Bright White Light Emission from Ultrasmall Cadmium Selenide Nanocrystals

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1 SUPPORTING INFORMATION Bright White Light Emission from Ultrasmall Cadmium Selenide Nanocrystals Teresa E. Rosson 1, Sarah M. Claiborne 1, James R. McBride 1, Benjamin S. Stratton 1,3, and Sandra J. Rosenthal *,1,2 1 Department of Chemistry, Vanderbilt University, VU Station B Box , Nashville, Tennessee 37235, United States 2 Department of Physics and Astronomy, Department of Pharmacology, Department of Chemical and Biomolecular Engineering, and The Vanderbilt Institute of Nanoscale Science and Engineering, Vanderbilt University, Nashville, Tennessee, United States; Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States. 3 Current Address: Columbia University, Mudd Building Room 801, New York, New York 10027, United States * Corresponding author: sandra.j.rosenthal@vanderbilt.edu CdSe Nanocrystal Synthesis. The nanocrystals were prepared by slightly modified methods from Bowers et al. 1 For the synthesis of ultrasmall white light-emitting CdSe nanocrystals, 4 g hexadecylamine (HDA), 6 g trioctylphosphine oxide (TOPO), 0.5 g dodecylphosphonic acid (DDPA), and g (1mmol) cadmium oxide (CdO) were combined in a 100 ml three-neck round-bottom flask on a stir-plate with a heating mantle. A temperature probe was inserted into one of the side necks, and the other side was closed with a rubber septum. The center of the flask was attached to a self-washing bump trap, with argon gas flowing through the flask to purge and maintain an inert atmosphere. The reaction was heated to 150 C while purging, the purge needle was removed, and heating continued to 330 C. When the solution changed from opaque brown to clear and colorless, 5 ml of 0.2 M selenium tributylphosphine solution (Se:TBP) was injected through a 12-gauge needle through the septum into the reaction. At the first sign of a yellow color in the solution, occurring about five to eight seconds after adding the Se:TBP, 20 ml butanol was injected through an 18-gauge needle. The flask was immediately cooled to below 90 C with compressed air to prevent further growth. The nanocrystals were precipitated with methanol in four 50 ml centrifuge tubes and collected by centrifugation at 6000 rpm for three minutes. The nanocrystal pellets were dried in the centrifuge tubes and then redispersed in 6 ml hexanol per tube and centrifuged at 6000 rpm for 20 min. The supernatant containing the nanocrystals was decanted into clean tubes, precipitated with methanol, and collected by centrifugation at 6000 rpm for 20 min. The final solid nanocrystals were dried, dissolved in toluene, and stored in the dark. S1

2 Spectral Analysis of White-Light Nanocrystals. Absorption and emission spectra for white-light CdSe nanocrystals are shown in Figure S1. The size of nanocrystals from a single batch can be calculated from the band edge absorption wavelength using calculations from Yu, et al. 2 The white-light emission spectrum exhibits three peaks in the spectrum at approximately 440, 488, and 550 nm, extending over the visible spectrum. The first peak in the spectrum has been shown to be related to the surfacepassivating phosphonic acid ligand. 3 This peak is pinned at 440nm in CdSe nanocrystal diameters of 1.7 nm or smaller, instead of blue-shifting with decreasing diameter. 3 This blue peak is a direct result of the phosphonic acid ligands on the nanocrystal, and it is absent when a different ligand is used in place of the phosphonic acid or is shifted depending on the alkyl chain length of the phosphonic acid. 3,4 The origin of the second peak at 488 nm is unknown, though it is hypothesized to be related to the surface state at the Se atoms on the nanocrystal. 3 The third broad peak at about 550 nm is a result of deep trap emission. 3,5 The peak close to 750 nm is the second order diffraction peak, which is an effect of the diffraction grating in the fluorometer. Fig. S1 Absorbance spectrum (blue line) of white-light nanocrystals with band edge absorption at 409 nm. Emission spectrum (red line) of white-light nanocrystals with quantum yield of 8.2% and CIE coordinates of 0.302, (Spectral intensities are normalized.) Quantum Yield Calculation. Quantum yield calculations were performed on original white-light nanocrystals and on treated nanocrystals. In order to track emission changes accurately, the measurements were made within a few hours of treatment, and the original and treated nanocrystals were measured together with both samples in toluene. The white-light nanocrystals were usually made one or two days before treatment. Coumarin 152A (in hexanes) and Coumarin 153 (in ethanol) were both used on different samples as the reference dye. The optical density or absorbance of the samples and dyes were adjusted using a UV-Vis spectrometer. Solutions of the original sample, treated sample, and S2

