1g Mode Research Articles

Defect-Driven MoS2 Nanosheets toward Enhanced Sensing Sensitivity.

In this study, S-deficient MoS2 was prepared using proton irradiation and then applied as sensing materials for the detection of NO2 gas. First, bulk MoS2 was treated by ultrasonics to produce 2D nanosheets of MoS2, which were subsequently bombarded by a flux of high-energy protons, resulting in the appearance of structural defects throughout MoS2. The proton fluxes were adjusted to different densities of 1 × 1011, 1 × 1012, 1 × 1013, and 1 × 1014 ions/cm2. The effects of proton irradiation on the defects, also referred to as atomic vacancies, were systematically investigated using Raman measurements to locate the E1 2g and A1g modes and X-ray photoelectron spectroscopy to determine the binding energy of Mo 3d and S 2p orbitals. It was revealed that the density of proton irradiation greatly affects the degree of S atom vacancies in irradiated MoS2, while also enhancing the n-type semiconducting behaviors of MoS2. The vacancy-rich MoS2 was then demonstrated to exhibit a higher response to NO2 gas compared to that of nonirradiated MoS2, showing a 4-fold increase in response within a concentration range from 1 to 20 ppm. These results could pave the way for new approaches to fabricating sensing materials.

Giant Magneto-Optical Effect in van der Waals Room-Temperature Ferromagnet Fe3GaTe2

Abstract The discovery of ferromagnetic two-dimensional (2D) van der Waals (vdWs) materials provides an opportunity to explore intriguing physics and to develop innovative spin electronic devices. However, the main challenge for practical applications of vdWs ferromagnetic crystals lies in the weak intrinsic ferromagnetism and small perpendicular magnetic anisotropy (PMA) above room temperature. Here, we report the intrinsic vdWs ferromagnetic crystal Fe3GaTe2, synthesized by the self-flux method, exhibiting a Curie temperature (T C) of 370 K, a high saturation magnetization of 33.47 emu/g, and a large PMA energy density of approximately 4.17 × 105 J/m3. Furthermore, the magneto-optical effect is systematically investigated in Fe3GaTe2. The doubly degenerate E 2g (Γ) mode reverses the helicity of incident photons, indicating the existence of pseudoangular-momentum (PAM) and chirality. Meanwhile, the non-degenerate non-chiral A 1g(Γ) phonon exhibits a significant magneto-Raman effect under an external out-of-plane magnetic field. These results lay the groundwork for studying phonon chirality and magneto-optical phenomena in 2D magnetic materials, providing the feasibility for further fundamental research and applications in spintronic devices.

Evidences for local non-centrosymmetricity and strong phonon anomaly in EuCu2As2: a Raman spectroscopy and lattice dynamics study

Phonon modes and their association with the electronic states have been investigated for the metallic EuCu2As2 system. In this work, we present the Raman spectra of this pnictide system which clearly shows the presence of seven well defined peaks above 100 cm−1 that is consistent with the locally non-centrosymmetric P4/nmm crystal structure, contrary to that what is expected from the accepted symmorphic I4/mmm structure. Lattice dynamics calculations using the P4/nmm symmetry attest that there is a commendable agreement between the calculated phonon spectra at the Γ point and the observed Raman mode frequencies, with the most intense peak at ∼232 cm−1 being ascribed to the A 1g mode. Temperature dependent Raman measurements show that there is a significant deviation from the expected anharmonic behaviour around 165 K for the A 1g mode, with anomalies being observed for several other modes as well, although to a lesser extent. Attempts are made to rationalize the observed anomalous behavior related to the hardening of the phonon modes, with parallels being drawn from metal dichalcogenide and allied systems. Similarities in the evolution of the Raman peak frequencies with temperature seem to suggest a strong signature of a subtle electronic density wave instability below 165 K in this compound.

