Raman-active Vibrational Modes Research Articles

Giant barocaloric effects in sodium hexafluorophosphate and hexafluoroarsenate

Solid-state refrigeration using barocaloric materials is environmentally friendly and highly efficient, making it a subject of global interest over the past decade. Here, we report giant barocaloric effects in sodium hexafluorophosphate (NaPF6) and sodium hexafluoroarsenate (NaAsF6) that both undergo a cubic-to-rhombohedral phase transition near room temperature. We have determined that the low-temperature phase structure of NaPF6 is a rhombohedral structure with space group R3¯ by neutron powder diffraction. There are three Raman active vibration modes in NaPF6 and NaAsF6, i.e., F2g, Eg, and A1g. The phase transition temperature varies with pressure at a rate of dTt/dP = 250 and 310 K GPa−1 for NaPF6 and NaAsF6. The pressure-induced entropy changes of NaPF6 and NaAsF6 are determined to be around 45.2 and 35.6 J kg−1 K−1, respectively. The saturation driving pressure is about 40 MPa. The pressure-dependent neutron powder diffraction suggests that the barocaloric effects are related to the pressure-induced cubic-to-rhombohedral phase transitions.

Electric field emission in GdNiO3 microflower

GdNiO3 microflowers were synthesized by hydrothermal method with unit sizes of about 2.2–3.5 μm and petal sizes of ∼75–100 nm. The X-ray diffraction pattern (XRD) and Raman spectra reveal an orthorhombic crystal structure with the space group Pbnm and various Raman active vibrational modes i.e., Ag, B1 g, B2 g associated with rare earth nickelates. The total density of states (TDOS) was calculated by density functional theory (DFT) study using a hybrid functional (HSE06) and the volume of TDOS near the Fermi level confirms the semiconducting nature of GdNiO3 with band gap ∼0.688 eV. The local bulk work function (φ) was estimated theoretically at ∼5.35 eV from DFT calculations which is one of the essential parameters for the field enhancement factor (β). Field emission current density (J) vs. applied electric field (E) divulges a low turn-on field (Eto)∼4.6 V/μm@1 μA/cm2 and a threshold field (Eth)∼5.2 V/μm@10 μA/cm2 respectively. The Fowler-Nordheim (F-N) model was introduced to calculate β ∼1637 and long-term field emission current stability was observed experimentally > 4 h at the preset values of current 2 μA and 10 μA, respectively. The easy preparedness, low turn-on field, and long-term current stability without significant decay suggest this material to be an efficient electron field emitting material in the rare-earth nickelates series.

Band Gap Engineering of MoxW1-xS2 Alloy Monolayers with Wafer-Scale Uniformity.

Modulating the band gap of two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors is critical for their application in a wider spectral range. Alloying has been demonstrated as an effective method for regulating the band gap of 2D TMDC semiconductors. The fabrication of large-area 2D TMDC alloy films with centimeter-scale uniformity is fundamental to the application of integrated devices. Herein, we report a liquid-phase precursor one-step chemical vapor deposition (CVD) method for fabricating a MoxW1-xS2 alloy monolayer with a large size and an adjustable band gap. Good crystalline quality and high uniformity on a wafer scale enable the continuous adjustment of its band gap in the range of 1.8-2.0 eV. Density functional theory calculations provided a deep understanding of the Raman-active vibration modes of the MoxW1-xS2 alloy monolayer and the change in the conductivity of the alloy with photon energy. The synthesis of large-area MoxW1-xS2 alloy monolayers is a critical step toward the application of 2D layered semiconductors in practical optoelectronic devices.

Recent advances on Raman spectroscopy of graphene: towards biosensing applications

Graphene displays extraordinary electronic, optical, and mechanical properties, generating substantial scientific interest and presenting vast potential across various applications. Raman spectroscopy is a versatile tool for identifying and characterizing the chemical and physical properties of newly discovered carbon-based materials like graphene, carbon nanotubes, and their artificial lattices. The Raman-active vibrational modes of graphene-related materials are sensitive to structural, electrical, and interfacial modifications. This sensitivity allows Raman spectroscopy to effectively study the interaction between graphene and other materials, such as molecules and bio-related samples, towards biosensing applications. This review summarizes the recent advances in Raman spectroscopy applied to graphene and its related materials, encompassing perspectives from fundamental research to practical applications, especially bio-related applications.

