Longitudinal Optical Phonon Modes Research Articles

Study of cytotoxic performance of green-synthesized Co doped NiO nanoparticles over human breast cancer cells

The present paper presents a green method for the synthesis of pure and 1, 3, and 5% Co doped NiO nanoparticles (Co-NiO-NP) by using the aqueous extract of Salvadora persica. The synthesized nanoparticles were confirmed through PXRD, UV–Vis, FESEM, EDX, and Raman analyses. The results of PXRD, EDX, and UV–Vis affirmed the existence of well doped cobalt within the nickel oxide structure, which caused an increase in its particle size. According to PXRD finding, NiO have a face-centered cubic (FCC) structure. FESEM images showed pseudo-cubic form for nickel oxide nanoparticles which this morphology changes with doped process. Raman outcome were assigned to the longitudinal optical (LO) phonon mode of NiO. UV–Vis outcomes presented a blue shift in the adsorption peak of NiONP after the doping of cobalt ions into nickel oxide structure. According to the performed MTT test on the toxicity effect of synthesized nanoparticles on human breast cancer (MCF-7) and human umbilical vein endothelial (HUVEC) cells, cobalt doped nanoparticles displayed a higher cytotoxic activity than pure nanoparticles. Therefore, we can approve the applicability of our synthesized nanoparticles in biological implementations and cancer treatment.

Computational approach to enhance thermoelectric performance of Ag2Se by S and Te substitutions.

Designing an n-type thermoelectric material with a high thermoelectric figure of merit at near room temperature is extremely challenging. Generally, pristine Ag2Se reveals unusually low thermal conductivity along with a high electrical conductivity and Seebeck coefficient, which leads to high thermoelectric performance (n-type) at room temperature. Herein, we report a pseudo-ternary phase (Ag2Se0.5Te0.25S0.25) that exhibits significantly high thermoelectric performance (zT ∼ 2.1) even at 400 K. First-principles calculation reveals that the Rashba type of spin-dependent band spitting, which originates due to sulfur and tellurium substitution, helps to improve the thermopower magnitude. We also show that the intrinsic carrier mobility is not only controlled by the carrier effective mass but is substantially limited by longitudinal acoustic and optical phonon modes, which is an extension of the deformation potential theory. Locally off-center sulfur atoms, together with the increase in configurational entropy via substitution of Te and S atoms in Ag2Se, lead to a drastic reduction in the lattice thermal conductivity (klat ∼ 0.34 W m-1 K-1 at 400 K). The Rashba effect coupled with the configurational entropy synergistically results in a high thermoelectric figure of merit in the n-type thermoelectric material working in the near-room-temperature regime.

Evaluation of Anisotropic Biaxial Stress in Extremely-Thin Body (100) Silicon-Germanium-on-Insulator p-Type Metal-Oxide-Semiconductor Field-Effect-Transistor by Oil-Immersion Raman Spectroscopy

