Samples

The chemical structure of the two type of GQDs studied here is displayed on Fig. 1a. They are both made of 96 sp2 carbon atoms arranged in a triangular shape which leads to lateral sizes of ∼2 nm. Six alkyl chains (R = C 12 H 25 ) were introduced at the edges of the first GQD to enhance its solubility. The edges of the second GQD have been functionalized with chlorine atoms to modify the optical properties. In the following, we call the two structures C 96 and C 96 Cl GQDs, respectively. The details of the synthesis have been already reported3,11 and are described in the Supplementary Methods section. Briefly, the C 96 GQD is synthesized in two steps from the 1,3,5-triethynylbenzene and 2,5-diphenyl-3,4-di(4-dodecylphenyl)-cyclopentadien-1-one via Diels-Alder cycloaddition followed by oxidative cyclodehydrogenation in the presence of FeCl 3 (See Supplementary Fig. 1). The C 96 Cl GQD is synthesized in three steps from the 1,3,5-triethynylbenzene and 2,3,4,5-tetraphenyl-cyclopentadien-1-one via Diels-Alder cycloaddition followed by the oxidation of the dendrimer via the Scholl reaction and the chlorination of the bare C 96 particle by treatment with ICl in the presence of AlCl 3 (see Supplementary Fig. 1). The intermediate compounds are fully characterized and the GQDs are characterized by MALDI-TOF spectrometry to confirm the complete dehydrogenation and chlorination (See Supplementary Fig. 2). The GQDs are dispersed in 1,2,4-trichlorobenzene and then mixed with a solution of polystyrene (PS). The mixture is subsequently spin-coated on a coverslip to perform the optical experiments, using the experimental setup described in Fig. 1b. Complementary details about the sample preparation can be found in the Methods section.

Fig. 1 Photoluminescence of single GQDs. a Chemical structure of the C 96 and C 96 Cl GQDs. R stands for C 12 H 25 . b Scheme of the microphotoluminescence setup. c 20 × 20 μm2 PL map of the C 96 GQDs in polystyrene matrix. The color bar represents the number of counts per second on the APD. The scale bar is 3 μm. The zoom shows a diffraction limited spot that can be fitted with a 2D Gaussian function leading to a 1/e2 diameter of ∼600 nm. The scale bar is 200 nm. d Room temperature PL spectra of a single C 96 GQD (solid line) and of a single C 96 Cl GQD (dotted line). Polarization diagram in excitation (blue) and emission (red) Full size image

Photoluminescence spectroscopy

Figure 1c shows an example of a photoluminescence (PL) map of C 96 GQDs embedded in PS matrix for the highest dilution used in this study (see also Supplementary Fig. 10). One can observe bright spots (25 kcounts s−1 with 200 nW excitation) with diffraction limited size. The PL spectrum acquired on such a spot is displayed on Fig. 1d. For this particular C 96 GQD, the spectrum is composed of three lines, noted 1, 2, and 3, centered at 653, 719, and 797 nm, respectively. The full width at half maximum of the main line is of the order of 27 nm (80 meV). Note that the wavelength as well as the relative intensity of the PL lines slightly vary from one GQD to another, which is certainly due to differences in their local environments. Also, averaging over 25 GQDs gives an energy splitting of 170 ± 5 meV between lines 1 and 2, and 165 ± 10 meV between lines 2 and 3. Moreover, as mentioned in the introduction, one of the great potential of GQDs lies in the precise tuning of their electronic properties through the control of their structure. As a first example, the dotted curve in Fig. 1d shows the PL spectrum of a single C 96 Cl GQD whose emission wavelength has been shifted by the chemical functionalization of the edges with chlorine atoms. Here the functionalization leads to an almost 100 nm rigid redshift of the main PL line. This observation is in agreement with theoretical predictions that show a decrease of the optical gap of C 96 Cl in comparison with C 96 GQD due to the electronegativity of the chlorine atoms11. This result illustrates how the intrinsic properties of GQDs can be controlled by the synthesis.

