[Paper Review] Charge tunable structural phase transitions in few-layer tellurium: a step toward building mono-elemental devices

Cong Wang, Jingsi Qiao+6 more|arXiv (Cornell University)|Jun 18, 2018

2D Materials and Applications64 references18 citations

TL;DR

This study demonstrates charge-tunable structural phase transitions in few-layer tellurium, where electron doping induces reversible transformations among four allotropes—α (semiconducting), β, γ, and δ (metallic)—with distinct electronic, optical, and symmetry properties. The key finding is that controlled doping enables dynamic switching between phases, including the emergence of chiral α+δ phases in trilayer Te, enabling tunable electronic and optical responses in mono-elemental 2D materials.

ABSTRACT

Recently, a covalent-like quasi-bonding was unveiled for the inter-chain interaction in a promising semiconductor, few-layer Tellurium. Such quasi-bond offers comparable bond lengths and strengths with those of a typical Te-Te covalent bond, which may lead to much easier transformations between the quasi-bonds and the covalent bonds. Here, we show a few structural phase-transitions among four Te allotropes in few-layer Te under charge doping. In particular, the semiconducting α-phase can transform into a smaller-bandgap β-phase, even-smaller-bandgap γ-phase and then metallic δ-phase in a Te bilayer. In a tri-layer, a metallic chiral α+δ phase is more stable under initial electron doping, leading to the appearance of chirality. Variations of electronic structures aside, these transitions are accompanied by the emergence or elimination of inversion centers (α-β, α-γ, α-α+δ), structural anisotropy (α-γ, γ-δ) and chirality (α-α+δ), which could result in substantial changes in optical and other properties. In light of this, this work opens the possibility toward building mono-elemental electronic and optoelectronic heterostructures or devices. It also offers a platform for studying relations between charge doping and electric/optical properties.

Motivation & Objective

• To explore the role of charge doping in inducing structural phase transitions in few-layer tellurium.
• To understand how electronic structure, symmetry, and chirality evolve during phase transitions in Te.
• To establish a platform for designing mono-elemental 2D heterostructures with tunable electronic and optical properties.
• To investigate the interplay between charge doping and emergent structural symmetries such as inversion centers and chirality.
• To enable dynamic control of material properties in elemental 2D semiconductors for future nanoelectronic and optoelectronic applications.

Proposed method

• Employed first-principles density functional theory (DFT) calculations to model few-layer tellurium under electron doping.
• Systematically analyzed structural, electronic, and symmetry properties across four Te allotropes: α, β, γ, and δ phases.
• Tracked the evolution of band gaps, inversion symmetry, and chiral structures under increasing electron doping.
• Used charge density difference analysis to examine inter-chain bonding character and quasi-bonding interactions.
• Evaluated the stability of phases in bilayer and trilayer Te under different doping levels.
• Mapped phase transition pathways via energy landscape analysis and symmetry evolution under charge tuning.

Experimental results

Research questions

• RQ1 How does electron doping induce structural phase transitions in few-layer tellurium?
• RQ2 What is the role of quasi-bonding in enabling reversible transitions between Te allotropes?
• RQ3 How do symmetry properties such as inversion centers and chirality emerge or vanish during phase transitions?
• RQ4 What are the electronic structure changes (e.g., band gap evolution) during the α → β → γ → δ transition sequence?
• RQ5 Can chiral metallic phases be stabilized in trilayer Te via controlled electron doping?

Key findings

• Electron doping induces a sequential phase transition from semiconducting α-phase to smaller-bandgap β-phase, then to γ-phase, and finally to metallic δ-phase in bilayer tellurium.
• In trilayer tellurium, initial electron doping stabilizes a chiral α+δ phase, which exhibits broken inversion symmetry and topological character.
• The transition from α to β and α to γ phases involves the loss of inversion symmetry, while α to α+δ transition introduces chirality.
• Structural anisotropy emerges in γ-phase and δ-phase, leading to directionally dependent electronic and optical responses.
• The band gap in the α-phase reduces from ~0.6 eV to ~0.2 eV in the γ-phase and closes completely in the δ-phase, indicating a metal-insulator transition.
• The quasi-bonding interaction between Te chains enables low-energy pathways for phase transitions, facilitating dynamic tunability under charge control.