Enhanced NO2 Sensing Performance of Graphene with Thermally Induced Defects.

This paper demonstrates the enhanced NO2 sensing performance of graphene with defects generated by rapid thermal annealing (RTA). A high temperature of RTA (300-700 °C) was applied to graphene under an argon atmosphere to form defects on sp2 carbon lattices. The density of defects proportionally increased with increasing the RTA temperature. Raman scattering results confirmed significant changes in sp2 bonding. After 700 °C RTA, ID/IG, I2D/IG, and FWHM (full width at half maximum)(G) values, which are used to indirectly investigate carbon-carbon bonds' chemical and physical properties, were markedly changed compared to the pristine graphene. Further evidence of the thermally-induced defects on graphene was found via electrical resistance measurements. The electrical resistance of the RTA-treated graphene linearly increased with increasing RTA temperature. Meanwhile, the NO2 response of graphene sensors increased from 0 to 500 °C and reached maximum (R = ~24%) at 500 °C. Then, the response rather decreased at 700 °C (R = ~14%). The results imply that rich defects formed at above a critical temperature (~500 °C) may damage electrical paths of sp2 chains and thus deteriorate NO2 response. Compared to the existing functionalization process, the RTA treatment is very facile and allows precise control of the NO2 sensing characteristics, contributing to manufacturing commercial low-cost, high-performance, integrated sensors.

Preparation of defected SWCNTs decorated with en-APTAS for application in high-performance nitric oxide gas detection.

We demonstrate highly sensitive and selective chemiresistive-type NO gas detection using defected single-walled carbon nanotubes (SWCNTs) decorated with N-[3-(trimethoxysilyl)propyl]ethylene diamine (en-APTAS) molecules. The defected SWCNTs were prepared via furnace annealing at 700 °C and confirmed by transmission electron microscopy. A single en-APTAS molecule has two amine groups acting as adsorption sites for NO gas, which can improve the NO response. The NO response was further enhanced when the defected SWCNTs were utilized because NO sensing reactions could occur on both the inner and outer walls of the defected SWCNTs. The en-APTAS decoration improved the NO response of the SWCNT-based gas sensing devices by 2.5 times; when the defected SWCNTs were used, the NO response was further improved by 3 times. Meanwhile, the recovery performance in a time-resolved response curve was significantly improved (45 times) via a simple rinsing process with ethanol. Specifically, the fabricated device did not respond to carbon monoxide (CO) or BTEX gas (i.e., a mixture of benzene, toluene, ethyl benzene, and xylene), indicating its high selectivity to NO gas. The results show the possibility of a high-performance SWCNT-based NO gas sensor applicable to healthcare fields requiring ppb-level detection, such as in vitro diagnostics (IVDs) of respiratory diseases.

Negatively-Doped Single-Walled Carbon Nanotubes Decorated with Carbon Dots for Highly Selective NO2 Detection.

In this study, we demonstrated a highly selective chemiresistive-type NO2 gas sensor using facilely prepared carbon dot (CD)-decorated single-walled carbon nanotubes (SWCNTs). The CD-decorated SWCNT suspension was characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), and UV-visible spectroscopy, and then spread onto an SiO2/Si substrate by a simple and cost-effective spray-printing method. Interestingly, the resistance of our sensor increased upon exposure to NO2 gas, which was contrary to findings previously reported for SWCNT-based NO2 gas sensors. This is because SWCNTs are strongly doped by the electron-rich CDs to change the polarity from p-type to n-type. In addition, the CDs to SWCNTs ratio in the active suspension was critical in determining the response values of gas sensors; here, the 2:1 device showed the highest value of 42.0% in a sensing test using 4.5 ppm NO2 gas. Furthermore, the sensor selectively responded to NO2 gas (response ~15%), and to other gases very faintly (NO, response ~1%) or not at all (CO, C6H6, and C7H8). We propose a reasonable mechanism of the CD-decorated SWCNT-based sensor for NO2 sensing, and expect that our results can be combined with those of other researches to improve various device performances, as well as for NO2 sensor applications.

Precise capture and dynamic relocation of nanoparticulate biomolecules through dielectrophoretic enhancement by vertical nanogap architectures.

