Advances in piezoelectric thin films for acoustic biosensors,acoustofluidics and lab-on-chip applications

Recently, piezoelectric thin films including zinc oxide (ZnO) and aluminium nitride (AlN) have found a broad range of lab-on-chip applications such as biosensing, particle/cell concentrating, sorting/patterning, pumping, mixing, nebulisation and jetting. Integrated acoustic wave sensing/microfluidic devices have been fabricated by depositing these piezoelectric films onto a number of substrates such as silicon, ceramics, diamond, quartz, glass, and more recently also polymer, metallic foils and bendable glass/silicon for making flexible devices. Such thin film acoustic wave devices have great potential for implementing integrated, disposable, or bendable/flexible lab-on-a-chip devices into various sensing and actuating applications. This paper discusses the recent development in engineering high performance piezoelectric thin films, and highlights the critical issues such as film deposition, MEMS processing techniques, control of deposition/processing parametres, film texture, doping, dispersion effects, film stress, multilayer design, electrode materials/designs and substrate selections. Finally, advances in using thin film devices for lab-on-chip applications are summarised and future development trends are identified.     

Synergistic effects from graphene and carbon nanotubes enable flexible and robust electrodes for high-performance supercapacitors

Flexible and lightweight energy storage systems have received tremendous interest recently due to their potential applications in wearable electronics, roll-up displays, and other devices. To manufacture such systems, flexible electrodes with desired mechanical and electrochemical properties are critical. Herein we present a novel method to fabricate conductive, highly flexible, and robust film supercapacitor electrodes based on graphene/MnO(2)/CNTs nanocomposites. The synergistic effects from graphene, CNTs, and MnO(2) deliver outstanding mechanical properties (tensile strength of 48 MPa) and superior electrochemical activity that were not achieved by any of these components alone. These flexible electrodes allow highly active material loading (71 wt % MnO(2)), areal density (8.80 mg/cm(2)), and high specific capacitance (372 F/g) with excellent rate capability for supercapacitors without the need of current collectors and binders. The film can also be wound around 0.5 mm diameter rods for fabricating full cells with high performance, showing significant potential in flexible energy storage devices.     

Development of 3D printing technology for the manufacture of flexible electric double-layer capacitors

This study presents a novel process and manufacturing system for the fabrication of Electric Double-Layer Capacitors (EDLCs) as energy storage devices. It shows an approach for printing multilayer EDLC components using 3D printing technology. This process allows layers of activated carbon (AC) slurry, gel electrolyte, and composite solid filaments to be printed with high precision. The study describes the detailed process of deposition of the AC and gel electrolyte using the dual nozzle system. The performance of the flexible EDLCs manufactured by 3D printing in a rectilinear infill pattern has been investigated. It describes the energy storage performance of the printed supercapacitors in relation to the differences in thickness of the AC printed layers and the differences in density of gel electrolyte. A supercapacitor based on printed AC and composite materials displays a specific capacitance of 38.5?mF?g^?1 when measured at a potential rate change of 20?mV?s^?1 and a current density of 0.136?A?g^?1. The highest energy density value for the flexible EDLC was 0.019?Wh?kg^?1 and power density of 165.0?W?kg^?1 in 1.6?M H₂SO₄/PVA gel electrolyte.     

Fabrication of large area hexagonal boron nitride thin films for bendable capacitors

Highly reliable and bendable dielectrics are desired in flexible or bendable electronic devices for future applications. Hexagonal boron nitride （h-BN） can be used as bendable dielectric due to its wide band gap. Here, we fabricate high quality h-BN films with controllable thickness by a low pressure chemical vapor deposition method. We demonstrate a parallel-plate capacitor using h-BN film as the dielectric. The h-BN capacitors are reliable with a high breakdown field strength of -9.0 MV/cm. Tunneling current across the h-BN film is inversely exponential to the thickness of dielectric, which makes the capacitance drop significantly. The h-BN capacitor shows a best specific capacitance of 6.8 F/cm^2, which is one order of magnitude higher than the calculated value.     

