Preparation and Ultraviolet− Visible Luminescence Property of Novel

Nov 16, 2010 - School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, People's Republic of China, and School of ...
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J. Phys. Chem. C 2010, 114, 21165–21172

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Preparation and Ultraviolet-Visible Luminescence Property of Novel BN Whiskers with a Cap-Stacked Structure Bo Zhong,† Xiaoxiao Huang,† Guangwu Wen,*,†,‡ Long Xia,†,‡ Hongming Yu,† and Hongwei Bai† School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China, and School of Materials Science and Engineering, Harbin Institute of Technology at Weihai, Weihai 264209, People’s Republic of China ReceiVed: August 12, 2010; ReVised Manuscript ReceiVed: October 31, 2010

A new kind of boron nitride (BN) whisker with a novel cap-stacked structure and excellent ultraviolet-visible luminescence properties has been successfully prepared via a facile chemical vapor reaction route. SEM analysis shows that the BN whiskers are uniform with a mean diameter of 1 µm and a length up to at least 1 mm. TEM images and the electron diffraction patterns indicate that the BN whiskers are constructed by stacking numerous cap-shaped BN sheets along the axial direction. The BN sheets are almost perpendicular to the whisker axis. The chemical compositions and the bonding characteristics of the BN whiskers are confirmed by EELS, XPS, and FTIR results. The diameters of the BN whiskers could be controlled by tuning the reaction pressure and temperature. The growth mechanism and morphology evolution of the BN whiskers are explained on the basis of the vapor-liquid-solid mechanism. Both photoluminescence and cathodoluminescence spectra of the BN whiskers exhibit broad emission bands from 200 to 700 nm, indicating that they could be used as compact light emitters at both ultraviolet and visible wavelengths. 1. Introduction Low-dimensional boron nitride (BN) micro/nanostructures, including zero dimensional nanocrystals,1,2 one-dimensional (1D) nanotubes,3,4 nanowires,5 nanoribbons,6 fibers,7-9 and whiskers,10,11 as well as two-dimensional nanoplates,12,13 and nanosheets,14-16 are subjects of unwavering interest due to their unique properties and great potential electronic, optical, and mechanical applications. Among them, the 1D BN micro/ nanostructures hold considerable technological promise for device applications, but it has long been difficult to achieve compared with their carbon counterparts owing to the lack of proper starting materials and synthetic techniques. Boron nitride nanotubes (BNNTs) are by far the most thoroughly explored 1D BN nanostructures. BNNTs possess a series of extraordinary properties, such as superior Young’s modulus,17 high thermal conductivity,18 excellent electrical insulation,19 high chemical stability, and resistance to oxidation.20 In addition, BNNTs are transparent to visible light due to a wide band gap (around 5.8 eV) that is almost independent of tube chirality and morphology.19,21 Therefore, BNNTs are promising candidates for compact ultraviolet emitters, hydrogen storage media, biological probes, piezoelectric materials, and polymeric composites.22-25 Apart from the BNNTs, a variety of nonhollow 1D BN micro/ nanostructures have been successfully synthesized. BN fiber is one of the most important nonoxide composite reinforcements that can substitute for carbon fibers under electrical insulating, wave penetrant, or oxidative environments at high temperatures and was first developed by Economy et al. in the 1970s.7,26 Several other synthetic routes based on preceramic polymers have been proposed to further improve their mechanical properties. Starting from alkylaminoborazines, Toury et al. have * To whom correspondence should be addressed. E-mail: [email protected]. † Harbin Institute of Technology. ‡ Harbin Institute of Technology at Weihai.

