Determination of the exciton binding energy in single-walled carbon nanotubes

We report that measurements of the Raman intensity versus applied voltage are sensitive to filling of the density of states and enable us to measure the second band gap in specific semiconducting single-walled carbon nanotubes (SWNTs). Raman scattering preferentially selects sets of SWNTs whose excitonic transitions are resonant with the incident or scattered photon energies. Simultaneous measurement of the electronic gap and exciton resonance allows us to infer binding energies for the exciton of 0.49+/-0.05 and 0.62+/-0.05 eV for tubes of (10, 3) and (7, 5), respectively. Metallic SWNTs exhibit no excitonic feature.     

Raman spectroscopy of carbon nanotubes

The use of Raman spectroscopy to reveal the remarkable structure and the unusual electronic and phonon properties of single wall carbon nanotubes (SWNTs) is reviewed comprehensively. The various types of Raman scattering processes relevant to carbon nanotubes are reviewed, and the theoretical foundations for these topics are presented. The most common experimental techniques used to probe carbon nanotubes are summarized, followed by a review of the novel experimental findings for each of the features in the first order and second order Raman spectra for single wall carbon nanotubes. These results are presented and discussed in connection with theoretical considerations. Raman spectra for bundles of SWNTs, for SWNTs surrounded by various common wrapping agents, and for isolated SWNTs at the single nanotube level are reviewed. Some of the current research challenges facing the field are briefly summarized.     

Raman spectroscopy of SWNTs produced by a XeCl excimer laser ablation at high temperatures

Synthesis of single-wall carbon nanotubes (SWNTs) was carried out by an ablation method using a XeCl excimer laser. It was irradiated onto a graphite target containing Co and Ni at the temperatures of 1073, 1173, 1273, 1373, 1473, 1523 and 1623 K under the atmosphere (0.1 MPa) of Ar gas with the flow rate of 12 ml/min. The measurement by a scanning/transmission electron microscope and Raman spectroscopy found the formation of SWNTs with the diameter of about 1.3 nm and the length of about 2 μm in ablated carbonaceous soot. The ratio of peak intensity of 1590 cm⁻¹ (G band) to that of 1335 cm⁻¹ (D band) in the high frequency Raman spectra increased with increasing the ambient temperature. The radial breathing mode (RBM) in the low frequency Raman spectra shows that the mean diameter of SWNTs increased with increasing the ambient temperature.     

A detailed Raman study on thin single-wall carbon nanotubes prepared by the HiPCO process

The Raman spectrum of single wall carbon nanotubes (SWNTs) prepared by high pressure CO decomposition (HiPCO process) has been recorded at nine excitation laser energies ranging from 1.83 eV to 2.71 eV. The characteristic nanotubes features: G band, D band and radial breathing mode (RBM) have been analyzed and compared to those of an arc discharge SWNT material of similar diameter. A strong Breit-Wigner-Fano type (metallic) contribution to the G band was found in the spectra measured with green lasers, while spectra measured with red lasers indicate resonances of semiconducting SWNTs. Analysis of the energy dependence of the position of the D band revealed sinusoid oscillations superimposed on a linear trend. The validity of full DOS calculations for HiPCO materials has been confirmed by a match found between the estimated spectral contribution of metallic SWNTs as calculated from the components of the measured G band and as predicted by the (n, m) indexes of the major scatterers of DOS simulations. The RBM region of the HiPCO spectrum is more complex than usually observed for SWNTs. The analysis performed with a Gaussian distribution and improved fitting parameters leads to a mean diameter and variance of 1.05 nm and 0.15 nm, respectively. A bimodal Gaussian distribution had little influence on the error sum but reduced the standard error slightly. The major spectral features of the RBM could be interpreted using available resonance Raman theory. Received 5 February 2002 / Received in final form 3 April 2002 Published online 19 July 2002     

Controlling growth and Raman spectra of individual suspended single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) were prepared with ethanol chemical vapor deposition (CVD) on SiO₂ flat and pillar-patterned Si substrates. The effect of CVD temperatures from 600 to 800 °C on SWNTs yields was investigated. By virtue of its unperturbed environment, an individual suspended SWNT grown between two different SiO₂ pillars provides a possibility to study the phonon band structure of SWNT itself at a single-nanotube level. Raman spectra of individual SWNTs grown between pillars were investigated systematically.     