High-performance asymmetric supercapacitors based on reduced graphene oxide/polyaniline composite electrodes with sandwich-like structure

The sandwich-like structure of reduced graphene oxide/polyaniline(RGO/PANI) hybrid electrode was prepared by electrochemical deposition. Both the voltage windows and electrolytes for electrochemical deposition of PANI and RGO were optimized. In the composites, PANI nanofibers were anchored on the surface of the RGO sheets, which avoids the re-stacking of neighboring sheets. The RGO/PANI composite electrode shows a high specific capacitance of 466 F/g at 2 m A/cm~2 than that of previously reported RGO/PANI composites. Asymmetric flexible supercapacitors applying RGO/PANI as positive electrode and carbon fiber cloth as negative electrode can be cycled reversibly in the high-voltage region of 0–1.6 V and displays intriguing performance with a maximum specific capacitance of 35.5 m F cm~(-2). Also, it delivers a high energy density of 45.5 m W h cm~(-2) at power density of 1250 m W cm~(-2). Furthermore, the asymmetric device exhibits an excellent long cycle life with 97.6% initial capacitance retention after 5000 cycles.Such composite electrode has a great potential for applications in flexible electronics, roll-up display,and wearable devices.     

Bacterial cellulose membranes coated by polypyrrole/copper oxide as flexible supercapacitor electrodes

The developments of flexible supercapacitors are of great importance to the growing demand of portable electronic products. In the present work, we have successfully prepared bacterial cellulose (BC) membranes coated by polypyrrole (PPy) and copper oxide (CuO) as flexible composite electrodes for supercapacitor applications. The highest electrical conductivity value of 7.4 S cm^?1 was achieved using copper acetate aqueous solution with concentration of 1 wt%. Electrochemical measurements proved that the supercapacitors using the PPy/CuO/BC electrodes had a specific capacitance of 601 F g^?1 with an energy density of 48.2 Wh kg^?1 and a power density of 85.8 W kg^?1 at a current density of 0.8 mA cm^?2. The specific capacitance was kept at 385 F g^?1 after 300 cycles. The introduction of the CuO nanoparticles gave rise to the improved capacitance.     

Flexible Zn2SnO4/MnO2 core/shell nanocable-carbon microfiber hybrid composites for high-performance supercapacitor electrodes

We demonstrate the design and fabrication of a novel flexible nanoarchitecture by facile coating ultrathin (several nanometers thick) films of MnO2 to highly electrical conductive Zn2SnO4 (ZTO) nanowires grown radially on carbon microfibers (CMFs) to achieve high specific capacitance, high-energy density, high-power density, and long-term life for supercapacitor electrode applications. The crystalline ZTO nanowires grown on CMFs were uniquely served as highly conductive cores to support a highly electrolytic accessible surface area of redox active MnO2 shells and also provide reliable electrical connections to the MnO2 shells. The maximum specific capacitances of 621.6 F/g (based on pristine MnO2) by cyclic voltammetry (CV) at a scan rate of 2 mV/s and 642.4 F/g by chronopotentiometry at a current density of 1 A/g were achieved in 1 M Na2SO4 aqueous solution. The hybrid MnO2/ZTO/CMF hybrid composite also exhibited excellent rate capability with specific energy of 36.8 Wh/kg and specific power of 32 kW/kg at current density of 40 A/g, respectively, and good long-term cycling stability (only 1.2% loss of its initial specific capacitance after 1000 cycles). These results suggest that such MnO2/ZTO/CF hybrid composite architecture is very promising for next generation high-performance supercapacitors.     

添加致孔剂制备树脂基活性炭及电容性能研究

以碱性条件下合成的热固性酚醛树脂(PF)为原料,聚乙烯醇缩丁醛(PVB)和聚乙烯二醇(PEG)为致孔剂,采用聚合物共混炭化活化法制备双电层电容器用活性炭材料.通过热重(TG)分析探讨了PF,PF与PVB、PEG的共混物在炭化过程中的热解行为.考察了活化温度和活化时间对所得活性炭的收率、BET比表面积、孔径分布和比电容的影响,并进一步探讨了以这种活性炭材料作电极的双电层电容器的电容性能.结果表明,随着活化温度的升高,活化温度对活性炭收率的影响更为显著,所得活性炭的收率下降.聚合物PEG较PVB更适合作为成孔剂来控制活性炭的中孔孔径分布.酚醛树脂基活性炭电极比电容在850℃活化1 h为79.2F/g,而聚乙烯二醇/酚醛、聚乙烯醇缩丁醛/酚醛混合树脂基活性炭电极比电容则分别高达130.5和145.6F/g.     

