Effect of Sulfur Evaporation Rate on Screw Dislocation Driven Growth

Nov 2, 2016 - dislocation driven growth of two-dimensional MoS2 that contains unprecedented atomic ...... patterns along with the respective analysis ...
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Effect of Sulfur Evaporation Rate on Screw Dislocation Driven Growth of MoS2 with High Atomic Step Density Pawan Kumar and B. Viswanath* School of Engineering, Indian Institute of Technology Mandi, Mandi, Himachal Pradesh 175001, India S Supporting Information *

ABSTRACT: We report the sulfur evaporation rate controlled screw dislocation driven growth of two-dimensional MoS2 that contains unprecedented atomic step density. Screw dislocation assisted growth of atomic thin MoS2 on amorphous SiO2 or a crystalline and conducting Si substrate paves the way to form spiral morphology of two-dimensional materials without the need for single crystalline miscut substrates to initiate the line defects. The unique spiral morphology promoted by screw dislocation is typically observed at a high nucleation rate induced by an abrupt increase in sulfur concentration. Screw dislocation assisted spiral MoS2 growth leads to a high step density with an ∼5 fold increase in the total edge length. Statistical analysis from detailed atomic force microscope phase imaging of flat and spiral MoS2 flakes reveals the linear increase in total edge length from ∼470 nm to ∼2325 nm with respect to variation in the number of steps available at the spiral structure in the range of 2−8. High resolution transmission electron microscopy imaging reveals the local atomic structure of the ledge that separates the individual layers in spiral MoS2 structure and aids the development of growth mechanism. The optimized spiral growth of MoS2 provides countless active sites for hydrogen energy generation applications. Hydrogen evolution performance of as-grown MoS2 on p-type Si substrates decorated with high ledge density provides active adsorption sites leading to hydrogen evolution at a lower potential with a higher current density. The developed strategy of increasing the sulfur evaporation rate to induce a spike in the nucleation rate promotes screw dislocation driven growth of MoS2 with spiral morphology. Such sulfur evaporation rate controlled spiral growth is important for surface engineering of two-dimensional materials to harvest the benefits of active edge sites for hydrogen evolution, catalyst, and sensor applications.

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oxidation.13 Aberration corrected high resolution transmission electron microscopy (TEM) imaging of high index facets in platinum (Pt) revealed that a high density of surface defects such as atomic steps with low co-ordination numbers found to be responsible for the very high chemical reactivity and catalytic activity for oxygen reduction and electro oxidation of organic fuel molecules.14 The availability of an ultra large surface to volume ratio in atomically thin 2D materials also provides additional opportunities to tune their properties to a greater extent via surface engineering of their atomic structure and defects.3 Molybdenum disulfide (MoS2) has attracted considerable attention due to its unusual optical, mechanical, and electrical properties compared to their bulk forms. Many interesting properties like induced magnetism, spin alignment due to metal doping, strain driven activity on the MoS2 basal plane,15,16 and vacancy induced catalytic activity on basal plane of MoS217,18 have been observed earlier. Such surface engineered MoS2 with a high density of defects also offers excellent semiconducting

INTRODUCTION Research on two-dimensional (2D) materials evolved as a paradigm shift in the field of nanotechnology with the discovery of 2D materials such as graphene and transition metal disulfides due to their superior properties with reduced layer thickness.1 Properties of 2D materials have been largely explored as described earlier for graphene that the “low hanging grapes are mostly harvested”.2 Probably the current challenge is to push the boundaries further by advancing nanomanufacturing aspects in 2D materials with the ability to enhance their properties to their full potential by engineering their surfaces and interfaces. Controlling the growth3−5 at the atomic scale is critical for tuning the properties of thin films, 2D materials, and heterostructures. This also plays a vital role in several technological applications such as catalysis, microelectronics, sensors, energy, and environmental processes.6−11 In the field of heterogeneous catalysis, specific high index crystal planes and high density of defects are known to enhance the turnover frequency of the catalytic reactions, e.g., high density of atomic steps and kinks observed in the case of nanoporous gold found to be responsible for its high catalytic activity for oxidation reactions.12 Similarly, dendritic palladium (Pd) with a high step density shows superior catalytic activity for formic acid © 2016 American Chemical Society

