The Effect of Surface Functionalization of Graphene on the Electrical Conductivity of Epoxy-based Conductive Nanocomposites Behnam Meschi Amoli 1,2,3,4, Josh Trinidad 1,2,3,4, Norman Y. Zhou 1,3,5, Boxin Zhao 1,2,3,4 1 Waterloo Institute for Nanotechnology (WIN), 2 Institute for Polymer Research (IPR), 3 Center for Advanced Materials Joining, 4 Department of Chemical Engineering, 5 Department of Mechanical Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada Extended Abstract During the last two decades, considerable efforts have been made to explore new generations of interconnecting materials and printed lines to replace the traditionally used toxic lead-based solders in electronic industries [1,2]. Electrically conductive adhesive (ECA) which consists of conductive metallic particles and a polymeric matrix has attracted lots of attentions as one of the most promising alternative materials. The conventional ECAs are typically made of silver micro flakes and epoxy resin. The low electrical conductivity of these ECAs is their main drawback compared to traditional lead-based solders which hinders their wide applications in today s blooming electronic industries. Enormous research works have been conducted to enhance the electrical conductivity of the conventional ECAs [2,3]. The recent progress in polymer and nanotechnology has helped material scientists to precisely design nanomaterials with different morphologies and surface chemistry for their application in polymers to make advanced nanocomposite materials. Owing to this capability, incorporation of nano-sized conductive fillers with different natures, morphologies, and surface properties inside the conventional formulation of ECAs has drawn considerable attentions to overcome the common drawbacks of conventional ECAs. Introduction of conductive nanomaterials into the conventional ECAs can have positive effects on the electrical conductivity of ECAs if their size, morphology, surface chemistry and the ratio between nanofiller and silver flakes be carefully taken under consideration. In this presentation, we will present some of our recent progress on the development of a new generation of hybrid ECAs by introducing graphene into the conventional formulation of ECAs with a particular focus on comparing the effects of two different approaches of surface modifications of graphene. [4,5] Experimental Two approaches were implemented to stabilize graphene inside the epoxy composites. In the first approach, we decorated graphene surface with silver nanoparticles (Ag NPs), which were functionalized with a thiocarboxylic acid (mercaptopropionic acid). The Ag NP-decorated graphene was prepared as reported before by our group [4]. In the second approach, we used an ionic surfactant (sodium dodecyl sulfate) to stabilize graphene. 1.5 wt% of each type of these functionalized graphene were added to the conventional ECAs and their electrical resistivity were measured and compared to those of conventional ECAs and hybrid ECAs with non-modified graphene. Results and Discussion
Figure 1A and 1B show representative TEM images of graphene before and after decoration with Ag NP. The black arrow in Figure 1B indicates the graphene edge. As observed in this image, spherical Ag NPs with an average size of 9.1 nm ± 3.1 nm formed on the graphene surface. The inserted image shows a HRTEM image of the selected area of Figure 4B. The amorphous structure over the surface of the NPs (as indicated by red arrows) clearly shows the surface coverage of Ag NPs by MPA. Figure 1C displays a SEM image of the Ag NP-decorated graphene showing two layers of the decorated graphene beside one another [4]. To investigate the effectiveness of the Ag NP-decorated graphene as a co-filler inside the electrical network, we measured the electrical resistivity of a thin-film of silver flakes and the decorated graphene thin-film and compared to those of pure silver flakes and silver flakes with non-modified graphene thin-films at varied temperatures to examine the inter-filler interaction. The results are shown in Figure 2. The electrical resistivity of hybrid filler system with both the decorated graphene (Ag flakes & Gr-Ag NPs) and non-modified graphene (Ag flakes & Gr) were decreased over the pure silver flake films (Ag flakes) for all measured temperatures. Due to its 2-D structure and high aspect-ratio, graphene provides more surface area for electron transportation inside the network and displays improved electrical conductivity with the hybrid filler system. As can be observed in Figure 2, the electrical resistivity of all three film samples decreased as the temperature increased. However, the electrical conductivity improvement for hybrid filler film with the decorated graphene was more significant at higher temperatures than the other two because of the sintering of the Ag NPs in the system [4]. A B Graphene edge C Figure 1. A) a representative TEM image of non-modified graphene; B) a representative TEM image of the decorated graphene with MPA-functionalized Ag NP, the edge of graphene single layer are pointed by a black arrow, C) a representative SEM image of the decorated graphene; D) a HRTEM images of the selected area of Figure B, showing the presence of an amorphous structure (pointed by arrows) of MPA on Ag NPs surface. [4]
Bulk resistivity (Ω.cm) 7.00E-04 6.00E-04 5.00E-04 4.00E-04 3.00E-04 2.00E-04 1.00E-04 Ag flakes & Gr-Ag NPs Ag flakes Ag flakes & Gr 0.00E+00 0 25 50 75 100 125 150 175 200 225 250 Temperature ( C) Figure 2. The effect of temperature on the electrical conductivity of conductive fillers thin-films before their addition into epoxy. [4] Hybrid ECAs were fabricated by adding a small amount of the Ag NP-decorated graphene (1.5 wt%) to the conventional ECAs consisting of silver micron flakes and epoxy. The decorated graphene was incorporated into conventional ECAs at two different silver contents, i.e., before and after the percolation threshold (60 wt% and 80 wt%, respectively) and the final composites were cured at 150 C. Figure 3A shows the bulk resistivity of the samples cured at 150 C. As can be seen in this figure, the resistivity of the hybrid ECAs with both decorated and non-modified graphene are less than that of the conventional ECAs for the both filler concentrations. The bulk resistivity of the hybrid ECA with the decorated graphene at 60 wt% of total silver content is 7.6 10-4 Ω.cm, which shows 90 % reduction compared to the bulk resistivity of conventional ECA with the same filler content (1.1 10-2 Ω.cm). This resistivity is close to that of the conventional ECA with 80 wt% of silver flakes. Also, the resistivity of the hybrid ECA with the decorated graphene at 80 wt% showed 67 % reduction compared to that of the conventional ECA with an equivalent silver content. To shed further light on the effect of Ag NPs on the electrical properties of the hybrid ECAs, the samples with 80 wt% of silver flakes were cured at different temperatures. The bulk resistivity data, shown in Figure 3B, revealed a much more significant resistivity reduction for the hybrid ECAs with the decorated graphene than for conventional ECAs and hybrid ECAs with non-modified graphene as curing temperature increased. A low bulk resistivity of 4.6 10-5 Ω.cm was achieved for the hybrid ECAs with the decorated graphene cured at 220 C, which is comparable to that of a typical eutectic solder (2 10-5 Ω.cm) [5]. It demonstrates a great potential of this hybrid ECA as an alternative electrical interconnect material. These results suggest that the Ag NPs on the surface of graphene improve the electrical conductivity of conventional ECA at low curing temperatures, and their influence is more pronounced at higher curing temperatures. The presence of Ag NPs at low temperatures may increase the number of contact points in the filler system that in turn increase the contact resistance [6]. However, when the temperature increases they start to sinter to each other and to the flakes so as to provide direct metallurgical contacts between the silver flakes and graphene to form flake-np-graphene-np-flake conduction paths inside ECAs. This situation makes electron transportation more convenient through the electrical network and decreases the contact resistance among fillers.
