Two-dimensional tantalum disulfide: controlling structure and properties via synthesis

The partial filling of 'd' orbitals in '1T' layered compounds enables fascinating structural and electronic behaviors [1]. For instance, the Peierls transition associated with charge density wave (CDW) instability is a unique feature of these 1T materials. CDW prompts new orbital ordering, which leads to unusual behavior like superconductivity or metal–insulator transition (MIT). Unlike the more heavily studied transition metal dichalcogenides (TMDs), 1T-TaS₂ is metallic and exhibits an intriguing relationship of electron-lattice interactions [2]. The undistorted 1T-TaS₂ is trigonal ($p\overline{3}m1$) and exhibits a metallic characteristic (figure 1(a) (band structure, (BS)) and figure 1(b) (density of states (DOS))) even when thinned to its monolayer limit [3]. However, below a critical temperature, the Fermi-surface instability spontaneously breaks the lattice symmetry of the crystal and the material forms the so-called 'Star-of-David' commensurate CDW (CCDW) phase with an in-plane periodic distortion of $\sqrt{13}\times \sqrt{13}$R  =  13.5°. During this transition, bulk 1T-TaS₂ exhibits interesting electronic properties; it behaves as a band insulator with an in-plane gap (Γ–M–K–Γ direction; see figure 1(c)) of nearly 200 meV but it shows metallic characteristics in the out-of-plane direction (Γ–A direction) (figures 1(c) (BS) and (d) (DOS)). This strongly implies that the interlayer coupling, along with the CCDW formation, plays a major role in opening the in-plane bandgap in CDW multilayer TaS₂ irrespective of the Coulombic repulsion in the crystal [1]. Similar to 1T-TaS₂, undistorted hexagonal 2H-TaS₂ (P6₃/mmc) also exhibits a metallic behavior (figures 1(e) and (f)). In the case of 2H, as the temperature is reduced, it undergoes a 3  ×  3 CDW superlattice transformation [2]. However, the energy lowering of the CDW distortion is quite small (approximately 2 meV per formula unit -f.u.) along with small atomic displacements (0.05 Å), indicating a similar electronic behavior to the undistorted one [4]. The calculation results for distorted 2H-TaS₂ can be found in the supplemental material (SI) (stacks.iop.org/TDM/5/025001/mmedia).

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Recent progress in TaS₂ relies on chemical vapor transport (CVT), followed by mechanical exfoliation onto different substrates [5–9]. With this route, stoichiometric Ta and S is sealed in a quartz tube and heated at elevated temperatures (>850 °C) for 10–14 d to create the bulk crystals [10, 11]. Besides the long growth time, the contamination during exfoliation and the transfer processes can degrade the material properties. Low yield and unpredictable flake size are also common problems. Thus, integrating TaS₂ into large-scale complex structures is highly desirable, yet challenging to realize. In this paper, we demonstrate the powder vapor (PV) deposition (figure 2(a)) method for direct and controllable synthesis of both 1T-TaS₂ and 2H-TaS₂ films.

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Powder vapor deposition (PV) (figure 2(a)) enables the controlled growth of 2H and 1T-TaS₂. This is evident in figure 2(b), where multiple Ta precursors are evaluated with sulfur (S) vapors to realize TaS₂. Tantalum films produced by thermal evaporation are 100% transformed into polycrystalline 2H-TaS₂ with nano-sized domains. On the other hand, transformations from tantalum oxide (Ta₂O₅), tantalum carbide (TaC) or tantalum nitride (TaN) powders lead to very limited sulfide formation due to the high thermal and chemical stability of these compounds. In order to increase chemical reactivity, a TaO_x film (with thickness smaller than 1 nm) is first deposited via atomic layer deposition (ALD). Sulfurizing ALD TaO_x, using an Ar/H₂ carrier gas, leads to successful transformation of the layer to 1T-TaS₂ with triangular or hexagonal domains (figure 2(c)), but with lateral sizes smaller than 200 nm. The small lateral size is limited due to the starting ALD oxide that is amorphous or nanocrystalline, as well as the very low surface diffusion rates of Ta. In order to increase domain size, vaporization of the precursors is utilized rather than simple deposition and sulfurization of Ta-based films. The term 'PV' is utilized for this technique due to the inherent lack of control compared to true chemical vapor deposition (CVD). Directly sulfurizing Ta powder does not yield any growth due to the high chemical reaction barrier between Ta and S. Therefore, we introduce the use of tellurium (Te) to catalyze the vaporization and mobility of Ta in the PV process. By mixing Te with Ta as a solid precursor, the growth of TaS₂ is greatly promoted as reported for several other TMDs [12, 13]. This is because the Te forms a binary eutectic with transition metals and thus lower the eutectic melting point significantly [12]. Additionally, Te also forms binary compounds with S that can more strongly reduce the transition metal. As a result, the reactivity between the chemical agents is largely increased [14]. Therefore, significantly larger TaS₂ domains were achieved on the substrates with the help of Te catalyst compared to domains without the Te addition.

