The effects of surface termination, stoichiometry, and strain on the optical properties of bulk-like ZnSe/ZnS core–shell nanocrystals

Why tiny glowing crystals matter

Flat-panel displays, medical scanners, and bio‑imaging tools increasingly rely on “quantum dots” – nanometer‑sized crystals that can be tuned to shine in very pure colors. Industry would like bright blue quantum dots that avoid toxic metals such as cadmium. Zinc selenide (ZnSe) nanocrystals with a thin zinc sulfide (ZnS) shell are a leading candidate, but experiments show puzzling differences in color even when dots seem to be the same size and made from the same materials. This study digs into the atomic‑scale details to explain why seemingly similar particles can glow differently, and how to deliberately tune their color.

Building safe blue light sources

The authors focus on relatively large, “bulk‑like” ZnSe quantum dots and on core–shell structures where a ZnSe core is wrapped in a ZnS shell. These heavy‑metal‑free particles are attractive because they combine strong blue emission with good chemical stability. Making them large helps shift the color into the desired deep‑blue range and suppresses unwanted processes that sap brightness. But large particles consist of tens or even hundreds of thousands of atoms, making standard quantum‑mechanical calculations impractical. To handle this, the team uses an atomistic tight‑binding method: an efficient, yet detailed, approach that can follow how electrons and holes move in a crystal built atom by atom.

How surface makeup changes the color

A key message of the work is that what happens at the surface of a quantum dot matters enormously, especially for smaller particles. Even if two nanocrystals have the same overall diameter and chemical formula, they can have different numbers of positively charged zinc ions and negatively charged selenium ions, depending on exactly how the spherical dot is carved out of the crystal lattice. The outermost atomic layer may also end up made almost entirely of one type of ion. The simulations show that such subtle shifts in the surface balance move the energies of electrons and holes by tenths of an electron‑volt, enough to noticeably alter the wavelength of emitted light. Zinc‑rich surfaces tend to push the emission toward higher energy (bluer light), while selenium‑rich surfaces pull it toward lower energy (redder light). As dots grow beyond about 10 nanometers, the surface makes up a smaller fraction of the total, and these stoichiometry‑driven shifts largely fade.

What happens when a shell is added

The team next examines ZnSe cores coated with ZnS shells of different thicknesses. In a simple picture, adding a shell increases the overall particle size, which should loosen the confinement of electrons and holes and therefore shift the color toward the red. The calculations confirm this behavior for small cores: wrapping a tiny ZnSe dot in ZnS can lower the emission energy by around half an electron‑volt. For medium‑sized dots, the effect weakens and eventually reverses. For large cores, adding a ZnS shell actually raises the emission energy, meaning the light becomes bluer. The detailed simulations also show that once a shell thicker than about one nanometer is present, variations in surface composition have a much smaller effect on the color, especially for bigger cores.

Strain as an invisible tuning knob

Why does a shell that usually softens confinement make large dots emit bluer light? The answer lies in strain. ZnS and ZnSe have slightly different natural lattice spacings, so forcing them to fit together stretches the shell and compresses the core. The authors compare calculations that include this strain with ones that artificially switch it off. Without strain, adding a shell always leaves the emission the same or pushes it to the red. With strain, the story changes: for medium and large cores, growing a thicker ZnS shell steadily raises the energy of the lowest electron state in the core, overpowering the red‑shifting effect of reduced confinement. The hole behaves differently, spreading somewhat into the shell but experiencing only modest energy changes. Together, these shifts produce a net blueshift that matches recent experimental observations.

Take‑home message for blue devices

This work shows that the color of light from ZnSe/ZnS quantum dots is controlled not only by their size, but also by the precise makeup of their surfaces and the hidden strains locked into their cores. For small dots, surface chemistry and overall size dominate, and adding a shell tends to produce redder emission. For large, bulk‑like dots favored in high‑performance blue LEDs, mechanical strain from the ZnS shell becomes the main player, nudging the emission toward the blue even when the interface is perfectly clean and free of defects. By capturing these effects in a predictive atom‑by‑atom model, the study offers a practical roadmap for designing bright, cadmium‑free blue emitters simply by choosing the right combination of core size, shell thickness, and surface termination.

Citation: Zieliński, M., Gajewicz-Skretna, A. The effects of surface termination, stoichiometry, and strain on the optical properties of bulk-like ZnSe/ZnS core–shell nanocrystals. Sci Rep 16, 10003 (2026). https://doi.org/10.1038/s41598-026-40051-2

Keywords: quantum dots, blue light emission, ZnSe/ZnS nanocrystals, core–shell nanoparticles, strain engineering