B1g Mode Research Articles

On the role of disorder produced by manganese vacancy at the B site on the structural and magnetic properties of [formula omitted] nanocrystalline

Single-phase perovskite-type manganese oxides La0.67Ba0.33Mn1−xO3 with x = 0.00–0.08 have been prepared by the modified sol–gel method (Pechini) and the influence of the vacancy concentration in the B-site on their structures and magnetic properties has systematically been investigated. The structural elucidation of the synthesized materials was carried out by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy. Rietveld analysis of fitted and observed X-ray diffraction patterns exhibited the single-phase nature of all the studied materials, which crystallize in R3¯c space group. The average crystallite size was found to be around 56 nm. FTIR spectra present the prominent absorption peak of the in-phase stretching mode (B2g mode) rising from the vibration of MnO bond length. Raman spectra at room temperature reveal a gradual change in phonon modes with increasing B-site vacancy concentration. All the samples undergo a ferro-to paramagnetic transition, at temperatures that vary from 335 to 216 K as the manganese vacancy increases. As follows from the results of structural and magnetic analyses, creating a vacancy at the Mn site (increase of chemical disorder) induces the formation of Mn4+ ions, which results in the decrease of unit cell volume and leads to a shortage of 3d electrons of the transition metal.

Sol–Gel Grown SnO2 Nanoparticles: Evaluation of Structure, Morphology, and Raman Spectra

Tin oxide (SnO2) nanoparticles (TONPs) were prepared using sol–gel method under different growth conditions. The influence of calcination temperature (450°C and 600°C) and molecular weight of polyethylene glycol (PEG 300 and PEG 4000) on the nanocrystallinity, surface morphology, and Raman spectra of as-prepared TONPs were evaluated. Variation of calcination temperature and dopant (sulfosuccinic acid, SA) was found to affect considerably the structure, surface morphology, and Raman activities of the TONPs. The size of TONPs estimated using Scherrer equation was discerned to be in the range of 15–32 nm. The observed intensity enhancement in the Raman vibrational modes at lower calcination temperature was attributed to the enlargement of TONPs size. The absorption of molecules at the TONPs surface led to a quenching in the A 2 g and Eu Raman peaks. Raman peaks centered around 673 cm−1, 799 cm−1, 640 cm−1 , and 432 cm−1 corresponding to A1g, B2g, A1g, and Eg modes, respectively have manifested highest peaks intensity. Furthermore, the enhancement of the Eg mode due to the addition of SA dopant was ascribed to the Jahn–Teller distortion mechanism.

Modifications of the structure and magnetic properties of ceramic YCrO3 with Fe/Ni doping

In this manuscript, we have investigated the effect of Fe and Ni doping on the structure and magnetic properties of YCr1−xMxO3 ceramics (M = Fe, Ni and x = 0, 0.1). X-ray diffraction analysis of the samples accompanied with, Rietveld refinement suggested no change in the structure upon doping, with structure of the samples being orthorhombic (space group: Pnma). Raman spectroscopic analysis of the samples revealed that doping induced disorder leads to broadening of the certain Raman modes of the system. While, both B3g(5) and B1g(3) modes are broadened in Ni and Fe doped samples, in addition Fe doped samples also show broadening of B1g(4) mode. In doped samples a new mode, A1g(3) appears due to the induced lattice disorder. Temperature dependent magnetic measurements suggested a negative value of Curie–Weiss temperature (θcw) indicating that all the samples are antiferromagnetic. However, the Neel temperature (TN) increased for Fe doping and decreased with Ni doping. These changes in the Neel temperature upon doping can be correlated to the changes in the nearest neighbor and next nearest neighbor exchange interactions.

Raman Spectroscopic Observation of Gradual Polymorphic Transition and Phonon Modes in CuPc Nanorod

A Raman scattering investigation of a single α-CuPc nanorod is demonstrated here within the temperature range 300–770 K. From the typical thermal dispersion of Raman shift and phonon line width of in-plane B1g mode at 1526 cm–1, the metastable α polymorph is observed to undergo unambiguous phase transition to thermally stable β phase. The phase transition temperature (456 K) is observed to be significantly lower than that reported for bulk samples. Extensive computation of normal modes associated with both α- and β-CuPc reveal that the “fingerprint” domain across 1300–1600 cm–1 is particularly sensitive to this polymorphic transition where a silent B2g mode appears at 1303 cm–1 in the spectral vicinity of molecular edge vibration sensitive Ag–Bg Davydov doublet.

