Precursors for Atmospheric Plasma‐Enhanced Sintering: Low‐Temperature Inkjet Printing of Conductive Copper

Abstract Bidentate diamine and amino‐alcohol ligands have been used to form solid, water‐soluble, and air‐stable monomeric copper complexes of the type [Cu(NH2CH2CH(R)Y)2(NO3)2] (1, R=H, Y=NH2; 2, R=H, Y=OH; 3, R=Me, Y=OH). The complexes were characterized by elemental analysis, mass spectrometry, infrared spectroscopy, thermal gravimetric analysis, and single‐crystal X‐ray diffraction. Irrespective of their decomposition temperature, precursors 1–3 yield highly conductive copper features [1.5×10−6 Ω m (±5×10−7 Ω m)] upon atmospheric‐pressure plasma‐enhanced sintering.


Introduction
The market for printed electronics has seen steady growth over the last decade, particularly as lower processing temperatures have facilitated am ove to low-cost flexible materials. [1] High-quality and, therefore, low-electrical-resistance printed features that can replace electrical wiring in ar ange of devices, particularly the shunting lines in organic light-emitting diodes (OLEDs), interconnectsf or photovoltaics, and the antennae in radio-frequency identification (RFID) devices,a re in urgent need. Inkjet printing of metallicf eatures is fairly common place industrially;t ypicallyn anoparticle (NP)-based formulations are used. [2] These "inks" consist of metaln anoparticles dispersed in as olvent that also contains stabilizinga gents and other substancest oc ontrol the viscosity,c onsistency,a nd wettability of the ink. [3] Sophisticated conditions are often required for their deposition onto substrates, as well as high temperatures for sintering( > 150 8C), which raises the cost andl imits the range of suitable substrates. [4,5] Metal-organic decomposition (MOD) inks, as an alternative, provide potentialf or highere conomic feasibility [6] and more widespreadu se. Owing to its lower cost and tendency for electromigration, with the passage of highe lectron current densities, [7] copper is an attractive alternative to other highly conductive metals,f or example, silver.C opper MOD inks typically consist of ac opper precursor,t hat is, am etal ion, oxidized and bound to ligands, such that upon specific treatment (usually heating) [2,8] the metal centeri sr educed and the ligandsd ecompose cleanly,leaving conductive metal features. Complexation of copper(II) with different ligandsg ives rise to intricate decomposition properties, owing to the varied stabilization offered by differentc ovalent andn on-covalent interactions. Strategic variation of the organic ligands affords the ability to tune the precursor properties:the lower its decomposition temperature, the highert he potential for use in conductive inks that can be printed on low-cost substrates.
Copper(II) nitrate and copper(II) formate have received much attention as viable precursors in both the chemical vapor deposition( CVD) and inkjet printing of metallic copper,d espite the latter's propensity to evolve formic acida nd, therefore, degrade the ink when mixed with protic additives. [9] The current state-of-the-art copper(II) formate ink formulations consist of in situ copper(II) formate solutions with co-complexing agents, usually amines and their derivatives. Yabuki et al. reported the deposition of films of conductive coppert hrough the thermal decomposition of mixtures of copper(II) formate and n-octylamine under an itrogen (inert) atmosphere. [10] The film with the lowest resistivity (2 10 À5 W cm) waso btaineda fter heatingf or 60 min at 140 8C. [11] Inks have been reported with improved results when ar atio of different amines are used. [12] The best performing ink contained ab lend of amines in a2 0mol %d ibutylamine-80 mol %o ctylamine mixture. This formulation demonstrateds ignificant improvement( resistivity of 5 10 À6 W cm), although sintering at 140 8Cs till took 30 min. Ac opperf ilm of comparable resistivity to this was produced by Kim et al. with af ormulation of copper(II) formate and hexylamine. [13] Ther esistivity of the film measured 5 10 À6 W cm at as intering temperatureo f2 00 8Cf or 2min. This demonstrates the on-going compromise that must be made between curing temperature and time.
