Quantitative Analysis of Gold NanorodAlignment after Electric Field-AssistedDepositionWaqqar Ahmed, E. Stefan Kooij,* Arend van Silfhout, and Bene Poelsema

Solid State Physics, MESA+ Institute for Nanotechnology, UniVersity of Twente,P.O. Box 217, NL-7500AE Enschede, The Netherlands

Received June 19, 2009; Revised Manuscript Received August 19, 2009

ABSTRACT

We have studied the alignment of colloidal gold nanorods, deposited from solution onto well-defined substrates in the presence of an AC electricfield generated by micrometer spaced electrodes. The field strengths employed in our experiments are sufficiently large to overcome Brownianmotion and induce accumulation and alignment of the nanorods in the region near the electrodes with their long axis parallel to the field. However,despite the large fields, we find that the degree of alignment is considerably smaller than what was previously reported for field-induced nanorodalignment in suspension. We show that hydrodynamic interactions and capillary effects during drying, as well as friction of nanorods on the substratesurface, to not play a major role. The limited alignment of nanorods is ascribed to the different experimental configuration and the correspondinglylarger density of nanorods. The mutual interactions of nanorods give rise to a disturbance of the local electric field and therewith their orientation.For sufficiently large field strengths, these interactions lead to the formation of nanorod chains that ultimately bridge the electrode gap. Furthermore,for small electrode spacing, the nanorods accumulate on the electrode surface, and the screening of their mutual interactions results into considerablyimproved alignment.

Electric field-assisted transport has emerged as a promisingapproach for manipulation of micro- and nanoparticles.1-6

One way to employ such fields is referred to as dielectro-phoresis (DEP): a nonuniform field gives rise to translationalmotion of dipolar particles in the direction of field gradients.7

The dipolar nature of the particles may be permanent, such asin relatively larger magnetic entities, or induced by the externalfield. The latter generally occurs in dielectrophoresis experi-ments. This technique has been used extensively for theprocessing of carbon nanotubes,8,9 biopolymers,10-13 biologicalcells, and viruses.14-19 More recently, considerable interestis in the field of metallic nanoparticles owing to their abilityto assemble into nanowires and microwires, making them apotential candidate for applications in for example nano-electronics.20-22

For the description of a theoretical basis, we approximatethe rod-shaped particles used in this work as prolate ellipsoidalentities. The dielectrophoretic force experienced by a prolateellipsoid with its R-axis parallel to field direction,23 is given by

where a and b are the radii in the long and short directions,respectively, εm is the permittivity of the medium in which the

particles are suspended, E is the electric field strength, and KR

is the Clausius-Mossotti factor, given by

in which R ) S, L designate values pertaining to the short andlong directions, respectively. The dielectric functions of theparticle and the suspending medium in eq 2 are in generalcomplex quantities εp,m* ) εp,m + σp,m/iω where εp,m representthe permittivity of particle and medium, respectively, andσp,m are the conductivities, which constitute the imaginarypart of the dielectric function. The frequency of appliedelectric field is given by ω. The depolarization factors alongthe short and long axis of a prolate ellipsoid, AS and AL,respectively, are expressed in terms of the eccentricity e2 )(1 - b2/a2) and given by23

For dielectric particles, it is well known that the dielec-trophoretic force FDEP depends on frequency. For this classof materials, the Clausius-Mossotti factor KR (eq 2) maychange sign at relatively low frequencies. However, when

* To whom correspondence should be addressed. E-mail: e.s.kooij@tnw.utwente.nl.

