Ion adsorption promotes Frank-van der Merwe growth of 2D transition metal tellurides

Summary

Reliable synthesis methods for high-quality, large-sized, and uniform two-dimensional (2D) transition-metal dichalcogenides (TMDs) are crucial for their device applications. However, versatile approaches to growing high-quality, large-sized, and uniform 2D transition-metal tellurides are rare. Here, we demonstrate an ion adsorption strategy that facilitates the Frank-van der Merwe growth of 2D transition-metal tellurides. By employing this method, we grow MoTe₂ and WTe₂ with enhanced lateral size, reduced thickness, and improved uniformity. Comprehensive characterizations confirm the high quality of as-grown MoTe₂. Moreover, various characterizations verify the adsorption of K⁺ and Cl⁻ ions on the top surface of MoTe₂. X-ray photoelectron spectroscopy (XPS) analysis reveals that the MoTe₂ is stoichiometric without K⁺ and Cl⁻ ions and exhibits no discernable oxidation after washing. This top surface control strategy provides a new controlling knob to optimize the growth of 2D transition-metal tellurides and holds the potential for generalized to other 2D materials.

Graphical abstract

Highlights

• •Ion adsorption strategy facilitates the Frank-van der Merwe growth
• •The tellurides exhibit larger sizes, enhanced uniformity, and superior crystallinity
• •The MoTe₂ is stoichiometric and exhibits no discernable oxidation

Physics; Condensed matter physics; Materials science

Introduction

The scale-up synthesis of high-quality, large-sized, and uniform two-dimensional (2D) transition-metal dichalcogenides (TMDs) is of utmost importance for their device applications, such as nanoelectronics, photonics, and quantum computing.^{1,2,3,4,5,6,7} To address this requirement, several chemical vapor deposition methods have been developed to facilitate the Frank-van der Merwe growth, enabling layer-by-layer growth of TMDs.^8,9,10 For instance, strategies such as substrate design to modulate interfacial interactions,^11,12 utilizing organic molecules to promote planar growth,^13,14 and adding salts to enhance precursor vapor pressure and adjust reaction pathways have all been explored.^15,16,17,18 While much attention has been given to controlling the underlying surface during TMD growth, manipulating the top surface presents a novel avenue for promoting the Frank-van der Merwe growth. A breakthrough has been achieved by employing a MoS₂-OH bilayer approach to fabricate inch-sized monolayer MoS₂ on various substrates.¹⁹ However, effectively controlling the top surface is rare and remains challenging, as absorption and growth need to coincide at the same location.

2D transition-metal tellurides have garnered significant attention due to their unique physicochemical properties and promising applications.^{20,21,22,23,24} For instance, 2H and 1T′ MoTe₂ can be synthesized and integrated simultaneously in a single step, creating high-performance 2D devices.^25,26 Additionally, 1T′-WTe₂ exhibits spontaneous ferroelectric switching, challenging the conventional notion that ferroelectric properties cannot exist in metallic systems.^27,28 Consequently, there is a pressing need to obtain large crystal sizes and uniform 2D transition-metal tellurides. However, most existing methods primarily focus on 2D transition-metal sulfides and selenides,^18,29,30 leaving a dearth of versatile approaches for growing large-sized and uniformly layered 2D transition-metal tellurides.

In this study, we have developed an ion absorption method to promote the Frank-van der Merwe growth of 2D transition-metal tellurides. Our approach enables the synthesis of MoTe₂ with enhanced quality, larger sizes, reduced thickness, and improved uniformity when compared to growth without this method. Moreover, we have grown uniform monolayer MoTe₂ and WTe₂ films by extending the growth time. The growth process involves simultaneous MoTe₂ formation, K⁺ ion absorption, and Cl⁻ ion adsorption at the growth site. These ions impede vertical growth while promoting the Frank-van der Merwe growth of 2D transition-metal tellurides. This ion adsorption-enabled top surface tuning offers a novel strategy for synthesizing large-sized and uniform 2D metal tellurides, potentially applicable to other 2D TMDs.

