A Thrifty Liquid-Phase Exfoliation (LPE) of MoSe2 and WSe2 Nanosheets as Channel Materials for FET Application.

Two-dimensional materials are trending nowadays because of their atomic thickness, layer-dependent properties, and their fascinating application in the semiconducting industry. In this work, we have synthesized MoSe2 and WSe2 nanosheets (NSs) via a liquid-phase exfoliation method and investigated these NSs as channel materials in field-effect transistors (FET). The x-ray diffraction (XRD) pattern revealed that the synthesized NSs have a 2H phase with 0.65 nm d-spacing which belongs to the (002) Miller plane. Transmission electron microscopy (TEM) studies revealed that MoSe2 and WSe2 have a nanosheet-like structure, and the average lateral dimensions of these NSs are ~ 25 nm and ~ 63 nm, respectively. From Raman spectra, we found that the intensity of the A1g vibrational mode decreases with the reduction in the number of layers. UV-visible spectroscopy revealed that the bandgap values of MoSe2 and WSe2 NSs are 1.55 eV and 1.50 eV, respectively, calculated using the Tauc equation. The output and transfer characteristics of the FET devices reveals that the fabricated FETs have good ohmic contact with the channel material and an ON/OFF current ratio of about 102 for both devices. This approach for the fabrication of FET devices can be achieved even without sophisticated fabrication facilities, and they can be applied as gas sensors and phototransistors, among other applications.

Lattice assisted dielectric relaxation in four-layer Aurivillius Bi5FeTi3O15ceramic at low temperatures.

We have investigated magnetic, structural and dielectric properties of Bi5FeTi3O15(BFTO) in the temperature range 5K-300 K. Using diffraction, Raman spectroscopy and x-ray absorption fine structure measurements, iso-structural modifications are observed at low temperatures (≈100 K). The analysis of dielectric constant data revealed signatures of dielectric relaxation, concomitant with these structural modifications in BFTO at the same temperatures. Further, employing complementary experimental methods, it is shown that the distribution of Fe/Ti ions in BFTO is random. With the help of techniques that probe magnetism at various length and time scales, it is shown that the phase-pure BFTO is non-magnetic down to the lowest temperatures.

Femtometer atomic displacement, the root cause for multiferroic behavior of CuO unearthed through polarized Raman spectroscopy.

Recently, CuO has been proposed as a potential multiferroic material with high transition temperature. Competing models based on spin current and ionic displacements are invoked to explain ferroelectricity in CuO. The theoretical model based on ionic displacement predicted very small displacement (∼10-5Å) along thebaxis. Experimentally detecting displacements of such a small amplitude in a particular direction is extremely challenging. Through our detailed angle resolved polarized Raman spectroscopy study on single crystal of CuO, we have validated the theoretical study and provided direct evidence of displacement along thebaxis. Our study provides important contribution in the high temperature multiferroic compounds and showed for the first time, the use of the polarized Raman scattering in detecting ionic displacements at the femtometer scale.

Strain healing of spin-orbit coupling:a cause for enhanced magnetic moment in epitaxial SrRuO3 thin films.

Enhanced magnetic moment and coercivity in SrRuO3 thin films are significant issues for advanced technological usages and hence are researched extensively in recent times. Most of the previous reports on thin films with enhanced magnetic moment attributed it to the high spin state. Our magnetization results show high magnetic moment of 3.3 µB/Ru ion in the epitaxial thin films grown on LSAT substrate against 1.2 µB/Ru ion observed in bulk compound. Contrary to the previous reports the Ru ions are found to be in low spin state and the orbital moment is shown to be contributing significantly in the enhancement of magnetic moment. We employed x-ray absorption spectroscopy and resonant valance band spectroscopy to probe the spin state and orbital contributions in these films. The existence of strong spin-orbit coupling responsible for the de-quenching of the 4d orbitals is confirmed by the observation of the non-statistical large branching ratio at the Ru M2,3 absorption edges. X-ray magnetic circular dichroism studies performed at the Ru M2,3 edges provided direct evidence of significant contribution of orbital moment in the film grown on LSAT. The relaxation of orbital quenching by strain engineering provides a new tool for enhancing magnetic moment and strain disorder is shown to be an efficient mean to control the spin-orbit coupling.

