Ga2O3 Peapodded Nanowires and Their

Aug 17, 2010 - Gold-peapodded Ga2O3 nanowires were fabricated successfully in a well-controlled manner by thermal annealing of core−shell gold−Ga2...
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Direct Observation of Au/Ga2O3 Peapodded Nanowires and Their Plasmonic Behaviors Po-Han Chen, Chin-Hua Hsieh, Sheng-Yu Chen, Chen-Hwa Wu, Yi-Jen Wu, Li-Jen Chou,* and Lih-Juann Chen Department of Materials Science and Engineering, National Tsing Hua University, No. 101, Section 2, Kuang-Fu Road, Hsinchu, Taiwan 300, Republic of China ABSTRACT Gold-peapodded Ga2O3 nanowires were fabricated successfully in a well-controlled manner by thermal annealing of core-shell gold-Ga2O3 nanowires. During the heating process, the core gold nanowires were broken up into chains of nanoparticles at sufficiently high temperature by the mechanism of Rayleigh instability. In addition, the size, shape, and interspacing between the particles can be manipulated by varying the annealing time and/or the forming gas. The plasmonic behaviors of these nanostructures are investigated by optical spectroscopy. A single nanowire optical device was designed, and its photonic characteristics were investigated. A remarkably high on/off photocurrent ratio in response to a 532 nm Nd:YAG laser light was found. As the size of the particle (pea) increases, the corresponding spectra are red-shifted. In addition, morphological changes of the peas lead to a distinct spectral response. The results may usher in the diverse applications in optoelectronics and biosensing devices with peapod nanostructures. KEYWORDS Rayleigh instability, plasmonic behaviors, optoelectronics and biosensing devices, core-shell, gold-peapodded Ga2O3 nanowires

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synthetic method via a two-step process in situ to produce the one-dimensional Au/Ga2O3 nanowires enhanced by silica and postannealing. The Au/Ga2O3 core-shell nanowires were synthesized by a reaction of Ga, Au, and silica substrates via VLS growth mechanism. Subsequently, the Au/ Ga2O3 core-shell nanowires were annealed at the same temperature for an additional 3-30 min. During the growth of the oxide nanowires, gold was drawn into the Ga2O3 nanotube through capillarity attraction.19 Subsequent annealing of the as-synthesized core-shell nanowires led to the formation of gold nanoparticles periodically encapsulated in the monoclinic single crystal Ga2O3 nanowires by the mechanism of Rayleigh instability. The concept of Rayleigh instability comes from the instability of liquid cylinders which is attributed to the surface tension. A liquid cylindrical wire tends to undulate its free surface with the wavelength of the perturbation. The driving force of the surface undulation is to decrease the surface area and thus the total surface energy.14 Cu and Au nanowires on a SiO2 substrate were observed to break up into chains of nanospheres due to Rayleigh instability with gradually increased annealing temperature.20,21 It is of great interest to further investigate the optical properties of these nanostructures, such as the plasmonic behavior. The mechanism of surface plasmon polarizations in subwavelength metal particles is that when the incident photon frequency is resonant with the coherent oscillation of the conduction electrons in nanoparticle of the metal, surface plasmons can be excited. It radiates an electromagnetic wave which results in the unique color that is not produced from the bulk metal. This phenomenon is known

he embedded noble metal nanoparticles in dielectric matrices have attracted a great deal of attention over the last few decades due to their phenomenal optical and electrical properties.1 Metal-dielectric nanocomposites show nonlinear2-4 and fast optical response near the surface plasmon resonance (SPR) frequency.5 In addition to the metal-dielectric nanocomposites that are mainly prepared in thin film or bulk form, a one-dimensional (1-D) noble metal nanoparticle chain is thought to possess more functionality, with a wide range of applications in optoelectronics and biosensing.6-8 In the past several years, various 1-D metal-peapodded nanostructures have been successfully synthesized and reported.9-12 Hu et al. found that Au nanopartilces encapsulated in silica nanowires exhibit a wavelength-dependent photoresponse due to the SPR effect which can be applied to the optical switching devices. Hsieh et al.13 reported the fabrication of gold in Ga2O3 peapodded nanowires where the Au peas were induced by the formation of twinned structures in the Ga2O3 pods. Qin et al. described the formation of Cu nanoparticle chains encapsulated in Al2O3 nanotubes with well-controlled manner by Rayleigh instability.9,14 Key parameters to be controlled are the size, shape, interparticle distance, and the surrounding matrix of the nanomaterials. Hence, precise control of the assembly of nanoparticles is a crucial step. Several approaches, such as the colloidal chemical method, lithographical method, and template agents have been applied to prepare these ordered structures.15-18 In this paper, we demonstrate a simple * Corresponding author, [email protected]. Received for review: 02/18/2010 Published on Web: 08/17/2010 © 2010 American Chemical Society

