Table of contents

Research on flexible electronics has grown exponentially over the last decade. Researchers around the globe are developing a wide range of flexible systems, including displays [1, 2], sensors [3–5], RFID tags [6, 7] and other similar devices [8]. Innovations in materials have been key to the increased research success in this field of research in recent years [9]. Transistors, interconnects, memory cells, passive components and other assorted devices all have challenging material demands for flexible electronics to become a reality. Nanomaterials of various kinds have been found to represent a tremendously powerful tool, with nanoparticles [10], nanotubes, nanowires [3, 11] and engineered organic molecules [12, 13] contributing to the realization of high-performance semiconductors, dielectrics and conductors for flexible electronics applications.

Nanomaterials offer tunability in terms of performance, solution processability and processing temperature requirements, which makes them very attractive as building blocks for flexible electronic systems. Indeed, such systems represent some of the largest families of commercially produced nanomaterials today, and numerous commercial products based on nanoparticle formulations are widely available.

This special issue focuses on the rapidly blossoming field of flexible electronics, with a particular focus on the use of nanotechnology to facilitate flexible electronic materials, processes, devices and systems. Contributions to the issue describe the development of nanomaterials—including nanoparticles, nanotubes, nanowires and carbon-based thin films—for use in conductors, transparent electrodes, semiconductors and dielectrics. The articles feature innovations in nanomanufacturing and novel materials, as well as the application of these technologies to advanced flexible devices and systems.

As flexible electronics systems move rapidly towards successful commercial deployment, it is extremely likely that they will exploit nanomaterials as building blocks. Developments in the field will help to leverage the power of these materials to realize novel functionalities in flexible form factors. This special issue provides a view of the state of the art in these technologies, and gives a vision of the coming innovations that will make flexible electronics a reality.

References

[1] Gelinck G H et al 2004 Flexible active-matrix displays and shift registers based on solution-processed organic transistors Nature Mater.3 106–10

[2] Zhou L, Wanga A, Wu S C, Sun J, Park S and Jackson T N 2006 All-organic active matrix flexible display Appl. Phys. Lett.88 083502

[3] Fan Z, Ho J C, Jacobson Z A, Razavi H and Javey A 2008 Large-scale, heterogeneous integration of nanowire arrays for image sensor circuitry Proc. Natl Acad. Sci.105 11066

[4] Sekitani T et al 2009 Organic nonvolatile memory transistors for flexible sensor arrays Science326 1516–9

[5] Mannsfeld S C B et al 2010 Highly sensitive flexible pressure sensors with microstructured rubber dielectric layers Nature Mater.9 859–64

[6]Subramanian V, Frechet J M J, Chang P C, Huang D C, Lee J B, Molesa S E, Murphy A R, Redinger D R and Volkman S K 2005 Progress toward development of all-printed RFID tags: materials, processes, and devices Proc. IEEE93 1330–8

[7] Jung M et al 2010 All-printed and roll-to-roll-printable 13.56 MHz-operated 1 bit RF tag on plastic foils IEEE Trans. Electron. Devices57 571–80

[8] Kim D-H et al 2011 Epidermal electronics Science333 838–43

[9] Wagner S and Bauer S 2012 Materials for stretchable electronics MRS Bull.37 207

[10] Grouchko M, Kamyshny A and Magdassi S 2009 Formation of air-stable copper–silver core–shell nanoparticles for inkjet printing J. Mater. Chem.19 3057–62

[11] Takei K et al 2010 Nanowire active-matrix circuitry for low-voltage macroscale artificial skin Nature Mater.9 821–6

[12] Sekitani T, Zschieschang U, Klauk H and Someya T 2010 Flexible organic transistors and circuits with extreme bending stability Nature Mater.9 1015–22

[13] Park S, Wang G, Cho B, Kim Y, Song S, Ji Y, Yoon M and Lee T 2012 Flexible molecular-scale electronic devices Nature Nanotechnol.7 438–42

The development of flexible electronic systems has been extensively researched in recent years, with the goal of expanding the potential scope and market of modern electronic devices in the areas of computation, communications, displays, sensing and energy. Uniquely, the use of soft polymeric substrates enables the incorporation of advanced features beyond mechanical bendability and stretchability. In this paper, we describe several functionalities which can be achieved using engineered nanostructured materials. In particular, reversible binding, self-cleaning, antireflective and shape-reconfigurable properties are introduced for the realization of multifunctional, flexible electronic devices. Examples of flexible systems capable of spatial mapping and/or responding to external stimuli are also presented as a new class of user-interactive devices.

