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Synthesis and luminescence of nanodiamonds from carbon black
Materials Science and Engineering B 157 (2009) 11–14
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Synthesis and luminescence of nanodiamonds from carbon black Shengliang Hu a,∗ , Fei Tian b , Peikang Bai a , Shirui Cao a , Jing Sun b , Jing Yang c a
School of Materials Science and Engineering, North University of China, Taiyuan 030051, People’s Republic of China School of Materials Science and Engineering, Tianjin University, Tianjin 300072, People’s Republic of China c Tianjin Petroleum Vocational and Technical College, Tianjin 301607, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 17 July 2008 Received in revised form 1 December 2008 Accepted 3 December 2008 Keywords: Diamond Optical properties Laser beams Carbon
a b s t r a c t Dispersed nanodiamonds just several nanometers in diameter have been successfully synthesized using carbon black as the carbon source by a long-pulse-width laser irradiation in water at room temperature and normal pressure. The produced nanodiamonds can emit strong visible light after simple surface passivation. The light emission is attributed to the surface states related to linkage groups formed on nanodiamond surface. The surface-passivated nanodiamonds with stable photoluminescence have high potential application in bioimaging and medicine. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Diamond has been widely used in cutting, grinding, optics and microelectronics for its attractive physical and chemical properties [1–3]. Many synthesis methods have been developed since diamond was firstly synthesized by the high-pressure high-temperature process (HPHT) in the 1950s [4]. Spherical nanometre-sized diamonds (commercial available nanodiamond powder) were prepared by the detonation method [5]. However, two disadvantages are often observed in detonation nanodiamonds [6,7]. One is that their surface molecular groups, which constitute approximately 10–20% of the total nanodiamond mass, are difficult to remove; the other is their aggregation during synthesis and subsequent treatments. Further deagglomeration and dispersion are required to facilitate their application in various fields. Therefore, the best method is to develop a new route to the synthesis of well dispersed and pure nanodiamonds. Pulsed laser ablation has attracted intensive attention for its promising application in laser-based material processing including thin solid film preparation, nanocrystal growth, surface cleaning, and microelectronic device fabrication [8,9]. Because the highpower laser ablation in liquid is advantageous for the extreme conditions of high pressure and high temperature, pulsed laser ablation in liquid (PLAL) can be used to synthesize high-pressure phase nanomaterials such as diamond and related materials [9–12]. For the previous method mentioned above, however,
∗ Corresponding author. Tel.: +86 351 3557423; fax: +86 351 3557519. E-mail address: [email protected] (S. Hu). 0921-5107/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2008.12.001
the nanodiamond yield was rather low and the size of nanodiamonds was over 30 nm [10–13]. To date, we developed a novel method that a long-pulse-width laser with low power density was employed to irradiate graphite suspension at room temperature and normal pressure. The obtained nanodiamonds mostly ranged between 3 nm and 6 nm and were dispersed easily [3,14]. Quantum-confined particles or quantum dots have promising application in biology and medicine as fluorescent probes for their unique optical and electronic properties [15,16]. Comparing with semiconductor quantum dots, diamond nanoparticles are chemically inert, environmental benign, and can be surfacefunctionalized easily with water-soluble polymers and carboxyl groups [17]. Therefore, luminescent diamond nanoparticles are interesting alternatives as fluorescent probes. It is well known that the doped diamond can emit visible light [17,18]. However, few report that the diamond nanoparticles without doping can emit strong visible light so far. Since the nanodiamond formation results from the carbon cluster condensation in the vapor plume produced by laser irradiation, it is independent of the microstructure of the starting materials [3]. Carbon black, with a quasi-zero-dimensional structure and very small size, is cheap and easy to remove as an impurity in products compare to graphite [19]. Thereby, carbon black should be an ideal carbon source for preparing pure nanodiamonds. In this study, we report a new method to prepare nanodiamonds, i.e. irradiating carbon black in water using a long-pulse-width laser with low power density. The synthesized nanodiamonds with several nanometers are dispersive and can give strong luminescence through surface passivation. Moreover, the possible luminescent mechanism of nanodiamonds is also given.
