Hamer Matthew J, Zultak Johanna, Tyurnina Anastasia V, Zólyomi Viktor, Terry Daniel, Barinov Alexei, Garner Alistair, Donoghue Jack, Rooney Aidan P, Kandyba Viktor, Giampietri Alessio, Graham Abigail, Teutsch Natalie, Xia Xue, Koperski Maciej, Haigh Sarah J, Fal'ko Vladimir I, Gorbachev Roman V, Wilson Neil R
School of Physics and Astronomy , University of Manchester , Oxford Road , Manchester , M13 9PL , U.K.
National Graphene Institute , University of Manchester , Oxford Road , Manchester , M13 9PL , U.K.
ACS Nano. 2019 Feb 26;13(2):2136-2142. doi: 10.1021/acsnano.8b08726. Epub 2019 Jan 28.
Atomically thin films of III-VI post-transition metal chalcogenides (InSe and GaSe) form an interesting class of two-dimensional semiconductors that feature a strong variation of their band gap as a function of the number of layers in the crystal and, specifically for InSe, an expected crossover from a direct gap in the bulk to a weakly indirect band gap in monolayers and bilayers. Here, we apply angle-resolved photoemission spectroscopy with submicrometer spatial resolution (μARPES) to visualize the layer-dependent valence band structure of mechanically exfoliated crystals of InSe. We show that for one-layer and two-layer InSe the valence band maxima are away from the Γ-point, forming an indirect gap, with the conduction band edge known to be at the Γ-point. In contrast, for six or more layers the band gap becomes direct, in good agreement with theoretical predictions. The high-quality monolayer and bilayer samples enable us to resolve, in the photoluminescence spectra, the band-edge exciton (A) from the exciton (B) involving holes in a pair of deeper valence bands, degenerate at Γ, with a splitting that agrees with both μARPES data and the results of DFT modeling. Due to the difference in symmetry between these two valence bands, light emitted by the A-exciton should be predominantly polarized perpendicular to the plane of the two-dimensional crystal, which we have verified for few-layer InSe crystals.
III-VI 后过渡金属硫族化合物（InSe 和 GaSe）的原子级薄膜构成了一类有趣的二维半导体，其带隙随晶体层数的变化而显著变化，特别是对于 InSe，预计会从体材料中的直接带隙转变为单层和双层中的弱间接带隙。在这里，我们应用具有亚微米空间分辨率的角分辨光电子能谱（μARPES）来可视化机械剥离的 InSe 晶体的层依赖价带结构。我们表明，对于单层和双层 InSe，价带最大值远离 Γ 点，形成间接带隙，已知导带边缘位于 Γ 点。相比之下，对于六层或更多层，带隙变为直接带隙，与理论预测吻合良好。高质量的单层和双层样品使我们能够在光致发光光谱中分辨出带边激子（A）与涉及一对更深价带中孔的激子（B），它们在 Γ 点简并，其分裂与 μARPES 数据和 DFT 建模结果均相符。由于这两个价带之间对称性的差异，A 激子发射的光应主要垂直于二维晶体平面偏振，我们已对少层 InSe 晶体进行了验证。
