Synthetic two-dimensional polymers, or bottom-up nanosheets, are ultrathin polymeric frameworks with in-plane periodicity. They can be synthesized in a direct, bottom-up fashion using atomic, ionic, or molecular components. However, few are based on carbon-carbon bond formation, which means that there is a potential new field of investigation into these fundamentally important chemical bonds. Here, we describe the bottom-up synthesis of all-carbon, π-conjugated graphdiyne nanosheets. A liquid/liquid interfacial protocol involves layering a dichloromethane solution of hexaethynylbenzene on an aqueous layer containing a copper catalyst at room temperature. A multilayer graphdiyne (thickness, 24 nm; domain size, >25 μm) emerges through a successive alkyne-alkyne homocoupling reaction at the interface. A gas/liquid interfacial synthesis is more successful. Sprinkling a very small amount of hexaethynylbenzene in a mixture of dichloromethane and toluene onto the surface of the aqueous phase at room temperature generated single-crystalline graphdiyne nanosheets, which feature regular hexagonal domains, a lower degree of oxygenation, and uniform thickness (3.0 nm) and lateral size (1.5 μm).

The contact properties between metal and graphene were examined. The electrical measurement on a multiprobe device with different contact areas revealed that the current flow preferentially entered graphene at the edge of the contact metal. The analysis using the cross-bridge Kelvin structure (CBK) suggested that a transition from the edge conduction to area conduction occurred for a contact length shorter than the transfer length of ~1 μm. The contact resistivity for Ni was measured as ~5×10 -6 Ωcm 2 using the CBK. A simple calculation suggests that a contact resistivity less than 10 -9 Ωcm 2 is required for miniaturized graphene field effect transistors.Graphene-based devices are promising candidates for future high-speed field effect transistors (FETs). An increase in the on/off current ratio (I on /I off ) is one of the critical issues to realize the graphene FETs. Although the contact properties are important in terms of an increase in I on , only a small number of experiments [1][2][3][4][5][6] have addressed this matter compared to the bandgap engineering for a decrease in I off . 7,8 In fact, an ohmic contact is obtained without any difficulty due to the lack of a bandgap, but it is concerned that a very small density of states (DOS) for graphene might suppress the current injection from the metal to graphene. Recently, we reported that the contact resistivity for a typical Cr/Au electrode was high and that it varied by several orders of magnitude. It has been suggested that the contact resistivity might significantly mask the outstanding performance of the monolayer graphene channel. 5,6 Although a lower contact resistivity was reported for a Ti/Au electrode, it was described in the units of either Ωμm or Ωμm 2 , 1,2,4 because the current flow path at the graphene/metal contact was not revealed. Furthermore, the actual contact resistivity required for FET applications has not yet been discussed. In this study, we first reveal the current flow path at the graphene/metal contact by using a multiprobe device with different contact areas. Then, the contact resistivities required for the miniaturized graphene FETs are quantitatively assessed based on the contact resistivity obtained experimentally by the cross-bridge Kelvin (CBK) method. Finally, the graphene/metal contact is discussed from the viewpoint of metal work function of contact metals employed.Graphite thin films were mechanically exfoliated from Kish graphite onto 90 nm SiO 2 /p + -Si substrates. The number of layers was determined by the optical contrast and Raman spectroscopy. 9 Electron-beam lithography was utilized to pattern electrical contacts onto graphene. The contact metals Cr/Au (~10/20 nm), Ti/Au (~10/20 nm), and Ni (~25 nm) were thermally evaporated on the resist-patterned graphene in a chamber with a background pressure of 10 -5 Pa and were subjected to the lift-off process in warm acetone. To remove the resist residual, graphene devices were annealed in a H 2 -Ar mixture at 300°C for 1 hour. The electrical measurements were p...

2D van der Waals ferroelectrics have emerged as an attractive building block with immense potential to provide multifunctionality in nanoelectronics. Although several accomplishments have been reported in ferroelectric switching for out-of-plane ferroelectrics down to the monolayer, a purely in-plane ferroelectric has not been experimentally validated at the monolayer thickness. Herein, an in-plane ferroelectricity is demonstrated for micrometersize monolayer SnS at room temperature. SnS has been commonly regarded to exhibit the odd-even effect, where the centrosymmetry breaks only in the odd-number layers to exhibit ferroelectricity. Remarkably, however, a robust room temperature ferroelectricity exists in SnS below a critical thickness of 15 layers with both an odd and even number of layers, suggesting the possibility of controlling the stacking sequence of multilayer SnS beyond the limit of ferroelectricity in the monolayer. This work will pave the way for nanoscale ferroelectric applications based on SnS as a platform for in-plane ferroelectrics.

Hexagonal boron nitride (BN) is widely used as a substrate and gate insulator for two-dimensional (2D) electronic devices. The studies on insulating properties and electrical reliability of BN itself, however, are quite limited. Here, we report a systematic investigation of the dielectric breakdown characteristics of BN using conductive atomic force microscopy. The electric field strength was found to be ~12 MV/cm, which is comparable to that of conventional SiO2 oxides because of the covalent bonding nature of BN. After the hard dielectric breakdown, the BN fractured like a flower into equilateral triangle fragments. However, when the applied voltage was terminated precisely in the middle of the dielectric breakdown, the formation of a hole that did not penetrate to the bottom metal electrode was clearly observed. Subsequent I-V measurements of the hole indicated that the BN layer remaining in the hole was still electrically inactive. Based on these observations, layer-by-layer breakdown was confirmed for BN with regard to both physical fracture and electrical breakdown. Moreover, statistical analysis of the breakdown voltages using a Weibull plot suggested the anisotropic formation of defects. These results are unique to layered materials and unlike the behavior observed for conventional 3D amorphous oxides.