We report on plasmon induced optical switching of electrical conductivity in two-dimensional (2D) arrays of silver (Ag) nanoparticles encapsulated inside nanochannels of porous anodic aluminum oxide (AAO) films. The reversible switching of photoconductivity greatly enhanced by an array of closely spaced Ag nanoparticles which are isolated from each other and from the ambient by thin aluminum oxide barrier layers are attributed to the improved electron transport due to the localized surface plasmon resonance and coupling among Ag nanoparticles. The photoconductivity is proportional to the power, and strongly dependent on the wavelength of light illumination. With Ag nanoparticles being isolated from the ambient environments by a thin layer of aluminum oxide barrier layer of controlled thickness in nanometers to tens of nanometers, deterioration of silver nanoparticles caused by environments is minimized. The electrochemically fabricated nanostructured Ag/AAO is inexpensive and promising for applications to integrated plasmonic circuits and sensors.
©2010 Optical Society of America
Dielectrics with embedded noble metal nanoparticles have attracted considerable research interest due to their unique optical and electrical properties [1–3]. These dielectrics have promising applications, e.g., surface-enhanced Raman spectroscopy (SERS) and plasmon enabled optoelectronic circuits, and biophotonic sensors. When closely spaced noble metal nanoparticles are illuminated by light of proper wavelengths, surface plasmon resonance and plasmonic coupling among nanoparticles are induced [4,5]. The collective oscillation of surface plasmons results in significantly increased absorption/scattering cross-section of nanoparticles and an enhanced local electric field [6,7]. Recent studies demonstrated that metal-dielectric nanocomposites exhibited nonlinear and fast optical responses near the frequency of surface plasmon resonance (SPR) due to the enhanced third-order optical susceptibility and thus had significant potential application to nanoscale integrated optoelectronic systems below the diffraction limit of light [8–10]. In addition, TiO2-based nanocomposites [11,12], molecule-linked gold (Au) nanoparticles [13,14], Au-nanoparticle-encapsulated silica nanowires , Au-in-Ga2O3 nanowires , Au-coated C60 wires , and Au-deposited GaN/InGaN/GaN quantum well cells  have been studied for applications to plasmon-enhanced energy conversion, electrical conduction, photocurrent and nanophotonic switches. Low dimensional nanostructures such as nanowires and nanotubes are expected to exhibit excellent performances, such as high sensitivity and instant response, in comparison with their bulk counterparts due to their small sizes and high surface-to-volume ratios. However, fabrication of useful nanowires and nanotubes based integrated devices and circuits usually requires complicated, low-yield, and high-cost processes, which have seriously hindered the development of large-scale nanowire and nanotube circuits.
Porous anodic aluminum oxide (AAO) films are characteristic of nanochannels with controllable diameters, good mechanical strength and thermal stability. AAO templates have been used for the fabrication of nanowires and nanotubes of various materials [19–21]. It has also been used to prepare self-assembled arrays of metals, alloys, and multilayers that can be incorporated into biochemical sensors, solar cells, and magneto-optical storage and recording media [22–24]. Furthermore, AAO nanochannels of very high aspect ratios (length to diameter ratio) can be fabricated for demanding applications such as photonic crystals and tunable filters . Ag nanoparticles have been grown or deposited inside the pores of AAO for applications to SERS biosensors. However, research on 2D Ag nanoparticles arrays encapsulated in AAO for photo-induced electrical conductance, photoresponse, photodetection is lacking. In this paper, we present our studies on photoconductivity and photoresponse of a well-arranged and closely spaced array of Ag nanoparticles encapsulated inside a highly ordered and large-area (1.27 cm2) matrix of AAO nanochannels, for which a simple and cost-effective fabrication technique was employed [4,26].
The Ag/AAO nanostructured film has a large area of photo responsive 2D nanostructured array of silver nanoparticles which are isolated from the ambient environments by a thin and inert alumina layer. This study reveals a remarkable on/off ratio of switched photoconductivity in response to alternating illumination by a 405-nm laser, corresponding to the strongest SPR absorption by the embedded Ag nanoparticles. Additionally, the whole nanostructure exhibits incident laser power-dependent characteristics. Electrochemical process for the fabrication of Ag/AAO is inexpensive and provides a promising solution for manufacturing AAO films incorporated with various hybrid materials. The wavelength-dependent, power-controllable and SPR assisted optical switching of electrical photoconductivity, operating in the transition from the capacitive to conductive coupling regimes between a closely spaced 2D array of Ag nanoparticles encapsulated in alumina dielectric will be reported.
