No abstract

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.

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.

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.

Interpolymer self-assembly of bottom-up graphene nanoribbons (GNRs) has been realized by using fluorinated anthracene trimer precursors (HFH-DBTA) deposited onto heated Au(111) substrate. Whereas polymers derived from conventional precursor [10,10'-dibromo-9,9'-bianthryl (DBBA)] are adsorbed on Au(111) without apparent close packing, poly-HFH polymers derived from HFH-DBTA are densely self-assembled and require a long annealing time for cyclo-dehydrogenation because of the steric hindrance. First-principles calculations based on density functional theory revealed that the partially fluorinated edges of HFH-DBTA make molecular-substrate interaction weaker than that of DBBA, accelerate desorption, and leave islands of accumulated and locally aligned polymers. The partially fluorinated precursors also induce templating effects in interpolymer stacking because of H-F hydrogen bonding and F-F repulsion. The statistical analysis revealed that 84% of GNRs is parallel to the adjacent GNRs in the case of HFH-DBTA precursors. Field-effect transistors (FETs) were fabricated using such locally aligned multiple GNRs as channels. It has been found that on average, the on-current of the FETs is three times larger than that of FETs using less-aligned GNR channels made from the conventional DBBA precursors.

We study the electronic transport properties of graphene nanoribbons (GNRs) on SiO2/Si with O-terminated (siloxane) or OH-terminated (silanol) surfaces. The channel length and SiO2 thickness are 9.91 and 0.45 nm, respectively. The GNR shows p-type conduction on both the SiO2/Si surfaces. More holes are injected from OH groups in the silanol SiO2 to GNRs. The on/off current ratio for both GNRs on SiO2/Si is 103–105, which is consistent with recent experiments, and smaller by a factor of 108 than those of freestanding GNRs. The ratio is even smaller on the p side for the silanol SiO2.

We propose and analyze a heterojunction backward diode for millimeter- or terahertz-wave detection using edge-modified graphene nanoribbons (GNRs). According to the electron-affinity difference between a hydrogen-terminated GNR and a fluorine-terminated GNR, it is possible to construct a staggered-type lateral heterojunction diode. First-principles calculations reveal that because of band-to-band tunneling, the diode has a nonlinear current of the order of kA/m. The small junction area contributes to the reduction of the intrinsic junction capacitance. Equivalent-circuit analyses show that when the total capacitance is reduced below 100 aF, the diode exhibits a voltage sensitivity of 3.79 × 103 V/W at 300 GHz.