Atomic Layer Deposition Of Cobalt Oxide And Lanthanum Cobalt Oxide Thin Film And Coating Properties Study On 3-dimensional Structure

In recent year, atomic layer deposition(ALD) has attracted much attention due to its precise film thickness control in atomic scale, large scale film uniformity and excellent conformity in high aspect ratio substrates. One of the applications of ALD deposited film is used as catalytic layers. Due to the self-limiting nature, ALD is uniquely suited to the deposition of conformal catalytic thin film layers on nanostructure substrate. By controlling the number of deposition cycles, the thickness of the catalytic layer can be tuned accurately. To optimize the application of this technique, two ways need to be developed. First is thoroughly study the growth behavior of novel catalytic thin film, which can give guidance for further application. Second is by choosing proper template materials to evaluate the coating conformity of the catalytic layer onto three dimensional nanostructures.In this work, we focus on the developing and optimizing the deposition of two kinds of catalytic thin films. One is cobalt oxide film; the other is the lanthanum cobalt oxide film. To evaluate the coating conformity and film properties, we choose three kinds of template materials: anodic aluminum oxide(AAO), anodic titanium oxide(Ti O2 NTs) and nano porous gold(NPG) as the three dimensional substrates. The main results are as follows:1. Three kinds of template materials(AAO, Ti O2 NTs and NPS) were successfully synthesized. The detailed morphology and structure features of templates were characterized by SEM and TEM and the structure differences are discussed. Based on the traditional through-hole AAO template, we optimized the experimental condition and demonstrate the formation of “fish-bone” like AAO nanochannels and show its superior hydrophilicity which allows improved pore filtration for template synthesis.2. Ultrathin cobalt oxide(Co Ox) films(<10 nm) have been prepared on both planar and three dimensional substrates by ALD using Co(Cp)2 and O3 as precursors. The optimal temperature window was 150~250 °C with a saturated growth rate of ~0.37 ?/cycle. Although the thicker film is verified to be Co3O4 by Raman spectrum, the compositional analysis of ultrathin film by X-ray photoelectron spectrum(XPS) shows the mixture of Co3O4 and Co O forms at the initial growth stage. Atomic force microscopy(AFM) and high-resolution transmission electron microscope(HR-TEM) characterization reveal that high deposition temperature will lead to the formation of crystalline islands and cause uneven coating, whereas the smooth and uniform coating can be obtained at a lower temperature in the temperature window. The relatively low density of nucleation sites and fast crystallization speed at elevated deposition temperature are the main origins of the observed uneven morphology. These results could guide us to explore the potential applications of ultrathin cobalt oxide films.3. Composite Co3O4/Ti O2 nanotube arrays(NTs) were fabricated via ALD of Co3O4 thin film onto well-aligned anodized Ti O2 NTs. The microscopic morphology, composition and interfacial plane of the composite structure were characterized by scanning electron microscopy, energy dispersion mapping, X-ray photoelectron spectra, and high-resolution transmission electron microscopy. It was shown that the ultrathin Co3O4 film uniformly coat onto the inner wall of the high aspect ratio(>100:1) Ti O2 NTs with its thickness precisely controlled by the number of ALD deposition cycles. The composite structure with ~4 nm Co3O4 coating revealed optimal photoelectrochemical(PEC) performance in the visible-light range(?>420 nm). The photocurrent density reaches as high as 90.4 ?A/cm2, which is about 14 times that of the pristine Ti O2 NTs and 3 times of impregnation method. The enhanced PEC performance could be attributed to the finely controlled Co3O4 coating layer that enhances the visible light absorption, maintains large specific surface area to the electrolyte interface and facilitates the charge transfer.4. ALD of lanthanum oxide film were synthesized on silicon and AAO substrates using lanthanum 2,2,6,6-tetramethyl-3,5- heptanedione(La(thd)3) and O3 as precursor. The effect of ALD parameters(including pulse times, substrate temperature) on the growth rate and film composition was investigated. Besides, AAO template was used to evaluate the coating uniformity. The XPS results demonstrate that the film contains nearly no impurity elements. EDX mapping measurements show a homogeneous deposition of lanthanum oxide on the on AAO with aspect ratio as high as 500:1. Finally, with the base of ALD cobalt oxide and ALD lanthanum oxide, we synthesized the lanthanum cobalt oxide films by using alternate pulse method. By tuning the pulse ratio, lanthanum cobalt oxide film with different La:Co ratio range of 4:1 to 1:4.2 was adjusted. The surface roughness of lanthanum cobalt oxide film increases with the increase of Co content.