ARTICLE pubs.acs.org/JPCC
Surface Polarity-Dependent Cathodoluminescence in Hydrothermally Grown ZnO Hexagonal Rods Won Woo Lee, Seong Been Kim, Jaeseok Yi, William T. Nichols, and Won Il Park* Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Korea
bS Supporting Information ABSTRACT: Vertically oriented ZnO hexagonal rod arrays were produced by site-specific hydrothermal growth. These ZnO nanostructures possess well-defined surfaces composed of Zn-terminated (0001) planes (for the top surface) and nonpolar {1010} planes (for the rectangular side faces), thereby providing an opportunity to investigate the correlation between surface structures and local optical properties. The cathodoluminescence (CL) spectra of various sized ZnO rods revealed that the ratio of the deep-level emission (DLE) to the nearband-edge emission (NBE) peak intensities increased continuously with increasing diameter, which is contradictory to the general trend that the DLE increases by enhancing the surface-to-volume ratio. From the CL spectral mapping, significant NBE quenching was observed at the Zn-terminated (0001) surface, whereas the DLE was bright for every surface. Based on these observations, the anomalous behavior observed in our ZnO rods can be attributed to the surface polarity-dependent NBE characteristics. We have also shown that the surface defects associated with the NBE quenching could be eliminated by appropriate thermal annealing.
I. INTRODUCTION One-dimensional semiconductor materials have become the focus of intensive research owing to their unique electrical and optical properties and potential applications in electronic and optoelectronic devices.13 Although nanowires or rods composed of various compound semiconductors, including GaAs, GaN, and InP, have been studied,4,5 ZnO has received heightened attention because of its unique properties.6,7 For example, ZnO is a direct and wide bandgap (Eg = 3.37 eV) semiconductor with an exceptionally large exciton binding energy of 60 meV, making ZnO a promising candidate for applications in optoelectronic devices, such as short-wavelength lasers and light-emitting diodes.8,9 Yet, despite the recent progress toward controlled synthesis, anomalous optical properties often emerge in ZnO at the nanoscale, and the fundamental issues relating to these anomalous behaviors are still under debate.1012 For instance, Shalish et al. and Pan et al. reported that the intensity ratio of the deep-level emission (DLE) to the near-band-edge emission (NBE) in ZnO nanorods increased with decreasing diameter, which was attributed to the strong surface effect on the deep level defects.13,14 On the other hand, Chang et al. observed the opposite result—the ratio was significantly larger in thicker nanorods—and considered bulk defects as the origin of the DLE.15 Although these contradictory results might be partially due to different synthesizing conditions, understanding the surface structure, especially the surface polarity, is a critical aspect of determining the optical properties.10 However, to our knowledge, no particular consideration was paid r 2011 American Chemical Society
to the surface polarity of ZnO at the small scale associated with surface-related recombination. Here we have performed the spatially and spectrally resolved investigation on hydrothermally grown ZnO rod arrays using cathodoluminescence (CL). From the epitaxially grown ZnO hexagonal rods with various diameters, the intensity ratio of the DLE to the NBE was found to increase continuously with increasing diameter, which is contradictory to the previous results observed by both photoluminescence and CL.13,14 These anomalous behaviors, combined with spatially resolved CL mapping images taken at a fixed wavelength, can be correlated to surface polarity-dependent luminescent properties.
II. EXPERIMENTAL SECTION Size- and position-controlled ZnO rod arrays were hydrothermally grown using a prepatterned poly(methyl methacrylate) (PMMA) mask on a ZnO seed layer. A hexagonal-shaped ZnO rod array was achieved using epitaxial ZnO thin films on Al2O3 (0001) substrates as seed layers. Deposition of the epitaxial ZnO thin films was performed by pulsed laser deposition using a XeCl excimer laser. Following the seed layer growth, a PMMA layer with a thickness of 200 nm was coated onto the samples, and a regular array of circular holes with a diameters ranging from 500 nm to 5 μm was created by electron beam lithography. After patterning Received: October 13, 2011 Revised: December 5, 2011 Published: December 22, 2011 456
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Figure 1. (a) SEM image of the ZnO rod array. Scale bar, 5 μm. Inset: top view of the SEM image shows the hexagonal-shaped ZnO rods. Scale bar, 100 nm. (b) CL spectrum of ZnO rods (blue curve) that can be decomposed into four Gaussian components (red curves) centered at 334, 382, 515, and 650 nm.
