Vikas Berry

Biocompatible, robust free-standing paper composed of a TWEEN/graphene composite.

Nonspecific binding (NSB), a random adsorption of biocomponents such as proteins and bacteria on noncomplementary materials,isoneofthebiggestproblemsinbiological applications including biosensors, protein chips, surgical instruments, drug delivery, and biomedicine. Polyoxyethylene sorbitan laurate (TWEEN), a commercially available chemical with aliphatic ester chains, has shown promise as a medical material and in overcomingproblems associated withNSB. [1‐4] However,stability during solution-based processing and uniformity of the materials that have TWEEN coating on flat substrates or nanomaterials using the selfassembled-monolayer (SAM) method has been an important issue. Further, biocompatible materials with high strength are important for several medical applications including stents, nail implants, and strong invasive instruments. Here, we present the production of a free-standing ‘‘paperlike’’ material composed of TWEEN and reduced graphene oxide (RGO) platelets and obtained by simple filtration of a homogeneous aqueous colloidal suspension of TWEEN/RGO hybrid. The ‘‘TWEEN paper’’ was highly stable in water without leakage of TWEEN and is compliant and sufficiently robust to be handled by hand without breaking. Furthermore, the TWEEN paper was noncytotoxic to three mammalian cell lines and biocompatible, inhibiting nonspecific binding of Gram-positive bacteria. [5] In contrast, RGO paper without TWEEN showed nonspecific bacterial binding. TWEEN is composed ofthree chemical parts (Fig. 1a): aliphatic esterchains that can prevent NSB ofbiomolecules, three-terminal hydroxyl groups that are hydrophilic and can be chemically modified for further applications, and an aliphatic chain that can easily be adsorbed on a hydrophobic surface by noncovalent interaction. Protein microarrays on flat substrates with SAM of TWEEN [4] and highly sensitive biosensors, [1‐3] built using field-effect transistor (FET) behavior of individual carbon nanotube (CNT) strands coated with TWEEN, have demonstrated that TWEEN can be effectively used to overcome NSB.

Controlled, Defect-Guided, Metal-Nanoparticle Incorporation onto MoS2 via Chemical and Microwave Routes: Electrical, Thermal, and Structural Properties

Ultrathin (0.3-3 nm) metal dichalcogenides exhibit confinement of carriers, evolution of band-structure and photophysical properties with thickness, high on/off rectification (in MoS2, WS2, and so forth) and high thermal absorption. Here, we leverage the stable sulfur/nobel-metal binding to incorporate highly capacitive gold nanoparticles (Au NPs) onto MoS2 to raise the effective gate-voltage by an order of magnitude. Functionalization is achieved via both diffusion limited aggregation and instantaneous reaction arresting (using microwaves) with selective deposition on crystallographic edges (with 60° displacement). The electrical, thermal, and Raman studies show a highly capacitive interaction between Au NP and MoS2 flakes (CAu-MoS2 = 2.17 μF/cm(2)), a low Schottky barrier (14.52 meV), a reduced carrier-transport thermal-barrier (253 to 44.18 meV after Au NP functionalization), and increased thermal conductivity (from 15 to 23 W/mK post NP deposition). The process could be employed to attach electrodes to heterostructures of graphene and MoS2, where a gold film could be grown to act as an electron-tunneling gate-electrode connected to MoS2.

How Do the Electrical Properties of Graphene Change with its Functionalization

Functionalization of graphene is essential to interface it with other moieties to expand the scope of its electrical/electronic applications. However, chemical functionalization and/or molecular interactions on graphene sensitively modulate its electrical properties. To evaluate and take advantage of the properties of functionalized graphene, it is important to understand how its electrical attributes (such as carrier scattering, carrier concentration, charge polarity, quantum-capacitance enhanced doping, energy levels, transport mechanisms, and orbital hybridization of energy-bands) are influenced by a change in carbons structural conformation, hybridization state, chemical potential, local energy levels, and dopant/interface coupling induced via functionalization or molecular interactions. Here, a detailed and integrated model describes factors influencing these electrical characteristics of functionalized graphene (covalent bonds, adsorption, π-π bonds, and lattice incorporation). The electrical properties are governed via three mechanisms: (a) conversion of carbons hybridized state, (b) dipole interactions enhanced via quantum capacitance, and (c) orbital hybridization with an interfacing molecule. A few graphenic materials are also identified where further studies are essential to understand the effect of their functionalization.

