Ink-jet printing of graphene for flexible electronics: An environmentally-friendly approach

Solid State Communications ∎ (∎∎∎∎) ∎∎∎–∎∎∎

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Ink-jet printing of graphene for flexible electronics: An environmentally-friendly approach A. Capasso 1, A.E. Del Rio Castillo 1, H. Sun, A. Ansaldo, V. Pellegrini, F. Bonaccorso n Istituto Italiano di Tecnologia, Graphene Labs, I-16163 Genova, Italy

art ic l e i nf o

Keywords: A. Graphene ink B. Liquid phase exfoliation D. Conductive stripes E. PET substrates.

a b s t r a c t Mechanical flexibility is considered an asset in consumer electronics and next-generation electronic systems. Printed and flexible electronic devices could be embedded into clothing or other surfaces at home or office or in many products such as low-cost sensors integrated in transparent and flexible surfaces. In this context inks based on graphene and related two-dimensional materials (2DMs) are gaining increasing attention owing to their exceptional (opto)electronic, electrochemical and mechanical properties. The current limitation relies on the use of solvents, providing stable dispersions of graphene and 2DMs and fitting the proper fluidic requirements for printing, which are in general not environmentally benign, and with high boiling point. Non-toxic and low boiling point solvents do not possess the required rheological properties (i.e., surface tension, viscosity and density) for the solution processing of graphene and 2DMs. Such solvents (e.g., water, alcohols) require the addition of stabilizing agents such as polymers or surfactants for the dispersion of graphene and 2DMs, which however unavoidably corrupt their properties, thus preventing their use for the target application. Here, we demonstrate a viable strategy to tune the fluidic properties of water/ethanol mixtures (low-boiling point solvents) to first effectively exfoliate graphite and then disperse graphene flakes to formulate graphenebased inks. We demonstrate that such inks can be used to print conductive stripes (sheet resistance of
13 kΩ/□) on flexible substrates (polyethylene terephthalate), moving a step forward towards the realization of graphene-based printed electronic devices. & 2015 Elsevier Ltd. All rights reserved.

1. Introduction Printed and flexible electronics is emerging as the next ubiquitous platform for the electronics industry [1]. The realization of electronic devices with performances akin to that of rigid-based platforms, but in lightweight, foldable, and flexible designs, would enable entirely new applications such as conformal and transparent electronics [1–4]. Printing technologies can also play a key role for the realization of rigid ultra-compact devices with tight assembly of components [5,6] conveying inherent advantages such as reduced cost and large electronic system integration by using novel mass manufacturing approaches, unavailable from more traditional platforms [7]. There is a clear market pull and flexible and printed electronics [8] bring advantages into devices and components such as transistors [4], solar cells [9], organic light-emitting diodes [10], and sensors [11]. However, the real revolution is still to come due to a number of technological challenges. In particular, commercial printed

n

Corresponding author. E-mail address: [email protected] (F. Bonaccorso). 1 These authors contributed equally to this work.

electronics should be electrically, optically and mechanically robust, with materials and components meeting essential performance criteria, such as low resistivity or transparency, under mechanical deformation [12]. Moreover, materials should be environmentallyfriendly. These requirements are unbearable to combine with any existing low-cost mass-manufacturing approach. Graphene is expected to play a role here, and graphene-based technology might deliver benefits in terms of both cost advantage and uniqueness of properties and performance [12]. Amongst printing and coating technologies [13], such as spray or rod-coating, gravure, flexographic and screen printing as well as laser patterning [14,15], ink-jet printing [5] is a well-suited technique for the direct deposition of novel nanomaterial-based inks. In an ink-jet process, it is mandatory to obtain a regular jetting from the print-head nozzles preventing printing instability, such as satellite drops and jetting deflection [16,17]. The realization of printable inks made of nanomaterials is thus a very challenging task, since the various rheological properties such as density (ρ), surface tension (γ), and viscosity (ν) have a strong effect on the printing process [18]. These properties, along with the nozzle size, need to be carefully evaluated and tuned ondemand for the proper formation and ejection of droplets from the nozzles [19]. The morphological properties (the lateral size in

