Fabrication of Metallic Microtubes Using Self-Rolled Polymer Tubes as

Apr 20, 2009 - We present a novel approach for fabricating single and bimetallic (gold, titanium) (Au, Ti, Au/Ti) microtubes with very high aspect rat...
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Fabrication of Metallic Microtubes Using Self-Rolled Polymer Tubes as Templates Kamlesh Kumar,*,† Bhanu Nandan,† Valeriy Luchnikov,‡ E. Bhoje Gowd,† and Manfred Stamm*,† †

Department of Nanostructured Materials, Leibniz Institute of Polymer Research Dresden, Hohe Strasse 6, 01069 Dresden, Germany, and ‡Insitut de Sciences des Mat eriaux de Mulhouse - IS2M, LRC 7228 - CNRS, 15, rue Jean Starcky BP 2488, 68057 Mulhouse Cedex, France Received January 26, 2009. Revised Manuscript Received March 24, 2009 We present a novel approach for fabricating single and bimetallic (gold, titanium) (Au, Ti, Au/Ti) microtubes with very high aspect ratio from self-rolled polymer templates. The polymer microtubes used as the template were generated by self-rolling of thin polymer bilayer films (polystyrene/poly(4-vinylpyridine) (PS/P4VP) gradually released from a solid substrate. The self-rolling was introduced in the polymer bilayer by swelling the bottom P4VP layer in dodecylbenzenesulfonic acid (DBSA) solution, which was opposed by a stiff top PS layer. The inner wall of the tube was metallized by depositing a thin layer of desired metal on top of the bilayer by physical vapor deposition. The polymer template was then removed by pyrolysis, resulting in pure metal microtubes, which were characterized by optical microscopy, scanning electron microscopy (SEM), infrared spectroscopy (IR), energy-dispersive X-ray (EDX), and X-ray photoelectron spectroscopy (XPS). The tube diameter was tailored by changing the metal layer thickness on the polymer bilayer. The approach described here is general and could be used to fabricate any type of single or multimetallic tube. The metal microtubes reported in this method have potential application in drug delivery systems, microelectronics, microfluidic devices, enzyme bireaction, and chemical and biochemical sensing devices.

I. Introduction Nano- and microtubes have large number of potential applications in microelectronics, cell analysis, microfluidics, waveguides, and chemical and biochemical sensors.1-6 Specific functions performed by microtubes may include encapsulation, chromatography, liquid and catalyst carriers, flow control, heat exchange, reinforcement, detection, filtration, sensing, optical waveguiding, and so forth. A decade ago, a simple and versatile approach was introduced for production of micro- and nanotubes via straindriven self-rolling of multilayer thin films released in a controlled *Corresponding author. E-mail: [email protected]; [email protected]. Tel: +493514658632. Fax: +49-3514658281. (1) Pool, R. Science 1990, 247, 1410. (2) Thurmer, D. J.; Deneke, C.; Mei, Y. F.; Schmidt, O. G. Appl. Phys. Lett. 2006, 89, 223507. (3) Tegenfeldt, J. O.; Prinz, C.; Cao, H.; Huang, R. L.; Austin, R. H.; Chou, S. Y.; Cox, E. C.; Sturm, J. C. Anal. Bioanal. Chem. 2004, 387, 1678. (4) Takei, K.; Kawashima, T.; Kawano, T.; Takao, H.; Sawada, K.; Ishida, M. J. Micromech. Microeng. 2008, 18, 035033. (5) Kipp, T.; Welsch, H.; Strelow, C.; Heyn, C.; Heitmann, D. Phys. Rev. Lett. 2006, 96, 77403. (6) Sparks, D.; Cruz, V.; Najafi, N. Sens. Actuators A: Phys. 2007, 135, 827. (7) Prinz, V. Ya.; Seleznev, V. A.; Gutakovsky, A. K.; Chehovsky, A. V.; Preobrazhenski, V. V.; Putyato, M. A.; Gavrilova, T. A. Physica E 2000, 6, 828. (8) Prinz, V. Ya Physica E 2004, 23, 260. :: (9) Prinz, V. Ya.; Grutzmacher, D.; Beyer, A.; David, C.; Ketterer, B.; Deckardt, E. Nanotechnology 2001, 12, 399. (10) Prinz, V. Ya.; Chehovskiy, A. V.; Preobrazhenskii, V. V.; Semyagin, B. R.; Gutakovsky, A. K. Nanotechnology 2002, 13, 231. (11) Vorob’ev, A. B.; Prinz, V. Ya. Semicond. Sci. Technol. 2002, 17(6), 614. (12) Seleznev, V.; Yamaguchi, H.; Hirayama, Y.; Prinz, V. Ya Jpn. J. Appl. Phys., Part 2 2003, 42, L791. :: (13) Schmidt, O. G.; Schmarje, N.; Deneke, C.; Muller, C.; Jin-Phillipp, N. Y. Adv. Mater. 2001, 13(10), 756. (14) Deneke, C.; Muller, C.; Jin-Phillipp, N. Y.; Schmidt, O. G. Semicond. Sci. Technol. 2002, 17, 1278. (15) Schmidt, O. G.; Deneke, C.; Kiravittaya, S.; Songmuang, R.; Nakamura, Y.; Zapf-Gottwick, R.; Muller, C.; Jin-Phillipp, N. Y. IEEE J. Sel. Top. Quantum Electron. 2002, 8, 1025. (16) Songmuang, R.; Denke, C. H.; Schmidt, O. G. Appl. Phys. Lett. 2006, 89, 223109.

