Horizontal growth of MoS₂ nanowires by chemical vapour deposition

Abstract

We describe a single step route for the synthesis of MoS₂ wires using a chemical vapour deposition (CVD) method. By tuning the CVD growth parameters, the horizontally oriented MoS₂ nanowires on SiO₂/Si substrate can be synthesized successfully. The MoS₂ nanowire has height of about 93 nm and width of about 402 nm with multilayer structure. Good local photoluminescence (PL) properties can be observed for these horizontal MoS₂ nanowires. The successful fabrication and prominent PL effect of the horizontal MoS₂ nanowires provide potential applications for the MoS₂-based in planar devices.

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Introduction

Extensive attention has been paid to the field of nanofabrication due to its huge number of potential applications in future nanodevice technology. Two major concerns which need to be addressed for practical applications are the synthesis of useful nanomaterials and the assembly of the synthesized nanostructures on the substrate with desired orientations.

On the other hand, semiconductor nanowires have attracted scientific and technological interest owing to their demonstrated potential for high-performance nanoscale devices such as transistors,¹ sensors,² solar cells,³ and thermoelectric systems.⁴ The ultra-thin bodies of nanowires allow excellent electrostatic control which is required for high performance electronic devices. The one-dimensional (1D) morphology of nanowires enables straightforward implementation of semicylindrical gate or gate-all-around architectures. Wafer-scale integration with current planar processing technology remains a challenge for the commonly grown out-of-plane nanowire geometry. Ex situ assembly methods are required to transfer and coarsely align the nanowires in plane on a functional device substrate.^5,6 Therefore, in order to extend the device integrations and viable applications of nanowires, deterministic fabrication method for horizontal growth of nanowires is also required for planar devices.⁷

MoS₂, a typical and the most abundant semiconductor materials, owns a structure of S–Mo–S stacking layers, weakly bounded by van der Waals interactions. The band structure of MoS₂ varies with the number of layer. With the layer number decreasing, the lowest band in the conduction band moves upward, increasing the overall bandgap,⁸ making it advantageous compared to graphene-based logic device applications.⁹ Moreover, the optical properties of layered materials are strongly dependent on morphology and quality of pristine materials.^10,11 Efforts have been made in developing reliable and flexible approaches for obtaining thin layer MoS₂ (ref. 12–14) or 1D nanostructures of MoS₂ such as nanotubes,¹⁵ nanoribbons¹⁶ and nanorods.¹⁷ In this paper, a simple and precise method to synthesize horizontal growth of MoS₂ nanowires has been proposed, which can provide new opportunities for further widespread applications of MoS₂-based in planar devices.

Experimental

In this paper, we report horizontal growth of MoS₂ nanowires on SiO₂/Si substrate by chemical vapour deposition (CVD), as shown in Fig. 1. Briefly, a two temperature zone tube furnace was used to provide accurate temperature control of the solid MoO₃ and sulfur (S), respectively. An alumina boat with S powders (99.9% purity) was placed upstream in the low-temperature zone. Another alumina boat containing MoO₃ (99.9% purity) was placed downstream in the high-temperature zone. A piece of Si wafer with 300 nm SiO₂ layer substrate was placed face down to the MoO₃ powder. The distance between these two alumina boats is 19 cm. The temperature of S and the substrate was increased concurrently to 200 °C and 750 °C in the Ar flow atmosphere, respectively. The deposition process was conducted by evaporating S and MoO₃ powders simultaneously with the stoichiometric S to Mo ratio of 4 : 1 in Ar flow rate of 30 sccm. After 1 minute, the furnace was cooled down naturally to room temperature. The sample structure was examined by using a Rigaku X-ray diffractometer (XRD) with Cu Kα radiation. The morphology of the MoS₂ nanowires was examined by using Park system atomic force microscopy (AFM). Raman spectra were collected by a Horiba Jobin Yvon Raman microscopic system. The solid-state excitation laser has a wavelength of 532 nm. A 100× objective was used to focus the laser beam. The photoluminescence (PL) measurements were also performed with the same laser by using the PL mode of the Raman microscopic system.

Fig. 1 Schematic illustration of the CVD system for MoS₂ nanowires fabrication.

Results and discussion

The optical image of MoS₂ nanowires on the SiO₂/Si substrate synthesized by this CVD system with pure Ar carrier gas flow rate of 30 sccm is presented in Fig. 2(a). The darker-contrast nanowires on the surface signify the MoS₂ nanowires, and the surrounding regions correspond to the SiO₂/Si substrate. A lot of nanowires are clearly visible over the surface of the substrate. Fig. 2(b) illustrates the morphology of MoS₂ nanowire by AFM measurements. A well-defined wire-like structure of MoS₂ wire can be observed clearly. Fig. 2(c) is the cross sectional AFM images of MoS₂ wires on the SiO₂/Si substrate. The height profile of this MoS₂ nanowire is shown in Fig. 2(d). From the height profile, it can be found that the MoS₂ nanowire has the height of about ∼93 nm and width of about ∼402 nm. Our previous works also demonstrated that the physical properties and morphologies of MoS₂ can be controlled by adjusting the chemical environment during growth.^18,19 Here, the MoS₂ nanowires are synthesized by evaporating S and MoO₃ powders simultaneously with the stoichiometric S to Mo ratio of 4 : 1 in Ar flow rate of 30 sccm. The higher partial pressure of gaseous S greatly increases the volatilization speed of MoO₃, forming a large amount of gaseous intermediate MoO_3−x in a short time.²⁰ Therefore, the high S sufficient atmosphere may promote the mass transfer process, which contributes to the increase in the crystal growth rate. Under the S sufficient atmosphere condition, the MoS₂ crystals are more likely to grow under “kinetic” conditions rather than thermodynamic ones, which is typical for nanocrystals with high precursor feedstock. Therefore, the increase in precursor concentration has an effect of promoting the crystal growth rate, resulting in the formation of MoS₂ nanowire, which is not favorable for the production of high-quality two-dimensional crystal.^18,19

