Hari Mohan Rai

Qualitative Evolution of Asymmetric Raman Line-Shape for NanoStructures

A qualitative evolution of an asymmetric Raman line-shape function from a Lorentzian line-shape is discussed here for application in low dimensional semiconductors. The step-by-step evolution reported here is based on the phonon confinement model which is successfully used in literature to explain the asymmetric Raman line-shape from semiconductor nanostructures. Physical significance of different terms in the theoretical asymmetric Raman line-shape has been explained here. Better understanding of theoretical reasoning behind each term allows one to use the theoretical Raman line-shape without going into the details of theory from first principle. This will enable one to empirically derive a theoretical Raman line-shape function for any material if information about its phonon dispersion relation, size dependence, etc., is known.

Study of Porous Silicon Prepared Using Metal-Induced Etching (MIE): a Comparison with Laser-Induced Etching (LIE)

Porous silicon (p-Si), prepared by two routes (metal induced etching (MIE) and laser induced etching (LIE)) have been studied by comparing the observed surface morphologies using SEM. A uniformly distributed smaller (submicron sized) pores are formed when MIE technique is used because the pore formation is driven by uniformly distributed metal (silver in present case) nanoparticles, deposited prior to the porosification step. Whereas in p-Si, prepared by LIE technique, wider pores with some variation in pore size as compared to MIE technique is observed because a laser having gaussian profile of intensity is used for porosification. Uniformly distribute well-aligned Si nanowires are observed in samples prepared by MIE method as seen using cross-sectional SEM imaging. A single photoluminescence (PL) peak at 1.96 eV corresponding to red emission at room temperature is observed which reveals that the Si nanowires, present in p-Si prepared by MIE, show quantum confinement effect. The single PL peak confirms the presence of uniform sized nanowires in MIE samples. These vertically aligned Si nanowires can be used for field emission application.

Role of metal nanoparticles on porosification of silicon by metal induced etching (MIE)

Abstract Porosification of silicon (Si) by metal induced etching (MIE) process has been studied here to understand the etching mechanism. The etching mechanism has been discussed on the basis of electron transfer from Si to metal ion (Ag + ) and metal to H 2 O 2 . Role of silver nanoparticles (AgNPs) in the etching process has been investigated by studying the effect of AgNPs coverage on surface porosity. A quantitative analysis of SEM images, done using Image J, shows a direct correlation between AgNPs coverage and surface porosity after the porosification. Density of Si nanowires (NWs) also varies as a function of AgNPs fractional coverage which reasserts the fact that AgNPs governs the porosification process during MIE. The Raman and PL spectrum show the presence of Si NSs in the samples.

Observation of large dielectric permittivity and dielectric relaxation phenomenon in Mn-doped lanthanum gallate

Polycrystalline LaGa1−xMnxO3 (x = 0, 0.05, 0.1, 0.15, 0.2 and 0.3) samples were prepared via the solid-state reaction method. These samples were characterized using synchrotron-based X-ray diffraction (XRD) and the X-ray absorption near edge structure (XANES). XRD studies confirm the orthorhombic structure for the prepared samples whereas XANES analysis reveals the co-existence of Mn3+ and Mn4+ in all Mn-doped samples. Dielectric relaxation is observed for all Mn-doped samples whereas a large dielectric constant (e′) is perceived in samples with higher Mn doping (x = 0.2 and x = 0.3). Occurrence of a large e′ is attributed to the huge decrease in impedance with increasing Mn doping which is governed by the hopping charge transport and extrinsic interface effects, whereas at high frequencies, this effect is observed possibly due to dipolar effects associated with the possible off-centrosymmetry of the MnO6 octahedron which is indicated by the pre-edge feature (Mn K-edge) in XANES and validated through P–E measurements. The appearance of dielectric relaxation was credited to the dipolar effects associated with the flipping of the Mn3+/Mn4+ dipole i.e., with the hopping of charge carriers between Mn3+ and Mn4+ under an external electric field. The value of activation energy (Ea = 0.36 eV), extracted from temperature-dependent dielectric data, reveals the polaron hopping mechanism.

Room temperature magnetodielectric studies on Mn-doped LaGaO3

Polycrystalline samples of LaGa1−xMnxO3 (0 ≤ x ≤ 0.3) were prepared by the solid-state reaction route. The phase purity of these samples was confirmed by powder x-ray diffraction experiments carried out on BL-12 at the Indus-2 synchrotron radiation source. The sample with x = 0.2 shows significant change in the value of capacitance with the application of a magnetic field. The observed results were understood by systematically analyzing magnetocapacitance (MC), magnetoresistance (MR), and dielectric loss as a function of frequency. Our results and analysis suggest that the observed magnetodielectric (MD) coupling may be due to the MR effect of the Maxwell–Wagner type and/or field-induced dipolar relaxation. Further, it is observed that oxygen stoichiometry plays a very crucial role in the observed MD coupling.

