1 Introduction
Carbon quantum dots (CQDs) generally refer to zero-dimensional carbon materials with a particle size of less than 10 nm, but there are also reports that fluorescent carbon nanomaterials with a particle size of 60 nm are called CQDs
[1]. According to different preparation conditions (such as carbonization temperature, etc.), CQDs are composed of crystalline or amorphous carbon cores hybridized by sp
2 or sp
3 and functional groups attached to their surfaces
[2,3]. At the same time, graphene quantum dots (generally 1 to 4 layers) with a complete crystal structure have similar structural and optical properties to CQDs, and are also commonly referred to as CQDs.
CQDs were first reported by Scrivens et al. In 2004. In the process of purifying single-walled carbon nanotubes prepared by arc discharge method, they found CQDs that could emit bright fluorescence
[4]. According to the different carbon sources and synthesis principles, the synthesis methods of CQDs can be generally classified into two types: top-down method and bottom-up method. The so-called top-down method is to strip bulk materials (such as graphite, plastic and wood) into CQDs with particle size less than 10 nm by etching or electrolysis
[5⇓~7]. The bottom-up approach is to polymerize small molecular carbon sources (such as citric acid, glucose, sucrose and amino acids) into CQDs by microwave, hydrothermal or combustion methods
[8⇓⇓~11]. Recently, the synthesis of CQDs by template method (such as MOF or SiO
2 as template) and low temperature self-polymerization method (such as room temperature aldol condensation method) have been reported
[12][13].
Compared with inorganic semiconductor quantum dots (such as CdSe, GaIn, etc.), CQDs have relatively low toxicity, which can avoid potential harm and damage to the environment. It has the characteristics of excellent optical performance, stable and adjustable luminous performance (for example, Sun and others can greatly improve the optical performance of CQDs after passivating the surface of CQDs with polyethylene glycol).Due to their good biocompatibility and unique electronic structure, CQDs have been widely used in the construction of biological imaging probes, the development of optoelectronic devices, energy storage and conversion, the preparation of biological composite materials, photocatalysis and biosensing
[14][15⇓~17][18⇓~20][21⇓~23][24⇓~26][27⇓~29][30⇓⇓~33].
In order to more precisely control the optical properties of CQDs, better realize the application of CQDs in different fields, and further develop the new application scope of CQDs, the key is to clarify the Photoluminescence (PL) mechanism of CQDs. Various mechanisms have been reported to explain the luminescence process of CQDs, such as quantum confinement effect, surface state emission, carbon core and fluorescent molecule emission, polycyclic aromatic hydrocarbon molecule emission, slow solvent relaxation and solvation effect, self-trapped exciton model, surface dipole emission center and multiple emission center. However, the above luminescence mechanisms of CQDs can only explain part of the PL phenomena of CQDs to a certain extent, and there is no mechanism that can fully explain all the PL phenomena of CQDs, which seriously restricts the regulation of the optical properties of CQDs. For example, (1) It has been reported that PL originates from carbon core, surface functional groups or defect States, but in the process of CQDs luminescence, it is usually accompanied by electron transfer, changes in internal structure and special reactions.The unique structure of CQDs may dominate or influence the PL emission process to a great extent, such as the formation of dipoles, electron-hole complexes and so on, which may be the PL emission centers of CQDs
[33][34]. (2) The relaxation effect of the solvent can also affect the luminescence mechanism of CQDs. The relaxation effect of the solvent generally exists with the protonation or deprotonation process of CQDs, which makes the luminescence mechanism of CQDs more complex
[35,36]. (3) Solid-state luminescent CQDs have also been reported, and this special luminescent phenomenon is related to the formation of hydrogen bonds between molecules and the special aggregation structure
[37][38][39].
At present, most of the review articles on CQDs focus on the preparation methods, properties and applications, but few review the PL mechanism of CQDs. In this paper, the optical properties and different PL mechanisms of CQDs are classified, summarized and summarized, in order to further clarify the PL mechanism and provide a reference for the controllable adjustment of the optical properties of CQDs
[11,17,22,56].
2 Optical properties of CQDs
CQDs have a variety of optical properties, such as UV-visible absorption, photoluminescence, electroluminescence and up-conversion fluorescence. Among them, UV-visible absorption and photoluminescence are the most common optical properties of CQDs, which have been widely studied because they are easy to control.
2.1 Ultraviolet-visible absorption characteristic
CQDs generally show strong absorption in the ultraviolet region (200-400 nm). Due to different preparation conditions, in some cases, the absorption of CQDs can be broadened to the visible region, e.g., up to 600 nm (Fig. 1A). The absorption spectra of CQDs generally show two absorption peaks with different intensities, located at 200 ~ 260 nm and 280 ~ 350 nm, respectively. The CQDs synthesized under certain conditions also showed obvious absorption peaks in the visible light region (400 ~ 600 nm)
[23]. The strong absorption peak in the range of 200 ~ 260 nm is generally attributed to the π-π
* transition of the C = C bond (carbon core, sp
2domain) hybridized by the sp
2 in CQDs
[42⇓~44]. The particle size of CQDs has a great influence on the position of the absorption peak, and the absorption peak has a significant red shift with the increase of particle size
[45,46].
