Capacitive touch sensors

Touch-sensing devices using PP fibres coated with SLG and FLG) grown by CVD, and with solution processed graphene films obtained by liquid-exfoliation (LEG) are shown in Figs. 1f and 2a. They were produced using the R2R-compatible method (Fig. 1a). The graphene coatings were characterised by means of Raman spectroscopy (Supplementary Fig. S1, S2 and S3), and optical transmittance (Supplementary Fig. S4a), confirming the presence of the different types of graphene on the surface of the PP textile fibres. Extensive microscopic studies of the surface of graphene-coated fibres along with detailed characterisation of the electrical and optical properties were published in refs. 22,23. These studies included characterisation techniques, such as atomic force microscopy, scanning electron microscopy, scanning thermal microscopy, surface profilometry, Raman spectroscopy, electrical conductivity measurements, optical transmittance measurements and measurements of the mechanical properties.

Fig. 2 Touch sensors: a Photographs of sensors with single layer graphene (SLG), few-layer graphene (FLG) fabricated using the roll-to-roll (R2R), solution processed graphene films obtained by liquid-exfoliation blade cut (LEG) and using the R2R compatible method. The LEG sensors using the standard patterning is also shown. b Sketch of the impedance response with and without touching. c Impedance upon touching versus time for flat FLG device. d Impedance upon touching versus time with repeated touching for FLG device. e Impedance upon touching versus time with repeated bending for FLG device. f Difference in performance between touch sensors patterned using the R2R compatible method and sensors patterned using reactive ion etching (RIE) for LEG. g Photo and schematics of position sensitive arrays. h Voltage drop upon touching vs. time for position sensitive arrays. i Time-resolved rise and fall times upon touching for position sensitive arrays. The various on/off intervals in c, d, e, f, h are due to the difference in the intensity and duration of the user’s finger touching the device Full size image

There are two main methods of touch-sensing34: resistive, where a change in resistance is measured as signal; and capacitive, where this signal comes from a change in capacitance in between the electrodes. We have implemented an approach involving the measurement of the impedance which offers the multi-functionality of switching between resistance and capacitance measurement to detect touch. A polar graph of the impedance is shown in Fig. 2b, with the impedance modulus of 60 MΩ and the phase φ = 89°, for the untouched device. Upon touching, the modulus reduces by an order of magnitude and the phase drops to φ = 68°, due to the finger shorting the interdigitated electrodes. When the finger is pressed (ON state) the impedance drops, and when released (OFF state) the impedance increases back to its original value with great stability, as can be seen for a FLG device (Fig. 2c). Even for atomically thin graphene active material, the excellent performance of the device is retained upon 500 touching cycles, with a clear distinction between the OFF/ON/OFF states (Fig. 2d). We assessed the flexibility of the touch-sensor by subjecting it to mechanical stress and, owing to the exceptional flexibility of graphene, the OFF/ON/OFF device performance remains unchanged upon 500 bends (Fig. 2e). These results are consistent, with great reproducibility (Supplementary Fig. S6 for SLG). To illustrate advantage of our novel R2R approach over standard RIE patterning, we compared it directly with a RIE-based device, as is demonstrated in Fig. 2f, showing that a larger ON/OFF ratio is achieved in the R2R-compatible patterning then in the standard RIE-based method. This is due to the inherently thicker nature of LEG when compared with CVD-grown samples, which makes more difficult to fully remove LEG by RIE. Indeed, after several RIE runs, the films are still conductive in the OFF state, meaning that the graphene was not been fully etched away using RIE. Moreover, the resistance increases after every touch, which is an indication that with every touch the finger is removing graphene flakes from the channel, hence erasing percolation pathways and changing the conductivity of the sample. This inhomogeneity makes it less suitable for sensor purposes. This effect is not shown in LEG with R2R approach because the photoresist prevented graphene deposition. An even simpler and more scalable method was also shown, using blade-cutting to define the pattern, with results comparable with the R2R method (Supplementary Fig. S7).