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particular for two-dimensional materials – 2DMs–) of the nanoparticles/nanotubes/flakes dispersed in the ink as well as the formation of aggregates in the ink and their accumulation on the print-head can also contribute to printing instability. It has been found [20] that limiting the lateral dimensions of the dispersed nanomaterials to
1/50 of the nozzle diameter can largely reduce these detrimental effects. Several inks based on nanomaterials have been produced so far, ranging from organic semiconductors [21] to metallic nanoparticles (MNPs) [22] and carbon nanotubes (CNTs) [23–26]. However, all these nanomaterials suffers several limitations. For example, organic semiconductors [27], used mainly for the realization of thin film transistors (TFTs), have low mobility (m) of charge carriers (
1 cm2 V  1 s  1) [11] while metallic nanoparticles [28,29] are mostly based on costly materials (silver and copper) that are not stable in most common solvents (e.g., water, isopropyl alcohol, acetone, and many others), thus requiring stabilizing agents [18,30]. Moreover, once printed, MNP-based inks tend to oxidize [18,30]. Carbon nanotubes in theory have the advantage, over other nanomaterial, of being electrically heterogeneous (can be both metallic and semiconducting) in nature [31]. However, this in turn emerged as their main limitation for practical applications, requiring a selective growth [32] and/or a sorting process for their separation [33–37] to exploit in full the CNT electronic properties. Thanks to their exceptional and complementary properties, graphene [38] and other 2DMs [15,39–41] are being exploited as functional materials for ink formulation [16,42–45]. The flakes of these 2DMs can be dispersed in various solvents, both aqueous [46] and organic [47] by liquid phase exfoliation (LPE) as a first step to produce printable inks [16,45,48]. In particular, thanks to their versatility, graphene-based inks have been exploited also for 3D printing, which holds great potential for the fabrication of fully-customized, ondemand designs [49]. First examples of extrusion-based 3D printing of graphene-based structures with sub-micron diameter were recently reported: Kwon's group fabricated freestanding nanowires made of reduced graphene oxide (RGO) [50], while Hersam's group printed a composite of graphene and polylactide-co-glycolide (a biocompatible elastomer) demonstrating potential applications in electronic and bio-medical devices [51]. To date, the most-effective solvents for the production of graphene-based inks, such as N-Methyl-2-pyrrolidone (NMP), Dimethylformamide (DMF), Dimethyl sulfoxide (DMSO), just to cite a few, are not environmentally benign [52] and this is posing a severe limitation for the development of a graphene-based printing technology. Moreover, all the aforementioned solvents have high boiling point (4170 1C) [53]. Non-toxic and low boiling point solvents such as water and alcohols, which would be crucial to develop a fully environmentally-compatible ink-jet printing process, however, require the addition of stabilizing agents like polymers or surfactants for the dispersion of graphene and 2DMs. Nevertheless, the presence of such stabilizers in the ink compromises the graphene and 2DMs (opto)electronic properties once printed on the target substrate. In this work, after a brief overview on background and state-ofthe-art of graphene inks formulation and ink-jet printing parameters optimization, we demonstrate the printability of graphene inks in environmentally-friendly solvent, i.e., ethanol/water (EtOH/ H2O) mixture, on flexible substrate such as polyethylene terephthalate (PET). The printed features do not need any postprocess treatment (as opposed to the case of graphene oxide (GO) [54] and RGO [55]), and, contrarily to NMP inks, the ink-jet printing process can be carried out at low temperature (up to 60 1C). Further optimization will enable the scaling up of production/formulation of environmentally-friendly graphene-based EtOH/H2O inks and their deposition on flexible substrates.

2. Background on printable inks of pristine graphene The first step in the production of graphene inks is the dispersion of the flakes in solvents to create homogeneous, in term of lateral size and thickness, and stable dispersions [56,57]. Solvents such as Nmethyl-2-pyrrolidone (NMP) and Dimethylformamide (DMF) are mostly used for the production and processing of graphene dispersions as well as for their inks formulation, as they possess minimal interfacial tension with the graphitic flakes [58], a condition that eases the dispersion of graphitic flakes. However, NMP and DMF are far from being optimal solvents for the fabrication of devices by inkjet printing owing to the limited sustainability of the process, since NMP and DMF are included in the candidate list of substances of very high concern [52] and may have teratogenic effects [53,59,60]. Moreover, the high boiling point (4170 1C) [53] of such solvents makes difficult their removal after the coating/printing process, especially for plastic substrates due to the limited resistance to the evaporation process requiring high temperature (4150 1C). This, in turn, influences both the morphology of the printed pattern and the optical/electrical performances (i.e., sheet resistance – Rs- and transmittance – Tr). The exploitation of low boiling-point solvents [47], such as acetone, chloroform, isopropanol, etc. can be an alternative. However, the γ of these solvents is too low (
25 mN m  1) for the exfoliation of graphite, making the yield (i.e., percentage of single layer graphene -SLG-) as well as the concentration of the dispersed flakes in the solvent by far too low [47] compared to the ones achieved with, for example, NMP [16,56,61]. Water has a γ
72 mN m  1 [62], too high (
30 mN m  1 higher than NMP) for the dispersion of graphitic flakes [63]. In this case, the exfoliated flakes can be stabilized against re-aggregation by Coulomb repulsion using surfactants (e.g., sodium dodecylbenzenesulfonate [57], sodium cholate [64] and sodium deoxycholate [58,65]), and/or polymers (e.g., pluronic [46]). However, having the dispersant molecules wrapped around the graphene/ graphitic flakes [34] is not the best option in view of the realization of conductive channels since the presence of these molecules can decrease the inter-flake connectivity and consequently the electrical conductivity of the printed patterns [15,66]. As a consequence, there is a clear need for alternative solvents with appropriate rheological properties for safety and sustainability of the printing process of graphene and other 2DMs. Finding environmentally-friendly, inexpensive and low-boiling point solvents would also make the up-scaling of the process viable towards industrial production, where safety considerations related to toxicity generally translate into cumbersome and costly countermeasures (e. g., safety equipment, fume hoods, exhausts, etc.). An approach often used to avoid the use of unwanted solvents involves the production of inks based on GO rather than pristine graphene, since GO can be easily dispersed in safe and clean solvents such as water [55,67]. However, GO is an insulating material and needs a thermal or chemical treatment for its reduction, i.e., the removal of oxygen groups [68–70]. Such post-processing treatments, usually, require harmful chemicals, such as hydrazine [71], or experimental conditions that are incompatible with the fabrication of electronic devices [72]. Moreover, RGO does not fully regain the pristine graphene electrical conductivity [15,73], thus limiting the possible applications of such printable inks in flexible electronics [1]. Several recent research efforts have been focussed on the development of printable inks of pristine graphene flakes. One promising approach entails the use of a co-solvent formulation to increase the affinity between solvent and pristine graphene flakes, as well as other 2DMs, by using a mixture of solvents [74], e.g., water/isopropyl alcohol [75], water/ethanol [74,75], etc. By adjusting the relative concentration of the co-solvents it is possible to tune the rheological properties (i.e., γ, ν and ρ) [76] of the mixture “on-demand”. However, the concentration of the graphitic flakes

