Two-dimensional (2D) layered materials, transition metal dichalcogenides and black phosphorus, have attracted much interest from the viewpoints of fundamental physics and device applications. The establishment of new functionalities in anisotropic layered 2D materials is a challenging but rewarding frontier, owing to their remarkable optical properties and prospects for new devices. Here, we report the anisotropic optical properties of layered 2D monochalcogenide of germanium sulfide (GeS). Three Raman scattering peaks corresponding to the B3g, A 1 g , and A 2 g modes with strong polarization dependence are demonstrated in the GeS flakes, which validates polarized Raman spectroscopy as an effective method for identifying the crystal orientation of anisotropic layered GeS. Photoluminescence (PL) is observed with a peak at around 1.66 eV that originates from the direct optical transition in GeS at room temperature. Moreover, determination of the polarization dependent characteristics of the PL and absorption reveals an anisotropic optical transition near the band edge of GeS, which is also supported by the density functional theory calculations. This anisotropic layered GeS presents the opportunities for the discovery of new physical phenomena and will find applications that exploit its anisotropic properties.

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Edge-fluorinated graphene nanoribbons are predicted to exhibit attractive structural and electronic properties, which, however, still need to be demonstrated experimentally. Hence, to provide further experimental insights, an anthracene trimer comprising a partially fluorinated central unit is explored as a precursor molecule, with scanning tunneling microscopy and X-ray photoelectron spectroscopy analyses, indicating the formation of partially edgefluorinated polyanthrylenes via on-surface reactions after annealing at 350 °C on Au(111) under ultrahigh-vacuum conditions. Further annealing at 400 °C leads to the cyclodehydrogenation of partially edge-fluorinated polyanthrylenes to form graphene nanoribbons, resulting in carbon−fluorine bond cleavage despite its high dissociation energy. Extensive theoretical calculations reveal a defluorination-based reaction mechanism, showing that a critical intermediate structure, obtained as a result of H atom migration to the terminal carbon of a fluorinated anthracene unit in polyanthrylene, plays a crucial role in significantly lowering the activation energy of carbon−fluorine bond dissociation. These results suggest the importance of transient structures in intermediate states for synthesizing edge-fluorinated graphene nanoribbons.

Bottom-up synthesis of graphene nanoribbons (GNRs) may open new possibilities in future electronic devices owing to their tunable electronic structure, which depends strongly on their well-defined width and edge geometry. For instance, armchair-edged GNRs (AGNRs) exhibit width-dependent bandgaps. However, the bandgaps of AGNRs synthesized experimentally so far are relatively large, well above 1 eV. Such a large bandgap may deteriorate device performance due to large Schottky barriers and carrier effective masses. Here, we describe the bottom-up synthesis of AGNRs with smaller bandgaps, using dibromobenzenebased precursors. Two types of AGNRs with different widths, namely 17 and 13 carbon atoms, were synthesized on Au(111), and their atomic and electronic structures were investigated by scanning probe microscopy and spectroscopy. We reveal that 17-AGNRs have the smallest bandgap, as well as the smallest electron/hole effective mass, among bottom-up AGNRs reported so far. The successful synthesis of 17-AGNRs is a significant step toward the development of GNR-based electronic devices.

In this study, high-purity single-chirality (5,4) and (6,4) single-wall carbon nanotubes (SWCNTs) were efficiently extracted using gel chromatography, and their optical properties were investigated. In the photoluminescence spectrum, the (5,4) SWCNTs exhibited multiple emission peaks, such as E11 * at 968 and E11 *− at 1096 nm, in addition to the original E11 peak at 829 nm for the E22 excitation. Although the (6,4) SWCNTs also exhibited similar emission peaks, the intensity of the additional peaks relative to the E11 peak was approximately four times lower than that of (5,4) SWCNTs, even though the same separation procedure was used for both species. The results from density functional theory calculations indicated that both the E11 * and E11 *− emissions originated as emissions from the oxide states. Because no oxidation treatments were performed on the samples, these oxides were most likely spontaneously produced during the sample preparation process. Theoretical results indicated that the (5,4) SWCNT possesses a more stable ether-a-O structure that could cause the E11 * emission. The difference in the stability of the oxide states for (5,4) SWCNT and (6,4) SWCNT strongly suggests that the (5,4) SWCNT is more likely to be oxidized by weak stimulation. These results indicate that the (5,4) SWCNT remained near the outside of the robust structure due to its strong curvature effects.

We have used first-principles methods to study the geometries and electronic structures of hydrogen (H), fluorine (F), chlorine (Cl), and hydroxyl (OH) terminated armchair graphene nanoribbons (H-AGNRs, F-AGNRs, Cl-AGNRs, and OH-AGNRs) with ribbon widths N = 7 and 19. The most stable geometries of H-AGNRs have planar configurations, but those of F-, Cl-, and OH-AGNRs have rippled edges. The ripples stem from steric hindrances between neighboring pairs of terminal atoms or groups, and the ripples are strongly localized to the edges. The most stable termination occurs with F atoms owing to strong C-F bonds despite their rippled edge structures. The energy band gaps of F- and Cl-AGNRs are narrower than those of H-AGNRs. This is due to structural deformations rather than chemical effects. For OH-AGNRs, chemical interactions between neighboring OH groups further reduce the band gaps.

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A two-dimensional (2D) layered SnS 2 film synthesized by the thermal-chemical vapor deposition method is utilized for detecting formaldehyde (HCHO), which causes a sick building syndrome. A back-gated field-effect transistor (FET)-based SnS 2 sensor successfully detects HCHO with concentrations down to 1 ppb in a nitrogen atmosphere. Sensing measurements performed under dry air conditions also exhibit a clear response to 20 ppb of HCHO, which is more sensitive than the previously reported sensors based on other 2D-layered materials. Moreover, it is found that the sensor possesses a high selectivity for HCHO over other organic species. Theoretical calculations suggest that native sulfur vacancies existing in n-type SnS 2 crystals play an important role in HCHO detection. Actually, oxygen atoms that are unexpectedly detached from HCHO molecules are found to fill the vacancies, giving rise to p-type doping in SnS 2 . As a result, decrease in the drain current of SnS 2 -FET can be found as a signal of HCHO detection. Furthermore, considering the future mass-production of sensors, we demonstrate large-scale growth of the SnS 2 film by means of magnetron-sputtering deposition and subsequent annealing in a diluted hydrogen sulfide atmosphere. The sputtered film is also found to exhibit a good sensing ability to HCHO.