Supporting Information. Single-Crystalline Copper Nano-Octahedra

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1 Supporting Information Single-Crystalline Copper Nano-Octahedra Shu-Chen Lu, Ming-Cheng Hsiao, Mustafa Yorulmaz, Lin-Yung Wang, Po-Yuan Yang, Stephan Link, Wei-Shun Chang, Hsing-Yu Tuan * Department of Chemical Engineering, National Tsing Hua University, 101, Section 2, Kuang-Fu Road, Hsinchu, Taiwan 30013, ROC. Department of Chemistry, Rice University, Houston, Texas 77005, United States. *Corresponding Author Hsing-Yu Tuan, Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan, Tel.: Fax: address: hytuan@che.nthu.edu.tw 1

2 Experimental section Chemicals: All chemicals were used as received without extra purification. Copper(I) chloride(99.99%) and trioctylphosphine (TOP, 90%) were purchased from Alfa Aesar. Oleylamine (OLA, 70%) and anhydrous toluene were purchased from Sigma-Aldrich. Synthesis of Cu Nanoctahedra: In a typical synthesis of Cu nanoctahedra, precursor solution was preapared by mixing 0.2 mmol CuCl and 2 ml TOP into a vial preheated at 200 C for 2 hours. 18 ml oleylamine (OLA) was added into a three-necked flask and kept under a flow of high-purity argon gas for 40 minutes with strong magnetic stirring. After the purge process was completed, OLA was then heated from room temperature to 335 C before precursor solution was injected. During the reaction, the color of the solution changed from bright yellow to red-brown, indicating the formation of Cu 0 species. After 20 minutes, the resulting colloidal products were cooled to room temperature rapidly with a water-bath and were purified by centrifugation and washing with toluene to remove excess reagents. Characterization: Scanning electron microscopy (SEM) images were taken by a HITACHI-SU8010 field-emission SEM operating at an accelerating voltage of 10kV. Transmission electron microscopy (TEM) images were acquired by a Hitachi H-7100 electron microscope with an accelerating voltage of 75kV. High-resolution TEM (HRTEM) and selected area electron diffraction (SAED) images were obtained by a JEOL JEM 2100F electron microscope with an accelerating voltage of 200kV. X-ray diffraction (XRD) was measured with a Rigaku Ultima IV X-ray diffractometer using Cu Kα radiation. The Fourier-transform infrared (FTIR) spectrum was measured by a Perkin- Elmer Spectrum RXI FTIR spectrometer with a resolution of 4cm -1. The high-resolution X-ray Photoelectron Spectrum (HRXPS) was recorded by a ULVAC-PHI Quantera SXM. UV-visible spectra were measured by a Hitachi U-4100 spectrophotometer in an ambient 2

3 environment. Single-Particle Scattering Spectroscopy: Single-particle scattering spectroscopy of Cu nanoparticles was performed on a home-built microscope consisting of a dark-field microscope (Axio Observer D1 with a dark-field condenser N.A. = 1.4) and a spectrograph equipped with a CCD camera (Princeton Instruments, Acton SpectraPro 2150i with Pixis 400BR thermoelectrically-cooled back-illuminated CCD) mounted on a programed translational stage (Newport Linear Acuator model LTA-HL). 1 A halogen lamp was used as an excitation source in a dark-field geometry. The scattered light from the nanoparticles was collected by an air-spaced objective (Zeiss, 50X, N.A. = 0.8) and directed to the spectrograph located at the first image plane of the microscope. The hyperspectral images were taken by moving the slit of the spectrograph across the scattered image while taking spectra. An indexed substrate was used for the correlation of the high-resolution SEM images and single-particle scattering spectroscopy on the same single Cu nanoparticles (Figure S10a). The analysis of the hyperspectral images was performed using a homewritten Matlab program. Each pixel in the image (Figure S10b) corresponds to one spectrum. In order to retrieve the scattering spectrum of individual nanoparticles, spectra from 5 x 5 pixels around the maximum intensity spectrum were averaged. The same procedure was performed on an area where there was no nanoparticle to create a background spectrum. The individual nanoparticle spectrum was obtained by subtracting the background spectrum from the particle spectrum, and dividing this obtained spectrum by the lamp spectrum to account for the spectral shape of the lamp (Figure S10c). Finite-Difference Time-Domain (FDTD) Calculations: The scattering spectrum of a Cu nanoparticle was modeled using a commercially available FDTD package which solves the classical Maxwell s equations in the time domain. 2 The model geometry is shown in Figure 3