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manner from the substrate.7-22 This micromechanical effect was explored for the formation of semiconductor nanotubes7-17,22,23 as well as polymer and hybrid polymer/metal microtubes.18-21 The bending moment in the films can be caused by such different factors as a misfit of crystal lattices of the top and the bottom layers,7-17,22-24different coefficients of thermal expansion,25 or selective swelling of cross-linked polymers in solvents.18-21 Recently, Mei et al. demonstrated the formation of tubular microand nanostructures from different materials by precisely releasing and rolling-up nanomembranes on polymers.26 In this paper we demonstrate the application of the self-rolling approach to the formation of metallic microtubes with high aspect ratio. Because of their electronic properties, such as high electrical and thermal conductivity as well as optical reflectivity, metallic microtubes are promising for a number of advanced applications, for instance wave-guiding of far- and mid-infrared radiation and chemical sensing.27 In our previous work it was shown that hybrid polymer/gold tubes can be formed by rolling of a polymer bilayer film capped by a metal layer.18 Here, we describe the combination of the self-rolling approach with pyrolysis of the polymer component in order to obtain pure (17) Nastauschev, Y. V.; Prinz, V. Ya.; Svitasheva, S. N. Nanotechnology 2005, 16, 908. (18) Luchnikov, V.; Sydorenko, O.; Stamm, M. Adv. Mater. 2005, 17, 1177. (19) Luchnikov, V.; Stamm, M. Physica E 2007, 37, 236. (20) Luchnikov, V.; Kumar, K.; Stamm, M. J. Micromech. Microeng. 2008, 18, 35041. (21) Kumar, K.; Luchnikov, V.; Nandan, B.; Senkovskyy, V.; Stamm, M. Eur. Polym. J. 2008, 44(22), 4115. :: (22) Mendach, S.; Schumacher, O.; Heyn, Ch.; Schnull, S.; Welsch, H.; Hansen, W. Physica E 2004, 23, 274. (23) Huang, M.; Boone, C.; Roberts, M.; Savage, D. E.; Lagally, M. G.; Shaji, N.; Qin, H.; Blick, R.; Nairn, J. A.; Liu, F. Adv. Mater. 2005, 17, 2860. (24) Li, X. J. Phys. D: Appl. Phys. 2008, 41, 193001. (25) Timoshenko, S. J. Opt. Soc. Am. 1925, 11, 233. :: (26) Mei, Y.; Huang, G.; Solovev, A. A.; Urena, E. B.; Monch, I.; Ding, F.; Reindl, T.; Fu, R. K. Y.; Chu, P. K.; Schmidt, O. G. Adv. Mater. 2008, 20, 4085. (27) Harrington, J. A. Fiber Integr. Opt. 2000, 19, 211.

