Construction of AlN-Based Core–Shell Nanocone Arrays for

May 23, 2011 - The application of one-dimensional AlN nanostructures as field emitters is still difficult because of their poor conductivity and tende...
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Construction of AlN-Based Core Shell Nanocone Arrays for Enhancing Field Emission Weijin Qian, Yongliang Zhang, Qiang Wu,* Chengyu He, Yu Zhao, Xizhang Wang, and Zheng Hu Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China

bS Supporting Information ABSTRACT: The application of one-dimensional AlN nanostructures as field emitters is still difficult because of their poor conductivity and tendency toward easy oxidization and hydrolyzation. In this study, by coating preformed AlN nanocone arrays, we successfully prepared AlN-based core shell nanostructures of AlN C, AlN CN, and AlN BCN, as confirmed by characterization using electron microscopy, Raman spectroscopy, and compositional analysis. The field-emission performances of the core shell nanostructures were effectively enhanced compared to that of the pristine AlN arrays, in the order AlN BCN > AlN CN > AlN C > AlN, because of the enhanced conductivity, the lower work function arising from the coating layer, and the synergetic effect between the inner core and the outer shell. The thickness of the coating layer is also an important factor influencing the field emission, and a thin coating layer is preferred for optimization. The results indicate that the construction of core shell nanostructures can efficiently improve the fieldemission performance of AlN-based nanocones, which is a promising route for the practical application of AlN-based field emitters.

1. INTRODUCTION It is well known that an excellent field emitter should exhibit some unique features such as large aspect ratio, low work function, good conductivity, and high stability, which are favorable to attain larger current densities under lower electric fields.1 In most cases, a single-component nanostructure could not have all of these advantages itself. Composite nanomaterials with heterogeneous or hybrid structures could exhibit intriguing properties arising from the synergism of different components that are beneficial for various applications, for example, fieldemission devices,2,3 light-emitting diodes,4 and field-effect transistors.5 Constructing core shell nanostructures is an efficient way to generate abundant heterogeneous or hybrid interfaces and has thus become a strategy to enhance the fieldemission (FE) performance of single-component nanostructures in recent years.2 In this respect, core shell nanostructures are usually prepared based on the following considerations: Field emitters that are sensitive to oxidation are usually protected by being coated with a surface shell of a resistance layer,2a,b whereas FE materials with low work functions, negative electron affinities, or good conductivities are coated on the nanostructures to improve the electron flow from the inner core to the coating shell for easier emission to vacuum by tunneling.2c,d In addition, coating semiconducting materials on conducting nanowires, such as metals and conductive polymers, can form triple junctions of conductor semiconductor vacuum, thereby leading to superior FE properties.2e,f In the past few years, one-dimensional AlN nanostructures have attracted increasing attention not only for their abundant r 2011 American Chemical Society

morphologies6 but also for the integration of large aspect ratios with attractive properties such as excellent thermal conductivity and hardness, high melting point, and very low or even negative electron affinity.7 These characteristics have encouraged the exploration of the FE properties of one-dimensional AlN nanostructures, and a notable emission current with a moderate turn on field has been observed.6c,8 Moreover, it was found that the FE performance could be enhanced effectively by the patterned growth of AlN nanocones to decrease the screening effect.9 Despite these promising advances, as a single-component material, the application of one-dimensional AlN nanostructures as field emitters is still difficult because of a wide band gap of 6.2 eV, resulting in poor conductivity, and a tendency toward easy oxidization and hydrolyzation.6a,b,10 To overcome these drawbacks, a worthwhile approach is to construct onedimensional nanostructures of AlN-based core shell composites. In this study, by using a two-step chemical vapor deposition method, we successfully coated preobtained AlN nanocone arrays with three different sheaths of C, CN, and BCN and characterized the FE properties of the resulting materials. In addition to the protection function, these AlN-based core shell nanostructures present much improved FE performances in comparison with the uncoated counterparts, suggesting a promising route for the practical application of AlN-based field emitters. Received: March 12, 2011 Revised: May 4, 2011 Published: May 23, 2011 11461

