www.sciencemag.org/cgi/content/full/science.aan6814/DC1

Supplementary Materials for

Robust epitaxial growth of two-dimensional heterostructures, multiheterostructures, and superlattices

Zhengwei Zhang,* Peng Chen,* Xidong Duan,† Ketao Zang, Jun Luo, Xiangfeng Duan†

*These authors contributed equally to this work.

†Corresponding author: E-mail: xidongduan@hnu.edu.cn; xduan@chem.ucla.edu

Published 3 August 2017 on Science First Release DOI: 10.1126/science.aan6814

This PDF file includes:

Materials and Methods Supplementary Text Figs. S1 to S8 Tables S1

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Materials and methods.

Growth of lateral heterostructures

The lateral heterostructures, superlattices and multiheterostructures were synthesized with

multi-step growth method. The lateral heterostructures were synthesized in a chemical vapor

deposition (CVD) system using thermally evaporated vapor-phase reactants from solid source

materials at atmospheric pressure via a sequential growth process. WS2, WSe2, MoS2, MoSe2

powders were directly used as the solid source for each growth step. The thermal CVD system is

designed so that the gas flow direction can be switched (reversed) during the temperature swing

period and growth period (Fig. 1A). To prevent cross contamination, the growth of each material

uses a separate CVD system. For example, to grow WS2-MoS2 heterostructures, monolayer WS2

was first grown in a first thermal CVD system dedicated for WS2 growth. The WS2 powder was

placed in a quartz boat located at the center heating zone of the furnace with 1-inch quartz tube,

and the SiO2 (300nm)/Si substrate was placed at downstream end of the furnace as the growth

substrate. The center-heating zone was then heated to 1150 ℃ under ambient pressure with a

reverse flow of 300 sccm argon. After reaching the desired growth temperature, the chemical

vapor source was carried downstream by a forward flow of 50 sccm argon gas for a growth

period of 10 minutes. The growth process was then terminated by reversing the argon flow and

the furnace was cooled down naturally. For the second step epitaxial growth of MoS2, the as-

grown WS2 domains on SiO2/Si substrate is placed at downstream in another similar CVD

system dedicated for MoS2 growth，and the source is changed to MoS2 powders. During the

temperature ramping up stage, a reverse flow of 300 sccm cold argon gas from the substrate to

source continuously flushes the existing WS2 crystals to minimize the thermal induced

degradation. After reaching the desired epitaxial growth temperature of MoS2 (1180 ℃), the flow

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direction is switched with a forward flow of 100 sccm to carry the MoS2 vapor source to the

growth substrate for the epitaxial growth of WS2-MoS2 heterostructure. The heating zone was

kept for 5 minutes for the epitaxial growth of MoS2. At last, the growth was terminated and the

sample was naturally cooled down to room temperature with the protection of argon gas.

Similarly, other lateral heterostructures can be readily grown using a similar protocol, and the

growth parameters are summarized in Table S1.

Growth of lateral superlattices and multiheterostructures

By employing the reverse flow during the temperature swing stage in the sequential growth

processes, the existing 2D crystals can be effectively cooled using by the continuous cold argon

flush to prevent undesired thermal degradation and uncontrolled homogeneous nucleation, thus

ensuring robust block-by-block epitaxial growth. The strategy described above can be repeated

multiple times for the growth more complex lateral heterostructures, including lateral

superlattices (e.g., WS2-WSe2-WS2-WSe2-WS2) and multiheterostructures (e.g., WS2-MoS2-

WS2, and WS2-WSe2-MoS2). Moreover, the width of each block can be controlled by varying the

growth time and/or growth temperature.

Characterization

The microstructures and morphologies of the nanostructures were characterized by optical

microscope, AFM and STEM. The micro-Raman and micro-PL studies were conducted using a

Renishaw confocal Raman system with 532 nm laser excitation. The WSe2-WS2 heterojunction

devices were fabricated using electron-beam lithography followed by electron-beam deposition

of metal thin film electrode. Ti/Au (5/50 nm) were used for the contact electrodes for WSe2 and

WS2. The electrical measurements were conducted by using an Agilent B2912A semiconductor

parameter analyzer in the atmosphere at room temperature.

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Supplementary Text

Challenges in typical sequential growth process

The step-by-step epitaxial growth of monolayer heterostructure is challenging due to

extremely delicate nature of the atomically thin crystals and the highly sensitive growth

conditions. The atomically thin 2D crystals can be easily degraded between sequential growth

steps due to the temperature and ambient variations (fig. S1). Additionally, with the single flow

direction used in the typical synthetic approaches reported previously, the chemical vapor is

supplied before reaching the ideal growth temperature, which can lead to uncontrolled nucleation

of new 2D crystals at low temperature and prevent robust lateral epitaxial growth (fig. S2).

SEM characterization of the ultra-narrow lateral multiheterostructures

Ultra-narrow epitaxial growth is particularly important for the growth of short period

superlattices or ultra-narrow lateral multiheterostructures. With the atomically sharp transition

across the lateral heterostructure interface, ultra-narrow epitaxy can in principle be achieved by

controlling the growth temperature and time. Owing to the resolution limits (~300 nm) of optical

microscope, we characterized the ultra-narrow epitaxy samples with scanning electron

microscope (SEM). As shown in Fig. S7, the monolayer WS2 and WSe2 show different contrast

in SEM image, with WSe2 clearly brighter than WS2. We found the growth temperature could

significantly influence the growth speed. At a relatively high temperature, the epitaxy width is

significantly wider than that obtained at a relatively low temperature with the same growth

duration, indicating a much slower growth speed at lower temperature (fig. S7A-C). By using

low temperature epitaxy growth strategy, we have, for the first time, successfully synthesized the

ultra-narrow WS2-WSe2-WS2 multiheterostructure with the width of WSe2 block down to

nanometer scale (fig. S7D-F).

