| Two-dimensional van der Waals layered crystal（2D vd Ws crystal）is a fascinating nanomaterial,whose surface is super smooth and lack of dangling band.It is an ideal platform for exploring original quantum phenomena.Also,2D vd Ws crystal owns good modifiability,and has great potential in the applications of energy storage,catalysis and so on.There are a large number of members in the 2D vd Ws crystal family,processing rich physical properties.Previous researches have shown that the atomically thin vd Ws crystals,such as graphene,monolayer h-BN,and monolayer Mo S2,can be used as the basic component materials of future transistor.However,it is still too difficult to realize the wafer-scale synthesis of above 2D vd Ws crystals with high quality.In addition,the stability of most studied 2D vd Ws semiconductors in air environment is not good enough,so it is necessary to explore new kinds of 2D vd Ws semiconductor with better atmospheric stability.In this paper,the preparations of large-area 2D WS2 films and BiOI crystals were systematically investigated,and the main experimental results are listed as follows:Large-area uniform WS2 films with thickness less than 3 nm were prepared by atomic layer deposition（ALD）combined with sulfurization process.The ALD process for the deposition of ultrathin WO3 films by using O2 plasma and（tBu N）2（Me2N）2W was systematically studied,and the ALD temperature window was found to be 150～220°C.The high uniformity of ultrathin WO3 film make it very suitable to be sulfurized to ultrathin WS2 film.Moreover,the sulfurization process of ultrathin WO3 film was improved.By introducing H2 into the sulfurization system at 500°C,the WO3 was reduced to WOx（x<3）,as a result,the sublimation of the ultrathin oxide film was suppressed.Finally,a continuous and uniform atomically thin WS2film can be obtained via sulfurization.The Raman spectra show that higher quality of WS2 film would be obtained at higher sulfurization temperature.A linear relationship can be found between the thicknesses of WS2 film and WO3 film,indicating that the layer number of final WS2 film can be controlled by adjusting the ALD cycle number.The transistor based on WS2 film was fabricated,showing a device mobility of 4.5 cm2?V-1?s-1 and an on/off ratio of 105.Few-layers BiOI nanosheets were grown on mica substrates using BiI3 and O2 as reactants.By stacking mica,a confined reaction space was created,which is favorable to the 2D nucleation and growth of BiOI crystals.The confined space plays a role in regulating the concentration field of reactants,and the main feature of the kinetic of space-confined chemical vapor deposition（SPCVD）is the nearly stagnant transport of reactants in confined space.The suitable temperature for the growth of 2D BiOI crystals is in the range of 290～370°C.As the 2D BiOI crystal is connected with mica substrate by van der Waals force,it can be easily transferred by a polymer film.The A1g Raman peak of 2D BiOI moves toward higher wave number with increasing thickness of BiOI,indicating the layer-dependent characteristic of Raman spectra.The contact between pristine 2D BiOI and four kinds of metal electrodes（Ti,Ni,Cr and Pd）is Schottky-type,and the work function of BiOI is measured to be 5.56 e V.Under ultraviolet irradiation,the iodine content in BiOI decreased,and the contact between modified BiOI nanosheet and Pd electrodes is also changed to be Ohmic-type.Under 473 nm,BiOI based device show a photoresponsivity of 26 m A?W-1,an external quantum efficiency of 6.9%,and a detectivity of 8.2×1011 Jones with a bias of 10 V.Above data show that BiOI owns a high sensitivity for 473 nm light.In order to prepare large-size 2D BiOI crystals,the concentration field of reactants in CVD system were further improved using vapor-trapping method and sacrificial agent.The use of sacrificial agent not only reduces the concentration of O2,but also increases the concentration of BiI3,resulting in a low nucleation density of BiOI.Consequently,millimeter-size 2D BiOI single crystals were successfully prepared.It was found that some BiI3 crystals were deposited on mica substrate at lower temperature（<320°C）during the growth process.The 2D BiOI ribbons are formed via the process of layer-by-layer oxidation and delamination.The apex angles of BiOI nanosheets are often sharpened,this phenomenon can be attributed to the 2D fractal growth of BiOI nanosheet.The linear thermal expansion coefficient of BiOI crystal is～2.2×10-4 K-1 at 370°C.The 2D BiOI based top-gate FET exhibits a n-channel depletion,and the on/off ratio of above device reaches up to 104.Additionally,the optical properties of 2D BiOI films were investigated by spectroscopy ellipsometry（SE）.In the visible region,the ellipsometry angleΔof few-layers BiOI is independent of its layer number,while the ellipsometry angleΔdecreases with the increasing thickness of BiOI.The optical dispersion model,including three Tauc-Lorentz oscillators,fits the SE data well.Further,the Tauc optical band gap of BiOI is found to increase from 1.99±0.03 e V to 2.33±0.05 e V as the thickness of BiOI decreases from 7.6 nm to 1.9 nm,this phenomenon can be attributed to the quantum confinement effect in 2D materials.In the visible region,typical refractive index of few-layer BiOI is 3.5.In 2.0～3.0 e V,the absorption coefficient of 8-layer BiOI is in the range of 8.4×104～2.6×105 cm-1.Moreover,the micro-zone absorption spectra of few-layers BiOI also show that the bandgap energy increases with decreasing thickness of 2D BiOI. |
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