Fluorescence “Giant” Red Edge Effect
8.3 Optical properties of carbon quantum dots
8.2.5 Hydrothermal and solvothermal synthesis
Hydrothermal carbonization has been introduced as the most effective way for the large-scale synthesis of surface-functionalized CQDs [29 33]. Different types of precursors such as small molecules of polymers or food commodities (e.g., milk, fruit) to wastes (waste microorganisms) can be used in this technique[33]. Size dis- tribution controllability and surface modifications of CQDs are the predominant characters of CQDs synthesized by this method, which have great importance for their applications[29].Fig. 8.5shows the proposed formation mechanism of CQDs in this process. Furthermore, the solvothermal method can be applied to synthesize the CQDs with full-spectrum emission. Due to the effect of solvent-related reac- tions, the tuning in the bandgap and PL are predicted[25,34,35].
8.3.1 Optical absorption
The UV visible optical absorption of CQDs has been reported to have prominent UV absorption (230 320 nm) with a tail extending into the visible region due to theπ-conjugated electrons in the core of CQDs and their surface chemical groups [8 10,37 40]. The broad peak atB230 nm could be assigned to theπ π transi- tion of aromatic C5C bonds, whereas the n π transition of C5O bonds or other surface groups may result in the shoulder atB300 nm[8 10]. Based on the quan- tum confinement effect, with the increase in size of CQDs a gradual redshift in excitonic absorption band from blue to red can be achieved and shown inFigs 8.6 and 8.7. InFig. 8.6A, daylight photographs and UV excited fluorescence images of multicolor bandgap fluorescent CQDs are presented. InFig. 8.6B, UV visible opti- cal absorption can be found. InFig. 8.6C, D, the normalized PL spectra and time- resolved luminescence measurement of the CQDs are respectively illustrated. The changes of HOMO and LUMO energy levels as a function of the CQDs size are shown inFig. 8.6E. InFig. 8.7A and B, the daylight photographs, UV excited fluo- rescence images of the narrow bandwidth emission triangular CQDs ethanol solu- tion are indicated, respectively.Fig. 8.7Cshows the normalized UV visible optical absorption andFig. 8.7Dindicates the normalized luminescence spectra of the blue, green, yellow, and red narrow bandwidth emission triangular CQDs, respectively.
Furthermore, surface functional groups on CQDs, i.e., C OH, C5O, O C5O, C5N, and C5S can also contribute to their absorption features. Studies showed that the surface functional groups provide the surface states formation that their energy levels are between π andπ states of C5C. These surface states thereby induce new absorption bands for possible electron transitions [8 10]. The red- shifted absorption bands can play a crucial role in the tunable multiwavelength emissions from surface passivated CQDs.
8.3.2 Photoluminescence emissions from ultraviolet to near- infrared regions
The wavelength-tunable fluorescence across the entire visible to NIR spectrum makes CQDs promising materials for a wide range of applications. The physical mechanisms underlying the PL of the CQDs are mainly due to their size, shape, and surface functionalization[33 47,83 85].
8.3.2.1 Photoluminescence emission due to quantum confinement effect
The size-related quantum confinement effect has been introduced as a main PL mechanism of CQDs[46,47,83 85]. Yuan et al. have reported full spectrum bright multicolor fluorescent CQDs originated from their bandgap transitions [84].
Fig. 8.6shows the redshift in fluorescence emission of CQDs with the size distribu- tion of 1 7 nm, indicating the quantum confinement effect and the red-shifted
excitonic absorption bands of the CQDs [84]. Furthermore, the Yuan group has reported the synthesis of triangular CQDs with high color purity, narrow bandwidth, and tunable wavelength emission across the whole visible spectrum [85].Fig. 8.7 depicts the excitonic absorption and highly tunable emission peaks originated from band-edge transitions in the triangular CQDs.
Figure 8.6 (A) Daylight photographs (left) and UV excited fluorescence images (right) of multicolor bandgap fluorescent CQDs. (B) UV visible optical absorption, (C) normalized PL spectra, and (D) time-resolved luminescence measurement of the CQDs. (E) The changes of HOMO and LUMO energy levels as a function of the CQDs size.CQDs, Carbon quantum dots.
Source: Reproduced with permission from F. Yuan, et al., Bright multicolor bandgap fluorescent carbon quantum dots for electroluminescent light-emitting diodes.Advanced Materials, 29 (3) (2017) 1604436.
8.3.2.2 Photoluminescence emission due to surface passivation and functionalization effect
The surface characteristics engineering of CQDs is one of the main remaining chal- lenges with the significant influences on their optical properties, especially tunable full-color fluorescence emission [33 46]. Wang et al. reported that in the sol- vothermal synthesis method, the formed fluorophores by the solvent-related reac- tions could lead to the full-spectrum emission of CQDs by PL bandgap tuning[34].
Ding et al. have reported production of the CQDs with tunable emission across the entire visible spectrum by mainly controlling the content of graphitic nitrogen and oxygen-containing surface functional groups[35]. Shao et al. have presented exper- imental and theoretical evidence demonstrating that the LUMO2HOMO energy gap of the amino-functionalized CQDs may decrease with an increase in the num- ber of amino (NH2) surface groups, resulting in the PL redshift and full-spectrum emission [36]. Jiang et al. have reported that tuning the surface state of CQDs by increased surface oxidation and carboxylation can result in the redshift and multi- color PL emission[37]. Xu et al. have shown that the formation of different surface level states by hydrogen bonds between the surfaces of the CDs and different sol- vents can result in multicolor luminescent CQDs [38]. Furthermore, it has been reported that multi-emission luminescence in carbon dots can originate from multi-energy states assigned to the bandgap state, surface defect state, and molecu- lar state [40]. The optical images of multicolor fluorescent CQDs with different molecular weights and at various oxidation times and corresponding fluorescence Figure 8.7 (A) Daylight photographs, and (B) UV excited fluorescence images of the narrow bandwidth emission triangular CQDs ethanol solution. (C) The normalized
UV visible optical absorption and (D) normalized luminescence spectra of the blue, green, yellow, and red narrow bandwidth emission triangular CQDs, respectively.
