Nanocrystals have a great potential for future materials with tunable bandgap, due to their optical properties that are related with the material used, their sizes and their surface termination. Here, we concentrate on the silicon–tin alloy for photovoltaic applications due to their bandgap, lower than bulk Si, and also the possibility to activate direct band to band transition for high tin concentration. We synthesized silicon–tin alloy nanocrystals (SiSn-NCs) with diameter of about 2–3 nm by confined plasma technique employing a femtosecond laser irradiation on amorphous silicon–tin substrate submerged in liquid media. The tin concentration is estimated to be\documentclass[12pt]{minimal}
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				\begin{document}$$\sim 17\%$$\end{document}∼17%, being the highest Sn concentration for SiSn-NCs reported so far. Our SiSn-NCs have a well-defined zinc-blend structure and, contrary to pure tin NCs, also an excellent thermal stability comparable to highly stable silicon NCs. We demonstrate by means of high resolution synchrotron XRD analysis (SPring 8) that the SiSn-NCs remain stable from room temperature up to\documentclass[12pt]{minimal}
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				\begin{document}$$400\,^{\circ }\text {C},$$\end{document}400∘C,with a relatively small expansion of the crystal lattice. The high thermal stability observed experimentally is rationalized by means of first-principle calculations.