Article pubs.acs.org/JPCC
Local Electrical Investigations of Nitric Acid Treatment Effects on Carbon Nanotube Networks Huiseong Jeong and Ji-Yong Park* Department of Physics and Department of Energy Systems Research, Ajou University, Suwon 443-749, Korea ABSTRACT: Nitric acid is a commonly used chemical in preparing and functionalizing carbon nanotube (CNT) networks. It is also used to enhance the conductivity of CNT-based networks or films, which is usually attributed to ptype doping by NO3− ions. We used local electrical characterization methods based on atomic force microscopy to elucidate the effects of nitric acid treatments on sparse and random CNT networks. IV characteristics of CNT network devices before and after nitric acid treatments are obtained along with local electrical characteristics with both scanning gate microscopy and electrostatic force microscopy. Our results show both p-type doping of CNTs and reduction of CNT−CNT junction resistances after nitric acid treatments.
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INTRODUCTION There have been ongoing efforts to utilize carbon nanotubes (CNTs) for various electronic applications such as thin film transistors,1,2 transparent conducting electrodes (TCEs),3−5 and various gas or electromechanical sensors6−8 with their desirable characteristics such as relatively large bandgap (for semiconducting CNTs), high conductivity, large charge sensitivity, and flexibility. Due to the difficulties in controlled fabrications and need for larger operating currents, networks or thin films of CNTs are often used for the electronic applications. In preparing CNT networks or films, nitric acid is often used in various stages. For example, refluxing nitric acid is a common purification method of as-produced CNTs.9,10 Metal catalysts and some amorphous carbons can be removed by nitric acid treatments. Also, nitric acid is known to induce ptype doping due to intercalation of NO3− ions.11,12 Such chemical doping is used to control optical absorption and electrical conductivity of CNT TCEs.13−15 The doping effect of nitric acid has been studied with Raman,16 XPS,12 optical absorption,13 and IV transport measurements17 which reveal almost complete bleaching of first and second interband transitions for semiconducting CNTs and enhancement of conductivity in CNT TCEs. Although these measurements have convincingly shown the p-type doping effect of nitric acid on CNTs, other mechanisms may also play a role in changing the electrical properties of random CNT networks. Also it is difficult to probe or understand what is happening to individual CNTs and CNT−CNT junctions comprising CNT networks when they are exposed to nitric acid with typical bulk measurements. Therefore, localized measurement can help to better understand mechanisms of changes in electrical properties.18 In this contribution, we performed local electrical characterizations of sparse and random networks of CNTs to assess the © 2015 American Chemical Society
effect of nitric acid treatment. We chose electrostatic force microscopy (EFM) and scanning gate microscopy (SGM) to study and visualize local electrical changes associated with nitric acid treatment by carrying out measurements on the same CNT network devices before and after exposure to nitric acid vapor. We focus on the sparse networks of single-walled CNTs below the percolation threshold since it is easier to study localized effects associated with nitric acid treatments. The conducting mechanisms of CNT networks and their changes with nitric acid treatment can be better understood by correlating observed local electrical changes with the bulk IV measurements.
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EXPERIMENTAL SECTION Catalyst solutions [Fe(NO)3·9H2O in isopropyl alcohol solution] are dip-coated on the device substrate [n++ doped Si (
