Photoluminescence and wettability control of NiFe/ZnO heterostructure bilayer films

Abstract

The controllable photoluminescence and wettability of NiFe/ZnO heterostructure bilayer films have been demonstrated by applying an ultrathin NiFe capping layer onto ZnO films by radio-frequency magnetron sputtering at room temperature without introducing any oxygen gas during the deposition process. High quality crystalline ZnO(002) textured films were fabricated at first and displayed a remarkable near-band-edge emission peak located at around 370 nm with a bandgap of 3.35 eV confirmed by room temperature photoluminescence spectra. Once the ZnO films were capped with a single NiFe layer, ranging from 5 to 20 nm in thickness, the intensity of their near-band-edge emission peak decreased and the emission band shifted to 414 nm. On the other hand, the contact angle of the uncapped ZnO film increased from 88° to 101° with the addition of a 10 nm thick NiFe capping layer. This means that the ultrathin NiFe layer acted as a surfactant layer. The surface wettability could be switched from hydrophilic to hydrophobic due to the varied surface free energy caused by the controllable grain morphology of the NiFe/ZnO heterostructures. This work demonstrates that a direct NiFe capping layer can effectively control the optical, surface and magnetic characteristics in NiFe/ZnO heterostructures depending on the bimetallic NiFe thickness and provide valuable multifunctional behaviors for potential novel magnetoelectric applications.

---

1. Introduction

In recent years interest in the metal/oxide nanocomposites have attracted much attention continuously because of their potential in many different applications such as medical implant construction, microelectronics and renewable energy.^1–5 In particular, multifunctional nanomaterials with the combination of magnetic metal and oxide thin films play a key role in the development of multifunctional nanomaterials. The main challenges of the modern control types, including optical-, magnetic-, and electrical-driven nanodevices such as the magnetic tunneling junction, have been widely explored and multifunctional nanomaterials have been used in magnetic storage medium and the field of spintronics. There are several groups that have proposed technical applications of magnetic metal and oxide thin films.^6–8 Parkin et al. reported that (100)-oriented MgO tunnel structures have a spin polarization above 85% at low temperature in CoFe/MgO/CoFe magnetic tunnel junctions.⁹ Yuasa et al. observed that the coherence of spin-polarized tunneling electrons can be conserved after multiple reflections in the electrode of NiFe/Al–O/Cu/Co magnetic tunnel junctions.¹⁰ Seki et al. reported no significant change of the magnetic coercivity (H_c) in an FePt/AlO_x system, in contrast to the claimed coercivity change and that observed in the FePt/MgO system.¹¹ Most of these works have focused on the MgO or Al–O systems, and there is still quite limited research work about magnetic metals/ZnO heterostructures.^9–11 Even Lin et al. reported the magnetism modulation of the Fe/ZnO system by Fe interface oxidation via direct current heating, but the corresponding evolution of the optical and wetting behavior is still deficient.⁸ Although a heterostructure is a such a complicated system, but it provides a benefit to device designers due to the fact that the energy-band structure of the semiconductor can be modified.^12,13 However, it can be realized that heterostructures are the building blocks for many of the most advanced oxide semiconductor materials from the references as mentioned. Among all advanced oxide semiconductor materials, ZnO has been considered an important candidate due to its direct wide bandgap (3.37 eV), high exciton binding energy (60 meV), and optical transparency for visible light, and transition metal-doped ZnO is predicted to act as a semiconductor with ferromagnetic behavior at room temperature (RT) for Curie temperature (T_C) applications. Therefore, ZnO is an expected material for many novel applications such as piezoelectric transducers, transparent thin film transistors, chemical gas sensors or biosensors,^14–17 solar cells, ultraviolet (UV) detectors^18–20 and spin light-emitting diodes (LEDs).²¹ For the above various suitable applications of ZnO, the combination of ZnO with other functional materials has attracted considerable interest, especially the combination with some magnetic metal. However, a lack of work has been reported on bimetallic magnetic metal/ZnO nanocomposites such as FePt, FePd and NiFe. Gu et al. successfully synthesized FePt–CdS multifunctional nanoparticles, exhibiting ferromagnetism and luminescence.²² Zhou et al. successfully synthesized FePt@ZnO multifunctional nanoparticles with a core and shell nanostructure, and they also observed luminescence in the ferromagnetic nanocomposite FePt@ZnO.²³ However, many kinds of nanocomposites composed of different nanocrystals with controlled interaction in an assembly provide a new way to finely tune the physical and chemical characteristics of materials design including the magnetic, electrical, optical and mechanical breakthroughs. Ni₈₀Fe₂₀ (permalloy) has excellent soft magnetic properties and unique magnetostrictive effect that is widely used in magnetic-based electronic devices such as magnetoresistive random-access memory (MRAM).²⁴ It is interesting that the electrical and magnetic properties could be changed with the various composite ratio of nickel and iron, for instance, when the content of nickel is 35%, permalloy exhibits high electrical resistivity (ρ) but low susceptibility. On the other hand, when the content of nickel is 50% and 80%, permalloy exhibits high saturation magnetization (M_s) and low coercivity without any magnetostrictive effect.²⁵

According to the above reasons, the combination of magnetic bimetallic NiFe with the wide bandgap ZnO is attracting considerable interest. In the present work, we investigate the structure, optical characteristics, magnetism and surface wettability of bimetallic NiFe combined with ZnO as heterostructure bilayer films deposited at room temperature by the magnetron sputtering system. The transparency of all the NiFe/ZnO bilayer thin films with various NiFe thicknesses ranging from 5 to 20 nm was also measured. The surface wettability of the NiFe/ZnO bilayer thin films was examined by water contact angle measurements. This research not only extends the scope of potential applications for NiFe/ZnO heterostructure bilayer films but also provides potential development of multifunctional nanomaterials.

