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J. Phys. Chem. C 2007, 111, 17506-17511
Fabrication of Single-Crystalline β-Ga2O3 Nanowires and Zigzag-Shaped Nanostructures Yuehui Wang,† Li Hou,† Xiujuan Qin, Shaodan Ma, Bo Zhang, Huiyang Gou, and Faming Gao* Department of Chemical Engineering, Yanshan UniVersity, Qinhuangdao 066004, China ReceiVed: July 8, 2007; In Final Form: September 18, 2007
Monoclinic gallium oxide (β-Ga2O3) nanowires and zigzag-shaped nanostructures were synthesized successfully by heat treating the pre-prepared GaN powder at 900 °C in dry nitrogen/oxygen atmosphere. The structures, morphologies, and compositions of the calcined products have been characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), and energy dispersive X-ray (EDX) analysis. The results revealed that the as-synthesized β-Ga2O3 nanowires and zigzag-shaped nanostuctures were pure, with high crystallinities and lengths of up to tens of micrometers. Here, The formation of diameter-modulated zigzag β-Ga2O3 nanostructures is reported for the first time. A possible growth mechanism for the formation of different morphologies of β-Ga2O3 is proposed. Besides, the optical property of β-Ga2O3 was observed in the photoluminescence (PL) spectra, which showed that the as-prepared product emits strong visible luminescence at 416 and 580 nm (λ ex ) 250 nm).
1. Introduction One-dimensional (1D) nanomaterials in the form of rods, tubes, wires, and belts have attracted significant attention over the past decade because of their interesting geometries, unique properties, and novel potential applications in many fields.1-4 Among several kinds of 1D nanostructures, the zigzag-shaped 1D structures represent an unusual group of nanomaterials, which process an additional geometry-driven property tenability within the building blocks of nanoscale devices. Several kinds of 1D zigzag nanostructures including InP,5 AlN,6 SiC,7 GaN,8 SnO2,9 and so forth have been synthesized already. These zigzag structures were explained by the small change in growth kinetics. Duan et al.10 discussed the formation of the SnO2 zigzag structure, and they thought it was due to the growth direction change. However, there are rare studies on the synthesis and characterization of zigzag-shaped β-Ga2O3 nanostructures. As a wide-band gap (Eg ) 4.9 eV) semiconductor, monoclinic gallium oxide (β-Ga2O3) possesses conduction and luminescence properties and thus has potential applications in optoelectronic devices including flat-panel displays, solar energy conversion devices, optical limiter for ultraviolet, and high-temperature stable gas sensors.11-13 So far, the synthesis of one-dimensional β-Ga2O3 nanostructures have been achieved successfully through various methods including physical evaporation,14 arc discharge,15 laser ablation,16 chemical vapor deposition,17 ball milled methods,18 and so on. However, the synthesis of β-Ga2O3 one-dimensional nanostructures from thermal annealing GaN powders has rarely been reported. In this paper, the straight and long β-Ga2O3 nanowires were synthesized successfully by thermal annealing GaN powders in dry nitrogen/oxygen atmosphere, due to the instability of the GaN material at high temperature. The as-synthesized β-Ga2O3 nanowires shows high crystallinity and most are up to tens of micrometers long. In addition, we present the synthesis of peculiar geometrical structures of β-Ga2O3 in the form of zigzag-shaped nanostruc* Corresponding author. E-mail:
[email protected]. † These authors contributed equally to this work.
