Facile growth of carbon nanotubes using microwave ovens: the emerging application of highly efficient domestic plasma reactors

The facile growth of carbon nanotubes (CNTs) using microwave radiation reveals a new way for the cost-effective synthesis of CNTs for a wide range of applications. In this regard, domestic microwave ovens can be used as convenient plasma reactors to grow CNTs in a very fast, simple, energy-saving and solvent-free manner. The special heating mechanism of microwaves can not only accomplish the fast growth of high-density CNT brushes within tens of seconds, but also eliminate the need for a flammable gaseous carbon source and an expensive furnace. By carefully selecting the substrate and catalyst, low-temperature growth of CNTs can also be achieved on low-melting point organic polymers at atmospheric pressure. Highly localized heating near the catalyst nanoparticles was observed under microwave irradiation, and this phenomenon can be utilized to grow CNTs at desired locations on the substrate to fabricate CNT-based nanoelectronics in situ. Finally, the microwave growth of CNTs is highly adaptive to different carbon sources, substrates and catalysts, showing enormous potential to generate functionalized CNT-based composites for emerging advanced applications.


Introduction
Carbon nanotubes (CNTs) have attracted signicant research interest since their rst discovery in 1991 and the representative CNT structures are shown in Fig. 1. 1 Based on the number of the graphene walls, CNTs can be categorized into single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs). As a well-known allotrope of fullerene and graphene, CNTs represent a totally new structural dimension of nanocarbons. Extraordinary structural, mechanical, electrical, thermal and optical properties have been derived from the unique one-dimensional (1D) nanostructure of CNTs. [2][3][4][5][6] For example, fullerenes can be encapsulated inside CNTs to form Dr Yang Liu graduated from Donghua University, Shanghai, China, in 2009 with a bachelor's degree in textile chemistry. He then joined Dr Xinyu Zhang's laboratory in Auburn University as a master's student and mainly conducted research in the synthesis and carbonization of conducting polymer nanostructures for sensory and energy-storage applications. He graduated from Auburn University with academic excellence in 2013 and joined Prof. John H. Xin's group at Hong Kong Polytechnic University to study the interfacial physics of nanostructured carbons and semiconductors as a PhD student. Now he is a research associate professor in Sun Yat-sen University, Guangdong, China.
Mr Naishun Guo was born in Shandong province, east of China in 1995. He received his bachelor's degree in biomedical engineering from Weifang Medical College in 2018. Now he is studying for a master's degree in the School of Biomedical Engineering at Sun Yat-sen University, focusing on the rapid synthesis of carbon nanotubes using microwaves under the supervision of Dr Yang Liu and Prof. Chao Zhang. His research interest also includes the mechanism study of the improved mechanical properties of polylactic acid with the addition of carbon nanotubes. nomenclature of the word "CNT" arises from the characteristic hollow-tube-like structure of these 1D nanocarbons, as graphene sheets are aligned parallel to the hollow-tube core and stacked to form the side-wall structure of the CNTs. The diameter of the hollow-tube core of CNTs is essentially dependent on the size of the catalyst nanoparticles and the growth conditions, e.g., temperature and time. Provided that the growth is conducted below the liquefaction temperature of the metal catalyst, the tube diameter is directly proportional to the catalyst particle size. [17][18][19] As the growth temperature is higher than the liquefaction temperature of the catalyst, the tube diameter may instead exhibit a universal Gauss-like distribution. 17,20 The mechanical strength of the as-synthesized CNTs, e.g., Young's modulus, is qualitatively related to the defects in the tube walls, [21][22][23] and the tube diameter, number of walls and chirality may have little effects on Young's modulus. 24 Similar to the case that one single CNT can be grown on one metallic nanoparticle, CNT bundles and forests can be grown on patterned arrays of metal nanoparticles as well. As the gaseous carbon species diffuse onto the surface of the metallic nanoparticles under the high growth temperature, CNTs may grow either by a "base-growth" or "tip-growth" pathway, depending on the synthesis techniques used. It was reported that large catalyst particles (>5 nm) favor the "tip-growth" mechanism to generate MWNTs while small catalyst particles (<5 nm) favor the "base-growth" mechanism to generate SWNTs and FWNTs (<7 walls). 25 The major conventional techniques involved in the synthesis of CNTs include arc-discharge, laser ablation and chemical vapor deposition (CVD) (Fig. 4). 