Structural and photoluminescence studies on catalytic growth of silicon/zinc oxide heterostructure nanowires
© Chong et al.; licensee Springer. 2013
Received: 23 January 2013
Accepted: 2 April 2013
Published: 17 April 2013
Silicon/zinc oxide (Si/ZnO) core-shell nanowires (NWs) were prepared on a p-type Si(111) substrate using a two-step growth process. First, indium seed-coated Si NWs (In/Si NWs) were synthesized using a plasma-assisted hot-wire chemical vapor deposition technique. This was then followed by the growth of a ZnO nanostructure shell layer using a vapor transport and condensation method. By varying the ZnO growth time from 0.5 to 2 h, different morphologies of ZnO nanostructures, such as ZnO nanoparticles, ZnO shell layer, and ZnO nanorods were grown on the In/Si NWs. The In seeds were believed to act as centers to attract the ZnO molecule vapors, further inducing the lateral growth of ZnO nanorods from the Si/ZnO core-shell NWs via a vapor-liquid-solid mechanism. The ZnO nanorods had a tendency to grow in the direction of  as indicated by X-ray diffraction and high resolution transmission electron microscopy analyses. We showed that the Si/ZnO core-shell NWs exhibit a broad visible emission ranging from 400 to 750 nm due to the combination of emissions from oxygen vacancies in ZnO and In2O3 structures and nanocrystallite Si on the Si NWs. The hierarchical growth of straight ZnO nanorods on the core-shell NWs eventually reduced the defect (green) emission and enhanced the near band edge (ultraviolet) emission of the ZnO.
One-dimensional semiconductor nanowires (NWs) have gained tremendous attention owing to their unique optical and electrical properties, which can be applied in nanophotonics and nanoelectronics [1, 2]. Among the semiconductor NWs, silicon (Si) and zinc oxide (ZnO) NWs are leading in numerous energy-related applications, especially in the fields of optics [3, 4], photovoltaic [5, 6], and field emission [7, 8]. Si exhibits an indirect band gap of 1.12 eV, which prevents it from emitting visible light. However, nanocrystalline Si as well as Si NWs can produce red emission due to the quantum confinement effect [9, 10]. This makes them applicable in photonics . ZnO nanorods (NRs) are also known to exhibit efficient ultraviolet (UV) and visible green emissions at room temperature . The UV emission is attributed to the near band edge emission of ZnO [12, 13] (Eg approximately 3.37 eV), while the green emission is generally known to be a defect emission due to oxygen vacancies or oxide antisite in ZnO NRs [14–16].
The combination of Si NWs and ZnO nanostructures to form nanoparticle (NP)-decorated core-shell and branched hierarchical NWs could significantly improve the shortcomings of each individual Si or ZnO nanostructures. One interesting approach is to obtain white emission by combining the different emission regions of both Si and ZnO nanostructures. A flat and broad range of visible light emission ranging from approximately 450 to 800 nm were independently demonstrated using a porous Si/ZnO core-shell NWs  and ZnO/amorphous Si core-shell NWs . Meanwhile, tunable photoluminescence (PL) from visible green to UV emission can be achieved by varying the thickness of SiO2 layer for ZnO/SiO2 core-shell NRs . Another example is the enhancement of the electron field emission properties, where an extremely low turn-on field <1 V/μm and field enhancement factor of approximately 104 were obtained from an ultrathin ZnO film (approximately 9 nm) coated Si nanopillar arrays . Similar field enhancement results were also obtained by several groups using ZnO NP-decorated Si NWs  and ZnO NWs/Si nanoporous pillar arrays .
To date, there are several studies using different techniques in regards to the synthesis of the heterostructured Si/ZnO core-shell NWs and hierarchical NWs [17, 20–27]. In general, the growth of Si NWs core and ZnO nanostructures shell was carried out by means of a two-step deposition. Most of the studies focused on the top-down method to fabricate Si NW arrays via a dry reactive etching [20, 23] and a wet metal-assisted etching [17, 21, 22, 24–27] techniques. It is important to note that this method of producing Si NWs is usually accompanied by surface defects and impurity issues [28, 29]. The Si/ZnO core-shell NWs can be formed by the settling of a ZnO layer on the Si NWs using atomic layer deposition [20, 21, 24], pulsed laser deposition , or metal-organic chemical vapor deposition . Meanwhile, three-dimensional hierarchical NWs with Si wire/NWs core and ZnO NRs branches were successfully grown using vapor transport and condensation [22, 25] or hydrothermal growth [26, 27] methods. In order to form the hierarchical heterostructured NWs, the interspacing between Si NW cores must be large enough (in other words, the density of Si NWs on the substrate must be low enough) to provide enough space for the lateral growth of ZnO NRs from the Si NWs. In this particular case, chemical vapor deposition method is a better approach to obtain the Si NWs array due to its capability of producing NWs with lower density and larger gaps compared to the metal-assisted etching method .
