Transfer-printing of as-fabricated carbon nanotube devices onto various substrates

Universal Transfer Printing of Micelle-Templated Nanoparticles Using Plasma-Functionalized Graphene

Nanostructure incorporation into devices plays a key role in improving performance, yet processes for preparing two-dimensional (2D) arrays of colloidal nanoparticles tend not to be universally applicable, particularly for soft and oxygen-sensitive substrates for organic and perovskite-based electronics. Here, we show a method of transferring reverse micelle-deposited (RMD) nanoparticles (perovskite and metal oxide) on top of an organic layer, using a functionalized graphene carrier layer for transfer printing. As the technique can be applied universally to RMD nanoparticles, we used magnetic (γ-Fe₂O₃) and luminescent (methylammonium lead bromide (MAPbBr₃)) nanoparticles to validate the transfer-printing methodology. The strong photoluminescence from the MAPbBr₃ under UV illumination and high intrinsic field of the γ-Fe₂O₃ as measured by magnetic force microscopy (MFM), coupled with Raman measurements of the graphene layer, confirm that all components survive the transfer-printing process with little loss of properties. Such an approach to introducing uniform 2D arrays of nanoparticles onto sensitive substrates opens up new avenues to tune the device interfacial properties.

High-performance printed electronics based on inorganic semiconducting nano to chip scale structures

The Printed Electronics (PE) is expected to revolutionise the way electronics will be manufactured in the future. Building on the achievements of the traditional printing industry, and the recent advances in flexible electronics and digital technologies, PE may even substitute the conventional silicon-based electronics if the performance of printed devices and circuits can be at par with silicon-based devices. In this regard, the inorganic semiconducting materials-based approaches have opened new avenues as printed nano (e.g. nanowires (NWs), nanoribbons (NRs) etc.), micro (e.g. microwires (MWs)) and chip (e.g. ultra-thin chips (UTCs)) scale structures from these materials have been shown to have performances at par with silicon-based electronics. This paper reviews the developments related to inorganic semiconducting materials based high-performance large area PE, particularly using the two routes i.e. Contact Printing (CP) and Transfer Printing (TP). The detailed survey of these technologies for large area PE onto various unconventional substrates (e.g. plastic, paper etc.) is presented along with some examples of electronic devices and circuit developed with printed NWs, NRs and UTCs. Finally, we discuss the opportunities offered by PE, and the technical challenges and viable solutions for the integration of inorganic functional materials into large areas, 3D layouts for high throughput, and industrial-scale manufacturing using printing technologies.

Enhancing the Viscoelastic Performance of Carbon Fiber Composites by Incorporating CNTs and ZnO Nanofillers

Carbon fiber reinforced plastic composites (CFRPs) possess superior elastic mechanical properties. However, CFRPs lack sufficient viscoelastic performance, such as damping and creep resistance. In an effort to improve these properties, in this study, hybrid multiscale composites with various combinations of zinc oxide nanorods (ZnO) and carbon nanotubes (CNTs) were deposited at the interface of carbon fiber laminae. The viscoelastic properties of the corresponding composites were characterized via dynamic mechanical analysis (DMA) during both temperature and frequency sweeps. The creep activation energy for each composite configuration was also calculated. The DMA temperature sweep analysis reported that the composite incorporating both ZnO and CNTs exhibited the highest improvements in all viscoelastic properties. This composite also attained better creep resistance, evident by the highest activation energy. The DMA frequency sweep analysis revealed that composites incorporating a single nanofiller improves the viscoelastic properties more than the combined nanofiller composite. Despite these improvements in the viscoelastic properties, the non-uniform dispersion and agglomerations of the nanofillers affected some of the elastic properties negatively, such as the storage modulus.

Transfer Printing and its Applications in Flexible Electronic Devices

Flexible electronic systems have received increasing attention in the past few decades because of their wide-ranging applications that include the flexible display, eyelike digital camera, skin electronics, and intelligent surgical gloves, among many other health monitoring devices. As one of the most widely used technologies to integrate rigid functional devices with elastomeric substrates for the manufacturing of flexible electronic devices, transfer printing technology has been extensively studied. Though primarily relying on reversible interfacial adhesion, a variety of advanced transfer printing methods have been proposed and demonstrated. In this review, we first summarize the characteristics of a few representative methods of transfer printing. Next, we will introduce successful demonstrations of each method in flexible electronic devices. Moreover, the potential challenges and future development opportunities for transfer printing will then be briefly discussed.

