Chemicals. Tin (II) chloride (SnCl2, 99%), bismuth (III) iodide (BiI3, 99.9%), bismuth (III) bromide (BiBr3, 99.9%), bismuth (III) chloride (BiCl3, 99.9%), and hexacarbonyl tungsten (W(CO)6, 97%) were purchased from Alfa Aesar. Hexamethyldisilazane(HMDS, 99%) was purchased from Sigma-Aldrich. Oleylamine (OAm, 70%) and 1-octadecene (ODE, 90%) were purchased from Sigma-Aldrich, and were degassed under vacuum for 1 h at 120 °C prior to use. N-hexane (>97%) and ethanol (>99.7%) were purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. Commercial Bi (50 nm,99.9%) was purchased from Shanghai BoWei Applied Material Technology Co., Ltd.Synthesis of Sn nanoparticles (NPs). According to a reported procedure,[1] W(CO)6

(10 mg) was added into a three-neck round-bottom flask equipped with a condenser, thermometer adapter, thermometer, and rubber septum. Then, this round-bottom flask was degassed under vacuum at room temperature for 1 h. SnCl2 (40 mg) was dissolved into 1 mL of OAm and 9 mL of ODE by sonication. Then, this mixed solution and 0.75 mL of HMDS were injected into the three-neck round-bottom flask under argon flow.The mixture was heated to 210 °C (~20 °C/min) from room temperature and held for 10 min. The detailed reaction phenomena are illustrated in Figure S3. The flask was quickly cooled by an ice water bath, after which a black solution was obtained. Hexane (12.5 mL) and ethanol (25 mL) were added into the solution to precipitate the NPs by centrifugation at 4000 rpm for 5 min. The precipitate was further purified three times using a hexane/ethanol mixture with a volumetric ratio of 1:1, and finally dissolved into hexane for further characterizations.

Synthesis of Sn1-xBix alloy NPs. The same procedure as for Sn NPs was followed by using SnCl2 and BiI3 as precursors. By increasing the amount of BiI3 (2, 5, and 10 mg), a series of Sn1-xBix alloy NPs (Sn0.93Bi0.07, Sn0.86Bi0.14 and Sn0.80Bi0.20, respectively)were obtained. The detailed reaction phenomena are illustrated in Figure S3. Ligand removal. The 10 mg of as-prepared Sn and Sn1-xBix alloy NPs was dispersed in 10 mL of n-butylamine and stirred for at least 12 h. This mixture was separated and purified several times by adding 10 mL of hexane with centrifugation at 4000 rpm for5 min. Finally, NP powders were obtained by drying under vacuum at 60 °C for at least 1 h.