The absorption peak in the range of 280 ~ 350 nm is usually attributed to the n-π
* transition of oxygen-containing functional groups (such as — C = O, — OH, etc.) On the surface of CQDs and other surface groups
[42⇓~44]. Generally speaking, the position and intensity of the absorption peak are affected by many factors, such as the oxygen content of the surface functional group, the type of solvent and the pH value of the medium
[47⇓~49]. An increase in the oxygen content of the surface functional group usually leads to a decrease in the band gap, resulting in a red shift in the position of the absorption peak and an increase in the absorption intensity, which is also used to regulate the absorption characteristics of CQDs. The effect of solvent type on the absorption peak is mainly due to the different dipole moment produced by the interaction of polar solution with CQDs, which leads to the red shift of the absorption peak. The more polar solvents red-shift the absorption peak to the visible region (fig. 1b). The influence of medium pH on the absorption characteristics of CQDs is mainly due to the protonation and deprotonation of some functional groups on the surface of CQDs caused by the change of pH, which leads to the change of the absorption peak position.For example, in alkaline environment, the surface groups of CQDs, such as some oxygen-containing functional groups, will deprotonate, resulting in the increase of surface negative charge and the red shift of the absorption peak.
There are still some disputes about the attribution of the absorption peaks in this region, although many literatures have reported that the absorption peaks in this region are attributed to the n-π
* transition of the surface groups, but there are also different views.For example, it is reported that the absorption peak originates from the π-π
* transition at the edge of the carbon core and the n-π
* transition of the surface group, and it is also reported that the absorption peak originates from the molecular fluorescent group on the surface of CQDs (such as 5-oxo-1,2,3,5-tetrahydroimidazo [1,2-α] pyridine-7-carboxylic acid (IPCA) and citrazinic acid 3,5-substituted derivatives, etc.), which are generated during the synthesis of CQDs. For example, IPCA can be generated on the surface of CQDs when citric acid and ethylenediamine are used as precursors, and citrazinic acid and its 3,5-substituted derivatives can be generated on the surface of CQDs when citric acid and tetramethylenediamine are used as precursors
[50,51][52⇓~54][52,53][54]. Therefore, the attribution of the absorption peak in this region needs to be further explored and clarified.
The absorption peak in the visible region is usually caused by the surface aggregation of CQDs, and the intensity of this absorption peak increases with the increase of CQDs concentration (Fig. 1C)
[55]; However, some studies have shown that the introduction of some functional groups (such as —NH
2) on the surface of CQDs by passivation may also produce absorption peaks in this region (Fig. 1D)
[23].
2.2 Fluorescence characteristics (photoluminescence characteristics)
The fluorescence characteristic of CQDs, namely photoluminescence (PL), means that after being irradiated by a certain external light source, the electrons in CQDs will be excited by the action of photons and jump to the excited state with higher energy. The electrons in the excited state are not stable and will return to the energy level with lower energy. This process will release part of the energy in the form of luminescence (Figure 2a). The excellent PL properties of CQDs are one of the important reasons for the wide application prospects of CQDs. Compared with inorganic perovskite quantum dots and metal quantum dots, the fluorescence properties of CQDs are more stable and resistant to photobleaching, and easy to control. These characteristics make CQDs have broad application prospects in the fields of optics and biology.
One of the remarkable fluorescence characteristics of CQDs is that their PL emission has a unique excitation light dependence, that is, with the change of excitation light wavelength, the wavelength and intensity of their emission light also change
[14,56]. When the wavelength of the excitation light is increased, the wavelength of the emission light is red-shifted; Before the optimal excitation wavelength, the intensity of the emitted light increases with increasing excitation wavelength and then decreases with increasing excitation wavelength (fig. 2b). Therefore, the emission wavelength of CQDs can be red-shifted to the near-infrared region by adjusting the excitation wavelength, and then the polychromatic emission of CQDs can be realized, which provides the possibility for polychromatic imaging technology in biological and optical fields.
