Paper surfaces for metal nanoparticle inkjet printing

Applied Surface Science 259 (2012) 731–739

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Paper surfaces for metal nanoparticle inkjet printing Thomas Öhlund a,∗ , Jonas Örtegren a , Sven Forsberg b , Hans-Erik Nilsson b a b

Mid Sweden University, Digital Printing Center, Järnvägsgatan 3, SE-89118 Örnsköldsvik, Sweden Mid Sweden University, Electronics Division, Department of Information Technology and Media, Holmgatan 10, SE-852 30, Sundsvall, Sweden

a r t i c l e

i n f o

Article history: Received 4 January 2012 Received in revised form 27 June 2012 Accepted 22 July 2012 Available online 27 July 2012 Keywords: Printed electronics Inkjet printing Paper substrates Flexible substrates Nanoparticles Conductive inks

a b s t r a c t The widespread usage of paper and board offer largely unexploited possibilities for printed electronics applications. Reliability and performance of printed devices on comparatively rough and inhomogenous surfaces of paper does however pose challenges. Silver nanoparticle ink has been deposited on ten various paper substrates by inkjet printing. The papers are commercially available, and selected over a range of different types and construction. A smooth nonporous polyimide film was included as a nonporous reference substrate. The substrates have been characterized in terms of porosity, absorption rate, apparent surface energy, surface roughness and material content. The electrical conductivity of the resulting printed films have been measured after drying at 60 ◦ C and again after additional curing at 110 ◦ C. A qualitative analysis of the conductivity differences on the different substrates based on surface characterization and SEM examination is presented. Measurable parameters of importance to the final conductivity are pointed out, some of which are crucial to achieve conductivity. When certain criteria of the surfaces are met, paper media can be used as low cost, but comparably high performance substrates for metal nanoparticle inks in printed electronics applications. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Printing methods for text and graphic applications, in which colorant inks are applied to the substrate via contact- or contactless deposition, are well known. Functional printing can be seen as a generalized concept in which the deposited material has some functional property other than visual in the given context, for example conducting, insulating, magnetic, optical or biological. Applications include printed active [1] and passive [2] electronic components, micro-lenses [3] and photovoltaic devices [4]. Several of the traditional printing methods are used in functional applications; among the most common ones are screen [5,6], flexography [7–9], gravure [10–12] and inkjet [13,14]. The choice of printing method depends on substrate, ink and application. Screenand flexography methods are suitable for higher viscosity inks, which may become necessary if high concentrations of functional material or thick layers are required. Inkjet differs from the methods mentioned above in that it is a non-contact method. It has therefore large flexibility in the type of geometries and applications it can be used in. Inkjet inks have low viscosity, making the substrate properties very important for controlling ink spreading and reaching sufficient levels of print definition and performance. Resolution is typically restricted to 20–40 ␮m [15,16]; however

∗ Corresponding author. Tel.: +46 (0) 73 061 1973; fax: +46 (0) 660 57860. E-mail addresses: [email protected], [email protected] (T. Öhlund). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.07.112

precision can be further improved by methods of controlling ink flow such as embossing [17] or chemical modification of the substrate [18,19]. In functional inkjet applications, piezoelectric print heads are traditionally used since they allow wider flexibility in the actuating waveform. Additionally, ink formulation is more flexible as there is no requirement for a volatile component. Nevertheless, thermal inkjet is successfully being used for printing of living cells [20] and other biological material [21]. For electrically conducting inks, generally solutions of conductive polymers, metal-oxides or metal nanoparticles are used. With metal nanoparticle inks, very high conductivity, exceeding 50% of bulk metal conductivity, can be reached [22]. Traditionally, particles of gold or silver have been used. Gold nanoparticles are non-reactive and can be synthesized in sizes of a just a few nanometers [23], making low temperature sintering possible. Cost is however a problem for widespread use of gold nanoparticle inks. Silver is the most common choice, offering high conductivity and reactivity low enough to be used in ambient conditions. However, concerns have been raised about antiseptic properties with possible human and environmental effects [24]. Copper and aluminum nanoparticle formulations are highly interesting alternatives, as low cost is crucial for large scale printed electronics manufacturing. Achieving good ambient oxidation stability is very challenging, although stable copper nanoparticle inks have been made [25]. To maintain a stable dispersion of particles in the carrier fluid, a stabilizing agent is necessary. This is often polymers that adhere to the particle surfaces, preventing particles from coalescence by

