Thermodynamics and Kinetics Synergetic Phase-Engineering of

Subscriber access provided by - Access paid by the | UCSB Libraries

Thermodynamics and Kinetics Synergetic Phaseengineering of CVD Grown Single Crystal MoTe2 Nanosheets Xiaosa Xu, Xiaobo Li, Kaiqiang Liu, Jing Li, Qingliang Feng, Lin Zhou, Fangfang Cui, Xing Liang, Zhibin Lei, Zong-Huai Liu, and Hua Xu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01624 • Publication Date (Web): 06 Apr 2018 Downloaded from http://pubs.acs.org on April 8, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Crystal Growth & Design

Thermodynamics and Kinetics Synergetic Phase-engineering of CVD Grown Single Crystal MoTe2 Nanosheets Xiaosa Xu,† Xiaobo Li,† Kaiqiang Liu,‡ Jing Li,† Qingliang Feng,§ Lin Zhou,ǁ Fangfang Cui,† Xing Liang,† Zhibin Lei,† Zonghuai Liu,† and Hua Xu*† †

Shaanxi Key Laboratory for Advanced Energy Devices; Shaanxi Engineering Lab for

Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, China ‡

Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education,

School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710119, China §

Key Laboratory of Space Applied Physics and Chemistry, Ministry of Education, and

Shaanxi Key Laboratory of Optical Information Technology, School of Science, Northwestern Polytechnical University, Xi’an 710072, China ǁ

Department of Electrical Engineering and Computer Sciences, Massachusetts

Institute of Technology, Cambridge, Massachusetts 02139, United States

ACS Paragon Plus Environment

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Molybdenum ditelluride (MoTe2), which is stabilized in both semiconducting hexagonal (2H) and metallic distorted octahedral (1T') phases, has attracted increasing attention owing to their attractive properties for promising wide applications. Exploring the full potential of this emerging material requires a reliable synthesis approach to precise control its phase structure. Here, we report on the growth of high crystallinity MoTe2 nanosheets with controlled phase via the tellurization of CVD-grown MoO2 nanosheets. The single crystal MoO2 nanosheets with rhombus shape were converted into MoTe2 single crystal nanosheets with morphology maintained well through the tellurization process. The phase structure of as-grown MoTe2 is determined by the synergetic effect of thermodynamics and kinetics in the crystal growth process, which is based on the difference in the thermodynamic stability and the lattice strain between 2H and 1T' phases. Low growth temperature combines with slow tellurization rate (low Te content) is in favor of growing 2H-MoTe2, while high growth temperature together with fast tellurization rate (high Te content) is benefit to grow 1T'-MoTe2. A phase diagram based on the thermodynamics and kinetics of MoTe2 growth was drawn, which provides significant guidance for future synthesizing of 2D materials with controlled phase structure.

KEYWORDS: Molybdenum ditelluride (MoTe2), phase-engineering, kinetics, thermodynamics, single crystal, CVD growth


Page 2 of 22

Page 3 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Introduction Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted considerable interest owing to their rich physics and promising potential applications.1-9 Interestingly, group VI TMDs have different electronic structures depending on their crystal structures: the trigonal prismatic structure (2H phase) is a semiconductor, and the octahedral and distorted octahedral (1T and 1T' phases) are metals.10, 11 Molybdenum ditelluride (MoTe2) is quite unique among the TMDs family since it is the only material that can stable in both 2H and 1T' forms, allowing to prepare these two types materials and even their heterojunctions based on phase-engineering.12 The semiconducting 2H-MoTe2 has lower thermal conductivity and higher Seebeck coefficient, which might be commendable in thermoelectricity.13, 14

Especially, the band gap of 1.1 eV in 2H-MoTe2, which is near to that of bulk

silicon, together with the strong absorption throughout solar spectrum and spin-orbit coupling make it highly desired for future electronic and optoelectronic device applications.15-17 In addition, the metallic 1T'-MoTe2 has extremely large magnetoresistance,18 pressure-driven superconductivity,11 and quantum spin Hall effect19. Therefore, the scalable production of large-area, highly crystalline 2D MoTe2 with controlled phase is highly desirable for their promising wide applications. However, the low chemical reactivity of Te together with the small electronegativity difference between Te and Mo/W makes the synthesis of Te-based TMDs (MoTe2 and WTe2) to be much more difficult than that of S- and Se-based TMDs, such as MoS2, WS2, MoSe2 and WSe2, etc.20-23 Furthermore, the ground-state



Page 4 of 22

energy difference per formula unit between 2H and 1T' phases of monolayer MoTe2 (35 meV) is much smaller than the comparable energy difference in other TMDs materials,24 posing great challenges to the controllable synthesis of a pure crystal phase MoTe2.18 Recently, several pioneering works have reported the successful synthesis of 2D MoTe2 by using chemical vapor deposition (CVD) growth.13, 25-28 For example, Lee and Kong's group reported the synthesis of few-layer MoTe2 film with different phases via the tellurization of pre-deposited Mo or MoO3 films.13,

