Photo-switching operation of MoS2field effect transistor by photoisomerization of azobenzene in solution delivered on a microfluidic platform.

Combining the photoisomerization of molecules with an electrical device is important for developing optoelectronic devices. Field effect transistors (FETs) with atomically thin channels are suitable for this purpose because the FET properties respond to chemical changes in molecules. Since the photoisomerization wavelength of the switching molecules can be tuned, complex logic operations can be realized if a specific molecule is delivered to the target FET of an integrated circuit. However, conventional techniques for transferring molecules, such as drop casting and sublimation, cannot efficiently realize this goal. In this study, we fabricated a MoS2FET device combined with a microfluidic platform, wherein the MoS2channel was in contact with the flow of an azobenzene solution in isopropyl alcohol as the solvent. UV radiation (365 nm) and thermal relaxation realize the cycle of trans- and cis-azobenzene states and the switching of the substantial FET properties. This study demonstrated the feasibility of using the solution for optical switching of the MoS2-FET, which can realize quick phase changes in the molecule and the delivery of the molecule to the target FET by a microfluidic platform.

Enhanced photocatalytic activity of Cu and Ni-doped ZnO nanostructures: A comparative study of methyl orange dye degradation in aqueous solution.

Heterogeneous photocatalysis has been considered one of the most effective and efficient techniques to remove organic contaminants from wastewater. The present work was designed to examine the photocatalytic performance of metal (Cu and Ni) doped ZnO nanocomposites in methyl orange (MO) dye degradation under UV light illumination. The wurtzite hexagonal structure was observed for both undoped/doped ZnO and a crystalline size ranging between 8.84 ± 0.71 to 12.91 ± 0.84 nm by X-ray diffraction (XRD) analysis. The scanning electron microscope (SEM) and energy dispersive X-ray (EDX) revealed the irregular spherical shape with particle diameter (34.43 ± 6.03 to 26.43 ± 4.14 nm) and ensured the purity of the individual elemental composition respectively. The chemical bonds (O-H group) and binding energy (1021.8 eV) were identified by Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) results respectively. The bandgap energy was decreased from 3.44 to 3.16 eV when Ni dopant was added to the ZnO lattice. The comparative photocatalytic activity was observed in undoped and doped nanocomposites and found to be 76.31%, 81.95%, 89.30%, and 83.39% for ZnO, Cu/ZnO, Ni/ZnO, and Cu/Ni/ZnO photocatalysts, respectively, for a particular dose (0.210 g) and dye concentration (10 mg L-1) after 180 min illumination of UV light. The photocatalytic performance was increased up to 94.40% with the increase of pH (12.0) whereas reduced (35.12%) with an increase in initial dye concentration (40 mg L-1) using Ni/ZnO nanocomposite. The Ni/ZnO nanocomposite showed excellent reusability and was found 81% after four consecutive cycles. The best-fitted reaction kinetics was followed by pseudo-first-order and found reaction rate constant (0.0117 min-1) using Ni/ZnO nanocomposite. The enhanced photodegradation efficiency was observed due to decreases in bandgap energy and the crystalline size of the photocatalyst. Therefore, Ni/ZnO nanocomposite could be used as an emerging photocatalyst to degrade bio-persistent organic dye compounds from textile wastewater.

A microfluidic approach for the detection of uric acid through electrical measurement using an atomically thin MoS2 field-effect transistor.

There is a demand for biosensors working under in vivo conditions, which requires significant device size and endurance miniaturization in solution environments. We demonstrated the detection of uric acid (UA) molecules, a marker of diseases like gout, whose continuous monitoring is required in medical diagnosis. We used a field effect transistor (FET) composed of an atomically thin transition metal dichalcogenide (TMD) channel. The sensor detection was carried out in a solution environment, for which we protected the electrodes of the source and drain from the solution. A microfluidic channel controls the solution flow that can realize evaporation-free conditions and provide an accurate concentration and precise measurement. We detected a systematic change of the drain current with the concentration of the UA in isopropyl alcohol (IPA) solvent with a detection limit of 60 nM. The sensor behavior is reversible, and the drain current returns to its original value when the channel is washed with pure solvent. The results demonstrate the feasibility of applying the MoS2-FET device to UA detection in solution, suggesting its possible use in the solution environment.

Chemistry of the photoisomerization and thermal reset of nitro-spiropyran and merocyanine molecules on the channel of the MoS2 field effect transistor.