Determination of Crystal Axes in Semimetallic T′‐MoTe2 by Polarized Raman Spectroscopy

Distorted octahedral T′ phase of MoTe2 has recently attracted significant interest due to its predicted topological states and novel charge transport properties. Here, we report a nondestructive method for determining the crystal orientation of few‐layer T′‐MoTe2 flakes by polarized Raman spectroscopy. The experimentally observed Raman modes are assigned to eigenmodes of vibrations predicted by density functional theory calculations. Polarized Raman measurements reveal four distinct types of angle‐dependent intensity variations. From group theory, it can be deduced that the intensity of the Bg mode reaches a maximum in the configuration when the polarization vector of the incident light is either parallel or orthogonal to the metal–metal zigzag chain direction. The intensity variation of the Bg mode cannot be used to unambiguously determine the crystal orientation. Using electron diffraction analysis, it is demonstrated that the intensity of the Ag mode at around 162 cm−1 reaches a maximum when the polarization vector of the incident light is parallel to the metal–metal chain direction in the configuration. Furthermore, a simple method is proposed for identifying crystal orientation in nonpolarized Raman spectroscopy.

Temperature-Dependent Raman Responses of the Vapor-Deposited Tin Selenide Ultrathin Flakes

Tin selenide (SnSe) is a newly emerging layered material. SnSe with low dimensionality has been reported as an appealing material with a diverse range of applications such as rechargeable lithium-ion batteries, memory switching devices, solar energy conversion, thermoelectric energy conversion, and near-infrared optoelectronic devices. Here we synthesized SnSe ultrathin flakes on SiO2/Si substrates through simple vapor deposition route and investigated its temperature-dependent Raman spectroscopy behavior with 532, 633, and 785 nm excitation wavelengths. It was found that the Ag2, Ag3, and B3g modes soften as temperature increases from 98 to 298 K under all wavelengths. Our results revealed anharmonic phonon properties of SnSe, and they will benefit the advanced study of its thermal properties. This approach will also have large application in characterizing the optical, electronic, and thermal properties of other novel layered materials. Moreover, our results can be utilized to measure the temperature of...

In‐situ isothermal micro‐Raman spectroscopy reveals the activation energy of dehydration in α‐FeOOH

A real‐time, isothermal micro‐Raman spectroscopy was developed as a single technique for monitoring the solid state kinetics of dehydration and measuring its activation energy. The technique comprises of acquisition and numerical analysis of a set of isothermal time‐dependent Raman spectra. α‐FeOOH transformation to Fe2O3 in N2 atmosphere was continuously monitored for up to 16 h at three temperatures. Dehydration activation energy of 1.4(1) eV was estimated. The Raman band evolution of two modes with different symmetries (B1g ~ 240 cm−1 and Ag ~ 400 cm−1) and the intensity decay of B1g mode at ~540 cm−1 are analyzed independently utilizing kinetic model‐free approach. The study opens new possibilities for developing versatile solid state chemical and bio‐chemical Raman sensors and advances the field of analytical micro‐Raman spectroscopy. Copyright © 2017 John Wiley & Sons, Ltd.

Effect of Fe or Ni doping on the structure and magnetic properties of YCrO<sub>3</sub> ceramics

In this report, the effect of dopant on the structure and magnetic properties of YCr1-xMxO3 (M = Fe, Ni and x = 0, 0.1) have been investigated by room temperature Raman scattering, X-ray diffraction and temperature variation of magnetic measurements. Compositional modifications have been made by doping of Fe/Ni at Cr-site. Both for Fe and Ni doping, the Rietveld refinement of X-ray diffraction pattern showed that structure is orthorhombic with Pnma space group. In the magnetic measurement, the negative value of Curie-Weiss temperature (θcw) indicates all are antiferromagnetic system. The Neel temperature (TN) increases for Fe doping while decreases with Ni doping which in effect Fe doping increases effectively both the J1 and J2 exchange interactions between the Cr and Cr ions, on the other hand, Ni doping decrease both the exchange interactions. In the Raman study, doping induced structural disorder is clearly reflected as in the form of broadening of the certain Raman modes of the system; when Ni is doped, B3g(5) and B1g(3) modes become broader and in case of Fe-doped sample shows significant broadening in B3g(5), B1g(4) and B1g(3) modes whereas A1g(3) mode appear due to the induced lattice disorder.

