Graphene nanoribbons (GNRs) are considered one of the most promising materials for next generation electronics, however a reliable and controllable synthesis method is still lacking. Here, we report the CVD growth of GNRs on a copper surface and the corresponding mechanisms of growth. One-dimensional GNR growth is enabled by a vapor-liquid-solid (VLS) graphene growth guided by on-surface propagation of a liquid catalyst particle. Controlling the suppression of vapor-solid-solid (VSS) graphene growth along the width direction of the GNR by tuning the flow of H2 during growth gives rise to a spear head-shaped graphene that we term graphene nanospears (GNSs). The real-time visual and spatially resolved observations confirm the VSS growth of graphene can be fully suppressed and lead to GNR formation on Cu surface. These findings reveal key insight into the growth mechanism of graphene and open a door for achieving a facile and scalable method of synthesizing free standing GNRs.
Recently, atomically well-defined cove-shaped graphene nanoribbons have been obtained using bottom-up synthesis. These nanoribbons have an optical gap in the visible range of the spectrum which make them candidates for donor materials in photovoltaic devices. From the atomistic point of view, their electronic and optical properties are not clearly understood. Therefore, in this work we carry out ab-initio density functional theory calculations combine with many-body perturbation formalism to study their electronic and optical properties. Through the comparison with experimental measurements, w…
Deterministic bottom-up approaches for synthesizing atomically well-defined graphene nanoribbons (GNRs) largely rely on the surface-catalyzed activation of selected labile bonds in a molecular precursor followed by step growth polymerization and cyclodehydrogenation. While the majority of successful GNR precursors rely on the homolytic cleavage of thermally labile C-Br bonds, the introduction of weaker C-I bonds provides access to monomers that can be polymerized at significantly lower temperatures, thus helping to increase the flexibility of the GNR synthesis process. Scanning tunneling micro…
We unveil the nature of the structural disorder in bottom-up zigzag graphene nanoribbons along with its effect on the magnetism and electronic transport on the basis of scanning probe microscopies and first-principles calculations. We find that edge-missing m-xylene units emerging during the cyclodehydrogenation step of the on-surface synthesis are the most common point defects. These "bite'' defects act as spin-1 paramagnetic centers, severely disrupt the conductance spectrum around the band extrema, and give rise to spin-polarized charge transport. We further show that the electronic conduct…
Graphene nanoribbons (GNRs) synthesized using a bottom-up technique potentially enable future electronic devices owing to the tunable electronic structures depending on the well-defined width and edge geometry. For instance, armchair-edged GNRs (AGNRs) exhibit width-dependent bandgaps. However, the bandgaps of AGNRs synthesized experimentally thus far are relatively large, well above 1 eV. Such a large bandgap may deteriorate device performances due to large Schottky barriers and carrier effective masses. We describe the bottom-up synthesis of AGNRs with a smaller bandgap using dibromobenzene-…
Bottom-up assembled nanomaterials and nanostructures allow for the studies of rich and unprecedented quantum-related and mesoscopic transport phenomena. However, it can be difficult to quantify the correlations between the geometrical or structural parameters obtained from advanced microscopy and measured electrical characteristics when they are made into macroscopic devices. Here, we propose a strategy to connect the nanomaterial morphologies and the device performance through a Monte Carlo device model and apply it to understand the scaling trends of bottom-up synthesized armchair graphene n…
van der Waals nanomaterials supporting phonon polariton quasiparticles possess unprecedented light confinement capabilities, making them ideal systems for molecular sensing, thermal emission, and subwavelength imaging applications, but they require defect-free crystallinity and nanostructured form factors to fully showcase these capabilities. We introduce bottom-up-synthesized α-MoO3 structures as nanoscale phonon polaritonic systems that feature tailorable morphologies and crystal qualities consistent with bulk single crystals. α-MoO3 nanoribbons serve as low-loss hyperbolic Fabry-Pérot nanor…
In this work the tight binding model calculations are carried out for AA-Bilayer Graphene nanoribbons as an example of bilayer systems. The effects of edges, NNN hopping, and impurities of a single layer are introduced numerically as a change in the elements of the relevant block diagonal matrix appearing in the direct diagonalization method. The direct interlayer hopping between the top and the bottom single layers is constructed in the generalized direct diagonalization method by the off-diagonal block matrices in which the strength of the interlayer hopping is included.