25, 29

Subsequently, Zhou and Liu' group illustrated the direct CVD growth of monolayer 1T'-MoTe2 film using MoO3 or MoO3-MoCl5-Te mixed source as Mo precursor.26, 27 These groundbreaking works have given great promotion to the synthesis of MoTe2 film, however, a method of controlled synthesis of highly crystalline 2H MoTe2 and 1T’ MoTe2 with defined geometry and thickness are still lacking. In particular, the structural metastability of MoTe2 makes its phase structure is sensitive to the subtle variation of the growth parameters in CVD process, which poses great obstacle to realize the phase control in CVD growth of MoTe2. In addition, the low thermostability of MoTe2 makes it decompose easily at high growth temperature, and thus causes large amount of Te defects in its lattice structure.12 The lattice defects, which degrades the quality of as-grown MoTe2, can also induce the phase transition from 2H to 1T' MoTe2. Thus, developing a synthesis method to obtain high crystallinity MoTe2 with controlled phase is challenging but crucial for its promising wide applications and rich physics study. In this work, we present an effective approach to synthesize 2D MoTe2 single


Page 5 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


crystal microplates with controlled phase (2H and 1T') via tellurization of CVD-grown MoO2 nanosheets (Figure 1a). The obtained MoTe2 nanosheets exhibit regular rhombus shape with thickness of ～30 nm and edge length up to ～40 µm, and have complete single crystal feature. Both 1T' and 2H-MoTe2 nanosheets can be controlled grown via tuning the thermodynamics and kinetics of the MoTe2 crystal growth. The low growth temperature combines with slow reaction rate (low partial pressure of Te) is in favor of growing 2H-MoTe2, while high growth temperature together with fast reaction rate (high partial pressure of Te) is benefit to the growth of 1T'-MoTe2. Furthermore, the inner-tube used during the tellurization stage, which can effectively prevent the Te deficiency, is critical to the phase-controlled growth of high quality MoTe2. Results and Discussion The synthesis strategy of MoTe2 nanosheets is schematically illustrated in Figure 1a, which is similar to previous reported work on the synthesis of MoS2 via tellurization of MoO2 but have some difference in several key points.30 Briefly, single crystal MoO2 nanosheets were first grown on SiO2/Si substrate by reducing MoO3 powder with H2 at 800 ºC in a CVD furnace. Then, the MoO2 nanosheets were converted into MoTe2 nanosheets via a tellurization reaction. Figure 1b schematically shows the approach of the tellurization stage, where MoO2 nanosheets at the hot centre of furnace was annealed in the tellurium vapor carried by Ar and H2 for 1.0 h. The evaporation temperature of Te powder (T1) and the tellurization temperature (T2), which are critical to control the phase of MoTe2 as discussed below, were controlled at



550-700 °C and 600-830 °C, respectively. Compared with previous synthesis methods, an inner-tube was used during the tellurization stage, which is important to prevent the Te deficiency as discussed below. Figure 1c shows the optical microscopy (OM) image of the CVD-grown MoO2 nanosheets, which have a rhombus shape with thickness about 30 nm (Figure S1) and edge length up to ～40 µm. The formation of rhombus shape MoO2 nanosheet should be attributed to its anisotropic growth with (1 0 0) and (1 0 -2) facets have larger surface area and lower surface energy, and to the relative low surface energy of SiO2/Si substrate which favor the growth of MoO2 at the 2D direction. The composition and crystal structure of obtained MoO2 nanosheets were confirmed by Raman spectroscopy X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM) (Figure S2). Furthermore, selected-area electron diffraction (SAED) patterns collected at several different positions of a MoO2 nanosheet exhibit the same direction (Figure S2), demonstrating its single crystal feature. After tellurization of MoO2, the color of the sample seen from OM image changes from the original pink into the light yellow (Figure 1d), and its rhombus shape is well maintained. Raman spectroscopy was utilized to monitor the structure variation of the samples during the tellurization process (Figure 1e). The as-obtained MoO2 plates show four prominent Raman peaks at 125, 204, 365, and 745 cm-1 under a 532 nm excitation laser. After tellurization, the peaks of MoO2 were displaced by the peaks of 2H-MoTe2, which shows several Raman peaks between 100 and 400 cm-1: the prominent peak of the in-plane E2g mode at ~ 234 cm-1, as well as the out-of-plane A1g and B2g mode at ~171 and ~289 cm-1, respectively. The uniform


Page 6 of 22

Page 7 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


intensity distribution in Raman mapping of a nanosheet before and after tellurization indicates that the conversion processes homogeneously. Furthermore, the energy dispersive X-ray spectroscopy (EDX) mapping of individual MoTe2 nanosheet shows that Mo and Te are homogeneously distributed throughout the entire nanosheet (Figure 1f). These results demonstrate that the MoO2 nanosheets were completely converted into 2H-MoTe2 nanosheets.