In the following, we focus on C 96 GQDs containing the C 12 H 25 alkyl chains. First we investigate the nature of the quantum states at the origin of the three PL lines. The energy splitting between each line is ∼170 meV very close to the C=C stretching vibration mode. Moreover, theoretical works on this family of objects indicate the existence of low energy dark states, the lowest one being called the α band in the Clar’s notation12,13. The coupling with the vibrations of the lattice can lead to a brightening of these low energy states14. Here, we attribute the first PL line to the zero-phonon emission line of the α band followed up with vibronic replicas, with a quantum of vibration ħΩ ∼ 170 meV. We also performed photoluminescence excitation (PLE) experiments in solution (See Supplementary Fig. 4). The PLE curves detected on the two highest energy PL lines of GQD superimpose well with the absorption spectrum of the solution. Besides, the PLE highlights the existence of two lines at 580 and 630 nm. The energy splitting between those states is also ∼170 meV revealing the Franck–Condon series of vibronic lines. The intense absorption line at high energy could be tentitatively attributed to the β band in the Clar’s notation12,13. Finally, the polarization response of a single GQD is shown in the inset of Fig. 1d. Here, the emission polarization profile is recorded on the line 1 at ∼650 nm. It is linearly polarized at a fixed direction certainly related to the geometry of the GQD. Likewise the excitation diagram is also linearly polarized in the same direction than the emission one. Therefore, it can be concluded that absorption and main emission dipoles are parallel. Further investigations, including theoretical modeling, are needed to explain this experimental observation.

Second-order correlation function

In order to identify the number of emitters associated with such diffraction limited spot and spectrum, we measured the second-order correlation function (g(2)(τ)) at room temperature integrating over the entire spectrum. As displayed in Fig. 2a, the strong antibunching observed at zero delay, g(2)(0) < 0.1, is a proof that a single emitter is detected. This is in strong contrast with the results of such experiments performed on top–down GQDs where no antibunching is observed9. In these earlier studies, the absence of antibunching is interpreted as a consequence of the extrinsic nature of the states at the origin of the luminescence, leading to multidefect sites emitting in an uncorrelated manner9. We have performed measurements on more than 30 GQDs, all of them leading to g(2)(0) < 0.1 (see examples on Supplementary Fig. 5). Moreover, these correlation measurements being performed by integrating all the wavelengths on the detector, implies that the spectrum described above actually arises from a single GQDs and not from several objects. Moreover, the weak value observed for the g(2)(0) is an indication of the good purity of single-photon emission associated with single GQD. This result enforces GQD as an interesting alternative to other single emitters15, such as defects in WSe 2 16,17,18,19,20, in h-BN21,22,23 or in carbon nanotubes24.

Fig. 2 Photophysics of a single GQD. a Second-order correlation function g(2)(τ) recorded from a diffraction limited spot such as the one of Fig. 1 (black dots), showing a strong antibunching. A fit (red line) with a function \(1 - (1 - b)e^{ - |\tau |/\tau _1}\) yields g(2)(0) = 0.05 ± 0.05 and a characteristic time of τ 1 ∼ 3.5 ns (FWHM of the IRF of the detector ∼0.9 ns). b Time-resolved PL of a single GQD (black dots) detected on the whole spectrum, fitted by a mono-exponential decay (red line) with a time constant τ = 5.37 ns. c Saturation curve of a single GQD (black dots) as a function of the pump power. A fit by Eq. (1) (red line) leads to a saturation power density of 28 kW cm−2 and a saturation intensity I sat ∼ 9.7 Mcounts/s. d PL time trace of a stable GQD over 100 min with a binning time of 200 ms. Fluctuations are due to setup instabilities. Zooms are shown on shorter timescale with a binning time of 10 ms Full size image

Photophysical properties

The fact that we are actually observing single objects allows us to characterize other important properties. First, another figure of merit of a single quantum emitter is its brightness. A saturation curve of a single GQD is displayed on Fig. 2c. The intensity is fitted by

$$R = R_{{\mathrm{sat}}}{\mathrm{/}}\left( {1 + \frac{{I_{{\mathrm{sat}}}}}{{I_{{\mathrm{exc}}}}}} \right),$$ (1)

