Manu Jaiswal

Toward Wafer Scale Fabrication of Graphene Based Spin Valve Devices

We demonstrate injection, transport, and detection of spins in spin valve arrays patterned in both copper based chemical vapor deposition (Cu-CVD) synthesized wafer scale single layer and bilayer graphene. We observe spin relaxation times comparable to those reported for exfoliated graphene samples demonstrating that chemical vapor deposition specific structural differences such as nanoripples do not limit spin transport in the present samples. Our observations make Cu-CVD graphene a promising material of choice for large scale spintronic applications.

High-gain graphene-titanium oxide photoconductor made from inkjet printable ionic solution.

As an alternative to such layer-by-layer assembled fi lms, which require multiple dipping cycles in different solutions, it is interesting to consider whether an optically transparent and electrically conducting matrix comprising of homogeneously dispersed TiO 2 and graphene can be synthesized. Ideally this composite blend would be derived from a homogenized mixture that acts as a precursor source and that can be spin-coated or inkjet-printed onto any substrate to be thermally sintered into a transparent conductor or light-harvesting photoconductor fi lm. Here, we show that a conventional sol-gel chemistry approach used previously for the formation of graphene-silica composites [ 6 ] is not applicable to GO-TiO 2 systems. Instead, we report a strategy based on blending GO sheets with a titanium hydroxide-based ionic salt to produce a chemically tunable graphene-TiO 2 composite, which can be used as a printable photodetector. The fabricated structure exploits the desirable charge injection and separation properties at dispersed heterojunctions, and thus opens a widely applicable fabrication strategy for graphene-based composites in photoconductors, sensors, and photovoltaics. Looking at their chemical structure, GO sheets are wellsuited to blending with titanium alkoxide precursors in solgel synthesis because of their water solubility and hydrogenbonding ability (Schematic 1 a). GO sheets are composed of planar, graphene-like aromatic domains and the basal planes and edges are decorated by hydroxyl, epoxy, ether, or carboxylic groups. [ 7 ] These hydroxyl functionalities in GO can participate in oxoor hydroxobridges with metal centers. The formation of titanium oxide in sol-gel synthesis involves interconnecting

A bioelectronic platform using a graphene-lipid bilayer interface.

The electronic properties of graphene can be modulated by charged lipid bilayer adsorbing on the surface. Biorecognition events which lead to changes in membrane integrity can be monitored electrically using an electrolyte-gated biomimetic membrane-graphene transistor. Here, we demonstrate that the bactericidal activity of antimicrobial peptides can be sensed electrically by graphene based on a complex interplay of biomolecular doping and ionic screening effect.

Controlled Hydrogenation of Graphene Sheets and Nanoribbons

The electronic properties of graphene sheets and nanoribbons with different degrees of hydrogenation have been investigated using a combination of charge transport and Raman spectroscopy experiments. The field-effect transistor mobility of graphene is shown to be highly sensitive to the treatment time during atomic hydrogen dose and follows an exponential decrease with time. Raman spectroscopy demonstrates linearly increasing defect-band intensity, and when considered together with transport data, the relationship between graphene mobility and the crystalline size of intact sp(2) carbon regions can be derived. Further, the increase in width of the voltage plateau for monolayer and bilayer graphene points to the formation of midgap states. For partially hydrogenated graphene, the temperature-dependent transport in these states shows a weak insulating behavior. A comparison of Raman spectrum and conductivity data of partially hydrogenated monolayer and bilayer graphene suggests that the latter is also quite susceptible to adsorption of hydrogen atoms, despite a stiffer lattice structure.

Correlation of morphology and charge transport in poly(3,4-ethylenedioxythiophene)–polystyrenesulfonic acid (PEDOT–PSS) films

A wide variation in the charge transport properties of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) films is attributed to the degree of phase segregation of the excess insulating polyanion. The results indicate that the charge transport in PEDOT-PSS can vary from hopping to the critical regime of the metal-insulator transition, depending on the subtle details of morphology. The extent of electrical connectivity in the films, directly obtained from a temperature-dependent high-frequency transport study, indicates various limiting factors to the transport, which are correlated with the phase separation process. The low temperature magnetotransport further supports this morphology-dependent transport scenario.

Graphene transport at high carrier densities using a polymer electrolyte gate

We report the study of graphene devices in Hall-bar geometry, gated with a polymer electrolyte. High densities of 6 × 10 13 /cm 2 are consistently reached, significantly higher than with conventional back-gating. The mobility follows an inverse dependence on density, which can be correlated to a dominant scattering from weak scatterers. Furthermore, our measurements show a Bloch-Gruneisen regime until 100 K (at 6.2 × 10 13 /cm 2 ), consistent with an increase of the density. Ubiquitous in our experiments is a small upturn in resistivity around 3 × 10 13 /cm 2 , whose origin is discussed. We identify two potential causes for the upturn: the renormalization of Fermi velocity and an electrochemically enhanced scattering rate. editors choice Copyright c EPLA, 2010

Giant spin Hall effect in graphene grown by chemical vapour deposition

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 20 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.

Electronic Properties of Nanodiamond Decorated Graphene

The electronic properties of graphene sheets decorated with nanodiamond (ND) particles have been investigated. The chemical fusion of ND to the graphene lattice creates pockets of local defects with robust interfacial bonding. At the ND-bonded regions, the atoms of graphene lattice follow sp(3)-like bonding, and such regions play the role of conduction bottlenecks for the percolating sp(2) graphene network. The low-temperature charge transport reveals an insulating behavior for the disordered system associated with Anderson localization for the charge carriers in graphene. A large negative magnetoresistance is observed in this insulating regime, and its origin is discussed in the context of magnetic correlations of the localized charge carriers with local magnetic domains and extrinsic metal impurities associated with the ND.

Charge transport in transparent conductors: A comparison

A comparative investigation of charge transport properties is presented, for polymeric [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)], single-wall carbon nanotube (SWNT) and inorganic (indium tin oxide, ITO), transparent conducting electrodes. The polymeric and nanotube systems show hopping transport at low temperatures, in contrast with the disordered-metal transport in ITO. The low temperature magnetotransport (up to 11 T) and high electric-field transport (up to 500 V/cm) indicate the significant role of nanoscopic scale disorder for charge transport in polymer and nanotube based systems. The results show that characteristic length scales like localization length correlates with the nanomorphology in these systems. Further, the high frequency conductivity measurements (up to 30 MHz) in PEDOT:PSS and SWNT follow the extended pair approximation model [σ(ω)=σ(0)[1+(ω/ω0)s].

Charge transport in transparent single-wall carbon nanotube networks

We report the electric-field effects and magnetotransport in transparent networks of single-wall carbon nanotubes (SWNT). The temperature dependence of conductance of the network indicates a 2D Mott variable-range hopping (VRH) transport mechanism. Electric field and temperature are shown to have similar effects on the carrier hops and identical exponents for the conductance of the network are obtained from the high electric field and temperature dependences. A power-law temperature dependence with an exponent 3/2 for the threshold field is obtained and explained as a result of the competing contributions from electric field and phonons to the carrier hop. A negative magnetoresistance (MR) is observed at low temperatures, which arises from a forward interference scattering mechanism in the weak scattering limit, consistent with the VRH transport.