Integrated nanophotonic hubs based on ZnO-Tb(OH)3/SiO2 nanocomposites
© Lin et al; licensee Springer. 2011
Received: 16 May 2011
Accepted: 22 August 2011
Published: 22 August 2011
Optical integration is essential for practical application, but it remains unexplored for nanoscale devices. A newly designed nanocomposite based on ZnO semiconductor nanowires and Tb(OH)3/SiO2 core/shell nanospheres has been synthesized and studied. The unique sea urchin-type morphology, bright and sharply visible emission bands of lanthanide, and large aspect ratio of ZnO crystalline nanotips make this novel composite an excellent signal receiver, waveguide, and emitter. The multifunctional composite of ZnO nanotips and Tb(OH)3/SiO2 nanoparticles therefore can serve as an integrated nanophotonics hub. Moreover, the composite of ZnO nanotips deposited on a Tb(OH)3/SiO2 photonic crystal can act as a directional light fountain, in which the confined radiation from Tb ions inside the photonic crystal can be well guided and escape through the ZnO nanotips. Therefore, the output emission arising from Tb ions is truly directional, and its intensity can be greatly enhanced. With highly enhanced lasing emissions in ZnO-Tb(OH)3/SiO2 as well as SnO2-Tb(OH)3/SiO2 nanocomposites, we demonstrate that our approach is extremely beneficial for the creation of low threshold and high-power nanolaser.
KeywordsZnO Tb(OH)3/SiO2 nanocomposite lasing
Semiconductive photonic nanostructures have attracted increasing attention for its many possible applications, such as laser, solar cell, biosensor, and photoelectric conversion [1–4]. Among all the semiconductor materials, zinc oxide is of great interest for photonic applications due to its wide bandgap (3.37 eV) and efficient emission . The optoelectronic properties of zinc oxide depend critically on its defect structure and rich morphologies. ZnO nanostructures have been made into diverse morphologies, such as nanoparticles, nanorods, nanowires, nanobelts, and nanotubes [6–9]. Of these, ZnO nanorods have received the greatest attention and have shown to be a good laser emitter, an electron emitter, and a photoelectric converter. Their excellent optical behaviors are due to the fact that ZnO cannot only be a good gain medium but also can present good confinements for both photons and electrons. Numerical calculations have concluded that ZnO nanorods provide high lateral photonic confinement and are excellent waveguides . Light intensity losses occur only at the end faces, and this makes longer nanorods higher Q resonators. In addition, nanostructures like ZnO nanorods coupled with photonic nanomaterials can lead to newer applications.
When another optical nanostructure is coupled with ZnO, the integrated optical phenomenon can be demonstrated. We would like to study the coupling of ZnO and the strong luminescent nanomaterials of lanthanide hydroxide. Due to the unique electronic, optical, and magnetic properties arising from the 4f electrons, lanthanide hydroxides are very attractive in various applications, including catalysts, laser materials, biolabels, and magnetic resonance imaging . Previously, lanthanide-doped nanoparticles have been fabricated mainly by ion implantation , sol-gel method , and sonochemical synthesis . Unfortunately, the obtained size is often not uniform. Recently, we reported a one-pot synthesis of monodispersed core/shell Tb(OH)3/SiO2 colloids . The Tb(OH)3/SiO2 colloidal particles self-assembled into a 3-D photonic crystals (PCs), giving a pronounced optical gap depending on the particle size. Many efforts have been made on applications of PCs, such as the resonators, sensors, and reflectors [16–18]. To expand more applications of Tb(OH)3/SiO2 with other materials and nanostructures, semiconductor nanowires were chosen because they can be used as waveguides when attached to other luminescent materials . Based on the monodispersed Tb(OH)3/SiO2 core/shell nanoparticles, we report a novel composite with ZnO nanotip on Tb(OH)3/SiO2 core/shell nanoparticle (ZnO-Tb(OH)3/SiO2), which can be used to manipulate the emissions from inside the PCs. Due to the confinement effect of PC, emissions can escape only from the nanotips of ZnO. We found that the light output can be greatly enhanced by two orders of magnitude. To optimize this effect, SnO2 nanowires were selected to show the enhanced lasing emission of Tb(OH)3/SiO2 PCs by growing them on Tb(OH)3/SiO2 PCs of 130 nm which can perform better lasing action at 380 nm. Therefore, these novel composites act like directional light fountains, i.e., the light confined underneath the surface of the photonic crystal can be extracted only through the specially designed semiconductor nanotips. We show that this unique property is very useful to create low threshold and high-power nanolasers.
