A1g Vibration Research Articles

Quantitative Defect Analysis in CVD‐Grown Monolayer MoS2 via In‐Plane Raman Vibration

ABSTRACTThe synthesis of two‐dimensional transition metal dichalcogenide (2D‐TMD) materials gives rise to inherent defects, specifically chalcogen vacancies, due to thermodynamic equilibrium. Techniques such as chemical vapor deposition (CVD), metal‐organic chemical vapor deposition (MOCVD), atomic layer deposition (ALD), flux growth method, and mechanical exfoliation produce large‐scale, uniform 2D TMD films, either in bulk or monolayers. However, defects on the film surface impact its quality, and it is necessary to measure defect density. The phonon confinement model indicates that the first‐order Raman band frequency shift depends on defect density. Monolayer Molybdenum disulfide (MoS2) exhibits three phonon dispersions at the Brillouin zone edge (M point): out‐of‐plane optical phonon vibration (ZO), in‐plane longitudinal optical phonon vibration (LO), and in‐plane transverse optical phonon vibration (TO). The LO and ZO modes overlap with Raman in‐plane vibration (𝐸12g) and Raman out‐of‐plane vibration (𝐴1g), respectively, causing peak broadening. In the presence of defects, the Raman 𝐸12g vibration energy decreases due to a reduced restoring force constant. The Raman 𝐴1g vibration trend is random, influenced by both restoring force constant and mass. The study introduces a quantitative defect measurement technique for CVD‐grown monolayer MoS2 using Raman 𝐸12g mode, employing sequential data processing algorithms to reveal defect density on the film surface.

Effect of 10 MeV electron irradiation on structural and magnetic properties of Ti- and Al- substituted strontium hexaferrite SrFe11.3Ti0.4Al0.3O19

The irradiation stability of Ti- and Al-substituted SrFe11.3Ti0.4Al0.3O19 strontium hexaferrite to 10 MeV electron irradiation with the fluences of 1014 and 2·1017 cm−2 was investigated using various characterization techniques such as scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectrum microanalysis, atomic force microscopy, Raman spectroscopy, vibrating sample magnetometry and magnetic force microscopy. Notable changes in lattice vibrations, magnetic domain microstructure and magnetization behavior were obtained after the electron irradiation with the fluence of 2·1017 cm−2. Phonon hardening observed by Raman spectroscopy confirms the increasing disorder in crystalline structure and the presence of spin-phonon coupling in the samples after electron irradiation with the fluence 2·1017 cm−2. It was found that the strongest spin-phonon interactions correspond to A1g vibration modes at trigonal bipyramidal 2b, 2a octahedral, 4f1 tetrahedral sites and E1g mode at 4f2 octahedral site in electron-irradiated SrFe11.3Ti0.4Al0.3O19 hexaferrite, while the most radiation-resistant phonon mode is A1g vibration of the Fe–O bonds at the 4f2 octahedral site. No significant change was observed in magnetic saturation magnitude after electron irradiation. The increase in coercivity and significant decrease of magnitude of the dM/dH derivative of magnetization curves with increasing radiation fluence is attributed to the increase in uniaxial magnetic-crystalline anisotropy. It is found that the fractal dimension of hierarchical magnetic domain structure in SrFe11.3Ti0.4Al0.3O19 continuously decreases with increasing fluence of 10 MeV electron irradiation, which can be caused by the distortion in uniaxial magnetocrystalline anisotropy and irradiation-induced defects.

