A Electron transport study on ultrathin armchair graphene nanoribbone (original) (raw)
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Molecular bandgap engineering of bottom-up synthesized graphene nanoribbon heterojunctions
Nature Nanotechnology, 2015
Bandgap engineering is used to create semiconductor heterostructure devices that perform processes such as resonant tunnelling 1,2 and solar energy conversion 3,4 . However, the performance of such devices degrades as their size is reduced 5,6 . Graphene-based molecular electronics has emerged as a candidate to enable high performance down to the single-molecule scale 7-17 . Graphene nanoribbons, for example, can have widths of less than 2 nm and bandgaps that are tunable via their width and symmetry . It has been predicted that bandgap engineering within a single graphene nanoribbon may be achieved by varying the width of covalently bonded segments within the nanoribbon 20-22 . Here, we demonstrate the bottom-up synthesis of such width-modulated armchair graphene nanoribbon heterostructures, obtained by fusing segments made from two different molecular building blocks. We study these heterojunctions at subnanometre length scales with scanning tunnelling microscopy and spectroscopy, and identify their spatially modulated electronic structure, demonstrating molecular-scale bandgap engineering, including type I heterojunction behaviour. First-principles calculations support these findings and provide insight into the microscopic electronic structure of bandgap-engineered graphene nanoribbon heterojunctions.
Electronic structure changes during the surface-assisted formation of a graphene nanoribbon
The Journal of Chemical Physics, 2014
High conductivity and a tunability of the band gap make quasi-one-dimensional graphene nanoribbons (GNRs) highly interesting materials for the use in field effect transistors. Especially bottom-up fabricated GNRs possess well-defined edges which is important for the electronic structure and accordingly the band gap. In this study we investigate the formation of a sub-nanometer wide armchair GNR generated on a Au(111) surface. The on-surface synthesis is thermally activated and involves an intermediate non-aromatic polymer in which the molecular precursor forms polyanthrylene chains. Employing angle-resolved two-photon photoemission in combination with density functional theory calculations we find that the polymer exhibits two dispersing states which we attribute to the valence and the conduction band, respectively. While the band gap of the non-aromatic polymer obtained in this way is relatively large, namely 5.25 ± 0.06 eV, the gap of the corresponding aromatic GNR is strongly reduced which we attribute to the different degree of electron delocalization in the two systems.
Electronic Structure of Spatially Aligned Graphene Nanoribbons on Au(788)
Physical Review Letters, 2012
We report on a bottom-up approach of the selective and precise growth of subnanometer wide straight and chevron-type armchair nanoribbons (GNRs) on a stepped Au(788) surface using different specific molecular precursors. This process creates spatially well-aligned GNRs, as characterized by STM. High-resolution direct and inverse photoemission spectroscopy of occupied and unoccupied states allows the determination of the energetic position and momentum dispersion of electronic states revealing the existence of band gaps of several electron volts for straight 7-armchair, 13-armchair, and chevron-type GNRs in the electronic structure.
Electron transport channels and their manipulation by impurity in armchair-edge graphene nanoribbons
Carbon, 2014
Under the scheme of the nonequilibrium Green's function combined with the tight-binding approximation, we study electron transport properties in different atomic chains of an armchair-edged graphene nanoribbon (AGNR) and their manipulation using a single substitutional impurity atom. By calculation and analysis of the local bond currents between nearest atom sites in the AGNR, we find that electron transport along two armchair-edged chains is more active than that along other chains for any clean AGNR. For a metallic AGNR, interestingly, there exists a series of parallel distributed major channels for the low-energy electron transport. Further, the transport properties of these channels can be manipulated by a single substitutional impurity atom with different strength and locating position, e.g., a suitable impurity can cause a selected channel to be closed completely while others still open. However, in the high-energy regime these independent channels disappear, and a metallic AGNR becomes entirely metallic in this case. The findings here suggest that an AGNR may be used as a multi-channel plane material in the future nanoelectronic technology.
2007
We report combined first-principle and tight-binding (TB) calculations to simulate the effects of chemical edge modifications on structural and electronic properties. The C-C bond lengths and bond angles near the GNR edge have considerable changes when edge carbon atoms are bounded to different atoms. By introducing a phenomenological hopping parameter t1t_{1}t1 for nearest-neighboring hopping to represent various chemical edge modifications, we investigated the electronic structural changes of nanoribbons with different widths based on the tight-binding scheme. Theoretical results show that addends can change the band structures of armchair GNRs and even result in observable metal-to-insulator transition.
Carbon, 2017
Recent advances in bottom-up production of atomically precise armchair graphene nanoribbons (AGNRs) and their structural and electronic characterization through scanning tunneling microscopy (STM) and spectroscopy (STS) present an opportunity and a challenge for their interpretation and inter-correlation, especially in view of several seemingly conflicting results for their electron distribution and gap size, sometimes by more than 300%. Such large discrepancies, which threaten to undermine the extraordinary achievements of their synthesis, are threefold: Experiment vs. theory; experiment vs. experiment; and theory vs. theory. Here we illustrate that by using many-body corrections through time-dependent (TD) density functional theory (DFT), and proper identification of the STS gap, we can reproduce all known, and predict new as yet unknown, experimental data for such AGNRs. Furthermore, we can rationalize and suggest ways to reconcile practically all known ACCEPTED MANUSCRIPT 2 discrepancies. We demonstrate that besides the width measured by the number N of carbon atoms across, the length and the length-variation of the gap properties, which reveal a semiconductor-metal transition, is an important factor which is usually overlooked in the literature. This, together with inherent problems of DFT for accurate gap determination, on top of experimental STS difficulties, are the main sources of such discrepancies.
Electronic Structure Evolution during the Growth of Graphene Nanoribbons on Au(110)
2017
Surface-assisted polymerization of molecular monomers into extended chains can be used as the seed of graphene nanoribbon (GNR) formation, resulting from a subsequent cyclo-dehydrogenation process. By means of valence-band photoemission and ab-initio density-functional theory (DFT) calculations, we investigate the evolution of molecular states from monomer 10,10'-dibromo-9,9'bianthracene (DBBA) precursors to polyanthryl polymers, and eventually to GNRs, as driven by the Au(110) surface. The molecular orbitals and the energy level alignment at the metal-organic interface are studied in depth for the DBBA precursors deposited at room temperature. On this basis, we can identify a spectral fingerprint of C-Au interaction in both DBBA single-layer and polymerized chains obtained upon heating. Furthermore, DFT calculations help us evidencing that GNRs interact more strongly than DBBA and polyanthryl with the Au(110) substrate, as a result of their flatter conformation.