Home
Search
Collections
Journals
About
Contact us
My IOPscience
Exceptionally strong and robust millimeter-scale graphene–alumina composite membranes
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 355701 (http://iopscience.iop.org/0957-4484/25/35/355701) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 132.203.227.63 This content was downloaded on 05/03/2015 at 09:45
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 439501 (1pp)
doi:10.1088/0957-4484/25/43/439501
Corrigendum: Exceptionally strong and robust millimeterscale graphene–alumina composite (2014 Nanotechnology 25 355701) Maria Berdova1, Alexander Pyymaki Perros2, Wonjae Kim2, Juha Riikonen2, Tuomo Ylitalo3, Jouni Heino4, Changfeng Li2, Ivan Kassamakov3,4, Edward Hæggström3, Harri Lipsanen2 and Sami Franssila1 1
Department of Materials Science and Engineering, School of Chemical Technology, Aalto University, PO Box 16200, FI-00076, Finland 2 Department of Micro- and Nanosciences, School of Electrical Engineering, Aalto University, PO Box 13500, FI-00076, Finland 3 University of Helsinki, Department of Physics, PO Box 64, FI-00014, Helsinki, Finland 4 Helsinki Institute of Physics, PO Box 64, FI-00014, Helsinki, Finland E-mail: maria.berdova@aalto.fi and alexander.pyymaki.perros@aalto.fi Received 12 September 2014 Accepted for publication 12 September 2014 Published 9 October 2014
(Some figures may appear in colour only in the online journal) There is an error in the axis of figure 3(b). The corrected figure is shown below:
0957-4484/14/439501+01$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
Nanotechnology Nanotechnology 25 (2014) 355701 (7pp)
doi:10.1088/0957-4484/25/35/355701
Exceptionally strong and robust millimeterscale graphene–alumina composite membranes Maria Berdova1,5, Alexander Pyymaki Perros2,5, Wonjae Kim2, Juha Riikonen2, Tuomo Ylitalo3, Jouni Heino4, Changfeng Li2, Ivan Kassamakov3,4, Edward Hæggström3, Harri Lipsanen2 and Sami Franssila1 1
Department of Materials Science and Engineering, School of Chemical Technology, Aalto University, PO Box 16200, FI-00076, Finland 2 Department of Micro- and Nanosciences, School of Electrical Engineering, Aalto University, PO Box 13500, FI-00076, Finland 3 University of Helsinki, Department of Physics, PO Box 64, FI-00014, Helsinki, Finland 4 Helsinki Institute of Physics, PO Box 64, FI-00014, Helsinki, Finland E-mail: maria.berdova@aalto.fi and alexander.pyymaki.perros@aalto.fi Received 24 April 2014, revised 3 July 2014 Accepted for publication 9 July 2014 Published 12 August 2014 Abstract
Graphene has attracted attention as a potential strengthening material and functional component in suspended membranes as utilized in micro and nanosystems. Development of a practical and scalable fabrication process is a necessary step to allow the exceptional material properties of graphene to be fully exploited in composite structures. Using standard and scalable microfabrication processes, we fabricated free-standing chemical vapor deposition monolayer graphene-reinforced Al2O3 composite membranes, 0.5 mm in diameter, that are strong and robust. Bulge tests revealed that the graphene reinforcement increased the membrane fracture strength by a factor of at least three and maximum sustainable strain from 0.28% to at least 0.69%. We show that the graphene-reinforced membranes are even tolerant to significant cracking without loss of membrane integrity. The graphene composite membranes’ freestanding area of ∼200 000 μm2 is almost a thousand times larger than suspended graphene membranes reported elsewhere. The presented graphene composite membranes may be seen as representing an interesting new class of durable composite materials warranting further study and having potential for broad applicability in a variety of fields. S Online supplementary data available from stacks.iop.org/NANO/25/355701/mmedia Keywords: CVD graphene, atomic layer deposition, suspended membrane, mechanical strength, bulge test, aluminum oxide (Some figures may appear in colour only in the online journal) 1. Introduction
properties [1–4]. Monolayer graphene exhibits 1 TPa Young’s modulus and 130 ± 10 GPa fracture strength giving it the distinction of being the world’s strongest material [4]. Graphene is flexible and can withstand strains exceeding 20% [4–6]. These properties make graphene attractive for many applications: a transparent conductor in flexible displays, a strengthening gas-barrier layer in thin film windows, or an
Graphene has attracted attention primarily directed at harnessing its exceptional electrical properties for electronics and photonics but it also possesses extraordinary mechanical 5
Authors contributed equally to this work.
0957-4484/14/355701+07$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
Nanotechnology 25 (2014) 355701
M Berdova et al
Figure 1. Process flow for the practical and scalable fabrication of graphene–alumina composite membranes. (a) Al2O3 is deposited by ALD on a silicon substrate, (b) graphene transfer, (c) Al seed layer evaporation and second ALD Al2O3 deposition, and (d) photolithography and release-etch from the backside. Schematic cross-sectional view of membrane: alumina (70 nm)/graphene (monolayer)/alumina (30 nm) (e) and optical image of composite membrane (top view) (f).
embedded sensing layer in suspended thin film structures [7–11]. These and other studies on graphene-based composites demonstrate graphene’s potential for practical multifunctionality in different industrial applications [6, 10, 11]. Research on graphene-based composites has thus far primarily focused on exfoliated graphene [12–16]. Nevertheless, CVD graphene is of particular interest for industrial use because exfoliated graphene is impractical for real-world applications that require large-area high-quality graphene [17–19]. CVD graphene has been considered to feature low mechanical quality due to the presence of grain boundaries [20]. However, theoretical and experimental studies indicate that the grain boundaries in CVD graphene can reach the characteristics of pristine graphene provided post CVD processing avoid damaging the graphene layer [21, 22]. Recently, potential breakthroughs in the growth of pristine wafer-scale single-crystalline monolayer CVD graphene on germanium (110) could make the previous concerns a thing of the past [23, 24]. Being merely one atom thick, graphene is susceptible to damage during transfer to a substrate and posttransfer processing [9, 25, 26]. Suspended CVD graphene membranes are still small, 384 μm2 being the largest cavity fabricated to our knowledge [27]. These small dimensions preclude the use of graphene in large-area windows. Many composites can be produced by atomic layer deposition (ALD), a technique for growing conformal, uniform, and pinhole-free thin layers on complex structures [28–33]. This technique achieves sub-nm precision in layer thickness allowing, for example, construction of nanolaminates or nanometer-thick suspended membranes [34, 35]. ALD alumina (Al2O3) in particular has attracted a lot of attention since it meets the requirements of flexible large-area membranes intended as gas diffusion barriers or sensing
elements [36–38]. ALD Al2O3 is also commonly used as a top gate dielectric in graphene field effect transistors [39, 40]. Significant performance improvement and new functionality could be achieved by combining graphene with ALD layers. Here we incorporate ALD Al2O3 and monolayer CVD graphene to fabricate strong and large area free-standing membranes, whose ∼200 000 μm2 area is almost a thousand times larger than those reported previously [27]. These graphene–alumina composites exhibit improved mechanical properties in free-standing membranes compared to those of unreinforced ALD Al2O3 membranes. Additionally, using standard microfabrication processes and CVD graphene, our approach avoids problems related to graphene handling, for example by reducing the risk of membrane damage during transfer to a holey substrate. This is achieved by releasing the membrane by plasma etch only as the very last step of the processing. Additionally, as the graphene layer is sandwiched between two protective alumina layers, post-transfer damage is minimized. In light of the recent advances in producing single-crystalline large-area graphene, our results gain more significance as the exploitation of graphene’s exceptional mechanical properties can be expected to attract more attention in large-area applications [23, 24].
