Principles Density Functional Theory Research Articles

The effect of Pt and IrO2 interlayers on enhancing the adhesion of Ti/SnO2 interface: A first principles density functional theory study

With potential applications from gas sensors to anode electrodes, Ti/SnO2 interfacial systems play a crucial role in the detection of harmful gases and the destruction of refractory organic pollutants via electrochemical advanced oxidation processes. However, there is an issue with the stability of the Ti/SnO2 interface, in which having an interlayer of another metal or ceramic may work to improve it. First principles density functional theory calculations were performed to study how the addition of Pt and/or IrO2 interlayers affect the stability and adhesion of Ti/SnO2. The calculations showed that Ti/SnO2 interfaces are destabilized by the movement of oxygens from the surface of SnO2 into the Ti phase, causing the adhesion between the newly oxidized Ti phase to be weak with the surface Sn atoms. In contrast, both Pt and IrO2 were found to form stable interfaces with both Ti and SnO2. Further investigation found that Ti/Pt/SnO2 and Ti/IrO2/SnO2 formed stable systems that promoted adhesion between all phases. However, having both Pt and IrO2 combined as interlayers did not form a stable system. Overall, the results show that both Pt and IrO2 may serve as potential interlayer for Ti/SnO2 that promotes adhesion, but combined together they do not.

Chemical Modification of Aluminum Nitride Nanotubes (AlNNT) using-OH, C=O, R-SH functional groups: First Principle's study

Based on the First Principle's density functional theory, we have investigated the structural and electronic properties of pristine aluminum nitride nanotube (AlNNT), which is chemically modified with various biologically available functional groups such as hydroxyl (-OH), thiol (R-SH) and carbonyl (C=O). The structural stability is examined through minimum total energy difference and binding energy by repeated random perturbation. Structural analysis shows that functional groups were found to induce sp3 transition in AlNNT as reflected in the p character. Our finding shows that, (5, 5) AlNNT possesses a wide band gap semiconducting behavior with an energy bandgap of 3.27 eV. Chemical functionalization reduces the band gap of armchair AlNNT (5, 5). The chemical potential of the pristine AlNNT is found to be -3.94 eV. Chemical potential analysis shows a blue shift for the bio complex system with respect to the pristine system. The ionic character and Gibbs free energy of solvation determine the structure's solubility in aqueous media. Chemical tuning of AlNNT with functional groups induces polar covalent characteristics which makes them to be used as possible carriers for drug delivery applications.

Electronic and Optical Properties of Single-Walled Carbon Nanotube Functionalized by CH<sub>3</sub>COOH

Organic functionalization of carbon nanotubes (CNTs) plays very important role in the development of electrochemical biosensors. In this study, pristine (5,5) carbon nanotube was functionalized by Ethanoic Acid (CH3COOH) using First Principles Density Functional Theory (DFT). It was found that the encapsulation of CH3COOH into the (5,5) CNT is endothermic due to the small diameter of the tube. However, interacting it outside the sidewall of the tube gives an exothermic process indicating a stable geometry. Accordingly, additional electronic bands and peaks are observed in the electronic structures of the functionalized CNT. Further, it was shown that that the p orbitals of the oxygen atoms and carbon atoms of the acid are the main contributors of the additional peaks in the valence and conduction regions, respectively. Finally, there were observed optical transitions in the functionalized CNT caused by the hybridization of the armchair CNT. Evidently, this study provided insights on more potential applications of carbon nanotubes as biosensors.

