Growth Kinetics of Two-Dimensional Hexagonal Boron Nitride Layers on Pd(111)

Direct Observation of Structural Phase Transformations during Phosphorene Formation on Cu(111)

Blue phosphorene, a two-dimensional, hexagonal-structured, semiconducting phosphorus, has gained attention as it is considered easier to synthesize on metal surfaces than its allotrope, black phosphorene. Recent studies report different structures of phosphorene, for example, on Cu(111), but the underlying mechanisms of their formation are not known. Here, using a combination of in situ ultrahigh vacuum low-energy electron microscopy and in vacuo scanning tunneling microscopy, we determine the time evolution of the surface structure and morphology during the deposition of phosphorus on single-crystalline Cu(111). We find that during the early stages of deposition phosphorus intermixes with Cu, resulting in copper phosphide structures. With the increasing surface concentration of phosphorus, the phosphide phase disappears, and a blue phosphorene layer forms, followed by the self-assembly of highly ordered phosphorus clusters that eventually grow into multilayer islands. We attribute the unexpected transformation of stable phosphide to a phosphorene layer to the presence of a large concentration of P2 dimers on the surface. Our results constitute direct evidence for a growth mode leading to a flat phosphorene layer via an intermediary phase, which could underpin the growth of other 2D materials on strongly interacting substrates.

Chemical vapor deposition of hexagonal boron nitride on germanium from borazine

The growth of hexagonal boron nitride (hBN) directly onto semiconducting substrates, like Ge and Ge on Si, promises to advance the integration of hBN into microelectronics. However, a detailed understanding of the growth and characteristics of hBN islands and monolayers on these substrates is lacking. Here, we present the growth of hBN on Ge and Ge epilayers on Si via high-vacuum chemical vapor deposition from borazine and study the effects of Ge sublimation, surface orientation, and vicinality on the shape and alignment of hBN islands. We find that suppressing Ge sublimation is essential for growing high quality hBN and that the Ge surface orientation and vicinality strongly affect hBN alignment. Interestingly, 95% of hBN islands are unidirectionally aligned on Ge(111), which may be a path toward metal- and transfer-free, single-crystalline hBN. Finally, we extend the growth time and borazine partial pressure to grow monolayer hBN on Ge and Ge epilayers on Si. These findings provide new insights into the growth of high-quality hBN on semiconducting substrates.

Understanding epitaxial growth of two-dimensional materials and their homostructures

The exceptional physical properties of two-dimensional (2D) van der Waals (vdW) materials have been extensively researched, driving advances in material synthesis. Epitaxial growth, a prominent synthesis strategy, enables the production of large-area, high-quality 2D films compatible with advanced integrated circuits. Typical 2D single crystals, such as graphene, transition metal dichalcogenides and hexagonal boron nitride, have been epitaxially grown at a wafer scale. A systematic summary is required to offer strategic guidance for the epitaxy of emerging 2D materials. Here we focus on the epitaxy methodologies for 2D vdW materials in two directions: the growth of in-plane single-crystal monolayers and the fabrication of out-of-plane homostructures. We first discuss nucleation control of a single domain and orientation control over multiple domains to achieve large-scale single-crystal monolayers. We analyse the defect levels and measures of crystalline quality of typical 2D vdW materials with various epitaxial growth techniques. We then outline technical routes for the growth of homogeneous multilayers and twisted homostructures. We further summarize the current strategies to guide future efforts in optimizing on-demand fabrication of 2D vdW materials, as well as subsequent device manufacturing for their industrial applications.

In Situ Imaging of Two-Dimensional Crystal Growth Using a Heat-Resistant Optical Microscope

Revealing low-dimensional material growth dynamics is critical for crystal growth engineering. However, in a practical high-temperature growth system, the crystal growth process is a black box because of the lack of heat-resistant imaging tools. Here, we develop a heat-resistant optical microscope and embed it in a chemical vapor deposition (CVD) system to investigate two-dimensional (2D) crystal growth dynamics. This in situ optical imaging CVD system can tolerate temperatures of ≤900 °C with a spatial resolution of ∼1 μm. The growth of monolayer MoS2 crystals was studied as a model for 2D crystal growth. The nucleation and growth process have been imaged. Model analysis and simulation have revealed the growth rate, diffusion coefficient, and spatial distribution of the precursor. More importantly, a new vertex-kink-ledge model has been suggested for monolayer crystal growth. This work provides a new technique for in situ microscopic imaging at high temperatures and fundamental insight into 2D crystal growth.

