Connecting the particles in the box--controlled fusion of hexamer nanocrystal clusters within an AB₆ binary nanocrystal superlattice

Unravelling three-dimensional adsorption geometries of PbSe nanocrystal monolayers at a liquid-air interface

The adsorption, self-organization and oriented attachment of PbSe nanocrystals (NCs) at liquid-air interfaces has led to remarkable nanocrystal superlattices with atomic order and a superimposed nanoscale geometry. Earlier studies examined the NC self-organization at the suspension/air interface with time-resolved in-situ X-ray scattering. Upon continuous evaporation of the solvent, the NC interfacial layer will finally contact the (ethylene glycol) liquid substrate on which the suspension was casted. In order to obtain structural information on the NC organization at this stage of the process, we examined the ethylene glycol/NC interface in detail for PbSe NCs of different sizes, combining in-situ grazing-incidence small-and-wide-angle X-ray scattering (GISAXS/GIWAXS), X-ray reflectivity (XRR) and analytical calculations of the adsorption geometry of these NCs. Here, we observe in-situ three characteristic adsorption geometries varying with the NC size. Based on the experimental evidence and simulations, we reveal fully three-dimensional arrangements of PbSe nanocrystals at the ethylene glycol-air interface with and without the presence of rest amounts of toluene.

Binary Assembly of PbS and Au Nanocrystals: Patchy PbS Surface Ligand Coverage Stabilizes the CuAu Superlattice

Self-assembly of two sizes of nearly spherical colloidal nanocrystals (NCs) capped with hydrocarbon surface ligands has been shown to produce more than 20 distinct phases of binary nanocrystal superlattices (BNSLs). Such structural diversity, in striking contrast to binary systems of micron-sized colloidal beads, cannot be rationalized by models assuming entropy-driven crystallization of simple spheres. In this work, we show that the PbS ligand binding equilibrium controls the relative stability of two closely related BNSL structures featuring alternating layers of PbS and Au NCs. At an intermediate size ratio, as-prepared PbS NCs assemble with Au NCs into CuAu BNSLs featuring orientational coherence of PbS NCs across the lattice. Measurement of interparticle separations within CuAu and modeling of the structure reveal that PbS inorganic cores are nearly in contact through (100) NC surfaces in the square tiling of the CuAu basal plane. On the other hand, AlB₂ BNSLs with PbS NCs packed in random orientations were found to be the dominant self-assembly product when the same binary NC solution was evaporated in the presence of added oleic acid (OAH). Solution nuclear magnetic resonance titration experiments confirmed that added OAH binds to PbS NCs, implicating ligand surface coverage as an important factor influencing the relative stability of CuAu and AlB₂ BNSLs at the experimental size ratio. From these results, we conclude that as-prepared PbS NCs feature sparsely covered (100) surfaces and thus effectively flat patches along NC x-, y-, and z-directions. Such anisotropic PbS-PbS interactions can be efficiently screened by restoring effectively spherical NC shape via addition of OAH to the binary assembly solution. Our findings underscore the important contribution of NC surfaces to superlattice phase stability and offer a strategy for targeted BNSL assembly.

Self-Assembly of Colloidal Nanocrystals: From Intricate Structures to Functional Materials

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.

Multi-component superstructures self-assembled from nanocrystal building blocks

More than three decades of intensive study to make high-quality nanocrystals have created a unique toolbox for building multi-component superstructures, which have been recognized as a new generation of metamaterials important to both fundamental sciences and applied technologies. This minireview summarizes recent advances in this exciting field. We will focus our discussion on the synthetic strategies and superstructures of this multi-component metamaterial, and highlight their novel properties and potential applications. Additionally, some perspectives on possible developments in this field are offered at the end of this review. We hope that this minireview will both inform and stimulate research interests for the design and fabrication of these nanocrystal-based multi-component metamaterials for diverse applications in the future.

A feasibility study of unconventional planar ligand spacers in chalcogenide nanocrystals

The solar cell efficiency of chalcogenide nanocrystals (quantum dots) has been limited in the past by the insulation between neighboring quantum dots caused by intervening, often long-chain, aliphatic ligands. We have conducted a computationally based feasibility study to investigate the use of ultra-thin, planar, charge-conducting ligands as an alternative to traditional long passive ligands. Not only might these radically unconventional ligands decrease the mean distance between adjacent quantum dots, but, since they are charge-conducting, they have the potential to actively enhance charge migration. Our ab initio studies compare the binding energies, electronic energy gaps, and absorption characteristics for both conventional and unconventional ligands, such as phthalocyanines, porphyrins and coronene. This comparison identified these unconventional ligands with the exception of titanyl phthalocyanine, that bind to themselves more strongly than to the surface of the quantum dot, which is likely to be less desirable for enhancing charge transport. The distribution of finite energy levels of the bound system is sensitive to the ligand's binding site and the levels correspond to delocalized states. We also observed a trap state localized on a single Pb atom when a sulfur-containing phenyldithiocarbamate (PTC) ligand is attached to a slightly off-stoichiometric dot in a manner that the sulfur of the ligand completes stoichiometry of the bound system. Hence, this is indicative of the source of trap state when thio-based ligands are bound to chalcogenide nanocrystals. We also predict that titanyl phthalocyanine in a mix with chalcogenide dots of diameter ∼1.5 Å can form a donor-acceptor system.

Oleic Acid-Induced Atomic Alignment of ZnS Polyhedral Nanocrystals

Ordered two-dimensional (2D) superstructures of colloidal nanocrystals (NCs) can be tailored by the size, shape, composition, and surface chemistry of the NC building blocks, which can give directionality to the resulting superstructure geometry. The exact formation mechanism of 2D NC superstructures is however not yet fully understood. Here, we show that oleic acid (OA) ligands induce atomic alignment of wurtzite ZnS bifrustum-shaped NCs. We find that in the presence of OA ligands the {002} facets of the ZnS bifrustums preferentially adhere to the liquid-air interface. Furthermore, OA ligands induce inter-NC interactions that also orient the NCs in the plane of the liquid-air interface, resulting in atomically aligned 2D superstructures. We follow the self-assembly process in real-time with in situ grazing incidence small-angle X-ray scattering and find that the NCs form a hexagonal superstructure at early stages after which they come closer over time, resulting in a close-packed NC superstructure. Our results demonstrate the profound influence that surface ligands have on the directionality of 2D NC superstructures and highlight the importance of detailed in situ studies in order to understand the self-assembly of NCs into 2D superstructures.