Figure 1. (a) Schematic of the synthesis of MoTe2 via tellurization of MoO2 nanosheets. (b) Schematic diagram of the chemical vapor deposition (CVD) growth method for the tellurization of MoO2 nanosheets. OM images of (c) as-grown MoO2 nanosheets and (d) converted into MoTe2 nanosheets through a tellurization process. The insets in (c) and (d) show the AFMs image of the CVD-grown MoO2 and MoTe2, respectively. (e) Typical Raman spectra of the MoO2 (black) and MoTe2 nanosheets (red). Inset: Raman intensity mapping of MoO2 (at ~125 cm-1) and MoTe2 (at ~234 cm-1). (f) EDX mapping of the MoTe2 nanosheet, green for Mo and red for Te, scale bar is 1.0 µm.

To reveal the factors determining the phase structure of MoTe2 growth, we first studied the effect of growth temperature on its phase structure by fixing T1 at 600 ºC,



while changing the growth temperature from 600 to 830 ºC (Figure 2a). The Raman spectra of MoTe2 samples grown at different T2 (700, 750 and 800 ºC) are shown in figure 2b. Obviously, only the characteristic peaks of 2H-MoTe2 were observed when the tellurization temperature was 700ºC, while the characteristic peaks of 1T'-MoTe2, such as Au (~107 cm-1), Ag ( ~127 cm-1), Bg ( ~163 cm-1) and Ag ( ~257 cm-1), were observed at 800 ºC. Interestingly, both the characteristic peaks of 2H-MoTe2 and 1T'-MoTe2 were observed from the samples with 750 ºC of tellurization. The evolution of Raman modes of MoTe2 samples grown at temperature range from 600 to 800 ºC is shown in Figure S3. The results indicate that 2H-MoTe2 is easily obtained when the growth temperature below ~700 ºC, while 1T'-MoTe2 is obtained when grown above ~800 ºC. The mixed phases MoTe2 are usually obtained at temperature range from 700-800 ºC. The statistical percentage of two phases (2H and 1T') as a function of the growth temperature show the amount of 2H phase decreases along with the 1T' phase increases as T2 increasing (Figure 2c). Furthermore, it is found that 1T' (and 2H) phase MoTe2 can be grown at 800 ºC (and 700 ºC) in just 10 min or an even shorter time (Figure S4), suggesting that the initial phase structure is determined directly by the growth temperature. We also noticed that the partial pressure of Te is critical to the temperature-controlled phase engineering of MoTe2 growth as discussed below. X-ray diffraction spectra were used to further confirm the phase structure of MoTe2 grown at different tellurization temperatures. Figure 2d shows the XRD patterns of the MoTe2 samples grown at 700, 750 and 800 ºC. The main peaks at (0002), (0004),


Page 8 of 22

Page 9 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(0006) and (0008) planes were detected in all of three samples, which matches well with the standard spectra of MoTe2 crystal.10, 18 It needs to be noted that only a single (0004) peak for samples grown at 700 and 800 ºC is observed, while it splits into two peaks when grown at 750 ºC, which are corresponding well with that of 2H and 1T' MoTe2 (Figure 2e), respectively.13, 18 These results are in good agreement with the results

obtained

from

above

Raman

analysis,

demonstrating

the

temperature-modulated phase engineering of MoTe2 synthesis. This can be understood in terms of the difference of thermodynamics stability between both phases MoTe2 as shown schematically in Figure 2f. It is well known that the 2H phase MoTe2 is more stable than that of 1T' phase in thermodynamics,31 as indicated from their ground-state energy difference (35 meV). In this case, the 1T'-MoTe2 is more energetically favorable grown at higher T2, while a relative lower energy (lower T2) is benefit to grow 2H-MoTe2. Notably, the temperature window for the mix phase spans about 100 ºC, which is reasonable in view of the much small energy difference between these two phases. Besides the growth thermodynamics, the growth kinetics is another important factor determining the phase structure of MoTe2 synthesis. Considering the transformation of MoO2 crystal to MoTe2 crystal should undergo a drastic structural reconfiguration process, large strain would be induced in the crystal structure. Both theoretical and experimental works indicate that 1T'-MoTe2 is more stable than 2H-MoTe2 under the mechanical strain conditions.24, 32 In this case, overly rapid tellurization rate (at high T2) would make the strain can't be released in time, which favors MoTe2 growth towards the 1T' phase. In contrast, the strain was



expected to be released timely during a slow tellurization process (at a low T2, ≤ 700 ºC), and thus 2H-MoTe2 tend to be grown. In addition, recent study has found 1T'-MoTe2 is always tend to be grown when starting from Mo precursor compared with MoO3, which is also attribute to the grater strain induced from its larger volume expansion from Mo.21 Therefore, from both the thermodynamics and the kinetics perspectives, high growth temperature favors growing 1T'-MoTe2, and vice versa.