Received: September 15, 2016 Revised: October 24, 2016 Published: November 2, 2016 7145

DOI: 10.1021/acs.cgd.6b01367 Cryst. Growth Des. 2016, 16, 7145−7154

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Figure 1. AFM analysis confirms screw dislocation driven growth of 2D MoS2 flakes on SiO2/Si substrate. (a) Representative AFM image of uniformly grown MoS2 flakes with a very high coverage area. (b) A clear view of spiral structure of as-grown AFM phase morphology. (c) High resolution AFM phase morphology captured for single spiral structure of as-grown MoS2 flake. (d) AFM height image and corresponding (e) line profile (line starts from outer edge and terminates at center of flake) capturing the thickness of individual MoS2 layers and step height.

properties and countless reactive edge sites. Recently, the field of semiconducting MoS2 based electronic devices19 and MoS2 based sensor20 and energy storing devices21 has been steadily growing. In addition to controlling the number of layers in MoS2, engineering the surface structure or stacking of individual atomic layers to expose active edge sites has been found to be useful to enhance their electrocatalytic properties.22,23 Recent studies show 2D MoS2 nanostructures found to enhance catalytic activities in hydrogen evolution reactions due to the higher availability of accessible active sites24−26 for hydrogen adsorption−desorption processes. Although the surface of MoS2 primarily consists of catalytically inert basal plane, edges of MoS2 layers show very high catalytic activity toward hydrogen evolution reactions.27 Density functional calculations also show the hydrogen binding energy on the (100) MoS2 edge is comparable to the binding energies of noble metal catalysts including Pt.28 Hence controlling the growth of 2D materials to achieve such reactive sites to trigger new properties is one of the challenging research topics. Chemical vapor deposition (CVD) is extensively used for the growth of 2D materials such as MoS2, WS2, graphene, and 2D heterostructures by stacking alternate layers of 2D materials.29,30 CVD growth optimizations consisting of controlling the various parameters are routinely carried out to achieve monolayer, bilayer, and large domain size on various substrates.31,32 On the basis of classical crystal growth theories, screw dislocation driven (SDD) growth is predicted to occur at

very low supersaturation, while layer by layer growth or dendritic growth proceeds at relatively higher supersaturation conditions. At very low supersaturation, SDD growth continuously supplies step edges where atoms can be incorporated easily and promote the spiral growth below the required supersaturation to form 2D nuclei. Such SDD growth at very low supersaturation has been demonstrated in a variety of materials with different morphologies.33−35 Recently, a similar approach has been extended to 2D materials such as Bi2Se3 to achieve SDD growth at low supersaturation by the polyol method with precise control of precursor concentration and pH value.36 Detailed observation of SDD growth in 2D nanoplates such as zinc hydroxy sulfate also revealed the correlation between the growth step velocities at the dislocation core versus outer edges and supersaturation to explain the different morphologies.37 While the spiral growth by screw dislocation is fairly clear, the emergence of screw dislocation only at certain supersaturation conditions is not well understood. Interestingly, seeding of screw dislocation has been demonstrated by introducing a spike in supersaturation in the initial stage of CVD growth of MoS2 layers with spiral morphology.38 In this work, we report sulfur evaporation rate controlled, screw dislocation driven CVD growth of MoS2 flakes with a spiral structure that provides enhanced active sites due to increased edge length by approximately 5 fold. We also demonstrate screw dislocation assisted growth of MoS2 on 7146

