Controlled Vapor-Phase Synthesis of VSe₂ via Selenium-Driven Gradual Transformation of Single-Crystalline V₂O₅ Nanosheets

Abstract

We report a gas-phase precursor modulation strategy for the controlled synthesis of 1T-phase vanadium diselenide (VSe₂) from vanadium pentoxide (V₂O₅) nanosheets by systematically adjusting the vapor pressure of selenium. By controlling the selenium vapor pressure, selenium-free vapor transport of vanadium dioxide led to the spontaneous oxidation and formation of tens-of-micrometer-sized rectangular V₂O₅ crystals, while moderate selenium introduction produced intermediate oxygen-rich phases with trapezoidal crystal facets, and a highly selenium-rich environment yielded trigonal VSe₂ crystals. Raman scattering measurements confirmed the stepwise transformation from V₂O₅ to VSe₂, and atomic force microscopy revealed well-defined layered morphologies and distinct conformation within an atomically thin regime. Additionally, high-resolution transmission electron microscopy validated the orthorhombic and trigonal crystal structures of V₂O₅ and VSe₂, respectively. This work demonstrates the versatility of fine-tuned vapor-phase growth conditions in vanadium-based layered compounds, providing useful platforms to optimize structural composition with atomic precision.

1. Introduction

Nanostructured transition metal dichalcogenides (TMDCs) with two-dimensional (2D) layered structures have attracted substantial interest due to their unique electronic, optical, and catalytic properties. These materials exhibit exotic phenomena, such as charge density waves (CDWs), superconductivity, and phase transitions, making them attractive for future computing applications, including field-effect transistors, optoelectronic devices, and energy storage systems [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]. Among TMDCs, vanadium diselenide (VSe₂) stands out because of its intriguing physical properties. In its 1T phase, VSe₂ is metallic and exhibits a CDW transition at low temperatures below ~110 K. Despite the promises for potential applications such as in memristor and neuromorphic hardware, the electronic structure of 1T VSe₂ is often sensitive to external factors such as air exposure and point defects, requiring thorough understanding of its synthesis protocol with controlled stoichiometry and composition [17,18,19,20]. Meanwhile, vanadium pentoxide (V₂O₅), a dissimilar layered vanadium compound, is widely utilized in electrochemical energy storage, catalysis, and optoelectronics [21,22,23,24,25,26,27,28]. It possesses a stable orthorhombic crystal structure and a high oxidation state of vanadium, making it a suitable template for transformation into lower-oxidation-state vanadium chalcogenides [29,30,31]. The precision transformation from atomically thin V₂O₅ to VSe₂ involves the reduction of vanadium and the incorporation of selenium, leading to versatile material platforms.

Here, we demonstrated a vapor-phase synthesis approach for VSe₂ through the selenium-induced transformation of single-crystalline V₂O₅ nanosheets. By precisely tuning the selenium vapor pressure, we directed the stepwise transformation from V₂O₅ to VSe₂ via intermediate oxygen-rich vanadium selenide phases. Our findings offer insights into the role of precursor modulation in TMDC growth and pave the way for designing high-quality layered materials with tailored electronic properties.