Fingerprint Raman spectroscopy for two-dimensional MoS2x Se2(1−x) alloys

Two-dimensional transition metal dichalcogenides (TMDs) and alloys based on them, is a promising platform for creating opto- and nanoelectronic devices. For layered alloys, there is a strong need to theoretically determine the frequencies of vibrational modes and dependences of their energies on the stoichiometric composition. By comparing experimentally measured Raman modes with theoretical predictions, it becomes possible to determine the stoichiometric composition of the manufactured alloys. In this work, we investigated the vibrational properties of monolayer MoS2x Se2(1−x) alloys utilizing density functional theory method and confirmed them experimentally by Raman spectroscopy. The dependence of A1ʹ and E1 2g ‘dactylographic’ modes on the stoichiometric composition of alloys has been calculated. When in pure MoSe2 structure Se atoms are substituted by S atoms, the in-plane E1 2g (S–Mo), E1 2g (Se–Mo) and out-plane A1ʹ (S–Mo) modes shifted linearly in x to higher frequencies, while the out-plane A1ʹ (Se–Mo) mode did the same in nonlinear way. We also identified the E1 2g (Se–Mo–S) mode, which specific for the MoS2x Se2(1-x) alloys and does not appear in pure two-component TMDs.

Raman study of layered breathing kagome lattice semiconductor Nb3Cl8

Niobium chloride (Nb3Cl8) is a layered two-dimensional semiconducting material with many exotic properties including a breathing kagome lattice, a topological flat band in its band structure, and a crystal structure that undergoes a structural and magnetic phase transition at temperatures below 90 K. Despite being a remarkable material with fascinating new physics, the understanding of its phonon properties is at its infancy. In this study, we investigate the phonon dynamics of Nb3Cl8 in bulk and few layer flakes using polarized Raman spectroscopy and density-functional theory (DFT) analysis to determine the material’s vibrational modes, as well as their symmetrical representations and atomic displacements. We experimentally resolved 12 phonon modes, five of which are A 1g modes while the remaining seven are Eg modes, which is in strong agreement with our DFT calculation. Layer-dependent results suggest that the Raman peak positions are mostly insensitive to changes in layer thickness, while peak intensity and full width at half maximum are affected. Raman measurements as a function of excitation wavelength (473–785 nm) show a significant increase of the peak intensities when using a 473 nm excitation source, suggesting a near resonant condition. Temperature-dependent Raman experiments carried out above and below the transition temperature did not show any change in the symmetries of the phonon modes, suggesting that the structural phase transition is likely from the high temperature P 1 phase to the low-temperature R phase. Magneto-Raman measurements carried out at 140 and 2 K between −2 and 2 T show that the Raman modes are not magnetically coupled. Overall, our study presented here significantly advances the fundamental understanding of layered Nb3Cl8 material which can be further exploited for future applications.

High-pressure response of vibrational properties of b-As x P1–x : in situ Raman studies

The structural evolution of black arsenic-phosphorous (b-As x P1–x ) alloys with varying arsenic concentrations was investigated under hydrostatic pressure using in situ Raman spectroscopy. High-pressure experiments were conducted using a diamond anvil cell, which revealed pressure-induced shifts in vibrational modes associated with P–P bonds (A 1 g , A 2 g , ), As–As bonds (A 1 g , A 2 g , ), and As–P bonds in b-As x P1–x alloys. Two distinct pressure regimes were observed. In the first regime (region I), all vibrational modes exhibited a monotonic upshift, indicating phonon hardening due to hydrostatic pressure. In the second regime (region II), As0.4P0.6 and As0.6P0.4 alloys displayed a linear blueshift (or negligible change in some modes) at a reduced rate, suggesting local structural reorganization with less compression on the bonds. Notably, the alloy with the highest As concentration, As0.8P0.2, exhibited anomalous behavior in the second pressure regime, with a downward shift observed in all As–As and As–P Raman modes (and some P–P modes). Interestingly, the emergence of new peaks corresponding to the Eg mode and A 1g mode of the gray-As phase was observed in this pressure range, indicating compressive strain-induced structural changes. The anomalous change in region II confirms the formation of a new local structure, characterized by elongation of the P–P, As–As, and As–P bonds along the zigzag direction within the b-As x P1–x phase, possibly near the grain boundary. Additionally, a gray-As phase undergoes compressive structural changes. This study underscores the significance of pressure in inducing structural transformations and exploring novel phases in two-dimensional materials, including b-As x P1–x alloys.

Helicity-resolved Raman scattering of MoS2 bulk crystal.

Helicity-resolved Raman spectroscopy (HRRS) can effectively distinguish the Raman modes of two-dimensional (2D) layered materials by phonon symmetry. In this paper, we systematically investigated the phonon helicity selection of basal and edge planes of MoS2 bulk by HRRS. We find that the symmetry of the crystal structure changes the helicity selection of the E1g, E1 2g, and A1g modes in the edge plane. The theoretical calculation results confirm that the E1 2g and A1g modes of the basal plane exhibit a perfect helicity exchange, and the helicity selections of the E1 2g and A1g modes of the edge plane are eliminated or weakened. Our study provides references for phonon helicity selection of 2D layered materials represented by MoS2.