2. Experimental procedures

The NiFe/ZnO heterostructure thin films deposited onto D263T glass substrates were employed by a radio-frequency (RF) magnetron sputtering system, and all the substrates were placed parallel to the bimetallic NiFe and ceramic ZnO targets. The NiFe target is composed of 80 at% Ni and 20 at% Fe, respectively. the ZnO target is composed of 99.99% purity pressed ZnO powder, and all the target sizes are 0.075 m in diameter and 0.006 m in thickness. All the substrates were rinsed in deionized water, ultrasonically cleaned in ethanol and acetone to remove organic contamination and then dried in hot air before they were loaded into the sputtering vacuum chamber. The sputtering chamber was pumped down to a base pressure of 3 × 10⁻⁷ torr. Argon was filled into the sputtering chamber sequentially with the low working pressure of 5 mtorr. The NiFe thin films were deposited with direct current (DC) power fixed at 15 W and the ZnO thin films were deposited with RF power fixed at 100 W, respectively. The deposition rates of NiFe and ZnO were 1.8 and 5.6 nm min⁻¹, respectively. The crystalline structure of the NiFe/ZnO heterostructure thin films was characterized by ex situ X-ray diffraction (XRD) with Cu K_α radiation (λ = 1.54 Å) in the range of 2θ = 20–60°. The surface morphology of the NiFe/ZnO heterostructure thin films was observed by field emission scanning electron microscopy (FE-SEM). The surface topography and roughness values of the NiFe/ZnO heterostructure thin films were further analyzed by atomic force microscopy (AFM). The magnetic properties were measured by a vibrating sample magnetometer (VSM) with an applied field of 2000 Oe at room temperature. The photoluminescence (PL) and UV-vis-NIR spectra were recorded at room temperature to study the optical properties of the NiFe/ZnO heterostructure bilayer films. The wettability of NiFe/ZnO heterostructure thin films was estimated from the contact angle θ of water droplets onto each NiFe/ZnO sample surface. The water contact angle measurement error was influenced by image quality and the built-in curve-fitting function, which was estimated to be ±1 degree. On the other hand, the corresponding surface free energy (SFE) could also be calculated and obtained at the same time.

3. Results and discussion

Fig. 1 shows the X-ray diffraction patterns for the NiFe/ZnO heterostructure bilayer films deposited onto glass substrates capped with different thicknesses of a NiFe thin film ranging from 5 to 20 nm. It can be clearly observed that all samples exhibit a strong peak located at 2θ = 34.5° originating from the 100 nm thick ZnO underlayer film. After capping with the NiFe layer ranging from 5 to 20 nm in thickness, the XRD patterns show peaks for ZnO and NiFe located at 2θ = 34.5° and 43.5°, which correspond to the ZnO(002) plane (JCPDS Card: 361451) and NiFe(111) plane (JCPDS Card: 471417). The above results indicate that the NiFe/ZnO heterostructure bilayers have highly textured growth, and the NiFe and ZnO are in face-centered cubic (fcc) and hexagonal close-packed (hcp) phase structures, respectively. The intensity of the NiFe(111) diffraction peak increased with the increasing thickness of the NiFe thin film, which is due to the increase in thickness of the NiFe thin film and improvement of the film crystallinity. The strong signal intensity of the (002) diffraction peak from the (002) plane is due to the lowest surface energy of the (002) basal plane in the ZnO phase, leading to a preferred orientation along the [001] crystalline direction.²⁶ Therefore, the NiFe/ZnO heterostructure bilayer films have been successfully deposited onto glass substrates at room temperature, and the crystalline quality degree of ZnO still exhibits a highly c-axis orientation while capped with the NiFe thin film.

Fig. 1 XRD patterns for the NiFe/ZnO heterostructure bilayer films deposited onto glass substrates at room temperature capped with various thicknesses of a NiFe layer ranging from 5 to 20 nm.

