Photoluminescence of polycrystalline hexagonal boron nitride (hBN) was measured by means of time- and energy-resolved spectroscopy methods. The observed bands are related to donor-acceptor pair transitions, impurities, and structural defects. The excitation of samples by high-energy photons above 5.4 eV enables a phenomenon of photostimulated luminescence (PSL), which is due to distantly trapped conduction band electrons and valence band holes. These trapped charges are metastable and their re-excitation with low-energy photons results in anti-Stokes photoluminescence. The comparison of photoluminescence excitation spectra and PSL excitation spectra allows band analysis that supports the hypothesis of Frenkel-type exciton in hBN with a large binding energy. DOI: http://dx.doi.org/10.1103/PhysRevB.78.155204 © 2008 The American Physical Society

Due to surface effects and quantum size effects, nanomaterials have properties that are vastly different from those of bulk materials due to surface effects. The particle size distribution plays an important role in chemical and physical properties. The measurement and control of this parameter are crucial for nanomaterial synthesis. Dynamic light scattering (DLS) is a fast and non-invasive tool used to measure particle size, size distribution and stability in solutions or suspensions during nanomaterial preparation. In this review, we focus on the in situ sizing of nanomaterial preparation in the form of colloids, especially for metal oxide nanoparticles (MONs). The measuring principle, including an overview of sizing techniques, advantages and limitations and theories of DLS were first discussed. The instrument design was then investigated. Ex-situ and in situ configuration of DLS, sample preparations, measurement conditions and reaction cell design for in situ configuration were studied. The MONs preparation monitored by DLS was presented, taking into consideration both ex situ and in situ configuration.

Near band-gap luminescence $(h\nu ⩾5\mathrm{eV})$ of hexagonal boron nitride has been studied by means of the time- and energy-resolved photoluminescence spectroscopy method. Two emissions have been observed at 5.5 and $5.3\mathrm{eV}$ . The high-energy emission at $5.5\mathrm{eV}$ is composed of fixed subbands assigned to bound excitons at 5.47, 5.56, and $5.61\mathrm{eV}$ . The nonstructured low-energy emission at $5.3\mathrm{eV}$ undergoes a large blueshift (up to $120\mathrm{meV}$ ) with a linear slope $\Delta E_{\mathrm{lum}}∕\Delta E_{\mathrm{exc}}<1$ with increasing excitation energy $E_{\mathrm{exc}}$ . At $E_{\mathrm{exc}}⩾5.7\mathrm{eV}$ , the band position is fixed and marks the transition from the Raman to the photoluminescence regime. We assign the $5.3\mathrm{eV}$ band to quasi-donor-acceptor pair (q-DAP) states due to electrostatic band fluctuations induced by charged defects. The shift is explained by photoinduced neutralization of charged defect states. The absence of contribution to the q-DAP luminescence from exciton suggests the existence of a large exciton binding energy, which is qualitatively consistent with theoretical predictions.