Solution-based Exfoliation For Large-sized Transition Metal Dichalcogenides Nanosheets And Their Phase Transition Investigation

Two-dimensional transition metal dichalcogenides(TMD)with atomically thin thickness are attracting numerous research interest and focus,owing to their abundant and unique Physical and Chemical properties.As a matter of fact,in order to achieve their real applications in electronics and energy storage fields,as well as scalable production,the synthesis of two-dimensional TMD nanosheets with high quality and large size becomes a critical bottleneck challenge.In this way,solution processing exfoliation has always been an effective way to produce two-dimensional nanomaterials with low cost as well as high yields.However,exfoliation methods currently used usually involve high energy input to drive the exfoliation process by overcoming the Vander Waals forces,such as sonication.The high energy input would inevitably cause damages to the in-plane chemical bonds of layered TMD,which greatly hinders the size and quality of the final production of two-dimensional nanomaterials.Hence,it still remains challenging to synthesize two-dimensional TMD nanosheets with large size and high crystallinity via solution processing exfoliation methods.This dissertation focuses on two-dimensional TMD,aims to develop solution-based top-down strategy to obtain various very large-sized TMD nanosheets of high crystallinity with gentle driving forces in mild condition.We have looked into the inherent relationship between exfoliation effects(such as yields,sizes and quality etc.)and their microstructures(such as lattice strains,electronic states etc.).Based on these results,new methodologies and ideas were specially developed catering for different applying conditions.Specifically,this dissertation has designed tree exfoliation methods based on solution processing,including fast total single crystal chemical exfoliation,acid-assisted exfoliation and programmed exfoliation,to obtain TMD nanomaterials for different utilizations in energy storage and electronic.Detailed content includes the following three points:1.First of all,we have designed a total single crystal exfoliation strategy based on solution processing.Taking TaS2 as an example,through partial lithium intercalation and with the help of hydration effect,the inter space of TaS2 single crystal was greatly expanded,and thus only mild manual shaking was adequate to trigger the lamination,achieving TaS2 monolayers with 90%yields.Of note,the obtained TaS2 monolayers exhibited very large lateral sizes,even up to sub-micrometers,and maintained extremely high crystallinity as well as the phase structure of the bulk crystal.Moreover,we uncovered the periodic behavior of nanosheet sizes among different TMD materials,of which the effect of lattice strain on nanosheet sizes was revealed.As a result,the tensile and compressive strains correspond to large and small sizes respectively.This work has been expected to realize massive production of these nanomaterials and provide important inspiration for novel exfoliation strategies.2.Then,we have developed an acid-assisted exfoliation method,simultaneously achieving the exfoliation of large-sized TMD monolayers and structural functionalization,to obtain metallic TaS2 monolayers with controllable sub-nano pores.In the acid-assisted exfoliation,the hydrogen ions not only violently react with the inserted lithium atoms,but also etch the in-plane chemical bonds,which in return achieve effective exfoliation and structure modification at the same time.The homogeneous sub-nano pores and the metallic conductivity allow fast ion transport and good electron conductivity respectively,which leads to excellent supercapacitor performance of the acid-assisted exfoliated TaS2 monolayers with volumetric capacitance of 508 F/cm3 and energy density of 58.5 Wh/L.3.Finally,we realized very large-sized TaX2(X=S,Se)1T-2H planner homojunctions via solution-based exfoliation method.The homojunctions exhibited atomically smooth interfaces,with electronic device performance on phase transition further investigated.Specifically,partial lithiation was employed to introduce phase transition from 1T to 2H in TaS2 crystals,followed with H2O and acid solution treatment to trigger exfoliation of 1T and 2H domains.Thus,lateral 1T-2H TMD homojunctions were successfully produced via top-down method.By analyzing the surface potential,the atomically smooth interfaces and the close Fermi level of the 1T and 2H phases owns enhanced carrier efficiency between the metal-semiconductor interfaces.The decreased barrier in 1T-2H homojunction helps to reduce the electric field strength for triggering the CDW transition,which would greatly decrease the power consumption of these devices and accelerate the electronics investigation and applications.