Manifestation of anharmonicities in terms of Fano scattering and phonon lifetime of scissors modes in α-MoO3.

α-MoO3 exhibits promising potential in the field of infrared detection and thermoelectricity owing to its exceptional characteristics of ultra-low-loss phonon polaritons (PhPs). It is of utmost importance to comprehend the phonon interaction exhibited by α-MoO3 in order to facilitate the advancement of phonon-centric devices. The intriguing applications of α-MoO3 for phonon-centric technology are found to be strongly dependent on scissors Raman modes. In this study, we have investigated the temperature-dependent asymmetric Raman line-shape characteristics of two scissors modes, Ag(1) and B1g(1), in the orthorhombic phase of bulk α-MoO3 within a temperature range spanning from 138 K to 498 K at 633 nm excitation wavelength. The Fano-Raman line-shape function was employed to analyze the asymmetry in terms of electron-phonon coupling strength, which varies from 0.050 to 0.313 and -0.017 to -0.192 for Ag(1) and B1g(1) modes, respectively, with temperature. This asymmetric behavior of Ag(1) and B1g(1) scissors modes are attributed to interference between the electronic energy continuum and discrete TO and LO phonon states, respectively. Therefore, the line-shape asymmetry in two scissors modes with increasing temperature stemming from the Fano resonance is also consistent with a 488 nm excitation wavelength. Additionally, anharmonicity caused by temperature results in redshift, and linewidth broadening of these two scissors modes through cubic-phonon decay has been observed. Moreover, the ultrashort lifetime of these optical phonons diminishes from ∼1.37 ps to ∼0.53 ps with increasing temperature due to the dominance of cubic-phonon decay over quartic-phonon decay. Our findings strongly emphasize the significance of investigating anharmonic interactions with Fano resonance to acquire an extensive comprehension of the vibrational characteristics of α-MoO3 for novel functionalities.

Elucidating the Role of Electron Transfer in the Photoluminescence of MoS2 Quantum Dots Synthesized by fs-Pulse Ablation.

Herein, MoS2 quantum dots (QDs) with controlled optical, structural, and electronic properties are synthesized using the femtosecond pulsed laser ablation in liquid (fs-PLAL) technique by varying the pulse width, ablation power, and ablation time to harness the potential for next-generation optoelectronics and quantum technology. Furthermore, this work elucidates key aspects of the mechanisms underlying the near-UV and blue emissions the accompanying large Stokes shift, and the consequent change in sample color with laser exposure parameters pertaining to MoS2 QDs. Through spectroscopic analysis, including UV-visible absorption, photoluminescence, and Raman spectroscopy, we successfully unraveled the mechanisms for the change in optoelectronic properties of MoS2 QDs with laser parameters. We realize that the occurrence of a secondary phase, specifically MoO3-x, is responsible for the significant Stokes shift and blue emission observed in this QD system. The primary factor influencing these activities is the electron transfer observed between these two phases, as validated by excitation-dependent photoluminescence and XPS and Raman spectroscopies.

Resonance Raman spectroscopy of twisted interfaces in turbostratic multilayer graphene.

Turbostratic multilayer graphene presents a unique system with a large number of twisted interfaces with variable twist angles. In this work, we have systematically studied the laser excitation energy dependence of the Raman modes of turbostratic graphene. The combination of 4 different laser energies is shown to be important to reveal the twist angles ranging from 5∘to 30∘present at the same lateral position of the sample. Rotational or R-modes and D-like modes are observed, which directly arise from additional momentum transfer from the potential of corresponding superlattices. Trends in their dispersion and intensity are discussed. The resonant window for laser excitation indicates lowered positions of the van Hove singularities. Furthermore, an anomalous broadening factor of 0.17-0.265 eV is estimated for the resonance window when compared to the literature on isolated twisted bilayer graphene. Interestingly, a weak dependence of the R-modes on the laser wavelength is also observed. Finally, the dispersion of the 2D modes is also presented.

