Structural and optical behaviors of 2D-layered molybdenum disulfide thin film:Experimental and ab-initio insights

The two-dimensional (2D) layered molybdenum disulfide (MoS2) material represents a nominee potent for optoelectronic devices application. In this research work, the experimental characterizations of 2D- MoS2 thin films are reported in terms of various microscopic and spectroscopic techniques. The synthesized MoS2 thin films are grown by employing the pulsed laser deposition (PLD) procedure on SiO2/Si substrates. In order to monitor the deposition rates of ablated films, the buffer argon-gas pressures are varied during the pulsed laser deposition at substrate temperature of 700 °C. The field emission scanning electron microscopy and atomic force microscopy analyzes revealed a change in the surface morphology of MoS2 films when the buffer Ar-gas pressure is varied between 0 and 100 mTorr. For all samples, a 2H-phase is revealed from X-ray diffraction patterns, indicating a reflection (2θ) around 14.85°. By varying the deposition pressure of laser-ablated MoS2 films, the X-ray photoelectron spectroscopy divulged the chemical compositional elements and valence states of Mo and S on the surface of MS2 films with low density of defects. Analysis of the photoluminescence spectroscopy illustrated emission bands spanning from the visible (Vis) to near-infrared (NIR) regimes in the deposition pressures range ~ 0–100 mTorr. This is mainly owing to the change in the recombination of electron–hole pairs and charge transfer between the deposited MoS2 films and SiO2 substrate surface under various buffer gas pressures. Additionally, first-principles electronic structure calculations are performed to qualitatively examine the effect of native point-defect species (sulfur-monovacancy and sulfur-divacancy defects) on the electronic structure and optical properties of 2D- MoS2 sheets. It is unveiled that the variation of compositional sulfur-vacancy defect in MoS2 monolayer creates an in–gap defect levels above the valence states, leading to an acceptor character. Importantly, the enhancement in the optical absorption spectra divulged a shift in the optical gap from Vis-NIR window with the increase of sulfur vacancy contents in MoS2 single-layer. The identification of intrinsic point defects may be beneficial for photovoltaic energy conversion at higher wavelengths by designing next generation 2D-semiconductors, which could be of vital significance for growing 2D layers and multilayers into practical technologies

Synthesis of nanocomposite films based on conjugated oligomer-2D layered MoS2 as potential candidate for optoelectronic devices

In this investigation, we have analyzed the structural, electrical, and optical behaviors of pure and composite thin films which are obtained from 2D monolayer molybdenum disulfide (MoS2) flakes, conjugated oligomer (CO) 1,4-Bis(9-ethyl-3-carbazo-vinylene)-9,9-dihexyl-fluorene (BECV-DHF), and by combining CO (BECV-DHF) with MoS2 in forms of CO/MoS2 composites. All the samples are coated on SiO2/Si substrates using the spin coating procedure where a spin-coating solution has been obtained by dispersing CO and MoS2 in ethanol or toluene. The structural morphology of MoS2 films and CO/MoS2 films of various thicknesses are analyzed using field emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM), and profilometer. These experimental results confirm the formation of MoS2 layer composite with oligomer nanocrystals. The optical properties of MoS2, CO, and CO/MoS2 films showed that the increased film thickness shifted the spectral peaks towards near infrared (NIR) and ultraviolet?visible (UV) regions of the electromagnetic spectrum. Moreover, devices such as solar cells, flexible memory cell and MOSFET were designed. The I-V characteristics of these devices show that CO/MoS2 composite films could serve as potential candidates for organic-inorganic nano-electronic device applications. ? 2021 The Author(s). Published by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)

Anisotropic permittivity of ultra-thin crystalline Au films: Impacts on the plasmonic response of metasurfaces

