NANO LETTERS
Photoelectrochemical Performance of CdSe Nanorod Arrays Grown on a Transparent Conducting Substrate
2009 Vol. 9, No. 9 3262-3267
Martin Schierhorn,† Shannon W. Boettcher,† Stephan Kraemer,‡ Galen D. Stucky,*,† and Martin Moskovits*,† Department of Chemistry and Biochemistry, Materials Department, UniVersity of California, Santa Barbara, California 93106 Received May 13, 2009; Revised Manuscript Received August 17, 2009
ABSTRACT Perpendicularly aligned semiconducting CdSe nanorod arrays were fabricated on ITO-coated glass substrate using porous aluminum oxide (PAO) as a hard template. Nanorod lengths were varied between 50 and 500 nm, while keeping the diameter at 65 nm. The electrochemical photovoltaic performance was found to depend critically on nanorod length and crystallinity. Arrays of rods annealed at 500 °C showed an order of magnitude improvement in white light power conversion efficiency over unannealed samples. The largest power conversion efficiency of 0.52% was observed for nanorods 445 ( 82 nm in length annealed at 500 °C. The technique described is generally applicable to fabricating highly aligned nanorods of a broad range of materials on a robust transparent conductor.
Solar power is a promising candidate as a sustainable energy supply. However, at present the cost of solar energy conversion per watt by photovoltaic or photoelectrochemical cells is prohibitive for widespread use.1-3 As a result, much effort has been put into developing inexpensive materials and architectures that reduce cost and/or increase efficiency.4-8 For optimal conversion efficiency, solar devices must absorb most of the incident light and the resulting charges must be utilized before they recombine.9 Because in many inexpensive materials the exciton or minority carrier diffusion lengths are much smaller than their absorption length, device design must be focused on decoupling the two parameters. One solution is to fabricate interpenetrating, charge-separating junctions such as “bulk heterojunctions”.4 Most such architectures5,8,10-18 involve isotropic blends with convoluted conduction pathways that can lead to charge trapping and decreased carrier mobilities.5 Higher efficiencies have been observed for systems with more ordered, anisotropic geometries.19,20 Ideal bulk heterojunctions possess straight conduction pathways toward the electrodes, as would be the case for arrays of nanorods aligned perpendicular to a conductive substrate.4,21,22 Such geometries can be achieved by using porous aluminum oxide (PAO) as the hard template for materials synthesis.23-32 PAO permits precise control over the dimensions of the templated materials and it is compatible with many materials. In most cases, PAO is fabricated by * To whom correspondence should be addressed. E-mail: (M.M.)
[email protected]; (G.D.S.)
[email protected]. † Department of Chemistry and Biochemistry. ‡ Materials Department. 10.1021/nl901522b CCC: $40.75 Published on Web 08/25/2009
2009 American Chemical Society
anodizing a sheet of aluminum,23,24,30,33 which is then fashioned with an electrode32,34,35 enabling electrodeposition of materials inside its nanopores. The major drawback of this synthesis method is the poor mechanical integrity of the resulting materials system once the PAO matrix is removed. Stability can be gained by affixing the templates to a solid support. However, finding the right adhesive that is both transparent and compatible with subsequent processing steps such as high temperature annealing and incorporation of other materials can be challenging. A more robust route to synthesize functional arrays of nanostructures using PAO involves the anodization of aluminum directly on self-supporting conducting substrates such as Si wafers,36-38 Ti-coated substrates,39 and ITO-coated glass.40 For photovoltaic and photoelectrochemical applications, transparent conducting substrates such as ITO are of special interest. Anodization of aluminum directly on ITO is challenging, however, because strong electric fields in the presence of the acidic electrolyte can dissolve the ITO film and cause PAO delamination.40 Isolating the ITO surface with a material that prevents destruction is thus essential for large scale PAO formation.41 In the present study, we demonstrate the use of PAO hard templates directly formed on transparent conducting substrates as a general, robust route to fabricating large areas of functional arrays of semiconducting nanorods aligned normal to the surface. CdSe was chosen as the model semiconductor because of its favorable opto-electronic characteristics for photovoltaic applications.19,34
Figure 1. Fabrication of CdSe nanorod arrays on ITO-coated glass. (1) ITO-coated glass slides were coated with a 30 nm layer of TiO2 in an electron-beam evaporator, followed by an annealing step at 500 °C for 1 h. (2) 300-800 nm of aluminum was deposited in an electron-beam evaporator. (3) The aluminum was anodized in 0.3 M oxalic acid at 40 V and 2 °C using a graphite counter electrode to form PAO. (4) Subsequent etching in 5% phosphoric acid for 65 min to widen the channels and remove the barrier layer. (5) Electrochemical deposition of CdSe. (6) Removal of PAO in 0.1 M NaOH.