Farraj et al. [14] evaluated ar ange of amino-hydroxyl complexesa sc omplexing ligandsw ith copper(II) formate. Amino hydroxyl complexes provide several advantages over others, including an improved stability duringp rocessing and storage (up to 3months), ad ecomposition which occurs at sufficiently low temperatures and good solubility of their copper complexes in glycol ether solvents. Compatibility with glycol ether solvents is advantageous, as they are commonlyu sed as a liquid vehicle in inkjet printing, owing to their use in tuning viscosity of the ink. [14,15] It was found that use of 2-amino-2methyl-1-propanol (AMP) led to both ad ecrease in the decompositiontemperature and the achievement of the best conductivity for the copper metal produced. It was suggested by Shin et al. [15] that the presence of the tertiary carbon, af eature unique to AMP compared to the other complexes tested,c ontributes to the formation of smallv olatile byproducts. The hindered carbon, which is not present in other amino-hydroxyl ligands,i nhibits the formation of stable carbamate ions, thereby restricting polymerization reactions between carbon dioxide and other organic byproducts. [16] Shin et al. [15] conducted asimilar investigationu sing copper(II) formate and AMP whilst also including ac o-complexing agent,o ctylamine, to increasep hysical contact between copperp articles followings intering and, therefore, improvec onductivity of the films produced. It was shown that the addition of both the amine andt he amino-hydroxyl complexes, together,d id result in smaller,m ore densely packed particles;h owever, the sinteringt ime and temperature (i.e. 30 min, 300 8C) of the complexes reported are high. In most cases, thermal sintering has been used to form conductive copper; however,i ts hould be noted that using UV [17,18] or microwave [19] irradiation, an argon ion LASER beam, [20] lowpressure plasma, [21] or pulse electric current [22] has also previously been reported. In particular,Farraj et al. recently reported the use of an in situ copper(II) formatea nd AMP ink with low pressure plasma [23] to form conductive copper features. The plasma-enhanced sinteringp rocedure was operated in the absence of other external heatinga nd under 0.2 mbar vacuum. The MOD ink needed to be exposed to 160 Wp lasma for 8min to yield conductive copper( 7 10 À6 W cm).
Despitet hesei nvestigations into the types of "in situ" ink formulations, which have shown much promise in depositing conductive copper,n on otable attempts have been made to design,s ynthesize, and isolatec opper(II) complexes. Isolated copper(II) complexes that can be further formulated into precursor inks to circumnavigate the shelf-life issues associated with using reactive in situ mixtures. Indeed, the production of solid ands table MOD inks would considerably improvet he inkjet printing of conductive features by preventing excessive and detrimentalw etting of the substrates, including paper,a s well as early decomposition. [24] From the coordination of bidentate diamine and amino-alcohol ligandsw ithc opper(II) nitrate to produce isolated precursors, this work aims to contribute towardagreater understandingo ft he relationship between functionality of ligands and resultant ink performance, with the hope that this new knowledgew ill assist in the selection of more effective directions for the development of ink precursors for the low-temperature plasma-enhanced sintering of highly conductive metallic features. [25] These new isolated copperp recursors, which are solid, water-soluble, and stable under ambient conditions, have been investigated with the atmospheric-pressure plasma-assisted inkjet printing that was recently demonstrated as av ersatile room-temperature approach that can be upscaled for the immediate conversion of MOD inks (Scheme 1). [24]

