Fabrication and Thermal Analysis of Submicron Silver Tubes Prepared

The thermal conductivity of the silver submicron tubes was found to differ depending on the method .... Metallization of electrospun polyacrylonitrile...
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Langmuir 2007, 23, 795-801

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Fabrication and Thermal Analysis of Submicron Silver Tubes Prepared from Electrospun Fiber Templates Frederick Ochanda and Wayne E. Jones, Jr.* Department of Chemistry and Institute for Materials Research, State UniVersity of New York at Binghamton, Vestal Parkway East, Binghamton, New York 13902 ReceiVed May 16, 2006. In Final Form: October 13, 2006 Submicron silver tubes have been synthesized by a polymer-based template approach. Two different approaches to metallization, electroless deposition and exchange plating, were evaluated within the template approach. Silver films with average thickness ∼50-100 nm were deposited on polycarbonate fibers ∼400 nm in diameter by each technique, resulting in tubes with a diameter between 450 and 500 nm after thermal degradation of core fibers. These nanomaterials were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and scanning thermal microscopy. The thermal conductivity of the silver submicron tubes was found to differ depending on the method of preparation, with tubes from electroless plating possessing relative thermal conductivity values that were 1 order of magnitude higher than that from exchange plating, 3000 W/m·K and 660 W/m·K, respectively. Interestingly, these results indicate that silver submicron tubes possess higher thermal conductivity than the bulk metal. This observation is discussed in the context of the continuous conduction path of the tubes and their high surface areato-volume ratio.

1. Introduction Metal nanostructures have developed into an important research focus due to their unusual electrical, optical, and thermal properties induced by quantum and surface energy-based variations from bulk materials. Silver nanostructures are of particular interest for applications in the electronics industry because the bulk metal exhibits the highest bulk electrical and thermal conductivity among all metals. In particular, silver nanostructures would be valuable for electronics system designers working in thermal management of electronic packages where power dissipation demands at the device level continue to increase.1 Thus, there is a need to develop new materials and methods which fabricate silver and other low-dimensional nanostructures as components of thermal interface materials. One-dimensional systems, such as nanowires and nanotubes, are the smallest dimension structures that can be used for efficient transport of electrons and/or phonons and thus are expected to be critical to the function and integration of nanoscale devices.2 One-dimensional nanostructures could be exploited in many applications, including nanoelectronics, super strong and tough composites, functional nanostructured materials, and novel probe microscopy tips.3-12 An area of current interest is the possible use of nanotubes and nanowires with high thermal conductivity * E-mail: [email protected], fax: (607) 777-4478, phone: (607) 777-2421. (1) National Technology Roadmap for Semiconductors: Technology Needs, 2002. http://www.nemi.org/roadmapping/status.html. (2) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15 (5), 353. (3) Yang, P.; Lieber, C. M. Science 1996, 273, 1836. (4) Yang, P.; Lieber, C. M. Appl. Phys. Lett. 1997, 70, 3158. (5) Yang, P.; Lieber, C. M. J. Mater. Res. 1997, 12, 2981. (6) Dai, H.; Hafner, J. H.; Rinzler, A. G.; Colbert, D. T.; Smalley, R. E. Nature (London) 1996, 384, 147. (7) Wong, S. S.; Harper, J. D.; Lansbury, P. T.; Lieber, C. M. J. Am. Chem. Soc. 1998, 120, 603. (8) Wong, S. S.; Joselevich, E.; Woolley, A. T.; Cheung, C; Lieber, C. M Nature (London) 1998, 394, 52. (9) Wong, S. S.; Harper, J. D.; Lansbury, P. T.; Lieber, C. M. J. Am. Chem. Soc. 1998, 120, 8557. (10) Wong, S. S.; Woolley, A. T.; Odom, T. W.; Huang, J. L.; Kim, P.; Vezenov, D. V.; Lieber, C. M. Appl. Phys. Lett. 1998, 73, 3465. (11) Lieber, C. M. Solid State Commun. 1998, 107, 607.

