Supplementary Figure S1. High-resolution XPS spectra in the Cu 2p region and Cu LMM spectra are shown in (A) and (B) respectively for CdSe NCs

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1 Supplementary Figure S1. High-resolution XPS spectra in the Cu 2p region and Cu LMM spectra are shown in (A) and (B) respectively for CdSe NCs treated with 651 Cu + ions per NC and in (C) and (D) respectively for unexchanged NCs.

2 Supplementary Figure S2. The actual amount of copper incorporated in the NCs (Supplementary Table S1) was detected at three points along the conversion from CdSe to Cu 2 Se using inductively coupled plasma-optical emission spectroscopy (ICP-OES). Absorption spectra signifying the degree of conversion for each of the samples are shown in (A). The starting NCs show a Cd to Se ratio of 1:1. With 399 Cu + ions per NC added, nearly all the available Cu + ions appears to be incorporated within the NCs, with a proportional decrease in the number of Cd 2+ ions. When the concentration reaches 1141 Cu + ions per NC, the NCs are predominantly copper selenide with an average stoichiometry of Cu 1.86 Se, with some residual Cd, possibly in the form of dopants in the NCs or leftover cadmium hydroxide, which is known to appear in XPS at a Cd 3d 5/2 binding energy of ev. 36, 37 XPS with Cd 3d 3/2 and 3d 5/2 peaks are shown in (B) for CdSe NCs treated with different amounts of Cu + ions from a methanolic solution. All XPS spectra were calibrated relative to the C 1s peak.

3 Defect PL Intensity Energy (ev) Supplementary Figure S3. In a control experiment, 200 µl of methanol was added to a solution of CdSe nanocrystals (NCs). A photoluminescence (PL) spectrum taken after 70 min showed that no defect luminescence emerged as a result of methanol addition. The duration of 70 min is representative of the timescale of each titration point in our experiments.

4 Supplementary Figure S4. High resolution transmission electron microscopy (HRTEM) verifies the crystalline nature of NCs throughout the cation exchange process. HRTEM images were taken using a JEOL 2010 LaB 6 operating at 200 kv. Four representative images each are shown at four stages of the conversion from CdSe to Cu 2 Se. At all stages of the conversion, images show lattice fringes with a spacing of 0.35 nm in a majority of cases (although a few nanocrystals were identified with other lattice orientations at all stages of the conversion). This lattice spacing matched the longest lattice spacing measured in electron diffraction patterns (shown in the main text) for all samples from CdSe through Cu 2 Se. Since the lattice spacings for the unexchanged CdSe and the Cu 2 Se NCs are similar, HRTEM cannot be used to characterize the degree of exchange for individual NCs. Samples were prepared by a slow titration of CdSe nanocrystals in toluene with increasing amounts of a methanolic solution of Cu +, similar to the titration experiments described in the main text figures. At various points in the titration, a small aliquot was drop-casted onto an ultrathin carbon TEM grid and washed with methanol several times. The total amount of Cu + added is indicated below each image in the form of number of ions per NC, estimated as described in the Methods section.

5 Supplementary Figure S5. High-angle annular dark field scanning electron transmission microscopy (HAADF-STEM) displays NC size and shape retention throughout cation exchange. Top panel shows representative HAADF-STEM images for NCs at various stages of conversion from CdSe to Cu 2 Se. NC sizes were estimated manually in Image J from several representative images by measuring the diameter of the NC using the line tool. Data from ca. 200 NCs was used to make the size histograms shown in the bottom panel. Average diameters were determined by Gaussian fitting of the histograms. At all stages, the nanocrystals are 3.9 nm in size and quasi-spherical in shape. Samples were prepared by a slow titration of CdSe nanocrystals in toluene with increasing amounts of a methanolic solution of Cu +, similar to the titration experiments described in the main text figures. At various points in the titration, a small aliquot was drop-casted onto an ultrathin carbon TEM grid and washed with methanol several times. The total amount of Cu + added is indicated below each image in the form of number of ions per nanocrystal, estimated as described in the Methods section. HAADF-STEM images were acquired on a JEOL 2010F at 200 kv with a 0.5-nm size beam.

6 Supplementary Figure S6. The absorption spectra for CdSe NCs exchanged with Ag + are presented in uncorrected (A) and baseline corrected (C) form. The defect luminescence spectra are presented in uncorrected (B) and baseline corrected (D) form. The method used for baseline correction was identical to that used for titrations with Cu +. The cleft feature at 1.79 ev in the corrected (D) and uncorrected (B) PL spectra is an instrumental artifact.

7 Supplementary Table S1. ICP-OES elemental analysis showing actual amount of copper incorporated in the NCs at three titration points along the conversion from CdSe to Cu 2 Se Cu + ions per NC Cd:Se ratio Cu:Se ratio Se:Se ratio Supplementary Table S2. Concentration calculations for Cu + exchange titrations Addition Number [Cu + ] added (moles) Total [Cu + ] (moles) Cu + ions per NC E E E E E E E E E E E E E E E E E E E E E E E E E E E E Supplementary References 36 Lee, W.; Kim, H.; Jung, D., Nahm, C.; Lee, J.; Kang, S.; Lee, B.; Park, B.; An effective oxidation approach for luminescence enhancement in CdS quantum dots by H 2 O 2, Nanoscale Res. Lett. 2012, 7, Zhang, D.E.; Pan, X.D.; Zhu, H.; Li, S.Z.; Xu, G.Y.; Zhang, X.B.; Ying, A.L.; Tong, Z.W.; A simple method to synthesize cadmium hydroxide nanobelts, Nanoscale Res. Lett. 2008, 3, 284.