Chair for Multicomponent Materials

In-situ tuning and control of the composition of alloy nanoparticles

Alloy nanoparticles are of particular research interest due to their promising catalytic and optical properties. In contrast to monometallic nanoparticles, the potential to tailor the nanoparticle composition adds another possibility to control their properties.

In 2017 Dr. Alexander Vahl introduced the multicomponent target approach in a gas aggregation cluster source (GAS). A multicomponent target consists out of a conventional sputtering target of a first material and one or more wires of a second material, which are embossed into a concentric trench at the location of the target erosion zone (Figure 1). This enables to tune the composition in-situ in the GAS by adjusting the inert gas flow and in turn the pressure. The pressure changes the width of the erosion zone and so the erosion zone is at high pressures smaller and at low pressures broader. This is schematically depicted in Figure 1. So for high pressures more wire material is sputtered and for low pressure more from the bulk target material. [1]

Figure 1: Photograph of the AgAu target after usage (top left) together with a schematic cross-section through the target (bottom left). The working principal for high and low gas pressures inside the GAS are shown (right). [1]

 

After several studies it turned out that due to redeposition of nanoparticles onto the target, a “target history” effect comes into play. That means that the composition is not adjustable linearly with a change in pressure because when the pressure is reduced redeposited material is also sputtered. To solve this issue M.Sc. Jonas Drewes and Dr. Alexander Vahl developed in 2020 an in-situ control approach for the multicomponent target approach in a GAS. They used in-situ optical emission spectroscopy together with ex-situ XPS to find a fitting function, which could be used in the end to determine the composition of the nanoparticles precisely (Figure 2). By that they solved the unwanted effect of redeposition onto the composition of the deposited nanoparticles. This method has the potential to be combined with a feedback loop in a way, that the operator only has to set the desired composition and then the flow is adjusted in a way that the desired composition of nanoparticles will be produced. [2]

Figure 2: Shown are Au concentration from fit vs. the Au concentration from the XPS. The calibration measurements are shown in blue and the test samples are shown in red. [2]

The multicomponent target approach has many advantages:

  • in-situ control of nanoparticle composition without the need for target replacement.
  • Reduce material costs, because the more expensive material can be placed directly in the center of the erosion zone.
  • After calibration, the OES can be used to determine the true composition in-situ without further composition measurements.
  • Enables detailed studies on the properties of alloy nanoparticles which are depending on their composition.
  • Compared to other gas aggregation related approaches for the deposition of alloy nanoparticles, such as the use of multiple magnetrons inside the GAS, the multicomponent target approach is a single plasma process. No effects of interacting plasmas from different sources have to be taken into consideration.

Today, this approach is routinely applied in the production of a variety of functional nanocomposites and multicomponent materials, including memristive devices and sensors. Also different target wire material combinations are tested meanwhile. AuAg was the first analyzed combination and afterwards also AgPt, CuNi and InCu have followed (Figure 3).

 

 

Figure 3: The figure shows four different target wire combinations. In the pictures background corresponding SEM images of the obtained nanoparticles are shown, a photograph of the targets and the tunable composition range. The red composition range indicates the range obtainable by adjusting the pressure and the blue range by adjusting the magnetic field.

 

Publications

[1] Single target sputter deposition of alloy nanoparticles with adjustable composition via a gas aggregation cluster source
A. Vahl, J. Strobel, W. Reichstein, O. Polonskyi, T. Strunskus, L. Kienle, F. Faupel
Nanotechnology 28 (17), (2017) 175703 (8pp)
DOI: 10.1088/1361-6528/aa66ef

[2] Enhancing composition control of alloy nanoparticles from gas aggregation source by in operando optical emission spectroscopy
J. Drewes, A. Vahl, N. Carstens, T. Strunskus, O. Polonskyi, F. Faupel

Plasma Process Polym. (2020), e2000208
DOI: 10.1002/ppap.202000208