Systematic study of CVD-growth parameters in NaCl-assisted growth of MoSe₂ nanostructures: nanoribbons, dendrites and spirals

Abstract

MoSe₂, a two-dimensional (2D) transition metal dichalcogenide (TMDC), has garnered significant interest in recent years due to its semiconducting properties and tunable band gap with potential applications in optoelectronics, photocatalysis and atomically thin devices. In this study, we report the controlled synthesis of MoSe₂ nanocrystals using a custom 12-zone atmospheric pressure chemical vapour deposition (APCVD) system. NaCl is used as a seed promoter to facilitate the growth of monolayer, bilayer, and multilayer films, often as large as 200 μm. Additionally, the morphological evolution of the MoSe₂ nanostructures is controlled by tuning different growth parameters based on insights, which we discuss in detail. The growth of dominant nanostructures, which include nanoribbons, snowflakes, monolayers and multilayer domains, among others, are discussed. High-resolution optical microscopy, field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) are used to image the as-grown crystals. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) are used to verify the quality and elemental composition of our as-grown MoSe₂ crystals. Our findings brighten the prospect of growing novel 1D and 2D TMDC nanostructures with sufficient control to make them suitable for advanced optoelectronic and catalytic devices.

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1. Introduction

Over the last decade, two-dimensional layered transition metal dichalcogenides (TMDCs), in general, have attracted significant attention because of their exceptional electronic and optical characteristics.^1–4 Of these, MoS₂ and MoSe₂ have been particularly popular because of the promise they hold for a range of applications. Within these, monolayer or few-layer MoSe₂ nanostructures exhibit superior behaviour to MoS₂ due to their unique band structures.^5–8 Few-layer 2D TMDCs, including MoSe₂, offer enhanced mechanical and chemical stability compared to their monolayer counterparts, making them more suitable for use in FETs. This stability is crucial, as it allows these materials to withstand the harsh conditions encountered during device fabrication. An increased number of layers also provides a larger number of transport channels and results in higher on-state current and carrier mobility.⁹

Chemical vapor deposition (CVD) has been widely used to synthesize highly crystalline and large-area MoSe₂ films. The successful growth of MoSe₂ on SiO₂/Si substrates using CVD was first reported by Shaw et al.¹⁰ Since then, there have been several reports of large-area MoSe₂ nanocrystals grown by CVD on many other substrates.^11–14 Chang et al.⁷ compared the optical and electronic properties of CVD-grown MoS₂ and MoSe₂ and found that the number of defects was lower in the latter. This was in part explained by Wang et al.,¹⁵ who initiated a discussion of the growth mechanisms involved in CVD-grown MoSe₂.

Two of the important growth mechanisms are worth noting. With the metal oxide precursor and substrate conventionally placed in a high-temperature zone, and the chalcogen in a relatively low-temperature region¹² crystals grow in the vapor–solid–solid (VSS) mode, wherein a solid crystal is deposited directly from the vapor phase onto a solid substrate. Vapor phase precursors directly adsorb and react on the substrate surface to form nuclei, which subsequently grow into solid, typically triangular or hexagonal, domains. These shapes are dictated primarily by the free energy of the crystal edges and the surface diffusion kinetics of the precursors. However, the morphologies produced by this route are limited.

In contrast, the vapor–liquid–solid (VLS) mode, facilitated by the introduction of alkali halide salts, e.g. NaCl or KCl, alters the free energy of the crystal edges^16,17 and opens out several new possibilities. This mechanism involves vapor-phase precursors adsorbing onto a catalytic liquid metal droplet, dissolving therein, and later precipitating at the liquid–solid interface to produce a range of nanostructures such as nanowires and nanoribbons. Several van der Waals layered compounds, including BN,¹⁸ NiCl₂,¹⁹ SnS₂²⁰ and Bi₂Se₂,²¹ have been synthesized by this method. However, control of the morphology and quality of the resulting MoSe₂ nanostructures continues to be an outstanding challenge.

