(Left) Scanning tunnelling microscope image of bottom-up zigzag graphene nanoribbons. (Right) Spin-density in the vicinity of a 'bite' defect in a zigzag graphene nanoribbon. Image: Empa/EPFL (adapted with permission from J. Phys. Chem. Lett. 2021,12, 4692-4696, Copyright 2021 American Chemical Society).Graphene nanoribbons (GNRs) are narrow strips of single-layer graphene that possess interesting physical, electrical, thermal and optical properties because of the interplay between their crystal and electronic structures. These novel characteristics have pushed GNRs to the forefront in the search for ways to advance next-generation nanotechnologies.
While bottom-up fabrication techniques now allow the controlled synthesis of a broad range of graphene nanoribbons that feature various edge geometries, widths and other atoms, the question of whether or not structural disorder is present in these atomically precise GNRs, and to what extent, is still subject to debate. The answer to this riddle is of critical importance for any potential applications or resulting devices.
A collaboration between Oleg Yazyev's Chair of Computational Condensed Matter Physics theory group at the Ecole Polytechnique Fédérale de Lausanne (EPFL) and Roman Fasel's experimental nanotech@surfaces laboratory at the Swiss Federal Laboratories for Materials Science and Technology (Empa), both in Switzerland, has now produced two papers that look at this issue in armchair-edged and zigzag-edged graphene nanoribbons.
"In these two works, we focused on characterizing 'bite-defects' in graphene nanoribbons and their implications on GNR properties," explains Gabriela Borin Barin from Empa's nanotech@surfaces lab. "We observed that even though the presence of these defects can disrupt GNRs' electronic transport, they could also yield spin-polarized currents. These are important findings in the context of the potential applications of GNRs in nanoelectronics and quantum technology."
The paper in 2D Materials specifically looks at nine-carbon-atom-wide armchair graphene nanoribbons (9-AGNRs). The mechanical robustness, long-term stability under ambient conditions, easy transferability onto target substrates, scalability of fabrication and suitable band-gap width of these GNRs has made them one of the most promising candidates for integration as active channels in field-effect transistors (FETs). Indeed, among the graphene-based electronic devices realized so far, 9-AGNR-FETs display the highest performance.
The detrimental role of defects in GNRs on electronic devices is well known. But so-called Schottky barriers, which are potential energy barriers for electrons formed at metal-semiconductor junctions, both limit the performance of current GNR-FETs and also prevent experimental characterization of the impact of defects on device performance. In the 2D Materials paper, the researchers report combining experimental and theoretical approaches to investigate defects in bottom-up AGNRs.
Using scanning-tunnelling and atomic-force microscopies, the researchers were first able to determine that missing benzene rings at the edges are a very common defect in 9-AGNRs, and to estimate both the density and spatial distribution of these imperfections, which they have dubbed 'bite' defects. They quantified the density and found that these defects have a strong tendency to aggregate. Using first-principles calculations, they then explored the effect of such defects on quantum charge transport, finding that these imperfections significantly disrupt charge transport at the band edges by reducing conductance.
By generalizing these theoretical findings to wider nanoribbons in a systematic manner, the researchers were able to establish practical guidelines for minimizing the detrimental role of these defects on charge transport, an instrumental step towards the realization of novel carbon-based electronic devices.
In a paper in the Journal of Physical Chemistry Letters, the same team of researchers reports combining scanning probe microscopy experiments and first-principles calculations to examine structural disorder and its effect on magnetism and electronic transport in so-called bottom-up zigzag GNRs (ZGNRs).
ZGNRs are unique because of their unconventional metal-free magnetic order that, according to predictions, is preserved up to room temperature. They possess magnetic moments that are coupled ferromagnetically along their edges and antiferromagnetically across them, and it has been shown that their electronic and magnetic structures can be modulated to a large extent, such as via charge doping, electric fields, lattice deformations or defect engineering.
This combination of tunable magnetic correlations, sizable band gap width and weak spin-orbit interactions has made these ZGNRs promising candidates for spin-logic operations. This study specifically looked at six-carbon-atom-wide zigzag graphene nanoribbons (6-ZGNRs), the only width of ZGNRs that has been produced with a bottom-up approach so far.
Again using scanning-tunnelling and atomic-force microscopies, the researchers first identified the presence of ubiquitous carbon vacancy defects located at the edges of the nanoribbons and then resolved their atomic structure. Their results indicated that each vacancy comprises a missing m-xylene unit, producing a similar 'bite' defect to those seen in AGNRs. This defect is created by the scission of carbon-carbon bonds during the synthesis reaction. The researchers estimate that the density of 'bite' defects in 6-ZGNRs is larger than in bottom-up AGNRs.
The researchers again theoretically examined the effect of these bite defects on the electronic structure and quantum transport properties of 6-ZGNRs. Similar to the case with AGNRs, they found that the defects cause a significant disruption to the conductance. However, in this nanostructure, these unintentional defects also induce sublattice and spin imbalance, causing a local magnetic moment. This, in turn, gives rise to spin-polarized charge transport that makes defective zigzag nanoribbons optimally suited for applications in all-carbon logic spintronics at the ultimate limit of scalability.
A comparison between ZGNRs and AGNRs of equal width shows that transport across the former is less sensitive to the introduction of both single and multiple defects than the latter. Overall, this research provides a global picture of the impact of these ubiquitous 'bite' defects on the low-energy electronic structure of bottom-up graphene nanoribbons. According to the researchers, future research might focus on investigating other types of point defects experimentally observed at the edges of such nanoribbons.
This story is adapted from material from Empa, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.