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Long Pu, Abdullah Saud Abbas, and Vivek Maheshwari*
Nanoparticles and their arrays present a diverse range of electrical behavior with characteristic coulomb blockade, coulomb staircase, transition from insulator to metal properties and sensitivity to gating.[1–6] The wide range of characteristics are the result of phenomena such as single electron charging effects due to the low capacitance of an isolated nanoparticle hence requiring high charging energy, quantum confinement due to tunneling barriers between adjacent nanoparticles and coupling between neighbouring particles due to their close proximity.[2,5,7–11] Au nanoparticles are used commonly for formation of such arrays due to their, well-controlled size, effective assembly using thiols and anions, and metallic nature.[12,13] The presence of defects in these nanoparticle arrays leads to observation of single electron behaviour and coulomb blockade at room temperature.[7,13] The highly sensitive characteristics of these arrays (on the scale of single electron) presents an opportunity for their application in devices and sensors. Developing methods or hybrid materials that can control or modulate the characteristic behavior of these arrays will therefore advance new applications for these systems. For example, recently it has been shown that nanoparticle arrays functioning as transistors in aqueous medium can detect cellular activity on interfacing with live cells.[14,15] Similarly by interfacing with nanomaterials such as ZnO nano-rods that are photosensitive and piezoelectric, it should be possible to modulate the array characteristics using light and stress stimuli.[16] Therefore a strategy for interfacing the arrays with other kind of nanomaterials will be of significant interest to further advance these systems. Here we demonstrate in-situ electrochemical synthesis of a parallel ZnO nano-rod interface on Au nanoparticle array that leads to photo-modulation of the array characteristics. Au nanoparticles, the basic building block of these array systems are catalytic in nature and are also electrochemically active.[17–20] Combining the electrical conduction of the array with the catalytic and electrochemical nature of its constituent Au nanoparticles, ZnO nano-rods are formed directly on the array by electrochemical synthesis. The diameter of the ZnO rods (20–30 nm) is on the scale of the size of Au nanoparticles (10–12 nm) and spatially their synthesis is confined by the extent of the array L. Pu, A. S. Abbas, Prof. V. Maheshwari Dept. of Chemistry Waterloo Institute of Nanotechnology University of Waterloo 200 University Ave. West, Waterloo, ON, N2L 3G1 Canada E-mail: [email protected]
DOI: 10.1002/adma.201402034
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Electrochemical Synthesis on Nanoparticle Chains to Couple Semiconducting Rods: Coulomb Blockade Modulation Using Photoexcitation
(1–2 µm). The process opens the use of such nanoparticle arrays as electrochemical systems, significantly expanding their potential use in sensors, multifunctional materials, development of single electron transistors and nano-scale energy systems. The ZnO-Au hybrid system presented here is a multifunctional material, combining the individual characteristics of the Au nanoparticle array and the ZnO nano-rod pathway and also intermodulation between the two materials. Behavior characteristic of Au nanoparticle array such as coulomb blockade at room temperature, single electron charging effects and a power law dependence in current-voltage are observed. The hybrid system also shows Schottky behavior and photocurrent generation due to the characteristics of the ZnO nano-rods.[21–23] Besides the individual characteristics, the hybrid system also has coupling between the Au nanoparticles and ZnO rods. This behavior is observed in the modulation of the threshold voltage of the Au array due to the generation of photo excited electronhole pairs in the ZnO rods. In complimentary fashion the conduction of the electrons in the Au array leads to modulation of the decay time for the photocurrent in the ZnO rods. Micrometer long chain networks of Au nanoparticles (10–12 nm in size) self-assembled in solution using divalent Ca2+ ions[12,15,24] are deposited between micron scale electrodes functionalized with silane, forming a conductive pathway (see Supporting Information, SI). As a result of the self-assembly, the UV-Vis absorption spectrum due to the surface plasmon resonance of the Au nanoparticles red shifts to ∼600 nm from 525 nm, as seen in Figure 1a. The red shift is attributed primarily to the plasmon coupling between neighbouring particles as they assemble into network of chains. The shoulder at ∼530 nm in the Ca-Au chains (Figure 1a) is attributed to the plasmon resonance along the shorter axis ∼ 10–12 nm, the diameter of the nanoparticles, similar to unassembled nanoparticles. Transmission electron microscopy (TEM) images of Figure 1b clearly show the assembly of the micron long arrays consisting of Au chain networks. The inset shows a typical chain structure with the spacing between the nanoparticles being on the scale of 1–2 nm (magnified images in SI, Figure S-2,S-3). The assembly is also monitored by using light scattering and zeta potential measurements. Prior to assembly a size of ∼10 nm and a zeta potential of ∼ −40 mV is recorded for the Au nanoparticles. After the addition of Ca2+ ions (see SI, Figures S-4–S-8) the size increases significantly and the zeta potential remains close to −40 mV. The length scale of chains can be controlled by the time provided for the assembly and the concentration of divalent ion used, shorter time lead to shorter chains (see SI, Figure S-9&S-10). We believe that the separation between
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Figure 1. (a) The assembly of the Au nanoparticles due to Ca2+ ions shifts the plasmon resonance peak from ∼520 nm to 610 nm. A noise in the equipment is observed at ∼650 nm. (b) The TEM image clearly shows the assembly of the nanoparticles into branched chain structures. The higher magnification image in the inset shows that Au nanoparticles are separated by 1–2 nm. (c) The chains upon deposition bridge the gap between two Au pads forming a conductive pathway. In white are highlighted the bottlenecks with 1-D chains. The higher magnification image in the inset shows that the assembly is similar to the TEM image of (b). (d) Typical characteristics of coulomb blockade and oscillations in conductance (inset) are observed from the array shown in (c).