DOI: 10.1021/acs.cgd.6b01367 Cryst. Growth Des. 2016, 16, 7145−7154

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observed at different growth conditions such as a low sulfur evaporation rate. The adopted growth conditions for spiral and flat MoS2 are listed in the Table-ST1 (see Supporting Information). With the knowledge of growth conditions, it is possible to estimate the growth driving forces for 2D nucleation and continuous growth by taking surface energy and monatomic step height into account and establishing the growth mechanisms. Since the 2D material growth is very sensitive to several vapor phase growth and reaction parameters, it is difficult to quantify the growth driving forces precisely. Nevertheless it is possible to explain the growth process by taking the qualitative values of growth driving forces in the light of classical crystal growth theories.43 Typically crystals growing at a low driving force have to rely on a layer by layer growth mechanism or on dislocation driven growth. Predicting the growth of crystals from the melt during solidification and growth from solution during a chemical reaction is possible considering the thermodynamic driving force available for growth.44 Earlier we reported symmetry breaking aspects and the formation of several 2D nanostructures via layer by layer growth at low driving forces in the absence of screw dislocation.45,46 When the driving force of the chemical reaction is matched with the driving force of layer by layer growth, the resulting crystals take 2D shapes. On the contrary, a continuous growth mechanism leading to the formation of 3D shapes is expected when the chemical driving force is approximately 3 times higher.44 It is possible to quantify the critical driving forces, Gcrit2D < σg/a and Gcrit3D > πσg/a for layer by layer growth and continuous growth, respectively. Here “σ” refers to surface energy, “g” is a measure of diffuseness of interface which is taken as 1 for sharp interfaces and “a” is monatomic step height. Molar volume of crystalline solid (VM = VcellA/Z), where symbols are Avogadro’s constant (A) and the number of formula units per unit cell Z is used for quantifying the driving forces for MoS2. We estimated the growth driving force by taking surface energy of basal plane and mono atomic step height of MoS2 to aid the mechanistic understanding of MoS2 growth. We have taken the reported values of the basal plane surface energy of ∼250 mJ/m2 and a monatomic step height of 0.68 nm for MoS2.23,47,48 The step height used for the estimation is also in agreement with measured step height from our AFM imaging and analysis. The calculated values of driving forces for layer by layer growth (2D) and continuous growth (3D) of MoS2 are 11.77 and 36.95 kJ/mol, respectively. The corresponding 2D nanostructure growth by lateral movement of atoms is expected at low driving force conditions (ΔG < 11.77 kJ/mol), while continuous growth leading to threedimensional shapes are favorable at high driving forces (ΔG > 36.95 kJ/mol). Accordingly, we observed the growth of flat MoS2 flakes at relatively low driving force conditions. This suggests that the flat MoS2 flakes grow at low driving force conditions set by a low evaporation rate of sulfur at 180−220 °C. However, we did not observe three-dimensional shapes of MoS2 for continuous growth; rather spiral growth is observed with the sulfur evaporation rate at 350−400 °C. Further growth optimization has been carried out by varying sulfur (S) concentration (S/MoO3 ratio), evaporation rate, and temperatures of MoO3 and sulfur zones. We achieved the formation of flat MoS2 at various combinations of low temperature, low sulfur concentration, and low evaporation rate, while the spiral structure of MoS2 resulted for high temperature, high sulfur concentration, and high evaporation rate.

various substrates such as 300 nm amorphous SiO2/Si, 30 nm amorphous Si3N4/Si, heavily doped Si (100) substrates along with reduced graphene oxide (rGO) to make heterostructures. This opens up newer possibilities to achieve spiral growth of 2D materials, decorated with atomic steps by controlling the evaporation rate of precursors on any required substrates. We find that increasing the sulfur evaporation rate increases the nucleation rate, aids incorporation of screw dislocation, and promotes spiral growth of MoS2. We have also optimized the growth conditions to increase the number of catalytically active ledges in 2D MoS2 and tested their hydrogen evolution performance for energy applications.

