Raman-scattering Measurements Research Articles

Iso-structural phase transition in pyrochlore iridates (Sm _{1-x}$Bix)2Ir2O7 (x = 0, 0.02, and 0.10): high-pressure Raman and x-ray diffraction studies

The pyrochlore iridates,A2Ir2O7, show a wide variety of structural, electronic, and magnetic properties controlled by the interplay of different exchange interactions, which can be tuned by external pressure. In this work, we report pressure-induced iso-structural phase transitions at ambient temperature using synchrotron-based x-ray diffraction (up to ∼20 GPa) and Raman-scattering measurements (up to ∼25 GPa) of the pyrochlore series (Sm_{1-x}Bix)2Ir2O7(x= 0, 0.02, and 0.10). Our Raman and x-ray data suggest an iso-structural transition in Sm2Ir2O7atPc∼ 11.2 GPa, associated with the rearrangement of IrO6octahedra in the pyrochlore lattice. The transition pressure decreases to ∼10.2 and 9 GPa forx= 0.02 and 0.10, respectively. For all the samples, the linewidth of three phonons associated with Ir-O-Ir (A1gandEg) and Ir-O (T2g4) vibrations show anomalous decrease up toPc, due to decrease in electron-phonon interaction.

Highly Controllable Fabrication of Gold Composite Nanostructures by Multiple Electroplating for High‐Performance Electrochemical and Raman Scattering Measurements

Morphology‐controlled nanostructures are important functional materials in multidisciplinary research fields, such as sensing, imaging, catalysis, and energy harvesting. However, existing nanofabrication methods have limitations in terms of controllability, diversity, cost, and efficiency. Here, a novel nanofabrication approach is proposed to construct Au nanostructures with high controllability, cost‐efficiency, and high‐performance sensing and energy harvesting. The fabrication incorporates electric field‐assisted hydrothermal growth of zinc oxide nanorods (ZnO NRs) with two‐step gold electroplating. The ZnO NRs serve as a dissolvable template during the electroplating formation of gold nanostructures for eventually synthesizing pure gold nanostructures on electrodes. The synthesized gold nanostructures take unique composite shape of nanospheres (hundreds of nanometers) covered with tinier nanostructures (tens of nanometers) like floccules, grains, lamellas, or rods, which are highly controlled by regulating electroplating parameters. The gold composite nanostructures provide high‐performance sensing and energy harvesting, such as enhanced electrochemical sensing interface (electric double‐layer impedance decrease up to 97.1%), excellent surface‐enhanced Raman scattering (SERS enhancement factor up to ), and preferable visible–near‐infrared light harvesting (absorptivity near to 90%). For the first time, the application of tumor cell photothermal therapy with nanostructured electrodes and in situ bimodal monitoring of single‐cell impedance and SERS‐based molecular expression during therapy is demonstrated.

Pressure-induced topological crystalline insulating phase in TlBiSe2 : Experiments and theory

We report in situ high-pressure studies on three-dimensional topological insulator ${\mathrm{TlBiSe}}_{2}$ using Raman scattering and synchrotron x-ray-diffraction experiments corroborated with the first-principles theoretical calculations. The phonon modes of a rhombohedral phase of ${\mathrm{TlBiSe}}_{2}$ show a systematic increase in frequencies under hydrostatic pressure up to $\ensuremath{\sim}7.0\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Interestingly, the linewidth of the ${A}_{1g}, N$, and ${E}_{g}$ phonon modes show clear anomalies at $\ensuremath{\sim}2.5\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ which is indicating the isostructural electronic transition. With the help of calculated electron-phonon coupling constant $\ensuremath{\lambda}$, anomalies in the Raman linewidth of ${E}_{g}$ mode are attributed to electron-phonon coupling changes. Moreover, our theoretical results reveal that the observed phonon anomalies are due to pressure-induced band inversion at the $F$ points of the Brillouin zone which leads to the changes in electronic topology reflected in the mirror Chern number ${n}_{M}$ and ${\mathbb{Z}}_{2}$ topological invariant. Therefore, the phonon anomalies and change in mirror Chern number ${n}_{M}$ confirm the pressure-induced topological crystalline insulator phase in ${\mathrm{TlBiSe}}_{2}$ at $\ensuremath{\sim}2.5\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Further, a reversible structural phase transition has been observed above $\ensuremath{\sim}7.0\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ from both synchrotron x-ray-diffraction and Raman-scattering measurements. Finally, our studies suggest the use of hydrostatic pressure as a potential pathway for exploring the topological crystalline insulating phase in strong spin-orbit coupling compounds, such as the thallium-based III-V-$\mathrm{V}{\mathrm{I}}_{2}$ ternary chalcogenide $\mathrm{TlBi}{X}_{2}$ ($X=\mathrm{S}$, Se, Te) family.

