A1g Mode Research Articles

Synthesis of Sn1−x NixO2 nanoparticles: Observation of room temperature structural, optical and magnetic behavior

Ni-doped SnO2 (Sn1−xNixO, x = 0.03, 0.05, 0.10, 0.12, and 0.15) powder samples were synthesized in a chemical auto combustion method and followed by calcination at 850 ºC for 5 h. Initially, their structural characteristics were investigated through X-ray diffraction (XRD) which reveals that the prepared samples are nanocrystalline rutile tetragonal structure similar to host SnO2. The surface morphological image analyses were done by scanning electron microscopy (SEM) which revealed the spherical nature of grains along with the presence of voids. From Raman spectroscopy study, the presence of fundamental vibration modes of SnO2 like A1g modes at 634 cm−1, B2g modes at 773 cm−1 and Eg modes at 473 cm−1 along with the formation of defects i.e. oxygen vacancies in the sample have been confirmed. Similarly, Fourier transforms infrared spectroscopy (FTIR) shows the prominent peaks of Sn-O and Sn-O-Sn vibrations around 560 cm−1and 615 cm−1 respectively, and confirms the incorporation of Ni ions into pure SnO2 lattice by the small shifting in the positions of the peaks. Furthermore, from UV–visible spectroscopy, the decrease of bandgap with increasing Ni concentration has been noticed. The existence of room temperature ferromagnetism in Ni-doped SnO2 nanoparticles has been confirmed from the hysteresis loop of the vibrating sample magnetometer measurement (VSM) measurement. From all the above characterizations, it has been confirmed that up to x = 0.12 of Ni has been incorporated into host material SnO2.

Dirac semimetal PdTe[formula omitted] temperature-dependent quasiparticle dynamics and electron–phonon coupling

Dirac semimetal PdTe2 single-crystal temperature-dependent ultrafast carrier and phonon dynamics were studied using ultrafast optical pump-probe spectroscopy. Quantitative analysis revealed a fast relaxation process (τf) occurring at a subpicosecond time scale originating from electron–phonon thermalization. This rapid relaxation was followed by a slower relaxation process (τs) on a time scale of ∼ 7-9.5 ps which originated from phonon-assisted electron–hole recombination. Two significant vibrational modes resolved at all measured temperatures. These modes corresponded to in-plane (Eg), and out-of-plane (A1g), Te atoms motion. Test results suggested that pure dephasing played an important role in the relaxation processes. Analysis of the electron–phonon coupling constant suggested that the A1g mode contributes greatly to the superconductivity, and high-frequency phonons are also involved forming of Cooper pairs. Our observations should improve the understanding of complex superconductivity of PdTe2.

Investigation of phonon modes in 2H-TaX2 (X = S/Se) flakes with electrostatic doping

Electrostatic doping-assisted phonon softening has been intensively investigated in semiconducting transition metal dichalcogenides (TMDCs), in which strong electron–phonon interactions can be achieved in thin layers. Though electron–phonon coupling plays an important role in structure phase transitions for metallic 2H-TaX2 (X = S, Se), it has been rarely studied under electrostatic doping. Here, the effect of electrostatic doping on the vibrational phonon modes of 2H-TaSe2 and 2H-TaS2 thin films is studied with ionic liquid gating by the electric-double-layer gating technique. Under a gate bias (VG) of 1.5 V, 2H-TaSe2 has a blue shift of 7 cm−1 and a red shift of 3 cm−1 for the in-plane (E2g) and out-of-plane vibration (A1g) modes, respectively, indicating a strong electron–phonon interaction in both intralayer and interlayer. By varying the thickness of the flakes, it has been found that there exists a threshold VG value for the stiffening of the E2g mode and the softening of the A1g mode because of the strong electrostatic screening effect at lower doping density. As the intensity of the E2g mode decreases with VG, an anomalous enhancement of the A1g phonons can be achieved, which is further convinced in TaS2. The asymmetric phonon evolution behavior is very different with that in the semiconducting TMDCs, which may provide useful information for understanding electron–phonon interplay in metallic layer materials.

