The present invention relates to a method for preparing metal nanowires on insulating surfaces and, more particularly, to the electrodeposition of lithographically patterned nanowires.
Electron beam lithography (EBL), developed in the early 70's, provides a means for patterning polycrystalline metal nanowires as small as 20 nm in diameter onto surfaces. The applicability of EBL, however, has been limited to research and development applications because it is a serial patterning technology. In 1990, a parallel version of EBL was described, but space charge “blurring” prevented this technique from approaching the resolution of direct-write EBL. By using, as a template, semiconductor surfaces with atomically-defined grooves and troughs, sub-10 nm metal nanowires have been prepared using vapor deposition. A variant of this approach has been used to create high density arrays of linear, 10 nm diameter, platinum nanowires. We previously demonstrated that ensembles of 30 nm antimony nanowires can be prepared by electrochemical step edge decoration on graphite surfaces coupled with etching, but no control of nanowire position on the surface or inter-wire pitch has been possible using this method.
It is desirable to provide a method for preparing nanowires with the ability to control the position on the surface the nanowire is formed, as well as the inter-wire pitch.
Embodiments described herein are directed to a new method for preparing metal nano-wires that are as small as about 20 nm in width, and patterning these wires over large areas of the surface of an insulator to create patterned metal nanowires. In a preferred embodiment, the method preferably involves optical lithography coupled with the electrodeposition of metal. Nanowire fabrication methods can be classified either as “top down”, involving photo- or electron beam lithography or “bottom-up”, involving the synthesis of nanowires from molecular precursors. Lithographically patterned nanowire electrodeposition (LPNE) combines attributes of photolithography with the versatility of bottom-up electrochemical synthesis. Photolithography is employed to define the position of a sacrificial nanoband electrode, preferably formed of nickel, copper, silver, gold and other metals, which is stripped using electrooxidation or a chemical etchant to advantageously recess the nanoband electrode between the substrate surface and the photoresist to form a trench defined by the substrate surface, the photoresist and the nanoband electrode. The trench acts as a “nanoform” to form an incipient nanowire during its electrodeposition. The width of the nanowire is determined by the electrodeposition duration while the height of the nanowire is determined by the thickness of the nanoband electrode.
Removal of the remaining photoresist and electrode layer material reveals a nanowire—preferably composed of a metal such as gold, platinum, palladium, cadmium, bismuth, or the like or a mettalloid, such as, e.g., silicon, germanium and the like, with a rectangular cross section and a height and width that can be independently controlled as a function of trench height and electrodeposition duration, down to about 20 nm in width and 6 nm in height. The polycrystalline nanowires synthesized by LPNE can be continuous for more than about 2 cm. These nanowires show a metal-like temperature dependent resistance.
a) is a graph showing oxidation current versus time during the removal of nickel from lithographically patterned surfaces by potentiostatic electrooxidation (reaction: Ni→Ni2++2e−) at −0.10 V vs. SCE in aqueous 0.1 M KCl at pH=1.0.
b) and (c) are graphs showing cyclic voltammogram for Ru(NH3)63+ at 5 mV s−1 in aqueous 0.1M NaCl before the removal of nickel and after the removal of nickel by electrooxidation resulting in the formation of a horizontal trench about 200 nm in depth terminated by a nickel nanoband about 40 nm in height.
a) is a graph showing electrodeposition current versus time for the potentiostatic growth, and overgrowth, of a palladium nanowire at +0.225 V vs. SCE.
b) is a SEM image showing a gold nanowire with a rectangular cross-section obtained from trench-confined growth for 100 s.
c) is a SEM image of a gold nanowire deposited for 1000 s showing “blooming” from the edge of the photoresist.
a) is an atomic force microscope (AFM) image showing the flat wire profile of a gold nanowire on glass prepared using the LPNE process.
b) is a graph showing nanowire height, measured by AFM, versus the thickness of the nickel layer deposited in step 1 of the LPNE process depicted in
c) is a graph showing nanowire width, measured by SEM, as a function of the electrodeposition time. For both
a) is a graph showing current versus voltage curves acquired using four evaporated electrodes (inset) for two gold nanowires prepared by the LPNE process. These nanowires had dimensions of 20 nm(h)×233 nm(w), and 100 nm(h)×166 nm(w)—the isolated wire length in both cases was 400 μm.
b) is a graph showing temperature dependence of the wire resistivity, p, normalized to the resistivity at 300K, ρ300, from 10 K to 350 K, for the same two nanowires shown in (a), and the resistivity of bulk gold.
Embodiments described herein are directed to a method of lithographically patterned nanowire electrodeposition (LPNE) for preparing metal nanowires and patterning these nanowires over large areas of the surface of an insulator such as glass, oxidized silicon or the like. As depicted in
The LPNE process 10 shown in
The nickel film-covered glass squares were then coated with a positive photoresist layer 16 (Shipley 1808) by spin coating (Step 2). This involved the deposition of about a 1 mL aliquot of photoresist onto each square and the rotation of the square at about 2500 rpm for 40 s. This produced a photoresist layer 16 having a thickness (after soft baking) of about 1 um. The freshly coated squares were soft-baked at about 90° C. for 30 minutes. After cooling to room temperature, a transparent contact mask (not shown) was pressed onto the photoresist 16 with a quartz plate (not shown) and the masked surface was exposed to UV light having a wavelength of about 365 nm and output power of about 0.5 mW cm2 for 2 minutes. To remove unexposed portions of the photoresist 16 and expose the nickel layer 14, the slide was then soaked first in a developer/water solution (1 part Shipley MF-351 to 4 parts water) for 20 s, and then in pure water for 1 minute, before drying in a stream of ultra-high purity (UHP) N2 (Step 3). The exposed nickel 14 was then electrochemically removed (Step 4) and palladium, platinum, or gold 20 was electrodeposited (Step 5) onto the nickel nanoband electrodes 14 produced by this process.