The top view FE-SEM images for the NiFe/ZnO bilayer films deposited onto glass substrates at room temperature with different thicknesses of the NiFe thin film ranging from 5 to 20 nm are shown in Fig. 2(a)–(d). A clear variation in the surface morphology of the NiFe/ZnO bilayer films can be observed and varied with the NiFe thickness and the average grain size increased with the increasing capping layer thickness of NiFe. Fig. 2(a)–(d) show two different types of grain structures denoted as the sub-nanograin and nanograin cases, respectively. The different grain structures were formed and caused by a suitable thickness of the NiFe capping layer during the deposition process. The sub-nanograin structure of the NiFe capping layers can be observed at a lower thickness (5 and 10 nm) as shown in Fig. 2(b) and (c). On the other hand, the nanograin structure could be observed in the pure ZnO thin film (without NiFe) and with a larger NiFe thickness (20 nm) as shown in Fig. 2(a) and (d), respectively. The insets in Fig. 2(a)–(d) present the grain size histograms for evaluating the average size and its distribution for the NiFe/ZnO heterostructures. The grain size fraction of the pure ZnO thin films with a 100 nm thickness ranged from 15 to 47 nm, with an average size of 31.2 ± 16.2 nm as shown in the inset of Fig. 2(a); the grain size fraction of the capping NiFe thin films with a 5 nm thickness ranged from 10 to 27 nm, with an average size of 18.5 ± 8.8 nm as shown in the inset of Fig. 2(b); the grain size fraction of the capping NiFe thin films with a 10 nm thickness ranged from 14 to 39 nm, with an average size of 26.8 ± 12.8 nm as shown in the inset of Fig. 2(c); and the grain size fraction of the capping NiFe thin films with a 20 nm thickness ranged from 16 to 44 nm, with an average size of 30.5 ± 14.4 nm as shown in the inset of Fig. 2(d). It can be observed that the grain size of the NiFe/ZnO heterostructures capped with 20 nm NiFe is similar to that of the pure ZnO thin film (without NiFe). When the capping thickness of NiFe is 5 nm, the mixed sub-nanograin and nanograin structures of NiFe/ZnO are coexisting in the microstructure as shown in Fig. 2(b). The above results could be attributed to the increased NiFe thickness that leads to the transformation of grain morphology, and the microstructure of the NiFe/ZnO heterostructure bilayers could be controlled by a suitable thickness of the bimetallic NiFe capping layer. It can be understood that with increasing the thickness of the NiFe capping layer, the nanograins of NiFe with multiple domains coalesce and connect to each other to conjoin into a big grain as shown in Fig. 2(d). The apertures could trap air and form air pockets inside, which was due to the surface of NiFe acting as a surfactant in the NiFe/ZnO heterostructure bilayers. Fig. 2(e)–(g) show the cross-sectional micrographs for the NiFe/ZnO bilayer films deposited onto glass substrates at room temperature with different thicknesses of the NiFe thin film ranging from 5 to 20 nm. It can be observed that a clear interface (red line) formed in NiFe/ZnO bilayer films. All samples show a typical self-assembled columnar structure of the ZnO films in [001] orientation perpendicular to the glass substrate, and these images also confirmed the corresponding thickness value of NiFe/ZnO bilayer films are close to 105, 110, and 120 nm.

Fig. 2 Top view FE-SEM images for the NiFe/ZnO heterostructure bilayer films deposited onto glass substrates capped with various thicknesses of a NiFe layer ranging from (a) 0 nm, (b) 5 nm, and (c) 10 nm to (d) 20 nm. The insets show the grain size histograms for evaluating the average size and its distribution for the NiFe/ZnO heterostructure bilayer films with different capping thicknesses of the NiFe layer. (e), (f) and (g) are the cross-sectional micrographs for the NiFe/ZnO heterostructure bilayer films, and the corresponding interface region is marked with a red line, respectively.

Self-cleaning is a special mechanism of a surface property; it relates to the chemical composition and the surface morphology. It has been focused for many potential applications such as environmental cleanup²⁷ and optoelectronic devices.²⁸ However, as reported in many references, there are many methods to prepare hydrophobic surfaces such as surface modification²⁹ or forming surface nanostructures to increase the surface roughness,³⁰ and therefore, the surface wettability of the NiFe/ZnO heterostructure is interesting to realize. Fig. 3(a)–(d) show the surface wetting images for the water contact angle of the NiFe/ZnO heterostructure bilayers. The values of the water contact angles (CAs) are 88.3 ± 1°, 98.9 ± 1°, 101.2 ± 1° and 97 ± 1° for each of the NiFe/ZnO heterostructure bilayers capped with 0, 5, 10 and 20 nm thick NiFe, respectively. The pure 100 nm thick ZnO film presents hydrophilic wetting, which is due to the smooth surface filled with the nanograin structure of ZnO. The larger grain size gets fewer apertures that cannot provide trapping of air as shown in Fig. 3(a). While a single 5 nm thick NiFe capping layer acted as a surfactant layer, the heterostructure surface contains NiFe sub-nanograin and ZnO nanograin mixed structures and shows hydrophobic behavior, which is due to the smaller grain size with more apertures to provide more trapping of air, leading to a reduction in the contact area between the water droplet and the smooth surface as shown in Fig. 3(b). The largest CA value, shown in Fig. 3(c), is observed for the heterostructure capped with 10 nm thick NiFe. The heterostructure surface contained ZnO nanograins and was covered with a NiFe sub-nanograin structure, which could provide many more opportunities for air trapping, leading to a reduction in the contact area between the water droplet and the smooth surface. This result, due to the surface free energy of the surfactant NiFe (10.47 mJ m⁻²), is much lower than that of ZnO (17.32 mJ m⁻²). A small contact angle of the NiFe/ZnO heterostructure (20 nm NiFe) is observed, as shown in Fig. 3(d), which is due to the grain structure of NiFe transforming from the sub-nanograin to the nanograin state. This transformation of grain morphology caused the CA to decrease, which was due to fewer apertures forming while the NiFe capping layer belonged to a continuous film.³¹

