Prof. Junjie Qi
Prof. Junjie Qi
University of Science & Technology Beijing, China
Title: Interface Engineering and Force Tuning Electrical Transport Behavior of Nano-devices Based on Atomic Layered MoS2

High-performance piezoelectricity in monolayer semiconducting transition metal dichalcogenides is highly desirable for the development of nanosensors, piezotronics and photopiezotransistors. Here we report the experimental study of the theoretically predicted piezoelectric effect in triangle monolayer MoS2 devices under isotropic mechanical deformation. The experimental observation indicates that the conductivity of MoS2 devices can be actively modulated by the piezoelectric charge polarization-induced built-in electric field under strain variation. These polarization charges alter the Schottky barrier height on both contacts, resulting in a barrier height increase with increasing compressive strain and decrease with increasing tensile strain. The underlying mechanism of strain-induced in-plane charge polarization is proposed and discussed using energy band diagrams. In addition, a new type of MoS2 strain/force sensor built using a monolayer MoS2 triangle is also demonstrated. Our results provide evidence for strain-gating monolayer MoS2 piezotronics, a promising avenue for achieving augmented functionalities in next-generation electronic and mechanical– electronic nanodevices. 

Developing nanoelectronics that utilize the vertical sub-nanometer thickness of two-dimensional materials is desirable for achieving miniaturization of circuit elements. However, only a few researches have been studied in the vertical transport of atomic layered materials so far. Here, the vertical electrical transport behavior of bilayer MoS2 under coupling of photons and force is exploredby the use of conductive atomic force microscopy. We found that the current-voltage behavior across the tip-MoS2-Pt junction is a tunneling current which can be well fitted by a Simmons approximation. Among which, direct tunneling is dominated at low bias voltages whereas Fowler-Nordheim tunneling is happened at high bias voltages. The bias voltages in transition point between direct and Fowler-Nordheim tunneling are matched with the tunneling barrier height of energy band shape difference.Furthermore, the source-drain current dropping surprisingly appears when we continually increase force in which the dropping point is altered by light provided.The potential mechanism is responsible for the tuning in tunneling barrier height and width by force and photons. These results provide a new way to design devices that takes advantage of ultrathin two-dimensional materials. Such ultrashort channel length electronic components that possess tunneling current are important for establishing high-efficiency electronic and optoelectronic systems. 

Key words: MoS2, piezoelectricity, tunneling current, force, tuning


Junjie Qi is the full Professor of University of Science & Technology Beijing, China. She received her Ph.D. degree from Department of Materials Science at University of Science & Technology Beijing in 2002. From Jan. 2003 to Dec. 2004, she had been with the Tsinghua University, where she was a Post-Doctoral researcher. She joined the Device Research Laboratory in Electrical Engineering Department at University of California, Los Angeles as a visiting scholar through Dec. 2013 to Dec. 2014. She is the author of more than 100 publications including “Nature Communicatios”, “Advanced Materials”. Her current research interests include semiconductor nanomaterials, electronic and opto-electronic properties and devices of low-dimensional materials. 

【Selected Publications】 

[1] Junjie Qi, Yann-Wen Lan, Adam Z. Stieg, Juyn-Hong Chen, Yuan-Liang Zhong, Lain-Jong Li, Chii-Dong Chen, Yue Zhang, Kang L. Wang. Piezoelectric effect in chemical vapour deposition-grown atomic-monolayer triangular molybdenum disulfide piezotronics. Nature Communications, 2015, 6: 7430. 

[2] Junjie Qi, Xiaofeng Hu, Zengze Wang, Xin Li, Wang Liu, Yue Zhang. A Self-powered ultraviolet detector based on a single ZnO microwire/p-Si film with double heterojunctions. Nanoscale, 2014, 6: 6025-6029. 

[3] Feng Li, Junjie Qi, et al. Layer Dependence and Light Tuning Surface Potential of Two-Dimensional MoS2 on Various Substrates. Small, 2017, 1603103 (Impact Factor: 8.3). 

[4] Zhao Wu, Junjie Qi, et al. The coupling influence of UV illumination and strain on the surface potential distribution of a single ZnO micro/nano wire. Nano research, 2016, 9(9): 2572–2580 [5] Junjie Qi, Yue Zhang, Yunhua Huang, Qingliang Liao, and Juan Liu, Doping and defects in the formation of single-crystal ZnO nanodisks,Appl. Phys. Lett., 2006, 89, 252115