Anisotropic Ordering in 1Tâ² Molybdenum and Tungsten Ditelluride

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Anisotropic Ordering in 1T# Molybdenum and Tungsten Ditelluride Layers Alloyed with Sulphur and Selenium Junhao Lin, Jiadong Zhou, Sebastian Zuluaga, Yu Peng, Meng Gu, Zheng Liu, Sokrates T. Pantelides, and Kazu Suenaga ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b08782 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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Anisotropic Ordering in 1T′ Molybdenum and Tungsten Ditelluride Layers Alloyed with Sulphur and Selenium Junhao Lin1*, Jiadong Zhou2, Sebastian Zuluaga3, Peng Yu2, Meng Gu3, Zheng Liu2, Sokrates T. Pantelides4,5, and Kazu Suenaga1,6 1

National Institute of Advanced Industrial Science and Technology (AIST), AIST Central 5,

Tsukuba 305-8565, Japan 2

Centre for Programmed Materials, School of Materials Science and Engineering, Nanyang

Technological University, Singapore 639798, Singapore 3

Department of Material Science and Engineering, South University of Science and

Technology, Shenzhen 518055, China 4

Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA

5

Department of Electrical Engineering and Computer Science, Vanderbilt University,

Nashville, TN 37235, USA 6

Department of Mechanical Engineering, The University of Tokyo, Tokyo 113-8656, Japan

* Correspondence and requests for materials should be addressed to: [email protected]

ABSTRACT: Alloying is an effective way to engineer the bandgap structure of two-dimensional transitionmetal dichalcogenide materials. Molybdenum and tungsten ditelluride alloyed with sulphur or selenium layers (MX2xTe2(1-x), M=Mo, W and X=S, Se) have a large bandgap tunability from metallic to semiconducting due to the 2H-to-1T′ phase transition as controlled by the alloy concentrations, whereas the alloy atom distribution in these two phases remain elusive. Here, combining atomic resolution Z-contrast scanning transmission electron microscopy (STEM)

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imaging and density functional theory (DFT), we discovered that anisotropic ordering occurs in 1T′ phase, in sharp contrast to the isotropic alloy behavior in the 2H phase under similar alloy concentration. The anisotropic ordering is presumably due to the anisotropic bonding in 1T′ phase, as further elaborated by DFT calculations. Our results reveal the atomic anisotropic alloyed behavior in 1T′ phase layered alloys regardless of their alloy concentration, shining light on fine tuning their physical properties via engineering the alloyed atomic structure.

KEYWORDS: anisotropic ordering, 1T′ phase layered materials, molybdenum ditelluride alloy, tungsten ditelluride alloy, MoS2xTe2(1-x), WSe2xTe2(1-x), STEM

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) have been substantially investigated due to their fascinating properties arising from the monolayer nature.1–3 For instance, semiconducting monolayer MoS2 has direct bandgap transition which leads to substantial enhancement in photoluminescence (PL), while its multilayer counterpart is a semiconductor with indirect bandgap, giving rise to trivial PL efficiency.4,5 To diversify the 2D TMDs materials as functional building blocks for electronic applications, it is necessary to tune their electronic structures in a wide range. Numerous techniques have been proposed to modify the electronic structures of the 2D TMDs materials, such as surface decoration,6,7 defect engineering,8–12 impurities doping13–15 and etc. Among all these techniques, alloying serves as an effective way to controllably engineer the intrinsic electronic structure of 2D TMDs while preserving their structural integrity. Monolayer alloys with wide-range tunable band gap has been achieved by alloying either the cation or anion in ternary 2D alloy such as MoxW1-xS2,16–18 MoS2xSe2(1-x),19,20 and most recently reported in quaternary alloy MoxW1−xS2ySe2(1−y).21