FDEF ) 2πab2

3εmRe(KR)∇(E2) (1)

KR ) ( εp* - εm*

(εp* - εm*)AR + εm* ) (2)

AL ) 1 - e2

2e3 [ln(1 + e1 - e) - 2e] and AS )

(1 - AL)

2(3)

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10.1021/nl901968e CCC: $40.75 2009 American Chemical SocietyPublished on Web 08/31/2009

the conductivity and permittivity of the particle are muchhigher than that of the medium, which is generally the casefor metallic entities in an aqueous medium, KR does notchange sign and can be approximated as AR

-1. In this case,the dielectrophoretic force drives the particles to regions ofhigher field strengths. This situation is commonly referredto as positive dielectrophoresis. Negative dielectrophoresisoccurs if either the conductivity or the permittivity of themedium exceeds that of the particle. For gold particles inwater, negative dielectrophoresis occurs at frequencies higherthan 1018 Hz.21

Apart from the translational motion toward the high fieldregion, a nonspherical particle in an electric field (see Figure1) also experiences a torque that tends to align one of themajor axes in the direction of the electric field. This torquearises from the fact that the induced dipole moment, whichstrongly depends on the particle geometry, prefers to alignalong the field lines. Consider a nanorod, approximated bya prolate ellipsoid, placed in electric field E. If θ is the anglebetween the electric field and the long axis, along which thelargest dipole moment is induced (see Figure 1), the averagealignment torque along the long axis is given by23

It is interesting to note that, in contrast to the dielectro-phoretic force, which depends on the gradient of square ofthe field strength, the alignment torque depends on the squareof the field strength.

In addition to control over their spatial position, variousenvisaged applications of nanorods, such as sensors andoptical devices, demand control over their collective orienta-tion in suspension and also after deposition at the interface.Aligned arrays of gold nanorods may be technologicallyrelevant as the different plasmon bands can be selectivelyexcited with polarized electromagnetic waves. In this respect,high-density multidimensional data storage using surfaceplasmons in gold nanorods has been described. Moreover,well-defined arrays of aligned nanorods have also beeninvestigated for Raman scattering enhancement.24-27 Electricfields have been used extensively for aligning anisotropicparticles such as biological cells, semiconductor nanorods,

nanowires, and carbon nanotubes.28-32 Surprisingly, relativelylittle attention is paid to the electric field-induced alignmentof metallic nanorods, despite their potential applications.There is a report on the electric field-induced alignment ofgold nanorods in suspension using parallel plate electrodes,29

but a quantitative study of gold nanorods alignment onsurfaces has not yet been reported to the best of ourknowledge.

In this letter, we present results of a study into the electricfield driven accumulation and alignment of gold nanorodson surfaces patterned with micrometer spaced electrodes. Theaccumulation, alignment, and chaining of nanorods is inves-tigated. A novel aspect in our results encompasses thequantitative analysis of the nanorod orientation after dryingin relation to the electric field strength. The remarkabledifference between the degree of alignment of depositednanorods, as compared to alignment experiments in suspen-sion are described. We discuss our results in terms of thestrengths of the electric field, field gradients, and the mutualinteraction of nanorods. For a more qualitative assessmentof these quantities and to enable analysis of the experimentaldata, electric field simulations are performed using finite ele-ment methods.

Gold nanorods were prepared by a standard three-stepseed-mediated protocol developed by Murphy et al.33 witha slight modification (see Supporting Information). The as-prepared gold nanorod solution (1 mL) was centrifuged twiceat 7000 rpm for 6 min to remove excess CTAB and minimizethe ion concentration in solution and redispersed in 15 mLof Millipore water (resistivity ) 18.2 MΩ cm). Figure 2depicts a transmission electron micrograph (TEM) of thesenanorods. Analysis of five such TEM images gave a meanrod diameter of 26.8 ( 4.5 nm, length 396 ( 39 nm, and anaspect ratio of 15 ( 2. Using eqs 3 and 4, this leads to valuesof the depolarization factors AL ) 0.010 ( 0.003 and AS )0.495 ( 0.001.

Standard optical lithography techniques were used to makethe platinum electrodes on a silicon substrate with a 120 nmoxide layer. The platinum electrodes are 500 µm long, 20µm wide, and 200 nm thick. In different designs, spacingsranging from 3.5-12 µm were used.

