Size dependent characteristics of plasma synthesized carbonaceous nanoparticles

Size dependent characteristics of plasma synthesized carbonaceous nanoparticles Eva Kovacevic, Johannes Berndt, Thomas Strunskus, and Laifa Boufendi Citation: J. Appl. Phys. 112, 013303 (2012); doi: 10.1063/1.4731751 View online: http://dx.doi.org/10.1063/1.4731751 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v112/i1 Published by the American Institute of Physics.

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JOURNAL OF APPLIED PHYSICS 112, 013303 (2012)

Size dependent characteristics of plasma synthesized carbonaceous nanoparticles Eva Kovacevic,1 Johannes Berndt,1 Thomas Strunskus,2 and Laifa Boufendi1 1

GREMI Universite´ d’Orle´ans, Polytech’Orleans, F-45067 Orleans Cedex 2, France Institute of Material Science, Christian-Albrechts-University of Kiel, D-24143 Kiel, Germany

2

(Received 22 February 2012; accepted 31 May 2012; published online 6 July 2012) Low temperature plasmas with their strong non equilibrium character offer unique possibilities for the production of nanoparticles. This contribution deals with size dependent properties of nanoparticles synthesized in a capacitively coupled discharge operated in mixtures of argon and acetylene. X-ray absorption measurements show that the particle properties dramatically change during the growth process. For nanoparticles under 10 nm in diameter, near edge x-ray absorption fine structure spectroscopy shows a sp2 rich graphite-like material. The bonding situation changes with the increasing size of the dust particles, showing the formation of a sp2 poor mantle around the sp2 rich core. This phenomenon can be explained in terms of the nucleation and growth process of nanoparticles, i.e., due to differences in the heating of small nanoparticles (nuclei) and due to differences in the gas phase species involved in the nucleation phase and the surface growth phase. C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4731751] V

I. INTRODUCTION

The formation of nanoparticles in plasmas is a rather ubiquitous phenomenon and can be observed both in terrestrial and cosmic environments. Nano- or dust particles have been observed in fusion plasmas,1,2 in extraterrestrial environments as for example the interstellar medium,3,4 and in processing plasmas used for thin film deposition5–11 or the manufacturing of microelectronic devices as first reported in Ref. 12. Due to their unique chemical and physical properties, nanoparticles are extremely interesting for the rapidly growing field of nanotechnology. They are used as building units for nanoassemblies13 and for the synthesis of novel nanocomposite materials.14 The common request for technological applications is the synthesis of nanoparticles with adjustable characteristics. The non-equilibrium character of low temperature plasmas allows in a simple way the production of nanoparticles and the controlled modification of surfaces. Particle properties as size, shape, chemical composition, and morphology can be controlled by the plasma parameters and by the choice of the precursor material. However, the formation of particles in a low temperature plasma is a rather complex process involving a great variety of different species. Various kinds of positive and negative ions as well as several neutral species can contribute to this process. Moreover the formation of particles and their charging has a severe impact on the plasma itself: the basic plasma parameters as electron temperature and electron density are changing as well as the density of excited states, the spatial profiles of these values, the chemical composition of the plasma, the heating mechanism, the sheath properties, or the plasma impedance.10,15 An essential requirement for a tailored production of dust particles is therefore the understanding and control of the mechanisms that are involved in their formation and growth. 0021-8979/2012/112(1)/013303/5/$30.00

In this contribution, we focus on the formation of nanoparticles in a capacitively coupled hydrocarbon discharge. The formation of nanoparticles in hydrocarbon plasmas is of great technological importance since the various allotropes of carbon can be used as building blocks for nanotechnology and biomedical applications.14,16,17 The research on nanoparticle formation in hydrocarbon plasmas has a long history that has involved till now many researchers (see, e.g., Refs. 18–25, as well as Refs. 10 and 26 and references therein). The research has been focused on both, physical phenomena related, for example, to the response of the plasma to the formation of particles and on chemical aspects dealing with the mechanisms responsible for the particle formation.22,23,27 Despite the great effort made in this field there are still many unresolved issues. Beside the details of the nucleation process one important point concerns the question how the particle properties are changing during the growth process in hydrocarbon plasmas. It is known from discharges operated in silane that even non thermal plasmas can favour the (homogeneous) production of small nanocrystals. In 1994, Boufendi and Bouchoule15 reported about the use of a capactively coupled discharge (operated in a mixture of argon and silane) for the production of silicon nanoparticles that are formed from small (2 nm sized) primary nanocrystallites. Roca i Cabarrocas and co-workers28 used the plasma based formation of thin films and nanocrystals for the production of amorphous silicon films containing inclusions of small silicon nanocrystals. More recently, Kortshagen and co workers29 reported about the nonthermal plasma synthesis of size-controlled, monodisperse, and freestanding germanium nanocrystals. In this paper, we concentrate on the homogeneous formation of carbonaceous nanoparticles. In order to answer the central question how far the particle properties depend on their size, i.e., how much they change during the growth

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Downloaded 25 Sep 2012 to 194.167.30.129. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

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process, we employed the technique of near edge x-ray absorption fine structure spectroscopy (NEXAFS) for the ex situ analysis of particles collected at different stages of their growth. NEXAFS is a very sensitive and element specific diagnostic technique that can be used even for the analysis of molecular monolayers (in particular organic molecules).30 The information depth of this method is in the range