Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX
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Deep-Ultraviolet Emissive Carbon Nanodots Shi-Yu Song,† Kai-Kai Liu,*,† Jian-Yong Wei,† Qing Lou,† Yuan Shang,*,‡ and Chong-Xin Shan*,† †
Henan Key Laboratory of Diamond Optoelectronic Materials and Devices, Key Laboratory of Material Physics, Ministry of Education, School of Physics, Zhengzhou University, Zhengzhou 450052, China ‡ Super Computer Center, Smart City Institute, Zhengzhou University, Zhengzhou 450001, China
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S Supporting Information *
ABSTRACT: Deep-ultraviolet (DUV) emissive carbon nanodots (CNDs) have been designed theoretically and demonstrated experimentally based on the results of first-principles calculations using the density functional theory method. The emission of the CNDs is located in the range from 280 to 300 nm, which coincides well with the results of theoretical calculation results. The photoluminescence (PL) quantum yield (QY) of the CNDs is up to 31.6%, and the strong emission of the CNDs originates from core-state (π−π*) carriers’ radiative recombination and surface passivation. Benefiting from the core-state emission and surface group passivation, the emission of the CNDs is independent of the excitation wavelength and ambient solvent. DUV light-emitting diodes (LEDs) have been fabricated based on the DUV emissive CNDs, and the LEDs can be used as the excitation source to excite blue, green, and red CNDs, indicating their potential application in DUV light sources. This work may provide a clue for the designing and realizing of DUV emissive CNDs, thus promising the potential application of CNDs in DUV light-emitting sources. KEYWORDS: Carbon nanodot, deep-ultraviolet, emission, density functional theory as excitation sources in versatile fields, such as sterilization, photocuring, anticounterfeit, and so on. Thus, developing efficient DUV emissive CNDs is of great importance and significance. Herein, DUV emissive CNDs have been designed theoretically based on the quantitative structure−activity relationship analysis method and demonstrated experimentally. The PL QY of the CNDs is 31.6%, which is the highest value ever reported in this region. It has been proved that the strong emission of the CNDs originates from core-state radiative recombination surface group passivation; thus emission of the CNDs is independent of excitation wavelength and ambient solvent. Light-emitting diodes (LEDs) have been fabricated employing the CNDs as the fluorescent phosphors, and obvious DUV emission can be detected from the LEDs, promising the potential applications of the CNDs in DUV light sources. Results and Discussion. Three kinds of configurations based on the o-phenylenediamine (OPDA) and methyl red, mphenylenediamine (MPDA) and methyl red, and p-phenylenediamine (PPDA) and methyl red are shown in the top of Figure 1. The highest occupied molecular orbital (HOMO)
D
eep-ultraviolet (DUV) light sources with a wavelength shorter than 300 nm have found various applications in the field of water sterilization, disinfection, confidential communication, and so on.1 Currently, the most frequently used DUV light sources are mercury, xenon, or deuterium lamps, but these gaseous lamps are bulky and fragile. Thus, the development of solid state DUV light sources is highly desired. Many materials have been investigated to this end.2,3 Nevertheless, efficient DUV light sources are still rare. Fluorescent carbon nanodots (CNDs) have attracted broad scientific interest in recent years because of their high efficiency, high chemical stability, antiphotobleaching, biocompatibility, low toxicity, etc.4−8 The above characters make CNDs a burgeoning luminescent nanomaterial that may be applied in many fields, such as biosensing,9−12 bioimaging,13,14 photothermal conversion therapy,15 drug delivery,16,17 optoelectronic devices,18−22 etc. To date, CNDs with an emission peak in visible regions have been reported extensively.23−30 Especially, blue CNDs with a photoluminescence (PL) quantum yield (QY) over 90%,31 green luminescent CNDs with PL QY higher than 70%,32 and red luminescent with PL QY higher than 86%,33 and UV emissive graphene quantum dots (GQDs) with PL QY of 11%, 34,35 have been demonstrated. However, CNDs with emission in the DUV region have not been reported yet. The emission of the CNDs is located in the DUV region, and the emitted photons have a high energy. As a result, the DUV emissive CNDs can be used © XXXX American Chemical Society
Received: May 22, 2019 Revised: June 27, 2019 Published: July 5, 2019 A
DOI: 10.1021/acs.nanolett.9b02093 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 1. Three kinds of configurations based on OPDA, MPDA, PPDA, and methyl red (top). HUMO and LUMO of the three configurations (middle). Calculated spectra of the three configurations (bottom).
