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Tunable Synthesis and in Situ Growth of Silicon-Carbon Mesostructures Using Impermeable Plasma


Tunable Synthesis and in Situ Growth of Silicon-Carbon Mesostructures Using Impermeable Plasma

Alireza Yaghoubi et al. Sci Rep.


In recent years, plasma-assisted synthesis has been extensively used in large scale production of functional nano- and micro-scale materials for numerous applications in optoelectronics, photonics, plasmonics, magnetism and drug delivery, however systematic formation of these minuscule structures has remained a challenge. Here we demonstrate a new method to closely manipulate mesostructures in terms of size, composition and morphology by controlling permeability at the boundaries of an impermeable plasma surrounded by a blanket of neutrals. In situ and rapid growth of thin films in the core region due to ion screening is among other benefits of our method. Similarly we can take advantage of exceptional properties of plasma to control the morphology of the as deposited nanostructures. Probing the plasma at boundaries by means of observing the nanostructures, further provides interesting insights into the behaviour of gas-insulated plasmas with possible implications on efficacy of viscous heating and non-magnetic confinement.


Figure 1
Figure 1. A few of many meso and nanostructures synthesized using IPS.
a) Sharp transition from microcrystals to nanocrystals at the boundary layers is clearly seen. b) A magnified view of microcrystals showing their perfectly faceted structure. c) Spatially well-defined growth of nanodendrites in the vicinity of ionization region is due to the presence of fast neutrals and diamagnetic properties of this layer. d) A magnified view of the nanodendrites with a depth of several microns. e) Uniform growth of graphitic carbon on an as-synthesized SiC crystal in the core region. Exfoliation and the consequent multi layer graphene are visible. f) Spherical quantum dots synthesized by adding carbon dioxide to the argon atmosphere. Large size of some of the dots (~200 nm) is associated with subtle concentration of impurities (see Fig. 2.a). g) The exotic nano-octopus structures synthesized under oxygen atmosphere show strong charging effects because of their large oxide content. Intricacy of this particular morphology makes it a good radiation absorbent. h) Pyramidal quantum dots synthesized by cyclic exposure of SiC microcrystals to boundary plasma. Size tuning of these structures should be possible by controlling the permeability during the second exposure. Note the well-defined interface and the unreacted region to the right. i) Nanowires formed through treatment of nanocrystals with boundary plasma have a uniform diameter of ~10 nm.
Figure 2
Figure 2. Energy dispersive spectra of selected structures manifests morphology- and size-dependency on composition.
The insets are secondary electron images of the corresponding structures. a, b) EDS for large (~200 nm) and small dots (~40 nm) are shown. Note the presence of impurities in large dots. c) Spatially well-defined growth of nanodendrites with large carbon contents indicates how the screening effect confined the carbon ions within the core. d) Significant oxygen content of nano-octopus structures is clearly detected. Lack of large concentration of carbon can be related to heavy oxidation.
Figure 3
Figure 3. Advantage of impermeable plasma over permeable plasma: All samples are 2 cm in diameter.
a) IPS at 100A exhibits great yield despite of low current discharge. The secondary source of energy, viscous heating was dominant. The boundary acted as a physical barrier and the influx of cold gas pushed the plasma to the opposite side. See Appendix H for a schematic diagram. b) With a negative influx, the plasma is largely permeable and therefore no viscous heating takes place. Even though a larger discharge current of 300A was applied, the yield is minimal. Note the reaction zone is nearly centred and the thick boundary regions are not well-defined meaning that neutrals could readily penetrate deep into the core. c) Raman spectrum of SiC crystals (thicker line) in the boundary region versus that of carbon nanodendrites of the ionization region is given. In situ growth over as-synthesized crystal is evident. The inset depicts removal of carbon nanostructures to reveal the SiC crystals beneath.

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