The circuit used for applying the voltage to induce nanorodalignment is schematically shown in Figure 2b. The elec-

Figure 1. Direction of the applied electric field relative to theorientation of a prolate ellipsoidal particle. The principle axes a,b, and c are oriented in x, y, and z directions, respectively. Theelectric field is oriented at an angle θ with respect to the majoraxis of the ellipsoid.

⟨τ⟩ ) 23

πab2εm(AS - AL)Re[KLKS]E2 sin 2θ (4)

Figure 2. (left) TEM image of the gold nanorods used in this work,obtained using a Philips CM 300ST system. The average aspectratio amounts to 15. (right) A schematic representation of the circuit(connected to four sets of parallel electrodes with different spacing)used for nanorod alignment experiments is also shown.

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trodes are shunted by a parallel resistance R1 (12 kΩ); thisensures a more constant applied voltage, independent ofsolution resistance. A series resistance R2 (15kΩ) wasincluded in case of short-circuit of the electrodes due tochaining. The voltage was applied using a function generator(Wavetek, model 22); in all cases we indicate the root-mean-square (rms) values. To avoid any effects due to double layerpolarization, all experiments were performed at a frequencyof 50 kHz. For each experiment, 10 µL of gold nanorodsolution was placed on the device by manual pipetting andwas allowed to dry in the presence of the field. Afterdeposition/drying, electron microscopy images were obtainedusing a SEM (Zeiss 1550).

Results of experiments, performed by drying a drop ofcolloidal nanorod suspension on our electrode structure whileapplying an external voltage, are summarized in Figures3a-d. SEM images are shown for electrode gaps of 5, 7, 9,and 12 µm, respectively; at an applied root-mean-squarevoltage of 5 V. As can be seen in these images, the electricfield-assisted deposition gives rise to accumulation of nano-rods predominantly within the space between the electrodeand preferentially near the electrode edges. The number ofaccumulated nanorods increases with decreasing electrodespacing, which corresponds to an increase of the effectivefield strength between the electrodes. Additionally, forsmaller gaps between the electrodes the alignment alsoimproves, as can be concluded from the angular distributionhistograms shown as insets in Figure 3. Nanorods arepredominantly oriented in the direction of electric field, thatis, perpendicular to the electrode edges. The effective electricfield can also be varied by changing the voltage for fixed

electrode spacing. Very similar results have been obtained,for intermediate electrode spacings. For the smallest electrodespacing, markedly different behavior has been observed,which will be described later.

As a control experiment, to verify the influence of dryingphenomena on the alignment and spatial distribution of thenanorods, we performed a similar measurement by drying adrop on the electrode geometry, but now without any voltageapplied, that is, in the absence of an electric field. The resultis shown in Figure 4. Since in this case, in contrast to thesituation with finite applied field, the nanorods do notpreferentially accumulate near the electrodes but are scatteredacross the entire surface, we used a much higher density(approximately 2 orders of magnitude) of nanorods insuspension. As can be seen in Figure 4, the distribution ofnanorods is random, while their density after drying isidentical on the metallic electrodes, as well as in the areasbetween them, and amounts to 2.8 nanorods per µm2. Despitethe much lower nanorod concentration in the suspensionsused for the dielectrophoresis experiments, the number ofaccumulated nanorods between the electrodes to which apotential is applied clearly indicates that dielectrophoreticmotion plays a dominant role.

Furthermore, the orientation of the nanorods is completelyrandom. Close examination of Figure 4 reveals that in someplaces a few nanorods are very close, that is, touching, withtheir long axes aligned in the same direction. In these cases,we assume that capillary effects during drying have led tosuch configurations. Nevertheless, since this occurs veryrarely, combined with the markedly lower nanorod concen-trations used in our actual dielectrophoretic experiments, has

Figure 3. SEM images showing electric field-induced alignment and accumulation of gold nanorods. The applied voltage (rms) amountsto 5 V, while the electrode separation distance varies: (a) 5 µm, (b) 7 µm, (c) 9 µm, and (d) 12 µm. The insets show histograms indicatingthe angular distribution of nanorods in each situation, obtained by determining the orientation of more than 150 nanorods.

