Controlled etching of silica nanospheres monolayer for template application: a systematic study

Monolayers of silica nanospheres (SNs), via self- or guided-assembly has been extensively used for template fabrication in thin films, employed in the areas of plasmonics, photonic crystals, and solar cells. We report on a versatile, rapid, and controllable process to obtain non-close-packed structure by restructuring the SNs geometry at two-particle level. A geometrical model has been proposed to quantify parameters that control the final morphology of the monolayer. SNs of different sizes (viz. 140 nm, 170 nm, and 220 nm) were self-assembled as a close-packed monolayer on a silicon substrate using a three-step spin coating method and then sintered at 950�C before being exposed to an etchant. We investigate the dependence of particle radius, neck (formed due to sintering) parameters and distance between the SNs, on etching time and etchant concentration. The intermediate and final morphology of the restructured monolayer is used as a template to grow silicon nanowires using metal-assisted chemical etching. We provide quantitative estimates of the parameters pertaining to the restructuring of the monolayer of SNs, which can be used as tunable templates for the growth of nanowires. The optimized process can be scaled-up for industrial application because of its faster and controllable rate of production.by Utsava Sakshum Khanna, Sagar Paneliya, Abhijit Ray, Indrajit Mukhopadhyay and Rupak Banerjee

Experimental and CFD analysis on heat transfer and fluid flow characteristic of a tube equipped with variable pitch twisted tape

In this paper, we report an experimental and Computational Fluid Dynamics (CFD) investigation on the overall heat transfer coefficient, friction factor and effectiveness of a tube equipped with classic and variable twisted tape inserts. Constant (Y = 4.0) and variable pitch twisted inserts (Y = 4-3-4) made of Stainless Steel and Aluminium were used during the experiments due to their ease in machinability. Resistance thermometers were used to measure temperature of the working fluid at the inlet and outlet of the test section. It was observed that the twisted tape in the tube imposes the turbulence in the fluid and enhances the heat transfer due to swirling flow. The results showed that the overall heat transfer coefficient and effectiveness of aluminium variable pitch twisted tape were higher than stainless steel inserts. The experimental data obtained were validated using Computational Fluid Dynamics (CFD) simulations. The constant pitch inserts for aluminium and stainless steel showed an enhancement of 67.54% and 66.65% as compared to plain tube. Whereas aluminium variable pitch inserts showed an enhancement of overall heat transfer coefficient by 18.15% as compared to constant pitch insert and stainless-steel variable pitch insert showed enhancement of 15.25% as compared to constant pitch insert

Systematic investigation of close-packed silica nanospheres monolayer under sintering conditions

In this paper, we have quantified and investigated the effect of various sintering temperature on close-packed Silica Nanospheres (SNs) monolayers. SNs with diameters of 140, 175 and 220 nm were fabricated by an effective and reliable spin-coating technique. The fabricated SNs monolayers were sintered up to 1200 °C and were analyzed from FESEM to investigate in details for local and extended transformations in their structural and morphological properties. A distinct "neck-formation" was observed and was quantified with different particle size distribution as well as surface packing density. It was observed that SNs monolayer undergoes intra-particle reformation in the form of shrinkage in individual SNs and compactification of growth domains, followed by inter-particle sintering. A geometrical model was developed to determine the curvature radius and interpenetration depth thus enabling us to quantify the parameters that dominate the dynamics of the sintering process for such non-porous SNs.by Sakshum Khanna, Utsav Harsh Chaliyawala, Sagar Paneliya, Debmalya Roy, Kingsuk Mukhopadhyay, Rupak Banerjee and Indrajit Mukhopadhya