Anharmonic Phonon Vibrations of Ag and Cu for Poor Lattice Thermal Conductivity in Superionic AgCrSe2 and CuCrSe2

Anharmonic Phonon Vibrations of Ag and Cu for Poor Lattice Thermal Conductivity in Superionic AgCrSe₂ and CuCrSe₂

Enhancing photodetector performance of MoS2 thin films by nitrogen ion irradiation

In this work, MoS2 thin films were grown on a Si substrate using RF sputtering, and subsequently subjected to nitrogen ion irradiation with varying ion fluence. The Raman spectroscopy observed the oxygen doping into MoS2 lattice and induced shift in E2g and A1g vibration modes, along with a decrease in the interdefect distance (LD) after ion irradiation. The evolution of oxygen into MoS2 sample after ion irradiation was confirmed by XPS. The incorporation of oxygen in the MoS2 is due to the bond breaking of the surface layers of the material due to ion irradiation, which creates dangling bonds. These bonds in the MoS2 are more reactive with the oxygen and lead to the formation of MoOx species on the surface of the material. The Field Emission Scanning Electron Microscopy (FESEM) provides the morphological study of post-irradiation samples. As ion fluence increases, the optical bandgap decreases from 1.46 eV to 1.33 eV. Using current-voltage (I–V) characteristics, the electrical properties of the irradiated films were characterized, along with photodetector measurements. This combined analysis provided a comprehensive understanding of the film's electrical behaviour, shedding light on their post-irradiation performance. Our results demonstrate that due to oxygen doped into MoS2/Si thin film significantly affects the barrier height, ideality factor, and carrier concentration in I–V characteristics. Taking silicon-based and n-type MoS2 heterojunction photodetectors, its photoresponsivity can reach ∼14.7 mA/W at 7.19 × 1016 ion-cm−2 ion fluence. Our findings provide insights into the tunability of the electrical properties of MoS2/Si thin films through ion irradiation, which has implications for the development of novel electronic and optoelectronic devices based on MoS2/Si thin film.

Dynamic strain coupling driven by structural phase transition in mixed-dimensional 2H-MoS2/VO2 van der Waals heterointerfaces

Mixed-dimensional van der Waals (vdW) heterostructures, integrated two-dimensional (2D) atomic crystals with three-dimensional (3D) functional materials, offer a powerful means to manipulating physical properties and generating unprecedented functionalities. Understanding interfacial couplings at those hetero (homo)-interfaces is indispensable for exploring new optical and electronic devices. Herein, we investigated dynamically phase-transition-driven strain coupling across a vdW heterointerface through integrating 2D layered 2H-MoS2 nanoflakes onto 3D phase-change VO2 epitaxial thin films. The Raman peak positions of the in-plane and out-of-plane vibration modes E2g1 and A1g from the 2H-MoS2 nanoflakes show a phonon softening and reversible hysteresis loop as a function of temperature in this mixed-dimensional vdW 2H-MoS2/(1¯11)-VO2/(11¯02)-Al2O3 heterostructure, originating from the co-action of temperature-dependent anharmonicity in 2H-MoS2 and reversible structural phase transition (SPT)-induced in-plane tensile strain from the VO2 thin film. Accordingly, the integrated Raman scattering intensity of these two feature peaks of the 2H-MoS2 nanoflakes increased (decreased) as the temperature increased (decreased), exhibiting a hysteresis loop in the SPT and metal–insulator transition region of VO2. Additionally, the peak integrated intensity enhancement ratio of the E2g1 and A1g vibration modes was approximately 2.3 and 2.8, respectively. These results indicate that the dynamically SPT-driven in-plane tensile strain from the bottom VO2 layer interfacially couples with the adjacent 2H-MoS2 nanoflakes and results in a reduction in the electronic transition energy, leading to an enhancement in the Raman scattering intensity of 2H-MoS2. Our work holds promise for dynamic strain control of lattice dynamics and electron–phonon interaction of 2D materials for functional electronic and photoelectronic devices.