We have explored the chemical reaction of the photoisomerization and thermal reaction of the photochromic spiropyran (SP) 1',3'-Dihydro-1',3',3' trimethyl-6-nitrospiro[2H-1 benzopyran-2,2'-(2H)-indole] molecule deposited on the atomic thin channel of a MoS2 field-effect transistor (FET) through the analysis of the FET property. With four monolayers of SP molecules on the channel, we observed a clear shift of the threshold voltage in the drain-current vs gate-voltage plot with UV-light injection on the molecule, which was due to the change of the SP molecule to merocyanine (MC). A complete reset from MC to SP molecule was achieved by thermal annealing, while the injection of green light could revert the FET property to the original condition. In the process of change from MC to SP, two types of decay rates were confirmed. The quick- and slow-decay components corresponded to the molecules attached directly to the substrate and those in the upper layer, respectively. The activation energies for the conversion of MC to SP molecules were estimated as 71 kJ/mol and 90 kJ/mol for the former and latter, respectively. Combined with DFT calculations, we concluded that the Id-Vg shift with photoisomerization from SP to MC is due to the upper layer molecules and the dipole moment in the surface normal direction. Based on the estimated activation energy of 90 kJ/mol for the reset process, we calculated the conversion rate in a controllable temperature range. From these values, we consider that the chemical state of MC can be maintained and switched in a designated time period, which demonstrates the possibility of this system in logical operation applications.

Sensor behavior of MoS2 field-effect transistor with light injection toward chemical recognition.

The application of field-effect transistor (FET) devices with atomically thin channels as sensors has attracted significant attention, where the adsorption of atoms/molecules on the channels can be detected by the change in the properties of FET. Thus, to further enhance the chemical sensitivity of FETs, we developed a method to distinguish the chemical properties of adsorbates from the electric behavior of FET devices. Herein, we explored the variation in the FET properties of an MoS2-FET upon visible light injection and the effect of molecule adsorption for chemical recognition. By injecting light, the drain current (I d) increased from the light-off state, which is defined as (ΔI d)ph. We examined this effect using CuPc molecules deposited on the channel. The (ΔI d)ph vs. wavelength continuous spectrum in the visible region showed a peak at the energy for the excitation from the highest occupied orbital (HOMO) to the molecule-induced state (MIS). The energy position and the intensity of this feature showed a sensitive variation with the adsorption of the CuPc molecule and are in good agreement with previously reported photo-absorption spectroscopy data, indicating that this technique can be employed for chemical recognition.

In situ study of sensor behavior of MoS2 field effect transistor for methyl orange molecule in ultra high vacuum condition.

We investigate the sensor behavior of the MoS2 field effect transistor (FET) device with the deposition of methyl orange (MO) molecule which is widely used as a chemical probe. The channel of the FET is made of the single layer of MoS2 which makes it highly sensitive to the molecule adsorption, but at the same time the behavior depends much on the surface conditions of the MoS2 channel. In order to make the channel-surface conditions more defined, we prepare an in situ experimental system in which the molecule deposition and the surface- and electrical-characterization of the MoS2 FET are executed in a single ultra-high vacuum chamber. This system makes it possible to examine the change of the FET properties with precise control of the molecule coverage in the sub-monolayer region without the effect of the atmosphere. We detected the shift of the I d-V g curve of the MoS2-FET device with the increase of the molecule coverage (θ) of the MO molecule, which is quantitatively analyzed by plotting the threshold voltage (V th) of the I d-V g curve as a function of θ. The V th shifts towards the negative direction and the initial change with θ can be expressed with an exponential function of θ, which can be accounted for with the Langmuir type adsorption of the molecule for the first layer and the charge transfer from the molecule to the substrate. The V th versus θ curve shows a kink at a certain θ, which is conserved as the starting of the second layer growth. We detected the adsorption of MO far less than monolayer and the phase change from the first layer to the second layer growth, which is realized by the benefit of the in situ UHV experimental condition.

Microfluidic tank assisted nicotine sensing property of field effect transistor composed of an atomically thin MoS2 channel.

We investigated the sensor behavior of a field effect transistor, the channel of which is made of atomically thin MoS2 layers, focusing on the interaction of the MoS2 channel with the solution containing target molecules. For this purpose, we made a newly designed device in which the mask covered the electrodes of the source and the drain in order to make the solution contact only with the channel. In addition, a micro-fluid tank was fabricated above the channel as a solution reservoir. We examined the FET properties of this device for the sensing of the nicotine molecule for the development of a detection system for this molecule in the human body under in vivo conditions. We detected the sensor behavior both for the drop-cast process and for the condition where the channel contacts with the solution. The drain-current vs. gate-voltage variation of the MoS2-FET with the attachment of the nicotine molecule was clearly observed for both cases. For the latter case, the threshold voltage shifted in the negative gate-voltage direction with the increase of the concentration of the nicotine in the solution. This can be explained by the electron transfer from the molecule to the MoS2 channel, which was further confirmed by analyzing the X-ray photoemission spectroscopy and Raman spectroscopy together with the DFT calculation. The sensor can detect the variation of the nicotine concentration in the IPA solution by detecting the Vth change of the MoS2-FET.