Increasing Irradiation Time Research Articles

Catalytic studies of NiFe2O4 nanoparticles prepared by conventional and microwave combustion method

A simple and rapid synthesis has been developed to synthesize NiFe2O4 nanoparticles with tunable optical, magnetic and catalytic properties by using Hibiscus rosa sinensis plant extract in the current study. The crystalline structure of the synthesized NiFe2O4 nanoparticles was confirmed by X-Ray diffraction (XRD) and Rietveld refinement analysis. Morphological, elemental, magnetic and optical properties of the synthesized nanoparticles were interpreted by fourier transform infra-red spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), UV–Visible diffused reflectance spectroscopy (UV-DRS), photoluminescence (PL) studies, and vibrating sample magnetometer (VSM). XRD analysis showed the single phase crystalline nature with average crystallite size of 50–70 nm. SEM and EDX analysis recognize the spherical shape nanoparticles and phase purity of the samples. The optical band gap is determined using UV-DRS spectra and is observed to decrease with increasing the calcination temperature. VSM studies obviously demonstrate the ferromagnetic property of the samples. The value of saturation magnetization has decreased with an increase in the calcination temperature and the coercivity is decreased. Additionally, NiFe2O4 nanoparticles were investigated for the catalytic reduction capability for different dyes like Rhodamine-B (Rh-B), Methylene blue (MB), Rose Bengal (RB), and Congo red (CR). It is found that the dye solution was degraded with increasing irradiation time. Hence, the prepared nanoparticles have been used as high potential catalysts for the reduction of organic dyes in the application of waste water treatment. NiFe2O4 is an active agent in catalyzing the oxidation of glycerol to formic acid with good conversion percentage and high selectivity. This catalytic system in the polar solvents, such as, acetonitrile provides the enhanced conversion results. The very important role played by a variety of reaction parameters in deciding the catalytic effectiveness is well established and every parameter has an optimal value to acquire the maximum catalytic activity.

A novel pathway for the photooxidation of catechin in relation to its prooxidative activity

In the present study, we evaluated the prooxidative mode of action of photoirradiated (+)-catechin at 400 nm in relation to reactive oxygen species generation and its possible application to disinfection. Photoirradiation of (+)-catechin at a concentration of 1 mg/mL yielded not only hydrogen peroxide (H2O2) but hydroxyl radical (·OH) in a total amount of approximately 20 μM in 10 min. As a result, photoirradiated catechin killed Staphylococcus aureus, and a > 5-log reduction in viable bacteria counts was observed within 20 min. Liquid chromatography-high-resolution-electrospray ionization-mass spectrometry showed that photoirradiation decreased the (+)-catechin peak (molecular formula C15H14O6) whilst it increased two peaks of a substance with the molecular formula C15H12O6 with increasing irradiation time. Nuclear magnetic resonance analysis revealed that the two C15H12O6 peaks were allocated to intramolecular cyclization products that are enantiomers of each other. These results suggest that photoirradiation induces oxidation of (+)-catechin resulting in the reduction of oxygen to generate H2O2. This H2O2 is then homolytically cleaved to ·OH, and alongside this process, (+)-catechin is finally converted to two intramolecular cyclization products that are different from the quinone structure of the B ring, as proposed previously for the autoxidation and enzymatic oxidation of catechins.

Optical Properties of ZnSe Nanocrystals (NCs) Prepared by Microwave Irradiation Method at Different pH and Different Irradiation Times