Results and discussion

In our experimental design, we employ K₂TeO₃ and MoCl₅ as precursors to grow MoTe₂ and generate K⁺ and Cl⁻ ions simultaneously. To ensure optimal and homogeneous dispersion of K₂TeO₃, we initially dissolved K₂TeO₃ powder in deionized water. Subsequently, the K₂TeO₃ solution is spin-coated onto fluorophlogopite mica. The excellent hydrophilicity of freshly cleaved mica facilities the even dispersion of K₂TeO₃ on mica. The deposited K₂TeO₃ is then dehydrated at 60°C, leading to its drying and immobilization on mica substrate (Figure S1). During the growth process, the predeposited K₂TeO₃ vaporizes and reacts with the metal chloride precursor at the growth temperature. The chemical reaction equation for growing MoTe₂ can be represented as:

2MoCl₅+4K₂TeO₃+13H₂ → 2MoTe₂+8KCl+12H₂O+2HCl

Importantly, the generation of ions and the growth of MoTe₂ occur simultaneously at the same location (Figure 1A). After a brief period of growth, uniform monolayer MoTe₂ flakes emerge on mica substrate (Figure 1B). As growth time prolongs, monolayer flakes continue to grow without the appearance of thicker flakes (Figure 1C). With further extension of time, dispersed monolayer MoTe₂ flakes merge to form a continuous and uniform MoTe₂ film (Figure 1D). Atomic force microscopy (AFM) analysis reveals that the thickness of the film is ∼0.73 nm with slight roughness (∼0.12 nm), indicating a smooth monolayer MoTe₂ film (Figure 1E).

Figure 1.

The schematic diagram and optical image of the MoTe₂ synthesis process

(A) The schematic diagram of the ion adsorption promoted MoTe₂ synthesis.

(B–D) Typical optical microscopy (OM) image of CVD-grown MoTe₂ under 1 min (B), 3 min (C), and 5 min (D).

(E) AFM image of CVD-grown monolayer MoTe₂ film.

(F) Typical optical microscopy (OM) image of CVD-grown MoTe₂ under 6 min.

The morphology of synthesized MoTe₂ is highly influenced by the growth temperature and growth time (Figure S2). Scattered monolayer MoTe₂ flakes are grown at 600°C. As the reaction temperature rises, the crystal size of MoTe₂ gradually increases. At 650°C, a relatively uniform monolayer MoTe₂ film is obtained, exhibiting a lateral dimension that reaches the millimeter scale. Further raising the growth temperature results in the uniform growth of a second layer of MoTe₂ on the monolayer film (Figure S2). Moreover, the second layer MoTe₂ flakes grows on the monolayer film when increasing the growth time (Figure 1F). These observations strongly suggest that the growth mode in our method follows the Frank-van der Merwe growth,³¹ characterized by a layer-by-layer deposition process.

K₂TeO₃ is crucial for achieving large-sized and uniform 2D MoTe₂. To investigate the role of K₂TeO₃, we conducted growth experiments without K₂TeO₃ while keeping all other growth parameters identical. The absence of K₂TeO₃ results in small MoTe₂ flakes with a wide thickness distribution (Figure 2A). The lateral size of flakes falls within the range of 10–20 μm. Their thicknesses vary from 0.7 nm to 25 nm. In contrast, larger-sized and uniform MoTe₂ flakes are obtained by introducing K₂TeO₃ (Figure 2B). Uniform monolayer MoTe₂ flakes are grown, as evidenced by the thickness of ∼0.73 nm (Figure 2C). The thickness distribution is concentrated below 5 nm (Figure 2D), indicating that K₂TeO₃ effectively reduces the thickness and thickness variation of MoTe₂. Notably, with the addition of K₂TeO₃, MoTe₂’s lateral size increases to approximately 30–50 μm, demonstrating the efficient promotion of lateral growth (Figure 2E). The quantity of K₂TeO₃ is important for the growth of MoTe₂. Inadequate K₂TeO₃ do not exhibit a noticeable impact on the MoTe₂ growth. Conversely, excessive K₂TeO₃ impede the growth of large-sized and uniform monolayer MoTe₂ (Figure S2). Furthermore, the Raman spectrum of as-grown MoTe₂ with K₂TeO₃ shows characteristic peaks at 102, 115, 130, 161, and 252 cm⁻¹, corresponding to monolayer 1T′-MoTe₂.^32,33 The sharp peak with a full width at half-maximum (FWHM) of 6.3 cm⁻¹ at the representative A_g mode (161 cm⁻¹) suggests the high crystallinity of monolayer 1T′-MoTe₂ (Figure 2G).³³

Figure 2.