Bulk resistivity (Ω.cm) 1.20E-02 1.00E-02 8.00E-03 6.00E-03 4.00E-03 A 1.10E-02 Conventional ECA Hybrid ECA with non-modified graphene Hybrid ECA with decorated graphene T = 150 C B 2.00E-03 2.40E-03 0.00E+00 7.60E-04 5.50E-04 4.00E-04 1.80E-04 60 80 Silver content (wt%) Figure 3. A) The comparison between the electrical conductivity of conventional and hybrid ECAs at different filler concentrations of 60 and 80 wt%, B) The effect of curing temperature on the electrical conductivity of ECAs [4] As the second approach for the stabilization of graphene, we used sodium dodecyl sulfate (SDS) to stabilize graphene inside the ECAs. The schematic of SDS modification of graphene is shown in Figure 4A. The mechanical energy (provided by bath or horn sonication) breaks the van der Waals interactions between graphene layers and at the same time, surfactant molecules are adsorbed onto the graphene layers surface resulting in the exfoliation of layers and preventing their re-stacking via steric repulsions. Figure 4B and 4C are representative TEM images of graphene before and after SDS modification, respectively. The dark feature of graphene before treatment shows that graphene layers are stacked together which prevent electrons to pass through the sample. However, after SDS modification, we see a bright and transparent graphene layer with a single distinguishable edge, confirming the successfulness of SDS modification approach in exfoliation of graphene. A Sonication B C Figure 4. A) SDS modification of graphene leads to exfoliation of graphene flakes, B) a representative TEM image of a non-modified small graphene, C) a representative TEM image of SDS-stabilized small graphene [5] SDS-modified graphene (1.5 wt%) with two different sizes were added into the conventional formulation of ECAs as a conductive co-filler and the bulk resistivity of hybrid ECAs containing SDS-
Resistivity (Ω cm) modified graphenes were measured and compared with that of conventional ECA, hybrid ECAs with nonmodified graphenes, and the hybrid ECA with Ag NP-decorated graphene. Figure 5 shows a comparison between the electrical resistivities of all these ECAs. Comparison between the electrical resistivity of hybrid ECAs with SDS-stabilized graphene with that of hybrid ECAs with non-modified graphenes confirms the effectiveness of SDS treatment on the electrical conductivity of improvement of ECAs. Figure 5 also shows that larger graphene are more effective to decrease the bulk resistivity of ECAs than smaller ones. As can be seen in Figure 5, the lowest electrical resistivity belongs to the hybrid ECA with SDS-modified large graphene (1.6 10-5 Ω.cm) which is even lower than that of lead-based solders [6]. The low bulk resistivity of this type of hybrid ECA is because of the bridging effect of the graphene layers between the silver flakes [7,8]. Figure 5 also shows that stabilization of graphene with SDS is noticeably more effective approach to harness the electrical properties of graphene for ECA application than decorating it with Ag NPs. 7.00E-04 6.00E-04 5.50E-04 5.00E-04 4.00E-04 3.00E-04 2.00E-04 1.80E-04 1.00E-04 7.00E-5 3.50E-5 3.00E-05 1.60E-05 0.00E+00 Conventional ECA Non-modified small Gr non-modified large Gr AG NP decorated Gr SDS-Stabilized Gr (small) SDS Stabilized Gr (large) Figure 5. A comparison between bulk resistivity of conventional ECAs, hybrid ECAs with non-modified small and large graphenes, hybrid ECAs with SDS-modified small and large graphenes, and hybrid ECA with Ag- NP-decorated graphene [4,5] References [1] S.P. Gumfekar, B. Meschi Amoli, A. Chen, B. Zhao, J. Polym. Sci. Part B Polym. Phys. 51 (2013) 1448. [2] B. Meschi Amoli, S. Gumfekar, A. Hu, Y.N. Zhou, B. Zhao, J. Mater. Chem. 22 (2012) 20048. [3] B. Meschi Amoli, E. Marzbanrad, A. Hu, Y.N. Zhou, B. Zhao, Macromol. Mater. Eng. 299 (2014) 739. [4] B. Meschi Amoli, J. Trinidad, A. Hu, Y.N. Zhou, B. Zhao, J. Mater. Sci. Mater. Electron. (2014).
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