CRYSTAL GROWTH & DESIGN
Growth of ZnO Nanoneedle Arrays with Strong Ultraviolet Emissions by an Electrochemical Deposition Method Bingqiang Cao,* Yue Li, Guotao Duan, and Weiping Cai* Key Lab of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, 230031, Anhui, Peoples Republic of China and The Graduate School of Chinese Academy of Sciences, Beijing, 100039, Peoples Republic of China
2006 VOL. 6, NO. 5 1091-1095
ReceiVed June 2, 2005; ReVised Manuscript ReceiVed October 13, 2005
ABSTRACT: We report a soft and template-free electrochemical deposition method to prepare wafer-scale high-quality ZnO nanoneedle arrays on oriented gold-film-coated silicon substrates at temperatures as low as 70 °C. SEM, XRD, and TEM data show that the ZnO nanoneedles are of single-crystal wurtzite structure and preferentially oriented along the c-axis perpendicular to the substrates. Such ZnO nanoneedle arrays exhibit strong ultraviolet emissions at room temperature but very weak defect-related visible emissions. The blue shift of the low-temperature ultraviolet emission compared with that at room temperature further confirms its excitonic origin. The nanoneedle growth mechanism can be attributed to formation of {0001}-oriented ZnO nuclei on the oriented gold-coated silicon substrate and then faster growth along the 〈0001〉 direction in terms of minimum interface and surface energy. 1. Introduction Zinc oxide (ZnO), a II-VI compound semiconductor with a direct band gap of 3.37 eV and a relatively high exciton binding energy (60 meV) at room-temperature, displays excellent piezoelectric, catalysis, and novel optical properties.1 Since the demonstration of room-temperature lasing actions in the highly ordered ZnO nanowire arrays,2 many methods have been developed to obtain one-dimensional (1D) ZnO nanostructure arrays including nanowire, nanorod, nanoneedle, and nanotube arrays. The most direct route to prepare 1D nanostructure arrays is the template-assisted method, in which the nanochannels of anodic aluminum oxide membrane were filled by the electrochemical deposition (ECD),3 sol-gel,4 or vapor deposition method.5 However, the ordered arrays may be destroyed when the template is removed. Another popular strategy is a vaporphase method, such as the (metal-catalyzed) thermal vapor transport and sedimentation method,6-8 metal-organic vaporphase epitaxy (MOVPE) method,9 and hydrogen-treated ZnO film method.10 Besides the limitations of a high preparation temperature and energy-consuming experiment facilities, these vapor-phase methods usually need expensive and/or insulting substrates, such as sapphire,2 titanium nitride (TiN),8 or gallium nitride (GaN),9 for epitaxial growth. On the contrary, solution approaches are attractive due to their low cost and high yield. Yang11 and Vayssieres12 developed the seeds-induced hydrothermal method to prepare ZnO nanowire arrays on silicon or glass substrates, which were performed under high pressure in an autoclave. Thus, the preparation of ZnO 1D nanostructure arrays on widely used silicon substrate by a soft and templatefree method at a low cost is still appealing for their potential application in nanodevices. The most recent progress was made by Han et al., who reported an effective fabrication method at low temperature for ZnO nanorod and nanotube arrays on ZnO film-coated substrates.13 The ECD method from dissolved precursors, especially in aqueous solutions, is a low-cost and scalable method that is usually well suited for mass production of semiconductor thin films. ECD of polycrystalline ZnO film was first demonstrated on conductive glass substrates by Izaki14 and Peulon15 10 years * To whom all correspondence should be addressed. Fax: +86-5515591434. E-mail:
[email protected] (B.Cao) or
[email protected] (W.Cai).
