Highly enhanced gas sensing in single-walled carbon nanotube-based thin-film transistor sensors by ultraviolet light irradiation
© Chen et al.; licensee Springer. 2012
Received: 5 October 2012
Accepted: 16 October 2012
Published: 23 November 2012
Single-walled carbon nanotube (SWCNT) random networks are easily fabricated on a wafer scale, which provides an attractive path to large-scale SWCNT-based thin-film transistor (TFT) manufacturing. However, the mixture of semiconducting SWCNTs and metallic SWCNTs (m-SWCNTs) in the networks significantly limits the TFT performance due to the m-SWCNTs dominating the charge transport. In this paper, we have achieved a uniform and high-density SWCNT network throughout a complete 3-in. Si/SiO2 wafer using a solution-based assembly method. We further utilized UV radiation to etch m-SWCNTs from the networks, and a remarkable increase in the channel current on/off ratio (Ion/Ioff) from 11 to 5.6 × 103 was observed. Furthermore, we used the SWCNT-TFTs as gas sensors to detect methyl methylphosphonate, a stimulant of benchmark threats. It was found that the SWCNT-TFT sensors treated with UV radiation show a much higher sensitivity and faster response to the analytes than those without treatment with UV radiation.
KeywordsSingle-walled carbon nanotubes Gas sensor UV radiation Thin-film transistor
Recently, there are many reports on the use of single-walled carbon nanotubes (SWCNTs) for fabricating a chemical sensor . The SWCNTs with every atom on the surface are expected to exhibit excellent sensitivity toward absorbates [2, 3]. SWCNTs also possess good environmental stability, excellent electronic properties, and ultrahigh ratio of surface to volume . These features make SWCNTs ideal sensing materials for compact, low-cost, low-power, and potable chemical sensors . Multiple types of SWCNT-based chemical sensors, such as chemiresistors [6, 7], chemicapacitors , and field-effect transistors (FETs) , have been developed for sensing application, but the SWCNT-FETs offer several advantages for sensing including the ability to amplify the detection signal with the additional gate electrode . Since Kong et al. first made use of SWCNT-FETs to detect NO2 and NH3, the SWCNT-FETs have been successfully used to detect a variety of gases or chemical vapors, such as formaldehyde , O2, organophosphor [6, 7], and TNT vapors . It has been shown that the SWCNT-FETs exhibit significant conductance change upon exposure to absorbed molecules. Despite these significant progresses made in this field, the fabrication of high-performance SWCNT thin-film transistor (TFT) sensors still faces a big challenge due to the coexistence of metallic and semiconducting nanotubes (denoted as m-SWCNTs and s-SWCNTs, respectively) within the conducting channels . In these unsorted SWCNT-TFTs, the electrical properties of the SWCNT networks are dominated by the m-SWCNTs in which small shifts of the Fermi level do not result in a substantial change in the density of state at the Fermi level, thus providing a lower electronic response upon analyte interaction with nanotubes [10, 11].
Several approaches have been developed for obtaining SWCNT-FETs with pure s-SWCNT in the channels such as selective synthesis of s-SWCNTs [15–18], post-treatment [19–21], and selective etching of m-SWCNTs [22, 23]. Gomez et al. have reported the method of using light irradiation to induce the metal-to-semiconductor conversion of SWCNTs for transistors based on both aligned and individual nanotube devices . This method increased the channel current on/off ratio up to 105 in the SWCNT transistors. Roberts et al. reported a self-sorting method to enrich and align the s-SWCNT on the surface of silicon and polymeric films by controlling the substrate surface chemistry . It was found that the aligned nanotube networks enriched with s-SWCNT showed a much higher sensitivity to analytes than those fabricated with random networks.
Random nanotube networks are easily deposited from the solution and represent an attractive path to large-scale device manufacturing [26–28]. However, they suffer from the mixture of s-SWCNTs and m-SWCNTs in the network, often resulting in a low on/off ratio due to the m-SWCNTs dominating the charge transport. Wang et al.  reported on the deposition of uniform and high-density s-SWCNT random networks from a s-SWCNT solution onto a wafer using an assembly method. These s-SWCNT-TFTs show on/off ratios larger than 104. However, this technology requires pre-separating 95% enriched s-SWCNTs. Here, we reported on the deposition of random SWCNT networks from the SWCNT solution on the wafer and then radiated the nanotube networks utilizing UV to fabricate SWCNT-TFT sensors with high performance. A remarkable increase in the channel current on/off ratio from approximately 11 to 5.6 × 103 was observed after UV irradiation. We also used these SWCNT-TFTs to detect methyl methylphosphonate (DMMP), a stimulant of benchmark threats. The SWCNT-TFTs treated with UV radiation show a much higher sensitivity to the analytes than those without treatment with UV radiation.
