Efficient Inkjet Printing of Graphene

Abstract

An efficient and mature inkjet printing technology is introduced for mass production of coffee-ring-free patterns of high-quality graphene at high resolution (unmarked scale bars are 100 μm). Typically, several passes of printing and a simple baking allow fabricating a variety of good-performance electronic devices, including transparent conductors, embedded resistors, thin film transistors, and micro-supercapacitors. Organic and printed electronics are now in the market and have great potential for further growth.1 Inkjet printing of graphene2-11 is a promising research direction in these fields because it combines the attractive features of inkjet printing (additive patterning, direct writing, low cost and scalability to large area manufacturing)12 with the unique electronic, optical and mechanic properties of graphene.13-15 The printed graphene or reduced graphene oxide (rGO) has shown promise in a variety of applications such as sensors,2, 3, 8 antennas,4 transparent conductors,7, 10 thin film transistors,7 and supercapacitors.9, 11 Inkjet printing fabricates functional devices or films through well-controlled ejection of droplets of liquid-phase materials (inks) from nozzles onto various substrates. Despite progresses in producing liquid-phase graphene dispersions,16-19 to date, there has been no sophisticated inkjet printing technology to allow efficient and reliable mass production of high-quality graphene films/patterns for practical applications. As a matter of fact, a number of challenges still need to be overcome. First, a good ink should possess proper fluidic properties, in particular the viscosity and surface tension.20 In practice, inkjet printing typically requires an ink viscosity7, 21 of approximately 10 cP. However, ideal solvents for graphene dispersions such as dimethylformamide (DMF) and N-methylpyrrolidone (NMP) are often of very low viscosity (<2 cP), which severely degrades the jetting performance.7, 22 Second, the graphene concentration in these solvents is often quite low (<0.1 mg mL−1) so that several tens of print passes are required to obtain functional films,3, 4, 7, 22 reducing efficiency of the technique. Third, graphene flakes easily aggregate in inks or during solvent evaporation,7 which decreases the ink stability and/or degrades the film/device performance. Finally, DMF and NMP are toxic so that their corresponding inks cannot be used in an open environment. On top of these issues, most studies published thus far on inkjet printing of graphene are actually based on graphene oxide (GO) inks, not graphene inks.3-11, 22 The techniques require reduction processes which struggle to regain the electrical performance of pristine graphene.7 Therefore, it is desirable to formulate graphene inks with proper fluidic properties and high graphene concentration, and free from flake aggregation and toxicity, which therefore utilize the superior properties of pristine graphene. Here, we propose a method that addresses all issues summarized above. The approach is based on our previously published distillation-assisted solvent exchange technique to prepare high-concentration graphene dispersions.23 Graphene is first exfoliated from graphite flakes in DMF, and then DMF is exchanged by terpineol through distillation by virtue of the large difference between their boiling points (Supporting Information, Figure S1). Therefore, graphene can be significantly concentrated if terpineol is of much lower volume than DMF. More importantly, the solvent is changed from low-viscosity and toxic DMF to high-viscosity (about 40 cP at 20 °C) and environmentally-friendly terpineol. However, the short stable period (the dispersion of 0.4 mg mL−1 graphene can only be stable for about 10 h)23 and the severe flake aggregation during solvent evaporation7 prevent the dispersions from being practical inks. In this work, we have improved the ink formulation mainly through polymer stabilization.24 Before distillation, a small amount of polymer (ethyl cellulose) is added into the harvested graphene/DMF dispersion to protect the graphene flakes from agglomeration.24 After printing, the stabilizing polymers can be effectively removed through a simple annealing process (baking on a hot plate at 300–400 °C in air for about 1 h). Benefiting from the polymer stabilization, we have prepared graphene/terpineol dispersions with a concentration around 1 mg mL−1 and a stable period of at least several weeks. Before printing, ethanol is added to tailor the viscosity and surface tension to make the dispersions compatible with inkjet printing. The inks provide well-directed and constant jetting out of all nozzles at an even velocity (Supporting Information, Figure S2), which is comparable to the performance of commercially available inks,22 a prerequisite for reliable and efficient printing of large-scale devices. Figure 1a–c shows the printed droplets, lines, and films, respectively. All patterns are of great uniformity and free from misplaced drops, confirming the printing reliability and efficiency. Furthermore, after the solvent drying, the patterns still retain excellent uniformity, as shown by the optical micrographs (Figure 1d,g) and the atomic force microscopy (AFM) images (Figure 1e,h) of the droplets and lines. The cross-sectional profile analyses of the AFM images (Figure 1f,i) quantitatively confirm the pattern uniformity. In Figure 1f,i, the patterns are globally uniform in any direction, and the local drastic