Applied Science

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.

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.

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-ona-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.

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.

A detailed analysis of the thermal expansion mechanism of graphite oxide to produce functionalized graphene sheets is provided. Exfoliation takes place when the decomposition rate of the epoxy and hydroxyl sites of graphite oxide exceeds the diffusion rate of the evolved gases, thus yielding pressures that exceed the van der Waals forces holding the graphene sheets together. A comparison of the Arrhenius dependence of the reaction rate against the calculated diffusion coefficient based on Knudsen diffusion suggests a critical temperature of 550°C which must be exceeded for exfoliation to occur. As a result of their wrinkled nature, the functionalized and defective graphene sheets do not collapse back to graphite oxide but are highly agglomerated. After dispersion by ultrasonication in appropriate solvents, statistical analysis by atomic force microscopy shows that 80% of the observed flakes are single sheets.

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 alloy's 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 alloy's 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.

Micellar coatings of surfactants at solid-liquid interfaces can provide colloidal stability, 1,2 corrosion inhibition, 3 and boundary lubrication. Understanding the dynamics of such coatings is necessary to the optimization of self-healing, a characteristic of micellar structures. Self-healing rates that are sufficient to counter the rate of defect formation due to physical or chemical attack are required for successful engineering applications. Although some dynamic parameters of bulk and surface processes have been reported, 7-10 the time scale for micellar surface self-assembly remains unknown. In this work, we present a novel atomic force microscopy (AFM) technique that allows us to visualize this selfhealing process over millisecond time scales at nanometer spatial resolution. No traditional technique used to detect surface adlayers, including neutron reflectometry, 11 optical reflectometry, 12,13 ellipsometry, 14 and Fourier transform infrared spectroscopy (FTIR) 10 combines such high temporal and spatial resolutions. Using our new method, we show that nanometer-sized defects in a crystalline array of surface micelles recover flawlessly in less than ∼6 ms.

We investigate Raman spectra of graphite oxide and functionalized graphene sheets with epoxy and hydroxyl groups and Stone−Wales and 5−8−5 defects by first-principles calculations to interpret our experimental results. Only the alternating pattern of single−double carbon bonds within the sp 2 carbon ribbons provides a satisfactory explanation for the experimentally observed blue shift of the G band of the Raman spectra relative to graphite. To obtain these single−double bonds, it is necessary to have sp 3 carbons on the edges of a zigzag carbon ribbon.

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.

Surfactant micelles form oriented arrays on crystalline substrates although registration is unexpected since the template unit cell is small compared to the size of a rodlike micelle. Interaction energy calculations based on molecular simulations reveal that orientational energy differences on a molecular scale are too small to explain matters. With atomic force microscopy, we show that orientational ordering is a dynamic, multimolecule process. Treating the cooperative processes as a balance between van der Waals torque on a large, rodlike micellar assembly and Brownian motion shows that orientation is favored.

We measure fluorescence lifetimes of emitters embedded in isolated single dielectric nanospheres. By varying the diameters of the spheres from 100 nm to 2 m and by modifying their dielectric surrounding, we demonstrate a systematic change of paradigm in the spontaneous emission rate, as we cross the border from the superwavelength regime of Mie resonances to the nanoscopic realm of Rayleigh scattering. Our data show inhibition of the spontaneous emission up to 3 times and are in excellent agreement with the results of analytical calculations. Spontaneous emission can be described by the interaction of an atomic dipole with the electromagnetic vacuum field [1]. It is therefore possible to modify the decay rate of an atomic excited state by placing it in a confined geometry where the vacuum fluctuations are altered due to reflections from boundaries . Drexhage demonstrated this in 1970 by examining a flat mirror very close to a thin fluorescent layer [3] while many other groups have modified the radiative decay rate of emitters by putting them between two flat reflectors [4], between mirrors of high-finesse optical cavities , and in whispering gallery mode resonators . In addition to superwavelength geometries, one might also expect that the presence of nanometer scale material could lead to the scattering of the vacuum field and therefore modification of the spontaneous emission rate. Indeed, several theoretical reports have predicted this phenomenon for an atom in the near field of nanoscopic spheroids, sharp tips, and substrates with lateral nanostructures [7], as well as for atoms inside subwavelength spheres . Experimentally, a few reports have indicated that the fluorescence lifetime of chromium ions in ruby microcrystals [10], of color centers in diamond nanocrystals , and of emitters in ensembles of colloidal particles [12,13] differ from their bulk values. However, quantitative and conclusive investigations have been lacking. In this Letter we present a systematic demonstration that the spontaneous emission rate of ions placed in dielectric spheres is substantially reduced as one crosses the border from the superwavelength regime of Mie resonances to the nanoscopic realm of Rayleigh scattering.

