LETTER pubs.acs.org/NanoLett
Electrochemical Crystallization of Plasmonic Nanostructures Andreas B. Dahlin,*,† Takumi Sannomiya,‡ Raphael Zahn,† Georgios A. Sotiriou,§ and Janos V€or€os† Laboratory of Biosensors and Bioelectronics, Swiss Federal Institute of Technology Z€urich, Switzerland
bS Supporting Information ABSTRACT: We show how gold recrystallizes when under the influence of electrochemical potentials. This “cold annealing” occurs without charge transfer reactions and preserves nanoscale structural features. By performing the process on plasmonic nanostructures, grain growth is monitored noninvasively by optical spectroscopy. In this way, the influence from crystal structure on plasmon resonances can be investigated independently. Observed spectral changes are in excellent agreement with analytical models and changes in electron relaxation time and plasma frequency are calculated. KEYWORDS: Crystal, grain, plasmon, gold, electrochemistry
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ollowing the advancement of nanotechnology and associated fabrication techniques, several areas of application have emerged based on optical or electrical phenomena that occur on the nanoscale. Indeed, understanding and controlling physics and chemistry on the nanoscale is clearly a prerequisite for further miniaturization of electronic devices. The possibility to couple light into metal nanostructures by plasmon excitation has already found several applications such as biosensing,1,2 catalysis,3,4 waveguiding,5 superlenses,6 and photonic circuits.7 The success of such future nanooptical/electrical devices depends heavily on understanding and controlling how material properties influence the application under consideration. In particular, optical and electrical properties of metals are expected to depend on crystal structure8-11 and the presence of grain boundaries.12 However, to investigate such effects, particularly in nanoscale devices, it must be possible to crystallize the metal in a controllable manner, that is, without damaging or altering the structure or surrounding materials. Most metal layers in nanodevices are physically deposited by evaporation under vacuum, which results in fine crystalline (grain size 0.8 V other effects due to dissolution of the metal starts to occur.34,37 This could be observed as a strong red shift, which for holes is due to the lower thickness of the Au film31 and for disks due to the increased aspect ratio.30 We emphasize that all other imaginable electrochemical processes in this system give red shifts and therefore cannot explain the observed blue shift. For the data in Figure 2, potential pulses of U = 0.5 V with a duration of 2 min were repeatedly applied and the surface was kept at U = -0.2 V for 1 min in between. Switching U between a positive and a negative potential was not necessary for crystallization but increased the grain growth rate. The immediate peak shifts observed in response to switching between high and low potential are attributed to the capacitive charging of the metal as we34 and others32 have described quantitatively for metal colloids.38 For holes, extracting or adding electrons to the surface Au atoms alters the dispersion relation for the surface plasmons launched by the holes27,28 and a peak shift is thus expected.26 During the crystallization process of holes (Figure 2D), clear variations (up to 50%) could be observed in the magnitude of the immediate peak shift response (insets in Figure 2C,D) to a potential pulse, while for disks these variations were much smaller ( 0.99) even when using absolute values for the cross section and allowing only τ and ωp to change, resulting in parameter values of τ = 1.50 ( 0.03 fs and ωp = 12.07 ( 0.18 PHz. These numbers are in agreement with the expected initial metal, that is, very small grains. However, the literature is not consistent in this respect due to the difficulty of distinguishing crystal grain size influence from other effects such as surface roughness, voids in the film, impurities, and so forth.9-11,41,42 Therefore, we cannot fully evaluate whether the model still contains a small error in the absolute values. For instance, the rough top disk surface is not taken into account and probably makes the model predict a lower τ,44 but there is actually no “true“ value to compare with. It should also be noted that although a small error cannot be excluded, a relative comparison, that is, relating spectral changes to changes in τ and ωp, should be highly accurate. By changing τ and ωp in the model for the metal (eq 1), the spectral dependence of each parameter can be observed, as shown by the green and purple spectra in Figure 4. These are indeed the only parameters expected to change due to altered crystallinity. All other parameters should remain constant when there are no changes in nanostructure dimensions. Clearly, changing ωp results in large peak shifts, but hardly any change in peak width. On the contrary, increasing τ sharpens the peak significantly but results in practically no change in peak position. A complete explanation of spectral changes (Figure 2A) can thus only be achieved if the crystallization modifies not only dephasing 1340