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2. Experimental Commercial carbon black with a purity of over 99.9% and a particle size around 200 nm were used as precursors. Carbon black particles were mixed with water, and then the water suspension was irradiated by a Nd:YAG pulsed laser with power density of 9 × 106 W/cm2 . The wavelength length, frequency, pulse width, and irradiation time were 1064 nm, 20 Hz, and 0.4 ms, 4 h, respectively. The irradiated product was purified by boiling in perchloric acid. Then 600 mg poly-(ethylene glycol) HO (CH2 CH2 O)n H (PEG2000N ) was mixed with the nanodiamonds obtained by purification in water. The mixture was heated to 120 ◦ C, held for 70 h, and then cooled to room temperature, followed by centrifuging (6000 rpm) for 40 min. The photoluminescence (PL) spectra were measured by a Hitachi F4500 fluorescence spectrophotometer. FEI Tecnai G2 F20 transmission electron microscopy (TEM) with a field emission gun was employed to analyze the structure of nanodiamonds. Laser Raman spectra were taken using a Renishaw MKI-2000 laser Raman spectrometer at an excitation wavelength of 632.8 nm. Absorption spectrum was measured by using ultraviolet–visible absorption spectrophotometer (model U-1800, Hitachi Company). 3. Results and discussion 3.1. Synthesis of nanodiamond The bright field TEM image of the product without purification after laser irradiation shows some nanocrystals beside carbon black (see Fig. 1). The observed nanocrystals are very small, having diameters between 2 nm and 6 nm. After removing carbon black by boiling in perchloric acid, a great amount of nanoparticles were obtained (see Fig. 2a). To give a reliable prevalence size distribution of the purified products, we recorded more than 280 nanocrystals and carried out a statistical analysis of the different size, as shown in Fig. 2c. Gaussian fitting indicates that the maximal probability for crystallite diameter is around 3.6 nm. Selected area electron diffraction (SAED) was made to investigate the crystal structure of nanopartices. Fig. 2b displays the SAED of the purified product. The ratio of the square of the ring radius is 3:8:11:16:19:24:27. . ., which indicates that the structure is diamond type, and the rings correspond to the {1 1 1}, {2 2 0}, {3 1 1}, {4 0 0}, {3 3 1}, {4 2 2}, and {5 1 1} . . .
Fig. 2. (a) TEM bright-field image on the purified product, the inset shows HRTEM image of a diamond nanocrystal. (b) Selected-area-electron-diffraction pattern of the purified product and (c) the size distribution of diamond nanoparticles.
Fig. 1. TEM images of the product without purification after laser irradiation.
planes of the diamond structure [3,13]. In addition, the interplanar distances measured directly on the enlarged images of nanoparticles are about 0.206 nm (see the inset of Fig. 2a), which accords with the (1 1 1) plane of the cubic diamond. Raman spectroscopy is widely used to characterize the phonon modes of the different allotropes and noncrystalline phases of
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13
Fig. 3. Raman spectra of carbon black and sample obtained in our experiment.
