Temperature-dependent Raman Spectra Research Articles

Study of the Substrate-Induced Strain of As-Grown Graphene on Cu(100) Using Temperature-Dependent Raman Spectroscopy: Estimating the Mode Grüneisen Parameter with Temperature

The strategy of using the thermal expansion of copper single crystal can provide an approach of homogenous strain along the basal plane to study the strain characteristic of graphene with temperature. Using an in-situ Raman measurement under a ultra-high vacuum (UHV) environment, the ability to remove contaminations allowed the direct observation of the strain property in as-grown chemical vapor deposition (CVD)-graphene with temperature on a Cu(100) substrate. In this study, the strain coefficients of G and G’ band with temperature were investigated from the in-situ temperature-dependent Raman spectra of the as-grown CVD-graphene on the single crystal of Cu(100) under UHV. By eliminating the minor contributions of the lattice expansion and anharmonic phonon scattering effects, we were able to estimate the strain coefficient of the G and G’ bands of graphene over a wide temperature from 100 K to 800 K. Based on the strain coefficients and the correlation map of the G- and G’-band frequencies, we made a re...

Spatially Resolved Thermometry of Resistive Memory Devices

The operation of resistive and phase-change memory (RRAM and PCM) is controlled by highly localized self-heating effects, yet detailed studies of their temperature are rare due to challenges of nanoscale thermometry. Here we show that the combination of Raman thermometry and scanning thermal microscopy (SThM) can enable such measurements with high spatial resolution. We report temperature-dependent Raman spectra of HfO2, TiO2 and Ge2Sb2Te5 (GST) films, and demonstrate direct measurements of temperature profiles in lateral PCM devices. Our measurements reveal that electrical and thermal interfaces dominate the operation of such devices, uncovering a thermal boundary resistance of 28 ± 8 m2K/GW at GST-SiO2 interfaces and an effective thermopower 350 ± 50 µV/K at GST-Pt interfaces. We also discuss possible pathways to apply Raman thermometry and SThM techniques to nanoscale and vertical resistive memory devices.

Phase transition and negative thermal expansion of HfMnMo3O12

A novel compound HfMnMo3O12 with negative thermal expansion (NTE) was synthesized using solid state method. The crystal structure, fracture morphology and NTE properties of the material were investigated by temperature-dependent X-ray diffraction (XRD), scanning electron microscope (SEM) and temperature-dependent Raman spectra. Our experimental results show that the compound adopts a monoclinic symmetry below room temperature and transforms to an orthorhombic phase above 348K. In the orthorhombic phase the compound exhibits NTE property with a linear coefficient of thermal expansion (CTE) −3.8×10−6K−1 from 400K to 700K. The physical mechanism of the NTE is discussed and elucidated according to our experiments.

Ultrathin 2D GeSe2 Rhombic Flakes with High Anisotropy Realized by Van der Waals Epitaxy

AbstractAs a layered p‐type semiconductor with a wide bandgap of 2.7 eV, GeSe2 can compensate for the rarity of p‐type semiconductors, which are desired for the production of high‐integration logic circuits with low power consumption. Herein, ultrathin 2D single crystals of β‐GeSe2 are produced using van der Waals epitaxy and halide assistance; each crystalline flake is ≈7 nm thick and shaped as a rhombus. The optical and electrical properties of the flakes are studied systematically, and the temperature‐dependent Raman spectra of the flakes reveal that the intensity of the Raman peaks decrease with increasing temperature. Low‐temperature electrical measurements suggest that the variable‐range hopping model is best for describing the electrical transport at 20–180 K; meanwhile, optical‐phonon‐assisted hopping can account for the transport behavior at 180–460 K. Impressively, the angle‐resolved polarized Raman measurements indicate strong in‐plane anisotropy of the rhombic GeSe2 flake under a parallel polarization configuration, which may result from the low symmetry of the monoclinic crystal structure of GeSe2. Furthermore, a photodetector based on a rhombic GeSe2 flake is constructed and shown to exhibit a high responsivity of 2.5 A W−1 and a fast response of ≈0.2 s.

