Interfaces of Mixed Crystallographic Planes of NiO Nanograins - The

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Interfaces of Mixed Crystallographic Planes of NiO Nanograins Sung-Wei Yeh,† Yi-Jen Ji,† Hsing-Lu Huang,‡ Dershin Gan,*,† and Pouyan Shen† Department of Materials and Optoelectronic Science; Center for Nanoscience and Nanotechnology, National Sun Yat-Sen UniVersity, Kaohsiung 80424, Taiwan, and Department of Mechanical Engineering, Chinese Military Academy, 83059 Kaohsiung, Taiwan ReceiVed: April 11, 2010; ReVised Manuscript ReceiVed: September 14, 2010

A nanofilm method has been developed to study the formation of low-energy interfaces by the rotation of nanograins. Ion-beam sputtering was used to prepare very thin epitaxial NiO nanofilms on the (100), (110), (111), and (112) surfaces of NaCl single crystals. When two different NiO nanofilms were overlapped at different angles and annealed at 300 °C, the nanograins were found to rotate until a stable interface was reached. The stable interfaces were analyzed by transmission electron microscopy. Several previously unreported interfaces were found that belong to the asymmetric [110] tilt boundaries on different low-index planes. The orientation relationships and the structures of the interfaces were analyzed. 1. Introduction Crystalline grains have been shown to be able to rotate in thin films or bulk materials. In a Au thin film, Au grains about 66 nm in size were observed to rotate at 250-400 °C.1,2 When two Au grains rotated to the same orientation, they merged into a single grain.1,2 In composite bulk oxides of NaCl- or fluoritetype structure, small intragranular oxide particles of micrometer size were also found to rotate to specific orientation relationships with oxide matrixes upon annealing at high temperatures near 1000 °C.3–6 As the intragranular oxide particles were not spherical and were tightly embedded in an oxide matrix of different composition or crystal structure, interfacial diffusion was apparently necessary during rotation to accommodate the shape and orientation changes.3–6 The rotation of metallic particles of nanometer or micrometer size has been used to study low-energy twist boundaries. Epitaxial Au particles were shown to rotate at high temperature on an epitaxial Au film, prepared by evaporating Au onto the NaCl single-crystal (001) surface, to reach stable twist boundaries with specific misorientations of 0°, 37°, and 45°.7–9 The 37° boundary is a low-energy Σ5 boundary, whereas that at 45° is a Σ29a boundary of high symmetry and low interfacial energy.7,10 Twist boundaries have also been studied experimentally or by simulation on the (100)/(100), (111)/(111), and (110)/ (110) interfaces of Au, Ag, Cu, and Si.7,9,11–14 In addition to the (100)/(100) 37° (Σ5) boundary, two more twist boundaries with very low interfacial energy, namely, a (111)/(111) boundary with 22° misorientation (Σ7 boundary) and a (110)/(110) boundary also with 22° misorientation (Σ3 boundary), have been identified. In fact, many other twist boundaries with higher interfacial energies and Σ numbers larger than 9 have also been recognized.9,12–14 Theoretical simulations showed relaxation energies of 0.5 J/m2 for the (100)/(100) 37°(Σ5) twist boundary12 and 0.35 J/m2 for the (111)/(111) 22° (Σ7) twist boundary.13 Other researchers concerned with the strengths of the above boundaries used bicrystal experiments and theoretical analyses to evaluate the behavior of the boundaries under stress or * Corresponding author. Tel.: +886 7 5252000 ext. 4054. Fax: +886 7 5254099. E-mail: [email protected]. † National Sun Yat-Sen University. ‡ Chinese Military Academy.