ago. ZnO films electrodeposited on gold-film-coated silicon substrates showing both UV and defect-related emissions with comparable intensities were reported.16 Epitaxial growth of ZnO nanorod arrays on polycrystalline zinc foil substrate has been developed by Wong et al.17 However, the as-synthesized samples were doped with chloride impurities and no ultraviolet (UV) emissions were observed. In this paper, we report a soft and template-free ECD method to prepare ZnO nanoneedle arrays with high crystal quality on oriented gold-film-coated silicon substrate. More importantly, the as-synthesized ZnO nanoneedle arrays show strong UV emissions at room temperature but very weak defect-related visible emissions, indicating that such ZnO nanoneedle arrays may have potential applications in future silicon-based optoelectronic nanodevices at a low industrial cost. 2. Experimental Section Gold film-coated silicon (Au/Si) substrates were prepared by thermal evaporation of gold at a vacuum better than 1 × 10-5 Pa. The deposition rate was about 0.5 Å/min controlled by a film thickness monitor (FTM-V, Shanghai). A gold layer about 50 nm thick was deposited on doped single-crystal silicon (100) with a resistivity smaller than 10 Ω‚cm. Galvanostatic cathodic deposition was employed on the Au/Si substrates at a current of 0.9 mA. Zinc sheets (99.99% purity) acted as the anode electrode, and the electrolyte solution was zinc nitrate aqueous solution (0.05 M). The pH value of the solution was about 6. The same cathodic deposition was also performed on silicon substrates for reference. The deposition temperature was fixed at 70 °C by a water bath, and the deposition time was 2 h. The samples were characterized by a field emission scanning electron microscope (FE-SEM, FEI Sirion 200), X-ray diffraction (XRD, Philips X’Pert, Cu KR line, 0.15419 nm), energy dispersive X-ray spectroscopy (EDS, Inca Oxford), and a high-resolution transmission electron microscope (HRTEM, JEOL-2010, equipped with EDS). Roomtemperature photoluminescence (RTPL) spectra were recorded on a LABRAM-HR Micro-Raman spectrometer (Jobin-Yvon) excited with a 325 nm He-Cd laser. The PL spectrum at 13 K was measured on a home-built low-temperature photoluminescence spectrometer equipped with a cryogenic refrigeration system utilizing liquid nitrogen.
3. Results and Discussion 3.1. Structure Characterization. Figure 1 shows the general morphology of the as-synthesized samples on Au/Si substrate. A layer of well-aligned nanowire arrays with high density was
10.1021/cg050246l CCC: $33.50 © 2006 American Chemical Society Published on Web 03/31/2006
1092 Crystal Growth & Design, Vol. 6, No. 5, 2006
Cao et al.
Figure 2. (A) XRD spectrum of ZnO nanoneedle array on Au/Si substrate. (Inset) θ Rocking curves of ZnO (0002) peak. (B) XRD spectrum of the Au/Si substrate. (Inset) θ Rocking curves of Au (111) peak. Figure 1. (A) Tilted SEM images of well-aligned ZnO nanoneedle arrays grown on Au/Si substrates. (B) Magnified SEM image of A showing the needlelike tips.
grown on the substrate surface erectly (Figure 1A). From its magnified FESEM image (Figure 1B) we can see that the nanowires exhibit needlelike tips, which we thus call nanoneedles. The nanoneedles electrodeposited in 0.05 M Zn(ON3)2 precursor solution under a current of 0.9 mA for 2 h show, typically, a mean length of about 2 µm with the tip tens of nanometers in diameter. Figure 2A shows the XRD pattern corresponding to the sample shown in Figure 1. Only four peaks are observed, which can be assigned to ZnO(002) and (004) (JCPDS#36-1451), Au(111), and Si(400). The absence of any other peaks confirms the good alignment of these nanoneedles with c-axis along the nanoneedle axis and almost perpendicular to the substrate, as observed by FESEM, Figure 1. Meanwhile, XRD θ rocking curve measurements were also performed to investigate the degree of alignment to the normal direction of the surface. The inset of Figure 2A shows the rocking curve of the ZnO (0002) peak, indicating a full width at half-maximum (fwhm) value of 7.5°, which is comparable to that of the ZnO nanoneedle (3-10°) arrays grown on silicon substrates18 or ZnO nanowire arrays (∼6.5°) grown on ZnO film substrates by the vaporphase epitaxy method.7 Figure 2B shows the XRD spectrum of the Au/Si substrate, showing its strong (111) orientation. The rocking curve of the Au(111) peak with an fwhm of 8° indicates that the Au (111) planes are not absolutely parallel to the silicon substrate, which leads to some rotation of the ZnO nanoneedle array, as can be seen in Figure 1. Figure 3A shows the TEM image of a single ZnO nanoneedle gently transferred onto a copper grid by ultrasonication. HRTEM examination indicated that the ZnO nanoneedle is of singlecrystal structure without visible defects, as shown in Figure 3B.