as fillers in thermal interface materials. While carbon nanotubes have been discussed extensively, metals, by virtue of their excellent thermal conductivity, are also potential candidates in this field. The goal would be to develop metallic tubes that can be synthesized and then dispersed into a polymer matrix or grown off one of the surfaces, such as the chip or the heat sink, prior to adhesion. We have been exploring the preparation of metal-based nanowires and nanotubes by a polymer fiber template approach.13 Metallization of polymers and polymer-based materials are used today in a large variety of technological applications ranging from the fabrication of printed circuits to decorative coatings in general manufacturing.14-21 For many common applications, metallization is achieved on polymer substrates by an electroless deposition process. This typically involves surface treatment to improve adhesion, surface seeding with an electroless catalyst (generally a palladium containing compound), and subsequent immersion in an electroless bath. We have successfully demonstrated this approach for a variety of metals including Ni, Au, and Cu. Characterization of the thermal properties of nanomaterials is also a fundamental challenge. Several attempts have been made to measure thermal conductivity of nanomaterials. Berber et al.22 recently reported an attempt to measure the thermal conductivity (12) Bockrath, M.; Cobden, D. H.; McEuen, P. L.; Chopra, N. G.; Zettl, A.; Thess, A.; Smalley, R. E. Science 1997, 275, 1922. (13) Ochanda, F.; Jones, W. E., Jr. Langmuir 2005, 21, 10791-10796. (14) Charbonnier, M.; Romand, M. Int. J. Adhes. Adhes. 2003, 23, 277. (15) Mittal, K. L.; Susko, J. R. Metallized plastics 1: fundamental and applied aspects; Plenum Press: New York, 1989. (16) Sacher, E.; Pireaux, J. J.; Kowakczyk, S. P. Metallization of polymers; American Chemical Society: Washington, DC, 1990. (17) Mittal, K. L. Metallized plastics 2: fundamental and applied aspects; Plenum Press: New York, 1991. (18) Mittal, K. L. Metallized plastics 3: fundamental and applied aspects; Plenum Press: New York, 1992. (19) Mittal, K. L. Metallized plastics: fundamental and applied aspects; Marcel Dekker: New York, 1998. (20) Mittal, K. L. Metallized plastics 5 & 6: fundamental and applied aspects; VSP: Utrecht, The Netherlands, 1998. (21) Mittal, K. L. Metallized plastics: fundamental and applied aspects; VSP: Utrecht, The Netherlands, 2000. (22) Berber, S.; Kwon, Y. K.; Tomanek, D. Phys. ReV. Lett. 2000, 84, 4613.

10.1021/la061385n CCC: $37.00 © 2007 American Chemical Society Published on Web 11/23/2006

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of a mat of carbon nanotubes. The measured thermal conductivity was found to be much lower than predicted by theoretical reports. They attributed this to weak coupling between the carbon nanotubes. Kim et al.23 have developed a microdevice containing two adjacent silicon nitride membranes suspended with two thermal isolation legs. A nanowire bridged the two suspended islands, each containing a Pt resistor used as both a heater and a thermometer. One of the islands is heated to a temperature Th, which is determined by measuring the resistance of the Pt wire on the island. Conduction through the nanowire heated the sensing island to a temperature Ts, measured by resistance thermometry. Noting the power dissipated by the heater, Qh, and estimating the thermal conductance of the suspended legs connected to the islands in the absence of a nanowire, the conductance of the nanowire Gn was calculated. This approach was used to determine the thermal conductivity of multiwall carbon nanotubes to be 3000 W/m·K at room temperature. Here we report the first efforts to prepare, fabricate, and characterize nanotubular materials of silver. Building on our previously successful polymer template-based approach,13 we have combined electrospinning with two methods of metal deposition in order to optimize the process further. We describe the direct comparison of electroless plating and exchange plating as convenient methods for preparing high surface area conducting metal tubes. We also report the thermal conductivity investigation of the silver submicron tubes using a calibration methodology based on scanning thermal microscopy (SThM) technique and a new calibration technique to determine the relative thermal conductivity of these nanomaterials. 2. Experimental Section 2.1. Reagents. Polycarbonate (PC) pellets, methylene chloride, N,N-dimethylformamide, palladium chloride, stannous chloride, disodium salt of EDTA, silver nitrate, cobalt(II) nitrate, copper(II) sulfate hexahydrate, and sodium hypophosphite dihydrate (all from Aldrich) and hydrochloric acid (J. T. Baker) were used as received from the manufacturer. 2.2. Template Fiber Sample Preparation. A polymer solution, 180 mg/mL, was prepared by dissolving polymer pellets (64 000 mw) in a CH2Cl2/DMF (0.65/0.35) solvent mixture. Electrospinning was done with an applied voltage of 20 kV and a distance of 20 cm between the collector screen and the spinneret.13 2.3. Metal Plating. The electrospun fibers were peeled from the aluminum foil collector after soaking in 1 M HCl solution for 5 min and then rinsed in deionized water. The fiber surface was first sensitized and activated by 20 min immersion in 3.0 mM SnCl2 aqueous solution and 3.0 mM PdCl2 aqueous solution containing 0.01M HCl respectively before electroless plating. 2.3.1. Exchange Plating. Copper plating was done at 60 °C by a single addition of 0.1 M aqueous sodium hypophosphite solution as a reductant copper plating solution (0.05 M). The metal plating was allowed to proceed for 30 min after the addition of the reducing agent. Copper polycarbonate composite fibers were then immersed in 0.05 M silver nitrate solution and the reaction temperature set at 60 °C. The reaction was allowed to proceed for 30 min, and the silver-coated fibers were rinsed with copious amounts of deionized water. 2.3.2. Electroless Plating. The electroless silver-plating solution contained (M) the following: Ag(I) 0.05, Co(II) 0.05, disodium EDTA 0.03 with cobalt(II) chloride containing a few drops of concentrated ammonia solution as the reducing agent. The electrospun fibers were first sensitized and activated in Sn2+ and Pd2+ before being put in the electroless plating bath as explained above. The temperature of the bath was maintained at 60 °C and plating proceeded for 30 min. The composite fibers were then subjected to thermal (23) Kim, P.; Shi, L.; Majumdar, A.; MacEuen, P. L. Phys. ReV. Lett. 2001, 87, 215502.

