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Langmuir 1999, 15, 866-871
Electrochemical Charge Injection into Immobilized Nanosized Gold Particle Ensembles: Potential Modulated Transmission and Reflectance Spectroscopy Theo Baum, Donald Bethell, Mathias Brust, and David J. Schiffrin*,† Department of Chemistry, University of Liverpool, L69 7ZD, U.K. Received July 23, 1998. In Final Form: November 19, 1998 Self-assembled multilayer thin films of nanometer-sized gold particles linked with organic dithiols have been prepared on glass, indium tin oxide, and gold substrates. The gold particles within these structures retain their integrity and no sintering occurs as demonstrated by their optical absorbance, ellipsometry, and potential modulated transmission and reflectance spectroscopy. The optical response to electrochemical charge injection into the outermost layer of gold particles is completely different from that of bulk gold electrodes, and it is concluded that the particles have to be regarded as discrete, immobilized quantumdots.
Introduction A fascinating aspect of small-particle research is the prospect of designing new materials whose properties are controlled by quantum-size effects that operate on the level of their constituents, i.e., metal or semiconductor nanoparticles. The preparation of stable particles that do not lose their integrity even in densely packed arrangements is a prerequisite which has been achieved in a number of cases.1 The use of small gold particles and clusters has led to particularly promising results in the past. Using a phosphine ligand and gold colloids, Schmid and Lehnert first described a material that could be isolated from solution and fully redispersed, indicating clearly that the particles maintain their integrity in the solid state.2 The same group had previously reported the Au55 cluster,3 a pure chemical compound, which has led to a significant increase in the understanding of the chemical and physical properties of small metal particles. Recently, the preparation of extremely stable thiolderivatized gold clusters has been reported.4 These new materials can be easily handled like simple organic compounds, crystallize into two- and three-dimensional quantum-dot superlattices,4f and are readily available for further chemical modifications of their ligand shell.4g,i Murray, Whetten, et al. have demonstrated that such particles, dissolved in an electrolyte solution, exhibit electrochemical coulomb-staircase charging due to their extremely small absolute capacitance leading to the quantization of the particle potential.4k,m Owing to their unusual stability, thiol-derivatized particles lend themselves to the study of closely packed ensembles of particles for which the emergence of collective electronic properties may be expected. For example, Heath †
E-mail:
[email protected].
(1) See for example: (a) Giersig, M.; Mulvaney, P. Langmuir 1993, 9, 3408. (b) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (c) Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Adv. Mater. 1995, 7, 795. (d) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (e) Weller, H. Angew. Chem., Int. Ed. Engl. 1996, 35, 1079 and references therein. (f) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1996, 270, 1335. (g) Pileni, M. P. Langmuir 1997, 13, 3266. (h) Feldheim, L.; Keating, C. D. Chem. Soc. Rev. 1998, 27, 1 and references therein. (2) Schmid, G.; Lehnert, A.; Angew. Chem., Int. Ed. Engl. 1989, 28, 780. (3) Schmid, G.; Pfeil, R.; Boese, R.; Bandermann, F.; Meyer, S.; Calis, G. H. M.; van der Velden, J. W. A. Chem. Ber. 1981, 114, 3634.
