  Recent experimental synthesis of ambient-stable MoSi2N4 monolayer have
garnered enormous research interests. The intercalation morphology of MoSi2N4 -
composed of a transition metal nitride (Mo-N) inner sub-monolayer sandwiched by
two silicon nitride (Si-N) outer sub-monolayers - have motivated the
computational discovery of an expansive family of synthetic MA2Z4 monolayers
with no bulk (3D) material counterpart (where M = transition metals or alkaline
earth metals; A = Si, Ge; and N = N, P, As). MA2Z4 monolayers exhibit
interesting electronic, magnetic, optical, spintronic, valleytronic and
topological properties, making them a compelling material platform for
next-generation device technologies. Furthermore, heterostructure engineering
enormously expands the opportunities of MA2Z4. In this review, we summarize the
recent rapid progress in the computational design of MA2Z4-based
heterostructures based on first-principle density functional theory (DFT)
simulations - a central \emph{work horse} widely used to understand the
physics, chemistry and general design rules for specific targeted functions. We
systematically classify the MA2Z4-based heterostructures based on their contact
types, and review their physical properties, with a focus on their performances
in electronics, optoelectronics and energy conversion applications. We review
the performance and promises of MA2Z4-based heterostructures for device
applications that include electrical contacts, transistors, spintronic devices,
photodetectors, solar cells, and photocatalytic water splitting. This review
unveils the vast device application potential of MA2Z4-based heterostructures,
and paves a roadmap for the future experimental and theoretical development of
MA2Z4-based functional heterostructures and devices.