Electrochemical characterization of carbon nanotube forests grown on copper foil using transition metal catalysts

Growth of vertical, multiwalled carbon nanotubes (CNTs) on bulk copper foil substrates can be achieved by sputtering either Ni or Inconel thin films on Cu substrates followed by thermal chemical vapor deposition using a xylene and ferrocene mixture. During CVD growth, Fe nanoparticles from the ferrocene act as a vapor phase delivered catalyst in addition to the transition metal thin film, which breaks up into islands. Both the thin film and iron are needed for dense and uniform growth of CNTs on the copper substrates. The benefits of this relatively simple and cost effective method of directly integrating CNTs with highly conductive copper substrates are the resulting high density of nanotubes that do not require the use of additional binders and the potential for low contact resistance between the nanotubes and the substrate. This method is therefore of interest for charge storage applications such as double layer capacitors. Inconel thin films in conjunction with Fe from ferrocene appear to work better in comparison to Ni thin films in terms of CNT density and charge storage capability. We report here the power density and specific capacitance values of the double layer capacitors developed from the CNTs grown directly on copper substrates.     

Hybrid Copper‐Nanowire–Reduced‐Graphene‐Oxide Coatings: A “Green Solution” Toward Highly Transparent,Highly Conductive,and Flexible Electrodes for (Opto)Electronics

This study reports a novel green chemistry approach to assemble copper‐nanowires/reduced‐graphene‐oxide hybrid coatings onto inorganic and organic supports. Such films are robust and combine sheet resistances (<30 Ω sq^?1) and transparencies in the visible region (transmittance > 70%) that are rivalling those of indium–tin oxide. These electrodes are suitable for flexible electronic applications as they show a sheet resistance change of <4% after 10 000 bending cycles at a bending radius of 1.0 cm, when supported on polyethylene terephthalate foils. Significantly, the wet‐chemistry method involves the preparation of dispersions in environmentally friendly solvents and avoids the use of harmful reagents. Such inks are processed at room temperature on a wide variety of surfaces by spray coating. As a proof‐of‐concept, this study demonstrates the successful use of such coatings as electrodes in high‐performance electrochromic devices. The robustness of the electrodes is demonstrated by performing several tens of thousands of cycles of device operation. These unique conducting coatings hold potential for being exploited as transparent electrodes in numerous optoelectronic applications such as solar cells, light‐emitting diodes, and displays.     

炭化温度对酚醛树脂基活性炭孔结构及电化学性能的影响研究

以酚醛树脂为炭前驱体,KOH作活化剂,通过调节炭化温度在相同活化条件下制备了具有不同孔隙结构的活性炭材料.N_2吸附测试表明随着炭化温度降低,活性炭材料比表面积先增大后减小,孔容则不断增大.其中,550℃炭化样品与KOH反应活性最佳,可制得比表面积为2983m~2/g,总孔容为1.58cm~3/g,中孔孔容达到0.59cm~3/g的活性炭材料.采用直流充放电法、交流阻抗法和循环伏安法测定以上述多孔炭为电极材料的双电层电容器的电化学性能,结果表明,PF550活性炭材料电容性能最佳,在有机电解液中100mA/g充放电时,比电容达到160F/g,电流密度增大50倍容量保持率达到82%,显示出良好的功率特性;活性炭材料中存在一定比例的中孔不仅可以改善电极材料的功率特性,而且可以提高微孔的利用率.     

Fabrication of Transistors on Flexible Substrates: from Mass‐Printing to High‐Resolution Alternative Lithography Strategies

In this report, the development of conventional, mass‐printing strategies into high‐resolution, alternative patterning techniques is reviewed with the focus on large‐area patterning of flexible thin‐film transistors (TFTs) for display applications. In the first part, conventional and digital printing techniques are introduced and categorized as far as their development is relevant for this application area. The limitations of conventional printing guides the reader to the second part of the progress report: alternative‐lithographic patterning on low‐cost flexible foils for the fabrication of flexible TFTs. Soft and nanoimprint lithography‐based patterning techniques and their limitations are surveyed with respect to patterning on low‐cost flexible foils. These show a shift from fabricating simple microlense structures to more complicated, high‐resolution electronic devices. The development of alternative, low‐temperature processable materials and the introduction of high‐resolution patterning strategies will lead to the low‐cost, self‐aligned fabrication of flexible displays and solar cells from cheaper but better performing organic materials.     

