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1 One-Pot Synthesis of Cu 1.94 S CdS and Cu 1.94 S Zn x Cd 1 x S Nanodisk Heterostructures Michelle D. Regulacio, Chen Ye,, Suo Hon Lim, Michel Bosman, Lakshminarayana Polavarapu, Wei Ling Koh, Jie Zhang, Qing-Hua Xu*, and Ming-Yong Han*,, Institute of Materials Research and Engineering, Agency for Science Technology and Research, 3 Research Link, Singapore , Department of Chemistry, National University of Singapore, Singapore , and Division of Bioengineering, National University of Singapore, Singapore Supporting Information Experimental Details All starting materials were purchased from commercial sources and used without further purification. Synthetic Procedure Cu(S 2 CNEt 2 ) 2. Synthesis of copper(ii) diethyldithiocarbamate was based on previously published procedures. CuCl 2.2H 2 O (1 mmol) was dissolved in 10 ml H 2 O producing a clear, blue solution. In a separate flask, Na(S 2 CNEt 2 ).3H 2 O (2 mmol) was dissolved in 20 ml H 2 O. The Cu 2+ solution was then added dropwise (with stirring) to the ligand solution resulting in the precipitation of the dark brown complex. The product was isolated through filtration, washed with H 2 O and air-dried. Cd(S 2 CNEt 2 ) 2. Following the procedure above, cadmium(ii) diethyldithiocarbamate was synthesized using CdCl 2 as the Cd 2+ source. The resulting complex is white in color. Cu 1.94 S nanodisks. The precursor solution was prepared by dissolving 0.1 mmol of Cu(S 2 CNEt 2 ) 2 in 0.5 ml trioctylphosphine (TOP). A solvent mixture consisting of 5 ml hexadecanethiol (HDT) and 5 ml trioctylamine (TOA) was degassed at 100 C for 20 to 30 min prior to heating under Ar to the desired synthetic temperature. Rapid injection of the precursor solution to the solvent mixture was done when the temperature reached 250 C. With vigorous stirring, the solution immediately turned from orange to dark brown. The resulting mixture was kept at 250 C under Ar for 5 min before placing in a H 2 O bath, allowing the mixture to cool to 40 C. Ethanol was added to precipitate the nanodisks, and this is followed by centrifugation. The solid obtained was washed thoroughly with hexane and ethanol. S1

2 Cu 1.94 S CdS heterostructured nanodisks. A solution of 0.1 mmol of Cu(S 2 CNEt 2 ) 2 in 0.5 ml TOP was swiftly injected into a solvent mixture that consists of 5 ml HDT and 5 ml TOA at 250 C (Note: Solvent mixture was degassed at 100 C for min prior to heating under Ar to 250 C). The resulting dark brown mixture was kept at 250 C for 5 min before rapidly injecting the Cd-precursor solution (0.05 mmol of Cd(S 2 CNEt 2 ) 2 in 0.5 ml TOP). The mixture turned golden brown and was maintained at 250 C for 2.5 min before cooling to room temperature using a H 2 O bath. The two-component nanodisks were precipitated using ethanol, isolated by centrifugation, and washed with hexane and ethanol. Similar product is obtained when the Cdprecursor solution used is 0.05 mmol of Cd(CH 3 COO) 2 in 0.5 ml TOP. Cu 1.94 S Zn x Cd 1 x S heterostructured nanodisks. The procedure is generally the same as the procedure employed in the synthesis of Cu 1.94 S CdS heterostructured nanodisks. The Zn x Cd 1 x S- precursor solution consists of Zn(S 2 CNEt 2 ) 2 and Cd(S 2 CNEt 2 ) 2 dissolved in 0.5 ml TOP. The amounts of Zn- and Cd-precursor used are: (a) Zn = and Cd = mmol, and (b) Zn = and Cd = mmol. Characterization X-ray Diffraction (XRD) patterns were collected on a Bruker GADDS D8 Discover diffractometer using Cu Kα radiation ( Å). The patterns were compared with those published in the Joint Committee of Powder Diffraction Standards (JCPDS) database for bulk materials. Bright-field Transmission Electron Microscopy (TEM) images were taken using a JEOL 2100 electron microscope operated at an accelerated voltage of 200 kv. In preparing the specimens, a drop of nanocrystals dispersed in hexane was placed on the surface of a lacey formvar/carbon 300-mesh Cu grid. The following electron microscopy measurements were done using an FEI Titan TEM with a Schottky electron source and objective polepiece with C s value of 1.3 mm, operated at 200 kv. High-angle annular dark-field scanning TEM (HAADF-STEM) images were acquired using a 0.2 nm electron probe, a convergence semi-angle of 10 mrad and a collection angle from 50 to 100 mrad. Energy-dispersive X-ray (EDX) measurements were done in STEM mode using a probe of nm in diameter. Energy-filtered TEM (EFTEM) images were obtained with the three-window method, where electrons from 30 ev wide energy windows were imaged at the Cd M 4,5 edge ( ev loss), and the Cu L 2,3 edge ( ev loss). For the copper EFTEM map, a color table from black to red was used to code the intensity; the cadmium map was coded blue. S2