processes of the CNDs involve a solvothermal process and chromatography. The CNDs from the carbonization of configurations 1, 2, and 3 are named CNDs, CND-2, and CND-3, respectively. The preparation process of the CNDs is used as an example (Scheme 1); the original solution was purple in color after hydrothermal treatment. Clear and transparent solution was obtained after chromatography. Detailed information was presented in the experimental section. All other CNDs were synthesized according to the above procedures. The atomic force microscope (AFM) images of the CNDs (Figure 2a) reveal their height is approximately 2.7 nm. The TEM image of the CNDs reveals that they are uniform in size, as shown in Figure 2b. A large-scale scan of the CNDs further indicates that they have a homogeneous size. The average size of the CNDs is about 6.0 nm from the size distribution shown
and lowest unoccupied molecular orbital (LUMO) of the configurations are achieved based on the first-principles calculations using density functional theory (DFT)-based methods (in the middle of Figure 1). This is consistent with the energy relative to the peak maximum at wavelengths, and thus, it can be used as an easy and straightforward method to accurately estimate emission spectra. The energy gap between HOMO and LUMO of configurations 1, 2, and 3 is 4.47, 4.69, and 4.18 eV, respectively, from which we can predict that the emission locates in DUV region. Additionally, we present theoretical calculation spectra of the three kinds of configuration molecules, as shown in the bottom of Figure 1. DUV emissive CNDs can be achieved by using the proposed configurations as the skeletons. The synthesis experiments of the DUV emissive CNDs are designed based on the theory calculation, and the preparation B
DOI: 10.1021/acs.nanolett.9b02093 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 2. (a) AFM image of the CNDs. The inset is the height distribution of the CNDs along the white line. (b) TEM image. The inset is the particle size distribution of the CNDs along the yellow line. (c) HRTEM image and (d) selected-area diffraction image. (e) XRD pattern of the CNDs. (f) FTIR spectrum of the CNDs.
in the inset of Figure 2b. The structure characterization of the CNDs indicates the CNDs have a disk-like shape, and the carbon core crystalline lattice structure of the CNDs can be observed clearly, as shown in Figure 2c. The interlayer spacing of the CNDs is 0.21 nm, which corresponds to the d spacing of the graphene (100) planes. Figure 2d is the selected-area diffraction pattern of the CNDs, further verifying their crystalline nature. The average particle sizes are 2.25 ± 0.24 nm and 2.12 ± 0.1 nm for CND-2 and CND-3, respectively (Figure S1). The XRD pattern of the CNDs (Figure 2e)
displays a broad peak centered at about 25°, which is a typical peak of CNDs. In the FTIR spectrum of the CNDs shown in Figure 2f, the stretching vibrations of COH at 3430 cm−1, NH at 3412 cm−1, stretching vibrational absorption band of CC at 1513 cm−1, and stretching vibrational absorption band of CO/CN at 1635 cm−1, and CN at 1290 cm−1 were observed, indicating the formation of carbon core structures and the presence of some functional groups.36,37 Abundant functional groups were also observed for CND-2 and CND-3 samples, indicating the formation of polyaromatic C
DOI: 10.1021/acs.nanolett.9b02093 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 3. (a) XPS spectrum of the CNDs. (b) Schematic diagram of XPS characterization of the CNDs without bombardment by argon. (c) Highresolution XPS C 1s, N 1s, and O 1s. (d) XPS spectrum of the bombarded CNDs. (e) Schematic diagram of XPS characterization of the DUV CNDs bombarded by argon ions for 6 s. (f) High-resolution XPS C 1s, N 1s, and O 1s spectra of the bombarded CNDs.
