Water-Soluble Copper Ink for the Inkjet Fabrication of Flexible Electronic Components

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Article Water-Soluble Copper Ink for the Inkjet Fabrication of Flexible Electronic Components

Nabi S. Shabanov 1,2, Kamil Sh. Rabadanov 1, Sagim I. Suleymanov 1, Akhmed M. Amirov 1 , Abdulgalim B. Isaev 2, Dinara S. Sobola 2,3,4,* , Eldar K. Murliev 1 and Gulnara A. Asvarova 1

1 Analytical Center for Collective Use, Dagestan Federal Research Centre of the Russian Academy of Sciences, 367001 Makhachkala, Russia; [email protected] (N.S.S.); [email protected] (K.S.R.); [email protected] (S.I.S.); [email protected] (A.M.A.); [email protected] (E.K.M.); [email protected] (G.A.A.) 2 Department of Inorganic Chemistry and Chemical Ecology, Dagestan State University, St. M. Gadjieva 43-a, Dagestan Republic, 367015 Makhachkala, Russia; [email protected] 3 Department of Ceramics and Polymers, Faculty of Mechanical Engineering, Brno University of Technology, Technická 2, 616 69 Brno, Czech Republic 4 Department of Physics, Faculty of Electrical Engineering and Communication, Brno University of Technology, Technická 2848/8, 616 00 Brno, Czech Republic * Correspondence: [email protected]

Abstract: The aim of this work is preparation and investigation of copper conductive paths by printing with a different type of functional ink. The solutions based on copper-containing complex compounds were used as inks instead of dispersions of metal nanoparticles. Thermal characteristics

of synthesized precursors were studied by thermogravimetry in an argon atmosphere. Based on the comparison of decomposition temperature, the dimethylamine complex of copper formate was

Citation: Shabanov, N.S.; Rabadanov, found to be more suitable precursor for the formation of copper layers. Structure and performance K.S..; Suleymanov, S.I.; Amirov, A.M.; of this compound was studied in detail by X-ray diffraction, test of wettability, printing on ﬂexible Isaev, A.B.; Sobola, D.S.; Murliev, E.K.; substrate, and electrical measurements. Asvarova, G.A. Water-Soluble Copper Ink for the Inkjet Fabrication of Keywords: organometallic compound; conductivity; thermogravimetry Flexible Electronic Components. Materials 2021, 14, 2218. https:// doi.org/10.3390/ma14092218 1. Introduction Academic Editor: Miguel Monge The use of printing in electronics manufacturing is attracting increasing attention in the ﬁeld of materials science and technology [1–4]. Unlike traditional methods of electronics Received: 26 February 2021 manufacturing, such as vacuum deposition [5], electroplating [6], and photolithography [7], Accepted: 21 April 2021 Published: 26 April 2021 printing technology is an additive approach to electronics manufacturing. This approach consists of the formation of structures by applying functional material to the required area

Publisher’s Note: MDPI stays neutral of the substrate, which greatly simplifies the technological process, due to direct printing with regard to jurisdictional claims in of the required materials with individual sizes and shapes [8,9]. This makes it possible to published maps and institutional affil- manufacture electronic circuits and assemble devices much faster and in a more economical iations. and environmentally friendly way. Moreover, printing technology is compatible with a variety of plastic substrate materials [10], providing electronic devices with additional advantages, such as lightness and flexibility [11]. This technology includes three main stages: preparing materials for printing, printing, and post processing [12]. Conductive materials in particular, which are main parts of Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. electronic devices, are used for printing to create conductive patterns, which provides This article is an open access article great prospects for their use [13]. Obviously, only metallic inks, such as gold (Au) [10], distributed under the terms and silver (Ag) [14], copper (Cu) [15], aluminum (Al) [16], and nickel (Ni) ones [17] are able conditions of the Creative Commons to provide low resistance within the range from 10 to 100 µOhm·cm. Among them, Ag Attribution (CC BY) license (https:// ink has the highest conductivity. However, it should be noted that along with the market creativecommons.org/licenses/by/ expansion, high cost and scarcity of Ag have hindered its use in large-scale production of 4.0/). printed electronics. As Cu has high conductivity (comparable to that of Ag), but it is much