Ochanda and Jones treatment in N2 atmosphere from room temperature to 650 °C for 30 min and annealed at this temperature for 30 min before cooling to room temperature. 2.4. Instrumentation. The morphology of the resulting metal tubes was examined by scanning electron microscopy (SEM) using a Hitachi S-570LB using an accelerating voltage of 10 kV. The surface of the samples was coated with a thin film of gold/palladium (Au/Pd) for SEM observations. Thermal conductivity images were obtained with a Nanonics multi View NSOM 400 microscope system in intermittent contact (IC) mode. A constant current of 7.0 mA was passed through the probe for all experiments, which affords resistive heating and heat flux between the tip and sample. The interaction was monitored through the output voltage measured across one leg of a Wheatstone bridge. The cantilever of the probe had a reference force of 479.0 nN, a resonance frequency of 49.0 kHz, tip radius of 250 nm, and resistance of 55.0 Ω. While the probe was scanned across the sample at constant force, variations in heat flow from the probe are measured by monitoring the bridge voltage, which can be used to create a thermal contrast image. All measurements are taken at the same current and gain to allow easy comparison. The relative thermal conductivity values were calculated from the images by a calibration method described below. 2.5. Calibration Standards. Calibration of the probe output was done by scanning a variety of metal samples with known thermal conductivities. The calibration standard set consisted of a zinc wire, a silver wire, an aluminum sheet, and a platinum sheet, with size range of mounted in a polished epoxy mold. In preparing the mold, an industrial epoxy resin-to-hardener ratio of 100:12 by weight was used. The metals were held in place during curing with ceramic holders, and the set up was allowed to cure for 24 h. One end of the hardened epoxy resin was ground using standard polishing techniques, ending with a 6-µm diamond finish. The polished surface was sonicated in ethyl alcohol solution. The rinsing was to remove pieces of silicon carbide, which stick in the grain boundary of the metals.

3. Results and Discussion The fabrication of submicron silver tubes involves a combination of two steps in order to achieve a uniform metallic tube; electrospinning for the generation of fibers employed as templates, the seeding of the template fibers. The seeding process serves two purposes. First, they make the fiber surface catalytic and create nucleation sites that facilitate silver growth since the template fibers are nonconductive. Second, there is large difference in surface free energy between the untreated fibers and the metal being deposited. Seeding increases the surface free energy of the fibers to more closely match that of the metal. 3.1. Electrospinning. The electrospinning process involves the application of an electrostatic field to a capillary connected to a reservoir containing a polymer solution or melt. Under the influence of the electric field, a hanging droplet of the polymer solution or melt at the capillary tip is deformed into a conical shape (Taylor cone).24,25 If the voltage surpasses a threshold value, the electrostatic forces overcome the surface tension, and a fine charged jet is ejected. The jet moves toward the counter electrode rapidly as a solution. Given the high solution viscosity and the presence of intermolecular polymer entanglements, the jet remains stable and does not transform into spherical droplets as expected for a liquid cylindrical thread. The result is the deposition of a thin, dry, polymer fiber on a substrate located on the counter electrode since the solvent evaporates during (24) (a) Huang, Z.-M.; Zhang, Y.-Z.; Kotaki, M.; Ramkrishna, S. Compos. Sci. Technol. 2003, 63, 2223. (b) Reneker, D. H.; Chun, I. Nanotechnology 1996, 7, 216. (c) Frenot, A.; Chronakis, I. S. Curr. Opin. Colloid Interface Sci. 2003, 8, 64. (25) (a) Shin, M. Y.; Hohman, M. M.; Brenner, M. P.; Rutledge, G. C. Appl. Phys. Lett. 2001, 78, 1149. (b) Reneker, D. H.; Yarin, A. L.; Fong, H.; Koombhongse, S. J. Appl. Phys. 2000, 87, 4531. (c) Taylor, G. Proc. R. Soc. London A 1969, 313, 453-475.

