Journal of The Electrochemical Society, 165 (8) B3163-B3167 (2018) B3163

JES FOCUS ISSUE ON UBIQUITOUS SENSORS AND SYSTEMS FOR IOT

Investigation of Printing Properties on Paper Substrate
Tonmoy Kumar Saha,1 Tyler Nathan Knaus,1 Ajit Khosla, 2,∗
and Praveen Kumar Sekhar 1,∗,z
1 Nanomaterials and Sensors Laboratory, School of Engineering and Computer Science, Washington State University
Vancouver, Vancouver, Washington 98686, USA
2 Department of Mechanical System Engineering, Graduate School of Science and Engineering, Yamagata University,
Yonezawa, Yamagata 992-8510, Japan

In this article, the optimum printing parameters were found when using silver nanoparticle ink to print on Kodak 4-Star photo paper
substrate. Fujifilm Dimatix 2831 was used as the inkjet printer. The printing parameters of interest included the number of printing
layers, the drop spacing, and curing temperature of the ink. Analysis of Variance (ANOVA) analysis of the experimental data reveals
sintering temperature to be significant (p < 0.05) to improve the conductivity. Pattern conductivity and surface roughness were used
to identify the optimum printing parameters. The optimum printing parameters were found to be 15 μm drop spacing, two printing
layer, and a sintering temperature of 90◦ C. The best conductivity measured under the above mentioned condition was found to be
5.56 × 106 −1 m−1 . Further, the bending test indicated that the printed patterns were unaffected (in terms of conductivity) when
flexed around a cylindrical support indicating excellent stability under stress. This study paves the way for developing mechanically
robust flexible devices with excellent electrical properties for Internet of Things (IoT) applications.
© The Author(s) 2018. Published by ECS. This is an open access article distributed under the terms of the Creative Commons
Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any
medium, provided the original work is properly cited. [DOI: 10.1149/2.0211808jes]

Manuscript submitted February 5, 2018; revised manuscript received May 4, 2018. Published May 16, 2018. This paper is part of
the JES Focus Issue on Ubiquitous Sensors and Systems for IoT.