Nanoparticles of Tb(OH)3 were encapsulated inside silica as core/shell structures with an outer diameter of 250 nm by a one-pot synthesis method reported in our previous paper . The monodispersed nanoparticles were self-assembled on glass or Si (100) substrate by a slow evaporation method, resulting in self-organized packing as photonic crystals. After coating with gold nanoparticles (20 mA, 20 s), Tb(OH)3/SiO2 nanoparticles were used as templates in a vapor-liquid-solid process to grow ZnO nanowires on the nanospheres. The mixed C/Zn powders were placed in an alumina boat, which was loaded in the center of a tube furnace. The gold-coated lanthanide nanosphere substrate was placed in the same boat but apart from the mixed powders for about 3 cm. Argon was then introduced into the system with a flow rate of 200 sccm as the carrier gas. Afterwards, the tube was heated to 980°C at a rate of 40°C/min. The reaction lasted about 60 min. After the furnace cooled down, white color products formed on the surface of the lanthanide nanosphere substrate. For SnO2 nanowire growth, the C/Zn powders were replaced with C/Sn powders then follow the above steps. Cathodoluminescence (CL) experiments were performed at room temperature with a scanning electron microscopy (JSM 6500, JEOL Ltd., Tokyo, Japan). Excitation spectra were gathered by a PMT detector with a CL system (Gatan instrument, MonoCL3, Gatan, Inc., Pleasanton, CA, USA).
Results and discussion
Fabrication of ZnO-Tb(OH)3/SiO2 composites
Photonic bandgap and CL spectra of Tb(OH)3/SiO2 PCs
As the electron beam with a spot size of 1 μm to approximately 2 μm in diameter was focused on the Tb(OH)3/SiO2 PCs surface, CL spectra showed broadened bands of intra-4f transitions under a small excitation current. As the current reached 5 × 10-8 A, two sharp peaks of the CL spectrum appeared at 543 and 551 nm (Figure 5a), possibly due to the Stark effect of stimulated emission . With increasing current, the two peaks are more resolved, and the corresponding intensity increases nonlinearly as shown in Figure 5b. The nonlinear relationship between CL intensity and excitation current revealed a threshold current of 5 × 10-8 A, indicating a stimulated emission behavior. PCs have been used as a lasing cavity to stimulate the confined emission inside . The low threshold may arise from the release of optical resonance between emissions near the stop band and the cavity of PCs.
Lasing action of rice paddy-like ZnO-Tb(OH)3/SiO2 PCs nanocomposites
With the designed ZnO-Tb(OH)3/SiO2 nanocomposite, a multifunctional integrated nanophotonic hub has been created. We have shown that growing ultra tapered ZnO nanotips on Tb(OH)3/SiO2 PCs can yield good control of emission out of PCs, in which the radiation confined underneath the PC surface can be well guided by the attached ZnO nanotips and escape from a designed direction. Similarly, SnO2 nanowires act as a directional light fountain, which may be very useful for the creation of ultra low threshold and high-power nanolaser. In view of the novel properties discovered here, the semiconductor Tb(OH)3/SiO2 composites pave a new way for the realization of applications of nanophotonics.
scanning electron microscopy
This work was supported by National Science Council of the Republic of China.
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