Modulation of electronic properties of MoS2 thin films by benzyl viologen treatment for IR detection

In this research, benzyl viologen (BV) is incorporated to enhance the n-type carriers in rapid thermally grown MoS2 thin films. A (002) oriented XRD peak, around 2θ = 14.1°, has been observed in BV treated MoS2 thin films. Raman spectra depicted the presence of two characteristics peaks around 389 cm−1 and 408 cm−1 corresponding to E12g and A1g modes of MoS2, respectively. The ideality factor and built-in voltage for the MoS2/Si heterojunction, treated with 25 mM and 6 coats of BV solution, were found to be 1.27 and 0.30 V, respectively for annealing temperature of 300 °C. The carrier concentration, calculated from Mott-Schottky analysis, is found to rise with the increase in BV concentration and annealing temperature up to 300 °C. The BV treated MoS2/Si heterojunction revealed the current On/Off ratio, rise and fall time of 520, 1.9 s and 1.7 s, respectively for the exposure of 940 nm IR light. With 850 nm IR illumination, the current On/Off ratio, rise and fall time decreased to 322, 1.4 s and 1.3 s, respectively. The results obtained provide the preliminary information for the fabrication of efficient next generation IR detectors.

Phase transformation and enhanced blue photoluminescence of zirconium oxide poly-crystalline thin film induced by Ni ion beam irradiation

Swift heavy ions (SHI) irradiation of Nickel (Ni) beam with different ions fluence bring the modifications in the functional properties of radio frequency (RF) grown zirconium oxide (ZrO2) nanocrystalline thin films. X-ray diffraction analysis affirms the monoclinic to tetragonal phase transformation and diminishing of peak at higher fluence 1 × 1014 and 2 × 1014 ions/cm2 induced by electronic excitation caused by SHI. Zirconium oxide thin films exhibit the same thickness (195 nm) of virgin and irradiated samples and whereas the nanocrystalline thin films have the elemental composition in proper stoichiometry (1:2) as analyzed by rutherford backscattering spectroscopy (RBS). Photoluminescence measurements confirm the blue emission of virgin and irradiated sample recorded at excitation wavelength 270 to 310 nm. The intensity of obtained emission bands varies with fluence which is interpreted in terms of generation and annihilation of defect centers. The characteristic Ag and Bg Raman modes of monoclinic and tetragonal ZrO2 are obtained at different positions. Moreover, the nanocrystalline ZrO2 thin films exhibits the most prominent absorption phenomenon in the visible range and the irradiation cause significant decrease in band gap to 3.69 eV compare to the virgin ZrO2 sample (3.86 eV). XPS analysis indicates the shifting of the core levels Zr 3d and O 1s towards higher binding energy and spin—orbit splitting of different states. The findings in this research justify that the irradiated thin films can be a potential candidate for designing of new materials, intense radiation environments, nuclear reactors, nuclear waste systems, clean energy sources.

Excited state absorption induced optical limiting action of MoS2-rGO nanocomposites

Molybdenum sulphide-reduced graphene oxide nanocomposite made of microsphere MoS2 with varying concentration of layered (15, 30, 45 wt%) rGO was prepared by hydrothermal method. XRD confirms the molecular formation of mixed phase MoS2 having rich semiconducting hexagonal phase with traces of metallic tetragonal phase (1T@2H MoS2) and reduced graphene oxide (2θ = 25.7°, (002)). Further EDS also ascertains the reduction of Mo and S with increase in C content with respect to rGO concentration. As the rGO concentration increases (15–45 wt%), strengthening of characteristic Raman peaks of rGO (G and D band) and weakening of MoS2 (E1g, E2g1 and A1g modes) in composite was observed. Estimated ID/IG (1.04) and I2D/IG (0.62) ratio suggest the formation of 2–3 layers of graphene. SEM depicts layered morphology of rGO, floret like microstructure of MoS2 and floret microspheres MoS2 decorated rGO layers of MoS2 – rGO nanocomposite. Optical studies show absorption peaks at 266 nm (π-π* transition of the CC aromatic bonds of graphene), 370 nm (and n-π* transition of the CO bond of graphene) and 330 nm (direct transitions in valence to conduction band of MoS2). Liquid dispersion of both pristine and composite depicted strong absorption in UV–Visible region and available electronic state favours the opportunity for two-photon absorption. Intensity dependent Z-Scan with nano-pulsed green laser excitation shows the existence of excited state absorption involved two-photon absorption. Variation of saturation intensity and nonlinear absorption coefficient with on-axis peak intensity (620 –2160 GW/cm2) confirms the involvement of sequential 2PA process (1PA + ESA). MoS2-(30 wt%)rGO nanocomposite exhibited higher 2PA coefficient (9.42 × 10−10 m/W) and lower onset optical limiting threshold (3.15 × 1012 W/m2) than its pristine counterparts and other well-known TMDC’s and graphene derivatives. Enhanced NLO response is due to synergistic effects of rGO (higher defect induced states, extended conjugation for charge transfer) and MoS2 (floret morphology, uniform decoration and strong visible absorption). Observed excited state absorption induced energy absorbing optical limiting action of MoS2-rGO nanocomposite with augmented NLO coefficients open up potential horizon for developing laser goggles against most sensitive and widely used nanopulsed green laser.

