High quality and high quantity ultra-long KNbO3 microwires with diameters of 200 nm to a few microns and lengths up to several milimeters have been synthesized by using a simple solvothermal method. The high purity, ideal length and good transferability make it easy to be transferred to the quartz substrate for further linear and nonlinear optical measurements. Functionalized by plasmonic Au nanoparticles, the as-prepared KNbO3 sub-microwires exhibited enhanced anisotropy of nonlinear absorption that the polarization ratio of individual Au nanoparticles coated KNbO3 sub-microwire (ρ=0.32) is two times more than the uncoated one. It is demonstrated that this ultra-long KNbO3 microwire is an ideal candidate for nanolasers, polarization sensitive photodetector and photo-thermal nanodevices.
© 2012 Optical Society of America
In the past decades, micro/nanoscale inorganic materials with unique geometries and novel properties have attracted numerous studies in electronics, energy harvesting, optics and opto-electronics [1–5]. In particular, many researchers use Far-Field Optical Microscopy to examine the linear and nonlinear optical properties of individual micro/nano structures, such as absorption [6–8], photoluminescence [9, 10], and second- and third-harmonic generation (SHG and THG) in the individual nanowire, particle or quantum dot [11–13]. These have many potential uses in nanodevices, such as nanolaser [4, 14], and frequency doublers . Recently, the hybrid semiconductor and noble metal nanostructures have attracted attention because of greatly enhanced optical characteristics as a consequence of a unique optical property called surface plasmon resonance (SPR) [16–18]. Thus, there is an impetus to study the nonlinear optical properties of individual hybrid nanostructures, which is capable of providing both modified optical properties and fundamental information on the optical coupling effect between noble metals and semiconductors.
Micro/nanoscale potassium niobate is one of the hot spots for their various potential applications in nano-electromechanical systems (NEMS) , photocatalytic water-splitting and organic dye degradation [20, 21], and especially, nonlinear optics [15, 22]. Although several kinds of potassium niobates nanostructures have been prepared such as nanoparticles, nanofilms, and nanoneedles, there are still very few works focusing on high quality KNbO3 micro/nanowires with aspect-ratio higher than 100, except the one by Magrez and his copartners . Unfortunately, in Magrezs process, it usually took more than 6 days to obtain the single-crystalline nanowires, which makes it unsuitable for real applications when considering the large-scale production at low cost. Considering the superior optoelectronic properties of 1-D nanostructures due to the directional charge transfer and the higher electron mobility, it is still highly desirable to develop an efficient and simple means to synthesize high quality potassium niobate micro/nanowires and then systematically investigate their interesting nonlinear properties.
In this work, by using Nb2O5 as a cheap, inert source of niobium, we reported the synthesis of ultra-long KNbO3 sub-microwires via a simple solvothermal method on a large scale. The as-prepared products have widths ranging from 200 nm to a few microns and lengths up to several millimeters, which can be clearly discerned by the naked eye shown in Fig. 1(c). To our best knowledge, this is the highest length-diameter ratio achieved up to now . Functionalized by plasmonic Au nanoparticles, the as-prepared KNbO3 sub-microwires exhibited greatly enhanced anisotropy of nonlinear absorption than pure sub-microwires, which shows great potential in applications of nanolasers, polarization sensitive photodetector and photo-thermal nanodevices.
All reagents were of analytical grade and used as received without further purification. Distilled water was used throughout. In a typical procedure, 1 mmol Nb2O5 and 60 mmol KOH were dissolved in 17.5 ml distilled water. Then 7.5 ml diethylene glycol (or ethanol) was added into the mixture. After vigorously stirring for 45 min, the mixed solution was transferred into a 40 ml autoclave container which was sealed and maintained at 240 °C for 24 h. After the autoclave was air-cooled to room temperature naturally, the final products were collected by filtration, washed with distilled water and absolute alcohol several times, dried at 60 °C and kept for further characterization.
