Session E16: Graphene and Graphene Nanoribbons

Observation of dopant-induced impurity states in bottom-up graphene nanoribbons

Graphene nanoribbons (GNRs) provide a means for inducing energy gaps in graphene and are a promising candidate for many nanotechnological applications.~New bottom-up fabrication techniques allow the structure of GNRs to be tuned with atomic precision, thus providing new opportunities for modifying their electronic structure. Here we report the synthesis of bottom-up armchair GNRs (AGNRs) with isolated substitutional boron-dopant centers; thus creating localized impurity states in the GNR. These impurities are realized via dilute doping of pristine n$=$7 AGNRs with sparse boron-containing monomer units, resulting in uniform-width n$=$7 AGNR segments where only two carbon atoms have been substitutionally replaced by boron atoms. Scanning tunneling microscopy (STM) and spectroscopy (STS) were performed to study the electronic structure of these AGNR impurity systems, enabling us to observe localized mid-gap impurity states.   

Bottom-up Synthesis of N$=$13 Sulfur-doped Graphene Nanoribbons.

Substitutional doping of graphene nanoribbons (GNRs) with heteroatoms is a principal strategy to fine-tune the electronic structure of GNRs for future device applications. Up to now, however, edge-doping in bottom-up fabricated GNRs has exclusively relied on the introduction of nitrogen heteroatoms in the form of pyridine and pyrimidine rings along the edges of chevron GNRs. Here we report the bottom-up synthesis and characterization of atomically-precise N$=$13 armchair graphene nanoribbons (S-13-AGNRs) wherein alternating (CH)$_{\mathrm{2}}$ groups lining the edges of the GNRs have been replaced by sulfur atoms. We study the resultant GNR with scanning tunneling microscopy (STM) and spectroscopy (STS). Our experimental results are consistent with first-principles simulations of the S-13-AGNR electronic structure.   

Bottom-up fabrication and characterization of boron doped N$=$7 armchair graphene nanoribbons

Graphene nanoribbons (GNRs) have recently attracted great interest because of their novel electronic and magnetic properties, as well as the significant potential they have for device applications. Although several top-down techniques exist for fabricating GNRs, only bottom-up synthesis of GNRs from molecular precursors yields nanoribbons with atomic-scale structural control. Here we report the successful bottom-up fabrication boron doped N$=$7 armchair graphene nanoribbons. Substitutional boron atoms were incorporated into the GNRs' central backbone, thus placing boron's empty p-orbital in conjugation with the extended pi system of the GNR. Topographic and local electronic structure characterization was performed via STM and CO-tip-functionalized nc-AFM, and compared to DFT simulations.   

Structure and Electronic Properties of Polymer Chains and Graphene Nanoribbon Formed by Molecular Self-Assembly on Au(111).

Graphene nanoribbons (GNRs) with bandgaps are promising building blocks for ultra-fast electronics. Bottom-up synthesis of GNRs from aromatic hydrocarbon molecules has been proven to be an effective way to control GNR's width with atomically precise edge structures. Using scanning tunneling microscopy (STM), we study the formation of both linear polymer chains and narrow GNRs in the bottom-up self-assembly process with the DBBA molecules as the precursor on Au(111). The linear polymer chains are formed after the deposition of DBBA and 200 \textdegree C annealing for 30 min. The polymers can be converted to 7-AGNRs (seven-carbon wide armchair GNRs) after 400 \textdegree C annealing. Interestingly, second-layer polymer is seen to survive on the GNRs during the annealing process. This result indicates that the Au(111) substrate plays an important role in the dehydrogenation process and the formation of GNRs, which is confirmed by our DFT calculations. Electronically, the polymers show a bandgap of 3.4 eV, much larger than that of GNRs. After annealing at 500 \textdegree C for 30 min, wider GNRs can form: 14-AGNR, 21-AGNR. The 7-AGNR shows a typical edge state at -1.1 eV, while for 14-AGNR it is at -1.35 eV. Moreover, junctions of GNRs with different widths can be formed with pronounced boundary states.   

Graphene Nanoribbons Anchored to a SiC Substrate.