26 These techniques generally utilize a high-energy source (e.g., high-voltage electric arc, laser beam, hot-lament, and plasma) to create highly dissociated or decomposed carbonaceous nanoclusters which subsequently diffuse onto the surface of catalyst nanoparticles and interact to induce CNT growth. In addition to the high-energy source, high growth temperatures (>873 K) are also required in these techniques to obtain CNTs with good crystallinity. For the arcdischarge and laser ablation techniques, the growth temperature can exceed 2000 K, leading to the formation of CNTs with high crystallinity, conductivity and mechanical strength. [27][28][29] Inert-gas protection is also required during the arc-discharge and laser ablation processes, as free oxygen may react with the forming carbonaceous species and transform them into carbon dioxide at the high growth temperatures. Explosive gases such as acetylene, hydrogen and carbon monoxide are frequently used in the CVD growth of CNTs; in turn they add high explosion risks and raise serious safety concerns during the CNT growth process. The arc-discharge, laser ablation and CVD techniques use radiant and resistive heating mechanisms to grow CNTs, which means that the objects (e.g., precursors, gases, substrates, and furnace walls) in the exposure range will be non-selectively heated simultaneously, and the optimization of the growth process and energy consumption is hard to achieve simultaneously. The CVD techniques also utilize an inductive heating mechanism, but the selectivity is limited to the conductive materials. 30 Another problem associated with the non-selective heating process is the obscurity of the localized growth of the CNTs in the selected area. This problem is rather critical in the in situ site-selective fabrication of the CNT-based nanodevices on low-meltingpoint substrates, as the in situ growth of CNTs at the selected locations is speculated as the most efficient approach to establish nanoscale electrical contact and to manipulate the functions of nanoelectronics. [31][32][33] The major drawbacks of conventional techniques are listed as follows: (a) High-energy source requirement, (b) Inert-gas protection, (c) High growth temperature, (d) High energy consumption and non-selective heating, (e) Usage of explosive and toxic gases. Different from conventional techniques, the heating process induced by microwave radiation does not rely on the external high voltage (e.g., arc-discharge), high-energy laser beam (e.g., laser ablation) and high-temperature furnace (e.g., CVD). In contrast, it utilizes the internal microwave absorption properties of the substrate materials and catalysts to realize a selective and localized heating process for CNT growth. [34][35][36][37] The selective heating for the CNT growth process can be realized by differentiating the microwave absorption properties, or more specically the difference of the penetration depths between the substrate and the catalyst. For the absorbed microwave power in the unit volume of a material, there is: where P is the absorbed power per unit volume, f is the frequency, 3 00 is the complex permittivity, s is the conductance and E is the electric eld intensity. However, the value of P decreases as the penetration depth d p decreases, and d p is inversely proportional to the conductance of the material. For d p , there is: where 3 0 is the dielectric constant and m is the permeability of the material. Therefore, the localized heating for CNT growth can be realized by choosing substrate materials with a low conductance and catalyst materials with a high conductance. For example, one can consider the case of Fe nanoparticle catalysts deposited on a SiO 2 substrate. As microwave radiation is applied, the electromagnetic waves are preferentially absorbed by the catalyst nanoparticles and penetrate through the insulating substrate due to its large d p , resulting in the selective heating of the catalyst particles and the temperature rising in the local environment near the catalysts. On the other hand, the microwave transparent substrate can also dissipate the heat generated from the catalyst particles and maintain relatively low temperatures in the reaction vessel, making the lowtemperature growth of CNT possible. Due to the volumetric, localized heating of the catalyst particles, the growth process of CNTs under microwave irradiation is very fast, which is typically in the range from 5 seconds (s) to 3 minutes (min). [34][35][36][38][39][40][41] Besides the heating mechanism, microwave methods are substantially different from CVD methods in terms of the product, reactor design, and processing parameters. CVD methods, which are regarded as the most widely used techniques for CNT synthesis, are ideal to produce SWNTs, FWNTs and MWNTs with long lengths. 