ionized oxygen vacancy (V•o) and can generate a V•• o center that is doubly positively charged with respect to the neutral lattice. In addition to the oxygen vacancies, the presence of water, hydrogen, or OH groups on the surface of ZnO has been considered as a possible origin of the DLE.20,21 Since these defects exist mainly in the surface layer and the tunneling rate drastically decreases with distance from the surface, the recombination process dominants near the surface.12 On the other hand, the emission band with a central wavelength near 650 nm was not observed from the ZnO rods grown on other Si or SiO2 substrates (see Figure S2), similar to the UV peak at 334 nm, and therefore it may be strongly related to the Al2O3 substrate. A similar red band emission was previously detected in an α-Al2O3 crystal by the CL technique.22 To determine if there is a correlation between the optical properties of the ZnO rods and their sizes, hydrothermal growth was performed using a PMMA growth mask layer composed of circular hole arrays with radii ranging from 0.25 to 2.5 μm. The resulting ZnO nanostructures are displayed in Figure 2a, which shows that the diameter of the ZnO rods increased with increasing hole size while both a hexagonal pillar shape and vertical alignment were maintained.16 In particular, these rods had a consistent height of ∼1.5 μm, allowing the relative area of the top surface to be intentionally tuned. This ability to precisely control the height provides an opportunity to study the effects of the surface structure on the CL of the ZnO crystals. Figure 2b shows the CL spectra obtained by collecting the emissions with orientations parallel to the top surface and normal to the ZnO rods. Consequently, the CL spectra of the ZnO hexagonal rods were found to be strongly dependent on rod size: the relative intensities of the NBE peaks decreased (i.e., the DLE peaks increased) with increasing diameter of the ZnO hexagonal rods (see Figure S3). To further clarify this point, the size-dependent CL spectra were normalized to a constant intensity of the DLE emission peaks. The ratio of the DLE to NBE peak intensity (IDLE/INBE) was plotted as a function of the radii (r) of the ZnO rods, as shown in Figure 2c, which illustrates that the IDLE/INBE ratio is almost linear with the radii of the ZnO rods. This result is contradictory to the previous reports that claim that the DLE intensity increased for smaller diameter ZnO nanorods or wires.13,14 In these cases, the enhanced surface area to volume ratio was suggested to explain the phenomenon.13,14 Considering that the DLE is strongly correlated to defects (such as oxygen vacancies) that reside mainly in the surface layer, reducing the diameter of the ZnO
the growth masks, the samples were placed in an aqueous solution containing 0.025 M zinc nitrate hexahydrate (Zn(NO3)2 3 6H2O, Sigma-Aldrich) and 0.025 M hexamethylenetetramine (C6H12N4, Sigma-Aldrich) and then held at 85 °C for 68 h. The detailed procedure for the hydrothermal growth of ZnO rods is described elsewhere.16 To investigate the optical properties of individual ZnO rods, CL measurements were carried out at room temperature using a Gatan Mono-CL3 system with a high-sensitivity photomultiplier tube (PMT) attached to a field-emission scanning electron microscopy (FE-SEM, S-4300SE, Hitachi). In our CL measurement system, the paraboloidal mirror having a hole at the center of the mirror was used for efficient luminescence collection and electron beam penetration. Light emitted from the specimens as a result of electron bombardment was collected in a very efficient manner and collimated along one axis of the mirror attached to the PMT (see Figure S1). All the CL measurements were performed using an accelerating voltage of 15 kV.17
III. RESULTS AND DISCUSSION Figure 1a depicts the general morphology and the CL image of the vertical ZnO rod array selectively grown on a ZnO epilayer using a PMMA mask that consisted of a regular array of circular holes with radii of 250 nm and an interdistance spacing of 5 μm. Highmagnification SEM images confirmed that the individual ZnO rods had single crystal structures with well-defined hexagonal facets (see Figure 1a, inset).16 The radius and height of each ZnO hexagonal rod were ∼250 nm and ∼1.5 μm, respectively. Spectrally resolved CL measurements were performed to analyze the optical properties of the ZnO nanostructures. As shown in Figure 1b, the CL emission spectrum of the ZnO rods can be decomposed into four Gaussian components centered at 334, 382, 515, and 650 nm. Considering the bandgap of ZnO (∼3.3 eV at room temperature), the ultraviolet (UV) emission peak at 382 nm can be attributed to the NBE associated with free exciton recombination, whereas the other UV peak at 334 nm was observed only in the samples grown on Al2O3 substrates (see Figure S2) and thus can be attributed to the F+-center emission (an oxygen vacancy with a single trapped electron) of sapphire.18 The green emission band with central wavelengths near 515 nm is common in ZnO and is generally referred to a DLE. The DLE relies on the presence of defects such as ionized oxygen vacancies or impurities.11,12,19 For instance, in the presence of oxygen vacancies, a photoinduced hole can easily be trapped by a singly 457
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Figure 2. (a) SEM images of ZnO rods with various sizes. The radii of the ZnO rods from left to right are 250 nm, 500 nm, 1 μm, and 2.5 μm, respectively. Scale bars, 500 nm. (b) Room temperature CL spectra of the ZnO hexagonal rods depending on their size. (c) IDLE/INBE as a function of the ZnO rod radii.