Exploring the possible interlinked structures in single‐wall carbon nanotubes under pressure by Raman spectroscopy

High‐pressure Raman measurements on single‐wall carbon nanotubes (SWNTs) have been carried out in a diamond anvil cell by using two wavelength lasers: 830 and 514.5 nm. Irrespective of using a pressure transmitting medium (PTM) or not, we found that nanotubes undergo similar transformations under pressure. The pressure‐induced changes in Raman signals at around 2 and 5 GPa are attributed to the nanotube cross‐section transitions from circle to ellipse and then to a flattened shape, respectively. Especially with pressure increasing up to 15–17 GPa, we observed that the third transition takes place in both the Raman wavenumber and the linewidth of G‐band. We propose explanations that the interlinked configuration with sp³ bonds forms in the bundles of SWNTs under pressure, which was the cause for the occurrence of those Raman anomalies, similar to the structural‐phase transition of graphite above 14 GPa. Our TEM observations and Raman measurements on the decompressed samples support this transition picture. Copyright © 2012 John Wiley & Sons, Ltd.     

Dielectric response of aligned semiconducting single-wall nanotubes

We report measurements of the full intrinsic optical anisotropy of isolated single-wall carbon nanotubes (SWNTs). By combining absorption spectroscopy with transmission ellipsometry and polarization-dependent resonant Raman scattering, we obtain the real and imaginary parts of the SWNT permittivity from aligned semiconducting SWNTs dispersed in stretched polymer films. Our results are in agreement with theoretical predictions, highlighting the limited polarizability of excitons in a quasi-1D system.     

DNA-wrapped carbon nanotubes aligned in stretched gelatin films: Polarized resonance Raman and absorption spectroscopy study

We present the study of DNA-wrapped single-walled carbon nanotubes (SWNTs) embedded in the stretched gelatin film by the polarized resonance Raman spectroscopy and visible-NIR optical absorption. The polarized dependent absorption spectra taken along and normal to the stretching direction demonstrate a comparatively high degree of the alignment of isolated SWNTs in the gelatin matrix. The analysis of Raman spectra of isolated SWNTs in the gelatin stretched films showed that the degree of the alignment of carbon nanotubes along the stretching direction is about 62%. The dependence of the peak position of G⁺-band in Raman spectra on the polarization angle θ between the polarization of the incident light and the direction of the stretching of films was revealed. This shift is explained by the different polarization dependence of the most intensive A and E₁ symmetry modes within the G⁺-band. The performed studies of embedded DNA-wrapped nanotubes in the gelatin film show the simple method for obtaining the controlled ordered biocompatible nanotubes inside a polymer matrix. It can be used for manufacturing sizable flexible self-transparent films with integrated nanoelectrodes.     

以Y/Ni为催化剂制备的单壁碳纳米管的拉曼光谱研究

采用电弧放电法以Y/Ni为催化制备了单壁碳纳米管（SWNTs），对样品进行了扫描电镜、透射电镜和拉曼光谱的研究。所制备的样品中单壁碳纳米管的含量较高。对单壁碳纳米管的共振拉曼散射增强效应进行了观察，随激光波长的不同，单壁碳纳米管的拉曼光谱也随之变化，尤其是低频区径向呼吸模的变化比较明显。利用布里渊区折叠法计算了单壁碳纳米管的电子态密度曲线，根据SWNTs电子态密度尖峰之间的能量差、管子的直径和呼吸模频率建立了一个图表，并对SWNTs的呼吸模进行了归属。分析结果表明：样品中单壁碳纳米管的直径分布在0.79-1.76nm范围，金属管和半导体管均存在，并且直径在1.45nm附近的碳管居多。     

Static dielectric properties of carbon nanotubes from first principles

We characterize the response of isolated single-wall (SWNT) and multiwall (MWNT) carbon nanotubes and nanotube bundles to static electric fields using first-principles calculations and density-functional theory. The longitudinal polarizability of SWNTs scales as the inverse square of the band gap, while in MWNTs and bundles it is given by the sum of the polarizabilities of the constituent tubes. The transverse polarizability of SWNTs is insensitive to band gaps and chiralities and is proportional to the square of the effective radius; in MWNTs, the outer layers dominate the response. The transverse response is intermediate between metallic and insulating, and a simple electrostatic model based on a scale-invariance relation captures accurately the first-principles results. The dielectric response of nonchiral SWNTs in both directions remains linear up to very high values of applied field.     