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RESULTS AND DISCUSSION A spiral structure of MoS2 flakes consisting of atomically thin layers stacked with the aid of screw dislocation has been grown using an in-house developed CVD system. MoS2 flakes with spiral morphology on amorphous SiO2 (300 nm)/Si substrate were first analyzed using atomic force microscope (AFM) as shown in Figure 1. AFM with scan assisted imaging is used to distinguish the single atomic layer thin flakes of MoS2 with multistep topography. The AFM phase imaging of MoS2 clearly shows the spiral structure topology due to SDD growth phenomena. Such spiral structures arising from screw dislocation are commonly observed in various 2D materials and nanostructures.39,40 It is interesting to note that the inert basal plane in MoS2 is known to be molybdenum-rich, while the active prismatic plane is terminated with sulfur atoms.41 The observed spiral morphology of MoS2 results in projecting the sulfur terminated edge sites. However, for realizing the property enhancement, the atomic scale control over the edge sites in MoS2 has to be grown in a large scale. Here we demonstrate the optimization of such growth in 1 cm2 area of amorphous SiO2/Si substrate. Figure 1a shows a representative AFM image of a large area coverage of SDD growth of MoS2 flakes. Further we achieved the controlled SDD growth of MoS2 with spiral morphology of a similar dimension with various areal coverages (see Supplementary Figure S1). It is evident from the AFM phase image that the as-grown MoS2 on SiO2/Si substrate is decorated with a high density of atomic steps as shown in Figure 1b. A zoomed AFM phase image of single MoS2 also reveals the atomic steps of spiral topography in as-grown MoS2 flakes (Figure 1c). The thickness of each atomic layer of this unique spiral structure of MoS2 is confirmed by AFM height profile as shown in Figure 1d. The step height measured from the individual MoS2 layer is ∼0.7 nm, which is very close to actual interlayer spacing of 0.67 nm.42 This suggests that the spiral growth of stepped monolayer indeed arises due to the SDD growth with atomic layer spacing of MoS2 (Figure 1d). This SDD growth of MoS2 is typically formed with left- or/and right-handed geometry (see Supplementary Figure S2). Since the MoS2 spiral growth with high density of steps require substantially higher S content, the growth carried out at high S evaporation rate results in spiral MoS2 structures exclusively. The formation of an SDD spiral structure has a very high (>95%) population in terms of the ratio to mixture of flat and spiral morphology obtained in the same sample. We have also achieved the growth of flat MoS2 flakes without any spiral morphology. Smooth surface topography without any observable surface defects is evident from the AFM and FESEM images of flat, trilayer MoS2 (see Supplementary Figure S3). This trilayer MoS2 (see Supplementary Figure S4) formation without spiral structure is mainly 7147

DOI: 10.1021/acs.cgd.6b01367 Cryst. Growth Des. 2016, 16, 7145−7154

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Figure 2. Growth evolution of screw dislocation driven spiral MoS2 flakes. (a−e) AFM phase images of a different growth stage (micron marker is same for all images) and (f) corresponding schematics explaining the growth phenomena using screw dislocation. Approximate growth durations for each stage are as (a−b) ≤ 1 min, (c) ∼2 min, (d−e) ∼5 min.

Figure 3. Screw dislocation driven growth of spiral MoS2. (a) Large flake of MoS2 showing a flat microstructure without spiral structure formation in the absence of a spike in the sulfur concentration. (b) MoS2 nucleating on top of another one with an incomplete edge after introducing a spike in the sulfur evaporation rate, (c) fully grown spiral MoS2 flake, (d) high resolution AFM image of as-formed spiral MoS2 shown in panel (c) with signatures of an accidental defect.

Layer by layer growth has to rely on both step formation and the lateral movement of the formed steps. In the absence of any defect or dislocation, the growth rate is determined by the formation of 2D nuclei which is the case for the observed flat MoS2 through layer by layer growth. On the contrary, the spiral growth rate is determined by step velocity and not the formation of steps as the presence of screw dislocations acting as a nonvanishing source for providing atomic steps.49 Typically, spiral growth is promoted by introducing vertical offset by means of screw dislocation on the growing surface of the 2D material.43 Such a ledge becomes an attractive site for atomic species due to the decreased energy barrier for

attachment. Once screw dislocation is introduced to the 2D layer, a step will be formed. Then the growth species can be easily incorporated to the already developed kink sites resulting in lateral movement of steps. Formation of different spiral structures mainly depends on the number of dislocation lines formed and propagation velocity of center point and of edge. Because of the non-centrosymmetric (AA stack) ordering and inversion symmetry,38 the hexagonal lattice structure of grown 2D MoS2 forms usually a triangular or hexagonal planar shape with edges parallel to each other during the formation of a spiral structure. However, it is not fully clear how the screw dislocations are generated in the MoS2 2D layer during the 7148

