AVS2016 Session PS+2D-TuA: Plasma Processing for Nanomaterials and 2D Materials

The remarkable stability of microplasmas at atmospheric pressure and their non-thermal operation facilitate the introduction of liquids such as water for water treatment, medical, and material applications. Recently, there has been interest in the reduction of metal salts in aqueous solutions by microplasmas to produce colloidal nanoparticles (NPs). It is generally accepted that some active species from the plasma react with the solution phase and either directly reduce the metal cation or produce a reducing species. However, it remains unclear how exactly the metal cation is reduced to produce NPs.

In this study, we carried out experiments to understand the formation of silver (Ag) and gold (Au) NPs from their respective metal salt precursors with or without stabilizing capping molecule by reactions at the interface of a microplasma and the aqueous solution phase. The NPs were characterized after synthesis by ultraviolet-visible (UV-vis) absorption spectroscopy and transmission electron microscopy (TEM), and the chemical composition of the solution was characterized before and after microplasma treatment by ionic conductivity, electrochemical potential, and UV-vis absorption spectroscopy.

Our results show that both Ag and Au NP formation are directly proportional to the plasma current and process time. The calculated reduction efficiency based on the number of electrons injected and the number of Ag⁺ reduced is only ~50% while the reduction efficiency for the Au precursor was ~25%. Another difference between the two metals is that plasmon band for Au was found to increase even after the plasma treatment was stopped. This was corroborated by a measured decrease in the concentration of the Au complex, confirming that reduction continues to occur without the plasma, presumably because of a long-lived reducing species generated in solution.

Assuming electrons are the important charge carriers, electrons can reduce metal cations, but can also reduce water to form OH radicals which in turn react to form hydrogen peroxide (H₂O₂). H₂O₂ is known to be a weak reducing agent and could also reduce the metal salts but based on our experiments and reduction potentials, we believe that H₂O₂ could only induce the formation of Au NPs.

To gain more insight into the different mechanisms involved, the gas phase above the plasma-liquid system was analysed by optical emission spectroscopy (OES); a variety of species were observed (OH, O, H, NO, N₂) and ultimately linked to the reactions occurring in the liquid phase.

This work was supported by the Belgian Federal Government (IAP research project P7/34 – Physical Chemistry of Plasma Surface Interactions).

Titanium nitride is a refractory material with optical properties similar to those of gold. It has therefore attracted significant interest, since TiN nanoparticles are expected to show localized surface plasmon resonance in the visible/near-infrared range, all while overcoming the cost and thermal stability limitations of gold. For instance, they are a very attractive substitute of gold nanoparticles in biomedical applications [1]. Most of the methods involving TiN nanopowder synthesis use effective but complicated chemical routes [2,3]. In this contribution, we present a highly scalable method for the production of TiN nanoparticles using a non-thermal plasma process. A low-pressure non-thermal plasma reactor is used to continuously nucleate and grow crystalline TiN nanoparticles starting from a mixture of ammonia and titanium tetrachloride. Besides achieving a remarkable production rate (~50 mg/h), we were also able to control the particle size and stoichiometry with great precision by tuning process parameters such as gas composition and plasma input power. This finding is of paramount importance because the plasmonic peak position is highly dependent on these two parameters [4]. Absorption measurements of the as-synthesized particles show clear plasmonic resonance in the near-infrared region, ranging between 800 and 1000nm when dealing with the largest and smallest particles, respectively. XRD and high resolution TEM/EDS characterization also provides insight on the nitrogen content of the samples and its close correlation to particle size. The role of process parameters on the surface of the particles, which in turn affects their plasmonic properties, will be discussed extensively.

References

[1] L.R. Hirsch, R.J. Stafford, J. a Bankson, S.R. Sershen, B. Rivera, R.E. Price, et al., Nanoshell-mediated near-infrared thermal therapy of tumors under magnetic resonance guidance., Proc. Natl. Acad. Sci. U. S. A. 100 (2003) 13549–54. doi:10.1073/pnas.2232479100.

[2] F. Liu, Y. Li, Y. Yao, H. Zhang, W. Shao, Y. Kang, et al., Preparation of titanium nitride nanoparticles from a novel refluxing derived precursor, J. Wuhan Univ. Technol. Sci. Ed. 26 (2011) 429–433.

[3] S. Kaskel, K. Schlichte, G. Chaplais, M. Khanna, Synthesis and characterisation of titanium nitride based nanoparticles, J. Mater. Chem. 13 (2003) 1496.

