Montaje e integración de nanotubos de carbono para aplicaciones

Assembly/Placement/Integration of Multiple CNTs

Integrating multiple CNTs is essential for the realization of large-scale device applications. This has proved challenging due to the need for precise control and positioning of the fabricated CNTs with respect to other device elements. In this section, we focus on some of the existing techniques that are used in the process of batch level control, fabrication of multiple CNTs and their subsequent integration onto the substrates.

Batch Level Control

Catalyst Patterning

During CVD, a catalyst is often dispersed on the substrate from a solution containing a suspension of the nanoparticles. This is done by spin coating the substrate or by dipping the substrate into the catalyst solution. Alternatively, catalysts can also be deposited on the substrates by evaporation to create thin films. In order to position the catalysts at specific locations, different lithographic techniques like photolithography and microprinting have been reported.

Photolithography is used to pattern the catalyst which leads to growth of CNT thin films after lift-off. In one of the methods, controlled growth of CNTs with diameters of 0.5–1.5 nm was reported using Fe salt as catalyst. In this work, photolithography produced liquid catalyst islands on polymethyl methacrylate (PMMA) and alumina substrates. However, most of the CNTs grown were randomly oriented [184, 185]. Self-assembled masks can also be used to pattern catalysts in solution in order to control the positioning and alignment of nanotubes [186]. Another work reported the controlled growth of CNT thin films in certain regions by catalyst particle patterning using self-assembled monolayers. Here, a thick silicon substrate was thermally oxidized and positive photoresist mesas where CNT thin films were formed were patterned [187]. In a recent work, the growth of SWCNTs with diameters in the range of 0.7 nm to 2.6 nm using Prussian blue analog (PBA)-based bimetallic catalysts was reported [188]. Control on the overall catalyst size and properties was possible by synthesising PBA nanoparticles with narrow size distribution. Silicon wafers coated with an oxide layer were used as substrates. On these, a self-assembled monolayer of silane molecules (having a pyridine group at the ends) was deposited in order for the bond formation with the PBA nanoparticles to occur. Catalyst precursor reduction and the SWCNT growth were done via CVD with CH₄ (Fig. 24).

Schematic of the steps followed in the methane CVD growth of SWCNT using PBA-based bimetallic nanoparticle catalysts. In this technique, SWCNTs with diameters in the range of 0.7 nm to 2.6 nm were grown on silicon substrates coated with an oxide layer onto which self-assembled silane molecules were deposited. Adapted from [188]

Nano-imprint lithography (NIL) is another technique for patterning the catalyst [189]. This technique can be used to produce CNTs (in the form of both individual tubes and arrays or forests) with sufficient degree of control over diameters, length and quality [190, 191]. NIL uses silicon molds/stamps with different patterns of nanoscale features to imprint a desired pattern onto a polymer-based thermal resist. After this, required pressure and ultraviolet (UV) light are applied to solidify the polymer resist and form desired circuit patterns. In some cases, temperature can also be applied to the photoresist instead of UV light. Later, the stamp is removed from the resist which leaves behind an imprint of the desired patterns on the substrate. The residual layer of polymer is removed by plasma etching, thereby exposing the substrate onto which the catalyst is deposited. This substrate is loaded into CVD to grow patterns of CNTs. An example of this step-by-step procedure and the corresponding scanning electron microscope (SEM) images of CNTs grown using NIL is shown in Fig. 25 [192, 193].

un Schematic of steps involved in the growth of CNT arrays using NIL. Adapted from [193]. b , c SEM images of CNTs grown using NIL. b CNTs arranged in a word format reading ‘Nano imprint’. Scale bar equals 20 μm. Adapted from [192]. c An array of CNTs with 10 μm spacing. Inset shows the tip of an individual MWCNT grown using Ni catalyst. Adapted from [192]

New techniques using nanolithography like nanowriting with nanopipettes [194] and dip-pens [195] help in the growth of CNTs at predetermined locations. For example, in the dip-pen method, the tip of an atomic force microscope (AFM) is usually dipped in an ‘ink’ that can subsequently be transferred to a substrate with nanometer-scale precision. Similarly, nanowriting provides direct and precise control over surface patterning without requiring complex lithographic processing [196].

Controlled production of large-area SWCNT networks can also be done using precise nanometer-scale catalyst patterning resulting in desired alignment of individual SWCNTs on silicon [197]. In this method, the catalysts act as a breadboard that connects the nanotubes with desired alignments. Here, a colloidal mask was used to pattern catalyst nanoparticles using polystyrene spheres that were deposited from liquid suspension and allowed to self-assemble during drying into hexagonal close-packed monolayer regions as shown in Fig. 26.

un Schematic of steps involved in fabrication of patterned catalyst array on undoped Si substrates using colloidal lithography. The spheres represent polystyrene spheres with a diameter of 450 nm. Adapted from [197]. b SEM image of the individual SWCNTs connected between catalyst patterned nanoparticle arrays. Adapted from [197]. c , d AFM image of individual SWCNTs with diameter of ~ 2 nm. Adapted from [197]. Green line shows the d corresponding cross-section. Adapted from [197]

Additionally, catalyst patterning can also be used to control the growth orientation of CNTs during CVD by patterning the catalyst layer on slanted surfaces etched using potassium hydroxide (KOH) as shown in Fig. 27 [198]. In this technique, the catalyst is patterned fully or partially on slanted trenches fabricated via KOH etching. After this, the patterning of a catalyst layer (of 1 nm Fe and 10 nm Al₂ O ₃ ) is carried on the sidewalls using lift-off and e-beam evaporation. Then, CVD is used to grow CNTs with the following conditions; growth was carried out at 775 °C for ~ 5 or 15 min).

Schematic of a catalyst patterning technique which involves the control of growth direction of CNTs by partially patterning the catalyst layer on slanted KOH-etched edges. The corresponding SEM image of CNT pillars grown on the pyramid inside the KOH-etched microchannel is shown. Adapted from [198]

Electric Field, Gas Flow and Substrate-Assisted Growth

Controlled synthesis of CNTs can be achieved by growing them on the SiO₂ /Si substrates in electric fields established across patterned metal electrodes [199]. In this technique, Si wafers were used with thermally grown SiO₂ as substrates. Molybdenum (Mo) metal electrodes with a gap of 10 mm were used to establish electric fields on the substrates. Then, the desired catalyst was patterned on top of the two opposing Mo electrodes leading to the growth of aligned SWCNTs across the gap between the electrodes in the direction of the applied electric field. Figure 28 [199] shows the AFM images of randomly grown nanotubes in the absence of an electric field and aligned nanotubes grown in the presence of an electric field.

