Morphology-Controlled Three-Dimensional Nanoarchitectures

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Morphology-Controlled Three-Dimensional Nanoarchitectures Produced by Exploiting Vertical and In-Plane Crystallographic Orientations in Hydrothermal ZnO Crystals Won Woo Lee,† Jaeseok Yi,† Seong Been Kim,† Yoon-Ho Kim,§ Hong-Gyu Park,§ and Won Il Park*,† †

Division of Materials Science and Engineering, Hanyang University, Seoul 133 791, Korea Department of Physics, Korea University, Seoul 136 713, Korea

§

ABSTRACT: Herein we describe the morphology-controlled synthesis of three-dimensional (3D) ZnO nanoarchitectures via a facile hydrothermal route. In this approach, vertical and inplane crystallographic orientations of ZnO crystals were tuned by appropriate patterning of growth masks and seed layer control, which enabled the formation of three types of ZnO nanoarchitectures. If polycrystalline ZnO layers with poor c-axis orientation were used as a seed, flower-like ZnO nanoarchitectures composed of radially oriented ZnO nanorods were achieved. In the case of c-axis oriented ZnO seeds, polygonal ZnO pillars grew vertically at the center of the growth holes. Even in the latter case, multidomain columnar joint structures or single domain crystal structures with well-defined hexagonal facets were achieved based on the existence of six-fold in-plane symmetry of the ZnO seed layers. These morphology-controlled ZnO nanoarchitectures exhibited clear differences in light propagation characteristics, which could be ascribed to strong light guiding in the one-dimensional nanostructures.

I. INTRODUCTION The ability to build nanocrystals with a desired position and shape is essential not only to diversify their potential applications in electronics and optoelectronics but also to enhance their performance.1 4 In an effort to assemble nanomaterials into a desired and highly ordered architecture, a variety of driving forces such as noncovalent interactions, electrical fields, capillary forces, and shear forces have been exploited.5 7 Further, the elucidation of growth mechanisms has readily enabled construction of threedimensional (3D) architectures via anisotropic crystal growth with control over the size and position of the nanocrystals.1,8 However, most of the work done so far remains at the level of simple nanoarchitectures consisting of periodic arrays of nanowires/nanorods grown vertically on the substrates. Although diverse materials have been examined as building blocks, nanostructured ZnO has been attracting particular interest owing to its unique optical, semiconducting, and piezoelectric properties.9 12 The fabrication of patterned ZnO nanostructured arrays has already been achieved with various techniques such as chemical vapor deposition, metal organic chemical vapor deposition, and a hydrothermal method using patterned substrates via nanosphere lithography, nanoimprint lithography, or electron beam lithography.8,13 16 In particular, the hydrothermal method has been widely adopted due to its potential advantages in rapid, large-scale, and low-cost synthesis. Indeed, the use of lithographically defined mask patterns on seeds has enabled the growth of periodic structures of ZnO nanorods. However, to r 2011 American Chemical Society

date, control of the shape of three-dimensional (3D) nanoarchitectures is rarely achieved and remains challenging. Here we report the successful control of 3D nanoarchitecture morphology achieved by exploiting the crystal growth behavior in a hydrothermal process. In our approach, the use of different types of seeds in terms of crystallographic orientation, after patterning with periodic circular holes, resulted in three types of ZnO nanoarchitectures: nanoflowers, columnar joints, and hexagonal pillars. On the basis of the observed relationship between the ZnO crystal orientations and X-ray diffraction results, we concluded that the crystal structure of ZnO seed layers influences the vertical and in-plane orientations of the c-axis grown wurtzite phase, thereby determining the overall ZnO crystal morphology. Moreover, through experimental and electromagnetic (EM) modeling results, we demonstrated that the strong guiding of light in these one-dimensional (1D) nanostructures resulted in unique light propagation characteristics depending on the nanostructure morphology.17,18

II. EXPERIMENTAL SECTION ZnO Seed Layer Growth. Our experiments began with the predeposition of ZnO seed layers on different substrates, and this process is schematically illustrated in Figure 1. In order to obtain three Received: June 25, 2011 Published: September 22, 2011 4927