Figure 2. Thermodynamics controlled growth of MoTe2 via turning the growth temperature. (a) Schematic of the temperature variation of Te source and MoO2 nanosheets during the growth process. (b) Raman spectra of MoTe2 grown at different growth temperatures. (c) Plot of the statistical percentage of two phases (2H and 1T') as a function of the growth temperature. (d) XRD patterns of MoTe2 nanosheets grown at different temperatures. (e) XRD patterns near the (0004) planes of MoTe2. (f) Schematic diagram of the energy variation during the reaction of MoO2 tellurization into MoTe2 with different phases.

The chemical composition and the crystallographic structure of the CVD-grown MoTe2 nanosheets were further characterized by XPS and TEM. The XPS full


Page 10 of 22

Page 11 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


spectrum reveals the presence of Mo and Te elements which originate from the MoTe2 nanosheet (Figure S5). The prominent Mo 3d peaks at 228.5 eV (3d5/2) and 231.7 eV (3d3/2) are assigned to Mo-Te bonds (Figure 3a). The Te 3d5/2 and 3d3/2 peaks, located at 573.2 eV and 583.6 eV, are also attributed to Mo-Te bonds (Figure 3b). These features are consistent well with the XPS spectra obtained from MoTe2 bulk crystal. Additionally, the atomic ratio between Mo and Te elements is 1:1.98, showing that the CVD-grown MoTe2 nanosheet is near to stoichiometric. Figure 3c,d show the high-resolution transmission electron microscopy (HR-TEM) images of the 2H-MoTe2 and 1T'-MoTe2 nanosheets, respectively. The distinct crystalline lattices can be clearly seen in the HR-TEM images of 2H-MoTe2 and 1T'-MoTe2, which highlight the high crystal quality of the CVD-grown sample. The inset of each image shows their corresponding SAED pattern. The SAED pattern of 2H-MoTe2 shows a hexagonal shape, while that of 1T'-MoTe2 displays a rectangular shape. In addition, the crystal plane distances of 3.2 Å and 1.8 Å corresponding to the (101̅0) and (112̅0) planes of 2H-MoTe2, those of the (100) and (010) planes for the 1T'-MoTe2 are determined to be 6.4 Å and 3.5 Å. These distinct results manifest the formation of two different phases MoTe2 with growth temperature control. Notably, the SEAD patterns collected from different positions of the same 2H (1T') MoTe2 nanosheet exhibit same orientation (Figure 3e,f), indicating the as-grown 2H (1T') MoTe2 nanosheets are single-crystalline. It is important to realize the conversion of one single crystal (MoO2) into another single crystal (MoTe2) with morphology maintained well, which provides an effective route for the synthesis of other 2D atomic layer materials with high



crystallinity.

Figure 3. Structure and composition characterization of the CVD-grown MoTe2 nanosheets. XPS spectra of (a) Mo 3d core levels and (b) Te 3d core levels for the CVD-grown MoTe2 film. HRTEM images of (c) 2H phase and (d) 1T' phase MoTe2 nanosheet, which are grown at 700 ºC and 800 ºC, respectively. Low-resolution TEM and SAED patterns of (e) 2H phase and (f) 1T' phase MoTe2, the SEAD patterns are collected at three different positions of each nanosheet.

To further confirm the kinetics role in the phase engineering of MoTe2 synthesis, we studied the effect of partial pressure of Te, which is in proportion to the tellurization rate, on its phase structure. For this purpose a series of experiments were performed by fixing T2 at 700 ºC, while changing T1 from 600 to 700 ºC (Figure 4a). In this case, the partial pressure of Te increases sharply with the T1 increases due to its low


Page 12 of 22

Page 13 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


sublimation temperature (400 ºC), which can greatly speed up the tellurization rate (Figure S6). As a results, the percentage of 2H-MoTe2 decreases obviously with the increase of T1 from 600 to 700 ºC, and only 1T'-MoTe2 is obtained when T1 rises near to 700 ºC (Figure 4b). Note that we can regain 2H-MoTe2 grown at T1=700 ºC by covering a thick layer of molecular sieve on the surface of Te source, which can effectively reduce its volatilization rate. These results demonstrate that the tellurization rate can be modulated by tuning the partial pressure of Te, and the faster tellurization rate (higher partial pressure of Te) is in favor of growing 1T'-MoTe2, while the slower tellurization rate (lower partial pressure of Te) is benefit to the growth of 2H-MoTe2. Note that the partial pressure of Te can't be too low, otherwise 1T'-MoTe2 with much rough (or inhomogeneous) surface would tend to be obtained (Figure S7). This phenomenon is common in our experiments and previously reported works, which is due to the generation of defects caused by the sublimation of Te in 2H-MoTe2.12, 13, 25 Thus, the inner-tube used during the tellurization stage is a key element of our method, which prevents the Te deficiency that would otherwise arise as a result of the fast drain away of Te with the carrier gas stream (see detailed discussion in Figure S7). Furthermore, we also investigated the effect of tellurization temperature on the phase structure of MoTe2 grown under an excessive Te content by fixing T1 at 700 ºC, while changing T2 from 580-800 ºC. It was found that only 1T'-MoTe2 can be obtained when grown at T2 above 650 ºC, while 2H-MoTe2 can be observed until T2 down to 580 ºC (Figure 4c). Obviously, the threshold temperature (T2) for growing