Recent advancements in printed electronics has enabled the de- In this context, the article reports on optimum printing parameters
velopment of flexible devices and IoT products,1 antenna,2 sensors,3 when using silver nanoparticle ink to print patterns on Kodak 4-Star
solar cells,4 organic resistor,5 and organic transponder,6 to name a few. photo paper substrate. The Compound Microscope, Scanning Electron
The field of flexible devices and wearables is growing fast primarily Microscope (SEM), Optical Profilometer, multimeter and four point
due to its eco-friendly and cost-effective fabrication methodologies. probe were used to inspect the samples for surface roughness and
Modern inkjet printing is advanced which utilizes conductive ink conductivity.
to fabricate devices without iterations in photolithographic mask de-
sign or etching methods.7 Inkjet printing can be performed using any
type of conductive, resistive, and biological inks, with wide varieties
of substrates including paper, PET film, textiles, fibers, etc. The elec- Experimental
trical and mechanical properties of the printed samples depend on the
Printing.—The printer used for this project was the Fujifilm’s Di-
type and quality of ink, as well as the surface characteristics of the
matix 2831 Inkjet Printer. The DMP printer offers drop-on-demand
substrate. Fujifilm’s commercially available Dimatix 2831 printer has
(DOD) piezoelectric ink-jet nozzles to ensure precision drop place-
been frequently used to print patterns on different substrates under
ment. It uses 10 pL drop-size cartridges (model: DMC-11610) for this
various printing conditions.8 The paper based substrate has become
experiment, which hold 1.5 mL of ink.
popular to realize devices due to its versatile used and properties that
During filling of the cartridge, it is prescribed by manufacturer
include possibility of microwave frequency applications, reel-to-reel
to avoid air, and keep the cartridge minimum 30 minutes in the idle
printing capability, low cost in nature, availability of hydrophobic or
state. It is recommended the cartridge be loaded in the printer carriage
fire retardant type, low surface profile, multilayer printing options, and
during this time, before giving the printing command.
compatible with inkjet printing.9 The hydrophobic and resin coated,
The ink used in this experiment was Ag Nanoparticle Inkjet 9104
Kodak photo paper is a common choice due to its conductive single
Ink, purchased from Methode Electronics. The viscosity, density,
layer traces allowing multiple layers of printing capability.10
and surface energy are as follows: 9 cps, 1.3 g/ml, 33 dynes/cm
There are numerous research papers regarding inks, where many
respectively.17 These ink properties ensure 25 m/ is the print-
researchers have claimed to have founded excellent device perfor-
ing electrical resistance. The optimum fluid physical characteristics
mance using either gold11 or copper12 ink. However, when devices
include a viscosity of 10–12 cps and a surface tension of 28–36
are implemented utilizing such inks, they quickly convert into their
dynes/cm.8 Patterns sampled were first created using the ANSYS EM
oxidized form (Au2 O3 or CuO) rendering them insulating. In addi-
simulation software, and saved as a .dxf file, which then were con-
tion, the required curing temperatures of gold and copper exceed the
verted to a bitmap file using the ACE3000 V7 software, and finally
temperature that the paper substrate can withstand. Silver nanoparti-
uploaded to the Dimatix program. Using the bitmap file uploader on
cle ink (AgNP) is preferred over Au or Cu due to its lower melting
the DMP, the resolution, drop spacing, layers, leader bar width, and
temperature,13 lower cost,14 minimal resistance to corrosion,10 and
reference point were set. A leader bar was used by printing a vertical
low reactivity in air.15 Apart from the ink, the critical factor that in-
line to the left of the pattern, to pre-jet nozzles and keep their drop
fluences the conductivity of the printed samples is sintering. The sin-
velocity uniform.
tering process can dry out the ink swiftly and remove the bubbles.16
To find the optimum printing technique, the drop spacing was
The sintering temperature needs to carefully chosen as over-sinter or
varied from 5 μm to 20 μm. If the drop spacing is too close, it causes
under-sintering can break the printed lines and degrade the overall
excess ink to form uneven lines in the print, resulting in poor print
quality of printed devices.
quality and conductivity. In contrast, too far of a drop spacing results
in broken connections in the drops being ejected. Through a visual
inspection, the optimum drop spacing resulting in uniform print lines
∗ Electrochemical Society Member. was found to be 15 μm. The optimal printing parameters for this
z
E-mail: Praveen.sekhar@wsu.edu research are given in Table I.

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 B3164 Journal of The Electrochemical Society, 165 (8) B3163-B3167 (2018)