carbon [20–22]. Fig. 3 shows the Raman spectra of the starting materials (carbon black) and the purified products (sample) obtained by the present method. The first-order spectrum of carbon black generally exhibits two broad and strongly overlapping peaks with intensity maxima at around 1350 cm−1 and around 1585 cm−1 , which corresponds to the defect band (D band) and the graphite band (G band) [20]. By comparing the Raman spectra of sample with that of carbon black there are two new intense peaks at 1331 cm−1 and 1611 cm−1 in the sample Raman spectrum. The peak at 1331 cm−1 is the first-order diamond Raman line, which proves the existence of diamond powders; the peak at 1611 cm−1 possibly results from the paired three-fold coordinated defects [21,22]. It should be pointed out that due to resonance effects the Raman cross-section for sp2 clusters is much greater than that for sp3 bonded structures, and scattering from the former often dominates the Raman spectrum, usually swamping the signal from the sp3 bonded fraction [22]. Therefore, the obtained products should be high purity. We reported the synthesis of nanodiamonds in graphite suspension by pulsed-laser irradiation [3,14]. The synthesis of nanodiamonds from carbon black should follow a similar mechanism. As the laser beam is focused through the liquid onto carbon black surface, the surface of carbon black and a small amount of the surrounding liquid are vaporized to form bubbles within the liquid [23]. It is believed that the species within the bubbles are subjected to temperatures of thousands of Kelvin and pressures of several gigapascal, which allows nanodiamonds with high-pressure-phase structure to be synthesized [24]. The structure of carbon black is of typical onion-like aggregates; this structure requires a large number of pentagon and heptagon defects to be present in the graphene lattice [25]. The introduction of pentagons, heptagons and pentagon–heptagon pair into a graphene layer modifies the topology of the structure. Generally, the insertion of a pentagon (or a heptagon) into a hexagonal network introduces positive (or negative) curvature to the graphene structure [25]. Although the insertion of a pentagon–heptagon pair into a hexagonal network introduces no net curvature the pair results in many topological features. Therefore, carbon black with such special characteristic, which is composed of many defects, is prone to vaporize under laser irradiation into carbon clusters compared to the graphite powders [19]. The enhanced high concentration of carbon clusters provides more opportunities for the formation of nanodiamonds, suggesting that the high yield rate of nanodiamonds is easily gained using carbon black as the carbon source. Moreover, carbon black remaining in the asprepared sample as a carbon impurity is easier to remove because defects such as pentagons and heptagons in their graphene lay-
Fig. 4. The absorption spectrum (labeled by ABS) and luminescence emission spectra with 400 nm excitation of water suspension of nanodiamonds and water suspension of surface-passivated nanodiamonds.
ers facilitate oxidation, thus resulting in high purity diamond nanoparticles. 3.2. Luminescence of nanodiamonds Both the as-prepared nanodiamond suspension and the mixture of the nanodiamond suspension and PEG2000N are transparent and do not emit visible light. After incubated for 70 h at 120 ◦ C, the mixture shows yellow color and gives strong visible light emission. The luminescence emission spectra of nanodiamonds before and after surface passivation are shown in Fig. 4. The peak position of photoluminescence (PL) spectrum is around 510 nm with 400 nm exciation. The absorption spectrum shown in Fig. 4 reveals the first absorption band at about 300 nm. Evidently, a huge red shift exists between emission and absorption spectrum. To investigate the PL stability of nanodiamonds, nanodiamonds upon surface passivation with PEG2000N were irradiated under UV lamp with the wavelength of 365 nm for 3 days. Whereafter, the PL spectrum was remeasured with the excitation light of 400 nm. It can be seen from Fig. 5b that the PL intensity decreases slightly, which implies that the luminescence of the passivated nanodiamonds is stable. Commercial diamond nanoparticles prepared by the detonation method were also purified by boiling in perchloric acid to get rid of the graphitized diamonds and then passivated by PEG2000N via the same incubation route. The PL spectra of detonation nanodiamonds and nanodiamonds obtained in the present work were acquired at the same parameters and compared in Fig. 5b. The PL intensity of detonation nanodiamonds is very low, which can be attributed to the aggregation of diamond nanoparticles. Fig. 5a shows the high resolution TEM of the purified detonation nanodiamonds. It can be clearly seen that diamond nanoparticles are aggregative although their size is also several nanometers. It should be pointed out here that the oxidative acid treatment was carried out in our experiment to get high purity diamond nanoparticles. Such treatment is known to introduce –COOH groups to the carbon nanomaterials surface, for example carbon nanotubes, nanodiamonds, and carbon nanoonions [2628]. Therefore, the obtained well-dispersed nanodiamonds with the large surface-to-volume ratio should have large numbers of carboxyl groups on their surfaces. Under the given temperature incubation, diamond nanoparticles with carboxyl groups may react with organic molecules (e.g. PEG2000N ) and combines with each
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covalent linkages or chemical adsorption [26–29]. Based on above consideration, we believe the surface states related to the linkage groups on the nanodiamond surface instead of band edge recombination are responsible for the visible light emission. Of course, the PL mechanism of the passivated nanodiamonds is far from clear and needs detained investigation in the further research. 4. Conclusions TEM, SAED and Raman spectra demonstrate that dispersed nanodiamonds are successfully synthesized using carbon black as the carbon source by laser irradiation in de-ionized water; the size of synthesized nanodiamonds mostly ranges between 2 nm and 6 nm. After surface passivation with PEG2000N , those diamond nanoparticles can emit strong visible light. The luminescence of surface-passivated nanodiamonds could be attributed to the surface states related to the linkage groups formed on the nanodiamond surface. Due to the low toxicity and biological compatibility, the luminescent nanodiamonds might be directly applied to biology and medicine. Acknowledgements This work is financially supported by the Natural Science Foundation of Tianjin city (No. 06YFJZJC 01200) and by the Natural Science Foundation for Young Scientists of North University of China, China. References [1] [2] [3] [4] [5] [6] [7] [8]
Fig. 5. (a) TEM image of detonation nanodiamonds purified by perchloric acid. (b) PL spectra of the passivated nanodiamonds before (short dot line) and after UV lamp irradiation for 3 days (dash line); the solid line shows the PL spectrum of the aggregated nanodiamonds from the detonation method via the same passivation route.
