Two-Dimensional to Three-Dimensional Growth of Transition Metal Diselenides by Chemical Vapor Deposition: Interplay between Fractal, Dendritic, and Compact Morphologies

Abstract

We investigate the role of growth temperature and metal/chalcogen flux in atmospheric pressure chemical vapor deposition growth of MoSe 2 and WSe 2 on Si/SiO 2 substrates. Using scanning electron microscopy and atomic force microscopy, we observe that the growth temperature and transition metal flux strongly influence the domain morphology, and the compact triangular or hexagonal domains ramify into branched structures as the growth temperature (metal flux) is decreased (increased). The competition between adatom attachment to the domain edges and diffusion of adatoms along the domain boundary determines the evolution of the observed growth morphology. Depending on the growth temperature and flux, two different branched structures fractals and dendritesgrow. The fractals (with a dimension of ∼1.67) obey a diffusion-limited aggregation mechanism, whereas the dendrites with a higher fractal dimension of ∼1.80 exhibit preferential growth along the symmetry-governed directions. The effect of chalcogen environment is studied, where a Se-rich condition helps restrict Mo-rich nucleus formation, promoting lateral growth. For a Se-deficient environment, several multilayer islands cluster on two-dimensional domains, suggesting a transition from lateral to vertical growth because of insufficient Se passivation. X-ray photoelectron spectroscopy analysis shows a near perfect stoichiometry (Mo/Se = 1:1.98) of MoSe 2 grown in a Se-rich environment, whereas in the Se-deficient condition, a ratio of Mo/Se = 1:1.68 is observed. This also supports the formation of metal-rich nuclei (Mo 1+x Se 2−x ) under Se-deficient conditions, leading to threedimensional clustering. Tuning the growth temperature and metal/chalcogen flux, we propose an optimized CVD growth window for synthesizing large-area Mo(W) selenide.

Two-Step Growth of Uniform Monolayer MoS2 Nanosheets by Metal–Organic Chemical Vapor Deposition

“…quartz tube and positioned at the center of a single-zone Lindberg/Blue M furnace. 32 A second boat containing S (99.98% Sigma-Aldrich CAS: 7704-34-9) was placed upstream in the tube outside the central heating zone of the furnace, and a separate coil heater was attached to apply heat to the S boat. The system was pumped down to base pressure (∼1.5 mTorr) and purged three times using ultrahigh pure N 2 at 200 sccm.…”

“…A clean double-side-polished Si/SiO 2 (300 nm, thermally grown oxide) substrate was placed on an alumina combustion boat/crucible containing 8.5 mg of MoO 3 (99.5% pure Alfa Aesar, CAS 1313-27-5) and placed inside a 1 in. quartz tube and positioned at the center of a single-zone Lindberg/Blue M furnace . A second boat containing S (99.98% Sigma-Aldrich CAS: 7704-34-9) was placed upstream in the tube outside the central heating zone of the furnace, and a separate coil heater was attached to apply heat to the S boat.…”

Hydrophilicity and Hydrophobicity Control of Plasma‐Treated Surfaces via Fractal Parameters

“…[16,17] It is not uncommon for the nanocomposites of this type to feature very complex surface, internal and interfacial structures that may in some instances form a fractal-like morphology. [18,19] Our understanding of the physical and chemical processes that lead to the development of such morphologies during material fabrication remains somewhat limited, hindering our ability to tailor the morphologies to suit desired applications. [20] In part, this is because the fabrication of such complex hierarchical structures relies on the use of sophisticated processes and complex environments such as, e.g., low-temperature plasma [1] and in particular, the highly-reactive oxygen plasma.…”

A mini-review focusing on ambient-pressure chemical vapor deposition (AP-CVD) based synthesis of layered transition metal selenides for energy storage applications

“…The most commonly used techniques to prepare TMSes and their nanohybrids are the hydrothermal/solvothermal method, chemical vapor deposition (CVD), microwave irradiation, chemical bath deposition, ion-exchange, chemical exfoliation, etc. 84,[92][93][94] Here we only highlight the CVD-based routes (especially examples of ambient pressure CVD growth) for preparing TMSes and discuss their implementation in energy storage separately.…”