Fig. 3 The CA values of the NiFe/ZnO heterostructure bilayer films capped with various thicknesses of a NiFe layer ranging from (a) 0 nm, (b) 5 nm, and (c) 10 nm to (d) 20 nm, respectively. The insets show the AFM 3D images for the NiFe/ZnO heterostructure bilayer films related to (a)–(d), respectively. (e) Schematic diagram illustrating the NiFe/ZnO heterostructure bilayer films with different scales of grain morphology induced by different thicknesses of NiFe. (f) The surface roughness and surface free energy of the NiFe/ZnO heterostructure bilayer films deposited onto glass substrates as a function of thickness of the NiFe capping layer.

The insets shown in Fig. 3(a)–(d) are the three-dimensional (3D) AFM micrographs (1 μm × 1 μm). The surface topography images in 3D mode show that the tendency of the average surface roughness values (root-mean-square, RMS) of the NiFe/ZnO heterostructure bilayers is to increase with increasing the thickness of the NiFe capping layer. The average RMS values are 2.32, 2.42, 2.72, and 2.51 nm for each of the NiFe/ZnO heterostructure bilayers with different thicknesses of the NiFe capping layer ranging from 0, 5, and 10 to 20 nm, as shown in Fig. 3(f). The above results indicate that the pure highly c-axis orientated ZnO(002) thin films could provide nanoscaled surface roughness to demonstrate aperture formation which can trap air and become air pockets. After capping a NiFe layer with the sub-nanograin structure, many more apertures are produced to trap air and cause the surface wettability to be hydrophobic. This is in accordance with natural lotus leaves, which demonstrate a water contact angle as high as 160° due to their particular nanoscaled surface structures.^32–36 Hence the surface microstructure should play an important role in the wettability for bulk solid materials. Hydrophobic materials are usually prepared by modifying their surface with low surface energy for forming nanostructures. Our present results indicate that the pure ZnO thin films with low surface energy have been successfully modified and controlled by capping with a sub-nanograin structure of NiFe. This kind of nanostructure was not only applied for surface modification, but also could be applied for insertion into some potential electronic devices to improve the resistance switching behavior.^37–39

The surface free energy (SFE) for all the NiFe/ZnO heterostructure bilayers was calculated using the Fowkes–Girifalco–Good theory.^40,41 According to the Fowkes method analysis, the considered critical interaction is the dispersive force or van der Waals force across the interface that exists between the droplet and the solid surface. The Fowkes equation can be described as:

γ_ls = γ_s + γ_l − 2(γ^d_sγ^d_l)^1/2 (1)

where γ^d_l and γ^d_s are the dispersive portions of the surface tension for the liquid and solid surface, respectively. Combining Young’s equation⁴² into eqn (1) and using a selected nonpolar liquid such as deionized water (72.8 mJ m⁻²), γ^d_l can be equal to γ_l, hence the Girifalco–Good–Fowkes–Young (GGFY) equation can be converted as:where θ is a constant that represents the CA on a smooth surface, and γ^d_s is the surface free energy of the measured samples. We used the GGFY equation to describe a surface state by calculating the SFE of all the NiFe/ZnO samples rather than just considering the contact angle value.

Fig. 3(e) is a schematic diagram illustration for the different types showing the state of the water droplet onto various surface nanostructures of the NiFe/ZnO heterostructure films. At first, there is a smooth surface structure with hydrophilic wetting behavior for the pure ZnO thin films, which is denoted as the nanograin type and has high surface energy (17.32 mJ m⁻²). Once the ZnO thin films capped with a 5 and 10 nm NiFe layer combine with a sub-nanograin structure, two different types of grain structure grow onto the glass substrates and form more apertures that could provide trapping of air and become air pockets, which is denoted as a mixed nanograin of ZnO and sub-nanograin of NiFe, called step 2 as shown in Fig. 3(e). At the same time, these two kinds of nanostructures with a 5 and 10 nm NiFe layer offered relatively lower surface energy (11.56 mJ m⁻² and 10.47 mJ m⁻²), which means that a larger CA value could be obtained as shown in Fig. 3(f). The ZnO thin film capped with a 20 nm NiFe layer belongs to the nanograin structure and has higher surface energy (12.47 mJ m⁻²), which is due to the formation of a uniform nanograin microstructure that gives more opportunities for the water droplet to contact with the surface area of the NiFe/ZnO thin films, denoted as the nanograin type and called step 3 as shown in Fig. 3(e).