High carrier mobility in organic cations intercalated multilayer MoS2

Two-dimensional semiconductors, such as MoS2, have demonstrated great potential applications in post-Moore electronic and optoelectronic devices, and organic cations intercalation has been widely utilized to modulate their physical properties. However, the correlation between the conductivity, carrier mobility, carrier density, and structure of organic cations intercalated MoS2 is still unclear. In this Letter, we systematically investigated the structural and electrical transport properties of pristine MoS2 and MoS2 intercalated with various organic cations such as tetradecyltrimethyl-ammonium, tetraheptyl-ammonium, and cetyltrimethyl-ammonium. Semimetal bismuth (Bi) was used as electrodes to make Ohmic contact with MoS2, and four-probe measurements were employed to obtain the intrinsic conductivity of MoS2. The intercalated organic cations greatly expand interlayer spacing and strongly dope MoS2 up to an electron concentration of 6.1 × 1013 cm−2 depending on the size and intercalation amount of organic cations. The severe electron doping constrains the out-of-plane A1g vibration mode and screens the Coulomb scattering, such that the intercalated MoS2 has enhanced Hall mobility of >50 cm2 V−1 s−1 at room temperature and even >1700 cm2 V−1 s−1 at 5 K. The intercalated MoS2 responds much faster than pristine MoS2 when functioning as a phototransistor. Our work provides insight for understanding the electrical transport properties of MoS2 and designing more efficient electronic and optoelectronic devices.

Synthesis and characterization of hybrid nanocatalyst (AgNPs-Fe2O3) for catalytic remediation of hazardous 2,4-dinitrophenol

Hybridized nanocatalysts (AgNPs-Fe2O3) were prepared with silver nitrate and ferric chloride in the presence of sodium borohydride. The synthesized nanocatalyst was calcined, milled, and sieved for further applications. The nanocatalyst was characterized using Fourier transform infrared spectroscopy (FTIR) for the identification of functional moieties involved in the synthesis of the nanocatalyst. The vibrations of Fe and Ag were found at 566, 790 and 892 cm-1. The crystalline nature of the nanocatalyst was determined using X-ray diffraction (XRD) studies. The diffraction peaks for Ag and Fe can be seen at 2θ values of 30.42, 44.36, 62.45, 42.24 and 53.25. The morphology and elemental analysis of nanocatalysts were studied using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDAX). The particles are found to be spherical in shape. The signal for Ag and Fe can be found at 3.0 keV and 0.7, 6.3, 7.0 keV, respectively. The synthesized nanocatalyst was employed in the remediation of 2,4-dinitrophenol. The AgNPs-Fe2O3 nanocatalyst completely reduced (97%) the 2,4-dinitrophenol to 2,4-diaminophenol within 24 h.
 KEY WORDS: Nanocatalyst, Chemical precipitation, Impregnation methodology, Milling, Sieving, Remediation
 Bull. Chem. Soc. Ethiop. 2024, 38(1), 113-121. DOI: https://dx.doi.org/10.4314/bcse.v38i1.9

Chemical assist rapid sonochemical synthesis of Cr doped SnO2 nanoparticles for the hydrogen gas sensing application

An effective chemical assist Sonochemical method was used to synthesize facile and homogenous undoped and Cr (3, and 6 at. %) doped SnO2 nanoparticles within aqueous solution. Different characterization methods, including X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-ray (EDX), UV–vis spectroscopy Fourier Transform Infrared Spectroscopy (FTIR), Raman Spectroscopy and gas sensing were used to examine the structural, optical and chemical behavior of the synthesized SnO2 nanoparticles. The XRD results showed that the synthesized SnO2 nanoparticles exhibit high crystallinity and rutile phase. The synthesized SnO2 nanoparticles had an average crystallite size of 7.8 ± 2 nm. Crystallite size is found to be decreased when chromium is doped in SnO2 at different concentrations.The morphology of SnO2 nanoparticles was studied by FESEM, and the results showed that the developed SnO2 nanoparticles are uniform, spherical, and evenly dispersed. The SnO2 nanoparticles were investigated by EDS's chemical technique. The chromium, oxygen peak and Sn peak are present, according to the EDS spectra. The doping concentrations of Cr (3 % and 6% wt.) confirmed from the EDS spectrum. The Optical band gap of Undoped and Cr doped SnO2 nanoparticles were evaluated by UV–Vis Spectroscopy. The results demonstrated that band gap of the SnO2 nanoparticles increased with addition of the Cr contents. Fourier Transform Infrared Spectroscopy was used to analyze the chemical characteristics of SnO2 nanoparticles. The Sn-O band stretching was confirmed from the FT-IR investigation. Further, the rutile tetragonal phase confirmed the A1g vibration mode of the Rutile SnO2 symmetry from Raman spectroscopy. Finally, the gas sensing characteristics of the SnO2 nanoparticles were studied at 270 °C the resistances versus ambient gas time. It has been observed that sensitivity of the prepared sample was higher than reported in the literature.