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Electrical characterization of the synthesized lateral heterostructure

To avoid possible short circuit path, the concentric triangular domain was lithographically

cut into rectangular heterostructure, with two Ti/Au electrodes deposited on WSe2 and WS2

region near the heterostructure interface. Figure S8A shows the optical microscopy image of a

typical WSe2-WS2 heterojunction device. Current (IDS) versus voltage (VDS) measurements

showed a rectification ratio up to 105, consistent with the presence of a monolayer lateral p-n

junction (Fig. S8, C and D). Because of the 2D nature of our device, the transport properties

exhibited strong gate voltage modulation behavior, with the on-state current increasing rapidly

with increasing negative gate voltage.

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Fig. S1. The excessive thermal degradation in a typical sequential growth process. (A)

Optical microscopy image of the degraded WSe2-WS2 heterostructures prepared by traditional

growth method (single flow direction without reverse flow during the temperature ramping up

stage). (B) Optical microscopy image of the degraded WS2-WSe2 heterostructures prepared by

traditional growth method. It is difficult for the traditional method to provide a suitable

temperature for the epitaxial growth. High temperature can easily damage the monolayer

samples. Scale bars, 5 μm.

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Fig. S2. Uncontrolled nucleation at low temperature. (A) Optical microscopy image of the

WS2-WSe2 heterostructure synthesized at low temperature (1000℃). (B) Optical microscopy

image of the WSe2-MoS2 heterostructure synthesized at low temperature (1050℃). (C) Optical

microscopy image of the WS2-MoS2 heterostructure synthesized at low temperature (1100℃).

There is apparently considerable undesired, uncontrolled nucleation happening at the low

temperature growth, preventing robust, clean epitaxial growth of lateral epitaxial growth of

lateral heterostructures with clean interfaces. Scale bars, 10 μm.

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Fig. S3. Optical micrographs of the synthesized lateral heterostructures,

multiheterostructures and superlattices. (A) optical microscope image of the resulting A-B

heterostructures; (B) optical microscope image of A-B-C multi-heterostructures; (C) optical

microscope image of A-B-A-B-A superlattices. Scale bars, 5 μm.

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Fig. S4. AFM characterization of the synthesized heterostructures. (A) AFM image and line

profile of the WS2-WSe2 heterostructure. (B) AFM phase image of the WS2-WSe2

heterostructure. (C) AFM image and line profile of the WS2-WSe2-WS2 heterostructure. (D)

AFM phase image of the WS2-WSe2-WS2 heterostructure. The thickness of the synthesized

samples ranges from 0.7 to 0.8 nm, indicating monolayer crystals. Scale bars, 2 μm.

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Fig. S5. PL spectra evolution at the interface of lateral heterostructures. (A, B) Optical

microscopy image and PL spectra line scan of WS2-WSe2 heterostructure. (C, D) Optical

microscopy image and PL spectra line scan of WSe2-MoS2 heterostructure. The PL peaks at

interface show a simple addition of the PL features of the inner and outer materials, suggesting

there is no apparent alloy formation at the interface. Scale bars, 5 μm.

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Fig. S6. (A1) Optical image of the WSe2-MoS2 heterostructure. (A2) Photoluminescence (PL)

spectra of WSe2-MoS2 heterostructure. The green curve is obtained from the centre region,

showing the characteristic PL peaks of WSe2; and the blue curve is obtained from the peripheral

region, showing the characteristic PL peaks of MoS2. (A3-A4) Spatially resolved PL mapping

images at 630 nm and 760 nm, showing characteristic PL emission of WSe2 and MoS2 in the

center and peripheral regions of the triangular domain. (B1-B4) Optical image, PL spectra and

image of monolayer WS2-MoS2 heterostructure. (C1-C4) Optical image, PL spectra and image of

monolayer WSe2-MoSe2 heterostructure. (D1-D4) Optical image, PL spectra and image of

monolayer WS2-MoSe2 heterostructure. Scale bars, 5 μm.

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Fig. S7. SEM characterization of the ultra-narrow lateral multiheterostructures. (A, B, C)

SEM image of the lateral WS2-WSe2 heterostructure grown at different temperature, with same

growth duration (30 seconds). (D, E, F), SEM image of the lateral WS2-WSe2-WS2 multi-

junction with ultra-narrow WSe2 block (~50 nm width in D obtained with a growth duration of

15 seconds, ~35 nm width in E obtained with a growth duration of 10 seconds, and ~13 nm

width in F obtained with a growth duration of 5 seconds), at same growth temperature (1020℃).

Scale bar, A, 500 nm, B-F, 200nm.

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Fig. S8. Electrical characterization of the monolayer lateral p-n junction. (A) Optical image

of the lateral WSe2-WS2 heterojunction device. The yellow dashed line indicates the position of

interface. Scale bar, 5 μm. (B) Schematic diagram of the band alignment of WS2-WSe2 at

interface. (C) IDS-VDS curves at different back gate voltage Vg. (D) semi-log plot of IDS-VDS

curves.

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Table S1. Epitaxial growth parameters for various 2D materials.

Ramping stage Isothermal stage

Reverse Ar flow

(sccm)

Forward Ar flow

(sccm) Temperature (℃) Growth Time

(min)

WS2 300 50 1120 5

WSe2 300 120 1080 3

MoS2 300 100 1180 5

MoSe2 300 200 1200 3