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Size-Dependent InAlO3(ZnO)m Nanowires with a Perfect Superlattice Structure D. L. Huang, L. L. Wu, and X. T. Zhang* Heilongjiang Key Laboratory for Low-Dimensional System and Mesoscopic Physics, School of Physics and Electronic Engineering, Harbin Normal UniVersity, Harbin 150025, People’s Republic of China ReceiVed: April 1, 2010; ReVised Manuscript ReceiVed: May 29, 2010
InAlO3(ZnO)m superlattice nanowires are successfully synthesized via chemical vapor deposition, using Au catalyst. High-resolution transmission electron microscopy observations indicate that these nanowires consist of an alternative stacking of InO2- layers and AlO(ZnO)m+ blocks along the [0001] direction. The periodicity of these nanowires is proportional to their diameter, which is determined by the Au alloy particle size at the tip of the nanowire. The linear relationship between the periodicity and the diameter is well explained by a cylindrical configuration model. Our results indicate that controlled synthesis of InAlO3(ZnO)m superlattice nanowires becomes possible. The photoluminescence properties of InAlO3(ZnO)m superlattice nanowires were studied. Introduction
Experimental Procedures
Transparent oxide semiconductors (TOSs) are optically transparent in the whole visible region and have good controllability for carrier generation.1,2 To date, one-dimensional (1D) TOS nanostructures have received considerable attention, because 1D nanostructures are nanoscale building blocks and could enable diverse applications in nanoelectronics and photonics through bottom-up assembly.3 In addition, a superlattice structure in 1D materials could greatly increase the versatility and power of the material in nanoscale electronic and photonic applications. Lieber thinks that the growth of 1D superlattice structures represents the primary focus of the new work now.4 The homologous compound InAlO3(ZnO)m (IAZO) (for m ) integer numbers), as an important n-type TOS, has a natural superlattice structure, in which alternate stacking of InO2- and AlO(ZnO)m+ layers forms periodic layered structures along the c axis. The space group is assigned as R3m (for m ) odd numbers) or P63/mmc (for m ) even numbers).1,5 Such a multilayered structure is similar to the conventional artificial superlattices; it is therefore regarded as a “natural superlattice”, which could give rise to some unique physical properties. For example, the superconducting state in layer-structured La2-xBaxCuO4, YBa2Cu3O7, and BiSr2CaCuO8 originates from the strong correlation among the confined electrons in the layers.6 Therefore, rational design and synthesis of 1D IAZO superlattice nanowires has become an active research area in recent years. Exciting developments in the growth of homologous compound In2O3(ZnO)m nanowires with modulated structures have been reported.7-11 Recently, the growth of InGaO3(ZnO)m nanowires12,13 as well as similar homogeneous structures such as higher manganese silicide nanowires14 was also reported. However, no report is available on IAZO nanostructures. In this paper, we report the fabrication of sizedependent IAZO superlattice nanowires via the vapor-liquidsolid (VLS) mechanism. The periodicity is linearly proportional to the diameter of individual nanowires, and a possible reason is proposed.
IAZO superlattice nanowires were prepared by the chemical vapor deposition method. A mixture of ZnO, In, and Al powders was placed into one end of an alumina boat. The Au thin film was precoated on Si substrates by sputtering with a thickness of 2 nm around and placed at the downstream of the mixture. The boat was housed inside an alumina tube that was inserted into a horizontal furnace, and the mixture was located at the center of the furnace. The furnace was heated to 1000 °C in 40 min and kept for 2 min under a N2 (carrier gas) flow rate of 200 sccm. The partial pressure was kept at 500 Pa. The furnace was sequentially heated to 1300 °C within 20 min, held for 20 min, and then naturally cooled. The as-prepared nanostructures were then characterized by a transmission electron microscope (TEM PHILIPS Tecnai 20), equipped with an energy-dispersive X-ray (EDX) spectrometer, indicating that about half of the assynthesized materials have superlattice structure. Photoluminescence (PL) spectra were measured using a microlaser Raman system (J-Y HR800, France). A He-Cd laser operated at a wavelength of 325 nm was used as the excitation source.
