Strong Visible Absorption and Broad Time Scale

Transcription

1 Supporting Information for: Strong Visible Absorption and Broad Time Scale Excited-State Relaxation in (Ga 1-x Zn x )(N 1-x O x ) Nanocrystals Chi-Hung Chuang, Ying-Gang Lu, Kyureon Lee, Jim Ciston, and Gordana Dukovic *, Department of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, Colorado 839, United States National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 9472, United States Corresponding Author: Gordana Dukovic Gordana.Dukovic@Colorado.edu Table of Contents I. Experimental: Sample Preparation and Characterization....S2 II. XRD Patterns of Oxynitride Nanocrystals and Crystallite Size Analysis....S4 III. Measurements of Sizes of Solubilized Oxynitride Nanocrystals by TEM..S6 IV. Estimation of Absorption Coefficient (α) of (Ga.27 Zn.73 )(N.27 O.73 ) Nanocrystals.....S6 V. Stability of Oxynitride Nanocrystals During TA Measurements. S7 VI. Pump-Power Dependence of TA Kinetics S7 VII. Rise of Bleach Signal of (Ga.27 Zn.73 )(N.27 O.73 ) Nanocrystals. S8 VIII. Kinetic Parameters for TA Kinetic Decays on the Short Timescale S8 IX. Bleach Signal in the TA Spectra of ZnO Nanocrystals.... S9 X. TA Kinetics of (Ga.27 Zn.73 )(N.27 O.73 ) Nanocrystals Over the 1 fs 1 µs Time Window......S1 XI. References..S11 S1

2 I. Experimental: Sample Preparation and Characterization Chemicals. Zinc acetylacetonate (Zn(acac) 2, %, Sigma-Aldrich), Gallium acetylacetonate (Ga(acac) 2, 99.99%, Sigma-Aldrich), 1,2-hexadecanediol (9%, Sigma-Aldrich), Oleylamine (7%, Sigma-Aldrich), Oleic acid (99%, Sigma-Aldrich), Benzyl ether (98%, Sigma-Aldrich), Zinc chloride (98%, Sigma-Aldrich), 1,2-ethanediol (99.%, Macron fine chemicals), 3-mercaptopropionic acid (3-MPA, 99.%, Sigma-Aldrich), Tetramethyl ammonium hydroxide (97%, Sigma-Aldrich), Ammonia (99.99%, Airgas), octadecyltrimethoxysilane (ODTMS, 9%, Sigma-Aldrich), butylamine (99.5%, Sigma-Aldrich), Sodium hydroxide (99.3%, Fisher Scientific), toluene (99.5%, Sigma-Aldrich), hexane (99%, Sigma-Aldrich), methanol (99.9%, Fisher Scientific), ethanol (95%, Decon Labs), and 2-propanol (99.9%, Fisher Scientific) were used as purchased. Synthesis of Oxynitride and ZnO Nanocrystals. We followed previously described procedures 1 to synthesize oxynitride and ZnO nanocrystals. Preparation of Soluble Nanocrystals. The as-prepared (Ga 1-x Zn x )(N 1-x O x ) nanocrystals were surface functionalized with ODTMS (chemical structure shown below) using a modification of previously described procedures. 2 2 mg of as-prepared (Ga 1-x Zn x )(N 1-x O x ) powder was dispersed in 6 ml of toluene with continuous sonication. Then, ODTMS (~.4 ml) was added into the solution with a small amount of butylamine (~.15 ml) as a catalyst. The sonication was continued for several hours. Reaction temperature was kept at around 3 C using a water bath. The (Ga 1-x Zn x )(N 1-x O x ) solution was then washed by adding 2-propanol to precipitate the solubilized particles. The resulting ODTMS-(Ga 1-x Zn x )(N 1-x O x ) nanocrystals were soluble in toluene and remained stable and well-dispersed for months. Soluble ODTMS-ZnO nanocrystals were prepared in the same way. Characterization. UV-vis absorption spectra were recorded using an Agilent 8453 UV vis spectrophotometer. Bright field TEM Images were obtained using a Phillips CM1 TEM operating at 8 kv equipped with a bottom-mounted 4 megapixel AMT v6 digital camera. To obtain measure nanocrystal size, a total number of >2 nanocrystals was analyzed using ImageJ. High resolution STEM image was recorded using the aberration-corrected TEAM.5 Titan microscope S2