First Principle Calculations of Magnetic Proximity Effect Induced Valley Splitting on Heterointerface WS<sub>2</sub>/CoO(111)

The first principle density functional theory calculation has been done for calculating electronic and valleytronic properties on WS2/CoO(111) heterointerface. We have performed the structural optimization of the WS2/CoO(111) heterointerface and obtained the most stable structures by evaluating the binding energy between WS2 and CoO(111) surface. Electronic and valleytronic properties can be understood by band structures, density of states, and spin texture of the crystal models. The results show that non-magnetic WS2 becomes ferromagnetic because of the interfacial effect with the CoO(111) surface. The presence of CoO(111) near the WS2 gives magnetic induction which breaks the time-reversal symmetry preserved on non-magnetic WS2. Accordingly, the largest valley degeneracy is observed at Q and Q’ point in the unoccupied state, with the valley splitting up to 186 meV. Furthermore, the out-of-plane spin texture has also been calculated and the results show that spin configuration at Q and Q’ have opposite signs (up and down, respectively) indicating that the valley couple occurs on the heterointerface. Our results suggest that WS2/CoO(111) is a promising candidate for valleytronic applications.

First principles prediction of yield strength of body centered cubic structured high entropy alloys

High entropy alloys (HEAs) with a body centered cubic (bcc) crystal structure have emerged as potential high-performance structural materials due to their high strength at room and elevated temperatures. In this study, a computational approach, based on the revised Peierls-Nabarro model and using the inputs solely calculated from the first principles density functional theory, has been developed to predict the yield strength of bcc HEAs. Examining its accuracy and reliability, the developed computational approach was applied to four different types of bcc HEAs. The yield strength was predicted to be 1034 MPa for MoNbTaW, 1489 MPa for MoNbTaV, 1356 MPa for AlCoCrFeNi, and 1740 MPa for AlCoCrFeNiZr0.3 alloy, respectively. These computational predictions are found to agree well with available experimental data. Moreover, the developed computational approach accurately quantifies the changes in the yield strength from MoNbTaW to MoNbTaV with a change of constituent W to V, and from AlCoCrFeNi to AlCoCrFeNiZr0.3 with addition of 6 at% Zr. Therefore, this first-principles based computational approach provides a way for expedite optimization on the mechanical properties of bcc HEAs across vast composition space.

Nucleation of Co and Ru Precursors on Silicon with Different Surface Terminations: Impact on Nucleation Delay.

Early transition metals ruthenium (Ru) and cobalt (Co) are of high interest as replacements for Cu in next-generation interconnects. Plasma-enhanced atomic layer deposition (PE-ALD) is used to deposit metal thin films in high-aspect-ratio structures of vias and trenches in nanoelectronic devices. At the initial stage of deposition, the surface reactions between the precursors and the starting substrate are vital to understand the nucleation of the film and optimize the deposition process by minimizing the so-called nucleation delay in which film growth is only observed after tens to hundreds of ALD cycles. The reported nucleation delay of Ru ranges from 10 ALD cycles to 500 ALD cycles, and the growth-per-cycle (GPC) varies from report to report. No systematic studies on nucleation delay of Co PE-ALD are found in the literature. In this study, we use first principles density functional theory (DFT) simulations to investigate the reactions between precursors RuCp2 and CoCp2 with Si substrates that have different surface terminations to reveal the atomic-scale reaction mechanism at the initial stages of metal nucleation. The substrates include (1) H:Si(100), (2) NHx-terminated Si(100), and (3) H:SiNx/Si(100). The ligand exchange reaction via H transfer to form CpH on H:Si(100), NHx-terminated Si(100), and H:SiNx/Si(100) surfaces is simulated and shows that pretreatment with N2/H2 plasma to yield an NHx-terminated Si surface from H:Si(100) can promote the ligand exchange reaction to eliminate the Cp ligand for CoCp2. Our DFT results show that the surface reactivity of CoCp2 is highly dependent on substrate surface terminations, which explains why the reported nucleation delay and GPC vary from report to report. This difference in reactivity at different surface terminations may be useful for selective deposition. For Ru deposition, RuCp2 is not a useful precursor, showing highly endothermic ligand elimination reactions on all studied terminations.