Impermeable Graphenic Encasement of Bacteria

Transmission electron microscopy (TEM) of hygroscopic, permeable, and electron-absorbing biological cells has been an important challenge due to the volumetric shrinkage, electrostatic charging, and structural degradation of cells under high vacuum and fixed electron beam.(1-3) Here we show that bacterial cells can be encased within a graphenic chamber to preserve their dimensional and topological characteristics under high vacuum (10(-5) Torr) and beam current (150 A/cm(2)). The strongly repelling π clouds in the interstitial sites of graphenes lattice(4) reduces the graphene-encased-cells permeability(5) from 7.6-20 nm/s to 0 nm/s. The C-C bond flexibility(5,6) enables conformal encasement of cells. Additionally, graphenes high Youngs modulus(6,7) retains cells structural integrity under TEM conditions, while its high electrical(8) and thermal conductivity(9) significantly abates electrostatic charging. We envision that the graphenic encasement approach will facilitate real-time TEM imaging of fluidic samples and potentially biochemical activity.

Nanotomy-based production of transferable and dispersible graphene nanostructures of controlled shape and size

Because of the edge states and quantum confinement, the shape and size of graphene nanostructures dictate their electrical, optical, magnetic and chemical properties. The current synthesis methods for graphene nanostructures do not produce large quantities of graphene nanostructures that are easily transferable to different substrates/solvents, do not produce graphene nanostructures of different and controlled shapes, or do not allow control of GN dimensions over a wide range (up to 100 nm). Here we report the production of graphene nanostructures with predetermined shapes (square, rectangle, triangle and ribbon) and controlled dimensions. This is achieved by diamond-edge-induced nanotomy (nanoscale-cutting) of graphite into graphite nanoblocks, which are then exfoliated. Our results show that the edges of the produced graphene nanostructures are straight and relatively smooth with an I(D)/I(G) of 0.22-0.28 and roughness <1 nm. Further, thin films of GN-ribbons exhibit a bandgap evolution with width reduction (0, 10 and ~35 meV for 50, 25 and 15 nm, respectively).

High‐Throughput, Ultrafast Synthesis of Solution‐ Dispersed Graphene via a Facile Hydride Chemistry

Graphene is a single-atom-thick two-dimensional macromolecule with sp-bound carbon atoms arranged in a honeycomb lattice. Recently, graphene has emerged as an attractive candidate for several applications, including ultrafast nanoelectronic devices, tunable spintronics, ultracapacitors, transparent conducting electrodes, single-molecule chemical sensors, ultrasensitive biodevices, and nanomechanical devices. These applications have evolved from its atypical properties, such as weakly scattered ballistic transport of charge carriers behaving as massless fermions at room temperature, magneto-sensitive transport, tunable bandgap, quantum Hall effect at room temperature, tunable optical transitions, exceptional mechanical strength, megahertz characteristic frequency, carrier collimation, and ultrahigh stiffness. Graphene can be 1) synthesized on-substrate, 2) deposited on-substrate via mechanical processes, or 3) deposited onsubstrate from solution. On-substrate synthesis includes hightemperature (>1000 8C) epitaxial growth on SiC, ruthenium or chemical vapor deposition on nickel and copper, while mechanical deposition includes adhesive-tape exfoliation of highly oriented pyrolytic graphite (HOPG) and the ensuing transfer. The third process, which is based on onsubstrate deposition from a graphene suspension, has several advantages including the large-scale production of reduced graphene oxide (RGO) and easy-to-apply chemical and physical manipulations for functionalization and directed deposition. Graphene suspension synthesis methods include 1) p–p intercalation or graphite intercalation compound (GIC)-based exfoliation of graphite flakes into graphene sheets, and 2) in-solution reduction of graphite oxide prepared by Hummers method with hydrazine. The p–p intercalation and GIC-based methods produce highquality graphene; however, the yield is low with relatively low stability of the graphene solution, in which the graphene sheets have a tendency to settle down. The graphene suspension

Electron-tunneling modulation in percolating network of graphene quantum dots: fabrication, phenomenological understanding, and humidity/pressure sensing applications.