This HRTEM image clearly shows the (0001) crystal planes perpendicular to the axis of the nanoneedle, indicating that 〈0001〉 is the preferred growth direction of the wurtzite ZnO nanoneedles. The inset of Figure 3B gives the corresponding selected area electron diffraction (SAED) pattern, which further proves the single-crystal nature of the nanoneedles and their 〈0001〉 growth direction. The EDS result of a single ZnO nanoneedle, performed on HRTEM, is illustrated in Figure 4. In addition to the carbon and copper signal from carbon-coated copper grid, the atomic ratio of zinc to oxygen is about 1.05, which is near the stoichiometric composition. 3.2. PL Measurements. PL measurements of the ZnO nanoneedle arrays were conducted under excitation of a 325 nm He-Cd laser at room temperature and low temperature (13 K), respectively. Figure 5A shows the RTPL spectra excited with different power densities. The full laser exciting power intensity (I0) is about 2 kW/cm2. A dominant emission peak centered at about 380 nm (3.26 eV), near band edge, was observed for all spectra. With increasing excitation power density, the integrated intensity of the UV PL peak increases linearly, as shown in the inset of Figure 5A. However, the UV peak does not show any detectable shift. This indicates that the sharp interband emission originates from recombination of free or bound excitons.19,20 The low-temperature PL spectrum is shown in Figure 5B together with the RT result for reference. At 13 K the excitonic emission blue shifts to 3.34 eV from the result at 300 K, which is in accordance with the prediction by Varshni’s formula and confirms the excitonic origin of the UV emissions.21 In the previous reports on ZnO nanorod film by ECD methods,17 zinc chloride were selected as electrolyte and other chlorides, such as KCl or CaCl2, were used as the supporting electrolyte resulting in the as-synthesized samples being doped with chloride impurities and no UV emission observed, which is in accordance with the PL results of ZnO
Growth of ZnO Nanoneedle Arrays
Crystal Growth & Design, Vol. 6, No. 5, 2006 1093
Figure 5. (A) RTPL spectra of ZnO nanoneedle arrays on the substrate excited by a 325 nm He-Cd laser under different excitation power intensities. The curves from bottom to top correspond to the power intensities: 0.01I0, 0.1I0, 0.25I0, and 1.00I0, respectively. (Inset) integrated PL intensity versus excitation power intensity. Magnified part: 10× (450 nm-550 nm) region of the top PL spectrum. (B) Comparison of UV excitonic peaks in LTPR (13 K) and RTPL (300 K) under excitation power density of I0. Figure 3. (A) TEM image of a single ZnO nanoneedle. (B) HRTEM image of the nanoneedle and its corresponding selected area electron diffraction pattern (inset).
to the weak green emission also indicates that the ZnO nanoneedle arrays prepared by the ECD method are of good crystal quality. 3.3. Formation of Nanoneedle Arrays. Now let us discuss the growth mechanism of the ZnO nanoneedle arrays on oriented gold-film-coated silicon substrate. From the viewpoint of chemical reactions, the one-step electrochemical deposition reactions to prepare ZnO film are proposed as follows24
Zn(NO3)2 f Zn2+ + 2NO3-
(1)
NO3- + H2O + 2e- f NO2- + 2OH-
(2)
Zn2+ + 2OH- f Zn(OH)2
(3)
