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We report the fabrication and nanoscale optical characteristics of polychromatic ZnO/CdxZn1-xO composite nanorods prepared by simple hydrothermal and sol-gel chemical methods. Hydrothermally grown ~300 nm diameter and ~3.5 µm long ZnO nanorods were coated, using the sol-gel method, with a thin CdxZn1-xO layer having a spatially varying Cd mole fraction, where x ranged from x = 0 to 1. Full control of the emission color, including white emission, was achieved by simply varying the local Cd mole fraction along the single ZnO/CdxZn1-xO nanorod. The continuous variation of the optical band gap energy along the single nanorod was visualized using nanoscale confocal absorption spectral imaging.
© 2014 Optical Society of America
1. Introduction
One-dimensional semiconductor nanostructures, such as rods, wires, belts, and tubes, have attracted considerable attention due to their unique optical and electronic properties, which are potentially useful in electronic and photonic applications [1–3]. ZnO is a widely used functional material with a wide direct band gap, a large exciton binding energy, and excellent chemical and thermal stability [4]. Due to these unique properties, its potential applications include nanogenerators [5,6], nanowire field effect transistors [7], piezo-electric diodes [8], and chemical sensors [9]. Among the various physical and chemical methods, the hydrothermal method is an inexpensive and environmentally friendly technique suitable for the growth of ZnO nanostructures, especially at low growth temperatures (80-100 °C) [10,11].
Reliable UV emission from ZnO-based device structures has been previously demonstrated both at low temperatures and at room temperature [12–14]. However, for practical applications, it is important to develop an efficient technique for tuning the emission wavelength by engineering the band gap of the nanomaterial. Band gap engineering is of cardinal importance in a multitude of applications where a specific band gap value is needed [15], for example in laser diodes, solar cells, heterojunction bipolar transistors, and many more. Tuning of the band gap can be achieved by one or more of the following methods [15]: (a) varying the chemical composition such as doping or alloying, (b) size confinement on the nanometer scale. The band gap tuning of ZnO through the addition of dopants has been extensively described in the literature [13,14,16–20]. Doping is important for band gap tuning of ZnO, however, several issues remain to be clarified, such as the possibility of doping nanostructures through a cost-effective and efficient process to tune their chemical and physical properties by incorporating dopants into the ZnO lattice. A critical element for the development of ZnO based optical devices is the ability to tune its band gap through alloying, leading to the formation of both energy barriers and quantum wells that unlock the route toward more effective quantum confinement and increased internal quantum efficiency. Moreover, the ability to tune the ZnO band gap would enable light emission over a broad spectrum, i.e., from the deep UV to the visible regime, which greatly increases the perspective applications for this material system.
MgxZn1-xO compounds with band gaps of over 6 eV [21,22] have already been developed by alloying with dielectric MgO which has a room temperature band gap of 7.9 eV [22]. Among the materials used to reduce the band gap of ZnO, the ternary semiconductor CdxZn1-xO, an alloy of ZnO with CdO, is most promising [23–25] for the following reasons: (1) Cd and Zn ions have similar ionic radii of ~0.74 and 0.64 Å, respectively, and (2) CdO has a direct band gap of ~2.3 eV at room temperature, ~1.0 eV narrower than the direct band gap of ZnO [26]. However, studies of the band gap engineering of ZnO by Cd have been limited to single color emission [27–30]. Multi-color-emission organic microtubes and CdSSe nanowires were reported [31,32], however, the organic microtubes and inorganic nanowires show only one or two color emission in the visible range, and multi-color-emission nanostructures required very high laser pump power using nonlinear materials [33].
Nanoscale optical characterization has enabled the visualization of the novel emission properties of light emitting nanomaterials on the single nano object level [34,35], allowing the characterization of their optical and structural properties and therefore control over their emission characteristics. In addition to nanoscale PL and Raman spectral imaging, confocal absorption spectral mapping has recently been used to probe the local absorption spectra of single nanomaterials, for example, by detecting the local crystal orientation of organic single rubrene nanoplates [36].