Background and ObjectiveSilicon germanium (SiGe)-on-insulator (SGOI) p-MOSFETs are expected for obtaining low power consumption and high current drive of CMOS since SiGe has higher hole career mobility than Si and the compressive strain can be induced in the SiGe channel without complicated heterostructures. In addition, the buried oxide between the Si substrate and the SiGe layer realizes suppression of leak current, and an extremely thin body (ETB) SGOI prevents short-channel effects. Strain engineering is important for improving the performance of p-MOSFETs because the hole mobility increases due to the compressive strain in the channel region. In previous studies, we have reported that anisotropic strain in ETB GOI channels of p-MOSFET improved electrical properties [1]. However, there are few reports regarding the detailed stress evaluation in ETB SGOI p-MOSFETs. The strain state has changed complicatedly by the SiGe device process. Therefore, a detailed stress measurement is indispensable to determine career mobility enhancement in the actual channel fabrication. In this study, we evaluated the anisotropic biaxial stress in the channel region of the ETB (100) SGOI p-MOSFETs by oil-immersion Raman spectroscopy.Experiments We prepared (100) SGOI p-MOSFETs along <110> with a channel thickness of approximately 10 nm by applying the Ge condensation process with slow cooling on the 60 nm-thick Si0.75Ge0.25 films epitaxially grown on SOI substrates with 10 nm-thick superficial Si layer [2]. We used the Ge condensation process that combined oxidation and intermixing annealing in order to suppress strain relaxation in the Ge-rich region [3] and obtained Ge-rich SiGe layer (Ge fraction: from 65 to 70%). For the evaluation of anisotropic biaxial stress in the SiGe channel region, we measured longitudinal optical (LO) and transverse optical (TO) phonon modes of Raman spectra for Ge-Ge vibration mode derived from SiGe by using oil-immersion Raman spectroscopy. In the oil-immersion Raman measurements, the wavelength of the excitation light source was 532 nm, the laser power at the sample surface was less than 1 mW, and the focal length of the spectrometer was 2,000 mm. The numerical aperture of the oil-immersion lens was 1.4, and the reflective index of oil was 1.5. We measured p-MOSFETs with different channel widths to examine the channel width dependence.Results and DiscussionFigure 1(a) and (b) show the Raman spectra of the LO and TO phonon modes obtained from the SiGe channel region at channel width (Wch) = 738 nm, respectively. We observed that Raman peaks of LO phonon mode shifted toward the higher wavenumber side compared to the Raman peak of strain-free SiGe. These results suggest that the high compressive strain is induced in the SiGe channel.Figure 1(c) shows the calculated transverse stress (σxx) and longitudinal stress (σyy) of the samples using LO and TO Raman shift for four different channel widths under the assumption of the anisotropic biaxial stress and Ge fraction 68% in the SiGe layer. From Fig. 1(c), we observed transverse stress (σxx) decreases with narrowing channel width. On the other hand, the longitudinal stress (σyy) did not significantly change regardless of channel width. These results suggest that the strain states in the SiGe channel region became closer to the uniaxial strain with narrowing the channel width. These changes are in good agreement with the previous electrical characteristics studies of the GOI p-MOSFETs [1], and it is expected that the compressive strain enhances the hole mobility in the ETB SGOI p-MOSFETs as well.AcknowledgementsThis work was supported by JSPS KAKENHI Grant Number 22H00208, Japan. A part of this work was conducted at Takeda Sentanchi Supercleanroom, The University of Tokyo, supported by "Nanotechnology Platform Program" of MEXT, Japan, Grant Number JPMXP09F-20-UT-0007. Reference [1] C. -T. Chen et al., IEEE Trans. Electron Devices 69, 1 (2022).[2] C. -T. Chen et al., Proc. IEEE Symp. VLSI Technology (2021).[3] K. -W. Jo et al., Appl. Phys. Lett. 114, 062101 (2019). Figure 1

AlScN-on-SiC Diaphragm Multimode Micromechanical Resonators for High-Temperature Sensing Applications

We demonstrate circular diaphragm multimode micromechanical resonators made of heterostructure thin film of aluminum scandium nitride (AlScN) sputtered on cubic silicon carbide (3C-SiC). We systematically characterize the multimode resonators from room temperature up to 600ºC high-temperature environment. We observe clear consistency in resonances measured in heating up and cooling down processes, validating that the AlScN/SiC diaphragm resonators can operate robustly in high-temperature environment up to 600°C without observable degradation. Raman spectroscopy results indicate that the turning points of the peak positions of the longitudinal optical (LO) phonon modes of both 3C-SiC and AlScN occur in almost the same temperature region where the turning point of temperature coefficient of resonance frequency (TCf) is observed. We calibrate the device temperature by measuring Raman peak of the silicon (Si) substrate of the chip, yielding a crystal lattice temperature of 410ºC at the heater setting temperature being 600ºC. The heating efficiency can be improved by clamping the chip using a clip or jig with lower thermal conductivity.

