Basados en WSe 2

Spray pyrolysis is a chemical deposition process, which is based on thermal decomposition of the initial material. The decomposed material will oxidize on the substrate and form the desired layer material. To ensure the deposition of the desired material, pyrolysis should take place only at the surface of substrate and it is also necessary to keep the temperature of the initial material below the decomposition temperature. This can be achieved by dissolving the initial material (precursor) in a solvent through precursor solution, atomizing it into fine droplets (aerosol) and carrying these droplets to the hot substrate with a carrier gas.

The pyrolysis process can take place in different ways depending on various factors including: substrate temperature, temperature gradient at substrate surface, nature of solution species, and size and speed of the aerosol droplets. The influences of substrate temperature and initial droplet size on deposition processes is schematically represented

in Fig. 4.7. For clarity, four potential paths of the droplet towards the substrate labeled

as A - D are represented with the influence of change in substrate temperature Fig. 4.7

(a), and change in starting droplet size Fig. 4.7 (b).

Figure4.7(a) shows the influence of change in substrate temperature on transformation of precursor solution droplet to its final state, while keeping the initial droplet size the same [164].

• At low temperature in process A, the droplet splashes on the substrate in liquid form. The solvent vaporizes and leaves a dry precursor precipitate in which decomposition under solid form occurs.

• At moderate temperature in process B, the solvent evaporates before the droplet reaches the substrate surface and the precursor reaches the substrate surface in solid form and the precipitate impinges upon the surface.

• For an average temperature in process C, the solvent vaporizes as the droplet approaches the substrate, then the solid melts and vaporizes (or sublimes) and the vapor diffuses to the substrate to undergoes a heterogeneous reaction there. In this case, we are in chemical deposition mode in vapor phase and is labeled as a true chemical vapor depostion (CVD) process.

4.1 Film production

• At high temperature in process D, the metallic compound vaporizes before it reaches the substrate surface and the chemical reaction takes place in the vapor phase and form particles.

Figure 4.7: Schematic representation of modifications on spray pyrolysis droplets as they are

transported from the atomizing nozzle to the substrate. There are four potential paths which the droplet can take as it moves towards the substrate and are labeled as A-D in the figure. The two main factors,(a) the influence of change in substrate temperature and (b) change in initial droplet size are represented. Inspired by the works of Vigui and Spitz [164] and Siefert [165]

.

The ideal transportation of droplets to the substrate would be when the droplet ap- proaches the substrate surface just as the solvent and then vaporized entirely. However, since in the generation of droplets obtaining a uniform droplet size is strenuous and the thermal behavior of the droplets depends on their masses, different deposition processes are possible depending on the size of the droplets. Figure4.7(b) show various deposition processes that occur above the required decomposition temperature depending on the droplet size [165] .

• In a process A, the droplet is so large that the heat absorbed from the surroundings will not be sufficient to vaporize entirely the solvent on the way to the substrate. The droplet hits the substrate, where the solvent is entirely vaporized leaving a dry precipitate; the temperature has now increased above the boiling point of the solvent and decomposition occurs. Because the vaporization of the solvent locally removes a lot of heat, the substrate temperature decreases at this point. This affects adversely the kinetics of the reaction, i.e. equalization of the particle

concentrations does not occur. The surface becomes rough and the specular transmission decreases markedly.

• Process B is distinguishable in that the droplet dries up entirely before reaching the substrate and then hits the surface in a statistical distribution. Some of the particles evaporate and condense in the gaps between the particles where the surface reaction starts. In this process also, the vaporization of the particle locally removes a lot of heat, but not to the same extent as in process A.

• Process C includes the classical chemical vapour deposition process leading to the optimum film properties. In this process, the solvent is entirely vaporized short of the substrate. Before the particle reaches the substrate, there is sufficient time for it to warm up to ambient temperature. The particle then melts and vaporizes and undergoes a heterogeneous reaction. This reaction is divided into the following steps: (i) diffusion of the reactant molecules to the surface; (ii) adsorption of one or several reactant molecules at the surface; (iii) surface diffusion, chemical reaction, incorporation into the lattice; (iv) desorption of product molecules from the surface; (v) diffusion of product molecules away from the surface into the vapor space.

• The behavior of the smallest droplets is shown in process D. In this process the solvent is already completely vaporized far away from the substrate. The particle melts and vaporizes and a chemical reaction will occur in the vapor phase. This is a homogeneous reaction, because all reactant molecules and product molecules are in the vapor phase. The molecules condense as microcrystallites, which form a powdery precipitate on the substrate. This powder disturbs the formation of the layer and leads to a reduction in transmission. In addition the homogeneous reaction diminishes the deposition efficiency of this procedure.

As highlighted in the description above, the size of the droplets affects the morphology and adhesion of the layer on the substrate surface. Therefore, care must be taken during aerosol generation. Aerosols are generated from the precursor solution and commonly three main techniques are used to generate them: pneumatic spraying, electronic spraying and ultrasonic spraying (used during this work).

Pneumatic-based spraying systems use a stream of pressurized air or gas (e.g. nitrogen or argon) that breaks up the precursor solution into droplets at the narrow nozzle jet [166]. In electrostatic spraying, electrostatic field is used to atomize the precursor solution and the chemicals will undergo a controlled chemical reaction and deposited on a substrate [167].

Ultrasonic spraying gives full control over the most important process parameters, such as ultrasonic amplitude, precursor solution, precursor composition/viscosity, flow rate,

4.1 Film production

and deposition temperature [168]. Moreover, this method allows to obtain a well defined and monodispersed drop diameter, which ensures an identical deposition mechanism for each droplet.

The working principle of ultrasonic spraying is described as follows: ultrasound is generated from mutifrequency disc shaped piezoceramic transducers in the precursor solution. These produce oscillations of the free surface of the solution, called capillary waves. These capillary waves have chessboard like pattern as shown inFig. 4.8(a). This phenomenon occurs when the vibration amplitude A exceeds a threshold value. On further increase of the amplitude, breakup of a drop from precursor solution follows and droplets are hurled from the crests of capillary waves [169]. In this way production of liquid fogs of droplets is possible.

Figure 4.8: Illustration for atomizations of precursor solution. (a) Schematic illustration

of capillary wave atomization, where d is the diameter of the drop, λ the wavelength of the capillary wave, and A the amplitude of the vibrational frequency (Redrawn from [168]); (b) Photograph of capillary wave instability during droplet formation (Reprinted from[170], with the permission fo AIP Publishing).

The droplet size is governed by ultrasonic parameters of transducers, including: trans- ducer frequency, transducer amplitude, physical properties of precursor solution, and by the viscosity of the solution. The average droplet diameter d is then proportional to the wavelength λ of the capillarity wave. This relation has been experimentally established by Lang [171] and is represented by the following equation:

d = 0.34λ = 0.34

8πT

ρf2

1₃

(4.3)

Where, λ is the wavelength of capillary wave, T is the liquid surface tension, ρ is the liquid density, and f is the excitation sound frequency in cps. Spraying takes place above a vibration threshold amplitude of the piezoelectric transducer, and this depends on f,