Fig. 2 (a) Optical microscope image for MoS₂ nanowires deposited on SiO₂/Si substrate; (b) and (c) a typical AFM image for MoS₂ nanowire; (d) height profile for MoS₂ nanowire.

In order to clarify the structure and quality of the MoS₂ nanowires, XRD and Raman spectroscopy measurements had been performed. Fig. 3(a) shows the XRD pattern of MoS₂ nanowires deposited on SiO₂/Si substrate. No diffraction peaks from impurities are observed in the XRD pattern. All the diffraction peaks in the pattern can be indexed as hexagonal-phase MoS₂. Moreover, the XRD pattern is matched against a simulated XRD pattern generated by using Java Electron Microscopy Simulation (JEMS) software.²¹ As shown in Fig. 3(b), the lattice constants of MoS₂ calculated from this XRD pattern are a = b = 3.17 Å and c = 12.36 Å. It shows an obvious increase in comparison with the standard values of MoS₂ (a = b = 3.16 Å, c = 12.29 Å). The results indicate that the distance between neighbouring MoS₂ layers increases and the in-plane structure of MoS₂ is slightly changed. The slight change in MoS₂ nanostructures might be associated with the defects formed in the growth of MoS₂ nanowires. Fig. 3(b) shows the Raman spectra of MoS₂ nanowires. It can be found that the two characteristic Raman vibration modes E_2g and A_1g can be seen in the spectra of MoS₂ nanowires. The E_2g mode is associated with the in-plane vibrations, i.e., two S atoms are displaced in one direction and the Mo atom is displaced in the opposite direction. The A_1g mode is related to the perpendicular vibrations, i.e., two S atoms are displaced in opposite directions while the Mo atom does not move. These two Raman modes, E_2g and A_1g, exhibit a sensitive layer thickness dependence,²² which provides convenient and reliable means for determining the layer thickness with atomic precision. Due to the alterations in the interlayer coupling,²³ the Raman frequency of the former decreases and that of the latter increases with the increasing thickness. It can be found that the in-plane E_2g and out-of-plane A_1g modes of vibration are at 384 cm⁻¹ and 408 cm⁻¹ for the MoS₂ nanowires on the SiO₂/Si substrate, respectively. A Δ of 24 cm⁻¹ between these two modes suggests the fabricated MoS₂ nanowires are the multilayers structure.

Fig. 3 (a) XRD patterns of MoS₂ nanowires; (b) schematic image of MoS₂ unit cells; (c) typical Raman spectra of MoS₂ nanowires.

Fig. 4 presents the PL spectrum for the MoS₂ nanowires with 532 nm laser source at room temperature. PL measurements were performed with the same laser excitation source and conditions as that for Raman spectroscopy. In contrast to the silent PL effect of the bulk MoS₂ indirect semiconductor, two strong peaks can be found in the PL spectrum for the MoS₂ nanowires, 684 nm and 631 nm. The PL emission of the MoS₂ nanorods could be related to the transition of PL emission process from trion recombination to exciton recombination, which is assisted with the defects in the materials.²⁴ Because the MoS₂ nanowires are synthesized in high S sufficient atmosphere with the stoichiometric S to Mo ratio of 4 : 1, under the S sufficient atmosphere condition, the MoS₂ crystals are more likely to grow under “kinetic” conditions rather than thermodynamic ones. In this case, instability may occur as atoms do not have enough time to move into the right lattice locations, where crystal domains could have the lowest surface free energy, and the probability of defect formation increases. Therefore, there could be many defects formed in the MoS₂ nanowires. The excitons localized at the defect sites generally have much larger binding energy,^25,26 which may suppress the thermally activated nonradiative recombination including defect trapping, and resulting in the observed PL properties.

Fig. 4 Typical PL spectrum of MoS₂ nanowires.

Conclusion

It is meaningful to develop a sample and precise method to synthesize horizontal MoS₂ nanowires for planar device application. In this paper, we report horizontal growth of MoS₂ nanowires on SiO₂/Si substrate by CVD. The MoS₂ nanowire has height of about 93 nm and width of about 402 nm. Good local PL properties of MoS₂ nanowires can be observed. The successful fabrication and prominent PL effect of the horizontal MoS₂ nanowires provide potential applications for the MoS₂-based in planar devices.

Acknowledgements

This work is supported by National Natural Science Foundation of China (Grant Nos 11164008, 51461019, 51361013, 11174226, and 51371129), the Project for Young Scientist Training of Jiangxi Province (20153BCB23016) and the Natural Science Foundation of Jiangxi Province (20151BAB202004).

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