**2. Langmuir‐Blodgett methodology**

If one dimension is restricted, we have two‐dimensional (2D) or layered shape material. An interesting example of this kind of materials is graphene. This material is a monolayer of carbon atoms tightly packed into 2D honeycomb lattice that has attracted worldwide attention since it was discovered in 2004 [1, 2]. This new material has emerged with a promising future due to its amazing properties such as transparency, high‐charge mobility, thermal conductivity, and mechanical resistance. Due to these unique properties, graphene has been proposed as a good candidate for manufacturing transparent‐conducting electrodes, transistors, hydrogen‐ storage devices, and gas sensors [3, 4]. The growing interest on graphene has highlighted the importance of another 2D material in technological applications such as transition metal

Several 2D materials can be obtained by exfoliation of a layered bulk crystal. However, this procedure is often difficult because the van der Waals interactions between adjacent layers must be overcome. Mechanical exfoliation provides good results, but mostly applied for fundamental research because it is arduous and expensive to produce the material at industrial scale by this way. Other methodologies such as solvent‐assisted ultrasound exfoliation [5] or chemical synthesis [6] allow obtaining large amounts of materials at low cost, although they present several disadvantages against mechanical exfoliation. One important disadvantage is related with the deposition of materials onto solids. Hence, for several technological applica‐ tions, it is necessary to support 2D materials onto solid substrates [7, 8] and since the properties of 2D materials deposited onto solids are strongly affected by the film morphology, a deposi‐ tion methodology becomes necessary, which allows a great control of the material density and packing. Several techniques such as drop casting [9] or spin coating [10] have been used to integrate these materials onto novel devices; however, they often lead to nonuniform films or films with aggregated materials due to uncontrolled capillary flow and dewetting processes during solvent evaporation. These aggregates decrease the specific properties of each material [2, 11]. An illustrative example of the aggregation produced by solvent evaporation can be seen in **Figure 1**. The figure shows a field emission scanning electron microscopy (FESEM)

image of a graphene oxide (GO) film deposited onto silicon by drop casting.

**Figure 1.** FESEM image of graphene oxide deposited onto silicon by drop casting.

chalcogenides (TMC) and layered ionic solids.

22 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

The LB methodology consists on the transfer process from the air‐water interface onto a solid substrate of a monomolecular layer of amphiphilic material adsorbed at the air‐water interface [12]. The amphiphilic material is dissolved in a volatile solvent and dropped on the air‐water interface. For optimal results, the solvent should have a positive spreading coefficient and be insoluble in the subphase [28]. After the spreading, the solvent evaporates and the material forms a monolayer. When the monolayer reaches the thermodynamic equilibrium, it is symmetrically compressed by using two barriers. The sequential isothermal compression changes the structure of the monomolecular film, which passes through a series of 2D states, referred to as gas, expanded and compressed liquids, and solid state. Consequently, knowing the 2D phase diagram of the film, it is possible to control its structure and associated physical and chemical properties. To transfer the film onto the solid, a flat substrate is immersed into the aqueous subphase and then extracted in a controlled way with the film adsorbed onto it, see **Figure 2a**. The transfer process can be repeated many times to obtain multilayers [12, 29] of different thickness and composition. During the transfer process, the surface pressure is maintained constant by barrier compression in order to compensate the loss of molecules transferred onto the solid. One variant of this methodology is the horizontal deposition technique, referred as Langmuir‐Schaefer (LS) method. In the latter, the solid substrate is placed parallel to the air‐water interface and deposition is done by contacting the substrate horizontally with the floating monolayer, see **Figure 2b**.

**Figure 2.** Langmuir‐Blodgett (a) and Langmuir‐Schaefer (b) transfer processes.

When the monolayer is transferred, its structure is often modified; therefore, to construct high‐ quality films, a careful control of experimental parameters, such as stability and homogeneity of the monolayer, subphase properties (composition, pH, presence of electrolytes, and temperature), substrate nature, speed of immersion/emersion of substrate, surface pressure during the deposition process, and the number of transferred monolayers, is required [12, 29].
