Yanhong Tong

Gas Dielectric Transistor of CuPc Single Crystalline Nanowire for SO2 Detection Down to Sub-ppm Levels at Room Temperature

Sulfur dioxide (SO 2 ) is one of the most dangerous air pollutants that impair environment and human health. SO 2 in air is released during the burning of fossil fuels and plays one major role in the formation of acid rain. The repeated exposure to low levels of SO 2 can cause permanent pulmonary impairment for humans. [ 1 ] The long and short-term exposure limits for SO 2 gas are 2 and 5 ppm, respectively, [ 1 , 2 ] and the U. S. Environmental Control Agency has set the acceptable limit for SO 2 in ambient air at a level of 0.5 ppm. [ 3 ] It is of great importance to measure low concentration of SO 2 in air accurately and fast for human health protection and air-quality monitoring. Semiconductor resistor sensors are, for more than 30 years, one of the most common used gas sensors to detect toxic and fl ammable gases such as NO 2 , H 2 S, CO, NH 3 , and H 2 because of their distinguished merits such as low cost, long lasting, high sensitivity, and good reliability, etc. [ 4 ] Organic fi eld-effect transistor (OFET) is another alternative semiconductor sensing technology with advantages over resistors, and has attracted much attention only recently. [ 5 ] The sensitivity can be dramatically enhanced by changing the source-drain current ( I SD ) of OFET when operating the sensors in the sub-threshold regime, as a result of the current modulation by the extra gate electrode. [ 6 ] Another advantage of OFET sensors is that the sensing response can be enhanced by integrating them in oscillator and adaptive amplifi er circuits. [ 7 ] These advantages combined with low cost and light weight of organic semiconductors ensure that OFET sensors have attracted much attention for the detection of a wide range of gases. [ 8 ]

One-Dimensional Nanostructure Field-Effect Sensors for Gas Detection

Recently; one-dimensional (1D) nanostructure field-effect transistors (FETs) have attracted much attention because of their potential application in gas sensing. Micro/nanoscaled field-effect sensors combine the advantages of 1D nanostructures and the characteristic of field modulation. 1D nanostructures provide a large surface area-volume ratio; which is an outstanding advantage for gas sensors with high sensitivity and fast response. In addition; the nature of the single crystals is favorable for the studies of the response mechanism. On the other hand; one main merit of the field-effect sensors is to provide an extra gate electrode to realize the current modulation; so that the sensitivity can be dramatically enhanced by changing the conductivity when operating the sensors in the subthreshold regime. This article reviews the recent developments in the field of 1D nanostructure FET for gas detection. The sensor configuration; the performance as well as their sensing mechanism are evaluated.

Conductive SnO2:Sb nanobelts as electrodes for detection of NO2 in ppb level with ultrahigh sensitivity

We report a SnO2 nanobelt field effect transistor sensor, with the SnO2:Sb nanobelts serving as the source and drain electrodes. An anomalous feature of the device is that the threshold voltage shows the negative shift upon exposure to NO2. The comparative results from the other two types of devices, including SnO2 nanobelt with metal film electrodes and SnO2:Sb nanobelt with metal film electrodes, reveal that the interface between the SnO2:Sb nanobelt electrodes and the SnO2 nanobelt is responsible for the improved carrier injection efficiency and the negative shift in the threshold voltage. Such a response mechanism results in the detection limit for NO2 down to 10 ppb, with a sensitivity as high as 7.16 × 105 % at room temperature.

High ON/OFF ratio single crystal transistors based on ultrathin thienoacene microplates

The high ON/OFF ratios of organic single crystal field-effect transistors (FETs) are obtained based on dinaphtho[3,4-d:3′,4′-d′]benzo[1,2-b:4,5-b′]dithiophene (Ph5T2) ultrathin microplates. The ON/OFF ratio is over 108 with a small sweep range of gate voltage (<45 V) in air at room temperature when the dielectric is modified with octadecyltrichlorosilane (OTS). The high ON/OFF ratio is related to the decreased off-state current by molecular design and the increased on-state current by dielectric modification. The ON/OFF ratio up to 1.3 × 107 can also be realized on a flexible transparent substrate. The ultrathin Ph5T2 single crystal provides stable performance even though the flexible device experiences a bending/recovering test for 200 times. The high ON/OFF ratio combined with the high mobility up to 0.51 cm2 V−1 s−1 and the good flexibility of the ultrathin organic single crystals show their promising potential in electronic applications.

