Substrate Effect Induced Growth of Various Single-Crystalline Zn and

Substrate Effect Induced Growth of Various Single-Crystalline Zn and Zn/ZnO Core-Shell Polyhedrons with Tunable Photoemission Wei-Yu Chen,† Ruey-Chi Wang,‡ and Chuan-Pu Liu*,† Department of Materials Science and Engineering, National Cheng Kung UniVersity, Tainan 70101, Taiwan, Center for Micro/Nano Technology Research, National Cheng Kung UniVersity, Tainan 70101, Taiwan, and Department of Chemical and Materials Engineering, National UniVersity of Kaohsiung, Kaohsiung, 81148, Taiwan

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2248–2255

ReceiVed September 11, 2007; ReVised Manuscript ReceiVed March 18, 2008

ABSTRACT: Single-crystalline Zn and Zn/ZnO core-shell polyhedrons of various types were synthesized by chemical vapor deposition on Si(111) and Si(100) followed by different oxidation treatments. The core-shell polyhedrons started with the growth of single-crystalline Zn polyhedrons, nucleated on a Si substrate, and the ZnO layers were formed on the surface of the Zn polyhedrons upon the oxidation treatments. The thickness of the oxide layers could be controlled precisely by adjusting the temperature during the oxidizing process. High-resolution transmission electron microscopy and focus ion beam show various polyhedrons were grown with different crystallographic orientations vertical to the Si substrate and exhibit special mechanical properties in tolerating strain induced by the lattice mismatch between Zn and Si. The strain energy as well as the surface and interfacial energy determine the number ratio of different polyhedrons on Si(111) and Si(001), and a thermodynamic model was proposed to explain the mechanism. Room-temperature cathodoluminescence measurements show that the Zn/ZnO core-shell polyhedrons exhibit tunable ultraviolet emissions and controllable green emission with different oxidation conditions, which were ascribed to the surface and strain effect. The growth of various Zn polyhedrons on Si provides evidence for understanding strain induced substrate effects and distinguishing strain tolerance between metal and oxide nanostructures.

1. Introduction ZnO is an important functional material owing to its large exciton binding energy, wide band gap in the near-ultraviolet, piezoelectric properties,1 semiconductivity,2 and pyroelectric properties.3 For industrial applications, it is necessary to synthesize large area single-crystalline ZnO on the important substrate of Si for opto-electric devices, such as light emitting diodes,4 or as templates for the growth of other nanomaterials. However, owing to the structural difference between ZnO and Si, most ZnO films grown on Si are polycrystalline5 or highly textured6 at best. Furthermore, the study of the crystallographic relationship of ZnO with Si is very limited.6 Recently, most of the single-crystalline ZnO objects on Si are one-dimensional structures, such as nanowires7,8 or nanotubes,9 etc. Fan et al.10 and Sulieman et al.11 have synthesized microsized ZnO microcages and Zn/ZnO microspheres on Si, respectively. However, they are randomly oriented structures, and the crystallographic relationships of ZnO with Si have not been reported. Wang et al. reported the synthesis of ZnO nanoflowers12 and microboxes13 on Si(100) and discovered the specific crystallographic relationship between single-crystalline hexagonal ZnO sheets and Si(100) substrate at the nucleation stage. However, the epitaxy of ZnO on Si would be disturbed for thicker films to form large-sized non-single-crystalline particles due to the lattice mismatch between ZnO and Si. Although the strain theory provides a logical explanation for the phenomenon, direct experimental evidence is deficient so far. Additionally, morphological revolution of micro/nanostructures due to lattice mismatch is an important issue for the substrate effect plays a significant role in the formation of many novel micro/nano* Author to whom correspondence should be addressed. E-mail: cpliu@ mail.ncku.edu.tw. † Department of Materials Science and Engineering and Center for Micro/ Nano Technology Research, National Cheng Kung University. ‡ Department of Chemical and Materials Engineering, National University of Kaohsiung.

