Xue et al. Nanoscale Research Letters 2013, 8:295 http://www.nanoscalereslett.com/content/8/1/295 NANO EXPRESS Open Access Gold nanoarray deposited using alternating current for emission rate-manipulating nanoantenna Jiancai Xue1, Qiangzhong Zhu1, Jiaming Liu1, Yinyin Li2, Zhang-Kai Zhou1*, Zhaoyong Lin1, Jiahao Yan1, Juntao Li1 and Xue-Hua Wang1* Abstract We have proposed an easy and controllable method to prepare highly ordered Au nanoarray by pulse alternating current deposition in anodic aluminum oxide template. Using the ultraviolet–visible-near-infrared region spectrophotometer, finite difference time domain, and Green function method, we experimentally and theoretically investigated the surface plasmon resonance, electric field distribution, and local density of states enhancement of the uniform Au nanoarray system. The time-resolved photoluminescence spectra of quantum dots show that the emission rate increased from 0.0429 to 0.5 ns−1 (10.7 times larger) by the existence of the Au nanoarray. Our findings not only suggest a convenient method for ordered nanoarray growth but also prove the utilization of the Au nanoarray for light emission-manipulating antennas, which can help build various functional plasmonic nanodevices. Keywords: Anodic aluminum oxide template, Au nanoarray, Emission rate, Nanoantenna, Surface plasmon PACS: 82.45.Yz, 78.47.jd, 62.23.Pq Background Owing to the self-organized hexagonal arrays of Excited by an incident photon beam and provoking a uniform parallel nanochannels, anodic aluminum oxide collective oscillation of free electron gas, plasmonic (AAO) film has been widely used as the template for materials gain the ability to manipulate electromagnetic nanoarray growth [26-29]. Many distinctive discoveries field at a deep-subwavelength scale, making them play a have been made in the nanosystems fabricated in AAO major role in current nanoscience [1-5]. The plasmonic films [30-34]. As increasing emphasis is placed on low metallic nanostructures have presented a vast number of cost, high throughput, and ease of production, AAO potential applications in various prospective regions template-assisted nanoarray synthesis is becoming the such as plasmon lasers [6-8], optical tweezers [9,10], and method of choice for the fabrication of nanoarrays [35]. biochemical sensing platforms [11-13]. In particular, the However, due to the existence of a barrier layer, it is nanoarray structure attracts extensive research efforts impossible to grow nanoarrays instantly after the AAO for its strong coupling between adjacent nanorods, template has been prepared via a two-step anodization which leads to dramatic field enhancement and high process using direct current (DC). Some complicated energy transfer efficiency [14,15], resulting in a diverse processes must be included, such as the Al foil removing, range of plasmonic devices such as subwavelength color the barrier layer etching, and the conducting layer making. imaging, plasmonic waveguide, functional metamaterials The pregrowth processes dramatically increase the diffi- [15-19], as well as nanoantennas for manipulating the culty of AAO template-assisted nanoarray synthesis light emission of various nanoemitters, which is of great especially in the case that a thin AAO film with a few importance for quantum science, nano-optical commu- micrometer is required [18]. On the other hand, it is nication, and light-harvesting devices [20-25]. reported that alternating current (AC) can get across the barrier layer and implement direct metal array deposition * Correspondence: [email protected]; [email protected] [36-38]. However, using the AC method, it is difficult to 1State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-Sen (Zhongshan) University, Guangzhou grow the nanoarray as ordered as that using DC, which 510275, People’s Republic of China leads to poor field enhancement and broad surface Full list of author information is available at the end of the article © 2013 Xue et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Xue et al. Nanoscale Research Letters 2013, 8:295 Page 2 of 8 http://www.nanoscalereslett.com/content/8/1/295 plasmon resonance (SPR) peaks [18,36-38]. This flaw pre- electron microscopy (TEM) was performed using a JEOL vents the AC growth method from being widely used. 2010HT (Akishima-shi, Japan) operated at 100 kV. The In this paper, we propose a pulse AC metal nanoarray TEM samples were prepared by dissolving the AAO tem- growth method, which can cut off some inevitable com- plate containing Au nanoarrays in NaOH solution. The plicated processes in AAO DC deposition and easily extinction spectra were recorded using an ultraviolet– fabricate