Deep-Ultraviolet and Helicity-Dependent Raman Spectroscopy for Carbon Nanotubes and 2D Materials.

Recent progress of Raman spectroscopy on carbon nanotubes and 2D materials is reviewed as a topical review. The Raman tensor with complex values is related to the chiral 1D/2D materials without mirror symmetry for the mirror in the propagating direction of light, such as chiral carbon nanotube and black phosphorus. The phenomenon of complex Raman tensor is observed by the asymmetric polar plot of helicity-dependent Raman spectroscopy using incident circularly-polarized lights. First-principles calculations of resonant Raman spectra directly give the complex Raman tensor that explains helicity-dependent Raman spectra and laser-energy-dependent relative intensities of Raman spectra. In deep-ultraviolet (DUV) Raman spectroscopy with 266 nm laser, since the energy of the photon is large compared with the energy gap, the first-order and double resonant Raman processes occur in general k points in the Brillouin zone. First-principles calculation is necessary to understand the DUV Raman spectra and the origin of double-resonance Raman spectra. Asymmetric line shapes appear for the G band of graphene for 266 nm laser and in-plane Raman mode of WS2 for 532 nm laser, while these spectra show symmetric line shapes for other laser excitation. The interference effect on the asymmetric line shape is discussed by fitting the spectra to the Breit-Wigner-Fano line shapes.

Tungsten Nitride (W5N6): An Ultraresilient 2D Semimetal.

Two-dimensional transition metal nitrides offer intriguing possibilities for achieving novel electronic and mechanical functionality owing to their distinctive and tunable bonding characteristics compared to other 2D materials. We demonstrate here the enabling effects of strong bonding on the morphology and functionality of 2D tungsten nitrides. The employed bottom-up synthesis experienced a unique substrate stabilization effect beyond van-der-Waals epitaxy that favored W5N6 over lower metal nitrides. Comprehensive structural and electronic characterization reveals that monolayer W5N6 can be synthesized at large scale and shows semimetallic behavior with an intriguing indirect band structure. Moreover, the material exhibits exceptional resilience against mechanical damage and chemical reactions. Leveraging these electronic properties and robustness, we demonstrate the application of W5N6 as atomic-scale dry etch stops that allow the integration of high-performance 2D materials contacts. These findings highlight the potential of 2D transition metal nitrides for realizing advanced electronic devices and functional interfaces.

Interacting Phonons between Layers in Raman Spectra of Carbon Nanotubes inside Boron Nitride Nanotubes.

We present the resonant Raman spectra of a single-wall carbon nanotube inside a multiwall boron nitride nanotube (SWNT@BNNT). At EL = 1.58 eV, SWNT@BNNT exhibited resonant Raman spectra at 807 (ωBN) and 804 cm-1 (ωGr). Their intensities almost disappeared at EL = 2.33 eV. We assigned ωBN to the out-of-plane BN phonon mode that coupled with ωGr. At EL = 4.66 eV, the G+ and G- bands of the SWNT@BNNT red-shifted 3.8 cm-1 compared with the SWNT, suggesting the interwall interactions between the in-plane modes of SWNT and BNNT. Moreover, the E2g mode of the BNNT in SWNT@BNNT appeared at 1370.3 ± 0.1 cm-1, which is undistinguishable for EL < 3 eV because of the overlap with the D band frequency. The assignment of the present Raman spectra was confirmed through the first-principles calculations.

Effect of heat stress on oxidative damage and antioxidant defense system in white clover (Trifolium repens L.).