translation.15–19 Twist boundaries between different metals, especially the isostructural Ag/Ni system, have also been studied experimentally and theoretically.20–24 The above review shows that the twist boundaries between identical crystallographic planes, that is, (100)/(100), (110)/(110) and (111)/(111), have been well studied. However, studies of the twist boundaries on mixed low-index planes have rarely been reported. Recently, ZrO2 nanoparticles were found to coalesce on the (100)/(110) interface with a specific orientation relationship.25 The purpose of the research reported herein was to explore further the low-energy boundaries between mixed planes of NiO of the rock-salt structure by a nanofilm rotation method. The method was to grow very thin epitaxial NiO nanofilms of (100), (110), (111), and (112) surfaces on the corresponding surfaces of NaCl single crystals. Two films of different surfaces were then overlapped with a specific misorientation angle and annealed until they rotated to form a stable interface. The interfaces and orientation relationships were determined by transmission electron microscopy (TEM). The structures and approximate coincidence site lattices (CSL) of the interfaces were analyzed. An annealing temperature of 300 °C was chosen deliberately because it is a quite high temperature for the present nanofilm rotation experiments. The overlapped thin films have been observed to rotate significantly even at room temperature after a month’s storage. At 300 °C, each pair of overlapped films rotated rapidly to the same final interface irrespective of the initial angle difference. Other interfaces, which can be found only by annealing at lower temperatures of 100-200 °C and with a proper initial angle difference, obviously have higher interfacial energies, that is, local energy minima. Because the purpose of this work was to report only the most stable interfaces between the various mixed surfaces, a high annealing temperature of 300 °C was used. Other interfaces will be addressed separately in another report. 2. Experimental Methods NiO films were grown with a radio-frequency ion-beam sputtering system. The target was a 7.5-cm-diameter Ni disk of 99.9% purity. The NaCl (001) surface was prepared by cleaving, whereas the (110), (111), and (112) surfaces were prepared by

10.1021/jp103244g  2010 American Chemical Society Published on Web 10/20/2010

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Figure 1. SAD patterns of epitaxial NiO thin films on the NaCl (a) (100), (b) (110), (c) (111), and (d) (112) surfaces. (e) BFI, (f) DFI, and (g) lattice fringe image of the NiO thin film in the [001] zone axis.

cutting and polishing of a NaCl single crystal at the appropriate angle. The films were grown under the conditions of 5 × 10-4 Torr working pressure, 700 V acceleration voltage, 50 mA beam current, 350 °C substrate temperature, and 10 sccm oxygen flow rate. The deposition time was 20 min, and the thickness of each film was about 10 nm. The specimen was immersed in distilled water to dissolve the NaCl substrate. The thin NiO film floating on the water surface was then scooped up with another NiO film of different crystallographic surface but still on the NaCl substrate. An angle difference of more than 15° was made between the two NiO thin films for an original nonepitaxial relationship. The overlapped specimen was placed in a furnace to anneal in air at 300 °C for 1 h to allow the nanograins to rotate to a stable orientation relationship. The specimen was again immersed in distilled water to dissolve the NaCl substrate and leave the composite NiO film floating on the surface. It was then scooped up with a Cu grid covered with a carbon-coated collodion film for TEM observations. The selected-area diffraction (SAD) pattern, brightfield image (BFI), and dark-field image (DFI) were used to analyzetheorientationrelationships,interfaces,andmicrostructures. 3. Results 3.1. Epitaxial NiO Films on NaCl Surfaces. Epitaxial NiO (001), (110), (111), and (112) films were successfully prepared on the NaCl (001), (110), (111), and (112) surfaces, as shown by the diffraction patterns in parts a-d, respectively, of Figure 1. The typical BFI and DFI (parts e and f, respectively, of Figure 1) show that the film was uniform and that the grains were about 10 nm in size. These nanograins were well aligned, as indicated by the diffraction patterns in Figure 1a-d, and had a parallel orientation relationship with respect to the NaCl substrate surface. Because the specimens were about 10 nm thick, the thin films were monograin thick, as evidenced by the lack of Moire´ fringes in Figure 1e,f and the clear lattice fringes in Figure 1g. 3.2. Interfaces between Mixed Surfaces. Figure 2a shows the initial state of overlapped (1j11) and (001) surfaces. The final state obtained after annealing at 300 °C for 1 h is shown in