Table I. Optimal printing parameters. thickness (t). Resistivity is given by:19

Jetting Voltage 13.5 V
A W×t
Jetting Frequency 4.5 kHz ρ=R =R , (.cm) [2]
Drop Size 10 pL L L
Drop Spacing 15 μm
Layers 2 To quantify the resistivity of the pattern, the resistance of the
Printing Print Height 750 μm sample was measured using two-point probe (Fluke 87 V industrial
Cartridge Temperature 38◦ C multimeter) and the results verified with Jandel’s four-point probe.
Platen Temperature 30◦ C Then, thickness of the layer was measured using both Scanning Elec-
tron Microscope (Quanta 3D 200i)20 and Optical Profiling System
(Veeco WYK0 NT 1100).21 In total, five printed samples were tested
After waiting at least 30 minutes, the Drop Manager software was at each condition and the reported resistivity is the average of the five
used to inspect the drop quality for the Ag nanoparticle ink. The key is samples.
to create uniform drops, falling with matching drop velocity, without
a tail or any satellite drops lagging behind, to be formed in sequential Flexibility test.—Mechanical reliability has been a primarily lim-
jets. The DMP requires the jets chosen for printing to be sequential, iting factor toward realizing a vast array of flexible devices. To test
and for this reason four sequential uniform drop-jets were chosen. To the effect of bending, an earlier study22 conducted several bending
inspect the printed pattern, and to set the printing origin, the printer tests, using the varied diameter of cylindrically shaped pattern, and
utilizes the Fiducial Camera, which has a resolution of 2.54 μm/pixel. deformed it using their hands. A similar approach was followed in
Drop spacing is varied by adjusting the cartridge angle. The Fiducial this article and the resistivity was evaluated (as a function of bending)
Camera is also used to do a drop offset, consisting of a 10 mm line in and compared with the resistivity of previously recorded non-bended
the X direction and a single dot 1 mm next to the line. results.
Cleaning cycle is another important setting which helps to avoid
jets clogging and ink settlings. Cleaning cycle settings include three Surface roughness measurement and surface
functions: purge, spit, and blot. After filling a new cartridge, it is characterization.—The sharpness of the edges, smoothness of
important to run a cleaning cycle with purge, pushing air and debris the printing, and ink distribution were checked using the Fiducial
out, clearing the nozzles. In this experiment, while printing, the group Camera of DMP and CD microscope (Nikon MM-400). Surface
used a cleaning cycle consisting of blot, using a non-contact absorption roughness was measured using the Optical Profiling System.23 On
pad, every 6 lines to maintain a clean print head. Once successfully the other hand, SEM (Quanta 3D 200i)24 was used to investigate the
printed, the samples were inspected using the DMP’s Fiducial Camera. thickness of the layer and surface morphology.
A sample printed pattern is shown in Fig. 1.
Results and Discussion
Sintering (Heat treatment).—Among various types of the sinter-
Sintering and resistivity results.—The resistivity of the printed
ing techniques, the most straightforward technique is thermal sinter-
pattern as a function of sintering temperature is shown in Table II.
ing. The experiment was conducted using Thermo Fisher Scientific’s
The resistivity of air dried patterns were exceptionally high due to the
digital hot plate (Model: HP88857100). Silver nanoparticles fuse to-
printed particles forming porous thin films over another porous paper
gether at much lower temperature than bulk Ag. If Tmelt is the melting
substrate. The measured resistivity gradually decreased with increase
point of metal nanoparticles, Tbulk is the melting point of solid metal,
in sintering temperature. The concentration of silver nanoparticle ink
σ is the characteristic parameter, and R is the radius of metal particles.
increases with an increase in the number of printed layers. Hence,
Then, the relationship among these variables is given below:18
the three layer printed pattern will have the highest conductivity. As
 σ inferred from Table II and Figure 3, the three layer printed patter
Tmelt (R) = Tbulk 1 − [1] was found to have cracks after sintering which in turn reduces the
R
conductivity. The optimal conductivity of the sample is obtained by
Based on Equation 1 and the limitation of the paper substrate, the investigating the drop spacing and sintering temperature.
printed layers (single, double and triple) were cured at 60◦ C, 75◦ C, A change of color of the printed pattern from yellowish-brown
90◦ C, and 120◦ C for 30 minutes. The conductivity of the printed to tan was observed with increasing sintering temperature. Further,
patterns were evaluated and compared with plain air drying. 15 μm drop spacing provided higher conductivity results than com-
pared to 10 μm drop spacing.
Resistivity measurements.—Electrical resistivity describes appro- Firstly, to figure out an optimum drop spacing, this experiment con-
priate signal carrying capability of a conducting path. It depends not sidered many from 5 μm to 20 μm and the above table compared the
only upon the chemical properties of the silver nanoparticles, but data for 10 μm and 15 μm drop spacing respectively. Irrespective of
also the geometry of the printed pattern: length (L), width (W), and the number of printed layers, the resistivity of the patterns plummeted
after curing. Theoretically, the triple layer should have the highest
conductivity. However, the double layer resistivity is lower compared
to the triple layer at 120◦ C. Such an anomaly might be speculated due
to the possibility miniscule cracks and discontinuities of ink explored
further in this article. The resistivity versus temperature curve is given
in Fig. 2.
Figure 2 shows the drastic fall of resistivity of printed samples
for sintering temperatures above 60◦ C. The best conductivity was
found for double layer printed samples cured at 90◦ C, (resistivity:
1.8 × 10−5 -cm, and conductivity: 5.56 × 106 −1 m−1 ). The
resistivity values after 90◦ were almost the same for all printing
thicknesses. To analyze the significance of sintering temperature on
pattern resistivity, one-way analysis of variance (ANOVA) analysis
was pursued. The ANOVA results (Table III) indicate with a level of
Figure 1. Image of the sample printed pattern acquired with a) Digital camera, significance (α = 0.05) that the sintering temperature is a significant
b) Fiducial Camera of DMP. factor affecting resistivity.