* To whom correspondence
[email protected].
should
be
addressed.
E-mail:
Results and Discussion Figure 1a shows a low-magnification TEM image of the assynthesized materials. It demonstrates the wirelike morphology of the product. The diameter of these nanowires ranges from 10 to 70 nm, and the length is about several tens of micrometers. Dark particles are located at the tip of most nanowires (as marked by the blue arrows in Figure 1a). Alternating bright/ dark contrast appears in a periodic manner along the [0001] growth direction. This is a typical superlattice feature for the IAZO system, in which the dark contrast corresponds to the InO2- layers and the bright contrast to the AlO(ZnO)m+ blocks (see below). The atomic arrangements in the IAZO system are similar to those in the In2O3(ZnO)m system.7 The selected area electron diffraction (SAED) pattern taken from a typical nanowire is shown in the inset in Figure 1a. A series of satellite diffraction spots (as marked by red arrows) are located between two adjacent diffraction spots (as marked by green arrows), indicating the existence of a superlattice structure. EDX spectra (Figure 1b) taken from the tip (black curve) and the wire (red
10.1021/jp102964p 2010 American Chemical Society Published on Web 06/22/2010
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Figure 1. (a) Low-magnification TEM image of the InAlO3(ZnO)m nanowire with a dark particle at the tip. Alternating bright/dark contrast appears in a periodic manner along the growth direction. The SAED pattern (shown in the inset) taken from the nanowire region in (a) confirms the existence of a superlattice structure. (b) EDX spectra taken at the tip (black line) and wire (red line) regions of the nanowires shown in (a).
Figure 3. (a-c) Low-magnification TEM images of InAlO3(ZnO)m nanowires showing periodical bright/dark contrast, with diameters of 34.2 nm (m ) 10), 38.1 nm (m ) 16), and 56.5 nm (m ) 15), respectively. (HR-a-HR-c) Corresponding HRTEM images of the InAlO3(ZnO)m nanowires shown in (a)-(c), respectively. Their lattice constants (c) are shown.
Figure 2. HRTEM image of a nanowire showing a perfect periodical superlattice structure with exactly 16 (m ) 15) Al/Zn-O layers between two adjacent In-O layers.
curve) regions indicate the composition to be a Au-Zn alloy and IAZO, respectively. The Cu signal comes from the TEM grid. The high-resolution TEM (HRTEM) image (Figure 2) of individual nanowires is very similar to those of In2O3(ZnO)m and InFeO3(ZnO)13 reported in previous papers.15 InO2- and AlO(ZnO)m+ layers are denoted as the In-O layer and Al/Zn-O block, respectively. The HRTEM image shows a better periodical superlattice structure with exactly 16 (m + 1, m ) 15) Al/ Zn-O layers between the two adjacent In-O layers. From Figure 2, it is obvious that the alternative stacking of In-O layers and Al/Zn-O blocks forms a periodic structure. Note that, for about 50 superlattice nanowires examined, the lattice constant (c) of the nanowire varies with the changing diameter (D) of the nanowire. We found that most of the nanowires meet the linear relationship between the periodicity and the diameter. Here, when m is even, c equals two period lengths. However,
Figure 4. Variation of c as a function of the nanowire diameter (D) based on the experimental results shown in Figure 3. The error of c is determined to be (0.2 nm.
when m is odd, c becomes three period lengths.16 Parts a-c of Figure 3are typical low-magnification TEM images of such nanowires with diameters ranging from 34 to 60 nm with a (2 nm error. The error is mainly due to the fact that the surfaces of these nanowires are not smooth at the atomic scale. The c of these nanowires in Figure 3a-c is 6.4 (m ) 10), 9.4 (m ) 16), and 13.2 (m ) 15) nm, respectively, which was determined by the corresponding high-resolution TEM images with better measurement accuracy (parts HR-a-HRc of Figure 3). The error of c is determined to be (0.2 nm. The variation of c as a function of D is plotted in Figure 4. An almost linear relationship can be identified between them; i.e., c is approximately proportional to D.
