Zn-Doped Gallium Nitride Nanotubes with Zigzag Morphology

Jul 28, 2009 - A single-crystalline zigzag nanotube consists of two building blocks with equivalent ... two peaks at 2.97 and 2.16 eV, respectively. I...
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J. Phys. Chem. C 2009, 113, 14633–14637

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Zn-Doped Gallium Nitride Nanotubes with Zigzag Morphology Xing Xie, Guan Zhong Wang,* Zhi Bin Shao, and Da Peng Li Hefei National Laboratory for Physical Sciences at Microscale, and Department of Physics, UniVersity of Science and Technology of China, Hefei, Anhui, 230026, People’s Republic of China ReceiVed: April 3, 2009; ReVised Manuscript ReceiVed: June 14, 2009

Zn-doped gallium nitride (GaN) nanotubes with zigzag morphology have been synthesized by a chemical vapor deposition method. A single-crystalline zigzag nanotube consists of two building blocks with equivalent growth directions, hexagonal cross sections, and about 10 nm wall thicknesses. The formation of the nanotube is attributed to the reduction of electrostatic interaction energy caused by the polar side surfaces of GaN. A room temperature photoluminescence spectrum of Zn-doped GaN nanotubes with zigzag morphology shows two peaks at 2.97 and 2.16 eV, respectively. Introduction Over the past decades, one-dimensional (1D) nanomaterials have attracted significant attention due to their interesting geometries, novel properties, and potential applications.1-10 Among 1D nanomaterials, those with zigzag morphology represent an unusual group of nanostructures, which may provide an additional dimension to tune properties of nanoscale devices assembled by this type of building block.11 To date, several kinds of 1D zigzag nanostructures of Ga2O3, GaN, AlN, In2S3, InP, and Zn3P2 have already been synthesized.12-17 Otherwise, another unique feature for 1D nanomaterials is the tubular structure, which is excited by the find of carbon nanotubes.1 After many materials have already been fabricated to zigzag morphology or tubular structure,12-27 nanomaterials combining these two features have fundamental importance and an advantage of application in the nanoscale devices with feasible tunability of properties. GaN is a very important semiconductor with a direct wide bandgap of 3.39 eV at room temperature, which has the significant advantage of applying in blue and ultraviolet light emitters and in high-powered electronic devices.28,29 A tubular structure of GaN was first observed in epitaxially grown GaN materials.30 Then by using an epitaxial casting method, Goldberger et al.31 fabricated ordered GaN nanotubes. Hu et al. reported the synthesis of hexagonal GaN (h-GaN) nanotubes through the conversion of amorphous gallium oxide nanotubes via a two-stage controllable process32 and cubical GaN (c-GaN) nanotubes via a template-free high-temperature route.33 However, there is neither report on the synthesis of GaN nanotubes with zigzag morphology nor report on Zn-doped GaN nanotubes. Moreover, Zn-doped GaN materials have attracted much attention in recent years.34,35 Here we demonstrate the synthesis of Zn-doped GaN nanotubes with zigzag morphology via a chemical vapor deposition method. Such zigzag nanotubes have single-crystalline structures, hexagonal cross sections, and wall thicknesses of about 10 nm. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns indicate that these nanotubes have alternate {01j11} polar side surfaces. This zigzag morphology is a result of the building blocks alternately * To whom correspondence should be addressed. Tel.: +86-55136003323. Fax: +86-551-3606266. E-mail: [email protected].