Figure 2b. A rotation of about 18° occurred, and the orientation relationship reached on the (1j11)/(001) interface in Figure 2b was [1j11]//[001] (zone axis), [110](1j11)// [110](001), and [1j12j](1j11)// [1j10](001). This orientation relationship is denoted as relationship 1. Similarly, Figure 3a shows the initial state of overlapped (1j11) and (1j10) surfaces. The final state obtained after annealing at 300 °C for 1 h is shown in Figure 3b. A rotation of about 22° occurred, and the (1j11)/(1j10) interface followed an orientation relationship of [1j11]//[1j10] (zone axis), [110](1j11)//[110](1j10), and [1j12j](1j11)//[001j](1j10) (relationship 2). Figure 4 shows that, after annealing, the final orientation relationship of the (001)/(1j10) interface was [001]//[1j10] (zone axis), [110](001)//[110](1j10), and [1j10](001)//[001j](1j10) (relationship 3). This interface is the same as the interface observed in ZrO2 nanoparticles of fluorite structure reported previously in a crosssectional view.25 Figure 5 shows that the orientation relationship of the (1j12)/(001) interface in a top view is [1j12]//[001] (zone axis), [110](1j12)// [110](001), and [1j11j](1j12) //[1j10](001) (relationship 4). Figure 6 shows that the orientation relationship of the (1j12)/ (1j11) interface is [1j12]//[1j11] (zone axis), [110](1j12)// [110](1j11), and [1j11j](1j12)// [1j12j](1j11) (relationship 5). The orientation relationship of the (1j12)/(1j10) interface, shown in Figure 7 is [1j12]//[1j10] (zone axis), [110](1j12)// [110](1j10), and [1j11j](1j12)// [001j](1j10) (relationship 6). The observed orientation relationships are listed in Table 1. From the table, it is clear that the relationships 2 and 4 are the same but have different interfaces; the same is true for relationships 1 and 6. From the table, it is also apparent that [110]//[110] is obeyed for all six orientation relationships. Figure 8 shows the typical BFI and DFI of the overlapped films after annealing at 300 °C for 1 h to reach a stable interface. No sintering or grain growth of the nanograins could be observed. The ease of preparing various surfaces, the capability of the nanograins to rotate at low temperatures, and the convenient result analysis by TEM SAD patterns make the present method uniquely suitable for finding various interfaces.

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Figure 2. (a) Initial state of the overlapped (1j11) and (001) surfaces. (b) Stable interface reached after annealing at 300 °C for 1 h (relationship 1).

Figure 3. (a) Initial state of the overlapped (1j11) and (1j10) surfaces. (b) Stable interface reached after annealing at 300 °C for 1 h (relationship 2).

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Figure 6. Stable (1j12)/(1j11) interface reached after annealing at 300 °C for 1 h (relationship 5).

Figure 7. Stable (1j12)/(1j10) interface reached after annealing at 300 °C for 1 h (relationship 6).

TABLE 1: Orientation Relationships of the Most Stable Interfaces Found after Annealing at 300 °C for 1 h, along with Angle Differences ∆θ of the Tilt Boundaries relationshipa

Figure 4. Stable (001)/(1j10) interface reached after annealing at 300 °C for 1 h (relationship 3).

