Browsing by Author "Amish G. Joshi"

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Bandgap Engineering and Signature of Ferromagnetism in Ti1−xMnxO2 Diluted Magnetic Semiconductor Nanoparticles: A Valence Band Study
(Wiley-VCH Verlag, 2019) Brijmohan Prajapati; Somnath Roy; Subhash Sharma; Amish G. Joshi; S. Chatterjee; Anup K. Ghosh
Diluted magnetic semiconductor Ti1−xMnxO2 (0.0 ≤ x ≤ 0.06) nanoparticles have been synthesized by sol–gel technique. Phase purity, structural, micro-structural, and vibrational properties of the samples have been studied by X-ray diffraction, transmission electron microscopy (TEM), high-resolution TEM, and Raman spectroscopy. UV–Vis and photoluminescence spectroscopy clearly indicate the tuning of bandgap and appearance of different defect states (oxygen vacancies) with Mn-doping, respectively. Chemical states and surface stoichiometry of the samples have been probed by X-ray photoemission spectroscopy (XPS). Shifting of binding energy of Ti2p toward lower value and appearance of Mn2+, Mn3+, and Mn4+confirm Mn doping into TiO2 and also indicate that Mn-doping reduces the number of oxygen vacancies in the system. Valence band studies have been done by XPS and ultraviolet photoemission spectroscopy (UPS) valence band spectra. Combined result of valence band spectra and optical data reveals shortening of HOMO–LUMO gap with increasing Mn-concentration. Room temperature ferromagnetism, originating from oxygen vacancies, has been explained on the basis of the bound magnetic polaron (BMP) model. Resistivity measurements have been conducted to examine the semiconducting behavior and to study the electrical conduction mechanism. It is revealed that the thermally activated conduction (Arrhenius) mechanism is valid in the high temperature region whereas Mott's variable-range hopping (VRH) mechanism is applicable in low temperature region. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Effect of chemical pressure at the boundary of mott insulator to itinerant electron limit transition in spinel vanadates
(American Scientific Publishers, 2015) P. Shahi; A. Kumar; Rahul Singh; Ripandeep Singh; P.U. Sastry; A. Das; Amish G. Joshi; A.K. Ghosh; A. Banerjee; Sandip Chatterjee
The effect of chemical pressure on the structural, transport, magnetic and electronic properties of ZnV2O4 has been investigated by doping Mn and Co onto the Zn sites of ZnV2O4. With Mn doping the V-V distance increases and with Co doping it decreases. The resistivity and thermoelectric power data indicate that, as the V-V distance decreases, the system moves towards quantum phase transition. The transport data also indicate that the conduction is due to small polaron hopping. The chemical pressure shows a non-monotonous behaviour of charge gap and activation energy. On the other hand, when Ti is doped on the V-site of ZnV2O4, the metal-metal distance decreases and, at the same time, TN also increases. © 2015 by American Scientific Publishers.
Electronic structure study of wide band gap magnetic semiconductor (La0.6Pr0.4)0.65Ca0.35MnO3 nanocrystals in paramagnetic and ferromagnetic phases
(American Institute of Physics Inc., 2016) G.D. Dwivedi; Amish G. Joshi; Shiv Kumar; H. Chou; K.S. Yang; D.J. Jhong; W.L. Chan; A.K. Ghosh; Sandip Chatterjee
X-ray circular magnetic dichroism (XMCD), X-ray photoemission spectroscopy (XPS), and ultraviolet photoemission spectroscopy (UPS) techniques were used to study the electronic structure of nanocrystalline (La0.6Pr0.4)0.65Ca0.35MnO3 near Fermi-level. XMCD results indicate that Mn3+ and Mn4+ spins are aligned parallel to each other at 20 K. The low M-H hysteresis curve measured at 5 K confirms ferromagnetic ordering in the (La0.6Pr0.4)0.65Ca0.35MnO3 system. The low temperature valence band XPS indicates that coupling between Mn3d and O2p is enhanced and the electronic states near Fermi-level have been suppressed below TC. The valence band UPS also confirms the suppression of electronic states near Fermi-level below Curie temperature. UPS near Fermi-edge shows that the electronic states are almost absent below 0.5 eV (at 300 K) and 1 eV (at 115 K). This absence clearly demonstrates the existence of a wide band-gap in the system since, for hole-doped semiconductors, the Fermi-level resides just above the valence band maximum. © 2016 Author(s).
