Nano Focus: Block copolymers enable nano-scale patterning of metal oxides
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ano Focus Block copolymers enable nanoscale patterning of metal oxides
T
he size scale for semiconductor components such as metal oxide is soon expected to dip below the 10-nmsize range. Current top-down fabrication
resulting spatial shift of the ions leads to a net polarization. Any optical domain wall that was not also a polar domain wall was found to disturb the ideal stacking for tellurium octahedra. Most inorganic materials that have large optical activity have so far only shown chirality without polarity. NTO is the first known material that shows both. “Our findings unveil the rich coupling nature of chiral and polar order parameters and provide new insights into understanding and engineering domains in functional chiral and polar materials,” the group states in their article. This understanding of the fundamental properties
of polar domain boundaries is key to the development of new economically important materials according to Chris Stock of The University of Edinburgh, who adds, “This has been evidenced by fundamental developments in disordered ferroelectrics (such as the lead-based relaxors) resulting in recent applications of memory storage such as FRAM [ferroelectric random-access memory]. The observation of these highly structured domains is really a breakthrough in materials physics and will lead to new studies on similar materials and eventually new applications.” Vineet Venugopal
techniques such as lithography are being stretched to their limits. A bottom-up approach, such as self-assembly of a block copolymer (BCP) avoids some of the challenges inherent to lithography; however, current BCP materials and processes cannot produce ultrasmall features. Now, a joint research team including Morgan
Schulze, a graduate student working with Marc Hillmyer at the University of Minnesota in collaboration with Christophe Sinturel from the Université d’Orléans, France, has surmounted these challenges, producing ordered arrays of metal oxide particles with diameters as small as 6 nm—all without the use of traditional lithography techniques. The key advance, reported in ACS Macro Letters (DOI: 10.1021/ acsmacrolett.5b00458), was the novel block polymer design and synthesis (see Schematic). “Our choice of blocks allowed us to not only achieve small sizes, but also allowed us to selectively incorporate metal oxide precursors into hydrophilic domains that enabled a simple pattern transfer process,” Hillmyer says. No top-down patterning is needed because the block copolymers self-assemble into the desired array of domains due to the incompatibility of the two polymer blocks. The process is analogous to the formation of an oil–water emulsion, except that because the polymer blocks are covalently joined, the domain size is very small (nanoscale). The problem, according to Hillmyer, is that “smaller feature sizes require smaller molecules, but the smaller the block polymer, the more incompatible the individual segments need to be.” Motivated by this challenge, the team developed a route for the synthesis of highly i
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