Substrate-Versatile Direct-Write Printing of Carbon Nanotube-Based Flexible Conductors,Circuits, and Sensors

Manufacturing of printed electronics relies on the deposition of conductive liquid inks, typically onto polymeric or paper substrates. Among available conductive fillers for use in electronic inks, carbon nanotubes (CNTs) have high conductivity, low density, processability at low temperatures, and intrinsic mechanical flexibility. However, the electrical conductivity of printed CNT structures has been limited by CNT quality and concentration, and by the need for nonconductive modifiers to make the ink stable and extrudable. This study introduces a polymer-free, printable aqueous CNT ink, and, via an ambient direct-write printing process, presents the relationships between printing resolution, ink rheology, and ink-substrate interactions. A model is constructed to predict printed feature sizes on impermeable substrates based on Wenzel wetting. Printed lines have conductivity up to 10 000 S m⁻¹. The lines are flexible, with <5% change in DC resistance after 1000 bending cycles, and <3% change in DC resistance with a bending radius down to 1 mm. Demonstrations focus on i) conformality, via printing CNTs onto stickers that can be applied to curved surfaces, ii) interactivity using a CNT-based button printed onto folded paper structure, and iii) capacitive sensing of liquid wicking into the substrate itself. Facile integration of surface mount components on printed circuits is enabled by the intrinsic adhesion of the wet ink.     

自还原体系银导电墨水的制备与低温印制导电图形的研究

总结了开发一种简便快捷的可印制在挠性基板上于低温条件实现烧结形成高导电性线路的自还原型银导电墨水的制备方法。该导电墨水通过混合二乙醇胺溶液和银氨溶液制备得到。印制在塑料基材上后,在75℃下固化形成导电线路。这是因为在碱性环境与高于50℃的温度下,二乙醇胺可分解生成甲醛并自发地与银氨溶液发生反应。随后还原出银原子并让其吸附在基材上形成银薄膜。用该方法印制得到的银线其导电率可达到金属银导电率的20％,通过继续改进可应用于各种印制电子技术中。     

Streptavidin‐Functionalized Silver‐Nanoparticle‐Enriched Carbon Nanotube Tag for Ultrasensitive Multiplexed Detection of Tumor Markers

A streptavidin‐functionalized silver‐nanoparticle‐enriched carbon nanotube (CNT/Ag NP) is designed as trace tag for ultrasensitive multiplexed measurements of tumor markers using a disposable immunosensor array. The CNT/Ag NP nanohybrid is prepared by one‐pot in situ deposition of Ag NPs on carboxylated CNTs. The nanohybrid is functionalized with streptavidin via the inherent interaction between the protein and Ag NPs for further linkage of biotinylated signal antibodies to obtain tagged antibodies. The functionalization process greatly improves the dispersibility of the nanohybrid in water. The immunosensor array is prepared by covalently immobilizing capture antibodies on chitosan‐modified screen‐printed carbon electrodes. Through a sandwich‐type immunoreaction on the immunosensor array, numerous Ag NPs are captured onto every single immunocomplex and are further amplified by a subsequent Ag NP‐promoted deposition of silver from a silver enhancer solution to obtain the sensitive electrochemical‐stripping signal of the Ag NPs. Using carcinoembryonic antigen and α‐fetoprotein as model analytes, this proposed multiplexed immunoassay method shows acceptable precision and wide linear ranges over four orders of magnitude with detection limits down to 0.093 and 0.061 pg mL^?1, respectively. The assay results of serum samples with the proposed method are in acceptable agreement with the reference values. The newly designed strategy and the functionalized tag avoid cross‐talk and the requirement of deoxygenation for electrochemical immunoassay, and thus provide a promising potential in clinical application.     

A Pyrenylpropyl Phosphonic Acid Surface Modifier for Mitigating the Thermal Resistance of Carbon Nanotube Contacts

Efforts to utilize the high intrinsic thermal conductivity of carbon nanotubes (CNTs) for thermal transport applications, namely for thermal interface materials (TIMs), have been encumbered by the presence of high thermal contact resistances between the CNTs and connecting materials. Here, a pyrenylpropyl‐phosphonic acid surface modifier is synthesized and applied in a straight forward and repeatable approach to reduce the thermal contact resistance between CNTs and metal oxide surfaces. When used to bond nominally vertically aligned multi‐walled CNT forests to Cu oxide surfaces, the modifier facilitates a roughly 9‐fold reduction in the thermal contact resistance over dry contact, enabling CNT‐based TIMs with thermal resistances of 4.6 ± 0.5 mm² K W^?1, comparable to conventional metallic solders. Additional experimental characterization of the modifier suggests that it may be used to reduce the electrical resistance of CNT‐metal oxide contacts by similar orders of magnitude.     

