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1 Supporting Information: Columnar Self-assembly of Cu 2 S Hexagonal Nanoplates Induced by Tin (IV)-X Complex Inorganic Surface Ligand Xiaomin Li, Huaibin Shen, Jinzhong Niu, Sen Li, Yongguang Zhang, Hongzhe Wang *, Lin Song Li * Key Laboratory for Special Functional Materials of Ministry of Education, Henan University, Kaifeng , P. R. China lsli@henu.edu.cn; whz@henu.edu.cn I. Experimental Section: Materials. 1-dodecanethiol (DDT, 98%) was purchased from Aldrich. 2, 4-pentanedione (98%), triethylamine (99%), hexanes (analytical grade), methanol (analytical grade), ethanol (analytical grade), acetone (analytical grade), copper acetate (analytical reagent), and tin tetrachloride (analytical reagent) were obtained from Beijing Chemical Reagent Ltd., China. All chemicals were used as received without any further purification. Synthesis of copper (II) acetylacetonate [Cu(acac) 2 ] and tin (IV) bis(acetylacetonate) bichloride [Sn(acac) 2 Cl 2 ]. In a typical synthesis of Cu(acac) 2, 20 mmol Cu(Ac) 2 was dissolved in 10 ml deionized water. Under magnetic stirring, 2, 4-pentanedione (5 ml, 50 mmol) was added and kept stirring for 15 minutes. Cu(acac) 2 was precipitated after appropriate amount of triethylamine was added in the solution. Then Cu(acac) 2 was washed for several times by ethanol and water, it was dried in vacuum at 50 o C for further use. The synthesis of Sn(acac) 2 Cl 2 is similar to that of Cu(acac) 2 (SnCl 4 was used to synthesize Sn(acac) 2 Cl 2 ). Route 1: Synthesis of Cu 2 S nanospheres. The synthesis of Cu 2 S naospheres was accomplished by directly heating Cu(acac) 2 in DDT under nitrogen protection. Typically, g (0.2 mmol) of Cu(acac) 2 powder was dispersed in 5 ml DDT with the aid of magnetic stirring and heated to 200 o C under a nitrogen flow for ~60 min. The nanospheres can be separated from the solution after adding a large amount of acetone followed by centrifugation. The precipitates, which can be well redispersed in hexanes, were used for subsequent characterization. Route 2: Synthesis of Sn-X capped hexagonal Cu 2 S nanoplates with 1D columnar self-assembly. The synthesis of hexagonal Cu 2 S nanoplates with columnar self-assembly was all the same as the synthesis of Cu 2 S nanospheres except the introduction of 0.1mmol Sn(acac) 2 Cl 2 along with Cu(acac) 2. Route 3: Synthesis of Sn-X capped hexagonal Cu 2 S nanoplates with 3D columnar self-assembly. First, a mixture of Sn(acac) 2 Cl 2 (0.0388g, 0.1mmol) and 5 ml DDT was loaded in a 25 ml three-neck flask and heated to 200 o C under nitrogen to obtain a clear Sn-X complex solution. After it was cooled down to room temperature, g (0.2 mmol) of Cu(acac) 2 powder was dispersed in it, the temperature was re-heated to 200 o C under a nitrogen flow for ~60 min. Finally, hexagonal Cu 2 S nanoplates with 3D columnar self-assembly were obtained. Device fabrication. Characteristics of conductive test were fabricated with a layered structure composed of Al/Cu 2 S/ITO. The fixed amount of Cu 2 S nanocrystal S1

2 solution (dispersed in hexanes) was applied directly to pre-cleaned ITO-coated glass substrates ( 40 Ω/sq.) by drop-casting and formed uniform nanocrystal thin films. Finally, Al electrodes were deposited by RF magnetron sputtering through a metal mask to form a sandwich structure device. Characterization: Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) observations were performed using a JEOL JEM-2010 microscope with an accelerating voltage of 200 kv. X-ray diffraction (XRD) studies of Cu2S nanocrystals were carried out at room temperature with a X-ray diffractometer (Philips X Pert Pro) using Cu Kα radiation (wavelength = 1.54 Å). Fourier transform infrared (FTIR) spectra were recorded on a Nicolet AVATAR 360 spectrometer. X-ray photoelectron spectrum (XPS) measurements were performed on an AXIS ULTRA X-ray photoelectron spectroscope, using monochromatised Al Kα radiation with an anode voltage of 15 kv and emission current of 3 ma. The TGA data were collected by EXSTAR 6000 instrument at heating rates of 10K/min under nitrogen atmosphere. I V measurements were performed using Keithley II. Results and discussion on Cu2S nanospheres, 1D and 3D columnar self-assembly of hexagonal Cu2S nanoplates. In route 1, without the participation of Sn-X complex, nearly monodisperse circular Cu2S nanocrystals were synthesized and shown in Figure S1. Figure S1B shows the HRTEM image of a single nanocrystal. It can be seen that the lattice spacing is 2.69 Å, corresponding to (1000) plane d spacing of monoclinic primitive Cu2S. From the TEM and HRTEM images we can not conclude whether the circular Cu2S nanocrystals are disk-like or spherical-like shape. TEM tilting experiment was introduced to further reveal the morphology of the as-synthesized Cu2S nanocrystals. Figure S1C and D show the TEM images of Cu2S nanocrystals taken by horizontal tilting at 0 and 20. The circular shape is maintained and only some overlap between two adjacent nanocrystals exist during the process of tilting. If the particle was disk-like shape, the nanocrystals would not overlap and it should become elliptic with tilting. So we conclude that the circular Cu2S shown in Figure 1A are spherical shape (i. e. nanospheres). Figure S1. TEM (A) and HRTEM (B) images of as-synthesized DDT capped Cu2S nanospheres synthesized by S2

