supplementary materials for...the lateral heterostructures were synthesized in a chemical vapor...
TRANSCRIPT
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: [email protected]; [email protected]
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
2
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
3
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.
4
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).
5
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.
6
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.
7
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.
8
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.
9
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.
10
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.
11
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.
12
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.
13
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.
14
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