(38e) Gas Phase Coating of Germanium Nanoparticles with Silicon

Nanocrystalline silicon (Si) and germanium (Ge) are promising materials for the application in printable electronics (Holman et al., 2010). Nanoparticles (NPs) of these materials can be dispersed in organic liquids and deposited as thin functional films. Especially hybrid structures made of both materials provide a great opportunity to manipulate the material properties. In previous works we have already shown the coating of Si NPs with Ge to produce defined patches on the particles (Mehringer et al., 2014). In this work we report our progress in coating Ge NPs with silicon. A major problem of Ge NPs is the high surface reactivity. Ge oxidizes very quickly and the oxide has a poor chemical stability. GeO₂ e.g. is highly dissolvable in water in contrast to SiO₂. Therefore, passivation of Ge NPs with silicon is a promising strategy to ensure high device performances (Hunter et al. 2017).

For the experiments, a two stage hot-wall reactor (HWR) setup combined with an external seeding unit is used. This unit consists of a hot wire generator (HWG) producing small metallic seed particles which are essential for the growth of well-defined Ge NPs in the first stage (HWR I). The seed particle stream is directly mixed with the precursor monogermane and injected into HWR I. In the second stage (HWR II) monosilane is added for coating the Ge NPs with silicon. In the first stage, the temperature is kept constant at 800 °C while in stage 2 it is varied in the range of 600‑1000 °C. At the reactor exit the aerosol is quenched by nitrogen. The particles sampled via a low-pressure impactor by deposition directly on substrates or via a filter.

The HWG offers the great advantage of precise control over the seed particle properties and thus over the final Ge NPs. The seed particles suppress the homogenous nucleation which would otherwise lead to high nucleation rates and small agglomerated particles. With the external seeding unit the spherical Ge NP synthesized in HWR I show a narrow size distribution (geometric standard deviation GSD < 1.1). The mean particle diameter can be varied in the range of 25 to 50 nm. These particles serve as cores for silicon coating in the second stage. The morphology of the coated particles is investigated by SEM or TEM images. On the germanium cores patches of silicon are visible instead a complete layer. This can be explained by energetically favoured sites for the initial island growth. Due to the slight crystal lattice misfit, the Si grows preferentially on itself and only partially around the Ge core. An increasing synthesis temperature leads to only one big patch because of the higher mobility of the adatoms. Therefore, the patch formation can be divided into two regimes. On the one hand a diffusion dominated regime at high temperatures and on the other hand a nucleation dominated regime at low temperatures. Furthermore, the patch number and size strongly depends on the Si concentration. A higher concentration leads to a decrease in the number of patches while simultaneously increasing the patch volume.

The progress on coating Ge NPs with silicon will be demonstrated, regarding the influence of the process parameters e.g. temperature, pressure, precursor concentration etc. Furthermore the growth mechanism will be discussed in detail.

This work was supported by the German Research Council (DFG) and the Cluster of Excellence “Engineering of Advanced Materials” (EAM).

C.Holman, Z. and Kortshagen, U.R. (2009). Solution-processed germanium nanocrystal thin films as materials for low-cost optical and electronic devices. Langmuir, 25(19):11883–11889.

Mehringer, C., Wagner, R., Jakuttis, T., Butz, B., Spiecker, E., and Peukert, W. (2014). Gas phase synthesis of anisotropic silicon germanium hybrid nanoparticles. J. Aerosol Sci., 67:119–130.

Hunter, K.I., Held, J.T., Mkhoyan, K.A., and Kortshagen, U.R. (2017). Nonthermal Plasma Synthesis of Core/Shell Quantum Dots: Strained Ge/Si Nanocrystals. ACS Appl. Mater. Interfaces, acsami.6b16170.