Phonon Raman spectroscopy of nanocrystalline multinary chalcogenides as a probe of complex lattice structures

Abstract

Ternary (I-III-VI) and quaternary (I-II-IV-VI) metal-chalcogenides like CuInS2 or Cu2ZnSn(S,Se)4 are among the materials currently most intensively investigated for various applications in the area of alternative energy conversion and light-emitting devices. They promise more sustainable and affordable solutions to numerous applications, compared to more developed and well understood II-VI and III-V semiconductors. Potentially superior properties are based on an unprecedented tolerance of these compounds to non-stoichiometric compositions and polymorphism. However, if not properly controlled, these merits lead to undesirable coexistence of different compounds in a single polycrystalline lattice and huge concentrations of point defects, becoming an immense hurdle on the way toward real-life applications. Raman spectroscopy of phonons has become one of the most powerful tools of structural diagnostics and probing physical properties of bulk and microcrystalline I-III-VI and I-II-IV-VI compounds. The recent explosive growth of the number of reports on fabrication and characterisation of nanostructures of these compounds must be pointed out as well as the steady use of Raman spectroscopy for their characterisation. Interpretation of the vibrational spectra of these compound nanocrystals (NCs) and conclusions about their structure can be complicated compared to bulk counterparts because of size and surface effects as well as emergence of new structural polymorphs that are not realizable in the bulk. This review attempts to summarise the present knowledge in the field of I-III-VI and I-II-IV-VI NCs regarding their phonon spectra and capabilities of Raman and IR spectroscopies in the structural characterisations of these promising families of compounds.

Structural and Optical Characterisation of Size-Selected Glutathione-Capped Colloidal Cu–In–S Quantum Dots

“…The Raman measurements of CuInS 2 QDs with the orthorhombic structure and with the average size of about 15 nm [12] revealed a rich Raman spectrum with rather narrow well resolved bands in the interval 70-350 cm −1 . Smaller-size nanoparticles, mostly with the tetragonal structure, similarly to our case, demonstrate broader, often overlapping bands [13,25,28,34,38,[41][42][43][44][45]54]. The broadening and asymmetry of features in the Raman spectra of small nanocrystals is, in particular, due to the phonon confinement-related scattering by nonzero-wavevector phonons, as well as surface phonon scattering [55,56].…”

“…A similar band appeared in the Raman spectra of copper-rich Cu-In-S films ([Cu] : [In]=1.39) [49] and polycrystalline stoichiometric CuInS 2 films [57]. A weak band near 420 cm −1 is also known for orthorhombic CuInS 2 QDs [12,54]. Note that a relatively broad band near 435 cm −1 was reported to be in the Raman spectra of quaternary Cu-Zn-Sn-S QDs, where it is related to the formation of secondary phases of Cu 𝑥 S type [54,60].…”

“…A weak band near 420 cm −1 is also known for orthorhombic CuInS 2 QDs [12,54]. Note that a relatively broad band near 435 cm −1 was reported to be in the Raman spectra of quaternary Cu-Zn-Sn-S QDs, where it is related to the formation of secondary phases of Cu 𝑥 S type [54,60]. In such case, it is understandable why this band was not observed in most of the Raman studies of stoichiometric or copper-deficient Cu-In-S QDs.…”

Molecular Precursor Approach to Sulfur-Free CuInSe2: Replacing Thiol Coordination in Soluble Metal Complexes

“…The A 1 mode vibration in CuIn­(S,Se) 2 originates from the vibrations of the anions in the crystal structure . The corresponding peak in Raman spectroscopy will appear around 174 cm –1 for selenium-related vibrations and around 290 cm –1 for sulfur-related vibrations, though the exact location of each peak can be shifted by changes in the surrounding atoms and the presence of defects. − The CISe precursor film from the new metal–selenium-containing ink was compared to a film made from a traditional ink-containing copper thiolates and indium thiolates (referred to as the CIS precursor film). While the CIS film shows the formation of CuInS 2 with the observed A 1 peak at 294 cm –1 , the CISe precursor film confirmed the presence of CuInSe 2 through the Raman peak observed at 179 cm –1 (Figure ).…”

Glutathione-Capped Quaternary Ag–(In,Ga)–S Quantum Dots Obtained by Colloidal Synthesis in Aqueous Solutions

“…In all cases, the observed first-order spectra were approximated by a superposition of overlapping vibrational peaks, their number ranging from 4 [38] up to 10 [41]. Interestingly, Raman scattering in Ag-In-S QDs is studied much more widely [21,34,38,39,41,46,57,59] than in bulk AgInS 2 crystals, for which, to our knowledge, the only experimental Raman data currently available refer to unpolarized spectra of polycrystalline AgInS 2 [60].…”