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Low interfacial contact resistance of Al-graphene composites via interface engineering
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 215603 (http://iopscience.iop.org/0957-4484/26/21/215603) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 128.227.24.141 This content was downloaded on 12/06/2017 at 04:05 Please note that terms and conditions apply.
You may also be interested in: Uniform dispersion of graphene oxide in aluminum powder by direct electrostatic adsorption for fabrication of graphene/aluminum composites Zan Li, Genlian Fan, Zhanqiu Tan et al. Transport and thermoelectric properties of polyaniline/reduced graphene oxide nanocomposites Rakibul Islam, Roch Chan-Yu-King, Jean-François Brun et al. A facile one-step process for 3D N-doped noncovalent functionalization PS/rGO composites Weiqi Huang, Hua Wang, Zheng Su et al. Synergistic interactions between silver decorated graphene and carbon nanotubes yield flexible composites to attenuate electromagnetic radiation Shital Patangrao Pawar, Sachin Kumar, Shubham Jain et al. Elastomeric composites based on carbon nanomaterials Sherif Araby, Qingshi Meng, Liqun Zhang et al. Enhanced electrical conductivity and hardness of silver-nickel composites by silver-coated multi-walled carbon nanotubes Dongmok Lee, Jeonghyun Sim, Wonyoung Kim et al. Programmable high crystallinity carbon patterns Xuewen Wang, Hong Wang, Yang Gu et al. Processable 3-nm thick graphene platelets of high electrical conductivity and their epoxy composites Qingshi Meng, Jian Jin, Ruoyu Wang et al.
Nanotechnology Nanotechnology 26 (2015) 215603 (7pp)
doi:10.1088/0957-4484/26/21/215603
Low interfacial contact resistance of Algraphene composites via interface engineering Myung Gwan Hahm1, Jaewook Nam2, Minseok Choi3, Chi-Dong Park4, Byungjin Cho1, Sanada Kazunori5, Yoong Ahm Kim6, Dong Young Kim7, Morinobu Endo8, Dong-Ho Kim1, Robert Vajtai9, Pulickel M Ajayan9 and Sung Moo Song10 1
Department of Advanced Functional Thin Films, Surface Technology Division, Korea Institute of Materials Science, 797 Changwondaero, Sungsan-Gu, Changwon, Gyeongnam 642-831, Korea 2 School of Chemical Engineering, Sungkyunkwan University, 300 Cheongcheon-dong, Suwon,Gyeonggido, 440-746, Korea 3 Advanced Characterization and Analysis Group, Korea Institute of Materials Science, 797 Changwondaero, Sungsan-Gu, Changwon, Gyeongnam 642-831, Korea 4 Research Center for Exotic Nanocarbons, Shinshu University, Faculty of Engineering, 4-17-1 Wakasato, Nagano, 380-8553, Japan 5 Department of Mechanical Engineering, Kyoto University, Yoshidahonmachi, Sakyo Ward, Kyoto, 6068501, Japan 6 Polymer & Fiber System Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju, 500-757, Korea 7 Materials Research Division, R&D Center, LG Display Co. LTd., 245 Lg-ro, Wollong-Myeon, Paju-Si, Gyeonggi-Do, 413-779, Korea 8 Research Center for Exotic Nanocarbons, Shinshu University, Faculty of Engineering, 4-17-1 Wakasato, Nagano, 380-8553, Japan 9 Department of Materials Science and NanoEngineering, Rice University, 6100 Main Street, Houston, Texas, 77005, USA 10 Department of Mechanical Engineering, Shinshu University, Faculty of Engineering, 4-17-1 Wakasato, Nagano, 380-8553, Japan E-mail: [email protected], [email protected] and [email protected] Received 10 March 2015, revised 30 March 2015 Accepted for publication 8 April 2015 Published 6 May 2015 Abstract
Al-based composites incorporating multilayered graphene sheets were developed via a facile approach. The multilayered graphene sheets were fabricated from the expanded graphite via a simple mechanical exfoliation process. The facile extrusion molding process with Al powder and graphene sheets exfoliated from expended graphite afforded Al-based graphene composite rods. These composites showed enhanced thermal conductivity compared to the pristine Al rods. Moreover, the Al-based multilayered graphene sheet composites exhibited lower interfacial contact resistance between graphene-based electrodes than the pristine Al. With increasing degrees of dispersion, the number of exposed graphene sheets increases, thereby significantly decreasing the interfacial contact resistance between the composite and external graphite electrode. S Online supplementary data available from stacks.iop.org/nano/26/215603/mmedia Keywords: graphene, aluminum, contact resistance, exfoliation, composites (Some figures may appear in colour only in the online journal)
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© 2015 IOP Publishing Ltd Printed in the UK
M G Hahm et al
Nanotechnology 26 (2015) 215603
1. Introduction Graphene has been used in many applications, including electronics [1, 2], sensors [3, 4], membranes [5, 6], energy storage [7, 8], and nano-composites [9, 10], owing to its large number of extraordinary physical properties such as high carrier mobility at room temperature (∼10000 cm2V−1s−1), good thermal conductivity (∼5000 Wm−1K−1), large specific surface area (∼3000 m2g−1), and high Young’s modulus (∼1 TPa) attributed to an ideal atomic-layered two-dimensional (2D) system of graphene [11–14]. However, maintaining its intrinsic characteristics for practical applications is extremely difficult; a number of limitations in fundamental scientific and technological applications exist because of its high cost and inborn disorders. The realization of the approaching limits has inspired a worldwide effort to develop alternative approaches for graphene-based device technologies. Various approaches have been attempted for many years and are still actively