*3.1. Evaluation of the Velocity Profile and Pressure in the Head-Substrate Gap*

Figure 2a shows the velocity profile obtained for a geometry in which the deposition gap was fixed at 150 μm, a value that is commonly used in real depositions and using the gas flow values previously mentioned. The profile shows the expected flow from the gas outlets to the exhausts, but it shows a

non-zero value on the lateral outflow regions of the simulation, which would indicate that the exhaust at the injection head may not be as efficient as required. The simulation shows a maximum velocity of ~2 m·s−<sup>1</sup> at the narrowest point on the fluid's path from the inlet to the exhaust. The pressure profile is also shown in Figure 2b and it presents a gradient of pressure from the corner of the gas outlets to the corner of the exhausts. For comparison, Reference [17] presents a similar study on the deposition gap of another close-proximity system and concludes that a 2 mm gap with a "low pumping" at the exhausts would achieve a good spatial separation of the reactants. While their system also relies on inlets, exhausts and a deposition gap, the geometry is not planar, as opposed to our system. It is also important to notice that their system works at a pressure lower than the atmospheric pressure, which prevents confinement of the gaseous flows, and that the geometry of the system may enhance diffusion processes, and therefore, enhance intermixing when working at close-proximity. In our case, the pressure achieved by the geometry (especially in the corners close to the exhaust) is helpful to induce a flow towards the exhaust, hence improving the convective flow of the reactants to the exhaust and reducing the diffusive flow. This in turn contributes to preventing precursor intermixing.

**Figure 2.** Computational Fluid Dynamics calculation made with Comsol Multiphysics® to represent the flow of the region of interest in the SALD geometry: (**a**) The velocity increases in the deposition gap, given the close proximity, and (**b**) the pressure increases under each of the outlets as it enters the deposition gap.
