Nucleation of titanium nanoparticles in an oxygen-starved environment, I: Experiments

TL;DR

This study demonstrates that titanium nanoparticles can be synthesized in an oxygen-starved environment using pulsed hollow cathode sputtering with high helium partial pressure, avoiding oxidation and enabling stable nanoparticle growth.

Contribution

It introduces a novel method for titanium nanoparticle growth without oxygen, utilizing helium cooling and detailed experimental mapping of process parameters.

Findings

Nanoparticles formed in helium environment without oxygen.

Growth limited below 200 Pa due to low dimer formation.

Nanoparticle production ceases at high argon flows due to increased temperature.

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 ({\alpha}Ti) 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 where 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 [1]. 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.