Abstract
The dielectric function {boldsymbol{varepsilon }}{boldsymbol{=}}{{boldsymbol{varepsilon }}}_{{bf{1}}}{boldsymbol{+}}{bf{i}}{{boldsymbol{varepsilon }}}_{{bf{2}}} of monolayer molybdenum diselenide (MoSe2) is obtained and analyzed at temperatures from 31 to 300 K and at energies from 0.74 to 6.42 eV. The sample is a large-area, partially discontinuous monolayer (submonolayer) film of MoSe2 grown on a sapphire substrate by selenization of pulsed laser deposited MoO3 film. Morphological and optical characterizations verified the excellent quality of the film. The MoSe2 data were analyzed using the effective medium approximation, which treats the film and bare substrate regions as a single layer. Second derivatives of ε with respect to energy were numerically calculated and analyzed with standard lineshapes to extract accurate critical-point (CP) energies. We find only 6 CPs for monolayer MoSe2 at room temperature. At cryogenic temperatures 6 additional structures are resolved. The separations in the B- and C-excitonic peaks are also observed. All structures blue-shift and sharpen with decreasing temperature as a result of the reducing lattice constant and electron-phonon interactions. The temperature dependences of the CP energies were determined by fitting the data to the phenomenological expression that contains the Bose-Einstein statistical factor and the temperature coefficient.
Highlights
TMDs are semiconducting materials, as opposed to graphene, which is metallic
A krypton fluoride (KrF) excimer laser was used to deposit a thin film of MoO3 on the polished side of a sapphire substrate
We report and analyze the intrinsic dielectric response ε of monolayer MoSe2 for energies from 0.74 to 6.42 eV and temperatures from 31 to 300 K
Summary
TMDs are semiconducting materials, as opposed to graphene, which is metallic. Their bandgaps are tunable depending on the number of layers, tensile strain, and external electric field[3,4,5]. Several workers have reported dielectric functions of monolayer MoSe2 using SE at room or high temperature, but these spectra are broad and transition peaks difficult to resolve[13,14]. Absorption data from 77 to 300 K in the energy region of the A- and B-excitonic peaks have been published, a systematic analysis of ε of monolayer MoSe2 from cryogenic to room temperature has not been reported[18,19]. We report ε data for monolayer MoSe2 from 0.74 to 6.42 eV at temperatures from 31 to 300 K. The separation of the C-excitonic peaks, which has not reported so far, results from the spin-orbit splitting of the top valence band[24]. The temperature dependences of the CP energies were determined by fitting the data to the phenomenological expression that contains the Bose-Einstein statistical factor and the temperature coefficient
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