Soft-Template Synthesis of Carbon Nanodots for Electronic Applications

Abstract

This thesis reports “soft-template” synthesis of organic-soluble CNDs and their application in electronic devices including transistors, lighting devices, and organic solar cells. The soft-template is self-assembled by emulsifiers such as bis(2-ethylhexyl) sulfosuccinate sodium salt (Aerosol OT), 1-octanol, hexadecylamine, oleylamine, and so on. Water containing a molecular precursor (typically carbohydrates such as sugars, organic acids, hydroxy acids, and etc.) is “dispersed” in a heavy lipid solvent (e.g. decane, octadecene, and etc.) as the form of nano-to-micrometer-sized water droplets with the aid of the emulsifier, so called a “water-in-oil” emulsion. This method would have several advantages over the previous methods: (1) Narrow size distribution and size tunability(2) No undesired aggregation and high product yield(3) Excellent photoluminescence and high quantum yield(4) Long-term air, thermal, and chemical stability (5) High solubility in common organic solventsFor practical use, the CNDs are finally utilized in electronic devices including transistors, lighting devices, and organic solar cells. The CND-based transistors show ambipolar transport with the electron and hole mobilities as high as 8.49 × 10–5 and 3.88 × 10–5 cm2 V–1 s–1, respectively. The electron mobilities are consistently 2-4 times larger than the hole mobilities due to a larger density of states for electrons. The mobilities decrease exponentially with the increase of the ligand length (the barrier width), consistent with the Miller-Abrahams approximation for nearest-neighbor hopping. The CNDs produce bright visible light under UV illumination so that would be worth utilizing in phosphor applications. To realize the potential, freestanding films of the CNDs are fabricated based on a poly(methyl methacrylate) matrix. The polymer matrix can not only provide mechanical support but also disperse the CNDs to prevent solid-state quenching. By combining these films and an InGaN blue light-emitting diode with the peak wavelength of 400 nm, it can be produced white light that the correlated color temperature is about 5000 K (identical to that of conventional tubular fluorescent lamps). The color of the light can be controlled from blue to yellow by regulating the in-film concentration of the CNDs, the thickness of the films, and the forward-bias current. Lastly, the CNDs are used as electron acceptors in organic solar cells. There are various energy levels in the CNDs where the LUMO levels may be mainly distributed from 3.80 to 2.80 eV. Hence, some low-lying LUMO levels can serve as electron acceptors in organic solar cells employing poly(3-hexylthiophene) as an electron donor. In this regard, it is formed a bulk-heterojunction of the CNDs and poly(3-hexylthiophene) as a photoactive layer for organic solar cells. The CND-based organic solar cell displays the overall efficiency of 0.23% which is ca. 10% of the reference organic solar cell. Such low performance is mainly ascribed to the insulating ligand molecules surrounding the CNDs. To further improve the efficiency, it should be required introduction of more low-lying energy levels and removal of the insulating ligand layer. It is believed that these results would pave the way for widespread application of the CNDs in electronic and optoelectronic devices.