Few‐layered films of WS2, synthesized by chemical vapor deposition on quartz, are successfully used as light sensors. The film samples are structurally characterized by Raman spectroscopy, atomic force microscopy, scanning electron microscopy, and high‐resolution transmission electron microscopy. The produced samples consist of few layered sheets possessing up to 10 layers. UV–visible absorbance spectra reveals absorption peaks at energies of 1.95 and 2.33 eV, consistent with the A and B excitons characteristic of WS2. Current–voltage (I–V) and photoresponse measurements carried out at room temperature are performed by connecting the WS2 layered material with Au/Ti contacts. The photocurrent measurements are carried out using five different laser lines ranging between 457 and 647 nm. The results indicate that the electrical response strongly depends on the photon energy from the excitation lasers. In addition, it is found that the photocurrent varies non‐linearly with the incident power, and the generated photocurrent in the WS2 samples varies as a squared root of the incident power. The excellent response of few‐layered WS2 to detect different photon wavelengths, over a wide range of intensities, makes it a strong candidate for constructing novel optoelectronic devices.
We report on the electrochemical charge storage behavior of few-layered flakes of molybdenum disulfide (MoS2) obtained by liquid phase exfoliation of bulk MoS2 powder in 1-dodecyl-2-pyrrolidinone. The specific capacitances of the exfoliated flakes obtained using a 6 M KOH aqueous solution as an electrolyte were found to be an order of magnitude higher than those of bulk MoS2 (∼0.5 and ∼2 mF cm(-2) for bulk and exfoliated MoS2 electrodes, respectively). The exfoliated MoS2 flakes also showed significant charge storage in different electrolytes, such as organic solvents [1 M tetraethylammonium tetrafluoroborate in propylene carbonate (Et4NBF4 in PC)] and ionic liquids [1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6)]. The values of specific capacitances obtained using Et4NBF4 in PC and BMIM-PF6 were ∼2.25 and ∼2.4 mF cm(-2), respectively. An analysis of electrochemical impedance spectroscopy using an equivalent circuit modeling was performed to understand the charge storage mechanism of these exfoliated MoS2 flakes using different electrolytes. Our findings indicate that liquid phase exfoliation methods can be used to produce large quantities of electrochemically active, two-dimensional layers of MoS2 and can act as an ideal material in several applications related to electrochemistry.
2-Dimensional structures with swift optical response have several technological advantages, for example they could be used as components of ultrafast light modulators, photo-detectors, and optical switches. Here we report on the fast photo switching behavior of thin films of liquid phase exfoliated MoS2, when excited with a continuous laser of λ = 658 nm (E = 1.88 eV), over a broad range of laser power. Transient photo-conductivity measurements, using an optical pump and THz probe (OPTP), reveal that photo carrier decay follows a bi-exponential time dependence, with decay times of the order of picoseconds, indicating that the photo carrier recombination occurs via trap states. The nature of variation of photocurrent with temperature confirms that the trap states are continuously distributed within the mobility gap in these thin film of MoS2, and play a vital role in influencing the overall photo response. Our findings provide a fundamental understanding of the photo-physics associated with optically active 2D materials and are crucial for developing advanced optoelectronic devices.
I. Y-function Method for estimating contact resistance. Y-function method, as first demonstrated by Ghibaudo 1 and modified by Chang et.al, 2 to include gate dependent Schottky barrier, was used to evaluate low-field mobility (μ0) and estimate effective contact resistance (RC) in strong inversion region (Vg >> Vd). Detailed derivation of Yfunction method can be found in ref 2. For low-field bias condition (Vg >> 0.5Vd), drain current can be writes as Equation S.I.1, (S.I.1) where all symbols carry their usual meaning and θ is effective attenuation factor, express as θ = θ0 + μ0.RC.Cox.W/L and θ0 is first-order mobility attenuation coefficient. In general case, Y-function, which is defined as Id/√gm (gm is transconductance, ∂Id/∂Vg) is given by Equation S.I.2,
Continuous and real-time detection of protein biomarker using a microfluidic graphene-based transistor functionalized with thrombin-binding aptamers.
