High-resolution inkjet printing of conductive carbon nanotube twin lines utilizing evaporation-driven self-assembly

Accepted Manuscript High-resolution inkjet printing of conductive carbon nanotube twin lines utilizing evaporation-driven self-assembly Nghia Trong Dinh, Enrico Sowade, Thomas Blaudeck, Sascha Hermann, Raul D. Rodriguez, Dietrich R.T. Zahn, Stefan E. Schulz, Reinhard R. Baumann, Olfa Kanoun PII:

S0008-6223(15)30290-6

DOI:

10.1016/j.carbon.2015.09.072

Reference:

CARBON 10342

To appear in:

Carbon

Received Date: 19 May 2015 Revised Date:

14 September 2015

Accepted Date: 19 September 2015

Please cite this article as: N.T. Dinh, E. Sowade, T. Blaudeck, S. Hermann, R.D. Rodriguez, D.R.T. Zahn, S.E. Schulz, R.R. Baumann, O. Kanoun, High-resolution inkjet printing of conductive carbon nanotube twin lines utilizing evaporation-driven self-assembly, Carbon (2015), doi: 10.1016/ j.carbon.2015.09.072. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT High-resolution inkjet printing of conductive carbon nanotube twin lines utilizing evaporation-driven selfassembly Nghia Trong Dinha, Enrico Sowadeb, Thomas Blaudeckb,c,e, Sascha Hermannc, Raul D. Rodriguezd, Dietrich R. T. Zahnd, Stefan E. Schulzc,f , Reinhard R. Baumannb,f, Olfa Kanouna,∗ a

Technische Universität Chemnitz, Measurement and Sensor Technology, 09107 Chemnitz, Germany Technische Universität Chemnitz, Digital Printing and Imaging Technology, 09107 Chemnitz, Germany c Technische Universität Chemnitz, Center for Microtechnologies, 09107 Chemnitz, Germany d Technische Universität Chemnitz, Semiconductor Physics, 09126 Chemnitz, Germany e Linköping University, Organic Electronics, 60174 Norrköping, Sweden f Fraunhofer Institute for Electronic Nano Systems (ENAS), 09126 Chemnitz, Germany

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ABSTRACT

Article history: Received Received in revised form Accepted Available online

We report about the inkjet printing of multi-walled carbon nanotubes (MWCNTs) for conductive tracks. The MWCNTs were grown by chemical vapor deposition allowing a defined length and diameter. An inkjet printable ink formulation was prepared by dispersing the MWCNTs in water. Inkjet-printed high resolution patterns were obtained by printing the prepared ink formulation on silicon wafers utilizing evaporation-driven self-assembly processes. After the deposition of the ink, the solvent evaporation induced material flow within the liquid moving the MWCNTs. The induced flow makes MWCNT position preferably to the edges of the printed patterns as well as to the print starting position where they assemble. Atomic force microscopy (AFM) reveals a preferential orientation of the deposited MWCNTs. The resulting deposit pattern is well-known as coffee-ring effect which is used here to enable high resolution printing and self-ordering of the MWCNTs. Depending on different print parameters such as drop spacing or substrate temperature, conductive track widths can be tuned in the range of 5 to 15 µm were achieved with a electrical resistivity of about 3.9·10-3 to 5.6·10-3 Ω·m measured by currentsensitive AFM.

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Keywords: Carbon nanotubes Self-ordering Inkjet printing Atomic force microcopy

∗ Corresponding author. Tel.: +49-371-531-36931; fax: +049-371-531-836931; e-mail: [email protected]

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The alignment of the CNTs improves the electrical conducACCEPTED MANUSCRIPT

Among the liquid deposition technologies, printing technologies allow the deposition of the CNT networks in patterns. The realization of CNT networks is easy [1] and it’s convenient for the low-cost sector or for a large area application. Inkjet printing enables a direct patterning on the substrate without the need of a mask or any lithography process. Compared to other printing methods, only a low amount of ink is required. Commonly, metallic inks such as silver [2] ink and since a couple of years also copper [3-5] inks are used for inkjet-printed electronics. However, the long-term stability of the ink is limited due to oxidation and electro-migration processes [6]. CNTs were also used in many examples in literature for the manufacturing of electronics by inkjet printing. In contrast to metal inks, CNTs do not oxidize and do not show electro-migration effects. Depending on the targeted application, the relatively inexpensive multi-walled CNTs (MWCNTs) are more attractive than single-walled CNTs (SWCNTs). Among other applications, MWCNTs were used for conductive electrode patterns [7], shielding [8], and strain measurements [9].

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Materials and methods

2.1. MWCNTs

A low number of wall defects (e.g. vacancy or topology defects) and a well- defined length [15] is required to investigate the alignment of MWCNTs. The defects can increase the radius of curvature along the tubes [23] which makes it difficult to study their orientation. Additionally, the quality of the walls also defines the electrical properties of the MWCNTs [24]. For these reasons, MWCNTs were grown by chemical vapor deposition (CVD) at 400 °C on p-type silicon using titanium-cobalt as catalyst. Further information about the growing process is found in Hermann et al. [25]. The length and the diameter of the MWCNTs were measured using SEM and TEM. Fig. 1 a) shows the length distribution of the grown MWCNTs ranging from about 500 nm to 2.6 µm. Most of the tubes have a length of about 2 µm. Fig. 1 b) depicts a TEM image of a MWCNT.