AFM images of CNTs grown using CVD technique between two Mo electrodes which are shown on top and bottom of the images. un Randomly grown CNTs in the absence of an electric field. Adapted from [199]. b Aligned grown in the presence of an electric field (a bias voltage of 10 V bias applied between the electrodes having a gap of 10 μm). Adapted from [199]

Another method of controlling the growth of CNTs is based on rapid heating (900 °C for 10 min) of catalyst nanoparticles (Fe/Mo) in the presence of feeding gas (CO/H₂ ) [200,201,202]. SWCNTs were grown parallel to the direction of feeding gas flow. This work reported directional control of the CNTs grown by positioning the substrate based on the gas flow direction. The location and length of SWCNTs was controlled by using photolithography to deposit the catalysts. This method produced ultra-long, well-aligned and well-isolated SWCNTs with length of few mm (Fig. 29) in contrast to an earlier work that reported that long SWCNTs (in the range of mm) either bend or form loops [203]. Here, the growth of long and straight SWCNTs was attributed to the above described growth process also termed as a kite-based growth mechanism [201].

SEM images of (a ) catalyst pattern seen on oxide coated Si wafer prior to the growth of SWCNTs. Adapted from [202]. b Long, well-oriented SWCNTs grown using fast-heating growth process with Fe/Mo nanoparticles, CO/H₂ 900 °C for 10 min. Inset shows the magnified image of the SWCNT arrays formed. Adapted from [202]. c Well aligned arrays of SWCNTs (~ 5 SWCNTs μm ⁻¹ ) formed by CVD growth on a ST-cut quartz substrate. Adapted from [207]. d AFM image of aligned SWCNTs grown on r-plane (1 1 0 2) crystalline surfaces of sapphire. Adapted from [206]

Large scale, highly aligned SWCNTs arrays can also be grown by interactions between SWCNTs and the substrate [204,205,206,207] using atomic arrangement-programmed growth. Highly aligned SWCNTs could also be grown using Y-cut single-crystal quartz or ST-cut quartz substrates using CVD of methane at 900 °C and Fe catalyst. As shown in Fig. 29c, SWCNTs with diameters of 1 ± 0.5 nm were grown on quartz using this technique. In this method, SWCNTs were synthesized using CVD of methane at 900 °C with Fe clusters that were dispersed on the sapphire substrates. Alternatively, use of a-plane and r-plane sapphire (Al₂ O ₃ ) substrates led to guided growth along specific lattice directions due to the attractive interactions between nanotubes and Al atoms that are oriented in specific crystalline directions on the substrates (Fig. 29d).

CNT Forest Growth

CNT forests (or CNT arrays) are arrays of vertically aligned CNTs that offer applications in the field of sensors [208], gecko tapes [209], strong fibers [210] and electrical interconnects due to their ability to carry high current densities of ~ 10 ⁸ A cm ⁻¹ [211, 212]. Various techniques such as use of plasma CVD, nanopatterning and flying carpets have been proposed as means of growing CNT forests [210, 213, 214]. Of these, CVD is regarded as one of the most efficient techniques to grow vertically aligned CNTs. Precise diameter control of CNT forests using pre-growth conditioning and catalyst engineering is one of the crucial parameters that influences its use in various applications [215,216,217]. In addition, control of number of layers and length of the tubes grown is also essential [218].

Growth of aligned MWCNT forests with diameters of ~ 8–15 nm was achieved using CVD in a quartz tube. Here, a Si wafer was used as a substrate, onto which a 5-nm-thick film of Fe (acting as a catalyst) was deposited using e-beam evaporation [219]. An alternate method for the fabrication of a closely packed MWCNT forest was reported using a multi-step growth method based on plasma-induced CVD technique [220]. The growth mechanism involves the following steps. First, very high-density plasma-induced catalytic nanoparticles were formed from a pre-deposited catalytic thin film. It was observed that these catalytic nanoparticles tend to aggregate at higher temperatures. In order to avoid this, the CNT nucleation process (immobilization procedure of catalytic nanoparticles) was performed and showed minimal aggregation of nanoparticles covered with graphitic carbon film. Finally, closely packed MWCNT forests were grown at a temperature of 450 °C (Fig. 30) [221, 223].

un Critical stages of CNT forest growth from a pre-deposited catalytic thin film using the multi-step plasma-induced CVD technique:formation of high-density plasma-induced nanoparticles, nucleation, and CNT forest formation. Adapted from [221]. b SEM image of the closely-packed MWCNT forest grown using plasma-induced CVD. Adapted from [222]

The direction of alignment of CNTs with respect to the substrate plays a crucial role in controlling the properties of CNTs grown. CNTs aligned vertically (i.e. perpendicular) to the substrate can be produced using porous silicon substrates with a catalyst patterned by electron-beam evaporation through shadow masks to produce MWCNT blocks. Another technique to produce these CNTs is using water-assisted CVD that produced SWCNT arrays in the order of few millimeters [135]. Here, large-scale production of dense CNT forests was reported to be possible with the help of CVD synthesis where the performance and lifetime of the catalysts was enhanced with the help of water. SWCNTs in the form of well-ordered pillars with a height of ~ 1 mm were grown from lithographically patterned catalyst islands as shown in Fig. 31 below. Alternatively, large-scale production of vertically aligned SWCNT forests is also possible using the oxygen-assisted CVD method. It was reported that the hydrogen species present during hydrocarbon CVD growth method may cause difficulties in the formation of new SWCNTs and can also potentially etch preformed SWCNTs. The introduction of oxygen during the growth process provides great control over the carbon and hydrogen ratio which is responsible for the growth of vertically aligned SWCNTs [223]. More recently, it was also shown that SWCNT forests could be grown using thermal CVD without a rectated etchant gas [224].

un , b SEM images of SWCNT forests grown using water assisted CVD. un SWCNT cylindrical pillars with 150 μm radius and ~ 1 mm height. Insert shows the root of a pillar. Adapted from [135]. b SWCNT sheets of 10 μm thickness. Adapted from [135]. c High-resolution TEM image of the SWCNTs grown using this method. Adapted from [135]

