**4. The Langmuir‐Blodgett films of 2D materials: QDs and nanowires**

Nanoparticles of CdSe Quantum Dots are semiconductors which show size dependence in their optoelectronic properties with attractive applications in the fabrication of solar cells or light‐emitting diodes (LEDs) due to their band‐gap tunability.

The most important optical advantages are a broad and continuous absorbance spectrum and a narrow emission spectrum whose maximum position and dynamic emission properties depend on its QD size. However, optoelectronic device applications based on nanoparticles require QDs assembly in controllable architecture to avoid the deterioration of the quantum film efficiency. Therefore, the thickness and uniformity of the assembled QD films are crucial factors in the emission properties of films [7, 55–58].

In the particular case of CdSe QDs, the hydrophobic nanoparticles present the highest quantum efficiency. However, when these nanoparticles are transferred from the air‐water interface onto substrates such as glass, silicon, or mica without treatment to become the solid surface, hydrophobic, low coverage and nanoparticle agglomeration have been observed [59, 60]. These undesirable results decrease the quantum yield of nanoparticle films. To solve this problem, some approaches have been proposed. One of the most widely used strategies consists of mixing nanoparticles with surfactants or polymers and then transferring the mixture from the air‐water interface onto the solid substrate [61–63]. This approach seeks to control the assembly of hydrophobic nanoparticles at the air‐water interface. With this purpose, we have proposed three amphiphilic molecules of distinct nature, the copolymers poly(octadecene‐co‐maleic anhydride), PMAO, and poly(styrene‐co‐maleic anhydride) partial 2‐butoxyethyl ester cumene terminated, PS‐MA‐BEE, and the Gemini surfactant ethyl‐bis(dimethyl octadecylam‐ monium bromide), 18‐2‐18. All these molecules present surface activity and can anchor to substrates such as mica, glass, or silicon, through their hydrophilic moieties [13, 14] favoring the QDs' adhesion across its hydrophobic part. We have chosen the polymer PMAO because it interacts effectively with hydrophobic nanoparticles leading to excellent stability by avoiding 3D aggregation [64, 65]. In the case of the polymer PS‐MA‐BEE, it was chosen because it is a good component to organize hybrid nanomaterials used in submicrometric electronic devices [66]. This is due to its mechanical rigidity and good adhesion on solids [67]. Finally, the Gemini surfactant was chosen since it has been proposed in combination with DNA for biotechnological applications [68, 69].

Results in **Figure 6a** show that the electrical conductivity increases as the Csp2 percentage. Moreover, the highest conductivity value is obtained for graphene oxide functionalized with the zwitterionic surfactant. In addition, samples with the lowest percentage of C**-**O and COOH groups, see **Figure 3**, correspond with reduced graphene oxides functionalized with the surfactant DDPS. All these facts suggest that the surfactant molecules can eliminate high amount of O‐groups of samples increasing the electrical conductivity of flakes as can be seen

On summarizing, the LB technique can be presented as a good methodology of building graphene oxide films because it renders high‐coverage and ordered films. On the other hand, the conductivity of our surfactant‐functionalized RGO samples is higher than the values found in the literature for paper‐like films of reduced graphene oxide [5, 54] functionalized with ionic surfactants, although more efforts must be done to improve the solid coverage and to increase

**4. The Langmuir‐Blodgett films of 2D materials: QDs and nanowires**

Nanoparticles of CdSe Quantum Dots are semiconductors which show size dependence in their optoelectronic properties with attractive applications in the fabrication of solar cells or

The most important optical advantages are a broad and continuous absorbance spectrum and a narrow emission spectrum whose maximum position and dynamic emission properties depend on its QD size. However, optoelectronic device applications based on nanoparticles require QDs assembly in controllable architecture to avoid the deterioration of the quantum film efficiency. Therefore, the thickness and uniformity of the assembled QD films are crucial

In the particular case of CdSe QDs, the hydrophobic nanoparticles present the highest quantum efficiency. However, when these nanoparticles are transferred from the air‐water interface onto substrates such as glass, silicon, or mica without treatment to become the solid surface, hydrophobic, low coverage and nanoparticle agglomeration have been observed [59, 60]. These undesirable results decrease the quantum yield of nanoparticle films. To solve this problem, some approaches have been proposed. One of the most widely used strategies consists of mixing nanoparticles with surfactants or polymers and then transferring the mixture from the air‐water interface onto the solid substrate [61–63]. This approach seeks to control the assembly of hydrophobic nanoparticles at the air‐water interface. With this purpose, we have proposed three amphiphilic molecules of distinct nature, the copolymers poly(octadecene‐co‐maleic anhydride), PMAO, and poly(styrene‐co‐maleic anhydride) partial 2‐butoxyethyl ester cumene terminated, PS‐MA‐BEE, and the Gemini surfactant ethyl‐bis(dimethyl octadecylam‐ monium bromide), 18‐2‐18. All these molecules present surface activity and can anchor to substrates such as mica, glass, or silicon, through their hydrophilic moieties [13, 14] favoring the QDs' adhesion across its hydrophobic part. We have chosen the polymer PMAO because it interacts effectively with hydrophobic nanoparticles leading to excellent stability by

the electrical conductivity values of graphene oxide films.