Published on Web 04/20/2009

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Figure 1. Schematics of gold tube formation by the self-rolling approach: (a) deposition of P4VP/PS polymer bilayer; (b) cross-linking of bilayer by UV radiation; (c) deposition of thin layer of metal (nm) by magnetron sputtering; (d) mechanical cutting of layers by a sharp object; (e) rolling of layers in DBSA solution; (f) PS/P4VP tubes with enrolled metal; (g) metal tubes after removal of polymer by pyrolysis at 500 °C.

free-standing metallic or multimetallic tubes. The polymer plays an auxiliary role in this fabrication scheme, and is removed from the final product. The main advantage of this approach is the possibility of the tube formation from eventually any single metal or multimetal films, even when they do not have the ability to selfroll by themselves. Moreover, the approach enables, in principle, the fabrication of arbitrary metallic patterns on the tube’s inner walls, by means of photolithography.19 This approach can be used for producing complex free-standing three-dimensional (3D) micro-objects via release of patterns from the polymer support by pyrolysis.

II. Experimental Section Materials. Poly(4-vinylpyridine) (P4VP) {Mn = 45 900, Mw = 82 500} and polystyrene (PS) {Mn = 600 000, Mw = 654 000} were obtained from Polymer Source. Inc. Highly polished singlecrystal silicon wafers of {100} orientation were purchased from Semiconductor Processing Co. and used as substrates. Solvents were purchased from Acros Organic and used as received. Fabrication of the Tube. The silicon wafers were cleaned with dichloromethane in an ultrasonic bath for 20 min and then further in a mixture of 30% ammonium hydroxide, 30% hydrogen peroxide, and 40% water (warning: this solution is extremely corrosive and should not be stored in tightly sealed containers) for 1.5 h at 65 °C, finally rinsed several times with water, and dried in an argon flow. The bilayer of PS (50 nm) and P4VP (70 nm) was deposited on the cleaned silicon wafer from toluene and chloroform solution, respectively. Dip coater was used for the deposition of bilayer. The bilayer was cross-linked by UV radiation, which was emitted through a UV lamp (G8T5, TecWest, Inc., U.S.A.) which has a 2.5 W output at 254 nm. A thin film of gold or titanium was deposited on the cross-linked bilayer using direct current (DC) sputtering at a pressure of 2.6  10-2 mbar and an operating voltage of -440V. The scratches were made by a sharp blade. Sample with microstructures was immersed in an aqueous solution of 4 wt % DBSA. The samples were pyrolyzed at 500 °C for 3 h in an oven to remove the polymers. Characterization of the Tube. The characterization of the sample was done by optical microscopy, scanning electron microscopy (SEM), infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray (EDX). The optical images of the microtubes were observed by optical microscopy in transmission mode. SEM micrographs of the tubes were captured by a Phenom electron microscope. A Fourier transform 7668 DOI: 10.1021/la900327v

Figure 2. Optical micrograph of the PS/P4VP tube. infrared (FTIR) spectrometer IFS 66v/s (Bruker, Germany) was used to obtain the IR spectrum of the polymer tube as well as the metal tube. The spectra was recorded in the transmission mode for the sample on a silicon wafer. The FTIR spectrum of the sample was obtained after subtraction of the neat silicon wafer spectra. XPS spectra of tubes were recorded by an Axis Ultra (Kratos Analytical, England) spectrometer with a mono-Al KR X-ray source of 300 W at 20 mA, and the takeoff angle was 0 ° between the surface normal and the electron-optical axis of the spectrometer, which provided a maximum information depth of about 8-10 nm. EDX of the samples was carried out in a Philips XL30 scanning electron microscope.

III. Results and Discussion In a two-step process, self-rolled polymer bilayers are used as the template, where inner walls are metallized with desired metals and the polymer template is removed by pyrolysis after tube formation. In this paper we restrict ourselves to gold (Au), titanium (Ti), and Au/Ti bimetallic tubes fabrication, although, in principle, any metal, sufficiently stable at ambient conditions, can be used. Figure 1 schematically illustrates the fabrication procedure. A bilayer of PS and P4VP, with P4VP as the bottom layer, is deposited on a silicon wafer by dip-coating. Subsequently, a desired metal or a combination of metals is deposited by physical vapor deposition on top of the bilayer. Although the Langmuir 2009, 25(13), 7667–7674

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Figure 3. Optical micrographs of gold and titanium tubes before pyrolysis (a,c,e) and after pyrolysis (b,d,f). Panels a and b are Au tube and thickness of film was 50 nm. Panels c and d are the Ti tube and thickness of film was 10 nm. Panels e and f are the bimetallic (Ti/Au) tube and thickness of layer was 40/10 nm. All the figures show the tube rolled in the direction from left to right.

