**3.2. Electrodes**

Several works report E-ALD of semiconducting materials over a polycrystalline substrates, although very interesting, this films cannot be analyzed by means of the SXRD technique. In this context, very common substrates are Au or Ag single crystal with almost every low index facets as electrodic surface. We focus on the last, which is less noble, but less expensive, even though it requires a specific treatment of the surface to enable its use as a substrate. Moreover, as shown by our group, it is possible to correctly orient its crystals to low index planes with an easy and inexpensive procedure. Silver single crystal spheres can be grown in a graphite crucible according to the Bridgman technique. The crystal is strongly etched exposing the low index facets with a good contrast at the macroscale and a visual detection of the crystal orientation can be made [38]. These procedure is quite reliable, and most common problems are related to the growth step with the Bridgman technique. This step leads to very pure materials, but the resulting silver single crystal could have a concentration of defects unsuitable for the SXRD setups. So, the growth of the crystal needs to be carried very carefully in order to ensure the necessary quality of the electrodes. After the cut, the electrodic surface is polished with emery paper and successively finer grades of alumina powder down to 0.05 μm. Before each measurement, the electrodes are cleaned with water in an ultrasonic bath for 15 min and chemically polished using a patented procedure based on CrO3 . The surface roughness can be improved in ultra-high vacuum by performing several sputtering-annealing cycles.

### **3.3. Automated system**

An automated system for the exchange of solution in the electrochemical cell was first built at the University of Florence and European synchrotron radiation facility (ESRF) workshops, enabling the deposition of several layers, up to 120 E-ALD, in few hours. The automated system has been implemented at the ID03 and ID32 beamlines of the European synchrotron radiation facility (ESRF) in Grenoble, France. The apparatus consists of pyrex solution reservoirs, solenoid valves, a distribution valve, and a flow-cell. The whole is under fully automatic computer control [12].

#### **3.4. The flow cell**

Improvements with respect to thin-layer cells have been made designing a cell with suitable windows and flow channels enabling the fast exchange of the solutions [12]. The working electrode is placed at the bottom part of the cell that is directly fastened to the sample holder of the diffraction beamline front end. This position is particularly convenient for the alignment of the electrode surface and for exposure to the X-ray radiation. Although a careful choice of the material has to be done, the electrochemical cell can be built with Teflon, Kelef, or other chemically resistant plastics. Among others, PEEK is gaining favor in the field of operand SXRD measurement due to its resistance to hard X-ray radiation. However, the PEEK suffers for the presence of some crystalline domains, and without an adequate design of the cell walls, the powder and amorphous patterns from the cell windows can hinder with the X-ray signal from the sample itself. We report the design defined by the joint work of the University of Florence and ESRF workshops in **Figure 2**.

Operando Structural Characterization of the E-ALD Process Ultra-Thin Films Growth http://dx.doi.org/10.5772/67355 41

**3.2. Electrodes**

CrO3

sputtering-annealing cycles.

matic computer control [12].

of Florence and ESRF workshops in **Figure 2**.

**3.4. The flow cell**

**3.3. Automated system**

Several works report E-ALD of semiconducting materials over a polycrystalline substrates, although very interesting, this films cannot be analyzed by means of the SXRD technique. In this context, very common substrates are Au or Ag single crystal with almost every low index facets as electrodic surface. We focus on the last, which is less noble, but less expensive, even though it requires a specific treatment of the surface to enable its use as a substrate. Moreover, as shown by our group, it is possible to correctly orient its crystals to low index planes with an easy and inexpensive procedure. Silver single crystal spheres can be grown in a graphite crucible according to the Bridgman technique. The crystal is strongly etched exposing the low index facets with a good contrast at the macroscale and a visual detection of the crystal orientation can be made [38]. These procedure is quite reliable, and most common problems are related to the growth step with the Bridgman technique. This step leads to very pure materials, but the resulting silver single crystal could have a concentration of defects unsuitable for the SXRD setups. So, the growth of the crystal needs to be carried very carefully in order to ensure the necessary quality of the electrodes. After the cut, the electrodic surface is polished with emery paper and successively finer grades of alumina powder down to 0.05 μm. Before each measurement, the electrodes are cleaned with water in an ultrasonic bath for 15 min and chemically polished using a patented procedure based on

40 X-ray Characterization of Nanostructured Energy Materials by Synchrotron Radiation

. The surface roughness can be improved in ultra-high vacuum by performing several

