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.

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Two-dimensional black phosphorus has sparked enormous research interest due to its high carrier mobility, layer-dependent direct bandgap and outstanding in-plane anisotropic property. It is one of the few 2D materials where it is possible to tune the bandgap over a wide energy range from the visible to the IR spectrum.

When a few atomic layers of BP are exposed to an electric field, a physical phenomenon known as the Stark effect is observed. In atomic spectra, the Stark effect is the shifting and splitting of atomic energy levels under the influence of an externally applied electric field. Similarly, this Stark effect causes the conduction and valence band to shift towards each other, resulting in the reduction of the bandgap of few-layer BP.

There is an ultimate limit for electronic miniaturization imposed by electron-electron interactions and by the Pauli exclusion principle that forbids two electrons to occupy the same quantum state. This will eventually prevent the development of denser circuitry, as well as of multiplexed or parallel schemes using electrons as information carrier units. Photons, however, can share a same logical gate, without interacting among one another, unless mediated by the supporting material and its nonlinear optical properties. This suggests that, in the near future, technology based on all-optical photonic devices will partially take over electronics and, possibly, extend classical computing in use today to include new quantum protocols and techniques. In this photon controlling photon type of devices, materials that provide efficient nonlinear optical interaction will indubitably play a central role.

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