Abstract:
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 from a metal such as nickel, copper, silver, gold or the like, which is stripped using electrooxidation or a chemical etchant to advantageously recess the nanoband electrode between a 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 its height is determined by the height of the nanoband electrode.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for preparing metal nanowires on insulating surfaces and, more particularly, to the electrodeposition of lithographically patterned nanowires. 
     BACKGROUND 
     Electron beam lithography (EBL), developed in the early 70&#39;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. 
     SUMMARY 
     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. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
         FIG. 1  is a schematic showing a seven step process flow for metal nanowire fabrication using lithographically patterned nanowire electrodeposition (LPNE). 
         FIG. 2(   a ) is a graph showing oxidation current versus time during the removal of nickel from lithographically patterned surfaces by potentiostatic electrooxidation (reaction: Ni→Ni 2+ +2e−) at −0.10 V vs. SCE in aqueous 0.1 M KCl at pH=1.0. 
         FIGS. 2(   b ) and ( c ) are graphs showing cyclic voltammogram for Ru(NH 3 ) 6   3+  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. 
         FIG. 3(   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. 
         FIG. 3(   b ) is a SEM image showing a gold nanowire with a rectangular cross-section obtained from trench-confined growth for 100 s. 
         FIG. 3(   c ) is a SEM image of a gold nanowire deposited for 1000 s showing “blooming” from the edge of the photoresist. 
         FIG. 4  are semi-images of examples of patterned nanowires prepared using LPNE process depicted in  FIG. 1 . 
         FIG. 5(   a ) is an atomic force microscope (AFM) image showing the flat wire profile of a gold nanowire on glass prepared using the LPNE process. 
         FIG. 5(   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  FIG. 1 . 
         FIG. 5(   c ) is a graph showing nanowire width, measured by SEM, as a function of the electrodeposition time. For both  FIGS. 5  ( b ) and ( c ), error bars represent +1σ for at least ten measurements of each nanowire. 
         FIG. 6(   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. 
         FIG. 6(   b ) is a graph showing temperature dependence of the wire resistivity, p, normalized to the resistivity at 300K, ρ 300 , from 10 K to 350 K, for the same two nanowires shown in (a), and the resistivity of bulk gold. 
     
    
    
     DESCRIPTION 
     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  FIG. 1 , the LPNE process  10  involves the preparation of a sacrificial nanoband electrode  14 , formed of nickel, copper, silver, gold or other metals, using optical lithography, and the subsequent electrodeposition at the sacrificial nanoband electrode  14  of a metal nanowire  20 , preferably formed of a noble metal such as gold, platinum or palladium (Au, Pt, or Pd) but may include other metals, such as, cadmium, bismuth and the like or metalloids, such as, e.g., silicon, germanium and the like. The nanoband electrode  14  is preferably recessed between the surface of the insulator  12  and a photoresist layer  16  forming a trench  18  defined by the surface of the insulator  12 , the electrode  14  and the photoresist  16 . The electrodeposition of a nanowire  20  into this “nanoform” produces a wire  20  with a generally rectangular cross-section where the height and width are independently controllable as a function of trench height and electrodeposition duration within a precision of about 5 nm, down to about 20 nm in width and 6 nm in height. Since optical lithography is used to define the position of the nanowires  20  on the surface of the insulator  20 , the spacing between nanowires  20  is constrained by the optical diffraction limit. 
     The LPNE process  10  shown in  FIG. 1  is described in greater detail as follows with regard to an example of an implementation of the process. One of skill in the art will readily recognize that the materials used were chose for exemplary purposes only. The LPNE process  10  was implemented as follows: Soda lime glass microscope slides were cleaned in aqueous Nochromix solution, air dried, and diced into 1″×1″ glass squares  12 . Onto each glass square  12 , a nickel film  14  (ESPI, 5N purity), 20-100 nm in thickness, was deposited by hot filament evaporation at a rate of 0.5-1.5 Å s −1  (Step 1). The film thickness and evaporation rate were monitored using a quartz crystal microbalance (Sigma Instruments). 
