石墨，MoTe2，及PtSe2材料之光譜性質研究
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2022
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Three-dimensional (3D) graphite, Na-doped graphite, MoTe2 single crystals, and two-dimensional (2D) PtSe2 thin films have received substantial research attention because of their intriguing physical and chemical properties and potential practical applications. Understanding the optical and vibrational properties in wide frequency and temperature ranges of these materials will benefit for technological development at different photon energies and temperatures. Spectroscopic ellipsometry is one of the non-destructively techniques that provide critical information on optical properties from various materials in bulk and thin-film forms. Furthermore, Raman scattering spectroscopy is asensitive tool for exploring the lattice dynamics of novel materials. In this thesis, we investigated the electronic and vibrational excitations of graphite and Na-doped graphite single crystals, 2H- and 1T'-MoTe2 single crystals, and bilayer and multilayer PtSe2 thin films using spectroscopic ellipsometry and Raman scattering spectroscopy. At room temperature, the complex dielectric function of graphite exhibited broad spectra in the deep ultraviolet energy region. The Breit–Wigner–Fano (BWF) line shape analysis of the optical absorption spectrum demonstrated that two BWF line shapes were fitted with central energies at 4.85 ± 0.01 eV and 6.21 ± 0.03 eV. We assigned these two features as interference between an excitonic transition at the saddle point (M) in the band structures and collective excitations of the surface plasmons. Analysis of the temperature-dependent asymmetric BWF line shape indicated that the peak position redshifted, the linewidth narrowed, and the intensity enhanced under increasing temperatures. The temperature-dependent optical absorption for graphite is opposite to that of the interband absorptions observed in silicon. This behavior is associated with the asymmetric factor 1/q BWF which decreased under increasing temperatures. Combining with the first-principles calculations, we verified the presence of two resonant conditions in the deep ultraviolet energy region. The lifetime of surface plasmons was expected to decrease with increased temperatures, leading to a decrease of the 1/q BWF. Furthermore, the D mode was only observed with 785-nm laser excitation in graphite, which was associated with the disorder resulting from the double-resonance Raman scattering process.The room temperature optical absorption spectra confirmed that 2H-MoTe2 had an indirect band gap at 0.89 ± 0.01 eV, whereas 1T′-MoTe2 possessed semimetal behavior. Furthermore, 2H-MoTe2 exhibited three distinct intralayer A1s, A2s, and B1s excitons and one interlayer AIL exciton at 1.136 ± 0.002, 1.181 ± 0.001, 1.469 ± 0.006, and 1.209 ± 0.010 eV at 4.2 K. By contrast, 1T′-MoTe2 displayed two broad intralayer A1s and B1s excitons at 0.778 ± 0.002 and 1.447 ± 0.001 eV. The spin-orbit splittingenergy of 2H- and 1T′-MoTe2 were 333 and 669 meV. Strikingly, the intralayer A1s exciton of 1T′-MoTe2 exhibited a blueshift, whereas its intralayer B1s exciton exhibited a redshift with an increase in temperature. The temperature-dependent properties of all optical transitions indicated strong electron-phonon interactions. Raman scattering measurement confirmed that the 𝐸2𝑔1 mode of 2H-MoTe2 is more prominent with laser excitations of 488 and 532 nm. By contrast, 1T′-MoTe2 revealed the dominated 3Ag mode using all laser excitation wavelengths.The room temperature refractive index spectra of PtSe2 thin films exhibited several anomalous dispersion features below 1000 nm and approached a constant value in the near-infrared frequency range. At a wavelength of 1200 nm, the thermo-optic coefficients of bilayer and multilayer PtSe2 were (4.31 ± 0.04) ☓ 10-4/K and (-9.20 ± 0.03) ☓ 10-4/K. The optical absorption spectra confirmed that bilayer PtSe2 had an indirect band gap of 0.75 ± 0.01 eV, whereas multilayer PtSe2 revealed semimetal behavior at room temperature. The suppression of electron-phonon interactions induced an increase in the band gap value of bilayer PtSe2 to 0.83 ± 0.01 eV at 4.5 K. Furthermore, the frequency shifts of Raman-active Eg and A1g phonon modes of both thin films in the temperature range between 10 and 500 K accorded with the predictions of the anharmonic model.We have studied the comprehensive optical and vibrational properties of graphite and MoTe2 single crystals, and PtSe2 thin films. These results not only offer important insight into the electronic and vibrational excitations and but also provide valuable information for technological development in optoelectronic and photonic devices of these materials.