Structural transformations in single-wall carbon nanotubes under high pressure

Single-wall carbon nanotubes (SWNTs) under high pressure exhibit high structural stability and a series of structural transitions up to 35 GPa. As theoretically predicted, the irreversible transformation of SWNTs in the pressure range of 10–30 GPa can be attributed to the polymerization of nanotubes. The electrical conductivity of SWNTs is studied at high pressures up to 35 GPa using a diamond anvil cell (DAC) with electrically conductive anvils of the “rounded cone-plane” type made of synthetic carbonado-type diamonds. SWNTs are studied before and after the application of high pressure using the Raman confocal microscopy technique. Analysis of Raman spectra and pressure dependences of the SWNT resistance shows that the observed structural changes in SWNTs are reversible and no polymerization or collapse are observed.     

Raman spectral features of longer polyynes HC2 nH ({\sf n=4}–8) in SWNTs

Size-selected linear hydrocarbon molecules, polyynes HC_2nH, were contacted in solutions with single-wall carbon nanotubes (SWNTs) prepared from laser-ablated metal/carbon composite rods (Rh/Pt/C) to produce polyyne-encapsulating SWNTs, HC_2nH@SWNT(RhPt). New Raman spectral features were observed at 2120, 2061, 2017, 1982, and 1963 cm^-1 for five polyynes of n=4–8, respectively, and identified as the vibrational excitation of symmetric stretching modes of the molecules inside the SWNTs. The Raman spectra were compared with those observed for polyynes on Ag islands (SERS) and in solutions. The filling factor was investigated from the concentration dependence of the Raman intensity for HC₁₀H@SWNT(NiCo) to give an estimate of one polyyne molecule per ~350 carbon atoms of SWNTs, providing a picture for head-to-tale filling of aligned C₁₀H₂ molecules inside the SWNTs.     

Electrochemical functionalisation of SWNTs with poly(3,4‐ethylenedioxythiophene) evidenced by anti‐Stokes/Stokes Raman spectroscopy

The capability of anti‐Stokes/Stokes Raman spectroscopy to evaluate chemical interactions at the interface of a conducting polymer/carbon nanotubes is demonstrated. Electrochemical polymerisation of the monomer 3,4‐ethylenedioxythiophene (EDOT) on a Au support covered with a single‐walled carbon nanotube (SWNT) film immersed in a LiClO₄/CH₃CN solution was carried out. At the resonant optical excitation, which occurs when the energy of the exciting light coincides with the energy of an electronic transition, poly(3,4‐ethylenedioxythiophene) (PEDOT) deposited electrochemically as a thin film of nanometric thickness on a rough Au support presents an abnormally intense anti‐Stokes Raman spectrum. The additional increase in Raman intensity in the anti‐Stokes branch observed when PEDOT is deposited on SWNTs is interpreted as resulting from the excitation of plasmons in the metallic nanotubes. A covalent functionalisation of SWNTs with PEDOT both in un‐doped and doped states takes place when the electropolymerisation of EDOT, with stopping at +1.6 V versus Ag/Ag⁺, is performed on a SWNT film deposited on a Au plate. The presence of PEDOT covalently functionalised SWNTs is rationalised by (1) a downshift by a few wavenumbers of the polymer Raman line associated with the symmetric C C stretching mode and (2) an upshift of the radial breathing modes of SWNTs, both variations revealing an interaction between SWNTs and the conjugated polymer. Raman studies performed at different excitation wavelengths indicate that the resonant optical excitation is the key condition to observe the abnormal anti‐Stokes Raman effect. Copyright © 2010 John Wiley & Sons, Ltd.     

