Shift Of Phonon Modes Research Articles

“Yb modification of bismuth Ferrite: Unveiling Optical, Dielectric, and electrochemical properties for advanced applications”

This study explores the detailed impact of Yb3+ substitutional doping on the structural, optical, dielectric, and electrochemical characteristics of bismuth ferrite. Rigorous structural analysis conducted via X-ray diffraction confirmed the formation of a rhombohedral distorted perovskite structure, within the R3C space group. The investigation of crystallite size showed a reducing trend as Yb3+ content increased. Morphological studies revealed the presence of finely dispersed nanocrystalline grains, exhibiting particle size within the range from 281 to 197 nm. Raman spectroscopy unveiled notable phonon mode shifts towards higher wavenumbers, correlating with increased Yb3+ doping concentrations. Additionally, a discernible reduction in bandgap energy, from 2.15 to 1.98 eV, was observed with increasing doping levels. An improved dielectric response was observed with Yb3+ doping. Electrochemical assessments, employing cyclic voltammetry and galvanostatic charge–discharge techniques, revealed the superior electrochemical performance of the 30 % Yb-doped bismuth ferrite-modified electrode, demonstrating a specific capacitance of 575 Fg−1 at a scan rate of 10 mVs−1. These findings underscore the tailored engineering of Yb-doped bismuth ferrite nanoparticles, positioning them as promising candidates for the next generation of supercapacitor devices.

Spin-order-dependent magneto-elastic coupling in two dimensional antiferromagnetic MnPSe3 observed through Raman spectroscopy

Layered antiferromagnetic materials have recently emerged as an intriguing subset of the two-dimensional family providing a highly accessible regime with prospects for layer-number-dependent magnetism. Furthermore, transition metal phosphorus trichalcogenides, MPX3 (M = transition metal; X = chalcogen) provide a platform on which to investigate fundamental interactions between magnetic and lattice degrees of freedom and further explore the developing fields of spintronics and magnonics. Here, we use a combination of temperature dependent Raman spectroscopy and density functional theory to explore magnetic-ordering-dependent interactions between the manganese spin degree of freedom and lattice vibrations of the non-magnetic sub-lattice via a Kramers-Anderson super-exchange pathway in both bulk, and few-layer, manganese phosphorus triselenide (MnPSe3). We observe a nonlinear temperature-dependent shift of phonon modes predominantly associated with the non-magnetic sub-lattice, revealing their non-trivial spin-phonon coupling below the Néel temperature at 74 K, allowing us to extract mode-specific spin-phonon coupling constants.

Moiré engineering in 2D heterostructures with process-induced strain

We report deterministic control over a moiré superlattice interference pattern in twisted bilayer graphene by implementing designable device-level heterostrain with process-induced strain engineering, a widely used technique in industrial silicon nanofabrication processes. By depositing stressed thin films onto our twisted bilayer graphene samples, heterostrain magnitude and strain directionality can be controlled by stressor film force (film stress × film thickness) and patterned stressor geometry, respectively. We examine strain and moiré interference with Raman spectroscopy through in-plane and moiré-activated phonon mode shifts. Results support systematic C3 rotational symmetry breaking and tunable periodicity in moiré superlattices under the application of uniaxial or biaxial heterostrain. Experimental results are validated by molecular statics simulations and density functional theory based first principles calculations. This provides a method not only to tune moiré interference without additional twisting but also to allow for a systematic pathway to explore different van der Waals based moiré superlattice symmetries by deterministic design.

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.

Evaluating strain and doping of Janus MoSSe from phonon mode shifts supported by ab initio DFT calculations.