Electrophoretically deposited reduced graphene oxide platform for food toxin detection
(2013) Saurabh Srivastava; Vinod Kumar; Md. Azahar Ali; Pratima R. Solanki; Anchal Srivastava; Gajjala Sumana; Preeti Suman Saxena; Amish G. Joshi; B.D. Malhotra
Reduced graphene oxide (RGO) due to its excellent electrochemical properties and large surface area, has recently aroused much interest for electrochemical biosensing application. Here, the chemically active RGO has been synthesized and deposited onto an indium tin oxide (ITO) coated glass substrate by the electrophoretic deposition technique. This novel platform has been utilized for covalent attachment of the monoclonal antibodies of aflatoxin B1 (anti-AFB1) for food toxin (AFB1) detection. The electron microscopy, X-ray diffraction, and UV-visible studies reveal successful synthesis of reduced graphene oxide while the XPS and FTIR studies suggest its carboxylic functionalized nature. The electrochemical sensing results of the anti-AFB1/RGO/ITO based immunoelectrode obtained as a function of aflatoxin concentration show high sensitivity (68 μA ng -1 mL cm-2) and improved detection limit (0.12 ng mL -1). The association constant (ka) for antigen-antibody interaction obtained as 5 × 10-4 ng mL-1 indicates high affinity of antibodies toward the antigen (AFB1). © The Royal Society of Chemistry 2013.
Enhancement in electrical and magnetic properties with Ti-doping in Bi0.5La0.5Fe0.5Mn0.5O3
(American Institute of Physics Inc., 2017) Rahul Singh; Prince Kumar Gupta; Shiv Kumar; Amish G. Joshi; A.K. Ghosh; S. Patil; Sandip Chatterjee
In this investigation, we have synthesized Bi0.5La0.5Fe05Mn0.5-xTixO3 (where x = 0 and 0.05) samples. The Rietveld refinement of X-ray diffraction (XRD) patterns shows that the systems crystallize in the orthorhombic phase with the Pnma space group. The observed Raman modes support the XRD results. The appearance of prominent A1-3 and weak E-2 modes in Bi0.5La0.5Fe0.5Mn0.45Ti0.05O3 indicates the presence of chemically more active Bi-O covalent bonds. Ferromagnetism of Bi0.5La0.5Fe0.5Mn0.5O3 is enhanced by Ti doping at the Mn-site, indicating that these particular samples might be interesting for device applications. © 2017 Author(s).
Existence of the multiferroic property at room temperature in Ti doped CoFe 2O 4
(2012) G.D. Dwivedi; Amish G. Joshi; H. Kevin; P. Shahi; A. Kumar; A.K. Ghosh; H.D. Yang; Sandip Chatterjee
The appearance of ferroelectricity has been observed in magnetically ordered Co(Fe 1-xTi x) 2O 4 at room temperature. Magnetization and dielectric constant is found to increase with Ti doping. It is observed from an X-ray Photoemission Spectroscopy study that Ti goes to the octahedral site with (4) ionic state. An MH hysteresis curve at room temperature shows the ferrimagnetic ordering and a PE loop at room temperature clearly indicates the existence of ferroelectricity. © 2011 Elsevier Ltd. All rights reserved.