The fabrication of the PAO is summarized schematically in Figure 1 (a detailed experimental procedure is given in the Supporting Information). Hu and co-workers showed that large areas of uniform PAO can be produced if the ITO surface is equipped with a thin Ti layer.41 While this method is suitable for fabricating uniform PAO films, one must also assess the effect of this new layered structure on the photovoltaic or photoelectrochemical behavior of the ultimate device fabricated in such templates. We and others have shown that TiO2 acts as a charge recombination barrier during device operation.34 We thus chose to deposit a thin layer of TiO2 as an aid to anodization and to improve device performances. The anodization process was controlled by monitoring the current as a function of time (Figure 2a). At time zero, the current is very high due to the large surface of highly conducting aluminum available for oxidation. The current then drops rapidly as the Al becomes oxidized to form an insulating barrier.26,42 As the PAO channels grow, the barrier layer thickness decreases and the current slightly rises to a steady state of ∼2 mA/cm2. When the anodization is almost complete (after ∼8 min), the current decreases to 1 mA/ cm2. The spike at 8 min presumably results when the electrolyte comes into contact with the underlying titania layer. Under the present conditions (e.g., electrolyte concentration, temperature, and applied voltage) the PAO channels formed at a rate of 32.5 nm/min; for a film of 300 nm in thickness the process was terminated after ∼10 min. Scanning electron microscopy (SEM) images of the anodized samples revealed the formation of pores 17 ( 4 nm in diameter and roughly 300 nm in height (Figure 2b). The barrier layer of ∼15-30 nm in thickness at the aluminaTiO2 interface was removed by treating the samples in 5 wt % H3PO4 for ∼65 min. (Figure 2c). The etching procedure, Nano Lett., Vol. 9, No. 9, 2009
Figure 2. (a) A typical current voltage curve for a 300 nm thick film of aluminum deposited on an ITO-coated glass slide with an intermediate TiO2 layer. (b) SEM images of PAO before and (c) after etching in 5 wt % H3PO4. Images on the left depict cross sections while images on the right show a top view of the samples. Scale bars are 200 nm. (d) Array of CdSe nanorods imaged at a 40° angle (left) and top view (right). The average length of the rods is 240 ( 45 nm. The scale bars are 500 nm. All insets are photographs of the respective samples. Scale bar in the inset is 1 cm.
however, also causes the diameter of the channels to increase to ∼65 ( 12 nm. The transparent templates are highly uniform on a macroscopic scale. Areas over 2 cm2 were routinely generated, with no evidence of delamination of the films in any of the samples (insets Figure 2b and c). Cadmium selenide nanorods were grown electrochemically from a solution containing 0.7 mM SeO2, 0.3 M CdSO4, and 0.25 M H2SO4 (Figure 1, step 5).43,44 Triton X (0.25% v/v) was added to ensure the complete infiltration of the electroplating solution into the nanochannels. The potential was swept between -0.357 and -0.757 V versus a Ag/AgCl counter electrode at a rate of 0.75 V/s for 500-4000 cycles using a potentiostat. A Pt mesh served as the counter electrode. Sweeping the voltage ensures a roughly 1:1 stoichiometry for cadmium and selenium (Supporting Information, Figure S1).34,44 The length of the nanorods was found to be proportional to the number of sweep cycles used, growing at a rate ∼0.1 nm/cycle (Supporting Information, Figure S2). SEM images of a typical array containing nanorods 240 ( 45 nm long and 60 ( 5 nm wide are shown in Figure 2d (left, 40° angle; right, top view). The density of nanorods varied between 8.0 × 109 and 9.0 × 109 rods/ cm2. The crystallinity of the CdSe nanorods was found to improve with annealing, as indicated by transmission electron micrcoscopy (TEM) imaging (Figure 3a-c) and selected area 3263
Figure 3. Top row: TEM images of CdSe nanorods that were (a) unannealed (as-made), (b) annealed at 400 °C, and (c) at 500 °C. For each sample type, the image on the left was obtained in bright field mode and the image on the right in dark field mode. Bottom: SAED patterns of the corresponding nanorods depicted above, which were obtained (d) on an as-made rod, (e) on a rod annealed at 400 °C, and (f) at 500 °C.