Size-Dependent InAlO3(ZnO)m Nanowires
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The IAZO superlattice nanowires were grown via the VLS mechanism, as suggested by the experimental observation that an alloy tip is always present at one end of the nanowires. This is a typical characteristic of the VLS growth mechanism. All of the nanowires grow along the [0001] direction. The nanowires have a relatively large size distribution. It may also be explained by another characteristic of the VLS mechanism, that the diameter of the nanowire is determined by the Au alloy particle size at the tip.17 The almost linear relationship between the c axis lattice constant and the nanowire diameter may be explained using a simple model by assuming a cylindrical configuration of the nanowires with cross-sectional D. According to the previous analysis,18 as trace In and Al were introduced into the hexagonal wurtzite structure of ZnO, the In and Al randomly substituted for the Zn atoms to form an InO4 or AlO4 tetrahedron, respectively. This could inevitably lead to the introduction of strain, because the ionic radius (0.081 nm) of In3+ is larger than that (0.074 nm) of Zn2+ while the ionic radius (0.051 nm) of Al3+ is smaller than that of Zn2+. As the strain accumulates, the In, Al, and O atoms in the tetrahedral sites cannot maintain their original sites and are forced to shift in three directions, forming either AlO5 (Al atoms are 5-fold coordinated) trigonal bipyramids or InO6 (In atoms are 6-fold coordinated) octahedron segregation.19,20 The elastic energy per unit length (E) of the cylinder can be written as E ) 4πC/D,21 where C is a constant describing the elastic properties and the surface energy of the specific material. Governed by the VLS mechanism, the IAZO phase would continuously precipitate out from the liquid Au alloy, allowing storage of the elastic energy (E) in the small volume of the solid IAZO phase. To lower the elastic energy, InO6 octahedrons often build up single In-O octahedral layers, and the AlO5 trigonal bipyramids are proposed to constitute an inverse flat plane in the Al/Zn-O blocks,19,20 forming the superlattice structure. The stored elastic energy can be released by the creation of a superlattice structure, which is associated with a superlattice structure energy (γ), leading to the relation of Ec ≈ γ. This results in the linear dependence of the crystal constant (c) on the nanowire diameter:
c≈
γD 4πC
(1)
Nevertheless, various kinetic growth factors could affect superlattice formation and introduce unavoidable fluctuations of the periodicity in individual nanowires or between nanowires with approximately the same diameter.21,22 Therefore, the c axis lattice constant or m can be controlled by the diameter of the nanowire, which is determined by the catalyst Au nanoparticle size.9 To study the optical properties of the as-synthesized IAZO nanowires, their PL spectra were taken at room temperature. PL spectra exhibit two peaks; one is from the near-band edge (NBE) emission, and the other is from the broad visible emission. Figure 5 is a representative PL spectrum of IAZO samples with different sizes. From Figure 5, a strong NBE emission peak centered at 380 nm, with a full width at halfmaximum (fwhm) of 160 meV, originating from the recombination of the free excitons,7 and a weak green emission band centered at 560 nm, originating from oxygen vacancies,23 were observed. The fluctuating green emission band results from the microlaser Raman system. We deduce that NBE emission peaks of all the superlattice nanowires with different m values should
Figure 5. Representative PL spectrum of the InAlO3(ZnO)m nanowires.
