Unraveling the Fluorescence Mechanism of Carbon Dots with Sub-Single-Particle Resolution

Abstract

Unlike quantum dots, photophysical properties of carbon dots (CDs) are not strongly correlated with particle size. The origin of CD photoluminescence has been related to sp2 domain size and the abundance of oxidized surface defects. However, direct imaging of surface-accessible spatially localized oxidized defects is still lacking. In this work, solvothermal-synthesized CDs are fractionated into different colors by polarity-based chromatography. We then study the mechanism of CD fluorescence by directly imaging individual CDs with subparticle resolution by scanning tunneling microscopy. Density of states imaging of CDs reveals that the graphitic core has a large bandgap that is inconsistent with observed fluorescence wavelength, whereas localized defects have smaller electronic gaps for both red-emitting dots (rCDs) and blue-emitting dots (bCDs). For individual bCDs within our laser tuning range, we directly image optically active surface defects (ca. 1–3 nm in size) and their bandgaps, which agree with the emission wavelength of the ensemble from which the bCDs were taken. We find that the emissive defects are not necessarily the ones with the smallest gap, consistent with quantum yields less than unity (0.1–0.26). X-ray photoelectron spectroscopy and pH-dependent fluorescence titration show that oxygen-containing surface-accessible protonatable functional groups (e.g., phenolic −OH, −COOH) define the chemical identity of the defects. This observation explains why we detect neither long-lived optical excitation of the core nor a correlation between size and emission wavelength. Instead, control over the number of oxygen-containing defects defines the emission wavelength, with more oxidized defects at the surface producing redder emission wavelengths.

Excitation‐Independent Emission of Carbon Quantum Dot Solids

“…Fu and coworkers proposed that PAHs of different sizes within the carbogenic core account for the excitation-dependent photoluminescence (PL) of CQDs [8]. e dependence of PL on the excitation wavelength is wildly observed in CQDs [6,9,10], even on a single CQD [11], and it could be attributed to the existence of multiple emitters within CQDs [12,13], especially to MFs [14][15][16]. On the contrary, Wang [17] and Yuan [18] groups reported that the emission spectra of CQDs, except for emission intensity, are independent on the excitation wavelength, probably due to simplicity in CQDs' structure [18].…”

Oil-Dispersible Green-Emitting Carbon Dots: New Insights on a Facile and Efficient Synthesis

“…In fact, after the separation, both the types of CDs show similar absorption bands in the visible range, distinct from the chemical groups or structures forming the CDs, in agreement with [24]. However, the possibility of safely inferring the exact origin of the electronic transitions giving rise to these bands is not trivial: similar absorption bands above 300 nm are often related to the n-π * transitions of the surface-related chemical groups of the CDs that form a system of multiple discrete energy states [29,43,44,[47][48][49]. However, other reports [24,25,50,51] ascribe them to the π-π* and n-π* transitions, respectively, of the characteristic interconnected heteroaromatic units of the CDs.…”

“…The UV-Vis absorption spectra ( Figure 1b) clearly display a peak at around 260 nm, and two faint bands at 360 and 420 nm. In the literature, for CDs synthesized with diverse approaches, the signal below 300 nm in the absorption spectrum is commonly ascribed to the characteristic π-π* transitions of the carbogenic cores of the CDs, due to the conjugated sp 2 -carbon domains [27,29,[42][43][44][45]. Moreover, in such a spectral range, the π-π* absorption transitions of the polyheteroaromatic structures derived from pyridinium units of the CPC head-group can also occur.…”

Perception on aggregation induced multicolor emission and emission centers in carbon nanodots using successive dilution, anion exchange chromatography, and multi-way statistics

“…Mohsen Kompany-Zareh 1,2* & Saeed Bagheri 1 Exploration in the way of understanding the optical behavior and structure of carbon nanodots has been increased due to their vast application. Their emission dependency on excitation wavelengths is the more prevalent and controversial subject.…”

“…Since its emersion, fluorescence carbon nanodots (CND) entice the subject of material science, due to their exceptional optical and electronic properties, the availability of precursor materials, and ease of their synthesis [1][2][3][4][5][6][7][8][9][10] . In some cases, CNDs are considered among the quantum dots (QD) which are mainly synthesized by the heavy elements 3,6,9 .…”