Results and Discussion
While targeting the preparation of solid and stable copper MOD precursorsf or the plasma-assisted inkjet printing of copper, as olution of copper nitrate trihydrate andt wo equivalents of ethylenediamine in methanolw ere stirred overnight to form [Cu(NH 2 CH 2 CH 2 NH 2 ) 2 (NO 3 ) 2 )] (1)i ns olution. After the solution was filtered and left at À5 8Cf or 1week, bright violet crystals of as uitable quality for single-crystal X-ray diffraction (SCXRD) formed, whichc onfirmed the isolation of 1 consistent with the literature. [26] For the synthesis of the isostructural complexes 2 and 3,a solution of copper nitrate trihydrate wasm ixed with two equivalents of ethanolamine( 1)o ra mino-2-propanol (A2P) (2) in methanol (1)o rethanol ( 2), and refluxed overnight. After filtering the solutions, the filtrates were left at À5 8Cf or 7days, after which bright blue crystals of [Cu(NH 2 CH 2 CH 2 OH) 2 (NO 3 ) 2 )] (2)a nd [Cu(NH 2 CH 2 CH(Me)OH) 2 (NO 3 ) 2 )] (3)h ad formed. Isolation of complexes 2 and 3 was confirmed by using elemental analysis( EA), mass spectrometry (MS), and infrared spectroscopy (IR), and recrystallization afforded suitablec rystals for SCXRD.T he structure of these complexes consists of centrosymmetric units crystallizedi natriclinic P1 space group (   (Table 1). In 3,t hese are slightly longer[ 1.458(2) for O1-C1( 1); 1.452(2) for O2-C4 (2)],o wing to the presence of the methyl group on theser espectivec arbons. Examinationo ft he bond angles around the central copperi n3 reveal that the CuO 2 N 2 plane is slightly distorted from planarity;f or example, the O1-Cu1-O2 bond angle is 176.69(5)8 rather than 1808,a sw ould be expected for ap erfectly square planar base. As imilar observation can be made on the N1-Cu1-N2 bond angle.T he nitrate group with central N3 is closer to the copper than the second nitrate with central N4 (2.435 for Cu1-O3; 2.536 for Cu1-O6). As it is closer to the complex ion, O5 on the nitratec an form as tronger hydrogen bond with hydrogen atoms on N1, than O8 can with hydrogen atoms on N2. As ar esult,t he rest of the atoms in the centralc omplex are distorted in one direction, making the complex asymmetric. The asymmetry of 3 reduces its stability and, therefore, itsd ecomposition temperature, adding to its potentialf or use as ac opperp recursor.F urther crystallographic data for complexes 2 and 3 are provided in Ta ble 2.
Interestingly,a ttempts to use AMP as al igand to isolate an isostructural octahedrally coordinated complex of the type [Cu(NO 3 ) 2 (AMP) 2 ]t ou se as ap recursor were not successful;i nstead, only the less reactive compound [Cu(OCH 2 C(CH 3 ) 2 NH 2 ) 2 ·H 2 O] could be isolated, which has been previously reported. [27] When considering the suitability of these complexes as precursors to copperm etal, their structure is mosti mportant, as metal-ligand bond length and, therefore, strength is driving the decomposition temperature. Complexes 1-3 all maintain their copper-to-nitrate ligand bond and the diamine/amino-alcohol ligands are coordinated, exhibiting aJ ahn-Te ller distorted octahedral geometry;t he coordination length of the nitrate groups throught he oxygen atoms is slightly longer than that of the nitrogen atoms in the diamine bidentate ligands. The diamine molecules are in the gauche configuration; therefore, the CH 2 groups in the ligand are asymmetrical about the CuN 4 plane.O verall, the high copperw eight% of these complexes (1:2 0.65 %; 2:2 0.52 %; 3:1 8.81 %) suggests that they may be good copper precursors. Additionally,t he presence of the counter-ion nitrate groups suggestst hat the interactions with the amine groups may be weaker and, therefore, the decomposition temperatureo ft hese complexes should be low.

Thermal Studies
The thermogravimetric analysis( TGA) of complexes 1-3 provides useful information concerningt he decomposition temperaturea nd profile of each complex that can be used to predict the thermal sintering behavior of the inks (Figure 2). Complex 1 begins to lose mass at approximately 75 8Cf ollowed by three more stages, likely owing to the dissociation of the ligands;d ecompositiont o3 0% of starting mass is complete by 230 8C. The change from ad iamine ligand to amino-alcohol is evident when comparing the TGA data from complex 1 with complexes 2 and 3,w hich are very similart oe ach other.F or 2 and 3,t he onset of decompositionb egins at approximately 90 and 100 8C, respectively.I ne ach case, the decomposition proceeds in two steps, with the first decomposition stage yielding a30% loss of the starting mass and the second one completed at 450 8C(70 %) for 2 and 495 8Cf or 3 (80 %).

