Temperature-dependent Raman Spectroscopy Research Articles

Atomistic insight into lithospheric conductivity revealed by phonon\u2013electron excitations in hydrous iron-bearing silicates

Amphiboles are essential components of the continental crust and subduction zones showing anomalous anisotropic conductivity. Rock properties depend on the physical properties of their constituent minerals, which in turn depend on the crystal phonon and electron density of states. Here, to address the atomic-scale mechanism of the peculiar rock conductivity, we applied in situ temperature-dependent Raman spectroscopy, sensitive to both phonon and electron states, to Fe2+-rich amphiboles. The observed anisotropic resonance Raman scattering at elevated temperatures, in combination with density-functional-theory modelling, reveals a direction-dependent formation of mobile polarons associated with coupled FeO6 phonons and electron transitions. Hence, temperature-activated electron-phonon excitations in hydrous iron-bearing chain and layered silicates are the atomistic source of anisotropic lithospheric conductivity. Furthermore, reversible delocalization of H+ occurs at similar temperatures even in a reducing atmosphere. The occurrence of either type of charge carriers does not require initial mixed-valence state of iron or high oxygen fugacity in the system.

Temperature-Dependent Phonon Shifts in van der Waals Crystals.

In recent years, there has been an explosive increase in the research on van der Waals (vdW) crystals because of their great potential applications in many optoelectronic devices. It is necessary to determine their temperature-dependent lattice vibration characteristics because their thermal and electrical transport are closely related to the anharmonic phonon effect, which will affect the performance of the devices. We review the temperature-dependent Raman spectroscopy of vdW crystals, systematically introduce the thermal behavior of optical phonons, and summarize their shift with temperature. Upon analyzing the theoretical models and summarizing the reported experimental data, it is found that the phonon shifts of vdW crystals have a "quasi-linear" relationship with temperature, which is widely described with first-order temperature (FOT) coefficients obtained through a linear fit. Thus, subsequently, the phonon shifts of monolayer materials, different-thickness crystals, suspended and supported samples, in-plane and out-of-plane modes in the same vdW materials, as well as heterostructures and alloys are discussed through comparative analysis of FOT coefficients.

Back-bonding driven negative thermal expansion along the one dimensional Hg[sbnd]C[tbnd]N linkage in [HgCN](NO3) probed by Raman spectroscopy

In this paper, the mechanistic aspect for anisotropic negative thermal expansion (NTE) in [Hg(CN)](NO3) is correlated to back-bonding along a one dimensional HgCN network using temperature dependent Raman spectroscopy. Signatures of back-bonding are contraction of the HgC bond (~299 cm−1), expansion of the CN bond (~2249 cm−1) and bending of the HgCN bond. These are clearly observed in the Raman spectra, correlating to a continuous increase in the HgCN bending frequency (~346 cm−1) with an increase in temperature. This bending of the HgCN bond may be responsible for an expansion of the Hg lattice along the ab-plane and a reorientation of the NO3− ions, as seen by the change in frequency variation of the 1637 cm−1 mode and the reduction of its intensity with the increase in temperature. All these effects are in agreement with the reported single crystal X-ray data. Thus, the present model of back-bonding electron transfer from the metal d-orbital to the π*-orbital of the cyanide ligand and the effect of temperature changes on it can be explored to understand the NTE behaviour in metal cyanide systems and more broadly in multifunctional materials.

Thermal expansion coefficient of few-layer MoS2 studied by temperature-dependent Raman spectroscopy

The thermal expansion coefficient is an important thermal parameter that influences the performance of nanodevices based on two-dimensional materials. To obtain the thermal expansion coefficient of few-layer MoS2, suspended MoS2 and supported MoS2 were systematically investigated using Raman spectroscopy in the temperature range from 77 to 557 K. The temperature-dependent evolution of the Raman frequency shift for suspended MoS2 exhibited prominent differences from that for supported MoS2, obviously demonstrating the effect due to the thermal expansion coefficient mismatch between MoS2 and the substrate. The intrinsic thermal expansion coefficients of MoS2 with different numbers of layers were calculated. Interestingly, negative thermal expansion coefficients were obtained below 175 K, which was attributed to the bending vibrations in the MoS2 layer during cooling. Our results demonstrate that Raman spectroscopy is a feasible tool for investigating the thermal properties of few-layer MoS2 and will provide useful information for its further application in photoelectronic devices.