Coherent Anti-Stokes Raman Scattering Measurements of Time and Length Scales of Temperature Fluctuations in a Turbulent Flame.

The ability to derive temporal and spatial scales of "instantaneous" local temperature variations in a turbulent flame by means of coherent anti-Stokes Raman scattering (CARS) spectroscopy is demonstrated, for the first time to our knowledge. The measurements employed two CARS spectrometers with synchronized nanosecond pulse-repetitive lasers. The system was enabling to record, with a high temporal resolution of about 10 ns, series of single laser shot CARS spectra of N2 molecules from two spatially overlapped or displaced probe volumes as small as 0.03 × 0.03 × 2 mm3. The spectra were being recorded at a variable delay between two sequential shots, following each other in pairs at a repetition rate of 10 Hz. The series of 500 coupled measurements, at the delays in the range 1 μs-10 ms and the displacements up to 2.5 mm, have been performed in a few points of an open premixed methane-air flame of a laboratory burner with the time-averaged temperatures in the range 1200-1800 K. From the spectra, "instantaneous" temperatures, at the given delay and probe volume distance, have been derived. This allowed the auto-correlation coefficients of temperature fluctuations versus the delay and the displacement to be calculated. These dependences enabled to evaluate temperature correlation times and lengths under various mixture flow rates and equivalence ratios.

Gold Nanocone Array with Extensive Electromagnetic Fields for Highly Reproducible Surface-Enhanced Raman Scattering Measurements.

Surface-enhanced Raman scattering (SERS) is a technique used to distinguish the constitution of disease-related biomarkers in liquid biopsies, such as exosomes and circulating tumor cells, without any recognition elements. Previous studies using metal nanoparticle aggregates and angular nanostructures have achieved the detection of various biomarkers owing to strong hot spots and electromagnetic (EM) fields by localized surface plasmon resonance (LSPR). Although these SERS platforms enable significant enhancement of Raman signals, they still have some problems with the fabrication reproducibility of platforms in obtaining reproducible SERS signals. Therefore, highly reproducible fabrication of SERS platforms is required. Here, we propose the application of a polymer-based gold (Au) nanocone array (Au NCA), which extensively generates an enhanced EM field near the Au NCA surface by LSPR. This approach was experimentally demonstrated using a 785 nm laser, typically used for SERS measurements, and showed excellent substrate-to-substrate reproducibility (relative standard deviation (RSD) < 6%) using an extremely simple fabrication procedure and very low laser energy. These results proved that a Au NCA can be used as a highly reproducible SERS measurement to distinguish the constitution of biomarkers.

Raman Scattering Measurement of Suspended Graphene under Extreme Strain Induced by Nanoindentation.

Graphene is known as a superstiff and extremely strong material. Hence, applying strains greater than 1% to graphene and simultaneously measuring changes in its physical properties has been challenging because of the limited methodologies for measuring both high strain and other physical properties. Here, Raman scattering measurement of suspended graphene under extremely high biaxial strain as large as 6.1% using an atomic force microscopy (AFM)-Raman spectroscopy measurement tool is reported. Nanoindentation is performed using AFM tips machined to have a flat top and a hole shape, resulting in a strained graphene area sufficiently large to enable the acquisition of a Raman signal. At the same time, the laser light is focused on the strained flat area of the graphene membrane. The Raman signals of the G and 2D bands of graphene are redshifted by 282 and 684 cm-1 , respectively, which is unprecedented for graphene. This measurement technique provides an effective methodology to measure variations in the physical properties of atomically thin materials under superhigh strain.

