MoS2 Core-Shell Structures on an

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Synthesis of Vertical MoO/MoS Core-Shell Structures on an Amorphous Substrate via Chemical Vapor Deposition Taejin Park, Mirine Leem, Hyangsook Lee, Wonsik Ahn, Hoijoon Kim, Jinbum Kim, Eunha Lee, Yong-Hoon Kim, and Hyoungsub Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08171 • Publication Date (Web): 22 Nov 2017 Downloaded from http://pubs.acs.org on November 28, 2017

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The Journal of Physical Chemistry

Synthesis of Vertical MoO2/MoS2 Core-Shell Structures on an Amorphous Substrate via Chemical Vapor Deposition

Taejin Park,†,‡ Mirine Leem,§ Hyangsook Lee,§,∥ Wonsik Ahn,§ Hoijoon Kim,§ Jinbum Kim,†,‡ Eunha Lee,∥ Yong-Hoon Kim,§,⊥ and Hyoungsub Kim*,§,⊥ †

Semiconductor R&D Center, Samsung Electronics, Hwaseong 18488, Republic of Korea

‡

Department of Semiconductor and Display Engineering, Sungkyunkwan University, Suwon

16419, Republic of Korea §

School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon

16419, Republic of Korea ∥

Samsung Advanced Institute of Technology, Suwon 16678, Republic of Korea

⊥

SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 16419, Republic of Korea

*Tel.: +82-31-290-7363. E-mail: [email protected]

ACS Paragon Plus Environment

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Abstract Vertical MoO2/MoS2 core-shell structures were synthesized on an amorphous surface (SiO2) by chemical vapor deposition at a high heating rate using a configuration in which the vapor-phase was confined. The confined reaction configuration was achieved by partially covering the MoO3containing boat with a substrate, which allowed rapid build-up of the partially reduced MoO3-x crystals at an early stage (below 680 °C). Rapid temperature ramping to 780 °C enabled spontaneous transition of the reaction environment from sulfur-poor to sulfur-rich, which induced a sequential phase transition from MoO3-x to intermediate MoO2, and finally to MoO2/MoS2 core-shell structures. The orthorhombic crystal structure of MoO3-x contributed to the formation of vertical crystals on the amorphous substrate, whereas the non-volatility of the subsequently formed MoO2 enabled layer-by-layer sulfurization to form MoS2 on the oxide surface with minimal re-sublimation loss of MoO2. By adjustment of the sulfurization temperature and time, excellent control over the thickness of the MoS2 shell was achieved through the proposed synthesis method.


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Introduction Owing to its unique material properties, MoS2 has been considered to be one of the exciting two-dimensional transition metal di-chalcogenides for realizing various novel electronic, optoelectronic, and photonic devices.1−3 Although initial studies have focused primarily on exfoliated MoS2,1−3 extensive reports on the growth of mono- or few-layer MoS2 films on various substrates have since emerged.4−10 Furthermore, significant effort has been also devoted to the modification of the morphology of MoS2 for use as a catalyst and in hydrogen storage and Li-ion batteries.11−21 In this respect, increasing the density of the electro-chemically active sites (edges)11−21 on MoS2 is one of the main approaches for achieving enhanced performance (i.e., enhanced reaction rate or charge capacity) when applied in devices. Among the many proposed nanostructures,11−21 vertically-aligned MoS2 crystals are promising candidates for achieving a significant increase in the number of electro-chemically active sites of MoS2 on a projected area.17−21 Li et al.19 reported the formation of vertical MoS2 nanosheets during growth by chemical vapor deposition (CVD), where the nanosheets were formed via mechanical collision or distortion of planar MoS2 islands. However, controlling the thickness, active site density per unit area, and spatial localization of the vertical MoS2 crystals might be difficult using this method, as the vertical MoS2 nanosheets were formed randomly at the borders between at least two laterally growing MoS2 islands.19 Lan et al.20 grew vertically aligned MoS2 triangles by exploiting the poor wettability of MoS2 on a SiC substrate during the nucleation/growth stage. Most recently, DeGregorio et al.21 formed vertical MoO2/MoS2 coreshell nanoplates via epitaxial interaction with a given substrate (SiC or sapphire). Although these synthesis routes may be favorable for obtaining vertical crystals with greater process control, the


