Van Der Waals Epitaxial Growth And Properties Study Of Two-Dimensional Non-Layered Materials

As an emerging materials system,two-dimensional（2D）materials have great application potential in electronic,optoelectronic and spintronic research fields due to their novel physical and chemical properties such as ultrathin atomic-scale thickness,dangling-bond free surface and diversity.Meanwhile,they are also important material support for extending Moore’s Law and are known as the best material candidate in the"post-Moore"era.However,at present,the research of 2D materials mainly focuses on the developments,device fabrications and applications of 2D layered materials.In recent years,the emergence of a large number of 2D non-layered materials has greatly enriched the 2D material system.2D non-layered materials may exhibit distinct properties from their bulk counterparts due to the low-dimensional nature and strong confinement effect.However,in contrast to the weak van der Waals interactions between layered materials,these non-van der Waals structures with isotropic covalent bonding hinder the preparation of their ultrathin 2D nanostructure.In recent years,van der Waals epitaxy technology is considered as an effective way to obtain ultrathin 2D structures from non-layered crystals.Van der Waals epitaxy method is to use 2D layered materials,such as mica,graphene,and molybdenum disulfide as the growth substrate for material growth process.The advantages of van der Waals epitaxial technology are mainly reflected in the following points:（1）2D layered material has no dangling-bond,which avoids the problem of lattice matching between epitaxial layer and substrate;（2）The epitaxial layer is completely relaxed without excessive strain;（3）The chemically inert 2D layered material surface can promote the rapid migration of reactive atoms and ensure the anisotropic growth of epitaxial layers.In this thesis,we focus on the epitaxial growth,device preparation,performance study and tuning of 2D non-layered materials,and realize the controllable preparation or doping of three kinds of 2D non-layered materials（including 2D non-layered narrow-bandgap PbSe semiconductor,2D single-element Co metal and Ni-doped CoO semiconductor）via van der Waals epitaxial technology,and systematically study their electrical,optoelectronic,and magnetic properties by experimental and theoretical means.The main research contents and conclusions are as follows:1.Van der Waals epitaxial growth of 2D PbSe and its high-performance heterostructure devicesIn view of the low optical absorption coefficient of 2D materials and poor photoelectric conversion efficiency of photodetectors prepared,we successfully synthesized 2D PbSe semiconductors with a large lateral dimension and ultrathin thickness via van der Waals epitaxy.The fabricated 2D PbSe device exhibits good electrical conductivity and superior multi-wavelength photoresponse performance with high responsivity（～10³ A/W）and impressive detectivity(～2×10¹¹ Jones).Furthermore,we demonstrate that 2D PbSe nanosheets can serve as component units for constructing high-performance heterostructure devices.With our strategy,ultrahigh current on/off ratio（～10⁸）and rectification ratio（～10⁶）,as well as high responsivity（～3×10³ A/W）and detectivity(～7×10¹² Jones),can be achieved in PbSe/Mo S₂ back-gated transistors.This work provides a valuable case for the controllable preparation of 2D narrow-band gap semiconductors with high optical absorption coefficients and the construction of high-performance heterojunction devices.2.Van der Waals Epitaxy Growth of 2D Single-Element Room-Temperature FerromagnetWe have successfully synthesized 2D elementary cobalt nanosheets with large lateral size（≈130μm）and ultrathin thickness（≈6 nm）via van der Waals epitaxy using a facile dihalides reduction process.Density functional theory calculations reveal their intrinsic ferromagnetic nature and epitaxial mechanism:that is,the synergistic effect between van der Waals interactions and surface energy minimization dominates the growth process.The as-grown cobalt nanosheets exhibit an ultrahigh blocking temperature above 710 K and in-plane magnetic anisotropy.Electrical transport measurements further reveal that cobalt nanosheets have significant magnetoresistance（MR）effect,and could realize a unique coexistence of positive MR and negative MR under different magnetic field configurations,which can be attributed to the competition and cooperation effect among ferromagnetic interaction,orbital scattering and electronic correlation.These results provide a valuable case for synthesizing 2D elementary metal crystals with pure phase and room-temperature ferromagnetism and pave the way for investigating new physics and related applications in spintronics.3.Tuning 2D magnetism in cobalt monoxide nanosheets via in situ nickel-dopingWe have successfully synthesized high-quality of 2D non-layered nickel-doped CoO nanosheets via in situ van der Waals epitaxy method.High-resolution transmission electron microscopy confirmed that nickel atoms were doped at the intrinsic cobalt atom sites.The nickel doping concentration was stable at around 15%.Magnetic measurements showed that pristine cobalt monoxide is nonferromagnetic,whereas nickel-doped cobalt monoxide exhibits robust ferromagnetic behavior with Curie temperature of≈180 K.Density functional theory calculations reveal that nickel atoms could improve the internal ferromagnetic correlation,giving rise to significant ferromagnetic performance of cobalt monoxide nanosheets.These results provide a valuable case for tuning the competing correlated states and magnetic ordering by substitution doping in 2D nonlayered oxide semiconductors.