tures. To the best of our knowledge, no β-Ga2O3 nanostructure with a diameter-modulated zigzag shape has been reported. The investigations of the morphology and structure via SEM and TEM hint that the formation of the zigzag β-Ga2O3 nanostructures is accomplished through repeated changing the stacking direction of the (002) plane. The zigzag-shaped 1D nanostructures represent an unusual group of nanoscale structures, which may provide an additional dimension of tunability in their properties as building blocks for the assembly of nanoscale devices. 2. Experimental Section All of the manipulations were carried out in a dry glove box with N2 flowing. In the typical process, NaN3 (1.5 g) and GaCl3 (1.65) were put into a stainless-steel autoclave of 50 mL capacity, and then the autoclave was filled with 25 mL anhydrous dimethylbenzene up to 60% of the total volume. The autoclave was sealed and maintained at 300 °C for 24 h in a furnace and then allowed to cool to room-temperature naturally. The products were collected and washed with absolute ethanol and distilled water several times to remove the impurities. Then the obtained product was dried in vacuum at 80 °C for 2 days. The dried product was used in the growth of Ga2O3 nanowires under oxygen flow (0.5% mixed with N2) at 900 °C in the tubular-stove. The as-grown products were retrieved for structural and optical characterizations. An X-ray powder diffraction (XRD) pattern was carried out on a D/max-2500/PC X-ray diffractometer with Cu KR radiation (λ ) 0.15418 nm) to study the phases present. The morphologies of the sample were characterized by transmission electron microscopy (TEM) using a JEM-2010 transmission electron microscope paralleled with energy dispersive spectroscopy (EDS) and a scanning electron microscope (SEM; XL30 ESEM FEG). Transmission electron diffraction (TED) was used to investigate the phase structure of the powder. Room-temperature PL measurement of the dry powder was recorded with a FL311 fluorescence spectrophotometer using an excitation wavelength of 250 nm and a slit wavelength of 325 nm.
10.1021/jp0753068 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/06/2007
Single-Crystalline β-Ga2O3 Nanowires
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Figure 1. XRD pattern of the synthesized sample (a) before calcination; (b) after calcination.
3. Results and Discussion XRD measurements were performed following the reaction process to assess the crystal structure and phase purity of the products obtained before and after calcination in dry nitrogen/ oxygen atmosphere. Figure 1a shows the XRD pattern of the products that were not calcined in the tubular stove. The two intense diffraction peaks at 32.91 and 56.37 can be indexed to (101) and (103) reflections of hexagonal GaN (JCPDS, 76-703). Figure 1b is a typical XRD pattern of the product obtained from annealing the sample with Figure 1a at 900 °C. All diffraction peaks in the pattern can be indexed to a monoclinic structure, in excellent agreement with the reported data of β-Ga2O3 crystals (a ) 12.23, b ) 3.04, c ) 5.80 Å, β ) 103.7°, and JCPDS 43-1012). No diffraction peaks for other phases or materials (such as GaN or Ga) are observed in XRD patterns, indicating the high purity and crystallinity of the final products. The XRD results confirm that the obtained GaN completely converted into β-Ga2O3 crystals in nitrogen/oxygen atmosphere via a N-O metathesis reaction, which occurs according to our design. The chemical reaction mechanism we employed can be formulated as19
GaCl3 + NaN3 f GaN + 3NaCl + N2
(1)
2GaN + (3/2 + x)O2(g) f Ga2O3 + 2NOx(g)
(2)
During the second process as indicated by formula 2, the replacement of N by O atoms is faciliated by the fact that the N atom has a weaker electronegativity than the O atom and N tends to escape from GaN upon thermal annealing. Thus, the vacancies and dangling bonds on the surface of GaN nanocrystals supply a favorable environment for O2 molecules to form bonds. Upon elevating the temperature, chemical bonding of O and Ga formed. At the same time, N atoms could be easily oxidized into NOx gas flowing out the stove.20 The size and morphology of the sample were examined by FESEM and TEM. As shown in Figures 2 and 3, the resulting sample contained mainly two different 1D nanostructres, the
