Letter pubs.acs.org/NanoLett
Sensing and Energy Harvesting of Fluidic Flow by InAs Nanowires Ying Chen,† Dong Liang,‡ Xuan P. A. Gao,*,‡ and J. Iwan D. Alexander*,†,§ †
Department of Mechanical and Aerospace Engineering and ‡Department of Physics, Case Western Reserve University, Cleveland, OH 44106, United States § School of Engineering, University of Alabama at Birmingham, Birmingham, AL 35924, United States S Supporting Information *
ABSTRACT: Indium arsenide (InAs) nanowire (NW) field effect transistors (FETs) were incorporated into a microfluidic channel to detect the flow rate change as well as to harvest fluid flow energy for electric power generation. Discrete changes in the electric current through InAs NW FETs were observed upon flow rate changes at steps of 1 mL/h (equivalent to ∼3 mm/s change in average linear velocity). The current also showed a sign change upon reversing flow direction. By comparing the response of the device with and without a driving voltage between source-drain electrodes, we conclude that the dominant contribution in the response is the streaming potential tuned conductance of NW. In the absence of sourcedrain voltage, we further demonstrate that the ionic flow could enable generation of an ∼mV electrical potential (or ∼nA electrical current) inside the InAs NW per mL/h increase of flow rate, most likely due to the charge dragging effect. KEYWORDS: Sensor, fluid flow, energy conversion, nanowire, indium arsenide the device. This opens up the possibility of exploiting fluid flow for electric power generation or self-powered nanoelectronics. In this experiment, InAs NWs were grown on Si(100) substrates in a custom built vapor deposition system via the vapor−liquid−solid (VLS) mechanism with 40 nm commercial Au nanoparticles (Ted Pella, Inc.) as the catalyst.43 A high temperature tube furnace (Lindberg Blue M) was used to control the evaporation temperature of the InAs powder source (Alfa Aesar) and the growth temperature at the Si substrate. To functionalize the growth substrate with Au nanoparticles, the substrates were treated with poly-L-lysine (Ted Pella, Inc.) for 8.5 min and covered with 40 nm of Au catalyst solution (the Au catalyst solution was diluted 1:6 by volume in deionized water (DI water) from the as received Au nanoparticle solution) for 5 min successively. The Au nanoparticle functionalized Si substrate was placed at a distance of 13.5 cm from the source. A maximum flow rate of 100 sccm (standard cubic centimeters per minute) of Ar and H2 (10%) mixture carrier gas was introduced after the system was pumped down to the base pressure of 8 mTorr. When the temperature of the furnace was increased to 660 °C, the flow rate was decreased to 4 sccm. The pressure was kept at 1.5 Torr for 5 min and then changed to 100 mTorr. InAs NWs were grown under these conditions for 3 h with growth substrate at ∼480 °C. To transfer randomly oriented NWs from the growth wafer onto a Si/SiO2 substrate with desired density and alignment for device fabrication, a lubricant assisted contact printing transfer was adopted.44 The donor (NW growth wafer) substrate was cut into the desired dimension, and then gently brought into
E
lectrical detection of chemical and biological species using novel nanomaterial devices, such as nanowir (NW) and carbon nanotube (CNT) field effect transistors (FETs), has attracted significant attention during the past decade because the nanomaterial devices can act as real-time and ultrasensitive biological and chemical sensors.1−19 Advances in micro- and nanofabrication techniques have made micro- and nanofluidics more accessible experimentally.20−25 A number of theoretical26−28 and experimental studies19,29−33 of the interaction between flow and nanostructure/substrate have demonstrated that a net conductance and surface potential difference can be generated in nanostructures in the presence of fluid flow. Previously studied nanoscale flow sensors are mostly based on CNT, graphene, or silicon nanowire (Si-NW).19,21,29,33 For semiconductor NWs, small bandgap III−V compounds such as InAs seem to be another promising candidate as a sensing material due to their high electronic transport performance and the existence of an electron surface accumulation layer that could be sensitive to surface potential changes caused by molecule adsorption on the surface or ionic flow in the environment.22,34−40 To the best of our knowledge, there are no reports of InAs NW’s flow sensing properties. Furthermore, beyond sensing the fluid flow, there has been significant interest in using nanodevices to convert fluid flow energy into electricity.29,33,41 However, whether an ionic flow environment can indeed generate an electrical current or voltage in nanomaterials still remains controversial.27,42 Here, we report that FETs of either individual or multiple parallel aligned InAs NWs can be used to locally sense the flow of an ionic liquid with different directions, flow rates, and pH via the streaming potential modulated conductance effect. Significantly, we further found that current can be generated in an InAs NW FET by fluid flow even without any electrical voltage driving © XXXX American Chemical Society
Received: June 16, 2013 Revised: July 26, 2013
