Direct visualization of structural defects in 2D semiconductors

Yutuo Guo(郭玉拓)， Qinqin Wang(王琴琴)， Xiaomei Li(李曉梅)， Zheng Wei(魏爭(zhēng))， Lu Li(李璐)，Yalin Peng(彭雅琳)， Wei Yang(楊威)， Rong Yang(楊蓉)，3， Dongxia Shi(時(shí)東霞)，Xuedong Bai(白雪冬)， Luojun Du(杜羅軍)， and Guangyu Zhang(張廣宇)，3，?

1Beijing National Laboratory for Condensed Matter Physics and Institute of Physics，Chinese Academy of Sciences，Beijing 100190，China

2School of Physical Science，University of Chinese Academy of Sciences，Beijing 100049，China

3Songshan-Lake Materials Laboratory，Dongguan 523808，Guangdong Province，China

Keywords: structural defects，direct visualization，molybdenum disulfide，anisotropic etching，edges

1. Introduction

Structural defects in two-dimensional (2D) semiconductors， such as zero-dimensional (0D) point defects and onedimensional (1D) grain boundaries， play important roles on modulation of their mechanical，electrical，optical，and chemical properties.[1–6]For examples， the vacancies and grain boundaries in 2D transition metal dichalcogenides (TMDs)can offer effective catalytic active sites and thus enhance the catalytic performance.[7–9]On the other hand， structural defects usually act as additional scattering centers and degrade the electric performance， such as mobility and on/off ratio.[2，6，10，11]As a consequence， it is essential to know the details of structural defects in 2D semiconductors on a large scale，such as the density and the location，which can provide key information to deeply understand their properties and to guide the further optimization.

Arguably the atomic resolution microscopy tools，such as high-resolution transmission electron microscopy (HRTEM)and scanning tunneling microscopy(STM)，[12–15]can directly visualize the structural defects in 2D semiconductors. However， these atomic resolution techniques not only cannot give large-scale information (usually only a few tens of nanometers)， but also require ultra-high vacuum， complex setups or specialized sample preparation， making them impractical to be a usual basis to characterize structural defects in 2D semiconductors. Remarkably， because of the high reactivity，atomic-scale structural defects can be enlarged into macroscopic structures after being etched， offering the possibility for direct visualization on a large scale through atomic force microscope (AFM) or even optical microscope.[16–18]Recently， multiple methods have been developed to directly image the structural defects in 2D semiconductors on a large scale， for example， via enlarging the defect sites by selective etching or decorating them with nanoparticles.[18–20]However，these methods either introduce new defects or lead to surface modifications.[2，9，16，19，21–29]Damage to the surface and quality of 2D semiconductors hinders their further applications.

In this paper，we introduce a non-invasive，facile method for large-scale assessment of structural defects in monolayer molybdenum disulfide (ML-MoS2)， a prototypical 2D semiconductor. Weak wet etching is utilized to enlarge those structural defects in ML-MoS2for direct visualization under optical microscope or AFM. The weak etching only etches the defected sites while keeping the perfect lattice of ML-MoS2intact， as confirmed by TEM， AFM and photoluminescence(PL) spectroscopy. Using this technique， we systematically investigated the defects in different types of ML-MoS2samples， providing key information to understand their electrical performance.

2. Methods

2.1. Sample fabrications

All monolayer MoS2samples on sapphire substrates were grown in a three temperature-zone chemical vapor deposition(CVD)system.For detailed information on the growth of MLMoS2large single domain，large domain ML-MoS2films，and small domain ML-MoS2，please refer to Refs.[33，35]，respectively. For thick MoS2flakes，they were mechanically exfoliated from bulk crystals onto scotch-tape，and then transferred onto SiO2/Si substrates. For the ML-MoS2films on SiO2/Si substrate， the ML-MoS2films was etched off substrates by KOH solution and transferred onto the SiO2/Si substrates.

2.2. Wet etching of the sample

The MoS2on substrates were immersed in calcium hypochlorite solution for the desired time. After etching，the sample is rinsed in deionized water to remove chemical residues and terminate the etching process，dried with N2gas，and then characterized with optical microscopy.

2.3. Sample characterizations

Raman and PL spectra were carried out by a micro-Raman spectrometer (Horiba LabRAM HR Evolution) in a confocal backscattering configuration with an excitation laser wavelength of 532 nm， a laser power of 1 mW and spot size of 1 μm2. The PL mapping was obtained with WiTec(Alpha 300R) Raman microscope with a laser power of 1 mW and step size of 1 μm. AFM imaging was performed by Veeco Multimode III under ambient conditions. STEM imaging was performed by an aberration-corrected JEOL Grand ARM 300 CFEG operated at 80 kV.

