| Along with technological innovation,optoelectronic devices are gradually evolving towards miniaturization and high performance.In this process,the size and performance of optoelectronic materials are becoming more and more demanding.Two-dimensional materials are expected to replace conventional semiconductors as popular candidates for future nanoscale optoelectronic devices due to their excellent physical properties.In recent years,twodimensional transition-metal dichalcogenides(TMDs)have received much attention due to their excellent optical and electrical properties and potential applications in electronic devices,optoelectronic devices,energy conversion,and storage.Previous studies have shown that the optical properties of 2D materials are susceptible to factors such as atomic doping and the external environment,which provide a new dimension in the regulation of optical properties of 2D materials.The investigation of the optical properties of two-dimensional TMDs and the mechanism of their modulation is essential for the design and optimization of TMDs-based optoelectronic devices.Among the means to characterize the optical properties of materials,spectroscopic ellipsometry is a non-contact,non-destructive,and highly accurate characterization method,which is particularly suitable for the study of optical properties of 2D materials.In this study,the fundamental optical properties of several transition-metal dichalcogenides are analyzed using ellipsometry in combination with other test methods(e.g.,Raman spectroscopy,absorption spectroscopy,etc.)and first-principles computational simulations based on density functional theory,and the effects of substrate,temperature,number of layers,crystal orientation and other factors on their optical properties and the mechanisms of regulation are investigated.The main elements and results of this study are as follows.1.Centimeter-scale molybdenum disulfide(MoS2)films were prepared on sapphire substrates by chemical vapor deposition and subsequently transferred to SiO2/Si substrates to investigate the influence of the substrate on the optical properties of the monolayer MoS2 films and the mechanism of modulation.Raman spectroscopy analysis showed that about 0.41%of uniaxial stress was released and weak n-type doping was introduced during the transfer process.The refractive indices,extinction coefficients,and dielectric functions of monolayers of MoS2 on sapphire and SiO2/Si substrates were obtained using ellipsometric spectroscopy,based on which their electron leap energies were calculated.The physical origins of the electron leap peaks in the monolayer MoS2 films were obtained in combination with first-principle theoretical calculations based on density functional theory.On sapphire substrates,the valence band of MoS2 splits due to spin-orbit coupling,leading to the splitting of the electron leap peaks,with the difference in energy between the centers of the two split peaks being about 0.14 eV.However,the monolayer MoS2 films do not exhibit spin-orbit coupling on SiO2/Si substrates,probably due to the good inversion symmetry of SiO2.Combined with the theoretical calculations based on the first-principle calculations,it can be analyzed that the weakened spinorbit coupling of MoS2 on SiO2/Si substrates is the main reason for the change in the optical properties of MoS2.2.Continuous films of centimeter-scale platinum diselenide(PtSe2)were grown using chemical vapor deposition,and the effect of temperature on the optical properties of PtSe2 films and the modulation mechanism were investigated.The absorption spectra of PtSe2 films were first obtained using a spectrophotometer,and the optical bandgap of PtSe2 was obtained by combining with the Tauc formula.It was found that the optical band gap increased with the increase of the number of layers.The optical constants and dielectric functions of PtSe2 films were obtained by ellipsometric spectroscopy.In particular,the effect of temperature on the optical constants of PtSe2 was investigated using variable temperature ellipsometric spectroscopy.It was found that the refractive index of PtSe2 films was almost constant in the wavelength range of 400 nm-500 nm and showed a small decrease in refractive index with increasing temperature in the wavelength range of 500 nm-800 nm.This phenomenon can be attributed to the increase in electron-phonon interactions with rising temperature.The thermal stability of thicker PtSe2 films is better than that of thinner PtSe2.The thermo-optical coefficients of the PtSe2 films were calculated.Between 400 nm-500 nm the thermo-optical coefficient is around the zero axis,indicating good thermo-optical stability in this wavelength band.The minimum value of the thermo-optical coefficient for both thicknesses of PtSe2 films occurs at around 680 nm.The thermo-optical coefficient is-6.7×10-5/K for the 4-layer sample and-5.6×10-5/K for the 6-layer sample.The negative thermo-optical coefficient of PtSe2 may be related to the semi-metallicity exhibited by the multilayer PtSe2.3.Continuous centimeter-scale palladium diselenide(PdSe2)films of different thicknesses between 3 and 15 layers were grown using chemical vapor deposition,and the effect of the number of layers on the optical properties of the PdSe2 films and the modulation mechanism were investigated.The surface morphology and composition of the samples were studied using Atomic Force Microscopy and X-ray Photoelectron Spectroscopy.The effect of the number of layers on the Raman spectrum of PdSe2 was investigated.The results showed that the Raman peak of PdSe2 shifted red with the increase of the number of layers due to the interlayer coupling effect.The mechanism of layer number modulation on the optical band gap of PdSe2 was analyzed and it was found that the bandgap of PdSe2 decreases with increasing layer number,which can be attributed to the interlayer coupling caused by increasing layer number which in turn leads to orbital hybridization.The effect of layer number on the optical properties of PdSe2 was systematically investigated using spectroscopic ellipsometry and a significant correlation between the optical properties of PdSe2 and the number of layers was found.This is related to the alternating dominance of exciton binding energy and complex interlayer interactions on the photoelectric properties of PdSe2.4.Micrometer rhenium disulfide(ReS2)and rhenium diselenide(ReSe2)thin sheets were prepared using the mechanical stripping method.The effect of crystal orientation on the optical properties of ReS2 and ReSe2 sheets and the mechanism of modulation were investigated.Firstly,a micro-area imaging ellipsometer with a lateral resolution of 2 μm was built to address the problem that mechanically exfoliated micron-scale 2D materials cannot be analyzed using conventional commercial ellipsometric spectrometers.The optical properties of the ReS2 and ReSe2 flakes were analyzed using this micro-area imaging ellipsometer.The results show that the ReS2 and ReSe2 sheets are optically anisotropic,the optical properties of the ReS2 and ReSe2 sheets in the a-axis and b-axis directions were obtained,and the mechanism by which the crystal orientation regulates the optical properties of the ReS2 and ReSe2 sheets were analyzed. |
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