From the ZnO Hollow Cage Clusters to ZnO Nanoporous Phases: A First-Principles Bottom-Up Prediction

Abstract

A family of Zn k O k (k = 12, 16) cluster-assembled solid phases with novel structures and properties has been characterized utilizing a bottom-up approach with density functional calculations. Geometries, stabilities, equation of states, phase transitions, and electronic properties of these ZnO polymorphs have been systematically investigated. First-principles molecular dynamics (FPMD) study of the two selected building blocks, Zn 12 O 12 and Zn 16 O 16 , with hollow cage structure and large HOMO−LUMO gap shows that both of them are thermodynamically stable enough to survive up to at least 500 K. Via the coalescence of building blocks, we find that the Zn 12 O 12 cages are able to form eight stable phases by four types of Zn 12 O 12 −Zn 12 O 12 interactions, and the Zn 16 O 16 cages can bind into three phases by the Zn 16 O 16 −Zn 16 O 16 links of H′, C′, and S′. Among these phases, six ones are reported for the first time. This has greatly extended the family of ZnO nanoporous phases. Notably, some of these phases are even more stable than the synthesized metastable rocksalt ZnO polymorph. The hollow cage structure of the corresponding building block Zn k O k is well preserved in all of them, which leads to their low-density nanoporous and high flexibility features. In addition the electronic integrity (wide-energy gap) of the individual Zn k O k is also retained. Our calculation reveals that they are all semiconductor with a large direct or indirect band gap. The insights obtained in this work are likely to be general in II−VI semiconductor compounds and will be helpful for extending the range of properties and applications of ZnO materials.

What is the best or most relevant global minimum for nanoclusters? Predicting, comparing and recycling cluster structures with WASP@N

“…The 2 Â 2 Â 6 D 2d conguration of alternating atoms, labelled DM01 in Fig. 6, was found to be the IP-GM for (LiI) 12 . One can imagine that this cuboid conguration could be cut from the NaCl rock salt structure, and thus it is not surprising that this structural motif was not generated for (GaAs) 12 .…”

“…Labelled DM01 in Fig. 5, the D 3d barrel was found to be the IP-GM for (SrO) 12 , whereas for (LiI) 12 and (GaAs) 12 it was ranked fourth and second, respectively. The 2 Â 2 Â 6 D 2d conguration of alternating atoms, labelled DM01 in Fig.…”

“…Using the hashkey, which provides a unique identier for each structural motif, we were able to reduce this initial set to just over 100 unique structural motifs (which we will refer to as the DM-set). If the database contained entries for alkali halides, (XY) 12 , and alkaline earth oxides, (ZO) 12 , for X ¼ Li to Cs, Y ¼ F to I, and Z ¼ Mg to Ba, then potentially there would be a maximum reduction of 96%. The determination of this reduced set (calculation and comparison of hashkeys) is orders of magnitude faster to perform than the additional structural relaxations (using standard algorithms within an electronic structure code) that would have been necessary if we could not determine equivalent structures.…”

“…Using our DM-set of unique LM, we now investigate three different compounds that were not included in the initial dataset taken from the Hive, namely (LiI) 12 , Table 2 Parameters for the shell model for ions X, where Q and Y are the point-charges of the core and shell, which are connected by a spring with constants k 2 and k 4 . The Coulomb contribution to the energy between point-charges of an individual ion X is replaced with the energy associated with the spring, 1/2k 2 x 2 + 1/4k 4 x 4 , where x is the distance between the core and shell.…”

“…The Coulomb contribution to the energy between point-charges of an individual ion X is replaced with the energy associated with the spring, 1/2k 2 x 2 + 1/4k 4 x 4 , where x is the distance between the core and shell. Note that the strontium cation is treated as a rigid ion and therefore only has one parameter (SrO) 12 and (GaAs) 12 . As the main focus of this article is the methodology as opposed to the physical and electronic properties of the predicted nanoclusters, we have chosen to present new IP-LM structures, i.e.…”