Making use of plasma-based methods for synthesizing nanoparticle offers a number of advantages like avoidance of agglomeration of nanoparticles in the gas phase, direct deposition on substrates maintaining their single particle properties or flexibility in the choice of materials as both chemical and physical means can be used to provide source material for the growth of nanoparticles. However, a major problem of using a sputter deposition technique for synthesizing nanoparticles is the slow growth rate of nanoparticles. By fostering growth utilizing ions, we showed that the growth rate can be increased by a factor of 100 [1] offering a strategy to overcome the slow growth rates of nanoparticles in sputter sources.
The potential to increase the growth rate of nanoparticles using ions is a consequence of the interaction of nanoparticles with the plasma environment; a nanoparticle embedded in a plasma attains a negative potential which in turn increases the probability for positively charged ions being collected by a nanoparticle. For fostering growth by collection of ions, the degree of ionization needs to be increased in order to provide a high density of ions and therewith making collection of ions a predominant process.
Low temperature plasmas contain electrons, ions and neutral atoms with, typically, a low degree of ionization (i.e., ratio between ions and neutral atoms). For sputtering, the plasma is used to generate ions that are accelerated to the cathode (i.e., target), and upon collision with the target surface atoms and ions are removed. The sputtered atoms and ions provide then the source material for nanoparticle growth. The nanoparticles grow by collection of both neutral atoms and ions; while the collection probability of neutral atoms is given by the cross section (area) of the nanoparticle, the cross-section for collection of (positively charged) ions is a function of the (negative) potential of a nanoparticle and can by a factor of 100 be larger than the cross section of neutrals. We could show that the growth rate can be increased by fostering collection of ions to a value of 470 nm/s compared to a value of 3 nm/s for collection of neutrals [1]. This was achieved by employing a pulsed operation of the plasma with high power pulses to increase the ionization degree. Further, we could show that the pulsing can be used as an external control parameter for controlling the size of nanoparticles [2]. Thus, the utilization of ions for nanoparticle growth offers not only a faster growth rate and therewith an option for a more efficient consumption of source material, but also an additional control as the motion of ions is easier to control compared to neutral atoms.
A need for a process where the nanoparticle growth and, hence, their properties can be controlled is generated by the variety of applications where nanoparticles have been utilized nowadays and found to improve the performance. Those applications cover a diverse field that ranges, e.g., from energy applications like fuel cells and solar cells over sensors to medical applications. For each application, the characteristic features that a nanoparticle should posses differ and need to be tailored for the specific requirements that the application imposes on them. In order to address this challenge, a synthesis method that is flexible enough to produce nanoparticles with different characteristic properties requires that nanoparticles made of different materials and with control over their size, shape and material compositions can be produced.
A gas phase technique like sputtering offers these requirements. Size and shape can be controlled via controlling the growth environment. The circumstance that the nanoparticles become negatively charged prevents agglomeration in the gas phase; this allows for deposition of single nanoparticles on substrates, which is an advantage when the single particle properties of nanoparticles need to be retained. The material range is very flexible because the target material can easily be changed or reactive gases can be added. In principle, there is a possibility for scale-up since the technique is well-known for deposition of thin films on an industrial scale.
The goal of further work is to deepen the understanding of the growth conditions and how the growth of nanoparticles by collection of ions can be manipulated. A control over the growth environment will enable one to synthesize complex nanoparticles whose properties can be tailored. This in turn would allow to synthesize novel materials with unique properties that can be utilized in applications in the future.
Further reading
[1] I. Pilch, D. Söderström, M. I. Hasan, U. Helmersson, and N. Brenning, ”Fast growth of nanoparticles in a hollow cathode plasma through orbit motion limited ion collection,” Appl. Phys. Lett. 103, 193108 (2013); http://dx.doi.org/10.1063/1.4828883
[2] I. Pilch, D. Söderström, N. Brenning, and U. Helmersson, “Size-controlled growth of nanoparticles in a highly ionized pulsed plasma,” Appl. Phys. Lett. 102 , 033108 (2013) ; http://dx.doi.org/10.1063/1.4788739