26,[42][43][44][45] With increasing growth temperatures from 500 C to 1200 C, the lengths of the CNTs synthesized by CVD methods can change from 0.1 mm to 10 5 mm. 26 On the contrary, microwave methods are more suitable to grow shorter CNTs with lengths ranging from 1 mm to 20 mm. 37,39,46,47 To speak further, CVD methods are capable of producing SWNTs in the diameter range from 0.6-4.0 nm and MWNTs in the diameter range from 10-240 nm; 48 while currently, microwave methods are able to generate MWNTs with diameters ranging from 10-200 nm. 37,39,41,46,47 In this regard, different perspectives in applications can be envisioned for the CVD-grown CNTs and microwave-grown CNTs. The high aspect ratios of the CVD-grown CNTs make them ideal for the fabrication of polymer composites, conductive llers, CNT bers, etc. [49][50][51] On the other hand, the lower aspect ratios of the microwave-grown CNTs make them suitable for surface modi-cation and functionalization. 37,46,47 The growth temperatures of CNTs in CVD techniques are typically in the range from 700 C to 900 C. 26 Microwave methods may utilize higher growth temperatures of around 1000 C, but the heating effect is highly localized in the proximity of the surface of the catalyst, resulting in a minimum increase of temperature in the surrounding environment. 46,47 The CNT growth process of microwave methods can proceed by using mixtures of metallocenes and substrate materials, while the essential catalyst preparation steps in the CVD methods can be omitted. It thus makes microwave methods especially suitable for growing CNT brushes on thin, conductive and porous micro-and nanodimensional substrates such as powders, bers, graphene and MXenes. 46,47,52,53 2. Fast synthesis of CNTs by microwave irradiation

Processes and instrumentation
By using microwave radiation, high-quality CNTs can be synthesized in a very fast manner. The reported synthesis time (t) for microwave radiation ranges from 5 s to 3 min, and t < 60 s is enough to produce long and entangled CNT forests according to most of the reported methods. [38][39][40][41]54 Typical CNT forests prepared from microwave irradiation methods are shown in Fig. 5. In additional to the very short synthesis time, the postsynthesis processes of the microwave-generated CNTs such as separation and purication are also very simple and timesaving. Unlike the slow cooling steps involved in CVD as a result of the long time required for the heat dissipation of the furnace at elevated synthesis temperatures, cooling of the CNTs synthesized by microwave radiation can be instantaneously triggered as the microwave power ceases, as a result of the internal heating mechanism of microwave; moreover, the microwave transparent reaction vessels also help dissipate the heat generated by the microwave absorbing materials quickly, which in sum makes the cooling process feasible under ambient conditions and can be completed in a few minutes. 53-55 Aer cooling, the as-obtained CNTs can then be separated from the carbon impurities by sonication and centrifugation.
Various instruments can be used to perform the synthesis of CNTs under microwave irradiation, including a single-mode microwave system, laboratory microwave oven and the domestic microwave oven, as shown in Fig. 6. High microwave power (2.45 GHz, up to 2000 watt), inert gas protection and pressure control can be equipped on the single-mode microwave system. In the meantime, single-mode reactors, monitoring cameras, and temperature and pressure controls are equipped in most of the laboratory microwave ovens. 56 However, for the laboratory microwave ovens (e.g., CEM dis-cover®), the shape and volume of the reaction chamber are restricted, and this substantially limits the customization of reaction and large-scale production. On the other hand, the microwave ovens designed for large-scale synthesis (e.g., Ultra Clave) are expensive and unsuitable for customized reactions. Such drawbacks can be circumvented by using domestic microwave ovens which provide reaction chambers with both high volumes that are compatible with high throughput synthesis and free space for customizing the experimental setups. Reaction vessels with different shapes can be placed into the domestic microwave oven and a variety of experimental congurations can be realized by the integration of vials, chambers and reactors, providing substantial capabilities of customization and control over the CNT growth process. Furthermore, the power and time control for the domestic microwave oven is facile and visual, making the parallel experiments highly reproducible. To further extend the synthesis functions of the domestic microwave oven, e.g., inert gas protection and pressure and temperature control, special add-ons such as the Art Box™ furnace can be integrated inside the chamber to provide more sophisticated engineering control over the synthesis parameters to satisfy the needs for the fabrication of CNTs and the related nanocomposites. 58 By customizing the microwave reactor with advanced functions for controlling the detailed parameters, a microwave synthesis system that is capable of producing CNTs with controlled sidewall structures and alignments, e.g., SWNTs, FWNTs, and vertically aligned CNTs, can be realized. But in most cases, a pristine domestic microwave oven is sufficient for the synthesis of high-quality MWNTs for a wide range of applications.