Submicron SilVer Tubes

transit between the electrodes. It has been found that the deposition rate as well as the fiber diameter can be controlled over a broad range.26a By proper selection of the solvent, polymer concentration, intrinsic solution conductivity, viscosity, and the surface tension of the solution, one obtains fibers with diameters as small as a few tens of nanometers. Previous studies have successfully electrospun polymers ranging from insulators to electrically conducting polymers such as polyaniline.26b-d 3.2. Electrospun Fiber Seeding. Metal plating on electrically insulating surfaces requires sensitization and activation of the interface, typically through metal nuclei (e.g. Pd) implanted in the surface of the matrix as functional sites.27 On these sites, continuous metal deposition is initiated in the presence of electrochemical plating solutions. For example, in solution, surface bound Sn2+ is oxidized to Sn4+ and Pd2+ is reduced to Pd. The resulting nanoscopic Pd particles act as nucleation sites and catalysts for deposition and growth of silver in the subsequent electroless deposition.28 3.3. Electroless Plating. Electroless coating29 chemistry has emerged as one of the leading growth areas in surface engineering, metal finishing, etc. Electroless coatings have unique physicochemical and mechanical properties. Some of the properties that render them usable include uniformity, excellent corrosion resistance, mechanical resistance to wear, solderability, high hardness, amorphous or microcrystalline forms, low coefficient of friction, high reflectivity, tunable resistivity, and magnetic properties, among others.30 In electroless (autocatalytic) plating, metallic coatings are formed as a result of a chemical reaction between the reducing agent and metal ions present in solution. The localization of the redox reaction on the surface to be plated is ensured by the catalytic properties of the metallic phase formed at any dopants on the surface.31 The major advantage of electroless plating is that it is nondestructive and can uniformly coat the fibers with metals to yield conductive substrates. Compared to conventional metal evaporation, electroless deposition covers the entire fiber surface with the metal and not just the portion of the fiber in a direct line of sight path from the evaporation source. In addition, no electrolytic current, high temperature, or ultrahigh vacuum is needed in the electroless process. This makes the process simpler and faster. In the silver electroless bath the silver nitrate is the silver precursor, with cobalt(II) nitrate acting as the reducing agent, while the complexing agent, ethylenediaminetetraacetic acid disodium salt (Na2EDTA), forms a chelate with the silver ions forming [AgEDTA]3-. This allows the subsequent reaction to be carried out at the surface of the fibers (eq 1). The chelating process prevents the reduction of silver ions to form insoluble compounds immediately upon addition of the reducing agent to the solution. In addition, chelating agents inhibit undesirable metal-catalyzed reactions by forming complexes with the metal ions. The chelate deactivates the metal ion and prevents it from reacting with other components of the system. The balanced (26) Hong, D.; Nyame, V.; MacDiarmid, A. G.; Jones, W. E. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 3934. (b) Aussawasathien, D.; Dong, J.-H.; Dai, L. Synth. Met. 2005, 154 (1-3), 37-40. (c) Zhou, Y.; Freitag, M.; Hone, J.; Staii, C.; Johnson, A. T.; Pinto, N. J.; MacDiarmid, A. G. Appl. Phys. Lett. 2003, 83 (18), 3800-3802. (d) Li, M.; Guo, y.; Wei, Y.; MacDiarmid, A. G.; Lelkes, P. I. Biomaterials 2006, 27 (13), 2705-2715. (27) Ohno, L.; Wakabayashi, O.; Haruyama, S. J. Electrochem. Soc. 1985, 132 (10), 2322. (28) Pap, A. E.; Kordas, K.; Jantunen, H.; Haapaniemi, E.; Leppavuori, H. Polym. Test. 2003, 22, 657. (29) Horkans, J.; Sanbucetti, C.; Markovich, V. IBM J. Res. DeV. 1984, 28 (6), 690. (30) (a) Hart, A. Mater. World 1996, 4 (5), 265. (b) Chang, S.-Y.; Lin, J.-H.; Lin, S.-J.; Kattamis, T. Z. Metallurg. Mater. Trans. A 1999, 30A, 1119. (c) Jackson, B.; Macary, R. Shawhan, G. Trans. 1nst. Met. Finish. 1990, 68, 75. (31) Brenner, A.; Riddel, G. J. Res. Nat. Bur. Std 1946, 37, 31.