MAIN CONCLUSION: This study leads to advances in the field of heat tolerance among different plant species. We concluded that a coordinated, increased antioxidant defense system enabled white clover to reduce heat-induced oxidative damage. The rise in global ambient temperature has a wide range of effects on plant growth, and, therefore, on the activation of various molecular defenses before the appearance of heat damage. Elevated temperatures result in accelerated generation of reactive oxygen species (ROS), causing an imbalance between ROS production and the ability of scavenging systems to detoxify and remove the reactive intermediates. The aim of this study was to determine the role of antioxidant defense systems in the alleviation of heat stress (HS) consequences in white clover (Trifolium repens L.), which is cultivated worldwide. We evaluated how temperature and time parameters contribute to the thermotolerance of white clover at different growth stages. We revealed HS protection in white clover from 37 to 40 °C, with 40 °C providing the greatest protection of 3-day-old seedlings and 28-day-old adult plants. Heat-provoked oxidative stress in white clover was confirmed by substantial changes in electrolyte leakage, malondialdehyde (MDA), and chlorophyll content, as well as superoxide anion (O2·-) and hydrogen peroxide (H2O2) production. Furthermore, superoxide dismutase (SOD) and ascorbate peroxidase (APX) as well as a high level of GSH non-enzymatic antioxidant were the most responsive, and were associated with acquired thermotolerance through the regulation of ROS generation. We demonstrated, by studying protoplast transient gene expression, direct genetic evidence of endogenous antioxidant-related genes that confer HS tolerance in white clover. Our present study clearly establishes that oxidative stress ensues from HS, which triggers the induction of antioxidant defense systems for ROS scavenging in white clover.

Optical Studies on the Phase Transitions in YBaMn2O6 Single Crystals.

The double perovskite YBaMn2O6 exhibited complex structural, magnetic, and charge/orbital ordering phase transitions. In this paper, we investigated the correlation between the temperature-dependent optical response and complex phase transitions of YBaMn2O6 single crystals through spectroscopic ellipsometry and Raman scattering spectroscopy. The room temperature optical absorption spectrum of YBaMn2O6 revealed three bands of approximately 1.50, 4.05, and 5.49 eV. The lowest optical absorption band was assigned to on-site d-d transitions in Mn ions, whereas the other two optical features were assigned to charge-transfer transitions between the 2p states of O and 3d states of Mn. The room temperature Raman scattering spectrum revealed 25 phonon modes. Notably, the MnO6 octahedral tilting and bending modes between 360 and 440 cm-1 increased in intensity at temperatures <200 K. Furthermore, several new phonon peaks appeared at temperatures <200 K, which were associated with charge ordering. Additionally, the magnetic order-induced changes were observed in the breathing modes, with reduced intensity of the 620 cm-1 and a substantial enhancement of the 644 cm-1 phonon peaks. The Jahn-Teller mode at approximately 496 cm-1 exhibited strong hardening at temperatures <200 K, which indicated a linear correlation with the square of the magnetic susceptibility and thus revealed the occurrence of spin-phonon coupling. Anomalies in the phonon frequency, line width, and intensity observed near the phase transition temperatures highlighted the importance of lattice-charge-spin interactions in YBaMn2O6.

Strain engineering of optical properties in transparent VO2/muscovite heterostructures.

Transparent VO2/muscovite heterostructures have attracted considerable attention because of their unique chemical and physical properties and potential practical applications. In this paper, we investigated the influence of uniaxial mechanical strain on the optical properties of VO2/muscovite heterostructures through Raman scattering and optical transmittance measurements. Under applied strain, linear shifts in peak positions of Raman-active phonon modes at approximately 340, 309, and 391 cm-1 were observed. The extracted Grüneisen parameter values were approximately between 0.44 and 0.57. Furthermore, a pronounced strain-induced change in the metal-insulator transition (MIT) temperature was observed, which decreased under compressive strain and increased under tensile strain. The rates of MIT temperature variation reached 4.5 °C per % and 7.1 °C per % at a wavelength of 1200 nm during heating and cooling processes, respectively. These results demonstrate that the modulation of the optical properties of VO2/muscovite heterostructures is controllable and reversible through strain engineering, opening up new opportunities for applications in flexible and tunable photonic devices.

Temperature-dependent optical properties of CuFeO2 through the structural phase transition.