growing along two equivalent directions of [112j3] and [1j1j23], which are coplanar with the [0001] direction and taking it as the symmetrical line. The formation of these nanotubes is attributed to the reduction of the electrostatic interaction energy caused by the {01j11} polar side surfaces. These novel zigzag and tubular nanostructures would enrich the family of gallium nitride nanostructures and possibly offer new opportunities for diverse applications. Experimental Section Zn-doped GaN nanotubes with zigzag morphology were fabricated in a horizontal furnace with a quartz tube. In a typical experiment, well-mixed Ga2O3 and ZnO powder (weight ratio 8:1) served as the source was put into an alumina boat, which was positioned in the middle of a horizontal quartz tube held in an electric furnace. A 2.5-nm portion of Au coated single crystal Si (111) or ceramic substrate was put on the alumina boat over the source with the Au-coated side facing downward. The whole system was pumped to about 10-1 torr and then heated at the rate of 80 °C per minute. When the temperature reached 900 °C, 50 standard cubic centimeters per minute (sccm) Ar and 50 sccm NH3 were introduced into the quartz tube. The pressure of the quartz tube was kept at 25 Torr for 30 min by continuous pumping and inletting at the same time. The asprepared products on the substrate were characterized by field emission scanning electron microscopy (FE-SEM JEOL JSM6700F), X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM JEOL 2010), energy dispersive spectrometer (EDS), and Xe lamp photoluminescence (PL). Results and Discussion A typical SEM image of as-prepared products fabricated on Si (111) substrate is shown in Figure 1a. These quasi 1D zigzag nanostructures are several micrometers in length and about 100 nanometers in width. All nanostructures have spherical particles (Au catalyst) on their ends, which is an evidence of the vapor-liquid-solid (VLS) growth mechanism. XRD pattern of the as-prepared products is shown in Figure 1b. With the exception of the peak for Si (111) at about 29°, other peaks can be indexed to hexagonal wurtzite GaN (Joint Committee on Powder Diffraction Standards (JCPDS) File No. 76-0703). No peaks from other materials (such as Zn and Ga2O3) are observed.

10.1021/jp903079c CCC: $40.75  2009 American Chemical Society Published on Web 07/28/2009

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Figure 1. (a) Typical SEM image, and (b) the XRD pattern of the as-prepared products.

Figure 3. (a) TEM image of a nanotube, the inset shows the corresponding SAED pattern obtained from the circled part. (b) Enlarged map of a corner in the nanotube shown in part a. (c) HRTEM image of the quadrate area in part b. (d) EDS pattern of the circled area in part a shows the doping of Zn element.

Figure 2. TEM image of (a) a GaN nanotube with zigzag morphology prepared on Si substrate, and (b) a GaN nanotube prepared on ceramic substrate. (c) SEM image of nanotubes showing hexagonal cross sections (as pointed by arrows). (d) SEM image of a nanotube with zigzag morphology, and (e) corresponding structural illustration.

As shown in Figure 2, parts a and b, the hollow tubular structure of the GaN nanotubes is clearly visible due to the contrast brightness variations between the core and walls in the TEM images. Different from the results reported previously,31-33 the GaN nanotubes are not straight, but zigzagged. As shown by white arrows in Figure 2a, these zigzag nanostructures consist

of two alternate blocks, which connect with a fixed angel. As shown by arrow A in Figure 2a, there are additional convex corners between two blocks. Although the additional convex corners break the high symmetry of the zigzag nanotube, their side surfaces are parallel to other blocks’ side surfaces. The EDS spectrum of the circled area shown in Figure 2a (See Supporting Information Figure S1) indicates that the black dot is an Au particle, which may be left inside during the VLS growth of the zigzag GaN nanotube. There is an amorphous layer on the surface of the nanotube, as pointed by arrow B in Figure 2a. Different from the sample prepared on Si substrate, as shown in TEM image of Figure 2b, there is no amorphous layer on the nanotube prepared on a ceramic substrate. In this nanotube, as shown by arrows in Figure 2b, a thin plate connecting two sides of the walls is also parallel to side surfaces. In order to know more about the 3D structure of these nanotubes, we broke up these nanotubes by scuffing the substrate using a knife, and found by SEM that they have hexagonal cross sections (shown in Figure 2c). Figure 2, parts d and e, shows that the nanotube has a ridge parallel to side surfaces at the middle instead of smooth front surface when the angle between two alternate side surfaces is about 124°. As shown in Figure 3a, the TEM image of a GaN nanotube prepared on Si substrate indicates that the pitch distance (L, as defined in Figure 3a) is ranging from 40 to 80 nm, and the diameter of the zigzag (D) is about 100 nm along the whole nanotube. The SAED pattern of the selected area inside Figure 3a is the same as hexagonal GaN (viewed from the [21j1j0]

