NANO LETTERS
Fabrication of Suspended Silica Glass Nanofibers from Polymeric Materials Using a Scanned Electrospinning Source
2004 Vol. 4, No. 11 2105-2108
Jun Kameoka,‡,§ Scott S. Verbridge,‡ Haiqing Liu, David A. Czaplewski,† and H. G. Craighead* School of Applied & Engineering Physics and the Nanobiotechnology Center, Cornell UniVersity, Ithaca, New York 14853 Received July 21, 2004; Revised Manuscript Received September 17, 2004
ABSTRACT We report on the fabrication of suspended silicon dioxide nanofibers using a scanned electrospinning source and a calcination process. We measured the mechanical oscillations of individual suspended fibers, driven by a piezoelectric actuator. The scanned electrospinning method was utilized to extrude polymeric nanofibers from a blended polymeric solution and deposit oriented nanofibers on patterned surfaces to form suspended structures. The deposited polymeric nanofibers were converted to silicon oxide by calcination without changing their morphologies. By utilizing this technique, a suspended nanofiber with a diameter of 120 nm was fabricated with a resonant frequency of 10.8 MHz and a mechanical quality factor of 1600. Because of the simplicity of the process steps to create organized inorganic nanofibers and the ability to produce suspended structures, this approach opens new opportunities in the study and device use of inorganic nanofibers.
The study of nanofibers and nanostructure-based sensors is an active area of research. Nanofiber devices have been used for a variety of sensor applications. Chemical sensing,1 biological molecular detection,2 and charge detection3 have been accomplished. We have been studying nanomechanical and nanofiber devices for a variety of sensor applications including detection of cells, viruses, and biochemicals.4-8 The sensors used in these studies operate with high sensitivity by monitoring the change in physical properties of the nanostructures resulting from interactions with specific species of interest. The coupling of grown or self-assembled nanostructures to microstuctures can be difficult, and the use of lithographic approaches can be challenging in terms of resolution and material limitations. As a result, we have been studying processes to combine non-lithographic nanostructures with microfabricated structures. In this letter we describe the formation and mechanical evaluation of silica glass fibers. While we have evaluated the mechanical properties of the fibers fabricated with this process, the same process could be used to form optical nanostructures as sensors or for optical coupling in more complex nanosystems. * Corresponding author. † Current address: Nanostructure and Semiconductor Physics Department Sandia National Laboratories, Albuquerque, NM 87185. ‡ These two authors contributed equally. § Current address: Electrical Engineering Department, Texas A&M University, College Station, Texas 77843-3128. 10.1021/nl048840p CCC: $27.50 Published on Web 09/29/2004
© 2004 American Chemical Society
Electrospinning technology, or electrostatic field assisted fiber deposition, has received increased attention in past years because of its utility for making organic nanoscale fibers. These organic nanofibers normally have a cylindrical cross section and smooth surface for lengths greater than 100 µm.9-11 In addition to organic nanofibers, titania nanofibers have been fabricated by converting organic nanofibers via calcination.12 Although the electrospinning technology has been used for fabricating a wide range of organic and inorganic nanofibers, the orientation of nanofibers deposited on a substrate has typically been random. Therefore, it has been difficult to produce nanodevices by incorporating individual electrospun nanofibers as building blocks. To organize the fiber morphology, we have developed a scanned electrospinning system. This system is capable of depositing oriented nanofibers as well as integrating them with microfabricated structures on a substrate for nanodevice fabrication. By employing this method, we have developed nanofiberbased devices such as nanofiber electronics, chemical sensors, and nanofluidic channels.13-16 In this letter, we report a new and direct process to form inorganic suspended nanofiber mechanical structures. Without utilizing electron beam lithography and multilayer thin film deposition processes, suspended silica glass nanofiber structures were fabricated with diameters ranging from 70 to 150 nm and lengths up to 30 µm. The resonant frequencies
Figure 1. Schematic diagram showing the process steps used to fabricate the nanomechanical resonators. (a) Nanofibers were extruded from the source and deposited on the silicon chip with microfabricated trenches. (b) Deposited composite nanofibers (green color) were heated in the oven at 850 °C for 3 h to convert to silica nanofibers. (c) Silica nanofibers (yellow color) were tightly bound to the silicon substrate.
