Strategic Synthesis and Unveiling the Critical Intermediate Stage of Two-Dimensional Selenium Compounds

Abstract

The discovery of two-dimensional (2D) materials is advancing at a remarkable pace. From graphene to recent 2D metals, the 2D world has witnessed new materials emerging generation by generation. Among them, selenium-based compounds have drawn considerable attention. In this thesis, the structures and properties of 2D materials are first introduced, along with top-down and bottom-up preparation methods and transfer techniques-including wet transfer, dry transfer, and ice-assisted transfer. The applications in electronics and optoelectronics are then illustrated. Since these 2D materials exhibit distinct application potentials, efficient preparation becomes crucial. Of all methods, chemical vapor deposition (CVD) enjoys high reputation due to its ease of operation, low cost, and controllability. Here, the thesis proceeds to three main research sections:

1. Basic CVD synthesis and characterization of the bismuth selenide family. Practically, the 2D materials Bi₂O₂Se, Bi₃Se₂O₂, and Bi₂Se₃ can all be synthesized following the instructions provided. Furthermore, the growth of these materials depends critically on the Se/Bi₂O₃ ratio. 2D Bi₂O₂Se forms when the Se/Bi₂O₃ ratio is below 22.30; 2D Bi₃Se₂O₂ forms when the ratio is above 22.30 but below 55.27; and 2D Bi₂Se₃ dominates when the ratio exceeds 55.27. Regarding catalytic growth, the introduction of hydrogen gas has an effect analogous to that of the Se/Bi₂O₃ ratio-lower H₂ flow yields 2D Bi₂O₂Se, whereas higher H₂ flow results in 2D Bi₃Se₂O₂ and Bi₂Se₃. The introduction of O₂, which reacts with Bi₂Se₃, shows the inverse effect compared with H₂, that low O₂ dosage favors Bi₃Se₂O₂ growth, and high O₂ dosage favors Bi₂O₂Se growth. Substrates such as SrTiO₃ also enable lattice-matched growth of 2D Bi₂O₂Se. Furthermore, a study of the odd/even layer-number preference of the 2D bismuth selenide family has been conducted, revealing that only 2D Bi₂Se₃ exhibits an odd-layer-number preference.

2. The strategic CVD growth of 2D bismuth oxyselenide. As discussed, among various synthesis methods, CVD stands out as an optimal approach for fabricating 2D Bi₂O₂Se, owing to its scalability and controllability, cost-effectiveness, and high efficiency. Despite its potential, understanding of the growth kinetics of 2D Bi₂O₂Se via CVD remains limited. In this study, we systematically investigate the CVD growth behavior under a range of conditions, including substrate selection, growth duration, precursor ratio modulation (Se/Bi₂O₃), a comparison between one-step and two-step growth processes, and substrate orientation (horizontal vs. vertical). We successfully achieve epitaxial growth of 2D Bi₂O₂Se on SrTiO₃ (100) substrates and explore key factors such as the relationship between growth time and lateral flake size, the influence of substrate roughness and wettability (hydrophilicity/hydrophobicity), and the effect of substrate orientation on growth morphology. Notably, we demonstrate a two-step CVD strategy involving Bi₂O₃ pre-deposition followed by selenization, which enables the formation of 2D Bi₂O₂Se, 2D Bi₃Se₂O₂ and 2D Bi₂Se₃ flakes. Our findings provide valuable insights into the CVD synthesis of 2D Bi₂O₂Se and offer a foundation for advancing its integration into next-generation electronic and optoelectronic devices.

3. Unveiling the critical intermediate stages during CVD of 2D ReSe₂. Here we apply atomic-resolution scanning transmission electron microscopy (STEM) to resolve the intermediate stages during CVD synthesis of 2D ReSe₂. Contrary to the conventional growth models proposed previously, stable intermediate species-namely, molecular metal chalcogenide clusters, are experimentally unveiled. These molecular clusters prevailing in the chemical vapor deposition chamber can significantly alter the growth kinetics, mass transport, and surface anchoring sites. Both new layer nucleation and the resulting flake morphology are influenced. Our work resolved the critical question of whether the nucleation occurs in the gas phase or on the solid surface. Besides, additional experiments show that the hydrogen environment in CVD chamber can mitigate the aggregation problem of clusters, which is decisive for obtaining uniform 2D continuous films. Combined with density functional theory (DFT) calculations, the key reaction steps during growth are identified. Here, we present clear insights into the long-debated growth mechanisms of 2D Se compounds, expected to facilitate further optimization of CVD growth conditions for stable mass production.

Date of Award	20 May 2026
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Thuc Hue LY (Supervisor)