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relatively low density of active sites in the vertical crystals and the need for a specific substrate with an epitaxial relationship are drawbacks for achieving highly efficient or high-chargecapacity energy-storage devices. In this study, we propose a single CVD process utilizing a configuration in which the vapor-phase is confined,22 in combination with a high heating rate, for the synthesis of vertical MoO2/MoS2 core-shell structures. This approach enables the dense growth of vertical MoO2/MoS2 core-shell structures on an amorphous substrate (SiO2) with excellent control over the thickness of the MoS2 shell. Furthermore, we present a detailed discussion of the kinetics of the vertical-growth process based on various experimental measurements.

Experimental Synthesis of MoO2/MoS2 core-shell structure. As schematically illustrated in Figure 1(a), MoO2/MoS2 crystals were grown in a quartz tube furnace (4 cm diameter). A SiO2(300 nm)/Si substrate was placed face-down on an alumina boat containing a stack of MoO3 powder (30 mg, 99.97%, Aldrich) in the middle. The boat had a 0.3-cm-diameter hole at one end and a 1.0-cmgap that was not covered by the substrate to permit access of sulfur vapor (see Figure 1(a) and 1(b)). Sulfur powder (120 mg, 99.98%, Aldrich) was loaded in a separate alumina boat located 20 cm away from the MoO3-containing boat. When a delayed supply of sulfur was needed, the sulfur-containing boat was moved 23 cm away from the MoO3-containing boat. For the extended reduction experiment at 890 °C, the amount of sulfur powder was increased to 200 mg. After loading the precursors and substrate, the quartz tube was evacuated to a base pressure below 8×10-3 Torr, and Ar (500 sccm, 99.999%) gas was then introduced. The working pressure was


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intrinsically maintained at 1.0–1.2 Torr during the entire growth process. To obtain a high heating rate during the CVD process, the hottest central zone of a heater pre-heated to different temperatures was quickly shifted to directly above the MoO3-containing boat after sample loading. Temperature measurements using a thermocouple inserted in the quartz tube indicated that the heating rate for the MoO3-containing boat and substrate region was around 52 to 54 °C min-1. In all of the experiments, the heater was quickly removed when all of the sulfur had vaporized. Characterization methods. The features of the as-grown crystals and their identities were analyzed using scanning electron microscopy (SEM, JEOL, JSM-6390A at 15 kV), high resolution scanning transmission electron microscopy (STEM, FEI, FEI Chemi-STEM at 200 kV), X-ray diffraction (XRD, Bruker, D8 Advance with a Cu Kα X-ray source), and Raman spectroscopy (WITEC, Alpha 300R). For preparation of the STEM specimens using a focused ion beam system, two protective capping layers (C and Pt) were deposited in series to minimize physical damage to the grown crystals. Quantification by energy-dispersive X-ray spectroscopy (EDS) image mapping was carried out using Bruker Esprit software. Thermal analysis of the precursors (MoO3-only and MoO3 mixed with 10 wt% sulfur, 12 mg) was conducted using thermogravimetry/differential thermal analysis (TG/DTA, Seiko, Exstar 6000) at a ramping rate of 80 °C min-1 under N2 atmosphere (200 sccm without pumping).