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dispersed as well as the percentage of SLG, obtained by the exfoliation process of graphite in such co-solvent mixtures is, up to date [74,75] [0.39 mg mL  1 (for 2-propanol/1,2-dichlorobenzene) [77] lower than the ones achieved in NMP [16] [up to 1 mg mL  1][78] and water-surfactant dispersions [1 mg mL  1] [57,65,79]. Moreover, the stability of co-solvent mixtures [74–76], mostly based on water and alcohols, is an issue. Indeed, γ changes exponentially after the addition of alcohols to water [76] and is very sensitive to solvent evaporation [75]. Moreover, all the rheological properties of alcohol-based co-solvents are temperature-sensitive [76]. This is a problem both for the processing (the ultrasonication causes a local temperature increase of the dispersion even if the process is thermalized) and for the shelf-life of the dispersions/inks. For the ink-jet printing of graphene flakes, the challenge is represented by the requirements concerning the choice of solvents able to disperse the flakes in addition to the aforementioned constrains related to the ink printability (e.g., a printable ink should have a γ between 28 and 33 mN m  1 [80]), thus limiting the selection/choice of suitable solvents. Table 1 reports the Rs and Tr literature values obtained for stripes printed with graphenebased ink in different solvents and printing/post processing conditions. The earliest attempts [16,45] exploited graphene inks prepared in NMP [16] and DMF (subsequently exchanged to terpineol [45]), respectively, to print conductive stripes reaching Rs ¼30 kΩ/□ on glass slides. Later on, graphene conductive stripes with Rs o15 kΩ/□ were reported, at the best of our knowledge, four times in literature [43,44,48,81] . In Refs. [48,81] the authors printed graphene inks on rigid substrates (SiO2 [81] and glass [48]) previously treated with hexamethyldisilazane, to prevent undesired coffee-ring effect of the printed features. In both cases the asprinted graphene stripes were thermally post-annealed at temperatures higher than 250 1C, achieving Rs in the 1–3 kΩ/□ range [48,81]. Ref. [44] exploited graphene ink in ethylene glycol mixed with a copolymer of N-vinyl-2-pyrrolidone and vinyl acetate to print on “FS3” papers (a glossy, polymer-coated paper specifically designed to increase the wettability), achieving Rs
1–2 kΩ/□. Ref. [43] used a graphene ink in NMP to print conductive stripes on PET foils coated with aluminium oxide and polyvinyl alcohol (such coating reduces substrate-related drying problems [43]), reaching Rs ¼2 kΩ/□. Building on the existing literature, we explore the use of low boiling point and environmentally-friendly co-solvents (ethanol/water

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mixture), for the printing of electrically conductive graphene stripes on PET without any pre- or post-treatments and at low temperature.

3. Experimental 3.1. Materials Graphite flakes (þ100 mesh, Z75% min), NMP (99.5% purity) and ethanol (absolute alcohol, without additive, Z99.8%) were purchased by Sigma-Aldrich and used without further purification. 3.2. Preparation of the inks We exploited liquid phase exfoliation of graphite [56] to produce the graphene inks [16] in NMP and in the mixture EtOH/H2O. For the NMP-based ink, 1 g of graphite flakes (Sigma Aldrich) was dispersed in 100 mL of NMP and ultrasonicated (Bransons 5800) for 6 h. The obtained dispersion was then ultracentrifuged at
16,000g (in a Beckman Coulter Optima™ XE-90 with a SW41Ti rotor) for 30 min at 15 1C, exploiting sedimentation-based separation (SBS) to remove thick flakes and un-exfoliated graphite [82,83]. After the ultracentrifugation process, we collected the supernatant by pipetting. The optimization of ink-jet printing ideally requires highly concentrated inks [16,45,48]. In order to achieve such a target, the supernatant extracted after the first ultracentrifugation process was further ultracentrifuged at
200,000g for 60 min at 15 1C. The high g force value promotes the sedimentation of the graphene flakes at the bottom of the ultracentrifuge tubes taking advantage of the higher density of the graphene flakes (
2.1 g/cm3) [84] in comparison with the solvent (ρNMP ¼1.03 g/cm3) [85]. The pellet (deposited graphene flakes) is collected and the supernatant is discarded. The pellet was re-suspended in 3 mL of pure NMP using an ultrasonic bath for 10 min. This sonication time was sufficient to re-disperse the graphene flakes, thus obtaining a stable (for more than 2 months) ink. The EtOH/H2O ink was then prepared as follows. 1 g of graphite was dispersed in an EtOH/H2O mixture [1:1 in volume] by ultrasonication for 6 h (Bransons 5800). The mixture was centrifuged at 670g for 10 min (in a Beckman Coulter Optima™ XE-90 with a SW41Ti rotor), longer centrifugation time or higher speed endorses the precipitation of the flakes in dispersion together with non-exfoliated graphitic material. After the centrifugation process,

Table 1 Printing tests with inks made of LPE graphene in various solvents. Authors, year

Rs [kΩ/ □]