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For ternary 2D alloy such as MoxW1-xS2 and even quaternary MoxW1−xS2ySe2(1−y), the resulting 2D alloy also maintains the same 2H phase as its alloy precursors such as MoS2 or MoSe2. The distribution of S and Se atoms in these 2D alloys is randomized, presumably due to the large phase miscibility.16,22 However, recent report show that WSe2 with 2H phase and WTe2 with 1T′ phase can also form stable layered WSe2xTe2(1-x) alloys, with a 2H-to-1T′ phase transition as controlled by the alloying concentration.23 Such phase transition enables a wide range tunability of electronic structures from semiconducting to metallic, while mixing Se into the Te matrix increases the ambient stability of the 1T′ phase. Nevertheless, due to the phase incompatibility, the Te-based alloy in both 2H and 1T′ may have different atomic alloy structures, whereas no investigation has been conducted. Specifically, alloy structure may give rise to extraordinary physical properties, such as spontaneous vertical dipoles induced by ordered distribution of Se and S atoms in Janus MoS2xSe2(1-x) monolayer.24 Therefore, revealing structure of the 2D Te-based alloys severs as the important step to explore their physical properties.

Here, using atomic resolution low-voltage scanning transmission electron microscopy (STEM) imaging with atom-by-atom analysis, we examined the atomic alloy behavior of several molybdenum and tungsten ditelluride monolayers alloyed with sulphur or selenium (MX2xTe2(1-x), M=Mo, W and X=S, Se) in both 2H and 1T′ phases and with different alloy concentrations. Through statistical analysis, we found that isotropic alloying still persists in 2H phase despite of the large lattice mismatch, while anisotropic ordering of sulphur or selenium atoms occurs in the 1T′ phase. Sulphur or selenium atoms preferably occupy the atomic sites near the metal atoms in the 1T′ phase regardless of the Te concentration. Combined with density functional theory (DFT) calculations, we demonstrated that the anisotropic ordering in the 1T′ phase originates from the anisotropic atomic sites of different


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bonding environments -- the alloyed sulphur or selenium atoms form more stable bonding when they are near the metal atoms in order to stabilize the alloyed structure. Such anisotropic ordering also changes the electronic structure of the alloy substantially as predicted by DFT calculations.

RESULTS AND DISCUSSION In order to accurately investigate the atomic structure of Te-based alloy by quantitative statistical analysis, we first examined the intensity distribution of atomic columns in STEM imaging of different phases in the WSe2xTe2(1-x) material system. While WSe2 and WTe2 result in pure 2H and 1T′ phases, phase mixing exists at moderate Te alloy concentration (50%~70%) in WSe2xTe2(1-x) layered alloys,23 which is explained by DFT calculations in detail as shown in Fig. S1 in the Supplementary Information. Figure 1 shows the histogram of the intensity distribution through large-scale atom-by-atom mapping (over thousand atoms) in 2H WSe2, 2H WSe2xTe2(1-x), 1T′ WSe2xTe2(1-x) and WTe2. Anion and cation sites are mapped separately, as indicated by the markers shown on the representative STEM images (the larger view of the images are shown in Fig. S2). The intensity of each atomic column in the Zcontrast STEM imaging is directly related to the atomic number of the imaged species,25 and number of atoms inside the column. Note that the intensity distribution is a broad Gaussian peak instead of a sharp delta function, consistent with previous reports,19,21 in which the variance is caused by aberration of the probe, thermal vibration of the atoms, peak finding error, inhomogeneously distributed absorbates or surface debris and etc., but different chemical species can still be distinguished on a statistical level when their atomic weight differs.