The reason why the emission of CQDs is affected by its excitation light has not been explained in a unified way. It has been reported that the excitation light-dependent fluorescence characteristics are related to the wide particle size distribution of the prepared CQDs, and the wide particle size distribution will have multiple energy gaps, resulting in multiple transition modes
[57]; It has also been reported that this dependence is caused by the functional groups on the surface of CQDs, because different functional groups will lead to the existence of multiple emission centers, so at a certain excitation wavelength, some emission centers will dominate
[58]. Similar to its absorption performance, the PL performance of CQDs is also affected by its particle size, the amount of oxygen, the type of solvent and the pH value of the medium
[45⇓⇓~48]. (1) With the increase of the particle size of CQDs, the PL emission wavelength of CQDs is red-shifted, which is also the basis for the realization of multi-color emission of CQDs; (2) The change of oxygen-containing groups on the surface of CQDs will affect the distribution of surface charges, and further change the size of the surface electronic transition band gap. For example, the increase of surface oxygen content will not only make the PL emission of CQDs red-shift, but also increase its PL intensity, on the contrary, the decrease of surface oxygen content will lead to the blue shift of PL emission and the decrease of its intensity; (3) The influence of solvent on the emission of CQDs is mainly caused by the relaxation effect of solvent and the dipole recombination effect caused by the interaction between solvent and CQDs. The increase of solvent polarity usually leads to the red shift of PL emission, which is also one of the ways to realize the polychromatic emission of CQDs. (4) The change of pH value of the medium will cause the protonation and deprotonation of CQDs, for example, the increase of pH value will cause the deprotonation of CQDs surface, thus accumulating a large number of negative charges on its surface.However, there is also an opposite view that the increase of pH value will lead to the blue shift of PL emission. For example, the PL emission of CQDs prepared by He and others using citric acid and basic fuchsin as precursors has a significant blue shift with the increase of pH value
[29].
2.3 Lectroluminescent characteristic
In addition to the usual absorption and photoluminescence properties, CQDs exhibit excellent electroluminescence properties. Electroluminescence (EL) refers to the phenomenon that under the action of a certain electric field, the electrons in CQDs are excited to transition back to a lower energy level and release energy to produce fluorescence.
Chi et al. First reported that CQDs have EL properties
[59]. They prepared CQDs by electrolyzing graphite rods and loaded them on the surface of Pt electrodes, and found that there was no EL response, but the loaded CQDs showed obvious EL response (Fig. 3A), indicating that CQDs could produce EL phenomenon. Because the observed EL phenomenon is similar to the EL phenomenon of quantum dots previously studied, they proposed that the EL mechanism of CQDs is that under the action of an electric field, electrons with negative charge (R
·-) and positive charge (R
·+) collide and annihilate to produce photons, forming excited state CQDs(R
*). When the excited state CQDs return to the ground state, part of the energy is emitted in the form of photons to produce fluorescence. Subsequently, it was reported that CQDs prepared by microwave treatment of a mixed solution of polyethylene glycol and glucose (fructose) and nitric acid etching of activated carbon could also produce EL phenomenon, and a similar explanation for the EL mechanism was given, which verified Chi et al
[60,61].
There are still some controversial views about the EL mechanism. Feng et al. compared the EL responses of oxidized CQDs (o-CQDs, with a high degree of surface oxidation) and reduced CQDs (r-CQDs, with a low degree of surface oxidation) in a K
2S
2O
8 solution system, and found that r-CQDs had a low EL activity, so they proposed that the EL was related to the surface state of CQDs
[62]. They believe that the "loose shell" of o-CQDs with oxygen-containing groups contributes to the generation of electrons from o-CQDs radicals, while the reduction process of S
2 in solution releases a strong oxidant,
radicals, which accept electrons from o-CQDs radicals to form EL emitters, resulting in higher EL activity of o-CQDs than r-CQDs.
Studies have shown that EL performance can be tuned by changing the voltage, resulting in higher output brightness and multicolor emission (Fig. 3B)
[63]. Therefore, the application of CQDs in the field of optoelectronic devices is widened.
2.4 Upconversion luminescence
Usually, when an electron absorbs energy and jumps to a higher energy level, it will release part of its energy in the process of returning to a lower energy level, so the energy of the emitted light will be lower than that of the excited light. However, subsequent studies have shown that some special CQDs can be excited by low-energy light and emit high-energy light, which is up-conversion luminescence. In 2007, Sun et al. First discovered that CQDs obtained by laser etching of carbon targets and surface passivation with polyethyleneimine-ethyleneimine as a passivator could emit visible light under 800 nm laser
[64]. Therefore, the up-conversion luminescence property of CQDs was confirmed. Zong et al. also reported that CQDs obtained by calcining citric acid at high temperature using mesoporous SiO
2 as a template had the similar luminescence phenomenon mentioned above, and showed obvious excitation wavelength dependence. When the excitation wavelength increased from 700 nm to 800 nm, the emission wavelength of CQDs shifted from 393 nm to 468 nm (Figure 4)
[66].
