Bulk nanocomposite made of ZnO lamellae embedded in the ZnWO₄ matrix: growth from the melt

Zinc oxide has received much attention in recent years because it exhibits a wide range of favorable properties. The lack of a center of symmetry and large electromechanical coupling give rise to strong piezoelectric and pyroelectric properties and the resulting use of ZnO in mechanical actuators and piezoelectric sensors [1]. Moreover, ZnO is a wide band gap (3.37 eV) semiconductor that is suitable for short wavelength optoelectronic applications, and its high exciton binding energy (60 meV) can ensure efficient excitonic emission at room temperature [2]. ZnO is also biocompatible, biodegradable, and biosafe for medical and environmental application and shows good photocatalytic activity [3]. Hence, ZnO may be considered for an extensive range of applications such as energy storage, sensors, cosmetic products, optoelectronic and electronic devices, among others [4]. Recently, ZnO nanostructures have received broad attention due to their unique properties [5]. In particular, they have been widely used for sensing applications because of their high sensitivity to the chemical environment. ZnO nanowires have the advantage of a high surface area and have demonstrated high sensitivity, even at the room temperature, whereas ZnO thin film gas sensors often need to be operated at elevated temperatures [6]. Lasing in ZnO nanowire arrays grown by a variety of methods has been observed by different research groups, and single nanowire light-emitting diode was presented as well [7]. Moreover, well-aligned single-crystalline nanowires were proposed for use as high-aspect-ratio probes for atomic force microscopy (AFM) [7]. Recently, ZnO nanostructured films have been also used in dye-sensitized solar cells as an electrode material [6].

High-quality crystals can, in principle, be grown from the melt. However, despite several trials [8], it is not really possible, since ZnO decomposes into atomic components below the melting point, 1975 °C, at atmospheric pressure [9]. However, this can be overcome with a eutectic transition, where from a one-phase melt a two-solid-phases composite material is crystallized at much lower melting temperature than its constituent phases, enabling growth of phases like the ZnO.

Directional solidification of eutectics has been considered as a promising method to fabricate bulk, highly crystalline and self-organized nano-/microstructures out of various materials combinations, with a plethora of geometrical motifs, controllable structure refinement and sharp interfaces [10].

Recently, directional solidification of eutectics was utilized for energy materials such as solid-oxide fuel cells [11, 12] and electrodes for hydrogen generation [13, 14], and optical materials such as photonic crystals [15, 16]; THz polaritonic materials [17, 18]; metamaterials [19‐23] and nanoplasmonic materials [24‐26]. It has been also shown that eutectic composites could exhibit unusual geometries when properly engineered [27‐29], in addition to being grown as bulk materials [30] even on the industrial scale and also printed [31].

In this work, we utilize the eutectic directional solidification for manufacturing of well-ordered ZnO layers directly from the melt. The ZnO ca. 250-nm-thick layers are embedded in the ZnWO₄ matrix forming a bulk ZnO-ZnWO₄ material. The growth of the composite by the micro-pulling down method is presented. The optical response of the obtained nanostructured ZnO composites was assessed by cathodoluminescence studies.

In order to demonstrate the possibility of utilizing directional solidification of eutectics for manufacturing ZnO nanostructured materials in a bulk form directly from the melt, we have grown rods with the eutectic composition of 65 mol % ZnO and 35 mol % of WO₃, marked on the WO₃-ZnO phase diagram in Fig. 1d [33]. According to Shchenev et al., the ZnO-WO₃ equilibrium phase diagram encloses only one ternary compound ZnWO₄ and two eutectic invariant points [33]. The ZnO-ZnWO₄ eutectic at 65 mol% of ZnO and 35 mol% of WO₃ molar ratio allows the solidification of ZnO as one of the component phases. The melting temperature at the eutectic point is about 1187 °C, which is much lower than the melting point of ZnO (1975 °C) [4] and WO₃ (1473 °C) [33]. The melting point is low enough to allow the coexistence of solid and liquid phases of ZnO and, under these conditions, the material does not decompose. Eutectic rods were obtained with a pulling rate of 0.1 and 0.5 mm/min. At the initial stage of growth, when the thermodynamic conditions were not stable, large ZnO inclusions (400 × 40 µm) could be observed within the eutectic rod core as shown in the SEM image (Fig. 1h). After stabilization of eutectic growth conditions, large inclusions of ZnO were not observed.

The XRD demonstrated two phases in the obtained rods, Fig. 2a. The diffraction peaks match well the standard diffraction pattern of wurtzite ZnO phase (JCPDS-04–015-4060) and wolframite ZnWO₄ phase (JCPDS-04–009-8448) with no evidence of any extra phase within the XRD detection limits. The lattice parameters deduced by Rietveld refinement of the XRD profile of ZnO-ZnWO₄ eutectic are a = 3.25 Å and c = 5.21 Å for ZnO and a = 4.69 Å, b = 5.72 Å, c = 4.93 Å, β = 90.64° for ZnWO₄ and match well the previously reported values [34, 35]. The plane orthogonal to the growth direction was determined from bulk crystal samples. They correspond to (10\(\overline{1}\)0) (2θ = 32°) and (11\(\overline{2}\)0) (2θ = 56°) for the ZnO and (001) (2θ ~ 36° and 2θ ~ 72°) and (013) (2θ ~ 57°) for ZnWO₄, the last one giving rise to a weaker diffraction signal.

The SEM images and EDS studies Fig. 2b, c revealed a eutectic micro/nanostructure consisting of ZnO lamellae/layers embedded in the ZnWO₄ matrix in the entire volume of the eutectic rods regardless of the thermodynamic conditions of growth, Fig. 3a–c. In the whole volume of sample, eutectic grains as big as 100 × 100 µm² and more were observed. Within one grain, ZnO lamellae were parallel to each other and their thickness was approx. 250 nm.