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The intensity distribution of the cation sites appears as a single Gaussian peak since alloying only occurs in the anion sites, i.e., W atoms are the only chemical species at the cation sites being imaged across the four materials. We aligned the peak of the cation intensity distribution to the same position among all the images obtained for four materials, in order to avoid the possible influence of surface adsorbates and probe aberrations to the anion sites, and to allow us to compare the intensity distribution quantitatively. Two anion sites of the 2H phase overlaps while being displaced in 1T′. Therefore, two distinct distribution peaks appear in WSe2, corresponding to single Se (monoselenium vacancy) atom and Se2 column whereas only one peak in WTe2 which corresponds to displaced Te atoms. In 1T′ WSe2xTe2(1-x), the intensity distribution of Se and Te are well separated, and the peak positions are also consistent with single Se atom in WSe2 and Te in WTe2. While single Se and Te atom maintain a smaller atomic weight than W, the total atomic number of Se+Te and Te2 atomic columns overwhelm the W columns in 2H WSe2xTe2(1-x), giving rise to three distinct Gaussian peak distribution. We noted that the increase of intensity vs. atomic weight does not simply follow the relation of ~Z1.7 as established in light elements25 even after background subtraction, presumably due to the different cross section of heavy elements. We also notice that intensity of atomic columns containing two atoms or more is not a simple addition of each. Nevertheless, Figure 1 validates the fact that, though intensity variance exists for the same specie being imaged, the chemical identity and composition of each atomic column can still be directly mapped out by quantitative analysis based on the intensity of the cation or anion sites in Te-based alloy materials.

We can now directly extract the atomic models from the STEM images. Furthermore, our theory suggests the coexistence of 2H and 1T′ at moderate Te alloy concentration is not limited to WSe2xTe2(1-x), and can be generalized to Te-based MX2xTe2(1-x) (M=Mo, W and


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X=S, Se) alloy materials (detail shown in Fig. S1). Therefore, we examined several molybdenum and tungsten Te-based layered alloys obtained from different growth methods (see methods section) to investigate the generalized alloy structures of 2H and 1T′ phase. We picked up MoSe2xTe2(1-x) and WSe2xTe2(1-x) layered alloys as representative materials for 2H phase and MoS2xTe2(1-x) and WSe2xTe2(1-x) for 1T′ phases. Figure 2 and 3 shows the monolayer regions of these representative alloys in 2H (Fig. 2) and 1T′ phase (Fig. 3), where individual atomic columns can be resolved clearly. The 2H phase shows hexagonal patterns, while the 1T′ phase exhibits alternating chain-like structures, as confirmed by the fast Fourier transformation (FFT) pattern showing the morphology of the unit cell structure (Fig. S3 in Supplementary Information). Line intensity profile show different chemical specie in these materials (Fig. S3). For instance, the brightest spots in Fig. 2a are Te2 columns, followed by Te+Se columns, Se2 columns, Mo columns and Te or Se monovacancy ranked by their intensity. We then extract the atomic structure and chemical distribution in these four regions and reconstruct the corresponding 3D atomic models. Simulated image based on the extracted models are in excellent agreement with the experiments, as shown in Fig. 2 and 3, further confirming the accuracy of our quantitative intensity analysis. Moreover, the alloy concentration of the shown regions can be directly deduced by the ratio of the alloyed atoms, which are MoSe1.04Te0.96 and WSe1.52Te0.48 in Fig. 2, and MoS0.86Te1.14 and WSe0.44Te1.56 in Fig. 3.

We investigated the alloying behavior of the atoms based on the extracted atomic models. A rough inspection of the models revealed that the 2H phase of Te-based alloy preserves an isotropic distribution of the alloy atoms. However, both sulphur atoms in MoS2xTe2(1-x) alloy and selenium atoms in WSe2xTe2(1-x) tend to stay at the atomic sites close to the metal chain in the 1T′ phase, resulting in partially directional sulphur or selenium chains inside the lattice.