Extraordinary magnetic properties of double perovskite Eu2CoMnO6wide band gap semiconductor
(Institute of Physics Publishing, 2020) Mohd Alam; Prajyoti Singh; Khyati Anand; Arkadeb Pal; Surajit Ghosh; A.K. Ghosh; Ranjan K. Singh; Amish G. Joshi; Sandip Chatterjee
Some novel magnetic behaviours in double perovskite Eu2CoMnO6 (ECMO) have been reported. The x-ray photoemission spectroscopy study shows the presence of mixed valence states of transition metal ions. The UV-visible absorption spectroscopic study suggests that the ECMO has a direct wide band gap. A second-order magnetic phase transition as a sudden jump in the magnetization curve has been observed around 124.5 K. The large bifurcation between the zero field cooling and field cooling, suggests existence of strong spin frustration in the system. The inverse DC susceptibility confirms the presence of the Griffiths like phase. Sharp steps in magnetization have been observed in the M-H curve at 2 K, which vanishes on increasing temperature. The AC susceptibility study demonstrates the Hopkinson like effect as well as the presence of volume spin-glass-like behaviour. The temperature dependent Raman spectrum shows the presence of spin-phonon coupling. © 2020 IOP Publishing Ltd.
Identification of point defects on Co-Ni codoping in SnO2 nanocrystals and their effect on the structural and optical properties
(American Institute of Physics Inc., 2019) S. Roy; Brijmohan Prajapati; A. Singh; Amish G. Joshi; S. Chatterjee; Anup K. Ghosh
Sn 0.97 - yCo 0.03Ni yO 2 (0 ≤ y ≤ 0.04) nanocrystals, with the average crystallite size in the range from 7.3 nm (for y = 0.00) to 5.6 nm (for y = 0.04), have been synthesized using a pH-controlled chemical coprecipitation technique. All the nonstoichiometric and stoichiometric point defects arising in the nanocrystals on codoping have been identified, and their effect on structural and optical properties of the nanocrystals have been extensively studied. It has been observed, using X-ray photoelectron spectroscopy (XPS), that on increasing the Ni codoping concentration (y), the nonstoichiometric Sn defect, Sn Sn ″, increases in compensation of the existing defect Sn i ⋯ for y = 0.00 nanocrystals. High-resolution transmission electron microscopy also confirms the existence of Sn Sn ″. Regarding the stoichiometric Frenkel defect, XPS results have indicated that the concentration of V O and O i, manifested in the form of dangling bond related surface defect states, increases with increase in y. Temperature dependent magnetization measurements of the nanocrystals confirm the charge state of V O. The point defects have been found to affect the structural properties in a way that the distortion in the octahedral geometry of a complete Sn-O octahedron effectively reduces, whereas the distortion in the trigonal planar coordination geometry of oxygen increases. A direct effect of the O related Frenkel defect has been observed on the blue luminescence of the nanocrystals such that the spectral contribution of blue luminescence in the total emission intensity increases by ≈ 72% for y = 0.04 as compared to y = 0.00. © 2019 Author(s).
Investigation of multi-mode spin-phonon coupling and local B-site disorder in Pr2CoFeO6 by Raman spectroscopy and correlation with its electronic structure by XPS and XAS studies
(Institute of Physics Publishing, 2019) Arkadeb Pal; Surajit Ghosh; Amish G. Joshi; Shiv Kumar; Swapnil Patil; Prince K. Gupta; Prajyoti Singh; V.K. Gangwar; P. Prakash; Ranjan K. Singh; Eike F. Schwier; M. Sawada; K. Shimada; A.K. Ghosh; Amitabh Das; Sandip Chatterjee
Electronic structure of Pr2CoFeO6 (at 300 K) was investigated by x-ray photoemission spectroscopy (XPS) and x-ray absorption spectroscopy techniques. All three cations, i.e. Pr, Co and Fe were found to be trivalent in nature. XPS valance band analysis suggested the system to be insulating in nature. The analysis suggested that Co3+ ions exist in low spin state in the system. Moreover, Raman spectroscopy study indicated the random distribution of the B-site ions (Co/Fe) triggered by same charge states. In temperature-dependent Raman study, the relative heights of the two observed phonon modes exhibited anomalous behaviour near magnetic transition temperature TN ∼ 270 K, thus indicating towards interplay between spin and phonon degrees of freedom in the system. Furthermore, clear anomalous softening was observed below TN which confirmed the existence of strong spin-phonon coupling occurring for at least two phonon modes of the system. The line width analysis of the phonon modes essentially ruled out the role of magnetostriction effect in the observed phonon anomaly. The investigation of the lattice parameter variation across TN (obtained from the temperaturedependent neutron diffraction measurements) further confirmed the existence of the spin- phonon coupling. © 2019 IOP Publishing Ltd Printed in the UK.
