Vol 439 5 January 2006 doi: 10. 1038/nature04414 nature LETTERS Structural diversity in binary nanoparticle superlattices Elena V. Shevchenko 2*+, Dmitri V. Talapin *t, Nicholas A Kotov, Stephen O'Brien& Christopher B. Murray Assembly of small building blocks such as atoms, molecules anoparticles of different materials(Fig. 1). Coherently packed noparticles into macroscopic structures-that is, bottom domains extend up to 10 um in lateral dimensions, and can displa ssembly-is a theme that runs through chemistry, biology and well defined facets(Supplementary Fig. 1). In many cases, several aterial science. Bacteria, macromolecules and nanoparticles' BNSL structures form simultaneously on the same substrate, under can self-assemble, generating ordered structures with a precision identical experimental conditions. The same nanoparticle mixture that challenges current lithographic techniques. The assembly of can assemble into BNSLs with very different stoichiometry and nanoparticles of two different materials into a binary nanoparticle packing symmetry. For example, 11 different BNSL structures were to a large variety of materials(metamaterials) with precisely nanoparticles(Supplementary Fig. 2). We also observe that, in controlled chemical composition and tight placement of the general, BNSLs tolerate much broader y ranges than hard sphere omponents Maximization of the nanoparticle packing density example, AlB2-type BNSLs assembled from different comb has been proposed as the driving force for BNSL formation, nations of PbSe, PbS, Au, Ag, Pd, Fe2O3, CoPt3 and Bi nanoparticles and only a few BNSL structures have been predicted to be n a broad y range( Supplementary Fig. 3). Further, we observe thermodynamically stable. Recently, colloidal crystals with BNSLs that could not be identified as isostructural with specific micrometre-scale lattice spacings have been grown from oppo- intermetallic compounds( Supplementary Fig. 2). This observed itely charged polymethyl methacrylate spheres,. Here we structural diversity of BNSLs defies traditional expectations, and demonstrate formation of more than 15 different BNSL struc- shows the great potential of modular self-assembly at the nanoscale tures, using combinations of semiconducting, metallic and mag. The formation of binary structures with packing density signifi netic nanoparticle building blocks. At least ten of these colloidal cantly lower than the density of single-phase f.c.c. close packing particles determine BNSLstod: ses on sterically stabilized nano- ordering. Moreover, van der Waals, steric or de e for nanoparticle crystalline structures have not been reported previously. We (0.7405)rules out entropy as the main driving force for nanopartic demonstrate that electrical char rticle iometry; additional contributions interactions are not sufficient to explain why these low density BNSLs der Waals, steric and dipolar forces stabilize form, instead of their constituents separating into single-compo the variety of BNSL structures. uperlattices. Opposite electrical charges on nanoparticles could Face-centred-cubic(fcc )ordering of monodisperse hard spheres impart a specific affinity of one type of particle(for example, dispersed in a liquid permits larger local free space available for each dodecanethiol-capped Au, Ag, Pd)for another (typically PbSe sphere compared to the unstructured phase, resulting in higher PbS, Fe2O3, CoPt, and so on, capped with long chain carboxylic translational entropy of the spheres. When the volume fraction of acids). If nanoparticles are oppositely charged, the Coulomb hard spheres approaches -55%, this ordering enhances the total potential would stabilize the BNSl while destabilizing the single- entropy of the system and drives the ordering phase transition. component superlattices. The electrical charges might be Entropy-driven crystallization has been studied in great detail both present on sterically stabilized nanoparticles even in non-polar theoretically and experimentally on monodisperse latex particles, solvents whose behaviour can be approximated by hard spheres 4. In a charges on the nanoparticles that form our BNSI mixture containing spheres of two different sizes(radi Rsmall and studied the electrophoretic mobility of PbSe and Au nanocrystals Rlarge ) the packing symmetry depends on the size ratio of the small Laser Doppler velocimetry allows the distribution of electrophoretic assembly of hard spheres into binary superlattices isostructural electrical charge(Z, in units of e)of a spherical particle in a low ith NaCl, AlB 2 and NaZn13 can be driven by entropy alone without dielectric solvent in absence of electrolyte can be calculated from the any specific energetic interactions between the spheres..Indeed, electrophoretic mobility (ue) where ue= Ze/(rna), n is the viscosity NaZn13- and AlB2-type assemblies of silica particles were found in of the solvent and a is the hydrodynamic diameter of a particle". with natural Brazilian opals and can be grown from latex spheres"?. In aa=10 nm, we obtain He 0.27X10-4Zcm2V-Is-1. These calcu certain y range, the packing density of these structures either exceeds lated values agree well with the peaks in the experimental mobili or is very close to the density of the close-packed f.c. c. lattice distribution for 7.2-nm-diameter PbSe nanocrystals in chloroform (0.7405), while structures with lower packing densities are predicted ( Fig. 2a). Owing to the organic coat (oleic acid), the effective to be unstable, Is hydrodynamic radius of PbSe nanocrystals extends beyond the Despite these predictions, we observed an amazing variety of crystalline core by 1-2 nm, depending on the density of surface BNSLs that self-assemble from colloidal solutions of nearly spherical coverage. The peaks in the mobility distribution curve indicate the arbor, Michigan 48109, USA +Present ading. 