The KNbO3 wires were coated with Au nanoparticles by repeatedly dipping in Au colloidal solution. This Au colloidal solution was prepared according to the previous literature . In a 1 L flask heated by a reflow oven, 250 mL 1 mM HAuCl4 was heated to boiling with vigorous stirring. Then 25 mL 38.8 mM sodium citrate was added rapidly into the above solution. The mixture changed from pale yellow to burgundy. After that, the cooling solution was pre-filtrated through a 0.8 m micro-pore filter membrane. The expected particle size of the Au was 13 nm±1.7 nm. Our experimental results (Inset in Fig. 2(b)) are in accord with this. After rinsing repeatedly with the Au colloid solution, the KNbO3 wires were supposed to be covered with Au nanoparticles. A Single Au-nanoparticle coated KNbO3 wire was transferred on to the quartz plate for the following optical measurement.
The phase structure and purity of the obtained products were measured on X-ray diffractometer (XRD, X’Pert PRO, PANalytical B. V., Netherlands) using CuKα radiation at a scan speed of 15 °/min. The particles’ size and morphology were observed by a scanning electron microscope (SEM, JSM-6701F and TEM, JEOL-4000EX). For optical characterization, a tungsten halogen lamp with a continuous spectrum (3200 K) was used as testing and illuminating source in linear and nonlinear optical measurements, respectively. The femtosecond laser system consisted of a mode-locked Ti/Sapphire oscillator and a regenerative amplifier (Spitfire, Spectra-Physics, 800 nm, 50 fs, 1 kHz) was used as the linearly polarized light excitation source. The beams are focused and re-collimated by a pair of objectives (40×, NA: 0.65, Olympus). An individual KNbO3 sub-microwire with coated Au particles, lying flat on a quartz substrate (10 mm×10 mm×0.3 mm) was mounted on an XY stage (MVP-25XA, Newport) at the focus of the objectives. The transmitted white beam and laser pulse were received by a monochrometer and photomultiplier tube (PMT) and double-phase lock-in amplifier (SR830, Stanford Research System), respectively. The polarized angle with respect to the long-axis, θ, is defined, which is controlled by rotating a linear polarizer (Polarizer, 400 nm∼780 nm, Zeiss) and a 1/2 wave-plate at 800 nm.
3. Results and discussion
The crystal structure of orthorhombic KNbO3 is shown in Fig. 1(a). With a typical layered perovskite structure, it consists of octahedral units of corner/edge-sharing NbO6, which form a two-dimensional layered structure. When viewing each layer as a whole, it is negatively charged, and K ions are located between the layers to compensate for the negative charges of the layers. Figure 1(b) shows the XRD pattern of the KNbO3 products obtained in our experiments and all peaks can be indexed to the pure orthorhombic phase of KNbO3 (JCPDS Card No.32-822 space group: Cm2m, a=5.6950 Å, b=5.7213 Å, c=3.9739 Å). No characteristic peaks from other crystalline impurities were found in this pattern, indicating the purity of our sample.
Figure 1(c) is a digital photograph of the KNbO3 products prepared in the ethanol/water solution with the Nb2O5: KOH ratio of 1:60, produced in a 5 cm inner diameter autoclave. Wires with extremely large quantity can be easily obtained from the present method and the KNbO3 wires can be easily discerned by naked eyes. Besides, the as-prepared KNbO3 wires arrays can reach as long as 6 mm, indicating their possible extremely high aspect ratios. Figure 1(d)–1(f) give the optical microscope image and SEM images of the produced KNbO3 wires, from which we can easily deduced that typical KNbO3 wires have diameters of 200 nm to a few microns and lengths up to several milimeters with smooth surfaces.
To systematically investigate the nonlinear properties of the as-obtained KNbO3 sub-microwires, Au nanoparticles were used to functionalize the wires via a solution process and the corresponding TEM image of a typical functionalized KNbO3 sub-microwire is shown in Fig. 2(a). The inset in Fig. 2(a) shows the digital image of the Au colloidal solution containing KNbO3 wires. From the TEM image, it can be seen that Au nanoparticles are randomly distributed on the surface of the wire in the form of clusters with a larger dimension along the long-axis, indicating the anisotropy of Au clusters. HRTEM image of the Au nanoparticles is shown in Fig. 2(b), indicating that the nanoparticles are in a crystalline state. The interplanar d-spacing of 0.2 nm and 0.24 nm correspond to the (200) and (111) lattice planes respectively. A closer examination indicates that the particle is inter-twinned with an icosahedron .