structures. The growth mechanisms of novel structures will be realized and controlled further if more understanding of the lattice mismatch is established. Instead of preparing a large area single-crystalline ZnO template, we fabricated various single-crystalline Zn polyhedrons by substrate effect and transformed the Zn to singlecrystalline Zn/ZnO core-shell polyhedrons step by step via different annealing treatments. The crystallographic relationships of various polyhedrons with Si(111) and Si(100) were studied by focus ion beam (FIB) milling and high-resolution transmission electron microscopy (HRTEM). The percentage of formation of various typed polyhedrons was determined by the degree of crystallographic symmetry, lattice mismatch, and surface energy. We also directly demonstrated the strain at the polyhedron/substrate interface region, and the stress relief and deformation of a polyhedron before and after being sliced by FIB for the first time. This supports an assumption that although Zn has a larger lattice mismatch with Si than ZnO, no microsized ZnO crystals could be formed directly on Si, which is attributed to the different strain tolerance and bonding types between metals and ceramics. Furthermore, we fabricate Zn/ZnO core-shell polyhedrons with different thicknesses of ZnO at different annealing temperatures. The cathodoluminescence (CL) results show obvious emission with tunable wavelength and intensity by adjusting the annealing temperature systematically. Although various Zn/ZnO heterostructures were reported,11,14–16 we report on the systematic shift of UV emission by adjusting the thickness of ZnO for the first time. The polyhedrons are promising for single-crystalline ZnO templates and nanooptoelectrical applications.

2. Experimental Section The zinc polyhedrons were synthesized by a catalyst-free thermal chemical vapor deposition in a horizontal quartz tube. Metallic zinc powder (purity 99.9%, 100 mesh) was transferred into the heating zone of the system, and cleaned Si substrates were located downstream at a

10.1021/cg700871f CCC: $40.75  2008 American Chemical Society Published on Web 06/14/2008

Zn and Zn/ZnO Core-Shell Polyhedrons

Crystal Growth & Design, Vol. 8, No. 7, 2008 2249

Figure 1. SEM images of (a) the Zn polyhedrons grown on Si (111), (b) a type I Zn polyhedron, (c) a type II Zn polyhedron, (d) a type III Zn polyhedron, and (e) typical EDS of the Zn polyhedrons. position where the temperature was maintained at around 300 °C during the growth process. The distance between the source and substrate was 10 cm. During the experiments, the zinc powder was heated at a rate of 20 °C/min from room temperature carried by argon with a flow rate of 65 sccm. Once the temperature was raised to 1000 °C, oxygen was introduced into the chamber with a flow rate of 19.1 sccm. The working pressure was kept at 10 Torr. After the sample was heated at 1000 °C for 1 h, the system was slowly cooled down. The as-grown Zn polyhedrons were then oxidized into Zn/ZnO core-shell structures by annealing under the nitrogen/oxygen flow (50 sccm/50 sccm) with a chamber pressure of 26 Torr at a temperature of 400, 450, 500, and 550 °C, respectively. The morphology and composition were characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDS). The microstructures and crystallographic orientations of the structures were characterized by HRTEM. The TEM samples were prepared by FIB milling. The optical properties were measured by room-temperature CL spectroscopy.

3. Results and Discussion 3.1. Zn Polyhedrons. 3.1.1. Morphology and Characterization of Zn Polyhedrons. Figure 1a shows a low-magnification SEM image of uniformly distributed Zn polyhedrons grown on Si(111). Apparently, the polyhedrons exhibit 6-fold symmetry with different orientations with Si. Figure 1b shows the SEM close-up image of a hexagonal polyhedron with crystallographic c-axis perpendicular to the substrate (classified as type I). The polyhedron of this type is the most common type to be synthesized on Si(111), and the size is in the range of 1-6 µm. Figure 1c demonstrates another typed polyhedron with the crystallographic c-axis parallel to the substrate (type II). Figure 1d shows the third type polyhedron (type III). The polyhedron also exhibits 6-fold symmetry and is constructed with stacked hexagonal sheets with [0002] orientation inclined to the normal of the substrate. In addition to Si(111), the three types of Zn polyhedrons could also be synthesized on Si(001). Interestingly, the number ratio of polyhedrons from type I to type III was estimated to be around 72:26:2 for Si(111) and 30:40:30 for Si(001), indicating that type I predominates only on Si(111). Furthermore, the variation of the size of Zn polyhedrons also affects the ratio of crystals with different orientation on Si(111). For polyhedrons of smaller size (diameter: 1-3 µm), the ratio of type II polyhedrons is around 28%. However, for polyhedrons