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more abundant (1000 times as much as Ag) and much cheaper (about 1% of the cost of Ag), copper ink is gaining increasing attention. Over the past decade, numerous efforts have been made to develop stable copper paints and highly efﬁcient sintering techniques to create copper patterns at low temperatures and achieve high conductivity and high stability. Although a number of studies have been published on achievements of Cu chemistry and nanotechnology [18–20], their authors have mainly focused on synthesis of Cu nanoparticles. A comprehensive review on these materials reliability and stability was recently given by W. Li et.al. [21]. For instance, copper ink composed of 45 nm particles formed layers with resistivity of 17.2 µOhm·cm at 325 ◦C[22], and the use of particles of about 3.5 nm gave resistivity values of 30.0 µOhm· cm at temperatures above 200 ◦C[23]. Disadvantages of this type of ink are sedimentation of nanoparticles (ink stability), relatively high probability of clogging of printing head nozzles, and high temperature of subsequent annealing associated with a higher melting point of Cu. However, it is necessary to use a lower temperature, not exceeding 150 ◦C, to match plastic substrates [24]. In particular, oxidation of the surface of Cu nanoparticles leads to the formation of stable and undesirable oxide layers (Cu2O, CuO), which strongly hinder mutual diffusion of Cu atoms between the particles and reduce the formation of percolation paths, and it leads to low conductivity of printed patterns [25]. A solution to the problems described above can be development of inks based on complex compounds, which at relatively low temperatures form functional copper layers directly on the substrate. This paper presents the results obtained in studies of the process of creating copper conductive paths by printing with a different type of functional ink. The purpose of this work is to show the impact of using the solutions based on copper-containing complex compounds, free from disadvantages of dispersions of metal nanoparticles.

2. Materials and Methods 2.1. Synthesis Procedure The copper compounds were prepared for their further investigation as precursors in functional inks.

(1) Copper formate Cu (OOCH)2 was prepared by adding 5 mL of 85% formic acid to 10 g of copper hydrocarbonate. (2) Ammonia complex of copper formate [Cu(NH3)2](OOCH)2 was prepared by dissolving 1 g of copper formate Cu(OOCH)2 in 2 mL of 25% aqueous ammonia solution. (3) Dimethylamine complex of copper formate [Cu(C2H6NH)2](OOCH)2 was prepared by dissolving 1 g of copper formate Cu(OOCH)2 in 2 mL of 33% aqueous solution of dimethylamine.

2.2. Characterization Techniques The analysis of the processes of reduction of copper formate complex-based inks to Cu was performed by X-ray diffraction (XRD) (Shimadzu XRD-6000, Kyoto, Japan) and Raman spectroscopy (Confocal Raman microscope «Senterra» Bruker, Bremen, Germany). Additionally, thermogravimetric analysis was performed on a synchronous thermal ana- lyzer (STA 449 F3 Jupiter “NETZSCH”, Selb, Germany), by examining ink solution samples (100 µL) at a heating rate of 15 ◦C/min. Elemental analysis was performed by energy- dispersive X-ray spectroscopy (EDX) (ASPEX Express electron microscope equipped by the OmegaMax EDX detector, Sydney, Australia) at accelerating voltage 20 kV. The films were printed by a commercial inkjet printer EPSON 110 (Epson, Nagano, Japan). Standard piezoelectrically-driven printing head was used with 20 µm diameter of a nozzle. The polyethylene terephthalate (PET) films were chosen as the substrates. The surface of the substrates was processed by an oxygen plasma for increasing of hydrophilicity. The films were printed in the shape of straight strips in numbers from one up to six. These structures were heated in air at the temperatures of 60–160 ◦C at a temperature-controlled hotplate. The electrical resistance of the prepared films was studied by the standard 2-probe method (Keithley 2002 multimeter). Electrical measurements were carried out for different Materials 2021, 14, x FOR PEER REVIEW 3 of 12

