Introduction

The embedding of metal nanocrystals (NCs) in dielectric matrix is of considerable interest for the widely potential application in nonlinear optical device and nano-electronics [1–3]. The properties depend on mainly the size and shape of the nanoparticles, the embedding environment, and so on [4, 5]. However, the controllable fabrication of nanostructure remains the daunting challenge for many deposition methods, including sol–gel [6], atom beam sputtering [7], and pulsed-laser deposition (PLD) [8, 9]. Another attractive method is referred to as a “self-organized” growth, in which the strain force would drive the three-dimensional (3D) island to form in the lattice mismatched growth process [10, 11]. Such “self-organization” growth process has been performed to fabricate the quantum structure in semiconductor devices successfully, such as InAs on GaAs, SiGe on Si [12]. To our knowledge, there is seldom report about the self-organization process of metal NCs in oxide matrix. In this work, the laser molecular beam epitaxy (L-MBE) was used to embed the Ni NCs in the BaTiO3 epitaxial film. The fabrication of the Ni-BaTiO3 nanocomposite system is interesting and significant for both fundamental and application aspects. Such composite films offer an combination of ferroelectric and ferromagnetic characteristics. Furthermore, another important application for Ni-BaTiO3 system is the nano-electronics such as the base-metal-electrode multilayered ceramic capacitors (BME-MLCC) [13].

Experimental

The Ni:BaTiO3 epitaxial films were prepared on SrTiO3 (001) substrate with BaTiO3/SrTiO3 buffer layers by L-MBE. The experimental parameters were listed in detail in Table 1. The deposition process involved a number of pulses on the Ni target in ultra high vacuum, followed by the epitaxial growth of BaTiO3 layer. After the completion of every BaTiO3 layer,the sample was annealed about 30 min in the oxygen ambient pressure. Such procedure was repeated up to 8 times to grow 300-nm-thick composite film. During the deposition process, the in situ reflection high-energy electron diffraction (RHEED) monitoring was performed in anti-Bray condition using 25-keV electron beam under a grazing incidence of 10–30 toward the surface. The microstructure and crystallinity of the nanocomposite films were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD), respectively.

Table 1 The experimental parameters for the Ni-BaTiO3 film fabrication
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Results and Discussion

Figure 1a–1d recorded the variation of RHEED pattern along the [100] azimuth in the self-organization course of Ni NCs on BaTiO3 (001) surface. With the increasing of Ni deposition pulses, the streak pattern of BaTiO3 disappears gradually, while the spot pattern of Ni NCs becomes dominant. It is considered that the Ni islands formed on BaTiO3 (001) surface, giving rising to bulk diffraction spots. Due to the large lattice mismatch between Ni and BaTiO3 (>9%), the in-plane lattice of Ni is subjected to large tensile strain to match the in-plane lattice of BaTiO3 at the initial stage. With the continuing deposition, the increasing strain energy is reduced by the formation of 3D islands with the enlargement surface. Therefore, the strain acts as a source of driving force for the self-organization of Ni NCs [10, 11]. In such growth process, the lattice of Ni NCs was relaxed to the value of Ni metal bulk. The curves II and III in Fig. 1e represent the diffraction intensities along the horizontal spacing and the vertical spacing, respectively, for Fig. 1c. If defining the distance between the (0 0), (1 0) BaTiO3 diffraction orders as b shown in curve I, both the horizontal distance and the vertical distance for the Ni bulk diffraction spots equal to 2.25b. This corresponds in real space to a lattice constant of 3.55 Å in cubic structure, close to the crystal structure of bulk Ni metal (3.52 Å) [14]. As well, the same crystal structure for Ni NCs was obtained according to the RHEED pattern along the [110] azimuth. From the patterns along the [100] and [110] azimuths, it demonstrates that the Ni NCs maintain the (001) orientation with in-plane Ni NCs/BaTiO3 epitaxial relationships of [200]Ni‖[100] BTO and [220]Ni ‖[110]BTO during the growth process in 650 °C.

Figure 1
figure 1

Evolution of RHEED patterns along the <100> azimuth during the self-organized of Ni NCs process: a BaTiO3 surface; b 100 pulses Ni; c 300 pulses Ni; d 600 pulses Ni; e The intensity spacing scan for Fig. 1c: Streak horizontal line scan (I), spot horizontal line scan (II), and spot vertical line scan (III)

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The RHEED intensity variation during the BaTiO3 growth on Ni NCs layer is displayed in Fig. 2. The inserts show that the spotty RHEED pattern transforms to the streaky pattern at the initial stage, indicating the recovering of two-dimensional BaTiO3 interface for the subsequent growth. Then, the overall RHEED intensity is enhanced and presents a strong oscillation behavior. It is evident that the BaTiO3 separation layer continued to grow in a layer-by-layer growth mode with the improving of surface smoothness. The appearance of RHEED intensity oscillation provided an effective way for controlling the accurate thickness of separation layer. Figure 3 is the cross-sectional TEM image of Ni:BaTiO3 nanocomposite film. In combination with the TEM result, it was determined that one period of oscillation corresponded to the growth of three BaTiO3 unit cells. The BaTiO3 separation layers were grown repeatedly in perfect layer-by-layer mode on the irregular interface of strained NCs layers, which smoothed the irregular growth front formed in the self-assembled process of Ni NCs and provided a flat substrate for the next strained Ni NCs layer. As Fig. 3 shows, the nanocomposite film consists of eight Ni NCs layers alternating with BaTiO3 separation layers. The sizes of Ni nanocrystals were estimated in the range from 3 to 5 nm. And the BaTiO3 separation layers have the uniform thickness of 30 nm or so. This confirms that the Ni NCs were embedded in BaTiO3 matrix, by the alternating of Ni NCs self-organization process and BaTiO3 epitaxial growth. Since the RHEED pattern is very sensitive to the surface microstructure [15–17], the microstructure and the period of such quantum dot superlattice can be engineered with the in situ monitoring of RHEED.