Enhancement in Thermoelectric Performance in Ti-doped Yb0.4Co4Sb12 Skutterudites via Carrier Optimization and Phonon Anharmonicity.

Yb0.4Co4Sb12, being a well-studied system, has shown notably high thermoelectric performance due to the Yb filler atom-driven large concentration of charge carriers and lower value of thermal conductivity. In this work, the thermoelectric performance of YbzCo4-xTixSb12 (where z = 0, x = 0 and z = 0.4, x = 0, 0.04, and 0.08) upon Ti doping prepared by the melt-quenched-annealing followed by spark plasma sintering (SPS) has been studied in the temperature range of 300-700 K. Addition of Yb and doping of donor Ti at the Co site simultaneously increase the electrical conductivity to 1453.5 S/cm at 300 K, which ultimately boosts the power factor as high as ∼4.3 mW/(m·K2) at 675 K in Yb0.4Co3.96Ti0.04Sb12. Adversely, a significant reduction in thermal conductivity is obtained from ∼7.69 W/(m·K) (Co4Sb12) to ∼3.50 W/(m·K) (Yb0.4Co3.96Ti0.04Sb12) at ∼300 K. As a result, the maximum zT is achieved as ∼0.85 at 623 K with high hardness of 584 HV for the composition of Yb0.4Co3.96Ti0.04Sb12, which demonstrates it to be an efficient material suitable for intermediate temperature thermoelectric applications.

Biosynthesis, Spectroscopic, and Antibacterial Investigations of Silver Nanoparticles.

Silver nanoparticles can be produced by an array of procedures, such as chemical, physical, and biological processes. The process of biosynthesis is more economical and significantly more environmentally friendly. We describe an environmentally compatible method (biosynthesis) of producing silver nanoparticles (Ag: NPs) with the capping component Artocarpus heterophyllus in this research work. Powder-X-ray crystallography (P-XRD), Fourier Transform Infrared (FT-IR), UV-visible (UV-Vis), Photoluminescence (PL), Field emission scanning electron microscopy (FE-SEM), and an antimicrobial test were all used to examine the synthesized samples. The P-XRD analysis revealed that the produced NPs have an FCC form with a typical particle size of 23 nm. FT-IR spectra further demonstrate the availability of the functional groups in the synthesized nanoparticles. The absorbance and transmittance spectra of the UV-Vis study have shown substantial transparency and less absorbance of the Ag: NPs in the entire visible region. The bandgap of the Ag: NPs was found to be 3.25 eV using the Tauc relation. In the PL study, an emission peak at 472 nm was found, suggesting the fluorescence emission of Ag: NPs. The FE-SEM micrographs provide confirmation of the surface-wide aggregate of nanostructural homogeneities. The FE-SEM micrographs illustrate that Ag: NPs are homogeneous aggregates of very small spheres. Variations in particle size and surface area-to-volume ratios of synthesized NPs have been proven to be responsible for the antibacterial activities. According to the antibacterial study, Ag: NPs restrain the development of both normal and harmful bacteria and so have the potential to be utilized for coating surgical equipment for aseptic operators in the healthcare industry.

Observation of positive trions in α-MoO3/MoS2 van der Waals heterostructures.

Mono-layer transition metal dichalcogenides (TMDCs) have emerged as an ideal platform for the study of many-body physics. As a result of their low dimensionality, these materials show a strong Coulomb interaction primarily due to reduced dielectric screening that leads to the formation of stable excitons (bound electron-hole pairs) and higher order excitons, including trions, and bi-excitons even at room temperature. van der Waals (vdW) heterostructures (HSs) of TMDCs provide an additional degree of freedom for altering the properties of 2D materials because charge carriers (electrons) in the different atomically thin layers are exposed to interlayer coupling and charge transfer takes place between the layers of vdW HSs. Astoundingly, it leads to the formation of different types of quasi-particles. In the present work, we report the synthesis of vdW HSs, i.e., α-MoO3/MoS2, on a 300 nm SiO2/Si substrate and investigate their temperature-dependent photoluminescence (PL) spectra. Interestingly, an additional PL peak is observed in the case of the HS, along with A and B excitonic peaks. The emergence of a new PL peak in the low-energy regime has been assigned to the formation of a positive trion. The formation of positive trions in the HS is due to the high work function of α-MoO3, which enables the spontaneous transit of electrons from MoS2 to α-MoO3 and injection of holes into the MoS2 layer. In order to confirm charge transfer in the α-MoO3/MoS2 HS, systematic power and wavelength-dependent Raman and PL studies, as well as first-principle calculations using Bader charge analysis, have been carried out, which clearly validate our mechanism. We believe that this study will provide a platform towards the integration of vdW HSs for next-generation excitonic devices.