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equation for the reaction of silver electroless process is shown below, where hexaamminecobalt(II) ions is the reducing agent and ethylenediaminetetraacetic acid (EDTA) is complexed to silver ions.

Co(NH3)62+(aq) + 2[AgEDTA]3-(aq) + OH-(aq) f 2Ag(s) + Co(NH3)63+(aq) + 2EDTA4-(aq) + H2O(l) (1) During electroless deposition of silver, cobalt(II) ions were oxidized to cobalt(III) while silver(I) ions reduced to silver(0) by accepting an electron from the reducing agent. The cobalt(III)-cobalt(II) redox couple32-35 provides a large variety of thermodynamic and kinetic features due to numerous possible complex species of various stability and structure. The redox potential of cobalt(III)-cobalt(II) couple varies in a wide range from 1.86 V for hydrated cobalt ions to negative values for systems having amines as ligands for cobalt ions forming complexes of high stability with cobalt(III). The spontaneity of metal ion reduction by cobalt(II) is determined from the redox potential values: the potential of the cobalt(III)-cobalt(II) couple is more negative than that of the silver ion-silver couple. In addition, the redox potential of CoIII-CoII and AgI-Ag couple is pH dependent and shifts to more negative values with increase in pH. The redox potential values depend on the cobalt(II), cobalt(III), and the deposited metal ion complex stability. 3.4. Exchange Plating. Exchange plating36 occurs under conditions when a less noble metal surface is immersed in an electrolyte containing a more noble metal. When a less noble metal surface is submerged into a solution containing the more noble metal, the surface ions will dissolve into solution. As this happens, the electronegative metal surface becomes negatively charged and attracts the positively charged more noble metal ions. The result is a thin homogeneous coating of the more noble metal on the substrate. Exchange plating is similar to electroless plating in that it uses a chemical reaction to apply the thin coating. However, the difference is that the reaction is caused by the metal substrate rather than by mixing two chemicals into the plating bath. In this plating process, the electrons are supplied by the base metal, as the reducing agent. The base metal donates electrons to the metal being plated; the base metal itself dissolves into solution, which is why it is called a replacement reaction. A key characteristic of the exchange reaction bath is that it is self-limiting, which means that once the base metal is covered, the plating ceases. This is distinguished from electroless plating where the deposition of the metal being plated continues to deposit as long as the substrate remains in solution. This results in a very thin, dense, nonporous coating. In general, the exchange process is temperature dependent, with increased plating occurring at higher temperatures. This process, like electroless plating, depends primarily on the shift in the equilibrium constant due to differences between the free energy at room temperature, ∆F°25°C and that at plating solution temperature, ∆F°25°+∆T°C. In most cases involving wellformulated solutions, the rate is primarily under chemical control. Unlike electroless plating, where the driving force is a reducing agent present in solution, the driving force here is due to the local (32) Vaskelis, A.; Norkus, E.; Rozovskis, G.; Vinkevicius, J. Trans. 1nst. Met. Finish. 1997, 75, 1. (33) Vaskelis, A.; Norkus, E. Electrochim. Acta 1999, 44, 3667. (34) Vaskelis, A.; Jagminiene, A.; Juskenas, R.; Matulionis, E.; Norkus, E. Surf. Coat. Technol. 1996, 82, 305. (35) Vaskelis, A.; Jaciauskiene, J.; Jagminiene, A.; Norkus, E. Solid State Sci. 2002, 4, 1299. (36) Gutfeld, R. J. V.; Romankiw, L. T.; Acosta, R. E. IBM J. Res. DeVelop 1982, 26 (2), 136.

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Ochanda and Jones

Ag2+ + Cu f 2Ag + Cu2+∆G ) -89.24 kJ mol-1 (4)