With the study of Janus monolayer transition metal dichalcogenides, in which one of the two chalcogen layers is replaced by another type of chalcogen atom, research on two-dimensional materials is advancing into new areas. Yet only little is known about this new kind of material class, mainly due to the difficult synthesis. In this work, we synthesize MoSSe monolayers from exfoliated samples and compare their Raman signatures with density functional theory calculations of phonon modes that depend in a nontrivial way on doping and strain. With this as a tool, we can infer limits for the possible combinations of strain and doping levels. This reference data can be applied to all MoSSe Janus samples in order to quickly estimate their strain and doping, providing a reliable tool for future work. In order to narrow down the results for our samples further, we analyze the temperature-dependent photoluminescence spectra and time-correlated single-photon counting measurements. The lifetime of Janus MoSSe monolayers exhibits two decay processes with an average total lifetime of 1.57 ns. Moreover, we find a strong trion contribution to the photoluminescence spectra at low temperature which we attribute to excess charge carriers, corroborating our ab initio calculations.

Observation of room-temperature amplitude mode in quasi-one-dimensional charge-density-wave material CuTe

We have performed in-plane electrical transport and polarized Raman spectroscopy measurements on layered material CuTe to characterize the quasi-one-dimensional charge density wave (CDW). Along with the CDW formation below TCDW ∼346 K, the a-axis resistivity shows a huge hump, but the b-axis resistivity exhibits no evident anomaly, manifesting the quasi-one-dimensional character of the CDW. Concomitantly, a rapid rise in positive Hall coefficient is observed due to partial gaping out of the Fermi surface. Based on our Raman scattering measurement, we observe two Raman-active phonon modes in the non-CDW state and, additionally, one collective amplitude mode and four zone-folded modes in the CDW state. The phonon-mode shift with temperature clearly deviates from the expected anharmonic model upon approaching TCDW, suggesting strong electron–phonon coupling in driving the CDW transition. The amplitude mode even survives at room temperature and above, whose evolution can be described by a modified mean-field model, indicating a well-defined CDW order. These results, in addition to the simple formula and easy-to-exfoliate layered structure, promise CuTe as a model system to study the Peierls-like CDW physics and hold great potentials for CDW-based applications.

Quantum confinement and effective masses dependence in black phosphorus quantum dots and phosphorene

In the present study, quantum confinement effect in phosphorene and black phosphorus quantum dots is extensively studied from experimental and theoretical viewpoints. These quantum structures were prepared with the help of Benzonitrile solvent using Liquid Phase Exfoliation and solvothermal-assisted Liquid Phase Exfoliation, respectively. On one hand, Fourier-transform infrared spectroscopy shows no sign of phosphorus oxidation in both phosphorene and black phosphorus quantum dots solutions. Furthermore, Raman spectroscopy showed a shift in B2gandAg2 phonon modes for phosphorene and black phosphorus quantum dots as compared to bulk black phosphorus. On the other hand, structural characterization via high-resolution transmission electron microscopy imaging and electron diffraction patterns showed a high degree of crystallinity in both quantum structures with no sign of aggregations. Optical properties characterization showed the expected increase in the bandgap value for both quantum structures, which were supported by theoretical calculations using density functional theory and effective mass approximation. Interestingly, we demonstrate that the quantum confinement observed in phosphorene is weakened to the expected extent, relative to that in BPQDs, by the loss of two confinement dimensions. Appropriate models, describing the bandgap and effective mass dependence as a function of the confinement regime, are presented.

Subsiding strain-induced In-Ga intermixing in InAs/InxGa1−xAs sub-monolayer quantum dots for room temperature photodetectors

