Sequential conversion from line defects to atomic clusters in monolayer WS₂

Abstract

Introduction

A formation of structural defects in two dimensional (2D) materials affects their intrinsic properties (Topsakal et al. 2008; Faccio and Mombrú 2012; Han et al. 2015). Various types of defects have been studied, like zero dimensional (0D) defects (Komsa et al. 2002; Azizi et al. 2014) and one dimensional (1D) defects (Lahiri et al. 2010; Botello-Mendez et al. 2011; Liu et al. 2012; van der Zande et al. 2013; Zhou et al. 2013; Enyashin et al. 2013; Lin et al. 2015; Barja et al. 2016). Defect theories of monolayer TMDs such as MoS₂ and WS₂ (Radisavljevic et al. 2011; Nourbakhsh et al. 2016; Liu et al. 2016) sheet with semiconducting properties for electronic and optoelectronic devices have been established, which explains that S atoms are easier to eject than W and Mo atoms. S loss leads to increasing vacancies and line defects that changes electrical properties, and as the width and length of the line defects increase, the transition from semiconductor to metallic properties (Ryu et al. 2016; Wang et al. 2016).

Generally, the large number of vacancies formed in a material are energetically disadvantageous, so they do not exist individually and tend to agglomerate together (Smallman and Bishop 1999). In two dimensional (2D) materials composed of two elements, holes with different aspects such as shape and edge termination are formed depending on knock-on thresholds of the materials (Ryu et al. 2016; Ryu et al. 2015; Park et al. 2015; Girit et al. 2009; Bieri et al. 2009; Farimani et al. 2014; Kotakoski et al. 2010). For edges of holes, they are similar to the hole surfaces in bulk materials, and linear defects including dislocations, grain boundary, and line defects are limited to 2D. In a case of monolayer hexagonal boron nitride, boron vacancies have been observed to diffuse and merge into extended triangular holes to reduce the surface energy of a larger hole. (Ryu et al. 2015; Alem et al. 2009; Alem et al. 2011). For a monolayer MoS₂, the increasing density of S vacancies develops into long line defects and extended holes with MoS nanowires (Liu et al. 2013; Sang et al. 2018) and Mo clusters (Ryu et al. 2016).

Previous works in TMDs have demonstrated that chalcogen atom vacancies are easily formed and transition metal atoms tend to aggregate at edges of extended holes (Chen et al. 2018; Komsa et al. 2013; Le et al. 2014). However, it is relatively difficult to form line defects than WS₂ due to the high energy barrier of S vacancy migration in MoS₂. Here, we report details on the conversion process from line defects into W clusters through a crystalline intermediate phase in a monolayer WS₂ sheet. ADF-STEM images are used to study atomic dynamics and are acquired in a clean area of WS₂ to observe the atomic dynamics of the overall formation process.

Results and discussion

If an electron beam is irradiated onto a synthesized monolayer WS₂ sheet using an acceleration voltage of 80 kV, long line defects are generally formed, and holes are formed at the point where the line defects collide (Fig. 1a and b). The knocking-off rate of S atoms is getting high as the hole is extended (Fig. 1c). This is because the S vacancies are linearly aligned to form the line defects and rapidly migrate to the exteneded hole. In addition, the line defects are often atomically uniform with a periodic lattice structure and they are absorbed into the hole leaving small clusters at edges of the holes (Fig. 1d). ADF-STEM images are used to investigate the exact atomic dynamics of the formation mechanism on the clusters. As shown in Fig. 1e-f, accumulated S vacancies are directly related to the formation of line defects, and when the line defects collide, topological holes are formed, leaving clusters at the edge of the holes.

Open in a separate window

Fig. 1

a–c ADF-STEM images showing the formation of atomic clusters at edges of a hole in the monolayer WS₂. d ADF-STEM image showing extended holes with line defects and clusters. e–h Simple successive schematics showing a whole process for the formation of clusters and holes from line defects in the monolayer WS₂ sheet

When a prolonged electron beam irradiates on the WS₂ sheet, holes are formed in the WS₂ due to the high concentration of S vacancies. The initiation point for the hole formation usually occurs at the intersection of long line defects and W clusters are formed at the edge of the hole or at the ends of the line defects. Figure 2 shows successive images of colliding line defects (Fig. 2a and b), and then a hole begins to open and extend (Fig. 2c−h). In Fig. 2h, the large hole absorbs the long line defects on the left, and the remaining line defects relate to the newly formed line defects. Once the hole is opened (Fig. 2c), the W cluster is adjusted, binding to the edge of the hole, and the hole also rapidly extends. Newly formed line defects near the existing holes are also absorbed by the hole when connecting and the hole is extended. The WS₂ sheet surrounding the holes is still basically a pristine lattice because S vacancies quickly diffuse into the holes. W clusters are attached to the edges of the holes with bright contrast.

