Growth temperature dependence of VLS-grown ultra-long ZnS nanowires prepared by CVD method
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The synthesis of ultra-long high-quality zinc sulfide (ZnS) nanowires of uniform size on heterogeneous substrates is highly desirable for investigating the fundamental properties of ZnS nanowires and for fabricating integrated functional nanodevices. The present study developed a novel technique for growing ultra-long ZnS nanowires on thin-catalyst-coated substrates. ZnS nanowires were synthesized by chemical vapor deposition on a silicon substrate deposited by gold (~ 5 nm in thickness) as the catalyst at 550, 600, 700, 750 and 800 °C. The structural properties of the samples were investigated by X-ray diffraction, and the results showed that the fabricated nanowires have both wurtzite and zinc blend structures. The morphological properties of the nanowires were determined by scanning electron microscopy, and the results show that the substrate is thoroughly coated with 10 µm of zinc sulfide nanowires. Increasing the substrate temperature from 600 to 800 °C increased the diameter of the nanowires and decreased the length. The growth mechanism of the nanowires was vapor–liquid–solid. The EDX spectra of this sample showed an absence of contamination, confirming the high purity of the ZnS nanowires.
KeywordsZinc sulfide (ZnS) Nanowire Chemical vapor deposition Morphology
Semiconductor nanostructures have attracted great interest for studding the quantum confinement effect and thermodynamic structures of these materials [1, 2, 3]. Varieties of physical properties such as mechanical strength, plasticity , melting [5, 6],sintering and alloying ability , diffusivity , chemical reactivity , as well as the mode of crystal growth (self-assembly) , have been found dependent upon particle size. Zinc sulfide (ZnS) with a wide direct optical band gap (Eg ~ 3.72 eV) for cubic zinc blend (ZB) and Eg ~ 3.77 eV for hexagonal wurtzite (WS) phase is an important II–VI semiconductor with a number of interesting properties. Nanostructured zinc sulfide materials have been intensively studied due to their wide potential applications in high-performance devices, such as optoelectronics, sensors, transducers and biomedical sciences . As a one-dimensional nanostructure, ZnS has been synthesized as nanowires, nanobelts and nanocombs . One of the most attracting features of nanowires (NWs) is that lattice mismatch or strain in NWs can be significantly relaxed due to their high surface/volume ratio and small lateral size. Different synthetic approaches have been used for the synthesis of ZnS nanomaterial, especially nanowires and nanorods; these methods include laser ablation , vapor transport in the presence  or in the absence of  catalyst, chemical vapor deposition (CVD) , electrochemical deposition  and solvothermal method . Among them, CVD has got a number of advantages such as simplicity and the ability to use a wide variety of precursors such as liquid precursors, gaseous precursors, solid precursors which include halides, hybrids, metal–organic and organic. It also enables the usage of a variety of substrates and allows materials growth in a variety of forms, such as powder, thin or thick films aligned or entangled straight or coiled nanotubes. It also offers better control on the growth process of materials . CVD allows proper control on the deposition rate and pressure temperature so as to prepare and maintain the desired structure, composition and size of the materials. Growth of nanowires via the vapor–liquid–solid (VLS) mechanism is an effective method, in which the diameter and length of nanowires are also affected by other growth conditions such as the growth ratio and temperature. . In ZnS nanowire preparation by VLS, a thin layer of Au is used as the catalyst by thermal evaporation of ZnS powder . However, separate evaporation of sulfur and zinc powder for ZnS nanowire preparation, which is more controllable by sulfur activity, is rarely reported. In the present work, we have preferred this route of synthesis and successfully prepared very long ZnS nanowires.
After closing the rotary pump, the tube was filled with high-purity Ar gas to eliminate any O2 in the tube, and the furnace was turned on to heat up. When the furnace reached the process temperature (950 °C), the terminal opposite to the gas flow system opened and was kept open during the process to allow the gas to flow out at a rate of 50 SCCM (sample A) and 80 SCCM (sample B). The pressure inside the tube was increased to ~ 2×10−1 mb, and the Ar carrier gas was fed into it. The values for flow rate and dynamic vacuum were kept constant until the end of the experiment. The surface morphology of the nanowires was investigated by means of SEM. To investigate the chemical composition of the ZnS nanowires, elemental analysis of the samples was performed using energy-dispersive X-ray (EDX) spectroscopy.
