Nucleation of titanium nanoparticles in an oxygen-starved environment. I: experiments

Abstract

A constant supply of oxygen has been assumed to be necessary for the growth of titanium nanoparticles by sputtering. This oxygen supply can arise from a high background pressure in the vacuum system or from a purposely supplied gas. The supply of oxygen makes it difficult to grow metallic nanoparticles of titanium and can cause process problems by reacting with the target. We here report that growth of titanium nanoparticles in the metallic hexagonal titanium (αTi) phase is possible using a pulsed hollow cathode sputter plasma and adding a high partial pressure of helium to the process instead of trace amounts of oxygen. The helium cools the process gas in which the nanoparticles nucleate. This is important both for the first dimer formation and the continued growth to a thermodynamically stable size. The parameter region, inside which the synthesis of nanoparticles is possible, is mapped out experimentally and the theory of the physical processes behind this process window is outlined. A pressure limit below which no nanoparticles were produced was found at 200 Pa, and could be attributed to a low dimer formation rate, mainly caused by a more rapid dilution of the growth material. Nanoparticle production also disappeared at argon gas flows above 25 sccm. In this case the main reason was identified as a gas temperature increase within the nucleation zone, giving a too high evaporation rate from nanoparticles (clusters) in the stage of growth from dimers to stable nuclei. These two mechanisms are in depth explored in a companion paper. A process stability limit was also found at low argon gas partial pressures, and could be attributed to a transition from a hollow cathode discharge to a glow discharge.

Nucleation of titanium nanoparticles in an oxygen-starved environment. II: theory

“…The theoretical understanding of how and influence the nucleation process, marked with question marks, is the main subject of the present paper. (c) Four different types of limits for nucleation of nanoparticles, marked (1), (2), (3), (4), as experimentally found in ( , ) surveys [4] [6]. Figure 1 (a) illustrates five process parameters that will be discussed in this paper: the argon gas flow Ar , the helium gas flow He , the pressure , and the fluxes Ti and Ti + , into zone 1, of growth material.…”

“…The role of helium in the discharge is interesting. Adding helium is necessary because, without helium, no nanoparticles are produced [4]. However, also argon is needed, since a discharge in pure helium gives no nanoparticles without oxygen.…”

“…However, also argon is needed, since a discharge in pure helium gives no nanoparticles without oxygen. A mix of the two gases is consequently needed, but this also causes problems in the form of a discharge instability which limits the process window [4].…”

“…We therefore call this oxygen-assisted nucleation, and call the limit marked (1) in the ( , Ar ) survey the "oxygen limit" for nucleation. In the experiment in [5] the oxygen limit for nucleation had the form Ar >1.3 [Pa/sccm] (1) In the UHV system studied in the experimental companion paper [4], the nanoparticles is produced at such low impurity concentration that they obtain the metallic Ti-phase, far below the oxygen limit for nucleation (1) found in [5]. Here, two other types of limits for nucleation are identified in the ( , Ar ) survey.…”

“…The analysis here is based on results from two experiments, one in a high vacuum system [5] with nucleation in an oxygen-containing environment, and one in a ultra-high vacuum (UHV) system [4]. In the latter, helium replaces oxygen as being necessary for nucleation.…”