2. Methods 2.1. Practical and scalable fabrication method
Double side polished silicon substrates were coated with ALD Al2O3 on both sides of the wafers as depicted in figure 1(a) (70 nm on the wafer topside and 20 nm on the backside). The alumina on top serves as the lower part of the membrane whereas alumina on wafer backside serves as the mask layer 2
Nanotechnology 25 (2014) 355701
M Berdova et al
Figure 2. Bulge plots for suspended membranes subjected to differential pressure. (a) 3D bulge plots obtained by SWLI as a function of
increasing differential pressure from 0 to 2 bar across a composite membrane suspended over a cavity. (b) The corresponding 2D displacement h as a function of differential pressure P (total measurement time >1 h).
for the membrane release etch. ALD Al2O3 is an ideal etch mask and etch stop due to its low dry etch rate, ≪1 nm min−1 [41, 42]. Following the ALD step, monolayer graphene was transferred as described in detail elsewhere (see also supporting information) [43]. Monolayer CVD graphene was synthesized utilizing photo-thermal CVD [43]. Next, a ∼2 nm thick aluminum layer was evaporated onto graphene. This thin seeding layer was rapidly oxidized to Al2O3 in air [44]. This was followed by the second ALD Al2O3 (30 nm) layer. The resulting ∼100 nm thin sandwich membrane comprised three layers: 70 nm alumina/graphene (monolayer)/30 nm alumina. The surface morphology of the membrane was measured by atomic force microscopy after deposition of the 30 nm alumina film. The mean surface roughness of the composite membrane is 3.1 nm. This roughness is higher than ALD Al2O3 deposited on atomically flat silicon wafers and is attributed to the underlying ∼2 nm thick aluminum layer evaporated onto graphene. The membranes were released by deep reactive ion etching (DRIE) using the Bosch cyclic etch chemistry. The DRIE process generated a pattern of 0.5 mm diameter holes in the silicon substrate. Care was taken to ensure that the graphene layer completely covered the suspended region and that it extended well beyond the cavity to avoid non-uniform strain at the graphene edges [14]. The fabrication is described in detail in the supporting information. Reference membranes were fabricated as described above but without graphene. It should be noted that while the total aluminum oxide thickness of the composite and reference membranes is identical, the composite membrane is composed of two thinner (70 nm and 30 nm) Al2O3 membranes separated by the graphene layer. This division of the Al2O3 into thinner layers is not trivial as thinner alumina features a higher strain threshold for cracking [45]. Fabricating a structure that avoids dividing the Al2O3 layer by placing the graphene layer only on the top of the alumina
layer was not considered a practical option due to risks associated with adhesion loss, poor stress transfer at high strains, and exposing the unprotected graphene layer to damage [14, 46–48]. All of these could diminish the membrane quality. 2.2. Bulge method
The bulge method was used to determine Young’s modulus and residual stress in the composite membranes [49]. This technique is based on measuring the displacement h of the membrane center as a function of differential pressure P, see figure 2. Increasing overpressure was achieved by feeding argon gas into the cavity of the test structure (see supporting information). With gradually increasing overpressure (discrete steps) the displacement increased consistently. The membrane center deflection was measured by a scanning white light interferometer (SWLI) after applying increasing overpressure by feeding argon gas into the cavity of the test structure [50]. Each interferometer scan lasted ∼8 min. The calculation of effective Young’s modulus is based on the spherical cap model for a suspended film clamped at its edges [51, 52]. The effective modulus E is determined by the slope of the linear region of the stress–strain curve and the intercept with the σ-axis is defined as the residual stress. The strain ε was calculated as the fractional increase in arc length of the bulged membrane across the membrane diameter (2a): ε=
2h2 3a 2
(1)
and the residual stress σ was defined as: σ=
where t is the film thickness. 3
Pa2 , 4ht
(2)
Nanotechnology 25 (2014) 355701
M Berdova et al
Figure 3. Comparison of unreinforced alumina and graphene–alumina composite membranes. (a) Deflection h of two graphene–alumina
composite membranes and reference alumina membranes versus differential pressure P as measured during the bulge test. (b) The corresponding stress versus strain for the two graphene–alumina composite membranes and reference alumina membranes. The composite membranes were intact after being subjected to 2.0 bar differential pressure whereas the reference alumina membranes were destroyed already at ∼0.6 bar differential pressure.
3. Results and discussion
3.2. Fracture strength of the membranes
In an earlier study, 75 nm thick ALD Al2O3 membranes of 0.5 mm in diameter withstood 3 bar shock pressure [50]. This is in contrast to the present result where the Al2O3 reference membranes failed consistently at ∼0.6 bar. The discrepancy is due to the different pressure ramp rates used in the two experiments. This is known as time-dependent failure; the fracture response of ceramic materials varies with load rate [58–61]. When the ramp rate is small, as in the current experiment, the material’s apparent strength diminishes due to slow crack propagation. In the present work each measured deflection was maintained for ∼8 min (total measurement time >1 h), whereas in the fracture shock pressure experiments the entire measurement was carried out in a mere 30 s (i.e. at fast-fracture conditions) [58, 61].
3.1. Effective Young’s modulus and residual stress of the membranes
The membranes were flat before and after each interferometer scan series, indicating the presence of tensile residual stress typical of ALD Al2O3 [50, 53]. The composite membranes deflected 23–26 μm at 2 bar, corresponding to 0.67–0.69% strain, see figure 3. For comparison, the equally thick reference Al2O3 membranes reached a maximum sustainable deflection of 16–20 μm already at ∼0.6 bar (maximum fracture pressure), corresponding to 0.26% strain. For these alumina membranes, the effective Young’s modulus was 161 ± 23 GPa and the residual stress was 298 ± 61 MPa based on the measured stress–strain curves. These values are in line with well-established literature values: the reported Young’s modulus for Al2O3 is 154–220 GPa and the residual stress is 320–500 MPa (for films grown at 177–220 °C) [35, 50, 53–57]. For our composite membranes the corresponding values were 180 ± 15 GPa and 352 ± 59 MPa, respectively. Here the instrument-related uncertainty in the results is negligible as the SWLI resolution is ±20 nm (versus ∼20 μm deflection) and the pressure control is ±10 mbar (versus 0.6–2.0 bar test pressures). The validity of the ‘ruleof-mixtures’ model (based on volume fractions of composite components) used to determine the effective Young’s modulus for the composite membrane is valid, even in the case of monolayer graphene. In our case the volume fraction of graphene in the composite membrane is merely ∼0.35%, i.e., practically negligible. Therefore, the Young’s modulus and residual stress of the composite membranes should be nearly equal to those of the reference membranes as also is evident in our measurement results.