高精度数码喷墨打印技术及在印刷电子上的应用

文章介绍了高精度数码喷墨打印技术的设备,材料和打印工艺及其在印刷电子上的应用。重点介绍了纳米银墨水的结构、性能、烧结条件和电性能以及打印性能及其在制备导电线路上的应用,探讨了喷头孔径及基材的表面性能对打印线路的影响。最后,介绍了挠性PCB的打印。     

Influence of Thiol Self‐Assembled Monolayer Processing on Bottom‐Contact Thin‐Film Transistors Based on n‐Type Organic Semiconductors

The performance of bottom‐contact thin‐film transistor (TFT) structures lags behind that of top‐contact structures owing to the far greater contact resistance. The major sources of the contact resistance in bottom‐contact TFTs are believed to reflect a combination of non‐optimal semiconductor growth morphology on the metallic contact surface and the limited available charge injection area versus top‐contact geometries. As a part of an effort to understand the sources of high charge injection barriers in n‐channel TFTs, the influence of thiol metal contact treatment on the molecular‐level structures of such interfaces is investigated using hexamethyldisilazane (HMDS)‐treated SiO₂ gate dielectrics. The focus is on the self‐assembled monolayer (SAM) contact surface treatment methods for bottom‐contact TFTs based on two archetypical n‐type semiconductors, α,ω‐diperfluorohexylquarterthiophene (DFH‐4T) and N,N′bis(n‐octyl)‐dicyanoperylene‐3,4:9,10‐bis(dicarboximide) (PDI‐8CN₂). TFT performance can be greatly enhanced, to the level of the top contact device performance in terms of mobility, on/off ratio, and contact resistance. To analyze the molecular‐level film structural changes arising from the contact surface treatment, surface morphologies are characterized by atomic force microscopy (AFM) and scanning tunneling microscopy (STM). The high‐resolution STM images show that the growth orientation of the semiconductor molecules at the gold/SAM/semiconductor interface preserves the molecular long axis orientation along the substrate normal. As a result, the film microstructure is well‐organized for charge transport in the interfacial region.     

Carbon nanotubes and silver flakes filled epoxy resin for new hybrid conductive adhesives

Combining conductive micro and nanofillers is a new way to improve electrical conductivity. Micrometric silver flakes and nanometric carbon nanotubes (CNTs) exhibit high electrical conductivity. A new type of hybrid conductive adhesives filled with silver flakes and carbon nanotubes (DWCNTs or MWCNTs) were investigated. High electrical conductivity is measured as well as improved mechanical properties at room temperature. Small agglomerates and free MWCNTs dispersed in the silver/epoxy composites improve the electrical conductivity and a synergistic effect between MWCNTs and micro sized silver flakes is observed in hybrid composites. Glassy and rubbery storage moduli of the hybrid composites increase with increasing silver loading at fixed CNTs volume fraction. High value of the storage modulus, measured in DWCNTs/μAg hybrid composites at rubbery state, is caused by strong agglomeration of DWCNTs bundles. The electrical and mechanical properties are consistent with the morphologies of the hybrid composites characterized by SEM.     

Enhanced field emission from printed CNTs by high-temperature sintering and plasma bombarding in hydrogen

A novel post-treatment, including high-temperature sintering and plasma bombarding in hydrogen ambient, was performed intentionally to improve the field emission capability and stability of screen-printed carbon nanotubes (CNTs). With this treatment, the organic bonding material covering on the CNTs was preferentially removed. And the CNTs were wetted and filled by silver at the interface between the CNTs and silver electrode so that a reliable ohmic contact was achieved. These effects improved the field emission capability and stability of CNT cathode. Moreover, the equal-high CNT emitters, after the removal of the excessively protruded CNT tips/or arches, would improve the luminescence uniformity. It is suggested that a high-temperature sintering and plasma bombarding in hydrogen may be a key technology for the commercialization of CNT field emission display.     

