Article pubs.acs.org/JPCC
Exciton Formation at Solid−Solid Interfaces: A Systematic Experimental and ab Initio Study on Compressed MgO Nanopowders Andreas Sternig,†,‡ David Koller,§ Nicolas Siedl,‡ Oliver Diwald,*,†,‡ and Keith McKenna*,∥ †
Cluster of Excellence - Engineering of Advanced Materials (EAM), and ‡Institute of Particle Technology, Friedrich-Alexander University Erlangen-Nuremberg, Cauerstrasse 4, 91058 Erlangen, Germany § Institute of Materials Chemistry, Vienna University of Technology, Getreidemarkt 9 1060 Wien, Austria ∥ Department of Physics, University of York, Heslington, York YO10 5DD, United Kingdom S Supporting Information *
ABSTRACT: An important and so far neglected class of structural elements affecting the overall properties of metal oxide nanopowders are interfaces between individual nanocrystals. In this work, we show experimentally that these defects inside a powder of compressed MgO nanocubes are subject to photoexcitation in the UV light range. In particular, we identify a so far unobserved photoluminescence emission process at 2.5 eV. First-principles calculations of the optical properties of nanocrystal interfaces provide plausible candidates for both light absorbing and emitting sites, which involve different types of interface features. It was found that edge dislocations that arise from interfaces between nanocube edges and terraces induce a significant electrostatic perturbation of the interfacial electronic states. This leads to exciton generation and luminescence at even lower energies than those related to corners and edges of MgO nanocubes.
1. INTRODUCTION A great deal of research on oxide nanocrystals has focused on structure−property relationships, which are framed in terms of the properties of individual nanocrystals.1 However, in most practical applications, one deals with an ensemble of nanocrystals, which may exhibit properties that are quite different from those of individual nanocrystals. For example, oxide nanocrystal powders contain heterogeneous distributions of nanocrystal size, shape, and composition.2−5 Heterogeneity is often cited as an important factor in determining the properties of such powders.6,7 However, another important factor is the presence of interfaces between nanocrystals, which can also modify functionality.8−11 In the context of nanocrystalline ceramics, the critical role of interfaces (i.e., grain boundaries) for ionic and electronic conduction processes is well appreciated.12,13 However, much less is known about the role of solid−solid interfaces inside nanocrystal networks of ionic insulators.14 As well as being an important issue for catalysis, sensing, and solid state ionics, understanding the role of interfaces is important for materials characterization.15−17 For example, to probe the electronic properties of oxide nanocrystals spectroscopically, as-formed powders are often pressed into pellets.18−21 However, pressing nanopowders is also associated with the generation of additional solid−solid interfaces. In turn, these interface defects may introduce additional spectroscopic features, which may not be representative of the properties of © 2012 American Chemical Society
the loose powder. Therefore, nanocrystal interfaces can complicate interpretation of the link between single nanocrystal structure and properties. In a recent letter, we highlighted these issues by demonstrating that compression of MgO powders to high density (∼50% of the theoretical maximum) leads to changes in their optical absorption spectra, which can be attributed to interfaces between MgO nanocrystals.3 To overcome the complexities in examining and understanding solid−solid interface effects, it is desirable to have a system of nanocrystals that exhibit narrow distributions of size, structure, and morphology. This allows one to test the effect of powder density and, thus, the concentration of solid−solid interfaces on the overall electronic, chemical, and optical ensemble properties. We achieve this using a chemical vapor synthesis (CVS) procedure.22 The MgO nanocrystals formed in this way are cubic, exposing low index (001) facets with low coordinated ions at edges, 4-fold coordinated (4C), and corners, 3-fold coordinated (3C). MgO nanopowders have also been the subject of numerous previous investigations18,23−26 and are perhaps one of the best understood ceramic nanomaterials.9,27,28 In previous work, a variety of spectroscopic techniques18,29−32 as well as first-principles theoretical calculations2,3,33−35 have been used to investigate Received: February 15, 2012 Revised: April 3, 2012 Published: April 4, 2012 10103
dx.doi.org/10.1021/jp3015222 | J. Phys. Chem. C 2012, 116, 10103−10112
The Journal of Physical Chemistry C
Article
and kept at this temperature for 5 h until full dehydroxylation of the sample surface is achieved. This procedure forms lowdensity powders (less than 1% of the bulk MgO density, ρMgO = 3580 kg m−3), and transmission electron microscopy (TEM) demonstrates they consist of agglomerated cubic nanocrystals. A defined mass of the powder is transferred into a 0.05 cm3 cavity, and a hydraulic press is used to compact the powder. By varying the pressure applied, compressed powders of densities up to 50% can be obtained in a controlled and reproducible way (Table 1). After being pressed, the MgO pellets were
the electronic properties of MgO nanocrystals formed by CVS. UV diffuse reflectance spectroscopy reveals two absorption bands far below the bulk absorption threshold of MgO, which have been attributed to corner (4.6 eV) and edge sites (5.2 eV). Photoluminescence spectroscopy detects two closely spaced emission bands at 3.4 and 3.2 eV, which result from photoexcitation of corners and edges, respectively.36 On the basis of these results, it is usually assumed that photons with sub-band gap energy exclusively excite low coordinated surface sites.18,32,33,36 In this Article, we build upon our previous work by investigating systematically how the optical absorption spectra of compressed MgO nanopowders change as the powder density is varied between less than 1% and 50%. For the first time, we report a so far unknown photoluminescence emission process that results from the photoexcitation of nanocrystal interfaces. Together with complementary theoretical calculations, this study provides a detailed picture of the electronic and optical properties of nanocrystal interfaces in MgO powders. Taking a different perspective, systematically increasing the density of a powder in a controlled way allows one to monitor changes in structure and properties as one transforms the system from a collection of loosely bound nanocrystals to a dense nanocrystalline ceramic. Therefore, this approach can provide insight into the structural and electronic properties of extended defects in such systems that are suspected to play an important role in electronic and ionic transport properties.
Table 1. Nanocrystalline MgO Pellets Produced by Applying Different Uniaxial Pressuresa pressure/Pa
ρMgO/%
0 5 × 107 1 × 108 1.7 × 108
