Highly Crumpled All-Carbon Transistors for Brain Activity Recording

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Highly crumpled all-carbon transistors for brain activity recording Long Yang, Yan Zhao, Wenjing Xu, Enzheng Shi, Wenjing Wei, Xinming Li, Anyuan Cao, Yanping Cao, and Ying Fang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b03356 • Publication Date (Web): 08 Dec 2016 Downloaded from http://pubs.acs.org on December 11, 2016

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Highly crumpled all-carbon transistors for brain activity recording Long Yang1, Yan Zhao2, Wenjing Xu3, Enzheng Shi3, Wenjing Wei1, Xinming Li1, Anyuan Cao3, Yanping Cao2* and Ying Fang1,4*

1

CAS Center for Excellence in Nanoscience, National Center for Nanoscience and

Technology, Beijing 100190, P. R. China. 2

Department of Engineering Mechanics, Tsinghua University, Beijing 100084, P. R. China.

3

Department of Materials Science and Engineering, College of Engineering, Peking

University, Beijing 100871, P. R. China. 4

CAS Center for Excellence in Brain Science and Intelligence Technology, 320 Yue Yang

Road, Shanghai 200031, P. R. China. *Corresponding authors. E-mail: [email protected]; [email protected]

KEYWORDS. graphene, carbon nanotubes, transistors, neural probes, crumpling

ABSTRACT. Neural Probes based on graphene field-effect transistors have been demonstrated. Yet, the minimum detectable signal of graphene transistor-based probes is inversely proportional to the square root of the active graphene area. This fundamentally limits the scaling of graphene transistor-based neural probes for improved spatial resolution in brain activity recording. Here, we address this challenge using highly crumpled all-carbon transistors formed by compressing down to 16% of its initial area. All-carbon transistors, 1

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chemically synthesized by seamless integration of graphene channels and hybrid graphene/carbon nanotube (hGC) electrodes, maintained structural integrity and stable electronic properties under large mechanical deformation, whereas stress-induced cracking and junction failure occurred in conventional graphene/metal transistors. Flexible, highly crumpled all-carbon transistors were further verified for in vivo recording of brain activity in rats. These results highlight the importance of advanced material and device design concepts to make improvements in neuroelectronics.

Neural probe technologies have contributed significantly to our understanding of brain function1, and are currently applied for diagnosis and treatment of brain disorders such as epilepsy2, Parkinson's3, and depression4. Over the past decades, there has been continuous effort to develop neural probes with improved capabilities in terms of spatial resolution5, 6, density7, 8, and tissue integration9, 10. One area of growing interest is the development of transistor-based neural probes11-13 in which the bioelectrical neural activity modulates the conductance of the semiconductor channel through capacitive coupling. In particular, the mechanical and electrical properties of graphene14-18 make it an attractive candidate for flexible transistor-based neural probes. However, similar to metal microelectrodes19, the minimum detectable signal of graphene transistor-based probes is inversely proportional to the square root of the active surface area.20, 21 As a result, reduction of the active graphene area leads to decreased sensitivity, which poses a fundamental challenge to improve the spatial resolution of graphene transistor-based neural probes. This difficulty is traditionally addressed in metal microelectrodes by applying high-surface-area coatings, such as spongy platinum black or conductive polymers, on the metal surfaces to decrease the impedance and 2


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thermal noise at the electrode/electrolyte interface22,

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. However, this method is not

applicable to graphene transistor-based neural probes which are limited by the 1/f noise in the active graphene channel.20 Until now, there still lacks an efficient and robust route to effectively improve the spatial resolution of graphene transistor-based neural probes.