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Nano Letters time but also the plasma frequency. Changes in ωp are not surprising due to reduced scattering at grain boundaries and may also have a small contribution from strain relaxation or lattice constant changes. The values that best describe the spectral changes observed during crystallization when performed as in Figure 2 are an increase in τ of 50 ( 10% and an increase in ωp of 5.4 ( 1.1%, which are reasonable changes considering the observed increase in grain size.11,12 For comparison, a spectrum generated using ε data from Johnson and Christy10 is shown in Figure 4 (keeping all other parameters equal). This commonly used data set provides a poor fit and much better results are achieved for data that has lower values of τ and ωp. Although our values approach those of Johnson and Christy10 upon grain growth, they cannot be fully reached. In fact, Johnson and Christy10 data is known to have the most extreme values for these parameters9 so the disagreement with experiment for this data set is not so surprising, although we consider it worth highlighting. We now turn back to the nanoholes to test if the material changes predicted for disks are consistent with the spectral changes observed for this structure. The full spectrum of nanoholes is complicated to model, primarily due to the resonance being an effect of both individual holes28 and their spacing2,27,32 inducing excitation of surface plasmon polaritons in the thin film. However, the dispersion relation between wavevector and frequency kSP(ω) for the symmetric bound surface plasmon mode in a thin metal film can be acquired by solving Maxwell’s equations for three layers, taking retardation effects into account.27,28 Further, resonances in nanohole arrays are geometrical effects in the sense that (the real part of) kSP at resonance is defined by the diameter28 and separation27 of holes. Indeed, these holes act as zero mode grating couplers, through their short-range order, for excitation of the bonding surface plasmon mode, although there is also a localized resonance associated with the extinction minimum (reference to parallel work submitted at the time of writing). The precense of holes in the film merely causes an offset error in the dispersion relation calculation. Thus, since the electrochemical crystallization preserves the nanostructure dimensions, the shift in peak position (converted to Δω) can be predicted as illustrated in Figure 5. The dispersion relation is then recalculated using new values for the material parameters τ and ωp, so that the same value of kSP corresponds to a new resonance frequency. Our model predicts a blue shift of 10 nm, assuming that the Au film with holes crystallizes in the same manner (same changes in τ and ωp) as the nanodisks when exposed to the same potential pulses. The typical observed saturated blue shift was indeed 10 nm for nanoholes, although with higher experimental variations ((4 nm). This is most likely related to another interesting observation, namely that the peak position was never monotonically decreasing but always changed into a red shift for some time before saturation (Figure 2D). In contrast, the nanodisk blue shift always approached a saturated response of ∼50 nm and experimental variations were only observed in the time for saturation to occur. Our interpretation is that this is related to the fact that ωp and τ influence the nanohole resonance differently, causing either blue shift or red shift, respectively (Figure 5). This again shows that an increase in τ alone cannot explain the results. In principle, the faceting of the Au film could also contribute to the spectral changes through increased roughness.25 However, the roughness measured by AFM remained the same for short correlation lengths and the influence on kSP is in any case expected to be small for the roughness
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Figure 5. Modeling the spectral changes for nanoholes. The calculated dispersion relation for surface plasmons in a 30 nm Au film on glass (refractive index 1.52) in contact with 100 mM NaCl (refractive index 1.334) is shown in terms of the real and imaginary parts of the wavevector. The solution is for the bonding mode28 (the antibonding mode is leaky into the glass). Photon lines for air, water, and glass are shown in brown. The influence of dephasing time and plasma frequency is investigated, using the changes calculated from the crystallization of nanodisks in Figure 4. The physical rationale behind the model is that the wavevector at resonance is not changed, because the geometry of the structure remains the same after crystallization. The lower plot shows the imaginary part of the wavevector for the four different sets of material parameters.