the nanoparticles should be dependent on the ion used for assembly. More detailed investigation in this aspect is being performed. The chains form a conductive pathway for electron transport on application of bias across the Au electrodes, Figure 1c, field emission scanning electron microscopy (FESEM) images. The inset, a high magnification FESEM image more clearly shows the nature of the Ca-Au chain pathway (also Figure S-3 in SI). A typical I−V (current-voltage) response of the chains is shown in Figure 1d. We observe the characteristic coulomb blockade (threshold voltage VT) and the single electron charging effects that lead to oscillations in the differential conductance (inset Figure 1d). The data points of the I−V response are fit to a power law model (as explained later), providing a threshold voltage, VT, of 4.87 V for the array and a power law scaling with 2.6. Both these values are consistent with the observed conduction through 2-D array of nanoparticles in presence of defects.[6,7,9,12] Typical defects such as a series of single Au nanoparticle channels in the pathway are bottlenecks for the flow of electrons (highlighted in white in Figure 1c). These defects lead to the observation of the coulomb blockade at room temperature. Following the formation of this conductive nanoparticle array, electrochemical synthesis of ZnO is performed on the array in a microfluidic cell using a three electrode system (details in SI),[25–29] schematic shown in Figure 2a. CVD and
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flame transport approach have also been used recently for growing heterogeneous ZnO nanostructures.[30,31] As a result, ZnO nano-rods grow on the Au nanoparticle chains. This transformation is shown in the FESEM images of Figure 2b. The initial Au nanoparticle chain array (inset of Figure 2b) is the specific location for the growth of the ZnO rods. The adjacent SiO2/Si areas are devoid of any ZnO (highlighted in black in the figure and in white in the inset) (more images in SI). Further in the higher magnification image of Figure 2c we observe that the diameter of the ZnO rods is ∼20–30 nm and they have hexagonal cross sectional area, typical for the growth of the ZnO rods along the 0001 plane. In the FESEM image of Figure 2d clearly visible underneath the rods is the presence of Au nanoparticles, seen as bright silhouette in the image. This confirms that Au nanoparticle chains serve two primary purposes: 1. Their metallic nature and ability to conduct electrons results in electrochemical synthesis on their surface upon the application of a potential. 2. The Au nanoparticles are well known to act as preferential growth sites for ZnO, this is confirmed by the observation that the diameter of the grown ZnO rods matches well with the size of the Au nanoparticles. This is also in contrast to ZnO rods synthesized in similar manner on continuous planar surfaces that have a diameter of ∼150–250 nm.[29] This hybrid device of Au nanoparticles and ZnO is then characterized for its current-voltage response and photo
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COMMUNICATION Figure 2. (a) The three electrode set-up for synthesis of ZnO on Au-chains. (b)Following the synthesis, ZnO nano-rods are clearly observed on the Au-chain array. Inset shows the Au-chain array prior to synthesis. Highlighted in, white in the inset and in black in the figure, clearly observed is the absence of the rods where there are no Au nanoparticles. Scale in the inset is 500 nm. (c) The ZnO nanorods are ∼20–30 nm in size and are growing directly from the nanoparticles. (d) We can clearly see the underlying Au-nanoparticle array below the ZnO nano-rods. The scale bar is 100 nm.
excitation properties. The I−V response from a typical device under dark conditions is shown in Figure 3a. The curve shows typical oscillations in the differential conductance (Figure 3b), indicative of current flow through the underlying Au nanoparticle pathway.[2,4] The device has a very similar current response under positive and negative bias (Figure 3a). This is also true for devices with only Au chains. The device is then illuminated with light from a solar simulator (1.5 solar mass) with progressively increasing intensity. As observed in Figure 3c, there is a corresponding increase in current with increasing light intensity. This response indicates that the ZnO rods also form an electron conduction pathway. This is further
confirmed by illuminating a device with only Au nanoparticles chains with light in which case no photo response is observed. Furthermore when a long pass filter of 420 nm is placed in the light path no photo response is observed from the hybrid device. This confirms that UV light, as required for the excitation of ZnO rods due to its reported bandgap of ∼3.2 eV, is necessary to generate the photocurrent.[16,22] The I−V response from the hybrid device is then modelled as consisting of two pathways, a Schottky equation for the flow of current from Au pads to ZnO rods to Au pads and an Au nanoparticle array which follows power law dependence (schematic in Figure 3d) (Equation (1)).
Figure 3. (a) The I−V characteristics of the hybrid device shows a non-linear response also observed are the periodic oscillations in current. (b) The oscillations in conductance are indicative of transport through the nanoparticle array. (c) On illumination, the current through the hybrid device increases. The current characteristics are accurately fit by Equation (1), electron transport though both nanoparticles and ZnO rods. (d) Schematic showing the two conduction pathways, through Au-nanoparticles in yellow and through nano-rods in grey. (e) Increasing light intensity decreases the threshold voltage, VT for the nanoparticle array; while α, it’s dimensionality remains unchanged. (f) Gating of a purely Au nanoparticle array device leads to a progressive decrease in the VT with increasing gate voltage.
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⎛V ⎞ I = P1 [ exp (P2V )] + K ⎜ − 1⎟ ⎝ VT ⎠
α
(1)
Here the first term is the Schottky current through the Au electrodes and the ZnO pathway with P1 being the saturation current and P2 combines thermal energy, electron charge and the ideality factor for ZnO pathway. P1 is also related to the thermionic emission over the barrier. This can be used to calculate the barrier height.[22,23] The second term is the power law for the current through the Au nanoparticle pathway. K, characterizes the current through a single nanoparticle channel in the pathway; VT is the threshold voltage required for the start of electron conduction and α is the scaling exponent that relates to the dimensionality of the nanoparticle array.[5] The fit of this equation to the data from the device both under dark and illumination is shown in Figure 3c. The points are actual data and the line is the fit from the equation, in these plots. The equation fits the data very well. The influence of the ZnO on the Au chains is illustrated by observing the effect of light intensity on the parameters VT & α. Typically for 2-D arrays α is ∼5/3 from theoretical models.[5] However due to local defects and conduction along single nanoparticle pathways, 1-D systems, the exponent increases and can typically range up to 2–3.5.[3,6–10,14,32,33] As shown in Figure 3e, we observe that the exponent is ∼2.45, which matches with the array being a 2-D matrix of nanoparticles with 1-D bottlenecks for conduction. These 1-D restrictions are the result of the basic nature of the Ca-Au chains that form a branched 1-D structure. On illumination, with increasing intensity of light no significant change is observed in α. This is expected as the underlying dimensionality of the Au matrix is not affected by the generation of the electron-hole pairs in the ZnO rods. The threshold voltage VT decreases significantly on photoexcitation of the ZnO rods, as seen in Figure 3e. This effect can be considered as similar to the gating of these pathways by an external electric field. On illumination, electron hole pairs are generated in the ZnO rods and the holes are trapped by the surface defects on the rods.[34–36] These holes lead to the local gating of the Au nanoparticles and hence modulate the threshold for the conduction along the Au array. The decrease in VT stabilizes at higher light intensities. This is expected as the generation of the electron-hole pairs saturates at higher intensities, hence so does the effect of gating due to ZnO. The modulation of VT occurs due to the close proximity of the ZnO, a photosensitive material, to the Au-array. The decrease in the threshold (VT) with increasing light intensity is qualitatively similar to the gating behavior in plain Au-Ca chain arrays (see Figure 3f), where a decreasing trend in VT with increasing gate voltage is also observed. Here the I−V behavior of Ca-Au chains recorded at increasing gate voltages shows a linear decrease in the threshold voltage, VT.[2,15] The VT of a hybrid device is ∼1.3 V, for a plain Au chain device this is ∼3.6 V in dark (Figure 3e,f). The difference arises as ZnO rods have a surface potential due to the presence of defects and the arrangement of the constituting ions (Zn2+ and O2−). Hence the formation of the ZnO rods directly on the Au chains results in their gating and the observed change in VT. However α remains relatively