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

light‐emitting diodes (LEDs) due to their band‐gap tunability.

factors in the emission properties of films [7, 55–58].

in **Figure 6b**.

Our results demonstrated that the QD aggregation is avoided by the addition of these polymer and surfactant molecules. Attractive interactions between the chains of these molecules and the hydrophobic moieties of the QD stabilizer, trioctylphosphine oxide (TOPO), favor the adsorption of QDs on the matrices, while the hydrophilic groups of polymer or surfactant molecules increase the QDs' adhesion in solid substrates, avoiding the nanoparticle agglom‐ eration.

We also found two different film features depending on the film composition. To illustrate this behavior, **Figure 7** collects some AFM, transmission electron microscopy (TEM), and scanning electron microscopy (SEM) images of QD films prepared with different matrix compositions.

**Figure 7.** (a) AFM image of a Gemini/QD film at the surfactant mole fraction of 0.98, (b) SEM image of a PMAO/QD LB film onto mica at the polymer mole fraction of 0.50. The Langmuir monolayers were transferred at the surface pressure of 30 mN m-1, (c) TEM image of mixed PS‐MA‐BEE/QD LB film of polymer mole fraction of 0.5 and deposited at the surface pressure of 14 mN m-1, and (d) AFM image of a mixed PS‐MA‐BEE/QD film of polymer mole fraction of 0.96 and deposited at the surface pressure of 30 mN m-1. The inset corresponds to the TEM image of the film.

Images in **Figure 7** show two different morphologies, hexagonal networks and domains of different shapes, depending on the film composition. Thus, mixed films of QDs and PS‐MA‐ BEE of high polymer mole fraction, *XP* ≥ 0.95, and deposited at 30 mN m-1 [22] and PMAO/QDs films are constituted by hexagonal networks [21], **Figure 7b** and **d**. It is interesting to note that the height of rims around the holes was 4 nm, which is compatible with the diameter of the nanoparticles dissolved in chloroform (3.4 nm). This result indicates that QDs are mainly confined in rims and do not form 3D aggregates. On the other hand, all the Gemini/ QDs films and PS‐MA‐BEE/QDs films of polymer mole fraction below 0.95 deposited at low‐ surface pressure (14 mN m-1) are constituted by domains of different morphologies, **Fig‐ ure 7a** and **c**. The domain height determined by AFM measurements (∼3 nm) is consistent with the diameter of QDs dissolved in chloroform. This fact indicates that there is no 3D aggregation in these films.

Differences between film morphologies were interpreted according to dewetting mechanisms [21, 22]. The two dewetting mechanisms considered in these cases are known as nucleation, growth, and coalescence of holes [70] and spinodal [71]. In the former, the gravity contribution predominates and the dewetting process starts with the nucleation of holes at film‐defect sites followed by the material displacement away from the nucleus. The material is accumulated in the rims of holes delimiting a mosaic [70]. Conversely, in the spinodal dewetting mechanism, the capillary waves break the film into nanostructures when the amplitude of the capillary waves exceeds the thickness of the film. Taking into account that the molecular weight of the polymer PMAO is around 50 times higher than the surfactant one, it becomes clear that the gravitational effect prevailed over the capillary waves even in films with small amount of the polymer PMAO. Therefore, the PMAO/QDs film morphology is driven by the mechanism of nucleation, growth, and coalescence of holes, while spinodal dewetting mechanism prevails in Gemini/QD films [21]. In the case of PS‐MA‐BEE/QD films, the interpretation of the behavior observed is not so evident and it is necessary to analyze the balance between the driving forces involved in the surface arrangement: gravitational and capillary forces. Thus, the elasticity values go through a minimum for PS‐MA‐BEE/QDs monolayers at the surface pressure of 30 mN m-1 and for polymer mole ratio above 0.95, while it reaches maximum values for mono‐ layers at the surface pressure value of 14 mN m‐-1 and *XP* < 0.95 [22]. Taking into account that the damping coefficient passes through a maximum at low elasticity values and decreases when the elasticity modulus increases [22], it is easy to understand that in PS‐MA‐BEE/QD films of low elasticity values (*π* = 30 mN m-1 and *XP* ≥ 0.95), the capillary waves are quickly damped and the film breaks in domains separated by holes due to gravitational effects. Conversely, the capillary waves for monolayers with the highest elasticity values (*π* = 14 mN m-1 and *XP* < 0.95) do not damp so quickly and they drive the dewetting mechanism. In these situations, the spinodal dewetting mechanism predominates against the growth of holes process leading to QD domains of different shapes [22].

