Dynamics of the formation of carbon nanotube serpentines.

Recently, Geblinger et al. [Nat. Nanotechnol. 3, 195 (2008)] reported the experimental realization of carbon nanotube S-like shaped nanostructures, the so-called carbon nanotube serpentines. We report here results from multimillion fully atomistic molecular dynamics simulations of their formation. We consider one-µm-long carbon nanotubes placed on stepped substrates with and without a catalyst nanoparticle on the top free end of the tube. A force is applied to the upper part of the tube during a short period of time and turned off; then the system is set free to evolve in time. Our results show that these conditions are sufficient to form robust serpentines and validates the general features of the "falling spaghetti model" proposed to explain their formation.

In situ atomic force microscopy tip-induced deformations and Raman spectroscopy characterization of single-wall carbon nanotubes.

In this work, an atomic force microscope (AFM) is combined with a confocal Raman spectroscopy setup to follow in situ the evolution of the G-band feature of isolated single-wall carbon nanotubes (SWNTs) under transverse deformation. The SWNTs are pressed by a gold AFM tip against the substrate where they are sitting. From eight deformed SWNTs, five exhibit an overall decrease in the Raman signal intensity, while three exhibit vibrational changes related to the circumferential symmetry breaking. Our results reveal chirality dependent effects, which are averaged out in SWNT bundle measurements, including a previously elusive mode symmetry breaking that is here explored using molecular dynamics calculations.

The use of a Ga+ focused ion beam to modify graphene for device applications.

In this work, we clarify the features of the lateral damage of line defects in single layer graphene. The line defects were produced through well-controlled etching of graphene using a Ga(+) focused ion beam. The lateral damage length was obtained from both the integrated intensity of the disorder induced Raman D band and the minimum ion fluence. Also, the line defects were characterized by polarized Raman spectroscopy. It was found that graphene is resilient under the etching conditions since the intensity of the defect induced Raman D peak exhibits a dependence on the direction of the lines relative to the crystalline lattice and also on the direction of the laser polarization relative to the lines. In addition, electrical measurements of the modified graphene were performed. Different ion fluences were used in order to obtain a completely insulating defect line in graphene, which was determined experimentally by means of charge injection and electric force microscopy measurements. These studies demonstrate that a Ga+ ion column combined with Raman spectroscopy is a powerful technique to produce and understand well-defined periodic arrays of defects in graphene, opening possibilities for better control of nanocarbon devices.

Quantifying defects in graphene via Raman spectroscopy at different excitation energies.

We present a Raman study of Ar(+)-bombarded graphene samples with increasing ion doses. This allows us to have a controlled, increasing, amount of defects. We find that the ratio between the D and G peak intensities, for a given defect density, strongly depends on the laser excitation energy. We quantify this effect and present a simple equation for the determination of the point defect density in graphene via Raman spectroscopy for any visible excitation energy. We note that, for all excitations, the D to G intensity ratio reaches a maximum for an interdefect distance ∼3 nm. Thus, a given ratio could correspond to two different defect densities, above or below the maximum. The analysis of the G peak width and its dispersion with excitation energy solves this ambiguity.

Highly efficient siRNA delivery system into human and murine cells using single-wall carbon nanotubes.

Development of RNA interference (RNAi) technology utilizing short interfering RNA sequences (siRNA) has focused on creating methods for delivering siRNAs to cells and for enhancing siRNA stability in vitro and in vivo. Here, we describe a novel approach for siRNA cellular delivery using siRNA coiling into carboxyl-functionalized single-wall carbon nanotubes (SWCNTs). The CNT-siRNA delivery system successfully demonstrates nonspecific toxicity and transfection efficiency greater than 95%. This approach offers the potential for siRNA delivery into different types of cells, including hard-to-transfect cells, such as neuronal cells and cardiomyocytes. We also tested the CNT-siRNA system in a non-metastatic human hepatocellular carcinoma cell line (SKHep1). In all types of cells used in this work the CNT-siRNA delivery system showed high efficiency and apparent no side effects for various in vitro applications.

Synthesis, electronic structure, and Raman scattering of phosphorus-doped single-wall carbon nanotubes.

Substitutional phosphorus doping in single-wall carbon nanotubes (SWNTs) is investigated by density functional theory and resonance Raman spectroscopy. Electronic structure calculations predict charge localization on the phosphorus atom, generating nondispersive valence and conduction bands close to the Fermi level. Besides confirming sustitutional doping, accurate analysis of electron and phonon renormalization effects in the double-resonance Raman process elucidates the different nature of the phosphorus donor doping (localized) when compared to nitrogen substitutional doping (nonlocalized) in SWNTs.

