NEMS applications

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Contents lists available at ScienceDirect

Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee

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Review Article

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Graphene and carbon nanotube (CNT) in MEMS/NEMS applications

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Xining Zang ⇑, Qin Zhou, Jiyoung Chang, Yumeng Liu, Liwei Lin

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Mechanical Engineering Department, Berkeley Sensor and Actuator Center, University of California at Berkeley, United States

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a r t i c l e

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i n f o

Article history: Received 28 July 2014 Received in revised form 20 October 2014 Accepted 26 October 2014 Available online xxxx

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Keywords: Graphene Carbon nanotube (CNT) Microelectromechanical Systems (MEMS) Nanoelectromechanical Systems (NEMS) Integration from nano to micro Local synthesis Transfer process

a b s t r a c t Carbon based nanomaterials, including one-dimensional carbon nanotubes (CNTs) and two-dimensional graphene, have attracted great research interests in recent years for various potential applications using the unique mechanical, electrical, optical and chemical properties. Specifically, the large surface area-tovolume ratio of these carbon-based material make them prime candidates for sensing applications in MEMS (Microelectromechanical Systems) and NEMS (Nanoelectromechanical Systems) devices. Here, we review the key electrical and mechanical properties of graphene with the focuses on their applications in sensors and actuators. State-of-art synthesis processes for graphene and CNTs are discussed since continuous advancements in their fabrication process are vital for the commercialization of graphene- or CNT-based products. Examples of graphene and CNT in MEMS/NEMS applications such as electronic components, mass/gas sensors, supercapacitors, and others are introduced and the advantages and challenges for graphene and CNTs based devices are discussed. Before these materials can be successfully utilized in MEMS/NEMS systems, effective integration processes with high yield have to be established. Approaches and discussions and are briefed in the future prospects of the paper. Ó 2014 Published by Elsevier B.V.

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1. Introduction

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‘‘There’s Plenty of Room at the Bottom’’ was a famous lecture given by Richard Feynman (Nobel Laureate in Physics) in 1959 as it inspired numerous research projects, commercial products and creative imaginations in science and engineering toward miniaturizations [1]. Over the past 50 years, many advanced technological developments in miniaturization have led to successful commercial products benefiting everyone’s daily life. For example, modern microelectronics has been shrinking the device sizes continuously as predicted by Moore’s law [1]. Stemming from the development of microelectronics, the area of MEMS (Microelectromechanical Systems) has seen continuous developments for the last decades, with mature commercial products such as silicon pressure sensors, micro accelerometers, micro gyroscopes widely used in automobiles, mobile phones, and video games [2–4]. The key technology driving force in MEMS is the utilization of the top-down manufacturing processes in microelectronics to make tiny mechanical and non-electrical components as the sensing elements with coupled/ integrated microelectronics to convert mechanical, chemical, biological or other sensing responses to electrical signals for a variety of applications. Today, MEMS is considered as a well-established field with good manufacturing infrastructures and many emerging

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Q3

⇑ Corresponding author.

commercial products and devices are continuously being explored and developed. On the other hand, further miniaturization of MEMS devices down to sub-micron or smaller scale goes to the regime of nanotechnology, which has been heavily promoted in the past decade. The combination of nanotechnology and microelectronics is often classified as the field of Nanoelectromechanical Systems (NEMS) [5,6]. In contrast to the field of MEMS, which are mostly based on the top-down fabrication processes, the key materials and processes in NEMS are often coming from the bottom-up processes to make key components in a variety of systems, such as nanowires, nanotubes, and two-dimensional nanostructures, including graphene [7–9]. Among the numerous nanomaterials, carbon-based nanostructures such as graphene and carbon nanotubes (CNTs) are very attractive due to their unique properties and ultra-small dimensions. Therefore, this report focuses on the discussions of graphene and CNTs in MEMS/NEMS devices as both of these carbon-based nanostructures have been widely studied in the literature with strong potentials for practical applications. Synthesized by the bottom-up processes, graphene and CNT are allotrope of carbons with sp2 bonds. The theoretical works for CNT and graphene started in the 1950s and 1940s, respectively, while the modern experimental demonstrations of these two nanomaterials started in 1991 and 2004, respectively [10–13]. Unique mechanical properties of graphene and CNT include low mass, high Young’s

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modulus, high thermal conductivity, and high surface area-to-volume ratio. These characteristics make them attractive in MEMS/ NEMS applications [14,15]. However, without proper integrations with microelectronics, their applications are restricted. Therefore, in addition to the synthesis of these nanomaterials, this report also addresses key integration issues, including: (1) process compatibility with microelectronics as most bottom-up processes require high processing temperature; (2) cost-effective and reliable assembly procedures to assemble the independently synthesized nanostructures with silicon-based microelectronics. In this report, we first review the state-of-art graphene and carbon nanotube properties and their growth processes. The applications of graphene and CNT in the field of MEMS/NEMS are discussed afterwards with illustration examples. Finally, the report ends with thoughts and discussions on future prospects.