Green synthesis of Cr2O3 nanoparticles by Cassia fistula, their electrochemical and antibacterial potential

Chromium oxide (Cr2O3) nanoparticles (NPs) find applications in modern science and technology due to their chemical stabilities, high-density corners and magnetic/electrical/catalytic properties. Current studies were performed to produce the (Cr2O3)aq and (Cr2O3)et NPs by treating chromium acetate with the aqueous and ethanolic extracts, respectively of Cassia fistula leaves; the same reaction was also performed in the presence of NaOH to yield the (Cr2O3)aqNa and (Cr2O3)etNa NPs, respectively. The synthesized NPs were characterized by XRD, FTIR, Raman spectroscopy,UV–Vis spectroscopy, SEM, TGA and DSC analysis and examined for their electrochemical properties. Their antibacterial potential was tested by biofilm inhibition and agar well diffusion methods. XRD studies revealed that Cr2O3 NPs possessed hexagonal crystal structures with the crystallite sizes of 14.85 to 23.90 nm; the lowest size (14.85 nm) was possessed by (Cr2O3)et. FTIR and Raman spectroscopies verified the +3-oxidation state of chromium and corresponding Cr-O and Cr=O vibrations. Raman spectroscopy determined A1g vibration modes (540.50–557.07 cm−1) with rhombohedral Cr2O3 structure, high degree of crystallinity and existence of Cr3+ ions in octahedral coordination whereas Eg vibration modes were displayed at 306.04–350.87 cm−1 and 602.47–613.81 cm−1. UV–Visible spectroscopy has shown band gaps in the range of 4.06–4.40 eV. The SEM images demonstrated the spherical morphology with a high degree of agglomeration between fine particles. The synthesized NPs exhibited good thermal stabilities up to 600 °C. The CV curves displayed the oxidation–reduction peaks and reversible behavior whereas GCD curve indicated the possible energy storage applications of NPs. (Cr2O3)aqNa NPs showed the highest specific capacitance (234.35 mAhg−1) at 1 mA current density. The biofilm inhibitions of the investigated NPs were comparable to those of the standard antibacterial drug (ciprofloxacin); the activity of (Cr2O3)etNa was found even better than that of ciprofloxacin. The NPs were more active against Escherichia coli (Gram-negative) as compared to those against Staphylococcus aureus (Gram-positive).

Quantitative evaluation of metamictisation of columbite-(Mn) from rare-element pegmatites using Raman spectroscopy

