Synthesis of ZnS/Al₂O₃/TaSe₂ Core/Shell Nanowires Using Thin Ta Metal Film Precursor

Abstract

This study introduces a novel approach for fabricating ZnS/Al₂O₃/TaSe₂ heterostructured core/shell nanowires (NWs) through the selenization of a metallic Ta thin film precursor. The synthesis process involves a meticulously designed four-step protocol: (1) generating ZnS NWs on an oxidized silicon substrate, (2) encapsulating these NWs with a precisely controlled thin Al₂O₃ layer via atomic layer deposition (ALD), (3) applying a Ta precursor layer by magnetron sputtering, and (4) annealing in a Se-rich environment in a vacuum-sealed quartz ampoule to transform the Ta layer into TaSe₂, resulting in the final core/shell structure. The characterization of the newly produced NWs using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) was validated using the integrity and composition of the heterostructures. Our method not only establishes a new pathway for the synthesis of TaSe₂-based core/shell NWs but also extends the potential for creating a variety of core/shell NW systems with chalcogenide shells by adapting the thin film metal precursor approach. This versatility opens the way for future advancements in nanoscale material applications, particularly in electronics and optoelectronics where core/shell geometries are increasingly important.

1. Introduction

In the last decade, layered two-dimensional (2D) chalcogenide materials, transition metal dichalcogenides (TMDs) in particular, have attracted significant attention in the material science research community [1,2,3,4,5,6,7]. Layered or van der Waals transition metal dichalcogenides (TMDs), and characterized by the general formula MX₂ (where ‘M’ includes metals such as V, Mo, Hf, Ta, W, and Re among others, and ‘X’ stands for S, Se, and Te), they demonstrate semiconducting or metallic properties [8,9]. TaSe₂ polymorphs are a typical layered TMD material belonging to the hexagonal crystal family that exhibits multiple charge density wave (CDW) phase transitions [9]. TaSe₂ can exist in several polytypes: the 1T, 2H, and 3R [10] as well as 4Hb [11] structures, each with distinct symmetry and physical properties. 1T-TaSe₂ has a trigonal ${\mathrm{D}}_{3d}$ symmetry (space-group Nr. 164 or P$\overline{3}$m1), where each Ta atom is octahedrally coordinated by six Se atoms [10]. 2H-TaSe₂ and related 4Hb-TaSe₂, which are minimally different due to the relative shift of layers relative to each other, are more stable phases and have a ${\mathrm{D}}_{6h}$ symmetry (space-group Nr. 194 or P$6_3$/mmc), where each Ta atom is coordinated by six Se atoms in a trigonal prismatic arrangement [10]. The structure of the atomic environment of Ta atoms in 1T-TaSe₂ and 2H-TaSe₂ phases is shown in the picture (Figure S1). 1T-TaSe₂ is a metal with lattice parameters of a = 0.348 nm and c = 0.627 nm [12], while 2H-TaSe₂ and 4Hb-TaSe₂ are semiconductors with lattice parameters of $a=0.343$ nm and $c=1.27$ nm [13] and of a = 0.3455 nm and c = 2.5148 nm [11] for 2H-TaSe₂ and 4Hb-TaSe₂ phases, respectively.

Core-shell based designs featuring core-shell nanowires (NW) and including 1D/2D heterostructures, such as 1D NWs coated with 2D layered materials, are highly promising in various applications for harvesting solar energy [14]. These 1D/2D heterostructures also demonstrate aptness for use in photocatalytic processes, notably in the hydrogen evolution reaction [15,16]. Furthermore, they possess advantageous optical properties, such as improved photoelectric efficiency [17], rendering them potentially effective for use in imaging and sensing technologies [18]. Additionally, 1D/2D heterostructures can be used as a template for thin 2D materials, which bring mechanical stability to 2D materials and help to anchor 2D structures to the substrate. Thin, free-standing 2D materials are very soft and cannot be manipulated without a stiff substrate or template [19]. Moreover, combining layered two-dimensional materials with semiconducting nanowires facilitates the development of sophisticated core/shell heterostructures with enhanced optoelectronic properties, e.g., greatly increased photosensitivity and a faster photoresponse time [18].