Characterizations of CVD-grown MoTe₂ and WTe₂

(A) Optical image of MoTe₂ grown without K₂TeO₃.

(B) Optical image of MoTe₂ film grown with K₂TeO₃.

(C) AFM image of CVD-grown MoTe₂ flake.

(D and E) Thickness and lateral dimension distribution analysis of MoTe₂ flakes.

(F) Optical image of WTe₂ film grown with K₂TeO₃.

(G) Raman spectra of CVD-grown MoTe₂ and WTe₂ flakes on mica substrate.

(H and I) High-resolution XPS spectra of Mo 3 days (H) and Te 3 days (I) in MoTe₂.

Our strategy for MoTe₂ growth through ion adsorption is also effective for the growth of WTe₂ (Figures 2F and S3). Similarly, the absence of predeposited K₂TeO₃ leads to small and nonuniform WTe₂ flakes (Figure S3). In contrast, introducing K₂TeO₃ results in large-sized and uniform WTe₂ flakes (Figures 2F and S3). Moreover, distinct Raman active A₁ modes corresponding to WTe₂ are observed at 114, 132, 161, and 209 cm⁻¹, consistent with the previous reports on monolayer 1T′-WTe₂ (Figure 2G).^34,35

A series of characterizations verify the high quality of as-grown MoTe₂. X-ray photoelectron spectroscopy (XPS) survey spectrum reveals prominent Mo and Te peaks, while no discernible K or Cl peaks, indicating the CVD sample is free of K⁺ and Cl⁻ ions contamination after water washing treatment (Figure S4). Two prominent peaks located at 227.5 (Mo 3d_5/2) and 230.5 eV (Mo 3d_3/2) correspond to Mo-Te bonds (Figure 2H). The Te 3d_5/2 and Te 3d_3/2 peaks at 572.8 and 582.6 eV are also assigned to Mo-Te bonds (Figure 2I). Furthermore, the atomic ratio between Mo and Te is approximately 1:2, suggesting that the CVD-grown MoTe₂ is stoichiometric. All these features are consistent with previous XPS results for 1T′-MoTe₂.^33,34 Notably, both Mo 3 days and Te 3 days spectra show no discernable oxidation peaks related to Mo-O or Te-O bonds. This observation suggests that the as-grown MoTe₂ exhibits high resistance to degradation in ambient conditions, thus reflecting the high crystal quality of our MoTe₂ sample.

The high quality of as-synthesized MoTe₂ and WTe₂ can be further confirmed by transmission electron microscopy (TEM) and polarized Raman spectroscopy. The selected area electron diffraction pattern (SAED) taken on MoTe₂ flake shows a single set of rectangle patterns, consistent with the feature of monoclinic phase MoTe₂ (Figure 3B). High-resolution TEM (HRTEM) image reveals a nearly perfect periodic atom arrangement with a lattice spacing of 0.63 nm, corresponding to the (100) planes of MoTe₂ (Figure 3A).^33,36 The perfect lattice without noticeable defects suggests high quality of as-grown MoTe₂. The SAED of WTe₂ flake also displays a single set of rectangle patterns, corresponding to 1T′-WTe₂ (Figure S5). Moreover, 2D mapping of different vibration modes confirms strong in-plane anisotropy of MoTe₂ and WTe₂ (Figures 3F and S7). The corresponding polar plots of different vibration modes are depicted in Figures 3G, S6, and S7. The polar plot of Raman peak intensity at 161 cm⁻¹ shows a typical two-lobe pattern, indicating the Raman anisotropy of the MoTe₂ with 2-fold symmetry (Figure 3G). The maximum intensity direction (marked with yellow double arrows) of the A_g mode peaks corresponds to the Mo-Te chain direction.^37,38

Figure 3.

