Hierarchical Cd4SiS6/SiO2 Heterostructure Nanowire Arrays
© to the authors 2009
Received: 1 August 2009
Accepted: 14 October 2009
Published: 29 October 2009
Novel hierarchical Cd4SiS6/SiO2 based heterostructure nanowire arrays were fabricated on silicon substrates by a one-step thermal evaporation of CdS powder. The as-grown products were characterized using scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. Studies reveal that a typical hierarchical Cd4SiS6/SiO2 heterostructure nanowire is composed of a single crystalline Cd4SiS6 nanowire core sheathed with amorphous SiO2 sheath. Furthermore, secondary nanostructures of SiO2 nanowires are highly dense grown on the primary Cd4SiS6 core-SiO2 sheath nanowires and formed hierarchical Cd4SiS6/SiO2 based heterostructure nanowire arrays which stand vertically on silicon substrates. The possible growth mechanism of hierarchical Cd4SiS6/SiO2 heterostructure nanowire arrays is proposed. The optical properties of hierarchical Cd4SiS6/SiO2 heterostructure nanowire arrays are investigated using Raman and Photoluminescence spectroscopy.
One-dimensional (1D) nanostructures with modulated compositions have recently become of particular interest with respect to potential applications in nanoscale building blocks of future optoelectronic devices and systems [1–3]. Among them, core/shell(or sheath) nanostructure materials can prevent oxidation of semiconductor 1D nanostructures and thus forestall interference in the building blocks of complex nanoscale circuits [4, 5]. Therefore, core/shell(or sheath)structures, such as nanocables, have attracted recent research focused on nanodevice applications such as field-effect transistors (FET) and light-emitting diodes (LED) [6, 7]. Very recently, significant progress has been made on the synthesis of complex 1D core/shell(or sheath) nanostructures such as Zn/ZnS core/shell (or sheath) fibers , TiO2/SiO2 nanocables , ZnS/SiO2 core/shell(or sheath)nanowires [10, 11], and ZnO/SiO2 core/shell nanorods . Among these reports, most SiO2 based 1D core/sheath (or core/shell) nanostructures were synthesized via complicated methods, such as high-temperature thermal evaporation of mixed materials  or two-step process .
However, the extensive reports of 1D core/shell(or sheath)structure growth have revealed several challenges that remain before these tools may practically be applied to commercial or industrial needs . The first challenge is how to fabricate long nanostructures that can be easily integrated and manipulated post-synthesis through simple methods. The second challenge is how to organize the long nanostructures in ordered or aligned patterns and in high densities. The third challenge is how to form SiO2 protective layer which is coated on 1D semiconductor nanostructures forms core/shell structures in order to protect the active layer in nanoelectronic circuits and obtain the noteworthy properties of the corresponding nanodevices [13, 14].
Cd4SiS6, a wide band gap semiconductor (E. g = 2.50 eV at 300 K), is a potential material used in electroluminescent devices (ELD) as its emission of visible light. To date, only few works have been successfully completed on Cd–Si–S system. For example, Odin et al.  have reported the cathodoluminescence property of Cd4SiS6 crystal. Zhan et al.  have documented the synthesis and microstructure of Cd4SiS6/Si composite nanowires. In this paper, we report on the fabrication, structure, and properties of hierarchical Cd4SiS6/SiO2 heterostructure nanowire arrays via a simple one-step thermal evaporation of CdS powder. The present work may be a rational route for meeting the earlier mentioned three obstacles.
The as-grown hierarchical Cd4SiS6/SiO2 heterostructure nanowire arrays were synthesized using a high-temperature vacuum-tube furnace, as described in detail elsewhere . Briefly, CdS powder (1.5 g) was placed on an alumina boat in the center region of a quartz tube. Silicon substrates that were covered with Au thin film were placed in downstream to collect the products in the quartz tube. The tube was then pumped down to a base pressure of ~2×10−2 Torr (1 Torr ≈ 133 Pa). Argon gas was introduced into the tube at a constant flow rate of 200 sccm. The total pressure was kept at 2 × 10−2 Torr during the experimental process. The furnace was maintained at 900 °C for two hours before it was cooled to room temperature. A large yield of light yellow product was deposited on the silicon substrates.
The morphology of the samples was examined by scanning electron microscopy (SEM, FEI SIRION 200). The crystallographic structures of the as-grown product were investigated by X-ray diffraction (XRD, RIGAKU D/Max-2550), transmission electron microscopy (TEM, JEOL JEM-2010, operated at 200 kV), and high-resolution TEM (HRTEM, JEOL JEM-2010F, operated at 200 kV) equipped with an energy dispersive X-ray spectrometer (EDS) The Raman spectrum was recorded on Raman spectrometer (NEXUS-670) with 514.5 nm radiations at room temperature. Photoluminescence spectroscopy was performed using the Xe lamp spectrophotometer (970CRT) at 350-nm line.
Results and Discussions
Based on the earlier mentioned experimental result, we conducted the experiment with different parameters, that is, changed the position of silicon substrate, but kept the other parameters identical. And we obtained hierarchical of Cd4SiS6/SiO2 heterostructure nanowire arrays.
In summary, hierarchical Cd4SiS6/SiO2 heterostructure nanowires arrays have been prepared on silicon substrates via simple thermal evaporation with CdS powder. Studies indicate that the hierarchical Cd4SiS6/SiO2 nanowires heterostructure arrays consist of a single crystalline Cd4SiS6 core coated amorphous SiO2 and adjacent amorphous SiO2 nanowires. The formation process of the hierarchical nanowires arrays was also discussed on the basis of thermodynamic aspect. The properties of hierarchical Cd4SiS6/SiO2 heterostructure nanowires were characterized by RS and PL. The Raman peaks can be assigned to the modes of Cd–S and Si–S. In PL spectra, the peak at 483 nm can be identified as the near-band-edge emission of Cd4SiS6 cores. The long and uniform Cd4SiS6/SiO2 heterostructure nanowires arrays may be applied as potential building blocks in nanodevices.
This work was supported by the National Natural Science Foundation of China under Grant No. 20671018, 10775031, and 10835004.
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