It is worth noting that, at high temperatures in a corrosive atmosphere, (S, Se) ZnO NWs are insufficiently chemically stable, and the use of ZnS NWs is preferable.

ZnS is a direct bandgap semiconductor with two polytypes: the most stable zincblende (or sphalerite) cubic diamond-like phase (space group Nr. 216 or F$\overline{4}$3m) with a bandgap of 3.6 eV, and high-temperature hexagonal wurtzite phase (space group Nr. 186 or P$6_3$mc) with bandgap 3.9 eV (visual representation of crystal structure for both ZnS phases is shown in Figure S1a) [20]. For both compounds, zinc and sulfur are tetrahedrally coordinated. ZnS is nontoxic, environmentally friendly, and a low-cost compound made of earth-abundant elements. Zinc sulfide demonstrates transparency across a broad spectrum of wavelengths. This characteristic renders it highly suitable for use in electroluminescent devices [21]. Additionally, its properties are advantageous for a variety of applications in electronics, optoelectronics, and sensors [22].

A thin layer of Al₂O₃ can be precisely deposited by atomic layer deposition (ALD) with sub-angstrom accuracy [23,24]. It was demonstrated that a thin layer of amorphous Al₂O₃ deposited by ALD around NWs helps to protect the core material from fracture and allows it to withstand significantly higher deformations and stresses in comparison to uncoated NW [25]. Moreover, an Al₂O₃ layer is able to preserve core NWs morphology in a corrosive chalcogenide atmosphere at elevated temperatures and facilitate the smooth growth of TMDs shell around NWs [26].

Thin films or even monolayers of TaSe₂ can be grown by chemical vapor deposition (CVD) methods. For example, Shi et al. [27] demonstrated the growth of wafer-scale uniform monolayer 2H-TaSe₂ films on Au foil from TaCl₅ and selenium precursors at 930 °C in a mixed Ar/H₂ gas flow. It is important to note that the TaCl₅ precursor is atmosphere-sensitive and requires manipulation in an inert atmosphere (glovebox). An alternative method of Ta deposition for fabrication of core-shell nanostructures is based on the anodization process reported by Pligovka et al. [28]. Wang et al. reported the synthesis of 1T-TaSe₂ on a Si/SiO₂ substrate using Ta₂O₅/MgCl₂ selenium in a temperature range from 640 to 890 °C in a mixed Ar/H₂ gas flow [12]. For the synthesis of 1D core/shell nanostructures, the direct deposition of transition metal oxide WO₃, MoO₃, and ReO₃ precursors can be used, as was shown in [18,26,29]. However, in contrast to these transition metal oxides that have medium-high sublimation/melting temperatures in the range 500–750 °C [30,31], tantalum oxide Ta₂O₅ has a very high melting point of 1872 °C and high chemical stability [30]. The chalcogenization of Ta₂O₅ requires a rather long processing time due to the high chemical stability of tantalum pentoxide. For example, Li et al. produced TaS₂ nanotubes by the sulfurization of Ta₂O₅ nanotubes at 625 °C under H₂S gas flow during a 24 h long process [32].

Both TaS₂ and TaSe₂ nanomaterials are expected to be effective catalysts for the hydrogen evolution reaction (HER) [33,34,35]; however, the controllable fabrication of 1D arrays of TaSe₂ and TaS₂ nanostructures still remains challenging and slows down the progress on their possible applications in catalysis and HER.

This study reports the synthesis of ZnS/Al₂O₃/TaSe₂ core/shell NWs using their ZnS/Al₂O₃ NWs as a template. As an alternative Ta-based TMD precursor, a thin metallic Ta film is proposed, which can be deposited by magnetron sputtering and subsequently selenized at medium-high temperatures of 650–750 °C in a vacuum-sealed quartz ampoule using a Se precursor. The magnetron deposition of thin films is a scalable technology, which allows a significant decrease in the consumption of Ta and makes it more ecologically friendly. The approach described in the present work can be used for the synthesis of other core/shell NWs with a layered 2D chalcogenides shell (such as selenides and sulfides), using appropriate metal precursors, including those of other transition metals.

