Formation Of Graphene Nanoribbons Research Articles

Kinetic control over the chiral-selectivity in the formation of organometallic polymers on a Ag(110) surface

Methods to control chiral-selectivity in molecular reactions through external inputs are of importance, both from a fundamental and technological point of view. Here, the self-assembly of prochiral 6,12-dibromochrysene monomers on Ag(110) is studied using scanning tunneling microscopy. Deposition of the monomers on a substrate held at room temperature leads to the formation of 1D achiral organometallic polymers. When the monomers are instead deposited on a substrate held at 373 K, homochiral organometallic polymers consisting of either the left- or right-handed enantiomer are formed. Post-deposition annealing of room temperature deposited samples at >373 K does not transform the achiral 1D organometallic polymers into homochiral ones and thus, does not yield the same final structure as if depositing onto a substrate held at the same elevated temperature. Furthermore, annealing promotes neither the formation of 1D covalently-coupled polymers nor the formation of graphene nanoribbons. Our results identify substrate temperature as an important factor in on-surface chiral synthesis, thereby demonstrating the importance of considering kinetic effects and the decisive role they can play in structure formation.

Unzipping Carbon Nanotubes to Sub-5-nm Graphene Nanoribbons on Cu(111) by Surface Catalysis.

Graphene nanoribbons (GNRs) are promising in nanoelectronics for their quasi-1D structures with tunable bandgaps. The methods for controllable fabrication of high-quality GNRs are still limited. Here a way to generate sub-5-nm GNRs by annealing single-walled carbon nanotubes (SWCNTs) on Cu(111) is demonstrated. The structural evolution process is characterized by low-temperature scanning tunneling microscopy. Substrate-dependent measurements on Au(111) and Ru(0001) reveal that the intermediate strong SWCNT-surface interaction plays a pivotal role in the formation of GNRs.

Catalytic Growth of Ultralong Graphene Nanoribbons on Insulating Substrates.

Graphene nanoribbons (GNRs) with widths of a few nanometers are promising candidates for future nanoelectronic applications due to their structurally tunable bandgaps, ultrahigh carrier mobilities, and exceptional stability. However, the direct growth of micrometer-long GNRs on insulating substrates, which is essential for the fabrication of nanoelectronic devices, remains an immense challenge. Here, the epitaxial growth of GNRs on an insulating hexagonal boron nitride (h-BN) substrate through nanoparticle-catalyzed chemical vapor deposition is reported. Ultranarrow GNRs with lengths of up to 10µm are synthesized. Remarkably, the as-grown GNRs are crystallographically aligned with the h-BN substrate, forming 1D moiré superlattices. Scanning tunneling microscopy reveals an average width of 2nm and a typical bandgap of ≈1eV for similar GNRs grown on conducting graphite substrates. Fully atomistic computational simulations support the experimental results and reveal a competition between the formation of GNRs and carbon nanotubes during the nucleation stage, and van der Waals sliding of the GNRs on the h-BN substrate throughout the growth stage. Thisstudy provides a scalable, single-step method for growing micrometer-long narrow GNRs on insulating substrates, thus opening a route to explore the performance of high-quality GNR devices and the fundamental physics of 1D moiré superlattices.

Catalytic superlubricity: engineering superlubricity caused by the catalytic effect of gold on hydrogenated amorphous carbon structure

<p indent="0mm">Material and energy losses caused by friction and wear hinder the development of mechanical systems. An effective way to solve the problem is to reduce friction coefficient and wear. Here, we proposed <italic>in-situ</italic> formation of graphene nanoribbons <italic>via</italic> gold-catalyzed reconstruction of amorphous hydrogenated carbon matrix to achieve engineering superlubricity. And we called this method “catalytic superlubricity”. In detail, amorphous hydrogenated carbon (a-C:H) films were grown by plasma chemical vapor deposition (PECVD), and gold (Au) films on steel balls were obtained by electroless chemical deposition. Comparing study on the tribology properties of the couple pairs of a-C:H/Au and a-C:H/GCr15 was carried out <italic>via </italic>home-made Tribometer 3. The results show that, compared with the couple pairs of a-C:H/GCr15 (<italic>μ</italic>~0.032 and wear depth of <sc>128 nm</sc> ), the couple pairs of a-C:H/Au show a friction coefficient of 0.008 and wear depth of <sc>40 nm,</sc> respectively. The reason of superlubricity of the couple pairs of a-C:H/Au is that, based on the catalysis of gold, the graphene nanoribbons formed <italic>in-situ</italic> during friction. The ordered structure is conducive to the occurrence of incommensurate contact, and effectively reduces the interfacial shear force and thus lowers the friction coefficient.