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Deposition of CNTs by printing can be separated in two categories. The first is the printing of relatively large areas such as large conductive rectangles [6]. In that case, the anisotropic characteristics of the film are most important and the alignment of CNTs plays a minor part. The second category is the printing of micrometer-wide conductive electrodes. In this case, the conductivity depends strongly on the alignment of the CNTs and thus also on the printing process parameters such as printing direction. The printing resolution in inkjet printing is usually in the micrometer range and many efforts are being dedicated to increase the printing resolution, aiming for higher integration density of printed electronics. Towards this end, one possibility is to exploit the coffee-ring effect in single inkjet-printed drops. This effect was previously demonstrated for the controlled deposition of silver nanoparticle inks [10, 11] or SWCNTs [12] to achieve fewmicron wide lines.

tivity in a conductive film due to the lower number of junctions and the preferential electron mobility along the tube axis [20-22]. In contrast to Denneulin et al. [17], the focus of this investigation is the printing of thin twin-line MWCNTs conductive electrode paths. However, we avoid the use of the hydrophilic plasma treatment [14] aiming to exploit the coffee-ring effect for well aligned CNTs deposits. To our knowledge, there is no report on the alignment of MWCNTs by inkjet printing and the electrical characterization along the thin twin-line. The morphology of the printed line is investigated by optical microscope and atomic force microscopy (AFM), the electrical characteristics of the thin twin-lines are locally studied by conductive AFM (cAFM) and resistivity calculations. Additionally, based on our experimental results and in agreement with previous investigations on the orientation and alignment of nanostructures during the evaporation [14, 16, 17], we propose an empirical, qualitative model for the alignment mechanism of CNTs in printed lines.

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In this contribution, we exploit the usually unintended coffeering effect for the manufacturing of a few-micron wide electrode lines based on MWCNTs. This approach is based on the twinline deposition concept described by Bromberg et al. for the deposition of silver nanoparticle inks [2]. Similar methods were recently applied for graphene [13] and SWCNTs [14]. We study the self-assembly process of the MWCNTs resulting in deposits with preferential MWCNTs alignment. The alignment of MWCNTs is of high importance for the final electrical performance [14-16]. Denneulin et al. were already classifying different CNT alignment regimes for inkjet printing within a dried printed line [17]. They show that the determination of the electrical path in a printed CNT network is complex. The alignment and the distribution of CNTs in a printed pattern strongly depend on the composition of the ink, the printing process as well as the particulars of the evaporation process. An important parameter to control the CNT distribution in a printed pattern is the fine tuning of the interaction between ink and substrate. Song et al. reported about a more homogenous and random distribution of SWCNTs in a drop by increasing the surface energy between ink and glass using a plasma treatment [18] that suppressed the coffee-ring effect. Another factor to consider is the substrate temperature. Wang et al. printed MWCNTs on glass substrate and obtained a high MWCNT concentration on the drop periphery at room temperature. A more uniform distribution of MWCNTs was achieved by increasing the substrate temperature to 70°C [19].

Fig. 1: a) SEM image of CVD-grown MWCNTs on p-type silicon; b) TEM image of CVD- grown MWCNT The average outer and inner diameters were estimated to be about 20 nm and 5 nm, respectively. The average number of shells is 20. Based on an outer diameter of 20 nm and a length of ∼2 µm, the aspect ratio of the grown MWCNTs is of the order of 100. 2.2. MWCNT dispersion The grown MWCNTs were removed from the silicon substrate and dispersed in water to obtain a printable ink formulation. Triton X-100 (TX-100) from Sigma Aldrich was applied as surfactant in the ink formulation with 0.1 wt%. TX-100 is a nonionic surfactant with a benzene ring tail [26] and well known as dispersant for CNTs in literature [27]. Although TX-100 is not the most efficient surfactant to disperse CNTs [27], it is promising due to its low decomposition temperature starting at 150 °C

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line morphology was performed by an optical microscope (in ambient) [28]. This allows removing mostACCEPTED of the dispersant MANUSCRIPT (Olympus BX51) in dark-field mode to enhance the contrast after its deposition with inkjet printing. between the deposited MWCNTs and the silicon substrate. The To obtain individual dispersed MWCNTs, the ultrasonic sonoanalysis of the distribution and the electrical characterization of trode Bandelin HD 3200 was applied for 10 min at 30 W (pulsathe deposited MWCNTs were carried out with an AFM from tion mode: 0.5 s to 1 s). The MWCNT dispersion was cooled Agilent (5600LS AFM). In the non-contact mode a commercially during the ultrasonic treatment to avoid the increase of temperaavailable AFM Si tip (NSG01, from NT-MDT) was used. The tip ture. Finally, the obtained dispersion was filtered with a hydrohas a nominal radius of 6 nm. For the current sensing AFM philic syringe filter (pore size of 4.5 µm) to separate larger (CSAFM) measurement a platinum coated tip from NT-MDT MWCNT bundles from individual dispersed CNTs. To determine (CSG01/PT) with a nominal radius of 35 nm and a custom-made the concentration of the prepared MWCNT dispersion, commersmooth AFM gold tip were used to prevent the damage of the cially available MWCNTs with different concentrations in water MWCNT structure. An amplifier operating in GΩ-range with a were used as a reference. UV-VIS spectroscopy was carried out current limit of 10 nA was employed for the electrical resistance for the commercially available dispersions and the prepared measurement. Due to the high conductivity of the MWCNT lines, filtered and not filtered MWCNTs dispersion. The solids content a high ohmic resistor of 1.3 GΩ was positioned between the of the not filtered MWCNT dispersion was determined to about, electrode and the voltage-current converter (AC Mode III) in 0.01 wt% and the solids content of the filtered (4.5 µm syringe order to avoid current saturation. This chosen setup allows minifilter) MWCNT dispersion to about 0.0055 wt%. Thus, the final mization of measurement errors. The software Gwyddion was concentration used for inkjet printing was about 0.0055 wt%. All employed for the analysis of the AFM images, e.g. for the deterdetails about the spectroscopy are found in Supporting Informination of line width and line height. mation, Fig. S1. 3. Results and discussions 3.1. Inkjet printing deposition and alignment concept (twin-line deposition)

2.3. Inkjet printing

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The Autodrop System MD-P-801 (Microdrop Technologies, Germany) was applied for the deposition of the MWCNT dispersion. The system setup is based on a single piezoelectric inkjet nozzle (MD-K-130) with a nozzle diameter of about 70 µm. An optimized waveform for the piezoelectric printhead was designed consisting of one pulse with 17 µs pulse length and a maximum voltage of 87 V. With these parameters, stable drop jetting of the MWCNT dispersion was obtained as shown in Fig. S2 (Supporting Information).