changes due to grain boundaries.25,45 Also, increased roughness should result in a broader peak, in contrary to our results. Clearly, more information than the peak position is needed to determine both τ and ωp using only data from nanoholes. Looking at the sharpening of the nanohole resonance, this effect can be qualitatively explained by a reduced imaginary part of kSP. Indeed, this is what our model predicts both for increased τ and ωp (Figure 5), even though the parameters influence the peak position differently. When monitoring the nanohole peak width during crystallization, the kinetics indeed showed a monotonically decreasing graph, in contrast to the peak position. Still, a deeper analysis based on peak width requires establishing a quantitative relation to the imaginary part of kSP, which we consider beyond the scope of the present work. We will now look closer at why the plasma frequency ωp increases due to the crystallization. From the model in eq 2, the two principle options are either an increased free electron density Ne by ∼11% or a reduced effective mass meff by ∼10%. At first glance, it appears like the increase in ωp cannot be explained by an overall compression of the metal (an increase in Ne due to higher density), because the metal nanostructure dimensions were not altered after crystallization according to AFM and SEM data. However, this is under the assumption that each gold atom always has exactly one free electron, which probably holds true 1341
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Nano Letters only for an infinite ideal lattice. The presence of grain boundaries might influence the freedom of the electrons so that Ne is in fact effectively increased upon grain growth due to the reduced surface to volume ratio of the crystallites. An alternative explanation is that the recrystallization reduces the effective electron mass meff by altering the band structure. The effective mass represents the influence from the atomic lattice, that is, the fact that the electron is not entirely free (as in vacuum). Strictly, meff is defined from the curvature (second derivative) of the electron dispersion relation.46 Considering the changes in crystallinity observed experimentally, in particular the lattice constant change indicated by XRD, it seems plausible that the band structure of the metal changes. If meff is higher at grain boundaries, the recrystallization could lead to an overall reduced meff. Although many types of crystallinity changes in metals have been studied previously,11,12,14,15,17-19,39 we believe this study introduces the simplest way to induce grain growth so far presented, using capacitive electrochemical potentials. However, it is not straightforward to see why the applied potentials induce crystallization. It should first be noted that the grain growth kinetics do not show pulsed behavior, that is, the potential (not the current) seems to be the determining factor. Second, the observed grain growth cannot be explained by simple heating. The sample is kept at room temperature and it can be easily calculated that the energy in a potential pulse (product of potential, current and time) is not sufficient to heat the gold by more than ∼10 K maximum, assuming there is no cooling effect from the surroundings. Another hypothesis is that crystal grains grow due to impurities at exposed grain boundaries being dissolved into solution,18 but we consider this less likely since pure Au is used and the surface is thoroughly cleaned.36 We believe that the grain growth is instead related to counterion adsorption37 and the strong field in which ions in the the Stern layer are exposed.20,26 The mechanism is probably related to atomic reconstruction20,21 induced by strongly bound counterions.37,39 Since crystallinity has a small influence on the local electrochemical potential,14,19,39 there are nanoscale potential gradients along the surface. The grain boundaries may act as electrochemical cells, spontaneously transforming Au atoms into (111) order via reversible interactions with counterions. From this point of view, the applied potential has the function of shifting the system to a higher absolute energy level, thereby promoting the process. Strain in the initially small grains may cause the recrystallization to continue from the surface into the metal during relaxation,40 which is in agreement with the lattice constant change observed by XRD. In summary, we have shown how vacuum deposited gold films crystallize when the metal is under the influence of an electrochemical potential in a salt solution. Plasmon resonances in nanostructures provide a simple independent way of monitoring the grain growth with high resolution and calculating changes in material properties. The grain size upon saturation increases with the magnitude of the applied potential and the growth rate can be increased by switching between negative and positive potential. Nanoholes in an Au film and Au nanodisks on ITO appear to crystallize in the same manner with grains that span across the whole 30 nm film. Future studies may investigate how thick metal layers that can be crystallized as well as how the process occurs in other metals and when using other electrolytes. We show that both the dephasing time and the plasma frequency change with grain size and highlight the large
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influence on nanoparticle and surface plasmons simply due to crystallinity changes, which can be studied in a controllable manner using this system. In future work, changes in electrical properties could also be investigated, for example, by measuring conductivity in metallic nanowires.
