Well-ordered ZnO nanowires with controllable inclination on semipolar ZnO surfaces by chemical bath deposition

The morphology of ZnO grown by CBD on the $(10\bar{1}2)$ and $(20\bar{2}1)$ ZnO single crystals is shown by top-view and cross-sectional FESEM images in figures 3(a)–(h). On both $(10\bar{1}2)$ and $(20\bar{2}1)$ surfaces, a nanostructured layer is formed and the nanostructures appear to be uniformly tilted with respect to the substrate surface. The tilt angle between the growth direction and substrate surface was measured on more than 30 nanostructures by using cross-sectional FESEM images as presented in figures 3(c), (f) and (h), which were analyzed by ImageJ software.

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Elongated ZnO nanostructures are grown on top of the $(10\bar{1}2)$ ZnO single crystal, exhibiting hexagonal shapes with relatively well-defined top facets and sidewalls, which are typical of ZnO NWs, as revealed in figures 3(a)–(c). The ZnO NWs cover the whole substrate surface with a high number density of 56 ± 3 μm⁻². Their average length and diameter are about 315 nm and 60 nm, respectively, corresponding to an aspect ratio of about 5. The length distribution of nanostructures is relatively narrow, while a broad range of diameters occurs, as observed in figures 3(a) and (b). The nanostructures with a diameter larger than 100 nm likely result from coalescence effects due to the high nucleation rate. Their growth direction is uniform and tilted with respect to the substrate surface by an angle of 47.6 ± 1.4°, as shown in figure 3(c), implying that all ZnO NWs grow along the same direction. The present tilt angle corresponds to an angle of 42.4 ± 1.4° between the normal to the growth direction and the $(10\bar{1}2)$ top surface plane, which is remarkably close to the 42.77° angle between the c-planes and $(10\bar{1}2)$ planes of ZnO. This strongly suggests that the ZnO nanostructures grow along the polar c-axis. The top facet of ZnO NWs also appears perpendicular to the growth direction, and should accordingly be polar c-planes. Additionally, the sidewalls of ZnO NWs are parallel each other, indicating an in-plane orientation and a possible homoepitaxial growth.

A distinct growth behavior is observed on the $(20\bar{2}1)$ ZnO single crystal, as presented in figures 3(d)–(h). The low magnification FESEM image in figure 3(d) reveals the formation of a 2D layer with a low structural uniformity through the occurrence of distinct regions. A layer composed of elongated ZnO nanostructures, growing on the $(20\bar{2}1)$ surface, is observed on the high magnification FESEM images shown in figures 3(e) and (f). The ZnO nanostructures have a typical length and diameter of about 390 nm and 110 nm, respectively. The broad range of diameter is the result of the high nucleation rate and of coalescence effects. The growth direction of the elongated ZnO nanostructures is uniformly tilted with respect to the substrate surface by an angle of 14.5 ± 1.5°, as shown in figure 3(f). The corresponding 75.5 ± 1.5° angle between the normal to the growth direction and the $(20\bar{2}1)$ top surface plane is very close to the 74.88° angle between the c-planes and $(20\bar{2}1)$ planes of ZnO. This strongly indicates again that the elongated ZnO nanostructures grow along the polar c-axis. In contrast, other regions, as shown in figures 3(g) and (h), reveal the formation of a compact 2D layer with a significant roughness and a thickness of 210 nm. This likely originates from the coalescence of the previously observed elongated nanostructures. Interestingly, the smooth parts of the 2D layer top surface are parallel to the substrate surface and should therefore exhibit $(20\bar{2}1)$ semipolar planes. The formation of $(20\bar{2}1)$ semipolar planes is expected to be driven by thermodynamic considerations. The textured surface formed by the highly dense ZnO nanostructures shown in figures 3(e) and (f) is composed of the low surface energy m-planes and the high surface energy c-planes [25]. As growth proceeds, the formation of the $(20\bar{2}1)$ surface is favored by reducing its total surface energy as (i) the formation of a planar layer decreases the specific surface area, and (ii) the surface energy of the $(20\bar{2}1)$ plane is expected to be lower than that of the c-plane. The distinct regions formed on the $(20\bar{2}1)$ surface are likely due to inhomogeneities in the surface structural properties, leading to a local variation of the nucleation rate and hence to the formation of distinct elongated nanostructures or of a compact 2D layer.

