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

Abstract

The synthesis of (Cu,Ag)-Zn-Sn-S (CAZTS) and Ag-Zn-Sn-S (AZTS) nanocrystals (NCs) by means of “green” chemistry in aqueous solution and their detailed characterization by Raman spectroscopy and several complementary techniques are reported. Through a systematic variation of the nominal composition and quantification of the constituent elements in CAZTS and AZTS NCs by X-ray photoemission spectroscopy (XPS), we identified the vibrational Raman and IR fingerprints of both the main AZTS phase and secondary phases of Ag-Zn-S and Ag-Sn-S compounds. The formation of the secondary phases of Ag-S and Ag-Zn-S cannot be avoided entirely for this type of synthesis. The Ag-Zn-S phase, having its bandgap in near infrared range, is the reason for the non-monotonous dependence of the absorption edge of CAZTS NCs on the Ag content, with a trend to redshift even below the bandgaps of bulk AZTS and CZTS. The work function, electron affinity, and ionization potential of the AZTS NCs are derived using photoelectron spectroscopy measurements.

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

“…The authors of [48] systematically studied the evolution of resonant Raman spectra upon variation of the nominal and actual compositions of 3 nm Ag-Zn-Sn-S (AZTS) NCs and (Cu,Ag)-Zn-Sn-S (CAZTS) NCs synthesized by means of 'green' chemistry in aqueous solution. They succeeded to identify the vibrational fingerprints of both the main AZTS phase and secondary phases of Ag-Sn-S and Ag-Zn-S compounds (the latter were reported for the first time) in both Raman and IR spectra.…”

“…For the AZTS NCs an additional feature at about 160 cm −1 could also be distinguished. As the formation of Ag-S and Ag-Zn-S as secondary phases could not be excluded for this type of synthesis, their possible manifestation in the vibrational spectra was also considered in [48]. As the result of the analysis, the Ag-Zn-S phase, which has its bandgap in near infrared range, was found to be the reason of the observed non-monotonous variation of the absorption edge of CAZTS NCs as the function of Ag content.…”

“…The spectra of 15-20 nm Cu 2 ZnSn 1−x Ge x S 4 NCs reported in [225] (figure 31) exhibit a single mode in the whole compositional range, although the bulk crystals have two more strong peaks, around 300 and 400 cm −1 (for Ge-rich The range of second and third order overtones of the first-order peak are shown in the inset [48]. Reproduced from [48]. CC BY 4.0. composition), respectively, under the same (resonant) excitation with 514.5 nm [201].…”

“…Being sensitive to the bond ionicity, the frequencies of the phonon modes can give information about chemical bonding in a mixture of compound semiconductors, which is not accessible for other experimental methods. Far infrared (FIR) absorption by phonons is a valuable complementary method to Raman spectroscopy, but IR phonon spectra of NCs of I-III-VI and I-II-IV-VI compounds are scarce and were almost not studied yet, except several recent publications [25,48].…”

Spectroscopy and Theoretical Modeling of Phonon Vibration Modes and Band Gap Energy of Cu2ZnSn(SxSe1–x)4Bulk Crystals and Thin Films

“…This has become a new stimulus in finding ways to understand the mechanisms explaining the formation of these compounds and to control their synthesis for either pure CZTS and CZTSe or CZTSSe solid solutions. In the last few years, special attention has been paid not only to the change in the content of S to Se atoms − but also of Sn to Ge or Si and − Zn to Cd and other elements − for both films and bulk crystals. As is known, either partial or complete replacement of Cu, Zn, or Sn cations with bigger cations in the Cu–Zn and Cu–Sn planes can increase the formation energy of antisite defects on these planes and, thus, decrease the extent of the disorder. , This allows to deepen understanding of the fundamental properties of this family of compounds and to develop tandem TFSCs able to absorb solar energy in a wider range of wavelengths, increasing their efficiencies .…”