Investigation of the metal-to-insulator transition of N-doped VO2(M1) thin films

This work reports the effect of nitrogen doping on the electrical and optical properties of VO2 thin films. An innovative method based on a VN target and an O2 ambient gas was used to synthesize N:VO2 films on quartz substrates using reactive pulsed laser deposition (RPLD). The doping percentage was determined by elastic recoil distribution (ERD) measurements, while temperature-dependent Raman spectroscopy as well as electrical resistivity and infrared reflectance measurements were performed to investigate the effect of N-doping on the insulator monoclinic-to-metallic tetragonal phase transition (IMT) of VO2. A small doping percentage around 0.7 at.% was achieved, inducing a decrease of the transition temperature TIMT from 68 °C to 50 °C, with slightly reduced electrical contrast ΔR and optical contrast ΔA. By comparing the volume fraction of monoclinic phase, obtained from Raman data to the volume fraction of metallic phase deduced concurrently from both electrical and optical measurements, the IMT in our nitrogen-doped VO2 films occurs through a percolation process.

The mechanism of phase transitions and luminescence properties of azide perovskites

The temperature-dependent IR and Raman spectroscopy has been used to study the phase transitions in Na/K-chromium-azide frameworks with tetramethylammonium (TeMA+) cations, [(CH3)4N]2[NaCr(N3)6] and [(CH3)4N]2[KCr(N3)6], which crystallize in a perovskite-like structure. The phase transition occurring within 310–315 K in both compounds leads to a symmetry change from Pa-3 to Fm-3m and its major contributor is an order-disorder process of the organic cations followed by an adjustment of the azide bridges. The luminescence properties arise from the optically active Cr3+ ions and the studies reveal some admixture of low-symmetry component of the octahedral crystal environment of the Cr3+ sites.

Potential liquid crystal-based biosensor depending on the interaction between liquid crystals and proteins

Liquid crystal biosensors serve an essential role to detect biomolecules and chemical events as an effective, simple and early detection tool. The detection of Human serum albumin protein by a room temperature liquid crystal 4́-pentyl-4-biphenylcarbonitrile has been investigated using multiple techniques such as Polarizing optical microscope, Raman spectroscopy and molecular docking. The dynamics of director transfigurations of the liquid crystal molecule in the presence of protein through interactions are reported in the study. The change in the alignment of liquid crystal molecules during the nematic phase is observed under a polarizing optical microscope. The interactions through which the liquid crystal molecules bind with protein is depicted from the docking analysis. The residues in the active sites confirm their presence from docking studies. The spectral behaviour has been investigated by temperature-dependent Raman spectroscopy. The findings from Raman spectra for the interaction between these compounds correlates with the residues confirmed from molecular docking analysis.

Temperature dependent Raman spectroscopy of shear and layer breathing modes in bilayer MoS2

We report low-frequency Raman scattering results of bilayer (2L) MoS2 structures with 3R and 2H stacking orders. The stacking orders were identified by Raman mapping images of the low-frequency shear (S) and layer breathing (B) modes. Linear temperature coefficients (χ) were obtained from temperature-dependent frequency changes of the shear and layer breathing modes for the 3R- and 2H-stacked 2L MoS2: χ3RS=−0.011±0.006cm−1/K, χ2HS=−0.009±0.002cm−1/K, χ3RB=−0.009±0.003cm−1/K, and χ2HB=−0.010±0.002cm−1/K. Interestingly, the temperature coefficients were significantly reduced after the MoS2 samples were thermally heated: χ3RS=−0.004±0.001cm−1/K, χ2HS=−0.004±0.001cm−1/K, χ3RB=−0.002±0.001cm−1/K, and χ2HB=−0.002±0.001cm−1/K. Similar behaviors were also observed for the temperature coefficients of the high-frequency E2g1 and A1g phonons. The heating-induced modifications in the temperature coefficients are attributed to changes in the MoS2–substrate coupling. Our result further paves the way for optimal design strategies for low-dimensional device applications where thermal management plays a crucial role in device performance.