developed several spinnable polymers from which BN fibers with tensile strengths up to 0.69 GPa were obtained.8,27 BN fibers fabricated using this method usually possess larger diameters (several tens of micrometers) and form polycrystalline materials. BN nanowires with nanocrystalline structure have also been reported.5 Those nanowires are made up of many BN (002) planes in random orientations. In addition to the hollow BNNTs as well as the nonhollow polycrystalline BN fibers and nanowires, BN whiskers with different internal structures were also synthesized by Ishii et al.10 and more recently by Zhu et al.11 These whiskers process unique microstructures and are demonstrated to be promising materials for deep-blue and UV applications. The one-dimensional BN micro/nanostructures listed above exhibit a variety of morphologies and microstructures, which not only result in different physical or chemical performances that are suitable for various technological applications but also provide opportunities for testing and understanding basic principles about the roles of dimensionality and microstructure in the fundamental properties. In the present paper, we additionally report a new type of one-dimensional BN whisker with a special cap-stacked structure. To the best of our knowledge, BN whiskers with such a special microstructure have never been reported in the literature. The novel BN whiskers are demonstrated to exhibit excellent luminescent performance in both ultraviolet and visible bands, indicating that they are uniquely suited for fabricating luminescence devices operating over a wide spectrum, including both ultraviolet and visible bands. The structure and luminescent properties of the as-prepared products have been extensively characterized. The growth mechanism of the BN whiskers is also briefly discussed. 2. Experimental Section 2.1. Material Preparation. All the chemical reagents used in this study were analytical grade (Shanghai Chemical Reagent

10.1021/jp107628z  2010 American Chemical Society Published on Web 11/16/2010

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Figure 1. XRD pattern of the as-prepared BN whiskers. Inset: digital photograph of the products.

Corp.). The synthesis of the ammonia borane (AB, H3BNH3) that was used as a precursor for the fabrication of BN whiskers in the present work has been carried out based on a previous literature procedure28 employing sodium borohydride and ammonium sulfate as starting materials. BN whiskers were grown by means of a gas pressure furnace that allowed chemical vapor reaction of precursor gases under pressure. A graphite crucible with a capacity of about 3 L was used as a reactor with a piece of graphite paper suspended in the center as a substrate. In a typical procedure, 6.0 g of ammonia borane and 1.5 g of catalyst ferrocene were mixed and loaded into the reactor. The reactor was then put into the furnace chamber. A starting N2 pressure of 0.8 MPa was established right after pumping the chamber to a base pressure of about 0.1 Pa. The furnace was subsequently heated to a temperature of 1400 °C and held for 60 min before it was finally cooled to the room temperature. The products were obtained onto both sides of the graphite paper and were directly used for characterization without further purification. 2.2. Characterization Techniques. The morphology, crystal structure, and chemical composition of the as-prepared BN whiskers were characterized by a variety of techniques, including

Zhong et al. X-ray powder diffraction (XRD, Rigaku D/max-γB X-ray diffractometer with Cu K radiation (λ ) 0.154178 nm)), field emission scanning electron microscopy (FESEM, FEI Quanta 200, Hitachi S-4700 and MX2600EF equipped with energydispersive X-ray spectroscopy (EDX)), conventional and highresolution transmission electron microscopy (TEM and HRTEM, Philips Tecnai 20 and Tecnai F30 FEG equipped with electron energy loss spectroscopy (EELS), respectively), X-ray photoelectron spectroscopy (XPS, PHI 5700 ESCA System with a PC-ACCESS data analysis system (Physical Electronics Inc.)), and Fourier transformation infrared spectroscopy (FTIR, PerkinElmer spectrum one system by using pressed KBr disks). Cathodoluminescence (CL) measurements were performed using a Gatan MONOCL3+ system installed on a JSM-7000F FESEM. Photoluminescence (PL) studies were conducted using a Hitachi F-4500 fluorescence spectrophotometer equipped with a Xe lamp at room temperature. 3. Results and Discussion 3.1. Morphological and Structural Characterization. Figure 1 shows the XRD pattern of the as-synthesized products. The diffraction peaks around 26° and 42° can be, respectively, indexed to the (002) and (100) planes of the hexagonal phase of BN with lattice parameters of a ) b ) 2.504 Å and c ) 6.656 Å (h-BN, JCPDS card No. 34-0421). This suggests that the as-prepared white products have a dominant h-BN structure. Fe-containing species were not detected within the XRD detection limit. The amorphous peak evidenced in the XRD pattern indicates that the products are not well-crystallized. This may be attributed to a relatively lower synthetic temperature (1400 °C) that is not enough for h-BN crystallization. The inset of Figure 1 presents a digital photograph of the as-grown products. A bulk amount of white fibrous products could be observed on the substrate. Typically, 200 mg of BN whiskers could be obtained in one batch, but the process should be easily scalable to larger amounts.