AbstractRaman spectroscopic analysis was performed on columbite-(Mn) samples from a variety of previously studied rare-element pegmatites in Xinjiang, China, including the Jing'erquan No. 1 spodumene-subtype, Dakalasu No. 1 beryl–columbite-subtype and Kalu'an spodumene-subtype pegmatites, to quantify the relationship between the degree of metamictisation of columbite and Raman spectra. For all of the analysed columbites-(Mn), the position (p) and the full width at half maximum (FWHM) of the strongest band, A1g vibration mode related to the Nb/Ta–O bond, in the Raman spectra have a negative correlation. Combined with previously determined U–Pb isotopic data and major–minor-element data for the columbites-(Mn), the degree of metamictisation was quantified using the alpha-decay dose (D) and displacement per atom (dpa), both of which were corrected for effects caused by annealing. The results demonstrate that the columbite-(Mn) from Jing'erquan and Kalu'an are very crystalline, whereas those from Dakalasu are transitional between crystalline and amorphous stages. The main factor influencing the key parameters, i.e. band position and FWHM, of the strongest Raman band of columbite-(Mn) is metamictisation caused by radiation damage, whereas composition and crystal orientation have limited influence. A set of equations are established to quantify the degree of metamictisation of columbite using the band position and the full width at half maximum: FWHM = 8.309 × ln(aD) + 30.11 (R2 = 0.9861); p = –5.187 × ln(aD) + 867.09 (R2 = 0.966); FWHM = 8.1453 × ln(adpa) + 48.425 (R2 = 0.9822); and p = –5.078 × ln(adpa) + 855.67 (R2 = 0.9594).

Mismatched atomic bonds and ultralow thermal conductivity in Ag-based ternary chalcopyrites

Diamond like ternary chalcopyrites $MB{X}_{2}$ ($M=\text{Cu},\text{Ag}$; $B=\text{Ga},\text{In},\text{Tl}$; $X=\text{S},\text{Se},\text{Te}$) have attracted tremendous research interest in thermoelectric society due to the competitive performance and a great variety of transport properties. Interestingly, the lattice thermal conductivity of $\text{Ag}B{X}_{2}$ is systematically two to five times lower that of $\text{Cu}B{X}_{2}$, in spite of their similar crystal and band structures. Based on the careful theoretical analysis, we show that the ultralow thermal conductivity of $\text{Ag}B{X}_{2}$ originates in the mismatched atomic bonds between Ag-$X$ and B-$X$ pairs. Owing to the very expanded $4d$ orbital of Ag, the bonding strength of Ag-$X$ is much weaker than that of $B\text{\ensuremath{-}}X$. As a result, the vibrations of Ag-$X$ and B-$X$ are well separated within the low-frequency modes. The phonon density of states of $\text{Ag}B{X}_{2}$ exhibits four divided peaks, compared with the three peaks in $\text{Cu}B{X}_{2}$. The calculated joint phonon density of states reveals that such characteristic in $\text{Ag}B{X}_{2}$ would increase the scattering rate of low-frequency phonons significantly. This study provides essential understandings on the ultralow thermal conductivity of Ag-based ternary chalcopyrites and indicates a general strategy to suppress the thermal conductivity in ternary compounds.

Self-powered photodetector based on direct vapour transfer (DVT) method grown tin selenide (SnSe) crystals

The 2D-TMDC (transition metal dichalcogenides) formed thin films have attracted more attention due to their tuneable optoelectronic properties that improved the application of the photo-sensing device. SnSe single crystals were grown in the current work utilising the direct vapour transfer (DVT) technique. The elemental proportion and 2D surface structural analysis of grown crystal SnSe are carried out using Energy-dispersive X-ray analysis (EDAX) and Scanning electron microscopy (SEM). The orthorhombic crystal system of grown SnSe confirmed by high resolution transmission electron microscopy (HRTEM), and Powder X-ray diffractometer (XRD), the regular spot patter of HRTEM and Sharp pick in XRD graph indicated that the SnSe are highly crystalline in nature. The X-ray photoelectron spectroscopy (XPS) is carried out for confirming the binding states and chemical composition of grown SnSe single crystal. The A1g, β3g and A2g vibration mode of SnSe single crystal confirmed using Raman Spectroscopy. In the application part, SnSe exhibits high photodetection capability under the monochromatic and polychromatic light sources at self-bias mode (photoresponsivity of 71.20 µA/W and photo-detectivity of 3.45 × 108 Jones). The optoelectronic characteristics of SnSe devices including dependent photo-responses have also been carried out. Additionally, the important factors affecting photodetection characteristics are assessed, including photocurrent (Iph), sensitivity (S), photoresponsivity (R), Specific detectivity (D*), rise time, and decay time.

