Twin Defect Derived Growth of Atomically Thin MoS2 Dendrites - ACS

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Twin Defect Derived Growth of Atomically Thin MoS Dendrites Jingwei Wang, Xiangbin Cai, Run Shi, Zefei Wu, Weijun Wang, Gen Long, Yongjian Tang, Nianduo Cai, Wenkai Ouyang, Pai Geng, Bananakere Nanjegowda Chandrashekar, Abbas Amini, Ning Wang, and Chun Cheng ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b07693 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 18, 2017

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Twin Defect Derived Growth of Atomically Thin MoS2 Dendrites Jingwei Wang,1,2 Xiangbin Cai,2 Run Shi,1,2 Zefei Wu,2 Weijun Wang,1 Gen Long,2 Yongjian Tang,2,4 Nianduo Cai,1 Wenkai Ouyang,1 Pai Geng,1 Bananakere Nanjegowda Chandrashekar,1 Abbas Amini,3 Ning Wang,2,* Chun Cheng1,* 1.

Department of Materials Science and Engineering, and Shenzhen Key Laboratory of Nanoimprint Technology, Southern University of Science and Technology, Shenzhen 518055, P. R. China

2.

Department of Physics, and Center for 1D/2D Quantum Materials, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China

3.

Center for Infrastructure Engineering, Western Sydney University, Kingswood, NSW 2751, Australia

4.

Department of Physics, Cornell University, Ithaca, New York 14853, USA

*Corresponding author: [email protected], [email protected] ABSTRACT: Morphology management for tailoring the properties of monolayer transition metal dichalcogenides (TMDCs), i.e., molybdenum disulfide (MoS2), has attracted great interest for promising applications such as in electrocatalysis and optoelectronics. Nevertheless, little progress has been made in engineering the shape of MoS2. Herein, we introduce a modified chemical vapor deposition method to grow monolayer MoS2 dendrites by pretreating substrates with adhesive tapes. The as-grown MoS2 crystals are featured with hexagonal backbones with fractal shapes and tunable degrees. By characterizing the atomic structure, it is found that these morphologies are mainly initiated from the twin defect derived growth and controlled by the S: Mo vapor ratio. Due to the accumulated sulfur vacancies in the cyclic twin regions, strong enhancement of photoluminescence emission is localized, which determines the shape dependency of optical property. This work not only enriches the understanding of the twin defects derived crystal growth mechanism and extends its applications from nanomaterials to two-dimensional crystals, but also offers a robust and controllable protocol for shape-engineered monolayer TMDCs in electrochemical and optoelectronic applications. Keywords: molybdenum disulfide, dendritic crystal, twin crystal, snowflake, shape-engineering, chemical vapor deposition

