Temperature-dependent Raman Spectra Research Articles

Efficient thermal conductance in organometallic perovskite CH3NH3PbI3 films

Perovskite-based optoelectronic devices have shown great promise for solar conversion and other optoelectronic applications, but their long-term performance instability is regarded as a major obstacle to their widespread deployment. Previous works have shown that the ultralow thermal conductivity and inefficient heat spreading might put an intrinsic limit on the lifetime of perovskite devices. Here, we report the observation of a remarkably efficient thermal conductance, with a conductivity of 11.2 ± 0.8 W m−1 K−1 at room temperature, in densely packed perovskite CH3NH3PbI3 films, via noncontact time-domain thermal reflectance measurements. The temperature-dependent experiments suggest the important roles of organic cations and structural phase transitions, which are further confirmed by temperature-dependent Raman spectra. The thermal conductivity at room temperature observed here is over one order of magnitude larger than that in the early report, suggesting that perovskite device performance will not be limited by thermal conductance.

The intermediate phase and low wavenumber phonon modes in antiferroelectric (Pb0.97La0.02) (Zr0.60Sn0.40−yTiy)O3 ceramics discovered from temperature dependent Raman spectra

Optical phonons and phase transitions of (Pb0.97La0.02) (Zr0.60Sn0.40−yTiy)O3 (PLZST 97/2/60/40-100y/100y) ceramics with different compositions have been investigated by x-ray diffraction and temperature dependent Raman spectra. From the temperature dependence of low wavenumber phonon modes, two phase transitions (antiferroelectric orthorhombic to intermediate phase and intermediate phase to paraelectric cubic phase) were detected. The intermediate phase could be the coexistence one of antiferroelectric orthorhombic and ferroelectric rhombohedral phase. In addition, two modes (a soft mode and an anharmonic hopping central mode) were found in the high temperature paraelectric cubic phase. On cooling, the anharmonic hopping central mode splits into two modes in the terahertz range. Moreover, the antiferrodistortive mode appears in the antiferroelectric orthorhombic phase. Based on the analysis, the phase diagram of PLZST ceramics can be well improved.

Temperature-dependent μ-Raman investigation of struvite crystals

The effect of temperature on the vibrational properties of struvite crystals grown from silica gels was systematically studied by μ-Raman spectroscopy. The time-dependent Raman spectra recorded in the process of long time annealing of struvite crystal at 353K do not indicate structural changes in the struvite crystal with the time of annealing. The temperature-dependent Raman spectra recorded in the range 298–423K reveal a phase transition in struvite at about 368K. Above this characteristic temperature, some of bands assigned to vibrations of the PO4 and NH4 tetrahedra and water molecules observed in the Raman spectra in low temperatures (orthorhombic phase) change their spectral parameters or disappear, which indicates a transition to a higher symmetry structure of struvite in the range of high temperatures.

Study of temperature-dependent Raman spectroscopy and electrical properties in [001]-oriented 0.35Pb(In1/2Nb1/2)O3-0.35Pb(Mg1/3Nb2/3)O3-0.30PbTiO3-Mn single crystals

In this work, the temperature-dependent Raman spectra and electrical properties of the [001]-oriented 0.5 mol. % Mn-doped 0.35Pb(In1/2Nb1/2)O3-0.35Pb(Mg1/3Nb2/3)O3-0.30PbTiO3-Mn (PIMNT-Mn) single crystals were investigated. All the unpoled and poled PIMNT-Mn single crystals experience a ferroelectric tetragonal phase to paraelectric cubic phase transition (FET-PC) around 183 °C (TC), which exhibits a second-order transition behavior. Whereas, the poled PIMNT-Mn single crystals exhibit another two dielectric anomalies around 130 °C (TRM) and 148 °C (TMT), in which the ferroelectric rhombohedral phase to ferroelectric monoclinic phase (FER-FEM) and the ferroelectric monoclinic phase to ferroelectric tetragonal phase (FEM-FET) transitions take place, respectively. Both the two ferroelectric phase transitions exhibit a first-order transition behavior. The discontinuous change of the phase degree (θ) and frequencies (fr and fa) around TRM suggest the occurrence of the FER-FEM phase transition in the poled PIMNT-Mn single crystals. The narrowing of the 510 cm−1 and 582 cm−1 Raman modes around the TRM, TMT, and TC temperatures shown in the temperature-dependent Raman spectra suggests their increased ordering of the local structure. The intensity ratio of I272 cm−1/I801 cm−1 increases obviously around the phase transition temperatures (TRM, TMT, and TC), indicating the reduction of the long-range order. The anomalous broadening of the 272 cm−1 Raman mode around the TRM, TMT, and TC temperatures indicates the occurrence of the successive ferroelectric phase transitions (FER-FEM, FEM-FET, and FET-PC) with increasing temperature in the poled PIMNT-Mn single crystals.

