Tuning Physical Properties Of Several Two-dimensional Semiconductors By Isovalent Doping

Two-dimensional semiconductors have attracted tremendous attraction for their potential applications in next generation of high performance electronic devices,due to their unique crystal structure and excellent physical properties.Isovalent doping is a widely used method to adjust the crystal and electronic structure in these materials,thus tuning the physical properties,such as transport and magnetic properties.In this thesis,the applicant synthesized several transition-metal-based two-dimensional semiconductors through isovalent doping and characterized its effect on their structure and physical properties.The main results are summarized as follows:First,we demonstrated the synthesis of Hf(S_1-xTe_x)₂crystals using chemical vapor transport(CVT)method and solid phase reaction,achieving large-scale tuning of chemical composition.With increasing Te content(x),the lattice constants increases linearly,and the frequency of Raman modes decreases linearly,which suggests there is no structural transition.Meanwhile,the band gap decreases linearly with Te doping and reaches zero near x=0.448,indicating a semiconductor-semimetal transition.The characterization of photodetectors based on Hf(S_1-xTe_x)₂flakes show excellent photoelectric properties,such as high on-off ratio,high response rate,high response speed,and moderate room-temperature mobility.Furthermore,the absorption spectrum can be extended to infrared region,far more beyond those of common transition metal dichalcogenide semiconductors,which usually cover the visible to near-infrared region.Secondly,we explored the synthesis of isovalent transition metal doped two-dimensional ferromagnetic semiconductors and obtained Cr_1-xV_xI₃and Cr_1-xV_xOCl single crystals by CVT method successfully.Cr_1-xV_xI₃has the same structure and similar lattice parameters as the low-temperature phases of parent Cr I₃and VI₃.The temperature-dependent magnetic susceptibility of Cr_1-xV_xI₃shows it is ferromagnetic with transition temperature lying between those of the parent phases and there is a bifurcation at low temperature under a small applied field.Cr_1-xV_xOCl has the same structure and similar lattice parameters as parent Cr OCl and VOCl.The temperature-dependent magnetic susceptibility shows no obvious antiferromagnetic characteristics.At low temperature,there is a small hysteresis in M-H curve.These are significantly different from the antiferromagnetic Cr OCl and VOCl.The magnetic anisotropy and other magnetic properties of Cr_1-xV_xI₃and Cr_1-xV_xOCl monolayer will be conducted.