Incorporation of tungsten or cobalt into TaN barrier layers controls morphology of deposited copper

Progress in semiconductor devices, which has enabled the information and communications technology explosion of the 21st century, has been driven by Moore’s Law and the accompanying aggressive scaling of transistors. However, it is now acknowledged that the currently used copper interconnects are becoming a bottleneck in sub-nm scaling. Semiconductor devices require a diffusion barrier and a seed layer in the volume available to the interconnect metal. This then limits the minimum size of the interconnect and copper suffers from a preference to form 3D islands which are non-conducting rather than conducting films. Therefore there is a pressing need to either replace copper, which has its own difficulties, or to reduce the volume taken up by the diffusion barrier and liner; ideally finding a single material displaying both properties is needed. We have previously shown that incorporation of Ru into the surface layer of TaN is a strong alternative to the usual TaN/Ta or TaN/Ru stacks. In this work we study other possible metals that can be incorporated into TaN, namely Co and W, which are less expensive and critical than Ru and can potentially outperform it. Our first principles density functional theory results from static relaxations and ab initio molecular dynamics show that there are several compositions of both Co- and W-doped TaN which should promote growth of 2D copper interconnects without compromising the barrier properties of TaN. With this selection of materials it should be possible to design new experimental processes that promote downscaled copper interconnects for the next generation of electronic devices. Additionally, our work presents an improved method towards prediction of thin film morphology on a given substrate, which can be of use for a variety of materials science applications.

STRUCTURAL, ELECTRONIC AND OPTICAL PROPERTIES OF BaSnA3 (A=S or Se); A DFT study

Chalcogenide perovskites play an important role in the solar cell photovoltaic applications. A lot of researches in the present decade studied these materials, such as optical and photo-electric properties. Herein we are applied first principles density functional theory (DFT) to study the structural, electronic and optical properties of BaSnS3 and BaSnSe3 compounds. Gradient generalized approximations GGA-PBE and GGA-WC from the DFT was implemented in CASTEP (Cambridge Serial Total Energy Package) and usedin our calculations. Results indicated that both BaSnS3 and BaSnSe3 crystallize in the orthorhombic structure and have a semi-conductivity behavior with energy gap between 0,35 eV and 0,97 eV. This paper prospects more works in the perovskites based-Sn.

Charge polarization induced site preferences phenomena of ethylene hydrogenation via electronic metal support interactions

The catalytic activity of supported catalyst is closely related to the electronic metal-support interactions (EMSI). In this paper, first principles density functional theory study was conducted to understand the EMSI between Pd clusters (Pd4, Pd7, Pd10) and CeO2 substrate. When Pd was loaded on CeO2 surface, the electrons of Pd would transfer to the CeO2 substrate via Pd-O bonding interactions. Moreover, the charge redistribution on the Pd cluster leads to significant polarization of electron density. Consequently, the Pd atoms located at the metal-support interface (IF site) are positively charged, while the Pd atoms at the topmost (T site) are negatively charged. The H2 dissociation and ethylene hydrogenation at these sites were calculated with both low and high hydrogen coverage. It was found that the two types of sites should collaborate synergically to promote the hydrogenation of ethylene. In particular, the IF sites exhibit relatively high activity for H2 dissociation, while the T sites show superior catalytic efficiency for ethylene hydrogenation. Therefore, the EMSI induced charge polarization can tune the electronic structure and charge state of supported metal catalysts, which further determines the performance of different region on the catalyst.

Local hybridized states of adsorbed atomic Sn on WS2 substrate

We study the growth of Sn and the local electronic properties of Sn grown WS2 surfaces by in-situ scanning tunneling microscopy (STM), scanning tunneling spectroscopy, and first principles density functional theory calculations. Our investigations suggest substitutional doping of Sn at the S sites, with Sn atoms occupying a slightly elevated position of 80 pm on the WS2 surface. These results agree well with our STM observations on room temperature growth of atomic Sn, indicating commensurate or nearly commensurate adsorption at the top S sites with a buckling height of 40 ± 10 pm. Pristine WS2 without any S vacancies on the surface exhibits a bandgap of 1.39 eV, while ingap local density of states comprised of W d and S p orbitals are detected when S vacancies are considered. Upon adsorption of Sn atoms, we find the signature of modulated hybridized ingap electronic states of Sn p and W d orbitals with a spin orbit coupling interaction up to 38 meV at the K point of the Brillouin zone. These results throw important light on the future fabrication of Janus like SnWS vertical heterojunctions with atomic level doping accuracy, which will have enormous device applications.