2. Experimental setup
AAO films with sub-wavelength nanostructures and encapsulated arrays of silver nanoparticles were fabricated electrochemically. High purity (99.99%) annealed aluminum (Al) foils were electro-polished in a solution of HClO4 mixed with C2H5OH by the volume ratio of 1:5 for reducing the surface roughness [27,28]. These Al sheets were subsequently anodized in 0.3 M oxalic acid under a constant applied DC voltage of 40 V at the temperature of 4°C. After the anodization process, these AAO substrates were etched in 5% phosphoric acid at room temperature for 40 minutes to enlarge the diameter of nanochannels in the AAO, for achieving the desired size and distance between nanochannels, in which Ag nanoparticles are deposited . Ag nanoparticles were electrochemically deposited at the bottom of the nanochannels by means of an electrodeposition process in a solution of 0.006 M silver nitrate and 0.165 M magnesium sulfate with an applied AC voltage of 15 V. After depositing Ag, the underlying Al was etched away in a saturated HgCl2 solution in order to expose the back-end AAO barrier layer. Finally, two tungsten electrodes were fabricated on the back-end barrier layer for subsequent measurements of photoconductivity. The length, width, thickness, and inter-space of these two electrodes were 600, 200, 1, and 20 μm, respectively.
Figure 1(a) shows the scanning electron microscope (SEM, JEOL JSM-7001) photograph of a fully processed Ag/AAO film. The shown back-end (after the remaining aluminum is etching away) barrier layer is constructed of hexagonal close-packed (hcp) convexes. To expose encapsulated silver nanoparticles in the AAO template, the back-end barrier layer was etched away in 5% phosphoric acid at room temperature. The inset in Fig. 1(a) clearly exhibits Ag nanoparticles embedded inside alumina nanochannels. Silver nanoparticles are arranged in a hexagonal array with the average channel diameter, inter-particle distance, and particle size of approximately 80, 20, and 80 nm, respectively. Figure 1(b) shows the corresponding EDS spectrum of the film, indicating the presence of Ag nanoparticles in the AAO. The Al and O peaks came from the alumina template. The Pt peak came from the coated thin conductive Pt layer on AAO for clearer observation of the film surface by SEM.
Figure 1(c) displays the far-field extinction spectra of an AAO film without embedded Ag and an Ag/AAO film as measured by a UV/VIS/NIR spectrometer (U-3010, HITACHI). The spectrum for the Ag/AAO film reveals an extinction band with the maximum located at around 410 nm, while that for the AAO film without embedded Ag nanoparticles is a featureless straight line. The comparison indicates that the absorption band originates from the embedded array of Ag nanoparticles, in consistent with their characteristic plasmon resonance wavelength. Moreover, the extinction spectrum of the Ag/AAO film shows a red-shift as compared with the calculated data by Creighton et al . This is believed to be due to the fact that the AAO matrix surrounding Ag nanoparticles has a higher refractive index than that in air or vacuum. Lasers with wavelengths within the high absorption spectral region were chosen for studying the plasmon-induced optical switching of electrical conductivity [5,31,32].
2.2 The optical system setup
Figure 1(d) exhibits a schematic diagram of the experimental setup for the measurement of photoconductivity of Ag/AAO films. A He-Ne laser (λR = 633 nm) and two diode lasers (λG = 532 nm, and λB = 405 nm) were used as the light sources to study the wavelength dependent photoconductivity. Polarization and power of the incident laser beam were maintained by a half-wave plate (HWP) and a polarizer (PL). Each of three continuous wave laser beams was conducted through a reflective mirror (RM), a beam splitter (BS), and then focused vertically onto the sample surface by a microscope objective lens (Olympus 20✕, NA = 0.46). For photoconductivity measurement, a programmable Keithley 237 High Voltage Source-Measure Unit, SMU, which is capable of supplying a program selected voltage and simultaneously measuring the electrical current flowing through two electrodes via two tungsten probes were used. DC voltage (V bias) of 1-20V was applied between these two electrodes for measuring the photo induced electrical current. A charge coupled device (CCD) was employed to confirm the position of the focused laser beam to be centered between these two tungsten electrodes. The sample surface was intermittently (light-on/light-off cycles) illuminated by lasers within preset time intervals. Photoconductivity measurements were carried out at the room temperature. The dynamic variation of photo induced electrical current was recorded in real-time by a computer for subsequent analyses.