roughness on the patterns is likely caused by some protruding graphene flakes which are confirmed by the scanning electron microscopy (SEM) images below. We note that the well-known and usually undesired coffee-ring effect7, 12, 20, 25-29 (particles accentuated at the edges causing non-uniform ring-like patterns) is not significant for our inks. The effect is generally explained by the pinning of contact lines of a drying drop in which liquid evaporating from the edge is replenished by liquid from the interior. The resulting outward flow carries solutes to the edges.25 In the literature, the suppression of coffee-ring effect usually relies on the introduction of an inward Marangoni flow induced by surface-tension gradients.12, 28-30 In contrast, our inks eliminate the contact line pinning and hence effectively suppress the coffee-ring effect as follows. Shortly after printing, the film has good wetting on a solid surface (low contact angle, Figure 2a,b). However, as the ethanol evaporates, polymers are deposited onto the surface and the remaining terpineol has poor wetting on the polymer-modified surface (high contact angle, Figure 2c,d). The poor wetting may result from a strong terpineol/polymer interfacial free energy (γsl in Figure 2d), which can unpin the contact line. The unpinning of the contact line can be clearly observed under a microscope (Supporting Information, Videos 1–3). With increasing substrate temperature, γsl is reduced to establish new balance (Supporting Information, Video 2) and the remaining terpineol evaporates faster (Supporting Information, Video 3) so as not to contract to a high-contact-angle drop (like Figure 2c), both of which result in more uniform patterns (Figure 2e). Thus, our technique of mixing low- and high-contact-angle solvents together with polymers to unpin contact lines may offer a new and general strategy to suppress the coffee-ring effect. Inkjet printed graphene patterns. a–c) Optical images of as-printed patterns on glass slides: a) droplet matrix, b) lines, and c) a film corner. d,g) The dried patterns of (a) and (b), respectively. e) An AFM image of a dried droplet in (d). h) An AFM image of a dried line segment in (g). f,i) Cross-sectional profiles along three directions in (e) and (h), respectively. All unmarked scale bars are 100 μm. Evaporation processes of inkjet printed graphene films on silicon wafer. a) Optical image of an original liquid film, printed with a drop spacing of 20 μm. b) Schematic interfacial interactions of the liquid film in (a), corresponding to a small contact angle θ. c) Optical image of the finally dried pattern of the film in (a). During drying, the substrate is at room temperature (about 25 °C). The dark region in the center might be some terpineol which has poor wetting on the polymer and contracts to the center in the end. d) Schematic interfacial interactions of terpineol on top of the polymer, corresponding to a high contact angle θ. e) Optical image of the dried pattern of a uniform graphene film, for which the printing drop spacing is 15 μm and substrate temperature is 60 °C. In (b) and (d), γsv, γsl, and γlv represent the free energies between solid/vapor, solid/liquid, and liquid/vapor surface (interface), respectively. The complete evaporation processes can be seen in the movies in Supporting Information. To investigate the quality of the printed graphene, we have fabricated large-area centimeter-scale graphene thin films with various (1–6) printing layers on glass slides (Figure 3a). Note that we mainly use the term “layer” for a thin film obtained with one printing pass as opposed to graphene literature where it typically refers to a monoatomic layer of graphene. The SEM image in Figure 3b shows uniformly distributed protruding graphene flakes with lateral dimension on the order of 100–500 nm. Raman spectra in Figure 3c confirm that the thin films are composed of few-layer graphene flakes, with no evident contamination by the polymer residue. The intensity ratio between the D band (around 1350 cm−1) and G band (around 1590 cm−1) is about 0.7, suggesting reasonably low defect densities.23, 24 The signature of the Raman spectra remains almost constant with additional printing layers, implying the absence of severe flake aggregation during printing and ink drying. It has been well established that graphene is a promising material for transparent conductors.31 Hence, we examined the respective performance (transmittance versus sheet resistance) of the printed graphene films (Figure 3d). At approximately 80% transmittance (at a wave length of λ = 550 nm), the sheet resistance reaches 30 kΩ □−1, about 2–3 orders of magnitude lower than that of other transparent conductors based on inkjet printed graphene7 or rGO.10 The significant performance boost reflects the high quality of our printed graphene. In addition, the device performance is also comparable to our previous work on films made from vacuum filtrated polymer-free graphene dispersions,23 confirming that the polymer residue does not cause severe performance degradation. As a matter of fact, the achieved performance (sheet resistance about 200 kΩ □−1 at transmittance around 90%) suggests that the proposed inkjet printing technology could provide an effective and low-cost solution to some industrial applications such as transparent electrostatic dissipation.32-34 Inkjet printed graphene transparent conductive films. a) Photograph of the printed films with various printing layers (from 1 to 6 layers). b) An