Spin induced exchange bias and lattice modulation in Nd1−x Eu x CrO3

We report the evolution of coupled phonons and exchange bias (EB) in perovskite-type Nd1−x Eu x CrO3 (x = 0.0, 0.05, and 0.10) samples by means of temperature-dependent Raman spectroscopy and dc magnetization measurements. We observed a non-monotonic behavior of the EB field around the temperature T *, which lies between the antiferromagnetic ordering temperature (T N) and spin-reorientation transition temperature (T SR). The temperature dependence of phonon modes related to antistretching and bending of CrO6 octahedra and Nd3+/Eu3+ ion vibration below T N confirms the strong spin–phonon coupling. The T * found from the non-monotonicity of the EB is imprinted with the additional anomaly observed in the low-temperature spin–phonon behavior. The phonon modes and phonon anomaly are also verified using the density functional theory-based calculations.

Evidence for Dominant Phonon-Electron Scattering in Weyl Semimetal WP2

Topological semimetals have revealed a wide array of novel transport phenomena, including electron hydrodynamics, quantum field theoretic anomalies, and extreme magnetoresistances and mobilities. However, the scattering mechanisms central to these behaviors remain largely unexplored. Here we reveal signatures of significant phonon-electron scattering in the type-II Weyl semimetal WP$_{2}$ via temperature-dependent Raman spectroscopy. Over a large temperature range, we find that the decay rates of the lowest energy $A_{1}$ modes are dominated by phonon-electron rather than phonon-phonon scattering. In conjunction with first-principles calculations, a combined analysis of the momentum, energy, and symmetry-allowed decay paths indicates this results from intraband scattering of the electrons. The excellent agreement with theory further suggests that such results could be true for the acoustic modes. We thus provide evidence for the importance of phonons in the transport properties of topological semimetals and identify specific properties that may contribute to such behavior in other materials.

Hidden polymorphism of FAPbI3 discovered by Raman spectroscopy.

Formamidinium lead iodide (FAPbI3) can be used in its cubic, black form as a light absorber material in single-junction solar cells. It has a band-gap (1.5 eV) close to the maximum of the Shockley-Queisser limit, and reveals a high absorption coefficient. Its high thermal stability up to 320 °C has also a downside, which is the instability of the photo-active form at room temperature (RT). Thus, the black α-phase transforms at RT with time into a yellow non-photo-active δ-phase. The black phase can be recovered by annealing of the yellow state. In this work, a polymorphism of the α-phase at room temperature was found: as-synthesized (αi), degraded (αδ) and thermally recovered (αrec). They differ in the Raman spectra and PL signal, but not in the XRD patterns. Using temperature-dependent Raman spectroscopy, we identified a structural change in the αi-polymorph at ca. 110 °C. Above 110 °C, the FAPbI3 structure has undoubtedly cubic Pm3[combining macron]m symmetry (high-temperature phase: αHT). Below that temperature, the αi-phase was suggested to have a distorted perovskite structure with Im3[combining macron] symmetry. Thermally recovered FAPbI3 (αrec) also demonstrated the structural transition to αHT at the same temperature (ca. 110 °C) during its heating. The understanding of hybrid perovskites may bring additional assets in the development of new and stable structures.

Revealing a masked Verwey transition in nanoparticles of coexisting Fe-oxide phases.

The attractive electronic and magnetic properties together with their biocompatibility make iron-oxide nanoparticles appear as functional materials. In Fe-oxide nanoparticle (IONP) ensembles, it is crucial to enhance their performance thanks to controlled size, shape, and stoichiometry ensembles. In light of this, we conduct a comprehensive investigation in an ensemble of ca. 28 nm cuboid-shaped IONPs in which all the analyses concur with the coexistence of magnetite/maghemite phases in their cores. Here, we are disclosing the Verwey transition by temperature dependent (4–210 K) Raman spectroscopy.

Thermal expansion coefficient of monolayer MoS2 determined using temperature-dependent Raman spectroscopy combined with finite element simulations

Microstructures is an international peer-reviewed, open access, online journal. The journal of Microstructures publishes articles covering new research and developments, perspectives, and reviews related to the microstructures of materials, including, but not limited to metals and alloys, ceramics, polymers, composites, crystals, glasses, biomaterials, interfaces and nanomaterials.

Phase Transitions in Low-Dimensional Layered Double Perovskites: The Role of the Organic Moieties

Halide double perovskites are an interesting alternative to Pb-containing counterparts as active materials in optoelectronic devices. Low-dimensional double perovskites are fabricated by introducing large organic cations, resulting in organic/inorganic architectures with one or more inorganic octahedra layers separated by organic cations. Here, we synthesized layered double perovskites based on 3D Cs2AgBiBr6, consisting of double (2L) or single (1L) inorganic octahedra layers, using ammonium cations of different sizes and chemical structures. Temperature-dependent Raman spectroscopy revealed phase transition signatures in both inorganic lattice and organic moieties by detecting variations in their vibrational modes. Changes in the conformational arrangement of the organic cations to an ordered state coincided with a phase transition in the 1L systems with the shortest ammonium moieties. Significant changes of photoluminescence intensity observed around the transition temperature suggest that optical properties may be affected by the octahedral tilts emerging at the phase transition.

