6-Fold-Symmetrical AlN Hierarchical Nanostructures: Synthesis and

Feb 13, 2009 - In this article, we report the synthesis of the new 6-fold-symmetrical AlN hierarchical nanostructures including urchinlike and flowerl...
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J. Phys. Chem. C 2009, 113, 4053–4058

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6-Fold-Symmetrical AlN Hierarchical Nanostructures: Synthesis and Field-Emission Properties Fan Zhang,† Qiang Wu,*,† Xuebin Wang,† Ning Liu,† Jing Yang,† Yemin Hu,† Leshu Yu,† Xizhang Wang,† Zheng Hu,*,† and Jianmin Zhu‡ Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, China, and National Laboratory of Solid State of Microstructures, Department of Physics, Nanjing UniVersity, Nanjing 210093, China ReceiVed: December 30, 2008; ReVised Manuscript ReceiVed: January 20, 2009

The controllable synthesis of nanomaterials with unique morphologies and sizes has attracted increasing interest because of the shape-dependent properties of the nanomaterials. In this article, we report the synthesis of the new 6-fold-symmetrical AlN hierarchical nanostructures including urchinlike and flowerlike ones assembled by AlN nanoneedles through the chemical reaction between AlCl3 and NH3 with the vaporization temperature of AlCl3 between 120 and 165 °C and the reaction temperature higher than 1050 °C. The morphologies, sizes, and densities of the AlN nanostructures could be controllably modulated by changing the reaction temperature and the vapor pressure of AlCl3. The formation mechanism of the AlN hierarchical nanostructures has been discussed on the basis of the change in the AlCl3 vapor pressure and the morphological evolution of the intermediate products. The shape-dependent properties of the AlN products have been observed, and the urchinlike nanostructures showed better optical and field-emission properties in comparison with the flowerlike ones. These results indicate the potential applications of the 6-fold-symmetrical AlN hierarchical nanostructures in optoelectronic and field-emission devices. Introduction In recent years, the controllable synthesis of nanomaterials with unique morphologies and sizes has attracted increasing interest because of the shape-dependent properties of nanomaterials. The self-assembly of one-dimensional nanostructures into hierarchical nanoarchitectures is of particular importance and is fascinating because the nanoarchitectures are promising building blocks for nanoscale electronic and photonic structures.1 They could also produce more active sites or exhibit more exciting electrical, optical, and magnetic properties than other simple nanostructures.2 During the past few years, much effort has been devoted to this issue, and some hierarchical nanostructures such as multipods,2a,b snowflakes,3 and symmetrical arrays4-6 have been reported. Generally, the morphology of the hierarchical nanostructure relies on the crystalline symmetry of the corresponding material. For instance, 6-fold-symmetrical hierarchical nanostructures are obtained for hexagonal ZnO and ε-MnO2,4b,5 whereas 4-fold ones are generated for cubic tindoped indium oxide and MgO.6 As an important member of the group III nitrides, hexagonal AlN is well known for its unique properties such as a direct wide band gap (6.2 eV), high thermal conductivity, superior mechanical strength, a high piezoelectric response, small or even negative electron affinity, and so on. Some physical and chemical methods have been developed to prepare different AlN nanostructures such as nanowires,7 nanotubes,8 nanobelts,9 and nanocones,10 which could have different applications as field electron emitters,7c,10a,11 flexible pulse-wave sensors,12 and nanoscale mechanical resonators.13 Recently, a few AlN hier* Corresponding authors. E-mail: [email protected] (Z.H.), wqchem@ nju.edu.cn (Q.W.). Tel.: 0086-25-83686015. Fax: 0086-25-83686251. † School of Chemistry and Chemical Engineering, Nanjing University. ‡ Department of Physics, Nanjing University.