Interface modified MoS2/CNT with enhanced power factor via energy filtering effect for flexible thermoelectric applications

Transition metal dichalcogenides (TMDCs) has gained vast attraction in academia and industry due to their outstanding optical and electrical properties. Among them, molybdenum disulfide (MoS 2 ) is a promising material in the field of thermoelectric due to its high effective mass, van der Waal’s interaction and anisotropy. In this work, layered structure of molybdenum disulfide is prepared by hydrothermal method and coated on ITO/PET flexible substrate by doctor blade method. The thermoelectric properties of MoS 2 are improved with the effect of carbon nanotubes (CNTs) by fabricating the film via layer-by-layer assembly and heterojunction. The presence of ( E 1 2g and A 1g ) modes and (D, G and G ’ ) band from Raman spectra confirmed the formation of MoS 2 and CNT, respectively. Further the chemical composition is also confirmed from the presence of oxidation states XPS. The maximum achieved Seebeck coefficient ( S ), electrical conductivity (σ) and power factor ( S 2 σ ) values are -19.45 µV/K, 430 S/m and 160 nW/mK 2 at 315 K, respectively, for heterojunction. The achieved maximum power factor for heterojunction is due to the energy carrier filtering effect at the interface of MoS 2 and CNT. The current research work paves a new strategy to design a flexible thermoelectric device. • The MoS 2 and CNTs films was fabricated on the flexible PET/ITO substrate by layer-by-layer assembly and heterojunction using doctor blade method. • The energy filtering effect is a strategy for improving the electrical conductivity and Seebeck coefficient. • The heterojunction (HM) MoS 2 /CNTs film showed high Seebeck coefficient, electrical conductivity and power factor of -19.5 µV/K, 430 S/m and 162.8 nW/mK 2 .

Relationship between morphological and physical properties in nanostructured CuMnO2

In this study, crednerite CuMnO 2 nanostructures were prepared using a hydrothermal method at 100 °C with various amounts of NaOH mineralizator. Obtained nanostructured crednerite CuMnO 2 with monoclinic structure (space group C 2/ m ) exhibits two morphologies: nanobelts with lengths of 1–1.5 nm and thickness of 15–25 nm, and nanoplates with diameters of 50–70 nm. Comparative analyses of the prepared samples reveal a close relationship between morphological and physical properties of nanostructured CuMnO 2 . A low NaOH concentration promotes elongated crystal growth along the c-axis, resulting in nanobelt-shaped morphology. A strong base solution, on the other hand, promotes the formation of nanoplates. The different morphologies of nanostructured CuMnO 2 have different spectroscopic and magnetic properties. The Raman active A 1g mode at 637 cm −1 and a modified Curie-Weiss behavior characterize the nanobelt-shaped sample. This phase has two magnetic phase transitions: ferromagnetic at 9.2 K and antiferromagnetic at 42 K. The nanoplate-shaped sample, on the other hand, exhibits typical behavior as reported in the literature, namely the Raman active A 1g mode at 688 cm −1 and low-dimensional magnetism with antiferromagnetic ordering below 62 K. The variation in magnetic properties is presumably due to partial oxidation of Cu 1+ /Cu 2+ and Mn 3+ /Mn 4+ , as well as a change in Mn 3+ spin states from low-spin in nanobelt-shaped samples to high-spin in nanoplate-shaped samples. Both morphological and preferred crystal growth of nanostructured CuMnO2 can be tuned utilizing suitable amounts of NaOH mineralizator in the growth solution. A pure CuMnO2 phase of monoclinic structure has been prepared in the form of nanobelts with lengths up to 1.5 mm or nanoplates possessing 50–70 nm in diameter. The physical properties measurements imply distinct behaviour amongst these samples.The CuMnO2 nano-belts exhibit a Raman peak at a lower Raman number compared to that of the nanoplates. The nanobelt sample has been shown to be undergone two successive magnetic phase transitions: antiferromagnetically at 42 K and ferromagnetically at 9.2 K. The nanoplate samples have exhibited low-dimensional magnetic behaviour. The variation in the magnetic properties can be ascribed to the partial oxidation of Cu1+/Cu2+ and Mn3+/Mn4+. The variation of spin states of the Mn ions can be correlated with the change in the morphologies, i.e., from a low-spin state in the belt-shaped sample to a high-spin state in the plate-shaped samples. • Both morphological and preferred crystal growth of nanostructured CuMnO2 and then their physical properties can be tuned utilizing suitable amounts of NaOH mineralizator in the growth solution. Remarkably, a nanobelt CuMnO2 has been obtained for the first time. • Our experiments divulge that a pure CuMnO2 phase of monoclinic structure can been prepared in the form of nano-belts with lengths up to 1.5 mm or nanoplates possessing 50–70 nm in diameter. • The physical properties measurements imply distinct behaviour amongst these samples. The CuMnO2 nano-belts exhibits Raman peak at a lower Raman number compared to that of the nanoplates. • Regarding to the magnetic characteristics, the nanobelt samples have shown to be undergone into two successive magnetic phase transitions: antiferromagnetically at 42 K and ferromagnetically at 9.2 K. • The variation in the magnetic properties can be ascribed to the partial oxidation of Cu 1+ /Cu2+ and Mn 3+ /Mn4+. Remarkably, the variation of spin states of the Mn ions can be correlated with the change in the morphologies, i.e., from low-spin state in the belt-shaped sample to high-spin state in the plate-shaped samples.