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controlled hotplate. The electrical resistance of the prepared films was studied by the standard 2-probe method (Keithley 2002 multimeter). Electrical measurements were car- riednumbers out for ofdifferent the printed numbers passes of and the for printe the variousd passes processing and for the temperatures. various processing Optical charac-temperatures.terization Optical of the printedcharacterization materials of was the done printed by Optical materials microscope was done Levenhuk by Optical (Levenhuk, micro- scopeInc., Levenhuk Tampa, FL, (Levenhuk, USA) after Inc., the inkTampa, reduction FL, USA) to Cu. after the ink reduction to Cu. TheThe quantum quantum chemical chemical calculation calculation was was carried carried out byby thethe DFTDFT (density (density functional functional the- theory)ory) method. method. This This method method is is widely widely used used to to calculate calculate the the electronic electronic structure structure of of molecules mole- culesand and condensed condensed matter matter [26 [26–28].–28]. The The B3LYPB3LYP approximation is is based based on on a ahybrid hybrid func- func- tional,tional, in inwhich which the the exchange exchange energy energy is iscalcul calculatedated using using the the exact exact result result obtained obtained by by the the Hartree–FockHartree–Fock method. method. All All calculations calculations were were performed performed using using the the Gaussian Gaussian 09W 09W program program (Gaussian,(Gaussian, Inc., Inc., Wallingford, Wallingford, CT, CT, USA). USA). The The influence influence of ofthe the solvent solvent was was not not taken taken into into account.account. The The initial initial structures structures of ofthe the comple complexesxes were were selected selected according according to tothe the symmetry symmetry criteria.criteria. At Atthe the first first stage, stage, the the geometry geometry was was optimized optimized for for each each initial initial configuration configuration at atthe the theoreticaltheoretical level level B3LYP/6-31G B3LYP/6-31G (d, (d, p), whichwhich doesdoes not not require require large large computational computational resources. resources.Further, Further, the stability the stability of each of complex each complex was confirmed was confirmed by the by absence the absence of imaginary of imaginary frequen- frequenciescies in the in calculated the calculated IR spectra. IR spectra. Further Further optimization optimization of the of geometry the geometry and calculation and calcu- of thermodynamic potentials for the selected complexes was carried out at the level of the lation of thermodynamic potentials for the selected complexes was carried out at the level B3LYP/6-311++ G (d, p) theory without limitation in symmetry. of the B3LYP/6-311++ G (d, p) theory without limitation in symmetry. 3. Results and Discussion 3. Results and Discussion 3.1. Copper-Inks Characterizations 3.1. Copper-Inks Characterizations Due to high sensitivity of copper to atmospheric oxygen, the temperature of copper formationDue to high from sensitivity its precursors of copper is of great to atmospheric importance. oxygen, Therefore, the thetemperature main requirement of copper for formationthe formation from its of precursors high-quality is copperof great layers importance. is a low Therefore, temperature, the main which requirement is also necessary for theto formation ensure the of deposition high-quality of functionalcopper layers layers is a on low heat-sensitive temperature, polymer which is substrates. also necessary to ensureTo the determine deposition thermal of functional characteristics, layers on we heat-sensitive studied thermogravimetric polymer substrates. dependence of synthesizedTo determine precursors thermal in characteristics, an argon flow withwe studied a heating thermogravimetric rate of 10 ◦C/min. dependence As can be seenof synthesizedfrom Figure precursors1, decomposition in an argon of copper flow with formate a heating to copper rate andof 10 gaseous °C/min. compounds As can be seen takes fromplace Figure at a temperature1, decomposition of 200 of◦ Ccopper according formate to the to copper reaction and equation: gaseous compounds takes place at a temperature of 200 °C according to the reaction equation: Cu(OOCH)2 → Cu + CO↑ + CO2↑ + H2O↑ (1) Cu(OOCH)2 → Сu + CO↑ + CO2↑ + H2O↑ (1)

Cu(OOCH)2 [Cu(NH3)2](OOCH)2 100 [Cu(C2H6NH)2](OOCH)2

60 Mass/% 50

20 40 60 80 100 120 140 160 180 200 220 240 Temp./°C

FigureFigure 1. Thermogravimetry 1. Thermogravimetry of copper of copper precursors. precursors. The addition of ammonia shifts the decomposition temperature by about 40 ◦C and The addition of ammonia shifts the decomposition temperature by about 40 °C and a greater shift of 120–140 ◦C can be observed during decomposition of dimethylamine a greater shift of 120–140 °C can be observed during decomposition of dimethylamine complex of copper formate, which is already acceptable for the formation of ink on heat- sensitive substrates.

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Raman spectra were studied to analyze the processes of chemical transformations of copper precursors. Temperature ranges were selected according to the results of ther- rises up to 150 °C, the nitrogen peaks are practically not detected, however, the oxygen mogravimetric analysis. Raman spectra of copper formate at temperatures of 25–200 ◦C content also begins to increase and reaches values of 30%. This can be a result of oxidation are presented in Figure2a. The spectra show characteristic intense bands of stretching of the produced copper. vibrations of the formates (1380 cm−1 and 826 cm−1)[29].

(a)

(b)

Figure 2. Cont.

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(c)

Figure 2. Raman spectra of the samples: (a) Cu(OOCH)2; (b) Cu(NH3)2](OOCH)2; (c) [Cu(C2H6NH)2](OOCH)2. Figure 2. Raman spectra of the samples: (a) Cu(OOCH)2;(b) Cu(NH3)2](OOCH)2;(c) [Cu(C2H6NH)2](OOCH)2.