Self-powered photodetectors based on InxMo1-xS2 crystals

Transition metal dichalcogenides (TMDCs) have attracted global interest in recent years due to their optoelectronic characteristics and tunable bandgap energies. Due to their potential use in photovoltaic devices, photodetectors, and field effect transistor (FET) recording systems, single crystalline semiconductors of TMDC family have attracted a lot of attention. In the present article, the direct vapour transport technique was used to synthesis the InxMo1-xS2 (x = 0, 0.05, and 0.1) crystals. The purity and distribution of all the elements were investigated by EDAX mapping. The power X-ray diffraction technique examine the grown crystals have hexagonal lattice structure. The structure of all the crystals was investigated by scanning electron microscopy (SEM). The in-plane E2g and outer-plane A1g vibration modes were studied through Raman spectra. Photodetectors based on InxMo1-xS2(x = 0, 0.05, and 0.1) crystals show the self-powered photoresponse under white light source at 100 mW/cm2power intensity.

Influence of poly(vinyl alcohol)/poly(N-vinyl-2-pyrrolidone) polymer matrix composition on the bonding environment and characteristics of Ag nanoparticles produced by gamma irradiation

A series of silver nanoparticles (AgNPs)-poly(vinyl alcohol)/poly(N-vinyl-2-pyrrolidone) (PVA/PVP) hydrogel nanocomposites were synthesized by gamma irradiation to examine the matrix's impact on properties of Ag nanocrystals important for strain engineering in functional materials. SEM analysis and the swelling studies revealed nanocomposite morphology and fluid transport properties. UV–Vis absorption characterization of AgNPs has enabled an understanding of the cluster size effects. The gel content indicated inter-crosslinking of the two components of the polymer matrix, especially for equivalent (1:1) PVA/PVP ratio, affecting the investigated AgNPs structural parameters. The lattice parameters and the interface stress, analyzed using XRD, have the lowest negative magnitude from the bulk lattice constant for the PVA/PVP (2:1) and (1:1) ratio. The lowest values of the lattice strain and dislocation density, as mutually dependent parameters, are obtained in AgNPs embedded in a PVA/PVP (2:1) and (1:1) matrix. The XPS and FTIR data show a positive shift of the O1s and a redshift of the CO – Ag vibration band indicating charge transfer to the vacant orbital of Ag nanocrystals. The N atoms of the pyrrolidine ring are not directly involved in the electronic interactions, but over the p-π conjugation with carbonyl O. In addition, the tested Ag-(PVA/PVP) hydrogel nanocomposites showed significant potential as a broad-spectrum antibacterial material.

Evaluation of vibrational properties and local structure change during phase transition in Ge2Sb2Te5 and In3SbTe2 phase change materials

Temperature-dependent local structural details of In3SbTe2 (IST) and Ge2Sb2Te5 (GST) phase change materials are explored with the aid of Raman scattering and x-ray photoelectron spectroscopy (XPS) techniques. Significant temperature-induced changes occur in the local structure of the phase change material, facilitating the amorphous to crystalline phase transformation. These two phases exhibit a large resistivity contrast, which is utilized to store data in the phase change memory application. The Raman spectra recorded for IST material suggest that the as-deposited sample (amorphous phase) first crystallized around 300 annealing temperature with two binary phases InTe and InSb formations. InTe and InSb phases were obtained at ∼85.5 cm−1 and ∼180 cm−1 Raman shift, respectively, having vibration modes associated with B1g symmetry and transversal optical phonons with Г15 symmetry. Further annealing at a higher temperature of 400 , a ternary IST phase is obtained at ∼163 cm−1 Raman shift indicating the transformation to a fully crystalline state. On the other hand, the amorphous GST material forms a metastable face-centred cubic phase and stable hexagonal (HEX) phase upon crystallization. The Raman findings demonstrate the changes in vibration modes of Ge–Te and Sb–Te bonds during phase switching without any phase separation. Crystalline GST (HEX phase) comprises rising peaks of GeTe4 and GeTe4−n Ge n (n = 1, 2) corner-sharing tetrahedra with A1 mode at ∼104.5 cm−1 and ∼137 cm−1 Raman shift, respectively, and a declining Sb2Te3 peak at ∼175 cm−1, having A1g (2) vibration mode. Furthermore, the XPS analysis displays the changes in the bonding mechanism of the elements present in the phase change material during the amorphous to crystalline phase transition, firmly supporting the Raman observations.

