Sumit Saxena

Investigation of structural and electronic properties of graphene oxide

The local atomic structure of graphene oxide has been probed using synchrotron radiations. Detailed investigations of recently proposed simplistic model of graphene oxide using x-ray absorption near edge spectroscopy have been performed. X-ray diffraction measurements and calculations indicate loss of coherence between graphene-like layers. However, larger in-plane structural coherence is understood to be present. Selected area electron diffraction measurements indicate the presence of graphitic regions in graphene oxide which is expected to produce interesting confinement effects in graphene oxide which could be important for the development of tunable electronic and photonic devices.

Spectroscopic investigation of confinement effects on optical properties of graphene oxide

The effect of electron confinement due to the formation of graphitic islands in graphene oxide has been studied using optical techniques. Photoluminescence studies indicate a strong ultraviolet (UV) emission at ∼356 nm along with a broadened feature in the green region of the visible electromagnetic spectrum. Strong UV emission suggests probable application of graphene oxide in photodynamic therapy and nanophotonic devices.

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.

Interacting Quasi-Two-Dimensional Sheets of Interlinked Carbon Nanotubes: A High-Pressure Phase of Carbon

A novel quasi-two-dimensional phase of carbon and the formation of a metastable hexagonal phase of single-walled carbon nanotubes (SWCNTs) have been investigated using density functional theory (DFT) by subjecting the SWCNT bundles to hydrostatic pressure. The chirality of the nanotubes determines the breaking of symmetry of the nanotubes under compression. Interestingly SWCNTs are found to undergo a mixture of sp(2) and sp(3) hybridization and are found to form novel interacting quasi-two-dimensional sheets of interlinked SWCNTs under hydrostatic pressure. Symmetry breaking, leading to the formation of highly directional bonds at stressed edges, is found to play an important role in the interlinking of the nanotubes. (3n + 3, 3n + 3) SWCNTs are found to acquire a hexagonal cross-section when subjected to hydrostatic pressures. The opening of a pseudogap is observed for small as well as large diameter armchair SWCNTs in nanotube bundles. Equilibrium separations calculated using the Leonard-Jones potentials indicate excellent agreement with the predictions of density functional calculations and experimental observations.

Double Negativity in 3D Space Coiling Metamaterials.

Metamaterials displaying negative refractive index has remarkable potential to facilitate the manipulation of incident waves for wide variety of applications such as cloaking, superlensing and the like. Space-coiling approach is a recently explored technique to achieve extreme properties. The space coiling phenomena cause less energy absorption as compared to local resonating phenomena for obtaining extreme parameters. Here we show extreme properties in doubly negative 3D space coiling acoustic metamaterials. Frequency dispersive spectrum of extreme constitutive parameters has been calculated for 2D maze and 3D space coiling labyrinthine structure. This is in good agreement to the calculated acoustic band dispersion.

Optical properties of few layered graphene quantum dots

Quantum dots provide a unique opportunity to study the confinement effects of electronic wave function on the properties of materials. We have investigated the optical properties of graphene quantum dots synthesized using ultra-fast light–matter interactions followed by one step reduction process. Atomic-scale morphological information suggests the presence of both zigzag and armchair edges in these quantum dots. Optical characterizations were performed using absorption, photoluminescence, and infrared spectroscopy. A shift in the emission spectrum and disappearance of n → π* transition in the absorption spectrum on reduction of the ablated samples confirmed the formation of graphene quantum dots. First principles calculations are in good agreement with the experimentally reported infrared data.

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.

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.

Structural, optical investigations of graphene from graphene oxide using green method

Graphene nano sheets (GNS) are synthesized from Graphene Oxide (GO) using commercial sugar as a reducing agent. A green and facile approach is followed to synthesize chemically converted GNS using exfoliated GO as precursor. The merit of this method is that both the reducing agents themselves and the oxidized products are environmentally friendly. The prepared materials are characterized with X-ray diffraction (XRD), UV-Visible absorption spectroscopy, High resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). The results of XRD, UV-vis analysis provide a clear indication of removal of oxygen-containing groups from GO and the formation of GNS.