Three-dimensional (3D) graphite, Na-doped graphite, MoTe2 single crystals, and two-dimensional (2D) PtSe2 thin films have received substantial research attention because of their intriguing physical and chemical properties and potential practical applications. Understanding the optical and vibrational properties in wide frequency and temperature ranges of these materials will benefit for technological development at different photon energies and temperatures. Spectroscopic ellipsometry is one of the non-destructively techniques that provide critical information on optical properties from various materials in bulk and thin-film forms. Furthermore, Raman scattering spectroscopy is asensitive tool for exploring the lattice dynamics of novel materials. In this thesis, we investigated the electronic and vibrational excitations of graphite and Na-doped graphite single crystals, 2H- and 1T'-MoTe2 single crystals, and bilayer and multilayer PtSe2 thin films using spectroscopic ellipsometry and Raman scattering spectroscopy. At room temperature, the complex dielectric function of graphite exhibited broad spectra in the deep ultraviolet energy region. The Breit–Wigner–Fano (BWF) line shape analysis of the optical absorption spectrum demonstrated that two BWF line shapes were fitted with central energies at 4.85 ± 0.01 eV and 6.21 ± 0.03 eV. We assigned these two features as interference between an excitonic transition at the saddle point (M) in the band structures and collective excitations of the surface plasmons. Analysis of the temperature-dependent asymmetric BWF line shape indicated that the peak position redshifted, the linewidth narrowed, and the intensity enhanced under increasing temperatures. The temperature-dependent optical absorption for graphite is opposite to that of the interband absorptions observed in silicon. This behavior is associated with the asymmetric factor 1/q BWF which decreased under increasing temperatures. Combining with the first-principles calculations, we verified the presence of two resonant conditions in the deep ultraviolet energy region. The lifetime of surface plasmons was expected to decrease with increased temperatures, leading to a decrease of the 1/q BWF. Furthermore, the D mode was only observed with 785-nm laser excitation in graphite, which was associated with the disorder resulting from the double-resonance Raman scattering process.The room temperature optical absorption spectra confirmed that 2H-MoTe2 had an indirect band gap at 0.89 ± 0.01 eV, whereas 1T′-MoTe2 possessed semimetal behavior. Furthermore, 2H-MoTe2 exhibited three distinct intralayer A1s, A2s, and B1s excitons and one interlayer AIL exciton at 1.136 ± 0.002, 1.181 ± 0.001, 1.469 ± 0.006, and 1.209 ± 0.010 eV at 4.2 K. By contrast, 1T′-MoTe2 displayed two broad intralayer A1s and B1s excitons at 0.778 ± 0.002 and 1.447 ± 0.001 eV. The spin-orbit splittingenergy of 2H- and 1T′-MoTe2 were 333 and 669 meV. Strikingly, the intralayer A1s exciton of 1T′-MoTe2 exhibited a blueshift, whereas its intralayer B1s exciton exhibited a redshift with an increase in temperature. The temperature-dependent properties of all optical transitions indicated strong electron-phonon interactions. Raman scattering measurement confirmed that the 𝐸2𝑔1 mode of 2H-MoTe2 is more prominent with laser excitations of 488 and 532 nm. By contrast, 1T′-MoTe2 revealed the dominated 3Ag mode using all laser excitation wavelengths.The room temperature refractive index spectra of PtSe2 thin films exhibited several anomalous dispersion features below 1000 nm and approached a constant value in the near-infrared frequency range. At a wavelength of 1200 nm, the thermo-optic coefficients of bilayer and multilayer PtSe2 were (4.31 ± 0.04) ☓ 10-4/K and (-9.20 ± 0.03) ☓ 10-4/K. The optical absorption spectra confirmed that bilayer PtSe2 had an indirect band gap of 0.75 ± 0.01 eV, whereas multilayer PtSe2 revealed semimetal behavior at room temperature. The suppression of electron-phonon interactions induced an increase in the band gap value of bilayer PtSe2 to 0.83 ± 0.01 eV at 4.5 K. Furthermore, the frequency shifts of Raman-active Eg and A1g phonon modes of both thin films in the temperature range between 10 and 500 K accorded with the predictions of the anharmonic model.We have studied the comprehensive optical and vibrational properties of graphite and MoTe2 single crystals, and PtSe2 thin films. These results not only offer important insight into the electronic and vibrational excitations and but also provide valuable information for technological development in optoelectronic and photonic devices of these materials.
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none, Graphite, PtSe2, MoTe2, Optical properties