with R sat the count rate at saturation, I sat the incident power density at saturation and I exc the incident power density. The fit leads to R sat ∼ 9.7 Mcounts s−1, and I sat = 28 kW cm−2. This value of count rate at saturation can be compared to other new quantum emitters such as defects in 2D materials. In this context, L.J. Martínez et al. have compared on the same setup the count rate at saturation of N-V centers and single defects in h-BN22. We decided to employ the same method in order to quantitatively compare GQDs and single defects in h-BN (see Supplementary Note 3 for details). It turns that GQDs are, at least, as bright as the brightest single-photon source found in 2D materials. Therefore, it puts GQDs in the highest values of brightness among other quantum emitters15,22,23. Next, we address the question of the photostability of GQDs. In this perspective, GQDs have also good properties. Indeed, photostability up to hours have been observed for an incident power of 200 nW (0.12 kW cm−2) and an emission rate of ∼20 kcounts.s−1 (see Fig. 2d and Supplementary Note 5). This encouraging result could even be improved by engineering the surrounding matrix, as it has been done for small molecules25,26. Moreover, Fig. 2d shows that no blinking is observed on the time trace of the luminescence of this GQD. The histogram of intensity fits well to a normal distribution (see Supplementary Note 4 and Supplementary Fig. 7). This observation is in strong contrast with numerous single emitters that undergo quantum jumps between bright and dark states. Nevertheless, the luminescence of GQDs end up disappearing. Supplementary Fig. 8 shows two different time traces around the time of bleaching. In the first one, the luminescence drops down to zero sharply. On the contrary, the second one goes through an intermediate gray state before ending up to zero. These two behaviors are representative of what we observed on the GQDs. The discrete intensity jumps are also characteristic of single-emitter experiments. In order to get insights in the recombination dynamics, time-resolved photoluminescence (TR-PL) experiments on single GQDs have been performed (see Fig. 2b and Supplementary Fig. 9). Here, the signal can be fitted by a mono-exponential decay with a time constant τ ∼ 5.37 ns. We have performed such experiments on several GQDs. The TR-PL signal is always mono-exponential with relaxation times ranging from 3 to 5.5 ns.

Finally, in order to get more information on the photophysics of GQDS, we studied the g(2) function on a longer timescale. In particular, such measurement allows highlighting ISC dynamics between singlet and triplet states27. This approach is also supported by calculations on GQDs showing that at least one triplet state is lying few hundreds of meV below the singlet state28,29. The g(2) functions recorded under different excitation density both at short and long-time delays are shown, respectively, in Fig. 3a, b. At short delays, one observes an acceleration of the dynamics of the g(2) function when the pump is increased. Likewise, at longer delays photon bunching (g(2) > 1) is observed with a relaxation down to g(2) = 1 within tens of microseconds. These data are well fitted assuming a three-level system model, as shown in the inset of Fig. 3b where state 3 stands for the triplet state. It leads to the evaluation of the transition rates: k 21 ∼ 0.28 ns−1, k 23 ∼ 0.025 μs−1, and k 31 ∼ 0.057 μs−1 (see Supplementary Note 1 for the details of the photophysics). The ISC yield k 23 /k 21 being small (∼10−4) and the triplet lifetime being short (1/k 31 ∼ 18 μs), the effect of the triplet on the emission efficiency is very limited. It can explain both the high brightness of GQDs and the absence of measurable blinking (see also Supplementary Fig. 12). Finally, we also provide an estimation of the absorption cross section of the C 96 GQD (σ ≃ 1.0 × 10−14 cm2), as well as of the PL quantum efficiency η Q above 35% (see Supplementary Note 2).

Fig. 3 Photons bunching of a single GQD. a, b g(2) functions of a single GQD, for two different excitation powers, 2 μW (red square) and 10 μW (blue diamond). a Zoom on short delays; b Full timescale intensity correlation. The solid line is a fit to the function \(1 - (1 + a)e^{ - \lambda _1|\tau |}\) + \(ae^{ - \lambda _2|\tau |}\), convolved by the time response of the detector. The three-level system used as a model is shown Full size image

To conclude, the good properties of GQDs: single-photon emission, brightness, and photostability show that they are very promising materials for applications requiring single emitters. More generally, the results reported here demonstrate that the high potential of GQDs revealed by theory is now accessible experimentally. In particular, the observation of single GQD emission paves the way to studies linking their properties to their structure. In addition, our results show that the modification of the properties through the control of the structure is indeed achievable. Therefore, one can now really imagine to perform engineering of other properties such as the spin structure in order to rend it optically detectable and controllable.