Temperature-dependent Raman spectra and thermal conductivity of multi-walled MoS2 nanotubes

In this letter, we investigated the temperature dependence of the in-plane (E12g) and out-of-plane (A1g) Raman modes of multi-walled MoS2 nanotubes fabricated from anodic aluminum oxide (AAO) templates. The frequencies of these two phonon modes were found to vary linearly with temperature. In this linear region, the first-order temperature coefficients for the A1g and E12g modes equaled –(1.15 ± 0.06)×10−2 cm−1/K and –(1.11 ± 0.1)×10−2 cm−1/K, respectively. The thermal conductivity of the multi-walled MoS2 nanotubes at room temperature was estimated to be in the range of 4.8 ± 0.1 to 11.1 ± 0.2 W m−1 K−1.

Temperature dependent polymorphism of pyrazinamide: An in situ Raman and DFT study

The α and γ polymorphs of drug pyrazinamide have been detected with the help of temperature dependent Raman spectroscopic technique. Pyrazinamide is a very useful drug used for the treatment of tuberculosis (TB) and plays a significant role in destroying the dormant tubercle bacilli which are not destroyed by other common TB drugs. Temperature dependent Raman spectra suggest polymorphic phase change from α→γ form of pyrazinamide between 145 and 146°C. In situ Raman spectra of pyrazinamide between 145 and 146°C show the conversion of α→γ form by the shift in CO stretching vibration accompanied by several other changes. The phase change is characterized by the breaking of two linear NH⋯O type hydrogen bonds associated with CO stretching vibration in α dimer and formation of one linear NH⋯N type hydrogen bond along with a weak intramolecular CH⋯O type hydrogen bond in the γ dimer.

Electrical and Thermal Characteristics of the Insulator–Metal Transition in Crystalline $$\hbox {V}_{2}\hbox {O}_{5}$$ V 2 O 5 Films

The electrical and thermal properties with respect to the crystallization in $$\hbox {V}_{2}\hbox {O}_{5}$$ thin films were investigated by measuring the resistance at different temperatures and applied voltages. The changes in the crystal structure of the films at different temperatures were also explored using Raman measurements. The thermal diffusivity of the crystalline $$\hbox {V}_{2}\hbox {O}_{5}$$ film was measured by the nanosecond thermoreflectance method. The microstructures of amorphous and crystalline $$\hbox {V}_{2}\hbox {O}_{5}$$ were observed by SEM and XRD measurements. The temperature-dependent Raman spectra revealed that a structural phase transition does not occur in the crystalline film. The resistance measurements of an amorphous film indicated semiconducting behavior, whereas the resistance of the crystalline film revealed a substantial change near $$250\,{^{\circ }}\hbox {C}$$ , and Ohmic behavior was observed above $$380\,{^{\circ }}\hbox {C}$$ . This result was due to the metal–insulator transition induced by lattice distortion in the crystalline film, for which $$T_{\mathrm{c}}$$ was $$260\,{^{\circ }}\hbox {C}$$ . $$T_{\mathrm{c}}$$ of the film decreased from 260 $${^{\circ }}\hbox {C}$$ to $$230\,{^{\circ }}\hbox {C}$$ with increasing applied voltage from 0 V to 10 V. Furthermore, the thermal diffusivity of the crystalline film was $$1.67\times 10^{-7}\,\hbox {m}^{2}\cdot \hbox {s}^{-1}$$ according to the nanosecond thermoreflectance measurements.

Exploring the effect of hole localization on the charge–phonon dynamics of hole doped delafossite

For weak or moderate doping, electrical measurement is not suitable for detecting changes in the charge localization inside a semiconductor. Here, to investigate the nature of charge–phonon coupling in the presence of gradually delocalized holes within a weak doping regime (~1016 cm−3), we examine the temperature dependent Raman spectra (303–817 K) of prototype hole doped delafossite (x = 0/0.03, y = 0/0.01). For both and phonons, negative lineshape asymmetry and relative thermal hardening are distinctly observed upon and doping. Using Allen formalism, charge density of states at the Fermi level per spin and molecule, and charge delocalization associated to plane, are estimated to increase appreciably upon codoping compared to the -axis. We delineate the interdependence between charge–phonon coupling constant () and anharmonic phonon lifetime (), and deduce that excitation of delocalized holes weakly coupled with phonons of larger is the governing feature of observed Fano asymmetry () reversal.