of larger size (diameter: 3.1-6 µm), the ratio of type II polyhedrons is decreased to around 19%. The phenomenon could be explained by a thermodynamic model later. Figure 1e is the EDS spectrum of the as-synthesized polyhedrons, confirming that the polyhedrons are mainly composed of Zn. The TEM examination of the various polyhedrons is shown in Figure 2. Figure 2a shows a TEM bright-field cross section image of a type I Zn polyhedron on Si(111). The thin layer of darker contrast enclosing the contour of the polyhedron is amorphous carbon deposited to prevent damage from focused ion beam milling. The insets are the electron diffraction pattern of Zn and Fourier transform of Si lattice images. The data demonstrate that the Zn polyhedron is a single-crystalline wurtzite structure with the c-axis vertical to the Si(111) substrate and the epitaxial relationship is {111}Si || {0002}Zn and Si || Zn. Figure 2b shows a HRTEM image near the interface of a type I Zn polyhedron and Si(100), where the insets are the electron diffraction patterns of Zn and Si. It demonstrates that the Zn polyhedron is single-crystalline wurtzite structure with the c-axis vertical to the Si(001) substrate and the crystallographic relationship is {100}Si || {0002}Zn and Si || Zn. Figure 2c shows a TEM bright- field cross session image of a type II Zn polyhedron on Si(001), which possesses the crystallographic [011j0] orientation vertical to the Si(001) substrate, with the crystallographic relationship being {001}Si || {011j0}Zn, and Si || < 0002>Zn. Figure 2d shows a HRTEM image of the interface region between Si(001) and the type II polyhedron in Figure 2c, with the inset being the high resolution image after Fourier filter. The numerous dislocations and lattice distortions as indicated by the dashed circle near the interface may be induced by the lattice mismatch between Zn and Si. According to the TEM crosssectional images, the interface with larger lattice mismatch shows a higher degree of lattice distortion and dislocation density. 3.1.2. Atomic Stacking of the Zn/Si Interface. From the TEM characterization, all the Zn polyhedrons grown on Si substrates are single-crystalline wurtzite structures, where the atomic stacking at the interface for the crystallographic relationship is shown in Figure 3. For type I Zn polyhedrons on Si(111)

2250 Crystal Growth & Design, Vol. 8, No. 7, 2008

substrate in Figure 3a, the lattice mismatch between Si and Zn is calculated to be 30.7%, which exhibits 6-fold symmetry. On the other hand, the type II Zn polyhedrons on Si(111) have two possible orientations with the c-axis parallel to Si or Si, as shown in Figure 3b,c, respectively. The lattice mismatch along the aligned axes (Si and Zn) is 28.6% in Figure 3b and 30.7% in Figure 3c (Si and Zn). However, atoms are misaligned to a large degree in other axes. The asymmetric behavior of lattice mismatch may cause higher formation energy for type II polyhedrons on Si(111). For Si(001) substrate, the type I polyhedrons exhibit the asymmetric crystallographic relationship as shown in Figure 3d, with the lattice mismatch along the aligned axis (Si and Zn) being 30.7%. On the other hand, the type II polyhedrons exhibit the crystallographic relationship of 2-fold symmetry, as shown in Figure 3e, with the lattice mismatch being similar between two axes. 3.1.3. Growth Mechanisms of Zn Polyhedrons. To realize the growth mechanisms of Zn polyhedrons of different types, a thermodynamic model for the nucleation and growth of Zn on Si substrate has been established as follows. The nucleation of Zn nuclei is ascribed to immiscibility of Zn and Si which promotes the surface diffusion of Zn and the formation of Zn nuclei. The growth of Zn polyhedrons of different types is determined by the minimum free energy,17,18 consisting of surface, interfacial, and strain energy. The higher probability of forming a certain type is due to lower total free energy. If taking Si(111) substrate as an example and considering two differently oriented hexagonal polyhedrons with the same height and side length of a micrometer as shown in Figure 4, the difference in the Gibbs free energy between a type I and type II Zn polyhedron on Si(111) can be expressed as

Chen et al.

