Carter Kittrell

Large-Scale Growth and Characterizations of Nitrogen-Doped Monolayer Graphene Sheets

In-plane heteroatom substitution of graphene is a promising strategy to modify its properties. Doping with electron-donor nitrogen heteroatoms can modulate the electronic properties of graphene to produce an n-type semiconductor. Here we demonstrate the growth of monolayer nitrogen-doped graphene in centimeter-scale sheets using a chemical vapor deposition process with pyridine as the sole source of both carbon and nitrogen. High-resolution transmission microscopy and Raman mapping characterizations indicate that the nitrogen-doped graphene sheets are uniformly monolayered. The existence of nitrogen-atom substitution in the graphene planes was confirmed by X-ray photoelectron spectroscopy. Electrical measurements show that the nitrogen-doped graphene exhibits an n-type behavior, different from pristine graphene. The preparation of large-area nitrogen-doped graphene provides a viable route to modify the properties of monolayer graphene and promote its applications in electronic devices.

Toward the Synthesis of Wafer-Scale Single-Crystal Graphene on Copper Foils

In this research, we constructed a controlled chamber pressure CVD (CP-CVD) system to manipulate graphenes domain sizes and shapes. Using this system, we synthesized large (~4.5 mm(2)) single-crystal hexagonal monolayer graphene domains on commercial polycrystalline Cu foils (99.8% purity), indicating its potential feasibility on a large scale at low cost. The as-synthesized graphene had a mobility of positive charge carriers of ~11,000 cm(2) V(-1) s(-1) on a SiO(2)/Si substrate at room temperature, suggesting its comparable quality to that of exfoliated graphene. The growth mechanism of Cu-based graphene was explored by studying the influence of varied growth parameters on graphene domain sizes. Cu pretreatments, electrochemical polishing, and high-pressure annealing are shown to be critical for suppressing graphene nucleation site density. A pressure of 108 Torr was the optimal chamber pressure for the synthesis of large single-crystal monolayer graphene. The synthesis of one graphene seed was achieved on centimeter-sized Cu foils by optimizing the flow rate ratio of H(2)/CH(4). This work should provide clear guidelines for the large-scale synthesis of wafer-scale single-crystal graphene, which is essential for the optimized graphene device fabrication.

Carbon nanotube-enhanced thermal destruction of cancer cells in a noninvasive radiofrequency field

Single‐walled carbon nanotubes (SWNTs) have remarkable physicochemical properties that may have several medical applications. The authors have discovered a novel property of SWNTs—heat release in a radiofrequency (RF) field—that they hypothesized may be used to produce thermal cytotoxicity in malignant cells.

A seamless three-dimensional carbon nanotube graphene hybrid material.

Graphene and single-walled carbon nanotubes are carbon materials that exhibit excellent electrical conductivities and large specific surface areas. Theoretical work suggested that a covalently bonded graphene/single-walled carbon nanotube hybrid material would extend those properties to three dimensions, and be useful in energy storage and nanoelectronic technologies. Here we disclose a method to bond graphene and single-walled carbon nanotubes seamlessly during the growth stage. The hybrid material exhibits a surface area >2,000 m(2) g(-1) with ohmic contact from the vertically aligned single-walled carbon nanotubes to the graphene. Using aberration-corrected scanning transmission electron microscopy, we observed the covalent transformation of sp(2) carbon between the planar graphene and the single-walled carbon nanotubes at the atomic resolution level. These findings provide a new benchmark for understanding the three-dimensional graphene/single-walled carbon nanotube-conjoined materials.

Selective vibrational excitation by stimulated emission pumping

Stimulated emission pumping (SEP) is a new method for sub‐Doppler spectroscopy and efficient state‐selective preparation of highly excited rotational–vibrational levels of small molecules. The sensitivity of SEP has been examined by exciting the I2(B 0+u–X 1Σ+g) 17–0 P(38) line and stimulating the P(38) and R(36) lines of the 17–11, 17–10, and 17–9 bands. All observed effects when the two laser pulses were temporally separated could be accounted for by a master equation model.

Three-Dimensional Metal–Graphene–Nanotube Multifunctional Hybrid Materials

Graphene was grown directly on porous nickel films, followed by the growth of controlled lengths of vertical carbon nanotube (CNT) forests that seamlessly emanate from the graphene surface. The metal-graphene-CNT structure is used to directly fabricate field-emitter devices and double-layer capacitors. The three-dimensional nanostructured hybrid materials, with better interfacial contacts and volume utilization, can stimulate the development of several energy-efficient technologies.

Spectral diagnosis of atherosclerosis using an optical fiber laser catheter.

This communication demonstrates that fluorescence spectra of human aorta with good S/N ratios can be collected using an optical fiber laser catheter. The performance of this catheter is compared to a non-fiber optic collection system with an equivalent delivery/collection geometry. For a given sample, fluorescence lineshapes obtained using the two systems are identical; differences in peak fluorescence intensity are related to the different collection efficiencies of the two systems. It is shown that the fluorescence lineshape of arterial tissue depends on the delivery/collection geometry of the detection system, and that this is due to the interaction of absorption and fluorescence within the artery wall. This effect is investigated systematically using a specially designed collection system. Results are analyzed qualitatively using a simple, one-dimensional model of tissue fluorescence. With this analysis, we present design requirements for a collection system in which such geometric effects are eliminated, and show that our optical fiber laser catheter satisfies these requirements.

Single wall carbon nanotube fibers extruded from super-acid suspensions: Preferred orientation, electrical, and thermal transport

Fibers of single wall carbon nanotubes extruded from super-acid suspensions exhibit preferred orientation along their axes. We characterize the alignment by x-ray fiber diagrams and polarized Raman scattering, using a model which allows for a completely unaligned fraction. This fraction ranges from 0.17 to 0.05±0.02 for three fibers extruded under different conditions, with corresponding Gaussian full widths at half maximum (FWHM) from 64° to 44°±2°. FWHM, aligned fraction, electrical, and thermal transport all improve with decreasing extrusion orifice diameter. Resistivity, thermoelectric power, and resonant-enhanced Raman scattering indicate that the neat fibers are strongly p doped; the lowest observed ρ is 0.25 mΩ cm at 300 K. High temperature annealing increases ρ by more than 1 order of magnitude and restores the Raman resonance associated with low-energy van Hove transitions, without affecting the nanotube alignment.

Large-Area Bernal-Stacked Bi-, Tri-, and Tetralayer Graphene

Few-layer graphene, with Bernal stacking order, is of particular interest to the graphene community because of its unique tunable electronic structure. A synthetic method to produce such large area graphene films with precise thickness from 2 to 4 layers would be ideal for chemists and physicists to explore the promising electronic applications of these materials. Here, large-area uniform Bernal-stacked bi-, tri-, and tetralayer graphene films were successfully synthesized on a Cu surface in selective growth windows, with a finely tuned total pressure and CH(4)/H(2) gas ratio. On the basis of the analyses obtained, the growth mechanism is not an independent homoexpitaxial layer-by-layer growth, but most likely a simultaneous-seeding and self-limiting process.

Green carbon as a bridge to renewable energy.

A green use of carbon-based resources that minimizes the environmental impact of carbon fuels could allow a smooth transition from fossil fuels to a sustainable energy economy.