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.     

Temperature dependence of the Raman spectra of individual carbon nanotubes

Resonant Raman scattering (RRS) spectra of individual carbon nanotubes on a SiO2 substrate have been investigated first in the temperature range of 100-600 K (Phys. Rev. B 2002, 66, 115411). It was revealed by the intensity abnormality of the radial breathing mode (RBM) that the carbon nanotubes have a temperature-dependent density of electronic states. This means that the previously reported temperature coefficients of RBM of carbon nanotubes are smaller than their "real" ones for the bulk samples of single- or double-walled carbon nanotubes. Comparatively, the G line of individual nanotubes shows no observable difference relative to the bulk samples.     

单根(7,5)蛇形单壁碳纳米管的拉曼光谱(英文)

研究了单根(7,5)蛇形单壁碳纳米管的拉曼光谱特征,观察到了环呼吸振动峰(RBM)、环呼吸振动的倍频峰(2RBM)、介于中间频率的振动峰(IMF)、无规振动峰(D)、剪切振动峰(G)、中间频率振动峰(M)、剪切振动和环呼吸振动的和频峰(G+RBM)、面内横向光学声子和纵向声学声子的和频峰(iTOLA)、无规振动的二次共振峰(G′或者2D)以及其它一些归属不清楚的拉曼峰.不同激发波长和不同激发偏振拉曼光谱研究表明,这些拉曼光谱峰显示出了非常强的激发能量和激发偏振的选择性.     

Spectroelectrochemistry of carbon nanostructures.

This review is focused on charge-transfer reactions at carbon nanotubes and fullerenes. The spectroelectrochemistry of fullerenes deals with the spin states of fullerenes, the role of mono-anions and the reactivity of higher charged states in C60. The optical (Vis-NIR) spectroelectrochemistry of single-walled carbon nanotubes (SWNTs) follows changes in the allowed optical transitions among the Van Hove singularities. The Raman spectroelectrochemistry of SWNT benefits from strong resonance enhancement of the Raman scattering. Here, both semiconducting and metallic SWNTs are analyzed using the radial breathing mode (RBM) and G-modes as well as the second order (D, G') and intermediate frequency modes. Raman spectroelectrochemistry of SWNT allows the addressing of index-identified tubes and even single isolated nanotubes. Optical and Raman spectroelectrochemistry of fullerene peapods, C60@SWNT and C70@SWNT indicates effective shielding of the intratubular fullerene (peas). The most striking effect in the spectroelectrochemistry of peapods is the so-called "anodic Raman enhancement" of intratubular C60. Double-walled carbon nanotubes (DWNTs) give a specific spectroscopic response in Vis-NIR spectroelectrochemistry for the inner and the outer tube. They are better distinguishable by Raman spectroelectrochemistry which allows a precise tracing of the specific doping response of outer/inner tubes.     

High levels of electrochemical doping of carbon nanotubes: evidence for a transition from double-layer charging to intercalation and functionalization

We studied the transition from the electrochemical double-layer charging regime to intercalative doping of bundled single-walled carbon nanotubes (SWNT) in KCl and HCl aqueous solution. For this purpose we used high doping levels by applying constant potentials above 1000 mV approaching and slightly exceeding the oxidation potential for Cl(-) ions. At each potential in situ Raman measurements of the radial breathing mode (RBM), the high-energy tangential mode (HEM), and the disorder-induced (D) mode were performed. Furthermore, the conductivity and reflectivity of a set of SWNT samples were measured as a function of doping and subsequently the samples were examined by X-ray photoelectron spectroscopy (XPS). From a comparative analysis of the results we conclude that above 1000 mV a significant penetration of chlorine species into the interstitial channels of the SWNT bundles and possible covalent functionalization take place.     

