Formation and photoluminescence of one-dimensional SiO_x dot array–ZnO nanobelt heterostructures

Abstract

We designed and successfully synthesized novel SiO_x/ZnO heterostructures via one-step thermal reaction route. Orbicular SiO_x dots regularly and alternately arrange in the middle of the up/down surfaces of a nanobelt to form a one-dimensional SiO_x dot array, respectively, by their self-assembly. A high-resolution transmission electron microscopy study and a selected area electron diffraction pattern revealed that ZnO nanobelts are crystalline while SiO_x dots are amorphous. The formation process of such heterostructure is discussed in terms of the Stranski–Krastanow growth mode of semiconductor quantum dots. Strong white luminescence was observed by bare eyes when the photoluminescence spectrum was measured.

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One-dimensional (1D) semiconductor heterostructures have attracted considerable attention in contrast to their more limited single component counterparts because they have demonstrated great potential for fundamental research on the effect of dimensionality and size on optical and electrical properties, as well as for the electronic and optoelectronic nanodevice applications.^1–4 Several groups have achieved 1D semiconductor heterostructures by combining various functional materials, such as GaAs/GaP,⁵ Si/SiGe,⁶ InAs/InP,⁷ TiO₂/SnO₂,⁸ Si/Ge,⁹ GaN/ZnO,¹⁰ Si/CdSe,¹¹ Si/ZnSe,⁴ SiO_x/Al₂O₃,¹² and ZnO/ZnS.¹³

Silicon oxide (SiO_x) is a promising fluorescent material, which might be extensively applied in the integrated photoelectric circuit as optical devices.^14,15 As is known, the luminescence of SiO_x was strongly related to its intrinsic structural defects and extrinsic environment influences introduced during sample preparation.¹⁶ However, its shortcoming is instability.¹⁷ Zhang et al.¹⁸ recently discovered that this problem can be solved by heterostructures. On the other hand, ZnO is a wide band-gap (3.37 eV) semiconductor material with thermal and chemical stability, larger dielectric constant (8.75), and a large exciton binding energy (60 meV), which stabilizes the exciton even at room temperature. Thus, ZnO is an ideal candidate material for heterostructures. In addition, the fabrication of high-quality p-type ZnO is very difficult due to the existence of self-compensation effect which hinders ZnO devices from developing, although the research on ZnO materials has made an important progress in material fabrication in the past few years.¹⁹ As is known, formation of hybrid- or hetero-structures can obtain expectant material properties, which are not easily achievable with either material alone. Lastly, both fabricating ZnO materials at lower temperature (500–800 °C) and a higher wet-chemical etching rate make the integration of ZnO with Si technology highly feasible, and SiOx has been used as the principal material for Si-based technology. Based on considering the above three points, we designed a novel SiO_x/ZnO heterostructure. So far, reports on the 1D SiOx/ZnO heterostructures are very limited.²⁰ In this paper, we reported that novel SiO_x/ZnO heterostructures were successfully synthesized via the one-step thermal reaction route. The SiO_x/ZnO heterostructure is composed of a ZnO nanobelt and many orbicular SiO_x dots, which regularly and alternately arrange in the middle of the up/down surfaces of the ZnO nanobelt to form a 1D dot array, respectively, by their self-assembly. The formation mechanism of the heterostructures is discussed in terms of the Stranski–Krastanow growth mode of semiconductor quantum dots. This new structure provides a new process for forming various heterostructures that could have an interesting application in optoelectronics.

The novel SiO_x/ZnO heterostructures were synthesized by chemical vapor deposition. The mixture of ZnO (1.0 g) and trace amounts of In and Ga was loaded into one end of an alumina boat and Si substrates coated with Au were placed at the downstream of the precursor. The boat was placed inside an alumina tube that was inserted into a horizontal furnace and the precursor was located at the center of the furnace. The furnace was heated to a preset temperature (1500 °C) under a flow of 30 ml min⁻¹ of argon gas and a pressure of 400 Pa. The furnace temperature was kept for 10 min, and then, naturally cooled down to room temperature. The synthesized products were found covering the entire substrate. They were characterized by X-ray powder diffraction (XRD, Rigaku RU-300 with Cu Kα radiation), field-emission scanning electron microscopy (SEM, S-4800, Hitachi), and high-resolution transmission electron microscopy (HRTEM, Philips Tecnai 20), equipped with energy dispersive X-ray spectrometer. The room-temperature photoluminescence (PL) spectrum was measured using a micro-Raman spectrometer(J-Y, HR800, France) in a backscattering configuration, employing the 325 nm line of an He–Cd laser as the excitation source.

