Controllable Preparation Of Two-dimensional TMDs On TiO₂ Surface And Investigation Of The Properties

Due to the unique electronic structure and superior physicochemical properties,two-dimensional transition metal dichalcogenides(2D-TMDs)have shown applicable potentials in the fields of catalysis,energy strorage,electronics as well as optoelectronic devices.To fabricate the 2D-TMDs,chemical vapor deposition(CVD)method has been widely applied in laboratories,which gives the as-grown 2D-TMDs films properities of considerable size,controllable thickness,fast growth rate and high crystallinity.In the CVD progress,the choice of a suitable substrate is of importance since it not only serves as the support for the as-grown 2D-TMDs films but also shows effects on the various aspects of the 2D-TMDs,such as the lateral size,thickness,orientation and the crystallinity.Although the amorphous and insulating SiO2/Si substrate is the commonly used in the fabrication of 2D-TMDs,more and more researches have demonstrated that using the functional transition metal oxide(TMO)as the support for the 2D-TMDs can usually create outstanding novel properties beyond the individual material,wherein the interface interactions between the TMO and the 2D-TMDs were generally considered playing a vital role.However,because the TMD/TMO hybridized systems were usually prepared via either hydrothermal synthesis or wet-method transfer methods in most researches,the inevitable contaminations as well as the unclear interface structures formulated the severe obstacle for deepened understanding of these interface effects.Therefore,it becomes an urgent issue to establish proper model systems whose interface structures are atomically clear and also ideally clean.It is well known that the CVD technology can optimally ensure the cleanness of the fabrication due to the high synthetic temperature.Nevertheless,the highly reactive atomosphere may also ruin the atomic structures of the applied substrate,thus making it a challenging task for growing 2D-TMDs on the atomically flat TMO substrates.Based on these considerations,we chose to investigate the fabrication as well as the properties of 2D-TMDs on the rutile TiO2 single crystal surface,the latter being selected due to the wide interests over its applications in photocatalysics.In this thesis,we have applied conventional CVD technique to grow single layer MoS2 and WS2 on the differently orientated rutile TiO2 substrates.By carefully optimizing the synthetic parameters,we realized the fabrication of ideal MoS2/TiO2 and WS2/TiO2 model systems with clean,seamless and atomically flat interfaces.Based on these high quality samples,we have systematically investigated the built-in optical and electronic properties of the corresponding heterostructures and their relations to the termination of the TiO2 substrates.The main contents and results are shown as the following:1.The study of the effects of the TiO2 terminations on the PL of MoS2.Here,with the CVD method we have successfully grown monolayer MoS2 films with exactly the same quality on differently oriented TiO2,i.e.(100),(110)and(001)substrates.In all MoS2/TiO2 heterojunctions,we achieved the seamless and well-defined interfaces,as confirmed by the AFM,SEM and XPS characterizations.In particular,detailed XPS analyses reveal that on all samples electrons transfer from MoS2 to TiO2.Therefore,it results in the establishment of the interfacial dipole field following the order of E(MoS2/TiO2(100))>E(MoS2/TiO2(110))>E(MoS2/TiO2(001)).Because the interfacial charge transfer can effectively change the formation probability of the negative trion(A-),it can thus influence the PL of the MoS2 by modulating the ratios between neutral exciton(A0)and negative trion(A-)as well as the total PL intensity.The more charges transfer,the ratio of IA-/IA0 turn to smaller,and the intensity of the total PL get stronger.Therefore,on these samples the PL intensity follows the order of MoS2/TiO2(100)>MoS2/TiO2(110)>MoS2/TiO2(001)while the IA-/IA0 ratio takes the trend of MoS2/TiO2(100)<MoS2/TiO2(110)<MoS2/TiO2(001),which also agrees with the binding energy shifts as observed in XPS.The results demonstrate that the work function difference of the different TiO2 substrates is the origin for the varied luminescence of MoS2.2.We further explore electronic structure and electron-phonon coupling of MoS2/TiO2.After investigating the optical properities of MoS2/TiO2 heterojunctions,For the MoS2/TiO2(100)sample we firstly obtained the clear energy dispersions of the valence band of single layer MoS2.It is found that the valence band maximum(VBM)and the conduction band minimum(CBM)both appear at the K point of the hybridized system,which thus concludes that the as-grown MoS2 has a direct band gap of 2.0 eV.Afterwards,we performed detailed measurments over the CBM at the K point and found it presenting the characteristic "peak-dip-hump" spectrum,which indicated the formation of an interfacial Frolich polaron.This proposition was further examined by the control experiments with potassium deposition,whose doping effects significantly increased the electron density in the MoS2/TiO2(100)system and hence gradually weakened the electron-phonon interaction based on the dynamic screening effects.Finally,we resorted to theory for deepened understanding of the interface coupling.The molecular dynamics simulation results demonstrated that the interfacial Frohlich polaron stems from the coupling of the electrons in MoS2 with the longitudinal optical(LO)Eu phonons mode of the rutile TiO2 substrate.Such interfacial Fr?lich polaron was also observed in the MoS2/TiO2(110)heterstructure,indicative of the independence on the termination of the TiO2 substrate.The interface polaronic states can be very robust against the exposure to ambient,showing large potentials for real applications.3.The study of valley polarization and valley coherence of the the WS2/TiO2 heterostructure.We developed a confined-spaced CVD technique to synthesize WS2 nanosheets on the TiO2(110)substrate.The combination of AFM,SEM,and Raman characterizations clearly demonstrated the single layer thickness of the WS2 as well as the cleanness and the atomic flatness of the W S2/TiO2(110).On this well-defined model system,KPFM and XPS measurements directly revealed that the WS2 adlayer has relatively lower work function thus transfers electrons to the TiO2 substrate,which results in the formation of interface dipole field.We subsequently performed systematic varied photoluminescence measurements.It was found that at 6 K the PL spectrum of WS2/TiO2(110)contains only neutral exciton(A0)without any contributions from the negative trion(A-),indicating that as-grown WS2 has very low carrier concentration.Meanwhile,the circularly polarized PL and linealy polarized PL measurments at 6 K reveal that WS2/TiO2(110)has a significant valley polarization and valley conherence,the latter even preserving until at room temperature.We account this phenomenon partially to the extremely high quality of the as-grown WS2,which has drastically suppressed the defect and phonon induced inter-valley scattering and thus improve the valley polarization.On the other hand,the low carrier concentration of the as-grown WS2/TiO2(110)also decreases the electron-hole exchange interactions and thus largely reduces the probability of decoherence.These results may provide a novel strategy for developing the tunable valleytronics based on interface interfactions.Finally in Chapter 6,we give a summary of the three works in the whole thesis and also a perspective for the related works which can be performed in the near future.