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Figure 1. (a d) SEM, (e g) HRTEM, (h) EDS, and (i) Raman characterizations of the products: (a) pristine AlN nanocones, (b,e) AlN C nanocones, (c,f) AlN CN nanocones, (d, g) AlN BCN nanocones. Note: Parts e and g indicate that the inner AlN nanocone has the growth direction of [001] (see the Supporting Information, Figure S1). In part h, O signals come from the unavoidable surface oxidation6a,b due to exposure to air, and Cu signals come from the TEM grid.

2. EXPERIMENTAL SECTION Growth of AlN Nanocone Arrays. AlN nanocone arrays were prepared through a vapor solid growth procedure as described in our previous report.6c Briefly, anhydrate AlCl3 and Si substrate were placed in the low- and high-temperature zones, respectively, of a two-zone tubular furnace. After the O2 and H2O in the reactor had been removed by evacuation and flushing with Ar, the two zones were separately heated to 125 and 700 °C. The vaporized AlCl3 species was carried by Ar gas (300 sccm) to the high-temperature zone, where it reacted with NH3 gas (20 sccm) to produce AlN nanocone arrays on the Si substrate. The reaction lasted for 4 h, and the system was then cooled to room temperature under Ar protection (100 sccm). Coating of AlN Nanocone Arrays. B, B2O3, and C powders in a weight ratio of 2:4:1 were ground into the mixture and then placed into a crucible. The preobtained AlN nanocone samples were placed face down above the crucible at a distance of about 0.5 cm. The furnace was heated to 1140 °C, and then N2/NH3 (NH3, 4 vol %) gas was introduced into the furnace, and the reaction was allowed to proceed for 15, 20, or 30 min. AlN nanocones coated with a BCN layer were obtained and are denoted as AlN BCN. To coat the AlN nanocones with C and CN layers, benzene and pyridine, respectively, were used as the precursors. Briefly, the AlN nanocone sample was placed at the center of the furnace and then heated to 800 °C in an Ar flow (100 sccm). The precursor was introduced by a peristaltic pump at a feed rate of 5 μL 3 min 1. The reaction was allowed to proceed for a duration of 20 min, and then the furnace was cooled

to room temperature. AlN nanocones coated with C and CN layers were thus obtained, denoted as AlN C and AlN CN, respectively. Characterization. The morphologies and microstructures of the products were observed by scanning electron microscopy (SEM; Hitachi S-4800) and high-resolution transmission electron microscopy (HRTEM; JEM-2100). The structure and composition of the as-prepared products were characterized by X-ray diffraction (XRD; Philips X’pert Pro X-ray diffractometer), energy-dispersive X-ray analysis (EDS), and X-ray photoelectron spectroscopy (XPS; Thermo ESCALAB 250). Raman spectroscopy (Renishaw Invia Raman Microscope) was also applied to measure the vibration frequency of the coating layers on the AlN nanocones. FE properties were evaluated by using a parallelplate configuration in a vacuum chamber at a pressure of about 7  10 5 Pa.

3. RESULTS AND DISCUSSION Typical morphologies, microstructures and spectroscopic characterizations of the products are shown in Figure 1. From the SEM images, it can be seen that the pristine product was quasi-aligned AlN nanocones with diameters of about 10 nm at the tips and lengths of up to hundreds of nanometers (Figure 1a). After the nanocones had been coated with C, CN, and BCN, their morphologies and sizes did not exhibit any obvious changes in comparison with those of the pristine sample (Figure 1b d), and their core shell structures were quite clear, as observed in the corresponding HRTEM images (Figure 1e g). The coating 11462

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Figure 2. XPS spectra of the AlN C, AlN CN, and AlN BCN samples: (a) C 1s, (b) B 1s, (c) N 1s. Note: The C—C binding energy in graphite (284.8 eV) was used as the calibration standard. O signals come from the unavoidable surface oxidation6a,b due to exposure to air. The binding energies for the marked species are about 284.8 eV (C—C/CdC), 285.9 eV (C—N), 287.2 eV (C—O), 288.9 eV (CdO), 190.7 eV (B—N), 191.9 eV (B—O), 396.8 eV (N—Al), 398.1 eV (N—B), 398.9 eV (N—C), 398.9 eV (PN), and 401.0 eV (GN). PN and GN represent pyridinic N and graphitic N, respectively.