developing [15–20]. One plausible way to harness exceptional properties of graphene is to mix it with other materials such as a matrix or filler. This composite approach in combination with various materials has been developed for diverse applications, and some of the composites yielded successful results [10, 21–27]. Notwithstanding such achievements, the approach still has some barricades to utilizing the composites for electronic devices. A considerable contact resistance between metals and graphene is well known to significantly limit the outstanding performance of the graphene channel on graphene-based field-effect transistors (FETs). Previously, several studies reported to decrease the contact resistance between graphitic structure and metals [28, 29]. The interface condition between graphite and metal electrodes is important for determining the resistance. In this study, we considered the contact resistance between graphene sheets and metal-based electrodes. Aluminum (Al) was selected as a metal owing to its electrical properties and cost effectiveness, as it can be a strong candidate for various practical applications [30, 31]. Therefore, the contact resistance is strongly determined by the graphene/Al interface condition at the contact junction. In the absence of any other junk material from cleaning or other fabrication procedures near the junction, there are two possibilities: graphene/native oxide of Al and graphene/Al interfaces. Clearly, the electron transport through these two types of interfaces can be considerably different from the corresponding interfacial resistances. Native oxide of Al would impart a larger barrier between Al and graphene sheets. As shown in figure 1, the formation of the α-Al2O3 layer yields an energy barrier of 4 ∼ 5 eV between Al and graphene and is significantly difficult to overcome from the viewpoint of the electron. Only quantum mechanical tunneling is allowed when the layer is thin enough. In addition, defects are incorporated into the oxide, and they can determine the electrical property. According to the literature, O vacancies, Al vacancies, and Al interstitials are considerable defects for the Fermi-level position in the band gap of the α-Al2O3 (the Al-rich condition is assumed
Figure 1. Band alignment between Al and graphene (Gr). α-Al2O3 is considered a native oxide on top of the Al electrode. The Fermi-level position with respect to the vacuum level for Al is taken from a previous work [33], and the valence-band offset between Gr and Al2O3 is also from a previous work [34]. Defect levels for α-Al2O3 are taken from a previous works [32, 35].
here) [32]. O interstitials have high formation energy; thus their formation is unlikely. This study indicates that O vacancies are most likely to form in the native oxide layer because the Fermi level in the Al/Al2O3/graphene sheets structure would be positioned near the Fermi level of Al metal. The role of the vacancies in the electrical property of the Al/Al2O3/graphene structure is investigated in terms of the formation energy and their positions of defect level with respect to the Al and graphene band position. The formation energy of the 2+ charged oxygen vacancy is represented by the following equation: E f (V0+2 ) = Etot (V0+2 ) − Etot (Al2 O3 ) − μ0 + 2E F where Etot (Al2 O3 ) is the total energy of a supercell containing +2 charged O vacancies; Etot (V0+2 ) is the total energy of the pristine Al2O3 supercell; μ0 is the O chemical potential; and EF is the Fermi level referenced to the valence band maximum (VBM) of Al2O3. The defect level (q/q′) in figure 1 is defined as the EF position below which the same defect is stable in charge state q and above which the same defect is stable in charge state q′. For example, the (+2/+1) level of the O vacancy is evaluated from the formation energies using the following equation: (+2 +1) = ⎡⎣ E f (V0+2 )@VBM − E f (V0+1 )@VBM ⎤⎦ (2 − 1), where E f (V0+2 )@VBM is the formation energy of the +2 charged O vacancy when EF is at the VBM and E f (V0+1 )@VBM is that of the +1 charged O vacancy at VBM [32]. From the predictions, we conclude that O vacancies may act as charge traps, which increase contact resistivity when native oxide forms in between Al and graphene sheets. When graphene and Al electrodes are prepared separately, the formation of native defects in the oxide layer on top of Al cannot be excluded when it is exposed to air. To 2
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Nanotechnology 26 (2015) 215603
Figure 2. Fabrication procedure of Al-based multilayered graphene composites. (a) SEM image of expanded graphite. The scale bar is 100 μm. (b) SEM image shows multilayered graphene formed from the expanded graphite. The scale bar is 3 μm. (c) SEM of micrometer-sized Al powder. (d) The scale bar is 100 μm. The mixture of expanded graphite and Al powder was ball-milled to mix two different materials. (e) Hot extruding the mixture. (f) Optical image shows the Al-based multilayered graphene composites.
it did enhance them, particularly the interfacial contact resistance with other carbon-based electrodes.
exploit the relatively small Fermi-level barrier between pristine Al and graphene, the intact Al at the interface needs to be preserved, and the junction contact needs to be formed below the outside metal surface. In this regard, a simple way to achieve this is to fabricate Al-based graphene composites. In this study, we report a facile approach to create Al-based multilayered graphene composites with significantly improved contact interface between the composites and graphene.