∆G ) GtypeII - GtypeI ) ∆γsurface + ∆Ginterfacial +

{[

∆Gstrain ) a2 -γZn{101j0} + γZn{0002} γSi{111}

(

3√3 -1 2

)] [

3√3 + 2

+ Gi Zn{101j0}/Si{111} -

Gi Zn{0002}/Si{111}

3√3 2

]}

+

2 VZnK[ε2Zn{101j0}/Si{111} - εZn(0002)/Si{111} ] (1)

where VZn is the volume of polyhedrons, K is the bulk modulus of Zn, and jε is the mean strain induced by lattice mismatch in the interface. The surface energies of Zn(0002) and Si(111) are taken as 0.989 and 1.14 J/m2, respectively.19,20 The surface energy of Zn (011j0) should be around 1.766 J/m2 according to the Wulff construction of Zn.21 On the other hand, the interfacial energy of Zn{101j0}/Si{111} and Zn{0002}/Si{111} could be estimated to be half of the surface energy of Zn{101j0} and Si{111}, Zn{0002}, and Si{111}, respectively, in the condition of coherent assumption and not considering the binding energy. Consequently, the summation of differences in surface and interfacial energy is 1.31a2 × 10-12 J. As for the difference in strain energy ∆Gstrain, complete analysis of strain components in three dimensions is required. However, if the homogeneous strain in Zn polyhedrons is assumed for simplicity, the interfacial strain in type I should be less than type II zinc polyhedron due to a similar crystal symmetry as Si(111) and less lattice mismatch as shown in Figure 3. As a result, the difference in strain energy could reach 1.81a3 × 10-11 J even if the difference in the square of the mean strain is assumed to be as small as 0.01% and the bulk

Figure 2. TEM cross-sectional bright-field images of (a) a type I Zn polyhedron on Si(111) with the insets showing the corresponding electron diffraction pattern of Zn and Fourier transform of Si, (b) high-resolution image of the interface between a type I polyhedron and Si(100) with the insets showing the corresponding diffraction patterns of Zn and Si, (c) a type II polyhedron on Si(001), and (d) high-resolution image of the interface between a type II polyhedron and Si(001) with the inset showing the image after Fourier filter.



Figure 3. Schematic diagrams of the crystallographic relationships of Zn polyhedrons with Si substrate (a) type I polyhedrons on Si(111); (b,c) type II polyhedrons on Si(111); (d) type I polyhedrons on Si(001); (e) type II polyhedrons on Si(001).

Figure 4. Schematic diagram of type I and type II Zn hexagonal polyhedrons with the same height and side length of a micrometer grown on Si(111) substrate.

modulus of zinc is 70 GPa. Therefore, in terms of both surface and strain energy, type I zinc polyhedron is more energetically favorable. Nevertheless, the difference in total free energy between different types is not great and can be overcome by thermal energy fluctuations. Therefore, both types grow on Si(111) and the type I zinc polyhedron exhibits higher probability. Besides, since the difference of surface energy and strain energy is proportional to a2 and a3, respectively, the ratio of type II polyhedrons is less for the polyhedrons of larger size. Compared with our previous work,13 the smaller lattice mismatch between Si and ZnO induced the formation of ZnO hexagonal microboxes with similar size and symmetry, which possess distinct crystallography from the Zn hexagonal polyhedrons in that the ZnO microboxes were enclosed only by {0002} facets. However, the microsized Zn single-crystalline polyhedrons could be grown on Si without changing crystallographic orientation. The distinct behaviors between ZnO and Zn indicate that Zn possesses larger strain tolerance than ZnO, which may be due to the difference in bonding types. In addition, the composition of the polyhedrons is Zn rather than ZnO, although oxygen was injected into the tube during

the synthesis process. This is reasonable since the substrate temperature is around 300 °C, at which the oxidation rate is much lower than the Zn deposition rate. Figure 5 is the schematic diagram of the proposed growth mechanisms for Zn single-crystalline polyhedrons and ZnO microboxes synthesized previously.13 3.2. Zn/ZnO Core-Shell Polyhedrons. 3.2.1. Morphology and Characterization of Zn/ZnO Core-Shell Polyhedrons. The single-crystalline Zn polyhedrons could transform into Zn/ZnO core-shell polyhedrons with controllable thickness of the ZnO shell upon oxidation treatment at various temperatures. Figure 6, panels a-d are the SEM images of polyhedrons upon annealing at 400, 450, 500, 550 °C for 1 h, respectively. As shown in Figure 6a, the morphology of the polyhedron stays the same upon annealing at 400 °C. However, the polyhedron becomes expanded and rounded upon annealing at 450 °C as shown in Figure 6b. When the annealing temperature is increased to 500 °C, the top (0002) facet is often broken as shown in Figure 6c, probably due to the tensions from the {011j0} ZnO side planes. When the annealing temperature is further increased to 550 °C, the melted Zn inside could accelerate Zn vapor evaporation through the broken shells leading to the ZnO nanowire formation on the surface of the polyhedron as shown in Figure 6d. Figure 6e is the EDS spectrum of the polyhedron annealed at 500 °C as shown in Figure 6c, showing the effective formation of ZnO shells from the presence of the oxygen peak. Figure 6f shows that the O/Zn ratio increases from 3% to 21% as the annealing temperature arises from 400 to 550 °C, indicating the increase of the shell thickness. Figure 7a is the cross-sectional TEM bright-field image of a type I polyhedron upon annealing at 450 °C, which is composed of a ZnO shell on the surfaces and a single-crystalline Zn core


Chen et al.

