- 标题
- 摘要
- 关键词
- 实验方案
- 产品
-
Screening Effect of Ultrathin Gold Films on Excitons in Monolayer WS2
摘要： Screening plays an important role in determining the energy levels and binding energies of excitons in two-dimensional (2D) semiconductors. However, the photoelectric properties of 2D materials deposited upon metal films are often assumed to remain unaffected by the screening effect. Herein, we derive the reflectance contrast spectra of monolayer WS2 on different thicknesses of ultrathin gold films by using the transfer matrix method, and investigate the screening effect of ultrathin gold films on excitons in monolayer WS2. We show that the exciton binding energy of the ground state gradually decreases as the gold film thickness increases due to the enhanced screening effect. Nevertheless, it may be due to the impact of free-electron scattering on the permittivities of gold films: exciton resonance energies of the ground state and the first excited state gradually increase as the gold film thickness increases, which are different from previously reported variation trend of exciton energy levels with the thickness of screening media of 2D materials. As a consequence, the electronic bandgap of the exciton only presents a small variation range of less than 4 meV with thickening of the gold film from 0.0 to 40 nm. These findings shed light on understanding to what extent the screening effect of gold films affects the excitons in 2D materials.
关键词： Ultrathin gold film,Transfer matrix method,Excitonic properties,Monolayer WS2,Dielectric screening effect
更新于2025-09-23 15:22:29
-
quantum dots: Insights from time-dependent density functional theory
摘要： Colloidal quantum dots (QDs) of group III-V are considered as promising candidates for next-generation environmentally friendly light emitting devices, yet there appears to be only limited understanding of the underlying electronic and excitonic properties. Using large-scale density functional theory with the hybrid B3LYP functional solving the single-particle states and time-dependent density functional theory accounting for the many-body excitonic effects, we have identified the structural, electronic, and excitonic optical properties of InP, GaP, and GaInP QDs containing up to a thousand atoms or more. The calculated optical gap of InP QD appears in excellent agreement with available experiments, and it scales nearly linearly with the inverse diameter. The radiative exciton decay lifetime is found to increase surprisingly linearly with increasing the dot size. For GaP QDs we predict an unusual electronic state crossover at a diameter of around 1.5 nm, whereby the nature of the lowest unoccupied molecular orbital (LUMO) state switches its symmetry from (cid:2)5-like at a larger diameter to (cid:2)1-like at a smaller diameter. After the crossover, the absorption intensity of the band-edge exciton states is significantly enhanced. Finally, we find that Vegard’s law holds very well for GaInP random alloyed quantum dots down to ultrasmall sizes with less than a hundred atoms. The obtained energy gap bowing parameter of this common-cation compound in QD regime appears positive, size-dependent, and much smaller than its bulk parentage. The volume deformation, dominating over the charge exchange and structure relaxation effects, is mainly responsible for the QD energy gap bowing. The impact of excitonic effects on the optical bowing is found to be marginal. The present work provides a road map for a variety of electronic and optical properties of colloidal QDs in group III-V that can guide spectroscopic studies.
关键词： InP,excitonic properties,time-dependent density functional theory,GaInP,quantum dots,GaP
更新于2025-09-11 14:15:04