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unaffected between the hybrid device (∼2.45) and the Au chain device (∼2.86, Figure 1d). Considering the charging energy required for a single Au nanoparticle (10 nm in size) due to its capacitance, we can relate the threshold voltage, VT, to the number of such charging centers present in the device.[5,14,32] There are ∼ 9 charging centers present in the hybrid device. On illumination and the subsequent effect of gating by ZnO the charge centers are reduced to ∼ 5. This provides the ability to photo modulate the charging effects in the array on the scale of a single nanoparticle. The photocurrent generation upon excitation of the ZnO rods by UV light was recorded to characterize their response and also the effect of the underlying Au matrix on the rods. The net response from the hybrid was recorded at a series of applied constant bias, Figure 4a. The relative photocurrent as plotted is the ratio of the net current to the dark current. The application of bias also leads to the flow of current through the Au nanoparticle array and as the bias increases the magnitude of the current through the nanoparticles also increases. This occurs due to increased electron transport through the nanoparticles, hence a greater charging of the nanoparticles to sustain the current. A simple effect of this increase in charging will be the greater interaction of the Au nanoparticles with the surface trapped holes on the ZnO rods. This is illustrated by fitting the decay of the photocurrent in the hybrid after turning off the illumination. Similar to other observations, a bi exponential equation with time constants of T1 and T2 is fit to the decay (details in SI).[37] The change in the decay time constants with increasing bias is plotted in Figure 4b. As expected at higher voltages larger time constants are observed, consistent with the increased interaction with between the surface trapped holes in ZnO and the greater electron charging of the Au nanoparticles. This increases the time for the recombination of the electron-hole pairs in ZnO as the illumination is turned off. The photocurrent effect of the hybrid device and just the ZnO rods pathway is decoupled in Figure 4c, using the parameters from the model (details in SI). The net relative photocurrent through the device (curve in black) increase with applied bias and stabilizes at higher voltages. While considering only the current through the ZnO path (curve in blue) a maximum in relative photocurrent is observed at ∼7 V. This occurs as with increasing bias the base current over the Schottky junction of ZnO path increases, leading to a reduced contribution from the photo generate current.[36] The relative photocurrent reported in other devices consisting of only ZnO pathway is significantly higher.[36,37] The fit of the Schottky term in Equation (1) provides a value of ∼1.5 × 10−12 amperes for P1. This is used to calculate the barrier height, ΦB, between the ZnO and Au electrodes using the expression for the saturation current i.e., P1 = AT2exp(-ΦB/kT), where A is the Richardson constant, which for ZnO is estimated to be 32 Acm−2K−2, T is the temperature and k is the Boltzmann’s constant.[22] Typical device areas are ∼10 µm2 (as defined by the spacing between the Au electrodes and the actual area over which the Au chains deposit as seen in Figure 1c) and with T = 298 Kelvin, a barrier height of 0.67 eV is obtained. This matches well with the reported values in literature for Au-ZnO contacts.[35,38–41] The relative current in the two pathways of ZnO rods and Ca-Au chains is plotted in Figure 4d both in dark and also
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COMMUNICATION Figure 4. (a) The net photocurrent response from the hybrid device increases with applied bias. The illumination is at 150 Watts for all the biases (b) The decay of photocurrent fit to double exponential shows that the time constants for the decay increase with increasing bias across the device. (c) The net photocurrent, in black from the device follows a monotonically increasing trend, while considering only the ZnO circuit in blue, a peak is observed at ∼7 V. (d) The relative current between the two pathways shows that depending on the applied bias and light conditions the device can switch from being dominated by ZnO to Au-array to a hybrid combination, for the flow of the current.
under illumination (by using the model, details in SI). The data illustrates the effect of the applied bias on the functioning of the hybrid device. At low bias (less than VT or close to it), conduction through ZnO pathway is the dominant mechanism. In the range of 3–4 V, conduction through Ca-Au chains increases and under dark conditions becomes the dominant mechanism. At higher biases the fraction of current through ZnO recovers slowly. The functioning of the device hence can be controlled by the applied bias and light illumination as from being pre-dominantly a ZnO nano-rod device to Ca-Au chain one or a hybrid with contributions from both the pathways. In summary, hybrid devices composed of two distinct nanomaterials with characteristic individual properties are synthesized and characterized. The device is made of a self-assembled Au-nanoparticle chains array, which is then used as an electrochemically active system for synthesis of ZnO nano-rods. The use of such single electron sensitive templates to assemble semiconducting nano-rods as a direct and parallel interface leads to hybrid properties for the device. The combination of two distinct materials is possible due to the ability of Au nanoparticles to conduct electrons and also act as nucleation centers for the formation of the ZnO nano-rods. The device shows typical characteristics of both, the conduction through Au nanoparticle array and also ZnO nano-rods. The direct interfacing between these two parallel devices leads to mutual inter-modulation between the two pathways. This allows us to switch the device functioning between being dominated by the Au-Chains or ZnO rod network or as a hybrid combination. Further by using photo excitation of the ZnO rods the charging centers in
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the Au chain array pathway can be modulated on the scale of single nanoparticles providing a means to gate the device and alter the VT. Arrays of Au nanoparticle chains are room temperature single electron sensitive systems. The ability to use such nanoparticle chains as electrochemically active systems with both structural and spatial confinement of the synthesized material will significantly expand their application. Besides ZnO other semiconducting materials such as p-type Cu2O, energy storage materials such as MnO2, catalytic materials such as Pt, semiconducting polymers and biological systems such as cells and proteins can be interfaced with these arrays. These will be useful for development of nano scale electro-optical devices, electrochemical sensors, nano scale energy storage systems and diverse catalytic systems.[17–20,42–50]
Experimental Section Ca-Au chains are made by mixing 4 ml of Au nanoparticle solution (10 nm size, from BBI International) with 360 µL of 1 mg/mL CaCl2 solution (details in SI). Following their assembly after 8 hrs., these Ca-Au chains are deposited on silane functionalized chips (Si substrate with thermally grown SiO2 layer) that have inter-digited Au finger electrodes (details in SI). The ZnO nano-rods are electrochemically synthesized on the deposited Au chains by placing the chip in a microfluidic cell containing 1.25 mM ZnCl2 and 0.1 M KCl maintained at 80 °C. The synthesis is conducted in a three electrode set-up with a working potential of −0.955 V applied for 7–10 minutes (details in SI). Both the deposited Ca-Au chains and the ZnO Ca-Au hybrid devices are
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www.MaterialsViews.com characterized for their current-voltage and photoresponse behavior (details in SI). The assembly of the Ca-Au chains is also characterized by light scattering and zeta potential measurements (details in SI).