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steric and electrostatic stabilization. To increase conductivity to necessary levels, sintering is employed, in which the print is heated by some method to melt the polymers and merge particles. The heat required is dependent on several properties of the ink such as material, particle size, stabilizing polymer and carrier fluid, but also of the substrate which may have a profound influence on sintering and resulting conductivity of the particle layer [22]. Paper has evoked substantial interest as a substrate for printed flexible electronics [26–28]. Examples of printed electronics components and devices on paper reported in the literature include RFID antennas [9,14,29,30], transistors [31,32], displays [33,34], energy storage devices [35], photovoltaic cells [36] and sensors [32,37,38]. The reasons for the increased interest in paper as substrate for printed electronics applications may be attributed to low cost and environmental advantages, as compared to alternatives such as plastic films. The widespread use of paper makes attempts of direct printing of electronic functions on packaging and paper media worthwhile. Papers are complex porous materials with nonuniform physical and chemical properties, and as such present challenges for printed electronics applications [39]. Porosity can however also be an advantage, especially for inkjet printing where quick removal of carrier fluid is desirable. The influence of paper surface properties on the electrical conductivity has been reported in the literature. Conductivity measurement on flexographic silver ink applied onto solid bleached sulfate coated board showed that the resistivity decreases with decreasing surface roughness [40]. Denneulin et al. [41] used a rod coater to apply a PEDOT:PSS solution onto various paper grades and found that the measured sheet resistance increased with decreasing substrate smoothness and with increasing substrate surface energy. Screen printing of silver flake ink on gloss coated paper with a smooth surface yielded a lower resistivity as compared to printing on a clear matt film with a rougher and porous surface [6]. On the other hand, flexography printing of silver flake inks on paper and board resulted in higher conductivity for substrates with the higher surface roughness and the lower surface energy [8]. This was explained as being due to the greater ink transfer in the printing process onto the substrates with a higher surface roughness. It has been reported that calendering at high temperatures and pressures of flexographically printed lines can increase the conductivity when printing silver flake ink and nanosilver ink on label and board [42]. The conductivity of a printed pattern on paper is consequently influenced by factors such as the printing technology and the properties of the ink and the paper. The degree of influence of the physical properties on the conductive performance needs to be clarified. In this study, a silver nanoparticle ink was deposited with inkjet on a range of commercially available paper substrates of varying types and compositions, with subsequent analysis of the resulting conductivity. The surfaces and printed layers were examined in detail, and differences in conductivity are explained in terms of the physical and chemical properties of the surfaces in order to give insight in how to select or construct a paper for optimizing electrical performance in paper electronics applications.

Fig. 1. Nominal printing pattern. Nominal line width is 0.4 mm.The dots on the measurement pads show the 4-point probe measurement points.

glycol monoethyl ether); the viscosity and surface tension were measured to 12.9 cP and 37.3 mN/m respectively, with a Brookfield LVDV-+ viscometer and a Kruss K9 tensiometer, 2.2. Post treatment and resistance measurements A Keithley 2611 A source-meter, set to current source mode, was used for the resistance measurements. Four-point mode was selected to remove resistance contribution from contact points and cables. Furthermore, spring loaded probe tips were used for better control of contact pressure. Ten prints on each substrate were measured. Before measuring, the prints were thoroughly dried for 10 h at 60 ◦ C to make sure that all liquid had been evaporated. A second set of resistance measurements were performed after additional heating for 20 min at 110 ◦ C. This temperature was selected as the highest temperature the most heat-sensitive papers (1–4) could withstand before deformation in the PE barrier layers occured. The curing time 20 min was selected since longer times did not lower resistance further. The drying and additional heating was made in a heat chamber with air circulation fans. 2.3. Paper characterization 11 commercial substrates were selected. Of those, nine were coated papers of different construction, one uncoated paper, and one polyimide film. The substrates are described in Table 1. Papers 1 to 4 are mesoporous high performance inkjet photo papers. Papers 5 and 6 are inkjet photo papers of swellable type. In these, the coating is not porous but instead the printing performance relies on the carrier fluid being dissolved and diffusing into the coating layer, whereas the dye molecules are encapsulated and fixated by the coating. Papers 7 and 8 are lightweight coated papers, used mainly in offset printing of magazines. Paper 9 is a matte inkjet paper while 10 is a regular office copy-paper. The polyimide film, 11, is the only non-absorbing substrate in this study. All papers were conditioned at 23 ◦ C, 40% RH for 24 h before characterization. Material analysis was performed using both energy-dispersive spectroscopy (SEM/EDS, Jeol JSM-5800 LV/Oxford Link ISIS) and Fourier transform infrared spectroscopy (FTIR, Perkin Elmer Spectrum One/AutoImage) on the top layer of the surfaces. Contact angle measurements were performed according to test method Tappi T558 om-07, using a Fibro DAT 1100 drop absorption tester with 4 ␮L of distilled water and 2.5 ␮L of diiodmethane. Measurements were made as a function of time with first readings taken at 0.02 s (initial contact angle, immediately after reasonable geometrical stabilization of the drops). Surface energy components were calculated using the geometric-mean equation [43]:

2. Materials and methods

(1 + cos (l ))l = 2(s,d l,d )1/2 + (s,p l,p )1/2

2.1. Printing and inks

where  l is the surface tension of the liquid and  s is the surface energy of the substrate. Superscripts d and p denotes dispersive and polar component respectively, where  =  d +  p . For the reference liquids, tabulated values of the surface tension dispersive and polar components were used [44]. Liquid absorption was measured using a Bristow absorption tester (BAT) [45]. Main parts of the BAT is a rotating wheel with user selectable rotational speed, and a headbox with a slit that is

