Raghvendra Pratap Chaudhary

Stanene: Atomically Thick Free-standing Layer of 2D Hexagonal Tin

Stanene is one of most important of 2D materials due to its potential to demonstrate room temperature topological effects due to opening of spin-orbit gap. In this pursuit we report synthesis and investigation of optical properties of stanene up to few layers, a two-dimensional hexagonal structural analogue of graphene. Atomic scale morphological and elemental characterization using HRTEM equipped with SAED and EDAX detectors confirm the presence of hexagonal lattice of Sn atoms. The position of Raman peak along with the inter-planar ‘d’ spacing obtained from SAED for prepared samples are in good agreement with that obtained from first principles calculations and confirm that the sheets are not (111) α-Sn sheets. Further, the optical signature calculated using density functional theory at ~191 nm and ~233 nm for low buckled stanene are in qualitative agreement with the measured UV-Vis absorption spectrum. AFM measurements suggest interlayer spacing of ~0.33 nm in good agreement with that reported for epitaxial stanene sheets. No traces of oxygen were observed in the EDAX spectrum suggesting the absence of any oxidized phases. This is also confirmed by Raman measurements by comparing with oxidized stanene sheets.

3D Oxidized Graphene Frameworks for Efficient Nano Sieving.

The small size of Na+ and Cl− ions provides a bottleneck in desalination and is a challenge in providing alternatives for continuously depleting fresh water resources. Graphene by virtue of its structural properties has the potential to address this issue. Studies have indicated that use of monolayer graphene can be used to filter micro volumes of saline solution. Unfortunately it is extremely difficult, resource intensive and almost impractical with current technology to fabricate operational devices using mono-layered graphene. Nevertheless, graphene based devices still hold the key to solve this problem due to its nano-sieving ability. Here we report synthesis of oxidized graphene frameworks and demonstrate a functional device to desalinate and purify seawater from contaminants including Na+ and Cl− ions, dyes and other microbial pollutants. Micro-channels in these frameworks help in immobilizing larger suspended solids including bacteria, while nano-sieving through graphene enables the removal of dissolved ions (e.g. Cl−). Nano-sieving incorporated with larger frameworks has been used in filtering Na+ and Cl− ions in functional devices.

Plasmonic Micro Lens for Extraordinary Transmission of Broadband Light

Extraordinary transmittance and focusing of light in quasi far field region using miniaturized optical devices is a daunting task. A polarization independent, broadband, planar metallic transmissive micro aperture capable of achromatically focusing visible light in quasi far field region is proposed. The calculated enhancement factor of transmission efficiency was about ~2.2. The total transmission after the aperture is about 60%. This high throughput focusing device will open new avenues for focusing electromagnetic energy in the wide area of sensors and energy concentration.

One-step, subwavelength patterning of plasmonic gratings in metal–polymer composites

2D and 3D micro/nano fabrication based on two-photon polymerization (TPP) has emerged as a strong contender for additive manufacturing. Here we present direct patterning of gold nanostructures using a femto-second laser inside a commercially available photoresist matrix without incorporating any dye. The nanostructures are written directly by in situ reduction of gold precursor within the photoresist using femto-second laser irradiation. The photo-initiator triggers the reduction of the gold precursor and induces simultaneous polymerization of the photoresist based on two-photon absorption phenomenon. Diffraction gratings with varied loadings of gold precursor in the photoresist have been fabricated and their diffraction efficiencies have been measured in the infrared region. Further 2D and 3D gold loaded mesh structures with subwavelength linewidths have been fabricated with sturdy features which may find metamaterial and plasmonic applications. The effect of gold precursor loading & laser power on line widths have been studied. A minimum line width of 390 nm has been achieved for 5 wt% gold precursor loaded polymers enabling the fabrication of sub-wavelength 2D and 3D plasmonic nanostructures.

Optical properties of stanene

Successful synthesis of graphene has created a runaway effect in the exploration of other similar two-dimensional materials. These materials are important as they provide large surface areas and have led to the exploration of new physical phenomena. Even though graphene has exotic electronic properties, its spin-orbit coupling is very weak. Tin, being one of the heaviest elements in this group, is expected to have enhanced spin-orbit coupling in addition to other exotic properties of graphene. Here we report optical signatures of free standing stanene obtained using UV-vis absorption spectroscopy. Raman measurements were performed on a transmission electron microscope (TEM) grid. Interlayer spacing, phonon frequencies and the imaginary part of the complex dielectric function obtained using first principles methods are in good agreement with the experimental data. Occurrence of parallel bands suggests the possibility of the presence of excitonic effects in stanene.

Synthesis of self-assembled large area films of complex hierarchical PZT clusters

The ability to bridge nano-micro interface for applications in functional miniaturized devices is of fundamental interest. We have synthesized novel large area films of complex hierarchical micro-flower morphologies of piezo-ceramics using hydrothermal reactions. The overall size of the samples produced is ~cm2. The growth morphologies are found to be dependent on concentration and pressure inside the reaction chamber. This can be used to deterministically grow these complex multi-scaled microstructures over a large area. These results outline a strategy for growth of omni-directional microstructures by utilizing self assembly processes.