Effects of Al2O3 capping layers on the thermal properties of thin black phosphorus

We investigate the thermal properties of thin black phosphorus (BP) with Al2O3 capping layer using the temperature-dependent and polarized-laser power-dependent Raman spectroscopy. Compared to the BP samples without Al2O3 capping layer, the Al2O3 passivation layer significantly improves the thermal stability of BP by reducing the thermal coefficients of the Ag1, B2g, and Ag2 Raman modes from −0.0082, −0.0142, and −0.0145 cm−1/K to −0.0046, −0.0074, and −0.0088 cm−1/K, respectively, which are attributed to the compressive strain and strong Al-P and O-P bonds. Meanwhile, the thermal conductivity reaches to about 45.4 and 54.4 W/mK along the armchair and zigzag directions, greatly larger than those of the BP films without Al2O3 24.1 and 39.0 W/mK, respectively, owing to the large thermal conductivity of Al2O3 and the interface charges between Al2O3 and BP. Overall, this work will contribute to improve the BP-based device performances and extend the BP applications profoundly.

Influence of magnetic ion doping on structural, optical, magnetic and hyperfine properties of nanocrystalline SnO2 based dilute magnetic semiconductors

This article reports the structural, optical and magnetic properties of transition metal (Ni, Co, Mn and Fe) doped SnO2 nanoparticles prepared by modified Pechini sol–gel method. From the X-ray diffraction studies, it is obvious that all the synthesized samples show a phase purity of rutile tetragonal crystal structure of SnO2. The morphology was studied and the particle sizes were estimated from the field emission scanning electron microscopy. From photoluminescence spectra, we observed emission due to the presence of singly ionized oxygen vacancies. Raman spectroscopy shows dominant peaks at 644 and 782 cm−1 which were ascribed to A1g and B2g modes of the rutile structure. Isomer shifting due to dopant addition and large quadrupole splitting due to surface defects were observed in Mossbauer spectra. All the samples show ferromagnetic ordering up to 1 T. The relatively stronger ferromagnetic nature in Fe and Co doped SnO2 is due to the strong p–d exchange interaction. In case of Ni and Mn doped SnO2 samples, the lack of carrier-mediated interaction due to its inherent semiconducting nature reduces the total magnetic moment observed in these samples. The exchange coupling depends on the dopant type and its concentration.

Temperature-dependent phonon shifts in atomically thin MoTe2 nanosheets

The present investigations deal with temperature-dependent Raman spectroscopy of bi-layer and few-layer molybdenum telluride (MoTe2) nanosheets prepared by micromechanical exfoliation method on a 300nm SiO2/Si substrate. The temperature-dependent Raman spectroscopy was carried out using 633nm laser excitation over the wider temperature range of 123–523K. The temperature-dependent study illustrates the softening of A1g, E2g1 and B2g modes as temperature increases from 123K to 523K. The calculated temperature coefficients for A1g, E2g1 and B2g modes of the bi-layer MoTe2 nanosheet sample were found to be −0.00911cm−1K−1, −0.01297cm−1K−1 and −0.0159cm−1K−1, respectively. For few-layer MoTe2 nanosheet sample, the temperature coefficient values were found to be −0.0113cm−1K−1 (A1g), −0.0129cm−1K−1 (E2g1) and −0.0149cm−1K−1 (B2g), respectively. The present work highlights the temperature-dependent lattice vibration consequence of 2D MoTe2 nanosheet, which can be fundamentally pertinent in other promising and emerging 2D ultrathin layer and heterostructured materials to be used in future optoelectronic and nanoelectronic devices.

Raman spectroscopy of KxCo2−ySe2 single crystals near the ferromagnet–paramagnet transition

Polarized Raman scattering spectra of the KxCo2−ySe2 single crystals reveal the presence of two phonon modes, assigned as of the A1g and B1g symmetry. The absence of additional modes excludes the possibility of vacancy ordering, unlike in KxFe2−ySe2. The ferromagnetic (FM) phase transition at K leaves a clear fingerprint on the temperature dependence of the Raman mode energy and linewidth. For the temperature dependence looks conventional, driven by the thermal expansion and anharmonicity. The Raman modes are rather broad due to the electron–phonon coupling increased by the disorder and spin fluctuation effects. In the FM phase the phonon frequency of both modes increases, while an opposite trend is seen in their linewidth: the A1g mode narrows in the FM phase, whereas the B1g mode broadens. We argue that the large asymmetry and anomalous frequency shift of the B1g mode is due to the coupling of spin fluctuations and vibration. Our density functional theory (DFT) calculations for the phonon frequencies agree rather well with the Raman measurements, with some discrepancy being expected since the DFT calculations neglect the spin fluctuations.

