Kiran Dasari

Cadmium selenide (CdSe) thin films were deposited on indium tin oxide (ITO) coated glass substrates using pulsed laser deposition (PLD) technique under different growth temperatures. Samples were investigated for their structural, morphological, and optical properties through X-ray diffraction (XRD), atomic force microscopy (AFM), and UV-Vis-NIR spectroscopy. AFM analysis revealed that the surface roughness of the as-grown CdSe thin films increased with the increase in deposition temperature. The optical constants and film thickness were obtained from spectroscopic ellipsometry analysis and are discussed in detail. The optical band gap of the as-grown CdSe thin films, calculated from the Tauc plot analysis, matched with the ellipsometry measurements, with a band gap of ~1.71 eV for a growth temperature range of 150 °C to 400 °C. The CdSe thin films were found to have a refractive index of ~3.0 and extinction coefficient of ~1.0, making it a suitable candidate for photovoltaics.

We report a detailed study on the dynamical response of localized electron hopping and dipole relaxation in bulk polycrystals of Zn diluted Cuprospinel (Cu1 − x Zn x Fe2O4). The variations in the dielectric dispersion and ac-resistivity (ρ ac ) were analyzed over a wide temperature (77 K ≤T ≤ 823 K) and frequency (20 Hz ≤ f ≤ 20 MHz) window for a critical composition x c = 0.4. The variation of ϵ R (f, T) followed the Maxwell–Wagner type polarization mechanism in-line with the Koops phenomenological theory. Our analysis of ρ ac (T, f) provide strong evidence to the Mott’s variable range hopping of charge transport between the localized states at low temperatures, however, thermally-activated Arrhenius like behaviour was noticed at high temperatures with E A = 656 meV for x c = 0.4. Moreover, electric modulus spectroscopic studies (M *(f, T)) reveals two distinct types of relaxation phenomena: (i) the short-range oscillations of the charge carriers within the potential well of grains and (ii) the long-range movement of charge carriers across the grain boundaries. The depressed semi-circles of the Nyquist plots and lower values of non-exponential parameter extracted from M *(f, T) suggest the non-Debye type relaxation process present in the system with a widespread distribution of relaxation times. The frequency exponent (s(x, T)) study of Jonscher’s power law reveals that the ac-conductivity follows small-polaron tunnelling followed by the correlated-barrier-hopping mechanism for x < 0.1. However, for x ≥ x c reorientational hopping mechanism is predominant, except for T > 400 K, where thermally activated Arrhenius-type conduction of charge carriers is prevalent in this spinel system. Furthermore, the tetragonally (I41/amd) distorted systems (x ≤ 0.05) exhibit less activation energy (E A − VRH ) values as compared to those of cubic-spinel symmetry (Fd-3m) which saturates at 130 meV for 0.1 ≤ x ≤ 0.6. Compositional dependent tunability of the above discussed parameters may open a constructive approach to design low energy-loss and high-resistive electromagnetic elements for microwave devices which is the key significance of the present study.

