Raman spectroscopy of individual single-walled carbon nanotubes from various sources

Resonance Raman spectroscopy/microscopy was used to study individualized single-walled carbon nanotubes (SWNTs) both in aqueous suspensions as well as after spin-coating onto Si/SiO2 surfaces. Four different SWNT materials containing nanotubes with diameters ranging from 0.7 to 1.6 nm were used. Comparison with Raman data obtained for suspensions shows that the surface does not dramatically affect the electronic properties of the deposited tubes. Raman features observed for deposited SWNTs are similar to what was measured for nanotubes directly fabricated on surfaces using chemical vapor deposition (CVD) methods. In particular, individual semiconducting tubes could be distinguished from metallic tubes by their different G-mode line shapes. It could also be shown that the high-power, short-time sonication used to generate individualized SWNT suspensions does not induce defects in great quantities. However, (additional) defects can be generated by laser irradiation of deposited SWNTs in air, thus giving rise to an increase of the D-mode intensity for even quite low power densities (approximately 10(4) W/cm2).     

Infrared spectroscopy of single-walled carbon nanotubes

By using the spectral moments method, we calculate the infrared spectra of chiral and achiral single-walled carbon nanotubes (SWCNTs) of different diameters and lengths. We show that the number of the infrared modes, their frequencies, and intensities depend on the length and chirality of the nanotubes. Furthermore, the dependence of the infrared spectrum as a function of the size of the SWCNT bundle is analyzed. These predictions are useful to interpret the experimental infrared spectra of SWCNTs.     

Aggregation effects on the Raman spectroscopy of dielectrophoretically deposited single-walled carbon nanotubes

The effect of aggregation on surfactant-suspended individual single-walled carbon nanotube (SWNT) Raman spectroscopy has been explored in the context of dielectrophoretic separation. The Raman spectra of individual surfactant-suspended HiPco SWNTs deposited on a substrate and the same suspension deposited via dielectrophoresis were compared as a function of iterative aggregation states. The evolution of the samples' radial breathing modes and tangential modes at multiple excitation wavelengths (514, 633, and 785 nm) illustrates a direct correlation between changes in the Raman spectra and a broadening and downshifting of resonance transition energies. Dielectrophoresis samples exhibited Raman changes similar to control samples, indicating characterization of electronic separation is compromised by aggregation effects.     

Effects of KI encapsulation in single-walled carbon nanotubes by Raman and optical absorption spectroscopy

The effect of KI encapsulation in narrow (HiPCO) single-walled carbon nanotubes is studied via Raman spectroscopy and optical absorption. The analysis of the data explores the interplay between strain and structural modifications, bond-length changes, charge transfer, and electronic density of states. KI encapsulation appears to be consistent with both charge transfer and strain that shrink both the C-C bonds and the overall nanotube along the axial direction. The charge transfer in larger semiconducting nanotubes is low and comparable with some cases of electrochemical doping, while optical transitions between pairs of singularities of the density of states are quenched for narrow metallic nanotubes. Stronger changes in the density of states occur in some energy ranges and are attributed to polarization van der Waals interactions caused by the ionic encapsulate. Unlike doping with other species, such as atoms and small molecules, encapsulation of inorganic compounds via the molten-phase route provides stable effects due to maximal occupation of the nanotube inner space.     

Substrate-induced Raman frequency variation for single-walled carbon nanotubes

We report on the monotonic Raman frequency shift and intensity variation when a laser spot moves along the same single-walled carbon nanotube (SWNT) for both the radial-breathing mode (RBM) and the G-band. Our substrates are Si wafers coated with thermal oxide, and trenches with widths of 1-80 mum are etched in the SiO2 by photolithography and reactive ion etching. SWNTs are grown by chemical vapor deposition and lie on top of the SiO2 and across the trenches. When the laser spot moves from the middle of the trench to the SiO2 region along the nanotube, we observe a clear upshift in the RBM and G-band frequencies and a decrease of intensity. The effect is more significant with large ( approximately 2 nm) diameter nanotubes and appears to be chirality dependent. These studies provide important information about environmental effects on single-walled carbon nanotube resonant Raman spectroscopy.     

