Bis(acetylacetonato)nickel [Ni(acac)₂] (99.9%) and N,N,N^′,N^′-tetramethylethylenediamine (99%) were purchased from J&K Scientific Ltd. (Shanghai, China), and other reagents were obtained from Sinopharm Chemical Reagent Co., Ltd. (Zhengzhou, Henan, China). The synthesis of the complex followed the reported literature method [36], and all reagents were used without further purification. The detailed synthesis method is as follows.

Ni(acac)₂(tmeda): A 250 mL flask was charged with Ni(acac)₂ (7.707 g, 30 mmol) and 100 mL of n-hexane under air. Subsequently, a mixture of N,N,N^′,N^′-tetramethylethylenediamine (99%) (3.835 g, 33 mmol) and 20 mL of n-hexane was added dropwise at room temperature, and the reaction mixture was stirred at room temperature for 4 h. After filtration and concentration, the reaction mixture was placed at –30 °C for 12 h to yield blue-green crystals of Ni(acac)₂(tmeda) (9.738 g, 87%).

The home-made LPCVD apparatus was as described in the literature [37], as shown in Fig. 1. Thin films were deposited on SiO₂/Si(100) substrates, and the substrates were evenly arranged in the effective heating zone of the reaction chamber. To remove contaminants from the substrate surface, the substrates were sequentially ultrasonicated in acetone, isopropanol, and deionized water for 10 min each, followed by drying with N₂. The complex was heated at 100 °C, with deposition temperatures ranging from 280 to 480 °C and a deposition time of 30 min. The precursor vapor was transported to the reaction chamber via N₂. The N₂ included 5 mL/min of “N₂ + precursor” and 100 mL/min of “N₂ alone”, which were used as the dilution gas and carrier gas, respectively. Other parameters included a deposition pressure of 5 torr and an O₂ (reactive gas) flow rate of 100 mL/min.

Fig. 1.

The LPCVD device. LPCVD, low-pressure chemical vapor deposition.

The surface morphology of the deposited films was analyzed using atomic force microscopy (AFM, Bruker Dimension Icon, CA, USA) using a contact mode and field-emission scanning electron microscopy (SEM, Nova NanoSEM 450, FEI Company, Hillsboro, OR, USA) using a SE mode. The thickness of the deposited films was measured by an EOPTICS SE-100A spectroscopic ellipsometer (Wuhan Eoptics Technology Co., Ltd., Wuhan, Hubei, China) using the Cauchy optical model with an incident angle of 65° and a step size of 1 nm, and the value was calibrated using SEM. The composition of the films was determined using X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA) using a standard Al K$\alpha$ radiation (1486.6 eV) with a 1 $\times$ 10^-10 mbar base pressure. The crystalline structure was analyzed using X-ray diffraction (XRD, Bruker D8 Advance, CA, USA) using Cu K$\alpha$ radiation.

As described in Experimental section, the complex Ni(acac)₂(tmeda) was synthesized via a simple coordination reaction between Ni(acac)₂ and N,N,N^′,N^′-tetramethylethylenediamine at room temperature in air. The straightforward synthesis process ensures a low cost for the synthesis of complex. To further evaluate the stability of the selected complex against moisture and air, an air exposure experiment was conducted. The results indicated that the complex remained unaltered after 5 days, demonstrating its excellent resistance to moisture and air. This characteristic facilitates handling and utilization.

In our previous work, the thermal properties of the selected complex were investigated [32], and the results demonstrated that the synthesized complex possesses excellent volatility and thermal stability, meeting the requirements of CVD precursors.

To directly explore the applicability of the selected complex in fabricating thin films via CVD, this study conducted thin film deposition experiments using Ni(acac)₂(tmeda) as the CVD precursor. Through thin film deposition experiments, it was found that nearly no film was formed when the deposition temperature was below 280 °C. Although a film with a thickness of 45.7 nm was obtained at 280 °C, the film growth rate (1.52 nm/min) was relatively low. This may be attributed to the lower reactivity of the precursor at the lower deposition temperature. Based on this, the present work further investigated the film growth rates at deposition temperatures of 380 °C and 480 °C, and it is found that the film growth rate significantly increased with the rise in deposition temperature, with the film growth rates being 6.73 nm/min (film thickness of 202 nm) at 380 °C and 6.9 nm/min (film thickness of 207 nm) at 480 °C. Furthermore, the deposition at 380 °C for 60 min successfully prepared a film with a thickness of 423 nm.

