Charge transfer state emission dynamics in blue-emitting functionalized silicon nanocrystals

We explore the dynamics of blue emission from dodecylamine and ammonia functionalized silicon nanocrystals (Si NCs) with average diameters of ∼3 and ∼6 nm using time-resolved photoluminescence (TRPL) spectroscopy. The Si NCs exhibit nanosecond PL decay dynamics that is independent of NC size and uniform across the emission spectrum. The TRPL measurements reveal complete quenching of core state emission by a charge transfer state that is responsible for the blue PL with a radiative recombination rate of ∼5 × 10^7 s^(−1). A detailed picture of the charge transfer state emission dynamics in these functionalized Si NCs is proposed

Fabrication and characterization of MgB2 films by electrochemical deposition and post-annealing

Superconducting MgB2 films were prepared using magnesium chloride hexahydrate and boric acid electrolytes, by electrochemical deposition and post-annealing. Electrochemical deposition cell was composed of Ag plate as working electrode, Ag/Ag+ as reference electrode and Pt as counter electrode. Scanning electron microscopy showed the presence of hexagonal-shaped MgB2 crystallites and MgO phases in all the samples. X-ray diffraction analysis revealed 4 peaks of MgB2. The resistivity-temperature measurement showed the effect of annealing time on the critical transition temperature of the samples. The Tc of 39 K was observed in the films annealed at 300 degrees Centigrade for 120 minutes while Tc of 33K and 30 K were observed in samples annealed at 300 degrees Centigrade for 90 minutes

Size <i>vs</i> Surface: Tuning the Photoluminescence of Freestanding Silicon Nanocrystals Across the Visible Spectrum <i>via</i> Surface Groups

The syntheses of colloidal silicon nanocrystals (Si-NCs) with dimensions in the 3–4 nm size regime as well as effective methodologies for their functionalization with alkyl, amine, phosphine, and acetal functional groups are reported. Through rational variation in the surface moieties we demonstrate that the photoluminescence of Si-NCs can be effectively tuned across the entire visible spectral region without changing particle size. The surface-state dependent emission exhibited short-lived excited-states and higher relative photoluminescence quantum yields compared to Si-NCs of equivalent size exhibiting emission originating from the band gap transition. The Si-NCs were exhaustively characterized using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transformed infrared spectroscopy (FTIR), and their optical properties were thoroughly investigated using fluorescence spectroscopy, excited-state lifetime measurements, photobleaching experiments, and solvatochromism studies

Evolution of the Ultrafast Photoluminescence of Colloidal Silicon Nanocrystals with Changing Surface Chemistry

The role of surface species in the optical properties of silicon nanocrystals (SiNCs) is the subject of intense debate. Changes in photoluminescence (PL) energy following hydrosilylation of SiNCs with alkyl-terminated surfaces are most often ascribed to enhanced quantum confinement in the smaller cores of oxidized NCs or to oxygen-induced defect emission. We have investigated the PL properties of alkyl-functionalized SiNCs prepared using two related methods: thermal and photochemical hydrosilylation. Photochemically functionalized SiNCs exhibit higher emission energies than the thermally functionalized equivalent. While microsecond lifetime emission attributed to carrier recombination within the NC core was observed from all samples, much faster, size-independent nanosecond lifetime components were only observed in samples prepared using photochemical hydrosilylation that possessed substantial surface oxidation. In addition, photochemically modified SiNCs exhibit higher absolute photoluminescent quantum yields (AQY), consistent with radiative recombination processes occurring at the oxygen-based defects. Correlating spectrally- and time-resolved PL measurements and XPS-derived relative surface oxidation for NCs prepared using different photoassisted hydrosilylation reaction times provides evidence the PL blue-shift as well as the short-lived PL emission observed for photochemically functionalized SiNCs are related to the relative concentration of oxygen surface defects