Ozonized single-walled carbon nanotubes investigated using NEXAFS spectroscopy

The use of NEXAFS spectroscopy in studying the electronic structure and chemical composition of pristine, wet-air oxidized, and sidewall-ozonized nanotubes is illustrated.     

Raman spectroscopy and imaging of ultralong carbon nanotubes

Raman spectroscopy and confocal Raman imaging with 514 nm excitation was performed on recently developed ultralong carbon nanotubes grown by the "fast-heating" chemical vapor deposition (CVD) method. The ultralong nanotubes are found to consist of both semiconducting and metallic types, with spectra that are consistent with the nanotubes being single walled. Characterization of nanotube diameters shows that short nanotubes appearing near the sample catalyst region have a broader distribution than is observed for the ultralong nanotubes. The narrow diameter distribution is determined by uniformity of catalyst particle size and gives additional evidence for the proposed "kite" mechanism for long nanotube growth. Raman imaging was performed over large length scales (up to 140 microm). Imaging reveals the ultralong nanotubes to be of high quality, with a very low defect density. Variations in G-band frequencies and intensity demonstrate the occurrence of minor structural changes and variations in nanotube-substrate interaction along the length of the nanotubes. Evidence also demonstrates that larger structural changes resulting in a full chirality change can occur in these nanotube types to produce a metal-to-semiconductor intramolecular junction.     

Near-infrared fluorescence spectroscopy of single-walled carbon nanotubes and its applications

Semiconducting single-walled carbon nanotubes (SWCNTs) emit fluorescence at near-infrared (NIR) wavelengths that are characteristic of the specific diameter and the chiral angle. While providing a convenient method for structural identification of semiconducting SWCNTs, NIR fluorescence of SWCNTs also offers a powerful approach for sensor development and in vivo or real-time imaging of biological systems.This article provides an introductory overview of the approaches to obtaining individually dispersed semiconducting SWCNTs with reasonably good purity, which is a critical step in acquiring NIR fluorescence spectra. It also summarizes the progress since 2002 in sensor design and applications in bioimaging in vitro and in vivo using NIR fluorescence of semiconducting SWCNTs.     

Excitation energy dependence of the Raman spectrum of single-walled carbon nanotubes

We investigate the excitation energy dependence of Raman modes in the 600–1200 cm⁻¹ range. The main features are first, two lines around 860 and 1060 cm⁻¹ independent of the energy and second, energy-dependent couples of lines, each of them composed of one first-order mode and one substractive combination band. In each couple, the frequency of the lines is found to follow a linear, strong and opposite energy dependence.     

Surface-enhanced Raman scattering of single-walled carbon nanotubes on modified silver electrode

A roughed silver electrode modified with gold/silver nanoparticles is used as a substrate, on which high quality SERS of SWCNTs are obtained, indicating that the modified silver electrode is a high-quality SERS-active substrate for SWCNTs. Some new bands that indicate the structure of SWCNTs were obtained. The gold/silver nanoparticles modified on the roughed silver electrode surface can not only make sure the strong adsorption of SWCNTs in this system but also play an important role in magnifying the surface local electric field near the silver electrode surface through resonant surface plasmon excitation. From the rich information on the modified silver electrode obtained from the SERS and the potential dependent SERS, we may deduce the probable SERS mechanism in the process. The theory and experiment results indicate that it is can be used as a new technique for monitoring synthesis quality of SWCNTs. The probable reasons are given.     

Bioelectrochemical single-walled carbon nanotubes

Metalloproteins and enzymes can be immobilized on SWNTs of different surface chemistry. The combination of high surface area, robust immobilization and inherent nanotube electrochemical properties is of promising application in bioelectrochemistry.     

