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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. Bacteria 1 , macromolecules 2 and nanoparticles 3 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][4][5][6][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 formation 3,8,9 , 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 oppositely charged polymethyl methacrylate spheres 10,11 . Here we demonstrate formation of more than 15 different BNSL structures, using combinations of semiconducting, metallic and magnetic nanoparticle building blocks. At least ten of these colloidal crystalline structures have not been reported previously. We demonstrate that electrical charges on sterically stabilized nanoparticles 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 theoretically 12 and experimentally on monodisperse latex particles, whose behaviour can be approximated by hard spheres 13,14 . In a mixture containing spheres of two different sizes (radii R small and R large ), the packing symmetry depends on the size ratio of the small and large spheres (g ¼ R small /R large ) 3,8 . Calculations show that assembly of hard spheres into binary superlattices isostructural with NaCl, AlB 2 and NaZn 13 can be driven by entropy alone without any specific energetic interactions between the spheres 9,15 . Indeed, NaZn 13 -and AlB 2 -type assemblies of silica particles were found in natural Brazilian opals 16 and can be grown from latex spheres 17 . 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 unstable 8,15 .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). Coheren...

Chemical methods developed over the past two decades enable preparation of colloidal nanocrystals with uniform size and shape. These Brownian objects readily order into superlattices. Recently, the range of accessible inorganic cores and tunable surface chemistries dramatically increased, expanding the set of nanocrystal arrangements experimentally attainable. In this review, we discuss efforts to create next-generation materials via bottom-up organization of nanocrystals with preprogrammed functionality and self-assembly instructions. This process is often driven by both interparticle interactions and the influence of the assembly environment. The introduction provides the reader with a practical overview of nanocrystal synthesis, self-assembly, and superlattice characterization. We then summarize the theory of nanocrystal interactions and examine fundamental principles governing nanocrystal self-assembly from hard and soft particle perspectives borrowed from the comparatively established fields of micrometer colloids and block copolymer assembly. We outline the extensive catalog of superlattices prepared to date using hydrocarbon-capped nanocrystals with spherical, polyhedral, rod, plate, and branched inorganic core shapes, as well as those obtained by mixing combinations thereof. We also provide an overview of structural defects in nanocrystal superlattices. We then explore the unique possibilities offered by leveraging nontraditional surface chemistries and assembly environments to control superlattice structure and produce nonbulk assemblies. We end with a discussion of the unique optical, magnetic, electronic, and catalytic properties of ordered nanocrystal superlattices, and the coming advances required to make use of this new class of solids.

Initially poorly conducting PbSe nanocrystal solids (quantum dot arrays or superlattices) can be chemically "activated" to fabricate n- and p-channel field effect transistors with electron and hole mobilities of 0.9 and 0.2 square centimeters per volt-second, respectively; with current modulations of about 10(3) to 10(4); and with current density approaching 3 x 10(4) amperes per square centimeter. Chemical treatments engineer the interparticle spacing, electronic coupling, and doping while passivating electronic traps. These nanocrystal field-effect transistors allow reversible switching between n- and p-transport, providing options for complementary metal oxide semiconductor circuits and enabling a range of low-cost, large-area electronic, optoelectronic, thermoelectric, and sensing applications.

All nanomaterials share a common feature of large surface-to-volume ratio, making their surfaces the dominant player in many physical and chemical processes. Surface ligands - molecules that bind to the surface - are an essential component of nanomaterial synthesis, processing and application. Understanding the structure and properties of nanoscale interfaces requires an intricate mix of concepts and techniques borrowed from surface science and coordination chemistry. Our Review elaborates these connections and discusses the bonding, electronic structure and chemical transformations at nanomaterial surfaces. We specifically focus on the role of surface ligands in tuning and rationally designing properties of functional nanomaterials. Given their importance for biomedical (imaging, diagnostics and therapeutics) and optoelectronic (light-emitting devices, transistors, solar cells) applications, we end with an assessment of application-targeted surface engineering.