In this context, it is worth mentioning that the addition of alkali halides to the metal oxide enhances the growth rates and sizes of the resulting crystals (or continuous films) and reduces the growth temperature. Li et al.²² reported that large WSe₂ and WS₂ monolayer crystals could be grown at moderate temperatures (700–850 °C) using alkali metal halides as growth promoters. Li et al.²³ demonstrated a dramatic enhancement in the growth of 2D TMDC selenides and tellurides, namely MoSe₂ and WTe₂, using mixed transition metal salts and chalcogen salts. The addition of salts induced the wafer-scale growth of these TMDCs, forming a continuous monolayer film and a large gain size of 100–50 μm. Kim et al.²⁴ made the same observation with regard to MoS₂. Zhou et al.²⁵ subsequently demonstrated that molten-salt-assisted CVD can be broadly applied to produce atomically thin TMDC films. They also showed that salt assistance increases the overall reaction rate by decreasing the melting point of the reactants, which in turn facilitates the formation of intermediate products. For example, metaloxyhalides, formed in reactions between halides and metal oxides, evaporate at relatively low temperatures, enhancing 2D growth in the process. Chen et al.⁵ reported the controlled CVD growth of large-scale monolayer and multi-layer MoSe₂ films and nanoribbons. Li et al.²⁶ synthesised single-crystal, monolayer MoSe₂, with domain sizes reaching up to 250 μm, by employing NaCl-assisted CVD. Feng et al.²⁷ reported that increasing the growth temperature in the presence of NaCl leads to drastic changes in the morphology of the as-grown WSe₂ samples. Singh et al.²⁸ similarly grew MoS₂ on different substrates and demonstrated that NaCl facilitates the formation of seeding promoters, such as a water-soluble layer of Na₂S and/or Na₂SO₄ on the substrate, which aids in the 2D planar nucleation of MoS₂. Bay et al.²⁹ further systematised this approach by growing MoSe₂ nanocrystals, using five different water-soluble catalysts, namely sodium chloride (NaCl), zinc chloride (ZnCl₂), potassium hydroxide (KOH), potassium chloride (KCl), and potassium oxalate (K₂C₂O₄). These were applied as thin layers onto the SiO₂/Si substrates. Among these five salts, the use of NaCl resulted in the most uniform growth of the as-synthesised flakes.

Each TMDC morphology, including nanoribbons, dendrites, fractals, and spiral formations, offers distinct advantages for various applications. For example, 1D nanoribbons have shown significant promise in electronic, magnetic, and catalytic applications;^30–33 however, the controllable fabrication of MoSe₂ nanoribbons is found to be scant in the literature. Similarly, dendritic and fractal structures are particularly advantageous for catalytic applications owing to the high density of the active edge sites and defects they possess. Studies have shown that the rate of the hydrogen evolution reaction (HER) increases linearly with the density of these features.^34–37 The CVD growth of MoS₂ dendrites, as reported earlier by our group,³⁸ can be further enhanced by adding NaCl to the precursor. The formation of dendrites or fractals in MoSe₂ under optimised CVD-growth conditions, as reported here, would be suitable for HER catalytic activity. Additionally, spiral structures, which interlink the layers of MoSe₂, provide unique pathways for electrical current, enhancing the vertical conductivity of the devices. This unique morphology was found to notably increase the vertical conductivity in MoS₂,³⁹ enhancing the overall electrical conductivity and making these structures ideal for use in nanoelectronic devices.⁴⁰ Moreover, the non-inversion symmetry present in these nanostructures gives rise to interesting non-linear optical (NLO) effects, polarization and piezoelectricity, all of which promise to find applications in electronics and optoelectronics.^41–43

In this study, we systematically investigate the morphological evolution of MoSe₂ nanocrystals by varying a set of specific CVD-growth parameters. NaCl is employed as a seed promoter to enhance both the growth rate and size of the crystals. By meticulously tuning these parameters, a wide variety of nanocrystal morphologies were successfully synthesized. There are several studies on NaCl-assisted CVD growth of TMDCs available in the literature, as mentioned earlier; but in addition, there are substantial research gaps. Our studies are an attempt to address this gap by correlating the CVD growth of each of these different MoSe₂ nanostructures, including nanoribbons, dendrites, and spiral structures, with specific regions of parameter space. This is essential for the controlled growth of large-area crystals with desired morphologies. Initially, an optimal NaCl concentration was identified to facilitate the formation of large flakes. Under these conditions, nanoribbons emerged as the predominant nanostructure. Subsequently, by varying the growth temperature and the concentration of the chalcogen precursor, a diverse array of nanostructures was obtained. This comprehensive analysis defines the range of achievable morphologies and offers valuable insights into the underlying growth mechanisms driving their formation.

2. Experimental details

Fig. 1(a) depicts a schematic diagram of the (CVD) setup for MoSe₂ growth. The CVD experiments were carried out in a commercial APCVD system, QRYSTAL-CVD-1100,⁴⁴ manufactured by Quazar Technologies.⁴⁵ The furnace comprises twelve independent, 75 mm-long thermal zones, each equipped with its own heater and temperature sensor. The temperature of each zone is individually adjustable through an intuitive graphical user interface, allowing for seamless control. Adjacent zones can be integrated by synchronizing their set temperatures, enabling the formation of composite heating regions of variable lengths. These zones maintain a temperature uniformity of ±5 °C. This sophisticated system facilitates exceptional control over growth parameters, ensuring the high precision necessary for the demands of this study.

