K. Kohlhaas

Bio: K. Kohlhaas is an academic researcher from Northwestern University. The author has contributed to research in topic(s): Nanowire & Resonance. The author has an hindex of 6, co-authored 11 publication(s) receiving 11416 citation(s). Previous affiliations of K. Kohlhaas include University of South Carolina.

Graphene-based composite materials

Abstract: The remarkable mechanical properties of carbon nanotubes arise from the exceptional strength and stiffness of the atomically thin carbon sheets (graphene) from which they are formed. In contrast, bulk graphite, a polycrystalline material, has low fracture strength and tends to suffer failure either by delamination of graphene sheets or at grain boundaries between the crystals. Now Stankovich et al. have produced an inexpensive polymer-matrix composite by separating graphene sheets from graphite and chemically tuning them. The material contains dispersed graphene sheets and offers access to a broad range of useful thermal, electrical and mechanical properties. Individual sheets of graphene can be readily incorporated into a polymer matrix, giving rise to composite materials having potentially useful electronic properties. Graphene sheets—one-atom-thick two-dimensional layers of sp2-bonded carbon—are predicted to have a range of unusual properties. Their thermal conductivity and mechanical stiffness may rival the remarkable in-plane values for graphite (∼3,000 W m-1 K-1 and 1,060 GPa, respectively); their fracture strength should be comparable to that of carbon nanotubes for similar types of defects1,2,3; and recent studies have shown that individual graphene sheets have extraordinary electronic transport properties4,5,6,7,8. One possible route to harnessing these properties for applications would be to incorporate graphene sheets in a composite material. The manufacturing of such composites requires not only that graphene sheets be produced on a sufficient scale but that they also be incorporated, and homogeneously distributed, into various matrices. Graphite, inexpensive and available in large quantity, unfortunately does not readily exfoliate to yield individual graphene sheets. Here we present a general approach for the preparation of graphene-polymer composites via complete exfoliation of graphite9 and molecular-level dispersion of individual, chemically modified graphene sheets within polymer hosts. A polystyrene–graphene composite formed by this route exhibits a percolation threshold10 of ∼0.1 volume per cent for room-temperature electrical conductivity, the lowest reported value for any carbon-based composite except for those involving carbon nanotubes11; at only 1 volume per cent, this composite has a conductivity of ∼0.1 S m-1, sufficient for many electrical applications12. Our bottom-up chemical approach of tuning the graphene sheet properties provides a path to a broad new class of graphene-based materials and their use in a variety of applications.

Modulus, Fracture Strength, and Brittle vs. Plastic Response of the Outer Shell of Arc-grown Multi-walled Carbon Nanotubes

Abstract: The fracture strengths and elastic moduli of arc-grown multi-walled carbon nanotubes (MWCNTs) were measured by tensile loading inside of a scanning electron microscope (SEM). Eighteen tensile tests were performed on 14 MWCNTs with three of them being tested multiple times (3×, 2×, and 2×, respectively). All the MWCNTs fractured in the “sword-in-sheath” mode. The diameters of the MWCNTs were measured in a transmission electron microscope (TEM), and the outer diameter with an assumed 0.34 nm shell thickness was used to convert measured load-displacement data to stress and strain values. An unusual yielding before fracture was observed in two tensile loading experiments. The 18 outer shell fracture strength values ranged from 10 to 66 GPa, and the 18 Young's modulus values, obtained from a linear fit of the stress–strain data, ranged from 620 to 1,200 GPa, with a mean of 940 GPa. The possible influence of stress concentration at the clamps is discussed.

Mechanics of crystalline boron nanowires

Abstract: The mechanical response of crystalline boron nanowires was studied with the mechanical resonance method and tensile testing. The mechanical resonances of cantilevered boron nanowires were excited and their frequencies were used to obtain the Young’s modulus of the nanowires, according to simple beam theory. The influence of non-ideal boundary conditions on the nanowire’s resonance frequency was investigated and is presented. Tensile loading measurements on boron nanowires were performed to obtain the fracture strength and Young’s modulus. The modulus values from tensile tests are consistent with the set of values obtained from the mechanical resonance tests.

Partially graphitic, high-surface-area mesoporous carbons from polyacrylonitrile templated by ordered and disordered mesoporous silicas

Abstract: Ordered and disordered mesoporous carbons synthesized from polyacrylonitrile using a templating method were heated under argon atmosphere at ∼2470 K to partially graphitize them without the loss of mesoporosity. The high-temperature treatment led to a marked enhancement of graphitic ordering, which manifested itself in a narrowing of wide-angle XRD peaks, and in the appearance of domains of lateral dimensions 5–15 nm, consisting of stacked graphitic planes with interplanar spacing of ∼0.34 nm. Raman spectroscopy provided evidence for the increased content of graphitic sp 2 carbon structures. The specific surface areas and total pore volumes of the carbons were as high as 500–600 m 2 g −1 and 0.8–1.8 cm 3 g −1 , respectively. These carbons had essentially no microporosity and their surface properties were similar to those of a graphitized carbon black Carbopack X. For ordered mesoporous carbons, the high-temperature treatment led to the loss of nanoscale periodicity, broadening of the pore size distribution (PSD) and ∼40% decrease in the mesopore volume. In contrast, PSD and total pore volume of the disordered carbon were essentially unchanged. These results show that the high-temperature treatment of mesoporous carbons from PAN affords partially graphitic carbons with high specific surface areas and large pore volumes.