A high-performance supercapacitor electrode based on three-dimensional poly-rowed copper hydroxide nanorods on copper foam

Three-dimensional (3D) poly-rowed copper hydroxide [Cu(OH)₂] nanorods nanostructures have been synthesized on copper foam slices, using copper foam as copper source, current collector and 3D substrate, through a facile and scalable one-step anodization method. The synthesized products can be directly used as binder free electrodes of supercapacitors. The synthesized material exhibited a 3D connect and quasi connect network structure which can facilitate ion mobility and increase the amount of active sites for redox reactions. These merits together lead to a high areal specific capacitance of 1.889 F cm^?2 at a scan rate of 2 mV s^?1, excellent rate capability (75.39% retention upon increasing the current density by 15 times), low intrinsic resistance (0.792 Ω cm^?2) and good cycling stability (87.23% capacitance retention after 5000 cycles), demonstrating a good potential for high performance energy storage devices applications.     

基于MoS2/SiO2范德华异质结的VO2薄膜转移打印研究

近年来, 柔性电子器件由于在物联网、生物电子等领域的潜在应用引起了研究者的广泛关注。功能氧化物材料在柔性聚合物中的集成已被证明是实现高性能柔性电子器件的有效方式。由于功能氧化物薄膜通常需要高温制备, 直接在柔性聚合物基底上合成高质量的氧化物薄膜仍然是一个巨大的挑战。本研究提出了一种基于MoS₂/SiO₂范德华异质结转移打印大面积VO₂薄膜的方法, 即利用MoS₂和SiO₂薄膜亲疏水性能的不同, 可以仅使用去离子水解离MoS₂/SiO₂范德华异质结界面, 成功将Si/SiO₂/MoS₂/SiO₂/VO₂ 多层膜结构上的VO₂薄膜转印到Si、SiO₂/Si以及柔性基底上。X射线衍射(XRD)结果显示, 转印前后VO₂薄膜的晶体结构没有差异, 变温Raman光谱和变温红外反射光谱证明了转印前后VO₂薄膜良好的金属-绝缘体转变性能。本研究提供了一种有效的功能氧化物薄膜转印方法, 在不引入牺牲层和腐蚀性溶剂的条件下, 实现了VO₂薄膜在任意基底上的低温集成, 为柔性可穿戴电子器件的研制提供了一种新思路。     

基于四羟甲基甘脲前驱体的新型含氮富微孔活性炭制备及其性能研究

以四羟甲基甘脲为碳前驱体,采用KOH活化法处理得到双电层电容器用多孔活性炭材料。考察了不同碳化、活化温度对活性炭比电容的影响。结果表明,在850℃碳化,650℃活化处理时其电容性能最好,SEM和比表面与孔径分布测试说明TA-850-650表面富集微孔;恒流充放电与循环伏安测试结果表明TA-850-650的比电容在电流密度为0.2A/g时可达527F/g。     

碳布/聚吡咯储能包装膜材料的制备及表征

目的以甲壳素纳米纤维、多壁碳纳米管、碳布、吡咯为原料,制备柔性超级电容器复合电极薄膜。方法先利用化学氧化法提高碳布的表面粗糙度,再通过真空抽滤在碳布表面附着甲壳素纳米纤维和多壁碳纳米管,以增加碳布的负载空间,最后通过原位聚合吡咯来增加复合薄膜的电容性能。同时制备氧化碳布/聚吡咯复合薄膜作为对照组。结果制成的氧化碳布/甲壳素纳米纤维/多壁碳纳米管/聚吡咯复合薄膜在扫描速率为5 mV/s时,质量比电容达到了307 F/g,是氧化碳布/聚吡咯质量比电容(175 F/g)的1.75倍;在电流密度为2 A/g时,经过2000次循环后电容保留率为72.3%,库仑效率为73.8%。结论制备的氧化碳布/甲壳素纳米纤维/多壁碳纳米管/聚吡咯薄膜具有较高的比电容和循环稳定性,可以作为超级电容器电极材料应用于物联网行业的有源储能包装。     

Free‐Standing,Multilayered Graphene/Polyaniline‐Glue/Graphene Nanostructures for Flexible,Solid‐State Electrochemical Capacitor Application

