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Abstract

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experiments
  5. III. Results and Discussion
  6. IV. Conclusions
  7. Acknowledgments
  8. References

The BN and BCNO phosphors were prepared at 750°C using different methods and their structure and luminescent properties were investigated. All the prepared samples were turbostratic boron nitride structure. The SEM and high-resolution TEM images show that the BCNO phosphors are polycrystalline in nature and include some nanocrystals. The carbon and oxygen impurities have great effects on the excitation, emission, and absorption spectra of BN and BCNO phosphors. The first-principle calculations results indicate that the carbon and oxygen impurities will produce energy levels in the band gap, which can affect the spectra properties of BCNO phosphors. The spectra properties of BN and BCNO phosphors can be well explained by a simplified energy level diagram.

I. Introduction

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experiments
  5. III. Results and Discussion
  6. IV. Conclusions
  7. Acknowledgments
  8. References

Boron nitride (BN) especially hexagonal BN-based materials[1-3] have emerged as promising phosphors without rare-earth doping. To modulate the emission spectra of BN, much effort has been devoted to the development of boron carbon nitride (BCN) materials.[4-7] Both theoretical and experimental results show the adjustable luminescence properties of BCN materials.[8-10] However, the photoluminescence (PL) intensity and quantum efficiency of the BCN compounds are very low.[11] Recently, Okuyama[12] has synthesized boron carbon oxynitride (BCNO) phosphors with high quantum efficiency and tunable spectra using urea combustion method. The BCNO phosphors have many advantages[13] such as low sintering temperatures (below 900°C) under atmosphere pressure, low cost and nontoxicity, wide range of excitation from short ultraviolet to blue, and emission spectra from violet to near-red regions, respectively.

BCNO phosphors have great potential applications in general lighting, automobiles, white light-emitting diode, phosphorus pigments, biological imaging, and DNA labeling.[14-16] However, the luminescence mechanism of BCNO phosphor remains unclear at present. Up to now, there are two possible emission mechanisms for BCNO phosphors, one possible emission mechanism is that the emission of BCNO phosphors may be induced by the closed-shell BO and BO2 anions that act as high-efficiency luminescence centers.[17] The other possible emission mechanism is attributed to the impurity defects especially nitrogen vacancies in the BCNO nanocrystals which is responsible for the observed PL properties.[18] In addition to the nitrogen vacancies, the carbon and oxygen impurities also have great influence on luminescence properties of BCNO phosphors. In this study, the effects of carbon and oxygen impurities were investigated by experimental and theoretical points of view. In experimental part, BN was prepared using sodium borohydride (NaHB4) and ammonium chloride (NH4Cl) as raw materials (RM) without carbon and oxygen elements. In addition, BCNO phosphors were prepared by urea combustion method[12] using boric acid (H3BO3), urea (CON2H4), and polyethylene glycol (PEG) as RM. The spectra properties of BN and BCNO phosphors with different PEG masses were investigated. In theoretical part, the electronic structures of BN and BCNO were calculated with first-principle calculations and the effects of carbon and oxygen impurity levels were discussed. Finally, an energy level diagram was given to explain the effects of nitrogen vacancies, carbon and oxygen impurities on the luminescence properties of BCNO phosphors.

II. Experiments

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experiments
  5. III. Results and Discussion
  6. IV. Conclusions
  7. Acknowledgments
  8. References

BN was prepared at 750°C using NaBH4 and NH4Cl as RM. NaBH4 and NH4Cl were milled sufficiently and pressed into a disk. The disk was put into a crucible with a lid and placed in furnace when the furnace increased to 750°C. The disk was sintered at 750°C for 12 h. BN was obtained after milling and removing NaCl by washing the prepared compound, which was defined as S1. The BCNO phosphors were prepared by urea combustion method. First, the boric acid (B source), urea (N source), and PEG (H(EG)nOH with Mw = 10 000, C source) were dissolved in deionized water, and then the solution was stirred for 6 hours to form clear solution. Second, the clear solution was heated to over 100°C to evaporate water, and solid precursor was obtained. At last, the solid precursor was sintered at 750°C for 45 min under atmosphere. After washing and drying, the BCNO phosphors were obtained. In this study, the boric acid and urea were fixed at 0.02 and 0.10 mol, respectively, and the mass of PEG (10 000) was changed from 0 to 1.4 g. The BCNO phosphors were prepared with different masses of PEG (10 000) MPEG = 0, 0.2, 0.6, 0.9, 1.0, 1.1, and 1.4 g, which were defined as S2, S3, S4, S5, S6, S7, and S8, respectively.

