Direct Synthesis, Growth Mechanism, and Optical Properties of 3D AlN

Jan 2, 2009 - It was reported that the field electron emission of an array of Eiffel-tower-shape AlN nanotips have low turn-on and small fluctuation o...
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Direct Synthesis, Growth Mechanism, and Optical Properties of 3D AlN Nanostructures with Urchin Shapes Weiwei Lei, Dan Liu, Jian Zhang, Pinwen Zhu, Qiliang Cui,* and Guangtian Zou National Laboratory of Superhard Materials, Jilin UniVersity Changchun 130012 P. R. China

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 3 1489–1493

ReceiVed August 30, 2008; ReVised Manuscript ReceiVed NoVember 9, 2008

ABSTRACT: Aluminum nitride (AlN) nanostructures have shown novel physical and chemical properties that are essential for technological applications. We report a vapor-solid growth of novel three-dimensional (3D) AlN urchin-like nanostructure in DC arc plasma via the direct reaction between Al vapor and N2 gas without any catalyst or template. The as-prepared 3D AlN nanostructures which have urchin-like shapes consist of numerous microdaggers with sharp tips and lengths of up to several micrometers and widths of 0.5-2 µm. A growth mechanism of AlN nanostructures with urchin shapes was suggested and explained in detail. The optical properties of the AlN nanostructures with urchin shapes were also studied with photoluminescence spectrum, which reveals a broad emission, suggesting potential applications in electronic and optoelectronic nanodevices.

1. Introduction Since the discovery of carbon nanotubes (CNTs), onedimensional (1D) nanostructures with various morphologies, such as nanowires, nanorods, nanobelts, nanotips, and nanotubes, have attracted considerable attention due to their novel nanostructures, unique physical and chemical properties, and wide potential applications.1-4 In recent years, nanomaterials with novel nanostructures of other chemical compositions have also attracted research enthusiasm.5-7 Among them, the synthesis of complex three-dimensional (3D) nanostructures is of great interest, particularly for those composed of 1D nanostructures. They have a wide range of potential applications such as electron field emitter, sensors, catalysis, biomarkers, microelectronics, and energy storage.8-10 Moreover, understanding the formation mechanism of these structures with complex 3D nanostructures should lead to developments in advanced functional nanomaterials. To date, most methods of 3D nanostructures synthesis use catalysts or templates, and most of these nanostructures have been produced by stacking 1D or two-dimensional (2D) inorganic nanostructures with simple morphologies.11-15 Thus, simple, one-step, catalyst-free, and template-free methods are highly desirable for synthesis of complex 3D structures. III-nitride compounds with various structures and morphologies have in recent years attracted increasing attention due to their significant applications in optoelectronic and field-emission devices.16-19 Among them, aluminum nitride (AlN) has gained considerable interest due to its high band gap about 6.2 eV, large exciton binding energy, and various unique properties such as excellent thermal conductivity, high chemical resistance, and high melting point.20-23 Applications in field-emission (FE) of AlN materials have also increased because their electron affinity are rather small, ranging from negative values to 0.6 eV. It was reported that the field electron emission of an array of Eiffeltower-shape AlN nanotips have low turn-on and small fluctuation of FE current.24 Some researchers reported their studies on the photoluminescence (PL) properties of AlN nanostructures and indicated that the efficient visible luminescence of AlN in the 2-4 eV region could make it a promising material for light emitting applications.25 Hence, the synthesis of AlN nanostructures with controlled shapes and sizes is an important topic * Corresponding author. Tel: +86-431-85168346; fax: +86-431-85168346; e-mail: [email protected].

worthy of exploration.26-32 To our knowledge, there has not been any reports on the synthesis of high-quality complex 3D urchin-like AlN nanostructures. In this report, we present a method of direct synthesis of highquality 3D AlN nanostructures with urchin shapes through a chemical-vapor transport and condensation process in DC arc plasma without any catalyst or template. We also suggest a growth mechanism of the AlN nanostructures. Their photoluminescence spectrum has emission properties as expected, suggesting potential applications in light-emission nanodevices.

