  In this computational work based on density functional theory we study the
electronic and electron transport properties of asymmetric multi-layer MoSSe
junctions, known as Janus junctions. Focusing on 4-layer systems, we
investigate the influence of electric field, electrostatic doping, strain, and
interlayer stacking on the electronic structure. We discover that a metal to
semiconductor transition can be induced by an out-of-plane electric field. The
critical electric field for such a transition can be reduced by in-plane
biaxial compressive strain. Due to an intrinsic electric field, a 4-layer MoSSe
can rectify out-of-plane electric current. The rectifying ratio reaches 34.1 in
a model junction Zr/4-layer MoSSe/Zr. This ratio can be further enhanced by
increasing the number of MoSSe layers. In addition, we show a drastic sudden
vertical compression of 4-layer MoSSe due to in-plane biaxial tensile strain,
indicating a second phase transition. Furthermore, an odd-even effect on
electron transmission at the Fermi energy for Zr/$n$-layer MoSSe/Zr junctions
with $n=1, \, 2,\, 3, \,\dots,\, 10$ is observed. These findings reveal the
richness of physics in this asymmetric system and strongly suggest that the
properties of 4-layer MoSSe are highly tunable, thus providing a guide to
future experiments relating materials research and nanoelectronics.