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contact on the receiver substrate (Si with 300 nm thick SiO2 on the surface), pushed with a constant velocity of ∼20 mm/min by the pressure of a mounted weight at ∼15 g/cm2. An octane and mineral oil (2:1, v:v) mixture was used as a lubricant during the transfer process.44 Photolithography was used to pattern the electrode arrays for NW FETs. A 60 nm thick Ni was deposited as electrodes by electron beam (E-beam) evaporation after photolithography. In addition, 30 nm thick aluminum oxide was E-beam evaporated onto Ni electrodes to avoid current leakage in the fluid. The effectiveness of the aluminum oxide passivation was confirmed by measuring the leakage current in water between electrodes without the nanowire. Before starting the flow of liquid, the electrical transport properties of the devices were investigated using a probe station system (The Micromanipulator Co., Inc.) at room temperature in air. The I−V characteristic showed a linear characteristic at room temperature. This indicates Ohmic contact behavior. Figure 1 shows the experiment setup of InAs NW FETs integrated in a microchip, where the NW devices were laid
Figure 2. Effect of flow direction and rate v on the conductance of a single InAs nanowire field effect transistor with source-drain voltage of Vsd = 10 mV in DI water. The black (red) curve corresponds to flow direction the same as (opposite to) the current flow. The inset on the right shows a dark field optical image of a single InAs nanowire device connected to electrodes. The top left inset shows a schematic of the device’s response to flow rate change.
change. We attribute the 1−2 min transient period preceding the steady state to the time required to stabilize fluid flow in the system. It is also seen that the flow direction affects the sign of the conductance change. With increasing v, the conductance increases when the source-drain current is in the same direction as the flow and decreases if it is oppositely directed. This InAs NW FET device had a clear stepwise response and showed reliability and reversibility at least over the nearly 2 h of experiment period. In addition, as a good candidate for a flow sensor, the current signal response time of the InAs NW FET is about 1−2 min after v is changed. Similar sensing data from another device can be found in the Supporting Information (Figure S1). In prior flow sensing experiments with CNT, Si-NW, and graphene, the device’s response was attributed to a fluid flow induced streaming potential effect.21,36 When the Si/SiO2 substrate is in contact with liquid solutions with a pH value greater than the isoelectric point (∼2−3) of SiO2, the substrate surface becomes negatively charged due to the deprotonation of the silanol groups [SiOH(s): SiO−(s) + H+(aq)].21 With flowing liquid, the excess counterions in the electrical double layer (EDL) at the surface of the SiO2 substrate are swept downstream by the pressure-driven flow. This generates a steaming potential ΔVstr which offsets the water gate (or reference electrode) voltage and thus shifts the surface potential of InAs NW (Figure 3a).21 Since the sign of the generated streaming potential reverses upon the reversal of fluid flow direction, an opposite change of current (or conductance) in NW FET is expected at opposite flow direction, as shown schematically at the bottom of Figure 3a. This is consistent with the experimental observation here. According to the electrokinetic theory of streaming potentials based on the Smoluchowski equation, the steaming potential is a function of flow rate (v) and ionic concentration (C) and given by22,34−36
Figure 1. Experimental setup of InAs nanowire transistor devices enclosed in a microfluidic flow channel for flow sensing and power harvesting.
down horizontally on the Si/SiO2 substrate and covered by a polydimethylsiloxane (PDMS) flow channel. The main parts of source and drain electrodes16,36,45 were packaged and covered by the PDMS layer that reduces exposure of electrodes to water. The flow of the liquid through the PDMS microfluidic channel was controlled by a syringe pump (New Era Pump Systems, Inc.). In the flow sensing experiments, typically a 10 mV DC voltage was applied to the source and drain electrodes. The source-drain current Isd was converted to a voltage by a current amplifier (Stanford Research, model SR570) and recorded using a National Instrument data acquisition card and BNC terminal box. The two curves in Figure 2 show the relationship between time and conductance of an FET with a single InAs NW in the ∼2 μm long channel at different flow directions and rates v of DI water at a source-drain voltage of Vsd = 10 mV. The flow direction is the same as (opposite to) the source-drain current flow direction for the top or black (bottom or red) trace. Generally, once the flow velocity was set to change by a sudden change of pump rate, a gradual rise or drop of conductance followed by a stable steady state reading is observed in the InAs NW’s conductance response, as shown in the top left schematic. In Figure 2, the black arrows mark the steady state regime after the NW conductance stabilizes upon flow rate B
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Figure 3. (a) Schematic illustration of the fluid flow generated streaming potential. The slope of the generated streaming potential between the reference electrode and the nanowire FET changes sign as the flow direction reverses. (b) Plot of the InAs nanowire FET’s current (left axis) and equivalent surface potential change Δφ (right axis) vs flow rate for experiment in Figure 2. (c) Response of the InAs nanowire FET’s source-drain current to changes in water gate voltage Vg at a source-drain voltage of 10 mV. Linear fitting of Isd vs Vg yields a conversion factor of 0.45 nA/mV for the calculation of Δφ in part b.
