Discovery of robust yet reversibly switchable electric dipoles at reduced dimensions is critical in advancing nanoelectronics devices. Energy bands flat in momentum space generate robust localized states that are activated independently of each other. We determined flat bands exist and induce robust yet independently switchable dipoles exhibiting a unique ferroelectricity in HfO2. Flat polar phonon bands in HfO2 cause extreme localization of electric dipoles within its irreducible half-unit-cell-widths (~3 Å). Contrary to conventional ferroelectrics with spread dipoles, those intrinsically localized dipoles are stable against extrinsic effects such as domain walls, surface exposure, and even down-to-angstrom-scale miniaturization. Moreover, the sub-nm-scale dipoles are individually switchable without creating any domain-wall energy cost. This offers unexpected opportunities for ultimately-dense unit-cell-by-unit-cell ferroelectric switching devices directly integrable into silicon technology.

This certifies that the thesis of Chang Ki Moon is approved.

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Although intensive research for graphene oxide (GO) has been conducted, the broad size distribution of as‐synthesized GO flakes still remains a challenging issue since flake size has a direct impact on the material properties and related applications. Herein, we propose a facile and scalable approach for the size fractionation of GO flakes by manipulating the surface charge of GO with an organic solvent‐mediated aqueous solution. Both the experimental and simulation results confirm that the dispersion of GO flakes is notably influenced by the bond strength with solvent, which is attributed by the charge amount of oxygen in solvent molecule. From the understanding of the dispersion mechanism, an optimal solvent for the size fractionation of GO flakes was determined and GO flakes were successfully sorted into various size groups with a narrow distribution. The effect of size fractionation on the material property and related application was confirmed through various analyses. Our results could bring significant advances in producing two‐dimensional materials with reliable and reproducible material properties toward practical applications.

The recently discovered ferroelectricity in thin-film orthorhombic HfO2, which can be directly integrated into complementary metal–oxide semiconductor technology, has become an important research target. However, the use of orthorhombic HfO2 in practical devices has been limited by undesirable mixing with the monoclinic phase, which is nonpolar and thus degrades the ferroelectric properties. Here, we demonstrate that a Si dopant significantly stabilizes the ferroelectric phase because of its unique bonding characteristics, particularly its intrinsic tendency to form strong covalent bonds with O, thereby weakening the phase boundary to stabilize the ferroelectric orthorhombic phase over the nonpolar monoclinic phase, relatively. On the basis of our theoretical predictions, we conducted transmission electron microscopy measurements and confirmed that Si substitution doping indeed induced monoclinic structural components into the orthorhombic phase, which is a strong indication of the weakened phase boundary and subsequent facilitation of the ferroelectric transition. This work thus provides an atomic-scale picture for understanding the unique role of Si in promoting the ferroelectric phase and the dopant dependence on the wake-up effect in HfO2, offering a substantial advancement toward integrating ferroelectrics into practical devices.