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Bandgap engineering of atomically thin 2D crystals is critical for their applications in nanoelectronics, optoelectronics, and photonics. Here, we report a simple but rather unexpected approach for bandgap engineering of muscovite-type mica nanosheets (KAl 3 Si 3 O 10 (OH) 2 ) via controlled molecular thickness. Through density functional calculations, we analyze electronic structures in 2D mica nanosheets and develop a general picture for tunable bandgap narrowing induced by controlled molecular thickness. From conducting atomic force microscopy, we observe an abnormal bandgap narrowing in 2D mica nanosheets, contrary to wellknown quantum size effects. In mica nanosheets, decreasing the number of layers results in reduced bandgap energy from 7 to 2.5 eV, and the bilayer case exhibits a semiconducting nature with ∼2.5 eV. Structural modeling by transmission electron microscopy and density functional calculations reveal that this bandgap narrowing can be defined as a consequence of lattice relaxations as well as surface doping effects. These bandgap engineered 2D mica nanosheets open up an exciting opportunity for new physical properties in 2D materials and may find diverse applications in 2D electronic/optoelectronic devices. ■ INTRODUCTIONTwo-dimensional (2D) nanosheets with atomic or molecular thickness are emerging as important new materials because of their particular properties and potential applications in nextgeneration electronic devices. 1−12 One attractive aspect of these exfoliated nanosheets is that various nanostructures can be fabricated using them as 2D building blocks. Sophisticated functionalities or nanodevices may be designed through combining different nanosheets with a precise control over their arrangement on a molecular scale. The discovery of graphene can be considered a defining point in the research and development of such 2D material systems. 1,2,12 This breakthrough has opened up the possibility of exploring the fascinating properties of 2D nanosheets of other inorganic layered materials; 2−11 the reduction to single or a few atomic layers will offer new properties and novel applications. 13 To expand the utility of these 2D nanosheets, the electronic properties must be tailored through bandgap engineering and/ or doping process. Bandgap engineering of 2D nanosheets is particularly important for their applications in nanoelectronics, optoelectronics, and photonics. One key issue in the developments of 2D nanosheets is to produce semiconductor nanosheets with a narrow bandgap or a semiconductor-tometal transition, since it allows the use of field effect transistors (FETs) as well as the effective operation for low-energy absorptions and excitation of semiconductor optoelectronics. A possible indication of the bandgap engineering came from MoS 2 nanosheets, which exhibited a crossover behavior from an indirect to a direct-gap semiconductor in the monolayer limit. 14 However, the bandgap narrowing of nanomaterials is almost always difficult to achieve, since most nanomaterials would show ...

Complex perovskite oxides offer tremendous potential for controlling their rich variety of electronic properties, including high-T superconductivity, high-κ ferroelectricity, and quantum magnetism. Atomic-scale control of these intriguing properties in ultrathin perovskites is an important challenge for exploring new physics and device functionality at atomic dimensions. Here, we demonstrate atomic-scale engineering of dielectric responses using two-dimensional (2D) homologous perovskite nanosheets (CaNaNbO; m = 3-6). In this homologous 2D material, the thickness of the perovskite layers can be incrementally controlled by changing m, and such atomic layer engineering enhances the high-κ dielectric response and local ferroelectric instability. The end member (m = 6) attains a high dielectric constant of ∼470, which is the highest among all known dielectrics in the ultrathin region (<10 nm). These results provide a new strategy for achieving high-κ ferroelectrics for use in ultrascaled high-density capacitors and post-graphene technology.