Highly Selective H2S Gas Sensor Based on Ti3C2Tx MXene-Organic Composites.

Cost-effective and high-performance H2S sensors are required for human health and environmental monitoring. 2D transition-metal carbides and nitrides (MXenes) are appealing candidates for gas sensing due to good conductivity and abundant surface functional groups but have been studied primarily for detecting NH3 and VOCs, with generally positive responses that are not highly selective to the target gases. Here, we report on a negative response of pristine Ti3C2Tx thin films for H2S gas sensing (in contrast to the other tested gases) and further optimization of the sensor performance using a composite of Ti3C2Tx flakes and conjugated polymers (poly[3,6-diamino-10-methylacridinium chloride-co-3,6-diaminoacridine-squaraine], PDS-Cl) with polar charged nitrogen. The composite, preserving the high selectivity of pristine Ti3C2Tx, exhibits an H2S sensing response of 2% at 5 ppm (a thirtyfold sensing enhancement) and a low limit of detection of 500 ppb. In addition, our density functional theory calculations indicate that the mixture of MXene surface functional groups needs to be taken into account to describe the sensing mechanism and the selectivity of the sensor in agreement with the experimental results. Thus, this report extends the application range of MXene-based composites to H2S sensors and deepens the understanding of their gas sensing mechanisms.

Ultrafast photoresponse of vertically oriented TMD films probed in a vertical electrode configuration on Si chips.

Integrated photodetectors based on transition metal dichalcogenides (TMDs) face the challenge of growing their high-quality crystals directly on chips or transferring them to the desired locations of components by applying multi-step processes. Herein, we show that vertically oriented polycrystalline thin films of MoS2 and WS2 grown by sulfurization of Mo and W sputtered on highly doped Si are robust solutions to achieve on-chip photodetectors with a sensitivity of up to 1 mA W-1 and an ultrafast response time in the sub-µs regime by simply probing the device in a vertical arrangement, i.e., parallel to the basal planes of TMDs. These results are two orders of magnitude better than those measured earlier in lateral probing setups having both electrodes on top of vertically aligned polycrystalline TMD films. Accordingly, our study suggests that easy-to-grow vertically oriented polycrystalline thin film structures may be viable components in fast photodetectors as well as in imaging, sensing and telecommunication devices.

Room temperature and high response ethanol sensor based on two dimensional hybrid nanostructures of WS2/GONRs.

Here in this research, room temperature ethanol and humidity sensors were prepared based on two dimensional (2D) hybrid nanostructures of tungsten di-sulfide (WS2) nanosheets and graphene oxide nanoribbons (GONRs) as GOWS. The characterization results based on scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (ESD), Raman spectroscopy and X-ray diffraction (XRD) analysis confirmed the hybrid formations. Ethanol sensing of drop-casted GOWS films on SiO2 substrate indicated increasing in gas response up to 5 and 55 times higher compared to pristine GONRs and WS2 films respectively. The sensing performance of GOWS hybrid nanostructures was investigated in different concentrations of WS2, and the highest response was about 126.5 at 1 ppm of ethanol in 40% relative humidity (R.H.) for WS2/GONRs molar ratio of 10. Flexibility of GOWS was studied on Kapton substrate with bending radius of 1 cm, and the gas response decreased less than 10% after 30th bending cycles. The high response and flexibility of the sensors inspired that GOWS are promising materials for fabrication of wearable gas sensing devices.

Computational investigation of gas detection and selectivity on TiS3 nanoflakes supported by experimental evidence.

Titanium trisulfide (TiS3), a transition metal chalcogenide, bears the potential to replace silicon, when taking the form of nanoflakes, due to its favorable band gap and optical response. In this paper, we investigate the response of TiS3 nanoflakes to gas detection through a careful quantum computational approach and a few succinct measurements. The computations are benchmarked and compared with a relevant experiment at each step, where their results/conclusions are discussed. The most stable surface of TiS3 particles is determined as (001), in agreement with the literature. The adsorption of 5 gas molecules is characterized through formulating and estimating their adsorption intensity values, rather than using singled-out values of binding energies. This formulation, which is rooted in a statistical view of the gas adsorption process, distinguishes H2 and CH4 molecules from H2O and O2 explicitly and unambiguously through comparing their adsorption profiles. The difference in the adsorption intensities thus predicts and elucidates the difference in the sensing behaviour of TiS3 particles. This work suggests that the computationally obtained profile for the adsorption spectrum of gas molecules serves as a tool/criterion to predict the selectivity of their detection by TiS3.

