Thin films composed of carbon nanotubes (CNTs) are an emerging class of material with exceptional electrical, mechanical, and optical properties that can be readily integrated into many novel devices.[1–4] These features suggest that CNT films have potential applications as conducting or semiconducting layers in different types of electronic, optoelectronic, and sensor systems. To understand better how to obtain these films and how to fabricate devices using them, film-forming techniques and experimental work that reveals their collective properties are of importance from fundamental and applied viewpoints.Get more news about Carbon Nanotube Thin Film,you can vist our website!
CNTs are a well-known class of material, whose molecular structure can be considered as a series of graphene sheets rolled up in certain directions designated by pairs of integers.[5] In fact, the exceptional electrical, mechanical, optical, chemical, and thermal properties have terms with their unique quasi-one-dimensional structure, atomically monolayered surface, and extended curved π-bonding configuration.[6–10] For example, with a different chirality and diameter, an individual single-walled nanotube (SWNT) can be either semiconducting, metallic, or semimetallic, and they can be used as active channels in transistor devices because of their high mobilities (up to about 10000 cm2Vs–1 at room temperature),[11] or as electrical interconnectors, because of their low resistivities,[12,13] high current-carrying capacities (up to about 109 A cm–2),[14] and high thermal conductivities (up to 3500 W m–1 K–1).[15] With their unique structure, CNTs are stiff and strong, with Young’s moduli in the range of 1–2 TPa. Their fracture stresses can be as high as 50 GPa, exhibiting a density-normalized strength 50 times larger than that of steel wires.[16] In addition, the weight-normalized surface area of CNTs can be as high as 1600 m2 g–1,[17] thereby rendering them suitable for various sensor applications. CNTs can be used in many areas, ranging from nanoscale circuits,[18,19] to field-emission displays,[20] to hydrogen-storage devices,[21,22] to drug-delivery agents,[23,24] to light-emitting devices,[25,26] thermal heat sinks,[27,28] electrical interconnectors,[29] and chemical/biological sensors.
It should be noted that the electronic features of CNTs are among their most important properties. Because of their high mobilities and ballistic transport characteristics, CNT films have been considered as the best replacement for Si in future devices.[31,32] Although most CNT films show a structure of completely random networks, these films are still attractive in large-area-coverage electronics, such as macroelectronics[33], mechanical flexibility/stretchability, and optical transparency.
2. CNT thin-film synthesis
Formation of thin films of CNTs is a necessary step to their fundamental study and use in applications. For the different fabrication techniques, how to control the tube density, the overall spatial layouts, their lengths, and their orientations must be understood, because these parameters significantly influence the collective electrical, optical, and mechanical properties.
2.1. Chemical vapor deposition growth
Chemical vapor deposition (CVD) is a direct method to obtain CNT films on solid substrates. Generally, Fe and Co are used as catalysts with CO, ethylene, or ethanol as the feedstock. To prevent the pyrolysis of carbon to form soot,[34] some hydrogen is usually added. Typical processing conditions involve flowing H2 at 400–1000 sccm and CO at 200–1000 sccm with the temperature in the range 600–900 °C under argon.
CNT films formed using the CVD method show high levels of structural perfection, long average tube lengths, high purity, and relative absence of tube bundles. Moreover, the density, morphology, alignment, and position of tubes are also easily controlled in the CVD method. As is well known, the density value (D) is important because of its strong influence on the electrical properties of films. Experimental data demonstrate that the composition and flow rate of the feed gas can be used to control D. Compared with the case of methane, D for films obtained using ethanol as the carbon feedstock significantly increases. This is possibly because of the ability of OH radicals in ethanol to remove amorphous carbon seeds from catalytic sites in the early stages of growth.[35] The nature of the catalyst is also important. The multiple-component catalysts of Fe/Co/Mo [36–38] yield densities higher than those obtained from single Fe nanoparticles, because the former has an increased surface area, pore volume, and catalytic activity. In addition, the concentration of the catalyst, the size,[39–41] composition of the catalyst, growth temperature, pressure, and time can also affect properties such as D, diameter distributions, chiralities, and average tube length.
By using different driving forces from electrical fields,[43,44] laminar flow of feed gas,[45–48] and surface atomic steps,[49,50] as well as anisotropic interactions between CNTs and single-crystalline substrates,[51–53] high alignment can be obtained. For example, electric fields ( 1 V μm–1) can provide high torques, which are sufficiently large to limit thermal motions of growing CNTs, even with high-temperature growth conditions, thereby yielding field-aligned SWNTs. The degree of alignment is mainly controlled by the surface quality, cleanliness, and the physics of the underlying interactions. With catalysts patterned into small regions on a solid substrate, both perfect levels of alignment and the highest levels of D can be achieved; thus the tubes grow primarily in regions of the substrate with reacted catalyst particles.
