Scientists at Empa and EPFL, Switzerland, have identified a new type of defect as the
most common source of disorder in on-surface synthesized graphene nanoribbons, a novel
class of carbon-based materials that may prove extremely useful in next-generation electronic
devices.
The researchers identified the atomic structure of these so-called "bite" defects and investigated
their effect on quantum electronic transport.
These kinds of defective zigzag-edged nanoribbons may provide suitable platforms for certain
applications in spintronics.
Graphene nanoribbons (GNRs), narrow strips of single-layer graphene, have interesting
physical, electrical, thermal, and optical properties because of the interplay between
their crystal and electronic structures.
These novel characteristics have pushed them to the forefront in the search for ways to
advance next-generation nanotechnologies.
While bottom-up fabrication techniques now allow the synthesis of a broad range of graphene
nanoribbons that feature well-defined edge geometries, widths, and heteroatom incorporations,
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 to any potential applications or resulting
devices.
Collaboration between researchers at EPFL and Empa has produced two papers that look
at this issue in armchair-edged and zigzag-edged graphene nanoribbons.
The researchers say that in these two works, they focused on characterizing "bite-defects"
in graphene nanoribbons and their implications on GNR properties.
The researchers 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 “Quantum electronic transport across “bite” defects in graphene nanoribbons,”
recently published in the journal 2D Materials, specifically looks at 9-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.
While the detrimental role of defects on electronic devices is well known, Schottky barriers,
potential energy barriers for electrons formed at metal–semiconductor junctions, both limit
the performance of current GNR-FETs and prevent experimental characterization of the impact
of defects on device performance.
In the 2D Materials paper, the researchers combine experimental and theoretical approaches
to investigate defects in bottom-up AGNRs.
Scanning-tunnelling and atomic-force microscopies first allowed the researchers to identify
missing benzene rings at the edges as 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 they have a strong tendency to aggregate.
The researchers then used first-principles calculations to explore the effect of such
defects on quantum charge transport, finding that these imperfections significantly disrupt
it at the band edges by reducing conductance.
These theoretical findings are then generalized to wider nanoribbons in a systematic manner,
allowing the researchers 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 the paper “Edge disorder in bottom-up zigzag graphene nanoribbons: implications
for magnetism and quantum electronic transport,” recently published in The Journal of Physical
Chemistry Letters, the same team of researchers combines 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 the edge and antiferromagnetically
across it and it has been shown that the electronic and magnetic structures can be modulated to
a large extent by, for example, charge doping, electric fields, lattice deformations, or
defect engineering.
The combination of tunable magnetic correlations, sizable band gap width and weak spin−orbit
interactions has made these ZGNRs promising candidates for spin logic operations.
The study specifically looks at six-carbon zigzag lines wide graphene nanoribbons (6-ZGNRs),
the only width of ZGNRs that has been achieved with a bottom-up approach so far.
Again using scanning-tunnelling and atomic-force microscopies, the researchers first identify
the presence of ubiquitous carbon vacancy defects located at the edges of the nanoribbons
and then resolve their atomic structure.
Their results indicate that each vacancy comprises a missing m-xylene unit, that is, another
“bite” defect, which, as with those seen in AGNRs, comes from the scission of the C−C
bond that occurs during the cyclodehydrogenation process of the reaction.
Researchers estimate the density of “bite” defects in the 6-ZGNRs to be larger than that
of the equivalent defects in bottom-up AGNRs.
The effect of these bite defects on the electronic structure and quantum transport properties
of 6-ZGNRs is again examined theoretically.
They find that the introduction of the defect, similarly to AGNRs, causes a significant disruption
of the conductance.
Furthermore, in this nanostructure, these unintentional defects 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 in 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 in the latter.
Overall, the research provides a global picture of the impact of these ubiquitous “bite”
defects on the low-energy electronic structure of bottom-up graphene nanoribbons.
Future research might focus on the investigation of other types of point defects experimentally
observed at the edges of such nanoribbons.