4 S11a. The Cu nanoparticle was assumed to be an octahedron with an edge length equal to 155 nm in Figure 3c. One of the Cu nanoparticle faces was in contact with the substrate, as shown in the inset in Figure 3c and in Figure S11a. The energy-dependent optical constants of the Cu nanoparticle were adopted from the tabulated values for bulk Cu. 3 The substrate used in the experiment was approximated as SiO 2. 3 The wavelength dependent scattering cross sections were obtained by dividing the scattering power by the light intensity. The final spectrum was averaged over both p- and s-polarizations to take into account the unpolarized dark-field excitation geometry. The extinction spectra shown in Figure 3f were calculated based on the geometries shown in Figure S9 with an edge length of 130 nm. The extinction cross section was obtained by dividing the extinction power by the light intensity. The calculations were carried out using the Shared Tightly-Integrated Cluster (STIC) provided by Rice University. The minimum grid size used in this simulation was 2 nm. Default convergence criteria were used. Each job required a total of 64 CPUs with an approximately four-hour run time. 4

5 Figure S1. Large-area SEM images of Cu nano-octahedra. 5

6 Figure S2. Large-area TEM images of Cu nano-octahedra. 6

7 (a) (c) (d) Figure S3. Partially-enlarged TEM images of Cu nano-octahedra. 7

8 Figure S4. (a) XRD pattern recorded from Cu nano-octahedra drop-cast on Si wafer. (b) FTIR spectra of Cu nano-octahedra. XPS spectra of Cu nano-octahedra: (c) Cu 2p; (d) P 2p. 8

9 Figure S5. (a)-(d) TEM images and SAED patterns of a single Cu octahedron viewed along the [111] and [112] zone axes by a tilting experiment. (e) Schematic drawing of the image conversion tilted by 19.5 degrees. Figure S6. (a) SAED patterns of a single Cu octahedron viewed along the [110] zone axis. (b) Dark field TEM image recorded using a (111) reflected beam. 9

10 Figure S7. TEM image and HRTEM image of the edge of a single Cu nano-octahedron. 10

11 Figure S8. Scattering spectra of single Cu nano-octahedra measured by dark-field microscopy and their correlated SEM images shown in the insets. The scale bars in the insets all correspond to 50 nm. 11

12 Figure S9. Simulation geometries for the extinction spectra of a (a) dimer (b) trimer (c) quadrumer (d) heptamer. The propagation direction of the excitation is normal to these images while the polarization is indicated by the blue arrows. 12

13 Figure S10. (a) SEM image of a large area containing octahedral Cu nanoparticles. The predefined patterns shown in the SEM image were used for the correlation of electron and optical microscopy on the same individual nanoparticle. SEM imaging with low magnification was first performed to select the particles with octahedral shape as indicated by the small white boxes. (b) Hyperspectral image of the area highlighted by the blue box in (a). The two images shown in (b) are the same but with different color scaling to better show the good match of the spatial locations of the particles and the pattern, confirming that correlated measurements were performed on the same nanoparticles. (c) The scattering spectrum of one nanoparticle obtained by analyzing the hyperspectral image shown in (b). 13

14 Figure S11 (a) Schematic illustration of the FDTD calculation conditions. The wave vector k is parallel to the substrate. The red box represents the discrete Fourier transform (DFT) monitors, which record the scattering power. The green box defines the total-field-scatter-field (TFSF) light source. The simulation was performed under p- or s-polarization, in which the electric field was aligned parallel or perpendicular to the long axis of the octahedron. The perfect matching layer (PML) defines the total simulation volume (1000 nm 1000 nm 1000 nm). (b) Calculated scattering cross section (σ scat ) as a function of wavelength. 14

15 References: 1. Byers, C. P.; Hoener, B. S.; Chang, W.-S.; Yorulmaz, M.; Link, S.; Landes, C. F., J. Phys. Chem. B 2014, 118, FDTD Solutions v Lumerical Solutions, Inc: Vancouver, Canada, Palik, E. D., Handbook of Optical Constants of Solids Academic Press Orlando,