1

interface (1j11)/(001)

2

(1j11)/(1j10)

3

(001)/(1j10)

4

(1j12)/(001)

5

(1j12)/(1j11)

6

(1j12)/(1j10)

orientation relationship [110](1j11)//[110](001), [1j12j](1j11)//[1j10](001) [110](1j11)//[110](1j10), [1j12j](1j11)//[001j](1j10) [110](001)//[110](1j10), [1j10](001)//[001j](1j10) [110](1j12)//[110](001), [1j11j](1j12)//[1j10](001) [110](1j12)//[110](1j11), [1j11j](1j12)//[1j12j](1j11) [110](1j12)//[110](1j10), [1j11j](1j12)//[001j](1j10)

∆θb 54.7° 35.3° 90° 35.3° 19.5°, 90°c 54.7°

a Orientation relationships 2 and 4, although of different interfaces, are the same, as are orientation relationships 1 and 6. b ∆θ explained in Figure 11 below. c The (1j12)/(11j1) interface of ∆θ ) 90ο is equivalent to the (1j12)/(1j11) interface of ∆θ ) 19.5ο

Figure 5. Stable (1j12)/(001) interface reached after annealing at 300 °C for 1 h (relationship 4).

4. Discussion 4.1. Interfaces of NiO Mixed Surfaces. 4.1.1. Approximate Coincidence Site Lattices of the Interfaces. Figure 9a-d shows the atom positions of the NiO (001), (1j10), (1j11), and (1j12) surfaces. Figure 10a is a composite image of the (1j11)/(001) interface overlapped according to the orientation relationship in Figure 2b (relationship 1); similarly, Figure 10b is a composite

image of the (1j11)/(1j10) interface in Figure 3b (relationship 2), Figure 10c is a composite image of the (001)/(1j10) interface in Figure 4 (relationship 3), Figure 10d is a composite image of the (1j12)/(001) interface in Figure 5 (relationship 4), Figure 10e is a composite image of the (1j12)/(1j11) interface in Figure 6 (relationship 5), and Figure 10f is a composite image of the (1j12)/(1j10) interface in Figure 7 (relationship 6). In all of these images, there is a perfect match of oxygen atoms along the common [110] direction for all six relationships, apparently the most important and probably the fundamental factor in the formation of these orientation relationships. In the directions orthogonal to the [110] direction, there are various degrees of ion matching. Still, the match of the oxygen ions can be defined by the rectangular approximate CSLs indicated in the images.

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Figure 8. (a) BFI of the stable (1j11)/(1j10) interface in Figure 3 reached after annealing at 300 °C for 1 h. (b) DFI of the (1j10) thin film. (c) DFI of the (1j11) thin film.

Figure 9. Atom positions of the NiO (a) (001), (b) (1j10), (c) (1j11), and (d) (1j12) planes.

Figure 10. Overlapping of (a) the (1j11)/(001) interface of relationship 1 (Figure 2b), (b) the (1j11)/(1j10) interface of relationship 2 (Figure 3b), (c) the (001)/(1j10) interface of relationship 3 (Figure 4), (d) the (1j12)/(001) interface of relationship 4 (Figure 5), (e) the (1j12)/(1j11) interface of relationship 5 (Figure 6), and (f) the (1j12)/(1j10) interface of relationship 6 (Figure 7).

The [110]//[110] linear coherency is favored for all of the interfaces. One possible reason might be that, among the low-

index high-atomic-density planes in a cubic crystal, there are six {110} planes, compared to three {100} planes and four

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Figure 11. Traces of various crystallographic planes of NiO in the [110] zone.

{111} planes. In addition, the {110} planes are present in all of the four [100], [110], [111], and [112] zones, whereas the {100} planes are in only two zones ([100] and [110]) and the {111} planes are also in only two zones ([110] and [112]). Figure 10 shows that, although there is a perfect match along the [110] direction, the match in other directions is generally not as good. This indicates that, in these complex interfaces of mixed planes, a good match in a single direction might be a major factor in forming the observed orientation relationship. Several similar results were reported previously.26–28 In the η′Cu6Sn5 (monoclinic structure) and Cu system, the linear coherency of (4j02)η′//(110)Cu was found on both of the observed (204)η′/(001)Cu and (204)η′/(111)Cu interfaces. The mismatch in this direction was only -0.31%, a very low value, although the mismatch in other direction was much larger.26,27 Similarly, in the η-Cu6Sn5 (hexagonal structure) and Cu system, the linear coherency of (0001)η//(110)Cu was found on both of the observed (112j0)η/(001)Cu and (112j0)η/(1j11)Cu interfaces, as the mismatch was -0.27%.27 In the δ-AuSn (hexagonal structure) and Au system, the linear coherency of (0001)δ//(110)Au with a mismatch of -4.2% was found on all three of the observed (112j0)δ/ (001)Au, (11j00)δ/(1j10)Au, and (112j0)δ/(1j11)Au interfaces, regard-