Feasibility of ZrSiO4 as reference signature in naturally-occurring radioactive elements for the application of radioactivity monitoring

Radioactivity monitoring post-cold war has become more complex due to the nuclear fallout and the surge in use of radioactive materials. This requires novel methods to detect, trace and distinguish natural and anthropogenic radioactive sources in the environment. We explored the feasibility of using ZrSiO4 (Zircon), as a reference signature for radioactivity monitoring due to the unique phenomenon of metamictization. We investigated the variations in microstructural properties of Zircon samples collected from a proposed Uranium site to identify these signatures using analytical techniques such as Gamma-ray Spectroscopy, XRD and Raman spectrum analysis. Besides elevated levels of radioactivity, the samples exhibited distinct properties such as increased lattice parameters observed from the XRD analysis and dramatic broadening of A1g (439 cm−1) and B1g (1008 cm−1) vibrational modes in the Raman spectrum. Structural parameters were further analyzed by modeling the crystal from experimentally observed lattice parameters. Ab-initio calculations were then performed on the modeled structure providing more insight into the microstructural variations. Samples collected from proposed Uranium mines indicate an increase of 1.226% and 0.9389% in Si–O and Zr–O bond lengths of the Zircon crystal signifying the ongoing process of metamictization from radiation damage. By correlating radioactivity levels with the lattice parameters variations of the collected samples, the study establishes a linear relation between the degree of damage to a mineral's crystal structure and the amount of radioactivity. We propose to use the variations in damage found in a mineral's structure as a nuclear forensic signature for advanced assessment of accumulated radioactivity in a particular geographical location.

Relationship between Rattling Mg2+ ions and anomalous microwave dielectric behavior in Ca3-xMg1+xLiV3O12 ceramics with garnet structure

Ca3-xMg1+xLiV3O12 (0 ≤ x ≤ 1) ceramics with cubic-garnet-structure were synthesized by the solid-phase reaction. The replacement of Ca2+ with the smaller Mg2+ exerted an increasingly strong “rattling” effect at the A-site of Ca3-xMg1+xLiV3O12. With increased Mg2+, εr increased from 10.5 ± 0.1 to 15.4 ± 0.1 and εcorr increased from 11.0 ± 0.1 to 16.0 ± 0.1, whereas the theoretical εth decreased from 12.0 ± 0.1 to 10.5 ± 0.1. εcorr was higher than εth due to the “rattling” effect. The enhanced “rattling” effect caused τf to increase rapidly from –64.1 ± 1.0 ppm/°C to +267.2 ± 1.0 ppm/°C. With increased x, Q × f decreased from 74,700 ± 500 GHz to 15,370 ± 500 GHz, due to the decreased packing fraction, increased FWHM of the A1g modes, and enhanced “rattling” effect. Additionally, the chemical compatibility between Ca2.75Mg1.25LiV3O12 and Ag electrodes was confirmed, indicating this material’s potential for LTCC.

