Top-Down Fabrication of Luminescent Graphene Quantum Dots Using Self-Assembled Au Nanoparticles

Abstract

A new graphene quantum dot (GQD) fabrication method is presented, which employs a lithographic approach based on self-assembled Au nanoparticles formed by solid-state dewetting. The GQDs are formed by the patterned etching of a graphene layer enabled by Au nanoparticles, and their size is controllable through that of the Au nanoparticles. GQDs are fabricated with four different diameters: 12, 14, 16, and 27 nm. The geometrical features and lattice structures of the GQDs are determined using transmission electron microscopy (TEM). Hexagonal lattice fringes in the TEM image and G- and 2D-band Raman scattering evidence the graphitic characteristics of the GQDs. The oxygen content can be controlled by thermal reduction under a hydrogen atmosphere. In GQDs, the absorption peak wavelengths in the ultraviolet range tend to decrease as the size of the GQDs decreases. They also exhibit apparent photoluminescence (PL). The PL peak wavelength is approximately 600 nm and becomes shorter as the size of the GQDs decreases. The blue shift in the optical absorption and PL of the smaller GQDs is attributed to the quantum confinement effect. The proposed GQD fabrication method can provide a way to control the physical and chemical properties of GQDs via their size and oxygen content.

Figure 1

Illustration of the GQD fabrication process using self-assembled Au nanoparticles. (a) Substrate cleaning, (b) graphene transfer to the substrate, (c) deposition of the intermediate SiO₂ layer, (d) deposition of the Au thin film, (e) thermal annealing to form Au nanoparticles, (f) SiO₂ layer etching by CF₄ plasma, (g) graphene etching by O₂ plasma, and (h) removal of Au nanoparticles and SiO₂ layer.

Figure 2

Self-assembled Au nanoparticles. Scanning electron microscopy images of Au nanoparticles made from (a) 2 nm Au film annealed by 500 °C for 2 min, (b) 2.5 nm Au film annealed by 600 °C for 2 min, (c) 3 nm Au film annealed by 400 °C for 2 min, and (d) 5 nm Au film annealed by 300 °C for 30 s. (e) Histogram for the diameter of Au nanoparticles given in panels (a–d).

Figure 3

(a) Low-magnification TEM image of the fabricated GQDs dispersed on a TEM grid and (b) high-resolution TEM image of a single GQD. (c) FFT image for the area indicated by the white dotted box and (d) lattice fringes over the green dotted box of panel (b).

Figure 4

Raman spectra of (a) graphene and (b) as-fabricated GQDs.

Figure 5

Raman spectra of as-fabricated GQDs and thermally reduced GQDs.

Figure 6

Optical absorbance of graphene and GQDs with different diameters. The spectra are characterized by a blue shift for smaller diameters of the GQDs.

Figure 7

(a) Photoluminescence spectra taken at 10 K from GQDs with diameters of 12 and 14 nm. A shorter PL peak is observed in 12 nm diameter GQDs. (b) Trend of the PL peak energy depending on the diameter of GQDs. These results are consistent with the results in refs (−57). Reprinted in part from ref (55) with permission from Kim et al., ACS Nano 2012, 6, 8203–8208. Copyright 2012 American Chemical Society, from ref (56) with permission from Peng et al., Nano Lett. 2012, 12, 844–849. Copyright 2012 American Chemical Society, and from ref (56) with permission from Chhabra et al., RSC. Adv. 2018, 8, 11446–11454. Copyright 2018 Royal Society of Chemistry.