3788 Nano Lett., Vol. 9, No. 11, 2009

led us to conclude that capillary effects can be ruled out.Despite the fact that we are not able to define the actualfield distribution during the solvent evaporation process,apparently the field strengths are sufficiently large todominate position and orientation of the nanorods asobserved after drying of the substrates.

Generally, for the geometry of plane parallel electrodes,the field as well as the field gradient is larger near theelectrodes; for larger distances from the substrate on whichthe electrodes reside, the field strength also decreases. Toassess the actual magnitude of the three-dimensional distri-bution of the electric field strength, we performed simulationsusing finite element methods. These simulations only providea representation of a static situation, and do not take intoaccount the effects related to evaporation of the solvent, suchas decreasing liquid on the electrode configuration andaccompanying modification of the electric field strengths.Nevertheless, it serves as a benchmark for the initial situation,and provides an indication of the forces acting on thenanorods.

For this purpose, we employed commercially availablesoftware (FlexPDE) specifically designed to solve partialdifferential equations for one-, two- and three-dimensionalproblems. As the concentration of ions in solution is verylow, double layer effects were ignored, and the electrodepotential was set equal to the applied potential. Also, thelength of the electrodes is much larger than their width,thickness, and mutual spacing, so the dimension along thelength, that is, the z-axis, can be ignored without affectingthe accuracy of simulations. This reduces the problem to twodimensions. We only take into account the directions parallelto the gap, that is, the x-axis, and perpendicular to the

substrate, that is, the y-axis. The electrodes are consideredto be submerged under a 10 µm thick layer of water (ε )80ε0). Furthermore, the substrate consists of a bulk p-typesilicon substrate, covered with a 120 nm thick insulating layerof SiO2 (ε ) 5ε0).34 For our simulations, we set the potentialof the silicon substrate to 0 V.

The result of a typical simulation of the electric fieldstrength in the vicinity of electrodes spaced 5 µm apart withan applied potential of 5 V is shown as a contour plot inFigure 5a. Cross sections of the electric field strength betweenthe electrodes, as well as plots of the gradient along andperpendicular to the substrate are shown in Figure 5b fordifferent electrode spacing, as indicated in the legend.

It is evident from Figure 5 that for all gap widths the fieldstrength is largest near the electrodes. With increasingdistances from the electrodes, the field strength drops sharply.Moreover, close examination of the area between the planarelectrodes reveals that the insulating layer of SiO2 is not thickenough to completely shield the effects of the underlying Sisubstrate on the field above it in the liquid phase.34 Thisinteraction results in the creation of a region in the center ofthe gap separating the electrodes, where the y-componentof the electric field gradient (dEy ) ∂|E|2/∂y) is positive, thatis, where the gradient is directed away from the substratesurface, as shown in the bottom panel of Figure 5b. SincedEy is responsible for the dielectrophoretic force normal tothe substrate, nanorods experiences a repulsive force awayfrom the surface in the area between the electrodes.Moreover, when the same voltage is applied, the repulsiveforce decreases with increasing gap lengths. In other words,for larger electrode spacing, the repulsive nanorod-substrateinteraction decreases. Indeed, close examination of the SEMimages in Figure 3 reveals a slight increase in the density ofdeposited nanorods between the electrodes for larger separa-tion distances.

In addition to the repulsive force in the y-direction, thex-component of the electric field gradient (dEx ) ∂|E|2/∂x)exerts a horizontally oriented dielectrophoretic force on thenanorods, which drives them toward the electrodes. Therelatively high density of nanorods near the electrodes, asobserved for all SEM images in Figure 3, is a consequenceof the combined effect of these two forces.