fabrication of the tubes without the intermediate PS layer is also possible, we have found that inclusion of this layer in the fabrication scheme improves the uniformity of the tube’s shape. The obtained multilayer of polymer and metal is then microstructured via straight scratches made by a sharp razor blade. This opens access of the dodecylbenzenesulfonic acid (DBSA) to the bottom layer of the multilayer film. Supramolecular binding between DBSA and pyridine rings28 causes swelling of the P4VP and thus induces in-layer strain in the film, which is relaxed by rolling starting at the scratches. Note that upper layers (i.e., PS and/or metal) protect the P4VP layer from uncontrollable swelling apart at the front of rolling. The microtube thus formed has (28) Ikkala, O.; Ruokolainen, J.; Brinke, G.; Torkkeli, M.; Serimaa, R. Macromolecules 1995, 28, 7088.

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an inner wall made from metal, whereas the outer wall consists of polymer. The rate of rolling was found to be slow (a few micrometers per minute) and can be controlled by acid concentration and UV irradiation dose.21 Hence, it was possible to stop the rolling process when a desired number of rolls was achieved. Finally, the tube was pyrolyzed at 500 °C for 3 h in an oven in order to remove the entire polymer, and thus the tube left was made entirely of the deposited metal. We first fabricated purely polymeric self-rolled PS/P4VP microtubes having no metallic layer (Figure 2). These tubes served as reference ones, and are used to adjust the fabrication parameters, such as the concentration of DBSA, the UV irradiation dose, and the thickness of P4VP and PS layers. Some hybrid metal/polymer tubes are presented in Figure 3a,c,e. In most cases, we obtained tubes with perfect cylindrical shape, although DOI: 10.1021/la900327v

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Figure 4. SEM micrograph of a gold 20 nm thick layer, created by magnetron sputtering, before (a) and after (b) thermal annealing at 500 °C for 3 h.

sometimes they had an oval cross-section supposedly due to the action of capillary forces during the sample drying. Such shape distortions, which are due to capillary forces during sample drying, were reported also for self-rolled semiconductor tubes12,15 and represent a general problem in micro- and nanofabrication. Because of relatively high stiffness of the metal films compared to that of the polymer layer (the Young moduli (G) of gold, titanium, and PS are GAu∼ 78 GPa, GTi ∼ 110 GPa, and GPS ∼ 3 GPa respectively), the diameter of the tubes is sensitive to the thickness and the type of the metallic layer. Figure 3a shows the P4VP/PS/ Au tube, in which the Au layer is 50 nm thick, whereas in the P4VP/PS/Ti tube (Figure 3c) the thickness of the Ti layer is only 10 nm. However, under similar condition of rolling, the Au/ polymer and Ti/polymer hybrid tubes have comparable diameters of 12.5 and 8.8 μm, respectively, which is due to higher stiffness of Ti compared to that of gold. Figure 3e shows the P4VP/PS/Ti/Au tube, in which the thickness of the Au/Ti layer is 50 nm, as for the P4VP/PS/Au tube (Figure 3a). However, the polymer/bimetal tube has significantly larger diameter (∼37 μm), because the more stiff titanium constitutes 80% of the bimetal film thickness (approx. 40 nm). The metal microtubes obtained by pyrolysis from the hybrid tubes are shown in Figure 3b,d,f. The cylindrical shape of the tube was found to remain intact. However, since the polymer component was mostly removed, some shrinkage of the tube diameter is observed. Thus, the hybrid polymer/Au, polymer/Ti and polymer/Au/Ti microtubes discussed above shrink after pyrolysis to diameters of 10.2, 8.1, and 28.8 μm, respectively. The shrinking of the tube diameter could be explained considering the effect of pyrolysis on gold layer structure and on the strain relaxation in the film. In the sputtering process, gold is deposited in the form of nanoclusters. On the other hand, it is known that the melting point of nanoclusters is strongly depressed in comparison to the bulk material.29 According to the data of Buffet and Borel,29 the melting point of 3-4 nm wide Au particles falls below 500 °C. This melting of particles, which make part of a strained gold layer, may lead to complete or partial relaxation of mechanical strain, which arises in the gold layer during rolling. The change of the gold layer structure upon annealing is evident from the SEM micrographs, which represent the surface of the Au layer before (Figure 4a) and after (Figure 4b) heating at 500 °C for 3 h. One can say that nanocluster form of gold was (29) Buffet, P. H.; Borel, J. P. Phys. Rev. A. 1976, 13, 2287.

