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Publication Title | POLYMER NANOCRYSTALS A general route to nanocrystal kebabs periodically assembled on stretched flexible polymer shish

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A general route to nanocrystal kebabs periodically assembled on stretched flexible polymer shish

Hui Xu,1,2 Yuci Xu,3 Xinchang Pang,1 Yanjie He,1 Jaehan Jung,1 Haiping Xia,2* Zhiqun Lin1*

2015 © The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC). 10.1126/sciadv.1500025

Assembling nanoparticles into one-dimensional (1D) nanostructures with precisely controlled size and shape ren- ders the exploration of new properties and construction of 1D miniaturized devices possible. The physical proper- ties of such nanostructures depend heavily on the size, chemical composition, and surface chemistry of nanoparticle constituents, as well as the close proximity of adjacent nanoparticles within the 1D nanostructure. Chemical syn- thesis provides an intriguing alternative means of creating 1D nanostructures composed of self-assembled nano- particles in terms of material diversity, size controllability, shape regularity, and low-cost production. However, this is an area where progress has been slower. We report an unconventional yet general strategy to craft an exciting variety of 1D nanonecklace-like nanostructures comprising uniform functional nanodiscs periodically assembled along a stretched flexible polymer chain by capitalizing on judiciously designed amphiphilic worm-like diblock copolymers as nanoreactors. These nanostructures can be regarded as organic-inorganic shish-kebabs, in which nanodisc kebabs are periodically situated on a stretched polymer shish. Simulations based on self-consistent field theory reveal that the formation of organic-inorganic shish-kebabs is guided by the self-assembled elongated star-like diblock copolymer constituents constrained on the highly stretched polymer chain.


Owing to their structural anisotropy and unique optical, electrical, mag- netic, and catalytic properties, the synthesis of one-dimensional (1D) inorganic nanostructures, including nanorods, nanowires, nanobelts, and nanotubes (1–4), has emerged as an important scientific activity in nanomaterials and nanotechnology for use in solar cells (5, 6), su- percapacitors (7), hydrogen generation (8), photodetectors (9), light- emitting diodes (10), field-effect transistors (11), and biosensors (12, 13), to name but a few. 1D nanostructures are often produced by template- assisted electrochemical deposition (14), virus-enabled synthesis (15), chemical vapor deposition (16), and controlled colloidal synthesis (17, 18). In the latter context, effective colloidal synthetic approaches to 1D inorganic necklace-like morphology composed of self-assembled, periodically positioned nanocrystals with well-controlled dimension, chemical composition, and phase purity are comparatively few and limited in scope (19, 20).

The concept of organic molecular necklace was first proposed two decades ago for the description of a rotaxane containing many threaded a-cyclodextrins (a-CDs) (21). Inorganic nanonecklaces can be regarded as 1D nanostructures with desired functions and properties comprising well-defined nanoscopic building blocks. Many efforts in this area have been devoted to exploiting self-assembly techniques based on intrinsic interactions between the building blocks [for example, van der Waals force, electrostatic interactions (22), and dipole-dipole attraction (23)] or external fields [for example, a magnetic field (24, 25)] to achieve 1D nanonecklace-like nanostructures. However, most of the tech- niques are subject to certain limitations, such as non-uniform size and shape (26), and are difficult to generalize. It is worth noting that in comparison to the bulk reports on synthesis of nanotubes and nano-

1School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA. 2State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China. 3Department of Polymer Science and Engineering, Faculty of Materials Science and Chemical Engineering, Key Laboratory of Specialty Polymers, Ningbo University, Ningbo, Zhejiang 315211, China.

*Corresponding author. E-mail: (Z.L.); (H.X.)

wires, the preparation of nanonecklaces by chemical synthesis instead of by self-assembly or the application of external fields is surpris- ingly rare.

Here, we demonstrate a general and robust strategy for the in situ synthesis of a variety of 1D inorganic nanonecklaces composed of pe- riodically assembled, uniform nanocrystals with precisely controlled size and composition by using the rationally designed, amphiphilic, unimolec- ular, worm-like diblock copolymers poly(acrylic acid)-block-polystyrene (PAA-b-PS) as nanoreactors. First, nanoreactors are judiciously syn- thesized by sequential atom transfer radical polymerization (ATRP) of tert-butylacrylate (tBA) and styrene (St) from a polyrotaxane-based macro- initiator, yielding worm-like poly(tert-butylacrylate)-block-polystyrene (PtBA-b-PS), followed by the hydrolysis of PtBA into PAA (that is, yielding PAA-b-PS). The polyrotaxane-based macroinitiator is virtually prepared by forming an inclusion complex between a-CD and linear polyethylene glycol (PEG). Subsequently, the preferential coordination interaction between metal moieties of precursors and functional groups of PAA in worm-like PAA-b-PS creates PS-capped inorganic nanonecklaces in which a wide range of regularly spaced disc-like nanocrystals (here- after referred to as nanodiscs), including semiconductor CdSe, mag- netic Fe3O4, and ferroelectric BaTiO3, are threaded by the flexible yet stretched PEG chain. Quite intriguingly, because PEG acts as the central stem with nanodiscs periodically grown on it, such linearly assembled nanonecklaces resemble organic-inorganic nanohybrid shish-kebabs. The nanodiscs (kebabs) are oriented perpendicular to the PEG shish, although the latter cannot be identified under transmission electron microscope (TEM) because of small size and low electron density. The average distance between adjacent nanodisc kebabs is 2 nm, with an average nanodisc size of 4 nm. To elucidate the mechanism governing the formation of nanonecklaces, we performed simulations based on self-consistent field theory (SCFT) and calculated the dimensions of nanonecklaces (that is, diameter, thickness, and spacing) from different samples, which are in substantial agreement with the experimental results. Our work reveals the versatility of the strategy of capitalizing on amphiphilic worm-like diblock copolymer template for directing the formation of a wide diversity of periodically spaced nanocrystals.

Xu et al. Sci. Adv. 2015;1:e1500025 27 March 2015

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