Colloidal CdSe Quantum Rings

Transcription

1 Supporting Information Colloidal CdSe Quantum Rings Igor Fedin and Dmitri V. Talapin *,, Department of Chemistry and James Franck Institute, the University of Chicago, IL 60637, USA Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, USA S1

2 Chemicals Cadmium acetate hydrate (99.99%, Aldrich), cadmium nitrate tetrahydrate (99.999%, Aldrich), cadmium formate (MP Biomedicals), zinc acetate dihydrate (99.99%, Aldrich), myristic acid (99.0%, Aldrich), selenium (pellet or powder, 99.99%, Aldrich), tellurium (powder, 30 mesh, %, Aldrich), sulfur (99.98%, Aldrich), thioacetamide (99.0%, Aldrich), ammonium sulfide (40 48% in water, Aldrich), trioctylphosphine (TOP, 97%, Strem), tributylphosphine (TBP, mixture of isomers, 97%, Aldrich), oleic acid (OA, 90%, Aldrich), oleylamine (OAm, 70%, Aldrich), 1-octadecene (ODE, 90%, Aldrich), hexane (anhydrous, 95%, Aldrich), methylcyclohexane (MCH, anhydrous, 99%, Aldrich), toluene (anhydrous, 99.8%, Aldrich), ethanol (anhydrous, 99.5%, Aldrich), N-methylformamide (NMF, 99%, Alfa Aesar) were used as received. Synthesis of Nanomaterials CdE NPLs. We synthesized CdSe 512 NPLs from Cd myristate, Cd acetate, and Se powder in 1- octadecene (ODE) using procedures originally developed by S. Ithurria et al. and described in a work by M. Pelton et al. 1 We degassed 170 mg of cadmium myristate in 15 ml of ODE at 80 C for 10 min in a three-neck flask and then cooled the solution to room temperature. We weighed out 12 mg of Se powder in a nitrogen glovebox (GB) and added it to the degassed solution outside the GB. We then degassed the resulting mixture (cadmium myristate and Se in ODE) at 90 C for 30 min. After that, we heated up the mixture at a rate of C per min. In the meantime, we weighed out 40 mg of freshly ground Cd acetate dihydrate and added it to the solution once the temperature reached 190 C (the reaction mixture was orange-red at this point). We continued to rapidly heat the mixture until 215 C and then lowered heating rate to reach 240 C without overshooting the mark. We kept the reaction mixture at 240 C for 0 and 5 min (depending on the desired lateral dimensions of the NPLs). The next step is dangerous. We withdrew the hot reaction product and injected it into a solution of 2 ml OA in 10 MCH using a glass syringe with a steel needle. We let the solution sit for a few hours and centrifuged it at rpm (14000g) for 5 min to isolate the NPLs from QDs and other species in the solution. We discarded the supernatant, collected the precipitate, dispersed the latter in 4 ml of anhydrous MCH, and filtered the solution through a 0.2 μm PFTE filter. We also varied the duration of the growth at 240 C to vary the lateral dimensions of the NPLs. For short NPLs, we terminated the reaction when the temperature reached 225 C. 2 We synthesized CdS NPLs using X. Peng s recipe located in Section Synthesis of CdS quantum disks with their first absorption peak at 374 nm of ref. 3 CdTe NPLs emitting at 500 nm were synthesized from Cd acetate and TOPTe using a single-shot injection following the approach reported by S. Pedetti et al. 4 Transformation of NPLs into NRings. To synthesize of toroidal nanorings, we precipitated assynthesized CdSe NPLs emitting at 512 nm from their crude solution with ethanol to remove S2

3 leftover Cd(OAc)2 from the synthesis and redispersed NPLs in hexane. After the second precipitation and drying, the precipitate weighed 7.8 mg. We dispersed the precipitate into 3 ml of ODE and 1 ml of OAm in a three-neck flask. We did this easily by dispersing the NPLs in a small volume (50 μl) of MCH, dissolved the solution into the ODE and OAm, and then heated the contents at 80 C under nitrogen for 10 min to evaporate the MCH. We sonicated 7.9 mg of Se in 1 ml of OAm and added 0.2 ml of the resulting suspension into the flask. Then we degassed the contents of the flask at room temperature for 10 min, heated them up to 140 C under nitrogen and maintained this temperature for 10 min. Next, we injected 0.2 ml TBP, increased the temperature to 220 C, removed the heat mantle, and allowed the solution to cool to room temperature. At 37 C we added 10 mg of finely ground Cd formate and stirred the mixture for an hour. The ratio of NPLs to Se is important. Absorption spectroscopy is a convenient way to estimate the amount of NPLs. When a stock solution of NPLs diluted by a factor of 300 had absorbance of 0.2 at the first excitonic peak in a 1 cm cuvette, we used 1 ml of this stock solution for the perforation. While the solution s temperature was maintained at 140 C for 10 min, the color of the solution darkened and turned dark brown-red. If the solution is red (the absorption spectrum of an aliquot is clear past 600 nm), one should make another 0.2 ml injection of Se/TBP and track the time from the last injection. Characterization methods Transmission electron microscopy (TEM). The imaging was carried out using a 300 kv FEI Tecnai F30 microscope. Samples were prepared by drying μl of diluted nanocrystal solution in hexane onto a carbon-coated copper grid (Ted Pella). Optical absorption measurements. Absorption spectra for colloidal solutions were collected using a Cary 5000 UV-Vis-NIR spectrophotometer. Photoluminescence measurements. PL spectra in the wavelength range of nm were collected using a FluoroMax-4 or FluoroLog-3 spectrofluoremeter (HORIBA Jobin Yvon). S3