Fig. 1 (a) Schematic illustration showing the experimental set-up for CVD of MoSe₂; (b) and (c) the side view and the top view of the substrates placed on the alumina boat containing MoO₃ and NaCl, respectively; (d) the temperature profile for the APCVD growth of MoSe₂.

For the deposition of MoSe₂, high-purity molybdenum trioxide (MoO₃, ≥99.5%, Sigma Aldrich), selenium (Se, ≥99.5%, Sigma Aldrich), and sodium chloride (NaCl) powders (99.9%, CDH) were used as precursors, and 300 nm thermally oxidised SiO₂/Si wafers were used as substrates. An alumina boat containing 1 mg of MoO₃ and three substrates placed face down on top of it (labelled 1, 2 and 3 in Fig. 1(c)) were loaded into the quartz tube. A gap of 0.5 mm was kept between wafers 1 and 2, which acts as an outlet for the precursor vapor. This ensures that some of the vapors escape the alumina boat, and uniform deposition on wafers 4 and 5 occurs, which were placed face-up adjacent to the alumina boat (Fig. 1(c)). The boat containing Se powder (600 mg) was placed upstream at a distance of 31 cm away from the MoO₃ boat (Fig. 1(a)). For the NaCl-assisted growth experiments, a mixture of the MoO₃ and NaCl powder was used.

Prior to deposition, the substrates (10 mm × 10 mm) were cleaned ultrasonically in acetone and isopropyl alcohol (IPA), respectively, and then dried under Ar gas. The alumina boat containing the mixture of MoO₃ and NaCl and the substrates was then loaded into a quartz tube. The CVD furnace was configured into two heating zones: a chalcogen zone heated up to 500 °C for Se vaporisation and the growth zone comprising MoO₃, NaCl and the substrates, which was heated up to a higher temperature for MoSe₂ synthesis. The temperature profile of these zones is shown in Fig. 1(b). The CVD system was initially evacuated and purged. This procedure was repeated five times over a total period of 30 minutes, during which the chalcogen zone and the growth zone were maintained at room temperature and 150 °C, respectively. Subsequently, the gas flow rate was set to 10 sccm, which comprised Ar (8 sccm) and H₂ (2 sccm), with H₂ acting as a reducing agent. The growth zone temperature was then ramped up at a rate of 15 °C min⁻¹ until it reached 750 °C. Se was rapidly evaporated at 500 °C when the growth zone reached 725 °C. The system was maintained under this condition for 20 minutes. Finally, the system was cooled gradually with Ar flow at 200 sccm, stopping the H₂ flow.

2.1. Characterization

High-resolution optical images of the as-grown MoSe₂ flakes were captured using a Carl Zeiss Axio Lab. A1 microscope equipped with an AxioCam ERc5s camera. Raman spectra were recorded with a Horiba Jobin Yvon LabRAM HR spectrometer using a 532 nm excitation laser. X-ray photoelectron spectroscopy (XPS) measurements were conducted using a Thermo Fisher Scientific K-Alpha instrument. Field emission scanning electron microscopy (FESEM) images were acquired with an FEI Apreo LoVac microscope. Atomic force microscopy (AFM) measurements were performed using an Agilent LS5600 system.

3. Results and discussion

MoSe₂ crystals deposited using the standard recipe (without any NaCl) are triangular, with a maximum domain size of ∼25 μm as shown in Fig. 2(a). The crystals grown are uniform monolayers, as confirmed by Raman spectra (Fig. 2(b)) obtained at different points on the same crystal. The standard A_1g (out-of-plane vibrational mode) and E_2g (in-plane vibrational mode) peaks for monolayer MoSe₂ are observed at 239.7 cm⁻¹ and 286.2 cm⁻¹, respectively.⁴⁶ The combination modes of the longitudinal (LA(M)) and the transverse (TA(M)), originating from the vicinity of the high-symmetry M point of the MoSe₂ Brillouin zone, are observed at 247.4 cm⁻¹.^47,48Fig. 2(c) and (d) show high-resolution deconvoluted XPS spectra of Mo 3d and Se 3d core-level signals, respectively. The Mo 3d_3/2 and 3d_5/2 peaks are located at 232.5 eV and 229.3 eV, respectively, as depicted in Fig. 2(c). Those of Se are detected around 54 eV, as shown in Fig. 2(d). These data are in agreement with the respective values of MoSe₂ found in the literature.^14,49 These two Mo 3d peaks represent the oxidation charge state of MoSe₂. The Mo peak positions are shifted significantly from their hexavalent positions of ∼235.9 eV and 232.5 eV (Mo 3d_3/2 and 3d_5/2 core levels of MoO₃), indicating the reduction of Mo from Mo 6p to Mo 4p. The two comparatively weak peaks present in the Mo 3d plot are linked to Mo⁶⁺ 3d and Se²⁻ 3s states, respectively.⁵⁰ The presence of an Mo⁶⁺ peak indicates that a relatively minor amount of MoO₃ is expected to be present in the MoSe₂ flakes. The peak of Se around 54 eV relates to the doublet core level 3d_5/2 and 3d_3/2, with peak positions at 54.8 eV and 55.5 eV, respectively.