Spin-lattice correlation in Eu3+ doped antiferromagnet TmFeO3.

We report the physical properties of Eu-doped bulk TmFeO3 through X-ray diffraction, magnetic susceptibility (χ), Raman scattering and X-ray absorption spectroscopy (XAS) study, which shows a similar orthorhombic structure with the Pbnm space group as TmFeO3. Magnetic measurement on Eu-doped TmFeO3 provides evidence for spin reorientations of Fe3+. Further, the Raman spectra of Eu3+ doped TmFeO3 show significant changes in Raman modes as a function of temperature, which are evidence for strong spin-lattice interaction. From the XAS spectra, the L-edge of Fe provides information on the valence state of Fe, whereas the K-edge of oxygen shows that the compound has a strong influence on the hybridization of the O(2p) state with the 3d states of Fe.

Observation of magnetoelastic and magnetoelectric coupling in Sc doped BaFe12O19 due to spin-glass-like phase.

The present work reports magnetic, magnetoelastic and magnetoelectric (ME) response of scandium (Sc) doped barium hexaferrite, BaFe10Sc2O19. DC magnetization shows that partial substitution of non-magnetic Sc for Fe in barium hexaferrite results in a reduction of Curie temperature (T C) from 730 K known for the parent compound BaFe12O19 to 430 K. Magnetization measurements show that, in BaFe10Sc2O19, in addition to the magnetic transition at 250 K corresponding to longitudinal conical magnetic structure, another magnetic anomaly occurs in the vicinity of 50 K (T max). Ac susceptibility and magnetic relaxation show that the magnetic transition at T max is associated with spin glass like dynamics. Field dependence of this glassy transition temperature follows the Almeida-Thouless (A-T) line expected for spin glass-like behaviour. Unit cell volume obtained from the neutron diffraction (ND) measurements shows deviation from the Debye-Gruneisen behaviour below 50 K, revealing the magnetoelastic coupling. Existence of magnetoelastic coupling is also confirmed by Raman spectra as Raman modes show anomalous changes around 50 K and also indicates presence of lattice modulation. Further, the magnetic structure obtained from ND data shows that incommensurate longitudinal conical ferrimagnetic structure persists from 210 K to 3 K. The integrated intensity of (0 0 2) peak and magnetic moments undergoes a subtle change below 50 K that seems to favour coexistence of long range magnetic ordering and spin glass-like dynamics. Significant magneto-dielectric effect was observed around 50 K. Temperature dependent studies of dielectric constant and pyroelectric current indicate the presence of ferroelectricity even in zero magnetic field. Further, existence of ME coupling below 50 K is confirmed by temperature dependence of pyroelectric current under magnetic fields up to 70 kOe. In short, this work identifies a new magnetic anomaly around 50 K, which is spin-glass-like inducing magnetoelastic and ME anomalies, even in the absence of external magnetic fields.

Thermoelectric properties of Ag-doped CuS nanocomposites synthesized by a facile polyol method.

We report the first thermoelectric properties of Cu1-xAgxS, x = 0-0.75 nanocomposites, synthesized by using a facile polyol method. Systematic characterizations using powder XRD, Rietveld refinement of XRD, EDAX, XPS and Raman spectroscopy confirmed their single phase, hexagonal crystal structure with the space group P63/mmc, nominal elemental composition, valence states of the constituent elements and stoichiometric nature. The TEM images showing the CuS formation of nearly perfect hexagonal disk-like particles of average thickness 26.7 nm and breadth ranging in a few hundreds of nanometers with nanorods stacked from these hexagonal nanodisks (NDs) elongated along the c axis corroborate the FESEM images. Attributed to structural phase transition, an anomaly at 55 K is clearly observed in both the thermopower and Hall resistivity data. By increasing x, a systematic reduction in thermal conductivity was observed near 300 K. Consequently, a 50% enhancement in figure of merit was observed for Cu0.9Ag0.1S as compared to pure CuS at 300 K. These results therefore are expected to provide a new direction in improving ZT near 300 K.