Here, we design and build a graphene transistor-based neural probe with substantially compressed projected area while keeping the active graphene area unchanged to achieve both high sensitivity and improved spatial resolution. When subjected to large biaxial compressive forces, thin sheets of stiff materials tend to crumple, forming a network of high-aspect-ratio ridges that meet at vertices.24-26 The key challenge lies in material and device design to avoid not only cracking of the active semiconductor and its electrodes but also delamination failure of their junction under large mechanical deformation.27-29 Graphene has high fracture strain of approximately 13%,30 and crumples easily on a soft substrate when subjected to in-plane compression owing to its extremely small wrinkling strain.31, 32 As for the electrode materials, typical thin metal films, such as gold films, fracture at tensile strains below ~1%,33 which places severe constraints on the construction of highly crumpled electronics. In addition, the mechanical mismatch between graphene and metal electrodes could result in interfacial stress during deformation, increasing the risk of delamination failure at graphene-metal junctions. On the other hand, ultrathin CNT sheets have been favorably used as electrode materials in flexible electronics owing to their metallic conductivity and high flexibility.34 Hence, we develop a highly crumpled, but sufficiently robust neural probe based on all-carbon transistors. This conceptually new approach enables in vivo brain activity recording with high sensitivity and substantially improved spatial resolution. 3


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Figure 1a illustrates a schematic representation of an all-carbon transistor formed by the seamless integration of a graphene channel and hybrid graphene/CNT electrodes through chemical synthesis, as previously reported35. Briefly, an ultrathin and porous CNT film was patterned into source/drain electrodes and interconnects and transferred onto a copper substrate before the chemical vapor deposition (CVD) of graphene (Figure S1). During CVD, semiconducting graphene and metallic graphene/CNT hybrid (Figure 1b) were formed on the bare and CNT-coated copper surface, respectively, before coalescing into a seamless all-carbon transistor (Figure S2a-d). The all-carbon transistor was then transferred onto a biaxially pre-stretched elastomer substrate. When the biaxial pre-strain in the elastomer was simultaneously released, the all-carbon transistor was compressed and spontaneously developed into a highly crumpled structure. Figure 1c and 1d show representative scanning electron microscope (SEM) and optical images of a highly crumpled all-carbon transistor that has been compressed to 16% of its initial area. The microscopic morphology of the highly crumpled all-carbon transistor is characterized by a high density of ridges and vertices into which the strain energy is localized. Significantly, the all-carbon transistor maintained its structural integrity without any detectable crack upon aggressive biaxial compression. In addition, the crumpled graphene and hGC regions are morphologically indistinguishable, indicating the mechanical homogeneity throughout the device.

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Figure 1. Crumpling of all-carbon transistor. (a) Schematic illustration of an all-carbon transistor with a graphene channel and hGC electrodes. (b) Cross-sectional SEM image of a hGC electrode supported on 200 nm PMMA. (c) SEM image of a highly crumpled transistor. The active graphene channel was outlined by red box. (d) Optical image of the all-carbon transistor. The optical transparency of the graphene channel is higher than that of the hGC electrodes. (e) 3D confocal images of the all-carbon transistor under increased area strain, shown in top view, front sectional view, and perspective view. The active graphene channel was outlined by red boxes in the top view images. Scale bar, 100 µm. All images in c, d, and e were taken on the same all-carbon transistor. Three-dimensional (3D) confocal imaging was performed to monitor the morphology change of the all-carbon transistor under increased biaxial compressive strain (Figure 1e). Here, we define the in-plane area strain as || /, where A0 and A are the initial and projected area of the crumpled transistor, respectively. Upon the controlled release of the 5


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pre-stretched substrates, the all-carbon transistor gradually scales down in the lateral XY dimension but extrudes in the transverse Z dimension. Eventually, an initial two-dimensional (2D) transistor transforms into a 3D one consisting of high-aspect-ratio ridges and vertices, which greatly reduces the projected area of the transistor. We note that the formation of the highly crumpled structures is a self-organizing process that can be easily introduced over large areas at low cost.36, 37

We next evaluated the electrical integrity of the seamlessly integrated all-carbon transistors under compression. For comparison, we also tested analogous, layer-by-layer stacked graphene transistors with metal electrodes formed by conventional microfabrication techniques (Figure S2e and f). The metal electrodes, with the same layout as the hGC electrodes, consist of 50-nm-thick gold with 5-nm-thick chromium as adhesion layer. Prior to the transfer of the transistors, the elastomer was uniaxially pre-stretched by 400% perpendicular to the channel direction. When the pre-strain in the elastomer was relaxed, localized ridge instabilities formed in both the all-carbon and graphene/gold transistors (Figure 2a-h and Movie S1 and S2). The configurations of the ridges depend on—beside the elastic properties of the soft substrate—the mechanical properties of graphene and its electrodes, including moduli and thickness. Figure 2i summarizes the ridge height of both the graphene channels and the electrodes as a function of the compressive strain | |

