Crystal Growth And Properties Of Several Transition Metal Chalcogenides

The transition metal chalcogenides（TMDCs）are another attractive new types of two-dimensional materials after graphene,which attracted much attention because of their unique structures,excellent physical and chemical properties,bright prospects in the field of energy storage and optoelectronics.The preparation of large-size and high-quality crystals is an emphase and foundation of applications in the study of TMDCs.In this paper,a series of transition metal chalcogenides（MoS₂,MoSe₂ and MoTe₂）bulk single crystals were proposed to grow by the flux method,and saturable absorber for Q-switched laser,effect of defects on band structure,mand spectral properties were researched pertinently.Main contents and conclusions in this dissertation are as follows:Ⅰ.Flux method growth and characterization of MoS₂,MoSe₂ and MoTe₂ bulk crystalsSingle crystals of MoS₂,MoSe₂ and MoTe₂ were grown by flux method,and the bulk crystals with large size and high quality were obtained,during the experiment,the flux system and growth conditions were explored systematically.MoS₂ bulk crystal was grown by Sn flux method for the first time,and the MoS₂ bulk single crystal with an area of 3×4mm² and a thickness of 40-100μm was obtained.The optimal growth conditions were obtained by experiments:mole ratio of the starting materials is Mo:S:Sn=1:2:10,the cooling range varies from 1180℃ to 800℃,the cooling rate is 2-4℃/h.The experimental results show that MoS₂ crystal morphology is mainly controlled by the cooling rate.The basical structure and crystallinity of MoS₂ crystal were investigated,and the crystal growth mechanism of MoS₂ was studied.Spiral growth steps were observed on the surface of crystals by optical microscope and atomic force microscope（AFM）,which suggested that crystal growth process of MoS₂ by Sn flux method was controlled by the screw dislocation mechanism.MoSe₂ bulk crystal was also grown by Sn flux method.Through exploring the growth conditions,molar ratio of starting materials and cooling range played a decisive role in getting pure MoSe₂ crystals.Te was used as self flux to grow MoTe₂ crystals,which not only avoids the incorporation of other impurities in the system,but also simplifies the experimental process.MoTe₂ crystals in two different phases were obtained by using the molar ratio of raw materials in Mo:Te ＝1:15.Large-size 1T-MoTe₂ crystal can be obtained when the temperature is cooled in the high temperature region,and the large size 2H-MoTe₂ crystal can be grown successfully by enlarging the cooling range to a lower temperature.The crystal structures of MoTe₂ crystal in two phases are analyzed,2H-MoTe₂ has hexagonal structure similar to that of graphene and 2H-MoS₂,the space group is P63/mmc,and the 1T’-MoTe₂ has a distorted octahedral structure,the space group is P21/m.It is worth noting that Sn has also been used as a flux to grow MoTe₂,but large-size MoTe₂ crystal has not been obtained.In this thesis,MoS₂ and MoSe₂ bulk crystals were grown by Sn flux method,while large size MoTe₂ crystal was not got with Sn as fulx,it was grown by Te flux method.It indicates that Sn is a suitable flux for the growth of some TMDCs,but also has certain restriction.In the growth of TMDCs,we need to explore specific conditions according to different compounds.The research results of this paper show that the flux method is a feasible method for the growth of transition metal chalcogenides crystals,and high-quality and large-size bulk crystals can be obtained,while flux selection and growth conditions of each specific compound should be explored individually.Ⅱ.Exfoliation of MoS₂ crystal and applications in laser Q-switchingThe common exfoliation method for two-dimensional materials:mechanical exfoliation method and liquid ultrasonic method were used to exfoliate MoS₂ crystals grown by Sn flux method,and graphene-like MoS₂ nano-flakes were prepared successfully,which were characterized by SEM,TEM and AFM,indicating the large-area,nano MoS₂ flakes well-crystallized.The MoS₂ nano flakes prepared with liquid phase ultrasonic method Were applied in the laser Q-switching experiment as a saturable absorber.A passively Q-switched laser at a wavelength of 1064 nm were operated,the maximum laser output energy is 250mW,a maximum repetition can reach 524 kHz,the single pulse energy is 0.48J,and the pulse width is only 326 ns.The pulse width obtained in our experiment is narrower than that reported by the literature in which the MoS₂ was grown by PLD,suggesting that the high crystallinity of MoS₂ would promote the results of laser Q-switched and ultrathin nano-flake MoS₂ exfoliated from the flux-grown crystals is a promising candidate for use as the saturable absorber for Q-switched SSLs.