Reasonable designing interfacial charge transfer channels in 2D metal–semiconductor contact: A time-domain ultrafast dynamic study

The van der Waals (vdW) integration of two-dimensional (2D) metal and semiconductor materials holds great potential for realizing Ohmic contacts in electronic and optoelectronic devices, owing to the weak Fermi pinning effect resulting from their dangling-bond-free interface. However, the vdW gap leads to an additional tunneling barrier at the interface. The common tunneling barrier-lowering strategies by constructing interfacial covalent bonds may cause the recombination of photogenerated electrons and holes, thereby impacting the optoelectronic devices performance. Herein, inspired by 2D MSi2N4 (M = Cr, Hf, Mo, Ti, V, and Zr) semiconductors with band edge states being protected by the outlying Si–N sublayer, we construct semimetal/MSi2N4 heterostructure by decorating the surface of MSi2N4 with transition metal single-atoms to form covalent bonds with semimetal. The tunneling barrier is notably reduced, and the corresponding probability can reach up to 23.74%, higher than the 4.19% observed in vdW-type contacts. Meanwhile, for optoelectronic applications, benefitting from the protection of the outlying Si–N sublayer to the MSi2N4's band edge states, the photogenerated holes can migrate from MSi2N4 to the interfacial channel on a femtosecond timescale, which not only facilitates the trapping of electrons by the electrodes but also observably prolongs the lifetime of photogenerated carriers from 0.99 to 3.74 ns. Our work provides an effective way to advance the high-efficient 2D electronic and photoelectronic devices.