Single Crystal Synthesis And Transport Properties Of Ternary Niobium-based Chalcogenides

Transition metal dichalcogenides(TMDs),as one of the most representative layered materials family,provide diverse experimental and material research platforms for the field of electronics and photonics with their special crystal structures,electronic structures,and unique physical and chemical properties.With the increasingly stringent requirements for material performance in integrated highefficiency electronic devices,the research on sulfur compounds has gradually extended from binary systems to ternary systems.Ternary metal chalcogenides can be achieved using TMDs as the parent structure through intercalation,doping,element introduction,and other methods.They have been proven to have more extraordinary electromagnetic transport,optoelectronic,thermoelectric,mechanical,and electrochemical properties.However,due to the limitations of single crystal synthesis technology,many excellent ternary metal chalcogenides have not been fully explored,and research on the regulation and mechanism of their intrinsic transport properties is almost blank.In this paper,a series of high-quality single crystal samples of new ternary chalcogenide compounds based on niobium-based TMDs were synthesized,and their structures and transport behaviors were systematically studied.The main research results are as follows:(1)Single crystals of Ge-intercalated layered metal chalcogenides Ge0.33NbS2 and Ge0.26NbSe2 were successfully synthesized by chemical vapor transport methods,achieving effective regulations of their parent TMDs metals.They both show metallic electronic behaviors,with Ge0.33NbS2 exhibiting ultrahigh mobility and conductivity at room temperature.Its mobility at 300 K(83.40 cm2 V-1 s-1)is～24 times larger than that of binary NbS2,and its conductivity at 300 K(6.83 × 104 S·cm-1)is higher than those of all other metallic TMDs,making it competitive for the application of high-performance electrode material of two-dimensional electronic devices.Abnormal magnetoresistance behaviors were also observed in these two materials.The negative magnetoresistance of Ge0.26NbSe2 is related to the vacancy defects,and the linear magnetoresistance of Ge0.33NbS2 is related to the extraordinary non-trivial band crossings,which is confirmed by the first-principles calculations.These two Ge-intercalated materials with unique electronic properties can also be exfoliated to nanoflakes,which further prove their potential applications in next-generation atomically thin circuitry.(2)Nb2GeTe4 is a semiconductor material with excellent performance predicted by theoretical calculations,and has potential excellent quantum properties.In this work,high-quality and large size single crystal samples of Nb2GeTe4 were successfully prepared by chemical vapor transport method for the first time.Based on this sample,we confirmed its properties as a narrow band gap semiconductor through its resistivity and optical bandgap results.And its surprisingly high mobility(424.37 cm2 V-1 s-1)at room temperature is higher than most 2D high mobility semiconducting materials,including MoS2 and WSe2.In addition,the unsaturated negative magnetoresistance caused by the modulation structures of Nb2GeTe4 at low temperatures has also been observed(-31%at 5 K,5 T).These interesting findings indicate that Nb2GeTe4 has excellent physical properties and make it competitive for the design of 2D electronic devices.(3)Nb3GeTe6 and Ta3GeTe6 single crystals were successfully synthesized by the chemical vapor transport(CVT)method,and Ta3GeTe6 with this structure has not been reported in the previous literature.The resistivity,Hall resistivity and magnetoresistance were performed to explore their transport properties of the two materials.They both shows the metallic behaviors.The magnetoresistance of them starts to show a linear field dependence behavior under a very low magnetic field and show no obvious signs of saturation within measured field range,especially Nb3GeTe6,indicating their potential nontrivial topological properties.To further investigate their topological features,we studied the band structures of them by first-principles calculations and found that there are different types of non-trivial band crossings near the Fermi surface guaranteed by nonsymmorphic symmetry.Our results demonstrated that both Nb3GeTe6 and Ta3GeTe6 are potential systems for studying the band crossings and exotic quantum phenomena of topological semimetals.(4)The synthesis of heterojunction compound Nb2GeTe6 single crystals was achieved by a one-step flux method.Nb2GeTe6 forms a unique A-A-B type heterostructure of NbTe2-NbTe2-GeTe2 within the layer,which is first reported.It can be easily mechanically exfoliated and can be made into a 36 nm thin layer sample.First-principles calculations have confirmed that Nb2GeTe6 is a metal with potential high conductivity and transport anisotropy,which make it have great potential in electrocatalysis and the manufacturing of next-generation twodimensional electronic devices.