Due to their exceptional fundamental characteristics graphene nanoribbons play a major role in the development of future nano-technological applications. The high chemical reactivity of the graphene nanoribbon edges can be utilized to create modified materials. Using first principles simulations we explore this possibility to construct patterned systems composed of anchored ribbons of zigzag edges covalently bonded to a SiC substrate. The hybrid edge states are found to possess interesting electronic and magnetic properties, which alter the overall behavior of the entire system as compared to the behavior of the individual components. It is found that the van der Waals interactions are important for the overall stability and structure of the anchored ribbons. Also, spin-polarization effects play a profound role in the electronic structure and associated density of states. The hybrid graphene/SiC zigzag edges are analyzed in terms of their transport characteristics as well.   

Pseudospin dephasing in relaxed shape armchair graphene nanoribbons

In this presentation, we argue that the Zeeman pseudospin splitting energy might not be neglected for smaller widths of the graphene nanoribbons (GNRs). Mathematically valid study shows that the pseudospin splitting energy breaks the symmetry of degeneracy due to ripple induced Zeeman effect in GNRs originating from electromechanical effects. We estimate the spin relaxation time caused by in-plane phonon modes for possible application in straintronics and quantum information processing. We acknowledge Natural Sciences and Engineering Research Council of Canada and Canada Research Chair programs for their financial support.   

Decay patterns of edge states at reconstructed armchair graphene edges

Density functional theory calculations are used to investigate the electronic structures of localized states at reconstructed armchair graphene edges. We consider graphene nanoribbons with two different edge types and obtain the energy band structures and charge densities of the edge states. By examining the imaginary part of the wave vector in the forbidden energy region, we reveal the decay behavior of the wave functions in graphene. The complex band structures of graphene in the armchair and zigzag directions are presented in the first-principles framework.   

Orientationally Misaligned Zipping of Lateral Graphene and Boron Nitride Nanoribbons with Minimized Strain Energy and Enhanced Half-Metallicity

Lateral heterostructures of two-dimensional materials may exhibit various intriguing emergent properties. Yet when specified to the orientationally aligned heterojunctions of zigzag graphene and hexagonal boron nitride (hBN) nanoribbons, realizations of the high expectations on their properties encounter two standing hurtles. First, the rapid accumulation of strain energy prevents large- scale fabrication. Secondly, the pronounced half-metallicity predicted for freestanding graphene nanoribbons is severely suppressed. By properly tailoring orientational misalignment between zigzag graphene and chiral hBN nanoribbons, here we present a facile approach to overcome both obstacles. Our first-principles calculations show that the strain energy accumulation in such heterojunctions is significantly diminished for a range of misalignments. More strikingly, the half-metallicity is substantially enhanced from the orientationally aligned case, back to be comparable in magnitude with the freestanding case. The restored half-metallicity is largely attributed to the recovered superexchange interaction between the opposite heterojunction interfaces. The present findings may have important implications in eventual realization of graphene-based spintronics.   

First principles-based moir\'{e} model for incommensurate graphene on BN

Various properties of supported graphene films depend strongly on the exact positions of carbon atoms with respect to the underlying substrate. While density functional theory (DFT) can predict atom position in many systems, it cannot be applied straightforwardly to systems that are incommensurate or have large unit cells, such as graphene on a BN surface. We address these limitations by developing a simple moir\'{e} model with parameters derived from DFT calculations for systems strained into commensurate structures with manageable unit cell sizes. Our moir\'{e} model, which takes into account the flexural rigidity of graphene and includes the influence of the substrate, is able to reproduce the DFT-relaxed carbon positions with an accuracy of \textless 0.01 {\AA}. We then apply this model to the unstrained C/BN system and predict how structure and energy vary with azimuthal orientation of the graphene sheet with respect to the BN substrate.   