Pure copper formate is stable up to 150 ◦C. The changes at this temperature are N Cu − N Cu 1 Cu Cu O observed in theС frequencyO range upCu toCu 500 cm , as wellСO as a decrease of the intensity at С Cu Cu Cu −1 −1 о 826 cm and broadening of the peak at150 1380оС cm . We assume, that this is due150 toС a change 150оС in the crystal structure and the nature of interactions between the atoms of the copper

formate molecules. A decrease in the intensity of all characteristic peaks is observed at 200 ◦C which indicates a decomposition processes of the sample. 130оС 130оС −1 130оС Both characteristic bands and bands of NH group vibrations (3100–3300 cm ) are

observed in the Raman spectrum of the ammonia complex of copper formate (Figure2b).

Comparison of the spectra taken at temperatures of 25 and 100 ◦C show a decrease in

the intensity of the background curve. Thisо is due to the evaporation of unboundо water 110оС 110 С 110 С

Intensity and an increase of the crystallinity of the complex compound. The growth of the powder

◦ temperature up to 150 C leads to an increase in the intensity of the symmetric C-O and

a decrease in the intensity of symmetric O-C-O and N-H band vibrations. This effect can о о 25beоС associated with the rearrangement25 ofС the formate anion and appearance25 ofС volatile ammonium formate salts. An increase in temperature up to 180 ◦C leads to an intensive de-

composition of the complex. This is reﬂected in a decrease in the intensity of all absorption bands of the sample. 0 2000 4000 6000 8000 10000 0 2000 4000 6000 8000 10000Figure2c shows the Raman spectrum of the complex0 with 2000 dimethylamine. 4000 6000 8000 The 10000 assign- Energies (eV) Energies (eV) Energies (eV) ment of vibrations of dimethylamine molecules was carried out according to literature [30]. (a) The spectra of the complex( containb) bands of symmetric and asymmetric(c) stretching, bending −1 vibrations of CH2, CH3, and N-H groups. The characteristic bands at 900 cm can be Figure 3. Elemental EDX analysisattributed of the tosamples CN vibrations. after calcination The absorption to 150 °C: (a bands) Cu(OOCH) of the2 CO; (b)carboxyl [Cu(NH3) group2](OOCH) at 13802; (c) cm−1, [Cu(C2H6NH)2](OOCH)2. O-C-O at 816 cm−1 bonds are present. Heating the sample to 70 ◦C leads to a decrease in the intensity of the broad absorption bands, which are associated with the evaporation of Thus, the results of EDX analysis correlate well with the results of Raman and DTA unbound water and growth of the complex crystallinity. A further increase of the temper- data and allows us to provide assumptions about the mechanism of copper reduction. A ature up to 100 ◦C did not change the Raman spectra. However, the release of gaseous noticeable decrease in the copper reduction temperature depends on the composition of the complexing agent, due to the influence of nitrogen on the copper reduction process,

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reaction products is observed when the temperature was above 110 ◦C. It leads to the disappearance of the bands of that belong to CN and NH bonds vibrations in the spectra, as well as a decrease in the bands of CH2, CH3 groups vibrations. Vibrational Raman bands disappear at a temperature of 130 ◦C. Raman spectra show copper reduction temperatures depending on the complexing agent. The data conﬁrm the results of thermogravimetric analysis. However, Raman spectroscopy does not provide information on the content of metallic copper formed as a result of sintering. Therefore, additional studies were carried out by elemental EDX analysis of samples sintered to a 150 ◦C (Figure3) accompanied by SEM imaging of the analyzed areas (Table S1). This temperature is considered to be critical for most polymer substrates. Figure3 shows a relative decrease in the oxygen peak with an increase of the sintering temperature for all three samples. This dependence can be explained by the removal of adsorbed water up to 110–130 ◦C and by the decomposition of copper formate at higher temperatures. Another indicator of the processes in complex compounds is the change in the nitrogen content in the composition of the samples. Nitrogen is retained after sintering at 150 ◦C in an amount of 20% in the ammonia complex of copper formate. This is slightly higher than its stoichiometric content in the ammonia complex of the formic acid salt. This indicates the beginning of the decomposition process of the complex with the release of metallic copper. In the complex of copper formate with dimethylamine (Figure3c ), this process is observed at a temperature of 130 ◦C. In this case, the nitrogen content does not exceed 5%, and the copper content is most evident and reaches 70%. ◦ As the temperature rises up(c) to 150 C, the nitrogen peaks are practically not detected, however, the oxygen content also begins to increase and reaches values of 30%. This can be Figure 2. Raman spectraa result of the of samples: oxidation (a) Cu(OOCH) of the produced2; (b) Cu(NH copper.3)2](OOCH)2; (c) [Cu(C2H6NH)2](OOCH)2.