Brush-controlled oriented growth of TCNQ microwire arrays for field-effect transistors

We demonstrate a novel solution-based assembly method using a writing brush to realize the controllable fabrication of highly-oriented and large-scale TCNQ single-crystal microwire arrays. The arrays can not only be grown on conventional rigid substrates, such as Si, Si/SiO2 and low-cost glass, but also on nonconventional substrates, which include flexible polyethylene terephthalate (PET), curved glass hemispheres and commercially available plastic contact lenses. Their morphology is optimized by tuning solution concentration, substrate temperature, brush type, inclination angles and pressure of the brush. The length of the microwire arrays can extend to the millimeter level, and their preferential orientation is perpendicular to the lengthwise direction of the brush hair. The coverage area of microwire arrays with a consistent orientation can reach 1.5 × 2.0 mm2 and the success ratio is as high as 93%. Based on these microwire arrays, devices on different substrates, including rigid Si/SiO2 and flexible PET, can be easily realized in one step. The anisotropic transport of TCNQ crystals is studied with respect to the concentration controlled morphology. All these results illustrate the broad application prospects of this facile writing-brush method in the growth of large-scale, high-quality organic micro/nanowires for integration into flexible organic semiconductor devices and circuits.

Individual single-crystal nanowires as electrodes for organic single-crystal nanodevices

Conductive, transparent, and flexible SnO2:Sb single-crystal nanowires are shown as electrodes for F16CuPc single-crystal nanowire devices on flexible plastic, which includes anisotropic-transport OFETs, electrode-movable OFETs, and p–n junction photovoltaic devices. The SnO2:Sb nanowires provide a good energy level match and excellent soft contact with F16CuPc nanowires, leading to multifaceted applications of the SnO2:Sb nanowire in nanowire electronics and optoelectronics, as well as high device performance. Combined with their good size compatibility, these results show that the conductive SnO2:Sb single-crystal nanowire opens a window into the fundamental understanding of the intrinsic properties of highly ordered organic semiconductors, optimization and miniaturization of organic nanocircuits, and development of new-generation flexible organic nanodevices.

Photolithography-compatible conformal electrodes for high-performance bottom-contact organic single-crystal transistors

High-performance rubrene single-crystal field-effect transistors (SCFETs) with bottom-gate bottom-contact configuration were successfully fabricated on both planar and curved surfaces based on a photolithography-compatible conformal electrode. This electrode not only provides versatile precise patterns for device design, but also eliminates the device differences by the fabrication of multiple devices based on one single crystal, which is very favorable for studies of the intrinsic properties and integration of organic devices. The resulting rubrene SCFETs show excellent electrical properties with good device uniformity, zero hysteresis, a device yield as high as 92%, and a field-effect mobility of over 20 cm2 V−1 s−1 on different surfaces including a banknote, a pencil, and a 0.7 cm glass sphere. The high electrical performance in our bottom-contact devices can be attributed to the nondestructive interface contact and eliminated electrode steps. Such a soft coplanar electrode provides a preferred configuration for bottom-contact organic field-effect transistors (OFETs), facilitating the studies on the fundamental properties of organic transistors, and showing strong potential for the development of large-scale commercial organic transistor fabrication.

Flexible Organic Single-Crystal Field-Effect Transistor for Ultra-Sensitivity Strain Sensing

To date, the functional application of the flexible organic single-crystal devices in strain sensors has not been studied. In this letter, the excellent flexibility of the beltlike rubrene single crystals enables the rubrene single-crystal field-effect transistors to be bent inward and outward, and their electrical properties under compressive and tensile strain are demonstrated. The current and mobility of the device show the nearly linear changes under the tensile and compressive strain of <0.4%. The dynamic response of the strain presents good repeatability. The calculated sensitivity under tensile strain is two orders of magnitude higher than the previously reported organic thin-film-based strain sensor. It is demonstrated that the rubrene single-crystal device can effectively detect the movement of a human finger. These results exhibit the promising potential of our transistors in artificial intelligence and healthcare systems.

Gate-modulated transport properties and mechanism for nanowire cross junction based on SnO2 semiconductor

The transport properties and mechanism of the three-terminal field-effect nanowire cross junction have been systematically investigated. An interesting phenomenon, such as applied voltage bias on nanowire cross junction makes the ON/OFF current ratio of the transistor improved by over 2 orders of magnitude, has been observed. Different from the two-terminal nanowire cross junctions, the cross junction induced potential barrier in three-terminal counterparts is found to be capable to prevent the current of the top semiconductor nanowire from injecting into the bottom nanowire at off state, while to make the current of the top semiconductor nanowire contribute to the current of the bottom nanowire at on state, resulting in the current switch between on state and off state by the gate voltage modulation.

Plasma treatment introduced memory properties in MoS2 field-effect transistors

We present a facile method to obtain MoS2-based nonvolatile memory field-effect transistors by oxygen plasma treatment on the MoS2 surface that is in contact with a dielectric. The oxygen plasma treatment provides a way of introducing deep defects into the MoS2 surface. Only those deep defects located at the semiconductor/dielectric interface can behave as charge trapping sites to develop the memory capability. No memory properties can be observed when the MoS2 surface far from the conductive channel was treated with oxygen plasma. This method brings promising advantages to MoS2-based memory devices obtained using a simple fabrication method and small device dimensions.