     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 cm 2  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) N 2  (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(NO 3 ) 2 , 2 mM saccharine, and 0.1 M KCl. Platinum was electroplated from a solution containing 0.1 M KCl, and 1.0 mM K 2 PtCl 6  at 0.025 V vs. SCE. Gold was electroplated from aqueous commercial gold plating solution (Clean Earth Solutions, Carlstadt, N.J.—a 6 mM AuCl 3  solution), with added 1.0M KCl at −0.90 V vs. SCE. All aqueous solutions were prepared using Millipore MilliQ water (σ&gt;18.0 M           cm). A saturated calomel reference electrode (SCE) and a 2 cm 2  Pt foil counter electrode were also employed. The stripping and deposition were both carried out on a computer-controlled EG&amp;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 N 2 . 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 N 2 . The excess nickel film  14  was then removed by washing with dilute HNO 3  (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  FIG. 1 . In Fritsch et al., Individually addressable, submicrometer band electrode arrays; Fabrication from multilayered materials; Analytical Chemistry 70, 2902-2907 (1998), gold nanoband electrodes were prepared using a process similar to the first four steps shown in  FIG. 1 , but lacking in the formation and use of a trench  18  as described and illustrated herein. The process  10  depicted in  FIG. 1  advantageously removes exposed nickel or other nanoband electrode material  14  electrochemically at step 4 by applying an oxidizing potential to the material, which for nickel is in the range from about −0.085 to −0.10 V vs. SCE (see  FIG. 2   a ). Oxidation is continued until the electrode material  14  at the exposed edges  15  of the pattern is etched, preferably undercutting the photoresist  16  by about 100-300 nm ( FIG. 2   a ). Three transients are shown in  FIG. 2   a  for nickel layers  14  with approximate thicknesses of 40 nm, 70 nm, and 90 nm. For nickel films of these thicknesses, as shown, the etching current is approximately proportional to the nickel film thickness as the electrochemical undercutting occurs. “Over-etching” of the material produces a trench  18 , horizontally oriented in this instance, into which a metal nanowire  20  can be electrodeposited in step 5 of  FIG. 1 . 
     A recessed nickel nanoband  14  is both electrochemically reactive and accessible to redox species. To confirm this, the cyclic voltammetry of Ru(NH 3 ) 6   3+ , a metal complex, which undergoes a fast, reversible one electron reduction, was investigated. Before dissolution of the exposed nickel, as shown in  FIG. 2   b , the cyclic voltammogram shows current peaks and hysteresis of the forward and reverse scans; all consistent with the planar diffusion of this molecule to the relatively large nickel areas exposed by the dissolution of photoresist after lithography. After electrooxidation of the nickel and the formation of the horizontal trench, as shown in  FIG. 2   c , the Ru(NH 3 ) 6   3+  reduction current was reduced by a factor of more than 100 and a steady-state, sigmoidal CV showing no hysteresis was observed, qualitatively as expected for a nanoband electrode. This result demonstrated that the recessed nickel electrode was both electrochemically active and accessible to the dissolved redox species Ru(NH 3 ) 6   3+ , as required for nanowire electrodeposition. 
     The growth of the nanowire  20  into the trench  18  occurs in three phases as shown in  FIG. 3 . First, a plating voltage for a metal nanowire material  20  such as, e.g., palladium, is applied to an aqueous solution containing 2 mM Pd(NO 3 ) 2 , 2 mM saccharine, and 0.1 M KCl and immediately after the application of a plating voltage the current increases as the palladium nucleates along the entire length of the nickel nanoband electrode  14 . When this nucleation process was complete, a quasi-constant deposition current was observed during growth of the confined metal nanowire  20  because the wetted surface area of the confined wire  20  is constant, and because for such a kinetically-controlled reaction, the deposition current per unit area of the metal is constant. The quasi-constant current seen from 20 to 50 seconds is seen because for this kinetically-controlled deposition process current is proportional to the wetted surface area which remains constant while growth of the nanowire  20  is confined to the horizontal trench  18 . Terminating growth during this phase results in a nanowire  20  having a rectangular cross section, such as seen in  FIG. 3   b.    