Molecular self-healing mechanisms between C60-fullerene and anthracene unveiled by Raman and two-dimensional correlation spectroscopy.

The self-healing polymer P(LMA-co-MeAMMA) crosslinked with C60-fullerene has been studied by FT-Raman spectroscopy in combination with two-dimensional (2D) correlation analysis and density functional theory calculations. To unveil the molecular changes during the self-healing process mediated by the Diels-Alder equilibrium between 10-methyl-9-anthracenyl groups and C60-fullerene different anthracene-C60-fullerene adducts have been synthesized and characterized by time-, concentration- and temperature-dependent FT-Raman measurements. The self-healing process could be monitored via the C60-fullerene vibrations at 270, 432 and 1469 cm(-1). Furthermore, the detailed analysis of the concentration-dependent FT-Raman spectra point towards the formation of anthracene-C60-fullerene adducts with an unusual high amount of anthracene bound to C60-fullerene in the polymer film, while the 2D correlation analysis of the temperature-dependent Raman spectra suggests a stepwise dissociation of anthracene-C60-fullerene adducts, which are responsible for the self-healing of the polymer.

Graphitic carbon nanospheres: A Raman spectroscopic investigation of thermal conductivity and morphological evolution by pulsed laser irradiation

Graphitic carbon nanospheres (GCNSs) were prepared by a unique acidic treatment of multi-walled nanotubes. Spherical morphology with a narrow size distribution was confirmed by transmission electron microscopy studies. The room temperature Raman spectra showed a clear signature of D- and G-peaks at around 1350 and 1591 cm−1, respectively. Temperature dependent Raman scattering measurements were performed to understand the phonon dynamics and first order temperature coefficients related to the D- and G-peaks. The temperature dependent Raman spectra in a range of 83–473 K were analysed, where the D-peak was observed to show a red-shift with increasing temperature. The relative intensity ratio of D- to G-peaks also showed a significant rise with increasing temperature. Such a temperature dependent behaviour can be attributed to lengthening of the C-C bond due to thermal expansion in material. The estimated value of the thermal conductivity of GCNSs ∼0.97 W m−1 K−1 was calculated using Raman spectroscopy. In addition, the effect of pulsed laser treatment on the GCNSs was demonstrated by analyzing the Raman spectra of post irradiated samples.

Effects of crystal orientation on electronic band structure and anomalous shift of higher critical point in VO2 thin films during the phase transition process

The phase transition behaviour of vanadium dioxide (VO2) with different thicknesses has been investigated by temperature-dependent optical transmittance and Raman spectra. It is found that the crystal orientation has a great effect on the metal-insulator transition (MIT) of VO2 films. The x-ray diffraction (XRD) analysis shows that the films are polycrystalline and exhibit the characteristics of the monoclinic phase. The preferential growth crystal orientation (0 2 0) is converted to the ( 1 1) plane with the film thickness increasing. It is believed that the ( 1 1) plane is the reflection of a twinned structure with (0 1 1) crystal orientation, which will lead to the arrangements of oxygen atoms and vanadium atoms deviating from the pure monoclinic structure. It is found that the highest order transition (E3) is highly susceptible to the crystal orientation, whereas the lowest order transition (E1) is nearly unaffected by it. The E3 exhibits an anomalous temperature dependence with an abrupt blue-shift (∼0.5 eV) in the vicinity of the metal-insulator transition (MIT) for VO2 film with a thickness of 84 nm. The findings show that the empty band can be driven close to the Fermi level when the (0 2 0) orientation is converted to the ( 1 1) orientation. Compared to the VO2 films with thicknesses of 39 and 57 nm, the E3 decreases by 0.8 eV and the E2 increases by about 0.1 eV at the insulator state for the VO2 film with a thickness of 84 nm. The abnormal electronic transition and the variation of energy band is likely caused by the lattice distortion and V–V dimerisation deviation from the monoclinic axis.