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2. Properties and synthesis processes of graphene and carbon nanotube

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2.1. Key properties of graphene and CNT

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Key properties of carbon-based nanomaterials are summarized in Table 1 and compared with silicon and steel. Graphene has the highest theoretical value of Young’s modulus among these materials at 2 TPa and tensile strength at 130 GPa [16,17]. The Young’s Modulus of silicon is 169 GPa in the <1 1 0> direction and 130 GPa in the <1 0 0> direction or about 1/10th of that of graphene while its tensile strength is much lower than that of graphene [18]. Intrinsic single layer graphene has no band gap, while doping or patterning could open the band gap in graphene [19,20]. On the other hand, intrinsic silicon has the band gap of 1.12 eV [21]. Mobility is an important parameter in device physics and it is defined as the drift velocity to the applied electric field and high mobility is essential in high speed electrical devices, such as ultra-high speed transistors [22,23]. Graphene has the highest carrier mobility in Table 1 at 2  105 cm2 V1 s1 which is two orders of magnitude higher than that of silicon [24]. In terms of CNTs, both single-walled CNT (SWNT) and multi-walled CNT (MWNT) are listed in the Table 1 and they have lower Young’s Modulus and tensile strength as compared with graphene [25]. MWNT is metallic with no band gap and SWNT can have the configurations of zigzag, armchair or chiral depending on the chirality and their band gaps can vary from 0 to 2 eV. The mobility of SWNT and MWNT is in the same order of graphene [26,27]. Diamond is also listed in Table 1 as it is made of three-dimensional carbons based on sp3 structure. Intrinsic diamond without doping has a wide band gap of 5.5 eV, high Young’s Modulus of 1220 GPa, good tensile strength at 1.2 GPa, and good mobility at 0.79–1.2  105 cm2 V1 s1 [28,35]. Steel, on the other hand, is the most common structural material and is listed in Table 1 for comparisons with carbon-based materials. It can be observed that both the Young’s modulus and tensile stress of steel are much lower than those of graphene and CNT.

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2.2. Synthesis of graphene

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Various methods have been developed aiming to synthesis high quality graphene at low price, as summarized in Table 2. The first single-layer graphene was made by the method of mechanical cleavage of graphite in 2004 with a scotch tape [13]. It was peeled from graphite and transferred to another substrate with low defects in the form of single crystalline structure and high electron mobility [61]. Although mechanical cleavage can produce high quality graphene, the process requires tedious manual steps and it is not suitable for large scale productions. High quality graphene can also be grown epitaxially by decomposing SiC with mobility at 104 cm2 V1 s1 [47]. A high frequency transistor with a top graphene gate has been demonstrated with this method [46]. However, the cost of SiC wafer and the high processing temperature are two drawbacks for this synthesis technique. Another method to make graphene starts with oxidation of graphite to graphite oxide, which is ultrasonically separated as flakes and dispersed/deposited onto a substrate before reduced back to graphene [50,62]. This method has been used for applications requiring high density conductors such as supercapacitor electrodes [63]. However, this method is not desirable for making microelectronics as boundaries in separated graphene flakes reduce carrier mobility. Graphene has also been grown by carbon precipitations from metal (including Ni, Co or Ga) [60,64]. In this process, the low atomic percentage of carbon is dissolved into metal and precipitates onto the surface during the annealing process. Using this method, graphene has been grown on diamond and patterned as electrodes [60]. The most promising method to synthesize graphene is the Chemical Vapor Deposition (CVD) process by controlling the precursor, catalyst, pressure and temperature to produce high quality graphene [54]. For example, graphene has been synthesized on different metal materials including Cu, Ni, Au, Ag, etc. while copper and nickel are the two most common materials to synthesize graphene [65]. A recent study by the isotope labeling technique shows that graphene grown on Cu is a surface reaction process while graphene grown on Ni is a carbon segregation and precipitation process [66]. Since surface properties and grain boundaries greatly affect the quality of graphene synthesized by CVD [56,67], researchers have explored methodologies to improve the quality of graphene by thermal treatments [68,69]. For example, one approach is to eliminate grain boundaries by melting the metal catalysts in the synthesis process. As shown in Fig. 1a–g, Geng et al. carried out the CVD process on a large melted Cu droplet at 1120 °C (melting temperature of Cu is 1080 °C) and observed the formation and merge of graphene flakes into a continuous sheet [69]. In a recent work by our group, discontinuous metal droplets are formed under high temperature heating form an originally continuous thin film to investigate and control the growth and domain size of graphene as shown in Fig. 2a–d, [70]. Specifically, a 50 nm-thick nickel thin film is deposited on the silicon substrate with a thin oxide layer on top. After the CVD growth process at 1000 °C under 5 sccm of Methane and 50 sccm of Hydrogen for 10 min, single-layer graphene with typical size of 1 lm is synthesized nickel droplets with average diameter of

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Table 1 Properties of CNT, graphene, diamond, silicon and steel.

Graphene MWNT SWNT Diamond Silicon Steel

Young’s modulus (GPa)

Tensile strength (GPa)

Bandgap (eV)

Carrier mobility (cm2 V1 s1)

2000 ± 400 [16] 270–950 [25] 1000 [31] 1220 [34] 130–169 [18] 200 [40]

130 ± 10 [17] 11 [29]–150 [30] 13–53 [32] 1.2 [34] 7 [37] 0.25 [41]

0 [28] 0 0–2 [26] 5.5 [35] 1.12 [38] –

2  105 [24] – 0.79–1.2  105 [33] 156 [36] 1000 [39] –

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X. Zang et al. / Microelectronic Engineering xxx (2014) xxx–xxx Table 2 Summary of graphene synthesis processes. Quality Cleavage Epitaxial (SiC) Exfoliation CVD Precipitation

High Medium Poor High High

Size (lm2) 1000 100 100 Wafer 1000

Mobility (cm2 V1 s1) 5

2  10 104 1 10,000 1000

Temperature

Layer

Cost

Refs.

Room >1000 °C Room 1000 °C 1000 °C

Single Controllable Stacked Single Single

High Medium Medium Low High

[13,42,43] [44–48] [49–52] [53–56] [57–60]

Fig. 1. (a–g) Graphene grown on a single large droplet of melted liquid copper to reduce the grain boundaries of graphene. (a) The schematic of graphene formation on a big liquid copper droplet. (b–e) SEM images of graphene grown on liquid copper using CVD at 1120 °C for 30-, 38-, 38-min, and 2-h, respectively. (f and g) SEM images of graphene on liquid Cu via CVD at 1140 °C and 1160 °C, respectively. Copyright Ó 2012, National Academy of Sciences. (h–k) Continuous monolayer graphene grown on small discontinuous liquid nickel droplets. (h) A 50 nm-thick nickel layer is deposited onto the silicon substrate (with a top thermal oxide layer). (i) Formation of nickel droplets under high temperature and the growth of graphene on the droplets using methane as the CVD gas. (j) SEM of graphene grown on top of the nickel droplets. (k) Raman spectrum verifies the graphene characteristics as single layer (IG/I2D < 0.5) with low defects (ID is low).

