Preparation,characterization And Electrical Properties Of The Two-dimensional Materials

As the continuously miniaturization of transistor feature size,the traditional silicon-based semiconductor devices have approached their physical limits.The quantum tunneling effect will greatly increase the power consumption of the devices.Thus,safe and stable operation of the devices cannot be guaranteed.To deal with the growing demand of information,developing new materials,new principles,new architecture and new devices to continue Moore's law has become one of the hot issues.In recent years,two-dimensional transition metal dichalcogenides are promising as channel materials for future transistors due to their atomic-thin thickness and smooth surface.With the deepening of research,researchers are gradually focusing on regulation of intrinsic physical properties rather than the exploration of synthesis.The relationship between the intrinsic electronic structure and device performance is still worth exploring.On the other hand,studying spin-related transport phenomena and constructing new sptronics devices based on spin freedom is another strategy for post-moore information storage.Two-dimensional magnetic materials have rapidly become the focus of scientific research since they were discovered.They can keep the spontaneous magnetization down to the monolayer limit and exhibit thickness-dependent magnetic properties.This provides a new research platform to understand and regulate the low-dimensional magnetism.Additionally,it also opens up a new direction for the research and development of two-dimensional magnetism and related spintronics devices.However,most single two-dimensional materials are still limited in device applications.Therefore,how to realize the regulation of theis intrinsic properties and study its impact on electrical transport is still the focus of current research.In this paper,we focus on the two-dimensional van der Waals(vdW)layered materials,including the transition metal dichalcogenides and vdW magnetic materials.Controllable preparation of various two-dimensional materials was realized.The correlation between the physical properties and the electrical transport performance of two-dimensional materials was established by the characterizations of various techniques.The strategies of doped alloying and surface regulation were designed to effectively modulate the intrinsic physical properties of the transition metal dichalcogenides,and their effects on electrical properties were studied.Focusing on the newly emerging two-dimensional magnetic materials,we explore the characterization of magnetic properties and magnetoelectric transport in the low temperature.The main research contents of this paper are summarized as follows:(1)The ternary MS2(1-x)Se2x(M=Mo,W)alloys were synthesized with a band gap tunability of?170 meV by using a solid state reaction method.A series of characterization methods confirmed the effective incorporation of Se atoms.With the increasing of Se concentration,a shift to lower frequency of phonon vibration mode appeared in Raman spectrum.Moreover,the UV-vis-NIR absorption results confirmed the realization of tunable band gap in MS2(1-x)Se2x(M=Mo,W)alloys by changing the content of Se,establishing the corresponding relationship between doping concentration and optical properties.Finally,the back-gate FET devices were fabricated based on MoS2(1-x)Se2x alloy,and the influence of Se doping concentration on the electron transport performance of MoS2(1-x)Se2x alloy was studied.This kind of doped alloying provides a new way for broadening the types of materials and intrinsic regulation of two-dimensional materials.(2)Zero-dimensional graphene quantum dots GQDs/2D WSe2 composite structrue was constructed.An enhanced p-type electrical transport behavior was realized in a hybrid structure of GQDs/WSe2.By Raman spectrum and PL characterizations,we found the incorporation of GQDs onto the surface of thin layer WSe2 can effectively control on the electronic structure of WSe2 film,successfully triggers the charge transfer from WSe2 to GQDs due to the band alignment at the interface.Electrical device performance based on hybrid structure of GQD/WSe2 was optimized due to the improved the hole density.The carrier mobility showed the growth of the 50 times and the threshold voltage is greatly reduced.This surface modification provides an effective way to regulate the intrinsic electrical behavior of two-dimensional material systems.(3)The antiferromagnetic ordering in 2D MnPSe3 was explored by Raman spectroscopy.Thickness and temperature-dependent Raman spectroscopy confirms the antiferromagnetic ordering in MnPSe3 down to atomically thin layers with trivial thickness-dependent Neel temperature from multiple evidence:the enhanced phonon Raman in-tensities,significant deviations from the anharmonic contribution of phonon frequencies due to spinphonon coupling,and the occurrence of one-magnon Raman peak below the Neel temperature.Our work motivates the explorations of 2D novel antiferromagnet spintronic devices.(4)The thickness-dependent magnetic behavior in vdW ferromagnetic metal Fe3GeTe2 has been explored by anomalous hall effect.Its coercive field increases as the thickness decreases,which changes from soft magnetic material of bulk to hard magnetic material of thin layer.Combined with its available exfoliation feature,we prepared Fe3GeTe2 nanoflake with an artificial step feature.Fe3GeTe2 is divided into two regions with opposite magnetization directions.Thus the magnetization orientation in the step thickness region is continuously varying,which is similar to the domain wall behavior.We studied magnetoresistance across the thickness step and found the abnormal phenomena of antisymmetric magnetoresistance.The temperature-dependent magneto-transport properties reveal that the direction of current flow does not necessarily have to be perpendicular to the step direction to achieve antisymmetric MR.This orientation is distinct from the mutually perpendicular geometry in conventional epilayer structures.In addition,the origin of antisymmetric reluctance is analyzed by theoretical simulation.This work for the first time reveals the correlation between thickness step and magnetic transport in two-dimensional magnetic materials,showing great potential in next generation of advanced magnetic memory based on multi-resistive states.