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the University of Waterloo, Canada Foundation for Innovation, Ontario Research Fund and NSERC Canada. Received: May 6, 2014 Revised: July 4, 2014 Published online: August 14, 2014
[1] A. Zabet-Khosousi, P. E. Trudeau, Y. Suganuma, A. A. Dhirani, B. Statt, Phys. Rev. Lett. 2006, 96, 156403. [2] K. K. Likharev, Proc. IEEE 1999, 87, 606. [3] M. G. Ancona, W. Kruppa, R. W. Rendell, A. W. Snow, D. Park, J. B. Boos, Phys. Rev. B 2001, 64, 033408. [4] T. Sato, H. Ahmed, D. Brown, B. F. G. Johnson, J. Appl. Phys. 1997, 82, 696. [5] A. A. Middleton, N. S. Wingreen, Phys. Rev. Lett. 1993, 71, 3198. [6] H. E. Romero, M. Drndic, Phys. Rev. Lett. 2005, 95, 156801. [7] R. Parthasarathy, X. M. Lin, H. M. Jaeger, Phys. Rev. Lett. 2001, 87, 186807. [8] R. Parthasarathy, X. M. Lin, K. Elteto, T. F. Rosenbaum, H. M. Jaeger, Phys. Rev. Lett. 2004, 92, 076801. [9] H. O. Muller, K. Katayama, H. Mizuta, J. Appl. Phys. 1998, 84, 5603. [10] M. O. Blunt, M. Suvakov, F. Pulizzi, C. P. Martin, E. Pauliac-Vaujour, A. Stannard, A. W. Rushforth, B. Tadic, P. Moriarty, Nano Lett. 2007, 7, 855. [11] T. B. Tran, I. S. Beloborodov, J. S. Hu, X. M. Lin, T. F. Rosenbaum, H. M. Jaeger, Phys. Rev. B 2008, 78, 075437. [12] M. C. Daniel, D. Astruc, Chem. Rev. 2004, 104, 293. [13] V. Maheshwari, J. Kane, R. F. Saraf, Adv. Mater. 2008, 20, 284. [14] C. C. Yu, S. W. Lee, J. Ong, D. Moore, R. F. Saraf, Adv. Mater. 2013, 25, 3079. [15] E. H. Lee, S. W. Lee, R. F. Saraf, Acs Nano 2014, 8, 780. [16] U. Ozgur, H. Morkoc, Proc. IEEE 2010, 98, 1255. [17] Y. Lin, J. Ren, X. Qu, Adv. Mater. 2014, 26, 4200. [18] J. Suntivich, Z. C. Xu, C. E. Carlton, J. Kim, B. H. Han, S. W. Lee, N. Bonnet, N. Marzari, L. F. Allard, H. A. Gasteiger, K. Hamad-Schifferli, Y. Shao-Horn, J. Am. Chem. Soc. 2013, 135, 7985. [19] E. Katz, I. Willner, J. Wang, Electroanal. 2004, 16, 19. [20] O. Lioubashevski, V. I. Chegel, F. Patolsky, E. Katz, I. Willner, J. Am. Chem. Soc. 2004, 126, 7133. [21] A. Y. Polyakov, N. B. Smirnov, E. A. Kozhukhova, V. I. Vdovin, K. Ip, Y. W. Heo, D. P. Norton, S. J. Pearton, Appl. Phys. Lett. 2003, 83, 1575.
6496
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[22] D. A. Scrymgeour, J. W. P. Hsu, Nano Lett. 2008, 8, 2204. [23] O. Harnack, C. Pacholski, H. Weller, A. Yasuda, J. M. Wessels, Nano Lett. 2003, 3, 1097. [24] V. Maheshwari, D. E. Fomenko, G. Singh, R. F. Saraf, Langmuir 2010, 26, 371. [25] D. Pradhan, K. T. Leung, Langmuir 2008, 24, 9707. [26] S. Peulon, D. Lincot, Adv. Mater. 1996, 8, 166. [27] M. Izaki, T. Omi, Appl. Phys. Lett. 1996, 68, 2439. [28] M. H. Wong, A. Berenov, X. Qi, M. J. Kappers, Z. H. Barber, B. Illy, Z. Lockman, M. P. Ryan, J. L. Manus-Driscoll, Nanotechnology 2003, 14, 968. [29] J. Tam, S. Salgado, M. Miltenburg, V. Maheshwari, Chem. Commun. 2013, 49, 8641. [30] J. Song, S. Kulinich, J. Yan, Z. Li, J. He, C. Kan, H. Zeng, Adv. Mater. 2013, 25, 5750. [31] Y. K. Mishra, S. Kaps, A. Schuchardt, I. Paulowicz, X. Jin, D. Gedamu, S. Freitag, M. Claus, S. Wille, A. Kovalev, S. N. Gorb, R. Adelung, Part. Part. Syst. Charact. 2013, 30, 775. [32] A. Bezryadin, R. M. Westervelt, M. Tinkham, Appl. Phys. Lett. 1999, 74, 2699. [33] A. S. Cordan, Y. Leroy, A. Goltzene, A. Pepin, C. Vieu, M. Mejias, H. Launois, J. Appl. Phys. 2000, 87, 345. [34] O. Lupan, G. Chai, L. Chow, G. A. Emelchenko, H. Heinrich, V. V. Ursaki, A. N. Gruzintsev, I. M. Tiginyanu, A. N. Redkin, Phys. Status Solidi 2010, 207, 1735. [35] C. A. Mead, Phys. Lett. 1965, 18, 218. [36] C. Soci, A. Zhang, B. Xiang, S. A. Dayeh, D. P. R. Aplin, J. Park, X. Y. Bao, Y. H. Lo, D. Wang, Nano Lett. 2007, 7, 1003. [37] G. Gedamu, I. Paulowicz, S. Kaps, O. Lupan, S. Wille, G. Haidarschin, Y. K. Mishra, R. Adelung, Adv. Mater. 2014, 26, 1541. [38] H. Sheng, S. Muthukumar, N. W. Emanetoglu, Y. Lu, Appl. Phys. Lett. 2002, 80, 2132. [39] K. M. Tracy, P. J. Hartlieb, S. Einfeldt, R. F. Davis, E. H. Hurt, R. J. Nemanich, J. Appl. Phys. 2003, 94, 3939. [40] N. Ohashi, J. Tanaka, H. Haneda, M. Ozawa, T. Ohgaki, T. Tsurumi, J. Mater. Res. 2002, 17, 1529. [41] U. Ozgur, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Dogan, V. Avrutin, S. J. Cho, H. Morkoc, J. Appl. Phys. 2005, 98, 041301. [42] X. Zhao, B. M. Sanchez, P. J. Dobson, P. S. Grant, Nanoscale 2011, 3, 839. [43] A. S. Arico, P. Bruce, B. Scrosati, J. M. Tarascon, W. Van Schalkwijk, Nat. Mater. 2005, 4, 366. [44] T. M. Day, P. R. Unwin, N. R. Wilson, J. V. Macpherson, J. Am. Chem. Soc. 2005, 127, 10639. [45] J. S. Gu, S. M. Collins, A. I. Carim, X. G. Hao, B. M. Bartlett, S. Maldonado, Nano Lett. 2012, 12, 4617. [46] C. X. Ma, N. M. Contento, L. R. Gibson, P. W. Bohn, Acs Nano 2013, 7, 5483. [47] Z. W. Liu, W. B. Hou, P. Pavaskar, M. Aykol, S. B. Cronin, Nano Lett. 2011, 11, 1111. [48] J. B. Mu, C. L. Shao, Z. C. Guo, Z. Y. Zhang, M. Y. Zhang, P. Zhang, B. Chen, Y. C. Liu, Acs Appl. Mater. Interfaces 2011, 3, 590. [49] X. J. Duan, R. X. Gao, P. Xie, T. Cohen-Karni, Q. Qing, H. S. Choe, B. Z. Tian, X. C. Jiang, C. M. Lieber, Nat. Nanotech. 2012, 7, 174. [50] X. J. Huang, Y. K. Choi, Sensors and Actuators B 2007, 122, 659.
