Structural evolution of carbon dots during low temperature pyrolysis.

Hui Luo,Maria-Magdalena Titirici,Leonardo Lari,Liviu Cristian Tanase,Vlado K Lazarov,Servann Hérou,Hyunjeong Kim

doi:10.1039/d1nr07015k

Abstract

Carbon dots (CDs) are an emerging class of photoluminescent material. Their unique optical properties arise from the discrete energy levels in their electronic states, which directly relate to their crystalline and chemical structure. It is expected that when CDs go through structural changes via chemical reduction or thermal annealing, their energy levels will be altered, inducing unique optoelectronic properties such as solid-state photoluminescence (PL). However, the detailed structural evolution and how the optoelectronic characteristics of CDs are affected remain unclear. Therefore, it is of fundamental interest to understand how the structure of CDs prepared by hydrothermal carbonisation (HTC) rearranges from a highly functionalised disordered structure into a more ordered graphitic structure. In this paper, detailed structural characterisation and in situ TEM were conducted to reveal the structural evolution of CDs during the carbonisation process, which have demonstrated a growth in aromatic domains and reduction in oxidation sites. These structural features are correlated with their near-infrared (NIR) solid-state PL properties, which may find a lot of practical applications such as temperature sensing, solid-state display lighting and anti-counterfeit security inks.

Highlights

Carbon dots (CDs) are emerging luminescent nanomaterials for energy, bioimaging, optoelectronic and catalysis applications.[1,2,3,4,5] In particular, CDs resemble semiconductor materials with small energy gaps, excellent luminescence properties and high carrier mobilities and concentrations, making them good candidates to replace the more traditional materials such as semiconductor quantum dots and graphene
CDs were prepared via hydrothermal carbonisation of glucose in water at 200 °C for 12 hours,[21] denoted as HTC-CDs
Further carbonisation under different temperatures at 350 °C, 550 °C and 750 °C in N2 atmosphere was carried out to investigate the structural evolution of the CDs

Summary

Introduction

CDs are emerging luminescent nanomaterials for energy, bioimaging, optoelectronic and catalysis applications.[1,2,3,4,5] In particular, CDs resemble semiconductor materials with small energy gaps, excellent luminescence properties and high carrier mobilities and concentrations, making them good candidates to replace the more traditional materials such as semiconductor quantum dots and graphene. Those clusters have limited sizes but high local molecular orientation, which means those basic structural units are mostly parallel in extended domains as lamellae.[31,42] Upon heating to 750 °C, most of the oxygen groups have been removed, the remaining sp[3] carbon atoms undergo aromatisation to form small aromatic clusters, resulting in a more disordered structure in the matrix, while the basic structural units continue graphitisation process to form larger graphitic structure, i.e. CDs.[43] This process follows the two-stage graphitization process described by Oberlin et al, which demonstrate that the first step eliminates most noncarbon components and initiates formation of an aromatic skeleton consisting of a network of six-membered, planar rings of carbon.[44] It is followed by the second step consisting mostly of polymerization and structural rearrangement of the aromatic skeleton towards the thermodynamically stable layered graphitic structure.[45]

Results

Conclusion