Results and Discussion The confined vapor-phase reaction configuration was created by placing the SiO2/Si substrate face-down on top of a boat containing MoO3,22 with small openings at both ends of the


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boat (Figure 1(a) and 1(b)). The heating schedule for the CVD process at different temperatures is presented in Figure 1(c–e). At an early stage of the reaction, prior to sulfur melting (at a CVD temperature below 680 °C), the sulfur supply was limited, and therefore, a weakly reducing atmosphere could be established. In addition, a large amount of partially reduced MoO3-x could be deposited4,23 on the SiO2/Si substrate due to the small transport distance (less than 1 cm) between the substrate and MoO3 as well as the high partial pressure of MoO3-x in the confined configuration. With progress of the CVD process, melting of the sulfur powder was initiated (at a CVD temperature over 680 °C, which corresponds to a temperature exceeding 105 °C at the site of the sulfur-containing boat) and the reaction zone quickly changed to a highly reducing atmosphere under a sufficient sulfur-vapor supply. Under these highly reducing conditions, it was expected that most of the deposited MoO3-x would be quickly transformed to a non-volatile compound such as MoO224−26 before re-subliming, given that the ramping rate (over 50 °C min-1) was much higher than that used for conventional CVD processes (generally, below 20 °C min1 21,27–29

).


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(a)

(b) Heater Substrate

Substrate

Ar

S

MoO3

Alumina boat

Alumina boat with MoO3

Onset of Heater sulfur melting removed

1000

(c)

(d)

MoO3-position Temp.

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600 400 200 0

Sulfur-position Temp.

0

5

10 15 20 Time (min)

25

0

5

10 15 20 25 Time (min)

0

5

10 15 20 25 Time (min)

Figure 1. (a) Schematic diagram of the CVD system and (b) photograph of the MoO3-containing boat covered with a substrate in a confined vapor-phase reaction configuration. Temperatures measured at the sites of the MoO3- and sulfur-containing boat as a function of the process time for different CVD temperatures: (c) 730, (d) 780, and (e) 890 °C. The onset times for sulfur melting were determined by visual observation.

To verify the proposed mechanism of sulfur-induced reduction at a rapid heating rate, TG/DTA measurements were performed using a high ramping rate of around 80 °C min-1 with two different powders: MoO3-only and MoO3 mixed with 10 wt% sulfur (Figure S1 in the Supporting Information). The exact onset temperature for the possible phase transition or chemical reaction could not be confirmed due to the difference in the experimental conditions (pressure, environment, etc.) between the CVD and TG/DTA setups; however, the overall series of events that occur during rapid ramping remain valid. When the temperature was quickly increased to 750 °C, the MoO3 in the mixed sulfur-MoO3 powder underwent continuous sublimation after a short sulfur-vaporization step. The rate of sublimation of MoO3 in the mixed sulfur-MoO3 powder was similar to that of the pure MoO3 powder, and thus, was not notably


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affected by sulfur vapor possibly remaining in the atmosphere. In addition, the amount of remaining liquid sulfur might be small enough not to affect the rate of MoO3 sublimation considering the fast drop in the TG signal during the initial temperature-ramping stage. However, when the target temperature was increased to 800 °C, the mixed sulfur-MoO3 powder exhibited a much slower sublimation rate than that at 750 °C, whereas the rate of sublimation of the pure MoO3 powder increased significantly. This indicates that the vaporized sulfur largely suppressed sublimation of MoO3 by quickly transforming MoO3 to non-volatile species when the temperature approached 800 °C. MoO2 is thought to be the non-volatile compound formed during the TG/DTA measurement based on its formation temperature (775 °C in the case of sulfur-induced reduction of MoO3)30 and sublimation temperature (1100 °C).25,26 Consequently, it was expected that the non-volatile intermediate MoO2 formed during the CVD process would be sulfurized during the prolonged process at high temperature. Using the proposed CVD hardware configuration and by rapidly elevating the temperature of both the MoO3 source and substrate to 730, 780, or 890 °C, we obtained densely grown vertical MoO2 and MoO2/MoS2 crystals (a few micrometers in size) in the center of the substrate, as shown in the SEM images in Figure 2(a–c); note that reproducible results were obtained for more than ten replicate experiments. The higher CVD temperatures (780 and 890 °C) resulted in production of more vertical crystals. For all temperatures, there was a greater abundance of vertical crystals in the center of the substrate (right above the MoO3 powder) compared with the edges (Figure 2(d–f)), which is probably due to the effects of the substrate geometry on transport of the sublimed-MoO3-x vapor. At the very edge of the substrate near the two openings, only a small population of laterally grown crystals was observed (see Figure S2); this is attributed to the relatively lower partial vapor pressure of MoO3-x near the openings in the confined configuration.