Externally limited defect generation in multiwalled carbon nanotubes upon thermal annealing, and possible mechanism.

Structural defects in multiwalled carbon nanotubes (MWCNTs) are found to increase upon moderate thermal annealing below 1400 K in an argon atmosphere. The defects are estimated using the ID/IG ratio in Raman spectroscopy of MWCNTs and confirmed by a direct observation using high-resolution transmission electron microscopy (HRTEM). HRTEM shows that the structural defects are created due to large damage to the outer walls of the nanotubes, while inner walls do not sustain any damage. The generation of defects on MWCNTs is attibuted to mechanical abrasion between the MWCNTs in contact, augmented by the momentum transfer from the flow of hot gas. A possible mechanism is proposed and experimentally validated by means of modulating the chemical environment of annealing from argon to hydrogen.

SERS investigations on orientation of 2-bromo-3-methyl-1,4-dimethoxy-9,10-anthraquinone on silver nanoparticles.

Silver nanoparticles (Ag NPs) were prepared by solution combustion method with urea as fuel. Silver nanoparticles were characterized by UV-visible spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. Surface-enhanced Raman scattering (SERS) of 2-bromo-3-methyl-1,4-dimethoxy-9,10-anthraquinone (BMDMAQ) adsorbed on silver nanoparticles was investigated. The orientation of BMDMAQ on silver nanoparticles was inferred from nRs and SERS spectral features. Density functional theory (DFT) calculation was also performed to study the theoretical performance. The observed spectral features such as the high intensity of C-H out-of-plane bending mode and ring C-C stretching mode revealed that BMDMAQ adsorbed on silver surface through 'stand-on' orientation. Anthraquinone (AQ) derivatives have wide biomedical application which includes laxatives, antimalarials and antineoplastics used in the treatment of cancer. This present study would help to identify the interaction of drug molecules with DNA.

Adsorption of N-(1-(2-bromophenyl)-2-(2-nitrophenyl)ethyl)-4-methylbenzenesulfonamide on silver nanoparticles: SERS investigation.

SERS provides essential data regarding the interaction of molecules in drugs with DNA. In the present study silver nanoparticles were synthesized using a solution combustion method with urea as fuel. The prepared silver nanoparticles are rod like structure. Surface-enhanced Raman scattering (SERS) of N-(1-2-bromophenyl)-2-(2-nitrophenyl)ethyl)-4-methylbenzenesulfonamide (BrS) adsorbed on the silver nanoparticle was studied. The nRs and Raman spectral analysis reveal that the BrS adsorbed tilted orientation on the silver surface. Vibrational modes of nRs along with HF calculations are also performed to study the HOMO and LUMO behavior and vibrational features of BrS.

Orientation of N-(1-(2-chlorophenyl)-2-(2-nitrophenyl)ethyl)-4-methylbenzenesulfonamide on silver nanoparticles: SERS studies.

In the present study, the silver nanoparticles were synthesized using a solution combustion method with urea as fuel. The prepared silver nanoparticles show an FCC crystalline structure with particle size of 59nm. FESEM image shows the prepared silver is a rod like structure. The surface-enhanced Raman scattering (SERS) spectrum indicates that the N-(1-(2-chlorophenyl)-2-(2-nitrophenyl)ethyl)-4-methylbenzenesulfonamide (CS) molecule adsorbed on the silver nanoparticles. The spectral analysis reveals that the sulfonamide is adsorbed by tilted orientation on the silver surface. The Hatree Fock calculations were also performed to predict the vibrational motions of CS. This present investigation has been a model system to deduce the interaction of drugs with DNA.

DFT calculation and vibrational spectroscopic studies of 2-(tert-butoxycarbonyl (Boc) -amino)-5-bromopyridine.

