Mechanism of Defect Formation and Defect-Driven Growth of Two-Dimensional Materials

Sequential atomic resolution images of monolayer hBN showing how the shape and orientation of the holes are preserved after further growth. AR-TEM image of the same region as in Figure 28a after tens of seconds of electron beam irradiation. AR-TEM image from Figure 28a and simulated images of ABA and ABC stacking configurations.

Intensity profiles along the blue and red lines in the experimental image (left) and two simulated images (middle and right) of the 6′6′-N and 44-B structures, respectively. Intensity profiles along the AA′/AB stacking boundary in experimental images (purple), simulated images of the 6′6′ (red) and 558 (olive) configurations.

Introduction

Second, the formation of interlaced double-helix few-layer hBN driven by screw dislocations is reported. Pairs of helical dislocations initiated at the anti-phase boundaries (APBs) of monolayer domains form double helical structure. Unlike other 2D materials with single-helical structures, the double-helical structure enables the interlaced h-BN layers to maintain the most stable AA′ stacking configuration.

This study reveals the unique double helical growth of 2D hBN multilayers synthesized in a chemical vapor deposition (CVD) chamber. And finally, I will discuss the one-dimensional hBN conduction channel with 6'6' configuration and 558 embedded in AA'/AB stacking boundaries of few-layer hBN.

Research background

Aberration corrected transmission electron microscopy

Hole defects on 2D materials induced by electron beam irradiation

• Formation of defect by electron beam and knock-on thresholds of graphene, hBN and
• Hole defects on graphene
• Hole defects on hBN
• Hole defects on MoS 2

The size of the hole defect can be controlled by the irradiation time of the electron beam at a certain acceleration voltage. The graphene hole defect has a mixed armchair and zigzag edge atomic configuration. It gives a perspective in controlling the graphene hole defect edge for graphene nanopore devices, although the contamination effect still remains an issue to be resolved.

Meanwhile, high reactivity of the edges of graphene hole defect has been a major obstacle for the realization of graphene nanopore devices. The hole defect formed by prolonged electron beam irradiation at Figure 4a is more clearly defined as triangular shape (Figure 4b).

Stacking structure and Stacking boundary of hBN

• Stacking structures of hBN
• Identification of the stacking structure and the number of layers of hBN using DF-TEM
• Transition region at stacking boundary

The intensity of the DF-TEM image depends on the interference of electron waves produced by the specific lattice periodicity corresponding to the diffraction spot used in acquiring the DF-TEM image. As shown in Figure 7, for AA′ stacking, which has no lateral translation, the intensity of the families of first (Φ1) and second (Φ2) order diffraction spots increases with the number of layers, as the electron wave from one layer diffraction. layer always constructively interferes with the wave deflected from the other layers. For ABC stacking, the Ø1 diffraction spots exhibit completely destructive interference since the phases of the electron wave scattered by the AB and AC stacking are opposite101-102.

As discussed in the previous section, the bright contrast in the DF-TEM image implies constructive interference of waves diffracted from a given grating. The opposite directions of the red contours divided by the black dashed line indicate the phase change between [BN] and [NB]; the orientations of the triangular defects are in opposite directions in the [BN] and [NB] structures, since the electron beam always creates N-terminated triangular defects.

Atomic-scale dynamics of triangular hole growth in monolayer hBN

• Introduction
• Experimental section
• Results and discussion
• Conclusion

As summarized in Figure 11, the growth of a triangular hole appeared to be initiated by the removal of B and N atoms near the centers of the hole edges. As shown in Figure 14b, breaking a B-N bond perpendicular to the edge of a hole generated a chain of B and N atoms, as indicated by the yellow dotted box. Electron beam irradiation of the triangular hole (Figure 14c) resulted in the formation of a single chain consisting of B and N atoms (Figure 14d).

Subsequent spreading of the fluctuating region resulted in the formation of single chains of B and N atoms in the hole (Figure 15f), followed by the development of two triangular holes (Figure 15g). Such processes involve breaking B-N bonds perpendicular to the edges of the hole and result in the formation of single chains containing B and N atoms.

Screw dislocation-driven growth of double-spiral hBN

• Introduction
• Experimental Section
• Results and discussion
• Synthesis and characterization of double-spiral hBN
• Growth mechanism of screw dislocation-driven double spiral hBN
• Shear strain between hBN spiral clusters
• Conclusion

Because the triangular hole defects of hBN have N-terminated edges, the opposite orientation of the triangular defects between adjacent layers indicates that the hBN layers were AA′-stacked (i.e., [BN]/[NB]/[BN]/ [NB] Figure 1E), which is the most energetically favorable stacking configuration in bulk hBN (37-38). This is possible thanks to the unique double spiral system initiated at the APB. Figure 20b,c shows the characteristics of the triangular defects corresponding to the upper and lower growth respectively, including the layer boundary of the bilayer and the trilayer, for a comparison of the orientation of the triangular defects in the AR-TEM image in figure 20d.

The orientations of the purple triangles above the layer boundary are the same, demonstrating the highest growth of the hBN layer. Characteristically, hBN helical islands cluster along defect lines due to numerous screw dislocations located along the APB (Fig. 17a). An AR-TEM image of the white rectangular area including the dark line in Fig. 22a is shown in Fig. 22b.