Source: Reproduced with permission from F. Yuan, et al., Engineering triangular carbon quantum dots with unprecedented narrow bandwidth emission for multicolored LEDs.Nature communications, 9 (1) (2018) 1 11.
emission spectra of the CQDs under excitation of 360 nm are shown inFig. 8.8A.
In Fig. 8.8B, the influence of size of CQDs and their surface oxidation on the PL properties are shown.
It is evident that the surface states of CQDs have arisen from both sp3- and sp2- hybridized carbons and the surface functional groups[41,42]. Generally, the surface states-related fluorescence aspect may include the size of CQDs, their surface func- tional groups, and defects and heteroatom doping[41,42]. Experimental results indi- cate that the strong coupling interaction of carbon cores with carboxyl and carbonyl surface functional groups can alter the energy gaps of CQDs, resulting in the size- dependent PL ascribed to the surface states. It has been shown that diverse surface defects with different energy levels formed due to the oxygen-containing functional groups (epoxide, hydroxyl, carbonyl, and carboxyl). This kind of CQDs surface defect can result in multicolor emissions covering the visible light spectrum [41,42]. The process of tunable PL emission of CQDs due to oxygen-related surface defect state is shown in Fig. 8.9. In addition, the surface state emission intensity can be enhanced by the formation of the nitrogen-related surface functional group such as amino, pyridinic, hydrazine, or graphitic nitro. Also, they result in red- shifted emission and tunable multiwavelength PL from CQDs[41,42]. It was fig- ured out that the CQDs doping with a heteroatom such as sulfur, phosphorus, boron, and fluorine, has also been introduced as an effective way for significant enhance- ment of their fluorescence intensity and redshift of emission wavelength[41,42].
(A) (B)
a b c d e f g
350 400 450 500 550 600 650 700 750 Wavelength / nm
Surface-oxidation degree increases
Energy
Surface Core
Energy
Core
Surface Surface
Core Core Surface
Size increases Red-shift in the emission
Red-shift in the emission
hv1 hv3
hv1 hv2
Two pathways to tune the luminescence of C-dots
C-dots Core of C-dots Surface-oxidized groups of C-dots
Figure 8.8 (A) Top: Optical images of multicolor fluorescent CQDs with different molecular weights and at various oxidation times. Bottom: Corresponding fluorescence emission spectra of the CQDs under excitation of 360 nm. (B) The influence of size of CQDs and their surface oxidation on the PL properties.
Source: Reproduced with permission from L. Bao, et al., Photoluminescence-tunable carbon nanodots: surface-state energy-gap tuning.Advanced Materials, 27 (10) (2015) 1663 1667.
8.3.2.3 Up-conversion photoluminescence
In recent years, the CQDs with different surface functional groups have been introduced as effective spectral converters through broadband up-conversion of NIR/visible photons to higher energy photons[86 95]. The CQDs can exhibit multiple photon absorption in the three NIR windows (650 2000 nm), resulting in efficient visible light emission applicable for photoelectrode systems and bioimaging [87 90]. Fig. 8.10 shows the up-conversion fluorescence of CQDs. Under 800 nm fs-laser excitation, a transferring from the NIR light to visible light occurred [90]. Moreover, CQDs are confirmed to have the visible up-conversion PL property, capable of improving the visible/ultraviolet light photocatalytic performance of different materials[91 94]. It should be noted that the normal fluorescence due to the second-order grating diffraction of the exciting source may be incorrectly referred to as up-conversion fluorescence of CQDs[95].
8.3.3 Electroluminescence
The obtained wavelength-tunability from visible to NIR emission of CQDs has stimulated worldwide interest in developing efficient CQDs-based light-emitting devices for low-cost applications[48 53]. In light-emitting devices, an electrolumi- nescence (EL) process occurs due to the electronic excitation. When CQDs are used in the light-emitting devices, the emission is observed by PL photoexcitations at multiple wavelengths. It has been shown that the CQDs with EL emission covering the entire visible spectrum can act as the emissive layer for achieving high-performance white LEDs [48,49]. Zhang et al. have reported switchable Figure 8.9 (A) UV excited fluorescence images of CQDs with corresponding luminescence spectra. (B) A model that presents the influence of different degrees of surface oxidation on the tunable PL of CQDs.
Source: Reprinted (adapted) with permission from H. Ding, et al., Full-color light-emitting carbon dots with a surface-state-controlled luminescence mechanism.ACS Nano, 10 (1) (2016) 484 491. Copyright {2016} American Chemical Society.
multicolor blue, cyan, magenta, and white EL emissions from the same CQDs by controlling the applied voltage (6 9 V) and injecting current density[50]. Mo et al.
have presented low-voltage multicolor EL from CQDs/Si heterostructures including three distinct peaks at 438, 540, and 600 nm originating from the intrinsic CDs core, the C5O, and C N surface groups, respectively [51]. As seen in Fig. 8.11, the CQDs with multicolor bright bandgap EL emission across the whole visible spectrum can be applied as an active emissive layer for the monochrome LEDs[84].