Modular Design via Multiple Anion Chemistry of the High Mobility van der Waals Semiconductor Bi 4 O 4 SeCl 2 .

Making new van der Waals materials with electronic or magnetic functionality is a chemical design challenge for the development of two-dimensional nanoelectronic and energy conversion devices. We present the synthesis and properties of the van der Waals material Bi 4 O 4 SeCl 2 , which is a 1:1 superlattice of the structural units present in the van der Waals insulator BiOCl and the three-dimensionally connected semiconductor Bi 2 O 2 Se. The presence of three anions gives the new structure both the bridging selenide anion sites that connect pairs of Bi 2 O 2 layers in Bi 2 O 2 Se and the terminal chloride sites that produce the van der Waals gap in BiOCl. This retains the electronic properties of Bi 2 O 2 Se while reducing the dimensionality of the bonding network connecting the Bi 2 O 2 Se units to allow exfoliation of Bi 4 O 4 SeCl 2 to 1.4 nm height. The superlattice structure is stabilized by the configurational entropy of anion disorder across the terminal and bridging sites. The reduction in connective dimensionality with retention of electronic functionality stems from the expanded anion compositional diversity.