Photonic Sintering of Copper through the Controlled Reduction of

Subscriber access provided by University of Otago Library

Article

Photonic Sintering of Copper through the Controlled Reduction of Printed CuO Nanocrystals Francesco Paglia, Doojin Vak, Joel van Embden, Anthony S. R. Chesman, Alessandro Martucci, Jacek Jaroslaw Jasieniak, and Enrico Della Gaspera ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08430 • Publication Date (Web): 27 Oct 2015 Downloaded from http://pubs.acs.org on October 29, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Photonic Sintering of Copper through the Controlled Reduction of Printed CuO Nanocrystals Francesco Paglia,†,‡ Doojin Vak,† Joel van Embden,♯ Anthony S. R. Chesman,† Alessandro Martucci,‡ Jacek J. Jasieniak*,§, Enrico Della Gaspera *,† †

‡

CSIRO Manufacturing, Bayview Avenue, Clayton, Victoria 3168, Australia Dipartimento di Ingegneria Industriale Via Marzolo 9, Universita' di Padova - 35131

PADOVA, Italy ♯

School of Applied Science, RMIT University, Melbourne, Victoria 3000, Australia

§

Department of Materials Science and Engineering, Monash University, Clayton, Victoria

3800, Australia.

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 19

ABSTRACT The ability to control chemical reactions using ultra-fast light exposure has the potential to dramatically advance materials and their processing towards device integration. In this study, we show how intense pulsed light (IPL) can be used to trigger and modulate the chemical transformation of printed copper oxide features into metallic copper. By varying the energy of the IPL, CuO films deposited from nanocrystal inks can be reduced to metallic Cu via a Cu2O intermediate using single light flashes of 2 millisecond duration. Moreover, the morphological transformation from isolated Cu nanoparticles to fully sintered Cu films can also be controlled by selecting the appropriate light intensity. The control over such transformations enables for the fabrication of sintered Cu electrodes that show excellent electrical and mechanical properties, good environmental stability, and applications in a variety of flexible devices.

KEYWORDS Photonic curing, Intense Pulsed Light, Flash annealing, printed electronics, flexible electronics.


Page 3 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Intense pulsed light (IPL) curing has emerged recently as a powerful technique that enables sintering of metallic nanocrystals (NCs) leading to the formation of conductive coatings at low processing temperatures.1-5 The process relies on the absorption of highly energetic light pulses by a nanocrystalline coating deposited on insulating substrates, such as glass, plastic or paper. The energy that is absorbed cannot be dissipated efficiently given the poor thermal conductivity of the insulating substrate. Therefore, localized heat is generated within the absorbing material, without major detrimental effects on the substrate, even those that are temperature-sensitive like plastic or paper.6 This phenomenon opens up significant opportunities towards the development of various printed flexible electronic devices using inexpensive functional inks. The effect of IPL exposure has been reported for various metals, such as Ag1-3 and Cu,4,5 and even for semiconductors.7,8 These studies have shown that light-induced structural transformation of nanomaterials readily occur, namely through grain growth and sintering, resulting in electrical properties approaching those of bulk materials. While these transformations must occur through appropriate intermediates, a detailed understanding of these processes in many systems is surprisingly still lacking, including the archetypal transformation of copper oxide to copper. In this study, we show that a high energy light pulse can be used to fully control the chemical transformation of copper (II) oxide (CuO) NCs to metallic Cu NCs by reduction via a copper (I) oxide (Cu2O) intermediate under standard laboratory conditions (air atmosphere, room temperature). We demonstrate that the IPL-induced reduction of CuO can provide sintered copper coatings that exhibit conductivities suitable for printed circuit implementation (greater than 25% bulk Cu conductivity), with excellent environmental and mechanical stability.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 19

The transformation of CuO induced by high energy pulsed light described here exemplifies the distinctive advantages photonic curing has over thermal annealing in promoting the controlled reduction of the oxide and full sintering of the metallic phase. This study lays the foundation of a research effort directed towards understanding how high energy light pulses can be used to trigger and modulate chemical reactions in related systems.

