and dielectric nanospheres and quantum dots, and fabricated multilayer combinations. Additionally, the chemical synthesis of starting materials such as copolymers and liquid crystals is included. Growth as used here is specifically limited to the epitaxial growth of semiconductor materials; it also includes self-assembled arrays via nanoparticle formation, such as Stranski-Krastanov formation of quantum dots by the interplay between strain and surface tension. Fabrication refers to the creation of ensembles of nanostructures—for example, photonic crystals composed of nanoparticles.

Traditional top-down processing derived from the integrated circuit industry is reaching to scales of direct relevance to nanophotonics and beyond and certainly will be an important component of any nanophotonics fabrication suite. This is so in part as a result of the strong ability of traditional top-down processing to engineer hierarchical structures incorporating multiple, disparate length scales and the proven mass production capabilities of batch wafer processing. Self-assembly is bottom-up processing; a simple example is the assembly of colloidal nanoparticles into photonic crystal arrangements. A related approach is the use of multiphase systems, such as block copolymers1 along with surfactant-covered nanoparticles, to spontaneously form complex patterns driven by an external forcing function such as evaporation.

Increasingly, techniques are being developed that combine top-down and bottom-up approaches and blur the distinctions of the categories described above. One example is nanoscale crystal growth, in which a pattern is defined by fabrication and a subsequent growth process results in an array of nanoscale semiconductor structures. The fabrication can occur either by self-assembly (as, for example, porous anodization of a continuous metal film) or by traditional lithographic pattern definition and etching. As opposed to the subtractive etching of a large-area semiconductor film (the growth described above), this sequence allows much more flexibility and often produces materials with fewer defects and improved functionality. Generically, this combination is referred to as directed self-assembly.

Figure 3-1 shows a lithographic template that serves to guide the self-organizing crystals, block polymers, or colloids to pack and orient into a predetermined pattern, thus bringing the ultrasmall length scales achievable by molecular self-organization into play with the somewhat larger engineered structures to create hierarchical devices.

Synthesis

Nanoparticles

The need for improved optical materials has driven researchers in chemistry, materials science, and chemical engineering to create new synthetic pathways to afford better control over the composition, size, and shape of nanoparticles in order to produce new types of supramolecular building blocks. As mentioned in Chapter 2, metallic nanoparticles, such as gold nanostars, can be useful for localized surface plasmon resonance. Thus, it is important to control not only particle size but particle shape. Moreover, it is absolutely critical to have monodispersity in size and shape.

Researchers have found that absorbed surfactants can influence the relative growth rate of various facets (surfactant binding and size-dependent facet surface energies) resulting in the growth of complex-shaped nanoparticles. For example, Alivisatos and colleagues found that they could create tetrapod gold (Au) or cadium selenium (CdSe) nanoparticles. The Au particles are composed of a set of (111) twin

1

Block copolymers are made up of two or more homopolymer subunits linked by covalent bonds. Block copolymers with two (or three) distinct blocks are called diblock (or triblock) copolymers.



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