Phonon Spectra of Strongly Luminescent Nonstoichiometric Ag–In–S, Cu–In–S, and Hg–In–S Nanocrystals of Small Size

Abstract

We present a detailed analysis of Raman and infrared (IR) phonon spectra of strongly luminescent nonstoichiometric M−In−S (M = Cu, Ag, Hg) and core/shell M−In-S/ZnS nanocrystals (NCs) of small size (d ≈ 2−4 nm), formed by means of aqueous colloidal chemistry under mild conditions. Despite presumably similar factors determining position and broadening of the Raman and X-ray diffraction (XRD) peaks, phonon spectra are shown to be more sensitive to NC composition and crystals structure. The spectral Raman pattern of these strongly Mdeficient M−In-S NCs is different from that of the corresponding stoichiometric phases, e.g., CuInS 2 or AgIn 5 S 8 , and excludes its assignment to relevant binary sulfides, e.g., In 2 S 3 . Resonant behavior of relative peak intensities in Raman spectra is different from that of larger-size stoichiometric NCs and bulk samples studied before, while the temperature dependence reveals an analogous enhancement of the highest-frequency LO modes supporting an unambiguous assignment of the latter. Therefore, we conclude that the Raman spectra observed are characteristic of the specific structure of these highly nonstoichiometric small NCs. IR modes of these NCs occur in the same frequency range as the Raman ones but at higher frequencies than the IR phonons in bulk material. The IR spectra are less characteristic, compared to Raman ones, revealing more similarity among the three NC compounds and with the bulk counterparts.

Composition-Dependent Optical Band Bowing, Vibrational, and Photochemical Behavior of Aqueous Glutathione-Capped (Cu, Ag)–In–S Quantum Dots

“…The first-order Raman spectra of the CAIS QDs (Figure c) are basically similar to those reported earlier for both stoichiometric CuInS 2 QDs ,, and AgInS 2 QDs, , as well as nonstoichiometric CIS QDs ,,, and AIS QDs , with some variations in the band intensities, which can be related to differences in the QD composition, preparation technique, and different excitation wavelengths and confinement effects.…”

“…In several studies, Raman spectra for Zn-doped CIS QDs as well as core/shell CIS/ZnS , and AIS/ZnS ,,, nanostructures were presented. Our recent paper is devoted to a detailed study of Raman and infrared spectra of nonstoichiometric AIS, CIS, and Hg–In–S (HIS) QDs based on the phonon assignment with the account of data for bulk crystals. , …”

“…The addition of a strong electron acceptor (methylviologen (MV 2+ )) to the colloidal CAIS QD solutions enabled us to almost completely quench the intense broadband PL of the QDs. This effect originates from the efficient photoinduced electron transfer from the semiconductor QDs to MV 2+ reducing it to a MV •+ cation radical. − Recently, we used this effect to probe Raman vibrational properties of aqueous nonstoichiometric M I –In–S QDs as a function of the nature of the M I component (M = Ag, Hg, Cu). , …”

Intrinsic and Extrinsic Exciton Recombination Pathways in AgInS 2 Colloidal Nanocrystals

“…The relative Φ PL increase at cryogenic temperatures is consistent with SEC experiments at negative potentials, suggesting that electron trapping at the NC surface is phonon-assisted. By fitting the integrated PL intensity evolution with T to the function [45,51]…”

Bench tests for microscopic theory of Raman scattering in powders of disordered nonpolar crystals: Nanodiamonds and beyond

“…The good candidates are the inherently disordered NPs like CuInS 2 and Cu 2 ZnSnS 4 . [44,45] Due to the synthesis method, they do not obey the ideal stoichiometries, which leads to the presence of a number of vacancies, intersitials, anticites, and other defects with the concentration c imp $ 1. The Raman spectroscopy is a standard tool for characterization of such materials, and we shall show that the theoretical framework proposed in the present paper can provide some useful information about their intrinsic structure.…”