Preparation of multicolor-emissive carbon dots with high quantum yields and their epoxy composites for fluorescence anti-counterfeiting and light-emitting devices

Abstract

A facile solvothermal method for synthesizing full-color-emission carbon dots (CDs) was developed using p-phenylenediamine (PPD) and maleic anhydride (MAH) as raw materials under mild reaction conditions. By only changing the solvents, blue light-emitting carbon dots (B-CDs), green light-emitting carbon dots (G-CDs), orange light-emitting carbon dots (O-CDs) and red light-emitting carbon dots (R-CDs) can be realized at 180 °C for 4 h, and their emission wavelengths cover the visible spectrum ranging from 420 to 607 nm. The relative quantum yields for B-CDs, G-CDs, O-CDs and R-CDs are 55.6%, 76.5%, 59.5% and 30.9%, respectively. The different fluorescence features of these CDs are mainly attributed to various-sized sp² conjugated domains with N- and O-decorated surfaces. The as-prepared CDs have good thermal properties with an initial thermal decomposition temperature of 187–223 °C and significant pH-dependent fluorescence relationships. Even after heat treatment at 100 °C for 1 h and remaining at room temperature for 30 days, the four CDs still exhibit stable fluorescence performance. After CDs are introduced to epoxy resins (EP), the resulting CDs/EP composites with 0.25–1.0% CDs have enhanced tensile strengths (60–69 MPa) because of the higher conversion of epoxy groups and the excellent interface interaction between CDs and the EP matrix. CD/EP composites also display good thermal properties, high light transparency of over 80% and good photoluminescence stability. Using an ultraviolet chip as the excitation source combined with a CD/EP film, bright monochromatic light-emitting diodes (LEDs) and white LEDs are easily obtained. CD/EP composites can potentially be used in fluorescence anti-counterfeiting devices.