K. Dasari

Extended-area nanostructuring of TiO2 with femtosecond laser pulses at 400 nm using a line focus.

An efficient way to generate nanoscale laser-induced periodic surface structures (LIPSS) in rutile-type TiO(2) with frequency-converted femtosecond laser pulses at wavelengths around 400 nm is reported. Extended-area structuring on fixed and moving substrates was obtained by exploiting the line focus of a cylindrical lens. Under defined conditions with respect to pulse number, pulse energy and scanning velocity, two types of ripple-like LIPSS with high and low spatial frequencies (HSFL, LSFL) with periods in the range of 90 nm and 340 nm, respectively, were formed. In particular, lower numbers of high energetic pulses favour the generation of LSFL whereas higher numbers of lower energetic pulses enable the preferential creation of HSFL. Theoretical calculations on the basis of the Drude model support the assumption that refractive index changes by photo-excited carriers are a major mechanism responsible for LSFL. Furthermore, the appearance of random substructures as small as 30 nm superimposing low spatial frequency ripples is demonstrated and their possible origin is discussed.

Electrical conductivity and photoresistance of atomic layer deposited Al-doped ZnO films

Al-doped ZnO films were deposited by the atomic layer deposition (ALD) on both glass and sapphire (0001) substrates. The Al composition of the films was varied by controlling the Zn:Al pulse cycle ratios. The films were characterized by the atomic force microscopy, x-ray photoelectron spectroscopy, x-ray diffraction, and optical measurements. The Film resistivity was measured as a function of Zn:Al cycle ratios as well as temperature for films grown at various substrate temperature used for ALD deposition. The resistivity of the ALD grown films decreases significantly, and so as the increase in the carrier concentration as the cycle ratio increases. The systematic measurements of temperature dependence of resistivity of films at various cycle ratios clearly demonstrate the crossover of the metal–semiconductor–insulator phase with the function of temperature as well as the cycle ratios. The average transmission of all films is greater than 85% and the optical absorption increases significantly in the visib...

Low-temperature anomalous magnetic behavior of Co2TiO4 and Co2SnO4

We report the low-temperature anomalous magnetic behavior of ferrimagnetic spinels cobalt orthotitanate (Co2TiO4), which exhibits magnetic compensation behavior across 31.74 K, and cobalt orthostannate (Co2SnO4) exhibiting two sequential magnetic transitions, namely (i) ferrimagnetic to paramagnetic transition with Neel temperature TN ∼ 41 K and reentrant spin-glass behavior with glass transition temperature TSG ∼ 39 K. The Arrott plot (H/M versus M2) criterion has been used to extricate the order of sequential magnetic transitions occurring below TN. Negative slopes of the Arrott plots below 32 K, metamagnetic-like character of the M-H isotherms, anomalies in the specific-heat (CP T−1 versus T) below 15 K, and a zero-crossover of isothermal magnetic-entropy-change (ΔS) signify the presence of pseudo first-order discontinuous magnetic phase transition in the low-temperature regime 5 K ≤ T ≤ 32 K. The dc- and ac-susceptibilities of both Co2TiO4 and Co2SnO4 are interpreted in terms of frozen-spin-clusters, ...

Visible photoluminescence and room temperature ferromagnetism in high In-content InGaN:Yb nanorods grown by molecular beam epitaxy

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 Vegards 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 temperature, reducing luminescence amplitude temperature dependent fluctuation, and significant narrowing of excitonic emission band as compared to the undoped InGaN. The magnetic properties measured by superconducting quantum interference devices reveals room temperature ferromagnetism, which can be explained by the double exchange mechanism and magnetostriction.

Femtosecond-Laser Induced Nanostructures in TiO2

TiO2 nanostructures are important functional materials with a growing number of applications in fields like medicine, photochemistry or photovoltaics. We demonstrate that highly reproducible nanostructures can be generated on the surface of bulk as well as thin film material by exploiting the phenomenon of laser-induced periodic surface structures (LIPSS). The influence of key parameters like pulse number, laser fluence, wavelength, and surface quality on the formation of such nanoripples is discussed. The time-integrated theory of Drude and Sipe is extended by the nonlinear excitation of transparent materials into a transient metal-like state enabling for the effective generation of surface plasmon polaritons.

Generation of extended-area femtosecond laser induced periodic nanostructures on TiO2 by moving samples through a line focus

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.