In order to control the nucleation density and to clearly identify the morphology of ZnO nanostructures grown on the $(20\bar{2}1)$ ZnO single crystal, a SAG was performed by patterning the $(20\bar{2}1)$ surface with EBL. A low density of holes in the pattern was used along with a reduced growth time of 1 h to prevent coalescence effects resulting in the formation of a compact 2D layer. Top-view FESEM images of the SAG of ZnO by CBD are shown in figures 4(a) and (b). ZnO NWs clearly nucleate and grow from the holes in the pattern with high structural uniformity. Their average length and diameter are 1.6 μm and 330 nm, respectively, corresponding to an aspect ratio of about 5. No ZnO NWs are found to grow outside the holes, confirming that the PMMA layer efficiently inhibits the ZnO NW nucleation. The ZnO NWs appear to lie down on the substrate surface, similarly to the spontaneous growth on the $(20\bar{2}1)$ ZnO single crystal. All the ZnO NWs exhibit parallel m-plane sidewalls, revealing an in-plane orientation. The high uniformity over the whole domain further confirms that ZnO NWs effectively form on the $(20\bar{2}1)$ surface, even though this semipolar plane is highly tilted with respect to the polar c-plane.

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Both $(10\bar{1}2)$ and $(20\bar{2}1)$ semipolar ZnO surfaces are eventually of high chemical activity with regards to the growth of ZnO NWs by CBD. The FESEM images suggest that an homoepitaxial growth occurs with an elongation along the c-axis, resulting in uniformly tilted ZnO NWs.

2Theta/omega scans, omega scans and x-ray pole figures were collected in order to confirm the homoepitaxial growth of ZnO NWs and nanostructures along the polar c-axis on top of the $(10\bar{1}2)$ and $(20\bar{2}1)$ ZnO single crystals. The 2Theta/omega scan of the bare $(10\bar{1}2)$ and $(20\bar{2}1)$ ZnO single crystals, as presented in figures 5(a) and (d), reveal high intensity diffraction peaks pointed at 47.554° and 69.066°, respectively, which is in excellent agreement with the positions of the $(10\bar{1}2)$ and $(20\bar{2}1)$ diffraction peaks at 47.539° and 69.100°, respectively, as reported in the 00-036-1451 ICDD file of wurtzite ZnO. Both $(10\bar{1}2)$ and $(20\bar{2}1)$ diffraction peaks are sharp, with a full-width-at-half-maximum (FWHM) of 0.014° and 0.023°, respectively. After the growth of ZnO NWs on top of the $(10\bar{1}2)$ ZnO single crystal, the peak position at 47.535° and FWHM of 0.010° are not significantly changed, suggesting their homoepitaxy, while a slight broadening is observed at the base owing to the small dimensions of the ZnO NWs. Similarly, after the formation of a coalesced layer of elongated ZnO nanostructures on top of the $(20\bar{2}1)$ ZnO single crystal, the $(20\bar{2}1)$ diffraction peak remains largely unchanged with a peak position of 69.083° and a FWHM of 0.018°, suggesting the high crystalline quality and homoepitaxy of the coalesced ZnO layer.

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The omega scans recorded on the $(10\bar{1}2)$ and $(0002)$ diffraction peaks for the ZnO NWs grown on top of the $(10\bar{1}2)$ ZnO single crystal, as presented in figures 5(b) and (c), show a FWHM as low as 0.005° and 0.008°, respectively, which is in the same range as the bare $(10\bar{1}2)$ ZnO single crystal with a FWHM of 0.007° and 0.008°, respectively. For both omega scans, the rocking curve is slightly broader at the base, revealing that the ZnO NW orientation is slightly more dispersed as compared to the single crystal substrate. Similarly, the omega scans recorded on the $(20\bar{2}1)$ and $(0002)$ diffraction peaks for the coalesced ZnO nanostructures on top of the $(20\bar{2}1)$ ZnO single crystal, as presented in figures 5(e) and (f), both show a narrow 0.009° FWHM, in the same range as the 0.012° FWHM of the bare $(20\bar{2}1)$ ZnO single crystal. A small broadening is only noticeable on the omega scan recorded on the $(0002)$ diffraction peak. In both cases, the omega scans reveal an excellent ordering and structural uniformity of the ZnO NWs and nanostructures along the growth direction, strongly suggesting an homoepitaxial growth on the $(10\bar{1}2)$ or $(20\bar{2}1)$ ZnO single crystal substrate.