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The print layout was defined as a pattern of conventional, straight lines with a line width corresponding to the diameter of a drop spread on the solid surface and a line length of 100 to 140 drops deposited next to each other. The resulting pattern had a length between 8.4 mm and 11 mm depending on the chosen drop space. A minimum of two lines was printed for each parameter variation. To investigate the influence on the printing direction, two close together printing lines were oriented in opposite direction, respectively. The printing speed was about 1 mm/s and the center to center distance of the deposited drops (drop space) was varied from 60 µm to 110 µm (in steps of 10 µm). To improve the layer formation, the substrate holder was heated to 35 °C, 50 °C, or 60 °C. All printings were performed under ambient conditions. A p-type silicon wafer with a silicon oxide layer of 100 nm was used as printing substrate. The surface energy of the wafer was determined by the Owens-Wendt-Rabel-Kaelble (OWRK) method (see e.g. [29]) using a Krüss GH11 measurement system to 45 mN/m (27.2 mN/m disperse parts, 17.8 mN/m polar part). The contact angle of a sessile drop of the prepared MWCNT dispersion on the silicon wafer was about 35.5 ± 0.8 °. This low contact angle is mainly caused by the surfactant TX100. The contact angle of deionized water on the wafer is about 62 ± 4 °. 2.4. Characterization Methods

The printed and dry layers were heated after the deposition process on a hotplate at 300 °C for 20 min. This temperature treatment removes some expected remnants of the surfactant [28]. Without the surfactant, the electrical conductivity will be increased. Thus it allows also a higher contrast in the conductive atomic force microscopy experiment. The characterization of the

The twin line deposition concept applied for this research is shown in Fig. 2. Fig. 2 a) depicts a cross-sectional view of the inkjet-printed line and the piezoelectric inkjet printhead. Depending on the chosen drop spaces in printing direction and across (line feed) and the footprint defined by the surface energy of substrate and the surface tension of the ink formulation, vicinal drops may join and form either a line or a film, as reported recently by Belgardt et al. for the regime of evaporation with a floating contact line [30]. In this work the contact line of the deposited aqueous ink is pinned on the silicon substrate during evaporation, the solvent starts to evaporate and transport flows within the droplet are initiated. As a consequence, the area of the substrate covered by the printed ink remains constant but the contact angle between the substrate and the deposited drops decreases. An edge-enhanced evaporation of the water transports the MWCNTs to the periphery of the deposited film where they assembly and agglomerate. This phenomenon initially described by Deegan et al. [31] is well known as coffee-ring effect. After water evaporation, dry deposits of MWCNTs are formed mainly at the edges. Only a few MWCNTs are located between the twin-line configuration. Fig. 2 b) shows schematically a top view of the twin-line deposition concept. The digital printing pattern is indicated as a rectangle consisting of individual drops with a drop spacing dx = 60; 70; 80; 90; 100; 110 µm (for terminology, see e.g. [30]). The resulting liquid pattern shows that the ink spreads on the substrate resulting in a different pattern with a larger size than digitally defined. At the starting position, the lines are remarkably more widened than at other positions. These results in clearly visible bulge morphology and also in more MWCNTs agglomerated. We observed similar results when extending the small and simple line pattern to larger rectangles as shown in Fig. S3 in the Supporting Information. Therefore, a strong axial transportation flow seems to be established perpendicular to the printing direction as well as against the printing direction towards the print start position. These observations are in line with the report of Duineveld [32]. He assumed, that the bulging occurs due to a symmetry change of the liquid deposit starting from a truncated spherical symmetry as usual for drops to a truncated cylinder symmetry of the line [32]. The bulge attracts the liquid and the dispersed MWCNTs of the pinned liquid line

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tion flux along the printed line is established additionally to the due to an axial gradient in capillary pressure [2, 32]. In addition, MANUSCRIPT ACCEPTED axial material transportation flow to compensate for the solvent the evaporation process is by far more advanced at the print loss due to evaporation at the contact line. starting position compared to other positions due to the sequential nature of the inkjet deposition process. An enhanced evapora-

Fig. 2: Twin-line deposition concept based on inkjet printing as a) cross-sectional view and b) top view 3.2. Macroscopic line morphology of inkjet-printed MWCNTs

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Fig. 3 shows typical examples of the inkjet-printed line morphology obtained by optical microscopy in dark-field mode. The left line represents a starting position whereas the right line represents an end position of a printed line for Fig. 3 a), b) and c), respectively. The substrate temperature was varied from a) to c) and affects obviously the morphology of the deposits.

The twin-line width (Wt) of the uniform middle part of the deposited lines, where the twin lines are parallel, was determined by means of optical microscopy in dark-field mode. Fig. 4 a) shows the twin-line width as a function drop space and substrate temperature. The twin-line width decreased for all substrate temperatures with increasing drop space due to a higher distance between the deposited drops and thus a lower amount of material per line. A similar trend was shown by Seifert et al. [34].

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Next to variations of the substrate temperature, we also varied the drop space as presented in Fig. 3 d), e) and f). Similar printing tests were also performed by Wang et al. [19]. They investigated the line morphology as a function of drop space and the coffeering effect for printed MWCNT lines as a function of substrate temperature. A classification of the line morphology is introduced. Wang et al. divide in stable and unstable lines. Unstable lines show bulging along the length of the line due to a low drop space initiating a misbalance of transports flows in the printed line. Fig. 3 f) presents the corresponding line morphology with our MWCNT ink formulation deposited on silicon substrate. The bulging locations are separated by uniform regions with a certain periodicity [32, 33].

