Transition from Molecular Vibrations to Phonons in Atomically Precise Cadmium Selenide Quantum Dots

Transcription

1 Supporting Information for Transition from Molecular Vibrations to Phonons in Atomically Precise Cadmium Selenide Quantum Dots Alexander N. Beecher, Rachel A. Dziatko, Michael L. Steigerwald, Jonathan S. Owen, and Andrew C. Crowther* Department of Chemistry, Barnard College, New York, New York 10027, United States Department of Chemistry, Columbia University, New York, New York 10027, United States Table of Contents 1. Synthesis of Octadecylphosphonic Acid 2. Sample Absorbance and Photoluminescence Spectra 3. Full Raman Spectra of Clusters, Silicon Substrate, and Ligands 4. Cd(O 2 CPh) 2 (n-bunh 2 ) 2 Raman Spectra 5. Room-Temperature CdSe Cluster Peak Width Fitting Results 6. Models of Cd 20 Se 10 Cl 20 (NH 3 ) 20 with Different Ligand Arrangements Used for DFT Calculations 7. Variable-Temperature Raman Spectra 8. Variable-Temperature CdSe Cluster Surface Peak Fitting Results 9. Bulk Zincblende Unit Cell in CdSe (408 nm) 10. Fitting Approach and Fit Result Tables 11. References S1

2 1. Synthesis of Octadecylphosphonic Acid Adapted from Kosolapoff 1 and Marsi. 2 In a typical synthesis, g (0.346 mol.) of NaH (60% in oil by weight) and 250 ml of N,N-dimethylformamide are mixed under Ar and cooled to 0 C. Dimethylphosphite (33.58 ml, mol.) is added drop-wise and the flask is allowed to warm to room temperature and stir for an additional hour. A 250 ml solution of g (0.288 mol) 1- bromooctadecane in tetrahydrofuran is added drop-wise to the sodium dimethylphosphite solution and stirred for 1 hour before quenching with water and extracting three times with ethyl acetate. The organic layer is washed three times with a solution of brine, dried over MgSO 4, filtered, and concentrated under reduced pressure to yield a white solid, dimethyl octadecylphosphonate. This material is dissolved in 200 ml of methylene chloride to which 78.2 ml (0.605 mols) of bromotrimethylsilane is added drop-wise. After 3 hours, the reaction flask is evacuated, and the reaction is concentrated under reduced pressure to remove methylene chloride and the bromomethane co-product. Excess methanol is added to the dry precipitate, the mixture is heated to 40 C to dissolve the solid, and the product crystallizes overnight in a freezer. After filtration, the white, waxy solid is dissolved in 1L of hot toluene and precipitated with ~200 ml of hexane. This recrystallization procedure is repeated three times to ensure purity. The final product (~64 g, ~60% yield) is melted and dried overnight under vacuum. S2

3 2. Sample Absorbance and Photoluminescence Spectra Figure S1. Absorption (black solid line) and photoluminescence (blue dashed line) spectra of cluster samples investigated in this study. Red (λ exc = 633 nm) and green (λ exc = 532 nm) vertical dashed lines indicate the laser wavelengths used for Raman measurements. All cluster Raman spectra are nonresonant. S3

4 Figure S2: Absorption (black solid line) and photoluminescence (blue dashed line) spectra of wurtzite and zincblende nanocrystal samples investigated in this study. Red (λ exc = 633 nm ) and green (λ exc = 532 nm) vertical dashed lines indicate the laser wavelengths used for Raman measurements. Raman spectra measured with the 532 and 633 nm lasers are resonant and nonresonant, respectively. S4

5 3. Full Raman Spectra of Clusters, Silicon Substrate, and Ligands Figure S3. Full Raman spectra measured with (a) λ exc = 532 nm and (b) λ exc = 633 nm for the cadmium selenide clusters, Cd(O 2 CPh) 2 (n-bunh 2 ) 2, and the silicon substrate. Cd-Se, ligand, and silicon peaks are indicated with dashed lines or boxes that are blue, red, and black, respectively. S5