both phases MoTe2 at T1=700 ºC lower than that growing at T1=600 ºC. Above results demonstrate that the synergetic effect between thermodynamics and kinetics of the tellurization reaction determine the phase-engineering of the MoTe2 synthesis. Based on above results and analysis, a phase diagram for synthesizing MoTe2 is present as schematically shown in Figure 4d. The low growth temperature combine with slow tellurization rate is in favor of growing 2H-MoTe2, while the high growth temperature together with fast tellurization rate is benefit to grow 1T'-MoTe2. Furthermore, the defect-induced phase transition of MoTe2 from 2H to 1T' tends to occur when grown under the deficient partial pressure of Te. In fact, the conclusion obtained in our work is well consistent with the approach and results of growth of 2H and 1T' MoTe2 reported in recent work,33 indicating the universality of our approach for controlled growing MoTe2 and WTe2 with different phase structures.

Figure 4. (a) Schematic of the temperature variation of Te source. (b) Raman spectra of MoTe2


Page 14 of 22

Page 15 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


samples grown at T2 = 700 ºC with T1 variation from 600 to 700 ºC. (c) Raman spectra of MoTe2 samples grown at T2 ranges from 600 to 800 ºC, and T1 is fixed at 700 ºC. (d) Schematic phase diagram of MoTe2 growth based on the reaction thermodynamics and kinetics.

Conclusion We have successfully synthesized large-scale 1T' and 2H-MoTe2 nanosheets via a two-step CVD growth approach. The effective conversion from MoO2 single crystal into MoTe2 single crystal via a tellurization process provides a new route for the synthesis of high crystallinity 2D atomic layer materials. The phase structure is found to be sensitive to both the growth temperature and the tellurium concentration. The synergetic effect between thermodynamics and kinetics of the crystal growth process is found to be a critical factor in the phase-controlled synthesis of MoTe2. The phase engineering is based on the difference in thermodynamic stability and lattice strain between the two phases MoTe2. To obtain uniform 2H-MoTe2 nanosheets, low growth temperature combines with slow tellurization rate (low Te content) is essential. While high growth temperature together with fast reaction rate (high Te content) is benefit to grow 1T'-MoTe2. Finally, a phase diagram based on the reaction thermodynamics and kinetics for MoTe2 growth was drawn, which would be benefit for guiding the future phase-controlled synthesis of single layer MoTe2 and WTe2 film. Our study provides insights into the controllable synthesis and phase engineering of high quality MoTe2.



Experiment section Growth of MoO2 single crystal nanosheets. The MoO2 nanosheets were synthesized by using CVD growth method with MoO3 (purity 99.9%) as Mo sources and H2 as reducing agent. Before growth, a ceramic boat contained 1.5 mg MoO3 was put into the hot center of the quartz tube. The SiO2/Si (300 nm) substrate was placed face-down on the boat. Argon was used as the carrier gas with a flow rate of 20 sccm during the whole growth process. The system was heated from room temperature to 800 °C at 40 °C min−1. When the temperature reached 700 °C, 1 sccm H2 was introduced into the system to reduce MoO3. MoO2 was synthesized at 800 °C for 5 min under atmosphere pressure. Then, the as-grown MoO2 sample was take out for further characterization and tellurization synthesize MoTe2.

Growth of single crystal MoTe2 nanosheets. The synthesis method of MoTe2 is schematically shown in Figure 2b. An inner-tube (8 mm diameter) with a tiny hole (1 mm) was used during the tellurization stage. Before growth, the as grown MoO2 sample and 100 mg Te powder (purity 99.8%) were placed at the center and the upstream of the hot zone in the inner-tube, respectively. The mixed gas of H2/Ar with 4 sccm and 3 sccm was used as the carrier gas. To control the tellurization reaction, the temperatures of the Te zone (T1) and MoO2 film zone (T2) were controlled separately. T1 and T2 were heated from room temperature to 600-700 °C (with a ramping rate of ～17 °C min-1) and 580-830 °C (with a ramping rate of ～20 °C min-1), and then maintained for 1 h for the growth of MoTe2. The system was finally cooled down to room temperature by opening the furnace.