Local symmetry breaking in SnO2 nanocrystals with cobalt doping and its effect on optical properties
(Royal Society of Chemistry, 2018) S. Roy; Amish G. Joshi; S. Chatterjee; Anup K. Ghosh
X-ray photoemission spectroscopy (XPS), X-ray diffraction (XRD) and transmission electron microscopy (TEM) have been used to study the structural and morphological characteristics of cobalt doped tin(iv) oxide (Sn1-xCoxO2; 0 ≤ x ≤ 0.04) nanocrystals synthesized by a chemical co-precipitation technique. Electronic structure analysis using X-ray photoemission spectroscopy (XPS) shows the formation of tin interstitials (Sni) and reduction of oxygen vacancies (VO) in the host lattice on Co doping and that the doped Co exists in mixed valence states of +2 and +3. Using XRD, the preferential position of the Sni and doped Co in the unit cell of the nanocrystals have been estimated. Rietveld refinement of XRD data shows that samples are of single phase and variation of lattice constants follows Vegard's law. XRD and TEM measurements show that the crystallite size of the nanocrystals decrease with increase in Co doping concentration. SAED patterns confirm the monocrystalline nature of the samples. The study of the lattice dynamics using Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy shows the existence of many disorder activated forbidden optical phonon modes, along with the corresponding classical modes, signifying Co induced local symmetry breaking in the nanocrystals. UV-Vis spectroscopy shows that the optical band gap has red shifted with increase in doping concentration. The study of Urbach energy confirms the increase in disorder in the nanocrystals with Co doping. Local symmetry breaking induced UV emission along with violet, blue and green luminescence has been observed from the PL study. The spectral contribution of UV emission decreases and green luminescence increases with increase in doping. Using PL, in conjunction with Raman spectroscopy, the type of oxygen vacancy induced in the nanocrystals on Co doping has been confirmed and the position of the defect levels in the forbidden zone (w.r.t. the optical band gap) has been studied. © 2018 The Royal Society of Chemistry.
Magnetic and optical properties of Fe doped crednerite CuMnO2
(Royal Society of Chemistry, 2015) Kaushal K. Shukla; P. Shahi; S. Gopal; A. Kumar; A.K. Ghosh; Ripandeep Singh; Neetika Sharma; A. Das; A.K. Sinha; Amish G. Joshi; A.K. Nigam; Sandip Chatterjee
A geometrically frustrated magnetic CuMnO2 system has been investigated because of its rich magnetic properties. Neutron diffraction, synchrotron X-ray, magnetic, X-ray photoemission spectroscopy (XPS) and UV-Visible spectroscopy measurements have been carried out on CuMnO2 and 5% Fe doped CuMnO2 samples. Fe doping reduces the distortion. Moreover, Fe doping induces the ferromagnetic coupling between ab planes. The value of magnetization is increased with Fe doping but coercivity is decreased. These might be due to the direct Mn-Mn exchange and Mn-O-Cu-O-Mn super-super exchange interactions. The UV-Vis data indicate the appearance of new energy bands in these compounds. The XPS study indicates that Fe is in the 3+ state. © The Royal Society of Chemistry.