Identification of the structures of superlong oriented single-walled carbon nanotube arrays by electrodeposition of metal and Raman spectroscopy

In this Communication, we have demonstrated a facile and effective approach to identify the structure of the superlong well-aligned single-walled carbon nanotubes (SWNTs) by the combination of electrodeposition of metal (Ag) with Raman spectroscopy. The suitable density and the visibility of the Ag-deposited long oriented nanotubes make it possible to acquire Raman spectra from isolated individual nanotubes very easily. The results reveal that the well-oriented SWNT arrays on SiO2/Si wafer fabricated by EtOH chemical vapor deposition using Fe/Mo nanoparticles as catalyst exhibit a low percentage of metallic SWNTs (5%). Among other SWNTs about 62.3% are semiconducting SWNTs, and a small amount of nanotubes are quasimetallic. About 32% are a so-called quasi-insulator, which is caused inevitably by the defects during growth. Furthermore, the structural uniformity of the long SWNTs can be also evaluated by the deposition of Ag along the length and Raman spectroscopy. This method also provides an approach to deposit other metals on long SWNTs, which could have various potential applications such as for use as sensors, etc. More importantly, this facile method can be applied to long SWNT arrays fabricated from other different catalytic systems so that the relationship between the growth conditions and the structures of SWNTs are expected to be ruled out.     

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).     

The environmental effect on the radial breathing mode of carbon nanotubes. II. Shell model approximation for internally and externally adsorbed fluids

We have previously shown that the upshift in the radial breathing mode (RBM) of closed (or infinite) carbon nanotubes in solution is almost entirely due to coupling of the RBM with an adsorbed layer of fluid on the nanotube surface. The upshift can be modeled analytically by considering the adsorbed fluid as an infinitesimally thin shell, which interacts with the nanotube via a continuum Lennard-Jones potential. Here we extend the model to include internally as well as externally adsorbed waterlike molecules, and find that filling the nanotubes leads to an additional upshift of two to six wave numbers. We show that using molecular dynamics, the RBM can be accurately reproduced by replacing the fluid molecules with a mean field harmonic shell potential, greatly reducing simulation times.     

Growth of millimeter-long and horizontally aligned single-walled carbon nanotubes on flat substrates

Millimeter-long and well-aligned single-walled carbon nanotubes (SWNTs) have been produced on silica/silicon surfaces using the carbon monoxide chemical vapor deposition (CO-CVD) method. The orientation of the nanotube arrays can be well-controlled by the gas flow during the growth. The majority of the orientated SWNTs are straight and individual. The length of the nanotubes can be >2 mm for a 10 min growth. Furthermore, multidimensional crossed-networks of SWNT can be easily generated by multistep processes. These results present a great opportunity in the controllable production of organized SWNT arrays for large-scale carbon nanotube-based nanodevice fabrication.     

Influence of impurities on the x-ray photoelectron spectroscopy and Raman spectra of single-wall carbon nanotubes

The effect of impurities on the properties of single-wall carbon nanotubes (SWNTs) was investigated with multiple analytical techniques. Charge transfer is believed to occur between the impurities and the SWNTs as observed by combining the Raman scattering and x-ray photoelectron measurements. The impurity condition (type and level) was found to strongly affect the electronic and vibrational properties of the SWNT. The metal catalysts in the impurity usually behave as electron donors, which can downshift the graphitic (G) band as well as the radial breathing mode frequencies. The low temperature air oxidation of as-prepared SWNT material usually upshifts the radial breathing mode Raman peaks to higher frequencies.     

Raman and IR spectroscopy of chemically processed single-walled carbon nanotubes

IR and Raman spectroscopy has been used to study the evolution of the vibrational spectrum of bundled single-walled carbon nanotubes (SWNTs) during the purification process needed to remove metal catalyst and amorphous carbon present in arc-derived SWNT soot. We have carried out a systematic study to define the different outcomes stemming from the purification protocol (e.g., DO, DO/HCl, DO/HNO(3), H(2)O(2), H(2)O(2)/HCl), where dry oxidation (DO) or refluxing in H(2)O(2) was used in a first purification step to remove amorphous carbon. The second step involves acid reflux (HCl or HNO(3)) to remove the residual growth catalyst (Ni-Y). During strong chemical processing, it appears possible to create additional defects where carbon atoms are eliminated, the ring structure is now open, localized C=C bonds are created, and O-containing groups can be added to this defect to stabilize the structure. Evolution of SWNT skeletal disorder obtained via chemical processing was studied by Raman scattering. Higher intensity ratios of R- and G-band (I(R)/I(G)) are more typically found in SWNT materials with low D-band intensity and narrow G-band components. Using IR transmission through thin films of nanotubes, we can resolve the structure due to functional groups that were present in the starting material or added through chemical processing. After high-temperature vacuum annealing of the purified material at 1100 degrees C, IR spectroscopy shows that most of the added functional groups can be removed and that the structure that remains is assigned to the one- and two-phonon modes of SWNTs.     