For the purpose of understanding the wetting behavior of the NiFe/ZnO heterostructure bilayers, as suggested by Bormashenko et al.,^43,44 the typical Cassie–Baxter (CB) equation was considered that can be used as a principle of explanation in this work. In the typical CB equation, the drops are suspended onto a hydrophobic surface with air trapped underneath as an incomplete filling and the equilibrium contact angle (CA, θ) of a drop on a nanostructured film can be described as:

cos θ′ = f₁ cos θ + f₂, (3)

where f₁ and f₂ are the area fractions of the liquid–solid interface and liquid–air interface, respectively. When f₁ + f₂ = 1, eqn (3) can be converted to:

cos θ′ = f₁(cos θ + 1) − 1, (4)

where θ is a constant that represents the CA on a smooth surface, and θ′ is the CA on a rough surface. Based on eqn (4), it can be understood that θ′ increases with a decreasing area fraction of the liquid–solid interface (f₁), and the surface fraction will make a significant contribution to the CA. It is demonstrated that surface hydrophobicity improves when there is more air trapped between the liquid and solid surface exhibiting a larger CA. Therefore, more air is trapped between the water droplet and the surface of the NiFe/ZnO bilayer films. The CA value of the ZnO thin films will be greatly enhanced due to the fact that it prevents complete wetting of the surface. We explored and presented the morphology and grain size effect on the controllable wettability properties of NiFe/ZnO heterostructure bilayers, and the proposed mechanism was able to explain the phenomena.

Fig. 4 shows the transparency of the NiFe/ZnO heterostructure films deposited onto glass substrates at room temperature with different thicknesses of the NiFe capping layer ranging from 5 to 20 nm, respectively. From the transmittance spectra, it can be clearly observed that the NiFe/ZnO bilayers capped without and with 5 nm thick NiFe have good transparency with a visible light averaged transmittance value over 80%. On the other hand, the values of averaged transmittance in visible light decreased to 40% and 18% while the capping layer thicknesses of NiFe were 10 and 20 nm, respectively. It can be understood that a thicker NiFe capping layer has much more absorption and decreases the transmission strength of light for visible light. It is interesting that the NiFe/ZnO heterostructure bilayers capped with NiFe with a thickness of 5 nm have a high averaged transmission with low absorption above 82% in the range of 500 to 700 nm. It can be attributed to the 5 nm NiFe sub-nanograin structures that offer a path to lead more photonic scattering from forward light.^45,46 This work presented the functional NiFe/ZnO oxide-based heterostructures that could be applied to self-disinfection glass or smart windows combined with hybrid devices such as sensors and solar cells, and their high transmittance could play an important role in particular industrial products.

Fig. 4 Optical transmittance spectra for the pure bare glass substrate and the NiFe/ZnO heterostructure bilayer films with various thicknesses of the NiFe capping layer ranging from 0 to 20 nm, respectively.

The photoluminescence spectra of pure ZnO and the NiFe/ZnO heterostructure bilayers have been measured at room temperature as are shown in Fig. 5. The pure ZnO film exhibits a remarkable near-band-edge emission peak located at around 370 nm with a bandgap of 3.35 eV. When the NiFe single layer ranging from 5 to 20 nm in thickness was capped onto the ZnO film, the intensity of the near-band-edge emission peak decreased and the emission band shifted to 414 nm. This phenomenon was due to the oxygen vacancy effect and could be attributed to a rise of mismatch between Zn²⁺ and Fe³⁺. As proposed by Bhatti et al.,⁴⁷ the PL intensity decreasing for the near-band-edge emission band could be due to the impurity in ZnO that acts as a quencher; the impurity caused the transition between energy levels and led to the emission band shifting from 370 nm to 414 nm. The above result indicates that ZnO was doped with Fe atoms and the concentration of structural defects in the NiFe/ZnO heterostructure films was higher than that in the pure ZnO one.^48–51 Therefore, it is interesting that the emission band or bandgap of the NiFe/ZnO heterostructure can be easily controlled by the NiFe capping layer.

Fig. 5 Photoluminescence spectra of the NiFe/ZnO heterostructure bilayer films capped with various thicknesses of a NiFe layer ranging from 0 to 20 nm.

The Ni₈₀Fe₂₀ magnetic alloy is a well-known ferromagnetic material and possesses manifest soft magnetic hysteresis, and all the NiFe/ZnO heterostructure bilayer films with different thicknesses of the NiFe capping layer are characterized by VSM measurements at room temperature. The saturation magnetization (M_s) values as a function of the thickness of the NiFe capping layer for the NiFe/ZnO heterostructure bilayers are shown in Fig. 6. The M_s value of the NiFe/ZnO heterostructures was gradually varied from 18 to 546 emu cm⁻³ with an increasing capping layer thickness of NiFe ranging from 5 to 20 nm. The M_s value of the NiFe/ZnO heterostructures was reduced with a decreasing thickness of the NiFe capping layer accompanied with a clear thickness variation. The reduced magnetization value is due to the oxidation and magnetic dead layer effects of the NiFe film, and this fact almost exists in many soft and hard magnetic materials.^52–55 The ZnO was doped with Fe atoms that could diffuse from the interface between the bimetallic NiFe and ZnO and form FeO_x as a magnetic dead layer.⁵⁶ A very weak magnetization value is observed for the NiFe/ZnO heterostructures with a 5 nm thick NiFe capping layer. On the other hand, the thicknesses of the NiFe capping layer over 10 nm could exhibit a clear soft magnetic hysteresis loop as shown in the inset of Fig. 6. The magnetic character in the NiFe/ZnO heterostructures is also dependant on the thickness of the NiFe capping layer.