Electrically Addressable Hybrid Architectures of Zinc Oxide Nanowires Grown on Aligned Carbon Nanotubes

The fabrication and characterization of hybrid architectures of ZnO nanowires (ZNWs) grown on organized carbon nanotubes (CNTs), by a two‐step chemical vapor deposition (CVD) process involving CNT growth from a hydrocarbon source followed by ZNW growth using a Zn metal source, is reported. The ZNWs grow uniformly and radially from individual CNTs and CNT bundles, and the aligned morphology of the CNTs is not disturbed by the ZNW growth process. The nucleation and growth of ZnO crystals on CNTs are analyzed in relation to the classical vapor–solid mechanism. Importantly, the CNTs make uniform and distributed electrical contact to the ZNWs, with up to a 1000‐fold yield advantage over conventional ZNW growth on a flat substrate. Hybrid ZNW/CNT sheets are fabricated by scalable CVD, rolling, and printing methods; and their electrical properties, which are governed by transport through the anisotropic CNT network, are characterized. Functional interaction between the ZNWs and CNTs is demonstrated by photoconductive behavior and photocurrent generation of the hybrid material under UV illumination. There is significant future opportunity to extend these processing methods to fabricate other functional oxides on CNTs, and to build devices that harness the attractive properties of ZNWs and CNTs with high volumetric efficiency over large areas.     

Inkjet Process for Conductive Patterning on Textiles: Maintaining Inherent Stretchability and Breathability in Knit Structures

In this work, a novel technique of inkjet printing e‐textiles with particle free reactive silver inks on knit structures is developed. The inkjet‐printed e‐textiles are highly conductive, with a sheet resistance of 0.09 Ω sq^‐1, by means of controlling the number of print passes, annealing process, and textile structures. It is notable that the inkjet process allows textiles to maintain its inherent properties, including stretchability, flexibility, breathability, and fabric hand after printing process. This is achieved by formation of ultrathin silver layers surrounding individual fibers. The silver layers coated on fibers range from 250 nm to 2.5 µm, maintaining the size of interstices and flexibility of fibers. The annealing process, structure of fibers, and printed layers significantly influence the electrical conductivity of the patterned structures on textiles. Outstanding electrical conductivity and durability are demonstrated by optimizing print passes, controlling textile structures, and incorporating an in situ annealing process. The electrical resistance dependence on the strain rate of the textiles is examined, showing the ability to maintain electrical conductivity to retain light‐emitting diode use, stable more than 500 consecutive strain cycles. Most importantly, inkjet‐printed e‐textiles maintain their characteristic washability, breathability, and fabric hands for applications in wearable technology.     

Innovative scheme for selective carbon nanotubes integration in via structures

Thanks to their outstanding electrical properties [1] and [2], carbon nanotubes (CNTs) are promising candidate to replace Cu in advanced interconnects [3], [4], [5], [6], [7] and [8]. In damascene based CNT via integration scheme, CNTs growth occurs on the whole surface of the wafers: in vias, but also on top surfaces [5]. CNTs on top are subsequently removed by polishing. In this paper, an alternative integration scheme is proposed which avoids CNTs on top. Thanks to careful choice of top surface (TiN) and bottom electrode (doped silicon) materials, CNT growth occurs only in vias. Dense growth (6 × 10¹¹ CNTs/cm²) of small multi wall CNTs is achieved in vias over doped poly-silicon lines. Good encapsulation of CNTs is obtained with SACVD SiO₂ or ALD Al₂O₃ materials. Thanks to polishing of emerging CNTs, planarized CNT vias are obtained. Initial electrical measurements by conductive AFM show the conductivity of these CNT vias.     

A Unified View on Nanoscale Packing,Connectivity, and Conductivity of CNT Networks

The design of functional structures from primary building blocks requires a thorough understanding of how size, shape, and particle–particle interactions steer the assembly process. Specifically, for electrically conductive networks build from carbon nanotubes (CNTs) combining macroscopic characterization and simulations shows that the achievable conductivity is mainly governed by CNT aspect ratio, length dispersity and attractive interactions. However, a direct link between the actual 3D network topology that leads to the observed electrical conductivity has not been established yet due to a lack in nanoscale experimental approaches. Here it is shown experimentally for randomly packed (jammed) CNT networks that the CNT aspect ratio determines, as theoretically predicted, the contact number per CNT which in turn scales linearly with the resulting electrical conductivity of the CNT network. Furthermore, nanoscale packing density, contact areas, contact distribution in random and nonrandom configurations, and least resistance pathways are quantified. The results illustrate how complex nanoscale networks can be imaged and quantified in 3D to understand and model their functional properties in a bottom‐up fashion.     

