Raman Experiments Research Articles

Raman scattering study of two-dimensional magnetic van der Waals compound VI3**Project supported by the Science Fund from the Ministry of Science and Technology of China (Grant Nos. 2017YFA0302904 and 2016YFA0300504), the National Natural Science Foundation of China (Grant Nos. 11774419, U1932215, 11774423, and 11822412), the Fundamental Research Funds for the Central Universities, China, and the Research Funds of Renmin University

The layered magnetic van der Waals materials have generated tremendous interest due to their potential applications and importance in fundamental research. Previous x-ray diffraction (XRD) studies on the magnetic van der Waals compound VI3, revealed a structural transition above the magnetic transition but output controversial analysis on symmetry. In this paper we carried out polarized Raman scattering measurements on VI3 from 10 K to 300 K, with focus on the two Ag phonon modes at ∼ 71.1 cm−1 and 128.4 cm−1. Our careful symmetry analysis based on the angle-dependent spectra demonstrates that the crystal symmetry can be well described by C2h rather than D3d both above and below structural phase transition. We further performed temperature-dependent Raman experiments to study the magnetism in VI3. Fano asymmetry and anomalous linewidth drop of two Ag phonon modes at low temperatures, point to a significant spin–phonon coupling. This is also supported by the softening of 71.1-cm−1 mode above the magnetic transition. The study provides the fundamental information on lattice dynamics and clarifies the symmetry in VI3. And spin–phonon coupling existing in a wide temperature range revealed here may be meaningful in applications.

Probing Europa's subsurface ocean composition from surface salt minerals using in-situ techniques

The composition of Europa's subsurface ocean is of great interest for understanding the internal geochemistry and potential habitability of this icy body. However, current constraints on the ocean composition need to rely largely on its expression on the surface. In this work, we combine chemical divide modeling with cryogenic Raman and X-ray diffraction experiments to examine freezing of a simple putative brine system containing Na+, Mg2+, Cl−, and SO42− across a range of ionic concentrations and freezing rates to assess the feasibility of inferring the ocean's chemical composition via in-situ techniques. The results suggest that multiple hydrated salts not predicted by chemical models are frequently encountered in the final solid phase, making accurate quantification of the subsurface liquid composition via surface observables rather challenging. In addition, flash freezing of diluted brines often produces water ice together with vitreous Mg-bearing salt hydrates, which may significantly hinder their detection. These findings can help inform both analytical protocols for a Raman spectrometer onboard a Europa surface lander as well as potential locations for exploration, in order to best provide meaningful constraints on emplacement mechanisms and the composition of frozen salt minerals on the surface.

Phase diagram of hydrogen at extreme pressures and temperatures; updated through 2019 (Review article)

Hydrogen is expected to display remarkable properties under extreme pressures and temperatures stemming from its low mass and thus propensity to quantum phenomena. Exploring such phenomena remains very challenging even though there was a tremendous technical progress both in experimental and theoretical techniques since the last comprehensive review (McMahon et al.) was published in 2012. Raman and optical spectroscopy experiments including infrared have been extended to cover a broad range of pressures and temperatures (P—T) probing phase stability and optical properties at these conditions. Novel pulsed laser heating and toroidal diamond anvil techniques together with diamond anvil protecting layers drastically improved the capabilities of static compression methods. The electrical conductivity measurements have been also performed to much higher than previously pressures and extended to low temperatures. The dynamic compression techniques have been dramatically improved recently enabling ramp isentropic compression that allows probing a wide range of P–T thermodynamic pathways. In addition, new theoretical methods have been developed beyond a common DFT theory, which make them predictive and in better agreement with experiments. With the development of new theoretical and experimental tools and sample loading methods, the quest for metallic hydrogen accelerated recently delivering a wealth of new data, which are reviewed here.