30 Torrisi et al., 2012 [16]
3 Secor et al., 2012 [81] 30 Li et al., 2013 [45] Arapov et 1–2 al., 2014 [44] Finn et al.,
2 2014 [43] Gao et al.,
1 2014 [48]

Tr [%] or thickness [nm]

Ink type

Substrate

Posttreatment

80%

LPE graphene in NMP

Si/SiO2 and glass with hexamethyldisilazane (HMDS) and O2 treatment

170 1C for 5 min

140 nm

LPE graphene in ethanol and ethyl cellulose mixed in cyclohexanone/terpineol

HMDS-treated SiO2

250 1C for 30 min

80%

LPE graphene in DMF, exchanged to terpineol

Glass slides

800 nm

Ethylene glycol þ Plasdone S-630 (copolymer of N-vinyl-2-pyrrolidone and vinyl acetate ink)

FS3 paper (glossy, polymer-coated paper from Felix Schoeller) and LumiForte paper(rough paper without coating from Stora Enso)

375–400 1C for 30– 60 min None

160 nm

LPE graphene in NMP

PET coated with aluminium oxide and polyvinyl alcohol

None

60%

LPE graphene mixed with ethyl cellulose and cyclohexanone

HMDS-treated glass slides

None

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the supernatant was collected by pipetting. In order to get a concentrated graphene ink, the supernatant was ultra-centrifuged at
16,000g for 15 min at 15 1C. The supernatant was discarded and the pellets were re-suspended in 3 mL of pure EtOH/H2O [1:1] mixture, using an ultrasonic bath for 10 min, to re-disperse the graphene flakes, thus obtaining the final ink. After 1 week, there is the formation of sediments that are however easily re-dispersed in the same solvent by manual shaking of the bottle containing the ink.

3.3. Characterization of the inks 3.3.1. Electron microscopy As-prepared inks (both in NMP and EtOH/H2O [1:1] solutions) were characterized morphologically (i.e., lateral size and thickness) by a transmission electron microscopy (TEM) (JOEL JEM 1011). The as-prepared inks (diluted 1:100 in NMP and EtOH/H2O [1:1]) were dropped with a pipette on holey carbon 200 mesh grids and dried under vacuum overnight. The acceleration voltage used for the measurements was 100 kV.

3.3.2. Optical spectroscopy 3.3.2.1. Optical absorption spectroscopy. Optical absorption spectroscopy (OAS) of the graphene inks were performed in the range 300–1200 nm with a Cary Varian 6000i UV–vis–NIR spectrometer. The absorption spectra were acquired using a 1 mL quartz glass cuvette. The inks were diluted to 1:10 for EtOH/H2O and to 1:100 for NMP, to avoid scattering losses at higher concentrations. The corresponding solvent baseline was subtracted to each spectrum. The concentration of graphitic flakes is determined from the optical absorption coefficient at 660 nm, using A¼ αlc, where l [m] is the light path length, c [g L  1] is the concentration of dispersed graphitic material, and α [L g  1 m  1] is the absorption coefficient, with α
1390 L g  1 m  1 at 660 nm [57,58].

3.3.2.2. Raman spectroscopy. The as-prepared NMP- and EtOH/ H2O-based graphene inks were drop-cast onto a Si wafer with 300 nm thermally grown SiO2 (LDB Technologies Ltd.) and dried under vacuum. Raman measurements on both graphene inks were collected by a Renishaw inVia confocal Raman microscope using an excitation line of 532 nm (2.33 eV) with a 50  objective lens, and an incident power of
1 mW on the samples. We used Lorentzian functions to fit the peaks. For each sample we collected more than 20 spectra.

3.3.3. Rheological measurement The viscosity of the inks was measured with a Discovery HR-2 Hybrid Rheometer (TA instruments), using a double-wall concentric cylinders geometry (inner diameter of 32 mm and outer diameter of 35 mm), designed for low-viscosity fluids. The temperatures of the inks were set and maintained at 25 1C throughout all the measurements.

3.4. Deposition of the inks on flexible substrates The graphene inks (both in NMP and EtOH/H2O) were ink-jet printed on PET with a Fujifilm Dimatix 2800 printer. After the preparation and characterization phase, 2 mL of ink was loaded into the cartridge reservoir (fluid bag) via a syringe with a needle. The filled cartridge was then loaded in the printer.

3.5. Characterization of printed patterns 3.5.1. Electron microscopy The printed patterns were imaged by loading the printed PET samples in a field-emission scanning electron microscope (FESEM) (Joel JSM-7500 FA) without any pre-treatment. 3.5.2. Optical absorption spectroscopy Transmittance spectra of the printed patterns were performed in the 250–1200 nm range with a Cary Varian 6000i UV–vis–NIR spectrometer, using a 1 mm pinhole holder. The pristine PET substrate was used as a baseline. Each sample was measured 5 times and the average values were reported. 3.5.3. Raman spectroscopy Raman measurements on the printed stripes (both from NMPand EtOH/H2O based inks) are collected via a Renishaw inVia confocal Raman microscope using an excitation line of 532 nm (2.33 eV) with a 50  objective lens, and an incident power of
1 mW on the samples. We used Lorentzian functions to fit the peaks. For each sample we collected 20 spectra. 3.5.4. Electrical characterization All the electrical measurements have been performed with a Keithley Model 2612A Dual-channel System Source Meter in two wires configuration. The printed pattern (1 by 4 mm) was placed crossing two parallel palladium wires spaced by 1 mm. The crossing, between the printed pattern and the palladium wires, defines a 1 by 1 mm measuring area. A constant voltage of 1 V was applied.