Different parameters such as pH and microwave (MW) irradiation time (in MW-assisted synthesis methods) can affect the photo-physical properties of nano-materials via tuning the size of nanocrystals. In this research work, ZnSe semiconducting nanocrystals (NCs) were synthesized in aqueous medium at different pH. Growth of NCs was done employing microwave irradiation with the power of 1000 W at different irradiation times. According to the results of x-ray diffraction (XRD), no displacement was seen in the x-ray diffraction pattern peaks of ZnSe nanoparticles by altering the pH and microwave irradiation time. Generally, UV–Visible absorption spectra of the samples indicate that the increase of microwave irradiation time and the pH, resulted in the red shift of the absorption edge. The exact amount of the energy band gap of the nanoparticles was estimated by using the derivation of absorption spectrum fitting method (abbreviated as DASF) which was within the range of 3.488–3.721 eV depending on the pH and microwave irradiation time. Also, for the present NCs, using the values of energy gap, the optical charge carrier transition index (m) was evaluated, indicating crucially the direct gap nature. Depending to the microwave irradiation time, the size of the synthesized crystallites (obtained from XRD) were within the range of 1.74–2.18 nm and no general certain trend was founded against pH; employing the precise data of energy gap, effective mass approximation gives a reasonable increasing trend of NCs size with increasing pH except for pH = 12.2 Other purposes of this study are gaining the additional photo-physical properties of Urbach energy (Etail), dielectric constant (e), and the refractive index (n).

In Situ Tracking of Organic Reactions at the Vapor/Liquid Interfaces of Ionic Liquids.

The molecular structures of ionic liquids at interfaces play a crucial role in determining their chemical activities in applications. In situ X-ray photoelectron spectroscopy (XPS) was used to track the evolution of X-ray irradiation-induced chemical reactions in a series of ionic liquids ([Cn mim][AuCl4 ]; n=4, 6, 8, 10) on the Si (111) single-crystal surface. Analyses of microstructure and chemical bonding based on the XPS results indicated that reactions occurred at the vapor/liquid interfaces of the ionic liquids. The time-resolved XPS spectra revealed that with increasing irradiation time, the intensity of the peak corresponding to trivalent Au anion decreased for the four ionic liquids as Au was continually reduced to a lower chemical state and finally converted to gold nanoparticles. The rate and conversion of the reaction were associated with the length of the alkyl chain of the ionic liquids cation. Molecular dynamics simulations further revealed that the alkyl chain of the cation in the ionic liquids was oriented towards the vacuum environment at the vapor/liquid interface. Our results provide a real-time atomic-scale experimental evidence of organic reactions at the vapor/liquid interfaces of ionic liquids. The findings are important for understanding the roles of ionic liquids in catalysis, separation, electrochemistry, functional materials, and so on.

Silica nanoparticles embedded with water insoluble phthalocyanines for the photoinactivation of microorganisms.

Silica nanoparticles (SiNPs) embedded with Zn(II) 2,9,16,23-tetrakis(methoxy)phthalocyanine (SiNPZnPcOCH3), Zn(II) 2,9,16,23-tetrakis(4-pyridyloxy) phthalocyanine (SiNPZnPcOPy) and Zn(II) 2,9,16,23-tetrakis(t-butyl) phthalocyanine (SiNPZnPctBu) were synthesized in the nonpolar core of AOT/1-butanol/water micelles using triethoxyvinylsilane and 3-aminopropyltriethoxysilane. These SiNPs-Pc presented an average diameter of about 20-25 nm. UV-vis absorption spectra presented the characteristic Soret and Q bands of phthalocyanines embedded into the nanoparticles. Moreover, red fluorescence emission of SiNPs bearing phthalocyanines was detected in water. The SiNPs-Pc produced the photodecomposition of 2,2'-(anthracene-9,10-diyl)bis(methylmalonic acid), which was used to sense the singlet molecular oxygen O2(1Δg) generation in aqueous medium. Also, the formation of superoxide anion radical was detected by nitro blue tetrazolium reduction in the presence of NADH. Photoinactivation of microorganisms was investigated in Staphylococcus aureus and Candida albicans. In vitro experiments showed that photosensitized inactivation induced by SiNPZnPcOCH3 and SiNPZnPctBu improved with an increase of irradiation times. After 30 min irradiation, over 7 log reduction was found for S. aureus. Also, these SiNPs-Pc produced a decrease of 2.5 log in C. albicans after 60 min irradiation. In both cases, a lower photoinactivation activity was found for SiNPZnPcOPy. Studies of photodynamic action mechanism showed that the photokilling of microbial cells was protected in the presence of sodium azide and diazabicyclo[2.2.2]octane. Also, a reduction on the cell photodamage was found with the addition of D-mannitol. Therefore, the photodynamic activity sensitized by SiNPZnPcOCH3 and SiNPZnPctBu in microbial cells was mediated by a contribution of both type I and type II photooxidative mechanisms. Thus, silica nanoparticles are interesting materials to vehicle ZnPcOCH3 and ZnPctBu in aqueous media to photoeradicate microorganisms.

