Ligand-nanocrystal interaction under visible light irradiation

When designing photoelectric devices such as solar cells, photocatalysts and photodetectors, scientists usually give priority to materials that are stable and have adjustable properties. This allows them to precisely control the optical properties of the material and ensure that their properties are maintained over time under different environmental conditions.

Organic-inorganic nanocrystals consist of organic ligands connected to the surface of colloidal inorganic nanocrystals by coordination bonds and are promising in this respect. Organic ligands are known to exhibit enhanced stability because they form a protective layer around reactive inorganic nanocrystals. However, it has been found that the incorporation of organic ligands can reduce the conductivity and photon absorption efficiency of inorganic nanocrystals.

In a groundbreaking study on ligand-nanocrystal interactions, researchers from Japan have now demonstrated the quasi-reversible displacement of organic ligands on the surface of nanocrystals. Their findings, published in ACS Nano, shed new light on the widely held belief that organic ligands anchor to the surface of nanocrystals.

A research team led by Professor Yoichi Kobayashi of Ritsuekanate University in Japan found that it was possible to reversibly replace the coordination bond between Perylene bisimide (PBI) with carboxyl and inorganic zinc sulfide (ZnS) nanocystals by exposing the material to visible light.

To reveal the new behaviour of organic-inorganic nanohybrids, Professor Kobayashi said: "We explored the ligand properties of organic-inorganic nanohybrid systems -- ZnS) Perylene bisimide by using zinc sulfide (ZnS) NCs (PBI) with a carboxyl (PBI) coordination as a model system. Our results provide the first example of photoinduced aromatic ligand replacement with semiconductor nanocrystals."

In their study, the researchers conducted theoretical analysis and experimental studies to understand the unique light-induced properties of the material. They first performed density functional theory calculations to investigate the structure and orbits of PBI-ZnS ([PBI-Zn 25 S 31] -) in its ground state and first excited state.

Next, they performed time-resolved pulsed excited Raman spectroscopy, which excites the sample with a super-fast laser. This helped them analyze the corresponding Raman spectra and reveal the properties of PBI-ZnS excited states.

Experimental observations and calculations show that when photoexcited, the PBI molecule excites an electron, and the corresponding "hole" (vacancy formed due to the absence of electrons) quickly moves from the aromatic ligand (PBI) to ZnS. This results in the surfacing of long-lived negatively charged PBI ions from ZnS nanocrystals.

However, over time, the displaced ligands recombine with the surface defects of the ZnS nanocrystals, resulting in a quasi-retrolight-induced displacement of the coordination PBI. It is important to note that the dynamic behavior of the ligand molecules observed in this study is different from that observed during typical photoinduced charge transfer, in which holes are usually retained on the donor molecule, allowing it to quickly recombine with electrons.

Explaining the importance of these findings, Professor Kobayashi said: "An accurate understanding of ligand-nanocrystal interactions is important not only for basic nanoscience, but also for the development of advanced light-functional materials using nanomaterials. These include photocatalysts that use visible light to break down persistent chemicals and photoconductive microcircuit patterns for wearable devices."

In fact, the results of this study provide a promising avenue for enhancing the adjustability and functionality of inorganic materials with aromatic molecules. This, in turn, could have a significant impact on the field of basic nanoscience and photochemistry in the future.

Source: Laser Net