ΔVstr =
ε0εrζR Δv ηe(C + λ)μ
caused by the streaming potential developed over the macroscopic length and not related to carrier transport in CNT per se.27 In graphene devices immersed in hydrochloric acid (HCl), the flow of HCl solution was also found to generate more than 10 mV voltage.33 However, such an effect was explained alternatively by the interaction of electrode and ionic water.42 Therefore, it remains unclear if fluid flow can cause carriers in nanomaterial to generate a voltage or current via any intrinsic mechanism. Here, we found that water flow can indeed induce power generation in InAs NW FETs (∼1 nA or mV every mm/s flow rate increase). It is proposed that an intrinsic dragging effect is responsible for the observed phenomenon. Figure 4 illustrates the source-drain current through a second InAs NW device in response to flow rate changes without any external source-drain voltage to drive the NW (the setup is sketched as an inset). This measurement is in contrast to the sensing experiment, where nonzero Vsd was applied. Here,
(1)
where ε0 is the vacuum permittivity, εr is the relative dielectric constant of the electrolyte solution, ζ is the zeta potential of the ionic double layer, R is the hydraulic flow resistance estimated from Poiseulle’s law, η is the viscosity of the electrolyte solution,35 e is the electron charge, λ is the offset concentration that generated from the background concentration of ions, and μ is the effective ionic mobility of the electrolyte solution.34,36 According to eq 1, the streaming potential is proportional to v. In Figure 3b, we plot the source-drain current Isd against flow rate for data in Figure 2. Good linear dependence between the current and flow rate is seen for both flow directions. According to the streaming potential mechanism for nanoelectronic FET flow sensing, the streaming potential adjusts the surface potential of NW which modulates the conductance of NW through the field gating effect. Note that, using pH as another independent control of NW’s surface potential, we experimentally confirmed that our InAs NW FET shows a clear response to surface potential change (Supporting Information, Figure S2). We quantify the flow rate induced surface potential change Δφ at the surface of the InAs NW by comparing the current change in the sensing experiment to the device’s current vs water gate voltage Vg (or reference electrode voltage) in Figure 3c and show the corresponding Δφ by the axis on the right in Figure 3b. On the basis of this analysis, we find that every mL/h change in flow rate causes Δφ or streaming potential change ΔVstr ∼10 mV. Using the cross-sectional area (500 μm × 200 μm) to estimate the average flow velocity, this leads to an estimate of ΔVstr /Δv ∼3.5 mV/(mm/s). This is very close to the value obtained in Si-NW flow sensors at comparable ionic strength (μM).21 While our InAs NW FET flow sensor behaves consistently with other nanoscale FET flow sensors studied in the past, there is an outstanding question if nanomaterials or a device can indeed be used to harvest electrical power from fluid flow. In bundles of CNTs with macroscopic dimensions (∼mm), the flow of liquid can generate voltage on the order of mV along the flow direction.29 However, such an effect may also be
Figure 4. Fluid flow generated current through an InAs nanowire at different flow rates and directions without any source-drain voltage applied on the nanowire. The inset shows a schematic of experimental circuit. C
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although the flow rate change induces a streaming potential change ΔVstr, ΔVstr is not expected to cause any current flowing through the NW, since ΔVstr influences only the conductance of NW which converts to a current change only if a finite Vsd is applied. Without any source-drain voltage applied on the NW, we clearly see an ∼1 nA stepwise increase of current through the NW for every mL/h increase of flow of DI water. The generated current also reverses direction when the flow changes direction. A control experiment on a device without NW between the electrodes did not show any measurable current over a similar range of water flow rate. This attests to the good isolation of electrodes from water in the device and proves that the flow induced current in Figure 4 came from the InAs NW. Using the measured NW resistance of ∼1 MΩ, the equivalent electrical voltage generated inside the NW is estimated to be ∼1 mV for every mL/h increase of flow rate (corresponding to ∼pW power). A similar behavior and magnitude (∼1 mV per mL/h change) of flow induced electrical current (voltage) generation was observed in other InAs NWs with similar resistance (Supporting Information, Figure S3). In the literature, there are mainly two mechanisms for such flow induced voltage/current generation in material. The first one is pulsating asymmetric ratcheting29,41 in which a fluctuating Coulombic field of the liquid (caused by shear stress) imbalances local charge neutrality of ionic fluid, provides stochastic asymmetric potential which moves with flow, and drags free carriers to induce voltage. It is proposed29,41 that a fluctuating Coulombic potential arises due to local imbalances in charge neutrality of the ionic fluid. The unidirectional nature of the flow provides a bias that can be thought of as the deformation of the transient accumulation of charge. The pulsating asymmetric ratcheting resulted voltage is29,41 V ≅ V0 ×
1 − exp( −av) exp( −bv)
Figure 5. Sensing response over the flow rate and direction change of DI water for a FET device with multiple (∼6) horizontally aligned InAs nanowires between the electrodes. The source-drain voltage is 10 mV. The inset shows a dark field optical image of multiple horizontally aligned InAs nanowire devices connected on electrodes.