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The Raman spectra shown in Figure 2(g) reveal that the crystals grown on both the center and edges at 730 °C consisted mostly of MoO2. When the reaction temperature was further increased, the two typical MoS2 Raman peaks of the E12g and A1g states appeared around 383.3 and 405.4 cm-1, respectively.31 This indicates that MoS2 underwent a certain degree of transformation at the edges of the substrate at 780 °C and at both the center and edges at 890 °C, as shown in Figure 2(h) and 2(i). Detailed wide-range Raman spectra are also presented in Figure S3. The fast transformation to MoS2 on the substrate edges can be attributed to the geometrical effect of using the confined vapor-phase reaction configuration, because the substrate edges are close to the boat openings, and thus, sulfurization to MoS2 is more facile at the edges than the center.


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780 °C

(a)

890 °C

(b)

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5㎛

5㎛

5㎛

1㎛

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2g

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(i) MoO2

A1g E12g

Substrate edge Substrate center

360 380 400 420 Raman Shift (cm-1)

360 380 400 420 Raman Shift (cm-1)

360 380 400 420 Raman Shift (cm-1)

Figure 2. (a−f) SEM images and (g−i) Raman spectra of the crystals at the center and edges of the SiO2/Si substrate, grown using CVD at different temperatures: (a, d, g) 730, (b, e, h) 780, and (c, f, i) 890 °C. The SEM images in (a−c) and (d−f) were obtained from the center and edges of the substrate, respectively. The scale bar in the inset figures in (a−c) is 1 µm.

Based on these experimental results, it is proposed that the initially grown MoO3-x underwent a sequential phase change to MoO2 and then MoS2 as the reaction proceeded, which is similar to the two-step growth approach used for the CVD synthesis of MoS2.4,21,24,30 Figure 3(a)


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and 3(b) show schematic illustrations of the reaction paths, with three possible stages for the present CVD process. In Stage I, the atmosphere is weakly reducing before the sulfur melts. During this stage, a large number of MoO3-x crystals grow on the down-facing substrate due to the small distance between the substrate and MoO3-x and the high concentration of volatile MoO3-x molecules sublimed from MoO3 in the substrate-confined boat structure. Due to the orthorhombic crystal structure of MoO3-x,23,32,33 the probability of vertical growth on the substrate is high if sufficient MoO3-x vapor is retained in the confined configuration. In order to confirm the formation of MoO3-x in the early stages of deposition, we analyzed the substrate and remaining powder within the boat after interrupting the CVD process at 680 °C (just prior sulfur melting). As shown in Figure 3(c), stacks of perpendicularly grown plates were deposited on the substrate and the Raman spectra of these species verified the formation of various lower-valencestate Mo oxides34,35 (see Figure S3(d)). Furthermore, the XRD data (Figure 3(d)) for the blackcolored powder remaining in the boat indicated that most of the MoO3 powder was transformed into partially reduced MoO3-x phases, such as Mo9O26 and Mo4O11. It is noteworthy that the remaining powder also contained MoO3 and MoO2. Following Stage I, the CVD reaction proceeded to Stages II and III at high temperatures. Stage II provides a highly reducing environment with sulfur-enrichment in the reaction space. Due to the fast ramping rate, MoO3-x (grown in Stage I) is rapidly reduced to non-volatile MoO2 in this environment before it re-sublimes. Further reduction at 780 °C (Stage III) induces a phase transition from MoO2 to MoS2, as verified by the Raman spectra in Figure 2. The vertical MoO2 crystals formed in Stage II remain in the solid state, even up to 890 °C, due to the high MoO2 sublimation temperature of 1100 °C.25,26 In Stage III (above 780 °C), the sulfur-induced transition to MoS2 proceeds from the edges of the substrate to the center, as shown in Figure 2,


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demonstrating the possible dependence of this transition on the amount of sulfur supplied. Therefore, an increase in the number of MoS2 atomic layers is expected upon increasing the sulfurization time in Stage III.