Growth, characterization, and Raman spectra of the 1T phases of TiTe2, TiSe2, and TiS2

High-quality large 1T phase of TiX 2 (X = Te, Se, and S) single crystals have been grown by chemical vapor transport using iodine as a transport agent. The samples are characterized by compositional and structural analyses, and their properties are investigated by Raman spectroscopy. Several phonon modes have been observed, including the widely reported A 1g and Eg modes, the rarely reported Eu mode (∼183 cm−1 for TiTe2, and ∼185 cm−1 for TiS2), and even the unexpected K mode (∼85 cm−1) of TiTe2. Most phonons harden with the decrease of temperature, except that the K mode of TiTe2 and the Eu and “A 2u /Sh” modes of TiS2 soften with the decrease of temperature. In addition, we also found phonon changes in TiSe2 that may be related to charge density wave phase transition. Our results on TiX 2 phonons will help to understand their charge density wave and superconductivity.

Mode-selective excitation of an infrared-inactive phonon mode in diamond using mid-infrared free electron laser

The direct demonstration was achieved for the selective excitation of the infrared-inactive T 2g mode (1332 cm−1) in the single crystal diamond by two-photon excitation using a mid-infrared free electron laser (MIR-FEL) as pump light. Anti-Stokes Raman scattering light at around −1339 cm−1 was observed with cooling at 5 K only when MIR-FEL whose wavenumber was that of one-half of the T 2g mode, 666 cm−1, was irradiated. Update of the probe system, i.e. the introduction of the image intensified CCD, made it possible to probe the scattering signal with a lower laser power density compared to our previous study, and it relieved the surface damage on the sample accompanied by laser irradiations which induced a red-shift and a band broadening of the mode in the previous study.

Davydov Splitting, Resonance Effect and Phonon Dynamics in Chemical Vapor Deposition Grown Layered MoS2

We present comprehensive temperature dependent Raman measurements for chemical vapor deposition grown horizontally aligned layered MoS2 in a temperature range of 4–330 K under a resonance condition. Our analysis of temperature dependent phonon frequency shift and linewidth suggests a finite role of three and four phonon anharmonic effect. We observe Davydov splitting of the out-of-plane (A 1g ) and in-plane () modes for both three layer (3L) and few layer (FL) systems. The number of Davydov splitting components are found more in FL compared to 3L MoS2, which suggests that it increases with an increasing number of layers. Further, Davydov splitting is analyzed as a function of temperature. Temperature evaluation of the Raman spectra shows that the Davydov splitting, especially for A 1g mode, is very strong and well resolved at low temperature. We observe that A 1g mode shows the splitting at low temperature, while mode is split even at room temperature, which suggests a prominent role of A 1g mode in the interlayer interaction at low temperature. Further, an almost 60-fold increase in the intensity of the phonon modes at low temperature clearly shows the temperature dependent tuning of the resonance effect.