⁄ , where W0 is the initial channel width of the transistor and W is the projected channel width, of the two transistors under compression (see Figure S4 for more details on the statistics of ridge height). Importantly, the all-carbon transistor displays a relatively uniform distribution of ridge height throughout the entire device. The mismatch of ridge 6


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height between graphene and the hGC electrodes is limited to 30% under a large compressive strain of 56% (Figure 2j). In contrast, the mismatch of ridge height between graphene and the gold electrodes reaches 120% under 56% strain. Figure 2k presents the normalized resistance variation as a function of the compressive strain for the two transistors. There is no observable change in the source-drain resistance of the all-carbon transistors up to 56% strain. On the other hand, the graphene/gold transistor shows a pronounced rise of the source-drain resistance above 13% strain. Under 56% strain, the source-drain resistance of the graphene/gold transistor has been drastically increased by 450%, indicating a serious deterioration of its electrical performance.

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Figure 2. Comparison of all-carbon and graphene/gold transistors under compression. (a~d) Optical images of an all-carbon transistor under uniaxial compression. (e~h) Optical images of a graphene/gold transistor under uniaxial compression. Red arrows in a and e indicate the edges of the graphene channels. (i) Ridge height versus compressive strain for gold electrode (black), gold-contacted graphene channel (grey), hGC electrode (red), and hGC-contacted graphene channel (pink), respectively. Data points represent the average height of six random positions in each region, and error bars indicate the standard deviation. See also Figure S4. (j) Ridge height mismatch versus compressive strain. Black data points represent the mismatch between gold and graphene, and red data points represent the mismatch between hGC and graphene. (k) Source-drain resistance variation versus compressive strain for the all-carbon (red) and graphene/gold (black) transistors, respectively. To understand the loss of electrical integrity of the graphene/gold transistor under large compression, nonlinear finite element (FE) analysis was performed to track dynamically the deformation behavior of both gold and graphene films on strongly pre-stretched substrates.38 Figure 3a illustrates the evolution of wrinkling mode of both the gold and graphene films 8


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under in-plane compression with a uniaxial substrate pre-stretch of 100%. Upon the release of the pre-stretched substrate, the films are compressed and form wrinkles. With increased compressive strain, ridge mode occurs as a consequence of the altered nonlinearity in the substrate induced by the large pre-stretch. The formation of the ridges relaxes compression in its neighborhood, and the maximum tensile strain in the film concentrates along the longitudinal axes of the ridges. Figure 3a further summarizes the dependence of maximum tensile strains on the overall compression of both the gold and graphene films. The tensile strain increases with the compression until the ridges are fully developed. Due to the small fracture strain of the gold film (1%)33, cracking may occur with the formation of ridges. Our experimental results further reveal that the crumpling of the films was accompanied by their partial detachment from the substrate in response to interfacial stresses between film and substrate (Figure S5)31,

39

. Film-substrate detachment moves the neutral plane to the

mid-plane of the gold and graphene films (Figure S6). As a result, the tensile strain, which is mainly caused by bending, is slightly decreased in the gold films upon gold-substrate detachment (Figure 3a, i). Significantly, the tensile strain in the graphene film becomes trivial upon graphene-substrate detachment, owing to its atomically-thin thickness (Figure 3a, ii and Supplementary information-Mechanical analysis).

We note that the large height of the sharp ridges in the detached graphene film may decrease the coupling efficiency between graphene and neurons. Therefore, we performed further experiments to control the height of the ridges by increasing the adhesion between graphene and the substrate. As shown in Figure S7, 8 nm Cu or 15 nm SiO2 film was deposited as adhesion layer between graphene and the biaxially pre-stretched elastomer. No 9


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local sharp ridges were observed during the compression of the graphene film, and the average height of the ridges in the graphene film was decreased to ca. 4 µm.

