Controlled growth of a line defect in graphene and implications for gate-tunable valley filtering

Abstract

Atomically precise tailoring of graphene can enable unusual transport pathways and new nanometer-scale functional devices. Here we describe a recipe for the controlled production of highly regular "5-5-8" line defects in graphene by means of simultaneous electron irradiation and Joule heating by applied electric current. High-resolution transmission electron microscopy reveals individual steps of the growth process. Extending earlier theoretical work suggesting valley-discriminating capabilities of a graphene 5-5-8 line defect, we perform firstprinciples calculations of transport and find a strong energy dependence of valley polarization of the charge carriers across the defect. These findings inspire us to propose a compact electrostatically gated "valley valve" device, a critical component for valleytronics.2 Atomically-precise modification of low-dimensional materials such as graphene is exceedingly challenging since existing experimental techniques rarely achieve atomic precision. Nevertheless, if successful, atomic manipulations could have dramatic impact on graphene's electrical, magnetic, optical, mechanical, chemical, and thermal properties [1][2][3][4] , leading to novel functionalities that could be exploited in nanoscale devices.A recently emerging field is "valleytronics", a zero-magnetic-field analog to spintronics which exploits the quantum mechanical "valley" degree of freedom of charge carriers in graphene 2, 5-10 .At low energies the band structure of single-layer graphene is composed of two energetically degenerate valleys ("Dirac cones"), separated by ~30 nm -1 . 11 The intervalley coupling is quite weak in high quality graphene even at room temperature 12,13 , and hence this additional degree of freedom is a good quantum number. Valley polarization could be used for information processing much as the electron spin degree of freedom is used in spintronics, with the added benefit of temperature insensitivity.Generally, two approaches have been suggested for lifting the degeneracy and thus achieving graphene valley polarization: 1) application of external magnetic field, and 2) using local modifications of the crystalline lattice. The growth process of the 5-5-8 linear defect is intriguing, and we examine it in some detail here. The metastable 5-6 pair generated at the growth-leading end is the key to the growth mechanism. Figures 3a-3c illustrate critical formation steps as determined by TEM. Each experimental image is constructed from an average of 12 single shot TEM images taken in rapid succession to reduce the background noise and to include all possible configurations of the defect. As the line defect grows by one octagon, one carbon atom (marked by a blue dot in the illustrations) is ejected, and a new bond is formed between its nearest neighbors (marked by yellow dots). This process also creates a new 5-6 termination pair, which serves as a seed for continued growth. Since an 6 isolated pentagon cannot be sustained in otherwise ideal graphene 23 , an extended structural irre...

Bilayer graphene formed by passage of current through graphite: evidence for a three-dimensional structure

“…We do not rule out the possibility that sublimation/condensation may play some role in the structural transformation, but suggest that it is probably not the major factor. We note that recent in situ Joule heating experiments have shown that carbon sublimation can result in the formation of certain line defects in graphene [18].…”

Magnetoresistance of Mn-decorated topological line defects in graphene

“…Though it is not straightforward to realize this system, because line defects have to be created in a controlled fashion and then have to be decorated with the transition metal, the situation does not look hopeless. Pristine line defects have been observed in as-grown graphene [5] and can also be induced by electron irradiation [6,7]. In addition, substantial preferential bonding of the Mn atoms at octagonal hollow sites overcomes the problem that transition-metal atoms usually are very mobile on graphene.…”

“…This defect with fractional chirality was predicted previously for nanotubes [4] and, referring to the arrangement of the rings, is called 8-5-5-8 topological line defect. It has been observed in as-grown graphene [5] and can also be induced in a controlled fashion by electron irradiation [6,7]. In general, the intentional introduction of defects into graphene is a topic that currently receives enormous attention, because it can lead to a multitude of applications [8][9][10][11][12][13][14].…”

Polycrystalline graphene and other two-dimensional materials

“…1f) can be viewed as a degenerate GB as it has zero misorientation angle. Because of its topologically trivial structure, this defect can be engineered in a controlled way, as demonstrated by Chen et al 46 . Another work observed a different line defect in graphene oriented along the armchair direction 47 .…”