Highly Robust Reentrant Superconductivity in CsV3Sb5 under PressureSupported by the National Key Research and Development Program of China (Grant Nos. 2017YFA0304700, 2018YFE0202601, and 2016YFA0300600), the National Natural Science Foundation of China (Grant Nos. 51922105, 11804184, 11974208, and 51772322), the Chinese Academy of Sciences (Grant No. QYZDJ-SSW-SLH013), the Beijing Natural Science Foundation (Grant No. Z200005), and the Shandong Provincial Natural Science

We present the superconducting (SC) property and high-robustness of structural stability of kagome CsV3Sb5 under in situ high pressures. For the initial SC-I phase, its T c is quickly enhanced from 3.5 K to 7.6 K and then totally suppressed at P ∼ 10 GPa. With further increasing pressure, an SC-II phase emerges at P ∼ 15 GPa and persists up to 100 GPa. The T c rapidly increases to the maximal value of 5.2 K at P = 53.6 GPa and slowly decreases to 4.7 K at P = 100 GPa. A two-dome-like variation of T c in CsV3Sb5 is concluded here. The Raman measurements demonstrate that weakening of E 2g mode and strengthening of E 1g mode occur without phase transition in the SC-II phase, which is supported by the results of phonon spectra calculations. Electronic structure calculations reveal that exertion of pressure may bridge the gap of topological surface nontrivial states near E F, i.e., disappearance of Z 2 invariant. Meanwhile, the Fermi surface enlarges significantly, consistent with the increased carrier density. The findings here suggest that the change of electronic structure and strengthened electron-phonon coupling should be responsible for the pressure-induced reentrant SC.

Temperature-dependent Raman modes of MoS2/MoSe2 van der Waals heterostructures

Two-dimensional (2D) monolayer transition metal dichalcogenides (TMDs) show extra-ordinary properties compared to their bulk forms, which has inspired a large number of researchers to investigate these materials recently. Design and fabrication of different combinations of 2D TMDs layers can allow for high-performance and novel heterostructure-based devices, of which the performance will depend also on their thermal properties. On the other hand, the temperature-dependent behavior of such heterolayers and their interaction at different temperatures is still not comprehensively studied in a wide temperature range. In this work, we have performed a systematical temperature-dependent (83 K—483 K) Raman spectroscopic analysis of the MoS2/MoSe2 van der Waals (vdW) heterostructures and discussed their stability. After the transfer process of the MoS2 monolayers onto the MoSe2/SiO2/Si, we annealed the samples, which is a commonly used process to increase the crystallinity. Associatively, the thermal annealing process leads to a decrease in the thermal coefficients of the E1 2g and A1g modes of MoS2 and MoSe2 monolayers. Our study shows that the peak positions of the Raman modes in the heterostructures redshift with an increase in temperature. Furthermore, the full width at half maximum (FWHM) of the E1 2g and A1g modes of the layers broaden at higher temperatures. This phenomenon is attributed to increasing phonon–phonon interactions and thermal expansion effects with the ascending temperature. To the best of our knowledge, for the first time, temperature-dependent Raman analysis of MoS2/MoSe2 vdW heterostructures before and after annealing are carried out; and peak positions, FWHMs, and thermal coefficients of the layers are extracted. We do not observe any deformation in the heterobilayer structure even at very low (83 K) or very high temperatures (483 K). This is the first step to confirm the durability of the MoS2/MoSe2 heterolayered devices under extreme temperatures by studying their thermal properties.

γ-ray Irradiation-Induced Chemical and Structural Changes in CVD Monolayer MoS2

Two dimensional (2D) materials are the ideal choice for highly efficient, lightweight, low power consumption nanoelectronics in space science applications. Therefore, investigation of radiation hardness of 2D-materials is of great interest. Herein, we report the effect of gamma irradiation on chemical vapor deposition (CVD) synthesized crystalline monolayer (1 L) molybdenum disulfide (MoS2) flakes on the sapphire substrate at various doses, viz., 1–1000 kGy. We estimated that the temperature of MoS2 increases by 1.3 °C per kGy of gamma-ray dose. It was observed that up to 130 kGy (∼195 °C), gamma exposure has no significant impact on the morphology and chemical properties of MoS2. However, X-ray photoelectron spectroscopy (XPS) shows that from 275 kGy (∼385 °C), MoS2 starts converting into MoOx. Raman spectroscopy shows that at low gamma doses, the crystallinity of MoS2 increases while at higher doses, a traceable amount of MoS2 starts converting into amorphous MoS3. Our findings show that both E1 2g and A1g Raman mode of MoS2 blue shift as gamma dose increases owing to the combined effect of sulfur vacancies creation and strain caused by thermal expansion of MoS2.