Calculating high-pressure vibrational frequencies analytically with the extended hydrostatic compression force field approach.

We report the implementation of the analytical Hessian for the mechanochemical extended hydrostatic compression force field method in the Q-Chem program package. To verify the implementation, the analytical Hessian was compared with finite difference calculations. In addition, we calculated the pressure dependency of the Raman active vibrational modes of methane, ethane, and hydrogen, as well as all IR and Raman active modes of Buckminsterfullerene, and compared the results with experimental and theoretical data. Our implementation paves the way for the analysis of geometric points on a pressure-deformed potential energy surface and provides a straightforward model to calculate the vibrational properties of molecules under high pressure.

Hyper-Raman spectroscopy of biomolecules.

The hyper-Raman scattering (HRS) spectra of biologically significant molecules (D-glucose, L-alanine, L-arabinose, L-tartaric acid) in aqueous solutions are reported. The HRS spectra were measured using a picosecond laser at 532 nm operating at a MHz repetition rate. High signal to noise spectra were collected with a commercial spectrometer and CCD without resonant, nanoparticle, or surface enhancement. The HRS peak frequencies, relative intensities, band assignments, and depolarization ratios are examined. By comparing HRS to Raman scattering (RS) and infrared absorption spectra we verify that the IR-active vibrational modes of the target molecules are observed in HRS spectra but come with substantially different peak intensities. The HRS of the biomolecules as well as water, dimethyl sulfoxide, methanol, and ethanol were deposited into a data repository to support the development of theoretical descriptions of HRS for these molecules. Depositing the spectra in a repository also supports future dual detection RS, HRS microscopes which permit simultaneous high-spatial-resolution vibrational spectroscopy of IR-active and Raman-active vibrational modes.

Electronic phase transition, vibrational properties and structural stability of single and two polyyne chains under external electric field

Search for one dimensional (1D) van der Waals materials has become an urgent need to meet the demand as building blocks for high performance, miniaturized, lightweight device applications. Polyyne, a 1D atomic chain of carbon is the thinnest and strongest allotrope of carbon, showing promising applications in new generation low dimensional devices due to the presence of a band gap. A system of two carbon chains held together by van der Waals interaction has been theoretically postulated and shows band gap tunability under structural changes which finds applications in the realms of resistive switching and spintronics. In this study, we use first principles Density Functional Theory (DFT) to show a sharp semiconductor to metal transition along with the emergence of an asymmetry in the spin polarized density of states for single and two polyyne chains under a transverse electric field. The thermodynamic stability of the system has been substantiated through the utilization of Ab Initio Molecular Dynamics (AIMD) simulations, phonon dispersion curve analyses, and formation energy calculations. Furthermore, in addition to its dynamic stability assessment, phonon calculations have served to identify Raman-active vibrational modes which offers an invaluable non-destructive experimental avenue for discerning electronic phase transitions in response to an applied electric field. Our study presents a predictive framework for the prospective utilization of one and two polyyne chains in forthcoming flexible nano-electronic and spintronic devices. The future prospects of the system are contingent upon advancements in nano-electronics fabrication techniques and the precise construction of circuitry for harnessing spin-related applications.

Supercapacitor electrode properties of NiWO4 nanocrystals processed by thermal plasma and deposited on the FTO substrate