of these nanostructures were measured using laser interference techniques. The details of the scanned electrospinning system and technique were previously reported.14,17 The system has two components, a silicon arrow shaped deposition source and a rotating counter electrode. The polymer solution was prepared by dissolving 5 wt % poly(vinyl)pyrrolidone (PVP, MW:1 300 000) polymer in spin-on glass (SOG) intermediate coating solution (IC1-200 from Futurrex, Inc.) and dispensed onto the deposition source. PVP was blended with the SOG to increase the viscosity of the SOG solution, which must be viscous enough to create a continuous polymeric jet during the deposition process. The target substrate was attached to the rotating counter electrode for nanofiber deposition. When the electrostatic force applied between the source and the counter electrode overcame the surface tension of the
polymer solution, a polymeric jet was produced from the source. The polymer jet dried in transit to the target substrate and deposited as polymer nanofibers on the chip (Figure 1a). During the deposition process, the motion of the counter electrode, perpendicular to the electric field (deposition direction), controls the orientation of the nanofibers.17 Trenches with 10, 20, 30, and 40 µm widths and depths greater than 2 µm were fabricated on silicon chips using the standard microfabrication processes of contact photolithography and plasma etching. Probably because of electrostatic interactions between the surfaces of the nanofibers and the bottom of the trenches, the nanofibers tend to sag and rest on the bottom of the trench if either the trench depth is less than about 2 µm or the trench width is more than about 30 µm. A schematic diagram of deposited nanofibers as doubly clamped cylindrical nanomechanical structures can be seen in Figure 1b. Deposited polymer nanofibers were converted to chemical vapor deposition (CVD) grade SiO2 nanofibers by heating in air at 850 °C for 3 h without changing their suspended morphology (Figure 1c). Because the evaporation temperature of PVP is less than 200 °C, the PVP polymer was vaporized. During the calcination process, the SiO2 molecules were cross-linked. A scanning electron micrograph of a cylindrical polymeric nanofiber before the calcination process is shown in Figure 2a. The average diameter of the nanofiber was 107 nm. A scanning electron micrograph of the same SiO2 nanofiber after the calcination is shown in Figure 2b. The average diameter, post calcination, of the nanofiber is 85 nm. Due to the evaporation of organic materials and solvents, and cross-linking of the SiO2, the diameter of the nanofibers decreased by about 18%. A substantial decrease in PVP content during the calcination process has been verified by energy dispersive X-ray analysis (EDX). Using a scanning electron microscope equipped with an EDX detector, the carbon content of several fibers was examined. Fibers examined included freshly electrospun fibers that had not yet been baked and fibers that had gone through the calcination process on both evaporated gold and bare silicon substrates. The average carbon peak amplitude for the postcalcination samples, with fibers of diameter on the order of 1 µm examined, was 30% of the carbon peak amplitude for
Figure 2. Scanning electron micrographs of composite nanofibers (a) before the calcination process and (b) after calcination. The diameter of the nanofiber was 107 nm before and 85 nm after calcination. 2106
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Figure 3. Scanning electron micrographs of suspended silica nanofibers after calcination. The diameter and length of the resonators were (a) 160 nm and 10 µm, (b) 120 nm and 10 µm, (c) 70 nm and 10 µm. (d) Scanning electron micrograph of the cross section of a silica nanofiber deposited on the silicon surface.
the fibers that had not yet been baked. The average background carbon signal was 23% of the unbaked fiber carbon peak, due to carbon contamination of the substrate and detector equipment. This significant reduction in carbon content to near-background levels, as shown by a reduction of the average EDX carbon peak, is consistent with a significant reduction in fiber PVP concentration during the calcination process. Scanning electron micrographs of cylindrical suspended nanomechanical structures, created by the scanned electrospinning technique, after the calcination process, can be seen in Figures 3a, 3b, and 3c. The diameters of these silica cylinders are 160, 120, and 70 nm, respectively. A scanning electron micrograph of the cross section of a silica nanofiber, with a 160 nm diameter, deposited on a silicon surface is shown in Figure 3d. The cross section of the nanofiber is almost circular and the bottom of the nanofiber is tightly bound to the substrate. The system used to detect the nanomechanical oscillation in this study has been previously used to detect nanomechanical oscillation in a vacuum or in air.18-20 Our nanomechanical resonators were placed in a vacuum chamber and pumped down to pressures below 10-5 Torr. The chip containing the nanofibers was attached to a piezoelectric transducer inside the vacuum chamber for mechanical actuation. A 5 mW He-Ne laser was focused on the center of the cylinder through a microscope objective lens. The laser light, reflected from the cylinder and the bottom of trench, was directed into a photodiode to detect the oscillation. The location of the laser spot on the cylinder was adjusted to Nano Lett., Vol. 4, No. 11, 2004
Figure 4. Plot of the mechanical frequency response of a 10 µm long, 120 nm diameter oscillator showing a resonance at 10.8 MHz. The quality factor of the oscillator is approximately 1600.