The conventional confocal microscope configuration was used for the optical measurements shown in Fig. 3(a). In Fig. 3(b), it is shown an optical image of an individual KNbO3 sub-microwire with its long-axis along the X-axis. Furthermore, the polarized angle with respect to the long-axis, θ, is defined, which is controlled by rotating a linear polarizer and a 1/2 wave-plate at 800 nm. As shown in Fig. 3(c), the 2D laser scanning image exhibits a pattern with a length (X-axis) of several tens of micrometers but a much narrow width (Y-axis). In the case of nonlinear effect, the decrease of the transmitted power can be attributed to the nonlinear absorption. If a Gaussian beam spot scans across to the microwire, the largest signal can be obtained when the wire is located at the center of the focal spot (the red region). The yellow regions beside show the decrease process of the signal, while green regions relate to the quartz substrate with no nonlinear absorption signal under a low excitation of 3 GW/cm2. Thus, the wirelike signal observed can only be attributed to the individual KNbO3 sub-microwire. For a sample that is smaller than the spot size, the normalized change of the transmittance of the far field can be written as :
Figure 4 shows the normalized absorption curves (380 nm∼800 nm) of the individual pure KNbO3 sub-microwire (dot) and Au coated KNbO3 sub-microwire at different polarized angles of 0° (red), 45° (blue) and 90° (cyan). The flat curve of the KNbO3 sub-microwire indicates that it is transparent in the range from 400 nm to 800 nm, and is independent of the polarization of the light source. However, all curves of Au coated KNbO3 sub-microwire exibihts an absorption band in the same wavelength range (400 nm∼800 nm), which can only be ascribed to the SPR absorption of coated Au nanoparticles. Furthermore, this SPR absorption is strongly dependent on the polarized angle of the light source such that two SPR absorption peaks located at a wavelength of 545 nm and 720 nm are observed at θ = 0°, while only one absorption peak located at 605 nm can be observed at θ = 90°. For the absorption curve at θ = 45°, no absorption peak but a wide absorption band can be found. As shown in Fig. 2, the coated Au nanoparticles tend to assemble in clusters with a larger dimension along the long-axis. Thus, as the polarization direction is along the long-axis (θ = 0°), the strong absorption band (650 nm∼800 nm) due to the the longitudinal plasmon of the Au clusters can be observed, while the absorption band at 550 nm is attributed to the single Au nanoparticles are not so assembled. When the polarization is normal to the long-axis (θ = 90°), the resonant absorption of the transverse plasmon of the Au clusters in the range of 530 nm and 700 nm is observed. This wide absorption band could also include the contribution of the single Au nanoparticles so no individual absorption band at 540 nm is found. Moreover, as the polarization angle remains at 45°, all the contributions of longitudinal and transverse plasmon, as well as the single Au nanoparticles lead to a wide absorption band in the range of 400 nm and 800 nm.
Figure 5 shows the normalized change of transmittance of the femtosecond laser in the far field, −ΔT/T, as a function of the polarization angle, θ, of the individual pure (blue) and Au coated KNbO3 sub-microwire (red), respectively. At the low intensity of the excitation laser, the signal can only be attributed to the nonlinear absorption. Both curves exhibit oscillations with a period of π, indicating the anisotropy of optical nonlinearity of these samples. In this situation, wave vector k is parallel to the c-axis, the electric-field polarization is relative to  crystallographic axis in the crystal graphic xy plane. The electric-field polarization in the crystal graphic xy plane is given by E = E0(cosθx + sinθy). Since KNbO3 exhibits an orthorhombic structure, the intrinsic permutation symmetry leaves eight independent tensor components. For this specific geometry, the effect third-order susceptibility is :
Specifically, minimum values of −ΔT/T of the pure and Au coated KNbO3 sub-microwire are obtained at θ = 55° and θ = 90°, the maximum values of both samples are obtained when polarization angle is 0°, resulting in polarization ratios, ρ = (Imax − Imin)/(Imax + Imin), of 0.16 and 0.32, respectively. Thus, the value of −ΔT/T(θ = 0°) of Au coated KNbO3 sub-microwire is significantly larger than that of KNbO3 sub-microwire, while the value of −ΔT/T at θ = 90° of both samples are almost the same, indicating an enhancement of the anisotropy of optical nonlinearity of the Au coated KNbO3 wire with respect to the uncoated one.