The two-dimensional (2D) electron cloud, flexible carbon-carbon bonds, chemical modifiability, and size-dependent quantum-confinement and capacitance makes graphene nanostructures (GN) a widely tunable material for electronics. Here we report the oxidation-led edge-roughening and cleavage of long graphene nanoribbons (GNRs) (150 nm wide) synthesized via nanotomy (nanoscale cutting) of graphite (with 2 nm edged diamond knife) to produce graphene quantum dots (GQD). These GQDs (~100-200 nm) selectively interfaced with polyelectrolyte microfiber (diameter = 2-20 μm) form an electrically percolating-network exhibiting a characteristic Coulomb blockade signature with a dry tunneling distance of 0.58 nm and conduction activation energy of 3 meV. We implement this construct to demonstrate the functioning of humidity and pressure sensors and outline their governing model. Here, a 0.36 nm decrease in the average tunneling-barrier-width between GQDs (tunneling barrier = 5.11 eV) increases the conductivity of the device by 43-fold. These devices leverage the modulation in electron tunneling distances caused by pressure and humidity induced water transport across the hygroscopic polymer microfiber (Henrys constant = 0.215 Torr(-1)). This is the foremost example of GQD-based electronic sensors. We envision that this polymer-interfaced GQD percolating network will evolve a new class of sensors leveraging the low mass, low capacitance, high conductivity, and high sensitivity of GQD and the interfacial or dielectric properties of the polymer fiber.

Graphene Interfaced with Biological Cells: Opportunities and Challenges.

By interfacing the quantum mechanical properties of nanomaterials with the complex processes in biology, several bio/nano systems have evolved with applications in biosensors, cellular devices, drug delivery, and biophotoluminescence. One recent breakthrough has been the application of graphene, a two-dimensional (2-D) sheet of sp(2) hybridized carbon atoms arranged in a honeycomb lattice, as a sensitive platform for interfacing with biological cells to detect intra- and extracellular phenomena, including cellular excretion and cell membranes potential modulation. In this Perspective, we discuss the recent results on graphene/cell interfacial devices and the principles defining the modulation of charge-carrier properties in graphene and its derivatives via interaction with cellular membranes. Graphenes high sensitivity in these applications evolves from the π-carrier cloud confined within an atom-thick layer, quantum-capacitance-induced doping enhancement, closely spaced electronic bands, and a large surface area. We discuss the effect of the electronegativity of the cell wall and the dynamic changes in its chemical potential on doping specific carriers into graphene. Finally, we discuss the challenges and opportunities of graphene-interfaced biocellular systems.

Large-Area, Transfer-Free, Oxide-Assisted Synthesis of Hexagonal Boron Nitride Films and Their Heterostructures with MoS2 and WS2

Ultrasmooth hexagonal boron nitride (h-BN) can dramatically enhance the carrier/phonon transport in interfaced transition metal dichalcogenides (TMDs), and amplify the effect of quantum capacitance in field-effect gating. All of the current processes to realize h-BN-based heterostructures involve transfer or exfoliation. Rational chemistries and process techniques are still required to produce large-area, transfer-free, directly grown TMDs/BN heterostructures. Here, we demonstrate a novel boron-oxygen chemistry route for oxide-assisted nucleation and growth of large-area, uniform, and ultrathin h-BN directly on oxidized substrates (B/N atomic ratio = 1:1.16 ± 0.03 and optical band gap = 5.51 eV). These intimately interfaced, van der Waals heterostructures of MoS2/h-BN and WS2/h-BN benefit from 6.27-fold reduced roughness of h-BN in comparison to SiO2. This leads to reduction in scattering from roughness and charged impurities, and enhanced carrier mobility verified by an increase in electrical conductivity (5 times for MoS2/h-BN and 2 times for WS2/h-BN). Further, the heterostructures are devoid of wrinkles and adsorbates, which is critical for 2D nanoelectronics. The versatile process can potentially be extrapolated to realize a variety of heterostructures with complex sandwiched 2D electronic circuitry.

Synthesis and Characterization of Amphiphilic Reduced Graphene Oxide with Epoxidized Methyl Oleate

Amphiphilic reduced graphene oxide is obtained by oleo-functionalization with epoxidized methyl oleate (renewable feedstock) using a green process. The excellent diverse solvent-dispersivity of the oleo-reduced amphiphilic graphene and its reduction chemistry are confirmed in this study. Oleo-reduction of amphiphilic graphene is amenable to industrially viable processes to produce future graphene-based polymer composites and systems.