(146f) The Role of Van Der Waals Interactions In Predicting the Band Gap of BiI3

Bismuth tri-iodide (BiI₃) is a semiconductor material for potential use as a room temperature gamma-ray detection, primarily due to its intermediate band gap, high density, and high effective atomic number. At room temperature, BiI₃ has a rhombohedral crystal structure (space group R-3, No. 148) with a layered structure of highly ionic Bi-I bonds in the layers and weak van der Waals bonding between layers. The band gap energy of the semiconductor material determines the number of electron-hole pairs generated under the radiation, and affects the energy resolution of the radiation detector. Despite the need to describe accurately the electronic structure of BiI₃, the experimentally and computationally determined band gap energy of BiI₃ has been reported with values ranging from 1.67 eV to 2.2 eV. More surprisingly, the band gap is overestimated by standard density functional theory (DFT), which is well known to underestimate band gaps. In this work, the Grimme’s potential for van der Waals interactions is modified and incorporated within DFT software. The validity of the DFT-D3 model is discussed and comparisons of standard DFT and DFT-D3 results are made for various materials containing van der Waals interactions. The inclusion of van der Waals interactions on the predicted band gap in BiI₃ is also presented.