Raman spectroscopy of free-standing individual semiconducting single-wall carbon nanotubes

The radial breathing modes and tangential modes have been systematically measured on a large number of individual semiconducting single-wall carbon nanotubes (thin bundles) suspended between plots (free-standing single-wall carbon nanotubes). The strong intensity of the Raman spectra ensures the precision of the experimentally determined line shapes and frequencies of these modes. The diameter dependence of the frequencies of the tangential modes was measured. This dependence is discussed in relation with recent calculations. The present data confirm/contradict some previous interpretations.     

Tuning the Raman resonance behavior of single-walled carbon nanotubes via covalent functionalization

We present a systematic Raman study over a range of excitation energies of arc discharge single-walled carbon nanotubes (SWCNTs) covalently functionalized according to two processes, esterification and reductive alkylation. The SWCNTs are characterized by resonance Raman spectroscopy at each step of the functionalization process, showing changes in radial breathing mode frequencies and transition energies for both semiconducting and metallic tubes. Particular attention is given to a family of tubes clearly identified in the Kataura plot for which we continuously tune the excitation energy from 704 to 752 nm. This allows us to quantify the energy shift occurring in the spacing of the van Hove singularities. We demonstrate that, independently of the functionalization technique, the type of chain covalently bound to the tubes plays an important role, notably when oxygen atoms lie close to the tubes, inducing a larger shift in transition energy as compared to that of other carbonaceous chains. The study shows the complexity of interpreting Raman data and suggests many interpretations in the literature may need to be revisited.     

Surface-enhanced Raman scattering of single-walled carbon nanotubes on silver-coated and gold-coated filter paper

Surface-enhanced Raman scattering (SERS) spectra of single-walled carbon nanotubes (SWCNTs) on metal-coated filter paper are reported for the first time. Experimental results show that the metal-coated filter paper is very effective and active. The SERS spectrum not only shows that all Raman bands of SWCNTs in normal Raman scattering have been generally enhanced, but also shows many new bands, which characterize the structure of SWCNTs and the interaction between SWCNTs and silver/gold nanoparticles, arising from symmetry lowering and selection rule relaxing of SWCNTs induced by the silver/gold surface. In our case, it is difficult to separate the contributions of the electromagnetic and chemical mechanisms to the great enhancement of the Raman signal. The analysis shows that the SERS spectra of SWCNTs on the metal-coated filter paper provide convenience for probing the sample molecules with fine structures related to defects of SWCNTs, the diameter of SWCNTs, and the SERS mechanism of SWCNTs deposited on metal-coated filter paper. Moreover, this can be used as a probe technique for monitoring the synthesis quality of SWCNTs with significant higher sensitivity than other methods, which has promise of being a new technique for monitoring synthesis quality of SWCNTs.     

Characterization of the chemical interaction between single-walled carbon nanotubes and titanium dioxide nanoparticles by thermogravimetric analyses and resonance Raman spectroscopy

Raman spectroscopy is a powerful technique that is used to characterize or observe alterations in the structure or properties of carbon nanotubes and its composites. This method can provide information about electronic changes or quantify them. We used Raman spectroscopy to study the chemical and electronic changes in a composite formed by titanium dioxide nanoparticles and single-walled carbon nanotubes. This composite was characterized by scanning electron microscopy to investigate the morphology and by thermogravimetric analyses to assess the thermal stability of the isolated carbon nanotubes as compared with the nanotubes by titanium dioxide nanoparticles. The Raman results showed that the modification of the nanotubes with the TiO₂ nanoparticles generates a new material with different structure of the nanotubes, resulting in a decrease in defects. The charge transfer from the TiO₂ nanoparticles to the nanotubes alters the electronic properties of both moieties in the hybrid material. The interaction between the nanotubes and nanoparticles decreases the CC bound order of the nanotubes and decreases their thermal stability.     