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Transitional-metal dichalcogenides (TMDCs) with the formulas of MX2 (M refers to Mo or W; and, X to S, Se, or Te) have attracted great attention among two-dimensional (2D) materials due to their distinct physical and chemical properties.1 Possessing a direct band-gap at single layer,2 spin and valley physics3 and active sites at edges4 make TMDCs ideal candidates for solar cells,5 gas sensors,6 nonlinear and electro-optical devices,7-9 and, for valleytronics,10,11 optoelectronics12,13 and hydrogen evolution reactions,14-16 etc. The morphology of TMDCs materials correlates with the crystallinity, crystal orientation, edge structure, and lattice defect plays a crucial role in the property modification of 2D crystalline materials. Thus, controlling the morphology of high quality 2D MX2 can be an effective approach for exploring promising applications such as in electrocatalysis.17 As a representative of TMDCs, the growth of monolayer molybdenum disulfide (MoS2) has been greatly promoted by using the chemical vapor deposition (CVD) method. Many achievements have been made in growth mechanism,18 layer number,19,20 orientation,21-23 coverage,24,25 domain size26-29 and position control,30,31 while the progress in morphology control is far from researchers’ expectations. Limited by the structure symmetry, the most common shapes of CVD grown monolayer MoS2 are dominated by compact triangles. The mature growth mechanisms of traditional nanostructures may provide useful hints for the shape-engineering of monolayer MoS2. Besides the intrinsic symmetry of crystals, structural defects, such as hetero-phases, screw dislocations, twins, etc., have been reported in engineering the morphology of nanostructures.32-34 Among these, the twinned setup can be an effective method to connect building blocks for intentionally organizing complex nanostructures. Recently, the formation of monolayer MoS2 crystals in polygonal shapes has been reported by merging the triangular domains across faceted tilt and mirror twin boundaries, including tetragon, pentagon, hexagon and butterfly shapes.35 These polygonal monolayer MoS2 flakes present a shape dependent photoluminescence behavior due to their high dense defects accumulated in twin boundaries.36 Therefore, twinned structures in the atomic crystals of MoS2 not only create abundant morphologies, but also strongly affect the electronic, magnetic, optical and mechanical properties of MoS2. However, there are few robust approaches in controlling twin defects during the growth of monolayer MoS2 and the understanding of the twin defects derived growth mechanism is desired. Considering the above facts, it is essential to develop simple and facile methods for reliable shape-engineering of CVD-grown monolayer MoS2. Herein, we report the CVD grown monolayer MoS2 dendrites obtained from a facile substrate treatment. The as-grown MoS2 with various fractal shapes possesses hexagonal backbones which differentiate from the shapes ever reported.37-41 We found that the shape evolution is strongly correlated with the S: Mo vapor concentration ratio. These shapes are considered to be attributed to the twin defect derived growth mechanism according to the transmission electron microscope (TEM) studies. Optical characterizations showed that strong photoluminescence emissions are localized in the cyclic twin regions of the dendrites which displays a shape dependent optical property. 2


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The feasibility of the proposed substrate pretreatment for engineering the shape was successfully shown by tailoring monolayer MoS2 from compact six-pointed star configuration to the irregular fractal dendrites by utilizing different tapes. RESULTS AND DISCUSSION

Figure 1 (a) Schematic illustration of MoS2 dendrites synthesis by introducing the adhesive seed from screen protector on SiO2/Si substrate; (b) SEM image of hexagonal symmetric MoS2 flakes, scale bar: 100 µm; (c) SEM image of snowflake-like MoS2 flake, scale bar:10 µm. Figure 1a schematically illustrates the synthesis process of dendritic MoS2 on a SiO2/Si substrate. In brief, a tiny amount of organic adhesive from the tape was mechanically exfoliated on the substrate before the growth. A relatively low temperature (600 °C) atmospheric pressure chemical vapor deposition (APCVD) process was applied for the atomic layer growth (for details, refer to the method part). Typical scanning electron microscopy (SEM) image of as-grown MoS2 is shown in Figure 1b. Different from the triangle shape, mostly reported in previous work, the MoS2 flakes exhibit a dominant hexagonal symmetry which resembles the CVD grown graphene and some transition metal phosphide crystals.42-46 Interestingly, this hexagonal symmectric MoS2 can be tailored to fractal, forming snowflake-like MoS2 dentrites which has not been reported so far. As shown in Figure 1c, the snowflake-like MoS2 consists of many side branches stretching out from the six symmetrical backbones. All these side branches keep 60° with their backbones, indicating a specific growth mechanism governed. Here, we apply the fractal dimension D to evaluate the shape complexity of MoS2 crystals. By using the standard box counting evaluation,47 the D value of snowflake-like MoS2 is deduced to 1.88 which is much lower than that of compact triangle (D = 2) (see Figure S2).