2. Experimental Section 2.1. Sample Preparation. The synthesis was carried out in an improved direct arc discharge plasma setup.33,34 In a typical run, aluminum (purity 99.999%) metal and N2 gas (purity 99.999%) were used as aluminum and nitrogen sources, respectively. This novel morphology was found in the sample produced by direct nitrification of the aluminum metal in DC arc plasma with N2 as the working medium. An aluminum column was used both as the evaporation source and as the deposition substrate. Before the direct current arc was ignited, the chamber pressure was evacuated to less than 1 Pa, and then N2 gas was introduced into the chamber. The N2 pressure was selected from 5 to 35 kPa. When the direct current arc was ignited, the input current was maintained at 90 A and the voltage was a little higher than 40 V. In the plasma, the process of nitridation involves the evaporation of aluminum, decomposition of N2, and nucleation of AlN. The formation of AlN microurchins was controlled by the reaction time and the pressure of N2. After growth for 100 min, the substrate was covered by a gray-colored crust. Finally, the products were passivated for about 24 h in pure Ar gas at 80 kPa. 2.2. Characterization. The crystal structures of AlN microurchins were characterized by X-ray diffractometry (XRD, Rigaku D/Max-A, Cu KR). The chemical composition of these microurchins was determined by energy-dispersive X-ray spectroscopy (EDS). The morphology of the complex 3D AlN microurchins was characterized by field-emission scanning electron microscopy (SEM, XL 30 ESEM FEG), transmission electron microscopy (TEM, Hitachi-8100), and high-resolution transmission electron microscopy (HRTEM, JEM-3010). Micro-Raman spectrum was measured by Renishaw inVia (excited with an Ar+ line at 514 nm). Photoluminescence spectrum was measured with an JY-T800 Raman spectrometer (excited with a He-Cd line at 325 nm). All measurements were performed at room temperature.

3. Results and Discussion After the synthesis, the gray-colored crust was collected on the aluminum anode substrate. The crystal structure of the

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Figure 1. The X-ray powder diffraction pattern of the as-synthesized AlN microurchins.

products was examined by X-ray diffraction (XRD) measurements. The XRD pattern of the as-synthesized AlN microurchins is shown in Figure 1. All observed peaks can be indexed to a pure hexagonal phase [space group: P63mc (186)] of AlN (JCPDS file No. 08-0262) with calculated lattice constants a ) b ) 3.107 Å, and c ) 4.973 Å. No peaks of any other phases or impurities were detected. The morphologies of the sample can be seen in the SEM images in Figure 2. The sample has an interesting urchin-like morphology, and a large number of complex 3D AlN microurchins were formed on the substrate as shown in Figure 2a. It can be seen that these microurchins have hemispherical 3D structures with diameters ranging from 80 to 100 µm. Such a 3D microurchin structure of AlN has not been reported before. As shown in the enlarged image of an individual microurchin (Figure 2b), the morphology of the urchin-like sphere is composed of many AlN microdaggers. The AlN microdaggers radiate from the center of the crystals forming a spherical urchinshaped structure. Figure 2c shows the high magnification SEM image of a typical AlN nanostructure with urchin shapes. The AlN microdaggers are about 0.5-2 µm in width and several micrometers in length. The surfaces of the microdaggers are very smooth. The radii of the tips of the microdaggers are 20-60 nm. The chemical composition of these microurchins was further determined by energy-dispersive X-ray spectroscopy (Figure 2d). Only peaks of the elements Al and N are present in the EDS spectrum with an approximate ratio of 1:1, implying the stoichiometry of AlN. To determine the crystal structure of the sample, systematic transmission electron microscopy (TEM) imaging, high-resolution TEM (HRTEM) imaging, and fast Fourier transform (FFT) analysis were conducted. Figure 3a is a TEM image of a single microdagger of ∼3 µm in length and 1 µm in width. The lattice spacing is measured to be 0.267 nm, which agrees well with that of the (100) plane of hexagonal AlN (Figure 3b). This also shows that the microdagger grows along the [100] direction. The inset in Figure 3b shows a fast Fourier transform (FFT) of the image which confirms the growth direction of the microdagger to be [100]. The vibrational properties of the AlN microurchins were investigated by Raman scattering techniques at room temperature. Six Raman-active modes may be predicted by group theory, that is, 1A1(TO) + 1A1(LO) + 1E1(TO) + 1E1(LO) + 2E2, because AlN belongs to the space group P63mc. Figure 4 shows the Raman spectrum of the as-grown AlN microurchins. Three distinct peaks centered at 614.3 cm-1, 659.5 cm-1, and 670.3 cm-1 are correlated to the first-order vibrational modes of A1(TO), E2(high), and E1(TO), respectively. The low-intensity

