Oxygen vacancy (OV) controlled hydrophobicity of self-assembled TiO2 nanorods (NRs) on chemically etched Si pyramids is investigated by irradiating with 50 keV Ar+-ions at room temperature. Apparent contact angle (CA) is found to increase from 122° to 141° up to a fluence of 1 × 1015 ions/cm2, followed by a gradual reduction to 130° at 1 × 1017 ions/cm2. However, the drop in apparent CA is found to be associated with the decrease in fractional surface area via transformation of NRs to an amorphous layer above 1 × 1015 ions/cm2, though it is still higher than that of as-grown one. Detailed X-ray photoelectron spectroscopy and electron paramagnetic resonance measurements suggest that the control of hydrophobic behavior is related to the suppression of surface free energy via migration of OVs into the voids in TiOx layers. © 2016 American Chemical Society.

Aloke Kanjilal

Physics

(SoNS) School of Natural Sciences

Saini C.P.

Barman A.

Das D.

Satpati B.

Bhattacharyya S.R.

Kanjilal D.

Ponomaryov A.

Zvyagin S.

<p>Shiv Nadar University is a comprehensive, multidisciplinary, research-focused, and student-centric University offering a full range of academic programs at the undergraduate, postgraduate and doctoral level. With state-of-the-art infrastructure, the University comprises of academic wings, sophisticated labs, international standard sports facilities, amphitheaters, auditorium, conference rooms and smart classrooms. The University&rsquo;s goal is to become internationally recognized for the quality of its research and creative endeavors and their applicability to improving quality of life, generating new insights and expanding the boundaries of human knowledge creativity. Committed to excellence in teaching, research and service, the University aims to serve the higher education needs of India and the world beyond.</p>

Shiv Nadar University

Journal of Physical Chemistry C

Simultaneous superhydrophobic and visible range antireflective properties are demonstrated in hydrothermally grown ZnO nanorods (NRs) on chemically textured Si surfaces. A drastic transformation of Si micro-pyramids from hydrophobic to superhydrophobic is observed by securing the formation of polycrystalline ZnO NRs at surfaces, showing an increment of apparent contact angle from ∼102° to ∼157° and further explained in the light of a decrease in solid fractional surface area, according to the Cassie-Baxter model. Moreover, wurtzite phase of ZnO NRs is confirmed by the X-ray diffraction and Raman analysis. Such hierarchical structures further exhibit a large visible range antireflection (∼5\%), and correlated it to the variation in aspect ratio of Si pyramids in conjunction with the formation of graded refractive index. The combined superhydrophobic and large antireflection phenomena in the present dual-scale structures is believed to be useful for Si-based photovoltaic applications. © 2019 IOP Publishing Ltd.

Fulltext

Materials Research Express

Tailoring superhydrophobic ZnO nanorods on Si pyramids with enhanced visible range antireflection property

The efficacy of 50 keV Ar+-ion irradiation toward the interfacial and stoichiometric engineering of strained Al-doped ZnO (AZO)/TiO2 heterostructure is systematically investigated using a variety of experimental techniques, notably by cross-sectional transmission electron microscopy. Glancing-angle X-ray diffraction evidences the release of in-plane compressive stress from the as-grown AZO/TiO2 bilayer structure at a critical fluence of 1 × 1016 ions/cm2, and we discuss in the light of microcracks and voids formation combined with the dewetting phenomenon. Ion irradiation also leads to an improvement of stoichiometry in both top AZO and underneath amorphous TiO2 layers, as manifested by depth-dependent energy dispersive X-ray spectroscopy owing to the large diffusion of oxygen toward the AZO/TiO2 interfacial region through the AZO defect sites. Such ion beam induced self-healing in stoichiometry of AZO/TiO2 heterostructure has been attributed to a conjunction of sputtering and diffusion phenomena involving the constituent elements (Zn, Ti, and O). Further increase in ion fluence up to 5 × 1016 ions/cm2 causes a complete deterioration of the heterostructure with the formation of a graded layer via intermixing of these elements, followed by the evolution of voids. © 2018 Author(s).

Journal of Applied Physics

Probing the impact of energetic argon ions on the structural properties of ZnO:Al/TiO2 heterostructures

Temperature-dependent photoluminescence (PL) of titanium oxide (TiO2) shows an evolution of blue emission when exposed to 50 keV Ar+ ions. The origin of observed PL has been examined by X-ray absorption near-edge spectroscopy (XANES) at Ti-K,L and O-K edges, revealing the reduction of ligand field splitting owing to the formation of oxygen vacancies (OVs) by destroying TiO6 octahedral symmetry. Detailed PL and XANES analyses suggest that the fluence (ions/cm2) dependent increase in OVs not only boosts the conduction electrons but also increases the density of holes in localized self-trapped exciton (STE) states near the valence band. Based on these observations, we propose a model in which doped conduction electrons are recombining radiatively with the holes in STE states for blue light emission. © 2017 American Chemical Society.