Table 1. Photovoltaic Figures of Merit Obtained from Curves Such As Those Presented in Figure 4a sample
Isc (mA/cm2)
Voc (V)
as-made 0.48 ( 0.05 0.24 ( 0.02 400 °C 1.54 ( 0.12 0.32 ( 0.03 500 °C 2.48 ( 0.13 0.35 ( 0.02 a The power of the incident light was ∼100 mW/cm2. The power of the
electron diffraction (SAED) analysis (Figure 3d-f). SAED (Figure 3d) shows the unannealed nanorod (Figure 3a) to be highly polycrystalline. Broad rings with only few defined spots resulting from the (101), (110), and (201) planes of hexagonal CdSe are observed. Dark-field TEM imaging (Figure 3a, right side) in which crystallites aligned along a given crystal axis appear bright against a dark background, shows a multitude of randomly oriented crystallites with sizes in the range 7 to 20 nm with the occasional larger grain. Annealing at 400 °C increased the size of the crystallites as evidenced by the formation of more distinct diffraction spots and the disappearance of continuous rings (Figure 3e). Likewise, the TEM image (Figure 3b) shows larger grains than in the as-made sample. Finally, the sample annealed to 500 °C shows only diffraction spots in SAED patterns (Figure 3f), and the TEM image indicates only a few large randomly oriented crystallites, some spanning the entire diameter of the nanorod (Figure 3c). Annealing was also found to have a profound effect on the photoelectrochemical light conversion efficiency of the nanorod devices in an aqueous electrolyte containing 1 M NaOH, Na2S and S. This polysulfide (Sn2-) redox mediator is known to form charge-separating junctions with CdSe and to suppress photo-oxidation.45 Nanorods were exposed by dissolving the PAO in 0.1 M NaOH for 30 min (Figure 1, step 6). The samples were isolated with inert epoxy to yield an exposed area of about 0.15 cm2. The light beam was focused to an area of 0.025 cm2 to ensure that all of the incident light fell on the nanorods. Current density versus voltage (J-V) characteristics were measured under chopped 3264
FF
η (%)
0.26 ( 0.01 0.03 ( 0.01 0.35 ( 0.04 0.16 ( 0.02 0.36 ( 0.05 0.30 ( 0.07 light at 550 nm was 3.32 mW/cm2.
IPCE at 550 nm (%) 4.6 ( 0.5 11.7 ( 0.8 19.3 ( 1.5
white light with an intensity of ∼100 mW/cm2, and IPCE measurements were recorded by illuminating the samples with monochromatic light calibrated to a silicon photodiode. To show the effects of annealing we tested samples with nanorods with a fixed length of ∼240 ( 45 nm (Figure 3a,b). The short circuit current (ISC), open circuit voltage (VOC), the fill factor (FF), and the white light power conversion efficiency (η) were determined from current density versus voltage (J-V) analysis. The average values are listed in Table 1. Curves shown in Figure 4a were obtained from the best performing device of each type of sample. The ISC increased more than 3-fold from 0.48 ( 0.05 to 1.54 ( 0.12 mA/cm2 when samples were annealed at 400 °C and increased more than 5-fold to 2.48 ( 0.13 mA/cm2 upon annealing at 500 °C. The VOC increased from 0.24 ( 0.02 V (as-made) to 0.32 ( 0.03 (400 °C) to 0.35 ( 0.02 V (500 °C). Annealing also affects the FF with values increasing from 0.26 ( 0.01 (as-made) to 0.35 ( 0.04 (400 °C) up to 0.36 ( 0.05 (500 °C). Standout FFs of up to 0.45 were observed at 500 °C. That is, the overall power conversion efficiency of the asmade (0.03 ( 0.01%) could be increased by an order of magnitude (0.30 ( 0.07%) by annealing to 500 °C. Incident-photon-to-current-efficiency (IPCE) measurements (Figure 4b) show the onset of photocurrent at around 757 nm which implies a band gap of ∼1.64 eV. The value is in good agreement with the absorption onset at 1.65 eV (Figure 4c, red circles) and is slightly lower than the optical bandgap of bulk CdSe of 1.74 eV.46 IPCE values measured at 550 nm (at a monochromatic light power of 3.32 mW/ cm2) increased from 4.62 ( 0.46% for as-made samples Nano Lett., Vol. 9, No. 9, 2009
average nanorod height (and hence of the film thickness). As a result, its absorption coefficient is expected to be more or less independent of the film’s thickness (because the diameters of the nanorods and their inter-rod distances are smaller than the wavelengths involved). Therefore the dielectric function of the medium consisting of the nanorods plus surrounding matrix is given to an acceptable level approximation by an effective medium formula such as that of Maxwell-Garnett47 which would only be dependent on the volume fraction and not the film thickness. This approximation is likely to be true in any effective medium expression used for the dielectric function of the composite so long as the volume fraction occupied by the CdSe is independent