archical nanostructures of nanoflowers,14 comblike nanoarchitectures,1a and multiple-nanotip nanorods15 have also been prepared. However, all of these AlN hierarchical nanostructures have random morphologies, and symmetrical nanostructures have not been obtained to date. Here we report the first synthesis of the 6-fold-symmetrical AlN hierarchical nanostructures assembled by AlN nanoneedles via the reaction of AlCl3 and NH3. By regulating the vaporization temperature (VT) of AlCl3, the hierarchical nanostructures could evolve from flowerlike to urchinlike while retaining the 6-fold symmetry. The formation mechanism has been discussed on the basis of the change in the AlCl3 vapor pressure and the morphologic evolution of the intermediate products. The promising photoluminescence and field-emission properties of the products were observed and compared with each other, which indicate their potential in the fields of light emission and field emission. Experimental Section The synthesis of AlN hierarchical nanostructures was conducted in a horizontal tubular furnace with two temperature zones (Supporting Information). Typically, 0.5 g of anhydrous AlCl3 was placed in the low-temperature zone, and a Si(111) substrate was placed at the center of the high-temperature zone for the deposition of product. Under the protection of Ar gas, AlCl3 was vaporized in the low-temperature zone at the designed temperature below its sublimation point (180 °C). The vapor was subsequently transported by Ar flow (180 cm3 min-1) to the high-temperature zone to react with NH3/N2 at 240 cm3 min-1 (NH3, 4 vol %) for about 3 h. AlN hierarchical nanostructures were deposited on Si substrates. The morphologies and structures of the products were characterized by scanning electron microscopy (SEM, LEO1530VP), transmission electron microscopy (TEM, JEM-1005),

10.1021/jp811484r CCC: $40.75  2009 American Chemical Society Published on Web 02/13/2009

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Figure 1. Typical SEM and TEM images of the AlN products deposited on Si substrates at different RTs while keeping the VT of AlCl3 at or below 130 °C. (a) SEM image of AlN nanorods (RT ) 950 °C, VT ) 130 °C). (b) SEM image of AlN flowerlike nanostructures (RT ) 1050 °C, VT ) 130 °C). The inset is the enlargement of an individual nanoflower. (c) TEM image of an individual nanoflower in image b. The inset is the sketch of an individual nanoflower. (d) SEM image of AlN flowerlike nanostructures (RT ) 1100 °C, VT ) 130 °C). The inset is the corresponding TEM image. (e) Enlargement of an individual nanoflower in image d. (f) SEM images of AlN flowerlike nanostructures (RT ) 1200 °C, VT ) 120 °C).

high-resolution transmission electron microscopy (HRTEM, JEM-40001X), and X-ray diffraction (XRD, Philips X’pert Pro X-ray diffractometer with 1.5406 Å Cu KR radiation). The optical properties of the products were studied by photoluminescence spectrometry (PL, Amino; Bowman series-2 spectrometer, excited with a He-Cd laser line at 325 nm) at room temperature. Field-emission measurements were carried out using a parallel-plate diode configuration in a test chamber maintained at a pressure of 7 × 10-5 Pa. Results and Discussion Figure 1 shows typical SEM and TEM images of the AlN products deposited on the Si substrate in the reaction temperature (RT) range of 950-1200 °C while keeping the VT of AlCl3 at 130 °C (Figure 1a-e) and 120 °C (Figure 1f). It is seen that, at an RT of 950 °C, the products are all short nanorods with a diameter of about 100 nm (Figure 1a), in agreement with our previous results.10a Upon increasing the RT to 1050 °C, quantities of flowerlike AlN nanostructures were grown on the substrate (Figure 1b). The individual nanoflower has a length dimension of about 1000 nm, which is assembled by nanoneedles with a diameter of ca. 15 nm at the root and a length of 300-600 nm (Figure 1b,c). The multiple rows of nanoneedles protruding from the center are 6-fold-symmetrical, and the angle between the neighboring rows is close to 60°, as illustrated in the sketch on the left inset of Figure 1c. Further increasing the RT to 1100 °C could obviously increase the density of the

product, whereas the morphology and symmetry of the product change very little (Figure 1d,e). When increasing the RT to 1200 °C but lowering the VT to 120 °C, the flowerlike nanostructures were still obtained (Figure 1f). To learn about the growth direction of the nanoneedles, HRTEM characterization was performed on an individual nanoneedle separated from the nanoflower obtained at an RT of 1100 °C, as shown in Figure 2. The interlayer spacing of 0.183 nm corresponds to the d102 value of h-AlN (Figure 2a). Accordingly, the growth direction of the nanoneedle could be determined to be [101]. The selected-area electron diffraction (SAED) patterns also illustrate the growth direction of the nanoneedles along [101] (Figure 2b). The morphologies of the products were further regulated by changing the reaction conditions. When keeping the RT at 1100 °C while increasing the VT of AlCl3 to 140 °C, the product still has the 6-fold-symmetrical flowerlike morphology (Figure 3a). In addition, some AlN microcrystals with truncated octahedral shapes were observed together with the AlN nanoflowers (inset of Figure 3a). When the VT of AlCl3 was further increased to 165 °C, the product kept the symmetrical nanoflower morphology while the number of nanoneedles in the six rows forming the nanoflower increased (Figure 3b,c) and some nanoneedles began to grow on the microcrystals (inset of Figure 3b). By keeping the VT at 165 °C while increasing the RT to 1200 °C, a large quantity of nanoneedles grew on the facets of the microcrystals, giving rise to the formation of the urchinlike

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Figure 2. (a) HRTEM image of an individual nanoneedle. (b) TEM image of the 6-fold AlN nanoflower. Insets are the SAED patterns corresponding to area 1 (upper left) and area 2 (bottom right).

