26 novembre 2019

[Thin-Films] – Epitaxially-grown molybdenum oxide advances as a bulk-like 2D dielectric layer

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Crystal structure of Mo3 nanosheet

Since the successful isolation of graphene from bulk graphite, remarkable properties of graphene have attracted many scientists to the brand-new research field of 2D materials. However, despite excellent carrier mobility of graphene, direct application of graphene to field-effect transistors is severely hindered due to its gapless band structure. Alternatively, semiconducting transition metal dichalcogenides (TMDCs) have been focused intensively over the last decade. However, wide bandgap 2D materials with > 3 eV have been required for UV-related optoelectronic devices, power electronics, and dielectric layers.

One of promising candidates is transition metal oxides (TMOs), which has large bandgap, structural diversity, and tunable physical/chemical characteristics. Nevertheless, the scalable growth of atomically-thin TMOs remains challenging until now since it is very prone to the lattice-mismatch strain and strong substrate clamping during growth.

Recently, the research team led by Prof. Gwan-Hyung Lee of Seoul National University overcame the issue by employing the van der Waals (vdW) epitaxial growth method. The research team reported a novel method for scalable growth of orthorhombic molybdenum oxide(α-MoO3) nanosheets on the graphene substrate. An important question in this work is what the effect of thickness on the electrical and physical properties is. To figure out this, comprehensive atomic force microscopy (AFM) studies were performed to explore structural and electrical properties of MoO3 layers with various thickness.

Interestingly, AFM study revealed that MoO3 nanosheets retain bulk-like structural and electrical properties even when MoO3 nanosheets are thicker than 2 ~ 3 layers (1.4 ~ 2.1 nm in thickness).

Especially, the thickness-sensitivity of friction is very small compared to other hexagonal 2D materials. This intriguing result is attributed to the doubled octahedral planes of monolayer MoO3 with exceptionally small interatomic separation. Additionally, work function and dielectric constant are also thickness-independent, along with invariant electronic band structure regardless of the thickness. Besides, the team showed that MoO3 nanosheets obtain large current gap and high dielectric constant, emphasizing that MoO3 can be used as a promising 2D dielectric materials.

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