Metal Seed Layer Thickness-Induced Transition From Vertical to

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Metal Seed Layer Thickness-Induced Transition From Vertical to Horizontal Growth of MoS2 and WS2 Yeonwoong Jung, Jie Shen, Yanhui Liu, John M Woods, Yong Sun, and Judy J Cha Nano Lett., Just Accepted Manuscript • Publication Date (Web): 18 Nov 2014 Downloaded from http://pubs.acs.org on November 19, 2014

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Metal Seed Layer Thickness-Induced Transition From Vertical to Horizontal Growth of MoS2 and WS2

Yeonwoong Jung1,3, Jie Shen1,3, Yanhui Liu1, John M. Woods1,3, Yong Sun2 and Judy J. Cha1,3 1

Department of Mechanical Engineering and Materials Science, Yale University, New Haven,

Connecticut 06511, United States 2

Micro/Nano Fabrication Laboratory, PRISM, Princeton University, Princeton, New Jersey 08540,

United States 3

Energy Sciences Institute, Yale University West Campus, West Haven, Connecticut 06477, United

States

RECEIVED DATE:

CORRESPONDING AUTHOR FOOTNOTE: *

Corresponding author. E-mail: [email protected]

Telephone number: +1 (203) 737-7293, Fax number: (203) 432-6775

ACS Paragon Plus Environment

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ABSTRACT:

Two-dimensional (2D), layered transition metal dichalcogenides (TMDCs) can grow in two different growth directions, i.e., horizontal and vertical. In the horizontal growth, 2D TMDC layers grow in planar direction with their basal planes parallel to growth substrates. In the vertical growth, 2D TMDC layers grow standing upright on growth substrates exposing their edge sites rather than their basal planes. The two distinct morphologies present unique materials properties suitable for specific applications, such as horizontal TMDCs for opto-electronics and vertical TMDCs for electrochemical reactions. Precise control of the growth orientation is essential for realizing the true potential of these 2D materials for large-scale, practical applications.

In this Letter, we investigate the transition of vertical-to-horizontal growth

directions in 2D molybdenum (or, tungsten) disulfide and study the underlying growth mechanisms and parameters that dictate such transition. We reveal that the thickness of metal seed layers plays a critical role in determining their growth directions. With thick (> ~ 3 nm) seed layers, the vertical growth is dominant, while the horizontal growth occurs with thinner seed layers. This finding enables the synthesis of novel 2D TMDC heterostructures with anisotropic layer orientations and transport properties. The present study paves a way for developing a new class of 2D TMDCs with unconventional materials properties.

KEYWORDS:

layered material, two-dimensional material, metal dichalcogenide, vertical growth, MoS2, WS2


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MANUSCRIPT TEXT Transition metal dichalcogenides (TMDCs) consist of two-dimensional (2D) molecular layers weekly bonded by van der Waals (vdW) attraction. Owing to the nature of anisotropic vdW bonding, a large number of 2D TMDCs present unique materials properties distinct from their bulk counterparts.1, 2 Amongst them, MoS2 and WS2 are particularly interesting due to their extraordinary opto-electronic and electrochemical properties.1,

2

To explore these properties,

mechanical exfoliation of individual 2D layers from bulk crystals has initially been developed, while various chemical synthesis approaches have also been pursued.3-9 Conventional chemical synthesis has focused on growing horizontally-stacked 2D molecular layers; MoS2 (or WS2) horizontally grows lying on growth substrates exposing its basal planes (Figure 1a). Recently, it was found that these 2D TMDCs can also grow vertically under certain growth conditions10-12; 2D layers grow vertically standing on growth substrates exposing their edge sites (Figure 1b). Depending on the growth directions, these materials present unique materials properties suitable for specific applications.

Horizontally-grown 2D MoS2 and WS2 are promising for opto-

electronics due to their indirect-to-direct bandgap transition 13-15 and large on/off ratios (>105) as transistors.16,

17

Vertically-grown 2D MoS2 and WS2 are being investigated for various

electrochemical applications due to their chemically reactive edge sites.10,

12

This vertical

orientation also allows for direct top-to-down atomic-level characterizations of vdW gaps, therefore, presents unique opportunities for fundamental sciences (i.e., correlation of layer spacing on transport properties).

Given their promising applications, precise tuning of the

growth orientations of these 2D TMDCs is critically important to realize their desired materials properties and device functionalities.