Fig. 3 d) shows a stacked coins structure [33] appearing due to the high drop spacing and the elevated substrate temperature (60 ºC). In this case, in comparison to the other drop spaces, the amount of deposited material is lower and evaporation occurs at a faster rate, e.g. individually drop by drop so that complete drop coalescence will not take place. These inhomogeneities in the printed line can be avoided. A drop space of 90 µm was found as an optimum value to print stable, uniform lines without bulging effects as presented in Fig. 3 e). To investigate the morphology of the printed lines in more detail, inkjet printing of the MWCNTs ink was performed for different drop spaces (60 µm up to 110 µm in steps of 10 µm), and at different substrate temperatures (35 °C, 50 °C, 60 °C).

Fig. 3: Microscopic images of print starting (left line in each image) and ending (right line in each image) positions at a drop space of 90 µm as a function of substrate temperature: a) 35 °C, b) 50 °C, c) 60 °C d) to f) are microscopic images of the middle regimes of the printed lines at constant substrate temperature of 60 °C and varied drop space of d) 110 µm, e) 90 µm f) 60 µm In addition, the twin-line width also increases with decreasing substrate temperature. The higher substrate temperature results in

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position of the print was also determined. Fig. 4 b) shows the faster evaporation of the liquid so that spreading of the deposited MANUSCRIPT ACCEPTED maximum line width - which means the twin-line width at the drops is much lower than on substrates at lower temperature. The starting position of the printing (Ws) - as a function of drop space same observation was made by von den Berg et al. [35] for printed droplets and lines. This observation can be related to the influand substrate temperature. We found the same trend as seen ence of vertical temperature gradients to a liquid pattern (droplet before in Fig. 4 a). Additionally, the twin-line width at the end [30, 36, 37], line or bead [33, 38], or two-dimensional films at position (We) was determined. At this position, we could see a interfaces [39]. The macroscopic morphology of the deposits line thinning as indicated in the scheme in Fig. 2 and Fig. 3 a) to after evaporation can be explained by the induced Marangoni Fig. 3 c). However, there was no clear trend on drop space or flows that are governed by processes described by the Kelvin substrate temperature. The average twin-line width at the print equation (Supporting Information S4, equation S.4.1). end position was about 127 ± 10 µm.

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Microscopic images of all the lines are found in Fig. S5 in the Supporting Information. The width of the bulge at the starting

Besides the overall line width, the width of the individual twin lines forming the MWCNT sediment was investigated. Therefore, the width of a single-line (Wl) of the twin-line was determined as a function of drop space and substrate temperature in the uniform middle part (Fig. S6, Supporting Information). The same trend as noticed before in Fig. 4 was observed.

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Heuristically, von den Berg et al. simply explain the trend of reduction of spreading at higher temperature with the faster evaporation of the solvent of the droplet, finally minimizing the line width. Indeed, in our study (Fig. 4 a)), the lowest line width (133 ± 4 µm) was obtained at 60 °C substrate temperature (drop space: 110 µm).

Fig. 4: a) Twin-line width at the middle part (Wt) and b) twin-line width at the print start position (WS) of the inkjet-printed MWCNTs ink as a function of drop space and substrate temperature 3.3. Micro- and nanoscopic line morphology of inkjet-printed MWCNTs

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The self-ordering of the MWCNTs at the edge of a printed line, that implies one individual line of the twin-lines, is discussed by means of AFM investigations. All samples were fabricated with the optimized printing parameters introduced before, i.e. a drop space of 90 µm and a substrate temperature of 60 °C (see also Fig. 3 e)). Single-pass (the substrate is passed one time beneath the printhead) and multi-pass (substrate is passed several times beneath the printhead) printing were performed. Samples with two and three consecutively printed lines were prepared. In these cases, the second and the third line were deposited at exactly the same position as the first one enabling overprinting.

and the time constants of the wetting and solvent evaporation have to be considered.

3 . 3 . 1 . M WC N T s i n g l e -l i n e m o r p h o l o g y i n t h e m i d d l e p o s i t i o n o f a p r i n t e d l i n e (s i n g l e -p a s s p r i n t i n g ) The alignment of the MWCNTs in a single-line (single-pass) printing can be seen in Fig. 5, where “a” is the uncovered substrate area, “b” the MWCNT deposit marking the edge (liquid-air periphery) and “c” is the area between the two edges of a printed line (see Fig. 2 b)). The average single-line width (Wl) and height of area b in Fig. 5 was 3.5 ± 0.3 µm and 37 ± 12 nm, respectively. It is clearly observed that in area “c” are almost no MWCNTs, whereas in area “b” a dense network of MWCNTs is detected. Area “b” is established in the “equilibrium stage” [40-43]. For a discussion of the observed phenomena, the geometric directions

Fig. 5: Alignment of MWCNTs in a printed single-line (single-pass, white arrow indicates printing direction, black arrow indicates the paraxial transportation flow based on the coffeering effect) In the regime of a pinned contact line, the liquid-air peripheries on the edges remain for a longer time in the same position and a continuous capillary flow of the ink (solvent including dispersed MWCNTs) proceeds from the middle of the line towards the two edges, compensating for the liquid evaporation at the three-phase boundary. According to Talbot et al., the lifetime (i.e., duration of evaporation) of an aqueous ink droplet in picoliter range on silicon oxide is in the order of a few seconds [44]. This figure is two orders of magnitude longer than the time that passes (ca. 90 ms) until the subsequent inkjet-printed droplet reaches the sur-