Stretchable transparent composites have been synthesized consisting of a silver nanowire (AgNW) network embedded in the surface layer of a crosslinked poly(acrylate) matrix. The interpenetrating networks of AgNWs and the crosslinked polymer matrix lead to high surface conductivity, high transparency, and rubbery elasticity. The presence of carboxylic acid groups on the polymer chains enhances the bonding between AgNWs and the polymer matrix, and further increases the stretchability of the composites. The sheet resistance of the composite electrode increases by only 2.3 times at 50% strain. Repeated stretching to 50% strain and relaxation only causes a small increase of the sheet resistance after 600 cycles. The morphology of the composites during reversible stretching and relaxation has been investigated to expound the conductivity changes.

Carbon nanotube (CNTs) inks may provide an effective route for producing flexible electronic devices by digital printing. In this paper we report on the formulation of highly concentrated aqueous CNT inks and demonstrate the fabrication of flexible electroluminescent (EL) devices by inkjet printing combined with wet coating. We also report, for the first time, on the formation of flexible EL devices in which all the electrodes are formed by inkjet printing of low-cost multi-walled carbon nanotubes (MWCNTs). Several flexible EL devices were fabricated by using different materials for the production of back and counter electrodes: ITO/MWCNT and MWCNT/MWCNT. Transparent electrodes were obtained either by coating a thin layer of the CNTs or by inkjet printing a grid which is composed of empty cells surrounded by MWCNTs. It was found that the conductivity and transparency of the electrodes are mainly controlled by the MWCNT film thickness, and that the dominant factor in the luminance intensity is the transparency of the electrode.

We describe the use of semiconductor nanomaterials, advanced fabrication methods and unusual device designs for a class of electronics capable of integration onto the inner and outer surfaces of thin, elastomeric sheets in closed-tube geometries, specially formed for mounting on the fingertips. Multifunctional systems of this type allow electrotactile stimulation with electrode arrays multiplexed using silicon nanomembrane (Si NM) diodes, high-sensitivity strain monitoring with Si NM gauges, and tactile sensing with elastomeric capacitors. Analytical calculations and finite element modeling of the mechanics quantitatively capture the key behaviors during fabrication/assembly, mounting and use. The results provide design guidelines that highlight the importance of the NM geometry in achieving the required mechanical properties. This type of technology could be used in applications ranging from human–machine interfaces to 'instrumented' surgical gloves and many others.

Graphene, a two-dimensional one-atom-thick planar sheet of carbon atoms densely packed in a honeycomb crystal lattice, has attracted appreciable attention due to its extraordinary mechanical, thermal, electrical, and optical properties. One of these properties, graphene's outstanding tensile strength, allows graphene-based electronic and photonic devices to be flexible, stretchable, and foldable. In this work, we propose a novel platform technology and architecture of graphene-based flexible photonic devices for the development of high-performance flexible devices and components. We investigated the characteristics of the graphene-based plasmonic waveguide for the development of high-performance optical interconnection in flexible human-friendly optoelectronic devices. We concluded that graphene-based photonic devices have huge potential for the development of next-generation human-friendly flexible optoelectronic systems.

Wireless power transmission to inexpensive and disposable smart electronic devices is one of the key issues for the realization of a ubiquitous society where sensor networks such as RFID tags, price tags, smart logos, signage and sensors could be fully interconnected and utilized by DC power of less than 0.3 W. This DC power can be provided by inductively coupled AC from a 13.56 MHz power transmitter through a rectenna, consisting of an antenna, a diode and a capacitor, which would be cheap to integrate with inexpensive smart electronic devices. To integrate the rectenna with a minimum cost, a roll-to-roll (R2R) gravure printing process has been considered to print the rectenna on plastic foils. In this paper, R2R gravure printing systems including printing condition and four different nanoparticle based inks will be reported to print the rectenna (antenna, diode and capacitor) on plastic foils at a printing speed of 8 m min⁻¹ and more than 90% device yield for a wireless power transmission of 0.3 W using a standard 13.56 MHz power transmitter.

Roll-to-roll lamination is one promising technique to produce large-area organic electronic devices such as solar cells with a large through output. One challenge in this process is the frequent electric point shorting of the cathode and anode by the excess or concentrated applied stress from many possible sources. In this paper, we report a method to avoid electric point shorting by incorporating insulating and hard barium titanate (BaTiO₃) nanoparticles (NPs) into the active layer to work as a spacer. It has been demonstrated that the incorporated BaTiO₃ NPs in poly(3-hexylthiophene):[6,6]-phenyl-c-61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction solar cells cause no deleterious effect to the power conversion process of this type of solar cell. The resulting laminated devices with NPs in the active layer display the same efficiency as the devices without NPs, while the laminated devices with NPs can sustain a ten times higher lamination stress of over 6 MPa. The flexible polymer solar cell device with incorporated NPs shows a much smaller survivable curvature radius of 4 mm, while a regular flexible device can only sustain a bending curvature radius of 8 mm before fracture.