Effect of solution plasma process with hydrogen peroxide on the degradation and antioxidant activity of polysaccharide from Auricularia auricula

Solution plasma process (SPP) in combination with hydrogen peroxide (H2O2) were adopted to the degradation of Auricularia auricula polysaccharide (AAP). The results showed that SPP irradiation with H2O2 was effective for the AAP degradation. The intrinsic viscosity ([η]) of AAP solution decreased exponentially with increase in irradiation time. The degradation reaction closely fitted to the first order kinetics. The degradation rate constant after SPP irradiation with H2O2 was approximately 4.2 times that with SPP irradiation alone. GPC analysis showed that the molecular weight distribution of AAP was narrowed during degradation. FTIR and 13C NMR analysis revealed that primary structure of APP was not changed by the combination method. In vitro antioxidant activity of the polysaccharides was evaluated by determining their reducing power and radical (ABTS radical and superoxide radical) scavenging abilities. It was found that the degraded AAP possessed the higher antioxidant activity. The results suggested that SPP with H2O2 was an effective means for AAP degradation.

Experiment on the temporal evolution characteristics of a CCD multilayer structure irradiated by a 1.06 μm continuous laser.

In this study, an experiment of a CCD detector irradiated by a 1.06μm continuous laser is established. Factors including the microlens focusing beam, microlens carbonization at high temperature, shading aluminum film opening rate, low shading aluminum film absorption rate, and different materials' absorption to the laser are comprehensively considered. By combining the irradiation time, the output image of the CCD detector, surface morphology of the damage area, the optical micrograph, and regional energy spectrogram, the damage mechanism of the CCD is explored. In addition, the threshold time and the damage morphology of the multilayer structure of the CCD detector are investigated. The results show that when the irradiation time increases, the damage starts from the microlens due to melting, which is represented as point damage. Subsequently, the aluminum film melts and is separated from the SiO2 by stress and melting damage, causing vertical bright linear damage. Without the protection of the shading aluminum film, the silicon electrode heats up and reaches the melting point, causing damage to the wiring circuit, which is represented as horizontal dark linear damage. Eventually, the N-Si layer in the silicon substrate melts and the clock signal is destroyed, which means that the optical signal is not converted into an electrical signal. The CCD detector gets completely damaged.

Tuning Transport and Photoelectric Performance of Monolayer MoS2 Device by E‐Beam Irradiation

AbstractHere, the effect of E‐beam irradiation on the transport properties of the monolayer MoS2 is investigated in situ using scanning electron microscope (SEM). The results show that the source drain current is increased under E‐beam irradiation; the current gradually increases to a saturation value with an increase in irradiation time, and the saturation value increases with an increase in E‐beam intensity. After E‐beam irradiation, X‐ray photoelectron spectroscopy (XPS) results show that the stoichiometry of MoS2 is modified, while the Raman and PL results reveal that the optical performance of the monolayer MoS2 is not deteriorated. The source–drain current and electron mobility of the MoS2 transistor are reduced by E‐beam irradiation, while the on–off current ratio of the transistor almost unchanges. However, the photoresponse performance, such as the light dark current ratio, response, and release speed of the device are improved. The results are discussed by the creation of defects which vary the injected and trapped charges of the sample by E‐beam irradiation. The results provide a new method to improve the optoelectronic performance of two dimensional (2D) materials devices.

