Controllable Preparation And Properties Of Two-dimensional Platinum-based Transition Metal Chalcogenides

Two-dimensional(2D)transition metal chalcogenides(TMDs)exhibit broad application prospects in the fields of nanoelectronics,optoelectronics,catalysis,and sensors due to their atomic-scale layered crystal characteristics.In addition to the widely studied semiconductor TMDs(for example,MoS2 and WSe2),group 10 platinum-based chalcogenides(PtX2)have attracted extensive attention from researchers owing to their unique structure and novel physical properties.Unlike most common TMDs with few d electrons,the d orbitals of 2D PtX2 materials are almost completely occupied by electrons.And the pz orbitals corresponding to the interlayer chalcogen atoms are highly hybridized,resulting in strong layer dependence behavior and interlayer interaction.Therefore,the 2D PtX2 materials exhibit many excellent characteristics,including thickness-dependent tunable band gap,high carrier mobility and good air-stability.As emerging materials,2D PtX2 have many excellent properties and broad application prospects.However,the realization of large-area and controllable synthesis is a prerequisite for further exploration of fundamental investigations and practical applications.The present research on PtX2 is mainly limited to mechanical exfoliation with low productivity,small crystal domain size and uncontrollable number of layer,which can not meet the needs of large-scale electronic device applications.Chemical vapor deposition(CVD)has been considered to be an effective method for the synthesis of high-quality 2D TMDs materials(MoS2,WS2 and ReS2).Therefore,in order to solve the epitaxial difficulty problems of PtX2 materials,this thesis developed an effective CVD growth method to realize the controllable preparation of large-area,high-quality 2D PtX2 materials,followed by systematic studies of their properties and applications.The specific research content of this thesis is as follows:(1)Controllable growth of ultrathin single crystal PtS2 on the surface of Au substrateAiming at the problems of epitaxial difficulty and preferred vertical growth trend of PtS2 material,Au foil with low surface barrier was selected as the substrate.Using its catalytic effect on the growth of 2D materials,the controlled growth of single crystal PtS2 was firstly achieved by CVD.The influence of substrate type and growth temperature on the morphology and size of PtS2 was systematically studied.In addition,by constructing PtS2 field-effect transistors(FETs)with different thickness,the transition from metal to n-type semiconductor was observed.Under illumination conditions,the thin-layer PtS2 exhibited obvious photoelectric response characteristics.In consideration of the high conductivity,thick-layer PtS2 was applied to the electrochemical hydrogen evolution reaction(HER).The Tafel slope is as low as 40 mV/dec(Pt electrode:31 mV/dec).This result proved that as-grown PtS2 can be used as efficient HER catalyst.(2)Au(001)crystal plane step induced growth of PtSe2 nanobeltsBased on the controllable preparation of PtS2,the CVD synthesis of PtSe2 on Au substrate was further explored.By controlling the annealing temperature close to the melting point,the Au substrate with main(001)crystal plane was obtained.Using the metal steps on the surface,PtSe2 nanobelts with controllable thickness were prepared on Au(001)crystal plane.The "step-induced growth" mode of the(001)crystal plane was explored through DFT calculations.It was confirmed that the growth of PtSe2 nanobelts at the edge of the(001)crystal plane step is more favorable than on the terrace,due to the Pt atoms at the edge are more easily passivated by Au.The nanobelt structure of PtSe2 material exposes abundant active sites on the edges,which can be further used for HER.The Tafel slope as low as 37 mV/dec,which is comparable to that of the Pt electrode.Additionlly,by constructing PtSe2 FETs with different thickness,the basic electrical and optoelectronic properties were explored.(3)Controllable epitaxy of 2D Dirac semi-metal PtTe2 and its terahertz photodetectionFor the problems of strong interlayer coupling effect and low mobility of Pt and Te atoms,PtTe2 usually suffered small crystal domain size on the current substrates.This thesis developed a eutectic-assisted CVD growth method,cleverly using the characteristic of Te and Au to form binary eutectic,and the melting point of Au is effectively reduced.The molten Au substrate can provide low surface barrier for atom migration,which is beneficial to the epitaxial growth of 2D ultrathin PtTe2 crystals.By controlling the amount of Te precursor during the growth of PtTe2,rectangular and triangular PtTe2 were obtained on solid and molten Au substrates,respectively.The unique isotropic and anisotropic growth modes were revealed.In addition,the metal-PtTe2-metal structure photodetector realizes broadband detection from 0.04 to 0.3 THz at room temperature,and shows high responsivity and fast response speed.