Fabrication of thermally stable graphene quantum dots incorporated siloxane hybrid composite materials

Abstract

Over the past several decades, quantum dot (QD) has been spotlighted as a luminescent material in many applications such as transistors, solar cells, LEDs and diode lasers due to its unique luminescent properties (luminescence characteristics depending on size, narrow emission peak, and high photoluminescence quantum yield (PL QY)). Among them, graphene quantum dot (GQD), nano-sized graphene fragment is low in toxicity, source abundant, low-cost, and easy to attach many functional groups, can be dissolved in various solvents and easy to modulate its luminescent properties as advantages compared with inorganic semiconductor quantum dot. In addition, GQD is also attracting attention as a PL film for display because it emits blue light, which is difficult to obtain in inorganic semiconductor QD. However, due to high surface area of nano materials and the nature of the organic material, it has disadvantage that it is vulnerable to high temperature and humidity environment. Therefore, thermal and chemical stability are important factors, because luminescent properties of quantum dot should not change during manufacturing process.

Inorganic-organic hybrid material (hybrimer) is a composite material synthesized by sol-gel process where organic and inorganic components are chemically bonded as a molecular unit, so it has uniform properties, and optical transparency of organic material, and mechanical and thermal stability of inorganic material at the same time. In addition, that it is possible to produce material with desired properties according to the type of organic functional group of precursor, is the advantage of inorganic-organic hybrid material. Recently, in our laboratory, red CdSe/ZnSeS QD, dispersed in siloxane hybrimer through in-situ sol-gel reaction and photo-curing, could have chemical stability and stability under harsh conditions (85℃/5%RH and 85℃/85%RH conditions).

In this thesis, we tried to enhance the thermal stability of GQD by passivating it in siloxane hybrimer through in-situ sol-gel process, and to compare the thermal stability according to the type of matrix, we fabricated GQD composite film by differentiating functional group of hybrimer as methacrylate and phenyl methacrylate hybrimer, and also conventional acrylate polymer film. After long-term thermal stability test (85℃/5%RH, 120℃/5%RH and 85℃/85%RH), in conventional acrylate polymer film, due to the oxidation inside GQD, PL QY and PL intensity decreased, while in hybrimer film, the initial emission wavelength could maintain even after thermal stability test, because hybrimer could block the outside oxygen. Especially, in siloxane hybrimer containing phenyl group the PL intensity increased up to 2 times higher than the initial value after exposure to the high temperature environment($120^\circ C$). In addition, methacrylate phenyl hybrimer were also able to maintain the initial PL QY value after chemical stability tests (acetone, ethanol, and 0.5 N HCl). Thus, phenyl methacrylate hybrimer could secure both thermal and chemical stability of graphene quantum dot.