3. Results and discussion
The dependence of dynamic variation of photo induced electrical current on the wavelength of the incident light is first investigated. Figure 2(a) and 2(b) depict the photo induced electrical currents (I AAO and I Ag/AAO) measured between two electrodes of the same size and distance on an AAO film without embedded Ag nanoparticles and an Ag/AAO film as a function of time under alternately laser-on and laser-off illumination. With a spot diameter of around 22 μm, the intensity (P R,G and B) of illuminating light is about 2.63 μW/μm2 for all three wavelengths applied in this series of experiments. Under the illumination by a 405-nm laser (B), the photo induced electrical current of the Ag/AAO film is measured to be about two and half times of what is measure with a 532-nm laser (G), and about five times of that with 633-nm laser (R). However, the photo induced electrical current for the AAO film without embedded Ag nanoparticles does not vary much when the illuminating lasers are turned on and off for all three lasers of different wavelengths. For the Ag/AAO film, the photo induced electrical current sharply rises upon the laser illumination, fluctuates slightly, and then quickly returns to the initial value after the laser source is turned off. By comparison of these results, Fig. 1(c) implies that the variation of induced photocurrent for the Ag/AAO film can be attributed to the presence of an array of Ag nanoparticles and their corresponding plasmon resonance excitation. In addition, the higher electrical current (15 pA) induced by the illumination light B than those by the illumination light R and G suggests that surface plasmons of silver nanoparticles depend on the wavelength of the illuminating light, which plays an important role in the induction of photoconductivity.
Enhancement of photoconductivity was reported for Au nanoparticle-embedded-Ga2O3 nanowires and Au nanoparticle monolayer arrays, in which the hot electron tunneling via the oxide barrier and the photothermal effect were proposed as possible electrical conduction mechanisms for the measured photoconductance [14,15]. Hence, in order to clarify the mechanism which dominates the photoconductance of the Ag/AAO films, variation of photo induced electrical current for an AAO film without embedded Ag nanoparticles and that for an Ag/AAO film upon R, G, and B illumination at the power level of P R,G and B = 2.63 μW/μm2 for 20 minutes was measured. Figure 2(c) and 2(d) show measured photo induced electrical current under an extended period of laser illumination. It is found that the photo induced electrical current is stable (does not vary with the illumination time) for both the AAO film and the Ag/AAO film. It indicates that the measured photo induced electrical current on the Ag/AAO film is not caused by the accumulative photo-thermal effect. Consequently, the enhanced photo-response for the Ag/AAO film possibly originates from hot electron generation, and will be discussed later.
Figure 3(a) shows the photoresponses of an Ag/AAO film under R, G, and B illumination with varied laser power density (P R,G and B = 0.526~5.26 μW/μm2). The linear relationship between the photocurrent and the laser power density suggests that the space charge effect of the Ag/AAO film is almost negligible . The photocurrent, I ph, is defined as I ph = I Ag/AAO-I d, where I d and I Ag/AAO are the dark current and total electrical current, respectively. The current responses upon R, G, and B illumination are proportional to the laser power density ranging from 0.526 to 5.26 μW/μm2, but with different slopes equaling to 0.6(R), 1.2(G), and 3(B) pA μm2/μW, respectively. It is obvious that the largest increase in photo induced electrical current at any specific power density of laser illumination occurs when the Ag/AAO is exposed to B illumination. This phenomenon should have its roots from the light-illumination-induced plasmons on the Ag/AAO film according to the strongest SPR absorption measured under B illumination, as shown in Fig. 1(c). Figure 3(b) exhibits the reversible photoresponse curves, showing the maximum photocurrent of 4.6, 9, and 23 pA with on/off ratios of 35, 70, and 185 under R, G, and B illumination, respectively. An enlarged view of a typical on/off cycle of the three laser lights is shown in Fig. 3(c). The measured response and recovery time are 0.15 and 0.1 s, respectively. Similar response and recovery time are also observed for other 6 samples with similar structure. It should be noted that, the probe station, device geometry, and photocurrent measurement circuit used in this work are not optimized for high-speed switching measurements. Precise timing measurement of the photoconductivity is underway and will be published in subsequent papers. Since the photo-response switching is controllable by the laser illumination, 2D optoelectronic devices and customized photodetectors can be realized. For example, focused ion beam (FIB) can be applied to fabricate the desired photo induced electrical routes, systems or chips based on AAO with embedded properly selected metal nanoparticles [34,35].
