Carbon Micro/Nano Machining toward Miniaturized Device: Structural Engineering, Large-Scale Fabrication, and Performance Optimization

With the rapid development of micro/nano machining, there is an elevated demand for high-performance microdevices with high reliability and low cost. Due to their outstanding electrochemical, optical, electrical, and mechanical performance, carbon materials are extensively utilized in constructing microdevices for energy storage, sensing, and optoelectronics. Carbon micro/nano machining is fundamental in carbon-based intelligent microelectronics, multifunctional integrated microsystems, high-reliability portable/wearable consumer electronics, and portable medical diagnostic systems. Despite numerous reviews on carbon materials, a comprehensive overview is lacking that systematically encapsulates the development of high-performance microdevices based on carbon micro/nano structures, from structural design to manufacturing strategies and specific applications. This review focuses on the latest progress in carbon micro/nano machining toward miniaturized device, including structural engineering, large-scale fabrication, and performance optimization. Especially, the review targets an in-depth evaluation of carbon-based micro energy storage devices, microsensors, microactuators, miniaturized photoresponsive and electromagnetic interference shielding devices. Moreover, it highlights the challenges and opportunities in the large-scale manufacturing of carbon-based microdevices, aiming to spark further exciting research directions and application prospectives.

On-chip integration of bulk micromachined three-dimensional Si/C/CNT@TiC micro-supercapacitors for alternating current line filtering

Three-dimensional (3D) micro-supercapacitors (MSCs) with superior performances are desirable for miniaturized electronic devices. 3D interdigitated MSCs fabricated by bulk micromachining technologies have been demonstrated for silicon wafers. However, rational design and fabrication technologies of 3D architectures still need to be optimized within a limited footprint area to improve the electrochemical performances of MSCs. Herein, we report a 3D interdigitated MSC based on Si/C/CNT@TiC electrodes with high capacitive properties attributed to the excellent electronic/ionic conductivity of CNT@TiC core–shells with a high aspect ratio morphology. The symmetric MSC presents a maximum specific capacitance of 7.42 mF cm⁻² (3.71 F g⁻¹) at 5 mV s⁻¹, and shows an 8 times areal capacitance increment after material coating at each step, fully exploiting the advantage of 3D interdigits with a high aspect ratio. The all-solid-state MSC delivers a high energy density of 0.45 μW h cm⁻² (0.22 W h kg⁻¹) at a power density of 10.03 μW h cm⁻², and retains ∼98% capacity after 10 000 cycles. The MSC is further integrated on-chip in a low-pass filtering circuit, exhibiting a stable output voltage with a low ripple coefficient of 1.5%. It is believed that this work holds a great promise for metal-carbide-based 3D interdigitated MSCs for energy storage applications.

A novel fabrication strategy for the realization of a bulk micromachined 3D Si/C/CNT@TiC micro-supercapacitor is experimentally demonstrated.

Advances in wearable textile-based micro energy storage devices: structuring, application and perspective

The continuous expansion of smart microelectronics has put forward higher requirements for energy conversion, mechanical performance, and biocompatibility of micro-energy storage devices (MESDs). Unique porosity, superior flexibility and comfortable breathability make the textile-based structure a great potential in wearable MESDs. Herein, a timely and comprehensive review of this field is provided according to recent research advances. The following aspects, device construction of textile-based MESDs (TMESDs), fabric processing of textile components and smart functionalization (e.g., mechanical reliability, energy harvesting, sensing, self-charging and self-healing, etc.) are discussed and summarized thoroughly. Also, the perspectives on the microfabrication processes and multiple applications of TMESDs are elaborated.

Lipid nanotubes as an organic template for the fabrication of carbon nanostructures by pyrolysis

We demonstrate the fabrication of carbon nanoribbons with a width of 40 nm based on fixation and pyrolysis of an organic template, lipid nanotubes. To our best knowledge, this is the smallest feature size achieved by pyrolysis of surface-patterned organic templates. Such a pyrolytic carbon nanostructure can be used for electronics and sensing applications in future.

