According to theoretical studies, narrow graphene nanoribbons with atomically precise armchair edges and widths of o2 nm have a bandgap comparable to that in silicon (1.1 eV), which makes them potentially promising for logic applications. Different top-down fabrication approaches typically yield ribbons with width 410 nm and have limited control over their edge structure. Here we demonstrate a novel bottom-up approach that yields gram quantities of high-aspect-ratio graphene nanoribbons, which are only B1 nm wide and have atomically smooth armchair edges. These ribbons are shown to have a large electronic bandgap of B1.3 eV, which is significantly higher than any value reported so far in experimental studies of graphene nanoribbons prepared by top-down approaches. These synthetic ribbons could have lengths of 4100 nm and self-assemble in highly ordered few-micrometer-long 'nanobelts' that can be visualized by conventional microscopy techniques, and potentially used for the fabrication of electronic devices.

Narrow graphene nanoribbons (GNRs) constructed by atomically precise bottom-up synthesis from molecular precursors have attracted significant interest as promising materials for nanoelectronics. But there has been little awareness of the potential of GNRs to serve as nanoscale building blocks of novel materials. Here we show that the substitutional doping with nitrogen atoms can trigger the hierarchical self-assembly of GNRs into ordered metamaterials. We use GNRs doped with eight N atoms per unit cell and their undoped analogues, synthesized using both surface-assisted and solution approaches, to study this self-assembly on a support and in an unrestricted three-dimensional (3D) solution environment. On a surface, N-doping mediates the formation of hydrogen-bonded GNR sheets. In solution, sheets of side-by-side coordinated GNRs can in turn assemble via van der Waals and π-stacking interactions into 3D stacks, a process that ultimately produces macroscopic crystalline structures. The optoelectronic properties of these semiconducting GNR crystals are determined entirely by those of the individual nanoscale constituents, which are tunable by varying their width, edge orientation, termination, and so forth. The atomically precise bottom-up synthesis of bulk quantities of basic nanoribbon units and their subsequent self-assembly into crystalline structures suggests that the rapidly developing toolset of organic and polymer chemistry can be harnessed to realize families of novel carbon-based materials with engineered properties.

demonstrated ten-fold improvement of charge carrier mobility in graphene-based field-effect transistors (FETs) when a conventional Si/SiO 2 substrate is replaced by an epitaxial lead zirconate titanate Pb(Zr,Ti)O 3 (PZT) film. Several other studies also showed that graphene-based FETs on ferroelectric substrates have nonvolatile memory properties. [3][4][5][6][7][8] While these studies demonstrate some practical characteristics of graphene-ferroelectric FETs (FeFETs), their electrical properties are not yet completely understood. In particular, these devices exhibit an unusual antihysteresis of electronic transport, which contradicts the hysteretic polarization dependence of PZT. [3][4][5][6] The electronic behavior of graphene FeFETs is schematically illustrated by Figure 1. Figure 1A shows the scheme of a typical graphene FeFET considered in prior studies, as well as in this work. It consists of a graphene channel bridging the source (S) and drain (D) electrodes on a PZT film covering a back gate (G) electrode. When sufficient positive voltage is applied to the gate electrode (V G ), the polarization of ferroelectric is pointing upward, and with sufficient negative V G the polarization is pointing downward. In these experiments, the drain-source current (I DS ) is measured as a function of gate voltage V G . When a certain V G is applied, it creates an electric field E, which affects the polarization of PZT, P. The resulting dielectric displacement D, which changes the Fermi level of graphene and thus I DS , can be expressed as D = ε o E + P. Figure 1B shows a general hysteretic P-V G dependence for PZT. The condition for the charge neutrality in graphene where its conductivity is the lowest is D = 0, which occurs when P = −ε o E. As shown schematically in Figure 1B, there are two gate voltages (shown as green dots) at which this condition is met. This means that in cyclic I DS -V G dependences of graphene FeFETs two points of minimum conductivity, or Dirac points (V DP ), should be observed-one when V G is scanned forward (V DP,F ), and another one when V G is scanned back (V DP,B ). The expected I DS -V G dependence for a graphene FeFET is schematically shown in Figure 1C. In order to describe the effect of a ferroelectric polarization on conductivity of graphene we will define the V DP shift as ΔV DP =V DP,B − V DP,F . For the expected, i.e., polarization-related I DS -V G hysteresis Ferroelectric field-effect transistors (FeFETs) employing graphene on inorganic perovskite substrates receive considerable attention due to their interesting electronic and memory properties. They are known to exhibit an unusual hysteresis of electronic transport that is not consistent with the ferroelectric polarization hysteresis and is previously shown to be associated with charge trapping at graphene-ferroelectric interface. Here, an electrical measurement scheme that minimizes the effect of charge traps and reveals the polarization-dependent hysteresis of electronic transport in graphene-Pb(Zr,Ti)O 3 FeFETs is demonstrated....

Atomically precise graphene nanoribbons (GNRs) of two types, chevron GNRs and N = 7 straight armchair GNRs (7-AGNRs), have been synthesized through a direct contact transfer (DCT) of molecular precursors on Au(111) and gradual annealing. This method provides an alternative to the conventional approach for the deposition of molecules on surfaces by sublimation and simplifies preparation of dense monolayer films of GNRs. The DCT method allows deposition of molecules on a surface in their original state and then studying their gradual transformation to polymers to GNRs by scanning tunneling microscopy (STM) upon annealing. We performed STM characterization of the precursors of chevron GNRs and 7-AGNRs, and demonstrate that the assemblies of the intermediates of the GNR synthesis are stabilized by π-π interactions. This conclusion was supported by the density functional theory calculations. The resulting monolayer films of GNRs have sufficient coverage and density of nanoribbons for ex situ characterization by spectroscopic methods, such as Raman spectroscopy, and may prove useful for the future GNR device studies.