3.3. Crack tolerance of the composite membrane
Scanning electron microscopy (SEM) was utilized to confirm the composite membrane integrity after bulge testing. SEM images, figure 4, indicate channeling type cracks in the Al2O3 layer of composite membranes brought to 2 bar differential pressure during testing [62]. The existence of cracks has significant implications: normally onset of cracking is in the suspended ceramic membrane cavity area and does not extend from the membrane edge. Using graphene to increase the suspended membrane tolerance against cracking may provide an interesting path for detailed study of cracking failure in alumina free-standing membranes. 3.4. Raman analysis of the composite membrane
Raman analysis of the graphene after membrane processing (i.e. after releasing the free-standing window) displays a typical Raman fingerprint of high quality monolayer graphene 4
Nanotechnology 25 (2014) 355701
M Berdova et al
Figure 4. Cracking of the composite membrane. SEM micrographs of cracks in a composite membrane after bulge measurements that brought the differential pressure up to 2 bar. Composite membrane remains intact despite the presence of significant cracking.
Figure 5. Raman analysis of the membrane area with Al2O3 cracking after bulge testing to 2 bar. (a) Optical and (b)–(d) Raman mapping images of D, G, and 2D-line. (e), (f) Raman images of D/G and 2D/G. (g) Raman spectra corresponding to the points indicated by Arabic numbers in (d). The Raman spectra are offset for clarity and scale bars correspond to 9 μm.
in figure S1. This indicates the embedded CVD graphene incurred no damage during processing. Moreover, the detailed Raman characterization performed even after bulge testing to 2 bar reveals no impairment of the graphene film as shown in figure 5. Particularly noteworthy is that even in and around the cracked regions of the alumina the graphene showed no signs of damage as evidenced by a very low D/G ratio in Raman maps and practically non-existent D-band in the Raman spectra at around 1250 cm−1. Mapping of the spectral position of the 2D band reveal no significant variation, particularly no correlation with the cracking (not shown here). Considering that the
2D band position in graphene is susceptible to strain, it can be concluded that cracking has not induced additional strain to graphene [63]. In fact, a slight increase in the 2D peak can be observed in figure 5(d) around the cracks which we speculate might be related to a loss of adhesion of the alumina. This observation is of great importance as it implies that despite severe cracking of the alumina the embedded CVD graphene film remains intact even at the grain boundaries and is capable of maintaining the integrity of the membrane. These results indicate good stitching of the grain boundaries in the embedded CVD graphene [21, 22].
5
Nanotechnology 25 (2014) 355701
M Berdova et al
is still of high quality. (4) The fabrication process is straightforward, scalable, and easy to adapt to other ALD materials and to more complex nanolaminate structures. (5) Our approach also permits implementing additional components to the structure since all processing is carried out on standard silicon wafers. As graphene responds electrically to deformation, further processing could, e.g., include fabrication of on-chip read-out circuitry for sensing applications. (6) Due to the substantially improved mechanical robustness demonstrated in this work, these graphene-sandwich composite membranes can potentially be viewed as a new class of robust thin films.
Acknowledgments Figure 6. Lower differential pressure bulge testing of composite
This work was undertaken at the Micronova Nanofabrication Center of Aalto University. The research leading to these results has received funding from the European Union Seventh Framework Programme under grant agreement n° 604391 Graphene Flagship. This research project was also supported by Academy of Finland under the research project (project number 263566) two-dimensional programmable materials for optical and electronic applications (TODIMA). The authors also acknowledge additional support from Academy of Finland and TEKES—the Finnish Funding Agency for Technology and Innovation. M B is funded by Aalto School of Chemical Technology PhD student grant and by The Finnish National Graduate School in Nanoscience (NGS Nano).
membrane. Deflection behavior of a graphene–alumina composite membrane and reference alumina membranes as measured by bulge test. No cracking was observed in the composite membrane brought to 1.2 bar. 3.5. Lower differential pressure bulge testing of composite membrane
To investigate how the composite membranes would behave under lower differential pressure, another graphene–alumina composite was bulge tested to 1.2 bar, see figure 6. SEM inspection revealed no cracking in the alumina in the bottom or top side of the membrane. The results suggest that graphene reinforcement is a promising facilitator for constructing large vacuum-tight thin film windows. Current state-of-the-art large-area windows are based on low-pressure CVD Si3N4 thin films, which require a mesh support that occupies a significant fraction, ∼23%, of the window region [64]. It is envisioned that incorporating graphene into this and similar kinds of structures will increase the sensing area by allowing a reduction of mesh area. This would increase transmission and aperture by reducing grid-induced collimation.