Figure 2. (a, b) Low-magnification SEM images of the as-grown BN whiskers showing the abundance and lengths of the products. (c, d) Highmagnification SEM images showing typical morphologies of the BN whiskers.

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Figure 3. (a) A combined SEM image of a single BN whisker. (b-d) High-magnification SEM images taken from the locations framed in part a showing the morphology evolution of the BN whisker. (e) Representative tip of the BN whiskers. All scale bars represent 1 µm unless otherwise indicated.

Figure 4. EDX spectra collected from representative (a) tips and (b) whiskers, showing the presence of the Fe element on the whisker tips.

Interesting morphologies of the as-prepared products were characterized by SEM. As can be seen from the overview images (Figure 2a,b), the BN whiskers show uniform diameters of about 1 µm and lengths up to at least 1 mm. A careful inspection of Figure 2b reveals the presence of two distinct morphologies. One is a necklace-shaped morphology (Figure 2c); the other is a cylinder-shaped morphology (Figure 2d). A deeper insight into the relationship between the two morphologies was gained by evaluating the shape evolution along one individual whisker. Figure 3a is a combined SEM image showing a single BN whisker. Magnified SEM images of different locations along the whisker, as framed in Figure 3a, are shown in Figure 3b-d. One observes that the growth of the BN whisker begins with a knob (Figure 3b). As the growth proceeds, the morphology of the BN whisker changes into a necklace shape and then a

cylinder shape (Figure 3c,d). Because the BN whisker is very long, it is hard to trace the other end of the same whisker, but instead, we could find another type of tip other than that observed in Figure 3b, as shown in Figure 3e, which is believed to represent the front end of the BN whiskers. The chemical compositions of the tips as well as the whiskers themselves were qualitatively analyzed using EDX spectroscopy, as shown in Figure 4a,b, respectively. Pronounced B and N signals are detected on both positions. The weak O, C, and Au signals are attributed to the moisture capture, conducting resin, and thin conductive Au layer deposited on the sample, respectively. It is noted that Fe signals are detected on the tips of the BN whiskers. Despite the limitations of the EDX technique, we can conclude that the growth of the BN whiskers might be catalyzed by Fe-containing species via the well-known vapor-

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Figure 5. (a) Combined TEM images of a single BN whisker. (b, c) Magnified TEM images of different segments of the BN whisker with schematic drawings showing the cap-stacked structures. (d) High-magnification TEM image of the whisker. (e) HRTEM image taken from the area framed in part d. Inset of part b: SAED pattern of the BN whisker.

liquid-solid (VLS) growth mechanism, similar to that occurring in the growth process of carbon whiskers and ZnS nanowires.29,30 It is also worth noting that no Fe-containing species could be detected by XRD; this is because the concentration of the catalyst particles in the sample is below the detection limit of the XRD technique. The detailed growth mechanism will be further discussed in later sections. To further clarify the detailed crystal structure of these BN whiskers, TEM analyses have been undertaken. Figure 5a depicts combined TEM images showing a segment of the whiskers. One can verify that the cylinder-shaped and necklaceshaped whiskers grow together. The catalyst particle responsible for the growth of the whisker is also observed. The diameter of the whisker is around 1 µm, which is consistent with the SEM observations. Magnified TEM images displayed in Figure 5b,c suggest that the whisker is constructed of numerous cap-shaped BN sheets stacking along the whisker axis with the BN sheets almost perpendicular to the axis. The selected-area electron diffraction (SAED) pattern taken from the whisker is depicted in the inset of Figure 5b. Sharp diffraction spots from h-BN (002) planes are readily observed, indicating that the whiskers are composed of h-BN. A high-magnification TEM image taken from the whisker, as marked in Figure 5c, is shown in Figure 5d. BN caps with a thickness of about 20 nm are definitely observed. The HRTEM image displayed in Figure 5e clearly suggests that the caps are constructed of well-defined BN layers with a d-spacing of about 0.34 nm, which is typical for h-BN (002) planes. To further pin down the chemical composition of the whiskers, we performed EELS analysis on the as-prepared samples. The EELS spectrum recorded from a typical whisker is displayed in Figure 6. The pronounced peaks centered around 188 and 401 eV correspond to the characteristic K-shell ionization edges of B and N atoms, respectively. The sharp π* and σ* peaks identified from the profile are typical for the sp2 bonding configuration, which is commonly observed in the EELS spectra of h-BN. Quasi-quantitative analysis by EELS gives a B/N atomic ratio of 47/53, which is close to the stoichiometric chemical composition of h-BN. FTIR analysis elucidates the bonding characteristics of the as-prepared products, as shown in Figure 7. The spectrum shows

