In our work, we use a particular electrochemical technique—known as electrochemical atomic layer epitaxial deposition (ECALE)—to manufacture high-quality monolayers and multilayers of metal sulfide materials. With ECALE, we can exploit the underpotential deposition (UPD) methodology (i.e., the property of some materials to be deposited as a monolayer before massive electrochemical deposition). In addition, ECALE allows the composition, morphology, and structure of an aqueous solution at low temperature to be controlled. When the growth from ECALE—either from UPD processes or from any surface limited reaction—cannot be rigorously considered as epitaxial, it can be referred to as electrochemical atomic layer deposition (E-ALD). The major goal of our research is to demonstrate that ECALE can be used to efficiently fabricate high-quality monolayers and multilayers of pseudo-2D materials.
We grow the compound materials on metallic surfaces by depositing (using the UPD process) alternating monolayers of the different components (see Figure 1). We are also involved in developing a method to grow homostructures (i.e., monolayer and multilayers) and heterostructures (multilayers made of different layers of pseudo-2D materials). These techniques are being optimized in several laboratories. Indeed, we have used ECALE to successfully obtain a number of binary and ternary semiconductor thin films. These compounds include cadmium sulfide (CdS),7–11 cadmium telluride (CdTe),12 cadmium selenide (CdSe),13 zinc sulfide (ZnS),14 zinc selenide (ZnSe),14 nickel sulfide (NiS),15 lead sulfide (PbS),16–18 copper sulfide (CuS),19, 20 and indium arsenide (InAs).21
A specific need in our field of work is for the structural characterization—with the use of opportunistic analytical techniques—of the ultra-thin films that are obtained from the electrochemical deposition techniques. We have thus performed in situ surface x-ray diffraction (SXRD) measurements to investigate the growth of CuS films. We conducted our experiments at the ID03 beamline at the European Synchrotron Radiation Facility in Grenoble, France. In these tests, we monitored the growth of the film by examining the evolution of the Bragg peaks after each E-ALD step. The results of our SXRD measurements are illustrated in Figure 2. These highlight the occurrence of a self-standing film, with a definite crystal structure, after 15 E-ALD cycles. After the first observation of the Bragg reflections, we registered only minor subsequent changes to the structural arrangement of the film.
We also conducted additional SXRD measurements to monitor the influence of the applied electric potential on the stability of the electro-deposited crystal structure. We performed these SXRD measurements during the switch-off of the potential. Our results showed that a structural change was indeed registered. Furthermore, we found that this structural change was correlated with the occurrence of the stable phases under conventional laboratory conditions.
Our research involves the use of the ECALE deposition technique for the growth of ultra-thin films of metal sulfide,22–25 and other, materials. So far we have been able to successfully develop several films of materials, including CdS, CdTe, CdSe, ZnS, ZnSe, NiS, PbS, CuS, and InAs. Such films can be used for the manufacture of semiconductors for solar cells. Our electrochemical approach can meet the requirements for low-cost and efficient fabrication methods. We have also experimentally demonstrated that SXRD can be used to successfully characterize the structure of the fabricated films.26, 27 Our future research will be aimed primarily at the construction of an entire solar cell, as well as electrodeposition of p and n semiconductors via the E-ALD technique.
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15. G. Pezzatini, S. Caporali, M. Innocenti, M. L. Foresti, Formation of ZnSe on Ag(111) by electrochemical atomic layer epitaxy, J. Electroanal. Chem. 475, p. 164-170, 1999.
16. F. Loglio, M. Innocenti, A. Jarek, S. Caporali, I. Pasquini, M. L. Foresti, Nickel sulfur thin films deposited by ECALE: electrochemical, XPS, and AFM characterization, J. Electroanal. Chem. 638, p. 15-20, 2010.
17. V. C. Fernandes, E. Salvietti, F. Loglio, M. Innocenti, L. Mascaro, M. Foresti, Electrodeposition of PbS multilayers on Ag(111) by ECALE (electrochemical atomic layer epitaxy), ECS Trans. 11, p. 279-286, 2007.
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19. M. Innocenti, I. Bencistà, S. Bellandi, C. Bianchini, F. Di Benedetto, A. Lavacchi, F. Vizza, M. L. Foresti, Electrochemical layer by layer growth and characterization of copper sulfur thin films on Ag(111), Electrochim. Acta 58, p. 599-605, 2011.
20. I. Bencistà, F. Di Benedetto, M. Innocenti, A. De Luca, G. Fornaciai, A. Lavacchi, G. Montegrossi, et al., Phase composition of CuxS thin films: spectroscopic evidence of covellite formation, Eur. J. Mineral. 24, p. 879-884, 2012.
21. M. Innocenti, F. Forni, G. Pezzatini, R. Raiteri, F. Loglio, M. L. Foresti, Electrochemical behavior of As on silver single crystals and experimental conditions for InAs growth by ECALE, J. Electroanal. Chem. 514, p. 75-82, 2001.
22. F. Di Benedetto, I. Bencistà, S. Caporali, S. Cinotti, A. De Luca, A. Lavacchi, F. Vizza, M. M. Miranda, M. L. Foresti, M. Innocenti, Electrodeposition of ternary CuxSnySz thin films for photovoltaic applications, Prog. Photovolt. 22, p. 97-106, 2014.
23. M. Innocenti, L. Becucci, I. Bencistà, E. Carretti, S. Cinotti, L. Dei, F. Di Benedetto, et al., Electrochemical growth of Cu-Zn sulfides, J. Electroanal. Chem. 710, p. 17-21, 2013.
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25. S. Caporali, A. Tolstogouzov, O. M. N. D. Teodoro, M. Innocenti, F. Di Benedetto, S. Cinotti, R. A. Picca, M. C. Sportelli, N. Cioffi, Sn-deficiency in the electrodeposited ternary CuxSnySz thin films by ECALE, Solar Energy Mater. Solar Cells 138, p. 9-16, 2015.
26. F. Carlà, F. Loglio, A. Resta, R. Felici, E. Lastraioli, M. Innocenti, M. L. Foresti, Electrochemical atomic layer deposition of CdS on Ag single crystals: effects of substrate orientation on film structure, J. Phys. Chem. C 118, p. 6132-6139, 2014.
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