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face. It is admitted that this argumentation holds only for not MANUSCRIPT Also the roughness of the deposited MWCNTs contribute to the ACCEPTED heated substrates. According to the Kelvin-Thomson formalism, formation of the boarder. As a result, the contact line pinning of the evaporation rate is indirectly proportional to the temperature the second and third printed layer takes mainly place inside the difference between droplet and environment, hence the chosen area confined by the twin-lines of the first printing pass. Only substrate temperatures (in our case: 60 °C) are expected to reduce some water will penetrate into the deposited layer of the first the droplet lifetime by a much lower factor. This means that any pass. This could result in a partial re-distribution of the deposited coalescence with the next droplet (an equivalent phenomenon to MWCNTs of the first pass. These considerations are the reason ‘dot touch’ in graphical printing) will inevitably lead to forwhy a lower degree of alignment is obtained at two and three mation of an elongated ‘bead’ [30, 33] in which now two perpenprinting passes as shown in Fig. 6. dicular directions of solvent fluxes due to evaporation at the 3 . 3 . 3 . M WC N T s i n g l e -l i n e m o r p h o l o g y a t t h e p r i n t contact line emerge. According to Zhang et al., such a difference e n d p o s i t i o n o f a p r i n t e d l i n e (s i n g l e -p a s s p r i n t i n g ) in evaporative pressure gradients can be result of a significant As already shown in Fig. 2 b), a MWCNT gradient is formed difference between the net curvatures. In our case, this situation between the print end position and the print start position of the occurs between the peripheries and between the print start posiline. A line thinning appears at the print end position whereas a tion and print end position of the contact line [45]. In a singular line widening is obtained for the print start position. Here, the droplet or in beads with low aspect ratio, the MWCNTs will start focus is set on the print end position where the line thinning takes to align in the direction of the radial capillary flow towards the place. Fig. 7 shows AFM images depicting the morphology of the edge of the printed line [32]. In this position, they have the MWCNTs at the end of the lines. Close to the line end the smallest flow resistance [16]. Once they reach sediments at the amount of MWCNTs is reduced and thus the single-line width edge, they turn their position and self-align perpendicular to the lower than compared to the parallel regime explained in the secflow direction. The turning process of long CNTs from paraxial tion above (3.1). The closer the distance to the parallel middle to perpendicular is not well understood but explained phenomeregime, the higher the single-line width as shown in Fig. 7 b) and nologically in earlier reports [14, 17]. For longer lines, a solvent Fig. 7 c). flux due to the evaporative compensation along the printed line will become more relevant. Towards the end of this process, even It can be shown, that in the narrow end position a depletion area in the regime of a pinned contact line, the footprint of an almost of MWCNTs arises. The depletion can be explained by the gradidry drop (at the print end position of the line) is reduced by 10ent in the capillary pressure along the printed line [2, 32] origi30 % [44] as reflected in the scheme of an evaporating printed nating from the different states of evaporation in each singular line in Fig. 2 b). part of the printed line due to the sequential nature of inkjet printing [45]. Hence the MWCNTs are transported along the printing 3 . 3 . 2 . M WC N T l i n e m o r p h o l o g y i n t h e m i d d l e p o s i direction towards the place where the evaporation has progressed t i o n o f a p r i n t e d l i n e ( m u l t i -p a s s p r i n t i n g ) the most. Due to the depletion of MWCNTs in the print end of Fig. 6 presents the MWCNT line morphology of a printed line the line, some gaps between MWCNTs are observed. Conductivifor a) two and b) three printing passes. The preferential orientaty measurements (current-sensitive AFM) in the print end of the tion of the individual MWCNTs along the printing direction is line prove that the both single-lines (edges) of a printed line are not as good as in case of single-pass printing where only one not electrically connected. layer was deposited. In multi-pass printing new line zones with MWCNTs enrichment are established. Obviously, the numbers of line zones corresponds to the number of passes/deposited layers as shown in Fig. 6. Fig. 6 b) shows that the line width seems to increase with each printing pass, respectively. Between two enriched zones, a zone of depletion with less MWCNTs can be noticed. We assume that the deposited and agglomerated MWCNT line of the first pass forms a partial border for the wet deposits of following passes. The border is mainly established based on repelling forces appearing between the minor polar component of the surface energy of the MWCNTs and the high polar component of the surface tension of the water-based ink formulation.

Fig. 6: AFM images of printed MWCNT lines obtained with multi-pass printing; a) two printing passes b) three printing passes; dotted lines indicate the individual linear MWCNTs deposits of each pass

Fig. 7: a) Microscopic images of the line morphology at the end position (arrow shows printing direction) and AFM images b) close to the line end position and c) close to the parallel middle part; images indicate a reduction of the amount agglomerated MWCNTs along the printing direction

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3 . 3 . 4 . S u m m a r y o f M W C N T s i n g l e -l i n eACCEPTED morphology along the printed line length

vline = 0.6·vdrop is obtained. Both velocity vectors contribute to the MANUSCRIPT total velocity vector (vtotal).

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Fig. 8 depicts the average single-line heights and average singleline width of the MWCNT sediments as a function of the print position. The green dashed lines indicate the measurement position at the inset. All measurements were performed by AFM. The printing parameters for the sample are 60 °C substrate temperature and a drop space of 90 µm.

Fig. 9: Flow velocity vectors calculation a) AFM image for the calculation b) calculated angle

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Fig. 8: Single-line height and single-line width of the MWCNT sediments as a function of print position

Based on our experimental results and the results from Li et al. [16] and Denneullin et al. [17], we propose the following empirical, qualitative model for the alignment mechanism on the periphery (Fig. 10). In this model, the two water influxes and vander-Waals forces between the nanotubes are the driving factors for the alignment. Due the superposition of the compensating solvent fluxes, one MWCNT is transported from the middle of the drop angularly to the periphery (Fig. 10 a)). As vline and vdrop are reached, the MWCNT starts to turn until the MWCNT is in the direction of the total velocity vector (vtotal). The turn direction depends on the MWCNT orientation at the beginning. The shortest way to achieve a position parallel to the total velocity vector will be favored.