Graphene/polyaniline multilayered nanostructures (GPMNs) are prepared using a straightforward process through which graphite is physically exfoliated with quaternary polyaniline (PANI)‐glue. This is only accomplished by sonication of the graphite flakes in an organic solvent to form continuous films with PANI. During the sonication, the conductive PANI‐glue is spontaneously intercalated between the graphene sheet layers without deterioration of the sp² hybridized bonding structure. The resultant free‐standing, flexible films are composed of a network of overlapping graphene sheets and are shown to have a long‐range structure. The effects of different PANI content ratios and different interfacial energies (depending on the dispersion solvent) on the morphology and properties of the resulting GPMN are examined. It is found that GPMNs dispersed in water have a maximum specific capacitance of 390 F g⁻¹ in a three‐electrode configuration. Importantly, the unique structural design of GPMNs enables their use as electrode materials for the fabrication of flexible, solid‐state electrochemical capacitors, which show an enhanced performance compared to graphene‐only devices. They exhibit a high specific capacitance of 200 F g⁻¹, a cycling stability with capacitance retention of 82% after 5000 charge/discharge cycles, and, moreover, superior flexibility.     

Mesoporous activated carbon as electrode for electric double layer capacitor

Mesoporous activated carbon with high surface area (AC-Y-A) was prepared by carbonization followed by activation of vinylidene chloride copolymer containing yttrium acetylacetonate. Their performances as electrodes for electric double layer capacitor (EDLC) in tetraalkylammonium tetrafluoroborate (R₄NBF₄)/propylene carbonate (PC) or acetonitrile (AN) solutions were evaluated and compared with those of microporous activated carbons [AC and AC(CO₂)]. The specific EDLC capacitances of the mesoporous AC-Y-A are almost the same as those of microporous AC at low discharge current. However, at high discharge current, the specific capacitance of AC-Y-A is much higher than those of both AC and AC(CO₂) in PC and AN solutions. The capacitances decreased with increasing alkyl chain length of R₄ N⁺ ions and discharge current. These results suggest that EDLC capacitance depends on pore size of activated carbons and smooth movement of R₄ N⁺ ions in pores.     

Conductive polypyrrole incorporated nanocellulose/MoS2 film for preparing flexible supercapacitor electrodes

Conductive films have emerged as appealing electrode materials in flexible supercapacitors owing to their conductivity and mechanical flexibility. However, the unsatisfactory electrode structure induced poor output performance and undesirable cycling stability limited their application. Herein, a well-designed film was manufactured by the vacuum filtration and in-situ polymerization method from cellulose nanofibrils (CNFs), molybdenum disulfide (MoS₂), and polypyrrole. The electrode presented an outstanding mechanical strength (21.3 MPa) and electrical conductivity (9.70 S·cm⁻¹). Meanwhile, the introduce of hydrophilic CNFs induced a desirable increase in diffusion path of electrons and ions, along with the synergistic effect among the three components, further endowed the electrode with excellent specific capacitance (0.734 F·cm⁻²) and good cycling stability (84.50% after 2000 charge/discharge cycles). More importantly, the flexible all-solid-state symmetric supercapacitor delivered a high specific capacitance (1.39 F·cm⁻² at 1 mA·cm⁻²) and a volumetric energy density (6.36 mW·h·cm⁻³ at the power density of 16.35 mW·cm⁻³). This work provided a method for preparing composite films with desired mechanical and electrochemical performance, which can broaden the high-value applications of nanocellulose.     

Inorganic Semiconductors for Flexible Electronics

This article reviews several classes of inorganic semiconductor materials that can be used to form high‐performance thin‐film transistors (TFTs) for large area, flexible electronics. Examples ranging from thin films of various forms of silicon to nanoparticles and nanowires of compound semiconductors are presented, with an emphasis on methods of depositing and integrating thin films of these materials into devices. Performance characteristics, including both electrical and mechanical behavior, for isolated transistors as well as circuits with various levels of complexity are reviewed. Collectively, the results suggest that flexible or printable inorganic materials may be attractive for a range of applications not only in flexible but also in large‐area electronics, from existing devices such as flat‐panel displays to more challenging (in terms of both cost and performance requirements) systems such as large area radiofrequency communication devices, structural health monitors, and conformal X‐ray imagers.