Experimental and theoretical study of ultraviolet-induced structural/optical instability in nano silicon-based luminescence

Nano silicon is emerging as an active element for UV applications due to quantum confinement-induced widening of the Si bandgap, amenability to integration on Si, and less sensitivity to temperature. NanoSi-based UV applications include deep space exploration, high temperature propulsion, solar photovoltaics, and particle detection in high energy accelerators. However, the viability of the technology is limited by a complex nanoSi optical quenching instability. Here, we examined the time dynamics of UV-induced luminescence of sub 3-nm nanoSi. The results show that luminescence initially quenches, but it develops a stability at ∼50% level with a time characteristic of minutes. Upon isolation, partial luminescence recovery/reversibility occurs with a time characteristics of hours. To discern the origin of the instability, we perform first principles atomistic calculations of the molecular/electronic structure in 1-nm Si particles as a function of Si structural bond expansion, using time dependent density functional theory, with structural relaxation applied in both ground and excited states. For certain bond expansion/relaxation, the results show that the low-lying triplet state dips below the singlet ground state, providing a plausible long-lasting optical trap that may account for luminescence quenching as well as bond cleavage and irreversibility. Time dynamics of device-operation that accommodates the quenching/recovery time dynamics is suggested as a means to alleviate the instability and allow control of recovery, which promises to make it an effective alternative to UV-enhanced Si or metal-based wide-bandgap sensing technology.