New nanocrystals with surface treatments shine brighter and scientists explain why

10.09.2025

The nanocrystals are being hit by a laser light ray. Image: Marco Vega.

Ag2S nanocrystals were improved with surface passivation treatments and studied under different pump fluences and photon energies.
Transient absorption spectroscopy reveals that the treated nanostructures exhibit faster multi-exciton recombination dynamics, highlighting the positive impact of the surface passivation.

Madrid, September 10^th, 2025. A research collaboration between researchers at IMDEA Nanociencia and ICMM-CSIC has unveiled new insights into the mechanisms that could enhance the light-emitting properties of nanocrystals. They studied silver sulfide (Ag₂S) nanocrystals, tiny particles that emit infrared light—an area key for biomedical imaging and telecommunications. These nanomaterials, while promising, typically present low emission due to imperfections on their surface. The team tackled this challenge by experimenting with different surface treatments, including the addition of selenium protective coatings. These approaches significantly boosted the crystals' ability to emit light, making them more viable for use in real-world applications like infrared sensors or laser sources.

To understand how these tiny particles behave under different conditions, the researchers used ultrafast laser pulses to test the inner workings of the nanocrystals. They discovered that surface-treated particles not only became brighter but also responded more efficiently to high-energy light. At lower energy levels, the nanocrystals behaved in predictable ways, but as the energy of the incoming light increased, more complex processes emerged, some of which could interfere with the light emission. Víctor Vega-Mayoral, principal author of the study, explains, “If a material is to be used for light-based applications like lasers, we need to know how it behaves when excited at high fluences—and what processes compete with the desired light emission.”

Plain Ag₂S nanocrystals and surface-passivated nanostructures were studied under different pump fluences and photon energies. At low fluences, plain Ag₂S nanocrystals show dynamics dominated by exciton trapping and recombination at defects. In contrast, when particles are coated with passivating layers, these coatings minimize the contribution of defects to the emission. At low exciton densities, transient absorption spectroscopy reveals that the excited-state dynamics are governed mainly by free exciton thermalization and recombination. At higher exciton densities, fluence-dependent studies show clear signatures of biexciton formation and Auger recombination, which appear at much lower fluences than in plain nanocrystals. Overall, surface-passivated nanostructures display faster multi-exciton recombination dynamics, underscoring the strong influence of effective selenium and zinc combined coatings.

Beyond improving their emission, the team's findings also open the door to practical uses: because the nanocrystals’ light emission changes with temperature, they could serve as ultra-small temperature sensors—so-called "nanothermometers"—for tracking conditions inside the human body. Their infrared emission also fits perfectly within the window used in fiber-optic communications, making them strong candidates for future tech in both healthcare and telecommunication fields. The study was published in the journal Nanoscale.

This work is a collaboration among researchers at IMDEA Nanociencia, led by Víctor Vega-Mayoral, and ICMM-CSIC, led by Beatriz H. Juárez, and is partially funded by the accreditation Excellence Severo Ochoa awarded to IMDEA Nanociencia (CEX2020-001039-S).

Glossary:

Nanocrystal: crystalline material with size of few nanometers.
Surface passivation: a technique that uses a coat of protective material to create a shield and enhance the chemical, thermal or optoelectronic properties of materials.
Exciton densities: the number of bound states of an electron and a hole per unit volume.
Auger recombination: a non-radiative process occurring when an electron and hole recombine, but instead of producing light, either an electron is raised higher into the conduction band or a hole is pushed deeper into the valence band. The process is a significant factor in limiting the efficiency of devices like solar cells and LEDs, particularly at high excitation levels.

Reference

Victor Vega-Mayoral,* Saül Garcia-Orrit, Peijiang Wang, Rafael Morales-Márquez, Emma Martín Rodríguez, Beatriz H. Juárez*and Juan Cabanillas-Gonzalez. Exploring many-body phenomena: biexciton generation and auger recombination in Ag2S-based nanocrystals. Nanoscale 17, 15697 (2025). DOI: 10.1039/d5nr00511f

Link to IMDEA Nanociencia Repository https://repositorio.imdeananociencia.org/handle/20.500.12614/3624

Contact:

Victor Vega Mayoral
Optoelectronic properties of two-dimensional materials
https://www.victorvegamayo.com/
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Oficina de Divulgación y Comunicación en IMDEA Nanociencia
divulgacion.nanociencia [at]imdea.org

Source: IMDEA Nanociencia

IMDEA Nanociencia Institute is a young interdisciplinary research Centre in Madrid (Spain) dedicated to the exploration of nanoscience and the development of applications of nanotechnology in connection with innovative industries.