Tracking the Source of Carbon Dot Photoluminescence: Aromatic Domains versus Molecular Fluorophores

Abstract

Carbon dots (CDs) are an intriguing fluorescent material; however, due to a plethora of synthesis techniques and precursor materials, there is still significant debate on their structure and the origin of their optical properties. The two most prevalent mechanisms to explain them are based on polycyclic aromatic hydrocarbon domains and small molecular fluorophores, for instance, citrazinic acid. Yet, how these form and whether they can exist simultaneously is still under study. To address this, we vary the hydrothermal synthesis time of CDs obtained from citric acid and ethylenediamine and show that in the initial phase molecular fluorophores, likely 2-pyridone derivatives, account for the blue luminescence of the dots. However, over time, while the overall size of the CDs does not change, aromatic domains form and grow, resulting in a second, faster decay channel at similar wavelengths and also creating additional lower energetic states. Electrophoresis provides further evidence that the ensemble of CDs consists of several subsets with different internal structure and surface charge. The understanding of the formation mechanism enables a control of the chemical origin of these emitters and the ensuing optical properties of the CDs through synthetic means.

Solvent‐Controlled Synthesis of Highly Luminescent Carbon Dots with a Wide Color Gamut and Narrowed Emission Peak Widths

“…The results (Figure f; Table S2, Supporting Information) show that they can be fitted by a dual‐exponential formula, involving a short‐lived component τ 1 (about 1 ns) and a long‐lived component τ 2 (about 10 ns), which are assigned to the recombination processes in the core states and the surface states, respectively . It is clear that from B‐CDs to R‐CDs, as the average lifetime decreases from 10.2 to 2.1 ns, the τ 1 proportion increases from 40% to 88%, indicating that the role of core states in the radiative lifetime of these CDs increases with their PL redshift . The above results demonstrate that our CDs possess excellent and uniform optical properties, and their tunable PL emission can be ascribed to both the carbon cores and the surface states which are controlled by different reaction solvents.…”

Customizing the Electrochemical Properties of Carbon Nanodots by Using Quinones in Bottom‐Up Synthesis

“…[57][58][59] Thee mission is attributed to core and surface state related emission, in particular when Values were calculated from the onset of the reduction/oxidation peak as described in the Supporting Information. [57][58][59] Thee mission is attributed to core and surface state related emission, in particular when Values were calculated from the onset of the reduction/oxidation peak as described in the Supporting Information.…”

Manipulating the Optical Properties of Carbon Dots by Fine‐Tuning their Structural Features

“…This absorbance change in o-CDs is caused by partial destruction of the sp 2 hybridization network on oxidation, which decreases the probability of p-p*t ransitions, in analogy to systems with wide energyg ap. [15,19,36] Steady-stateP Lm easurements as af unctiono fe xcitation wavelength werep erformed, and the 3D spectral maps are shown in Figure 3c-e. [32][33][34][35] Energy-gap changes depending on the size of sp 2 domains can be found in previouse xperimental and theoretical studies.…”

“…The excitation wavelength was increasedf rom 250 to 530 nm in 20 nm increments. [11,36,[42][43][44][45][46] Thist rend in F is an indicatoro ft he strong impact of chemical treatment on the optical properties of HTC-CDs. [16,[37][38][39][40] Therefore, PL quantum yields F were recorded with 370 nm excitation versus ar eference solution of quinines ulfate.…”

“…Ta ble 2l ists the results from our least-squares fitting with the stretched exponential function DA~expÀ(t/t)b,i nw hich b is the stretching exponent, t the decay time constant, and t the time. [11,36,42,43,46] To analyze the excitation-wavelength dependence of the excited-state properties of the CDs, as seen in the PL spectra in FigureS7i nt he Supporting Information, we recorded the transient absorption spectra of the CDsw ith 310 nm excitation. Such ultrafast excited-state decay dynamics is typical for graphene and graphene oxide systems and recently investigated by temperature-dependent PL studies on triangularc arbonq uantum dots, all of which identified the main nonradiative relaxation mechanism as phonon-assisted relaxation to the ground state through the density of states of the sp 2 carbon network.…”