Fig. 6 The saturation magnetization (M_s) values of the NiFe/ZnO heterostructure bilayer films as a function of the thickness of the NiFe capping layer. The inset shows the in-plane hysteresis loops of the NiFe/ZnO heterostructures with various thicknesses of the NiFe capping layer ranging from 0 to 20 nm.

4. Conclusions

Multifunctional NiFe/ZnO heterostructure bilayer films can be successfully fabricated onto amorphous glass substrates at room temperature. The surface wettability of pure ZnO films was controlled by capping a single NiFe layer in the NiFe/ZnO heterostructure bilayers, and the NiFe/ZnO heterostructures showed a maximum CA value (101 ± 1°) via capping with 10 nm thick NiFe due to its lowest surface free energy (10.47 mJ m⁻²). All the NiFe/ZnO heterostructure bilayers exhibited good crystallinity with tunable magnetic properties. Pure ZnO(002) textured films showed a high visible transparency (over 80%), and the visible transparency decreased with the increasing thickness of the NiFe capping layer as confirmed from the transmittance spectra due to there being much more absorption with a thicker NiFe layer. The PL spectra show that the pure ZnO film has a remarkable near-band-edge emission peak located at around 370 nm, and the intensity of the near-band-edge emission peak decreased and the emission band shifted to 414 nm while capping with a single NiFe layer. Therefore, a simple method is achieved and presented here that the surface wettability and optical properties of a ZnO phase can be effectively controlled by capping with a bimetallic NiFe layer. The NiFe/ZnO heterostructure bilayer films exhibit many novel and valuable magnetoelectric applications in future due to their multifunctional behaviors.

Acknowledgements

We acknowledge financial support of the main research projects of the Ministry of Science and Technology (MOST) under Grant No. 104-2221-E-027-006 and formerly the National Science Council (NSC) of Republic of China under Grant No. 101-2622-E-027-003-CC2 and 101-2221-E-027-042, respectively.

