Abhay V. Thomas

High Sensitivity Gas Detection Using a Macroscopic Three-Dimensional Graphene Foam Network

Nanostructures are known to be exquisitely sensitive to the chemical environment and offer ultra-high sensitivity for gas-sensing. However, the fabrication and operation of devices that use individual nanostructures for sensing is complex, expensive and suffers from poor reliability due to contamination and large variability from sample-to-sample. By contrast, conventional solid-state and conducting-polymer sensors offer excellent reliability but suffer from reduced sensitivity at room-temperature. Here we report a macro graphene foam-like three-dimensional network which combines the best of both worlds. The walls of the foam are comprised of few-layer graphene sheets resulting in high sensitivity; we demonstrate parts-per-million level detection of NH3 and NO2 in air at room-temperature. Further, the foam is a mechanically robust and flexible macro-scale network that is easy to contact (without Lithography) and can rival the durability and affordability of traditional sensors. Moreover, Joule-heating expels chemisorbed molecules from the foams surface leading to fully-reversible and low-power operation.

Graphene Nanoribbon Composites

It is well established that pristine multiwalled carbon nanotubes offer poor structural reinforcement in epoxy-based composites. There are several reasons for this which include reduced interfacial contact area since the outermost nanotube shields the internal tubes from the matrix, poor wetting and interfacial adhesion with the heavily cross-linked epoxy chains, and intertube slip within the concentric nanotube cylinders leading to a sword-in-sheath type failure. Here we demonstrate that unzipping such multiwalled carbon nanotubes into graphene nanoribbons results in a significant improvement in load transfer effectiveness. For example, at ∼0.3% weight fraction of nanofillers, the Youngs modulus of the epoxy composite with graphene nanoribbons shows ∼30% increase compared to its multiwalled carbon nanotube counterpart. Similarly the ultimate tensile strength for graphene nanoribbons at ∼0.3% weight fraction showed ∼22% improvement compared to multiwalled carbon nanotubes at the same weight fraction of nanofillers in the composite. These results demonstrate that unzipping multiwalled carbon nanotubes into graphene nanoribbons can enable their utilization as high-performance additives for mechanical properties enhancement in composites that rival the properties of singlewalled carbon nanotube composites yet at an order of magnitude lower cost.

Defect-induced plating of lithium metal within porous graphene networks

Lithium metal is known to possess a very high theoretical capacity of 3,842 mAh g(-1) in lithium batteries. However, the use of metallic lithium leads to extensive dendritic growth that poses serious safety hazards. Hence, lithium metal has long been replaced by layered lithium metal oxide and phospho-olivine cathodes that offer safer performance over extended cycling, although significantly compromising on the achievable capacities. Here we report the defect-induced plating of metallic lithium within the interior of a porous graphene network. The network acts as a caged entrapment for lithium metal that prevents dendritic growth, facilitating extended cycling of the electrode. The plating of lithium metal within the interior of the porous graphene structure results in very high specific capacities in excess of 850 mAh g(-1). Extended testing for over 1,000 charge/discharge cycles indicates excellent reversibility and coulombic efficiencies above 99%.

Epoxy Nanocomposites with Two-Dimensional Transition Metal Dichalcogenide Additives

Emerging two-dimensional (2D) materials such as transition metal dichalcogenides offer unique and hitherto unavailable opportunities to tailor the mechanical, thermal, electronic, and optical properties of polymer nanocomposites. In this study, we exfoliated bulk molybdenum disulfide (MoS2) into nanoplatelets, which were then dispersed in epoxy polymers at loading fractions of up to 1% by weight. We characterized the tensile and fracture properties of the composite and show that MoS2 nanoplatelets are highly effective at enhancing the mechanical properties of the epoxy at very low nanofiller loading fractions (below 0.2% by weight). Our results show the potential of 2D sheets of transition metal dichalcogenides as reinforcing additives in polymeric composites. Unlike graphene, transition metal dichalcogenides such as MoS2 are high band gap semiconductors and do not impart significant electrical conductivity to the epoxy matrix. For many applications, it is essential to enhance mechanical properties while also maintaining the electrical insulation properties and the high dielectric constant of the polymer material. In such applications, conductive carbon based fillers such as graphene cannot be utilized. This study demonstrates that 2D transition metal dichalcogenide additives offer an elegant solution to such class of problems.

Controlled Crumpling of Graphene Oxide Films for Tunable Optical Transmittance

The delamination buckling approach provides a facile means to dynamically control the optical transmittance of extremely flexible and stretchable graphene oxide coatings with fast response time. Such graphene oxide coatings can be deposited by scalable solution-processing methods for potential applications in dynamic glazing.

Graphene Drape Minimizes the Pinning and Hysteresis of Water Drops on Nanotextured Rough Surfaces

Previous studies of the interaction of water with graphene-coated surfaces have been limited to flat (smooth) surfaces. Here we created a rough surface by nanopatterning and then draped the surface with a single-layer graphene sheet. We found that the ultrasheer graphene drape prevents the penetration of water into the textured surface thereby drastically reducing the contact angle hysteresis (which is a measure of frictional energy dissipation) and preventing the liquid contact line from getting pinned to the substrate. This has important technological implications since the main obstacle to the motion of liquid drops on rough surfaces is contact angle hysteresis and contact line pinning. Graphene drapes could therefore enable enhanced droplet mobility which is required in a wide range of applications in micro and nanofluidics. Compared to polymer coatings that could fill the cavities between the nano/micropores or significantly alter the roughness profile of the substrate, graphene provides the thinnest (i.e., most sheer) and most conformal drape that is imaginable. Despite its extreme thinness, the graphene drape is mechanically robust, chemically stable, and offers high flexibility and resilience which can enable it to reliably drape arbitrarily complex surface topologies. Graphene drapes may therefore provide a hitherto unavailable ability to tailor the dynamic wettability of surfaces for a variety of applications.

Reduced stability of copper interconnects due to wrinkles and steps on hexagonal boron nitride substrates

There is great scientific and technological interest in the use of chemical-vapor-deposition grown hexagonal boron nitride dielectric substrates for microelectronics applications. This interest stems from its superior heat spreading capability compared to silicon dioxide as well as the lack of surface dangling bonds or charge traps in hexagonal boron nitride which results in superior performance for graphene based electronics devices. However, surface heterogeneities, such as wrinkles or steps, are ubiquitous in such devices due to the fabrication and processing of chemical vapor deposition grown hexagonal boron nitride. In this study, we characterize the effect of such surface heterogeneities on the stability of copper interconnects used in microelectronics devices. Based on the theoretical thermo-physical properties of the constituent thin film layers, our simulations predict that copper interconnects deposited on hexagonal boron nitride can withstand ∼1.9 times more power than on a silicon dioxide subs...