Electronic and optical properties of BaTiO3 across tetragonal to cubic phase transition: An experimental and theoretical investigation

Temperature dependent diffuse reflectance spectroscopy measurements were carried out on polycrystalline samples of BaTiO3 across the tetragonal to cubic structural phase transition temperature (TP). The values of various optical parameters such as band gap (Eg), Urbach energy (Eu), and Urbach focus (E0) were estimated in the temperature range of 300 K to 480 K. It was observed that with increasing temperature, Eg decreases and shows a sharp anomaly at TP. First principle studies were employed in order to understand the observed change in Eg due to the structural phase transition. Near TP, there exist two values of E0, suggesting the presence of electronic heterogeneity. Further, near TP, Eu shows metastability, i.e., the value of Eu at temperature T is not constant but is a function of time (t). Interestingly, it is observed that the ratio of Eu (t=0)/Eu (t = tm), almost remains constant at 300 K (pure tetragonal phase) and at 450 K (pure cubic phase), whereas this ratio decreases close to the transition temperature, which confirms the presence of electronic metastability in the pure BaTiO3. The time dependence of Eu, which also shows an influence of the observed metastability can be fitted with the stretched exponential function, suggesting the presence of a dynamic heterogeneous electronic disorder in the sample across TP. First principle studies suggest that the observed phase coexistence may be due to a very small difference between the total cohesive energy of the tetragonal and the cubic structure of BaTiO3. The present work implies that the optical studies may be a sensitive probe of disorder/heterogeneity in the sample.

Fe doped LaGaO3: good white light emitters

A polycrystalline sample of LaGa1−xFexO3 has been prepared by solid state reaction. The structural, optical and electronic structure of Fe doped LaGaO3 have been investigated. It is observed that with Fe doping the lattice parameter of the prepared samples systematically increases whereas the optical band gap decreases. First-principles density functional theory (DFT) calculations were carried out to understand the effect of Fe doping on the electronic structure and to understand the origin of systematic decrease in optical band gap. DFT studies suggests that there exists an indirect band gap to direct band gap transition with Fe doping which is further confirmed using photoluminescence measurements. Photoluminescence studies suggest that Fe doped LaGaO3 are potential candidates as white light emitting materials.

Observation of room temperature magnetodielectric effect in Mn-doped lanthanum gallate and study of its magnetic properties

Polycrystalline samples of Mn-doped LaGa1−xMnxO3 (LGMO) with 0 ≤ x ≤ 0.2 have been prepared via solid-state reaction method. The structural phase purity of all these samples was confirmed by powder X-ray diffraction experiments carried out at the BL-12 beamline of the Indus-2 synchrotron radiation source. Room-temperature (RT) dielectric measurements were performed in the absence and presence of a magnetic field. A noticeable magnetodielectric (MD) effect, i.e., a change in the value of the dielectric constant owing to the application of a low magnetic field, was observed in the LGMO sample with x = 0.2 (LG8M2O). In order to separate the intrinsic and resistive contributions present in the observed RT MD effect, magnetoresistance impedance spectroscopy (MRIS) was performed at RT. The present MRIS analysis suggests that at frequencies corresponding to the grain contribution (≥105 Hz for the present samples), the observed MD phenomenon appears to be an intrinsic property of the presently studied samples, whereas at lower probing frequencies (<105 Hz) the observed change appears to be dominated by MR (considering frequency-dependent resistance), which was possibly due to the coexistence of Mn3+ and Mn4+. The coexistence of Mn3+ and Mn4+ was revealed by XANES (Mn K-edge) spectroscopy. Moreover, RT and low-temperature magnetization–magnetic field (M–H) measurements, along with M–T measurements in FC and ZFC modes, were performed to investigate the state of magnetic ordering. The appearance of a narrow M–H loop indicates the presence of some magnetic ordering at RT. Furthermore, a ferromagnetic (FM) transition observed around 36 K and a normal M–H loop with saturated magnetization recorded at 5 K confirm FM ordering at low temperatures, whereas a bifurcation in FC-ZFC curves indicates competing FM and antiferromagnetic (AFM) interactions at low temperatures.

Interplay between phonon confinement and Fano effect on Raman line shape for semiconductor nanostructures: Analytical study

Abstract Theoretical Raman line shape functions have been studied to take care of quantum confinement effect and Fano effect individually and jointly. The characteristics of various Raman line shapes have been studied in terms of the broadening and asymmetry of Raman line shapes. It is shown that the asymmetry in the Raman line-shape function caused by these two effects individually does not add linearly to give asymmetry of line-shape generated by considering the combined effect. This indicates existence of interplay between the two effects. The origin of interplay lies in the fact that Fano effect itself depends on quantum confinement effect and in turn provides an asymmetry. This can not be explained by considering the two effects contribution independent of each other.

Possibility of spin-polarized transport in edge fluorinated armchair boron nitride nanoribbons

We predict the possibility of spin based electronic transport in edge fluorinated armchair boron nitride nanoribbons (ABNNRs). The structural stability, electronic and magnetic properties of these edge fluorinated ABNNRs have been systematically analyzed by means of first-principles calculations within the local spin-density approximation (LSDA). Regardless of their width, ABNNRs with F-passivation at only the edge-B atoms are found to be thermodynamically stable and half-metallic in nature. The spin polarized states are found to be ∼0.4 eV more stable than that of spin compensated states. Further, upto 100% spin polarization is expected in ABNNRs with F-passivation at only the edge-B atoms as indicated by the giant splitting of spin states which is observed in the corresponding band structures, DOS and transmission spectrum. The existence of half-metallicity is attributed to the localization of electronic charge at unpassivated edge-N atoms as revealed from the analysis of Bloch states and projected density of states (PDOS). Importantly, present stability analysis suggests that the possibility of experimental realization of spin polarized transport in ABNNRs is more promising via F-passivation of ribbon edges than that of H-passivation. The observed half-metallic nature and large difference in the energies (∼0.4 eV) of spin polarized and spin compensated states projects these half-metallic ABNNRs as potential candidates for inorganic spintronic applications.