通过NMR确定单壁碳纳米管的电子结构
1Department of Physics and Astronomy and Curriculum in Applied and Materials Sciences, University of North Carolina, Chapel Hill, NC 27599-3255, USA.
Science (New York, N.Y.)
|April 25, 2000
概括
单壁碳纳米管表现出不同的核自旋行为. 金属纳米管表现出快速放松,而半导体的放松缓慢,两者都受到氧气暴露的影响.
科学领域:
- 材料科学 材料科学 材料科学
- 固态物理 固态物理
- 频谱学是一种光谱学.
背景情况:
- 单壁碳纳米管 (SWCNTs) 是具有独特电子特性的关键纳米材料.
- 了解它们的电子结构,特别是费米级的状态密度 (DOS),是它们应用的关键.
- 核磁共振 (NMR) 光谱为探测这些特性提供了一种强大,非破坏性的方法.
研究的目的:
- 通过碳-13 (13C) NMR,研究金属和半导体SWCNT的电子特性.
- 为了将不同SWCNT直径的自旋格子放松率与费米水平的状态密度相关联.
- 评估氧气暴露对SWCNT电子特性和放松动态的影响.
主要方法:
- 使用13C NMR光谱学研究SWCNTs.
- 分析不同的13C核旋转群体的旋转放松时间 (T1).
- 与SWCNT直径和电子特征相关联的NMR数据.
主要成果:
- 确定了两个不同的13C核自旋群体,具有不同的自旋格子放松率.
- 归因于金属SWCNTs的快速放松组件,表现出与金属系统一致的放松行为.
- 费米水平的状态密度随着金属SWCNT直径的减少而增加.
- 缓慢放松的组件,可能来自半导体SWCNT,在费米水平上显示出明显较低的DOS.
- 暴露在氧气中大大改变了金属和半导体SWCNT组件的放松率.
结论:
- 基于它们的放松动态,13C NMR有效地区分金属和半导体SWCNT.
- 这项研究证实了SWCNT直径,电子结构和放松行为之间的关系.
- 氧气显著影响SWCNTs的电子特性,影响它们的NMR放松特性.
相关概念视频
01:13¹H NMR: Complex Splitting
A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first.
00:52¹H NMR of Conformationally Flexible Molecules: Temporal Resolution
At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...
01:10Carbon-13 (¹³C) NMR: Overview
Carbon-13 is a naturally occurring NMR-active isotope of carbon with a low natural abundance of 1.1%. In contrast, carbon-12 is the most abundant isotope of carbon with zero nuclear spin. Therefore, it is NMR inactive. The gyromagnetic ratio of carbon-13 is smaller than that of protons. As a result, carbon-13 resonance is about 6000 times weaker than proton resonance. For a given magnetic field strength, the resonance frequency of carbon-13 is about one-fourth of the resonance frequency for...
01:12Two-Dimensional (2D) NMR: Overview
The 1D NMR spectrum of large and complex molecules like natural products has complicated splitting patterns and overlapping signals, which can be easily interpreted using 2-dimensional (2D) NMR. Unlike 1D NMR, 2D NMR has two frequency axes that provide the coupling information between the nucleus A and nucleus B in a molecule. The process from which 2D spectra are obtained has four steps.
The first step is the preparation period, during which nucleus A is excited with a radiofrequency pulse.
The first step is the preparation period, during which nucleus A is excited with a radiofrequency pulse.
01:15¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR
The axial and equatorial protons in cyclohexane can be distinguished by performing a variable-temperature NMR experiment. In this process, except for one proton, the remaining eleven protons are replaced by deuterium. The deuterium substitution avoids the possible peak splitting caused by the spin-spin coupling between the adjacent protons. The remaining proton flips between the axial and equatorial positions.
01:04¹³C NMR: ¹H–¹³C Decoupling
The probability of having two carbon-13 atoms next to each other is negligible because of the low natural abundance of carbon-13. Consequently, peak splitting due to carbon-carbon spin-spin coupling is not observed in spectra. However, protons up to three sigma bonds away split the carbon signal according to the n+1 rule, resulting in complicated spectra.
A broadband decoupling technique is used to simplify these complex, sometimes overlapping, signals. Broadband decoupling relies on a...
A broadband decoupling technique is used to simplify these complex, sometimes overlapping, signals. Broadband decoupling relies on a...