Tunneling transport in a few monolayer-thick WS2/graphene heterojunction
  This paper demonstrates the high-quality tunnel barrier characteristics and layer number controlled tunnel resistance of a transition metal dichalcogenide (TMD) measuring just a few monolayers in thickness. Investigation of vertical transport in WS2 and MoS2 TMDs in graphene/TMD/metal heterostructures revealed that WS2 exhibits tunnel barrier characteristics when its thickness is between 2 to 5 monolayers, whereas MoS2 experiences a transition from tunneling to thermionic emission transport with increasing thickness within the same range. Tunnel resistance in a graphene/WS2/metal heterostructure therefore increases exponentially with the number of WS2 layers, revealing the tunnel barrier height of WS2 to be 0.37 eV. 
Takehiro Yamaguchi, Rai Moriya, Yoshihisa Inoue, Sei Morikawa, Satoru   Masubuchi, Kenji Watanabe, Takashi Taniguchi, and To...
cond-mat.mes-hall