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Adv. Mater. 2014, 26, 6491–6496
Long Pu, Abdullah Saud Abbas, and Vivek Maheshwari*
Nanoparticles and their arrays present a diverse range of electrical behavior with characteristic coulomb blockade, coulomb staircase, transition from insulator to metal properties and sensitivity to gating.[1–6] The wide range of characteristics are the result of phenomena such as single electron charging effects due to the low capacitance of an isolated nanoparticle hence requiring high charging energy, quantum confinement due to tunneling barriers between adjacent nanoparticles and coupling between neighbouring particles due to their close proximity.[2,5,7–11] Au nanoparticles are used commonly for formation of such arrays due to their, well-controlled size, effective assembly using thiols and anions, and metallic nature.[12,13] The presence of defects in these nanoparticle arrays leads to observation of single electron behaviour and coulomb blockade at room temperature.[7,13] The highly sensitive characteristics of these arrays (on the scale of single electron) presents an opportunity for their application in devices and sensors. Developing methods or hybrid materials that can control or modulate the characteristic behavior of these arrays will therefore advance new applications for these systems. For example, recently it has been shown that nanoparticle arrays functioning as transistors in aqueous medium can detect cellular activity on interfacing with live cells.[14,15] Similarly by interfacing with nanomaterials such as ZnO nano-rods that are photosensitive and piezoelectric, it should be possible to modulate the array characteristics using light and stress stimuli.[16] Therefore a strategy for interfacing the arrays with other kind of nanomaterials will be of significant interest to further advance these systems. Here we demonstrate in-situ electrochemical synthesis of a parallel ZnO nano-rod interface on Au nanoparticle array that leads to photo-modulation of the array characteristics. Au nanoparticles, the basic building block of these array systems are catalytic in nature and are also electrochemically active.[17–20] Combining the electrical conduction of the array with the catalytic and electrochemical nature of its constituent Au nanoparticles, ZnO nano-rods are formed directly on the array by electrochemical synthesis. The diameter of the ZnO rods (20–30 nm) is on the scale of the size of Au nanoparticles (10–12 nm) and spatially their synthesis is confined by the extent of the array L. Pu, A. S. Abbas, Prof. V. Maheshwari Dept. of Chemistry Waterloo Institute of Nanotechnology University of Waterloo 200 University Ave. West, Waterloo, ON, N2L 3G1 Canada E-mail: [email protected]
DOI: 10.1002/adma.201402034
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Electrochemical Synthesis on Nanoparticle Chains to Couple Semiconducting Rods: Coulomb Blockade Modulation Using Photoexcitation
(1–2 µm). The process opens the use of such nanoparticle arrays as electrochemical systems, significantly expanding their potential use in sensors, multifunctional materials, development of single electron transistors and nano-scale energy systems. The ZnO-Au hybrid system presented here is a multifunctional material, combining the individual characteristics of the Au nanoparticle array and the ZnO nano-rod pathway and also intermodulation between the two materials. Behavior characteristic of Au nanoparticle array such as coulomb blockade at room temperature, single electron charging effects and a power law dependence in current-voltage are observed. The hybrid system also shows Schottky behavior and photocurrent generation due to the characteristics of the ZnO nano-rods.[21–23] Besides the individual characteristics, the hybrid system also has coupling between the Au nanoparticles and ZnO rods. This behavior is observed in the modulation of the threshold voltage of the Au array due to the generation of photo excited electronhole pairs in the ZnO rods. In complimentary fashion the conduction of the electrons in the Au array leads to modulation of the decay time for the photocurrent in the ZnO rods. Micrometer long chain networks of Au nanoparticles (10–12 nm in size) self-assembled in solution using divalent Ca2+ ions[12,15,24] are deposited between micron scale electrodes functionalized with silane, forming a conductive pathway (see Supporting Information, SI). As a result of the self-assembly, the UV-Vis absorption spectrum due to the surface plasmon resonance of the Au nanoparticles red shifts to ∼600 nm from 525 nm, as seen in Figure 1a. The red shift is attributed primarily to the plasmon coupling between neighbouring particles as they assemble into network of chains. The shoulder at ∼530 nm in the Ca-Au chains (Figure 1a) is attributed to the plasmon resonance along the shorter axis ∼ 10–12 nm, the diameter of the nanoparticles, similar to unassembled nanoparticles. Transmission electron microscopy (TEM) images of Figure 1b clearly show the assembly of the micron long arrays consisting of Au chain networks. The inset shows a typical chain structure with the spacing between the nanoparticles being on the scale of 1–2 nm (magnified images in SI, Figure S-2,S-3). The assembly is also monitored by using light scattering and zeta potential measurements. Prior to assembly a size of ∼10 nm and a zeta potential of ∼ −40 mV is recorded for the Au nanoparticles. After the addition of Ca2+ ions (see SI, Figures S-4–S-8) the size increases significantly and the zeta potential remains close to −40 mV. The length scale of chains can be controlled by the time provided for the assembly and the concentration of divalent ion used, shorter time lead to shorter chains (see SI, Figure S-9&S-10). We believe that the separation between
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Figure 1. (a) The assembly of the Au nanoparticles due to Ca2+ ions shifts the plasmon resonance peak from ∼520 nm to 610 nm. A noise in the equipment is observed at ∼650 nm. (b) The TEM image clearly shows the assembly of the nanoparticles into branched chain structures. The higher magnification image in the inset shows that Au nanoparticles are separated by 1–2 nm. (c) The chains upon deposition bridge the gap between two Au pads forming a conductive pathway. In white are highlighted the bottlenecks with 1-D chains. The higher magnification image in the inset shows that the assembly is similar to the TEM image of (b). (d) Typical characteristics of coulomb blockade and oscillations in conductance (inset) are observed from the array shown in (c).