obtain the maximum reflected light modulation detected by the photodiode. A spectrum analyzer was used to observe the frequency response of the fiber oscillation and to determine the resonant frequencies of the nanofiber. A plot of the mechanical response of a 120 nm diameter nanofiber as a function of actuation frequency is shown in Figure 4. The resonant oscillation frequency of this device was 10.8 MHz with a quality factor of approximately 1600. This peak is the fundamental resonant frequency, and this frequency 2107
is consistent with SOG doubly clamped beams made by electron beam lithography.21 In conclusion, we have developed a nanomaching process for fabricating silica nanofibers. Suspended fibers were formed by combining a scanning electrospinning deposition method with contact photolithography and a calcination process to convert polymeric nanofibers to inorganic nanofibers. The organic materials contained in the composite nanofibers were vaporized by a heating process, leaving behind the cylindrical silica nanofibers that we used as nanomechanical resonators. This fabrication approach offers an alternative to the conventional method of thin film deposition, electron beam lithography, and plasma etching for the formation of nanomechanical devices. We have demonstrated the use of these nanofibers as mechanical resonators, but similar geometries could be used for optical or other nanomechanical devices. Acknowledgment. This work was supported by the Nanobiotechnology Center (NBTC), an STC Program of the National Science Foundation under Agreement No. ECS9876771. The photolithography was performed at the Cornell Nano-Scale Science & Technology Facility (a member of the National Nanofabrication Users Network). This research made use of the Cornell Center for Materials Research Shared Experimental Facilities supported through the NSF MRSEC program (DMR-0079992). One of the authors (S.V.) thankfully acknowledges a GAANN fellowship given through the Cornell Center for Materials Research. References (1) Lavrik, N.; Datskos, P. G. Appl. Phys. Lett. 2003, 82(16), 26972699.
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(2) Cui, Y.; Wei, Q. Q.; Park, H. K.; Lieber, C. M. Science 2001, 293, 1289-1292. (3) Cleland, A. N.; Roukes, M. L. Nature 1998, 392, 160-162. (4) Ilic, B.; Czaplewski, D. A.; Zalalutdinov, M.; Craighead, H. G.; Neuzil, P.; Campagnolo, C.; Batt, C. J. Vac. Sci. Technol. 2001, Vol. B19(6), 2825-2828. (5) Ilic, B.; Czaplewski, D.; Craighead, H. G.; Neuzil, P.; Campagnolo, C.; Batt, C. Appl. Phys. Lett. 2000, 77, 450. (6) Ilic, B.; Craighead, H. G.; Krylov, S.; Senaratne, W.; Ober, C.; Neuzil, P. J. Appl. Phys. 2004, 95, 3694-3703. (7) Liu, H.; Kameoka, J.; Czaplewski, D. A.; Craighead, H. G. Nano Lett. 2004, 4, 671-675. (8) Ilic, B.; Yang, Y.; Craighead, H. G. Appl. Phys Lett., in press. (9) Shin, Y. M.; Hohman, M. M.; Brenner, M. P.; Rutledge, G. C. Appl. Phys. Lett. 2001, 78, 1149. (10) Doshi, J.; Reneker, D. H. J. Electrost. 1995, 35, 151. (11) Deitzel, J. M.; Kleinmeyer, J. D.; Hirvonen, J. K.; Beck Tan, N. C. Polymer 2001, 42, 8163. (12) Li, D.; Xia, Y. Nano Lett. 2003, 3, 555-560. (13) Kameoka, J.; Czaplewski, D.; Liu, H.; Craighead, H. G. J. Mater. Chem. 2004, 14, 1503-1505. (14) Kameoka, J.; Orth, R.; Yang, Y.; Czaplewski, D.; Mathers, R.; Coates, G.; Craighead, H. G. Nanotechnology 2003, 14, 1124-1129. (15) Czaplewski, D. A.; Kameoka, J.; Craighead, H. G. J. Vac. Sci. Technol. B 2003, 21, 2994-2996. (16) Czaplewski, D. A.; Kameoka, J.; Mathers, R.; Coates, G. W.; Craighead, H. G. Appl. Phys. Lett. 2003, 83, 4836-4838. (17) Kameoka, J.; Craighead, H. G. Appl. Phys. Lett. 2003, 83, 371373. (18) Evoy, S.; Olkhovets, A.; Sekaric, L.; Parpia, J. M.; Craighead, H. G.; Carr, D. W. Appl. Phys. Lett. 2000, 77, 2397-2399. (19) Sekaric, L.; Zalalutdinov, M.; Bhiladvala, R. B.; Zehnder, A. T.; Parpia, J. M.; Craighead, H. G. Appl. Phys. Lett. 2002, 81, 26412643. (20) Carr, D. W.; Craighead, H. G. J. Vac. Sci. Technol. 1997, B15, 27602763. (21) Tannenboum, D. M.; Olkhovets, A.; Sekaric, L. Vac. Sci. Technol. 2001, B19, 2829-2833.
NL048840P
Nano Lett., Vol. 4, No. 11, 2004