4. Results and discussion The production of the environmentally-friendly graphene ink in ethanol and water mixture was carefully designed to obtain a stable ink, having the required parameters in term of ρ, γ and ν for the optimal ink-jet printing process [19]. In this context, graphene flakes are neither stable in ethanol nor in water due to the mismatch between the γ of these two solvents (72 mN m  1 for water [62], and 24.5 mN m  1 for ethanol [50]) and the surface energy of graphite [15]. According to Wang et al., in order to obtain a stable dispersion of graphene flakes, γ should be close to
46.7 mN m  1 [63] (for comparison γ of NMP is
41 mN m  1) [56]. Some groups [47,86– 89] also exploited the Hansen solubility parameters (HPs) [90] to study the stability of graphene flakes in diverse solvents. The HPs [90] are three: the energy from dispersion forces between molecules (δD), the energy from dipolar intermolecular force between molecules (δP) and the energy from hydrogen bonds between molecules or electron exchange parameter (δH) [90]. These parameters generally describe the solubility of a molecule in a solvent [90]. In our case the HPs describe the graphene dispersion stability in different solvents [47,86–88,90]. The required HPs of a solvent to stabilize graphene flakes are: δD
18 MPa1/2, δP
10 MPa1/2, δH
7 MPa1/2 [47,86–88] (for comparison HPs of pure NMP are: δD
17.4 MPa1/2, δP
13.7 MPa1/2, δH
11.3 MPa1/2 [47,86–88,90]). These three parameters can be treated as coordinates for a point in three dimensions also known as the Hansen space [90]. According to Hansen's Handbook [90], the distance between two points in the Hansen space is known as Hansen distance (Ra), expressed in MPa1/2. Thus, an ideal solvent for dispersion of a molecule or material is the solvent which has the HP located in the Hansen space closer to the coordinates of the molecule or material [90]. Consequently, shorter is the Ra, between the Hansen coordinates of the solvent and the Hansen coordinates of the material or molecule, stronger will be the solvent/material interaction [90].

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Contrary, if the Ra is large, the solvent is not adequate to disperse or stabilize the material. In our particular case the Hansen distance can be defined as [90]:
2
2
2 ð1Þ Ra 2 ¼ δPG  δPS þ 4 δDG  δDS þ δHG  δHS where the subscripts G and S refers to graphene and the solvent mixture, respectively. The HP values for the mixture EtOH/H2O are calculated as follows:[90]

δPS ¼ τδPEtOH þ ð1  τÞδPH2 O

ð2Þ

δDS ¼ τδDEtOH þ ð1  τÞδDH2 O

ð3Þ

δHS ¼ τδHEtOH þ ð1  τÞδHH2 O

ð4Þ

where τ is the volume fraction of ethanol. Table 2 shows the γ adapted from Ref. [91] and the calculated HPs for different ratios of EtOH/H2O mixtures and their respective Ra to the graphene Hansen coordinates. The calculated Ra corresponding to the best γ for graphene (
46.7 mN m  1 [63]), reported in Table 2, indicates that the EtOH/ H2O (20:80) mixture provides the best condition to obtain stable graphene dispersion. The calculated value agrees with results reported by other research groups [47,86–88,90] (for comparison, the calculated Ra between NMP and graphene is 5.7 MPa1/2) [86,90]. In principle the EtOH/H2O (20:80) mixture should represent the ideal combination to perform exfoliation and dispersion of the graphitic flakes. However, considering the fluid formulation guidelines for the Fujifilm Dimatix 2800 printer, the optimal γ values should be in the range of 28–33 mN m  1 [80]. Values in this order can be met with a 50% volume ratio EtOH:H2O (30.90 mN m  1). As reported in Table 2, the 50% EtOH:H2O volume ratio shows a good compromise between γ and HP, thus we produced the ink starting from this volume ratio of the two solvents. The rheological properties of the as-formulated inks were characterized by means of OAS and viscosity measurements under shear conditions. The concentration of the two inks was calculated by OAS. Fig. 1a shows the absorption spectra of NMP ink diluted 100 times and the EtOH/H2O ink diluted 10 times. The peak at
266 nm, for both inks, is a signature of the van Hove singularity in the graphene density of states [92]. The asymmetry of the UV peak in both spectra, with a high-wavelength tail, is attributed to excitonic effects [93]. The obtained concentrations were 3.32 mg mL  1 for NMP ink and 0.62 mg mL  1 for EtOH/H2O ink. Concerning the ν of our inks, Fig. 1b shows the ν of the inks under shear stress (the ν values at the constant shear rate of 10 s  1 are reported as an inset). The NMP ink has ν ¼3.14 mPa s which is almost twice the one of NMP solvent (ν ¼1.59 mPa s). By contrast, the EtOH/ H2O ink has ν ¼ 2.47 mPa s, which is only 8.5% higher than the EtOH/ H2O solvent mixture's one (2.26 mPa s) [94]. ν of the NMP-based ink decreases with increasing shear rates (Fig. 1b), showing shear thinning (also known as pseudoplastic) properties, as expected for a structured fluid (i.e., a colloidal suspension) [95,96]. The ν of the EtOH/H2O-based ink is instead almost independent of the shear rate, at least in the shear rate range here investigated, a behaviour typical of Newtonian fluid (i.e., a fluid in which the viscosity arising from its flow is linearly proportional to the strain rate, such as water) [97]. These different trends for the two inks are directly linked to their concentrations in the respective solvents, being the NMP-based ink five times more concentrated than the EtOH/H2O ink. Indeed, by diluting the NMP ink (concentration value of
0.6 mg mL  1, i.e., comparable with the EtOH/H2O one) we obtained ν values independent on shear rate, i.e., a Newtonian flow behaviour. These results demonstrates that the ν values achieved with the EtOH/H2O- and NMP-based ink are within the range required for inkjet printing (up to 12 mPa s) [80].