Temperature-dependent phonon dynamics of Ag3PO4 microcrystals

In this work, we present the study of the temperature-dependent behavior of silver orthophosphate (Ag3PO4) microcrystals using in situ Raman scattering. The Ag3PO4 as-synthesized microcrystals were prepared by the precipitation method and characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman and infrared spectroscopy, and differential scanning calorimetry (DSC). Temperature-dependent phonon dynamics were performed on Ag3PO4 microcrystals and pointed to a first-order phase transition in the temperature range 500–515 °C: Phase I (25–500 °C) → Phase II (515–590 °C). The phase transition is reversible and a temperature hysteresis was observed during the heating – cooling process: Phase II (590–470 °C) → Phase I (455–25 °C). The reversible phase transition is related to the distortion of the tetrahedral symmetry of PO4 caused by the decrease in the crystalline order. DSC analysis confirmed the results of temperature-dependent Raman spectroscopy.

Effects of Cu-doping on the vibrational and electronic properties of epitaxial PrNiO3 thin films

We have synthesized a series of epitaxial PrNi1-xCuxO3 (x = 0–0.1) thin films (12 nm) on single crystal LaAlO3 substrate by pulsed laser deposition (PLD) method and studied the effects on vibrational and electronic properties of the prepared films. Raman spectroscopy emerges as an alternate tool to indicate an increment in octahedral tilt angles due to Cu-doping. However, this change in the tilting angle does not change the overall structural symmetry in the series. For a comparative study by temperature-dependent Raman spectroscopy, two films with x = 0 (undoped) and 0.10 (maximally doped) were chosen. The presence of anisotropic Grüneisen parameter in the system indicates that Raman frequencies associated with NiO6 octahedral vibrations are more sensitive to temperature as compared to that of NiO bond stretching frequencies. Moreover, the system undergoes more thermal expansion with temperature after inclusion of Cu-doping. Temperature-dependent Raman line width fitted with Balkanski model signifies that the anharmonicity in the system increases with Cu-doping. All the films exhibit steady metallicity below 300 K. The theoretical fits to the temperature-dependent resistivity indicate the existence of Non-Fermi Liquid transport, where all fitting parameters vary depending on Cu-doping.

Anisotropic thermoelectric transport in textured Sb1.5Bi0.5Te3 nanomaterial synthesized by facile bottom-up physical process

High anisotropy in thermal conductivity (κ) coupled with anisotropy in Seebeck coefficient (S) has resulted in peak zT value of 0.85 at 150 °C in the textured Sb1.5Bi0.5Te3 nanomaterial in perpendicular to the preferential ab-plane direction. Further, strong and opposite temperature dependences in electrical conductivity (σ) and thermal conductivity, in the ranges of 1.6–1.8 and 1.4–1.8 respectively, have resulted in better average zT value of 0.72 in the 50°C–250 °C temperature range along this direction, where zT = (S2σ/κ)T. The high anisotropy in Seebeck coefficient in the range of 0.82–0.84 is peculiar, which may be attributed to differential scattering for holes and electrons by oxide interface on the ab-planes. Sample temperature and laser power dependent Raman spectroscopy have revealed that Eg(2) mode is dominant phonon transfer mode in this material and therefore, scattering of Eg(2) mode phonons may be critical in obtaining thermal conductivity reduction. The nanomaterial has been synthesized by a facile bottom-up physical synthesis process and consolidated by direct current hot pressing. Our synthesis process requires controlled melting of ingredient metals at just above their melting points-rocking and air quenching. The synthesized nanomaterial hardly experiences its melting point in this process. This energy efficient process does not require any kind of milling and produces single phase nanomaterial in unique plate-like morphology, which easily results in high texturing. This texturing has been studied by XRD analysis as well as SEM images and is also correlated with texture factor in the thermoelectric measurements. HRTEM image has shown high grain boundary density within the plates, but these have not been able to scatter phonons for thermal conductivity reduction within ab-plane. This is possibly due to smaller than optimum size of these randomly oriented grains. Such features also offer possibility of zT enhancement along preferential ab-plane direction in the textured specimen. Further, the study of anisotropy in power output density and thereby, engineered power factor under practical temperature gradients is also presented. The process also offers simplicity in obtaining further thermal conductivity reduction in perpendicular to the preferential ab-plane direction by use of tellurium or semiconductor interface layer or by use of metallic cluster in the ab-plane direction.