other by esterification or absorption [28]. Thus the surfaces of diamond nanoparticles are passivated or functionalized by organic molecules. The more dispersive nanodiamonds are, the better surface passivation becomes. Because in aggregation the interior diamond nanoparticles are difficult to oxidate during the oxidative acid treatment, there are less diamond nanoparticle surfaces to be passivated by organic molecules, leading to low PL intensity. The bulk diamond has the band gap of 5.5 eV. As the size of nanodiamonds decreases to several manometers, the band gap should expand due to quantum confinement effect, and hence the intrinsic luminescence arising from band edge emission should give UV light rather than the visible light. On the other hand, neither the as-prepared nanodiamonds nor the mixture of the purified nanodiamond suspension and PEG2000N emit visible light, the visible light emission could be only obtained after the mixture was incubated at 120 ◦ C. Under such condition, the diamond nanoparticle surfaces are passivated by organic molecules (PEG2000N ) via either
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
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Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Synthesis and luminescence of nanodiamonds from carbon black Shengliang Hu a,∗ , Fei Tian b , Peikang Bai a , Shirui Cao a , Jing Sun b , Jing Yang c a
School of Materials Science and Engineering, North University of China, Taiyuan 030051, People’s Republic of China School of Materials Science and Engineering, Tianjin University, Tianjin 300072, People’s Republic of China c Tianjin Petroleum Vocational and Technical College, Tianjin 301607, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 17 July 2008 Received in revised form 1 December 2008 Accepted 3 December 2008 Keywords: Diamond Optical properties Laser beams Carbon
a b s t r a c t Dispersed nanodiamonds just several nanometers in diameter have been successfully synthesized using carbon black as the carbon source by a long-pulse-width laser irradiation in water at room temperature and normal pressure. The produced nanodiamonds can emit strong visible light after simple surface passivation. The light emission is attributed to the surface states related to linkage groups formed on nanodiamond surface. The surface-passivated nanodiamonds with stable photoluminescence have high potential application in bioimaging and medicine. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Diamond has been widely used in cutting, grinding, optics and microelectronics for its attractive physical and chemical properties [1–3]. Many synthesis methods have been developed since diamond was firstly synthesized by the high-pressure high-temperature process (HPHT) in the 1950s [4]. Spherical nanometre-sized diamonds (commercial available nanodiamond powder) were prepared by the detonation method [5]. However, two disadvantages are often observed in detonation nanodiamonds [6,7]. One is that their surface molecular groups, which constitute approximately 10–20% of the total nanodiamond mass, are difficult to remove; the other is their aggregation during synthesis and subsequent treatments. Further deagglomeration and dispersion are required to facilitate their application in various fields. Therefore, the best method is to develop a new route to the synthesis of well dispersed and pure nanodiamonds. Pulsed laser ablation has attracted intensive attention for its promising application in laser-based material processing including thin solid film preparation, nanocrystal growth, surface cleaning, and microelectronic device fabrication [8,9]. Because the highpower laser ablation in liquid is advantageous for the extreme conditions of high pressure and high temperature, pulsed laser ablation in liquid (PLAL) can be used to synthesize high-pressure phase nanomaterials such as diamond and related materials [9–12]. For the previous method mentioned above, however,
∗ Corresponding author. Tel.: +86 351 3557423; fax: +86 351 3557519. E-mail address: [email protected] (S. Hu). 0921-5107/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2008.12.001