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as localized surface plasmon resonance (LSPR), or particle plasmon. It is due to the charge density oscillation and confined between the metal nanoparticles as the particle size is significantly smaller than the wavelength of incident light. As long as localized surface plasmons are excited, intense light absorption, light scattering, and local electromagnetic field enhancement of the metal particles would occur. The features strongly depend on the composition, size, shape, and the surrounding local dielectric environment of the particles. In the present study, Au/Ga2O3 core-shell nanowires were synthesized with thermal annealing treatment. The fabrication of well-organized nanoparticle chains confined in nanotubes was observed by in situ transmission electron microscopy (TEM). A single nanowire optical device was designed, and its photonic characteristics were investigated. A remarkably high on/off photocurrent ratio as a response to a Nd:YAG laser light with a 532 nm wavelength was found. As the size of the particle (pea) increases, the corresponding spectra are red-shifted. In addition, morphological change of the peas leads to a distinct spectral response. The results may usher in the diverse applications in optoelectronics and biosensing devices with peapod nanostructures To form Au-in-Ga2O3 peapods, Au/Ga2O3 core-shell nanowires were prepared on an amorphous SiO2 substrate. The quartz tube was heated in a horizontal furnace to 600-800 °C with a ramping rate of 20 °C/min. Subsequently, core-shell nanowires were placed into the quartz tube. The reaction process was held at a pressure of 1 × 10-2 Torr for 3-30 min. For high resolution TEM (HRTEM) analysis, the as-synthesized nanowires were ultrasonically dispersed in a methanol solution. A few drops of the mixture were dripped on a carbon-coated Cu TEM grid. The morphologies and crystal structures were analyzed by fieldemission scanning microscopy (FESEM, JSM-6500F) and transmission electron microscopy (FETEM, JEM-3000F). The FETEM operated at 300 kV and having a point-to-point resolution of 0.17 nm was equipped with a high-angle angular dark field (HAADF) detector and an energy dispersive spectrometer (EDX). For in situ TEM observation, a JEOL 2000 V ultrahigh vacuum (UHV)-TEM (base pressure 3 × 10-10 Torr) was used in the present study. The samples were heated in a double tilt heating holder inserted into the electron beam path of the TEM and the real time behavior was video recorded. All optical imagings were performed with an optical microscope (BX51M, Olympus, Japan) with the numerical aperture of the UM Plan Fl 100X set at 0.90. To obtain a plasmon resonance spectrum, unpolarized light from a halogen lamp source is incident on the nanowires using dark field illumination. The studies of photoresponse were carried out using a double frequency Nd:YAG laser (Laser Century Technology, model GL532T) externally modulated at 1 kHz by a chopper controller (Stanford Research Systems, SR540) as an excitation source, with a wavelength of 532 nm and maximum power of 1 W. In order to © 2010 American Chemical Society

FIGURE 1. (a) HAADF image of an individual core-shell nanowire and (b) energy-dispersive spectra indicating the outer shell and the core are Ga2O3 and gold, respectively.