Another interesting example is the preparation of silver nanowire films for manufacturing modern devices such as photovoltaic cells, touch panels, and light‐emitting diodes. Although the development of new materials is mainly by the requirements of each application [72], high transparency and electrical conductivity always constitute required requisites.

Indium tin oxide (ITO) currently dominates the field of transparent conductive electrodes as a result of its excellent optoelectronic properties [73]; however, it suffers important limitations due to the scarcity of indium, brittleness of its electrodes, and high manufacturing cost. Several materials such as carbon nanotubes [74, 75], graphene films [76, 77], conducting polymers, and metal nanowires [72, 78] are being analyzed to replace ITO. However, the properties of these materials, in terms of electrical resistance and transparency, are still inferior to ITO [78]. Among all, the silver nanowires arouse great interest due to the high conductivity of silver (6.3 × 10<sup>7</sup> S m-1) [79]. Since the nanowires are usually synthesized in solution, an important issue is the control of the transfer process from solutions onto the substrate. This is because to achieve low electrical resistance and high transparency, it is necessary to optimize the morphology, the placement of nanowires, and the junction resistance between them in the network. As commented previously, spin‐coating and drop‐casting methodologies present several disad‐ vantages since water evaporation leaves discontinuous films with typical coffee rings that significantly decrease the quality of AgNW films [80, 81]. To overcome these limitations, we have reported a strategy based on the Langmuir‐Schaefer methodology to transfer hydropho‐ bic AgNW from the air‐water interface onto Lexan polycarbonate substrate in an ordered and controlled way [27].

QDs films and PS‐MA‐BEE/QDs films of polymer mole fraction below 0.95 deposited at low‐ surface pressure (14 mN m-1) are constituted by domains of different morphologies, **Fig‐ ure 7a** and **c**. The domain height determined by AFM measurements (∼3 nm) is consistent with the diameter of QDs dissolved in chloroform. This fact indicates that there is no 3D

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

Differences between film morphologies were interpreted according to dewetting mechanisms [21, 22]. The two dewetting mechanisms considered in these cases are known as nucleation, growth, and coalescence of holes [70] and spinodal [71]. In the former, the gravity contribution predominates and the dewetting process starts with the nucleation of holes at film‐defect sites followed by the material displacement away from the nucleus. The material is accumulated in the rims of holes delimiting a mosaic [70]. Conversely, in the spinodal dewetting mechanism, the capillary waves break the film into nanostructures when the amplitude of the capillary waves exceeds the thickness of the film. Taking into account that the molecular weight of the polymer PMAO is around 50 times higher than the surfactant one, it becomes clear that the gravitational effect prevailed over the capillary waves even in films with small amount of the polymer PMAO. Therefore, the PMAO/QDs film morphology is driven by the mechanism of nucleation, growth, and coalescence of holes, while spinodal dewetting mechanism prevails in Gemini/QD films [21]. In the case of PS‐MA‐BEE/QD films, the interpretation of the behavior observed is not so evident and it is necessary to analyze the balance between the driving forces involved in the surface arrangement: gravitational and capillary forces. Thus, the elasticity values go through a minimum for PS‐MA‐BEE/QDs monolayers at the surface pressure of 30 mN m-1 and for polymer mole ratio above 0.95, while it reaches maximum values for mono‐ layers at the surface pressure value of 14 mN m‐-1 and *XP* < 0.95 [22]. Taking into account that the damping coefficient passes through a maximum at low elasticity values and decreases when the elasticity modulus increases [22], it is easy to understand that in PS‐MA‐BEE/QD films of low elasticity values (*π* = 30 mN m-1 and *XP* ≥ 0.95), the capillary waves are quickly damped and the film breaks in domains separated by holes due to gravitational effects. Conversely, the capillary waves for monolayers with the highest elasticity values (*π* = 14 mN m-1 and *XP* < 0.95) do not damp so quickly and they drive the dewetting mechanism. In these situations, the spinodal dewetting mechanism predominates against the growth of holes

Another interesting example is the preparation of silver nanowire films for manufacturing modern devices such as photovoltaic cells, touch panels, and light‐emitting diodes. Although the development of new materials is mainly by the requirements of each application [72], high