Diffuse phase transition of sol-gel deposited BaFexTi2-xO5 thin films

BaFexTi2-xO5 (x = 0, 0.004, 0.008, 0.012) thin films were prepared on Pt/Ti/SiO2/(100)Si substrates via sol-gel process. The phase transition behavior of the BaTi2O5 thin film was investigated using temperature dependent dielectric measurements in conjunction with temperature dependent Raman spectra. Diffuse phase transition was found in BaTi2O5 thin film which exhibited a broad peak without frequency dependence on the ε versus T, ε(T), curves around 370 °C. As the Fe content increased, a broad near-plateau dielectric permittivity maximum occurred in a wide temperature range. For x = 0.012, the value of the broad permittivity maximum was 875 at 100 kHz in the temperature range of 150–300 °C. It means that the Fe-doped BaTi2O5 thin films exhibited enhanced diffuse phase transition behavior. The wide temperature stability of dielectric permittivity was obtained in the Fe-doped BaTi2O5 thin films, which are promising in the high-temperature application.

Spin-phonon coupling in hexagonal Sr0.6Ba0.4MnO3

We have probed the magnetic properties of hexagonal Sr0.6Ba0.4MnO3 (SBMO) and its correlations with the vibrational spectra. Magnetization measurements reveal two antiferromagnetic transitions at 325K and at 227K. Temperature dependent Raman spectra of SBMO suggest that though structural symmetry is maintained across the transition temperatures, imprints of these magnetic transitions are noticed in anomalous behavior of Mn displacement E1g mode position in the same temperature range. A1g mode related to non-magnetic oxygen ion does not show such anomaly and follows an anharmonic behavior. Deviation from the anharmonic behavior of the E1g mode and its absence in A1g highlight the spin-phonon coupling in SBMO. The spin-phonon coupling parameter is evaluated to be 2.2cm−1.

Ferroelectric domains and phase evolution in (Fe:) KTa1−xNbxO3 crystals

The domain structures and phase evolution in mixed ferroelectric (Fe): KTa1−xNbxO3 (KTN) crystals were investigated. Temperature dependent Raman spectra show that Curie temperatures of KTN and Fe: KTN are far below room temperature, but the ferroelectric domain switching was still visualized by scanning probe microscopy at room temperature. These observed domains origin from the nano-regions near the grain boundaries. In addition, the intrinsic domains (triangle for KTN and straight line/stripe for Fe: KTN) could only be observed at low temperature by transmission electron microscopy. Three phase transitions in Fe: KTN crystals were found by Raman spectroscopy and dielectric testing: 175K for Rhombohedral-to-Orthorhombic (R–O), 210K for Orthorhombic-to-Tetragonal (O–T) and 250K for Tetragonal-to-Cubic (T–C), which is consistent with the domain behavior.

Phase Transformation, Vibrational and Electronic Properties of K2Ce(PO4)2: A Combined Experimental and Theoretical Study.

Herein we report the high-temperature crystal chemistry of K2Ce(PO4)2 as observed from a joint in situ variable-temperature X-ray diffraction (XRD) and Raman spectroscopy as well as ab initio density functional theory (DFT) calculations. These studies revealed that the ambient-temperature monoclinic (P21/n) phase reversibly transforms to a tetragonal (I41/amd) structure at higher temperature. Also, from the experimental and theoretical calculations, a possible existence of an orthorhombic (Imma) structure with almost zero orthorhombicity is predicted which is closely related to tetragonal K2Ce(PO4)2. The high-temperature tetragonal phase reverts back to ambient monoclinic phase at much lower temperature in the cooling cycle compared to that observed at the heating cycle. XRD studies revealed the transition is accompanied by volume expansion of about 14.4%. The lower packing density of the high-temperature phase is reflected in its significantly lower thermal expansion coefficient (αV = 3.83 × 10-6 K-1) compared to that in ambient monoclinic phase (αV = 41.30 × 10-6 K-1). The coexistences of low- and high-temperature phases, large volume discontinuity in transition, and large hysteresis of transition temperature in heating and cooling cycles, as well as drastically different structural arrangement are in accordance with the first-order reconstructive nature of the transition. Temperature-dependent Raman spectra indicate significant changes around 783 K attributable to the phase transition. In situ low-temperature XRD, neutron diffraction, and Raman spectroscopic studies revealed no structural transition below ambient temperature. Raman mode frequencies, temperature coefficients, and reduced temperature coefficients for both monoclinic and tetragonal phases of K2Ce(PO4)2 have been obtained. Several lattice and external modes of rigid PO4 units are found to be strongly anharmonic. The observed phase transition and structures as well as vibrational properties of both ambient- and high-temperature phases were complimented by DFT calculations. The optical absorption studies on monoclinic phase indicated a band gap of about 2.46 eV. The electronic structure calculations on ambient-temperature monoclinic and high-temperature phases were also carried out.

