Surface Oxidation of Tin Chalcogenide Nanocrystals Revealed by ¹¹⁹Sn–Mössbauer Spectroscopy

Abstract

Narrow band gap tin(II) chalcogenide (SnS, SnSe, SnTe) nanocrystals are of high interest for optoelectronic applications such as thin film solar cells or photodetectors. However, charge transfer and charge transport processes strongly depend on nanocrystals' surface quality. Using (119)Sn-Mössbauer spectroscopy, which is the most sensitive tool for probing the Sn oxidation state, we show that SnS nanocrystals exhibit a Sn((IV))/Sn((II)) ratio of around 20:80 before and 40:60 after five minutes exposure to air. Regardless of the tin or sulfur precursors used, similar results are obtained using six different synthesis protocols. The Sn((IV)) content before air exposure arises from surface related SnS(2) and Sn(2)S(3) species as well as from surface Sn atoms bound to oleic acid ligands. The increase of the Sn((IV)) content upon air exposure results from surface oxidation. Full oxidation of the SnS nanocrystals without size change is achieved by annealing at 500 °C in air. With the goal to prevent surface oxidation, SnS nanocrystals are capped with a cadmium-phosphonate complex. A broad photoluminescence signal centered at 600 nm indicates successful capping, which however does not reduce the air sensitivity. Finally we demonstrate that SnSe nanocrystals exhibit a very similar behavior with a Sn((IV))/Sn((II)) ratio of 43:57 after air exposure. In the case of SnTe nanocrystals, the ratio of 55:45 is evidence of a more pronounced tendency for oxidation. These results demonstrate that prior to their use in optoelectronics further surface engineering of tin chalcogenide nanocrystals is required, which otherwise have to be stored and processed under inert atmosphere.

Interrelations between technology for obtaining quantum dots and optoelectronic properties of semiconductors [Invited]

“…NCs formed through synthesis of SnS are of spherical form because the theoretical dependences are computed following the GLSWD. One can see from the fragments a and b that the theoretical curves are well fitting the experimentally obtained histograms for SnS, which correspond to the protocols A and E [41]. It puts in evidence that growth of QDs obtained by applying the chemical techniques is governed at the OR stage by two mechanisms, viz.…”

A Facile Peroxo-Precursor Synthesis Method and Structure Evolution of Large Specific Surface Area Mesoporous BaSnO3

“…The structure evolution of the BS-120 precursor to the BS-T samples is investigated by 119 Sn Mossbauer spectra, from which the chemical state of Sn on a atomic scale can be detected with a high sensitivity based on the obtained values of isomer shift (IS) and quadrupole splitting (QS). 45 The IS value is closely related to the valence state and atomic spacing, while the QS value is sensitive to the symmetry of the charge around the Sn atom. 46 The experimental and fitted Mossbauer spectra data for BS-T and S-120 at room temperature are shown in Figure 6, and the corresponding parameters are given in Table 2.…”

Sn Cation Valency Dependence in Cation Exchange Reactions Involving Cu2-xSe Nanocrystals

“…The laser source (nominal power 0.5 mW) was focused on the sample through a 100× objective, with an integration time of 120 s. In the case of SnSe and Cu 2- x Se/SnSe NCs the measurements were performed under inert atmosphere in order to avoid oxidation phenomena. 33 For this purpose, nitrogen was fluxed through a closed chamber (from Linkam) coupled with a Renishaw InVia spectrometer. Data were acquired at λ = 632.8 nm with a 50× objective using a nominal power of 25 mW and an integration time of 30 s.…”