An automated system for the exchange of solution in the electrochemical cell was first built at the University of Florence and European synchrotron radiation facility (ESRF) workshops, enabling the deposition of several layers, up to 120 E-ALD, in few hours. The automated system has been implemented at the ID03 and ID32 beamlines of the European synchrotron radiation facility (ESRF) in Grenoble, France. The apparatus consists of pyrex solution reservoirs, solenoid valves, a distribution valve, and a flow-cell. The whole is under fully auto-

Improvements with respect to thin-layer cells have been made designing a cell with suitable windows and flow channels enabling the fast exchange of the solutions [12]. The working electrode is placed at the bottom part of the cell that is directly fastened to the sample holder of the diffraction beamline front end. This position is particularly convenient for the alignment of the electrode surface and for exposure to the X-ray radiation. Although a careful choice of the material has to be done, the electrochemical cell can be built with Teflon, Kelef, or other chemically resistant plastics. Among others, PEEK is gaining favor in the field of operand SXRD measurement due to its resistance to hard X-ray radiation. However, the PEEK suffers for the presence of some crystalline domains, and without an adequate design of the cell walls, the powder and amorphous patterns from the cell windows can hinder with the X-ray signal from the sample itself. We report the design defined by the joint work of the University

**Figure 2.** Technical specification for an electrochemical flow cell used in operando SXRD experiments.

The internal vessel is a cylinder with an internal diameter of 6.7 mm and a height of 40 mm. The electrochemical cell volume (1.5 mL) was delimited by the working electrode on one side and the counter electrode on the other side. The inlet and the outlet for the solutions were placed on the side walls of the cylinder. The counter electrode was a gold foil, and the reference electrode made of a small Ag/AgCl (KCl sat.) is placed in the outlet pipe. The attenuation of the X-rays along the typical path for an SXRD experiment can be estimated considering the absorbed radiation through a typical optical path as reported in **Figure 2**. Considering an incident angle with the wall and the electrolyte very close to 90°, **Figure 3** reported different thickness and material for the X-ray window in the cell, while the optical path in the electrolyte does not change in the different setups taken into consideration. The X-ray beam propagates through two walls 100 μm–1 mm thick and roughly 10 mm of water. In the following paragraphs we report two SXRD setups, one involving a Teflon cell (for CdS experiments, Section 4.1) and then a PEEK cell (for Cu<sup>2</sup> S experiment, Section 4.2). For the first CdS experiment, it has been reported that a cell was made of Teflon, with a wall of 1 mm. At 20 keV, for this setup, **Figure 3** reports an overall transmission across the X-ray windows and electrolyte of roughly 40%, which is well matching the data reported in the literature (50%) [12]. PEEK ensures a lower attenuation of diffracted signal. Moreover, PEEK windows can be reduced to 100 μm thanks to its better mechanical proprieties. Hence, as depicted in **Figure 3**, this setup ensures a transmission very close to 70% at 24 keV. It is worth noticing that PEEK has a strong X-ray diffraction, reducing the thickness of the X-ray window by a factor of 10, makes completely undetectable the signal diffracted by PEEK.

**Figure 3.** Transmission of X-ray at different energy through different surface.

We should report that the state-of-the-art E-ALD systems are also commercially available for the deposition of bigger electrode up to 4 cm2 , mainly used for polycrystalline substrates.

## **4. E-ALD and the sulfides structure**

In this work, we report the intensity with respect to a coordinate system referred to as the pseudohexagonal surface unit cell of the Ag(111) substrate, in which the surface unit-cell parameters (a, b, c, α, β, γ) are defined so that the a and b vectors lay on the sample surface along the standard fcc [1 −1 0] and [−1 1 0] directions, while the c vector is perpendicular to the surface and parallel to the fcc [111] direction. The amplitude of the three vectors is given by the following relation together with the main surface cell angles.

$$a = \beta = 90^{\circ}, \gamma = 120^{\circ}, \, |c\rangle = \sqrt{3} \, a\_{\vartheta} \, |a\rangle = |b| \, = \frac{a\_{\vartheta}}{\sqrt{2}} \tag{3}$$

where *a*<sup>0</sup> is the lattice parameter of the cubic fcc cell of Ag. In the following, we adopted a reciprocal space metrics where *h*, *k*, and l are parallel to the a\*, b\*, and c\* vectors of the reciprocal surface cell. In the following, we present two case studies on the CdS and Cu2 S comparing the results and the different applications of the SXRD due to different scientific questions that have to be answered for a better understanding of the E-ALD growth of these two materials.