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An additional figure of merit that was proposed in the past to account for the drop formation and give an indication on the printability of an ink is the Z number, which is defined as the inverse of the Ohnesorge number Oh=η/(γρα)1/2 [16]. Inks with Z values in the range 1 oZ o14 were originally observed, see Ref. [16] for an overview on this subject, to guarantee the best performance in terms of regular drop formation, jetting accuracy, and attainable jetting frequency; however, Torrisi et al. reported NMP-based graphene printable inks with Z number as high as 24 [16]. In our case, considering the η values reported in Fig.1b, the literature values of γ [53], the measured ρ and the the a value (21μm), the Z values are 9.6 and 10.3 for the NMP- EtOH/H2Obased ink, respectively. Both the Z values are well within the “restricted” (1-14) optimal Z range. We carried out the characterization of the structural and morphological properties of the flakes dispersed in the two inks by Raman spectroscopy, which is a fast and non-destructive technique widely used to identify number of layers, defects, doping, disorder and chemical modifications of graphene [98,99]. Fig. 2a plots (black curve) the typical spectrum, with excitation wavelength of 532 nm, of graphite deposited on Si/SiO2. The Raman spectra of representative flakes of NMP- and EtOH/H2O-based graphene inks are plotted in orange and green lines, respectively. The G peak corresponds to the E2g phonon at the Brillouin zone centre [100]. The D peak is due to the breathing modes of sp2 rings and requires a defect for its activation by double resonance [98,99,101]. The 2D peak is the second order of the D peak [98]. This is a single peak in monolayer graphene, whereas it splits in multi-layer graphene, reflecting the evolution of the band structure [98]. The 2D peak is always seen, even when no D peak is present, since no defects are required for the activation of two phonons with the same momentum, one backscattered from the other [98]. Double resonance can also happen as intra-valley process, i.e. connecting two points belonging to the same cone around K or K0 [98]. This process gives rise to the D0 peak. The 2D0 is the second order of the D0 . From statistical analysis, based on 20 measurements for each sample, we find the 2D peak position (Pos (2D)) peaked at
2695 cm  1 and
2700 cm  1 for NMP- and EtOH/ H2O graphene inks, respectively. The full width at half maximum of 2D (FWHM(2D)) (Fig.2b) varies from 62 to 76 cm  1 and from 64 to 78 cm  1 NMP- and EtOH-based graphene inks, respectively. The I (2D)/I(G) ranges from 0.45 to 0.7 for NMP-based ink and from 0.4 to 0.75 for EtOH/H2O-based ink (Fig. 2d). This is consistent with the samples being a combination of SLG and few-layer graphene (FLG) flakes.[82] The Raman spectra show significant D and D0 peaks intensity, with I(D)/I(G) ranging from 0.1 to 0.5 and from 0.4 to 1.6 for EtOH/H2O and NMP-based inks, respectively (Fig. 2e). This is attributed to the edges of our sub-micrometre flakes [102] rather than to the presence of a large amount of structural defects within

Table 2 γ values were taken and adapted from Ref. [91]. The HPs for different EtOH:H2O volume ratios were calculated. γ (mN m  1)

% Vol EtOH

H2O

0 10 20 30 40 50 60 70 80 90 100

100 90 80 70 60 50 40 30 20 10 0

72.72 51.80 41.47 36.25 33.18 30.90 29.00 27.43 26.25 25.37 24.55

Hansen solubility parameters (MPa1/2) δD

δP

δH

Ra

16.70 16.61 16.52 16.43 16.34 16.25 16.16 16.07 15.98 15.89 15.80

18.70 17.71 16.72 15.73 14.74 13.75 12.76 11.77 10.78 9.79 8.80

16.70 16.97 17.24 17.51 17.78 18.05 18.32 18.59 18.86 19.13 19.40

19.96 18.41 16.96 15.62 14.45 13.47 12.72 12.27 12.13 12.32 12.82

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Fig. 1. (Colour online) (a) Absorption spectra of graphene inks in EtOH/H2O (green curve) and NMP (orange curve) diluted 1:10 and 1:100 respectively. The concentrations are 0.62 mg mL  1 for EtOH/H2O ink and 3.32 mg mL  1 for NMP ink. (b) ν of the produced inks vs shear rate for EtOH/H2O (green curve) and NMP (orange curve) inks. In the inset-table the ν values of the inks and of the solvents are reported, measured at 25 1C for a constant shear rate of 10 s  1 (each value was averaged over 100 measurements).

