  We present a comprehensive study of the band alignments of two-dimensional
(2D) semiconducting materials and highlight the possibilities of forming
momentum-matched type I, II and III heterojunctions; an enticing possibility
being atomic heterostructures where the constituent monolayers have band edges
at the zone center. Our study, which includes the Group IV and III-V compound
monolayer materials, Group V elemental monolayer materials, transition metal
dichalcogenides (TMD) and transition metal trichalcogenides (TMT) reveals that
almost half of these materials have conduction and/or valence band edges
residing at the zone center. Using first-principles density functional
calculations, we present the type of the heterojunction for 903 different
possible combination of these 2D materials which establishes a periodic table
of heterojunctions.