2. Experimental Details

2.1. Materials and Methods

The fabrication of ZnS/Al₂O₃/TaSe₂ core/shell NWs was accomplished using a four-step method (see Figure 1):

• In the initial step, ZnS NWs were synthesized on top of the oxidized silicon wafers Si/SiO₂ (Si(100) wafer, 50 nm of thermal oxide, Semiconductor Wafer, Inc., Hsinchu, Taiwan) using the Au nanoparticles (50 nm in diameter, water suspension, BBI International, Grand Forks, ND, USA) as a catalyst for vapor–liquid–solid (VLS) growth. ZnS powder 0.4 g (>97%, Sigma Aldrich, St. Louis, MO, USA) was thermally sublimated in a quartz tube reactor at a temperature of 950 °C for 30 min, followed by natural cooling. The ZnS vapor was carried downstream by a Ar/H₂ 35% gas mixture to the substrate to grow ZnS NWs.
• Subsequently, a thin Al₂O₃ layer was deposited on the NWs using the ALD technique in a Savannah S100 reactor. The deposition process, carried out at 150 °C, involved 66 cycles (Al₂O₃ thickness ∼6 nm) of alternating Trimethylaluminum (TMA) and H₂O as precursors, with N₂ serving as the inert carrier gas.
• A Ta metallic layer, approximately 15 nm thick on flat substrates, was deposited over the ZnS/Al₂O₃ NWs using direct current (DC) magnetron sputtering from a Ta target in an Ar atmosphere ($4\times {10}^{-3}$ torr, 30 sccm Ar gas flow at 100 W DC power). It is noteworthy that, due to geometrical factors, the actual thickness of the Ta film on the vertical NWs might be less than 15 nm.
• The final step was the annealing of the coated NWs in a selenium environment. The samples underwent a 50-min anneal at 650 °C within a vacuum-sealed quartz ampoule to transform the metallic Ta layer into TaSe₂. The procedure involved placing the Si/SiO₂ wafer with ZnS/Al₂O₃/Ta NWs or a Ta thin film on Si/SiO₂ inside the ampoule, which was then evacuated using a turbo pump (vacuum better than ${10}^{-3}$ torr) and hermetically sealed. Selenium pellets (50 mg, Sigma Aldrich) and Ta foil (100 mg, GoodFellow, Huntingdon, UK) were also introduced into the ampoule to maintain a stable vapor pressure of TaSe₂ and to minimize the vapor’s transport to cooler areas of the ampoule. The length of ampoule was tailored to align with the hot zone of the oven, ensuring that its ends remained cooler, which, in turn, allowed for the condensation of any unreacted selenium (Figure S2, Supplementary data).

2.2. Characterisation

The phase composition of the samples was determined through X-ray diffraction (XRD), conducted on a Rigaku MiniFlex 600 X-ray powder diffractometer. This equipment employs Bragg–Brentano $\theta$–$2\theta$ geometry and is equipped with a 600 W Cu anode, utilizing Cu K$\alpha$ radiation ($\lambda =1.5406$ Å). Rietveld Analysis was performed using the BGMN program with a graphical user interface Profex [36]. For verifying the chemical composition of the NW samples, X-ray photoelectron spectroscopy (XPS) was conducted using an ESCALAB Xi spectrometer from ThermoFisher. The excitation source was an Al K$\alpha$ X-ray tube, operating at an energy of 1486 eV. The examination area of the samples measured 650 $\mathsf{\mu }$m × 100 $\mathsf{\mu }$m, and the sample surface was positioned at a 90° angle to the analyzer. Sputter-cleaning before the measurements was not used. An electron gun was used to perform charge compensation. The base pressure during the spectra acquisition was better than ${10}^{-5}$ Pa.

