Theoretical Insights into Chemical Functionalization of Carbon Nanotubes: from Chirality Separation to Photoexcited Dynamics

Svetlana V Kilina（北达科他州立大学法戈分校）、Dmitri S Kilin（南达科他州立大学法戈分校）和Andrei Kryjevski（北达科他州立大学法戈分校）获得了由大分子、超分子和纳米化学（MSN）项目、刺激竞争性研究实验项目办公室（EPSCoR）和化学理论、模型和计算方法（CTMC）项目共同资助的奖项。该项目是使用计算技术来研究碳纳米管（CNTs），这是一种由碳连接在一起的小管。一系列的碳碳键可以绕管轴旋转，不同的螺旋速率表示不同类型的CNTs。这些管可以用小有机分子或聚合物的附着物来装饰（功能化），并且可以选择附着物和位置，以便设计，例如，当外部光线照射到碳纳米管上时，会发出什么光以及它的强度如何。我们追求的另一个目标是利用计算优化每种碳纳米管的分子修饰，以便利用化学过程分离具有不同螺旋速率的粘性碳纳米管束，这是碳纳米管的通常生产方式。将它们分离是开展碳纳米管实验的理想选择，因为这样实验可以提供更多的鉴别信息。对修饰或功能化碳纳米管的计算使用量子力学和经典力学来解释和预测与实验相关的性质。本研究旨在促进对功能化碳纳米管中太阳能捕获和转化过程的基础控制，因此将与化学、材料科学和工程相关。小有机表面活性剂和共轭聚合物对碳纳米管的功能化可以改善其光电和输运性质的化学控制，并为获得具有类似手性的碳纳米管样品的分离技术提供了希望。本研究的目的是对不同类型的共轭低聚物和聚合物功能化碳纳米管的形态、电子结构和激发态动力学进行系统的理论研究。利用计算模型，研究人员正在努力理解发生在有机-无机界面上的基本物理过程，并控制基于碳纳米管的光电和光伏器件的操作，如激子弛豫和重组，电荷转移，以及从一个吸收光子产生几个激子（载流子倍增）。密度泛函理论（DFT）和时变DFT （TDDFT）被用于精确计算聚合物-碳纳米管杂化结构的电子结构和光响应。此外，量子经典非绝热动力学方法被用于模拟声子介导的动力学，包括这些系统中的电荷和能量转移。基于量子场论中的多体微扰理论增强的DFT，一种计算量子效率（光子产生的激子数）的新方法正被应用于功能化碳纳米管中。各种光谱观测值的计算允许与超快激光光谱数据（由其他实验室）直接比较和验证，探测功能化碳纳米管的基本电子动力学。同样重要的是，这些计算为实验测量中出现的关键现象提供了明确的解释和预测。

Svetlana V Kilina (North Dakota State University Fargo), Dmitri S Kilin (University of South Dakota) and Andrei Kryjevski (North Dakota State University Fargo) are supported in an award co-funded by the Macromolecular, Supramolecular and Nanochemistry (MSN) Program, the Office of the Experimental Program to Stimulate Competitive Research (EPSCoR) and the Chemical Theory, Models and Computational Methods (CTMC) program. The project is to use computational techniques to investigate carbon nanotubes (CNTs), which are small tubes with carbon connected together. Series of carbon-carbon bonds may spiral around the axis of the tube, and different spiraling rates indicate different types of CNTs. The tubes can be decorated with attachments of small organic molecules or polymers (functionalization), and the choices of which attachments and where can be customized so as to design, for example, what light comes out and how intense it is when an outside light shines on a CNT. Another goal being pursued is to use computations to optimize the molecular decoration for each type of CNT so that chemical processes may be used to separate sticky bundles of CNTs with different rates of spiral, which is the usual way CNTs are produced. Having them separated is ideal for carrying out experiments on CNTs, since experiments can then provide much more discriminating information. The computations for the decorated or functionalized CNTs use both quantum and classical mechanics to interpret and predict properties relevant to experiments. This research aims to facilitate control over the processes of solar energy capture and conversion in functionalized CNTs at the fundamental level, and, thus, will be relevant to chemistry, materials science and engineering.Functionalization of CNTs by small organic surfactants and conjugated polymers can improve chemical control of their optoelectronic and transport properties as well as offer promise in separation techniques for obtaining CNT samples with similar chiralities. The objective of this research is a systematic theoretical investigation of morphology, electronic structure, and excited state dynamics of CNTs functionalized by different types of conjugated oligomers and polymers. Using computational modeling, the investigators are working to understand fundamental physical processes occurring at organic-inorganic interfaces and governing the operation of CNT-based optoelectronic and photovoltaic devices such as exciton relaxation and recombination, charge transfer, and creation of several excitons from one absorbed photon (carrier multiplication). Density functional theory (DFT) and time dependent DFT (TDDFT) are being used for computing accurate electronic structure and optical response of polymer-CNT hybrids. Furthermore, quantum-classical non-adiabatic dynamics methods are being used to model phonon-mediated dynamics including charge and energy transfer in these systems. A novel approach to the calculation of quantum efficiency, the number of excitons generated by a photon, based on DFT augmented by the many body perturbation theory technique of quantum field theory is being applied to functionalized CNTs. Calculations of a variety of spectroscopic observables permit direct comparison and validation with ultrafast laser spectroscopic data (by other lasboratories) probing the fundamental electronic dynamics in functionalized CNTs. Just as importantly, these calculations provide explicit interpretations and predictions of the key phenomena occurring in the experimental measurements.