Delafossite CuFeO2 has recently attracted considerable attention because of its complex phase transitions and practical applications. A thorough understanding of the optical properties of CuFeO2 is essential for its further exploration. In this paper, we investigated the temperature-dependent optical properties of CuFeO2 single crystals through Raman scattering spectroscopy and spectroscopic ellipsometry. The room temperature Raman scattering spectrum exhibited six phonon modes at approximately 352, 509, 692, 1000, 1052, and 1171 cm-1. Upon cooling across 11 K, which is the rhombohedral to monoclinic structural phase transition temperature, a softening of the E g-symmetry 352 cm-1 mode and a hardening of the A 1g-symmetry 692 cm-1 mode were observed. Moreover, analysis of the temperature-dependent real part of the dielectric function and direct band gap revealed anomalies at 11 K. These results demonstrate a profound connection between the structural phase transition, lattice dynamics, and electronic structure of CuFeO2 and provide key information for CuFeO2-based device design and fabrication.

Temperature-dependent optical and vibrational properties of PtSe2 thin films.

PtSe2 has received substantial research attention because of its intriguing physical properties and potential practical applications. In this paper, we investigated the optical properties of bilayer and multilayer PtSe2 thin films through spectroscopic ellipsometry over a spectral range of 0.73-6.42 eV and at temperatures between 4.5 and 500 K. At room temperature, the spectra of refractive index exhibited several anomalous dispersion features below 1000 nm and approached a constant value in the near-infrared frequency range. The thermo-optic coefficients of bilayer and multilayer PtSe2 thin films were (4.31 ± 0.04) × 10-4/K and (-9.20 ± 0.03) × 10-4/K at a wavelength of 1200 nm. Analysis of the optical absorption spectrum at room temperature confirmed that bilayer PtSe2 thin films had an indirect band gap of approximately 0.75 ± 0.01 eV, whereas multilayer PtSe2 thin films exhibited semimetal behavior. The band gap of bilayer PtSe2 thin films increased to 0.83 ± 0.01 eV at 4.5 K because of the suppression of electron-phonon interactions. Furthermore, the frequency shifts of Raman-active Eg and A1g phonon modes of both thin films in the temperature range between 10 and 500 K accorded with the predictions of the anharmonic model. These results provide basic information for the technological development of PtSe2-based optoelectronic and photonic devices at various temperatures.

Temperature-dependent optical constants of monolayer [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text]: spectroscopic ellipsometry and first-principles calculations.

The temperature-dependent ([Formula: see text]) optical constants of monolayer [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] were investigated through spectroscopic ellipsometry over the spectral range of 0.73-6.42 eV. At room temperature, the spectra of refractive index exhibited several anomalous dispersion features below 800 nm and approached a constant value of 3.5-4.0 in the near-infrared frequency range. With a decrease in temperature, the refractive indices decreased monotonically in the near-infrared region due to the temperature-dependent optical band gap. The thermo-optic coefficients at room temperature had values from [Formula: see text] to [Formula: see text] for monolayer transition metal dichalcogenides at a wavelength of 1200 nm below the optical band gap. The optical band gap increased with a decrease in temperature due to the suppression of electron-phonon interactions. On the basis of first-principles calculations, the observed optical excitations at 4.5 K were appropriately assigned. These results provide basic information for the technological development of monolayer transition metal dichalcogenides-based photonic devices at various temperatures.

Anomalous boron isotope effects on electronic structure and lattice dynamics of CuB2O4.

Copper metaborate had a unique crystal structure and exhibited noteworthy magnetic phase transitions at 21 and 10 K. The electronic structure and lattice dynamics of copper metaborate Cu11B2O4 single crystals were investigated and compared with the optical properties of CuB2O4, to assess the boron isotope effect. The optical absorption spectrum at room temperature revealed two charge-transfer bands at approximately 4.30 and 5.21 eV with an extrapolated direct optical band gap of 3.16 ± 0.07 eV. Compared with the data on CuB2O4, the electronic transitions were shifted to lower energies upon the replacement of a heavier boron isotope. The band gap was also determined to be lower in Cu11B2O4. Anomalies in the temperature dependence of the optical band gap were observed below 21 K. Furthermore, 38 Raman-active phonon modes were identified in the room-temperature Raman scattering spectrum of Cu11B2O4, which were also observed in CuB2O4 with a shift to lower frequencies. No broadening caused by isotopic changes was observed. As the temperature decreased, phonon frequencies shifted to higher wavenumbers and the linewidth decreased. Anomalous softening in the Raman peaks below 21 K was also revealed.

