Recently, the research group led by Prof.Chen Feng, from the School of Physics, has made important progress in the field of ion beam material modification. They have demonstrated that Se-defects generated via Ga⁺ion irradiation reverse the direction of the out-of-plane charge transport in the MoSe_2-x/graphene heterostructure, resulting in the selectable positive and negative photoresponse.

This study titled "Defect Engineering of Out-of-Plane Charge Transport in van der Waals Heterostructures for Bi-Direction Photoresponse" was published in ACS Nano, where the doctoral student Liu Yanran from the School of Physics is the first author, and Professor Tan Yang, Associate Professor Qu Yuanyuan and Professor Chen Feng are the co-corresponding authors.

Defects are ubiquitous in two-dimensional (2D) transition-metal dichalcogenides (TMDs), generated by the initial growth- or the post-processing. However, these defects will reduce the properties of 2D materials, resulting in the decrease of mechanical strength, carrier mobility, and other adverse effects. Therefore, it is very necessary to conduct defect engineering research on 2D materials. In this work, Se-defects were generated via Ga⁺ion irradiation on the top of MoSe₂/graphene heterostructure. The results demonstrate that the Se-vacancy defects in the MoSe₂ side of the MoSe₂/graphene heterostructure can modify the surface potential of MoSe_2-x and switch the direction of out-of-plane charge transport. Photoresponse spectra showed that defects led to the barrier reverse between graphene and MoSe_2-x and switches of the photoresponse from the negative to the positive. The photo responsivity (R) of the irradiated heterostructure is of the same order of magnitude as the pristine one. Besides, the irradiated MoSe₂/graphene heterostructure has a higher specific detectivity (D*) because of the reduced dark current by Se-vacancies. Due to the appearance of mid-gap states by defects at heterostructure, as demonstrated by the density functional theory calculation and scanning tunneling microscope results, the irradiated heterostructure has a broadband response ranging from 450 nm to 1064 nm and exhibits comparable or higher positive responsivity to the negative one of the pristine MoSe₂/graphene heterostructure. Based on defect-engineered heterostructures, optoelectronic OR and AND logic devices with a broadband operation are constructed. This work provided a meaningful research idea for 2D optic neural network simulation.

This work is supported by the National Natural Science Foundation of China, the Mount Tai Scholars' Climbing Program of Shandong Province, and the State Key Laboratory of Crystal Materials.

Paper publication link: https://pubs.acs.org/doi/10.1021/acsnano.1c06238