Another technique involves the fabrication of the CNTs within the pores or channels of a nanoporous template [160, 225, 226]. The commonly used templates are track-etch membranes, porous alumina (AAO) templates as well as various other nanoporous structures. Template-based synthesis allows the preparation of nanomaterials with a desired shape. A template is basically a structure in which the CNT networks form. Once the template is removed, it exposes a filled cavity with features similar to those of the template. After deposition, the nanotubes may be allowed to remain inside the pores of the templates, or they can be collected as a group of free nanoparticles. The template-based deposition process commonly makes use of CVD techniques, in which hydrocarbon precursors like pyrene and ethylene are exposed to elevated temperatures inside alumina templates. As the thermal deposition of the gas occurs over the entire surface of the pores, this method offers considerable control over the length and diameter of the tubes. One example of the growth of highly ordered arrays of parallel CNTs inside a hexagonally close-packed nanochannel alumina template is described below. At first, the anodization of high purity alumina was carried out, and led to the creation of a nanochannel alumina template made up of hexagonal array of channels with a diameter of 32 nm and length of 6 μm. Next, a small amount of catalyst (Co) was deposited electrochemically into the bottom of the template channels. By heating these templates at 600 °C for 4–5 h in a tube furnace in the presence of CO, then in a mixture of acetylene in nitrogen at 650 °C for 2 h, growth of highly ordered array of nanotubes was observed (Fig. 32) [160, 227]. Periodic array of nanotubes with diameters ranging from 10 nm to several hundred nm were grown by pyrolysis of acetylene on Co at a temperature of 650 °C.

un Mechanism of growth of highly-ordered arrays of parallel CNTs using nanochannel anodic alumina templates. The process begins with the anodization of high-purity aluminum on the substrate. By varying the anodization conditions, hexagonal close-packed arrays of varying diameters, densities and lengths can be formed. Adapted from [227]. b SEM image of the resulting hexagonally ordered array of CNTs fabricated using the method in (a ). Adapted from [227]

Another method in the growth of SWCNT forests, involves the introduction of gas shower system to deliver water vapor and carbon source gas from the top of the forests, rather than traditional way of delivering from the side. This method enables parallel flow of water vapor and carbon source gas to the CNTs within the forest. This technique enables mass production of CNT forests with increased growth yield, height, carbon efficiency and scalability for large area growth [214, 228].

Macrostructure Fabrication

Fabrication and assembly of CNTs on a macroscale is of utmost importance in order to enhance its practical applications [179]. A commonly reported technique is using spin coating. This process of fabricating CNT films involves depositing droplets of CNT-based suspensions at the center of a substrate and then spinning the substrate to a very high velocity such that the CNT dispersion spreads out on the surface of the substrate, forming a thin film (Fig. 33a) [229, 230]. This process is due to the centrifugal force on the CNT dispersion. The thickness of the film depends on the viscosity of the dispersion, angular speed, spin time and the concentration of CNTs. Before the CNT dispersion is formed, it is important to overcome the Van der Waals forces between the tubes, else they will tend to aggregate and form clumps on the film. Amphiphilic surfactants are usually added to enhance the dispersibility of the tubes; conventional surfactant will not suffice because there is a strong charge repulsion between the CNT complexes and the surfactant, which prohibits the deposition of CNTs onto the substrate. Additionally, high-power ultra-sonication or strong-acid treatment is usually performed on the tubes. After the CNT film is deposited, the surfactant is usually either washed off or vaporized from the surface of the substrate. Spin coating is not applicable for large substrates because they cannot be spun at a sufficiently high rate to form an even layer. Additionally, the process lacks material efficiency as a lot of the material is flung off during spinning [229]. Alternatively, electrophoresis process can also be used for the film deposition of CNTs [231]. Some of the other relevant techniques include using composites [232], foams [233], yarns [210] and aerogels [234] as discussed below.

un Schematic of spin coating process for the fabrication of thin film of CNTs. Adapted from [230]. The corresponding SEM images of the SWCNT film prepared by single deposition of spin coating the substrate with CNT solution in an organic solvent called quinquethiophene-terminated poly (ethylene glycol). Adapted from [229]. b A schematic of a CNT-polymer composite fabrication via solution-based processing. Adapted from [232]

CNT Composites

Due to the various properties that CNT’s exhibit, especially being superior in stiffness and strength, they can be widely used in structural applications. Furthermore, researchers are showing increased interest in tapping CNT’s properties by fabricating them into a polymer matrix. Some of the commonly used CNT-based polymer composites are made up of polystyrene, ultra-high molecular weight polyethylene (UHMWPE), PMMA, epoxy and phenolic resin [62, 235,236,237]. Various techniques have been proposed for fabricating CNT-polymer composites based on the material combinations used. Some of the common methods like solution casting, melt mixing and in-situ polymerization are discussed below.

Solution casting, also referred as solvent casting, facilitates CNT dispersion, where CNTs are suspended in the desirable polymer solution via magnetic stirring or sonication (energetic agitation process). The solvent is then allowed to evaporate to produce CNT-polymer composites. Polymers such as polyvinyl alcohol (PVA) [238], PMMA [239] and epoxy [236] have been used in this method. One such manufacturing technique is shown in Fig. 33. Here, PVA/CNT composite fibers of ~ 50 μm diameter were synthesized using the following steps. First, CNTs were dispersed in ethanol, then a PVA-coated polyester strip was dipped in a solution containing the CNT mixture. Then, the CNT-coated PVA on the polyethylene terephthalate (PET) strip were delaminated. Finally, this delaminated layer was stretched and twisted to form a composite fiber (Fig. 33a). Studies involving different solutions and methods have demonstrated that the selection of solvent for nanotube dispersion had an influence on the properties of synthesized nanocomposites and the type of solvent is dependent on the solubility of polymer in it [240]. A major shortcoming of this method is that the nanotubes may agglomerate if the solvent evaporation process is slow. This may be mitigated by increasing the rate of evaporation [241]. This leads to inhomogeneous distribution of CNT’s in the polymer matrix [242].

Another commonly used method for the fabrication of CNT-polymer nanocomposites is called melt mixing. It is mostly used for manufacturing of thermoplastics and thermistors. Thermoplastic polymers melt/soften when heated to high temperatures thus making them less viscous. This causes the substrate to be less viscous and the need for high shear forces to disrupt the nanotubes bundle. In this method, composites of various polymers (such as polystyrene and polypropylene) are formed with CNTs. To start with, the selected polymers are mixed with nanotubes in a high sheer mixer. Post this, composite films are formed using compression molding [6]. Further studies in this area has enabled different techniques such as extrusion, compression molding, injection molding, etc. for fabricating samples of various shapes [6, 243, 244].