CNDs, the samples of CNDs are bombarded for 6 s by argon ions prior to the XPS measurement (illustrated in Figure 3e), and the corresponding XPS spectrum is shown in Figure 3d. The inner structure of the CNDs is mainly composed of carbon (87.48%), nitrogen (3.23%), and oxygen (9.29%). Obviously, the peak of O 1s decreases greatly, while the peaks of C 1s and N 1s increase compared with the XPS spectrum of the outer surface, implying that the carbon content inside the CNDs is much higher than that of the surface. The highresolution spectra of C 1s, N 1s, and O 1s are shown Figure 3f. The CC/CC bonds increase along with the decrease of the CO and CO, indicating the inner of the CNDs is mainly composed of sp2/sp3 carbons and the outer surface of the CNDs has some functional groups.42 The XPS and FTIR data reveal that all CND samples contain a carbon core and some functional groups. The functional groups on the surface of the CNDs passivate the carbon core of the CNDs, making them stable in all kinds of solvents, which we will discuss later. The excitation and PL spectra of the CNDs are displayed in Figure 4a, three excitation peaks at around 250, 270, and 278 nm can be observed. In the PL spectra, the strongest emission peak of the CNDs in ethanol solution is centered at 290 nm,
structures in the CNDs during the reaction process, as shown in Figure S2. It can be speculated that the core of all of the CNDs samples is made of sp2/sp3 carbons (CC/CC) with some functional groups on the surface. The survey XPS spectrum of the CNDs is presented in Figure 3a, and the schematic diagram of the XPS characterization is illustrated in Figure 3b. From the XPS spectrum, three peaks including C 1s (285 eV), N 1s (399.8 eV), and O 1s (531 eV) can be observed, which are mainly related to the surface elements of the CNDs.38,39 XPS analysis reveals that the surface layer of the CNDs is mainly composed of carbon, nitrogen, and oxygen. The high-resolution spectra of C 1s, N 1s, and O 1s are shown in Figure 3c. The presence of sp2/sp3 carbons (CC/CC, 284.5 eV), nitrous carbons (CN, 285.7 eV), and carbonyl carbons (CO, 288.2 eV) is observed from the C 1s spectrum. The high-resolution spectrum of N 1s displays two peaks at 399.7 and 401.1 eV, which can be attributed to CNC and NH groups, respectively. The O 1s spectrum can be divided into two peaks: the main peak with the binding energy of 532.2 eV can be attributed to the CO, and another peak at 530.8 eV can be attributed to CO.40,41 To study the inner structure of the D
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Figure 4. (a) PL excitation (black) and emission (purple) spectra of the CNDs in the ethanol solution (0.1 mg/mL). (b) Calculated PL spectrum of configuration 1. (c) Experimental PL spectrum of the CNDs. (d) Absorption spectra of the CNDs (0.05 mg/mL) in different polar protic solvents. The inset is the magnified spectra in the range of 220−300 nm. (e) Temperature-dependent PL spectra of the CNDs. (f) PL spectra of the CNDs (0.1 mg/mL) in different polar solvents.
Figure 5. Time-resolved luminescence decay curves of the CNDs collected at (a) 282, (b) 290, and (c) 300 nm for CNDs in different polar solvents: water, DMSO, isopropanol, methanol, DMF, ethanol. (d) The luminescence decay curves for the CNDs in ethanol. (e) Schematic diagram of the radiative recombination transitions in the CNDs.
and two shoulder peaks at 282 and 300 nm can also be observed. Notably, the self-absorption of the CNDs is quite small because of their small overlap between the absorption and emission spectrum, which is beneficial for DUV emission.43 The PL QY of the CNDs is 31.6% under 250 nm excitation. The calculated PL spectrum of configuration 1 and the PL spectrum of the CNDs experimentally measured are shown in Figure 4b and Figure 4c. Three emission peaks
can be observed from the calculated spectrum of configuration 1, which can also be observed from the PL spectrum of the CNDs. Configuration 1 and CNDs have similar emissive properties, indicating the core of CNDs is mainly composed of configuration 1. Note that the calculated spectrum of the configuration 1 hypochromatic shifts 20 nm to align its maximum with that of the experimentally measured curve. The calculated PL spectra of configuration 2 and configuration 3 E
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Figure 6. (a) Emission spectrum of the LED (red), PL excitation spectrum of the CNDs (black). (b) The emission spectra of the DUV emissive CND-based LED under different driven currents. (c) The spectra of the LED working for different times. (d) Emission variations of the LED versus working times.