and co-workers studied distance-dependent metalinsulator transitions in two-dimensional superlattices of similarly stable thiol-coated silver particles and discovered an abrupt but reversible onset of metallic properties when the particles are in close proximity while maintaining their discrete character.5 Self-assembled multilayers and bulk materials of gold particles either stabilized or linked by alkanethiols exhibit nonmetallic electronic properties and charge transport via activated electron hopping between the individual particles.1c,4c,6 In the present paper, the optical function of such materials is examined based on Mie theory,7 and it is demonstrated that the transmittance and reflectance of nanostructured thin films can be modulated by electrochemical charge injection. The resulting changes in reflectance and transmittance of the material are explained in terms of electron-density oscillations in each individual particle, for which the (4) For representative refererences see: (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (b) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. J. Chem. Soc., Chem. Commun. 1995, 1655. (c) Terrill, R. H.; Postlethwaite, T. A.; Chen, C.-H.; Poon, C. D.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, D.; Falvo, M.; Johnson, C. S.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537. (d) Ohara, P. C.; Leff, D. V.; Heath, J. R.; Gelbart, W. M. Phys. Rev. Lett. 1995, 75, 3466. (e) Badia, A.; Singh, S.; Demers, L. M.; Cuccia, L. A.; Brown, G. R.; Lennox, R. B. Chem. Eur. J. 1996, 96, 2657. (f) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Adv. Mater. 1996, 8, 428. (g) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212. (h) Badia, A.; Cuccia, L.; Demers, L.; Morin, F. G.; Lennox, R. B. J. Am. Chem. Soc. 1997, 119, 2682. (i) Ingram, R. S.; Hostetler, J. M.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 9175. (j) Ingram, R. S.; Hostetler, J. M.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Whetten, R. L.; Bigioni, T. P.; Guthrie, D. K.; First, P. N. J. Am. Chem. Soc. 1997, 119, 9279. (k) Fink, J.; Kiely, C. J.; Bethell, D.; Schiffrin, D. J. Inst. Phys. Conf. Ser. No. 153 1997, 601. (l) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17. (m) Schmitt, H.; Badia, A.; Dickinson, L.; Reven, L.; Lennox, R. B. Adv. Mater. 1998, 10, 475. (n) Chen, S. W.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2098. (5) (a) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978. (b) Shiang, J. J.; Heath, J. R.; Collier, C. P.; Saykally, R. J. J. Phys. Chem. B 1998, 102, 3425. (6) Brust, M.; Bethell, D.; Kiely, C. J.; Schiffrin, D. J. Langmuir, in press. (7) van de Hulst, H. C. Light Scattering by Small Particles; Dover Publications Inc.: New York, 1981.
10.1021/la980930k CCC: $18.00 © 1999 American Chemical Society Published on Web 01/13/1999
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relative changes in electron density are much larger than in bulk metals. Experimental Section Preparation of Colloidal Gold. “Naked” colloidal gold solutions in toluene were prepared by a modification of a twophase synthesis method previously described.1c,4a,8,9 Tetra-noctylammonium bromide was employed as a phase-transfer reagent to make toluene solutions of AuCl4- and the Au colloids were made by reduction with borohydride. Particle size was determined by transmission electron microscopy. The preparations had a mean particle diameter of 6 nm. Preparation of Electrodes. The indium tin oxide (ITO, Phosphor Products Company, U.K.) used had an electrical sheet resistance of 6 Ω sq. The electrodes were cleaned by immersion in boiling 1 M KOH in ethanol for 20 min. (CAUTION: This cleaning mixture can cause severe burns.) The surfaces were rendered acidic prior to derivatization by refluxing for 5 min in 0.1 M nitric acid, then rinsed with abundant triply distilled water and dried at 120 °C. The slides were derivatized with (3mercaptopropyl) trimethoxysilane (MPS, Aldrich) by immersion in a boiling solution of 5 mL of MPS and 5 mL of H2O in 200 mL of 2-propanol.10 The slides were rinsed with 2-propanol, blown dry in a stream of nitrogen, and finally dried in an oven at 85105 °C for 8 min. Exposure of the derivatized electrodes to temperatures above 110 °C reduced the activity of the surfacebound thiol groups, as indicated by the low absorbance of the films obtained after colloidal gold attachment. Derivatization with gold clusters was carried out by immersion of the ITO electrodes for several hours in gold solution in the toluene. After being washed with toluene, the slides were introduced into a 2 mM solution of 1,4-benzenedimethanethiol (Aldrich) for several hours, then washed with abundant toluene and immersed again in the colloidal gold solution. The above sequence of operations was repeated to incorporate an increasing number of layers of nanoparticles on the conductive substrate. Care