Figure 5. Schematic diagram of the proposed growth mechanisms of Zn polyhedrons and ZnO microboxes.13

Figure 6. SEM images of the annealed polyhedron at different temperature for 1 h: (a) 400 °C, (b) 450 °C, (c) 500 °C, (d) 550 °C; (e) EDS spectrum of the polyhedrons annealed at 500 °C; (f) relationship between oxygen concentration and annealing temperature

inside. The average thickness of the shell on ZnO (0002) and (011j0) facets were around 56 and 30 nm, respectively, implying that the oxidation rate on {0001} facets is higher, which may be attributed to higher chemical activation of {0001} facets. Examining Figure 7a, there is detachment of the ZnO shell from the Zn core, implying that oxidation preceded the melting and spheroidizing of the core. Figure 7b is the cross-sectional TEM bright-field image of a type I polyhedron upon annealing at 500 °C. The Zn/ZnO polyhedrons under such conditions usually undergo cracking in the shells, which results in sink of the top {0001} facet as shown in the figure. In particular, part of the Zn/ZnO polyhedron is detached from the Si substrate, implying the instability of the interface at this temperature. Figure 7c shows a TEM image of the interface region of a Zn/ZnO polyhedron. The thickness of the ZnO shell was measured to be 72.3 nm. Figure 7d shows the relationship between the

thickness of the ZnO shell and annealing temperature. The ZnO thickness of top facet increases from 19 to 131 nm as the annealing temperature rises from 400 to 550 °C. Figure 7e is the high-resolution image of the Zn/ZnO interface (as shown in Figure 7c). The outer ZnO and inner Zn are both singlecrystalline structures. The right and left insets show the diffraction pattern of Zn and Fourier transform of ZnO, respectively, demonstrating that the ZnO is parallel to the Zn. However, the [0002] directions of both Zn and ZnO are not parallel, which is in contrast to the previous report.22 3.2.2. Growth Mechanisms and Stress Relief of Zn/ ZnO Core-Shell Polyhedrons. The microsized single-crystalline Zn polyhedrons formed on Si, in spite of a large lattice mismatch between Zn and Si, has been attributed to the higher strain tolerance of the metal bonds of Zn compared with the ionic/covalent bonds of ceramic ZnO. Figure 8a is the cross-



Figure 7. (a) TEM cross-sectional bright-field images of (a) a type I polyhedron annealed at 450 °C; (b) a type I polyhedron annealed at 500 °C; (c) the surface region of the polyhedron in (b); (d) relationship between the ZnO shell thickness and annealing temperature; (e) high-resolution image of the Zn/ZnO interface region with the insets showing the corresponding Fourier transform of ZnO and electron diffraction pattern of Zn.

Figure 8. Cross-sectional images of Zn/ZnO core-shell polyhedrons at different stages: (a) SEM image of a Zn/ZnO core-shell polyhedron before FIB slicing; (b) SEM image of another Zn/ZnO polyhedron being sliced in half by FIB; (c) TEM bright-field image of the Zn/ZnO polyhedron as in (b); (d) schematic diagram of the proposed formation mechanism of Zn/ZnO core-shell polyhedrons.

sectional SEM image of a type I polyhedron upon annealing at 500 °C, showing a crack at the interface as indicated, which may be ascribed to the difference in thermal expansion coefficient. However, the crack becomes larger and propagates along the interface after half of the polyhedron has been sliced by focus ion beam, as shown in Figure 8b, due to less restriction from the surrounding materials. Furthermore, the TEM image in Figure 8c shows complete detachment of the polyhedron after

a TEM thin foil has been made. More interestingly, polyhedrons underwent morphology deformation during the slicing process and the height/width ratio of the polyhedron is increased from 0.60 in Figure 8b to 0.71 in Figure 8c, in accordance with a large degree of strain restored resulting from the lattice mismatch of Zn/Si and Zn/ZnO. The results provide direct evidence of the formation of epitaxial micro/nanostructures by lattice strain. In these core-shell Zn/ZnO polyhedrons on Si, if the annealing


Chen et al.