The surface morphology of the deposited films was characterized using SEM and AFM. As shown in Fig. 2, the films were composed of granular structures, exhibiting continuous and uniform surfaces without cracks. Additionally, it was observed that the particle size of the deposited films gradually increased with increasing deposition temperature. Similarly, the surface roughness of the films was found to increase with increasing deposition temperature, which is consistent with the results obtained from SEM. This may be attributed to enhanced atomic diffusion and lattice reconstruction at higher deposition temperatures, which leads to an increase in both particle size and surface roughness. Moreover, the deposited film exhibits high adhesion to the substrate, with no film delamination observed after attempting to scratch.

Fig. 2.

The SEM and AFM images of the deposited films at 280 °C (a), 380 °C (b), and 480 °C (c). SEM, scanning electron microscopy; AFM, atomic force microscopy; RMS, root mean square roughness. Scale bar = 500 nm.

To obtain the composition and crystalline structure of the deposited films, XRD analysis was conducted on films deposited at different temperatures. As shown in Fig. 3, films deposited at 380 °C and 480 °C exhibited diffraction peaks at 2$\theta$ = 37.0°, 43.1°, and 62.6°, corresponding to the (111), (200), and (220) crystallographic directions of NiO [38], respectively. It was obvious that the film deposited at 380 °C exhibited preferential growth in the (200) direction, while the film deposited at 480 °C exhibited preferential growth in the (111) direction. Moreover, it is worth noting that no diffraction peaks were observed for the film deposited at 280 °C, indicating an amorphous structure. These results demonstrated that the deposited films not only exhibit changes in crystallinity but also preferential growth in different crystallographic directions with increasing deposition temperature. This is likely because the change in deposition temperature significantly influences the chemical reaction pathways of the precursor, the surface diffusion capability of atoms, and the surface reaction rates and nucleation density, leading to the preferred orientation of the deposited films.

Fig. 3.

The XRD patterns of the deposited films at 380 °C (a) and 480 °C (b). XRD, X-ray diffraction.

To further analyze the chemical composition and bonding states of the elements in the deposited films, the film deposited at 380 °C was selected as a representative sample for XPS analysis. The XPS survey spectra of the films are shown in Fig. 4. As depicted, the films primarily consist of Ni, O, and a certain amount of C. After argon ion etching of the films to a depth of 10 nm, the carbon content decreased to 2.1%, indicating that the C element mainly originates from environmental contamination.

Fig. 4.

The XPS survey spectra of the deposited film at 380 °C. XPS, X-ray photoelectron spectroscopy.

Fig. 5 presents the narrow-scan spectra of Ni and O elements in the deposited films. According to the Gaussian-Lorentzian equation, the Ni 2p_2/3 region can be divided into seven peaks, corresponding to the compositions of NiO and Ni(OH)₂ in the films [39]. The peaks at 854.4 and 856.3 eV, resulting from peak multiplet splitting, can be attributed to Ni²⁺ in NiO; and the peak at 861.7 eV is a satellite peak of Ni²⁺ in NiO, arising from the monopolar charge transfer process accompanying the ionization of Ni 2p electrons [39]. Additionally, the peak at 856.6 eV, and the satellite peak at 862.4 eV can be attributed to Ni²⁺ in Ni(OH)₂, also resulting from peak multiplet splitting and the monopolar charge transfer process [39]. The narrow-scan spectrum analysis of the O element is consistent with that of the Ni 2p narrow-scan spectrum. As shown, the O 1s region can be divided into two peaks at 529.8 eV and 531.7 eV, corresponding to O^2- in NiO and Ni(OH)₂ [39], respectively. It is worth noting that hydroxylation is a common phenomenon for oxide materials [40, 41, 42], and hydroxylation occurs upon exposure of the deposited films to air, resulting in the formation of hydroxides. Obviously, the XPS analysis results are consistent with the XRD analysis results, and all these results strongly demonstrate that the complex Ni(acac)₂(tmeda) meets the requirements of a CVD precursor and can successfully fabricate nickel oxide nano-films.