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Figure 2 Morphology evolution of MoS2 dendrites. (a) Optical images of as-grown products on the Si wafer with 300 nm oxidation layer. The S: Mo vapor ratio increases along the sulfur flow direction; (b-f) Optical images of dendritic MoS2 at positions marked in Figure 2a. The insets illustrate the crystal morphology of single MoS2 from each treatment with a scale bar of 10 µm; (g) The evolution of fractal dimension verses the S: Mo vapor concentration ratio; here, only five representative shapes are shown. According to our results, the S: Mo ratio of CVD vapor has a great impact on the final morphology of MoS2 flakes. To demonstrate the effect of vapor concentration on shape evolution, we separate the substrate to several areas along the direction of gas flow and mark them with b-f in Figure 2a. In experiments, MoO3 precursor was placed at the downstream from the SiO2/Si substrate (Figure S1), so, the S: Mo ratio continuously increases from b to f while the sulfur vapor remains at a constant concentration. The corresponding optical images of as-obtained products for each deposition area are presented in Figure 2b-f, respectively. It is found that the MoS2 flakes undergo a regular shape transformation from triangle to six-pointed star as the distance between MoO3 precursor and deposition area increases. In area b and downstream locations, the dominated shape of MoS2 crystals ends up with compact triangles and the size of ≤5 µm (Figure 2b). This is due to the high nucleation density caused by the high dense MoOx vapor. When the deposition area is 4


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located away from MoO3 precursor, the decreasing density of nucleation leads to the increase in MoS2 flake size. At the upstream sites (c area), however, polygonal MoS2 flakes emerge. As shown in Figure 2c, tetragonal, pentagonal and hexagonal MoS2 flakes are dominant while the number of triangular MoS2 declines gradually. In area d, the hexagonal MoS2 flakes evolve into specific fractal morphology, in which six main branches slightly shrink (compared to those in Figure 2c) and several side branches appear (inset Figure 2d). In area e, more tiny side branches randomly emerge from the hexagonal MoS2 flakes and six main branches gradually evolve to dagger-like shapes. In this area, the relatively high dense sulfur vapor is accumulated; this phenomenon along with the decrease in Mo source triggers the fast kinetically driven growth of crystals. Consequently, MoS2 dendritic snowflakes are present as the dominant morphology (Figure 2e). In area f, the density of MoS2 flakes reduces further and the domain size decreases to 1~ 2 µm. Interestingly, the side branches vanish and only six main sharp backbones remain, forming six lobed MoS2 (inset Figure 2f). It is suggested that the extremely low concentration of Mo precursor slow down the growth rate and shift the growth from the kinetic regime to the thermodynamic regime again.48 Therefore, the growth of side branching flakes was entirely depressed. Based on the above results, the relationship between the S: Mo ratio and shape complexity (D) is illustrated in Figure 2g. It can be seen that the increase in S: Mo vapor ratio lead to a continuous decrease in D and turn the growth process from the thermodynamic regime to kinetic regime due to the increase in growth rate. However, when the S: Mo ratio reaches an extremely high level, the relatively low concentration of Mo switches the growth back to the thermodynamic control and, thus, reduces the crystal shape complexity as a result of the suppression of side branch growth.

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Figure 3 (a) Bright-field and (b) false color dark-field TEM images of six-lobed MoS2 (framed diffraction point in Figure 3c); (c) and (d) are diffraction patterns of marked areas in (a); (e) Bright-field and (f) false color dark-field TEM images of side branch structure (framed diffraction point in Figure 3h); (g) and (h) are diffraction patterns of marked areas in (e); the insets in (c, d, g, h) show the diffraction intensity of framed spots and the vectors indicate the two families, KS and KMo; (i) Schematic drawing of thorns generation along the branches, the twin boundary is simplified. In order to identify the crystalline structure and interpret the growth mechanism of dendritic MoS2, we transferred the as-grown crystals onto carbon film coated copper grids for TEM characterization. The initial growth can be studied by analyzing the structure of small crystals. Figure 3a shows the bright-field TEM image of a tiny six lobed MoS2 appeared in Figure 2f. Corresponding selected-area electron diffraction (SAED) patterns obtained on two lobe areas of e and f in Figure 3a reveals the similar set of hexagonal diffraction spots (Figure 3c and 3d) without any rotation between them, indicating the highly crystalline structure of as-grown sample, ruling out the possibility of distortion caused by organic heterogeneity. Figure 3b shows the false color dark-field TEM image by extracting the circled first-order spot in Figure 3c. The contrast asymmetry reveals that the six lobed MoS2 flake is formed by two sets of mirror-twin crystalline regions which has also been reported by other groups.35,37,49 It is known that the intensity of diffraction spots pointed towards the sulfur sub-lattice (Ks) is lower compared to that of the spots towards the molybdenum sub-lattice (KMo).35 Thus, by comparing the diffraction intensities of neighbor spots, one can determine the lattice orientation as well as the outer atom type of a certain 6