At present, the general explanation for the mechanism of the upconversion fluorescence property of CQDs is a multi-photon excitation process. After absorbing two or more photons, CQDs acquire more energy, so when the electrons return to a lower energy level, they will emit photons with higher energy, resulting in a shorter emission wavelength than the excitation wavelength in a short time (anti-Stokes type) (Figure 5)
[63⇓⇓~66]. CQDs with up-converted fluorescence have broad application prospects in biological imaging probes because of their strong deep tissue penetration (weak light scattering), less damage to cells (low energy) and weak tissue absorption.
3 PL mechanism for CQDs
In order to make better use of the optical properties of CQDs and explain their PL phenomena, several PL mechanisms of CQDs have been reported, including quantum confinement effect, surface state emission, carbon core and fluorescent molecule emission, polycyclic aromatic hydrocarbon molecule emission, self-trapped exciton emission, surface dipole emission center, multiple emission center, slow solvent relaxation and solvation effect. Since CQDs (i.e., carbon dots) and graphene quantum dots sometimes follow different PL mechanisms, in the following discussion, we will only focus on the PL mechanism of CQDs unless otherwise specified.
3.1 Quantum confinement effect
The typical particle size distribution of CQDs (< 10 nm) is comparable to that of conventional quantum dots. Therefore, it has been reported that the PL emission of CQDs also originates from the quantum confinement effect
[68⇓⇓⇓~72]. The quantum confinement effect means that when the size of the particle reaches the nanometer level, the electronic energy level near the Fermi level splits into discrete energy levels from the continuous state, and the energy gap is related to the size of the particle.
Tan et al. synthesized CQDs emitting different fluorescence
[68]. TEM showed that the particle sizes of blue (B-), green (G-), yellow (Y-), orange (O-) and red (R-) CQDs were 1.95, 2.41, 3.78, 4.90 and 6.68 nm respectively, and the maximum PL emission peaks of B-, G-, Y-, O- and R- CQDs were located at 430, 513, 535, 565 and 604 nm respectively (Figure 6a). According to the absorption spectra and UPS calculation, the band gaps of the corresponding CQDs are 3.02, 2.61, 2.49, 2.30 and 2.12 eV, respectively, indicating that the increase of particle size will lead to the decrease of their band gaps, so the different fluorescence wavelengths emitted by CQDs are caused by their different band gaps (Fig. 6B). Li et al. Used electrochemical method to electrolyze graphite rod to prepare CQDs, and also observed that the PL emission was affected by the band gap of CQDs
[69].
Density functional theory (DFT) calculations using graphene quantum dots as a model show that the particle size can significantly affect the wavelength of their fluorescence emission (Fig. 6C)
[73]. The band gap of graphene quantum dots with single benzene ring to 20 benzene rings decreases from 7 to 2 eV (Fig. 6d), indicating that the band gap of graphene quantum dots decreases with the increase of particle size
[74]. This conclusion is also applicable to CQDs, which further confirms that the emission of CQDs originates from the quantum confinement effect.
3.2 Surface state emission
Although the view that PL emission originates from quantum confinement effect has been reported in many literatures, Xiong et al. Proposed that PL originates from surface state emission
[75]. In order to verify this idea, they used urea and o-phenylenediamine to prepare CQDs by hydrothermal method, and separated and purified them by silica gel column chromatography technology to obtain CQDs emitting different fluorescence. Through a variety of characterization methods to analyze CQDs emitting blue, green, yellow and red fluorescence, the following conclusions were drawn: (1) From the TEM image analysis, it can be seen that the four CQDs have similar particle size distribution, with an average particle size of about 2.6 nm. (2) From the FT-IR and XPS spectra, the intensity of — COOH peak increases and the intensity of — OH peak decreases with the emission of CQDs from blue to red, which indicates that — COOH on the surface can dominate the red emission, while — OH dominates the blue emission. With the increase of oxygen content, the band gap of CQDs decreases gradually (as shown in Fig. 7A, B), so they propose the following emission mechanism: the emission center is located on the surface of CQDs, mainly composed of conjugated carbon atoms and bonded oxygen atoms.
Chen et al. Also reported the contribution of surface States to PL emission. They synthesized CQDs with different emission wavelengths by changing the precursors (p-phenylenediamine, o-phenylenediamine, m-phenylenediamine), and had similar particle size distribution. Subsequently, tartaric acid (TA) was added to m-phenylendiamine or o-phenylendiamine solution as a precursor to observe the red shift of PL emission
[76]. After the addition of tartaric acid, the content of carbonyl and amide groups on the surface increased, indicating that such groups can realize the fluorescence emission of CQDs in the long wavelength region (fig. 7c). The above results reveal that the PL is related to the surface state and the emission wavelength depends on the type of surface groups, which is also confirmed by the relevant literature
[77⇓⇓~80].
