Core/Shell Nanoheterostructures

Transcription

1 High Temperature Photoluminescence of CdSe/CdS Core/Shell Nanoheterostructures Benjamin T. Diroll and Christopher B. Murray Department of Chemistry and Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, Figure S1. (a,b) Transmission electron microscopy, (c) absorption, and (d) photoluminescence (PL) data collected for a sample of CdSe nanorods (NRs) before and after performing a heating cycle for temperature-dependent PL measurements. The sample was heated to 600 K, then returned to 300 K and dissolved in hexanes to perform measurements. Scale Bars are 50 nm. (e) Steady-state PL from the same CdSe NRs at 300 K, 600 K, and returned to 300 K after heating. (f) Time-resolved photoluminescence signals from the same sample measured at the same temperatures. The results show strong PL quenching and short lifetimes at elevated temperatures similar to previous results. 1,2

2 Figure S2. Black open circles show the experimental values for the change in the energy maximum of the PL for each dot-in-rod and rod-in-rod sample measured in this paper, defined as the change from the maximum at 300 K. The black dashed line is the expected result based upon previous literature findings. 3,4 Blue squares show the change in the bandwidth of sample PL, defined as change in the FWHM compared to the FWHM measured at 300 K. The blue dashed line is the approximate broadening found in the literature. We have further assumed in this fitting that the samples show instrinsic FWHM of ~40 mev from 0 K to 300 K and that the band-gap change from 0 K to 300 K is 50 mev, both consistent with literature findings. 3 Figure S3. A reproduction of the plots from Figure 3 of the main text replacing the heating curve measurements of PL intensity with the cooling curve PL intensity measurements.

3 Figure S4. (a) PL intensity as a function of temperature plotted for three samples of sphere-insphere type CdSe/CdS nanocrystals (NCs) with their corresponding TEM images (b) and absorption spectra (c). Scale bars are 50 nm. Figure S5. (a,b) Scatter plots of the quantum yield measured at room temperature in solution with 450 nm excitation and the PL preserved at 600K and upon cooling back to 300 K. (c,d) Volume fraction of the core as a percentage of the total heterostructure volume, determined by

4 TEM, plotted against the preserved PL at 600 K and the PL intensity upon cooling back to 300 K. The difference symbols and colors represent samples made with different core samples. Figure S6. Enlarged selection from time-dependent PL data collected at 412 K showing the exponential rise in PL intensity fitted with the red line. Figure S7. Solution fluorescence lifetime data extracted for CdSe/CdS core/shell nanocrystals plotted against the fraction of the heterostructure volume occupied by the CdSe core. Lifetime data is extracted empirically according to literature methods. 5 Aggregated data are derived from samples studied in this work and previous work by the authors. 6

5 Figure S8. (a) Arrhenius plots of the non-radiative rate (using the quantum yield of the sample at room temperature weighted by the normalized temperature-dependent intensity and the radiative lifetime from TRPL measurements). Fit lines are linear fits to the data. (b) Extracted parameters from fits of (a), the attempt frequency and the activation energy. Sample numbers are derived from the table below. Figure S9. PL maxima of isotropic CdSe/CdS heterostructures made using seeded-growth techniques. Colors represent different samples.

6 Figure S10. Relative PL intensity in heating and cooling curves of CdSe/CdS dot-in-rod sample that has been overcoated with a CdZnS shell at 190 C. Approximately 90% of the PL is quenched upon returning to 300 K. Figure S11. TEM micrographs of samples used in this study. (a-c) large core dot-in-rod samples; (d) medium core dot-in-rod; (e-g) small core dot-in-rod samples; (h) rod-in-rod sample; samples of dot-in-rod (i) and rod-in-rod (j) synthesized at low temperature and not through seededgrowth. Scale bars are 50 nm (a-h), 10 nm (i), and 20 nm (j). Table of Seeded Core/Shell NRs Used in this Work Sample Core Length Width Volume PL@600K 325K 300K Τ QY (nm) (nm) (nm 3 ) return return (ns) 1 L L L M S S S Rod

7 Photoselection Anisotropy Full details of photoselection methods to determine fluorescence anisotropy can be found elsewhere. In short, anisotropy is defined as 2 in which V and H denote (1 st ) excitation polarizer position (vertical or horizontal) and (2 nd ) emission polarizer position. The factor corrects for the differential throughput of vertical or horizontal polarizations in instrumentation. Photoselection as described here presumes a randomly dispersed sample in which dipoles co-aligned with the incident polarization are selectively excited. This subpopulation of the ensemble is then responsible for emission of light and so any rotation of the sample after excitation decreases the measured anisotropy according to 1 in which τ is the radiative lifetime and is the rotational constant of the fluorophore. Decreasing the solvent viscosity results in smaller, but experiments in this paper are made under conditions such that for all temperatures measured. Typical fluorescence lifetimes of CdSe/CdS NRs are ~10-30 ns and typical rotational constants for the same are >1 μs. 7 References (1) Zhao, Y.; Riemersma, C.; Pietra, F.; Koole, R.; Donegá, C. de M.; Meijerink, A. High- Temperature Luminescence Quenching of Colloidal Quantum Dots. ACS Nano 2012, 6, (2) Rowland, C. E.; Schaller, R. D. Exciton Fate in Semiconductor Nanocrystals at Elevated Temperatures: Hole Trapping Outcompetes Exciton Deactivation. J. Phys. Chem. C 2013, 117, (3) Rainò, G.; Stöferle, T.; Moreels, I.; Gomes, R.; Kamal, J. S.; Hens, Z.; Mahrt, R. F. Probing the Wave Function Delocalization in CdSe/CdS Dot-in-Rod Nanocrystals by Time- and Temperature-Resolved Spectroscopy. ACS Nano 2011, 5, (4) Valerini, D.; Cretí, A.; Lomascolo, M.; Manna, L.; Cingolani, R.; Anni, M. Temperature Dependence of the Photoluminescence Properties of Colloidal CdSe ZnS Core/Shell Quantum Dots Embedded in a Polystyrene Matrix. Phys. Rev. B 2005, 71,

8 (5) She, C.; Demortière, A.; Shevchenko, E. V.; Pelton, M. Using Shape to Control Photoluminescence from CdSe/CdS Core/Shell Nanorods. J. Phys. Chem. Lett. 2011, 2, (6) Diroll, B. T.; Koschitzky, A.; Murray, C. B. Tunable Optical Anisotropy of Seeded CdSe/CdS Nanorods. J. Phys. Chem. Lett. 2014, 5, (7) Kamal, J.; Gomes, R.; Hens, Z.; Karvar, M.; Neyts, K.; Compernolle, S.; Vanhaecke, F. Direct Determination of Absorption Anisotropy in Colloidal Quantum Rods. Phys. Rev. B 2012, 85,