3 route 1, i.e. without the participation of Sn-X complex. TEM images of stacked nanocrystals tilted at (C) 0 and (D) 20. In route 2 and 3, no matter if the Sn-X complex was formed in advance (route 3) or simultaneously with the formation of Cu2S (route 2), columnar self-assembly of Cu2S hexagonal nanoplates could be obtained. From Figure S2 and S3, it can be seen that the columnar self-assembly of Cu2S hexagonal nanoplates are ubiquitous. Furthermore, when the Sn-X complex was formed in advance (route 3), the Cu2S hexagonal nanoplates could assemble into 3D columns in large area (Figure S3). The statistics on the size of the platelets of the 1D and 3D columnar self-assembled hexagonal Cu2S nanoplates are shown in Figure S4A. Figure S2. Low (A) and high (B, C) magnification TEM images of as-synthesized hexagonal Cu2S nanoplates with 1D columnar self-assembly synthesized by route 2 with the participation of Sn-X complex. S3

4 Figure S3. Low (A) and high (B, C) magnification TEM images of as-synthesized hexagonal Cu2S nanoplates with 3D columnar self-assembly synthesized by route 3 with the participation of Sn-X complex. Figure S4C shows the XRD patterns of Cu2S nanospheres and hexagonal Cu2S nanoplates with 1D and 3D columnar self-assembly. It can be seen that all the peak positions matched well with the theoretical values of monoclinic primitive Cu2S. However, they are quite different in intensity due to the selective growth of some planes. Take the peaks at 37.6o and 48.7o which correspond to the (804) and (1204) plane of monoclinic primitive Cu2S as an example, the two peaks gradually weakened from bottom to top and almost disappeared in 3D columnar self-assembled Cu2S nanoplates. It was calculated that the (804) and (1204) plane in a platelet correspond to [201] and [301] crystallographic directions as shown in the structural sketch model (Figure S4D). In the figure, we can see that [201] and [301] crystallographic directions happen to be the partly reflection of the thickness of platelets. So we can conclude that the thicknesses from the thickest to the thinnest are Cu2S nanospheres, 1D columnar self-assembled Cu2S nanoplates, and 3D columnar self-assembled Cu2S nanoplates, respectively. The results are consistent with TEM images; and the statistics are shown in Fig. S4A. S4

5 Figure S4. (A) The evolution of the diameter and thickness of the 1D and 3D columnar self-assembled Cu 2 S nanoplates. (B) The evolution of the distance between two adjacent face-to-face nanoplates synthesized by route 2 and route 3. (C) XRD patterns of Cu 2 S nanospheres and hexagonal Cu 2 S nanoplates with 1D and 3D columnar self-assembly. The bottom lines represent the XRD patterns of monoclinic primitive structured of bulk Cu 2 S (JCPDS: ). (D) Structural sketch model of a single hexagonal Cu 2 S platelet and the lattice parameters of monoclinic primitive Cu 2 S. Figure S5. TEM images and platelet diameter distribution analysis of as-synthesized 2D Cu 2 S nanoplates with columnar self-assembly sampled at 15 min (A, D), 30 min (B, E), and 60 min (C, F). The results of the platelet diameter distribution analysis show that the diameter of the platelet was increased with the reaction prolonging. III. Results and discussion on Sn-X complex. Figure S6 shows the FTIR spectra of the complex solution which was obtained by heating 10 mmol Sn(acac) 2 Cl 2 and excess DDT at 200 o C for 15min. By comparing with pure DDT, it can be seen that they both have the sharp bands at 2923 and 2857 S5