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transformed into polycrystalline structure. It is very likely that any macroscopic mechanical strain in the film is relaxed upon such a profound structure transformation. Shrinking of the tube’s diameter upon pyrolysis may be also due to melting of gold nanoclusters and recrystallization. In this process, the gaps between the clusters are filled, and the material became more dense, which might lead to diminishing of the layer dimensions. Hence, melting of gold nanoclusters and recrystallization might lead to densification of the material and its shrinking in all directions, thus leading to a decrease of the tube’s diameters. Figure 5a,b,c shows SEM micrographs of a single Au tube while an array of Au tubes is shown in Figure 5d,e. As proven by the side-view of the tube opening (Figure 5b,c), pyrolysis conserves almost perfect cylindrical tube’s shape. Not only single tubes, but also arrays of tubes can be fabricated on the same substrate (Figure 5d,e). Tube length was limited by substrate dimensions (2-3 cm). As discussed above, the tube diameter can be tailored by varying the thickness of the metal layer. Figure 5f,g shows SEM images of gold tubes obtained from 20 and 50 nmthick gold films on polymer bilayers, respectively. SEM micrographs of Ti and Au/Ti bimetallic tubes are shown in Figure 6a and Figure 6b, respectively. FTIR was used to monitor the polymer removal process during pyrolysis. Figure 7 shows the FTIR spectrum of microtubes before and after pyrolysis. For reference, the FTIR spectrum of a simple PS/P4VP polymer tube is also shown. FTIR spectra of samples before pyrolysis show the characteristic peaks of aromatic C-C out of plane bending at 700 cm-1, C-H bending vibration at 833 cm-1, aromatic C-C stretch at 1493 cm-1, and aromatic overtone band at 1640 cm-1 and 1719 cm-1. The stretching bands of the CH2 group at 2856, 2927, and 3083 cm-1 were also clearly seen in the spectra of tube before pyrolysis. After pyrolysis at 500 °C, as expected, the organic moiety was removed, and only a silica fingerprint region at 1100 cm-1 is observed, which comes from the silicon wafer. Hence, it was concluded that the entire polymer was burned out during pyrolysis, and only metal remains at this stage. The metallic tubes were further characterized by XPS and EDX. Figure 8I shows the XPS spectra of a polymer tube, a polymer/gold hybrid tube, and the gold tube obtained after pyrolysis. It can be clearly observed that the characteristic peak of the polymer vanishes after the pyrolysis. The characteristic signals of gold (Au 4d3/2 and 4d5/2) are clearly observed in the XPS spectra. Similar results were observed with titanium tubes and, for reason of brevity, are not shown here. Langmuir 2009, 25(13), 7667–7674

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The XPS analysis of a Au/Ti bimetallic tube is more complex because the limited penetration depth of X-rays makes it difficult

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to get information about both Au and Ti in the tube. Hence EDX was used in this case. EDX was performed on the 50  50 μm2

Figure 5. SEM micrographs of gold tubes: (a) Image with rolled part and unrolled part. The rolling direction of the tube was from bottom to top in the SEM image. (b ,c) Clearly visible open end of tube. (d) Arrays of gold tubes (top view). (e) Arrays of gold tubes (side view); shows open end of tubes. (f) Gold tube formed from 20 nm layer thickness. (g) Gold tube formed from 50 nm gold layer thickness. Langmuir 2009, 25(13), 7667–7674

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Figure 6. SEM images of metal tubes: (a) titanium tube; (b) bimetallic (Au/Ti) tube.

Figure 7. FTIR spectra of a PS/P4VP tube, polymer with an inner gold layer, and the tube after pyrolysis (only gold remains).