First-principles calculations to investigate the dielectric and optical anisotropy in two-dimensional monolayer calcium and magnesium difluorides in the vacuum ultraviolet

Anisotropic dielectric and optical properties of two-dimensional (2D) calcium and magnesium difluorides were investigated in the vacuum ultraviolet (VUV) region of the electromagnetic spectrum (EM) using the first principles density functional theory (DFT). The anisotropy between the in-plane and out-of-plane directions shows that these materials are uniaxial, exhibiting optical and dielectric anisotropy. The optical functions of these anisotropic materials-optical absorption, photoconductivity, refractive index, reflection and extinction coefficients, and electron energy loss (EEL) spectra-are calculated in the framework of DFT. The low refractive index values and relatively small extinction coefficient make these materials alternative low-index 2D materials for the long wavelengths in the VUV region of the EM spectrum. The reflection and transmission spectra indicate the antireflective property of these materials. The calculated EEL function shows less energy loss of fast-traveling electrons in the material's medium. The maxima in the EEL spectrum are the main feature of plasma oscillations. The dissipation in the incident light radiation energy propagating through the dielectric medium is estimated with the dielectric loss tangent (tanδ). The magnesium difluoride is identified as a less dielectric loss medium than calcium difluoride in the VUV region. The present results suggest that these 2D materials are promising in low refractive index, high reflective, and antireflective coating materials in optoelectronic device applications. Also, electronic studies revealed that these are excellent materials for gate insulators in field-effect transistors based on 2D electronic materials.

Composition engineering of lead-free double perovskites towards efficient warm white light emission for health and well-being

Lead-free halide double perovskites (DPs) [A2B(I)B(III)X6] (A: monovalent cation, B(I): monovalent cation, B(II): trivalent cation, X: halide anion) has drawn tremendous attention because of their environmental-friendliness and high stability compared to traditional lead-based perovskites ABX3 (A: monovalent cation; B: Pb2+; X: halide anion). The optoelectronic properties of DPs are, however, compromised. This work provides a methodology for improving the photophysical properties of DPs and achieving high-efficiency warm white light emission. First principles density functional theory (DFT) was used in material design and the ligand-free synthesis of halide DP Cs2Ag1−xKxIn0.875Bi0.125Cl6 (x ≤ 0.60) via a facile recrystallization process was demonstrated. The compositional tuning of K+ to Ag+ resulted in an improvement in photophysical properties. Independent of K+ content, all the samples exhibit strong absorption at a wavelength of λmax ∼ 375 nm with a direct bandgap which is consistent with our DFT calculation results. Broadband photoluminescence (PL) emission profiles covering the spectra from blue to deep red (405–820 nm at λmax ∼ 629 nm) were observed. Besides, with an optimized K+ alloying content of 0.20, the optical performance of these halide DPs was impressively regulated, demonstrating improved photoluminescence quantum yield (PLQY) of host Cs2AgInCl6 crystals from ∼ 2.70 % (x = 0) to ∼15.96 % (x = 0.20), due to increased radiative recombination of self-trapped excitons. the highest report for such structures. It also shows high stability over 180 days. Furthermore, the use of blue-emitting CsPbBr3 and DP Cs2Ag0.80K0.20In0.875Bi0.125Cl6 as color conversion layers in white light-emitting diodes (LEDs) demonstrates that high-quality white light emission with the CIE color coordinate of (0.387, 0.385) and a correlated color temperature (CCT) of 3878 K, color rendering index (CRI) of 85, and luminous efficacy of radiation (LER) of 214 lm/W. Hence, the research work provides insight into realizing environmentally friendly and high-efficiency white light emission with high color quality by using earth-abundant elements and the low-cost synthesis method.

