Manu Jaiswal

Recombination of photo-generated charges is one of the most significant challenges in designing efficient photo-anode for photo electrochemical water oxidation. In the case of TiO2, mixed phase (anatase-rutile) junctions often shown to be more effective in suppressing electron-hole recombination compared to a single (anatase or rutile) phase. Here, we report the study of bulk and surface recombination process in TiO2 multi-leg nanotube (MLNTs) anatase-rutile (A-R) junctions decorated with reduced graphene oxide (rGO) layers, through an analysis of the photo-current and impedance characteristics. To quantify the charge transport/transfer process involved in these junctions, holes arriving at the interface of semiconductor/electrolyte were collected by adding H2O2 to the electrolyte. This enabled us to interpret the bulk and surface recombination process involved in anatase/rutile/rGO junctions for photo-electrochemical water oxidation. We correlated this quantification to the electrochemical impedance spectroscopy (EIS) measurements, and showed that in anatase/rutile junction the increase in PEC performance was due to suppression in electron-hole recombination rate at the surface states that effectively enhances the hole transfer rate to the electrolyte. On the other hand, in rGO wrapped A-R MLNTs junction it was due to both phenomenon i.e decrease in bulk recombination rate as well as increase in hole transfer rate to the electrolyte at the semiconductor/electrolyte interface.

Harnessing the solar energy and producing clean fuel hydrogen through efficient photoelectrochemical water splitting has remained one of the most challenging endeavors in materials science. The core problem is to develop a suitable photo-catalyst material that absorbs a significant part of the solar spectrum and produces electron-hole pairs that can be easily separated without recombination. In the recent times, the composite of Titanium dioxide with graphene have been investigated to explore the advantages of both class of materials. Here we report on the photo-electrochemical properties of reduced graphene oxide functionalised TiO2 whiskers. The TiO2 whiskers are obtained from potassium titanium oxide (KTi8O16) synthesized through hydrothermal technique followed by ion exchange method and heat treatment. Graphene oxide was deposited on the as repared TiO2 whiskers using hydrothermal method. As formed samples were characterized by Raman pectroscopy to confirm the presence of reduced graphene oxide (RGO) attached to TiO2 whiskers. Comparative photo electrochemical studies were carried out for TiO2 and reduced graphene oxide modified TiO2 whiskers. Among these, RGO modified TiO2 whiskers show significantly higher photo current density possibly due to enhancement in charge separation ability and longer electron life times.

Graphene's extremely small intrinsic spin-orbit (SO) interaction makes the realization of many interesting phenomena such as topological/quantum spin Hall states and the spin Hall effect (SHE) practically impossible. Recently, it was predicted that the introduction of adatoms in graphene would enhance the SO interaction by the conversion of sp 2 to sp 3 bonds. However, introducing adatoms and yet keeping graphene metallic, that is, without creating electronic (Anderson) localization, is experimentally challenging. Here, we show that the controlled addition of small amounts of covalently bonded hydrogen atoms is sufficient to induce a colossal enhancement of the SO interaction by three orders of magnitude. This results in a SHE at zero external magnetic fields at room temperature, with non-local spin signals up to 100 Ω; orders of magnitude larger than in metals. The non-local SHE is, further, directly confirmed by Larmor spin-precession measurements. From this and the length dependence of the non-local signal we extract a spin relaxation length of ∼1 μm, a spin relaxation time of ∼ 90 ps and a SO strength of 2.5 meV. © 2013 Macmillan Publishers Limited. All rights reserved.

The temperature-dependent thermal expansion coefficient of graphene is estimated for as-grown chemical vapor deposited graphene using temperature-dependent Raman spectroscopy. For as-grown graphene on copper, the extent of thermal expansion mismatch between substrate and the graphene layer is significant across the entire measured temperature interval, T = 90-300 K. This mismatch induces lattice strain in graphene. However, graphene grown on copper substrates has a unique morphology in the form of quasi-periodic nanoripples. This crucially influences the profile of the strain in the graphene membrane, which is uniaxial. An estimate of the thermal expansion coefficient of graphene α(T) is obtained after consideration of this strain profile and after incorporating temperature-dependent Grüneisen parameter corrections. The value of α(T), is found to be negative (average value, -3.75 × 10-6 K-1) for the entire temperature range and it approaches close to zero for T < 150 K. For graphene wet-transferred to three kinds of substrates: copper, poly-dimethylsiloxane, and SiO2/Si, the Raman shifts can largely be modeled with lattice expansion and anharmonic contributions, and the data suggests limited interfacial interaction with the substrate.

Graphene and h-BN are known to be permeating for thermal protons, molybdenum disulphide (MoS2) is found to block them. We report permeation of highly energetic hydrogen atoms produced from electron beam irradiation of hydrogen silsesquioxane resist through MoS2 layers on graphene platform. Single-layer graphene has been used as a detector to estimate permeation. Extent of hydrogenation of graphene, estimated from Raman spectroscopy studies, was used to quantify the permeation of hydrogen atoms through monolayer, tri-layer and bulk MoS2. A decrease in defect density induced in graphene was observed with increasing layer numbers of MoS2. This data was used to estimate the absorption coefficient of MoS2 using the defect density n D , with a value close to 0.06 Å-1. The percentage transmission of hydrogen atoms was found to be 65% per layer.