Computational study of bottom-up fabrication of armchair graphene nanoribbons

Graphene nanoribbons have promising electronic properties for nanotechnology. They can be fabricated using a bottom-up procedure with specific molecular precursors, which results in atomically precise nanoribbon structures. Herein we demonstrate the formation of a 7-atomic-layer pristine armchair graphene nanoribbon from the 10,10'-dibromo-9,9'-bianthracene precursor through polymerization followed by cyclodehydrogenation. We performed calculations using density functional theory with van der Waals corrections to study how Au(111) and Cu(111) substrates affect the energy profiles for the reaction process. To further understand the effects of the substrate on the cyclodehydrogenation procedure, we also considered double-layer ribbon structure on top of the substrate. These results may provide insight into the use of different precursors for producing both polymers and ribbons with novel electronic properties.   

A first-principles study on Magnetic and Electronic properties of Graphane vacancies Dots.

Graphane is the end product of the complete hydrogenation of graphene. The incomplete hydrogenation of graphene produces hydrogen vacancies in graphane. Hydrogen vacancies can alter the electronic structure of graphane and therefore tune the electronic, magnetic, and optical properties of the composite. In this paper, we use a first-principles density functional calculation to investigate a variety of well-separated clusters of hydrogen vacancies in graphane with magnetism, including the geometrical shapes of triangles, parallelograms, and rectangles. The results indicate that energy levels caused by the missing H are generated in the broad band gap of pure graphane. All triangular clusters of H vacancies are magnetic, the larger the triangle the higher the magnetic moment. The defect levels introduced by the missing H in triangular and parallelogram clusters are spin-polarized and can find application in optical transition. Parallelograms and open-ended rectangles are antiferromagnetic and can be used for nanoscale registration of digital information.   

Influence of Spatial Inhomogeneity on Electronic and Magneto Transport in Graphene.

We discuss room temperature electronic and magnetotransport measurements of polycrystalline graphene, grown by chemical vapor deposition, on a SiO$_{\mathrm{2}}$ dielectric. The measured graphene devices are intentionally spatially inhomogeneous such that the length of the sample is much greater (\textgreater 1000 times) than the average grain size. At magnetic field $B \quad =$ 0 T the electronic transport is well described by a diffusive transport model with contributions from grain boundary scattering significantly larger in the high charge carrier density limit. We find the largest percent change in the magnetoresistivity occurs at the film's Dirac point where the magnetotransport is largely dependent upon charge disorder. Away from the Dirac point we find a modified expression for the charge carrier density dependence of the magnetoresistivity with respect to the case of single-crystal graphene.   

Spontaneous and Thermally Enhanced Charge Transfer at Graphene-Silica Interface

Low dimensional carbon materials undergo spontaneous hole doping in the ambient conditions. Thermal annealing enhances the degree of the charge transfer in silica-supported graphene exposed to oxygen and water vapor. In this work, we investigated the mechanisms responsible for the charge transfer using Raman spectroscopy and water contact angle measurements. Mechanically exfoliated graphene samples were annealed at various temperatures in a range of 100 to 1000 $^{\mathrm{o}}$C to induce the hole doping. While the annealing-induced charge density of graphene increased with increasing annealing temperature up to 700 $^{\mathrm{o}}$C, it decreased as increasing the temperature further higher. Graphene samples prepared in a low humidity condition lead to significantly decreased hole doping suggesting that water contained in the samples plays a key role. We will propose and discuss a charge transfer mechanism that involves thermal hydroxylation and rehydroxylation of silica surfaces.   

Scrolling of Suspended CVD Graphene Sheets

Carbon Nanoscrolls, one dimensional spiral forms of graphitic carbon, have attracted recent interest due to their novel proposed properties [1]. Although various production methods and studies of carbon nanoscrolls have been performed, low yield and poor controllability of their synthesis have slowed progress in this field. Suspended graphene membranes and carbon nanotubes have been predicted as promising systems for the formation of graphene scrolls [2]. We have suspended chemical vapor deposition (CVD)-grown graphene over large holes in a Si/SiO2 substrate to make suspended membranes upon which nanotubes are placed. Initial experiments have been performed showing that tears or cuts of the suspended sheet can initiate scrolling. Our latest progress towards carbon nanotube initiated formation of graphene scrolls and suspended CVD graphene scrolling, along with measurements of these novel structures will be presented. [1] E. Perim et al., Front. Mater., 1:31 (2014); [2] E. Perim et al., J. Appl. Phys. 113, 054306 (2013)