Pawan Kumar Srivastava

Band Gap Engineering of ZnO using Core/Shell Morphology with Environmentally Benign Ag2S Sensitizer for Efficient Light Harvesting and Enhanced Visible-Light Photocatalysis

Band gap engineering offers tunable optical and electronic properties of semiconductors in the development of efficient photovoltaic cells and photocatalysts. Our study demonstrates the band gap engineering of ZnO nanorods to develop a highly efficient visible-light photocatalyst. We engineered the band gap of ZnO nanorods by introducing the core/shell geometry with Ag2S sensitizer as the shell. Introduction of the core/shell geometry evinces great promise for expanding the light-harvesting range and substantial suppression of charge carrier recombination, which are of supreme importance in the realm of photocatalysis. To unveil the superiority of Ag2S as a sensitizer in engineering the band gap of ZnO in comparison to the Cd-based sensitizers, we also designed ZnO/CdS core/shell nanostructures having the same shell thickness. The photocatalytic performance of the resultant core/shell nanostructures toward methylene blue (MB) dye degradation has been studied. The results imply that the ZnO/Ag2S core/shell nanostructures reveal 40- and 2-fold enhancement in degradation constant in comparison to the pure ZnO and ZnO/CdS core/shell nanostructures, respectively. This high efficiency is elucidated in terms of (i) efficient light harvesting owing to the incorporation of Ag2S and (ii) smaller conduction band offset between ZnO and Ag2S, promoting more efficient charge separation at the core/shell interface. A credible photodegradation mechanism for the MB dye deploying ZnO/Ag2S core/shell nanostructures is proposed from the analysis of involved active species such as hydroxyl radicals (OH(•)), electrons (e(-)(CB)), holes (h(+)(VB)), and superoxide radical anions (O2(•-)) in the photodegradation process utilizing various active species scavengers and EPR spectroscopy. The findings show that the MB oxidation is directed mainly by the assistance of hydroxyl radicals (OH(•)). The results presented here provide new insights for developing band gap engineered semiconductor nanostructures for energy-harvesting applications and demonstrate Ag2S to be a potential sensitizer to supersede Cd-based sensitizers for eco-friendly applications.

Eliminating defects from graphene monolayers during chemical exfoliation

Graphene layers with and without defects have been grown by chemically exfoliating graphite in organic solvents and characterized by different spectroscopic techniques. It has been shown that defects can be controlled in graphene layers while intercalating different organic molecules in graphite. The transfer characteristics of transistors fabricated on graphene monolayers exfoliated using organic solvent with low dielectric constant and low boiling point show almost no shift of minimum conductivity point, i.e., Dirac point, indicating defect free pristine graphene.

Controlled Doping in Graphene Monolayers by Trapping Organic Molecules at the Graphene–Substrate Interface

We report controlled doping in graphene monolayers through charge-transfer interaction by trapping selected organic molecules between graphene and underneath substrates. Controllability has been demonstrated in terms of shifts in Raman peaks and Dirac points in graphene monolayers. Under field effect transistor geometry, a shift in the Dirac point to the negative (positive) gate voltage region gives an inherent signature of n- (p-)type doping as a consequence of charge-transfer interaction between organic molecules and graphene. The proximity of organic molecules near the graphene surface as a result of trapping is evidenced by Raman and infrared spectroscopies. Density functional theory calculations corroborate the experimental results and also indicate charge-transfer interaction between certain organic molecules and graphene sheets resulting p- (n-)type doping and reveals the donor and acceptor nature of molecules. Interaction between molecules and graphene has been discussed in terms of calculated Mulliken charge-transfer and binding energy as a function of optimized distance.