The internal crystalline structure of core/shell NWs, placed on a Lacey Cu Transmission Electron Microscope grid (Agar Scientific, Essex, UK), was examined using a Transmission Electron Microscope (Tecnai GF20, FEI, Hillsboro, OR, USA) functioning at an acceleration voltage of 200 kV. To analyze the morphology of these core/shell NWs, scanning electron microscopy (SEM, Lyra, Tescan, Brno, Czech Republic) was employed at an accelerating voltage of 12 keV, complemented by high-resolution SEM at 5 keV using a Helios 5 UX (Thermo Fisher Instruments, Waltham, MA, USA).

3. Results and Discussion

The phase composition of the selenized ZnS/Al₂O₃/Ta NWs on oxidized Si/SiO₂ substrates was studied by XRD using the Rietveld method, and the result of the refinement is shown in Figure 2. The main 4Hb-TaSe₂ characteristic peak (004) was identified [11], confirming the successful selenization of the Ta coating. The wurtzite ZnS phase ([37] and ICDD 36-1450 [38]) for the NWs’ core was also detected [39]. It is important to note that no ZnSe peaks can be seen in the XRD pattern, which means the selenization of the ZnS NWs core did not occur. On the other hand, a $\beta$-Ta₂O₅ (space group Nr. 49 or Pccm and lattice parameters of $a=0.6217$, $b=0.3677$ and $c=0.7794$ nm [40,41]) (001) peak was detected, possibly due to some oxidation of the Ta coating when exposed to air and subsequent crystallization during high-temperature processing in the ampoule [42]. Nonetheless, no peaks corresponding to Al₂O₃ were detected, likely due to the amorphous nature and high crystallization temperature of the ALD coating. Additional Bragg peaks identified in the XRD patterns were ascribed to the Si(100) substrate (the forbidden Si(200) reflection at 2$\theta$≈ 33°), and the gold (ICDD 04-0784 [43,44]) nanoparticles employed in the VLS growth process. Crystallographic data for Rietveld analysis taken from refs. [11,37,40,44] were used for 4Hb-TaSe₂, Zn, Ta₂O₅, and Au, respectively.

XPS analysis further validated the chemical states within the shell of the heterostructured NWs, as depicted in Figure 3 (survey scan presented in Figure S3). High-resolution spectra of the Ta 4f and Se 3d peaks were acquired and calibrated relative to the adventitious C 1s peak at 284.8 eV. No other elements, except the constituents of the SiO₂ substrate, were detected due to the tantalum selenide coating and carbon layer. A Ta 4f scan revealed a peak consisting of two doublets. To mitigate the error during the peak fitting, a spin-orbit splitting ${\Delta }_{7/2-5/2}=1.91$ eV was fixed between the Ta 4${\mathrm{f}}_{7/2}$ and 4${\mathrm{f}}_{5/2}$ components in each doublet and the area ratio was held at 4:3. The first doublet with a Ta 4${\mathrm{f}}_{7/2}$ peak at 25.2 eV corresponded well to the Ta valence state $4^{+}$ in the TaSe₂ compound [27,45]. The other Ta 4${\mathrm{f}}_{7/2}$ peak, shifted to higher energies at 27.0 eV, was attributed to the Ta chemical state with valence $5^{+}$ [27,42], as in the Ta₂O₅ compound. The presence of the $\beta$-Ta₂O₅ phase was also confirmed by the XRD measurements. TaSe₂ is known to be susceptible to oxidation in ambient conditions, especially in its nanostructured form, thus the material should be handled and stored in an oxygen-free environment to increase its stability. The Se 3${\mathrm{d}}_{5/2}$ peak was located at 55.6 eV (spin-orbit splitting ${\Delta }_{5/2-3/2}=0.86$ eV), matching tantalum selenide [45].

To determine the geometry and morphology of the core/shell NWs, SEM was used (Figure 4). According to the SEM images, NWs have a straight shape and a length in the order of 10 microns. The preservation of the straight shape of NWs is important because the shape and integrity of NWs can be compromised at high temperatures and in an aggressive corrosive atmosphere. As mentioned before, a thin layer of Al₂O₃ was used to cover and protect NWs during the selenization procedure, similar to in our previous work [26].