Yeh et al. less of the mismatch in the other directions.28 Furthermore, the invariant-line concept has been used to describe the irrational crystallographic relationship of precipitates in close-packed or nearly close-packed matrixes.29 4.1.2. Asymmetric [110] Tilt Boundaries. Table 1 shows that the [110]//[110] linear coherency is present in all of the interfaces observed. This suggests that these interfaces belong to the so-called [110] tilt boundaries, although the asymmetric ones. The symmetric [110] tilt boundaries include six boundaries, namely, Σ3 (111) (∆θ ) 109.5°), Σ3 (112) (∆θ ) 70.5°), Σ11 (113) (∆θ ) 50.5°), Σ11 (332) (∆θ ) 129.5°), Σ9 (114) (∆θ ) 38.9°), and Σ9 (221) (∆θ ) 141.1°) boundaries, where the plane is the interface and ∆θ is the misorientation angle.30–32 These interfaces can be more easily understood with the help of Figure 11, which shows the related planes in the [110] zone. For example, joining of the (1j11) and (11j1) surfaces, which have an angle of 109.5°, forms the Σ3 (111) boundary. These interfaces have been studied experimentally and subjected to high-resolution TEM analysis.30–32 The usual asymmetric [110] tilt boundary, formed by replacing the symmetric interface with a vicinal surface inclined at a small angle to it, has also been well studied experimentally and theoretically.33–36 However, the new interfaces found in this work appear to be special types of asymmetric [110] tilt interfaces that have not been recognized before. These interfaces of mixed surface planes and the misorientation angles (∆θ) are also compiled in Table 1, which can also be easily visualized with the help of Figure 11 by selecting the respective surfaces and calculating the angle difference. For example, the (1j11)/(001) interface of relationship 1 can form a [110] asymmetric tilt boundary with ∆θ ) 54.7°. The (1j12)/(1j11) interface (relationship 5) is equivalent to the (1j12)/(11j1) interface and thus has two possible angles. Of these interfaces, only one (relationship 3) was reported previously.25 4.1.3. Cross-Sectional View. Nanoparticles have been frequently observed to have low-index planes as surfaces and to

Figure 12. Diffraction patterns and structures of the interfaces of (a) the (001)/(1j10) interface of relationship 3 viewed from the [1j10](001) and [001j](1j10) directions, (b) the (001)/(1j11) interface of relationship 1 viewed from the [1j10](001) and [1j12j](1j11) directions, (c) the (1j11)/(1j10) interface of relationship 2 viewed from the [001j](1j10) and [112j](1j11) directions, (d) the (1j12)/(001) interface of relationship 4 viewed from the [1j10](001) and [1j11j](1j12) directions, (e) the (1j12)/(1j11) interface of relationship 5 viewed from the [1j12j](1j11) and [1j11j](1j12) directions, and (f) the (1j12)/(1j10) interface of relationship 6 viewed from the [001j](1j10) and [1j11j](1j12) directions.

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Figure 13. Other (001)/(1j10) interfaces reached after annealing at 100 °C for 1 h: (a) and (b), (c) and (d), and (e) and (f) are the (a,c,e) SAD patterns and (b,d,f) corresponding overlapping figures of the interfaces of relationships (a,b) 3′, (c,d) 3′′ (an asymmetric [001] tilt boundary), and (e,f) 3′′′.