3.3 Fluorescent Molecules and Carbon Nuclei
In order to explain the effect of temperature on the formation process of CQDs, Giannelis et al. First proposed that the PL mechanism of CQDs obtained by pyrolysis is related to the pyrolysis temperature
[81]. CQD is prepared by pyrolyzing a mixture of citric acid monohydrate and ethanolamine. When the pyrolysis temperature is low (for example, 180.degree. C.), CQDs cannot be formed, but CQDs precursors are formed because CQDs particles are not observed by both dynamic light scattering and transmission electron microscopy, and the PL emission of the sample shows excitation-independent characteristics. The precursor contains some organic molecular fluorophores, and the organic molecular fluorophores have influence on the PL, and the fluorescence quantum yield reaches 50%; When the pyrolysis temperature reaches 230 ℃, the PL quantum yield decreases to 15%, and the dependence of excitation wavelength appears. When the temperature increases to 300 or even 400 ℃, the CQDs precursors polymerize and grow to produce carbon nuclei, and the fluorescence quantum yield continues to change. Elemental analysis showed that the carbon content continued to increase with increasing temperature. They proposed the following mechanism: when the carbonization temperature is low, the emission comes from the molecular fluorophore, and when the carbonization temperature is higher (400 ℃), the carbon core-dominated luminescence becomes the main emission mechanism. When the carbonization temperature is between these two temperatures, both the carbon core and the molecular fluorophore affect the luminescence of CQDs (Fig. 8 a).
Unlike the high-temperature pyrolysis method described above, Yang et al. Synthesized CQDs by a hydrothermal method using citric acid and ethylenediamine
[82]. In the case of low hydrothermal temperature (below 150 ℃), a fluorescent molecule IPCA was found. The characterization results show that IPCA is the PL emission center, and the high quantum yield of this type of CQDs may be due to the presence of IPCA molecule. At elevated temperatures (150 ~ 200 ℃), CQDs are shown to be a mixture of IPCA, polymer, and carbon core, at which time all three structural States contribute to PL. At higher temperatures (above 200 ° C), the fluorescent molecules and polymer intermediates are difficult to capture, and the characterization shows that the PL emission originates mainly from the carbon core (Fig. 8B). In addition, the mechanism of fluorescence induced by fluorescent molecules and carbon nuclei has also been reported and confirmed in the relevant literature
[83⇓⇓~86].
3.4 Molecular emission of polycyclic aromatic hydrocarbon
In order to explain the origin of PL from a more microscopic structural level, the emission mechanism of polycyclic aromatic hydrocarbons has been reported in the literature after the report of fluorescent molecular emission
[87⇓⇓~90]. Polycyclic aromatic hydrocarbons (PAHs) refer to hydrocarbons that contain two or more benzene rings and are arranged in linear, angular or cluster forms. The most common ones are anthracene, acenaphthene and pyrene. Urban et al. Confirmed the molecular emission mechanism of polycyclic aromatic hydrocarbons through experiments. CQDs used in the experiments were synthesized by hydrothermal reaction of citric acid and ethylenediamine
[87]. They performed single-particle tests on CQDs and showed excitation-dependent PL properties, indicating the presence of multiple chromophores in a single CQDs. Because the absorption and PL spectra of anthracene (3 rings), pyrene (4 rings), and acenaphthene (5 rings) overlap with those of CQDs, and their structures are relatively simple (Fig. 9a, B). According to previous reports on the structure of CQDs, they chose these three basic PAHs to simulate the sp
2C region of CQDs, chose polymethyl methacrylate as the sp
3 hybrid carbon matrix, and fixed the molecular ratio of anthracene, pyrene, perylene and polymethyl methacrylate at 10:10:1:20. It can be seen that the absorption spectra of the scaled PAHs films exhibit a similar shape to that of the actual CQDs (Fig. 9c), and it is observed that the PL spectra of the PAHs films are very similar to those of CQDs at excitation wavelengths of 420, 440 and 480 nm, respectively, suggesting a contribution of PAHs to the PL emission.
They gave the following explanation for the emission mechanism of PAHs-induced luminescence: photoexcited excitons can induce the distortion or reorientation of PAHs; The exciton and the distorted structure combine to form a polaron. The exciton becomes locally stabilized to form a self-trapped exciton, thereby dominating the generation of PL (fig. 9d). The excitation dependence of CQDs is also related to the presence of PAHs, PAHs with smaller band gaps emit longer wavelength PL, and different PAHs absorb different wavelengths of excitation light differently.Under short wavelength excitation, the self-trapped exciton in anthracene molecule is preferentially excited and produces fluorescence, while under long wavelength excitation, acenaphthene and pyrene are excited, resulting in red shift of fluorescence, thus showing different degrees of Stokes shift.