the nanodiamond yield was rather low and the size of nanodiamonds was over 30 nm [10–13]. To date, we developed a novel method that a long-pulse-width laser with low power density was employed to irradiate graphite suspension at room temperature and normal pressure. The obtained nanodiamonds mostly ranged between 3 nm and 6 nm and were dispersed easily [3,14]. Quantum-confined particles or quantum dots have promising application in biology and medicine as fluorescent probes for their unique optical and electronic properties [15,16]. Comparing with semiconductor quantum dots, diamond nanoparticles are chemically inert, environmental benign, and can be surfacefunctionalized easily with water-soluble polymers and carboxyl groups [17]. Therefore, luminescent diamond nanoparticles are interesting alternatives as fluorescent probes. It is well known that the doped diamond can emit visible light [17,18]. However, few report that the diamond nanoparticles without doping can emit strong visible light so far. Since the nanodiamond formation results from the carbon cluster condensation in the vapor plume produced by laser irradiation, it is independent of the microstructure of the starting materials [3]. Carbon black, with a quasi-zero-dimensional structure and very small size, is cheap and easy to remove as an impurity in products compare to graphite [19]. Thereby, carbon black should be an ideal carbon source for preparing pure nanodiamonds. In this study, we report a new method to prepare nanodiamonds, i.e. irradiating carbon black in water using a long-pulse-width laser with low power density. The synthesized nanodiamonds with several nanometers are dispersive and can give strong luminescence through surface passivation. Moreover, the possible luminescent mechanism of nanodiamonds is also given.
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S. Hu et al. / Materials Science and Engineering B 157 (2009) 11–14
2. Experimental Commercial carbon black with a purity of over 99.9% and a particle size around 200 nm were used as precursors. Carbon black particles were mixed with water, and then the water suspension was irradiated by a Nd:YAG pulsed laser with power density of 9 × 106 W/cm2 . The wavelength length, frequency, pulse width, and irradiation time were 1064 nm, 20 Hz, and 0.4 ms, 4 h, respectively. The irradiated product was purified by boiling in perchloric acid. Then 600 mg poly-(ethylene glycol) HO (CH2 CH2 O)n H (PEG2000N ) was mixed with the nanodiamonds obtained by purification in water. The mixture was heated to 120 ◦ C, held for 70 h, and then cooled to room temperature, followed by centrifuging (6000 rpm) for 40 min. The photoluminescence (PL) spectra were measured by a Hitachi F4500 fluorescence spectrophotometer. FEI Tecnai G2 F20 transmission electron microscopy (TEM) with a field emission gun was employed to analyze the structure of nanodiamonds. Laser Raman spectra were taken using a Renishaw MKI-2000 laser Raman spectrometer at an excitation wavelength of 632.8 nm. Absorption spectrum was measured by using ultraviolet–visible absorption spectrophotometer (model U-1800, Hitachi Company). 3. Results and discussion 3.1. Synthesis of nanodiamond The bright field TEM image of the product without purification after laser irradiation shows some nanocrystals beside carbon black (see Fig. 1). The observed nanocrystals are very small, having diameters between 2 nm and 6 nm. After removing carbon black by boiling in perchloric acid, a great amount of nanoparticles were obtained (see Fig. 2a). To give a reliable prevalence size distribution of the purified products, we recorded more than 280 nanocrystals and carried out a statistical analysis of the different size, as shown in Fig. 2c. Gaussian fitting indicates that the maximal probability for crystallite diameter is around 3.6 nm. Selected area electron diffraction (SAED) was made to investigate the crystal structure of nanopartices. Fig. 2b displays the SAED of the purified product. The ratio of the square of the ring radius is 3:8:11:16:19:24:27. . ., which indicates that the structure is diamond type, and the rings correspond to the {1 1 1}, {2 2 0}, {3 1 1}, {4 0 0}, {3 3 1}, {4 2 2}, and {5 1 1} . . .
