Synthesis and characterization of carbon-nanowalls from a single-source metal-organic precursor

Nicolas Wöhrl, Universität Duisburg-Essen, Duisburg, Germany
Andre Giese, Universität Duisburg-Essen, Duisburg, Germany
Volker Buck, Universität Duisburg-Essen, Duisburg, Germany
Axel Lorke, Universität Duisburg-Essen, Duisburg, Germany

Carbon nanowalls (CNWs) can be described as vertically aligned carbon sheets with a thickness ranging from several hundred nanometers down to a monolayer of graphene. CNWs have unique characteristics, including high aspect ratio, large surface area, and high chemical stability and mechanical strength. With this combination of properties, CNWs have great potential to be used in devices such as electron field emitters, capacitor electrodes, or as matrix material for catalytic applications.

In this work, well-aligned CNWs with adjustable morphology, density and thickness were synthesized from aluminum acetylacetonate (Al(acac)3) on different substrate materials without the need for a catalyst. The precursor is low-cost, non acidic and does not require the usage of hydrogen and organic gases for the carbon deposition. Hence, the presented method can also be promising for industrial applications.

The CNWs were deposited on silicon, nickel, aluminum and steel substrates using inductively coupled plasma-enhanced chemical vapor deposition. The solid aluminum acetylacetonate (Al(acac)3) precursor was sublimated in an evaporation source at a constant temperature of 127 °C and then transported to the reaction chamber using argon as a carrier gas. Different deposition conditions were realized by systematically varying the process parameters like the argon gas flow rate (28 sccm to 55 sccm), the substrate temperature (250 °C to 665 °C) and the substrate bias (0 V to -100 V).

The morphology of the films was characterized by scanning electron microscopy (SEM). To investigate the bond-structure of the films, Raman spectroscopic measurements were carried out using a laser operating at 633 nm. The chemical properties of the films were investigated by Auger spectroscopy, and transmission electron microscopy (TEM) was used to evaluate the microstructure of the films.

It was found that the growth rate as well as wall density and thickness can be adjusted by the process parameters. The deposition parameters are correlated with the resulting structure of the carbon nanowalls, so that a first growth model can be established. The aluminum is present inside the carbon nanowall matrix in the form of well-crystallized nanosized Al4C3 and Al2O3 precipitates.

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