Monolithic carbon structures including suspended single nanowires and nanomeshes as a sensor platform
© Lim et al.; licensee Springer. 2013
Received: 14 October 2013
Accepted: 15 November 2013
Published: 20 November 2013
With the development of nanomaterial-based nanodevices, it became inevitable to develop cost-effective and simple nanofabrication technologies enabling the formation of nanomaterial assembly in a controllable manner. Herein, we present suspended monolithic carbon single nanowires and nanomeshes bridging two bulk carbon posts, fabricated in a designed manner using two successive UV exposure steps and a single pyrolysis step. The pyrolysis step is accompanied with a significant volume reduction, resulting in the shrinkage of micro-sized photoresist structures into nanoscale carbon structures. Even with the significant elongation of the suspended carbon nanowire induced by the volume reduction of the bulk carbon posts, the resultant tensional stress along the nanowire is not significant but grows along the wire thickness; this tensional stress gradient and the bent supports of the bridge-like carbon nanowire enhance structural robustness and alleviate the stiction problem that suspended nanostructures frequently experience. The feasibility of the suspended carbon nanostructures as a sensor platform was demonstrated by testing its electrochemical behavior, conductivity-temperature relationship, and hydrogen gas sensing capability.
KeywordsSuspended carbon nanostructure Pyrolysis C-MEMS Nanomesh
The advantageous physicochemical properties of many of the different carbon microstructures have attracted a wide range of research interests and a large variety of carbon allotropes ranging from graphene sheets to carbon nanotubes (CNTs), diamond-like coatings, and glassy carbon have been investigated intensively [1–4]. Glassy carbon is one of the carbon allotropes of particular interest in this study; it exhibits a wide electrochemical stability window, excellent biocompatibility, superior thermal and chemical stability, low gas permeability, and high thermal conductivity . The low reactivity and gas impermeability of glassy carbon has been explained by a fullerene-related model that holds that glassy carbon contains primarily non-graphitizing sp2-bonded carbons . Glassy carbon has been explored for applications in solar cell systems , Li-ion batteries , optical memory devices , and electrochemical sensing platforms .
To enable these listed applications, several research groups are working towards low-cost carbon fabrication processes. Interesting three-dimensional (3D) glassy carbon shapes can often be obtained simply by patterning certain polymer precursors into the desired geometry and heating it at high temperature in an inert atmosphere or in vacuum, i.e., by pyrolysis or carbonization . Based on this general fabrication scheme, various types of polymer patterning processes and pyrolysis process variations are combined to extend the applications of glassy carbon devices. Polyfurfuryl alcohol (PFA) [12–14] and photosensitive polymers [5, 10, 15, 16] are widely used as polymeric precursors for glassy carbon. Glassy carbon nanowires were fabricated, for example, by the pyrolysis of poly furfuryl alcohol nanowires polymerized in the pores of a nanoporous alumina template and subsequent template removal . These nanowires exhibited semiconductor-type electrical properties as also found in semiconducting amorphous materials . However, with a technique like this, it is difficult to position carbon nanowires at desired locations of pre-existing structures for the completion of micro/nanodevices or for realizing reliable ohmic contacts with the nanowire at desired points along the nanowires. A more versatile fabrication method called carbon microelectromechanical systems (C-MEMS) was developed; it is capable of generating monolithic 3D carbon micro/nanostructures, inclusive of ohmic contacts, by pyrolyzing photosensitive micro/nanopolymer structures pre-patterned using any type of lithography including UV lithography and e-beam lithography [8, 16]. Especially when UV lithography is used to pattern the polymer structures, C-MEMS constitutes a simple and relatively low-cost fabrication method [5, 10, 15]. During pyrolysis, the polymer precursor experiences dramatic volume shrinkage and that shrinkage is isometric and predictable. The smaller the original polymer feature size, the more dramatic the shrinkage, and micrometer-sized features shrink as much as 85% to 90% . In an interesting variation on this process, suspended carbon nanowires between walls and posts were fabricated using a combination of UV lithography and electrospinning . The electrospun nanowires were pyrolyzed together with the UV lithographically patterned SU-8 photoresist ensuring good ohmic contact between walls/posts and wires [19, 20]. The reason these authors wanted to fabricate suspended carbon nanowires was to avoid deleterious substrate effects and to enhance mass transport in gases and liquids to the sensing element.