Strain – induced intermixing in sub – monolayer (SML) quantum dots (QDs) affects primarily the dot size distribution in an erratic way and also the inter – dot coupling efficiency for carrier transport. In this study, we have explored the role of strain and carrier confinement effects in order to control the dot size distribution by varying InAs QDs coverage (X: 0.3 and 0.5 ML) and stacks (Y: 4, 6, and 8) simultaneously. The size variation affects the position of localized levels inside QDs primarily as it is the inter – dot coupling that decides the luminescence efficiency. Next, through multiple stacking of QD layers, strain variation becomes inhomogenous that facilitate a higher degree of In – Ga intermixing which necessitates to estimate the amount of Indium (%) present inside such In–rich islands to progress for a superior optical performance. Hence, we combine and address these above concerns in great detail. The 20 K ground-state (GS) photoluminescence (PL) energy for 0.3 and 0.5 ML samples were centered at 1.24–1.33 eV, and 1.11–1.19 eV respectively. The PL linewidth and activation energy variation with temperature validated that there exists two possible thermal escape pathways for carrier recombination. Besides, PL – excitation (PLE) studies comprehended the influence of phonon and excited states (ES) in the samples. Raman analysis helped us calculate the In% inside QDs by figuring out the shift in InAs QD phonon modes (amount of compressive strain). Furthermore, the structural analysis by high-resolution X-ray diffraction (HRXRD) was done to calculate the strain moderated defect density and also the effect of in – plane compressive strain to the reduction in quantum confinement. The increase in compressive strain inside QDs caused more In–Ga intermixing in 0.3 ML samples and higher defect densities. Altogether we see that sample with 0.5 ML, 4 stacks showed vertically-correlated dot growth (from HR-XTEM) with minimum strain (−0.03383 a.u.), dislocation density (4.887 x 1011 cm−2) and higher In content (62.41%). This high degree of excitonic carrier confinement and strain tuning makes SML QDs an attractive candidate for room-temperature (RT) operable photodetector (PD) devices.

Probing phase separation in Nd1−xSrxMnO3 (x ≈ 0.4, 0.5) polycrystals through temperature dependent magnetic and Raman spectroscopy studies

The manuscript reports temperature-dependent magnetic transitions correlated to variations in the phonon modes in Nd1−xSrxMnO3. Synthesis of x = 0.4 & 0.5 compounds by the solid-state reaction method was followed by structural/microstructural and electronic characterizations. The crystal structure of both compositions is orthorhombic with Pbnm symmetry, as confirmed by x-ray diffraction. The XPS data confirm the mixed Mn valance with the desired stoichiometry. The phase-separated nature is manifested through the occurrence of various phases, e.g., spin-glass (SG), charge order (CO), ferromagnetic (FM), and paramagnetic (PM), as the temperature is varied from 20 to 300 K. The temperature-dependent Raman measurements reveal that although the material is structurally stable in the studied temperature range of 80–440 K, noticeable discontinuities in the phonon mode shifts were noticed. These phonon mode shifts are found to directly correspond to the magnetic transitions. The change in the frequency of the Raman modes with temperature, which causes the observed shift, could be attributed to various factors such as lattice expansion, anharmonic interactions, spin-phonon coupling, electron-phonon interaction, etc. Here, we have used spin-phonon coupling along with lattice expansion as well as anharmonic interactions to describe the behavior of modes in the FM regime and lattice expansion and anharmonic interaction in the PM phase.

Effects of Density and Diameter on Surface Optical Phonon Modes in GaAs Nanowire Bundles.

We report the systematic investigation of the surface optical phonon modes in Au-catalyzed GaAs nanowires grown on an Au pre-patterned GaAs(111)B substrate using μ-Raman spectroscopy. We employed electron-beam dose rate as a control parameter during the substrate patterning step for adjusting the nanowire base diameter and coverage, which are independent from the nanowire growth conditions. We have experimentally studied the effect of the fill factor and average diameter on the surface optical phonon modes and explained the red-shift and broadening of the surface optical phonon frequencies by employing the dielectric continuum model. The surface optical phonon mode shift is exhibited to be sensitive to fill factor, rather than base diameter. The decrease in the average diameter from 280 nm to 180 nm results in the asymmetric broadening and red-shift of the surface optical phonon frequency (~1.83 cm-1) but the theoretical calculation from the isolated single nanowire-based dielectric continuum model cannot solely explain the behaviors of the surface optical phonon mode. In contrast, the change in the fill factor from 0.01 to 0.83 results in a shift of the surface optical phonon frequency (~6.5 cm-1) from the GaAs bulk value. The red-shift and asymmetric broadening of the surface optical phonons, in an agreement with the Maxwell-Garnett approximation, are consequences of dipolar interaction of randomly aligned neighboring nanowires and the polar nature of GaAs nanowire bundles. This work suggests the pre-patterning parameter dependent surface optical phonon characteristics of GaAs nanowire bundles which are of great importance in the nondestructive characterization of low-dimensional opto-electronic materials and devices.