References [1] Geim A K and Novoselov K S 2007 Nat. Mater. 6 183 [2] Bonaccorso F, Sun Z, Hasan T and Ferrari A C 2010 Nat. Photonics 4 611 [3] Koppens F H L, Chang D E and García de Abajo F J 2011 Nano Lett. 11 3370 [4] Lee C, Wei X, Kysar J W and Hone J 2008 Science 321 385 [5] Liu F, Ming P and Li J 2007 Phys. Rev. B 76 064120 [6] Young R J, Kinloch I A, Gong L and Novoselov K S 2012 Compos. Sci. Technol. 72 1459 [7] Bunch J S, Verbridge S S, Alden J S, van der Zande A M, Parpia J M, Craighead H G and McEuen P L 2008 Nano Lett. 8 2458 [8] Nair R R, Blake P, Grigorenko A N, Novoselov K S, Booth T J, Stauber T, Peres N M R and Geim A K 2008 Science 320 1308 [9] Li X, Zhu Y, Cai W, Borysiak M, Han B, Chen D, Piner R D, Colombo L and Ruoff R S 2009 Nano Lett. 9 4359 [10] Singh V, Joung D, Zhai L, Das S, Khondaker S I and Seal S 2011 Prog. Mater. Sci. 56 1178 [11] Huang X, Qi X, Boey F and Zhang H 2012 Chem. Soc. Rev. 41 666 [12] Kim H, Miura Y and Macosko C W 2010 Chem. Mater. 22 3441 [13] Potts J R, Dreyer D R, Bielawski C W and Ruoff R S 2011 Polymer 52 5 [14] Young R J, Gong L, Kinloch I A, Riaz I, Jalil R and Novoselov K S 2011 ACS Nano 5 3079
4. Conclusion Our work highlights the significant improvements that the incorporation of a monolayer of CVD graphene can confer to ALD Al2O3 membranes with mm-scale diameter. (1) Strengthening of the membrane structure, (2) the division of aluminum oxide into two layers results in a sandwich structure with a higher strain threshold than the unreinforced membrane, (3) improved tolerance against cracking: the composite structure remains intact even at high strain in the presence of significant cracking. The graphene reinforced membranes sustain more than three times higher differential pressure and more than twice the strain compared to corresponding unreinforced Al2O3 membranes. It should be noted that even though cracks developed in the composite, it still maintained the differential pressure which combined with the Raman analysis indicates that the monolayer CVD graphene 6
Nanotechnology 25 (2014) 355701
M Berdova et al
[43] Riikonen J, Kim W, Li C, Svensk O, Arpiainen S, Kainlauri M and Lipsanen H 2013 Carbon 62 43 [44] Kim S, Nah J, Jo I, Shahrjerdi D, Colombo L, Yao Z, Tutuc E and Banerjee S K 2009 Appl. Phys. Lett. 94 062107 [45] Jen S-H, Bertrand J A and George S M 2011 J. Appl. Phys. 109 084305 [46] Gong L, Kinloch I A, Young R J, Riaz I, Jalil R and Novoselov K S 2010 Adv. Mater. 22 2694 [47] Gong L, Young R J, Kinloch I A, Riaz I, Jalil R and Novoselov K S 2012 ACS Nano 6 2086 [48] Hashimoto A, Suenaga K, Gloter A, Urita K and Iijima S 2004 Nature 430 870 [49] Bromley E I 1983 J. Vac. Sci. Technol. B 1 1364 [50] Berdova M, Ylitalo T, Kassamakov I, Heino J, Törmä P T, Kilpi L, Ronkainen H, Koskinen J, Hæggström E and Franssila S 2014 Acta Mater. 66 370 [51] Beams J W 1959 Structure and Properties of Thin Films (New York: Wiley) [52] Small M K and Nix W D 1992 J. Mater. Res. 7 1553 [53] Ylivaara O M E et al 2013 Thin Solid Films 552 124 [54] Tapily K, Jakes J E, Stone D S, Shrestha P, Gu D and Baumgart H Elmustafa A A 2008 J. Electrochem. Soc. 155 H545 [55] Miller D C et al 2009 J. Appl. Phys. 105 093527 [56] Miller D C, Foster R R, Jen S-H, Bertrand J A, Cunningham S J, Morris A S, Lee Y-C, George S M and Dunn M L 2010 Sensors Actuators A 164 58 [57] Krautheim G, Hecht T, Jakschik S, Schröder U and Zahn W 2005 Appl. Surf. Sci. 252 200 [58] Krausz A S and Krausz K 1986 Fracture Mechanics of Ceramics ed R C Bradt, A G Evans, D P H Hasselman and F F Lange (New York: Springer) pp 333–40 [59] Jadaan O M, Nemeth N N, Bagdahn J and Sharpe W N Jr 2003 J. Mater. Sci. 38 4087 [60] Nemeth N N, Jadaan O M and Gyekenyesi J P 2005 Lifetime Reliability Prediction of Ceramic Structures Under Transient Thermomechanical Loads (Cleveland: NASA) NASA/TP—2005-212505 [61] Nemeth N N, Jadaan O M, Palfi T and Baker E H 2005 Fract. Mech. Ceram. 14 555 [62] Hutchinson J W 1996 Stress and Failure Modes in Thin Films and Multilayers (Notes for a Dcamm Course) (Lyngby: Technical University of Denmark) [63] Mohiuddin T et al 2009 Phys. Rev. B 79 205433 [64] Törmä P T et al 2013 IEEE Trans. Nucl. Sci. 60 1311
[15] Srivastava I, Mehta R J, Yu Z-Z, Schadler L and Koratkar N 2011 Appl. Phys. Lett. 98 063102 [16] Wang J, Li Z, Fan G, Pan H, Chen Z and Zhang D 2012 Scr. Mater. 66 594 [17] Kim K S, Zhao Y, Jang H, Lee S Y, Kim J M, Kim K S, Ahn J, Kim P, Choi J and Hong B H 2009 Nature 457 706 [18] Li X et al 2009 Science 324 1312 [19] Bae S et al 2010 Nat. Nanotechnology 5 574 [20] Huang P Y et al 2011 Nature 469 389 [21] Grantab R, Shenoy V B and Ruoff R S 2010 Science 330 946 [22] Lee G-H et al 2013 Science 340 1073 [23] Wang G et al 2013 Sci. Rep. 3 2465 [24] Lee J-H et al 2014 Science 344 286 [25] Kang J, Shin D, Bae S and Hong B H 2012 Nanoscale 4 5527 [26] Regan W, Alem N, Alemán B, Geng B, Girit C, Maserati L, Wang F, Crommie M and Zettl A 2010 Appl. Phys. Lett. 96 113102 [27] Smith A D et al 2013 Nano Lett. 13 3237 [28] Leskelä M and Ritala M 2002 Thin Solid Films 409 138 [29] Saarenpää H, Sariola-Leikas E, Pyymaki Perros A, Kontio J M, Efimov A, Hayashi H, Lipsanen H, Imahori H, Lemmetyinen H and Tkachenko N V 2012 J. Phys. Chem. C 116 2336 [30] Hakola H, Perros A P, Myllyperkiö P, Kurotobi K, Lipsanen H, Imahori H, Lemmetyinen H and Tkachenko N V 2013 Chem. Phys. Lett. 592 47 [31] Miikkulainen V, Leskelä M, Ritala M and Puurunen R L 2013 J. Appl. Phys. 113 021301 [32] Knez M, Nielsch K and Niinistö L 2007 Adv. Mater. 19 3425 [33] Puurunen R L 2005 J. Appl. Phys. 97 121301 [34] Leskelä M, Kemell M, Kukli K, Pore V, Santala E, Ritala M and Lu J 2007 Mater. Sci. Eng. C 27 1504 [35] Wang L, Travis J J, Cavanagh A S, Liu X, Koenig S P, Huang P Y, George S M and Bunch J S 2012 Nano Lett. 12 3706 [36] George S M 2010 Chem. Rev. 110 111 [37] Carcia P F, McLean R S, Reilly M H, Groner M D and George S M 2006 Appl. Phys. Lett. 89 031915 [38] Liang X, King D M, Groner M D, Blackson J H, Harris J D, George S M and Weimer A W 2008 J. Membr. Sci. 322 105 [39] Schwierz F 2010 Nat. Nanotechnology 5 487 [40] Kim W, Riikonen J, Li C, Chen Y and Lipsanen H 2013 Nanotechnology 24 395202 [41] Dekker J, Kolari K and Puurunen R L 2006 J. Vac. Sci. Technol. B 24 2350 [42] Sainiemi L and Franssila S 2007 J. Vac. Sci. Technol. B 25 801
7
Search
Collections
Journals
About
Contact us
My IOPscience
Exceptionally strong and robust millimeter-scale graphene–alumina composite membranes
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 355701 (http://iopscience.iop.org/0957-4484/25/35/355701) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 132.203.227.63 This content was downloaded on 05/03/2015 at 09:45
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 439501 (1pp)
doi:10.1088/0957-4484/25/43/439501
Corrigendum: Exceptionally strong and robust millimeterscale graphene–alumina composite (2014 Nanotechnology 25 355701) Maria Berdova1, Alexander Pyymaki Perros2, Wonjae Kim2, Juha Riikonen2, Tuomo Ylitalo3, Jouni Heino4, Changfeng Li2, Ivan Kassamakov3,4, Edward Hæggström3, Harri Lipsanen2 and Sami Franssila1 1
Department of Materials Science and Engineering, School of Chemical Technology, Aalto University, PO Box 16200, FI-00076, Finland 2 Department of Micro- and Nanosciences, School of Electrical Engineering, Aalto University, PO Box 13500, FI-00076, Finland 3 University of Helsinki, Department of Physics, PO Box 64, FI-00014, Helsinki, Finland 4 Helsinki Institute of Physics, PO Box 64, FI-00014, Helsinki, Finland E-mail: maria.berdova@aalto.fi and alexander.pyymaki.perros@aalto.fi Received 12 September 2014 Accepted for publication 12 September 2014 Published 9 October 2014