Thickness-Dependent Terahertz Permittivity of Epitaxially Grown PbTe Thin Films

The exceptional thermoelectric properties of PbTe are believed to be associated with the incipient ferroelectricity of this material, which is caused by strong electron–phonon coupling that connects phononic and electronic dynamics. Here, we have used terahertz time-domain spectroscopy measurements to generate complex permittivity spectra for a set of epitaxially grown PbTe thin films with thicknesses between 100 and 500 at temperatures from 10K to 300K. Using a Drude–Lorentz model, we retrieved the physical parameters of both the phononic and electronic contributions to the THz permittivity. We observed a strong decrease, or softening, of the transverse optical phonon mode frequency with decreasing temperature, determining a thickness-independent negative ferroelectric-transition critical temperature, while we found a thickness-dependent anharmonic phonon decay lifetime. The electronic contribution to the permittivity was larger in thinner films, and both the carrier density and mobility increased with decreasing temperature in all films. Finally, we detected a thickness-dependent longitudinal optical phonon mode frequency, indicating the presence of plasmon–phonon coupling.

Morphology-dependent optical properties of a 3D CH3NH3PbBr3 based metal − organic framework

More recently, Metal–Organic Frameworks (MOFs) have displayed extensive promise in the fields of energy storage, solar cells, catalysis, super capacitors, and optoelectronic devices. This work investigates the nonlinear thermo-optical responses of CH3NH3PbBr3 (MAPbBr3) and mix MAPbBr3 with UiO-66-NH2 (MAPbBr3 + MOF) thin films using the Z-scan technique with different incident power of laser ranging from 10 mW to 50 mW. The results show that the third-order nonlinear optical coefficients of MAPbBr3 + MOF thin films were improved obviously than MAPbBr3 thin films. Furthermore, nonlinear optical properties of MAPbBr3 + MOF thin film is evaluated to be improved from 12.5 × 10-8 to 38.2 × 10-8 by decreasing the input power of laser from 50 mW to 10 mW. This treatment repeated for MAPbBr3 thin film, as well. Considering Kubelka-Munk equation, energy gap) Eg for the MAPbBr3 and MAPbBr3 + MOF thin films deposited on glass film is calculated to be 2.565 and 2.530 eV. Due to analyze the optical characterization of the synthesized samples, optical reflection, transmission and absorption were studied. Kramers-Kronig relations and the reflectance data were used for calculation of complex refractive index and dielectric constant. The transverse and the longitudinal optical phonon modes of MAPbBr3 and MAPbBr3 + MOF thin films locate at about 548 and 598 nm, and 590 and 640 nm, respectively. This research opens a new window for wide scale optoelectronic devices in Q-switching for the generation of pulsed lasers including MAPbBr3 with UiO-66-NH2 by detecting the remarkable nonlinear optical characteristics in lower incident power.

Effect of Mn concentration on the polar optical phonons frequencies in Cd1− xMnxTe thin films

Cd1−xMnxTe (CMT) is a wide bandgap semiconductor that stands out among the ternary compounds in several industrial applications, such as optoelectronic and solar cell devices. Therefore, it is worth understanding the mechanisms of light interaction with CMT produced by growth techniques. In this work, we investigate the room temperature Raman scattering by longitudinal optical phonon modes CdTe-like (LO1) and MnTe-like (LO2) in CMT thin films grown on Si(111) by molecular-beam epitaxy. The well-known linear dependence of LO1 and LO2 frequencies on x is observed in Raman spectra when the excitation photon energy is above the bandgap energy. As expected, for the excitation energy of the incident laser line near the fundamental gap of CMT, the resonance becomes evident in the Raman results. However, for a specific resonance condition due to tuning of the bandgap energy dependence on Mn concentration, the frequencies of the LO1 and LO2 phonon modes remain constant as the average manganese concentration increases to values x≳ 0.30. From micro-photoluminescence spectroscopy investigations, we concluded that for Mn concentration above 0.3, a broad range of optical transitions provides the required conditions for a resonant Raman scattering selected by the incident photon energy. It introduces a resonant selectivity of regions where the incident excitation energy coincides with a bandgap energy associated with a specific value of x, even for samples with nominally different compositions. The results show the sensitivity of resonant Raman to express diagnostics of ternary compound growth.