area, which comprises both the tube as well as the unrolled film area. The EDX spectra of Au/Ti tube after pyrolysis (Figure 8II) revealed the presence of Si, Au, and Ti peaks with an atomic ratios of 88.89:9.39:1.72, respectively. It can be concluded from the EDX spectrum that the polymer is removed from the tube and finally a Au/Ti bimetallic tube is obtained. In the EDX spectrum of tube, the following peaks are observed: three peaks of gold (MR and Mβ at 2.1 keV, LR at 9.7 keV and Lb at 11.4 keV) and two Ti peaks (KR at 4.5 keV and Kβ at 4.9). A silicon peak is also present at 1.7 keV which is a contribution from the silicon wafer. Besides the Si peak, a small peak of oxygen is also obtained at 0.452 keV, which may be due to limited oxidation of Ti and Si during the pyrolysis process. One of the interesting aspects of the self-rolling fabrication process is the possibility to control the tube dimensions. As discussed above, the diameter of the tubes can be preset by varying the thickness of the deposited metal film, whereas the thickness of the tube’s walls (or, equivalently, the number of rolls) can be tailored by stopping the rolling at some stage simply by removing the sample from DBSA solution. Hence, it is essential to know the rate of rolling for the films with different thickness of the metal layer. Detailed investigation of polymer tube formation was reported elsewhere.21 In the present work we undertake a detailed study on the rate of rolling as the thickness of the gold layer is varied. Figure 9a shows the dependence of the rate of rolling on the gold layer thickness, while the polymer bilayer thickness is kept constant. Rolling rate was found to decrease drastically as the Au layer thickness increased above 20 nm. Such a variation in the rate of rolling might be related to slower relaxation of the internal stresses via plastic deformation of the gold layer. 7672 DOI: 10.1021/la900327v

Figure 8. (I) XPS wide-scan spectra of (a) a PS/P4VP tube; (b) a PS/P4VP/gold tube; (c) an only gold tube. (II) EDX spectra of a Au/Ti tube.

Similarly, the diameter of the tube (D) was found to increase with the thickness in gold layer (h) as shown in Figure 9b. Such a variation in rate of rolling and tube diameter is obvious from the Langmuir 2009, 25(13), 7667–7674

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where E is the Young modulus of gold. However, it is known that gold forms nanoscale clusters in the course of magnetron sputtering deposition. This may lead to deviation of a and q from the values for the bulk material. Equating now the free energies of polymer and gold layers per unit tube length, we obtain a 3 hq 1 1 ¼ Kp 2 R R0, p R

!2 ð2Þ

Resolving this equation for R gives RðhÞ ¼ R0, p ð1 þ B 3 hq=2 Þ

ð3Þ

or, for diameters of the tubes, DðhÞ ¼ D0, p ð1 þ B 3 hq=2 Þ

Figure 9. (a) Plot depicting rate of rolling vs thickness of gold layer and (b) plot depicting variation of tube diameter with thickness of gold layer. Solid curve in panel b represents the theoretical fit of the experimental data using eq 4.

fact that the increase in the gold layer thickness makes the film stiffer, which not only results in slow rolling but also allows only small curvatures during the bending process. It must also be noted that the variation shown in Figure 9 is specific for gold. In general, the stiffer the metallic layer, the slower the rolling process and the smaller will be the curvature of the tube. A complete quantitative analysis of the D(h) dependence is not yet possible, since it should involve many unknown parameters, such as bending stiffness of both the polymer and the metallic layers, and take into account possible dependence of these parameters on the thicknesses of the layers. We still do not have enough data to elucidate these dependencies. Nevertheless, good theoretical fit of the experimental data can be obtained in frames of the following simple model. According to linear elasticity theory, the bending energy (F) of a layer of elastic material, forced to curl to some curvature radius (R), can be written as K F ¼ 2

Z 

1 1 R R0

2 dA

ð1Þ

where κ is the bending rigidity of the layer, and R0 is the equilibrium curvature, which pertains to the layer in the absence of external forces.30 In our simplified model, we suppose that the equilibrium curvature radius of the gold layer is very large compared to that of the polymer layer, and the corresponding curvature C0.Au= 1/R0,Au can be neglected. During the rolling process, the bending energy of the polymer Fp decreases, and that of the gold layer, FAu, increases. The equilibrium curvature radius can be found from the equality of the absolute values of the two energies. Let κp be the bending rigidity of the polymer layer, and R0,p be the equilibrium radius of the polymer layer, which it demonstrates after swelling in the DBSA solution. We suppose that the dependence of the bending rigidity κAu of the gold layer on the thickness has the form κAu = a 3 hq, where a and q are constants. In the classical elasticity theory, a = E/12 and q = 3, (30) Landau, L. D.; Lifshitz, E. M. Publishing: London, 1999; Chapter 18.