Observation of structural change-driven Griffiths to non-Griffiths-like phase transformation in Pr2-xSrxCoFeO6 (x = 0 to 1)
(Elsevier B.V., 2022) Arkadeb Pal; Khyati Anand; Dheeraj Kumar; Amish G. Joshi; Peter Tsung-Wen Yen; Shin-Ming Huang; H.D. Yang; A.K. Ghosh; Sandip Chatterjee
The study of crystal structure, electronic structure, transport, and magnetic properties of heterovalent Sr2+ doped Pr2-xSrxCoFeO6 (x = 0.0 to 1.0) system have been done. Crystal structure study reveals an occurrence of structural change from orthorhombic (Pnma) to tetragonal (I4/m) phase above x = 0.6. A sudden transformation of the Griffiths-like to non-Griffiths-like magnetic phase is observed as the system changes its crystal structure from Pnma to I4/m. The X-ray photoemission spectroscopy (XPS) study suggests for the existence of mixed oxidation states of the B-site ions viz., Co3+/Co4+ and Fe3+/Fe4+, and it also indicates an increase in the mean oxidation states owing to the hole substitution (Sr2+). The temperature variation of the electrical resistivity of the studied systems follows two different transport mechanisms, such as the variable range hopping (VRH) (in the lower temperature region) and small polaron hoping (SPH) (in the higher temperature region) models. Dc magnetization study shows that a local competing ferromagnetic (FM) exchange interaction increases with Sr doping. Finally, the ac susceptibility study reveals breaking of the long-range-ordering in the system x = 1.0, which appears to be related to the structural change and enhanced spin frustration due to increased competing local FM exchange interactions. In addition, electronic density of states (DOS) calculations of PrSrCoFeO6 (i.e. x = 1.0) using the density functional theory (DFT) have been performed for various Co/Fe atomic distributions. For most of the Co/Fe atomic distributions studied, the calculations show that the total energy of the system with FM coupling among spins has slightly lower energy than that for antiferromagnetic (AFM) coupling. © 2022
Structural, magnetic, magneto-transport properties, and electronic structure study of charge-ordered (La0.4Pr0.6)0.65Ca0.35MnO3
(Elsevier Ltd, 2017) G.D. Dwivedi; Satyam Kumar; Amish G. Joshi; Shiv Kumar; A.K. Ghosh; H. Chou; H.D. Yang; Sandip Chatterjee
Structural, magnetic and magneto-transport properties of nanocrystalline (La0.4Pr0.6)0.65Ca0.35MnO3have been investigated along with their electronic structures. Temperature dependent magnetization measurement shows different magnetic phases in different temperature regions. (La0.4Pr0.6)0.65Ca0.35MnO3completely transforms into a ferromagnet below 50 K. The temperature dependent resistivity measurement of (La0.4Pr0.6)0.65Ca0.35MnO3system displays semiconducting behavior up to 45 K under zero magnetic field. Application of 1 T magnetic field enforces semiconductor to metal transition around 65 K during cooling and around 115 K during warming. This shows that (La0.4Pr0.6)0.65Ca0.35MnO3still behaves as itinerant ferromagnet under applied magnetic field. X-ray photoemission spectra of Mn2p and Mn3s core-level confirm dual valence states of Mn (Mn3+and Mn4+), which is responsible for the magnetic behavior of the (La0.4Pr0.6)0.65Ca0.35MnO3system. High-resolution ultraviolet photoemission spectra near Fermi-edge confirms the presence of finite electronic states at Fermi-level, which explains the observed low-temperature metallic behavior of the (La0.4Pr0.6)0.65Ca0.35MnO3system. © 2016
Structural, transport and optical properties of (La0.6Pr0.4)0.65Ca0.35MnO3 nanocrystals: A wide band-gap magnetic semiconductor
(Royal Society of Chemistry, 2015) Satyam Kumar; G.D. Dwivedi; Shiv Kumar; R.B. Mathur; U. Saxena; A.K. Ghosh; Amish G. Joshi; H.D. Yang; Sandip Chatterjee
(La0.6Pr0.4)0.65Ca0.35MnO3 system has been synthesized via a sol-gel route at different sintering temperatures. Structural, transport and optical measurements have been carried out to investigate (La0.6Pr0.4)0.65Ca0.35MnO3 nanoparticles. Raman spectra show that Jahn-Teller distortion has been decreased due to the presence of Ca and Pr in A-site. Magnetic measurements provide a Curie temperature around 200 K and saturation magnetization (MS) of about 3.43μB/Mn at 5 K. X-ray photoemission spectroscopy study suggests that Mn exists in a dual oxidation state (Mn3+ and Mn4+). Resistivity measurements suggest that charge-ordered states of Mn3+ and Mn4+, which might be influenced by the presence of Pr, have enhanced insulating behavior in (La0.6Pr0.4)0.65Ca0.35MnO3. Band gap estimated from UV-Vis spectroscopy measurements comes in the range of wide band gap semiconductors (∼3.5 eV); this makes (La0.6Pr0.4)0.65Ca0.35MnO3 a potential candidate for device application. This journal is © The Royal Society of Chemistry 2015.
