuators

We describe the fabrication of photopatterned actuators, composed of stimuli-responsive hydrogel bilayers made from N-isopropyl-acrylamide (NIPAm), acrylic acid (AAc), and poly-ethylene oxide diacrylate (PEODA). The hydrogels were deposited by spin coating and casting and were patterned by non-contact photolithography. We investigated the swelling behavior of the individual photopatterned hydrogels in aqueous solutions of varying pH and ionic strength (IS). By combining materials with optimal swelling responses, bilayer structures were triggered via changes in pH and IS to actuate into three dimensional (3D) structures. We also used these hydrogel bilayers as hinges to actuate integrated structures composed of rigid polymeric SU-8 panels, patterned to resemble the shape of a Venus Flytrap. This system provides a straightforward way to design and fabricate actuator hinges composed entirely of polymers.     

Polymer‐derived SiCN cellular structures from replica of 3D printed lattices

In comparison with metals and polymers, ceramics and/or carbon are more difficult to process into well‐defined cellular architectures (e.g., cubic, tetrakaidecadehron, etc.) using Additive Manufacturing techniques. The present work reports a simple method for generating complex and precise SiCN ceramic lattices using a preceramic polymer and applying the replica approach to structures fabricated using stereolithography of plastic materials, with the associated ease of fabrication. Three‐dimensional printed plastic lattices impregnated with a polysilazane were converted to SiCN by pyrolysis at 1000°C in inert atmosphere. In spite of the high amount of mass loss (~58%) and volume shrinkage (~65%), the impregnated structures did not collapse during pyrolysis, leading to highly porous (total porosity ~93 vol%) components possessing suitable strength for handling and potential use as lightweight components.     

Two microcell flow-injection analysis (FIA) platforms for capacitive silicon-based field-effect sensors

pH- and penicillin-sensitive electrolyte-insulator-semiconductor (EIS) sensors (Al/Si/SiO₂/Ta₂O₅ and penicillinase) have been successfully integrated into a commercial flow-injection analysis (FIA) system. For the FIA set-up, a miniaturised home-made flow-through cell of plexiglass with a variable internal volume from 12 to 48 μl has been developed. As a second platform, an SU-8 based miniaturised flow-through cell, also combined with a pH-sensitive EIS structure, has been fabricated at wafer level. The application of SU-8 as a photoresist with good qualities (distinct chemical resistance, high aspect ratio, good mechanical and dielectric behaviour) is favoured. Different channel/sensor layouts have been realised with sensitive areas of the EIS sensors of 1.7-8.2 mm² and sample volumes of 0.3-4 μl, respectively. FIA parameters (sample volume, flow rate, distance between injection valve and sensor) have been optimised in terms of high throughput, minimum dispersion, fluid consumption, sensitivity and reproducibility.     

热解Ni/Co-ZIF-8制备碳纳米管桥连多孔碳及其在超级电容器中的应用

金属-有机骨架(MOFs)衍生碳材料具有丰富的孔道结构和超高的比表面积,在超级电容器等储能领域展现出巨大潜力。以环保型ZnO纳米球为模板,通过水热法制备核壳结构ZnO@Ni/Co-ZIF-8前体。将其在四种温度(700、800、900、950℃)下热解,获得不同形貌的Ni、Co及N掺杂的MOFs衍生碳材料Ni/Co-CN,并探究了煅烧温度对其储能性能的影响。结果表明,随着煅烧温度升高,Ni/Co-CN逐渐由多孔碳变为碳纳米管桥连多孔碳结构。当热解温度为900℃时,Ni/Co-CN-900的比电容最大。在1 mol/L的KOH电解液中对其进行循环伏安测试,曲线对称性良好,表明其具有优异的电化学可逆性。通过计算该过程电荷存储的电容贡献和扩散贡献占比可知,Ni/Co-CN的储能主要来自多孔碳的双电层吸附,少量来自N掺杂导致的法拉第反应。在0.5 A/g的电流密度下,Ni/Co-CN-900的比电容高达273.5 F/g。在10.0 A/g的电流密度下进行5000次恒流充放电后,其比电容保持率高达93.8%,展现出良好的电化学性能。     

热解Ni/Co-ZIF-8制备碳纳米管桥连多孔碳及其在超级电容器中的应用

金属-有机骨架(MOFs)衍生碳材料具有丰富的孔道结构和超高的比表面积,在超级电容器等储能领域展现出巨大潜力。以环保型ZnO纳米球为模板,通过水热法制备核壳结构ZnO@Ni/Co-ZIF-8前体。将其在四种温度(700、800、900、950℃)下热解,获得不同形貌的Ni、Co及N掺杂的MOFs衍生碳材料Ni/Co-CN,并探究了煅烧温度对其储能性能的影响。结果表明,随着煅烧温度升高,Ni/Co-CN逐渐由多孔碳变为碳纳米管桥连多孔碳结构。当热解温度为900℃时,Ni/Co-CN-900的比电容最大。在1 mol/L的KOH电解液中对其进行循环伏安测试,曲线对称性良好,表明其具有优异的电化学可逆性。通过计算该过程电荷存储的电容贡献和扩散贡献占比可知,Ni/Co-CN的储能主要来自多孔碳的双电层吸附,少量来自N掺杂导致的法拉第反应。在0.5 A/g的电流密度下,Ni/Co-CN-900的比电容高达273.5 F/g。在10.0 A/g的电流密度下进行5000次恒流充放电后,其比电容保持率高达93.8%,展现出良好的电化学性能。