Abstract

In this study, the energy loss near edge structure (ELNES) of two-dimensional transition metal carbides and nitrides, namely ‘MXenes’, has been calculated in the density functional theory using the full-potential linearized augmented plane wave method. Calculations of band structure demonstrate that, in agreement with other studies, although MAX phases (bulks) and pristine MXenes (monolayers) are all metallic, some functionalized MXenes are semiconductors. For the first time, we have calculated the ELNES spectra of carbon and nitrogen K-edges in MAX M₂AlX phases, pristine M₂X MXenes and functionalized MXenes M₂XT₂ (M = Sc, Hf; X = C, N; T = F, OH, O) at magic angle conditions. Compared to the M₂AlX bulks, the differences of energy between the main peaks in carbon and nitrogen K edges of M2X monolayers increase due to the slightly smaller bond lengths that can be used as a fingerprint for monolayers. The structures in ELNES spectra are sensitive to the chemical nature and the location of the T groups on the MXene surface. The energy position of the first main peak and the separation between the main peaks in the ELNES spectra of M₂XT₂ increase from T = O to T = OH and to T = F surface groups, respectivelyshows that the M-X bond length decreases from T = O to T = OH and to T = F. This short bond in M₂XF₂ leads to enhanced M-X interaction. The main features in carbon and nitrogen K edges correspond to the peaks in the unoccupied densities of states (DOS), namely σ* and π* states; however, the contribution of σ* states is dominant. Furthermore, for accurate insight into the excitonic effects (electron–hole coupling) in core edges, we have used the solving of the equation of motion of the two-particle Green’s function, the Bethe–Salpeter equation (BSE) to determine these effects on the K x-ray absorption near edge structure (XANES) of semiconductors Sc₂CF₂, Sc₂C(OH)₂, Sc₂CO₂ and Hf₂CO₂ MXenes. Calculations have been performed in two different levels of theory, with and without considering the excitonic effects. The excitonic effects considerably result in shifting the spectral features to the lower energies and changing in the intensity and overall shapes of the absorption spectra.

中文翻译：

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从第一性原理计算终止的Scan和MXene单层的电子能量损失结构

摘要

在这项研究中，已经在密度泛函理论中使用全势线性化增强平面波方法计算了二维过渡金属碳化物和氮化物的近边缘结构（ELNES）的能量损失，即“ MXenes”。能带结构的计算表明，与其他研究一致，尽管MAX相（本体）和原始MXene（单层）都是金属的，但某些官能化的MXene是半导体。首次，我们计算了MAX M ₂ AlX相，原始M ₂ X MXene和功能化MXene M ₂ XT _2中的碳和氮K边缘的ELNES光谱（M = Sc，Hf； X = C，N； T = F，OH，O）在幻角条件下。与M ₂相比AlX块，M2X单层碳和氮K边缘的主峰之间的能量差增加，这是因为可以用作单层指纹的键长略短。ELNES光谱中的结构对MXene表面的化学性质和T基团的位置敏感。M ₂ XT ₂的ELNES光谱中的第一个主峰的能量位置和主峰之间的间隔从T = O到T = OH和到T = F表面基团分别增加，表明MX键长从T减小= O到T = OH到T =F。M ₂ XF _2中的此短键导致增强的MX交互。碳和氮K边缘的主要特征对应于空位密度（DOS）的峰值，即σ *和π *状态。但是，σ *状态的贡献占主导地位。此外，为了准确了解核心边缘的激子效应（电子-空穴耦合），我们使用了求解两粒子格林函数的运动方程，Bethe-Salpeter方程（BSE）来确定这些效应的方法。半导体Sc ₂ CF ₂，Sc ₂ C（OH）₂，Sc ₂ CO ₂和Hf ₂的K x射线吸收近边缘结构（XANES）CO ₂二甲苯。在考虑和不考虑激子效应的情况下，以两种不同的理论水平进行了计算。激子效应会导致光谱特征转移到较低的能量，并改变吸收光谱的强度和整体形状。