N Cu N Cu Cu Cu O СO Cu Cu СO С Cu Cu Cu о 150оС 150 С 150оС

130оС 130оС 130оС

о о 110оС 110 С 110 С Intensity

о о 25оС 25 С 25 С

0 2000 4000 6000 8000 10000 0 2000 4000 6000 8000 10000 0 2000 4000 6000 8000 10000 Energies (eV) Energies (eV) Energies (eV) (a) (b) (c)

Figure 3. Elemental EDX analysis of the samples after calcination to 150 °C:◦ (a) Cu(OOCH)2; (b) [Cu(NH3)2](OOCH)2; (c) Figure 3. Elemental EDX analysis of the samples after calcination to 150 C: (a) Cu(OOCH)2;(b) [Cu(NH3)2](OOCH)2;(c) [Cu(C2H6NH)2](OOCH)2. [Cu(C2H6NH)2](OOCH)2.

Thus,Thus, the results of of EDX EDX analysis analysis correlate correlate well well with with the the results results of Raman of Raman and and DTA DTA datadata andand allowsallows us to provide provide assumptions assumptions about about the the mechanism mechanism of ofcopper copper reduction. reduction. A A noticeablenoticeable decrease in in the the copper copper reduction reduction temperature temperature depends depends on onthe the composition composition of of thethe complexingcomplexing agent agent,, due due to to the the influence inﬂuence of of nitrogen nitrogen on on the the copper copper reduction reduction process, process, which acts as a bridge for the transfer of an electron from hydrogen of a formate anion to copper, showing, in fact, a catalytic reaction mechanism. To conﬁrm this mechanism, we

calculated Mulliken atomic charges and thermodynamic potentials of reactions by the DFT Materials 2021, 14, x FOR PEER REVIEW 7 of 12

Materials 2021, 14, 2218 7 of 12 which acts as a bridge for the transfer of an electron from hydrogen of a formate anion to copper, showing, in fact, a catalytic reaction mechanism. To confirm this mechanism, we calculated Mulliken atomic charges and thermodynamic potentials of reactions by the method with the B3LYP/6-311++G(d, p) functional. Figure4 shows the calculated atomic DFT method with the B3LYP/6-311++G(d, p) functional. Figure 4 shows the calculated charges in the Cu(OOCH)2 (A), [Cu(NH3)2](OOCH)2 (B) and [Cu(C2H6NH)2](OOCH)2 (C) atomic charges in the Cu(OOCH)2 (A), [Cu(NH3)2](OOCH)2 (B) and compounds. [Cu(C2H6NH)2](OOCH)2 (C) compounds.

(a) (b) (c)

Figure 4. Calculated thermodynamic potentials to estimate the probability of various concurrent reactions for Cu(OOCH)2 Figure 4. Calculated thermodynamic potentials to estimate the probability of various concurrent reactions for Cu(OOCH)2 (a), [Cu(NH3)2](OOCH)2 (b) and [Cu(C2H6NH)2](OOCH)2 (c) compounds. (a), [Cu(NH3)2](OOCH)2 (b) and [Cu(C2H6NH)2](OOCH)2 (c) compounds.

As cancan bebe seen seen from from the the Table Table1, the 1, reactionsthe reactions of the of formation the formation of complex of complex compounds com- ofpounds a copper of cationa copper with cation ammonia with and ammonia dimethylamine and dimethylamine molecules compete molecules with compete side reactions with ofside the reactions formation of the of Cu(OH)formation2, whichof Cu(OH) the oxide2, which CuO the producesoxide CuO upon produces heating, upon and heating, when aand dimethylamine when a dimethylamine molecule molecule is used as is aused ligand, as a theligand, probability the probability of primary of primary reaction reac- (1) increases.tion (1) increases.

Table 1.1. ThermodynamicThermodynamic potentialspotentials ofof concurrentconcurrent reactions.reactions.