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Figure 1. Schematic illustrating the hydrothermal process for the growth of three types of ZnO nanoarchitectures via the appropriate patterning of growth masks and seed layer selection. seed layers, the deposition of ZnO thin films was carried out on Si(100) and Al2O3(0001) substrates, respectively, by PLD using a XeCl excimer laser with a 308 nm wavelength in an oxygen atmosphere. The typical growth temperature was 450 °C and growth time was 80 min, which produced c-axis oriented ZnO films with a typical thickness of ∼200 nm. Patterning of Polymer Growth Masks. In order to achieve position-controlled ZnO nanoarchitecture growth, a prepatterned polymethylmethacrylate (PMMA) mask was defined on the ZnO seed layers using an e-beam lithography technique. A 200-nm-thick PMMA layer was coated onto the sample, and a regular array of circular holes with diameters ranging from 100 nm to 2 μm was patterned by electron beam lithography with electron beam irradiation at a dose of 300 350 μC cm 2. Hydrothermal Growth of ZnO NRs. Following growth mask patterning on the three types of preseeded substrates, hydrothermal growth of ZnO nanostructures was performed while holding all other possible conditions constant in order to assess the effect of each type of seed layer on crystal growth behavior. During the hydrothermal process, the preseeded substrates were placed in an aqueous solution containing zinc nitrate hexahydrate (Zn(NO3)2.6H2O, Sigma Aldrich) and hexamethylenetetramine (C6H12N4, Sigma Aldrich) in a 1:1 molar ratio (0.025 M) at 85 °C for 6 8 h. The detailed procedure for ZnO hydrothermal growth is described elsewhere.14

Figure 2. SEM images of three types of ZnO nanoarchitectures: (a) nanoflower array on type I seed; (b) columnar joint array on type II seed; and (c) hexagonal pillar array on type III seed. different types of seed layers, we diversified not only the substrates but also the growth method. The three growth method substrate combinations we used were sputtering ZnO seed layer (type I), pulsed laser deposition (PLD) ZnO seed layer on Si(100) (type II), and PLD ZnO seed layer on Al2O3(0001) (type III). The first type of ZnO seed layer was deposited at 400 °C by radio frequency (RF) magnetron sputtering of a stoichiometric ceramic ZnO target. During the deposition, an Ar/O2 mixture (12% O2) was introduced into a chamber where the total working pressure and RF power were kept at ∼1 10 mtorr and 100 W, respectively. For the type II and III

III. RESULTS AND DISCUSSION Three types of ZnO nanostructure arrays are shown in Figure 2, which demonstrates that morphology, size, and orientation of ZnO crystals were strongly affected by the ZnO seed layers.16,19 In the ZnO nanostructures on type I seed layers (sputtering ZnO layer on SiO2/Si substrate), the closely packed ZnO nanorods grew out of the mask holes and spread out like chestnut burrs, which led to flower-like ZnO nanorod bundles with a typical diameter of ∼2.5 μm for 1-μm-diameter mask holes and ∼4 μm for 2-μm-diameter mask holes. The individual nanorods had a diameter ranging from 40 to 50 nm and a length of up to ∼3 μm. In the PLD ZnO seed layers (types II and III), polygonal ZnO pillars grew vertically at the center of the growth mask holes. Notably, even on the PLD ZnO seed layers, 4928

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Crystal Growth & Design multidomain columnar joint structures or single-domain crystal structures with well-defined hexagonal facets formed, depending on the underlying substrate. To better understand the effect of seed layer crystallinity on ZnO crystal formation, we investigated the crystal structures of the seed layers using high-resolution X-ray diffraction (XRD) (Figure 3).20 23 Distinctly different features were noted. First, the XRD θ 2θ scan pattern of the type I seed revealed an extremely weak peak, assigned to diffraction from the (0002) planes of ZnO, which appeared only when multiplied by 200. In contrast, ZnO (000l) peaks were distinctly observed for the type II and III seeds (Figure 3a, green and blue lines). Second, the XRD θ-rocking curve of the type I seed was broad with a full

Figure 3. X-ray diffraction analyses of three types of ZnO seed layers. (a, b) XRD θ 2θ scan patterns (a) and θ-rocking curves (b). (c, d, e) XRD pole figures through the ZnO (1011) plane of type I (c), type II (d), and type III (e) seed layers.