TEM microscopy was used to investigate the morphology and inner structure of core/shell NWs in detail. Typical images of two core/shell NW are presented in Figure 5. At the ends of every NW, Au nanoparticles can be seen, which is typical for the VLS growth mechanism. Au NPs appear as bright dots in the SEM images (Figure 4b) and as black dots in the TEM images (Figure 5a,b). Upon the selenization of the Ta shell, a TMD layer of TaSe₂ can be seen around NWs, which appears as black parallel lines (Figure 5b). The shell thickness varies from ∼10 layers to ∼20, with interlayer distance measured around 0.627 nm, which corresponds very well to the 0.627 nm interlayer distance in the 1T-TaSe₂ [12], 0.635 nm in the 2H-TaSe₂ [13], and 0.6287 in the 4Hb-TaSe₂ [11] materials. A layer of non-crystalline carbon was discovered atop the TaSe₂ surface, potentially formed through catalysis from carbon present in the atmosphere. More TEM images of selenized ZnS/Al₂O₃/Ta NWs can be found in Figure S3 in the Supplementary Data file. It was impossible to achieve the atomic resolution of the Al₂O₃ interlayer, which covers ZnS NWs; however, the thickness of the amorphous spacing between ZnS and TaSe₂ corresponds well to the expected Al₂O₃ thickness.

A similar approach was tested for ZnO/Al₂O₃/Ta NWs at the same temperature and selenization procedure duration. The ZnO NWs core completely sublimated during the selenization procedure; however, the Al₂O₃ shell was intact and TaSe₂ layer successfully produced, which can potentially be used for the preparation of hollow core tube-like Al₂O₃/TaSe₂ 1D heterostructures. The TEM images of the selenized ZnO/Al₂O₃/Ta NWs can be found in Figure S5 in the Supplementary Data file.

It is interesting to note that attempts to selenize Ta thin films and core-shell structures at atmospheric pressure using gas flow, all conducted at the same temperature, were unsuccessful. Additionally, the in-ampoule selenization of other metals, namely T and V, was tested within the same temperature range, yielding positive results. The vapor pressure of these refractory metals at temperatures between 650–700 °C is relatively low. This opens an additional option for the selenization of sandwich-like composite structures. However, further dedicated research is required to validate this hypothesis.

4. Conclusions

This study presents the development of a robust method for fabricating core/shell NWs with a two-dimensional transition metal dichalcogenide (TMD) shell by selenizing a thin film of tantalum. The process entails a four-step synthesis routine: (1) synthesis of ZnS NWs on a silicon wafer, (2) deposition of a thin Al₂O₃ layer by atomic layer deposition (ALD), (3) magnetron sputtering of a Ta layer, and (4) annealing of the ZnS/Al₂O₃/Ta NWs in a selenium atmosphere within a vacuum-sealed quartz ampoule at 650 °C to form the core/shell structure. Through this technique, complete transformation of the metallic Ta layer into a well-defined TaSe₂ shell around the ZnS/Al₂O₃ NWs was achieved. Characterization of the synthesized core/shell ZnS/Al₂O₃/TaSe₂ NWs by XRD, XPS, TEM, and SEM confirmed the formation of the desired core/shell structure with the integrity of the NWs preserved throughout the selenization process.

The significance of this work lies in its introduction of a scalable and versatile method for fabricating core/shell NWs with 2D TMD shells based on selenization of thin metal Ta film. This technique extends beyond TaSe₂, applicable to a range of 2D chalcogenides, like selenides and sulfides, using various metal precursors, including Ti and other refractory metals. Such versatility in the fabrication process expands the potential for creating a diverse range of core/shell NW systems, which is crucial for advancing material applications at the nanoscale. The unique properties of 2D materials as well as the method presented in the paper open ways for future research and development in nanostructures, with potential breakthroughs in energy harvesting, sensing, and nanodevice fabrication.