Giant photostriction of CaCu3Ti4O12 ceramics under visible light illumination

Ferroelectrics with a perovskite structure, exhibiting strong coupling between photons, carriers, and phonons, have been demonstrated to show a strong photostrictive response. Limited by the optical bandgap, most of the reported ferroelectric perovskites respond only to UV light or weakly respond to visible light below 550 nm. Herein, the photostrictive performance of CaCu3Ti4O12 (CCTO) ceramics, with a low bandgap of ∼1.5 eV (ideal for solar spectrum absorption), is comprehensively investigated. Although the compound presents no ferroelectricity, in general, a giant photostriction of ∼10−3 with a large photostrictive efficiency of ∼10−11 m3/W in CCTO ceramics is observed by applying purple (405 nm), green (520 nm), and red (655 nm) laser illumination. Evidenced by the XRD peak shifts under external laser illumination and the phonon mode shifts via the power-dependent Raman spectrum, the light-induced non-thermal deformation in CCTO is attributed to the enhanced stretching of O–Ti–O chains and the distortions of TiO6 octahedrons. The excellent and uniform photostrictive performance of CCTO ceramics in the visible-light region makes them potential candidates for practical applications in next-generation photo-driven electromechanical devices.

'Corrigendum: Role of Ni2+(d8) ions in electrical, optical and magnetic properties of CdS nanowires for optoelectronic and spintronic applications (2018 Nanotechnology 29 265602)'.

For the first time, 1D Ni ions doped CdS nanowires (NWs) were synthesized via chemical vapour deposition (CVD). The synthesized Cd0.886Ni0.114S NWs were single crystalline. We have reported here, investigation of optical, electrical and magnetic properties of prepared NWs for optoelectronic and spintronic applications. Successful incorporation of Ni ions in an individual CdS NW has been confirmed through several characterization tools: significant higher angle and phonon modes shift were observed in the XRD and Raman spectra. SEM-EDX and XPS analysis also confirmed the presence of Ni2+ ions. Room-temperature photoluminescence (RT-PL) showed multi-peaks: two emission peaks in the visible region centered at 517.1 nm (green), 579.2 nm (orange), and a broad-band near infrared (NIR) emission centered at 759.9 nm. Herein, the first peak showed 5 nm red-shift upon Ni2+ doping hinting the formation of exciton magnetic polarons (EMPs), and broad-NIR emission was observed in both chlorides and bromides which was assigned to d-d transition of Ni ions whose energy levels lie at 749.51 nm (13342 cm-1) and 750.98 nm (13316 cm-1) are very close to NIR emission. Orange emission was not only remained at same peak position but also its PL intensity significantly enhanced at 78 K and is assigned to d-d transition (3A2g → 1Eg) of Ni2+ ions. It was observed that 11.4% Ni2+ ions doping enhance the conductivity of our sample around 20 times and saturation magnetization (Ms) increases from 7.2 × 10-5 Am2/Kg to 1.17 × 10-4 Am2/Kg which shows promise for optoelectronic and spintronic applications.

The aggregation of Mn2+, its d-d transition in CdS:Mn(II) nanobelts and bound magnetic polaron formation at room temperature

Tuning the photoluminescence (PL) and magnetic properties of 1D semiconductor nanostructures is extremely important in processing light, improving the speed and storage capacity for optoelectronic and spintronic applications. Here, we have reported the 1D Cd1−xMnxS (x = 0–0.102) nanobelts (NBs) and investigated their optical and magnetic properties. These NBs were synthesized by chemical vapor deposition method. The successful incorporation of Mn ions into an individual CdS NB has been confirmed through several characterization tools: SEM-EDX analysis, significant higher angle and phonon mode shifts were observed in the XRD and Raman spectra. Room-temperature PL showed two emission peaks at the near band edge. The first peak is related to exciton magnetic polaron (EMP) and the second one appeared on the low-energy side of the band edge emission and showed very large red-shift (∼33 nm) compared to EMP, which is attributed to bound magnetic polaron (BMP). BMP emission was detected for the first time in CdS low-dimensional nanostructures. Our study showed that Mn ions tuned CdS emission more than 400 nm (from 512 to 929 nm) covering the whole visible spectral region up to the near infrared region for the first time, and significantly boosted the room-temperature ferromagnetism, which shows promise for optoelectronic and spintronic applications.