Graphene nanoribbons (GNRs) make up an extremely interesting class of materials. On the one hand GNRs share many of the superlative properties of graphene, while on the other hand they display an exceptional degree of tunability of their optoelectronic properties. The presence or absence of correlated low-dimensional magnetism, or of a widely tunable band gap, is determined by the boundary conditions imposed by the width, crystallographic symmetry and edge structure of the nanoribbons. In combination with additional controllable parame-ters like the presence of heteroatoms, tailored strain, or…
We report on the experimental demonstration and electrical characterization of N = 7 armchair graphene nanoribbon (7-AGNR) field effect transistors. The back-gated transistors are fabricated from atomically precise and highly aligned 7-AGNRs, synthesized with a bottom-up approach. The large area transfer process holds the promise of scalable device fabrication with atomically precise nanoribbons. The channels of the FETs are approximately 30 times longer than the average nanoribbon length of 30 nm to 40 nm. The density of the GNRs is high, so that transport can be assumed well-above the percol…
We have used molecular dynamics to calculate the thermal conductivity of symmetric and asymmetric graphene nanoribbons (GNRs) of several nanometers in size (up to ~4 nm wide and ~10 nm long). For symmetric nanoribbons, the calculated thermal conductivity (e.g. ~2000 W/m-K @400K for a 1.5 nm {\times} 5.7 nm zigzag GNR) is on the similar order of magnitude of the experimentally measured value for graphene. We have investigated the effects of edge chirality and found that nanoribbons with zigzag edges have appreciably larger thermal conductivity than nanoribbons with armchair edges. For asymmetri…
Ballistic quantum transport calculations based on the non-equilbrium Green's function formalism show that field-effect transistor devices made from chevron-type graphene nanoribbons (CGNRs) could exhibit negative differential resistance with peak-to-valley ratios in excess of 4800 at room temperature as well as steep-slope switching with 6 mV/decade subtheshold swing over five orders of magnitude and ON-currents of 88$μ$A/$μ$m. This is enabled by the superlattice-like structure of these ribbons that have large periodic unit cells with regions of different effective bandgap, resulting in miniba…
Bottom-up approaches allow the production of ultra-narrow and atomically precise graphene nanoribbons (GNRs), with electronic and optical properties controlled by the specific atomic structure. Combining Raman spectroscopy and ab-initio simulations, we show that GNR width, edge geometry and functional groups all influence their Raman spectra. The low-energy spectral region below 1000 cm-1 is particularly sensitive to edge morphology and functionalization, while the D peak dispersion can be used to uniquely fingerprint the presence of GNRs, and differentiates them from other sp2 carbon nanostru…
The fabrication of atomically precise structures with designer electronic properties is one of the emerging topics in condensed matter physics. The required level of structural control can either be reached through atomic manipulation using the tip of a scanning tunneling microscope (STM) or by bottom-up chemical synthesis. In this review, we focus on recent progress in constructing novel, atomically precise artificial materials: artificial lattices built through atom manipulation and graphene nanoribbons (GNRs) realized by on-surface synthesis. We summarize the required theoretical background…
Precision control of interfacial structures and electronic properties is the key to the realization of functional heterostructures. Here, utilizing the scanning tunneling microscope (STM) both as a manipulation and characterization tool, we demonstrate the fabrication of a heterostructure in a single atomically precise graphene nanoribbon (GNR) and report its electronic properties. The heterostructure is made of a seven-carbon-wide armchair GNR and a lower band gap intermediate ribbon synthesized bottom-up from a molecular precursor on an Au substrate. The short GNR segments are directly writt…
We report a detailed analytic investigation of the interplay between size quantization and local scattering centers in armchair graphene nanoribbons, as seen in the conductance. The scattering property of a local scattering center is dependent on if it is located on one sublattice (A-site impurity) or both (impurity situated at neighboring carbon atoms, A-B-site impurity). The A-site impurity scatters in a similar way as a localized impurity in a one-dimensional channel made from a two-dimensional electron gas. On the other hand, the A-B-site impurity introduces A- to B-sublattice scattering, …
Bottom-up synthesized graphene nanoribbons (GNRs) are quantum materials that can be structured with atomic precision, providing unprecedented control over their physical properties. Accessing the intrinsic functionality of GNRs for quantum technology applications requires the manipulation of single charges, spins, or photons at the level of an individual GNR. However, experimentally, contacting individual GNRs remains challenging due to their nanometer-sized width and length as well as their high density on the metallic growth substrate. Here, we demonstrate the contacting and electrical chara…
The influence of substrate steps on the bottom-up synthesis of atomically precise graphene nanoribbons (GNRs) on an Au(111) surface is investigated. A straight surface step is found to promote the assembly of long and compact arrays of polymers with enhanced interchain pi-pi stacking interaction, which creates a steric hindrance effect on cyclodehydrogenation to suppress the H passivation of polymer ends. The modified two-stage growth process results in periodic arrays of GNRs with doubled average length near step edges.
