Acc. Chem. Res. 2002, 35, 1035-1044 Carbon Nanotubes: Synthesis, Integration, and Properties HONGJIE DAI* Department of Chemistry, Stanford University, Stanford, California 94305 Received January 23, 2002 ABSTRACT Synthesis of carbon nanotubes by chemical vapor deposition over patterned catalyst arrays leads to nanotubes grown from specific sites on surfaces. The growth directions of the nanotubes can be controlled by van der Waals self-assembly forces and applied electric fields. The patterned growth approach is feasible with discrete catalytic nanoparticles and scalable on large wafers for massive arrays of novel nanowires. Controlled synthesis of nano- tubes opens up exciting opportunities in nanoscience and nano- technology, including electrical, mechanical, and electromechanical properties and devices, chemical functionalization, surface chem- istry and photochemistry, molecular sensors, and interfacing with soft biological systems. Introduction Carbon nanotubes represent one of the best examples of novel nanostructures derived by bottom-up chemical synthesis approaches. Nanotubes have the simplest chemi- cal composition and atomic bonding configuration but exhibit perhaps the most extreme diversity and richness among nanomaterials in structures and structure-prop- erty relations.1 A single-walled nanotube (SWNT) is formed by rolling a sheet of graphene into a cylinder along an (m,n) lattice vector in the graphene plane (Figure 1). The (m,n) indices determine the diameter and chirality, which FIGURE 1. (a) Schematic honeycomb structure of a graphene sheet. Single-walled carbon nanotubes can be formed by folding the sheet are key parameters of a nanotube. Depending on the along lattice vectors. The two basis vectors a1 and a2 are shown. chirality (the chiral angle between hexagons and the tube Folding of the (8,8), (8,0), and (10,-2) vectors lead to armchair (b), axis), SWNTs can be either metals or semiconductors, with zigzag (c), and chiral (d) tubes, respectively. band gaps that are relatively large (∼0.5 eV for typical ∼ diameter of 1.5 nm) or small ( 10 meV), even if they have charging, Luttinger liquid, weak localization, ballistic nearly identical diameters.1 For same-chirality semicon- transport, and quantum interference.2-4,7,8 On the other ducting nanotubes, the band gap is inversely proportional hand, semiconducting nanotubes have been exploited as to the diameter. Thus, there are infinite possibilities in novel building blocks for nanoelectronics, including tran- the type of carbon tube ªmoleculesº, and each nanotube sistors and logic, memory, and sensory devices.2-6 could exhibit distinct physical properties. Nanotube characterization and device explorations The past decade has witnessed intensive theoretical have been greatly facilitated by progress in nanotube and experimental effort toward elucidating the extreme synthesis over the years. At the same time, many aspects sensitivity of the electronic properties of nanotubes to of basic research and practical application requirements 2-6 their atomic structures. It has been revealed that have been driving and motivating synthetic methods for metallic and semiconducting nanotubes exist in all ma- better and more homogeneous materials. This comple- terials synthesized by arc-discharge, laser ablation, and mentary cycle continues, and it is now at a time when chemical vapor deposition methods. Metallic SWNTs have nanotube synthesis has evolved from enabling growth into become model systems for investigating the rich quantum consciously controlling the growth. While the extreme phenomena in quasi-1d solids, including single-electron sensitivity of nanotube structure-property relations has led to rich science and promises a wide range of applica- tions, it poses a significant challenge to chemical synthesis Hongjie Dai was born in Shaoyang, Hunan, P. R. China, in 1966. He received his B.S. from Tsinghua University in 1989, his M.S. from Columbia University in 1991, in controlling the nanotube diameter and chirality. Un- and his Ph.D. from Harvard University with Charles Lieber in 1994. After derstanding how to synthesize nanotubes with predictable postdoctoral work with Richard E. Smalley at Rice University, he joined the faculty of the Chemistry Department at Stanford University in 1997. * E-mail: [email protected]. 