TEM and Raman characterization of MoTe₂ grown with K₂TeO₃

(A) High-resolution TEM image of a MoTe₂ flake.

(B) SAED pattern of a MoTe₂ flake.

(C–E) HAADF-TEM image of a MoTe₂ flake and corresponding EDS element mapping of Te and Mo elements.

(F) 2D mapping of Raman spectra regarding the incident light’s polarization angle (θ).

(G) Polar plots for the Raman peak intensity at 161 cm⁻¹ with respect to θ.

(H and I) Optical image of MoTe₂ flake and corresponding Raman intensity map of the 161 cm⁻¹ peaks.

The homogeneity of MoTe₂ and WTe₂ are evaluated through energy dispersive spectroscopy (EDS) and Raman mapping. EDS spectrum confirms the presence of Mo (W) and Te elements in the MoTe₂ (WTe₂) sample, without any detectable elemental impurities (Figure S8). Elemental maps demonstrate uniform colors across the flake region, indicating a homogeneous spatial distribution of both Te and Mo (W) elements (Figures 3C–3E and S5). Furthermore, Raman intensity maps at 161 cm⁻¹ (Figures 3H and 3I) display uniformly distributed color across the flake, indicating the uniformity of as-grown MoTe₂.

To verify the ion adsorption growth of MoTe₂, we conducted XPS and time-of-flight secondary ion mass spectrometry (TOF-SIMS) on as-grown monolayer MoTe₂ (Figure 4A). XPS fine-scan spectra exhibit Cl and K peaks, indicating the existence of K⁺ and Cl⁻ ions in the MoTe₂ sample (Figures 4B and 4C). Moreover, we identify ions distribution of the MoTe₂ sample using TOF-SIMS in negative modes. Low-resolution cross-sectional images (Figures 4D–4F) demonstrate that Cl⁻ and KCl⁻ ions locate above Te⁻ ion, suggesting K⁺ and Cl⁻ ions adsorb on the top surface of monolayer MoTe₂ during growth. 3D images of depth profiles (Figures 4G and 4H) visually represent the vertical distribution patterns of Cl⁻ and KCl⁻. Furthermore, the normalized TOF-SIMS depth profile indicates that the Cl, K, Te, Mo, and Al ions (from mica substrate) are distributed from top to bottom, verifying K⁺ and Cl⁻ ions adsorption on the top surface of monolayers MoTe₂ (Figure 4I). In contrast to conventional methods that rely on promoting in-plane growth to inhibit vertical growth and achieve uniform monolayers, we present a novel approach utilizing surface ion adsorption to suppress vertical growth and facilitate in-plane growth. This approach offers improved control over the thickness and uniformity of 2D transition metal tellurides.

Figure 4.

XPS and TOF-SMIS characterizations of MoTe₂ grown with K₂TeO₃

(A) Schematic diagrams of CVD-grown MoTe₂ with Cl⁻ and K⁺ ions on mica substrate.

(B and C) High-resolution XPS spectra of MoTe₂ with Cl (B) and K (C) elements in Si/SiO₂ substrate.

(D–F) Low-resolution cross-section image of Cl⁻, KCl⁻, and Te⁻.

(G and H) Low-resolution 3D image of Cl⁻ and KCl⁻.

(I) Normalized TOF-SIMS depth profile of the MoTe₂ sample.

Conclusions

In summary, we have developed an ion adsorption method to promote the Frank-van der Merwe growth of 2D transition metal tellurides. Through this method, the as-grown transition metal tellurides exhibit notable improvements, including larger sizes, reduced thickness, and enhanced uniformity compared to those grown without this technique. Moreover, the as-grown MoTe₂ exhibits stoichiometric composition, superior crystallinity, and homogeneity. This research broadens the synthesis methods for achieving high-quality, large-sized, and uniform 2D transition metal tellurides. We anticipate this tuning top surface strategy can be generalized to synthesizing other 2D materials.

Limitations of the study

We have developed an ion adsorption method to promote uniform growth of two-dimensional layered materials. However, the applicability of ion adsorption in synthesizing non-layered materials remains unexplored, which also serves as our future research direction.