Carbon Nanotubes as Etching Masks for the Formation of Polymer Nanostructures

We investigate the interaction of carbon nanotubes (CNTs) embedded in a polymer matrix [poly(methyl methacrylate) (PMMA)] with Ar plasma, which results in the formation of PMMA nanostructures, as CNTs act as an etching mask. Because of the large differences in the Ar ion sputtering yields between CNTs and PMMA, PMMA lines with the width comparable to that of CNTs and as high as 20 nm (for single-walled CNTs) or 80 nm (for multiwalled CNTs) can be obtained after repeated exposure of CNT/PMMA films to Ar plasma. We also follow the etching process by investigating changes in the IV characteristics and Raman spectra of CNTs after each exposure to Ar plasma, which shows progressive defect generations in CNTs while they maintain structural integrity long enough to act as the etching mask for PMMA underneath. We demonstrate that the PMMA nanostructure patterns can be transferred to a different polymer substrate using nanoimprinting.

Transfer printing gold nanoparticle arrays by tuning the surface hydrophilicity of thermo-responsive poly N-isopropylacrylamide (pNIPAAm)

For the transfer of 2-D gold nanoparticle arrays between different substrates, we have developed a new method using thermo-responsive poly-N-isopropylacrylamide (pNIPAAm). By tuning the degree of surface hydrophilicity of pNIPAAm between 5 °C and 50 °C, we demonstrate the transfer of arrays extending over micron-scale areas with preservation of array properties.

Direct Observation of Indium Conductive Filaments in Transparent, Flexible, and Transferable Resistive Switching Memory

Electronics with multifunctionalities such as transparency, portability, and flexibility are anticipated for future circuitry development. Flexible memory is one of the indispensable elements in a hybrid electronic integrated circuit as the information storage device. Herein, we demonstrate a transparent, flexible, and transferable hexagonal boron nitride (hBN)-based resistive switching memory with indium tin oxide (ITO) and graphene electrodes on soft polydimethylsiloxane (PDMS) substrate. The ITO/hBN/graphene/PDMS memory device not only exhibits excellent performance in terms of optical transmittance (∼85% in the visible wavelength), ON/OFF ratio (∼480), retention time (∼5 × 10⁴ s) but also shows robust flexibility under bending conditions and stable operation on arbitrary substrates. More importantly, direct observation of indium filaments in an ITO/hBN/graphene device is found via ex situ transmission electron microscopy, which provides critical insight on the complex resistive switching mechanisms.

Recent Progress in Materials and Devices toward Printable and Flexible Sensors

Printable electronics present a new era of wearable electronic technologies. Detailed technologies consisting of novel ink semiconductor materials, flexible substrates, and unique processing methods can be integrated to create flexible sensors. To detect various stimuli of the human body, as well as specific environments, unique electronic devices formed by "ink-based semiconductors" onto flexible and/or stretchable substrates have become a major research trend in recent years. Materials such as inorganic, organic, and hybrid semiconductors with various structures (i.e., 1D, 2D and 3D) with printing capabilities have been considered for bio and medical applications. In this review, we report recent progress in materials and devices for future wearable sensor technologies.

Alignment and Patterning of Ordered Small-Molecule Organic Semiconductor Micro-/Nanocrystals for Device Applications

Large-area alignment and patterning of small-molecule organic semiconductor micro-/nanocrystals (SMOSNs) at desired locations is a prerequisite for their practical device applications. Recent strategies for alignment and patterning of ordered SMOSNs and their corresponding device applications are highlighted.

Silica-gold bilayer-based transfer of focused ion beam-fabricated nanostructures

The demand for using nanostructures fabricated by focused ion beam (FIB) on delicate substrates or as building blocks for complex devices motivates the development of protocols that allow FIB-fabricated nanostructures to be transferred from the original substrate to the desired target. However, transfer of FIB-fabricated nanostructures is severely hindered by FIB-induced welding of structure and substrate. Here we present two (ex and in situ) transfer methods for FIB-fabricated nanostructures based on a silica-gold bilayer evaporated onto a bulk substrate. Utilizing the poor adhesion between silica and gold, the nanostructures can be mechanically separated from the bulk substrate. For the ex situ transfer, a spin-coated poly(methyl methacrylate) film is used to carry the nanostructures so that the bilayer can be etched away after being peeled off. For the in situ transfer, using a micro-manipulator inside the FIB machine, a cut-out piece of silica on which a nanostructure has been fabricated is peeled off from the bulk substrate and thus carries the nanostructure to a target substrate. We demonstrate the performance of both methods by transferring plasmonic nano-antennas fabricated from single-crystalline gold flakes by FIB milling to a silicon wafer and to a scanning probe tip.