Graphene Oxide/Silver Nanoparticles Platforms for the Detection and Discrimination of Native and Fibrillar Lysozyme: A Combined QCM and SERS Approach.

We propose a sensing platform based on graphene oxide/silver nanoparticles arrays (GO/AgNPs) for the detection and discrimination of the native and toxic fibrillar forms of an amyloid-prone protein, lysozyme, by means of a combination of Quartz Crystal Microbalance (QCM) and Surface Enhanced Raman Scattering (SERS) measurements. The GO/AgNPs layer system was obtained by Langmuir-Blodgett assembly of the silver nanoparticles followed by controlled adsorption of GO sheets on the AgNPs array. The adsorption of native and fibrillar lysozyme was followed by means of QCM, the measurements provided the kinetics and the mechanism of adsorption as a function of protein concentration as well as the mass and thickness of the adsorbed protein on both nanoplatforms. The morphology of the protein layer was characterized by Confocal Laser Scanning Microscopy experiments on Thioflavine T-stained samples. SERS experiments performed on arrays of bare AgNPs and of GO coated AgNP after native, or fibrillar, lysozyme adsorption allowed for the discrimination of the native form and toxic fibrillar structure of lysozyme. Results from combined QCM/SERS studies indicate a general construction paradigm for an efficient sensing platform with high selectivity and low detection limit for native and amyloid lysozyme.

Superconductivity and electron–phonon interaction in Sr x Bi2Se3 under pressure

Pressure-induced superconductivity has been widely explored and observed in Bi2Se3-based topological materials to hunt for topological superconductors. Although their triggered superconductivity has a close connection to their pressure-induced structural phase transitions, the quest for the electron pairing mechanism of these superconducting semiconductors in both their initial rhombohedral and high-pressure phases remains unknown. In this work, we present a systematic study of the pressure effect on superconducting properties and lattice dynamics using a combination of electrical transport, Raman-scattering, and synchrotron x-ray diffraction measurements using diamond anvil cells. One key finding is our observation of a cooperative connection between the strength of the electron–phonon interaction (EPI) generated by optical branches and the pressure-tunable superconductivity in rhombohedral Sr x Bi2Se3 crystal. The underlying suppression mechanism of the T c by pressure is ascribed to the weakening of the electrons’ interaction with the optical phonon modes in the rhombohedral phase. In the intermediate monoclinic phase, the T c value underwent a sharp increase with carrier density accumulation accompanying the concurrent enhanced EPI. This is intuitively unusual since it is expected that the EPI shall be weakened by inducing more conducting carriers in a normal metal. In the tetragonal phase, the superconductivity is interpreted within BCS theory, since it is fully metallized and obeys the adiabatic Born–Oppenheimer approximation well. Our findings are important to fully understand unconventional superconductivity and the unusual pairing mechanism in the layered rhombohedral Bi2Se3-based superconductors.

Drug preconcentration and direct quantification in biofluids using 3D-Printed paper cartridge

Drug detection in biofluids has always been great importance for clinical diagnosis. Many detection technologies such as chromatography-mass spectrometry, have been applied to the detection of drugs. However, these technologies required multi-step operations, including complicated and time-consuming pretreatment processes and operations of bulky detection instruments, significantly limiting development of drug detection in clinical diagnosis. Herein, a portable 3D-printed paper cartridge was fabricated for fast sample preconcentration and direct drugs quantitative detection in biofluids by a portable Raman spectrometer. This cartridge contained both paper tip with silver nanowires to preconcentrate samples and achieve surface-enhanced Raman Scattering (SERS) measurement, and 3D-printed cartridge to build enclosed environment for the improvement of detection, which cost only one dollar. The preconcentration ability of the cartridge was up to 18.13-fold fluorescence enhancement and compared to the non-preconcentration method, it achieved 9.93-fold improvement of SERS performance. The anticancer drug of epirubicin hydrochloride, cyclophosphamide and their mixtures were quantitatively detected in the bovine serum or artificial urine. The integrated detection procedure required only 1 h, including sample pretreatment and preconcentration, drying, SERS measurements, and quantification analysis. This 3D-printed paper cartridge constituted a portable detection platform that would be potentially a practical and point-of-care detection tool for drug preconcentration and quantification on the clinical diagnosis.