By fitting atomic models with different shear strain widths and magnitudes (Figures 23 and Figure 24), I replicated the experimental dimensions of the strained region using a shear strain value of 1 hBN unit cell (2.50 Å) on the top layer of the AA'-stacked hBN bilayer, with width of about 10 nm along the zigzag direction (Fig. 22d). The region highlighted by red dotted lines along the zigzag direction shows a square moiré pattern instead of the original hexagonal hBN lattice with AA' stacking configuration. ΔEED = G × (a/b)2 × (h × b × a) layers in hBN.

The relationship between the stride distance and ΔEvdW. The value of α can then be estimated as the average of the vdW energy difference with the step distance:. 3), the total increase in energy attributed to the shear deformation can be obtained from b, i.e. the width of the shear region (Figure 22f). This study provides understanding of the growth mechanism of helical hBN multilayers and explains the shear strain between helical clusters.

Atomically sharp AA′/AB stacking boundary of hBN as one-dimensional conducting

Introduction

Experimental section

To elucidate the atomic and electronic structure of the twin boundary of BN nanoribbons, DFT calculation was performed within the generalized gradient approximation (GGA) using VASP183-185. For the Brillouin zone interaction, a (9x1x1) and (20x1x1) lattice was used for atomic relaxation and calculation of band states in the Gamma-centered special k-point scheme, respectively. To investigate the temperature dependence of the structural deformation behaviors of the twin boundary of BN nanoribbons, MD simulations were performed at temperatures of 10 ~ 1000 K.

Results and discussion

• Synthesized few-layer films of hBN with AA′ and AB stacking structures
• Twin boundary at the AA′ and AB stacking boundaries
• Stability of exposed and sandwiched 6′6′ twin boundary
• Formation mechanism of atomically sharp twin boundaries and EK edges

Magnified atomic images on each side of the insets in Fig. 28a clearly show the AA′ and AB stacking structures on the left and right sides of the image. The colors change from green to blue as the number of layers decreases from 3 L to 0 L (vacuum) in each of the stacked regions AA′ and AB. The deduction of atomic configurations in Fig. 28d,e starts with four possible structures of the AA′A/ABA stacking boundary along the zigzag direction (Fig. 31).

Therefore, an atomically sharp twin boundary, with N atoms as the mirror plane and new elongated hexagons (6′6′), is formed at the middle layer of the AA′A/ABA stacking boundary (Figure 28e). Each top-view image is from the middle layer of the stack structure represented below it. The N atoms in the mirror plane of the 6'6'-N structure are doubly coordinated with a dangling bond.

The optimized structures (c,f,g) of the four possible initial structures in the stacking boundary (a,b,d,e) show the construction of 558 configurations as a twin boundary. However, the red EEL spectrum taken from the atomically sharp stacking boundary shows a gradual decrease in leading edge intensity with no steep drop in the x -axis of 2 eV. Another example of the EELS line scan along the stacking boundary (Figure 41) also shows the gradual lowering of the leading edge of the peak to 2 eV at the stacking boundary.

To summarize, the sandwiched 6′6′ twin boundary is very stable for a long time with electron beam irradiation and even when some atoms are knocked out of the surface, compared to AA′ and AB stacked intragrain regions. Compared with the relative positions of the hexagonal and triangular patterns (orange lines) along light contrast at the 1|2 layer boundary, the experimental image matches with the AC stacking structure. The atomic model of the 1|2 layer boundary in Figure 46b is shown with the stacking notation (Figure 46c) and the first layer ([NB]-A) is removed (Figure 46d).

Now suppose that a second [BN]-A' layer has grown on top of the AA' stacking monolayer region (upper part of Fig. 46e). If the B-atom edge EK of the second layer meets the second layer of AA′-stacked hBN, [NB]-AC forms an atomically sharp 6′6′-B double boundary (Figure 48b).

Conclusion

Conclusion

S.; Hong, S.; Lee, Z., Atomic scale dynamics of triangular hole growth in monolayer hexagonal boron nitride under electron irradiation. Yin, L.-C.; Cheng, H.-M.; Saito, R., Triangular defect states of hexagonal boron nitride atomic layer: Density functional theory calculations. S.; Li, L.-J.; Kong, J., Synthesis of few-layer hexagonal boron nitride thin films by chemical vapor deposition.

F.; Dresselhaus, M.; Palacios, T.; Kong, J., Synthesis of monolayer hexagonal boron nitride on Cu foil using chemical vapor deposition. C.; Hofmann, S., In situ observations during chemical vapor deposition of hexagonal boron nitride on polycrystalline copper. G.; Zett, A., Formation and dynamics of electron irradiation-induced defects in hexagonal boron nitride at elevated temperatures.

Sutter, P.; Lahiri, J.; Zahl, P.; Wang, B.; Sutter, E., Skaalbare sintese van uniforme paar-laag seskantige boornitried diëlektriese films. Ruoff, "Sinthesis of Aligned Simmetrical Multifaceted Monolayer Hexagonal Boron Nitride Single Crystals on Resolidified Copper", Nanoscale, 8(4), Jan 2016. Lee*, "Atomic-scale dynamics of triangular hole growth in monolayer hexagonal boor nitride under electron irradiation, ” Nanoskaal 7, pp.

Lee, “Screw Dislocation-Driven Growth of Double-Helical Hexagonal Boron Nitride”, International Microscopie Conference 19, september Sydney, Australië (poster). Lee, “Screw-Dislocation Driven Growth of Multilayer Hexagonal Boron Nitride”, 2018 Korean Society of Microscopy Conference, Jun Jeju, Korea (poster). Prijs voor beste poster, ‘Screw-Dislocation Driven Growth of Multilayer Hexagonal Boron Nitride’, 2018 Korean Society of Microscopy Conference, juni Jeju, Korea.