Compounds of ternary (I-III-VI) and quaternary (I-II-IV-VI) metal chalcogenides are promising materials for photodetectors and absorber layers of thin-film solar cells of the new (third) generation, fluorophores and low-toxic fluorescent labels for biomedical applications, photocatalytic devices, supercapacitors, and other energy conversion and storage devices.
The advantages of these compounds are the cheap and non-toxic components (chemical elements), as well as the possibility of partial substitution of any of the elements, which allows the properties of the material to be modified in a wide range. These materials are characterized by a combination of several important physical parameters, such as high optical absorption coefficient, the spectral position of the absorption edge in the range of solar radiation, good electrical and thermal conductivity, advantageous position of the band edges in terms of charge transfer to other components.
The production of these materials in the form of colloidal nanocrystals (NC) has certain advantages over other methods. In particular, the formation of a certain crystal structure and component composition of the material is carried out independently of the material of the (future) substrate, its morphology, etc. Compatibility with inkjet technology and other technologies of inexpensive large-scale production of thin films is an additional benefit of application of colloidal NC.
The phonon spectra of I-III-VI and I-II-IV-VI NCs were little studied before this work was started, because the crystal structure and component composition of these compounds significantly depend on the technological conditions of production. Even among the available data, there were inconsistencies in interpretation.
This thesis aims to establish the features of the phonon spectra of small-size I-III-VI and I-II-IV-VI NCs; the relationship of phonon spectra with the structure of the crystal lattice and component composition; the interaction between electron and phonon subsystems, in particular in the resonance Raman scattering spectra.
There following scientific results were obtained in this thesis:
1. Raman spectra of Hg-In-S compound and non-stoichiometric Ag-In-S NCs, low-temperature and second-order spectra for Ag-In-S and Cu-In-S NCs, core-shell (ZnS) NCs for all three compounds were studied for the first time. The combination of these data was crucial for the assumption that combinations of first-order phonons can form a second-order scattering spectrum.
2. Spectra of IR absorption by phonon in Ag-In-S, Hg-In-S, Cu-In-S NC, core-shell (ZnS) NC based on them, as well as Cu-Zn-Sn-S and Ag-Zn-Sn-S NCs of different component composition were studied for the first time.
3. The vibrational spectra of Ag-Zn-S and Ag-Sn-S compounds were studied for the first time.
4. One of the main contradictions in the literature on Raman spectra of I-III-VI compounds is solved, namely it is shown that Raman phonon modes in the range 340-350 cm-1 are caused by LO modes of the tetragonal structure of MInS2 type, and not by modes of spinel impurity phase, in agreement with XRD results.
5. With the help of EPR studies the existence of Cu2+ in non-stoichiometric CZTS NCs is experimentally confirmed for the first time, and corroborated by the results of electrical measurements.
6. It is established that in contrast to the previously established possibility of obtaining single-phase CZTS NCs by the method of aqueous low-temperature synthesis, (Cu,Ag)-Zn-Sn-S NCs obtained by this method retain the structure of CZTS only at low Ag content (several %). With increase of Ag content and transition to Ag-Zn-Sn-S, a combination of the main quaternary (AZTS or ACZTS) and impurity ternary phase Ag-Zn-S is formed. The presence of the latter phase can explain the non monotonous dependence of the optical absorption edge of Ag-Cu-Zn-Sn-S NC in a certain range of Ag/Cu content, which was observed also earlier in other works but did not find an explanation.
The practical significance of the obtained results consists of the development of a method of structural diagnostics of multicomponent NCs of nonstoichiometric component composition based on the analysis of their phonon Raman spectra. A method for obtaining Raman scattering spectra from highly luminescent NCs by controlled functionalization with a strong electron acceptor has also been developed. For thin films formed from Cu-Zn-Sn-S NCs of different component compositions, the relationship between the conditions of formation and heat treatment of films and their electrical properties is established.