We report on the performance of supercapacitor devices fabricated using 1-pyrenecarboxylic acid (PCA)-functionalized graphene electrodes. The specific capacitances obtained (using 6 M KOH aqueous solution as an electrolyte) was found to be an order of magnitude higher than nonfunctionalized graphene electrodes (∼30 and ∼200 F/g for pure graphene and PCA-functionalized graphene electrodes, respectively). The analysis of electrochemical impedance spectroscopy using equivalent circuit modeling as well as electrolyte wettability measurements shows a substantial increase in the electrical double layer capacitance due to the PCA functionalization. These findings indicate that PCA functionalization of graphene can significantly enhance the capacitive storage ability of graphene-based electrodes and can act as superb electrode materials in electrical energy applications.
Adsorption of gas molecules on the surface of atomically layered two-dimensional (2D) materials, including graphene and transition metal dichalcogenides, can significantly affect their electrical and optical properties. Therefore, a microscopic and quantitative understanding of the mechanism and dynamics of molecular adsorption and desorption has to be achieved in order to advance device applications based on these materials. However, recent theoretical calculations have yielded contradictory results, particularly on the magnitude of the adsorption energy. Here, we have experimentally determined the adsorption energy of oxygen molecules on graphene and 2D tungsten disulfide using temperature-programmed terahertz (THz) emission microscopy (TPTEM). The temperature dependence of THz emission from InP surfaces covered with 2D materials reflects the change in oxygen concentration due to thermal desorption, which we used to estimate the adsorption energy of oxygen molecules on graphene (~0.15 eV) and tungsten disulphide (~0.24 eV). Furthermore, we used TPTEM to visualize relative changes in the spatial distribution of oxygen molecules on monolayer graphene during adsorption and desorption. Our results provide much insight into the mechanism of molecular adsorption on the surface of 2D materials, while introducing TPTEM as a novel and powerful tool for molecular surface science.The successful isolation of monolayer graphene in 2004 and its remarkable properties found subsequently have paved the way for a new research field of two-dimensional (2D) atomic layer materials [1][2][3][4] . Many other 2D materials have since been discovered with a wide range of characteristics, from metallic to semiconducting to insulating, opening up exciting new opportunities for the development of devices based on monolayers, bilayers, and heterostructures of 2D materials [5][6][7][8] . However, since these materials typically consist of one or a few atomic layers, their properties are extremely susceptible to perturbations from their environment. Exposure to gases, for example, has been shown to drastically affect their electrical and optical properties [9][10][11][12][13][14][15] , which means that in order to realize 2D-materials-based devices, it is crucial to understand and control the influence of gas adsorption and desorption dynamics on their properties.Of the possible gas adsorbates/contaminants, oxygen (O 2 ) is one of the most important because not only it significantly alters the properties through doping, it is also the second most abundant gas in the atmosphere and is therefore highly likely to affect the performance of devices in practical applications. Though theoretical simulations proved to be useful in understanding the interaction of O 2 molecules and/or O atoms with 2D materials, conflicting results for the adsorption energies were obtained due to the inability of the approximation functionals used to properly describe the dispersion forces [16][17][18][19][20][21][22][23] . Knowing the correct value of the adsorption ...
Here, we report on the ac conductivity [σ (ω); 10 mHz < ω < 0.1 MHz] measurements performed on atomically thin, two-dimensional layers of MoS 2 grown by chemical vapor deposition (CVD). σ (ω) is observed to display a "universal" power law, i.e., σ (ω) ∼ ω s measured within a broad range of temperatures, 10 K < T < 340 K. The temperature dependence of ''s" indicates that the dominant ac transport conduction mechanism in CVD-grown MoS 2 is due to electron hopping through a quantum mechanical tunneling process. The ac conductivity also displays scaling behavior, which leads to the collapse of the ac conductivity curves obtained at various temperatures into a single master curve. These findings establish a basis for our understanding of the transport mechanism in atomically thin, CVD-grown MoS 2 layers.
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