In this study, we fabricated polychromatic ZnO/CdxZn1-xO composite nanorods using simple hydrothermal and sol-gel chemical methods. Fine tuning of the emission color throughout the visible wavelength range was achieved by controlling the mole fractions of Cd and Zn in the ZnO/CdxZn1-xO composite nanorod. White light emission, achieved by laterally varying the local mole fraction of Cd along a single nanorod, was demonstrated to show the fine tailorability of local emission color in a single ZnO/CdxZn1-xO composite nanorod. The continuous variation of the local optical band gap along the single white-emitting ZnO/CdxZn1-xO composite nanorod was visualized by a nanoscale confocal absorption spectral imaging technique.
2. Experimental details
Undoped ZnO nanorods were grown on a Si substrate using the known hydrothermal method [37]. For ZnO seed layer, sol-gel precursor solution was prepared by dissolving 0.1 M zinc acetate dihydrate [Zn(CH3COO)22H2O] in 0.1 M 2-methoxyethanol [CH3OCH2CH2OH] solvent. For the growth of the ZnO nanorods on the ZnO seed layer by hydrothermal method, an aqueous solution containing 0.01 M zinc nitrate hexahydrate [Zn(NO3)26H2O] and 0.01 M hexamethylenetetramine (HMT) [C6H12N4] was prepared. The ZnO nanorods were grown at 95 °C for 12 h and then treated with UV light (LDLS Eq. (-99)FC, ENERGETIQ Corp.) for 2 h to convert the surface from hydrophobic to super-hydrophilic [38,39].
We used the spin-coating method to prepare ZnO/CdxZn1-xO composite nanorods by coating the surface of the ZnO nanorods with CdxZn1-xO sol solution with x ranging from 0.25 to 1. We controlled the Cd mole fraction, x, by varying the amounts of the cadmium acetate dihydrate [Cd(CH3COO)22H2O] and the zinc acetate dihydrate in CdxZn1-xO sol solution. By using the limited amount of sol solution in the spin-coating process and due to the hydrophilicity of the ZnO nanorod surface, ZnO nanorods can be only partially coated by CdxZn1-xO layers starting from the bottom, and the length (height) of CdxZn1-xO coating on the ZnO nanorod can be controlled by the sol solution amount of CdxZn1-xO used in spin-coating.
For the polychromatic ZnO/CdxZn1-xO composite nanorods having increasing Cd mole fraction from the top to bottom of the nanorod, first the sol solution of 40 µl Cd0.25Zn0.75O was spin-coated onto the super-hydrophilic surfaces of the standing ZnO nanorod array. Subsequently, the sol solutions of 30 µl Cd0.5Zn0.5O, 20 µl Cd0.75Zn0.25O, and 10 µl CdO were spin-coated on the same sample. Between each spin-coating process, 300 °C heating treatment for 10 mins. were conducted for curing of the CdxZn1-xO layer. During the spin-coating, the samples were constantly irradiated with UV light to maintain the super-hydrophilicity of the ZnO nanorod surface. Spin-coating and the subsequent curing process were repeated three times for each CdxZn1-xO solution. For the final crystallization, the ZnO/CdxZn1-xO composite composite nanorods underwent a post-heat treatment in a furnace at ambient 420 °C for 1 h. Samples were dispersed on a glass substrate (thickness of ~0.17 mm) for individual inspection using optical microscopes and electron microscopes.
For the confocal PL spectroscopy measurements, a lab-made laser confocal microscope with a spectrometer was used. With oil emulsion objective lens with a 1.3 NA, the focused spot diameter of the laser light was approximately 300 nm. Scattered light was collected using the same objective and guided to a 30 cm long monochromator equipped with a cooled CCD through an optical fiber with a 100 µm core diameter, which acted as a confocal detection pinhole. The excitation lasers were the 355 nm laser line of a diode-pumped solid-state laser and a 405 nm semiconductor laser, with a typical laser power applied to the sample of 500 µW and an acquisition time of 5 ms per pixel. Nanoscale confocal absorption spectral imaging was conducted using the same system with a tungsten-halogen lamp as the light source. The experimental details of the confocal absorption spectral imaging are explained in the supplementary document of the previous paper [36].