Figure 9. CL properties of the type I Zn/ZnO core-shell polyhedrons after annealing at different temperatures: (a) room-temperature CL spectra; (b) plot of the UV emission energy with reciprocal of the ZnO shell thickness.

temperature is below 500 °C, the ZnO thickness is too thin to affect the Zn/Si interface, so the ZnO shell tends to detach from the rounded Zn polyhedron. On the other hand, when the annealing temperature is higher than 500 °C, the ZnO thickness is thick enough to break down the Zn/Si interface, causing collapse on the top and detachment on the bottom of a polyhedron. Figure 8d shows the schematic diagrams of the formation and deformation of Zn/ZnO core-shell polyhedrons. 3.3. Tunable CL Properties of Zn/ZnO Core-Shell Polyhedrons. The Zn/ZnO polyhedrons with controllable ZnO shell thickness formed under different annealing treatments possess tunable emission properties. Figure 9a shows the room temperature CL spectra of type I Zn/ZnO core-shell polyhedrons annealed at different temperatures. The spectra exhibit relatively sharp ultraviolet (UV) emissions at around 366 nm and broad green emissions at around 485 nm. The UV emission is attributed to the near band edge excitonic recombination, while the green emission may be the result of the defects related transitions.23–27 The intensity of both UV and green emissions increases with the annealing temperature and thus the ZnO shell thickness. An enlarged view of all the UV emission peaks after normalizing UV intensity is shown in the inset of Figure 9a. The UV peak of the polyhedrons shifts from 364 to 369 nm gradually as the annealing temperature increases from 400 to 550 °C, corresponding to the ZnO shell thicknesses of 19-131 nm, which are larger than the Bohr radius of ZnO. Therefore, the peak shift here is not caused by the quantum confinement effect. To further examine the origin of the shift, the emitted UV energy is plotted with the inverse of the ZnO shell thickness in Figure 9b, which exhibits a nonlinear behavior with a slightly larger slope at a higher thickness of ZnO. Since the blue shift should not be caused by the quantum confinement effect, surface effect and strain induced band gap shift should be considered in the transition energy given as

∆Eg ) ∆Eg surface + ∆Eg strain

(2)

According to the mechanism proposed by Chen et al.,28 the emission is ascribed to the contribution of excitonic recombination and subsequent emission of a photon from the surface band, which is of higher energy than that of the bulk band. Therefore, the energy shift ∆Eg surface depends on the change in the surfaceto-volume ratio, which is estimated to be “4tO/d” for a nanorod, where tO represents the effective thickness of the surface recombination layer and d is the diameter of a nanorod. As a result, the energy shift is proportional to 1/d due to surface

effect. For the case of the Zn/ZnO polyhedrons, the surfaceto-volume ratio could be considered as tO/t, where t represents the thickness of the ZnO shell. Consequently, the energy shift due to the surface effect should also be proportional to 1/t, which is expressed by a dashed line in the figure. Apparently, while the emission energy of the three thicker ZnO films exhibits a linear relationship with 1/t, the polyhedron with the thinnest ZnO shell thickness exhibits a large deviation of -0.04 eV. The further energy shift could be ascribed to the strain induced energy shift, since only the polyhedrons annealed at 400 °C still possess epitaxial Zn/ZnO interfaces, while ZnO shells have been detached and relaxed from Zn crystals for other polyhedrons according to TEM cross-sectional images. Therefore, the strain induced energy shift for the sample annealed at 400 °C should be considered. According to Davydov et al.,29 change in the band gap of a strained hexagonal crystal could be given by

∆Eg strain ) R(exx + eyy) + βezz ) (2R + βq)exx ) [2(a2 + b2) + (a1 + b1)q]exx (3) where q ) -2C13/C33. By substituting the deformation potentials (a1, a2, b1, b2) and elastic stiffnesses (C13, C33) of ZnO,30 the formula for ZnO could be simplified to be ∆Eg strain ) 11.64exx. By measuring the d spacing of the ZnO(01j11) planes as shown in Figure 6e, the exx of the ZnO shell is calculated to be -0.35%. Consequently, the resulting energy shift due to lattice strain is -0.041 eV, which is in good agreement with the additional energy shift of -0.04 eV in the figure.