In addition to photoresponses, we also measured wavelength-dependent variation in photocurrent and the dark current as a function of applied bias voltage to investigate the electronic transport properties of Ag/AAO films. The source/drain current-voltage (I-V) curves measured at P R,G and B = 2.63 and 5.26 μW/μm2 are shown in Fig. 4(a) and 4(b), respectively. For both laser power densities, the Ag/AAO film exhibits an almost linear behavior under the bias voltage ranging from 1 V to 20 V. Illumination with RGB causes an increase in the electrical conductivity (reduced electrical resistance). Since the conductivity and photoresponse manifested in the Ag/AAO film are relevant to the SPR absorption, the photocurrent is believed to derive from the large localized electromagnetic (EM) field interactions between closely spaced sliver nanoparticles [11,31]. The Ag/AAO film presents a wavelength-dependent photoconductance. The calculated slopes, representing electrical resistance of the Ag/AAO film, of the linear I-V curves shown in Fig. 4(a) and 4(b) are 0.705(B1), 0.275(G1), 0.135(R1), 1.090(B2), 0.410(G2), and 0.210(R2) pA/V, respectively. The resistance change under B illumination at a power density of 2.63 μW/μm2 is nearly 2.56 and 5.22 times higher than those under G and R illumination, respectively. Also, the resistance change under B illumination at a power density of 5.26 μW/μm2 is about 2.66 and 5.20 times larger than those under G and R illumination, respectively. Hence, the Ag/AAO film under in-resonance illumination exhibits a smaller electrical resistance than that under off-resonance illumination.
Several possible means of increasing the induced photocurrent include, for example, enlarging the width of illuminated conducting path for photocurrent, and the optimization of the dimensions of the AAO nanostructures. Our simulation results (not shown) indicate that plasmon induced local electromagnetic field can be increased by one order of magnitude when the thickness of alumina barrier layer is reduced to 35 nm, the inter-particle space is reduced to 20 nm, and the particle size is reduced to 25 nm. The higher EM field would also significantly increase the photo-induced electrical current through the AAO/Ag film.
Alumina is a good electrical insulator with a high resistance as indicated by the I-V measurements. The enhanced conductivity by light illumination originates from the greatly enhanced electromagnetic field in the vicinity of the surfaces of silver nanoparticles, which enhances the third-order nonlinear susceptibility of the silver nanoparticles. The incident light excites surface plasmon polaritons at the Ag/AAO interface. Thus, the electromagnetic field of surface plasmon polaritons decays exponentially into the AAO barrier layers between neighboring Ag nanoparticles. Tunneling and drifting of hot electrons generated by the surface plasmon resonance and surface plasmon coupling contribute to the enhanced photocurrent [15,16]. The photo-response is determined by the generation rate of hot electrons and by their tunneling rate through the alumina barrier. The number of generated hot electrons is proportional to the intensity of the incident light of proper wavelengths within the experimental parameter space of this work. This laser intensity dependent photocurrent is shown in Fig. 3(a). A constant illumination results in a stable surface plasmon strength and thus constant resistance, which is shown in Fig. 4 as an Ohmic (linear) I-V relationship.
We have investigated 2D orderly spaced AAO nanochannels with encapsulated Ag nanoparticles which exhibited significant photoconductance. The photoconductance of Ag/AAO films is strongly enhanced by surface plasmon resonance. The photocurrent, based on localized surface plasmon couplings among closely spaced Ag nanoparticles which are separated by thin AAO barrier layers, can be exploited to control optoelectronic properties of plasmonic circuits. Upon illumination with R, G, and B light, the Ag/AAO films exhibit wavelength-dependent, power-controllable and reversible photoresponses, which are consistent with SPR absorption of silver nanoparticles. It clearly shows that the photo-induced surface plasmons and surface plasmon couplings notably increase electron transportation in the Ag/AAO film. The advantage of the electrochemical fabrication process is inexpensive and simple for producing large-area uniform and almost periodic Ag/AAO composite nanostructures. The AAO barrier layer protects encapsulated Ag nanoparticles from environment induced deterioration. The 2D nanostructure is expected to enable applications which cannot be achieved by one-dimensional photoconductivity.
We gratefully acknowledge the support by Taiwan National Science Council via grants 98-3114-M-006-001 and 96-2221-E-006-286-MY3.
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