Recently, melt-mixing method was used in the manufacturing of positive temperature coefficient (PTC) thermistors using CNTs as conductive fillers in high-density polyethylene (HDPE) composites. Traditional PTCs manufactured using carbon black (CB) filled high-density polyethylene composites suffer from disadvantages like low thermal stability and poor processability. Use of CNTs in the manufacturing process can help in overcoming these challenges as the CNT-based thermistors showed ~ 129% increase in hold current and hold voltage, in comparison with the CB-filled composites [245]. These CNT/HDPE microstructure composites were prepared by melt-mixing, hot-pressing and packaging methods as shown in Fig. 34a. CNT/HDPE composites showed a decrease in electrical resistivity with an increase in the CNT loading. Figure 34b shows a comparison of current vs time plot obtained for CNT/HDPE and CB/HDPE thermistors.

un Schematic of the fabrication of CNT/HDPE thermistors using melt mixing. The process consists of melt-mixing, hot-pressing and packaging. Adapted from [245]. b Graph comparing the current vs time plots of CNT/HDPE and CB/HDPE thermistors. The comparison of the current decay between the CNT/HDPE and CB/HDPE thermistors with the same resistances under peak applied voltages shows a quick response of current delay of the CNT/HDPE thermistors. Adapted from [245]

CNTs can also be fabricated with monomers or polymers of higher molecular weight using in-situ polymerization. In general, this method can be used in the fabrication of almost any polymer composite with CNT to polymer matrix that is either covalently or non-covalently bounded [246, 247]. In addition, this method makes the grafting of larger polymer molecules to the walls of CNT possible. This enables the fabrication using insoluble and thermally unstable polymers that cannot be achieved by solution or melt processing. A stronger interaction of CNTs with polymers during the growth stage due to π-bonding was observed in this technique [248]. One such schematic of fabrication of CNTs using in-situ polymerization is shown below in Fig. 35 [249].

Schematic of formation of SWCNT/P3HT/PPy or SWCNT/P3HT/PEDOT conductive films and fibers using in-situ polymerization. The SWCNT/P3HT monomer dispersions were prepared and then coated onto a glass slide to form a film or extruded from a syringe to form fibers. Adapted from [249]

Many other lesser known methods to fabricate nanocomposites have also been proposed such as twin screw pulverization [250], latex fabrication [251], coagulation spinning [252] and electrophoretic deposition [253].

Foams, Yarns and Fibers

Three-dimensional porous networks made of polymers and CNTs have a great potential to be used in various applications like energy storage [254] and sensing [255]. Here, CNTs are used as modifiers into structures made of different porous materials like carbonaceous aerogels, foams and sponges [219, 234, 256]. Fabrication of CNT foams and aerogels can be done using several techniques like c-CVD, phase separations, polymerization reactions, etc. Creation of CNT foams and aerogels can be done using a bottom-up method consisting of phase separations induced by thermal phase transition. Here, a three-dimensional (3D) network of CNTs is initially formed in a solution, post which the liquid part is removed without disturbing this network. This technique is commonly referred as freezing as it involves a solid-liquid phase separation process. However, it was reported that CNTs grown using the various freeze-drying techniques lead to agglomerations [257].

Another technique of CNT aerogel formation is using a gas-liquid phase separation process, commonly referred to as foaming as shown in Fig. 36 [258]. Fabrication of CNT foams with controlled pore structures using this foaming process can be done using PVA, polystyrene, polyurethane, etc. [259, 260] An alternate route to the fabrication of CNT foams is using CVD, wherein, by controlling the growth parameters like type of catalyst and source of carbon, vertically aligned CNT array foams could be formed [233, 260]. Recently, aligned CNT foam structures of desired size were fabricated by stacking sheets of CNTs and infiltrating the stacked sheet assembly with pyrolytic carbon (PyC) [233]. Here, vertically aligned MWCNTs were grown using CVD technique with FeCl₂ as catalyst and acetylene as the carbon precursor. These aligned CNTs were drawn from arrays and collected around two rotating parallel glass rods in order to have less compacted macro-porous structures. The CNT foams were then coated with 20 to 100 cycles of alumina buffer layers (that pins the catalyst particles on the surface of nanotubes) to help in the formation of secondary CNTs leading to creation of junctions, or branched CNT networks [260, 261]. In this method, prior to CVD, a buffer layer made of Al₂ O ₃ is deposited onto the porous structure to promote secondary CNT growth (in the form of straight CNTs or coiled-CNTs) at the surface of the primary nanotubes and within the pores of the CNT foams [261].

CNT foam formation using various techniques. The figure depicts various phase separation processes being performed on a CNT suspension to produce the CNT foam. The phase separation process is usually done via a gas-liquid phase separation or a solid-liquid phase separation as shown in the figure. Adapted from [258]

MWCNTs shaped into yarns have potential to be used in multifunctional applications like artificial muscles and actuators, in electronic textiles and as fiber-based supercapacitors [219]. Electromechanical actuators formed using sheets of SWCNTs (as electrolyte-filled electrodes of a super capacitor) are reported to have shown higher stresses than natural muscle. Like natural muscles, the macroscopic actuators are assemblies of billions of individual nanoscale actuators and are shown to have a great potential in various applications [39, 262, 263]. In addition, by twisting CNTs from a MWCNT forest, CNT yarns could be formed [219, 264]. Here, MWCNTs with a diameter of ~ 10 nm were drawn from a MWCNT forests (grown using CVD by using Fe catalyst) and twisted with a motor running a variable speed of ~ 2000 rpm [219]. This twisting causes the MWCNTs to form small individual bundles consisting of few CNTs. Further, by allowing the twisted yarns to relax (untwist in an opposite twist direction), knitted and knotted yarns were formed (Fig. 37a–c) [265,266,267]. Alternatively, MWCNT yarns could be synthesized from aerogels during CVD [268]. MWCNTs can also be drawn into sheets from MWCNT forests synthesized using CVD with ~ 3 nm iron film as a catalyst and acetylene or ethylene as carbon source [269]. These sheets were formed as a highly anisotropic electronically conducting aerogels that could be drawn into sheets with ~ 50 nm thickness as shown in Fig. 37d [270].

SEM and optical images of CNT yarns. un SEM image of a part of CNT yarn from the nanotube forest being drawn and twisted. Dry spinning of the CNT yarns from the forests (vertically grown CNT array) is shown. Adapted from [266]. b SEM image of a knotted CNT yarn. Adapted from [265]. c Optical image of a MWCNT knitted yarn, with a glass rod through the center. The average diameter of the MWCNT knitted yarn was measured to be 34.09 ± 2.86 μm. Adapted from [267]. d SEM image of MWCNTs in a forest being drawn into a sheet. Adapted from [270]

Recently, electrically conducting 3D fabrics made from hybrid Spandex (SPX)–MWCNT yarns showed excellent stretchable properties with a potential to be used as artificial muscles (Fig. 38). They were fabricated in a knitting machine into which a CNT aerogel sheet drawn from CNT forest, wrapped around continuous supply of SPX filament was fed. These fabrics exhibited breaking strains of 600% to 900%, tensile strengths in the range 75 to 86 MPa and an approximate resistance of 3.0 kΩ m ⁻¹ [271]. They offer unique advantages such as large tensile actuation, high repeatability, scalability and stretchability due to which they can be used in various applications like medical sensors/devices and smart clothing.