RESULTS AND DISCUSSION We deposited CuO strips on PET substrates using a commercial CuO nanocrystal ink and a custom-made slot-die coater, resulting in homogeneous and highly reproducible coatings (see Experimental Methods for details). We chose slot-die coating because it is a widely adopted industrial process which is designed to produce more uniform layers on a larger scale compared to other common deposition techniques. Ink-jet printing is another viable method that enables the fabrication of potentially any pattern design, as will be presented at the end of this manuscript; however, it is not ideal for the deposition of uniform layers suitable to assess intrinsic materials properties. Following exposure of the printed films to one single light pulse of different energies, which was controlled by varying the pulse duration and supplied voltage (Figure S1), we used x-ray diffraction (XRD) to investigate the crystalline phases as a function of the energy of the pulsed light. As expected, the diffraction peaks of the starting material could be assigned to CuO (ICDD No. 48-1548), with no additional crystalline phases detected (Figure 1a, Figure S2). The broadening of the peaks is consistent with the CuO crystalline domains being in the nanometer size regime. This phase was maintained following exposure to low light energies (< 800 J). However, by increasing the IPL energy the CuO NCs were converted to Cu2O, as signified by the appearance of a diffraction peak centered at ~36.5° (ICDD No. 05-0667). With a further increase in IPL energy, the oxide phases were fully converted to


Page 5 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


metallic Cu (ICDD No. 04-0836). By analyzing the evolution of the crystalline phases (Figure 1b), a concomitant increase in the proportion of Cu2O relative to CuO was observed with an increase in IPL energy to ~1000 J/pulse. When the IPL energy was increased further, Cu2O started to be reduced to metallic Cu, and the full transformation was achieved with ~1300 J/pulse. The reduction process can be attributed to a combination of photoreduction of copper oxides and the presence of proprietary reducing agents in the commercial ink.9

Figure 1. a) Evolution of XRD patterns for CuO films subjected to IPL exposure of different energies. The predicted peak positions for CuO (ICDD No. 48-1548, black), Cu2O (ICDD No. 05-0667, blue) and Cu (ICDD No. 04-0836, red) are reported at the bottom. b) Evolution of the proportion of the detected crystalline phases as evaluated from the XRD data. The error bars represent the signal to noise ratio and the lines are just guides for the eye.

The exposure of the Cu-based coatings to IPL also has a dramatic effect on their optical properties, as can be observed in Figure 2. The CuO NC ink and the as-deposited films are deep brown in color. This color is retained after IPL exposure with pulse energies lower than ~800 J because CuO is the only phase present. When CuO is reduced to Cu2O (light pulse of 970 J), the color of the samples changed to dark yellow. This change is ascribed to the shift in the absorption onset, arising from the difference between the band gaps of the two copper oxides (reported values for bulk crystals are ~1.4 eV for CuO and ~2.2 eV for Cu2O).10 Consequently, the transformation of CuO to Cu2O reduces the amount of light absorbed by the printed coating. By integrating the lamp emission spectrum averaged across the absorption spectrum of the two oxide films, we observed that these materials



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 19

individually absorb ~93% and ~73% of the initial pulse energy, respectively (see Figure S3). Therefore, the partially reduced Cu2O films are still able to absorb sufficient light from the pulses to promote the subsequent chemical transformation to Cu. An additional increase in IPL energy beyond ~1000 J/pulse caused the emergence of metallic Cu phases. Interestingly, while XRD provided evidence for only a simple single-step transition from Cu2O to Cu, UV-vis spectroscopy allowed us to determine that there were two discrete stages within this process (Figure 2a). First, isolated metallic Cu NCs are formed at pulse energies of 1100-1200 J. This was identified by the deep green color of the coating, which arises from the presence of a localized surface plasmon resonance (LSPR) peak that is typical of metallic nanoparticles, such as Au, Ag and Cu.11 The LSPR peak is centered at slightly longer wavelengths compared to the expected position for metallic Cu NCs. Such a shift can be ascribed to either (i) the close proximity of NCs in the dry deposit, causing the coupling of the plasmonic resonances,12,13 and/or (ii) the presence of an oxide shell, which provides a high refractive index environment.14,15 By further increasing the IPL energy to values >1300 J, metallic Cu NCs start to neck and sinter together to give bulk-like metallic coatings, which are very reflective and therefore show a transmittance below 10% in the visible range. Increasing the IPL energy beyond ~1500 J/pulse caused mechanical failure of the Cu layers due to unsustainable thermal stresses. This arises from a series of factors, which include differences in coefficient of thermal expansion between substrate and coating, abrupt removal of organic compounds present underneath the sintering film, and the vaporization of Cu (Figure S4). The light-induced chemical transformations of the printed coatings were also monitored using FTIR spectroscopy (Figure 2b). The spectra of the as-deposited, untreated films clearly show the CuO vibrational peaks at ~500-550 cm-1,16 and the presence of