Suspended single-walled carbon nanotube fluidic sensors

In this paper, we demonstrate the fabrication of liquid flow sensors employing partially suspended single-walled carbon nanotubes (SWNTs). We have found that the sign of the conductance change in SWNT flow sensors is not influenced by the direction of water flow for both supported and suspended devices. Therefore, the streaming potential is not the principal mechanism of the SWNT sensor response. Instead, the conductance change is more likely due to a reduction in the cation density in the electrical double layer, whose equilibrium conditions are determined by the liquid flow rate. More importantly, we have found that the sensitivity of suspended SWNT devices is more than 10 times greater than that of supported SWNT devices. A reduced screening effect and an increase in effective sensing volume are responsible for the enhanced sensitivity, which is consistent with the ion depletion model. We also have measured conductance as a function of gate bias at different flow rates and have determined the flow-rate dependent effective charge density, which influences the electrostatic configuration around SWNT devices.

A high-yield two-step transfer printing method for large-scale fabrication of organic single-crystal devices on arbitrary substrates

Single-crystal organic nanostructures show promising applications in flexible and stretchable electronics, while their applications are impeded by the large incompatibility with the well-developed photolithography techniques. Here we report a novel two-step transfer printing (TTP) method for the construction of organic nanowires (NWs) based devices onto arbitrary substrates. Copper phthalocyanine (CuPc) NWs are first transfer-printed from the growth substrate to the desired receiver substrate by contact-printing (CP) method, and then electrode arrays are transfer-printed onto the resulting receiver substrate by etching-assisted transfer printing (ETP) method. By utilizing a thin copper (Cu) layer as sacrificial layer, microelectrodes fabricated on it via photolithography could be readily transferred to diverse conventional or non-conventional substrates that are not easily accessible before with a high transfer yield of near 100%. The ETP method also exhibits an extremely high flexibility; various electrodes such as Au, Ti, and Al etc. can be transferred, and almost all types of organic devices, such as resistors, Schottky diodes, and field-effect transistors (FETs), can be constructed on planar or complex curvilinear substrates. Significantly, these devices can function properly and exhibit closed or even superior performance than the device counterparts fabricated by conventional approach.

Highly sensitive integrated pressure sensor with horizontally oriented carbon nanotube network

This paper presents a functionalized, horizontally oriented carbon nanotube network as a sensing element to enhance the sensitivity of a pressure sensor. The synthesis of horizontally oriented nanotubes from the AuFe catalyst and their deposition onto a mechanically flexible substrate via transfer printing are studied. Nanotube formation on thermally oxidized Si (100) substrates via plasma-enhanced chemical vapor deposition controls the nanotube coverage and orientation on the flexible substrate. These nanotubes can be simply transferred to the flexible substrate without changing their physical structure. When tested under a pressure range of 0 to 50 kPa, the performance of the fabricated pressure sensor reaches as high as approximately 1.68%/kPa, which indicates high sensitivity to a small change of pressure. Such sensitivity may be induced by the slight contact in isolated nanotubes. This nanotube formation, in turn, enhances the modification of the contact and tunneling distance of the nanotubes upon the deformation of the network. Therefore, the horizontally oriented carbon nanotube network has great potential as a sensing element for future transparent sensors.

The selective transfer of patterned graphene

We demonstrate a selective microcleaving graphene (MG) transfer technique for the transfer of graphene patterns and graphene devices onto chosen targets using a bilayer-polymer structure and femtosecond laser microfabrication. In the bilayer-polymer structure, the first layer is used to separate the target graphene from the other flakes, and the second layer transfers the patterned graphene to the chosen targets. This selective transfer technique, which exactly transfers the patterned graphene onto a chosen target, leaving the other flakes on the original substrate, provides an efficient route for the fabrication of MG for microdevices and flexible electronics and the optimization of graphene's performance. This method will facilitate the preparation of van der Waals heterostructures and enable the optimization of the performance of graphene hybrid devices.

A review of carbon nanotube- and graphene-based flexible thin-film transistors

Carbon nanotubes (CNTs) and graphene have attracted great attention for numerous applications for future flexible electronics, owing to their supreme properties including exceptionally high electronic conductivity and mechanical strength. Here, the progress of CNT- and graphene-based flexible thin-film transistors from material preparation, device fabrication techniques to transistor performance control is reviewed. State-of-the-art fabrication techniques of thin-film transistors are divided into three categories: solid-phase, liquid-phase, and gas-phase techniques, and possible scale-up approaches to achieve realistic production of flexible nanocarbon-based transistors are discussed. In particular, the recent progress in flexible all-carbon nanomaterial transistor research is highlighted, and this all-carbon strategy opens up a perspective to realize extremely flexible, stretchable, and transparent electronics with a relatively low-cost and fast fabrication technique, compared to traditional rigid silicon, metal and metal oxide electronics.