Materials and Structures Laboratory, ‡Materials Research Center for Element Strategy, and #Frontier Research Center, Tokyo Institute of Technology , Yokohama 226-8503, Japan.
J Am Chem Soc. 2014 Oct 22;136(42):14959-65. doi: 10.1021/ja507890u. Epub 2014 Oct 10.
β-BaZn2As2 is known to be a p-type semiconductor with the layered crystal structure similar to that of LaZnAsO, leading to the expectation that β-BaZn2As2 and LaZnAsO have similar bandgaps; however, the bandgap of β-BaZn2As2 (previously reported value ~0.2 eV) is 1 order of magnitude smaller than that of LaZnAsO (1.5 eV). In this paper, the reliable bandgap value of β-BaZn2As2 is determined to be 0.23 eV from the intrinsic region of the temperature dependence of electrical conductivity. The origins of this narrow bandgap are discussed based on the chemical bonding nature probed by 6 keV hard X-ray photoemission spectroscopy, hybrid density functional calculations, and the ligand theory. One origin is the direct As-As hybridization between adjacent [ZnAs] layers, which leads to a secondary splitting of As 4p levels and raises the valence band maximum. The other is that the nonbonding Ba 5d(x(2)-y(2)) orbitals form an unexpectedly deep conduction band minimum (CBM) in β-BaZn2As2 although the CBM of LaZnAsO is formed mainly of Zn 4s. These two origins provide a quantitative explanation for the bandgap difference between β-BaZn2As2 and LaZnAsO.
β-BaZn2As2 是一种 p 型半导体，具有与 LaZnAsO 相似的层状晶体结构，这使得人们期望 β-BaZn2As2 和 LaZnAsO 具有相似的能带隙；然而，β-BaZn2As2 的能带隙（之前报道的值约为 0.2eV）比 LaZnAsO（1.5eV）小一个数量级。在本文中，通过电导率的温度依赖性的本征区域确定 β-BaZn2As2 的可靠能带隙值为 0.23eV。基于 6keV 硬 X 射线光电子能谱探测的化学成键性质、杂化密度泛函计算和配体理论，讨论了这种窄能带隙的起源。一个起源是相邻 [ZnAs] 层之间的直接 As-As 杂化，这导致了 As4p 能级的二次分裂，并提高了价带最大值。另一个起源是尽管 LaZnAsO 的导带底（CBM）主要由 Zn4s 形成，但非键合的 Ba5d(x(2)-y(2))轨道在 β-BaZn2As2 中形成了出乎意料的深导带底（CBM）。这两个起源为 β-BaZn2As2 和 LaZnAsO 之间的能带隙差异提供了定量解释。
