Investigation Of Charge And Energy Transfer Behavior In Two-Dimensional Heterostructure Nanomaterials

In recent decades,the science was advanced step by step,while the electronic industry developed following the famous Moore's law which tells that the amount of the component device is doubled so that the corresponding performance is also doubled,within a period of 18 to 24 mouths.It is undisputed a very fast developing speed,and also represents a fastest speed of the.However,such a nice situation will end very soon.With the further shrink of the size,the physical properties of the component device are significantly variated due to the more and more obvious quantum effect.Keeping increase the amount of the component device will bring about many calculation mistakes.Therefore,the fast developing of the electronic industry will find a saturation,while the Moore's law will also be invalid.How to inherit the Moore's law with high developing speed,will be the common wish and the purpose that should be devoted by whole human.The emergence of the nanomaterials gives the new expectation of the scientists.In contrast to the traditional materials,nanomaterials have smaller size and mass but larger specific surface area.In addition,the ultra-small size at some dimensions of nanomaterials bring about many features of the quantum effect that we can directly investigate,and thereby achieving novel photonic and quantum devices.These new devices will continue the further developing of the integrated device performance,also have potential to achieve new generation devices with low cost,high computing power,fast communication.Two-dimensional nanomaterials with quantum confinement at one dimension have many features with both novelty and interestingness.In 2004,the discovery of the graphene opened the door of the new two-dimensional materials.However,because the dirac cone of the band structure,graphene lacks the bandgap and thereby faces a big challenge in application of the photonics.Hence,new two-dimensional materials which have bandgap such as boron nitride with big bandgap,phosphorene with small bandgap as well the transition metal dichalcogenides and other layered materials which have the bandgap covering visible to near infrared wavelength,are emerged.Furthermore,two-dimensional materials can be stacked to form heterostructure by relatively weak van der waals force.Because of the closely compact of the surface in each layer,the coupling within the layers is really close,and thus bringing about many coupling interactions such as charge and energy transfer.Just these intriguing interlayer coupling interactions,van der waals heterostructures developed many novel features that beyond their individual layers,and realized many devices like photodetector and light emitting diode.There have been many works reported these charge and energy transfer processes,however,their mainly focus on the mechanism of the transfer at the interface,while there is no work reported the modulation and engineering of these coupling processes.This thesis,based on the two-dimensional van der waals heterostructures,mainly investigated the charge and energy transfer behaviors and subsequently realized the modulation and engineering of these behaviors.We aim at to further improve the performance of the device or even to engineer novel devices by investigating and engineering these coupling interactions between different layers.Main achievements are given as follows:1.By a simple two-step chemical vapor deposition method,we integrated the traditional layered PbI₂ into the transition metal dichalcogenides to form new van der waals heterostructure for the first time.By using the WS₂ and WSe₂ powder as the growth source,we prepared monolayer-dominated WS_2?1-x?Se₂ alloys with big size and tunable x.At the second step,we deposited PbI₂ to the monolayer WS_2?1-x?Se₂ and subsequently demonstrated the PbI₂/WS_2?1-x?Se_2x heterobilayers.Furthermore,the controlling of the thickness of PbI₂/WS₂ heterostructure?keeping WS₂ as monolayer?was also be achieved by adjusting the growth time and position of the PbI₂ in the second growth step.2.By tuning the alloying degree of the bottom WS_2?1-x?Se_2x layer in PbI₂/WS_2?1-x?Se_2x heterostructures,we tuned the absolute band position with a large range,and thus realized the transition from the type-I to type-II of their band alignment.The change of the band alignment also leads the distinction of the charge transfer behavior.It also demonstrates that we achieved the control of the charge transfer behavior by alloying the bottom layer.Meanwhile,we also realized the separation and aggregation of the carrier by controlling the charge transfer,and thus controlling the quenching and enhancement of the PL of the bottom layer in heterostructure.3.By utilizing the type-I band alignment in PbI₂/WS₂ heterostructure,we probed the carrier interlayer transport behavior in PbI₂ mutilayers with WS₂ acting as a carrier extracting layer of the PbI₂.By performing the time-resolved photoluminescence experiments,we studied carrier decay dynamic of the PbI₂ and WS₂ in PbI₂/WS₂ heterostructure.A one-dimensional diffusion model described the carrier interlayer transport very well.We obtained a diffusion coefficient of the electron and hole with values of 0.039 and 0.032 cm² s^-1.It is the first time to report the carrier interlayer transport behavior in multilayer materials,giving many fundamental physical information for improving the performance of van der waals device and even engineering new device.4.Through constructing type-I PbI₂/WS₂ heterostructure,both the electrons and holes can be injected from the PbI₂ layers to WS₂ layer.After optimizing the thickness of PbI₂ in morphology,we obtained a largest population of carrier injection in WS₂.Meanwhile,the carriers after injection process in WS₂ circumvents band nesting states which is dominated by nonradiative recombination,thus induced the large improvement of the quantum yield of WS₂.We finally obtained a maximal PL enhancement of 106-fold in WS₂.This is significant for the improvement of the poor light emission property of transition metal dichalcogenides induced by the low absorptivity and low quantum yield.5.We engineered a certain graded bandgap of the donor in CdS_x Se_1-x lateral heterostructure nanoplates by controlling the alloying degree.Because of the energy funneling effect,this graded bandgap can efficiently and directionally drive the carriers from donor to the hetero-interface for the energy transfer.This strategy largely improved the extreme low efficiency of transport process before the energy transfer process in heterostructure,also largely improved the total energy transfer efficiency.Through this bandgap engineering,we achieved an energy import efficiency up to¹09.3%of the acceptor.This novel bandgap engineering opens a new door for improving the energy transfer in nano-heterostructure,shows a significant potential in improving the performance of relevant device.