The carboxylic acid-functionalized SWCNTs were obtained from Carbon Solutions Inc. (Riverside, CA, USA). The Si/SiO2 wafer used is an n-doped Si (100) wafer with 300-nm oxide layer on the top of the Si wafer. DMMP and 3-aminopropyltrimethysilane (APS) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Deionized water, ethanol, acetone, and toluene were used throughout all processes for cleaning. All reagents were analytical grade and were used without further purification.
SWCNT network deposition and sensor fabrication
To fabricate our gas sensors, the Si/SiO2 wafer substrate was ultrasonically rinsed with toluene, acetone, ethanol, and deionized water, followed by cleaning with a piranha solution (98% H2SO4:30% H2O2 = 3:1 (v/v)) for 2 h at 80°C. This clean wafer was immersed in APS aqueous solution (1.5 mM) for 2 h and then kept in a vacuum evaporator at 120°C for 1 h to form the amino-terminated monolayer on the surface of the Si/SiO2 substrate. The purified SWCNTs were ultrasonically dispersed in deionized water for 2 h. The pretreated Si/SiO2 substrate modified with an APS monolayer was immersed in the SWCNT suspension, followed by rinsing with ethanol and deionized water and drying with the aid of nitrogen flow. The morphology of the deposited SWCNTs was characterized by scanning electron microscopy (SEM; JSM-7401F, JEOL Ltd., Akishima, Tokyo, Japan).
The SWCNT-TFT sensors were fabricated using the standard microfabrication procedures in our previous report . The interdigitated electrode fingers were made by sputtering 10 nm Cr and 180 nm Au onto the patterned photoresist mold. We introduced a lift-off process to remove the photoresist. Finally, the electrodes were ultrasonically rinsed in acetone and ethanol repeatedly, washed with deionized water thoroughly, and then dried by nitrogen flow before they were used. The UV light radiation wavelength utilized is 250 to approximately 360 nm, and the SWCNT-TFT sensor was exposed to the UV light for 0 to 1 h with a distance of 10 cm.
Sensor testing system
The homemade sensor testing system was established as in the previous reports . Nitrogen, used as carrier gas, flowed through a porous glass-disc bubbler containing liquid DMMP to form DMMP vapor, and then, this DMMP vapor was mixed with the diluting N2 in a stainless steel. The output flow ratio of the diluted DMMP vapor was controlled by mass flow controllers. The different concentrations of DMMP were produced by regulating the flow ratio of the dilution gas to the flow rate of the carrier gas. The DMMP vapor was delivered into the sensing chip to test the sensor performance. Before testing, the sensor was cleaned by UV for about 2 min to remove the absorbed impurity from the SWCNT network (meanwhile, the conductance was monitored to avoid etching SWCNTs) and then exposed to the DMMP.
The electrical signal of the sensor was monitored using a semiconductor parameter analyzer (Agilent 4156C, Agilent Technologies Inc., Santa Clara, CA, USA). After a stable baseline electrical signal was obtained, the DMMP vapor with the required concentration was introduced, and all sensing measurements were carried out at room temperature. The DMMP was desorbed from the SWNT surface by N2 blowing together with illumination with a lamp (the wavelength was 710 nm, and the power was 200 W).
Results and discussion
The transfer characteristic (Ids versus Vg) of the sensor was measured using the Si substrate as a back gate. The measurement was operated at a constant source-drain voltage of 0.5 V and gate voltage between −20 and +20 V. As shown in Figure 3, the SWCNT device shows a p-type semiconductor behavior with holes as the majority carriers, evidenced by the decreasing electrical conductance when gate voltage (Vg) was swept from negative to positive values. The on/off ratio of the untreated SWCNT-TFTs is low (approximately 11) due to the contribution of the m-SWCNTs. After 5 to 20 min of light irradiation, the SWCNT-TFTs retained a p-type semiconductor behavior, but the on/off ratios increased from approximately 11 to approximately 5.6 × 103, suggesting that most of the m-SWCNTs were etched from the channels.
We reported the deposition of uniform and high-density SWCNT networks from the SWCNT solution on the wafer and then used these nanotube networks to fabricate SWCNT-TFT sensors. UV irradiation was utilized to etch m-SWCNTs and (or) convert m-SWCNTs to s-SWCNTs. A remarkable increase in the channel current on/off ratio from approximately 11 to 5.6 × 103 was observed after UV irradiation. We also used these SWCNT-TFT sensors to detect DMMP. It was found that the irradiated SWCNT-TFT sensors show a much higher sensitivity to the analytes than those without treatment with UV radiation. Our work provides a simple and efficient approach to a large-scale fabrication of SWCNT-TFT sensors and solves the problem of low on/off ratio in the nanotube-based TFTs when the random SWCNT network is used.