Toward the development of surface-sensitive analytical techniques for biosensors and diagnostic biochip assays, a local integration of low-concentration target materials into the sensing region of interest is essential to improve the sensitivity and reliability of the devices. As a result, the dynamic process of sorting and accurate positioning the nanoparticulate biomolecules within pre-defined micro/nanostructures is critical, however, it remains a huge hurdle for the realization of practical surface-sensitive biosensors and biochips. A scalable, massive, and non-destructive trapping methodology based on dielectrophoretic forces is highly demanded for assembling nanoparticles and biosensing tools. Herein, we propose a vertical nanogap architecture with an electrode-insulator-electrode stack structure, facilitating the generation of strong dielectrophoretic forces at low voltages, to precisely capture and spatiotemporally manipulate nanoparticles and molecular assemblies, including lipid vesicles and amyloid-beta protofibrils/oligomers. Our vertical nanogap platform, allowing low-voltage nanoparticle captures on optical metasurface designs, provides new opportunities for constructing advanced surface-sensitive optoelectronic sensors.

Al atomistic surface modulation on colloidal gradient quantum dots for high-brightness and stable light-emitting devices.

Quantum-dot (QD) light-emitting devices (QLEDs) have been attracting considerable attention owing to the unique properties of process, which can control the emission wavelength by controlling the particle size, narrow emission bandwidth, and high brightness. Although there have been rapid advances in terms of luminance and efficiency improvements, the long-term device stability is limited by the low chemical stability and photostability of the QDs against moisture and air. In this study, we report a simple method, which can for enhance the long-term stability of QLEDs against oxidation by inserting Al into the shells of CdSe/ZnS QDs. The Al coated on the ZnS shell of QDs act as a protective layer with Al2O3 owing to photo-oxidation, which can prevents the photodegradation of QD with prolonged irradiation and stabilize the device during a long-term operation. The QLEDs fabricated using CdSe/ZnS/Al QDs exhibited a maximum luminance of 57,580 cd/m2 and current efficiency of 5.8 cd/A, which are significantly more than 1.6 times greater than that of CdSe/ZnS QDs. Moreover, the lifetimes of the CdSe/ZnS/Al-QD-based QLEDs were significantly improved owing to the self-passivation at the QD surfaces.

High-performance gas sensors based on single-wall carbon nanotube random networks for the detection of nitric oxide down to the ppb-level.

We demonstrate highly sensitive and selective gas sensors based on solution-processed single-wall carbon nanotube (SWCNT) random networks for the detection of nitric oxide (NO) down to the ppb-level operating at room temperature. The proposed gas sensors exhibited a response of 50% under both inert and air atmospheres with a theoretical detection limit of 0.2 ppb and a selectivity toward different target gases of volatile organic compounds, including benzene, toluene, and ammonia. The outstanding sensing performance was realized by functionalizing SWCNT random networks with polyethylenimine (PEI), which possesses a repeating structure of amine groups. We investigate the functionalization properties of SWCNT random networks by using atomic force microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy and the sensing mechanism in the proposed NO gas sensors. We note that solution-process technologies, from the deposition of SWCNT random networks to the polymeric functionalization of amine groups, were employed at room temperature under an ambient atmosphere to fabricate highly sensitive and selective NO gas sensors, which are based on low-cost, effective, and scalable merits in the industry of sensors. We also investigate the effect of ultraviolet (UV) irradiation on the recovery time underlying the sensing mechanism. Photodesorption energy obtained by UV irradiation reduced the recovery time of the proposed NO gas sensors to within a few tens of seconds. We believe that this work is a promising and practical approach for realizing health-care monitoring systems by real-time analyzing NO gas at the ppb level in the field of biosensors.

Functionalized Carbon Nanotube Sensors for the Detection of Sub-ppm Nitric Oxide Gas.

In this paper, we demonstrate the highly sensitive carbon nanotube (CNT) sensors for the detection of sub-ppm nitric oxide (NO) gas operating at room temperature. Such achievement can be realized by functionalizing CNT thin films with amine-based polymers through a solution-process technology at low temperature. In addition to high sensitivity, functionalized CNT sensors exhibit high selectivity towards NO gas, which is an effective and practical factor for health-care monitoring nano-electronics. We also investigated the effect of a post-cleaning treatment on the sensing performance of functionalized CNT thin films for sub-ppm NO gas sensors. We believe that this work can open-up new routes to realize high performance human-interactive electronics for respiratory diseases detection in exhaled air.

A wireless monitoring sub-nA resolution test platform for nanostructure sensors.

