Nanotube coalescence-inducing mode: A novel vibrational mode in carbon systems
Nanotube coalescence-inducing mode: A novel vibrational mode in carbon systems
复制标题
DOI：
10.1002/smll.200600087
复制
发表时间：
2006-08-01
期刊：
影响因子：
13.3
通讯作者：
Dresselhaus, Mildred S.
中科院分区：
文献类型：
--
作者：
Endo, Morinobu;Kim, Yoong Ahm;Dresselhaus, Mildred S.
我们报告了位于1855 cm-1处的共振拉曼模式的出现，该模式与线性碳链的振动有关[1]，并被观察到为高度纯化的双壁碳纳米管（DWNT）合并的前体。这种模式，被称为“聚结诱导模式”（CIM），它启动的聚结过程，是由热退火引起的，其效果是增强硼掺杂。CIM模式产生于在相邻管之间共价建立的短1D碳链（例如，3-7个原子长）的产生。分子动力学计算表明，线性碳链首先建立在管和这些链触发纳米管聚结通过拉链模型。实验上我们注意到，当管合并时，CIM振动消失。对振动模式的进一步计算也表明，这个特定的频率对应于只含有几个碳原子的线性链。虽然拉曼光谱已经被证明是表征碳纳米管的有力工具，[2]这项技术已被广泛用于研究热处理单壁碳纳米管（SWNT），[3-7]纳米角，[8，9]豆荚，[10]和双壁碳纳米管（DWNT）的结构信息（例如，管直径和结构完善程度）。[11]在这项研究中，我们报告的拉曼活性模式在1855 cm-1，突然出现在DWNTs的热处理温度Thtt达到一个临界值的观察。该模式的开始触发DWNT聚结。因此，这种模式被称为聚结诱导模式（CIM）。的CIM振动的发病显示发生在一个低得多的热处理温度时，硼添加，显示硼作为催化剂或焊接剂的聚结过程。一旦在硼的存在下相邻的DWNT之间的聚结过程正在进行中，CIM拉曼模式消失，硼被纳入六方碳网络，这导致在无序诱导的D带的外观。的拉曼光谱的依赖性的Thtt和硼掺杂的研究，结合高分辨率透射电子显微镜（HRTEM）的研究相同的样品时，提供了清晰的洞察到聚结机制。CIM振动被确定为与小的线性sp链的碳原子共价连接相邻的DWNTs在聚结过程中。当管合并时，CIM模式消失，因为管完全合并并且没有碳链在高温下存活。因此，拉曼光谱-ACHTUNGTRENNUNGcopy现在能够监测DWNT中纳米管聚结过程的起始，并且能够识别参与DWNT聚结早期阶段的1D碳链。值得注意的是，sp碳链也出现在其他类型的碳相关系统中[1]，但尚未得到系统的研究。通过催化化学气相沉积（CCVD）方法合成高度纯化的DWNT束，如前所述，在水平反应器炉的一侧使用调节催化剂，在炉的中间使用纳米管催化剂。[12-14]为了获得纯的DWNTs，对合成产物应用了一种新的优化的两步纯化工艺。[12]仔细的HRTEM观察证实了高产量的DWNT（95%）成束，[12]和磁化率研究证实了高DWNT样品纯度通过其抗磁性行为。[15]这里的讨论集中在观察…
We report on the appearance of a resonant Raman mode located at 1855 cmÀ1, which is related to vibrations of linear carbon chains [1] and is observed as a precursor to the merging of highly purified double-walled carbon nanotubes (DWNTs). This mode, termed the “coalescence-inducing mode”(CIM), which initiates the coalescence process, is induced by thermal annealing and its effect is enhanced by boron doping. The CIM mode arises from the generation of short 1D carbon chains (eg, 3–7 atoms long) established covalently between adjacent tubes. Molecular dynamics calculations demonstrate that linear carbon chains are first established between the tubes and these chains trigger the nanotube coalescence via a zipper model. Experimentally we noted that as the tubes coalesce, the CIM vibration disappears. Additional calculations on the vibrational modes also show that this specific frequency corresponds to linear chains containing only a few carbon atoms. While Raman spectroscopy has already been shown to provide a powerful tool for the characterization of carbon nanotubes,[2] this technique has been widely utilized for studying structural information (eg, tube diameter and the degree of structural perfection) of thermally treated singlewalled carbon nanotubes (SWNTs),[3–7] nanohorns,[8, 9] peapods,[10] and double-walled carbon nanotubes (DWNTs).[11] In this study, we report the observation of a Raman-active mode at 1855 cmÀ1 that appears suddenly in DWNTs as the heat-treatment temperature Thtt reaches a critical value. The onset of this mode triggers DWNT coalescence. This mode is therefore called the coalescence-inducing mode (CIM). The onset of the CIM vibration is shown to occur at a much lower heat-treatment temperature upon boron addition, showing boron to act as a catalyst or welding agent for the coalescence process. Once the coalescence process between adjacent DWNTs in the presence of boron is underway, the CIM Raman mode vanishes, and boron is incorporated within the hexagonal carbon network, which results in the appearance of the disorder-induced Dband. Study of the dependence of the Raman spectra on Thtt and boron doping, when combined with high-resolution transmission electron microscopy (HRTEM) studies of the same samples, provides clear insights into the coalescence mechanism. The CIM vibration is identified with small linear sp chains of carbon atoms that covalently connect adjacent DWNTs during coalescence. As the tubes coalesce, the CIM mode disappears because the tubes coalesce completely and no carbon chains survive the high temperatures. Therefore, Raman spectros-ACHTUNGTRENNUNGcopy is now able to monitor the initiation of the nanotube coalescence process in DWNTs, and is capable of identifying the 1D carbon chains that participate in the early stages of DWNT coalescence. It is noteworthy that sp carbon chains also appear in other types of carbon-related systems,[1] but have not yet been studied systematically. Highly purified DWNT bundles were synthesized by a catalytic chemical vapor deposition (CCVD) method using a conditioning catalyst on one side of the horizontal reactor furnace and a nanotube catalyst in the middle of the furnace as described previously.[12–14] In order to obtain pure DWNTs, a novel optimized two-step purification process was applied to the synthesized products.[12] Careful HRTEM observations confirmed a high yield of DWNTs (95%) in bundles,[12] and magnetic susceptibility studies confirmed the high DWNT sample purity through their diamagnetic behavior.[15]The discussion here is focused on the observation of the …