SEM image of CNT/SPX8 knitted textile and the corresponding mechanical properties. un SEM image of CNT/SPX8 knitted textile. Adapted from [271]. b Stress–strain curves of the CNT textile material. Adapted from [271]. c Strain versus time for this textile, stretched for 1000 cycles at 0.2 Hz. Adapted from [271]

Alignment/Placement on Substrates

Most methods of CNT mass production result in randomly oriented CNT bundles. The placement and alignment of the tubes can either be done while they are being grown or after they have been grown. In the former, the CNTs are grown in a given direction and position within the substrate or normal to it, while in the post-growth method, the tubes are first isolated in a dispersion and then the dispersed CNTs are aligned and selectively positioned in a certain direction by applying external forces. Various methods have been used for aligning and placement of CNTs on substrates, as discussed below.

Photolithography

Photolithography has been successfully used to prepattern either catalysts or substrates for site-selective growth of CNTs. In such a case, the photolithography setup consists of a substrate (glass), a patterned resist and a photomask. The process involves the following steps; first, direct photolithographic patterning is performed on the metal-containing photoresist. Next, metal oxides are heated and then reduced by hydrogenation to patterned metal nanoparticles, which act as catalysts from which aligned CNTs are grown, via the hydrocarbon pyrolysis of FePc (Fig. 39a, b) [272]. Alternatively, catalyst patterning using photolithography can be done by creating a template with desired patterns of photoresist, followed by molding of PET stamps which are used in transfer printing of CNTs. However, this technique suffers from certain limitations which include not having the necessary resolution to grow nanometer-scale patterns. In addition, growth of CNTs may be hindered/affected due to contamination that may be introduced through these masks [195].

un Schematic flow diagram of micropattern formation of vertically aligned MWCNT arrays by direct photolithography process. Adapted from [272]. b SEM images of patterned films of vertically aligned MWCNT arrays formed by the pyrolysis of FePc onto a photolithographically patterned quartz substrate. The resolution of the figure is in the micrometer scale. Adapted from [272]. c Step-by-step illustration of transfer printing of SWCNTs onto various substrates by contact stamping. Adapted from [273]

Transfer Printing

Currently, most nanoelectronics devices are being made on flat, rigid, smooth surfaces. This poses a problem because future nanoelectronics applications will require for nanomaterials to be placed on flexible substrates, such as plastic, or nonplanar materials. Due to this, methods such as transfer printing, which allow for the fabrication of a device on a conventional substrate and then transfer-printing of the entire working device onto target substrates, are being developed [273, 274]. This precludes the need for performing harsh synthesis and fabrication methods on the target substrate. The mechanism for transfer printing CNTs is as follows; first, the tubes are stabilized by mixing them with surfactant molecules, next the surfactant-stabilized CNTs are deposited on a variety of substrates from the solution. The most common process for the deposition of the tubes with wide area coverage is based on controlled flocculation. The controlled flocculation approach uses a spinning mechanism, in which the suspension of tubes and a solvent (methanol) are applied at the same time, to a spinning substrate. The methanol or any other hydrophilic solvent displaces the surfactant from the nanotubes, by bonding to them thereby causing the tubes to precipitate from the solution, to the edges of the substrate where they are deposited on a high-resolution polydimethylsiloxane (PDMS) stamp. The tubes form thick films on the PDMS stamps, and the stamp can be pressed on the target substrate to transfer the tube patterns from the raised regions of the stamp (Fig. 39c) [273]. One problem with this technique is the requirement to pattern the stamps each time based on the design [275].

Template-Based Deposition

Patterning in thin film CNTFET channels can be achieved with an inexpensive solution-based self-assembled colloidal mask that leads to devices with large on/off ratios while maintaining carrier mobility and sub-threshold performance. By guiding SWCNT thin film formation, the colloidal mask approach allows an ordered nanoscale CNT network with more consistent and accessible channel surface area resulting in patterned thin film FET channels whose structure and properties can be tuned for different applications (such as sensors). In one of the template-based deposition techniques, the patterned SWCNT thin films were fabricated using colloidal lithography [241]. This method employed a liquid-based self-assembly of colloidal sphere monolayers as masks to create ordered nanoscale arrays and films. Here, heavily doped silicon wafers with ~ 100-nm-thick SiO₂ layer was used as a substrate and patterning of Au/Ti source-drain electrodes with ~ 40 nm thickness and 1–3 μm spacing on the surface of wafer was carried using photolithography. SWCNT powder (1.2–1.5 nm diameter and 2–5 μm length) was dispersed in chloroform with SDS surfactant to form aqueous suspensions, which was mixed with an aqueous suspension of silica or polystyrene colloidal spheres to form patterned nanotube networks. The overall process flow for patterning the SWCNT thin films and the corresponding results are illustrated in Fig. 40a–c.

un Schematic of fabrication process for patterned CNTFET networks. b Optical microscope image of patterned electrode substrates after self-assembled colloidal mask assembly. c SEM image of interconnected nanotube ring patterns formed during the self-assembly process and gate voltage-dependent I - V characteristics of resulting CNTFET. The center to center distance of the circles is around 500 nm. (Inset) shows the corresponding magnified view of the interconnected nanotube ring patterns. [Y. Chen and C. Papadopoulos, unpublished results]

Another technique to control the placement, alignment and spacing of CNTs is achieved using DNA-origami structures [276,277,278]. Here, various geometrical shaped (L-, T-, Y-, rectangular, triangular, etc.) nanotube structures are formed on DNA-origami templates using self-assembly process (Fig. 41), where the CNTs were organized into several patterns, with control over the inter-tube angles. In one of the techniques, CNTs solubilized by wrapping with ssDNA reacted with the DNA origami constructs forming linear arrays of ssDNA that lead to immobilization of the CNTs onto the DNA origami scaffold. This was due to the strong π–π interaction existing between the bases of ssDNA and CNTs. SWCNTs with lengths ranging from ~ 92 ± 24 nm (termed as short SWCNTs) and ~ 314 ± 249 nm (termed as long SWCNTs) were obtained in this work [276].

