Monolithic carbon structures including suspended single nanowires and nanomeshes as a sensor platform
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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, 2, 3, 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, 13, 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.
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