Ultrafast carrier dynamics in single-walled carbon nanotubes probed by femtosecond spectroscopy

Ultrafast carrier dynamics in individual semiconducting single-walled carbon nanotubes was studied by femtosecond transient absorption and fluorescence measurements. After photoexcitation of the second van Hove singularity of a specific tube structure, the relaxation of electrons and holes to the fundamental band edge occurs to within 100 fs. The fluorescence decay from this band is dependent on the excitation density and can be rationalized by exciton annihilation theory. In contrast to fluorescence, the transient absorption has a distinctly different time and intensity dependence for different tube structures, suggesting a branching to emissive and trap states following photoexcitation.     

Theoretical study of the 13C NMR spectroscopy of single-walled carbon nanotubes

The 13C NMR spectroscopy of armchair and zigzag single-walled carbon nanotubes has been investigated theoretically. Spectra for (4,4), (5,5), (6,6), (6,0), (9,0), and (10,0) nanotubes have been simulated based on ab initio calculations of model systems. The calculations predict a dominant band arising from the carbon atoms in the "tube" with smaller peaks at higher chemical shifts arising from the carbon atoms of the caps. The dominant band lies in the range of 128 and 138 ppm. Its position depends weakly on the length, width, and chirality of the tubes. The calculations demonstrate how structural information may be gleaned from relatively low-resolution nanotube 13C NMR spectra.     

Direct electrochemistry of uric acid at chemically assembled carboxylated single-walled carbon nanotubes netlike electrode

Carboxylated single-walled carbon nanotubes (SWCNT) chemically assembled on gold substrate was employed as netlike electrode to investigate the charge-transfer process and electrode process kinetics using uric acid as an example. The electrochemical behavior of uric acid in carboxylated SWCNT system was investigated using cyclic voltammetry, chronoamperometry, and single potential time-based techniques. The properties of raw SWCNT electrode were also studied for comparison purpose. Uric acid has better electrochemical behavior whereas ascorbic acid has no effective reaction on the carboxylated SWCNT electrode. Cyclic voltammograms indicate that the assembled carboxylated SWCNT increases more active sites on electrode surface and slows down the electron transfer between the gold electrode and uric acid in solution. The charge-transfer coefficient (alpha) for uric acid and the rate constant (k) for the catalytic reaction were calculated as 0.52 and 0.43 s(-1), respectively. The diffusion coefficient of 0.5 mM uric acid was 7.5 x 10(-6) cm2 x s(-1). The results indicate that electrode process in the carboxylated SWCNT electrode system is governed by the surface adsorption-controlled electrochemical process.     

Soluble ultra-short single-walled carbon nanotubes

Soluble, ultra-short (length < 60 nm), carboxylated, single-walled carbon nanotubes (SWNTs) have been prepared by a scalable process. This process, predicated on oleum's (100% H2SO4 with excess SO3) ability to intercalate between individual SWNTs inside SWNT ropes, is a procedure that simultaneously cuts and functionalizes SWNTs using a mixture of sulfuric and nitric acids. The solubility of these ultra-short SWNTs (US-SWNTs) in organic solvents, superacid and water is about 2 wt %. The availability of soluble US-SWNTs could open opportunities for forming high performance composites, blends, and copolymers without inhibiting their processibility.     

Studies of bromine modified single-walled carbon nanotubes using photoelectron spectroscopy and density-functional theory

Many applications based on single-walled carbon nanotubes (SWNTs) require chemical modification of carbon nanotube to optimize the functionalities of the device. In this contribution we discuss the properties of SWNTs immersed in a hydrobromic acid (HBr) solution. Changes of atomic and electronic structures of bromine modified SWNTs were investigated using photoelectron spectroscopy (PES). Spectra of SWNTs before and after immersion in the HBr solution exhibit different features. To understand the mechanism of interaction between SWNTs and bromine, we performed density-functional theory calculations to reveal the structural changes, adsorption energy and chemical bonding information of SWNTs interacting with bromine. In addition, based on the Gelius model, from the molecular orbitals (MOs), we calculated ultraviolet photoelectron spectra (UPS) of SWNTs with and without functionalizing and compared them with the experiment. The present study is a first step in the understanding of the functionalization mechanism of carbon nanotubes.