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To further confirm the existence of anisotropic ordering in the 1T′ phase but not in the 2H phase, we performed statistical analysis on sub-regions of the sample. Since the metal framework in the unit cell between the 2H and the 1T′ phase is similar (indicated in the atomic model in Fig. 4a), we divided the alloyed chalcogen atoms into two groups alternatively based on the unit cell of the metal atoms as highlighted by different colors in Fig. 4a, to further compare the alloying ordering between the two phases. For isotropic alloying since the distribution of the alloy atom is random, there is no preferable sites for sulphur or selenium atom to occupy, whereby the sulphur or selenium atoms should be found to have equal occupancy in the two groups; otherwise if they are found to occupy mostly in either one of the two group, anisotropy ordering should have been confirmed. Figure 4a shows the statistical ratio of sulphur or selenium atoms inside group I sampled from different regions for the four monolayer alloys examined previously, as a function of the Te concentration. Representative images of these regions for the four alloys are shown in Fig. S4 and S5 in Supplementary Information. As expected, the 2H phase does not show any ordering as the ratio is around 50%, i.e., random distribution of the alloyed atoms. In contrast, the1T′ phase shows obvious anisotropic ordering where over 95% of the sulphur or selenium atoms are found in group I, i.e., the atomic sites near the metal atom chain. Figure 4a also suggests that such anisotropic ordering is independent with the Te concentration but solely determined by the phase of the alloy. Figure 4b shows a schematic when the anisotropy ordering occurs in 50% alloy concentration, suggesting the S (or Se) atoms could form separated 1D channels in the 1T′ phase. Note that Fig. 4b is only an ideal model, and atomic diffusion inevitably occurs due to the thermodynamical equilibrium in the growth process even at the precise concentration.


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To further understand the origin of the anisotropic ordering of sulphur or selenium atom in 1T′ telluride alloys, we performed DFT calculations on different atomic configurations on alloys that consist 50% Te concentration and optimized their structures by DFT structural relaxation. Figure 5 shows the energy of different atomic configurations in 2H and 1T′ phases for all 4 alloys. For 2H phase, all configurations show nearly the same energy without any preferential atomic sites for sulphur or selenium. However, there is a large energy difference in the 1T′ phase depending on the anion configurations. The energy difference indicates sulphur or selenium atoms prefer to bond near the metal chain to maintain lower energy of the structure, leading to the anisotropic ordering observed in experiment. The origin of such energy difference is straightforward to understand. In 1T′ MoTe2 or WTe2 the structure can be considered as quasi-one-dimensional due to the distortion of the metal atoms which forms two distinct positions for Te atoms, i.e., they can bond either close or far from the metal atoms in chains.26 However, sulphur or selenium forms shorter bond length when bonding with metal atoms compared to Te. When they are alloyed in the 1T′ phase, the bonding is more stable when the bond length is close to the 2H phase. We compared the bond lengths between the sulphur and the Mo atoms in MoS2xTe2(1-x) (also true for selenium) for the two distinct sites (near and far from the metal, corresponding to group I and II atomic sites in Fig. 4) in the 1T′ phase, which is 2.40Å and 2.57Å. The former is very close to the Mo-S bond length in the 2H case (2.42Å). In another word, if the sulphur atoms occupy the atomic site that is far from the metal atoms, they are highly strained which makes the structure unstable. Moreover, we calculate the total density of states (TDOS) of each configuration for the four systems, as summarized in Fig. S6, where the anisotropic ordering leads to new states near the fermi level. The new state below the conduction band seems to come from the valence band below. Even though this is counter intuitive (one would expect that such shift in the DOS would lower the stability of the system), however, in general, such assumptions are not valid since


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all the changes in the valence band have to be taken into account. Therefore, the anisotropic ordering is important to understand the electronic structure emerging in these Te-based layered alloy.

CONCLUSIONS In conclusion, we found that in molybdenum and tungsten ditelluride alloyed with sulphur or selenium atomic layers with the 2H-to-1T′ phase transition, though the 2H phase maintains an isotropic distribution of the alloyed atoms, highly anisotropic ordering of the sulphur and selenium atoms are observed in the 1T′ phase, which preferentially occupy the atomic sites next to the metal chain. DFT calculations further explain that such anisotropic ordering originates from the anisotropic bonding between different atomic sites in 1T′ phase, where sulphur or selenium atoms are much more stable when occupying the sites near the metal chains (group I atomic sites in Fig. 4). Given the highly selective occupation in specific atomic sites, it is possible to form structural modulating material by controlling the alloy concentration and the growth conditions, where quasi-one-dimensional sulphur or selenium strips interweaving with Te chains in 1T′ molybdenum and tungsten ditelluride layered alloy can be achieved.