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 Journal of The Electrochemical Society, 165 (8) B3163-B3167 (2018) B3165

Table II. Resistivity results at different curing conditions and drop spacing.

Resistivity ∗ 10−4 -cm

Single Layer Double Layer Triple Layer

Sl. No. Sintering Temperature (◦ C) Cooling Period min 10 ds 15 ds 10 ds 15 ds 10 ds 15 ds

1 air 720 11800 2880 8300 1300 9580 970
2 60 30 84.87 26.32 37.1 19.8 29.9 7.86
3 75 30 21.71 6.8 3.6 5.4 3.24 2.62
4 90 30 1.13 0.283 0.54 0.18 0.262 0.262
5 120 30 0.47 0.189 0.18 0.18 0.262 0.262

Table III. ANOVA Analysis.

Source SS DF MS F0 Fcritical P-Values

Sintering Temperature 6.316E-06 3 2.1E-06 9.528 4.066 p<0.05
Error 1.768E-06 8 2.2E-07
Total 8.084E-06 11

Surface characterization.—Imaging the corner side of the printed may increase, decreasing the conductivity and eventually deteriorating
patterns was a priority as these are the most failure prone areas after the performances of the devices. Figs. 3c and 3d shows the images
printing.21 Figure 3 presents CD microscopy images of three different for double layer printed pattern. A significant improvement in printing
layers of printed samples. Images were taken at 10x magnification. quality has been observed: sharp edges, uniform ink depositions, fewer
Figs. 3a and 3b shows the single layer printed pattern cured at open air holes, and increased bonding with the substrate.
and 90◦ C respectively. A lot of pinholes, as well as non-continuous A closer observation of the image reveals coffee stain effects,
printed lines can be seen before curing. After curing at 90◦ C, the particularly after curing. This effect can be controlled by decreasing
edge quality improved and brought ink more closer. Yet, the pattern the voltage of the droplets further in the cartridge settings.
contained many holes. The size of the hole of the substrates overtime For the three layer printed pattern (Fig. 3e) which is air cured, there
are relatively no holes, ink is distributed evenly. But, there seems to
be some satellite spots, which were not present for the single and
Resistivity versus sintering temperature plot double layer prints. When cured to 90◦ C and above, miniscule cracks
100 appeared to form. At 120◦ C, there were even more cracks present
(Fig. 3f). The probable cause is speculated to be that the increased
Resistivity (x10-4 Ω-cm)
(in logarithmic scale)

10
amount of silver nanoparticles, as well as the associated impurities
disintegrate at higher sintering temperatures. However, for single or
1L
double layers of printed patterns, such cracks or fatigues were absent.
1 2L Figure 4 shows the thickness of the double layer printed samples
3L cured at 90◦ C for 30 minutes using 15 drop spacing. The thickness
of the double layer printed pattern was found to be 1.8 μm. Table IV
0
60 75 90 105 120
Sintering Temperarture (0C)
Table IV. Layer thickness of the printed patterns.