(Some figures may appear in colour only in the online journal) There is an error in the axis of figure 3(b). The corrected figure is shown below:
0957-4484/14/439501+01$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
Nanotechnology Nanotechnology 25 (2014) 355701 (7pp)
doi:10.1088/0957-4484/25/35/355701
Exceptionally strong and robust millimeterscale graphene–alumina composite membranes Maria Berdova1,5, Alexander Pyymaki Perros2,5, Wonjae Kim2, Juha Riikonen2, Tuomo Ylitalo3, Jouni Heino4, Changfeng Li2, Ivan Kassamakov3,4, Edward Hæggström3, Harri Lipsanen2 and Sami Franssila1 1
Department of Materials Science and Engineering, School of Chemical Technology, Aalto University, PO Box 16200, FI-00076, Finland 2 Department of Micro- and Nanosciences, School of Electrical Engineering, Aalto University, PO Box 13500, FI-00076, Finland 3 University of Helsinki, Department of Physics, PO Box 64, FI-00014, Helsinki, Finland 4 Helsinki Institute of Physics, PO Box 64, FI-00014, Helsinki, Finland E-mail: maria.berdova@aalto.fi and alexander.pyymaki.perros@aalto.fi Received 24 April 2014, revised 3 July 2014 Accepted for publication 9 July 2014 Published 12 August 2014 Abstract
Graphene has attracted attention as a potential strengthening material and functional component in suspended membranes as utilized in micro and nanosystems. Development of a practical and scalable fabrication process is a necessary step to allow the exceptional material properties of graphene to be fully exploited in composite structures. Using standard and scalable microfabrication processes, we fabricated free-standing chemical vapor deposition monolayer graphene-reinforced Al2O3 composite membranes, 0.5 mm in diameter, that are strong and robust. Bulge tests revealed that the graphene reinforcement increased the membrane fracture strength by a factor of at least three and maximum sustainable strain from 0.28% to at least 0.69%. We show that the graphene-reinforced membranes are even tolerant to significant cracking without loss of membrane integrity. The graphene composite membranes’ freestanding area of ∼200 000 μm2 is almost a thousand times larger than suspended graphene membranes reported elsewhere. The presented graphene composite membranes may be seen as representing an interesting new class of durable composite materials warranting further study and having potential for broad applicability in a variety of fields. S Online supplementary data available from stacks.iop.org/NANO/25/355701/mmedia Keywords: CVD graphene, atomic layer deposition, suspended membrane, mechanical strength, bulge test, aluminum oxide (Some figures may appear in colour only in the online journal) 1. Introduction
properties [1–4]. Monolayer graphene exhibits 1 TPa Young’s modulus and 130 ± 10 GPa fracture strength giving it the distinction of being the world’s strongest material [4]. Graphene is flexible and can withstand strains exceeding 20% [4–6]. These properties make graphene attractive for many applications: a transparent conductor in flexible displays, a strengthening gas-barrier layer in thin film windows, or an
Graphene has attracted attention primarily directed at harnessing its exceptional electrical properties for electronics and photonics but it also possesses extraordinary mechanical 5
Authors contributed equally to this work.
0957-4484/14/355701+07$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
Nanotechnology 25 (2014) 355701
M Berdova et al
Figure 1. Process flow for the practical and scalable fabrication of graphene–alumina composite membranes. (a) Al2O3 is deposited by ALD on a silicon substrate, (b) graphene transfer, (c) Al seed layer evaporation and second ALD Al2O3 deposition, and (d) photolithography and release-etch from the backside. Schematic cross-sectional view of membrane: alumina (70 nm)/graphene (monolayer)/alumina (30 nm) (e) and optical image of composite membrane (top view) (f).
embedded sensing layer in suspended thin film structures [7–11]. These and other studies on graphene-based composites demonstrate graphene’s potential for practical multifunctionality in different industrial applications [6, 10, 11]. Research on graphene-based composites has thus far primarily focused on exfoliated graphene [12–16]. Nevertheless, CVD graphene is of particular interest for industrial use because exfoliated graphene is impractical for real-world applications that require large-area high-quality graphene [17–19]. CVD graphene has been considered to feature low mechanical quality due to the presence of grain boundaries [20]. However, theoretical and experimental studies indicate that the grain boundaries in CVD graphene can reach the characteristics of pristine graphene provided post CVD processing avoid damaging the graphene layer [21, 22]. Recently, potential breakthroughs in the growth of pristine wafer-scale single-crystalline monolayer CVD graphene on germanium (110) could make the previous concerns a thing of the past [23, 24]. Being merely one atom thick, graphene is susceptible to damage during transfer to a substrate and posttransfer processing [9, 25, 26]. Suspended CVD graphene membranes are still small, 384 μm2 being the largest cavity fabricated to our knowledge [27]. These small dimensions preclude the use of graphene in large-area windows. Many composites can be produced by atomic layer deposition (ALD), a technique for growing conformal, uniform, and pinhole-free thin layers on complex structures [28–33]. This technique achieves sub-nm precision in layer thickness allowing, for example, construction of nanolaminates or nanometer-thick suspended membranes [34, 35]. ALD alumina (Al2O3) in particular has attracted a lot of attention since it meets the requirements of flexible large-area membranes intended as gas diffusion barriers or sensing
elements [36–38]. ALD Al2O3 is also commonly used as a top gate dielectric in graphene field effect transistors [39, 40]. Significant performance improvement and new functionality could be achieved by combining graphene with ALD layers. Here we incorporate ALD Al2O3 and monolayer CVD graphene to fabricate strong and large area free-standing membranes, whose ∼200 000 μm2 area is almost a thousand times larger than those reported previously [27]. These graphene–alumina composites exhibit improved mechanical properties in free-standing membranes compared to those of unreinforced ALD Al2O3 membranes. Additionally, using standard microfabrication processes and CVD graphene, our approach avoids problems related to graphene handling, for example by reducing the risk of membrane damage during transfer to a holey substrate. This is achieved by releasing the membrane by plasma etch only as the very last step of the processing. Additionally, as the graphene layer is sandwiched between two protective alumina layers, post-transfer damage is minimized. In light of the recent advances in producing single-crystalline large-area graphene, our results gain more significance as the exploitation of graphene’s exceptional mechanical properties can be expected to attract more attention in large-area applications [23, 24].
