Multianalytical investigation of Fe-bearing nanokuramite

In the present study, we report the results of a multianalytical study (SEM/EDS, XRD, Electron Paramagnetic Resonance, SQUID magnetometry, X-ray Absorption Spectroscopy and Diffuse Reflectance Spectroscopy) carried out on different materials belonging to the Cu-Zn-S and Cu-Fe-Zn-Sn-S systems, related to the kesterites, Cu2ZnSn(S,Se)4, in the light of their potential application to solar cell production. In recent years, in fact, kesterites attracted a worldwide interest because they encompass reduced production costs and low environmental risks. Samples are obtained by two different synthetic routes. Fe- and Zn-bearing kuramite nanoparticles are obtained through solvothermal syntheses, whereas Cu-Zn-S thin films analysed in the present study were realised through the E-ALD (Electrochemical Atomic Layer Deposition) technique. Both Fe- and Zn- bearing nanokuramite samples crystallise in a tetragonal pseudocubic structural arrangement, affine to that of the ZnS sphalerite. The pseudocubic arrangement is confirmed by the EXAFS results, according to which we can attribute the lack of tetragonal reflections to the random occupancy of cations (Cu, Sn). From a microanalytical point of view, nanocrystals exhibit different particle morphologies, accompanied by own characteristic chemical compositions. That is, Fe appears inhomogeneously distributed within the powders, whereas Zn appears more regularly distributed over the lattice. As far as thin films are concerned, a wide degree of heterogeneity, involving the morphological organisation of the films, as well as their phase composition, is revealed. E-ALD is proposed to operate a progressive and conventional coverage of the Ag (111) surface through a nanometric polycrystalline film consisting of oriented microcrystals. The insertion of a unit of ZnS in the deposition sequence apparently induces the formation of nanowhiskers on the film surface. From a structural point of view, Cu and Zn follow completely different paths in the crystallisation of the film. The XAS data, in fact, point to a poorly ordered low coordinated structure for Cu as in the mineral chalcocite. Conversely, Zn forms a crystalline structure in excellent agreement with the sphalerite model compound.