Supporting Information. Oleic Acid-Induced Atomic Alignment of ZnS Polyhedral Nanocrystals

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1 Supporting Information Oleic Acid-Induced Atomic Alignment of ZnS Polyhedral Nanocrystals Ward van der Stam, 1 Freddy T. Rabouw, 1 Sander J. W. Vonk, 1 Jaco J. Geuchies, 1 Hans Ligthart, 1 Andrei V. Petukhov, 2,3 and Celso de Mello Donega* 1 1. Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands 2. Physical and Colloid Chemistry, Debye Institute for Nanomaterials Science, Padualaan 8, 3584 CH Utrecht, The Netherlands 3. Laboratory of Physical Chemistry, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Corresponding Author *c.demello-donega@uu.nl S1

2 Experimental Section Materials. Copper (II) sulfate pentahydrate (CuSO 4.5H 2 O, 98%), Zinc chloride (ZnCl 2, 98%), 1- dodecanethiol (DDT, 98%), Oleylamine (OLAM, tech. 70%), 1-octadecene (ODE, 90%), trioctylphosphine (TOP, 90%), Oleic acid (OA, 90%), Ethylene glycol (EG, anhydrous, 99.8%), Toluene (anhydrous, 99.8%), Methanol (MeOH, anhydrous, 99.8%), 1-Butanol (BuOH, anhydrous, 99.8%) were purchased from Sigma Aldrich. OLAM and ODE were degassed at 120 C for 2 hours before use, all other chemicals were used as received. Bifrustrum-shaped colloidal Cu 2-x S nanocrystals were synthesized according to a method modified from Lu et al. 1 Our adaptation included the removal of the Indium salt and the replacement of copper(ii) nitrate by copper(ii) sulfate pentahydrate. First, 200 mg of CuSO 4.5H 2 O is mixed in 7.5 ml DDT and 6.0 ml OA. Next, the mixture is heated to 200 C for two hours, during which the Cu 2-x S nanocrystals form. The reaction was quenched by removal of the heating mantle and the addition of ~5 ml toluene at 70 C. Afterward, the Cu 2-x S nanocrystals were washed by precipitation with 5 ml of an 1:1 MeOH:BuOH mixture and finally redispersed in ~5 ml toluene. This washing cycle was repeated twice. For the final washing step, 1 ml of the NC solution in toluene were redispersed in ~2 ml trioctylphosphine (TOP) by precipitation with the same MeOH:BuOH mixture and centrifugation. Cation exchange reactions of Cu + for Zn 2+ were performed as described by Li et al. 2 First, 1 mmol ZnCl 2 was dissolved in 3 ml ODE and 2 ml OLAM (both previously degassed) at 250 C. Subsequently, the Cu 2-x S NCs dispersed in TOP were injected at that temperature (concentration ~10-8 M). The mixture was allowed to react for 5 min under heating and stirring, after which the temperature was lowered to 70 C, followed by the addition of ~5 ml toluene. The NCs were precipitated by adding a 1:1 MeOH:BuOH solution. Afterward, the NCs were isolated by centrifugation and finally redispersed in ~3 ml toluene. This cycle was repeated twice. S2

3 Self-assembled 2D NC superlattices were obtained according to the method described by Chen et al. 3 First, the NC solution in toluene was diluted 4 times (final concentration ~3*10-9 M, assuming stoichiometric reaction and no purification losses) and 0.5 ml of this diluted solution was brought onto a dense liquid surface (ethylene glycol, EG, 28 ml, in a Teflon petri dish with an inner diameter of 64 mm). Subsequently, the toluene was allowed to evaporate at room temperature over the course of an hour, resulting in the formation of a continuous membrane at the liquid-air interface. Finally, these 2D superstructures were scooped onto a TEM-grid and dried overnight prior to further ex-situ investigation with TEM and ED. In the experiments where additional ligands were added, 100 µl of a 0.03 M ligand in ethylene glycol stock-solution, prepared by dissolving 0.09 mmol ligand in 3 ml ethylene glycol, was added to the liquid ethylene glycol substrate. This gives a nominal ligand/nc ratio of 2x10 6, which corresponds to roughly 3.5x10 2 added ligands per surface Zn site (assuming that 50% of the surface sites are Zn atoms and a spherical NC of 11 nm diameter). In-situ Grazing Incidence Small Angle X-ray Scattering (GISAXS) and Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) measurements were carried out at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, at the ID10 beamline. A photon energy of 8 kev was used and the angle of incidence was fixed at 0.3. All GIWAXS signals were collected with a Pilatus- 300K photon detector, with dimensions of 487 x 619 pixels and a pixelsize of 172x172 µm 2. The distance from the liquid cell to the GIWAXS detector was approximately 20 cm. The 2D detector used for the GISAXS measurements was a Maxipix CCD sensor, with dimensions of 256 x 1296 pixels and a pixelsize of 52x52 µm 2. The sample-to-detector distance was 50 cm. The GISAXS and GIWAXS detectors were calibrated as follows. The positions of the X-ray detectors relative to the middle of the sample cell were calibrated using reference samples of Ag-behenate (for GISAXS) and α-al 2 O 3 (for GIWAXS). In the analysis, however, we have to take into account that the sample cell has a diameter of 6.4 cm, and the X-ray beam comes in at a very shallow angle. Therefore, a slight variation in the liquid level or in the curvature of the liquid surface may change S3

