On the Atomistic Simulation Approach Towards the Structural Stability of the ZnS Nanoparticles

• Author / Creator

  Khalkhali, Mohammad

• Recently, ZnS quantum dots have attracted a lot of attention since they can be a suitable alternative for cadmium-based quantum dots, which are known to be highly carcinogenic for living systems. Suitable optoelectronic properties and non-toxic nature of ZnS quantum dots capacitate exiting applications for these nanomaterials especially in the field of biomedical imaging. The ability to tune the optoelectronic properties of quantum dots based solely on size of these nanoparticles, which is due to the quantum confinement effect, has raised significant interest both in experimental and computational studies. Nevertheless, the structural stability of nanocrystalline ZnS seems to be a challenging issue since they potentially prone to autonomous structural evolutions in ambient conditions. Thus, it is essential to build an understanding about governing factors controlling structural changes of ZnS nanoparticle before they can be safely implemented, especially for in vivo applications. Using the molecular dynamics technique, we have studied the structural evolution of ZnS nanoparticles at bare and hydrated states. Accuracy of molecular dynamics simulation highly depends on the reliability of the empirical potential it uses. Although multiple empirical potentials have been suggested for ZnS, there was no comprehensive study on comparing the performance of these potentials. Hence, this study started with a through review of available empirical potentials of ZnS in literature. The performance of each potential is tested through comparing the ZnS properties calculated using empirical potentials with experimental or higher level first principle calculation results. Based on the obtained results and the nature of our study which is focused on the noncrystalline ZnS, we chose the proper potential. The study of the structural evolution of 1 to 5 nm freestanding ZnS nanoparticles in vacuum revealed that relaxed configurations of bare ZnS nanoparticles larger than 3 nm consist of three regions: a) a crystalline core, b) a distorted network of 4-coordinated atoms surrounding the crystalline core, and c) a surface structure entirely made of interconnected 3-coordinated atoms. Decreasing the size of the ZnS nanoparticle to 2 nm causes the crystalline core to disappear. Further reducing the size makes all of the atoms to become 3-coordinated and adopt a bubble-like structure. The simulation results also showed that polarity of nanoparticles is also affected by their structural evolutions. The non-polar ideal initial structures change to polar structures after relaxation at 300 K. For NPs smaller than 3 nm, where surface structure is predominant, magnitude of dipole moment of zinc-blende and wurtzite nanoparticles are similar due to the similarity of their surface structures. Increasing the size makes the crystalline core dominant so, dipole moments converge to the bulk values. The bulk wurtzite structure at 300 K shows a natural dipole moment of 0.3855 D per ZnS along the unit cell c direction because of the slight C3v-distortion of the elementary ZnS tetrahedron. On the other hand, zinc-blende lattice does not have a polar nature due to the Td symmetry. As a result, increasing the size makes bare zinc-blende and wurtzite nanoparticles less and more polar, respectively. Structural analyses of ZnS nanoparticle in water showed that the 3-phase structure of bare nanoparticles is not formed in the hydrated state. Bulk of hydrated nanoparticles has more crystalline structure, however, the inhomogeneity in their surface relaxation makes them more polar comparing to bare nanoparticles. This inhomogeneity is more severe in hydrated wurtzite nanoparticles, causing them to show larger dipole moments. Analysing the structure of water in the first hydration shell of the surface atoms show that water is mainly adsorb to the nanoparticles' surface through Zn-O interaction. This interaction causes the structure of water in the first hydration shell to be discontinuous and positions of water molecules have the same pattern as positions of Zn atoms on the surface of nanoparticles. Long residence time of water molecules in the first hydration shell of surface Zn atoms, can affect the interaction of nanoparticle with other nanoparticles and the arouse solution.

• Subjects / Keywords

  • ZnS
  • Molecular-dynamics
  • Nanoparticles
  • Structural stability

• Graduation date

  Fall 2015

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3ST7F80W

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.