Department of Physics

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We studied the influence of annealing conditions on magnetic and thermal properties of Finemet-type alloys. The evolution of the Curie temperature, Tc, and the change of heat capacity in the vicinity of this magnetic transformation, ΔCp Tc, were investigated using Differential Scanning Calorimetry, DSC. Relaxation of atomic structure of amorphous phase during the annealing was accompanied by an increase in Tc and decrease in ΔCp Tc. Two relaxation processes with different mechanisms take place in amorphous state: topological ordering and chemical ordering. The study of the kinetics of relaxation process allows to estimate the apparent values of activation energy of both relaxation processes: 29 and 87 kJ/mol for ribbon, 37 and 112 kJ/mol for microwire. Precipitation of α-Fe nanocrystallites in amorphous phase leads to gradual degradation of the Tc peak. The Tc peak reduction is correlated to decrease of the amorphous phase as well as lowering of its magnetostriction. The influence of internal stress on the position and shape of the Curie peak was determined by comparing data for the ribbon-shaped alloys and glass coated microwires. Considerable magnetic softening and giant magnetoimpedance effect under heat treatment is observed in the studied Finemet-type microwires. Changes in magnetic properties after heating are associated with devitrification and stresses relaxation. We also study the influence of internal stresses induced by glass coating on the magnetic properties of as-prepared and annealed microwires. The results confirm the previously made conclusion that changes in the thermal and magnetic properties of Finemet-type alloys during heating are interrelated, since they are reflections of the fundamental characteristics of the material. © 2019 Elsevier B.V.

World University root node

Journal of Magnetism and Magnetic Materials

The effect of heat treatment on magnetic and thermal properties of Finemet-type ribbons and microwires

Specialty optical fiber for specific applications have become a new paradigm in guided wave photonics. In this invited presentation, we would review some of our application-specific specialty fiber designs for parabolic pulse generation to overcome nonlinear detriments, and all-fiber light sources for mid-IR and THz photonics. © 2019 URSI. All rights reserved.

2019 URSI Asia-Pacific Radio Science Conference, AP-RASC 2019

Application-specific specialty optical fibers: A new paradigm in fiber designs

Designing a molecule with pH, temperature and ion concentration sensitive luminescence properties is always fascinating. Tin disulfide (SnS2), a member of layered metal dichalcogenides (LMDs)family, is explored much for their commendable applications, however, its fluorescence property is still less explored. Present study reports a facile and eco-friendly bottom up synthesis route for tin disulfide quantum dots (SnS2-QDs). Transmission electron microscopic (TEM), and Atomic force microscopic (AFM)analysis revealed an average particle size of ∼3.6 nm. Our reported synthesis method provide the in-situ functionalization of quantum dots (QDs)making it highly sensitive to its environment. SnS2-QDs are found to be pH sensitive and hence a detailed pH dependent luminescence study has been performed. Interestingly it was found that SnS2-QDs possess ∼16 fold enhanced luminescence intensity in acidic condition (pH ∼ 1, QY = 5.32%)rather its basic condition (pH ∼ 12, QY = 1.17%). To explain this pH dependent behavior of SnS2-QDs, a mechanism has been proposed where this luminescence switching is mainly supposed due to protonation and deprotonation between –NH2 and –COOH groups. We believe present study may provide an insight for the development of pH sensor using SnS2-QDs for practical applications. © 2019 Elsevier B.V.

