**2.2. Copper-based sulfides**

growth. The charge deposited for the first Te layer on Ag(111) is equal to that found on the first UPD of Te on Au(111), for which a (12 × 12) structure was revealed by STM images as well as by low-energy electron diffraction (LEED) patterns. LEED measurements after emersion at a potential corresponding to the second UPD of Te also showed that the (12 × 12) structure originated from a (3 × 3) structure [17]. The structure formed in correspondence with the sole UPD of Te observed on Ag(111) is expected to have the same structure [18]. However, no morphological or structural analysis has been performed on such substrate in order to confirm the expectation. Eventually, E-ALD also gives the possibility of growing ternary and quaternary

grown on Ag(111) [19–21]. The stoichiometry of the ternary chalcogenides grown by E-ALD can be controlled by the ZnX/CdX (X = S, Se) ratio. Still, CdX deposition seems to be favored with respect to ZnX deposition, hence the ratio between Cd and Zn cycles in the E-ALD sequence does not correspond to the stoichiometric ratio of Cd and Zn in the ternary compounds. Zn-deficiency is a general trend in ternary compounds deposited by means of E-ALD. The authors reported that the most likely explanation for the experiments is a lower deposition rate for Zn. However, electrochemical and XPS characterization confirmed the layer-by-layer growth and the 1:1 ratio between metals and sulfide ions. Eventually, the thickness seems to decrease while decreasing the Cd content. Analogous behavior, although referred to films as

The potential shifts exploited by the SLR involved in the E-ALD steps are related to the driving force of the process. As discussed in the first part of this text, the definition of the driving

The first term probably dominates the first stages of the growth, while for films with a thickness of several E-ALD cycles the second and third terms become predominant. Experimental results on similar electrodeposition process reported by Golan showed that the growth of such thin films can be either stopped [23, 24] or develop overgrowing quantum dots due to their cumulative strain. Hence, for a qualitative analysis on thin films of practical interest, we

*ΔE* = *ΔEf* + *ΔEmis* (1)

the cumulative elastic energy of the overgrowth due to the mismatch with the substrate.

*n ao*

is the chemical energy related to the formation of the CdX compound, *ΔE*misis

related to the free energy of formation).

Zn1−xS and Cd<sup>x</sup>

Zn1−xSe

(2)

compounds, and here we describe the case of ternary chalcogenides Cdx

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

thick as 1–5 μm, was found for the compounds grown by the dip technique [22].

force is an open question for the E-ALD, and we can divide it in three terms:

*2.1.3. An overview on thermodynamics and lattice mismatches*

consider the potential shift related to the following energy term:

Considering m:n supercells structure, the mismatch is the following:

*<sup>M</sup>* <sup>=</sup> *<sup>n</sup> ao* - *ma* \_\_\_\_\_\_\_\_\_\_\_\_*<sup>s</sup>*

**1.** The interaction with the surface.

where Δ*Ef*

**2.** The formation of a compound (ΔE<sup>f</sup>

**3.** The cumulative elastic energy (ΔEmis).

Copper sulfides are considered very interesting for their particular transport and electronic proprieties. Several phases in the Cu-S compositional field have been reported to have a band gap suitable for photovoltaics. Moreover, covellite (CuS) is a natural superconductor and chalcocite (Cu<sup>2</sup> S) is a superionic conductor. In the attempt to modulate the band gap of Cu2 S, a Zn-doped compound semiconductor (Cux Zn<sup>y</sup> S) has been deposited. Hence, recently, the research focused on Cu<sup>2</sup> S and Cu<sup>x</sup> Zn<sup>y</sup> S with some operando SXRD studies. In some cases, the epitaxial growth of thin films or 2D structure has been characterized. Revealing the peculiar structural proprieties of these materials. Moreover, these studies confirmed the advantage in using E-ALD as a production techniques for these systems in terms of growth control and quality of the resulting structure.