be near 380 nm. Naghavi et al.24 studied the optical characterization of the In2O3-ZnO thin films and concluded that the optical band gap (Eg) could be estimated from optical spectra as a function of the Zn/(Zn + In) ratio. From their results, when 0.82 e Zn/(Zn + In) e 1, Eg ≈ 3.37 eV, the crystal has a ZnOtype wurtzite structure. For our experiment, 0.83 e Zn/(Zn + In + Al) e 0.88; thus, we think that it is reasonable that NBE emission peaks are located at ∼380 nm, although there is a slight difference in the peak position for the NBE emission of IAZO superlattice nanowires with different m values. In addition, the larger fwhm likely results from the overlapping of the three NBE emission peaks. Conclusion In summary, IAZO superlattice nanowires were grown by the VLS mechanism, and their periodicity was determined by the diameter of the nanowire. The linear relationship between the periodicity of the superlattice and the diameter is well understood by a simple cylindrical configuration model. The PL spectrum consists of an intensive NBE emission peak and a weak green emission band. Acknowledgment. This work is partly supported by the National Natural Science Foundation of China (Grant 60776010), the Science Foundation for Distinguished Young Scholars of Heilongjiang Province (Grant JC200805), the Natural Science Foundation of Heilongjiang (Grants A2007-03, A200807, and F200828), the Education Bureau of Heilongjiang Province (Grants 11531225 and 11531227), the Project of Overseas Talent, Personnel Bureau, Heilongjiang Province, the Excellent Leader of Subjects, Bureau of Science and Technology of Harbin, Heilongjiang Province (Grant 2007RFXXG028), and the Graduate Students’ Scientific Research Innovation Project of Heilongjiang Province (Grant YJSCX2009-256HLJ). References and Notes (1) Nomura, K.; Kamiya, T.; Ohta, H.; Ueda, K.; Hirano, M.; Hosono, H. Appl. Phys. Lett. 2004, 85, 1993. (2) Ohta, H.; Hosono, H. Mater. Today 2004, 7, 42. (3) Lieber, C. M. Sci. Am. 2001, 285, 58. (4) Lieber, C. M. Nano Lett. 2002, 2, 81. (5) Li, C.; Bando, Y.; Nakamura, M.; Onoda, M.; Kimizuka, N. J. Solid State Chem. 1998, 139, 347. (6) Ohta, H.; Nomura, K.; Orita, M.; Hirano, M.; Ueda, K.; Suzuki, T.; Ikuhara, Y.; Hosono, H. AdV. Funct. Mater. 2003, 13, 139. (7) Zhang, X. T.; Lu, H. Q.; Gao, H.; Wang, X. J.; Xu, H. Y.; Li, Q.; Hark, S. K. Cryst. Growth Des. 2009, 9, 364. (8) Jie, J. S.; Wang, G. Z.; Han, X. H.; Hou, J. G. J. Phys. Chem. B 2004, 108, 17027. (9) Na, C. W.; Bae, S. Y.; Park, J. J. Phys. Chem. B 2005, 109, 12785. (10) Aleman, B.; Fernadez, P.; Piqueras, J. Appl. Phys. Lett. 2009, 95, 013111.
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Huang et al. (18) Wu, L. L.; Zhang, X. T.; Wang, Z.; Liang, Y.; Xu, H. Y. J. Phys. D 2008, 41, 195406. (19) Yan, Y. F.; Da Silva, JL. F.; Wei, S. H.; Al-Jassim, M. Appl. Phys. Lett. 2007, 90, 261904. (20) Silva, J. L. D.; Yan, Y. F.; Wei, S. H. Phys. ReV. Lett. 2008, 100, 255501. (21) Li, Q.; Gong, X.; Wang, C.; Wang, J.; Ip, K.; Hark, S. K. AdV. Mater. 2004, 16, 1436. (22) Wang, Z. L.; Dai, Z. R.; Bai, Z. G.; Gao, P. R.; Gole, J. Appl. Phys. Lett. 2000, 77, 3349. (23) Zhang, X. T.; Liu, Y. C.; Zhang, J. Y.; Lu, Y. M.; Shen, D. Z.; Fan, X. W.; Kong, X. G. J. Cryst. Growth 2003, 254, 80. (24) Naghavi, N.; Marcel, C.; Dupont, L.; Rougier, A.; Leriche, J. B.; Gue´ry, C. J. Mater. Chem. 2000, 10, 2315.
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