2. Results and Discussion

2.1. Vapor-Phase Control of Selenium for Synthesis of O-Rich and Se-Rich Crystals

The controlled synthesis of VSe₂ was achieved by progressive precursor modulation, increasing the vapor pressure of selenium and maintaining that of VO₂ (Figure 1a). First, in the absence of selenium, rutile VO₂ was synthesized in a chemical vapor deposition system. Layered V₂O₅ nanosheets with a lateral size of tens of micrometers were grown by the spontaneous oxidation of VO₂ vapor, forming rectangular-faceted single crystals (Figure 1b), presumably due to diffusion-limited growth, where oxygen diffusion to the surface controlled the rate of oxidation. Ar/H₂ mixture gas also acted as a reductant, helping to reduce VO₂ towards a lower oxidation state. Then, the selenium-driven transformation of V₂O₅ nanosheets was achieved by introducing the vapor pressure of selenium (280~370 °C) and maintaining other growth parameters (total pressure, amount of precursor powder, flow rate of Ar/H₂) during the reactions, resulting in the gradual modification of facets from rectangles to trapezoids, as shown in Figure 1c. We identified these crystals as intermediate states between V₂O₅ and VSe₂. Under the selenium-rich environment (selenium at 370 °C), trigonal VSe₂ crystals revealed triangular facets on sapphire substrate (top) and SiO₂ substrates (bottom) (Figure 1d), highlighting a complete transformation. The gradual transformation was may attributed to the vapor pressure of selenium gradually substituting the oxygen atoms within the lattice, thereby modifying the vanadium oxidation state. This gradual transformation from V₂O₅ to VSe₂ was verified by Raman characterization. To confirm the gradual transformation with consistency, Raman scattering spectra (Figure 1d) were measured from four individual batches. With considerable Se addition, a dramatic change in the peak positions was also identified via Raman spectroscopy (Figure 1e and

Table 1. Summary of Raman peaks and related vibration modes at 532 nm laser excitation.

Sample	Raman Shift (cm−1)	Vibration Mode
VSe2	200.3	A1g
2L-VSe2	206.1	A1g
V2O5	103.7	A1g
V2O5	145.6	B1g/B3g
V2O5	196.8	A1g/B2g
V2O5	284.7	B1g/B3g
V2O5	304.4	A1g
V2O5	405.4	A1g
V2O5	484.1	A1g
3 nm thick V2O5	102.3	A1g
3 nm thick V2O5	190.2	A1g/B2g
3 nm thick V2O5	303.2	A1g
3 nm thick V2O5	401.5	A1g

). Under the growth condition without selenium vapor, which was not heated in a separate zone, the rectangular crystal showed seven prominent scattering peaks [103.7 cm⁻¹ (A_1g), 145.6, 284.7 cm⁻¹ (B_1g/B_3g), 196.8 cm⁻¹ (A_1g/A_2g), 304.4, 405.4, and 484.1 cm⁻¹ (A_1g)]. Under the condition with selenium at <280 °C, those peaks were not noticeable, showing new peaks at ~255 cm⁻¹, which was attributed to V₂O₃, and 195 cm⁻¹, indicative of an intermediate selenization stage. Sufficient amounts of Se at 365~370 °C resulted in a clear peak of A_1g (200.6 cm⁻¹) vibration modes, confirming the formation of trigonal VSe₂.

2.2. Material Characterization of Atomically Thin VSe₂ and V₂O₅ Nanosheets

Se-rich and O-rich vanadium compounds were further optimized to thin down the thickness of the crystals. Alkali halide (potassium iodide) was introduced to act as a nucleation inhibitor to suppress the nucleation density. The alkali halide is known for its function as a surfactant, reducing surface energy and thereby limiting excessive nucleation events. To characterize the atomically thin nanosheets of VSe₂ and V₂O₅ obtained under two different conditions using the controlled vaporization of selenium, we performed Raman mapping at a wavelength of 532 nm. In Figure 1a, a rectangular crystal with additional vertically overgrown layers is revealed in the Raman mapping images. This crystal showed four Raman peaks (102.3 cm⁻¹, 190.2 cm⁻¹, 303.2 cm⁻¹, and 401.5 cm⁻¹), as shown in Figure 2b, which were partially red-shifted, as indexed in Table 1 [32,33]. These V₂O₅ nanosheets were characterized by atomic force microscopy (Figure 2c), displaying clean surface and edges in the AFM scan images even at a thickness of 3 nm. Atomically thin VSe₂ showed weak intensities in the Raman mapping image (Figure 2e). It had a sole vibration mode at ~206 cm⁻¹ that was blue-shifted, indicating a reduced layer thickness compared to the reference bulk crystals. The surface of VSe₂ was contaminated, as we observed oxidized chunks at the crystal edges in the AFM mapping image with the step height of a bilayer.