Figure 3. Cracking in graphene/gold transistor. (a) Simulated maximum tensile strain versus compression curves of both the gold (i) and graphene (ii) films with (blue) and without (red) substrate detachment. The substrate was uniaxially pre-stretched by 100%. The inset figures show the strain maps of three typical deformation states in the two films: (1) the occurrence of a single mountain ridge; (2) a fully-developed mountain ridge; (3) the formation of new mountain ridges. (b) SEM images and FE simulation of graphene/gold transistor under biaxial compression. (i), SEM image of a highly crumpled graphene/gold transistor with an area compression of 84%. (ii), Enlarged SEM view of small cracks around the vertices of the crumpled gold electrode. (iii), Simulation of tensile strain map in gold film under biaxial compressive strain of 20%. The substrate was eqi-biaxially pre-stretched by 100% during simulation. (iv), Enlarged SEM view of a long crack in the crumpled gold electrode. (v), Enlarged SEM view of the graphene channel. No cracking was observed in graphene. Under biaxial compression, our FE simulations reveal the formation of high-aspect-ratio ridges and vertices in the gold film, and the tensile strain in the film has been dramatically 10


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increased compared with that of uniaxial compression (Figure 3b). The maximum strain locates at the vertices in the gold films and reaches as high as 9.2% even considering gold-substrate detachment (Figure 3b, iii). As a result, small cracks extensively emerge around the vertices of the crumpled gold electrodes (Figure 3b, ii). Moreover, cracks can easily grow along the ridges with high stress concentrations. Long cracks, propagating up to tens of micrometers, have been observed in the crumple gold electrodes (Figure 3b, iv). These long cracks cause a pronounced rise of electrical resistance in the graphene/gold transistor. For comparison, the highly crumpled graphene channel maintains its structural integrity without any crack under large deformation (Figure 3b, v), owing to both its high fracture toughness30 and atomically-thin thickness.

Another important process that contributes to the loss of electrical conductivity in the graphene/gold transistor is the junction failure between graphene and gold layers under large deformation (Figure 2 and Figure S8). Due to mismatched mechanical properties between graphene and gold, concentrated interfacial stress appears at their junction during deformation, as illustrated by our simulation results (Figure S9). As the interfacial stress surpasses the adhesion strength between the graphene and gold layers, junction delamination occurs, leading to a further deterioration of the electrical conductivity of the graphene/gold transistor. In contrast, junction failure has been effectively prevented in the chemically synthesized all carbon transistors due to both their seamless structure and the mechanical similarity between graphene and carbon nanotubes.

To examine the ability of the highly crumpled all-carbon transistors to probe ionic signals, 11


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their electrical properties have been systematically characterized in a liquid-gated configuration under controlled compression. In a typical measurement, all-carbon transistors were immersed into phosphate buffer (PB) solution at pH 7. To modulate the carrier density in graphene, a Ag/AgCl reference electrode was used to apply liquid-gate voltage through the double-layer capacitance at the graphene-electrolyte interface. Figure 4a illustrates the transfer curves of a representative all-carbon transistor subjected to incremental biaxial compressive strain. The semiconductor channel of the transistor has an original lateral dimension of 100 µm×100 µm with a sensitivity of 5 µV in the electrolyte solution (Figure S10). As summarized in Figure 4b, both the on-current value and the transconductance of the all-carbon transistor are relatively stable under aggressive area strain. The all-carbon transistor retains 90% of its on-current value and 80% of its transconductance even under an area strain of 80%. These results indicate that the all-carbon transistor maintained its active semiconductor area accessible to the electrolyte solution under aggressive compression.

Figure 4. Electrical performance of highly crumpled all-carbon transistor in ionic solution. (a) Source-drain current (I) versus liquid-gate voltage (Vg) curves of an all-carbon transistor under incremental biaxial compression. The source-drain bias voltage was 60 mV. (b) On-current (Ion, red balls) and normalized transconductance of hole carries (gm, black balls) as functions of the area strain. The on-current was measured at Vg-VDirac=-0.35 V.