Temperature induced phonon behaviour in germanium selenide thin films probed by Raman spectroscopy

Here we report a detailed study of temperature-dependent phonon properties of exfoliated germanium selenide thin films (several tens of nanometers thick) probed by Raman spectroscopy in the 70–350 K temperature range. The temperature-dependent behavior of the positions and widths of the Raman modes was nonlinear. We concluded that the observed effects arise from anharmonic phonon–phonon interactions and are explained by the phenomenon of optical phonon decay into acoustic phonons. At temperatures above 200 K, the position of the Raman modes tended to be linearly dependent, and the first order temperature coefficients χ were −0.0277, −0.0197 and −0.031 cm−1 K−1 for B3g, Ag(1) and Ag(2) modes, respectively.

Anomalously enhanced thermal stability of phosphorene via metal adatom doping: An experimental and first-principles study

Atomically thin black phosphorus, also known as phosphorene, is an emerging two-dimensional (2D) material, which has attracted increasing attention due to its unique electronic and optoelectronic properties. However, the reduced thermal stability of phosphorene limits its suitability for high-temperature fabrication processes, which could be detrimental for the performance of phosphorenebased devices. Here, we investigate the impact of doping by Al and Hf transition metal adatoms on the thermal stability of phosphorene. The formation of Al–P covalent bonds was found to significantly improve the thermal coefficients of the A g 1 , B2g, and A g 2 phonon modes to 0.00044, 0.00081, and 0.00012 cm–1·°C–1, respectively, which are two orders of magnitude lower than those observed for pristine P–P bonds (~0.01 cm–1·°C–1). First-principles calculations within the density functional theory framework reveal that the observed thermal stability enhancement in the Al-doped material reflects a significantly higher Al binding energy, due to the stronger Al–P bonds compared to the weak van der Waals interactions between adjacent P atoms in the undoped material. The present work thus paves the way towards phosphorene materials with improved structural stability, which could be promising candidates for potential nanoelectronic and optoelectronic applications.

Metallic monoclinic phase in VO2 induced by electrochemical gating: In situ Raman study

We report in situ Raman scattering studies of electrochemically top gated VO2 thin film to address metal-insulator transition (MIT) under gating. The room temperature monoclinic insulating phase goes to metallic state at a gate voltage of 2.6 V. However, the number of Raman modes do not change with electrolyte gating showing that the metallic phase is still monoclinic. The high-frequency Raman mode Ag(7) near 616 cm−1 ascribed to V-O vibration of bond length 2.06 Å in VO6 octahedra hardens with increasing gate voltage and the Bg(3) mode near 654 cm−1 softens. This shows that the distortion of the VO6 octahedra in the monoclinic phase decreases with gating. The time-dependent Raman data at fixed gate voltages of 1 V (for 50 minutes, showing enhancement of conductivity by a factor of 50) and 2 V (for 130 minutes, showing further increase in conductivity by a factor of 5) show similar changes in high-frequency Raman modes Ag(7) and Bg(3) as observed in gating. This slow change in conductance together with Raman frequency changes show that the governing mechanism for metalization is more likely due to the diffusion-controlled oxygen vacancy formation due to the applied electric field.

Giant Anisotropic Raman Response of Encapsulated Ultrathin Black Phosphorus by Uniaxial Strain

The giant anisotropic Raman response of encapsulated ultrathin black phosphorus (BP) is reported by uniaxial strain. A modified bending technique is employed to apply precise uniaxial tensile strain along the zigzag or armchair direction of the ultrathin BP encapsulated by a layer of polymethyl methacrylate. The Raman shift rates of the A 1 g, B 2g, and A 2 g modes are significantly distinct for strain applied along different directions. For the strain applied along zigzag direction, the Raman shift rate of the B 2g mode can reach a remarkable value of ≈−11 cm−1/% strain. In addition, the Grüneisen parameter is as high as ≈2.5, which is the largest among all the reported common 2D materials. Density functional perturbation theory calculations are performed to understand the exceptional anisotropic strain response discovering that not only the bond lengths but also the bond angels are changed in the strained ultrathin BP, which lead to the giant anisotropic Raman response. Furthermore, an alternative method based entirely on the strained ultrathin BP and nonpolarized Raman spectroscopy is demonstrated to determine the crystallographic orientations of ultrathin BP. This work paves a way to study the strain‐induced anisotropic electrical conductance and magnetotransport properties of BP.