SEM image of the 6-layer (6L) film in (a). c) Raman spectra of the 6 films in (a). d) Transmittance (at λ = 550 nm) versus sheet resistance for 4 films (from 3 layers to 6 layers) in (a). The sheet resistance of one-layer (1L) and two-layer (2L) films in (a) is beyond our measurement range and is not included in (d). The transmittance spectra of all the 6 films in (a) are shown in Supporting Information, Figure S3. In printed electronics, it is of great interest to print homogeneous narrow lines.4, 21, 26 As shown in Figure 4, the reliability and reproducibility of our technology enables us to print smooth, even, straight and long graphene lines with a width around 80 μm on both plastic substrates (Figure 4a) and silicon wafers (Figure 4d). In Figure 4, although each line has been overwritten with multiple (up to 20) passes, the desired line pattern is retained. The integration of the graphene lines with silver electrodes, also fabricated by inkjet printing, enables us to study their electrical properties. Figure 4b plots the current-voltage curves of 250-μm-long graphene lines with 10, 15 and 20 printing layers. The curves exhibit ohmic characteristics. Figure 4c displays the resistance (R) dependence on the device length (l). An excellent linearity in the measurements suggests that the graphene line resistance follows R = RC + ρl, where RC is the graphene/silver contact resistance and ρ is the line resistivity. RC is negligible (Figure 4c) for all of the three printing layers, and ρ = 4.5 ± 0.3, 1.37 ± 0.06, and 0.83 ± 0.06 kΩ μm−1 for 10, 15, and 20 printing layers, respectively. By adjusting the graphene line length, width, and thickness (proportional to the number of printing passes), our technology may offer a simple solution to small-area embedded resistors with broad resistance values (from a few kΩ to several MΩ) on flexible substrates which are essential components for printed analog circuits.1 Inkjet printed graphene narrow-line resistors on Kapton and TFTs on Si wafer. a) Optical images of an as-printed 15-layer graphene lines (upper) and the final resistors with vertical silver electrodes on Kapton (lower). b) Current-voltage curves of the printed line resistors of length about 250 μm and with different printing layers. c) Resistance versus device length for resistors with different printing layers. d) Optical images of an as-printed six-layer graphene lines (upper) and the final TFTs with vertical silver source/drain electrodes on a silicon wafer (lower). e) Transfer characteristics of a six-layer TFT with channel length about 480 μm. For each scanning, gate voltage increases from –30 V to 30 V and then goes back to –30 V. For the second scanning, the gate cable is not connected. f) Dependence of ON and ratio on channel length for the six-layer graphene All scale bars are 100 μm. the of graphene, the effect and devices have particular with promising applications in there has been increasing interest in thin film with graphene or as source/drain electrodes and/or channel Here, we study the performance of TFTs with inkjet printed graphene and printed silver source/drain We used a silicon substrate with a layer of that as the gate Transfer characteristics versus gate with a of = 1 V and back gate voltage from –30 V to 30 V and back to –30 V The in the measurement is small with the the the gate voltage range for these devices, the effect is confirmed the gate the gate (the second lines in Figure The characteristics versus also confirm the effect (Supporting Information, Figure The in Figure of about because of the absence of the in the our devices have lower ratio than some printed graphene TFTs (about are still comparable to other for the the effective is lower than that of the printed TFTs in other comparable to that of inkjet printed An of these graphene TFTs is that the ratio is to the channel length the is almost to Figure Supporting Information, Figure is a from inkjet printed on because of the of The printed graphene TFTs exhibit which is a prerequisite for applications. the of these graphene TFTs is by their poor there is great potential for as resistors for analog or for with other inkjet materials to the performance. Graphene has been as a promising material for because of high electrical and large surface which can be with electronic and Inkjet printing is as a technology to for applications in flexible 11 It is superior to other direct such as because it can not only the graphene electrodes, also the and even the integration of onto flexible electronic In this work, silver first printed as the in an (Figure graphene printed onto the as the electrodes (Figure to and (Figure the printed silver possess of at a of 100 from the curves (Figure Supporting Information, Figure in However, with only 8 printing layers of graphene, the increases by about 3 to (Figure With a lower the is even at 10 Figure SEM images that graphene flakes from the surface after the films have been which make the of large surface area and to the improved performance of the To further out the efficiency of graphene, an where only one of by 4 layers of graphene (Supporting Information, Figure The curves clearly exhibit suggesting graphene an in the performance. In addition, the graphene have a low of obtained from the in Figure corresponding to a of As for a the printed exhibit of at low and at high should thus be as a in Figure For this the is = where and are the and of the respectively. At the and out and = The plots (Figure of the and of and that the graphene