(a) I. Growth of MoO3-x crystals (Orthorhombic)

II. Strong reduction to MoO2 (Partial re-sublimation)

III. Sulfurization to MoS2 (Surface transition)

Highly reducing

Highly reducing

Substrate

Weakly reducing MoO3-x

1000

MoO3

(b)

800

Mo4O11

MoO2

III

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II I

400 200 0

Mo9O26

(d)

(c)

100 ㎛ 0

5

10 15 20 Time (min)

25

10

20

30

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Figure 3. (a) Schematic illustration of the proposed three-step CVD growth in a confined vaporphase reaction configuration. (b) CVD temperature as a function of the process time, indicating the three steps for the 890 °C process; the inverted red triangles indicate the onset time for sulfur melting observed visually. (c) SEM images of the crystals grown on the substrate and (d) XRD spectrum of the remaining powder in the MoO3-containing boat (see the inset photograph) unloaded immediately prior to sulfur melting (Stage I). The scale bar in the inset figure in (c) is 20 µm. The XRD peaks were assigned based on the JCPDS data (MoO3: #01-089-7112, Mo9O26: #00-012-0753, Mo4O11: #01-072-0448, and MoO2: #01-073-1249).


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We believe that the fast ramping rate with timely supply of sufficient sulfur vapor (by its melting) to generate a reducing atmosphere before re-sublimation of the initially grown vertical crystals is one of the most critical experimental parameters for successfully conducting the threestage CVD process. When the ramping rate was decreased (5 °C min-1 at a CVD temperature of 780 °C) or when sulfur melting was delayed (by placing the sulfur source far from the hottest zone (23 cm away from the substrate)), formation of the vertical crystals was largely suppressed, as shown in Figure S4 and S5. Notably, for the experiment with a delayed sulfur supply, triangular and star-shaped mono- or bi-layered MoS2 crystals larger than 15 µm were grown laterally at 780 and 890 °C; this is typically observed in the one-step MoS2 growth mode,4,27–29 where growth is based on a gas-phase reaction between sublimed MoO3-x and sulfur vapor.4 The summary on the effects of various process parameters can be found in Table S1. TEM analyses were performed to clearly observe the formation of the vertical crystals and their transition to MoS2 with variation of the sulfurization temperature and time. The TEM measurements show that the thickness of the vertical crystals was approximately 80–100 nm. As inferred from the STEM images in Figure 4(a) and 4(b), vertical MoO2 crystals without MoS2 were formed at 730 °C and mono-layer MoS2 started to form on the surface of the vertical MoO2 crystals at 890 °C. When the sulfurization time was extended by 6 min, with an increased sulfur amount (from 120 to 200 mg) and at an identical temperature of 890 °C, MoS2 grew to generate a tri-layered film (see Figure 4(c)). One notable observation here is that the possible formation of extended MoS2 islands overlying MoO2 was only observed with a large increase in the CVD temperature (see Figure S6). These results strongly indicate that the number of atomic MoS2