Figure 6. EELS profile of the as-prepared BN whiskers.

Figure 7. FTIR spectrum of the as-prepared BN whiskers.

strong vibrations at 1342, 1503, 811, and 773 cm-1. The absorption band at ∼1342 cm-1 is attributed to the transverse optical (TO) mode of h-BN sheets, whereas the absorption band at ∼1503 cm-1 is ascribed to the stretching of the h-BN network, which corresponds to the longitudinal optical (LO) mode of the h-BN sheets and shows up only when the curvature of the BN sheets induces a strain on the h-BN networks.31,32 These two bands are softened with a downward shift in wavenumber compared with the corresponding bands of BNNTs. This is because of the relatively smaller curvatures of the BN caps than that of the BNNTs, which induces smaller strain on the BN sheets.32 The weak absorption bands at ∼811 and ∼773 cm-1 are associated with the out-of-plane buckling mode where boron

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J. Phys. Chem. C, Vol. 114, No. 49, 2010 21169 1D micro/nanostructure.35 In the VS mechanism, 1D micro/ nanostructures are formed directly from the source vapors without the help of molten catalyst particles. Now that Fecontaining catalyst particles are found on the tips of the BN whiskers, the growth process could be considered within the general scenario of the tip-growth rather than base-growth VLS mechanism. In the initial stage, catalyst particles were formed (Figure 10, stage a) from the decomposition of the ferrocene according to the following reaction36

Fe(C5H5)2 f Fe + H2 + CH4 + C5H6 + · · ·

Figure 8. Wide scan, N 1s (inset, upper left) and B 1s (inset, upper right) XPS spectra recorded from the as-prepared BN whiskers.

and nitrogen atoms are moving upward and downward across the plane of BN sheets.32,33 A deeper insight into the chemical composition of the asprepared BN whiskers was gained by XPS measurements. The full range survey XPS spectrum displayed in Figure 8 suggests that the products contain N and B elements. Carbon and oxygen signals primarily arose from atmospheric exposure, which is common for XPS analysis. High-resolution N 1s and B 1s spectra displayed in the insets of Figure 8 are nearly of Gaussian type and are not decomposed further. The N 1s peak at a binding energy of 398.2 eV and the B 1s peak at a binding energy of 190.3 eV can be assigned to the N and B atoms in h-BN, respectively.34 The B/N atomic ratio calculated according to the XPS spectra is around 1/0.86. Both the binding energies and the atomic ratio of boron and nitrogen atoms are close to that of h-BN. As far as applications are concerned, it is essential to be able to control the diameters of the BN whiskers. To achieve this, a series of controlled experiments were carried out. Attempts were first made to increase the reaction pressure. It was found that the diameters of the BN whiskers could be reduced by increasing the nitrogen pressure. Figure 9a,b displays typical SEM images of the BN whiskers fabricated under 1.5 MPa nitrogen pressure (about twice of the original pressure) at 1400 °C, showing that the diameters of the whiskers have been reduced from the 1-2 µm to 500-800 nm. On the other hand, reaction temperatures were also tuned to control the growth of the BN whiskers. It is revealed from the SEM measurements (Figure 9c) that, upon increasing the reaction temperature to about 1500 °C (under 0.8 MPa nitrogen pressure), the diameters of the BN whiskers are greatly reduced to about 200-500 nm. However, careful examinations by TEM (Figure 9d) suggest that the “BN whiskers” evidenced in the SEM image are hollow in nature. The BN whiskers are transformed into bamboo-shaped BNNTs at higher temperatures! The fundamental formation mechanism behind the above observations will be elucidated in the following section. 3.2. Growth Mechanism. Further advancement of this approach to the fabrication of BN whiskers requires a clear understanding of the growth mechanism. To date, the vaporliquid-solid (VLS) and vapor-solid (VS) mechanisms are two dominant growth mechanisms that can well interpret the formation of most 1D micro/nanostructures. The VLS mechanism involves two steps: the source vapors first react with a molten metallic particle, which acts as a catalyst, and form a supersaturated eutectic alloy droplet, and then the dissolved atoms would precipitate from the liquid-state droplet to form a