nanowires and the zigzag-shaped nanostructures, while the particles in cluster can also be identified as the β-Ga2O3 crystal. According to the observation of SEM, the estimated yield of the nanowires is about 40%. Figure 2 shows the representative images of the as-prepared nanowires. The range of the diameter of as-grown nanowires spans from 60 to 150 nm, and length is several to tens of micrometers as shown in Figure 2a. Figure 2b shows a representative TEM image of a free-standing nanowire, which is about 60 µm long. (There is connected picture of 12 TEM images in Figure 2b, which cannot been depicted in one picture under a low multiple scanning apparatus). To the best of our knowledge, such long β-Ga2O3 nanowires synthesized by this method have not been found before. The detailed structure investigation of β-Ga2O3 nanowires have been performed by SAED and HRTEM. A typical SAED pattern, which was recorded with the electron beam perpendicular to the long axis of the wire as shown in Figure 2c, can be indexed to the [101] zone axis of β-Ga2O3. It can be seen that the SAED pattern consists of a set of diffraction spots, which is consistent with single-crystalline nature. Further evidence for the formation of single-crystalline nanowires could be found in the HRTEM image shown in Figure 2d. The HRTEM image was taken from the blue circled part in Figure 2b and clearly shows the lattice fringes, which are consistent throughout the crystal. The measured spacing between adjacent lattice planes is 0.283 nm, corresponding to the distance between (002) planes of asprepared β-Ga2O3. In addition, the angle between the (002) plane and the long axis of wire is about 75.8°, which indicates that the wire axis is parallel to the [001h] crystalline orientation of Ga2O3, and further confirming that the nanowire preferably grew along the [001h] direction. It also reveals that the examined region is free from dislocation and stacking faults, whereas the surface of the wire is terminated with thin (about 0.5 nm) and smooth amorphous layers. However, vacancy or interstitial defects may not be visible in the HRTEM observation. EDX measurement was used to estimate the stoichiometry of the wires. From the spectra as shown in Figure 2e, it can be seen that the nanowires are composed mainly of O and Ga atoms, (Cu peaks are due to the copper grid used for TEM measurements). The A% data of the O and Ga atoms are 60.42% and 39.58%, which make stoichimetric Ga2O3 clearly. Figure 3 shows the typical images of the as-prepared zigzagshaped nanostructures. On the basis of the observation of TEM, the estimated yield of the zigzag-shaped nanostructures is about 10%. It is worth noting that all of these nanostructures are free from aggregating together, presenting a free-standing appearance as shown in Figure 3a and b. The two FESEM images indicate that the as-prepared zigzag nanostructures are normally straight and ununiform in diameter (100-600 nm) and their length can very from a few to several hundred of nanometers by changing the growth direction. The TEM image of one type of zigzagshaped 1D β-Ga2O3 nanostructure (Figure 3c) clearly shows that this form of Ga2O3 nanostructure consists of alternate segments A and B and the angles formed between the two segments are almost the same, approximately 104.5°. Interestingly, these structures are also similar to those found in the Si zigzag nanostructure.21 The length (100-500 nm) of the two segments is varied, but it is much larger than its thickness. The bends or junctions remarkably repeat themselves in a regular pattern and the segments vary in length along the entire micrometer length of the structure. The inserted SAED pattern (Figure 3c), which was taken from the thin part of a zigzag-shaped nanostructure, is consistent with the single-crystalline nature. The reflections with strong intensities are determined to belong to the [010]
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Figure 2. (a) FESEM image of β-Ga2O3 nanowires. (b) A TEM of one ultra-long single-crystalline β-Ga2O3 nanowire. (c) The SAED pattern of nanowires. (d) The corresponding HRTEM image of the nanowire. (e) The EDX image of the β-Ga2O3 nanowire.