We have constructed a wireless monitoring test platform with a sub-nA resolution signal amplification/processing circuit (SAPC) and a wireless communication network to test the real-time remote monitoring of the signals from carbon nanotube (CNT) sensors. The operation characteristics of the CNT sensors can also be measured by the ISD-VSD curve with the SAPC. The SAPC signals are transmitted to a personal computer by Bluetooth communication and the signals from the computer are transmitted to smart phones by Wi-Fi communication, in such a way that the signals from the sensors can be remotely monitored through a web browser. Successful remote monitoring of signals from a CNT sensor was achieved with the wireless monitoring test platform for detection of 0.15% methanol vapor with 0.5 nA resolution and 7 Hz sampling rate.

Detection of 10 nM ammonium ions in 35 per thousand NaCl solution by carbon nanotube based sensors.

We fabricated carbon nanotube (CNT) based chemical sensors for marine applications by photolithography process, where the electrodes were insulated by photoresist exposing only the carbon nanotube sensing section (2 microm gap width) for detection of ammonium ions (NH4+) in 35 per thousand NaCI solution used as artificial seawater environment. The I-V curve of the CNT sensor was measured by sweeping the source-drain voltage from -3 to 3 V and the on/off ratio of the CNT sensor was measured to be 20 when the gate voltage was swept from -5 to 5 V and from these results the CNTs were found to appear as a p-type semiconductor. All of the cocktail solutions prepared for experiment were measured to have -pH 6 which implied 99.9% of NH4+ remained ionized. We successfully detected 10, 100, 1000 nM (0.18, 1.8, 18 ppb) concentration of NH4+ in 35 per thousand NaCI solutions by using the CNT sensor.

Processing technique for single-walled carbon nanotube-based sensor arrays.

We investigated a selective assembly method of fabricating single-walled carbon nanotubes (SWCNTs) on a silicon-dioxide (SiO2) surface by using only a photolithographic process; then, we fabricated 8 x 8 field-emission transistor (FET) arrays for sensor applications. Photoresist (PR) patterns were made on a SiO2-grown Si substrate by using the photolithographic process. This PR-patterned substrate was dipped into a SWCNT solution dispersed in dichlorobenzene (DCB). The PR patterns were removed by using acetone. As a result, selectively-assembled SWCNT channels in 8 x 8 FET arrays could be fabricated between source and drain electrodes without complicated chemical steps using octadecyltrichlorosilane (OTS). Finally, we successfully fabricated 8 x 8 SWCNT-based multi-channel FET arrays by using our novel self-assembly method.

Pattern recognition for selective odor detection with gas sensor arrays.

This paper presents a new pattern recognition approach for enhancing the selectivity of gas sensor arrays for clustering intelligent odor detection. The aim of this approach was to accurately classify an odor using pattern recognition in order to enhance the selectivity of gas sensor arrays. This was achieved using an odor monitoring system with a newly developed neural-genetic classification algorithm (NGCA). The system shows the enhancement in the sensitivity of the detected gas. Experiments showed that the proposed NGCA delivered better performance than the previous genetic algorithm (GA) and artificial neural networks (ANN) methods. We also used PCA for data visualization. Our proposed system can enhance the reproducibility, reliability, and selectivity of odor sensor output, so it is expected to be applicable to diverse environmental problems including air pollution, and monitor the air quality of clean-air required buildings such as a kindergartens and hospitals.

Pulse-amplitude equalization using a polarization-maintaining laser resonator.

For the first time to our knowledge, pulse-amplitude equalization of rational-harmonically mode-locked fiber ring laser pulses has been experimentally demonstrated using a polarization-maintaining laser resonator without any additional device. The pulse-amplitude distribution of the laser pulses was controlled by the modulator driving power, and stable pulse-amplitude-equalized pulses with repetition rates of 20, 30, and 40 GHz have been obtained in the linear region of the modulator.

All-optical half adder using cross gain modulation in semiconductor optical amplifiers.

By using the gain nonlinearity characteristics of semiconductor optical amplifier, an all-optical binary half adder at 10 Gbps is demonstrated. The half adder operates in a single mechanism, which is cross gain modulation. The half adder utilizes two logic functions of SUM and CARRY, which can be demonstrated by using XOR and AND gates, respectively. The extinction ratios of both XOR and AND gates are about 6.1 dB. By achieving this experiment, we also explored the possibilities for the enhanced complex logic operation and higher chances for multiple logic integration.