Ruthenium single‐atom modulated Ti3C2Tx MXene for efficient alkaline electrocatalytic hydrogen production

AbstractSingle‐atoms (SAs) supported on various substrates have emerged as a new form of electrocatalysts for hydrogen evolution reaction (HER). The exfoliated MXenes possess rich defects/vacancies and surface oxygen groups, can be favorably utilized to anchor SAs. Here, we take advantage of the exfoliated Ti3C2Tx to anchor Ru‐SAs on Ti3C2Tx through a wet‐chemistry impregnation process. The obtained RuSA@Ti3C2Tx possesses excellent HER activity, especially under high current densities. Remarkably, RuSA@Ti3C2Tx can readily attain high current densities of 1 and 1.5 A cm−2 at low over potentials of 425.7 and 464.6 mV, respectively, demonstrating its potential for practical applications. The A1g vibration frequency shift of the Raman spectrum is innovatively used to probe the surface OH coverage on Ti3C2Tx, providing critical information for mechanistic studies. The experimental and theoretical studies reveal that the superior HER electrocatalytic activity of RuSA@Ti3C2Tx results from the Ru‐SAs enhanced H2O adsorption and dissociation, and promoted H2 formation.image

Phonon anharmonicity in multi-layered WS2 explored by first-principles and Raman studies

Anharmonicity is an essential property of two-dimensional materials due to its significant impact on lattice dynamics – involving interactions between phonons, thermal expansion of the structure, and interaction with the substrate. The effects manifest in temperature-dependent redshifts of the phonon frequencies and reduced lifetime. In this work, we investigate phonon anharmonicity in supported 1–5 layered WS2 by performing Raman measurements in the temperature range of 80–500 K and explore the results by ab-initio DFT study. An ab-initio model including three-phonon interaction processes and thermal expansion of all geometrical degrees of freedom (in-plane lattice constant, interlayer distance, and monolayer thickness) well describes the experimental temperature trends of the fundamental E2g1 (in-plane) and A1g (out-of-plane) phonon modes, giving a great promise of using simulation to quantitatively analyze phonon propagation and heat transport in multi-layered two-dimensional materials. The models predict a noticeable increase in the temperature-induced slope of A1g vibrations’ frequency with the number of layers – primarily due to the thermal expansion of interlayer distance. Additionally, we study the impact of the substrate on the phonon properties – concerning cumulated effects of induced strain and electron doping. We show that the effect of biaxial strain is the most significant in the E2g1 phonon mode and independent of the number of layers, while the excess charge is more significant in thin films and affects mainly the A1g mode. The presented studies based on a combination of ab-initio and experimental approaches can be extended to quantitatively analyze phonon evolution with structural modification and external stimuli.