3. Results and discussion Figure 3 shows the multilayered graphene sheets fabricated from the expanded graphite via a simple pulverizing process. Clearly, the simple pulverizing process is very effective to produce multilayered graphene sheets. Moreover, the size and number of graphene sheet layers was easily controlled by the pulverization time at the same rpm. The size range of graphene sheets pulverized for 10 s is from 100 μm to 600 μm (figure 3(a)) and is larger than the size of the sheets (400 to 800 nm) pulverized for 1000 s (figure 3(c)). The high-magnification inset images clearly show the size difference caused by the two pulverization times. To evaluate the number of graphene sheet layers, transmission electron microscopy (TEM) was used. As shown in figures 3(b) and (d), the TEM images clearly show that the pulverization time controls the number of layers of graphene sheets as well. The shortest pulverizing process (10 s) resulted in multilayered graphene sheets (figure 3(b)), whereas the longest process (1000 s) afforded trilayered graphene sheets (figure 3(d)). From these observations, one can conclude that the mechanical energy transferred from the pulverization process not only disintegrated the sheets, decreasing their size, but also exfoliated them, decreasing the number of layers. To cross-check the effect of the pulverization time on the number of layers, Raman spectroscopy was performed. The Raman spectrum recorded for the expanded graphite shows a sharp G-band, as shown in figure 3(e), compared to that of the expanded graphite (figure 3(e): dark green and red). The disorder-induced D-band indicates the presence of disorder in sp2-hybridized
2. Methods The two key processes for incorporating multilayered graphene sheets in the pristine Al are (1) the production of multilayered graphene sheets and 2) the extrusion molding of Al-based multilayered graphene (Al-MG) composites, as shown in figure 2. First, multilayered graphene sheets were disunited from expanded graphite (figure 2(a)) using a pulverizer. Expanded graphite (1 g) was pulverized with 25,000 rpm for 10 and 1000 s to investigate the size effect of graphene in Al-MG composites. N-methylpyrrolidone (NMP)-based pulverized graphite solution was stirred and sonicated for 24 h. Then the resulting solution was centrifuged with 500 rpm for 45 minutes and vacuum filtered, resulting in multilayered graphene sheets as shown in figure 2(b). As shown in figure 2, micrometer-sized pristine Al powder was compounded with multilayered graphene sheets (1 wt%) by ball-mill (100 rpm) for 1 h. The Al-MG mixture powder was then pre-sintered under 300 MPa at 600 °C for 10 min. The composites were heated at 500 °C for 10 min and hot-extruded under 600 MPa. The filler, graphene, did not deteriorate the matrix materialʼs physical properties; however, 3
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Nanotechnology 26 (2015) 215603
Figure 3. SEM image shows multilayered graphene fabricated from expanded graphite with 10 s pulverizing time. The scale bar is 10 μm. The inset is a high-magnification SEM of the graphene sheet. The scale bar is 500 nm. (b) High-resolution TEM image of graphene sheet pulverized for 10 s. The scale bar is 5 nm. (c) SEM of multilayered graphene sheet pulverized from expanded graphite with 1000 s pulverizing time. The scale bar is 10 μm. The inset is a high-magnification SEM of the graphene sheet. The scale bar is 500 nm. (d) Highresolution TEM image shows the multilayered graphene sheet. The scale bar is 5 nm. (e) Raman spectra recorded from expanded graphite (blue), graphene sheet pulverized for 10 s (green), and graphene sheet pulverized for 1000 s. (f) The G′ spectra for three graphitic structures as a function of the number of layers.
in the matrix even with the same amount of filler, as shown in figures 4(c) and (d). Raman spectra of the pristine Al (blue), large-sized graphene sheets-Al composites (dark green), and the small-sized graphene sheets-Al composites (red) are shown in figure 4(e). Raman spectra of the composites clearly show the disorder-induced band (D-band), stretching of the carbon-carbon bond (G-band), and second-order process (G′ -band), and the presence of these peaks indicate that the composites have graphitic structures. We investigated the thermal and electrical properties of the Al-based graphene sheet composites. The thermal conductivities of the pristine Al and Al-based graphene composites were measured by the standard laser flash analysis method. The measured thermal conductivity of the pristine Al was 179 Wm−1K−1 (figure 5(a)). Al-based 10 s and 1000 s pulverized graphene sheets had higher thermal conductivities than that of the pristine Al, as shown in figure 5(a). Graphene is well known to have outstanding thermal conductivity; Balandin et al. reported that the thermal conductivity of single-layered graphene ranged between 4840 and 5300 Wm−1K−1 [11]. Notwithstanding, low graphene content in the composites functioned as a thermal conductivity enhancer for the Al matrix. The results clearly show that well-dispersed graphene sheets (1000 s pulverized graphene sheets) showed better thermal conductivity than the less dispersed ones (10 s pulverized graphene sheets). The Al-based graphene composites also showed slightly higher volume resistivity (4 ∼ 5 nΩm) than the pristine Al, as shown in figure 5(b); however, those values are comparable with the pristine Al. Graphene sheets inside the matrix may enhance the electrical properties of composites; however, there will be a trade-off between the
carbon structures [36]. Figure 3(f) shows the G′-band recorded from the expanded graphite and two multilayered graphene sheets pulverized with 10 and 1000 s. The G′-band (2500 ∼ 2800 cm−1) and G-band (1580 cm−1) are attributed to the sp2 graphitic structures. The G′-band is activated by double-resonance processes, and a second-order process related to the phonon near the Kpoint in the graphitic structure occurred [37]. The G′-band in particular is used for evaluating the number of layers of graphene with AB interlayer stacking [37]. With an increasing number of layers, the number of double-resonance scattering process increases [37]. The G′ -bands show a blue shift for the expanded graphite, and the full width at half maximums (FWHM) of two sheets increased as the process time increased from 10 to 1000 s as shown in figure 3(f). The results of peak fitting are related to the four and two possible double-resonance scattering [37]. Clearly, a long pulverizing process decreases the number of graphene sheet layers. Next, we focused on the structural analysis of Al-based multilayered graphene composites. Microstructures of Al rods incorporating 1 wt% of multilayered graphene sheets clearly show the size effect of graphene sheets inside the Al matrix (figure 4). A relatively large graphene sheet (pulverized for 10 s) made of tens of thousands of large holes, resulted in rough microstructures. In general, the graphene sheets act as the impurities in the Al matrix, and their low wettability with Al formed such microstructures. As shown in figure 4(b), a scanning electron microscopy (SEM) image overlaid with an energy-dispersive x-ray spectroscopy (EDS) map shows sparsely, but irregularly, distributed graphene sheets in the Al matrix. In contrast, small-sized graphene sheets caused by longer pulverization formed well-distributed graphene sheets 4
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Nanotechnology 26 (2015) 215603
Figure 4. (a) SEM image of the microstructure of Al-based graphene composites fabricated with 10 s pulverized graphene sheets. The scale
bar is 10 μm. The inset is the high-magnification SEM of the microstructure. (b) Overlaid carbon EDS map on SEM image shows sparse population of carbon in Al matrix. Red dots indicate the graphene sheets in the Al matrix. The scale bar is 500 nm. (c) The SEM image shows the microstructure of Al-based graphene composites fabricated with 1000 s pulverized graphene sheets. The scale bar is 10 μm. The inset is a high-magnification SEM of the microstructure. (d) Overlaid EDS map of Al-graphene composites fabricated with 1000 s pulverized graphene shows well-distributed graphene in Al-matrix. The scale bar is 500 nm. (e) Raman spectra recorded from bare Al (blue), Al-based composites fabricated with 10 s (dark green) and 1000 s (red).