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Obtaining sensitive and stable readout of brain activity is of great importance for both basic and applied neuroscience. Penetrating intracortical microelectrodes have been extensively used to record single-unit activity of neurons. Unfortunately, these hard microelectrodes suffer from poor long-term stability due to their mechanical mismatch with soft tissues.40 Recently, syringe-injectable mesh electronics have been demonstrated for intracortical single-unit recordings, which allows tight tissue integration and minimal immunoreactivity

in

the

brain.10

As

an

alternative

to

single-unit

recordings,

electrocorticography (ECoG)41 measures local population-level activity from the surface of the brain. ECoG is less invasive than penetrating microelectrodes, and has been widely used in epileptic patients to delineate seizure foci. Next, flexible, highly crumpled all-carbon transistors were applied as ECoG probes to record penicillin-induced epileptic activity in rats (Figure 5a and b). In an acute study, an anaesthetized rat was placed in a stereotactic frame, and a craniotomy was performed to allow access to the left cortex of the rat brain. Highly crumpled all-carbon transistors, compressed to 18% of the initial area, were placed over the cortical surface to record the ECoG activity (Figure 5c and Figure S11). The highly crumpled transistors closely conform to the brain surface owing to its flexible nature. Penicillin G sodium was intraperitoneal injected to induce epilepsy in the rat. Penicillin prevents GABA-mediated inhibitory control of the pyramidal neurons by blocking the GABA receptors and leads to rhythmic epileptiform discharges. A representative real-time recording of induced-epilepsy activity is illustrated in Figure 5d, and Figure 5e is the normalized time-frequency spectral analysis of the time-series data. Three periods can be clearly identified: basal activity, latent period, and epileptiform activity period (Figure 5d). The latent 13


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period appeared immediately after penicillin injection and lasted for several minutes, during which the delta waves (0-4 Hz) were suppressed (Figure 5g). After the induction of the epileptiform activity, population spikes were recorded by the graphene transistor-based probe (Figure 5h and Figure S12). The population spikes were generated by synchronously firing of neuronal clusters. There was a marked increase in spectral power between 5-25 Hz, consistent with former studies.42 The recording of the epileptiform discharges lasted for roughly three hours before the signal gradually declined to the basal level. In addition, multichannel mapping of the ECoG activity was further performed using flexible, highly crumpled all-carbon transistor arrays (Figure S13).

Figure 5. In vivo brain activity recording with highly crumpled all-carbon transistors. (a) Schematic illustration of a 4X4 array of all-carbon transistors. (b) Optical image of a 4X4 array of biaxially crumpled all-carbon transistors. (c) Optical image of the biaxially crumpled all-carbon transistor array placed over the left cortical surface of rat brain. (d) Real-time recording trace by a highly crumpled all-carbon transistor under 82% area strain. Black arrowhead indicates the time point of penicillin injection. (e) Normalized time-frequency spectral analysis of the time-series data in d. (f~h) Enlarged view during basal activity period, latent period, and epileptiform activity period, respectively. 14