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The deposited lines have varying heights and widths over their length as a result of the two different solvent fluxes introduced before. The lowest line height and line width was obtained at the print end position. The line width reaches its maximum at the print start position where a bulge was formed. Also the line height seems to increase at the reverse printing direction although it was found that the middle position has in average the largest line height. The line height at the print start position could be lower due to the larger width of the line. This will allow the MWCNTs to agglomerate on a larger area and limiting at the same time the line height. The results show clearly the dependency of line height and width over their length and thus as a function of print position. This observation is of high importance. Similar behavior is expected also for other materials deposited by inkjet printing on non-absorbent substrates. Compensation methods have to be considered to allow the deposition of homogenous lines regarding height and width [46]. 3 . 3 . 5 . Al i g n m e n t m e c h a n i s m o f M WC N T s

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As explained above, in a printed line, there are two perpendicular directions of compensating solvent fluxes. The force invoked by the radial flux (fdrop) goes from the middle of the drop to the peripheries. The second force invoked by the solvent flux (fline) along the printed line goes from the subsequent printed drops to the previous printed drops. The both forces will cause two perpendicular velocity vectors vdrop and vline, respectively. From AFM images showing the orientation angle of MWNCTs close to the edge, the direction of this total velocity vector can be investigated. In the following analysis (Fig. 9), only well separated single MWCNTs or MWCNT bundles are taken into account. In Fig. 9 a), the white arrow indicates the printing direction. Due the hydrophilic wetting behaviour and the coffee-ring effect, the total velocity vector points infallibly to the periphery. Therefore, a 180° coordinate system is needed. From a sequential enumeration of 13 MWCNTs or MWCNT bundles, an average angle of 120.7° with respect to vline (30.7° with respect to vdrop) (Fig. 9 b)) can be evaluated. With consideration of trigonometry, a relation

(a)

(b)

fline

P3 vdrop P1

P2

fline

1

fdrop

f31 P2 f21 P1 f11 vtotal

fline 2

fdrop

f32

v line

vtotal P3 P2 P1 vtotal

P3

f22 fline fdrop

P3 fdrop f11>f21>f31

fline fdrop

P2 3

P1 f12

y x

4

5

f12=0: fat=fre f22>f32 Printing direction

Fig. 10: Model for the alignment of carbon nanotubes on the periphery. Fig. a) shows the alignment mechanism of MWCNTs, when they flow to the periphery. 1) MWCNT start to turn in the direction of vtotal; 2) MWCNTs are in the direction of vtotal and flow to the periphery; 3) MWCNTs turn parallel to the periphery due to vdrop = 0. Fig. b) shows the alignment mechanism with the additional Van der Waals force. 4) MWCNTs are in the direction of vtotal and flow to the periphery; 5) MWCNTs turn parallel to the periphery due to vdrop = 0 and the Van der Waals force. When the first end (P1) of the MWCNT arrives to the periphery the velocity vector in x-direction will be zero, and inevitably the total velocity vector will be parallel to the periphery (Fig. 10 a), no. 3). Thus the MWCNT will align parallel to the total vector and hence parallel to the printing direction. When some MWCNTs are already on the periphery (Fig. 10 b)), the van-derWaals attraction acts additional to the alignment. The van-der-

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Waals force is strongly depends on the distanceACCEPTED [47] between two MANUSCRIPT To produce an appropriate electrical contact for the measurement tubes and acts within a small distance of several nm [48]. In Fig. one end of the printed line was sputtered with about 100 nm 10 b) No. 5 one MWCNT is quasi in touch with the other silver. The other end of the printed single-line was contacted with MWCNT in position 1 (P1). Due to the equilibrium between the the AFM tip used as a movable electrode. repulsive and attractive forces the arriving MWCNTs will be A negative voltage of -0.5 V was applied to the silver contact. stack on this position. When the sum of the forces fdrop and the The current along the MWCNT line was measured by cAFM. van-der-Waals force is larger than the force fline, then the incomThe measurement setup is depicted in Fig. 11 b). From the curing MWCNT will align parallel to the previous tube. rent sensing scan (Fig. 11 a)), an approximately 5 µm wide conductive line is detected. It is proven that the MWCNTs in the center part (between two single lines) are not electrical connected 3.4. Electrical characteristics of the printed MWCNT lines with the MWCNTs on the edges. In this section, the conductivity paths of the MWCNTs deposit The focus of the second investigation is the evaluation of the on the edges were investigated (Fig. 11 a)). The aim is to study resistance at line edges. As mentioned before, a custom-made the influence of the printing passes on the conductivity. smooth AFM gold tip was used in these experiments. Reference

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current-voltage (I-V) curves on different positions of the silver electrode were measured. From the measurements a reference average curve and the reference resistance (from silver electrode until AC Mode III) were calculated. Afterwards we have verified the conductivity on the print end position (see Fig. 7 a)) to be sure that both edges (single lines) are not connected. Using the positionable stage the I-V curves dependency on the distance to the silver electrode was obtained. In order to increase the result’s reliability, ten measurement sweeps for each position were performed. The average resistances from I-V curves for each position were calculated. The subtracted I-V curves (without reference resistance) for the single-pass and multi-pass printings on the same distance are depicted in Fig. 12 a).

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Fig. 11: a) cAFM image of the deposited MWCNTs in a single line which forms a measureable current path b) Conductivity measurement setup

Fig. 12: a) I-V curves at a distance of 1.5 mm from silver electrode and b) resistance of the printed MWCNT on the peripheries as a function of distance.

All I-V curves show a linear characteristic, which indicate a metallic behavior. Due to the small difference of the current a noisy signal was obtained for each curve. However, the different resistances as a function of printing passes are clearly visible. The higher the current difference (in comparison to the reference resistance) the higher the resistance. Fig. 12 b) presents the resistance of the lines as a function of distance to the silver pad. The resistance values were calculated by subtracting the resistance of the MWCNTs and the reference resistance of the silver electrode. A linear increase of resistance as a function of the distance to the silver electrode was found.