Fig. 2 (a) FESEM image of the as-grown MoSe₂ crystal on the SiO₂/Si substrate using the standard recipe (without NaCl) at a growth temperature of 750 °C. (b) Raman spectrum of MoSe₂. High-resolution deconvoluted XPS spectra of (c) Mo 3d and (d) Se 3d.

3.1. Variation in growth parameters

NaCl plays a crucial role in influencing the morphology and distribution of the MoSe₂ flakes. The intermediate species formed in the reaction of NaCl with MoO₃ lowers the energy barrier and decreases the reaction temperature, thereby facilitating the synthesis. The following chain of chemical reactions occurs:

2MoO₃ + 2NaCl → MoO₂Cl₂↑ + Na₂MoO₄

3MoO₃ + 2NaCl → MoO₂Cl₂↑ + Na₂Mo₂O₇

The MoO₃–NaCl mixture has a large weight loss at ∼550 °C,⁵¹ much lower than the melting points of MoO₃ and NaCl (T_m(MoO₃) = 795 °C, T_m(NaCl) = 801 °C). Volatile MoO₂Cl₂ is one product of the reaction,⁵² which sublimates at a relatively low temperature of 175 °C. The vapors of these metal oxychlorides can travel long distances to the substrates and initiate VSS (vapor–solid–solid) growth, resulting in early nucleation of TMDCs. Furthermore, some reports indicate that metal oxychlorides may undergo selenisation at a significantly faster rate than metal oxides, which enhances the reaction rate during the growth process.^25,53 Thus, the reactions of intermediate metal oxychlorides (MoO₂Cl₂), Se, and H₂ are faster and eventually facilitate the nucleation and layered growth by lowering the energetic barrier. In contrast, Na reacts with the transition metal oxides to form non-volatile molten salts (Na_xMoO_y) with high melting points (>600 °C) on the growth substrate without vapor phase transportation.⁵⁴ After reaction with chalcogen vapor, MoSe₂ crystals are grown from liquid melts in the VLS (vapor–liquid–solid) mode. The coexistence of the VSS and VLS induced by oxychlorides and sodium metal oxides results in an increased growth rate, large-area crystals and different morphological nanostructures. We investigated the morphological variation of the as-grown MoSe₂ nanocrystals as the CVD-growth parameters were changed in a methodical manner.

3.1.1. Amount of NaCl. MoSe₂ crystals were synthesised by mixing 1 mg of MoO₃ powder with different amounts of NaCl, varying it from 0.5 mg to 3 mg. With the increase in the amount of NaCl, the size of the as-grown nanocrystals increased, as shown in Fig. 3. Monolayer domains were found to be predominant up to 1.3 mg of NaCl. On further increase of the amount, a large area of multilayered MoSe₂ regions was deposited. The optimal amount of NaCl was found to be 1.7 mg, which produced multilayered-triangular crystals with large sizes ≥100 μm. At higher NaCl concentrations (≥1.7 mg), the lateral size of the crystals decreases significantly to 5–7 μm. NaCl acts as a fluxing agent, releasing Na and Cl species that interact with MoO₃ and Se precursors during the CVD process. The intermediate volatile species obtained in the reactions enhance the surface diffusion of Mo and Se adatoms, promoting anisotropic growth. The decrease in size at a concentration >1.7 mg suggests an oversaturation effect. This is because the growth environment with NaCl-derived species favours rapid nucleation, leading to suppressed lateral growth. The variation of domain size as a function of the amount of NaCl is represented graphically in Fig. 4(b), depicting a clear trend in the variation of the size of the crystals. This variation is summarised in Table S1 (ESI†). Hence, using the optimal amount of NaCl, it should be possible to deposit large-area, high-quality 2D TMDCs.