Silver Surface Iodination for Enhancing the Conductivity of Conductive Composites

The electrical conductivity of a silver microflake‐filled conductive composites is dramatically improved after a filler surface treatment. By a simple iodine solution treatment, nonstoichiometric silver/silver iodide nanoislands form on the silver filler surface. Evidence of the decrease of surface silver oxide species is provided by TOF‐SIMS and the redox property of the nanoclusters is studied using cyclic voltammetry and TOF‐SIMS depth profile analyses. The redox property of the nanoclusters on silver flakes helps enhance the electrical conductivity of the conductive composites. The electrical resistivity of the improved conductive composites is measured by four‐point probe method; the reliability of the printed thin film resistors is evaluated by both the 85 °C/85% relative humidity moisture exposure and the −40 ∼ 125 °C thermal cycling exposure. The conductive composite printed radio frequency identification (RFID) antennas with 27.5 wt% of the modified silver flake content show comparable performance in the RFID tag read range versus copper foil antennas, and better than those commercial conductive adhesives that require much higher silver content (i.e., 80 wt%). This work suggests that a surface chemistry method can significantly reduce the percolation threshold of the loading level of the silver flakes and improve the electrical conductivity of an important printed electronic passive component.     

Electrophoretically‐Deposited Metal‐Decorated CNT Nanoforests with High Thermal/Electric Conductivity and Wettability Tunable from Hydrophilic to Superhydrophobic

A single‐step, room‐temperature, and scalable electrophoretic deposition process is reported to form nanocomposites on any electrically conductive surface with metal nanoparticle decorated carbon nanotubes (CNTs). The contact angles (CAs) can be easily tuned from ≈60° to 168° by varying the deposition voltage, while hydrophobicity and superhydrophobicity surprisingly arise from the hydrophilic CNTs being deposited. The relatively high voltage tends to vertically align CNTs during deposition, leading to architectural micro/nanoscale roughness on the surface. The combination of the multiscale roughness along with the low surface energy of hydrocarbon functional groups on the CNT surface has enabled facile wettability control, including the Petal and Lotus effects. Further, the relatively vertical orientation of the CNTs, without any coating, allows for current and heat transfer along their axis with superior conductivity. Similar behavior in terms of CA control is seen for all three divalent metal ions in the deposition solution (i.e., Cu²⁺, Ni²⁺, and Zn²⁺) that are used to charge the CNTs while eventually getting co‐deposited. This implies that this method could possibly be extended to other metals by selecting appropriate charging salt. A patterning technique is also demonstrated for facile fabrication of superhydrophobic CNT‐metal islands surrounded by hydrophilic CNT coating.     

Covalently Bonded Graphene–Carbon Nanotube Hybrid for High‐Performance Thermal Interfaces

The remarkable thermal properties of graphene and carbon nanotubes (CNTs) have been the subject of intensive investigations for the thermal management of integrated circuits. However, the small contact area of CNTs and the large anisotropic heat conduction of graphene have hindered their applications as effective thermal interface materials (TIMs). Here, a covalently bonded graphene–CNT (G‐CNT) hybrid is presented that multiplies the axial heat transfer capability of individual CNTs through their parallel arrangement, while at the same time it provides a large contact area for efficient heat extraction. Through computer simulations, it is demonstrated that the G‐CNT outperforms few‐layer graphene by more than 2 orders of magnitude for the c‐axis heat transfer, while its thermal resistance is 3 orders of magnitude lower than the state‐of‐the‐art TIMs. We show that heat can be removed from the G‐CNT by immersing it in a liquid. The heat transfer characteristics of G‐CNT suggest that it has the potential to revolutionize the design of high‐performance TIMs.     

Exploring the Performance Limit of Carbon Nanotube Network Film Field‐Effect Transistors for Digital Integrated Circuit Applications

Carbon nanotube (CNT) network thin film field‐effect transistors (TFTs), which used to be considered as low cost and low performance transistors for display driving or flexible electronics, have recently been used to construct digital integrated circuits (ICs). However, few studies have focused on exploring how optimal CNT TFTs can be achieved according to transistor standards in digital applications. In this work, sub‐micrometer TFTs based on high‐quality and high‐purity solution‐derived CNT films are fabricated and the potential performance restriction due to the switching‐off property of these transistors is explored. Specifically, subthreshold swing (SS) severely degrades upon scaling down the channel length or increasing the CNT density in TFTs, and a tradeoff between peak transconductance (g_m) and SS in CNT TFTs due to the random orientation distribution of CNTs has been observed in experiments and proven by theoretical simulations. A well‐designed balance between g_m and SS is necessary to build CNT TFTs with SS of 120 mV dec^?1 and g_m of 150 µS µm^?1 to meet device requirements in digital ICs powered by a supplied voltage, V_DD, lower than 2.0 V.     