6 cm -1, which can be assigned to the asymmetric methyl stretching and asymmetric and symmetric methylene stretching modes, whereas a new band at 1695 cm -1 belonging to C=O vibrations is shown during the formation of Sn-X complex. We conclude that Sn(acac) 2 Cl 2 reacts with excess DDT to form a Sn-X complex where acetylacetonate groups release to form acetylacetone. Figure S6. FTIR spectra of pure DDT (b) and the complex solution which was obtained by heating 10 mmol Sn(acac) 2 Cl 2 and excess DDT at 200 o C for 15min (a). The process of the reaction was shown in the bottom. As seen in Figure S7A, there is a large decrease (almost disappearance) of absorption features of organic groups in the IR spectra, it almost has no C H, S H, and C C bonds in the sample of columnar self-assembled hexagonal Cu 2 S. As shown in Figure S7B, it shows broad bulge absorption between 400 nm and 600 nm for columnar self-assembled hexagonal Cu 2 S (C). By comparing the absorption position of Sn-X complex, we attribute the broad bulge in C to the absorption overlap of Cu 2 S nanocrystals and Sn-X complex. From the inset of Figure S7B, the nanoplates were deposited after 3 hours standing, which indicate the Sn-X complex capped Cu 2 S hexagonal nanoplates were poorly soluble in organic solvent. Figure S7. (A) FTIR spectra of columnar self-assembled hexagonal Cu 2 S (c), Cu 2 S nanospheres (b), and pure DDT (a). (B) UV-vis spectra of self-assembled hexagonal Cu 2 S (c), Cu 2 S nanospheres (b), and the complex solution which was obtained by heating 10 mmol Sn(acac) 2 Cl 2 and excess DDT at 200 o C for 15min (a). The inset (in B) shows the photographs of as-synthesized columnar self-assembled hexagonal Cu 2 S (dispersed in hexanes) before (left) and after (right) 3 h standing. S6

7 XPS was used to characterize the composition of as-synthesized columnar self-assembled Cu 2 S hexagonal nanoplates (Figure S8). The results corroborated the presence of Sn(IV) element. After washing many times with polar solvents, the columnar self-assembly of hexagonal nanoplates collapsed and XPS quantitative analysis showed that Sn (IV) element contents decreased to a lower level. Due to such low concentration of Sn-X complex, it may lead to the force between two adjacent platelets weakening, so the columnar self-assembled structure will eventually collapse. So we conclude that the Sn-X complex is indeed capped on the Cu 2 S hexagonal nanoplates. Figure S8. XPS survey spectra (A), High-resolution XPS (HR-XPS) spectra of the copper 2p (B), HR-XPS spectra of the sulfur 2p (C), and HR-XPS spectra of the tin 3d (D) of as-synthesized hexagonal Cu 2 S nanoplates with columnar self-assembly. The binding energies of copper 2p3/2 and 2p1/2 for Cu 2 S were ev and ev, and no shake-up peaks were found in the higher binding energy direction, which was consistent with the standard reference XPS spectrum of Cu 2p in Cu 2 S. The sulfur 2p3/2 and 2p1/2 peaks in the spectra are located at and ev, which are consistent with the ev range expected for S in sulfide phases. The tin HR-XPS spectrum shows two narrow and symmetric peaks at and ev, indicative of Sn(IV) with a peak splitting of 8.4 ev. (E) TEM image of as-synthesized hexagonal Cu 2 S nanoplates after it was washed by the mixture of acetone, methanol, and DI water many times. (F) Cu/Sn molar ratio in as-synthesized hexagonal Cu 2 S nanoplates before and after collapse determined by XPS. In order to further investigate on ligand effect related to Sn-X complex and evaluate the potential device applications with such columnar self-assembled Cu 2 S hexagonal nanoplates, we studied the electric transport properties of Cu 2 S nanocrystals with and without the participation of Sn-X complex. The results are shown in Figure S9, an increase of its conductivity has been observed which was induced by the decrease of the interparticle spacing. S7

8 Figure S9. I-V characteristics of Cu2S nanospheres (black) and columnar self-assembled hexagonal Cu2S (red) films. The inset shows the schematic of fabricated sample used for electrical measurement. In Figure S10, it can be seen that when SnCl4 was used as precursor (the same as route 2), the columnar self-assembled structure was maintained but not as good as when Sn(acac)2Cl2 was used as the precursor. Figure S11 shows thermogravimetric analysis of Cu(acac)2 and Sn(acac)2Cl2. It can be seen that they all exhibit a similar single major thermal event after the temperature reached 200 oc. The weight loss processes basically in synchrony and occurred very fast, so we considered that this synchrony may result in the better columnar-shape self-assembly of Cu2S nanoplates when Sn(acac)2Cl2 was used as precursor. Therefore, both one-pot (route 2) and two-step (route 3) methods could be accomplished successfully in our synthesis system. Figure S10. TEM images of as-synthesized Sn-X capped columnar self-assembled Cu2S nanoplates by using Sn(acac)2Cl2 (A) and SnCl4 (B) as Sn(IV) precursor, respectively. S8

9 Figure S11. Thermogravimetric analysis of Cu(acac) 2 and Sn(acac) 2 Cl 2. S9