Mesoscale Self-assembly Research Articles

Self-assembly of doped semiconductor nanocrystals leading to the formation of highly luminescent nanorods

Abstract Meso-scale self-assembly of doped semiconductor nanocrystals leading to the formation of monocrystalline nanorods showing enhanced photo- and electro-luminescence properties are reported. Polycrystalline ZnS: Cu + –Al 3+ nanoparticles of zinc-blended (cubic) structure with an average size of ∼4 nm were aggregated in aqueous solution and grown into nanorods of length ∼400 nm and aspect ratio ∼12. Transmission electron microscope (TEM) images indicate crystal growth mechanisms involving particle-to-particle oriented-attachment assisted by sulphur–sulphur catenation leading to covalent-linkage. The nanorods exhibit self-assembly dependant luminescence properties such as quenching of the lattice defect-related emissions accompanied by enhancement of dopant-related emission, efficient low-voltage electroluminescence (EL) and super-linear voltage-brightness EL characteristics. This study demonstrates the technological importance of aggregation based self-assembly in doped semiconductor nanosystems.

Making Things by Self-Assembly

AbstractSelf-assembly—the spontaneous generation of order in systems of components—is ubiquitous in chemistry; in biology, it generates much of the functionality of the living cell. Self-assembly is relatively unused in microfabrication, although it offers opportunities to simplify processes, lower costs, develop new processes, use components too small to be manipulated robotically, integrate components made using incompatible technologies, and generate structures in three dimensions and on curved surfaces. The major limitations to the self-assembly of micrometer- to millimeter-sized components (mesoscale self-assembly) do not seem to be intrinsic, but rather operational: selfassembly can, in fact, be reliable and insensitive to small process variations, but fabricating the small, complex, functional components that future applications may require will necessitate the development of new methodologies. Proof-of-concept experiments in mesoscale self-assembly demonstrate that this technique poses fascinating scientific and technical challenges and offers the potential to provide access to hard-to-fabricate structures.

Enhanced electroluminescence properties of doped ZnS nanorods formed by the self-assembly of colloidal nanocrystals

Aggregation based meso-scale self-assembly of doped semiconductor nanocrystals leading to the formation of monocrystalline nanorods showing enhanced photo- and electro-luminescence properties is reported. ∼4 nm sized, polycrystalline ZnS nanoparticles of zinc-blende (cubic) structure, doped with Cu +–Al 3+ have been aggregated in the aqueous solution and grown into nanorods of length ∼400 nm and aspect ratio ∼12. Transmission electron microscopic (TEM) images indicate crystal growth mechanisms involving particle-to-particle oriented-attachment assisted by sulphur–sulphur catenation leading to covalent-linkage. The nanorods exhibit self-assembly dependant luminescence properties such as quenching of the lattice defect-related emissions accompanied by the enhancement in the dopant-related emission, efficient low-voltage electroluminescence (EL) and super-linear voltage–brightness EL characteristics. This study demonstrates the technological importance of aggregation based self-assembly in doped semiconductor nanosystems.

Higher-order organization by mesoscale self-assembly and transformation of hybrid nanostructures.

The organization of nanostructures across extended length scales is a key challenge in the design of integrated materials with advanced functions. Current approaches tend to be based on physical methods, such as patterning, rather than the spontaneous chemical assembly and transformation of building blocks across multiple length scales. It should be possible to develop a chemistry of organized matter based on emergent processes in which time- and scale-dependent coupling of interactive components generate higher-order architectures with embedded structure. Herein we highlight how the interplay between aggregation and crystallization can give rise to mesoscale self-assembly and cooperative transformation and reorganization of hybrid inorganic-organic building blocks to produce single-crystal mosaics, nanoparticle arrays, and emergent nanostructures with complex form and hierarchy. We propose that similar mesoscale processes are also relevant to models of matrix-mediated nucleation in biomineralization.