Subsequently, Urban et al. Further improved the molecular emission mechanism of polycyclic aromatic hydrocarbons, and they believed that the aromatic domain structure formed by the combination of polycyclic aromatic hydrocarbons and the fluorescent molecular precursors dominated the PL emission at different stages
[88].
In the initial stage of the hydrothermal reaction, the fluorescent molecular groups (such as 2-pyridone derivatives) dominate the emission of blue fluorescence. With the increase of hydrothermal time, the fluorescent molecular groups continue to polymerize to form polycyclic aromatic hydrocarbons, and eventually form aromatic domain structures, which dominate the formation of PL (Fig. 10). The electrophoresis results also show that there is a transition from fluorescent molecular group to aromatic domain structure in different stages of hydrothermal process.
3.5 Self-trapped exciton emission
Recent reports indicate that exciton self-trapping plays an important role in the emission process of CQDs. For example, Sun et al. Revealed the emission mechanism of CQDs by polarization anisotropy spectroscopy, electric field modulation spectroscopy and time-resolved PL measurements, and provided evidence that the emission comes from self-trapped excitons
[91]. They believe that the self-trapped exciton model can reasonably explain some emission characteristics of CQDs, including steady-state and time-resolved spectral data.
Polarization spectroscopy is used to study momentum and energy relaxation dynamics. When a sample is irradiated by linearly polarized light, the hot carriers of momentum will align, resulting in an anisotropic momentum distribution, which eventually leads to linearly polarized emission. However, the decay process after excitation will randomly distribute the carrier set by collision or emission of phonons, resulting in isotropy. Therefore, only the hot carriers with higher energy will show anisotropy. The emission anisotropy versus photon energy spectra of the CQDs films show a high emission anisotropy for the CQDs samples (Fig. 11A), suggesting the possible presence of hot carriers in the CQDs.
It has been reported that the electric field can cause the separation of hot carriers, resulting in modulated energy States or non-radiative decay. In addition, the electric field may cause a spatially moving carrier of heat, thereby reducing radiative recombination. However, the electric field modulation experiment showed unusual results, with the intensity and line shape of the emission spectra being almost the same in both the presence and absence of an electric field (fig. 11B). This indicates that the emission center has a strong localization property, which can provide a strong local potential field, so that the high electric field does not "affect" the hot carriers and cause fluorescence quenching. Power-dependent PL measurements also show that the PL intensity increases linearly with power (Fig. 11c). Since the number of excitons is proportional to the excitation power and the defects tend to saturate, the emission of CQDs originates from self-trapped exciton recombination rather than structural defects.
For a self-trapped exciton, a smaller charge distance R has a larger wave function overlap with the transition matrix, and they recombine preferentially. This results in a shift of the emission to longer wavelengths with increasing time and therefore an increase in fluorescence lifetime. Time-resolved spectra show a similar pattern, which confirms the emission of a self-trapped exciton. The authors represent the electronic structure of CQDs by combining the processes of photoexcitation, relaxation and recombination (Fig. 11d). Over time, the self-trapped exciton will emit photons of smaller energy after undergoing the relaxation process.
The excitation-dependent PL properties of CQDs can also be explained by the self-trapped exciton model. Since the excitation self-trapping energy of a monochromatic light source is equal to the exciton state energy of the excited photon, the energy decay process causes the emission to start at the excitation wavelength. When the excitation wavelength is changed, the photon excites a self-trapped exciton state determined by the energy of the excitation photon, and then in the same way performs a suppressed decay process, resulting in emission starting at the new excitation wavelength. Self-trapped exciton emission has also been confirmed by some reports
[92~94].
3.6 Surface dipole emission center
In order to reveal the mechanism of single particle emission and its structural information, Chizhik et al. Used the combined spectroscopy technology to realize the relevant determination of PL, including the crystal structure and the shape of single CQDs, by combining high-resolution transmission electron microscopy with PL imaging spectroscopy
[95]. They synthesized CQDs by a microwave-assisted hydrothermal method using sucrose as a carbon source and polyethylene glycol as a passivating agent. High resolution transmission electron microscopy revealed the presence of two types of carbon nanoparticles: one with an onion-like structure, as shown in the four images on the left of fig. 12A. The onion-like CQDs range in diameter from 7 to 20 nm (aspect ratio from nearly 1.5 to 1), but have the same lattice spacing of about 0.35 to 0.36 nm, corresponding to the (002) plane. As shown in the picture on the right of Fig. 12A, the other structure is a fully crystalline particle with a diameter ranging from 2 to 5 nm and a lattice constant of 0.24 nm, corresponding to the (100) plane. By using two Gaussian functions to fit the emission spectra of single CQDs, it is found that the splitting energy is in the range of 70 ~ 150 meV and is independent of the emission peak. Subsequently, based on the crystal structure and energy splitting information of single CQDs provided by HRTEM, they calculated the photons involved in the emission of CQDs using the density functional tight-binding method, and proposed three types of optical phonons, of which 98 meV phonons were involved in the emission of onion-shaped CQDs, while 112 and 113 meV phonons were involved in that of fully crystallized CQDs. The lower energy satellite peaks can therefore be assigned to the excitation of photons and radiative recombination coupled to carriers. Interactions between electrons and lattice vibrations result in bound States, which result in reduced energy compared to non-interacting systems. This is manifested in a decrease in the energy of the emitted photons
[96].