Zn-Doped Gallium Nitride Nanotubes

Figure 4. (a) Atomic structure of h-GaN viewed along the [21j1j0] direction. (b) The fundamental building block of the nanotube with zigzag morphology. (c) The schematic structure of GaN nanotube with zigzag morphology.

direction). Moreover, from the relationship between the SAED pattern and TEM image of the zigzag nanotube, it is found that the zigzag nanotube grows along [0001] direction, which is consistent with the GaN nanotubes already reported.31,32 Figure 3b is an enlarged map of a corner of the nanotube in Figure 3a, and the wall thickness of the nanotube is about 13 nm. Because the incident electron beam is parallel to [21j1j0] of h-GaN, as well as the angle between two lines is 124° (shown in Figure 3b), exactly the same value between (01j11) and (011j1) planes of h-GaN, these two alternate side surfaces of the nanotube are confirmed to be equivalent (01j11) and (011j1) planes.36 Figure 3c is the HRTEM image of the quadrate area from a joint of the nanotube in Figure 3b. The clear crystal fringes in the image indicate the joint is not a twin but single crystalline. The layer distance is about 0.52 nm, the same as the distance of {0001} planes of h-GaN. Combining the fact that the SAED patterns obtained from two different parts of a GaN nanotube are exactly the same (Figure S2 of the Supporting Information), we conclude that the whole nanotube is a single-crystalline. As shown in Figure 3d, the EDS spectrum of the nanotube indicates

J. Phys. Chem. C, Vol. 113, No. 33, 2009 14635 that the nanotube is Zn doped at the level of about 1021 cm-3. In addition, Si signal was detected in the EDS spectrum of the nanotube coating with amorphous layer, which was prepared on Si substrate. However, there is neither an amorphous layer on the surface nor an Si signal detected in the EDS spectrum of the GaN nanotubes prepared on ceramic substrates (Figures 2b and S3 of the Supporting Information). This suggests that the amorphous layer on the surface of the GaN nanotube prepared on Si substrate is silica. Si atoms could evaporate from Si substrate and form amorphous layers in the form of silica on the surface of GaN nanotubes. The schematic structure of GaN nanotubes with zigzag morphology is shown in Figure 4. The nature of {01j11} planes can be understood from Figure 4a.36 Besides (0001) planes, {01j11} are also polar surfaces in hexagonal structure. The fundamental building block of the nanotube with zigzag morphology is shown in Figure 4b. The block has hexagonal cross-section and grows along the [112j3] direction, with the two end surfaces being ( (0001), side surfaces being nonpolar {101j0}, Ga-terminated +(01j11) (or +(1j011)) and N-terminated -(01j11) (or -(1j011)). According to Zhou et al.’s13 explanation of their zigzag GaN nanowires, as well as the structure information provided in Figures 2 and 3, we conclude that these zigzag nanotubes consist of two alternate building blocks by changing growth direction from [112j3] to [1j1j23] or vice versa, as constructed in Figure 4c. These nanotubes have alternate positive charges of Ga-terminated and negative charges of N-terminated polar side surfaces of {01j11}. Compared to the nanotubes with straight polar surfaces, the distribution of the alternate positive and negative charges on the surfaces of the polar nanotubes, formed due to the zigzag morphology, can reduce the electrostatic interaction energy caused by the polar surfaces. A similar result has been found in helical ZnO nanowires by Yang et al.36 They also suggested a model in which the growth is led by changing growth directions to reduce the electrostatic interaction energy caused by the ( {101j1} polar surfaces of the nanowires. Figure 5 shows schematic structures of GaN nanotubes with and without regular zigzag morphology. As shown in Figure 5a, the regular zigzag morphology of a nanotube is formed as the building blocks alternately grow along two equivalent directions of [112j3] and [1j1j23], which are coplanar with [0001] direction and take it as the symmetrical line. Viewed in TEM, only two groups of parallel sidelines can be found because the building blocks grow in two equivalent directions. Therefore,

Figure 5. Schematic structure of GaN nanotubes (a) with, and (b) without regular zigzag morphology.

