Letter to the EditorSelective synthesis of SiO2 NWs on Si substrate and their adjustable photoluminescence
Introduction
One dimensional semiconductor nanostructures (SCNS) have aroused intense research interests due to their excellent properties and potential applications in optical-electronic device manufactories. A large number of nanostructures, taking nanorodes (NRs) and nanowires (NWs) for an example, of ZnO, GaN, Si and silica, have been synthesized over the past decade year. The vapor-liquid-solid (VLS) mechanism has been developed to explain the growth mechanism of these nanostructures. Meanwhile, some novel nanostructures, taking ZnO for an example, are grown on carbon nanotubes by thermal evaporation [1]. Owing great deal to their unique properties, nowadays, semiconductor nanowires are extensively being investigated as components for future integrated circuits (IC). A variety of electronic device prototypes have also been realized using NWs, such as field-effect transistors [2] and photodiodes to biosensors. Although, the SCNS can drastically reduce the cost and size and probably improve the device efficiency, these devices cannot easily be compatible with modern VLSI processes.
Amorphous SiO2 coating has been widely used in IC manufactory because of its excellent insulation property. Recently, it is accepted more and more that SiO2 nanostructures, such as nanoparticles and NWs, are stable luminescent phosphors [3]. The potentials of developing silicon-based nanostructures in optoelectronics device manufactories have been predicted from the significant discoveries on the luminescent performance in amorphous SiO2 nanostructures [4]. So far, the SiO2 NWs have been prepared by several approaches including thermal evaporation [5], [6], laser ablation [7], [8], sol-gel [9] and chemical vapor deposition [10]. Meanwhile, the SiO2 NWs exhibit complicated photoluminescence properties. For instance, SiO2 NWs formed with the assistance of Mg [11] as catalyst have three peaks at 380 nm, 414 nm and 434 nm. Yet, SiO2 nanowires prepared with the assistance of Ni [12] as catalyst have a UV band ranged from 350 nm to 420 nm, and those synthesized by thermal evaporation [6] without any catalyst show a UV band centered at 400 nm with two additional green emission bands at 510 nm and 560 nm, respectively. These properties present the brilliant to the future micro-optoelectronic device, but the mechanism of these peaks is still divergent, and needs further study. For instance, there is an argument that whether these photoluminescence emissions intensively depend on the SiOx defects, such as neutral oxygen vacancies (O≡Si–Si≡O), intrinsic diamagnetic defect centers (Si–O–Si), nonbonding oxygen hole centers (≡ Si–O), impurity atoms and interface state [13], [14], [15]. In addition, it is reported that the hydrogen was used industrially to dangling silicon bonds or interface traps [16]. Although the photoluminescence property has been reported in the literatures, the photoluminescence efficiency is still not high enough for practical application.
Hereby, we report a simple yet efficient method to selectively grow amorphous SiO2 NWs by annealing the ultra-thin Au film coated silicon wafers found by accident. A growth model based on an extended vapor-liquid-solid (VLS) mechanism is discussed. Meanwhile, it will be worth pointing out that the photoluminescence performance can be tunable by changing the growth conditions, and the mechanism of it is also further discussed according to the experiment conditions.
Section snippets
Experimental procedure
The experimental procedure is described bellow: Firstly, the n-Si (111) wafers were cleaned for 5 min by chemical etching of HF (5%) and then rinsing for about 15 min in a sonicating bath of acetone, ethanol and deionized water, respectively. Secondly, a thermal oxidation process was carried out at 1100 °C to form a silicon oxide film of 500 nm in thickness on the Si wafers, and then SiO2 film was patterned by means of conventional photolithography process. Thirdly, an ultra-thin Au film, 5 nm in
Results
Fig. 1 shows the typical SEM images of SiO2 NWs grown on the silicon wafer. The low magnification SEM image (Fig. 1(a)) displays the SiO2 NWs selectively grown on the surface where the Au film was coated. The SiO2 NWs growth matches well with the photo pattern in high fidelity. Fig. 1(b) and (c) shows SEM images of the sample at a higher magnification to reveal more details. The SiO2 NWs are of smooth surface, and no nano-sized metal particles are observed at the tip of the SiO2 NWs. According
Discussion
According to the results presented in the former section, we propose the schematically process for SiO2 NWs formation, which is depicted in Fig. 4(a)–(c). Fig. 4(a) shows a preparative stage of the substrate. In the initial stage, some molten Au may shrink into nano-size small drops and bigger spheres, as shown in Fig. 4(b). It should be noted that we do not have a gaseous Si precursor to maintain the growth of the SiO2 NWs, so the Si wafer is the only Si source. The Au–Si phase diagram shows a
Conclusion
In conclusion, SiO2 NWs have been selectively synthesized through a simple annealing process with silicon wafers coated an ultra-thin Au film. The SiO2 NWs have diameters of 20 ~ 200 nm, and lengths of up to several micrometers. The SiO2 NWs show the blue emission band ranging from 460 nm to 480 nm due to SiOx clusters, and the PL intensity can be adjusted by changing the oxygen.
Acknowledgements
The authors would like to show our strong thanks to Mrs. Hui Wang, and the analytical testing center of Sichuan University for the SEM characterization.
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