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Theory of Elasticity; Reed Elsevier

ð4Þ

where B = (κp/a)1/2. From experiment on rolling the polymer bilayer without the gold layer, we know the parameter D0,p ≈ 4.3 μm. Numerical fitting of the free parameters of the model, B and q, to experimental data gives the values B ≈ 1.1  10-4 and q ≈ 4.99 (note that the thickness is measured in nanometers, the diameter is measured in microns, and the dimensionality of B is the inverse to that of hq/2). The experimental data and the fitting curve are shown in Figure 9b. We have no clear explanation of why the parameter q exceeds almost twice that of the classical theory. Probably, the bending rigidity of a material composed of clusters may increase faster with the thickness of the film than the rigidity of a monocrystalline material. Special study is needed to elucidate this question. It should be noted here that there is a possibility to make a gold tube without an assisted polymer bilayer due to the presence of lateral strain in the gold layer. The presence of lateral strain in the gold layer was confirmed by a control experiment (details are given in the Supporting Information). Nevertheless, the use of an assisting polymer bilayer for the tube formation has several important advantages: (a) The strain developed in the polymer due to swelling can be much more considerable than the one in the bare metallic layer. This permits one to obtain metallic tubes of small diameters with the use of relatively thick metallic layers, which by themselves roll up in large tubes. For instance, as shown by Figure 5f, a 20nm thick Au layer can be rolled in an approximately 5 μm thick tube, whereas the same layer, used in the control experiment, rolls in the 50 μm thick tube (Figure S1). But, for many applications, it might be necessary to have sufficiently thick shells of the metallic tubes, to provide them better mechanical robustness, electrical and thermal conductivity, and so forth. (b) The degree of swelling of the polymer, and hence the strain in it and the diameter of the tubes, can be easily controlled by concentration of DBSA in the solution. The strain in the bare metallic tubes, created by thermal evaporation or magnetron vacuum sputtering, cannot be controlled so easily. (c) Although we do not demonstrate it in the present paper (but it is argued elsewhere18), the use of the polymer bilayer as the assisting layer in the rolling process potentially enables the creation of complex 3D metallic patterns, created by photo- or electron lithography on the top of the polymer bilayer. Such objects as spirals, springs, rings, and so forth and their dimensions can be easily programmed by the direction of rolling and the dimensions of the template polymer tubes. The hollow interior and good mechanical properties of metallic tubes may find interesting applications in microfludic devices. In many practical cases, fluids can be introduced in the tubes simply DOI: 10.1021/la900327v

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Figure 10. Optical microscopy image of the flow of glycerine in a titanium tube: h1 and h2 indicate the stage of flow at an 8 s interval of time.

by capillary forces. To demonstrate this, we used the titanium tube and put a drop of glycerin at the tube opening. Spreading of the liquid inside the tube was tracked by optical microscopy. Figure 10 shows the position of glycerine meniscus inside the tube at an 8 s time interval.

IV. Conclusions We have reported a simple and quite inexpensive two-step method for the fabrication of gold, titanium, and bimetallic microtubes. The metal layer is enrolled as part of the hybrid polymer/metal tube in the first step, and polymer is removed by pyrolysis in the second step. The diameter of the tube can be tailored by the thickness of the metal layer, whereas the thickness of the tube walls can be controlled by the rolling rate and time of the tube’s formation. The polymer bilayer plays the role of the support for the metallic layers, and allows the formation of tubular architectures of arbitrary single-metal or multimetal thin films. Moreover, the use of a rolling polymer support potentially enables fabrication of complex free-standing metallic objects via enrolling of lateral metallic patterns, which can be created by

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photolithography on the top of the polymer bilayer film. Metallic microtubes fabricated by the self-rolling approach may find potential application as waveguides, or elements of microfluidic devices. :: Acknowledgment. We thank Dr. Martin Muller and Gudrun Adam for their help in FTIR measurements, and Dr. Frank Simon for the XPS measurement. The authors also thank Stefan Hoffmann, P. Scheppan, and Dr. U. Burkhardt from Max Planck Institute for Chemical Physics of Solids, Dresden, for their help in EDX analysis. We are grateful to Prof. I. K. Varma for proof reading the manuscript. This work was financially supported by DFG (Project No. STA 324/41.1) and E.B.G. acknowledges financial support from the Alexander von Humboldt foundation. Supporting Information Available: The presence of lateral strain in the gold layer is shown without the assistance of polymer bilayer. This material is available free of charge via the Internet at http://pubs.acs.org.

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