Figure 5. Measured thermal conductivities of 10 s pulverized graphene-Al composites (blue), 1000 s pulverized graphene sheets-Al
composites (red), and pristine Al (brown). (b) Volume resistivities of 10 s pulverized graphene-Al composites (blue), 1000 s pulverized graphene sheets-Al composites (red), and bare Al (brown). (c) Contact resistances measured from 10 s pulverized graphene-Al composites, 1000 s pulverized graphene sheets-Al composites, and pristine Al. (d) Mean contact resistances of 10 s pulverized graphene-Al composites, 1000 s pulverized graphene sheets-Al composites, and pristine Al. 5
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Nanotechnology 26 (2015) 215603
Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2013M3A6B1078872).
enhanced electrical properties and high contact resistance of graphene with metal. Next, the contact resistances for the graphitic structures/ pristine Al and graphitic structure/composites interfaces were investigated. To measure the contact resistance, three equally spaced graphene sheet electrodes were coated on the pristine Al and two Al-based graphene sheets composites (see the detailed information for the measurement of the contact resistances in the supporting information). The measured contact resistances for the graphitic structure/pristine Al interface were ∼2 kΩ (1.73 ∼ 2.82 kΩ). For the graphitic structure/composite interface, however, the contact resistances significantly decreased, as shown in figure 5(c). The mean contact resistances were 2.099, 0.491, and 0.266 kΩ for the pristine Al, 10-s and 1 000-s pulverized graphene composites, respectively (figure 5(d)). The graphene sheets inside the Al matrix clearly decreased the contact resistance. Well-dispersed composites have lower interfacial contact resistance than less dispersed composites because of an increase in the number of conduction modes of the exposed graphene sheets of composites to the external graphitic structure.
References [1] Avouris P, Chen Z and Perebeinos V 2007 Nat. Nanotechnology 2 605–15 [2] Novoselov K S, Fal ko V I, Colombo L, Gellert P R, Schwab M G and Kim K 2012 Nature 490 192–200 [3] Kaniyoor A, Imran Jafri R, Arockiadoss T and Ramaprabhu S 2009 Nanoscale 1 382–6 [4] Liu Y, Dong X and Chen P 2012 Chem. Soc. Rev. 41 2283–307 [5] Garaj S, Hubbard W, Reina A, Kong J, Branton D and Golovchenko J 2010 Nature 467 190–3 [6] Lv W, Xia Z, Wu S, Tao Y, Jin F M, Li B, Du H, Zhu Z P, Yang Q H and Kang F 2011 J. Mater. Chem. 21 3359–64 [7] Pumera M 2011 Energy Environ. Sci. 4 668–74 [8] Xu C, Xu B, Gu Y, Xiong Z, Sun J and Zhao X S 2013 Energy Environ. Sci. 6 1388–414 [9] Huang X, Qi X, Boey F and Zhang H 2012 Chem. Soc. Rev. 41 666–86 [10] Stankovich S, Dikin D A, Dommett G H, Kohlhaas K M, Zimney E J, Stach E A, Piner R D, Nguyen S T and Ruoff R S 2006 Nature 442 282–6 [11] Balandin A A, Ghosh S, Bao W, Calizo I, Teweldebrhan D, Miao F and Lau C N 2008 Nano Lett. 8 902–7 [12] Lee C, Wei X, Kysar J W and Hone J 2008 Science 321 385–8 [13] Novoselov K S, Geim A K, Morozov S, Jiang D, Zhang Y, Dubonos S, Grigorieva I and Firsov A 2004 Science 306 666–9 [14] Stoller M D, Park S, Zhu Y, An J and Ruoff R S 2008 Nano Lett. 8 3498–502 [15] Cho D H, Wang L, Kim J S, Lee G H, Kim E S, Lee S, Lee S Y, Hone J and Lee C 2013 Nanoscale 5 3063–9 [16] Essig S et al 2010 Nano Lett. 10 1589–94 [17] Fang Z, Liu Z, Wang Y, Ajayan P M, Nordlander P and Halas N J 2012 Nano Lett. 12 3808–13 [18] Feng J, Li W, Qian X, Qi J, Qi L and Li J 2012 Nanoscale 4 4883–99 [19] Kang J, Shin D, Bae S and Hong B H 2012 Nanoscale 4 5527–37 [20] Yan Z et al 2013 ACS Nano 7 58–64 [21] Fan Z, Yan J, Zhi L, Zhang Q, Wei T, Feng J, Zhang M, Qian W and Wei F 2010 Adv. Mater. 22 3723–8 [22] Han T H, Lee W J, Lee D H, Kim J E, Choi E Y and Kim S O 2010 Adv. Mater. 22 2060–4 [23] Huang X, Li S, Huang Y, Wu S, Zhou X, Li S, Gan C L, Boey F, Mirkin C A and Zhang H 2011 Nat. Commun. 2 292 [24] Jahan M, Bao Q, Yang J X and Loh K P 2010 J. Am. Chem. Soc. 132 14487–95 [25] Ramanathan T et al 2008 Nat. Nanotechnology 3 327–31 [26] Wang Y, Li Z, Hu D, Lin C T, Li J and Lin Y 2010 J. Am. Chem. Soc. 132 9274–6 [27] Zhu J, Zhu T, Zhou X, Zhang Y, Lou X W, Chen X, Zhang H, Hng H H and Yan Q 2011 Nanoscale 3 1084–9 [28] Kim Y L, Jung H Y, Kar S and Jung Y J 2011 Carbon 49 2450–8 [29] Xia F, Perebeinos V, Lin Y m, Wu Y and Avouris P 2011 Nat. Nanotechnology 6 179–84 [30] Lee H M, Choi S Y, Jung A and Ko S H 2013 Angew. Chem. 125 7872–7
4. Conclusion In summary, we developed Al-based composites incorporated with multilayered graphene via facile mechanical pulverization and extrusion molding processes. The multilayered graphene sheets were fabricated by a simple pulverizing process from expanded graphite. The as-prepared graphene sheets consisted of a few layered-graphene sheets (3 ∼ 8 layers). The facile extrusion molding process with Al powder and graphene sheets afforded Al-based multilayered graphene composite rods. These composites showed enhanced thermal conductivity compared to the pristine Al rods. Low graphene sheet contents functioned as a thermal conductivity enhancer inside the Al matrix. Moreover, the Al-based multilayered graphene sheets composites exhibited lower interfacial contact resistance than the pristine Al. With an increasing degree of dispersion, the number of exposed graphene sheets increases, thereby significantly decreasing the interfacial contact resistance between the composite and external graphite electrode.