In the current study, we prepared monolithic suspended carbon nanostructures, including nanowires and nanomeshes, which were patterned by two successive UV exposure processes and a single pyrolysis process. The microstructure of the carbon nanowire and the development of stress along the wire were explored using a focused ion beam (FIB) milling process, scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). The intrinsic tensile stress along the nanowire and its bent supports mitigated stiction problem and this structural advantage was explored by executing photolithography, metal deposition, wet etching, and electrochemical experiment on an approximately 200-nm-diameter suspended carbon nanowire. In order to confirm the feasibility of suspended carbon nanostructures as nanosensors, their electrical, electrochemical, and thermal properties were characterized experimentally and through simulations. Moreover, the carbon nanowire was selectively coated with palladium using a lift-off process and its functionality as a hydrogen gas sensor was tested.
The shape and microstructure of the suspended carbon nanostructures were characterized using a SEM (Quanta 200, FEI company, Hillsboro, OR, USA), a HRTEM (JEM-2100 F, JEOL Ltd., Tokyo, Japan), a FIB (Quanta 3D FEG, FEI company, Hillsboro, OR, USA), and a Raman spectroscopy systems (alpha 300R, WITec GmbH, Ulm, Germany). The crystallinity of the pyrolyzed carbon was analyzed by comparing the HRTEM diffraction patterns of a suspended nanowire and the Raman spectroscopy results of bulk carbon structures. The change in the composition of the SU-8 structures after pyrolysis was confirmed using XPS (K-Alpha, Thermo Fisher Scientific Inc., Waltham, MA, USA).
The temperature-dependent resistivity change was recorded using a Keithley 2400 SourceMeter (Keithley Instruments Inc., Cleveland, OH, USA) while varying the temperature of the suspended carbon nanowire in a natural-convection oven (ON-02GW, JEIO TECH CO., Ltd., Seoul, South Korea). The samples were equilibrated for 2,000 s at each temperature to ensure that the temperature of the carbon nanowire coincided with the oven temperature. The applied current value was limited to ≤10 μA to avoid nanowire temperature increase due to Joule heating.
Electrochemical properties were established using a multichannel potentiostat (CHI-1020, CH Instruments, Inc., Austin, TX, USA) for recording cyclic voltammograms of single suspended carbon nanowires in a 10 mM ferricyanide (Sigma-Aldrich Co. LLC., St. Louis, MO, USA) and 0.5 M KCl (BioShop Canada Inc., Burlington, ON, Canada) solution. The voltage was scanned from 0.6 V to −0.2 V at a ramp rate of 0.05 V · s−1 against an Ag/AgCl reference electrode, and a Pt wire was used as a counter electrode. Diffusion-limited currents from a suspended carbon nanowire and a non-suspended wire (planar on a solid substrate) were calculated and compared to each other using COMSOL Multiphysics (ver. 4.2a, COMSOL, Stockholm, Sweden) software to confirm the effects of geometry of the suspended structures on the electrochemical current signal.
The feasibility of a single suspended carbon nanowire as a hydrogen gas sensor was tested by surface functionalization with palladium. A single carbon nanowire was coated with a 5-nm-thick palladium layer using an e-beam evaporation process subsequent to a photolithography process in which only a suspended region of the carbon structure was exposed in the metallization process. After metal deposition, the photoresist layer was stripped off using a wet process. The resistance change of the palladium-coated carbon nanowire in response to the concentration change of hydrogen gas mixed with air was recorded.