Change Your Partner, Do-Si-Do

Like square dancers following the caller's directions, the little plastic pieces (shown, insets) regroup themselves in response to changes in their environment. These two-way jigsaw puzzles are among the many aspects of mesoscale self-assembly being explored by the group of chemistry professor George M. Whitesides at Harvard University [ J. Am. Chem. Soc. , 124 , 14508 (2002)]. Each geometrical component—measuring just under 1 cm across—is made of the hydrophobic polymer poly(dimethylsiloxane) doped with alumina. Certain faces of each have been oxidized to make them hydrophilic. (Thick lines in the shapes shown in the insets represent hydrophobic faces; thin lines, hydrophilic.) Depending on their surroundings, capillary interactions between the hydrophobic faces can be quite strong, while those between the hydrophilic are negligible, or vice versa. Thus, the assemblies shown on the left form at the interface between water and the dense hydrophobic liquid perfluorodecalin. But when the density of the aque...

Dissections: self-assembled aggregates that spontaneously reconfigure their structures when their environment changes.

This Communication describes a new strategy for the design of adaptive structures based on reconfigurable mesoscale self-assembly. Several sets of millimeter-scale objects have been designed that can self-assemble into two different, regular aggregates at the interface between an aqueous solution and perfluorodecalin; the choice between the two aggregates is determined by the density of the aqueous phase.

Simulation of indentation fracture in crystalline materials using mesoscale self-assembly.

A new physical model based on mesoscale self-assembly is developed to simulate indentation fracture in crystalline materials. Millimeter-scale hexagonal objects exhibiting atom-like potential functions were designed and allowed to self-assemble into two-dimensional (2D) aggregates at the interface between water and perfluorodecalin. Indentation experiments were performed on these aggregates, and the stresses and strains involved in these processes were evaluated. The stress field in the aggregates was analyzed theoretically using the 2D elastic Hertz solution. Comparison of the experimental results with theoretical analysis revealed that fracture develops in regions subjected to high shear stress and some, albeit low, tensile stress. The potential for the broader application of the model is illustrated using indentation of assemblies with point defects and adatoms introduced at predetermined locations, and using a two-phase aggregate simulating a compliant film on a stiff substrate.

Mesoscale organization of metal nanocrystals

Abstract Nanocrystals of metals covered by alkanethiols organize themselves in two-dimensional arrays. We discuss such arrays of metal nanocrystals at length, with focus on the dependence of the structure and the stability of the arrays on the particle diameter and the distance between the particles. Three-dimensional superstructures of metal nanocrystals obtained by the use of alkanedithiols are examined. These ordered two- and three-dimensional structures of thiolized metal nanocrystals are good examples of mesoscale self-assembly. The association of metal nanocrystals to give rise to giant clusters with magic nuclearity provides an even more graphic demonstration of mesoscale self-assembly.

Self-assembly of 10-microm-sized objects into ordered three-dimensional arrays.

This paper describes the self-assembly of small objects--polyhedral metal plates with largest dimensions of 10 to 30 microm--into highly ordered, three-dimensional arrays. The plates were fabricated using photolithography and electrodeposition techniques, and the faces of the plates were functionalized to be hydrophobic or hydrophilic using self-assembled monolayers (SAMs). Self-assembly occurs in water through capillary interactions between thin films of a hydrophobic liquid (a liquid prepolymer adhesive) coated onto the hydrophobic faces of the plates; coalescence of the adhesive films reduces the interfacial free energy of the system and drives self-assembly. By altering the size and surface-patterning of the plates, the external morphologies of the aggregates were varied. Curing the adhesive furnished mechanically stable aggregates that were characterized by scanning electron microscopy (SEM). For assemblies formed by plates partially composed of a sacrificial material, a subsequent etching step furnished fully open, three-dimensional microstructures. This work validates the use of capillary interactions for three-dimensional mesoscale self-assembly in the 10-microm-size regime and opens new avenues for the fabrication of complex, three-dimensional microscructures.