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In summary, we have introduced a new strategy for achieving both high sensitivity and substantially improved spatial resolution of transistor-based neural probes through aggressive in-plane compression. We have shown that chemically integrated, seamless all-carbon transistors maintain their structural and electrical integrity upon aggressive crumpling, permitting a six-fold increase in spatial resolution. We have further demonstrated in vivo recording of brain activity using flexible, highly crumpled all-carbon transistors. Our approach, characterized by a self-organizing process, presents a direct and economic route to the formation of high-aspect-ratio electronics over large areas, and could be extended to other 2D materials, such as transition metal chalcogenides43. Reference (1) Nicolelis, M. A. L. Nat. Rev. Neurosci. 2003, 4, 417-422. (2) Guy M. McKhann II; Julie Schoenfeld-McNeill; Donald E. Born; Michael M. Haglund; George A. Ojemann. Journal of Neurosurgery 2000, 93, 44-52. (3) Limousin , P.; Krack , P.; Pollak , P.; Benazzouz , A.; Ardouin , C.; Hoffmann , D.; Benabid , A.-L. New England Journal of Medicine 1998, 339, 1105-1111. (4) Mayberg, H. S.; Lozano, A. M.; Voon, V.; McNeely, H. E.; Seminowicz, D.; Hamani, C.; Schwalb, J. M.; Kennedy, S. H. Neuron 2005, 45, 651-660. (5) Kozai, T. D. Y.; Langhals, N. B.; Patel, P. R.; Deng, X.; Zhang, H.; Smith, K. L.; Lahann, J.; Kotov, N. A.; Kipke, D. R. Nature Mater. 2012, 11, 1065-1073. (6) Duan, X.; Gao, R.; Xie, P.; Cohen-Karni, T.; Qing, Q.; Choe, H. S.; Tian, B.; Jiang, X.; Lieber, C. M. Nature Nanotechnol. 2012, 7, 174-179. (7) Eversmann, B.; Jenkner, M.; Hofmann, F.; Paulus, C.; Brederlow, R.; Holzapfl, B.; Fromherz, P.; Merz, M.; Brenner, M.; Schreiter, M.; Gabl, R.; Plehnert, K.; Steinhauser, M.; Eckstein, G.; Schmitt-Landsiedel, D.; Thewes, R. IEEE J. Solid-State Circuits 2003, 38, 2306-2317. (8) Viventi, J.; Kim, D.-H.; Vigeland, L.; Frechette, E. S.; Blanco, J. A.; Kim, Y.-S.; Avrin, A. E.; Tiruvadi, V. R.; Hwang, S.-W.; Vanleer, A. C.; Wulsin, D. F.; Davis, K.; Gelber, C. E.; Palmer, L.; Van der Spiegel, J.; Wu, J.; Xiao, J.; Huang, Y.; Contreras, D.; Rogers, J. A.; Litt, B. Nature Neurosci. 2011, 14, 1599-1605. (9) Kim, D.-H.; Viventi, J.; Amsden, J. J.; Xiao, J.; Vigeland, L.; Kim, Y.-S.; Blanco, J. A.; 15


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(31) Zang, J.; Ryu, S.; Pugno, N.; Wang, Q.; Tu, Q.; Buehler, M. J.; Zhao, X. Nature Mater. 2013, 12, 321-325. (32) Leem, J.; Wang, M. C.; Kang, P.; Nam, S. Nano Lett. 2015, 15, 7684-7690. (33) Lacour, S. P.; Wagner, S.; Huang, Z. Y.; Suo, Z. Appl. Phys. Lett. 2003, 82, 2404-2406. (34) Zhang, M.; Fang, S. L.; Zakhidov, A. A.; Lee, S. B.; Aliev, A. E.; Williams, C. D.; Atkinson, K. R.; Baughman, R. H. Science 2005, 309, 1215-1219. (35) Shi, E.; Li, H.; Yang, L.; Hou, J.; Li, Y.; Li, L.; Cao, A.; Fang, Y. Adv. Mater. 2015, 27, 682-688. (36) Kim, J. B.; Kim, P.; Pegard, N. C.; Oh, S. J.; Kagan, C. R.; Fleischer, J. W.; Stone, H. A.; Loo, Y.-L. Nat. Photonics 2012, 6, 327-332. (37) Cao, C.; Chan, H. F.; Zang, J.; Leong, K. W.; Zhao, X. Adv. Mater. 2014, 26, 1763-1770. (38) Cao, Y.; Hutchinson, J. W. J. Appl. Mech. 2012, 79, 031019-031027. (39) Mei, H.; Landis, C. M.; Huang, R. Mech. Mater. 2011, 43, 627-642. (40) Chestek, C. A.; Batista, A. P.; Santhanam, G.; Yu, B. M.; Afshar, A.; Cunningham, J. P.; Gilja, V.; Ryu, S. I.; Churchland, M. M.; Shenoy, K. V. J. Neurosci. 2007, 27, 10742-50. (41) Crone, N. E.; Miglioretti, D. L.; Gordon, B.; Lesser, R. P. Brain 1998, 121, 2301-2315. (42) Canan, S.; Ankarali, S.; Marangoz, C. Epilepsy Research 2008, 82, 7-14. (43) Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Nature Nanotechnol. 2012, 7, 699-712. Acknowledgements Y.F. acknowledges support from the Excellent Young Scholar Program of the National Natural Science Foundation of China (21322302) and the CAS’s Strategic Priority Research Program for Brain Sciences (XDB02050008). Y.C. acknowledges the financial support from the National Science Foundation of China (Nos. 11172155 and 11432008). Supporting information available Detailed description of experimental methods and additional figures are available free of charge via the Internet at http://pubs.acs.org.