Characterizations of MoO2 and MoTe2 nanosheets. The prepared samples were systematically characterized by using optical microscopy (Olympus BX51), Raman spectroscopy (Renishaw, 532 nm laser), AFM (Bruker Dimension ICON) and TEM (Tecnai G2 F20; acceleration voltage, 200


Page 16 of 22

Page 17 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


kV). For the TEM measurements, both MoO2 and MoTe2 samples were transferred from SiO2/Si substrate onto the TEM grid by using the poly(methyl methacrylate) (PMMA)-mediated transfer method with buffered oxide etchant.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Morphology and structure characterization of CVD-grown MoO2 nanosheets (Figure S1 and Figure S2), Raman spectra of MoTe2 grown at different T2 (Figure S3), different T1 (Figure S6) and different tellurization time (Figure S4), XPS spectra of the as-grown MoTe2 nanosheets (Figure S5), OM and Raman of MoTe2 grown via the CVD method with and without the inner-tube (Figure S7). AUTHOR INFORMATION

Corresponding Author * E-mail: [email protected].

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (51502167 and 21473110), and the fundamental Research Funds for the Central Universities (GK201802003).



REFERENCES (1) Bhimanapati, G. R.; Lin, Z.; Meunier, V.; Jung, Y.; Cha, J.; Das, S.; Xiao, D.; Son, Y.; Strano, M. S.; Cooper, V. R.; Liang, L.; Louie, S. G.; Ringe, E.; Zhou, W.; Kim, S. S.; Naik, R. R.; Sumpter, B. G.; Terrones, H.; Xia, F.; Wang, Y.; Zhu, J.; Akinwande, D.; Alem, N.; Schuller, J. A.; Schaak, R. E.; Terrones, M.; Robinson, J. A. Recent advances in two-dimensional materials beyond graphene. ACS Nano 2015, 9, 11509-11539. (2) Lv, R.; Robinson, J. A.; Schaak, R. E.; Sun, D.; Sun, Y.; Mallouk, T. E.; Terrones, M. Transition metal dichalcogenides and beyond: Synthesis, properties, and applications of single- and few-layer nanosheets. Acc. Chem. Res. 2015, 48, 56-64. (3) Butler, S. Z.; Hollen, S. M.; Cao, L. Y.; Cui, Y.; Gupta, J. A.; Gutiérrez, H. R.; Heinz, T. F.; Hong, S. S.; Huang, J. X.; Ismach, A. F.; Johnston Halperin, E.; Kuno, M.; Plashnitsa, V. V.; Robinson, R. D.; Ruoff, R. S.; Salahuddin, S.; Shan, J.; Shi, L.; Spencer, M. G.; Terrones, M.; Windl, W.; Goldberger, J. E. Progress, challenges, and opportunities in two-dimensional materials beyond graphene. ACS Nano 2013, 7, 2898-2926. (4) Geim, A. K.; Grigorieva, I. V. Van der waals heterostructures. Nature 2013, 499, 419-425. (5) Cui, F. F.; Wang, C.; Li, X. B.; Wang, G.; Liu, K. Q.; Yang, Z.; Feng, Q. L.; Liang, X.; Zhang, Z. Y.; Liu, S. Z.; Lei, Z.; Liu, Z. H.; Xu, H.; Zhang, J. Tellurium-assisted epitaxial growth of large-area, highly crystalline ReS2 atomic layers on mica substrate.

Adv. Mater. 2016, 28, 5019-5024. (6) Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P.; Zhang, H. The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets. Nat Chem

2013, 5, 263-275. (7) Xu, M. S.; Liang, T.; Shi, M. M.; Chen, H. Z. Graphene-like two-dimensional materials. Chem. Rev. 2013, 113, 3766-3798. (8) Yu, Y. F.; Huang, S. Y.; Li, Y. P.; Steinmann, S. N.; Yang, W. T.; Cao, L. Y. Layer-dependent electrocatalysis of MoS2 for hydrogen evolution. Nano Lett. 2014,