4. Conclusions In this paper, we report on several types of single-crystalline Zn and Zn/ZnO core-shell polyhedrons synthesized step by step by CVD followed by designed oxidizing treatments. HRTEM and FIB show that various types of polyhedrons were grown with specific crystallographic orientations on Si. The number ratios of different polyhedrons on Si(111) and Si(100) were analyzed and explained by a proposed thermodynamic model. Direct evidence of apparent deformation of polyhedrons after stress relief was provided to prove the lattice mismatch effect. Room-temperature CL measurements show the polyhedrons exhibit tunable UV and green emissions with different oxidation conditions, which were attributed to surface and strain effects. The Zn/ZnO polyhedrons are promising for single-


crystalline ZnO templates and opto-electronic nanodevice applications. Acknowledgment. The work was supported by Research Grant No. NSC95-2221-E-006-080-MY3 and No. NSC 962221-E-390-016 National Science Council of Taiwan. The authors thank the Center for Micro/nano Technology Research, National Cheng Kung University and the Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Taiwan, for the provision of HRTEM and FESEM.

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Crystal Growth & Design, Vol. 8, No. 7, 2008 2255 (13) Wang, R. C.; Liu, C. P.; Huang, J. L. Appl. Phys. Lett. 2006, 89, 173121. (14) Kong, X. Y.; Ding, Y.; Wang, Z. L. J. Phys. Chem. B 2004, 108, 570. (15) Lin, C. C.; Liu, K. H.; Chen, S. Y. J. Cryst. Growth 2004, 269, 425. (16) Zeng, H. B.; Liu, P. S.; Cai, W. P.; Cao, X. L.; Yang, S. K. Cryst. Growth Des. 2007, 7, 1092. (17) Yang, Y. H.; Wang, B.; Yang, G. W. Cryst. Growth Des. 2007, 7, 1242. (18) Yang, Y. H.; Wang, B.; Yang, G. W. Nanotechnology 2006, 17, 5556. (19) Vitos, L.; Ruban, A. V.; Skriver, H. L.; Kolla´r, J. Surf. Sci. 1998, 411, 186. (20) Messmer, C.; Bilello, J. C. J. Appl. Phys. 1981, 52, 4623. (21) Arnstein, G. M.; Bolsaitis, P.; Armstrong, R. W. Acta Crystallogr., Sect. A 1972, 28, 344. (22) Gao, P. X.; Lao, C. S.; Ding, Y.; Wang, Z. L. AdV. Funct. Mater. 2006, 16, 53. (23) Vanhausden, K.; Warren, W. L.; Seager, C. H.; Tallant, D. R.; Voigt, J. A.; Gnade, B. E. J. Appl. Phys. 1996, 79, 7983. (24) Meng, X. Q.; Shen, D. Z.; Zhang, J. Y.; Zhao, D. X.; Lu, Y. M.; Dong, L.; Zhang, Z. Z.; Liu, Y. C.; Fan, X. W. Solid State Commun. 2005, 135, 179. (25) Chen, Y. Q.; Jiang, J.; He, Z. Y.; Su, Y.; Cai, D.; Chen, L. Mater. Lett. 2005, 59, 3280. (26) Ng, H. T.; Chen, B.; Li, J.; Han, J.; Meyyappan, M.; Wu, J.; Li, S. X.; Haller, E. E. Appl. Phys. Lett. 2003, 82, 2023. (27) Chen, Z.; Wu, N.; Shan, Z.; Zhao, M.; Li, S.; Jiang, C. B.; Chyu, M. K.; Mao, S. X. Scr. Mater. 2005, 52, 63. (28) Chen, C.-W.; Chen, K. -H.; Shen, C.-H.; Ganguly, A.; Chen, L.-C.; Wu, J.-J.; Wen, H.-J.; Pong, W. F. Appl. Phys. Lett. 2006, 88, 241905. (29) Davydov, V. Yu.; Averkiev, N. S.; Goncharuk, I. N.; Nelson, D. K.; Nikitina, I. P.; Polkovnikov, A. S.; Smirnov, A. N.; Jacobson, M. A.; Semchinova, O. K. J. Appl. Phys. 1997, 82, 5097. (30) Stiens, J.; Vounckx, R.; Veretennicoff, I.; Voronko, A.; Shkerdin, G. J. Appl. Phys. 1997, 81, 1.

CG700871F

Substrate Effect Induced Growth of Various Single-Crystalline Zn and

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