Using Te-assisted PV, consistent growths are achieved on multiple substrates (sapphire, SiO₂/Si, epitaxy graphene, etc). A typical flake shape is shown in figure 2(d). The average flake size is 5–15 µm. Due to different crystal formation energy between 1T- and 2H-TaS₂ [15], temperature is the key to control the reactant phases during PV. Figure 2(e) summarizes the averaged phase ratios from each growth as a function of temperature. Since 1T-TaS₂ has a higher formation energy compared with 2H-TaS₂ [16], higher temperatures favor 1T-TaS₂ growth [17]. The transition from 2H to 1T-TaS₂ growth occurs gradually from 850 °C to 975 °C, with a minimum of 975 °C to ensure a fully formed 1T phase. To stabilize the 1T phase following high temperature synthesis, the samples are cooled at a high rate (~60 °C/min) to  <300 °C; otherwise the phase transforms to the 2H phase during the cooling cycle.

Growth of TaS₂ (both 1T and 2H) via Te-assisted PV enables the formation of crystalline domains with different chemical signatures. This is evidenced by high resolution scanning transmission electron microscopy (HRTEM) (figures 3(a) and (b)), where an intensity line is drawn along the Ta–Ta direction to highlight the Ta-S signal contrast. As shown, there is a stronger S signal from the 2H-TaS₂ image than from the 1T-TaS₂ image. This is due to difference in their crystal structure. In 1T-TaS₂, S is octahedrally coordinated with Ta while in 2H-TaS₂, S is trigonal pristimatically coordinated to Ta. As a result, S from both layers adds to the total intensity in the 2H-TaS₂ image, while in the 1T-TaS₂ the S signal is only collected from one layer resulting in a weaker contrast. X-ray photoemission spectroscopy (XPS) (figures 3(c) and (d)) of the Ta 4f peaks from 1T-TaS₂ and 2H-TaS₂ shows the peak is broader in 1T-TaS₂ (figure 3(c)) than 2H-TaS₂ (figure 3(d)). The peak broadening can be attributed to differeneces in their CDW properties. Although both are CDW materials, they have different CDW structures and transition temperatures. For 1T-TaS₂, at room temperature, its structure is already partially distorted and forms a 'David Star' (this is known as the nearly commensurate CDW (NCCDW) phase) [9]. As a result, Ta atoms do not preserve a 'pure' chemical envionment [18, 19]. On the contrary, 2H-TaS₂ remains in its pristine crystal structure until 75 K [20]. Below 75 K, it directly transitions into the commenrate CDW (CCDW) structure and forms a 3  ×  3 superstructure. As a result, at room temperature, all the Ta atoms share the same chemical bonding environment and result in sharp Ta 4f peaks. High resolution XPS survey scans (figure 3(e)) also confirm that no Te exists within the XPS detection limit.

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TaS₂ is highly sensitive to the ambient environment [21]. Both XPS and cross-section TEM reveal heavy oxidation of as-grown TaS₂ is possible within hours following growth. As observed in XPS, in addition to the Ta 4f peaks from the TaS₂, a large fraction of the Ta 4f peak signal from Ta₂O₅ is detected. After 10–20 h exposure to ambient without capping or any other protection, cross-section TEM was conducted on these flakes (figure 3(f)). As shown in figure 3(f), the top 2–3 nm of the TaS₂ is oxidized. Furthermore, after 3 d the as-grown TaS₂ is nearly fully oxidized, leaving column-shaped oxidation traces vertically through the TaS₂ (SI). This directly indicates that the oxidation process is not self-limiting, and that care needs to be taken when considering the use of TaS₂.

