Semiconducting electronic bandgap at boundaries

18 Feb 2015 NUS professors discovered semiconducting electronic bandgap tunability at grain boundaries in single atomic layer inorganic compound.

Graphene has captured the attention of the materials community in the past decade since the first detailed study of its physical properties in 2004. Graphene, an atomically thin layer of carbon, is a semi-metal and hence has limited applications for semiconductor electronic devices. There are other two-dimensional materials with properties similar to graphene but are semiconducting, thus overcoming graphene’s zero band gap problem that limits it applications in electronics. Transition metal dichalcogenides (TMDs), such as molybdenum disulphide (MoS₂), are semiconductors with tunable direct bandgaps that depend on the number of atomic layers, thus opening up potential electronic and optoelectronic applications.

A team led by Prof Andrew WEE from the Department of Physics in NUS has investigated the structural and electronic properties of monolayer MoS₂ in greater detail in order to enable device applications. In this work, the team has obtained the first high resolution atomic images of monolayer, bilayer and trilayer MoS₂ using scanning tunneling microscopy (STM). They measured its bandgap energies as a function of number of atomic layers, and observed tunable bandgaps at grain boundaries (see Figure). This work allows them to evaluate new possibilities for flexible electronic and optoelectronic devices with tunable bandgaps that utilize both the control of two-dimensional layer thickness and grain boundary engineering.

Figure (a) Large-scale STM shows a MoS2 flake containing single-layer (SL), bilayer (BL) and trilayer (TL) thickness (150 × 150 nm2;, V_Tip = 2.4 V). The inset shows the atomically resolved STM image of SL MoS₂ (3 × 3 nm2; V_Tip = 1.2 V). (b) dI/dV spectra taken at the SL, BL and TL MoS₂ respectively, reveals the bandgap decrease with the increasing thickness (set point: V_Tip = 1.5 V, I_Tip = 80 pA). (c) and (d) are bias-dependent images recorded at the grain boundary (GB) with misorientation angle of 18˚ (8 × 4 nm2; c, V_Tip = 1.2 V; d, V_Tip = -0.5 V). (e) A schematic diagram of the bandgap (E_g) change with respect to the distance (d) from GBs. The red one is for 18˚ misorientation, and the blue one with 3˚ misorientation. [Image credit: Andrew Wee]

Reference

YL Huang, Y Chen, W Zhang, SY Quek, CH Chen, LJ Li, WT Hsu, WH Chang, YJ Zheng, W Chen, ATS Wee. “Bandgap tunability at single-layer molybdenum disulphide grain boundaries” Nature Communications 6 (2015) 6298