Interplay of piezoresponse and magnetic behavior in Bi0.9A0.1FeO2.95 (A = Ba, Ca) and Bi0.9Ba0.05Ca0.05FeO2.95 co-doped ceramics.

Extensive piezoresponse force microscopy (PFM) and magnetic force microscopy (MFM) measurements in conjunction with piezoresponse spectroscopy have been carried out on pellets of Bi0.9A0.1FeO2.95 (A = Ba, Ca) and Bi0.9Ba0.05Ca0.05FeO2.95 co-doped ceramic samples in order to characterize their ferroelectric and magnetic nature and correlate the findings with our recent far-infrared spectroscopic studies on these samples. We are able to clearly discern the switching behavior of the 71° and 109° ferroelectric domains as distinct from that of the 180° domains in both pristine and Ba-doped bismuth ferrite samples. While substitution of Ba at the Bi site in bismuth ferrite does not affect the ferroelectric and magnetic properties to a great extent, Ca-doped samples show a decrease in their d 33 values with a concomitant increase in their magnetic behavior. These results are in agreement with the findings from our far-infrared studies.

Enhanced photo-fenton and photoelectrochemical activities in nitrogen doped brownmillerite KBiFe2O5.

Visible-light-driven photo-fenton-like catalytic activity and photoelectrochemical (PEC) performance of nitrogen-doped brownmillerite KBiFe2O5 (KBFO) are investigated. The effective optical bandgap of KBFO reduces from 1.67 to 1.60 eV post N-doping, enabling both enhancement of visible light absorption and photoactivity. The photo-fenton activity of KBFO and N-doped KBFO samples were analysed by degrading effluents like Methylene Blue (MB), Bisphenol-A (BPA) and antibiotics such as Norfloxacin (NOX) and Doxycycline (DOX). 20 mmol of Nitrogen-doped KBFO (20N-KBFO) exhibits enhanced catalytic activity while degrading MB. 20N-KBFO sample is further tested for degradation of Bisphenol-A and antibiotics in the presence of H2O2 and chelating agent L-cysteine. Under optimum conditions, MB, BPA, and NOX, and DOX are degraded by 99.5% (0.042 min-1), 83% (0.016 min-1), 72% (0.011 min-1) and 95% (0.026 min-1) of its initial concentration respectively. Photocurrent density of 20N-KBFO improves to 8.83 mA/cm2 from 4.31 mA/cm2 for pure KBFO. Photocatalytic and photoelectrochemical (PEC) properties of N-doped KBFO make it a promising candidate for energy and environmental applications.

g-C3N4/Ca2Fe2O5 heterostructures for enhanced photocatalytic degradation of organic effluents under sunlight.

g-C3N4/Ca2Fe2O5 heterostructures were successfully prepared by incorporating g-C3N4 into Ca2Fe2O5 (CFO). As prepared g-C3N4/CFO heterostructures were initially utilized to photodegrade organic effluent Methylene blue (MB) for optimization of photodegradation performance. 50% g-C3N4 content in CFO composition showed an enhanced photodegradation efficiency (~ 96%) over g-C3N4 (48.15%) and CFO (81.9%) due to mitigation of recombination of photogenerated charge carriers by Type-II heterojunction. The optimized composition of heterostructure was further tested for degradation of Bisphenol-A (BPA) under direct sunlight, exhibiting enhanced photodegradation efficiency of about 63.1% over g-C3N4 (17%) and CFO (45.1%). The photoelectrochemical studies at various potentials with and without light illumination showed significant improvement in photocurrent response for g-C3N4/Ca2Fe2O5 heterostructures (~ 1.9 mA) over CFO (~ 67.4 µA). These studies revealed efficient solar energy harvesting ability of g-C3N4/Ca2Fe2O5 heterostructures to be utilized for organic effluent treatment.

