Optical Properties of Nanocrystal Interfaces in Compressed MgO Nanopowders

Abstract

The optical properties and charge trapping phenomena observed on oxide nanocrystal ensembles can be strongly influenced by the presence of nanocrystal interfaces. MgO powders represent a convenient system to study these effects due to the well-defined shape and controllable size distributions of MgO nanocrystals. The spectroscopic properties of nanocrystal interfaces are investigated by monitoring the dependence of absorption characteristics on the concentration of the interfaces in the nanopowders. The presence of interfaces is found to affect the absorption spectra of nanopowders more significantly than changing the size of the constituent nanocrystals and, thus, leading to the variation of the relative abundance of light-absorbing surface structures. We find a strong absorption band in the 4.0−5.5 eV energy range, which was previously attributed to surface features of individual nanocrystals, such as corners and edges. These findings are supported by complementary first-principles calculations. The possibility to directly address such interfaces by tuning the energy of excitation may provide new means for functionalization and chemical activation of nanostructures and can help improve performance and reliability for many nanopowder applications.

Surface-specific visible light luminescence from composite metal oxide nanocrystals

“…The theoretical calculations of optical absorption spectra help to explain this effect as resulting from electronic transitions involving Ba ions located at low-coordinated surface sites. The excited state calculations are performed using time-dependent density functional theory and the B3LYP hybrid density functional theory which has been shown to predict optical excitation energies in MgO for a wide range of defects, in good agreement with experiment [30,41]. These calculations show that incorporation of Ba leads to a red-shift in the optical absorption in semiquantitative agrement with the experimental results.…”

“…The structural models of the surface Ba sites are relatively simple-involving substitution of Mg for Ba at highly symmetrical structural features such a nanocube corners and edges. Here, we do not consider the possibility that Ba segregation may induce a reconstruction of surface features, nor do we consider more complex features such as interfaces [30,31]. A more complete model would need to consider a statistically representative distribution of complex Ba-modified surface/interfaces features.…”

“…Excitation energies are also calculated by time-dependent density functional theory. This type of approach has proved extremely reliable for predicting the electronic, optical, and chemical properties of MgO nanopowders and a wide range of defects [11,12,24,30,31,[37][38][39]. In our implementation, Mg and O ions in the quantum cluster are described at the all electron level using a Gaussian 6-311G** basis set.…”

“…Since the luminescence in these materials has a surface origin, scaling particle size to the nanoscale can improve quantum yield while also reducing multiple scattering. On the other hand, tuning and optimizing absorption and luminescence properties of nanoparticles will require precise control over the surface structure and compositon of nanoparticles as well as interfaces between nanoparticles which is challenging [30,31].…”

Defects in Metal Oxide Nanoparticle Powders

“…As a matter of fact, when discussing why the spectroscopic properties and surface reactivity of nanocrystal ensembles differ from those of macroscopic materials, their high specific surface area, the size and shape of the nanoparticles, and the high concentration of surface elements are commonly taken into account. The influence of particle-particle interfaces [60,[66][67][68], the concentration of which is significantly enhanced in nanocrystalline systems, is, however, strongly neglected in the discussion of their properties. Despite substantial electrochemical evidence for annealing-induced stoichiometry changes, molecular spectroscopy studies that address a quantitative description of point defects participating in the overall mechanism are scarce [60].…”