◦ ◦ ConcurrentConcurrent Reactions Reactions ∆rH∆r,HH°, H ∆∆rGrG°,,H H Cu2+ 2+ + 2NH3 + 2H2O = Cu(OH)2 + 2NH+ 4+ −0.547 −0.516 Cu + 2NH3 + 2H2O = Cu(OH)2 + 2NH4 −0.547 −0.516 2+ + Cu2+ + 2(CH3)2NH + 2H2O = Cu(OH)2 + 2(CH3)2NH+ 2 −0.604 −0.574 Cu + 2(CH3)2NH + 2H2O = Cu(OH)2 + 2(CH3)2NH2 −0.604 −0.574 2+ 3 2 2 2 3 2 2+ Cu2+ + 2NH + 2H O = [Cu(H O) (NH 2+) ] −0.537 −0.483 Cu + 2NH3 + 2H2O = [Cu(H2O)2(NH3)2] −0.537 −0.483 Cu2+ + 2(CH3)2NH + 2H2O = [Cu(H2O)2((CH3)2NH)2]2+ −0.565 −0.503 2+ 2+ Cu + 2(CH3)2NH + 2H2O = [Cu(H2O)2((CH3)2NH)2] −0.565 −0.503 Cu2+ + 2NH3 + 2NH4+ + 2OH− = Cu(NH3)42+ + 2H2O −1.168 −1.116 2+ + − 2+ − − Cu2+ + 2(CHCu +3)2 2NHNH3 ++ 2(CH 2NH43)2NH+ 2OH2+ + 2OH= Cu(NH− = Cu((CH3)4 +3 2H)2NH)2O 42+ + 2H2O 1.168−1.245 −1.1161.179 Cu2+ + 2(CH ) NH + 2(CH ) NH + + 2OH− = Cu((CH ) NH) 2+ + 2H O −1.245 −1.179 Thus, due3 2 to the lowest3 2 decomposition2 temperature,3 2 4 the dimethylamine2 complex of copper formate is the most preferable precursor for the formation of copper layers; therefore, Thus,this compound due to the was lowest chosen decomposition as the object of temperature, our further theresearch. dimethylamine complex of copperTo study formate the structure is the most and preferablecomposition precursor of degradation for the products formation of ofthe copper copper layers; dime- therefore,thylamine this complex, compound we obtained was chosen X-ray as diffraction the object ofpatterns our further of the research. complexes processed at 120, 140,To study and 160 the structure°C for 10 andmin composition and they are of shown degradation in Figure products 5. of the copper dimethylamine complex, we obtained X-ray diffraction patterns of the complexes processed at 120, 140, and 160 ◦C for 10 min and they are shown in Figure5.

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o ° 630 160 C ○ – Cu ● – CuO

420 – Cu2O

• ° 210 • ♦ ° • • • • • • ♦ 0

o 450 140 C

300

Intensity 150

330 120oC

220

110

20 25 30 35 40 45 50 55 60 65 70 75 80 Angle (2θ)

Figure 5. Diffraction patterns of degradation products of the copper dimethylamine complex at Figure 5. Diffraction patterns of degradation products of the copper dimethylamine complex at variousvarious temperatures temperatures of of processing. processing. As can be seen from the diffraction patterns, an increase in the temperature of process- As can be seen from the diffraction patterns, an increase in the temperature of pro- ing of the copper complex leads to a noticeable increase in the proportion of by-products in cessing of the copper complex leads to a noticeable increase in the proportion of by-prod- the form of copper oxides, which strongly hinder mutual diffusion of Cu atoms between ucts in the form of copper oxides, which strongly hinder mutual diffusion of Cu atoms the particles and reduce the formation of percolation paths, and it leads to low conductivity between the particles and reduce the formation of percolation paths, and it leads to low of printed patterns. At a temperature of 120 ◦C, copper peaks appear clearly. Oxide peaks conductivity of printed patterns. At a temperature of 120 °C, copper peaks appear clearly. are hardly observed, however, there appears to be a noisier character of the background Oxide peaks are hardly observed, however, there appears to be a noisier character of the curve, which is probably due to the content of residual copper complex in the form of background curve, which is probably due to the content of residual copper complex in the amorphous inclusions. Thus, a temperature of 120–130 ◦C can be considered optimal for post-processingform of amorphous of the inclusions. ink based onThus, the a dimethylamine temperature of complex. 120–130 °C can be considered op- timalThe for ink post-processing was prepared of by the dissolving ink based dimethylamine on the dimethylamine complex incomplex. dimethyl sulfoxide at a concentrationThe ink was of 300prepared mg/mL. by dissolving dimethylamine complex in dimethyl sulfoxide at a Theconcentration choice of theof 300 solvent mg/mL. is conditioned by a number of its advantages, including inactivityThe inchoice relation of the to thesolvent ink components,is conditioned the by optimal a number boiling of its point advantages, (190 ◦C) ensuringincluding stabilityinactivity preventing in relation premature to the ink inkcomponents drying, and, the viscosityoptimal boiling of 2 mPa point·cm (190 (millipascal °C) ensuring per centimeter)stability preventing being optimal premature for inkjet ink drying, technology. and viscosity In addition, of 2 mPa·cm we investigated (millipascal the degreeper cen- oftimeter) wetting being of glass optimal and plastic for inkjet surfaces technology. (Figure6 In), whichaddition, is crucial we investigated in the printing the degree process, of ensuringwetting of continuity glass and of plastic the pattern surfaces when (Figure printing 6), which small parts,is crucial as well in the as printing adhesion process, of the formedensuring material continuity to the of substrate. the pattern when printing small parts, as well as adhesion of the formed material to the substrate.