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Figure 6. PL spectrum of the GaN nanotubes.

viewed along [21j1j0] direction, the side surfaces of the nanotubes are {01j11} planes and the angle between side surfaces of part 1 and part 2 is 124°, which is consistent with the result shown in Figure 3. There is a ridge parallel to side surfaces at the middle of the front surface, consistent with the result shown in Figure 2, parts d and e. However, as shown in Figure 5b, if the building blocks grow along [112j3], then [1j1j23], and then [2j113] (these are all equivalent [112j3] directions), different from the growth pattern in Figure 5a, the surface sidelines in part 1 are not parallel to those in part 3 when the nanotube is viewed along [21j1j0]. This indicates that alternately growing along [112j3] and [1j1j23] of building blocks is necessary for the formation of these regular nanotubes with zigzag morphology that we prepared. Zn doping can provide efficient radiation recombination centers in GaN material,37 which is most promising for applications in light-emitting devices, due to the capability of covering even the blue part of the spectrum. A room temperature PL spectrum from 350 to 650 nm of the as-prepared products using Xe lamp as the excited source is exhibited in Figure 6. For a spectrum excited with a 350 nm wavelength, there are two peaks located at 2.16 and 2.97 eV, respectively. Because the silica layers do not contribute to the PL emission,38,39 the two peaks should come from Zn doping GaN nanotubes. In Zndoped GaN, Zn acts as an acceptor impurity. According to Monemar et al.’s report,40 the doping of Zn into GaN causes the formation of four different acceptor levels (such as ZnGa and ZnN-) with binding energies of 0.4, 0.7, 1.0, and 1.4 eV. Among these acceptor levels, the level with a binding energy of 0.4 eV has attracted the most attention due to its contribution to blue luminescence (BL). In our case, the strong broad peak of BL is at 2.97 eV, which could be attributed to the acceptor level of Zn complex or ZnGa. Compared with the result reported by Monemar et al., the peak at 2.97 eV has a blue shift of 0.1 eV. Due to the wall thickness of about 13 nm, which is close to the Bohr radius of exciton in GaN (∼11 nm), we suggest that the blue shift originates from the quantum confinement effect. The broad peak centered at 2.16 eV is called the yellow luminescence (YL), which is attributed to acceptor level of the ZnN-.40 Conclusions In summary, Zn-doped GaN nanotubes with zigzag morphology have been synthesized by a chemical vapor deposition method. These GaN nanotubes are single-crystalline structures with wall thicknesses about 10 nm. A nanotube is formed as the building blocks alternatively grow along two equivalent directions of [112j3] and [1j1j23], which are coplanar with [0001] direction and take it as the symmetrical line. The building block has an hexagonal cross-section and grows along [112j3], with the two end surfaces being ( (0001), side surfaces being nonpolar {101j0}, Ga-terminated +(01j11) (or +(1j011)), and

