“Mutation is the key to unraveling the mystery of evolution. The path of development from the simplest organism to the dominant biological species lasts for millennia. But every hundred thousand years there is a leap forward in evolution” (Charles Xavier, X-Men, 2000). If we discard all the sci-fi elements that are present in comics and films, then the words of Professor X are quite true. The development of something proceeds evenly most of the time, but sometimes there are jumps that have a huge impact on the whole process. This applies not only to the evolution of species, but also to the evolution of technology, the main engine of which is people, their research and inventions. Today we will get acquainted with the study, which, according to its authors, is a real evolutionary leap in nanotechnology. How did scientists from Northwestern University (USA) manage to create a new two-dimensional heterostructure, why were graphene and borophene chosen as the basis, and what properties can such a system have? The report of the research group will tell us about this. Go.
Research basis
We have heard the term "graphene" many times already - this is a two-dimensional modification of carbon, consisting of a layer of carbon atoms 1 atom thick. But "borofen" is extremely rare. This term refers to a two-dimensional crystal consisting solely of boron (B) atoms. For the first time, the possibility of the existence of borophene was predicted back in the mid-90s, but in practice this structure was obtained only by 2015.
The atomic structure of borophene consists of triangular and hexagonal elements and is a consequence of the interaction between two-center and multicenter intraplanar bonds, which is very typical for elements with an electron deficit, to which boron belongs.
*By two-center and multicenter bonds are meant chemical bonds - interactions of atoms that characterize the stability of a molecule or crystal as a single structure. For example, a two-center two-electron bond occurs when 2 atoms share 2 electrons, and a two-center three-electron bond occurs when 2 atoms and 3 electrons, etc.
From a physical point of view, borophene may be more durable and flexible than graphene. It is also believed that borophene structures can be an effective addition to batteries, as borophene has a high specific capacitance and unique electronic conduction and ion transport properties. However, this is just a theory at the moment.
Being trivalent element*, boron has at least 10 allotropes*. In a two-dimensional form, similar polymorphism* is also observed.
Trivalent element* is able to form three covalent bonds, the valency of which is three.
Allotropy* - when one chemical element can be represented in the form of two or more simple substances. As an example, carbon is diamond, graphene, graphite, carbon nanotubes, etc.
Polymorphism* - the ability of a substance to exist in different crystal structures (polymorphic modifications). In the case of simple substances, this term is synonymous with allotropy.
Given such a wide polymorphism, there is a suggestion that borophene may be an excellent candidate for creating new two-dimensional heterostructures, since various configurations of boron bonds should weaken the requirements for crystal lattice matching. Unfortunately, earlier this issue was studied exclusively at the theoretical level due to the difficulties in the synthesis.
For conventional 2D materials obtained from bulk layered crystals, vertical heterostructures can be realized using mechanical stacking. On the other hand, two-dimensional lateral heterostructures are based on bottom-up synthesis. Atomically precise lateral heterostructures have great potential in solving heterojunction functionality control problems, however, due to covalent bonding, imperfect lattice matching typically results in wide and disordered interfaces. Therefore, there is potential, but there are also problems in its implementation.
In this work, the researchers succeeded in integrating borophene and graphene into one two-dimensional heterostructure. Despite the mismatch of crystallographic lattices and symmetries between borophene and graphene, successive deposition of carbon and boron on an Ag(111) substrate in ultrahigh vacuum (UHV) results in almost atomically precise lateral heterointerfaces with predictable lattice alignments, as well as vertical heterointerfaces.
Preparation for research
Before studying the heterostructure, it had to be fabricated. The growth of graphene and borophene was carried out in an ultrahigh vacuum chamber with a pressure of 1x10-10 millibars.
The Ag(111) single-crystal substrate was cleaned by repeated cycles of Ar+ sputtering (1 x 10-5 mbar, energy 800 eV, 30 minutes) and thermal annealing (550 °C, 45 minutes) until an atomically clean and flat Ag(111) surface was obtained. .
Graphene was grown by electron-beam evaporation of a pure (99,997%) graphite rod 2.0 mm in diameter on an Ag (750) substrate heated to 111 °C at a heating current of ~ 1.6 A and an accelerating voltage of ~ 2 kV, which gives an emission current of ~ 70 mA and carbon flux ~40 nA. The pressure in the chamber was 1 x 10-9 millibars.
Borophene was grown by electron-beam evaporation of pure (99,9999%) boron rod on submonolayer graphene on Ag (400) heated to 500–111°C. The filament current was ~1.5 A and the accelerating voltage was 1.75 kV, which gives an emission current of ~34 mA and a boron flux of ~10 nA. The pressure in the chamber during the growth of borophene was about 2 x 10-10 millibars.
Results of the study
Image #1
On the image 1А shown STM* snapshot of grown graphene, where the graphene domains are best visualized with a map dI/dV (1V), Where I и V are the tunnel current and sample displacement, and d - density.
STM* – scanning tunneling microscope.
dI/dV sample maps made it possible to see a higher local density of states of graphene compared to the Ag(111) substrate. In accordance with previous studies, the surface state of Ag (111) has a stepped characteristic shifted towards positive energies by dI/dV graphene spectrum (1S), which explains the higher local density of states of graphene on 1V at 0.3 eV.
On the image 1D we can see the structure of single layer graphene where the honeycomb lattice is clearly visible and moiré superstructure*.
Superstructure* - a feature of the structure of a crystalline compound, which is repeated at a certain interval and thus creates a new structure with a different alternation period.
Moire* - the imposition of two periodic mesh patterns on top of each other.
At lower temperatures, growth leads to the formation of dendritic and defective graphene domains. Due to weak interactions between graphene and the underlying substrate, the rotational alignment of graphene relative to the underlying Ag(111) is not unique.