of the CNDs.48,49 The temperature-dependent PL spectra of the CNDs are shown in Figure 4e. The PL intensity of CNDs shows a slight decrease with an increase of temperature, and the decrease can be ascribed to thermally activated exciton dissociation and nonradiative trapping. Importantly, the PL intensity of the CNDs with an increase of temperature (from 10 to 302 K) is less than 22%, indicating the CNDs have minimal nonradiative recombination centers or defects due to the surface passivation. Additionally, the emission behavior of the CNDs remains unchanged in different polar solvents (Figure 4f, Figure S4a,b), indicating the high ambient stability of all CNDs. To further clarify the luminescence mechanism of the CNDs, luminescence decay curves of the CNDs in different solvents are shown in Figure 5. Time-resolved luminescence decay curves collected at 282, 290, and 300 nm for the CNDs show the same decay trend in different polar solvents, as shown in Figure 5a−c, indicating the carriers of the CNDs have an identical dynamics process, which can be assigned for the intrinsic transition of the CNDs. The decay curves at 282, 290, and 300 nm can be best fitted with a single-exponential function, giving a lifetime of 6.01, 5.96, and 6.00 ns, respectively (Figure 5d). For the CND-2 and the CND-3, time-resolved luminescence decay curves show the same decay trend in different polar solvents. The CND-2 and the CND-3 also have a single-exponential decay property, as shown in Figure S7a,b. The single-exponential decay property of the CNDs confirms that they have one emission center, which comes from the intrinsic properties of the CNDs instead of the functional groups on the surface. Based on the above analysis, the DUV emissive CNDs have three typical emission behaviors: excitation-independent, polar-independent, and single-exponential decay. A possible fluorescence mechanism has been proposed: the CNDs mainly contain a carbon core with some functional groups on the surface, and the DUV emission is attributed to the radiative recombination transition of π−π* of the carbon-core state. The surface groups passivate the carbon core of the CNDs and thus decrease the defect
are also compared with those of CND-2 and CND-3, as shown in Figure S3. The CND-2 shows an intense DUV emission (λem = 290 nm) and UV emission (λem = 330 nm) under the excitation of 250 nm. The DUV emission region of the CND-2 is substantially consistent with the calculated fluorescence spectrum of configuration 2. For CND-3, the emission locates in the range from 280 to 360 nm, and a dominant peak and shoulder peak from 280 and 320 nm can be observed in the spectrum. The shape of the calculated curve agrees well with that of the experimental curve, and the range of the emission is almost identical. The PL spectra of the CNDs with different concentrations were collected as shown in Figure S5. It can be seen that the spectra have identical peaks, while the intensity increases with the increase of concentration in the range from 0.05 to 1.00 mg/mL. A similar result can also be discovered in CND-2 and CND-3, as shown in Figure S5b,c. In order to further explore the DUV emission mechanism of the CNDs, the absorption spectra of the CNDs in different polar solvents were recorded, as shown in Figure 4d. The spectra show distinct absorption bands in the DUV region with prominent peaks at around 240−278 nm, which can be attributed the charge-transfer transition of π−π* of the CNDs.36 With the increase of the polarity of the protic solvent, no obvious shift can be observed in absorption spectra. This is because the carbon-core transition involving the π−π* transition is inside the CNDs, thus making them suffer a little from the ambient solvent.44,45 Similar phenomena can also be observed in CND2 and CND-3, as shown in Figure S4c,d. It has been suggested that high-energy absorption of the CNDs (∼280 nm) is attributed to sp2-hybridized carbon nanodomains of graphenelike flakes embedded in a matrix comprising of sp3-hybridized carbon, while the oxygen/nitrogen containing functional groups on the surface passivate the carbon core of the CNDs.46,47 Excitation-independent PL behavior can be observed in the PL spectra of the CNDs, as shown in Figure S6. It is reported that excitation-dependent emission behavior is related to the molecular state; thus the DUV emission originates from the core-state carrier radiative recombination F