Printing and Plasma-Enhanced Sintering
As evidenced by TGA, the three complexes reported in this work decompose at fairly high temperatures,t hat is, 230 to 495 8C. Consequently,t he thermals intering of inks formulated from the studied complexes implies the use of temperature in the same range to yield metallicc opper.T hese temperatures preventthe use of numerouss ubstrates, for example, polymers and paper.A sr eported in severalw orks, the temperature limitation raised by the thermals intering approachc an be circumvented by using ap lasma to enhancet he sintering. [23,24] However,p lasma-enhanced sintering does not resolve all of the complications, such as wetting and early decomposition. Recent studies on the atmospheric-pressure plasma-assisted inkjet printing of highlyc onductive silver features on paper have notably highlighted the wetting and stabilityi ssues encountered when working on porous substrates at ambient conditions. [24] Thus, solid and air-stable MOD complexes,w hich can be made into aqueous inks, are highly desirable for simple room-temperature plasma-assisted inkjet printing of functional devices.C onveniently,t he three copper complexes reported above are all solid, water-soluble, and stable under the laboratory atmosphere,m aking them suitable candidates for inkjet printing and atmospheric plasma-enhanced sintering. Complexes 1-3 were made up to 0.14 m aqueous solutions to formulate inks 1-3,a nd weres ubsequently used to print tracks onto glass substrates. Following their deposition, the films of precursor inks were plasma-enhanced sintered to reduce the copper ions and yield ap ure metal film. An atmospheric-pres-  sure dielectric barrier discharge (AP-DBD) configuration operated at atmosphericp ressure and supplied with an argon-hydrogen mixture was selected, owing to its suitability for the treatment of large surfaces. [28] A9 7.5:2.5 argon-hydrogen plasma gas composition was selected to maximize the concentration of reactive hydrogen species [29] that favor the low-temperature reduction of MOD ink [30] and promote an on-thermald esorption of the ligands. [31] In contrastt oo ur previous work, where the plasma-assisted inkjet printing of silver required numerous fast and short inkjet printing/plasma-enhanced sintering cycles (ca. 2s)t op revent the decomposition of the silver MOD ink to silver oxide,t he present work involves only three cycles.T hese cycles consist of af ast inkjet printings tep followed by a plasma-enhanced sinterings tep that was set to 40 min to ensure the full conversion of the inks and to produce thin films with decent thicknesses (2-4 mm) for surface analysis. Conductive copper [1.5 10 À6 W m( AE 5 10 À7 W m)],w ith a resistivity 100 times highert han bulk copper( 1.68 10 À8 W m) was achieved for the three ink solutions and the same deposition procedure. X-ray photoelectrons pectroscopy (XPS) analysis of the copper coatings revealed ah igh relative weightc oncentration of Cu, that is, 79.3, 75.2, and 69.7 wt %f or conductive copperc oatings elaborated from inks 1 to 3,r espectively. It is interesting to note that, despite adecomposition temperature close to that of complex 3 (i.e. 2:4 50 8Ca nd 3:4 95 8C), complex 2 yields as ignificantly higher copperc ontent of the produced coating. Quite strikingly,t his copper content (75.2 wt %) is very close to that of the coating elaborated from complex 1,whichpossesses am uch lower decomposition temperature (i.e. 1:2 30 8Cv s. 2:4 50 8C), but shares as imilar initial copperc ontent in the complexes (1:2 0.65 %; 2:2 0.52 %; 3: 18.81 %). Thus, when designingn ew precursors for the atmospheric-pressure plasma-assisted inkjet printing of metallicc oatings, one may not exclusively target the lowest decomposition temperature, but theu se of smaller ligands that can desorb more readily.
For all of the preparedc oatings, and irrespective of the MOD ink, XPS analysis revealed two peaks at 951.78 and 931.98 eV,c orresponding to the 2p 1/2 and 2p 3/2 peaks of metallic Cu, respectively (Figure 3a). The positions of the Cu 2p spin-orbit components,s eparated by approximately 20 eV,i s consistentw ith the formation of metallic copper.X -ray diffraction (XRD) analysis (Figure 3b)o ft he copperd eposited from inks 1-3 confirms the formation of copper,w ith no evidence of impurities (copper oxidesoru nconverted precursor).
Scanning electron microscopy (SEM) images reveal the rather dense morphology of the copper coatings composed of particles ranging from 50 to 200 nm (Figure 4). It is during the sinteringp rocess that the metal particles form connections between themselves, creating ac ontinuous percolating network throughout the film. [32] The low resistance of the copper coatings coupled to their rather dense morphology compared to other MOD ink-based coatings underline the suitability of the synthesized complexesf or the proposed plasma approach. The use of plasma rather than the conventional thermalh eating methods highlights the potential future for targeted precursor design and synthesis, resulting in low-temperature processing.
Indeed,t he AP-DBD configuration employed in the present study produces non-thermal plasma that induces ar educed increase in the substrate temperature. Even for long processing times, the temperature did not exceed 70 8C. On the other hand, AP-DBD produces aw ide variety of reactive species, that is, electrons, ions, radicals, metastables, and photons, which all possessasignificant energy that can easily breakc hemicals bonds. [33,34] As an example, in AP-DBD, the mean electron  energy is in the range of 1-10 eV.I nt he present work, precursors 1-3 undergo reduction to metallic copperw hen they are exposed to an argon-hydrogen atmospheric plasma. This result can be partly attributedt ot he longera nd, therefore, weaker bond between the copper(II) centera nd its ligands. In addition, the molecules' asymmetry,w hichc auses dipole moments within the complex, induces stronger intermolecular interactions [35] that increaset he oligomerization of the complexes, resulting in ar educed volatility and improvingt heir suitability as precursors. [36]

