1. Introduction

Zinc oxide (ZnO), an important II-VI semiconductor with a direct wide band gap (3.37 eV) and a large exciton binding energy (60 meV at room temperature) [1], has become one of the key technological materials of considerable interest due to its biosafety and biocompatibility [2] and its wide applications including piezoelectric devices [3], gas sensors [4], photocatalyst [5], light-emitting diodes [6], photodetectors [7], optical modulator [8], laser diodes with waveguide structure [9], and solar cells [10]. Previously, hexagonal cylindrical ZnO nanowires have been examined as a waveguide due to the transverse nanoscale and longitudinal microscale dimensions [11]. Well-defined faceting nature of such nanostructures enable the observation of unique optical confinement [12]. However, in classical waveguides, different transverse optical modes can be sustained within waveguides of different cross-section [13]. In our previous research, we obtained a pearl-like ZnO NW with a hybrid structure, where a crystal structure transformation occurred [14]. It is thus interesting to examine the propagation of light within ZnO nanowires with cross-sections other than hexagon.

In this letter, we would pay a close attention to the propagation of light within pearl-like nanowires with hybrid structures. Pearl-like ZnO NW was obtained through an electrical breakdown method by increasing longitudinal bias beyond the toleration of individual ZnO NWs and polycrystalline structure was demonstrated in the pearl part. Room temperature Raman and photoluminescence (PL) were performed to compare the physical properties of different structure. Multi zone light emission was observed from the pearl parts and a possible mechanism based on finite element analysis was proposed.

2. Experimental sections

Our experiments are based on ZnO NWs synthesized through a conventional chemical vapor deposition method [15]. After cooling down to room temperature, samples were dispersed into ethanol and then dripped on Si substrate covered with a 300 nm thick SiO₂. Pearl-like ZnO NWs were realized inside a scanning electron microscope (JEOL 6490) equipped with a nanomanipulator module (Zyvex S100). The crystal structure was certified by high-resolution transmission electron microscope (HRTEM; JEM 2100). We conducted selected-area Raman spectra experiments (He-Ne laser 634 nm) and selected-area PL spectra experiments (He-Cd laser 325 nm) at room temperature in the atmosphere by employing Jobin-Yvon HR800, by focusing laser on the pearl or on the pole. Electromagnetic field distributions in the two-dimensional cross section were simulated by finite element method.

3. Results and discussion

Pearl-like ZnO NWs were achieved through an electrical breakdown method described as follows. For an individual ZnO NW, two terminal I-V measurements were conducted by applying a longitudinal bias. When the bias reached the threshold value, pearl-like ZnO NW was obtained with a decline of current in the I-V curve, as illustrated in Fig. 1 . The SEM images in the figure compared the morphologies of the samples before (left) and after (right) the failure process. Polycrystalline ZnO was demonstrated in the pearl parts while the pole part maintained single crystal structure, as illustrated in HRTEM images. A more detail characterization of crystal structure was given in [14].

 

Fig. 1 I-V curve recorded during the nanodamage process, insert SEM and HRTEM images compare the morphology and crystal structures before and after the failure process.

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Figure 2 shows the position of the E₂ (high) mode for pearl and pole parts. The E₂ (high) = 437 cm⁻¹ mode of the single-crystal ZnO in pole is at almost the same position as the ZnO standard [16]. However, for polycrystalline ZnO in pearl, a Raman shift of 1 cm⁻¹ had taken place. Lo et al have observed Raman shift with ZnO morphologies, owing to the tensile stress. An increase in the E₂ phonon frequency is related to compressive stress, whereas a decrease in the E₂ photon frequency is ascribed to tensile stress [17]. For single crystal ZnO, as a wurtizite-type crystal, there is the competition between the long-range electrostatic forces and the short-range forces, due to anisotropy in the interatomic force constants on the vibrational spectrum while polycrystalline ZnO reveals isotropy [18]. Therefore, the presence of different in-plain stress/strain in the single crystal and polycrystal is believed to contribute to the Raman shift. The smaller differences in peak position probably represent real differences between single crystal ZnO and polycrystalline ZnO, which tends to confirm the view that the phonon spectrum of the crystal is largely determined by near-neighbor interactions [19].

 

Fig. 2 Enlarged room temperature Raman spectra at E₂ (high) for pearl and pole parts.