Figure 6(a) and 6(c) show the laser scanning image of the uncoated and Au coated KNbO3 sub-microwire to the same scale of 50 μm×100 μm at θ = 0°, respectively. The strong nonlinear absorption response of the Au coated KNbO3 sub-microwire with respect to the uncoated one indicates an enhancement of the optical nonlinear absorption of the Au coated KNbO3 sub-microwire. In addition, the related scan curves along Y-axis are shown in Fig. 6(b) and 6(d), respectively. If the sample is located at the focus plane (z=0) and move along the Y-axis, we need to integrate the absorption coefficient with the intensity of the beam over one dimension (X-axis):Fig. 4. Specifically, at θ = 0°, the strong absorption band (650 nm∼800 nm) due to the longitudinal plasmon of the Au clusters enhances nonlinear absorption in the infrared region. While the excitation laser is located at the off-resonance region of transverse plasmon band (530 nm∼700 nm) at θ = 90°, implying that no obvious enhancement of optical nonlinearity occurs.
In conclusion, high quality and high quantity ultra-long KNbO3 microwires with diameters of 200 nm to a few microns and lengths up to several milimeters with smooth surfaces have been synthesized via a simple solvothermal method. The polarization ratio of Au nanoparticles coated KNbO3 sub-microwires (ρ=0.32) is two times more than the uncoated one, indicating an enhanced anisotropy of optical nonlinear absorption. It makes an ideal candidate for nanolasers, polarization sensitive photodetector and photo-thermal nanodevices.
This work was supported by National Natural Science Foundation of China ( 60925021, 11104095, 51002059, 21001046), and the 973 Program under grant 2010CB923203, 2011CBA00703, 2011CB933300, the Program for New Century Excellent Talents of the University in China (Grant NCET-11-0179). Special thanks to the Analytical and Testing Center of HUST and the Center of Micro-Fabrication and Characterization (CMFC) of WNLO for using their facilities.
References and links
1. R. X. Yan, D. Gargas, and P. D. Yang, “Nanowire photonics,” Nat. Photonics. 3, 569–576 (2009). [CrossRef]
2. G. Z. Shen, J. Xu, X. F. Wang, H. T. Huang, and D. Chen, “Growth of directly transferable In2O3 nanowire mats for transparent thin-film transistor applications,” Adv. Mater. 23, 771–775 (2011). [CrossRef] [PubMed]
5. K. Wang, J. Zhou, L. Y. Yuan, Y. T. Tao, J. Chen, P. X. Lu, and Z. L. Wang, “Anisotropic third-order optical nonlinearity of a single ZnOmicro/nanowire,” Nano Lett. 12, 833–838 (2012). [CrossRef] [PubMed]
6. L. Y. Cao, J. S. White, J. S. Park, J. A. Schuller, B. M. Clemens, and M. L. Brongersma, “Engineering light absorption in semiconductor nanowire devices,” Nature Mater. 8, 643–647 (2009). [CrossRef]
7. J. Giblin, M. Syed, M. T. Banning, M. Kuno, and G. Hartland, “Experimental determination of single CdSe-nanowire absorption cross section through photothermalimaging,” ACS Nano 4, 358–364 (2010). [CrossRef] [PubMed]
8. G. Brönstrup, N. Jahr, C. Leiterer, A. Csáki, W. Fritzsche, and S. Christiansen, “Optical properties of individual silicon nanowires for photonic devices,” ACS Nano 4, 7113–7122 (2010). [CrossRef] [PubMed]