In this work, we present a study on the growth of ZnO nanostructures on Si NWs using an In catalyst. Tapered Si NW arrays were first synthesized by following a vapor-liquid-solid (VLS) mechanism using In catalyst and a hot-wire chemical vapor deposition . In seeds were then coated on the as-grown Si NWs using the same system. This was followed by the synthesis of ZnO nanostructures using vapor transport and condensation. The method was carried out by way of a thermal evaporation of graphite-mixed ZnO powder . The ZnO nanostructures formed at different growth time were then studied. Structural, compositional, and optical properties of the as-grown samples were characterized using field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), energy dispersive X-ray (EDX), X-ray diffraction (XRD), and PL spectroscopy methods.
Si NWs were synthesized on a p-type Si(111) substrate using a home-built plasma-assisted hot-wire chemical vapor deposition system . In catalysts with sizes ranging from 40 to 100 nm were coated on the substrate prior to the synthesis of Si NWs. Silane gas diluted in hydrogen (H2) gas in a ratio of 1:20 (5:100 sccm) was used as the Si source for the growth of Si NWs. The details of the deposition process and parameters have been previously described [31, 34–37]. The as-grown Si NWs were first coated with a layer of In seeds using the same system. Next, 1.3 ± 0.1 mg of In wire was hung on a tungsten filament 3 cm above the Si NWs substrate. The In wire was evaporated at filament temperature of approximately 1,200°C under a hydrogen plasma environment to produce nano-sized In seeds . The H2 flow rate and rf power of the plasma were fixed at 100 sccm and 40 W, respectively. The In seed-coated Si NWs (In/Si NWs) substrate was then transferred into a quartz tube furnace for the ZnO nanostructures deposition.
ZnO nanostructures were deposited onto the In/Si NWs via a vapor transport and condensation process. A mixture of ZnO and graphite (1:1) powders with a total weight of approximately 0.2 g was placed at the hot zone center of the quartz tube. One end of the quartz tube was sealed and connected to N2 gas inlet, while the other end remained open. The In/Si NWs substrate was then inserted through the open end and placed at approximately 12 cm from the evaporation source. The mixed powder was heated to approximately 1,100°C at different durations starting from 0.5 to 2 h. The sample substrates placed downstream of the quartz tube resulted in a gradient temperature change of 600 to 500°C from the center towards the opened end.
Morphologies of the samples were observed from a Hitachi SU 8000 FESEM (Chiyoda-ku, Japan). An EDAX Apollo XL SDD detector EDX spectroscopy (Mahwah, NJ, USA) attached to the FESEM was utilized for the composition analysis of the samples. TEM and HRTEM micrographs as well as the fast Fourier transform (FFT) electron diffraction patterns of the samples were studied using a JEOL JEM 2100F HRTEM (Akishima-shi, Japan). A SIEMENS D5000 X-ray diffractometer (Munich, Germany) was used to obtain the XRD pattern of the samples. The measurements were performed at a grazing angle of 5°. PL spectra were recorded using a Renishaw InVia PL/Raman spectrometer (Wotton-under-Edge, UK) under an excitation He-Cd laser source of 325 nm.
Results and discussion
The initial growth stage of the ZnO NRs can be observed from the FESEM and TEM micrographs (Additional file 1: Figure S1). Catalyst particles can be clearly seen on the tip of the ZnO NRs (white circles in Additional file 1: Figure S1a). This suggests that a VLS growth mechanism was involved in the growth of ZnO NRs [39, 40]. The observed large variation of the ZnO NR lengths (Figure 2c,d) is also indicative of a catalytic growth process for the ZnO NRs. Due to the different sizes of the In catalyst seeds, the nucleation time as well as the growth rate of the ZnO NRs can vary . Thus, in this case, In seeds have two roles: first is to act as a center to attract vaporized molecules/atoms to form the ZnO shell layer covering the Si NWs, and second is to catalyze the growth of ZnO NRs when the amount of ZnO reaches a certain critical point. Similar to tin (Sn), In is one of the rare materials which forms alloy with Zn and exists at low eutectic temperature of approximately 150°C at 3% of Zn . Several studies have revealed that Sn could catalyze the growth of ZnO NRs via a VLS growth mechanism [43, 44]. Our results showed that In carried out the same role as well. A lattice-resolved HRTEM image was taken at the interface ZnO and In structures as shown in Additional file 1: Figure S2. In contrast to the single crystalline structure of ZnO NR, the In seed showed an amorphous structure. This could be due to the incorporation of oxygen and Zn elements into the In seeds, thus forming Zn-doped In2O3 structure during the ZnO deposition process .