Figure 3. Typical SEM images and XRD patterns of the AlN products obtained by successive regulation of the RT and VT. (a) RT ) 1100 °C and VT ) 140 °C. (b) RT ) 1100 °C and VT ) 165 °C. (c) Enlargement of an individual nanoflower in image b. (d) RT ) 1200 °C and VT ) 165 °C. (e) Local enlargement of image d. (f) XRD patterns of the AlN products in Figures 1d and 3d. Insets in images a and b are enlargements of an individual microcrystal.

hierarchical nanostructures that retain the beautiful 6-fold symmetry (Figure 3d,e). XRD diffraction peaks of all of the products could be indexed to h-AlN as typically shown in Figure 3f. Owing to the fact that no catalyst was involved, vapor-solid growth was proposed for this synthesis.16 The formation mechanism for the flowerlike and urchinlike nanostructures could be determined by analyzing the change in AlCl3 vapor pressure versus VT and the morphologies of the intermediate products obtained after 1 h of reaction. The plot of AlCl3 vapor pressure versus VT is shown in Figure 4a. It is deduced from the vapor-pressure equation of log P(mmHg) ) A - B/T, where A ) 16.24 and B ) 6006.17 The vapor pressure of AlCl3 remains at a very low level below 140 °C. In this case, the products remain the flowerlike nanostructures even though RT is varied

from 1050 to 1200 °C (Figure 1b-f). The low vapor pressure of the AlCl3 precursor brings about the deficiency of the AlN species, thus RT has little influence on the morphologies of the products. From the SEM image of the intermediate product after 1 h of growth at VT ) 130 °C and RT ) 1050 °C (Figure 4b), some nanometer-sized nuclei formed on the Si substrate, as marked by the arrows. Their further growth along six equal growth directions leads to the formation of the flowerlike nanostructures18 (Figure 3b). At high RT over 1050 °C, increasing VT from 120 to 165 °C results in the morphologic evolution (Figure 1b-f, Figure 3a-e) due to the dramatic increase in the AlCl3 vapor pressure (Figure 4a). Specifically, AlN microcrystals began to appear at VT ) 140 ° in addition to the flowerlike nanostructures (Figure 3a). It should result from sufficient AlN species for the

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Figure 4. (a) Plot of AlCl3 vapor pressure vs VT. (b) SEM image of the AlN intermediate product after 1 h of reaction at VT ) 130 °C and RT ) 1050 °C. White arrows indicate the nanometer-sized nuclei. (c) SEM image of the AlN intermediate product after 1 h of reaction at VT ) 165 °C and RT ) 1200 °C. (d) Enlargement of an individual AlN microcrystal in image c.

Figure 5. PL spectra of the flowerlike AlN nanostructures (RT ) 1100 °C and VT ) 130 °C) and urchinlike AlN nanostructures (RT ) 1200 °C and VT ) 165 °C).

high AlCl3 vapor pressure. By further increasing VT to 165 °C with higher AlCl3 vapor pressure, a large number of AlN microcrystals formed with some nanoneedles growing on their facets (inset of Figure 3b). Keeping VT at 165 °C and elevating RT to 1200 °C could accelerate the growth of the nanoneedles on the facets of the AlN microcrystals and could cover them to form urchinlike nanostructures. This is supported by SEM observation of the intermediate products after 1 h of reaction under this condition, which shows the initial growth of the nanoneedles on the microcrystals (Figure 4c,d). Light-emitting properties of the two kinds of AlN hierarchical nanostructures were characterized by PL spectroscopy. As shown in Figure 5, a strong blue emission band centered at 479 nm (2.59 eV) was observed for the urchinlike nanostructures in comparison with the weak, broad emission for the flowerlike ones. Apparently, this emission does not originate from the band emission. It may be related to the nitrogen vacancy or the oxygen impurities,19 much similar to the emission spectra from the AlN nanocones10a and nanowhiskers.20 The larger PL intensity of the urchinlike nanostructures may result from the better alignment of the AlN nanoneedles.21 The intensive PL emission from the urchinlike nanostructures indicates their potential application as blue light-emitting diodes.