Fig. 3 High-resolution OM images showing variation in the size of the MoSe₂ crystal on the SiO₂/Si substrate with different amounts of NaCl. (a)–(l) Ranging from 0 to 3 mg at a growth temperature of 750 °C.

Fig. 4 (a) Optical image of the as-grown MoSe₂ crystal surrounded by solidified liquid droplets. (b) The domain size of MoSe₂ as a function of the amount of NaCl. (c) The domain size of MoSe₂ as a function of the distance between the metal oxide precursor and NaCl precursor to the region of deposition. (d) Raman spectrum of a monolayer MoSe₂ grown using NaCl.

As the size of the crystals increased with the amount of NaCl, it was observed that the large area crystals were surrounded by solidified liquid droplets along the edges. This is depicted in the OM image of Fig. 4(a). These droplets attach themselves to the energy favourable sites along the edges of the as-grown crystals, serving as catalysts, enhancing the mass transport along the edges, thereby increasing the lateral size of the crystals.^17,55 In Fig. 4(c), the domain size of the crystals was plotted as a function of the distance between the metal precursor (MoO₃ + NaCl) and the position of the as-grown crystals on the substrate. Zero denotes the position of the metal oxide precursor. It is evident from the graphs that the addition of NaCl increases the size of the crystals considerably. The reduction in the size of the crystal as the distance from the source precursor increases is evident in Fig. 4(c). This is due to a concentration gradient of the Mo vapors across the substrate. The presence of such a concentration gradient is due to the fact that the local Mo : Se ratio varies along the length of the growth substrate. The crystal quality of the NaCl-assisted MoSe₂ crystal was determined by Raman spectroscopy (Fig. 4(d)). A_1g and E_2g peaks for monolayer MoSe₂ are observed at 239.9 cm⁻¹ and 28 cm⁻¹, respectively. The separation, Δ, between the A_1g and E¹_2g peaks is 45.1 cm⁻¹, further confirming the monolayer nature of the crystals.⁵⁶

XPS experiments were carried out to study the chemical composition of the MoSe₂ grown using NaCl. Peaks of Mo, Se, and O elements can be clearly seen in the XPS survey spectrum (Fig. 5a). Fig. 5b shows the binding energy of Na 1s located at 1072.7 eV, indicating that Na from NaCl combines with the precursor, forming intermediates such as Na₂MoO₄.⁵⁷Fig. 5c displays energy core-level peaks located at 229.4 eV and 232.6 eV, which are attributed to Mo 3d_5/2 and Mo 3d_3/2 of Mo⁴⁺, respectively. The Mo⁶⁺ 3d peak is not observed in the spectrum, indicating that the films are high-quality MoSe₂ and the addition of NaCl does not compromise the quality of the crystal. The peaks located at 54.9 eV and 55.8 eV correspond to the binding energies for Se 3d_5/2 and Se 3d_3/2, representing the Se²⁻ oxidation state (Fig. 5d).

Fig. 5 XPS spectra of MoSe₂ monolayer grown on SiO₂/Si. (a) XPS survey spectrum. (b–d) Core-level spectra of Na 1s, Mo 3d, and Se 3d, respectively.

3.1.2. Growth temperature. After fixing the optimal concentration of NaCl at 1.7 mg and keeping all the other parameters constant, the variation in growth temperature was studied. Fig. 6 depicts the morphological evolution of MoSe₂ flakes deposited at different growth temperatures. Monolayer MoSe₂ crystals were predominant at growth temperatures of 675 °C and 700 °C, along with the formation of some multilayered islands. At a higher temperature of 750 °C, large-area multilayered crystals with lateral sizes of ∼200 μm were observed. The nucleation density was found to increase progressively with the rise in temperature. On further increasing the temperature to 800 °C, the size of the crystals reduced drastically, and a continuous multilayered film was deposited.

Fig. 6 High-resolution OM images showing variation in the morphology of MoSe₂ on SiO₂/Si at different growth temperatures using 1.7 mg of NaCl (a)–(e) ranging from 675–800 °C. (f) The domain size of MoSe₂ as a function of growth temperature.