Conclusions
Three solid, water-soluble, and air-stable copperc oordination complexes have been synthesized, characterized, and successfully investigated for the low-temperature deposition of metallic copperl ayers by atmospheric-pressure plasma-assisted inkjet printing. In particular, the use of isolated copperc oordination complexes,whose crystallographic data are reported for the first time, provided ac onvenienta pproach to preventt he early decomposition of the inks. More importantly,t he plasmaassisted inkjet printing of thesed esigned precursors 1-3 has been shown to be an effective alternative to thermal sintering. Irrespective of the decomposition temperature of complexes 1 to 3 (230 to 495 8C), it yields highly conductive copperf eatures at temperatures lower than 70 8C. This work could be further extendedi nt he future to deposit highly conductive copper features onto ar ange of different substrates, including paper and plastics, such as previously demonstrated for the plasmaassisted inkjet printingo fs ilver. [24] Experimental Section Crystallographic/refinement data for complexes 2 and 3 (CCDC1864245-1864246) can be found online. [37] Mass spectrometry was performed on aT hermo Finnigan MAT900 XP operating in electron impact (EI) and chemical ionization (CI) modes. Singlecrystal X-ray diffraction datasets were collected on aS uperNova (dual source) Atlas diffractometer,u sing either monochromated Cu K a radiation (l = 1.54184 )o rm onochromated Mo K a (l = 0.71073 ).

General Procedures
All reagents and solvents were procured from Sigma Aldrich and used as received with no further purification. All IR spectra were recorded by using aS himadzu FTIR-8200 spectrometer,o perating in the region of 4000-400 cm À1 .E Aw as carried out by using an elemental analyzer (CEÀ440) (Exeter Analytical Inc.). The X-ray singlecrystal diffraction (SCD) experiments were carried out by using an Atlas diffractometer.Asuitable crystal was selected and mounted on an ylon loop on aS uperNova, Dual, Cu/ Mo. The crystal was kept at 150 Kd uring data collection. Using Olex2, [38] the structure was solved with the Superflip [39] (2, 3)s tructure solution program, using Charge Flipping and refined with the olex2.refine [40] (2)o r ShelXL [41] (3)r efinement package with Gaussian-Newton or Least Squares minimization, respectively.T he instrument used for simultaneous thermal analysis was aN etzsch STA4 49C. All measurements were carried out with the precursor sample in an aluminum crucible. The data were recorded from room temperature (20 8C) to 600 8C. XRD patterns were recorded with aB ruker D8 Discover Xray diffractometer by using monochromatic Cu K a1 and Cu K a2 radiation of wavelengths 1.54056 and 1.54439 ,r espectively,e mitted in an intensity ratio of 2:1w ith av oltage of 40 kV and ac urrent of 40 mA. The diffraction pattern was taken over 2q = 10-66. SEM was performed with aP hilips XL30 FEG instrument operating in plan mode with an electron-beam accelerating energy of 30 kV and an instrument magnification of 50000 .X PS was performed with a Thermo Scientific K-Alpha XPS system by using monochromatic Al K a radiation at a1 486.6 eV X-ray source. CasaXPS software was used to analyze the data with binding energies referenced to an adventitious C1sp eak at 284.8 eV.