Discovery of pressure‐induced monoclinic to monoclinic phase transition above 60 GPa in single crystal NaAlSi2O6 jadeite

AbstractAs one typical aluminum‐ and sodium‐bearing silicate mineral in the crust, the high‐pressure behaviors of NaAlSi2O6 jadeite are always of interest for investigation. However, the phase transition of jadeite at lower mantle remains elusive; thus, further static compression experiments and systematic theoretical calculations are highly needed. Here, we report a new monoclinic to monoclinic phase transition of jadeite with a volume collapse of 3.17% at around 68 GPa by in situ Raman experiments and first‐principles calculations at high pressures. Such transition leads to the increase of coordination numbers of Si from four‐ to five‐fold coordination, because of Bader charge transfer between Si─O and Al─O bonds. The physical mechanism behind this transition provides important value to understand the minerals in the lower mantle.

Raman and ab initio analyses of ion pairs in concentrated K[B(OH)4] droplets.

In this study, microscopic Raman spectroscopy and Ab initio quantum chemical calculation were used to determine the structural details of ion pairs and their transformation in concentrated K[B(OH)4] droplets. The Raman experiment shows that the vsym-B(OH)4- undergoes a downward shift with the decrease of WSR. The contact ion pairs (CIPs) change to solvent shared ion pairs when the molar water-to-solute ratio (WSR) is bigger than 6; CIPs are the dominant species when 1.33<WSR<6, where K+ bonds to [B(OH)4-] in bidentate form (CIP-II); the CIPs quickly dehydrate and associate to triple ion pairs (TIPs) when WSR<5. Raman experiment and ab initio quantum chemical calculation show that TIPs are mainly present in "anionic" type such as {[B(OH)4-]K+[B(OH)4-](H2O)n}, where K+ bonds to two [B(OH)4-] in bidentate or/and tridentate form (TIP-a-II or/and TIP-a-III). When WSR <1.33, most TIPs convert to complex clusters such as chain-like structure. The remaining TIPs associate to six-membered ring structure [B3O3(OH)4-] and the relative content increases from 0 to 20% when the WSR ranges from 1.33 to 0.55.

Enhanced visible-light photocatalytic activity of anatase-rutile mixed-phase nano-size powder given by high-temperature heat treatment.

Nano-size EVONIK AEROXIDE® P25 titanium dioxide, TiO2, powder was heat-treated at temperatures, 700–900°C, in air. An X-ray diffraction study showed that the P25 powder is composed of approximately 20 and approximately 80 mass% of rutile and anatase phases, respectively. It was also shown that the transformation from anatase to rutile induced by high-temperature heat treatment was almost completed at 750°C, whereas a small amount (less than 3 mass%) of anatase phase was still left even in the powder heat-treated at 900°C. The transformation behaviour was consistent with results obtained by Raman scattering spectroscopy. Raman experiments also indicated that high-temperature heating induced the formation of oxide ion vacancies. Powders were dispersed in methyl orange (MO) aqueous solution, and the bleach rate of MO was measured to evaluate photocatalytic activity under ultraviolet (UV)- and visible-light irradiation. After the heat treatment, the UV-light photocatalytic performance sharply deteriorated. Interestingly, visible-light photocatalytic activity was enhanced by high-temperature heating and reached the highest performance for an 800°C-heated sample, indicating that the P25 powder obtained high visible-light photocatalytic performance after heat treatment. Even after 900°C heat treatment, the photocatalytic performance was higher than that of as-received powder. Enhancement of photocatalytic activities was discussed in relation to visible light absorption and charge carrier transfer.

Single particle Raman spectroscopy analysis of the metal-organic framework DUT-8(Ni) switching transition under hydrostatic pressure.