the nanoparticles should be dependent on the ion used for assembly. More detailed investigation in this aspect is being performed. The chains form a conductive pathway for electron transport on application of bias across the Au electrodes, Figure 1c, field emission scanning electron microscopy (FESEM) images. The inset, a high magnification FESEM image more clearly shows the nature of the Ca-Au chain pathway (also Figure S-3 in SI). A typical I−V (current-voltage) response of the chains is shown in Figure 1d. We observe the characteristic coulomb blockade (threshold voltage VT) and the single electron charging effects that lead to oscillations in the differential conductance (inset Figure 1d). The data points of the I−V response are fit to a power law model (as explained later), providing a threshold voltage, VT, of 4.87 V for the array and a power law scaling with 2.6. Both these values are consistent with the observed conduction through 2-D array of nanoparticles in presence of defects.[6,7,9,12] Typical defects such as a series of single Au nanoparticle channels in the pathway are bottlenecks for the flow of electrons (highlighted in white in Figure 1c). These defects lead to the observation of the coulomb blockade at room temperature. Following the formation of this conductive nanoparticle array, electrochemical synthesis of ZnO is performed on the array in a microfluidic cell using a three electrode system (details in SI),[25–29] schematic shown in Figure 2a. CVD and
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flame transport approach have also been used recently for growing heterogeneous ZnO nanostructures.[30,31] As a result, ZnO nano-rods grow on the Au nanoparticle chains. This transformation is shown in the FESEM images of Figure 2b. The initial Au nanoparticle chain array (inset of Figure 2b) is the specific location for the growth of the ZnO rods. The adjacent SiO2/Si areas are devoid of any ZnO (highlighted in black in the figure and in white in the inset) (more images in SI). Further in the higher magnification image of Figure 2c we observe that the diameter of the ZnO rods is ∼20–30 nm and they have hexagonal cross sectional area, typical for the growth of the ZnO rods along the 0001 plane. In the FESEM image of Figure 2d clearly visible underneath the rods is the presence of Au nanoparticles, seen as bright silhouette in the image. This confirms that Au nanoparticle chains serve two primary purposes: 1. Their metallic nature and ability to conduct electrons results in electrochemical synthesis on their surface upon the application of a potential. 2. The Au nanoparticles are well known to act as preferential growth sites for ZnO, this is confirmed by the observation that the diameter of the grown ZnO rods matches well with the size of the Au nanoparticles. This is also in contrast to ZnO rods synthesized in similar manner on continuous planar surfaces that have a diameter of ∼150–250 nm.[29] This hybrid device of Au nanoparticles and ZnO is then characterized for its current-voltage response and photo
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COMMUNICATION Figure 2. (a) The three electrode set-up for synthesis of ZnO on Au-chains. (b)Following the synthesis, ZnO nano-rods are clearly observed on the Au-chain array. Inset shows the Au-chain array prior to synthesis. Highlighted in, white in the inset and in black in the figure, clearly observed is the absence of the rods where there are no Au nanoparticles. Scale in the inset is 500 nm. (c) The ZnO nanorods are ∼20–30 nm in size and are growing directly from the nanoparticles. (d) We can clearly see the underlying Au-nanoparticle array below the ZnO nano-rods. The scale bar is 100 nm.
excitation properties. The I−V response from a typical device under dark conditions is shown in Figure 3a. The curve shows typical oscillations in the differential conductance (Figure 3b), indicative of current flow through the underlying Au nanoparticle pathway.[2,4] The device has a very similar current response under positive and negative bias (Figure 3a). This is also true for devices with only Au chains. The device is then illuminated with light from a solar simulator (1.5 solar mass) with progressively increasing intensity. As observed in Figure 3c, there is a corresponding increase in current with increasing light intensity. This response indicates that the ZnO rods also form an electron conduction pathway. This is further
confirmed by illuminating a device with only Au nanoparticles chains with light in which case no photo response is observed. Furthermore when a long pass filter of 420 nm is placed in the light path no photo response is observed from the hybrid device. This confirms that UV light, as required for the excitation of ZnO rods due to its reported bandgap of ∼3.2 eV, is necessary to generate the photocurrent.[16,22] The I−V response from the hybrid device is then modelled as consisting of two pathways, a Schottky equation for the flow of current from Au pads to ZnO rods to Au pads and an Au nanoparticle array which follows power law dependence (schematic in Figure 3d) (Equation (1)).
Figure 3. (a) The I−V characteristics of the hybrid device shows a non-linear response also observed are the periodic oscillations in current. (b) The oscillations in conductance are indicative of transport through the nanoparticle array. (c) On illumination, the current through the hybrid device increases. The current characteristics are accurately fit by Equation (1), electron transport though both nanoparticles and ZnO rods. (d) Schematic showing the two conduction pathways, through Au-nanoparticles in yellow and through nano-rods in grey. (e) Increasing light intensity decreases the threshold voltage, VT for the nanoparticle array; while α, it’s dimensionality remains unchanged. (f) Gating of a purely Au nanoparticle array device leads to a progressive decrease in the VT with increasing gate voltage.
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⎛V ⎞ I = P1 [ exp (P2V )] + K ⎜ − 1⎟ ⎝ VT ⎠
α
(1)
Here the first term is the Schottky current through the Au electrodes and the ZnO pathway with P1 being the saturation current and P2 combines thermal energy, electron charge and the ideality factor for ZnO pathway. P1 is also related to the thermionic emission over the barrier. This can be used to calculate the barrier height.[22,23] The second term is the power law for the current through the Au nanoparticle pathway. K, characterizes the current through a single nanoparticle channel in the pathway; VT is the threshold voltage required for the start of electron conduction and α is the scaling exponent that relates to the dimensionality of the nanoparticle array.[5] The fit of this equation to the data from the device both under dark and illumination is shown in Figure 3c. The points are actual data and the line is the fit from the equation, in these plots. The equation fits the data very well. The influence of the ZnO on the Au chains is illustrated by observing the effect of light intensity on the parameters VT & α. Typically for 2-D arrays α is ∼5/3 from theoretical models.[5] However due to local defects and conduction along single nanoparticle pathways, 1-D systems, the exponent increases and can typically range up to 2–3.5.[3,6–10,14,32,33] As shown in Figure 3e, we observe that the exponent is ∼2.45, which matches with the array being a 2-D matrix of nanoparticles with 1-D bottlenecks for conduction. These 1-D restrictions are the result of the basic nature of the Ca-Au chains that form a branched 1-D structure. On illumination, with increasing intensity of light no significant change is observed in α. This is expected as the underlying dimensionality of the Au matrix is not affected by the generation of the electron-hole pairs in the ZnO rods. The threshold voltage VT decreases significantly on photoexcitation of the ZnO rods, as seen in Figure 3e. This effect can be considered as similar to the gating of these pathways by an external electric field. On illumination, electron hole pairs are generated in the ZnO rods and the holes are trapped by the surface defects on the rods.[34–36] These holes lead to the local gating of the Au nanoparticles and hence modulate the threshold for the conduction along the Au array. The decrease in VT stabilizes at higher light intensities. This is expected as the generation of the electron-hole pairs saturates at higher intensities, hence so does the effect of gating due to ZnO. The modulation of VT occurs due to the close proximity of the ZnO, a photosensitive material, to the Au-array. The decrease in the threshold (VT) with increasing light intensity is qualitatively similar to the gating behavior in plain Au-Ca chain arrays (see Figure 3f), where a decreasing trend in VT with increasing gate voltage is also observed. Here the I−V behavior of Ca-Au chains recorded at increasing gate voltages shows a linear decrease in the threshold voltage, VT.[2,15] The VT of a hybrid device is ∼1.3 V, for a plain Au chain device this is ∼3.6 V in dark (Figure 3e,f). The difference arises as ZnO rods have a surface potential due to the presence of defects and the arrangement of the constituting ions (Zn2+ and O2−). Hence the formation of the ZnO rods directly on the Au chains results in their gating and the observed change in VT. However α remains relatively