**Table 1.** Mismatches for Cd chalcogenides on different epitaxial relationship for Ag(111) and Au(111) substrates.

#### *2.2.1. Copper sulfide (Cu<sup>2</sup> S)*

Copper sulfide thin films were grown on Ag(111) by means of E-ALD, hence alternating the UPD of S and Cu. A reliable assignment of the structure and stoichiometry for these semiconductor compounds is very difficult because of the variety of stable known structures in the Cu-S compositional field. Moreover, the electrochemical behavior of Cu on the Ag(111) substrate is quite complex. In fact, Cu cannot be deposited by means of UPD on Ag(111), [26], however the compound can be obtained by deposition of Cu on the S covered Ag(111) surface. Moreover, the electrochemistry of Cu(II) in ammonia buffer involves a number of interconnected reactions that must be taken into account [26]. It has been found that there are clearly predominant processes in this set of four possible reactions whose equilibrium changes according to the applied potential. It is reported that on silver Cu(II) is immediately reduced to Cu(I), and the formation of Cu(0) from Cu(I) reduction occurs at less negative potentials than that from Cu(II) [26]. Hence, experimental data reported by Innocenti et al. support the hypothesis of an SLR deposition as a result of the competitive process between these four reactions [26]. Like for CdTe and CdSe, such complex chemistry of the SLR hinders the strict assignment of the E-ALD step as an UPD process. However, even depositing Cu and S using surface-limited depositions, more complex in nature, the electrochemical and compositional analyses confirmed that we succeeded in growing multilayers of Cu<sup>2</sup> S. Regarding the valence state of the elements in the compound, Cu(I) is present in both Cu2 S and CuS, which are the endmembers of the Cu-S compositional field. Hence, one cannot easily distinguish the two phases by means of an analysis of the binding energy as measured with XPS. In fact, Cu(I) and Cu(II) have almost coincident energy in the XPS, while both sulfide and disulfide peaks have been found in the spectra [27]. Hence, on this basis, the assessment of the stoichiometry associated with the Cu2 S cannot be safely discussed. Moreover, the short-range structural analysis carried by means of extended X-ray absorption fine structure (EXAFS) reported a first shell compatible with chalcocite structure (Cu<sup>2</sup> S) for similar systems (Cu<sup>x</sup> Zn<sup>y</sup> S) [28]. The electrochemical characterization also allows to establish that the same amount of compound is deposited in each deposition cycles, thus indicating the layer-by-layer growth mechanism that was the goal to achieve. Eventually, AFM analysis for 20 E-ALD is able to evidence the low roughness values of our deposits (4 Å), still higher than CdS [26].

#### *2.2.2. Cux Zny S*

Although mixed Cu-Zn sulfides are supposed to be obtained as a solid solution [5], just few studies claimed to have synthesized CuZnS and (Cu, Zn)S, respectively [29, 30], without a conclusive proof. It is known that the electrodeposition of alloys and compounds allows to deposit metastable phases, thus the E-ALD approach can be a good way to grow Cux Zn<sup>y</sup> S films. The E-ALD process for Cux Zn<sup>y</sup> S follows the alternate deposition of Cu<sup>x</sup> S and ZnS layers. The films obtained applying the general sequence Ag/S/[(Cu/S)/(Zn/S)m)]n with several different (m,n) were fully investigated by means of electrochemical characterizations. Chemical composition of the film obtained with (n,m) = (1,40) E-ALD cycles indicated a Cu/ Zn ratio of about 6. Thus confirming the low contribution of Zn in ternary compound already evidenced in the previous studies [6, 19, 31]. Eventually, Cux Zn<sup>y</sup> S was grown by E-ALD with a stoichiometry changing linearly with m at constant S layers (n x m); hence, the stoichiometry can be tuned by the ZnS/CuS deposition sequence. By means of extrapolation, the authors report that the 1:1 ratio can be achieved with *n* = 13 meaning a strong deficiency of Zn in the sulfides. This could be explained by a partial redissolution of zinc during the deposition of copper, which could also cause a rearrangement in the deposited materials. This is known to happen in similar methods such as the selective electrodesorption-based atomic layer deposition (SEBALD) method [32]. It is worth notice that at least two prevailing morphologies and two different crystal structures were highlighted by SEM-EDX and XAS investigations [28]. If grown alone, the Cu<sup>2</sup> S and ZnS films reveal thin film morphology. For Cux Zn<sup>y</sup> S, it is reported that the occurrence of nanowires can be attributed to the lack of miscibility between the Cu and Zn sulfides [28, 33–35]. On the basis of the experimental results, E-ALD is proposed to progressively cover the Ag (111) surface with a nanometric polycrystalline film consisting of oriented microcrystals [2, 5–7, 27, 31]. However, in some case, it is possible to obtain polycrystalline phases when depositing ternary compounds or solid solutions of binary compounds that are not completely miscible [28].

