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Study on Growth of Two-Dimensional Transition Metal Dichalcogenides on Graphene: The Interface-

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Moreover, in the case of transition metal dichalcogenides (TMDs), the transition from the direct band gap to the indirect band gap is evident as the thickness increases due to the interfacial effect. Due to the diffusion-limited growth regime on the ncG template, the synthesized WSe domain shows a fractal.

Introduction

Introduction

Basic concepts for research

  • Transition metal dichalcogenides (TMDs)
  • Line defects in 2D TMDs

Furthermore, advanced CVD methods allow us to fabricate in-plane heterostructure for tuning electronic and optical properties, resulting in the formation of in-plane heterostructure boundaries. Heterostructure boundary in the plane | In-plane heterostructure has enormous potential for application in high-performance and flexible electro-optical devices by combining different building blocks.

Figure 1.1. Various polytypes of single-layer  and stacked TMDs. (a) 1H phase, (b) 1T phase, (c)  distorted  (2  ×  1)  1T  phase  (denoted  as  1T′),  (d)  2H  phase,  and  (e)  3R  phase
Figure 1.1. Various polytypes of single-layer and stacked TMDs. (a) 1H phase, (b) 1T phase, (c) distorted (2 × 1) 1T phase (denoted as 1T′), (d) 2H phase, and (e) 3R phase

Objective and outline of the dissertation

Investigation of defects using modern transmission electron microscopy (TEM)

Modern TEM

  • Aberration corrected TEM
  • Electron energy loss spectroscopy with enhanced energy resolution

Atomic-scale characterization (e.g., bonding, composition, atomic configuration) of all atoms in a sample has long been a goal of electron microscopy. -hexapole system has been successfully applied in conventional TEM and scanning TEM (STEM) systems with several advantages.

Figure 2.1. Multipole lenses. Electron travel path is into the page. This figure 64  is reprinted from A
Figure 2.1. Multipole lenses. Electron travel path is into the page. This figure 64 is reprinted from A
  • Temperature (cryo / heating holder)
  • Stress (mechanical holder)

An in situ tensile testing platform using MEMS push-to-pull (PTP) device is developed and allows to understand the relationship between atomic scale structural characterization and mechanical properties with uniform uniaxial loading of a freestanding 2D material in TEM84. Meanwhile, the stress-strain graph and breaking strength can be measured using in-situ tensile testing.

Figure 2.4. Phase transition of 1T-TaS  according to the temperature using in situ cryo / thermal
Figure 2.4. Phase transition of 1T-TaS according to the temperature using in situ cryo / thermal

Defect study using modern TEM

  • Stacking order and dislocation analysis using dark-field TEM
  • Identification of low angle GB using artificial moiré pattern
  • Electron beam irradiated defect generation

Metallic anti-phase boundary of TMDs grown on the pristine graphene

APBs in TMDs

First, e-beam irradiation in TEM manipulates the dense networks of APBs from the chalcogen-deficient environment due to the low threshold voltage of chalcogen atoms in TMDs. These experimental results are strengthened by the DFT calculations to investigate the energetically stable atomic configurations of APBs.

Experimental methods

To investigate the synthesized W(Mo)S2, scanning electron microscopy (SEM) measurements were performed in secondary electron (SE) and backscattered electron (BSE) modes using a cold field emission SEM (Hitachi SU-8220) operated at an accelerating voltage of 1 kV. Dark-field mode was used to analyze accurate orientation of W(Mo)S2 and confirm the global features of APB.

Structural analysis of two types of APBs

  • Characterization of orientation and overall structure of APB in synthesized WS 2
  • Atomic configurations of WS 2 APBs
  • Structural stability of APB configurations

Using the different intensities at and in the SAED pattern (Figure 3.1b) we can determine the exact orientation of each grain. It is striking that the W-facing APB in Figure 3.4a has the sawtooth shape with discrete facets, while the S-facing APB in Figure 3.4b has the broadly straight shape. In the local image of the S-facing APB, the direction of APB (Figure 3.6b) is rotated about 17° with respect to the S-ZZ line, which coincides with the results from the previous reports34,38.

Each facet is rotated about 22° from the N-WW line and consists of Srhomb and W-S-W (Fig. 3.8e and Fig. 3.9), which are identical to the atomic configuration of the S-facing APB in Fig. 3.6a. The calculation result shows that the W-edges are the most stable for the entire ∆mS range, which corresponds to the experimental result (Figure 3.1 and Figure 3.2). To obtain a stable APB configuration, the energy of APB formation (gAPB) was calculated using the following equation 110,125 (Figure 3.12).

As shown in Figure 3.17c-e, there is no migration or reconfiguration of APBs, which represents a high migration barrier for APBs.