After boron deposition, scanning tunneling microscopy (1E) showed the presence of a combination of borophene and graphene domains. The image also shows regions inside graphene, which were later identified as graphene intercalated by borophene (indicated in the image Gr/B). Linear features are also clearly visible in this area, oriented in three directions and separated by an angle of 120° (yellow arrows).
Image #2
Snapshot on 2АAs 1E, confirm the appearance of localized dark depressions (depressions) in graphene after boron deposition.
In order to better examine these formations and find out their origin, another image was taken of the same area, but using maps |dlnI/dz| (2B), where I - tunnel current, d is the density, and z — tip-sample separation (gap between microscope needle and sample). The use of this technique makes it possible to obtain images with a high spatial resolution. You can also apply CO or H2 on the microscope needle for this.
Picture 2S is an STM image whose needle was coated with CO. Image comparison А, В и С shows that all atomic elements are defined as three adjacent bright hexagons pointing in two non-equivalent directions (red and yellow triangles in the images).
Enlarged images of this area (2D) confirm that these elements are in agreement with the boron dopants occupying two graphene sublattices, as indicated by the superimposed structures.
The CO coating of the microscope tip made it possible to reveal the geometric structure of the borophene sheet (2E), which would be impossible if the needle was standard (metal) without CO coating.
Image #3
Formation of lateral heterointerfaces between borophene and graphene (3А) should occur when borophene grows near graphene domains that already contain boron.
Scientists remind that lateral heterointerfaces based on graphene-hBN (graphene + boron nitride) have lattice consistency, and heterojunctions based on transition metal dichalcogenides have symmetry consistency. In the case of graphene/borophene, the situation is slightly different - they have minimal structural similarity in terms of lattice constants or crystal symmetry. However, despite this, the graphene/borophene lateral heterointerface demonstrates almost perfect atomic consistency, with the directions of the boron row (B-row) aligned with the zigzag (ZZ) directions of graphene (3А) On the 3V an enlarged image of the ZZ region of the heterointerface is shown (blue lines mark interfacial elements corresponding to boron-carbon covalent bonds).
Since the growth of borophene occurs at a lower temperature compared to graphene, the edges of the graphene domain are unlikely to have high mobility during the formation of a heterointerface with borophene. Therefore, the almost atomically precise heterointerface is likely the result of various configurations and characteristics of boron's multicenter bonds. Scanning tunneling spectroscopy spectra (3S) and differential tunneling conductivity (3D) show that the electronic transition from graphene to borophene occurs at a distance of ~5 Å without visible interface states.
On the image 3E three scanning tunneling spectroscopy spectra taken along the three dotted lines in 3D are shown, which confirm that this short electronic transition is insensitive to local interfacial structures and is comparable to that at the borophene-silver interfaces.
Image #4
Graphene intercalation* previously also extensively researched, however, the transformation of intercalants into true 2D sheets is relatively rare.
Intercalation* - reversible inclusion of a molecule or group of molecules between other molecules or groups of molecules.
The small atomic radius of boron and the weak interaction between graphene and Ag (111) suggest a possible intercalation of graphene with boron. On the image 4А evidence is presented not only for boron intercalation, but also for the formation of vertical borophene-graphene heterostructures, especially triangular domains surrounded by graphene. The honeycomb lattice observed on this triangular domain confirms the presence of graphene. However, this graphene exhibits a lower local density of states at -50 meV compared to the surrounding graphene (4V). Compared to graphene directly on Ag (111), the absence of signs of a high local density of states in the spectrum dI/dV (4C, blue curve) corresponding to the Ag(111) surface state is the first evidence of boron intercalation.
Also, as expected for partial intercalation, the graphene lattice remains continuous along the entire lateral interface between graphene and the triangular region (4D - corresponds to a rectangular area on 4Аcircled in red dotted line). A picture using CO on the microscope needle also confirmed the presence of substituting boron impurities (4E - corresponds to a rectangular area on 4Аcircled in yellow).
During the analysis, microscope needles without any coating were also used. In this case, signs of one-dimensional linear elements with a periodicity of 5 Å were revealed in the intercalated graphene domains (4F и 4G). These one-dimensional structures resemble the boron series in the borophene model. In addition to the set of points corresponding to graphene, the Fourier transform of the image on 4G displays a pair of orthogonal dots corresponding to a 3 Å x 5 Å rectangular lattice (4Н), which is in excellent agreement with the borophene model. In addition, the observed triple orientation of the array of linear elements (1E) agrees well with the same predominant structure observed for borophene sheets.
All these observations provide strong evidence for the intercalation of graphene by borophene near the Ag edges, which consequently leads to the formation of vertical borophene-graphene heterostructures, which can be predominantly realized by increasing the initial coating of graphene.
4I is a schematic representation of a vertical heterostructure on 4H, where the direction of the boron row (pink arrow) is closely aligned with the zigzag direction of graphene (black arrow), thus forming a rotationally commensurate vertical heterostructure.
For a more detailed acquaintance with the nuances of the study, I recommend looking at и to him.
Finale
This study showed that borophene is quite capable of forming lateral and vertical heterostructures with graphene. Such systems can be used in the development of new types of two-dimensional elements used in nanotechnology, flexible and wearable electronics, as well as in new types of semiconductors.
The researchers themselves believe that their development can be a powerful push forward for technologies related to electronics. However, it is still difficult to say for sure that their words will become prophetic. At the moment, there is still a lot to be explored, understood and invented so that those science fiction ideas that fill the minds of scientists become a full-fledged reality.
Thank you for your attention, stay curious and have a good week everyone guys. 🙂
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Source: habr.com