A topological insulator is a new state of matter, possessing gapless spin-locking surface states across the bulk band gap which has created new opportunities from novel electronics to energy conversion. However, the large concentration of bulk residual carriers has been a major challenge for revealing the property of the topological surface state via electron transport measurement. Here we report surface state dominated transport in Sb-doped Bi2Se3 nanoribbons with very low bulk electron concentrations. In the nanoribbons with sub-10nm thickness protected by a ZnO layer, we demonstrate complet…
Bottom-up synthesized GNRs and GNR heterostructures have promising electronic properties for high performance field effect transistors (FETs) and ultra-low power devices such as tunnelling FETs. However, the short length and wide band gap of these GNRs have prevented the fabrication of devices with the desired performance and switching behaviour. Here, by fabricating short channel (Lch ~20 nm) devices with a thin, high-k gate dielectric and a 9-atom wide (0.95 nm) armchair GNR as the channel material, we demonstrate FETs with high on-current (Ion >1 uA at Vd = -1 V) and high Ion/Ioff ~10^5 at …
We predict enhanced electron-hole superfluidity in two coupled electron-hole armchair-edge terminated graphene nanoribbons separated by a thin insulating barrier. In contrast to graphene monolayers, the multiple subbands of the nanoribbons are parabolic at low energy with a gap between the conduction and valence bands, and with lifted valley degeneracy. These properties make screening of the electron-hole interaction much weaker than for coupled electron-hole monolayers, thus boosting the pairing strength and enhancing the superfluid properties. The pairing strength is further boosted by the q…
Graphene is a truly two-dimensional atomic crystal with exceptional electronic and mechanical properties. Whereas conventional bulk and thin-film materials have been studied extensively, the key mechanical properties of graphene, such as tearing and cracking, remain unknown, partly due to its two-dimensional nature and ultimate single-atom-layer thickness, which result in the breakdown of conventional material models. By combining first-principles ReaxFF molecular dynamics and experimental studies, a bottom-up investigation of the tearing of graphene sheets from adhesive substrates is reported…
Three-dimensional topological insulators (TIs) host helical Dirac surface states at the interface with a trivial insulator. In quasi-one-dimensional TI nanoribbon structures the wave function of surface charges extends phase-coherently along the perimeter of the nanoribbon, resulting in a quantization of transverse surface modes. Furthermore, as the inherent spin-momentum locking results in a Berry phase offset of $π$ of self-interfering charge carriers an energy gap within the surface state dispersion appears and all states become spin-degenerate. We investigate and compare the magnetic field…
Graphene nanoribbons (GNRs) have attracted considerable interest as their atomically tunable structure makes them promising candidates for future electronic devices. However, obtaining detailed information about the length of GNRs has been challenging and typically relies on low-temperature scanning tunneling microscopy. Such methods are ill-suited for practical device application and characterization. In contrast, Raman spectroscopy is a sensitive method for the characterization of GNRs, in particular for investigating their width and structure. Here, we report on a length-dependent, Raman ac…
Graphene nanoribbons (GNRs) are a family of one-dimensional (1D) materials carved from graphene lattice. GNRs possess high mobility and current carrying capability, sizable bandgap, and versatile electronic properties tailored by the orientations and open edge structures. These unique properties make GNRs promising candidates for prospective electronics applications including nano-sized field-effect transistors (FETs), spintronic devices, and quantum information processing. To fully exploit the potential of GNRs, fundamental understanding of structure-property relationship, precise control of …