Experimental in situ observations of phase coexistence in switchable metal-organic frameworks are reported to provide a fundamental understanding of dynamic adsorbents that can change their pore structure in response to external stimuli. A prototypical flexible pillared layer framework DUT-8(Ni) (DUT = Dresden University of Technology) was studied under hydrostatic pressure by in situ Raman spectroscopy on single crystals. The closing transition of the open pore phase (op) containing DMF in the pores in silicon oil as a pressure transmitting fluid, as well as the closed pore phase (cp) to op transition under pressure in methanol, were studied. Phase coexistences during both transitions were observed.

Alkylphosphates as low-cost inhibitors in silver tarnishing

Tarnishing is a process taking place in silver surface causing corrosion. This is due to the presence in the atmosphere, even in small amounts, of reactive species like water, sulphidric acid and carbonyl sulphide which can be physisorbed onto the silver surface thus reacting with it. We propose the use of alkylphosphates to protect the silver surface against corrosion: the polar head group should interact with the silver surface with the formation of a thin film of opportunely oriented surfactant molecules. This would shield the silver surface from the attack of reactive agents present in the atmosphere and from the adsorption of humidity water helped by the surfactant scarce hydrophilicity. The joint Raman and XPS experiments showed the protective action exerted by dibutylphosphate or bis (2-etylhexyl) phosphate according to the different sulphidizing conditions. Our discovery deserves to be tailored in cultural heritage where silver artefacts are generally precious and inimitable objects, but also in modern microelectronics where silver joints and conductors are used.

Crystal Structure and Stability of Ammonium Azide Under High Pressure

Due to its potential applications as a high-energy density material, the high-pressure polymorphs of ammonium azide (AA) have received much attention recently. However, the crystal structure of phase II (AA-II), stable above 3.0 GPa, has remained elusive until now. By combining X-ray diffraction and Raman experiments with first-principles calculations, we determine that AA-II is a hydrogen (H)-bonded structure of ammonium and azide ions with monoclinic symmetry, space group P2/c. The latter is comprised of alternating molecular layers of ammonium and azide ions and mostly differs from AA-I by its denser packing of molecular planes while preserving the hydrogen bond network. First-principles calculations show that phase II has the lowest enthalpy among all other considered structures from 4.9 to 102.6 GPa, pushing the phase transitions to the previously predicted hydronitrogen solids to higher pressures. Raman data to 85.0 GPa at room temperature confirm the absence of phase transition and agree very well with the pressure evolution of the Raman modes of AA-II predicted by our calculations.

Covalent Organic Framework (COF-1) under High Pressure.

COF‐1 has a structure with rigid 2D layers composed of benzene and B3O3 rings and weak van der Waals bonding between the layers. The as‐synthesized COF‐1 structure contains pores occupied by solvent molecules. A high surface area empty‐pore structure is obtained after vacuum annealing. High‐pressure XRD and Raman experiments with mesitylene‐filled (COF‐1‐M) and empty‐pore COF‐1 demonstrate partial amorphization and collapse of the framework structure above 12–15 GPa. The ambient pressure structure of COF‐1‐M can be reversibly recovered after compression up to 10–15 GPa. Remarkable stability of highly porous COF‐1 structure at pressures at least up to 10 GPa is found even for the empty‐pore structure. The bulk modulus of the COF‐1 structure (11.2(5) GPa) and linear incompressibilities (k [100]=111(5) GPa, k [001]=15.0(5) GPa) were evaluated from the analysis of XRD data and cross‐checked against first‐principles calculations.

Covalent Organic Framework (COF‐1) under High Pressure

AbstractCOF‐1 has a structure with rigid 2D layers composed of benzene and B3O3 rings and weak van der Waals bonding between the layers. The as‐synthesized COF‐1 structure contains pores occupied by solvent molecules. A high surface area empty‐pore structure is obtained after vacuum annealing. High‐pressure XRD and Raman experiments with mesitylene‐filled (COF‐1‐M) and empty‐pore COF‐1 demonstrate partial amorphization and collapse of the framework structure above 12–15 GPa. The ambient pressure structure of COF‐1‐M can be reversibly recovered after compression up to 10–15 GPa. Remarkable stability of highly porous COF‐1 structure at pressures at least up to 10 GPa is found even for the empty‐pore structure. The bulk modulus of the COF‐1 structure (11.2(5) GPa) and linear incompressibilities (k[100]=111(5) GPa, k[001]=15.0(5) GPa) were evaluated from the analysis of XRD data and cross‐checked against first‐principles calculations.

