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.
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.
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