Raman and X-ray Photoemission Identification of Colloidal Metal Sulfides as Potential Secondary Phases in Nanocrystalline Cu₂ZnSnS₄ Photovoltaic Absorbers

Abstract

The aim of this study is to establish reliable spectroscopic fingerprints of compounds that may form as secondary phases in Cu2ZnSnS4 (CZTS) nanocrystals (NCs) synthesized by “green” colloidal chemistry directly in aqueous solutions or during post-processing of NC films for photovoltaic applications. For this purpose, we investigated a series of binary and ternary compound NCs synthesized under the same conditions as the quaternary CZTS NCs. The capabilities of combined Raman and X-ray photoemission (XPS) spectroscopy are used to identify these compounds formed separately and define spectral fingerprints for distinguishing them as possible secondary phases in the spectra of CZTS NCs. Besides the conventional analysis of element ratios and chemical shifts of the core-level peaks in the XPS spectra, the careful analysis of Auger lines and modified Auger parameters is applied to distinguish otherwise similar spectral contributions of different compounds. In the case of Cu x S NCs, the binding energy separation between the Cu2p3/2 and S2p3/2 core-levels is used as the additional fingerprint. As a criterion of a certain crystal structure in Raman spectroscopy, we rely not only on frequency positions of particular phonon modes but also on selective probing of different compounds at different (resonant) excitation wavelengths. The reasons of controversial previous reports on Raman spectra of Cu x S are revealed, and characteristic Raman spectra of Sn-poor Cu–Sn–S and Sn-poor Zn–Sn–S are proposed. For Cu–Zn–S, a mixture of Cu x S and ZnS is formed under the given mild conditions rather than ternary compounds or alloys.

Raman and X-ray Photoelectron Spectroscopic Study of Aqueous Thiol-Capped Ag-Zn-Sn-S Nanocrystals

“…Figure 4a shows Raman spectra of a series of AgxZnSnS4 with a nominal Ag content (x) between 0 and 30. The spectrum at xAg = 0 coincides with the Zn-Sn-S NC spectrum reported by us for the first time recently [29]. With an increase of the nominal Ag content in AZTS NCs, the features at 210 and 260 cm −1 increase in intensity with respect to the 336 cm −1 one.…”

Raman and X-ray Photoelectron Spectroscopic Study of Aqueous Thiol-Capped Ag-Zn-Sn-S Nanocrystals

“…Figure 4a shows Raman spectra of a series of AgxZnSnS4 with a nominal Ag content (x) between 0 to 30. The spectrum at xAg=0 coincides with the Zn-Sn-S NC spectrum reported by us for the first time recently [29]. With an increase of the nominal Ag content in AZTS NCs, the features at 210 and 260 cm-1 increase in intensity with respect to the 336 cm -1 one.…”

“…Using Raman spectroscopy with different exc has proven to be a powerful tool for selectively probing both compositional and structural heterogeneities of small colloidal NCs [20,[28][29][30][31][32]57]. In the previous subsections (Figure 6b and Figure 7), we already discussed some of the resonant effects and referred to Raman spectra at different exc (Figure S2-S3).…”

“…Raman spectroscopy has proved to be a generally accepted structural characterization technique for CZTS and many related compounds, including colloidal NCs [14][15][16][17][18][19][20][21][22][23][24][25][26][27]. Besides high sensitivity to secondary (impurity) phases [20,[28][29][30][31][32], Raman spectroscopy requires only tiny amounts of the material for analysis (unlike XRD) and no sample preparation (unlike TEM, which is in addition quite challenging for tiny aqueous NCs). Furthermore, it can probe NCs directly in the as-synthesized solutions.…”

Ultra-small aqueous glutathione-capped Ag–In–Se quantum dots: luminescence and vibrational properties

“…In the present paper we discuss the most efficient ways of affecting the composition, size, and crystallinity of GSH-capped AISe QDs as well as special features that can be observed in their light absorption and emission as well as vibrational Raman spectra. Besides providing the vibrational (phonon) spectra themselves, resonant Raman spectroscopy is an efficient tool of probing both the lattice structure of semiconductor QDs and local electronic resonances related either with intentional heterogeneity, like in core/shell structures, 52 or spontaneous formation of secondary phases, 53 common in complex chalcogenide QDs but hardly detectable by other structural techniques. 54 Based on our previous ndings of distinct vibrational patterns of aqueous II-VI 55 and M-In-S QDs 56 of ultrasmall size, an investigation of similar small Ag-In-Se is additionally motivated.…”