Silicon-Based 3D All-Solid-State Micro-Supercapacitor with Superior Performance

The large-scale fabrication of high-performance on-chip micro-supercapacitors (MSCs) is the footstone for the development of next-generation miniaturized electronic devices. In practical applications, however, MSCs may suffer from a low areal energy density as well as a complicated fabrication strategy that is incompatible with semiconductor processing technology. Herein, we propose a scalable fabrication strategy for the realization of a silicon-based three-dimensional (3D) all-solid-state MSC via the combination of semiconductor-based electrode processing, chemical vapor deposition, and hydrothermal growth. The individual Si/C/MnO₂ electrode shows a maximum specific capacitance of 223.74 mF cm^-2, while the symmetric electrodes present a maximum areal energy density of 5.01 μWh cm^-2 at the scan rate of 1 mV s^-1. The full 3D Si/C/MnO₂ MSC delivers a high energy density of 2.62 μWh cm^-2, at a power density of 117.82 μW cm^-2, as well as a long cycle life with capacitance retention >92% after 4000 cycles. Our proposed method enables the fabrication of 3D MSCs based on a thick silicon interdigitated electrode array, holding a great promise for the development of 3D on-chip microscale energy storage devices.

Surface Engineering of Carbon-Based Microelectrodes for High-Performance Microsupercapacitors

In this research, the enhancement in electrochemical performance of pyrolyzed carbon microelectrodes by surface modification is investigated. For the proposed microfabrication process, pyrolyzed carbon microelectrodes with multi-walled carbon nanotubes (MWCNTs) on their surface are obtained by developing GM-1060 photoresist in mixture of propylene glycol methyl ether acetate (PGMEA) and CNTs, and following pyrolysis of a micropatterned photoresist. Polyvinyl alcohol (PVA)/H2SO4 electrolyte (1 M) was applied to assemble this carbon/CNT microelectrode-based all-solid-state microsupercapacitor (carbon/CNT-MSC). The carbon/CNT-MSC shows a higher electrochemical performance compared with that of pyrolyzed carbon microelectrode-based MSC (carbon-MSC). The specific areal and volumetric capacitances of carbon/CNT-MSC (4.80 mF/cm2 and 32.0 F/cm3) are higher than those of carbon-MSC (3.52 mF/cm2 and 23.4 F/cm3) at the scan rate of 10 mV/s. In addition, higher energy density and power density of carbon/CNT-MSC (2.85 mWh/cm3 and 1.98 W/cm3) than those of carbon-MSC (2.08 mWh/cm3 and 1.41 W/cm3) were also achieved. This facile surface modification and optimization are potentially promising, being highly compatible with modern microfabrication technologies and allowing integration of highly electrically conductive CNTs into pyrolyzed carbon to assemble MSCs with improved electrochemical performance. Moreover, this method can be potentially applied to other high-performance micro/nanostructures and microdevices/systems.

In Situ Growth of the Ni3V2O8@PANI Composite Electrode for Flexible and Transparent Symmetric Supercapacitors

Because of the poor specific capacitance of transparent and flexible supercapacitors reported recently, exploring electrode materials with high electrochemical properties for such devices is still a big challenge. We reported that the Ni₃V₂O₈@PANI composite has been synthesized using an in situ chemical bath method. It was found that the synthesized Ni₃V₂O₈@PANI composite has outstanding electrochemical behaviors (specific capacitance value of 2565.7 F/g at 5 mV/s, wide potential window, good rate capability), which are much superior to those of Ni₃V₂O₈ and PANI electrodes. The improved electrochemical behaviors result from the synergistic effect between Ni₃V₂O₈ and PANI. A symmetric flexible and transparent supercapacitor was fabricated using the Ni₃V₂O₈@PANI composite as the working electrode. The device demonstrated a maximum areal capacitance of 58.5 mF/cm² at 5 mV/s and an energy density of 20.8 μW h/cm² with 1.6 V potential window. Furthermore, the capacitance retained nearly 88% of its original value after 20 000 galvanostatic charging and discharging cycles. These results favor the promising potential of the Ni₃V₂O₈@PANI composite as the electrode material for the application in flexible and transparent supercapacitors.

In Situ Synthesis of Vertical Standing Nanosized NiO Encapsulated in Graphene as Electrodes for High-Performance Supercapacitors

NiO is a promising electrode material for supercapacitors. Herein, the novel vertically standing nanosized NiO encapsulated in graphene layers (G@NiO) are rationally designed and synthesized as nanosheet arrays. This unique vertical standing structure of G@NiO nanosheet arrays can enlarge the accessible surface area with electrolytes, and has the benefits of short ion diffusion path and good charge transport. Further, an interconnected graphene conductive network acts as binder to encapsulate the nanosized NiO particles as core-shell structure, which can promote the charge transport and maintain the structural stability. Consequently, the optimized G@NiO hybrid electrodes exhibit a remarkably enhanced specific capacity up to 1073 C g-1 and excellent cycling stability. This study provides a facial strategy to design and construct high-performance metal oxides for energy storage.