We report the development of a near-field optical nanolithography method using a roll-type phase-shift mask. Sub-wavelength resolution is achieved using near-field exposure of photoresist through a cylindrical phase mask, allowing dynamic and high throughput continuous patterning. As an application, we present the fabrication of a transparent electrode in the form of a metallic wire grid by using the roller-based optical lithography method. To fabricate a mesh-type metal pattern, a specific phase-shift mask was designed and critical experimental parameters were also studied. As a result, a transparent conductor with suitable properties was achieved with a recently built cylindrical phase-shift lithography prototype designed to pattern on 100 mm² of substrate area.

Two near-ultraviolet (UV) sensors based on solution-grown zinc oxide (ZnO) nanowires (NWs) which are only sensitive to photo-excitation at or below 400 nm wavelength have been fabricated and characterized. Both devices keep all processing steps, including nanowire growth, under 100 °C for compatibility with a wide variety of substrates. The first device type uses a single optical lithography step process to allow simultaneous in situ horizontal NW growth from solution and creation of symmetric ohmic contacts to the nanowires. The second device type uses a two-mask optical lithography process to create asymmetric ohmic and Schottky contacts. For the symmetric ohmic contacts, at a voltage bias of 1 V across the device, we observed a 29-fold increase in current in comparison to dark current when the NWs were photo-excited by a 400 nm light-emitting diode (LED) at 0.15 mW cm⁻² with a relaxation time constant (τ) ranging from 50 to 555 s. For the asymmetric ohmic and Schottky contacts under 400 nm excitation, τ is measured between 0.5 and 1.4 s over varying time internals, which is ∼2 orders of magnitude faster than the devices using symmetric ohmic contacts.

To reduce the driving voltage, and hence enhance the power efficiency of OLEDs, the mobility of the various carrier transport layers needs to be increased. Buckminsterfullerene (C₆₀) has been proposed to be one possible alternative conductive electron transport layer (ETL) to enhance the power efficiency in OLEDs, due to its high conductivity and the formation of an ohmic contact with the LiF/Al cathode. The optical properties of a nanocomposite of C₆₀ with LiF (C₆₀:LiF) and its potential as an efficient ETL in OLEDs was studied. With proper optimization of the device structure, a more than 50% improvement in the power efficiency, without sacrificing the high EQE, in optimized fluorescent OLEDs with the use of C₆₀:LiF nanocomposite ETL was achieved.

The excellent electronic and material properties of single walled carbon nanotubes (SWNTs) makes this nanomaterial very attractive for incorporation into flexible and stretchable electronics. However, the widespread application of SWNTs in electronic devices is still limited. To purify, process and place SWNTs, appropriate solvents for dispersion are needed. However, a fundamental understanding of the reasons why certain solvents are capable of dispersing SWNTs is still missing. Here we report on two new potential solvents containing amidine moieties, 1,8-diazabicycloundec-7-ene (DBU) and 1,5-diazabicyclo(4.3.0)non-5-ene (DBN). Even though these solvents' molecular structures differ by only two –CH₂– groups, we found that DBU is capable of dispersing SWNTs, while DBN is not. We carried out density functional theory (DFT) calculations to investigate the interaction between DBU and DBN, and we elucidated the reasons for the different performances of the two solvents. DBU has a preferential edge-on interaction with the SWNT, thus allowing for a higher solvent coverage than DBN. In addition, the CH₂–SWNT interaction present for DBU substantially increases the adsorption energy compared to DBN. Our results point to the important interplay between the interaction of pi electrons, nitrogen lone pairs and the –CH₂– groups present in aprotic solvent molecules and the delocalized pi electrons in SWNTs.

Laser-assisted, one-step direct nanoimprinting of metal and semiconductor nanoparticles (NPs) was investigated to fabricate submicron structures including mesh, line, nanopillar and nanowire arrays. Master molds were fabricated with high-speed (200 mm s⁻¹) laser direct writing (LDW) of negative or positive photoresists on Si wafers. The fabrication was completely free of lift-off or reactive ion etching processes. Polydimethylsiloxane (PDMS) stamps fabricated from master molds replicated nanoscale structures (down to 200 nm) with no or negligible residual layers on various substrates. The low temperature and pressure used for nanoimprinting enabled direct nanofabrication on flexible substrates. With the aid of high-speed LDW, wafer scale 4 inch direct nanoimprinting was demonstrated.