Role of Ni2+(d8) ions in electrical, optical and magnetic properties of CdS nanowires for optoelectronic and spintronic applications

For the first time, 1D Ni ion doped CdS nanowires (NWs) were synthesized via chemical vapour deposition (CVD). The synthesized Cd0.886Ni0.114S NWs were single crystalline. We have reported here the investigation of optical, electrical and magnetic properties of prepared NWs for optoelectronic and spintronic applications. Successful incorporation of Ni ions in an individual CdS NW has been confirmed through several characterization tools: significantly higher angle and phonon mode shift were observed in the XRD and Raman spectra. SEM-EDX and XPS analysis also confirmed the presence of Ni2+ ions. Room temperature photoluminescence (RT-PL) showed multiple peaks: two emission peaks in the visible region centered at 517.1 nm (green), 579.2 nm (orange), and a broad-band near infra-red (NIR) emission centered at 759.9 nm. The first peak showed 5 nm red shift upon Ni2+ doping, hinting at the formation of exciton magnetic polarons (EMPs), and broad NIR emission was observed in both chlorides and bromides, which was assigned to d−d transition of Ni ions whose energy levels lying at 749.51 nm (13 342 cm–1) and 750.98 nm (13 316 cm–1) are very close to NIR emission. Orange emission not only remained at same peak position—its PL intensity was also significantly enhanced at 78 K; this was assigned to d−d transition (3A2g → 1Eg) of Ni2+ ions. It was observed that 11.4% Ni2+ ion doping enhanced the conductivity of our sample around 20 times, and saturation magnetization (Ms) increased from 7.2 × 10−5 Am2/Kg to 1.17 × 10−4 Am2/Kg, which shows promise for optoelectronic and spintronic applications.

Second-harmonic phonon spectroscopy of α -quartz

We demonstrate midinfrared second-harmonic generation as a highly sensitive phonon spectroscopy technique that we exemplify using $\alpha$-quartz (SiO$_2$) as a model system. A midinfrared free-electron laser provides direct access to optical phonon resonances ranging from $350\ \mathrm{cm}^{-1}$ to $1400\ \mathrm{cm}^{-1}$. While the extremely wide tunability and high peak fields of an free-electron laser promote nonlinear spectroscopic studies---complemented by simultaneous linear reflectivity measurements---azimuthal scans reveal crystallographic symmetry information of the sample. Additionally, temperature-dependent measurements show how damping rates increase, phonon modes shift spectrally and in certain cases disappear completely when approaching $T_c=846\ \mathrm{K}$ where quartz undergoes a structural phase transition from trigonal $\alpha$-quartz to hexagonal $\beta$-quartz, demonstrating the technique's potential for studies of phase transitions.

Hierarchical self‐assembly of black phosphorus quantum dots with quantum confinement effects to a centimeter‐scale membrane

The self‐assembly of simple building blocks to complex superstructures is significant for engineering materials with enhanced and synergistic properties. In this work, black phosphorus quantum dots (BPQDs) showing quantum confinement effects were prepared. The BPQDs are hierarchically self‐assembled to paper‐like membranes as large as in the centimeter range in two main stages: aggregation driven by Brownian motions due to minimization of the surface free energy and an interfacial self‐concentrating process by capillary interactions. Moreover, from the absorption band edges, the band gap of BPQDs is estimated to be about 1.3 eV, which is evidently larger than the 0.8 eV of bulk black phosphorus. The remarkable extension of the band gap indicates the quantum confinement effects of the electrons. The Raman spectrum of BPQDs shows that all the phonon modes shift to lower frequencies and broaden dramatically compared with that of the bulk BP, which suggests phonon confinement effects.

