NANO LETTERS
Simple Route to Large-Scale Ordered Arrays of Liquid-Deposited Carbon Nanotubes
2004 Vol. 4, No. 4 603-606
Marcus D. Lay,† James P. Novak,† and Eric S. Snow* NaVal Research Laboratory, Washington, D.C. 20375 Received December 25, 2003; Revised Manuscript Received February 17, 2004
ABSTRACT Carbon nanotube (CNT) transistors, sensors, and field emitters have recently attracted much attention. However, it is difficult to form ordered submonolayer arrays of these molecular wires because of their high aspect ratio and strong inter-nanotube van der Waals interactions. A tractable procedure for the formation of horizontally aligned CNT arrays will assist in incorporating them into devices that can be mass produced. Such a method is described here. Large-scale, ordered arrays of CNTs were deposited at room temperature from aqueous suspensions onto silanized SiO2 surfaces. When these arrays were used as the source-drain channel for field-effect transistors, device performance showed a strong dependence on CNT orientation.
Introduction. Carbon nanotubes (CNTs) have been the focus of intense research interest since their report in 1991.1 Because of their high tensile strength, extraordinarily high electron mobility,2 and nanometer-scale dimensions, there is considerable interest in incorporating these 1D wires into a wide variety of devices, including transistors and sensors. Field-effect transistors with individual p-type semiconducting single-walled carbon nanotubes (SWNTs) bridging metal source-drain contacts have been reported.3-5 These devices exhibited a higher field-effect mobility and transconductance per unit channel width than comparable devices composed of crystalline silicon. Yet, multiple SWNTs, side-by-side, are needed to provide the necessary current density to surpass the drive of current Si devices. Additionally, recent advances in producing n-type SWNTs6-8 present the possibility of creating more complex electronic devices composed entirely of cross-linked SWNTs. One of the major obstacles to the mass production of these devices is the lack of a method to deposit SWNTs on a wide variety of substrates with precise control over the density, position, and orientation of the SWNTs. One way to circumvent this difficulty is the use of electrically continuous random arrays of SWNTs, as previously reported by Snow and co-workers.9 Transistors composed of low densities of these random arrays (∼1 µm-2) were found to have fieldeffect mobilities of ∼10 cm2/Vs, exceeding by an order of magnitude the performance of amorphous Si and organic thin-film transistors. Additionally, Heath and co-workers have reported that SWNT ropes, up to 20 times longer than * Corresponding author. E-mail:
[email protected]. † National Research Council Postdoctoral Associate, Naval Research Laboratory. 10.1021/nl035233d CCC: $27.50 Published on Web 03/13/2004
© 2004 American Chemical Society
the SWNTs composing them, could be formed into crossbar arrays via the self-assembly of single SWNTs at lithographically patterned electrodes.10 Another method of device fabrication involves the use of an atomic force microscope (AFM) tip to position individual CNTs onto source and drain electrodes. Although effective, this technique is too protracted for mass production. This report presents a next step in improving CNT device fabrication; devices were formed from large-scale parallel arrays of SWNTs deposited from an aqueous dispersion. Parallel arrays of CNTs are highly anisotropic conductors of heat and electricity.11 Furthermore, horizontal arrays of SWNTs have shown improved field-emission properties (such as lower onset and threshold electric fields) over vertically aligned SWNTs because emission occurs from the tips (which are sometimes capped by residual catalyst particles, reducing emission) as well as from defects along the body.12,13 Additionally, SWNT-encapsulated ferromagnetic metals have shown a high magneto-resistance effect14 and may be aligned to form future high-density magnetic storage devices. A method to align CNTs in large numbers may also facilitate the mass production of SWNT field-effect transistors. Therefore, there is significant interest in techniques that involve depositing aligned SWNTs onto a wide variety of substrates. There have been reports of several high-temperature techniques, such as thermal CVD, used to grow horizontally aligned SWNTs.15,16 However, the substrates that could be used were limited because of the high temperatures necessitated by the growth process. Therefore, various lowtemperature techniques have been investigated. Lieber et al. have reported that inorganic semiconducting nanowires were
assembled into ordered arrays with up to micrometer-scale order using a combination of fluidics and surface patterning,17 yet attempts to deposit ordered arrays of CNTs from suspension have met with limited success.18,19 Suh and coworkers used dc electric fields to deposit SWNTs onto an interdigitated array of electrodes and found a small fraction bridging the electrodes.20 A recent report by Hong et al. demonstrated that large-scale arrays of aligned CNTs could be fabricated using dip pen nanolithography or microcontact printing to create arrays of polar regions on Au-coated surfaces.21 SWNTs were then deposited onto these polar regions from organic suspensions and were found to align around the patterned areas with a high degree of disorder because there was little directional control over individual SWNTs. The Langmuir-Blodget method has also been used to deposit single and multiple layers of functionalized CNTs22-24 but has not proven useful in depositing the lowdensity arrays needed for many electronic applications. Our method, which aligns the SWNTs in a flowing solution prior to deposition, results in significantly better alignment. In typical AFM images, over 90% of the SWNTs were rotated by less than (5°. Experimental Section. SWNTs were obtained from CNI Technologies (purified) and Carbolex (as-produced). Solutions of varying SWNT concentrations were prepared in a 1% sodium dodecyl sulfate (SDS) solution via probe sonication, followed by centrifugation at 14 000 g. The upper three-quarters of the supernatant was then carefully removed. The substrates were Si/SiO2 substrates with a 250-nm thermal oxide. These substrates were functionalized to produce an amine-terminated surface by exposure to a solution containing 3-aminopropyl-triethoxysilane. This functionalized substrate was inverted and grazed against the surface of the SWNT solution such that a layer of solution remained on the surface. While still coated with this solution, the substrate was blown dry in a stream of N2. This resulted in an optically homogeneous thin film of SDS + SWNTs. The substrate was then rinsed with deionized H2O and dried again. This process was repeated to yield a desired resistance. AFM imaging of samples prepared in this manner showed that the deposited SWNTs were principally aligned in the N2 streamflow direction (Figure 1). Results and Discussion. Apparently, the SWNTs in the solution that coated the silanized Si/SiO2 surface were driven forward by the orientational shear flow of the solution, as propelled by the N2 stream, and aligned parallel to the flow direction to reduce resistance. As the solution dried, SWNTs diffused to the surface and adhered through strong van der Waals interactions. The SDS was then rinsed away, leaving the aligned SWNTs behind. Because of slight fluctuations in the angles of the deposited SWNTs with respect to the drying direction, there is significant end-to-end registry, in contrast to a previous report involving Si nanowires deposited using the LangmuirBlodget method.25 Additionally, a small fraction (
