This invention relates to nanowires fabricated with a reduced diameter and aligned configuration and to methods of making same.
Nano-scale wires, such as small-sized carbon nanotubes on the order of 1-100 nanometers in diameter and 0.1-100 xcexcm in length, have received considerable attention in recent years. See Liu et al., Science, Vol. 280, p. 1253 (1998); Ren et al., Science, Vol. 282, p. 1105 (1998); Li et al., Science, Vol. 274, p. 1701 (1996); Frank et al ., Science, Vol. 280, p. 1744 (1998); J. Tans et al., Nature, Vol. 36, p. 474 (1997); Fan et al., Science, Vol. 283, p. 512 (1999); Collins et al, Science, Vol. 278, p. 100 (1997); Kong et al., Nature, Vol. 395, p. 878 (1998); and Ebbesen et al., Nature, Vol. 382, p. 54 (1996).
Carbon nanotubes exhibit unique atomic arrangements, nano-scale structures, and interesting physical properties such as one-dimensional electrical behavior, quantum conductance, and ballistic transport characteristics. The ballistic transport in carbon nanotubes, as reported by Frank et al, allows the passage of huge electrical currents in electronic circuits, with the magnitude of current density comparable to or better than those in some superconductors. Carbon nanotubes are one of the smallest dimensioned nanowire materials with generally high aspect ratio and small diameter, e.g., single-wall nanotubes may be made with diameters of xcx9c1 nm and multi-wall nanotubes with diameters of less than xcx9c50 nm {see Rinzler et al, Applied Physics, Vol. A67, p. 29 (1998); Kiang et at, J. Physical Chem., Vol. 98, p. 6612 (1994), and Kiang et al, Physical Review Letters, Vol. 81, p. 1869 (1998)}, although the single-walled nanotubes are generally prepared as bundles to result in an overall increased diameter, as described below.
High-quality single-walled carbon nanotubes are typically grown as randomly oriented, needle-like or spaghetti-like, tangled nanowires by laser ablation or arc techniques (a chemical purification process is usually needed for arc-generated carbon nanotubes to remove non-nanotube materials such as graphitic or amorphous phase, catalyst metals, etc). Chemical vapor deposition (CVD) methods such as used by Ren et al., Fan et al., and Li et al tend to produce multiwall nanowires attached to a substrate, often with a semi-aligned or an aligned, parallel growth perpendicular to the substrate. As described in these articles, catalytic decomposition of hydrocarbon-containing precursors such as ethylene, methane, or benzene produces carbon nanotubes when the reaction parameters such as temperature, time, precursor concentration, flow rate, are optimized. Nucleation layers such as thin coatings of Ni, Co, Fe, etc. are often intentionally added to the substrate surface to nucleate a multiplicity of isolated nanowires. Carbon nanotubes can also be nucleated and grown on a substrate without using such a metal nucleating layer, e.g., by using a hydrocarbon-containing precursor mixed with a chemical component {such as ferrocene, (C5H5)2Fe} which contains one or more of these catalytic metal atoms. During the chemical vapor decomposition, these metal atoms serve to nucleate the nanotubes on the substrate surface. See Cheng et al., Chem. Physics Letters, Vol. 289, p. 602 (1998), and Andrews et al., Chem. Physics Letters, Vol. 303, p. 467 (1999).
The modem trend in electronic circuit design, interconnection, and packaging is toward use of finer features, such that submicron feature sizes have been reached in recent years. In producing ultra-high density electronic packaging, a small width of the circuit lines is important, as well as a three-dimensional, multi-layer configuration with vertically-integrated circuit layers. The small dimensions of electrically-conducting nanowires such as carbon nanotubes make them useful as nano-scale, vertically-connecting wires between circuit device layers as well as in-plane connecting wires between adjacent electrical pads.
The diameter of the nanowires can affect the bandgap of semiconducting carbon nanotubes with accompanying changes in electrical conductivity. See Martel et al, APP. Physics Letters, Vol. 73, p. 2447 (1998). Control of the nanowire diameter and orientation is important for many device applications. In addition to nano-scale circuit interconnections, carbon nanotubes are useful for field emission devices such as flat panel field emission displays and microwave amplifiers. Conventional field emission cathode materials typically have been made of metal (such as Mo) or semiconductor material (such as Si) with sharp tips of submicron size. However, with these materials the control voltage required for emission is relatively high (around 100 V), because of high work functions and insufficiently sharp tips. To significantly enhance local fields and result in an overall reduced voltage requirement for operating field emission devices, it would be advantageous to provide new cathode materials (e.g., comprising carbon nanotubes) with small diameters and sharp tips.
