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Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. We demonstrate a method for synthesizing free standing silicon nanocrystals in an argon/silane gas mixture by using a remote expanding thermal plasma. Transmission electron microscopy and Raman spectroscopy measurements reveal that the distribution has a bimodal shape consisting of two distinct groups of small and large silicon nanocrystals with sizes in the range 2-10 nm and 50-120 nm, respectively. We also observe that both size distributions are lognormal which is linked with the growth time and transport of nanocrystals in the plasma. Average size control is achieved by tuning the silane flow injected into the vessel. Analyses on morphological features show that nanocrystals are monocrystalline and spherically shaped. These results imply that formation of silicon nanocrystals is based on nucleation, i.e., these large nanocrystals are not the result of coalescence of small nanocrystals. Photoluminescence measurements show that silicon nanocrystals exhibit a broad emission in the visible region peaked at 725 nm. Nanocrystals are produced with ultrahigh throughput of about 100 mg/min and have state of the art properties, such as controlled size distribution, easy handling, and room temperature visible photoluminescence.
We report a rigorous analytical approach based on one-particle phonon confinement model to realize direct detection of nanocrystal size distribution and volume fraction by using Raman spectroscopy. For the analysis, we first project the analytical confinement model onto a generic distribution function, and then use this as a fitting function to extract the required parameters from the Raman spectra, i.e., mean size and skewness, to plot the nanocrystal size distribution. Size distributions for silicon nanocrystals are determined by using the analytical confinement model agree well with the one-particle phonon confinement model, and with the results obtained from electron microscopy and photoluminescence spectroscopy. The approach we propose is generally applicable to all nanocrystal systems, which exhibit size-dependent shifts in the Raman spectrum as a result of phonon confinement.
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Conventional photoelectrochemical (PEC) cells are based on planar photoelectrodes supported on glass substrates and liquid electrolytes. Only few recent studies have examined an alternative PEC design which is robust and scalable, where the key elements are polymeric electrolyte membranes and porous photoelectrodes. This work aims to give further insights on the operation of such cells utilizing titania photoelectrodes and proton and hydroxide ion conducting membranes. Two families of photoelectrodes were developed on Ti porous substrate; TiO2 nanotubes grown by anodization and subsequent oxygen annealing, and TiO2 layers developed under oxygen annealing. Initial screening of the photoanodes for water splitting and (poly)alcohol photo-oxidation took place in conventional PEC cells. We found that the annealing temperature affects the performance of the photoanodes, evidenced by a monotonic increase in the activity for water photo-oxidation with increasing annealing temperature. Moreover it was demonstrated that anatase phase is predominantly active for the (poly)alcohol electro-oxidation, while there is a synergy between rutile and anatase which is beneficial for water splitting. In addition, the most promising photoanodes for water splitting were evaluated in our polymeric electrolyte membrane photoelectrochemical (PEM-PEC) cell during gas phase operation. It was found that PEM-PEC operation is more efficient when OHconducting membranes are used, while the nature of the carrier gas does not significantly influence the activity. Overall, PEM-PEC operation is more promising than conventional PEC in both acidic and alkaline media, since comparable (or even at some cases higher) photocurrents were obtained while liquid pumping systems are not required for PEM-PEC devices.
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