diffraction of β-Ga2O3. A further investigation on the microstructure of the nanostructures was performed by HRTEM. From Figure 3d, which was taken from the blue circled section of the zigzag-shaped nanostructure in Figure 3c, it can be seen that the average distance between the neighboring fringes is about 0.28 nm, matching with XRD, and the lattice plane is (002) ,which is in good accordance with the strongest peak in Figure 1b. In addition, the angle between the (002) plane and the A axis is about 75.9° and that between the (002) plane and the B axis is about 29.6°, which indicates that the axes of segments A and B are [001h] and [1h01h], respectively. The angle between the [001h] and [1h01h] directions is 103.7° in theory, which correlates well with the experimental value (104.5°). Combining the HRTEM image and the SAED pattern, it can be concluded that this type of zigzag nanostructure is formed by changing the growth directions from [001h] to [1h01h] or vice versa and the growth plane of both segments is (002). The other zigzag-shaped 1D β-Ga2O3 nanostructure shown in Figure 3e is straight and has a periodidically and uniform jagged morphology along its axis. The zigzag nanostructure has a modulated diameter (75150 nm), and the period (distance from valley to valley) varies from 50 to 200 nm as revealed in our TEM observation. However, the lengths of the two segments (A and B or A′ and B′) are equal to each other in each repeated unit. The corresponding selected-area electron-diffraction pattern (inset in Figure 3e) reveals that the whole zigzag-shaped 1D nanostructure is a single crystal. The spots on the pattern can be attributed to the [010] zone axis of a β-Ga2O3. The HRTEM image (as shown in Figure 3f) obtained from the blue circled area of the nanostructure in Figure 3e further confirms that the nanostructure is a single crystal, and the spacing between visible fringes is about 0.28 nm, which agrees with the (002) interplanar spacing of β-Ga2O3. Also, high-resolution images verify that
the nanostructure axes are paralled to the [103h] direction. The growth direction of the β-Ga2O3 crystal changes alternately from segment B to A in this zigzag nanostructure, along [001h] and [101h], respectively, and the growth plane of both segments is (002). In theory, the angle between [001h] and [101h] or [001] and [1h01] is 126.21°, whereas the measured angle between the two edges is 127° from the corner part of HRTEM in Figure 3f. The experimental results accord with the theoretical analysis within the admissible error. On the basis of the above experimental results, we propose that both straight-like and zigzag-shaped β-Ga2O3 nanostructures grow by self-stacking of (002) planes because the 1D nanostructures are periodic or nearly periodic. Figure 4 shows the schematic for the formation models. The schematic models consist of mainly two elements: the brown atom is gallium, and the red atom is oxygen. The (001) plane of β-Ga2O3 with an interplanar distance d100 ) 0.564 nm is employed as the basal repeat unit, which is equal to two (002) planes of β-Ga2O3. Figure 4a presents a ball and stick structual model for the straight β-Ga2O3 nanowires. It clearly indicates that the straight-like nanowires should self-assemble the (002) plane as it grows along the [001h] direction, finally leading to the formation of the straight β-Ga2O3 nanostructure as shown in Figure 2a. Figure 4b is the structual model of the first type of zigzag nanostructure as shown in Figure 3c. The growth direction of the β-Ga2O3 crystal changes alternately from left to right in a zigzag nanostructure, along [001h] and [1h01h] directions, and the growth plane of both segments is also (002). The structure model of another type of zigzag nanostructure is shown in Figure 4c. It can be seen that the nanostructure grows along two directions, [001h] and [101h] in turn, and consequently forms the 1D zigzagshaped nanostructure around the nominal axis of [103h]. The angles between the [001h] and [1h01h] directions and that between
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Figure 3. (a and b) Two FESEM images of zigzag-shaped β-Ga2O3 nanostructures. (c) TEM image of a zigzag-shaped β-Ga2O3 nanostructure with the different lengths of the two segments (A and B). The inset shows a (010) SAED pattern. (d) The corresponding HRTEM image taken from the blue circled part in c. (e) TEM image of a zigzag-shaped β-Ga2O3 nanostructure with the same length of the two segments (A and B). The inset shows the corresponding SAED pattern. (f) The corresponding HRTEM image taken from the blue circled part in e.