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Despite the technological and scientific significance, the underlying growth mechanisms and parameters responsible for such growth transitions have not been fully explored. A kinetically-driven growth mechanism was originally proposed to explain the vertical growth of MoS210, 12; At a relatively low temperature, the sulfur diffusion rate along the vdW gaps of 2D layers is much higher than the diffusion rate through the layers, leading to the faster growth in the vertical orientation. Low growth temperature (550 oC- 600 oC) was considered to be the key factor to drive this kinetically-driven growth. However, under similar growth conditions, quite distinct morphologies of 2D MoS2, such as all vertically-aligned layers,10, 12 sparsely verticallyaligned layers,11, 18 and horizontally-stacked layers19 have been reported. This variation suggests a presence of additional critical factors that more significantly contribute to govern the 2D layer growth orientation. Clear understanding of these variables is critical to precisely control the morphology of these 2D TMDCs during their growth. Such knowledge could further allow for the synthesis of novel 2D TMDC heterostructures that consist of both vertically and horizontallygrown 2D layers. In this Letter, we investigate the morphology of MoS2 and WS2 films grown by sulfurizing elemental metal (Mo or W) seed layers. Specifically, we focus on the effect of the metal seed layer thickness - a largely unexplored parameter for 2D TMDCs growth - on the orientation of 2D layers. We reveal that the metal seed layer thickness is a critical factor to determine the distinct growth directions and observe a transition of vertical-to-horizontal growth with decreasing layer thickness. Based on this observation and a new understanding of 2D growth mechanisms, we realize novel TMDC heterostructures composed of alternating vertical/horizontal 2D layers with anisotropic transport properties.


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We grew MoS2 and WS2 films by sulfurizing Mo (or, W)-deposited SiO2/Si substrates with varying Mo (or, W) layer thicknesses (Refer to the Experimental Section for growth details). Figure 2(a) shows the color evolution of as-grown MoS2 on SiO2/Si substrates, initially deposited with varying thicknesses of Mo from 20 nm to 0.3 nm. All samples were grown under identical growth conditions. The color evolution indicates the different thicknesses of grown MoS2 films. Similar color evolution is also observed for WS2 growth with varying W seed layer thicknesses (Supporting Information, S1). The effect of the seed layer thickness on the film morphology is characterized with transmission electron microscopy (TEM).

Figure 2(b)-2(e) compare the

morphology of MoS2 films grown with 10 nm, 4 nm, 1 nm, and 0.3 nm Mo, respectively. In Figure 2(b), TEM on the 10 nm sample shows a large-area, textured MoS2 film (Figure 2(b), left) whose structure is poly-crystalline revealed by the corresponding selective area electron diffraction (SAED) pattern (left inset). High resolution TEM (HRTEM) image (Figure 2(b), right) reveals that the film is composed of highly dense, all vertically-aligned 2D MoS2 layers. Some grains consist of a large number (> 40) of self-assembled MoS2 layers with an intermolecular spacing of ~0.63 nm 10 (yellow arrow). In Figure 2(c), TEM on the 4 nm sample reveals a largely horizontally-grown MoS2 film with regions of vertically-aligned MoS2 layers. The SAED pattern (left inset) reveals poly-crystalline diffraction spots, similar to those of the 10 nm sample. The density of the vertically-aligned layers is much smaller with fewer (typically 510) aligned layers in each grain, in comparison to the 10 nm sample. As the Mo seed layer thickness further decreases, a morphological transition from vertical-to-horizontally grown MoS2 layers becomes more pronounced. Figure 2(d) shows a HRTEM image of a MoS2 film grown with 1 nm thick Mo seed layer. The film is poly-crystalline on a large-area without any vertically-aligned layers.