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Figure 4. Histograms of total edge length obtained from (a) individual flat MoS2 flake and (b) spiral MoS2 flake. Representative AFM phase images show the dimensions of edge lengths for (c) flat MoS2 and (d) spiral MoS2. (e) Plot showing the correlation between the MoS2 flake thickness, number of steps, and total edge length for individual spiral MoS2 nanostructures.

initial stages of growth. It is proposed that intersecting adjacent domains may generate screw dislocations after second layer extension on the uplifted edge for the SDD growth in the case of WSe2.50 While the earlier reports correlate the SDD growth solely with low supersaturation, precise understanding of screw dislocation origin has not been addressed. As noted in CVD growth conditions (Table-ST1), flat MoS2 growth is observed at a low evaporation rate of sulfur at the temperature range of 180−220 °C, while the high temperature range of 350−400 °C with increased sulfur evaporation rate results in SDD spiral growth of MoS2. While it is difficult to quantify the supersaturation in the vapor transport growth, it can be described qualitatively based on the sulfur evaporation rate. In addition, the followed synthesis protocol with a higher ratio (20) of MoO3 and sulfur also results in achieving high supersaturation for the case of SDD spiral growth of MoS2. The rate of nucleation is sensitive to the degree of supersaturation; typically, the nucleation rate is zero up to a certain value of supersaturation. Beyond this critical supersaturation, the nucleation rate increases sharply by many orders of magnitude. Such an abrupt increase in the nucleation rate may cause accidental growth causing defective edges seeding the screw

dislocation. We carried out detailed AFM imaging to capture the growth mechanism of MoS2 with spiral geometry. Figure 2a−e reflects the growth sequence of the formation of the spiral structure of 2D MoS2 flakes as captured by the AFM phase signal. Although it is difficult to capture the exact morphology of growth evolution sequence as a function of growth time, the presented images show snapshots of their evolution. At the initial stage of layer formation, local changes in the arrival rate of species such as sulfur may introduce step separating domains of different atomic thicknesses. This leaves one layer uplifted against another layer in the same MoS2 flake and forms spiral geometry during the growth propagation. Here spiral morphology formed after the dislocation is introduced in base/second layer and a further layer grows on top of it. We hypothesize that the variation in the arrival rate of chemical species is directly related to the nucleation rate of MoS2 and plays an important role in the formation of dislocation and spiral growth. To test our hypothesis, we induced local heterogeneity in the nucleation rate by introducing a spike in the sulfur evaporation rate. Such a spike in the evaporation rate is achieved by moving the sulfur precursor into a high temperature zone abruptly for a short interval of 30 s using 7149