Page 18 of 22

Page 19 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


14, 553-558. (9) Wang, H.; Liu, F. C.; Fu, W.; Fang, Z. Y.; Zhou, W.; Liu, Z. Two-dimensional heterostructures: Fabrication, characterization, and application. Nanoscale 2014, 6, 12250-12272. (10) Sankar, R.; Narsinga Rao, G.; Muthuselvam, I. P.; Butler, C.; Kumar, N.; Senthil Murugan, G.; Shekhar, C.; Chang, T. R.; Wen, C. Y.; Chen, C. W.; Lee, W. L.; Lin, M. T.; Jeng, H. T.; Felser, C.; Chou, F. C. Polymorphic layered MoTe2 from semiconductor, topological insulator, to weyl semimetal. Chem. Mater. 2017, 29, 699-707. (11) Qi, Y. P.; Naumov, P. G.; Ali, M. N.; Rajamathi, C. R.; Schnelle, W.; Barkalov, O.; Hanfland, M.; Wu, S. C.; Shekhar, C.; Sun, Y.; Süß, V.; Schmidt, M.; Schwarz, U.; Pippel, E.; Werner, P.; Hillebrand, R.; Förster, T.; Kampert, E.; Parkin, S.; Cava, R. J.; Felser, C.; Yan, B.; Medvedev, S. A. Superconductivity in weyl semimetal candidate MoTe2. Nat. Commun. 2016, 7, 11038. (12) Cho, S.; Kim, S.; Kim, J. H.; Zhao, J.; Seok, J. B.; Keum, D. H.; Baik, J.; Choe, D. H.; Chang, K. J.; Suenaga, K.; Kim, S. W.; Lee, Y. H.; Yang, H. J. Phase patterning for ohmic homojunction contact in MoTe2. Science 2015, 349, 625-628. (13) Park, J. C.; Yun, S. J.; Kim, H.; Park, J. H.; Chae, S. H.; An, S. J.; Kim, J. G.; Kim, S. M.; Kim, K. K.; Lee, Y. H. Phase-engineered synthesis of centimeter-scale 1T'- and 2H-molybdenum ditelluride thin films. ACS Nano 2015, 9, 6548-6554. (14) Balendhran, S.; Walia, S.; Nili, H.; Ou, J. Z.; Zhuiykov, S.; Kaner, R. B.; Sriram, S.; Bhaskaran, M.; Kalantar-zadeh, K. Two-dimensional molybdenum trioxide and dichalcogenides. Adv. Funct. Mater. 2013, 23, 3952-3970. (15) Lezama, I. G.; Arora, A.; Ubaldini, A.; Barreteau, C.; Giannini, E.; Potemski, M.; Morpurgo, A. F. Indirect-to-direct band gap crossover in few-layer MoTe2. Nano Lett.

2015, 15, 2336-2342. (16) Ruppert, C.; Aslan, O. B.; Heinz, T. F. Optical properties and band gap of singleand few-layer MoTe2 crystals. Nano Lett. 2014, 14, 6231-6236. (17) Pradhan, N. R.; Rhodes, D.; Feng, S.; Xin, Y.; Memaran, S.; Moon, B.-H.; Terrones, H.; Terrones, M.; Balicas, L. Field-effect transistors based on few-layered ACS Paragon Plus Environment


MoTe2. ACS Nano 2014, 8, 5911-5920. (18) Keum, D. H.; Cho, S.; Kim, J. H.; Choe, D. H.; Sung, H. J.; Kan, M.; Kang, H.; Hwang, J. Y.; Kim, S. W.; Yang, H. J.; Chang, K. J.; Lee, Y. H. Bandgap opening in few-layered monoclinic MoTe2. Nat Phys 2015, 11, 482-486. (19) Qian, X.; Liu, J.; Fu, L.; Li, J. Quantum spin hall effect in two-dimensional transition metal dichalcogenides. Science 2014, 346, 1344-1347. (20) Lu, X.; Utama, M. I. B.; Lin, J.; Gong, X.; Zhang, J.; Zhao, Y.; Pantelides, S. T.; Wang, J.; Dong, Z.; Liu, Z.; Zhou, W.; Xiong, Q. Large-area synthesis of monolayer and few-layer MoSe2 films on SiO2 substrates. Nano Lett. 2014, 14, 2419-2425. (21) Zhou, L.; Xu, K.; Zubair, A.; Liao, A. D.; Fang, W.; Ouyang, F.; Lee, Y.-H.; Ueno, K.; Saito, R.; Palacios, T.; Kong, J.; Dresselhaus, M. S. Large-area synthesis of high-quality uniform few-layer MoTe2. J. Am. Chem. Soc. 2015, 137, 11892-11895. (22) Chang, Y. H.; Zhang, W. J.; Zhu, Y.; Han, Y.; Pu, J.; Chang, J. K.; Hsu, W. T.; Huang, J. K.; Hsu, C. L.; Chiu, M. H.; Takenobu, T.; Li, H.; Wu, C.-I.; Chang, W. H.; Wee, A. T. S.; Li, L. J. Monolayer MoSe2 grown by chemical vapor deposition for fast photodetection. ACS Nano 2014, 8, 8582-8590. (23) Gao, Y.; Liu, Z. B.; Sun, D. M.; Huang, L.; Ma, L. P.; Yin, L. C.; Ma, T.; Zhang, Z. Y.; Ma, X. L.; Peng, L. M.; Cheng, H. M.; Ren, W. C. Large-area synthesis of high-quality and uniform monolayer WS2 on reusable au foils. Nat. Commun. 2015, 6, 8569. (24) Duerloo, K.-A. N.; Li, Y.; Reed, E. J. Structural phase transitions in two-dimensional Mo- and W-dichalcogenide monolayers. Nat. Commun. 2014, 5, 4214. (25) Zhou, L.; Zubair, A.; Wang, Z. Q.; Zhang, X.; Ouyang, F. P.; Xu, K.; Fang, W. J.; Ueno, K.; Li, J.; Palacios, T.; Kong, J.; Dresselhaus, M. S. Synthesis of high-quality large-area homogenous 1T' MoTe2 from chemical vapor deposition. Adv. Mater. 2016,