Diameter dependence of the dielectric constant for the excitonic transition energy of single-wall carbon nanotubes.

The measured optical transition energies Eii of single-wall carbon nanotubes are compared with bright exciton energy calculations. The Eii differences between experiment and theory are minimized by considering a diameter-dependent dielectric constant kappa, which comprises the screening from the tube and from the environment. Different kappa dependencies are obtained for (E11(S), E22(S), E11(M)) relative to (E33(S), E44(S)). A changing environment changes the kappa diameter dependence for (E11(S), E22(S), E11(M)), but for (E33(S), E44(S)) the environmental effects are minimal. The resulting calculated exciton energies reproduce experimental E_{ii} values within +/-70 meV for a diameter range (0.7

Mechanism of near-field Raman enhancement in one-dimensional systems.

We develop a theory of near-field Raman enhancement in one-dimensional systems, and report supporting experimental results for carbon nanotubes. The enhancement is established by a laser-irradiated nanoplasmonic structure acting as an optical antenna. The near-field Raman intensity is inversely proportional to the 10th power of the separation between the enhancing structure and the one-dimensional system. Experimental data obtained from single-wall carbon nanotubes indicate that the Raman enhancement process is not significantly influenced by the specific phonon eigenvector, and is mainly defined by the properties of the nanoplasmonic structure.

Decarboxylation of oxidized single-wall carbon nanotubes.

A classical protocol widely used in organic chemistry of aromatic and polyaromatic molecules has been successfully applied in this work for the decarboxylation of oxidized single-wall carbon nanotube (SWNT) to rend C-H SWNT derivatives. SWNT produced by arc discharge method have been oxidized during a purification process using strongly oxidant agents, such as hydrogen peroxide and nitric acid. The decarboxylation of oxidized SWNT has been conduced with copper(I) oxide in a 50:50 solution of N-methylpyrrolidone and quinoline. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and acid-base potentiometric titration analyses were carried out to characterize quali and quantitatively the changes in the chemical environment on the SWNT surface in each step of the purification and the decarboxylation process. Those techniques showed the appearance of mainly carboxylic and phenolic groups after the purification process and the disappearance of the carboxylic groups after the decarboxylation reaction. Fourier transform infrared spectroscopy analysis indicated also the formation of aliphatic and aromatic C-H groups. X-ray photoelectron spectroscopy and potentiometric titration results determined an efficiency higher than 90% for our decarboxylation procedure. The purity and structural quality of the SWNT sample used in the decarboxylation process were evaluated by thermogravimetry and Raman spectroscopy. Thermogravimetric analysis identified a purified sample with approximately 80 wt% of SWNT, in fractions distributed in highly structured SWNTs (25 wt%), with distribution in composition, length and structural quality (35 wt%) and with very defective and short tubes (25 wt%). The damages on the purified SWNT walls were characterized by the Raman scattering analysis.

Nanoscale mapping of carbon oxidation in pyrogenic black carbon from ancient Amazonian anthrosols.

Understanding soil organic matter is necessary for the development of soil amendments, which are important for sustaining agriculture in humid tropical climates. Ancient Amazonian anthrosols are uniquely high in black recalcitrant carbon, making them extremely fertile. In this study, we use high-resolution electron microscopy and spectroscopy to resolve the oxidation process of carbon in the nanoscale crystallites within the black carbon grains of this special soil. Most alkali and acid chemical extraction methods are known to cause chemical modifications in soil organic matter and to give poor or no information about the real spatial structure of soil aggregates. However, here we show that carbon-oxygen functional groups such as phenol, carbonyl, and carboxyl dominate over different spatial regions, with areas varying from over tens to hundreds of nm(2). The chemical maps show that in the nanoscale grain, the surface has a tendency to be less aromatic than the grain core, where higher oxidative-degradation levels are indicated by the presence of carbonyl and carboxyl groups. A deep understanding of these structures could allow artificial reproduction of these natural events.