Acknowledgments M G H acknowledges financial support from the Fundamental Research Program (PNK4060) of the Korean Institute of Materials Science (KIMS) and Basic Science Research Program by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2014R1A1A1006214). J N is grateful for the support from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (Grant No. NRF2013R1A1A1004986). M C was supported by the Global 6
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[34] Huang B, Xu Q and Wei S H 2011 Phys. Rev. B 84 155406 [35] de Walle C V, Choi M, Weber J, Lyons J and Janotti A 2013 Microelectron. Eng. 109 211–5 [36] Dresselhaus M and Eklund P 2000 Adv. Phys. 49 705–814 [37] Dresselhaus M S, Jorio A, Hofmann M, Dresselhaus G and Saito R 2010 Nano Lett. 10 751–8
[31] Qiu Y X, De-Li L, Ge S and Bing-Yun A 2011 Chin. Phys. B 20 017102 [32] Choi M, Janotti A and van de Walle C G 2013 J. Appl. Phys. 113 044501 [33] Shiota T, Morita M, Umeno M, Tagawa M, Ohmae N and Shima N 1999 Phys. Rev. B 60 16114–9
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Low interfacial contact resistance of Al-graphene composites via interface engineering
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 215603 (http://iopscience.iop.org/0957-4484/26/21/215603) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 128.227.24.141 This content was downloaded on 12/06/2017 at 04:05 Please note that terms and conditions apply.
You may also be interested in: Uniform dispersion of graphene oxide in aluminum powder by direct electrostatic adsorption for fabrication of graphene/aluminum composites Zan Li, Genlian Fan, Zhanqiu Tan et al. Transport and thermoelectric properties of polyaniline/reduced graphene oxide nanocomposites Rakibul Islam, Roch Chan-Yu-King, Jean-François Brun et al. A facile one-step process for 3D N-doped noncovalent functionalization PS/rGO composites Weiqi Huang, Hua Wang, Zheng Su et al. Synergistic interactions between silver decorated graphene and carbon nanotubes yield flexible composites to attenuate electromagnetic radiation Shital Patangrao Pawar, Sachin Kumar, Shubham Jain et al. Elastomeric composites based on carbon nanomaterials Sherif Araby, Qingshi Meng, Liqun Zhang et al. Enhanced electrical conductivity and hardness of silver-nickel composites by silver-coated multi-walled carbon nanotubes Dongmok Lee, Jeonghyun Sim, Wonyoung Kim et al. Programmable high crystallinity carbon patterns Xuewen Wang, Hong Wang, Yang Gu et al. Processable 3-nm thick graphene platelets of high electrical conductivity and their epoxy composites Qingshi Meng, Jian Jin, Ruoyu Wang et al.