Journal of Luminescence

pH dependent luminescence switching of tin disulfide quantum dots

The development of highly responsive, ultrathin, and cost-effective 0D-2D nanocomposite photodetectors, in which light absorption and carrier transportation may be realized separately and independently, has garnered considerable attention. In the present work, we demonstrate the fabrication of atomically thin UV photodetectors based on a hybrid structure (0D-2D) of semiconducting WS2 quantum dots (0D) with graphene (2D) on SiO2/Si substrate. Graphene and WS2 quantum dots (WS2-QDs) are synthesized through chemical vapor deposition (CVD) and hydrothermal processes, respectively. The proposed photodetector offers a remarkable response to ultraviolet (UV) light of ∼365 nm, owing to the high absorption efficiency of WS2-QDs and excellent charge mobility of graphene. The photodetector exhibits high responsivity of ∼1814 A W-1 under illumination of UV light (365 nm, power density of 50.74 μW cm-2) and a high photodetectivity of ∼7.47 × 1012 Jones (cm Hz1/2 W-1). The photodetector fabricated in this work shows a fast photoresponse time of ∼2 s (rise time) and ∼2.9 s (fall time). We have also elucidated the working principle of the proposed photodetector. Outcomes of the present work are comparable or better than other results available in the literature. Our findings suggest that this nanocomposite structure of WS2-QDs with graphene sheets is a prospective candidate for high-performance optoelectronic devices. © 2019 American Chemical Society.

ACS Appl. Nano Mater.

WS2 Quantum Dot Graphene Nanocomposite Film for UV Photodetection

We have studied the Mukundpura CM2 meteorite for magnetic properties as a function of temperature and magnetic field, as well as its Mössbauer spectrum, at room and low temperatures (up to 5 K). We find that the high temperature paramagnetic phase is followed by two magnetic transitions: a weak transition near 125 K and a strong transition at 8 K. The weak (125 K) magnetic phase can be attributed to complex Fe2+–Fe3+ constituents present in the meteorite. The absence of the characteristic sextet corresponding to magnetite in Mossbauer spectrum indicates that this magnetic phase is not magnetite, which, if present, must be in insignificant amount. The 8 K magnetic ordering is superimposed with weak ferromagnetic ordering, showing spin-glass transition. The Mössbauer spectrum taken at 5 K substantiates the observed spin-glassy nature, as very large hyperfine field ~32 T is recorded, causing localized subordering leading to spin-glass behavior. The Mössbauer spectra also confirm that iron is mainly present in serpentine-group minerals, both in ferrous and ferric states. The complete serpentinization of basic silicates indicates aggressive hydrous alteration. These results show that the observed spin-glass signature is a characteristic feature of the cronstedtite phase in CM meteorites. This feature is unique to carbonaceous CM chondrites and could be used for nondestructive, quick, and independent classification of this rare class of meteorites. Furthermore, the absence of olivine and the presence of cronstedtite in Mossbauer spectra show that the degree of aqueous alteration observed is the most severe in Mukundpura CM2 meteorite, as compared to many other CM2 meteorites. The degree of aqueous alteration in CM2 carbonaceous chondrites increases in the sequence: Paris, Murchison, Murray, Mighei, Nogoya, Cold Bokkeveld, and Mukundpura. © The Meteoritical Society, 2019.

Meteoritics and Planetary Science

Glassy magnetic cronstedtite signatures in Mukundpura CM2 chondrite based on magnetic and Mössbauer studies

Mössbauer analysis is carried out on CZTS samples, subjected to a low temperature processing at 300 °C (S1) and high temperature processing at 550 °C under sulfur environment (S2). Loss of Sn is observed in sample S2 due to high temperature thermal treatment. The isomer shifts obtained in the Mössbauer spectra confirms the existence of Sn at its 4 + valence state in both the samples. Relatively high quadriple splitting value is observed in S1 with respect to S2, suggesting dislocations and crystal distortion present in S1, which are reduced drastically by high temperature annealed S2 sample. The fabricated solar cell with S1 and S2 absorbers showed significant improvement in efficiency from ∼0.145% to ∼1%. The presence of excess Sn in S1 allows enhanced recombination and the diode ideality factor shows larger value of 4.23 compared to 2.17 in case of S2. The experiments also validate the fact that S1 with Sn rich configuration shows lower acceptor carrier concentration as compared to S2 because of enhanced compensating defects in S1. © 2018 Elsevier B.V.