In this communication, nickel tungstate (NiWO4) nanocrystals were prepared using the coprecipitation method at 90 °C for 6 h and processed by thermal plasma (TP) at 600 °C for 1 h. X-ray diffraction (XRD), Rietveld refinement data, micro-Raman, and Fourier transform infrared (FT-IR) spectroscopies were employed to analyze their structure and optical data. The morphology was observed through field emission scanning electron microscopy (FE-SEM) images. Moreover, their supercapacitor electrode properties were investigated in detail. XRD patterns, Rietveld refinement, micro-Raman/FTIR spectra, and FE-SEM images confirm pristine NiWO4 nanocrystals with a monoclinic structure, symmetric and antisymmetric stretch vibrations of ONiO and OWO bonds, which are related to IR- and Raman-active vibrational modes, spherical shape with average crystal sized ~29.2 nm. These NiWO4 nanocrystals present a high specific capacitance of 351.26 F g−1 at 40 mV s−1, in comparison with NiWO4 nanocrystals processed in heat treatment conventional (120 F g−1 at 40 mV s−1), which can be applied as a stable cathodic electrode. These results suggest that NiWO4 nanocrystals processed by TP and deposited on FTO substrate are good candidates for use as electrodes for energy storage devices in the next generation of batteries.

A Comparative and Critical Analysis for In Vitro Cytotoxic Evaluation of Magneto-Crystalline Zinc Ferrite Nanoparticles Using MTT, Crystal Violet, LDH, and Apoptosis Assay.

Zinc ferrite nanoparticles (ZFO NPs) are a promising magneto-crystalline platform for nanomedicine-based cancer theranostics. ZFO NPs synthesized using co-precipitation method are characterized using different techniques. UV-visible spectroscopy exhibits absorption peaks specific for ZFO. Raman spectroscopy identifies Raman active, infrared active, and silent vibrational modes while Fourier transforms infrared spectroscopic (FTIR) spectra display IR active modes that confirm the presence of ZFO. X-ray diffraction pattern (XRD) exhibits the crystalline planes of single-phase ZFO with a face-centered cubic structure that coincides with the selected area electron diffraction pattern (SAED). The average particle size according to high-resolution transmission electron microscopy (HR-TEM) is 5.6 nm. X-ray photoelectron spectroscopy (XPS) signals confirm the chemical states of Fe, Zn, and O. A superconducting quantum interference device (SQUID) displays the magnetic response of ZFO NPs, showing a magnetic moment of 45.5 emu/gm at 70 kOe. These ZFO NPs were then employed for comparative cytotoxicity evaluation using MTT, crystal violet, and LDH assays on breast adenocarcinoma epithelial cell (MCF-7), triple-negative breast cancer lines (MDA-MB 231), and human embryonic kidney cell lines (HEK-293). Flow cytometric analysis of all the three cell lines were performed in various concentrations of ZFO NPs for automated cell counting and sorting based on live cells, cells entering in early or late apoptotic phase, as well as in the necrotic phase. This analysis confirmed that ZFO NPs are more cytotoxic towards triple-negative breast cancer cells (MDA-MB-231) as compared to breast adenocarcinoma cells (MCF-7) and normal cell lines (HEK-293), thus corroborating that ZFO can be exploited for cancer therapeutics.

Impact of luminescent-ion doping on the crystallographic and photo-physical properties of the CaMoO4 nanoparticles.

Luminescent lanthanide (Ln3+ = Pr, Nd, Sm, Eu, and Tb)-ions doped calcium molybdate(CaMoO4) nanoparticles(NPs) were prepared by the polyol wet-chemical route. X-ray diffraction (XRD) pattern of all samples showed the formation of a single-phase scheelite type tetragonal structure with an average crystalline size over 21.6-33.4nm. Thermal stability was evaluated to study the surface-anchored functional groups by weight loss measurement. Fourier transform infrared (FTIR) spectra were recorded to identify the adsorbed functional groups. Aqueous dispersibility and colloidal stability were recorded with the help of the UV/visible absorption spectra. These nanocrystals formed semi-transparent colloidal solutions after being evenly disseminated in aqueous media. The doping of the luminescent ions significantly affects the crystal structure and photoluminescence (PL) properties of the CaMoO4:Ln3+ NPs. In a comparative analysis of the absorption spectra, bandgap, Raman-active modes, and luminescent properties, they were greatly influenced by altering the dopant ion due to the variation in the atomic radius of the element. The doping of smaller atomic radius Ln3+-ions distorts the unit cell, and, subsequently, bond angle/length alters the symmetry of the host crystal. The distorted crystal lattice affects the crystalline, size, lattice parameter, band gap values, Raman active vibrational modes, and luminescent efficiency. The distorted crystal structure of the host lattices facilitates the movement of the oxygen vacancies through charge transfer, resulting in efficiently suppressed emission efficiency.Graphical abstract.