At the same time, ammonia borane was decomposed into gaseous borazine (BHNH)3, hydrogen (H2), and the solid residue containing B, N, and H (BNxHy) based on the following route37

BH3NH3(s) f (BHNH)3(g) + H2(g) + BNxHy(s) where s and g denote solid and gas phases, respectively. The gaseous borazine might dissolve in the melted catalyst particles at high temperatures and then precipitate as BN sheets. As the precipitation proceeded, numerous BN sheets were produced and stacked together to form the BN whiskers, as illustrated in Figure 10, stages b and c. Considering that the reaction temperature and pressure are relatively high (1400 °C and 0.8 MPa, respectively), the slight variations in diameters of the BN whiskers (see Figures 2c, 5c, and 10d) can be ascribed to the instability of the external experimental conditions, such as the unavoidable fluctuations in the pressure, temperature, and/or vapor concentration. Similar observations have been frequently reported in the literature.38 We now turn to the interpretation of the influences of the experimental parameters on the growth of the BN whiskers. As stated above, higher nitrogen pressure results in thinner BN whiskers. This is because the increased number of nitrogen molecules lowered the collision probability of the ferrocene molecules and, therefore, reduced the diameters of the catalyst particles. Upon increasing the reaction temperature to 1500 °C, the morphologies of the BN whiskers were found to be drastically altered. The BN whiskers are transformed into bamboo-shaped BNNTs with catalyst particles in the knobs. The formation mechanism of this type of nanotubes is welldocumented and can be generally explained within the stressinduced growth model.39 The B- and N-containing vapors first reacted with the molten catalyst particles to form supersaturated droplets (Figure 10, stage e). Subsequently, the BN layers would precipitate from the droplets. During this process, the newly formed BN layers would exert a stress on the formerly formed BN shells and the molten catalyst particle and lead to the extrusion of the molten catalyst particle outside of the BN shell (Figure 10, stage f). These processes repeatedly continued and formed the bamboo-shaped BNNTs (Figure 10, stage g). Another pivotal issue that remains to be addressed is the reason why the extrusion process did not occur at the lower reaction temperature (1400 °C) at which we harvest the BN whiskers. A possible interpretation is that, at lower temperature, the catalyst particles are half-melted and enclose hard cores inside. Therefore, it is hard to expect that the catalyst particles without reasonable liquidity could be extruded. This seems the key point that governs the formation of the novel cap-stacked BN whiskers. 3.3. Luminescence Properties. To investigate the luminescence properties of the BN whiskers, both photoluminescence

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Figure 9. SEM and TEM images of BN whiskers fabricated under controlled experimental conditions: (a, b) 1.5 MPa nitrogen pressure, 1400 °C and (c, d) 0.8 MPa nitrogen pressure, 1500 °C, showing that the diameters of the whiskers could be reduced by increasing the nitrogen pressure and that increasing the reaction temperature would lead to bamboo-shaped BN nanotubes.

Figure 10. Schematic illustration of the formation processes of the BN whiskers and tubes.