500 West 120th Street, New York, New York 10027, USA. Department of Chemical University of Michigan, Ann dress: The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 2006 Nature Publishing Group© 2006 Nature Publishing Group Structural diversity in binary nanoparticle superlattices Elena V. Shevchenko1,2*†, Dmitri V. Talapin1 *†, Nicholas A. Kotov3 , Stephen O’Brien2 & Christopher B. Murray1 Assembly of small building blocks such as atoms, molecules and nanoparticles into macroscopic structures—that is, ‘bottom up’ assembly—is a theme that runs through chemistry, biology and material science. Bacteria1 , macromolecules2 and nanoparticles3 can self-assemble, generating ordered structures with a precision that challenges current lithographic techniques. The assembly of nanoparticles of two different materials into a binary nanoparticle superlattice (BNSL)3–7 can provide a general and inexpensive path to a large variety of materials (metamaterials) with precisely controlled chemical composition and tight placement of the components. Maximization of the nanoparticle packing density has been proposed as the driving force for BNSL formation3,8,9, and only a few BNSL structures have been predicted to be thermodynamically stable. Recently, colloidal crystals with micrometre-scale lattice spacings have been grown from oppo￾sitely charged polymethyl methacrylate spheres10,11. Here we demonstrate formation of more than 15 different BNSL struc￾tures, using combinations of semiconducting, metallic and mag￾netic nanoparticle building blocks. At least ten of these colloidal crystalline structures have not been reported previously. We demonstrate that electrical charges on sterically stabilized nano￾particles determine BNSL stoichiometry; additional contributions from entropic, van der Waals, steric and dipolar forces stabilize the variety of BNSL structures. Face-centred-cubic (f.c.c.) ordering of monodisperse hard spheres dispersed in a liquid permits larger local free space available for each sphere compared to the unstructured phase, resulting in higher translational entropy of the spheres. When the volume fraction of hard spheres approaches ,55%, this ordering enhances the total entropy of the system and drives the ordering phase transition. Entropy-driven crystallization has been studied in great detail both theoretically12 and experimentally on monodisperse latex particles, whose behaviour can be approximated by hard spheres13,14. In a mixture containing spheres of two different sizes (radii Rsmall and Rlarge), the packing symmetry depends on the size ratio of the small and large spheres (g ¼ Rsmall/Rlarge)3,8. Calculations show that assembly of hard spheres into binary superlattices isostructural with NaCl, AlB2 and NaZn13 can be driven by entropy alone without any specific energetic interactions between the spheres9,15. Indeed, NaZn13- and AlB2-type assemblies of silica particles were found in natural Brazilian opals16 and can be grown from latex spheres17. In a certain g range, the packing density of these structures either exceeds or is very close to the density of the close-packed f.c.c. lattice (0.7405), while structures with lower packing densities are predicted to be unstable8,15. Despite these predictions, we observed an amazing variety of BNSLs that self-assemble from colloidal solutions of nearly spherical nanoparticles of different materials (Fig. 1). Coherently packed domains extend up to 10 mm in lateral dimensions, and can display well defined facets (Supplementary Fig. 1). In many cases, several BNSL structures form simultaneously on the same substrate, under identical experimental conditions. The same nanoparticle mixture can assemble into BNSLs with very different stoichiometry and packing symmetry. For example, 11 different BNSL structures were prepared from the same batches of 6.2 nm PbSe and 3.0 nm Pd nanoparticles (Supplementary Fig. 2). We also observe that, in general, BNSLs tolerate much broader g ranges than hard spheres: for example, AlB2-type BNSLs assembled from different combi￾nations of PbSe, PbS, Au, Ag, Pd, Fe2O3, CoPt3 and Bi nanoparticles in a broad g range (Supplementary Fig. 3). Further, we observe BNSLs that could not be identified as isostructural with specific intermetallic compounds (Supplementary Fig. 2). This observed structural diversity of BNSLs defies traditional expectations, and shows the great potential of modular self-assembly at the nanoscale. The formation of binary structures with packing density signifi- cantly lower than the density of single-phase f.c.c. close packing (0.7405) rules out entropy as the main driving force for nanoparticle ordering. Moreover, van der Waals, steric or dipolar interparticle interactions are not sufficient to explain why these low density BNSLs form, instead of their constituents separating into single-component superlattices. Opposite electrical charges on nanoparticles could impart a specific affinity of one type of particle (for example, dodecanethiol-capped Au, Ag, Pd) for another (typically PbSe, PbS, Fe2O3, CoPt3 and so on, capped with long chain carboxylic acids). If nanoparticles are oppositely charged, the Coulomb potential would stabilize the BNSL while destabilizing the single￾component superlattices. The electrical charges might be present on sterically stabilized nanoparticles even in non-polar solvents18–20. To measure charges on the nanoparticles that form our BNSL, we studied the electrophoretic mobility of PbSe and Au nanocrystals. Laser Doppler velocimetry allows the distribution of electrophoretic mobilities within an ensemble of nanoparticles to be measured. The electrical charge (Z, in units of e) of a spherical particle in a low dielectric solvent in absence of electrolyte can be calculated from the electrophoretic mobility (me) where me ¼ Ze=ð3phaÞ; h is the viscosity of the solvent and a is the hydrodynamic diameter of a particle21. With a ¼ 10 nm, we obtain me < 0:27 £ 1024Z cm2 V21 s 21: These calcu￾lated values agree well with the peaks in the experimental mobility distribution for 7.2-nm-diameter PbSe nanocrystals in chloroform (Fig. 2a). Owing to the organic coat (oleic acid), the effective hydrodynamic radius of PbSe nanocrystals extends beyond the crystalline core by 1–2 nm, depending on the density of surface coverage. The peaks in the mobility distribution curve indicate the LETTERS 1 IBM Research Division, T. J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, New York 10598, USA. 2 Department of Applied Physics & Applied Mathematics, Columbia University, 200 SW Mudd Building, 500 West 120th Street, New York, New York 10027, USA. 3 Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA. †Present address: The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA. *These authors contributed equally to this work. Vol 439|5 January 2006|doi:10.1038/nature04414 55