Some regions (yellow box, (a)) are composed of bilayers of


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horizontally-grown MoS2 as revealed by its typical Moiré fringes (right, top inset) consistent with previous studies.5, 18 In other regions (yellow box, (b)), monolayer, single-crystalline grains are observed (right, bottom inset). The mixed crystalline structure of mono and a few layer MoS2 is also indicated by the fast-Fourier transform of the corresponding HRTEM image (left, top inset). Figure 2(e) shows a TEM image of a large-area (> 20 μm in lateral dimension), freestanding 2D MoS2 film grown with a 0.3 nm Mo seed layer. A clear band contour originating from diffraction contrasts in the film reflects the wrinkled nature of the film. The SAED pattern (bottom left inset) reveals that it is a single-crystalline, monolayer MoS2. The corresponding HRTEM image (bottom right inset) shows hexagonal single-crystalline lattice fringes of MoS2 oriented in (001) zone axis, consistent with other studies.3, 4, 6 Atomic force microscopy (AFM) was used to measure the thickness of the films, which further confirms the growth of singlecrystalline, monolayer 2D MoS2 (Supporting Information, S2). We typically observe mono-to-a few layer MoS2 grow horizontally with Mo seed layer thickness ≤ 0.5 nm, while verticallyaligned layers start to grow with Mo ≥ ~3-4 nm in thickness. In the intermediate seed layer thickness regime, poly-crystalline MoS2 films composed of a large number of horizontal 2D layers are dominant (typically, ~1- 2 nm Mo), while a few layer 2D MoS2 are also observed (typically, ~0.5- 1 nm Mo). Such a transition from horizontal (mono-to-a few layers) to vertical growth direction also occurs for WS2 (Supporting Information, S3). The morphology of the vertically-aligned 2D TMDCs with different metal seed layer thicknesses is quantitatively analyzed. The density of the vertically-aligned layers increases with increasing seed layer thickness. Figure 3(a) shows the coverage ratio of vertically-aligned MoS2 and WS2 layers as a function of Mo and W seed layer thickness. The ratio was determined by analyzing the top-view representative TEM images taken at identical magnification. The number


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of the vertically-aligned layers constituting each grain increases with seed layer thickness (Figure 3(b)). These results are in a qualitative agreement with the TEM images in Figure 2(b) and 2(c). The plots in Figure 3(c) compare the thicknesses of as-grown TMDC films in the vertical-growth regime as a function of seed layer thickness. A linear relationship was obtained for both MoS2 and WS2 yielding a slope of ~3.5 from the linear fitting. This value indicates the extent of the lateral growth (i.e, volume expansion) of 2D basal planes through the sulfurization of metal seed layers. Similar slopes for MoS2 and WS2 indicate similar growth rates of their individual basal planes. The value of the slope is ~ 2-3 times larger than those obtained from the growth of horizontal, vdW-stacked 2D MoS2 (or, WS2) with varying metal seed layer thicknesses.19, 20 This indicates that a significant volume expansion occurs during the lateral growth of 2D basal planes. The seed layer thickness-dependent morphology of the 2D TMDCs is further characterized by Raman spectroscopy. Figure 3(d) and 3(e) show the Raman spectra of MoS2 and WS2 films grown with three different seed layer thicknesses of 20 nm, 3 nm, and 0.3 nm, respectively. The vertical dashed lines in each plot denote the position of E12g and A1g Raman peaks for the films grown with a 20 nm seed layer. In Figure 3(d) for MoS2, E12g and A1g peaks are assigned to be ~383 cm-1 and ~408 cm-1, respectively, yielding A1g ─ E12g = ~ 25 cm-1. These values are consistent with those observed for bulk MoS2 in the literature.21, 22 As Mo thickness decreases, downward and upward shifts of E12g and A1g are observed respectively, which becomes particularly obvious with Mo ≤ ~ 3 nm thickness. With MoS2 films grown with 0.3 nm Mo (red plot), E12g and A1g peaks are positioned at ~386 cm-1 and ~406 cm-1 yielding A1g ─ E12g = ~ 20 cm-1. Such decrease of A1g ─ E12g with decreasing Mo layer indicates a bulk-like to mono (or, a few) layer transition in MoS2, similar to the Raman characteristics of mechanically exfoliated MoS2 layers with varying layer numbers.21, 22 This observation is also qualitatively


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consistent with the previous studies on poly-crystalline, horizontal MoS2 films grown with different seed layer thicknesses.20 For the Raman spectra of WS2 (Figure 3(e)), a slight decrease of A1g ─ E12g (from ~ 66 cm-1 to ~ 65 cm-1) is observed with decreasing W seed layer thickness, also consistent with previous studies.20, 23 One unique feature observed in our Raman spectra is that the intensity ratio of E12g/A1g increases with decreasing seed layer thickness for both MoS2 and WS2. For the MoS2 with ~100 % yield of vertically-aligned layers grown with 20 nm Mo, the value for E12g/A1g is as small as ~0.21. This small value indicates a pronounced out-of-plane vibration (A1g) mode over in-plane vibration (E12g) reflecting the dominantly exposed MoS2 edge sites, which is a signature of vertical growth.10 Meanwhile, the MoS2 grown with 0.3 nm Mo displays a larger value (~0.39) of E12g/A1g. Such transition is more obvious for WS2 where the values for E12g/A1g are 0.77 and 3.1 for the films grown with 20 nm and 0.3 nm W, respectively. The enhancement of E12g/A1g values further supports the suppression of vertical growth with decreasing seed layer thickness, consistent with the TEM results in Figure 2. The seed layer thickness dependent growth suggests opportunities for realizing novel 2D TMDC heterostructures with precisely defined growth orientations. In order to further verify that the growth transition is solely governed by the metal seed layer thickness, we prepared an array of W line patterns with alternating W layer thicknesses (Figure 4(a)). We, then, sulfurized the substrates and grew patterned WS2 films. Figure 4(b) shows an optical image of a patterned WS2 film where the periodic colors of yellow and blue represent WS2 grown from 7 nm and 3 nm W, respectively. The inset image reveals clear color dispersion on an entire substrate, suggesting a high periodicity of the patterned WS2. Figure 4(c) shows a bright-field TEM image of a large-area, patterned WS2 film, which reveals a periodic imaging contrast reflecting the