On-Surface Synthesis of Graphene Nanoribbons on Two-Dimensional Rare Earth-Gold Intermetallic Compounds.

Here, we demonstrate two reliable routes for the fabrication of armchair-edge graphene nanoribbons (GNRs) on TbAu2/Au(111), belonging to a class of two-dimensional ferromagnetic rare earth-gold intermetallic compounds. On-surface synthesis directly on TbAu2 leads to the formation of GNRs, which are short and interconnected with each other. In contrast, the intercalation approach-on-surface synthesis of GNRs directly on Au(111) followed by rare earth intercalation-yields GNRs on TbAu2/Au(111), where both the ribbons and TbAu2 are of high quality comparable with those directly grown on clean Au(111). Besides, the as-grown ribbons retain the same band gap while changing from p-doping to weak n-doping mainly due to a change in the work function of the substrate after the rare earth intercalation. The intercalation approach might also be employed to fabricate other types of GNRs on various rare earth intermetallic compounds, providing platforms to tailor the electronic and magnetic properties of GNRs on magnetic substrates.

Preparation of graphene nanoribbons (GNRs) from twisted structure carbon nanotubes using unzipping technique

This work deals with the preparation of graphene nano ribbons (GNRs), which are small bars or strips of graphene with narrow range width less than 50 nm and one atom thick sheet (approximately 140 °A). This material has many applications in electronics, polymer composite, contrast agent bio-imaging and others. This material was prepared from twisted structure of carbon nanotubes CNTs using oxidation process by acids and breaking down by high frequency ultrasonication (sonochemical unzipping). Pre-prepared twisted CNTs were characterized by scanning electron microscope (HRSEM) before treatment. The size, morphology, crystallinity of prepared graphene nanoribbons was also investigated using transmission electron microscope, high resolution transmission electron microscope, and X-ray diffraction and particle size analyzer. The results showed of formation of GNRs with narrow width as lower than 29 nm and uniform sizes. X-ray diffraction reveals Bragg reflections corresponding to the lattice planes (002), (100), (101), (004), and (110) matched with hexagonal system of graphite.

Coronene-Based Graphene Nanoribbons Insulated by Boron Nitride Nanotubes: Electronic Properties of the Hybrid Structure.

We present a theoretical study on the formation of graphene nanoribbons—via polymerization of coronene molecules—inside the inner cavity of boron nitride nanotubes. We examine the electronic property of the hybrid system, and we show that the boron nitride nanotube does not significantly alter the electronic properties of the encapsulated graphene nanoribbon. Motivated by previous experimental works, we examine graphene nanoribbons with two different widths and investigate probable scenarios for defect formation and/or twisting of the resulting graphene nanoribbons and their effect on the electronic properties of the hybrid system.

Structural evolution dynamics in fusion of sumanenes and corannulenes: defects formation and self-healing mechanism