References

1. C. T. Campbell, Ultrathin metal films and particles on oxide surfaces: structural, electronic and chemisorptive properties, Surf. Sci. Rep., 1997, 27, 1.
2. H. L. Mosbacker, Y. M. Strzhemechny, B. D. White, P. E. Smith, D. C. Look, D. C. Reynolds, C. W. Litton and L. J. Brillson, Role of near-surface states in ohmic-Schottky conversion of Au contacts to ZnO, Appl. Phys. Lett., 2005, 87, 012102.
3. P. Li, Z. Wei, T. Wu, Q. Peng and Y. D. Li, AuZnO hybrid nanopyramids and their photocatalytic properties, J. Am. Chem. Soc., 2011, 15, 5660–5663.
4. Y. Li, L. Zang, X. Xiang, D. P. Yang and F. Li, Engineering of ZnCo-layered double hydroxide nanowalls toward high-efficiency electrochemical water oxidation, J. Mater. Chem. A, 2014, 2, 13250–13258.
5. W. H. He, Y. Yang, L. R. Wang, J. J. Yang, X. Xiang, D. P. Yan and F. Li, Photoelectrochemical water oxidation efficiency of a core/shell array photoanode enhanced by a dual suppression strategy, ChemSusChem, 2015, 8, 1568–1576.
6. E. Popova, J. F. Vincent, C. Tiusan, C. Bellouard, H. Fischer, M. Hehn, F. Montaigne, M. Alnot, S. Andrieu and A. Schuhl, Epitaxial MgO layer for low-resistance and coupling-free magnetic tunnel junctions, Appl. Phys. Lett., 2002, 81, 1035–1037.
7. W. G. Wang, M. G. Li, S. Hageman and C. L. Chien, Electric-field-assisted switching in magnetic tunnel junctions, Nat. Mater., 2012, 11, 64–68.
8. W. C. Lin, P. C. Chang, C. J. Tsai, T. C. Hsieh and F. Y. Lo, Magnetism modulation of Fe/ZnO heterostructure by interface oxidation, Appl. Phys. Lett., 2013, 103, 212405–212410.
9. S. S. P. Parkin, C. Kaiser, A. Panchula, P. M. Rice, B. Hughes, M. Samant and S. H. Yang, Giant tunnelling magnetoresistance at room temperature with MgO (100) tunnel barriers, Nat. Mater., 2004, 3, 862–867.
10. S. Yuasa, T. Nagahama and Y. Suzuki, Spin-polarized resonant tunneling in magnetic tunnel junctions, Science, 2002, 297, 234–237.
11. Y. Kikuchi, T. Seki, M. Kohda, J. Nitta and K. Takanashi, Voltage-induced coercivity change in FePt/MgO stacks with different FePt thicknesses, J. Phys. D: Appl. Phys., 2013, 46, 285002.
12. G. Burns, Solid State Physics, Academic Press, Orlando, 1985, ch. 10.
13. Q. Y. Meng, Y. X. Kang, X. Y. Zhai, Z. W. Yin and D. P. Peng, Nonlinear Electrical Conductivity Properties of Au Films Prepared by Sputtering, J. Nanomater., 2014, 2014, 437082.
14. J. J. Hassan, M. A. Mahdi, C. W. Chin, H. Abu-Hassan and Z. Hassan, A high-sensitivity room-temperature hydrogen gas sensor based on oblique and vertical ZnO nanorod arrays, Sens. Actuators, B, 2013, 176, 360–367.
15. C. L. Hsu, K. C. Chen, T. Y. Tsai and T. J. Hsueh, Fabrication of gas sensor based on p-type ZnO nanoparticles and n-type ZnO nanowires, Sens. Actuators, B, 2013, 182, 190–196.
16. Y. C. Liang, W. K. Liao and S. L. Liu, Performance enhancement of humidity sensors made from oxide heterostructure nanorods via microstructural modifications, RSC Adv., 2014, 4, 50866–50872.
17. Y. C. Liang and W. K. Liao, Annealing induced solid-state structure dependent performance of ultraviolet photodetectors made from binary oxide-based nanocomposites, RSC Adv., 2014, 4, 19482–19487.
18. Y. N. He, W. Zhang, S. C. Zhang, X. Kang, W. B. Peng and Y. L. Xu, Study of the photoconductive ZnO UV detector based on the electrically floated nanowire array, Sens. Actuators, A, 2012, 181, 6–12.
19. C. H. Chao, M. Y. Chen, C. R. Lin, Y. C. Yu, Y. D. Yao and D. H. Wei, Postannealing Effect at Various Gas Ambients on Ohmic Contacts of Pt/ZnO Nanobilayers Toward Ultraviolet Photodetectors, Int. J. Photoenergy, 2013, 2013, 372869.
20. N. K. Hassan and M. R. Hashim, Flake-like ZnO nanostructures density for improved absorption using electrochemical deposition in UV detection, J. Alloys Compd., 2013, 577, 491–497.
21. S. Jana, B. B. Strivastava, S. Jana, R. Bose and N. Pradhan, Multifunctional doped semiconductor nanocrystals, J. Phys. Chem. Lett., 2012, 3, 2535–2540.
22. H. W. Gu, R. K. Zeng, X. X. Zhang and B. Xu, Facile one-pot synthesis of bifunctional heterodimers of nanoparticles: A conjugate of quantum dot and magnetic nanoparticles, J. Am. Chem. Soc., 2004, 126, 5664–5665.
23. T. J. Zhou, M. H. Lu, Z. H. Zhang, H. Gong, W. S. Chin and B. Liu, Synthesis and characterization of multifunctional FePt/ZnO core/shell nanoparticles, Adv. Mater., 2010, 22, 403–406.
24. G. Nahrwold, J. M. Scholtyssek, S. Motl-Ziegler, O. Albrecht, U. Merkt and G. Meier, Structural, magnetic, and transport properties of Permalloy for spintronic experiments, J. Appl. Phys., 2010, 108, 013907.
25. Y. S. Liu, J. C. Zhang, L. M. Yu, G. Q. Jia, C. Jing and S. X. Cao, Magnetic and frequency properties for nanocrystalline Fe–Ni alloys prepared by high-energy milling method, J. Magn. Magn. Mater., 2005, 285, 138–144.
26. N. Fujimura, T. Nishihara, S. Goto, J. F. Xu and T. Ito, Control of preferred orientation for ZnO_x films: Control of self-texture, J. Cryst. Growth, 1993, 130, 269.
27. D. H. W. Li, J. C. Lam, C. C. S. Lau and T. W. Huan, Lighting and energy performance of solar film coating in air-conditioned cellular offices, Renewable Energy, 2004, 29, 921.
28. Z. Y. Huang, M. Chen, S. R. Pan and D. H. Chen, Effect of surface microstructure and wettability on plasma protein adsorption to ZnO thin films prepared at different RF powers, Biomed. Mater., 2010, 5, 054116.