The green defect emission is normally observed for the ZnO nanostructures in addition to the near band edge emission. Although the origin of the green emission remains questionable, it is generally attributed to the transition of donor-acceptor pair related to the oxygen vacancies [14–16, 50–52]. A number of studies suggested that the green emission is a surface-related process, where the radiative recombination of electrons with photogenerated holes trapped in singly ionized oxygen vacancies takes place in the surface structures of the ZnO [51, 52]. In other words, this defect emission can be enhanced due to the large surface area of ZnO nanostructures under oxygen deficient conditions. Moreover, covering the surface of the ZnO nanostructures with surfactant or dielectric layers will eventually reduce or suppress the defect emission [53, 54]. These findings correlate well with the results from our study. The high intensity of green to UV emission (approximately seven times) could be a feature of the defective states created by large quantities of ZnO NPs formed on the In/Si NWs. Only a minute increase in the green to UV peak intensity ratio was observed due to the volume expansion of the ZnO NPs by increasing the ZnO growth time from 0.5 to 1 h. The great increase in the surface area of ZnO by the hierarchical growth of ZnO NRs from the core-shell NWs resulted in the development of the green emission. Similar observation was reported by Wang et al.  in the comparison of PL properties of hierarchical grown ZnO NWs with ZnO NWs. Furthermore, our initial growth of ZnO NRs shows significant amount of kinking and bending structures. This indicates that there is a certain number of defect structures due to the nonstoichiometric (oxygen or zinc vacancies) ZnO which could be responsible for the defect emission.
Conversely, a reduction in the defect emission in conjunction with enhancement in the near band edge emission was also observed by further increasing the ZnO growth time to 2 h. The FESEM and TEM results showed the highly c-axis-oriented straight (no kinking) ZnO NRs growing from the core-shell NWs. The reduction of the defect emission can thus be explained by the improvement in the ZnO crystal lattices which minimizes the defect states of oxygen vacancies in ZnO. It is commonly known that the enhancement in the ZnO near band edge emission could be related to the size effect  and/or crystalline structure quality  of the ZnO NRs. Larger size of the ZnO NRs (diameter ≥70 nm) is always required to provide enough recombination center for the strong near band edge emission . This is relevant to our case, where longer ZnO growth time increases the condensation of ZnO molecules, thus forming large sizes of ZnO NRs. According to our experiment, the branches of ZnO NRs with a diameter approximately 45 ± 13 nm and lengths of approximately 400 nm to 1 μm are sufficient for the enhancement in the near band edge emission. The UV emission peak of ZnO (centered at approximately 380 nm) was fitted using a Gaussian function to study the relation of PL peak width with the ZnO growth time. Full width at half maximum (FWHM) of the ZnO near band edge emission peak reduced from approximately 27 to 20 nm with the increase in ZnO growth time. The reduction in the FWHM value of the UV emission peak indicates an improvement in the crystalline structures of the ZnO NRs formed at longer growth time, which is consistent with the observations from XRD and HRTEM. This can be partly due to the annealing effect of the sample while increasing the ZnO growth time.
The growth of ZnO nanostructures on In/Si NWs was studied using a vapor transport and condensation method. The results showed that a controllable morphology of ZnO nanostructures from ZnO NPs decorated to core-shell and hierarchical core-shell NWs can be achieved by controlling the condensation time of the ZnO vapors. The ZnO NRs which were hierarchically grown on the In/Si NWs were produced using In as a catalyst. XRD and HRTEM results indicated that the ZnO NPs had a tendency to be in (100) and (101) crystal planes, while the ZnO NRs on the Si/ZnO NWs advance along the  direction. The Si/ZnO core-shell NWs revealed a broad range of PL at spectral range of 400 to 750 nm due to the combined emission of nanocrystallite Si, oxygen deficiency in In2O3 and oxygen-related defects in ZnO. Further, the growth of ZnO NRs from the core-shell NWs suppressed those defect emissions and enhanced the near band edge emission of ZnO.
This work was supported by the UM/MOHE High Impact Research Grant Allocation of F000006-21001, the Fundamental Research Grant Scheme (FRGS) of KPT1058-2012 and the University Malaya Research Grant (UMRG) of RG205-11AFR.
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