Fig. 2. (a) TEM bright-field image on the purified product, the inset shows HRTEM image of a diamond nanocrystal. (b) Selected-area-electron-diffraction pattern of the purified product and (c) the size distribution of diamond nanoparticles.
Fig. 1. TEM images of the product without purification after laser irradiation.
planes of the diamond structure [3,13]. In addition, the interplanar distances measured directly on the enlarged images of nanoparticles are about 0.206 nm (see the inset of Fig. 2a), which accords with the (1 1 1) plane of the cubic diamond. Raman spectroscopy is widely used to characterize the phonon modes of the different allotropes and noncrystalline phases of
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Fig. 3. Raman spectra of carbon black and sample obtained in our experiment.
carbon [20–22]. Fig. 3 shows the Raman spectra of the starting materials (carbon black) and the purified products (sample) obtained by the present method. The first-order spectrum of carbon black generally exhibits two broad and strongly overlapping peaks with intensity maxima at around 1350 cm−1 and around 1585 cm−1 , which corresponds to the defect band (D band) and the graphite band (G band) [20]. By comparing the Raman spectra of sample with that of carbon black there are two new intense peaks at 1331 cm−1 and 1611 cm−1 in the sample Raman spectrum. The peak at 1331 cm−1 is the first-order diamond Raman line, which proves the existence of diamond powders; the peak at 1611 cm−1 possibly results from the paired three-fold coordinated defects [21,22]. It should be pointed out that due to resonance effects the Raman cross-section for sp2 clusters is much greater than that for sp3 bonded structures, and scattering from the former often dominates the Raman spectrum, usually swamping the signal from the sp3 bonded fraction [22]. Therefore, the obtained products should be high purity. We reported the synthesis of nanodiamonds in graphite suspension by pulsed-laser irradiation [3,14]. The synthesis of nanodiamonds from carbon black should follow a similar mechanism. As the laser beam is focused through the liquid onto carbon black surface, the surface of carbon black and a small amount of the surrounding liquid are vaporized to form bubbles within the liquid [23]. It is believed that the species within the bubbles are subjected to temperatures of thousands of Kelvin and pressures of several gigapascal, which allows nanodiamonds with high-pressure-phase structure to be synthesized [24]. The structure of carbon black is of typical onion-like aggregates; this structure requires a large number of pentagon and heptagon defects to be present in the graphene lattice [25]. The introduction of pentagons, heptagons and pentagon–heptagon pair into a graphene layer modifies the topology of the structure. Generally, the insertion of a pentagon (or a heptagon) into a hexagonal network introduces positive (or negative) curvature to the graphene structure [25]. Although the insertion of a pentagon–heptagon pair into a hexagonal network introduces no net curvature the pair results in many topological features. Therefore, carbon black with such special characteristic, which is composed of many defects, is prone to vaporize under laser irradiation into carbon clusters compared to the graphite powders [19]. The enhanced high concentration of carbon clusters provides more opportunities for the formation of nanodiamonds, suggesting that the high yield rate of nanodiamonds is easily gained using carbon black as the carbon source. Moreover, carbon black remaining in the asprepared sample as a carbon impurity is easier to remove because defects such as pentagons and heptagons in their graphene lay-
Fig. 4. The absorption spectrum (labeled by ABS) and luminescence emission spectra with 400 nm excitation of water suspension of nanodiamonds and water suspension of surface-passivated nanodiamonds.
