Strongly Fluorescent Quaternary Cu-In-Zn-S Nanocrystals Prepared from Cu 1-x InS 2 Nanocrystals by Partial Cation Exchange

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1 SUPPORTING INFORMATION FOR: Strongly Fluorescent Quaternary Cu-In-Zn-S Nanocrystals Prepared from Cu 1-x InS 2 Nanocrystals by Partial Cation Exchange Inductively Coupled Plasma and emission analysis In:Cu precursors In:Cu In:Cu Zn:In In:S 1 : 1 1 : : : 1 1 : : : : : 1 1 : : 1 2 : : :1 1 : : : : : 1 1 : : : 1 6 : : 1 1 : : : 1 9.5: : 1 1 : 2.4 Table S1. In:Cu ratios were measured using Inductively Coupled Plasma(ICP). The In:S ratio is very close to 1:2 in all the CIS samples and was therefore rounded to 1:2 (typical values are in the range :1). ICP chemical quantifications are affected by a systematic error of about 5%. In:Cu PL QY λem (nm) precursors λ em (nm) PL QY (%) (%) 1 : : : : : : Table S2. λ em is the wavelength corresponding to the maximum of the emission peak exciting at 450 nm. The QY was measured using Rhodamine 101 as reference standard. S1

2 X-ray Powder Diffraction (XRD) Figure S1. XRD patterns measured for samples along with the bulk pattern of roquesite. Time Resolved Photoluminescence (TRPL) Samples PL decay times (ns) CuInS (28) Cu 0.45InS (33) Cu 0.38InS (34) Cu 0.91InZn 1.1S Cu 0.32InZn 0.9S Cu 0.17InZn 0.8S Table S3. PL decay times. Short components, if present, are indicated in brackets. S2

3 STEM-EDX and EFTEM analysis The elemental quantifications were performed also via STEM-EDX analysis considering many areas containing one hundred of NPs. In Figure S2 are reported STEM-EDX spectra of two representative samples (precursors In:Cu=1:1) of CIS (Figure S2a) and CIZS (Figure S2b). The experimental spectra exhibited S Kα, In Lα and Cu Kα peaks for CIS and S Kα, In Lα Cu Kα and Zn Kα peaks for CIZS. The chemical quantifications were consistent with roquesite and quaternaty phase with roquesite-like structure, respectively. Figure S2. STEM-EDX spectra of two representative CIS (a) and CIZS (b) samples showing the peaks of S Kα, In Lα and Cu Kα and. S Kα, In Lα Cu Kα and Zn Kα, respectively. The elemental quantifications (atomic percentage) yielded chemical compositions of Cu 26%, In 27%, S 48% for CIS NPs and Cu 19%, In 19%, Zn 16%, S 46% for CIZS NPs In Figure S3 we reported, as explicative example, two STEM-EDX analyses of CIZS sample performed acquiring EDX spectra from a single nanocrystal (A) and from one hundred of NPs (B), under the same experimental condidions. The X-ray signals coming from the single NC (size of 3 nm), formed by atoms, exhibited a poor peak/background ratio that made the A EDX spectrum not interpretable in term of stoichiometry. In this case the signals of In Lα, Cu Kα and Zn Kα were negligible and below their detection limit. On the contrary, in the B EDX spectrum S Kα, In Lα, Cu Kα and Zn Kα signals exhibited a good peak/background ratio yielding a chemical composition Cu 19% (±0.24%), In 19% (±0.92%), Zn 15% (±1.5%) and S 46% (±0.7%) that was consistent with a stoichiometry of CuInZn 0.8 S Figure S3. HAADF STEM image shwoing EDX analysis of single NC (A) and one hundred NCs (B) of CIZS. S3