One-pot synthesis of iron–nickel–selenide nanorods for efficient and durable electrochemical oxygen evolution

Fe–Ni–Se nanorods have been synthesized by a facile one-pot method and they acted as efficient and durable OER catalysts.

Rapid, all dry microfabrication of three-dimensional Co3O4/Pt nanonetworks for high-performance microsupercapacitors

On-chip electrochemical energy storage devices have attracted growing attention due to the decreasing size of electronic devices. Various approaches have been applied for constructing the microsupercapacitors. However, the microfabrication of high-performance microsupercapacitors by conventional and fully compatible semiconductor microfabrication technologies is still a critical challenge. Herein, unique three-dimensional (3D) Co₃O₄ nanonetwork microelectrodes formed by the interconnection of Co₃O₄ nanosheets are constructed by controllable physical vapor deposition combined with rapid thermal annealing. This construction process is an all dry and rapid (≤5 minutes) procedure. Afterward, by sputtering highly electrically conductive Pt nanoparticles on the microelectrodes, the 3D Co₃O₄/Pt nanonetworks based microsupercapacitor is fabricated, showing a high volume capacitance (35.7 F cm^-3) at a scan rate of 20 mV s^-1 due to the unique interconnected structures, high electrical conductivity and high surface area of the microelectrodes. This microfabrication process is also used to construct high-performance flexible microsupercapacitors, and it can be applied in the construction of wearable devices. The proposed strategy is completely compatible with the current semiconductor microfabrication and shows great potential in the applications of the large-scale integration of micro/nano and wearable devices.

Carbon-MEMS-Based Alternating Stacked MoS2 @rGO-CNT Micro-Supercapacitor with High Capacitance and Energy Density

A novel process to fabricate a carbon-microelectromechanical-system-based alternating stacked MoS₂ @rGO-carbon-nanotube (CNT) micro-supercapacitor (MSC) is reported. The MSC is fabricated by successively repeated spin-coating of MoS₂ @rGO/photoresist and CNT/photoresist composites twice, followed by photoetching, developing, and pyrolysis. MoS₂ @rGO and CNTs are embedded in the carbon microelectrodes, which cooperatively enhance the performance of the MSC. The fabricated MSC exhibits a high areal capacitance of 13.7 mF cm^-2 and an energy density of 1.9 µWh cm^-2 (5.6 mWh cm^-3 ), which exceed many reported carbon- and MoS₂ -based MSCs. The MSC also retains 68% of capacitance at a current density of 2 mA cm^-2 (5.9 A cm^-3 ) and an outstanding cycling performance (96.6% after 10 000 cycles, at a scan rate of 1 V s^-1 ). Compared with other MSCs, the MSC in this study is fabricated by a low-cost and facile process, and it achieves an excellent and stable electrochemical performance. This approach could be highly promising for applications in integration of micro/nanostructures into microdevices/systems.

Polynanocrystalline Graphite: A New Carbon Anode with Superior Cycling Performance for K-Ion Batteries

We synthesized a new type of carbon-polynanocrystalline graphite-by chemical vapor deposition on a nanoporous graphenic carbon as an epitaxial template. This carbon is composed of nanodomains being highly graphitic along c-axis and very graphenic along ab plane directions, where the nanodomains are randomly packed to form micron-sized particles, thus forming a polynanocrystalline structure. The polynanocrystalline graphite is very unique, structurally different from low-dimensional nanocrystalline carbon materials, e.g., fullerenes, carbon nanotubes, and graphene, nanoporous carbon, amorphous carbon and graphite, where it has a relatively low specific surface area of 91 m²/g as well as a low Archimedes density of 0.92 g/cm³. The structure is essentially hollow to a certain extent with randomly arranged nanosized graphite building blocks. This novel structure with disorder at nanometric scales but strict order at atomic scales enables substantially superior long-term cycling life for K-ion storage as an anode, where it exhibits 50% capacity retention over 240 cycles, whereas for graphite, it is only 6% retention over 140 cycles.