A whose two faces are made of different atoms – selenium on top and sulfur underneath, for example – named after the two-faced Roman god. Because top and bottom differ, the sheet carries a built-in electric field from one face to the other, which a symmetric layer cannot have. That lopsidedness allows effects a symmetric sheet forbids, such as producing a voltage when it is squeezed from above.
Going deeper
Swap one face of a layer for a different element and it is no longer the same seen from above and below. The broken mirror symmetry gives the sheet a built-in field and lets it respond to pressure from above with a voltage – effects a symmetric layer cannot show.
Two faces, one field
In an ordinary MoS2 layer the sulfur sheets above and below the molybdenum are identical, so the layer looks the same from either side. Replace the top sheet with selenium and that is gone. Sulfur holds on to electrons slightly more strongly than selenium, so charge shifts a little towards the sulfur face and the layer becomes a permanent electric dipole: calculations put the step in electrostatic potential across it at several hundred millielectronvolts.
The broken symmetry switches on effects that are forbidden in the symmetric parent. The layer answers pressure from above with a voltage across its thickness, electrons moving through it feel a -type splitting of their , and light at twice the frequency is generated with a component pointing out of the sheet. Its , about 1.7 eV in MoSSe, sits between those of MoS2 and MoSe2.
Making one face different
A Janus layer cannot simply be grown the way its parents are, because the two faces must end up different rather than mixed. The first ones, reported in 2017, were made by converting one face of a finished monolayer. One route strips the top sulfur of MoS2 with a hydrogen plasma and then supplies selenium to take its place; another heats MoSe2 in sulfur vapour under conditions where only the upper selenium sheet is exchanged. Later methods make the swap at room temperature, which spares the fragile layer and allows Janus layers to be built into stacks.
The hard part is proving the result. A layer with selenium and sulfur mixed at random on both faces is an , not a Janus layer, and telling the two apart from or emission spectra alone takes care. Measurements that respond to the missing mirror symmetry, such as out-of-plane , or atom-by-atom imaging, settle it.
Janus by nature
A few crystals are Janus without any conversion step. In the rhodium chalcohalide RhSeCl every layer has all its selenium on one face and all its chlorine on the other, and the layers stack so that the whole crystal keeps the same polarity. Such crystals can be grown large and exfoliated like any other layered material, down to single layers – a far easier source of Janus than converting monolayers one at a time – and their lack of an makes them unusually strong sources of second-harmonic light.
Computational screens have proposed many more Janus layers, from other pairs of to layers with a halogen on one face, but only a handful have been made so far.
For specialists
A monolayer in which the two outer atomic planes are different elements, as in MoSSe, where one chalcogen sheet is S and the other Se. Mirror symmetry through the metal plane is broken (D3h → C3v for the 1H structure), giving an intrinsic out-of-plane dipole, out-of-plane , Rashba-type spin splitting and an out-of-plane second-harmonic response. Most are made by replacing one face of a parent monolayer; RhSeCl is a rare bulk crystal whose layers are Janus by nature.