Figure 2
figure 2

RHEED intensity oscillations recorded on spot (curve I) and streak (curve II), respectively, during the BaTiO3 epitaxial growth. The inserts are the RHEED patterns collected at 10, 30, and 50 s

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Figure 3
figure 3

The cross-sectional TEM image of Ni:BaTiO3 nanocomposite film. It consists of eight Ni NCs layers alternating with BaTiO3 separation layers

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To identify the growth mechanism of the nanocomposite film, a simple schematic cross-sectional view of Ni NCs layer shown in Fig. 4, with a dislike Ni island strained on the BaTiO3 matrix. After the relaxation of lattice strain for Ni NCs, the in-plane lattice of Ni on region B is stretch near to that of BaTiO3, whereas the in-plane lattice of Ni on region C is relaxed to that of bulk Ni metal. The average in-plane lattice strain at the island surface (region B) is less than the region C away from the island, thus the layer is considered to be uniformly strained. If further growth of BaTiO3 layer, the lattice of the BaTiO3 layer is then distorted above region C, while it is less of distortion above region A (BaTiO3 natural surface) and region B. The epitaxial growth of BaTiO3 in layer-by-layer mode occurs more rapidly above region A and region B with less lattice strain, then forms an atomically flat interface for the subsequent epitaxial growth [18, 19]. Only the relaxation of lattice strain above the well-developed 3D Ni islands is significant, while above the other islands, the strain relaxation is less. Therefore, the 2D growth of BaTiO3 was sustained although the local growth was perturbed by the Ni islands.

Figure 4
figure 4

A schematic diagram of a strained Ni NCs on flat BaTiO3 surface showing the variation of in-plane lattice strain occurring near the growth surface

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Figure 5 compares the structural characterizations of the pure BaTiO3 epitaxial film and the Ni:BaTiO3 nanocomposite films by XRD. Besides the (00 l) peaks for the SrTiO3 substrate, Fig. 5a consists of two peaks corresponding to BaTiO3 (001) and BaTiO3 (002), respectively, meaning a single phase in the pure BaTiO3 film. In Fig. 5b and 5c, the observed systematic shift of the (00 l) peaks toward to the lower diffraction angles indicates an increasing of the out-of-plane parameter of BaTiO3 due to the embedding of Ni NCs. The elongating of the out-of-plane lattice is caused by the large compressive strain at the Ni:BaTiO3 interface for the in-plane lattice match. The stronger strain was introduced for the higher Ni NCs concentration with the more obvious shift of the (00 l) peaks in Fig. 5c. Furthermore, the extra(h 00)peaks exhibit in Fig. 5b and 5c, which is a typical polydomain pattern containing the c domains with the c axis normal to the surface and the a domains with the a axis normal to the surface. The domain formation in BaTiO3 epitaxial films is a mechanism that relaxes the total strain energy as a result of the great lattice mismatch [20]. The angles between the surface normal [001] of (001) oriented domains and [100] of (100)-oriented domains is defined as α angle, α = {2arctan(c/a)–90°}, where a and c denote the lattice parameters of the a and c axes [21]. The total internal angles of a/c domains were estimated to be approximately 1.19° and 1.34° for Fig. 5b and 5c, respectively. Both the values are larger than the theoretical value for the strain-relaxed BaTiO3 powder (0.7°) [22], which illustrates that the c/a/c/a polytwin relieved the internal strain only partially. The XRD characterization confirmed the RHEED result that the epitaxial growth of BaTiO3 was not hindered by the embedding of Ni NCs. However, the strain from the lattice mismatch between Ni and BaTiO3 altered the microstructure of such nanocomposite film.

Figure 5
figure 5

XRD θ-2θ scans of BaTiO3 film, filled square correspond to diffraction peaks from SrTiO3 (001) substrates. a pure BaTiO3 film; b Ni: BaTiO3 nanocomposite film (300 Ni pulses per layer); c Ni: BaTiO3 nanocomposite film (600Ni pulses per layer); d an expand view around the 002 BaTiO3 peak with the circle label in Fig. 5b; e an expand view around the 002 BaTiO3 peak with the circle label in Fig. 5c

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Magnifying the labeled broad peak, a weak peak was observed beside the BaTiO3 (002) peak shown as the inserts in Fig. 5d and 5e, which was supposedly the Ni (111) peak. In comparison with the in situ RHEED results, there is a transformation of the preferred out-of-plane orientation for Ni NCs from [200]Ni‖[100]BTO to [111]Ni‖[100]BTO after annealing. The fcc metal tends to exhibit preferential crystallographic orientation with (111), its closet packed plane, achieving minimum surface energy. The Ni NCs were speculated to remain metallic, which may be attributed to be well protected by the BaTiO3 separation layer. Moreover, Jiang et al. found that oxygen atoms in NiO intermediate layer migrated to the BaTiO3 layer at 800°C [23]. Those results suggest that the co-exist system of metal and orientation-preferred ferroelectric oxides was available by the self-organized growth of metal NCs in ferroelectric oxides.

Conclusion

In summary, the Ni NCs formed in the epitaxial BaTiO3 film by L-MBE. The alternate growth of Ni NCs layer and epitaxial BaTiO3 film was well controllable for the desired quality by the monitoring of RHEED. For the large lattice mismatch between Ni and BaTiO3, the strain drived the self-organization growth of Ni NCs. And the BaTiO3 remained the layer-by-layer growth on the strained Ni NCs layer. The strain was relaxed only partially by the emergence of the (001)/(100) polydomain structure in the BaTiO3 epitaxial film with the elongating of c axis.