Hannes C. Schniepp

Functionalized graphene sheets for polymer nanocomposites

Polymer-based composites were heralded in the 1960s as a new paradigm for materials. By dispersing strong, highly stiff fibres in a polymer matrix, high-performance lightweight composites could be developed and tailored to individual applications. Today we stand at a similar threshold in the realm of polymer nanocomposites with the promise of strong, durable, multifunctional materials with low nanofiller content. However, the cost of nanoparticles, their availability and the challenges that remain to achieve good dispersion pose significant obstacles to these goals. Here, we report the creation of polymer nanocomposites with functionalized graphene sheets, which overcome these obstacles and provide superb polymer-particle interactions. An unprecedented shift in glass transition temperature of over 40 degrees C is obtained for poly(acrylonitrile) at 1 wt% functionalized graphene sheet, and with only 0.05 wt% functionalized graphene sheet in poly(methyl methacrylate) there is an improvement of nearly 30 degrees C. Modulus, ultimate strength and thermal stability follow a similar trend, with values for functionalized graphene sheet- poly(methyl methacrylate) rivaling those for single-walled carbon nanotube-poly(methyl methacrylate) composites.

Methods of graphite exfoliation

For applications of two-dimensional graphene, commercially viable sources are necessary. Exfoliation from bulk, stacked graphite is the most economical way to achieve large quantities of single layer graphene. A number of methods have been developed to achieve exfoliation of graphite, each with advantages and disadvantages. In this review, we describe current exfoliation methods and techniques used to produce single layer materials from graphite precursors.

Bending Properties of Single Functionalized Graphene Sheets Probed by Atomic Force Microscopy

We probe the bending characteristics of functionalized graphene sheets with the tip of an atomic force microscope. Individual sheets are transformed from a flat into a folded configuration. Sheets can be reversibly folded and unfolded multiple times, and the folding always occurs at the same location. This observation suggests that the folding and bending behavior of the sheets is dominated by pre-existing kink (or even fault) lines consisting of defects and/or functional groups.

Shear-Induced Self-Assembly of Native Silk Proteins into Fibrils Studied by Atomic Force Microscopy

Noncontact mode atomic force microscopy was used to investigate native silk proteins prepared in different ways. Low protein concentrations revealed that single protein molecules exhibit a simple, round shape with apparent diameters of 20-25 nm. Shearing the native protein solutions after extraction from the gland and prior to drying led to a beads-on-a-string assembly at the nanometer scale. Protein concentration had a significant effect on the morphology of the protein assemblies. At higher protein concentrations, shear-induced alignment into nanofibrils was observed, while lower concentrations lead to the formation of much thinner fibrils with a width of about 8 nm.

Large Scale Thermal Exfoliation and Functionalization of Boron Nitride

However, for large-scale applications, such as nanofi llers in polymer composites, exfoliation is a more economically attractive route to single sheet hBN. Previous attempts to produce hBN nano fi llers include the work of who Zhi used the sonication of hBN in N, Ndimethylformamide (DMF) to produce layered hBN with the functionality of the sheets not determined due to diffi culties in obtaining FTIR spectra. [ 5 ]

Stabilization of graphene sheets by a structured benzene/hexafluorobenzene mixed solvent.

Applications requiring pristine graphene derived from graphite demand a solution stabilization method that utilizes an easily removable media. Using a combination of molecular dynamics simulations and experimental techniques, we investigate the solublization/suspension of pristine graphene sheets by an equimolar mixture of benzene and hexafluorobenzene (C(6)H(6)/C(6)F(6)) that is known to form an ordered structure solidifying at 23.7 °C. Our simulations show that the graphene surface templates the self-assembly of the mixture into periodic layers extending up to 30 Å from both sides of the graphene sheet. The solvent structuring is driven by quadrupolar interactions and consists of stacks of alternating C(6)H(6)/C(6)F(6) molecules rising from the surface of the graphene. These stacks result in density oscillations with a period of about 3.4 Å. The high affinity of the 1:1 C(6)H(6)/C(6)F(6) mixture with graphene is consistent with observed hysteresis in Wilhelmy plate measurements using highly ordered pyrolytic graphite (HOPG). AFM, SEM, and TEM techniques verify the state of the suspended material after sonication. As an example of the utility of this mixture, graphene suspensions are freeze-dried at room temperature to produce a sponge-like morphology that reflects the structure of the graphene sheets in solution.

Inhibition and Promotion of Copper Corrosion by CTAB in a Microreactor System

We report on an optical microscopy technique for the analysis of corrosion kinetics of metal thin films in microreactor systems and use it to study the role of cetyltrimethylammonium bromide surfactant as a corrosion inhibitor in a copper-gold galvanic coplanar microsystem. A minimum in the dissolution rate of copper is observed when the surfactant concentration is approximately 0.8 mM. To explain why the inhibitory role of the surfactant does not extend to higher concentrations, we use zero resistance ammetry with separated half cells and show that while the surfactant inhibits cathodic reactions on gold, it also promotes the corrosion of copper because of the catalytic action of bromide counterions. These two competing processes lead to the observed minimum in the dissolution rate.

Tuning of structural color using a dielectric actuator and multifunctional compliant electrodes.

We have developed electrically conducting silicone elastomer nanocomposites that serve both as compliant electrodes in an electrostatic actuator and, at the same time, as optically active elements creating structural color. We demonstrate the capabilities of our setup by actuating an elastomeric diffraction grating and colloidal-crystal-based photonic structures.

Spontaneous emission in nanoscopic dielectric particles

We report on theoretical studies of the inhibition of the spontaneous emission process in subwavelength dielectric media. We discuss the modification of the spontaneous emission rate as a function of the size and shape of the medium as well as the position of the emitter in it.

Surface oxide net charge of a titanium alloy: Comparison between effects of treatment with heat or radiofrequency plasma glow discharge

In the current study, we have compared the effects of heat and radiofrequency plasma glow discharge (RFGD) treatment of a Ti6Al4V alloy on the physico-chemical properties of the alloys surface oxide. Titanium alloy (Ti6Al4V) disks were passivated alone, heated to 600 °C, or RFGD plasma treated in pure oxygen. RFGD treatment did not alter the roughness, topography, elemental composition or thickness of the alloys surface oxide layer. In contrast, heat treatment altered oxide topography by creating a pattern of oxide elevations approximately 50-100 nm in diameter. These nanostructures exhibited a three-fold increase in roughness compared to untreated surfaces when RMS roughness was calculated after applying a spatial high-pass filter with a 200 nm-cutoff wavelength. Heat treatment also produced a surface enrichment in aluminum and vanadium oxides. Both RFGD and heat treatment produced similar increases in oxide wettability. Atomic force microscopy (AFM) measurements of metal surface oxide net charge signified by a long-range force of attraction to or repulsion from a (negatively charged) silicon nitride AFM probe were also obtained for all three experimental groups. Force measurements showed that the RFGD-treated Ti6Al4V samples demonstrated a higher net positive surface charge at pH values below 6 and a higher net negative surface charge at physiological pH (pH values between 7 and 8) compared to control and heat-treated samples. These findings suggest that RFGD treatment of metallic implant materials can be used to study the role of negatively charged surface oxide functional groups in protein bioactivity, osteogenic cell behavior and osseointegration independently of oxide topography.