2. Methods 2.1. Practical and scalable fabrication method
Double side polished silicon substrates were coated with ALD Al2O3 on both sides of the wafers as depicted in figure 1(a) (70 nm on the wafer topside and 20 nm on the backside). The alumina on top serves as the lower part of the membrane whereas alumina on wafer backside serves as the mask layer 2
Nanotechnology 25 (2014) 355701
M Berdova et al
Figure 2. Bulge plots for suspended membranes subjected to differential pressure. (a) 3D bulge plots obtained by SWLI as a function of
increasing differential pressure from 0 to 2 bar across a composite membrane suspended over a cavity. (b) The corresponding 2D displacement h as a function of differential pressure P (total measurement time >1 h).
for the membrane release etch. ALD Al2O3 is an ideal etch mask and etch stop due to its low dry etch rate, ≪1 nm min−1 [41, 42]. Following the ALD step, monolayer graphene was transferred as described in detail elsewhere (see also supporting information) [43]. Monolayer CVD graphene was synthesized utilizing photo-thermal CVD [43]. Next, a ∼2 nm thick aluminum layer was evaporated onto graphene. This thin seeding layer was rapidly oxidized to Al2O3 in air [44]. This was followed by the second ALD Al2O3 (30 nm) layer. The resulting ∼100 nm thin sandwich membrane comprised three layers: 70 nm alumina/graphene (monolayer)/30 nm alumina. The surface morphology of the membrane was measured by atomic force microscopy after deposition of the 30 nm alumina film. The mean surface roughness of the composite membrane is 3.1 nm. This roughness is higher than ALD Al2O3 deposited on atomically flat silicon wafers and is attributed to the underlying ∼2 nm thick aluminum layer evaporated onto graphene. The membranes were released by deep reactive ion etching (DRIE) using the Bosch cyclic etch chemistry. The DRIE process generated a pattern of 0.5 mm diameter holes in the silicon substrate. Care was taken to ensure that the graphene layer completely covered the suspended region and that it extended well beyond the cavity to avoid non-uniform strain at the graphene edges [14]. The fabrication is described in detail in the supporting information. Reference membranes were fabricated as described above but without graphene. It should be noted that while the total aluminum oxide thickness of the composite and reference membranes is identical, the composite membrane is composed of two thinner (70 nm and 30 nm) Al2O3 membranes separated by the graphene layer. This division of the Al2O3 into thinner layers is not trivial as thinner alumina features a higher strain threshold for cracking [45]. Fabricating a structure that avoids dividing the Al2O3 layer by placing the graphene layer only on the top of the alumina
layer was not considered a practical option due to risks associated with adhesion loss, poor stress transfer at high strains, and exposing the unprotected graphene layer to damage [14, 46–48]. All of these could diminish the membrane quality. 2.2. Bulge method
The bulge method was used to determine Young’s modulus and residual stress in the composite membranes [49]. This technique is based on measuring the displacement h of the membrane center as a function of differential pressure P, see figure 2. Increasing overpressure was achieved by feeding argon gas into the cavity of the test structure (see supporting information). With gradually increasing overpressure (discrete steps) the displacement increased consistently. The membrane center deflection was measured by a scanning white light interferometer (SWLI) after applying increasing overpressure by feeding argon gas into the cavity of the test structure [50]. Each interferometer scan lasted ∼8 min. The calculation of effective Young’s modulus is based on the spherical cap model for a suspended film clamped at its edges [51, 52]. The effective modulus E is determined by the slope of the linear region of the stress–strain curve and the intercept with the σ-axis is defined as the residual stress. The strain ε was calculated as the fractional increase in arc length of the bulged membrane across the membrane diameter (2a): ε=
2h2 3a 2
(1)
and the residual stress σ was defined as: σ=
where t is the film thickness. 3
Pa2 , 4ht
(2)
Nanotechnology 25 (2014) 355701
M Berdova et al
Figure 3. Comparison of unreinforced alumina and graphene–alumina composite membranes. (a) Deflection h of two graphene–alumina
composite membranes and reference alumina membranes versus differential pressure P as measured during the bulge test. (b) The corresponding stress versus strain for the two graphene–alumina composite membranes and reference alumina membranes. The composite membranes were intact after being subjected to 2.0 bar differential pressure whereas the reference alumina membranes were destroyed already at ∼0.6 bar differential pressure.
3. Results and discussion
3.2. Fracture strength of the membranes
In an earlier study, 75 nm thick ALD Al2O3 membranes of 0.5 mm in diameter withstood 3 bar shock pressure [50]. This is in contrast to the present result where the Al2O3 reference membranes failed consistently at ∼0.6 bar. The discrepancy is due to the different pressure ramp rates used in the two experiments. This is known as time-dependent failure; the fracture response of ceramic materials varies with load rate [58–61]. When the ramp rate is small, as in the current experiment, the material’s apparent strength diminishes due to slow crack propagation. In the present work each measured deflection was maintained for ∼8 min (total measurement time >1 h), whereas in the fracture shock pressure experiments the entire measurement was carried out in a mere 30 s (i.e. at fast-fracture conditions) [58, 61].
3.1. Effective Young’s modulus and residual stress of the membranes
The membranes were flat before and after each interferometer scan series, indicating the presence of tensile residual stress typical of ALD Al2O3 [50, 53]. The composite membranes deflected 23–26 μm at 2 bar, corresponding to 0.67–0.69% strain, see figure 3. For comparison, the equally thick reference Al2O3 membranes reached a maximum sustainable deflection of 16–20 μm already at ∼0.6 bar (maximum fracture pressure), corresponding to 0.26% strain. For these alumina membranes, the effective Young’s modulus was 161 ± 23 GPa and the residual stress was 298 ± 61 MPa based on the measured stress–strain curves. These values are in line with well-established literature values: the reported Young’s modulus for Al2O3 is 154–220 GPa and the residual stress is 320–500 MPa (for films grown at 177–220 °C) [35, 50, 53–57]. For our composite membranes the corresponding values were 180 ± 15 GPa and 352 ± 59 MPa, respectively. Here the instrument-related uncertainty in the results is negligible as the SWLI resolution is ±20 nm (versus ∼20 μm deflection) and the pressure control is ±10 mbar (versus 0.6–2.0 bar test pressures). The validity of the ‘ruleof-mixtures’ model (based on volume fractions of composite components) used to determine the effective Young’s modulus for the composite membrane is valid, even in the case of monolayer graphene. In our case the volume fraction of graphene in the composite membrane is merely ∼0.35%, i.e., practically negligible. Therefore, the Young’s modulus and residual stress of the composite membranes should be nearly equal to those of the reference membranes as also is evident in our measurement results.