2.3. Crystal Structure of Orthorhombic V₂O₅ and Trigonal VSe₂

The atomic structures of the VSe₂ and V₂O₅ were characterized by transmission electron microscopy (TEM). Due to the inability to transfer V₂O₅ grown on SiO₂ substrates using hydrofluoric acid or potassium hydroperoxide etching, the focused ion beam (FIB) was exploited to pick up V₂O₅/SiO₂ stacks. The prepared V₂O₅ specimen had an orthorhombic crystal structure, which was classified as a Pmmn space group, as shown in Figure 3a. The plan-view V₂O₅ showed rectangular atomic arrangements with corresponding (020) diffraction spots, as shown in the selected-area electron diffraction (SAED) pattern in Figure 3b,c, although vanadium atoms were only attributed to this TEM image. Additionally, high-resolution bright-field TEM indicated that VSe₂ multilayers (Figure 3d) exhibited a trigonal structure with (10-10) diffraction spots (Figure 3e,f) despite the substantial contamination along the edges of the crystal that was observed in the AFM scan image [20]. The observed diffraction patterns confirmed the integrity of the synthesized layers, with no detectable alloying effects between VSe₂ and V₂O₅.

3. Materials and Methods

3.1. Chemical Vapor Deposition (CVD) of Atomically Thin Vanadium Compounds

The synthesis of V₂O₅ and VSe₂ was carried out using a CVD approach. V₂O₅ nanosheets were synthesized via the vapor-phase oxidation of vanadium dioxide (VO₂). High-purity VO₂ powder (99.99%) was placed in a ceramic boat at the center of a 3 in. quartz tube furnace, and a 1 cm × 1 cm sapphire or 2 cm × 2 cm SiO₂/Si substrate was positioned downstream with a 1–1.5 cm gap from the solid powder. The furnace was heated to 850 °C under an Ar/H₂ (20:15 sccm) flow and maintained for 5 min, promoting the spontaneous oxidation of VO₂ into layered V₂O₅ nanosheets. The furnace was naturally cooled down to room temperature. The chamber pressure was held around 600–700 torr. For VSe₂, selenium vapor was additionally introduced to generate a product of VSe₂. Selenium powder (99.99%) was separately heated to temperatures ranging around 370 °C to control the vapor pressure. The reaction proceeded at 820 °C under a reducing atmosphere of Ar/H₂ (23:11 sccm) for a duration of 5 min, enabling a gradual substitution of oxygen with selenium. The chamber pressure was held around 600–700 torr.

3.2. Gradual Selenium Control Mechanism

To investigate the effect of selenium’s vapor pressure, we performed a series of reactions with varying selenium source temperatures. At lower temperatures (<280 °C), selenium diffusion was limited, leading to the formation of oxygen-rich intermediate phases. These structures exhibited trapezoidal crystal facets, indicative of an incomplete conversion process. At moderate temperatures (280–365 °C) in the selenium region, increased selenium vapor pressure facilitated a higher degree of substitution, forming partially selenized vanadium oxyselenide compounds. At a selenium temperature of 370 °C, a complete phase transition to trigonal VSe₂ was achieved.

4. Conclusions

In summary, we have demonstrated a controlled vapor-phase synthesis strategy for VSe₂ via the selenium-driven transformation of atomically thin V₂O₅ nanosheets. By precisely tuning the vapor pressure of selenium, we directed a stepwise transition from V₂O₅ nanosheets to VSe₂ nanosheets through intermediate phases of oxygen-rich vanadium selenides. Comprehensive structural characterization using Raman spectroscopy, atomic force microscopy, and high-resolution transmission electron microscopy confirmed the phase transformation, layered morphology, and high crystallinity of the synthesized materials. This work highlights the importance of vapor phase precursor modulation in tailoring phase transitions within vanadium-based layered chalcogenides, offering a versatile platform for tuning electronic properties at the nanoscale. Potentially, similar vapor-phase modulation methods could be extended to other vanadium-based chalcogenides, such as VS₂ or VTe₂. The insights provide further advancements in the precision synthesis and integration of VSe₂ for future electronic, optoelectronic, and neuromorphic computing applications.