Results and discussion
In summary, we demonstrated the simple batch nanofabrication of monolithic suspended carbon nanostructures, including single nanowires and nanomeshes of scalable dimensions and user-defined designs depending on conditions in UV lithography and pyrolysis. The conversion from the microscale polymer wires to nanoscale carbon wires resulted from volume reduction of negative photoresist structures during pyrolysis under vacuum conditions. The suspended nanowire bridging carbon posts demonstrated perfect ohmic contact due to the monolithic structures. The transverse gradient of the longitudinal tension and the bridge-shaped geometry with thick bent supports of the carbon nanowire ensures high resistance to device failure due to a stiction phenomenon that limits reproducibility and applications of suspended nanostructure-based nanodevices. Furthermore, the overall density of suspended nanowire array could be enhanced by modulating the geometry of the nanowire structures from straight nanowire arrays aligned in parallel to nanomeshes that neither conventional bottom-up nor top-down nanofabrication processes can realize easily. The linked structure of the nanomeshes ensured better structural robustness than that of a linearly aligned nanowire array. We believe that the advantageous properties of the suspended carbon nanostructures including cost-effective batch nanofabrication procedure, semiconductor type electrical conductivity, electrochemical sensing capability, easy surface functionalization, structural robustness, and suspended geometry will enable those nanostructures to work as platforms for a variety of nanodevices such as gas sensors, biosensors, and nanogenerators that can be implemented by simply coating functional materials on the suspended carbon nanostructures.
This research was supported by SK Innovation Breakthrough Technology Research Program, the development program of local science park funded by the Ulsan Metropolitan City and the MSIP (Ministry of Science, ICT and Future Planning), and Basic Science Research Program through the National Research Foundation of Korea (2009–0077340). We are grateful for technical assistance from the staff members at UCRF (UNIST Central Research Facilities) in UNIST and support from the PLSI supercomputing resources of KISTI and UNIST.
- Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, Grigorieva IV, Firsov AA: Electric field effect in atomically thin carbon films. Science 2004, 306: 666–669. 10.1126/science.1102896View Article
- Iijima S: Helical microtubules of graphitic carbon. Nature 1991, 354: 56–58. 10.1038/354056a0View Article
- Lin Y, Zhang L, Mao H-K, Chow P, Xiao Y, Baldini M, Shu J, Mao WL: Amorphous diamond: a high-pressure superhard carbon allotrope. Phys Rev Lett 2011, 107: 175504.View Article
- Cowlard FC, Lewis JC: Vitreous carbon - a new form of carbon. J Mater Sci 1967, 2: 507–512. 10.1007/BF00752216View Article
- Wang C, Jia G, Taherabadi LH, Madou MJ: A novel method for the fabrication of high-aspect ratio C-MEMS structures. J Microelectromech Syst 2005, 14: 348–358.View Article
- Harris PJF: Fullerene-related structure of commercial glassy carbons. Philos Mag 2004, 84: 3159–3167. 10.1080/14786430410001720363View Article
- Imoto K, Takahashi K, Yamaguchi T, Komura T, Nakamura J, Murata K: High-performance carbon counter electrode for dye-sensitized solar cells. Sol Energy Mater Sol Cells 2003, 79: 459–469. 10.1016/S0927-0248(03)00021-7View Article
- Wang C, Madou M: From MEMS to NEMS with carbon. Biosens Bioelectron 2005, 20: 2181–2187. 10.1016/j.bios.2004.09.034View Article
- Tian H, Bergren AJ, McCreery RL: Ultraviolet–visible spectroelectrochemistry of chemisorbed molecular layers on optically transparent carbon electrodes. Appl Spectrosc 2007, 61: 1246–1253. 10.1366/000370207782597094View Article
- Heo JI, Shim DS, Teixidor GT, Oh S, Madou MJ, Shin H: Carbon interdigitated array nanoelectrodes for electrochemical applications. J Electrochem Soc 2011, 158: J76-J80. 10.1149/1.3531952View Article