(PL) and cathodoluminescence (CL) spectra were recorded from the as-prepared sample at room temperature. The most striking property of the BN whiskers is that they emit intensive and stable light at both ultraviolet and visible bands, which could be utilized to fabricate wide-waveband luminescence devices. It is not straightforward to interpret the occurrence of these emission bands because of the lack of theoretical calculations for the electronic structure of such BN whiskers. However, these BN whiskers could be viewed as distorted h-BN in terms of the crystalline structure and are expected to exhibit similar optical properties to those of materials composed of deformed h-BN sheets, such as multiwalled BNNTs. Therefore, the origins of the emission bands from the BN whiskers could be reasonably explained on the basis of both luminescence studies and theoretical calculations dealing with the optical properties of BNNTs and h-BN. Figure 11a shows the PL spectrum of a concentrated ensemble of the BN whiskers excited at 200 nm. Apart from the strong signals at 400 and 600 nm that stem from the harmonic overtones of the excitation light (200 nm), a series of emission

bands arising from the BN whiskers within both ultraviolet and visible wavelengths could be clearly observed. The broad luminescence band around 330 nm is commonly observed in luminescence spectra of multiwalled BNNTs, BN nanorods, and BN whiskers and can be assigned either to impurity levels (possibly carbon or oxygen impurities) and defect centers or to radiative excitonic dark states of h-BN.40,41 Examining the spectrum in more detail, we can identify a series of peaks centered at 441, 453, 470, 484, and 495 nm (2.81, 2.74, 2.64, 2.56, and 2.51 eV in photon energy, respectively) with basically regular energy intervals. The 441 nm band is typical for BN nanostructures and is generally attributed to impurities, defects, or B and N vacancies in the nanostructures.42,43 Nevertheless, peaks with regular intervals in this energy range have never been reported in the literature. We tentatively attribute these peaks to an electronic transition at E0 ) 2.81 eV (corresponding to 441 nm) and its coupling with a phonon mode of the BN whiskers. In the inset of Figure 11a, we plot the energy of each phonon replica against the number of peaks. The slope of the linear dependence is ∼0.08 eV (645 cm-1) per phonon, which is close to the phonon mode at ∼773 cm-1 in the IR spectrum (Figure 7). Interestingly, a strong phonon replica within another energy range (3.0-4.2 eV) has recently been observed for BNNTs.44,45 In addition, a small shoulder at 546 nm could be observed in Figure 11a. Similar luminescence bands have also been reported previously by Su et al. in their PL spectrum of BNNTs, but they attribute this peak to adventitious species (AlN or silicate formed in the growth process) rather than the BNNTs themselves.41 Other luminescence studies on Si-doped as well as Eu-doped BNNTs suggest that the emission bands around 546 nm stem from the BNNTs and may be induced by the silicon or rare-earth element doping in the BNNTs.46,47 To decide between the two different explanations of the origin of the 546 nm band and to explore the nature of the luminescence lines of the BN whiskers more deeply, a cathodoluminescence (CL) spectrum has been recorded on an individual

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Figure 11. (a) Photoluminescence (PL) and (b) cathodoluminescence (CL) spectra of the as-prepared BN whiskers.