4 The effect of experimental conditions. The transformation of CdSe NPLs into NRings upon treatment with Se OAm complex is sensitive to the ratio of NPLs to Se, the manner of the introduction of Se, and the duration of the treatment at 140 C. We took aliquots of the mixture as the reaction progressed at 140 C and after the injection of TBP as the system was heated to 220 C. The perforation did not begin until the temperature reached 140 C. After the first minute, Se grew crowns around the NPLs. At the fourth minute, the NPLs were in the middle of the perforation, which was nearly over at the end of the eighth minute. No visible change of the nanostructures (both in terms of absorption spectra and TEM not shown here) happened after the injection of TBP at 10 min and when the system was heated to 220 C. This is likely due to lowered Se reactivity resulting from the formation of TBPSe (Figure S1). Figure S1. TEM images and spectra of aliquots taken in a typical treatment. The labels indicate time elapsed at 140 C. In the case of a lack of Se, perforated NPLs were transient between the second and sixth minutes. They collapsed into dumbbell-like structures by the tenth minute (Figure S2). Se began to dissolve in ODE at C, which sets 140 C close to the lower boundary for the perforation reaction. We attempted several reactions at higher temperatures (150 C and 160 C) and observed issues with run-to-run reproducibility of the ring formations. S4

5 Figure S2. TEM and absorption spectra of aliquots from a different batch of CdSe 512 NPLs treated with Se at 140 C. S5

6 Figure S3. Examples of CdSe 512 NPLs perforated with thioacetamide at 140 C. Figure S4. CdSe NPLs treated with S in OAm at 140 C. Sulfur in OAm only partly etches the middle part of NPLs. S6

7 Figure S5. (A) Treatment of CdSe NPLs with thioacetamide in the absence of OAm at 140 C. In the absence of OAm, thioacetamide does not perforate NPLs. (B) In the absence of Se, OAm and TBP do not perforate NPLs. Intensity θ (deg) Figure S6. Powder X-ray (Cu Kα radiation) diffraction pattern of CdSe nanorings shown in Figure 1c. The vertical lines show positions and relative intensities of the diffraction peaks for bulk zinc blende CdSe phase. S7

8 Figure S7. One can vary the lateral dimensions of the initial CdSe 512 NPLs. 2 Transmission electron microscope images of CdSe NPLs with 1.2 nm thickness and different lateral dimensions: (a) short (b) medium (c) long. Coating CdSe NRings with Cd 1-x Zn x S shells. We grew three monolayers of the shell using c- ALD, Method B reported by S. Ithurria et al. 5, with slight modifications. In the first cycle, we introduced pure Cd(OAc)2/NMF at the cation stage. In the second cycle, we introduced Cd(OAc)2 and Zn(OAc)2, 7:3. In the third cycle, we introduced Cd(OAc)2 and Zn(OAc)2, 1:1. We observed an expected red shift in the absorption and emission spectra of the core shells. The holes were visible in TEM. Figure S8. (A) Absorption and PL spectra of original CdSe nanorings and resulting CdSe/Cd1-xZnxS (3 ML) core shells. (B,C) TEM images of the core shells. S8

9 Figure S9. (A) Absorption and PL spectra of original CdSe double ring nanostructures and resulting CdSe/Cd1-xZnxS (2 ML) core shells. (B.C) TEM images of the core shells. 5 nm Figure S10. High-resolution TEM image of CdSe/Cd1-xZnxS (2 ML) double ring core shell nanostructures showing their single-crystalline nature. S9

10 Extending the treatment to CdS and CdTe NPLs. CdS and CdTe NPLs, like the family of CdSe NPLs emitting at 462 nm, have large lateral dimensions and are thus sheet-like structures. Se powder introduced to CdS NPLs6 with the excitonic peak at 386 nm created holes that grew and caused some NPLs to tear into stripe-like fragments. CdTe NPLs emitting at 500 nm4 behaved similarly they developed holes that propagated and eventually reached the NPL edges. 50 nm 50 nm Figure S11. TEM images of CdTe NPLs perforated with Se powder at 140 C. Supporting references S1. Pelton, M.; Ithurria, S.; Schaller, R. D.; Dolzhnikov, D. S.; Talapin, D. V. Nano Lett. 2012, 12, S2. She, C.; Fedin, I.; Dolzhnikov, D. S.; Dahlberg, P. D.; Engel, G. S.; Schaller, R. D.; Talapin, D. V. ACS Nano 2015, 9, S3. Li, Z.; Qin, H.; Guzun, D.; Benamara, M.; Salamo, G.; Peng, X. Nano Res. 2012, 5, S4. Pedetti, S.; Nadal, B.; Lhuillier, E.; Mahler, B.; Bouet, C.; Abecassis, B.; Xu, X.; Dubertret, B. Chem. Mater. 2013, 25, S5. Ithurria, S.; Talapin, D. V. J. Am. Chem. Soc. 2012, 134, S6. Bouet, C.; Laufer, D.; Mahler, B.; Nadal, B.; Heuclin, H.; Pedetti, S.; Patriarche, G.; Dubertret, B. Chem. Mater. 2014, 26, S10