3. Results

To directly visualize the structural defects in ML-MoS2，we first use a calcium hypochlorite solution to etch the samples at room temperature grown by CVD.The reaction process of MoS2and hypochlorite ion is as follows:[30]

Since the defects have relatively high chemical reactivity， the etching process starts at the defects first. After the amplification by etching， 0D point defects and 1D grain boundaries transformed into triangular pits and trenches， respectively (Fig. 1(a)). This opens the possibilities to directly observe the structural defects in ML-MoS2by an optical microscope. Figure 1(b)shows the typical optical image of a single crystal ML-MoS2domain on sapphire substrates after etching with a 2-wt%calcium hypochlorite solution for 20 seconds at room temperature. Obvious pits appeared in the crystal domain after etching. The triangular shape of the etched pits indicating that the etching process has strong anisotropy. Alluringly， the etched triangular pits are in the same direction as the domains. For the CVD grown ML-MoS2grown under sulfur-rich conditions，the edges are S-terminated zigzag edges(ZZS).[10，31–33]The etched triangular pits in the same direction as the domains should have zigzag edges terminated with molybdenum(ZZMo)，[24，34]as shown in the inset of Fig.1(b).Figure 1(c)shows a typical optical image of merged ML-MoS2domains on sapphire substrates after etching. It can be seen that the grain boundaries have been etched to be trenches，enable the direct visualization. Figure 1(d) presents the AFM image of the etched trenches，indicating that the edges are very smooth and the surface is clean.

Fig. 1. Anisotropic and non-invasive etching of ML-MoS2 by calcium hypochlorite solution: (a)schematic diagram of wet etching;(b)optical image of ML-MoS2 large single domain after etching. The inset shows the schematic diagram of the atomic geometry of ML-MoS2 large single domain;(c) optical image of merged ML-MoS2 domains after etching with calcium hypochlorite solution; (d) close-up AFM image of the areas surrounded by lines in panel(c);(e)the evolution of the number of triangular pits in different batches of MoS2 with the etching time;(f)PL spectra of MoS2 with different etching times. Scale bar: 10 μm in panels(b)and(c)，1 μm in panel(d).

Figure 1(e)shows the evolution of the number of triangular pits in three triangular domains from different batches with etching time. The number of triangular pits of all the three samples reached saturation after about 200 seconds. This indicates that the etching process of ML-MoS2by hypochlorite ion only starts from the inherent defects，and does not generate new defects. This is further confirmed by the perfect atomic structure after etching，please see Fig.S1 for more details.The increase in the number of triangular pits over time in the initial stage of etching may stem from the different chemical reactivity of different defects.[12，13]Figure 1(f) shows that the PL spectra of ML-MoS2after etching remained unchanged，even if the etching time was extended to 600 seconds. This indicates that our method is non-invasive and significantly different from previous etching methods.[16，18，21，22]Please see Fig. S2 for more details of the PL spectra in Supporting information.

Fig.2. Defect density of different types of treated MoS2 samples: (a)AFM image of mechanically exfoliated MoS2 etched by calcium hypochlorite solution; (b) optical image of CVD-grown ML-MoS2 films with small grain size etched by calcium hypochlorite solution; (c) close-up AFM image of the areas surrounded by lines in panel(b);(d)optical image of CVD-grown ML-MoS2 films with large grain size etched by calcium hypochlorite solution.The white arrow points to the location of the triangular pits;(e)close-up optical image of the areas surrounded by lines in panel (d); (f) statistics of average pits densities of various MoS2 samples. Scale bar:1 μm in panel(a)，10 μm in panel(b)，300 nm in panel(c)，10 μm in panel(d)，2 μm in panel(e).

Using our non-invasive techniques，we further studied the internal defects of different types of MoS2，namely，mechanically exfoliated MoS2，CVD-grown ML-MoS2single domain，and CVD-grown ML-MoS2films with small and large grain size. After 60 seconds of etching， triangular pits appeared in mechanically exfoliated MoS2， as shown in Fig. 2(a). The density of triangular pits is～4.50×106cm-2. For CVDgrown ML-MoS2films with small grain size， as shown in Figs. 2(b) and 2(c)， the density of triangular pits is much higher than that of mechanically exfoliated MoS2and can reach～1.18×109cm-2.Notably，the triangular pits of CVDgrown ML-MoS2films with small grain size have two orientations. This is because the film is stitched by two oriented domains: 0°and 60°.[35]For CVD-grown ML-MoS2single domain and ML-MoS2films with large grain size，the densities of triangular pits are～4.47×105cm-2and～5.61×105cm-2，respectively， which are much lower than that of CVD-grown ML-MoS2films with small grain size and also more than one order of magnitude lower than that of mechanically exfoliated MoS2. Statistics of average pits densities of various MoS2samples are shown in Fig. 2(f). The ultralow defect density in CVD-grown ML-MoS2films with large grain size provides key information for understanding the excellent electrical properties recently observed.[33]It is worth noting that in order to avoid the mutual fusion phenomenon of the pits during the etching process，see Fig.S3 for details，the etching time for CVD-grown ML-MoS2films with small grain size and mechanically exfoliated MoS2is only 60 seconds. This is different from the situations of CVD-grown ML-MoS2single domain and ML-MoS2films with large grain size where the etching has reached saturation(etching time exceeds 200 seconds). This means that the defect densities in mechanically exfoliated MoS2and CVD-grown ML-MoS2films with small grain size may be higher than that shown in Fig.2(f).