Thickness-Dependent Domain Relaxation Dynamics Study in Epitaxial K0.5Na0.5NbO3 Ferroelectric Thin Films.

We explored the time dependence of the nanoscale domain relaxation mechanism in epitaxial K0.5Na0.5NbO3 (KNN) thin films grown on La0.67Sr0.33MnO3/SrTiO3 (001) substrates over the thickness range 20-80 nm using scanning probe microscopy. Kelvin probe force microscopy (KFM) and piezoresponse force microscopy were performed on pulsed-laser-deposition-deposited KNN thin films for studying the time evolution of trapped charges and polarized domains, respectively. The KFM data show that the magnitude and retention time of the surface potential are the maxima for 80 nm-thick film and reduce with the reduction in the film thickness. The charging and discharging of the samples reveal the easier and stronger electron trapping compared to hole trapping. This result further indicates the asymmetry between retention of the pulse-voltage-induced upward and downward domains. Furthermore, the time evolution of these ferroelectric nanodomains are found to obey stretched exponential behavior. The relaxation time (T) has been found to increase with increase in thickness; however, the corresponding stretched exponent (ß) is reduced. Moreover, the written domain can retain for more than 2300 min in KNN thin films. An in-depth understanding of domain relaxation dynamics in Pb-free KNN thin films can bridge a path for future high-density memory applications.

Development of short and long-range magnetic order in the double perovskite based frustrated triangular lattice antiferromagnet Ba[Formula: see text]MnTeO[Formula: see text].

Frustrated magnets based on oxide double perovskites offer a viable ground wherein competing magnetic interactions, macroscopic ground state degeneracy and complex interplay between emergent degrees of freedom can lead to correlated quantum phenomena with exotic excitations highly relevant for potential technological applications. By local-probe muon spin relaxation ([Formula: see text]SR) and complementary thermodynamic measurements accompanied by first-principles calculations, we here demonstrate novel electronic structure and magnetic phases of Ba[Formula: see text]MnTeO[Formula: see text], where Mn[Formula: see text] ions with S = 5/2 spins constitute a perfect triangular lattice. Magnetization results evidence the presence of strong antiferromagnetic interactions between Mn[Formula: see text] spins and a phase transition at [Formula: see text] = 20 K. Below [Formula: see text], the specific heat data show antiferromagnetic magnon excitations with a gap of 1.4 K, which is due to magnetic anisotropy. [Formula: see text]SR reveals the presence of static internal fields in the ordered state and short-range spin correlations high above [Formula: see text]. It further unveils critical slowing-down of spin dynamics at [Formula: see text] and the persistence of spin dynamics even in the magnetically ordered state. Theoretical studies infer that Heisenberg interactions govern the inter- and intra-layer spin-frustration in this compound. Our results establish that the combined effect of a weak third-nearest-neighbour ferromagnetic inter-layer interaction (owing to double-exchange) and intra-layer interactions stabilizes a three-dimensional magnetic ordering in this frustrated magnet.

Study of Thermometry in Two-Dimensional Sb2Te3 from Temperature-Dependent Raman Spectroscopy.