the [001h] and [101h] are 103.7° and 126.2°, respectively. The experimental results of about 104.5 ° and 127 ° correlate well with these values. 1D nanostructures grow by the simple selfstacking of the closest-packed (001) or (002) plane and the repeated alternation of growth orientations, which can lead to the formation of the zigzag-shaped β-Ga2O3 nanostructure as shown in Figure 3c and e. Ideally, the zigzag growth changes from one direction to another keeping the same periodicity all the time; the final morphology could be predicted to be a symmetrical and regular nanostructure. However, one side of the zigzag nanostructure grows along one direction more than the other side; that is, the growth direction of the two sides of
the zigzag nanostructure is not exactly the same, which leads to the formation of diversiform morphologies of the whole zigzag-shaped nanostructure. Figure 5a presents a representative FESEM image of the zigzag nanostructure without uniform or symmetrical morphology. On the basis of the above analysis, it is easy to construct the formation model as shown in Figure 5b. All of the angles in the schematic are very consistent with the experimental result measured from the FESEM image, which further confirms the validity of our growth mechanism for the zigzag-shaped β-Ga2O3 nanostructures. The optical property of as-synthesized samples was investigated by PL measurement. Figure 6 presents the photolumi-
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Figure 4. Schematic of the structural ball and stick models for (a) the straight β-Ga2O3 nanowires; (b) the first type of zigzag-shaped nanostructure as shown in Figure 3c; and (c) another type of zigzag-shaped nanostructure as shown in Figure 3e, which consist of two elements: the brown atom is gallium, and the red atom is oxygen.
Figure 5. (a) Representative FESEM images of the zigzag-shaped β-Ga2O3 nanostructures without uniform or symmetrical morphologies. (b) The corresponding structural model.
Figure 6. Room-temperature PL spectra of β-Ga2O3 with a UV fluorescent light excitation of 250 nm and a filter wavelength of 325 nm.
nescence spectra of a product at room temperature with UV fluorescent light excitation of 250 nm and a filter wavelength of 325 nm. For the as-prepared β-Ga2O3 crystals, there is a stronger characteristic peak at 416 nm and a weaker characteristic peak around 365 nm. Compared with the PL feature of
bulk β-Ga2O3 powders,22 the PL of β-Ga2O3 nanocrystals increases largely in intensity and also has a distinct blueshift about several tens of nanometers. As for the PL mechanism of metal oxides (such as Ga2O3 and In2O3), previous studies suggested that it originates mainly from the recombination of an electron on a donor formed by oxygen vacancies and a hole on an acceptor formed by metal vacancies. In the photoexcitation process, the electron in a donor oxygen vacancy can be captured by the excited hole on an acceptor, and then a blue photon is emitted via the radiative recombination process.22 That may be the reason that blue emission bands with peaks at 416 nm appeared in our PL spectra. Interestingly, there is a visiblelight peak at 580 nm in spectra, which was not reported before. It is believed that that the luminescence characteristics are related to the size and structure of the materials. Considering the inevitable defects occurring in the crystallization process of zigzag-shaped nanostructures, a mass quantity of oxygen vacancies would be created in the growth of Ga2O3 nanomaterials. These oxygen vacancies generally act as deep defect donors in semiconductors and would induce the formation of new energy levels in the band gap.23 Thus, the emission at 580 nm may be tentatively assigned to the defect-trapped states (vacancy-type defect) of β-Ga2O3 nanostructures. The interesting
Single-Crystalline β-Ga2O3 Nanowires optical properties increase largely in the β-Ga2O3 band frequency, making it suitable for visible and other optical devices. 4. Conclusions In summary, 1D β-Ga2O3 nanostructures with straight or zigzag morphologies were synthesized successfully by heat treating the pre-prepared GaN powder at 900 °C in dry nitrogen/ oxygen atmosphere. The straight nanowires grown by the simple self-stacking of the closest-packed (002) plane along the [001h] direction are in well crystallinity. The possibility of forming zigzag-shaped nanostructures can be discovered by changing periodically the stacking direction of the (002) plane. Compared with the conventional β-Ga2O3 1D nanostructures with smooth surfaces and uniform diameters, the unusual zigzag nanostructures exhibit unique structural characteristics and novel optical properties, which may find applications in optoelectronic devices. Further work to investigate the growth mechanism of the zigzag-shaped nanostructures, especially the driving force for the changing of the growth direction, and to improve the yield or control of the growth process is now underway. References and Notes (1) Bjork, M. T.; Ohlsson, B. J.; Sass, T.; Persson, A. I.; Thelander, C.; Magnusson, M. H.; Deppert, K.; Wallenberg, L. R.; Samuelson, L. Nano Lett. 2002, 2, 87. (2) Ma, C.; Moore, D.; Li, J.; wang, Z. L. AdV. Mater. 2003, 3, 228. (3) Jun, Y. W.; Lee, S. M.; Kang, N. J.; Cheon, J. W. J. Am. Chem. Soc. 2001, 123, 5150. (4) Duan, X. F.; Huang, Y.; Cui, Y.; Wang, J.; Liber, C. M. Nature 2001, 409, 66. (5) Shen, G. Z.; Bando, Y. S.; Liu, B. D.; Tang, C. C.; Golberg, D. J. Phys. Chem. B 2006, 110, 20129.