PV deposited 1T-TaS₂ layers exhibit similar temperature-dependent properties compared to mechanically exfoliated 1T-TaS₂. Low-temperature Raman indicates that the 1T-TaS₂ transforms from the NCCDW phase to the CCDW phase below 180 K [22], where all the Ta atoms group into a $\sqrt{13}\times \sqrt{13}$ superstructure (figure 1(c)), which are commensurate with the underlying lattice. This new structure changes the Raman features dramatically [3]. At room temperature, 1T-TaS₂ has a characteristic sharp peak at 70 cm⁻¹, while 2H-TaS₂ has three broad peaks located at 200 cm⁻¹, 310 cm⁻¹ and 395 cm⁻¹ (figures 4(a) and (b)). While decreasing temperature, there are no significant changes for the 1T-TaS₂ Raman peaks down to 180 K; however, once the temperature goes below 180 K, the main peak splits into two sub-peaks abruptly, while a group of small but sharp peaks appear near 100 cm⁻¹. This peak evolution is readily tracked by Raman spectroscopy and is used to represent the CDW phase transitions in 1T-TaS₂ [9]. On the contrary, within the available measurement temperature range of 80–300 K, there are no new peaks detected from 2H-TaS₂ Raman. The noticeable change is the slow disappearance of A₁ mode while lowering temperature, which shows the decreasing intensity from the two phonon scattering process [23]. On the other hand, the increasing intensity ratio from A₂ mode to E mode also suggests enhanced inter-layer interactions [4].

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Electronic transport of TaS₂ confirms that the transition of 1T-TaS₂ from NCCDW to CCDW leads to a large increase in resistivity, while the 2H phase remains highly conductive. This is confirmed via two-terminal TaS₂ devices fabricated on sapphire substrates (figure 5(a)). Figure 5(b) plots resistivity change as a function of temperature. The resistivity is derived from $~\rho =\frac{RWt}{L}$, where R is the extracted resistance, t is the TaS₂ thickness, and W/L the width to length ratio of the device. The 1T-TaS₂ displays the CCDW to NCCDW transition at ~180 K, with a hysteresis of 20 °C during the cooling cycle returning from the NCCDW to CCDW state. This hysteresis is 4×  lower than that reported from similar measurements for exfoliated 1T-TaS₂ [24], however, there is also a 2.5×  reduction in the magnitude of the transition compared to the CVT flakes. The reduction in magnitude is indicative of a higher concentration of defects within PV grown flakes (See supplemental images) compared with CVT materials [25, 26], which can negatively impact the electronic transition. Importantly, electrical switching of the PV grown 1T-TaS₂ is also possible while the temperature remains below 130 K (figure 5(c)). We have also observed an abrupt, reversible insulator/metal transition (IMT) associated with the CCDW to NCCDW transition with a Δ_IMT of ~2×  . At 130 K, the metal-to-insulator (MIT) transition in the reverse sweep direction becomes more gradual due to the slower switching time in comparison to the IMT in the forward direction [24]. The 2H-TaS₂ device, however, does not display the abrupt IMT in either the ρ versus T or the I–V over these temperature ranges [20].

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We have introduced a single-step tellurium-assisted synthesis process for controllably creating high crystallinity 1T-TaS₂ and 2H-TaS₂ domains via powder vaporization. Synthesis via this technique is universal, and can be carried out on a variety of substrates (sapphire, SiO₂/Si, eG). The 1T-TaS₂ undergoes a NCCDW/CCDW phase transition at 180 K, leading to dramatic changes in the phononic and electronic properties, as measured by Raman and electrical characterization. This study provides a unique way to directly synthesize CDW TMDs where the corresponding transition metal oxides have very high melting temperatures and hard to be directly sulfurized. Our work may open up possibilities to directly integrate TaS₂ into further complex structures.

Density functional theory

This work was supported by NSF Grant No. EFRI-1433307. The authors acknowledge Dr Yi Wang for helpful calculation discussions.