Type and quality of the as-synthesized CNTs
MWNTs can be readily grown at atmospheric pressure by using a domestic microwave oven and ferrocene as the precursor, and a conversion efficiency (from ferrocene to CNT) as high as 82 wt% can be achieved. 47 The as-obtained MWNTs are observed as long, curly and entangled CNT bundles, as shown in Fig. 7a. The wall structure of the MWNTs is highly graphitized, and the lattice structure of the carbonized graphene layers can also be clearly observed (Fig. 7b). The growth direction of the graphene layers is aligned according to the residual catalyst nanoparticles that are stuck inside the cores of the MWNTs (Fig. 7c), and local curvatures are created by the nanoparticles along the tube axis which direct the growth of the MWNTs (Fig. 7d). By carefully tuning the growth conditions during microwave irradiation, e.g., substrate temperature, reaction pressure, time, feed gas, microwave power and catalyst size, vertically aligned FWNT forests can be obtained (Fig. 8). 59 However, the above-mentioned requirements involved in the synthesis control to generate the aligned FWNT forests will signicantly complicate the instrument conguration, resulting in a microwave synthesis system that is similar to the CVD system. The MWNTs produced by the CVD methods typically show I D /I G ratios in the range of 0.64-1.07. [60][61][62][63] On the other hand, the MWNTs produced by the microwave methods show a relatively wide-range distribution of the I D /I G ratios from 0.15 to 1.1, according to Raman spectroscopic characterization. 35,[37][38][39][40][41]47,58,64 The values of the I D /I G ratio of the microwavegrown MWNTs are similar to, or smaller than, those of the CVD-grown MWNTs, indicating that the quality of the microwave-grown MWNTs is comparable to that of the CVD ones. The presence of the gaseous carbon sources and inert gases is found to enhance the degree of graphitization of the microwave-grown MWNTs, while at the same time decrease their defect densities, resulting in reduced I D /I G ratios. For example, Odedairo et al. reported the growth of MWNT hybrids  in a microwave reactor (SAIREM) with the addition of syngas as the carbon source and argon as the protective gas; I D /I G ratios ranging from 0.15 to 0.37 can be achieved. 37 Yoon et al. was able to produce MWNTs with an I D /I G ratio of 0.41 by using activated carbon ber as the carbon source and argon as the protective gas. 38 Guo et al. also demonstrated that the presence of nitrogen protection can improve the I D /I G ratio of the microwave-grown MWNTs from 1.04 to 0.854. 64

The carbon yield
The yields of the MWNTs grown by microwave methods differ signicantly and are strongly dependent on the growth conditions. By using acetylene as the carbon source, Hong et al. were able to obtain MWNTs on carbon black-supported metal catalysts, i.e., Ni, Fe, and Co, in a commercial microwave oven with an applied power of 800 watt (W) and a reaction time of 30 min. Carbon yields between 200-350 wt% calculated based on the weight change of the catalyst before and aer growth can be obtained, depending on the type of metal catalysts. 35 By using syngas as the carbon source, Odedairo et al. managed to synthesize MWNT hybrids on ceria-supported bi-metal catalysts in a microwave reactor with a power of 500 W. The carbon yields of the process aer 14 hours of growth differed from 0.61 g to 0.67 g based on the type of catalyst. 37 By using ferrocene as the carbon source and catalyst, Nie et al. were able to synthesize MWNTs in 15-20 s in a domestic microwave oven with 1000 W applied power and a single carbon ber as the heating lament. The carbon yield was calculated to be 40 wt% based on the weight of ferrocene. 