Page 7 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


distinctive features arising from organic compounds. In detail, the broad band at 3000-3700 cm-1 is ascribed to hydroxyl groups present in residual solvent and adsorbed moisture,17 and peaks in the 2800-3000 cm-1 range are due to vibration of C–H bonds belonging to organic compounds.17,18 The peak at 1650 cm-1 can be assigned to –NH2 groups of either primary amines or amides,19 which are likely to be constituents of the reducing agents introduced into the ink formulation. Other minor peaks also ascribed to organic compounds are detected at ~1350 cm-1. Notably, strong vibrational peaks belonging to the substrate can be observed at ~800 cm-1 and 1070 cm-1 (with a shoulder at 1200 cm-1). These are consistent with Si–O–Si vibrations in SiO2,17,20 which is likely to be the material constituting the porous adhesion layer of the Novele™ PET substrate.4

Figure 2. UV-visible-NIR optical transmission spectra (a) and FTIR absorption spectra (b) of CuO coatings deposited on PET and exposed to one light pulse of different energies. The legend is valid for both panels. The inset in (b) shows a picture of the deposited strips (width = 5 mm) exposed to pulses of increasing energy from left to right.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 19

When the IPL energy was progressively increased, a decrease of the peak intensity ascribed to organics and hydroxyl groups was observed. This is consistent with the loss of residual solvents and surfactants. In addition, the CuO peak intensity decreased and a new peak appeared at ~600 cm-1, which is assigned to Cu–O vibrations in Cu2O,16 consistent with the results obtained from XRD. Interestingly, concurrent with the decrease in the intensity of the initial organic peaks, a new band emerged at ~2200 cm-1. This peak, although difficult to assign, may be due to the formation of intermediates (cyanates, azides) during the decomposition of the reducing agents initially present within the ink formulation. At higher pulse energies, when the conversion to Cu is complete, the films became highly reflective and no vibrational peaks could be detected in the FTIR spectrum. We analyzed the morphological differences in the deposited samples following IPL treatment at different energies using SEM (Figure 3 and Figure S5). The as-deposited films are porous and composed of polydisperse particles, with the morphology not changing after conversion of CuO to Cu2O (Figure 3c). However, the reduction of Cu2O to Cu caused a small transformation from highly polydisperse and faceted nanoparticles to more isotropic, spheroidal nanoparticles (Figure 3d). The biggest morphological change occurred upon sintering of copper NCs, which resulted in massive grain growth and full percolation of the metallic phase (Figure 3e). From the cross sectional images a marked thickness reduction can be observed after IPL (from ~600 nm for the as deposited films to ~200 nm of the fully sintered coatings), which stems from the reduced porosity of the sintered layers and from the removal of the organic compounds during the IPL treatment. It is worth noting that the sintered layers, even if fully percolated, show some residual porosity. In fact, the underlying porous adhesion layer of the substrate can be identified through small gaps in the copper film.


Page 9 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. SEM characterization in top view (a-e) and cross section (f,g) of the CuO coatings exposed to different energies of IPL. All scale bars are 500 nm. In the cross sectional images the substrate, CuO and Cu are color coded in green, brown and red, respectively.