Fig. 2. Electromechanical resonator made by suspended graphene for possible sensing applications. (a) Illustration of a graphene electromechancial resonator from a suspended graphene sheet, (b) SEM of the graphene resonator. Scale bar 1 lm. (c) Amplitude versus frequency plot of the graphene resonator which is about 15 nm in thickness. Inset, an optical image, scale bar 5 lm. (d) Amplitude versus frequency plot of a single-layer graphene resonator. Copyright Ó 2007, American Association for the Advancement of Science.

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Fig. 3. (a) Illustration of the flexible gas sensor with an embedded graphene FET. The graphene channel is open to the environment for gas sensing and parylene and PEI work as the gate dielectrics and channel dopant, respectively. The whole structure is sitting on top of a flexible polyimide substrate. (b) An array of 9  9 as-fabricated graphene gas sensors on the flexible substrate is held at the tip of a gripper.

Table 3 Application of graphene-based materials in various sensing application. GO – graphene oxide, R-GO reduced graphene oxide. Sensor

Detection element

Sensing materials

Detecting mode

Refs.

Physical

Mass Strain Pressure

Monolayer graphene Graphene/PDMS Monolayer graphene

Mix-down current AFM/resistance Resistance

[99,110] [111,112] [113,114]

Chemical

Gas (NH3, CO, H2O, NO2) Gas (O2) Heavy metal ions Heavy metal ions

Few layer graphene Monolayer R-GO/Au Graphene

FET resistance change Resistance change FET Ids Electrochemical

[115,116] [117] [118] [119]

Biosensor

DNA/protein DNA Protein Dopamine H2O2

Graphene/Al2O3 R-GO GO R-GO Pt/graphene

Conductance Electrochemical Fluorescence Electrochemical Electrochemical

[120] [121] [122] [123] [124]

Fig. 4. (a) Illustration of the strain induced by quantum–mechanical charge injection and electrostatic double-layer effects. (b) Simulation results of strain versus charge due to charge injection and charge injection + double-layer effects. Copyright Ó 2011, American Chemical Society.

Table 4 Graphene based materials in actuator applications. Actuator

Actuator materials

Driving model

Refs.

Physical

Monolayer graphene Graphene/PDA Graphene/SiN PDVF/graphene Graphene elastomer

Electrochemical Thermal Thermal Acoustic IR

[98] [130] [131] [132] [126,133,134]

GO/MWNT GO Graphene/GO fiber Graphene Graphene/PPY

Humidity/gas Liquid hydration Moisture Electrochemical Electrochemical

[135] [136] [137] [138] [139–141]

Chemical

196 197 198 199

0.5 lm. In this case, continuous graphene sheets can be synthesized on top of the discontinuous nickel droplets. By adding two top contact electrodes, an optical sensor can be readily achieved as the demonstration of a working device [70].

2.3. Synthesis of CNT

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In 1952, the synthesis of carbon tubes was first reported in a Russian journal but it was not until 1991 that the work by Iijima et al. using the arc-discharge evaporation method got the major interests [12,71]. Today, many methods have been developed to synthesize CNTs, including the three major techniques: arc-discharge, laser ablation, and chemical vapor deposition processes. The arc-discharge method has been used to make C60, and it is the practical method to make highly graphitized tubes [72,73]. In the arc-discharge chamber which is usually filled with He/Ar gas, a graphite anode for carbon is placed 1 mm away from the cathode [74,75]. Under a high DC voltage about 20 V, carbon plasma is resulted with a current density around 150 A/cm2 when graphite starts to pyrolyze into carbon gas to form multi-walled carbon nanotubes (MWNTs) on the cathode [72]. Single-walled carbon nanotube can be synthesized by the arc-discharge method with the assistance of metal catalysts. The purification of CNTs via oxi-

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Fig. 5. Illustration of using near-field electrospinning to directly write complementary FETs and junctions on a single substrate. (a) Two different kinds of polymer fibers are electrospun onto a monolayer graphene by near-field electrospinning to define the n- and p-type regions of graphene. The polymer fibers can dope the graphene as n-type or p-type, and also work as the mask to protect the graphene underneath during the following oxygen plasma etching process to remove the graphene in non-protected areas. Scale bar = 5 lm (b) cross-sectional view of different types of graphene-based transistors and a pn junction on the same substrate. (c) Cross sectional view of the electrospun PEO fiber. Scale bar = 500 nm. (d) Graphene channel after etching. Scale bar = 5 lm. (e) SEM image of the intersection area. Scale bar = 500 nm. (f) Illustration of the p-type Graphene FET, pn junction and npn junction on the same substrate. Copyright Ó 2014, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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dation is typically the following step, since oxidation at 450 °C will burn amorphous carbon without damaging CNTs [75–78]. The laser ablation method uses laser pulses on graphite with the assistance of catalysts to grow high quality and high purity CNTs [79–82]. Nd:YAG and CO2 lasers are commonly used to synthesize SWNTs in high temperature (1200 °C) furnaces [81,83]. The most frequently used catalysts by laser ablation are Ni, Co and Fe and it has been reported that the diameter of SWNT synthesized by laser ablation is affected by the furnace temperature and carrier gas flow rate. MWNTs can also be synthesized by the laser ablation process by using boron as the catalyst instead of the aforementioned metal catalysts [84,85]. Similar to the graphene synthesis processes, the CVD process is the most effective method to produce large-scale SWNTs and MWNTs [86–88]. Carbon precursors such as C2H2, C2H4, and C2H5OH decompose at temperature around 700 °C which is much lower than the temperatures used in the aforementioned laser ablation or arc-discharge processes. The decomposed carbon atoms can then dissolve into metal catalysts such as Ni, Co, or Fe and extend out to form carbon nanotubes. In general, the size of the metal catalyst determines the diameter of SWNTs or MWNTs. However, it is difficult to control the synthesis to be either the tip-growth or a root-growth process [86,87,89,90]. Furthermore, additional doping can be introduced by adding gases such as NH3 and N2 during the synthesis processes [91–97].