ers facilitate oxidation, thus resulting in high purity diamond nanoparticles. 3.2. Luminescence of nanodiamonds Both the as-prepared nanodiamond suspension and the mixture of the nanodiamond suspension and PEG2000N are transparent and do not emit visible light. After incubated for 70 h at 120 ◦ C, the mixture shows yellow color and gives strong visible light emission. The luminescence emission spectra of nanodiamonds before and after surface passivation are shown in Fig. 4. The peak position of photoluminescence (PL) spectrum is around 510 nm with 400 nm exciation. The absorption spectrum shown in Fig. 4 reveals the first absorption band at about 300 nm. Evidently, a huge red shift exists between emission and absorption spectrum. To investigate the PL stability of nanodiamonds, nanodiamonds upon surface passivation with PEG2000N were irradiated under UV lamp with the wavelength of 365 nm for 3 days. Whereafter, the PL spectrum was remeasured with the excitation light of 400 nm. It can be seen from Fig. 5b that the PL intensity decreases slightly, which implies that the luminescence of the passivated nanodiamonds is stable. Commercial diamond nanoparticles prepared by the detonation method were also purified by boiling in perchloric acid to get rid of the graphitized diamonds and then passivated by PEG2000N via the same incubation route. The PL spectra of detonation nanodiamonds and nanodiamonds obtained in the present work were acquired at the same parameters and compared in Fig. 5b. The PL intensity of detonation nanodiamonds is very low, which can be attributed to the aggregation of diamond nanoparticles. Fig. 5a shows the high resolution TEM of the purified detonation nanodiamonds. It can be clearly seen that diamond nanoparticles are aggregative although their size is also several nanometers. It should be pointed out here that the oxidative acid treatment was carried out in our experiment to get high purity diamond nanoparticles. Such treatment is known to introduce –COOH groups to the carbon nanomaterials surface, for example carbon nanotubes, nanodiamonds, and carbon nanoonions [2628]. Therefore, the obtained well-dispersed nanodiamonds with the large surface-to-volume ratio should have large numbers of carboxyl groups on their surfaces. Under the given temperature incubation, diamond nanoparticles with carboxyl groups may react with organic molecules (e.g. PEG2000N ) and combines with each
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S. Hu et al. / Materials Science and Engineering B 157 (2009) 11–14
covalent linkages or chemical adsorption [26–29]. Based on above consideration, we believe the surface states related to the linkage groups on the nanodiamond surface instead of band edge recombination are responsible for the visible light emission. Of course, the PL mechanism of the passivated nanodiamonds is far from clear and needs detained investigation in the further research. 4. Conclusions TEM, SAED and Raman spectra demonstrate that dispersed nanodiamonds are successfully synthesized using carbon black as the carbon source by laser irradiation in de-ionized water; the size of synthesized nanodiamonds mostly ranges between 2 nm and 6 nm. After surface passivation with PEG2000N , those diamond nanoparticles can emit strong visible light. The luminescence of surface-passivated nanodiamonds could be attributed to the surface states related to the linkage groups formed on the nanodiamond surface. Due to the low toxicity and biological compatibility, the luminescent nanodiamonds might be directly applied to biology and medicine. Acknowledgements This work is financially supported by the Natural Science Foundation of Tianjin city (No. 06YFJZJC 01200) and by the Natural Science Foundation for Young Scientists of North University of China, China. References [1] [2] [3] [4] [5] [6] [7] [8]
Fig. 5. (a) TEM image of detonation nanodiamonds purified by perchloric acid. (b) PL spectra of the passivated nanodiamonds before (short dot line) and after UV lamp irradiation for 3 days (dash line); the solid line shows the PL spectrum of the aggregated nanodiamonds from the detonation method via the same passivation route.
other by esterification or absorption [28]. Thus the surfaces of diamond nanoparticles are passivated or functionalized by organic molecules. The more dispersive nanodiamonds are, the better surface passivation becomes. Because in aggregation the interior diamond nanoparticles are difficult to oxidate during the oxidative acid treatment, there are less diamond nanoparticle surfaces to be passivated by organic molecules, leading to low PL intensity. The bulk diamond has the band gap of 5.5 eV. As the size of nanodiamonds decreases to several manometers, the band gap should expand due to quantum confinement effect, and hence the intrinsic luminescence arising from band edge emission should give UV light rather than the visible light. On the other hand, neither the as-prepared nanodiamonds nor the mixture of the purified nanodiamond suspension and PEG2000N emit visible light, the visible light emission could be only obtained after the mixture was incubated at 120 ◦ C. Under such condition, the diamond nanoparticle surfaces are passivated by organic molecules (PEG2000N ) via either
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