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thickness difference. The as-grown WS2 on a SiO2/Si substrate is further characterized by Raman spectroscopy. Figure 4(d) compares the Raman spectra primarily obtained from the thick (yellow) and thin (blue) regions of the substrate. The Raman peaks for E12g and A1g do not shift irrespective of probed positions. However, the Raman spectrum from the thin region displays a larger value of E12g/A1g (~1.8 times) over the thick region. The enhancement of E12g/A1g is consistent with the results in Figure 3(e), indicating the suppression of vertical growth in thin regions. The morphology of the patterned WS2 films is also characterized with TEM. Figure 5(a) shows a bright-field TEM image of a patterned WS2 where three different regions (red, blue, and green boxes) are inspected, shown in Figure 5(b)-5(d). In Figure 5(b), the zoomed-in TEM image of the red box region reveals the interface of the thick (dark) and thin (bright) regions. In Figure 5(b) left, it is clear that vertically-aligned layers are grown only on the region of 7 nm W, while horizontal films are observed on the thinner region. Figure 5(b) right shows a HRTEM image of an interface where vertically-aligned layers and horizontal films co-exist. The inset shows the zoomed-in HRTEM image of the yellow dotted box adjacent to the vertically-aligned layers, revealing the lattice fringes of a hexagonal, single-crystalline WS2. Figure 5(c) shows a HRTEM image obtained from a thin region away from interfaces (blue box in Figure 5(a)). The image reveals the poly-crystalline structure of large-area, horizontally grown WS2 without any vertically-aligned layers. A mixture of mono layer (red dotted box and its corresponding inset) and poly-crystalline, multiple layers are also observed. Figure 5(d) shows a HRTEM image obtained from the green box in Figure 5(a), which reveals highly dense, all vertically-aligned WS2 layers, in a sharp contrast to the horizontally-grown WS2 in Figure 5(c). The results clearly


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show that the thickness of the metal seed layer is the sole factor to govern the growth orientation of 2D TMDCs. To verify the effect of the growth orientations on the physical properties of TMDCs, we characterize the carrier transport properties of patterned WS2 films. Figure 6(a) shows the twoterminal electrical characterizations of a WS2 film with alternating vertical/horizontal heterostructures with two different metal contact configurations: parallel (red plot) or perpendicular (black plot) to the orientation of the WS2 line patterns. Highly linear currentvoltage (I-V) characteristics are observed for both configurations suggesting Ohmic transports. Meanwhile, higher current is observed for the perpendicular contact (black plot) over the parallel contact (red plot). This anisotropic transport originates from the different carrier transport characteristics in each vertical/horizontal region of the heterosturcture. In the region of vertically-aligned layers, the carrier transport is dominated by a cross-plane hopping process24, 25; carriers must travel through a large number of vdW gaps as opposed to the in-plane carrier transport for 2D horizontal films. In the case of the parallel contact, the carrier transport is limited by the interface of vertical/horizontal layers, thus, the cross-plane hopping in the vertically-aligned layers dominates. Meanwhile, the in-plane carrier transport on the horizontal films dominates for the perpendicular contact. The schematics in Figure 6(b) illustrate the different nature of carrier transport for different 2D layer orientations. We discuss the growth mechanism for metal seed layer thickness-dependent vertical-tohorizontal transition. During the growth of 2D TMDC, a significant volume expansion is anticipated as the metal seed layer becomes sulfurized. Figure 7(a) illustrates the body-centeredcubic unit cell of elemental Mo (left) and a top-view of monolayer MoS2 and its corresponding unit cell (right). The lattice constant of ~ 0.31 Å remains similar for both the elemental Mo and


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MoS2 with similar bonding lengths (~0.27 Å) for Mo-Mo and Mo-S after sulfurization. However, the number of Mo atoms per unit cell decreases (2 to 1).26 This suggests that a significant volume expansion must occur if most of the deposited Mo atoms participate in converting MoMo bonding to Mo-S bonding.