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2.4. Graphene in MEMS/NEMS applications

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The properties of low mass and high surface area-to-volume ratio make graphene a potentially excellent candidate for mass and gas sensing applications, respectively. Specifically, graphene-based

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electromechanical resonators have been demonstrated by using suspended graphene structures over silicon oxide trenches. Fig. 2a illustrates this concept and Fig. 2b is the SEM photo of the fabricated structure [98]. Experimental results have shown successful mechanical resonance of graphene (Fig. 2c) with a quality factor of 78 (Fig. 2d). This graphene-based device can be utilized as the foundation to sense the frequency shift due to mass changes, including mass changes due to the absorption of gasses as a gas sensor. Other works have extended the investigations by using graphene resonators to electrically transduce signals to sense mass, tension and charge concurrently [99]. Specifically, the best experimental results in graphene-base mass sensor show good sensing capability of 1 zg (1021 g), while the theoretical analyses indicate graphene could achieve the sensitivity of yg (1024 g) [100–102]. In addition to the aforementioned graphene sensors based on mechanical resonators, graphene-based chem-resistors and field effect transistors have also been utilized for gas sensing applications since the resistance of graphene is sensitive to gas molecules [103–105]. Here, we use a recent work from our group as an illustration example based on a flexible gas sensor with an embedded graphene FET [106]. Fig. 3a shows the cross-sectional view of the sensor. The channel is made of graphene and is exposed to the environment for gas sensing while the layers of parylene and PEI (Polyethylenimine) work as gate dielectrics and dopants for the channel, respectively. The whole structure is constructed on top of a flexible polyimide substrate and Fig. 3b shows a gripper holding the flexible graphene sensor structure having an array of 9  9 as-fabricated graphene gas sensors. Experimentally, a sensitivity of 0.00428 ppm1 has been demonstrated for ammonia from the resistance change measurements between the source and drain electrode (DR/RDS) [106].

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Fig. 6. Characterizations of several graphene-based devices fabricated with near-field electrospinning process. (a) The structures of graphene-based p-channel FET, pn and npn junctions are illustrated in Fig. 5f. The p-type graphene FET shows a Dirac Point when gate voltage is 78 V; the pn junction has local minimum drain currents at Vg = 35 V and Vg = 80 V; and the npn junction shows local minimums at Vg = 35 V, 10 V, and 78 V, respectively. (b) Band diagram of the npn junction. (c) KPFM (Kelvin probe force microscope) visualization of the surface potential of a graphene junction (before the oxygen plasma etching process). (d) An inverter based on the complementary graphene FETs. Scale bar = 5 lm. Ref. [144]. Copyright Ó, 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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Graphene has also been utilized in other applications with the combinations of a variety of materials. For example, in a recent work, a graphene-on-diamond structure has been built as a UV detector based on the carbon sp2–sp3 heterojunction [107]. Functionalized graphene, graphene oxide (GO), reduced graphene oxide (R-GO), and graphene composites have also been explored in sensing applications [108,109]. Table 3 summarizes various graphenebased materials in sensing and sensor applications. Graphene-based actuators have also attracted lots of research interests. For example, it has been theoretically proposed and numerically simulated that the injection of quantum–mechanical charge and electrostatic double-layer (DL) effects can both introduce strain into the graphene layers as shown in Fig. 3 [125]. This could be the foundation for graphene-based actuators. Fig. 4 recent research results include: a photomechanical actuator based on graphene nano platelet; an electrochemical actuator based on surface modified graphene; a bimorph micro actuator based on the graphene-epoxy structure [126–128]. In order to further increase the output efficiency and responsivity, one research report has stacked graphene flakes as ‘‘paper like’’ materials as well as combined graphene flakes with polymers for better actuating performances [129]. Table 4 summarized graphene-based materials in the area of actuators. Graphene based materials show high diffusion mobility, leading to high speed devices such as field effect transistors (FET) [142].

There are many published works in this area [143] and a recent work is introduced here as the illustration example. This graphene-based FET device used a simple yet versatile technique by means of near-field electrospinning to construct complementary FETs and junctions on a single graphene [144]. In this method, electrospun fibers are used to define and dope graphene into n- or p-type by using different polymers as shown in Fig. 5a [144]. Complementary FETs and junctions can be built on graphene after a single plasma etching process (Fig. 5b) while the electrospun fibers can have well-controlled deposition locations and profiles down to sub-micro meters (Fig. 5c). The oxygen plasma etching condition is 50 W for 7 s, and such short time etching process with weak power won’t affect the polymer’s chemical properties. The fabricated devices are illustrated in Fig. 5d and e as the examples of forming a p-type FET, pn-junction and npnjunction by the two electrospun fibers with a cross-shape is illustrated in Fig. 5f. The p-type graphene channel field effect transistor, pn junction and npn junction have been demonstrated by simply changing the electrical connections to test the finish device. Specifically, the prototype p-type graphene FET in Fig. 6a shows a single Dirac point at 78 V; the pn junction has two minimum current outputs under gate voltages of 80 V and 35 V; and the npn junction has local minimum currents at three gate voltages of 35, 10 V, and 78 V, respectively. The band structure analysis is illustrated in

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Fig. 7. Roll-to-roll production of graphene for transparent electrodes. (a) Photograph of the 7.5-inch quartz tube wrapped with copper foil for the CVD process. (b) A 30-inch long graphene sheet is transferred onto a 35-inch PET sheet. (c) A flexible touch panel is made by patterning silver paste onto the graphene/PET substrate. (d) Demonstration of the graphene based touch-screen panel. Copyright Ó 2010, Rights Managed by Nature Publishing Group.