Figure 14. Density of the approximate CSL per square nanometer as a function of the misorientation angle between the [110] directions on the (001) and (1j10) surfaces for the observed interfaces.

Figure 15. Schematic showing qualitatively the relative interfacial energies of relationships 3, 3′, 3′′, and 3′′′ of the observed (001)/(1j10) interfaces as a function of the angle between the [110] directions on the (001) and (1j10) surfaces.

be faceted. These particles can rotate relatively easily at low temperature due to their fine size to form the stable interfaces found in this report. Because the interfaces are generally observed by TEM in a cross-sectional view whereas the planeview results are presented in this report (Figure 10), it is helpful

Figure 16. Densities of the approximate CSLs per square nanometer of relationships 1-6 plotted in order of decreasing density.

to replot the interfaces in a cross-sectional view for comparison. For the view along the [1j10](001) and [001j](1j10) directions in Figures 4 and 10c (relationship 3), the oxygen-sharing interface and diffraction patterns are shown schematically in Figure 12a and the diffraction patterns are the same as those observed in ZrO2.25 Figure 12b shows the cross-sectional view and diffraction patterns of Figure 2b (relationship 1) from the [1j12j](1j11) and [1j10](001) directions; similarly, Figure 12c shows those of Figure 3b (relationship 2) from the [1j12j](1j11)and [001j](1j10) directions, Figure 12d shows those of Figure 5 (relationship 4) from the [1j11j](1j12)and [1j10](001) directions, Figure 12e shows those of Figure 6 (relationship 5) from the [1j11j](1j12)and [1j12j](1j11)directions, and Figure 12f shows those of Figure 7 (relationship 6) from the [001j](1j10)and [1j11j](1j12)directions. The interfaces shown in Figure 10 in plane view and in Figure 12 in cross-sectional view are unrelaxed interfaces. In the actual interfaces, some local relaxation must be present, which should be further analyzed by atomic-resolution TEM. Theoretical simulation is also necessary to predict the types of relaxation of the interfaces and to calculate the depths of the energy cusps. 4.2. Other (001)/(1j10) Interfaces. The [110] tilt boundaries listed in Table 1 are the most stable interfaces with the minimum

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Figure 17. Superimposed images of (a) the (001)NaCl and (001)NiO, (b) (110)NaCl and (110)NiO, (c) anion-exposed (111)NaCl and (111)NiO, and (d) (112)NaCl and (112)NiO planes.

interfacial energy of the possible interfaces in each group. Because the purpose of the present research was to find the most stable interfaces, a high annealing temperature of 300 °C was used. However, there are many other possible interfaces that can be found only by annealing at lower temperatures and by selecting a proper initial angle difference. Therefore, these interfaces must have higher interfacial energies, representing local energy minima. Take the (001)/(1j10) interface as an example. Three more interfaces in addition to relationship 3, denoted as relationships 3′, 3′′, and 3′′′, have been found experimentally by annealing specimens at 100 °C for 1 h, as shown in Figure 13. However, upon annealing of these specimens at 300 °C, only relationship 3 is obtained, indicating that relationship 3 has the minimum interfacial energy among them. It is to be noted that relationship 3′′, shown in Figure 13c,d, is actually an asymmetric [001] tilt boundary on the (001)/(1j10) interface. The area of the approximate CSL, indicated by dashed lines in Figures 10c, 13b, 13d, and 13f, can be used to evaluate the relative interfacial energies of the above interfaces. Because exact coincidence of atoms along different directions is generally not possible, a mismatch of 5% or less was used to determine the approximate CSL in these figures. A smaller area of the approximate CSL indicates a better interfacial match and, therefore, a decreased interfacial energy. Figure 14 shows the density of the approximate CSL per square nanometer, which is the inverse of the area of the approximate CSL, as a function of the misorientation angle between the [110] directions on the (001) and (1j10) surfaces for the observed interfaces. From Figure 14, the schematic in Figure 15 was plotted to show the probable relative interfacial energies of relationships 3, 3′, 3′′, and 3′′′. Similarly, Figure 16 shows the densities of the approximate CSL per square nanometer of interfaces of relationships 1-6 calculated from the areas of the approximate CSLs shown in Figure 10. Figure 16 indicates that the relative interfacial energies of the interfaces are probably in the order 3 < 4 < 1 ≈ 2 ≈ 6 < 5. The small approximate CSLs in Figure 10 can be explained by the fact that the distance between the oxygen ions in the [110] direction is less than those in the [100], [111], and [112] directions, as shown in Figure 9. Therefore, the reason that the [110] tilt boundaries of relationships 1-6 have the minimum interfacial energies is probably due to the exact match of the closely packed atoms along the [110] direction, which significantly decreases the area of the approximate CSL. In addition to the relationship 3 group, it is obvious that, in other relationships in Table 1, there are additional interfaces of higher interfacial energies than the asymmetric [110] tilt

