Synthesis of Nanoparticles Using Low Temperature Plasmas and Its Application to Solar Cells and Tracers in Living Body
Authors: Koga Kazunori , Seo Hyunwoong , Tanaka Akiyo , Itagaki Naho , Shiratani Masaharu 
Published: February 2020

Plasma is a promising method for synthesis of nanoparticles which are drawn much attention for opt-electric and medical applications such as solar cells and drug delivery systems. The method can control agglomeration of nanoparticles by utilizing their charge and can produce high purity nanoparticles. Wide variety of plasmas have been developed for synthesizing nanoparticles, allowing us to produce them of many kinds of materials and with a high yield. So far, we have employed three types of plasmas: multi-hollow discharge reactive plasmas for Si nanoparticles, high pressure rf sputtering for Ge ones, and plasma in water for In ones. We have firstly succeeded in demonstrating multiple exciton generation in Si and Ge nanoparticles to realize the third generation solar cells. Using In nanoparticles, we have realized high sensitive tracers for understanding nanoparticle kinetics in living body.
For the multi-hollow discharge reactive plasma [1, 2], discharges were sustained in 8 holes of 5 mm in diameter and 7 mm in length. Nanoparticles were nucleated and grow in the H2+SiH4 discharges and they were transported to the downstream region by gas flow. The discharge frequency and power were 60 MHz and 180 W. The crystallinity and size of Si nanoparticles were controlled by H2 dilution ratio and working pressure [3, 4]. We found crystalline Si nanoparticles can be formed at high H2 dilution ratio and larger size of them are produced for higher pressure. Crystalline Si nanoparticles of 4 nm in size were fabricated under SiH4 and H2 gas flow rate of 2 and 448 sccm, respectively, and pressure of 266 Pa. They were collected by stainless mesh grids located at the downstream region. Transparent conductive oxide/Si nanoparticles/Al structured Schottky cells were employed for characterization of quantum effects. Carrier generation evaluation showed quantum efficiency of the cells show 100 % or more for incident photon energy above two times larger than the bandgap energy of Si nanoparticles. Consequently, multiple exciton generation by synthesized Si nanoparticles was firstly demonstrated [4].
Ge nanoparticles have more outstanding quantum confinement effects than Si nanoparticles due to the larger excitonic Bohr radius of 24.3 nm for Ge nanoparticle than 4.9 nm for Si ones. Ge is also known to exhibit larger absorption coefficient of infrared light than Si. We have fabricated Ge nanoparticle composite films using a high pressure rf sputtering method [5]. X-ray diffraction (XRD) and Raman spectra of the films showed the films with high crystallinity of 0.8. Fourier transform infrared measurements revealed Ge-H and Ge-H2 stretching bonds in the films. These results suggest the crystalline Ge nanoparticles of 6 nm in size are covered by hydrogenated amorphous Ge. We fabricated Ge nanoparticles solar cells in the TiO2/Ge/polysulfide electrolyte system. We found that the incident photo-to-current conversion efficiency increases with decreasing the wavelength, and it reaches 10 % at 400 nm [6]. The result suggests that excitons generated in Ge nanoparticles were separated into electrons and holes, and such carriers successfully extracted to the outer circuit.
For medical applications of nanoparticles, their toxic effects are frequently pointed out. We have employed indium-contained nanoparticles for studying kinetics of nanoparticles in the living body because no In compound exists in living body. For In nanoparticle synthesis, In rod and plate electrodes were immersed into pure water. Discharges were generated by applying pulsed voltage between the electrodes. The discharge voltage and frequency was 9.2 kV and 10 kHz, respectively. The size of primary and secondary nanoparticles was 7 nm and 315 nm, respectively. The yield was 42 mg/min. Energy dispersive X-ray spectroscopy and XRD measurements showed that indium crystalline and indium hydroxide crystalline particles were generated with the mass ratio of 8:2. Preliminary subcutaneous administration of nanoparticles to mice shows that indium is transported from subcutaneous to blood. Comparing with In-contained nanoparticles of 150 nm in primary particle size, nanoparticles for smaller primary particle size indicated more prompt transportation. These results show that synthesized indium-containing nanoparticles are useful for analyzing kinetics of nanoparticles in living body [7].
In summary, plasma expands the capability of the nanoparticle synthesis towards opt-electronics and life science.
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