Multicolor Nitrogen-Doped Carbon Quantum Dots for Environment-Dependent Emission Tuning

Abstract

Carbon quantum dots (CQDs) have potential applications in many fields such as light-emitting devices, photocatalysis, and bioimaging due to their unique photoluminescence (PL) properties and environmental benignness. Here, we report the synthesis of nitrogen-doped carbon quantum dots (NCQDs) from citric acid and m-phenylenediamine using a one-pot hydrothermal approach. The environment-dependent emission changes of NCQDs were extensively investigated in various solvents, in the solid state, and in physically assembled PMMA-PnBA-PMMA copolymer gels in 2-ethyl-hexanol. NCQDs display bright emissions in various solvents as well as in the solid state. These NCQDs exhibit multicolor PL emission across the visible region upon changing the environment (solutions and polymer matrices). NCQDs also exhibit excitation-dependent PL and solvatochromism, which have not been frequently investigated in CQDs. Most CQDs are nonemissive in the aggregated or solid state due to the aggregation-caused quenching (ACQ) effect, limiting their solid-state applications. However, NCQDs synthesized here display a strong solid-state emission centered at 568 nm attributed to the presence of surface functional groups that restrict the π-π interaction between the NCQDs and assist in overcoming the ACQ effect in the solid state. NCQD-containing gels display significant fluorescence enhancement in comparison to the NCQDs in 2-ethyl hexanol, likely because of the interaction between the polar PMMA blocks and NCQDs. The application of NCQDs-Gel as a solid/gel state fluorescent display has been presented. This research facilitates the development of large-scale, low-cost multicolor phosphor for the fabrication of optoelectronic devices, sensing, and bioimaging applications.

Figure 1

(a) Hydrothermal synthetic route for the preparation of multicolor NCQDs, (b) photographs of NCQDs dispersed in water in daylight (left) and under the illumination of 365 nm (right), and (c) illustration showing the hydrogen bonding of polar protic and aprotic solvent molecules with surface functional groups of NCQDs.

Figure 2

(a) TEM image of a single NCQD at a higher magnification. (b) Powder XRD pattern of NCQDs.

Figure 3

Characterization of NCQDs: (a) FTIR spectrum, (b) XPS full survey, and high-resolution XPS spectra for (c) C 1s, (d) N 1s, and (e) O 1s.

Figure 4

(a) UV–visible spectrum of NCQDs in water and (b) the corresponding plot for (αhυ)² versus photon energy, E. The horizontal intercept of the tangent in (b) indicates the band gap of NCQDs. (c) PL emission spectra of NCQDs for different excitation wavelengths and (d) the corresponding normalized PL emission spectra.

Figure 5

(a) Absorption spectra of NCQDs in polar protic solvents; (b) normalized emission spectra of NCQDs in different solvents excited at a wavelength of 360 nm; (c) emission maxima as a function of dielectric constants of solvents; and (d) Lippert–Mataga relationship capturing orientational polarization of hydrogen bonding solvents versus Stokes shift.

Figure 6

CIE diagram of NCQD dispersion in (a) acetone, (b) tetrahydrofuran, (c) dimethylformamide, (d) methanol, and (e) water with different excitation wavelengths.

Figure 7

(a) Fluorescence intensity decay and model fit in different solvents (excitation at 360 nm and emission at 450 nm). (b) Solid-state absorption and PL emission spectrum of the NCQD powder. (c) CIE diagram of the NCQD powder. (d) Fluorescence emission photographs of the NCQD powder under the illumination of visible and UV 365 nm light.

Figure 8

Images of (a) NCQDs in 2-ethyl-1-hexanol, (b) pristine gel, (c) NCQDs-Gel, (d) NCQDs/PnBA, (e) NCQDs/PMMA, and (f, g) NCQDs-Gel in a three-dimensional (3D) printed mold under daylight and under 365 nm UV light.

Figure 9

Temperature-dependent PL spectra of (a) NCQDs in the 2-ethyl-1-hexanol solution, (b) NCQDs-Gel, (c) NCQDs-PnBA, and (d) NCQDs-PMMA.