Apart from the intrinsic defects，we also use our method to study the artificially introduced defects in MoS2. Chemical hydrogen plasma treatment can generate sulfur vacancies in MoS2.[36–38]After treating with calcium hypochlorite solution， MoS2treated with hydrogen plasma is quickly etched away. Note that before treating with calcium hypochlorite solution， the structure of MoS2treated with hydrogen plasma is intact (Fig. S4). Even if the etching time is shortened to about 0.5 seconds， there are still many pits on the surface of the MoS2sample， as shown in Fig. 3(a). The grooves in the image are attributed to the stress caused by the bombardment of hydrogen ions on the MoS2. This indicates that the calcium hypochlorite solution has a good response to the sulfur vacancies generated by hydrogen plasma. Argon plasma treatment is a physical process and can introduce phase transitions in MoS2.[39]After argon ion treatment of the CVD-grown MLMoS2films with large grain size，the density of triangular pits is increased by more than three orders of magnitude and can reach 4.70×108cm-2，as shown in Fig.3(b).Figure S5 shows details about the phase transition. The strongly enhanced defect density in samples treated by argon ion provides new insight to understand the physical mechanism of phase transitions triggered by argon plasma treatment.[40]

Fig. 3. Response of wet etch to artificial defects: (a) AFM image of MoS2 treated by hydrogen plasma after being etched by solution;(b)optical image of 1T phase MoS2 after being etched by solution. The inset is an AFM image of the sample. Scale bar: 100 nm in panel(a)， 10 μm in panel(b)， 1 μm in the insert of panel(b).

Significantly， the shape of the pit can be highly tunable by the concentration of the solution. As shown in Figs.4(a)–4(h)，as the concentration of calcium hypochlorite solution decreases， the shape of the pits evolves from triangles to truncated triangles， then to hexagons， and finally to inverse triangles. This indicates that the edges of the pit change from pure ZZMoto pure ZZSas the concentration of the solution decreases.[34]The etch rate of the two types of edge states varies with concentration as shown in Fig.4(i). In other word，our technique opens up the opportunities to obtain atomically sharp edge states on demand. This provides opportunities for different cut-off boundaries of ML-MoS2， showing great application potential in metamaterial preparation，[41]heterojunction epitaxy，[42，43]defect repair，[44，45]hydrogen evolution catalysis，[46]nonlinear optics，[47]and nanoelectronics.[48，49]In addition， the anisotropic etching of ML-MoS2by the calcium hypochlorite solution is not limited by the substrate.Figure S6 shows that for ML-MoS2on SiO2substrate， the pits also show triangle sharp， indicating the anisotropy of the etching. In addition to MoS2， the calcium hypochlorite solution can also be used to etch other 2D-TMDs， such as WSe2，MoSe2， and WS2(Fig. S7). This indicates that our method is a general technique for directly visualizing defects in 2DTMDs， and has the potential to be applied to other 2D semiconductors.

Fig. 4. The shape of the pits etched under different calcium hypochlorite concentrations: (a)–(h)optical images of MoS2 etched by different concentrations of calcium hypochlorite solutions.The blue and red sides of the triangles and truncated triangles in the illustration correspond to ZZMo and ZZS，respectively. (i)Etching rate of the two types of boundaries varies with concentration. ν1 and ν2 respectively correspond to the etching rate of Zigzag edges terminated with molybdenum (ZZMo) and sulphur (ZZS). Scale bar:10 μm in panels(a)–(h).

4. Summary

In summary， we have developed a simple and noninvasive method to directly visualize the structural defects in 2D-TMDs on a large scale. Utilizing this etching technique，we have investigated the intrinsic defects of four types of MoS2samples and found that CVD-grown ML-MoS2single domain and ML-MoS2films with large grain size have lowest defect density. This enable us to understand the relationship between structural defects and performance. Moreover， for artificially introduced defects， such as sulfur vacancies introduced by hydrogen plasma and phase transition triggered by argon plasma， our method also has a good response. Finally，the edges of by etched pits can be tunable by changing the concentration of the calcium hypochlorite solution，providing the opportunities to obtain edge states on demand.

Acknowledgments

Project supported by the Key-Area Research and Development Program of Guangdong Province， China (Grant No. 2020B0101340001)， the Strategic Priority Research Program of Chinese Academy of Sciences (CAS) (Grant No.XDB30000000)，and the National Natural Science Foundation of China(Grant Nos.61888102 and 11834017).