Temperature-dependent Raman Spectra Research Articles

Influence of Co Doping on the Structural, Dielectric and Raman Properties of Ba0.75Sr0.25Ti1−xCoxO3

We synthesized Ba1−xSrxTiO3 (BST) and Co doped Ba0.75Sr0.25CoxTi1−xO3 (BSCTx) compositions (x = 0.03, 0.05, 0.07) by the solid state reaction route and probed the effect of Co-doping on the structural, microstructural, dielectric and phonon behaviour. Room temperature x-ray diffraction patterns of sintered ceramics reveal perovskite type average cubic crystalline structure (space group Pm-3m) for all doped compositions, whereas, pure BST composition stabilizes in the tetragonal crystalline phase (space group P4mm). Absence of any impurity peaks in the doped samples reveals that Co-ions are entering into the lattice without any metal clustering. Quantitative full profile fitting using Reitveld refinement fits well in the cubic phase giving a small value of goodness of the fitting parameter (Rwp). The real part of the dielectric permittivity as well as dielectric loss (tan δ) decreases with Co-doping indicating that intrinsic defects get annealed with doping. \( (\varepsilon^{\prime}/\varepsilon^{\prime\prime} - T) \) plots clearly show the cubic to tetragonal phase transition in BST at around 50°C, whereas Co-doped samples show no such transition in the temperature range studied. Temperature dependent Raman spectra reveal the characteristic phonon modes associated with the tetragonal phase at room temperature, whereas, Co-doped samples exhibit only bands associated with local disorder in the cubic phase. The spectral features of tetragonal phase appear around − 10°C in the 3% Co-doped system indicating the decrease of transition temperature with Co-doping. The analysis of deconvoluted Raman parameters using two sub-lattice models clearly reveals the structural disorder due to doping with slightly distorted TiO6 octahedra getting almost perfect thus giving rise to a cubic phase in the doped system.

Doping induced dielectric anomaly below the Curie temperature in molecular ferroelectric diisopropylammonium bromide.

A dielectric anomaly induced by doping has been observed at about 340 K in chlorine-doped diisopropylammonium bromide. The dielectric anomaly has a switchable behaviour, which indicates potential applications on switches and sensors. Temperature-dependent Raman spectrum, X-ray diffraction and differential scanning calorimetry do not show any anomaly around the dielectric anomaly temperature, which prove that the dielectric anomaly does not come from structure phase transition and has no specific heat variety. It is assumed that this dielectric anomaly can be attributed to the freezing of ferroelectric domain walls induced by the pinning of point defects.

Temperature-dependent Raman spectroscopic study of ferroelastic K2Sr(MoO4)☆☆Project supported by the Natural Science Foundation of Anhui Province, China (Grant Nos. KJ2018A0588 and KJ2017A625).

Raman scattering measurements of K2Sr(MoO4)2 were performed in the temperature range of 25–750 °C. The Raman spectrum of the low-temperature phase α-K2Sr(MoO4)2 that was obtained by first-principle calculations indicated that the Raman bands in the wavenumber region of 250–500 cm−1 are related to Mo–O bending vibrations in MoO4 tetrahedra, while the Raman bands in the wavenumber region of 650–950 cm−1 are attributed to stretching vibrations of Mo–O bonds. The temperature-dependent Raman spectra reveal that K2Sr(MoO4)2 exhibits two sets of modifications in the Raman spectra at ∼150 °C and ∼475 °C, attributed to structural phase transitions. The large change of the Raman spectra in the temperature range of 150 °C to 475 °C suggests structural instability of the medium-temperature phase β-K2Sr(MoO4)2.

Temperature dependence of Raman responses of few-layer PtS2

Platinum disulfide (PtS2) is a newly emerging 2D material, which possesses relatively high carrier mobility, a widely tunable band gap from 0.25 to 1.6 eV, and ultra-high air stability, showing a potential in electronics and optoelectronics. Here, for the first time, we study the temperature-dependent Raman spectra on PtS2 with different thicknesses. It was found that with the temperature increase from 80 to 298 K, the and modes of all samples show linear softening. Moreover, the linear softening with temperature of PtS2 is much smaller than other 2D transition metal dichalcogenides, which could be attributed to the stronger interlayer coupling in PtS2. Our work gives fundamental temperature-dependent vibrational information of PtS2, which will be useful in future PtS2–based electronic devices.

