Self-assembled supracrystals and hetero-structures made from colloidal nanocrystals

Wet-chemistry approaches have especially been acknowledged for their capability to deliver a variety of nanocrystals systematically tailored with sub-nanometer level accuracy over a broad range of dimensional-morphological regimes. This is achieved by careful regulation of thermodynamic parameters and growth kinetics in liquid media with the assistance of selected solvents, ligands, surfactants or catalyst additives.^1,2

While this synthetic expertise is still susceptible to being refined, new challenges are presently being imposed on colloidal nanochemistry research to meet the rising demand for advanced classes of “smart” nanostructured materials that should not only exhibit novel and/or collective properties, but also demonstrate diversified capabilities to be simultaneously exploitable in multiple applications. In this regard, tremendous leaps forwards have been recently made with the solution-phase synthesis of multi-component heterostructured nanocrystals with a topologically defined distribution of their composition,^3–7 and with the assembly of preformed colloidal nanocrystals into one-, two- and three-dimensional superstructures with tunable composition and degree of ordering.^8–14

Understanding the formation of complex all-nanocrystal-made structures from their building blocks is essential for the design of hierarchically structured materials.^15,16 Much work has been devoted to the fabrication of colloidal nanoparticles with desired properties; however, the creation of large supracrystals or heterostructured nanocrystals is still not fully understood, nor controlled at the molecular level. An analogy between atoms in either nanocrystals or bulk materials, and nanocrystals in supracrystals is very often observed.¹⁷ We have to note that several million years ago, highly uniform magnetite particles of sub-micrometer size were assembled in 3D arrays in the early solar system, and silicates, in the same range of size, composed either of one or two different sizes were produced.^18,19

This CrystEngComm themed collection focuses on novel approaches and strategies for the assembly and hetero-structuring of colloidal nanocrystals, with a special emphasis on the underlying formation mechanisms. It offers wonderful examples of current studies in nanoparticle assembly and multimaterial nanocrystal formation. Although the two classes of nanomaterials seem different from a first look, this issue tries to bring them together and to demonstrate that their fabrication strategies and chemical–physical properties share some similarities.

The issue starts with a Highlight by A. Eychmüller et al. (DOI: 10.1039/C4CE00882K) describing the latest advances in mesocrystal formation and non-classical crystallization of pre-synthesized nanoparticles. It aims at providing a complete description of a number of complex mesocrystalline systems through examination of the crystallization mechanisms leading to their construction, of their properties and applications in optoelectronic, photonics, sensing, energy conversion and storage, and heterogeneous (photo)catalysis. In his Highlight, B. A. Grzybowski (DOI: 10.1039/C4CE00689E) discusses the crystallization of metal nanoparticles functionalized with self-assembled monolayers of ligands terminated by charged groups. These “nanoions” may be purposely exploited as universal organic–inorganic “surfactants”, capable of self-assembling into variable high-order structures ranging from two-dimensional coatings to three-dimensional supracrystals with superior potential as chemical amplifiers.

Several original research articles deserve to be highlighted. Ryan et al. (DOI: 10.1039/C4CE00679H) describe the assembly of various semiconductor metal–chalcogenide nanorods by different solution-based techniques and electrophoretic deposition, allowing the fabrication of one-, two- and there-dimensional supracrystals with local to device-scale ordering. A. R. Tao et al. (DOI: 10.1039/C4CE00681J) demonstrate a polymer-directed route to binary assemblies entailing noble-metal nanoparticles of different size, shape and composition. Y. Wang et al. (DOI: 10.1039/C4CE00646A) exploit ligand exchange to realize reversible self-assembly of colloidal plasmonic Cu_1.94S nanoparticles. J. Zhang et al. (DOI: 10.1039/C4CE00670D) show the fabrication of supracrystals made of plasmonic Cu₇Te₅ nanocuboids. Y. Hou et al. (DOI: 10.1039/C4CE00657G) report on the layer-by-layer assembly of magnetic FePt nanocrystals into films with significant perpendicular magnetic anisotropy. M. Striccoli et al. (DOI: 10.1039/C4CE01291G) study the formation of superlattices made of organic-capped Au and PbS nanoparticles by a combination of in situ and ex situ X-ray techniques.

Hetero-structured nanoparticles are also well represented in this issue. Two highlights from J. Kolny-Olesiak (DOI: 10.1039/C4CE00674G) and A. Nag et al. (DOI: 10.1039/C4CE00462K) discuss mechanistic aspects relevant to understanding hetero-structure formation in colloidal media. The first focuses on the pathways underlying the growth of hybrid nanocrystals and shape-controlled semiconductor nanocrystals starting from copper sulphide seeds. The second summarizes different synthetic strategies for the preparation of colloidal hetero-structured nanocrystals, namely, the seeded-growth, nanocrystal-fusion, ion-exchange and inorganic-ligand-mediated approaches.

Original research articles cover a broad variety of issues concerned with the synthesis, in-depth characterization and elucidation of the relevant formation mechanism on nanocrystals characterized by a complex spatial distribution of their composition and/or crystal structure. For example, Mokari et al. (DOI: 10.1039/C4CE00663A) identified conditions for the controllable growth PtCu alloy nanoparticles, while T. Nann et al. (DOI: 10.1039/C4CE00545G) studied the process of partial cation exchange in CuInS₂ nanocrystals. The generation of complex hetero-structured nanocrystals based on different material combinations has been reported by several groups. For example, H. Wang et al. (DOI: 10.1039/C4CE00601A) discuss a general approach to the tailored overgrowth of Pd shell on Au nanorods. Q. Wang et al. (DOI: 10.1039/C4CE00694A) developed a seedless, one-pot approach to Ag₂S–CdS heteronanostructures based on the co-pyrolysis of single-source precursors. M. Grzelczak et al. (DOI: 10.1039/C4CE00724G) report on a general route to polymer coating that affords polymer particles embedding one or multiple shape-controlled Au nanocrystals. U. Banin et al. (DOI: 10.1039/C4CE00822G) investigate the evolution of RuRh–Cu₂S and Rh–Cu₂S nanocages upon delicate transformation of solid Cu₂S faceted nanocrystals via heterogeneous nucleation and redox reactions.

Finally, the work by Pyun et al. (DOI: 10.1039/C4CE00680A) combines the fabrication of hetero-structures made of Co-tipped CdSe@CdS nanorods and their assembly into one-dimensional “colloidal inorganic polymers.”²⁰ Particular emphasis is put on the critical step of selective activation of the Pt tips in Pt-terminated CdSe@CdS nanorods, through which the growth of additional Co domains can be directed onto only one terminus per nanorod. This result paves the way to dipolar assemblies of matchstick-like heterostructures driven by the Co tips, whereby short CdSe@CdS nanorod side chains are arranged in an exclusive perpendicular orientation relative to a central long chain of Co nanoparticle tips.

Although this collection of papers focuses on the mechanisms leading to the formation of hetero-structures and the assembly of colloidal nanoparticles, the complex breeds of nanostructured materials introduced in these pages could be expected to show novel properties and functionalities that may find applications in a variety of fields, such as optoelectronics, sensing, catalysis, and energy storage and conversion.^6,21–23 However, it appears clear that only with a deeper understanding of the chemical–physical and structural features of complex nanocrystals and assemblies thereof, which may result from broadly dissimilar nanoscale growth mechanisms, nanomaterial–property relationships will be unambiguously addressable. We hope that the present collection will stimulate further interest and efforts towards the achievement of such an ambitious goal in nanomaterials research.

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