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crystalline area. Insets of Figure 3c and d show the diffraction intensities of framed spots. According to the hexagonal geometry of the lobed MoS2, the angle between two neighbor lobe areas is 60° (Figure 3a). The edge sites of these two areas are dominated by sulfur atoms while both of these two front tips are terminated by molybdenum atoms. We investigated all the six lobe areas of the MoS2 flake in Figure 3a and obtained the same result. Namely, the six lobes are grown along [100] direction with the front tips terminated with molybdenum atoms. The above results confirm that the formation of cyclic twin defects, at the very beginning stage, is responsible for the growth of hexagonal backbones and the further evolution of enlarged dendritic crystals. It is well known that the metal atom terminated surfaces along [0001] polar direction in Wurtzite nanocrystals, such as ZnO, ZnSe, GaN, etc., are catalytically active and, thus, act as the fast growth front.50,51 Here, [100] of monolayer MoS2 crystals is also a polar direction with alternative assembly of Mo and S zigzag. Thus, the fast growth at the Mo-terminated tips of the six lobes follows the self-catalytic growth mechanism in binary polar nanocrystals. It is worthy to notice that the relative growth speed along Mo-terminated [100] direction decreases with the increase of Mo concentration, so that the main lobes become much broader as the deposition area moves from f to b in Figure 2a. For the dendritic MoS2 flakes, the central crystal structure is similar to the structure of six lobed MoS2 while the branch area has specific features. Figure 3e and 3f shows the bright and false color dark-field TEM images of a representative MoS2 branch (for raw figures refer to Figure S3). Although the actual edge of the main branch is curved and consists of many sharp thorns with an angle of 120° to the growth front (Figure 3e), the uniform contrast of dark-field image (Figure 3f, blue) and diffraction pattern (Figure 3h) verify the single crystallinity nature of the fishbone main branch. According to the principle of crystal growth, the fast growth facet disappears first, and affects the final morphology of crystal.22 In our case, the growth rate of Mo-zigzag edge termination (VMo) is much faster than that of S-zigzag edge termination (Vs) as a result of excessive sulfur vapor. During the fast growth towards branch front, the growth of S-zigzag termination (red vectors in Figure 3i) cannot catch up with that of Mo-zigzag termination (blue vectors in Figure 3i). Thus, sharp thorns are formed along two sides of the branch with an angle of 120° to the VMo direction as illustrated in Figure 3i. This sulfur induced thorn shape can be also confirmed by the growth result shown in Figure S4. Moreover, side sub-branches that normally stretch out with an angle of 60° from main branches were studied (Figure 3e). Different contrasts in dark-field image (Figure 3f) reveals different orientations of crystal lattice while only single set of diffraction pattern exists for the combined area of side and main branch (Figure 3h). The only possibility is the twin crystal structure where two diffraction patterns can coincide with each other.52 This is reasonable because, during fast growth, the side edge of main branch is curved and not exact S-zigzag.48 So defects can be easily induced by the impurity left on the substrate to reduce the surface energy which is very common in crystal growth.45,53 In this case, twin crystals are preferred as their formation energy is normally one order of magnitude smaller than the other types of 7


