Session GT2: Plasmas and Nanotechnology I

High-power pulsed gas-flow sputter synthesis of nanoparticles, core/shell nanoparticles, and extended chain-like complexes

Nanoparticles play an important role in many industrial applications. Therefore, the development of new complex nanoparticles, and the methods for their synthesis, are very important research topics. Complex nanoparticles, including core/shell, core/satellite, composition gradient and Janus particles, can be designed to obtain a wide range of novel properties. The pulsed hollow cathode plasma-based method used in the present work is very flexible and can grow heterogenous particles from any combination of materials that can be sputtered and can be used in combination with reactive gases. The method is unique in that close to full ionization of the source material is achieved through the very high plasma-density achieved. The ionization leads to a high rate of material entrapment on the growing nanoparticles reducing the material loss to a minimum. In this presentation the method is exemplified by Ni/Pt alloys and Ni/Pt core/shell nanoparticles assembled into nanowires for use as electrodes in hydrogen generation. As an example, Ni nanoparticles (50 nm in diameter) are coated with Pt shells of controlled thickness in the 1-3 nm range.   

Control of Schottky barrier height for efficient fabrication of graphene nanoribbon-based quantum dot devices

Recently, we demonstrated the fabrication of graphene nanoribbon (GNR)-based quantum dot devices by advanced plasma CVD with one-dimensional Ni nanobar as a catalyst [1-2]. Detailed measurements at cryogenic temperatures using liquid helium revealed the formation of a fine structure in the centre of the GNRs, which has the characteristics of quantum dots. However, the use of a confinement barrier naturally formed within GNR made its reproduction difficult. In this study, the confinement barrier height of the device was actively controlled by introducing two-step electron-beam lithography, which can change the kinds of nanobar and electrode metal. Detailed measurements revealed that the Schottky barrier height between the electrodes and GNRs was varied by changing the metal species used for the electrodes. This suggests that the Schottky barrier height control between the electrodes and GNR may useful to increase the fabrication yield of GNR-based quantum dot devices.

    

Understanding of monolayer WS2 nucleation by in-situ monitoring CVD

Transition metal dichalcogenide (TMD), two-dimensional material, is expected to be promising candidates for novel transparent flexible devices because of their excellent electrical and optical properties. In terms of the industrial applications, there are still remained several issues to be solved in the growth stage of TMDs, which can be overcome by clarifying the synthesis mechanism of monolayer TMD. We have recently succeeded in developing the in-situ monitoring CVD, which can contribute to uncovering the detailed growth mechanism of TMD [1].

In this study, we improved the in-situ monitoring CVD and introduced auto-image analysis to quantitatively analyze the initial growth stage. Systematic investigations reveal that the growth of monolayer tungsten disulfide (WS₂) is mainly dominated by not vapor-solid (VS) transition but liquid-solid (LS) transition. The incubation time (?t_i), which is the time required for the nucleation of monolayer WS₂, was found to be strongly influenced by the substrate temperature. These results are in good agreement with the time-temperature-transformation (TTT) curve, which is known as a classical three-dimensional crystal growth model. Furthermore, detailed image analysis suggested that the nucleation of WS₂ follows two-step nucleation, which is known as a non-classical nucleation [2]. These findings can be useful to introduce plasma reaction in TMD synthesis for future high quality and single crystal synthesis of TMD in large scale.

    

Coagulation and Condensation Rates in Si Nanoparticle Growth at Different Feeding Durations of Feedstock Using Tandem Modulated Induction Thermal Plasmas

Numerical study was conducted on the distribution of temperature, gas flow velocity and Si nanoparticle density in tandem-type modulated induction thermal plasmas (tandem-MITP) with time-controlled feedstock feeding (TCFF) of Si at different feeding durations. This tandem-MITP+TCFF method has been confirmed for high efficient evaporation of feedstock during on-time and also high efficient nucleation and particle growth suppression during off-time. In this report, condensation and coagulation rates were paid attention for nanoparticle growth because it can be affected by feeding duration. These rates are very important factors in nanoparticle growth. It was shown that Si nanoparticles were more efficiently generated in tandem-MITP, and their growth due to coagulation could be suppressed under the condition of 40% duty factor of feedstock feeding.   

Highly efficient exosome capture by carbon nanowalls template

 Carbon nanowalls (CNWs) are wall-like nanostructured materials consisting of multi-layered graphene sheets vertically standing on substrates. The CNWs have unique properties such as large surface area and high electrical conductivity and are expected to be applied in biology.

 In this study, highly efficient capture of exosomes, one of the extracellular vesicles, was performed employing the CNWs template. the zeta potential of CNWs was also measured to discuss electrostatic interactions between the CNWs and exosomes.

 The zeta potential of CNWs was -23 mV, and that of exosomes is known to be negative. Therefore, it can be inferred that the exosome capture efficiency on the CNWs was reduced due to Coulomb repulsion. However, the actual capture efficiency of exosomes on the CNWs was over 90 %, which is higher than those in the previous studies. The maze-like structure of CNWs may have affected the capture efficiency of exosomes. In other words, the capture efficiency of exosomes on the CNWs can be further improved by shifting the zeta potential of CNWs positively by the plasma surface modification.