Inorganic Chemistry has published a study from researchers at the Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP), Chinese Academy of Sciences, reporting “Na‑assisted molecular beam epitaxy (Na‑MBE)” of monolayer MoS2 on Au(111). The team achieved single‑crystalline domains exceeding one micrometer—an order‑of‑magnitude larger than those grown by standard MBE on the same substrate—while preserving the atomic‑level control that makes MBE attractive for precisely doped two‑dimensional (2D) semiconductors. The work also demonstrates compositionally uniform , underscoring the method's relevance for doping‑controlled 2D materials. The advance addresses a longstanding bottleneck in 2D transition metal dichalcogenides (TMDs). MBE provides exquisite control over thickness, interfaces and substitutional doping, but MoS2 domains produced this way have typically remained below ~100 nm, creating grain boundaries that limit charge transport. By contrast, chemical vapor deposition (CVD) can yield millimeter‑ to centimeter‑scale domains using promoters such as sodium or NaCl, yet its gas‑phase fluctuations and thermal gradients can complicate atomically sharp heterostructures and precisely doped profiles. The study asks whether promoter strategies proven in CVD can translate to MBE's ultrahigh‑vacuum environment without sacrificing precision—and answers in the affirmative.
The researchers compared a literature‑adapted “standard MBE” flow with Na‑MBE on Au(111). Standard MBE—Mo deposition, low‑temperature sulfurization in H2S, then higher‑temperature annealing—produced MoS2 islands in the 5–60 nm range. With Na‑MBE, sodium was deposited first, followed by Mo and a brief sulfurization step. Scanning tunneling and electron microscopies visibly show the shift from nanometer‑scale islands to micron‑scale, single‑crystalline MoS2 domains at similar surface coverage; X‑ray photoemission spectra indicate near‑complete conversion to the 1H phase and a sharp low‑energy electron diffraction pattern confirms high crystalline order.
What changes the growth kinetics is a surface chemistry pivot around Na2S. In situ spectroscopy and microscopy revealed that Na reacts with H2S to form Na2S on the surface, which acts as a chalcogen reservoir and lowers the barrier for MoS2 incorporation at domain edges. Stepwise annealing experiments captured transient metallic Mo and Na2S, then their conversion back to MoS2 as temperature increased. The team even replaced H2S with Na2S powder and still obtained substantially enlarged domains, underscoring Na2S's central role. A schematic side‑by‑side mechanism visualizes how standard MBE relies solely on gas‑phase H2S, whereas Na‑MBE leverages surface‑enriched Na2S to boost sulfur flux and accelerate edge growth. Remarkably, depositing Na onto preformed, nanometer‑scale MoS2 islands and annealing also recrystallized them into micron‑scale domains, highlighting a post‑growth pathway for domain enlargement.
Scaling and uniformity were evaluated after transferring continuous Na‑MBE films onto SiO2. Optical images show a contiguous monolayer limited mainly by the sample holder size. Spatially resolved Raman spectra displayed the expected E2g and A1g modes with a monolayer‑typical separation and minimal variation across multiple points, while photoluminescence (PL) showed a sharp A‑exciton feature—both indicators of high optical quality and thickness uniformity. Angle‑resolved photoemission confirmed a monolayer MoS2 band structure akin to standard MBE films, and post‑growth analysis showed that Na evaporated during annealing, leaving no promoter residue; defect‑related in‑gap states appeared only beyond the films’ thermal stability window, helping define processing limits.
Crucially, the approach retained MBE’s hallmark control over composition and doping‑style substitutions. By co‑depositing Mo and W under Na promotion and brief sulfurization, the team synthesized a homogeneous monolayer with a 1:1 Mo/W ratio verified in situ by XPS and mapped ex situ by EDXS. Raman signatures contained both MoS2‑like and WS2‑like modes, and PL mapped uniformly over tens of micrometers, evidencing compositional and optical homogeneity. As the optical maps illustrate, the alloy's excitonic emission remained consistent across the field of view—an outcome that is directly aligned with device‑relevant uniformity requirements.
By uniting micron‑scale domain growth with atomic‑level stoichiometry and interface control, Na‑MBE offers a practical bridge between scalability and precision for 2D semiconductors. Larger single‑crystalline domains mean fewer grain boundaries and potentially higher mobility; atomic‑scale control supports programmable doping and band‑structure engineering. Because the promoter acts through a surface intermediate (Na2S) under UHV, the strategy may be extendable to other TMDs and heterostructures that demand both clean interfaces and on‑demand alloying. For CMOS‑like integration at sub‑nanometer nodes, such a pathway could simplify the road to wafer‑compatible, doping‑controlled TMD channels grown with the reproducibility of MBE.
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