Focused and defocused azimuthally polarized laser beams (APLB) can be used to measure and provide information about the excitation and emission transition dipole moments (TMD), respectively, which can be used to reveal the emission mechanism of CQDs. The results are shown in Figure 12B. Both excitation and emission transition dipole moment patterns show two close elliptical bright spots, indicating fixed linear excitation and emission transition dipole moments. The slight asymmetry of the double lobe pattern is due to the slight distortion of the excitation laser beam. A pattern with a fixed orientation indicates that the TDM has not been repositioned during the acquisition time. The fixed dipole moments of the excitation and emission transitions indicate that CQDs act as a dipole that can absorb, excite, and emit. The transition of the PL emission energy between the "on" and "off" States indicates that the observed emission originates from a single emitter.
While imaging using APLB can provide information about the TDM in the excited state, collective imaging can determine the orientation and size of the emitted TDM. PL imaging of different CQDs, which show different defocus values, as shown in Fig. 12 B, shows that the experimental images fit well to a model that assumes that the emission source is from a single fixed dipole. Excitation mode and defocused image measurements of the same single CQDs show that the excitation and emission transition dipoles of the particles are oriented parallel to each other within the measurement error.
All individual CQDs exhibit a narrower PL spectrum (12 C) compared to the whole CQDs, and show a random spectral shift from 50 to 650 nm (around 2.5 to 1.9 eV), so the individual CQDs have an obvious single emission characteristic
[95].
In order to further confirm the single emission characteristics of single CQDs, they tested whether the shape of PL spectra changed under different excitation wavelengths, as shown in Figure 12d. At the excitation wavelengths of 467 and 488 nm, the shape of PL spectra of five single CQDs was basically unchanged, indicating that single CQDs was a single emitter, and the excitation-dependent PL phenomenon was the result of heterogeneity. This is also supported by the fact that different individual CQDs have different quantum yields.
3.7 Polymer emission center
In general, fluorescence quenching occurs when the concentration of CQDs solution is high. However, recent studies have found that CQDs synthesized under certain conditions show different phenomena, such as increased fluorescence emission or red shift at higher concentrations. Therefore, it is believed that this is the result of the formation of aggregates at high concentrations of CQDs, and it has been reported that aggregates are preferentially formed even at lower concentrations, resulting in the PL phenomenon
[54,97⇓⇓⇓⇓~102]. According to the experimental results of Demchenko et al., the absorption spectra of CQDs prepared by microwave treatment of alanine were significantly different from the fluorescence excitation spectra. While considering CQDs as H aggregates, they compared and analyzed the different characteristics of the absorption and fluorescence excitation spectra of CQDs in detail
[97]. Due to exciton relaxation, H aggregates look very similar to J aggregates, and the distorted region is a dynamic defect, which can produce a corresponding absorption band in the excitation spectrum due to indirect optical transition. The ultrafast intraband exciton relaxation to the lowest exciton state leads to a significant extension of the H-band (Fig. 13 a). Excitation to any known band (H, J, or monomer band) produces nearly identical luminescence spectra (Fig. 13A).
In theory, the optical transition from the lowest exciton state to the ground state is strictly prohibited in an ideal molecular stack. Therefore, such aggregates should not have luminescent properties, and excitation to the ground state in a non-radiative relaxation state would effectively deactivate. However, after the H-aggregate absorbs the photon, the free exciton undergoes an ultrafast non-radiative in-band relaxation provided by exciton-lattice interaction and allows the H-aggregate to produce an emissive state. Therefore, due to the coupling between the exciton and the lattice, the exciton produces local distortion, and the mutual spatial arrangement of the monomers changes, and the angle between them may be less than 54.7 °. This distorted region looks like a J-aggregate (Fig. 13 B), allowing optical transitions from the lowest exciton state. The luminescence band of such J-aggregates was not observed because the exciton underwent stronger excitation (Fig. 13 a). However, the exciton will undergo a stronger relaxation when the barrier-free self-trapping occurs, thus creating a new polarization state below the bottom of the exciton band. Therefore, the local distortion of the H aggregates actually acts as an exciton trap, which recombines at this site and forms a luminescent self-trapped exciton, which is a common phenomenon in the formation of molecular aggregates. Xie et al. Also reported the phenomenon of aggregation-induced polychromatic emission, and believed that the formation of aggregates would lead to a decrease in the band gap, resulting in a red shift in emission (Fig. 13C)
[98].