at a given flow rate is also amplified by 3−4 times, due to the larger number of NWs between electrodes. Further work is being performed to test if such multi-NW devices can be exploited to generate larger current and power in the power generation configuration without any source-drain bias. A key46 to a nanosystem is how a nanogenerator or nanosensor could provide high output voltage and power. In this regard, multiNW devices have the potential to yield much greater power than the single NW device as the internal resistance of the device reduces. We also note that the initial result on the flow rate sensing by the multi-NW device presented here appears to show larger conductance fluctuations than single-NW devices. These fluctuations may be related to the low frequency 1/f noise in NW devices.47 In the future, it will be essential to determine how such fluctuations change from single NW device to multi-NW device, since they are intimately tied to the signalto-noise ratio in sensing applications or the output power stability in the energy harvesting. In conclusion, InAs NW FETs were demonstrated to show a linear response to the changing flow rate of water in a microfluidic channel. The observed fluid flow sensing effect is attributed to the streaming potential of a flowing ionic liquid. Besides, fluid flow is found to induce electrical current through the InAs NW FET even in the absence of a source-drain voltage. To the best of our knowledge, this is the first time a NW-based electronic device is shown to convert energy from fluid flow into electricity. We expect that the demonstrated InAs NW-based sensors and energy harvesters for liquid flow may find uses in applications ranging from analytical chemistry to biomolecular detection to self-powered nanosystems.
(2)
where V0, a, and b are constants and v is the flow velocity. Equation 2 predicts a sublinear relation between induced voltage and flow velocity. This appears to disagree with data in Figure 4 where a quasi-linear dependence is seen. Another possible explanation26 is that voltage generation arises due to a phonon wind that drags free carriers in InAs NW. That is the transfer of momentum from the flowing liquid molecules to the acoustic phonons as the phonon quasimomentum, which in turn drags free electrons in the NW. In this model, the voltage has a linear relationship to the flow velocity and liquid viscosity. The velocity dependence expected in the carrier drag mechanism seems to be in agreement with our observation here. In addition to studying single InAs NW devices, we also explored the possibility of scaling up the InAs NW FET flow sensor by preparing NW thin film FETs with multiple parallel InAs NWs between the electrodes using repeated contact printing transfers to increase the number and density of NWs between electrodes. Such a multi-NW device may provide a longer lifetime and better statistical performance characteristics compared to a single NW device. Our initial work showed that the multi-InAs NW device (with ∼6 NWs in each FET) still responds to flow rate in a similar fashion to the single NW device after repeated contact printing transfers, as shown in Figure 5. Compared to the single NW device in Figure 2, not only does the absolute value of conductance increase in the multi-NW device, but the flow induced the conductance change
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ASSOCIATED CONTENT
S Supporting Information *
Additional figures related to the following: flow sensing from an additional InAs nanowire device, InAs nanowire FET’s response to pH change, and power generation from an additional InAs nanowire device. This material is available free of charge via the Internet at http://pubs.acs.org. D
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (X.P.A.G.);
[email protected] (J.I.D.A.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS X.P.A.G. thanks CWRU startup fund and NSF CAREER award program (DMR-1151534) for funding support. Y.C. acknowledges Yuan Tian for providing some of the InAs nanowires during this study. Y.C. and J.I.D.A. gratefully acknowledge support from the US Department of Energy (DE-EE0000275) and the Great Lakes Energy Institute.
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