The phase structure of BN and BCNO phosphors was characterized by powder X-ray diffraction (XRD) (Rigaku Ultima IV, Tokyo, Japan). The morphology of the phosphors was measured by a scanning electronic microscope (SEM, S–4800, Hitachi, Tokyo, Japan). The microstructure was measured by a transmitted electronic microscope (TEM, JEM2100F, JOEL, Tokyo, Japan). The excitation and emission spectra of BN and BCNO phosphors were measured by a Hitachi F–7000 spectrophotometer (Tokyo, Japan). The ultraviolet–visible–near-infrared (UV–VIS–NIR) absorption spectra were measured by a spectrophotometer (Hitachi, U–4100). All the measurements were performed at room temperature.

III. Results and Discussion

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experiments
  5. III. Results and Discussion
  6. IV. Conclusions
  7. Acknowledgments
  8. References

Figure 1(a) shows the typical XRD patterns of S1 and S6 samples. The XRD patterns show the formation of turbostratic boron nitride (t-BN) structure with two dominated peaks at 26.6° and 43.6° corresponding to (002) and (10) ((101) and (100)) reflections, respectively. Moreover, there were two distinct peaks for S1 at 31.7° and 45.4° which corresponding to the crystal faces of cubic NaCl (JCPDS Card No. 05-0628) at (200) and (220), respectively. The two peaks at 31.7° and 45.4° were induced by the NaCl remainder without washing out in S1 sample. The XRD results indicated all the prepared samples with two methods are turbostratic boron nitride structure. Figure 1(b) shows the SEM image and high-resolution TEM image [inset of Fig. 1(b)] of S6 sample. The particle shape is irregular and its size changes from a hundred nanometers to several hundreds of nanometers. The high-resolution TEM image indicates that the BCNO phosphor is polycrystalline in nature and is composed of several distinct nanocrystals, each of which is approximately less than 5 nm in size.

image

Figure 1. (a) XRD patterns of specimens S1 and S6; (b) SEM image and high-resolution TEM image (inset) of S6 sample.

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Figure 2 shows the excitation spectrum monitored at 390 nm and emission spectra excited by 215, 280, and 350 nm of the S1 sample. As shown in Fig. 2, a very broad band centered at 335 nm appears and superimposes with the structured emission under a 215-nm excitation. For 280-nm excitation, a structured band centered at 318 and 333 nm is observed. The structured emission spectra between 300 and 350 nm excited by 280 and 215 nm are induced by the internal defects of BN, mostly by nitrogen vacancy.[19] There is also a broad band centered at 385 nm in the emission spectra under a 280-nm excitation, and the emission peak is located at 390 nm when the excitation wavelength is 350 nm. The emission peak centered at 390 nm is likely induced by the oxygen related defects.[20] For excitation spectra of S1 sample monitored by 390 nm, there are three distinct excitation peaks centered around 215-, 280-, and 350-nm emission, respectively. Although there are no carbon and oxygen elements in RM, oxidation is evidently inevitable in preparing S1 sample; therefore, both excitation and emission spectra are originated from B-, N-, and O-related defects. The 215-nm (~5.7 eV) excitation is corresponding to the band gap transition of BN, and the 280-nm (~4.4 eV) excitation is likely to be correlated with intrinsic defects of BN such as N vacancy, whereas the 350 nm (~3.5 eV) excitation may be correlated with the oxygen-related defects in BN.[21-23]

image

Figure 2. Excitation spectra monitored at 390 nm and emission spectra excited by 215, 280, and 350 nm for specimen S1.

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Figure 3 shows the excitation and emission spectra of S2 sample using boric acid and urea as RM. As there are carbon elements in urea and oxygen elements in boric acid, the effects of carbon and oxygen cannot be ruled out for S2 sample. In the excitation spectra monitored at 390 nm, there are two obvious peaks centered at 315 and 350 nm, and a shoulder peak located at 280 nm, respectively. Comparing with S1 sample, there is no 215-nm excitation peak for S2 sample, which is probably induced by the carbon element of urea. The little carbon may affect the electronic structure of BN, which can be reflected by UV–VIS–NIR absorption spectra (as shown in Fig. 5). In addition, a new excitation peak centered at 315 nm appears which may be correlated with carbon impurity defects.[24] The 280-nm excitation is originated from nitrogen-related defects and the 350-nm excitation is likely induced by oxygen-related defects. The emission spectra of S2 sample excited by 280 nm are also different from that of S1 sample. No structured emission band appears between 300 and 350 nm, and a broad band centered at 365 nm displays between 300 and 500 nm. The broad emission spectra can be decomposed into two Gaussian curves: one curve centered at 350 nm, which is mostly induced by carbon-related defects, and the other curve centered at 400 nm that may correspond to oxygen-related defects. The above phenomenon can be clearly seen in the emission spectra excited by 315 nm, and the spectra are composed of contributions of two luminescence centers. The emission spectra excited by 350 nm show one broad emission centered at 390 nm, which is induced by the oxygen-related defects. For 315-nm excitation, the emission spectrum is induced by the transition from nitrogen-related defects levels to carbon and oxygen-related defects levels. While for 350-nm excitation, the one broad emission spectrum is mainly originated from the transition from nitrogen-related defects levels to oxygen-related defects levels.

image

Figure 3. Excitation spectra monitored at 390 nm and emission spectra excited by 280, 315, and 350 nm for specimen S2.