A variety of device applications has been proposed using polyoxometalate-based ionic liquids. However, the assembly of large polyoxometalate ions on surfaces and the associated interfacial properties are not well understood, particularly since the assembly is influenced by steric effects and stronger ion-ion interactions. In this study, graphene transistors gated with a polyoxometalate-based ionic liquid were probed with in situ Raman spectroscopy and charge transport studies. The ionic liquid comprised Cu-substituted lacunary Keggin anions, [PW11O39Cu]5-, which were surrounded by tetraoctyl ammonium cations, (C32H68N)+. The application of gate voltage caused these ions to assemble at the interface with graphene, which resulted in a shift of the Fermi level of the graphene monolayer grown on a copper foil. The shift was determined by the quantum capacitance, Cq, of graphene in series with the electric-double layer capacitance. Estimates of the electric-double layer thickness, spatial density of the ions and temporal rate of the assembly of the electric double-layer were obtained. This study provides insights into the microscopic understanding of the electric double-layer formation at the graphene interface.

In this paper, a wet-dry hybrid technique to transfer patterned reduced graphene oxide (rGO) thin film to arbitrary substrates at predetermined locations without using any chemicals is reported. The transfer process involves water-assisted delamination of rGO, followed by dry transfer to an acceptor substrate using viscoelastic stamp. Patterned reduced graphene oxide films are transferred to silicon dioxide (SiO2/Si) substrate to begin with. Subsequently, the method is deployed to transfer rGO to different polymer substrates such as poly(methyl methacrylate) (PMMA), and crosslinked poly(4-vinylphenol) (c-PVP), which are commonly used as gate dielectric in flexible electronic applications. The credibility of the transfer process with precise spatial positioning on the target substrate leads to fabrication of freely suspended reduced graphene oxide membrane towards nanoelectromechanical systems (NEMS) based devices such as nanomechanical drum resonators.

Advances in large-area graphene synthesis via chemical vapour deposition on metals like copper were instrumental in the demonstration of graphene-based novel, wafer-scale electronic circuits and proof-of-concept applications such as flexible touch panels. Here, we show that graphene grown by chemical vapour deposition on copper is equally promising for spintronics applications. In contrast to natural graphene, our experiments demonstrate that chemically synthesized graphene has a strong spin-orbit coupling as high as 20â ‰meV giving rise to a giant spin Hall effect. The exceptionally large spin Hall angle ∼0.2 provides an important step towards graphene-based spintronics devices within existing complementary metal-oxide-semiconductor technology. Our microscopic model shows that unavoidable residual copper adatom clusters act as local spin-orbit scatterers and, in the resonant scattering limit, induce transverse spin currents with enhanced skew-scattering contribution. Our findings are confirmed independently by introducing metallic adatoms-copper, silver and gold on exfoliated graphene samples. © 2014 Macmillan Publishers Limited.

We report the swelling kinetics and evolution of the electrical charge transport in poly(3,4-ethylene dioxythiophene) polystyrene sulfonate (PEDOT:PSS) thin films subjected to water vapor. Polymer films swell by the diffusion of water vapor and are found to undergo structural relaxations. Upon exposure to water vapor, primarily the hygroscopic PSS shell, which surrounds the conducting PEDOT-rich cores, takes up water vapor and subsequently swells. We found that the degree of swelling largely depends on the PEDOT to PSS ratio. Swelling driven microscopic rearrangement of the conducting PEDOT-rich cores in the PSS matrix strongly influences the electrical charge transport of the polymer film. Swelling induced increase as well as decrease of electrical resistance are observed in polymer films having different PEDOT to PSS ratio. This anomalous charge transport behavior in PEDOT:PSS films is reconciled by taking into account the contrasting swelling behavior of the PSS and the conducting PEDOT-rich cores leading to spatial segregation of PSS in films with PSS as a minority phase and by a net increase in mean separation between conducting PEDOT-rich cores for films having abundance of PSS.

In this report we study the effect of annealing temperature on the structural stability, phase transformation and photo-electrochemical performance of TiO2 multi-leg nanotubes. Multi-leg nanotubes were synthesized using electrochemical anodization method, and as synthesized nanotubes were annealed in air at the temperatures ranging from 500 to 900 °C.We observed that multi-leg morphology is preserved upto 900 °C and a dominant rutile phase is observed at this temperature. X-ray diffraction and Raman spectra measurements were carried out to study the crystallite size, phase transformation and phonon confinement effects. The multi-leg nanotubes annealed at 900 °C shows enhanced photo-electrochemical performance; this is attributed to mixed phase, increase in crystallite size and decrease in defects.