6 4. Cd(O 2 CPh) 2 (n-bunh 2 ) 2 Raman Spectra Figure S4. Raman spectra measured with λ exc = 532 and 633 nm for Cd(O 2 CPh) 2 (n-bunh 2 ) 2, which has benzoate and n-butylamine groups to match the ligands of our CdSe clusters. 5. Room-Temperature CdSe Cluster Peak Width Fitting Results Figure S5: Interior and surface peak fwhm for both λ exc = 532 nm (empty) and λ exc = 633 nm (filled). Markers for each excitation wavelength are slightly offset horizontally for clarity. From left to right: CdSe (350 nm), CdSe (380 nm), CdSe (408 nm), CdSe (Wz NCs), and CdSe (Zb NCs). For the spherical nanocrystals, the edge length is the diameter. S6

7 6. Models of Cd 20 Se 10 Cl 20 (NH 3 ) 20 with Different Ligand Arrangements Used for DFT Calculations Figure S6. Models of Cd 20 Se 10 Cl 20 (NH 3 ) 20 with a (a) symmetric and (b) asymmetric arrangement of ligands. In the symmetric model (symmetric model #1 in Figure 5d), the vertex cadmium atoms each possess two ammonia ligands and one chloride ligand, the edge cadmium atoms each possess one ammonia ligand and one chloride ligand, and the remaining facial cadmiums each possess one chloride ligand resulting in an even distribution of ammonia and chloride ligands on each face. The asymmetric model has some faces enriched in ammonia ligands and some faces enriched in chloride ligands. Cadmium is indicated in green, selenium in orange, chloride in purple, nitrogen in blue, and hydrogen in white. S7

8 7. Variable-Temperature Raman Spectra Figure S7. Representative variable-temperature Raman spectra for CdSe (380 nm) and Cd(O 2 CPh) 2 (n-bunh 2 ) 2 with λ exc = 532 nm. The black, gray, and red curves are the data, individual Gaussian fits, and total fit, respectively. (a) For CdSe (380 nm) the vertical dashed line is centered on the interior peak at room temperature, to emphasize that this peak shifts to higher energy as CdSe (380 nm) cools. (b) The dashed lines in the ligand spectra are centered on a weak peak at 200 cm -1, which is likely the entire benzoate group vibrating against a surface cadmium atom, and a broad peak at ~100 cm -1, which we assign to a collection of low-frequency modes. S8

9 8. Variable-Temperature CdSe Cluster Surface Peak Fitting Results Figure S8. Temperature dependence of (a) frequency and (b) fwhm of the surface peak from measurements with λ exc = 532 nm. CdSe (350 nm), CdSe (380 nm), and CdSe (408 nm) are the black, red, and blue data, respectively. Solid lines are fits to the data. The fit equations are ω surf,350 = T + 178, ω surf,380 = 0.013T + 174, ω surf,408 = -0.05T + 197, fwhm surf,350 = T + 30, fwhm surf,380 = 0.014T + 27, and fwhm surf,408 = T Error bars are the standard deviation of the results from independent fits to multiple spectra. S9

10 9. Bulk Zincblende Unit Cell in CdSe (408 nm) Figure S9. The single bulk zincblende unit cell contained in the structure of CdSe (408 nm). Cadmium and selenium atoms in the unit cell are blue and purple, respectively. Dark blue cadmium atoms are at the corners of the unit cell. Four of these corner atoms are at the center of each face of the cluster. Light blue cadmiums are on the faces of the unit cell. S10

