Electrochemical Deposition of Manganese Oxides on Carbon Nano-Sheets

Abstract

We have electrodeposited Mn-oxide films on Carbon nano-sheets (CNS), thin (< 10 nm) vertical standing graphitic sheets, having in mind potential energy storage applications of this high-surface-area composite structure. We examined the deposition kinetics of Mn-oxides and physical-chemical properties of Mn-oxide/CNS composite films on substrates ranging from flat to high-aspect-ratio (HAR) Si pillars. We have also explored the possibility to integrate this process into production line typical for microelectronics industry.CNS was grown using RF-PECVD technique [1] onto both blanket 200 mm Si wafers coated with 70 nm thick PVD TiN, and wafer pieces with high-aspect-ratio Si pillars coated with 10 nm ALD TiN. CNS was grown on TiN from CH4/H2mixture [1] to a height of approximately 1 µm.Anodic electrodeposition of Mn-oxide, often also termed electrolytic manganese-dioxide (EMD), was performed on both blanket CNS/TiN/Si and patterned CNS/ALD TiN/Si samples using 0.3 M MnSO4·xH2O + 0.55 M H2SO4aqueous electrolyte. Plating experiments were performed using a three-electrode clip-on Teflon cell connected to a computer controlled Autolab potentiostat PGSTAT100 (Metrohm), with Ag/AgCl as a reference electrode (RE) and Pt mesh as a counter electrode (CE). Characterization of EMD/CNS films was performed using Scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy (RS), and X-ray photoelectron spectroscopy (XPS). Figure 1 shows SEM images of as-grown CNS (a) and EMD coverage as a function of deposition current (b and c). EMD is plated galvanostatically, and potential-time responses were analyzed to help optimize deposition parameters for desired film thickness and structural properties. The analysis was based on the theories of galvanostatic nucleation and growth [2], and proposed reaction mechanisms during galvanostatic deposition of EMD [3]. Further surface area enhancement was attempted through EMD/CNS deposition on HAR Si pillars coated with 10 nm ALD TiN. Figure 2 shows HAR ALD TiN/Si pillars before and after CNS and EMD deposition, respectively. CNS coating was applied to various ALD TiN/Si structures having different aspect ratios and spacing between pillars, and then followed by EMD deposition.A typical Raman spectrum of our EMD/CNS structure, shown in Figure 3, contained broad features in the range 450 to 730 cm-1 where characteristic EMD peaks are expected [4 and references therein], and showed characteristic CNS peaks in 1000 to 3500 cm-1range [5]. Thanks to its sensitivity to local structure/symmetry Raman spectroscopy proved to be a useful tool for characterization of disordered and defective EMD. We were able to detect EMD structural changes upon annealing at various temperatures and lithiation in anhydrous propylene carbonate solvent with 1 M LiClO4, and also monitor the effects of plating and post-plating processing on CNS.