3 For the following measurements, samples were dispersed in CHCl 3 and loaded into a quartz cuvette. Room-temperature absorption spectra were recorded using a Shimadzu UV-3150 UV- Vis-NIR spectrophotometer. Room-temperature photoluminescence spectra were measured at 400-nm excitation using Shimadzu RF-5301PC spectrofluorophotometer. Pump probe experiments were performed using a Spectra Physics femtosecond Ti:sapphire oscillator seeded amplifier laser system, which gives output laser pulse with a pulse energy of 2 mj at 800 nm and a repetition rate of 1 khz. The 800-nm laser beam was split into two portions. The larger portion of the beam was passed through a BBO crystal to generate the 400-nm pump beam by frequency-doubling. A small portion of the 800-nm pulses was used to generate a white light continuum, which acted as the probe beam. The pump beam is focused onto the sample with a beam size of 300 μm and overlaps the smaller probe beam (100 μm in diameter). A monochromator was placed before the photodiode to select the detection wavelength. The time delay between the pump and probe pulses was varied by using a computer-controlled translation stage. The transient absorption spectra were obtained by scanning the monochromator. Table S1. Resulting phase and morphology of Cu x S nanocrystals prepared by rapid injection of the precursor solution into varying solvents at 250 C.* Solvent/Solvent Mixture (Total Volume = 10 ml) A. HDT + TOA or DDT + TOA (5 ml each) Cu x S phase Morphology Cu 1.94 S hexagonal-shaped disks (see Figure S1) B. HDT or DDT (10 ml) Cu 1.94 S hexagonal-shaped disks with wider size distribution than those obtained using Solvent A C. HDT + OM (5 ml each) Cu 1.94 S huge faceted nanocrystals that are polydisperse in size and shape (see Figure S2) D. TOP or TOA (10 ml) or TOP + TOA (5 ml each) - precursor did not decompose readily at 250 C E. TOP (9 ml) + OM (1 ml) Cu 2 S (α or β depending on heating time) polydisperse in size and shape; etching of nanocrystals was observed (see Figure S2) *Precursor solution: 0.1 mmol Cu(S 2 CNEt 2 ) 2 in 0.5 ml TOP. Heated at 250 C for 5 min unless specified otherwise. HDT = hexadecanethiol; TOA = trioctylamine; DDT = dodecanethiol; OM = oleylamine; TOP = trioctylphosphine. Monoclinic α Cu 2 S (low-temperature chalcocite) was produced when the reaction mixture was heated at 250 C from 1 to 5 min. Longer heat treatment (15 to 40 min) gave hexagonal β Cu 2 S (high-temperature chalcocite). S3

4 (a) (b) 100 nm 100 nm (c) Cu 1.94 S θ (deg) Figure S1. TEM images of Cu 1.94 S nanodisks prepared by thermolysis of Cu(S 2 CNEt 2 ) 2 at 250 C in a solvent mixture of (a) HDT and TOA, and (b) DDT and TOA. Most of the disks stand on edge, perpendicular to the substrate. The figure in (c) displays the XRD pattern of the nanodisks shown in (b). Note: Red lines are from standard JCPDS file for monoclinic Cu 1.94 S [ ]. S4

5 (a) (d) Cu 1.94 S (b) 100 nm θ (deg) (e) α Cu 2 S (c) 100 nm θ (deg) (f) β Cu 2 S 50 nm θ (deg) Figure S2. TEM images (a c) and corresponding XRD patterns (d f) of Cu x S nanocrystals prepared by thermolysis of Cu(S 2 CNEt 2 ) 2 at 250 C in a solvent mixture of: (a, d) HDT and OM for 5 min; (b, e) TOP and OM for 5 min; (c, f) TOP and OM for 30 min. Note: Red lines are from standard JCPDS file for monoclinic Cu 1.94 S [ ]. Blue lines are from standard JCPDS file for monoclinic Cu 2 S [ ]. Olive lines are from standard JCPDS file for hexagonal Cu 2 S [ ]. S5