BN whisker, as shown in Figure 11b. The CL spectrum shows distinct emission bands centered at 330, 461, and 541 nm, which are basically similar to that recorded in PL spectroscopy. It is noteworthy that the luminescence band around 546 nm evidenced in the PL spectrum appears in the CL spectrum (corresponding to the 541 nm shoulder). This undoubtedly confirms that the shoulder around 546 nm in both PL and CL spectra originates from the BN whiskers rather than the adventitious species because the CL signals are collected directly from a single BN whisker. The emission mechanism might be related to the Fe doping, similar to what occurs in Si- or Eudoped BNNTs, because an Fe-containing species is used in the synthetic process. In the CL spectrum, except for the luminescence bands that possess corresponding counterparts in the PL spectrum, two new emission lines centered at about 212 and 666 nm (5.85 and 1.86 eV in photon energy, respectively) are detected. Considering that the band gap of h-BN is measured to be 5.8 eV,48 the deep UV emission at 212 nm could be indubitably attributed to a band-to-band optical transition, whereas the broad red emission at 666 nm should relate to the sp3 bonding in the BN whisker, presumably due to interlinked layers.40 Further theoretical calculations on the BN whiskers with various defects and dopants are required to understand more precisely the origins of the above observed luminescence bands, which are currently underway. It is of interest to note the differences between the PL and CL spectra of the BN whiskers. Although the two spectra are similar on the whole, there are still several differences between them. For example, two new emission bands emerge in the CL spectrum, and the phonon replica evidenced in the PL spectrum disappears in the CL spectrum. The PL and CL techniques fundamentally differ from each other in their excitation sources. The PL uses photons, whereas the CL uses electrons. One photon could produce only one electron-hole pair, but one electron can produce thousands of electron-hole pairs,49 which implies that more photons could be generated in the CL and that weak signals are more likely to be detected by CL. This explains the presence of the weak band-to-band transition band at 212 nm that appeared in the CL spectrum rather than in the PL spectrum. Another difference between the two excitation particles lies in that the electrons can penetrate deeper into the sample than the photons.49,50 Therefore, new emission bands associated with the defects in the deeper regions of the sample may appear only in the CL spectrum. This may be a reason why we have observed an emission band at 666 nm in the CL spectrum instead of the PL spectrum. It should be also noted that the PL spectrum was collected from an ensemble of BN whiskers, whereas the CL spectrum was recorded on an

individual one. This may also result in the differences between the two spectra. It seems that the occurrence of the phonon replica is dependent on individual whiskers and that not all the whiskers can emit a phonon replica around ∼470 nm. This accounts for the absence of the phonon replica in the CL spectrum that is collected from an individual, randomly selected BN whisker. 4. Conclusions Novel BN whiskers with a special cap-stacked structure have been successfully prepared by a facile chemical vapor reaction method. Combined XRD, SEM, TEM, EELS, FTIR, and XPS analyses indicate that the as-prepared products are BN whiskers constructed of stacking-cap-shaped BN layers along the whisker axis. The diameters of the BN whiskers are about 1 µm, and the lengths could reach 1 mm. By increasing the nitrogen pressure, the diameters of the BN whiskers could be reduced to 500-800 nm. At higher temperatures (1500 °C), the BN whiskers are transformed into bamboo-shaped BN nanotubes. The formation mechanisms of the BN whiskers and their variations as a function of the experimental parameters can be explained within the framework of the vapor-liquid-solid (VLS) growth model. The BN whiskers exhibit an excellent luminescence performance in both ultraviolet and visible wavelength regions and represent a promising candidate for wide-waveband luminescence devices. Acknowledgment. This work was supported by the National High-tech R&D Program (863 Program) (No. 2007AA03Z340) and the Program of Excellent Team in Harbin Institute of Technology. References and Notes (1) Hao, X. P.; Cui, D. L.; Shi, G. X.; Yin, Y. Q.; Xu, X. G.; Wang, J. Y.; Jiang, M. H.; Xu, X. W.; Li, Y. P.; Sun, B. Q. Chem. Mater. 2001, 13, 2457–2459. (2) Hou, L.; Gao, F.; Sun, G.; Gou, H.; Tian, M. Cryst. Growth Des. 2007, 7, 535–540. (3) Chopra, N. G.; Luyken, R. J.; Cherrey, K.; Crespi, V. H.; Cohen, M. L.; Louie, S. G.; Zettl, A. Science 1995, 269, 966–967. (4) Golberg, D.; Bando, Y.; Tang, C. C.; Zhi, C. Y. AdV. Mater. 2007, 19, 2413–2432. (5) Yong, J. C.; Hong, Z. Z.; Ying, C. Nanotechnology 2006, 17, 786– 789. (6) Chen, Z. G.; Zou, J.; Liu, G.; Li, F.; Wang, Y.; Wang, L. Z.; Yuan, X. L.; Sekiguchi, T.; Cheng, H. M.; Lu, G. Q. ACS Nano 2008, 2, 2183– 2191. (7) Economy, J.; Anderson, R. V. J. Polym. Sci., Part C 1967, 19, 283–297. (8) Toury, B.; Miele, P.; Cornu, D.; Vincent, H.; Bouix, J. AdV. Funct. Mater. 2002, 12, 228–234.

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