It has been determined by density functional theory (DFT) simulations that the extracted permittivities of ultra-thin crystalline gold (Au) films exhibit large anisotropies which are not predicted by classical models or previous experimental determinations of the dielectric function. The optical scattering characteristics of a periodic array of Au discs are simulated with the DFT extracted permittivity and contrasted against those obtained with several commonly used Au permittivity models. It is demonstrated that the DFT-based transmittance spectra for these plasmonic metasurfaces lead to significantly redshifted results when compared to those predicted by standard Drude and Johnson-Christy permittivity models. © 2013 AIP Publishing LLC

Size-dependent permittivity and intrinsic optical anisotropy of nanometric gold thin films: A density functional theory study

Physical properties of materials are known to be different from the bulk at the nanometer scale. In this context, the dependence of optical properties of nanometric gold thin films with respect to film thickness is studied using density functional theory (DFT). We find that the in-plane plasma frequency of the gold thin film decreases with decreasing thickness and that the optical permittivity tensor is highly anisotropic as well as thickness dependent. Quantitative knowledge of planar metal film permittivity's thickness dependence can improve the accuracy and reliability of the designs of plasmonic devices and electromagnetic metamaterials. The strong anisotropy observed may become an alternative method of realizing indefinite media. © 2013 Optical Society of America

Exploring thermoelectric materials for renewable energy applications: The case of highly mismatched alloys based on AlBi1-xSbx and InBi1-xSbx

The high throughput thermoelectric devices are considered promising futuristic energy source to control global warming and realize the dream of green energy and sustainable environment. The ability of the highly mismatched alloys (HMAs), to show the intriguing impact on the physical properties with controlled modifications, has extended their promise to thermoelectric applications. Here, we examine comprehensively the potential of the two prototypical HMAs such as AlBi1-xSbx and InBi1-xSbx for thermoelectric applications within density functional theory together with the Boltzmann transport theory. For comprehensive understanding, alloying of these materials has been performed over the entire composition range. From our calculations, we found, the replacement of Sb with Bi leads to a significant evolution in the energy band-gap and effective masses of the charge carriers that consequently deliver enhancement in thermoelectric response. Improvement of magnitude 1.25 eV and 0.986 eV has been respectively recorded in band-gaps of AlBi1-xSbx and InBi1-xSbx for the across composition alloying. Similarly, by the electronic-structure engineering of HMAs, thermoelectric properties such as, the Seebeck coefficients over Fermi-level were found to be improved from 82.90 µV/K to 107.52 µV/K for AlBi1-xSbx and 60.32 µV/K to 92.73 µV/K for InBi1-xSbx. As a result, the thermoelectric figure of merit (ZT) and power factor show considerable enhancement as a function of alloying composition for both alloys at room temperature. However, at a higher temperature, the thermal conductivity of these materials experience an exponential increase, results in lower ZT values. Overall, the observed evolution in the electronic structure and thermoelectric response for replacing Sb over Bi is significant in AlBi1-xSbx as compared to InBi1-xSbx. Hence, with the capability of significant and controlled evolution in electronic-structure and subsequent thermoelectric properties, HMAs particularly AlBi1-xSbx are believed potential candidates for thermoelectric applications

Vanadium Carbide (V4C3) MXene as an Efficient Anode for Li-Ion and Na-Ion Batteries

Li-ion batteries (LIBs) and Na-ion batteries (SIBs) are deemed green and efficient electrochemical energy storage and generation devices; meanwhile, acquiring a competent anode remains a serious challenge. Herein, the density-functional theory (DFT) was employed to investigate the performance of V4C3 MXene as an anode for LIBs and SIBs. The results predict the outstanding electrical conductivity when Li/Na is loaded on V4C3. Both Li2xV4C3 and Na2xV4C3 (x = 0.125, 0.5, 1, 1.5, and 2) showed expected low-average open-circuit voltages of 0.38 V and 0.14 V, respectively, along with a good Li/Na storage capacity of (223 mAhg?1) and a good cycling performance. Furthermore, there was a low diffusion barrier of 0.048 eV for Li0.0625V4C3 and 0.023 eV for Na0.0625V4C3, implying the prompt intercalation/extraction of Li/Na. Based on the findings of the current study, V4C3-based materials may be utilized as an anode for Li/Na-ion batteries in future applications. 2022 by the authors.This work was financially funded by the authors express their gratitude for the support of the Researchers Supporting Project Number (RSP-2021/267) King Saud University, Riyadh, Saudi Arabia. This work is also supported by Scientific Research Fund of Hunan Provincial Education Department (No. 21B0637).Scopu