Static and Dynamic Performance of Complementary Inverters Based on Nanosheet α-MoTe2 p-Channel and MoS2 n-Channel Transistors.

Molybdenum ditelluride (α-MoTe2) is an emerging transition-metal dichalcogenide (TMD) semiconductor that has been attracting attention due to its favorable optical and electronic properties. Field-effect transistors (FETs) based on few-layer α-MoTe2 nanosheets have previously shown ambipolar behavior with strong p-type and weak n-type conduction. We have employed a direct imprinting technique following mechanical nanosheet exfoliation to fabricate high-performance complementary inverters using α-MoTe2 as the semiconductor for the p-channel FETs and MoS2 as the semiconductor for the n-channel FETs. To avoid ambipolar behavior and produce α-MoTe2 FETs with clean p-channel characteristics, we have employed the high-workfunction metal platinum for the source and drain contacts. As a result, our α-MoTe2 nanosheet p-channel FETs show hole mobilities up to 20 cm(2)/(V s), on/off ratios up to 10(5), and a subthreshold slope of 255 mV/decade. For our complementary inverters composed of few-layer α-MoTe2 p-channel FETs and MoS2 n-channel FETs we have obtained voltage gains as high as 33, noise margins as high as 0.38 VDD, a switching delay of 25 µs, and a static power consumption of a few nanowatts.

Non-classical logic inverter coupling a ZnO nanowire-based Schottky barrier transistor and adjacent Schottky diode.

On a single ZnO nanowire (NW), we fabricated an inverter-type device comprising a Schottky diode (SD) and field-effect transistor (FET), aiming at 1-dimensional (1D) electronic circuits with low power consumption. The SD and adjacent FET worked respectively as the load and driver, so that voltage signals could be easily extracted as the output. In addition, NW FET with a transparent conducting oxide as top gate turned out to be very photosensitive, although ZnO NW SD was blind to visible light. Based on this, we could achieve an array of photo-inverter cells on one NW. Our non-classical inverter is regarded as quite practical for both logic and photo-sensing due to its performance as well as simple device configuration.

Molybdenum disulfide nanoflake-zinc oxide nanowire hybrid photoinverter.

We demonstrate a hybrid inverter-type nanodevice composed of a MoS2 nanoflake field-effect transistor (FET) and ZnO nanowire Schottky diode on one substrate, aiming at a one-dimensional (1D)-two-dimensional (2D) hybrid integrated electronic circuit with multifunctional capacities of low power consumption, high gain, and photodetection. In the present work, we used a nanotransfer printing method using polydimethylsiloxane for the fabrication of patterned bottom-gate MoS2 nanoflake FETs, so that they could be placed near the ZnO nanowire Schottky diodes that were initially fabricated. The ZnO nanowire Schottky diode and MoS2 FET worked respectively as load and driver for a logic inverter, which exhibits a high voltage gain of ∼50 at a supply voltage of 5 V and also shows a low power consumption of less than 50 nW. Moreover, our inverter effectively operates as a photoinverter, detecting visible photons, since MoS2 FETs appear very photosensitive, while the serially connected ZnO nanowire Schottky diode was blind to visible light. Our 1D-2D hybrid nanoinverter would be quite promising for both logic and photosensing applications due to its performance and simple device configuration as well.

A ZnO nanowire-based photo-inverter with pulse-induced fast recovery.

We demonstrate a fast response photo-inverter comprised of one transparent gated ZnO nanowire field-effect transistor (FET) and one opaque FET respectively as the driver and load. Under ultraviolet (UV) light the transfer curve of the transparent gate FET shifts to the negative side and so does the voltage transfer curve (VTC) of the inverter. After termination of UV exposure the recovery of photo-induced current takes a long time in general. This persistent photoconductivity (PPC) is due to hole trapping on the surface of ZnO NWs. Here, we used a positive voltage short pulse after UV exposure, for the first time resolving the PPC issue in nanowire-based photo-detectors by accumulating electrons at the ZnO/dielectric interface. We found that a pulse duration as small as 200 ns was sufficient to reach a full recovery to the dark state from the UV induced state, realizing a fast UV detector with a voltage output.