Fulltext

Materials Chemistry and Physics

Impact of excess and disordered Sn sites on Cu 2 ZnSnS 4 absorber material and device performance: A 119 Sn Mössbauer study

We used density functional theory based calculations to investigate the structural and optoelectronic properties of copper-based ternary chalcogenide Cu-M-X2 (M: Sb, Bi &amp; X: S, Se). These form orthorhombic crystallographic structure with Pnma space group. The relative thermodynamic stability of these structures is supported by their phonon band dispersions. The calculated electronic band structure is indirect for all these compounds in conjunction with a close direct band gap transition. The calculated effective mass of electrons and holes are (0.074, 0.732), (0.053, 0.297), (0.039, 0.655) and (0.031, 0.514) for CuSbS2, CuSbSe2, CuBiS2 and CuBiSe2, respectively. Interestingly, a very high optical absorption coefficient above 105 cm−1 above band gap values is noticed for these materials, making them suitable for ultrathin solar cell absorbers. © 2018 Elsevier Ltd

Solar Energy

Thermodynamic stability and optoelectronic properties of Cu(Sb/Bi)(S/Se)2 ternary chalcogenides: Promising ultrathin photoabsorber semiconductors

BiFe0.99Cr0.01O3 thin film is deposited on FTO glass using sol-gel technique. The deposited film is used in Ag/ BiFe0.99Cr0.01O3/FTO configuration to understand its potential as RRAM device. The device showed forming free bipolar RRAM characteristics with Ion/Ioff~80. The device showed non-volatile behaviour by maintaining its LRS and HRS states for 104 s. The device exhibits excellent reproducibility up to 100 cycles and reliability of Vset and Vreset voltage is observed. The endurance characteristics for 200 cycles ensures the stability of the device, further, conduction mechanism is attributed to filament formation and switching mechanism is assigned to migration of silver ion inside the film. © 2019 IEEE.

2019 IEEE 14th Nanotechnology Materials and Devices Conference, NMDC 2019

Reliable and forming free bipolar resistive switching in solution derived Ag/BiFe0.99Cr0.01O3/FTO device

Structural and electronic properties of pristine and transition metal doped ZnS monolayer are investigated within the framework of density functional theory. The pristine ZnS monolayer is showing direct band gap of about 2.8 eV. The investigated transition metal doping showed the transition from non-magnetic semiconductor to a magnetic system e.g. magnetic semiconductor for Co doped ZnS and half metal for Ni doped ZnS monolayers. The Co doped ZnS monolayer showed higher formation energy, confirming the strong bonding than that of Ni doped ZnS monolayer. The electron difference density shows the charge sharing between transition metal (Ni and Co) and S, confirming the covalent bond formation. © Springer Nature Switzerland AG 2019.

Springer Proceedings in Physics

Transition metal doped zns monolayer: The first principles insights

We studied the influence of various point defects such as vacancy, antisite and impurities at different ionic sites in CdO monolayer through the electronic and magnetic properties using density functional theory. Vacancy defect such as Cd, O, Cd/O, antisite defects Cd ↔ O and extrinsic B, C, and N impurity elements are considered in CdO monolayer. The thermodynamic stability of pristine CdO monolayer is studied using phonon band dispersion and that of CdO monolayer with defect is studied in terms of the formation energy. The oxygen vacancy defect OV introduces the indirect band gap of 1.4 eV while Cd/OV showed the metallic behavior. Further, CdV and Cd/OV vacancy defects showed the onset of ferromagnetism with 2.0 μΒ and 1.21 μΒ magnetic moment, respectively. In contrast, the direct band gap nature of CdO monolayer is persistent against antisite defects and also no magnetic behavior is observed. The extrinsic B and C doped CdO monolayer showed half-metallic behavior, whereas N doped CdO monolayer showed semiconducting nature. These extrinsic dopants showed the onset of ferromagnetic ordering with 0.98μΒ, 1.99 μΒ and 0.99 μΒ magnetic moments for BO, CO, and NO defects in CdO monolayer. © 2018 Elsevier B.V.