EDTA used in the silver plating reduced the plating rate and the amount of free precipitating particles. The elemental composition of the tubes was investigated using energy dispersive spectroscopy (EDS), and the spectrum is shown in Figure 3. From the spectrum, it is clear that we have successfully fabricated silver tubes via both the electroless and exchange plating approaches. The shoulder in the silver peak can be attributed to the beta peak (Kβ) of silver. The absence of a copper peak in the energy dispersive spectrograph is evidence of complete exchange of the copper surface. Palladium peak was not observed in the EDS spectrum which can be explained in terms of electronegativity of Pd relative to Ag. Pd metal is more electronegative than Ag; hence, in addition to Pd surface acting as nucleation site for Ag deposition through a chemical reducing agent, we propose a displacement reaction occurring in which Pd gains electrons from Ag ions and goes into solution. In that case, there will be no Pd peak after thorough rinsing. The spectrum also indicates complete removal of the electrospun fiber template through pyrolysis given that there is no carbon peak observed. 3.6. Thermal Analysis. Thermal transport in these materials was studied using scanning thermal microscopy (SThM).37-42 The SThM technique operates by bringing a sharp, thermally active tip into close proximity with the sample surface. Localized heat transfer between the heated tip and the sample surface changes the tip temperature and can be monitored as a change in the current necessary to maintain the tip at constant temperature. The change in the resistance can be measured as a change of the output voltage across a Wheatstone bridge circuit. By scanning the tip across the sample surface, a spatial distribution of the heat transfer is mapped out. The spatial resolution of the resulting image depends on the tip sharpness (30-150 nm), the tip sample heat transfer mechanism, and the thermal design of the probe. This technique provides a key path to understanding the thermal transport behavior of these new nanomaterials. 3.7. SThM Calibration. The output voltage from SThM and the thermal conductivity are closely related and proportional. However, more information and instrument calibration is necessary for the determination of the absolute thermal conductivity. This can be achieved using a variety of samples with known thermal conductivities and similar heat capacities. The thermal conductivity, K, is proportional to the heat flow or (Uout)2.43 Where Uout is the bridge output voltage. Using identical tip and operation conditions, the square of the output voltage (Uout)2 can be measured for a set of standard samples. Then from a graph of Uout2 versus thermal conductivity, K, for samples with known thermal conductivity, the absolute values for the unknown samples can be obtained. In these experiments a set of metal samples were embedded in an epoxy matrix as described in the Experimental Section. The sample surface was polished and then sonicated in ethyl alcohol to remove pieces of silicon carbide, which stick in the grain boundary of the embedded metals. This is important because during thermal conductivity mapping, a thermal probe scans the metal surface including the particulate impurities that are encountered. Given the small size of thermal scanning probe, it

3.5. SEM and EDS of Silver SubmicronTubes. The SEM images of the silver submicron tubes are shown in Figures 1 and 2 below. The average diameter of the silver submicron tubes from electroless plating is 500 nm while that from exchange plating is 450 nm and a wall thickness of ∼50-100 nm in each case. The difference in the tube diameters is consistent with the fact that the exchange plating process is self-limiting. The composite fiber morphology indicated uniform coating with isolated particles between the fibers. This indicates that the ligand

(37) Dekhter, R.; Khachatryan, E.; Kokotov, Y.; Lewis, A. Appl. Phys. Lett. 2000, 77 (26), 4425. (38) Dinwiddie, R. B.; Pylkki, R. J.; West, P. E. Therm. Conduct. 1993, 22, 668. (39) Brotzen, F. R.; Loos, P. J.; Brady, D. P. Thin Solid Films 1992, 207, 197. (40) Griffin, A. J.; Brotzen, F. R.; Loos, P. J. J. Appl. Phys. 1994, 75 (8), 3761. (41) Goodson, K. E.; Flik, M. I.; Su, L. T.; Antoniadis, D. A. IEEE Electron. DeVice Lett. 1993, 14 (10), 490. (42) (a) Nonnenmacher, M.; Wickramasinghe, H. K. Appl. Phys. Lett. 1992, 61, 168. (b) Ruiz, F.; Sun, W. D.; Pollak, F. H.; Venkatraman, C. Appl. Phys. Lett. 1998, 73, 1803. (43) Florescu et al. J. Appl. Phys. 2002, 91 (2), 15.

Figure 1. SEM image of silver submicron tubes by exchange plating.

Figure 2. SEM image of silver submicron tubes by electroless plating.

differences in the free energy ∆F° resulting from the heterogeneous nature of the substrate (i.e., differences in ∆F° for the intergranular regions and for the surfaces of crystallites). In the exchange plating process, the oxidation potential of silver (+0.80 V) was more noble to that of copper surface (+0.34 V), leading to precipitation of silver. In terms of free energy, half reaction for the reduction potential of silver is more negative than that of pure copper. Hence, the E°cell of the overall reaction with the half reaction of copper written in terms of oxidation potential is positive and the free energy negative, indicating that exchange plating of silver is favorable and spontaneous as written in eq 4. The half reactions and the overall cell reaction on the fiber surface are shown in the equations below.