Boosting photocatalytic dehydrogenation and simultaneous selective oxidation of benzyl alcohol to benzaldehyde over flower-like MoS2 modified Zn0.5Cd0.5S solid solution

A series of MoS2/Zn0.5Cd0.5S with varying concentrations of flower-like MoS2 is prepared based on a simple and practical method with highly efficient photocatalytic dehydrogenation and simultaneous selective oxidation of benzyl alcohol (BA) to benzaldehyde (BAD). A comparative study between the composites and their two components (pristine MoS2 and Zn0.5Cd0.5S) is carried out for various structural, optical, electrical, and photocatalytic characterization. The studies reveal that the size of Zn0.5Cd0.5S nanoparticles is in the order of 3∼5 nm and is distributed uniformly over the surface of the flower-like 350 nm sized MoS2. The optical research conducted using UV–vis diffuse reflectance spectroscopy reflecting an excellent photoresponse by the MoS2/Zn0.5Cd0.5S. The photoelectrochemical tests showed that the adding of proper ratio of flower-like MoS2 (6%) results in an efficient separation and migration of photogenerated carriers for pristine Zn0.5Cd0.5S nanoparticles. The photocatalytic dehydrogenation rate of BA for the 6%-MoS2/Zn0.5Cd0.5S is the highest at 3.03 mmol g−1 h−1, meanwhile the yield of BAD and the selective conversion of BA to BAD are 49.3% and 100%, respectively. The essential stages of the photocatalytic dehydrogenation of BA are ·C caused by the photo-generated holes, according to the EPR spectra of the PBN-carbon centered radical (PBN–C). Besides, the mechanism for improved photocatalytic dehydrogenation and synergistic selective oxidation of BA to BAD with flower-like MoS2 modified Zn0.5Cd0.5S nanoparticles was systematically studied through gas chromatography-mass spectrometry analysis and first principle density functional theory calculations. The highly efficient dehydrogenation and synergistic selective oxidation of BA to BAD for MoS2/Zn0.5Cd0.5S are mainly attributed to the introduction of MoS2 nanoflowers that can provide abundant active sites, and formed MoS2/Zn0.5Cd0.5S heterostructures could further promote the separation of photogenerated electron-hole pairs. Moreover, the valance band position of the 6%-MoS2/Zn0.5Cd0.5S is more positive, so the hole at the VB position has the stronger ability to oxidize BA. This study provides a feasible method for efficiently photocatalytic dehydrogenation synergism selective oxidation of organic substrates into valuable products.

SnO2 -polypyrrole scaffolds as high capacity anodes for rechargeable lithium-ion batteries: A cooperative density functional theory and experimental investigation

Recently, porous architectures have received enormous attraction in the field of electrochemical energy storage and conversion devices, especially next-generation metal-ion batteries. In this regard, SnO2-Polypyrrole nanocomposite is synthesized by hydrothermal method and the porous scaffolds are fabricated by freeze-drying method. Structural analyses confirm the nanocomposite formation. Scanning electron microscope image shows spherical shape morphology for SnO2-Polypyrrole nanocomposite and porous structure for scaffolds. The Brunauer-Emmett-Teller measurement result validates the presence of micro and mesoporous on SnO2-PPy scaffold with a surface area of 6 m3g-1. The scaffold anode delivers the initial and first discharge capacity of 2201 mAh g−1 and 1304 mAh g−1 at 100 mA g−1 respectively. First principle density functional theory simulation is performed to calculate the Li adsorption on SnO2-Polypyrrole and pristine SnO2. The result reveals, a layer of lithium-ions being chemisorbed to the highly porous SnO2-Polypyrrole and responsible for high capacity. The porous architecture of SnO2-Polypyrrole, with high discharge capacity, turns out to be best possible anode material for lithium-ion battery.

Optimization of Chemical Bonding through Defect Formation and Ordering─The Case of Mg7Pt4Ge4.