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Figure 1. Crumpling of all-carbon transistor. (a) Schematic illustration of an all-carbon transistor with a graphene channel and hGC electrodes. (b) Cross-sectional SEM image of a hGC electrode supported on 200 nm PMMA. (c) SEM image of a highly crumpled transistor. The active graphene channel was outlined by red box. (d) Optical image of the all-carbon transistor. The optical transparency of the graphene channel is higher than that of the hGC electrodes. (e) 3D confocal images of the all-carbon transistor under increased area strain, shown in top view, front sectional view, and perspective view. The active graphene channel was outlined by red boxes in the top view images. Scale bar, 100 µm. All images in c, d, and e were taken on the same all-carbon transistor. 145x145mm (150 x 150 DPI)


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Figure 2. Comparison of all-carbon and graphene/gold transistors under compression. (a~d) Optical images of an all-carbon transistor under uniaxial compression. (e~h) Optical images of a graphene/gold transistor under uniaxial compression. Red arrows in a and e indicate the edges of the graphene channels. (i) Ridge height versus compressive strain for gold electrode (black), gold-contacted graphene channel (grey), hGC electrode (red), and hGC-contacted graphene channel (pink), respectively. Data points represent the average height of six random positions in each region, and error bars indicate the standard deviation. See also Figure S4. (j) Ridge height mismatch versus compressive strain. Black data points represent the mismatch between gold and graphene, and red data points represent the mismatch between hGC and graphene. (k) Source-drain resistance variation versus compressive strain for the all-carbon (red) and graphene/gold (black) transistors, respectively. 156x137mm (150 x 150 DPI)


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Figure 3. Cracking in graphene/gold transistor. (a) Simulated maximum tensile strain versus compression curves of both the gold (i) and graphene (ii) films with (blue) and without (red) substrate detachment. The substrate was uniaxially pre-stretched by 100%. The inset figures show the strain maps of three typical deformation states in the two films: (1) the occurrence of a single mountain ridge; (2) a fully-developed mountain ridge; (3) the formation of new mountain ridges. (b) SEM images and FE simulation of graphene/gold transistor under biaxial compression. (i), SEM image of a highly crumpled graphene/gold transistor with an area compression of 84%. (ii), Enlarged SEM view of small cracks around the vertices of the crumpled gold electrode. (iii), Simulation of tensile strain map in gold film under biaxial compressive strain of 20%. The substrate was eqi-biaxially pre-stretched by 100% during simulation. (iv), Enlarged SEM view of a long crack in the crumpled gold electrode. (v), Enlarged SEM view of the graphene channel. No cracking was observed in graphene. 153x117mm (150 x 150 DPI)


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Figure 4. Electrical performance of highly crumpled all-carbon transistor in ionic solution. (a) Source-drain current (I) versus liquid-gate voltage (Vg) curves of an all-carbon transistor under incremental biaxial compression. The source-drain bias voltage was 60 mV. (b) On-current (Ion, red balls) and normalized transconductance of hole carries (gm, black balls) as functions of the area strain. The on-current was measured at Vg-VDirac=-0.35 V. 130x56mm (300 x 300 DPI)


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Figure 5. In vivo brain activity recording with highly crumpled all-carbon transistors. (a) Schematic illustration of a 4X4 array of all-carbon transistors. (b) Optical image of a 4X4 array of biaxially crumpled allcarbon transistors. (c) Optical image of the biaxially crumpled all-carbon transistor array placed over the left cortical surface of rat brain. (d) Real-time recording trace by a highly crumpled all-carbon transistor under 82% area strain. Black arrowhead indicates the time point of penicillin injection. (e) Normalized timefrequency spectral analysis of the time-series data in d. (f~h) Enlarged view during basal activity period, latent period, and epileptiform activity period, respectively. 157x95mm (150 x 150 DPI)


Highly Crumpled All-Carbon Transistors for Brain Activity Recording

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