The environmental effect on the radial breathing mode of carbon nanotubes in water

We investigate, using molecular dynamics, the effect on the radial breathing mode (RBM) frequency of immersion in water for a range of single-walled carbon nanotubes. We find that nanotube-water interactions are responsible for an upshift in the RBM frequency of the order of 4-10 wave numbers. The upshift is comprised of two components: increased hydrostatic pressure on the nanotube due to curvature effects, and the dynamic coupling of the RBM with its solvation shell. In contrast to much of the current literature, we find that the latter of the two effects is dominant. This could serve as an innovative tool for determining the interaction potential between nanotubes/graphitic surfaces and fluids.     

Direct evidence for root growth of vertically aligned single-walled carbon nanotubes by microwave plasma chemical vapor deposition

The root growth mode of extremely dense and vertically aligned single-walled carbon nanotubes (SWNTs) synthesized by microwave plasma chemical vapor deposition was clarified by a new method, marker growth, which does not require transmission electron microscopy. SWNT layers were grown intermittently on a substrate, and a line between the layers was used as a marker to identify the growth mode. Micro-Raman spectroscopy revealed that the SWNT layers have the same diameter distribution.     

Optical properties of ultrashort semiconducting single-walled carbon nanotube capsules down to sub-10 nm

Single-walled carbon nanotubes (SWNTs) are typically long (greater than or approximately equal 100 nm) and have been well established as novel quasi one-dimensional systems with interesting electrical, mechanical, and optical properties. Here, quasi zero-dimensional SWNTs with finite lengths down to the molecular scale (7.5 nm in average) were obtained by length separation using a density gradient ultracentrifugation method. Different sedimentation rates of nanotubes with different lengths in a density gradient were taken advantage of to sort SWNTs according to length. Optical experiments on the SWNT fractions revealed that the UV-vis-NIR absorption and photoluminescence peaks of the ultrashort SWNTs blue-shift up to approximately 30 meV compared to long nanotubes, owing to quantum confinement effects along the length of ultrashort SWNTs. These nanotube capsules essentially correspond to SWNT quantum dots.     

Electrochemical doping of chirality-resolved carbon nanotubes

Raman spectra of electrochemically charged single-wall carbon nanotubes (HiPco) were studied by five different laser photon energies between 1.56 and 1.92 eV. The bands of radial breathing modes (RBM) were assigned to defined chiralities by using the experimental Kataura plot. The particular (n,m) tubes exhibit different sensitivity to electrochemical doping, monitored as the attenuation of the RBM intensities. Tubes which are in good resonance with the exciting laser exhibit strong doping-induced drop of the RBM intensity. On the other hand, tubes whose optical transition energy is larger than the energy of an exciting photon show only small changes of their RBM intensities upon doping. This rule presents a tool for analysis of mixtures of single-walled carbon tubes of unknown chiralities. It also asks for a re-interpretation of some earlier results which were reported on the diameter-selectivity of doping. The radial breathing mode in strongly n- or p-doped nanotubes exhibited a blue-shift. A suggested interpretation follows from the charging-induced structural changes of SWCNTs bundles, which also includes a partial de-bundling of tube ropes.     

Multiplexed multicolor Raman imaging of live cells with isotopically modified single walled carbon nanotubes

We show that single walled carbon nanotubes (SWNTs) with different isotope compositions exhibit distinct Raman G-band peaks and can be used for multiplexed multicolor Raman imaging of biological systems. Cancer cells with specific receptors are selectively labeled with three differently "colored" SWNTs conjugated with various targeting ligands including Herceptin (anti-Her2), Erbitux (anti-Her1), and RGD peptide, allowing for multicolor Raman imaging of cells in a multiplexed manner. SWNT Raman signals are highly robust against photobleaching, allowing long-term imaging and tracking. With narrow peak features, SWNT Raman signals are easily differentiated from the autofluorescence background. The SWNT Raman excitation and scattering photons are in the near-infrared region, which is the most transparent optical window for biological systems in vitro and in vivo. Thus, SWNTs are novel Raman tags promising for multiplexed biological detection and imaging.     

