Adv. Mater. http://doi.org/p34 (2013)
Mixing compositionally identical but structurally different crystals in a controlled way creates a polytypic metamaterial with potentially useful optical or electronic properties. The prototypical mix is that of wurtzite and zincblende structured crystals of a III–V compound such as indium arsenide. In terms of the stacking of InAs bilayers, the (hexagonal) wurtzite structure is represented by the ABAB... sequence and (cubic) zincblende as ABCABC... (the sequences of letters indicate bilayers that are shifted relative to each other).
Nanowires, with their large aspect ratios, are at present the best platform for the controlled growth of polytypes. And now, Luying Li and colleagues have determined the local electric polarization fields in a polytypic InAs nanowire using electron-microscopy techniques — an exemplary piece of structure-property analysis.
From high-resolution images of the projected positions of atomic In and As columns (pictured), Li et al. were able to derive the spontaneous polarization throughout wurtzite structured InAs regions: these lack inversion symmetry, a prerequisite for polarization. The authors found that the polarization values vary significantly along the stacking direction, and attribute this variation to strain that develops from the wurtzite–zincblende interfaces.