Predicting the Initial Thermal Decomposition Path of Nitrobenzene Caused by Mode Vibration at Moderate-Low Temperatures: Temperature-Dependent Anti-Stokes Raman Spectra Experiments and First-Principals Calculations.

The lack of understanding of the initial decomposition micromechanism of energetic materials subjected to external stimulation has hindered its safe storage, usage, and development. The initial thermal decomposition path of nitrobenzene triggered by molecular thermal motion is investigated using temperature-dependent anti-Stokes Raman spectra experiments and first-principles calculations to clarify the initial thermal decomposition micromechanism. The experiment shows that the symmetric nitro stretching, antisymmetric nitro stretching, and phenyl ring stretching vibration modes are active as increasing temperature below 500 K. The DFT method is used to examine the effects of the three mode vibrations on the initial decomposition of nitrobenzene by relaxed scan for each relevant change in bond lengths and bond angles to obtain the optimal reaction channel leading to initial thermal decomposition of nitrobenzene. The results demonstrate that the initial thermal decomposition is the isomerization of nitrobenzene to phenyl nitrite. The optimal reaction channel leading to the initial isomerization is the increase or decrease of angle O-N-C from the antisymmetric nitro stretching vibration, which causes the torsion of nitro group and the subsequent oxygen atom attacking carbon atom. The scanning energy barrier related to angle O-N-C is about 62.1 kcal/mol, which is very consistent with the calculated activation barrier of isomerization of nitrobenzene. This proves the reliability of our conclusions.

Anharmonic Phonon Coupling in Single-Crystal Semiconducting and Metal-Like van der Waals In2Se3

We study the anharmonic phonon interactions in the single-crystal semiconducting (α) and metal-like (β) van der Waals In2Se3 layers, through the determination and analysis of temperature-dependent Raman spectra and thermal conductivities, supported by first-principles calculations of phonon band structures. Our results indicate strong lattice anharmonicity in β-In2Se3 giving rise to significant phonon peak broadening and a suppressed lattice thermal conductivity and reveal that the anharmonic phonon interactions are the main thermal transport-limiting mechanism in both phases. The low thermal conductivity combined with a large electrical conductivity makes the metal-like β-In2Se3 a potential efficient thermoelectric material.

Manipulating Behaviors from Heavy Tungsten Doping on Interband Electronic Transition and Orbital Structure Variation of Vanadium Dioxide Films.

Vanadium dioxide (VO2) with a metal-insulator transition (MIT) has been supposed as a candidate for optoelectronic devices. However, the MIT temperature ( TMIT) above room temperature limits its application scope. Here, high-quality V1- xW xO2 films have been prepared by pulsed laser deposition. On the basis of temperature-dependent transmittance and Raman spectra, it was found that TMIT increases from 241 to 279 K, when increasing the doping concentration in the range of 0.16 ≤ x ≤ 0.20. The interband electronic transitions and orbital structures of V1- xW xO2 films have been investigated via fitting transmittance spectra. Moreover, with the aid of first-principles calculations, an effective orbital theory has been proposed to explain the unique phenomenon. When the W doping concentration increases, the π* and dII orbitals shift toward the π orbital. Meanwhile, the energy gap between the π* and dII orbitals decreases at the insulator state. It indicates that the bandwidth is narrowed, which impedes MIT. In addition, the overlap of the π* and dII orbitals increases at the metal state, and more doping electrons occupy the π* orbital induced by increasing W doping concentration. It manifests that the Mott insulating state becomes more stable, which further improves TMIT. The present work provides a feasible approach to tune TMIT via orbital variation and can be helpful in developing the potential VO2-based optoelectronic devices.