minimize the illuminating spot size, an excited laser beam was coupled into a single-mode lens fiber and focused on the peapod nanowires. Under a constant bias of 2 V, the photocurrent produced by a single nanowire was enhanced in a programmable lock-in amplifier (Stanford Research Systems, SR830) with a constant load resistance of 56 MΩ. All measurements were performed at room temperature and under atmospheric pressure. The Au/Ga2O3 core-shell nanowires were synthesized by a reaction of Ga, Au, and silica substrates via a VLS growth mechanism. High-density Ga2O3 nanowires with a fairly uniform diameter were obtained. Analysis of the X-ray diffraction (XRD) spectrum indicates that the sample was composed of monoclinic β-Ga2O3. From the low-magnification HAADF image, the core-shell nanowires can be seen from the distinct contrast of outer shell and the fillings (Figure S1 in Supporting Information,). Analytical HRTEM techniques were used to obtain further insights of the core-shell nanowires. The corresponding high angle annular dark field (HAADF) image of individual Au-Ga2O3 coreshell nanowire is highlighted in Figure 1a with the relatively large difference in atomic numbers among Au, Ga, and O elements. From the compositional line profile, characterized by EDS and marked in Figure 1b, the bright interior and the outer shell were found to be Au and Ga2O3, respectively. The Au nanowires were observed to be fragmented by the thermal annealing process. To study the structural changes as a function of temperature, core-shell nanowires were annealed isothermally at 600 and 850 °C, each for 30 min. The central core was seen to preserve their cylindrical shape even after several hours of annealing at 600 °C. After annealing at 850 °C for 30 min, significant morphological change in the core-shell nanowire was observed. The nanowire was fragmented into nanoparticle chains along the whole length of the nanowire (see Figure S2 in Supporting Information). 3268

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FIGURE 3. SEM images of the gold-peapodded Ga2O3 nanowire with (a) 50 and (b) 76 nm embedded nanoparticles. The corresponding size histograms are given in the insets of parts a, and b, respectively. (c) Scattering spectra of gold-peapodded Ga2O3 nanowire with 50 and 76 nm embedded nanoparticles. FIGURE 2. (a-e) A sequence of in situ UHV-TEM images of Au-Ga2O3 core-shell nanowire with different annealing times as indicated in minutes and seconds at 850 °C. (f) Formation mechanism of gold nanoparticle chains.

In order to elucidate the detailed formation mechanism of nanopeapods, in situ UHV-TEM was applied to visualize the peapod formation processes in real time. The method is capable of providing detailed information about the reaction mechanisms of nanostructures in an ultrahigh vacuum environment. Images captured from the video were acquired to demonstrate the dynamic development of nanopeapods from Au-Ga2O3 core-shell nanowire. Panels a-e of Figure 2 show a series of images of a Au-Ga2O3 core-shell nanowire heated at 850 °C in the TEM. During the thermal treatment, we observed that some perturbations occurred, resulting in fragmentation after 110 s of annealing, as shown in Figure 2b. From Figure 2c, the Au core decays into a much shorter section. Finally, a row of spheres was formed after prolonged annealing, as shown in Figure 2d. From Figure 2e, we can clearly observe the remarkable growth in the embedded Au particles in 5 min at 850 °C under UHV. On the basis of the above in situ TEM observation, a schematic is plotted to illustrate the formation mechanism for the nanopeapods, as shown in Figure 2f. Upon heating at high temperature, the surface diffusion of gold atom of the central core results in an undulation. Further undulation then leads to the fragmentation of gold-core into rod-shaped, oval, and cylindrical fragments by Rayleigh instability. Oval or rodshaped fragments eventually transform into spheroids. Karim et al. have also demonstrated morphological evolution of gold nanowires, which was similar to our observation. However, gold was confined in the Ga2O3 matrix in our case. As a result, coarsening of gold nanoparticles occurs with increased heating time. The diffusion of gold atoms results in the increasing size of gold nanopartilces (final stage of Figure 2e). The above observation indicates that with the increasing annealing time, the embedded gold particles become larger. Furthermore, the size of embedded Au particle in the Aupeapodded nanowires could be controlled by varying the © 2010 American Chemical Society

FIGURE 4. (a, b) TEM and HAADF images of a single Au-peapodded Ga2O3 nanowire showing that the gold particles embedded in nanowires were rodlike with an average aspect ratio of 2 and (c) a corresponding scattering spectra of the nanowire.

annealing time (1 and 5 min) at 800 °C. (See Figure S3 in Supporting Information). Embedded Au nanoparticles with different average diameters were prepared as described above. SEM images of the Au-peapodded Ga2O3 nanowires with 50 and 76 nm embedded nanoparticles are shown in panels a and b of Figure 3, respectively. The shape of nanoparticles is nearly spherical. The size histograms of the 50 and 76 nm gold particles corresponding to the SEM pictures are given in the insets of panels a and b of Figure 3, respectively. Figure 3c shows the scattering spectra of two different sizes of gold nanoparticles. We find that with the increasing particle size, the corresponding spectra are red-shifted (λmax ) 532 and 550 nm for the 50 and 76 nm embedded particles). In other words, a strong size dependence of the plasmon absorption band has been found. In addition to the size dependence of LSPR wavelength, another interesting effect has also been observed in our investigation. Change in the shape of gold nanoparticles was found to affect significantly their light scattering spectrum. Shape-dependent light scattering for gold nanoparticles is shown qualitatively in Figure 4. Panels a and b of Figure 4 are TEM and HAADF images of one single gold-peapodded Ga2O3 nanowire, showing that the gold particles embedded in nanowires are rodlike with an average aspect ratio of 2, and a corresponding scattering 3269

