Single- and few-layer chromium triiodide (CrI₃), which has been intensively investigated as a promising platform for two-dimensional magnetism, is usually prepared by the mechanical exfoliation. Here, we report direct growth of single-layer CrI₃ using molecular beam epitaxy in ultrahigh vacuum. Scanning tunneling microscopy (STM), together with density functional theory (DFT) calculation, revealed that the iodine trimers, each of which consists of three I atoms surrounding a three-fold Cr honeycomb center, are the basic units of the topmost I layer. Different superstructures of single-layer CrI₃ with periodicity around 2–4 nm were obtained on Au(1 1 1), while only the 1 × 1 structure was observed on the graphite substrate. At an elevated temperature of 423 K, single-layer CrI₃ began to decompose and transformed into single-layer chromium diiodide. Our bias-dependent STM images suggest that the unoccupied and occupied states are spatial-separately distributed, consistent with the results of our DFT calculation. We also discussed the role of charge distribution in the super-exchange interactions among Cr atoms in single-layer CrI₃.

单层和少层三碘化铬 (CrI ₃ ) 作为二维磁性的有前途的平台已被深入研究，通常通过机械剥离制备。在这里，我们报告了在超高真空中使用分子束外延法直接生长单层 CrI _{3 。}扫描隧道显微镜 (STM) 结合密度泛函理论 (DFT) 计算表明，碘三聚体是最顶层 I 层的基本单元，每个碘三聚体由三个 I 原子围绕一个三重 Cr 蜂窝中心组成。单层CrI _{3的不同超结构}在 Au(1 1 1) 上获得了约 2-4 nm 的周期性，而在石墨基板上仅观察到 1 × 1 结构。在 423 K 的高温下，单层 CrI ₃开始分解并转变为单层二碘化铬。我们的偏置相关 STM 图像表明未占用和占用状态空间分离分布，与我们的 DFT 计算结果一致。我们还讨论了电荷分布在单层 CrI ₃中 Cr 原子间超交换相互作用中的作用。