In order to reconstruct the microstructure of ZnO-ZnWO₄ eutectic in three dimensions (3D), the FIB-SEM technique was used. In this technique, the sample is sequentially milled using an ion beam perpendicular to the specimen while imaging the newly exposed surface using an electron beam. A thorough analysis of sectional micrographs perpendicular to the growth axis recorded using FIB-SEM technique confirms that the ZnO-ZnWO₄ eutectic crystallizes in the broken-lamellar structure, Fig. 3d [36]. The characteristic feature of the eutectic with broken-lamellar structure is the small volume ratio of the lamellar phase to the matrix phase. The minor ZnO phase appears as broken lamellae in the 2D microstructure, but FIB-SEM shows that the ZnO phase is continuous and occurs as perforated plates with holes filled with ZnWO₄ matrix phase. This broken lamellar type microstructure obviously represents a structure dictated by a long-range force favoring lamellar morphologies. However, a short-range force (instability perhaps) clearly exists which “breaks” the lamellae [37]. As suggested by Beghi, the relatively large interlamellar spacings and thin lamellae observed in low-volume-fraction eutectics require large diffusion distances. Therefore, it seems quite possible that thermally induced local irregularities in the solute concentration or diffusional flow in the melt could produce the irregular “breaks” as seen in two dimensions or “holes” in three dimensions [38].

The ZnO-ZnWO₄ eutectics have been further studied using high-resolution TEM (HRTEM). The thin foil was extracted from a plane containing the growth axis. In order to study the crystal orientation relationship between the ZnO and ZnWO₄ and to define the interface planes, diffraction studies were performed (Fig. 4). Figure 4a shows a selected area electron diffraction (SAED) pattern of the ZnWO₄. When the thin foil is perpendicular to the electron beam, no simple zone axis can be obtained for the matrix. Only the [001]* reciprocal direction can be imaged, indicating that the (001) plane of the ZnWO₄ matrix is perpendicular to the surface of the FIB foil. With a few degrees tilt, the foil is oriented in [100] zone axis, Fig. 4a with the 001 reflection obtained by double diffraction (dd). For the same tilt angles, the ZnO lamella does not show a simple zone axis but the [0001]* reciprocal direction for ZnO (Fig. 4b). Figure 4c depicts diffraction patterns recorded at the interface, to superpose the two patterns. The (0002) planes of ZnO are parallel to the (020) planes of ZnWO₄ as their inter-planar distances are compatible (d₀₂₀. ~ 0.286 nm for ZnWO₄ and d₀₀₀₂. ~ 0.260 nm for ZnO). The HRTEM image of the interface between (010) ZnWO₄ and (0002) ZnO is shown in Fig. 4d. Based on the TEM observation, the orientation relationship for the ZnO lamellae and ZnWO₄ matrix is defined as [1\(\overline{1}\)00] ZnO almost //[100] ZnWO₄ (less than 5° mismatch) and planes (0001) ZnO // (010) ZnWO₄.

Figure 5a shows the CL spectra acquired for ZnO-ZnWO₄. The spectra consist of two emission peaks at about ~ 371 and ~ 498 nm. The peak at 371 nm (UV luminescence) is attributed to near band edge (NBE) emission [39], which originates from the recombination of free-excitons through an exciton–exciton collision process in ZnO. The other peak ~ 498 nm is usually attributed by intrinsic tungstate emission. The WO₆ octahedra and a slight deviation from perfect order in the crystal structure are responsible for blue light emission band of ZnWO₄ [40]. Only UV luminescence (~ 373 nm) was observed for ZnO lamellae, whereas the ZnWO₄ matrix exhibited a broad band emission centered at 498 nm. The study of luminescence properties of ZnO-ZnWO₄ eutectic component phases independently reveals that the blue/green luminescence originates only from the ZnWO₄ phase. No green and blue luminescence has been detected in ZnO. In contrast to our ZnO-ZnWO₄ eutectic composite, the photoluminescence (PL) studies on (ZnO)_1-x(ZnWO₄)_x (x = 0, 0.1,0.2,0.3,0.5,0.7, 0.9, 1) nanocomposite prepared by precipitation method revealed mainly broad blue-green emission band in the range of 440–550 nm and the PL intensity of ZnO was insignificant compared with that of ZnWO₄ [41]. Figure 5b, c shows CL intensity and SEM images taken at 371 nm which corresponds to NBE emission in ZnO. The strong excitonic emission, with negligible emission in the visible range in ZnO lamellae, indicates the good optical quality of these nanostructures. Although the actual recombination mechanisms for visible emission in ZnO are still not fully understood, the absence of the visible defect-related deep level (DL) emission reveals a lower density of such defects in ZnO lamellae within the obtained eutectic composite [42].

A new eutectic with 65 mol% ZnO and 35 mol% WO₃ composition has been grown, and its structural and optical properties have been studied. The presence of only two phases, ZnO and ZnWO₄, has been confirmed by XRD and SEM. The ZnO-ZnWO₄ eutectic tends to grow with a broken-lamellar structure, with the ZnO phase forming perforated nanolamellae embedded in the ZnWO₄ matrix. Growth of ZnO-ZnWO₄ eutectic is an excellent platform for achieving ZnO nanostructures with high crystal quality and low density of defects as assessed by cathodoluminescence. Such a bulk yet nanostructured ZnO material exhibiting ZnO lamellae/layers could work as optoelectronic elements which can be cut from the volumetric material and polished. Potential applications could include filters [43], polaritonic lasers [44], efficient second harmonic generators [45], ultraviolet photodetectors and optical switches [46].