has an lower than 11 for the silver confirming At lower than the can be to a RC In the low a is = where R is the resistance of the At the angle is to the RC as = For our graphene 80 (Figure corresponding to suggests that our printed graphene can much faster than 1 Note that the graphene has a lower than the silver 550 Figure which should result from the have resistance (Figure In our printed flexible graphene shows both high and and hence is promising for applications. Inkjet printed a) Photograph of silver for an which can also be as an with silver b) Photograph of a graphene is on top of silver In (a) and (b), the are long and by The large silver the are for electrical for the The effective area for each is about c) SEM images of silver and d) graphene e) The f) dependence of on and h) dependence of and angle on for with silver electrodes and graphene The of is an for The line in the angle of We note that this work on the efficiency of our inkjet printing technology for graphene, we not to the performance of the devices. For only 8 printing layers of graphene, the of the is comparable to from other it can be further if more layers of graphene are In addition, the materials and device used in this work are mainly in their Therefore, further performance are through other techniques into our printing For proper of can the sheet resistance of the printed transparent conductive films and the resistance range of the printed narrow-line By an gate can be for the printed graphene TFTs could be further by producing inks with graphene with other The printed graphene can be improved by as which in a voltage and hence energy In we have a mature simple technology for inkjet printing of high-quality few-layer graphene. The of solvent exchange and polymer techniques enables us to formulate stable graphene inks with high concentration and compatible fluidic characteristics for efficient and reliable inkjet printing. The of and solvents together with polymers in the inks the coffee-ring effect through unpinning of the contact lines during solvent The technology the of a variety of graphene devices, which in general only printing no more than 10 and annealing (at about °C for at most a few transparent conductive films a sheet resistance around 200 kΩ □−1 at a transmittance of about narrow-line resistors exhibit a resistance range from a few kΩ to several few-layer graphene thin film can be by the effect. a high of and a around As a the technology an efficient and low-cost method to a variety of graphene electronic devices with good performance and is a promising for applications in printed and flexible Graphene First, a of graphite flake mg in DMF for 40 the applications of or 20 other The at 10 for 30 to flakes and the To the of flakes, the mg viscosity 4 cP for in added to the harvested and the dispersion for 1 Finally, mg viscosity 22 cP for in added to obtain a stable graphene added to graphene/DMF dispersion DMF exchanged by terpineol through a vacuum distillation process a DMF to the dispersion to 80 °C and the reduced to 30 After DMF the remaining graphene/terpineol dispersion with the original DMF graphene in the final terpineol dispersion concentrated by about 60 In order to the graphene concentration, a small amount of dispersions with terpineol by 20 and the optical by a 100 at λ = nm. the = is the and the = the graphene to be mg mL−1 and mg mL−1 for of 20 and 40 respectively. to printing, the graphene/terpineol dispersion with ethanol at the volume ratio of to the viscosity and surface tension for inkjet printing. All the patterns and devices printed a with a 10 graphene inks and the commercially available silver inks The annealing process is a simple baking of the devices on a hot The transparent conductive films fabricated by printing the graphene inks onto glass slides at a drop spacing of 30 μm and a substrate temperature of 40 °C. The then on a hot plate at °C for 30 The sheet SEM images and Raman obtained a 100 a a and an 550 Raman respectively. The narrow-line resistors printed on Kapton Kapton μm Before printing, Kapton by in for First, the graphene inks printed multiple into narrow line patterns at drop spacing of μm and substrate temperature of 40 °C. The then at °C for 30 Finally, silver inks printed as electrodes and at °C for 30 The current-voltage curves The TFTs fabricated on silicon wafers with a The and to for narrow-line resistors, that the baking temperature for graphene and silver is °C and respectively. For the device in Figure the effective through = where 480 μm is the channel length, 80 μm is the channel width, = 1 V is the 20 is the gate and 0.4 is the The fabricated on First, a layer of silver printed in an as the at a drop spacing of 20 μm. The at °C for 30 (Figure For graphene different layers of graphene printed on the as electrodes at a drop spacing of 15 μm and a substrate temperature of 60 °C (Figure After printing, the with silver electrodes and with graphene at °C for 30 to further the solvents and then at °C for 1 to the polymer and/or the silver The properties in a with solution as the The curves an at from 10 to 100 a with V and 10 a range from 20 to 100 Supporting is available from the or from the The of and for and The and for in the measurements and and for in SEM and by the through the and the the through 3 in and in and the are As a to our and this by the materials are and may be for are not or issues from than should be to the The is not for the or of any by the than should be to the corresponding for the