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Self-Assembled Nanostructures

1. Self-Assembly of POM-POSS Janus Molecules

In collaboration with Prof. Wei Wangs group from Nankai University, we have studied the self-assembly of Janus type molecule consisting of a polyoxymetallate (POM) and a silsesquioxanes (POSS) interconnected by different organic linkers to understand the fundamental contributions controlling the self-assembled structure.

As one example, we have studied the supramolecular assembly of POM-C4-POSS molecules using low-dose HAADF-STEM and STEM tomography. Electron tomography revealed the formation of a hexagonal columnar self-assembled structure in agreement with coarse-grained simulations of the self-assembled structure. A more detailed analysis by low-dose STEM shows that the hexagonal structure consists of a core of POSS and the organic linker and the walls consist of a POM bilayer (Figure 1). However, the low-dose STEM images also reveal that the structure is not a regular hexagonal network, but a significantly distorted, tessellated structure (Figure 2). Moreover, also the arrangement of the POM clusters is varying within the bilayer. This is the results of structural frustration with phase separation favoring a hexagonal arrangement, whereas a regular packing of the POM layers would require straight interfaces. As the short organic linker only enables a very limited distance between the POM and POSS groups, the apparent solution is the distorted structure providing a local compromise between both controlling forces.

Low-dose STEM imaging of POM-C4-POSS HRSTEM image of POM-C4-POSS

Figure 1: Low-dose HAADF-STEM image of POM-C4-POSS showing a hexagonal columnar structure for suitably oriented crystals. The detailed image on the right reveals the individual POM clusters as bright dots arranged in a bilayer around the POSS core.

 

Low-dose STEM image of POM-C4-POSS with structural distotions

Figure 2: Details of the low-dose STEM image revealing the tessellated hexagonal and the distorted POM bilayer structure.

The low-dose STEM images further reveal characteristic defects close to the surface of the nanoparticles consisting of extended POM/POSS layers with only a single POM row at the surface (figure 3). These defects are presumable the starting point for the bicrystals often observed in electron tomography sharing a common {100} plane. 

Low-dose STEM image of defects in POM-C4-POSS 

Figure 3: Low-dose STEM image of characteristic defects close to the surface

 

 

Details of this study have been published and are available at

  • C. Ma et al.: An artificial honeycomb structured crystal formed by self-assembly of an asymmetric co-cluster, Angew. Chem. Int. Ed. Engl, 2016, Vol. 54, 15699-15704, DOI: 10.1002/anie.201507237 .