     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  FIG. 3   c.    
     The nanowires  120  produced by the LPNE process  10  shown in  FIG. 1  have, as depicted in  FIG. 5   a , an approximately rectangular cross section that is enforced by the dimensions of the trench  18 . The minimum dimensions for the width and height of these nanowires is about 40 nm and 20 nm, respectively. For example, a platinum nanowire  220  shown in the SEM image of  FIG. 4   a  is 46 nm in width, 39 nm in height and 1 cm in length. As shown in  FIGS. 4   b - d , large substrate areas exceeding 1 cm 2  can be lithographically patterned with nanowires; i.e., (b) parallel gold nanowires  320 , 1 cm in length and spaced by 9 μm; (c) a coiled nanowire  420  with total length of 2.7 cm; and (d) nanowire loops  520 . The nanowires produced show remarkable dimensional uniformity over the entire sample surface. The relative standard deviations of the nanowire width and height were about 10-12% and &lt;5%, respectively, for the patterns shown in  FIGS. 4   b - d . Of particular interest is the fact that corners, like those present in the pattern of the nanowire  420  shown in  FIG. 4   c , do not cause either narrowing or thickening of the nanowire—the latter expected for a diffusion-controlled deposition processes. This curvature invariance of the wire width is expected if the wire electrodeposition reaction occurs under conditions of kinetic, not diffusion, control. 
     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  FIGS. 5   b  and  5   c . The AFM-measured nanowire height increases linearly with the nickel layer thickness for nanowires composed of gold, palladium and platinum ( FIG. 5   b ). The slope of these data points is 0.98 indicating that the nanowire height equals the thickness of the evaporated nickel layer. The smallest attainable nanowire heights were in a range of about 18 nm for Au and Pt and somewhat larger for Pd. For a metal electrodeposition process occurring within a rectangular trench at a time-invariant current density, J dep , the width of the deposited nanowire, w, is given by:
 
 w ( t )= J   dep   t   dep   V   m   /nF   (1)
 
where t dep  is the deposition time, V m  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 t dep  for gold, palladium and platinum ( FIG. 5   c ) are all approximately linear in accordance with Eq. 1. The slopes of these plots are proportional to J dep  the magnitude of which depends on the concentration of metal complex present in the plating solution and on the electrodeposition potential.
 
     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  FIG. 6   a  (inset), we measured the electrical behavior of 4.00 μm long sections of two gold nanowires with cross-sections of 20 nm×233 nm, and 100 nm×166 nm. Two characteristics of metallic conduction are seen in these nanowires: First, the current voltage behavior was ohmic ( FIG. 6   a ) and second, a positive temperature-coefficient of resistivity, α, was observed ( FIG. 6   b ). However clear quantitative differences in the behavior of these nanowires, associated with their diminutive size, are also seen. The measured room temperature electrical resistivity of these nanowires was significantly higher than expected for bulk gold, and this disparity increased with decreasing temperature, down to 10K, just as reported for nanowires in several previous studies 14-18 . One well-understood source for the elevated electrical resistivity of the nanowires is boundary scattering: an increased frequency for the inelastic collision of conduction electrons with wire surfaces. But the contribution of boundary scattering to the total resistivity of our 20 nm nanowires, as estimated using Fuchs-Sondheimer theory, is approximately a factor of 2 in resistivity for the 20 nm nanowire and this enhancement is temperature independent. Two additional contributing factors are impurities in the electrodeposited gold, and a mismatch in the coefficients of thermal expansion for gold (κ=14×10 −6  K −1 , 300K) and the soda lime glass support (κ=9×11 −6  K −1 , 300K). This mismatch, which increases with decreasing temperature, means that the length of the gold nanowire shrinks more rapidly with decreasing temperature than the glass surface on which it is supported, placing it in tension. The total strain that accumulates in cooling from 300K to 80K, for example, is significant: approximately 0.7 μm for the 400 μm nanowires investigated here. 