The effect of battery charge levels of Mobile phone on the amount of Electromagnetic waves emission

Over the past decades, mobile phone usage have increased dramatically. Extensive development and use of mobile telecommunication services has increased exposure to radio frequency electromagnetic waves (RF-EMW) in the daily lives of humans, and concerns about the harmful effects of mobile phones have also increased on human health. Therefore, this study aimed to investigate the effect of battery charge levels of the mobile phone on electromagnetic waves emission. The mobile phone used in the current study was HTC One E9+ (0.181W/kg SAR) with a non-removable battery model Li-Po 2800 mAh. The power density was measured with the mobile phone set to operate at the 2G mode by a SMP2 Portable Electromagnetic Field Monitoring System. Power density was measured in Calling mode (50 sec), Called mode (40 sec) and Talking mode (360 sec) at the battery charge levels of 1, 5, 10, 15, 20, 30, 50, 60, 70, 80 and 100%. In Calling mode, the maximum electromagnetic waves were determined when the mobile phone had 1% battery charge and also while it was being charged. Contrary to Calling mode, there is no statistically significant difference between the power density emitted in Called mode and Talking mode at the various battery charge levels. Power density was found to be highest in the Called mode (29.11 μw/cm2), and to be higher in the Talking mode (23.005 μw/cm2) than in the Calling mode (10.27 μw/cm2). The data of the present study can be used to monitor the daily exposure of mobile phone users as well as to estimate exposure levels in the laboratory and non-laboratory studies. As long as a mobile phone that is in the standby mode remains within the geographic domain of the operator's service zone, the power density emitted from that phone will be virtually zero, and any background wave can be attributed to other sources.

Electrochemical Properties of Nano-Sized MoS2 Positive-Electrodes in Ester-Based Electrolyte Solutions for Magnesium Rechargeable Batteries

Magnesium rechargeable batteries (MRBs) have attracted attentions as next-generation secondary batteries. Mg metal is a promising negative-electrode material because it has a high volumetric capacity (3830 mAh cm-3) and a low standard potential (-2.37 V vs. NHE). In addition, MRBs ensure a higher safety compared to lithium metal rechargeable batteries, since Mg metal forms no dendrite. We previously reported that g-butyrolactone (GBL)-based electrolyte solutions allowed the electrochemical deposition/dissolution reactions of Mg and the insertion of Mg2+ into Chevrel phase Mo6S8 positive-electrodes. Mo6S8 is a prototypical active material that has a wide path for rapid diffusion of Mg2+. However, it works at extremely low potentials (1.1 V vs. Mg2+/Mg), and has a small theoretical capacity (129 mAh g-1). On the other hand, MoS2, a popular member of metal dichalcogenides, has a larger theoretical capacity (223.2 mAh g-1) at a higher working voltage around 1.8 V 1). In this work, we synthesized MoS2 nanoparticles with a short diffusion path of Mg2+, and investigated the charge and discharge properties of AZ31|MoS2 cells using 0.3 mol dm-3 Mg(CF3COO)2 in GBL as the electrolyte solution. MoS2 nanoparticles were synthesized by a solvothermal method 2). Mo(CO)6 powder and sulfur powder were mixed into 120 ml 2-propanol as a solvent. The solutions were stirred under heating at 80oC for 10 h, and then the solution was cooled to room temperature naturally. The dark powders were anneled at 800oC for 2h under Ar atmosphere. The resultant powder was characterized by an X-ray diffraction and Raman spectroscopies. The morphology was observed with a scanning electron microscope. Electrochemical measurements were conducted at a constant current of ca. 5-7 mA cm-2 at 100oC using a three-electrode cell. The counter and reference electrode were AZ31 foil (Mg alloy). After the initial discharge and charge, the MoS2 electrode was analyzed by Raman spectroscopy. The resultant MoS2 was impurity-free, and spherical in shape with a diameter of 80-100 nm. The lattice fringe of 0.623 nm corresponds to the spacing of the MoS2 (002) planes. The Raman bands at 378 cm-1 and 405 cm-1 were clearly observed, corresponding to in-plane vibrational E1 2g mode and out-of-plane vibrational A1g mode, respectively 3 ). The initial discharge capacity of AZ31|MoS2 cell was ca. 180 mAh g-1. After the intial discharge, the Raman bands shifted to higher frequencies of 382 cm-1 and 407 cm-1, which is quite similar to those for the insertion of Li+ into MoS2 4). After initial charge, each Raman band returned to its original position. These results indicated that the insertion/extraction reactions of Mg2+ at the MoS2 occur in the GBL-based electrolyte solutions. Reference s : 1) Liang, et al., Adv. Mater., 23, 640-643 (2011). 2) Tao, et al., CrystEngComm, 14, 3027-3032 (2012). 3) Lee, et al., ACN Nano, 4 (5), 2695-2700 (2010). 4) Wang, et al., PNAS, 49, 19701-119706 (2013)