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broad peak around 905.4 cm-1 is assigned to the overlap of the modes A1(LO) and E1(LO). These results are in good agreement with previously reported results for AlN nanobelts.35 The broad and asymmetrical Raman peaks can be explained by the nanosize effects and the internal stress of the microurchins.36 To explore the formation processes of the novel AlN microurchins, we investigated the influence of the reaction time on the growth of the urchin-like nanostructures. The corresponding samples were examined by SEM. Figure 5 shows SEM images of the as-obtained samples measured after 80 min with all other conditions being kept constant at the same time. As can be seen, the products were composed of many imperfect urchin-shaped AlN crystals with diameters ranging from 30 to 80 µm smaller than the perfect urchin-like nanostructures (as seen in Figure 2a). We speculate these microurchins may be in early stages of the growth process. Figure 5b,c shows the lowand high-magnification SEM images of the AlN nanostructure with imperfect urchin shapes. It can be seen that these microurchins were composed of aggregated particles and microcones. Figure 5d shows the high magnification SEM image of a single AlN microcone of microurchin. The surface of the microcone is coarse without any smooth areas. The above experimental results indicate that appropriate reaction time plays an important role in the formation of the AlN microurchins. On the other hand, the N2 pressure gradually decreased with the increase of the reaction time during the synthetic process. Generally speaking, the optimal conditions for nucleation for the synthesis of other nitride nanomaterials are low N2 pressure (5-20 kPa) and short reaction time (10-30 min) and the nitrides synthesized are mostly nanoparticles.33,34 However, in this experiment, only AlN nanowires were obtained when the N2 pressure, voltage, current, and reaction time are 30 kPa, 30 V, 80 A, and 10 min, respectively. To obtain novel nanostructures, we increased the N2 pressure and the reaction time simultaneously. The novel microurchins were obtained when the N2 pressure and the reaction time increased to 35 kPa and 100 min, respectively. Thus, a higher pressure of nitrogen is also very critical to the growth of AlN microurchins. At higher pressure of nitrogen, both evaporating aluminum and decomposing N2 are enhanced simultaneously, resulting in the formation of large aggregates of AlN crystals. The abundant supplies of Al and N atoms lead to large nucleations of 1D AlN crystals on the surface of the AlN aggregates, ensuring the growth of microurchins consisting of many microdaggers. Low pressure of nitrogen is not conducive for the formation of large-sized aggregates of AlN crystals essential for multiple nucleations of 1D AlN crystals which would lead to the eventual formation of microurchins. Small aggregates of AlN crystals often lead to single nucleation of AlN crystals, which result in the vaporsolid growth of AlN nanowires. This is similar to the formation of AlN nanoflowers.37 In general, the 1D AlN nanostructures growth involves a catalytic-assisted vapor-liquid-solid (VLS) or vapor-solid (VS) process.29,38,39 In this work, no metallic particle catalyst or template was used, nor were metal nanoparticles found on the tips of the individual microdaggers and microcones (Figure 6). Therefore, the formation of the individual AlN microdaggers and microcones should be related to the VS mechanism due to the intrinsic properties of AlN, such as the anisotropy of its hexagonal structure.26 On the basis of the above observations, we believe that the growth of the urchin-shaped AlN crystals could be divided into four steps as shown in Figure 7. In the first step, as the processing temperature increases, the thermal decomposition of N2 and the evaporation of Al column result

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Figure 2. (a) SEM image of an urchin-like nanostructured AlN substrate with crystal growth time of 100 min. (b) Low-magnification SEM image of typical AlN nanostructures with urchin shapes. (c) High-magnification SEM image of typical AlN nanostructures with urchin shapes. (d) EDS spectrum of the AlN nanostructures with urchin shapes.