Magic Nuclearity Giant Clusters of Metal Nanocrystals Formed by Mesoscale Self-Assembly

Magic nuclearity giant clusters formed by the mesoscale self-assembly of Pd nanocrystals of 2.5 nm diameter (nuclearity, ∼561) have been identified by transmission electron microscopy. The clusters...

Self-Assembly of Microscale Objects at a Liquid/Liquid Interface through Lateral Capillary Forces

Small (100−600 μm in width) hexagonal polymeric plates with faces patterned into hydrophobic and hydrophilic regions, and interacting through lateral capillary forces, were allowed to self-assemble at the perfluorodecalin/water interface. These plates were fabricated from photoresist and patterned by shadow evaporation of gold onto selected faces. The arrays that assembled from the 100 μm objects were similar in structure to those that assembled from millimeter-sized objects with analogous patterns of hydrophobic and hydrophilic faces, but with three important differences. (i) The contribution of buoyancy forces in establishing the level at which the 100 μm objects floated relative to the interface was small compared to the contribution of the vertical capillary forces. (ii) As a result, the designs of hydrophobic edges necessary to generate menisci useful in self-assembly were different for 100 μm objects than for millimeter-sized objects. (iii) The arrays that formed from the 100 μm objects had higher densities of defects than the arrays that formed from the millimeter-sized objects; these defects reflected the increase in the strength of the capillary forces (which favored assembly) relative to the shear forces (which disrupted assembly). This work adds two new elements to the study of mesoscale self-assembly: (i) It describes a new method of fabrication of plates with faces patterned into regions of different hydrophobicity that is applicable to small (perhaps <10 μm) objects, and (ii) it describes the self-assembly of 100 μm plates made by this method into ordered arrays. The work also established the contours of the menisci on the separate 100 μm and millimeter-sized plates. The scaling of the lateral and vertical capillary forces and buoyancy forces acting on millimeter-sized objects, relative to those acting on 100 μm objects, is described.

Selectivities among capillary bonds in mesoscale self-assembly

This letter describes the capillarity-driven self-assembly of mm-sized plates of poly(dimethylsiloxane) at the perfluorodecalin/H2O interface into complex arrays. The shape of the interacting menisci could be tailored by three strategies: (1) changing the shape of uniformly hydrophobic edges; (2) patterning the distribution of vertical hydrophobic strips on the edges and, in some cases, varying the density of the blocks; and (3) patterning the hydrophobic areas on a face to be chiral.

Mesoscale Self-Assembly of Hexagonal Plates Using Lateral Capillary Forces: Synthesis Using the “Capillary Bond”

This paper examines self-assembly in a quasi-two-dimensional, mesoscale system. The system studied here involves hexagonal plates (“hexagons”) of poly(dimethylsiloxane) (PDMS; 5.4 mm in diameter, 0.9−2.0 mm thick), with faces functionalized to be hydrophilic or hydrophobic, floating at the interface between perfluorodecalin (PFD) and H2O. The hexagons assemble by capillary forces originating in the interactions of the menisci at their hydrophobic and hydrophilic rectangular faces. The strength and directionality of the interactions can be tailored by manipulating the heights of the faces, the pattern of the hydrophobic faces, the pattern of hydrophobic regions on these faces, and the densities of the three interacting phases (organic liquid, aqueous liquid, polymeric solid). Examination of all 14 possible combinations of hydrophobic and hydrophilic faces on the hexagonal plates led to three outcomes: (i) the extension of the strategies of self-assembly from the molecular to the mesoscale, (ii) the demonstration of a system in which small objects can be designed to self-assemble into a variety of arrays, and (iii) the hypothesis that capillary forces between objects can, in some circumstances, be considered to form the basis for a “bond” between themthe capillary bondand be used in synthesis in a way analogous to that in which noncovalent bonds are employed in molecular-scale synthesis.

Three-Dimensional Mesoscale Self-Assembly

Three-Dimensional Mesoscale Self-Assembly