The electrochemical stripping of the nickel layer 14 and the electrodeposition of these metals 20 was carried out in a 50 mL, one compartment, three-electrode cell. Nickel dissolution was carried out in aqueous 0.1 M KCl containing 0.1 mL of concentrated HCl. Palladium was electroplated from an aqueous solution containing 2 mM Pd(NO3)2, 2 mM saccharine, and 0.1 M KCl. Platinum was electroplated from a solution containing 0.1 M KCl, and 1.0 mM K2PtCl6 at 0.025 V vs. SCE. Gold was electroplated from aqueous commercial gold plating solution (Clean Earth Solutions, Carlstadt, N.J.—a 6 mM AuCl3 solution), with added 1.0M KCl at −0.90 V vs. SCE. All aqueous solutions were prepared using Millipore MilliQ water (σ>18.0 Mcm). A saturated calomel reference electrode (SCE) and a 2 cm2 Pt foil counter electrode were also employed. The stripping and deposition were both carried out on a computer-controlled EG&G 273A potentiostat/galvanostat.
The removal of the sacrificial metal (nickel, silver, copper, gold, or the like) layer can be accomplished using either electrooxidation or chemical etching. As one preferred embodiment, the stripping of the nickel layer 14 is achieved by securing the photoresist 16—covered nickel film 14 with self-closing metal tweezers, and placing part of the exposed nickel film 14 in the nickel stripping solution. The film 14 was then held at a potential of −0.085 V vs. SCE for 1000 s. This removed all of the exposed nickel 14 and produced an “undercut” beneath the photoresist 16 at each edge of the nickel layer 14 produced by photolithography on the surface. The slide was then rinsed with Nanopure water, and placed into the palladium deposition solution. Palladium metal 20 was electrodeposited by holding the nickel film 14 at a potential of about +0.225 V vs. SCE for times ranging from about 25 to 400 s. The glass slide 12 was then rinsed with Nanopure water and dried with UHP N2. The photoresist 16 was then removed (Step 6) by rinsing the slide with electronic grade acetone (Acros), methanol (Fisher), then Nanopure water, respectively, before drying with UHP N2. The excess nickel film 14 was then removed by washing with dilute HNO3 (Step 7).
The position of the nanowires 20 on a surface of an insulator 12 such as glass is determined by the position of the nanoband electrodes 14 prepared in the first four steps of the LPNE process 10 shown in
A recessed nickel nanoband 14 is both electrochemically reactive and accessible to redox species. To confirm this, the cyclic voltammetry of Ru(NH3)63+, a metal complex, which undergoes a fast, reversible one electron reduction, was investigated. Before dissolution of the exposed nickel, as shown in
The growth of the nanowire 20 into the trench 18 occurs in three phases as shown in
If wire growth is continued, deposited metal fills the trench 18 and begins to emerge from it and the total current increases because the wetted surface area increases. This “blooming” 21 of the nanowire 20 as it emerges from the trench is undesirable since it produces a distribution of dendrites along one edge of the nanowire as shown in
The nanowires 120 produced by the LPNE process 10 shown in
The width and height of these nanowires can be independently controlled as a function of electrode thickness or trench height and electrodeposition duration. The electrode layer thickness fixes the height of the nanowires while the width of the nanowires is proportional to the duration of the electrodeposition step. This control is documented by the data shown in
w(t)=JdeptdepVm/nF (1)
where tdep is the deposition time, Vm is the molar volume of the deposited metal, n is the number of electrons required to reduce each metal complex ion, and F is the Faraday constant (96485 C eq.−1). Plots of w versus tdep for gold, palladium and platinum (
Previous measurements of the electrical properties of metal nanowires have employed wires ranging in length from 500 nm to 5 μm. Using a four-point probe shown in
While the invention is susceptible to various modifications, and alternative forms, specific examples thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the invention is not to be limited to the particular forms or methods disclosed, but to the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the appended claims.
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/US2007/076433 | 8/21/2007 | WO | 00 | 2/3/2009 |
Publishing Document | Publishing Date | Country | Kind |
---|---|---|---|
WO2008/024783 | 2/28/2008 | WO | A |
Number | Name | Date | Kind |
---|---|---|---|
6110354 | Saban et al. | Aug 2000 | A |
6662442 | Matsui et al. | Dec 2003 | B1 |
20020158342 | Tuominen et al. | Oct 2002 | A1 |
20040023253 | Kunwar et al. | Feb 2004 | A1 |
20040146560 | Whiteford et al. | Jul 2004 | A1 |
20040238367 | Penner et al. | Dec 2004 | A1 |
20050176228 | Fonash et al. | Aug 2005 | A1 |
20060097389 | Islam et al. | May 2006 | A1 |
20090020315 | Dutton | Jan 2009 | A1 |
Number | Date | Country | |
---|---|---|---|
20090197209 A1 | Aug 2009 | US |