boundary found experimentally. Because of the complex nature of the interfaces, it is also clear that detailed theoretical simulations should be performed to verify the results indicated by the approximate CSLs established experimentally by the present nanofilm approach. 4.3. NiO/NaCl Interfaces. The interfaces of the epitaxial NiO films and the corresponding NaCl substrate surfaces, overlapped according to the observed orientation relationships, are shown in Figure 17. Because the lattices of NiO (a0 ) 0.417 nm37) and NaCl (a0 ) 0.564 nm38) have a parallel orientation relationship of 〈100〉//〈100〉, the unconstrained misfit was calculated to be -26%. However, for every four NiO (100) planes and every three NaCl (100) planes, the misfit reduces to -1.2%. The square approximate CSL on the (001) interface, as outlined in Figure 17a, shows a good correspondence of the ions on the interface, although the effect of aliovalent charges on the linkage of ions requires further study. The approximate CSLs are rectangular for the other interfaces. 5. Conclusions A nanofilm method has been used to study the rotation of nanograins and the formation of stable low-energy interfaces. Epitaxial NiO nanofilms were prepared by ion-beam sputtering onto the (100), (110), (111) and (112) surfaces of NaCl single crystals. By overlapping these films of different surfaces and annealing at 300 °C, the nanofilms were induced to rotate until stable interfaces were reached. Many new interfaces on mixed planes, namely, (1j11)/(001), (1j11)/(1j10), (001)/(1j10), (1j12)/(001), (1j12)/(1j11), and (1j12)/ (1j10), with their respective orientation relationships, were found. These interfaces are the most stable ones of the possible interfaces in each group because of the high annealing temperature at which they were obtained. The [110]//[110] linear coherency was found in all of these interfaces, and the interfaces were asymmetric [110] tilt boundaries of mixed planes. The approximate coincidence site lattices of the interfaces were analyzed and discussed. The perfect match of ions in the [110] direction appears to be the most important factor in forming the orientation relationships. Other (001)/(1j10) interfaces of higher interfacial energies, found at an annealing temperature of 100 °C, were compared with the asymmetric [110] tilt boundary of the (001)/(1j10) interfaces. Acknowledgment. This work was financially supported by the National Science Concile of Taiwan under Contract NSC 96-2221-E-110-054 and by the Center for Nanoscience and Nanotechnology of National Sun Yet-Sen University.