Indium tin oxide (ITO) currently dominates the field of transparent conductive electrodes as a result of its excellent optoelectronic properties [73]; however, it suffers important limitations due to the scarcity of indium, brittleness of its electrodes, and high manufacturing cost. Several materials such as carbon nanotubes [74, 75], graphene films [76, 77], conducting polymers, and metal nanowires [72, 78] are being analyzed to replace ITO. However, the properties of these materials, in terms of electrical resistance and transparency, are still inferior to ITO [78]. Among all, the silver nanowires arouse great interest due to the high conductivity of silver (6.3 × 10<sup>7</sup>

S

transparency and electrical conductivity always constitute required requisites.

aggregation in these films.

process leading to QD domains of different shapes [22].

The first step for building LB films is to obtain stable monolayers of hydrophobic materials. Therefore, it is necessary to synthesize hydrophobic nanowires, since the commercial ones are water soluble since they use polyvinyl pyrrolidone molecules as capping agents. To synthesize hydrophobic AgNW, polyvinyl pyrrolidone was replaced by octyl thiol molecules. The surface modification is achieved through the surface ligand exchange procedure reported by Tao [82]. After the synthesis of AgNW, they were deposited at the air‐water interface and different surface states were transferred onto the solid substrate by the LS methodology. The surface states of nanowire monolayers are characterized by the surface compressional modulus, *ε,* calculated from the surface pressure isotherm and Eq. (1), and ε‐values are plotted against the surface concentration, *Γ*, in **Figure 8a**. As can be seen in **Figure 8a**, when the surface concen‐ tration is small, the elasticity modulus value is close to zero. In this region, named as low‐ surface density state (LD), nanowires are randomly orientated. When the surface density is further increased and *ε* reaches a value of 10 mN m-1, the monolayer is highly packed; we referred to this state as the high‐surface density state (HD) [83].

**Figure 8.** (a) Elasticity isotherm of silver nanowires capped with octyl thiol at 20°C, (b) FESEM image of a bilayer of AgNW of 645 mg m-2. Arrows indicate the orientation of the first (red) and second (blue) layers, and (c) variation of sheet resistance and transmittance with the nanowire surface concentration of LS films.

We have transferred AgNW Langmuir monolayers at LD and HD states by the Langmuir‐ Schaefer methodology. With the purpose of achieving a network of nanowires, a second layer in which the nanowires are oriented perpendicular to the first layer was deposited. In the first and second layers, the surface density of the transferred Langmuir monolayer was the same [27]. The surface density is controlled by the surface pressure value. **Figure 8b** shows a representative FE‐SEM image of a nanowire film obtained by this methodology.

The sheet resistance, *R*s, measured in Ω sq-1 and the transmittance measured at 550 nm are plotted against the surface concentration in **Figure 8c**. Data in **Figure 8c** show that the monolayers at the LD state present high *R*s values which decrease when the surface concen‐ tration increases, while the transmittance value is almost independent on surface concentration and remains constant at 88%. The behavior is opposite for films built from Langmuir mono‐ layers at the HD state. In this case, the sheet resistance is maintained at 8 Ω sq-1 while the transmittance value changes from 65 to 89% when the surface concentration was modified between 345 to 770 mg m-2. According to the resistance and transparency values, our AgNW films can be employed as substitutes for ITO as components of devices such as touch screens, electromagnetic shielding, and defrosted windows [27]. Moreover, our results proved that the Langmuir‐Schaefer methodology is a versatile technique, which allows modifying the transmittance keeping the sheet resistance or tuning the sheet resistance, maintaining the transparency of films constant by properly selecting the surface state and the nanowire mass transferred onto the solid substrate.

Results analyzed in this chapter allow us to discuss the ability of the Langmuir‐Blodgett and Langmuir‐Schaefer methodologies to build thin films of 2D materials such as graphene oxides, transition metal chalcogenide nanoparticles, CdSe Quantum Dots, and silver nanowires. We discuss the advantages of these methods against the most conventional ones such as drop and spin coating for built‐in 2D material films with applications in the fabrication of solar cells, LEDs, sensors, and transparent electrodes.

We also review some strategies for improving the solid coverage, avoiding the nanoparticle aggregation, and modulating the film morphology. All these issues are crucial for increasing the quality of films and to modulate its properties according to the properties required for each application.

Results analyzed in this chapter indicate that the Langmuir‐Blodgett and Langmuir‐Schaefer methodologies combined with self‐assembled materials can be proposed as a non‐template reproducible technique for patterning at the nanoscale. However, most efforts have to be done for achieving more homogeneous films, higher coverage, and a greater control of the material arrangements to build good‐quality films to be used in technological applications.