Te-based layered materials are attracting more and more attention recently due to their fascinating properties such as the non-saturating magnetoresistance.27 The anisotropic ordering of 1T′ phase reported in this work is also important to the promising phase-switching devices made by MoTe2 alloys which enabled by the trivial energy between the semiconducting 2H and metallic 1T′ phase,28,29 since such anisotropic ordering would also occur during the phase switching of the alloy. The present results are also likely to induce more effort in exploring how the anisotropic alloy structure modulates the physical properties


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of the 1T′ phase materials, such as their magnetoresistance. More importantly, our findings may stimulate research on other low-dimensional Te-based alloy materials, for instance, how the alloy structure behave in one-dimensional nanowires that can be directly fabricated from TMDs alloy monolayer materials.30,31

METHODS Sample preparation. The single crystals of WSe2xTe2(1-x) in 2H and 1T′ phases were grown by a self-flux method with Se and Te as the flux that described in the previous literature.23 The MoS2xTe2(1-x) in 1T′ phase and MoSe2xTe2(1-x) in 2H phase was grown by a CVD method assisted by molten salt. In brief, mixed powder of NaCl and MoO3 with weight of 1 mg and 6 mg in the alumina boat was placed in the center of the tube. Another alumina boat containing mixed powder (S and Te or Se and Te with different weight ratio) was placed in the upstream. The furnace was heated with a ramp rate of 50 oC/min to the growth temperature of 750 oC and held at this temperature for 15 mins before cooled down to room temperature naturally. The mixed Ar/H2 with a flow rate of 50/5 sccm was used as the carrier gas. The monolayer samples of WSe2xTe2(1-x) were prepared by mechanical exfoliation onto SiO2/Si substrate, then subsequently moved to an Au quantifoil TEM grid by a dry-transferred method.32 The CVDgrown MoS2xTe2(1-x) in 1T′ phase and MoSe2xTe2(1-x) in 2H phase monolayers were directly transferred onto Au TEM grid by a dry-etching method.33 The as-prepared TEM samples were stored in a vacuum chamber for future characterizations.

STEM characterization. The Z-contrast STEM imaging were done in a JEOL 2100F with delta probe corrector, which corrects the aberration up to 5th order, resulting in a probe size of 1.4 A. The imaging was conducted at an acceleration voltage of 60 kV. The convergent angle for illumination is about 35 mrad, with a collection detector angle ranging from 62 to 200


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mrad. A JEOL heating-holder is used to keep the sample at 500 °C to evaporate hydrocarbon contaminations introduced during the transfer, which can also effectively reduce carbon redeposition during the examination of the sample. All images have been low-pass filtering for better visibility.

DFT calculations. DFT calculations were carried out using Perdew-Burke-Ernzerhof generalized-gradient approximation (PBE-GGA).34 The ion-electron interaction is treated using the projector augmented-wave (PAW)35 technique as implemented in the Vienna abinitio simulation package (VASP).36,37 A plane-wave expansion with a kinetic-energy cutoff of 500 eV is used. A 21×21×1 Monkhorst Pack grid was used to sample the k points for small unit cells, while 5×5×1 grid is used for larger unit cell in some specific alloying concentrations. A vacuum of 10 Å was used in the z direction to avoid interactions between layers.

ASSOCIATED CONTENT The authors declare no competing financial interests.