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width at half-maximum (fwhm) value of 12°, whereas those of type II and III seeds had narrower fwhm values of 1.6° and 1.4°, respectively. On the basis of these results, it can be concluded that type I ZnO films sputtered on SiO2/Si substrates had relatively poor crystallinity and c-axis orientation compared with PLD films deposited on crystalline substrate surfaces (types II and III). Moreover, comparison of the XRD data with scanning electron microscopy (SEM) images in Figure 2 revealed a strong correlation between the c-axis orientation of the seed layers and the vertical orientation of hydrothermally grown ZnO crystals. In addition to the orientation of the (0001) ZnO plane in the substrate surface planes, there is still a rotational degree of freedom around the surface normal. X-ray pole figure analysis was performed in order to investigate the relationship between the type of seed layer and in-plane crystallographic orientation.19,21 The pole figures through the ZnO (1011) plane, shown in Figure 3b e, revealed a clear difference in crystal structures among the three types of seed layers. Random crystallographic vertical and in-plane orientations of the type I seed layer were confirmed from the pole figure, and this finding is consistent with the XRD θ 2θ scan result. Meanwhile, due to the c-axis orientation of the type II and III seeds, ZnO (1011) diffraction occurred only at ψ = 63°. While the type II layer exhibited ringlike patterns, the type III layer showed six strong poles separated by 60°, indicating a six-fold in-plane symmetry.21,23,24 Indeed, there was a 30° rotation of the poles with respect to those of the (1123) plane of sapphire (i.e., ZnO[1010]//sapphire[1120]), which was ascribed to the large lattice match (∼18%) between ZnO and sapphire (0001). An extended-domain matching model predicted that a combination of 30° rotation and extended domain matching can reduce the lattice strain to 1.4%.25 Conversely, an additional set of weak poles, labeled with circles, appeared with a 30° rotation with respect to strong poles. This originated from sapphire-matched ZnO domains without rotation having an

Figure 4. Comparison of the in-plane orientation between each domain in type II and III ZnO nanoarchitectures and their seed layers. (a, b) Top-view SEM images of type II, columnar joint (a) and type III, hexagonal pillar (b) ZnO nanoarchitectures. (c, d) Overlap of the histograms of the relative orientation of individual columns and ϕ scan profile (at ψ = 63°) of their seed layers for type II (c) and type III (d) nanoarchitectures. 4929

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Figure 5. SEM images showing the morphological evolution of types I (a), II (b), and III (c) ZnO nanoarchitectures depending on the diameter of the circular holes in the growth masks. The diameters of the holes from left to right are 100 nm, 0.5 nm, 1 μm, and 2 μm.

epitaxial relationship of ZnO[1010]/sapphire[1010]. These two kinds of crystallographic domains (hereafter referred to as “30°rotated domain” and “on-position domain”, respectively) were frequently observed in the epitaxial ZnO thin films on sapphire (0001) substrates.21,24 On the basis of the XRD analysis, we concluded that c-axis orientation and in-plane crystallographic orientation of the seed layers influence the overall morphologies of ZnO crystals. To emphasize this point, we statistically compared the relative orientations of each domain in type II and III ZnO nanostructures around the surface normal (0001) direction with respect to the in-plane crystallographic orientations of their seed layers. A top view SEM image of a type II sample revealed the formation of columnar joint structures composed of randomly oriented small hexagonal columns with diameters in the range of ∼100 300 nm (Figure 4a). The histogram of relative orientation of individual columns with respect to the substrate in Figure 4b clearly demonstrates the random distribution around the surface normal, which is similar to a ϕ scan profile (at ψ = 63°) for the type II seed layer (blue line). However, the type III sample exhibited the formation of well-faceted hexagonal pillars with diameters comparable to or slightly larger than that of the growth mask patterns (1 μm), and the hexagonal facets exhibited preferential in-plane alignment. In contrast, small columns protruding from the side facets of the main hexagonal pillars were also noted (Figure 4c, inset), and they were often rotated by 30° compared to the main pillars. According to the X-ray pole figure result, the main pillars and the other, 30°-rotated, protruding columns were closely related to the 30°-rotated domain and an on-position domain of the ZnO seed, respectively. The angular distribution of the ZnO hexagonal