Fig. 8. A single clamped CNT resonator as an atomic resolution mass sensor. (a) High resolution TEM of the double-walled CNT resonator. (b) Frequency changes with respect to time under extra mass loadings onto the CNT resonating sensor as compared with the change of a quartz crystal microbalance. Copyright Ó 2008, Rights Managed by Nature Publishing Group.

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Fig. 6b, where the n-p and p-n+ transition areas are highlighted. KPFM (Kelvin probe force microscopy) visualization of the surface potential of the graphene junction (before oxygen plasma etching process) is shown in Fig. 6c. The p-type graphene has the lowest potential and the n-type graphene has the highest potential, while the pristine graphene has the medium surface potential. Junction boundaries also show clear edges. A complementary graphene inverter is demonstrated with an on/off voltage ratio of 4.2 as shown in Fig. 6d, which is higher than that of the n-type graphene FET at 1.92 and p-type graphene FET at 3.42 [144]. Due to its low absorption of light (2.3%) over a very wide range of wavelength spectra, graphene has been proposed in the area of transparent electrodes. For example, a roll-to-roll process has been demonstrated to synthesis and transfer single layer graphene as large as 30 inches in length onto a flexible substrate to make a transparent electrode as shown in Fig. 7 [145]. In this work, a 7.5-inch quartz tube is wrapped with copper foil for the CVD

process (Fig. 7a). Afterwards, a 30-inch long graphene sheet is transferred onto a 35-inch PET (Polyethylene terephthalate) sheet (Fig. 7b) to make flexible touch panels (Fig. 7c). Successful operation of the device has been demonstrated in Fig. 7d. Four layers of p-doped graphene are stacked to make the film with as a sheet resistance of 30 X and the system is capable of sustaining up to 6% of mechanical strain.

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2.5. CNT in MEMS/NEMS applications

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Carbon nanotubes also has the key characteristics of graphene in terms of high stiffness, low density, and small cross-section area [146]. The applications of CNTs in MEMS/NEMS have plenty of examples. Specifically, in CNT-based resonators, the state-of-art highest mass sensing resolution is achieved by using CNT as the resonating mass detector [147]. As shown in Fig. 8a, a single clamped and double-walled CNT beam is used instead of single-walled CNT

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Fig. 9. (a–d) SWNT FET with ballistic transportation. (a) SEM and AFM image of CNT back gated SWNT FET with Pd contacts. (b) G (conductance) versus Vds for a 3 lm in channel length SWNT FET at different temperatures. Inset, Gon (conduction at ‘‘on’’ mode) versus temperature. (c) G versus Vds for a 300 nm in channel length SWNT FET at different temperatures. Inset, dIdS/dVds (differential conductance) measured at low temperature (T = 1.5 K). (d) Gon versus temperature data. (e–g) Arrays of aligned SWNT transistors. Copyright 2000, Macmillan Publishers Limited. (e) SEM image of highly oriented unipolar SWNTs on a quartz substrate by the CVD process. Pre-patterned Fe catalysts are between the CNT bundles. (f) Schematic illustration of transistors made by the SWNT arrays. (g) SEM image of the channel region of (f). Copyright Ó 2007, Rights Managed by Nature Publishing Group.

Fig. 10. The architecture of CNT + MEMS system for electrothermal gas sensing applications. (a) Schematic diagram showing the architecture and gas sensing principle of the suspended CNT. Heat generation in the CNT by the applied current is dissipated by heat conduction to gases (Wgas1, Wgas2), heat radiation (WR), and heat conduction to the contact electrodes (WC). (b) SEM image of a single MWNT electrothermal gas sensor. Copyright Ó 2007, American Chemical Society.

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to avoid issues related to the chirality of CNTs. Since carbon nanotube has smaller density than other materials fabricated by the traditional top-down lithographic techniques, lower resonance frequency is expected with similar physical dimensions. This should result in larger magnitude responses at a lower resonant frequency for sensing applications. Fig. 8b shows the example of a CNT-based mass sensor based on the resonator structure to achieve a sensitivity of 1.3  1025 kg Hz1/2, which equals to ‘‘0.4 gold atom’’ Hz1/2 – a better performance than the aforementioned graphene-based resonating sensor for mass sensing applications. The high carrier mobility of SWNT has been utilized in the demonstrations of CNT-based FETs [148]. For example, modified CNT contacts by using a high work function metal such as Pd and the post modification with hydrogen have helped to show CNT–FETs with conductance near the ballistic transportation limit of 4e2/h at room temperature with on/off ratio as high as 106 [149]. Fig. 9a shows the back gated SWNT FET in SEM and AFM images using Pd as the contact metal with different lengths. The conductance versus Vds measurements for the 3 lm and 300 nm-SWNT FETs at different temperature are recorded in Fig. 9b and c, respectively. The Gon

versus temperature data are illustrated in Fig. 9d. However, both the chirality and band gap (semiconductor of metallic) of SWNT can change the FET performances and control of these properties has not been achieved [150]. Furthermore, the metal contact is crucial to the device performance and improvements are still needed in this area [151]. As a result, the device-to-device reproducibility is still challenging for SWNT FETs as there is still lack of a good synthesis method to control SWNTs with homogeneous electrical properties [152]. Devices based on single SWNT can only be patterned by e-beam lithography which is not suitable for scalable fabrication and circuit integration. There have been many efforts trying to address the issue of large-scale manufacturing. For example, highly aligned SWNT transistors made from unipolar SWNTs have been demonstrated as illustrated in Fig. 9e–g [153]. In the fabrication process, both top gated and bottom gated transistors are patterned after the SWNTs are synthesized. Fig. 9e shows the SEM image of highly oriented SWNTs on a quartz substrate by the CVD process. Prepatterned iron catalysts are shown between the CNT bundles. The cross-sectional view device schematic Fig. 9f shows the source,