Wide-Range Thermometry at Micro/Nano Length Scales with In2O3 Octahedrons as Optical Probes.

We report the temperature-dependent photoluminescence and Raman spectra of In2O3 octahedrons synthesized by an evaporation-condensation process. The luminescence obtained here is due to the defect-related deep level emission, which shows highly temperature-dependent behavior in 83-573 K range. Both the position as well as the intensity varies with temperature. Similarly, Raman spectroscopy in 83-303 K range shows temperature-dependent variation in peak intensity but no change in the peak position. Interestingly, the variation of intensity for different peaks is consistent with Placzek theory which invokes the possibility of temperature sensing. We demonstrate the reversibility of peak intensity with temperature for consecutive cycles and excellent stability of the octahedrons toward cryogenic temperature sensing. Overall, both the temperature-dependent photoluminescence and Raman spectra can be explored to determine temperature in the cryogenic range at micro/nano length scales. As an example, we evaluate the temperature-dependent Raman spectra of WO3 that undergoes a phase transition around 210 K and temperature-dependent luminescence of Rhodamine 6G (Rh6G) where intensity varies with temperature.

High power and temperature luminescence of Y3Al5O12:Ce3+ bulky and pulverized single crystal phosphors by a floating-zone method

Yellow Y3Al5O12:Ce3+ single crystal phosphor was grown through a floating zone method. It showed a broader blue excitation and yellow emission spectra with lower quantum efficiency due to large internal reflections but higher thermal stability due to high thermal conductivity than the conventional powder. With pulverizing the as-grown single crystal down to micron powder, its quantum efficiency was enhanced, while its lumen maintenance at high temperature (200°C) was reduced due to a generation of more phonons, confirmed by temperature-dependent Raman spectra. The as-grown and pulverized single crystal samples were applied to a high-power blue laser diode, and thus they gave an excellent luminescence feasibility to a high-power lighting source.

Significant increase of Curie temperature and large piezoelectric coefficient in Ba(Ti0.80Zr0.20)O3-0.5(Ba0.70Ca0.30)TiO3 nanofibers

Ba(Ti0.80Zr0.20)O3-0.5(Ba0.7Ca0.3)TiO3 (abbreviated as BTZ-0.5BCT) is a piezoelectric ceramic with a high piezoelectric coefficient d33 (∼620 pC N−1) and has been regarded as one of the most promising candidates to replace PZT-based materials (200–710 pC N−1). However, its Curie temperature TC is relatively low (93 °C) limiting its application. In this letter, we found a temperature dependent Raman spectrum in BTZ-0.5BCT nanofibers (NFs), demonstrating a diffused tetragonal-to-cubic phase transition at 300 °C. This means that the TC of the NFs is nearly 207 °C higher than that of the normal bulk material. The increased TC is considered to be associated with the size effect of BTZ-0.5BCT nanoceramic subunits and the nanoporous nature of the fiber, resulting in discontinuous physical properties. The variation of the ferro/piezoelectricity over the fiber surface is attributed to the polycrystalline structure. The d33 (173.32 pm V−1) is improved in terms of the decreased Q factor result in an increase in d33 of 236.54 pm V−1 after polarization. With a high TC and a very large d33, BTZ-0.5BCT NFs are capable of providing electromechanical behavior used in moderate temperatures.