Preparation and properties of heat and ultraviolet‐induced bonding and debonding epoxy/epoxy acrylate adhesives

ABSTRACTHeat and ultraviolet (UV)‐induced bonding and debonding (BDB) adhesives were designed and prepared through blending an epoxy resin, diglycidyl ether of bisphenol A (DGEBA) with an epoxy acrylate resin, bisphenol‐A epoxy acrylate resin (BEA). The variation of the chemical structure of DGEBA and BEA in the sequential heat‐ and UV‐curing processes was characterized by Fourier transform infrared spectroscopy (FTIR). The FTIR results indicate that DGEBA and BEA successfully took part in both the heat‐curing and UV‐curing processes. The effects of the mass ratio of BEA to DGEBA, amount of heat‐curing agent, type of diluents, and UV irradiation time on the BDB properties of BDB adhesive were systematically investigated. The results show that the bonding strength increases with the decrease of the mass ratio of BEA to DGEBA and with the increase of the amount of heat‐curing agent in a certain range. The debonding strength decreases with the increase of the mass ratio of BEA to DGEBA. The mass ratio of BEA to DGEBA was set at 10 to ensure the ratio of the bonding strength to debonding strength greater than 10 times. The debonding strength of BDB adhesives also depends on the UV irradiation time, decreasing with the increase of UV irradiation time in a certain range. Based on the FTIR results and the dependence of the bonding and deboning strengths on the reaction conditions, a possible BDB mechanism of BDB adhesive was proposed. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46435.

Microwave irradiation’s effect on promoting coalbed methane desorption and analysis of desorption kinetics

To enhance desorption of coalbed methane and increase extraction efficiency, a technique using microwave irradiation is proposed. Methane desorption experiments without and with microwave irradiation were carried out in the laboratory using an experimental system developed to study methane desorption. The experimental results show that microwave irradiation causes the total quantity of methane desorbed to increase from 1.91 to 3.92 times larger than the quantity desorbed without MI. Two hour trials were performed, applying microwaves for 4, 8 or 16 min, equivalent to output energies of 192, 384 and 768 kJ. These results show that 1 kJ of microwave energy can cause methane desorption to increase by 0.0088 ml per gram of coal. Under successive microwave irradiations of 40 s, the desorption rate increases by a factor of 10.2. Kinetic analysis also found that microwave irradiation enhances the methane diffusion coefficient and decreases the attenuation coefficient. Regardless of which diffusion model is used to determine the diffusion coefficient, the quantity of gas desorbed increases as the microwave irradiation time increases. The pore and surface features of coal samples exposed to microwaves were determined by using mercury intrusion porosimetry and nitrogen adsorption and specimens were examined by scanning electron microscopy. Methane desorption evidently results from both the microwave thermal effect and structural damage to the coal. The results reported herein reveal the mechanisms that promote methane desorption by microwave irradiation and suggest a new field technique for extracting coalbed methane.

CO2 laser-assisted preparation of transparent Eu2Ti2O7 thin films

We present a laser-assisted preparation of transparent europium-titanate Eu2Ti2O7 thin films with tailored structural and optical properties. We have evaluated the effects of the irradiation time on the structural and the optical properties of the films. This approach allows the preparation of nanocrystalline crack-free films and micro patterns. The amorphous thin films were prepared by a sol-gel method. The films were annealed by a CO2 laser beam for various time intervals. The laser irradiation induced a crystallization process that resulted in the formation of Eu2Ti2O7 nanocrystals. The nanocrystals regularly grew with increasing irradiation time reaching the size from 25 nm to 45 nm. A film of a thickness 480 nm exhibited an optical transmission of 91.9% that is close to the maximal theoretical limit. The film's refractive index at 632 nm was 2.26. A micrometric pattern was prepared by a direct laser writing followed by a wet chemical etching. Feasibility of the demonstrated approach, together with the high film's quality, and europium-titanate chemical resistivity open up many opportunities for advanced applications. The approach can be used for a preparation of protective coatings and integrated photonic devices such as planar optical waveguides and couplers.

