Origin of humidity influencing the excited state electronic properties of silicon quantum dot based light-emitting diodes

One of the challenges of silicon quantum dots (Si QDs) in practical application as quantum dot-based light-emitting diodes is the irreversible degradation induced by humid conditions, revealing their excited state electronic properties strongly influenced by the surface water; however, the photoluminescence (PL) mechanism associated with the change of excited state electronic properties remains elusive. Here, we performed the time-dependent density functional theory calculations to investigate how the PL of Si₂₉H₃₆, as typical spherical Si QDs, is determined by dipole-dipole interactions between water molecules and different surface substituent groups. Relative to the hydrophobic group of pure hydrogen passivation, the substituent effect with a hydrogen atom replaced by a fluorine atom almost has no influence on the PL of Si QDs with the adsorption of water clusters. Interestingly, although a hydrophilic hydroxyl group substitution itself will partly change the surface state with the slight blue-shift of PL, the intensive dipole-dipole interaction between a hydroxyl group and water molecules can drastically induce the delocalized electrons to be localized, resulting in a dual-band peak observed in the PL spectra of Si₂₉H₃₅OH surrounded by four or five water molecules. This distinct PL mechanism originates from the adsorption of water molecules through dipole-dipole interactions inducing the existence of surface trap states. The presence of highly polarizable double-bonded oxygen will trigger the electron distribution centered on the silicon-oxygen double bond, resulting in the corresponding PL spectrum of Si₂₉H₃₅O unaffected by the water molecules. This study reveals that the PL of Si QDs with the substituent hydroxyl group is extremely sensitive to humidity and lays a foundation for the practical application of Si QDs as optoelectronic devices.