A Dimatix 2831 piezoelectric inkjet printer was used to print lines of nominal dimension 20 × 0.4 mm (Fig. 1). The drop volume was 10 pL and the nozzle voltage 24 V. Drop spacing was set to 20 ␮m with a nozzle- and platen temperature of 28 ◦ C. The silver nanoparticle ink DGP40LT-15C from Advanced Nano Products was used. This dispersion uses a polar solvent (triethylene

(1)

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Table 1 Substrates in the study. Classification based on data from manufacturers, Hg-porosimetry and SEM-EDS/FTIR material analysis. The numbers and group classifications will be used for reference throughout the article. Number

Trade name

Coating type

Main surface composition

1 2 3 4 5 6 7 8 9 10 11

Ilford gold fibre silk Canon PT101 Ilford smooth gloss Ilford smooth pearl Ilford classic gloss Ilford classic pearl Sappi LWC SCA LWC GC80 Mreal photo matt Data copy Kapton HN

Mesoporous Mesoporous Mesoporous Mesoporous Water soluble polymer Water soluble polymer Clay coating Clay coating Porous Uncoated Uncoated

Al2 O3 Al2 O3 Al2 O3 Al2 O3 Gelatin Gelatin CaCO3 , Caolin Clay CaCO3 , Caolin Clay Silica Cellulose, CaCO3 filler Polyimide

resting on the wheel with 0.1 MPa contact pressure. A strip of the substrate is attached to the wheel and a known amount of liquid is filled in the headbox. The time available for absorption, the contact time t of the liquid, is dependent on the peripheral speed and the width of the slit (1 mm). The amount of liquid transferred into the substrate can be expressed as Kr + Ka t 1/2

(2)

where Kr is a coefficient reflecting roughness (the amount of liquid to fill the surface at time zero) and Ka is the absorption rate. By a series of tests at different rotational velocities and corresponding measurements of transferred fluid area on the strips, transferred fluid amount is plotted against t1/2 and the absorption rate is determined from the slope of the fitted straight line. In this article, water was used as test liquid and contact times were in the range of 0.01–2 s. Surface roughness was measured with an optical profilometer (FRT MicroProf with CRT H0 sensor). The profilometer data was post-processed in custom FFT software to extract wavelength-dependent surface roughness information. An atomic force microscope (Nanosurf Easyscan2) was used for complementary analysis of small-scale surface roughness. The porosity was characterized with a Mercury porosimeter (Micromeritics Autopore IV 9500) according to ISO 15901-1. A pore size range of 10 ␮m down to approximately 5 nm (corresponding to the maximum pressure limit of 230 MPa) was examined. 3. Results and discussion 3.1. Paper characterization All the coated papers are coated two sides. For papers 1–6, the back coatings are different, used for purposes such as humidity protection and improvements in mechanical properties and printer feeding. For the LWC papers (7–8) and the matte inkjet paper (9), the coating on each side was verified by the manufacturers to be the same, apart from small differences between the wire side and felt side. 3.1.1. Material analysis Results from the surface material analysis and porosimetry can be seen in Table 1. The top coating of paper 1–4 consist of a mesoporous structure of aluminium oxide. All of them have a polyethylene barrier layer separating the coating from the paper base. The function of the barrier layer is generally to prohibit the ink carrier from penetrating the base paper and therefore minimizing physical deformations such as cockling. Paper 3,4 differs from 1,2 in that polymers (PVA and a surface active polymer similar to alkyl phenol polyglycol ether) are present in the coating of the former. Paper 5 and 6 have a coating of a water soluble polymer in the form of a protein gel (gelatine). Paper 7 and 8 are very similar

Group classification

Mesoporous

Swellable LWC Inkjet Coated Uncoated paper Polyimide

in coating material content. They have coatings consisting of calcium carbonate and kaolin clay, with styrene butadiene binders. No barrier layers are present. Paper 9 has a coating of silica gel particles, giving a comparably coarse structure, and contains no barrier layers. Paper 10 is an uncoated base sheet, produced at a grammage of 80 gsm, surface sized and thermo-calendered. The fiber composition is a mix of hardwood and softwood at an approximate ratio between 3:1 and 3:2. The calcium carbonate filler content was approximately 20%. Substrate 11 is a film of homogeneous, non-porous polyimide. 3.1.2. Contact angle measurements The calculated surface energy components can be seen in Fig. 2. Apparent total surface energies (polar and dispersive components added) are highest for the mesoporous metal oxide coatings (1–4) and for the swellable coatings (5–6). The swellable coatings have the largest polar component, reflecting its water soluble (hydrophilic) character. The surfaces of papers (7–10) are hydrophobic in character and have lower apparent surface energies. Note that measurements of contact angles are not trivial on heterogeneous and porous substrates such as paper, since factors such as surface roughness, chemical heterogeneity and absorption usually are not accounted for [46]. The suitability of using the standard test drop volumes of 2.5 ␮L and 4 ␮L on thin porous films on paper may also be questioned, since the liquid to some extent may penetrate into the underlying bulk during the measurement. At which time after drop impact, contact angle is measured, should be considered. It will take some time for the drop to stabilize geometrically. Before that, vibrations will be present and therefore fluctuations in the contact angle. On the other hand for some coatings, substantial absorption takes place shortly after impact. It was

Fig. 2. Calculated apparent surface energies based on contact angles at 0.02 s.