The x-ray pole figure of ZnO NWs on top of the $(10\bar{1}2)$ ZnO single crystal recorded on the $(0002)$ diffraction peak, as shown in figures 6(a) and (b), reveals one very high intensity peak located at χ = 42.9°, as expected from the $(10\bar{1}2)$ orientation of the ZnO single crystal substrate. The FWHM is as low as 2.2°, confirming the high crystallinity of the ZnO substrate and NWs. Three additional minor diffraction peaks, located at χ = 70.1°, 47.0°, and 70.0°, correspond to the $(01\bar{1}0),$ $(10\bar{1}0),$ and $(1\bar{1}00)$ reflections, respectively. They occur due to the resolution limit of the x-ray source and to the possible sample misalignment, leading to slight deviations from the $(0002)$ diffraction conditions. The x-ray pole figure recorded on the $(10\bar{1}2)$ diffraction peak, as presented in figures 6(c) and (d) reveals the expected high intensity peak at the center with a 1.6° FWHM, and its associated equivalent reflections with lower intensity located at χ = 39.9° and 72.3°. Therefore, for both x-ray pole figures, the expected diffraction peaks corresponding to the $(10\bar{1}2)$ ZnO single crystal substrate are visible. Since no other significant reflections occur, the ZnO NWs grown on the substrate surface are expected to have homoepitaxially nucleated.

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Similarly, x-ray pole figures of ZnO nanostructures on top of the $(20\bar{2}1)$ ZnO single crystal were recorded on the $(0002)$ and $(20\bar{2}1)$ diffraction peaks, as shown in figures 7(a), (b) and (c), (d), respectively. The $(0002)$ x-ray pole figure reveals one diffraction peak of high intensity at χ = 74.8°, which is as expected for the $(0002)$ diffraction peak of the $(20\bar{2}1)$ ZnO single crystal substrate. Three minor diffraction peaks with very low intensity and corresponding to the $\{10\bar{1}0\}$ planes are located at χ = 15.5° and 61.2° and occur due to deviations from diffraction conditions. The small 2.0° FWHM of the main diffraction peak is in agreement with the high crystallinity of the ZnO substrate and nanostructures. Since no other reflections are visible, the ZnO nanostructures on top of the $(20\bar{2}1)$ surface have the same crystallographic orientation as the substrate. This is also confirmed by the x-ray pole figure recorded on the $(20\bar{2}1)$ diffraction peak, which shows a high intensity peak at the center with a low FWHM of 1.3°. Several lower intensity equivalent reflections occur at χ = 30.5°, 57.6° and 66.7°, while four other minor diffraction peaks corresponding to the $\{11\bar{2}2\}$ planes are either located at χ = 32.0° or at χ = 55.5° and occur due to deviations from diffraction conditions.

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The ZnO bulk single crystals and nanostructures are crystallized into the wurtzite structure, belonging to the ${C}_{6\nu }^{4}$ space group. According to group theory, the optical modes in the Brillouin zone center are expressed following the irreducible representations: Γ_opt = A₁ + E₁ + 2E₂ + 2B₁ [56]. B₁ modes are silent. ${E}_{2}^{{\rm{low}}}$ and ${E}_{2}^{{\rm{high}}}$ are non-polar modes, which are Raman active only; they are related to vibrations of the Zn and O sub-lattices, respectively. A₁ and E₁ modes are polar modes, which are both Raman- and infrared-active. They split into longitudinal optical (LO) and transverse optical (TO) phonon components owing to macroscopic electrostatic forces within the polar crystal [56–58]. A₁ phonons are polarized parallel to the c-axis, while E₁ phonons are polarized in the basal plane. The Raman scattering spectra of ZnO NWs and nanostructures on both the respective $(10\bar{1}2)$ and $(20\bar{2}1)$ ZnO single crystals are shown in figure 8(a).