From the average gradients of the curves, a resistance of about 30.4 MΩ/mm for single-pass printed lines, 20.8 MΩ/mm for lines printed with two passes and 13.8 MΩ/mm for lines with three passes were calculated. In order to calculate the resistivity, the cross-sectional area of the deposited MWCNT lines was obtained from five AFM height distributed randomly along the printed line profiles and appropriate averaging. More details about the calculation of the line width and line height can be found in Fig. S7 in the Supporting Information. With the resistance vs distance dependency, the resistivity can be calculated by the following equation:

8

tion to the print origin as proposed by Duineveld [32]. Based on ACCEPTED MANUSCRIPT (1)

Whereby R is the measured resistance, b the line wide, h the line height and l is the line length. The average values and the calculated resistivity for different printed lines are listed in Table 1. Table 1: Dimensions of the printed conductive lines and their calculated resistivity two printing passes

three printing passes

Average line width (µm)

3.5

6.9

8.7

Average line height (nm)

36.7

32.4

46.8

Resistivity (Ω·m·10-3)

3.9±0.6

4.7±0.6

5.6±1.06

Table 2 compares resistivity values of different materials applied in inkjet printing such as silver, graphene, SWCNTs as well as a different MWCNT ink formulation. Table 2: Comparison of resistivity given in literature for selected inkjet-printed nanomaterials

(Ω·m)

Silver [2]

Graphene [49]

SWCNTs [18]

MWCNTs [6]

1·10-5 … 1·10-6

4·10-5

2·10-5

7·10-3

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Resistivity

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The resistivity of the printed MWCNTs have the highest resistance of all materials. The difference in resistivity of SWCNTs and MWCNTs seems to be about two orders of magnitude. However, the electrical performance of our measurements with about 3.9·10-3 Ω·m is better than 7·10-3 Ω·m shown by Kwon et al. [6]. In our case, a much higher single line resolution was obtained allowing the deposition of very narrow single lines with better electrical performance as obtained with larger film areas by Kwon [6]. A better resistivity was obtained mainly due to a higher degree or alignment of the deposited MWCNTs resulting in a lower number of contact resistances between the MWCNTs.

4.

The resistance of the lines was studied as a function of printing passes (5.63·10-3 Ω·m for three printing passes; 3.9·10-3 Ω·m for one printing pass) and the relation to the distance could be calculated (13.8 MΩ/mm for three printing passes; 30.4 MΩ/mm for one printing pass). It could be proven that the aligned MWCNTs of the first printed line will be re-dispersed by the subsequent printing lines in multi-pass printing. cAFM was used to characterize conductive lines of a single printing pass with a width of less than 5 µm.

Acknowledgments This work was partially made possible by financial support from the Deutsche Forschungsgemeinschaft (DFG) in the frameworks of the International Research and Training Group “Materials and Concepts for Advanced Interconnects and Nanosystems“ (GRK1215) and the Research Unit “Sensoric Micro- and Nanosystems” (FOR1713), further within the European Commission's Seventh Framework Programme TDK4PE (grant agreement no. 287682) and a postdoctoral scholarship from Linköping University, Department of Science and Technology (T. B., grant number ITN-2010-00018). The authors thank Susanne Hartmann for the support and helpful discussions about the AFM technique, Iris Höbelt for the SEM imaging, Andreas Morschhauser and Allyn Große for the microscope images, Dr. Steffen Schulze for the TEM analysis. Patrick Matthes, Dr. Christian Schubert, Dr. Holger Fiedler, Mrs. Andrea Arreba and Mr. Jun Ma (all TU Chemnitz) are acknowledged for support in sample preparation and printing.

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our experimental results and the results from earlier reports [16, 17], we suggest the introduction of two velocity vectors to describe the turning process from perpendicular to parallel to the periphery. Due to the axial gradient in the capillary pressure along a printed line during the evaporation, there are less MWCNTs situated in the end of a printed line compared to the printing origin. The MWCNTs content at the end position of a line is not sufficient to build a conductive path so that the both peripheries are only connected at the print start position of the printed line.

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R ⋅b ⋅ h l

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ρ=

Conclusions

In summary, we achieved high-resolution conductive twin-line electrode structures invoking a preferential alignment of MWCNTs at the three-phase boundary of inkjet-printed lines. This result obtained by optimizing printing parameters such as drop space and substrate temperature and using a custom-made ink formulation based on CVD-grown MWCNTs well dispersed in water by the help of surfactants. The preferential alignment at the contact line can be related to two components of convective transportation flows within an evaporating droplet. At the one hand, there is the well-known coffee-ring effect, but on the other hand we clearly observed another convective component in direc-

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SUPPORTING INFORMATION High-resolution inkjet printing of conductive carbon nanotube twin lines utilizing evaporation-driven self-assembly

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Nghia Trong Dinha, Enrico Sowadeb, Thomas Blaudeckb,c,e, Sascha Hermannc, Raul D. Rodriguezd, Dietrich R. T. Zahnd, Stefan E. Schulzc,f , Reinhard R. Baumannb,f, Olfa Kanouna,∗ a

Technische Universität Chemnitz, Measurement and Sensor Technology, 09107 Chemnitz, Germany Technische Universität Chemnitz, Digital Printing and Imaging Technology, 09107 Chemnitz, Germany c Technische Universität Chemnitz, Center for Microtechnologies, 09107 Chemnitz, Germany d Technische Universität Chemnitz, Semiconductor Physics, 09126 Chemnitz, Germany e Linköping University, Organic Electronics, 60174 Norrköping, Sweden f Fraunhofer Institute for Electronic Nano Systems (ENAS), 09126 Chemnitz, Germany

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Fig. S1: a) Absorbance at 500 nm of reference MWCNTs dispersions as a function of wt% of MWCNTs und used MWCNTs dispersions (green and red horizontal lines correspond to the measured absorbance of our MWCNTs dispersions); b) Absorbance from 400 nm to 1300 nm as a function of wavelength of the prepared MWCNTs dispersions in comparison to the reference dispersions

Fig. S2: Optimized jetting parameters result in stable ejection of ball-shaped droplets of the 0.0055 wt% MWCNT ink

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Fig. S3: Section of an inkjet-printed rectangle of MWCNTs of the size of 6 mm x 4 mm deposited at a drop space of 20 µm and an inkjet nozzle diameter of 21.5 µm; printing direction was unidirectional line by line from left to right and more material is obviously agglomerated at the starting position as indicated by the more intense black color in comparison to the other positions

S4: Influence of the temperature on the kinetics of a sessile droplet

The thermodynamics of a sessile droplet with extensions in the µm range can be

It reads

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approximated by the Kelvin equation that describes the vapor pressure over a curved surface.

