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.

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.

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.

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.

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.

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.

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.

Direct Growth of Wafer-Scale, Transparent, p-Type Reduced-Graphene-Oxide-like Thin Films by Pulsed Laser Deposition.

Reduced graphene oxide (rGO) has attracted significant interest in an array of applications ranging from flexible optoelectronics, energy storage, sensing, and very recently as membranes for water purification. Many of these applications require a reproducible, scalable process for the growth of large-area films of high optical and electronic quality. In this work, we report a one-step scalable method for the growth of reduced-graphene-oxide-like (rGO-like) thin films via pulsed laser deposition (PLD) of sp2 carbon in an oxidizing environment. By deploying an appropriate laser beam scanning technique, we are able to deposit wafer-scale uniform rGO-like thin films with ultrasmooth surfaces (roughness <1 nm). Further, in situ control of the growth environment during the PLD process allows us to tailor its hybrid sp2-sp3 electronic structure. This enables us to control its intrinsic optoelectronic properties and helps us achieve some of the lowest extinction coefficients and refractive index values (0.358 and 1.715, respectively, at 2.236 eV) as compared to chemically grown rGO films. Additionally, the transparency and conductivity metrics of our PLD grown thin films are superior to other p-type rGO films and conducting oxides. Unlike chemical methods, our growth technique is devoid of catalysts and is carried out at lower process temperatures. This would enable the integration of these thin films with a wide range of material heterostructures via direct growth.

Influence of Microstructure on the Nanomechanical Properties of Polymorphic Phases of Poly(vinylidene fluoride).

Poly(vinylidine fluoride) (PVDF) is a semicrystalline polymer which is known to exist in several polymorphic phases, namely, α, ß, and γ. Each one of these polymorphic phases is characterized by unique features such as spherulite formation in the case of the α and γ phases and the presence of large piezoelectric and ferroelectric activity in the ß phase. Despite being widely used as thin coatings in sensors, lack of reports on nanomechanical properties suggests that investigation of mechanical properties of PVDF, let alone those of its polymorphic phases, seems to have evaded the sight of the research community. Herein, we report the nanomechanical properties of the α, ß, and γ phases of PVDF. The modulus and hardness values were evaluated from nanoindentation experiments; it was found that the electroactive ß phase is the softest among the three polymorphic phases. This result was further confirmed by scratch experiments. We have attempted to establish a correlation between the microstructure and nanomechanical properties of these phases. This work sheds light on the mechanisms responsible for the observed mechanical behavior and the role of tie molecules and amorphous content in providing flexibility to the polymer.

Near Infrared Random Lasing in Multilayer MoS2.

We demonstrated room temperature near infrared (NIR) region random lasing (RL) (800-950 nm), with a threshold of nearly 500 µW, in ∼200 nm thick MoS2/Au nanoparticles (NPs)/ZnO heterostructures using photoluminescence spectroscopy. The RL in the above system arises mainly due to the following three reasons: (1) enhanced multiple scattering because of Au/ZnO disordered structure, (2) exciton-plasmon coupling because of Au NPs, and (3) enhanced charge transfer from ZnO to thick MoS2 flakes. RL has recently attracted tremendous interest because of its wide applications in the field of telecommunication, spectroscopy, and specifically in biomedical tissue imaging. This work provides new dimensions toward realization of low power on-chip NIR random lasers made up of biocompatible materials.

Photoactive Brownmillerite Multiferroic KBiFe2O5 and Its Potential Application in Sunlight-Driven Photocatalysis.

KBiFe2O5 (KBFO) is an upcoming promising brownmillerite-structured multiferroic photoactive material for next-generation photovoltaic and photocatalytic applications. In the present work, KBFO has been developed using multistep thermal treatment method to reduce the volatility of constituent elements and improve the stability of compound. The band gap of KBFO (found to be ∼1.68 eV) extends to the near-infrared region compared to traditional perovskite-structured multiferroics. The magnetic and dielectric transitions occur in the same temperature range (740 K-800 K), reflecting the existence of magneto-dielectric effect in the as-synthesized sample. It also shows promising photocatalytic activity by degrading organic effluents under natural sunlight compared to regular perovskite BiFeO3 photocatalyst (operating under visible light). A new application of brownmillerite multiferroic KBFO photocatalyst in environmental and energy applications has been explored by integrating the structural, optical, magnetic, and dielectric properties of the same.

ZnO@MnO2 Core-Shell Nanofiber Cathodes for High Performance Asymmetric Supercapacitors.

Asymmetric supercapacitors (ASCs) with aqueous electrolyte medium have recently become the focus of increasing research. For high performance ASCs, selection of cathode materials play a crucial role, and core-shell nanostructures are found to be a good choice. We successfully synthesized, ZnO@MnO2 core-shell nanofibers (NFs) by modification of high-aspect-ratio-electrospun ZnO NFs hydrothermally with MnO2 nanoflakes. High conductivity of the ZnO NFs and the exceptionally high pseudocapacitive nature of MnO2 nanoflakes coating delivered a specific capacitance of 907 Fg-1 at 0.6 Ag-1 for the core-shell NFs. A simple and cost-effective ASC construction was demonstrated with ZnO@MnO2 NFs as a battery-type cathode material and a commercial-quality activated carbon as a capacitor-type anode material. The fabricated device functioned very well in a voltage window of 0-2.0 V, and a red-LED was illuminated using a single-celled fabricated ASC device. It was found to deliver a maximum energy density of 17 Whkg-1 and a power density of 6.5 kWkg-1 with capacitance retention of 94% and Coulombic efficiency of 100%. The novel architecture of the ZnO@MnO2 core-shell nanofibrous material implies the importance of using simple design of fiber-based electrode material by mere changes of core and shell counterparts.

