Jihua Yang

On the Origin of Photoluminescence in Silicon Nanocrystals: Pressure-Dependent Structural and Optical Studies

A lack of consensus persists regarding the origin of photoluminescence in silicon nanocrystals. Here we report pressure-dependences of X-ray diffraction and photoluminescence from alkane-terminated colloidal particles. We determine the diamond-phase bulk modulus, observe multiple phase transitions, and importantly find a systematic photoluminescence red shift that matches the X(conduction)-to-Γ(valence) transition of bulk crystalline silicon. These results, reinforced by calculations, suggest that the efficient photoluminescence, frequently attributed to defects, arises instead from core-states that remain highly indirect despite quantum confinement.

Silicon nanocrystals at elevated temperatures: Retention of photoluminescence and diamond silicon to β-silicon carbide phase transition

We report the photoluminescence (PL) properties of colloidal Si nanocrystals (NCs) up to 800 K and observe PL retention on par with core/shell structures of other compositions. These alkane-terminated Si NCs even emit at temperatures well above previously reported melting points for oxide-embedded particles. Using selected area electron diffraction (SAED), powder X-ray diffraction (XRD), liquid drop theory, and molecular dynamics (MD) simulations, we show that melting does not play a role at the temperatures explored experimentally in PL, and we observe a phase change to β-SiC in the presence of an electron beam. Loss of diffraction peaks (melting) with recovery of diamond-phase silicon upon cooling is observed under inert atmosphere by XRD. We further show that surface passivation by covalently bound ligands endures the experimental temperatures. These findings point to covalently bound organic ligands as a route to the development of NCs for use in high temperature applications, including concentrated solar cells and electrical lighting.

Environmental photostability of SF6-etched silicon nanocrystals

We report on the long-term environmental stability of the photoluminescent (PL) properties of silicon nanocrystals (SiNCs). We prepared sulfur hexafluoride (SF(6)) etched SiNCs in a two-stage plasma reactor and investigated their PL stability against UV irradiation in air. Unlike SiNCs with hydrogen-passivated surfaces, the SF(6)-etched SiNCs exhibit no photobleaching upon extended UV irradiation despite surface oxidation. Furthermore, the PL quantum yield also remains stable upon heating the SF(6)-etched SiNCs up to 160 °C. The observed thermal and UV stability of SF(6)-etched SiNCs combined with their PL quantum yields of up to ~50% make them attractive candidates for UV downshifting to enhance the efficiency of solar cells. Electron paramagnetic spin resonance indicates that the SF(6)-etched SiNCs have a lowered density of defect states, both as-formed and after room temperature oxidation in air.

Atmospheric-pressure glow plasma synthesis of plasmonic and photoluminescent zinc oxide nanocrystals

In this paper, we present a large-volume (non-micro) atmospheric pressure glow plasma capable of rapid, large-scale zinc oxide nanocrystal synthesis and deposition (up to 400 μg/min), whereas in the majority of the literature, nanoparticles are synthesized using micro-scale or filamentary plasmas. The reactor is an RF dielectric barrier discharge with a non-uniform gap spacing. This design encourages pre-ionization during the plasma breakdown, making the discharge uniform over a large volume. The produced zinc oxide nanocrystals typically have diameters ranging from 4 to 15 nm and exhibit photoluminescence at ≈550 nm and localized surface plasmon resonance at ≈1900 cm−1 due to oxygen vacancies. The particle size can be tuned to a degree by varying the gas temperature and the precursor mixing ratio.

Effect of Initial Straining and γ Irradiation on Strength of Polyethylene

Uniaxially and biaxially oriented samples of polyethylene irradiated by γ rays have been investigated at room temperature. Both the tensile strength and the modulus of elasticity are found to increase as the degree of orientation induced by homogeneous deformation increases. But the tensile strength decreases as the dose of γ irradiation increases while the modulus of elasticity increases. The combined effects seem to indicate that the γ irradiation essentially destroys the macromolecular orientation and thus reduces the tensile strength to that of unoriented solid, but with relatively higher elastic modulus as a result of the occurrence of crosslinking.