Cu2+ + 2e- f Cu(+0.34V)∆Gfo ) -64.98 kJmol-1

(2)

Ag2+ + 2e-f 2Ag(+0.80 V)∆Gfo ) 2[-77.11] kJ mol-1 (3) Overall reaction

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Figure 3. EDS spectrum of silver submicron tubes showing complete selective removal of electrospun fibers used as templates. Table 1. Output Voltage and Bulk Thermal Conductivity of Metals Used in Calibration metal

output voltage (Uout), V

Uout2

bulk thermal conductivity, k (300 K)

platinum zinc aluminum silver

0.046 0.057 0.069 0.087

0.002 0.0033 0.0047 0.0076

70 116 237 429

was important to polish the metal surfaces so that they become flat and smooth. The goal was less than 10 nm RMS surface roughness as confirmed by AFM. The surfaces were dried using a stream of nitrogen gas and stored in a vacuum desiccator to prevent oxidation of the metals due to presence of water on their surfaces. The images were scanned with 10 ms integration at each point, a resolution of 256 data points per line and 256 lines. The time the tip takes at each point is greater than the response time of the probe bridge circuit (20 µs)44 and is sufficient for the heated volume to reach thermal equilibrium. In this condition, the system is assumed to be in steady state. The image analysis software provides topographical information, surface analysis, cross section information, and roughness information. Cross section analysis of the thermal image gives the output voltage, Uout, along a selected line in the image. The output voltage difference between any two points on this line can be measured. If the trace passes over two different materials, the output voltage difference between them can be used to calculate the relative thermal conductivities. Using the same tip and operating conditions the square of the output voltage (Uout)2 was measured for the calibration samples shown in Table 1 including platinum, zinc, aluminum, and silver. The square of output voltage (Uout)2 and the known bulk thermal conductivities were used to plot a curve, from which the absolute thermal conductivity values for the unknown samples could be obtained. The calibration plot for the standard metallic sample in Table 1 is shown in Figure 4 below. The equation for the linear fit of the plot in Figure 4 was used in the determination of the relative thermal conductivity of silver submicron tubes prepared in this experiment. From the equation, y corresponds to the thermal conductivity and x represents the square of output voltage, Uout2, derived from the thermal image. The Uout2 values can be substituted into the equation to determine the unknown thermal conductivity values for the synthesized samples. Topography, phase, or thermal images generated by this technique are shown in the figures below. (44) Rima, D.; Khachatryan, E.; Kokotov, Y.; Lewis, A. Appl. Phys. Lett. 2000, 77 (26), 25.

Figure 4. Calibration plot of metal wires illustrating a linear relationship between thermal conductivity and square of output voltages of known metals. The equation for the linear fit is employed in the determination of relative thermal conductivity of unknown specimen.

Figure 5. Phase image of silver submicron tubes by exchange plating showing surface roughness and porosity of the tubes.

When the probe comes into close proximity with regions of the surface that differ in their thermal properties, varying amounts of heat will flow from the probe to the sample. The required feedback voltage is used to create the contrast in the thermal image where regions absorbing more heat appear dark. The thermal images in Figures 6 and 8 show thermal contrast achieved by SThM as a result of the significant difference in thermal properties of the metal tubes and the substrate and a fairly uniform thermal conductivity distribution along the tubes as shown by the color scheme. However, there is more uniform thermal conductivity distribution in tubes from electroless plating than

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Figure 6. Thermal image of silver submicron tubes by exchange plating showing non uniform thermal conductivity variation along the tubes.

Figure 7. Topography image of silver submicron tubes by electroless plating, indicating tubes clearly separated from each other and a tube surface that appears smooth.

for tubes from exchange plating as is reflected in the thermal images. The power involved in the creation of the thermal image may be calculated from the bridge output voltage and is affected by factors such as the probe/sample contact area, the temperature difference between probe and sample, the heat capacity of the samples, and variations in the local thermal conductivity of the near-surface regions of the sample. 3.8. Thermal Transport in Nanotubes. The relative thermal conductivity of the submicron silver tubes fabricated by the electroless and exchange plating processes were 3000 W/m·K and 660 W/m·K, respectively. These data represent a significant increase over the thermal conductivity of bulk silver (429 W/m· K) which is used as one of the reference standards in our calibration curve. This is a very exciting result which suggests that at the decreased length scales found in these submicron tubes (∼50-100 nm wall thickness) there is a significant enhancement in the thermal properties beyond bulk values. (45) (a) Kim, P.; Shi, L.; Majumdar, A.; McEuen, P. L. Phys. ReV. Lett. 2001, 87 (21), 2155021. (b) Fisher, J. E.; Zhou, W.; Vavro, J.; Llaguno, M. C.; Guthy, C.; Haggenmueller, R. Casavant, M. J.; Walters, D. E.; Smalley, R. E. J. Appl. Phys. 2003, 93 (4), 2157.