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defects.54 As a consequence, the developed Mo-zigzag edge reacts immediately with surrounding sulfur and form a branch with an angle of 60° to the branch front direction which can be proved by the diffraction intensity in inset of Figure 3g. Similar intensity of neighboring diffraction spots in the inset of Figure 3h also evidences the two different lattice arrangements which further confirm our assumption. Thus, it can be concluded that the excessive sulfur vapor concentration leads to the dagger-like shape of MoS2, and the successive nucleation of twin crystals at the side edge is responsible for the sub-branch growth towards the direction of Mo-zigzag edge. The plentiful appearance of sub-branches finally gives rise to the formation of dendritic morphologies of MoS2 flakes.

Figure 4 (a-c) Raman spectra, AFM image and PL spectra of the dendritic MoS2, scale bar is 5 µm; (d-h) PL intensity mapping of different MoS2 morphologies. The intensities are normalized to the same range; (i) Schematic illustration of grain boundary evolution for different shapes. In order to characterize the crystal quality and optical properties, Raman spectroscopy, atomic force microscopy (AFM) and Photoluminescence spectroscopy (PL) were carried out on the dendritic MoS2 crystals. Figure 4a shows the typical Raman spectrum of as-grown MoS2 flakes. The distance in Raman shift between the in-plane E2g1 mode and out-of-plane A1g mode is about 20.9 cm-1 which indicates the predominant monolayer feature of MoS2 flakes.55 Additionally, the AFM graph in Figure 4b gives a line scan profile of an as-grown MoS2 flake, which shows the height of ~0.84 nm, confirming its monolayer (0.69 nm) nature. In Figure 4c, the PL peak positions are located at 677 nm (A peak) and 627 nm (B peak) which are almost identical to the mechanically exfoliated counterpart,56 indicating good crystalline quality of as-grown samples. 8


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When PL measurements are taken under an atmospheric condition, O2 and H2O molecules, adsorbed on the surface of MoS2, can transfer carriers and thus significantly enhances the PL intensity.57,58 In particular, the enhancement of PL intensity is more obvious at the sulfur vacancy sites such as twin boundary where more carriers can be transferred from gas molecule to MoS2 due to strong binding energy.57 Thus, the PL intensity mapping is a useful probe to identify the distribution and characters of twin grain boundaries of dendritic MoS2. Figure 4d-h show the contour maps of integrated PL intensity at 677 nm corresponding to various shapes of MoS2 flakes. Similar to our recent results,36 the triangular shape MoS2 in Figure 4d displays a relatively strong PL intensity at the vertices sites which is attributed to the lattice defect or edge dangling bonds. However, for the compact six-pointed star MoS2 (Figure 4e), the center part shows a stronger PL intensity than that of edges. This is due to the aggregation of twin grain boundaries which consist of numerous sulfur vacancies.37 As the six main lobes of MoS2 flakes become thinner (Figure 4f to 5h), the area of maximum PL intensity shrinks gradually to the center. This result agrees well with the fact that the cyclic twin boundary reduces as the shape of MoS2 flakes turns into dagger-like morphology (Figure 4i). Besides, the twin grain boundary at the side branch sites also contributes to the PL intensity enhancement shown in Figure 4f and 4g. In Figure 4h, there are no twin-defects induced sub-branches and only a small size of cyclic twins in the center of the MoS2 flake. As a consequence, the maximum PL intensity is limited within the center body area. Therefore, the shape dependent optical properties of as-grown MoS2 flakes shown by PL mapping are consistent with the grain boundary evolution as shown in Figure 4i. The above morphology strongly correlates with the optical property; this promisingly brings the possibility of localizing and modifying the PL emission through cyclic twin derived shape-engineering of monolayer MoS2.

Figure 5 Optical images of MoS2 grown on SiO2/Si substrates modified by (a) screen protector, (b) blue tape, (c) 3M magic tape and (d) 3M transparent tape; (e) Statistic comparison of fractal dimension corresponding to different tape treatments.