ACKNOWLEDGEMENT: J.L. and K.S. acknowledge JST-ACCEL and JSPS KAKENHI (JP16H06333 and P16823) for financial support. This work is also supported by the Singapore National Research Foundation under NRF RF Award No. NRF-RF2013-08, Tier 2 MOE2016-T2-2-153 and MOE2015-T22-007. Work at Vanderbilt was supported by the U.S. Department of Energy via grant No. DE-FG02-09ER46554 and by the McMinn Endowment (S.Z., S.T.P.).

Supporting Information


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Supporting Information Available: The file contains discussions on DFT calculations on 2Hto-1T′ phase transition in layered alloy MX2xTe2(1-x) (M=Mo, W and X=S, Se), and additional Figures S1-S6 referred in the main text. This material is available free of charge via the Internet at http://pubs.acs.org.

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FIGURES:

Figure 1: Histogram of the intensity mapping on atomic columns in WSe2, WSe2xTe2(1-x) and WTe2 monolayers. Intensity mapping and related histogram in 2H WSe2 (a), 2H WSe2xTe2(1-x) (b), 1T′ WSe2xTe2(1-x) (c) and WTe2 1T′ (d) monolayers. Cation and anion sites are mapped separately as highlighted by red and green markers, respectively. Larger view of


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the mapping region is shown in the Supplementary Information. The distribution of the intensity is fitted with Gaussian peaks. Distinct peaks are observed due to different atomic weight of the atomic columns and different number of atoms in the atomic column. The intensity distribution of the cations is aligned across all the images obtained from these materials in order to avoid the possible influence of surface adsorbate, and probe aberrations. The histogram validates the fact that we can directly assign the chemical identity of each atomic column based on their image intensity.


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Figure 2: Atomic resolution STEM images, simulation and corresponding atom-by-atom mapping of the representative selenium alloyed ditelluride monolayers in 2H phase. (a) MoSe2xTe2(1-x) monolayer. The alloyed concentration is determined by the ratio between the alloyed sulphur or selenium and tellurium atoms. The Te concentration in this region is calculated to be ~48% according to the atomic mapping; (b) WSe2xTe2(1-x) monolayer. Te concentration in this region is ~24%.


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Figure 3: Atomic resolution STEM images, simulation and corresponding atom-by-atom mapping of the representative sulphur and selenium alloyed ditelluride monolayers in 1T′ phase. (a) MoS2xTe2(1-x) monolayer. The Te concentration in this region is calculated to be ~57% based on the atom counting; (b) WSe2xTe2(1-x) monolayer in 1T′ phase. Te concentration ~78%.


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Figure 4: Anisotropic ordering of alloyed sulphur and selenium atoms in 1T′ phase. (a) All the chalcogen atoms are divided into two groups alternatively based on the positions of the metal atoms to compare the site-dependent atomic occupancy in 2H and 1T′ phases, as highlighted by red and green in the atomic models. The sulphur or selenium atoms are counted separately in regions labelled as Group I and Group II, then the atomic ratio of alloyed atoms in Group I can be defined as

%, which serves

as an indication parameter for anisotropic ordering. Each statistical data point is obtained by averaging the values calculated from several regions in the sample (over 5 regions and totally over 2000 atomic coordinates in each material), where the error bar indicates the standard deviation. The four data points corresponds to four kinds of samples shown in Fig. 2 and 3, respectively. (b) Ideal schematic of the randomized and anisotropic distribution of S/Se atoms in 2H and 1T′ phases with a 50% alloy concentration.


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Figure 5: DFT calculations of the energy for different alloyed atom configurations in 2H and 1T′ phases. All the calculated values have been normalized to set the 2H phases as the zero-reference line. For 1T′ phases, the sulphur or selenium atoms have much lower energy when bonding near the metal atoms (configuration 4), suggesting a preferential occupancy in these specific atomic sites which results in the anisotropic ordering. Whereas in 2H phase there is no energy difference regardless of different alloyed atomic configurations, indicating an isotropic distribution of the alloyed atoms in the 2H phase.


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For TOC only:


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Anisotropic Ordering in 1Tâ² Molybdenum and Tungsten Ditelluride

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