Figure 3. (a) J E curves and (b) F N plots of the pristine AlN nanocones and AlN C, AlN CN, and AlN BCN core shell nanocones with an electrode gap of 100 μm. The coating time was 20 min for the core shell samples.

shells were amorphous and had thicknesses of less than 5 nm. Taking the AlN BCN core shell nanocones as an example (Figure 1g), the components of the BCN shell could be detected from the corresponding EDS analysis, which showed signals corresponding to B, C, N, and Al, as expected (Figure 1h). Similar results were also found for the AlN C and AlN CN nanocones (Figure e,f,h). The Raman spectra showed an evolution of the intensity ratio of the D band to the G band (ID/IG) (Figure 1i) from 0.80 for AlN C through 0.92 for AlN CN to 0.96 for AlN BCN, which might result from increasing disorder upon doping with N or codoping with N and B.11 Further compositional analysis by XPS presented the anticipated signals from Al, N, C, and B for the corresponding samples, as displayed in Figure 2 (also see the Supporting Information, Figures S2 and S3), in agreement with the EDS results (Figure 1h). For the AlN C sample, a major signal from C— C/CdC bonds and a minor signal from C—O bonds were observed at 284.8 and 287.2 eV, respectively, in the C 1s spectrum,12 whereas for the AlN CN and AlN BCN samples, a new additional peak from C—N bonds due to the presence of the N dopant appeared at 285.9 eV (Figure 2a).13 A trace peak at 288.9 eV could also be noticed that might be due to CdO bonds.12 It is worth mentioning that the signal from C—B bonds at ∼283.5 eV14 could not be detected in the C 1s spectrum of the

AlN BCN sample. Correspondingly, only two peaks for B—N bonds at 190.7 eV and B—O bonds at 191.9 eV were present in the B 1s spectrum,14,15 and no signal for B—C bonds at ∼189.6 eV14 was observed (Figure 2b). These results suggest that B atoms do not directly bond with C atoms in the AlN BCN material. For the N 1s spectrum (Figure 2c), only the peak at 396.8 eV from AlN nanocones was observed for the AlN C sample, as expected.16 For the AlN CN sample, two additional peaks appeared at 398.9 and 401.0 eV corresponding to pyridinic N and graphitic N, respectively,11 arising from the CN coating layer, and for the AlN BCN sample, two additional peaks were observed at 398.1 and 398.9 eV corresponding to N—B and N—C bonds from the BCN coating layer.15 Based on the preceding experimental results, it was concluded that AlN-based core shell nanostructures were successfully prepared by coating a thin layer ( AlN CN > AlN C > AlN, because of the enhanced conductivity and the lower work function arising from the coating layer and the synergetic effect between the inner core and the outer shell. The thickness of the coating layer is also an important factor influencing the field emission, and thin coating layers are preferred for optimization; otherwise, the synergetic effect is weakened at the expense of the field-emission properties. These results indicate that the coating is an efficient approach to enhancing the field-emission performance of AlN-based field emitters, which might offer a promising route for promoting the practical application of AlN-based field emitters. ’ ASSOCIATED CONTENT

bS

Supporting Information. XRD pattern, XPS spectra, estimation on the atomic content of C in BCN coating, and estimation of the work function for BCN. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was financially supported by the “973” program (2007CB935503), NSFC (20525312, 20601013, and 20873057), and the program for Changjiang Scholars and Innovative Research Team in University (PCSIRT). 11464

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