Solvent-Mediated Affinity of Polymer-Wrapped Single-Walled Carbon Nanotubes for Chemically Modified Surfaces

Semiconducting
single-walled carbon nanotube (s-CNT) arrays are being explored for
next-generation semiconductor electronics. Even with the multitude
of alignment and spatially localized s-CNT deposition methods designed
to control s-CNT deposition, fundamental understanding of the driving
forces for s-CNT deposition is still lacking. The individual roles
of the dispersant, solvent, target substrate composition, and the
s-CNT itself are not completely understood because it is difficult
to decouple deposition parameters. Here, we study poly­[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(6,6′-[2,2′-{bipyridine}])]
(PFO-BPy)-wrapped s-CNT deposition from solution onto a chemically
modified substrate. We fabricate various self-assembled monolayers
(SAMs) to gain a greater understanding of substrate effects on PFO-BPy-wrapped
s-CNT deposition. We observe that s-CNT deposition is dependent on
both the target substrate and s-CNT dispersion solvent. To complement
the experiments, molecular dynamics simulations of PFO-BPy-wrapped
s-CNT deposition on two different SAMs are performed to obtain mechanistic
insights into the effect of the substrate and solvent on s-CNT deposition.
We find that the global free-energy minimum associated with favorable
s-CNT adsorption occurs for a configuration in which the minimum of
the solvent density around the s-CNT coincides with the minimum of
the solvent density above a SAM-grafted surface, indicating that solvent
structure near a SAM-grafted surface determines the adsorption free-energy
landscape driving s-CNT deposition. Our results will help guide informative
substrate design for s-CNT array fabrication in semiconductor devices.

半导体单壁碳纳米管（semiconducting single-walled carbon nanotube, s-CNT）阵列正被探索应用于下一代半导体电子器件领域。尽管已有大量旨在调控s-CNT沉积的取向与空间局域沉积方法问世，但学界对s-CNT沉积的核心驱动力仍缺乏基础认知。由于难以解耦各沉积参数间的耦合关系，分散剂、溶剂、目标衬底成分以及s-CNT本身各自所发挥的作用仍未被完全阐明。本研究针对经聚[(9,9-二辛基芴基-2,7-二基)-alt-共聚-(6,6′-[2,2′-联吡啶])]（PFO-BPy）包裹的s-CNT在化学改性衬底上的溶液沉积过程展开研究。我们制备了多种自组装单分子层（self-assembled monolayers, SAMs），以更深入地探究衬底对PFO-BPy包裹s-CNT沉积过程的影响。实验结果表明，s-CNT的沉积同时依赖于目标衬底与s-CNT分散溶剂。为补充实验研究，我们针对两种不同SAM衬底上PFO-BPy包裹s-CNT的沉积过程开展分子动力学模拟（molecular dynamics simulations），以从机理层面解析衬底与溶剂对s-CNT沉积的调控作用。我们发现，当s-CNT周围的溶剂密度最小值与SAM修饰表面上方的溶剂密度最小值相匹配时，会出现利于s-CNT吸附的全局自由能极小值位点，这表明SAM修饰表面附近的溶剂结构，决定了驱动s-CNT沉积的吸附自由能景观。本研究结果可为半导体器件中s-CNT阵列制备的衬底优化设计提供有力指导。