[4] U. Guler, S. Suslov, A. V. Kildishev, A. Boltasseva, V.M. Shalaev, Colloidal Plasmonic Titanium Nitride Nanoparticles: Properties and Applications, Nanophotonics. 4 (2015) 269–276. doi:10.1515/nanoph-2015-0017.

This presentation discusses the physics of plasma synthesis process. High photoluminescense quantum yields are achieved by careful surface functionalization through grafting alkene ligands to the nanocrystal surfaces. We also discuss the substitutional doping of silicon nanocrystals with boron and phosphorous using a nonthermal plasma technique. While the synthesis approach is identical in both cases, the activation behavior of these two dopants is found to be dramatically different. Finally, we present some experimental work on transport in films of highly phosphorous-doped nanocrystals, which indicates the approach to the metal-to-insulator transition.

This work was supported in part by the NSF Materials Research Science and Engineering Center under grant DMR-1420013, the DOE Energy Frontier Research Center for Advanced Solar Photophysics, and the Army Office of Research under MURI grant W911NF-12-1-0407.

Provided the fact that the monomer to deposit possesses free-radical polymerizable bonds, initiated PECVD (iPECVD) promotes conventional chain-growth polymerisation pathways, ensuring an excellent retention of the monomer structure. Several works have highlighted the radical polymerizability of the exo-pyrrole double bond of the porphyrin rings. The propagation reaction occurs at the beta-position of the porphyrin ring, leading to the formation of a polymer of reduced porphyrins, i.e. chlorin. Based on this wet-chemistry result, we recently investigated the use of zinc (II) meso-tetraphenylporphyrin (ZnTPP) building units in an iPECVD process and formed a new kind of metal-organic covalent network (MOCN) thin films.

Graphene is the lightest and strongest known material, and is also an ideal thermal and electrical conductor. Despite its unique properties, graphene has to be patterned to achieve its full engineering and nanotechnological potential. Recent experiments show that a monolayer of graphene deposited on an SiO2 substrate and subjected to hydrogen plasma treatment either undergoes (a) selective etching from the edges of the graphene sheet while leaving the basal plane intact, or experiences (b) etching of both the edges and basal plane of the graphene sheet which results in the formation of nanoscale holes in graphene. The plasma-etched holes in (b) can be either circular or hexagonal, suggesting that the etching process can be isotropic or anisotropic. Here, we model the hydrogen-plasma etching of monolayer graphene on SiO2 substrates across the range of plasma energies using scale-bridging molecular dynamics simulations. Our results uncover distinct etching mechanisms, operative within narrow hydrogen-plasma energy windows, which fully explain the differing plasma-graphene reactions observed experimentally. Specifically, our simulations reveal very sharp transitions in the etching mechanisms with increasing hydrogen ion energy: selective edge etching at ion energies of ~1 eV, isotropic basal plane etching at ion energies of between 2 and 5 eV, and anisotropic etching at ion energies > 7 eV. Understanding the complex plasma-graphene chemistry and the relationship to plasma process parameters opens up a means for controlled patterning of graphene nanostructures.

Graphene has been a research focus due to its numerous unique properties which have motivated vast interdisciplinary research in the search of materials for next-generation technologies. With its unique atomically thin nature, graphene has enabled a closer look at material surface interactions and highlighted the importance of surface interfaces, defects and adsorbates. Purposeful and native defects have demonstrated to have advantageous or adverse influences on the chemical, electrical, optical, mechanical and even magnetic properties of graphene. On the other hand, control of the localization of defects and their arrangement onto ordered and extended structures has enabled new graphene-based materials with novel properties. Surface functionalization has provided the ability to manipulate the material attributes, offering a range of opportunities for the chemically modified materials. It is clear that fundamental understanding of the modification of graphene relies on the understanding of the chemical functionalization dynamics, kinetic barriers, chemical transitions, and diffusion energies experienced by the added adsorbates with the graphene surface as well as the influence of the substrate on each.

Plasmas provide both ease and versatility by enabling a single tool process on a wide range of background gases. In particular, electron-beam generated plasmas can introduce different functional groups over a large coverage range with atomic layer precision, providing the ability to tailor the locality of the surface chemistry on graphene opening up a wide range of reactivity studies and synthesis capabilities. Such unique ability allows for precise nano-engineering of the surface chemistry effecting local electronic properties, electron transfer kinetics, and surface reactivity; opening up a wealth of opportunities in device performance, chemical sensing, bottom-up material growth, plasmonics, and catalysis applications.

This work is supported by the Naval Research Laboratory Base Program.