41 Since the theoretical carbon content in ferrocene is approximately 64.5 wt%, the conversion efficiency of the carbon element in ferrocene to MWNTs is approximately 62% in this case. By using ferrocene as the carbon source, graphite as the microwave absorber and carbon ber as the heating lament, Bajpai et al. were able to grow MWNTs in a commercial microwave oven (DEC18E2, ACP) with an applied  power of 1800 W and a time of 5 s. The carbon yield based on the weight of the precursor mixture was measured to be 26 AE 5 wt%. In this case, the conversion efficiency of the carbon element in ferrocene to MWNTs is approximately 66.5-98.2%. 47 However, the higher carbon yields in the latter case can be attributed to the addition of graphite which effectively absorbed the microwave power and generated substantial heat to trigger the fast decomposition of ferrocene for CNT growth.
The fast and cost-effective CNT synthesis process enabled by the domestic microwave oven can easily generate MWNTs with appropriate functions (e.g., crystallinity, conductivity, and aspect ratio) in high yields, with low instrumental and precursor requirements. The as-obtained MWNTs can be further used in a wide-range of applications including llers, eld emission displays, transistors and lithium-ion batteries. 64-67

Processes, substrates and products
By using the microwave radiation, CNT brushes can be directly grown on different substrates in situ, such as graphite, graphene, MXenes, Kevlar, carbon ber and y ash. 46,47,64,[68][69][70] In this regard, the domestic microwave oven can be used to realize the CNT growth on the various substrates and materials at atmospheric pressure without an external furnace. For example, MWNT brushes were grown on y ash and glass ber fabrics by using a domestic microwave oven (Fig. 9). The experimental set-ups were very simple and only required the mixing of the polypyrrole (Ppy) coated y ash (or glass ber fabrics) with ferrocene, and then 1250 W microwave power was applied on the mixture for 30 s by using a domestic microwave oven. The CNT growth process was completed in a single glass vial and no external heating devices and pressure vessels were required. However, intensive absorption of the microwave radiation by the mixture can increase its temperature to reach the growth temperature in a few seconds; the pressure inside the reaction vial is also increased quickly as ferrocene is decomposed into abundant gaseous carbonaceous compounds by the dielectric heating of microwave. Besides the fast and facile growth, CNTs can be grown on substrates of different dimensional characteristics, from nanosheets and micro-bers to macro-scale surfaces. For example, CNT brushes can be synthesized on the surface of 2D nanomaterials, such as MXenes, exfoliated graphite nanoplatelets and graphene (Fig. 10a-c); they can also grow on  micrometer-size materials, such as ceramic powders, graphite powders, Kevlar bers and glass bers (Fig. 10d-f) and on macro-scale materials' surface such as glass, Teon, and polycarbonate. [34][35][36] Besides the growth of CNT brushes on different substrates, simultaneous functionalization of the CNTs during growth can also be achieved by the introduction of functionalization reagents into the precursor mixture. For example, nitrogen-doped CNT brushes can be obtained by adding azobis(cyclohexanecarbonitrile). 68 Metal-decorated (e.g., Fe and Pd) magnetic CNT brushes were obtained by the incorporation of iron compounds into the precursor mixture. 68,70,71 Metal oxides/ CNT brushes/Fe hybrid nanostructures were also fabricated by the oxidation of MXenes under microwave irradiation. 52,69,72

The growth mechanisms
