Functional nanostructured materials based on self-assembly of block copolymers

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Introduction A block copolymer (BCP) is a polymer that is made up of blocks of chemically distinct monomers. For example, a polystyreneblock-polymethylmethacrylate (PS-b-PMMA) BCP consists of a PS chain covalently bonded to a PMMA chain. If the blocks are immiscible, the BCP can self-assemble into periodic nanostructures with dimensions ranging from a few nanometers to several 100 nanometers.1–5 The phase-separated structure consists of “microdomains” of each block whose geometry depends on the volume fraction of the block and whose periodicity depends on the length of the polymer chain.6 The periodicity of the structure can range from a few nanometers to hundreds of nanometers. A rich variety of nanostructures can be self-assembled based on the intrinsic properties of the blocks and the microdomain geometry, which is influenced by the polymer architecture, the annealing process (e.g., annealing by exposure to a solvent vapor environment or to an elevated temperature), and constraints such as those imposed by a patterned substrate.3,7 BCPs with two chemically distinct blocks (e.g., diblock copolymers [Figure 1a–c]) or three (e.g., triblock copolymers) or three (e.g., triblock terpolymers) chemically distinct blocks are most commonly used, but incorporating further blocks and various architectures such as linear, star, cyclic, or brush copolymers brings more diversity to both the properties

and the morphology of the resulting self-assembled structure (Figure 1d).8 BCPs can be used in a wide range of applications,9,10 constituting functional materials themselves by virtue of the properties of the blocks, or by selectively etching11 one block to form masks or templates for the synthesis of functional materials. BCPs can include organic coil-like blocks, liquid-crystalline blocks, Si- or metal-containing blocks, biological blocks, or conductive blocks, thereby providing a multitude of properties.12–15 The BCP nanostructure can be altered by several chemical methods such as sequential infiltration synthesis (SIS), which introduces metals or oxides from a precursor as in chemical vapor deposition,16 by salt complexation,17 or by nanoparticle incorporation.18 Many of these applications are based on thin films of BCPs, and one of the most widely studied applications is in nanoscale lithography, where one block is removed by etching, and the other is used as a mask to define features in a microelectronic device (Figure 1a). In this article, we will first discuss the applications of thin-film BCPs in lithography and other fields where a two-dimensional (2D) nanopatterned structure is required. We will then discuss three-dimensional (3D) BCP assemblies. We show that BCP nanostructures have applications in fabrication of integrated circuits,19,20 magnetic storage media,21–23 sensors,24,25 photonic crystals,26 photovoltaics,27 and separation membranes,28 as well as for making 3D nanostructures.4

W. Bai, Department of Materials Science and Engineering, Massachusetts Institute of Technology, USA; [email protected] C.A. Ross, Department of Ma

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