Figure 2. Resistivity result of samples with different curing temperatures. The Layer Single Double Triple
error bar represents standard deviation. (Number of printed layers, 1L: One
layer, 2L: Two Layer, and 3L: Three Layer). Thickness, (μm) 0.944 1.8 2.62

Figure 3. CD microscopy images to observe ink deposition, holes, crack, and coffee ring effect at 10X magnification level. The printed samples are: (a) single
layer, air cure, (b) single layer, cured at 90◦ C, (c) double layer, air cure, (d) double layer, cured at 90◦ C, (e) triple layer, air cure, and (f) triple layer, cured at 120◦ C.

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 B3166 Journal of The Electrochemical Society, 165 (8) B3163-B3167 (2018)

SEM micrographs of the surface of the different printed layers

is shown in Figure 6. Figures 6a, 6b, and 6c are the SEM pictures
of the single layer, double layer, and triple layers of printed patterns
respectively, cured at 90◦ C for 30 minutes. The number density of
holes seems to decrease gradually from Figs. 6a to 6c.
Due to the cracking and pinhole issues in triple and single layer
printing patterns observed earlier, we had chosen to focus on double-
layer pattern for further analysis. The double layer pattern seems to
deliver the best printing results of this particular substrate. It also cuts
down one-third of the cost of printing (ink, cartridge, cleaning pad,
drop watcher pads, etc.) related to material consumption compared to
three layer printed pattern. According to the Figs. 6d, 6e, and 6f, a
smooth surface morphology is observed for the double layer printed
patterns at different sintering temperatures. The silver nanoparticles
appeared to have fused together which helped boost up the conduc-
Figure 4. Thickness of double layer printed samples using SEM. tivity upon sintering at different temperatures.
Mechanically induced electrical degradation is a primary concern
for flexible devices. For each sample, five bending tests were sequen-
shows the thickness of single, double, and triple layer printed patterns tially performed: (a) flexed around 18 mm cylindrical shaped pattern,
cured at 90◦ C. As expected, the thickness increased with an increase (b) 12 mm cylindrical pattern, and (c) three random diameter stress
in layer thickness. tests using human hand. The number of bend cycles were limited to
The 2D surface topographies of printed samples with different 10. Fig. 7a and Fig. 7b shows the bending test. After repeated bending
layer thickness cured at 90◦ C is shown in Figure 5. These graphs were tests, the resistivity was measured using four-point probe method. The
obtained using the Veeco software associated with the Optical Profiler. maximum deviation found in resistivity of the samples after repeat-
The average roughness values of the different printed samples were ing bending was about ± 0.5%. No cracks were observed with naked
calculated using MATLAB. Table V shows the surface roughness val- eye and hence further imaging was not carried out. The thickness of
ues as a function of the number of layers of printed pattern. This table the paper substrate is 0.254 mm. The results from the bending tests
indicates that the increasing of the number of layers of printed pattern indicate the robustness of the printed patterns.
increased the average surface roughness, but not proportionally.
Conclusions
In this article, the electrical property and surface morphology of
Table V. Average roughness of 3 different printed layer samples.
different layers of ink-jet printed patterns on paper substrate were
investigated as a function of sintering temperature. ANOVA anal-
ysis indicated that the sintering temperature is a significant factor
Layer Single Double Triple
affecting conductivity of the samples. Among the three printed lay-
Roughness, (nm) 289 328 356 ers, the double layer printed pattern (cured at 90◦ C for 30 minutes)

Figure 5. Surface roughness analysis using the Optical Profiler of (a) Single layer, (b) Double layer, and (c) Triple layer printed samples.

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 Journal of The Electrochemical Society, 165 (8) B3163-B3167 (2018) B3167

Figure 6. SEM pictures for microstructural variations on the pattern surface as a function of number of printed layers and sintering temperature. The effect of
amount of layer printed and cured at 90◦ C for a (a) single layer, (b) double layer, and (c) triple layer sample. Surface morphology of double layer printed pattern
sintered at (d) 60◦ C, (e) 90◦ C, and (f) 120◦ C.

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