When very thin, thus discontinuous seed layers undergo

sulfurization, growth of large-area 2D horizontal films is energetically favorable over the growth of vertically-aligned layers. This is because horizontally-grown 2D films preferably expose basal planes with low surface energy (Figure 7(b)-1), while the vertical growth results in exposing edge sites with high surface energy (Figure 7(b)-2). It is noted that the surface energy of the edge sites is almost two orders of magnitude higher than that of the basal plane.27 In this case, the horizontal volume expansion can be easily accommodated due to the discontinuous nature of the initial seed layer. When continuous, thick seed layers undergo sulfurization, horizontal volume expansion cannot be easily accommodated as the metal seed layer is anchored to growth substrates. In this case, the horizontal growth of a large-area, continuous film will induce a significant amount of strain energy (Figure 7(c)-1). A more feasible scenario is that films grow vertically allowed by an unconstrained free volume expansion in the vertical direction. Releasing strain energy by expanding vertically can compensate for the enhanced surface energy owing to the newly exposed edge sites of vertically-aligned layers (Figure 7(c)-3). For the seed layers of intermediate thicknesses, both vertical and horizontal growths can occur. In this case, horizontally-grown 2D layers become discontinuous, bent and overlapped with each other forming poly-crystalline structures, which is to release strain energy (Figure 7(c)-2). To support the proposed growth mechanism, we have characterized the morphology of metal seed layers by AFM. The AFM topography images in Supporting Information, SI4, show morphology of Mo seeds deposited on SiO2/Si substrates with nominal, target thicknesses of 0.3 nm, 1 nm,


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and 2nm. The images show that Mo films below 1 nm are discontinuous, providing rooms for MoS2 to expand horizontally. This supports the correlation of Mo nominal thickness with the molecular layer growth direction. We note that our growth method based on elemental metal precursors is more scalable than the conventional approaches based on more widely used metal oxide precursors (MoO3 or WO3).4, 9 Although this method is scalable, growing monolayer-only MoS2 (or, WS2) films on a wafer-scale using this method is still challenging. Major challenge lies in preparing a thin metal seed layer with uniform coverage and thickness, which should maintain its uniformity during sulfurization. Nevertheless, we believe that the ability to uniformly deposit metal seeds via wellestablished physical vapor deposition techniques (e.g., sputtering) and their lithography compatibility suggests opportunities for patterned 2D TMDCs with compositional/structural heterogeneity. In conclusion, we demonstrate the transition of vertical-to-horizontal growth directions in 2D MoS2 and WS2. We reveal that the thickness of metal seed layers is a critical factor to dictate the growth orientation of these 2D layers. Based on this finding, we realize structurally heterogeneous 2D layers with anisotropic carrier transport properties.

We believe that the

present study should help better understand the growth mechanisms of various TMDCs and could be utilized to realize unique 2D materials heterostructures.

Experimental Substrate preparation and materials growth: Mo (or, W) with different thicknesses were deposited on a SiO2/Si substrate (300 nm SiO2 thickness) using magnetron sputtering system (AJA International). The deposition rate was 1.8 nm/min for both Mo and W, which was


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measured using SQM-160 Rate/Thickness monitor. The deposition was carried out with 50 W power and a working Ar pressure of 0.3 Pa, and the base pressure was below 10-5 Pa. All TMDC films were grown inside a single-zone horizontal tube furnace (Lindberg/Blue M). The metaldeposited SiO2/Si substrates were placed at the center zone of the growth furnace, and elemental sulfur powders (Sigma Aldrich) were placed at the upstream side. The furnace was pumped down to a base pressure of ~ 5 mTorr and flushed with Ar gas several times to remove residual organics and oxygen inside the tube. Subsequently, the furnace was heated to the growth temperature (750 oC- 800 oC) with a total ramping time of 20 min and was maintained for 15 min followed by a natural cooling. During the reaction, the flow rate of Ar carrier gas and the vapor pressure were maintained to be ~150 Standard Cubic Centimeters per Minute (SCCM) and ~ 300 mmTorr, respectively. The W line patterns with alternating thickness (Figure 4) were prepared by optical lithography (Heidelberg DWL66, 405 nm laser writer). Morphological/electrical characterizations:

The morphology of TMDC films was

characterized using TEM/STEM (FEI Tecnai Osiris 200kV) and Raman spectroscopy (HoribaJobin Yvon HR-800 equipped with a 532 nm green laser). For TEM sample preparation, asgrown TMDC films on SiO2/Si substrates were immersed in potassium hydroxide (KOH) solution at 80 oC. The KOH selectively etches the underlying SiO2 layer, and the dispersed TMDC films in KOH were subsequently transferred to carbon-coated TEM grids followed by cleaning with acetone, isopropyl alcohol and deionized water. Electrical characterizations were performed on large-area TMDC films transferred from growth substrates to heavily-doped SiO2/Si substrates.

As-grown TMDC films on growth substrates were deposited with

Polymethyl methacrylate (PMMA; Microchem) and cured. The substrates were, then, subsequently immersed in KOH for a selective etching of SiO2. Once the PMMA/TMDC films


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were lifted off, they were transferred to heavily-doped SiO2/Si substrates followed by cleaning. Metal electrodes (Cr/Au) were patterned and fabricated by e-beam lithography (Vistec EBPG 500+). All the electrical characterizations were performed with Agilent B1500A device parameter analyzer.

ASSOCIATED CONTENT Supporting Information; Color evolution of WS2 films grown with W of different thicknesses, AFM characterizations of monolayer MoS2 films, TEM characterizations of WS2 films grown with W of different thicknesses, AFM characterizations of Mo seed layers, Figure S1-S4. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT: Microscopy facilities used in this work were supported by the Yale Institute for Nanoscience and Quantum Engineering and National Science Foundation MRSEC DMR 1119826. The authors also acknowledge the use of facilities of the Micro/Nanofabrication Laboratory (MNFL) at the Princeton Institute for the Science and Technology of Materials (PRISM).


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Figure 1. Schematics for the horizontal and vertical growth of 2D TMDCs. (a) side-view (left) and projected-view (right) of horizontally-grown 2D layers. (b) side-view (left) and projectedview (right) of vertically-grown 2D layers.


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Figure 2. (a) A photography of as-grown MoS2 on SiO2/Si substrates grown with Mo seed layer of 20 nm, 15 nm, 10 nm, 7 nm, 5 nm, 2 nm, and 0.3 nm, respectively (from left to right). (b)-(e); TEM characterizations of MoS2 grown with (b) 10 nm Mo, (c) 4 nm Mo, (d) 1 nm Mo, and, (e) 0.3 nm Mo.


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Figure 3. (a) Coverage ratio of vertically-aligned MoS2 and WS2 layers as a function of metal seed layer thickness. (b) Number of vertically-aligned MoS2 and WS2 layers per each grain. (c) Comparison of the thickness of as-grown MoS2 and WS2 films as a function of metal seed layer thickness. (d) Raman spectra of MoS2 grown with different Mo layer thicknesses. (e) Raman spectra of WS2 grown with different W layer thicknesses.


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Figure 4. (a) A schematic to illustrate the growth of a patterned WS2 film with alternating WS2 thickness. (b) An optical microscopy image of a patterned WS2 grown on a SiO2/Si substrate. (c) A bright-field TEM image of a patterned WS2 film. (d) Raman spectra of a patterned WS2 film collected from the thick (red circle/plot) and thin (black circle/plot) areas.


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Figure 5. (a) Low-magnification TEM image of a patterned WS2 film with alternating WS2 thickness. Three areas (red, blue, and green boxes) are inspected for detailed structural characterizations. (b)-(d) TEM characterizations of the red, blue, and green boxed areas in (a) which correspond to the vertical/horizontal interface (b), horizontal (c), and vertical (d) WS2.


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Figure 6. (a) Two terminal I-V characteristics of a patterned WS2 film with horizontal/vertical heterostructures with parallel (red plot) and perpendicular (black plot) metal electrode contacts. The orange color in the schematics represents metal electrodes while the alternating yellow/green represents horizontal/vertical layers, respectively. (b) Schematics to illustrate the distinct carrier transport mechanisms for vertical (top) and horizontal (bottom) 2D layers.


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Figure 7. (a) Illustrations to visualize the unit cell of elemental Mo (left) and 2D monolayer of MoS2 and its corresponding unit cell (right). (b)-(c) Schematics to show the transition of verticalto-horizontal growths where thin (b) and thick (c) metal seed layers are used.


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SYNOPSIS TOC


Metal Seed Layer Thickness-Induced Transition From Vertical to

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