To quantify the nanorod alignment, we introduce the orderparameter S.30 If θ is the angle of a nanorod with respect tothe electric field (see Figure 1), the order parameter isdetermined by averaging the orientation angles of allnanorods according to

Two limiting values for the order parameter S are obviousfrom eq 5, (i) in the case of perfect alignment with allnanorods parallel to the direction of the electric field, S )1, while (ii) a value of S ) 0 corresponds to a completelyrandom orientation of all nanorods.

To obtain S from our experimental results, we determinedthe angle of orientation θ of at least 150 nanorods locatedin the gap between the electrodes with respect to the direction

Figure 4. SEM images showing the random spatial and randomorientation distribution of nanorods deposited in the absence of anapplied electric field. The density of nanorods amounts to 2.8 µm-2,while their orientation is completely random.

S ) ⟨2 cos2 θ - 1⟩ (5)

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of the electric field. In that way, we are able to quantify theaverage orientation of the nanorods from SEM images suchas those in Figures 3 and 4. For the control experiments atzero field (Figure 4), we had already mentioned that theorientation was random and did not depend on whether thenanorods are on top of or between the electrodes. This alsofollows from the orientation parameter, which amounts to S≈ 0. This value is consistent with the orientational distribu-

tion in the histogram in Figure 4, which demonstrates thatthere is no directional preference.

Using the procedure described above, we are now able toplot the order parameter S as a function of the electrodespacing for different voltages. The result is shown in thetop panel of Figure 6. With increasing distance between theelectrodes, the effective field between the electrodes de-creases and as expected the order parameter also becomessmaller. Furthermore, for the same distance between theelectrode, S decreases for smaller voltages, in line withexpectations.

In the bottom part of Figure 6 the S-values are plotted asa function of the average electric field in a region near theelectrode where the nanorods are accumulated. To estimatethis average electric field, we have used the simulations, suchas those shown in Figure 5, and averaged the field strengthwithin 1.5 µm from the electrode surface. As a function ofthe effective field, the data scale to a single curve. Thisindicates that the electric field experienced by the nanorodsis the predominant parameter that governs their alignmentwith respect to the field direction. All other potentiallyrelevant factors, such as capillary effects or convection duringdrying are of minor importance.

As is to be expected, Figure 6 confirms that low values ofthe effective field correspond to small S representing limitedalignment of the nanorods. The extent of competitionbetween the dielectrophoretic force and Brownian motionof the nanorods, which tends to randomize the nanorodmotion and their orientation, can be estimated. The criticalfield strength required to overcome Brownian motion is

Figure 5. Simulation of the electric field for electrodes spaced 5µm apart with an applied potential of 5 V. (a) Contour plot of across section through the electrodes; the color bar indicates the fieldstrength. (b) Electric field (top panel) and field gradients in x- andy-direction (middle and bottom panels, respectively) obtained fromthe simulation at a distance of 15 nm above the SiO2 surface, as afunction of distance r normalized to the gap size d.

Figure 6. (top) Order parameter S as a function of electrode spacingfor varying voltages, as indicated in the legend. The solid trianglespertain to the results shown in Figure 3. (bottom) Scalability of Sas a function of the averaged electric field in a region of 1.5 µmwidth near the electrodes. The dashed lines are a guide to the eye.