Lattice dynamics of the topological Dirac semimetal LaAgSb2 with charge density wave ordering

LaAgSb$_{2}$ is a rare material, which offers the opportunity to investigate the complex interplay between charge density wave (CDW) ordering and topology protected electronic band structure. As both of these phenomena are governed by the structural symmetries, a comprehensive study of the lattice dynamics is highly desirable. In this report, we present the results of temperature and pressure dependent Raman spectroscopy and x-ray diffraction in single crystalline LaAgSb$_{2}$. Our results confirm that Raman spectroscopy is a highly sensitive tool to probe CDW ordering phenomenon, particularly the low-temperature second CDW transition in LaAgSb$_{2}$, which appears as a very weak anomaly in most experiments. The crystal orientation-dependent measurements provide the evolution of Raman modes with crystallographic symmetries and can be further studied through group symmetry analysis. The low-temperature x-ray diffraction data show the emergence of structural modulations corresponding to the CDW instability. The combined high-pressure Raman spectroscopy and synchrotron x-ray diffraction reveal multiple structural phase transitions through lowering of crystalline symmetries, which are also expected to lead to electronic topological transitions.

Thickness dependence of antiferromagnetic phase transition in Heisenberg-type MnPS3

The behavior of 2-dimensional (2D) van der Waals (vdW) layered magnetic materials in the 2D limit of the few-layer thickness is an important fundamental issue for the understanding of the magnetic ordering in lower dimensions. The antiferromagnetic transition temperature TN of the Heisenberg-type 2D magnetic vdW material MnPS3 was estimated as a function of the number of layers. The antiferromagnetic transition was identified by temperature-dependent Raman spectroscopy, from the broadening of a phonon peak at 155 cm−1, accompanied by an abrupt redshift and an increase of its spectral weight. TN is found to decrease only slightly from ~78 K for bulk to ~66 K for 3L. The small reduction of TN in thin MnPS3 approaching the 2D limit implies that the interlayer vdW interaction is playing an important role in stabilizing magnetic ordering in layered magnetic materials.

Structural features of solid-solid phase transitions and lattice dynamics in U3O8

Triuranium octoxide $({\mathrm{U}}_{3}{\mathrm{O}}_{8})$ undergoes an orthorhombic to hexagonal structural phase transition near ${T}_{s}=305{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, and a separate nonstructural phase transition at ${T}_{c}=210{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. The later transition has previously been associated with temperature-induced fluctuations in the uranium oxidation state. A discontinuity in the slope of electrical conductivity versus temperature measurement at $210{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ has supported this idea. The orthorhombic phase has three crystallographic sites in two distinct oxidation configurations [2 U(V) and 1 U(VI)], whereas the hexagonal phase has one distinct uranium site. High-resolution x-ray diffraction measurements eliminate the possibility of superlattice Bragg reflections to less than 0.2 ${\mathrm{e}}^{\ensuremath{-}}$ scattering power and ${\mathrm{U}}_{3}{\mathrm{O}}_{8}$ is not metallic; consequently, the presence of oxidation fluctuations is required for charge balancing. Interestingly, the order-to-disorder transition occurs at a much lower temperature than the structural transition. Using temperature-dependent x-ray diffraction and Raman spectroscopy, we show anisotropic lattice expansion in the in-plane $b$ and $c$ lattice constants. A specific discontinuity in the temperature derivatives of the $b$ and $c$ lattice constants are the first reported structural signatures of the order-to-disorder transition, suggestive of a change in local U--O coordination. Phonon frequencies of ${\mathrm{U}}_{3}{\mathrm{O}}_{8}$ measured by Raman spectroscopy show significant temperature-dependent dynamics. Redshifting of several modes between 40 and $300{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ cannot be explained by unit cell expansion alone because the unit cell volume decreases in this region. Instead, we show that phonon frequencies are highly correlated with the anisotropic lattice expansion/contraction along specific crystallographic directions.