Synthesis of The Tangerine-based Cdots and AgNp-Cdots Composite

Silver nanoparticles (AgNp) have been widely developed, one of which is an antibacterial agent. The ability of AgNp to inhibit bacteria is unquestionable. These properties and characteristics can be studied further by adding defects. The defect used is Cdots from tangerines. Cdots are new materials that can be synthesized from natural materials and have unique properties. AgNp itself was synthesized by the chemical reduction method with sodium citrate as a reducing agent, while Cdots was synthesized by hydrothermal method. The results of the synthesis were analyzed using the Fourier Transform Infrared (FT-IR) method. Absorption appears at a wave number of 1637 cm-1 which indicates the presence of C=C, C=O bonds which are characteristic of Cdots. For AgNp, absorption appears at a wave number of 593.11 cm-1 which indicates the presence of Ag vibrations. Analysis with UV-Vis spectrophotometer gave the results at a wavelength of 431 nm absorbance AgNp-Cdots of 0.723 and a wavelength of 422 nm AgNp absorbance of 0.285. The results of the Particle Size Analyzer (PSA) analysis showed that AgNp-Cdots had an average size of 38.7 nm with a PI of 0.499 and pure AgNp of 51.8 nm with a PI of 0.566. Antibacterial activity of AgNp-Cdots against E. coli bacteria showed an inhibition zone of 2 mm while AgNp showed an inhibition zone of 5 mm.

Stability & durability of self-driven photo-detective parameters based on Sn1-β Sb β Se (β = 0, 0.05, 0.10, 0.15, 0.20) ternary alloy single crystals.

In the present investigation Sn1-β Sb β Se crystals are grown using the direct vapor transport method. The crystals after growth were analyzed by EDAX and XPS to confirm the elemental composition. The surface morphological properties were studied by scanning electron microscope, confirming a flat surface and layered growth of the Sn1-β Sb β Se crystals. The structural properties studied by X-ray diffraction and high-resolution transmission electron microscopy confirm the orthorhombic structure of the grown Sn1-β Sb β Se crystals. The Raman spectroscopic measurements evince the presence of B2g and Ag vibration modes. The PL intensity peak at ∼400 nm to 500 nm confirms the energy band gap. The indirect energy band gap of 1.18 eV was evaluated using Tauc plot by employing UV-visible spectroscopy making it a promising candidate for optoelectronic and photonic applications. The pulse photo response of pure and doped samples was studied under a monochromatic source of wavelength 670 nm and intensity of 30 mW cm-2 at zero biasing voltage firstly on day one and then the same samples were preserved for 50 days and the stability of the photodetectors was observed. Photodetector parameters such as rise time, decay time, photocurrent, responsivity, sensitivity, and detectivity were observed, and evaluated results are presented in this article.

Emphanisis in Cubic (SnSe)0.5(AgSbSe2)0.5: Dynamical Off-Centering of Anion Leads to Low Thermal Conductivity and High Thermoelectric Performance.

The structural transformation generally occurs from lower symmetric to higher symmetric structure on heating. However, the formation of locally broken asymmetric phases upon warming has been evidenced in PbQ (Q = S, Se, Te), a rare phenomenon called emphanisis, which has significant effect on their thermal transport and thermoelectric properties. (SnSe)0.5(AgSbSe2)0.5 crystallizes in rock-salt cubic average structure, with the three cations occupying the same Wycoff site (4a) and Se in the anion position (Wycoff site, 4b). Using synchrotron X-ray pair distribution function (X-PDF) analysis, herein, we show the gradual deviation of the local structure of (SnSe)0.5(AgSbSe2)0.5 from the overall cubic rock-salt structure with warming, resembling emphanisis. The local structural analysis indicates that Se atoms remain in off-centered position by a magnitude of ∼0.25 Å at 300 K along the [111] direction and the magnitude of this distortion is found to increase with temperature resulting in three short and three long M-Se bonds (M = Sn/Ag/Sb) within the average rock-salt lattice. This hinders phonon propagation and lowers the lattice thermal conductivity (κlat) to 0.49-0.39 W/(m·K) in the 295-725 K range. Analysis of phonons based on density functional theory (DFT) reveals significant soft modes with high anharmonicity which involve localized Ag and Se vibrations primarily. Emphanisis induced low κlat and favorable electronic structure with multiple valence band extrema within close energy concurrently give rise to a promising thermoelectric figure of merit (zT) of 1.05 at 706 K in p-type carrier optimized Ge doped new rock-salt phase of (SnSe)0.5(AgSbSe2)0.5.