9. J. F. Wang, M. S. Gudiksen, X. F. Duan, Y. Cui, and C. M. Lieber, “Highly polarized photoluminescence and photodetection from single indium phosphide nanowire,” Science 293, 1455–1457 (2001). [CrossRef] [PubMed]
10. P. Kukura, M. Celebrano, A. Renn, and V. Sandoghdar, “Imaging a single quantum dot when it is dark,” Nano Lett. 9, 926–929 (2009). [CrossRef]
11. J. P. Long, B. S. Simpkins, D. J. Rowenhorst, and P. E. Pehrsson, “Far-field imaging of optical second-harmonic generation in sgingle GaN nanowires,” Nano Lett. 7, 831–836 (2007). [CrossRef] [PubMed]
13. J. Butet, J. Duboisset, G. Bachelier, I. Russier-Antoine, E. Benichou, C. Jonin, and P. F. Brevet, “Opticalsecond harmonic generation of single metallic nanoparticles embedded in a homogeneous medium,” Nano Lett. 10, 1717–1721 (2010). [CrossRef] [PubMed]
14. R. Chen, B. Ling, X. W. Sun, and H. D. Sun, “Lasing: room temperature excitonic whispering gallery mode lasing from high-quality hexagonal ZnO microdisks,” Adv. Mater. 23, 2199–2204 (2011). [CrossRef] [PubMed]
15. Y. Nakayama, P. J. Pauzauskie, A. Radenovic, R. M. Onorato, R. J. Saykally, J. Liphardt, and P. D. Yang, “Tunable nanowire nonlinear optical probe,” Nature 447, 1098–1102 (2007). [CrossRef] [PubMed]
16. D. Pacifici, H. J. Lezec, and H. A. Atwater, “All-optical modulation by plasmonic excitation of CdSe quantum dots,” Nat. Photonics 1, 402–406 (2007). [CrossRef]
17. S. Savasta, R. Saija, A. Ridolfo, O. D. Stefano, P. Denti, and F. Borghese, “Nanopolaritons: vacuum rabisplitting with a quantum dots in the center of a dimer nanoantenna,” ACS Nano 11, 6369–6376 (2010). [CrossRef]
18. A. Vaneski, A. S. Susha, J. Rodríguez-Fernndez, M. Berr, F. Jäckel, J Feldmann, and A. L. Rogach, “Hybird colloidal heterostructures of anistropic semiconductor nanocrystals decorated with noble metals: synthesis and fuction,” Adv. Func. Mater. 21, 1547–1556 (2011). [CrossRef]
20. U. Unal, Y. Matsumoto, N. Tamoto, M. Koinuma, M. Machida, and K. J. Izawa, “Visible light photoelectrochemicalactivity of K4Nb6O17 intercalated with photoactive complexes by electrostatic self-assembly deposition,” Solid State Chem. 179, 33–40 (2006). [CrossRef]
21. G. K. Zhang, F. S. He, X. Zou, J. Gong, and H. J. Zhang, “Hydrothermal preparation and photocatalyticproperties of sheet-like nanometer niobate K4Nb6O17,” Phys. Chem. Solids 69, 1471–1474 (2008). [CrossRef]
24. K. C. Grabar, R. G. Freeman, M. B. Hommer, and M. J. Natan, “Preparation and characterization of Au colloid monolayers,” Anal. Chem. 67, 735–743 (1995). [CrossRef]
25. J. L. Gardea-Torresdey, J. G. Parsons, E. Gomez, J. Peralta-Videa, H. E. Troiani, P. Santiago, and M. J. Yacaman, “Formation and growth of nanoparticles inside live alfalfa plants,” Nano Lett. 2, 397–401 (2002). [CrossRef]
26. A. Arbouet, D. Christofilos, N. D. Fatti, and F. Vallée, “Direct measurement of the single-metal-cluster optical absorption,” Phys. Rev. Lett 93, 127401-1–127401-4 (2004). [CrossRef]
27. R. DeSalvo, M. Sheik-Bahae, A. A. Said, D. J. Hagan, and E. W. Van Stryland, “Z-scan measurements of the anisotropy of nonlinear refraction and absorption in crystals,” Opt. Lett. 18, 194–196 (1993). [CrossRef] [PubMed]