Electronic and vibrational properties of PbI2 : From bulk to monolayer

Using first-principles calculations, we study the dependence of the electronic and vibrational properties of multi-layered PbI 2 crystals on the number of layers and focus on the electronic-band structure and the Raman spectrum. Electronic-band structure calculations reveal that the direct or indirect semiconducting behavior of PbI 2 is strongly influenced by the number of layers. We find that at 3L-thickness there is a direct-to-indirect band gap transition (from bulk-to-monolayer). It is shown that in the Raman spectrum two prominent peaks, A 1g and E g , exhibit phonon hardening with increasing number of layers due to the inter-layer van der Waals interaction. Moreover, the Raman activity of the A 1g mode significantly increases with increasing number of layers due to the enhanced out-of-plane dielectric constant in the few-layer case. We further characterize rigid-layer vibrations of low-frequency inter-layer shear (C) and breathing (LB) modes in few-layer PbI 2 . A reduced mono-atomic (linear) chain model (LCM) provides a fairly accurate picture of the number of layers dependence of the low-frequency modes and it is shown also to be a powerful tool to study the inter-layer coupling strength in layered PbI 2 .

Influence of inverse spinel structured CuGa2O4 on microwave dielectric properties of normal spinel ZnGa2O4 ceramics

AbstractSpinel Zn1‐xCuxGa2O4 (x = 0‐0.15) ceramics were prepared by the conventional solid‐state method. Only a single phase was indexed in all samples. The continuous lattice contraction of ZnGa2O4 unit cell was caused by Cu2+ substitution, and the lattice parameter shows a linear correlation with the content of Cu. The refined crystal structure parameters suggest that Cu2+ preferentially occupies the octahedron site, and the degree of inversion of Zn1‐xCuxGa2O4 (x = 0‐0.15) ceramics almost equals to the content of Cu2+. The relative intensity of A*1g mode in Raman spectra confirm that the degree of inversion climbed with the growing content of Cu2+. The experimental and theoretical dielectric constant of Zn1‐xCuxGa2O4 ceramics fit well. Zn1‐xCuxGa2O4 (x = 0.01) ceramics sintered at 1400°C for 2 h exhibited good microwave dielectric properties, with εr = 9.88, Q × f = 131,445 GHz, tanδ = 6.85 × 10−5, and τf = −60 ppm/°C.

Cation distributions and microwave dielectric properties of Cu-substituted ZnGa2O4 spinel ceramics

Abstract Spinel Zn1-xCuxGa2O4 (x = 0–0.05) ceramics were prepared by the conventional solid-state method. Only a single phase was indexed in all samples. The relative density increased with Cu-substitution. Refined crystal structure parameters suggested that Cu2+ preferentially occupies the octahedron site and formed inverse spinel structure. The relative intensity of A*1g mode in Raman spectra confirmed that the inversion degree climbed with the growing content of Cu. The differences of ion dielectric polarizability resulted in the shift of absorption bands in Fourier transform infrared (FT-IR) spectroscopy. Ion dielectric polarizability and cell volumes differences affected permittivity (er) and increased er slightly during Cu-substitution. The temperature coefficient of resonant frequency (τf) kept steady, however, the quality factor (Q × f) value of Zn1-xCuxGa2O4 ceramics increased by 50% due to the Cu-substitution (from 85,824 GHz to 131,445 GHz). Zn0.99Cu0.01Ga2O4 ceramic s sintered at 1400 °C for 2 h exhibited good microwave dielectric properties, with er = 9.88, Q × f = 131,445 GHz, tan δ = 6.85 × 10−5,and τf = −60 ppm/°C.