Graphene nanoribbons (GNRs) have recently been shown by Cao, Zhao, and Louie [Cao, T.; Zhao, F.; Louie, S. G. Phys. Rev. Lett. 2017, 119, 076401] to possess distinct topological phases in general, characterized by a Z2 invariant. Cove-edged and chevron GNRs moreover are chemically and structurally diverse, quasi-one-dimensional (1D) nanostructures whose structure and electronic properties can be rationally controlled by bottom-up synthesis from precursor molecules. We derive the value of the topological invariant of the different types of cove-edged and chevron GNRs, and we investigate the ele…
Spin-ordered electronic states in hydrogen-terminated zigzag nanographene give rise to magnetic quantum phenomena that have sparked renewed interest in carbon-based spintronics. Zigzag graphene nanoribbons (ZGNRs), quasi one-dimensional semiconducting strips of graphene featuring two parallel zigzag edges along the main axis of the ribbon, are predicted to host intrinsic electronic edge states that are ferromagnetically ordered along the edges of the ribbon and antiferromagnetically coupled across its width. Despite recent advances in the bottom-up synthesis of atomically-precise ZGNRs, their …
Graphene's isolation launched explorations of fundamental relativistic physics originating from the planar honeycomb lattice arrangement of the carbon atoms, and of potential technological applications in nanoscale electronics. Bottom-up fabricated atomically-precise segmented graphene nanoribbons, SGNRs, open avenues for studies of electrical transport, coherence, and interference effects in metallic, semiconducting, and mixed GNRs, with different edge terminations. Conceptual and practical understanding of electric transport through SGNRs is gained through nonequilibrium Green's function (NE…
Knowledge of the topology of the electronic ground state of materials has led to deep insights to novel phenomena such as the integer quantum Hall effect and fermion-number fractionalization, as well as other properties of matter. Joining two insulators of different topological classes produces fascinating boundary states in the band gap. Another exciting recent development is the bottom-up synthesis (from molecular precursors) of graphene nanoribbons (GNRs) with atomic precision control of their edge and width. Here we connect these two fields, and show for the first time that semiconducting …
Atomic-scale imaging of graphene nanoribbons on graphene after polymer-free substrate transfer
Universidad de Granada（格拉纳达大学）；The Pennsylvania State University（宾夕法尼亚州立大学）；Max Planck Institute for Polymer Research（马克斯·普朗克聚合物研究所）；University of Bern（伯尔尼大学）；Johannes Gutenberg University Mainz（美因茨约翰内斯·古腾堡大学）
Synthesis of Metal-Semiconductor Heterojunctions in Single Graphene Nanoribbons
University of California, Berkeley（加州大学伯克利分校）；Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室）；Kavli Energy NanoSciences Institute at the University of California Berkeley and the Lawrence Berkeley National Laboratory（加州大学伯克利分校与劳伦斯伯克利国家实验室卡弗里能源纳米科学研究所）；Department of Chemistry, Columbia University（哥伦比亚大学化学系）；Department of Physics, University of Illinois Urbana-Champaign（伊利诺伊大学厄巴纳-香槟分校物理系）；Department of Physics, Cornell University（康奈尔大学物理系）；Department of Materials Science and Nanoengineering, Rice University（莱斯大学材料科学与纳米工程系）
Bottom-up prepared carbon nanostructures appear as promising platforms for future carbon-based nanoelectronics, due to their atomically precise and versatile structure. An important breakthrough is the recent preparation of nanoporous graphene (NPG) as an ordered covalent array of graphene nanoribbons (GNRs). Within NPG, the GNRs may be thought of as 1D electronic nanochannels through which electrons preferentially move, highlighting NPG's potential for carbon nanocircuitry. However, the π-conjugated bonds bridging the GNRs give rise to electronic cross-talk between the individual 1D channels,…