#### *2.2.3. CdS/Cux Zny S*

*2.2.1. Copper sulfide (Cu<sup>2</sup>*

tilayers of Cu<sup>2</sup>

(Cu<sup>2</sup>

*2.2.2. Cux*

present in both Cu2

S) for similar systems (Cu<sup>x</sup>

Å), still higher than CdS [26].

films. The E-ALD process for Cux

*Zny S* *S)*

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

Copper sulfide thin films were grown on Ag(111) by means of E-ALD, hence alternating the UPD of S and Cu. A reliable assignment of the structure and stoichiometry for these semiconductor compounds is very difficult because of the variety of stable known structures in the Cu-S compositional field. Moreover, the electrochemical behavior of Cu on the Ag(111) substrate is quite complex. In fact, Cu cannot be deposited by means of UPD on Ag(111), [26], however the compound can be obtained by deposition of Cu on the S covered Ag(111) surface. Moreover, the electrochemistry of Cu(II) in ammonia buffer involves a number of interconnected reactions that must be taken into account [26]. It has been found that there are clearly predominant processes in this set of four possible reactions whose equilibrium changes according to the applied potential. It is reported that on silver Cu(II) is immediately reduced to Cu(I), and the formation of Cu(0) from Cu(I) reduction occurs at less negative potentials than that from Cu(II) [26]. Hence, experimental data reported by Innocenti et al. support the hypothesis of an SLR deposition as a result of the competitive process between these four reactions [26]. Like for CdTe and CdSe, such complex chemistry of the SLR hinders the strict assignment of the E-ALD step as an UPD process. However, even depositing Cu and S using surface-limited depositions, more complex in nature, the electrochemical and compositional analyses confirmed that we succeeded in growing mul-

S. Regarding the valence state of the elements in the compound, Cu(I) is

Hence, one cannot easily distinguish the two phases by means of an analysis of the binding energy as measured with XPS. In fact, Cu(I) and Cu(II) have almost coincident energy in the XPS, while both sulfide and disulfide peaks have been found in the spectra [27]. Hence, on

cussed. Moreover, the short-range structural analysis carried by means of extended X-ray absorption fine structure (EXAFS) reported a first shell compatible with chalcocite structure

to establish that the same amount of compound is deposited in each deposition cycles, thus indicating the layer-by-layer growth mechanism that was the goal to achieve. Eventually, AFM analysis for 20 E-ALD is able to evidence the low roughness values of our deposits (4

Although mixed Cu-Zn sulfides are supposed to be obtained as a solid solution [5], just few studies claimed to have synthesized CuZnS and (Cu, Zn)S, respectively [29, 30], without a conclusive proof. It is known that the electrodeposition of alloys and compounds allows to deposit metastable phases, thus the E-ALD approach can be a good way to grow Cux

ers. The films obtained applying the general sequence Ag/S/[(Cu/S)/(Zn/S)m)]n with several different (m,n) were fully investigated by means of electrochemical characterizations. Chemical composition of the film obtained with (n,m) = (1,40) E-ALD cycles indicated a Cu/ Zn ratio of about 6. Thus confirming the low contribution of Zn in ternary compound already

this basis, the assessment of the stoichiometry associated with the Cu2

Zn<sup>y</sup>

Zn<sup>y</sup>

S and CuS, which are the endmembers of the Cu-S compositional field.

S follows the alternate deposition of Cu<sup>x</sup>

S) [28]. The electrochemical characterization also allows

S cannot be safely dis-

Zn<sup>y</sup> S

S and ZnS lay-

Recently, it has been reported that the first successful deposition of a CdS layer on the top of Cu<sup>x</sup> Zn<sup>y</sup> S deposits was over a Ag(111) electrode by means of E-ALD [36]. This is a remarkable achievement despite the process resulted as highly complex. Thus, the final attribution of a SLR deposition for Cd has been achieved in an unconventional way due to the overlap of the oxidative stripping potential of Zn and Cd. This conundrum has been overcome taking into account two deposition processes that were different with respect to the potential related to a small bump in the CVs: (1) potentiostatic in underpotential conditions and (2) potentiostatic in slightly overpotential conditions. The charge deposited by means of the two procedures to the deposition of the partial atomic layer of Cd over a sulfide substrate [8, 37] confirming the process as an effective SLR Cd deposition. Moreover, thanks to the XPS investigations, the occurrence of a complex sample composition (including the eventual presence of ZnS oxidation and/or the alloying between CdS and ZnS) is revealed. Finally, XPS measurements suggest that the structure is layered as expected from the E-ALD process applied to the growth of a p-n junction.