DOI: 10.1021/acs.cgd.6b01367 Cryst. Growth Des. 2016, 16, 7145−7154

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the magnetic arm attached to the CVD system. Interestingly, AFM imaging confirms the formation of the spiral MoS2 structure only when a spike in the sulfur evaporation rate is introduced (Figure 3). Growth evolution starting from the incomplete MoS2 layer and eventual formation of spiral MoS2 during the CVD growth with a sudden increase in the sulfur evaporation rate is shown in Figure 3b,c. AFM imaging of spiral MoS2 grown with a spike in the sulfur evaporation rate also reveals the signatures of accidental overgrowth at the MoS2 edges as pointed by the arrow mark in Figure 3d (see Supplementary Figure S5). Further analysis from the AFM phase signal of the MoS2 spiral structure reveals the correlation between edge length and number of atomic steps. Total edge length of MoS2 with spiral topography also increased significantly compared to the flat MoS2 flakes. Our optimized growth at a high sulfur evaporation rate yielded spiral MoS2 with an ∼5 fold increase in the active edge length in a single flake as compared to the flat MoS2. Statistical analysis of the edge length obtained from the AFM imaging of flat and spiral MoS2 is presented as a histogram in Figure 4, panels a and b, respectively. The total edge length averaged over the area is estimated from the entire spiral structure and compared with the outer boundary edge length of flat MoS2 flakes. While the total edge length of flat MoS2 is around 478 nm, the total spiral edge length is measured to be around 2320 nm. The observed 5 fold increase in the edge length in comparison to the same dimension of nonspiral flakes suggests the large increase in sulfur terminated prismatic edges. Figure 4c,d shows AFM images of flat and spiral MoS2 respectively, indicating their distribution. Detailed analysis of AFM phase images of spiral MoS2 flakes shows a direct correlation between number of atomic steps, total thickness of individual flakes, and total edge length. Figure 4e shows the linear increase in the number of steps varying from 2 to 8 for the increase of flake thickness that varies from ∼1 to 6 nm with their total edge length from ∼40 to ∼4400 nm. Raman spectroscopy has been employed to confirm the formation of MoS2 flakes. Comparative study displayed in Figure 5a shows the difference in Raman shift (Δk = 24.7) of in-plane vibration (E2g) and out of plane vibration (A1g) and clearly indicates the formation of atomically thin few layer MoS2 flakes. Considerable variation in the Raman shift is observed between spiral MoS2 and flat MoS2. This is consistent with the Raman shift trend with respect to the increasing number of layers reported earlier.51 As the number of layers increases and goes up to maximum of eight in spiral MoS2 flakes, their corresponding increased edge length and the Δk value also varies in a specified range. The observed A1g and E2g bands in MoS2 are inferred as the out-of-plane vibration of sulfur atoms along the c-axis and in-plane vibration of S−Mo−S atoms along the basal plane, respectively.52 For comparison, the actual position of Raman bands and shifts (24.71 cm−1) of spiral MoS2 flakes are shown in reference to the Si peak of 521 cm−1 (see Supplementary Figure S6). Electron microscope investigations have been carried out to provide more insights to the growth and microstructure of spiral MoS2 flakes. Secondary electron image obtained using FESEM captures the atomic steps in spiral MoS2 flakes as shown in Figure 5b. Atomic steps and spiral structure in several MoS2 flakes are clear from the topological contrast. It is also evident from the SEM observation that the MoS2 flakes are densely grown on substrate, undergo coalescence, and form an interconnected 2D structure on the substrate. Formation of such dense MoS2

Figure 5. Structural and microstructural investigation of screw dislocation assisted spiral growth of MoS2 flakes using Raman spectroscopy and electron microscopy. (a) E2g and A1g Raman modes of few layer spiral MoS2 flakes having a Raman shift, Δk = 24.7 in comparison to bulk (Δk = 26.73) and flat few layer (mostly trilayer, Δk = 24.8) (b) FESEM surface morphology of spiral MoS2 flakes (c) TEM bright field image of spiral MoS2 flakes directly grown on the Si3N4 membrane grid.

flakes with a spiral structure is again confirmed by a TEM bright field image shown in Figure 5c. MoS2 flakes also show considerable contrast inhomogeneity within a single platelet indicating the spiral structure formation with varying thickness. High magnification TEM bright field images of MoS2 flake clearly reveal the spiral structure of MoS2. A representative TEM bright field image of spiral MoS2 and the corresponding electron diffraction pattern recorded along the [001] zone axis are shown in Figure 6, panels a and b, respectively. It is evident from the TEM bright field image that the center of MoS2 appears darker compared to the outer region due to the variation in the thickness contrast. The variation in the thickness can be easily seen as the number of layers is increasing from the edge to center in agreement with the AFM 7150

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Figure 6. TEM analysis of few layered spiral MoS2 grown on Si3N4/Si membrane. TEM bright field image of MoS2 with spiral structure and the corresponding selected area diffraction pattern are shown in (a) and (b) respectively. (c) HRTEM image recorded from the edge of the layered structure along with their FFT (inset). Hexagonal atomic arrangement in the outermost single layer, double layer, and their FFT filtered images showing a moiré pattern (d and e). Zoomed regions from the single layer and double layer are also shown as insets in (d) and (e).