Results and discussion
Three distinct stages existed in the catalyst-assisted growth of ZnS nanowires. In the first stage, the Au film-coated Si substrate is annealed at ~ 700 °C. The Au film aggregated to form small islands because of the liquid-like mobility of the Au atoms at high temperature. In the second stage, called alloying and nucleation, the islands were exposed to Zn vapor carried in Ar gas and reacted with the vapor to form an Au–Zn alloy of increased volume. At 950 °C, the vapor pressure of zinc was > 103 mbar, which was much greater than the sulfur vapor pressure at 140 °C of ~ 10−1 mbar and system pressure of 2 × 10−1 mbar.
Figure 2c is a sample prepared under the same conditions as in Fig. 2a with the addition of a substrate at 600 °C. This shows that the synthesized products consisted of a small quantity of nanowires with diameters of about 60–100 nm and a length of several µm along with nanocombs and tetrapodal nanostructures.
The chemical potential of the component in the metal alloy catalyst increased as the size of the catalyst decreased because of the Gibbs–Thomson effect. Dissolving a vapor component into a liquid alloy became increasingly difficult as the size decreased, making it difficult to reach a sufficient supersaturation state to induce the growth of nanowires. It is known that the growth of nanowires with diameters of more than > 10 nm is feasible; however, growing those < 10 nm in size is difficult because of the thermodynamic limitation associated with the use of a catalyst.
According to the VLS mechanism, the size of the Au nanoparticles plays a significant role in determining the length and diameter of the ZnS nanowires. The reports show that as the substrate temperature increases, the merging of Au nanoparticles increases . It appears that the increase in average diameter and the decrease in average length relate to the increase in the diameter of Au nanoparticles.
Diffraction peaks with 2θ values of 28.50, 47.50 and 56.30 related to the (111), (220) and (311) of the cubic phase of ZnS appeared. Moreover, XRD peaks at 2θ values of 26.90, 30.50, 39.6 0 and 51.7 0 related to reflection from the (100), (101), (102) and (103) planes of wurtzite ZnS also appeared. The peak at 2θ of 32.90 relates to the Bragg reflection from the (200) plane of the silicon substrate. Two other features at 2θ values of 38.20 and 44.40 relate, respectively, to the (111) and (002) planes of the Au catalyst.
The small difference in Gibbs free energy  facilitates the phase transformation and is very sensitive to the reaction parameters. As the substrate temperature was quite high, both phases of the ZnS nanostructures were created in the mixture. The II–VI NWs entwined with the mixed ZB/WS structures can be commonly observed along the (111) direction, such as ZnS,  ZnSe  and ZnTe . It is known that the cubic to hexagonal phase transformation takes place at a temperature of < 1020 °C, which is the cubic–hexagonal phase transition temperature of bulk ZnS.
A novel successive multistep growth method was developed to synthesize very long ZnS nanowires of several hundred µm in length. A systematic study was carried out to determine the optimal parameters affecting the dimensions of the nanowires synthesized on a large scale. Ultra-long ZnS nanowires were grown successfully through thermal evaporation of zinc and sulfur powder at 600–800 and 140 °C, respectively. The ZnS nanowires were grown with a mixture of zinc blend and wurtzite structures. In VLS deposition, adjusting the synthesis conditions controls the formation of ZnS nanostructures as nanoflowers, nanowires and nanorods. Increasing the substrate temperature from 600 to 800 °C increases the nanowire diameter and decreases their length. The abundance of wurtzite structures in the mixture increased as the substrate temperature increased. XRD data analysis [as per the ratio of (100)–(002) peaks at different substrate temperatures] confirms that a cubic to hexagonal phase transformation has taken place at a lower temperature than that for bulk ZnS.
Authors would like to acknowledge from Imam Khomeini international university for their financial help.
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