The morphological evolution in the MoSe₂ domains on temperature can be explained by the diffusion rates of Mo, Se, and NaCl at elevated temperatures. The diffusion rate and the deposition of the MoO₃, Se, and NaCl vapors on the substrate surface vary with the change in temperature. At lower temperatures, the MoO₃ clusters deposited on the substrate's surface have a lower concentration and are small in size. At the onset of deposition, monolayers are formed on the substrate surface by reducing the MoO₃ vapors. Further deposition results in an increase in the domain size, and the MoSe₂ domains are thus formed with both monolayer and bilayer structures. In the case of growth at high temperatures, the high diffusion rate of the precursor results in the transport of a high concentration of nanoparticles on the surface of the substrate. This results in rapid selenisation and growth of the MoSe₂ crystals with the deposition of multilayer nuclei of the MoO_3−xSe_y nanoparticles. Thus causing higher vapor pressure of the metal precursors and enhanced reaction rate. The further growth of these nanoparticles induces the growth of larger crystals. As the temperature is increased beyond 750 °C, the flake size decreases along with predominant multilayer deposition. The domain sizes decrease due to enhanced desorption, favouring vertical growth. Thus, 750 °C was the optimal temperature for the growth of large-area MoSe₂ crystals.

The Raman spectrum in Fig. 7(a) further confirms the dependence of growth temperature on the thickness of the MoSe₂ films. At a growth temperature of 675 °C, the A_1g vibrational mode appears at 239.2 cm⁻¹, characteristic of monolayer MoSe₂. At 700 °C, a further shift to 240 cm⁻¹ is observed. Consequently, with increasing growth temperature, a blue shift in the A_1g peak is observed (as plotted in Fig. 7(b)), indicating an increase in the film thickness.^58,59 At 800 °C, a maximum shift of 3 cm⁻¹ is measured. This is consistent with the formation of multilayer MoSe₂. The low-intensity E_1g peak at ∼283.4 cm⁻¹ diminishes considerably with increasing thickness. The effects of the growth temperature on the morphology of the MoSe₂ crystals are summarised in Table S2 (ESI†).

Fig. 7 (a) Temperature-dependent Raman spectra of the MoSe₂ crystal depicting evolution of the Raman peak; different colors represent different temperatures. (b) Frequency peaks of the A_1g mode plotted w.r.t. growth temperature.

3.1.3. Amount of Se precursor. Besides growth temperature, the Mo : Se ratio is the primary criterion affecting the growth of TMDCs. Here, we investigated the effect of Se concentration on the growth of MoSe₂ films while keeping the NaCl amount and growth temperatures fixed at 1.7 mg and 750 °C, respectively. By controlling the Se vapor pressure, the as-grown atomic crystals undergo a three-stage shape transformation from fractals and dendrite deposition to 2D compact nanocrystals and eventually to nanoribbons, as illustrated by the flowchart in Fig. 8.

Fig. 8 Flowchart illustrating the change in morphology of MoSe₂ crystals with varied amounts of Se precursor.

At lower Se concentration (150 mg), fractal and dendritic crystals with thick branches were observed. The size of these crystals varies, with a maximum length of ∼100 μm. At higher Se concentrations (300 mg), the gaseous atmosphere becomes enriched with Se and Mo. More Se atoms reach the substrate, resulting in compact triangular crystal structures. When the concentration of Se is further increased to 600 mg, nanoribbons with lengths ≥100 μm were observed. Preferential growth of the nanoribbons along the edges of the triangular MoSe₂ domains was predominant. The variation in morphology with Se concentration is summarised in Table S3 (ESI†).

The analysis of XPS data to study the stoichiometry of these crystals is reported in the ESI† (Section S2). The high-resolution deconvoluted XPS spectra of Mo 3d and Se 3d peaks of crystals grown at 150 mg, 300 mg and 600 mg Se concentrations are shown in Fig. S1, S2 and S3 (ESI†), respectively. The Mo : Se ratio for the dendritic crystals was determined to be ∼0.47, corresponding to a Se-deficient condition. This sub-stoichiometric composition can also be ascribed to Se vacancies.⁵⁶ This ratio for the triangular crystals was found to be 0.5, which is congruent with the literature. Whereas, for the nanoribbons, the value of the Mo : Se ratio was ∼0.67, corresponding to a Se-rich environment. The details of these calculations are reported in Table S4 (ESI†).

3.2. Morphological evolution

On systematic variation of the growth parameters, diverse nanostructures were deposited. Among them, nanoribbons were dominant. The transition from fractals to snowflakes and dendrites was also prevalent. In addition, spiral growth on MoSe₂ triangular domains was found. The growth mechanism of these nanostructures is discussed in the following sub-sections.