Raman scattering and photoluminescence in In+ and As+ ion-implanted SiO2 layers encapsulated with Si3N4

Raman and photoluminescence spectra were investigated in the In+ and As+ ion-implanted SiO2 films encapsulated with Si3N4 layers as a function of annealing temperature. The optical phonon frequency, as a function of the InAs nanocrystal size, was also calculated within the confined phonon model. The Raman scattering band of around 231 cm−1, close to the low-frequency shifted longitudinal optical phonon mode in the InAs matrix, was observed as the annealing temperature increased to 900 °C. The InAs nanocrystal size of 3 nm was estimated. The strong room-temperature photoluminescence peaking at 550 nm (2.25 eV) was also obtained under the 473 nm wavelength excitation. Its intensity reached a maximum value as the annealing temperature increased to 1000 °C. Its peak position was blue-shifted as the excitation wavelength decreased. The direct irradiative electron and hole recombination in the InAs nanocrystals was proposed as a possible mechanism of this photoluminescence.

Growth of Cd1-xMnxTe crystals by vertical Bridgman method and analysis of composition dependence on the bandgap

This article reports the growth of cadmium manganese telluride (Cd1-xMnxTe, CMT) single crystal by vertical Bridgman method and the property measurements for three different Mn compositions (x = 0.05, 0.10 & 0.15). The single crystalline nature of the grown crystals was confirmed with Laue back reflection method. The lattice constants were calculated using single-crystal X-ray diffraction analysis. Phase formation and zinc blende crystal structure were confirmed with powder X-ray diffraction analysis. The shifting of the diffraction peak position indicates the reduction in lattice parameter of Cd1-xMnxTe crystals with Mn concentration. Raman spectra exhibited two mode behavior having CdTe and MnTe-like structures and their longitudinal optical phonon modes displaying peak shift with Mn concentration. The increase in the Mn concentration in CMT lattice made a blueshift in photoluminescence emission band leading to changes in the bandgap from 1.5776 to 1.7250 eV. The experimentally observed IR transmittance was found to be in line with theoretically observed transmittance. The observed Mn concentration dependent structural and optical properties confirm the formation of high quality CMT single crystals by vertical Bridgman technique.

Resonant Exciton Scattering Reveals Raman Forbidden Phonon Modes in Layered GeS.

Germanium monosulfide with an anisotropic puckered crystalline structure has recently attracted much attention due to its unique optical and electronic properties; however, exciton-phonon interactions were only superficially elucidated. We study the resonant Raman scattering and the photoluminescence of the optically active Γ-exciton in layered GeS flakes and evaluate the exciton and phonon responses on variations in the excitation energy, laser-light and emission polarizations, temperature, and laser power. A double-resonance mechanism allows for observing Raman forbidden (dark) first- and second-order longitudinal-optical phonon modes whose symmetries and energies are moreover calculated by density functional perturbation theory. For (quasi)-resonant exciton excitation, the selection rules become relaxed so that a fourth-order Fröhlich intraband process is mediated by the scattering of the electron with a longitudinal-optical and an acoustic phonon. Our results demonstrate considerable coupling between phonons and photogenerated carriers in GeS flakes and the high efficiency of multiorder scattering in optical processes.

Spectroscopic and electrical analysis of p–Si/n-ZnSxSe1−x (0.0 ≤ x ≤ 1.0) heterostructures for photodetector applications

The present paper focuses on the properties of the p–Si/ ZnSxSe1−x (0 ≤ x ≤ 1) heterojunctions in photodetector applications. The heterostructures were fabricated by depositing ZnSSe on Si wafer using the thermal co-evaporation technique with ZnS and ZnSe powders. The GIXRD study showed that films were in cubic phase, and the prominent peak was shifted with composition x. The maximum crystallite size of the films was found for x = 0.8. The presence of point defects and emission related to higher Zn content in the thin films was confirmed by Photoluminescence. Temperature-dependent Raman analysis reveals that the longitudinal optical phonon modes shift to the lower wavenumber side as temperature decreases, which describes the variation of lattice parameters with temperature. The barrier height and ideality factor were calculated by implementing the thermionic emission. The photoresponse of p–Si/ ZnSxSe1−x heterostructures was studied. The investigation showed that the sample with x = 0.8 exhibits high photosensitivity and is suitable for photodetector applications.