Selective Encaging of N2O in N2O-N2 Binary Gas Hydrates via Hydrate-Based Gas Separation.

The crystal structure and guest inclusion behaviors of nitrous oxide-nitrogen (N2O-N2) binary gas hydrates formed from N2O/N2 gas mixtures are determined through spectroscopic analysis. Powder X-ray diffraction results indicate that the crystal structure of all the N2O-N2 binary gas hydrates is identified as the structure I (sI) hydrate. Raman spectra for the N2O-N2 binary gas hydrate formed from N2O/N2 (80/20, 60/40, 40/60 mol %) gas mixtures reveal that N2O molecules occupy both large and small cages of the sI hydrate. In contrast, there is a single Raman band of N2O molecules for the N2O-N2 binary gas hydrate formed from the N2O/N2 (20/80 mol %) gas mixture, indicating that N2O molecules are trapped in only large cages of the sI hydrate. From temperature-dependent Raman spectra and the Predictive Soave-Redlich-Kwong (PSRK) model calculation, we confirm the self-preservation of N2O-N2 binary gas hydrates in the temperature range of 210-270 K. Both the experimental measurements and the PSRK model calculations demonstrate the preferential occupation of N2O molecules rather than N2 molecules in the hydrate cages, leading to a possible process for separating N2O from gas mixtures via hydrate formation. The phase equilibrium conditions, pseudo-pressure-composition (P-x) diagram, and gas storage capacity of N2O-N2 binary gas hydrates are discussed in detail.

Ferroelectric phase transition and electrical conduction mechanisms in high Curie-temperature PMN-PHT piezoelectric ceramics

Ferroelectric phase transition characteristic and electrical conduction mechanism of the high Curie-point (TC) 0.15Pb(Mg1/3Nb2/3)O3−0.4PbHfO3−0.45PbTiO3 (PMN-PHT) piezoelectric ceramics were studied by the temperature dependent Raman spectra and electrical properties. Sole first-order ferroelectric phase transition is demonstrated by the thermal hysteresis behavior of the temperature dependent dielectric constant and the dramatic drop of the derivative of inverse dielectric constant ξ= d(1/εr)/dT around TC in the PMN-PHT ceramics. The temperature dependent Raman spectroscopy not only provides further evidence for the ferroelectric to paraelectric phase transition appearing around TC in the PMN-PHT ceramics, but also reveals the successive phase symmetry changes of the polar nanoregions (PNRs), in which apparent anomalies appear in the Raman peaks' wavenumber, wavenumber distance, intensity, intensity ratio, and line width of some selected Raman modes upon heating. Typical sole cole-cole circle is obtained for the PMN-PHT ceramics in the temperature range of 440–560°C, based on which the activation energy (Ea) of the electrical conduction is calculated being ~1.2eV. Such low value of Ea indicates that the oxygen vacancies formed in the PHT-PMN ceramics induced by the evaporation of Pb during the sintering process dominate the high-temperature extrinsic electrical conduction.

Thermal Conductivity of M@C82 [M = Dy, Gd] Thin Films

Temperature dependent Raman spectra of a Dy@C82 thin film are investigated between 80 and 300 K. The first order temperature coefficient is evaluated for both Dy–C and C–C vibrational modes which reveals the presence of thermal and anharmonic contributions to the phonon hardening at low temperatures. The Dy–C mode broadens while decreasing the temperature revealing that the electron–phonon coupling is dominant over phonon–phonon coupling at low temperatures. In addition, laser-power dependent Raman spectra of Dy@C82 and Gd@C82 have been analyzed for estimating their first order power coefficients and the thermal conductivities.