Tape Type Screen protector tape

Defects at initial

Defects at

nucleation stage

growth stage

Cyclic twin

Twin

Dominant shape Dendrites with hexagonal backbones

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Blue tape

Cyclic twin

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No twin

Compact six-pointed stars

Scotch Magic tape

No twin

Twin

Dendrites with trigonal backbones

Scotch Transparent tape

No cyclic twin

Twin

Irregular dendrites

Table 1. Defects induced by four types of tapes during the MoS2 synthesis process along with the dominant synthesized shapes. To further explore the feasibility of substrate pretreatment for tailoring the shape through twin defects, we modified the substrates by four kinds of tapes with various adhesive compositions and carried out CVD growth of monolayer MoS2 under the same growth conditions (refer to Table S1). Noting that the growth condition for each case may not be best tuned, but since the same growth condition was used for all cases, the results here provide a qualitative evaluation for comparison purposes. Figure 5a shows the growth result on the modified Si substrate with “screen protector tape”. Most MoS2 dendrites possess hexagonal backbones and small side branches, indicating the twin defect formation at both initial nucleation (especially cyclic twin) and the following growth stages. As a common tape for the exfoliation of 2D materials, “blue tape” can modify the substrate differently due to its silicone-free adhesive. Figure 5b shows that the MoS2 grown on the modified substrate with “blue tape” forms compact six-pointed stars. This indicates that the cyclic twinning occurs at the nucleation stage while further twin derived branching at the growth stage is suppressed. By using the “Scotch Magic tape” for substrate pretreatment, triangle shaped MoS2 flakes were fabricated as the main products (Figure 5c). A small number of side branches are randomly formed along the main branches. All these side branches follow the twin crystal geometry which indicates that the twinning merely occurs at the later growth stage. “Scotch Transparent tape” resulted in asymmetric MoS2 dendrites as shown in Figure 5d. Distinctly different from other MoS2 flakes obtained in Figure 5a-c, the as-grown MoS2 dendrites do not have a symmetric backbone and their shapes are greatly diverse with branch directions randomly pointing outward. This result suggests that there are no cyclic twins occurring in the initial nucleation stage while a high density of twins is present in the whole MoS2 growth stage, resulting in the formation of large amounts of branches. Table 1 summarized the defects induced by four types of tapes during the MoS2 growth along with the dominated produced shapes. It is suggested that the adhesive seed affects the formation of nuclei or interrupts the growth of MoS2 crystal by inducing twinning as the impurity. Due to the difference in composition (or impurity type, see Table S1), different tapes have various abilities to promote the formation of twinning such as forming cyclic twin at nucleation stage or simple twin at the branch edge during growth process. Figure 6e illustrates the statistics results of corresponding D to different tape treatments by collecting 10 samples from each case. The D of resultant MoS2 crystals follows the following trend: (scotch transparent tape) < (scotch magic tape) < (screen protector) < (blue tape). This shows a good control on the fractal dimension of as-grown MoS2 crystals simply occurred by the substrate treatments. In addition, the fractal dimension can further be modulated by combining insulating 10