- Jenkins GM, Kawamura K: Structure of glassy carbon. Nature 1971, 231: 175–176. 10.1038/231175a0View Article
- Lentz CM, Samuel BA, Foley HC, Haque MA: Synthesis and characterization of glassy carbon nanowires. J Nanomater 2011, 2011: 129298.View Article
- Samuel BA, Rajagopalan R, Foley HC, Haque MA: Temperature effects on electrical transport in semiconducting nanoporous carbon nanowires. Nanotechnology 2008, 19: 275702. 10.1088/0957-4484/19/27/275702View Article
- Schueller OJA, Brittain ST, Whitesides GM: Fabrication of glassy carbon microstructures by soft lithography. Sens Actuators A 1999, 72: 125–139. 10.1016/S0924-4247(98)00218-0View Article
- Kostecki R, Schnyder B, Alliata D, Song X, Kinoshita K, Kotz R: Surface studies of carbon films from pyrolyzed photoresist. Thin Solid Films 2001, 396: 36–43. 10.1016/S0040-6090(01)01185-3View Article
- Du RB, Ssenyange S, Aktary M, McDermott MT: Fabrication and characterization of graphitic carbon nanostructures with controllable size, shape, and position. Small 2009, 5: 1162–1168.View Article
- Silva SRP, Carey JD: Enhancing the electrical conduction in amorphous carbon and prospects for device applications. Diamond Relat Mater 2003, 12: 151–158. 10.1016/S0925-9635(03)00016-5View Article
- Sharma CS, Sharma A, Madou M: Multiscale carbon structures fabricated by direct micropatterning of electrospun mats of SU-8 photoresist nanofibers. Langmuir 2010, 26: 2218–2222. 10.1021/la904078rView Article
- Maitra T, Sharma S, Srivastava A, Cho YK, Madou M, Sharma A: Improved graphitization and electrical conductivity of suspended carbon nanofibers derived from carbon nanotube/polyacrylonitrile composites by directed electrospinning. Carbon 2012, 50: 1753–1761. 10.1016/j.carbon.2011.12.021View Article
- Sharma S, Sharma A, Cho YK, Madou M: Increased graphitization in electrospun single suspended carbon nanowires integrated with carbon-MEMS and carbon-NEMS platforms. ACS Appl Mater Interfaces 2012, 4: 34–39. 10.1021/am2014376View Article
- Park BY, Taherabadi L, Wang C, Zoval J, Madou MJ: Electrical properties and shrinkage of carbonized photoresist films and the implications for carbon microelectromechanical systems devices in conductive media. J Electrochem Soc 2005, 152: J136-J143. 10.1149/1.2116707View Article
- Singh A, Jayaram J, Madou M, Akbar S: Pyrolysis of negative photoresists to fabricate carbon structures for microelectromechanical systems and electrochemical applications. J Electrochem Soc 2002, 149: E78-E83. 10.1149/1.1436085View Article
- Williams DB, Carter CB: Transmission electron microscopy: a textbook for materials science. New York: Springer; 2009.View Article
- Wang Z, Lu Z, Huang Y, Xue R, Huang X, Chen L: Characterizations of crystalline structure and electrical properties of pyrolyzed polyfurfuryl alcohol. J Appl Phys 1997, 82: 5705–5710. 10.1063/1.366434View Article
- Soukup L, Gregora I, Jastrabik L, Konakova A: Raman spectra and electrical conductivity of glassy carbon. Mater Sci Eng B 1992, 11: 355–357. 10.1016/0921-5107(92)90240-AView Article
- Sundberg P, Larsson R, Folkesson B: On the core electron binding energy of carbon and the effective charge of the carbon atom. J Electron Spectrosc Relat Phenom 1998, 46: 19–29.View Article
- Ranganathan S, McCreery R, Majji SM, Madou M: Photoresist-derived carbon for microelectromechanical systems and electrochemical applications. J Electrochem Soc 2000, 147: 277–282. 10.1149/1.1393188View Article
- Kuriyama K, Dresselhaus MS: Metal-insulator transition in highly disordered carbon fibers. J Mater Res 1992, 7: 940–945. 10.1557/JMR.1992.0940View Article
- Im Y, Lee C, Vasquez RP, Bangar MA, Myung NV, Menke EJ, Penner RM, Yun M: Investigation of a single Pd nanowire for use as a hydrogen sensor. Small 2006, 2: 356–358. 10.1002/smll.200500365View Article
- Choi J, Kim J: Highly sensitive hydrogen sensor based on suspended, functionalized single tungsten nanowire bridge. Sens Actuat B 2009, 136: 92–98. 10.1016/j.snb.2008.10.046View Article
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