Further to planar single-layer hexagonal structures, GaN and AlN can also form free-standing, single-layer structures constructed from squares and octagons. We performed an extensive analysis of dynamical and thermal stability of these structures in terms of ab initio finite-temperature molecular dynamics and phonon calculations together with the analysis of Raman and infrared active modes. These single-layer square-octagon structures of GaN and AlN display directional mechanical properties and have wide, indirect fundamental band gaps, which are smaller than their hexagonal counterparts. These density functional theory band gaps, however, increase and become wider upon correction. Under uniaxial and biaxial tensile strain, the fundamental band gaps decrease and can be closed. The electronic and magnetic properties of these single-layer structures can be modified by adsorption of various adatoms, or by creating neutral cation-anion vacancies. The single-layer structures attain magnetic moment by selected adatoms and neutral vacancies. In particular, localized gap states are strongly dependent on the type of vacancy. The energetics, binding, and resulting electronic structure of bilayer, trilayer, and three-dimensional (3D) layered structures constructed by stacking the single layers are affected by vertical chemical bonds between adjacent layers. In addition to van der Waals interaction, these weak vertical bonds induce buckling in planar geometry and enhance their binding, leading to the formation of stable 3D layered structures. In this respect, these multilayers are intermediate between van der Waals solids and wurtzite crystals, offering a wide range of tunability.

中文翻译：

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八角形结构的GaN和AlN的单层和多层：稳定性，电子特性和功能化

除了平面单层六边形结构，GaN和AlN还可以形成由正方形和八边形构成的独立式单层结构。我们从头开始对这些结构的动力学和热稳定性进行了广泛的分析有限温度分子动力学和声子计算，以及对拉曼和红外激活模式的分析。GaN和AlN的这些单层方形八边形结构显示方向性机械性能，并具有较宽的间接基带隙，该间隙小于它们的六边形对应间隙。但是，这些密度泛函理论的带隙在校正后会增加并变宽。在单轴和双轴拉伸应变下，基带隙减小并且可以闭合。这些单层结构的电子和磁性可以通过吸附各种原子或通过产生中性阳离子-阴离子空位来改变。单层结构通过选定的原子和中性空位获得磁矩。尤其是，局部间隙状态在很大程度上取决于空位的类型。通过堆叠单层而构造的双层，三层和三维（3D）分层结构的能量学，结合以及由此产生的电子结构受相邻层之间的垂直化学键影响。除了范德华相互作用之外，这些弱的垂直键还引起平面几何形状的弯曲并增强其结合力，从而导致形成稳定的3D分层结构。在这方面，这些多层位于范德华固体和纤锌矿晶体之间，具有广泛的可调性。通过堆叠单层构建的三维（3D）分层结构受相邻层之间的垂直化学键影响。除了范德华相互作用之外，这些弱的垂直键还引起平面几何形状的弯曲并增强其结合力，从而导致形成稳定的3D分层结构。在这方面，这些多层位于范德华固体和纤锌矿晶体之间，具有广泛的可调性。通过堆叠单层构建的三维（3D）分层结构受相邻层之间的垂直化学键影响。除了范德华相互作用之外，这些弱的垂直键还引起平面几何形状的弯曲并增强其结合力，从而导致形成稳定的3D分层结构。在这方面，这些多层位于范德华固体和纤锌矿晶体之间，具有广泛的可调性。