3.2.1. Nanoribbons. The growth of nanoribbons can be explained by the vapour–liquid–solid (VLS) mechanism.

In the synthesis process, the precursors evaporate and undergo a chemical reaction between the vapors of MoO₃ and NaCl, forming intermediate species like Na₂MoO₄/Na₂MoO₇.³³ A mixture of gas–liquid droplets is formed when these intermediate species diffuse and melt on the substrate. Vaporised Se dissolves into the droplet continuously and results in the formation of MoSe₂ seeds. As the droplet crawls on the surface, Se continues to dissolve into the droplet, inducing further growth and yielding ribbon-like structures.⁶⁰ In the XPS data (Fig. S4 and S5, ESI†), the Na 1s peak is observed at 1071.69 eV corresponding to Na₂MoO_x,⁶¹ which confirms the presence of intermediate Na–Mo compounds on the surface. This peak intensifies at higher growth temperatures, namely 750 °C. This was also the optimal temperature for the CVD growth of long MoSe₂ nanoribbons in our studies. This further gives evidence of the role of intermediate species in the growth of nanoribbons.

The average length of the nanoribbons was measured to be ∼70 μm, with a maximum length of ∼120 μm. The lengths vary with the amount of NaCl mixed with the MoO₃ precursor. It was found that the growth of nanoribbons was initiated with NaCl concentration greater than 1.3 mg. The length increases with increase in NaCl concentration, reaching a saturation point. Beyond this, there is no effect on the length of the nanoribbons with the addition of NaCl. This observation is congruent with the variation in the size of the crystals with NaCl concentration. The variation in the average nanoribbon length as a function of NaCl amount is shown in Fig. 9(e). The growth of nanoribbons observed in this study is predominantly along the edges of the underlying domain, as evident from Fig. 9(a) and (f).

Fig. 9 (a)–(c) are high-resolution OM images of MoSe₂ nanoribbons. (d) Raman data of the nanoribbons. (e) Plot showing variation in the length of the nanoribbons w.r.t. the amount of NaCl used. FESEM images showing (f) nanoribbons grown along the edges of the MoSe₂ crystals, indicated by yellow arrows; (g) zoomed-in image with an encircled nanoparticle; (h) AFM image and height profile (along the line AB) of the as-grown MoSe₂ nanoribbons.

The alignment of these ribbons is primarily determined by the orientation of the underlying crystal. Additionally, few-layered nanoribbons extending from the nucleation site on the MoSe₂ domain are observed as shown in the high resolution OM images of Fig. 9(b) and (c). These ribbons are locally aligned but exhibit occasional regular kinks, suggesting that the growth is guided either by the substrate or the crystal facets of the underlying MoSe₂ domain. It is observed that some of the nanoribbons are terminated with a nanoparticle. Fig. 9(g) shows a zoomed-in FESEM image of such nanoparticles. The size of these particles is measured to match with that of the width of the ribbons (∼0.14 μm). As mentioned earlier, these characteristic features of nanostructures result from the crawling mode of VLS growth. For a few nanoribbons, there were no nanoparticles at their ends. In some cases, the absence of the nanoparticle can be explained by the fact that the liquid droplet precursor might have been consumed during the growth of the ribbon. Fig. 9(d) shows the Raman measurement of the nanoribbons grown on triangular MoSe₂ domains. The difference between A_1g and E_2g (Δ = 42 cm⁻¹) in the data confirms that the multilayer nanoribbons are grown on few-layer MoSe₂ domains. AFM measurements were acquired to study the surface morphology and the thickness of the MoSe₂ nanoribbons. Fig. 9(h) depicts the AFM image and the height profile of a nanoribbon scanned in the non-contact mode, respectively. The contrast in the image indicates the difference in the heights of the structure. A height profile along the line AB denotes a thickness of ∼6 nm, corresponding to a thickness of around 10 layers.

3.2.2. Fractals-snowflakes-dendrites. A transition from compact triangles to fractal to dendritic deposition was observed under certain growth conditions. We observed the growth of fractals and snowflakes at a 1 mg concentration of NaCl (Mo : Se = 1 mg : 600 mg), but thick-branched dendrite structures were observed at a low Se concentration (Mo : Se = 1 mg : 150 mg). In Fig. 10(a) and (b), a clear transition from the compact island to fractals at the edges is observed.

Fig. 10 (a) FESEM image of a MoSe₂ crystal with fractal edges. (b) Zoomed-in image of the area enclosed in a red square showing the fractals near the edges in Fig. 9(a). (c) AFM image showing the fractal edge of a triangular crystal. (d) FESEM images of snowflakes, inset contains a high-resolution OM image of the same. (e) and (f) High-resolution OM images of the as-grown MoSe₂ dendrite structures.