4 Energy Filtered TEM (EFTEM) maps, obtained by filtering the Zn L edge (1020 ev), were not interpretable because of the faint and blurred Zn signal across the analyzed regions. There were two negative factors to take into account, i.e. the small scattering volume (few nm 3 ) of single NCs that limited the useful scattered signal and the low value of energy-edge/background ratio of the heavy elements like Zn. Both factors, in addition to mechanical drift during the maps acquisition, contributed to decrease and blur the Zn signal across the NCs making difficult elemental maps interpretation. Below are shown, as example, the S L edge (165 ev) and the Zn L (1020 ev) energy filtered maps. Figure S4. EFTEM images of showing the S L edge (165 ev) and Zn L edge (1020 ev) maps HAADF-STEM analysis Figure S5. HAADF STEM images of Cu 0.91 InZn 1.1 S 2.6 (a), Cu 0.32 InZn 0.9 S 2.5 (b) and Cu 0.17 InZn 0.8 S 2.7 (c) NCs. 3D atomic modeling Starting from the experimental observations of crystal size and shape, we built a 3D atomic model of a roquesite crystal which exhibits tetrahedral habit with 3 nm side (Figure S3). This crystal has a total volume of 3.12 nm 3 and contains about 180 atoms of Cu, 180 of In and 360 of S. If a {222} monolayer of ZnS sphalerite with a thickness of at least 0.16 nm, containing 91 atoms of Zn, coats all four facets of roquesite crystal, forming a continuous shell, the total amount of Zn atoms would be 364 atoms. In this case the atomic ratio Zn:In will be 2, but this value disagrees totally with the experimental atom ratio Zn:In of 0.8, calculated from the STEM-EDX quantifications. Figure S6 displays the atomic models of tetrahedral roquesite crystal (CuInS 2 ) and {222} sphalerite slice (ZnS), S4

5 respectively. Figure S7 shows schematically the growth process of a continuous ZnS shell on roquesite crystal with total covering. The only way to obtain a ratio Zn:In=1 would be the growth of a continuous shell or a patched shell, both formed by {222} ZnS islands 0.16 nm thick, that cover 50% of the whole crystal surface (Figure S8). In both cases the particles diameter would increase, but no size variation occurred via HAADF STEM image assessment. From a crystallographic point of view it is evident that the unit cell of ZnS sphalerite (a= 5.4 Å) is compatible with half unit cell of roquesite (a=b=5.52 Å, c=11.14 Å), as shown in Figure S9. In particular, both phases exhibit the same S anionic framework and equivalent cation crystallographic sites filled by atoms with ionic radius not so different (Cu 1+ =0.96Å, In 3+ =0.81 Å and Zn 2+ =0.74 Å). For these reasons the Zn atoms can give rise to a partial substitution of Cu and In atoms in roquesite producing a mixed Cu-In-Zn-S phase with chalcopyrite-like structure instead of a core/shell structure roquesite/sphalerite, as confirmed from all experimental characterizations carried out on Figure S6. Atomic 3D model of a tetrahedral crystal of roquesite (3 nm width) and a {222} slab of sphalerite (0.16 nm thick) Figure S7. Morphological schema showing the growth of a continuous shell with total covering; {222} ZnS slices in blue and roquesite crystal in green. S5

6 Figure S8. Morphological schema showing the growth of continuous (left) and patched (right) {222} ZnS shells with partial covering of 50% of the total crystal surface; ZnS in blue, roquesite in green. Figure S9. Unit cell atomic sketches of tetragonal roquesite (a=b=5.52 Å, c=11.14 Å) and sphalerite (a= 5.4 Å) XPS analysis Atomic concentrations (obtained by measuring the areas under the XPS peaks, normalized by the corresponding atomic sensitivity factor) are reported in Table S4. In:Cu precursors In:Cu In:Cu In:Zn In:S 1 : : : : 1 1 : : : : : 1 1 : 3 8 : : 1 7 : : 1 1 : 3.1 Table S4. In:Cu, In:Zn and In:S ratios measured through X-ray Photoemission Spectroscopy (XPS) in CIS and Zn-CIS samples. S6

7 Figure S10. High resolution XPS on CIS samples. Comparison of S 2p peaks measured on CuInS 2, Cu 0.45 InS 2 and Cu 0.38 InS 2 samples. The position of S 2p peak maximum was not affected by the In:Cu stoichiometry, and was always at (162.1 ± 0.1) ev. Figure S11. High resolution XPS on CIS and CIZS samples. Panel a: comparison of Cu 2p peaks measured on CuInS 2, Cu 0.45 InS 2 and Cu 0.38 InS 2 samples. Panel b: comparison of Cu 2p peaks measured on Cu 0.91 InZn 1.1 S 2.6, Cu 0.32 InZn 0.9 S 2.5 and Cu 0.17 InZn 0.8 S 2.7 samples. In all cases, no evidence of shake-up satellites in the Cu 2p spectra, typical of Cu(II) species. S7