probe-type-ii-band-alignment-one-dimensional-van-der-waals

Probe Type II Band Alignment in One-Dimensional Van Der Waals Heterostructures Using First-Principles Calculations

Computational tools based on density-functional theory (DFT) enable the exploration of the qualitatively new, experimentally attainable nanoscale ...

Southern University of Science and Technology

Polar metals based on binary and ternary compounds have been demonstrated in literature. Here, we propose a design principle for ferroelectric-like elemental polar metals and relate it to real materials. The design principle is that, to be an elemental polar metal, atoms should occupy at least two inequivalent Wyckoff positions in a crystal with a polar space group, where inversion symmetry is spontaneously broken. According to this rule, we propose the first class of potential ferroelectric-like elemental polar metals in a distorted α-La-like structure with a polar space group P6 mc in which two inequivalent Wyckoff positions 2a (0, 0, z) and 2b (1/3, 2/3, z) are occupied by group-V elements (phosphorus, arsenic, antimony, and bismuth). Analyses based on first-principles calculations indicate that the dynamically stable polar phase results from a lone pair driven polar distortion of the nonploar phase in P6/mmc symmetry where two inequivalent Wyckoff positions 2a (0, 0, 0) and 2c (1/3, 2/3, 1/4) are occupied. This ferroelectric-like transition involves a transition from a metallic state to a semimetallic state. These predicted polar phases are metastable with respect to their corresponding ground phases. Moreover, ionic bonding characters are found due to the inequivalence in Wyckoff positions between group-V atoms. Our work opens a route to single-element parity-breaking phases.

Parity-breaking in single-element phases: ferroelectric-like elemental polar metals.

Two-dimensional (2D) semiconductors SnP are predicted, from first-principles calculations, to host moderate band gaps (0.72 eV for monolayer and 1.07 eV for bilayer), ultrahigh carrier mobility (∼10 cm V s for bilayer), strong absorption coefficients (∼10 cm) and good stability. Moreover, the band gap can be modulated from an indirect character into a direct one via strain engineering. For experimental accessibility, the calculated exfoliation energies of monolayer and bilayer SnP are smaller than those of the common arsenic-type honeycomb structures GeP and InP. More importantly, a semiconductor-to-metal transition is discovered with the layer number N  >  2. We demonstrate, in remarkable contrast to the previous understandings, that such phase transition is largely driven by the correlation between lone-pair electrons of interlayer Sn and P atoms. This mechanism is universal for analogues phase transitions in arsenic-type honeycomb structures (GeP, InP and SnP).

Article	Year
Parity-breaking in single-element phases: ferroelectric-like elemental polar metals. Journal of physics. Condensed matter : an Institute of Physics journal\| PubMed ID: 30178760	2018
Multifunctional two-dimensional semiconductors SnP: universal mechanism of layer-dependent electronic phase transition. Journal of physics. Condensed matter : an Institute of Physics journal\| PubMed ID: 30378570	2018

Xing-Qiang Shi

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