Flexible organic solar cells (OSCs) composed of blended films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were fabricated and investigated with chemically doped multilayer graphene films as transparent and conducting electrodes on plastic substrates. The sheet resistance of the chemically doped graphene film was reduced to half of its original value, resulting in a significant performance enhancement of OSCs featuring doped graphene electrodes. Moreover, there was no substantial variation observed in the fill factor and power conversion efficiency values of the flexible OSCs under bending conditions. A power conversion efficiency of ∼2.5% for flexible OSCs with doped graphene electrodes was observed under bending conditions, even up to a 5.2 mm bending radius.

We demonstrate air-stable low voltage flexible nonvolatile memory transistors by embedding gold nanoparticles (Au NPs) in poly(methyl methacrylate) (PMMA) as the charge storage element. The solution processability of the nanocomposite is suitable for low-cost large area processing on flexible substrates. The memory transistor exhibits a memory window of 2.1 V, long retention time ( > 10⁵ s) with low operating voltage (≤5 V). The memory behavior has been tuned via varying the composition of the fillers (Au NPs), which offers relatively easy processability for different flexible electronics applications. The electrical properties of the memory devices are found to be stable under bending. These findings will be of value for low cost and low voltage advanced flexible electronics.

Flexible and high-aspect-ratio C₆₀ nanorods are synthesized using a liquid–liquid interfacial precipitation process. As-grown nanorods are shown to exhibit a hexagonal close-packed single-crystal structure, with m-dichlorobenzene solvent molecules incorporated into the crystalline structure in a C₆₀:m-dichlorobenzene ratio of 3:2. An annealing step at 200 °C transforms the nanorods into a solvent-free face-centred-cubic polycrystalline structure. The nanorods are deposited onto field-effect transistor structures using two solvent-based techniques: drop-casting and dip-coating. We find that dip-coating deposition results in a preferred alignment of non-bundled nanorods and a satisfying transistor performance. The latter is quantified by the attainment of an electron mobility of 0.08 cm ² V ⁻¹ s ⁻¹ and an on/off ratio of  > 10⁴ for a single-crystal nanorod transistor, fabricated with a solution-based and low-temperature process that is compatible with flexible substrates.

We demonstrate low-temperature growth and direct transfer of graphene–graphitic carbon films (G–GC) onto plastic substrates without the use of supporting materials. In this approach, G–GC films were synthesized on copper layers by using inductively coupled plasma enhanced chemical vapor deposition, enabling the growth of few-layer graphene (G) on top of Cu and the additional growth of graphitic carbon (GC) films above the graphene layer at temperatures as low as 300 °C. The patterned G–GC films are not easily damaged or detached from the polymer substrates during the wet etching and transfer process because of the van der Waals forces and π–π interactions between the films and the substrates. Raman spectroscopy reveals the two-dimensional hexagonal lattice of carbon atoms and the crystallinity of the G–GC films. The optical transparency and sheet resistance of the G–GC films are controlled by modulating the film thickness. Strain sensors are successfully fabricated on plastic substrates, and their resistance modulation at different strains is investigated.

We report the fabrication, at low-temperature, of solution processed graphene transistors based on carefully engineered graphene/organic dielectric interfaces. Graphene transistors based on these interfaces show improved performance and reliability when compared with traditional SiO₂ based devices. The dielectric materials investigated include Hyflon AD (Solvay), a low-k fluoropolymer, and various organic self-assembled monolayer (SAM) nanodielectrics. Both types of dielectric are solution processed and yield graphene transistors with similar operating characteristics, namely high charge carrier mobility, hysteresis free operation, negligible doping effect and improved operating stability as compared to bare SiO₂ based devices. Importantly, the use of SAM nanodielectrics enables the demonstration of low operating voltage ( < |1.5| V), solution-processable and flexible graphene transistors with tunable doping characteristics through molecular engineering of the SAM's molecular length and terminal group. The work is a significant step towards graphene microelectronics where large-volume and low-temperature processing are required.

In this study, we report a new method to fabricate a wire grid polarizer (WGP) that greatly relaxes the requirement on patterning and etching, and can be easily applied to produce flexible WGPs. The technique is to pattern a high aspect ratio and narrow linewidth grating by nanoimprint lithography followed by two angled aluminum depositions in opposite directions to produce the narrow spacing between the aluminum lines required for a visible band WGP. Anisotropic reactive ion etching is used to remove the aluminum deposited at the top of the grating but leave the aluminum layer on the grating sidewalls, thereby forming a metal wire grid with much smaller spacings than a lithographically defined grating. As a result, the fabricated WGP showed good performance in a wide range of visible wavelength.