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Figure 4(a) shows the normalized excitation spectra monitored at the emission peak position for specimens S3–S8, respectively. The excitation spectrum of S3 sample monitored at 385 nm shows a broad band between 280 and 360 nm. With increasing mass of PEG (10 000) (specimens S4 and S5), the excitation spectra show two obvious bands centered at 280 and 370 nm, respectively. The 280-nm excitation is related to the intrinsic defects of BCNO, mostly nitrogen vacancy, and the 370-nm excitation is mostly correlated with carbon- and oxygen-related defects. For further increasing PEG (10 000) mass (specimens S7 and S8), the 280- and 370-nm excitation peaks change to shoulder peaks, and a new shoulder peak centered at 235 nm appears for S8 which may be induced by the superfluous carbon source. Compared with specimens S1–S2, specimens S3–S7 have excitation peaks in 320–400 nm range which change from 350 to 370 nm. The change in excitation peak position is induced by the addition of PEG (10 000) in preparing BCNO. With addition of PEG, the electronic structure of BN is changed and carbon and oxygen impurity levels will appear in band gap, which can be verified by the calculation results (as shown in Fig. 6). Figure 4(b) shows the emission spectra of S3–S8 samples, and the inset of Fig. 4(b) shows the emission peak position as a function of PEG (10 000) mass. For S3 and S8 samples, the emission spectra are in the range between 300 and 500 nm, whereas the emission spectra for specimens S4–S7 are in the range between 400 and 600 nm. The peak position increases from 385 nm for S3 to 545 nm for S6 and then decreases to 390 nm for S8, the redshift of emission peak is induced by the composition of BCNO and especially the carbon impurity concentration,[25, 26] and the blueshift of emission peak is induced by the superfluous carbon impurity.[27] For S7 and S8 samples, the carbon concentration is saturated and the redundant carbon has negative effects on the luminescence properties of BCNO phosphors, which induces the blueshift of emission peak and decrease in emission peak intensity.[28] With further increasing carbon source content, there will be no emission peak for the BCNO phosphors. From the excitation and emission spectra of BCNO phosphors, it can be concluded that the spectra of BN is induced by the intrinsic defects of BN and oxygen impurity defects, whereas the spectra of BCNO phosphors are mainly induced by the carbon and oxygen impurity defects, which can be verified by the absorption spectra and first-principle calculations.

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Figure 4. (a) Normalized excitation spectra monitored at the emission peak position for specimens S3–S8, respectively; (b) Normalized emission spectra of S3–S8 samples, and the inset shows the emission peak position as a function of PEG(10 000) mass.

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Figure 5 shows the UV–VIS–NIR absorption spectra of specimens S1–S7 in the range 240–1400 nm. For S1 sample, the absorbance decreases with increasing wavelength and a shoulder peak located at 270 nm appears in absorption spectra (inset of Fig. 5), which is induced by the intrinsic defects of BN (mostly nitrogen vacancy). The band gap absorption of BN is not observed as the spectrum is tested from 240 nm. There is a shoulder peak around 400 nm, which is likely induced by the oxygen impurity level. For S2 sample, the tendency of absorption spectra is similar to that of S1, and an absorption peak centered at 260 nm appears, which is possibly induced by the nitrogen vacancy level. For S3 sample, there are a shoulder peak located at 265 nm and two peaks centered at 295 and 410 nm, respectively. The 265-nm absorption is related to the intrinsic defects level of BN. The 295-nm (~4.2 eV) absorption is induced by the transition between carbon impurity level and conduction band as carbon impurity will produce an energy level of ~4.1 eV below conduction band in BN.[29, 30] The absorption peak centered at 410 nm (~3.0 eV) is possibly induced by the carbon and oxygen impurity defects. The nitrogen vacancy has two different types, one type is three-boron center which introduces trapping levels at 1.0 eV below conduction band, and another type is one-boron center which introduces trapping levels at 0.7 eV below conduction band.[31, 32] The 410-nm absorption may be induced by the transition from carbon impurity level to nitrogen vacancy level, and oxygen impurity level may also have a contribution to the absorption.[33] With increasing MPEG, the 265- and 295-nm absorption peaks become not obvious, and a broad absorption band in 250–700 nm range appears. The broad absorption spectra of BCNO phosphors may be induced by the broadening of carbon and oxygen impurity defects levels.

image

Figure 5. UV–VIS–NIR absorption spectra of specimens S1–S7 in the range 240–1400 nm.