Spectroscopic study of phase transitions in ferroelectric Bi0.5Na0.5Ti1−xMnxO3−δ films with enhanced ferroelectricity and energy storage ability

Lead-free ferroelectric Bi0.5Na0.5TiO3 (BNT) has attracted considerable attention taking into account environment issues and applications in micro electro mechanical systems. The effects of manganese (Mn) substitution on microstructure, lattice dynamics, and optical properties of BNT films have been investigated by X-ray diffraction, Raman scattering and ellipsometric spectra. The phase transitions as a function of temperature have been systemically explored by temperature-dependent Raman and ellipsometric spectra. The anomalous temperature-dependent behavior of Raman-active modes suggests that there is an intermediate phase between ferroelectric rhombohedral and paraelectric tetragonal phase, which is confirmed by the temperature-dependent optical band gap (Eg) and extinction coefficient (κ) extracted from ellipsometric spectra analysis. And then, a phase diagram as a function of Mn composition has been proposed. Finally, we demonstrate that Mn-dopant is an effective approach to enhance ferroelectric properties, improve energy storage ability, and increase permittivity as well as suppress leakage current. Electrical dielectric responses indicate that there is a critical frequency and polarization relaxation behavior in the films. The present results will be helpful for the application of BNT-based lead-free multifunctional devices.

Temperature-dependent Raman spectroscopy of Cu2Sn1−xGexS3 thin films

Analysis on the temperature-dependent Raman spectra measured between 27–450 °C range in relation to various Ge compositional ratios of Cu2Sn1−xGexS3 thin-film alloys, which were prepared by closed-tube sulfurization technique, was carried out in detail. By XRD and Raman analyses, linear transformation of the lattice parameters for Cu2Sn1−xGexS3 thin-film alloys with respect to the Ge composition was confirmed to be obeying the Vegard’s law. By AC Hall effect measurement, reducing of carrier mobility was observed in the Cu2Sn1−xGexS3 thin-film alloys with higher Ge content. By the temperature dependent Raman spectroscopy analysis, it is observed that the red shifting of Raman vibration energy due to the thermal expansion is resembled to that of the temperature dependency of energy bandgap of semiconductors. Anisotropic thermal dilatation behavior was observed in the Cu2Sn1−xGexS3 thin-film alloy independent of its Ge composition.

Carrier Transport in Reduced Graphene Oxide Probed Using Raman Spectroscopy

In the present work, we have investigated the temperature-dependent electronic conduction in hydrazine-reduced graphene oxide in the temperature range of 300–5 K. Raman spectroscopy and transport measurements are used to probe the conduction mechanism. It is observed that the carrier transportation takes place via variable-range hopping. For variable-range hopping, various parameters such as density of states, hopping distance, and hopping energy have been calculated. The conduction behaviors across the temperature range show three different regions with three different characteristic temperatures. Temperature-dependent Raman spectra have been successfully studied for ID/IG ratio, which uncover the length scales of defect sites and number of defects. Further, the line width of the G band is studied to get an insight of electron–phonon interaction. Quantitative analysis of the transport mechanism is done using electron–phonon coupling and ID/IG ratio.

Spectroscopic Study of [(CH3)2NH2][Zn(HCOO)3] Hybrid Perovskite Containing Different Nitrogen Isotopes

We present a combined differential scanning calorimetry (DSC) and infrared (IR), Raman, electron paramagnetic resonance (EPR), and NMR spectroscopy study of dimethylammonium zinc formate frameworks [(CH3)2NH2][Zn(HCOO)3] containing 14N and 15N nitrogen isotopes. The DSC reveals very small differences in the phase-transition temperatures for both compounds. The IR and Raman spectroscopies indicate different frequencies of the vibrational modes that involve a nitrogen atom. The temperature-dependent IR and Raman spectra also show a tiny isotope effect on the properties and temperature of the phase transition. The EPR measurements are performed for both compounds doped with small amount of paramagnetic Mn2+ ions. The obtained continuous-wave Mn2+ EPR spectra of the disordered phase are very similar for both frameworks, whereas the temperature dependence of the zero-field splitting parameter indicates a slightly different distortion of the MnO6 octahedra in the low-temperature phase. The EPR line width analys...

Formation of “Nano-Ice” and Density Maximum Anomaly of Water

Abstract Water is still mysterious despite intensive and extensive studies over the years. Anomalous behavior of water as a liquid is yet to be fully comprehended. Here we show that the most generally known anomaly of water, the density maximum anomaly, is well accounted for by the formation of nanometer-size ice crystallite at low temperatures. We show spectroscopically that, in cold and super-cooled water, this nanometer-size ice crystallite is formed and coexists with the other two forms of water. Multivariate hyperspectral analysis of 140 temperature dependent Raman spectra in the range of −23∼45 °C determines the three distinct vibrational spectra of the three forms of water and their fractions at different temperatures. Simulation based on the determined fractions successfully reproduces the temperature dependence of density with a maximum at the right temperature. The mystery of the density maximum of water has thus been given an unequivocal solution. The nanometer-size ice crystallite might well be called “nano-ice”.