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unaffected between the hybrid device (∼2.45) and the Au chain device (∼2.86, Figure 1d). Considering the charging energy required for a single Au nanoparticle (10 nm in size) due to its capacitance, we can relate the threshold voltage, VT, to the number of such charging centers present in the device.[5,14,32] There are ∼ 9 charging centers present in the hybrid device. On illumination and the subsequent effect of gating by ZnO the charge centers are reduced to ∼ 5. This provides the ability to photo modulate the charging effects in the array on the scale of a single nanoparticle. The photocurrent generation upon excitation of the ZnO rods by UV light was recorded to characterize their response and also the effect of the underlying Au matrix on the rods. The net response from the hybrid was recorded at a series of applied constant bias, Figure 4a. The relative photocurrent as plotted is the ratio of the net current to the dark current. The application of bias also leads to the flow of current through the Au nanoparticle array and as the bias increases the magnitude of the current through the nanoparticles also increases. This occurs due to increased electron transport through the nanoparticles, hence a greater charging of the nanoparticles to sustain the current. A simple effect of this increase in charging will be the greater interaction of the Au nanoparticles with the surface trapped holes on the ZnO rods. This is illustrated by fitting the decay of the photocurrent in the hybrid after turning off the illumination. Similar to other observations, a bi exponential equation with time constants of T1 and T2 is fit to the decay (details in SI).[37] The change in the decay time constants with increasing bias is plotted in Figure 4b. As expected at higher voltages larger time constants are observed, consistent with the increased interaction with between the surface trapped holes in ZnO and the greater electron charging of the Au nanoparticles. This increases the time for the recombination of the electron-hole pairs in ZnO as the illumination is turned off. The photocurrent effect of the hybrid device and just the ZnO rods pathway is decoupled in Figure 4c, using the parameters from the model (details in SI). The net relative photocurrent through the device (curve in black) increase with applied bias and stabilizes at higher voltages. While considering only the current through the ZnO path (curve in blue) a maximum in relative photocurrent is observed at ∼7 V. This occurs as with increasing bias the base current over the Schottky junction of ZnO path increases, leading to a reduced contribution from the photo generate current.[36] The relative photocurrent reported in other devices consisting of only ZnO pathway is significantly higher.[36,37] The fit of the Schottky term in Equation (1) provides a value of ∼1.5 × 10−12 amperes for P1. This is used to calculate the barrier height, ΦB, between the ZnO and Au electrodes using the expression for the saturation current i.e., P1 = AT2exp(-ΦB/kT), where A is the Richardson constant, which for ZnO is estimated to be 32 Acm−2K−2, T is the temperature and k is the Boltzmann’s constant.[22] Typical device areas are ∼10 µm2 (as defined by the spacing between the Au electrodes and the actual area over which the Au chains deposit as seen in Figure 1c) and with T = 298 Kelvin, a barrier height of 0.67 eV is obtained. This matches well with the reported values in literature for Au-ZnO contacts.[35,38–41] The relative current in the two pathways of ZnO rods and Ca-Au chains is plotted in Figure 4d both in dark and also
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COMMUNICATION Figure 4. (a) The net photocurrent response from the hybrid device increases with applied bias. The illumination is at 150 Watts for all the biases (b) The decay of photocurrent fit to double exponential shows that the time constants for the decay increase with increasing bias across the device. (c) The net photocurrent, in black from the device follows a monotonically increasing trend, while considering only the ZnO circuit in blue, a peak is observed at ∼7 V. (d) The relative current between the two pathways shows that depending on the applied bias and light conditions the device can switch from being dominated by ZnO to Au-array to a hybrid combination, for the flow of the current.
under illumination (by using the model, details in SI). The data illustrates the effect of the applied bias on the functioning of the hybrid device. At low bias (less than VT or close to it), conduction through ZnO pathway is the dominant mechanism. In the range of 3–4 V, conduction through Ca-Au chains increases and under dark conditions becomes the dominant mechanism. At higher biases the fraction of current through ZnO recovers slowly. The functioning of the device hence can be controlled by the applied bias and light illumination as from being pre-dominantly a ZnO nano-rod device to Ca-Au chain one or a hybrid with contributions from both the pathways. In summary, hybrid devices composed of two distinct nanomaterials with characteristic individual properties are synthesized and characterized. The device is made of a self-assembled Au-nanoparticle chains array, which is then used as an electrochemically active system for synthesis of ZnO nano-rods. The use of such single electron sensitive templates to assemble semiconducting nano-rods as a direct and parallel interface leads to hybrid properties for the device. The combination of two distinct materials is possible due to the ability of Au nanoparticles to conduct electrons and also act as nucleation centers for the formation of the ZnO nano-rods. The device shows typical characteristics of both, the conduction through Au nanoparticle array and also ZnO nano-rods. The direct interfacing between these two parallel devices leads to mutual inter-modulation between the two pathways. This allows us to switch the device functioning between being dominated by the Au-Chains or ZnO rod network or as a hybrid combination. Further by using photo excitation of the ZnO rods the charging centers in
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the Au chain array pathway can be modulated on the scale of single nanoparticles providing a means to gate the device and alter the VT. Arrays of Au nanoparticle chains are room temperature single electron sensitive systems. The ability to use such nanoparticle chains as electrochemically active systems with both structural and spatial confinement of the synthesized material will significantly expand their application. Besides ZnO other semiconducting materials such as p-type Cu2O, energy storage materials such as MnO2, catalytic materials such as Pt, semiconducting polymers and biological systems such as cells and proteins can be interfaced with these arrays. These will be useful for development of nano scale electro-optical devices, electrochemical sensors, nano scale energy storage systems and diverse catalytic systems.[17–20,42–50]
Experimental Section Ca-Au chains are made by mixing 4 ml of Au nanoparticle solution (10 nm size, from BBI International) with 360 µL of 1 mg/mL CaCl2 solution (details in SI). Following their assembly after 8 hrs., these Ca-Au chains are deposited on silane functionalized chips (Si substrate with thermally grown SiO2 layer) that have inter-digited Au finger electrodes (details in SI). The ZnO nano-rods are electrochemically synthesized on the deposited Au chains by placing the chip in a microfluidic cell containing 1.25 mM ZnCl2 and 0.1 M KCl maintained at 80 °C. The synthesis is conducted in a three electrode set-up with a working potential of −0.955 V applied for 7–10 minutes (details in SI). Both the deposited Ca-Au chains and the ZnO Ca-Au hybrid devices are
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www.MaterialsViews.com characterized for their current-voltage and photoresponse behavior (details in SI). The assembly of the Ca-Au chains is also characterized by light scattering and zeta potential measurements (details in SI).