Reductive transformation of profenofos with nanoscale Fe/Ni particles.

Profenofos is an abundantly used organophosphate pesticide in agriculture but its excessive use may lead to hazardous effects on environment. Thus, the present study focused on the reductive transformation of this pesticide in the presence of Fe/Ni bimetallic nanoparticles by optimizing the process parameters such as stirring time, nanoparticles dose, pH, and initial pesticide concentration. The results of reductive transformation were compared with photodegradation studies. It was found that with the increase in UV irradiation time, the percent degradation was increased. After 660min, a 78% photodegradation of 100μM solution of pesticide was observed. On increasing the initial pesticide concentration to 200μM, the maximum degradation was achieved in 570min, but here, only 73% degradation was observed. The rates of photodegradation observed with 100 to 400μM solutions were 1.4 × 10-3, 1.5 × 10-3, 1.5 × 10-3, and 5 × 10-4min-1 respectively.In case of reductive transformation carried out in the presence of Fe/Ni bimetallic nanoparticles, the degradation was observed to be increased from 78 to 93.9% in only 180min. A further increase in pesticide concentration led to a decrease in degradation. Under these conditions, the rate of reaction was found to be 1.09 × 10-2min-1. A 93% degradation of profenofos was further increased to 98% when the quantity of nanoparticles was increased twice; hence, a significant reduction in time of irradiation was observed. Reductive transformation of pesticide thus provided an efficient and cheaper method for reducing the burden of profenofos from the environment.

Compatibility and bone bonding efficiency of gamma irradiated Hench's Bioglass-Ceramics

Samples of Hench's bioglass-ceramic derived from thermal heat treatment of parent Hench's bioglass of the normal composition (45SiO2−24.5Na2O-24.5CaO-6P2O5 wt%) were prepared. Combined collective characterization of the effects of both gamma irradiation with varying doses (1→10Mrad) and immersion times in SBF (1→4 weeks) through specific analytical techniques including FTIR, X-Ray diffraction and SEM investigations. The overall extended studies are focused on justifying the bone-bonding efficiency with the interference of both X-Ray diffraction and immersion times. Experimental data indicate the stability of FTIR spectra upon the increase of gamma irradiation but prolonged immersion times cause distinct variations which touch the vibrational bands in consequence to the ion exchange or preferential attack and replacement of modifiers by hydronium ions leading to the surface silica gel layer from the SBF and the final formation of hydroxyapatite as an indicator for bone-bonding ability. The FTIR spectra show some variations as visualized from the IR spectra of the only un-irradiated samples and that for irradiated samples upon prolonged immersion. X-ray diffraction patterns are found to change by gamma irradiation dose and/or immersion time in SBF. Hydroxyapatite and silicon phosphate crystalline phases are identified to be formed upon gamma irradiation alone or with combined immersion and gamma irradiation beside the formation of crystalline sodium calcium silicate phase formed with both the un-irradiated and un-immersed samples.

Controlling the size and magnetic properties of nano CoFe2O4 by microwave assisted co-precipitation method

In this report, cobalt ferrite nanoparticles synthesized using microwave assisted co-precipitation method was reported. Efforts have been made to control the particles size, distribution, morphology and magnetic properties of cobalt ferrite nanoparticles by varying the concentration of NaOH solution and microwave irradiation time. It was observed that the rate of nucleation and crystal growth was influenced by the tuning parameters. In that way, the average crystallite size of single phase cobalt ferrite nanoparticles was controlled within 9–11 and 10–12 nm with an increase of base concentration and microwave irradiation time, respectively. A narrow size distribution of nearly spherical nanoparticles was achieved through the present procedure. A soft ferromagnetism at room temperature with the considerable saturation magnetization of 58.4 emu g−1 and coercivity of 262.7 Oe was obtained for the cobalt ferrites synthesized with 2.25 M of NaOH solution for 3 and 7 min of microwave irradiation time, respectively. The cobalt ferrite nanoparticles synthesized with a shorter reaction time of 3–7 min was found to be advantageous over other methods that involved conventional heating procedures and longer reaction time to achieve the better magnetic properties for the technological applications.