Figure 3.2. Tungsten terminated growth of WS 2 . (a) DF-TEM image and SAED pattern (inset) of  WS 2 /graphene heterostructure
Figure 3.2. Tungsten terminated growth of WS 2 . (a) DF-TEM image and SAED pattern (inset) of WS 2 /graphene heterostructure

Electronic properties of X- and M-facing APBs

To study the collective effect of sawtooth APB on electrical performance, we fabricated a large number (>30) FET devices from pristine MoS2 flakes (denoted p-MoS2) and MoS2 with S-directed APBs (s- MoS2) and with Mo-targeted APBs (m-MoS2). To explain the difference in transport behavior, we first checked the Vth, which is a useful parameter for the estimation of the Fermi level shift due to the doping effects depending on the APBs (e.g. carrier density, n∝ΔVth). The obtained average Vth for s-MoS2, m-MoS2, and p-MoS2 devices are similar, indicating that the doping effects by APBs were negligible.

On the other hand, it could be noted that the line defect density (e.g., APB length) is proportionally related to the degradation of the electronic properties, including μFET20. In addition, the s-MoS2 devices do not show any detrimental effects on the transport behavior compared to the p-MoS220 device, indicating that the atomic defects composed of APBs are not strongly degraded by electrical transport. Combined with the increased kink density in APB facing W(Mo), we suggest that the reduced ion and μFET of m-MoS2 is due to a localized electron at a random disordered potential between the facets acting as intrinsic scattering centers, while s-MoS2 retained its electron transport behavior along an atomically faceted 1D metal wire channel.

The effects of APBs on electron transport properties. (a) The logarithmic and linear scale transfer (Ids-Vgs) curves at Vds = 1.0 V and (b) On-current (Ion) vs.

Figure  3.20.  MoS 2   FET  devices  for  investigating  effect  of  facet  in  TMDs’  APBs
Figure 3.20. MoS 2 FET devices for investigating effect of facet in TMDs’ APBs

Conclusion and outlook

Moreover, TMDs are considered long-term air-stable semiconductors, which is of great importance for applications. However, recently, starting from non-stoichiometric defect locations (e.g., chalcogen vacancy, GB), severe surface degradation of TMDs has been observed at ambient conditions. For example, the TMD/graphene heterostructure significantly improves air stability through the charge transfer-mediated doping effect.

Similar to the APB configuration, which has an energetically stable atomic configuration despite the different overall structure, in TMD two types of edges (i.e. sawtooth and straight) are observed, which are predicted to have energetically favorable edge configurations. Constrained edge configurations are very useful for manipulating desirable properties (e.g., magnetic moment, spin order), so further research into atomic-scale aging mechanisms, detailed atomic edge configurations, and edge engineering (e.g., edge decoration) is needed. Edges have different general structures to have energetically favorable edge configurations at the atomic scale, similar to the case of APB.

Figure 3.22. Two types of edge in degraded MoS 2 . Edges have  different overall structures to have  energetically favorable edge configurations at atomic scale, similar to the case of APB
Figure 3.22. Two types of edge in degraded MoS 2 . Edges have different overall structures to have energetically favorable edge configurations at atomic scale, similar to the case of APB

AB/AC stacking boundaries of TMDs grown on the wrinkled graphene

AB/AC SBs in 2D materials

Experimental section

Structural analysis of AB/AC SBs

  • AB/AC SBs in WS 2
  • Burgers vector analysis and atomic structure in AB/AC SBs in WS 2

Formation of AB/AC SBs on wrinkled graphene template

Buckling of AB/AC SBs and their electronic property

Conclusion and outlook

Edge rich multilayered TMDs grown on the nanocrystalline graphene

TMDs grown on the graphene defects

Defects in the template layer have a major influence on the nucleation and growth behavior of heterostructures, because the dangling bonds at defect sites (eg voids, folds and GBs) are more reactive with high chemical reactivity114. Despite their chance to produce heterostructure with new property by using the defects in the template layer, previous theoretical and experimental studies only considered the defect-free templates169. Previously, we demonstrated the impact of the defects in template layer on the heterostructure property using directly synthesized WS2 on polycrystalline graphene13.

There are two types of WS2/graphene heterostructure, (1) WS2 cored on graphene GBs (D-WS2) and (2) WS2 cored on the basal plane of graphene (B-WS2). Here, we show WSe2 flakes grown on graphene with giant defect sites as an extension of previous research. Interestingly, WSe2 flakes grown on SiO2/Si or pristine graphene show monolayer limited thickness based on lateral growth, WSe2 flakes grown on ncG film show an edge-rich multilayer structure due to high nucleation density at defect sites.

