[Paper Review] Electronic, optical and transport properties of van der Waals Transition-metal Dichalcogenides Heterostructures: A First-principle Study

Ke Xu, Yuanfeng Xu+9 more|arXiv (Cornell University)|Apr 7, 2018

2D Materials and Applications81 references16 citations

TL;DR

This first-principles study investigates the electronic, optical, and transport properties of van der Waals heterostructures composed of transition-metal dichalcogenides (MX₂, M = Mo, W; X = S, Se, Te). Using density functional theory with van der Waals corrections, the authors reveal that most heterostructures exhibit type-II band alignment with spatially separated electron-hole pairs, indirect bandgaps due to interlayer coupling, enhanced carrier mobilities, and strong ultraviolet absorption—making them promising for optoelectronic devices such as photodetectors and LEDs.

ABSTRACT

Two-dimensional (2D) transition-metal dichalcogenide (TMD) MX$_2$ (M = Mo, W; X= S, Se, Te) possess unique properties and novel applications. In this work, we perform first-principles calculations on the van der Waals (vdW) stacked MX$_2$ heterostructures to investigate their electronic, optical and transport properties systematically. We perform the so-called Anderson's rule to classify the heterostructures by providing the scheme of the construction of energy band diagrams for the heterostructure consisting of two semiconductor materials. For most of the MX$_2$ heterostructures, the conduction band maximum (CBM) and valence band minimum (VBM) reside in two separate semiconductors, forming type II band structure, thus the electron-holes pairs are spatially separated. We also find strong interlayer coupling at $Γ$ point after forming MX$_2$ heterostructures, even leading to the indirect band gap. While the band structure near $K$ point remain as the independent monolayer. The carrier mobilities of MX$_2$ heterostructures depend on three decisive factors, elastic modulus, effective mass and deformation potential constant, which are discussed and contrasted with those of monolayer MX$_2$, respectively.

Motivation & Objective

• To systematically investigate the electronic, optical, and transport properties of van der Waals-stacked MX₂ heterostructures (M = Mo, W; X = S, Se, Te).
• To classify heterostructures using Anderson’s rule and analyze their band alignment and energy band diagrams.
• To explore the impact of interlayer coupling on band structure, including indirect bandgap formation at the Γ point.
• To evaluate mechanical stability and carrier transport properties, particularly electron and hole mobilities.
• To analyze optical response, including absorption spectra and dielectric functions, for potential optoelectronic applications.

Proposed method

• Employed density functional theory (DFT) with the PBE functional and DFT-D2 dispersion correction to account for van der Waals interactions.
• Used the projector-augmented wave (PAW) method and a 12×12×1 k-point mesh for Brillouin zone sampling in structural and electronic structure calculations.
• Applied the HSE06 hybrid functional for accurate band gap and optical property calculations, especially to correct for self-interaction error.
• Calculated optical properties using the dielectric function ε(ω), with absorption coefficient α(ω) derived from ε₁(ω) and ε₂(ω).
• Computed carrier mobilities using the deformation potential theory, based on elastic modulus, effective mass, and deformation potential constant.
• Analyzed band structures and wavefunction overlaps to assess interlayer coupling and symmetry effects in AA and AB stacking configurations.

Experimental results

Research questions

• RQ1 How do the electronic band structures of MX₂ heterostructures vary with stacking order (AA vs AB) and material composition?
• RQ2 What is the nature of band alignment (type I, II, or III) in MX₂ heterostructures, and how does it affect carrier separation and excitonic properties?
• RQ3 To what extent does interlayer coupling induce indirect bandgaps or modify the effective mass and carrier mobility?
• RQ4 How do the optical absorption spectra of heterostructures compare to monolayer MX₂, and in which spectral range do they exhibit strong response?
• RQ5 What is the role of structural symmetry and interlayer interaction in determining the mechanical and transport stability of the heterostructures?

Key findings

• Most MX₂ heterostructures exhibit type-II band alignment, with conduction band maximum (CBM) and valence band minimum (VBM) localized in different monolayers, enabling spatially separated excitons.
• Interlayer coupling leads to indirect bandgaps at the Γ point in many heterostructures, even though the K-point band structure remains similar to isolated monolayers.
• The bandgaps of hetero-bilayers are generally smaller than those of the constituent monolayers, with a range between 0 and 2 eV depending on the material pair.
• Electron mobilities in hetero-bilayers are typically higher than in monolayer MX₂ due to increased elastic modulus, though hole mobilities are reduced in indirect-gap systems due to larger effective masses.
• Optical absorption spectra show strong absorption in the ultraviolet region (3.0–5.0 eV), with refractive indices ranging from 2.80 to 4.27, indicating suitability for UV optoelectronic devices.
• Dielectric functions and absorption spectra are nearly identical for AA and AB stacking configurations due to similar electronic contributions from the constituent monolayers, as predicted by Anderson’s rule.