was taken to ensure that the Au-terminated substrate was always kept wet with toluene; otherwise particle coalescence was observed. The electrodes for reflectance experiments were made of a polycrystalline Au disk fixed with epoxy resin at the end of a syringe barrel. This was placed at the center of an optical cell which had two windows at 45° from the plane of incidence. Experimental Equipment. The experimental setup used to carry out modulated electrotransmittance spectroscopy is shown schematically in Figure 1. A standard three-electrode electrochemical cell with two plane parallel windows was used; a Pt grid was the counter electrode, and the potentials were referred to a saturated calomel reference electrode (SCE). The ITO working electrode was clamped to the cell with an O-ring seal sandwiched between the electrode surface and the cell, to give an electrode area of 1.8 cm2. The potential was controlled with an in-housebuilt potentiostat and modulated using a square wave generator (LF1, Farnwell, U.K.). The working electrode served as the entrance window for the light beam. The light source was a 150 W Hg-xenon arc lamp powered by a stabilized power supply (Applied Photophysics, 4060, U.K.). The monochromator was driven by a stepping motor control unit (Applied Photophysics, 7602, U.K.) to vary the wavelength between 350 and 750 nm. The bandwidth of the monochromator was 2 nm; this and the calibration of the monochromator were checked with a He-Ne laser at 532.8 nm. The output of the monochromator was directed at the ITO surface as a parallel beam and the transmitted light intensity was measured with a photomultiplier tube (Applied Photophysics, R928, placed in a 7610 housing and using a Bentham 215, U.K., power supply) connected to a current amplifier (256 LF, Bentham, U.K.). The optical response to the potential modulation was extracted from the total transmitted light signal with a lock-in amplifier (5207 EG&G, Princeton Applied Research, USA) which was referenced to the modulation (8) Bethell, D.; Brust, M.; Schiffrin, D. J.; Kiely, C. J. J. Electroanal. Chem. 1996, 409, 137. (9) Fink, J.; Kiely, C. J.; Bethell, D.; Schiffrin, D. J. Chem. Mater. 1998, 10, 922. (10) Goss, C. A.; Charych, D. J.; Majda, M. Langmuir 1991, 63, 85.
Figure 1. Schematic diagram of the experimental setup used for modulated electrotransmittance spectroscopy. of the electrode potential. The signal from the lock-in amplifier, representing the electrochemical modulation of the intensity of transmitted light, ∆T, and values of the total intensity of the transmitted light, T, i.e., the signal obtained from the current amplifier, were stored in the PC that was used for controlling the experiment; 100-200 values of ∆T/T were averaged for each wavelength measured. After averaging, the value of ∆T/T was written to a file, the wavelength changed by the software and a new measurement cycle was started. All the data acquisition was carried out under computer control. The same instrumental arrangement was employed for the reflectance measurements, but in this case, the photomultiplier tube was placed so that it collected the light reflected from the electrode surface.
Results and Discussion Transmittance. The growth of multilayers of nanostructured materials by the stepwise technique6,8,11,12 in which the construction of the material is determined by directional chemical reactions, represents an alternative to the preparation of superlattices by direct crystallization.4f A general problem for the understanding of the properties of these nanostructures is the degree of sintering that can take place. Thiol-linked nanostructures appear to have a surprisingly high stability.6 The optical properties can give a very clear indication of the degree of sintering, i.e., formation of extended regions where the properties of bulk gold predominate. These effects can be visualized from a comparison of the transmittance of nanostructured thin films made by the self-assembly process described above and that for a continuous metal (11) Brust, M. Thesis, Liverpool, 1995. (12) Musick, M. D.; Keating, C. D.; Keefe, M. H.; Natan, M. J. Chem. Mater. 1997, 9, 1499.
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Figure 2. Comparison of the absorbance of a continuous gold film calculated from eq 1 (dashed line) with experimental data for a self-assembled thin film of 8 layers of ∼6-nm gold particles linked with 1,9-nonanedithiol (solid line).
thin film. The transmittance (T) of a film (2) supported on a substrate (3) for an angle of incidence of zero and in contact with medium (1) is given by13
()
T ) tt*
n3 n1
Figure 3. ∆-Ψ signature of layer-by-layer deposition of ∼6nm gold particles on mercaptosilanized glass linked with 1,4benzenedimethanethiol. The solid line represents a simulation of continuous film growth of a material with a refractive index of n ) 2.4 - 1.38i on glass (n ) 1.5).