Synthesis of Bis(ethanolamine)copper(II) Nitrate(2)
Cu(NO 3 ) 2 .3H 2 O( 6g,2 4.8 mmol) was dissolved in MeOH (30 mL), and then ethanolamine (3.06 g, 89.9 mmol) dissolved in MeOH (45 mL) was added. Upon addition of the ethanolamine, the solution turned from pale blue to dark blue. The mixture was refluxed and stirred overnight. After 21 h, the mixture was filtered by using gravity to remove ab lue precipitate and some solvent was removed from the clear blue solution with ar otary evaporator before it was left at À5 8Ct oa llow crystals to grow.A fter 7days in the freezer,d ark blue crystals were found. Experimental analysis for [Cu(EA) 2

Synthesis of Bis(amino-2-propanol)copper (II) Nitrate(3)
Cu(NO 3 ) 2 .3H 2 O( 2.03 g, 8.28 mmol) was dissolved in ethanol (10 mL), and then amino-2-propanol (A2P) (1.24 g, 16.6 mmol) dissolved in ethanol (15 mL) was added. Upon addition the solution turned from clear pale blue to dark blue. The mixture was heated to reflux and stirred overnight. After 22 h, the solution was removed from heat, cooled, and filtered by using gravity,s eparating ab lue solute from ab lue precipitate. The solution was left at À5 8Ct oa llow crystals to grow.A fter 12 days in the freezer,d ark blue crystals suitable for X-ray analysis had formed. Experimental Complex 1 (0.2301 g, 0.7 mmol, 0.14 m)w as dissolved in deionized water (5 mL) and stirred for 20 min in air.T he ink, which was a violet/blue color,w as filtered through a2 00 nm syringe filter before use, despite there being no visible precipitate.

Formulation of Ink 2
Complex 2 (0.2401 g, 0.7 mmol, 0.14 m)w as dissolved in deionized water (5 mL) and stirred for 20 min in air.T he ink, which was royal blue, was filtered through a2 00 nm syringe filter before use, despite there being no visible precipitate.

Formulation of Ink 3
Complex 3 (0.2421 g, 0.7 mmol, 0.14 m)w as dissolved in deionized water (5 mL) and stirred for 20 min in air.T he ink, which was royal blue, was filtered through a2 00 nm syringe filter before use, despite there being no visible precipitate.

DepositionofConductive Copper
Inks 1-3 were all used to achieve coatings of copper in as emi-industrial AP-DBD reactor composed of two high-voltage electrodes covered by ad ielectric and am oving stage. Copper inks 1-3 were inkjet-printed onto glass substrates by using aM icroMist ultrasonic nozzle operating at 120 kHz and coupled to an air-shaping system from Sono-Tek. The inkjet-printing step was performed in the dynamic mode (100 mm s À1 )a nd approximately (1 mL min À1 ) 83 mLcm À2 were deposited per cycle (three cycles of print/plasma in total). After printing, the coated substrate was exposed to an atmospheric-pressure plasma discharge ignited in am ixture of argon and hydrogen (97.5:2.5), using a1 0000 Hz (2 Wcm À2 )s inusoidal signal generated by aC orona generator 7010R from SOFTAL electronic GmbH for 40 min. The discharge gap between the high-voltage electrodes and the glass substrate was 1mm.