of the film’s height. Figure 4. (a) Current density vs voltage (J-V) curves obtained under chopped white light conditions from best devices of each sample type. (b) IPCE curves of best performing devices of each sample type. (c) Absorptance curve (red circles) and internal quantum efficiency (IQE) (blue triangles) of a sample with nanorod lengths of 240 ( 45 nm annealed at 500 °C. (d) Power conversion efficiencies obtained under white light conditions plotted as a function of nanorod lengths. The error bars indicate deviations in rod heights within a particular sample. For (b) and (d), black triangles represent the as-made samples, red circles symbolize samples annealed at 400 °C, and blue squares are measured on samples annealed at 500 °C.
(black triangles) to 19.27 ( 1.54% after annealing at 500 °C (blue squares). To determine the internal quantum efficiency (IQE) we measured the absorptance using an integrating sphere with the light incident on the glass/ITO/ TiO2 side The IQE of arrays of nanorods with lengths of 240 ( 45 nm annealed at 500 °C was determined to be ∼44% at 550 nm (Figure 4c, blue triangles). This value is a lower bound since we did not correct for the extinction by the TiO2 and ITO layers. Nevertheless, we estimate that this value is within 10% of the actual value. Possible factors which lower the IQE are charge transport problems along the nanorods, the formation of a Schottky junction between the CdSe and the TiO2 and the internal resistance of the TiO2. The increase in PEC performance with higher annealing temperatures can be attributed primarily to an increase in crystallite size and as a consequence improved charge carrier transport properties within the nanorods. The effect becomes apparent when considering the white light power conversion efficiency as a function of nanorod length (Figure 4d). Longer nanorods absorb more of the incident light leading to a higher value of η. The efficiencies of nanorods for all samples with lengths below 100 nm are similar (regardless of annealing) but they differ significantly for the longer rods. While asmade samples do not show a significant increase in efficiency as a function of rod size (black triangles), annealing at 400 °C (red circles) improves the performance for longer nanorods. Annealing at 500 °C yields an even greater increase in η of up to an order of magnitude when changing the rod length from 100 to 450 nm. These results (Figure 4d) can be rationalized in a simple way as follows. The composite medium that consists of the CdSe nanorods and the surrounding matrix has a volume fraction of CdSe that is approximately independent of the Nano Lett., Vol. 9, No. 9, 2009
The photovoltaic efficiency of the composite in converting photons to useful electrons is expected to be proportional to the quantity of light absorbed in the film. For a film of thickness h residing on a substrate the fraction, A, of the light incident on it that is absorbed is given by A ) 1 - T - R, in which T and R are the fractions transmitted and reflected. To the extent that the fraction reflected can be ignored and assuming that h is smaller than the wavelength of light, and further assuming normal incidence, A is given approximately by A ) 2πkh/λ, in which k is the effective absorption coefficient of the composite medium consisting of the CdSe nanorods and its matrix and λ is the vacuum wavelength of the incident light.48 Hence, to this level of approximation, the quantum efficiency of the composite is proportional to h × k. We assume that, in addition to the normal electron-hole recombination mechanisms occurring in the bulk, electron-hole recombination is enhanced at grain boundaries.49 If so the efficiency would be reduced proportionally to the average number of grain boundaries that a minority hole encounters as it diffuses from the center of a nanorod to its outer surface. It is difficult to determine the exact volume density of grains in our nanorods from the 2D TEM images in Figures 3a-c because of the complex geometries of individual grains. However, an order of magnitude analysis can be carried out as follows. Assuming that the volume density of grains in a nanorod is g, the average volume of a grain would be g-1, and its average radius is proportional to g-1/3. The average number of grain boundaries an electron will cross as it diffuses from the center of the nanorod to the surface will be ∝rg1/3, where r is the radius of the nanorod. Hence, for small enough crystallites in which the recombination events are dominated by processes at grain boundaries, the efficiency will be inversely proportional to g1/3. For a nanorod that is a single crystal g ) 1. The poorer the crystallinity, the larger the value of g would be. Hence, a plot of efficiency versus h is expected to be a straight line with slope proportional to g-1/3. Referring to Figure 4d, one sees that the efficiency is indeed approximately linear in h and with slope that increases sharply with annealing temperature. One cannot determine the values of g absolutely since we made use of proportionalities rather than equalities in our analysis but the relative values of g can be estimated from ratios of slopes (to the extent that one can rely on such a simple model). Taking ratios of the 3265