The molecular structure of 2-(tert-butoxycarbonyl (Boc) -amino)-5-bromopyridine (BABP) was optimized by the DFT/B3LYP method with 6-311G (d,p), 6-311++G (d,p) and cc-pVTZ basis sets using the Gaussian 09 program. The most stable optimized structure of the molecule was predicted by the DFT/B3LYP method with cc-pVTZ basis set. The vibrational frequencies, Mulliken atomic charge distribution, frontier molecular orbitals and thermodynamical parameters were calculated. These calculations were done at the ground state energy level of BABP without applying any constraint on the potential energy surface. The vibrational spectra were experimentally recorded using Fourier Transform-Infrared (FT-IR) and micro-Raman spectrometer. The computed vibrational frequencies were scaled by scale factors to yield a good agreement with observed experimental vibrational frequencies. The complete theoretically calculated and experimentally observed vibrational frequencies were assigned on the basis of Potential Energy Distribution (PED) calculation using the VEDA 4.0 program. The vibrational modes assignments were performed by using the animation option of GaussView 05 graphical interface for Gaussian program. The Mulliken atomic charge distribution was calculated for BABP molecule. The molecular reactivity and stability of BABP were also studied by frontier molecular orbitals (FMOs) analysis.

Synthesis, characterization and SERS activity of biosynthesized silver nanoparticles.

Silver nanoparticles were rapidly synthesized using Moringa oleifera flower extract as the reducing agent shows surface plasmon resonance peak at 439nm. The size and shape of the nanoparticles controlled by varying the concentration of M. oleifera flower extract in the reaction medium. The synthesized silver nanoparticles were well-dispersed spherical nanoparticles with the average size of 14nm. The retinoic acid present in M. oleifera flower extract used as reducing agent and proteins was responsible for capping of the bioreduced silver nanoparticles. The obtained nanoparticle shows size-dependent SERS activity. The SERS spectrum indicates that the pyridine adsorbed on the silver surface in a stand-on orientation via its nitrogen lone pair electrons.

Enhancement of the ferromagnetic metallic phase fraction by extrinsic disorder in phase separated La(5/8-y)Pr(y)Ca(3/8)MnO3 (y = 0.45) thin film.

Our study shows that extrinsic disorder plays a decisive role in shaping inhomogeneities at large length scales in phase separated systems. Epitaxial La5/8-yPryCa3/8MnO3 (y = 0.45) thin films grown on SrTiO3, LaAlO3 and NdGaO3 substrates exhibited comparable biaxial strain while showing markedly dissimilar extrinsic disorder. Compressively strained film on LaAlO3 is found to be free from extrinsic disorder and has a robust insulating phase with small phase separation while film grown on SrTiO3 shows huge extrinsic disorder due to the strain relaxation process which invokes phase separation at a large length scale that is sufficient to cross the percolation threshold and cause a metal-insulator transition.

SERS investigations of 2,3-dibromo-1,4-naphthoquinone on silver nanoparticles.

In the present study silver nanoparticles were synthesized using a solution combustion method with glycine as fuel. The prepared silver nanoparticles show an fcc crystalline structure with a particle size of 39 nm. Surface-enhanced Raman scattering (SERS) spectra of 2,3-dibromo-1,4-naphthoquinone (DBNQ) adsorbed on silver nanoparticles were investigated. The C-C stretching modes were enhanced and they were broaden in SERS spectrum with respect to normal Raman spectrum. The spectral analysis reveals that the DBNQ adsorbed flat-on orientation on the silver surface. DFT calculations are also performed to study the vibrational features of DBNQ.

Raman tensor and domain structure study of single-crystal-like epitaxial films of CaCu3Ti4O12 grown by pulsed laser deposition.

The local domain structure of a strain free, 150 nm thick, epitaxially grown single crystalline thin film of CaCu(3)Ti(4)O(12) is probed by polarized Raman spectroscopy. The polarization dependence of the Raman intensities of the observed bands as a function of varying angle between the domain axes and the polarization vector of the scattered laser photon is measured. Theoretical formulations involving the Raman tensor are presented, which enable determination of the domain structure from the observed polarized Raman spectra, and a single-crystal-like domain structure is found. The Raman tensor elements and domain orientation direction were determined by fitting the observed Raman intensities with theoretical calculations and by carrying out Raman mapping of the film. Our data show an absence of twin domain structure and twin domain boundaries in the single-crystal-like epitaxial thin films of CaCu(3)Ti(4)O(12).

An x-ray absorption spectroscopy study of Ni-Mn-Ga shape memory alloys.