The fusion processes of structures consisting of various combinations between sumanene and corannulene, leading to the formation of graphene nanoribbons (GNRs) under heating are simulated by density-functional-based tight-binding molecular dynamics. Distinct stages are unraveled in the course of GNR formation. Firstly, the carbon fragments coalescence into highly strained framework. Secondly, structural reconstruction invokes breaking most strained bonds to form a GNR structure containing numerous defects. Lastly, defects are remedied by the delicate ‘edge-facilitated self-healing’ process through two synergized edge-related effects: elevated mobility of defects and promoted structure reconstructions owing to the remarkable dynamics associated with edges. Importantly, detailed dynamics in the course of forming GNRs with defects and grain boundaries simulated in this work is valuable to provide better understanding at the atomistic scale of defect formation as well as self-healing in the context of the sp2 carbon network. In particular, edges play important roles in not only generating Stone–Wales (SW), 5-8-5 types of defects, 8-5-5-8 and pentagon–heptagon grain boundaries. In addition, our simulations predict the existence of one novel defect, coined as the Inverse SW defect, which is to be confirmed in future experimental studies. This study of dynamic structural evolution reveals that edges are prone to intrinsic and extrinsic modifications such as atomic-scale defects, structural distortions and inhomogeneity.

Electronic structure changes during the on-surface synthesis of nitrogen-doped chevron-shaped graphene nanoribbons

Utilizing suitable precursor molecules, a thermally activated and surface-assisted synthesis results in the formation of defect-free graphene nanoribbons (GNRs), which exhibit electronic properties that are not present in extended graphene. Most importantly, they have a band gap in the order of a few electron volts, depending on the nanoribbon width. In this study, we investigate the electronic structure changes during the formation of GNRs, nitrogen-doped (singly and doubly N-doped) as well as non-N-doped chevron-shaped CGNRs on Au(111). Thus we determine the optical gaps of the precursor molecules, the intermediate nonaromatic polymers, and finally the aromatic GNRs, using high-resolution electron energy loss spectroscopy and density functional theory calculations. As expected, we find no influence of N-doping on the size of the optical gaps. The gap of the precursor molecules is around 4.5 eV. Polymerization leads to a reduction of the gap to a value of 3.2 eV due to elongation and thus enhanced delocalization. The CGNRs exhibit a band gap of 2.8 eV, thus the gap is further reduced in the nanoribbons, since they exhibit an extended delocalized $\ensuremath{\pi}$-electron system.

Graphene Nanoribbon Synthesis with a Fluorinated Molecular Precursor

The surface-assisted reaction is a key process for graphene nanoribbons (GNRs) synthesis. So far, anthracene based compounds, as represented by 10,10’-dibromo-9,9’-bianthracene, are most-used molecular precursors for GNRs synthesis. However, even the simple anthracene based precursor molecules exhibit a variety of reactivity because the on-surface synthesis includes hierarchical reactions such as dehalogenation, polymerization, and cyclodehydrogenation. Considering that the reactivity of the small precursor molecules determines the final structure of GNRs, the key to success for synthesizing tailored GNRs is the deep understanding the relationship between the reactivity of the molecules and the geometry on the catalytic metal surface. Here, we investigated the reactivity of a fluorinated precursor molecule during the GNR formation. The fluorine atoms were introduced at 2,3,6,7-positions of an anthracene sandwiched by 9-bromoanthracenes in order that the effect of fluorine substitution at the edge positions of polyanthrylene and GNRs would be investigated. Scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) indicate the formation of the edge-fluorinated polyanthrylenes. Interestingly, the following cyclodehydrogenation yields GNRs, dissociating the carbon-fluorine bonds at the edge positions of polyanthrylenes. Namely, the strong C-F bond survived during the polymerization step, but the following cyclodehydrogenation cleaved the C-F bond. Theoretical calculation implies that the structure of the intermediate state during the cyclodehydrogenation played an important role in the fluorine dissociation. Thus, the lessons from the dissociation of the strong C-F bond, which is believed as one of the strongest bonding in the solution chemistry, would provide the synthetic access to the proper design of precursor molecules for edge-functionalized GNRs.