metallic nanoarrays as uniform as those by DC visible-near-infrared region (UV–vis-NIR) spectropho- deposition. The extinction spectra, the quantum dot (QD) tometer (PerkinElmer Lambda950, Waltham, MA, USA) emission rate manipulation measurement, as well as the using a p-polarized source with an incident angle of 70°. theoretical analysis of electric field distribution and local density of states (LDOS) confirm that the pulse AC-grown Optical experiments Au nanoarrays can be a good candidate for nanoantennas. The PL from the samples was collected by the reflection measurement. An s-polarized laser for the measurements Methods of PL was generated using a mode-locked Ti:sapphire Preparation of samples laser (MaiTai, Spectra Physics, Newport Corporation, The AAO templates were prepared by a two-step anodiza- Irvine, CA, USA) with a pulse width of approximately tion process [18,33]. First, the aluminum sheets (purity 150 fs and a repetition rate of 79 MHz. The wavelength of 99.999%) were degreased in acetone and electropolished the laser beam was tuned to 400 nm. The scattering noise under a constant current condition of 1.2 A for 3 min in a was filtered using a band-pass filter, followed by a 100- mixture of HClO4 and C2H5OH at 0°C to smooth the sur- mm-focal-length lens which was used to excite the sample face morphology. In the first and second anodization pro- at a Brewster angle θb ≈ 50°. The luminescence from the cesses, treated aluminum sheets were exposed to 0.3 M sample was collected using the focusing lens and a long- H2SO4 or H2C2O4 solution under a constant voltage of 19 wave pass filter before entering the liquid-nitrogen-cooled or 45 V in an electrochemical cell at a temperature of CCD (SPEC-10, Princeton Instruments, Trenton, NJ, about 4°C. The alumina layer produced by the first anodi- USA). The time-resolved PL decay traces were recorded zation process was removed by wet chemical etching in a using a time-correlated single-photon counting system mixture of phosphoric acid (0.15 M) and chromic acid (PicoQuant GmbH, Berlin, Germany). (0.60 M) at 60°C for 1 h. The barrier layer of AAO templates was thinned stepwise with reducing potential Computational simulations down to 6 V. The computational simulations were performed using The ordered Au nanoarrays were deposited in the the finite difference time domain (FDTD) method with nanopores of the AAO template by pulse AC (50 Hz) Bloch and perfectly matched layer (PML) boundary electrodeposition in an electrolyte containing HAuCl4 (10 conditions for the x- and y-axes and z-axis, respectively. 3 mM) and H2SO4 acid (0.03 M) with a Pt counter The cell size was 2 × 2 × 5 nm . The nanowires were electrode. The deposition was carried on instantly after hexagonally arrayed with a rod diameter of 34 nm, the completion of the AAO template using a common AC length of 150 nm, and spacing of 110 nm. The refractive power source (GW APS-9301, GW Instek, New Taipei index of Al2O3 was set to be 1.76, and the complex City, Taiwan) supplying a 4-s pulse of 16 V, followed by a dielectric constants of the gold were taken from the lit- growth potential of 9 V. There is no need to remove the erature of Johnson and Christy [39]. Al foil, etch the barrier layer, and make a conducting layer The photonic LDOS was obtained by calculating the before Au nanoarray growth, which makes the electrode- Green function with the help of the COMSOL software position very convenient. The normal AC deposition (version 4.2a). The hexagonal lattice of Au nanowires method was carried on in the same condition as the pulse was simulated with the scale of 7 × 7 arrays. The lattice AC, except for the 4-s pulse of 16 V. constant was set to be 110 nm. The length and the The quantum dots were commercial carboxyl CdSe/ZnS diameter of each Au nanowire were set to be 150 and 34 quantum dots, which were purchased from Invitrogen nm, respectively. The refractive index of the background Corporation (Carlsbad, CA, USA). In the time-resolved was 1.76. The dielectric constant of gold was taken from photoluminescence (PL) measurement of the QDs, the Al the literature of Johnson and Christy [39]. An electric foil was taken using CuCl2 solution, and QDs were point dipole is set 10 nm above the center of the arrays. dropped on the barrier side of the AAO template. A block with the size of 0.99 × 0.887 × 0.31 μm3 is set to separate the array and the PML. The PML is set to a Characterization of samples size of 1.65 × 1.547 × 1.15 μm3 with general type. To get Scanning electron microscopy (SEM) was performed a good mesh, a sphere with the radius of 4 nm is set to using a Zeiss Auriga-39-34 (Oberkochen, Germany) op- surround the dipole.