rods could cause more surface-defect luminescence (i.e., DLE) in ZnO crystals. This difference, i.e., the increase versus decrease in the INBE/IDLE with rod diameter reduction, could originate from different surface structures and measurement geometries.12 For instance, our samples consisted of ZnO hexagonal rods with a nearperfect vertical alignment that exhibited distinct geometry anisotropy, while previous studies mainly consisted of randomly oriented ZnO nanowires or rods. We also investigated the microscopic correlation between surface structures and luminescent properties using spatially resolved CL spectral mapping.23 Figure 3a shows a top-view secondary electron image (SEI) and corresponding CL images taken at 382 nm (NBE) and 515 nm (DLE). Bright areas in the CL images reveal the luminescence sites. Quite interestingly, the NBE is strong only at the tip edges, whereas the DLE is bright over the entire ZnO rod surface (see also Figure S4). Enlarged CL images obtained for two adjacent ZnO rods are also shown in Figure 3b, where one is standing vertically and the other is lying on the seed layer surfaces. Contrary to DLE, it is apparent that NBE is strongly surface-dependent: the side surface of the ZnO {1010} plane shows bright NBEs, while the emission is quenched at the top surface of the (0001) plane. These surface-dependent CL characteristics can also be applied to nanoscale ZnO. Figure 3c shows a ZnO nanorod bundle grown on a polycrystal ZnO seed layer, which consists of radially oriented ZnO nanorods with diameters ranging from 40 to 60 nm and lengths of ∼1.5 μm. The suppression of the NBE at the nanorod tips is distinctly observed by comparing the SEI with the CL maps for the NBE and DLE. The phenomenon of the strong NBE intensity near the hexagonal boundary is quite similar to whispering-gallery modes (WGMs) in dielectric resonant cavities.24 However, we have excluded WGM enhanced emission as a possible mechanism for the following reasons: (i) the observed phenomenon (bright NBE
Figure 3. (a) Top-view SEI and corresponding CL images for NBE and DLE. Bright areas in the CL images reveal the luminescence sites. Scale bars, 5 μm. (b) High-magnification SEI and CL images obtained for two adjacent ZnO rods, where one is standing vertically and the other is lying on the seed layer surfaces. Scale bars, 1 μm. (c) SEI and CL images of the ZnO nanorod bundle grown on a polycrystal ZnO seed layer. Scale bars, 200 nm. (d) Schematic of the ZnO hexagonal rods and corresponding crystalline structure illustrating the atomic arrangement. 458
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linearly with the radii of the ZnO rods, which is in agreement with our experimental results given in Figure 2c. In order to minimize the NBE quenching at the ZnO top surface, thermal annealing in an oxygen atmosphere was performed.27,28 Compared with the as-grown ZnO rod, the NBE intensity was significantly higher, while the DLE decreased by the thermal annealing (see Figure 4). Moreover, thermal annealing led to bright NBE even on the (0001) plane of the ZnO top surface, as confirmed by the CL images shown in the inset of Figure 4 (see also Figure S5). This result indicates that the thermal annealing process eliminated the defects of the ZnO (0001) plane, leading to the prevention of NBE quenching. Although no consensus has been reached for the origin of these defects and annihilation by thermal annealing, it is likely related with the adsorption/desorption of water, hydrogen, or OH groups on the surface. Considering that OH groups can easily adsorb on positively charged Zn-(0001) surface during hydrothermal process, NBE emission quenching observed at the Zn-(0001) surface can be attributed mainly to the presence of the OH groups. The enhanced NBE (and reduced DLE) after thermal annealing might originate from the desorption of those OH groups.20,21,29 Further study is needed to better understand the underlying mechanism. Furthermore, the disappearance of the bright NBE emission at the hexagonal edge of ZnO rods, together with intense and uniform NBE emission across the top surface after thermal annealing, supports the conclusion that the bright NBE emission at the hexagonal edge of as-grown ZnO rods is not associated with the WGM enhanced emission.