3.3. Crack tolerance of the composite membrane
Scanning electron microscopy (SEM) was utilized to confirm the composite membrane integrity after bulge testing. SEM images, figure 4, indicate channeling type cracks in the Al2O3 layer of composite membranes brought to 2 bar differential pressure during testing [62]. The existence of cracks has significant implications: normally onset of cracking is in the suspended ceramic membrane cavity area and does not extend from the membrane edge. Using graphene to increase the suspended membrane tolerance against cracking may provide an interesting path for detailed study of cracking failure in alumina free-standing membranes. 3.4. Raman analysis of the composite membrane
Raman analysis of the graphene after membrane processing (i.e. after releasing the free-standing window) displays a typical Raman fingerprint of high quality monolayer graphene 4
Nanotechnology 25 (2014) 355701
M Berdova et al
Figure 4. Cracking of the composite membrane. SEM micrographs of cracks in a composite membrane after bulge measurements that brought the differential pressure up to 2 bar. Composite membrane remains intact despite the presence of significant cracking.
Figure 5. Raman analysis of the membrane area with Al2O3 cracking after bulge testing to 2 bar. (a) Optical and (b)–(d) Raman mapping images of D, G, and 2D-line. (e), (f) Raman images of D/G and 2D/G. (g) Raman spectra corresponding to the points indicated by Arabic numbers in (d). The Raman spectra are offset for clarity and scale bars correspond to 9 μm.
in figure S1. This indicates the embedded CVD graphene incurred no damage during processing. Moreover, the detailed Raman characterization performed even after bulge testing to 2 bar reveals no impairment of the graphene film as shown in figure 5. Particularly noteworthy is that even in and around the cracked regions of the alumina the graphene showed no signs of damage as evidenced by a very low D/G ratio in Raman maps and practically non-existent D-band in the Raman spectra at around 1250 cm−1. Mapping of the spectral position of the 2D band reveal no significant variation, particularly no correlation with the cracking (not shown here). Considering that the
2D band position in graphene is susceptible to strain, it can be concluded that cracking has not induced additional strain to graphene [63]. In fact, a slight increase in the 2D peak can be observed in figure 5(d) around the cracks which we speculate might be related to a loss of adhesion of the alumina. This observation is of great importance as it implies that despite severe cracking of the alumina the embedded CVD graphene film remains intact even at the grain boundaries and is capable of maintaining the integrity of the membrane. These results indicate good stitching of the grain boundaries in the embedded CVD graphene [21, 22].
5
Nanotechnology 25 (2014) 355701
M Berdova et al
is still of high quality. (4) The fabrication process is straightforward, scalable, and easy to adapt to other ALD materials and to more complex nanolaminate structures. (5) Our approach also permits implementing additional components to the structure since all processing is carried out on standard silicon wafers. As graphene responds electrically to deformation, further processing could, e.g., include fabrication of on-chip read-out circuitry for sensing applications. (6) Due to the substantially improved mechanical robustness demonstrated in this work, these graphene-sandwich composite membranes can potentially be viewed as a new class of robust thin films.
Acknowledgments Figure 6. Lower differential pressure bulge testing of composite
This work was undertaken at the Micronova Nanofabrication Center of Aalto University. The research leading to these results has received funding from the European Union Seventh Framework Programme under grant agreement n° 604391 Graphene Flagship. This research project was also supported by Academy of Finland under the research project (project number 263566) two-dimensional programmable materials for optical and electronic applications (TODIMA). The authors also acknowledge additional support from Academy of Finland and TEKES—the Finnish Funding Agency for Technology and Innovation. M B is funded by Aalto School of Chemical Technology PhD student grant and by The Finnish National Graduate School in Nanoscience (NGS Nano).
membrane. Deflection behavior of a graphene–alumina composite membrane and reference alumina membranes as measured by bulge test. No cracking was observed in the composite membrane brought to 1.2 bar. 3.5. Lower differential pressure bulge testing of composite membrane
To investigate how the composite membranes would behave under lower differential pressure, another graphene–alumina composite was bulge tested to 1.2 bar, see figure 6. SEM inspection revealed no cracking in the alumina in the bottom or top side of the membrane. The results suggest that graphene reinforcement is a promising facilitator for constructing large vacuum-tight thin film windows. Current state-of-the-art large-area windows are based on low-pressure CVD Si3N4 thin films, which require a mesh support that occupies a significant fraction, ∼23%, of the window region [64]. It is envisioned that incorporating graphene into this and similar kinds of structures will increase the sensing area by allowing a reduction of mesh area. This would increase transmission and aperture by reducing grid-induced collimation.