Crystal structure, optical behavior and electrical conduction of the new organic–inorganic compound CH3NH3CdI3

A new organic–inorganic compound CH3NH3CdI3 (MACdI3) was prepared by solvent diffusion method. Single crystal diffraction results showed that MACdI3 had a monoclinic system with P21/c space group at room temperature. UV–Visible absorption spectra revealed that the optical band gap ($${E_g}$$) of 3.45 eV is in agreement with the theoretical value. Band structure and density of states calculations indicated that the valence band is mainly iodine 5p in character and the conduction band is the interaction between Cd 4d in character and iodine 5p states. The temperature dependent dielectric constant and alternating current (AC) conduction analysis displayed a phase transition at about 348 K, which could be confirmed by temperature dependent Raman spectra. AC conduction results demonstrated that the conduction in MACdI3 was attributed to correlated barrier hopping at 308–348 K and non-overlapping small polaron tunneling at 348–398 K.

Insights into the helix-sense inversion of poly(β-phenethyl-L-aspartate) by two-dimensional Raman correlation spectroscopy.

In this study, the transition process of the helix-sense inversion of poly(β-phenethyl-L-aspartate) was investigated by Raman scattering and 2-dimensional correlation spectroscopy. Temperature-dependent Raman spectra were obtained during the helix-sense inversion. The results of 2-dimensional correlation analysis in the spectral regions of 1600-1800 and 3200-3400cm-1 showed that the intensity changes of the side-chain ester C═O stretching bands occurred prior to those of amide A and amide I bands in the unwinding process of αR-helix on heating. The sequential order of the intensity changes for amide A, amide I, and the side-chain ester C═O stretching bands during the inversion process was determined. It was found that the conformation change of the side chain occurred prior to that of the main chain for the αR-helix on heating. Thus, we concluded that the transformation of the backbone chain from right-handed to left-handed is triggered by the conformational change of the side chains.

Structural symmetry changes in SmB6 - 2D correlation spectroscopy and principal component analysis

2D correlation analysis (2DCOS) and principal component analysis (PCA) have been performed on temperature-dependent Raman spectra from 12 K to 300 K of SmB6 single crystals grown from Boron with a purity of 99.9% (3N) or 99.9999% (6N). Significant differences in the Raman spectra of SmB6(3N) and SmB6(6N) are observed. The Raman active T2g, Eg, and A1g modes of SmB6(3N) are broader and shift to higher wavenumbers compared with those of SmB6(6N). The results of 2DCOS and PCA on temperature-dependent Raman spectra of SmB6(3N) and SmB6(6N) give us further information. We find that all the T2g, Eg, and A1g modes of SmB6(6N) are single bands, while those of SmB6(3N) show all doublet features. The appearance of multiple bands in Raman vibrational modes of SmB6(3N) sample implies that the structure of boron octahedra of the SmB6(3N) has lower symmetry than that of SmB6(6N). Our findings in 2DCOS and PCA of the Raman spectra strongly support the differences in the behavior of SmB6(3N) and SmB6(6N).

Local Bi–O bonds correlated with infrared emission properties in triply doped Gd2.95Yb0.02Bi0.02Er0.01Ga5O12 via temperature-dependent Raman spectra and x-ray absorption fine structure analysis

A correlation function between the Raman intensities and the nearest-neighbor mean-square relative displacement (MSRD) of local Bi–O bonds is successfully established based on x-ray absorption fine structure (XAFS) and temperature-dependent Raman spectra in the temperature range 77–300 K in amorphous and crystalline Gd2.95Yb0.02Bi0.02Er0.01Ga5O12. The structural symmetries of Gd2.95Yb0.02Bi0.02Er0.01Ga5O12 are described by using of local Bi–O bonds. More importantly, Gd2.95Yb0.02Bi0.02Er0.01Ga5O12 is found to show excellent infrared (IR) emission properties due to changes in Bi–O bonds, and the IR emission intensities are found to depend on , by using temperature-dependent photoluminescence spectroscopy. The maximum emission intensity at 1533 nm is obtained when at the lowest symmetry. This work shows that temperature-dependent Raman intensities can be used effectively to analyze the local covalent bonds around absorbing atoms as well as to study the emission properties of this visible-light-activated IR luminophor.