3.8 Multiple Launch Center
The PL mechanisms mentioned above all assume that the PL originates from the CQDs molecule itself or a single structure generated in the emission process. However, the actual emission process of CQDs is complex, and different structures may play a role in the PL emission. There have been some reports that the PL emission originates from the synergistic effect of multiple centers
[103~108].
Zeng et al. Observed the solvent-dependent PL emission of CQDs, and proved the existence of multiple emission centers (carbon core state, edge state and surface state) in CQDs through structural and optical characterization
[103]. In different solvents, PL originates from different emission centers (fig. 14 a). Li et al. Synthesized CQDs with full-band emission by one-step hydrothermal method using perylene-3,4,9,10-tetracarboxylic diglucoside and triethylamine, and separated the components with different emission wavelengths by silica gel column chromatography
[104]. The analysis of the components with different emission wavelengths still shows the rule of red shift of fluorescence emission wavelength with the increase of particle size, but it is observed that the emission wavelengths of the four components with similar average particle size also have a certain degree of red shift (about 4.2 nm, Fig. 14 B). Raman spectroscopy characterization shows that the I
D/I
G of the four samples gradually increase, and DFT calculation shows that the band gap of the four samples decreases in turn, so the defect content decreases, which also leads to the red shift of PL emission wavelength. These results strongly suggest that both carbon nuclei and surface defects dominate the occurrence of PL.
3.9 Slow Solvent Relaxation and Solvation Effect
In addition to the structure of CQDs themselves, Yu et al. Observed that solvents can also affect the PL emission of CQDs, with a red shift in the fluorescence emission wavelength in solvents with increased polarity (fig. 15A)
[109]. The solvatochromic effect is usually attributed to intramolecular charge transfer, so they believe that this solvent-dependent photoluminescence is the result of a dipole-induced change or transfer of surface electron States, with the presence of a large number of oxygen and nitrogen substructures on the surface of CQDs increasing the density of charge carriers and inducing charge transfer of electrons to the edge (Fig. 15 B). The stable excited state of this structure on the surface of CQDs is strongly influenced by the dipole moment of the solvent. In aprotic solvents, with the increase of solvent polarity, the dipole moment will lead to more surface electronic structures, resulting in a decrease in the energy gap and a red shift in the PL emission.
Fan et al also found that CQDs emit different wavelengths of fluorescence in different solvents, and then speculated that the strong blue light and excitation-dependent emission of CQDs originated from the inherent surface state through structural characterization and PL spectra.The long wavelength emission independent of excitation in DMSO, DMF and methanol is attributed to the solvent molecules attached to the surface of the CQDs by intermolecular hydrogen bonds
[110]. Solvent molecules bound to the surface of CQDs can form new energy levels such as C = O/S = O, resulting in a decrease in the band gap and a red shift in the fluorescence emission (Figure 15C). Solvent relaxation and solvatochromic theories have also been reported in other literature
[111~115]. In order to explore the origin of solvation discoloration, Iso et al. Designed a series of experiments to control solvation discoloration by surface modification
[111]. When different molecules were modified on the surface, such as decanoic acid (DA) and perfluorodecanoic acid (PA), respectively, the phenomenon of solvation discoloration was observed (fig. 15d). The PL emission of CQDs in two selected solvents, chloroform and methanol, showed a significant red shift. However, the PL emission of CQDs modified with DA showed only a weak red shift, while the PL emission of CQDs modified with PA showed an opposite blue shift. The shift of the PL emission depending on the surface group indicates that the solvatochromism is the result of the dipolar interaction between the surface state of CQDs and the solvent.
4 Conclusion and prospect
In this paper, the research progress of the optical properties and luminescence mechanism of CQDs is summarized, and the research in this area has been developed rapidly. The mechanism summarized in this paper explains the PL phenomenon from different aspects, which has important reference value for revealing the origin of PL, and provides direction for solving some basic problems, such as improving the optical properties and stability of CQDs in practical applications, and selecting synthesis conditions. However, due to the complexity of the luminescence process of CQDs, CQDs prepared by different methods may show different PL phenomena, and the improvement of characterization methods leads to the discovery of more PL phenomena, so the PL mechanism reported so far can only explain part of the PL phenomena. With the deepening of the study, the chemical processes or some special structures at a more microscopic level in the PL process may be observed, and the contribution of some free radicals to the PL process may also be further explored.In the future, we can further explain the PL process by focusing on the microstructure of the PL processes, in order to solve the practical application problems of CQDs. Therefore, we expect more convincing emission mechanisms to be proposed, and we hope that this review will help to realize the commercial application of CQDs as soon as possible.
National Natural Science Foundation of China(21675088)
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