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Fig. 3. Bristow absorption rate.

found that taking the measurement 0.02 s after impact was the best compromise between geometric instability and onset of absorption. We use Eq. (1) with the contact angles at 0.02 s and refer to the result as apparent surface energy. 3.1.3. Absorption tests When the absorption is assumed to be linear with the square root of fluid contact time, the absorption rate corresponds to the slope of the Bristow absorption curve [23]. This linear assumption was seen to hold true also for the mesoporous papers (1–4) with barrier layers, although only for short enough contact times (<0.5 s). Therefore, when calculating absorption rates for the quickly absorbing, barrier layer equipped mesoporous papers, the linear regression was performed only within the short contacttime, linear region of each curve. Calculated absorption rate values are shown in Fig. 3. Absorption rate, according to Bristow tests, is clearly largest for the mesoporous papers 1–4 and lowest for the lightweight coated papers 7–8. It was seen that for the fast absorbing papers 1–4, the curves enter a non-linear region when the contact time is too long. This can be explained by saturation of the coating layer, which is separated from the base paper by a barrier layer, prohibiting further absorption into the structure. For this reason the contact times prescribed in the standard Bristow test procedure were too long, and a series of shorter contact times had to be added to reach a linear region for all papers, permitting the fitting of a straight line and correct calculation of absorption rate. It should be noted that for the swellable, gel coated papers 5-6, the absorption mechanism is very different in that it is not a capillary driven absorption into a porous structure, but instead the water uptake is a solvation and diffusion driven process. Some caution in comparing the absorption rate values should therefore be taken. The swellable papers 5–6 do not have a nonlinear appearance of curves like the microporous papers 1–4. This indicates that the swellable coating can handle a higher amount of water before entering saturation. 3.1.4. Porosity measurements In the sample preparation for the coated papers, the back coatings were removed by adhesive tape, splitting the papers in two. The contribution from the remaining base paper will affect the result of the measurement, but is distinguishable from the coating due to the larger characteristic pore size of the base paper. Therefore the base paper contribution should not affect the interpretation of the porosity curves and the conclusions drawn from them. The differential pore volume distributions are shown in Fig. 4. Pore volume distributions of the mesoporous coated papers 1–4 are seen in Fig. 4a. Distributions of paper 2–4 are very narrow with a well-defined, dominating pore radius near 10 nm, at which the pore

Fig. 4. Pore size distributions. Upper figure: Narrow distributions and small characteristic pore sizes for the mesoporous papers (note the larger y-axis scale). Lower figure: Broad distributions with significant contribution above 1 ␮m for paper 9 and 10.

volume is very high. Paper 1 has a wider distribution and a larger dominating pore radius of 20 nm approximately. In Fig. 4a, the contributions seen between 0.8–1.5 ␮m pore radii arguably arise from intrusion into the base paper. This is not relevant for printing on top of the surface since the base paper is isolated from ink penetration by the barrier layer. In Fig. 4b, the distributions of paper 7–10 are seen. Note that the Y-axis scale is different here. The lightweight coated papers 7–8 have dominating pore radii around 60 nm and low porosity overall. The matte inkjet paper (9) and the copy paper (10) have the broadest distribution with significantly larger porosity above 1 ␮m than the rest of the papers. The matte inkjet paper however, has a much higher concentration of small pores compared to the copy paper and the lightweight coated papers. The polyimide film (11) is nonporous but was still measured as an extra means for testing the reliability of the measurement setup. As expected the intrusion was measured to approximately zero for all pressures. The result is not shown for clarity reasons. The polymer coatings of papers 5–6 are essentially nonporous in nature. Measurements of these showed a porosity contribution around 1 ␮m from the base paper, which again is not relevant from a printing perspective, since the gel coating, in normal inkjet conditions, will handle all the ink. There was a response for very small pore radii (high pressures), however there are reasons to believe that this is not due to actual mercury intrusion, but to compression of the elastic gel coating. The curves of these measurements have therefore been omitted for reasons of relevancy and clarity. 3.1.5. Surface roughness Optical profilometer measurements of surface variations as a function of wavelength (length-scale of variations) is shown in Fig. 5. Papers 9–10 have by far the largest surface roughness in the entire measurement range 7.5 ␮m–5 mm. Paper 1 is clearly

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paper 2 (mesoporous) was problematic to measure because of a semi-transparent character of the coating. Complementing Parker Print Surf (PPS) and AFM measurements show that paper 2 is similar in surface roughness to paper 3. PPS and AFM measurements show that the polyimide film has the lowest surface roughness of all the substrates. While very different in the longer scale roughness, paper 3 and 4 have very similar surface roughness at the short scale below 7.5 ␮m, as seen in Fig. 6(B, C). At this small scale, paper 1 still has the highest roughness among the mesoporous group, however with peaks well within 100 nm from the surface plane (Fig. 6A). Note the very smooth surface of the polyimide film (Fig. 6D).