Acknowledgment. We thank Mr. Gongpu Li for his assistance with the TEM experiments. This work was supported by the Natural Science Foundation of China (Grant No. 10574122, 50772110, 50721091), the National Basic Research Program of China (2006CB922000, 2009CB939901, 2007CB925202), and KJCX2.YW.W06-3 of CAS. Supporting Information Available: EDS spectrum of the circled area in Figure 2a; TEM and SAED patterns from different parts of a GaN nanotube prepared on ceramic substrate; EDS spectrum of the sample prepared on ceramic substrate. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Iijima, S. Nature 1991, 354, 56. (2) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208. (3) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (4) Hu, J. Q.; Bando, Y.; Liu, Z. W.; Zhan, J. H.; Golberg, D.; Sekiguchi, T. Angew. Chem., Int. Ed. 2004, 43, 63. (5) Nath, M.; Govindaraj, A.; Rao, C. N. R. AdV. Mater. 2001, 13, 283. (6) Goldberger, J.; He, R.; Zhang, Y.; Lee, S.; Yan, H.; Choi, H.; Yang, P. D. Nature 2003, 422, 599. (7) Xia, Y. N.; Yang, P. D.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (8) Dai, H. J. Acc. Chem. Res. 2002, 35, 1035. (9) Patzke, G. R.; Krumeich, F.; Nesper, R. Angew. Chem., Int. Ed. 2002, 41, 2446. (10) Zhang, R. Q.; Lifshitz, Y.; Lee, S. T. AdV. Mater. 2003, 15, 635. (11) Lieber, C. M. Solid State Commun. 1998, 107, 607. (12) Zhan, J. H.; Bando, Y.; Hu, J. Q.; Xu, F. F.; Golberg, D. Small 2005, 1, 883. (13) Zhou, X. T.; Sham, T. K.; Shan, Y. Y.; Duan, X. F.; Lee, S. T.; Rosenberg, R. A. J. Appl. Phys. 2005, 97, 104315. (14) Wang, H.; Liu, G.; Yang, W.; Lin, L.; Xie, Z.; Fang, J. Y.; An, L. J. Phys. Chem. C 2007, 111, 17169. (15) Datta, A.; Sinha, G.; Panda, S. K.; Patra, A. Cryst. Growth Des. 2009, 9, 427. (16) Shen, G. Z.; Bando, Y.; Liu, B. D.; Tang, C. C.; Golberg, D. J. Phys. Chem. B 2006, 110, 20129. (17) Shen, G. Z.; Chen, P. C.; Bando, Y.; Golberg, D.; Zhou, C. W. J. Phys. Chem. C 2008, 112, 16405. (18) Tenne, R.; Margulis, L.; Genut, M.; Hodes, G. Nature 1992, 360, 444. (19) Margulis, L.; Salitra, G.; Tenne, R.; Talianker, M. Nature 1993, 365, 113. (20) Chopra, N. G.; Luyken, R. J.; Cherrey, K.; Crespi, V. H.; Cohen, M. L.; Louie, S. G.; Zettl, A. Science 1995, 269, 966. (21) Spahr, M. E.; Bitterli, P.; Nesper, R.; Mu¨ller, M.; Krumeich, F.; Nissen, H. U. Angew. Chem., Int. Ed. 1998, 37, 1263. (22) Nath, M.; Rao, C. N. R. J. Am. Chem. Soc. 2001, 123, 4841. (23) Zhu, Y. Q.; Hsu, W. K.; Kroto, H. W.; Walton, D. R. M. J. Phys. Chem. B 2002, 106, 7623. (24) Nath, M.; Rao, C. N. R. Angew. Chem., Int. Ed. 2002, 41, 3451. (25) Albu-Yaron, A.; Arad, T.; Popovitz-Biro, R.; Bar-Sadan, M.; Prior, Y.; Jansen, M.; Tenne, R. Angew. Chem., Int. Ed. 2005, 44, 4169. (26) Li, Y.; Wang, J.; Deng, Z.; Wu, Y.; Sun, X.; Yu, D.; Yang, P. J. Am. Chem. Soc. 2001, 123, 9904. (27) Wu, Q.; Hu, Z.; Wang, X.; Lu, Y.; Chen, X.; Xu, H.; Chen, Y. J. Am. Chem. Soc. 2003, 125, 10176. (28) Zolper, J. C.; Shul, R. J.; Baca, A. G.; Wilson, R. G.; Pearton, S. J.; Stall, R. A. Appl. Phys. Lett. 1996, 68, 2273. (29) Chen, Q.; Khan, M. A.; Wang, J. W.; Sun, C. J.; Shur, M. S.; Park, H. Appl. Phys. Lett. 1996, 69, 794. (30) Liliental-Weber, Z.; Chen, Y.; Ruvimov, S.; Washburn, J. Phys. ReV. Lett. 1997, 79, 2835. (31) Goldberger, J.; He, R.; Zhang, Y.; Lee, S.; Yan, H.; Choi, H.; Yang, P. Nature 2003, 422, 599.

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