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To better understand the BCNO phosphor, we performed density functional theory calculations to explore the effects of the carbon and oxygen impurities on the band structure of BN bulk. Because of the known limitations of the ab initio calculation when applied to systems involving large number of atoms, a 5 × 5 × 2 supercell is chosen as a representative of BN bulk. On the basis of the present experimental results of BCNO, we replaced one nitrogen atom with carbon atom and the other near nitrogen with oxygen atom to model the partially carbon- and oxygen-doped BN [as shown in inset of Fig. 6(a)]. In our studies, the band gap for pristine BN is calculated to be 4.1 eV, which achieves good agreement with previous studies, indicating that the methodology adopted in this work can give a reliable description of electronic structure of BN bulk. As shown in Fig. 6(a), carbon- and oxygen-doped BN is still a semiconductor and substitutional impurity induces the occupied impurity states in the gap region which located below conduction band for ~3 eV. Therefore, it is expected that the carbon and oxygen substitutional doping will result in pronounced modification in the electronic structure and optical properties of BN systems. From the partial density of state [PDOS, Fig. 6(b)], the impurity band is not only contributed by carbon and oxygen atoms but also donated by the boron and nitrogen atoms near to the dopant.

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Figure 6. (a) Total density of state of BCNO and inset shows the configuration of BCNO; (b) partial density of state of BCNO; (c) schematic energy level diagram of BCNO phosphors.

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On the basis of above results, a simplified energy level diagram can be tentatively constructed to explain the luminescence mechanism for BN and BCNO phosphors, as shown in Fig. 6(c). For BN sample, there is no carbon-related levels, and the 215-nm (~5.8 eV) excitation for S1 is induced by the band gap transition. Nitrogen vacancy levels called three-boron center (VN3) and one-boron center (VN1) will appear below conduction band 1.0 and 0.7 eV, respectively, and the 280-nm (~4.4 eV) excitation is induced by the transition from valence band to nitrogen vacancy levels for all the samples. In addition, oxygen impurity is inevitable in preparing BN and it may produce an energy level of ~4.5 eV below the conduction band. The 350-nm (~3.5 eV) excitation for specimen S1 may be induced by the transition from oxygen impurity levels to nitrogen vacancy levels. For BCNO samples, when the carbon impurity concentration is little (S2), carbon-related defects will produce an energy level of ~4.1 eV below conduction band. Except 280- and 350-nm excitation peaks, a 300-nm (~4.1 eV) excitation peak appears, which is induced by the transition from carbon-related levels to conduction band. With increasing carbon source, the 300-nm excitation peak disappears, substituted by a broad excitation band between 280 and 350 nm, which is probably induced by the broadening of carbon-related levels. The emission spectra can also be explained by the energy level diagram. For S1 sample, the emission is mainly induced by the transition from nitrogen vacancy levels to oxygen impurity levels. For S2 sample, carbon impurity levels appear and the emission is mainly induced by the transition from nitrogen vacancy levels to carbon impurity levels. With further increasing carbon source, the carbon-related levels become broader and the carbon-related levels may shift with carbon impurity concentration in the range 2 ~ 3 eV, which results in the changeable emission spectra of BCNO phosphors.[21, 24, 34] The absorption spectra are also correlated with the nitrogen vacancy levels, carbon- and oxygen-related levels, which can be understood by the energy level diagram.

IV. Conclusions

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experiments
  5. III. Results and Discussion
  6. IV. Conclusions
  7. Acknowledgments
  8. References

In summary, the BN and BCNO phosphors were prepared and the effects of carbon and oxygen impurities on the luminescence properties were investigated. Both BN and BCNO samples were turbostratic boron nitride structure. The excitation spectra were changed from separated excitation peaks for BN to a broad excitation band in 250–400 nm for BCNO, and the emission peak position of BCNO shifted to long wavelength and then to short wavelength with increasing PEG mass. The absorption peak was narrow and located around 260 nm for BN, while it became not obvious for BCNO and a broad absorption band appeared in the range 250–700 nm with increasing PEG mass. The theoretical calculation and experimental results indicated that the carbon and oxygen might introduce impurity levels in the band gap of BN, which has great effects on spectra properties of BCNO. The spectral properties of BN and BCNO phosphors could be explained well by a tentatively simplified energy level diagram.

Acknowledgments

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experiments
  5. III. Results and Discussion
  6. IV. Conclusions
  7. Acknowledgments
  8. References

This work was supported by the National Natural Science Foundation of China (nos. 51172760, 51171056, 11104073, and 51272064) and Natural Science Foundation of Hebei Province of China (nos. E2011202012 and E2012202044) and Tianjin Key Technology R&G Program (11ZCKFGX01300).

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  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experiments
  5. III. Results and Discussion
  6. IV. Conclusions
  7. Acknowledgments
  8. References
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