On-surface synthesis of hydroxy-functionalized graphene nanoribbons through deprotection of methylenedioxy groups.

We demonstrate on-surface deprotection of methylenedioxy groups which yielded graphene nanoribbons (GNRs) with edges functionalized by hydroxy groups. While anthracene trimer precursors functionalized with hydroxy groups did not yield GNRs, it was found that hydroxy groups are first protected as methylenedioxy groups and then deprotected during the cyclo-dehydrogenation process to form GNRs with hydroxy groups. The X-ray photoemission spectroscopy and non-contact atomic force microscopy studies revealed that ∼20% of the methylenedioxy turned into hydroxy groups, while the others were hydrogen-terminated. The first-principles density functional theory (DFT) study on the cyclo-dehydrogenation process was performed to investigate the deprotection mechanism, which indicates that hydrogen atoms emerging during the cyclo-dehydrogenation process trigger the deprotection of methylenedioxy groups. The scanning tunneling spectroscopy study and DFT revealed a significant charge transfer from hydroxy to the Au substrate, causing an interface dipole and the HOMO being closer to the Fermi level when compared with hydrogen-terminated GNR/Au(111). This result demonstrates on-surface deprotection and indicates a possible new route to obtain GNRs with desired edge functionalization, which can be a critical component for high-performance devices.

Highly Sensitive NO2 Detection by TVS-Grown Multilayer MoS2 Films.

Two-dimensional layered materials have been investigated for sensor applications over the last decade due to their very high specific surface area and excellent electrical characteristics. Although grain boundaries are inevitably present in polycrystalline-layered materials used for real applications, few studies have investigated their effects on sensing properties. In this study, we demonstrate the growth of two distinct MoS2 films that differ in grain size by means of chemical vapor deposition (CVD) and thermal vapor sulfurization (TVS) methods. Transistor-based sensors are fabricated using these films, and their NO2 sensing properties are evaluated. The adsorption behavior of NO2 on MoS2 is considered in terms of the Langmuir isotherm, and the experimental results can be well fitted by the equation. The CVD-grown film exhibits electrical properties 1-2 orders of magnitude superior to those of the TVS-grown one, which is attributed to the large grain size of the CVD-grown film. In contrast, the sensitivity to NO2 is unexpectedly found to be higher in the TVS-grown film and is of the same order of a previously reported record value. Transmission electron microscopy observations suggest that the TVS-grown film consists of multiple rotationally oriented grains that are connected by mirror twin grain boundaries. Theoretical calculation results reveal that the adsorption of NO2 on the grain boundary that we modeled is equal to that on the ideal basal plane surface of MoS2. In addition, the porous structure in the TVS-grown film may also contribute to enhancing the sensor response to NO2. This study suggests that a highly sensitive MoS2 sensor can also be fabricated by using a polycrystalline film with small grain size, which can possibly be applied to other two-dimensional materials.

Optimization of the composition in a composite material for microelectronics application using the Ising model.

Tailored material is necessary in many industrial applications since material properties directly determine the characteristics of components. However, the conventional trial and error approach is costly and time-consuming. Therefore, materials informatics is expected to overcome these drawbacks. Here, we show a new materials informatics approach applying the Ising model for solving discrete combinatorial optimization problems. In this study, the composition of the composite, aimed at developing a heat sink with three necessary properties: high thermal dissipation, attachability to Si, and a low weight, is optimized. We formulate an energy function equation concerning three objective terms with regard to the thermal conductivity, thermal expansion and specific gravity, with the composition variable and two constrained terms with a quadratic unconstrained binary optimization style equivalent to the Ising model and calculated by a simulated annealing algorithm. The composite properties of the composition selected from ten constituents are verified by the empirical mixture rule of the composite. As a result, an optimized composition with high thermal conductivity, thermal expansion close to that of Si, and a low specific gravity is acquired.

Vacancy-Assisted Selective Detection of Low-ppb Formaldehyde in Two-Dimensional Layered SnS2.

A two-dimensional (2D) layered SnS2 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 SnS2 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 SnS2 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 SnS2. As a result, decrease in the drain current of SnS2-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 SnS2 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.

Graph Classification of Molecules Using Force Field Atom and Bond Types.

Classification of the biological activities of chemical substances is important for developing new medicines efficiently. Various machine learning methods are often employed to screen large libraries of compounds and predict the activities of new substances by training the molecular structure-activity relationships. One such method is graph classification, in which a molecular structure can be represented in terms of a labeled graph with nodes that correspond to atoms and edges that correspond to the bonds between these atoms. In a conventional graph definition, atomic symbols and bond orders are employed as node and edge labels, respectively. In this study, we developed new graph definitions using the assignment of atom and bond types in the force fields of molecular dynamics methods as node and edge labels, respectively. We found that these graph definitions improved the accuracies of activity classifications for chemical substances using graph kernels with support vector machines and deep neural networks. The higher accuracies obtained using our proposed definitions can enhance the development of the materials informatics using graph-based machine learning methods.

Effect of Edge Functionalization on the Bottom-Up Synthesis of Nano-Graphenes.

We demonstrate the effect of edge functionalization on the on-surface Ullmann coupling of nano-carbon materials. Unlike 10,10'-Dibromo-9,9'-bianthryl (DBBA), which is widely known to form anthracene polymers and armchair-edge graphene nanoribbons on Au(111), newly-developed precursor named 5-bromo-11(10-bromoanthracene-9-yl)anthra[2,3-b : 7,6-b']dithiophene (BABAT) with isomers, which has similar structure as DBBA with one anthracene substituted with anthradithiophene, was found to make intramolecular C-C bonding instead of long anthracene polymers after annealing at 200 °C on Au(111). The mechanism was investigated using first-principle density functional theory, which revealed that on-surface polymerization is not kinetically preferred in case of BABAT. The reaction rate of intramolecular C-C bonding of BABAT is ∼206 times faster than that of DBBA. The intramolecular C-C bonding in DBBA biradicals, on the other hand, do not take place because of faster reverse reaction. By referring to electron density of BABAT biradicals, it was concluded that thiophene functionalization modifies distribution of electron density in BABAT radicals and facilitates electrophilic addition, leading to intramolecular C-C bonding after 200 °C annealing. These results indicate that the design of radical moiety is particularly important in the on-surface Ullmann-type coupling.

Interpolymer Self-Assembly of Bottom-up Graphene Nanoribbons Fabricated from Fluorinated Precursors.

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.

Experimental and Theoretical Investigations of Surface-Assisted Graphene Nanoribbon Synthesis Featuring Carbon-Fluorine Bond Cleavage.

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 edge-fluorinated 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.