Nanotechnology Nanotechnology 26 (2015) 215603 (7pp)
doi:10.1088/0957-4484/26/21/215603
Low interfacial contact resistance of Algraphene composites via interface engineering Myung Gwan Hahm1, Jaewook Nam2, Minseok Choi3, Chi-Dong Park4, Byungjin Cho1, Sanada Kazunori5, Yoong Ahm Kim6, Dong Young Kim7, Morinobu Endo8, Dong-Ho Kim1, Robert Vajtai9, Pulickel M Ajayan9 and Sung Moo Song10 1
Department of Advanced Functional Thin Films, Surface Technology Division, Korea Institute of Materials Science, 797 Changwondaero, Sungsan-Gu, Changwon, Gyeongnam 642-831, Korea 2 School of Chemical Engineering, Sungkyunkwan University, 300 Cheongcheon-dong, Suwon,Gyeonggido, 440-746, Korea 3 Advanced Characterization and Analysis Group, Korea Institute of Materials Science, 797 Changwondaero, Sungsan-Gu, Changwon, Gyeongnam 642-831, Korea 4 Research Center for Exotic Nanocarbons, Shinshu University, Faculty of Engineering, 4-17-1 Wakasato, Nagano, 380-8553, Japan 5 Department of Mechanical Engineering, Kyoto University, Yoshidahonmachi, Sakyo Ward, Kyoto, 6068501, Japan 6 Polymer & Fiber System Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju, 500-757, Korea 7 Materials Research Division, R&D Center, LG Display Co. LTd., 245 Lg-ro, Wollong-Myeon, Paju-Si, Gyeonggi-Do, 413-779, Korea 8 Research Center for Exotic Nanocarbons, Shinshu University, Faculty of Engineering, 4-17-1 Wakasato, Nagano, 380-8553, Japan 9 Department of Materials Science and NanoEngineering, Rice University, 6100 Main Street, Houston, Texas, 77005, USA 10 Department of Mechanical Engineering, Shinshu University, Faculty of Engineering, 4-17-1 Wakasato, Nagano, 380-8553, Japan E-mail: [email protected], [email protected] and [email protected] Received 10 March 2015, revised 30 March 2015 Accepted for publication 8 April 2015 Published 6 May 2015 Abstract
Al-based composites incorporating multilayered graphene sheets were developed via a facile approach. The multilayered graphene sheets were fabricated from the expanded graphite via a simple mechanical exfoliation process. The facile extrusion molding process with Al powder and graphene sheets exfoliated from expended graphite afforded Al-based graphene composite rods. These composites showed enhanced thermal conductivity compared to the pristine Al rods. Moreover, the Al-based multilayered graphene sheet composites exhibited lower interfacial contact resistance between graphene-based electrodes than the pristine Al. With increasing degrees of dispersion, the number of exposed graphene sheets increases, thereby significantly decreasing the interfacial contact resistance between the composite and external graphite electrode. S Online supplementary data available from stacks.iop.org/nano/26/215603/mmedia Keywords: graphene, aluminum, contact resistance, exfoliation, composites (Some figures may appear in colour only in the online journal)
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© 2015 IOP Publishing Ltd Printed in the UK
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Nanotechnology 26 (2015) 215603
1. Introduction Graphene has been used in many applications, including electronics [1, 2], sensors [3, 4], membranes [5, 6], energy storage [7, 8], and nano-composites [9, 10], owing to its large number of extraordinary physical properties such as high carrier mobility at room temperature (∼10000 cm2V−1s−1), good thermal conductivity (∼5000 Wm−1K−1), large specific surface area (∼3000 m2g−1), and high Young’s modulus (∼1 TPa) attributed to an ideal atomic-layered two-dimensional (2D) system of graphene [11–14]. However, maintaining its intrinsic characteristics for practical applications is extremely difficult; a number of limitations in fundamental scientific and technological applications exist because of its high cost and inborn disorders. The realization of the approaching limits has inspired a worldwide effort to develop alternative approaches for graphene-based device technologies. Various approaches have been attempted for many years and are still actively developing [15–20]. One plausible way to harness exceptional properties of graphene is to mix it with other materials such as a matrix or filler. This composite approach in combination with various materials has been developed for diverse applications, and some of the composites yielded successful results [10, 21–27]. Notwithstanding such achievements, the approach still has some barricades to utilizing the composites for electronic devices. A considerable contact resistance between metals and graphene is well known to significantly limit the outstanding performance of the graphene channel on graphene-based field-effect transistors (FETs). Previously, several studies reported to decrease the contact resistance between graphitic structure and metals [28, 29]. The interface condition between graphite and metal electrodes is important for determining the resistance. In this study, we considered the contact resistance between graphene sheets and metal-based electrodes. Aluminum (Al) was selected as a metal owing to its electrical properties and cost effectiveness, as it can be a strong candidate for various practical applications [30, 31]. Therefore, the contact resistance is strongly determined by the graphene/Al interface condition at the contact junction. In the absence of any other junk material from cleaning or other fabrication procedures near the junction, there are two possibilities: graphene/native oxide of Al and graphene/Al interfaces. Clearly, the electron transport through these two types of interfaces can be considerably different from the corresponding interfacial resistances. Native oxide of Al would impart a larger barrier between Al and graphene sheets. As shown in figure 1, the formation of the α-Al2O3 layer yields an energy barrier of 4 ∼ 5 eV between Al and graphene and is significantly difficult to overcome from the viewpoint of the electron. Only quantum mechanical tunneling is allowed when the layer is thin enough. In addition, defects are incorporated into the oxide, and they can determine the electrical property. According to the literature, O vacancies, Al vacancies, and Al interstitials are considerable defects for the Fermi-level position in the band gap of the α-Al2O3 (the Al-rich condition is assumed
Figure 1. Band alignment between Al and graphene (Gr). α-Al2O3 is considered a native oxide on top of the Al electrode. The Fermi-level position with respect to the vacuum level for Al is taken from a previous work [33], and the valence-band offset between Gr and Al2O3 is also from a previous work [34]. Defect levels for α-Al2O3 are taken from a previous works [32, 35].
here) [32]. O interstitials have high formation energy; thus their formation is unlikely. This study indicates that O vacancies are most likely to form in the native oxide layer because the Fermi level in the Al/Al2O3/graphene sheets structure would be positioned near the Fermi level of Al metal. The role of the vacancies in the electrical property of the Al/Al2O3/graphene structure is investigated in terms of the formation energy and their positions of defect level with respect to the Al and graphene band position. The formation energy of the 2+ charged oxygen vacancy is represented by the following equation: E f (V0+2 ) = Etot (V0+2 ) − Etot (Al2 O3 ) − μ0 + 2E F where Etot (Al2 O3 ) is the total energy of a supercell containing +2 charged O vacancies; Etot (V0+2 ) is the total energy of the pristine Al2O3 supercell; μ0 is the O chemical potential; and EF is the Fermi level referenced to the valence band maximum (VBM) of Al2O3. The defect level (q/q′) in figure 1 is defined as the EF position below which the same defect is stable in charge state q and above which the same defect is stable in charge state q′. For example, the (+2/+1) level of the O vacancy is evaluated from the formation energies using the following equation: (+2 +1) = ⎡⎣ E f (V0+2 )@VBM − E f (V0+1 )@VBM ⎤⎦ (2 − 1), where E f (V0+2 )@VBM is the formation energy of the +2 charged O vacancy when EF is at the VBM and E f (V0+1 )@VBM is that of the +1 charged O vacancy at VBM [32]. From the predictions, we conclude that O vacancies may act as charge traps, which increase contact resistivity when native oxide forms in between Al and graphene sheets. When graphene and Al electrodes are prepared separately, the formation of native defects in the oxide layer on top of Al cannot be excluded when it is exposed to air. To 2
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Figure 2. Fabrication procedure of Al-based multilayered graphene composites. (a) SEM image of expanded graphite. The scale bar is 100 μm. (b) SEM image shows multilayered graphene formed from the expanded graphite. The scale bar is 3 μm. (c) SEM of micrometer-sized Al powder. (d) The scale bar is 100 μm. The mixture of expanded graphite and Al powder was ball-milled to mix two different materials. (e) Hot extruding the mixture. (f) Optical image shows the Al-based multilayered graphene composites.