Based on the intrinsic microwave absorption properties of the substrate, the CNT brushes can undergo either a "direct growth" pathway or a "medium growth" pathway. For the microwave absorbing substrates, such as Ppy, graphite, graphene oxide and carbon ber, the direct growth pathway is favorable since the substrate itself can effectively absorb microwaves and generate sufficient heat to reach high local temperatures on the surface to trigger growth. However, when microwave transparent substrates are used, such as Kevlar, Teon, y ash and glass ber, the medium growth pathway is more favorable since the substrate itself does not absorb microwaves and cannot generate enough heat under microwave irradiation. In this regard, an additional layer of microwave absorbing materials (e.g., polypyrrole, graphene, and metal suldes) can be introduced onto the surface of the substrates via a coating process, such as dip coating, spray coating and sputter coating. The microwave absorbing materials, i.e., the microwave absorbers shown in Table 1, are mediated between the microwave-transparent substrates and catalyst nanoparticles. On one hand, they can serve as the binder to combine both the substrate and the catalyst; on the other hand, they vigorously absorb the microwave power and generate substantial heat, raising the temperature of the precursor blend (e.g., the mixture of the catalyst, microwave absorber and substrate material) to the appropriate range that is suitable for the growth of CNTs. In several cases, the microwave absorbers also served as the carbon source to support CNT growth. 38,58 This phenomenon can be attributed to the intimate deposition of the catalyst nanoparticles on the surface of the microwave absorber, Graphene oxide Graphene oxide Atmospheric Graphene/Co/Ndoped CNT hybrids and the recrystallization of the amorphous carbon inside the microwave absorber under the growth conditions. In both of the "direct growth" and "medium growth" pathways, metal compounds such as ferrocene, iron acetate and cobalt sulde are oen selected as the catalysts due to their high dispersibility and reactivity. [34][35][36]46,52,[64][65][66][67][68][69][70][71][72] Before microwave irradiation, the catalyst is either directly mixed with the substrate materials or coated on the substrate surface (in the direct growth pathway), or can be mixed with/coated on the microwave absorbing material-coated substrate (in the medium growth pathway). During microwave irradiation, the metal compounds quickly decompose into metal nanoparticles, serving as the new catalytic centers for CNT growth; low-molecular-weight hydrocarbons are also generated by the decomposition of the metal compounds, and they can serve as the primary carbon source to support the growth.  (Fig. 11). Patterned CNT growth can also be realized on conductive substrates such as silicon by the selective heating mechanism. In this case, the catalyst nanoparticles should be carefully deposited and patterned on the substrate before the initiation of the catalytic growth process. Patterning of the catalyst nanoparticles can be achieved by either monolayer selfassembly, phase separation of block co-polymers or photolithography. 36,76,77 For example, iron nanoparticles were patterned on the silicon surface by using a self-assembled monolayer of noctadecyltrichlorosilane (OTS). The surface of the OTS layer was precisely oxidized using a conductive scanning force microscopy tip and the formed carboxylic groups that could bond with ferrous iron cations (Fe 2+ ). Aerwards a hydrazine vapor reduction process was applied on the OTS surface and patterned Fe nanoparticles were obtained. A microwave power of 300 W was used to grow CNTs on the Fe loaded and OTS coated silicon substrate. At this time, the Fe nanoparticles were heated by microwave irradiation selectively to induce CNT growth. As a result of the selective-heating mechanism, the growth process was able to proceed at very low environmental temperatures (130-150 C) and a short time (3 minutes) (Fig. 12).

