Evidence for Orientational Order in Nanothreads Derived from Thiophene

Nanothreads
are one-dimensional sp3 hydrocarbons that
pack within pseudohexagonal crystalline lattices. They are believed
to lack long-range order along the thread axis and also lack interthread
registry. Here we investigate the phase behavior of thiophene up to
35 GPa and establish a pressure-induced phase transition sequence
that mirrors previous observations in low-temperature studies. Slow
compression to 35 GPa results in the formation of a recoverable saturated
product with a 2D monoclinic diffraction pattern along (0001) that
agrees closely with atomistic simulations for single crystals of thiophene-derived
nanothreads. Paradoxically, this lower-symmetry packing signals a
higher degree of structural order since it must arise from constituents
with a consistent azimuthal orientation about their shared axis. The
simplicity of thiophene reaction pathways (with only four carbon atoms
per ring) apparently yields the first nanothreads with orientational
order, a striking outcome considering that a single point defect in
a 1D system can disrupt long-range structural order.

纳米线（nanothreads）是一类一维sp³杂化烃类，可组装于伪六角形晶格之中。据推测，这类纳米线沿自身轴线缺乏长程有序性，且纳米线间不存在配准关系。本研究针对压力最高达35 GPa的噻吩（thiophene）开展相行为研究，确立了其压力诱导的相变序列，该序列与此前低温研究中的观测结果高度吻合。以缓慢加压方式升至35 GPa时，可得到可回收的饱和产物，该产物沿(0001)方向呈现二维单斜衍射花样，其结果与噻吩衍生纳米线单晶的原子模拟结果高度一致。矛盾的是，这种低对称性的堆积方式反而体现出更高的结构有序度——因为其必然源自沿公共轴线具有一致方位角取向的结构单元。噻吩的反应路径较为简洁（每个环仅含4个碳原子），本研究显然得到了首批具有取向有序性的纳米线；考虑到一维体系中仅单个点缺陷即可破坏长程结构有序性，这一结果尤为引人注目。