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In conclusion, Au-peapodded Ga2O3 nanowires were synthesized by thermal annealing of core-shell Au-Ga2O3 nanowies at a sufficiently high temperature. From in situ TEM observation, the gold core inside the Ga2O3 nanowire was fragmented into nanoparticle chains along the whole length of the wire upon heating. Additionally, the coarsening of the gold particles can also be observed with increasing time. The formation mechanism is ascribed to Rayleigh instability together with diffusion between gold particles. Furthermore, gold particle size can be controlled by annealing time. Using a dark-field optical microscope modified with microregion spectroscopy, optical properties of localized surface plasmon resonance from single gold-peapodded Ga2O3 nanowires were also observed and studied. Light scattering from embedded gold nanoparticles is size-dependent, red-shifting with increasing size from 50 to 76 nm diameter particles. Moreover, the plasmon resonance peak splits into two peaks as embedded gold nanoparticles were not spherical, but rodlike instead. The blue peak is attributed to plasmon resonance perpendicular to the major (long) rod axis, while the red peak is along the major (long) rod axis, according to the experimental verification and the prediction from Gans theory. These results suggest that such controllable onedimensional Au-peapodded Ga2O3 nanowires are excellent candidates as functional building blocks for nanoscale wavelength-dependent photoswitches and optoelectronics. The developed simple and straightforward synthetic approach may be highly valuable for the diverse applications in optoelectronics and biosensing devices.

FIGURE 5. Reversible switching behaviors of gold-peapodded Ga2O3 nanowire from a nanodevice with and without illumination. The bias on the nanowire is 2 V.

spectra of the nanowire is shown in Figure 4c. While gold spheroids embedded in Ga2O3 nanowires have a single spectral peak wavelength at ∼530 nm (Figure 3c), the plasmon resonance peak splits into two peaks: located at the wavelengths of 500 and 640 nm, respectively. Compared to the results obtained by Roorda et al. on aligned gold nanorods in silica, the extinction spectra show surface plasmon resonance shifts characteristic for anisotropic gold nanorods.22 According to Gans’ theory, The absorption of a high-energy band at around 500 nm corresponds to the oscillation of the electrons perpendicular to the major (long) rod axis and is referred to as the transverse plasmon absorption. The other absorption band at lower energies is caused by the oscillation of the free electrons along the major (long) rod axis and is known as the longitudinal surface plasmon absorption. The monoclinic Ga2O3 with a wide band gap of 4.9 eV is considered to be a good insulator and is verified by the I-V measurements of a high resistance at around 19 GΩ for a single-crystalline Ga2O3 nanowire. (See Figure S4 in Supporting Information.) The enhanced photoconductivity and distinctly wave-selected photoresponse spectra in terms of the measured resistances of a peapod nanostructure are demonstrated in Figure 5. The improved electrical conductivity and optical response of a dielectric matrix with encapsulated Au nanoparticles have been reported by Hsieh et al. and was explained by the large electromagnetic field enhancement in the vicinity of the metal surface which enhances the third-order nonlinear susceptibility of the Au nanoparticles.13 The instant photoresponse of the present peapods under illumination over 5 s is illustrated by a sharply rising photocurrent followed by a small fluctuation. The electrons generated by the SPR effect drift to the Ga2O3 barrier and tunnel to the counterelectrode. © 2010 American Chemical Society

Acknowledgment. This work was financially supported by the National Science Council through Grants NSC 952221-E-007-245-MY2 and NSC 96-ET-7-007-002-ET and partially supported by Delta Electronics through Grants 98F2233EA and 97N2418E1. Supporting Information Available. Figures showing XRD spectrum of Au-Ga2O3 core-shell nanowires, SEM image of core-shell nanowires before and after annealing, Au nanoparticle chains after annealing, and SEM images of the nanowire device. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES AND NOTES (1) (2) (3) (4) (5) (6) (7)

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