Mixed Crystallographic Planes of NiO Nanograins References and Notes (1) Harris, K. E.; Singh, V.; King, A. K. Acta Mater. 1998, 46, 2623. (2) King, A. K.; Harris, K. E. Mater. Res. Soc. Symp. Proc. 1996, 403, 15. (3) Lee, W. H.; Shen, P. Mater. Sci. Eng. 2002, A338, 253. (4) Li, M.; Shen, P.; Hwang, S. Mater. Sci. Eng. 2003, A343, 227. (5) Wang, J. Y.; Shen, P. Mater. Sci. Eng. 2003, A359, 761. (6) Li, M. Y.; Shen, P. Mater. Sci. Eng. 2005, A398, 309. (7) Chan, S. W.; Ballufi, R. W. Acta Metall. 1985, 33, 1113. (8) Chan, S. W.; Ballufi, R. W. Acta Metall. 1986, 34, 2191. (9) Hermann, G.; Gleiter, H.; Baro, G. Acta Metall. 1976, 24, 353. (10) Merkle, K. L.; Thompson, L. J. Phys. ReV. Lett. 1999, 83, 556. (11) Kohyama, M.; Yamamoto, R. Phys. ReV. 1994, B49, 17102. (12) Wei, X. M.; Zhang, J. M.; Xu, K. W. Appl. Surf. Sci. 2006, 253, 854. (13) Jhang, J. M.; Wei, X. M.; Xin, H. Appl. Surf. Sci. 2005, 243, 1. (14) Th, J.; Hosson, M.; De; Vitec, V. Phil. Mag. 1990, 61, 305. (15) Wei, X. M.; Zhang, J. M.; Xu, K. W. Appl. Surf. Sci. 2007, 253, 4307. (16) Delogu, F. J. Phys.-Condens. Matter 2007, 19, 396005. (17) Monzen, R.; Futakuchi, M.; Suzuki, T. Scr. Metall. Mater. 1995, 32, 1277. (18) Sato, K.; Miyazaki, H.; Ikuhara, Y.; Kurishita, H.; Yoshinaga, H. Mater. Trans. 1990, 31, 865. (19) Kurishita, H.; kuba, S.; Kubo, H.; Yoshinaga, H. J. JIM 1983, 47, 539. (20) Zhang, J. M.; Xin, H.; Wei, X. M. Appl. Surf. Sci. 2005, 246, 14.

J. Phys. Chem. C, Vol. 114, No. 44, 2010 18893 (21) Xin, H.; Zhang, J. M.; Wei, X. M.; Xu, K. W. Surf. Interface Anal. 2005, 37, 608. (22) Allameh, S. M.; Dregia, S. A.; Shewmon, P. G. Acta Mater. 1994, 42, 3569. (23) Gao, Y.; Dregia, S. A.; Shewmon, P. G. Acta Mater. 1989, 37, 1627. (24) Maurer, R. Acta Metall. 1987, 35, 2557. (25) Yeh, S. W.; Huang, H. L.; Gan, D.; Shen, P. J. Phys. Chem. 2007, C111, 9437. (26) Wang, K. K.; Gan, D.; Hsieh, K. C.; Chiou, S. J. Thin Solid Films 2010, 518, 1667. (27) Wang, K. K. Ph.D. Thesis, National Sun Yat-sen University, Kaohsiung, Taiwan, 2009. (28) Wang, K. K.; Gan, D. Thin Solid Films 2010, 518, 6945. (29) Dahmen, U. Acta Metall. 1982, 30, 63. (30) Sawada, H.; Ichinose, H. Scr. Mater. 2001, 44, 2327. (31) Sawada, H.; Ichinose, H.; Kohyama, M. Scr. Mater. 2004, 51, 689. (32) Shibata, N.; Yamamoto, T.; Ikuhara, Y.; Sakuma, T. J. Electron Microsc. 2001, 50, 429. (33) Tschopp, M. A.; Mcdowell, D. L. Phil. Mag. 2007, 87, 3871. (34) Tschopp, M. A.; Mcdowell, D. L. J. Mater. Sci. 2007, 42, 7806. (35) Gokon, N.; Kajihara, M. Mater. Sci. Eng. 2008, A477, 121. (36) Goukon, N.; Yamada, T.; Kajihara, M. Acta Mater. 2000, 48, 2837. (37) Joint Committee on Powder Diffraction Standards (JCPDS) File 471049. (38) Joint Committee on Powder Diffraction Standards (JCPDS) File 050628.

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