28, 9526-9531. (26) Naylor, C. H.; Parkin, W. M.; Ping, J.; Gao, Z.; Zhou, Y. R.; Kim, Y.; Streller, F.; Carpick, R. W.; Rappe, A. M.; Drndić, M.; Kikkawa, J. M.; Johnson, A. T. C. Monolayer single-crystal 1T'- MoTe2 grown by chemical vapor deposition exhibits ACS Paragon Plus Environment

Page 20 of 22

Page 21 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


weak antilocalization effect. Nano Lett. 2016, 16, 4297-4304. (27) Zhou, J. D.; Liu, F. C.; Lin, J. H.; Huang, X. W.; Xia, J.; Zhang, B. W.; Zeng, Q. S.; Wang, H.; Zhu, C.; Niu, L.; Wang, X. W.; Fu, W.; Yu, P.; Chang, T. R.; Hsu, C. H.; Wu, D.; Jeng, H. T.; Huang, Y. Z.; Lin, H.; Shen, Z. X.; Yang, C. L.; Lu, L.; Suenaga, K.; Zhou, W.; Pantelides, S. T.; Liu, G. T.; Liu, Z. Large-area and high-quality 2D transition metal telluride. Adv. Mater. 2017, 29, n/a-n/a. (28) Empante, T. A.; Zhou, Y.; Klee, V.; Nguyen, A. E.; Lu, I. H.; Valentin, M. D.; Naghibi Alvillar, S. A.; Preciado, E.; Berges, A. J.; Merida, C. S.; Gomez, M.; Bobek, S.; Isarraraz, M.; Reed, E. J.; Bartels, L. Chemical vapor deposition growth of few-layer MoTe2 in the 2H, 1T', and 1T phases: Tunable properties of MoTe2 films.

ACS Nano 2017, 11, 900-905. (29) Zhou, L.; Xu, K.; Zubair, A.; Zhang, X.; Ouyang, F. P.; Palacios, T.; Dresselhaus, M. S.; Li, Y. F.; Kong, J. Role of molecular sieves in the CVD synthesis of large-area 2D MoTe2. Adv. Funct. Mater. 2017, 27, 10.1002/adfm.201603491. (30) Wang, X. S.; Feng, H. B.; Wu, Y. M.; Jiao, L. Y. Controlled synthesis of highly crystalline MoS2 flakes by chemical vapor deposition. J. Am. Chem. Soc. 2013, 135, 5304-5307. (31) Li, W. B.; Li, J. Ferroelasticity and domain physics in two-dimensional transition metal dichalcogenide monolayers. Nat. Commun. 2016, 7, 10843. (32) Song, S.; Keum, D. H.; Cho, S.; Perello, D.; Kim, Y.; Lee, Y. H. Room temperature semiconductor–metal transition of MoTe2 thin films engineered by strain.

Nano Lett. 2016, 16, 188-193. (33) Sung, J. H.; Heo H.; Si S.; Kim Y. H.; Noh, H. R.; Song, K.; Kim, J.; Lee, C. S.; Seo, S. Y.; Kim, D. H.; Kim, H. K.; Yeom, H. W.; Kim, T. H.; Choi S. Y.; Kim, J. S.; Jo, M. H. Coplanar semiconductor–metal circuitry defined on few-layer MoTe2 via polymorphic heteroepitaxy. Nat. Nanotech., 2017, 12, 1064-1071.



Thermodynamics and Kinetics Synergetic Phase-engineering of CVD Grown Single Crystal MoTe2 Nanosheets Xiaosa Xu,† Xiaobo Li,† Kaiqiang Liu,‡ Jing Li,† Qingliang Feng,§ Lin Zhou,ǁ Fangfang Cui,† Xing Liang,† Zhibin Lei,† Zonghuai Liu,† and Hua Xu*†

Table of Contents Graphic

High crystallinity MoTe2 nanosheets with controlled phase structure (2H and 1T') were synthesized via the tellurization of CVD-grown MoO2 nanosheets. The phase structure of grown MoTe2 is found to be determined by the synergetic effect of thermodynamics and kinetics in the crystal growth process, which is based on the difference in thermodynamic stability and lattice strain between 2H and 1T' phases.


Page 22 of 22

Thermodynamics and Kinetics Synergetic Phase-Engineering of

Recommend Documents