Ultrathin MoO2 nanosheets with good thermal stability and high conductivity

Exploration and development of new two-dimensional (2D) materials with good stability and remarkable physical properties have become the research hotspots. We report for the first time the monodispersity of ultrathin MoO2 nanosheets have been synthesized through an improved chemical vapor deposition (CVD) method using only molybdenum trioxide as precursor. The grown MoO2 nanosheets have an average thickness of ∼ 5 to 10 nm and exhibit good crystal-quality. Temperature-dependent Raman spectra show that the ultrathin MoO2 nanosheets have high thermal stability up to 503 K. In addition, the first order temperature coefficients of the MoO2 characteristic Raman modes O1–Mo and O2–Mo were firstly found to be -1.91×10-2 and -3.94×10-2 cm−1/K, respectively. Two-probe electrical measurements show that the as-fabricated ultrathin MoO2 nanosheets devices preserve a high electrical conductivity in ambient conditions, reaching up to 200 - 475 S/cm. The exceptionally high conductivity of individual MoO2 nanosheet is ascribed to the unique crystal structure. Our results demonstrate that the ultrathin MoO2 nanosheets show great potential applications in constructing new integrated electronic devices and systems.

Strong thermal stability of Lu3Al5O12:Ce3+ single crystal phosphor for laser lighting

Green-yellow single crystal Lu3Al5O12:Ce3+ (LuAG:Ce) as a color conversion phosphor for high-power laser lighting was grown through a floating zone method by an image furnace. It showed an excellent thermal stability, a lower quantum efficiency (QE), and a broader photoluminescence excitation (PLE) spectrum compared with the polycrystalline powder phosphor. The thermal stability was explained by a decrease of intrinsic defects and a less phonon generation at high temperature, confirmed by PLE spectra and temperature-dependent Raman spectra, respectively. To evaluate its feasibility for high-power laser lighting, the single crystal LuAG:Ce was applied to a 5W blue laser diode with a remote-phosphor structure, and thus it reached a lower equilibrium temperature at 62°C after 40min operating in comparison with the polycrystalline powder phosphor (95°C). Thus, the single crystal LuAG:Ce phosphor can achieve the extremely stable light emission without an efficiency loss under the high-power density of blue laser diode.

Temperature-dependent optical phonon behaviour of a spinel Zn2TiO4single crystal grown by the optical floating zone method in argon atmosphere

The spinel Zn2TiO4single crystals were grownviaoptical floating zone technology in an argon atmosphere for the first time. And temperature dependent Raman spectra were presented.

Intermediate phases during solid to liquid transitions in long-chain n-alkanes.

The solid to liquid phase transition of n-alkanes with more than ten carbon atoms is an interesting phenomenon relevant to many fields, from cosmetics to automotive. Here we report Raman spectroscopy of tetradecane, pentadecane and hexadecane as a function of temperature. In order to gain information on the structural changes that the hydrocarbons undergo during melting, and to determine the temperature and the speed at which the phase change occurs, their temperature-dependent Raman spectra are acquired. The spectra are analysed not only with respect to frequency shifts, band widths, and intensity ratio of certain bands, but also using a principal component analysis. The spectroscopic data suggest that the solid to liquid phase transition in hexadecane, differently from tetradecane and pentadecane, is almost instantaneous. Tetradecane shows a slightly faster transition than pentadecane. In addition, a rotator phase as an intermediate state between the liquid and crystalline solid phases is identified in pentadecane. Different characteristic features in the solid spectra of the hydrocarbons relate tetradecane and hexadecane to a tryclinic crystalline structure, and pentadecane to an orthorhombic structure.

Temperature and composition dependent phase transitions of lead-free piezoelectric (Bi0.5Na0.5)TiO3-BaTiO3 thin films.

Extensive studies on (Bi0.5Na0.5)TiO3-BaTiO3 piezoceramics have aroused growing interest in developing high-performance piezoelectric films based on this promising lead-free piezoelectric system. High quality (1 - x%) (Bi0.5Na0.5)TiO3-x% BaTiO3 (x = 0-15) thin films were synthesized on Pt(111)/Ti/SiO2/Si(100) substrates via a sol-gel method. The phase transitions as functions of composition and temperature were investigated by X-ray diffraction (XRD) and temperature-dependent Raman spectra analysis. A morphotropic phase boundary (MPB) located at 5 ≤ x ≤ 10 was found in the films, being identical to that in bulk materials. Additionally, complicated structural changes with temperature in a wide composition range were observed, which correspond to the evolution from a low-symmetry ferroelectric phase or a relaxor phase to high-symmetry relaxor phases. The ferroelectricity and piezoelectricity were also systematically studied with an emphasis on the influence of composition and phase structure. A phase diagram focusing on the above two kinds of phase transitions has been proposed in this work.