Ⅲ.Effects of defects to the band structure of MoS₂ crystalThe effects of defects were researched by theoretical calculation and experiment.Firstly,the band structures of bulk MoS₂ crystal before and after the introduction of sulfur vacancies were made a comparison by the first principle theory.Three new levels appear in the band gap of MoS₂ after sulfur vacancies are introduced,of which two new states located above the Fermi level are deep level,another state located below the Fermi level and slightly above valence band top,is a shallow level.Through the analysis of the local charge density,it confirms that two states above Fermi level are produced by the molybdenum atoms around the sulfur vacancies and and the new level below Fermi level is caused by sulfur.vacancies.Then,we further explored the effects by experiments.MoS₂ crystals were annealed in high vacuum and sulfur atmosphere for a long time,and the annealed samples were characterized by XRD,TEM,EDS,XPS and Raman spectra,which can be confirmed that MoS₂ crystals maintained good crystallinity and sulfur vacancies were introduced.Angle resolved photoelectron spectroscopy（ARPES）was used to analyze band structures in experiment,the ARPES can only describe the band structure below the Fermi level because the states below the Fermi level are occupied states.The experimental results showed that after introduction of sulfur vacancies,a new state appeared in the band gap of MoS₂ crystal,which located below the Fermi level and just above top of the valence band.It is consistent with the theoretical simulation results,and the top of the valence band is also slightly upward,closer to the Fermi level.IV.The structural phase transition behavior of MoTe₂ crystalMoTe₂ crystal can keep stable in two structures-2H-MoTe₂ and 1T’-MoTe₂ at room temperature,the phase transitions are prone to occur due to the small energy difference between the two phases,and MoTe₂ phase transitions induced by thermal and light were studied systematically in this thesis.In order to avoid the oxidation of crystal,samples were annealed in the high vacuum at a high temperature,and the following conclusions are obtained:2H-MoTe₂ crystal at 900℃ transfers to 1T’-MoTe₂,1T’-MoTe₂ turns back to 2H-MoTe₂ in cooling process,the phase transitions are reversible;1T’-MoTe₂ will transform.into 2H-MoTe₂ when the temperature rose to 530℃,and this phase transition is irreversible.In addition,the surface of crystal was irradiated by laser and it was found that the 2H-MoTe₂ crystal could be changed into 1T’-MoTe₂,but the phase transition of 1T’-MoTe₂ was not observed under the irradiation of the laser.V.The electromagnetic properties of MoTe₂ crystalTd-MoTe₂ crystals are widely concerned because of typeIIweyl semimetallic properties and the colossal magnetoresistance effect（MR）.The structure of 1T’-MoTe₂ crystal changed slightly when the temperature dropped to about 250K,and it turned into Td-MoTe₂.Here,low temperature electromagnetic properties 1T’-MoTe₂ grown by Te flux were characterized.The results show that Td-MoTe₂ has a positive colossal magnetoresistance,at the temperature of 5 K,the magnetic field intensity of 6T,the magnetic field is perpendicular to the surface of the crystal,we get the MR value of 19300%.The magnetoresistive effect of MoTe₂are temperature dependence and direction anisotropy,MR value decreases gradually with the increase of temperature,when the magnetic field is perpendicular to the crystal surface,the MR value obtained is larger one to two orders of magnitude than that of obtained when the magnetic field is parallel to the crystal surface.The previous reports show that gaint unsaturated magnetoresistance comes from the perfect electron-hole compensation,and our experimental results also verify this conclusion,however,by comparison tests between the as-grown and annealed crystals,we found that there are other mechanisms in 1T’-MoTe₂ competing with the electron-hole compensation,which is likely related to defects and temperature.Ⅵ.THz spectra of MoTe₂ crystalsTerahertz technology and two-dimensional materials are both research focuses in the scientific field in recent years,this paper will study from the combination of the two points.The spectrum from 0.4 to 2.2 THz of 2H-MoTe₂ crystal was obtained by terahertz time-domain spectroscopy,absorption coefficient,refractive index,dielectric constant and carrier concentration in this range can be extracted by following fast Fourier transform and mathematical calculation.We get the following conclusion:2H-MoTe₂ crystal has a weak absorption for the terahertz wave,the absorption coefficient in the range of 7-13 cm^-1;the index remained at around 4.2 in this range;the real part of permittivity is about 17 and the imaginary part of the dielectric decrease from 0.6 to 0.2 gradually.This experiment provides a base for the further application of 2H-MoTe₂ in THz range.