line profile analysis. Selected area electron diffraction pattern (SAED) recorded from such spiral MoS2 flakes with several layers also showing overlapping diffraction patterns consisting of more than two reflection sets (see Supplementary Figures S7 and S8). Note that the MoS2 layers were directly grown on amorphous Si3N4 membrane grids and show the actual growth microstructure of spiral MoS2 with overlapping layers. High resolution image recorded from spiral MoS2 edge reveals the local atomic structure of ledges. MoS2 lattice spacing of 2.7 Å for (100) plane is also confirmed by the line profile analysis of high resolution TEM images (Figure 6c). High resolution TEM imaging reveals the ledge which separates the bilayer and monolayer in spiral MoS2. FFT analysis across the ledge from both the bilayer and monolayer has been carried out, and the respective FFT patterns are shown as insets. Further analysis shows that the layers are stacked with some definite angular misorientation in the case of bilayer MoS2. From the FFT pattern, we observed a slight rotation (θ ≈ 18.7°) between the two individual layers. FFT from the bilayer region also shows a double reflection instead of a single spot confirming the overlapping MoS2 layers with spiral geometry. Such additional spots are not observed from the monolayer MoS2 which is confirmed by the FFT in agreement with the SAED pattern shown in Supplementary Figure S8. FFT filtered images from the bilayer and monolayer region also reveal the atomic structure. A hexagonal atomic arrangement is seen in the case of monolayer MoS2, while the moiré patterns are observed from the overlapping bilayer region (Figure 6, panels d and e respectively). It is the ledge that separates the MoS2 layers and acts as active sites for enhancing the hydrogen evolution reaction (HER) activity as described below. MoS2 directly grown on heavily doped, conducting Si substrate has been used for carrying out electrochemical HER. This approach enables the direct electrochemical measurement from as-grown MoS2 on Si without the need for MoS2 transfer on a glassy carbon by drop casting or coating. Figure 7 shows the linear sweep voltammograms for flat MoS2 and spiral MoS2 flakes with different areal coverage. For comparison, we have also investigated the HER activity of bare, heavily doped Si. As expected, bare Si does not have any HER activity, while other MoS2 nanostructures with different coverage show appreciable signatures of catalytic activity. Representative AFM images showing different areal coverage of spiral MoS2 and flat MoS2 on heavily doped Si substrate

Figure 7. HER activity of CVD grown spiral MoS2 with high density of active ledge structure in comparison to different areal coverage as MoS2 flat structure with 85% coverage, spiral MoS2 with 25% and 50% coverage on Si substrates.

which is used for HER activity are shown in Supplementary Figure S9. Highest HER activity is observed for spiral MoS2 with 50% coverage followed by trilayer, flat MoS2 with 85% coverage. It is interesting to note that spiral MoS2 with as low as 25% coverage is comparable to the activity of flat MoS2 with 85% coverage. This signifies the importance of dense atomic steps providing active sites for catalytic HER activity. The corresponding Tafel slope values vary from 198 mv/dec to 170 mv/dec when we compare the flat MoS2 with 85% coverage to the spiral MoS2 with 50% coverage. Along with low overpotential, high current density is also observed for the case of 50% coverage based spiral MoS2 in comparison to others. The observed enhancement in hydrogen evolution performance per geometrical area of spiral MoS2 is due to the high density of surface steps that are decorating the MoS2 grown on Si. We have calculated the geometrical area (0.8 cm2) by considering the dimension of Si substrate (1 cm × 0.8 cm dimension) with as-grown MoS2 nanostructures. It is reported that the MoS2 pyramids with a high density of steps show a decrease in catalytic performance with an increase in the number of layers due to poor electron transport across the vertical direction as explained by the electron hopping between 7151

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layers in relatively thick 2D structure.53,54 Hence it is important to expose the active edge sites of MoS2 keeping the thickness of 2D structure in the order of nanometers. Interestingly the SDD spiral morphology of MoS2 achieved in the present case provides a continuous pathway to charge carriers around the edges to reach from the top to bottom as they are connected through spiral structure as observed in AFM image (Figure 2e). Hence the role of ledge controlled growth of MoS2 with spiral morphology is 2-fold, provides sulfur terminated active edge sites for hydrogen adsorption, and enables the efficient transfer of charge carriers to the substrate.27 While active edge sites in SDD growth of MoS2 flakes are evident, improving the conductivity and transferring them to suitable electrodes such as glassy carbon may be needed to make a greater impact on hydrogen evolution reaction. Alternatively, we demonstrated the SDD driven spiral growth of MoS2 directly on a reduced graphene oxide surface to make 2D heterostructures (see Supplementary Figure S10). Development of such hybrid structures combining the benefit of graphene, MoS2 along with active edge sites is expected to show superior performance for hydrogen storage and energy applications.