it did enhance them, particularly the interfacial contact resistance with other carbon-based electrodes.
exploit the relatively small Fermi-level barrier between pristine Al and graphene, the intact Al at the interface needs to be preserved, and the junction contact needs to be formed below the outside metal surface. In this regard, a simple way to achieve this is to fabricate Al-based graphene composites. In this study, we report a facile approach to create Al-based multilayered graphene composites with significantly improved contact interface between the composites and graphene.
3. Results and discussion Figure 3 shows the multilayered graphene sheets fabricated from the expanded graphite via a simple pulverizing process. Clearly, the simple pulverizing process is very effective to produce multilayered graphene sheets. Moreover, the size and number of graphene sheet layers was easily controlled by the pulverization time at the same rpm. The size range of graphene sheets pulverized for 10 s is from 100 μm to 600 μm (figure 3(a)) and is larger than the size of the sheets (400 to 800 nm) pulverized for 1000 s (figure 3(c)). The high-magnification inset images clearly show the size difference caused by the two pulverization times. To evaluate the number of graphene sheet layers, transmission electron microscopy (TEM) was used. As shown in figures 3(b) and (d), the TEM images clearly show that the pulverization time controls the number of layers of graphene sheets as well. The shortest pulverizing process (10 s) resulted in multilayered graphene sheets (figure 3(b)), whereas the longest process (1000 s) afforded trilayered graphene sheets (figure 3(d)). From these observations, one can conclude that the mechanical energy transferred from the pulverization process not only disintegrated the sheets, decreasing their size, but also exfoliated them, decreasing the number of layers. To cross-check the effect of the pulverization time on the number of layers, Raman spectroscopy was performed. The Raman spectrum recorded for the expanded graphite shows a sharp G-band, as shown in figure 3(e), compared to that of the expanded graphite (figure 3(e): dark green and red). The disorder-induced D-band indicates the presence of disorder in sp2-hybridized
2. Methods The two key processes for incorporating multilayered graphene sheets in the pristine Al are (1) the production of multilayered graphene sheets and 2) the extrusion molding of Al-based multilayered graphene (Al-MG) composites, as shown in figure 2. First, multilayered graphene sheets were disunited from expanded graphite (figure 2(a)) using a pulverizer. Expanded graphite (1 g) was pulverized with 25,000 rpm for 10 and 1000 s to investigate the size effect of graphene in Al-MG composites. N-methylpyrrolidone (NMP)-based pulverized graphite solution was stirred and sonicated for 24 h. Then the resulting solution was centrifuged with 500 rpm for 45 minutes and vacuum filtered, resulting in multilayered graphene sheets as shown in figure 2(b). As shown in figure 2, micrometer-sized pristine Al powder was compounded with multilayered graphene sheets (1 wt%) by ball-mill (100 rpm) for 1 h. The Al-MG mixture powder was then pre-sintered under 300 MPa at 600 °C for 10 min. The composites were heated at 500 °C for 10 min and hot-extruded under 600 MPa. The filler, graphene, did not deteriorate the matrix materialʼs physical properties; however, 3
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Figure 3. SEM image shows multilayered graphene fabricated from expanded graphite with 10 s pulverizing time. The scale bar is 10 μm. The inset is a high-magnification SEM of the graphene sheet. The scale bar is 500 nm. (b) High-resolution TEM image of graphene sheet pulverized for 10 s. The scale bar is 5 nm. (c) SEM of multilayered graphene sheet pulverized from expanded graphite with 1000 s pulverizing time. The scale bar is 10 μm. The inset is a high-magnification SEM of the graphene sheet. The scale bar is 500 nm. (d) Highresolution TEM image shows the multilayered graphene sheet. The scale bar is 5 nm. (e) Raman spectra recorded from expanded graphite (blue), graphene sheet pulverized for 10 s (green), and graphene sheet pulverized for 1000 s. (f) The G′ spectra for three graphitic structures as a function of the number of layers.