6 50 nm 50 nm Figure S3. TEM images of Cu 1.94 S CdS nanodisks. C Cu S 1 Cu S Cu C Cu S Cd S Cd Cd Cd Energy (kev) Cu Cu Figure S4. HAADF-STEM image (top) and corresponding EDX spectra of a representative Cu 1.94 S CdS nanodisk. Two different regions of the nanodisk, one on each side of the interface, are labeled 1 and 2. The upper plot shows the EDX spectrum of region 1 whereas the bottom plot displays the EDX spectrum of region 2. The very strong C peaks in the spectra of both regions are attributed to the organic surfactants capping the disks and the carbon coating on the TEM grid. The weak Cu peaks in the spectrum of region 2 are possibly due to the Cu TEM grid used in the measurement. S6

7 (a) (b) 50 nm 50 nm (c) nm 55 2θ (deg) Figure S5. (a b) TEM images and (c) XRD pattern of Cu1.94S CdS nanodisks prepared using Cd(acetate)2 as the Cd precursor. Note: Red and blue lines in (c) are from standard JCPDS file for monoclinic Cu1.94S [ ] and wurtzite CdS [ ], respectively. (a) (b) 50 nm 50 nm Figure S6. TEM images of Cu1.94S ZnxCd1 xs nanodisks prepared using Zn:Cd molar ratio of (a) 1:9 and (b) 2:8. S7

8 (a) Intensity (a.u.) (b) (c) θ (deg) Figure S7. XRD pattern of Cu 1.94 S Zn x Cd 1 x S nanodisks prepared using Zn:Cd molar ratio of (a) 2:8 and (b) 1:9. The XRD pattern of Cu 1.94 S CdS nanodisks is shown in (c) for comparison. Note: Red and blue lines are from standard JCPDS file for monoclinic Cu 1.94 S [ ] and wurtzite CdS [ ], respectively. S8

9 Lattice parameter, c (Angstrom) (a) x = 0.07 (b) x = 0.20 Zn:Cd molar ratio used in synthesis c (Å) calculated from diffraction peaks Zn mole fraction, x actual alloy composition (a) 1: Zn 0.07 Cd 0.93 S (b) 2: Zn 0.20 Cd 0.80 S Zn mole fraction, x Figure S8. Vegard s law plot for alloyed Zn x Cd 1 x S (solid black line). The composition of the Zn x Cd 1 x S region in our two samples prepared using Zn:Cd molar ratio of (a) 1:9 and (b) 2:8 is determined to be Zn 0.07 Cd 0.93 S (x = 0.07) and Zn 0.20 Cd 0.80 S (x = 0.20), respectively. S9

10 Wavelength (nm) Figure S9. Absorption spectrum of CdS nanorods (synthesized based on previously published procedures). 1 The shoulder at ~480 nm is attributed to the first excitonic transition of CdS. A 2 (a.u.) Absorbance (a.u.) Photon Energy (ev) Figure S10. Plot of the square of absorbance vs. photon energy for Cu 1.94 S nanodisks. The optical band gap is determined by extrapolating the linear portion of the plot to the abscissa (red line). S10

11 ΔA/A (a) 0.02 Cu 1.94 S-CdS 0.2 ps 1 ps 50 ps ΔA/A Cu 1.94 S Wavelength (nm) Wavelength (nm) ΔA/A (b) Cu nm Cu nm Cu nm Delay time (ps) Figure S11. (a) Transient absorption spectra of Cu 1.94 S CdS and Cu 1.94 S (inset) nanodisks at different time delays. (b) Single-wavelength dynamics of Cu 1.94 S CdS and Cu 1.94 S nanodisks. Pump wavelength used is 400 nm. Probe wavelengths are indicated in the figure. Carrier Dynamics in Cu 1.94 S CdS Nanodisks The time evolution of the transient absorption spectra of Cu 1.94 S CdS nanodisks upon 400-nm excitation is shown in Figure S11a. Both Cu 1.94 S and CdS absorb strongly at 400 nm,* thus the optical response of the two components will contribute to the observed spectra. There are several features observed in the spectra: (i) at around 450 nm, there is a weak photoinduced absorption band that builds up over time; (ii) a dominant bleaching band that peaks at ~490 nm rapidly rises at early times but slowly decays at longer times; (iii) in the nm range, a photoinduced absorption band seen at an early time develops into transient bleaching; (iv) a broad ( nm) transient absorption band appears at an intermediate time and decays at longer times. Features (i) and (ii) have been previously observed in transient spectra of CdS nanocrystals. 2 Lupo et al. have attributed the photoinduced absorption at ~450 nm to the filling of surface trap states during hot-carrier relaxation. 2a The strong bleaching signal at ~490 nm is associated with the first excitonic transition of CdS. Note that the spectral position of this peak is in the range of the excitonic absorption of CdS nanorods in the spectrum shown in Figure S9. Feature (iii), wherein a broad absorption band partially overlaps with a transient bleaching signal, is believed to be a superposition of the contributions from both components. The observed transient absorption is derived from the Cu 1.94 S component. This is evidenced by the transient absorption spectra of Cu 1.94 S-only nanodisks (inset in Figure S11a), which exhibit a similar broad absorption band peaking at ~550 nm. A similar feature has been reported for stoichiometric Cu 2 S nanocrystals by Klimov and co-workers, who attribute the absorption to carrier trapping. 3 The spectral bleaching, on the other hand, is derived from the CdS component. S11