Growth of Highly Oriented (VNbMoTaW)S2 Layers

Compositional tunability, an indispensable parameter for modifying the properties of materials, can open up new applications for van der Waals (vdW) layered materials such as transition-metal dichalcogenides (TMDCs). To date, multielement alloy TMDC layers are obtained via exfoliation from bulk polycrystalline powders. Here, we demonstrate direct deposition of high-entropy alloy disulfide, (VNbMoTaW)S2, layers with controllable thicknesses on free-standing graphene membranes and on bare and hBN-covered Al2O3(0001) substrates via ultra-high-vacuum reactive dc magnetron sputtering of the VNbMoTaW target in Kr and H2S gas mixtures. Using a combination of density functional theory calculations, Raman spectroscopy, X-ray diffraction, scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy, we determine that the as-deposited layers are single-phase, 2H-structured, and 0001-oriented (V0.10Nb0.16Mo0.19Ta0.28W0.27)S2.44. Our synthesis route is general and applicable for heteroepitaxial growth of a wide variety of TMDC alloys and potentially other multielement alloy vdW compounds with the desired compositions.

Progress on the in situ imaging of growth dynamics of two-dimensional materials

One key issue to promote the industrialization of two-dimensional (2D) materials is to grow high-quality and large-scale 2D materials. Investigations of the growth mechanism and growth dynamics are of fundamental importance for the growth of 2D material, in which in situ imaging is highly needed. By applying different in situ imaging techniques, details for growth process, including nucleation and morphology evolution, can be obtained. This review summarizes the recent progress on the in situ imaging of 2D material growth, in which the growth rate, kink dynamics, domain coalescence, growth across the substrate steps, single-atom catalysis, and intermediates have been revealed.

Borazine Promoted Growth of Highly Oriented Thin Films

We report on a phenomenon, where thin films sputter-deposited on single-crystalline Al₂O₃(0001) substrates exposed to borazine─a precursor commonly used for the synthesis of hexagonal boron nitride layers─are more highly oriented than those grown on bare Al₂O₃(0001) under the same conditions. We observed this phenomenon in face-centered cubic Pd, body-centered cubic Mo, and trigonal Ta₂C thin films grown on Al₂O₃(0001). Interestingly, intermittent exposure to borazine during the growth of Ta₂C thin films on Ta₂C yields better crystallinity than direct deposition of monolithic Ta₂C. We attribute these rather unusual results to a combination of both enhanced adatom mobilities on, and epitaxial registry with, surfaces exposed to borazine during the deposition. We expect that our approach can potentially help improve the crystalline quality of thin films deposited on a variety of substrates.

Structure, Properties and Applications of Two-Dimensional Hexagonal Boron Nitride

Hexagonal boron nitride (h-BN) has emerged as a strong candidate for two-dimensional (2D) material owing to its exciting optoelectrical properties combined with mechanical robustness, thermal stability, and chemical inertness. Super-thin h-BN layers have gained significant attention from the scientific community for many applications, including nanoelectronics, photonics, biomedical, anti-corrosion, and catalysis, among others. This review provides a systematic elaboration of the structural, electrical, mechanical, optical, and thermal properties of h-BN followed by a comprehensive account of state-of-the-art synthesis strategies for 2D h-BN, including chemical exfoliation, chemical, and physical vapor deposition, and other methods that have been successfully developed in recent years. It further elaborates a wide variety of processing routes developed for doping, substitution, functionalization, and combination with other materials to form heterostructures. Based on the extraordinary properties and thermal-mechanical-chemical stability of 2D h-BN, various potential applications of these structures are described.