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Name Water DМSО

GLASS

PLASTIC

Figure 6. (Left): solution of complex compound in dimethyl sulfoxide. (Right): microphotographs of drops of water and Figure 6. (LeftDMSO): solution on glass and of plastic complex substrates. compound in dimethyl sulfoxide. (Right): microphotographs of drops of water and DMSO on glass and plastic substrates. The measurements show that dimethyl sulfoxide exhibits the glass surface wetting Theability measurements comparable to that show of water, that due dimethyl to a certain sulfoxide polarity of exhibits the molecules the glassin its com- surface wetting abilityposition, comparable while the to plastic that ofsurface water, wetting due abil to aity certain is noticeably polarity different—the of the molecules wetting an- in its compo- gle of DMSO on plastic is noticeably smaller than that of water. This effect is conditioned sition,by while the affinity the plasticof the solvent surface and wetting the substrate. ability is noticeably different—the wetting angle of DMSO on plastic is noticeably smaller than that of water. This effect is conditioned by the affinity3.2. ofRealization the solvent of Copper and Films the on substrate. Flexible Substrates The ink was printed using an EPSON 110 inkjet printer. Before use, the ink was pre- 3.2. Realizationfiltered with of a Coppersyringe filter Films with on a Flexible pore size Substrates of 0.5 μm. Printing was carried out on the surface of a flexible polymer film, and it became possible at a rather low post-processing Thetemperature ink was of the printed printed using ink. Figure an EPSON 7a shows 110 a microphotograph inkjet printer. of Beforethe surface use, of the ink was pre-filteredformed withcopper, a which syringe is characterized filter with aby pore a sufficiently size of 0.5highµ density.m. Printing On the was right carried there out on the surface(Figure of a 7b) flexible are conductive polymer paths film, 200 andμm wide. it became possible at a rather low post-processing temperatureThe scanning of the printedelectron microscope ink. Figure (SEM)7a im showsages (Figure a microphotograph 7c,d) show the microstruc- of the surface of ture of copper particles formed as a result of thermal decomposition of copper ink. The formed copper, which is characterized by a sufficiently high density. On the right there Materials 2021, 14, x FOR PEER REVIEWsizes of particles vary from 100 to 1000 nm. There is a porosity of the formed structures,10 of 12 (Figuredue7 b)to arethe gaseous conductive reaction paths products 200 releasedµm wide. during the chemical transformation of the ink components. At the same time, the structure is quite dense, and the particles are presented in the form of agglomerates that fused together and provide high electrical conductivity of the printed layers, which is a good indicator for the films annealed at 120 °C.

Figure 7. Microphotograph of the surface of the copper layer (a) and conductive paths on the surface of the ﬂexible polymer Figure 7. Microphotograph of the surface of the copper layer (a) and conductive paths on the surface of the flexible poly- substrate (b) and SEM images heated at 120 ◦C copper layers (c,d). mer substrate (b) and SEM images heated at 120 °C copper layers (c,d).

The main functional characteristic of copper-based printed layers is conductivity, and its values are presented in Figure 8. The left curve shows the dependence of conductivity on the number of printing passes of the inkjet printer and, as one can see from this figure, the first two passes cause a noticeable decrease in the film resistance, due to an increase in continuity of the copper layer on the substrate surface. An increase in the number of printing passes increases the density and thickness of the layers formed. The right curve shows conductivity versus post-processing temperature of the printed ink. At low temperatures (60–70 °C), resistance is high—about 2–3 powers of Ohm per square. At these temperatures, ink can be found in the form of complex compounds and exhibit predominantly ionic conductivity. As the temperature increases, there is a noticeable decrease in resistance associated with the appearance of electronic conductivity of the formed metal. However, an increase in temperature above 130–140 °C is accompanied by an increase in resistance of the formed layers, due to an increase in the proportion of by-products in the form of copper oxides, which greatly reduce the number of percolation paths, and it leads to low conductivity of the printed patterns.