催化剂比例对单壁碳纳米管制备的影响

采用电弧放电法以Y/Ni为催化剂制备了单壁碳纳米管 ,研究了不同催化剂比例对制备产物的影响。获得了不同激发波长下 (476 5～ 1 0 6 4nm)单壁碳纳米管的拉曼光谱 ,采用图表法对径向呼吸模的谱峰进行了认定。结果表明 :样品中碳管的直径分布在 1 2～ 1 6nm之间 ,直径在 1 43nm附近的碳管居多。催化剂的比例只是影响碳管的产额 ,对其直径分布的影响很小     

单层纳米碳管振动模的拉曼光谱研究

测量了单层纳米碳管的一级、二级拉曼光谱.在40—3300cm^-1范围内观测到18条一级拉曼谱线和7条二级拉曼谱线,理论所预言的谱线几乎全被观测到,谱峰位置和理论值符合得很好.通过和理论值的对照,对这些谱线作了初步标定.所观测到的单层纳米碳管的拉曼激活振动模数目及二级拉曼谱线条数都是迄今最多的. 关键词：     

碳纳米管晶格振动模及拉曼光谱的研究进展

本文介绍了碳纳米管的结构特征和晶格振动模的理论研究 ,综述了不同方法生长的多壁碳纳米管和单壁碳纳米管拉曼光谱的研究进展。另外 ,还简单描述了单壁碳纳米管的应用前景     

低温下单壁碳纳米管拉曼光谱温度效应研究

在单壁碳纳米管的低温拉曼光谱测量过程中,发现径向呼吸模(RBM)和正切拉伸模(GM)的拉曼频移在低温下的温度效应和在高温时的温度效应存在着很大的区别,在低温下拉曼光谱的频移和温度并不呈线性关系。而且,在温度为210K时,单壁碳纳米管内部的振动结构可能发生了变化。在低温下单壁碳纳米管拉曼峰的强度的变化是不可逆的。     

Surface‐enhanced Raman scattering studies of carbon nanotubes using Ag‐core Au‐shell nanoparticles

Surface‐enhanced Raman scattering from carbon nanotube bundles adsorbed with plasmon‐tunable Ag‐core Au‐shell nanoparticles (Ag@Au nps) was carried out for the first time. By utilizing nanoparticles whose plasmon resonance peak (541, 642 nm) closely matches the commonly used Raman excitation sources (532, 632.81 nm), we can observe a large enhancement in the Raman signatures of carbon nanotubes. We obtain greater enhancement in the Raman signal for the above case when compared to nanotubes adsorbed with conventional Ag, Au or other ‘off resonant’ Ag@Au nps. The power‐dependent SERS experiment on single‐walled nanotubes (SWNTs) with resonant Ag@Au nps reveals a linear behavior between the G‐band intensity and the photon flux density, which is in agreement with the vibrational pumping model of SERS. The observed enhancement by resonance matching is pronounced for carbon nanotubes and may lead to insights into understanding nanotube–nanoparticle interaction. Copyright © 2009 John Wiley & Sons, Ltd.     

1.66 μm和 1.55 μm光源对长距离光纤拉曼温度传感器测量时间影响的理论对比

通过理论分析,在综合考虑长距离光纤拉曼温度传感器中各光学器件波长相关特性的基础上,比较了1.66 μm和1.55 μm光源对传感器测量时间的影响.计算结果表明,由于各光学器件在1.5 μm波段的高性能和最大允许入纤脉冲峰值功率的增加,采用1.66 μm光源的光纤拉曼温度传感器,最高可获得约1.94倍的背向反斯托克斯信号.在相同空间分辨率和温度分辨率的情况下,测量时间可以降低约3/4.     

Catalytic activity of Fe, Co and Fe-Co-MCM-41 for the growth of carbon nanotubes by chemical vapour deposition method

Iron, cobalt and a mixture of iron and cobalt incorporated mesoporous MCM-41 molecular sieves were synthesised by hydrothermal method and used to investigate the rules governing their nanotube producing activity. The catalysts were characterised by XRD and N₂ sorption studies. The effect of the catalysts has been investigated for the production of carbon nanotubes at an optimised temperature 750 °C with flow rate of N₂ and C₂H₂ is 140 and 60 ml/min, respectively for a reaction time 10 min. Fe-Co-MCM-41 catalyst was selective for carbon nanotubes with low amount of amorphous carbon with increase in single-walled carbon nanotubes (SWNTs) yield at 750 °C. Formation of nanotubes was studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. Transmission electron microscope and Raman spectrum was used to follow the quality and nature of carbon nanotubes formed and the graphitic layers and disordered band, which shows the clear evidence for the formation of SWNTs, respectively. The result propose that the diameter of the nanotubes in the range of 0.78-1.35 nm. Using our optimised conditions for this system, Fe-Co-MCM-41 showed the best results for selective SWNTs with high yield when compared with Fe-MCM-41 and Co-MCM-41.