3. Results and discussion
Figures 1(a) and 1(b) show field-emission scanning electron microscopy (FE-SEM) images of the pure ZnO nanorods on a Si substrate grown by the hydrothermal method. The length of the ZnO nanorods was about 5 µm and the diameter ranged from 150 nm to 350 nm. Figures 1(c) and 1(d) show FE-SEM images of dispersed single polychromatic ZnO/CdxZn1-xO composite nanorod on glass substrate at low- and high-magnification, respectively. We note that a thin layer of CdxZn1-xO coats the ZnO nanorod due to super-hydrophilic surface caused by UV illumination. It was found that the UV illumination of the sample during the sol-gel process and the subsequent spin-coating process greatly improved the uniformity of the sol-gel thin films [40]. On the other hand, the CdxZn1-xO layer was only coated top of the ZnO nanorod array without UV illumination because of their hydrophobic surface (result not shown here).
Fig. 1 (a) Top-view and (b) cross-section view FE-SEM images of ZnO nanorods on a Si substrate grown by the hydrothermal method. (c) Low- and (d) high-magnification FE-SEM images of a dispersed single ZnO/CdxZn1-xO nanorod. (e) Schematic diagram of polychromatic ZnO/CdxZn1-xO composite nanorods with spatially varying Cd mole fraction. (f) Typical PL spectrum of the ZnO nanorods.
Figure 1(e) shows the fabrication scheme of the multi-color emitting ZnO/CdxZn1-xO composite nanorods made by coating CdZnO on ZnO nanorods. We controlled the local mole fraction of Cd and achieved the emission color of our choice or by using continuously varying Cd mole fraction along the nanorod we obtained multi-color-light and white-light emission. Figure 1(f) shows a representative PL spectrum of bare ZnO nanorods with 355 nm laser wavelength excitation. There were two emission peaks in the PL spectrum of the bare ZnO nanorods. The PL emission at 390 nm is explained by a near-band-edge transition of ZnO, and the broad PL band at 600 nm is attributed to the defects, such as interstitial defects and vacancies of oxygen in the ZnO crystals [41,42].
Figures 2(a) to 2(d) show fluorescence images of the various color emission of ZnO/CdxZn1-xO composite nanorods, in which the ZnO nanorods are half coated with CdxZn1-xO with x = 0.25, 0.5, 0.75, and 1. The fluorescence images of ZnO/CdxZn1-xO composite nanorods clearly exhibit variations in emission color according to the Cd mole fraction used in the sol-gel spin-coating process. Figure 2(e) shows representative PL spectra obtained from each ZnO/CdxZn1-xO composite nanorod at the position identified by the white arrow, excited with a 405 nm laser wavelength. The spectra show that the peak wavelength of the ZnO/CdxZn1-xO composite nanorods changes from 460 to 660 nm as the Cd mole fraction increases from x = 0.25 to 1, spanning the entire visible wavelength range.
Fig. 2 Fluorescence image of a half-coated emission ZnO/CdxZn1-xO composite nanorod with different Cd mole fraction, x: (a) x = 0.25, (b) x = 0.5, (c) x = 0.75, and (d) x = 1. (e) PL spectrum of the half-coated emission ZnO/CdxZn1-xO composite nanorod with different Cd mole fraction: (a) x = 0.25, (b) x = 0.5, (c) x = 0.75, and (d) x = 1.
To demonstrate the tailorability of the local Cd mole fraction and thus the local emission color, we fabricated white light-emitting ZnO/CdxZn1-xO composite nanorods that emit different colors of PL along the nanorod, resulting in the overall emission of white light. We changed the Cd mole fraction during the sol-gel process and thus coated CdxZn1-xO layer on ZnO nanorods with x varying along the nanorod. We varied the local Cd composition so that it increased from x = 0, 0.25, 0.5, 0.75 to 1 along the nanorods. An energy- dispersive spectroscopy (EDS) in FE-SEM was performed to investigate thechemical composition of the polychromatic ZnO/CdxZn1-xO composite nanorod. We obtained EDS spectra from 15 predetermined square regions in steps of 200 nm as shown in Fig. 3(a). Figure 3(b) displays EDS spectra obtained from 5 selected square regions located at 400, 1000, 1600, 2200, 2800 nm from the left end of the polychromatic nanorod. It clearly shows the trend that with the increasing distance from the left end of the nanorod, Cd element peak grows larger while Zn peak diminishes. The complete plot of Cd and Zn peak integrated intensity in EDS spectra as a function of the distance from the left end of the polychromatic nanorod is shown in Fig. 3(c), confirming this trend. The plot also shows that Cd and Zn compositions arewell maintained with the intended x values in CdxZn1-xO displaying clear plateau behaviour at each x value control. The other relatively weak EDS peaks without marking were C, K, Ti, Na, and Al, which might be attributed to unintended impurities of the sol-gel solution. We note in Fig. 3(b) a reduction of O peak with increasing Cd mole fraction, which is attributed to partial vaporization of oxygen from less stable CdO than ZnO during the post-heating process [43]. The strongest EDS peak at 1.73 keV was originated from Si of the glass substrate. The intensity of the unintentional EDS peaks was not largely changed in all samples. We note that it is quite challenging to fabricate the hybrid nanostructure using simple chemical method such as sol-gel method due to the lattice mismatch between the constituting materials. However, our coating technique using sol-gel method on the surface of nanostructures is found to be very effective to fabricate the hybrid or composite nanostructures.