EUV photofragmentation study of hybrid nonchemically amplified resists containing antimony as an absorption enhancer.

A detailed investigation to understand the mechanism of the resist action at a fundamental level is essential for future Extreme Ultraviolet Lithography (EUVL) resists. The photodynamics study of a newly developed hybrid nonchemically amplified 2.15%-MAPDSA–MAPDST resist using synchrotron radiation excitation at 103.5 eV (12 nm) is presented. Antimony was incorporated in the resist as a heavy metal absorption center in the form of antimonate (2.15%). The results showed the fast decomposition rate of the radiation sensitive sulfonium triflate. HR-XPS and sulfur L-NEXAFS spectra of the copolymer films revealed that after irradiation the Ar–S+–(CH3)2 sulfonium group bonded to the phenyl ring resisted the EUV excitation. Those results confirmed the polarity switching mechanism from hydrophilic sulfonium triflates to hydrophobic aromatic sulfides obtained in previous results. The inorganic component SbF6− included in the resist formulations as an EUV absorption enhancer was particularly illustrative of the photofragmentation process. F 1s and O 1s HR-XPS spectra showed that fluorine remains linked to the antimony, even after 15 min of irradiation. A change of the antimony oxidation state was also observed with an increase in irradiation time. The presence of the heavy metal may control the high energy deposited on the resist which finally led to very well resolved 20 nm isolated line patterns by EUVL. The 10 times improved sensitivity compared with previous poly-MAPDST resists studied in the past showed the potential of this class of hybrid resists for next generation semiconductor industry applications.

C-N-Codoped TiO2 Synthesis by using Peroxo Sol Gel Method for Photocatalytic Reduction of Cr(VI)

C-N-codoped TiO 2 (CNTO) photocatalysts were successfully synthesized by using “green” method peroxo sol-gel. XRD results revealed the presence of anatase phase only for all samples. The result of Scanning Electron Microscope-Electron Dispersive X-Ray (SEM-EDX) showed surface morphology of CNTO10 was spherical (44 nm) and homogeneous. Diffuse Reflectance Spectra UV-Vis (DRS UV-Vis) result showed red shift in absorbance indicating successful modification of TiO 2 by C and N. The result of photocatalytic activity in reducing Cr(VI) showed that the Cr(VI) reduction increase with the increase of irradiation time and photocatalyst’s mass. The highest Cr(VI) reduction was 90.07% for CNTO10 sample. This is due to the synergistic effects of C and N dopants that improves TiO 2 photocatalytic activity under visible light irradiation.

The effect of gamma irradiation on chemical, morphology and optical properties of polystyrene nanosphere at various exposure time

Irradiation of polymers causes structural, chemical and the optical properties changes. Polystyrene nanosphere was drop coated to substrates and the gamma irradiation was carried out in a Cesium-137 (Cs-137) source chamber at different time (1-5 hours) with constant dose of 30 kGy. Fourier transformation infrared spectroscopy (FTIR) and Raman spectroscopy were employed to characterize the chemical properties of irradiated polystyrene while Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) were used to study the surface morphological changes of the samples. The optical energy band gaps of the thin films were investigated and studied using transmittance and absorbance measurements. The results obtained revealed that as irradiation time increases the optical properties changes and polystyrene gradually undergoes crystal to carbonaceous from its amorphous state. The average particles diameter and roughness of the samples decreases with increasing irradiation time.