3.2. Conductivity of prints Fig. 5. Surface variations of substrates (substrate 2 and 11 were not measurable with optical profilometer).

the third roughest surface, with a tenfold reduction of variations in the 7.5–100 ␮m range compared to paper 9 and 10. It can also be seen when comparing the Ilford gloss (paper 3, paper 5) with the material-wise very similar Ilford pearl (4, 6) papers, surface roughness is higher in the measured range for the pearl papers. The difference is especially large for longer scale variations above 50 ␮m. Note that papers 7–8 (LWC) have smooth surfaces at the short scale with roughness occurring mainly at the longer scale. The mesoporous coatings 2–4 all have low roughness in the shortto midrange scale; paper 1 is different to the other mesoporous papers with its significantly higher roughness in the 7.5–300 ␮m range. The polyimide film (substrate 11) could not be characterized reliably by the optical profilometer due to its transparency. Also

The conductivity of the silver patterns and the standard deviation of measurements can be seen in Fig. 7. The conductivity was calculated from resistance measurements and determination of the pattern geometry (volume). The geometry determination was made on the substrate with the best line definition (substrate 2). The average pattern height on this substrate was measured to be 0.7 ␮m with an AFM. The width and length of the pattern was near the nominal values of 0.4 mm and 20 mm, respectively. This pattern volume was then assumed to be equal for all substrates since the printer will deposit an equal amount of ink for all patterns. In Fig. 7 the conductivity has been expressed as a percentage of the conductivity of bulk silver (6.30 × 107 S/m). Because of the heatsensitive nature of some of the substrates in this study, particularly paper 1–4, the curing temperature was limited to 110 ◦ C to avoid substrate deformation. At this temperature, visible neck formation and particle growth could not be seen in SEM micrographs.

Fig. 6. Short-range surface roughness of substrates, imaged with atomic force microscopy. (A) Paper 1 (mesoporous); (B) Paper 3 (mesoporous); (C) Paper 4 (mesoporous); (D) Substrate 11 (polyimide film).

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Fig. 7. Conductivity after two steps of heating. Error bars represent ±2 standard deviations. No prints on substrates 9 and 10 were conducting.

However, substantially increased conductivity levels resulted, as seen in Fig. 7. This suggests that the curing effect was obtained mainly by thermal degradation of organic compounds in the ink. It is common to express the conductive performance of thin layers as sheet resistance Rs =  × W/L where  is the volume resistivity and W, L is the conductor width and length, respectively. Therefore, the sheet resistance will also take the spreading of the ink on each substrate into account, since the conductor width will depend on the spreading. To determine the sheet resistance, the conductor width on each substrate was extracted using digital image acquirement of the printed patterns and calculating the line width according to the ISO 13660 standard, using in-house software. Images of printed conductors are seen in Fig. 8. The pattern on substrate 2 has a line width of 415 ␮m, which is the closest to the nominal value of 400 ␮m. The sheet resistance after curing is shown in Fig. 9. The lowest average sheet resistance, 150 m /sq, was achieved for paper 2. 3.2.1. Mesoporous coated papers It is seen that the conductivity after 110 ◦ C treatment is high for all mesoporous papers (1–4). Moreover, there were only minor variations between different samples (small standard deviation). Fig. 10A and B show the very well behaved character of the film formed on the mesoporous coating of paper 2. No cracks or gaps in the film are evident and the cross section show a dense particle layer with almost even thickness and no visible particle penetration. An interesting observation is that papers 3–4, although very similar in physical characteristics to papers 1–2, show a completely different result after the lower temperature treatment, where papers 3–4 show a very low conductivity with large sample variations. After additional 110 ◦ C treatment however, the conductivity for papers 3–4 rises abruptly, to values similar with papers 1–2. The observation that prints on two of the mesoporous coatings (3–4) show very low conductivity after drying at 60 ◦ C is interesting. The measured characteristics of low surface roughness, small pore size, high absorption rate and large contact angles are all very similar to coatings 1–2. The material analysis shows that papers 1–4 all have coatings based on aluminium oxide particles of similar size.

Fig. 9. Sheet resistance after curing. Error bars represent ±2 standard deviations. The sheet resistance on substrates 9 and 10 is infinite.