Impact of Self-Trapped Excitons on Blue Photoluminescence in TiO2 Nanorods on Chemically Etched Si Pyramids

Post-deposition annealing mediated evolution of self-decorated Au nanoparticles (NPs) on chemically etched Si pyramids is presented. A distinct transformation of Si surfaces from hydrophilic to hydrophobic is initially found after chemical texturing, showing an increase in contact angle (CA) from 58°to 98°(±1°). Further improvement of hydrophobicity with CA up to ∼118°has been established after annealing a 10 nm thick Au-coated Si pyramids at 400°C that led to the formation of Au NPs on Si facets along with self-ordering at the pyramid edges. Detailed x-ray diffraction studies suggest the evolution of crystalline Au NPs on strained Si facets. Microstructural studies, however, indicate no mixing of Au and Si atoms at the Au/Si interfaces, instead of forming Au nanocrystals at 400°C. The improved hydrophobicity of Si pyramids, even with Au NPs can be explained in the light of a decrease in solid fractional surface area according to Wenzel's model. Moreover, a sharp drop in specular reflectance from Si pyramids in the range of 300-800 nm, especially in the ultraviolet region up to ∼0.4\% is recorded in the presence of Au NPs by ultraviolet-visible spectroscopy, reflecting the possible use in photovoltaic devices with improved antireflection property. © 2016 AIP Publishing LLC.

Self-decorated Au nanoparticles on antireflective Si pyramids with improved hydrophobicity

Transformation of self-assembled crystalline TiO2 nanorods to amorphous layer, and the corresponding impact on optical-bandgap (Eg) on Si pyramids are investigated by irradiating with 50 keV Ar+-ions. Initially, Eg is found to be reduced from 3.23 to 2.94 eV up to a fluence of 1 × 1016 ions/cm2, and discussed in terms of the rise in oxygen vacancies (VO). However, a sudden increase in Eg to 3.38 eV is detected at a fluence of 1 × 1017 ions/cm2 through evolution of voids by over-saturating VO, manifesting the appearance of degenerate states by shifting the Fermi level above the conduction band minimum via Burstein-Moss effect. © 2016 AIP Publishing LLC.

Applied Physics Letters

Defect-engineered optical bandgap in self-assembled TiO2 nanorods on Si pyramids

Towards developing next generation scalable TiO2-based resistive switching (RS) memory devices, the efficacy of 50 keV Ar+-ion irradiation to achieve self-organized nano-channel based structures at a threshold fluence of 5 × 1016 ions/cm2 at ambient temperature is presented. Although x-ray diffraction results suggest the amorphization of as-grown TiO2 layers, detailed transmission electron microscopy study reveals fluence-dependent evolution of voids and eventual formation of self-organized nano-channels between them. Moreover, gradual increase of TiO/Ti2O3 in the near surface region, as monitored by x-ray photoelectron spectroscopy, establishes the upsurge in oxygen deficient centers. The impact of structural and chemical modification on local RS behavior has also been investigated by current-voltage measurements in conductive atomic force microscopy, while memory application is manifested by fabricating Pt/TiO2/Pt/Ti/SiO2/Si devices. Finally, the underlying mechanism of our experimental results has been analyzed and discussed in the light of oxygen vacancy migration through nano-channels. © 2015 AIP Publishing LLC.

Self-organized titanium oxide nano-channels for resistive memory application

Chemically textured Si with improved absorption in the complete range of solar spectrum is investigated by ultraviolet/visible/near-infrared (UV/Vis/NIR) spectroscopy, showing an average specular reflectance of ∼0.4\% in the wavelength of 500-3000nm. The pyramidal structures on such solar-blind Si can reduce the reflectance further below 0.1\% in the UV region by conformal growth of granular Al-doped ZnO (AZO) films. X-ray diffraction analyses suggest the growth of polycrystalline AZO on faceted-Si. Moreover, marginal increase in electrical conductivity of AZO is found on textured surfaces, whereas rise in leakage current in Schottky-like Ag/AZO/Si/Ag heterostructure devices is noticed with increasing Si surface area. © 2014 AIP Publishing LLC.