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the University of Waterloo, Canada Foundation for Innovation, Ontario Research Fund and NSERC Canada. Received: May 6, 2014 Revised: July 4, 2014 Published online: August 14, 2014
[1] A. Zabet-Khosousi, P. E. Trudeau, Y. Suganuma, A. A. Dhirani, B. Statt, Phys. Rev. Lett. 2006, 96, 156403. [2] K. K. Likharev, Proc. IEEE 1999, 87, 606. [3] M. G. Ancona, W. Kruppa, R. W. Rendell, A. W. Snow, D. Park, J. B. Boos, Phys. Rev. B 2001, 64, 033408. [4] T. Sato, H. Ahmed, D. Brown, B. F. G. Johnson, J. Appl. Phys. 1997, 82, 696. [5] A. A. Middleton, N. S. Wingreen, Phys. Rev. Lett. 1993, 71, 3198. [6] H. E. Romero, M. Drndic, Phys. Rev. Lett. 2005, 95, 156801. [7] R. Parthasarathy, X. M. Lin, H. M. Jaeger, Phys. Rev. Lett. 2001, 87, 186807. [8] R. Parthasarathy, X. M. Lin, K. Elteto, T. F. Rosenbaum, H. M. Jaeger, Phys. Rev. Lett. 2004, 92, 076801. [9] H. O. Muller, K. Katayama, H. Mizuta, J. Appl. Phys. 1998, 84, 5603. [10] M. O. Blunt, M. Suvakov, F. Pulizzi, C. P. Martin, E. Pauliac-Vaujour, A. Stannard, A. W. Rushforth, B. Tadic, P. Moriarty, Nano Lett. 2007, 7, 855. [11] T. B. Tran, I. S. Beloborodov, J. S. Hu, X. M. Lin, T. F. Rosenbaum, H. M. Jaeger, Phys. Rev. B 2008, 78, 075437. [12] M. C. Daniel, D. Astruc, Chem. Rev. 2004, 104, 293. [13] V. Maheshwari, J. Kane, R. F. Saraf, Adv. Mater. 2008, 20, 284. [14] C. C. Yu, S. W. Lee, J. Ong, D. Moore, R. F. Saraf, Adv. Mater. 2013, 25, 3079. [15] E. H. Lee, S. W. Lee, R. F. Saraf, Acs Nano 2014, 8, 780. [16] U. Ozgur, H. Morkoc, Proc. IEEE 2010, 98, 1255. [17] Y. Lin, J. Ren, X. Qu, Adv. Mater. 2014, 26, 4200. [18] J. Suntivich, Z. C. Xu, C. E. Carlton, J. Kim, B. H. Han, S. W. Lee, N. Bonnet, N. Marzari, L. F. Allard, H. A. Gasteiger, K. Hamad-Schifferli, Y. Shao-Horn, J. Am. Chem. Soc. 2013, 135, 7985. [19] E. Katz, I. Willner, J. Wang, Electroanal. 2004, 16, 19. [20] O. Lioubashevski, V. I. Chegel, F. Patolsky, E. Katz, I. Willner, J. Am. Chem. Soc. 2004, 126, 7133. [21] A. Y. Polyakov, N. B. Smirnov, E. A. Kozhukhova, V. I. Vdovin, K. Ip, Y. W. Heo, D. P. Norton, S. J. Pearton, Appl. Phys. Lett. 2003, 83, 1575.
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[22] D. A. Scrymgeour, J. W. P. Hsu, Nano Lett. 2008, 8, 2204. [23] O. Harnack, C. Pacholski, H. Weller, A. Yasuda, J. M. Wessels, Nano Lett. 2003, 3, 1097. [24] V. Maheshwari, D. E. Fomenko, G. Singh, R. F. Saraf, Langmuir 2010, 26, 371. [25] D. Pradhan, K. T. Leung, Langmuir 2008, 24, 9707. [26] S. Peulon, D. Lincot, Adv. Mater. 1996, 8, 166. [27] M. Izaki, T. Omi, Appl. Phys. Lett. 1996, 68, 2439. [28] M. H. Wong, A. Berenov, X. Qi, M. J. Kappers, Z. H. Barber, B. Illy, Z. Lockman, M. P. Ryan, J. L. Manus-Driscoll, Nanotechnology 2003, 14, 968. [29] J. Tam, S. Salgado, M. Miltenburg, V. Maheshwari, Chem. Commun. 2013, 49, 8641. [30] J. Song, S. Kulinich, J. Yan, Z. Li, J. He, C. Kan, H. Zeng, Adv. Mater. 2013, 25, 5750. [31] Y. K. Mishra, S. Kaps, A. Schuchardt, I. Paulowicz, X. Jin, D. Gedamu, S. Freitag, M. Claus, S. Wille, A. Kovalev, S. N. Gorb, R. Adelung, Part. Part. Syst. Charact. 2013, 30, 775. [32] A. Bezryadin, R. M. Westervelt, M. Tinkham, Appl. Phys. Lett. 1999, 74, 2699. [33] A. S. Cordan, Y. Leroy, A. Goltzene, A. Pepin, C. Vieu, M. Mejias, H. Launois, J. Appl. Phys. 2000, 87, 345. [34] O. Lupan, G. Chai, L. Chow, G. A. Emelchenko, H. Heinrich, V. V. Ursaki, A. N. Gruzintsev, I. M. Tiginyanu, A. N. Redkin, Phys. Status Solidi 2010, 207, 1735. [35] C. A. Mead, Phys. Lett. 1965, 18, 218. [36] C. Soci, A. Zhang, B. Xiang, S. A. Dayeh, D. P. R. Aplin, J. Park, X. Y. Bao, Y. H. Lo, D. Wang, Nano Lett. 2007, 7, 1003. [37] G. Gedamu, I. Paulowicz, S. Kaps, O. Lupan, S. Wille, G. Haidarschin, Y. K. Mishra, R. Adelung, Adv. Mater. 2014, 26, 1541. [38] H. Sheng, S. Muthukumar, N. W. Emanetoglu, Y. Lu, Appl. Phys. Lett. 2002, 80, 2132. [39] K. M. Tracy, P. J. Hartlieb, S. Einfeldt, R. F. Davis, E. H. Hurt, R. J. Nemanich, J. Appl. Phys. 2003, 94, 3939. [40] N. Ohashi, J. Tanaka, H. Haneda, M. Ozawa, T. Ohgaki, T. Tsurumi, J. Mater. Res. 2002, 17, 1529. [41] U. Ozgur, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Dogan, V. Avrutin, S. J. Cho, H. Morkoc, J. Appl. Phys. 2005, 98, 041301. [42] X. Zhao, B. M. Sanchez, P. J. Dobson, P. S. Grant, Nanoscale 2011, 3, 839. [43] A. S. Arico, P. Bruce, B. Scrosati, J. M. Tarascon, W. Van Schalkwijk, Nat. Mater. 2005, 4, 366. [44] T. M. Day, P. R. Unwin, N. R. Wilson, J. V. Macpherson, J. Am. Chem. Soc. 2005, 127, 10639. [45] J. S. Gu, S. M. Collins, A. I. Carim, X. G. Hao, B. M. Bartlett, S. Maldonado, Nano Lett. 2012, 12, 4617. [46] C. X. Ma, N. M. Contento, L. R. Gibson, P. W. Bohn, Acs Nano 2013, 7, 5483. [47] Z. W. Liu, W. B. Hou, P. Pavaskar, M. Aykol, S. B. Cronin, Nano Lett. 2011, 11, 1111. [48] J. B. Mu, C. L. Shao, Z. C. Guo, Z. Y. Zhang, M. Y. Zhang, P. Zhang, B. Chen, Y. C. Liu, Acs Appl. Mater. Interfaces 2011, 3, 590. [49] X. J. Duan, R. X. Gao, P. Xie, T. Cohen-Karni, Q. Qing, H. S. Choe, B. Z. Tian, X. C. Jiang, C. M. Lieber, Nat. Nanotech. 2012, 7, 174. [50] X. J. Huang, Y. K. Choi, Sensors and Actuators B 2007, 122, 659.
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