     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. 
     REFERENCES 
     
         
         1. Spicer, D. F., Rodger, a. C. &amp; Varnell, G. L. Computer-Controlled Pattern Generating System for Use with Electron-Beam Writing Instruments. Journal of Vacuum Science &amp; Technology 10, 1052-1055 (1973). 
         2. Varnell, G. L., Spicer, D. F. &amp; Rodger, a. C. E-Beam Writing Techniques for Semiconductor-Device Fabrication. Journal of Vacuum Science &amp; Technology 10, 1048-1051 (1973). 
         3. Vieu, C. et al. Electron beam lithography: resolution limits and applications. Applied Surface Science 164, 111-117 (2000). 
         4. Berger, S. D. &amp; Gibson, J. M. New Approach to Projection-Electron Lithography with Demonstrated 0.1 μM Linewidth. Applied Physics Letters 57, 153-155 (1990). 
         5. Berger, S. D. et al. Projection Electron-Beam Lithography—a New Approach. Journal of Vacuum Science &amp; Technology B 9, 2996-2999 (1991). 
         6. Liddle, J. A. et al. Space-charge effects in projection electron-beam lithography: Results from the SCALPEL proof-of-lithography system. Journal of Vacuum Science &amp; Technology B 19, 476-481 (2001). 
         7. Jorritsma, J., Gijs, M. A. M., Kerkhof, J. M. &amp; Stienen, J. G. H. General technique for fabricating large arrays of nanowires. Nanotechnology 7, 263-265 (1996). 
         8. Natelson, D., Willett, R. L., West, K. W. &amp; Pfeiffer, L. N. Fabrication of extremely narrow metal wires. Applied Physics Letters 77, 1991-1993 (2000). 
         9. Natelson, D., Willett, R. L., West, K. W. &amp; Pfeiffer, L. N. Geometry-dependent dephasing in small metallic wires. Physical Review Letters 86, 1821-1824 (2001). 
         10. Melosh, N. A. et al. Ultrahigh-density nanowire lattices and circuits. Science 300, 112-115 (2003). 
         11. Thompson, M. A., Menke, E. J., Martens, C. C. &amp; Penner, R. M. Shrinking nanowires by kinetically controlled electrooxidation. J Phys Chem B 110, 36-41 (2006). 
         12. Nagale, M. P. &amp; Fritsch, I. Individually addressable, submicrometer band electrode arrays. 1. Fabrication from multilayered materials. Analytical Chemistry 70, 2902-2907 (1998). 
         13. Bard, A. J. Electrochemical Methods: Fundamentals and Applications (Wiley &amp; Sons, New York, 2001). 
         14. Durkan, C. &amp; Welland, M. E. Size effects in the electrical resistivity of polycrystalline nanowires. Phys Rev B 61, 14215-14218 (2000). 
         15. Marzi, G. D., lacopino, D., Quinn, A. J. &amp; Redmond, G. Probing intrinsic transport properties of single metal nanowires: Direct-write contact formation using a focused ion beam. J Appl Phys 96, 3458-3462 (2004). 
         16. Yang, F. Y. et al. Large magnetoresistance of electrodeposited single-crystal bismuth thin films. Science 284, 1335-1337 (1999). 
         17. Yang, F. Y. et al. Large magnetoresistance and finite-size effect in electrodeposited bismuth lines. J Appl Phys 89, 7206-7208 (2001). 
         18. Chiu, P. &amp; Shih, I. A study of the size effect on the temperature-dependent resistivity of bismuth nanowires with rectangular cross-sections. Nanotechnology 15, 1489-1492 (2004). 
         19. Fuchs, K. The conductivity of thin metallic films according to the electron theory of metals. P Camb Philos Soc 34, 100-108 (1938). 
         20. Sondheimer, E. H. The Mean Free Path of Electrons in Metals. Adv Phys 1, 1-42 (1952).