Spectroscopic Characterization of Some Foil Alloys Based on Nickel

In this research, we carried out some structural and magnetic analyses of foil alloys in the form of NixCr1-x (x=0.8); NixCuyFez (x=0.65, y=0.33, z=0.02); NixFeyCuzMot (x=0.77, y=0.14, z=0.05, t=0.04); NixCryCoz(MoTiAlFe)t (x=0.58, y=0.19, z=0.14, t=0.09) and NixCoyCrzFet(NMoMn)w (x=0.13, y=0.425, z=0.20, t=0.20, w=0.045). In order to have knowledge about the magnetic properties, we use electron paramagnetic resonance spectroscopy which is effective to see the paramagnetism in the structure. For structural analyses, several techniques were studied. Fouirer transform infrared spectrophotometer was performed to understand the functional groups in the structures. Raman experiment was applied for determination of molecular contents and minerology of the samples. Energy dispersive spectrosopy was used to learn the elemental compositions of the foils. The surface morphologies were seen by scanning electron microscopy. The obtained magnetic, structural and elemental results of the nickel based foil alloys are discussed in detail.

Linear programming applied to polarized Raman spectroscopy for elucidating molecular structure at surfaces

We present a framework for using linear programming to solve a challenging problem in surface science, the elucidation of the structure and composition of adsorbed molecules from a mixture, using simulated data from polarized Raman experiments. In the past, methods applied in order to interpret such spectroscopic information were combinatorial approaches that are limited in scalability or accuracy. Quantum mechanical electronic structure calculations yield the optical response of a single molecule, from which spectra of a mixture can be determined by appropriate weighting. Furthermore, spectral obtained in different beam polarizations provide projections of the signal in the laboratory frame. We demonstrate that linear programming is an ideal tool for utilizing all of this information in order to provide the sought structural picture.

Influence of spatial dispersion on spectral tuning of phonon-polaritons

The field of nanophotonics has long sought to identify mechanisms to realize dynamical control of optical modes. In most approaches, the magnitude of tuning is dependent upon the degree to which the optical permittivity is malleable upon some material change, such as carrier concentration. Here, through a multiwavelength Raman spectroscopic examination of 4H-SiC nanopillars, momentum is identified as an alternative means to enhance spectral tunability of nanophotonic modes, owing to the spatial dispersion implicit in the infrared (IR) optical permittivity of polar semiconductors. Experimentally, this is deduced through the observation of a ``forbidden'' Raman mode at $\ensuremath{\approx}780\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ and the emergence of the surface-optical phonon polariton at $\ensuremath{\approx}950 {\mathrm{cm}}^{\ensuremath{-}1}$, which evolved with intensities dependent upon the nanopillar diameter and the wavelength of the incident light. The evolution of these modes is accompanied by a redshift and spectral narrowing of the longitudinal-optical plasmon coupled (LOPC) mode exhibiting a similar wavelength and diameter dependence. Mie resonances, identified using ultraviolet-visible spectroscopy and excited by the visible laser excitation of the Raman experiment, acted to vary the momentum sampled during the Raman experiment leading to these spectral dependencies. This was deduced by fitting the Raman response accounting for both the presence of the surface phonon and the overdamped LOPC mode under the Lindhard-Mermin approximation. This fitting not only explains the Raman response, but also clearly exhibits the spatially disperse permittivity of the SiC, which is shown to have a momentum-dependent sensitivity to carrier concentration. Such sensitivity, in turn, highlights the potential of spatial dispersion as a means to accentuate the performance of active IR nanophotonic approaches employing phonon polaritons.

