Computational Modeling of CdSe Quantum Nanosheets

Calculations of the structures of the 2D CdSe quantum nanosheets were performed using first‐principles DFT calculations implemented in the Vienna Ab‐initio Simulation Package (VASP). Specifically, projector augmented wave (PAW) potentials and the Perdew‐Burke Ernzerhof (PBE) generalized gradient approximation (GGA) were employed. The plane cutoff energy was set at 400 eV, and Gaussian smearing was used with the width of 0.1 eV. For all CdSe nanosheets, the convergence criterion was 10⁻⁵ eV for electronic energy, and the Hellmann–Feynman force was converged to 10⁻² eV Å⁻¹ for the ionic relaxation. A slab model and z‐direction vacuum were used to simulate the CdSe nanosheets. For wurtzite‐CdSe, the slab model consists of seven Cd–Se atomic monolayers (≈1.4 nm thickness) with the (112¯0) surface termination and each monolayer contains one Cd and one Se atomic layers. After the reconstruction of the nonpolar (112¯0) surface, supercell structures were created by forming Se defects in the wurtzite‐CdSe structures obtained by the surface reconstruction. For zinc blende‐CdSe, the slab model consists of 7 atomic monolayers, and each monolayer was a combination of one Cd and one Se atomic layers. After the (001) surface was reconstructed in the same way as described above, Se defects were formed to create supercell structures. The surface reconstruction processes were conducted by following the modified methods of previous works.^[76, ^77] Supercell structures of wurtzite‐CdSe nanosheets with Se defects (Figures S9 and S10, Supporting Information) and zinc blende‐CdSe nanosheets with Se defects (Figure S11, Supporting Information) were calculated with Monkhorst‐Pack 4 × 3 × 1 k‐points and 4 × 4 × 1 k‐points, respectively. To consider the computational cost and accuracy of the energy outcome, it was identified that a vacuum space greater than 10 Å along the z‐axis was the optimal value for the slab model. The distance between supercell structures was far enough that there was no electronic interference between them. In the wurtzite‐CdSe nanosheets, all atoms used in the calculation were not fixed, except for the atoms in the middle layer. By contrast, all atoms in other supercells were flexible. The Se vacancy formation energy was calculated to demonstrate that Se defects can be caused by electron beam irradiation. The vacancy formation energy (E_f) was calculated as per the Equation 1.
$$E_{\mathrm{f}}=E_{\mathrm{vac}}-\left(E_{\mathrm{bulk}}-{\mu }_{\mathrm{x}}\right)$$ where E_vac is the energy of supercells with defects and E_bulk is the energy of original structure before the formation of defects. The chemical potential µ_x of the Se atom was considered as the energy of the isolated atom.^[78]