29. L. Y. Lin, H. J. Kim and D. E. Kim, Wetting characteristics of ZnO smooth film and nanowire structure with and without OTS coating, Appl. Surf. Sci., 2008, 254, 7370–7376.
30. C. Y. Cao, Y. Y. Feng, J. F. Zang, G. P. López and Z. H. Zhao, Tunable lotus-leaf and rose-petal effects via graphene paper origami, Extreme Mechanics Letters, 2015, 4, 18–25.
31. J. Han and W. Gao, Surface wettability of nanostructure Zinc Oxide films, J. Electron. Mater., 2009, 38, 601–608.
32. R. Rosario, D. Gust, A. A. Garcia, M. Hayes, J. L. Taraci, T. Clement, J. W. Dailey and S. T. Picraux, Lotus effect amplifies light-induced contact angle switching, J. Phys. Chem. B, 2004, 108, 12640–12642.
33. T. L. Sun, L. Feng, X. F. Gao and L. Jiang, Bioinspired surfaces with special wettability, Acc. Chem. Res., 2005, 38, 644–652.
34. P. H. Yang, K. Wang, Z. W. Liang, W. J. Mai, C. X. Wang, W. G. Xie, P. Y. Liu, L. Zhang, X. Cai, S. Z. Tan and J. H. Song, Enhanced wettability performance of ultrathin ZnO nanotubes by coupling morphology and size effects, Nanoscale, 2012, 4, 5755–5760.
35. E. Pakdel, W. A. Daoud and X. G. Wang, Self-cleaning and superhydrophilic wool by TiO₂/SiO₂ nanocomposite, Appl. Surf. Sci., 2013, 275, 397–402.
36. K. Midtdal and B. P. Jelle, Self-cleaning glazing products: A state-of-the-art review and future research pathways, Sol. Energy Mater. Sol. Cells, 2013, 109, 126–141.
37. L. Shi, D. S. Shang, Y. S. Chen, J. Wang, J. R. Sun and B. G. Shen, Improved resistance switching in ZnO-based devices decorated with Ag nanoparticles, J. Phys. D: Appl. Phys., 2011, 44, 45530–455310.
38. C. Y. Liu, J. J. Huang and C. H. Lai, Resistive switching characteristics of a Pt nanoparticle-embedded SiO₂-based memory, Thin Solid Films, 2013, 529, 107–110.
39. M. J. Yun, H. D. Kim and T. G. Kim, Improved resistive-switching characteristics observed in Pt embedded nickel-nitride films prepared by radio-frequency magnetron sputtering, J. Vac. Sci. Technol., B: Nanotechnol. Microelectron.: Mater., Process., Meas., Phenom., 2013, 31, 060601.
40. M. Żenkiewicz, Methods for the calculation of surface free energy of solids, J. Achiev. Mater. Manuf. Eng., 2007, 24, 137–145.
41. D. B. Mahadik, A. V. Rao, V. G. Parale, M. S. Kavale, P. B. Wagh, S. V. Ingale and S. C. Gupta, Effect of surface composition and roughness on the apparent surface free energy of silica aerogel materials, Appl. Phys. Lett., 2011, 99, 104104.
42. K. G. Kabza, J. E. Gestwicki and J. L. McGrath, Contact angle goniometry as a tool for surface tension measurements of solids, using Zisman plot method. A physical chemistry experiment, J. Chem. Educ., 2000, 66, 63–65.
43. E. Bormashenko, Why does the Cassie–Baxter equation apply?, Colloids Surf., A, 2008, 324, 47–50.
44. G. Whyman, E. Bormashenko and T. Stein, The rigorous derivation of Young, Cassie–Baxter and Wenzel equations and the analysis of the contact angle hysteresis phenomenon, Chem. Phys. Lett., 2008, 450, 335–359.
45. N. Venugopal and A. Mitra, Optical transparency of ZnO thin film using localized surface plasmons of Ag nanoislands, Opt. Mater., 2013, 35, 1467–1476.
46. C. L. Tan, S. J. Jang and Y. T. Lee, Localized surface plasmon resonance with broadband ultralow reflectivity from metal nanoparticles on glass and silicon subwavelength structures, Opt. Express, 2012, 20, 17448.
47. H. S. Bhatti, R. Sharma and N. K. Verma, Fast photoluminescence decay processes of doped CaS phosphors, Radiat. Eff. Defects Solids, 2006, 161, 113–119.
48. M. H. Lu, H. Gong, J. P. Wang, H. W. Zhang and T. J. Zhou, Nanoparticles composites: FePt with wide-band-gap semiconductor, J. Magn. Magn. Mater., 2006, 303, 323–328.
49. Y. Guo, X. B. Cao, X. M. Lan, C. Zhao, X. D. Xue and Y. Y. Song, Solution-Based Doping of Manganese into Colloidal ZnO Nanorods, J. Phys. Chem. C, 2008, 112, 8832.
50. Z. X. Yang, W. Zhong, C. T. Au, X. Du, H. A. Song, X. S. Qi, X. J. Ye, M. H. Xu and Y. W. Du, Novel Photoluminescence Properties of Magnetic Fe/ZnO Composites: Self-Assembled ZnO Nanospikes on Fe Nanoparticles Fabricated by Hydrothermal Method, J. Phys. Chem. C, 2009, 113, 21269.
51. E. Oh, S. H. Jung, K. H. Lee, S. H. Jeong, S. G. Yu and S. J. Rhee, Vertically aligned Fe-doped ZnO nanorod arrays by ultrasonic irradiation and their photoluminescence properties, Mater. Lett., 2008, 62, 3456.
52. K. Oguz, P. Jivrajka, M. Venkatesan, G. Feng and J. M. D. Coey, Magnetic dead layers in sputtered Co₄₀Fe₄₀B₂₀ films, J. Appl. Phys., 2008, 103, 07B526.
53. D. S. Hung, Y. D. Yao, D. H. Wei, K. T. Wu, J. C. Hsu, T. Ding and Y. C. Chen, Permittivity study of multiferroic AlN/NiFe/AlN multilayer films, J. Appl. Phys., 2008, 103, 07E318.
54. Y. S. Cui, W. J. Yin, W. Chen, J. W. Lu and S. A. Wolf, Epitaxial τ phase MnAl thin films on MgO (001) with thickness-dependent magnetic anisotropy, J. Appl. Phys., 2011, 110, 103909.
55. D. H. Wei, P. W. Chi and C. H. Chao, Perpendicular magnetization reversal mechanism of functional FePt films for magnetic storage medium, Jpn. J. Appl. Phys., 2014, 53, 11RG01.
56. L. S. Huang, J. F. Hu and J. S. Chen, Critical Fe thickness for effective coercivity reduction in FePt/Fe exchange-coupled bilayer, J. Magn. Magn. Mater., 2012, 324, 1242–1247.

---