Polaron states of the full-configuration defects in metal halide perovskites

The systematical analysis for varieties of defects with different depths and lattice relaxation strengths in metal halide perovskites (MHPs) is a challenging task. Here, we study the energy shifts of the full-configuration defects due to the polaron effect based on the all-coupling variational method in MHPs, where these polaron states are formed stemming from different defect species coupling with the longitudinal optical phonon modes via Frhlich mechanism. We find that the polaron effect results in defect levels varying from tens to several hundreds of meV, which are very close to the correction of defect levels due to the defect-polaron effect, especially for these defects migration proved in the recent experiments in MHPs. These results provide the significant enlightenment not only for analyzing the radiation and non-radiation processes of carriers mediated by defects, but also for optimizing defect effect in the photovoltaic and photoelectric devices based on MHPs materials.

Strain evolution and confinement effect in InAs/AlAs short-period superlattices studied by Raman spectroscopy

Raman spectra of two series of InAs/AlAs short-period superlattices were measured at room temperature to investigate the impact of strain on the phonon modes taking into consideration the confinement effect and interface mode. The evolution of strain in the InAs layer and the AlAs layer was studied in (InAs)2/(AlAs)2 superlattices grown at various temperatures (400–550 °C). While the strain existed in the AlAs layer remained almost constant, the strain in the InAs layer varied significantly as the growth temperature increased from 500 to 550 °C. The confinement effect on the optical phonons was analyzed based on results from (InAs)n/(AlAs)n grown at 450 °C (n = 2, 3, 4, and 5). Additionally, the confinement effect was found to be stronger in shorter periods with higher interface quality. The interface phonon modes were resolved between the longitudinal optical and transverse optical phonon modes, which assist in the rough estimation of the thickness of the layers. The disorder-activated acoustic phonon modes at the low-frequency side were also addressed.

Heteroepitaxial chemistry of zinc chalcogenides on InP nanocrystals for defect-free interfaces with atomic uniformity

Heteroepitaxy on colloidal semiconductor nanocrystals is an essential strategy for manipulating their optoelectronic functionalities. However, their practical synthesis typically leads to scattered and unexpected outcomes due to the intervention of multiple reaction pathways associated with complicated side products of reactants. Here, the heteroepitaxy mechanism of zinc chalcogenide initiated on indium phosphide (InP) colloidal nanocrystals is elucidated using the precursors, zinc carboxylate and trialkylphosphine selenide. The high magnetic receptivity of 77Se and the characteristic longitudinal optical phonon mode of ZnSe allowed for monitoring the sequence of epilayer formation at the molecular level. The investigation revealed the sterically hindered acyloxytrialkylphosphonium and diacyloxytrialkylphosphorane to be main intermediates in the surface reaction, which retards the metal ion adsorption by a large steric hindrance. The transformation of adsorbates to the crystalline epilayer was disturbed by surface oxides. Raman scattering disclosed the pathway of secondary surface oxidation triggered by carboxylate ligands migrated from zinc carboxylate. The surface-initiated heteroepitaxy protocol is proposed to fabricate core/shell heterostructured nanocrystals with atomic-scale uniformity of epilayers. Despite the large lattice mismatch of ZnS to InP, we realised a uniform and interface defect-free ZnS epilayer (~0.3 nm thickness) on InP nanocrystals, as evidenced by a high photoluminescence quantum yield of 97.3%.

A remarkable third-order nonlinear optical behavior of single-crystal bromide organic-inorganic lead halide perovskite

The optical and structural aspects of methylamine lead bromide (MAPbBr3) single crystal (SC) are investigated. Based on Kramers–Kronig (KK) equations, the optical and dielectric parameters are investigated. The optical phonons Longitudinal optical (LO = 678 cm−1) and Transversal optical (TO = 578 cm−1) phonon modes are associated with the optical interaction with lattices in the visible region for MAPbBr3 SC. The Z-scan method is used to investigate the third-order nonlinear coefficients of SC hybrid organic–inorganic lead halide perovskites, MAPbBr3 with the 2.19 eV optical band gap energy. According to the nonlinear optical (NLO) properties, MAPbBr3 has a high third-order NLO response. The two-photon absorption (2PA) coefficient is found to be 3 × 10-4 cm W−1 using the transmission open aperture Z-scan technique. The nonlinear refraction (NLR) is obtained in the order of 10-9 cm2 W−1 utilizing a close aperture Z-scan, which revealed self-defocusing phenomena. This research shows that MAPbBr3 is a practical and low-priced nonlinear material for ultrafast photonics applications.

