The Design Of Novel Two-dimensional Semiconductors And Analysis Of Their Electronic Structures By First-principles Calculations

As the monolayer of graphite, graphene has been exfoliated in experiment, the research in two-dimensional（2D） materials has triggered the enthusiasm of researchers and engineers from various fields. Numerous 2D materials ranging from group IV element crystal: graphene, silicene, germanene, stanene, to group III borophene and group V phosphorene, then to molybdenum disulfide（MoS2） and tungsten diselenide（WSe2） these types of Transitional Metal Dichalcogenide（TMD）. These 2D materials not only have been synthesized in a short period of time, but also have been used to fabricate electronic devices such as field-effect transistors（FET）, transparent conductive electrodes etc. In order to enrich the number of types of 2D materials, this research aims to design and predict some other new 2D materials and explore their related electronic structure by first-principles calculation for the potential usage in the future electronic, optoelectronic and spintronic fields. Hopefully, we expect to propel the microelectronic devices to much thinner, smaller and much more efficient nano-electronic devices. The contents of research are listed below:1. This research work selected 5 elements of Be, Mg, Ca, Sr, Ba from group IIA and 3 elements of Zn, Cd, Hg from IIB of periodic table as cation and 4 elements of O, S, Se, Te from group VIA as anion to form binary compounds under the prototype of graphene. These total 32 types of two-dimensional binary compounds are optimized and computed by firstprinciples calculation which is based on Density Functional Theory（DFT）. According to the phonon spectra of these 32 types of compounds, there are 10 out of 32 showed little imaginary frequency, which means that these 10 types are dynamically stable. The stable compounds which have six-member ring character are BeO, MgO, CaO, ZnO, CdO, CaS, SrS, SrSe, BaTe, HgTe. To further check the stability given from the phonon spectra, we explored the Molecular Dynamics Simulation（MDS） under 1000 K for 10 ps. The stability results correspond to the results assessed from phonon spectra. In addition, from the MDS, there is a sign that the unstable structure like HgS tends to evolve from six-member ring into four-member ring. Inspired by this phenomenon, we further found another two stable 2D structures with four-member ring. They are HgS and BaS. Their band gaps are equal to the wavelength of 380 nm and 440 nm, respectively. This paper summarized the HSE band gap distribution in wavelength of these total 12 types of novel 2D materials and found that most of these band gaps of graphene-like materials lie in the field of ultraviolet, except CdO and HgTe. The band gap of CdO lies in visible light and the band gap of HgTe corresponds to the region of infrared ray. Intriguingly, the recent report showed that 2D HgTe is able to change into non-trivial topological insulator under the combination of Spin Orbital Coupling（SOC） and in-plane strain. Interestingly, the 2D SrSe has a very flat valence band, and it could become magnetic with 1 μB after extracting one electron from the primitive cell system. These new discoveries are very exciting and inspiring for the design and application for the future novel electronics, optoelectronics and spintronics.2. Compared with the graphene, the monolayer of boron with Kagome structure is the system that lacks electron. Therefore, the research is based on the concept that transitional metal donates electrons to balance the electrons in Kagome Boron layer. This work designed a sandwich structure of Transitional Metal Hexa-Boride（TMB₆）, which takes the two layers of Kagome Boron monolayer as “sliced bread” layers and Transitional Metal（TM） layer is placed between slices of “bread”. These two layers of “bread” are aligned perfectly and TM atoms sit rightly on the hexagonal center of the Boron Kagome. We explored 10 types of transitional metal within the sandwich prototype and found 4 of them are stable, which are MnB₆, FeB₆, ScB₆ and TiB₆. Among them, MnB₆, FeB₆ are semiconductors and ScB₆ and TiB₆ are metals. Most importantly, the MnB₆ behaves as an intrinsic magnetic semiconductor. In addition, the conduction band minimum（CBM） and valence band maximum（VBM） are both dominated by one type of spin of electron. This special character makes MnB₆ a potential candidate for the usage of single spin filter or spin transistor in the future spintronic devices.3. This part is inspired by the prototype of novel 2D materials MXene within which Ti2 C is the most typical one. This work aims to find 2D materials which could match well with current semiconductor industry mainly based on silicon. In this research, Ti atom of Ti2 C is replaced with conventional group IV semiconductor atoms like carbon, silicon, germanium and tin within the framework of MXene. Three stable semiconductor materials are discovered namely, Si2 C, Ge2 C and Sn2 C. In this MXene-like structure prototype, these 2D semiconductors atoms form a regular octahedron, and carbon atom lies in the center of octahedron. Since the outside plane is composed of conventional semiconductor atoms, these 2D materials have an inspiring potential usage for the future low scale semiconductor devices because of the element compatibility.These three parts of research work predicted the stability of three types of 2D materials, respectively. A series of dynamically stable 2D materials were found. Among these new 2D materials, some of them are very suitable for optoelectronic devices since they have direct band gaps; some others have intriguing properties. For example, 2D SrSe is able to become magnetic after hole doping; MnB₆ has intrinsic magnetism. All these intriguing properties are very inspiring for the future 2D electronic devices. The difference and changing laws between 2D materials and corresponding 3D materials are also very fascinating research topics for physicists...