Synthesis of Quasi-2D NiO by Dip-Coating and Langmuir Methods

The synthesis of low dimensional structures based on p-type oxides with controlled composition and morphology, as well as their heterostructures with other p- or n-type oxides, are go-to topics that can broaden the field of applicability of these materials in modern technology. NiO is one of the very few p-type oxides which combines wide-bandgap with good stability and interesting physical properties [1]. However, contrary to other van der Waals materials, the fabrication of this oxide in 2D form, membranes and ultrathin films, is still scarce and in most cases requires of complex chemical reactions or high energy-consumption synthesis routes. In that sense, the advent of modern 2D-device technology requires of adequate and scalable synthesis methods to fabricate NiO, and other metal oxides, in the 2D range. In this work, low-cost and scalable methods based on dip-coating and Langmuir films and synthesis at the liquid-liquid interfaces are proposed. These methods are based on the self-assembly of nanoparticles or molecules previously dispersed in suitable solvents [2]. The ultra-thin films can be easily transferred to a target substrate, adding the possibility to fabricate heterostructures layer-by-layer. In order to assess the advantages of these methods, alternative NiO ultrathin films fabricated by Pulsed Laser Deposition (PLD) method will be analysed for comparison.<br/>Owing to the optimization of the parameters involved in the synthesis of the dip-coating and Langmuir films, homogeneous films with variable thickness and properties were fabricated, using Si as substrate. The structural, compositional and physical properties of the NiO samples were analysed by means of diverse microscopy and spectroscopy techniques. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) confirm the formation of homogeneous films with low roughness and thickness in the range of few tens of nm, while Raman spectroscopy and energy dispersive x-ray spectroscopy (EDS) confirms the formation of rock-salt NiO. The optical and electrical properties of the samples were also analysed, and variations as a function of the thickness, doping or thermal treatments involved during the synthesis are discussed.<br/><br/>[1] M. Taeño, D. Maestre, A. Cremades. <i>J. Alloys and Compounds</i>, 881, 160654 (2021)<br/>[2] A. J. G. Zalbin. <i>Mater. Horiz</i>.,8, 1409-1432, (2021)