Plasma As a Tool for Growth of 1D and 2D Nanomaterials and Their Conversions
Authors: Cvelbar Uros , Filipič Gregor , Sunkara Mahendra Kumar 
Published: February 2020

The growth of 1D and 2D nanostructures in low pressure oxygen plasma is presented with the special stress on metal-oxide nanowires and their deterministic growth mechanisms. Since the resulting nanostructures not always have required properties for applications their modifications are required. Therefore their conversions into different oxides or sulphites/nitrides are required with either molecules, atoms, electrons or photons.
There has been tremendous interest and progress with plasma synthesis of 1D and 2D nanostructures like inorganic nanowires (NWs), nanotubes, nanowalls, nanosheets, etc. in recent years. The advantage of plasma growth is not only abundant quantity, but short time-scales of synthesis and many other advanced properties of materials, e.g. single-crystallinity of materials, superstructure of crystal lattices, pureness, etc. Moreover, such materials show improved performance to others nanomaterials synthesized by other methods when tested in devices.
Growing different nanostructures, especially more complex ones like 1D or 2D depends significantly on plasma parameters of interacting plasmas and surface conditions during the growth. The selectivity criteria of plasmas is based mostly on fluxes of charged species and neutral atoms to the surface as well as potential at the surface. All these are further influenced by recombination of plasma species on the surfaces and surface temperatures. Results are seen in creation of nuclei and seeds which lead either to creation of thin layers and nanowalls or nanowire growth. Moreover, in some cases we get also more exotic structures like nanospikes, nanotubes, etc. All this will be demonstrated for the case of different metal oxides (Fe2O3, CuO, etc.) nanostructure growth, especially nanowires where we can create various structures without any deposition from the gas phase but only with reactive gaseous plasma. The metallic substrates are typically exposed to low pressure weakly ionized and highly dissociated oxygen RF plasma at different plasma conditions. The nanowires and nanostructures are grown on the top of thin oxide layer through neutral species and ion recombinations. [1] Results indicate that different densities of the ion current to the side and top area of NW modify the NW growth in height and width. The NW growth is enhanced by presence of ions, and thus implies that it can be controlled by discharge power. [1,2]
However there are many drawbacks to resulting nanostructures connected to their properties or even material type. Looking at state of the art of 1D nanostructures like nanowires, much of the progress only resulted in NWs with diameters much greater than their respective quantum confinement scales, i.e. 10–100 nm. Even at this scale, NW-based materials offer enhanced charge transport and smaller diffusion length scales for improved performance with various electrochemical and photo-electrochemical energy conversion and storage applications. However, many times synthesized materials don’t meet specific properties for optimal performance in certain devices, e.g. band gap energies, or they are even hard to obtain. These deficiencies can be eliminated by additional plasma conversion processes, where we can substitute atoms in crystal networks or controllably release them and create defects. Here the NWs or 1D crystalline nanomaterials of metal oxides with diameters less than 100 nm provide a useful platform for creating new materials either as substrates for heteroepitaxy or through the phase transformation with reaction. Specific results with single crystal phase transformation of e.g. hematite (a-Fe2O3) to pyrite (FeS2) NWs will be presented to illustrate the viability of using NWs for creating new materials through plasma particle reformation. [3] Furthermore, the metal oxide NWs can be altered and modified also by other energetic particles like electrons, which cause irradiation damages and release atoms from crystal networks. [4] This process leads to metal oxides with reduced oxide states. All these processes lead to improved properties, tailored band gaps and increased performance of NWs in devices.

References

[1] G. Filipiè, O. Baranov, M. Mozetiè, U. Cvelbar, J. Appl. Phys. 117 (4) (2015) 04334.
[2] G. Filipiè, O. Baranov, M. Mozetiè, K. Ostrikov, U. Cvelbar, Phys. Plasmas 21 (11) (2014) 113506.
[3] D. R. Cummins, H. B. Russell, J.B. Jasinski, and M.K. Sunkara, Nano Lett., 13 (6) (2013) 2423.
[4] U. Cvelbar, Z. Chen, I. Levchenko, R.M. Sheetz, J.B. Jasinski, M. Menon, M.K. Sunkara Chem. Comm. 48 90 (2012) 11070.