Department of Chemistry, University of Pennsylvania , 231 South 34th Street, Philadelphia, Pennsylvania 19104-6323, United States.
Nano Lett. 2014 Feb 12;14(2):504-11. doi: 10.1021/nl403511s. Epub 2014 Jan 2.
We utilize individualized, length-sorted (6,5)-chirality enriched single-walled carbon nanotubes (SWNTs) having dimensions of 200 and 800 nm, femtosecond transient absorption spectroscopy, and variable excitation fluences that modulate the exciton density per nanotube unit length, to interrogate nanotube exciton/biexciton dynamics. For pump fluences below 30 μJ/cm(2), transient absorption (TA) spectra of (6,5) SWNTs reveal the instantaneous emergence of the exciton to biexciton transition (E11 → E11,BX) at 1100 nm; in contrast, under excitation fluences exceeding 100 μJ/cm(2), this TA signal manifests a rise time (τ rise ∼ 250 fs), indicating that E11 state repopulation is required to produce this signal. Femtosecond transient absorption spectroscopic data acquired over the 900-1400 nm spectral region of the near-infrared (NIR) region for (6,5) SWNTs, as a function of nanotube length and exciton density, reveal that over time delays that exceed 200 fs exciton-exciton interactions do not occur over spatial domains larger than 200 nm. Furthermore, the excitation fluence dependence of the E11 → E11,BX transient absorption signal demonstrates that relaxation of the E11 biexciton state (E11,BX) gives rise to a substantial E11 state population, as increasing delay times result in a concomitant increase of E11 → E11,BX transition oscillator strength. Numerical simulations based on a three-state model are consistent with a mechanism whereby biexcitons are generated at high excitation fluences via sequential SWNT ground- and E11-state excitation that occurs within the 980 nm excitation pulse duration. These studies that investigate fluence-dependent TA spectral evolution show that SWNT ground → E11 and E11 → E11,BX excitations are coresonant and provide evidence that E11,BX → E11 relaxation constitutes a significant decay channel for the SWNT biexciton state over delay times that exceed 200 fs, a finding that runs counter to assumptions made in previous analyses of SWNT biexciton dynamical data where exciton-exciton annihilation has been assumed to play a dominant role.
我们利用尺寸为 200 和 800nm 的个体化、长度排序的（6,5）手性富集单壁碳纳米管（SWNTs），飞秒瞬态吸收光谱和可变激发强度，调制每个纳米管单元长度的激子密度，来研究纳米管激子/双激子动力学。对于低于 30 μJ/cm(2)的泵浦强度，（6,5）SWNTs 的瞬态吸收（TA）光谱显示在 1100nm 处激子到双激子跃迁（E11 → E11,BX）的瞬间出现；相比之下，在激发强度超过 100 μJ/cm(2)的情况下，这个 TA 信号具有一个上升时间（τrise∼250fs），表明需要重新填充 E11 态来产生这个信号。在近红外（NIR）区域的 900-1400nm 光谱区域内，随着纳米管长度和激子密度的变化，获得的（6,5）SWNTs 的飞秒瞬态吸收光谱数据表明，在超过 200fs 的时间延迟后，激子-激子相互作用不会在大于 200nm 的空间区域发生。此外，E11 → E11,BX 瞬态吸收信号的激发强度依赖性表明，E11 双激子态（E11,BX）的弛豫会导致 E11 态的大量填充，随着延迟时间的增加，E11 → E11,BX 跃迁的振子强度也会相应增加。基于三态模型的数值模拟与一种机制一致，即通过在 980nm 激发脉冲持续时间内顺序激发 SWNT 的基态和 E11 态，可以在高激发强度下产生双激子。这些研究表明，激发强度依赖的 TA 光谱演化表明，SWNT 的基态到 E11 态和 E11 态到 E11,BX 态的激发是共振的，并提供了证据表明，在超过 200fs 的延迟时间内，E11,BX → E11 弛豫构成了 SWNT 双激子态的一个重要衰减通道，这一发现与以前分析 SWNT 双激子动力学数据时的假设相矛盾，以前的假设认为激子-激子湮灭起主导作用。