Selectivity control in Pt-catalyzed cinnamaldehyde hydrogenation

Chemoselectivity is a cornerstone of catalysis, permitting the targeted modification of specific functional groups within complex starting materials. Here we elucidate key structural and electronic factors controlling the liquid phase hydrogenation of cinnamaldehyde and related benzylic aldehydes over Pt nanoparticles. Mechanistic insight from kinetic mapping reveals cinnamaldehyde hydrogenation is structure-insensitive over metallic platinum, proceeding with a common Turnover Frequency independent of precursor, particle size or support architecture. In contrast, selectivity to the desired cinnamyl alcohol product is highly structure sensitive, with large nanoparticles and high hydrogen pressures favoring C=O over C=C hydrogenation, attributed to molecular surface crowding and suppression of sterically-demanding adsorption modes. In situ vibrational spectroscopies highlight the role of support polarity in enhancing C=O hydrogenation (through cinnamaldehyde reorientation), a general phenomenon extending to alkyl-substituted benzaldehydes. Tuning nanoparticle size and support polarity affords a flexible means to control the chemoselective hydrogenation of aromatic aldehydes

A first-principles study of electronic and magnetic properties of a quasi-one-dimensional organic ferromagnet

The magnetic properties of a trans-polyacetylene with either one or two side radicals containing unpaired electrons, have been studied within the density functional theory using the generalized gradient approximation. Results show that a $\pi$-electron spin-polarization cloud appears around the unpaired electrons with alternation of the sign and amplitude of the spin moment extending over the main chain. Furthermore we found that in order to obtain ferromagnetic order in this kind of material, the number of carbon atoms between the two carbon atoms with which the free radicals are connected, should be odd. Additionally, the system has the most stable ferromagnetic state, the strongest ferromagnetism and the highest Curie temperature when the number of the carbon atoms between the two carbon atoms with which the two free radicals are connected, is three. It is shown that dimerization would stabilize the high-spin ground state and hence enforce the ferromagnetism of the quasi-one-dimensional organic ferromagnet. It is also found that dimerization has almost no effect on the electric properties of the quasi-one-dimensional organic metallic-ferromagnet, and therefore, Peierls metal-insulator phase transition would not occur in this system

New plasmonic materials in visible spectrum through electrical charging

Due to their negative permittivity, plasmonic materials have found increasing number of applications in advanced photonic devices and metamaterials, ranging from visible wavelength through microwave spectrum. In terms of intrinsic loss and permittivity dispersion, however, limitations on available plasmonic materials remain a serious bottleneck preventing practical applications of a few novel nano-photonic device and metamaterial concepts in visible and nearinfrared spectra. To overcome this obstacle, efforts have been made and reported in literature to engineer new plasmonic materials exploring metal alloys, superconductors, graphene, and heavily doped oxide semiconductors. Though promising progress in heavily doped oxide semiconductors was shown in the near-infrared spectrum, there is still no clear path to engineer new plasmonic materials in the visible spectrum that can outperform existing choices noble metals, e.g. gold and silver, due to extremely high free electron density required for high frequency plasma response. This study demonstrates a path to engineer new plasmonic materials in the visible spectrum by significantly altering the electronic properties in existing noble metals through high density charging/discharging and its associated strong local bias effects. A density functional theory model revealed that the optical properties of thin gold films (up to 7 nm thick) can be altered significantly in the visible, in terms of both plasma frequency (up to 12%) and optical permittivity (more than 50%). These corresponding effects were observed in our experiments on surface plasmon resonance of a gold film electrically charged via a high density double layer capacitor induced by a chemically non-reacting electrolyte. © 2013 SPIE