New approaches to synthesize photostable thiol-capped CdTe nanocrystals are reported. Post-preparative sizeselective precipitation and selective photochemical etching have been developed as methods providing an increase of photoluminescence quantum efficiency of the nanocrystals of up to 40%. Some advantages of thiol-capping in comparison to conventional organometallic syntheses of quantum dots are discussed.

Single-crystal PbSe nanowires are synthesized in solution through oriented attachment of nanocrystal building blocks. Reaction temperatures of 190-250 degrees C and multicomponent surfactant mixtures result in a nearly defect-free crystal lattice and high uniformity of nanowire diameter along the entire length. The wires' dimensions are tuned by tailoring reaction conditions in a range from approximately 4 to approximately 20 nm in diameter with wire lengths up to approximately 30 microm. PbSe nanocrystals bind to each other on either {100}, {110}, or {111} faces, depending on the surfactant molecules present in the reaction solution. While PbSe nanocrystals have the centrosymmetric rocksalt lattice, they can lack central symmetry due to a noncentrosymmetric arrangement of Pb- and Se-terminated {111} facets and possess dipole driving one-dimensional oriented attachment of nanocrystals to form nanowires. In addition to straight nanowires, zigzag, helical, branched, and tapered nanowires as well as single-crystal nanorings can be controllably prepared in one-pot reactions by careful adjustment of the reaction conditions.

Highly monodisperse CdSe nanocrystals were prepared in a three-component hexadecylamine−trioctylphosphine oxide−trioctylphosphine (HDA−TOPO−TOP) mixture. This modification of the conventional organometallic synthesis of CdSe nanocrystals in TOPO−TOP provides much better control over growth dynamics, resulting in the absence of defocusing of the particle size distribution during growth. The roomtemperature quantum efficiency of the band edge luminescence of CdSe nanocrystals can be improved to 40−60% by surface passivation with inorganic (ZnS) or organic (alkylamines) shells.Chemically grown CdSe nanocrystals (also referred to as quantum dots) are probably the most extensively investigated object among semiconductor nanoparticles since the introduction of the concept of the "size quantization effect" in the earlier eighties. 1,2 This is caused to a large extent by the existence of a very successful preparation method for highquality CdSe nanocrystals, i.e., arrested precipitation in high boiling mixtures of trioctylphosphine oxide (TOPO) and trioctylphospine (TOP). 3,4 The term "high-quality quantum dots" has been recently defined as follows: 5 the achievement of desired particle sizes over the largest possible range, narrow size distributions, good crystallinity, desired surface properties, and in the case of luminescent materials, high quantum yield. CdSe nanocrystals prepared by the TOPO-TOP route and size-separated after synthesis meet all these requirements apart from the lastshigh luminescence quantum yield, which does not exceed 5-15% for as-prepared particles. 6-8 The luminescence quantum efficiency can be sufficiently improved by growing heteroepitactically an inorganic shell of the wide-band gap semiconductor around the particles. 6-9 The conventional techniques used for the wide-band gap shell growth, however, allow us to prepare only very small amounts of core-shell nanoparticles. Another problem of the TOPO-TOP synthesis is the irreproducibility of the growth dynamics and the shape of the CdSe nanocrystals conditioned by an uncertain composition of the coordinating solvent. Technical grade TOPO (90%, Strem or Aldrich), for instance, provides better conditions for the growth of CdSe nanocrystals than distilled TOPO. 10 Recent developments of the organometallic synthetic routes to II-VI semiconductor nanocrystals included an introduction of hexylphosphonic acid to the TOPO-TOP mixture 10-12 and use of hexadecylamine (HDA) as the capping agent for pure and doped ZnSe nanocrystals. 13,14 Taking into account the growing demand on highly luminescent semiconductor nanocrystals for light-emitting devices 15-17 and tagging applications, 18,19 we tried to improve the conventional organometallic TOPO-TOP synthesis by introducing an additional coordinating component (HDA) to the TOPO-TOP mixture. In this mixture focusing of the size distribution is observed during particle growth so that no postpreparative size-selective precipitation is required. The surface of as-prepared CdSe nanocrystals can be passivated wi...