Electronic structure and lattice dynamics of Ba2CuTeO6 single crystals.

The electronic structure and lattice dynamics of Ba2CuTeO6 single crystals were investigated through spectroscopic ellipsometry and Raman scattering measurements. The room-temperature optical absorption spectrum of Ba2CuTeO6 presented a direct optical band gap at approximately 1.04 eV and exhibited four bands at approximately 1.45, 3.43, 4.65, and 5.79 eV. The optical absorption band at 1.45 eV was attributed to on-site Cu2+ d-d transition. The other bands were attributed to charge-transfer transitions between the O 2p and Cu 3d or Te 5p states. The room-temperature Raman scattering spectrum of Ba2CuTeO6 exhibited 16 phonon modes at approximately 85, 97, 104, 119, 160, 194, 380, 396, 404, 409, 492, 568, 574, 606, 679, and 751 cm-1. When the temperature decreased to less than 287 K, which is the temperature at which structural phase transition occurs from the monoclinic phase to the triclinic phase, additional phonon modes appeared at approximately 124, 128, 152, and 601 cm-1. On further cooling to lower than 75 and 15 K, which are the temperatures at which short- and long-range antiferromagnetic phase transitions occur, respectively, the phonon modes at approximately 97, 104, 124, 128, 152, 160, 194, 380, 396, 409, 568, 574, 606, and 679 cm-1 exhibited softening, which indicates a coupling between the magnetic and lattice degrees of freedom. The stretching vibration of CuO6 octahedra located at 679 cm-1 had the largest spin-phonon coupling constant (1.67 mRy Å-2).

Observing quantum trapping on MoS2 through the lifetimes of resonant electrons: revealing the Pauli exclusion principle.

We demonstrate that the linewidth of the field emission resonance (FER) observed on the surface of MoS2 using scanning tunneling microscopy can vary by up to one order of magnitude with an increasing electric field. This phenomenon originates from quantum trapping, in which the electron relaxed from a resonant electron in the FER is momentarily trapped in a potential well on the MoS2 surface due to its wave nature. Because the relaxed electron and the resonant electron have the same spin, through the action of the Pauli exclusion principle, the lifetimes of the resonant electrons can be substantially prolonged when the relaxed electrons engage in resonance trapping. The linewidth of the FER is thus considerably reduced to as narrow as 12 meV. The coexistence of the resonant electron and the relaxed electron requires the emission of two electrons, which can occur through the exchange interaction.

Optical Study of the Electronic Structure and Lattice Dynamics of NdBaMn2O6 Single Crystals.

We investigated the electronic structure and lattice dynamics of double perovskite NdBaMn2O6 single crystals through spectroscopic ellipsometry and Raman scattering spectroscopy. The optical absorption band centered at approximately 0.88 eV was assigned to on-site d-d transitions in Mn, whereas the optical feature at approximately 4.10 eV was assigned to charge-transfer transitions between the 2p state of O and 3d state of Mn. Analysis of the temperature dependence of the d-d transition indicated anomalies at 290 and 235 K. The activated phonon mode, which appeared at approximately 440 cm-1 alongside with the enhancement of the 270 cm-1 phonon mode, coupled strongly to the metal-insulator transition at 290 K, which was associated with a charge/orbital ordering. Moreover, the MnO6 octahedral breathing mode at 610 cm-1 exhibited softening at a temperature lower than 235 K (temperature of the antiferromagnetic phase transition), which revealed the strong coupling between the lattice and magnetic degrees of freedom. The spin-phonon coupling constant obtained was λ = 2.5 cm-1. These findings highlight the importance of charge-orbital-spin interactions in establishing NdBaMn2O6 phases with novel properties.

