Present red phosphor converters provide spectra dominated by sharp lines and suffer from availability and stability issues which are not ideal for color mixing in display or solid state lighting applications. We examine the use of mono dispersed 3 nm silicon nanoparticles, with inhomogeneously broadened red luminescence as an effective substitute for red phosphors. We tested a 3-phase hybrid nanophosphor consisting of ZnS:Ag, ZnS:Cu,Au,Al, and nanoparticles. Correlated color temperature is examined under UV and LED pumping in the range 254, 365-400 nm. The temperature is found reasonably flat for the longer wavelengths and drops for the shorter wavelengths while the color rendering index increases. The photo stability of the phosphors relative to the silicon nanoparticles is recorded. The variation in the temperature is analyzed in terms of the strength of inter-band-gap transition and continuum band to band transitions.

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.

Silicon nanoclusters exhibit light emission with direct-like ns–µs time dynamics; however, they show variable synthesis and structure, optical, and electronic characteristics. The widely adopted model is a core–shell in which the core is an indirect tetrahedral absorbing Si phase, while the shell is a network of re-structured direct-like H–Si–Si–H molecular emitting phases, with the two connected via back Si–Si tetrahedral bonds, exhibiting a potential barrier, which significantly hinders emission. We carried out first-principles atomistic computations of a 1-nm Si nanoparticle to discern the variabilities. Enlarging the network reduces the potential barrier monotonically to a finite limit not sufficient for strong emission to proceed while inducing a path to quenching of emission via a conical crossing between the excited and ground states. However, enlarging the network is found to induce strain and structural instability, which causes structural relaxation that creates a direct path for emission without crossing the barrier. Following emission, the particle relaxes back to the indirect ground structure, which completes the cycle. The results also confirm the pivotal role of HF/H2O2 etching in synthesizing the core–shells and affording control over the molecular network. Measurements using synchrotron and laboratory UV excitation of thin films of 1-nm Si particles show good agreement with the simulation results. It is plausible that the relaxation is behind the stimulated emission, gain, or microscopic laser action, reported earlier in macroscopic distributions of 1- and 3-nm Si nanoparticles.

In bulk single crystal silicon, the semiconducting diamond to metallic ß-Sn phase transformation nucleates on defects, and is manifested by a sharp uptake in light absorption at a threshold of ~ 11 Gpa, accompanied with the creation of nanosized (20-50 nm) fragmentation domains. We report on the observation of linear uptakes in the absorption and in the luminescence, and with insignificant spectral change in ultrasmall 1 nm Si particles. We associate the gradual absorption uptake and luminescence yield with silicon-metal transformation on the surface. The insignificant change in the spectral content of the luminescence points to surface stability for particles, which are smaller than the bulk fragmentation domain. First-principle atomistic calculations yield absorption behavior that exhibits gradual uptake followed by sharp uptake at ~ 9-11 Gpa. The results point to the conclusion that two dimensional surface-like phase transformations are manifested by linear uptake in absorption and luminescence. a) m-nayfeh@illinois.edu 2

We used wet treatment to immobilize luminescent silicon nanoparticles on industrial glass fibers to impart optical and chemical functions to the fiber. Carpets or pads consisting of thousands of fibers are processed in parallel, enhancing the sensitivity of detection and the sampled volume. Treated pads exhibit strong luminescence, characteristic of the luminescence of the particles; showing no shift, broadening, or reduction of quantum efficiency. We demonstrate that drawing material by the pad due to physical adsorption can be reversed. We also demonstrate that allylamine can be covalently attached by photoinduced irradiation reactions, which results in imprinting the amine emission spectrum, providing spectral recognition. The imprint accompanied with a blue-shifting of the luminescence spectrum of the probe, allowing examination of the effect of termination on the nanoparticle structure. The shift is found to be consistent with an increase in the bandgap of the Si nanoparticle and is consistent with Quantum Monte Carlo calculations. In addition to sampling, the nano probe pad has the potential to enable a variety of biomedical applications through subsequent attachment.