Shailendra K. Saxena

Silicon nanowires prepared by metal induced etching (MIE): good field emitters

Micro-Raman scattering and electron field emission characteristics of silicon nanowires (SiNWs) synthesized by metal assisted chemical etching (MACE) are investigated. Scanning electron microscopy images reveal the growth of well aligned vertical SiNWs. Raman shift and size relation from bond-polarizability model has been used to calculate exact confinement sizes in SiNWs. The Si optical phonon peak for SiNWs showed a downshift and an asymmetric broadening with decreasing diameter of the SiNWs due to quantum connement of optical phonons. The field emission characteristics of these SiNWs are studied based by carrying out currentvoltage measurements followed by a theoretical analysis using FowlerNordheim equation. The electron field emission increased with decreasing diameter of SiNWs. Field emission from these SiNWs exhibits signicant enhancement in turn-on eld and total emission current with decreasing nanowire size. The reported results in the current study indicate that MACE is a simple technique to prepare well-aligned SiNWs with potentials for applications in field emission devices. ‡ Electronic mail: vivek@unitus.it ar X iv :1 40 5. 71 93 v1 [ co nd -m at .m es -h al l] 2 8 M ay 2 01 4 Micro-Raman and FE studies of SINWs... 2

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.

Interfacial redox centers as origin of color switching in organic electrochromic device

Fabrication and operation of simple solid state electrochromic devices using ethyl viologen diperchlorate in a polymer matrix is presented here. In-situ Raman and transmission/absorption studies have been done to establish the origin of bias induced color change, between a transparent and navy blue color, in the electrochromic device. The origin of bias induced color change has been attributed to the bias induced redox switching between its viologen dication and free redicle forms. Fundamental reason behind colour changes of viologen molecule has been established. In-situ UV-Vis spectra reveals that the navy blue color of the device under biased condition is not due to increase in the transparency corresponding to blue wavelength but due to suppression of the transparency corresponding to the complementary colors. Absorption modulation has been reported from the device with good ON/OFF contrast of the device.

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.

Effect of silicon resistivity on its porosification using metal induced chemical etching: morphology and photoluminescence studies

A comparison of porous structures formed from silicon (Si) wafers with different resistivities has been reported here based on the morphological studies carried out using scanning electron microscope (SEM). The porous Si samples have been prepared using metal induced etching (MIE) technique from two different Si wafers having two different resistivities. It is observed that porous Si containing well aligned Si nanowires are formed from high resistivity (1-20

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

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