Schematics of a rectangular DNA-origami. Adapted from [276]. b , c Site-specific immobilization of ssDNA-wrapped short and long CNTs onto the single rectangular DNA-origami scaffold. Adapted from [276]. d AFM images of DNA origami-nanotubes formed using self-assembly showing CNTs aligned at the edges. Adapted from [276]. e AFM images of DNA origami-nanotubes formed using self-assembly showing two single rectangular DNA-origamis aligning two long 6,5-SWCNTs. Adapted from [276]. f Cross-shaped SWCNTs. Scale bar equals 100 nm. Adapted from [276]

Solution-Based Deposition of CNTs

Most solution-based alignment or deposition methods of CNTs require the formation of CNT films from CNT suspensions. As such, there’s a need to form uniform dispersions of CNTs in a solvent. A challenge that is commonly encountered in dispersing CNTs is overcoming the strong inter-tube interaction of CNTs, which is attributed to van der Waals forces. When CNTs are placed in a solvent, they usually bundle together. This attractive force of the CNTs is usually overcome by binding the CNT walls to surfactant molecules (surfactant wrapping) to form stable solutions of CNTs. As such, these van der Waals forces are a major consideration when using solution-based methods for aligning CNTs [230]. Some of the solution-based alignment techniques are discussed below.

Inkjet printing is a precise method of patterning, and therefore, post-printing steps are not needed. This technique is advantageous because it does not require the use of a prefabricated template, thereby allowing for rapid printing at a low cost [279, 280]. Additionally, it is valuable because many layers of ink can be printed on top of one another. The carbon nanotube ink is prepared following certain steps; first, sonication is used to disperse the CNTs within the liquid. After dispersing the CNTs, the ink is centrifuged several times to separate the well-dispersed CNTs from the bundles, which could clog the printer nozzle. Next the supernatant is collected and filtered severally to remove any remaining CNT bundles. After the ink has been successfully produced, it is loaded into an inkjet cartridge and ready to be used for printing on substrates like glass and polymers.

Consumer inkjet printers are of two types; continuous and drop-on-demand. The former supplies a continuous stream of ink droplets, which are charged once they leave the nozzle, and are then deflected by voltage plates, such that the applied voltage determines if the droplet will be deposited on the substrate, while the drop-on-demand printer functions in a different manner, in that, the printer only ejects ink when required [279]. Carbon nanotube inkjet printing has been successfully used to deposit MWCNTs and SWCNTs. It has also been used to fabricate transistors, sensors and electroluminescent devices. The process is a useful development in electronics because it allows direct printing of the CNTs to make a pattern or a circuit on a suitable substrate, thereby enabling us to control the transparency and resistance of the printed patterns (Fig. 42a) [281]. In order to produce uniform networks of CNTs by inkjet printing, the jetting conditions need to be properly tuned to generate correctly directed droplets. Another important consideration is the temperature of the substrate. The substrates need to be heated in order to reduce the drying time for the printed CNT solution. For manufacturing purposes, quick drying of the dropped ink is desirable. However, the temperature should not be too high as it would cause the droplet to evaporate once it is released from the nozzle. A constraint in using this process is the difficulty of dispersing nanomaterials within the ink as CNTs typically bundle together in a solvent, due to their attractive van der Waals forces. The bundling of tubes needs to be prevented as it could cause clogging of the inkjet nozzle [279].

un Optical image of a CNT-based circuit on polyimide wrapped around a test tube. The circuit was fabricated using CNT inkjet printing. Adapted from [281]. b Schematic of the Langmuir-Blodgett (LB) process for the fabrication of films. The LB process is either performed using the b vertical dipping method or c horizontal lifting method. Adapted from [283]

Alternatively, spray-coating involves spraying CNTs dispersed in a suspension on a heated substrate such that every sprayed droplet undergoes pyrolytic decomposition when it reaches the hot substrate surface thereby forming a thin layer of CNTs film [282]. CNT films can be spray deposited on glass before being transferred to a flexible substrate or they could be directly sprayed onto the flexible substrate. Before the spraying of the CNTs can take place, the solution has to be prepared. First, CNTs in powdered form are dispersed into a solvent via sonication. The CNTs are usually either dispersed in an organic solvent or in a surfactant-based aqueous dispersion. Surfactant-based dispersion using SDS or carboxymethyl cellulose (CMC) is usually preferred because the surfactants can be rinsed off after the film is deposited, as these surfactants are soluble in water. The surfactant is first dissolved in water to form an aqueous solution, to which the CNTs are later added. This forms a surfactant based aqueous CNT solution. Sonication of the surfactant-based solution is then performed to evenly disperse the CNTs. Finally, the solution is centrifuged, and the supernatant is taken from the top to be used for deposition. Before spray deposition of the CNTs can occur, the substrate surface has to be treated and cleaned using solvents like acetone and isopropanol, followed by plasma cleaning. After treatment of the substrate, the colloidal suspension of CNTs is sprayed through an air atomizing spray gun, onto a heated substrate. The dispersing fluid (in this case, the surfactant) then evaporates due to the heating of the substrate to leave behind a uniform coating or layer of CNTs.

CNT films can also be produced using the Langmuir-Blodgett (LB) technique, in which a substrate is dipped in a solution containing well-dispersed CNTs, and then slowly pulled out either by horizontal lifting or by vertical dipping method as shown in Fig. 42b, c [283]. This technique produces a thin layer of CNTs aligned in the dipping direction on the substrate. The thickness of the CNT film is dependent on the concentration of CNTs in the solution, the pulling speed, and the number of dips. Generally, SWCNTs are dispersed in an amphiphilic polymer matrix, spread on a water surface [284]. This film is transferred to a substrate by dipping the substrate in it. The disadvantage of this method, as with most solution-based methods, is that the solution might require a solvent that is incompatible with some substrates, for example plastic.

Well-aligned, horizontal arrays of semiconducting nanotubes can also be created using nanoscale thermocapillary effects in thin-film organic coatings (Fig. 43a, b) [285]. In this method, metallic SWCNTs are selectively removed via thermal resist exposure and etching. SWCNTs, grown on quartz substrates comprise of both semiconducting and metallic nanotubes with diameters between 0.6 and 2 nm. By means of thermal evaporation, thin organic thermocapillary resist layer of a,a,a′-tris (4-hydroxyphenyl)-1-ethyl-4-isopropylbenzene is deposited on these SWCNT arrays. Post this, by etching metallic CNTs are removed from the arrays.

un Schematic illustration of steps involved in selective removal of metallic CNTs using thermocapillary effects. Adapted from [285]. b Corresponding AFM images of each step starting from growth of SWCNT arrays, followed by deposition of thermocapillary resist and etching of metallic SWCNTs. Adapted from [285]. c Schematic of the deposition of well-aligned CNTs on HfO₂ by means of the ion exchange process. Adapted from [286]. d Corresponding AFM image of the nanotubes deposited on a HfO₂ trench with a width of about 200 nm. Adapted from [286]

Alternatively, aligned CNTs were deposited on a HfO₂ trench with a density of 10 ⁹ cm ⁻² using self-assembly (Fig. 43c, d) [286]. This was done through the SDS-wrapped CNTs dispersed in water and the use of 4-(N -hydroxycarboxamido)-1-methylpyridinium iodide (NMPI) to form surface monolayer. This led to ion exchange between Na ⁺ in SDS and I ⁻ in NMPI. By controlling the concentrations of SDS surfactant and dimensions of the trenches, placement of CNTs inside the trench was randomly varied. With trenches of dimensions, 200 nm and 500 nm fabrication of a billion nanotubes per square centimeter is predicted. For example, one preliminary successful functioning CNT computer circuitry was self-assembled using selective surface chemistry [287].

