First-principles Investigation On The Catalytic Mechanism Of Hydrogen In Epitaxial Growth Of Single-crystalline Borophene

As a new member of the two-dimensional material family,borophene is highly expected to become another“magic nanomaterial”following graphene due to its superior mechanical,thermal,and electrical properties.Such as the high mechanical strength and flexibility,anisotropic metallic behaviors,and optical properties have made borophene a promising candidate material platform for high-speed,transparent and flexible electronics.At present,researchers have developed a series of synthetic methods of borophene,while the complexity of its structures,especially the multiphase coexistence in the synthesized samples,seriously restricts the wide application of borophene in the next-generation of electronic devices.The problem of the coexistence of multiphase borophene has become one of the most important worldwide problems in the condensed matter physics and material science.Recently,researchers in Northwestern University of United States prepared the first stable two-dimensional boron––borophane（hydrogen passivated borophene）.However,there are several significant scientific issues need to be answer,and further in-depth research is urgently needed,such as the existence of hydrogen in the initial nucleation and then the growth of borophene,what is the role of hydrogen?what is the effect of hydrogen in the coexistence of borophene phases.On the other hand,as the most abundant and simplest element in the universe,hydrogen exhibits extraordinary physical and chemical properties in many research fields,and currently,it is also one of the most important clean energy materials.However,in the field of catalysis,as compared with the metal catalysts in d-block and even p-block elements,the catalytic effect of hydrogen in s-block is largely ignored.Therefore,it is of irreplaceable significance to study the catalytic mechanism of hydrogen in specific reaction processes,such as the borophene growth.In this thesis,taking the epitaxial growth of two-dimensional borophene as an example,using the density functional based first-principles calculation,we demonstrate the s-block hydrogen catalyst have highly catalytic activity on the whole process of borophene growth,including the diffusion,nucleation,and the formation of large-area single-crystal borophene.Our results show that:First,hydrogen makes the B monomer dissolved into the subsurface of Ag（111）substrate form BH molecule and stably adsorbed on the substrate,which significantly lower the diffusion barrier of B atoms on the substrate,and thus significantly promotes the initial nucleation of B atoms.Second,the introduction of H significantly modulates the distribution of electronic states near Fermi between different polymorphs of borophene,then effectively reducing the degeneracy of borophene polycrystalline phases,and thus facilitate the large-area single-crystal borophene growth at low temperature.Our research in this thesis reveals the great potential of H elements as an efficient,clean and low-cost catalyst in the borophene growth,and provides some important theoretical guidance and implementation scheme for the low-temperature,large-area single crystal borophene growth.The chaptered contents of this thesis are described as follows:Chapter one,introduction,briefly reviewed the development of two-dimensional materials,focuses on the discovery process of the new two-dimensional material borophene from theoretical prediction to experimental synthesis,and discussed the electronic structure characteristics of borophene,bonding characteristics,multiphase coexistence phenomenon,and application fields.Chapter two,the basic theory and calculation methods used in the thesis.Chapter three,we study the initial nucleation of B atoms on the Ag（111）substrate and the then the growth of borophene,the dominate role of H catalyst in the initial nucleation of borophene is revealed.Hydrogen causes B monomer dissolved into the subsurface of Ag（111）to form BH molecule and thus stably adsorbed on the substrate surface,which significantly reducing the diffusion barrier of B atoms and then promoting the nucleation of B atoms on the substrate.Chapter four,our research show that the introduction of H effectively modulates the distribution of electronic states near the Fermi level between different borophene polymorphs,thus reducing the degeneracy of borophene polycrystalline phases,which is conducive to the large area growth of single crystal borophene at low temperature.Firstly,usingβ₁₂ andχ₃ phases as examples,we study the most stable adsorption sites for H passivation of borophene fragments.Then,the effective modulate of H passivation on the stability of the two phases in a series of B clusters is studied,for example,β₁₂ phase is significantly more stable thanχ₃ phase.Finally,we study the fundamental physical mechanism of H effectively reducing the phase degeneracy of borophene polymorphs,which provides an important theoretical basis for the large area,single crystalline borophene growth at low temperature.Chapter five,concludes the whole thesis and the future work is proposed.