J. Phys. Chem. C, Vol. 111, No. 47, 2007 17511 (6) Duan, J. H.; Yang, S. G.; Liu, H. W.; Gong, J. F.; Huang, H. B.; Zhao, X. N.; Zhang, R.; Du, Y. W. J. Phys. Chem. B 2005, 109, 3701. (7) Wu, R. B.; Pan, Y.; Yang, G. y.; Gao, M. X.; Wu, L. L.; Chen, J. J.; Zhai, R.; Lin, J. J. Phys. Chem. C 2007, 111, 6233. (8) Zhou, X. T.; Sham, T. K.; Shan, Y. Y.; Duan, X. F.; Lee, S. T.; Rosenberg, R. A. J. Appl. Phys. 2005, 97, 104315-1. (9) Huang, L. S.; Pu, L.; Shi, Y.; Zhang, R.; Gu, B. X.; Du, Y. W. Appl. Phys. Lett. 2005, 87, 163124. (10) Duan, J. H.; Yang, S. g.; Liu, H. W.; Gong, J. f.; Huang, H. B.; Zhao, X. N. J. Am. Chem. Soc. 2005, 127, 6180. (11) Zhang, J.; Liu, Z. G.; Lin, C. K.; Lin, J. J. Cryst. Growth 2005, 280, 99. (12) Patra, C. R.; Mastai, Y.; Gedanken, A. J. Nanopart. Res. 2004, 6, 509. (13) Baban, C.; Toyoda, Y.; Ogita, M. Thin Solid Films 2005, 484, 369. (14) Yang, Z. X.; Zhu, F.; Wu, Y. J.; Zhou, W. M.; Zhang, Y. F. Physica E 2005, 27, 351. (15) Park, G. S.; Choi, O. B.; Kim, J. M.; Choi, Y. C.; Lee, Y. H. J. Cryst. Growth 2000, 220, 494. (16) Zhang, H. Z.; Kong, C.; Wang, Y. Z.; Du, X.; Bai, Z. G.; Wang, J. J.; Yu, D. P.; Ding, Y.; Hang, Q. L.; Feng, S. Q. Solid State Commun. 1999, 109, 677. (17) Kim, H. W.; Kim, N. H. Appl. Phys. A 2005, 81, 763. (18) Lee, J. S.; Park, K. W.; Nahm, S.; Kim, S. W.; Kim, S. S. J. Cryst. Growth 2002, 244, 287. (19) Choi, Y. C.; Kim, W. S.; Park, Y. S.; Lee, S. M.; Bae, D. J.; Lee, Y. H.; Park, G. S.; Choi, W. B.; Lee, N. S.; Kim, J. M. AdV. Mater. 2000, 12, 746. (20) Gao, S. M.; Xie, Yi.; Zhu, L. Y.; Tian, X. B. Inorg. Chem. 2003, 42, 5442. (21) Ma, D. D. D.; Lee, C. S.; Lifshitz, Y.; Leea, S. T. Appl. Phys. Lett. 2002, 81, 3233. (22) Liang, C. H.; Meng, G. W.; Wang, G. Z.; Wang, Y. W.; Zhang, L. D. Appl. Phys. Lett. 2001, 78, 3202. (23) Fu, L.; Liu, Y. Q.; Hu, P. A.; Xiao, K.; Yu, G.; Zhu, D. B. Chem. Mater. 2003, 15, 4287.