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Fig. 11. (a) Schematics of the bottom-up and top-down hybrid method to fabricate 3D CNT complex structures on a single wafer. (b) SEM image of densified CNT plates. Scale bar 100 lm. (c) SEM image of CNT beams. Scale bar 2 lm. (d) Silicon pillars with CNT sheets and CNT beams suspended on them. Scale bar 1 lm. (e and f) SEM photos of suspended CNT cantilevers on silicon wafer with pre-etched trenches. Scale bar 2 lm. (g and h) SEM images of 3D CNT cantilevers. Scale bar 10 lm. Copyright Ó 2008, Rights Managed by Nature Publishing Group.

Fig. 12. (a) The schematic diagram of CNT-based pseudo-capacitor with uniformly decorated NiOx on to MWNT forests. The charge and discharge process of NiOx/CNT hybrid pseudo-capacitor causes the redox of Ni ions to help increasing the charge storage capacity. (b) SEM of CNT forest coated with Ni by electroplating for 20 s. Scale bar 300 nm. (c) SEM of CNT coated with Ni by electroplating for 2 min. Scale bar 300 nm. Copyright Ó 2013, American Chemical Society.

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drain and gate electrodes and the CNTs as the channel structures and SEM image in Fig. 9g is the fabricated device. Uncoated p-type SWNTs and PEI (Polyethylenimine) coated n-type SWNTs are utilized to achieve the complementary device architecture. The transistor has 2100 SWNTs with a measured trans-conductance of 1000 cm2 V1 s1 and a current output up to 1A. The structure based on the CNT array instead of a single SWNT reduces the influences of the differences in chirality and defects to improve the

uniformity and reproducibility of the device [154]. On the other hand, these horizontally aligned CNTs should go through an electrical breakdown process to eliminate the metallic CNTs, which makes it difficult to count the precise number of active CNTs in this multichannel FET device [155]. Further studies based on integrated CNT circuits have resulted in the first CNT-based computer in 2013 with the potential to replace silicon in MOSFETs for improved efficiency [156]. CNT

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Table 5 CNT based materials in MEMS/NEMS application (GCE: glassy carbon electrode, CP: carbon powder, Nf: Nafion, AOx: alcohol oxidase, PEI: polyethyleneimine). Materials

Working mode

Refs.

Sensor

Mass Strain Ionization Flow Organic chemical Gas (NO2, NH3) Gas (O2) PEI Nitrotoluene Biological (glucose) NADH Glucose

Double walled CNT MWNT MWNT film SWNT SWNT network SWNT SWNT SWNT SWNT Enzyme/SWNT CNT/teflon MWNT/chitosan

Resonator frequency AFM Electrical breakdown Columbic field Capacitance change Conductance change Resistance change FET conductance FET conductance FET conductance Electrochemical Electrochemical

[147] [172] [173] [174] [175] [176] [177] [178] [179] [180] [181] [182]

Actuator

Electromechanical

MWNT SWNT/nafion SWNT/ion liquid film MWNT/elastomer MWNT/SWNT SWNT SWNT MWNT yarn

Rotational Displacement Bimorph bending Nematic Nanotweezer Pneumatic Bimorph bending Torsional

[183] [184] [185] [186] [187] [188] [189] [190]

SWNT/LiCx MWNT MWNT/Si MWNT SWNT forest MWNT/NiOx MWNT/SiNx GCE/CP/MWNT/Nf/AOx/PEI Paraffin/diatomite/MWNTs SWNT

Lithium battery Lithium battery Lithium battery Supercapacitor Supercapacitor Pseudocapacitor Switch capacitor Biofuel cell Heat storage Hydrogen storage

[191] [192] [193] [194] [195] [196] [197] [198] [199] [200]

Electrochemical

Energy storage

Fig. 13. (a) The schematic diagram of an integrated nanostructure + MEMS + microelectronics system. In this approach, the state-of-art micro accelerometer by Analog Devices Inc. is used as the example. The MEMS structure is integrated with microelectronics by mass production and serves as the synthesis platform to allow local heating and local synthesis of nanostructures, such as nanowires, nanotubes and graphene using the proper catalyst and chemical vapor deposition gas. (b) SEM of 6 MWNTs grown by the local synthesis method. The bottom MEMS structure is used as the heating structure and the top MEMS structure is used as the local electrical bias structure to build up a local electrical field to guide the growth of MWNTs. (c) The schematic diagram showing the local synthesis of graphene via the MEMS structure. (d) Illustration of the local synthesis of MWNTs and electrical circuit illustrating the control of the assembly process by monitoring the potential changes at the second MEMS structure. Copyright Ó 2007, American Chemical Society.