the flakes. Indeed, if a large amount of defects are present in the basal plane of graphene the G and D0 peak become broader merging into a single wide band [101], which is not the case for the spectra of our inks. Moreover, there is not a clear correlation between I(D)/I(G) and FWHM(G) (see Fig. 2h), an indication that the major contribution to the D peak comes from the sample edges, for both NMP- and EtOH/ H2O-based ink. We can attribute this to disorder of the graphene edges rather that defects within flakes [102]. The low-resolution TEM bright field images (Fig. 3a and e for NMP- and EtOH/H2O-based inks, respectively) show that graphene flakes in the two inks have markedly different lateral size distributions, with an average lateral size peaked at
200 nm for the NMPbased ink and a much broader size distribution in the 100–2000 nm range for the EtOH/H2O-based ink (see histograms in Fig. 3b and f). The high-resolution images of isolated flakes in Fig. 3c (NMP) and Fig. 3g (EtOH/H2O) and the corresponding electron diffraction patterns (Fig. 3d and h) demonstrate that the flakes are crystalline. All the rings can be indexed as h,k, h k,0 reflections of an hexagonal lattice with a¼ 0.244(1) nm, in agreement with the graphene structure [103]. 4.1. Printing on PET When using a new ink, the printer settings need in general to be adjusted in order to obtain a regular and constant drop jetting [18,19]. In our case, we ran preliminary tests with both NMP- and EtOH/H2O-based ink, finding out a set of parameters for the two inks. Following this procedure, we were able to make a comparison of the printed patterns made with the two inks in the same printing conditions. Clearly, the printer parameters can be further optimized for each ink to improve the process to a greater extent, a study that is beyond the scope of this work. In particular, the main parameters to be set are the cartridge temperature, the nozzle's voltage waveform, the drop spacing and the platen temperature, as explained in details in the following. The cartridge temperature can be controlled to regulate the working ν of the fluid, if needed. In our case it was set to 35 1C for all the depositions. A voltage waveform controls the movements of the printhead nozzles. During the printing process, the nozzles need to draw fluid into the pumping chamber, hold the fluid for a certain time, and then eject the fluid out forming a drop [19]. These movements are driven and controlled by piezoelectric elements [19]. The voltage applied to the piezoelectric elements has a specific waveform that can be edited to regulate the drop ejection and speed. We designed a waveform that triggered a regular ejection of drops for both the inks. A low amplitude pulse was additionally given to the nozzles at a frequency of 2 kHz to prevent the accumulation of residual flakes on the nozzle outlet. The drop spacing is the centre-

to-centre distance in X and Y of the drops that the printer deposits on the target substrate to create the pattern. The optimization of the drop spacing is crucial to avoid possible “coffee ring” effects, which would limit the uniformity of the flakes spreading [104]. The “coffee ring” effect takes place during the solvent evaporation and is more or less marked on the account of the ink ν and the interface tension existing between the substrate and the solvent [20,105]. The “coffee ring” effect is responsible of an anisotropic deposition of the material on the ring of the drops and might reduce the homogeneity of the deposition [104]. After preliminary tests, the drop spacing was fixed to 25 mm, a value that limited the “coffee ring” effect and provided a reasonable uniformity in the deposition with both the NMP- and EtOH/H2O-based inks. After tuning the printer settings, the ejected drops came out in phase amongst each other, all with a regular round shape, as shown in Fig. 4 for the EtOH/H2O ink. 1–5 nozzles out of the 16 available on the printhead were used for printing. The use of more than 1 nozzle reduces the time required for the process, which can be lengthy for a high number of printing passes, but it deteriorates the reproducibility of the process, particularly relevant in the optimization phase of the inkjetting. We found a good compromise for the rheological properties of our inks with up to 5 nozzles. The PET substrate was kept at 60 1C (i.e., the maximum temperature possible of the printer's platen) to ease the solvent evaporation during the printing process. Rectangular stripes (5  2 mm2) were printed with a number of printing passes spanning from 3 to 25. We aimed at printing continuous layers of interconnected graphene flakes without voids or agglomerates in order to obtain an uniform coverage of the substrate within the whole pattern area. The continuity and uniformity of the printed graphene stripes was desired to lower their Rs as much as possible, keeping a constant Tr. The morphology of the printed flakes as well as the coverage obtained on the substrate after each print were characterized by SEM analysis. The SEM micrographs of the printed stripes in Fig. 5 highlight some differences in the deposition carried out with the two inks. After 3 printing passes, the area printed with the NMP ink is completely covered with graphene flakes (Fig. 5(a)), while with the EtOH/H2O ink the deposition is less homogeneous with areas covered with graphene flakes alternated with ones where the coating is not uniform (Fig. 5c). In these cases, such differences can be mainly accounted for by the different concentrations of the two inks (3.32 mg mL  1 for NMP ink and 0.62 mg mL  1 for EtOH/H2O ink). It is interesting to note, however, that the graphene flakes from NMP ink appear all embedded in the printed stripe and do not show well-defined boundaries between each other (identifiable instead in the EtOH/ H2O printed flakes): this may be caused by some NMP residues left

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Fig. 2. (Colour online) (a) Raman spectra of graphite and graphene inks in NMP (orange) and EtOH/H2O (green), (b) Pos(2D) for graphene ink in NMP and EtOH/H2O. Distribution of (c) FWHM(2D), (d) I(2D)/I(D), (e) ratios I(D)/I(G), (f) Pos(G), (g) FWHM(G) and (h) I(D)/I(G) ratios as a function of FWHM(G).

after the printing process, considering the high boiling point of the solvent (
202 1C) [58]. When increasing the number of printing passes to 12 or more, the stripes printed with both inks appear more uniform with interconnected graphene flakes (see the SEM micrographs in Fig. 5b and d), and therefore a continuous film is formed. Fig. 6 shows the Raman spectra of graphene patterns printed on PET with different number of printing passes. Up to 12 printing

passes, some Raman features of graphene (e.g., D0 ) are superimposed to the Raman peaks of the PET. With the increase of the number of passes only the graphene peaks are seen. Moreover, the D peaks and 2D peaks of graphene show no significant changes in shape or intensity ratios for printing passes higher than 18. The analysis of the Raman spectra in Fig. 6c–f highlights that by increasing the number of printed passes, the deposited material shows Raman features very close to those of

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Fig. 3. TEM analysis of graphene flakes cast on TEM grid from NMP- (upper row) and EtOH/H2O-based (lower row) inks: (a) and (e) low-magnification images; (b) and (f) histograms of statistical lateral size distribution; (c) and (g) high-magnification images of isolated flakes with (d) and (h) corresponding selected area diffraction patterns.