Dielectric environment as a factor to enhance the production yield of solvent exfoliated graphene

High yield production of high quality graphene is essential for its application in electronics, optoelectronics and energy storage devices. Liquid phase exfoliation based methods for obtaining graphene are becoming popular because of their versatility and scalability. These advantages are absent with other growth methods such as mechanical exfoliation using scotch tape and chemical vapor deposition. Here we present a sonication assisted, surfactant free method for liquid phase exfoliation of graphene using solvents with varying dielectric constants. We have shown that the method presented here is capable of producing high yields (1.22 wt%), and exceptionally large sizes (30–50 microns) with a high carrier mobility of 10 000 cm2 Vs−1 in monolayer graphene. Moreover, it is possible to obtain pristine as well as doped monolayer or bilayer or multilayer graphene with extreme controllability, on any solid substrate. It has been shown that choice of a solvent of a particular dielectric constant and sonication time are key parameters for liquid phase exfoliation. It is further shown that the exfoliation efficiency can be enhanced using solvents with high dielectric constant due to functionalization which has also been supported by density functional theory based electronic structure calculations. We have also tested this fact by using different solvents with similar dielectric constant. This method promises high-end industrial scale synthesis for potential applications in different types of devices, graphene based composites and liquid phase chemistry as well.

Relativistic nature of carriers: Origin of electron-hole conduction asymmetry in monolayer graphene

We report electron-hole conduction asymmetry in monolayer graphene. Previously, it has been claimed that electron-hole conduction asymmetry is due to imbalanced carrier injection from metallic electrodes. Here, we show that metallic contacts have negligible impact on asymmetric conduction and may be either sample or device-dependent phenomena. Electrical measurements show that monolayer graphene based devices exhibit suppressed electron conduction compared to hole conduction due to the presence of donor impurities which scatter electrons more efficiently. This can be explained by the relativistic nature of charge carriers in a graphene monolayer and can be reconciled with the fact that in a relativistic quantum system transport cross section does depend on the sign of scattering potential in contrast to a nonrelativistic quantum system.

Doping of graphene during chemical exfoliation

Graphene provides a perfect platform to explore the unique electronic properties in two-dimensions. However, most electronic applications are handicapped by the absence of a semiconducting gap in pristine graphene. To control the semiconducting properties of graphene, doping is regarded as one of the most feasible methods. Here we demonstrate that graphene can be effectively doped during chemical exfoliation of highly ordered pyrolitic graphite in organic solvents. Layered structure of graphene sheets was confirmed by confocal Raman spectroscopy and doping was probed by analyzing shift in Raman peak positions and transistor transfer (IDS-VGS) characteristics.

Effect of Dielectric Environment on Carrier Mobility in Chemically Exfoliated Graphene

We have studied the effect of dielectric environment on transport properties of graphene based field effect transistors where graphene layers were grown in various organic solvents with varying dielectric constant using chemical exfoliation. Electrical measurements of graphene transistors clearly indicate that dielectric constant of the solvents has no significant effect on charge carrier mobility in graphene.

How to Achieve High Quality Large Area Monolayer Graphene with Field Effect Mobility of 20,000 cm2/Vs

High quality graphene monolayers were grown using chemical exfoliation method by sonicating the highly ordered pyrolytic graphite in organic solvents. Properties of as exfoliated graphene layers were found to be strongly dependent on dielectric constant of the solvents. This was corroborated by confocal Raman spectroscopy and electrical measurements. Graphene samples exfoliated in solvents with low dielectric constant show diminutive D band intensities and excellent field effect mobility of 20,000 cm2/Vs and in solvents with high dielectric constant, D band intensity increases and mobility reduces to 11,000 cm2/Vs due to doping induced defects in graphene layers. Our results also show up shift in minimum conductivity point with doping level due to inhomogeneous potential induced by point defects near Dirac point.

Growing Monolayer, Bilayer and Trilayer Reproducibly By Chemical Exfoliation

A prerequisite for the development of graphene based electronics is how to obtain the desired no. of graphene layers reproducibly. Here we present a method for the synthesis of monolayer, bilayer and trilayer of graphene via chemical exfoliation of HOPG in various solvents. The graphene monolayer, bilayer and trilayer were confirmed by Transmission Electron Microscopy, Raman spectroscopy and universal optical transmittance spectroscopy.

Synthesis and Characterization of Single and Few Layer Graphene for Field Effect Transistor

Single and few-layer graphene sheets with sizes up to 100 micron were synthesized using physico-chemical method by dispersing highly ordered pyrolytic graphite (HOPG) flakes into N, N-dimethylformamide (DMF) solution. The layer structure and thickness of graphene sheets were characterized with transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM) and Raman spectroscopy. Synthesized graphene sheets were transferred to solid substrates for further processing. Field-effect transistor with individual graphene sheet was fabricated. In ambient conditions, graphene sheets were found to exhibit p-type behavior.