Magnetodielectric and magnetoelastic coupling in Co4Nb2O9 below Néel temperature

The dielectric constant of Co4Nb2O9 shows an obvious upturn below Néel temperature (TN), and the magnetic field enhances this anomaly. Thus the magnetodielectric effect exists in the entire temperature region below TN, not only near TN as reported previously. At the same time, the strain exhibits a drop below TN, and the magnetic field suppresses this anomaly, demonstrating magnetoelastic coupling in Co4Nb2O9. The origin of dielectric anomaly and magnetodielectric effect below TN may be related to the phonon mode shifts induced by magnetoelastic coupling.

Effects of residual stress on the electrical properties in PbZr0.52Ti0.48O3 thin films

In this paper, Pb(Zr0.52Ti0.48)O3 (PZT) thin films were prepared on Pt/Ti/SiO2/Si substrates via sol–gel method. The effects of residual stress on the electrical properties in PZT thin films were investigated. There is no significant difference in grain size and orientation for PZT thin films, indicating that the influences of grain size and orientation on electrical properties of PZT thin films can be neglected. The residual stresses in PZT thin films, which decrease with increase in the film thickness, were detected by the phonon mode shift of Raman scattering. In the thin films, the interface between PZT thin films and platinum electrodes is the main factor that affected dielectric properties. As the film thickness increases, the contribution of the interface is increasingly attenuated and the residual stress is the main factor. To relieve some of the residual stress, there are more 90° domains to form, which can improve the ferroelectric and piezoelectric properties of PZT thin films. The value of dielectric constants decreases at 1 MHz, and tan δ increases with the decrease in film thickness. In this paper, the processing was designed to control grain size and crystal orientation. Therefore, residual stress and interface are the main factors that affect the dielectric constant. For the sample with the thickness of 162 and 324 nm, there is a ~45 % increase in measured dielectric constant and this change appears to be too much to be accounted for by the residual stress alone. The main factor that affects dielectric constant in the thinner films is the interface which can dilute the effective dielectric constant. As the film thickness increases, the contribution of the interface is increasingly attenuated and there is only 4 % change in the dielectric constant, indicating that residual stress is the main factor that affected dielectric constant.

Temperature Dependent Variations of Phonon Interactions in Nanocrystalline Cerium Oxide

The temperature dependent anharmonic behavior of the phonon modes of nanocrystalline CeO2 was investigated in the temperature range of 80–440 K. The anharmonic constants have been derived from the shift in phonon modes fitted to account for the anharmonic contributions as well as the thermal expansion contribution using the high pressure parameters derived from our own high pressure experimental data reported previously. The total anharmonicity has also been estimated from the true anharmonicity as well as quasiharmonic component. In the line‐width variation analysis, the cubic anharmonic term was found to dominate the quartic term. Finally, the phonon lifetime also reflected the trend so observed.

Metallic and semi-conducting resistivity behaviour of La0.7Ca0.3−x K x MnO3 (x = 0.05, 0.1) manganites

The temperature dependence of electrical resistivity, ρ, of ceramic La0.7Ca0.3−x K x MnO3 (x = 0.05, 0.1) is investigated in metallic and semi-conducting phase. The metallic resistivity is attributed to be caused by electron–phonon, electron–electron and electron–magnon scattering. Substitutions affect average mass and ionic radii of A–site resulting in an increase in Debye temperature θ D attributed to hardening of lattice with K doping. The optical phonon modes shift gradually to lower mode frequencies leading to phonon softening. Estimated resistivity compared with reported metallic resistivity, accordingly ρ diff. = [ρ exp. − {ρ 0 + ρ e−ph (=ρ ac + ρ op)}], infers electron–electron and electron–magnon dependence over most of the temperature range. Semi-conducting nature is discussed with variable range hopping and small polaron conduction model. The decrease in activation energies and increase in density of states at the Fermi level with enhanced Ca doping is consistently explained by cationic disorder and Mn valence.