Transparent and flexible position-sensitive arrays of graphene-coated fibres were woven in a squared fabric by orthogonally intertwining conducting fibres separated by a poly(methyl methacrylate) (PMMA) dielectric layer, thus providing sensitive points at their intersections (Fig. 2g). The sensing mechanism, in this case, is purely self-capacitive, widely used across the electronics industry (Supplementary section S3). The touch-sensing performance, quantified as the voltage drop across the PMMA layer sandwiched between 2 orthogonal graphene fibres, is shown in Fig. 2h, with very stable ON/OFF states upon several touches. We analysed the speed of sensing (Fig. 2i) and found a rise and fall times of 1.4 ms regardless the type of graphene used. The rise and fall time of our devices are limited by the electronics used in the measurement, within the range of commercial devices and rival with the best values in the literature.27,28,29,30 In contrast to miniaturised conventional CMOS electronics that are mounted on flexible and textile substrtes,35 smart textiles attained by intertwined graphene fibres allow advanced detection schemes that can be readily implemented when complex functions such as simultaneous multi-touch features are required (Supplementary Video 1).

Light-emitting devices

Having demonstrated a novel technology for enabling sensing capabilities of PP fibres, we now proceed to considerably broaden the spectrum of applications by describing the development of graphene-enabled textile fibres with light-emitting functionalities and woven opto-electronic technologies. The ACEL device configuration was chosen as this technology uniquely enables the realisation of large-area flexible and foldable graphene light sources, with good contrast and uniform brightness.33 Furthermore, ACEL devices can display images with high resolution, can withstand mechanical shocks and a wide range of temperatures,36 making this technology a valuable candidate for smart textiles. ACEL devices were fabricated on individual graphene-coated PP fibres which served as bottom electrode. Graphene was subsequently covered by an emitter layer of commercially available Cu-doped zinc sulfide (ZnS:Cu), an insulating layer of BaTiO 3 and a top electrode (Fig. 1c, e). Further details about the coating techniques and optimisation of the devices can be found in Supplementary section S5 and Fig. S10–12 therein. Upon excitation with an AC voltage, light is emitted from the ZnS:Cu layer due to impact ionisation and recombination of electron-hole pairs.37 The emission spectra is in the visible, with an emission peak around 500 nm (Supplementary Fig. S11) and average light intensity dependent on the applied voltage, as typical for ACEL devices (Fig. 3a).30 Figures 1e and 3b, c show photos of light-emitting devices on individual fibres, as well as text displayed on fibre achieved by patterning the graphene electrode with features down to 100 µm.

Fig. 3 ACEL devices on textile fibres: a Intensity of the emitted light as a function of bias voltage fitted to the Alfrey–Taylor relation between ACEL brightness (L) and voltage (V): L = L 0 exp(−b/V)1/2, where L 0 and b are empirical constants, fitted with the with good agreement (R2 = 0.9982).34,35 Photograph of the device in bending b and torsion c. Change in emission as a function of: d the bending radius and corresponding fibre strain; e repeated bending cycles; f repeated twisting cycles. Two approaches to ACEL arrays and corresponding photos in light and dark conditions: g large pixels (scale bars: left 5 mm; right 20 mm) and h small pixels (scale bars: top 10 mm; bottom 1 mm) Full size image

To ascertain the mechanical properties of the light-emitting textile fibres we have subjected the fibres to different types of mechanical stress such as bending, torsion and cyclic loading, assessing the changes in the average intensity (ΔI) as compared to the average light intensity before applying the stress (I 0 ). As shown in Fig. 3d, the change in emission upon bending was negligible up to a 10 mm radius, similar to the radius of a human finger. The mechanical resilience of the device was studied in the form of repeated bending (Fig. 3b, e) and torsion tests (Fig. 3c, f) between the flat state and the bent or twisted state. For both types of stress, only slight changes were observed, demonstrating the suitability of these devices for smart textile technology.