The magnetic properties and spin–phonon coupling of Pr2CoMnO6 particles

Pr2CoMnO6 particles with different sizes (45 nm–1 μm) have been synthesized by a citrate-gel method with different annealing temperatures and investigated by x-ray diffraction, Raman spectroscopy, and magnetometry. It is found that at a lower annealing temperature related to sample synthesis, the samples with particle size D ≤ 200 nm show a higher ferromagnetic (FM) transition temperature Tc1, which can be attributed to the FM superexchange interaction of the ordered Co2+–O–Mn4+ pairs. As the annealing temperature increasing, a new FM transition originating from FM vibronic superexchange interaction of Co3+–O– Mn3+ pairs in the disordered phase appears at temperature Tc2 lower than Tc1. Raman spectra at room temperature show that both the antisymmetric stretching and symmetric stretching modes at around 490 and 637 cm−1, respectively, show a decrease in the linewidth and an increase in the intensity with the annealing temperature decreasing. All the results indicate that the ratio of the disordered phase to the ordered phase in the sample decreases as the annealing temperature decreasing. It is suggested that the annealing temperature dependent oxygen vacancies play a key role in the magnetic properties of the Pr2CoMnO6 particles. Furthermore, the temperature-dependent Raman spectrum for Pr2CoMnO6 particles reveals obviously softening of the phonon modes involving stretching vibrations of the (Co/Mn)O6 octahedra at Curie temperature, indicating a close correlation between magnetism and lattice in Pr2CoMnO6 particles. The Pr2CoMnO6 compound with smaller particle size retains a higher FM transition temperature and obvious spin–phonon coupling, which are of special interest for the application.

Temperature dependent raman and photoluminescence of an individual Sn-doped CdS branched nanostructure

Temperature dependent Raman spectra and photoluminescence, as well the Raman mapping of different parts in an individual Sn-doped CdS comb-like nanostructure reveal that the stronger electron–phonon coupling exist at the trunk-branch junctions than other parts. The Huang–Rhys factor was calculated and further confirms that the strong electron–phonon correlation at the junction. The localized deformation from the Sn dopant in the lattice leads to strong electron–phonon coupling at the local junction, which is proved by the scanning transmission electron microscope and energy dispersion spectrum. Moreover, the lifetime of near-band-edge emission and deep-level emission drastically increase with decreasing temperature, which both relate to the localized electron–phonon coupling and relaxation process. This work provides a clear description of the localized carrier correlation in the cross junction part of these branched nanostructures, which can be used to modulate the optoelectronic performance of micro/nanodevices.

Temperature dependent Raman and DFT study of creatine

Temperature dependent Raman spectra of creatine powder have been recorded in the temperature range 420–100K at regular intervals and different clusters of creatine have been optimized using density functional theory (DFT) in order to determine the effect of temperature on the hydrogen bonded network in the crystal structure of creatine. Vibrational assignments of all the 48 normal modes of the zwitterionic form of creatine have been done in terms of potential energy distribution obtained from DFT calculations. Precise analysis gives information about thermal motion and intermolecular interactions with respect to temperature in the crystal lattice. Formation of higher hydrogen bonded aggregates on cooling can be visualized from the spectra through clear signature of phase transition between 200K and 180K.

The role of atomic vacancies on phonon confinement in α-GeTe

Atomic defects and their dynamics play a vital role in controlling the behavior of non-volatile phase change memory materials used in advanced optical storage devices. Synthesis and structural analysis by XRD and Raman spectroscopy on α-GeTe single crystal with different sizes are reported. The spectroscopic measurements on micron and nano sized α-GeTe single crystal reveal the evolution of phonon confinement with crystal sizes of few hundred nanometers. The characteristic vibrational modes of bulk α-GeTe structure are found to downshift and asymmetrically broaden to lower frequency with decreasing the single crystal size. We attribute the observed downshift of Raman lines in α-GeTe is largely due to the presence of high concentration of atomic vacancies. The crystal size and temperature dependent Raman spectra provide explicitly the dynamics of vacancies on optical phonon confinement in α-GeTe structure. Thus, the observed large concentration of vacancies and their size dependency might influence the phase change phenomenon in GeTe based alloys.

Phase evolution and dielectric/ferroelectric properties of KTa0.67Nb0.33O3 single crystal

High quality KTa0.67Nb0.33O3 (KTN) single crystal has been grown by the Czochralski method. Three phase transitions Rhombohedral-to-Orthorhombic (R–O), Orthorhombic-to-Tetragonal (O–T) and Tetragonal-to-Cubic (T–C) were observed using temperature-dependent Raman spectra, thermal expansion, and dielectric measurements. The phase transition temperatures determined by the Raman spectra are close to those determined by thermal expansion, i.e., R–O at 175K, O–T at 210K, and T–C at 250K. While dielectric measurements showed that the R–O, O–T, and T–C phase transitions occur at 192, 223, and 265K, respectively. The dielectric properties of KTN obeys the Curie–Weiss law (T0=260K) above 265K. At 77K, typical polarization–electric (P–E) hysteresis loops were observed, and the largest remnant polarization is 15μC/cm2.

