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

Abstract

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 time characteristics of hours. To discern the origin of the instability, we perform first principle 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 (TD-DFT), 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 deviceoperation 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.

Strong Reduction in Ge Film Reflectivity by an Overlayer of 3 nm Si Nanoparticles: Implications for Photovoltaics

“…The process also thins the luminescent layer and reduces impurities, producing nearly defect-free and clean samples. The monohydride passivation is robust enough not to be susceptible to environmental attachments such as oxidation keeping the luminescence spectral content stable over time and exposure to the ambient conditions or mild heating …”

“…The process has been described in detail in previous work . The colloidal monodispersing consisted of unfunctionalized hydrogen-terminated Si-NPs with ∼3 nm diameter and a concentration of ∼3 μM. , The chemical procedure predominantly produces particles of dominant size of 2.9 nm in diameter; a histogram gives counts of 4, 56, 12, 9, and 2 at diameters of 3.5, 2.9, 2.2, 1.7, and 1 nm …”

“…51,52 The chemical procedure predominantly produces particles of dominant size of 2.9 nm in diameter; a histogram gives counts of 4, 56, 12, 9, and 2 at diameters of 3.5, 2.9, 2.2, 1.7, and 1 nm. 53 In general, anodization etching processes produce samples that are passivated with hydrogen but are plagued with a large degree of surface defects, such as dihydrides (Si−H 2 ) and trihydrides (Si−H 3 ), as well as impurities such as carbon or oxygen bearing species. 54 However, the procedure used in the present experiments incorporates the oxidizing agent H 2 O 2 in the anodization mixture, which results in ideal hydrogen passivation (Si−H).…”

“…The monohydride passivation is robust enough not to be susceptible to environmental attachments such as oxidation keeping the luminescence spectral content stable over time and exposure to the ambient conditions or mild heating. 53 The Si substrates used are (100) oriented, n-type P doped with a resistivity of ∼0.02 Ω cm. The high-quality Ge thin films are deposited using the two-step approach portrayed in.…”

Time–thermo-dynamics of anti-Stokes and Stokes scattering and luminescence in 1-nm silicon nanoparticles: Toward an optical nanorefrigerator

“…The existence of singlettriplet crossings was established using time-dependent density functional theory (TDDFT) atomistic calculations and simulations. [61][62][63] We approximate the particle by a restricted three-level (-state) system: ground singlet state, excited singlet state, and excited triplet state. The mechanism is based on the crossing of the lowest triplet state with the lowest singlet state as well as with the ground state (singlet).…”