Magnetic Iron Oxide Cores with Attached Gold Nanostructures Coated with a Layer of Silica: Easily Homogeneously Deposited New Nanomaterial for Surface-Enhanced Raman Scattering Measurements

Magnetic Iron Oxide Cores with Attached Gold Nanostructures Coated with a Layer of Silica: Easily Homogeneously Deposited New Nanomaterial for Surface-Enhanced Raman Scattering Measurements

Analysis of fluctuations in the Raman spectra of suspended and supported graphene films

The repeatability of the Raman scattering (RS) spectra was analyzed in monolayer and bilayer CVD-grown graphene films. Position and width of the two main RS lines (G-line and 2D-line) together with their intensity ratio I2D/IG were measured in samples where graphene film was supported by a SiO2/Si substrate and where the film was suspended over pits in the substrate. The results are presented as histograms averaged by a Gaussian with the width W, considered as a measure of non-repeatability. Relatively large fluctuations in the I2D/IG value and linewidth were observed upon repeated measurements at the same point both in supported and suspended samples. The RS measurements in crystalline graphite and mechanically exfoliated graphene deposited on hexagonal boron nitride showed higher repeatability which means that increased W in CVD-grown films reflects the sensitivity of RS processes in these films to changes in charge and deformation. In bilayer CVD samples, the values of W are higher due to additional fluctuations in the twist angle between two misoriented layers. The average position of the RS lines is shifted in opposite directions and makes it possible to distinguish between bilayer and monolayer films, despite the similarity in the RS spectra.

Kramers doublets, phonons, crystal-field excitations, and their coupling in Nd2ZnIrO6

We report comprehensive Raman-scattering measurements on a single crystal of double-perovskite Nd2ZnIrO6 in temperature range of 4-330 K, and spanning a broad spectral range from 20 cm-1 to 5500 cm-1. The paper focuses on lattice vibrations and electronic transitions involving Kramers doublets of the rare-earth Nd3+ ion with local C1 site symmetry. Temperature evolution of these quasi-particle excitations have allowed us to ascertain the intricate coupling between lattice and electronic degrees of freedom in Nd2ZnIrO6. Strong coupling between phonons and crystal-field excitation is observed via renormalization of the self-energy parameter of the phonons i.e. peak frequency and line-width. The phonon frequency shows abrupt hardening and line-width narrowing below ~ 100 K for the majority of the observed first-order phonons. We observed splitting of the lowest Kramers doublets of ground state (4I9/2) multiplets i.e. lifting of the Kramers degeneracy, prominently at low-temperature (below ~ 100 K), attributed to the Nd-Nd/Ir exchange interactions and the intricate coupling with the lattice degrees of freedom. The observed splitting is of the order of ~ 2-3 meV and is consistent with the estimated value. We also observed a large number of high-energy modes, 46 in total, attributed to the intra-configurational transitions between 4f3 levels of Nd3+ coupled to the phonons reflected in their anomalous temperature evolution.

Symmetry-breaking transitions in the early steps of protein self-assembly.

Protein misfolding and subsequent self-association are complex, intertwined processes, resulting in development of a heterogeneous population of aggregates closely related to many chronic pathological conditions including Type 2 Diabetes Mellitus and Alzheimer's disease. To address this issue, here, we develop a theoretical model in the general framework of linear stability analysis. According to this model, self-assemblies of peptides with pronounced conformational flexibility may become, under particular conditions, unstable and spontaneously evolve toward an alternating array of partially ordered and disordered monomers. The predictions of the theory were verified by atomistic molecular dynamics (MD) simulations of islet amyloid polypeptide (IAPP) used as a paradigm of aggregation-prone polypeptides (proteins). Simulations of dimeric, tetrameric, and hexameric human-IAPP self-assemblies at physiological electrolyte concentration reveal an alternating distribution of the smallest domains (of the order of the peptide mean length) formed by partially ordered (mainly β-strands) and disordered (turns and coil) arrays. Periodicity disappears upon weakening of the inter-peptide binding, a result in line with the predictions of the theory. To further probe the general validity of our hypothesis, we extended the simulations to other peptides, the Aβ(1-40) amyloid peptide, and the ovine prion peptide as well as to other proteins (SOD1 dimer) that do not belong to the broad class of intrinsically disordered proteins. In all cases, the oligomeric aggregates show an alternate distribution of partially ordered and disordered monomers. We also carried out Surface Enhanced Raman Scattering (SERS) measurements of hIAPP as an experimental validation of both the theory and in silico simulations.