Vibrational Properties of Pristine and Lithium‐Intercalated Black Phosphorous under High‐Pressure

AbstractStructural evolution of Li‐intercalated and pristine black phosphorous (BP) under high‐pressure (up to ≈8 GPa) is studied using in situ Raman spectroscopy. Even though both materials show a monotonic blueshift of the out‐of‐plane vibrational mode (A1g) with pressure, Li‐intercalated BP do not show a blueshift until a threshold pressure (2.4 GPa) is reached to compensate the structural expansion caused by intercalation. However, the in‐plane modes (B2g and A2g) in each sample respond differently. In the mid‐pressure region, they both show redshifts which in Li‐intercalated BP is also followed by abrupt blueshifts. Such behavior indicates pressure‐induced structural reorganizations inside the material. Computational modeling reveals the existence of a process of P─P bond breaking and reforming in the system due to the redistribution of intercalated Li atoms under pressure. This work shows the significance of combined effect of pressure and intercalation on structural changes in the search for new phases of BP and other two‐dimensional (2D) materials.

Exploring the structural, elastic, optical, dielectric and magnetic characteristics of Ca2+ incorporated superparamagnetic Zn0.5−xCa0.1Co0.4+xFe2O4 (x = 0.0, 0.05 & 0.1) nanoferrites

This work is aimed to understand influence of 10% Ca doping on the structural, optical, dielectric and magnetic behavior of half doped Zinc-Cobalt nanoferrites. Zn0.5−xCa0.1Co0.4+xFe2O4 (x = 0.0, 0.05 & 0.1) nanoferrites with crystallite size in 10–13 nm range were synthesized through a sol-gel auto-incinerated route. Rietveld profile refinement of PXRD patterns, observation of spinel ferrite type fcc Laue rings in SAED patterns, Raman modes (A1g,T2g & Eg) and metal-oxide (υ1 & υ2) bonds in 400–600 cm−1 IR range illustrated formation of single phase mixed valent spinel type fcc nanoferrites. In these nanoferrites Zn2+ ion resides at tetrahedral (TA) site, while Co2+ & Ca2+ ions preferentially fill the octahedral OB site and Fe3+ ion occupy both TA & OB sites. Moderate optical absorbance in blue color region and strong absorbance in the lower UV region is observed in these nanoferrites, which is owing to presence of mixed valent ions at the OB and TA sites respectively. 10% Ca doping in these half doped Zn-Co nanoferrites led to considerable decrement in the band gap energy (Eg) and turning them to semiconducting state (Eg in 2.5–2.3 eV range). In these nanoferrites hopping of electrons between divalent Fe2+, Ca2+, Zn2+, Co2+ & trivalent Fe3+ & Co3+ cations led to systematic decrease in the dielectric parameters with frequency. These nanoferrites exhibited superparamagnetism with a high magnetic moment (~ 2.9–3.0μB/f.u.) at room temperature. Systematic enhancements in saturation magnetization (Ms), coercivity (Hc), remanence (Mr) and squareness ratio at low temperatures revealed changing of superparamagnetic character to soft ferrimagnetic state of these nanoferrites. At 20K, highest value of Ms = 119 emu/g was observed for Zn0.5Ca0.1Co0.4Fe2O4 nanoferrite, while highest Hc = 2911 Oe & Mr = 61.0 emu/g were recorded for Zn0.4Ca0.1Co0.5Fe2O4 nanoferrite. Thus, a moderate 10% Ca2+ ion incorporation in Zn0.5−xCa0.1Co0.4+xFe2O4 (x = 0.0, 0.05 & 0.1) nanoferrites is emerged as an effective route for obtaining superparamagnetic nanoferrites with high magnetic moment and reduced band gap energy. Unique magneto-optical characteristics of Zn0.5−xCa0.1Co0.4+xFe2O4 (x = 0.0, 0.05 & 0.1) nanoferrites would project them as a good material for opto-magnetic, drug carrier, biomedical contrast agent and magnetic storage applications.