We thank for the financial support from the National Natural Science Foundation of China (No. 51272155) and the Foundation for SMC Excellent Young Teacher in Shanghai Jiao Tong University.
- Kauffman DR, Star A: Carbon nanotube gas and vapor sensors. Angew Chem Int Ed 2008, 47: 6550–6570. 10.1002/anie.200704488View ArticleGoogle Scholar
- Lu YJ, Meyyappan M, Li J: A carbon-nanotube-based sensor array for formaldehyde detection. Nanotechnology 2011, 22: 1–4.View ArticleGoogle Scholar
- Kauffman DR, Star A: Electronically monitoring biological interactions with carbon nanotube field-effect transistors. Chem Soc Rev 2008, 37: 1197–1206. 10.1039/b709567hView ArticleGoogle Scholar
- Dekker C: Carbon nanotubes as molecular quantum wires. Phys Today 1999, 52: 22–28.View ArticleGoogle Scholar
- Bondavalli P, Legagneux P, Pribat D: Carbon nanotube/polythiophene chemiresistive sensors for chemical warfare agents. Sens Actuators B 2009, 141: 304–318.View ArticleGoogle Scholar
- Wang F, Gu HW, Swager TM: Carbon nanotube/polythiophene chemiresistive sensors for chemical warfare agents. J Am Chem Soc 2008, 130: 5392–5393. 10.1021/ja710795kView ArticleGoogle Scholar
- Wei LM, Shi DW, Ye PY, Dai ZQ, Chen HY, Chen CX, Wang J, Zhang LY, Xu D, Wang Z, Zhang YF: Hole doping and surface functionalization of single-walled carbon nanotube chemiresistive sensors for ultrasensitive and highly selective organophosphor vapor detection. Nanotechnology 2011, 22: 425–501.Google Scholar
- Snow ES, Perkins FK, Houser EJ, Badescu SC, Reinecke TL: Chemical detection with a single-walled carbon nanotube capacitor. Science 2005, 307: 1942–1945. 10.1126/science.1109128View ArticleGoogle Scholar
- Novak JP, Snow ES, Houser EJ, Park D, Stepnowski JL, McGill RA: Nerve agent detection using networks of single-walled carbon nanotubes. Appl Phys Lett 2003, 83: 4026–4028. 10.1063/1.1626265View ArticleGoogle Scholar
- Roberts ME, LeMieux MC, Bao ZN: Sorted and aligned single-walled carbon nanotube networks for transistor-based aqueous chemical sensors. ACS Nano 2009, 3: 3287–3293. 10.1021/nn900808bView ArticleGoogle Scholar
- Kong J, Franklin NR, Zhou CW, Chapline MG, Peng S, Cho K, Dai HJ: Nanotube molecular wires as chemical sensors. Science 2000, 287: 622–625. 10.1126/science.287.5453.622View ArticleGoogle Scholar
- Wang R, Zhang D, Zhang Y, Liu C: Boron-doped carbon nanotubes serving as a novel chemical sensor for formaldehyde. J Phys Chem B 2006, 110: 18267–18271. 10.1021/jp061766+View ArticleGoogle Scholar
- Collins PG, Bradley K, Ishigami M, Zettl A: Extreme oxygen sensitivity of electronic properties of carbon nanotubes. Science 2000, 287: 1801–1804. 10.1126/science.287.5459.1801View ArticleGoogle Scholar
- Chen PC, Sukcharoenchoke S, Ryu K, De Arco LG, Badmaev A, Wang C, Zhou CW: 2,4,6-trinitrotoluene(TNT) chemical sensing based on aligned single-walled carbon nanotubes and ZnO nanowires. Adv Mater 2010, 22: 1900–1904. 10.1002/adma.200904005View ArticleGoogle Scholar
- Ding L, Tselev A, Wang JY, Yuan DN, Chu HB, Thomas PM, Li Y, Liu J: Selective growth of well-aligned semiconducting single-walled carbon nanotubes. Nano Lett 2009, 9: 800–805. 10.1021/nl803496sView ArticleGoogle Scholar
- Hong G, Zhang B, Peng BH, Zhang J, Choi WM, Choi JY, Kim JM, Liu ZF: The fabrication and gas-sensing characteristics of the formaldehyde gas sensors with high sensitivity. J Am Chem Soc 2009, 131: 14642–14643. 10.1021/ja9068529View ArticleGoogle Scholar
- Yao YG, Feng YG, Zhang J, Liu ZF: “Cloning” of single-walled carbon nanotubes via open-end growth mechanism. Nano Lett 2009, 9: 1673–1677. 10.1021/nl900207vView ArticleGoogle Scholar