Magnetic, structural and optical behavior of cupric oxide layers for solar cells

Room temperature ferromagnetic hysteresis is observed in CuO nano films synthesized through chemical strategy. The room temperature ferromagnetic behavior in CuO is quite a new observation without any doping, annealing or any other post preparation treatments. The observed magnetization value is 0.24 T which is obviously depicted that the nano-scale magnetic properties dependent on the shape and size of nano films. The morphological observations is depicted that the uneven ellipsoidal grains transformed to nano-sheet shaped grains. Atomic force microscopy (AFM) images are also strengthening the observation of shape variations due to bath temperature variation. The surface profile and grain contour images are verified the size and shape variations. Structural variations are precisely predicted by X-ray diffraction patterns and crystallinity of the films is discussed. The growth mechanism of nano sheet formation and physical phenomena are discussed. The luminescence properties variations are observed by photoluminescence (PL) spectroscopy and it has exposed that the bath temperature effects on the CuO films. X-ray photoelectron spectroscopy (XPS) spectra is depicted that Cu 2p3/2, Cu 2p1/2 and O1s related peaks at 929.50, 952.80 and 529.38 eV binding energy, respectively. In addition, the optical dielectric dispersion parameters such as effective mass (me), optical susceptibility (χe), carrier concentration (N), plasma frequency (ωp), relaxation time (τ) and frequency (f) are also evaluated. Raman scattering spectra revealed the Ag and Bg modes shift at 296 and 628 cm−1.

Ferroelastic nature of high pressure phase transition in MgF2

Using quantum theoretical simulations based on density functional and density functional perturbation theory we illuminate the nature of the phase transition occurring in magnesium fluoride (MgF2) at high pressure. At atmospheric pressure it is stable in the rutile structure. It was predicted by theory and later confirmed by experiments that a phonon soft mode driven phase transition transforms MgF2 to a CaCl2-type structure. The phase transition was connected with an optic phonon mode B1g. By simulating the behavior of phonons at and near the Γ point we prove that a lattice dynamical instability originates in an unstable acoustic phonon mode. The calculation of the elastic constants shows that the phase transition is a proper ferroelastic phase transition occurring under a homogenous deformation of the crystal lattice. These results further illustrate the behavior of fluoride materials under extreme conditions, such as pressure.

Raman scattering studies on the collapsed phase of CaCo2As2**Project supported by the National Basic Research Program of China (Grant No. 2012CB921701), the National Natural Science Foundation of China (Grant No. 11474357), and the Fundamental Research Funds for the Central Universities, and the Research Funds of Renmin University of China.

In this work, Raman scattering measurements have been performed on the collapsed phase CaCo2As2 crystals. At least 8 Raman modes were observed at room temperature though CaCo2As2 is structurally similar to other 122 compounds like BaFe2As2. Two Raman modes are assigned to the intrinsic A1g and B1g of this material system respectively. The other ones are considered to originate from the local vibrations relevant to cobalt vacancies. Careful polarized measurements allow us to clearly resolve the four-fold symmetry of the B1g mode, which put strong constraints on possible point group symmetries of the system with Co vacancies. The temperature-dependent measurements demonstrate that the anomalies in both frequency and width of the B1g mode occur around Neel temperature TN. The anomalies are considered to be related to the gap opening near the magnetic transition. The study may shed light on the structural and magnetic changes and their correlations with superconductivity in 122 systems.

(Invited) Strain Engineering in Ultrathin Black Phosphorus

We report the giant anisotropic strain response in vibrational modes of ultrathin black phosphorus (BP). A modified bending technique was employed to apply precise uniaxial tensile strain to zigzag or armchair directions of the BP flakes. The Raman shift rates of A1 g, B2g and A2 g modes of BP are significantly distinct for strain applied along zigzag or armchair directions. For the applied strain along zigzag direction, the Raman shift rate and the Grüneisen parameter of the B2g mode can reach a remarkable value of ~-11 cm-1/% strain and ~3.7 respectively, which are the largest among all the reported values in two-dimensional materials. The density functional theory calculations were performed to understand the exceptional anisotropic strain response in the vibrational modes of BP. It is found that not only the bond lengths but also the bond angels are changed in BP upon applied strain which lead to giant anisotropic response in mode B2g . Furthermore, we demonstrate a simple method based entirely on the strained BP and conventional Raman spectroscopy to determine the crystallographic orientations of BP without the aid of polarization Raman spectroscopy. This work may pave a way to study the strain-induced anisotropic electrical conductance and the magnetotransport properties of BP.