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To determine the variation of optical properties of pearl-like ZnO NWs, the room temperature PL spectra were collected by focusing laser on the pearl and pole, respectively, as illustrated in Fig. 3(a) . Inset is the investigated pearl-like ZnO NWs. Generally, the PL spectrum of typical single crystal ZnO nanostructures comprises the weak UV emission band and visible emission band. The visible emission band corresponds to a singly ionized oxygen vacancy in ZnO and results from the recombination of a photogenerated hole with the single ionized charge state of this defect. For the single crystal pole in this experiment, we only observed a strong peak at 375 nm, which indicates that these columnar NWs are free of oxygen vacancy. However, in the polycrystalline pearl, the imperfect boundaries and stacking faults may help the surface state to trap the impurities such as O₂, as the test is conducted in the atmosphere. Therefore, the weak emission band of the pearl at around 500~600 nm is attributed to the irradiative recombination of a delocalized electron close to the conduction band [17,20]. Furthermore, the band edge emission from polycrystalline ZnO was positioned at 3.25 eV with a redshift of 60 meV compared with the band edge emission of single crystal ZnO located at 3.31 eV. According to previous reports, the emission spectra of ZnO NWs beyond the quantum confinement region (on the base of uncertainty principle: ∆x≈5 nm for ZnO) showed a blueshift in the energy region with a decrease in the diameter of the nanorods, which was associated with the surface resonance effect caused by the enhanced surface to volume ratio [21]. However, redshift in the energy positions was observed in the emission spectra for polycrystalline ZnO with a smaller surface to volume ratio due to the spherical structure. The redshift in the room temperature PL of polycrystalline could be ascribed to various reasons: (i) different in-plain stress/strain in the single crystal and polycrystal [22], (ii) excitonic emissions and their phonon replicas [23], and (iii) laser heating effect [24]. The PL measurements were conducted under similar conditions for single crystal and polycrystal. Therefore, laser heating effect was not considered to be a cause because the laser power was kept very low (<2 mW) during the PL measurements. Ahn et al [25] and Gu et al [26] have demonstrated that the redshift in the energy position of the band edge emission in the room temperature is not related to an in-plain stress/strain, but associated with the different contributions of the excitonic emission and their phonon replicas which could be influenced by the surface states. Furthermore, the intrinsic ZnO NWs are free of oxygen vacancy as confirmed in the PL spectra and the impurities trapped by imperfect boundaries and stacking faults in polycrystal may modulate the surface state. Therefore, the redshift of PL peak position may be due to the different contributions of excitonic emissions and their phonon replicas. Room temperature PL images of the columnar ZnO NW (top) and the pearl-like ZnO NW (bottom) are shown in Fig. 3(b), by focusing laser on one end of the wires, respectively. Light emission in the pearl regions (marked by white circles) as well as the end was observed in the pearl-like NW, while only emission from the end is collected in the columnar NW. Yang et al have observed the UV emission from both ends of the ZnO ribbon and almost no signal can be detected from the side surfaces, a clear indication of wave guiding of UV emission by the nanoribbon itself, which is also demonstrated in smooth NW [27]. However, as shown in our experiment, strong light emission was also observed from the polycrystalline pearl part within the pearl-like NW, which was associated with crystal structure [28].

 

Fig. 3 (a) Room temperature PL spectra of single crystal and polycrystalline ZnO. Inset: the sample, the red and blue circles indicate the test location. (b) Photoluminescence images of columnar (top) and pearl-like ZnO (bottom) nanowires dispersed on a SiO_X/Si substrate, respectively. White circles point out the emission parts except the end.

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To explore the propagation of light within pearl-like nanowires with hybrid structures, we introduce a finite element analysis on the electromagnetic field distribution in the two-dimensional cross section of single crystal and polycrystalline ZnO waveguides, respectively, as illustrated in Fig. 4 . The confinement of light in components with nanoscale cross-sections significantly enhances the magnitude of the optical experienced by these components. Optical forces can be divided into two major categories: radiation pressure and transverse gradient forces. Radiation pressure effects can be understood as momentum exchange between photons and matter, so the force acts along the light propagation direction. The gradient force acts transversely to the propagation direction of the light and enables, for example, optical tweezing in free-space optics [29]. Because of the isotropic in the polycrystalline, the electromagnetic field reveals a concentric circle distribution, meaning the light could propagate in all directions (for the two-dimensional cross section, the light could propagate along radial directions). While in the single crystalline, the different dielectric constants along c axis and a axis for hexagonal ZnO single crystal lead to the confinement of light along the c axis [11,12]. For the nanowire hybrid structures, due to the light propagation in all direction in the polycrystalline parts, we could observe the multi-zone emission in the pearl-like NWs in Fig. 3(b). In addition, due to the biosafety and biocompatibility of ZnO materials [2], we consider that in their present form these nanowire hybrid structures could be exploited as optical nanobarcodes, which could be useful as labels for imaging [30]. Moreover, III-Nitride semiconductors, particularly InGaN and AlGaN based alloys, have the similar wurtzite crystal structure as ZnO [31,32]. Therefore, the similar concept may be applicable in InGaN and AlGaN based materials, which need a further work.

 

Fig. 4 Electromagnetic field distributions in the two-dimensional cross section of single crystalline and polycrystalline ZnO waveguides.

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4. Conclusion

This paper presents observation of multi-zone light emission in pearl-like ZnO nanowire hybrid structure waveguide. For polycrystalline parts, a Raman shift of 1 cm-1 (E₂(high)) had taken place compared with the single crystal ZnO, which is attributed to the presence of different in-plain stress/strain in the single crystal and polycrystalline. The imperfect boundaries and stacking faults in polycrystalline ZnO may contribute to the weak emission band at around 500~600 nm and the redshift of PL peak position. Furthermore, with a finite element analysis, we found the propagation of light within pearl-like nanowire hybrid structures is related to the different dielectric constants between single crystal and polycrystal. Strong emission of polycrystalline pearl parts in the pearl-like ZnO NW in room temperature PL image could be attributed to the light propagation in all direction in the polycrystalline parts. These findings provide a novel multi zone light emission structures and could be exploited as optical nanobarcodes for imaging.