Therefore the conductivity results should be expected to be similar, if not differences in coating chemical content is present and its chemical interaction with the ink is the reason for the large differences. SEM-EDS and FTIR analysis indeed confirmed the presence of polymer additives in the coatings of 3–4, whereas no polymer additives could be found in the coatings of 1–2. We believe that the large conductivity differences at low temperature is because coating additive polymers and molecules interact with the stabilizing polymers attached to the silver nanoparticles as to increase steric or electrostatic repulsion between particles. After being exposed at the higher temperature of 110 ◦ C the coating polymer has been melted and the interaction vanished, which explains that all mesoporous coatings 1–4 show similar results after 110 ◦ C treatment. 3.2.2. Swellable coated papers The swellable gel coated papers (5–6) showed low conductivities, both for the 60 ◦ C dried samples and the additional 110 ◦ C treated samples. The relative sample variation was very large. The coatings are not porous but the water uptake instead relies on that a water based ink carrier dissolves and diffuses into the polymer layer. When the solvent diffuses into the layer, the layer typically swells to several times its initial thickness, then slowly retaining its original thickness during the solvent evaporation. In graphical applications, swellable coatings are only used with dye-based inks in which the dye molecules are completely dissolved, and after deposition, encapsulated by the coating polymers for physical fixation and improved light-fastness. The silver nanoparticles are, like color pigments, too large to be encapsulated by the coating, thus a continuous smooth particle film on top of the coating with a working mechanism for fluid removal could possibly result. This was indeed the case in this study. With SEM it was seen that a continuous metal film is formed at the top of the coating without any significant particle penetration. While the ink used here is not water based, the carrier is polar and has some ability to dissolve the coating. However, the conductivity was low both before and after curing (Fig. 7). Large conductivity variations among the samples and low mechanical robustness were typical for prints on these papers. Possibly the main reason for the low conductivity of the films is seen in Fig. 10C. Small cracks are formed in the layer, which is also the probable explanation for the lower

Fig. 8. Printed patterns for different paper types. Substrate numbers are shown in the figure. The ink spreading is largest for the swellable (6) and the uncoated (10) type.

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Fig. 10. SEM micrographs of nanoparticle layer on different substrates. (A) mesoporous surface (2) at 30,000×; (B) cross section with nanoparticle film on top of mesoporous surface (2) at 45,000×; (C) swellable coating (5) at 30,000×; (D) cross section with nanoparticle layer on top of LWC paper (8) at 30,000×; (E) matte inkjet paper (9) at 5000× (the main aggregates seen are coating silica gel particles); (F) cross section of uncoated paper (10), ink penetration is evident through the entire paper (light microscopy).

mechanical robustness, since the cracks might easily be widened when subject to mechanical stress. It is reasonable to believe that the network of micrometer sized cracks are formed during the swelling and de-swelling phase, introducing movement and mechanical tension of the particle film during the film formation and drying. 3.2.3. Lightweight coated papers The lightweight coated papers 7–8 showed high values of conductivity, and small variations. SEM analysis of the particle film on the lightweight coated papers revealed a very similar appearance as with the mesoporous paper in Fig. 10A. This is true also for the cross section, showing that the coating is dense enough to prevent particle penetration. Fig. 10D shows a high magnification cross section, showcasing some small amplitude, high frequency surface roughness. As seen, the amplitude is small enough to be filled up and neutralized by the ink layer. 3.2.4. Silica-gel coated paper and uncoated paper The matte coated inkjet paper (9) and the uncoated paper (10) are the only substrates in this study that do not show any conductivity of the printed pattern. None of the ten samples printed on each of these papers had a measurable conductivity, neither at

60 ◦ C nor at additional 110 ◦ C treatment. The network of bare fibers in the uncoated paper creates steep walls and large voids all over the surface and it is intuitively understood that no continuous particle film is to be expected. Looking at the cross section in Fig. 10F it is confirmed that the ink particles penetrate deep into the paper interior. The surface of the matte coated inkjet paper is seen in Fig. 10E. Also here, no continuous particle film should be expected since the rather coarse silica gel particles create a dense array of steep voids and peaks, in the same size of order as the ink film thickness. Cross section microscopy showed considerable particle penetration 50–100 ␮m into the paper, similar with the uncoated paper. 3.2.5. Polyimide film The prints on the polyimide film (11) showed reasonable conductivity, although a factor 2-4 less than the highest performing papers. Sample variations were quite large and additional heat treatment at 110 ◦ C did not improve the results, but instead increased the resistance variation. The printed conductors had larger mean width and less definition compared to all paper substrates, due to large and non-uniform spreading of the ink. Coffee-staining [47] effects were visible. Since the polyimide film has no absorption, and a smooth and chemically homogenous surface, the combination of ink surface