Structural Stability of γ-Boron under High Pressure up to 126 GPa with Fine Pressure Increments

The structural stability of γ-boron is investigated using Raman spectra and DFT calculations under high pressures, up to 126 GPa, at ambient temperature. The pressure dependence of all the Raman-active modes of the γ-boron is reported. We also observe amusing changes within the B3g and B1g Raman-active vibrational modes, which result in the phenomenon of first merging and then separating the Raman peaks. In addition to the Raman measurements, the changes in crystal structure and force constants are calculated to reasonably explain the discrepancy between the two Raman modes in response to pressure. The results of the continuous shifts for all Raman modes and the unit-cell parameters, as well as volume with increasing pressure, indicate that there is no structural transformation of γ-boron below this pressure value, with no changes in either symmetry or structure.

Lattice Dynamics and Electron-Phonon Coupling in Double Perovskite Cs2NaFeCl6.

Phonon-phonon and electron/exciton-phonon coupling play a vitally important role in thermal, electronic, as well as optical properties of metal halide perovskites. In this work, we evaluate phonon anharmonicity and coupling between electronic and vibrational excitations in novel double perovskite Cs2NaFeCl6 single crystals. By employing comprehensive Raman measurements combined with first-principles theoretical calculations, we identify four Raman-active vibrational modes. Polarization properties of these modes imply Fm3̅m symmetry of the lattice, indicative for on average an ordered distribution of Fe and Na atoms in the lattice. We further show that temperature dependence of the Raman modes, such as changes in the phonon line width and their energies, suggests high phonon anharmonicity, typical for double perovskite materials. Resonant multiphonon Raman scattering reveals the presence of high-lying band states that mediate strong electron-phonon coupling and give rise to intense nA 1g overtones up to the fifth order. Strong electron-phonon coupling in Cs2NaFeCl6 is also concluded based on the Urbach tail analysis of the absorption coefficient and the calculated Fröhlich coupling constant. Our results, therefore, suggest significant impacts of phonon-phonon and electron-phonon interactions on electronic properties of Cs2NaFeCl6, important for potential applications of this novel material.

Asymmetry-enhanced 59Co NMR thermometry in Co(iii) complexes.

Design strategies for molecular thermometers by magnetic resonance are essential for enabling new noninvasive means of temperature mapping for disease diagnoses and treatments. Herein we demonstrate a new design strategy for thermometry based on chemical control of the vibrational partition function. To do so, we performed variable-temperature 59Co NMR investigations of four air-stable Co(iii) complexes: Co(accp)3 (1), Co(bzac)3 (2), Co(tBu2-acac)3 (3), and Co(acac)3 (4) (accp = 2-acetylcyclopentanonate; bzac = benzoylacetonate; tBu2-acac = 2,2,6,6-tetramethyl-3,5-heptanedionate and acac = acetylacetonate). We discovered 59Co chemical shift temperature sensitivity (Δδ/ΔT) values of 3.50(2), 3.39(3), 1.63(3), and 2.83(1) ppm °C-1 for 1-4, respectively, at 100 mM concentration. The values observed for 1 and 2 are new records for sensitivity for low-spin Co(iii) complexes. We propose that the observed heightened sensitivities for 1 and 2 are intimately tied to the asymmetry of the accp and bzac ligands versus the acac and tBu2-acac ligands, which enables a larger number of low-energy Raman-active vibrational modes to contribute to the observed Δδ/ΔT values.

Solvent signals in two-dimensional electronic spectroscopy

We report on properties of non-resonant signals of several commonly used solvents in two-dimensional electronic spectroscopy. The signals can be split into three types with different evolution in population time: an initial, rather strong coherent spike with non-periodic oscillatory phase behavior, periodic oscillatory signals mostly connected to Raman-active vibrational modes of the solvents and non-oscillatory long lived signals. The relative amplitudes of the contributions depend on the solvent. We propose a simple model for predicting spectral positions of the non-resonant solvent signals. The results can be used to facilitate the identification and distinction of solvent and sample cell signals from the sample signal in two-dimensional electronic spectroscopy.