Fig. 1 shows a representative X-ray diffraction pattern of the as-synthesized products. All diffraction peaks which can be indexed to the hexagonal wurtzite ZnO (JCPDS file No.05-0664.) within the experimental error, indicating that it is a ZnO nanostructure. Fig. 2a shows a representative SEM image of the ZnO nanostructures, which reveals that a peapod-like ZnO nanostructure is formed on the Si substrate. The peapod-like ZnO nanostructures have several tens of micrometers in length and 2–3 µm in width. The SEM image is further magnified, as shown in Fig. 2b. The peapod-like ZnO nanostructure consists of a nanobelt (pod) and many black orbicular dots (peas). Each nanobelt has a substantially even width and thickness throughout their lengths. Interestingly, these black orbicular dots decorate the nanobelt and are regularly and alternately interspersed in the middle of the up and down surfaces of the nanobelt to form a 1D dot array, respectively (Fig. 2c). The surface of the nanobelts is rough and there are some steps to be clearly observed on it. The diameter of the dots is about 1∼2.5 µm.

Fig. 1 XRD pattern of the as-synthesized SiO_x/ZnO heterostructures.

Fig. 2 SEM images of the as-synthesized SiO_x/ZnO heterostructures. (a) Low-magnification SEM image. (b) Enlarged SEM image. A smaller SiO_x dot is marked by an arrow head. (c) A side view SEM image of a SiO_x/ZnO heterostructure. SiO_x dots regularly and alternately arrange in the middle of the up/down surfaces of the nanobelt to form the 1D SiO_x dot array, respectively, by their self-assembly.

The micro-structures of the nanobelt and black dots were studied by TEM. Fig. 3a and b show the low magnification TEM images of peapod-like nanostructures. They are a bright-field and a dark-field TEM images recorded from the different nanostructures, respectively. The dots are clearly observed. Energy-disperse X-ray (EDX) microanalyses are also used to determine the chemical composition at the bare nanobelt part and the dot part (two selected areas as marked in Fig. 3b) of the nanostructures, and the results are shown in Fig. 3c and d. It is worthy of note that the nanobelt is composed of only Zn and O, and no other elements such as Si were detected, indicating that it is pure ZnO; and all the dots are composed of Si, O, and Zn (the Zn signal comes from the nanobelt below the dots), indicating that they were SiO_x. SA slected area electron diffraction (SAED) pattern (Fig. 3e) of the ZnO nanobelt (as marked in Fig. 3b) shows that the nanobelt has a single crystalline wurtzite internal structure and grows along the <101̄0> direction, in good agreement with the XRD results. HRTEM was employed to confirm the crystal structure of peapod-like heterostructures. Fig. 3g shows a typical HRTEM image taken from the end of the nanobelt along the <0001> zone axis. It displays clear lattice fringes, and the lattice spacing of 0.28 nm is consistent with the d values of {101̄0} planes. The ZnO nanobelt grows along the <101̄0> direction, being consistent with the SAED result. Based on the HRTEM study (Fig. 3g) and SAED pattern (Fig. 3e), the up surface of the nanobelt should be (0001) or (0001̄) crystal plane, and the side surface is a (112̄0) or (1̄1̄20) crystal plane. The SAED pattern (Fig. 3f) of the orbicular SiO_x dots, consisting of diffuse halo rings, indicates that SiO_x is amorphous.

Fig. 3 (a) A bright-field TEM image. A smaller dot is marked by an arrow head. (b) A dark-field TEM image. The letter A represents the bare ZnO nanobelt part while B does the SiO_x dot part; (c) and (d) EDX spectra of parts A and B, respectively; (e) and (f) SAED patterns of parts A and B, respectively; g) a HRTEM image taken from the end of a ZnO nanobelt.

The formation of the 1D SiO_x dot array–ZnO nanobelt heterostructures could result from a two-step growth process, on the basis of the information we have gathered. In the first step, since there is no center of inversion in the hexagonal crystal structure, an inherent asymmetry along the c-axis is present, which allows an anisotropic crystal growth along the [0001] direction. Under thermodynamic equilibrium conditions, the surface energy of the ±(0001) planes is higher than those of {11̄00} and {112̄0} planes, 1D nanowire along the c-axis growth can be easily obtained. However, such thermodynamic equilibrium state could be broken once by external conditions, such as the incorporation of impurities, which will change the surface energy of the as-synthesized materials. In our experiment, ZnO nanobelts are formed on the Si substrate (Fig. 4a), and grow along the <101̄0>direction, because the precursor contains trace amounts of metal as catalyst. In the second step, the formation of 1D SiO_x dot array on the ZnO nanobelt could be explained by the Stranski–Krastanow growth mode. Si in the substrates close to the precursor has a high chemical activity due to high temperature (1300∼1500 °C) and reacts with oxygen to form SiO_x, which may be expressed as follows:

Si(g) + O₂(g) → SiO_x(g)

subsequently, SiO_x vapor is transported downstream, and deposits on the surfaces of ZnO nanobelts to form a SiO_x thin layer (Fig. 4b). The interface strain between SiO_x and ZnO increases with the increase of the SiO_x thickness. When the SiO_x thin layer grows over some critical thickness, the transition from two-dimensional to three-dimensional growth will occur in order to relax the strain, leading to the formation of SiO_x quantum dots (Fig. 4c). The strain relaxation proceeds in a few minutes and the formation processes of the quantum dots almost terminate. Subsequently, the Ostwald ripening²¹ processes dominate the evolution of the quantum dots. The larger quantum dots adsorb smaller quantum dots, and then grow gradually into the micro-dots, leading to dots of uniform size and shape. However, there are still some small size dots to be observed in our experiment, as marked in Fig. 2b and 3a. We propose that the Stranski–Krastanow growth mode dominates the whole growth process of the SiO_x micro-dots. In fact, the observations of trace amounts of smaller dots and the steps on the surface of ZnO nanobelts, which could be the residual SiO_x track, support our conclusion to some extent. Finally, these SiO_x dots regularly and alternately arrange in the middle of the up/down surfaces of the ZnO nanobelt to form the 1D dot array, respectively, by their self-assembly (Fig. 4d), being subject to the minimum principle of the surface energy. As a matter of fact, there are many such things in nature, for example, the branches of many plants alternately growing along their stems.

Fig. 4 Schematic showing the formation process of SiO_x/ZnO heterostructures: (a) ZnO nanobelt; (b) a SiO_x thin layer formed on the ZnO nanobelt; (c) appearance of SiO_x quantum dots; (d) SiO_x micro-dots regularly and alternately arrange in the middle of up/down surfaces of a nanobelt to form 1D SiO_x dot array, respectively, by Ostwald ripening.

In order to investigate the optical properties of the SiO_x/ZnO heterostructures, their photoluminescence spectrum was measured at room temperature (black line in Fig. 5). A broad peak centered at 2.33 eV appears in the PL spectrum of the heterostructures. The observed wavelength covers the whole visible region from 400 to 700 nm. The strong white luminescence was observed by bare eyes when measuring. In comparison, the PL spectrum (red line) of the pure ZnO nanowires is shown in Fig. 5. It consists of a visible emission peak and an ultraviolet emission peak. The visible emission of the ZnO nanowires is observed at 2.53 eV, which belongs only to the green light emission. Thus, we deduce that the emission band centered at 2.33 eV originates from the SiO_x dot array rather than ZnO nanobelts and the luminescence centers are attributed to the native defects of SiO_x,²² which are formed during the heterostructure preparation. In addition, the near-band edge (NBE) emission centered at about 3.20 eV from the ZnO was not observed in the heterostructures. We think there are two possible reasons: (1) the visible light emission from the SiO_x native defects is much stronger than the NBE emission from ZnO. Furthermore, the visible emission band is very wide, maybe merge it; (2) the surface is very rough from the TEM observation, implying that more surface defect states are formed and become the local states. These defects capture a number of the excited electrons, leading to a big decrease of NBE emission intensity, even disappearance.²³

Fig. 5 Normalized PL spectra of the SiO_x/ZnO heterostructures (black line) and pure ZnO nanowires (red line).

In conclusion, we have synthesized novel SiO_x/ZnO heterostructures. The formation process of the heterostructures has been investigated, and is ascribed to the interface strain between SiO_x and ZnO. The growth model is in good agreement with the Stranski–Krastanow growth mode of semiconductor quantum dots. A strong white light emission of the heterostructures from the native defects of SiOx was observed. This new structure provides a new process for investigating optoelectronics of SiOx/semiconductor heterostructures.

Acknowledgements

This work was partially supported by the National Natural Science Foundation of China under Grant No. 60776010; the Science Foundation for Distinguished Young Scholars of Heilongjiang Province (JC200805); the Natural Science Foundation of Heilongjiang (A2007-03, A200807 and F200828); the Education Bureau of Heilongjiang Province (11531225 and 11531227); the Project of Overseas Talents, Personnel Bureau, Heilongjiang Province; and the Excellent Leader of Subjects, Bureau of Science and Technology of Harbin, Heilongjiang Province (2007RFXXG028).

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