3790 Nano Lett., Vol. 9, No. 11, 2009

obtained by comparing (i) the dipole energy in the electricfield with (ii) the thermal energy of the nanorods35

Rewriting the above equation yields an expression for theestimated critical field strength, at which the dipole energyexceeds the thermal energy

At room temperature (300 K) and taking into account thegeometry of our nanorods, we find a critical field value of 3× 104 V/m. Moreover, Van de Zande et al. observedexperimentally that nanorods, very similar to ours, insuspension almost perfectly align (S > 0.9) between flatelectrodes at field strengths exceeding 105 V/m.29 Theseresults seem to be in agreement with more theoretical work,36

in which the rotational energy due to the electric field isinserted into the Boltzmann distribution. From this analysis,it is concluded that at field strength exceeding 105 V/m theelectric field overcomes Brownian motion, and the orderparameter is generally expected to be larger than 0.9.36

Although there seems to be an initial regime in Figure 6in which the field has little effect, the field strengths used inour experiments are in all cases much higher than the criticalfield strength described above. This indicates that Brownianmotion is not a predominant parameter. Furthermore, thescaling observed in Figure 6 shows that the electric field isthe dominant parameter controlling the alignment, whilecapillary forces during drying as well as surface friction canbe neglected. The latter is assumed based on the fact thatBrownian motion is relatively pronounced, and even afterdrying the nanorods can still be rinsed off relatively easily.

In our deposition experiments, the order parameter satu-rates at values between S ) 0.6 and S ) 0.7 for field strengthsexceeding 2 × 106 V/m. Despite the qualitative agreementwith what is expected, that is, the degree of alignmentincreases with field strength, the alignment within the gapbetween the electrodes in our case is markedly lower evenat much higher field strengths. Considering the absence ofevaporation-induced effects and also that friction should notplay as major role, we ascribe this discrepancy between thedegree of alignment in our deposition experiments and thoseperformed in suspension to the large difference in experi-mental conditions. The mutual interaction between nanorodsalready adsorbed at the electrode and those being depositedunder the influence of the externally applied field plays avital role in our case. As discussed before, the larger fieldstrength near the electrode edges (Figure 5) gives rise to ahigh density of nanorods in these areas, as opposed to areasfurther away. In addition, the surface of a perfectly conduct-ing metallic nanorod, as we are using in all our experiments,represents an equipotential surface. In close vicinity of theelectrode and aligned parallel to the field, this gives rise to

an extension of the applied electric field into the gap betweenthe electrodes and focusing of the field lines to its outer end.37

To substantiate our aforementioned assumption, we visual-ize the effect of a single metallic nanorod near an electrodeusing another simulation obtained from FlexPDE; the resultis shown in Figure 7. Additionally, the result for a simulationinvolving multiple nanorods is shown as an inset. For thesefield simulations, a three-dimensional analysis was employedsince the finite width of the nanorod along the z-axis cannotbe ignored in this case.

As can be seen from Figure 7, the field is concentrated atthe outer end of the nanorod and is markedly reduced alongits length. The field gradient at the tip attracts subsequentlydeposited nanorods, while the gradient along the length givesrise to an effective repulsion. Overall, this results indielectrophoretic motion and gives rise to orientational forcesin directions different from that of the externally appliedelectric field generated via the electrodes. We assume thatthis additional interparticle interaction hinders completealignment of all nanorods, and results in a lower degree oforientation, that is, lower values of the ordering parameterS as compared to the case in which the field is uniform andparticles are well separated.

The simulation of a single nanorod presents an idealsituation. To assess the effect of multiple nanorods, both nextto each other at the electrode edges as well as further awayfrom the electrode, we have performed a simulation usingseveral nanorods. The result is shown in the inset (right) inFigure 7. It is clear from this contour plot that in the narrowregion between closely spaces nanorods, there is still a fieldinhomogeniety. Moreover, the disturbance of the field nearthe nanorods is even more pronounced than in the case of

23

πab2εmRe(KL)E2 g32

kBT

Ec ≈ 9kBT

4πab2εmRe(KL)(6)

Figure 7. Contour plot showing the disturbance of the electric fieldbetween the electrodes due to the presence of a single nanorod, aswell as multiple nanorods. Simulations were performed for 5 µmelectrode spacing gap at 5 V applied voltage; the dimensions ofthe nanorod were taken to be 30 nm × 400 nm. The left inset showsan enlargement of the outer end of the nanorod. The right insetshows the (×2 zoom) result of a simulation involving sevennanorods near the electrode surface.

