In spite of the technical importance of detecting environmental SOx and NOx gases, a selective detection has not been realized because of their similar chemical properties. In this report, adsorption and desorption of SO2 and NO2 gas on carbon nanotubes are investigated in terms of different humidity levels at room temperature. A random-network single walled carbon nanotube (SWCNT) resistor is constructed by a dip-pen method using a SWCNT/dichloroethane (DCE) solution. In the case of SO2 gas adsorption, the resistance increases at high humidity level (92%) and shows no obvious change at low humidity levels. On the other hand, in the case of NO2 gas adsorption, the resistance always decreases independent of moisture levels. Our density functional theory (DFT) calculations show that this selective behavior originates from cooperative charge compensation between the SO2–nH2O complex and the p-type CNT resistor. The change of response time and recovery time with different moisture levels is further investigated. This humidity-assisted gas reaction provides a simple route to detect these two gases selectively.
The lack of a bandgap makes bulk graphene unsuitable for room temperature transistors with a sufficient on/off current ratio. Lateral constriction of charge carriers in graphene nanostructures or vertical inversion symmetry breaking in bilayer graphene are two potential strategies to mitigate this challenge, but each alone is insufficient to consistently achieve a large enough on/off ratio (e.g. > 1000) for typical logic applications. Herein we report the combination of lateral carrier constriction and vertical inversion symmetry breaking in bilayer graphene nanoribbons (GNRs) to tune their transport gaps and improve the on/off ratio. Our studies demonstrate that the on/off current ratio of bilayer GNRs can be systematically increased upon applying a vertical electric field, to achieve a largest on/off current ratio over 3000 at room temperature.
Abstract While several approaches have been developed for sorting metallic (m) or semiconducting (s) single-walled carbon nanotubes (SWCNTs), the length of SWCNTs is limited within a micrometer, which restricts excellent electrical performances of SWCNTs for macro-scale applications. Here, we demonstrate a simple sorting method of centimetre-long aligned m- and s-SWCNTs. Ni particles were selectively and uniformly coated along the 1-cm-long m-SWCNTs by applying positive gate bias during electrochemical deposition with continuous electrolyte injection. To sort s-SWCNTs, the Ni coating was oxidized to form insulator outer for blocking of current flow through inner m-SWCNTs. Sorting of m-SWCNTs were demonstrated by selective etching of s-SWCNTs via oxygen plasma, while the protected m-SWCNTs by Ni coating remained intact. The series of source-drain pairs were patterned along the 1-cm-long sorted SWCNTs, which confirmed high on/off ratio of 10 4 –10 8 for s-SWCNTs and nearly 1 for m-SWCNTs.
Two-dimensional (2D) layered materials with properties such as a large surface-to-volume ratio, strong light interaction, and transparency are expected to be used in future optoelectronic applications. Many studies have focused on ways to increase absorption of 2D-layered materials for use in photodetectors. In this work, we demonstrate another strategy for improving photodetector performance using a graphene/MoS2 heterojunction phototransistor with a short channel length and a tunable Schottky barrier. The channel length of sub-30 nm, shorter than the diffusion length, decreases carrier recombination and carrier transit time in the channel and improves phototransistor performance. Furthermore, our graphene/MoS2 heterojunction phototransistor employed a tunable Schottky barrier that is only controlled by light and gate bias. It maintains a low dark current and an increased photocurrent. As a result, our graphene/MoS2 heterojunction phototransistor showed ultrahigh responsivity and detectivity of 2.2 × 105 A/W and 3.5 × 1013 Jones, respectively. This is a considerable improvement compared to previous pristine MoS2 phototransistors. We confirmed an effective method to develop phototransistors based on 2D materials and obtained ultrahigh performance of our phototransistor, which is promising for high-performance optoelectronic applications.
Amorphous InGaZnO/single-walled carbon nanotubes (a-IGZO/SWNTs) composite thin-film transistors were fabricated with sol-gel method. The SWNTs supply the enhanced-current path for carrier transportation, and the contact resistance was optimized by incorporating SWNTs as well. The threshold voltage (Vth) was modulated by adjusting the Ga content. High electrical performance was demonstrated, including a field-effect mobility of 132 cm2/V·s and a Vth of 0.8 V. We have fabricated large-scale working devices with channel lengths from 20 μm down to 0.7 μm. Moreover, the devices were stable over time. These results indicate that a-IGZO/SWNTs composite Thin-film transistors strongly sustain further investigation of their applicability
A simple and effective way to develop hybrid phototransistors with extraordinary optoelectronic properties is demonstrated by M. S. Jeong, Y. H. Lee and co-workers. On page 3653, they decorate singlewalled carbon nanotube (SWCNT) surfaces with semiconducting quantum dots. This hybrid structure shows a clear negative photoresponse and optical switching behavior, which could be further tuned by applying an external gate bias. Moreover, this hybrid structure shows an enhancement in the ‘optical Stark effect’ without applying any external electric field.