10.1021/ar0101640 CCC: $22.00 2002 American Chemical Society VOL. 35, NO. 12, 2002 / ACCOUNTS OF CHEMICAL RESEARCH 1035 Published on Web 08/07/2002 Carbon Nanotubes: Synthesis, Integration, and Properties Dai properties essentially requires exquisite control of atomic specific catalytic sites on surfaces.9 We have carried out arrangement along the tubes, which is an ultimate task such patterned growth for both multiwalled and single- for chemists in the nanotube field. walled nanotubes, exploited ways including self-assembly Arc-discharge, laser ablation, and chemical vapor depo- and active electric field control to manipulate the orienta- sition have been the three main methods used for carbon tion of nanotubes, and pursued several generations of nanotube synthesis.6 The first two employ solid-state catalysts ranging from powdery supported catalyst to carbon precursors to provide carbon sources needed for discrete catalytic nanoparticles. These works have led to nanotube growth and involve carbon vaporization at high ordered nanotube arrays or networks formed at the temperatures (thousands of degrees Celsius). These meth- synthesis stage of nanotubes. ods are well established in producing high-quality and (a) Ordered Arrays of Multiwalled NanotubessSelf- nearly perfect nanotube structures, despite large amounts Assembly by Intratube van der Waals Interactions. Our of byproducts associated with them. Chemical vapor earlier work has demonstrated the synthesis of regular deposition (CVD) utilizes hydrocarbon gases as sources arrays of ordered towers consisting of multiwalled nano- for carbon atoms and metal catalyst particles as ªseedsº tubes (MWNTs) by CVD growth (700 °C; carbon source, for nanotube growth that takes place at relatively lower C2H4; alumina-supported iron catalyst) on porous silicon temperatures (500-1000 °C).6 For SWNTs, none of the or silicon substrates patterned with iron particles in square three synthesis methods has yielded bulk materials with regions (Figure 2a).17 The nanotubes within each tower homogeneous diameters and chirality thus far. Nonethe- are well aligned along the direction perpendicular to the less, arc-discharge and laser ablation techniques have substrate surface. The alignment is a result of nanotubes produced SWNTs with impressively narrow diameter grown from closely spaced catalyst particles in each square distributions averaging ∼1.4 nm. CVD methods have come self-assembling into rigid bundle structures due to strong a long way from producing carbon fibers, filaments, and intratube van der Waals binding interactions. The rigidity multiwalled carbon nanotubes to the synthesis of of the assembly allows the nanotubes to self-orient and SWNTs6,9-14 with high crystallinity and perfection com- grow perpendicular to the substrate. The arrayed nano- parable to those of arc15 and laser16 materials, as revealed tubes exhibit excellent characteristics in electron field by electrical transport and microscopy and spectroscopy emission, opening up the possibility of spatially defined measurements. massive field emitter arrays derived by simple chemical routes for flat panel display applications.17 This Account presents our up-to-date research on controlling the growth of carbon nanotubes by CVD (b) Ordered Networks of Suspended Single-Walled s approaches. While arc-discharge and laser ablation meth- Nanotubes Self-Assembly by van der Waals Interactions ods produce only tangled nanotubes mixed randomly with with Substrates. We have synthesized suspended SWNT byproducts, we illustrate that site-selective CVD synthesis networks with well-defined orientations on substrates 18,19 on catalytic patterned substrates grows nanotube arrays containing lithographically patterned silicon posts. at controllable locations and with desired orientations on Contact printing is used to transfer catalyst materials onto ° surfaces. We further show successful chemical vapor the tops of pillars selectively, and CVD (900 C; carbon deposition of SWNTs on preformed discrete catalytic source, CH4; supported iron catalyst derived by a template nanoparticles. The results suggest that understanding the method from liquid precursors) on the substrates leads chemistry involved in the growth and controlling the to suspended SWNTs forming nearly ordered networks catalytic nanoparticles could eventually allow for the with the nanotube orientations directed by the pattern of - control of the diameter and chirality of nanotubes. We the silicon posts (Figure 2b d). The mechanism for also review elucidations of the interplay of electrical, nanotube self-orienting in this case is due to van der Waals mechanical, and chemical properties of nanotubes as interactions between nanotubes and the silicon posts. The facilitated by our progress in controlled materials synthe- nanotubes grown from the posts float and wave in the sis. Implications of nanotube functionalization, molecular gas until they are fixed by nearby posts. The suspended sensors, and interfacing with biological molecules are nanotube networks are difficult to obtain otherwise by, discussed. e.g., postgrowth assembly with arc- or laser-grown materi- als and are useful for building novel nanoelectromechani-