Study of band structure, transport and magnetic properties of BiFeO3–TbMnO3 composite
(Springer Nature, 2019) Prince K. Gupta; Surajit Ghosh; Arkadeb Pal; Somnath Roy; Amish G. Joshi; A.K. Ghosh; Sandip Chatterjee
Charge transfer across the interface of two materials in a composite can create reconstruction of bands near the interface which in turn brings multiple changes in physical properties of the materials. Thus, investigation of band structure experimentally is of immense importance in studying composite materials to understand their physical properties. Here, we have studied magnetoelectric multiferroic composite of two types of multiferroic (types I and II) consisting of BiFeO3 and TbMnO3 for enhanced magnetic and transport properties. The band structure was investigated with the help of UV–visible absorption spectrum, the valence band X-ray photoemission spectra (XPS), and ultraviolet photoemission spectra. The band structure thus obtained can successfully explain the magnetic and transport properties of the composite. The insulating behavior of the system is understood from the reconstruction of the energy bands at the interface and subsequent decrease in the band gap which happens due to lattice mismatch of the two materials. The large coercivity and the increase in the magnetization value are understood to be due to superexchange interaction between different Mn ions (Mn2+, Mn3+, and Mn4+). From the composition study of EDXA and core-level XPS, oxygen vacancy was found which in turn creates the mixed valence state of Mn to maintain the charge neutrality. © 2019, Springer Nature Switzerland AG.
Study of structural, dielectric, optical properties and electronic structure of Cr-doped LaInO3 perovskite nanoparticles
(Elsevier Inc., 2017) Shiv Kumar; G.D. Dwivedi; Amish G. Joshi; Sandip Chatterjee; A.K. Ghosh
This work reports the change in the structural, dielectric and optical behaviour of Cr-doped LaInO3 nanoparticles synthesized via sol-gel method. X-ray diffraction patterns confirm that systems crystallize in orthorhombic Pnma space group. Raman spectroscopy measurements exhibit blue-shift in Raman modes with Cr-doping due to the compressive stress introduced by smaller Cr3 + cations. Trivalent oxidation state of Cr was confirmed from Cr(2p) core-level X-ray photoemission spectroscopy measurements, while valence band spectra suggest the absence of electronic states near Fermi level. High-resolution Ultraviolet photoemission spectra near Fermi level shows the electronic density of states arises sharply near Fermi level. Room temperature dielectric constant decreases with increasing Cr-concentration, which shows that conductivity has been increased after Cr-doping. Ultraviolet–visible spectroscopy measurements provide band gap of 4.8 eV for LaInO3 and Cr-doped LaInO3nanoparticles.Moreover, Cr-doped LaInO3systems exhibit an intermediate band gap of 3 eV, which might have been created from Cr(3d) states. © 2017 Elsevier Inc.