Characterizing temperature-dependent optical properties of (MA0.13FA0.87) PbI3 single crystals using spectroscopic ellipsometry.

In this paper, we present spectroscopic ellipsometry measurements of (MA0.13FA0.87)PbI3 single crystals assessed at photon energies of 0.73-6.42 eV and at temperatures between 4.4 and 400 K. At room temperature, the refractive index was dispersed as a function of frequency, which is typical of a semiconductor. The absorption spectrum exhibited several electronic transitions. We estimated a room temperature direct band gap of 1.66 ± 0.02 eV and exciton binding energy of 40 meV. With decreasing temperature, the refractive index increased. The room-temperature thermo-optic coefficients were -1.7 × 10-4 and -2.5 × 10-4 K-1 at wavelength of 600 and 1200 nm. The exciton peak position and bandgap energy exhibited a redshift, which was attributed to a reverse ordering of the band structures. Additionally, an anomaly in exciton peak position and bandgap occurred at approximately 100-200 K due to the structural phase transition. This phenomenon was associated with the coexistence of MA/FA-disordered and MA/FA-ordered domains. Our results provide a foundation for the technological development of lead halide perovskites-based photonic devices at various temperatures.

Deep-ultraviolet Raman scattering spectroscopy of monolayer WS2.

Raman scattering measurements of monolayer WS2 are reported as a function of the laser excitation energies from the near-infrared (1.58 eV) to the deep-ultraviolet (4.82 eV). In particular, we observed several strong Raman peaks in the range of 700∼850 cm-1 with the deep-ultraviolet laser lights (4.66 eV and 4.82 eV). Using the first-principles calculations, these peaks and other weak peaks were appropriately assigned by the double resonance Raman scattering spectra of phonons around the M and K points in the hexagonal Brillouin zone. The relative intensity of the first-order [Formula: see text] to A1g peak changes dramatically with the 1.58 eV and 2.33 eV laser excitations, while the comparable relative intensity was observed for other laser energies. The disappearance of the [Formula: see text] peak with the 1.58 eV laser light comes from the fact that valley polarization of the laser light surpasses the [Formula: see text] mode since the [Formula: see text] mode is the helicity-exchange Raman mode. On the other hand, the disappearance of the A1g peak with the 2.33 eV laser light might be due to the strain effect on the electron-phonon matrix element.

Semiconducting Coordination Polymers Based on the Predesigned Ternary Te-Fe-Cu Carbonyl Cluster and Conjugation-Interrupted Dipyridyl Linkers.

A series of semiconducting cluster-incorporated Cu-based coordination polymers, namely, 1D zigzag polymers [{TeFe3 (CO)9 Cu2 }(L)]n (L=1,2-bis(4-pyridyl)ethane (bpea), 1; L=1,2-bis(4-pyridyl)ethylene (bpee), 5), 2D honeycomb-like polymers [{TeFe3 (CO)9 Cu}Cu(L)2.5 ]n (L=bpea, 2; L=bpee, 6), and 2D wave-like cation-anion polymer [{Cu2 (L)4 }({TeFe3 (CO)9 Cu}2 (L))]n (L=1,3-bis(4-pyridyl)propane (bpp), 4), as well as the macrocycle [{TeFe3 (CO)9 Cu2 }2 (bpp)2 ] (3) have been quantitatively synthesized via the liquid-assisted grinding from the pre-designed cluster [TeFe3 (CO)9 Cu2 (MeCN)2 ] with conjugated or conjugation-interrupted dipyridyl linkers. Notably, the most conjugation-interrupted bpp-bridged polymer 4 exhibited extraordinary semiconducting characteristics with an ultra-narrow bandgap of 1.43 eV and a DC conductivity of 1.5×10-2 Ω-1  cm-1 , which violates our knowledge, mainly attributed to the through-space electron transport via non-classical C-H⋅⋅⋅O(carbonyl) hydrogen bonds and aromatic C-H⋅⋅⋅π interactions. The incorporated Te-Fe-CO anions can not only provide numerous possibilities for secondary interactions within these Cu-based polymers but also serve as a redox-active coordination ligand to promote their conductivities. The intriguing structure-property relationships were studied by X-ray and DFT analyses and further demonstrated by significant change in the oxidation state of Cu atoms by XPS and Cu K-edge XANES.