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tension and substrate surface energy will arguably become much more important for controlling the spreading of the ink. Because evaporation is the only mechanism for fluid removal in this case, spreading will also depend on ink boiling point and substrate temperature. In this study, where no efforts have been made to specifically optimize the surface tension/surface energy combination, conductivity were lower than for most of the papers, in spite of a more ideal surface in the classical sense. This can be explained by the non-uniform distribution of silver nanoparticles, readily seen as coffee-staining patterns in the print. The presence of coffeestaining irregularities is in turn explained by the lack of absorption that would otherwise reduce ink spreading and dominate over evaporation. 3.3. Influence of surface properties The applicability of paper surfaces for metal nanoparticle inks requires a certain number of conditions to be met. A number of surface properties of importance have been analyzed in this study as discussed below. 3.3.1. Porosity A very important prerequisite is a surface pore structure fine enough to allow a sufficient number of particles to stay on top of the surface and form a physically connected path along the structure. If surface connected pores much larger than the particle diameter exist, particles will penetrate, leaving gaps in the film that will decrease or entirely disrupt conductivity, depending on size and number of gaps. Porosity is therefore one of the crucial factors that need to be controlled for applications with nanoparticle inks. Mercury porosimetry can be used as an effective method for characterizing pore size distribution and range of dominating pore size. The method does not directly reveal if pores are close to the surface or in interior parts, nor does it give the geometrical shape of pores. So, for a complete understanding of the porosity, these measurements need to be combined with supplementary analysis such as scanning electron microscopy (SEM). In this study, the lightweightcoated papers (7–8) have characteristic pore sizes in the 40–100 nm range which is small enough to allow a continuous conductive film of nanoparticles with an average diameter of approximately 50 nm. In contrast, the inkjet coated paper, where the coating consists of silica particles of up to several micrometers in diameter, the pore distribution shows a large contribution above 1 micrometer which does not allow a continuous film to be formed. The expected ideal porosity characteristic is represented by the mesoporous group of papers that have a narrow pore size distribution and a small characteristic pore size, typically around 10 nm, yet a large overall porosity and fast absorption. This allows a very fast ink carrier removal and setting of the particle film. 3.3.2. Absorption rate Absorption rate should ideally be large to allow quick transfer of ink carrier into the substrate interior. This will limit ink spreading and particles will settle near instantly, avoiding diffusive movement of particles toward edges known as coffee staining [47]. Indeed the highest conductivities were found for the mesoporous coated papers which are the fastest absorbing substrates in this study. While a quick absorption is certainly beneficial, there is no threshold absorption that is strictly necessary to achieve a conducting film. The non-absorbing polyimide film was usable and the prints showed reasonable conductivity, however the definition of prints as well as the mechanical robustness of the particle film suffered, compared to the absorbing substrates. We believe that lateral particle migration lowering the homogeneity of the film, as well as film thinning due to excessive ink spreading, are the main reasons

for this. Showcasing that absorption do not necessarily need to be fast are the lightweight coated papers 7–8, that despite a relatively slow absorption, show surprisingly large conductivity, almost at the same level as the mesoporous coated papers. 3.3.3. Surface roughness Surface roughness is, together with porosity, one of the two properties that we consider crucial for conductivity in this study. With that we mean that it needs to be below a certain threshold magnitude to give any conductivity at all. In contrast, other factors discussed here such as absorption rate, apparent surface energy and chemical properties of the surface; while certainly affecting electrical performance, unsuitable values usually do not disrupt conductivity totally. At which length-scale surface variations are dominant is important. Surface roughness of high frequency nature has large impact, since particle separation will then occur frequently over the surface. Therefore, relevant roughness measurements are preferably made with a high resolution profilometer or an atomic force microscope. Very small amplitude surface irregularities (much smaller than the nanoparticle layer thickness) are filled up and smoothed over by the dried layer (as can be seen in Fig. 10D), as to have little or no influence. Low frequency surface irregularities, at a ‘wavelength’ much larger than the layer thickness, either is smooth enough for the layer to continuously form around the irregularity; or if steep enough to break film connectivity at local areas, the perturbations usually are sufficiently spread out to preserve overall film conductivity. Consequently, longer scale variations can be comparably high amplitude and still not affect conductivity very much, as indicated by the conductivity results for paper 1, 4, 6, 7 and 8. 3.3.4. Surface energy The contact angle measurements and calculated apparent surface energies do not have a clear correlation with the measured conductivities in this study. It was seen that surface roughness, porosity and absorption dominate the influence of the apparent surface energy. As an example, the mesoporous substrate (1) has very similar apparent surface energy as the polyimide film (11), see Fig. 2, but has very different results in terms of ink spreading and resulting electrical conductivity (Fig. 7). For smooth, nonporous substrates such as plastic films or glass sheets, we expect the surface energy to become very important for achieved electrical conductivity since it then largely influence ink spreading and film formation. 4. Conclusion The results of this study suggest that surface roughness and porosity are the most crucial substrate parameters to control for achieving electrical conductivity of inkjet-printed metal nanoparticle inks on paper surfaces. Narrow scale, abrupt variations need to be much lower in magnitude than longer scale variations. As long as narrow scale surface variations, as well as characteristic surface pore sizes, are small enough, i.e. much less than the ink film thickness, a conducting film will likely form. Apparent surface energy measured on paper substrates has only a minor influence on conductivity, as it is largely dominated in importance by absorption and surface roughness. When absorption and roughness are both very low, appropriate matching of surface energy to ink surface tension becomes important or the print definition and conductivity will likely suffer, as indicated by the polyimide film in this study. For low temperature curing applications, chemical properties of the surface can be important. Surface molecules may interact with ink stabilizers as to either increase or decrease conductivity depending on the specific surface-ink combination. When the