Polarized raman study of aligned multiwalled carbon nanotubes

Polarized Raman spectra of high purity aligned arrays of multiwalled carbon nanotubes, prepared on silica substrates from the thermal decomposition of a ferrocene-xylene mixture, show a strong dependence of the graphitelike G band and the disorder-induced D band on the polarization geometry employed in the experiments. The experimental G-band intensity exhibits a minimum at straight theta(m) = 55 degrees in the VV configuration, in good agreement with theoretical predictions of a characteristic minimum at 54.7 degrees for A(1g) modes in single wall nanotubes, where straight theta(m) denotes the angle between the polarization direction and the nanotube axis.

Polarized raman study of single-wall semiconducting carbon nanotubes

Polarized Raman spectra were obtained from a rope of aligned semiconducting single-wall nanotubes (SWNTs) in the vicinity of the D band and the G band. Based on group theory analysis and related theoretical predictions, the G-band profile was deconvolved into four intrinsic SWNT components with the following symmetry assignments: 1549 cm(-1) [E2(E(2g))], 1567 cm(-1) [A(A(1g))+E1(E(1g))], 1590 cm(-1) [A(A(1g))+E1(E(1g))] and 1607 cm(-1) [E2(E(2g))]. The frequency shifts of the tangential G modes from the 2D graphitelike E(2g(2)) frequency are discussed in terms of the nanotube geometry.

Science and applications of single-nanotube Raman spectroscopy.

A review is presented of the resonance Raman spectra from individual isolated single-wall carbon nanotubes (SWNTs). A brief summary is given of how the measurements are made. Why the resonance Raman effect allows single-carbon nanotube spectra to be observed easily and under normal operating conditions is summarized. The important structural information that is provided by single-nanotube spectroscopy using one laser line is discussed, and what else can be learned from tunable laser experiments is reviewed. Particular attention is given to the determination of the nanotube diameter and of the energy of its van Hove singularities Eii. Applications of single-nanotube spectroscopy are emphasized, such as measurements of isolated SWNTs connected with circuit-based samples and of isolated SWNTs mounted on an atomic force microscope tip. A critical assessment of the opportunities and limitations of the resonance Raman method for structural (n, m) identification is presented. The trigonal warping effect, which is central to the (n, m) identification in resonance Raman spectroscopy, is discussed in simple terms, and the importance of this effect in nanotube science and applications is reviewed.

The concept of cutting lines in carbon nanotube science.

A review is presented of one-dimensional cutting lines that are utilized to obtain the physical properties of carbon nanotubes from the corresponding properties of graphite by the zone-folding scheme. Quantization effects in general low-dimensional systems are briefly discussed, followed by a more detailed consideration of one-dimensional single-wall carbon nanotubes. The geometrical structure of the nanotube is described, from which quantum confined states are constructed. These allowed states in the momentum space of graphite are known as cutting lines. Different representations of the cutting lines in momentum space are introduced. Electronic and phonon dispersion relations for nanotubes are derived by using cutting lines and the zone-folding scheme. The relation between cutting lines and singularities in the electronic density of states is considered. The selection rules for carbon nanotubes are shown to be directly connected with the cutting lines. Different experimental techniques are considered that confirm the validity of cutting lines and the zone-folding approach.

Factors associated with thrombocytopenia in severe leptospirosis (Weil's disease).

OBJECTIVE: This study was conducted to investigate factors associated with thrombocytopenia in a large cohort of patients with leptospirosis in an endemic area. METHODS: This retrospective study included 374 consecutive patients with leptospirosis who were admitted to tertiary hospitals in Fortaleza, Brazil. All patients had a diagnosis of severe leptospirosis (Weil's disease). Acute kidney injury was defined according to the RIFLE criteria. Thrombocytopenia was defined as a platelet count <100,000/mm3. RESULTS: A total of 374 patients were included, with a mean age of 36.1 ± 15.5 years, and 83.4% were male. Thrombocytopenia was present at the time of hospital admission in 200 cases (53.5%), and it developed during the hospital stay in 150 cases (40.3%). The patients with thrombocytopenia had higher frequencies of dehydration (53% vs. 35.3%, p=0.001), epistaxis (5.7% vs. 0.8%, p=0.033), hematemesis (13% vs. 4.6%, p=0.006), myalgia (91.5% vs. 84.5%, p=0.038), hematuria (54.8% vs. 37.6%, p=0.011), metabolic acidosis (18% vs. 9.2%, p=0.016) and hypoalbuminemia (17.8% vs. 7.5%, p=0.005). The independent risk factors associated with thrombocytopenia during the hospital stay were lengthy disease (OR: 1.2, p=0.001) and acute kidney injury (OR: 6.6, p=0.004). Mortality was not associated with thrombocytopenia at admission (12.5% vs. 12.6%, p=1.000) or during the hospital stay (12.6% vs. 11.3%, p=0.748). CONCLUSIONS: Thrombocytopenia is a frequent complication in leptospirosis, and this condition was present in more than half of patients at the time of hospital admission. Lengthy disease and acute kidney injury are risk factors for thrombocytopenia. There was no significant association between thrombocytopenia and mortality.