Nano Lett., Vol. 9, No. 11, 2009 3791

the single nanorods presented earlier. This substantiates ourassumptions that the dielectrophoretic forces “pulling” thenanorods from their aligned orientation along the externalfield direction are considerable and can well account for thelower order parameters observed in Figure 6 as comparedto results for freely suspended nanorods in solution.

Finally, as can be concluded from the bottom panel inFigure 5b, the length of the region between the electrodeswhere dEy is negative decreases as the electrode spacingbecomes smaller. For even smaller distances between theelectrodes, the spacing is not sufficient to accommodate thedeposition of nanorods in this region; their length becomescomparable to the width of the region where dEy < 0.Consequently, for sufficiently small electrode spacing, thenanorods will ultimately be repelled from the gap and willbe deposited on top of the electrodes. In Figure 8, the resultof a deposition experiment using electrodes spaced 3.5 µmapart at an applied rms-voltage of 5 V is shown. As expected,there are only a few nanorods in the region between theelectrodes. The few nanorods in this small area are mostlyoriented along the electrode edges.

Accordingly, there is a very high density of nanorods ontop of electrodes, which show very good alignment. Theorder parameter in this specific case amounts to S ) 0.85,indeed indicating a high degree of alignment as comparedto the results summarized in Figure 5. The electric field abovethe electrode has components in the x- and y-directions.Apparently, the component of the electric field parallel tothe substrate is sufficiently strong to align the nanorods.Moreover, the y-component is not negligible. However,during drying of the suspension, when a nanorod touchesthe electrode, its most stable orientation is parallel to thetangential x-component of the field.

Furthermore, it can be seen from Figure 8 that nanorodshave a higher density in a band 1.5-2 µm from the electrodeedge. As we move on top of the electrode away from thegap, the electric field strength, especially the x-component,decreases markedly (see Figure 5a). It is more favorable forthe nanorods to deposit near the electrode edge where thefield is relatively large. However, dEx, on top of the electrode,which in this case forces the nanorods into the gap, is hugenear the edge and decays very fast when we move awayfrom the edge. This makes the region in the vicinity of edgeunfavorable for nanorod deposition. The relative high densityof nanorods in 1.5-2 µm band from the electrode edge seemsto be a compromise of these two factors. The improvedalignment in this case is attributed to reduced interactionsbetween nanorods. This arises from the fact that the dipolefield of nanorods deposited on top of the electrodes isscreened by the metallic surface, that is, the electrode itself,and thus the externally applied field dominates all particle-substrate interactions. Consequently, the presumed interac-tions responsible for limiting of the alignment as outlinedin the previous section are absent, therewith enabling aconsiderably higher degree of orientation.

As discussed previously, the interaction of nanorodsdeposited next to each other gives rise to a lowering of theirorientation. With increasing field strengths, another interac-tion becomes important. The induced dipole moments of thenanorods will, at sufficiently strong fields, induce an attrac-tive interaction forcing the nanorods in a head-to-tailconfiguration. In fact, this is also obvious from the simulationresults shown in Figure 7. The electric field at the tip ofnanorod is 2 orders of magnitude larger than the fieldelsewhere within the region between the electrodes. Atsufficiently high field strengths, the chaining force willbecome dominant over the dielectrophoretic force drivingthe nanorods toward the electrodes.

Indeed, it can be seen from Figure 3 that at higher fieldstrengths (a,b) the nanorods start to assemble into chainlikestructures. In the specific situation of Figure 3, these nanorodchains are confined to the region where the field gradientdEy perpendicular to the substrate is negative. However, whenthe applied voltage is increased further, the increasingattractive interaction gives rise to the formation of longernanorod chains. At sufficiently high voltages the nanorodchains will be formed even in the regions where dEy > 0despite the repulsive substrate-nanorod interaction. Resultsfor different electrode spacings and applied potentials areshown in Figure 9. The chain length, that is, the number ofassembled nanorods, increases for larger effective fieldstrengths. Ultimately, nanorod chains of sufficient length willgive rise to bridging of the electrodes and eventually lead toa short-circuit.

