Controllable Preparation And Electronic Properties Of Novel Transition Metal Dichalcogenides And Their Heterostructures

In 2004,K.S.Novoselov and A.K.Geim realized to get single-layer graphene by micromechanical exfoliation and discovered its unique electronic properties,and thus won the 2010 Nobel Prize.In the band structure of single-layer graphene,the linear dispersion of K-point causes many new phenomena,such as abnormal room temperature quantum Hall effect,and opens up a new category of Fermi-Dirac physics.Meanwhile,graphene-based materials have been widely used in high-speed electrical and optical devices,energy storage,chemical sensors,and even DNA sequencing.Graphene has many fascinating properties,but its lack of electronic bandgap that stimulates interests in other two-dimensional materials.Two-dimensional transition metal dichalcogenides(TMDCs)can be easily exfoliated and present very interesting electrical and optical properties,making them one of the most compelling systems in solid state physics and device research.The TMDCs have the formula MX₂,where M is a transition metal atom(such as Moor W)and X is a chalcogen atom(such as S,Se or Te).At the same time,they have a rich chemical compositions and electronic structures,making them a powerful candidate for future device applications.The electronic structure of TMDCs depends largely on the d electron number of M.It can cause the Fermi level(E_F)of different groups'TMDCs to be in different positions(in gaps or bands),and so their electrical properties show diversity,from semiconductors,semi-metals to metals.Among them,2H-TMDCs based on group?B's Moand W exhibit typical semiconductivity,which have many interesting physical properties,such as layer-dependent indirect-direct band gap transition,valley polarization and light matter coupling.The semiconductor TMDCs have been widely used in electronic/photoelectric devices and energy conversion devices.Different from semiconductors,TMDCs based on group?B V,Nb,Ta and group?Pt,Pd are semimetallic with special physical characteristics,which have limited density of states at Fermi level,thus exhibiting charge density wave(CDW)or superconductivity,etc.At the same time,two-dimensional semimetallic TMDCs is an important component of manufacturing two-dimensional vd W heterostructure.Heterostructure based devices with different functions can be constructed by combining other two-dimensional materials with different conductivity(such as semimetal,semiconductor and insulator),which provides many unprecedented possibilities for physical research of two-dimensional devices.This thesis aims to design and exert the advantages of two-dimensional confinement effect of TMDCs and easy assembly of vd W heterostructures by using its correlated electronic system and light matter coupling to analyze different d-electron TMDCs systems.In this way,the performance of electrical transport,photodetection and photoluminescence can be improved,and a prerequisite for discovering new physics under the two-dimensional system can be created.The main research contents and results of this thesis are as follows:(1)Semimetallic transition metal dichalcogenides have unique electronic structure and surface topological properties,and have recently become a hot research topic in low-dimensional fields.Among them,the existence of type-II Dirac fermion in Group VIII Pt Te₂was experimentally confirmed,and the optical and electrical properties based on its special topological state have not been developed.Since the band structure and physical and chemical properties of such materials are closely related to their number of layers,it is particularly important to controllably obtain two-dimensional sample materials for their properties and application research.We successfully synthesized high quality ultra-thin Pt Te₂crystals by directed van der Waals(vd W)epitaxy.Impressively,the Pt Te₂ devices exhibit an excellent metallic feature with the extra-high electrical conductivity of up to 10⁷ S m^-1,3orders of magnitude higher than that of semimetallic 1T MoS₂.Meanwhile,the magnetoresistance effect at low temperatures reaches 800%in a field of 9.0 T.Furthermore,near-field nanooptical properties are performed on the Pt Te₂ for the first time.Considering the sub-wavelength effect,the plasmonic wavelength?_p?200 nm of 1T Pt Te₂ is obtained and a carrier concentration calculated from?_p is about 1.22×10¹⁵ cm^-2,which is 100-fold higher than that of MoTe₂ in the previous reports.We have demonstrated the formation mechanism of ultra-high conductivity with theoretical calculations,explored its application in the fields of electrical transport,near-field optics,etc.,and opened new insights into two-dimensional physical devices.(2)TMDCs semiconductors have the characteristics of strong light matter coupling and easy modulation,and have become a new research hotspot in the field of low-dimensional materials in recent years.Among them,the group IVB TMDCs materials represented by Hf S₂and Zr S₂ exhibit excellent field effect performance and photoelectric performance,and are expected to be applied to fabricate high-performance optoelectronic devices.Here,we report a facile strategy for controlled synthesis of high quality atomic layered Hf S₂ crystals by vd W epitaxy for the first time.Density functional theory calculations are applied to elucidate the systematic epitaxial growth process of the S-edge and Hf-edge.Impressively,the Hf S₂ back-gate field-effect transistors display a competitive mobility of 7.6 cm² V^-1 s^-1 and a ultrahigh on/off ratio exceeding 10⁸.Meanwhile,ultrasensitive near-infrared phototransistors based on the Hf S₂ crystals exhibits an ultrahigh responsivity exceeding 3.08×10⁵A W^-1,which is 10⁹-fold higher than 9×10^-5A W^-1 obtained from the multilayer MoS₂ in near-infrared photodetection.Moreover,an ultrahigh photogain exceeding 4.72×10⁵ and an ultrahigh detectivity exceeding 4.01×10¹² Jones,superior to the vast majority of the reported two-dimensional materials based phototransistors,imply a great promise in two-dimensional TMDCs-based electronic and optoelectronic applications.This discovery broadens the range of optoelectronic detection and opens up new possibilities for the field of phototransistors.(3)Two-dimensional heterostructures have shown great applied prospects in many fields such as optoelectronic devices,energy and catalysis.Among them,the interfacial heterostructures represented by MoS₂/h-BN and WS₂/h-BN have become new hotspots in the research field of low-dimensional materials due to their excellent field effect properties and photoelectric properties.At present,most interfacial heterojunction materials require mechanical exfoliation transfer.This method is not conducive to the large-scale preparation of MoS₂/h-BN heterojunction,and the transfer process will reduce the interlayer interaction of heterostructures.Thus,the scalable fabrication of TMDCs/h-BN heterostructures by direct chemical vapor deposition(CVD)growth is highly attractive.Here,we report a thoroughly direct CVD method to obtain TMDCs/h-BN vertical heterostructures without any intermediate transfer steps.This is attributed to the use of a nickel-based alloy with excellent sulfide-resistant properties and a high catalytic activity for h-BN growth.The strategy enables the direct growth of single crystal MoS₂ grains of up to 200?m² on h-BN,which is approximately 1 order of magnitude larger than that in previous reports.The direct band gap of our grown single-layer MoS₂ on h-BN is 1.85 e V,which is quite close to that for freestanding exfoliated equivalents.This strategy is not limited to MoS₂-based heterostructures and so allows the fabrication of a variety of TMDCs/h-BN heterostructures,suggesting the technique has promise for nanoelectronics and optoelectronic applications.Through theoretical calculations and detailed characterization methods,we have also confirmed this sulfide-resistant mechanism and provided new insights for designing new heterostructures.