Special roles of two-dimensional octahedral frameworks in photodynamics of Cs3Bi2Br9 nanoplatelets: Electron and lattice-wave localization

The low-dimensional cesium bismuth halides are intriguing wide-bandgap semiconductors with fruitful photophysics. However, their photodynamics is rather intricate and remains debated. We study the optical properties of the Cs3Bi2Br9 nanoplatelets (NPLs) by using the combined experimental and first-principles calculation methods. The results indicate that the exhibited dominant blue emission band and weak green band arise from two kinds of shallow color centers. The Cs3Bi2Br9 NPLs exhibit Raman active and inactive vibrational modes that are separately ascribed to the localized lattice waves propagating along the edges and interiors of the quantum well-like bromide–bismuth octahedral frameworks in Cs3Bi2Br9. These findings improve our understanding of the unique photodynamics of these multiple quantum well-like semiconductor nanocrystals.

Tailoring the structural and magnetic traits of copper modified BaFe12O19 nanostructured hexaferrites for recording media application

Barium nanohexaferrites, with a chemical composition of Ba1-0.5xCu0.5xFe12O19 (x = 0.0, 0.1, 0.2, 0.3) have been successfully produced using the sol–gel auto-combustion approach for tuning its structural, morphological, optical and magnetic properties. The current work has been carried out to control the coercivity of synthesized specimens, with the modification of copper ions at the barium sites of BaFe12O19 hexaferrites. The XRD patterns of synthesized specimens’ shows the formational of hexagonal-like crystal structure with P63/mmc space geometry. In the XRD diffraction patterns, the characteristic peaks (110), (008), (107), (114), (108), (203), (205), (206), (1011), (300), (217), (2011), (220), (2014)along with some impurity phases of hematite (α-Fe2O3) were observed. The crystallite size “D” was estimated from the observed XRD patterns using the Debye-Scherrer formula in the range of 41.94–83.91 nm respectively. The FESEM studies of the BM2 and BM3 specimens reveal the presence of agglomerated grains with a hexagonal shape and an average size of ∼ 277 and ∼ 469 nm respectively. The energy dispersive X-ray spectra of synthesized nanohexaferrites shows only peaks of copper, iron, barium, and oxygen which clearly confirms the higher purity of pure and doped barium hexagonal nanoferrites. The transmission electron micrograph of BM1 nanohexaferrite illustrates the production of the nanoparticles that are agglomeratedand spherical, and have an average particle size of 53.32 nm. The Raman spectrum of undoped and copper doped BaFe12O19 hexagonal nanoferrites shows the formation of seven Raman active vibrational modes in the frequency range of 200–800 cm−1. At room temperature (300 K), the magnetic traits of the produced barium hexagonal nanoferrites were measured, and using these measurements, a variety of magnetic parameters (Ms, Hc, Mr, and Ha) were reported. The value of saturation magnetization (Ms) increases from 37.75 emu/g to 59.97 emu/g with increased low copper content up to x = 0.1. In contrast, with the increasing copper content up to x = 0.3, the value of Ms shows a decline to 28.31 emu/g. In this work, admirable values of Ms (28.31–59.97 emu/g) and Hc (684–2563 Oe) were obtained, making these nanohexaferrites as beneficial candidates for the recording media and EMI shielding applications.

Vibrational Properties of LaNb0.8 M0.2 O4-δ (M=As, Sb, V, and Ta).

LaNb0.8 M0.2 O4-δ (where M=As, Sb, V, and Ta) oxides with pentavalent elements of different ionic sizes were synthesized by a solid-state reaction method. The vibrational properties of these oxides have been investigated. These studies revealed that the substituent element influences both Debye temperature value as well as the Raman active vibrational modes. Additionally, the low-temperature vibrational properties of LaNb0.8 Sb0.2 O4-δ have been determined to show the phase transition occurrence at 260 K which is lower than previously reported.