Emerging Applications and Challenges

Figure 44a gives an overall summary and comparison of the various CNT assembly techniques presented in this review. As seen throughout the preceding sections, CNTs have gained widespread interest for use in applications [1, 39, 52, 57, 65, 66, 276, 288]. Electronic devices, sensors, drug delivery, energy storage devices, crypto primitives and tissue engineering are just some of the potential areas where nanotubes can be employed (Fig. 44b).

un Summary description of CNT assembly and integration techniques. b Overview of various emerging applications of CNTs

CNTs are used in various commercial applications in the form of conductivity enhancement/control in plastics [289], in anti-static packaging and to enhance the strength of concrete, polymers, baseball bats, bicycles, etc. [62, 289]. In addition, use of CNTs for structural monitoring in aircraft structures is currently patented [290]. Successful use of CNTs in this application may greatly reduce the risk of an in-flight failure caused by structural degradation of aircraft.

CNTs are also finding use in biomedical applications such as imaging [291,292,293], tissue engineering as scaffolds for bone growth [294, 295] and scaffolds to guide neurite outgrowth [296] as shown in Fig. 45a–c. In addition, CNTs can also to be used in targeted drug delivery systems (Fig. 45d) [297, 298] due to their capabilities to interact with mammalian cells. Due to their ability to penetrate into cell membranes, CNTs can be used as carriers to deliver therapeutic agents into the cytoplasm of cells. CNTs that are gastrointestinally absorbed were lysosomotropic and could enter into the cells for targeted drug delivery.

un SEM image of bone-tissue sections observed 4 weeks after CNTs were implanted into tubal defects in mice. Adapted from [295]. b , c SEM images showing guided neurite growth along the MWCNT array pattern. The bottom image in part (c ) represents the magnified view of a selected region in the top image. Adapted from [296]. d Schematic of steps showing CNT-based drug delivery systems. CNTs of various lengths were used as drug carriers by making their ends open and allowing the drug to be loaded for targeted drug delivery. Adapted from [297]

As mentioned previously, a major class of applications of CNTs is in electronic devices like FETs [46, 48, 299] and to build integrated circuits (ICs) and flexible electronics [300, 301]. Some of the other applications include their use in batteries, where a CNT film was used as the current collector for a lithium ion battery [302], in transparent conductive films that can be used in flat panel displays such as laptops and cameras [57, 303], hydrogen storage to be used as fuel source [304], as interconnects [305, 306] that can potentially offer advantages over copper (due to their high thermal stability and large current carrying capacity), in non-volatile random access memory for molecular computing [307] and in thin-film solar cells made up of a semi-transparent thin film of nanotubes on a n-type crystalline silicon substrate, wherein CNTs films are used as photogeneration sites and are also used for enhancing conductivity (Fig. 46a, b) [308].

un Schematic of a thin-film CNT solar cell. Adapted from [308]. b J–V characteristics of a CNT solar cell with area = 2.15 cm ² (in red) and area = 0.06 cm ² (in black). Adapted from [308]. c Schematic of a randomly connected 2D nanotube array that can be used as a crossbar switch. Here, CNTs self-assemble in the HfO₂ trenches with width of 70–300 nm. Adapted from [314]. d Schematic of an s-SWCNT-based wearable array of electronic devices, consisting of memory units, capacitors and logic circuits that can be integrated into different circuits for day-to-day applications. Adapted from [317]

Due to their unique thermal and mechanical properties, CNTs have also found applications as nano additives in heat conductive materials made of CNT-based lubricating oils, nanoliquids, etc. [309] These additives can be used for heat dissipation in electronic devices like LEDs and computer processors [309,310,311]. The high thermal conductivity of CNTs also enables their use as thermally conductive composites that could potentially replace metallic parts in devices such as electric motors and generators [14, 312, 313].

In addition, CNTs have also been tested to create an unclonable electronic random structure to create cryptographic keys that can be a used to provide significantly higher level of security as compared to the conventional binary-bit architecture with the same key size [314, 315]. A schematic of generation of random bits based on two-dimensional (2D) carbon nanotube arrays is shown in (Fig. 46c). In this method, well-aligned CNTs are deposited on a HfO₂ trench, ~ 7 nm high using self-assembly. This was done through the SDS-wrapped CNTs dispersed in water and the use of NMPI to form a surface monolayer, which led to ion exchange between Na ⁺ in SDS and I ⁻ in NMPI. By controlling the concentrations of SDS surfactant and dimensions of the trenches, placement of CNTs inside the trench was randomly varied. Further, CNT thin films, also known as CNT mats, have shown potential to be used in radio-frequency signal transmission [316]. A combination of most of the above applications includes the use of CNTs in wearable electronics consisting of many memory units, capacitors, transistors and logic circuits, etc. (Fig. 46d) [317]. In one of the recent examples, stretchable and ultrathin wearable array of electronic devices with a dimension of ~ 3 μm showed a possibility to be integrated on the human skin. Techniques such as spin coating, thermal evaporation and photolithography were used to fabricate these devices based on the required application.

Use of aligned/organized CNTs in the form of yarns, forests and sheets can help in further development of CNTs for large-scale industrial applications. Some of the emerging applications of CNTs include their use as multi-functional coating materials, which in turn can be used as an alternative to environmentally hazardous paints containing biocides [288]. CNTs have also proven useful in anticorrosion coatings in metals (showing enhanced strength and coating stiffness) [318], and in transparent electronics like flexible displays. Due to their ability to be manufactured as metallic, semi-conducting or wide-band gap CNTs, they can also be used in manufacturing new generation electronic components and devices like batteries, supercapacitors, transistors, logic circuits, memristors, neuromorphic computing, sensors for optics, gas, pressure, glucose, tumor detection, electronic textiles, artificial muscles and electro-thermal actuators, etc. [1, 219, 297, 307, 319, 320]. CNTs have a potential to be used in other applications in the form of wires in nanoscale very-large-scale integrated (VLSI) circuits [306], to fabricate flexible and foldable sheets [321], fibers via wet spinning [264, 322], in energy and water treatment [323].