Raman non-coincidence effect of boroxol ring: The interplay between repulsion and attraction forces in the glassy, supercooled and liquid state

Temperature dependent Raman spectra of boric oxide have been measured in a temperature range covering the glassy, supercooled and liquid state. The shift of the isotropic band assigned to boroxol rings relative to the anisotropic component upon heating the glass is measured and attributed to the Raman non-coincidence effect. The measured shift is associated with the competition between attraction and repulsion forces with increasing temperature. The relation of dephasing and orientational relaxation times to the non-coincidence effect of the condensed phases has been examined. We discuss our results in the framework of the current phenomenological status of the field in an attempt to separate the attraction and repulsion contributions corresponding to the observed non-coincidence effect.

Anisotropic temperature‐dependence of optical phonons in layered PbI2

AbstractThe temperature‐dependence of wavenumbers of in‐plane E‐mode and out‐of‐plane A‐mode optical phonons of layered PbI2 were studied through analysis on its temperature‐dependent Raman spectra from 80 to 350 K. The temperature coefficient of the wavenumbers of in‐plane Eg phonon (−0.018 cm−1/K) and Eu phonon (−0.01 cm−1/K) are 3 and 10 times that of out‐of‐plane A1g phonon (−0.006 cm−1/K) and A2u phonon (−0.001 cm−1/K), respectively. Such anisotropy originated from the anisotropic thermal expansion of layered PbI2. These results will be beneficial for the design of PbI2‐based optoelectronic devices in the future.

Enhanced temperature stability in the R-T phase boundary with dominating intrinsic contribution.

In this study, 0.96KNNSx-0.01SZ-0.03BNZ ceramics (x = 0-0.08) were used as examples to illustrate the effects of phase boundaries on strain property and temperature stability. The addition of Sb5+ resulted in a rhombohedral-tetragonal (R-T) phase boundary at x = 0.05-0.07, as confirmed by temperature-dependent Raman spectra. In the R-T region, an improved piezoelectric constant (d33 = 390-440 pC N-1) and high unipolar strain (Suni = 0.14-0.15%) were observed due to the dominating intrinsic contribution. More importantly, a favorable temperature stability of Suni was observed in the ceramics with x = 0.05; for example, there was a slight variation of +5% to -13% when the temperature was increased from 20 °C to 180 °C. Through systematic investigations of composition and temperature-dependent strain, methods to improve Suni and consolidate its temperature stability in KNN-based ceramics were subsequently suggested. We believe that this study can promote the understanding and design of KNN-based ceramics with high strain and favorable temperature stability.

Anisotropic magnetoresistance across Verwey transition in charge ordered Fe3O4 epitaxial films

The anisotropic magnetoresistance (AMR) near the Verwey temperature $({T}_{\mathrm{V}})$ is investigated in charge ordered ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ epitaxial films. When the temperature continuously decreases below ${T}_{\mathrm{V}}$, the symmetry of AMR in ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$(100) film evolves from twofold to fourfold at a magnetic field of 50 kOe, where the magnetic field is parallel to the film surface, whereas AMR in ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$(111) film maintains twofold symmetry. By analyzing AMR below ${T}_{\mathrm{V}}$, it is found that the Verwey transition contains two steps, including a fast charge ordering process and a continuous formation process of trimeron, which is comfirmed by the temperature-dependent Raman spectra. Just below ${T}_{\mathrm{V}}$, the twofold AMR in ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$(100) film originates from uniaxial magnetic anisotropy. The fourfold AMR at a lower temperature can be ascribed to the in-plane trimerons. By comparing the AMR in the films with two orientations, it is found that the trimeron shows a smaller resistivity in a parallel magnetic field. The field-dependent AMR results show that the trimeron-sensitive field has a minimum threshold of about 2 kOe.