Cutting of single-walled carbon nanotubes by ozonolysis

Cutting of single-walled carbon nanotubes (SWNT) has been achieved by extensive ozonolysis at room temperature. Perfluoropolyether (PFPE) was selected as a medium for cutting SWNT due to its high solubility for ozone (O3). A mixture of 9 wt % of O3 in O2 was bubbled through a homogeneous suspension of pristine SWNT in PFPE, at room temperature. The intense disorder mode in the Raman spectra of ozonated SWNT indicates that extensive reaction with the sidewalls of SWNT occurs during ozonolysis. Atomic force microscopy (AFM) images of SWNT, before and after ozonolysis, provided a measure of the extent of the cutting effects. Monitoring of the evolved gases for both pristine and purified SWNT indicates CO2 was produced during the ozonolysis process with a dependence on both system pressure and temperature. During heating, FTIR analysis of gases released indicated that carbon oxygen groups on the sidewalls of SWNT are released as CO2. SWNT was found to be extensively cut after an ozone treatment with a yield of approximately 80% of the original carbon.     

Noncovalent functionalization of single-walled carbon nanotubes with water-soluble porphyrins

We have employed water-soluble porphyrin molecules [meso-(tetrakis-4-sulfonatophenyl) porphine dihydrochloride] to solubilize single-walled carbon nanotubes (SWNTs), resulting in aqueous solutions that are stable for several weeks. The porphyrin-nanotube complexes have been characterized with absorption and fluorescence spectroscopy and with AFM. We find that the porphyrin/SWNT interaction is selective for the free base form, and that this interaction stabilizes the free base against protonation to the diacid. Under mildly acidic conditions nanotube-mediated J-aggregates form, which are unstable in solution and result in precipitation of the nanotubes over the course of a few days. Porphyrin-coated SWNTs can be precisely aligned on hydrophilic poly(dimethylsiloxane) (PDMS) surfaces by combing SWNT solution along a desired direction and then transferred to silicon substrates by stamping. Parallel SWNT patterns have been fabricated in this manner.     

Spectroscopic Characteristics of Differently Produced Single‐Walled Carbon Nanotubes

Single‐walled carbon nanotubes (SWNTs) synthesized with different methods are investigated by using multiple characterization techniques, including Raman scattering, optical absorption, and X‐ray absorption near edge structure, along with X‐ray photoemission by following the total valence bands and C 1s core‐level spectra. Four different SWNT materials (produced by arc discharge, HiPco, laser ablation, and CoMoCat methods) contain nanotubes with diameters ranging from 0.7 to 2.8 nm. The diameter distribution and the composition of metallic and semiconducting tubes of the SWNT materials are strongly affected by the synthesis method. Similar sp² hybridization of carbon in the oxygenated SWNT structure can be found, but different surface functionalities are introduced while the tubes are processed. All the SWNTs demonstrate stronger plasmon resonance excitations and lower electron binding energy than graphite and multiwalled carbon nanotubes. These SWNT materials also exhibit different valence‐band X‐ray photoemission features, which are considerably affected by the nanotube diameter distribution and metallic/semiconducting composition.     

Effects of surfactant and boron doping on the BWF feature in the Raman spectrum of single-wall carbon nanotube aqueous dispersions

We examine the Breit-Wigner-Fano (BWF) line shape in the Raman spectra of carbon single-wall nanotubes (SWNTs) dispersed in aqueous suspensions. Bundling and electronic effects are studied by comparing undoped SWNTs (C-SWNTs) to boron-doped nanotubes (B-SWNTs) in a variety of different surfactant solutions. For SWNTs dispersed with nonionic surfactants that are less effective in debundling than ionic surfactants, the Raman spectra retain a large BWF feature. However, we demonstrate that even for SWNTs dispersed as isolated nanotubes by ionic surfactants the BWF feature may be present and that the intensity of the BWF is highly sensitive to the specific surfactant. In particular, surfactants with electron-donating groups tend to enhance the BWF feature. Also, modification of the SWNT electronic properties by boron doping leads to enhanced surfactant dispersion relative to undoped C-SWNTs and also to modification of the BWF feature. These observations are in agreement with reports demonstrating an enhancement of the BWF by bundling but also agree with reports that suggest electron donation can enhance the BWF feature even for isolated SWNTs. Importantly, these results serve to caution against using the lack or presence of a BWF feature as an independent measure of SWNT aggregation in surfactant dispersions.