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The lines at 100, 332, 410, and 438 cm⁻¹ are assigned to ${E}_{2}^{{\rm{low}}},$ ${E}_{2}^{\mathrm{high}}-{E}_{2}^{{\rm{low}}},$ E₁(TO), and ${E}_{2}^{{\rm{high}}}$ modes, respectively [59]. Second order weak features occur at 205 cm⁻¹ (2TA; $2{E}_{2}^{{\rm{low}}}$), 540 cm⁻¹ ($2{B}_{1}^{{\rm{low}}};$ 2LA), and 663 cm⁻¹ (TA + LO) [59]. Additional lines at higher wavenumber (not shown) include combinations of acoustical and optical modes and overtones. A last couple of lines is observed in both Raman spectra but at slightly different wavenumbers, depending on the substrate orientation. They occur at 397 cm⁻¹ (with a slight overlap with the E₁(TO) mode) and at 583 cm⁻¹ in the case of the $(10\bar{1}2)$ ZnO single crystal substrate. In contrast, they point at 381 and 588 cm⁻¹ in the case of the $(20\bar{2}1)$ ZnO substrate. In both spectra, the first line is situated between the A₁(TO) and E₁(TO) modes of bulk ZnO, the second one lying between the A₁(LO) and E₁(LO) modes [59]. According to Loudon's model for uniaxial crystals, modes with mixed A₁ and E₁ symmetry character can occur under certain propagation and polarization conditions [57, 58, 60, 61]. For phonon wave vectors parallel or perpendicular to the c-axis, only modes with pure A₁ and E₁ symmetry can be observed. In contrast, phonon wave vectors with an intermediate direction with respect to the c-axis give rise to mixed symmetry modes; they are termed quasi-modes. Quasi-TO (qTO) and quasi-LO (qLO) modes are observed between the A₁(TO) and E₁(TO) modes and between the A₁(LO) and E₁(LO) modes, respectively. Their wavenumber positions are given by the following equations:

$$\begin{eqnarray}&&{\omega }_{qTO}^{2}={\omega }_{{E}_{1}(TO)}^{2}{\cos }^{2}(\theta )+{\omega }_{{A}_{1}(TO)}^{2}{\sin }^{2}(\theta ),\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&{\omega }_{qLO}^{2}={\omega }_{{A}_{1}(LO)}^{2}{\cos }^{2}(\theta )+{\omega }_{{E}_{1}(LO)}^{2}{\sin }^{2}(\theta ),\end{eqnarray} \tag{ 2 }$$

where θ is the angle between the phonon wave vector and the c-axis, ${\omega }_{{A}_{1}(TO)},$ ${\omega }_{{E}_{1}(TO)},$ ${\omega }_{{A}_{1}(LO)}$ and ${\omega }_{{E}_{1}(LO)},$ are the respective positions of the A₁(TO), E₁(TO), A₁(LO), and E₁(LO) modes of bulk ZnO. In the case of the ZnO NWs and nanostructures grown on the $(10\bar{1}2)$ and $(20\bar{2}1)$ ZnO single crystals, the c-axis is inclined with respect to the laser beam propagation direction by an angle θ of 42.77° and 74.88°, respectively. The corresponding Raman scattering geometry is schematically illustrated in figure 8(b). The experimental and calculated positions of the qTO and qLO modes for both ZnO NWs and nanostructures are shown in table 1. The experimental values of the qTO and qLO modes correlate well with the values expected from Loudon's model, which therefore confirms further the orientation of the ZnO substrate and nanostructures with respect to the c-axis. The maximum difference of 2 cm⁻¹ between the calculated and experimental values can be ascribed to small experimental errors and possibly to some small sample misalignment.

Table 1.  Comparison of the experimental positions of the qTO and qLO modes for the ZnO NWs and nanostructures grown on top of the $(10\bar{1}2)$ and $(20\bar{2}1)$ ZnO single crystals with the corresponding positions calculated from the positions of pure symmetry modes given by Cuscó et al [59] for ZnO single crystals.

		qTO (cm−1)		qLO (cm−1)
	Angle between the c- and top surface planes (°)	Calculated	Experimental	Calculated	Experimental
$(10\bar{1}2)$ ZnO single crystal	42.77	396	397	581	583
$(20\bar{2}1)$ ZnO single crystals	74.88	380	381	589	588