Influence of the Crystal Structure of the Nucleus on the Morphology of t-ZnO Tetrapods
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STRUCTURE OF CRYSTALS
Influence of the Crystal Structure of the Nucleus on the Morphology of t-ZnO Tetrapods S. V. Avilova,*, A. V. Tuchina, A. N. Shebanova, and E. P. Domashevskayaa aVoronezh
State University, Voronezh, 394018 Russia *e-mail: [email protected]
Received June 25, 2018; revised June 25, 2018; accepted August 16, 2018
Abstract—A new approach to simulating the morphology of hierarchical nanocrystals has been proposed on the example of ZnO nanotetrapods. Within this approach, twinning is considered as a point of symmetric growth bifurcation under unstable conditions. The electronic structure has been calculated for elementary ZnO clusters of different symmetries by the molecular dynamics method to reveal the processes of symmetry transformations occurring under nonequilibrium conditions during the forming of the tetrapod nucleus. DOI: 10.1134/S1063774519020032
INTRODUCTION Zinc oxide is a nontoxic wide-gap semiconductor; its nanostructures are widely used in photocatalysis and electronics [1]. Currently, hierarchical zinc oxide nanostructures are of particular interest because they make it possible to improve the characteristics of photocells [2, 3], gas sensors [4], and lasers [5]. ZnO undergoes phase transitions between the equilibrium wurtzite (B4) and non-equilibrium sphalerite (B3) and stone salt (B1) structures, which may transform the symmetry of the nucleus under nonequilibrium conditions and lead to twinning during further crystal growth [6].
500 nm Fig. 1. An SEM image of t-ZnO nanocrystals [7].
The purpose of this study was to observe these processes as a symmetric bifurcation of the nanocrystal growth direction under unstable conditions. As an example, the nanoform of ZnO tetrapods (t-ZnO) that consists of four single crystals with the B4 structure growing from a common center (Fig. 1) has been investigated. RESULTS AND DISCUSSION The t-ZnO tetrapod is synthesized under nonequilibrium conditions by burning zinc and other methods (including the gas-transport method at elevated temperatures) [8–10]. Only particular cases of twinning in the t-ZnO nucleus were investigated using diffraction, spectroscopy, electron microscopy, and theoretical methods [11, 12]. However, the high reproducibility of this morphology, purity of the material obtained, and the random character of the deviation of tetrapod leg growth directions from the ideal-tetrahedron symmetry [13] have not been explained. The proposed t-ZnO growth models can be divided into two groups: (i) models suggesting polymorphism with a В3–B4 phase transition boundary and (ii) models of pure B4 phase twinning. The initial structures of ZnO clusters for models proposing polymorphism are B3 [14], B1 [11, 14], graphite [15], or a fullerene-like spheroid [16]. To reveal the most probable routes of t-ZnO nucleation, the total energy and electronic structure of elementary tetrahedra and basic clusters of various ZnO polymorphs were calculated using the density functional theory (DFT) and molecular dynamics (MD) methods. DFT calculatio
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