3 operated at 8 kv. The STEM convergence semi-angle was 3 mrad with a 49 mrad detector inner semi-angle and outer angle 247 mrad. X-ray diffraction patterns were obtained using a Rigaku Ultima IV diffractometer equipped with Cu Kα radiation. Elemental analysis by the ICP-OES was carried out using the ARL 341+ spectrometer. The TA setup was previously described in detail. 3 In brief, short timescale (1 fs 3 ns) TA measurements were conducted using an amplified Ti:sapphire laser (Solstice, Spectra-Physics, 8 nm, 1 fs), an optical parametric amplifier (TOPAS-C, Light Conversion), and the Helios spectrometer (Ultrafast Systems, LLC). The laser pulse train was split to generate a white light continuum probe pulse (34-8 nm) in a CaF 2 crystal and a tunable pump pulse. Pump power-dependent measurements were used to identify a regime were the TA kinetics were independent of power. For long timescale TA spectroscopy (3 ps 1 µs), EOS spectrometer (Ultrafast Systems, LLC) was used. All TA experiments were carried out in a 2 mm quartz cuvette sealed under Ar at room temperature. The samples were stirred during data collection. Absorption spectra of the oxynitride NCs were taken before and after the TA experiments and no significant change was observed (Figure S4). All the TA spectra were chirp-corrected using the instrument response function (IRF) determined by collecting the TA data of neat solvent. S3

4 II. XRD Patterns of Oxynitride Nanocrystals and Crystallite Size Analysis Figure S1. XRD patterns of (Ga 1-x Zn x )(N 1-x O x ) nanocrystals. Black: (Ga 1-x Zn x )(N 1-x O x ) powder before surface functionalization; Blue: solubilized ODTMS-(Ga 1-x Zn x )(N 1-x O x ) nanocrystals. The x values were determined using ICP-OES. Reference patterns for wurtzite ZnO (purple, JCPDS #5-664) and wurtzite GaN (green, JCPDS #2-178) are shown as vertical lines. To estimate the sizes of the nanocrystals before and after solubilization, we used the Scherrer method 4 as follows: D: Crystal size in diameter. D = k λ B cosθ k: A dimensionless shape factor. k has been determined to vary between.89 and 1.39, but is usually taken as close to unity. λ: Wavelength of the incident X-ray beam ( A for the Cu Kα). B: Band width of the X-ray pattern line at half peak-height (FWHM) in radians, where B 2 = B o 2 B i 2, B o is the observed X-ray peak width, B i is the width due to instrumental effects shown in Figure S2. X-ray pattern lines are fit with Gaussian functions. θ: Bragg s diffraction angle. The precision of crystallite-size analysis by this method is, at best, about ±1%. S4

5 Figure S2. XRD patterns of LaB 6. LaB 6 is the standard used to determine X-ray line width due to instrumental effects. Table S1. Determination of the crystallite sizes before and after surface functionalization using the Scherrer method. a All sizes are in units of nm. Size from (1) Size from (11) Size from (2) Average size peak peak peak Before ± 2.2 After ± 1.9 a k = 1. is used for the analysis. S5

6 III. Measurements of Sizes of Solubilized Oxynitride Nanocrystals by TEM Figure S3. Diameter distribution of solubilized (Ga.27 Zn.73 )(N.27 O.73 ) nanocrystals measured by low magnification TEM. IV. Estimation of Absorption Coefficient ( α ) of (Ga.27 Zn.73 )(N.27 O.73 ) Nanocrystals From the comparison of definitions of transmittance in solution (!!! = 1!!"# ) and in crystals (!!! = e!!" ), the absorption coefficient: α = 2.3 ε c. To estimate α for (Ga.27 Zn.73 )(N.27 O.73 ) at 45 nm, we use ε = 227 M -1 cation cm -1 (Figure 3) and c=69 M cation. The concentration (c) is estimated by assuming that the density of (Ga.27 Zn.73 )(N.27 O.73 ) is similar to density of ZnO, 5.6 g/cm 3. Based on this analysis, (Ga.27 Zn.73 )(N.27 O.73 ) has an absorption coefficient of ~36, cm -1 at 45 nm. S6