Our result could be easily applied and extended to other vdW 2D heterostructures and reveal the profound effect of the defective template on the structural properties of the heterostructure.

Experimental section

Growth behavior of WSe 2 grown on the nanocrystalline graphene

Raman spectra (Figure 5.2a) show that the WSe2 peak (degenerate E2g+A1g peak ~250 cm-1)172 is clearly observed, indicating that the WSe2 flakes have been successfully synthesized for both templates (ncG and SiO2/Si substrate). Although there is a significant difference in thickness between s-WSe2 and n-WSe2, as shown in the BSE-SEM image (Figure 5.1b,c), there is not any WSe2 peak shift, as it is not thickness dependent from WSe2. As shown in Figure 5.2b, as-grown ncG represents a significant D-band (~1354 cm-1) due to the occurrence of some form of disorder in the sp2 hybridized graphene matrix.

The D′ band (~ 1620 cm-1) is related to disordered structure and is commonly found in the nanocrystalline graphite, indicating that the ncG template has a large number of defect sites with nanocrystallinity. There is no peak shift of the G and 2D band between bare ncG and WSe2/ncG heterostructure, indicating that the ncG template is free from friction and strain effects. The corresponding height profile across the cluster (Insets in height image of Figure 5.3) shows that the maximum height at the center is approximately 12 nm, relative to the ncG.

The simultaneously acquired phase image shows the brightest contrast in the WSe2 cluster, compared to the exposed ncG surface, indicating that the different material has been successfully synthesized, which is WSe2.

Figure 5.2. Raman analysis of WSe 2 /ncG heterostructure. (a) Degenerated E 2g +A 1g  peak of WSe 2
Figure 5.2. Raman analysis of WSe 2 /ncG heterostructure. (a) Degenerated E 2g +A 1g peak of WSe 2

Edge rich multilayered WSe 2

WSe2 Rich Multilayer Cluster of Edge grown on ncG template. a,b) False-color STEM images of a WSe2 cluster, showing (a) the complete cluster and (b) a zoomed-in view. Compared to the W-terminated edge in the WS2/graphene heterostructure (as shown in Figure 3.1 and 3.2), n-WSe2 has the Se-terminated edge with several bends (Figure 5.6a; orange arrow) and indicates that n-WSe2 is grown under high-flux Se174 environment. WSe2 nanoribbon formation energy by Se chemical potential indicates the Se-terminated edge is the most stable edge configuration across the range of chemical environment conditions and much more stable in the Se-rich state176.

The corresponding intensity profile across the edge shows negligible intensity difference between the outermost edge and the 2Se pillar. From these results we can deduce that n-WSe2 has Se edge with 100% Se coverage, which is predicted to have magnetic moment from the unpaired electrons in the Se atom134. Interestingly, there are many discrete W atoms in the vicinity of the WSe2 domains (Figure 5.6a; green arrows), presumably due to the higher attachment rate than the edge and surface diffusion rate174.

The adatom distribution is much higher than the TMD grown on pristine graphene (Figure 5.6a; red arrows), suggesting that it is appreciable in the W atomic bridge, but further work, such as cross-section analysis, is required. .

Figure 5.4. Edge rich multilayered WSe 2  cluster grown on the ncG template. (a,b) False-colored  STEM images of one WSe 2  cluster, showing (a) whole cluster and (b) a magnified view
Figure 5.4. Edge rich multilayered WSe 2 cluster grown on the ncG template. (a,b) False-colored STEM images of one WSe 2 cluster, showing (a) whole cluster and (b) a magnified view

Electronic property of WSe 2 /ncG heterostructure

Conclusion and outlook

Conclusion

Our findings clarify the profound effects of substrate morphology and crystallinity on the structural features and corresponding properties of vdW heterostructures. Our understanding of interface-mediated novel defect structures and the relationship between the structure and the corresponding property could be significant for designing desirable properties and expanding the scope of vdW heterostructures via interface engineering.

Perspective and outlook

176 Addou, R.et al. One-dimensional metallic edges in atomically thin WSe2 induced by air exposure.

수치

Figure  2.2.  Ray  diagrams  for  TEM  and  STEM  with  image  and  probe  aberration  corrector,  respectively
Figure 2.4. Phase transition of 1T-TaS  according to the temperature using in situ cryo / thermal
Figure 2.5. Torn graphene with AC and ZZ edges after in situ tensile testing. Inset is diffraction  pattern, indicating the  orientation of  graphene
Figure 2.8. Bloch-wave simulation for various stacking configurations of WS 2 . Bottom panels are  intensity profiles along the red arrows in simulated diffraction patterns
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