4 the position on the sample cell where the X-ray beam hits. This position can shift from the middle of the sample cell to at most 3.2 cm back or forward along the direction of X-ray beam. We used the patterns recorded on the GIWAXS detector, where diffraction appears at known angles fixed by the wurtzite crystal structure of ZnS, to determine this shift. The shift determined in this way did not show abrupt changes as a function of time, and was at most 2.0 cm from the middle of the sample cell. All diffraction angles and scattering vectors given in the manuscript were corrected accordingly. Transmission Electron Microscopy (TEM) images were acquired using a FEI Tecnai-10 microscope operating at 100 kv. Samples for TEM imaging were prepared by scooping the NC superstructures onto a carbon-coated copper (400-mesh) TEM grid. Electron Diffraction (ED) patterns were obtained without diffraction aperture. S4

5 Figure S1. Comparison of atomic alignment of (a,d,g) small hexagonal bifrustum ZnS nanocrystals (NCs), (b,e,h) hexagonal ZnS bipyramids and (c,f,i) large hexagonal bifrustum ZnS NCs. For 2D NC superstructures of small bifrustum NCs, the {002} facets are all pointing upward (schematic representation in panel g), as evidenced by comparing the experimental GIWAXS pattern (a) and calculated GIWAXS pattern (d). For 2D NC superstructures of hexagonal bipyramid NCs, the {101} facets are pointing upward (schematic representation in panel h, dashed lines indicate the shape of the NC). In this case, the {002} facets are too small to adsorb to the liquid-air interface, due to the sharp tips of the bipyramids. The experimental GIWAXS pattern (b) closely matches the calculated GIWAXS pattern (e). When large hexagonal bifrustum NCs self-assemble into 2D NC superstructures, a coexistence of {002} and {101} facets pointing up is observed (schematic representation in panel i). (c) Diffraction spots originating from both orientations are present in the GIWAXS pattern and the experimental pattern (c) closely matches the calculated pattern (f). S5

6 Supporting Discussion Calculating the theoretical GIWAXS patterns of aligned nanocrystals The ZnS crystal structure is defined by a set of reciprocal lattice vectors {g hkl }, where hkl are the Miller indices that label the lattice vectors. In the experiment an X-ray beam with wave vector k in is incident on the sample. A nanocrystal can (elastically) diffract the beam if it is oriented such that one of the lattice vectors satisfies the conditions k out = k in + g hkl, (1) k out = k in, (2) where k out is the wave vector of the diffracted beam. We consider the situation of nanocrystals adsorbed to the liquid/air interface with a specific crystal facet, i.e. oriented with a specific lattice vector g up oriented upward, but with rotational freedom in the horizontal plane. Each reciprocal lattice vector hkl then has a fixed angle α with respect to the interface normal, but can freely rotate in the horizontal plane: g hkl = R(β)g hkl 0, (3) where R(β) is a rotation matrix for rotation in the horizontal plane over an angle β, and g hkl 0 is the reciprocal lattice vector at angle α with respect to g up but with some arbitrary orientation in the horizontal plane. We solve for the rotation angle β at which the diffraction condition (Eqs. 1 and 2) is matched. Knowing this angle, we can calculate k out (Eq. 1 with Eq. 3) and the azimuthal angle φ at which diffraction is expected: φ = arccos k out,y 2 k out,x +k2 out,y (4) Here k out,x is the component of k out parallel to the liquid/air interface but perpendicular to the incoming beam, and k out,y is the component normal to the interface. S6

7 Figure S2. (Left Panel) Scattering from the center of the sample cell gives the in-plane NC separation on the GISAXS detector as well as the atomic alignment on the GIWAXS detector. (Center panel) Scattering from a point along the incident X-ray beam other than the center, results in scattering at a different position on both the GISAXS and GIWAXS detectors. (Right panel) Scattering from 2D superstructure in which the NCs come closer together, results in scattering peaks at different positions on the GISAXS detector only. S7

8 Figure S3. (Top Panel) The small inter-nc separation observed in-situ (see main text) can be explained by buckling at the air-toluene interface, since this can cause a reduction from 13.6 nm to 10.4 nm for the planar structure vs. the buckled structure, even if the ligand layer separation is fixed at 2 nm. (Lower Panel) Close-packing of the NCs in a buckled superstructure at the air-toluene interface would result in small in-situ inter-nc separations. Supporting References 1. X. Lu, Z. Zhuang, Q. Peng, Y. Li, CrystEngComm 2011, 13, H. Li, M. Zanella, A. Genovese, M. Povia, A. Falqui, C. Giannini, L. Manna, Nano Lett. 2011, 11, A. Dong, J. Chen, P. M. Vora, J. M. Kikkawa, C. B. Murray, Nature 2010, 466, S8