Preparation,Structure And Optical Properties Of Two-dimensional Diselenide Nanofilms

The monolayer and few layer transition metal dichalcogenides(TMDCs)not only have a layered structure similar to graphene but also have band width of 1-2 eV to make up for the defect of graphene zero bandgap.So far,most studies have concentrated on monolayer MoS2 with a direct band gap of 1.8 eV,However,the synthesis and characterization of TSe2(MoSe2 and WSe2)remains largely unexplored,TSe2 with lower direct band gap value have some worthwhile advantages,for instance,monolayer MoSe2 and WSe2 both exhibited higher conversion efficiencies from solar to electrical energy,1.5eV of bandgap value in monolayer MoSe2 is more close to the ideal bandgap value of photo-electrochemical cells and single-junction solar cells.Suitable synthesis method to prepare a large area,high crystal quality single-layer or few layer TMDCs is the premise for widely used of TMDCs in the actual production and life.In this paper,we synthesis of monolayer and few layer TSe2 films in Si/SiO2 substrates by mechanical exfoliation and chemical vapor deposition(CVD).The shape and size of as-grown MoSe2 samples were observed using optical microscopy(OM)and scanning electron microscope(SEM)and the thickness of samples was exactly measured by Atomic force microscopy(AFM),the results of AFM were shown that the thickness of each step layer is around 0.8 nm.Raman and photoluminescence spectroscopy were employed to characterize and comparer the properties of monolayer and few layer MoSe2 films.With the increased in the layer number of material,the peak position of Raman modes and PL were shown a clear shifting,furthermore,the out-of-plane Raman Aig mode in triple layer,four-layer and five-layer film were shown splitting due to the so-called Davydov splitting.Except the typical triangular shape,pentagonal shape and hexangular shape monolayer MoSe2 films were also observed in as-grown samples,the Raman and PL spectrums of pentagonal shape and hexangular shape monolayer are consistent with the Raman and PL spectrum of triangular shape monolayer.