Various previous synthesis techniques produce nanowires having uncontrolled and often vastly varying diameters. At present, there is no well-established technique for obtaining a selected and desired nanowire diameter. Single-wall nanotubes (SWNTs) are typically prepared as bundles consisting of many tens of nanotubes stuck together by Van der Waals force with each bundle having an overall, large diameter (e.g., of about 20-100 nm), although each individual SWNT may have a small diameter of about 1 nm. Multi-wall nanotubes (MWNTs) typically have much larger diameters (e.g. of about 10-100 nm). However, for use in making nano-interconnections and field emission devices, desirably the nanowires would i) have a small overall diameter, e.g., on the order of a few nanometers, ii) be capable of existing separately without Van der Waals bundling, and iii) be capable of a parallel alignment along a certain desired direction instead of having the commonly observed, tangled spaghetti-like configuration.
In utilizing the carbon nanowires for circuit nano-interconnections, unaligned or tangled nanowires can create a risk of sideway shorting between adjacent contact pads or devices. In field emission devices, unaligned, random distribution of nanowires can cause inefficient electron emissions. In such devices, unaligned nanowires create inefficiencies, i.e., in the diode field emitter configuration, due to the varying distance and hence varying local electric fields between the cathode (comprised of emitting nanowire tips) and the anode, and in the triode configuration, due to varying distances between the cathode (nanowire tips) and the gate. In addition, when unaligned nanowires are used for emitters in a field emission device, an applied electric field between anode and cathode tends to bend the nanowires toward the field direction, the degree of which is dependent on the applied voltage. This bending causes uncontrollable and undesirable changes in the distance between cathode and gate, and hence alters the local field on different nanowires. In some cases, the bending causes outright electrical shorting between the nanowire tips and the gate. Nanowires pre-aligned in the direction toward the anode could prevent or at least reduce the likelihood of such a bending problem.
Nanowire alignment can be achieved by a number of different approaches, e.g., by using electric fields or by crowding with dense, closely-spaced nanowires, as described in the related applications, U.S. Ser. Nos. 09/405,641, 09/426,457, and 09/426,453, incorporated herein. In previous approaches, e.g., those used by Li et al., Kong et al., and Fan et al., there is no clear control of nanowire diameter, orientation, or periodicity. The diameter of the CVD synthesized carbon nanotubes may be limited by the size of the catalytic metal particles, and previously-used nanotube-nucleating catalytic particles are relatively large, e.g., on the order of 10-50 nm.
As may be appreciated from the foregoing, it would be advantageous to provide a method of controlling the diameter of nanowires and reducing the diameter to a select size. It would be particularly advantageous to provide a method for the synthesis. of small diameter nanowires, e.g., those having a diameter of about 10 nm or smaller. It also would be advantageous to provide nanowires that are aligned in parallel and untangled to allow for, among other things, nano-interconnection in an orderly manner without sideway shorting between adjacent contact pads or devices. This invention discloses nanowire structures and methods for fabricating small diameter nanowires that provide these advantages. Further advantageous may appear more fully upon considering the detailed description given below.
The invention embraces a nanowire structure that may be used to fabricate small diameter and aligned nanowires, e.g., having a diameter of less than 50 nm and more preferably, less than 10 nm. The structure includes an alloy substrate that is at least two-phased; it is comprised of at least a first phase and a second phase, wherein the first phase is catalytic and the second phase is less catalytic than the first phase (weakly or non-catalytic). A plurality of small diameter nanowires are grown from the first phase of the alloy substrate. Each one of the plurality of nanowires can be substantially vertically aligned relative to the substrate and preferably, the average deviation from full vertical alignment is less than 25 degrees, and more preferably less than 15 degrees. The alloy substrate is fabricated with an alloy system of catalytic and non (or less) catalytic elements that are phase separated, e.g., they may be spinodally decomposed or subjected to nucleation-and-growth transformation to achieve the phase-separated substrate comprising the first and second phases.
The invention further comprises a method for making small diameter nanowires comprising providing the phase-separated substrate including catalytic regions, elongating the substrate to reduce the diameters of the catalytic regions, optionally performing a subtractive or additive process to the substrate to define recessed pores therein whereby the reduced-diameter catalytic regions are disposed within the cavities of the recessed pores, and growing a plurality of nanowires from the reduced-diameter catalytic regions. The elongated substrate may be sectioned into alloy pieces before the nanowires are grown therefrom to define pieces having substantially equally-sized catalytic regions. The small diameter nanowires aligned on the substrate may be used for many applications, including microwave amplifier devices, field emission display devices, and lithium-rechargeable batteries.