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FULLY DIGITAL - NO PUBLIC : PhD Defence Yuqiang Gao | Tuning the electronic properties of two-dimensional systems

Tuning the electronic properties of two-dimensional systems

Due to the COVID-19 crisis measurements the PhD defence of Yuqiang Gao will take place online without the presence of an audience.

The recording of this defence will be added to the video overview of recent defences

Yuqiang Gao is a PhD student in the research group Computational Materials Science. His supervisor is prof.dr. P.J. Kelly from the Faculty of Science and Technology.

Nanotechnology is about making electronic devices with dimensions of the order of 1-100 nanometers. The smaller the size, the greater the integration and the more powerful the circuits. However, as the dimensions are reduced, the problems associated with heat dissipation and quantum size effect become more prominent that limiting the further miniaturization of electronic devices. New kinds of materials might make an important contribution. In 2004, the preparation of graphene, a single carbon atom nano sheet, opened a new area of two-dimensional (2D) materials and attracted huge attention for its potential in the miniaturization of electronic devices. The spectacular properties that do not exist in their 3D counterparts also provide opportunities for the design of novel electronic devices. In addition, 2D materials offer great flexibility in term of tuning their electronic properties using electric fields, functionalization, doping, and structural tuning.


In this thesis, I use first-principles density functional theory to investigate the electronic and magnetic properties of a number of two-dimensional systems, including the LaAlO3/SrTiO3 (LAO/STO) (001) n-type interface, monolayer of transition metal dichalcogenides (TMDs) and 2D-Xene (X=Si,Ge,Sn) and explore the possibility to tune them for practical application.

In Chapter 2, we study the ferroelectric field effect on the two-dimensional electron gas at the n-type LAO/STO (001) interface through a ferroelectric substrate BaTiO3 (BTO). We show that the carrier density at the LAO/STO interface can be reversely tuned by the ferroelectric field of the substrate. This originates from the ferroelectric field control of the intrinsic electric field in LAO.

In Chapter 3, we explore the possibility of making a semiconducting monolayer of MoS2 ferromagnetic by introducing holes into the narrow Mo d band that forms the top of the valence band. By substitutionally doping group VB elements (V,Nb,Ta) in monolayer of MoS2, we find the holes are fully polarized and pairs of such holes couple ferromagnetically unless the dopant atoms are too close. We analyse the mechanism behind the quenching and propose possible solutions to avoid the quenching by considering other TMD system such as monolayer of MoSe2 or MoTe2.

In Chapter 4, we carry out a systematic study of the structural and electronic properties of single acceptor and double acceptor (V, Nb, Ta, Ti, Zr, Hf) doped MX2 monolayers in the single impurity limit, including impurity binding energy, structure distortion, and single ion magnetic anisotropy. The effect of intrinsic defects such as vacancy and antisite are discussed.  The Curie temperature is evaluated in single acceptor doped monolayers of MX2.

In Chapter 5, we study the structural reconstruction at the bare zigzag edge of 2D-Xene (X=Si, Ge, Sn). An edge reconstruction with 3a periodicity is predicted, which opens a gap and shows non-magnetic ground state.