415 416 417 418 419 420

arrays are synthesized and transferred to a pre-defined bottom layer [157]. Source and drain regions are defined with e-beam lithography afterwards and unclamped CNTs are etched away by oxygen plasma. Electrical breakdown is performed to remove metallic CNTs which are undesirable in logic gate circuits [158]. In this work, basic computer functions such as counting and

integer-sorting; up to 20 different instructions have been accomplished. Similar to the aforementioned graphene applications in MEMS/ NEMS, CNTs are potentially good candidates in gas sensing applications [90]. Here, we utilize an example of an integrated CNT + MEMS system to illustrate several unique features and the

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potential benefit of CNT-based gas sensing systems. Fig. 10a shows the schematic diagram of the system consisting of two MEMS electrodes and a suspended MWNT [159]. When a current is applied via the MEMS electrode to the CNT, resistive heating occurs and the device works like a hot-wire sensor with probably the smallest hot wire in the world. Fig. 10b shows the SEM photo of a fabricated device with two MEMS electrodes made of single-crystalline silicon and a single MWNT suspended between them as the sensing element. The device works based on the principle of heat transfer processes. In the first application as a vacuum pirani gauge, it senses the gas pressure changes of the surrounding area. As the pressure is reduced, heat dissipation to the environment is reduced and the CNT temperature is increased. The increased temperature causes the resistance changes of the CNT as the pressure sensing mechanism. In the second application as a gas sensor, it can sense different types of gases. Specifically, different gases have different heat capacity which changes the heat dissipation amounts and will results in different temperature on the CNT sensor to be detected as the sensing mechanism. Both sensing mechanisms have been verified in the work and future improvements have also been discussed, including smaller CNT diameter and lower CNT heat conduction coefficient to reduce the heat losses to the MEMS electrode for better sensitivity [159]. The resistance changes of the CNT device from 200 K to 400 K have been measured and the TCR (temperature coefficient of resistance) has been characterized as 0.137% K1. Since the operation range of the sensor is below 150 °C, the structure is not expected to be damaged during the sensing operations [160]. In addition to CNT-based device applications, many works have focused on 3D wafer scale CNT structures [161]. As shown in Fig. 11a, the combination of a bottom-up and top-down hybrid fabrication methodology is used to make the 3D CNT structures. In the first step SWNTs are grown as the vertical CNT forest on predefined catalyst patterns by the CVD method on a silicon wafer [162]. The as-gown CNT forests are flattened in a liquid densification process. The wafer scale flattened CNT film can be patterned by the lithography method to further construct devices on the wafer. By using these parallel and scalable steps, different structures including CNT islands (Fig. 11b), beams (Fig. 11c), suspended CNT sheets (Fig. 11d), cantilevers (Fig. 11e and f), and 3D cantilevers (Fig. 11g and h) have been constructed onto a single wafer for various possible applications. On the other hand, vertically-aligned CNT forests have been applied as the electrode materials in electrochemical systems by taking the advantages of their high surface area and low contact resistance. For example, the as-grown MWNT forests have been shown to work as gas sensors [90]. These CNT forests with height Q5 of 100 lm have measured sheet resistance of 100 X/h with 0.5 ppm of resolution and 1 min of response time in the demonstrations for ammonia detections. CNT forests have also been

11

applied as supercapacitor electrodes since they have high surface area for large capacitance [163,164]. Densely packed CNT forests can facilitate ion exchanges in electrolyte as energy storage devices and CNT-based pseudo-capacitors with the capability of charge transfer in a redox reaction has also been implemented [165]. For example, a two-step process to fabricate CNT/NiOx hybrids electrode as a pseudocapacitor electrode has been demonstrated as illustrated in Fig. 12. The CNT forest is first grown by the CVD process [166], and nickel particles are decorated onto CNT forest by the electroplating process. Results shows that the specific capacitance of the CNT/NiOx hybrid electrode is 5.7 times higher than that of the bare CNT forest and the retention of capacitance reaches 94.2% of the original value after a 10,000-cyclic voltammetry test. There are other studies and architectures based on CNT for supercapacitor applications [167,168], such as a two-stage densification process to densify CNT for higher capacitance, a CNT-based flexible electrode, a silicon-coated CNT forest electrode, a CNT electrode conformal coated with RuO2 by atomic layer deposition, and a CNT electrode coated with conducting polymer [167–171]. Table 5 summarizes some published reports on CNT-based MEMS/NEMS applications, including sensors, actuators, and energy storage devices. Specifically, single SWNT/MWNT devices are generally demonstrated first and large scale devices and systems are developed next for better performance and multi-functionality. In the area of energy storage and electrochemical devices, CNTs are often assisted with other functional materials, such as metal oxide or conductive polymer to further increase their performances.

477

2.6. Future prospects

504

There have been many research progresses since graphene and CNT have been successfully constructed experimentally. It is clear that this report can only cover a small portion of these activities. For example, many efforts have concentrated on graphene and CNT-based basic electronic devices such as transistors but we only list a few examples as this report focuses more on the MEMS/NEMS applications. Beyond basic electronics, many innovated sensing and actuating systems based on graphene and CNT have been reported but this paper only covers some of them. Innovations and individual device performances are common emphases in these and other reports while integration and mass production are often neglected. This is clearly an issue for commercialization and practical applications. Therefore, this section concentrates on issues related to integration and commercialization. Integration of micro/nano device with microelectronics is essential for system-level practical applications and there are generally two approaches to accomplish integration: (1) integrated process to construct graphene or CNT together with other components and electronics on the same wafer; and (2) transfer processes to construct high quality graphene or CNT on a different substrate

505

Fig. 14. (a) The comparison of gas flow in a conventional CVD furnace and a micro CVD tube. (b) The local CVD process is used in this illustration for location controllable deposition of CNTs.

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Fig. 15. Integration of graphene resonators by the transfer process. (a) SEM of single layer graphene membrane over silicon/silicon oxide trenches. (b) Zoomed out SEM image of (a). (c) Optical image of monolayer graphene arrays, inset a schematic of the cross-section of graphene resonators. Copyright Ó 2010, American Chemical Society. (d) SEM image of a circular-shape graphene membrane. (e–g) Circular-shape graphene membranes are fabricated at the wafer scale by transferring a single layer graphene sheet onto a patterned wafer. (h) The membrane is driven and detected by optical interferometric motion. Copyright Ó 2011 American Chemical Society.

Fig. 16. Process of making the first CNT-based computer, including patterning the dielectric layer, transferring CNTs and patterning circuitry. Copyright Ó 2013, Rights Managed by Nature Publishing Group.