Experimental comparison of the effect of temperature on the vibrational and morphological properties of NixCo3-xO4 nanostructures

This work presents the experimental comparison of the effect of temperature and nickel addition on the synthesis of NixCo3-xO4 nanostructures x = 0.01, 0.04, 0.1, 0.16, using the sol–gel technique with final heat treatment at 600 °C and 800 °C. X-ray diffraction (XRD) analysis indicates the formation of pure Co3O4 at 600 °C and the appearance of an additional residual NiO phase at 800 °C. The presence of this phase evidences a deviation in the peaks of the diffraction angle to lower ones generated by the difference in ionic radii, which is related to the insertion of larger ions (Ni2+) doped in the matrix of Co3O4. Scanning electron microscope (SEM) analysis indicates that there is a densification of the grains at higher temperatures, possibly associated with the thermal effect that generates greater interdiffusion. Fourier-transform infrared spectroscopy (FTIR) spectra show two main absorption bands with vibrations Co2+¬O, Co3+¬O and Ni¬O in 456 cm−1. The effect of doping generates a displacement in the bands that are evident in the Raman analysis, where the vibrational mode A1g at 600 °C is translated 28.07 cm−1. The values of SBET specific surface area indicate that as temperature increases, the surface area and volume of micropores decreases.

Localized phase transition of TiO2 thin films induced by sub-bandgap laser irradiation

The ability to define the crystal phase of oxide semiconductors could benefit transparent electronics and catalysis. In this paper, laser-induced localized phase transitions of titanium dioxide (TiO2) thin films are reported. Irradiation was performed with a 532 nm continuous wave laser. Raman spectroscopy and micro-Raman mapping were used to identify the phase transformations. A Raman map of the anatase Eg mode (144 cm−1) and rutile Ag mode (608 cm−1) revealed the formation of crystalline microstructures due to the laser treatment. Laser irradiation under vacuum results in an anatase-to-rutile phase transition. Irradiating the rutile region in air changes the crystal structure back to anatase, despite the thermodynamic stability of rutile. The results suggest that irradiated photons are absorbed by defects, resulting in localized electronic excitation that leads to a mixture of amorphous and crystalline regions. The phase of the crystalline regions depends strongly on the ambient conditions (vacuum versus air).

Electron-Phonon and Spin-Lattice Coupling in Atomically Thin Layers of MnBi2Te4.

MnBi2Te4 represents a new class of magnetic topological insulators in which novel quantum phases emerge at temperatures higher than those found in magnetically doped thin films. Here, we investigate how couplings between electron, spin, and lattice are manifested in the phonon spectra of few-septuple-layer thick MnBi2Te4. After categorizing phonon modes by their symmetries, we study the systematic changes in frequency, line width, and line shape of a spectrally isolated A1g mode. The electron-phonon coupling increases in thinner flakes as manifested in a broader phonon line width, which is likely due to changes of the electron density of states. In 4- and 5-septuple thick samples, the onset of magnetic order below the Néel temperature is concurrent with a transition to an insulating state. We observe signatures of a reduced electron-phonon scattering across this transition as reflected in the reduced Fano parameter. Finally, spin-lattice coupling is measured and modeled from temperature-dependent phonon frequency.

Effect of cation distributions on structural and magnetic properties of Ni1-xCoxFe2O4 spinel ferrites

Single-phase polycrystalline samples of Ni1-xCoxFe2O4 (x = 0 – 0.50) were prepared by sol gel method and their structural and magnetic properties were studied. Room temperature X-ray diffraction patterns and Rietveld refinement confirm cubic crystal structure (Fd3-m). The lattice constant, saturation magnetization, magnetic anisotropy constant and coercivity of the samples are found to increase with Co concentration. An analytical quantum mechanical method was used to estimate the cation distributions in the prepared samples with the help of measured magnetic moment at T = 5 K. Even though, most of the doped Co ions occupy the octahedral site, considerable distribution of Co2+ ions at tetrahedral site is noticed. The observed magnetization is explained based on cation distribution worked out from quantum mechanical method. The red shifting of Raman active mode A1g and ESR results further confirm the presence of divalent cations in the tetrahedral sites.