Synthesis of CdSe QDs decorated ZIF-8 composite for visible light assisted degradation of methylene blue

Metal organic frameworks have proven to be a crucial facet for the enhancement of environmental research because of their excellent properties, especially, extensive surface area. In the present study, we propose the synthesis route of CdSe quantum dots (CdSe QDs) and zinc imidazole framework-8 (ZIF-8) composite for the effective and potential methylene blue (MB) degradation via photocatalysis in visible range which is kinetically a pseudo first-order reaction. ZIF-8 itself is a photocatalyst but it has large band gap (4.8–5.1 eV) in ultraviolet region. The doping of CdSe QDs into the ZIF-8 matrix is done with the aim of deviating the photocatalytic activity to visible range by lowering of the band gap of ZIF-8 which was validated by UV–vis–DRS. XRD results indicated the decrease in the d-spacing, crystallite size and also provided the information about the positive microstrain for all the components. FTIR and Raman spectra confirmed the composite synthesis and the CdSe longitudinal (LO) and surface optical phonon modes (SO) along with the fundamental modes of ZIF-8, respectively. The elemental state of the constituent was established by XPS analysis. The adsorption isotherm data and the kinetic studies indicate that the CdSe-ZIF-8 composite has a great potential as a catalyst.

Evaluation of Strained Group IV Semiconductor Devices by Oil-Immersion Raman Spectroscopy

1. Background and purpose Strain techniques are one of the principal technology boosters for the realization of high-performance logic devices composed of group IV semiconductors such as Si, Ge and their alloy. Recently, it is gradually difficult to measure strain states as miniaturization proceeds to the nanometer scale. Raman spectroscopy is one of the most powerful techniques for phonon measurements and can perform strain evaluation with high spatial resolution nondestructively [1]. However, conventional Raman spectroscopy is limited in strain information based on the Raman polarization selection rules [1].In this paper, we introduce oil-immersion Raman spectroscopy with the high-numerical-aperture (high-NA) lens to realize precise evaluation of strain states (for example, anisotropic strain) in group IV semiconductor devices. 2. Oil-immersion Raman spectroscopy In conventional Raman spectroscopy, it is impossible to excite transverse optical (TO) phonon mode due to Raman polarization selection rules under the backscattering geometry from the (001) group IV semiconductor surface. In other words, a uniaxial or isotropic biaxial stress measurements in group IV semiconductors can only be performed by exciting only the longitudinal optical (LO) phonon mode under a conventional (001) backscattering configuration.To realize anisotropic stress evaluation assuming the channel region of the group-IV semiconductor MOSFET, we used a high-NA immersion lens for the selective excitation of both LO and TO phonon modes. The high-NA and refractive index n of the oil were approximately 1.4 and 1.5, respectively. Using a high-NA immersion lens, a z-polarized light component can be effectively obtained owing to the aperture angle. Therefore, TO phonons can be excited on the basis of Raman selection rules for (001)-oriented group IV semiconductors [2, 3]. 3. Results and discussion As an example of oil-immersion Raman measurements results, we evaluated the strain states of extremely-thin-body Ge-on-insulator (ETB GOI) MOSFET channels fabricated by Ge condensation [4, 5] by oil-immersion Raman spectra. We observed that the Raman peak shifted toward a higher wavenumber, indicating that high compressive strain in the GOI channels. In addition, a clear splitting of LO and TO phonon modes was confirmed, implying the anisotropic biaxial strain states induced in the ETB GOI channel.Figure 1 shows the channel width (Wch) dependence of the anisotropic biaxial stress for ETB GOI MOSFET (channel thickness: 10.6 nm) obtained by the oil-immersion Raman spectroscopy. As shown in Fig. 1, the stress relaxation along the GOI channels was observed as the GOI channel width gets narrower. In other words, the uniaxial stress is applied into channels by narrow GOI channel patterning. These behaviors are in good agreement with the previous reports such as strained-SOI [6] and SiGe nano patterns [3][7].In conclusion, oil-immersion Raman spectroscopy is one of the most powerful strain measurements in Group IV semiconductor device structures. Acknowledgements This work was supported by JSPS KAKENHI Grant Number 22H00208, Japan. A part of this work was conducted at Takeda Sentanchi Supercleanroom, The University of Tokyo, supported by "Nanotechnology Platform Program" of MEXT, Japan, Grant Number JPMXP09F-20-UT-0007.

