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Spontaneous structural reconstructions and properties of ultrathin triangular ZnSe nanoplatelets

Alexander I. Lebedev

Abstract

Two-dimensional (2D) materials have revolutionized all areas of development of high-performance electronic devices. In particular, the unique electronic and optical properties of II--VI semiconductor nanoplatelets have been found to be very promising for optoelectronics. However, not all properties of this intriguing class of materials are yet known. A new, previously unknown hexagonal 2D structure of ZnSe nanoplatelets whose energy is lower than the energies of all previously studied systems is found from first-principles calculations. This structure appears as a result of spontaneous reconstruction of the wurtzite structure and differs from it by the stacking order of the bulk and near-surface Zn atomic layers. The phonon spectrum, electronic structure, and band gap of the obtained nanoplatelets are calculated. The phonon spectra of the nanoplatelets are in complete agreement with the spectra observed in experiment and differ strongly from the vibrational spectra of ZnSe nanoclusters. The adsorption of ZnCl$_2$ and $L$-cysteine molecules on the surface of the nanoplatelets is studied and is shown to be accompanied by yet another spontaneous reconstruction of the hexagonal structure into a tetragonal one and a new rearrangement of Zn atoms in the near-surface layers. Calculations of the natural optical activity of nanoplatelets covered with $L$-cysteine reveal an increase in the specific (calculated per chiral molecule) optical activity, which is especially strong for the Janus structures, as compared to the free $L$-cysteine molecule.

Spontaneous structural reconstructions and properties of ultrathin triangular ZnSe nanoplatelets

Abstract

Two-dimensional (2D) materials have revolutionized all areas of development of high-performance electronic devices. In particular, the unique electronic and optical properties of II--VI semiconductor nanoplatelets have been found to be very promising for optoelectronics. However, not all properties of this intriguing class of materials are yet known. A new, previously unknown hexagonal 2D structure of ZnSe nanoplatelets whose energy is lower than the energies of all previously studied systems is found from first-principles calculations. This structure appears as a result of spontaneous reconstruction of the wurtzite structure and differs from it by the stacking order of the bulk and near-surface Zn atomic layers. The phonon spectrum, electronic structure, and band gap of the obtained nanoplatelets are calculated. The phonon spectra of the nanoplatelets are in complete agreement with the spectra observed in experiment and differ strongly from the vibrational spectra of ZnSe nanoclusters. The adsorption of ZnCl and -cysteine molecules on the surface of the nanoplatelets is studied and is shown to be accompanied by yet another spontaneous reconstruction of the hexagonal structure into a tetragonal one and a new rearrangement of Zn atoms in the near-surface layers. Calculations of the natural optical activity of nanoplatelets covered with -cysteine reveal an increase in the specific (calculated per chiral molecule) optical activity, which is especially strong for the Janus structures, as compared to the free -cysteine molecule.

Paper Structure

This paper contains 9 sections, 5 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Computational Details
  3. Results and discussion
  4. Electronic structure of 2.5ML thick ZnSe nanoplatelets
  5. Structural transformation of wurtzite ZnSe(0001) nanoplatelets
  6. Electronic structure and optical transitions in tr-ZnSe nanoplatelets
  7. Phonon spectrum of 2ML thick tr-ZnSe nanoplatelets
  8. Adsorption of ZnCl$_2$ and $L$-cysteine on the surface of tr-ZnSe nanoplatelets
  9. Conclusions

Figures (5)

  • Figure 1: Side view of all 2ML ZnSe nanoplatelets studied in this work. (a) V-ZnSe, (b) t-ZnSe, (c) t2-ZnSe, (d) tr-ZnSe, (e) WZ100-ZnSe, and (f) WZ110-ZnSe. It should be noted that the WZ110 2ML structure spontaneously transforms to the t-ZnSe one when performing calculations using PAW pseudopotentials. Se atoms are shown in green and Zn atoms are gray.
  • Figure 2: Band structure of a 2.5ML thick wurtzite ZnSe nanoplatelet with the structure proposed in Ref. Materials.16.1073. The Fermi level $F$ is located deep in the valence band.
  • Figure 3: Dynamics of reconstruction of the initial wurtzite structure into the tr-ZnSe one in a 2ML thick nanoplatelet.
  • Figure 4: Phonon spectrum of a 2ML thick tr-ZnSe nanoplatelet.
  • Figure 5: Band structure of a 2ML thick tr-ZnSe nanoplatelet.