HoCrO3 and YCrO3: a comparative study.

A comparative study on structural, optical and magnetic properties of orthorhombic perovskite-like isostructural compounds HoCrO3 and YCrO3 was carried out, wherein Ho(3(+)) is a magnetic ion and Y(3+) is non-magnetic. We found almost identical structural parameters for both compounds, however a significant difference was observed in low wavenumber Raman active phonon modes. The effect of local field on optical transitions of Cr ion is studied by diffused reflectance spectroscopy. In order to understand the effect due to the Ho(3(+)) local magnetic field on the magnetic ordering of Cr(3(+)) in ACrO3 (A=Ho, Y), dc magnetization measurements were carried out in a temperature range of 5-300 K and several isotherms were analyzed. Magnetization measurements reveal that Cr(3+) magnetic ordering temperature is insensitive to local fields (due to Ho(3+)) and very sensitive to thermal cycling in the presence of an external applied magnetic field. This behavior is related to magnetic disorder near the phase transition due to weak ferromagnetism and/or structural distortions which we observed as thermal hysteresis. The importance of this work lies with the fact that both Ho(3+) (10.6 µB) and Y(3+) (0 µB) have identical ionic radii and the electron correlation effects in both the systems have been studied using different experimental techniques.

A codoping route to realize low resistive and stable p-type conduction in (Li, Ni):ZnO thin films grown by pulsed laser deposition.

We report on the growth of Li-Ni codoped p-type ZnO thin films using pulsed laser deposition. Two mole percent Li monodoped ZnO film shows highly insulating behavior. However, a spectacular decrease in electrical resistivity, from 3.6 × 10(3) to 0.15 Ω cm, is observed by incorporating 2 mol % of Ni in the Li-doped ZnO film. Moreover, the activation energy drops to 6 meV from 78 meV with Ni incorporation in Li:ZnO lattice. The codoped [ZnO:(Li, Ni)] thin film shows p-type conduction with room temperature hole concentration of 3.2 × 10(17) cm(-3). Photo-Hall measurements show that the Li-Ni codoped p-ZnO film is highly stable even with UV illumination. XPS measurements reveal that most favorable chemical state of Ni is Ni(3+) in (Li, Ni): ZnO. We argue that these Ni(3+) ions act as reactive donors and increase the Li solubility limit. Codoping of Li, with other transitional metal ions (Mn, Co, etc.) in place of Ni could be the key to realize hole-dominated conductivity in ZnO to envisage ZnO-based homoepitaxial devices.

Microstructural study of assorted ZnO nanostructures: nanocombs, nanocones and microspheres.

We report the microstructural study of ZnO nanostructures: nanocombs, nanocones and microspheres, synthesized using a simple thermal evaporation technique. While nanocombs require the presence of a catalyst, nanocones and hollow ZnO microspheres have been synthesized on silicon substrate in absence of any catalyst or template material. ZnO hexagonal nanocones have been synthesized on Si(100) substrates by directly evaporating zinc acetate dihydrate, at a low temperature of 400 degrees C, without using any carrier gas. The possible mechanism of formation of these structures has been discussed in brief. Chemical composition analysis by energy dispersive X-ray spectroscopy shows an O rich condition of ZnO nanocones and oxygen deficient condition of nanocombs.

Synthesis of novel ZnO hexagonal nanocones by direct thermal evaporation method.

Novel ZnO hexagonal nanocones have been synthesized on Si(100) substrate by directly evaporating zinc acetate dihydrate, at a low temperature of 400 degrees C, without using any carrier gas and catalyst. Clear formation of hexagonal nanocones was confirmed by high resolution scanning electron microscopy. Size of the hexagonal nanocones was found to be 600 nm. Structural analysis by X-ray diffraction pattern revealed that most of the nanocones were aligned along the c-axis (002). Chemical composition analysis by energy dispersive X-ray spectroscopy shows both zinc and oxygen are nearly equal in stoichiometry. Vapor-liquid-solid (VLS) mechanism was used to explain the growth of ZnO hexagonal nanocones. Diffused reflectance spectroscopy confirms the band gap of ZnO hexagonal nanocones as 3.34 eV. Photoluminescence of the ZnO hexagonal nanocones shows a near band-edge emission at 398 nm. No observable green-yellow emission, which corresponds to oxygen vacancies, was observed.

Structure and physical properties of undoped ZnO and vanadium doped ZnO films deposited by pulsed laser deposition.

Undoped ZnO films were deposited using pulsed laser deposition technique on Si and glass substrates in different O2 partial pressures (ranging from 10(-5) mbar to 3 mbar) and substrate temperatures. When the substrate temperature is 500 degrees C and O2 partial pressure (pp) approximately 3 mbar, randomly oriented ZnO hexagons were observed on glass substrate, whereas, dense ZnO hexagonal rod like structures (diameter ranging from 200-500 nm) were observed on Si substrate. The photoluminescence (PL) characterization of ZnO film grown on Si exhibited an intense defect free narrow excitonic emission in the UV region (Full width half maximum (FWHM) approximately 11.26 nm) as compared to broad emission (FWHM approximately 57.06 nm) from that grown on glass. The parent film emission was found to shift from UV to blue region on doping ZnO with Vanadium.