Enhancement in photocurrent conversion efficiency via recrystallization of zinc tin hydroxide nanostructures

Herein, the synthesis of cubic phase zinc tin hydroxide is reported by the hydrolysis assisted co-precipitation method followed by calcination in 150–850 °C. The recrystallization of zinc tin hydroxide (ZnSn(OH)6) to zinc tin oxide (Zn2SnO4) phase is observed with the intermediate amorphous phase (ZnSnO3) by increasing the calcination temperature. Increase in calcination temperature also causes the partial alteration in the cubic morphology and diminution in average particle size. The M-O-M and M-O vibration modes have distinct peaks in the FT-IR spectra below 1500 cm−1. Four Raman active vibration modes in ZnSn(OH)6 and two Raman active vibration modes in Zn2SnO4 phase are observed, while no Raman active mode is present in intermediate amorphous ZnSnO3. Chemical states of various elements and compositions are examined by X-ray photoelectron spectroscopy. The ZnSnO3 electrodes has the lowest charge transfer resistance and higher lifetime of the photogenerated charge carriers. Photoelectrochemical measurements rendered enhanced photocurrent density for ZnSnO3 and Zn2SnO4 phases, which is approximate 85 and 50 times higher as compared to ZnSn(OH)6 based photoelectrode, respectively. Due to crystalline nature, the Zn2SnO4 phase has better photo-stability.

Polarized Moiré Phonon and Strain-Coupled Phonon Renormalization in Twisted Bilayer MoS2

The ability to control the atomic-scale behavior in quasi-two-dimensional crystals via twisting the vicinal layers and generating patterned moiré potentials provides an additional degree of freedom to study correlated physics. Under the existence of macroscopic external parameters such as strain, we study the high-frequency Raman active vibrational modes of chemical vapor deposition (CVD)-grown monolayer, stacked (AA′ and AB), and twisted bilayer MoS2. The polarized Raman scattering, being an efficient characterization tool for these moiré structures, is employed to investigate the effects of strain on the different phonon modes. The recently discovered moiré phonons (FA1g), i.e., zone center phonons in a twisted bilayer MoS2, stem from the folding of the off-center phonons of its monolayer constituents and are found to be Raman polarized. Moreover, the Raman shift impressions of these moiré phonons in twist angle in the range between 0 and 30° are reproduced, showing the strain-coupled modifications of the previously reported sinusoidal behavior. Our findings establish an effective method to probe the strain-induced anisotropy in twisted van der Waals systems using polarized Raman spectroscopy showing future directions to moiré-related physics.

Effect of rare earth (Nd3+) metal doping on structural, morphological, optical and magnetic traits of Zn–Mg nano-ferrites

Herein, we report the synthesis of Zn0.7Mg0.3NdxFe2–xO4 (where, x = 0.0, 0.01, 0.02) ferrite nanoparticles by employing the sol–gel auto-combustion technique. The X-ray diffraction (XRD) pattern suggests the formation of a pure cubic structure, without any impurity phase, with an Fd3m space group at room temperature. With increasing doping amount, the crystallite size is reported as 35–41 nm, while the lattice parameters rise from 0.8381 to 0.8395 nm. Field emission scanning electron microscopy (FESEM) images show the formation of spherical grains with agglomerated morphology in all the samples, with grain sizes ranging from 49 to 103 nm. Energy dispersive X-ray spectroscopy (EDX) and elemental mapping investigation confirm the chemical purity of all the samples. Fourier transform infrared (FTIR) analysis shows the presence of two prominent peaks around 440 and 560 cm−1 that correspond to the octahedral and tetrahedral positions. In addition, the existence of five Raman active vibrational modes in all produced specimens again confirms the structural purity of all the samples. The M−H curve shows that saturation magnetization (Ms) first increases from 14.98 to 28.22 emu/g and then decreases to 18.98 emu/g with increasing doping amount. This is due to the A-B type super-exchange interaction for the synthesized samples. The variation in coercivity (Hc) and magnetic anisotropy (K1) suggest the thermal stability of all the samples and can be utilized in transformers and solenoids.