’ ASSOCIATED CONTENT
bS
Supporting Information. Description of nanofabrication and electrochemistry/spectroscopy experiments. Further descriptions of sample characterization by SEM, AFM, and XRD. Results of thermal annealing. Details on the modeling of the optical properties of nanodisks and nanoholes. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Addresses †
Institute of Biomedical Engineering, Gloriastrasse 35, 8092 Z€urich, Switzerland. ‡ Terabase Inc., Okazaki Institute for Integrative Bioscience, National Institutes of Natural Sciences, Okazaki, Japan. § Particle Technology Laboratory, Institute of Process Engineering, Department of Mechanical and Process Engineering, ETH Zurich, Sonneggstrasse 3, 8092 Zurich, Switzerland.
’ ACKNOWLEDGMENT This work was funded by the Swedish Research Council, Swiss National Science Foundation (200020-126694), and ETH Z€urich. The authors thank K. Kumar and Professor R. Spolenack at the Department of Materials Science for valuable help as well as Professor S.E. Pratsinis from the Particle Technology Laboratory for discussions and XRD support. ’ REFERENCES (1) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Nat. Mater. 2008, 7 (6), 442–453. (2) Jonsson, M. P.; Dahlin, A. B.; Jonsson, P.; Hook, F. Biointerphases 2008, 3 (3), Fd30–Fd40. (3) Larsson, E. M.; Langhammer, C.; Zoric, I.; Kasemo, B. Science 2009, 326 (5956), 1091–1094. (4) Novo, C.; Funston, A. M.; Mulvaney, P. Nat Nanotechnol. 2008, 3 (10), 598–602. (5) Lal, S.; Link, S.; Halas, N. J. Nat Photonics 2007, 1 (11), 641–648. (6) Zhang, X.; Liu, Z. W. Nat. Mater. 2008, 7 (6), 435–441. (7) Ozbay, E. Science 2006, 311 (5758), 189–193. (8) Tadjeddine, A.; Kolb, D. M.; Kotz, R. Surf. Sci. 1980, 101 (1-3), 277–285. (9) Aspnes, D. E.; Kinsbron, E.; Bacon, D. D. Phys. Rev. B 1980, 21 (8), 3290–3299. (10) Johnson, P. B.; Christy, R. W. Phys. Rev. B 1972, 6 (12), 4370– 4379. (11) Theye, M. L. Phys. Rev. B: Solid State 1970, 2 (8), 3060–3078. (12) Chen, K. P.; Drachev, V. P.; Borneman, J. D.; Kildishev, A. V.; Shalaev, V. M. Nano Lett 2010, 10 (3), 916–922. (13) Vemury, S.; Pratsinis, S. E.; Kibbey, L. J. Mater. Res. 1997, 12 (4), 1031–1042. (14) Chan, H. S. O.; Ho, P. K. H.; Zhou, L.; Luo, N.; Ng, S. C.; Li, S. F. Y. Langmuir 1996, 12 (10), 2580–2586. (15) Perdriel, C. L.; Arvia, A. J.; Ipohorski, M. J. Electroanal. Chem. 1986, 215 (1-2), 317–329. 1342
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