Influence of swift heavy ion irradiations on temperature dependent phononic behavior of epitaxial LaNiO3 thin film

The effects of Ag15+ (200 MeV) swift heavy ion irradiations on the structural and phononic properties of epitaxial LaNiO3 (LNO) thin film have been investigated using high resolution x-ray diffraction and Raman spectroscopy. After irradiation, the decrease in the out-of-plane lattice parameter of LNO toward its bulk value indicates the relaxation of epitaxial strain. The temperature dependency of phononic response for different ion irradiation doses was studied by performing the Raman measurements in a temperature range of 80−300 K. For pristine as well as irradiated samples of LNO, the observed phononic modes A1g and Eg shows softening with an increment in the temperature. The temperature coefficient of both modes varies with ion fluence. For the A1g mode, temperature coefficient increases from −0.087 cm−1 K−1 for pristine to −0.092 cm−1 K−1 for a maximum ion fluence of 1012 ions/cm2, while for the Eg mode, it decreases from −0.022 cm−1 K−1 for pristine to −0.015 cm−1 K−1 for 1012 ions/cm2. Raman frequency shift for both the modes shows non-linear behavior with temperature. This temperature dependent behavior was quantitatively analyzed by using a model which suggests that Raman shifts of the A1g mode emerged predominantly due to four phonon processes whereas, for the Eg mode, major contribution came from the thermal expansion effect. Ion irradiation did not change the dominating mechanism resulting in these temperature dependent Raman shifts, although the relative contribution of different processes was altered with ion fluence.

Davydov Splitting, Double-Resonance Raman Scattering, and Disorder-Induced Second-Order Processes in Chemical Vapor Deposited MoS2 Thin Films.

Atomically thin MoS2 hosts rich and distinct vibrational spectral features, which are prominent under selective excitation energies near the excitonic transitions. In this work, we have investigated the resonant Raman scattering of the MoS2 layers of different thicknesses, from monolayer to five-layer samples, measured near resonance with the A excitonic transition. We show that the near-resonance excitation (1.96 eV) resulted in a Davydov splitting of the out-of-plane A-like phonon mode (A1g) around 406 cm-1 caused by the weak interlayer interaction. The number of Davydov splitting components (N) equals the number of layers (NL) of the MoS2, suggesting that it can be used as a thickness indicator. The origin of various Davydov components is understood based on a simple nearest-interlayer interaction. We extend our investigation to identify some acoustic phonon modes associated with characteristic second-order double-resonance Raman and disorder-induced bands.

Semiconductor-to-metal transition in atomic layer deposition (ALD) of VO2 films using VCl4 and water

The semiconductor-to-metal transition of vanadium dioxide (VO2) films is studied using temperature-dependent Raman, optical, and electrical measurements. The VO2 films are deposited via an atomic layer deposition (ALD) process using alternate pulses of vanadium tetrachloride and H2O at 350 °C. A growth rate of 0.021 nm/cycle and a thickness of 33 nm of VO2 are obtained for all films studied. The phase of the film is determined using x-ray diffraction. The as-deposited films are amorphous and are transformed to the monoclinic phase with a post-deposition, forming gas anneal at temperatures ≥ 500 °C for 60 min. The purity of the films is determined using x-ray photoelectron spectroscopy and no evidence of residual chlorine is detected. The temperature-dependent Raman Ag mode of the monoclinic VO2 phase is observed to monotonically decrease from 25 °C to 78 °C; where no evidence of the Ag peak is observed in the film beyond 68 °C. The refractive index and extinction coefficient extracted from temperature-dependent ellipsometry confirm that, beyond 68 °C, free carriers are generated in the film. Electrical measurements performed on a fabricated p++Si/VO2/Ti/Au device show a semiconductor-to-metal transition behavior with a high resistance of 14701 ± 2284 Ω at 62 °C and a low resistance of 1064.1 ± 143 Ω at 67 °C. This work demonstrates that a halide-based ALD process provides a clean and robust approach to synthesizing high-quality VO2 films.

