Investigation Of The Controllable Synthesis And Interface Properties Of MoS₂ On The SrTiO₃ Single Crystals

The comprehension of functionalized semiconductor heterojunctions is of fundamental importance for innovative functionalization of the traditional materials.The key roles of the interface modulations in these heterojunctions have been commonly recognized and were found having close relevance with the fine structures of the interfaces.Therefore,the deep understanding of the interface modulation mechanism is becoming more and more important for both fundamental science and technical applications.Large number of researches have demonstrated that the two-dimensional(2D)VI group transition metal dichalcogenides(VI-TMDCs),such as molybdenum disulfide(MoS2),has unique interfacial interactions with the oxide substrates,which is helpful for improving their functionalities in nano-devices,catalytic transformation of energies.performances in electronic devices.Among these oxides,strontium titanate(SrTiO3)has been one of the most attractive substrates owing to its uniquely layered structure,high dielectric constant,and quantum mimicry,etc.Its support to the graphene,FeSe,and LaAlO3 films usually lead to novel properties of the hybridized systems.Hence,the composite system of MoS2 and SrTiO3 is also expected to show interesting properties which has recently provoked extensive research interests.Nevertheless,in most researches,the hybridized systems of 2D materials with the oxides were prepared through solvothermal,physical mixing or exfoliation-and-transfer methods,during which the interface contamination cannot be avoided.It thus remains a key issue to construct the well-defined model systems with ideally clean interfaces,so that the interfacial modulation mechanism can be revealed and the optimization of the hybridized system can be achieved.The chemical vapor deposition(CVD)with controlled reaction atmosphere under ambient pressure has been demonstrated as a resultful method to prepare VI-TMDCs/oxide model system with ideal interface.Through carefully optimizing the synthetic parameters,it is achievable to maintain the cleanness and atomic flatness of the oxide substrate while growing high quality 2D-TMDCs,and thus to realize the ideal control of the interface.Here in this thesis,we have successfully grown single layer MoS2 directly on the atomically flat SrTiO3 single crystals through the conventional CVD method.The established MoS2/SrTiO3 model system allowed us to perform a series of investigations on the heterojunction properties and their close relations of the surface/interface structures.Based on our study of the growth kinetics of MoS2,we also successfully fabricated high quality MoSe2 film on the SrTiO3 single crystals.All these results have brought us to a deeper understanding of the physical mechanism occurring at the Ⅵ-TMDCs/oxide interface.The main results achieved in this thesis are as follows:1.High quality single layer MoS2 nanosheets were directly grown on the atomically flat SrTiO3(111)and SrTiO3(100)single crystals through CVD process.The modulated effect of the SrTiO3 substrates on the optical property of MoS2 was systematically investigated.In this study,detailed microscopic characterizations of the sample surfaces were obtained with atomic force microscopy(AFM),scanning tunneling microscopy(STM)and scanning electron microscopy(SEM),which unambiguously confirmed the atomic lattice of MoS2,the cleanness and atomic flatness of the obtained MoS2/SrTiO3 interface.Raman spectroscopy(Raman),X-ray photoelectron spectroscopy(XPS)and photoluminescence spectroscopy(PL)were also employed to demonstrate the ideal interface structure of MoS2/SrTiO3 hybridized system.Based on these samples,the combination of ultraviolet photoelectron spectroscopy(UPS)and Kelvin probe force microscopy(KPFM)measurements clearly demonstrated the interfacial charge transfer from the MoS2 to the SrTiO3,which results in the establishment of an interfacial dipole field.What’s more,the temperature-dependent PL measurement revealed a significant enhancement of the trion in MoS2 by SrTiO3 substrate and the binding energy up to 100 meV.Systematic experiments demonstrated that such effect is stronger for(111)than the(100)SrTiO3 surface but completely quenches on the transferred samples,presenting the high sensitivity to the atomic structures of the interface.2.We investigated the electronic structure of MoS2/SrTiO3(111)by preparing the conductive samples.These samples were achieved on the Nb-doped SrTiO3(111)substrates through the same CVD process.The ideality of the interface was confirmed by a sequence of microscopic characterizations.On this sample,we have performed scanning tunneling spectroscopy(STS)and angle-resolved photoelectron spectroscopy(ARPES)measurements.The band structure of single layer MoS2 was clearly observed.Moreover,we found the bandgap of the system excessive increased with the decreasing of the temperature.Assisted by the theoretical calculations,we understand this phenomenon may originate from the structural changes of SrTiO3 during the temperature evolution.In addition,with the temperature decreasing,the SrTiO3 crystal gradually changes from the cubic phase into the tetragonal phase.Consequently,the charge transfer between MoS2 and SrTiO3 may changes with the evolution of the band-bending at the interface.As a result,the apparent band-edge shift could vary along with the temperature decreasing.3.We further examined the morphological effect of the SrTiO3 on the PL properties of MoS2.In this study,we first prepared SrTiO3 substrates into a trenched surface morphology,which composed of atomically flat terraces and high steps,and then successfully grew single layer MoS2 through the CVD process.A series of characterizations were applied to confirm the preservation of the trenched structure as well as the seamless contact between MoS2 and the SrTiO3 substrate.On such the sample surface,the tip enhanced photoluminescence(TEPL)measurements found that the PL of MoS2 completely quenching above the trenches in the SrTiO3 substrate.On the other hand,the tunneling atomic force microscopy(TUNA)and STM/STS measurements revealed that there is obvious bandgap reduction and electron enrichment around the SrTiO3 trenches.The locally accumulated electrons can account for the quenching of the photoluminescence in these regions.These results demonstrate that the surface corrugation plays an important role in affecting the properties of the TMDCs/oxide heterojunctions.4.Finally,by optimizing the CVD parameters,we realized the size control of MoS2 on the SrTiO3(111)single crystal surface.This could be achieved by adjusting the supply of MoO3 powder and S powder,the CVD reaction temperature and the specific distance between the source and substrate in the oven.We finally realized the fabrication of high quality and single layer MoS2 with larger size on all the aforementioned SrTiO3 substrates.This hence paves the route for the electron transportation studies of the MoS2/SrTiO3 system.Moreover,we applied the knowledge of MoS2 synthesis to fabrication of other TMDCs and successfully grew MoSe2 on the SrTiO3 substrates.And by using AFM,SEM,Raman,XPS measurements,we could confirm not only the successful construction of the MoSe2/SrTiO3 sample,but also the cleanness and atomic flatness of the interface.