Discovery of two-dimensional (2D) topological insulators (TIs) demonstrates tremendous potential in the field of thermoelectric since the last decade. Here, we have synthesized 2D TI, Sb2Te3 of various thicknesses in the range 65-400 nm using mechanical exfoliation and studied temperature coefficient in the range 100-300 K using micro-Raman spectroscopy. The temperature dependence of the peak position and line width of phonon modes have been analyzed to determine the temperature coefficient, which is found to be in the order of 10-2 cm-1/K, and it decreases with a decrease in Sb2Te3 thickness. Such low-temperature coefficient would favor to achieve a high figure of merit (ZT) and pave the way to use this material as an excellent candidate for thermoelectric materials. We have estimated the thermal conductivity of Sb2Te3 flake with the thickness of 115 nm supported on 300-nm SiO2/Si substrate which is found to be ~ 10 W/m-K. The slightly higher thermal conductivity value suggests that the supporting substrate significantly affects the heat dissipation of the Sb2Te3 flake.

Thermal transport across wrinkles in few-layer graphene stacks.

Wrinkles significantly influence the physical properties of layered 2D materials, including graphene. In this work, we examined thermal transport across wrinkles in vertical assemblies of few-layer graphene crystallites using the Raman optothermal technique supported by finite-element analysis simulations. A high density of randomly oriented uniaxial wrinkles were frequently observed in the few-layer graphene stacks which were grown by chemical vapor deposition and transferred on Si/SiO2 substrates. The thermal conductivity of unwrinkled regions was measured to be, κ ∼ 165 W m-1 K-1. Measurements at the wrinkle sites revealed local enhancement of thermal conductivity, with κ ∼ 225 W m-1 K-1. Furthermore, the total interface conductance of wrinkled regions decreased by more than an order of magnitude compared to that of the unwrinkled regions. The physical origin of these observations is discussed based on wrinkle mediated decoupling of the stacked crystallites and partial suspension of the film. Wrinkles are ubiquitous in layered 2D materials, and our work demonstrates their strong influence on thermal transport.

N+-ion implantation induced enhanced conductivity in polycrystalline and single crystal diamond.

With the 200 keV N+-ion implantation technique and a systematic variation of fluence, we report on the formation of highly conducting n-type diamond where insulator-to-metal transition (IMT) is observed above a certain fluence wherein the conductivity no longer obeys the hopping mechanism of transport rather, it obeys quantum corrections to Boltzmann conductivity at concentrations of n N ≥ 2 × 1020 cm-3. The conductivity for ultra-nanocrystalline diamond is found to be high, ∼650 Ω-1 cm-1 with thermal activation energy E a ∼ 4 meV. Interestingly, with gradual increase in fluence, the conductivity in polycrystalline diamond films has been seen to progress from the hopping mechanism of transport in the case of low fluence implantation to a semiconducting nature with medium fluence and finally a semi-metallic conduction is observed where percolation occurs giving an insulator-to-metal transition. XANES confirms that the long-range order in diamond films remains intact when implanted with low and medium fluences; while implantation at sufficiently high fluences >5 × 1016 cm-2 leads to the formation of a disordered tetrahedral amorphous carbon network leading to metallic conduction resembling a metallic glass behaviour. XPS confirms that the sp2 fraction increases gradually with fluence starting from only 6% in the case of low fluence implantations and saturates at 40-50% for implantation at high fluences. A similar observation can be made for single crystal diamond when implanted at high fluence; it retains long-range order but percolative transport takes place through defects or semi-amorphized regions.

Metal-insulator transition in epitaxial Ga-doped ZnO films via controlled thickness.

Understanding and tuning of metal-insulator transition (MIT) in oxide systems is an interesting and active research topics of condensed matter physics. We report thickness dependent MIT in Ga-doped ZnO (Ga:ZnO) thin films grown by pulsed laser deposition technique. From the electrical transport measurements, we find that while the thinnest film (6 nm) exhibits a resistivity of 0.05 Ω cm, lying in the insulating regime, the thickest (51 nm) has resistivity of 6.6 × 10-4 Ω cm which shows metallic type of conduction. Our analysis reveals that the Mott's variable range hopping model governs the insulating behavior in the 6 nm film whereas the 2D weak localization (WL) phenomena is appropriate to explain the electron transport in the thicker Ga:ZnO films. Magnetoresistance study further confirms the presence of strong localization in 6 nm film while WL is observed in 20 nm and above thicker films. From the density functional calculations, it is found that due to surface reconstruction and Ga doping, strong crystalline disorder sets in very thin films to introduce localized states and thereby, restricts the donor electron mobility.