Figure 4. CL spectra of the ZnO rods before (blue) and after (red) thermal annealing in an oxygen atmosphere. The inset images are the SEI and corresponding CL images for the NBE and DLE taken after thermal annealing. Scale bars, 100 nm.
emission at the hexagonal edge) is not size-dependent and observed even for very small diameter rods (D ∼ 500 nm); in contrast, the constructive interference condition for WGM in a hexagonal resonant cavity is strongly size-dependent (Figure S4). (ii) Even in the large diameter rods (D ∼ 5 μm) the luminescence intensity was uniformly distributed across the top surface for the DLE (Figure S4). The hexagonal wurtzite crystal structure for ZnO is unique for its noncentral symmetry and polar surfaces.10 As schematically described in Figure 3d, the c-axis oriented ZnO crystal is composed of closely packed O2 and Zn2+ layers stacked alternatively along the c-axis, creating a Zn-terminated (0001) surface along the top surface and nonpolar side faces of {1010} planes.10 These distinctly different surface structures were formed in our ZnO hexagonal rods and play key roles in determining the luminescent properties. The Zn-polar surface is more chemically reactive than the nonpolar or O-polar surface, which resulted in many surface defects.10,25 In particular, OH groups can easily adsorb on the positively charged Zn-(0001) surface of hydrothermally grown ZnO, and these defects might act as quenching sites for the radiative recombination of free excitons in the ZnO rod top surface.26 Assuming CL is predominant within a few nanometers from the surface, the volume associated with the DLE, VDLE, can be described by the summation of the top hexagonal layer and the six side walls as follows: pffiffiffi 3 3 2 r tDLE þ 6rhtDLE ð1Þ VDLE ¼ 2 where r, h, and tDLE represent the radius (i.e., one side of the hexagon), the height of the rods, and the thickness of the effective layer contributing to the DLE, respectively (see Figure 3d). On the other hand, since NBE occurs only near the side surfaces, the volume associated with the NBE, VNBE, can be expressed as VNBE ¼ 6rhtNBE
ð2Þ
where tNBE represents the thickness of the effective layer contributing to the NBE. If these volumes dominate the CL emission intensities, IDLE/INBE can be given by pffiffiffi ! IDLE tDLE 3 r ð3Þ ¼α 1 þ 4 INBE tNBE where α is the ratio of the efficiencies between the NBE and the DLE processes. Equation 3 shows that IDLE/INBE should increase
IV. CONCLUSION In conclusion, we have investigated the surface polaritydependent luminescent properties of size- and position-controlled ZnO hexagonal rods grown by a hydrothermal process. By correlating the size-dependent CL spectra and the spatially resolved CL microscopy, a strong correlation between the local surface luminescence and the surface polarities of the ZnO crystals was demonstrated. In particular, the NBE was strongly dependent on the surface polarity, whereas the DLE was bright at every surface without showing any dependence on the surface polarity. Additionally, we suggest that appropriate thermal annealing prevents NBE emission quenching by eliminating surface defects associated with the nonradiative recombination of free excitons. This result introduces the opportunity to control the surface luminescence in nanostructured materials, thereby furthering the advancement of nanoscale photonic devices. ’ ASSOCIATED CONTENT
bS
Supporting Information. Figures S1S5. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20090071357) and by a KIST research program (Grant 2E22121). 459
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