DOI: 10.1021/acs.nanolett.9b02093 Nano Lett. XXXX, XXX, XXX−XXX
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purchased from Qingdao Marine Chemical Factory. The N,Ndimethylformamide and dichloromethane were purchased from Sinopharm Chemical Reagent Co., Limited. All chemicals were used as received without further purification. The UV LED chips were purchased from Tianjin Anxinda Communication Technology Co., Limited. The water used in all experiments was purified with a Millipore system by a Waterjet laboratory dedicated ultrapure water machine. First-Principles Density Functional Theory Calculation. Optimization of the ground state geometries was done using the density functional theory (DFT) method and B3LYP functional with resolution of the identity approximation along with chain of spheres exchange method (RIJCOSX). For the consideration of the different noncovalent interactions, a Grimme’s dispersion (D3) correction with the Becker− Johnson (BJ) damping parameter was included during computational calculation. The time-dependent density functional theory (TDDFT) was applied to calculate the excitation electronic structure with ωB97X-D3 functional. The def2-SVP basis set and the corresponding auxiliary basis set were used in the optimization technique. The nature of all of the energy minimized structures (stationary points) was further confirmed by the frequency analysis. In all cases, no imaginary mode was observed. DFT calculations were performed for CND molecules using the ORCA package (version 4.1.2). Synthesis of the CNDs. The synthesis processes of the DUV emissive CNDs are as follows: The preparation of DUV emissive CNDs (named CNDs, CND-2, and CND-3 from OPDA, MPDA, and PPDA) can be readily accomplished. First, 1 g of OPDA (MPDA or PPDA) and 1 g of methyl red were added to 20 mL of DMF to form a dark red turbid solution. Then, the mixed solution was transferred into a 50 mL Teflonlined stainless-steel autoclave. The sealed autoclave was heated to 200 °C and kept for 10 h; after that, the reactor was cooled down to room temperature naturally. The obtained solution was purified via silica column chromatography using dichloromethane as an eluent. The color of the original solution was dark brown, but a transparent solution was obtained after purifying. The obtained CNDs were used for further characterizations and applications. Fabrication of CND-Based LEDs. The CND silica gel composites were prepared by mixing the CNDs and silica gel in ethanol solution under constant stirring for 12 h. The reaction mixtures were filtered to remove the unabsorbed CNDs, and the remaining solid blocks on filter paper were dried in an oven at 60 °C. The silica/CNDs composites were obtained by grinding the dried blocks. The composites were then kept in a vacuum oven for further applications. The CNDs and silica gel composites were dispersed in polyvinylpyrrolidone resin (PVP) with a mass ratio of 1:10, and then the mixture was dropped onto commercial UV LED chips (peak wavelength: 270 nm). After that, the silica and CNDs composite covered LEDs were then dried in air naturally to form CND LEDs. Characterization. The structural properties of the CNDs were characterized using a JEM-2010 transmission electron microscope (TEM), a Multimode 8 instrument (AFM), and Bruker D8 Discover (Germany) X-ray diffractometer (XRD). The fluorescence properties of the CNDs were assessed using a Hitachi F-7000 PC spectrophotometer. UV−vis absorption spectra of the CNDs were obtained using a Hitachi UH 4150 UV−vis spectrophotometer. The PL QY of the CNDs was measured using an FLS920 spectrophotometer with the