STAR★Methods

Key resources table

REAGENT or RESOURCE	SOURCE	IDENTIFIER
Chemicals, peptides, and recombinant proteins
Molybdenum pentachloride	Strem Chemicals	CAS: 10241-05-1
Tungsten chloride	Sigma-Aldrich	CAS: 13283-01-7
Tellurium powder	Sigma-Aldrich	CAS: 13494-80-9
Potassium tellurite	Sigma-Aldrich	CAS: 7790-58-1
Polymethyl Methacrylate	Sigma-Aldrich	CAS: 9011-14-7
Acetone	Sinopharm Chemical Reagent Co., China	CAS: 67-64-1
Software and algorithms
Origin 2018	Origin Lab Corporation	https://www.originlab.com
Cinema 4D	Maxon Computer	https://www.maxon.net

Resource availability

Lead contact

Further information and requests for resources and materials should be directed to and will be fulfilled by the lead contact, Lin Zhou. e-mail: linzhou@sjtu.edu.cn.

Materials availability

This study did not generate new unique reagents.

Data and code availability

• •All data reported in this paper in this paper is available from the lead contact upon request.
• •This study does not report the original code.
• •Any additional information required to reanalyze the data reported in this paper is available from the lead contact upon request.

Experimental models and study participant details

Our study does not use experimental models in the life sciences.

Method details

Synthesis and transfer of MoTe₂ and WTe₂

High-quality MoTe₂ and WTe₂ were synthesized by ambient pressure CVD growth. K₂TeO₃ powder (Sigma-Aldrich, 99.9%) was dissolved in deionized water. The K₂TeO₃ solution was spin-coated on mica substrate at 2000 rpm for 60 s and then baked on a heating plate at 60°C. The MoCl₅ (Strem Chemicals, 99.9%) or WCl₆ (Sigma-Aldrich, 99.9%) powder was placed in a quartz boat at the hot zone’s edge, where the temperature was ∼180°C. The Te powder (Sigma-Aldrich, 99.9%) was placed at the heating center of the tube furnace. Before heating, the CVD system was vacuumed and purged by Ar gas three times to remove water and air. Then, the furnace temperature was ramped to 650°C within 15 min, with a mixture of 250 sccm Ar and 5 sccm H₂ as the carrier gas and maintained at growth temperatures for 3 min. After growth, the quartz tube was removed from the furnace and rapidly cooled to room temperature. Moreover, the as-synthesized MoTe₂ or WTe₂ with different thicknesses and sizes can be obtained by adjusting different temperatures and growth times.

The as-grown MoTe₂ or WTe₂ were transferred onto target substrate (such as SiO₂/Si substrate and TEM grid) via polystyrene (PMMA)-assisted wet transfer approach for further structure and property characterization. First, the PMMA solution was spin-coated on mica substrate with as-grown MoTe₂ or WTe₂ at a speed of 2000 rpm for 60 s and then baked on a heating plate at 120°C for 30 min. Second, the PMMA film with MoTe₂ or WTe₂ was delaminated from the mica substrate with water assistance. Then, the target substrate picks up the PMMA film from water. Finally, the target substrate with PMMA film was heated at 120°C for 30 min and then immersed into acetone to remove PMMA.

Structure and spectral characterization

Optical images of MoTe₂ and WTe₂ were obtained by optical microscope (OM, Leica DM2700M). AFM (Bruker Multimode 8) was used to characterize the surface morphology and thickness of MoTe₂. XPS (Thermo Scientific NEXSA) was carried out to confirm the chemical composition and phase structure of MoTe₂. Atomic structure and element distribution were analyzed via high-resolution transmission electron microscopy (HRTEM, FEI Talos F200X) and EDS mapping (TEM, FEI Talos F200X). Raman spectra, Raman mapping, and angle-resolved polarization Raman spectra were acquired on Renishaw inVia Qontor with a 532 nm laser. The chemical composition imaging and depth analysis of MoTe₂ were carried out by time-of-flight secondary ion mass spectrometer (ION TOF TOF-SIMS 5100).

Quantification and statistical analysis

In Figures 2D and 2E, each thickness and lateral dimension value corresponds to the average value obtained from the measurements of various MoTe₂ on a mica substrate.

Published: March 2, 2024