boat inside the heating zone 2 using a magnetic arm. We have also growna spiral MoS2 structure decorated with a high step density on Si3N4 TEM grid, reduced graphene oxide, and heavily doped p-type Si substrate directly for the purpose of TEM investigation and HER activity measurements, respectively. Characterizations. As-synthesized MoS2 flakes were thoroughly characterized by atomic force microscope (AFM, Dimension Icon, Bruker, USA) imaging of 25 representative samples of MoS2 grown at different conditions. Phase contrast imaging and line profile analysis were used to reveal and measure the number of steps and thickness. Raman spectroscopy (LabRAM HR visible (400−1100 nm), Horiba Jobin Vyon, Poland) is used to characterize the various vibrational modes of MoS2 flakes. The size of laser spot was kept ∼1 μm using a 100× objective, and power was used as minimum as 0.2 mW to avoid any damage or local heating. Transmission electron microscopy (TEM, TECHNAI G2, FEI, USA) operated at 200 kV was used to characterize the as-grown MoS2 flakes. Field emission scanning electron microscope (FESEM, Nova Nano-SEM 450, FEI, USA) was used for secondary electron imaging. HER activity has been examined by using a three electrode based Electrochemical Workstation (Multichannel, Autolab, Potentiostat/Galvanostat, Metrohm, Netherlands). The three electrodes mainly consist of platinum as a counter electrode and Ag/AgCl as the reference electrode in the presence of 0.5 M H2SO4 electrolyte. MoS2 directly grown on heavily doped ptype Si substrate was used as the working electrode for evaluating HER performance.

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CONCLUSIONS We achieved CVD growth of MoS2 with spiral structures decorated with high density of active sites by controlling the sulfur evaporation rate on various substrates. Our findings provide a mechanistic understanding of how screw dislocation driven growth initiates and propagates to form a spiral MoS2 2D structure when the spike in the sulfur concentration is introduced. The demonstrated SDD driven MoS2 growth also provides newer pathways to grow 2D materials with a high density of active edge sites. The detailed AFM imaging revealed almost 5 times of increased active edge length for each MoS2 flake with sulfur terminated edges. High resolution TEM imaging and electron diffraction analysis revealed the atomic structure of the ledges that formed for every increment of MoS2 layer during the spiral growth. CVD grown spiral MoS2 with increased atomic step density connecting the entire structure shows enhanced HER. It is also desirable to have such atomically engineered 2D structures on various substrates to harvest their performance to greater extent. The investigated aspects of ledge controlled growth of MoS2 with high step density on Si substrate are relevant for making hybrid nanostructures by decorating MoS2 edges with quantum dots and other metal catalysts for sensor and energy applications. Surface engineered growth of MoS2 with high step density on CVD grown graphene might reduce the overpotential and enhance the HER activity further.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b01367. Additional AFM analysis, FESEM analysis, TEM images, and electron diffraction patterns along with the respective analysis (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the generous institute grant for financial support. We also thank the Advanced Materials Research Centre (AMRC) at IIT Mandi for imaging and other sophisticated characterization facilities. We are thankful to Dr. Aditi Halder of IIT Mandi for very useful discussion on HER.

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REFERENCES

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EXPERIMENTAL SECTION

CVD Growth of MoS2. Controlled growth of spiral MoS2 decorated with atomic steps and flat MoS2 are achieved by a custom built CVD system using a Thermo-Scientific quartz tube furnace (Model Lindberg Blue M). The CVD growth was carried out using MoO3 and sulfur powders as received from Alfa Aesar Ltd., USA (purity ∼99.99%). The typical experimental conditions of CVD growth for flat MoS2 and spiral MoS2 are described in Table-ST1. We have taken MoO3 powder in an alumina boat crucible and placed it at the center (heating zone 1) of a quartz tube furnace. Cleaned Si substrate was placed on the top of MoO3 (heating zone 1) containing an alumina boat-crucible. Another alumina crucible containing sulfur powder was placed on the outer edge of heating zone 2 of a tube furnace such that the environment for sulfurization of MoO3 can be achieved near growth temperature by moving the sulfur containing 7152

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