emission of the CNDs. The ground electrons can be excited to the π* orbits when the CNDs were excited by 250 nm wavelength and then relaxed to the minimum energy level of the excited state. After that, emission of the CNDs in the DUV region is emitted within about 6.0 ns (Table S1); the radiative recombination processes of π−π* of the CNDs are illustrated in Figure 5e. Since the CNDs exhibit a relatively high efficiency in the DUV region, DUV LEDs have been fabricated employing the CNDs as fluorescent phosphors. The PLE of the CNDs is centered at about 270 nm, a LED chip with an emission peaked at around 270 nm was selected as the excitation source, as shown in Figure 6a. The CNDs were mixed with silica gel in ethanol under constant stirring for 12 h until the ethanol evaporates completely; then the silica gel/CND composites were obtained by grinding the dried blocks in agate mortar. The PL excitation spectrum of the composites is the similar to the CNDs (Figure S8a). Furthermore, the composites still maintain the DUV emission behavior, as shown in Figure S8b. After coating the silica gel/CND onto the LED chip, a peak centered at 290 nm is visible, as shown in Figure 6b. The small peak at around 270 nm is derived from the LED chip, while the other emission peaks are from the CNDs. The spectra of the DUV LEDs (the current is 30 mA) working for different times were recorded, as shown in Figure 6c. It can be seen that the emission intensity of the DUV LEDs decreases a little after working for 25 h. The blue line is the result of fitting the experimental data using a linear decay formula (Figure 6d); the lifetime of the LED with an intensity down to 80% (T80) is 452.5 h, according to the fitting result. It is known that UV−B (UV−B, 280−300 nm) light is essential for the generation of vitamin D. The LED with emission in the UV−C (UV−C, 200−275 nm) region can be converted to ultraviolet B (UV− B, 280−300 nm) light by using the CNDs as phosphors. In our previous work, multicolor CND phosphors in the visible range have been prepared.50 In order to test whether the DUV LEDs can be used as an excitation source, the LEDs were employed as the excitation source for the multicolor CNDs. Figure S9 shows the images of the CNDs under sunlight and the CND LED illumination. Blue, green, and red fluorescence of the CND phosphors can be observed under illumination of the CND-LED, promising their potential applications in DUV light sources. Conclusions. In conclusion, DUV emissive CNDs have been demonstrated under the guidance of theoretical calculations, the PL QY of the CNDs is up to 31.6%, which is the highest value ever reported in this region. The strong DUV emission of the CNDs is derived from the radiative recombination of π−π* of the carbon-core state and surface groups passivation. Benefiting from the core-state emission and surface groups passivation, the emission of the CNDs shows excitation-independent and solvent-independent properties. Additionally, DUV LEDs have been fabricated based on the CNDs, promising their potential application in DUV lightemitting sources. Considering that solid DUV light sources with a high efficiency are highly desired, increasing the fluorescence efficiency of the DUV emissive CNDs will further promote their applications in DUV light-emitting sources. Methods. Chemicals and Materials. Methyl red (purity > 99.5%) was purchased from Tianjin Damao Chemical Reagent Factory, and the OPDA, MPDA, and PPDA (purity >99%) were purchased from Shanghai Maclean Biochemical Technology Co., Limited. The chromatography silica gels were G
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integrating sphere SFDA110 under 250 nm excitation. The Fourier transform infrared (FTIR) spectra of the CNDs were recorded on a Nicolet iS10 spectrophotometer. ESCALAB 250 X-ray photoelectron spectroscopy (XPS) with a mono X-ray source Al Kα excitation (1486.6 eV) was employed to measure the bonding state of the CNDs; note that the binding energy was calibrated based on C 1s at 284.6 eV.
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S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.9b02093.
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TEM images, FTIR spectra, PL spectra, emission spectra, time-resolved luminescence decay curves, fluorescence decay lifetime value of CND-2, CND-3 in different solvents, and the preparation process (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Kai-Kai Liu: 0000-0002-1811-6699 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51602288, U1604263, and 11804307), the Key Science and Technology Project of Henan Province (171100210600)
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REFERENCES
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DOI: 10.1021/acs.nanolett.9b02093 Nano Lett. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.nanolett.9b02093 Nano Lett. XXXX, XXX, XXX−XXX
