Electronic structures and properties of lanthanide hexaboride nanowires

The promising usage of lanthanide hexaboride nanowires as excellent electron emitter materials is generally attributed to the intrinsic low work functions of their bulk counterparts. Most analytical models for the field enhanced electron emission phenomenon adopt an underlying presumption of little or no change to the work function of the emission materials at the nanoscale. However, such a presumption is difficult to experimentally verify because current analytical models often employ empirical parameters such as the geometrically enhancement factors and the actual field emission areas are hard to determine. Herein, we report our density functional theory study of the size-dependence and element-specificity of the electronic structures and work functions of infinitely long lanthanide hexaboride nanowires constructed with n × n × ∞ unit cells (n = 1, 2, 3, and 4). Our modeling results reveal that the distinguished metal-like electronic properties and the low work function values of the sides of most examined nanowire systems are due to the abundant 4f and 5d states from the lanthanide metal atoms positioned at the Fermi level. These work function values are found to be weakly wire-size-dependent and element-dependent across the lanthanide series. They approach to the bulk values when their lateral wire-sizes are at or above 4-unit cell wide. The presence of abundance states at the Fermi level is found to be a common feature to rationalize the work functions of reported hexaboride systems.