Ion beam nanopatterning and micro-Raman spectroscopy analysis on HOPG for testing FIB performances.

This work reports Ga(+) focused ion beam nanopatterning to create amorphous defects with periodic square arrays in highly oriented pyrolytic graphite and the use of Raman spectroscopy as a new protocol to test and compare progresses in ion beam optics, for low fluence bombardment or fast writing speed. This can be ultimately used as a metrological tool for comparing different FIB machines and can contribute to Focused Ion Beam (FIB) development in general for tailoring nanostructures with higher precision. In order to do that, the amount of ion at each spot was varied from about 10(6) down to roughly 1 ion per dot. These defects were also analyzed by using high resolution scanning electron microscopy and atomic force microscopy. The sensitivities of these techniques were compared and a geometrical model is proposed for micro-Raman spectroscopy in which the intensity of the defect induced D band, for a fixed ion dose, is associated with the diameter of the ion beam. In addition, the lateral increase in the bombarded spot due to the cascade effect of the ions on graphite surface was extracted from this model. A semi-quantitative analysis of the distribution of ions at low doses per dot or high writing speed for soft modification of materials is discussed.

Defect characterization in graphene and carbon nanotubes using Raman spectroscopy.

This review discusses advances that have been made in the study of defect-induced double-resonance processes in nanographite, graphene and carbon nanotubes, mostly coming from combining Raman spectroscopic experiments with microscopy studies and from the development of new theoretical models. The disorder-induced peak frequencies and intensities are discussed, with particular emphasis given to how the disorder-induced features evolve with increasing amounts of disorder. We address here two systems, ion-bombarded graphene and nanographite, where disorder is represented by point defects and boundaries, respectively. Raman spectroscopy is used to study the 'atomic structure' of the defect, making it possible, for example, to distinguish between zigzag and armchair edges, based on selection rules of phonon scattering. Finally, a different concept is discussed, involving the effect that defects have on the lineshape of Raman-allowed peaks, owing to local electron and phonon energy renormalization. Such effects can be observed by near-field optical measurements on the G' feature for doped single-walled carbon nanotubes.

Factors associated with thrombocytopenia in severe leptospirosis (Weil's disease)

OBJECTIVE: This study was conducted to investigate factors associated with thrombocytopenia in a large cohort of patients with leptospirosis in an endemic area. METHODS: This retrospective study included 374 consecutive patients with leptospirosis who were admitted to tertiary hospitals in Fortaleza, Brazil. All patients had a diagnosis of severe leptospirosis (Weil's disease). Acute kidney injury was defined according to the RIFLE criteria. Thrombocytopenia was defined as a platelet count <100,000/mm3. RESULTS: A total of 374 patients were included, with a mean age of 36.1±15.5 years, and 83.4% were male. Thrombocytopenia was present at the time of hospital admission in 200 cases (53.5%), and it developed during the hospital stay in 150 cases (40.3%). The patients with thrombocytopenia had higher frequencies of dehydration (53% vs. 35.3%, p = 0.001), epistaxis (5.7% vs. 0.8%, p = 0.033), hematemesis (13% vs. 4.6%, p = 0.006), myalgia (91.5% vs. 84.5%, p = 0.038), hematuria (54.8% vs. 37.6%, p = 0.011), metabolic acidosis (18% vs. 9.2%, p = 0.016) and hypoalbuminemia (17.8% vs. 7.5%, p = 0.005). The independent risk factors associated with thrombocytopenia during the hospital stay were lengthy disease (OR: 1.2, p = 0.001) and acute kidney injury (OR: 6.6, p = 0.004). Mortality was not associated with thrombocytopenia at admission (12.5% vs. 12.6%, p = 1.000) or during the hospital stay (12.6% vs. 11.3%, p = 0.748). CONCLUSIONS: Thrombocytopenia is a frequent complication in leptospirosis, and this condition was present in more than half of patients at the time of hospital admission. Lengthy disease and acute kidney injury are risk factors for thrombocytopenia. There was no significant association between thrombocytopenia and mortality. .