12 Feature (iv) is likely due to the transient absorption of charge-separated species. The occurrence of a donor-acceptor charge transfer is supported by the succeeding discussion. Time traces at selected probe wavelengths are displayed in Figure S11b. The transient bleaching profile obtained for Cu 1.94 S CdS nanodisks at 490 nm is characteristic of CdS nanocrystals. 4 This is expected since the probe wavelength used to monitor the dynamics corresponds to the lowest energy transition of CdS. When the probe wavelength is set at 525 nm (peak of Feature (iii)), a more complicated profile, which exhibits an absorption component at early times and a bleaching component at longer times, is obtained. The bleaching dynamics seen at longer times matches the bleaching profile obtained at 490 nm, suggesting that they arise from the same mechanism (state-filling of CdS conduction band). Meanwhile, the transient absorption observed at early times is primarily attributed to Cu 1.94 S. For comparison, the transient absorption dynamics of Cu 1.94 S nanodisks is also shown in the figure (Note: The dynamics of Cu 1.94 S nanodisks observed at 525 nm and 550 nm are similar but only the data at 550 nm is shown because it has better signal-to-noise ratio). The initial absorption decay is found to be significantly faster for the heterostructure (τ = 0.51 ps) compared to pure Cu 1.94 S (τ = 1.41 ps). The reduction in the absorption decay time constant is due to electron transfer from Cu 1.94 S to the conduction band of CdS. The charge transfer rate is calculated to be 0.80 ps, which gives a charge separation efficiency of ~64%. The broad absorption feature (Feature (iv) in Figure S11a) that appears in the transient absorption spectra of the heterostructure at intermediate times on a time constant of ~0.7 ps is attributed to the absorption of the charge-separated species. *Although the most ideal and straightforward way to demonstrate electron transfer from Cu 1.94 S to CdS is to use a pump wavelength that only Cu 1.94 S can absorb (i.e., wavelengths longer than 550 nm), we have opted to use 400 nm for the following reasons. First, we are limited by our instrument as our amplifier laser system only offers 400 nm and 800 nm excitation source. Only Cu 1.94 S can absorb at 800 nm, but the absorption at this wavelength is very weak. Moreover, as already mentioned in page 3 of the paper, the very weak near-infrared absorption (seen at nm in Figure 4) exhibited by Cu 1.94 S is specifically due to absorption of free holes associated with the presence of Cu vacancies in the nonstoichiometric Cu x S phase. Thus, laser excitation at 800 nm will primarily result in intraband absorption of free holes in the valence band instead of excitonic absorption (i.e., excitation of electrons from the valence band to the conduction band). Since irradiating at 800 nm does not generate electron-hole pairs, this wavelength cannot be used for electron transfer and charge separation studies in our system. References (1) Yong, K-T.; Sahoo, Y.; Swihart, M. T.; Prasad, P. N. J. Phys. Chem. C 2007, 111, (2) (a) Lupo, M. G.; Sala, F. D.; Carbone, L.; Zavelani-Rossi, M.; Fiore, A.; Luer, L.; Polli, D.; Cingolani, R.; Manna, L.; Lanzani, G. Nano Lett. 2008, 8, (b) Klimov, V. I.; Haring- Bolivar, P.; Kurz, H.; Karavanskii, V. A. Superlatt. Microstruct. 1996, 20, (3) Klimov, V. I.; Karavanskii, V. A. Phys. Rev. B 1996, 54, (4) Peng, P.; Sadtler, B.; Alivisatos, A. P.; Saykally, R. J. J. Phys. Chem. C 2010, 114, S12