11 10. Fitting Approach and Fit Result Tables We fit CdSe (Wz NCs), CdSe (Zb NCs), and CdSe (Bulk) spectra to Voigt functions, which are a convolution of the Gaussian spectrometer resolution and the Lorentzian line shape. Fit results are in Table S1. We fit the more inhomogeneous CdSe (350 nm), CdSe (380 nm), and CdSe (408 nm) spectra to Gaussians in room-temperature (Table S1) and variable-temperature (Table S3) experiments. For temperature-dependent data, we determine peak widths by deconvoluting the Gaussian spectrometer resolution from the Gaussian fit. We did not deconvolute the Gaussian spectrometer resolution from the room-temperature Gaussian fits because of the minimal effect (< 1 cm -1 change ) of the spectrometer resolution on the width of such broad peaks. For each sample, the fit parameter value is the weighted average of independent fits to individual spectra on different samples on different days. The uncertainty is the standard deviation among the resulting set of fit parameters for a particular sample and laser wavelength. We also performed a global fit of the room-temperature spectra of each cluster size (Table S2). For example, we simultaneously fit all Raman spectra (λ exc = 532 and 633 nm) of CdSe (350 nm) to a single set of fitting parameters. The global fit uncertainties were unreasonably small because they do not take into account uncertainties in calibration and sample-to-sample heterogeneity. As a result, we conservatively assign the uncertainties in Table S2 to standard deviations of the combined set of individual fitting results for measurements with λ exc = 532 and 633 nm. However, the area ratio uncertainty does come directly from the global fit results, because the peak widths and heights had a large covariance in the individual fits, producing unreasonably large uncertainties. For all data, we use a linear background. For select room-temperature cluster measurements, we also tried addressing the background in other ways. We fit the ligand-only spectrum to a fifth order polynomial and used the derived function as the backround for the cluster spectra. We have also fit the low energy ligand feature and broad fluorescence with Gaussian functions. While the fit results were within the uncertainty of the linear background approach, these methods resulted in visually inferior fits, likely because our ligand control sample is not a perfect model for the ligand environment of our clusters. S11

12 Table S1: Individual Fit Results Room-Temperature Spectra Surface Peak Interior Peak Area Ratio Sample Laser ω (cm -1 ) fwhm (cm -1 ) ω (cm -1 ) fwhm (cm -1 ) A int /A surf CdSe (350 nm) 532 nm ± ± ± ± ± nm ± ± ± ± ± 0.86 CdSe (380 nm) 532 nm ± ± ± ± ± nm ± ± ± ± ± 1.98 CdSe (408 nm) 532 nm ± ± ± ± ± nm ± ± ± ± ± 0.35 Surface Optical Longitudinal Optical ω (cm -1 ) fwhm (cm -1 ) ω (cm -1 ) fwhm (cm -1 ) CdSe (Wz NCs) 532 nm Not Obs. Not Obs ± ± 0.6 N/A 633 nm ± ± ± ± 0.6 N/A CdSe (Zb NCs) 532 nm Not Obs. Not Obs ± ± 0.2 N/A 633 nm Not Obs. Not Obs ± ± 2.0 N/A Transverse Optical Longitudinal Optical ω (cm -1 ) fwhm (cm -1 ) ω (cm -1 ) fwhm (cm -1 ) CdSe (Bulk) 532 nm ± ± ± ± 4.8 N/A Table S2: Global Fit Results (λ exc = 532 and 633 nm spectra fit simultaneously) Surface Peak Interior Peak Area Ratio ω (cm -1 ) fwhm (cm -1 ) ω (cm -1 ) fwhm (cm -1 ) A int /A surf CdSe (350 nm) ± ± ± ± ± 0.01 CdSe (380 nm) ± ± ± ± ± 0.02 CdSe (408 nm) ± ± ± ± ± 0.06 Table S3: Individual Fit Results λ exc = 532 nm Temperature-Dependent Spectra Surface Peak Interior Peak Sample Temp (K) ω (cm -1 ) fwhm (cm -1 ) ω (cm -1 ) fwhm (cm -1 ) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.8 CdSe (350 nm) CdSe (380 nm) CdSe (408 nm) S12

13 11. References 1. Kosolapoff, G. M. J. Am. Chem. Soc. 1945, 67, Marsi, K. L. J. Am. Chem. Soc. 1969, 91, S13