Fig. 4. Drop formation of one single nozzle after the optimization of the ink-jet parameters for the EtOH/H2O ink.

Fig. 5. FE-SEM images of the surface of the patterns inkjet printed on PET with the two different graphene inks while varying the number of passes. NMP ink: (a) 3 and (b) 12 printing passes. EtOH-H2O ink: (c) 3 and (d) 15 printing passes.

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Fig. 6. Raman spectra of bare PET foil and patterns printed with inks in (a) NMP and (b) EtOH/H2O. Peak analysis of 12 and 25 printed passes, (c) FWHM(G), (d) I(D)/I(G), (e) Pos(2D), (f) FWHM(2D), (g) I(2D)/I(G) and (h) I(D)/I(G) vs. FWHM(G).

the corresponding graphene inks. In particular, the 2D peak still displays a FWHM(2D) distinctly different from that of graphite. This implies that the flakes are electronically decoupled, behaving as a collection of either SLG or FLG. Moreover, Raman analysis, and in particular the I(D)/I(G) ratio as function of the FWHM(G), does

not reveal structural defects on the deposited flakes for both inks (see Fig. 6). The Rs of the printed stripes vs. the Tr at 550 nm is reported in Fig. 7. The graphene stripes printed from NMP have a Rs ranging from 22 to 173 kΩ/□, with a corresponding Tr ranging from 5% to

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Fig. 7. Tr vs. Rs for 6, 12, 18 and 25 printing passes. Inset: optical image of printed stripes for NMP (left) and EtOH/H2O (right) inks after 6 and 25 printing passes on PET substrates, respectively.

37%. The Rs of the printed graphene films from EtOH/H2O ranges from 13 kΩ/□ to more than 10 MΩ/□, with a corresponding Tr of 22–70%, respectively. Due to the higher concentration of the NMP ink, the NMP films are on average less transparent than the EtOH/ H2O ones for same numbers of printing passes. The electrical measurements confirm that a good substrate coverage was obtained after 3 printing passes only with the NMP ink, while almost only isolated flakes were obtained with the EtOH/H2O in the same printing conditions. With 6 printing passes, the EtOH/H2O stripe has a Tr¼ 70% but the Rs is higher than 10 MΩ/□, meaning that the printed graphene flakes form a rather discontinuous percolation path for the charge carriers (as observed by SEM in Fig. 5 for 3 printing passes). With the same printing passes (6), the NMP stripe instead has Rs¼ 173 kΩ/□ (with Tr¼37%), as result of a better electrical percolation path. By increasing the number of printing passes to 12, the EtOH/H2O stripes have lower Tr (55% vs. 22%), but similar Rs value (
75 kΩ/□). At 18 printing passes and beyond, the stripes printed from EtOH/H2O have at the same time higher Tr and lower Rs than the NMPbased ones. These trends are remarkable as they demonstrate that by printing on PET at low temperature and without any post-annealing treatments, higher values of conductivity can be reached with an EtOH/H2O ink by depositing a smaller amount of material (the same number of printing passes, 18 or more, but with an ink
5.3 times less concentrated), keeping at the same time a higher Tr. The conductivity of the NMP stripes could most probably be increased by a postannealing treatment at high temperatures (as did in Ref. [16] at 170 1C) to further remove solvent residual on and between the graphene flakes, but such process would be of course destructive for the PET substrate. Different ranges of annealing temperatures in inert atmosphere could be explored to improve the NMP stripes conductivity/ transmittance ratio without compromising the substrate, but such optimization is beyond the scope of this work and, however, still requires post-processing treatments that we have demonstrated here to be overcome by the use of EtOH/H2O-based inks.

5. Conclusion We have demonstrated the use of low boiling point and environmentally-friendly solvents, such as ethanol and water to produce graphene- and few-layer graphene-based inks suitable for ink-jet printing processes on flexible substrates. A volume ratio of 1:1 between these solvents gives a surface tension of 30.9 mN m  1, which is in the required range for ink-jet printing (28–33 mN m  1). We were able to obtain inks of graphene and

few-layer graphene in EtOH/H2O, reaching concentrations of
0.6 mg mL  1. These inks were used to print conductive stripes with sheet resistance of
13 kΩ/□. Our work is the first attempt in the graphene printing technology that is at the same time: (1) based on environmentally-friendly solvents, (2) carried out on flexible substrates, and (3) does not require any pre- or posttreatments. Remarkably, the sheet resistance values obtained for our printed stripes compare with those reported by other groups exploiting not environmentally-friendly solvents and with postprocessing treatments. Currently, there is much room for improvement and many challenges have to be met to optimize the stability of the ink as well as the printing process. These inks have potential to be scaled up to the industrial level and exploited in the production of printed batteries, solar cells, super-capacitors and conductive flexible patterns, just to cite a few applications.

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