The austenite to martensite phase transition in Ni-Mn-Ga ferromagnetic shape memory alloys was studied by extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) spectroscopy. The spectra at all the three elements', namely, Mn, Ga and Ni, K-edges in several Ni-Mn-Ga samples (with both Ni and Mn excess) were analyzed at room temperature and low temperatures. The EXAFS analysis suggested a displacement of Mn and Ga atoms in opposite direction with respect to the Ni atoms when the compound transforms from the austenite phase to the martensite phase. The first coordination distances around the Mn and Ga atoms remained undisturbed on transition, while the second and subsequent shells showed dramatic changes indicating the presence of a modulated structure. The Mn rich compounds showed the presence of antisite disorder of Mn and Ga. The XANES results showed remarkable changes in the unoccupied partial density of states corresponding to Mn and Ni, while the electronic structure of Ga remained unperturbed across the martensite transition. The post-edge features in the Mn K-edge XANES spectra changed from a double peak like structure to a flat peak like structure upon phase transition. The study establishes strong correlation between the crystal structure and the unoccupied electronic structure in these shape memory alloys.

Evidence of the Fano resonance in a temperature dependent Raman study of CaCu3Ti4O12 and SrCu3Ti4O12.

Phononic excitations have been investigated using Raman scattering studies on CaCu(3)Ti(4)O(12) and SrCu(3)Ti(4)O(12) compounds as a function of temperature down to 10 K. Evidence of the Fano resonance effect is found in the A(g)(1) mode with an asymmetric phonon line shape that occurs because of composite electron-phonon scattering due to the onset of metallic fractions in the system. The evolution of the Fano line shape with temperature affirms the existence of nanoscale phase separation and the prominence of orbitally disrupted metallic regions above 100 K. Anomalies in the evolution of the line width of the A(g)(1) Raman mode with temperature are observed around 100 K where these compounds show an orbital order/disorder transition. These anomalies manifest mutual coupling of orbital degrees of freedom to lattice degrees of freedom.

Highly conducting phosphorous doped Nc-Si:H thin films deposited at high deposition rate by hot-wire chemical vapor deposition method.

In this paper, we report the synthesis of highly conducting phosphorous doped hydrogenated nanocrystalline silicon (nc-Si:H) films at substantially low substrate temperature (200 degrees C) by hot-wire chemical vapor deposition (HW-CVD) method using pure silane (SiH4) and phosphine (PH3) gas mixture without hydrogen dilution. Structural, optical and electrical properties of these films were investigated as a function of PH3 gas-phase ratio. The characterization of these films by low-angle X-ray diffraction, Raman spectroscopy and atomic force microscopy revealed that, the incorporation of phosphorous in nc-Si:H induces an amorphization in the nc-Si:H film structure. Fourier transform infrared spectroscopy analysis indicates that hydrogen predominately incorporated in phosphorous doped n-type nc-Si:H films mainly in di-hydrogen species (Si-H2) and poly-hydrogen (Si-H2)n bonded species signifying that the films become porous, and micro-void rich. We have observed high band gap (1.97-2.37 eV) in the films, though the hydrogen content is low (< 1.4 at.%) over the entire range of PH3 gas-phase ratio studied. Under the optimum deposition conditions, phosphorous doped nc-Si:H films with high dark conductivity (sigma Dark -5.3 S/cm), low charge-carrier activation energy (E(act) - 132 meV) and high band gap (- 2.01 eV), low hydrogen content (- 0.74 at.%) were obtained at high deposition rate (12.9 angstroms/s).

Evidence of orbital excitations in CaCu3Ti4O12 probed by Raman spectroscopy.

Raman scattering studies on CaCu(3)Ti(4)O(12) and SrCu(3)Ti(4)O(12) compounds provide evidence of the physics underlying the giant dielectric effect in the CaCu(3)Ti(4)O(12) compound. The temperature, polarization, and photon energy dependence of a broad Raman mode observed at high wavenumbers below ∼130 K indicates its origin from orbital excitations. The orbital order disorder transition observed around 100 K may be responsible for the conductivity changes required in the internal barrier layer capacitance model, hitherto used to explain the huge dielectric constant above 100 K in these compounds.