Spin glass behavior and oxidative catalytic property of Zn2MnO4 from a metathesis driven metastable precursor

The reaction of chloride salts of zinc and manganese with NaOH yielded a cubic spinel structured metastable precursor at room temperature, driven mainly by the salt elimination process's energetics. While classical drying processes failed to produce the monophasic oxide, recrystallization under the hydrothermal conditions yielded Zn2MnO4 in nano dimensions. The sample consisted of crystallites with an average 6 nm size and had a lattice dimension of 8.396 (13) Å. The selected area electron diffraction pattern reiterated the occurrence of cubic inverse spinel. The presence of fingerprint (A1g and F2g) modes of an inverse spinel at 663 and 561 cm−1 in the Raman spectrum further supported our finding. The TEM-EDS analysis confirmed the ratio of Zn: Mn as 1.95:1. The sample showed an optical bandgap of 2.54 eV. X-ray photoelectron spectral analysis established the existence of manganese in the IV oxidation state. The presence of Mn (IV) with small amounts of Mn (III) (up to 20%) was confirmed from the electron paramagnetic spectra recorded at room temperature and 77 K. An average oxidation state of 3.85 was deduced from the chemical redox titration experiments. The pseudocapacitive behavior of the sample was evident in cyclic voltammetric experiments. The sample exhibited paramagnetic behavior at 298 K within the applied magnetic field of ±50 kOe. In the temperature-dependent measurements, the zero-field and field cooled data points of Zn2MnO4 diverged at 13 K, suggesting a spin-glass behavior. An effective magnetic moment of 4.31 BM was deduced for the sample. The inverse spinel effectively catalyzed the oxidation of phenol. It facilitated nearly 100% degradation of bisphenol-A to salicylaldehyde and phenylethyl alcohol (as major products) in the presence of H2O2 and at a pH of 9.

Large-Area, Two-Dimensional MoS2 Exfoliated on Gold: Direct Experimental Access to the Metal-Semiconductor Interface.

Two-dimensional semiconductors such as MoS2 are promising for future electrical devices. The interface to metals is a crucial and critical aspect for these devices because undesirably high resistances due to Fermi level pinning are present, resulting in unwanted energy losses. To date, experimental information on such junctions has been obtained mainly indirectly by evaluating transistor characteristics. The fact that the metal–semiconductor interface is typically embedded, further complicates the investigation of the underlying physical mechanisms at the interface. Here, we present a method to provide access to a realistic metal–semiconductor interface by large-area exfoliation of single-layer MoS2 on clean polycrystalline gold surfaces. This approach allows us to measure the relative charge neutrality level at the MoS2–gold interface and its spatial variation almost directly using Kelvin probe force microscopy even under ambient conditions. By bringing together hitherto unconnected findings about the MoS2–gold interface, we can explain the anomalous Raman signature of MoS2 in contact to metals [ACS Nano. 7, 2013, 11350] which has been the subject of intense recent discussions. In detail, we identify the unusual Raman mode as the A1g mode with a reduced Raman shift (397 cm–1) due to the weakening of the Mo–S bond. Combined with our X-ray photoelectron spectroscopy data and the measured charge neutrality level, this is in good agreement with a previously predicted mechanism for Fermi level pinning at the MoS2–gold interface [Nano Lett.14, 2014, 1714]. As a consequence, the strength of the MoS2–gold contact can be determined from the intensity ratio between the reduced A1greduced mode and the unperturbed A1g mode.

First-principles calculations on ferroelectricity and lattice dynamics of Type-II multiferroic SmMn2O5

In this work, we report on the structural, electronic, and ferroelectric properties of SmMn2O5 by using first-principles density functional theory plus on-site Coulomb interaction (DFT + U) calculations. A thorough analysis was preformed to reveal the competing characteristics of different high-temperature (T) phases and the polarization mechanism in the low-T multiferroic phase. We show that the structural characteristics of the high-T phases have a strong influence on the low-T multiferroicity. In addition to the spin-induced lattice distortion that reduces substantially the purely electronic ferroelectricity, the dominant polarization mechanism in low-T SmMn2O5 still originates from the electronic polarization. By performing mode decomposition of the Hellmann–Feynman forces and the lattice distortion induced by the q = (0.5, 0, 0) magnetic order, we find that the Raman-active Ag mode characterized by the Mn4+O6 octahedron distortion and synergistic displacement of Mn3+ and Sm ions is of primary importance, while the infrared (IR)-active B2u mode plays a secondary role. These findings provide a theoretical foundation for future studies concerning the enhanced magnetoelectric effects of SmMn2O5 due to its pure exchange–striction mechanism.