References [1] Geim A K and Novoselov K S 2007 Nat. Mater. 6 183 [2] Bonaccorso F, Sun Z, Hasan T and Ferrari A C 2010 Nat. Photonics 4 611 [3] Koppens F H L, Chang D E and García de Abajo F J 2011 Nano Lett. 11 3370 [4] Lee C, Wei X, Kysar J W and Hone J 2008 Science 321 385 [5] Liu F, Ming P and Li J 2007 Phys. Rev. B 76 064120 [6] Young R J, Kinloch I A, Gong L and Novoselov K S 2012 Compos. Sci. Technol. 72 1459 [7] Bunch J S, Verbridge S S, Alden J S, van der Zande A M, Parpia J M, Craighead H G and McEuen P L 2008 Nano Lett. 8 2458 [8] Nair R R, Blake P, Grigorenko A N, Novoselov K S, Booth T J, Stauber T, Peres N M R and Geim A K 2008 Science 320 1308 [9] Li X, Zhu Y, Cai W, Borysiak M, Han B, Chen D, Piner R D, Colombo L and Ruoff R S 2009 Nano Lett. 9 4359 [10] Singh V, Joung D, Zhai L, Das S, Khondaker S I and Seal S 2011 Prog. Mater. Sci. 56 1178 [11] Huang X, Qi X, Boey F and Zhang H 2012 Chem. Soc. Rev. 41 666 [12] Kim H, Miura Y and Macosko C W 2010 Chem. Mater. 22 3441 [13] Potts J R, Dreyer D R, Bielawski C W and Ruoff R S 2011 Polymer 52 5 [14] Young R J, Gong L, Kinloch I A, Riaz I, Jalil R and Novoselov K S 2011 ACS Nano 5 3079
4. Conclusion Our work highlights the significant improvements that the incorporation of a monolayer of CVD graphene can confer to ALD Al2O3 membranes with mm-scale diameter. (1) Strengthening of the membrane structure, (2) the division of aluminum oxide into two layers results in a sandwich structure with a higher strain threshold than the unreinforced membrane, (3) improved tolerance against cracking: the composite structure remains intact even at high strain in the presence of significant cracking. The graphene reinforced membranes sustain more than three times higher differential pressure and more than twice the strain compared to corresponding unreinforced Al2O3 membranes. It should be noted that even though cracks developed in the composite, it still maintained the differential pressure which combined with the Raman analysis indicates that the monolayer CVD graphene 6
Nanotechnology 25 (2014) 355701
M Berdova et al
[43] Riikonen J, Kim W, Li C, Svensk O, Arpiainen S, Kainlauri M and Lipsanen H 2013 Carbon 62 43 [44] Kim S, Nah J, Jo I, Shahrjerdi D, Colombo L, Yao Z, Tutuc E and Banerjee S K 2009 Appl. Phys. Lett. 94 062107 [45] Jen S-H, Bertrand J A and George S M 2011 J. Appl. Phys. 109 084305 [46] Gong L, Kinloch I A, Young R J, Riaz I, Jalil R and Novoselov K S 2010 Adv. Mater. 22 2694 [47] Gong L, Young R J, Kinloch I A, Riaz I, Jalil R and Novoselov K S 2012 ACS Nano 6 2086 [48] Hashimoto A, Suenaga K, Gloter A, Urita K and Iijima S 2004 Nature 430 870 [49] Bromley E I 1983 J. Vac. Sci. Technol. B 1 1364 [50] Berdova M, Ylitalo T, Kassamakov I, Heino J, Törmä P T, Kilpi L, Ronkainen H, Koskinen J, Hæggström E and Franssila S 2014 Acta Mater. 66 370 [51] Beams J W 1959 Structure and Properties of Thin Films (New York: Wiley) [52] Small M K and Nix W D 1992 J. Mater. Res. 7 1553 [53] Ylivaara O M E et al 2013 Thin Solid Films 552 124 [54] Tapily K, Jakes J E, Stone D S, Shrestha P, Gu D and Baumgart H Elmustafa A A 2008 J. Electrochem. Soc. 155 H545 [55] Miller D C et al 2009 J. Appl. Phys. 105 093527 [56] Miller D C, Foster R R, Jen S-H, Bertrand J A, Cunningham S J, Morris A S, Lee Y-C, George S M and Dunn M L 2010 Sensors Actuators A 164 58 [57] Krautheim G, Hecht T, Jakschik S, Schröder U and Zahn W 2005 Appl. Surf. Sci. 252 200 [58] Krausz A S and Krausz K 1986 Fracture Mechanics of Ceramics ed R C Bradt, A G Evans, D P H Hasselman and F F Lange (New York: Springer) pp 333–40 [59] Jadaan O M, Nemeth N N, Bagdahn J and Sharpe W N Jr 2003 J. Mater. Sci. 38 4087 [60] Nemeth N N, Jadaan O M and Gyekenyesi J P 2005 Lifetime Reliability Prediction of Ceramic Structures Under Transient Thermomechanical Loads (Cleveland: NASA) NASA/TP—2005-212505 [61] Nemeth N N, Jadaan O M, Palfi T and Baker E H 2005 Fract. Mech. Ceram. 14 555 [62] Hutchinson J W 1996 Stress and Failure Modes in Thin Films and Multilayers (Notes for a Dcamm Course) (Lyngby: Technical University of Denmark) [63] Mohiuddin T et al 2009 Phys. Rev. B 79 205433 [64] Törmä P T et al 2013 IEEE Trans. Nucl. Sci. 60 1311
[15] Srivastava I, Mehta R J, Yu Z-Z, Schadler L and Koratkar N 2011 Appl. Phys. Lett. 98 063102 [16] Wang J, Li Z, Fan G, Pan H, Chen Z and Zhang D 2012 Scr. Mater. 66 594 [17] Kim K S, Zhao Y, Jang H, Lee S Y, Kim J M, Kim K S, Ahn J, Kim P, Choi J and Hong B H 2009 Nature 457 706 [18] Li X et al 2009 Science 324 1312 [19] Bae S et al 2010 Nat. Nanotechnology 5 574 [20] Huang P Y et al 2011 Nature 469 389 [21] Grantab R, Shenoy V B and Ruoff R S 2010 Science 330 946 [22] Lee G-H et al 2013 Science 340 1073 [23] Wang G et al 2013 Sci. Rep. 3 2465 [24] Lee J-H et al 2014 Science 344 286 [25] Kang J, Shin D, Bae S and Hong B H 2012 Nanoscale 4 5527 [26] Regan W, Alem N, Alemán B, Geng B, Girit C, Maserati L, Wang F, Crommie M and Zettl A 2010 Appl. Phys. Lett. 96 113102 [27] Smith A D et al 2013 Nano Lett. 13 3237 [28] Leskelä M and Ritala M 2002 Thin Solid Films 409 138 [29] Saarenpää H, Sariola-Leikas E, Pyymaki Perros A, Kontio J M, Efimov A, Hayashi H, Lipsanen H, Imahori H, Lemmetyinen H and Tkachenko N V 2012 J. Phys. Chem. C 116 2336 [30] Hakola H, Perros A P, Myllyperkiö P, Kurotobi K, Lipsanen H, Imahori H, Lemmetyinen H and Tkachenko N V 2013 Chem. Phys. Lett. 592 47 [31] Miikkulainen V, Leskelä M, Ritala M and Puurunen R L 2013 J. Appl. Phys. 113 021301 [32] Knez M, Nielsch K and Niinistö L 2007 Adv. Mater. 19 3425 [33] Puurunen R L 2005 J. Appl. Phys. 97 121301 [34] Leskelä M, Kemell M, Kukli K, Pore V, Santala E, Ritala M and Lu J 2007 Mater. Sci. Eng. C 27 1504 [35] Wang L, Travis J J, Cavanagh A S, Liu X, Koenig S P, Huang P Y, George S M and Bunch J S 2012 Nano Lett. 12 3706 [36] George S M 2010 Chem. Rev. 110 111 [37] Carcia P F, McLean R S, Reilly M H, Groner M D and George S M 2006 Appl. Phys. Lett. 89 031915 [38] Liang X, King D M, Groner M D, Blackson J H, Harris J D, George S M and Weimer A W 2008 J. Membr. Sci. 322 105 [39] Schwierz F 2010 Nat. Nanotechnology 5 487 [40] Kim W, Riikonen J, Li C, Chen Y and Lipsanen H 2013 Nanotechnology 24 395202 [41] Dekker J, Kolari K and Puurunen R L 2006 J. Vac. Sci. Technol. B 24 2350 [42] Sainiemi L and Franssila S 2007 J. Vac. Sci. Technol. B 25 801
7