Despite their numerous advantages and potential applications, several outstanding issues related to CNTs must still be addressed in order for their potential to be fully realized in the future (cf. Fig. 44a):For example, low mechanical strength and thermal conductivity of the typical as-grown CNT material has to be studied and improved. In addition, during the growth of long nanotubes tubes with closed ends are also formed due to the presence of pentagonal and heptagonal defects in the graphene lattice. This makes it difficult for their use in many applications where access to open-ended tubes is needed, e.g. to functionalize and/or contact the tube ends. Also, controlling chirality of the CNTs grown, which critically influences the electrical properties of the nanotubes is still considered a major challenge. Growing high purity, well-aligned semiconducting CNTs is one of the important steps for realization of various applications and thus far, growth of CNTs with very few metallic CNTs for thin films and devices is yet to be fully realized. Since the choice of catalyst plays a significant role in the growth of CNTs, further research on different materials that can be used as catalysts is necessary to understand issues arising due to the tube-catalyst interactions. A major obstacle to commercial implementation of CNT (or graphene) nanoelectronics is rooted in the lack of an efficient and reliable method of device production and integration (cf. silicon integrated circuits). In addition, CNT electronics will likely have to compete with the emergence of silicon nanowires and related structures for integrated circuits in the near future as the rapid pace of Moore’s Law continues along with evolution and advances in semiconductor electronics and CMOS technology. Continued focus on the other allotropes of carbon like graphene (for 3D CNT-graphene networks used in, e.g. thermal interfaces) and fullerenes is necessary which may help in wide scale production and/or improvement in the properties of existing CNTs. For applications requiring macroscopic amounts of nanotubes, well-ordered assemblies will allow optimization of desired properties with nanoscale control. Lastly, since CNTs have not yet been fully used in large-scale commercial applications, not much is known about their possible effects on the human health and the surrounding environment [324] both during processing, post-processing and disposal [325,326,327]. Studies focusing on these aspects will also be necessary.

Conclusions

The superb (and unique) properties of CNTs and their potential use in a wide range of applications has led researchers to consider nanotubes based on carbon as one of the emerging materials that may play key roles in the future of nanoscale-based applications. This is supported by the CNT-related patent statistics between 2000 and 2017 (Fig. 47). In this review, we have focused on the progress made in the field of CNT fabrication, purification, assembly and integration of single and multiple tubes for their use in various applications. We discussed how structural control of individual CNT properties such as chirality, diameters and junctions are important factors in determining their properties (structural, electrical, mechanical, thermal properties and cost) and to utilize them for various applications. Also included is information related to post-growth purification techniques of CNTs using methods such as selective surface chemistry, gel chromatography and density gradient centrifugation. Assembly and integration for multiple CNTs using forest growth, catalyst patterning and composites are discussed. Details of their alignment/placement onto different substrates using methods such as photolithography, transfer printing and different solution-based techniques are also included. Towards the end, we list some emerging applications like sensors, field-effect devices, energy storage devices, health monitoring in aircraft structures, crypto primitives, commercial applications in the form of conductivity enhancement/control in plastics, in bio-based applications like tissue engineering, drug delivery, etc. based on their unique properties, advantages and challenges that need to be understood for efficient utilization of these structures in the future.

Trends in CNT-related patents filed worldwide between 2000 and 2017. Data collected from European Patent Office’s webpage using keywords ‘carbon nanotubes or carbon nanotube or graphene’

Significant progress has been made over the past two decades in CNT fabrication and assembly, from individual tubes to large ensembles and commercial applications should continue to grow as well-organized CNT materials are introduced. In terms of nanoscale integration, precise control of CNT type, placement and orientation will enable many applications in electronics and photonics and beyond, and despite challenges, continued advances in CNTs, with their almost ideal one-dimensional structure and properties, and related structures will play an important role in future applications of nanotechnology.

Disponibilidad de datos y materiales

Not applicable.

Abreviaturas

1D:

One-dimensional

2D:

Two-dimensional

3D:

Tridimensional

AC:

Alternating current

AFM:

Microscopio de fuerza atómica

CB:

Carbon black

c-CVD:

Catalytic chemical vapor deposition

CMC:

Carboxylmethyl cellulose

CNTFET:

Carbon nanotube field-effect transistor

CNT:

Nanotubos de carbono

CVD:

Deposición de vapor químico

DC:

Direct current

DEP:

Dielectrophoresis

DGU:

Density gradient ultracentrifugation

DNA:

Deoxyribonucleic acid

DOS:

Density of states

FCCVD:

Floating catalyst chemical vapor deposition

FETs:

Field-effect transistors

HDPE:

High-density polyethylene

ICs:

Integrated circuits

IEX:

Ion-exchange chromatography

IR:

Infrared

LB:

Langmuir-Blodgett

LCDs:

Liquid crystal displays

m-SWCNTs:

Metallic single-walled carbon nanotubes

MWCNTs:

Multi-walled carbon nanotubes

N-CNTs:

Nitrogen-doped CNTs

NINGUNO:

Nano-imprint lithography

NMPI:

4-(N-hydroxycarboxamido)-1-methylpyridinium iodide

PBA:

Prussian blue analog

PDMS:

Polidimetilsiloxano

PECVD:

Deposición de vapor químico mejorada con plasma

PET:

Polyethylene terephthalate

PMMA:

Polimetacrilato de metilo

PTC:

Positive temperature coefficient

PVA:

Polyvinyl alcohol

PyC:

Pyrolytic carbon

SDS:

Sodium dodecyl sulfate

SEM:

Scanning electron microscopy/microscope

SPX:

Spandex

SSA:

Specific surface area

ssDNA:

Single-stranded-deoxyribonucleic acid

s-SWCNTs:

Semiconducting single-walled carbon nanotubes

SWCNTs:

Single-walled carbon nanotubes

TEM:

Microscopio electrónico de transmisión

THz:

Terahertz

TPU:

Thermoplastic polyurethane

UHMWPE:

Ultra-high molecular weight polyethylene

UV:

Ultraviolet

VHS:

Van Hove singularities

VLS:

Vapor-liquid-solid

VLSI:

Very-large-scale integrated

VPE:

Vapor phase epitaxy

VSS:

Vapor-solid-solid

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