Deposition of Ultrathin MgB₂ Films from a Suspension Using Cosolvent Marangoni Flow

Magnesium diboride (MgB₂) has demonstrated, theoretically and experimentally, promise as a candidate material for hydrogen storage and has thus attracted much contemporary research interest. To study hydrogen gas adsorption on MgB₂ thin films using a quartz crystal microbalance (QCM)─a workhorse apparatus for this specific experiment─MgB₂ must be deposited uniformly on the active surface of the QCM without damaging the quartz's performance. In work presented here, a wet-chemistry colloid synthesis and deposition process of a MgB₂ thin film on a gold (Au) surface was established to avoid the extreme conditions of conventional physical deposition methods. This process also counteracts the unwanted phenomena of drying droplets on a solid surface, particularly the coffee-ring effect. To verify the normal function of the QCM after MgB₂ deposition and its ability to obtain meaningful data, simple gas adsorption tests were conducted on the QCM, and the MgB₂ film on the QCM was characterized with X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) for elemental analysis and surface roughness, respectively. To obtain information about the thickness and the involvement of the coffee-ring effect, the same synthesis route was applied on a similar gold substrate─an evaporated Au film on glass. XPS characterization of the film and its precursor suspension shows the potential existence of both MgB₂ and its oxide forms. The film's thickness on evaporated Au was measured by scanning transmission electron microscopy (STEM) to be 3.9 nm. The resulting samples show mitigation of the coffee-ring effect through roughness measurements with AFM at two scan sizes of 50 × 50 and 1 × 1 μm².