7 V. Stability of Oxynitride Nanocrystals During TA Measurements 2. Absorbance Before After Wavelength / nm 7 Figure S4. UV-vis absorption spectra for the (Ga.27 Zn.73 )(N.27 O.73 ) nanocrystals recorded before and after the TA measurements. The sample was exposed to the laser over the course of 2 hours in the TA measurements and no significant change was observed in the absorption spectra. VI. Pump-Power Dependence of TA Kinetics (a) (b) 5.8 Normalized OD x1-3 Figure S5. Pump-power dependence of TA kinetics of (Ga.27 Zn.73 )(N.27 O.73 ) nanocrystals. (a) Kinetics at 455 nm normalized at 2.9 ns at several values of pulse energies. The values in the legend are in the units of nj/pulse. (b) Ratios of ΔOD values for early (.7 ps) and late (2.9 ns) delay time vs. pulse energy. The data suggest that the TA kinetics are independent of pulse energy below ~25 nj/pulse. TA kinetics in Figure 5a of the manuscript were measured with the pump set at 1 nj/pulse Ratio S Pulse energy / nj per pulse 5

8 VII. Rise of Bleach Signal of (Ga.27 Zn.73 )(N.27 O.73 ) Nanocrystals 2 x x nm nm x nm Figure S6. Rise of the bleach signal in the TA spectra of (Ga.27 Zn.73 )(N.27 O.73 ) at 365, 395, 425, and 455 nm (pump = 34 nm). The corresponding IRF values at those probe wavelengths are 19, 28, 41, and 43 fs, respectively. The rise kinetics are IRF-limited at all wavelengths. x nm 1 2 VIII. Kinetic Parameters for TA Kinetic Decays on the Short Timescale Table S2. Kinetic parameters for the TA decays of (Ga.27 Zn.73 )(N.27 O.73 ) nanocrystals (Figure 5a in the manuscript) at different spectral positions. Results were obtained from fits to biexponential decay functions. Excitation wavelength is 34 nm with the pulse energy of ~1 nj/pulse. Probe / nm τ 1 / ps τ 2 / ps ± 5. (.53) b 217 ± 384 (.47) ± 1.3 (.59) 3298 ± 586 (.41) ± 1.1 (.62) 2832 ± 265 (.38) ± 1.5 (.55) 313 ± 23 (.45) 425 a 32.4 ± 2.7 (.34) 3149 ± 254 (.66) a Pump at 45 nm with the excitation pulse energy of ~1 nj/pulse. b Normalized signal amplitudes are in parentheses. S8

9 IX. Bleach Signal in the TA Spectra of ZnO Nanocrystals τ 1 = 11 ps (.54) τ 2 = 6 ns (.46) Figure S7. The decay of the TA bleach feature of ZnO nanocrystal. Pump and probe wavelengths are 34 and 365 nm, respectively. Normalized signal amplitudes of the two time constants are in parentheses. S9

10 X. TA Kinetics of (Ga.27 Zn.73 )(N.27 O.73 ) Nanocrystals Over the 1 fs 1 µs Time Window 425 nm 455 nm 485 nm 515 nm 545 nm Time / µs Figure S8. TA kinetics at different wavelengths in the time window of 1-7 to 1 2 µs for (Ga.27 Zn.73 )(N.27 O.73 ) nanocrystals. The average lifetimes are 3 (425 nm), 25 (455 nm), 35 (485 nm), 4 (515 nm), and 31 (545 nm) µs. The integration limits in the average lifetime calculation (Eq. 1) were -1.87*1-6 and 1 µs. S1

11 XI. References 1. Lee, K.; Tienes, B. M.; Wilker, M. B.; Schnitzenbaumer, K. J.; Dukovic, G. Nano Lett. 212, 12 (6), Howgate, J.; Schoell, S. J.; Hoeb, M.; Steins, W.; Baur, B.; Hertrich, S.; Nickel, B.; Sharp, I. D.; Stutzmann, M.; Eickhoff, M. Adv. Mater. 21, 22 (24), Tseng, H. W.; Wilker, M. B.; Damrauer, N. H.; Dukovic, G. J. Am. Chem. Soc. 213, 135 (9), Suryanarayana, C.; Grant Norton, M., X-Ray Diffraction: A Practical Approach. Plenum Press: New York, S11