slopes of the lines in Figure 4d determined for the three samples such that the slopes of the as-made nanorods and that of the sample annealed at 400 °C are referred to the slope of the sample annealed at 500 °C, the two ratios are, respectively, ∼1300 and ∼21. In other words, the analysis implies that the crystallinity increases as a function of annealing temperature, so that the as-made samples would have a grain density ∼1300 times that of the sample annealed to 500 °C. This calculated increase in crystallite density is not inconsistent with what is observed (Figure 3). The goal of this study is to understand some of the key structural and materials parameters that contribute to good photovoltaic activity of a nanostructured electrode that would ultimately be integrated with organic semiconductors, such aspoly(3-hexylthiophene)(P3HT)toformaninorganic-organic hybrid solar cell with superior performance characteristics, rather than to achieve a high current efficiency as a liquid junction solar cell. The electrochemical photovoltaic performance was used as a test-bed to demonstrate how various structural and materials modifications of photovoltaically active arrays of semiconductor nanopillars fabricated using PAO as a hard template and perpendicularly aligned to a transparent conductive support, affect their performance. Liquid junction cells with higher power conversion efficiencies would be achieved with CdSe nanorods longer than 500 nm which would have increased light absorption. The thicker templates that such systems would require may pose additional fabrication difficulties to overcome. For the fabrication of organic-inorganic hybrid photovoltaic cells where the charge transport properties of the organic phase dictates the device thickness, the nanorod lengths used fall within the optimal range.4,22 In summary, arrays of CdSe nanorods were fabricated on ITO coated with a 30 nm thick TiO2 film. The TiO2 layer served both as a protective coating for the ITO aiding in the formation of large area of PAO on a solid substrate, and as recombination barrier in the final device. The electrochemical photovoltaic response of the nanorod arrays was found to depend sensitively on the crystallinity and the length of the nanorods. Longer and more crystalline nanorods exhibited an improved open circuit voltage, short circuit current, fill factor, and consequently an overall enhancement of the power conversion efficiency. Nanorod lengths could be varied by simply controlling the number of deposition cycles, while control over diameters is possible by varying the anodization potential, type of electrolyte and post anodization treatments.33 The structures presented here and the method for their synthesis are a convenient platform for the fabrication of devices where nanorods of a variety of materials (semiconductors, metals) aligned vertically on a transparent conducting substrate are required (e.g., large surface area electrodes and photoelectrodes, bulk-heterojunction devices). Acknowledgment. Funding from the Institute for Collaborative Biotechnologies through Grant DAAD19-03-D0004 from the U.S. Army Research Office, from Lawrence Livermore National Laboratories through a UCDRD grant, and the National Science Foundation (NSF) through Grants DMR 02-33728 and DMR 08-05148 are gratefully acknowl3266
edged. This work made use of MRL Central Facilities supported by the MRSEC Program of the National Science Foundation under Award No. DMR05-20415. A portion of the work was performed in the UCSB Nanofabrication Facility, a part of the NSF-funded NNIN (National Nanofabrication Infrastructure Network). We thank Elison Matioli for help with optical measurements. Helpful discussions with Nick Strandwitz and Anna Ivanovskaya are gratefully acknowledged. Supporting Information Available: Detailed experimental procedure, an EDS line scan, and a graph of deposition cycles versus nanorod length. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32)
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