Ochanda and Jones

Figure 8. Thermal image of silver submicron tubes by electroless plating, indicating uniform thermal conductivity variation along the tubes.

This effect has been suggested and observed previously for carbon nanotube systems,45a but not previously in metallic systems. The thermal conductivity in any conducting material can be attributed to a combination of electron and phonon (lattice vibrations) transport.45b In metal nanotubes, phonon states can play a significant role as the mode of heat conduction. This is because, as in carbon nanotubes, the density of electronic states is relatively small. So the electronic contribution to χ (and specific heat) is lower as compared to bulk metals at a given temperature. By comparison to the carbon nanotube studies, it is tempting to suggest that this is the basis for any enhanced thermal conductivity observed in these systems. Nanotube materials also differ from solid nanowires. A freestanding single wall nanotube has all the atoms on the surface, and the phonon modes can only propagate along the axial direction. This latter feature implies that unlike a solid nanowire in which the phonon modes inside the wire can hit the boundary (if the boundary scattering is strongly phase breaking), phonons on the carbon nanotube sheet have no boundaries to interact with. In addition, the strong modification of the phonon dispersion can also change the scattering mechanisms. These attributes of carbon nanotubes can be extended to metal tubes, in which there is electron transport in addition to phonon transport. Another potential reason for the observed enhancement in thermal conductivity is that in the nanometer scale regime, experiments have demonstrated that the close proximity of interfaces and extremely small volume of heat dissipation strongly modifies thermal transport.46 The difference in the relative values of the thermal conductivity for each sample can be explained by carefully examining the phase image of the silver tubes fabricated by exchange plating. While the electroless plating samples appear to be smooth, the phase image depicts the tubes from the exchange plating to be grainy, rough, and composed of particles that are uniform in size distribution. The grainy nature could have a negative effect on the thermal conductivity values in two ways. First, the particle to particle interactions can lead to additional electron and phonon scattering losses. Second, as the surface roughness increases, the thermal contact resistance between the sample and the sensing tip also increases. Consistent with this is the observation that the (46) Fourner, D.; Filloy, C.; Hole, S.; Roger, J. P.; Tessier, G. J. Phys. IV Fr. 2005, 125, 493.

Submicron SilVer Tubes

bright regions in the phase image represent a higher resistant force and hence more frictional force experienced by the tip. The corresponding regions in the thermal image recorded relatively lower thermal conductivity values, verifying the effect of thermal contact resistance. It is important to verify that the thermal images of submicron silver tubes were not due to topography-induced artifact or other types of artifacts. Throughout the study of the silver tubes, thermal images were taken at different applied currents, and the thermal signals were found to increase roughly linearly with the power input to the submicron tube. This indicates that the images obtained are not due to topographical artifacts, which are expected to be independent of power input. In addition, thermal images were usually taken for one tube sample mat under the same current conditions and at the same tip-sample contact force. The thermal images were found to be consistent with each other. This indicates that the tip-sample thermal coupling or thermal resistance remained constant for all the thermal images of one sample at the same contact force. Despite the impressive thermal conductivity value, additional work is underway to fully understand the heat transfer mechanism at both the nanoscale and a micron level by molecular dynamics simulation. We will try to understand if the metals used to develop the calibration standard, due to their size, may act as infinite heat sink. In this case, there would be differences in heat capacity and possibly

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temperature increases induced in the silver nanotubes by the tip, which did not occur for the calibration metals.

4. Conclusion Silver tubes have been prepared by both electroless deposition and exchange plating with submicron diameters and wall thicknesses of ∼50-100. The thermal conductivity of silver submicron tubes has been investigated by scanning thermal microscopy technique. The results indicate that silver submicron tubes possess higher thermal conductivity than the bulk metals. This has been attributed to the continuous conduction path of the tubes and their high surface area-to-volume ratio enabling rapid heat flow. The tubes prepared by exchange plating have lower relative thermal conductivity values. This has been explained in terms of the increased thermal contact resistance between the thermal tip and the substrate surface due to surface roughness. Acknowledgment. The Semiconductor Research Corporation (SRC) and Integrated Electronic and Engineering Center (IEEC) supported this work. We thank Henry Eichelberger of Biological Science Department for assistance with SEM, Debbie Dittrich for EDS collection, and Dr. Stanley Madan and Dr. Edmond Fey for useful discussion. LA061385N