In this article, we report a comparative analysis of various spectroscopic studies including low‐temperature ( 25K≤T≤300K ) Raman spectroscopy of cobalt‐orthotitanate ( Co2TiO4 ) and tricobalt‐tetraoxide ( Co3O4 ), and their solid solutions (1−x) Co3O4+x of Co2TiO4 ( 0≤x≤1 (100 wt.%)). For all the lower and intermediate compositions, five Raman‐active modes were recognized at 689, 618, 518, 480 , and 195cm−1 that are associated with A1g , Eg , and 3F2g phonon symmetries. Conversely, pure Co2TiO4 exhibits a broad spectrum of width ∼93.3cm−1 without any signatures of F2g (3) mode. At low‐temperatures (down to 25 K) the A1g and F2g peaks of both Co2TiO4 and Co3O4 shift toward the high‐frequency side with anomalies across the ferrimagnetic Néel temperature ( TN ∼48±5 K) and antiferromagnetic Néel temperature ( TN∼ 30 ± 10 K), respectively. All the investigated samples exhibit two distinct bands at 576 cm−1 ( B1 ) and 665 cm−1 ( B2 ) in the Fourier transform infrared spectra recorded at 300±10 K, associated with the vibrational stretching of the metal–oxygen bonds of length ∼ 195.8 pm (B–O) and ∼ 185.4 pm (A–O), respectively. The intensity of these sharp bands gradually decreases as the crystal structure transforms from normal‐spinel ( a=8.07 Å) to inverse‐spinel structure ( a=8.45 Å). The X‐ray photoelectron spectroscopy (XPS) studies revealed that the Ti was incorporated into the octahedral B‐sites of inverse‐spinel structure of Co2TiO4 . Interestingly, the XPS spectra of Co2TiO4 provide evidence of the trivalent character of Ti instead of tetravalent cationic configuration together with a weak Co3+ character at the octahedral sites. These results are discussed in terms of the binding‐energy (BE) difference between the O‐1s and Ti‐2p 3/2 ( Δ [O–Ti‐2p 3/2 ] = BE(O‐1s) −BE(Ti‐2p 3/2) ) and the mean chemical bond length l[Ti−O] . The peculiarities of all these results in consonance with the crystal‐structure (bond angles and bond lengths) and electron‐spin‐resonance studies are discussed in detail.

We report the growth of high indium content InGaN:Yb nanorods grown on c-plane sapphire (0001) substrates using plasma assisted molecular beam epitaxy. The in situ reflection high energy electron diffraction patterns recorded during and after the growth revealed crystalline nature of the nanorods. The nanorods were examined using electron microscopy and atomic force microscopy. The photoluminescence studies of the nanorods showed the visible emissions. The In composition was calculated from x-ray diffraction, x-ray photoelectron spectroscopy, and the photoluminescence spectroscopy. The In-concentration was obtained from photoluminescence using modified Vegard's law and found to be around 37% for InGaN and 38% for Yb (5 ± 1%)-doped InGaN with a bowing parameter b = 1.01 eV. The Yb-doped InGaN showed significant enhancement in photoluminescence properties compared to the undoped InGaN. The Yb-doped InGaN nanorods demonstrated the shifting of the photoluminescence band at room temp...

Nanorod of in situ Yb‐doped InGaN and undoped InGaN have been grown on (0001) sapphire substrates by plasma assisted molecular beam epitaxy (MBE). Selected regions on Yb‐doped InGaN sample show single dominant near band edge emission (NBE) in green, yellow or orange color due to the variation of In content. Temperature dependent PL peak energy of InGaN nanorod shows the characteristic S ‐shaped behavior indicating the presents of strong exciton localization energy in undoped InGaN nanorod. The exciton localization energy reduced significantly after incorporating Yb into InGaN, giving rise to damping of the S‐shape profile amplitude and narrowing of the PL line width from ∼20 meV to ∼12 meV at 11 K. It is proposed that the improved PL thermal stability and the PL line width in Yb‐doped InGaN nanorod is affected by the Yb gettering effect. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The combination of sample translation and line focusing by cylindrical optics is shown to be a convenient and highly effective way of generating laser induced coherent periodic surface structures (LIPSS) in TiO2 over significantly extended areas. Compared to known techniques based on a sample translation relative to a circular symmetric focus, the approach is much less time consuming and requires only a single translation stage. The capability of the method to form both high and low spatial frequency LIPSS (HSFL, LSFL) at the second harmonic wavelengths of a Ti:sapphire-laser (around 400 nm) at properly chosen scanning velocity and laser pulse energies is demonstrated. Structured multi-mm2 areas with periods of 80 nm and 325 nm were obtained corresponding to distinct sets of optimized parameters. Furthermore, the appearance of nano-bumps on 30 nm scale on the surface of the LSFL is reported. Basic technical issues are discussed and potential applications of LIPSS in rutile-type TiO2 like superwetting, friction control, catalysis and photovoltaic are proposed.