Carbide-Derived Nanoporous Carbon and Novel Core−Shell Nanowires

Abstract: Carbide-derived carbon (CDC) nanowires (NWs) have been synthesized by the high-temperature treatment of small-diameter β-SiC whiskers with Cl2/H2 A variety of physical measurements indicate that Si was extracted by exposure to Cl2 and that the C in the carbon nanowires is primarily sp2-bonded From BET measurements, the specific surface area of these carbon nanowires is 13 × 103 m2/g and they contain a network of nanopores Nanoindentation measurements indicate that the SiC-derived C is not a stiff material, the elastic modulus being 50 ± 12 GPa High-temperature treatment of the CDC nanowires under an inert gas significantly increases the degree of graphitization In addition, partial extraction was used to obtain core−shell structures having a thin and also very high surface area CDC shell; further treatment at high temperature was used to produce graphitized carbon shell−crystalline SiC core NWs

The rise of graphene

Abstract: Graphene is a rapidly rising star on the horizon of materials science and condensed-matter physics. This strictly two-dimensional material exhibits exceptionally high crystal and electronic quality, and, despite its short history, has already revealed a cornucopia of new physics and potential applications, which are briefly discussed here. Whereas one can be certain of the realness of applications only when commercial products appear, graphene no longer requires any further proof of its importance in terms of fundamental physics. Owing to its unusual electronic spectrum, graphene has led to the emergence of a new paradigm of 'relativistic' condensed-matter physics, where quantum relativistic phenomena, some of which are unobservable in high-energy physics, can now be mimicked and tested in table-top experiments. More generally, graphene represents a conceptually new class of materials that are only one atom thick, and, on this basis, offers new inroads into low-dimensional physics that has never ceased to surprise and continues to provide a fertile ground for applications.

Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide

Abstract: Reduction of a colloidal suspension of exfoliated graphene oxide sheets in water with hydrazine hydrate results in their aggregation and subsequent formation of a high-surface-area carbon material which consists of thin graphene-based sheets. The reduced material was characterized by elemental analysis, thermo-gravimetric analysis, scanning electron microscopy, X-ray photoelectron spectroscopy, NMR spectroscopy, Raman spectroscopy, and by electrical conductivity measurements.

Large-scale pattern growth of graphene films for stretchable transparent electrodes

Abstract: Problems associated with large-scale pattern growth of graphene constitute one of the main obstacles to using this material in device applications. Recently, macroscopic-scale graphene films were prepared by two-dimensional assembly of graphene sheets chemically derived from graphite crystals and graphene oxides. However, the sheet resistance of these films was found to be much larger than theoretically expected values. Here we report the direct synthesis of large-scale graphene films using chemical vapour deposition on thin nickel layers, and present two different methods of patterning the films and transferring them to arbitrary substrates. The transferred graphene films show very low sheet resistance of approximately 280 Omega per square, with approximately 80 per cent optical transparency. At low temperatures, the monolayers transferred to silicon dioxide substrates show electron mobility greater than 3,700 cm(2) V(-1) s(-1) and exhibit the half-integer quantum Hall effect, implying that the quality of graphene grown by chemical vapour deposition is as high as mechanically cleaved graphene. Employing the outstanding mechanical properties of graphene, we also demonstrate the macroscopic use of these highly conducting and transparent electrodes in flexible, stretchable, foldable electronics.

The chemistry of graphene oxide

Abstract: The chemistry of graphene oxide is discussed in this critical review Particular emphasis is directed toward the synthesis of graphene oxide, as well as its structure Graphene oxide as a substrate for a variety of chemical transformations, including its reduction to graphene-like materials, is also discussed This review will be of value to synthetic chemists interested in this emerging field of materials science, as well as those investigating applications of graphene who would find a more thorough treatment of the chemistry of graphene oxide useful in understanding the scope and limitations of current approaches which utilize this material (91 references)

Processable aqueous dispersions of graphene nanosheets

Abstract: Graphene sheets offer extraordinary electronic, thermal and mechanical properties and are expected to find a variety of applications. A prerequisite for exploiting most proposed applications for graphene is the availability of processable graphene sheets in large quantities. The direct dispersion of hydrophobic graphite or graphene sheets in water without the assistance of dispersing agents has generally been considered to be an insurmountable challenge. Here we report that chemically converted graphene sheets obtained from graphite can readily form stable aqueous colloids through electrostatic stabilization. This discovery has enabled us to develop a facile approach to large-scale production of aqueous graphene dispersions without the need for polymeric or surfactant stabilizers. Our findings make it possible to process graphene materials using low-cost solution processing techniques, opening up enormous opportunities to use this unique carbon nanostructure for many technological applications.