Open in a separate window

Fig. 2

Successive ADF-STEM images showing the formation of clusters initiated from line defects in the monolayer WS₂. Yellow dashed lines indicate long line defects 

Figure 3 shows a formation of an intermediate phase within a monolayer WS₂ sheet, removing the line defect or narrowing its width. In Fig. 3a and b, a line defect disappears and leaves the intermediate phase through the migration of W and S atoms composed of the line defect. Although the line defect did not contact the edge of the extended hole, it was converted into the intermediate phase. In Fig. 3c and d, other line defects have been converted to the intermediate phases, which are evident by tracing white dashed arrows. This shows that even if line defects are formed inside the WS₂ sheet, they can be converted to the intermediate phase.

Open in a separate window

Fig. 3

Formation of the intermediate phase of the atomic cluster inside the monolayer WS₂ sheet. a ADF-STEM image showing line defects before conversion into the intermediate phase (b). c–d ADF-STEM images showing that line defects can be narrowing that is followed by forming the intermediate phase

We also observed the presence of some freely migrating W atoms in the WS₂ sheet, due to the formation of line defects. For a hole to extend, both W and S atoms need to be ejected, but at low accelerating voltages, the knocking threshold of W is limited. This causes W atoms to accumulate at the edges of the hole and aggregate into clusters, which lowers the surface energy of this condition. Figure 4a shows the WS₂ sheet before forming holes. Line defects linked to a hole allows W atoms to migrate along the line defect indicated by yellow boxes in Fig. 4b and c. The transport of W atoms occurs when linear defects interact with the hole. The line defect leads to the hole aggregating W atoms with the intermediate phase. Transporting W atoms along the line defects also is converted to intermediate phases at the edge of the hole (Fig. 4d).

Open in a separate window

Fig. 4

Transport of W atoms along line defects. a ADF-STEM image showing the WS₂ sheet. b–c ADF-STEM images showing transport W atoms. Yellow boxes indicate transporting W atoms to form clusters. d Transition from the transporting atoms into the intermediate phase at the edge of the hole

The entire processe explains the hole formation and its connection with line defects and the intermediate phase. When line defects are connected to a hole, S atoms are ejected from the edge region by migrating along the line defects. This creates local S exhaustion at the edge and the edge is reconstructed into WS nanowires or W clusters. Eventually, line defects disappear by migrating toward extended holes is directly followed by extension of the the holes. During this process, some W and S atoms composed of the line defects migrate and aggregate at the edge of the holes and inside the WS₂ sheet, which is the end of the line defects or region where the line defects disappear. Here, the intermediate phase appears as shown in Fig. 5a-c. The intermediate phase has a uniform structure (Fig. 5d), which looks like a small sunflower-shaped. The W and S atoms composed of line defects migrate toward the hole and they reach the end of the line defects, where W clusters gradually grow on the WS₂ surface. To analyze the atomic structure of the intermediate phase, image simulation was performed according to the atomic model (Fig. 5e and f). This is consistent with the hexagonal lattice of the rotated bilayer WS₂, which is well matched with the experimental image of Fig. 5d, and the bright-contrast structures are due to a superposition of W and S atoms, with a rotation. As expected, the W cluster is mainly formed at the ends of the line defect contacting the holes, which leads to the phase on the WS₂ sheet. The phase is fully converted into the W cluster by prolonged electron beam irradiation.

Open in a separate window

Fig. 5.

Atomic structures of the intermediate phase. a ADF-STEM image showing a hole with clusters and a intermedate phase positioned independently inside WS2 sheet. b–c ADF-STEM images showing intermediate phases at the edge of holes. d Magnified image of a white box in (a). e Image simulation of the intermedate phase according to an atomic model (f)

Conclusions

We summarize that the line defects induced by electron beam irradiation migrate and diffuse into the holes, causing W atoms to aggregate at the hole edges of the monolayer WS₂ sheet. When the S vacancies are formed individually, they are arranged linearly by forming long line defects. And then, holes are formed at the points where the line defects collide each other, and W atoms aggregate at the edges of the holes. The holes grow to extended holes that absorb line defects with further knocking out the W and S atoms, and this process occurs repeatedly. At this point, before the W atoms form a complete cluster, they go through the intermediate phase that has been analyzed as a uniform crystalline, and this phase interacts with the migration of the line defects. Complete W clusters usually form at the edge of the hole, but this intermediate phase can form not only at the edge of the hole, but also inside the WS₂ sheet where line defects appeared. These results show the most detailed insights into the cluster formation mechanism and its association with topological defects activated in the TMD materials.

Methods

Acknowledgments

Abbreviations

Authors’ contributions

Corresponding E-mail Address: gh.ryu@gnu.ac.kr. The author(s) read and approved the final manuscript.

Funding

This research received no external funding.

Availability of data and materials

The datasets used and/or analyzed during the study are available from the corresponding author on reasonable request.

Competing interests

The authors declare that they have no competing interests.

Footnotes

References