Different behaviors in the transformation of PATP adsorbed on Ag or Au nanoparticles investigated by surface-enhanced Raman spectroscopy – A study of the effects from laser energy and annealing

In order to explore the key role of surface plasmon resonance (SPR) and active 3O2 for the chemical transformation to 4,4-dimercaptoazobenzene (DMAB) from p-aminothiophenol (PATP) adsorbed on Ag or Au NPs, we systematically investigated the laser wavelength and temperature dependent surface-enhanced Raman spectra of PATP capped Ag and Au NPs. DMAB can be easily observed at the 514.5nm laser for Ag NPs but at the 632.8nm laser for Au NPs, indicating that a suitable energy level is necessary for the formation of DMAB. The tendency is consistent with the wavelength dependent SPR properties of Ag or Au NPs accordingly. With the energy provided by annealing, the transformation of PATP to DMAB is much easier on Ag NPs at a lower temperature, and more DMAB can be observed at the same temperature, compared to the case of Au NPs under the same condition. It is mainly due to the active 3O2 on Ag surfaces could be more easily formed than that on Au surfaces.

Temperature dependent Raman spectra of CsCdBr3 and CsCdCl3 crystals

Temperature dependent Raman spectra of the crystals CsCdCl3 and CsCdBr3 grown by Bridgman method were measured in the temperature range of 5–300K.The description of the line-broadening and the line-shift of the observed Raman lines are mainly based on the anharmonicity due to the phonon–phonon interaction. The analysis of the temperature dependence of the Raman-lines that involve vibrational modes on common sites in both crystals make it possible to compare the anharmonicity of the lattice vibration in both crystals. The breathing mode in both CsCdCl3 and CsCdBr3 crystals is the strongest and comparable mode in this manner.

Investigation of the intrinsic magnetodielectric effect in La2CoMnO6: role of magnetic disorder

We present a large magnetodielectric (MD) effect of 65% at 100 kHz with a 5 T field in a B-site ordered La2CoMnO6 (LCMO) polycrystalline sample. Frequency and temperature dependent impedance, dielectric and Raman spectra studies under a magnetic field divulge both intrinsic and extrinsic origins for the observed MD effect.

Raman spectroscopic evidence of low temperature stability of d,l-glycolic and l-(+)-lactic acid crystals

Raman and infrared spectra of polycrystalline d,l-glycolic and l-(+) lactic acid are presented and assigned both by an ab initio calculation of normal modes of free conformers and by self-consistent-charge density-functional-theory computational program DFTB+. Temperature dependent Raman spectra from 295K to 10K reveal great stability of crystal lattices, since no soft modes and no band splittings that could be attributed to changes of the number of molecules per unit cell were observed. A semiempirical calculation with GULP program was used to estimate the strength of hydrogen bonds in crystals: in glycolic acid they have energies of −0.337eV/mol, −0.329eV/mol, −0.262eV/mol and −0.242eV/mol, while in lactic acid two hydrogen bonds have energies of −0.283eV/mol and −0.202eV/mol.

Spin-phonon coupling in BaFe12O19 M-type hexaferrite

The spin-phonon coupling in magnetic materials is due to the modulation of the exchange integral by lattice vibrations. BaFe12O19 M-type hexaferrite, which is the most used magnetic material as permanent magnet, transforms into ferrimagnet at high temperatures, but no spin-phonon coupling was previously observed at this transition. In this letter, we investigated the temperature-dependent Raman spectra of polycrystalline BaFe12O19 M-type hexaferrite from room temperature up to 780 K to probe spin-phonon coupling at the ferrimagnetic transition. An anomaly was observed in the position of the phonon attributed to the Fe(4)O6, Fe(5)O6, and Fe(1)O6 octahedra, evidencing the presence of a spin-phonon coupling in BaM in the ferrimagnetic transition at 720 K. The results also confirmed the spin-phonon coupling is different for each phonon even when they couple with the same spin configuration.