Field-emission measurements were carried out using a parallel-plate diode configuration in a test chamber maintained at a pressure of 7 × 10-5 Pa. The AlN nanostructures grown on the Si substrate were used as cathodes, and another plateshaped stainless steel electrode was used as an anode with a sample-anode distance of 100 µm. Figure 6a illustrates the field-emission current density as a function of the applied electric field (J-E) for the flowerlike and urchinlike nanostructures fabricated under VT ) 130 °C, RT ) 1100 °C and VT ) 165 °C, RT ) 1200 °C, respectively. The turn-on field and the threshold field are defined to be the electric fields that produce emission current densities of 10 µA/cm2 and 1 mA/cm2, respectively. The turn-on field of 9.3 V/µm and the threshold field of 22.1 V/µm for the urchinlike nanostructures are lower than the corresponding values of 14.9 and 29.0 V/µm for the flowerlike nanostructures. This indicates that the urchinlike nanostructures show better field-emission performance than do the flowerlike nanostructures. The error bars on the J-E curves represented the fluctuation ranges of the current density, showing that the AlN hierarchical nanostructures provide reasonable emission stability. The relationship between the current density, J, and the applied electric field, E, can be analyzed by the Fowler-Nordheim (F-N) equation22

J)

(

) (

Aβ2E2 Bφ3/2 exp φ βE

)

(1)

where J is the current density, E is the applied field, φ is the work function of the emitting material with a value of 3.7 eV for AlN,23 and A and B are constants with values of 1.56 × 10-10 (A eV V-2) and 6.83 × 103 (V eV-3/2 µm-1), respectively. β is the field -enhancement factor, which could be deduced by the F-N plots of ln(J/E2) versus 1/E as typically shown in Figure 6b. The linear characteristic of the F-N curves within the measurement range confirms that the electron emission from AlN nanostructures follows F-N behavior. Field-enhancement factor β can be estimated to be about 586 and 964 for the

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Figure 6. (a) Field-emission current density (J) as a function of the applied electric field (E) for the AlN urchinlike nanostructures and flowerlike nanostructure. (b) Corresponding F-N plots.

flowerlike and urchinlike nanostructures, respectively. Generally, the aspect ratio and the tip radius are the main factors influencing β. Meanwhile, the density and alignment of the emitters also have an important influence on it.24 Higher density would generate a greater screening effect to decrease the β value,25 whereas good alignment would be favored for a high β value.26 Taking the tip radius and aspect ratio of the nanoneedles of the two nanostructures to be similar, as shown in Figures 1e,f and 3d,e, the better alignment of the nanoneedles in the urchinlike nanostructures than in the flowerlike ones contributes to the higher β value. Actually, each urchin or flower could be considered to be an emission unit. From Figures 1d and 3d, the urchin density in the urchinlike nanostructures is less than the flower density in the flowerlike nanostructures. Hence, it is suggested that good alignment and appropriate density should be the two main factors in the better field-emission performance of the urchinlike nanostructures. Conclusions In summary, two new 6-fold-symmetrical AlN hierarchical nanostructures, including urchinlike and flowerlike ones assembled by AlN nanoneedles, have been controllably synthesized on Si substrates via the chemical reaction between AlCl3 and NH3 with the VT of AlCl3 between 120 and 165 °C and RT higher than 1050 °C. The formation mechanism of the AlN hierarchical nanostructures has been discussed on the basis of the change in the AlCl3 vapor pressure and the morphologic evolution of the intermediate products. The shape-dependent properties of the AlN products have been observed, and the urchinlike nanostructures showed better optical and fieldemission properties than the flowerlike ones. The turn-on and threshold fields for the urchinlike nanostructures are 9.3 and 22.1 V/µm respectively, obviously lower than the corresponding values of 14.9 and 29.0 V/µm for the flowerlike nanostructures. The results indicate that the morphological regulation is an effective way of optimizing the performance of nanostructures. Acknowledgment. This work was financially supported by the NSFC (grant nos. 20525312 and 20601013) and the National Basic Research Program of China (2007CB935503). Supporting Information Available: Sketch map of the horizontal tubular furnace with two temperature zones. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Yin, L. W.; Bando, Y.; Zhu, Y. C.; Li, M. S.; Li, Y. B.; Golberg, D. AdV. Mater. 2005, 17, 110. (b) Heath, J. R.; Kuekes, P. J.; Snider, G. S.;

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