At increased flux of metal precursor, the dominant morphology is fractal due to surface diffusion prevailing over edge diffusion.⁶² The adatoms reaching the edge of growing triangular domains do not contribute to the lateral growth. They diffuse randomly, which leads to the formation of fractals. The edge diffusion is highly suppressed, but the incoming adatoms attach themselves to the domain edges predominantly via diffusion-limited aggregation (DLA).⁶³ Adding NaCl as a seed promoter, there is an obvious increase in the metal flux because of the increase in volatile metal oxychlorides (MoO_xCl_y). Thus, the formation of fractals along the edges of a compact triangular domain and that of isolated fractals are promoted.

The transition from triangular to truncated, and further to fractal and dendritic MoSe₂ flakes, is attributed^64,65 to accelerated directional growth of either a Mo- or Se-terminated edge under a Se-deficient or Mo-rich growth environment, respectively. Thus, different dendritic shapes are induced, including snowflakes. Dendritic structures are observed at a low Se concentration, mainly at the substrate–boat contact interface, which creates a confined space leading to a high flux of metal precursors.^61,66 The abundance of MoO_3−x vapour in the confined reaction space also leads to fractal edges surrounding the compact triangular flakes, as shown in the AFM image of Fig. 10(c). The fractal edges observed in our experiment could be understood to be driven by kinetics rather than thermodynamics. This is congruent with our earlier work on MoS₂ dendrites.³⁸ These dendrites develop as atoms and diffuse preferentially along specific directions, forming the characteristic “branches” of the structure. From Fig. 10(d), it can be observed that most of the dendrites align with the three crystallographic directions typical of snowflakes. The larger dendrites have a greater number of branches, as shown in Fig. 10(e) and (f). The anisotropic influence of neighbouring atoms, combined with the tendency of dendrites to grow only along lower-energy directions, causes their preferential growth along specific directions.

3.2.3. Spiral growth. In addition to different morphologies, the growth of spiral MoSe₂ pyramids was observed. The contrast in the optical image of spiral MoSe₂ in Fig. 11 indicates that the thickness increases from the edge to the centre of spiral MoSe₂. The size of these domains ranges from 50 μm to 80 μm.

Fig. 11 (a)–(c) High-resolution OM images showing a spiral MoSe₂ pyramid.

The formation of the spiral growth depends on the MoSe₂ flux. At low MoSe₂ flux, monolayer MoSe₂ is formed, whereas, as the flux increases, both monolayer and spiral MoSe₂ are obtained. At a high flux of MoSe₂, thick spiral MoSe₂ with screw dislocations is formed. A diffusion-limited growth mechanism causes the formation of a domain when it is growing. During this growth, one grain boundary can elevate over another when two edges encounter a lattice mismatch within the same domain.⁴² A line of unsaturated chalcogen dangling bonds is formed on the lifted edge. A screw dislocation appears at the end of this line, which favours the growth of spirals. The generation of screw dislocations is thus increased, if the probability of a misaligned meeting of two exposed edges is increased. These accidental overgrowths occur frequently when the 2D growth rate increases. The rapid growth of certain edges is thus favoured over slower ones, when the growth rate is high. The spiral growth is observed in the as-grown MoSe₂ crystals on adding NaCl with amounts ≥1.3 mg at a growth temperature of 750 °C. The considerable increase in growth rate when adding salt promotes the formation of edge dislocations. As growth advances around the screw dislocation, the exposed edges trace the corners of the underlying layer edges. This leads to the formation of the spirals.

4. Conclusions

The controlled growth of diverse MoSe₂ nanostructures synthesised using an APCVD was systematically investigated, with NaCl serving as a seed promoter. Key CVD growth parameters, including the optimal NaCl concentration, growth temperature, and chalcogen precursor amount, were optimised to grow unique morphologies such as nanoribbons, dendrites, snowflakes, and spirals. A morphological mapping of a clearly defined parameter space is proposed to grow the desired nanostructures. This study not only establishes precise conditions for the reproducible synthesis of these unique structures but also provides insights into their growth mechanisms. This comprehensive study can be extended to the controlled CVD growth of various other TMDC nanostructures to be used in various next-generation applications.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We would like to express our sincere gratitude to Dr Krishnendu Chatterjee for his invaluable contribution to the development of the QRYSTAL CVD. We sincerely thank Mr Sandeep Kumar for his assistance in synthesizing CVD MoSe₂. Additionally, we extend our heartfelt thanks to Dr O. P. Thakur, Solid State Physics Laboratory (SSPL), DRDO, Timarpur, Delhi, for granting us access to their microscopy facility and to Dr Akhilesh Pandey, SSPL, DRDO, for providing the AFM facility. We thank Garima Gupta (SSPL, DRDO) for acquiring AFM data. We thank the SIF facility, BITS Pilani, and Pilani Campus, for providing FESEM, XPS and Raman facilities.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ma00121h

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