Spectroscopic and quantum chemical study of phthalocyanines with 1,4,7-trioxanonyl moieties

Abstract The spectral properties of new active in PDT anticancer and antiviral dye - 1,4,8,11,15,18,22,25-octakis(1,4,7-trioxanonyl)phthalocyanine and its metal complexes were investigated in terms of their spectroscopic properties. Three quantum chemical restricted (DFT), unrestricted (UDFT) and time-dependent (TD-DFT) models were applied to explain phenomena observed in the experimental data. The experimental UV–Vis-near-IR spectra were compared with the time-dependent density functional theory model PCM/ωB97X-D/6-31+G(d,p). The theoretical model revealed excellent compliance with measurements for magnesium complex and free metal ion phthalocyanine (relative errors were less than 5%). All observed electronic UV–Vis transitions were π→π* type. The Q band, which is crucial for photodynamic activity, presented HOMO to LUMO and HOMO to LUMO+1 character. The excited-state dynamics, including fluorescence lifetimes of studied phthalocyanines, were analyzed using the Time-Correlated Single Photon Counting (TCSPC) technique in DMF and DMSO. It was found that the fluorescence lifetimes measured for metallated phthalocyanines in DMF and DMSO are very similar, thus indicating their weak interaction with solvents. The fluorescence decays were associated with the type of metal-phthalocyanine complex. Moreover, metal-free phthalocyanine showed a stronger tendency to form aggregates in DMF and DMSO than it was observed for its metal ion complexes. The FTIR (400–4000 cm−1) and Raman (50–1600 cm−1) spectra were recorded, and the obtained data were applied in the quantum chemical study. The UB3LYP/6-31G(d,p) model applied for cobalt(II) and manganese(III) complexes allowed to assign the essential signals obtained in the computational study to these observed in FTIR and Raman experiments.

Carbon coating of air-sensitive insulating transition metal fluorides: An example study on α-Li3FeF6 high-performance cathode for lithium ion batteries

Li3FeF6 has been the focus of research of fluorine-based cathode materials for lithium-ion batteries. Because of the low electronic conductivity of Li3FeF6, the decrease of particle size, by an energy-consuming long-time ball milling process with carbon, is necessary to achieve a high electrochemical performance. The most successful method to enhance electrochemical activity, carbon coating, seemed to be impracticable, so far, for sensitive fluorides like Li3FeF6. In this work, carbon coating on Li3FeF6 particles has been successfully achieved for the first time, while avoiding both extended hydrolysis and Fe(III)-Fe(II) reduction. The heat treatment and atmosphere, yielding the maximal transformation of organic carbon to both graphitised and disordered carbon, has been determined. Carbon coating, with a thickness of approximately 2.5 nm, has been achieved by controlled thermal decomposition of glucose, under air, at 300 °C. Raman and X-ray photoelectron spectroscopy (XPS) experiments have proved the existence of carbon and Fe2O3 on the surface of Li3FeF6 nanoparticles. XPS spectroscopy indicates the presence of organic residues from glucose decomposition. Attempts to further reduce the organic carbon content results in a decrease of the amorphous carbon coating layer. Optimised carbon-coated Li3FeF6 nanoparticles deliver 122 mA h g-1 (85% of theoretical capacity) significantly higher than that of a non-coated sample (58 mA h g-1). Even more, a significant beneficial effect of carbon coating on both capacity retention and coulombic efficiency is observed.