The mechanism of selective heating and the role of metal catalysts
The selective heating of microwaves can realize the growth of CNTs at low temperatures and atmospheric pressure. It will therefore make the in situ growth of CNTs on plastics and sensitive electronics such as circuits possible. These CNTcontaining composites can be further used in exible electronics, sensors and eld-effect transistors. The selective heating mechanism of microwaves is based on the sharp difference between the microwave absorption properties of the catalysts and the substrates, which would result in the preferential absorption of the microwave power by the catalysts and raise the temperatures near the catalyst's surface. Therefore, the effect of selective heating induced by the microwaves can be maximized when the catalyst is microwave-absorbing and the substrate is microwave-transparent. It can be minimized when both the catalyst and substrate are microwave-absorbing. However, to improve the growth conditions of CNTs, it is better to choose metallic nanoparticles with higher microwave absorption properties for achieving higher temperatures on the surface of the nanoparticles. Due to the effect of heat dissipation induced by the microwave-transparent substrate, it is hard for the pure metal nanoparticles to reach the appropriate temperature range for CNT growth on their surface under the electromagnetic eld of microwaves. [34][35][36] To improve the microwave-absorption properties of the metal nanoparticles and make it sufficient to sustain CNT growth, several strategies can be employed besides sulfurization, including porosication, morphology engineering and particle size control. [78][79][80][81][82][83] For example, T. Liu et al. reported that the microporous Co nanoparticles derived from Co-Al alloy nanoparticles can have enhanced microwave absorption properties in terms of low reection loss (RL) and wide absorption bandwidth. 78 By converting the Ni compounds into a 3D owerlike structure, D. Liu et al. were able to obtain Ni microspheres with enhanced microwave absorption properties. 80 Wen et al. also proposed that the microwave absorption properties of cobalt powders can be enhanced by changing their morphologies from spherical to ake-like. 81 By controlling the sizes of the ferrite particles, Panwar et al. demonstrated that minimum RL can be obtained by changing the particle size ranges from 200-250 nm to 20-30 nm. 83

Conclusions and future work
The synthesis and growth of carbon nanotubes by using commercial or domestic microwave ovens present a new pathway for the fabrication of CNTs and related materials.
Microwave methods feature very short processing time, widerange adaptability to different substrates, non-solvent reaction and highly efficient selective-heating mechanism. They can signicantly reduce the processing requirements for CNT growth, e.g., time, equipment, feed gas and cost. The unique selective-heating mechanism of microwave can realize the growth of CNTs at low temperature and atmospheric pressure. By using these advantages, in situ growth of CNTs at selected locations on silicon wafers and chips, or low-melting-point substrates such as Teon, can be achieved by microwave methods, and they can be further utilized to fabricate and manipulate CNT-based nanoelectronics, circuits and sensors in situ. On the other hand, microwave methods have been proven to be highly adaptive to produce various CNT-based nanocomposites. Different kinds of carbon sources, catalysts, substrates and dopants can be simultaneously combined and irradiated in a domestic microwave oven which is turned into a simple but effective plasma reactor to synthesize new CNTbased composites and functionalized CNTs suitable for a wide-range of applications from lithium-ion batteries, eldeffect transistors, and thermoelectronics to photonics.
However, current CNTs produced by microwave methods suffer from drawbacks such as high defect density, random orientation and low structural homogeneity. To address these problems, the future microwave CNT synthesis techniques should strike a balance between the growth rate and microstructural control, e.g., to use longer synthesis time and lower microwave powers for more homogeneous growth. To achieve low-temperature CNT growth at atmospheric pressure, sulfurized metal nanoparticles such as Co 9 S 8 and FeS 2 can be used as high-reactivity catalysts instead of conventional metallic compounds and nanoparticles. Porosity, morphology and particle size engineering can also be applied for pure metal nanoparticles to enhance their microwave absorption properties. The defect density of the as-grown CNTs can be effectively lowered by the addition of reducing reagents while their orientation can be improved by using low-molecular-weight hydrocarbons such as methane, ethane and ethanol as the precursors. Aer all, the efforts for improving the microwave methods would eventually afford fast, large-scale, low-cost Fig. 12 (a and b) The n-octadecyltrichlorosilane monolayer was patterned using a conductive scanning force microscopy tip, which converted its methyl groups into carboxylic groups. (c) The carboxylic groups can act as the anchor sites for the ferrous cations, which were subsequently reduced to form iron nanoparticles as the catalysts. (d) The iron catalyst loaded n-OTS layer on a silicon substrate was placed on top of a quartz glass support filled with ethanol in a pressure vessel and irradiated with microwaves to grow CNTs. Reproduced from ref. 36 synthesis of high-quality oriented CNT brushes that are suitable for a wide range of applications.

Conflicts of interest
The authors declare no conict of interest.