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temperature is raised, these effects are likely to vanish as the nanoparticle stabilizing polymers thermally degrade and finally decompose. Acknowledgements Financial support from the Kempe Foundations and the Swedish Agency for Economic and Regional Growth is gratefully acknowledged. The funding sources had no involvement in this article. We would also like to thank Dr. Petru Niga for valuable discussions about this work. References [1] V. Subramanian, P.C. Chang, J.B. Lee, S.E. Molesa, S.K. Volkman, Printed organic transistors for ultra-low-cost RFID applications, IEEE Transactions on Components and Packaging Technologies 28 (2005) 742–747. [2] S.H. Ko, J. Chung, H. Pan, C.P. Grigoropoulos, D. Poulikakos, Fabrication of multilayer passive and active electric components on polymer using inkjet printing and low temperature laser processing, Sensors and Actuators A: Physical 134 (2007) 161–168. [3] P. Calvert, Inkjet Printing for Materials and Devices, Chemistry of Materials 13 (2001) 3299–3305. [4] C.N. Hoth, S.A. Choulis, P. Schilinsky, C.J. Brabec, High photovoltaic performance of inkjet printed polymer:fullerene blends, Advanced Materials 19 (2007) 3973–3978. [5] N.G. Patel, S. Meier, K. Cammann, G.C. Chemnitius, Screen-printed biosensors using different alcohol oxidases, Sensors and Actuators B: Chemical 75 (2001) 101–110. [6] M. Zveglic, N. Hauptman, M. Macek, M. Klanjsek Gunde, Screen-printed electrically conductive functionalities in paper substrates, Materiali in Tehnologije/Materials and Technology 45 (6) (2011) 627–632. [7] F.C. Krebs, J. Fyenbo, M. Jorgensen, Product integration of compact roll-toroll processed polymer solar cell modules: methods and manufacture using flexographic printing, slot-die coating and rotary screen printing, Journal of Materials Chemistry 20 (2010) 8994–9001. [8] R. Kattumenu, M. Rebros, M. Joyce, P.D. Fleming, G. Neelgund, Effect of substrate properties on conductive traces printed with silver-based flexographic ink, Nordic Pulp and Paper Research Journal 24 (2009) 101–106. [9] R. Kattumenu, M. Rebros, M. Joyce, E. Hrehorova, P.D. Fleming, Evaluation of flexographically printed conductive traces on paper substrates for RFID applications, TAGA Journal of Graphic Technology 5 (1) (2011) 19–41. [10] M. Pudas, N. Halonen, P. Granat, J. Vähäkangas, Gravure printing of conductive particulate polymer inks on flexible substrates, Progress in Organic Coatings 54 (2005) 310–316. [11] E. Hrehorova, A. Pekarovicova, V.N. Bliznyuk, P.D. Fleming, Polymeric materials for printed electronics and their interactions with paper substrates, in: Proc. NIP23: 23rd Int. Conf. on Digital Printing Tech., 2007, pp. 328–931. [12] E. Hrehorova, M. Rebros, A. Pekarovicova, P.D. Fleming, Suitability of gravure printing for high volume fabrication of electronics, in: Proc. NIP24/Digital Fabrication 2008: 24th Int. Conf. on Digital Printing Tech., 2008, pp. 260–264. [13] L. Adler-Abramovich, E. Gazit, Controlled patterning of peptide nanotubes and nanospheres using inkjet printing technology, Journal of Peptide Science 14 (2008) 217–223. [14] L. Yang, A. Rida, R. Vyas, M.M. Tentzeris, RFID Tag RF structures on a paper substrate using inkjet-printing technology, IEEE Transactions on Microwave Theory and Techniques 55 (12) (2007) 2894–2901. [15] S. Gamerith, A. Klug, H. Scheiber, U. Scherf, E. Moderegger, E.J.W. List, Direct ink-jet printing of Ag–Cu nanoparticle and Agprecursor based electrodes for OFET applications, Advanced Functional Materials 17 (2007) 3111–3118. [16] T. Öhlund, J. Örtegren, H. Andersson, H.-E. Nilsson, The Importance of surface characteristics for structure definition of silver nanoparticle ink patterns on paper surfaces, in: Proc. NIP26: 26th Int. Conf. on Digital Printing Tech., 2010, pp. 309–313. [17] J.Z. Wang, J. Gu, F. Zenhausern, H. Sirringhaus, Low-cost fabrication of submicron all polymer field effect transistors, Applied Physics Letters 88 (2006) 1335021–1335023. [18] C.W. Sele, T. von Werne, R.H. Friend, H. Sirringhaus, Lithography-free, selfaligned inkjet printing with sub-hundred-nanometer resolution, Advanced Materials 17 (2005) 997–1001. [19] N. Zhao, M. Chiesa, H. Sirringhaus, Y. Li, Y. Wu, B. Ong, Self-aligned inkjet printing of highly conducting gold electrodes with submicron resolution, Journal of Applied Physics 101 (064513) (2007) 1–6. [20] T. Xu, J. Jin, C. Gregory, J.J. Hickman, T. Boland, Inkjet printing of viable mammalian cells, Biomaterials 26 (2005) 93–99.

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