pillars exhibited a profile very similar to that of the ϕ scan profile of the seed layer (Figure 4d), as more than 80% of crystals had the same alignment with the 30°-rotated domain, while the remainder shared the same alignment as the on-position domain. The SEM and XRD results demonstrated that crystal structures of ZnO seed layers had a significant influence on the initial stage of ZnO growth, resulting in three different types of ZnO nanostructures in terms of crystal orientation, size, and morphology during the hydrothermal process. Along with the crystal structures of the seed layers, the existence of growth mask patterns affects the initial formation of ZnO nanocrystals. In order to investigate the mask size-dependent morphological evolution, hydrothermal growth was performed on three types of seed layers defined with circular hole arrays with diameters ranging from 100 nm to 2 μm (Figure 5). For type I seed layers, the total number of nanorods growing out of each hole decreased with reduced hole size, and individual nanorods remained the same diameter and length. Alternatively, single domain hexagonal pillars were achieved on type II and III seed layers as the diameter of holes became smaller than 100 nm. Meanwhile, by increasing the hole size, the morphology of ZnO crystals on type II seed layers changed to columnar joint structures consisting of many hexagonal columns with arbitrary inplane orientation. If the size of the hole was significantly larger than the diameter of each column, circularly shaped ZnO columns with a cross-sectional shape and diameter corresponding to that of hole were formed. Unlike the type II seed layers, a hexagonal pillar shape was maintained on the type III seed layers. In order to characterize the morphology-dependent optical properties, which are dictated by strong light confinement in the 4930

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Figure 6. (a, b) Schematic configurations of incident ray (R), sample (S), and detector (S) for diffuse (a) and specular (b) transmission measurements. (c, d) Experimental diffuse (black) and specular (red) transmittance spectra measured for hexagonal arrays of ZnO nanoflowers and pillars. The insets in (c) and (d) are simulation results of the cross-sectional E-field intensity distribution for the propagating EM wave (λ = 800 nm) in the nanoflower and pillar, respectively.

ZnO nanostructures, we examined transmission through the ZnO nanoflowers grown on glass (type I) or pillars grown on the bothside polished sapphire substrate (type III). Both diffused and specular transmission were measured; diffused transmission spectra were measured by collecting light from all angles using an integrating sphere (Figure 6a), whereas specular transmission spectra were obtained from light transmitted in the same direction as the incident light (Figure 6b).26,27 In a ZnO nanoflower array, the average diffused transmission was ∼77%, which is much higher than the specular transmission of ∼9% (Figure 6c). This result indicates that the light incident to the ZnO nanoflower structures was guided by individual ZnO nanorods and spread out radially after passing through the nanoflower.28,29 Conversely, incident light would be confined in vertically aligned ZnO pillars because of internal reflection, and thus most of the incident flux propagated along the vertical direction. In our measurement, the average specular transmission of the ZnO pillar array was ∼80%, which is quite close to the diffused transmission of ∼89% (Figure 6d). To further understand light propagation in the ZnO nanostructures, two-dimensional (2D) FDTD simulation was carried out. In the simulation, a plane wave was produced incident to the ZnO nanostructures with a refractive index (n) of 1.95. We modeled the nanoflower consisting of a radially oriented nanorod bundle (diameter of 100 nm, length of 3 μm, and maximum axial tilt angle of 50°) and pillar (diameter of 500 nm, length of 3 μm). The electric field intensity profiles (|E|2) showed that the EM waves were guided by the ZnO nanorods and pillars, thereby resulting in radial dispersion and vertical propagation of light for the nanoflower and pillar, respectively.17,18

morphology of 3D ZnO nanoarchitectures in a site-selective hydrothermal process. Comparison of the XRD and SEM results confirmed that the crystal structures of the ZnO seed layers influence the vertical and in-plane orientations of the c-axis grown wurtzite phase. The distinct differences in crystal growth behaviors, combined with the effect of selective nucleation and initial formation in the confined holes of the mask, determine the ultimate morphology of ZnO nanostructures. The resulting three types of ZnO nanoarchitectures exhibited a difference in light propagation behavior, mainly due to the strong guiding of light in the 1D nanorods and pillars. This observation suggests that the morphology-controlled ZnO nanoarchitectures can be used to increase light extraction and tailor the emission direction in light emitting diodes and displays.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was financially supported by a KIST research program (Grant no. 2E22121) and by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (2009-0071357). ’ REFERENCES

VI. CONCLUSION In conclusion, we studied the relationship between the crystallographic orientation of initial seed layers and the corresponding

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