in the matrix even with the same amount of filler, as shown in figures 4(c) and (d). Raman spectra of the pristine Al (blue), large-sized graphene sheets-Al composites (dark green), and the small-sized graphene sheets-Al composites (red) are shown in figure 4(e). Raman spectra of the composites clearly show the disorder-induced band (D-band), stretching of the carbon-carbon bond (G-band), and second-order process (G′ -band), and the presence of these peaks indicate that the composites have graphitic structures. We investigated the thermal and electrical properties of the Al-based graphene sheet composites. The thermal conductivities of the pristine Al and Al-based graphene composites were measured by the standard laser flash analysis method. The measured thermal conductivity of the pristine Al was 179 Wm−1K−1 (figure 5(a)). Al-based 10 s and 1000 s pulverized graphene sheets had higher thermal conductivities than that of the pristine Al, as shown in figure 5(a). Graphene is well known to have outstanding thermal conductivity; Balandin et al. reported that the thermal conductivity of single-layered graphene ranged between 4840 and 5300 Wm−1K−1 [11]. Notwithstanding, low graphene content in the composites functioned as a thermal conductivity enhancer for the Al matrix. The results clearly show that well-dispersed graphene sheets (1000 s pulverized graphene sheets) showed better thermal conductivity than the less dispersed ones (10 s pulverized graphene sheets). The Al-based graphene composites also showed slightly higher volume resistivity (4 ∼ 5 nΩm) than the pristine Al, as shown in figure 5(b); however, those values are comparable with the pristine Al. Graphene sheets inside the matrix may enhance the electrical properties of composites; however, there will be a trade-off between the
carbon structures [36]. Figure 3(f) shows the G′-band recorded from the expanded graphite and two multilayered graphene sheets pulverized with 10 and 1000 s. The G′-band (2500 ∼ 2800 cm−1) and G-band (1580 cm−1) are attributed to the sp2 graphitic structures. The G′-band is activated by double-resonance processes, and a second-order process related to the phonon near the Kpoint in the graphitic structure occurred [37]. The G′-band in particular is used for evaluating the number of layers of graphene with AB interlayer stacking [37]. With an increasing number of layers, the number of double-resonance scattering process increases [37]. The G′ -bands show a blue shift for the expanded graphite, and the full width at half maximums (FWHM) of two sheets increased as the process time increased from 10 to 1000 s as shown in figure 3(f). The results of peak fitting are related to the four and two possible double-resonance scattering [37]. Clearly, a long pulverizing process decreases the number of graphene sheet layers. Next, we focused on the structural analysis of Al-based multilayered graphene composites. Microstructures of Al rods incorporating 1 wt% of multilayered graphene sheets clearly show the size effect of graphene sheets inside the Al matrix (figure 4). A relatively large graphene sheet (pulverized for 10 s) made of tens of thousands of large holes, resulted in rough microstructures. In general, the graphene sheets act as the impurities in the Al matrix, and their low wettability with Al formed such microstructures. As shown in figure 4(b), a scanning electron microscopy (SEM) image overlaid with an energy-dispersive x-ray spectroscopy (EDS) map shows sparsely, but irregularly, distributed graphene sheets in the Al matrix. In contrast, small-sized graphene sheets caused by longer pulverization formed well-distributed graphene sheets 4
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Figure 4. (a) SEM image of the microstructure of Al-based graphene composites fabricated with 10 s pulverized graphene sheets. The scale
bar is 10 μm. The inset is the high-magnification SEM of the microstructure. (b) Overlaid carbon EDS map on SEM image shows sparse population of carbon in Al matrix. Red dots indicate the graphene sheets in the Al matrix. The scale bar is 500 nm. (c) The SEM image shows the microstructure of Al-based graphene composites fabricated with 1000 s pulverized graphene sheets. The scale bar is 10 μm. The inset is a high-magnification SEM of the microstructure. (d) Overlaid EDS map of Al-graphene composites fabricated with 1000 s pulverized graphene shows well-distributed graphene in Al-matrix. The scale bar is 500 nm. (e) Raman spectra recorded from bare Al (blue), Al-based composites fabricated with 10 s (dark green) and 1000 s (red).
Figure 5. Measured thermal conductivities of 10 s pulverized graphene-Al composites (blue), 1000 s pulverized graphene sheets-Al
composites (red), and pristine Al (brown). (b) Volume resistivities of 10 s pulverized graphene-Al composites (blue), 1000 s pulverized graphene sheets-Al composites (red), and bare Al (brown). (c) Contact resistances measured from 10 s pulverized graphene-Al composites, 1000 s pulverized graphene sheets-Al composites, and pristine Al. (d) Mean contact resistances of 10 s pulverized graphene-Al composites, 1000 s pulverized graphene sheets-Al composites, and pristine Al. 5
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Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2013M3A6B1078872).
enhanced electrical properties and high contact resistance of graphene with metal. Next, the contact resistances for the graphitic structures/ pristine Al and graphitic structure/composites interfaces were investigated. To measure the contact resistance, three equally spaced graphene sheet electrodes were coated on the pristine Al and two Al-based graphene sheets composites (see the detailed information for the measurement of the contact resistances in the supporting information). The measured contact resistances for the graphitic structure/pristine Al interface were ∼2 kΩ (1.73 ∼ 2.82 kΩ). For the graphitic structure/composite interface, however, the contact resistances significantly decreased, as shown in figure 5(c). The mean contact resistances were 2.099, 0.491, and 0.266 kΩ for the pristine Al, 10-s and 1 000-s pulverized graphene composites, respectively (figure 5(d)). The graphene sheets inside the Al matrix clearly decreased the contact resistance. Well-dispersed composites have lower interfacial contact resistance than less dispersed composites because of an increase in the number of conduction modes of the exposed graphene sheets of composites to the external graphitic structure.
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4. Conclusion In summary, we developed Al-based composites incorporated with multilayered graphene via facile mechanical pulverization and extrusion molding processes. The multilayered graphene sheets were fabricated by a simple pulverizing process from expanded graphite. The as-prepared graphene sheets consisted of a few layered-graphene sheets (3 ∼ 8 layers). The facile extrusion molding process with Al powder and graphene sheets afforded Al-based multilayered graphene composite rods. These composites showed enhanced thermal conductivity compared to the pristine Al rods. Low graphene sheet contents functioned as a thermal conductivity enhancer inside the Al matrix. Moreover, the Al-based multilayered graphene sheets composites exhibited lower interfacial contact resistance than the pristine Al. With an increasing degree of dispersion, the number of exposed graphene sheets increases, thereby significantly decreasing the interfacial contact resistance between the composite and external graphite electrode.
Acknowledgments M G H acknowledges financial support from the Fundamental Research Program (PNK4060) of the Korean Institute of Materials Science (KIMS) and Basic Science Research Program by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2014R1A1A1006214). J N is grateful for the support from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (Grant No. NRF2013R1A1A1004986). M C was supported by the Global 6
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