Low‐Temperature‐Processed Printed Metal Oxide Transistors Based on Pure Aqueous Inks

Additive patterning of transparent conducting metal oxides at low temperatures is a critical step in realizing low‐cost transparent electronics for display technology and photovoltaics. In this work, inkjet‐printed metal oxide transistors based on pure aqueous chemistries are presented. These inks readily convert to functional thin films at lower processing temperatures (T ≤ 250 °C) relative to organic solvent‐based oxide inks, facilitating the fabrication of high‐performance transistors with both inkjet‐printed transparent electrodes of aluminum‐doped cadmium oxide (ACO) and semiconductor (InO_x ). The intrinsic fluid properties of these water‐based solutions enable the printing of fine features with coffee‐ring free line profiles and smoother line edges than those formed from organic solvent‐based inks. The influence of low‐temperature annealing on the optical, electrical, and crystallographic properties of the ACO electrodes is investigated, as well as the role of aluminum doping in improving these properties. Finally, the all‐aqueous‐printed thin film transistors (TFTs) with inkjet‐patterned semiconductor (InO_x ) and source/drain (ACO) layers are characterized, which show ideal low contact resistance (R _c < 160 Ω cm) and competitive transistor performance (µ _lin up to 19 cm² V^?1 s^?1, Subthreshold Slope (SS) ≤150 mV dec^?1) with only low‐temperature processing (T ≤ 250 °C).     

环氧树脂–银粉复合导电银浆的制备

导电油墨(导电银浆等)是以全印制电子技术制作印制电路板的关键材料。研究了以环氧树脂为连结剂、自制超细银粉为填料、聚乙二醇等材料为添加剂的复合导电银浆配方及制备方法。研究获得的最佳配方为:w(银粉)为70%～80%,其他各组分之间的质量比ζ(环氧树脂∶四氢呋喃∶固化剂∶聚乙二醇)=1.00∶(2.00～3.00)∶(0.20～0.30)∶(0.05～0.10)。在最佳配方范围内,复合导电银浆室温固化后电阻率小于100Ω/cm,有机物挥发少,对环境友好,符合实际应用要求。     

The Relationship Between Contact Resistance and Contact Force on Au-Coated Carbon Nanotube Surfaces Under Low Force Conditions

Carbon nanotube (CNT)-coated surfaces are investigated to determine the electrical contact performance under low force conditions. The surfaces under investigation are vertically aligned multiwalled CNTs formed on a silicon substrate and coated with an Au film. These planar surfaces are mated with a hemispherical Au plated probe mounted in a nanoindentation apparatus. The maximum contact force used is 1 mN. The contact resistance of these surfaces is investigated as a function of the applied force and is also studied under repeated loading cycles. The surfaces are compared with a reference Au-Au contact under the same experimental conditions and the results compared to established contact theory. The results show that the vertically aligned multiwalled CNT surface provides a stable contact resistance. This paper shows the potential for the application of CNT surfaces as an interface in low force electrical contact applications.     

The Fusion of Imidazolium‐Based Ionic Polymer and Carbon Nanotubes: One Type of New Heteroatom‐Doped Carbon Precursors for High‐Performance Lithium–Sulfur Batteries

Rational design of sulfur host materials with high electrical conductivity and strong polysulfides (PS) confinement is indispensable for high‐performance lithium–sulfur (Li–S) batteries. This study presents one type of new polymer material based on main‐chain imidazolium‐based ionic polymer (ImIP) and carbon nanotubes (CNTs); the polymer composites can serve as a precursor of CNT/NPC‐300, in which close coverage and seamless junction of CNTs by N‐doped porous carbon (NPC) form a 3D conductive network. CNT/NPC‐300 inherits and strengthens the advantages of both high electrical conductivity from CNTs and strong PS entrapping ability from NPC. Benefiting from the improved attributes, the CNT/NPC‐300‐57S electrode shows much higher reversible capacity, rate capability, and cycling stability than NPC‐57S and CNTs‐56S. The initial discharge capacity of 1065 mA h g^?1 is achieved at 0.5 C with the capacity retention of 817 mA h g^?1 over 300 cycles. Importantly, when counter bromide anion in the composite of CNTs and ImIP is metathesized to bis(trifluoromethane sulfonimide), heteroatom sulfur is cooperatively incorporated into the carbon hosts, and the surface area is increased with the promotion of micropore formation, thus further improving electrochemical performance. This provides a new method for optimizing porous properties and dopant components of the cathode materials in Li–S batteries.