High-harmonic spectroscopy of coherent lattice dynamics in graphene

High-harmonic spectroscopy of solids is a powerful tool, which provides access to both electronic structure and ultrafast electronic response of solids, from their band structure and density of states to phase transitions, including the emergence of the topological edge states, to the PetaHertz electronic response. However, in spite of these successes, high-harmonic spectroscopy has hardly been applied to analyze the role of coherent femtosecond lattice vibrations in the attosecond electronic response. Here we study coherent phonon excitations in monolayer graphene to show how high-harmonic spectroscopy can be used to detect the influence of coherent lattice dynamics, particularly longitudinal and transverse optical phonon modes, on the electronic response. Coherent excitation of the in-plane phonon modes results in the appearance of sidebands in the spectrum of the emitted harmonic radiation. We show that the spectral positions and the polarization of the sideband emission offer a sensitive probe of the dynamical symmetries associated with the excited phonon modes. Our work brings the key advantage of high-harmonic spectroscopy---the combination of subfemtosecond to tens of femtoseconds temporal resolution---to the problem of probing phonon-driven electronic response and its dependence on the dynamical symmetries in solids.

Tuning the electronic, phonon, and optical properties of monolayer BX (X[dbnd]P and As) through the strain effect

Strain engineering has widely been recognized as a highly sensitive and effective way to modulate the physical properties of materials. Here, we thoroughly investigate the effects of biaxial strain-driven optoelectronic properties and thermodynamic stability of BX (XP and As) monolayers under a density functional framework. The electronic bandgap at the K-point of these materials increases with tensile (0 to +10%) strain and decreases in the presence of compressive (−10% to 0) strain. The phonon modes indicate that monolayer BP becomes thermodynamically unstable with larger than 4% compressive strain, and BAs loses stability at 4% compressive strain. However, both structures can show quite stable nature for up to + 8% tensile strain. In the presence of compressive strain, the longitudinal optical (LO) and transverse optical (TO) phonon modes become softening in nature. In contrast, it displays hardening behavior with the tensile strain in these materials. Also, superior light absorption near the infrared and visible light spectrum is feasible by the BP and BAs monolayer when the biaxial strain is incorporated. These results elucidate a promising way to manipulate the optical and electronic properties of monolayer BP and BAs via strain engineering and eventually make these materials promising for futuristic spintronics, electronics, and optoelectronic device applications.

Swift heavy ion irradiation induced modifications in electron beam deposited CdS thin films

The field of tailoring the physio-chemical properties by the way of radiation leads to the immense opportunities in the domain of advanced photonics and optoelectronic devices. In the present contribution, 120 MeV silver ions (Ag9+) induce electronic excitations which instigate modifications in crystalline cadmium sulphide (CdS) thin films deposited via electron beam evaporation are examined. X-ray Diffraction (XRD) reveals that the peak intensity and FWHM get affected as a consequence of swift heavy ions (SHI) irradiation while crystal phase remains preserved even after irradiation. Raman analysis exhibits the asymmetry in the fundamental longitudinal optical (LO) phonon mode which is directly related to the surface defects, reduction in particle size, and amorphization after irradiation. Field emission scanning electron microscopy (FESEM) images exhibit the reduction in particle size followed by highly strained as-deposited samples get stabilize with the deposited energy. The spherical shaped nanoparticles are observed for as-deposited and irradiated films through atomic force microscopy (AFM) and also, declared the nano-grain formation as resulted from negative surface tension induced at higher fluences. Photoluminescence (PL) spectra indicate the electronic excitations inspired additional yellow band due to generation of ICd-Vcd Frenkel pair defects along with the green and red emission.