Controllable conversion of quasi-freestanding polymer chains to graphene nanoribbons

In the bottom-up synthesis of graphene nanoribbons (GNRs) from self-assembled linear polymer intermediates, surface-assisted cyclodehydrogenations usually take place on catalytic metal surfaces. Here we demonstrate the formation of GNRs from quasi-freestanding polymers assisted by hole injections from a scanning tunnelling microscope (STM) tip. While catalytic cyclodehydrogenations typically occur in a domino-like conversion process during the thermal annealing, the hole-injection-assisted reactions happen at selective molecular sites controlled by the STM tip. The charge injections lower the cyclodehydrogenation barrier in the catalyst-free formation of graphitic lattices, and the orbital symmetry conservation rules favour hole rather than electron injections for the GNR formation. The created polymer–GNR intraribbon heterostructures have a type-I energy level alignment and strongly localized interfacial states. This finding points to a new route towards controllable synthesis of freestanding graphitic layers, facilitating the design of on-surface reactions for GNR-based structures.

Optical Unzipping of Carbon Nanotubes in Liquid Media

Optical unzipping of carbon nanotubes (CNTs) in liquid media is one of the most awaited technologies as it promises instant material transformation from CNTs to graphene nanoribbons (GNRs) and also an easy transfer of GNRs to arbitrary substrates. In the present article, we report the laser-induced optical unzipping of CNTs, dispersed in dimethylformamide (DMF) solvent. In a nutshell, laser unzipping of CNTs dispersed in liquid solvent is a photophysicochemical process where molecular interactions between CNTs and solvent are tuned by the laser irradiation and results in the formation of GNRs in a scalable manner. The proposed mechanism includes the creation of defects together with vacancies upon laser irradiation, followed by their migration toward the energetically favorable axis of the CNT—the longitudinal direction—finally leading to the unzipping/fragmentation of the nanotube. Distinct laser thresholds have been observed for each of the three events, namely, (a) the formation of the first defect, (b...

Intercalation-assisted longitudinal unzipping of carbon nanotubes for green and scalable synthesis of graphene nanoribbons.

We have demonstrated an effective intercalation of multi-walled carbon nanotubes (MWCNTs) for the green and scalable synthesis of graphene nanoribbons (GNRs) using an intercalation-assisted longitudinal unzipping of MWCNTs. The key step is to introduce an intercalation treatment of raw MWCNTs with KNO3 and H2SO4, making it promising to decrease the strong van der Waals attractions in the MWCNTs bundles and between the coaxial graphene walls of CNTs. Systematic micro Raman, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) characterizations suggest that potassium, nitrate, and sulfate ions play an important role in the CNT intertube and intratube intercalations during the pretreatment. Detailed scanning electron microscopy (SEM), transmission electron microscopy, XRD, and micro Raman characterizations indicate that the developed methodology possesses the ability to synthesis GNRs effectively with an improved CNT concentration in H2SO4 of 10 mg/ml at 70 °C, which is amenable to industrial-scale production because of the decreased amount of strong acid. Our work provides a scientific understanding how to enhance the GNR formation by accelerating the CNT longitudinal unzipping via suitable molecular intercalation.

Size-controlled ambipolar graphene nanoribbon transistors by an all-dry mask method

Conventional graphene nanoribbon (GNR) field-effect transistor (FET) fabrication involved the wet process with the separated nanoribbon formation and device fabrication. Here, we demonstrate one simple and novel non-solution method to integrate the GNR formation and the FET fabrication, where a gold film is used as mask for electrode deposition, following by using a SnO2 nanoribbon as mask for the formation of GNR. The channel length and width can be controlled by the widths of the gold film and the SnO2 nanoribbon, respectively. It is found that the GNR with the width up to 330nm presents the promising ambipolar field-effect properties in air ambient, the hole and electron mobilities are respectively as high as 904 and 703cm2V−1s−1, which benefits from the all dry process for both GNR fabrication and device fabrication.