3 reference dye were diluted to similar optical densities ( ) at a wavelength slightly shorter than that of the nanocrystal band edge ( nm). The emission spectra of the samples were taken from 400 nm to 800 nm on a fluorometer, using the chosen wavelength near the band edge as the excitation wavelength on the instrument. The quantum yield was then calculated using Equation S1. 4 ENC 2 A NC η NC QY NC = QYSTD E (1) STD ηstd ASTD where E NC and E STD are the integrated emission intensities of the nanocrystals and standard dye, respectively, A NC and A STD are the optical densities of the nanocrystals and standard at the excitation wavelength, η NC is the refractive index of toluene, η STD is the refractive index of the standard s solvent, and QY STD is the quantum yield of the standard from literature. The solvent refractive indices were for toluene, for hexanes, and for ethanol. The QY STD was 1.00 for Coumarin 152A and 0.38 for Coumarin ,7 Dodecanethiol Treatment The dodecanethiol (DDT) treatment to make nanocrystals warmer is currently simple and unrefined. The CdSe nanocrystals were prepared as though ready for a formic acid treatment: diluted in toluene, in a 50mL three-neck round-bottom flask with a stirbar. A solution of DDT in toluene was prepared in a 1.66*10-2 M concentration. The DDT solution was added via syringe in a molar excess to the nanocrystals. The solution was stirred at room temperature for 1-5 min, while the emission was monitored with a UV lamp set close to the flask. The emission color visibly changed from pale yellow-white (nanocrystal solution) to a warmer yellow-orange after the addition of DDT. The UV lamp was removed, and a small aliquot of the solution was taken for analysis before a formic acid treatment was performed. The effect of the DDT treatment is shown by emission spectra (Fig. S2) absorption spectra (Fig. S3) and CIE coordinates (Fig. S4). S3

4 Fig. S2 Emission spectra of original CdSe (blue) with quantum yield 10.2%, after DDT treatment (green) with quantum yield 4.2%, and after final formic treatment (red) with quantum yield 20.5%. Fig. S3 Absorption spectra of original CdSe (blue) with band edge absorption at 413 nm, after DDT treatment (green) with absorption at 424 nm, and after final formic treatment (red) with absorption at 414 nm. S4

5 Fig. S4 Visualization of CIE coordinates comparing a regular formic acid treatment (A, B) to a DDT and formic acid treatments (a, b, c). Table S1 CIE coordinates corresponding to Fig. S3. Sample CIE coordinates A Original CdSe 0.311, B Formic acid 0.237, a Original CdSe 0.312, b Dodecanethiol 0.428, c Formic acid 0.278, Quantum Yield of Treated White-light Nanocrystals over Time Three separate batches of white-light CdSe nanocrystals were treated with formic acid. For these three batches, the quantum yield increased on day 1, followed by a gradual decay to around 30% by days A plot of the average quantum yield measured over 18 days is shown in Figure S5. S5

6 Fig. S5 Plot showing the average quantum yield of three batches of treated white-light nanocrystals. After 18 days, the average quantum yield was ~ 30% Fluorescence of Hexanoic, Octanoic, and Oleic acid Treated White light CdSe Nanocrystals Fig. S6 Fluorescence of three different carboxylic acid-treated white light nanocrystals. The acetic acid spectrum was measured with an excitation wavelength at 365 nm. S6

7 Calculation of Predicted White-light Nanocrystal coated LED where, The power efficiency of a Nichia 385 nm UV LED = 15% The power efficiency of the nanocrystals, comparing radiant flux of the UV LED to radiant flux produced by the nanocrystals = The percentage of photons absorbed by the nanocrystals from the UV LED = (estimated using an integrating sphere) Quantum Yield of the nanocrystals = 0.4 S = stokes loss = 0.750, which is the energy difference between excitation and emission 0.39 The percentage of light emitted from the nanocrystals that escapes the polymer film = 250 lm/w = The luminous efficacy of radiation. This constant converts radiant flux to luminous flux. This constant takes into account the wavelength response of the human eye. The calculation of this constant is accomplished by assigning more priority to wavelengths our eyes are more sensitive to. The constant is calculated as follows: 8 where, K = The efficiency of the LED in lm/w K m = 683 lm/w and corresponds to the luminous efficacy of 555 nm monochromatic emission, the wavelength of light our eyes are most sensitive to. P(λ) = This is a function that describes the spectral response of the human eye to different wavelengths of light centered at 555 nm and normalized so that 555 nm is equal to one. 9 V(λ) = Fluorescence spectrum of the nanocrystal emission, specifically the one shown at the beginning of the paper. (1) Bowers II, M. J.; McBride, J. R.; Garrett, M. D.; Sammons, J. A.; Dukes Iii, A. D.; Schreuder, M. A.; Watt, T. L.; Lupini, A. R.; Pennycook, S. J.; Rosenthal, S. J. J. Am. Chem. Soc. 2009, 131, S7

8 (2) Yu, W. W.; Qu, L. H.; Guo, W. Z.; Peng, X. G. Chem. Mater. 2003, 15, (3) Dukes III, A. D.; Schreuder, M. A.; Sammons, J. A.; McBride, J. R.; Smith, N. J.; Rosenthal, S. J. J. Chem. Phys. 2008, 129, (4) Schreuder, M. A.; McBride, J. R.; Dukes, A. D.; Sammons, J. A.; Rosenthal, S. J. J. Phys. Chem. C 2009, 113, (5) Bowers II, M. J.; McBride, J. R.; Rosenthal, S. J. J. Am. Chem. Soc. 2005, 127, (6) Nad, S.; Kumbhakar, M.; Pal, H. J. Phys. Chem. A 2003, 107, (7) Jones II, G.; Rahman, M. A. J. Phys. Chem. 1994, 98, (8) Gosnell, J. D., Vanderbilt University, (9) Sharpe, L. T.; Stockman, A.; Jagla, W.; Jagle, H. Journal of Vision 2005, 5. S8