Vertically stacked two-dimensional van der Waals (vdW) heterostructures, used to obtain homogeneity and band steepness at interfaces, exhibit promising performance for band-to-band tunneling (BTBT) devices. Esaki tunnel diodes based on vdW heterostructures, however, yield poor current density and peak-to-valley ratio, inferior to those of three-dimensional materials. Here, we report the negative differential resistance (NDR) behavior in a WSe2/SnSe2 heterostructure system at room temperature and demonstrate that heterointerface control is one of the keys to achieving high device performance by constructing WSe2/SnSe2 heterostructures in inert gas environments. While devices fabricated in ambient conditions show poor device performance due to the observed oxidation layer at the interface, devices fabricated in inert gas exhibit extremely high peak current density up to 1460 mA/mm2, 3–4 orders of magnitude higher than reported vdW heterostructure-based tunnel diodes, with a peak-to-valley ratio of more than 4 at room temperature. Besides, Pd/WSe2 contact in our device possesses a much higher Schottky barrier than previously reported Cr/WSe2 contact in the WSe2/SnSe2 device, which suppresses the thermionic emission current to less than the BTBT current level, enabling the observation of NDR at room temperature. Diode behavior can be further modulated by controlling the electrostatic doping and the tunneling barrier as well.
Carbon nanotubes exhibit remarkable mechanical and electronic properties and are, therefore, being regarded as a new functional material for next generation electronics. Nevertheless, several obstacles still exist for an application in industry. The control of carriers in carbon nanotubes is of critical importance prior to an industrial application in transistors. As carbon nanotubes exhibit p-type behavior under ambient conditions, it is difficult to convert them from a p- to an n-type transistor. Also, doping control is a critical issue for applying traditional CMOS technology. Here, we discuss various approaches for preparing operating carbon nanotube transistors: i) impurity doping that employs conventional and interstitial insertion of group III or V materials, ii) chemical doping that induces charge transfer between chemicals and CNTs, iii) carrier control that utilizes the work function difference between metal and CNTs, iv) electrostatic doping that controls the carrier type by using a gate bias, and v) ambipolarity that does not use chemical doping. Advantages and drawbacks of these approaches will be discussed extensively in the text.
Abstract Vertical field effect transistors (VFETs) using graphene and transition metal dichalcogenides (TMDs) heterostructures are promising for downsizing the channel length to a monolayer TMD thickness of 0.65 nm. However, graphene/monolayer TMD/metal VFETs struggle with a low on/off ratio due to gate field screening by the graphene layer and a high off-state tunneling current caused by the large contact area. Here, we propose a 0.65 nm channel length VFET with a very high on/off current ratio made by cross-stacking top and bottom carbon nanotubes (CNTs) with a monolayer TMD in between. The ultra-narrow junction area in the CNT/monolayer TMD/CNT VFET can significantly reduce the off-state tunneling current. Additionally, the gate field is transmitted from the sidewall of the bottom CNT to the monolayer MoS2 vertical channel between the two CNTs without field screening, thus achieving very strong gate modulation. Unlike the BH change (< 92 meV) of the graphene/MoS2/metal junction, which is fully dependent on the Fermi level (EF) shift of graphene, the CNT/MoS2/CNT junction exhibits a larger BH change (370 meV) than the typical EF shift (20 meV with Vg = -30 ~ 20 V) of semi-metallic CNTs. As a result, our CNT/monolayer MoS2/CNT VFETs exhibit about 105 times higher on/off ratio (= 106), 105 times lower off current (= 10− 13 A), and 100 times lower SS (= 0.4 V.dec− 1) compared to graphene/monolayer TMD/metal VFETs. In the comparison between multilayer MoS2 and monolayer MoS2 VFETs, rigid multilayer MoS2 forms a large air gap at the multilayer MoS2/CNT/substrate heterostructure, which reduces electric field transmission. In contrast, monolayer MoS2 bends significantly along the sidewall of the CNT, resulting in minimal air gap formation and enhancing the electric field effect in the channel. As a result, CNT/monolayer MoS2/CNT VFET shows 10 times higher on-current saturation and on/off ratio compared to the CNT/multilayer MoS2/CNT VFET.
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