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$_2$ 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.

• The paper identifies a novel, energetically favored hexagonal ZnSe nanoplatelet phase emerging via barrierless surface reconstruction from the wurtzite configuration.
• It employs density functional theory to predict vibrational modes, direct band gaps (~3.785 eV for 2ML), and dipole-forbidden optical transitions with strong in-plane polarization.
• Surface adsorption by ZnCl2 and L-cysteine triggers further reconstruction, enhancing natural optical activity up to 11 times that of free cysteine for chiral photonic applications.

Spontaneous Structural Reconstructions and Properties of Ultrathin Triangular ZnSe Nanoplatelets

Introduction

This work presents a comprehensive first-principles investigation into the structure, vibrational, and electronic properties of ultrathin ZnSe nanoplatelets (NPLs) with triangular geometry. These II–VI semiconductor NPLs have emerged as promising candidates for optoelectronic and chiral photonic applications owing to their quantum confinement and shape-dependent properties. Despite extensive research on ZnSe-based colloidal NPLs, ambiguities persist regarding their precise atomic configuration, mechanisms of surface passivation, and the impact of surface adsorption on their physical properties. The study systematically resolves these issues, elucidating a novel, energetically favored hexagonal structure, establishing the impact of spontaneous surface reconstruction, and quantifying the effects of molecular adsorption on structural and chiroptical response.

New Energetically Favored 2D Structure and Reconstruction Mechanisms

Density functional theory calculations identified a previously unreported hexagonal 2D ZnSe nanoplatelet structure, hereafter denoted as tr-ZnSe, whose total energy per formula unit is the lowest among all 2D ZnSe structures to date. This tr-ZnSe phase emerges spontaneously from the wurtzite (WZ) [0001]-oriented configuration via a surface-driven reconstruction, characterized by a unique variation in stacking order of near-surface Zn layers versus bulk layers. Notably, the transformation is barrierless, occurring even in the absence of thermal activation, and yields a nonpolar centrosymmetric configuration with a fourfold Zn/Se coordination throughout the nanoplatelet. This contrasts with previous wurtzite and zinc-blende NPL models, which either suffer from metallicity due to unsaturated valencies or are dynamically less stable.

Analysis also revealed that increasing the NPL thickness beyond two monolayers (MLs) preserves the propensity for surface-driven reconstruction—either at both surfaces (for even-layered NPLs) or selectively at the thinner surface region (for thicker NPLs). The cohesive energies confirm that ligand protection is required to prevent energetically favorable stacking or aggregation of these reconstructed NPLs.

Electronic Structure and Selection Rules for Optical Transitions

The tr-ZnSe NPLs display a direct band gap at the $\Gamma$-point but exhibit nontrivial selection rules for optical transitions. Group theoretical analysis of the spinor wavefunctions demonstrates that, for 2ML tr-ZnSe, the lowest conduction subband (A$_{2u}$) shares parity with the highest valence subbands (E$_u$), rendering the fundamental transition at the band edge dipole-forbidden. Allowed transitions occur only for higher-energy valence subbands to conduction subbands of distinct symmetry (e.g., from HH2/LH2 (E$_g$) to CB2 (A$_{1g}$)), with strong matrix elements in the in-plane ($xy$) polarizations. Calculated fundamental band gaps using hybrid functionals are 3.785 eV (2ML) and 3.687 eV (4ML), with the energy of the first allowed transition exceeding the fundamental gap by 0.227 eV. Experimental optical absorption is at 4.23 eV, aligning with the theoretical predictions when accounting for DFT self-interaction errors and underestimation of quantum confinement effects.

Vibrational Properties and Experimental Correlation

The phonon spectrum of 2ML tr-ZnSe NPLs exhibits characteristic modes in full concordance with experimental IR and Raman measurements, specifically a robust TO mode at 207.8 cm$^{-1}$ ($E_u$) and an LO mode at 234.5 cm$^{-1}$ ($E_{xy}$). The numerically predicted frequencies are consistently lower than those of ZnSe nanoclusters, disqualifying nanocluster assignment for the observed triangular objects and unequivocally supporting their NPL identity. The close alignment between predicted and observed frequencies also reinforces the detailed configurational model advanced in this work.

Surface Chemistry: Molecular Adsorption and Further Reconstruction

Exposure to ZnCl$_{2u}$0 or $_{2u}$1-cysteine induces substantial surface-driven tetragonal reconstruction, converting the hexagonal NPL into a structure analogous to zinc-blende (with (001) orientation). This transformation is irreversible and is triggered by electronic charge transfer upon adsorption: for cysteine, S–H dissociation and hydrogen insertion into the NPL bulk stabilize the surface, while chloride induces pronounced rearrangement of surface Zn atoms. The calculated adsorption energies are substantial (ZnCl$_{2u}$2: 2.19 eV; cysteine: 1.54 eV), and the resulting structures are direct-gap semiconductors, with band structures sensitive to the adsorbate. For cysteine, mid-gap states arise, slightly shifting the valence band maximum.

Chiroptical Response and Natural Optical Activity

Calculations of the natural optical activity (NOA) based on the electric-magnetic response tensors reveal that tr-ZnSe NPLs decorated with $_{2u}$3-cysteine exhibit a marked enhancement in specific optical activity per chiral molecule compared to free cysteine. For Janus structures (cysteine adsorbed on one side), the induced NOA is 11 times higher than that of free cysteine and of opposite sign, a consequence of symmetry breaking and strong ligand-NPL electronic coupling. This has immediate implications for the design of chiral quantum photonic materials, where the handedness and magnitude of optical rotation can be modulated via surface chemistry and atomic structure. Theoretical comparisons with analogous CdSe NPLs indicate that the sign and magnitude of the optical response are sensitive to subtle structural differences, aligning with reported experimental trends in circular dichroism.

Conclusions

Detailed first-principles calculations have established a new structural ground state for ultrathin triangular ZnSe nanoplatelets, underpinned by spontaneous surface reconstruction from the wurtzite phase. This phase is dynamically stable, exhibits direct but symmetry-forbidden fundamental optical transitions, and offers vibrational and electronic properties in excellent agreement with experiment. Surface adsorption triggers additional reconstructive phenomena and substantially enhances natural optical activity, especially for Janus NPL-ligand complexes. These findings clarify ambiguities regarding the atomic structure of experimentally realized ZnSe NPLs and pave the way for engineered chiroptical functionalities via selective molecular functionalization. Future research should address experimental isolation and direct spectroscopic verification of the predicted Janus and reconstructed structures, exploration of their quantum emission properties, and the development of robust protocols to harness induced chirality for optoelectronic and quantum applications.

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Reference: "Spontaneous structural reconstructions and properties of ultrathin triangular ZnSe nanoplatelets" (2604.00636).