- Qian Y, Huang B, Gao FL, Wang CY, Ren GY: Preferential growth of semiconducting single-walled carbon nanotubes on substrate by europium oxide. Nanoscale Res Lett 2010, 5: 1578–1584. 10.1007/s11671-010-9679-xView ArticleGoogle Scholar
- Qiu HX, Maeda Y, Akasaka T: Facile and scalable route for highly efficient enrichment of semiconducting single-walled carbon nanotubes. J Am Chem Soc 2009, 131: 16529–16533. 10.1021/ja906932pView ArticleGoogle Scholar
- Zheng M, Jagota A, Strano MS, Santos AP, Barone P, Chou SG, Diner BA, Dresselhaus MS, Mclean RS, Onoa GB, Samsonidze GG, Semke ED, Usrey M, Walls DJ: Structure-based carbon nanotube sorting by sequence-dependent DNA assembly. Science 2003, 302: 1545–1548. 10.1126/science.1091911View ArticleGoogle Scholar
- Li PF, Xue W: Selective deposition and alignment of single-walled carbon nanotubes assisted by dielectrophoresis: from thin films to individual nanotubes. Nanoscale Res Lett 2010, 5: 1072–1078. 10.1007/s11671-010-9604-3View ArticleGoogle Scholar
- Zhang GY, Qi PF, Wang XR, Lu YR, Li XL, Ryan T, Sarunya B, David M, Zhang L, Dai HJ: Selective etching of metallic carbon nanotubes by gas-phase reaction. Science 2006, 314: 974–977. 10.1126/science.1133781View ArticleGoogle Scholar
- Yu B, Liu C, Hou PX, Tian Y, Li SS, Liu BL, Li F, Kauppinen EI, Cheng HM: Bulk synthesis of large diameter semiconducting single-walled carbon nanotubes by oxygen-assisted floating catalyst chemical vapor deposition. J Am Chem Soc 2011, 133: 5232–5235. 10.1021/ja2008278View ArticleGoogle Scholar
- Gomez LM, Kumar A, Zhang Y, Ryu K, Badmaev A, Zhou CW: Scalable light-induced metal to semiconductor conversion of carbon nanotubes. Nano Lett 2009, 9: 3592–3598. 10.1021/nl901802mView ArticleGoogle Scholar
- Roberts ME, LeMieux MC, Sokolov AN, Bao ZN: Self-sorted nanotube networks on polymer dielectrics for low voltage thin-film transistors. Nano Lett 2009, 9: 2526–2531. 10.1021/nl900287pView ArticleGoogle Scholar
- Li J, Lu YJ, Ye Q, Cinke M, Han J, Meyyappan M: Carbon nanotube sensors for gas and organic vapor detection. Nano Lett 2003, 3: 929–933. 10.1021/nl034220xView ArticleGoogle Scholar
- Wang C, Zhang JL, Ryu KM, Alexander B, Lewis GDA, Zhou CW: Wafer-scale fabrication of separated carbon nanotube thin-film transistors for display applications. Nano Lett 2009, 9: 4285–4291. 10.1021/nl902522fView ArticleGoogle Scholar
- Chen GG, Paronyan TM, Pigos EM, Harutyunyan AR: Enhanced gas sensing in pristine carbon nanotubes under continuous ultraviolet light illumination. Science Reports 2012, 2: 343.Google Scholar
- Song JW, Seo HW, Park JK, Kim JE, Choi DG, Han CS: Selective removal of metallic SWNTs using microwave radiation. Curr Appl Phys 2008, 8: 725–728. 10.1016/j.cap.2007.04.055View ArticleGoogle Scholar
- Zandian B, Kumar R, Theiss J, Bushmaker A, Cronin SB: Selective destruction of individual single walled carbon nanotubes by laser irradiation. Carbon 2009, 47: 1292–1296. 10.1016/j.carbon.2009.01.012View ArticleGoogle Scholar
- Priya BR, Byrne HJ: Quantitative analyses of microwave-treated HiPco carbon nanotubes using absorption and Raman spectroscopy. J Phys Chem C 2009, 113: 7134–7138. 10.1021/jp808913xView ArticleGoogle Scholar
- Mawhinney DB, Naumenko V, Kuznetsova A, Yates JT Jr: Infrared spectral evidence for the etching of carbon nanotubes: ozone oxidation at 298 K. J Am Chem Soc 2000, 122: 2383–2384. 10.1021/ja994094sView ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.