Nghiên cứu khả năng tạo gốc tự do trong nước bằng công nghệ plasma lạnh

This study was conducted to investigate factors that affected the free radicals concentration generated in aqueous solution by cold plasma technology. The factors such as solution flow through electrodes (from 1 to 5 L/min), voltage (from 12 kV to 16 kV), plasma irradiation time (from 10 to 60 min), organic compounds and water types were studied. In addition, the study also investigated the effects of indirect plasma and air flow rate on the free radical concentration. The results showed that the concentration of free radical decreased from 7.57×10-2 to 5.41×10-2 mM when increasing water flow rate from 1 to 5 L/min. Concentration of free radical increased from 6.89×10-2 to 7.77×10-2 mM when increasing the voltage from 12 to16 kV. Experiment data also showed that increasing irradiation time (10 to 60 min) would also increase concentration of free radical (7.52×10-2 to 8.89×10-2 mM). The highest concentration of free radical was obtained at condition of 1 L/min, 16 kV and 60 min irradiation. Furthermore, the study also found that the presence of organic compounds also strongly affected the free radical concentration.

Optical properties of polyvinyl alcohol film irradiated with Nd:YAG laser

Casting solution method has been used to prepare Polyvinyl alcohol (PVA) film. The prepared film has been irradiated with a laser beam of 6ns Q-switched Nd:YAG laser (Lotus II) operating at 10Hz and fourth harmonic wavelength, 266nm, at different irradiation times. Optical absorbance of the irradiated sample was studied in the wavelength region between 200 and 800nm. The absorption peak shift of 272nm towards the longer wavelengths region and the increase of absorption as the irradiation time increases, are related to the increase in crosslinking network. The fitting technique of the absorbance revealed an indirect allowed transition with optical energy gap of 4.22eV for the pristine PVA sample, and 2.85eV after 2h of laser irradiation. The index of refraction and the dielectric function showed dispersion in the studied wavelength range. The analysis of color parameters of PVA showed an observable change after laser irradiation.

Ultraviolet Wavelength-Dependent Optoelectronic Properties in Two-Dimensional NbSe2-WSe2 van der Waals Heterojunction-Based Field-Effect Transistors.

Atomically thin two-dimensional (2D) van der Waals (vdW) heterostructures are one of the very important research issues for stacked optoelectronic device applications. In this study, using the transferred and stacked NbSe2-WSe2 films as electrodes and a channel, we fabricated the field-effect transistor (FET) devices based on 2D-2D vdW metal-semiconductor heterojunctions (HJs) and systematically studied their ultraviolet (UV) wavelength-dependent electrical and photoresponse properties. Upon the exposure to UV light with a wavelength of 365 nm, the NbSe2-WSe2 vdW HJFET devices exhibited threshold voltage shift toward positive gate bias direction, a current increase, and a nonlinear photocurrent increase upon applying a gate bias due to the contribution of the photogenerated hole current. In contrast, for the 254 nm UV-irradiated FET devices, the drain current was decreased dramatically and the threshold voltage was negatively shifted. The time-resolved photoresponse properties showed that the device current after turning off the 254 nm UV light was completely and much more rapidly recovered compared with the case of the persistent photocurrent after turning off the 365 nm UV light. Interestingly, we found that the wettability of the WSe2 surface was changed with increasing irradiation time only after 254 nm UV irradiation. The measured wetting behavior on the WSe2 surface provided direct evidence that the experimentally observed UV-wavelength-dependent phenomena was attributed to the UV-induced dissociative adsorption of oxygen and water molecules, leading to the modulation of charge trap states on the photogenerated and intrinsic carriers in the p-type WSe2 channel. This study will help provide an understanding of the influence of environmental and electrical measurement conditions on the electrical and optical properties of 2D-2D vdW HJ devices for a variety of device applications through the stacking of 2D heterostructures.