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substrates, such as sapphire (c-plane) and fused silica (Figure S5), with the above tapes. In particular, the MoS2 dendrites grown on fused silica can obtain an impressive low fractal number (D = 1.72) than the previous reports.59 These results confirm the applicability of our strategy for reliable shape-engineering of 2D materials and a broad morphology modulation range (spanning D from 1.68 to 2.00). Compared with the recent reported dendritic MoS2 grown on symmetry-disparate substrates, such as SrTiO3 and LaAlO3,39-41 our method is more simple and robust which promotes the density of active sites not only by forming plenty of edges at fishbone structure, but also by creating abundant twin boundaries. Thus, these derived twin-defect 2D complex structures may provide a route for applications in optoelectronics, electrocatalysis, etc. CONCLUSIONS Shape-engineering of monolayer MoS2 dendrites was performed by intentionally introducing twin defects at the initial nucleation stage and/or the growth process. This was achieved by using the simple and robust substrate pretreatment with tapes. The obtained MoS2 crystals were featured with hexagonal backbones and tunable degrees of fractal shapes. The shape evolution was attributed to the synergistic result of twin defect nucleation derived by the adhesive seed and the variation of local S: Mo vapor ratio. In addition, strong and localized enhancement of photoluminescence emission was observed in the cyclic twin regions due to the accumulated sulfur vacancies. This work provides a robust strategy and simple protocol for synthesizing monolayer MoS2 with controllable shapes. It also contributes greatly to the understanding of twin defect growth mechanism and gives rise to electrocatalytic and optoelectronic applications. METHODS

Growth of dendritic MoS2. The MoS2 crystals were synthesized by using the atmospheric-pressure CVD method. The SiO2/Si substrates were cleaned with acetone, isopropanol and DI water, followed by 1 minute O2 plasma. A piece of tape (Screen protector in Figure 1) was gently pasted on the substrate and stripped away, leaving tiny amounts of adhesives on the surface. 2 mg MoO3 powder (≥99.5%, Sigma Aldrich) was placed in a quartz boat 18~ 20 cm away from the edge of the furnace. The substrate was located 0.2~ 0.5 cm upstream from the MoO3 source which provided a sulfur rich environment for the fast growth (see Figure S1). 80 mg sulfur powder (≥99.5%, Sigma Aldrich) was positioned near the edge of the furnace throughout the reaction. The system was pumped down to 10-3 torr for three times to remove oxygen and kept in atmospheric pressure with 8 sccm ultra-pure argon flow. Then, the furnace was heated up to 600 °C in 30 minutes and maintained for 5 minutes before natural cooling. TEM sample preparation. A Polymethyl methacrylate (PMMA, A2) layer was firstly spun onto the surface of substrate. 2M KOH solution was applied to etch the 11


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SiO2 layer, then, the PMMA support was rinsed in DI water for several times and fished by the TEM grid. By soaking the grid into acetone overnight, PMMA can be removed.

Characterizations. The SEM images were taken by Hitachi S-4800 with 1-5 kV. The surface morphology was examined with an atomic force microscope (AFM) (Asylum Research, MFP-3D Stand Alone). Raman and PL spectra were obtained by using a Raman spectroscope (Horiba, LabRAM HR Evolution) with a laser excitation wavelength of 532 nm. TEM characterizations were carried out on JOEL 2010F with 80 kV. Tapes information. Silicone-Free adhesive plastic films (Ultron Systems, Inc.), Commercial Screen Protector, 3M Scotch ® Transparent Film Tape 600, 3M Scotch® Magic™ tape 810. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Details of substrate treatment process and growth parameters, fractal dimension calculations, supplementary TEM images, temperature dependent snowflake-like morphology, vapor concentration dependent fishbone structure, impact of adhesive seed quantity on growth, fractal MoS2 dendrite on sapphire and silica, composition details of adhesives tapes.

ACKNOWLEDGEMENT This work was supported by the Guangdong-Hong Kong joint innovation project (Grant No. 2016A050503012), the National Natural Science Foundation of China (Grant No. 51406075 and 51776094), the Guangdong Natural Science Funds for Distinguished Young Scholars (Grant No. 2015A030306044), and the Training Program for Outstanding Young Teachers at Higher Education Institutions of Guangdong Province (Grant YQ2015151). The Student Innovation Training Programs (Grant SITP2016G02, SITP2016X04, SITP2016S18, pdjh2016b0444, pdjh2017b0446, pdjh2017c0009, pdjh2017c0020 and pdjh2017c0025) and starting grants from Southern University of Science and Technology are also acknowledged. REFERENCES 1.

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