Two-line ([formula omitted]/[formula omitted]) chemiluminescence technique for equivalence ratio mapping in turbulent stratified flames

Equivalence ratio fluctuations play an important role in exploring and controlling flame dynamic instabilities such as thermoacoustic combustion instabilities associated with lean premixed gas turbine (GT) engines. However, it is difficult to reliably measure local equivalence ratio, and so we often make estimates based on the ratios of chemiluminescent emissions. Recent studies have reported that equivalence ratio varies with chemiluminescence intensity ratio of CH* and CO2* in the visible spectrum (431 and 410 nm, respectively). A methodology based on this relationship is adopted in the present study to construct equivalence ratio maps for a series of turbulent premixed and stratified methane/air flames under globally lean conditions (mean or global equivalence ratio, ϕ¯=0.75), over a range of stratification with ϕ spanning 0.375–1.125 for the highest level of stratification. In contrast with the previously used CH*/OH* method, the current technique allows for a single visible range camera to be used for chemiluminescence imaging. A comparison of the current estimated equivalence ratio with previously calculated local equivalence ratio from Raman-scattering measurements shows that the two-line chemiluminescence technique can be reliably used to determine equivalence ratio in the instantaneous reaction zone of turbulent flames even at higher turbulence levels and mixture stratification conditions.

Determination of Molecular Orientation in Bi-analyte Mono-molecule Layer through Electrochemical Surface-enhanced Raman Scattering Measurements

Surface-enhanced Raman scattering measurements have been conducted using plasmonic dimer structures under electrochemical potential control. We have confirmed the potential dependence of the adsorp...

Chemical doping induced zone-edge phonon renormalization in single-layer MoS2

Chemical doping is an effective method for tailoring physical properties of semiconductors. It has been long known that trace-level doping can obviously influence vibrational properties of bulk semiconductors. Nevertheless, less is known about the effect of chemical doping on the lattice dynamics of two-dimensional (2D) semiconductors although electron-phonon interaction can significantly influence the electronic mobility of the 2D semiconductors. In this study, we carried out Raman-scattering measurements in combine with atom-resolved scanning transmission electron microscope characterization to unveil the effects of Re doping on the phonon dispersion of single-layer (1L) $\mathrm{Mo}{\mathrm{S}}_{2}$. Second-order Raman modes in a wide spectral range reveal the distinct softening of the longitudinal acoustic phonon branch of 1L $\mathrm{Mo}{\mathrm{S}}_{2}$ but with an imperceptible shift of other branches. Such zone-edge phonon renormalization mainly results from electron doping via strong electron-phonon coupling along with a minor contribution from the increased atomic weight from heavier Re dopants. This study enriches the understanding of dopant-host interactions in 2D materials in the aspect of phonons, which is important for tailoring physical properties of 2D materials, such as thermal and electrical transport.

Tuning of near infrared excitonic emission from InAs quantum dots by controlling the sub-monolayer coverage

Abstract In this paper, we present a combined optical and structural study of GaAs-hosted InAs sub-monolayer QD (SML-QD) vertical multi-stacks. The main feature of this paper is to demonstrate the feasibility of sub-monolayer InAs QD with as low coverage as 0.4 ML which shows all the characteristics of QD excitonic emission, emitting in the NIR region (1.496 eV). This emission energy could be precisely tuned successfully by systematically controlling InAs coverage fraction in the range of 0.4–0.8 with corresponding emission in the range of 1.406–1.496 eV. The luminescence efficiency (4 K) exhibited an increasing trend with the decrease in InAs coverage. This paper elaborately discusses the interdependence of structure, strain and emission characteristics through a combined study of high resolution x-ray diffraction, Raman Scattering and Photoluminescence measurement. Strain-induced growth of the dots with different vertical size distribution (height~2.3–1.4 nm) have been explored which were found to depend strongly on the InAs coverage at low temperature. The varying size distribution of the dot ensembles lead to different degree of carrier confinement, capture and localization, as determined from the low temperature (4 K) PL spectra. Significant enhancement of carrier localization inside almost 2D-like exciton was achieved by reducing InAs coverage well below one monolayer. From the relative temperature dependent photoluminescence measurements, it has been shown that the coupling and relaxation pathways of photo-carriers through the SML-QD multi-structure can be controlled by adjusting the InAs coverage.