The new phase Mg7Pt4Ge4 (≡Mg8□1Pt4Ge4; □ = vacancy) was prepared by reacting a mixture of the corresponding elements at high temperatures. According to single crystal X-ray diffraction data, it adopts a defect variant of the lighter analogue Mg2PtSi (≡Mg8Pt4Si4), reported in the Li2CuAs structure. An ordering of the Mg vacancies results in a stoichiometric phase, Mg7Pt4Ge4. However, the high content of Mg vacancies results in a violation of the 18-valence electron rule, which appears to hold for Mg2PtSi. First principle density functional theory calculations on a hypothetical, vacancy-free "Mg2PtGe" reveal potential electronic instabilities at EF in the band structure and significant occupancy of states with an antibonding character resulting from unfavorable Pt-Ge interactions. These antibonding interactions can be eliminated through introduction of Mg defects, which reduce the valence electron count, leaving the antibonding states empty. Mg itself does not participate in these interactions. Instead, the Mg contribution to the overall bonding comes from electron back-donation from the (Pt, Ge) anionic network to Mg cations. These findings may help to understand how the interplay of structural and electronic factors leads to the "hydrogen pump effect" observed in the closely related Mg3Pt, for which the electronic band structure shows a significant amount of unoccupied bonding states, indicating an electron deficient system.

Formation and transformation of metastable LPSO building blocks clusters in Mg-Gd-Y-Zn-Zr alloys by spinodal decomposition and heterogeneous nucleation

To study the formation and transformation mechanism of long-period stacked ordered (LPSO) structures, a systematic atomic scale analysis was conducted for the structural evolution of long-period stacked ordered (LPSO) structures in the Mg-Gd-Y-Zn-Zr alloy annealed at 300 °C∼500 °C. Various types of metastable LPSO building block clusters were found to exist in alloy structures at different temperatures, which precipitate during the solidification and homogenization process. The stability of Zn/Y clusters is explained by the first principles of density functional theory. The LPSO structure is distinguished by the arrangement of its different Zn/Y enriched LPSO structural units, which comprises local fcc stacking sequences upon a tightly packed plane. The presence of solute atoms causes local lattice distortion, thereby enabling the rearrangement of Mg atoms in the different configurations in the local lattice, and local HCP-FCC transitions occur between Mg and Zn atoms occupying the nearest neighbor positions. This finding indicates that LPSO structures can generate necessary Schockley partial dislocations on specific slip surfaces, providing direct evidence of the transition from 18R to 14H. Growth of the LPSO, devoid of any defects and non-coherent interfaces, was observed separately from other precipitated phases. As a result, the precipitation sequence of LPSO in the solidification stage was as follows: Zn/Ycluster+Mg layers→various metastable LPSO building block clusters→18R/24R LPSO; whereas the precipitation sequence of LPSO during homogenization treatment was observed to be as follows: 18R LPSO→various metastable LPSO building block clusters→14H LPSO. Of these, 14H LPSO was found to be the most thermodynamically stable structure.

Investigation of oxygen-vacancy complexes in diamond by means of ab initio calculations

Point defects in diamond may act as quantum bits. Recently, oxygen-vacancy related defects have been proposed to the origin of the so-called ST1 color center in diamond that can realize a long-living solid-state quantum memory. Motivated by this proposal we systematically investigate oxygen-vacancy complexes in diamond by means of first principles density functional theory calculations. We find that all the considered oxygen-vacancy defects have a high-spin ground state in their neutral charge state, which disregards them as an origin for the ST1 color center. We identify a high-spin metastable oxygen-vacancy complex and characterize their magneto-optical properties for identification in future experiments.