We demonstrate improved atomic force microscopic imaging of surfactant surface aggregates, featuring an increase in the topography contrast by several hundred percent with respect to all previous studies. Surfactant aggregates on rough gold surfaces, which could not be imaged previously because of low resolution, display substantially different morphologies when compared with atomically smooth materials.

A process is described to produce single sheets of functionalized graphene through thermal exfoliation of graphite oxide. The process yields a wrinkled sheet structure resulting from reaction sites involved in oxidation and reduction processes. The topological features of single sheets, as measured by atomic force microscopy, closely match predictions of first-principles atomistic modeling. Although graphite oxide is an insulator, functionalized graphene produced by this method is electrically conducting.

Surfactant micelles (cetyltrimethylammonium chloride) adsorbed on Au(111) exhibit orientational order dictated by the gold crystal axes. To explain this phenomenon, we take into account the ionic contribution to the dielectric response of the metal. Since the motion of an ion inside the metallic lattice is restricted by its neighbors in an anisotropic way, the total dielectric response of the metal acquires directional dependence. A crystalline substrate is thus able to generate both torque and attraction on geometrically asymmetric objects. Numerical calculations show that the resulting anisotropic van der Waals force is indeed capable of orienting rod-like dielectric micelles on a Au(111) surface.

A number of environmental and patient-related factors contribute to implant failure. A significant fraction of these failures can be attributed to limited osseointegration resulting from poor bone healing responses. The overall goal of this study was to determine whether surface treatment of a titanium-aluminum-vanadium alloy (Ti-6Al-4V) implant material in combination with a biomimetic protein coating could promote the differentiation of attached osteoblastic cells. The specific aims of the study were to investigate whether osteoprogenitor cells cultured on a rigorously cleaned implant specimen showed a normal pattern of differentiation and whether preadsorbed fibronectin accelerated or enhanced osteoblast differentiation. Ti-6Al-4V disks were rigorously cleaned, passivated in nitric acid, and dry heat- sterilized; some of the disks were then coated with 1 nmol/L fibronectin. MC3T3 osteoprogenitor cells were then cultured on the pretreated disks for several weeks. Quantitative real...

A process is described to produce single sheets of functionalized graphene through thermal exfoliation of graphite oxide. The process yields a wrinkled sheet structure resulting from reaction sites involved in oxidation and reduction processes. The topological features of single sheets, as measured by atomic force microscopy, closely match predictions of first-principles atomistic modeling. Although graphite oxide is an insulator, functionalized graphene produced by this method is electrically conducting.

PURPOSE: A number of environmental and patient-related factors contribute to implant failure. A significant fraction of these failures can be attributed to limited osseointegration resulting from poor bone healing responses. The overall goal of this study was to determine whether surface treatment of a titanium-aluminum-vanadium alloy (Ti-6Al-4V) implant material in combination with a biomimetic protein coating could promote the differentiation of attached osteoblastic cells. The specific aims of the study were to investigate whether osteoprogenitor cells cultured on a rigorously cleaned implant specimen showed a normal pattern of differentiation and whether preadsorbed fibronectin accelerated or enhanced osteoblast differentiation.MATERIALS AND METHODS: Ti-6Al-4V disks were rigorously cleaned, passivated in nitric acid, and dry heat- sterilized; some of the disks were then coated with 1 nmol/L fibronectin. MC3T3 osteoprogenitor cells were then cultured on the pretreated disks for several weeks. Quantitative real-time polymerase chain reaction was performed to measure changes over time in the mRNA levels of osteoblast genes.RESULTS: Fibronectin increased the peak expression of all analyzed osteoblast gene markers. "Early" genes that normally mark the proliferative phase (0 to 10 days) of osteoblastic development showed peak expression within the first 10 days after cell attachment to the titanium alloy. In contrast, "late" genes that normally mark the differentiation (10 to 20 days) and mineralization (20 to 36 days) phases of osteoblastogenesis achieved peak expression only after approximately 3 to 4 weeks of culture.CONCLUSIONS: Osteoprogenitors cultured on a rigorously cleaned Ti-6Al-4V alloy were found to demonstrate a normal pattern of osteoblast differentiation. Preadsorbed fibronectin was observed to stimulate osteoblast differentiation during the mineralization phase of osteoblastogenesis.

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 1. 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 2, 3, 4, 5, 6, 7, 8, 9, 10, 11. However, the cost of nanoparticles, their availability and the challenges that remain to ...