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Monochalcogenides enrich library of 2D crystals

The field of two-dimensional materials is possibly one of the fastest expanding fields in material science and condensed matter research worldwide. The interest on this class of materials was boosted by the fast development of ever more efficient methods to synthesize them at atomically thin level.

Within the ever-growing library of 2D crystals, layered group-IV monochalcogenides (MC) have become an increasingly important group of materials. In particular, the binary IV-VI compounds SnS, SnSe, GeS, and GeSe, which form a subgroup with orthorhombic structure. SnS can be found in nature: its orthorhombic α phase, also known as herzenbergite, is a naturally occurring (nontoxic) mineral with an optical band gap of ≈ 1.3 eV, in the range of optimal values for solar cells (1.1 to 1.5 eV). Such properties boosted experimental and theoretical research on SnS in recent years. 

At the CA2DM, Dr Lídia Gomes, Dr Alexandra Carvalho and Prof. A. H. Castro Neto have conducted theoretical research based mostly on first principles calculations to investigate properties of 2D group-IV MC. These binary materials are expected to reveal distinct intrinsic properties in monolayer form, as some of the lattice symmetry operations, including inversion, are only present in bulk and in even-numbered layer systems. 

In a first work on group-IV MC [1], the consequences of lower dimensionality and symmetry breaking on some of the properties of these materials are explored. From monolayer to bulk forms, these materials are always semiconducting with band gap energies covering most of the visible range. Significant spin-orbit splitting of the conduction-band minima is also predicte, a consequence of inversion symmetry breaking.

Their research has also shown that extremely high piezoelectric constants are expected for single-layers of some members of this group [2]. This is a rather interesting result since there are plenty of applications requiring efficient conversion between electrical and mechanical energy.

Yet another interesting property was found using a combination of DFT and analytic methods (k·p Hamiltonians) conducted by Dr Aleksandr Rodin to explore the band structure of tin (II) sulfide (SnS) [3]. Single-layer SnS has two pairs of valleys aligned perpendicularly to each other. It was found that these valley pairs can be optically pumped separately using linearly polarized light, allowing for write and read the valleys, making SnS a good candidate also for valleytronics applications.

The fast progress in synthesis of low-dimensional systems lead us very optimistic in the achievement of single-layers of the whole group-IV MC very soon. An indication of this is the recent isolation of SnS bilayers [4] and single-layers SnSe flakes [5].

[1] Phys. Rev. B, 2015, 92, 085406.
[2] Phys. Rev. B, 2015, 92, 214103.
[3] Phys. Rev. B, 2016, 93, 045431.
[4] J. Am. Chem. Soc., 2015, 137 (39), pp 12689–12696
[5] 2D Mater. 2017, 4, 021026.

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