Site preference-based luminescence studies in Eu doped calcium magnesium silicate phosphor: A combined experimental and DFT approach

The luminescence of Eu3+ doped calcium magnesium silicate (CMS: Eu3+) phosphor is studied experimentally by optimizing the single diopside structure in the solid state reaction method and theoretically verified using the first principle density functional theory (DFT) calculation. Field Emission Scanning Electron Microscopy (FESEM) analysis shows a transition from agglomerated morphology to particle formation at 900 °C. A combination of diopside and akermanite structures are formed at 800 °C, whereas a single diopside phase is optimized from 900 °C onwards. DFT calculation has been carried out to identify the lattice parameters of the diopside structure of calcium magnesium silicate (CMS) and Eu3+ doped CMS by replacing Eu3+ in the lattice site of Ca and Mg to identify the stable structure. The lattice expansion is minimal when Eu3+ replaces the Ca2+ position. Lattice parameters of the diopside phase calculated using Rietveld refinement of the XRD pattern are in agreement with DFT findings. FTIR analysis confirms a redshift corresponding to Ca–O and a blueshift corresponding to Mg–O stretching wavenumbers due to the structural modification of CMS due to Eu addition in the lattice. The stability and site preference of Eu3+ ion in CMS is determined from the mixing energy (ΔHm) of Eu doped in the Ca site and Mg site of CMS separately via DFT calculation and confirm that the mixing energy is minimum when Eu3+ replaces Ca lattice position. UV–visible, DRS spectroscopy confirms a bandgap of ∼6 eV for CMS and CMS: Eu3+. Luminescence studies confirm constant peak splitting in the 5D0→7FJ (J = 1,2,4) transitions due to the crystal field effect caused due to the incorporation of Eu3+ in the diopside structure with dodecahedral coordination. Eight coordinated site preference of Eu3+ in diopside is confirmed via DFT calculations. Modification in asymmetry ratio (R-ratio) with temperature also corroborates with modification in the intensity of photoluminescence spectra dictated by the reduction in inversion symmetry. CIE color chromaticity analysis shows a coordinate of (0.61, 0.39) in the red region of visible spectra with a color purity of 83.49% irrespective of annealing temperature.

Determination of the absolute Raman cross‐sections of α‐S8 film at ultralow frequencies pumped by 488 and 785 nm lasers

AbstractAbsolute Raman cross‐sections are widely used for quantitative analyses of condensed phases. However, solid thin films showing ultralow frequency (ULF) Raman characteristics at <100 cm−1 have not been studied deeply. Herein, we demonstrate an ULF Raman spectrometer equipped with 488 and 785 nm pumped lasers and determine the absolute Raman cross‐sections of 34‐μm‐thick α‐S8 film, which presents Raman peaks at approximately 27, 50, 83, 154, and 220 cm−1. These experimentally measured ultralow Raman frequencies and Raman cross‐sections were also confirmed via first principles density functional theory. Thus, our Raman cross‐section studies can be utilized as a quantitative standard for thin films showing ULF Raman characteristics.

Strain driven anomalous anisotropic enhancement in the thermoelectric performance of monolayer MoS2

First principles density functional theory based calculations have been performed to investigate the strain and temperature induced tunability of the thermoelectric properties of monolayer (ML) MoS2. Modifications in the electronic and phononic transport properties, under two anisotropic uniaxial strains along the armchair (AC) and zigzag (ZZ) directions, have been explored in detail. Considering the intrinsic carrier-phonon scattering, we found that the charge carrier mobility (μ) and relaxation time (τ) increase remarkably for strains along the ZZ direction. Concomitantly, strain along the ZZ direction significantly reduces the lattice thermal conductivity (κL) of ML-MoS2. The combined effect of shortened phonon relaxation time and group velocity, and the reduced Debye temperature is found to be the driving force behind the lowering of κL. The large reduction in κL and increase in τ, associated with the strains along the ZZ direction, act in unison to result in enhanced efficiency and hence, improved thermoelectric performance. Nearly 150% enhancement in the thermoelectric efficiency can be achieved with the optimal doping concentration. We, therefore, highlight the significance of in-plane tensile strains, in general, and strains along the ZZ direction, in particular, in improving the thermoelectric performance of ML-MoS2.