Surface-Assisted Reactions toward Formation of Graphene Nanoribbons on Au(110) Surface

Scanning tunneling microscopy and X-ray spectroscopy measurements are combined to first-principles simulations to investigate the formation of graphene nanoribbons (GNRs) on Au(110), as based on the surface-mediated reaction of 10,10′-dibromo-9,9′-bianthracene (DBBA) molecules. At variance with Au(111), two different pathways are identified for the GNR self-assembly on Au(110), as controlled by both the adsorption temperature and the surface coverage of the DBBA molecular precursors. Room-temperature DBBA deposition on Au(110) leads to the same reaction steps obtained on Au(111), even though with lower activation temperatures. For DBBA deposition at 470 K, the cyclodehydrogenation of the precursors preceds their polymerization, and the GNR formation is fostered by increasing the surface coverage. While the initial stages of the reaction are found to crucially determine the final configuration and orientation of the GNRs, the molecular diffusion is found to limit in both cases the formation of high-density...

Formation of graphene nanoribbons and Y-junctions by hydrogen induced anisotropic etching

Demonstrated formation of nanoribbons, and Y-junctions structures with controllable hydrogen-induced anisotropic etching of graphene. The distinct graphene edges of individual ribbon created 120° to form a Y-shape structure.

Tracking and Removing Br during the On-Surface Synthesis of a Graphene Nanoribbon

The fabrication of graphene nanoribbons (GNRs) requires a high degree of precision due to the sensitivity of the electronic structure on the edge shape. Using Br-substituted molecular precursors, this atomic precision can be achieved in a thermally induced two-step reaction following Br dissociation on a Au(111) surface. Using DFT, we find evidence that the Br atoms are bound to the intermediate polyanthrylene chains. We employ temperature-programmed desorption to demonstrate the associative desorption of HBr and molecular hydrogen during the final cyclodehydrogenation step of the reaction. Both processes are found to have similar activation barriers. Furthermore, we are able to remove Br atoms from the polyanthrylene chains by providing molecular hydrogen. The subsequent formation of GNR via a cyclodehydrogenation demonstrates that Br does not influence this part of the overall reaction.

IN SITU UNZIPPING OF CARBON NANOTUBES TO FORM GRAPHENE NANORIBBONS

We report the one step facile synthesis of graphene nanoribbons (GNRs) by unzipping carbon nanotubes (CNTs) from glucose ( C 6 H 12 O 6) precursor, using a simple chemical vapor deposition method. Some nanotubes are partially cut resulting in a GNR–CNT hybrid whereas others are fully cut to form GNRs. The average length of GNRs achieved by this method is typically in the range of 1–10 μm. The formation of GNRs is ascribed to the in situ oxygen-driven unzipping of CNTs. The process is free from aggressive oxidants and utilizes the in situ unzipping. This method offers an alternative approach for making GNRs, compared to previously used techniques to synthesize GNRs.

Graphene nanoribbons on vicinal SiC surfaces by molecular beam epitaxy

We present a method of producing a densely ordered array of epitaxial graphene nanoribbons (GNRs) using vicinal SiC surfaces as a template, which consists of ordered pairs of (0001) terraces and nanofacets. Controlled selective growth of graphene on approximately 10 nm wide (0001) terraces with 10 nm spatial intervals allows GNR formation. By selecting the vicinal direction of SiC substrate, [1$\overline{1}$00], well-ordered GNRs with predominantly armchair edges are obtained. These structures, the high-density GNRs, enable us to observe the electronic structure at $K$ points by angle-resolved photoemission spectroscopy, showing a clear band-gap opening of at least 0.14 eV.

Ordered and disordered packing of coronene molecules in carbon nanotubes

Monte Carlo simulations of coronene molecules in single-walled carbon nanotubes (SWCNTs) and dicoronylene molecules in SWCNTs are performed. Depending on the diameter D of the encapsulating SWCNT, regimes favoring the formation of ordered, one-dimensional (1D) stacks of tilted molecules (D ≤ 1.7 nm for coronene@SWCNT, 1.5 nm ≤ D ≤ 1.7 nm for dicoronylene@SWCNT) and regimes with disordered molecular arrangements and increased translational mobilities enabling the thermally induced polymerization of neighboring molecules resulting in the formation of graphene nanoribbons (GNRs) are observed. The results show that the diameter of the encapsulating nanotube is a crucial parameter for the controlled synthesis of either highly ordered 1D structures or GNR precursors.