Fig. 3 (a) FE-SEM image of a polychromatic ZnO/CdxZn1-xO composite nanorod. The white squares represent the 15 different regions inspected by EDS analysis. The scale bar is 500 nm. (b) EDS spectra obtained from selected 5 regions of the polychromatic ZnO/CdxZn1-xO composite nanorod shown in Fig. 3(a). (c) Integrated spectral intensities of Cd and Zn peaks in EDS spectra as a function of the spatial position along the nanorod. The dotted lines represent the average value for adjacent three regions.
Figures 4(a) and 4(b) show fluorescence images (top views) of the bare ZnO nanorods and of the polychromatic ZnO/CdxZn1-xO composite nanorods grown on Si substrates, respectively. The emission colors of polychromatic nanorods were seemingly very uniform, indicating achievement of good uniformity in optical characteristics. For the ZnO nanorods, a predominantly red emission was observed from the dispersed single nanorod, as shown in the inset of Fig. 4(a), because the band-edge UV emission was not detected by the camera. On the other hand, the polychromatic ZnO/CdxZn1-xO composite nanorods showed near-white emission, as shown in Fig. 4(b). The longitudinal variation in the color emission is also clearly exhibited in the fluorescence image of the single ZnO/CdxZn1-xO composite nanorod in the inset of Fig. 4(b). The total PL spectrum of the polychromatic ZnO/CdxZn1-xO composite nanorods is shown in Fig. 4(c), which spans the entire visible wavelength region from the near-UV to the near-IR. The white light emitted by the ZnO/CdxZn1-xO composite nanorod was assessed using the Commision International de I'Eclairage (CIE) x and y chromaticity diagram. The x and y values of the CIE chromaticity coordinate for the polychromatic ZnO/CdxZn1-xO composite nanorodswere 0.378 and 0.350, respectively, which are reasonably close to the center point of the CIE coordinate, as shown in Fig. 4(d), implying the successful demonstration of white light emission from the polychromatic CdxZn1-xO nanorod.
Fig. 4 Fluorescence image of (a) ZnO nanorods and (b) polychromatic ZnO/CdxZn1-xO composite nanorods grown on the Si substrate. The insets show the fluorescence images of a single ZnO nanorod and a polychromatic ZnO/CdxZn1-xO composite nanorod, respectively. (c) Representative PL spectrum of a polychromatic ZnO/CdxZn1-xO composite nanorod. (d) CIE x and y chromaticity diagram of the polychromatic ZnO/CdxZn1-xO composite nanorods.