Temperature- and roughness- dependent permittivity of annealed/unannealed gold films.

Intrinsic absorption and subsequent heat generation have long been issues for metal-based plasmonics. Recently, thermo-plasmonics, which takes the advantage of such a thermal effect, is emerging as an important branch of plasmonics. However, although significant temperature increase is involved, characterization of metal permittivity at different temperatures and corresponding thermo-derivative are lacking. Here we measure gold permittivity from 300K to 570K, which the latter is enough for gold annealing. More than one order difference in thermo-derivative is revealed between annealed and unannealed films, resulting in a large variation of plasmonic properties. In addition, an unusual increase of imaginary permittivity after annealing is found. Both these effects can be attributed to the increased surface roughness incurred by annealing. Our results are valuable for characterizing extensively used unannealed nanoparticles, or annealed nanostructures, as building blocks in future thermo-nano-plasmonic systems.

Anomalous lattice vibrations of monolayer MoS2 probed by ultraviolet Raman scattering.

We present a comprehensive Raman scattering study of monolayer MoS2 with increasing laser excitation energies ranging from the near-infrared to the deep-ultraviolet. The Raman scattering intensities from the second-order phonon modes are revealed to be enhanced anomalously by only the ultraviolet excitation wavelength 354 nm. We demonstrate theoretically that such resonant behavior arises from a strong optical absorption that forms near the Γ point and ½ΓK of the band structure and an inter-valley resonant electronic scattering by the M-point phonons. These results advance our understanding of the double resonance Raman scattering process in low-dimensional semiconducting nanomaterials and provide a foundation for the technological development of monolayer MoS2 in the ultraviolet frequency range.

The dielectric properties of La(Mg0.5Ti0.5)O3 ceramics studied by Raman-scattering, infrared reflectivity spectroscopy, and first-principles calculations.

La(Mg(0.5)Ti(0.5))O(3) (LMT) ceramics were prepared by either the solid-state reaction (LMT)(SS) or the citric-acid chemical method (LMT)(CA). A combination of Raman scattering, infrared reflectivity, and first-principles calculations was carried out to elucidate the correlation between lattice dynamics and the dielectric properties of these materials. Twelve Raman-active phonons are observed in both samples, displaying similar frequency positions. Interestingly, the A(g) phonon (g(11) mode) of (LMT)(SS) at about 717 cm(-1) involving the oxygen octahedron breathing vibrations demonstrates a narrower linewidth, suggesting its better crystallinity. Furthermore, an infrared-active u(2) phonon band due to the vibrations of O(I) and O(II) layers, which possesses the largest oscillator strength, exhibits stronger intensity for (LMT)(SS), as compared with those for (LMT)(CA). Additionally, the Q × f values (the product of dielectric Q values and measurement frequency) of (LMT)(SS) estimated from either microwave cavity or infrared spectroscopic measurements are larger than those of (LMT)(CA). These results indicate that the better coherence of lattice vibrations in (LMT)(SS) leads to its higher Q × f value, providing evidence for a strong connection between optical spectroscopic behavior and microwave dielectric characteristics in these materials.

The modulated structure and ferromagnetic insulating state in a nine-layer BaRuO(3).

We report the observation of a modulated structure and a ferromagnetic insulating state in a high quality single crystal of a nine-layer BaRuO(3). Using x-ray scattering, the modulated satellites were observed to double the unit cell along the c-axis at low temperature. The ferromagnetic insulating state is confirmed by magnetic and resistivity measurements. Analyzing the peak profiles from the modulation and host structure respectively, showed a lattice distortion at T∼55 K. These findings elucidate the intimate relationship between ferromagnetism and lattice distortion in a nine-layer BaRuO(3).