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and later transfer them to another substrate to accomplish integration. In the area of direct integration processes with microelectronics, our group has proposed and demonstrated a few integration examples by combined the top-down and bottom-up processes [201,202]. Specifically, the method of localized synthesis, assembly and integration is one key approach. The basic architecture utilizes the example of a prominent MEMS device – an integrated accelerometer by Analog Devices Inc. as illustrated in Fig. 13a. This stateof-art MEMS produce is fabricated by a mass production process combining both mechanical microstructures with microelectronics in large volume and very low price [203]. The chip at the right side of Fig. 13a is occupied mostly by microelectronics. However, there is a movable mechanical spring and mass microstructure at the center of the device as marked as ‘‘Micro,’’ which is the core of the mechanical sensor. The interface bridging the nano-to-micro world has attracted investigations in both fundamental and applied research [204]. Specifically, properties of nanomaterials

show abnormality as compared with their macro scale properties, such as melting point depression and quantum electromagnetic properties [70,205]. These should be considered when designing the integration process. Our idea is to use the MEMS structure to synthesize nanostructures such as nanowires, nanotubes at a very high temperature locally via joule heating without damaging the microelectronics. The local synthesis approach has been successfully demonstrated in the local fabrication of silicon nanowires and carbon nanotubes (Fig. 13b) as well as graphene (Fig. 13c) [203,206]. As illustrated in these figures, microstructures made by the top-down MEMS processes are used as the starting synthesis structure. With proper design via joule heating, the MEMS structures can be heated to high temperature and nanostructures can be synthesized locally. Furthermore, we have shown that by applying a local electrical field, one-dimensional nanostructures such as silicon nanowires and carbon nanotube can be grown to follow the electrical field

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direction [207,208]. Furthermore, for conductive materials such as MWNTs, it is possible to detect and monitor the assembly process as illustrated in Fig. 13d as the assembled MWNTs will change the output potential to be monitored as a scheme to control the number of assembled MWNTs. For example, this technique was used to assemble the aforementioned single MWNT gas sensor in Fig. 10b. The issues of nano-to-micro interface can be further explained in this case of MWNT and silicon structure [207,208]. It was reported that for the tip-grown MWNTs, they stopped their growth process when they reached the other cold silicon structure. On the other hand, for the base-grown MWNTs, they can continue to grow from the hot silicon structure [207,208]. The approach of local synthesis processes can potentially be utilized for scientific studies. For example, the ‘‘local CVD’’ process can be used as a tool to characterize and or improve the global CVD processes as the very small mass of the MEMS structures can enhance mass transport and reduce the reaction time during the synthesis processes [209]. For example, the conventional CVD system is compared with the micro CVD system in Fig. 14a. Analytically, the flow velocity of a tube with circular cross section is described by the parabolic velocity profile:

Using a similar process as illustrated in Fig. 16, the first CNT-based computer is made in 2013 [156]. Several groups have also demonstrated the ‘‘vertical’’ transfer of CNTs, which results in vertically aligned CNTs to a different substrate, typically for high surface area electrode applications [216]. To sum up, these and other transfer processes have made possible some of the important applications for both graphene and CNT in MEMS/NEMS systems and it is believed that innovated integration processes will eventually lead to the realization and practical applications of graphene and CNTs. This review starts with the discussions on the unique mechanical and electrical properties of graphene and CNTs and continues with their synthesis processes. Next, specific examples for MEMS/NEMS applications, including FETs, sensors, actuators, and supercapacitors are introduced. It is clear that scalable synthesis processes and integration methodologies have to be further developed for possible commercial applications of any of these devices. Analytically, graphene and CNTs have very high theoretical specific surface area and should be excellent materials in many applications. It is expected that future device innovations and process improvements can further improve the performances of devices based on graphene and CNTs.

625

582

mðrÞ ¼ 2mm ½1  ðr=r0 Þ2 

References

646

583

where r is the radial coordinate, mm is the mean gas velocity and r0 is the radius of the tube. The velocity gradient near the surface of the tube is:

559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579

626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645

580

584 585

ð1Þ

586 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624

@ mðrÞ 4mm ¼ j @r r¼r0 r0

ð2Þ

Therefore, by shrinking the size of conventional CVD system to a micro CVD system using MEMS technologies, one can expect enhanced mass transfer because gas velocity gradient near the reaction surface has about 104 increments in this example. These effects have been used in a local CVD process for graphene for better uniformity and the growth of SWNTs for controllable placements and depositions as shown in Fig. 14b as well as direct integration for SWNTs onto a paper substrate [210,211]. These are good examples of utilizing the MEMS technologies for the synthesis, assembly and integration of nanostructures. Further advancements in these and other efforts could lead to better understandings in the synthesis processes of nanostructures; better control in their assembly processes; and easier procedures for the integration. In the area of transfer processes for the integration with microelectronics, there are plenty of efforts. Here, two examples of wafer-level transfer processes for the fabrication of graphene resonators are utilized as demonstration examples. In Fig. 15a–c, wafer scale graphene grown by CVD is transferred onto a pre-patterned Si/SiO2 substrate and resonators are fabricated afterwards. Quality factor of the resonator is greatly increased to 9000 [212]. In Fig. 15d–h, a similar process is used to make circular-shape graphene resonators with measured quality factor of 2400 ± 300 [213]. Both examples illustrate the possibility of fabricating graphenebased devices by CVD process at large scale. As CVD graphene is commonly grown on copper-based metal foils at very high temperature and a transfer process is generally required afterwards to construct electrical or mechanical structures for possible applications. On the other hand, the transfer process for CNTs generally starts with the ultrasonically cutting of CNTs away from the synthesis wafer [214]. Afterwards, the CNT networks with randomly oriented CNTs can be transferred to other substrates for various applications [215]. In the state-of-art technology, CNT arrays can by aligned in certain direction during the transfer process which is essential for the integration of CNTs with other devices, as shown in Fig. 9e–g.

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