Although our results seem to indicate that for voltagesabove approximately 5 V the chaining becomes predominant,it is difficult to derive a threshold value for the electric fieldabove which chaining occurs. As mentioned above, thetransition from single nanorods at the electrode edge tochaining is a very gradual; chains are already observed inFigure 3, for example, while in Figure 9 also a number of

Figure 8. SEM image showing the deposition of aligned nanorods,deposited on top of the electrodes spaced 3.5 µm apart. The insetshows the angular distribution of the nanorods.

3792 Nano Lett., Vol. 9, No. 11, 2009

single nanorods are discernible. Moreover, the frequency isalso a relevant parameter. In principle, the threshold fieldfor chaining of spherical nanoparticles is proportional to thereciprocal value of the Clausius-Mossotti factor, Eth ∝ K-1,and therewith it should depend on the frequency.23 Althougha precise model for elliptical nanoparticles does not exist,we assume that a similar expression holds for our nanorods.As mentioned in the introduction, for metallic nanorods,characterized by a large dielectric function at frequencieswell below the visible optical range, the Clausius-Mossottifactor is nearly constant and only related to the depolarizationfactor. As such we do not expect a large influence of thefrequency in the range used in our experiments.

The nanorods suspensions used in our experiments do notonly contain nanorods, but are contaminated by irregularshaped nanoparticles and as well as spheres. The results inFigure 9 shows that these are built into the nanorods, orattach to their sides. Although we cannot be certain, weassume that these odd-shaped particles are also metallic. Ifthis would not be the case, they would not exhibit such stronginteraction with the nanorods and their chains. Moreover, inFigure 9b it seems that nanorod chains prefer to attach tothe large ill-defined particles near the electrode edges.Considering they are metallic, this is to be expected due tofield focusing effects similar to that shown in Figure 7.

Close examination of the images in Figure 9 reveals thatmany of the chains are a single nanorod wide. As expectedfor dipolar particles (permanent or induced), they experiencean attractive interaction for assembly into a head-to-tailconfiguration while the side-by-side orientation is governedby a repulsive interaction. In fact, the results in Figure 9seem to indicate that nanorod chains are well-separated dueto this repulsive interaction. Moreover, the density ofnanorods and chains in the direction parallel to the electrodelength (i.e., perpendicular to the E field) is markedly smallerin Figure 9 as compared to Figure 3.

Summarizing, we have demonstrated the results of a quan-titative study on the alignment of colloidal gold nanorods onwell-defined substrates under the influence of electric field.Despite the application of much higher field strengths, thedegree of alignment of nanorods was much smaller than forthose freely suspended in liquid between parallel electrodes.Finite element simulations revealed an intense modification ofthe local electric field due to the presence of nanorods, whichseemed to affect other nanorods and hinder their completealignment. At sufficiently large field strengths these nanorodsinteractions lead to the chaining of nanorods. The simulationresults also showed a region of repulsion away from thesubstrate for nanorods inside the gap. For adequately smallelectrode gap this gave rise to the potential deposition ofnanorods on electrode surface. The screening of dipole interac-tions by the metallic electrode in this case resulted into amarkedly higher degree of alignment.

Acknowledgment. The authors thank N. Izadi (Universityof Twente) for device fabrication, and B.C. Gierhart (Uni-versity of California) for assistance with the electric fieldsimulations. One of the authors (W.A.) acknowledges supportfrom the Higher Education Commission in Pakistan.

Supporting Information Available: Description of thesynthesis procedure of nanorods. This material is availablefree of charge via the Internet at http://pubs.acs.org.

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