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The scanning electron microscope (SEM) images (Figure7c,d) show the microstructure of copper particles formed as a result of thermal decomposition of copper ink. The sizes of particles vary from 100 to 1000 nm. There is a porosity of the formed structures, due to the gaseous reaction products released during the chemical transformation of the ink components. At the same time, the structure is quite dense, and the particles are presented in the form of agglomerates that fused together and provide high electrical conductivity of the printed layers, which is a good indicator for the films annealed at 120 ◦C. The main functional characteristic of copper-based printed layers is conductivity, and its values are presented in Figure8. The left curve shows the dependence of conductivity on the number of printing passes of the inkjet printer and, as one can see from this figure, Materials 2021, 14, x FOR PEER REVIEWthe first two passes cause a noticeable decrease in the film resistance, due to an increase11 of 12

in continuity of the copper layer on the substrate surface. An increase in the number of printing passes increases the density and thickness of the layers formed.

(a) (b)

FigureFigure 8. 8. ConductivityConductivity versus versus the the number number of of printing printing passes passes ( aa)) and and temperature temperature of proces processingsing of the printed ink (b).

4. ConclusionsThe right curve shows conductivity versus post-processing temperature of the printed ◦ ink.In At the low course temperatures of the research, (60–70 weC), obtained resistance the is ink high—about based on copper-containing 2–3 powers of Ohm organ- per ometallicsquare. Atcompounds these temperatures, for the formation ink can of beconduc foundtive in elements the form of of circuits complex by ink-jet compounds printing.and The exhibit process predominantly of copper reduction ionic conductivity. from the organometallic As the temperature complex increases, and the growth there is of a noticeable decrease in resistance associated with the appearance of electronic conductivity copper grains begins at temperatures above 110 °C and finishes completely◦ at 130 °C, whichof the formedis acceptable metal. for However, the formation an increase of electr in temperatureically conductive above 130–140 paths onC most is accompanied of flexible polymericby an increase materials. in resistance As an example, of the formed thin conductive layers, due copper to an increasepaths were in theformed proportion on a PET of plasticby-products film using in the a formstandard of copper EPSON oxides, 110 inkjet which printer. greatly reduce the number of percolation paths, and it leads to low conductivity of the printed patterns. Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Table S1. 4. Conclusions Images of scanning electron microscopy for the samples tested using the EDX analysis. In the course of the research, we obtained the ink based on copper-containing Author Contributions: Conceptualization, N.S.S. and S.I.S.; methodology, A.M.A., A.B.I., and organometallic compounds for the formation of conductive elements of circuits by ink-jet G.A.A.; validation, N.S.S., A.B.I. and K.S.R.; formal analysis, A.M.A., D.S.S. and E.K.M.; investiga- printing. The process of copper reduction from the organometallic complex and the growth tion, A.M.A. and K.S.R.; data curation, S.I.S. and N.S.S.; writing—original◦ draft preparation, N.S.S.◦ andof copper A.B.I.; writing—review grains begins at & temperaturesediting, D.S.S.; abovevisualization, 110 C K.S.R., and finishes D.S.S., E.K.M., completely and G.A.A.; at 130 C, supervision,which is acceptable N.S.S., S.I.S., for and the A.B.I.; formation resources, of electrically G.A.A. All conductiveauthors have paths read and on mostagreed of to flexible the publishedpolymeric version materials. of the As manuscript. an example, thin conductive copper paths were formed on a PET plastic film using a standard EPSON 110 inkjet printer. Funding: The research described in this paper was financially supported by Grant Agency of Czech Republic under project No. 19-17457S. A part of the work was carried out with the support of CEITEC Nano Research Infrastructure [grant ID LM2015041, MEYS CR, 2016–2019]. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data are available from the authors upon reasonable request. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Materials 2021, 14, 2218 11 of 12

Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/ma14092218/s1, Table S1. Images of scanning electron microscopy for the samples tested using the EDX analysis. Author Contributions: Conceptualization, N.S.S. and S.I.S.; methodology, A.M.A., A.B.I., and G.A.A.; validation, N.S.S., A.B.I. and K.S.R.; formal analysis, A.M.A., D.S.S. and E.K.M.; investigation, A.M.A. and K.S.R.; data curation, S.I.S. and N.S.S.; writing—original draft preparation, N.S.S. and A.B.I.; writing—review & editing, D.S.S.; visualization, K.S.R., D.S.S., E.K.M., and G.A.A.; supervision, N.S.S., S.I.S., and A.B.I.; resources, G.A.A. All authors have read and agreed to the published version of the manuscript. Funding: The research described in this paper was financially supported by Grant Agency of Czech Republic under project No. 19-17457S. A part of the work was carried out with the support of CEITEC Nano Research Infrastructure [grant ID LM2015041, MEYS CR, 2016–2019]. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data are available from the authors upon reasonable request. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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