Defect engineering of VO2 thin films synthesized by Chemical Vapor Deposition

Vanadium dioxide (VO2) has been a much sought after candidate for many electronics and photonics based applications. These applications demand CMOS compatibility and simple but scalable synthesis procedure. Herein we report the deposition of VO2 on CMOS compatible Si substrates via easily scalable Metal Organic Chemical Vapor Deposition (MOCVD) at different substrate temperatures between 520 and 550 °C. The morphology of the films deposited at different growth temperatures varied drastically despite retaining VO2 in the M1 phase. This was verified by XRD and Raman experiments. The thin films deposited at 535 °C showed sharp grain boundaries with a grain size of about 200 nm. Similarly, the electrical characteristics of the films deposited at 535 °C showed superior transition compared to the films deposited at other temperatures. However the films deposited at 550 °C retained the superiority in the transition strength, the transition width and the hysteresis of the transition increased at that temperatures. We understand that the reason for this observed behavior is the fraction of surface defects. It is seen from XPS measurements that vanadium was available in V3+, V4+ and V5+ oxidation states and the fractions of these individual components varied in all the thin films deposited at different temperatures. We believe that the defects fraction can give a handle to control the nature and quality of the transition in VO2.

Electric Field Control of Magnetism in Iron Oxide Nanoporous Thin Films

Voltage control of the magnetic properties of oxide thin films is highly appealing to enhance energy efficiency in miniaturized spintronic and magnetoelectric devices. Herein, magnetoelectric effects in electrolyte-gated nanoporous iron oxide films are investigated. Highly porous films were prepared by the evaporation-induced self-assembly of sol-gel precursors with a sacrificial block-copolymer template. For comparison, films with less porosity but analogous crystallographic structure were also prepared using an identical procedure except without the polymer template. The films were found to be 70-85 nm in thickness as measured by scanning electron microscopy and primarily hematite as determined by Raman spectroscopy. The templated (highly porous) films showed a very large magnetoelectric response with a maximum increase in magnetic moment at saturation of a factor of 13 and a noticeable (2-fold) increase of coercivity (after applying -50 V). The nontemplated films also exhibited a pronounced increase of magnetic moment at saturation of a factor of 4, although the coercivity remained unaffected over the same voltage range. Magnetoelectric effects in these latter films were found to be fully reversible in the voltage window ±10 V. The observed changes in magnetic properties are concluded to be magneto-ionically driven with oxygen ion exchange between the iron oxide and the liquid electrolyte, as evidenced from Raman and X-ray photoelectron spectroscopy experiments.

Si Nanopowder Based Anode Material for the Lithium Ion Battery Cell

We consider a silicon nanopowder based anode for a lithium ion battery cell. We present the design of the battery cell ready for in situ Raman and X-ray experiments and a technical procedure for the cell manufacturing. From the continuum mechanics point of view, this type of anode can be represented by a spherical nanoparticle surrounded by viscoelastic matrix. During the charging process this nanoparticle undergoes a chemical reaction. Based on the chemical affinity concept we describe how the mechanical properties of the matrix material influence the kinetics of the charging process. We study spherically symmetry problems numerically for different sets of matrix material parameters and show their influence on the reaction front kinetics.

High-pressure polymorphs of gadolinium orthovanadate: X-ray diffraction, Raman spectroscopy, and ab initio calculations

We present a study of the different high-pressure polymorphs of $\mathrm{GdV}{\mathrm{O}}_{4}$ and its stability. Powder x-ray diffraction and Raman experiments show a phase transition from a zircon- to a scheelite-type structure taking place at 6.8(4) GPa. Ab initio density functional theory calculations support this conclusion. The equations of state of these two phases are reported. In addition, we studied the pressure evolution of the Raman modes for the zircon and scheelite phases, showing good agreement between calculations and experiments. For the sake of completeness, we performed optical-absorption measurements up to 16 GPa, showing a band-gap collapse at the transition point. Beyond 20 GPa a second phase transition to a monoclinic fergusonite structure takes place as a consequence of a mechanical instability. A third transition is observed at around 29.3 GPa in Raman experiments. According to our calculations, this fourth polymorph corresponds to an orthorhombic structure described by space group Cmca. This phase involves an increase of the atomic coordination number of vanadium and gadolinium. The results are compared to those reported on isomorphic compounds.