Fig. 5.

The Ni 2p_2/3 (left) and O 1s (right) XPS survey spectra of the deposited film at 380 °C.

This work systematically investigated the synthesis, chemical properties of Ni(acac)₂(tmeda), and its performance as a CVD precursor for thin film deposition. All results indicate that Ni(acac)₂(tmeda) satisfies the requirements of CVD precursors regarding volatility, thermal stability, stability to moisture and air, and manufacturing cost, and has successfully enabled the preparation of NiO thin film materials via LPCVD. To more directly demonstrate the application advantages of the selected complex as a CVD precursor, a brief table comparing key properties (volatility, stability, cost) of Ni(acac)₂(tmeda) with other common Ni precursors is compiled. As shown in Table 1, the selected complex exhibits remarkable application advantages across three key parameters: volatility, stability, and manufacturing cost.

To elucidate the “off-on” structural characteristics of the selected complex, a speculative structural transformation process is proposed. As shown in Fig. 6, the Gibbs free energy change ($\mathrm{\Delta }$G) required for the transformation of the complex Ni(acac)₂(tmeda) (1) from a hexacoordinate to a pentacoordinate structure (2) at 298 K is 38.9 kJ/mol [36]. This indicates that under the provision of external energy, Ni(acac)₂(tmeda) may transform from a hexacoordinate to a pentacoordinate structure. As mentioned in the introduction, at room temperature, Ni(acac)₂(tmeda) exists in a hexacoordinate structure (off), with high stability to moisture and air. At elevated deposition temperatures, one nitrogen atom in the N,N,N^′,N^′-tetramethylethylenediamine (99%) ligand detaches from the metal center [Ni], resulting in a highly reactive pentacoordinate structure (on).

Fig. 6.

The transformation process of the complex Ni(acac)₂(tmeda) from hexacoordinate to pentacoordinate.

To further verify the “off-on” structural characteristics of the selected complex, a series of experiments was designed. Using Ni(acac)₂(tmeda) as the precursor and O₂ as the reactant gas, a film thickness of 45.7 nm (corresponding to film growth rate of 1.52 nm/min) was obtained at a deposition temperature of 280 °C (Fig. 7a). In contrast, no film deposition was observed when Ni(acac)₂(tmeda) was used as the precursor in the absence of O₂. This demonstrates that the CVD process at 280 °C primarily involves the gas-phase chemical reaction between Ni(acac)₂(tmeda) and O₂, and that Ni(acac)₂(tmeda) does not undergo thermal decomposition at this temperature. Further experiments were conducted under the same deposition conditions using Ni(acac)₂ as the precursor and O₂ as the reactant gas, resulting in a film thickness of 30.8 nm (corresponding to a film growth rate of 1.03 nm/min, Fig. 7b). This thickness is significantly lower than that obtained with Ni(acac)₂(tmeda), demonstrating that Ni(acac)₂(tmeda) exhibits higher reactivity than Ni(acac)₂ during the CVD process at 280 °C. This enhanced reactivity of Ni(acac)₂(tmeda) may be attributed to the fact that Ni(acac)₂ is a trimeric structure [36], which introduces significant steric hindrance around the nickel metal center, thereby reducing its reactivity. In contrast, Ni(acac)₂(tmeda) has a pentacoordinate, unsaturated intermediate structure, which enhances its reactivity. At room temperature, Ni(acac)₂(tmeda) exists in a hexacoordinate structure, exhibiting significantly lower reactivity than Ni(acac)₂. These results on some content demonstrate that the selected precursor Ni(acac)₂(tmeda) possesses high stability to moisture and air at room temperature (off) and high reactivity at elevated deposition temperatures (on).

Fig. 7.

The cross-section SEM images of the deposited films at 280 °C using Ni(acac)₂(tmeda) (a) and Ni(acac)₂ (b) as precursor. Scale bar = 500 nm.

In the present work, the proposed “off-on” structural characteristics of the selected compound were preliminarily investigated based on the comparative experiments and the literature reported structural transformation process. However, an in-depth research on “off-on” structural characteristics, simulation of deposition, optimization of conditions, and quantitative analysis of the film component and mechanism cannot be completed at this stage, and this will be carried out as the core work of the subsequent phase.