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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 ...

A simple and efficient method to enhance the quaternary nitrogen doping (N-doping) of graphene has been demonstrated. Recent studies have shown that quaternary N in the graphene network provides more efficient electrocatalytic activity. Therefore, a novel strategy to enhance the quaternary N-doping is currently in high demand. The strategy employed in this work was to modify graphene oxide (GO) prior to thermal annealing so as to provide a more efficient structure for quaternary N doping. GO was first chemically reduced with hydrazine to substantially increase the formation of C]C bonds and simultaneously decrease the atomic oxygen concentration. The reduced graphene oxide (RGO) was then annealed in the presence of NH 3 . Although N-doping via the replacement of oxygen is preferred, the probability of carbon being substituted with N dopants in the graphitic structure of RGO could increase due to the relatively higher content of C]C when compared to the atomic oxygen concentration. In addition, due to the decreased atomic oxygen concentration, the electro-conductivity was enhanced. Cyclic voltammograms (CVs) of 5 mM K 3 Fe(CN) 6 and 2 mM H 2 O 2 were used to examine the electrochemical response of the quaternary N-maximized RGO. An improvement in electrocatalytic reduction and a higher electro-conductivity were confirmed based on an analysis of the obtained CVs.

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Phone: þ82 2 2220 0501, Fax: þ82 2 2291 7395Liquid-phase pulsed laser ablation (LP-PLA) is a promising technique for the fabrication of various nanomaterials because this technique is very simple and it is easy to control the experimental parameters. This paper demonstrates the synthesis of phase-controlled iron oxide magnetic nanoparticles by laser ablation of a bulk a-Fe 2 O 3 target in the following liquid media: ethanol, D.I. water and acetone. Absorption spectra of the nanocolloidal solutions are measured by UV-Vis spectrophotometer. As-synthesized nanoparticles, extracted from the colloidal solutions, are characterized by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscope (TEM) equipped with X-ray energy dispersive spectrometry (EDX) and a vibrating sample magnetometer (VSM) to discover crystallinity, phase structure, morphology, elemen-tal compositions and magnetic properties in detail. The experimental results showed that the type of target and the magnitude of laser power play important roles in controlling the uniformity of iron oxide phase in the final product nanoparticles. Laser ablation of the iron oxide target in ethanol and acetone yields crystalline maghemite (g-Fe 2 O 3 ) nanoparticles, while that in D.I. water yields amorphous hematite (a-Fe 2 O 3 ). Use of an iron oxide (a-Fe 2 O 3 ) target for PLA in all three solvents is able to prevent the formation of metal iron and wustite phases in the final product nanoparticles. Moreover, our nanoparticles obtained in all three solvents possess magnetic behaviour, particularly that obtained in acetone, which has better saturation magnetization than those in ethanol and D.I. water.