Figure 3. (a) TEM image of a microdagger with sharp tip; (b) HRTEM image corresponding to the rectangular region in (a), indicating that the microdagger is single crystalline with a growth direction of [100]. The inset is the FFT pattern of the microdagger.

Figure 4. The corresponding Raman spectrum of the as-synthesized AlN microurchins.

in the formation of Al and N vapors, which are transported by N2 carrier gas to a zone with an appropriate temperature for

the growth AlN crystals through the following reaction: N(g) + Al(l) f AlN(s). During this step, the heat convection and temperature gradient produced by DC arc plasma provide a chemical-vapor transport and condensation process, which is responsible for the nucleation of AlN.40 The second step is the formation of aggregate of AlN crystals as the nucleation center of the microurchinin on the surface of the substrate (Figure S1, Supporting Information). The third step is the nucleation of short AlN microcones and their subsequent growth (Figure 5a). The nucleation and growth of these 1D nanostructures were attributed to the intrinsic anisotrpic properties of AlN.26 The last step is the formation of the AlN microurchins (Figure 2a). During this step, as the reaction time increases, the lateral growth of microcones becomes more pronounced, giving a smoother surface due to the adhesion of many adatoms at higher pressure of nitrogen. This phenomenon was observed previously in AlN nanotips.30 As a result, microurchins consisting of microdaggers with smooth surfaces are formed.

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Figure 5. (a) SEM image of an urchin-like nanostructured AlN substrate with a crystal growth time of 80 min. (b, c) Low magnification images showing underdeveloped urchin-like nanostructures. (d) Enlarged image showing a single microcone with coarse surface.

Figure 6. (a) High-magnification SEM image of a tip of microdagger, and (b) high-magnification SEM image of a tip of microcone. Figure 8. The room-temperature photoluminescence spectrum of the as-synthesized AlN microurchins.

in AlN nanocones and pine-shaped nanostructures.29,41 The PL emission indicates that the novel 3D AlN microurchins composed of microdaggers with sharp tips may have applications in light-emitting nanodevices.

4. Conclusions

Figure 7. Proposed growth process of the urchin-like nanostructures.

Figure 8 presents the photoluminescence spectrum of the AlN microurchins, which has a broad emission band centered at 553 nm. Obviously, this band is not the direct band gap emission, but is referred to as deep-level or trap-level emission. Clearly, the emission has generally been attributed to the nitrogen vacancy and the radiative recombination of a photon- (or electron-) generated hole with an electron occupying the nitrogen vacancy,32 This phenomenon has also been observed previously

In summary, we report the synthesis of a unique 3D AlN urchin-like nanostructure through direct nitrification of the aluminum metal in DC arc plasma with N2 as the working medium. In the formation process of the high-quality 3D AlN microurchins, no templates or catalysts were used. Compared with other AlN 3D nanostructures, these AlN nanostructures have urchin-like shapes which consist of many microdaggers with shape tips having lengths up to several micrometers and diameters of 0.5-2 µm. A possible growth mechanism was proposed to explain the formation of the AlN microurchins. The unique photoluminescence properties of these novel AlN microurchins suggest they may have applications in electronic and optoelectronic nanodevices.

AlN Nanostructures with Urchin Shapes

Acknowledgment. The authors are grateful to Keh-Jim Dunn for many useful discussions. This work was supported by The Postgraduate Innovative Foundation Program of Jilin University (MS20080217), Natural Science Foundation of China (No. 50772043), and National Basic Research Program of China (Nos. 2005CB724400 and 2001CB711201). Supporting Information Available: SEM images of the AlN nanostructures synthesized at shorter times and PL spectrum of the AlN microcones. This information is available free of charge via the Internet at http://pubs.acs.org.

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