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layers produced in the present CVD protocol can be manipulated in a facile way by controlling both the process temperature and time. In previous studies by Lan et al.20 and DeGregorio et al.,21 vertical crystal growth was achieved via epitaxial interaction with underlying substrates. However, in the present experiments, vertical crystal growth at an angle of nearly 90° to the substrate could be achieved directly on the amorphous SiO2 substrate; most of the vertical crystals were located adjacent to lateral crystals lying on the substrate (see the inset images in Figure 4(a–c)) with some isolated crystals (see Figure 4(d)). Because many of the vertical crystals were in contact with adjacent crystals, as revealed from the TEM image in Figure 4(e), it is possible that the initially formed vertical MoO3-x crystals grew epitaxially with the adjacent lateral crystals due to their orthorhombic crystal structure.23,32,33 It is noteworthy that the lateral crystals also exhibited a phase transition similar to that observed for the vertical crystals. As shown in Figure S7, a monolayer MoS2 film was formed on the lateral MoO2 crystal at a CVD temperature of 890 °C. In addition, the formation of a tri-layered MoS2 film with prolonged sulfurization was apparent from Figure 4(e); in this TEM image, the MoS2 film is shown to overlap with the vertically grown MoO2 because the TEM specimen was intentionally tilted to clearly observe the boundary between the vertical and lateral crystals. EDS elemental mapping images are shown in Figure 4(f), indicative of the phase transition to MoS2 on both sides of the vertical MoO2 crystals. The overlapping of the Mo and S EDS peaks hindered determination of the exact chemical composition.36 However, EDS mapping of O without any overlapping artifacts clearly showed the existence of an O-deficient layer and further confirmed the formation of a few-layered MoS2 film on the surface of the vertical MoO2 crystals.


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The observation that layer-by-layer conversion to MoS2 occurred only on the surface of MoO2 can be attributed to slow diffusion of sulfur into MoO2.24,37,38

(a)

(b)

(c) Mono-layer MoS2

Vertically standing crystal

5 nm

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200 nm

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Mo

Figure 4. HAADF-STEM images of vertically aligned crystals grown at (a) 730, (b) 890, and (c) 890 °C (extended sulfurization). The insets figures in (a−c) are low-magnification images of the vertical crystals in contact with adjacent lateral crystals; the scale bar is 300 nm. (d) HAADFSTEM image of an isolated vertical crystal on the SiO2/Si substrate. (e) TEM image of the interfacial region between vertical and lateral crystals taken from the red square region in panel (c). (f) HAADF-STEM and quantitative EDS mapping images of vertically aligned crystals grown at 890 °C (extended sulfurization).

Conclusion Vertically aligned MoO2/MoS2 core-shell structures were successfully synthesized on an amorphous substrate (SiO2) and the growth mechanism was investigated in detail based on


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various measurements. Utilizing a confined vapor-phase reaction configuration and high ramping rate facilitated spontaneous transition, with the initial growth of vertically aligned MoO3-x crystals and their subsequent transformation to non-volatile MoO2 crystals as the CVD environment changed from weakly to highly reducing. Further, sulfur-induced reduction at a higher temperature yielded the growth of mono- or tri-layered MoS2 on the surface of the vertical MoO2 crystals. The phase transition to MoS2 progressed in a layer-by-layer manner depending on the sulfurization temperature and time. Such dense and vertical MoO2/MoS2 core-shell structures deposited on amorphous SiO2 have prospective utility in highly efficient or highcharge-capacity energy devices.

Supporting Information. TG/DTA analyses of MoO3 powder without sulfur and mixed with sulfur, SEM images of lateral crystals, full-range Raman spectra of vertical crystals, analytical results for CVD growth under slow ramping and delayed sulfur supply, TEM images showing extended MoS2 growth (1050 °C) and core-shell formation on the lateral crystals, and summary on the influence of various process parameters.

Acknowledgements T. Park and M. Leem contributed equally to this work. This work was supported by a Basic Science Research program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (NRF2014R1A4A1008474).


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TOC Graphic Configuration of CVD system

1000 800

Heater

Ar

S

600 Substrate

400

MoO3 boat

200 0

M oO2 M oS2

0

5

10 15

20 25

Time (min)

MoO2 Mono-layer MoS2

MoO2 Tri-layer MoS2

5㎛


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MoS2 Core-Shell Structures on an

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