(S4.1)

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 p  2 ⋅ γ (T ) ⋅ Vm RT ⋅ ln k  = r  pv (T ) 

where pk is the vapor pressure over the flat surface (Kelvin pressure), pv is the vapor pressure

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over the curved surface, γ is the surface tension (liquid/gas), Vm the molar volume of the liquid, r the radius of curvature (i.e. virtual radius of the sphere comprising the spherical cap), R the specific gas constant and T the temperature. For the case of inkjet-deposited, sessile solvent droplets on a heated plate, it has to be noted that the surface tension itself is a function of temperature. This relation reads  T γ (T ) = γ C ⋅ 1 −  TC

  

n

(S4.2)

where γc is the surface tension at a certain critical temperature Tc (measured in Kelvin). In Equation (S4.2), according to Wang and Zhao [37], n is an empirical parameter. In numbers,

ACCEPTED MANUSCRIPT at a temperature variation from 68 °C to 97 °C, the authors calculated that the surface tension for a water droplet reduces from 56 mN/m down to 47 mN/m.

In Equation (S4.1), the calculation of pv / pk for sessile inkjet droplets similar to the ones used in this study has shown that the increase of the vapor pressure due to a changed curvature

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during evaporation is negligible [30]. Likewise, the change in the linear expansion coefficient of water in the temperature range 0 … 100 °C is smaller than 1 % [37] and can thus be neglected. Hence, the Kelvin equation (S4.1) can be transformed to

with K =

2 ⋅ γ C ⋅ Vm

p  R ⋅ ln k   pv 

n

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  

(S4.3)

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 T 1 − TC r = K⋅ T

representing a constant almost independent of temperature.

As expediently known from experiments [36, 37], heating of a sessile droplet induces a

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thermal Marangoni convection. When heated, the base of the droplet relaxes in a higher temperature than the other parts of the droplet that are in contact with the ambient environment. With the occurrence of the temperature gradient, the surface tension of the

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droplet varies along its direction. With that, the region with higher temperature including the three-phase boundary at the bottom has a lower surface tension than the cooler region at the apex. Hence the liquid flows upwardly on the peripheral surface and downwardly in the radial

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center of the droplet (“inverse chimney effect”). So the convection in the inkjet-printed droplets heated by a hot plate is expected to be upward at the periphery and downward in the center, and this effect is the more pronounced, the higher the temperature gradient is. The evaporation flux of the solvent scales with the nominal temperature of the substrate holder of the printer. This means, the higher the temperature difference between the base of the droplet (in contact with the heated substrate) and its apex (in contact with the surrounding atmosphere assumed at ambient conditions) are, the more pronounced evaporation fluxes are induced that will carry the constituents (in our case MWCNTs) to the three-phase boundary. This is the message also transported by the mathematical relationship given obtained in Equation (S4.3). Additionally, heuristically, the time at which the solvent concentration has reached a critical

ACCEPTED MANUSCRIPT value to transit from the slip to the stick condition e.g. by reaching a critical viscosity (the concentration changes with time of evaporating inkjet-printed droplets [30]) can be assumed shorter when the evaporation flux is higher, influencing the total spreading and hence the final distance between the twin lines.

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It has to be noted that the micro- and nanoscopic line morphology (discussed in Chapter 3.3) is not covered by this argumentation as it should follow e.g. the degree of turbulences invoked during the evaporation at the three-phase boundary. Here, we are referring solely to one set of printing parameters comprising a substrate temperature of 60 °C.

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REFERENCES [30] Christian Belgardt, Enrico Sowade, Thomas Blaudeck, Thomas Baumgartel, Harald Graaf, Christian von Borczyskowski, and Reinhard R. Baumann. Inkjet printing as a tool for the patterned deposition of octadecylsiloxane monolayers on silicon oxide surfaces. Phys. Chem. Chem. Phys., 15:7494–7504, 2013. [36] Hua Hu and Ronald G. Larson. Analysis of the effects of marangoni stresses on the microflow in an evaporating sessile droplet. Langmuir, 21(9):3972–3980, 2005. PMID: 15835963. [37] Ziqian Wang and Ya-Pu Zhao. In situ observation of thermal marangoni convection on the surface of a sessile droplet by infrared thermal imaging. Journal of Adhesion Science and Technology, 26(12-17):2177–2188, May 2012.

Substrate temperature 50 °C

Substrate temperature 60 °C

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Substrate temperature 35 °C

Fig. S5: Line morphology of inkjet-printed MWCNTs on silicon oxide surfaces as a function of substrate temperature and drop space

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Fig. S6: Measured single line width (width of the MWCNTs sediment at the line edge) of the inkjet-printed MWCNTs ink as a function of drop space and substrate temperature based on optical microscopy images. Here, a single line of about 7 ± 1 µm was obtained for 90 µm drop space and 60 °C substrate temperature based on measurements using optical microscopy images. In comparison to the results of Error! Reference source not found. (characterized by AFM), the single line width (Wl) here appears doubled. The reason is the higher accuracy of the AFM measurement in contrast to the lower resolution dark field optical microscopy. With the latter method, scattered light is mainly used for the image production and the intense scattering of the MWCNTs causes optically an increased feature size and thus an apparent increase of line width.

Fig. S7: Determination of the average line width and average line height; (a) the lines were scanned minimum at 5 different positions by AFM, (b) the average height of each position was calculated with the software Gwyddion