Figure 5(a) shows a typical confocal PL mapping image (total intensity) of the polychromatic ZnO/CdxZn1-xO composite nanorod, illustrating the spatial distribution of the different emission colors of the nanorod. We show the local PL spectra at four selected locations, indicated by arrows, as displayed in Fig. 5(b). On the end of the nanorod (x = 0, ①), the typical PL spectrum of the ZnO nanorod was observed. A stronger PL spectrum in the blue-green region appeared at point ②, as shown in Fig. 5(b). With increasing x, the intensity of the PL spectrum in the yellow-red region increased, as shown in Fig. 5(b)-③. The intensity of the PL spectrum in the yellow-red region gradually increased with further increases in x, as shown in Fig. 5(b)-④. The respective distribution of each emission color of the white-emitting ZnO/CdxZn1-xO composite nanorod is displayed in Fig. 5(c). The color indicates the integrated PL intensity of the spectra in the specified wavelength range at each position of the polychromatic ZnO/CdxZn1-xO composite nanorod. The intensity of the near- UV and violet emission was almost the same across the entire ZnO/CdxZn1-xO composite nanorod. However, the blue emission was much more intense on the right side of the nanorod, where relatively lower x was present, and the red emission was more intense on the left side of the nanorod, where the larger x was used in the sol-gelprocess. The green emission was slightly shifted toward the right side (lower x). Zhang et al. [31] introduced organic binary microtubes with multi-color emission prepared by a simple co-assembly method in which the visible red-green-blue colors were axially arranged according to the concentration gradient of dopants, resulting in color-tunable nanotubes, which suggested promising applications in high-speed and high-density photonic processing chips. Our polychromatic ZnO/CdxZn1-xO composite nanorods are comparable to organic binary microtubes in terms of size and performance but extend the materials range to inorganic materials, which provides the advantages of environmental stability under high-energy radiation, temperature, humidity, and oxidation compared with the organic microtubes [44].
Fig. 5 (a) Confocal PL mapping image of a dispersed single polychromatic ZnO/CdxZn1-xO composite nanorod. (b) PL spectrum of a single polychromatic ZnO/CdxZn1-xO composite nanorod measured at ①, ②, ③, and ④ in the confocal PL mapping image. (c) Rearranged confocal PL mapping image in the wavelength ranges of 360 to 450 nm, 450 to 495 nm, 495 to 570 nm, and 620 to 750 nm.
The white-light-emitting ZnO/CdxZn1-xO composite nanorod also possessed a varying optical band gap along the nanorod. Nanoscale confocal absorption spectral imaging was performed to visualize the longitudinal variation of the optical band gap of the polychromatic ZnO/CdxZn1-xO composite nanorod [36]. Figure 6(a) shows the confocal absorption mapping image of a single ZnO/CdxZn1-xO composite nanorod. The color indicates the integrated absorption intensity at each position of the image. As x, the mole fraction of Cd, increases along the nanorod, the absorption intensity monotonically increases due to the reduction in the optical band gap. To clearly show the variation in the optical band gap present in the polychromatic ZnO/CdxZn1-xO composite nanorod, Fig. 6(b) shows plots of (αhv)2 vs. the photon energy obtained from representative absorption spectra from the numbered locations on the polychromatic ZnO/CdxZn1-xO composite nanorod. The optical band gap can be determined by the extrapolation of the linear region from the plot near the onset of the absorption edge to the energy axis [45]. The measured local optical band gap varied from 3.3 eV in the pure ZnO region to 2.6 eV where the Cd mole fraction was a maximum at the end of the polychromatic nanorod.
Fig. 6 (a) Nanoscale confocal absorption intensity mapping image of a single polychromatic ZnO/CdxZn1-xO composite nanorod. The inset shows the optical band gap of the dispersed single polychromatic ZnO/CdxZn1-xO composite nanorod as a function of the Cd mole fraction. (b) Plot of (αhv)2 vs. the photon energy of the single polychromatic ZnO/CdxZn1-xO composite nanorod measured at locations ①, ②, ③, ④, and ⑤, marked in the confocal absorption mapping image (a).
4. Conclusions
We have created polychromatic ZnO/CdxZn1-xO composite nanorods displaying white-light and multi-color-light emission prepared by facile hydrothermal and sol-gel chemical methods. Nanoscale confocal PL and absorption spectral imaging were used to investigate the emission and absorption properties of single polychromatic ZnO/CdxZn1-xO composite nanorods. We were able to control the local mole fraction of Cd to achieve the full range of visible wavelength emission from the polychromatic ZnO/CdxZn1-xO composite nanorods, including white-light emission. In addition, the longitudinal variation in the optical band gap of a single white light-emitting ZnO/CdxZn1-xO composite nanorod was visualized using nanoscale confocal absorption spectral imaging.
Acknowledgments
This work was in part supported by the National Research Foundation grants (No. 2012R1A1A2A10043220).
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