Great blue-shift of luminescence of ZnO nanoparticle array constructed from ZnO quantum dots
© Wang et al; licensee Springer. 2011
Received: 11 March 2011
Accepted: 14 April 2011
Published: 14 April 2011
ZnO nanoparticle array has been fabricated on the Si substrate by a simple thermal chemical vapor transport and condensation without any metal catalysts. This ZnO nanoparticles array is constructed from ZnO quantum dots (QDs), and half-embedded in the amorphous silicon oxide layer on the surface of the Si substrate. The cathodoluminescence measurements showed that there is a pronounced blue-shift of luminescence comparable to those of the bulk counterpart, which is suggested to originate from ZnO QDs with small size where the quantum confinement effect can work well. The fabrication mechanism of the ZnO nanoparticle array constructed from ZnO QDs was proposed, in which the immiscible-like interaction between ZnO nuclei and Si surface play a key role in the ZnO QDs cluster formation. These investigations showed the fabricated nanostructure has potential applications in ultraviolet emitters.
Recently, ZnO has attracted very great attention because of its particular properties in broad fields. For example, it has a large direct band gap of 3.37 eV and exciton-binding energy of 60 meV, while the Bohr radius of exciton is as small as approx. 2.34 nm. Thus, ZnO is a promising candidate for the high efficient ultraviolet (UV) laser device [1–3]. Interestingly, when the size of ZnO nanoparticles is smaller than the Bohr radius (i.e., ZnO quantum dots, QDS), the quantum confinement has a notable influence on the band gap and further causes a series of novel characteristics such as the blue-shift of luminescence [4–7]. Therefore, there have been a variety of techniques to fabricate ZnO QDs [6–10]. Usually, the size of ZnO QDs is slightly larger than or just comparable with the exciton Bohr radius [8–13]. However, few research reports have been involved in the ZnO QDs, showing that their dimension is rigorously smaller than the Bohr radius [4–13].
In this study, we have fabricated the unique ZnO nanoparticle arrays that are constructed from ZnO QDs blocks on silicon substrates using a simple thermal chemical vapor transport and condensation without any metal catalysts. Importantly, we measure a great blue-shift of luminescence in the cathodoluminescence (CL) spectrum of the fabricated nanostructure, which implies that this ZnO QDs structure would be applicable to optoelectronic and spintronic applications.
The ZnO nanoparticle array is fabricated by a simple thermal vapor transport method, and the detailed experimental process has been reported in our previous study . Simply, Si wafers serving as substrates are loaded downstream in a quartz tube. Zinc oxide powders and graphite powders are mixed and heated to 1050°C under the argon gas flow at the rate of 50 sccm with a pressure of 9.0 × 104 Pa. Half-an-hour later, the source powders and the substrate are all taken out from the furnace and allowed to cool down to room temperature naturally. Field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM) coupled with electron-energy loss spectroscopy (EELS) are employed to characterize the morphologies and structures of the prepared samples. The CL measurement is carried out at room temperature using a Gatan Mono-CL system coupled to FESEM with the accelerating voltage of 10 kV.
Results and discussion
The EELS spectra of the Zn-L, O-K, and Si-L edges on the particle zone of the sample exhibited in Figure 2c,d,e show that the ZnO nanoparticles contain Zn, O, and Si elements. The sets of Zn-L edge with the peak centered at 1050 eV and the O-K edge with the feature peak at 538 eV demonstrate that the nanoparticles are zinc oxide, in accordance with reports and the analytic results shown above, while the spectra shift due to the native defects, such as Zn and O vacancies on the surface of ZnO QDs [14–17]. As we see the Si-L edge in Figure 2e, the distinct features are at 100, 107, 114, 127, and 157 eV, respectively. This Si-L edge is very similar with the spectrum of silicon monoxide that is overlapped by spectra of elemental silicon and of SiO2 whose onsets of the L2,3-edge are approx. 100 and 107 eV, respectively [18–23]. Thus, these results reveal that the ZnO QDs disperse in the silicon monoxide.
In summary, we have fabricated the ZnO nanoparticle array which is constructed from ZnO QDs on the Si substrate by the thermal chemical vapor transport and condensation without any metal catalysts. This fabricated ZnO nanostructure exhibited a great blue-shift of luminescence in the CL spectrum. These novel properties show that the ZnO nanoparticle array has potential applications in UV emitters.
electron-energy loss spectroscopy
field emission scanning electron microscopy
transmission electron microscopy
This study was supported by the NSFC (U0734004) and the Ministry of Education.
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