Growth of sillenite Bi12FeO20 single crystals: structural, thermal, optical, photocatalytic features and first principle calculations.

Ideal sillenite type Bi12FeO20 (BFO) micron sized single crystals have been successfully grown via inexpensive hydrothermal method. The refined single crystal X-ray diffraction data reveals cubic Bi12FeO20 structure with single crystal parameters. Occurrence of rare Fe4+ state is identified via X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The lattice parameter (a) and corresponding molar volume (Vm) of Bi12FeO20 have been measured in the temperature range of 30-700 °C by the X-ray diffraction method. The thermal expansion coefficient (α) 3.93 × 10-5 K-1 was calculated from the measured values of the parameters. Electronic structure and density of states are investigated by first principle calculations. Photoelectrochemical measurements on single crystals with bandgap of 2 eV reveal significant photo response. The photoactivity of as grown crystals were further investigated by degrading organic effluents such as Methylene blue (MB) and Congo red (CR) under natural sunlight. BFO showed photodegradation efficiency about 74.23% and 32.10% for degrading MB and CR respectively. Interesting morphology and microstructure of pointed spearhead like BFO crystals provide a new insight in designing and synthesizing multifunctional single crystals.

Study of absorption of radio frequency field by gold nanoparticles and nanoclusters in biological medium.

Gold nanoparticles (AuNPs) and gold nanoclusters (AuNCs) are gaining interest in medical diagnosis and therapy as they are bio-compatible and are easy to functionalize. Their interaction with radiofrequency (RF) field for hyperthermia treatment is ambiguous and needs further investigation. A systematic study of the absorption of capacitive RF field by AuNPs and AuNCs dispersed in phosphate-buffered saline (PBS) is reported here in tissue mimicking phantom. The stability of AuNPs and AuNCs dispersed in PBS was confirmed for a range of pH and temperature expected during RF hyperthermia treatment. Colloidal gold solutions with AuNPs (10 nm) and AuNCs (2 nm), and control, i.e. PBS without nanogold, were loaded individually in 3 ml wells in a tissue phantom. Phantom heating was carried out using 27 MHz short-wave diathermy equipment at 200 and 400 W for control and colloidal gold solutions. Experiments were conducted for colloidal gold at varying gold concentrations (10-100 µg/ml). Temperature rise measured in the phantom wells did not show dependence on the concentration and size of the AuNPs. Furthermore, temperature rise recorded in the control was comparable with the measurements recorded in both nanogold suspensions (2, 10 nm). Dielectric property measurements of control and colloidal gold showed <3% difference in electrical conductivity between the control and colloidal gold for both nanoparticle sizes. From the measurements, it is concluded that AuNPs and AuNCs do not enhance the absorption of RF-capacitive field and power absorption observed in the biological medium is due to the ions present in the medium.

Single-ion anisotropy driven splitting of spin wave resonances in BiFeO3 at low temperature.

The spin wave resonances of BiFeO3 ceramics have been followed at low temperature through far-infrared reflectance measurements. Following the scheme of Fishman et al (2015 Phys. Rev. B 92 094422) we have been able to assign all the spin wave modes observed. A complete lifting of the degeneracies of all these modes is seen at 250 K concomitant with the increase in single-ion anisotropy. For the first time, all the spin wave modes have been observed in the infrared spectra of BiFeO3. Correlated changes in the strength and frequencies of spin wave excitations with the reported magnetic transitions at low temperature are observed. A simultaneous increase in anharmonicity of the magnetic cycloid and single-ion anisotropy with decreasing temperature results in a partial suppression of the spin wave excitations. An increase in the magnetoelectric coupling is also observed below 150 K.