Determination of the small amount of proteins interacting with TiO2 nanotubes by AFM-measurement

Detecting the small amounts of proteins interacting effectively with the solid film electrodes surface still remains a challenge. To address this, in this work, a new approach was proposed by the combination of the adhesion forces and the molecular interaction measured with AFM. Cytochrome c (Cyt C) interacting effectively with TiO2 nanotube arrays (TNAs) was chosen as a probe. The amounts of Cyt C molecules interacting effectively on TNAs surface (CTNA) range from 5.5×10−12 to 7.0×10−12 mol/cm2 (68.2–86.8 ng/cm2) and they are comparable with the values obtained by the electrochemistry method in the literature, in evidence of the accuracy of this AFM-based approach. The reliability of the proposed approach was further verified by conducting Surface Enhanced Raman Scattering (SERS) measurements and estimating the enhancement factor (EF). This interaction-based AFM approach can be used to accurately obtain the small amounts of adsorbed substances on the solid film electrodes surface in the applications such as biosensors, biocatalysis, and drug delivery, etc.

High-pressure Raman scattering in bulk HfS2: comparison of density functional theory methods in layered MS2 compounds (M\u2009=\u2009Hf, Mo) under compression

We report high-pressure Raman-scattering measurements on the transition-metal dichalcogenide (TMDC) compound HfS2. The aim of this work is twofold: (i) to investigate the high-pressure behavior of the zone-center optical phonon modes of HfS2 and experimentally determine the linear pressure coefficients and mode Grüneisen parameters of this material; (ii) to test the validity of different density functional theory (DFT) approaches in order to predict the lattice-dynamical properties of HfS2 under pressure. For this purpose, the experimental results are compared with the results of DFT calculations performed with different functionals, with and without Van der Waals (vdW) interaction. We find that DFT calculations within the generalized gradient approximation (GGA) properly describe the high-pressure lattice dynamics of HfS2 when vdW interactions are taken into account. In contrast, we show that DFT within the local density approximation (LDA), which is widely used to predict structural and vibrational properties at ambient conditions in 2D compounds, fails to reproduce the behavior of HfS2 under compression. Similar conclusions are reached in the case of MoS2. This suggests that large errors may be introduced if the compressibility and Grüneisen parameters of bulk TMDCs are calculated with bare DFT-LDA. Therefore, the validity of different approaches to calculate the structural and vibrational properties of bulk and few-layered vdW materials under compression should be carefully assessed.

Measurement of a phonon-polariton dispersion curve by varying the excitation wavelength

Coupling between optical phonons and light leads to the formation of phonon polaritons, resulting in important changes to the material dispersion. Commonly, the phonon-polariton dispersion curve has been determined by transient-grating impulsive stimulated Raman-scattering measurements, but reported results for the prototypical dispersion curve of $\mathrm{LiNb}{\mathrm{O}}_{3}$ have led to some controversy. We present a procedure to measure the phonon-polariton dispersion curve by varying the pump wavelength with fixed probe wavelength that allows measurement over a broad range of wave vectors. We determine the lowest-frequency $E$-mode phonon-polariton dispersion curve for $\mathrm{LiNb}{\mathrm{O}}_{3}$ with measured wave vectors ranging from $1\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ to $19\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$, which is the broadest known optically measured phonon-polariton dispersion curve for $\mathrm{LiNb}{\mathrm{O}}_{3}$. The broad range offers improved understanding of the extracted material parameters. We also note some challenges that occur in interpreting the data at short pump wavelengths; these effects help explain controversial differences between previous transient-grating measurements of $\mathrm{LiNb}{\mathrm{O}}_{3}$ dispersion curves.