Accompanying such morphological, optical and chemical transformation, is a variation of the electrical properties of the deposited films. The printed strips composed of CuO or Cu2O are electrically insulating (sheet resistance >100 MΩ/□), regardless of the energy of the light pulse. This stems from the inherent poor conductivity of copper oxides, the presence of organic ligands and the high porosity of the films. Even when the oxides are fully reduced to Cu, but no sintering is achieved (see Figure 3d), the samples remain nonconductive. However, a dramatic change occurs after sintering, with sheet resistance values of 420±45 mΩ/□ being achieved for films that are less than 200 nm thick (average 190±36 nm). This corresponds to a resistivity (conductivity) of 7.98±0.87 µΩ·cm (1.25±0.14·107 S/m), which is ~20% of the conductivity of bulk copper (5.96·107 S/m). For completeness we note that the best samples prepared in this study reached a sheet resistance (conductivity) of 340 mΩ/□ (1.55·107 S/m), corresponding to more than 25% of the value of bulk copper. Notably, no difference in electrical properties was observed for the samples sintered directly from CuO coatings using a single high energy pulse (1400 J), and for samples pre-reduced to Cu2O using a low energy (970 J) light pulse that was followed by a second high energy flash 9 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 19

(1400 J) to further reduce and fully sinter the copper layers (Figure S6). This suggests that both copper oxides can be used as starting materials for preparing conductive copper coatings through photonic curing because they are both capable of absorbing a significant portion of the visible light and they both undergo complete reduction to give sintered Cu strips with comparable electrical conductivity. We note that if the IPL energy is too high (>1500 J), vaporization of metallic copper and “blow-off” issues are observed (see Figure S4), resulting in a loss of structural integrity of the copper coating which become electrically insulating. The electrical properties of the printed Cu strips can be improved by depositing 2 layers of CuO NCs and performing the IPL treatment (one pulse at 1400 J, full conversion to sintered Cu) after deposition of each layer. In this way, not only can thicker films be prepared (Figure 4a,b), but also more robust coatings, in regards to their electrical and mechanical properties, can be achieved. It is important to highlight that while thick films can be easily deposited in one single deposition by changing the printing conditions (ink concentration, printing speed, etc.), exposing thick films to IPL results in unreacted CuO at the interface with the substrate and delamination problems due to excessive stresses being generated during the IPL treatment (Figure S7). These results highlight the importance of achieving a balanced combination between IPL energy, sample thickness and processing steps in order to achieve homogeneous, highly conductive copper coatings. The coatings deposited with this 2-layer approach show lower sheet resistances compared to the single films, which arise from their higher thicknesses. In detail, double layers have an average thickness of 385±27 nm and an average sheet resistance of 185±11 mΩ/□. This value is less than half compared to that of the single layers. The conductivity associated with such a sheet resistance is 1.40±0.08 ·107 S/m, which corresponds to 23% of that of bulk Cu. Champion samples prepared with dual layers had the same conductivity as champion samples prepared with a single layer (1.53·107 S/m), confirming that the intrinsic


Page 11 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


material properties do not change for our best samples. However, the standard deviation on the sheet resistance values is reduced from ~11% to ~6%, confirming the greater reliability of the samples prepared with the double layer approach (see Figure S8).

Figure 4. Comparison between samples prepared from 1 layer or from 2 layers of CuO NCs: SEM images of the surface and of the cross section (inset) of a (a) 1-layer and (b) 2-layer sample. Scale bars are 1 µm for the main panel and 200 nm for the insets. Variation in resistance during storage (c,e) and variation in resistance during bending cycles (d, f) for samples kept in nitrogen (c,d) or air (e,f) atmosphere.

Ultimately, such copper features will be used in real-world devices where chemical stability and durability are critical parameters. To this end, we compared the stability of 1and 2- layered Cu strips stored in air (~40% relative humidity) with those stored in a nitrogen-filled glove box (water content

Photonic Sintering of Copper through the Controlled Reduction of

Recommend Documents