Investigation of Luminescent Properties of Carbon Nanodot Superparticles Assembled via Emulsion-Templated Assembly

Carbon Nanodots (CDs) are a family of carbon-based nanoparticles characterized by intense and tunable absorption–emission in the visible range, which makes them very promising for light-emitting devices, optoelectronics, photocatalysis, and more.

Emulsion-templated assembly has emerged as a scalable approach to fabricate the so-called superparticles (SPs), controlled three-dimensional structures with crystalline or amorphous organization. SPs are obtained by the hierarchical assembly of colloidal nanoparticles, which act as the building blocks, constituting artificial materials with new properties stemming from the crosstalk between individual nanoparticles. Unfortunately, the photophysics governing their optical response remains largely unclear. In particular, little information is available on the dynamics of photoexcited superparticles made from the assembly of CDs.

Here, we report the first synthesis of CD-based SPs by a water-in-oil self-assembly procedure. Before the assembly, CDs were purified by column chromatography to isolate a fraction with narrow size distribution. One type of SP was directly obtained by the spontaneous self-assembly of the purified CDs by a water-in-oil (toluene) procedure. Alternatively, we pegylated (P-CDs) the CDs before self-assembly in order to change the interparticle distance inside the superparticles (P-SPs).

Both SPs are characterized by 1-2 mm diameters, and they display luminescence even in a solid state after dropcasting the sample, which is not common for individual CDs. We have been able to collect the emission and the lifetime from a single SP, demonstrating that the emission depends dramatically on the distance between the CDs inside the superstructure. In fact, when decreasing the distance between CDs, we observed a redshift of the emission band and a shortening of the emission lifetime. This work lays the foundations for the synthesis of new luminescent materials which can be useful in several optoelectronic applications.