(1)
where t is the complex transmittance coefficient, t* is its complex conjugate, and n is the refractive index. The transmission coefficient for a thin film can be calculated using Airy’s equation,
t)
t12t23e-iφ 1 + r12r23e-2iφ
(2)
where tij and rij are the corresponding Fresnel coefficients for reflectance and transmittance across the i,j boundary, and φ is given by
φ)
2π nˆ d λ 2
(3)
In eq 3, λ is the wavelength of light in a vacuum, d is the film thickness and nˆ 2 is the complex refractive index (nˆ 2 ) n - ki). A comparison of the calculated film absorbance with experimental results is shown in Figure 2. The optical constants for gold were taken from ref 14, and the results are expressed as absorbances (A ) -log T) for convenience. It can be seen that the absorbance of a continuous film differs considerably from that of the nanostructured material. In the latter, the gold plasmon band is clearly visible, although its position is shifted to the red from the value of ∼520 nm observed in toluene solutions. The difference in the absorbance between continuous and discontinuous gold films has been known for a very long time. Faraday, in his classical paper in 1857,15 reported that “Gold wire deflagrated by explosions of a Leyden battery produces a divided condition very different to that presented by gold leaves” and that the ruby deposits thus formed could be transformed to the green transmitting form by applying pressure to them with a piece of agate. Ellipsometry. It has been shown recently that particles in self-assembled films prepared with dithiols as described (13) Yeh, P. Optical Waves in Layered Media; John Wiley & Sons: New York, 1988. (14) Innes, R. A.; Sambles, J. R. J. Phys. F: Met. Phys. 1987, 17, 277. (15) Faraday, M. Philos. Trans. R. Soc., 1857, 147, 145.
Figure 4. Ellipsometrical film thickness as a function of the number of 1,4-benzenedimethanethiol-linked gold particle layers deposited on mercaptosilanized glass.
here retain their integrity for long periods of time.6 A further indication of this structural characteristic was investigated by using ellipsometry to follow the different stages of growth of the gold nanostructure on glass. These results are presented in Figure 3 where the ∆-Ψ signature for the different self-assembled layers deposited on glass is shown. When analyzing ellipsometric angles for a film with a complex refractive index there is a general problem in trying to fit three parameters (n, k, and d) from two variables. This would not be possible for a single point in the ∆-Ψ plane. However, if the dielectric function of the layers formed are independent of the thickness, the ∆-Ψ trajectory can be used to calculate both the complex refractive index and film thickness by fitting the dependence of ∆ on Ψ to values of n, k, and d. The results of such a fit16 are shown as a solid line in Figure 3. The index for the gold-1,4-benzenedimethanethiol nanostructure found by this fitting procedure was nˆ ) 2.4-1.38i. The effective thicknesses for each layer calculated with this value are shown in Figure 4. The refractive index of the film is very different from that of bulk gold, for which Innes and Sambles found nˆ Au ) 0.197-3.45i.14 The refractive index found is similar for films in which the linker was 1,9-nonanedithiol (n ) 2.631.55i).6 Qualitatively, this is a clear indication that properties of the linker such as chain length and chemical (16) Greef, R. ELLGRAPH Software, Optichem, University of Southampton.
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dielectric function of the metal surface region by the applied potential due to the changes in the electronic density localized within the Thomas-Fermi screening length (δTF) at the metal surface.18,19 This length is of the order of less than 0.1 nm in metals; changes in surfacecharge density caused by the potential modulation are concentrated within it. The dielectric function of a metal (ˆ M) has contributions from free (f) and bound (b) electrons19
ˆ ) ˆ f + ˆ b - 1
Figure 5. Potential modulated transmittance spectrum of 4 layers of 1,4-benzenedimethanethiol-linked gold particles deposited on a mercaptosilanized ITO electrode in 0.1 M aqueous KCl. The electrode potential was set to 0 V vs SCE and modulated at 17.5 Hz with a peak-to-peak amplitude of 300 mV.
nature are of secondary importance in determining the optical properties of the film. Clearly, these are mainly influenced by the optical properties of the gold nanoparticles themselves. The effective thickness of the film shows an unusual dependence on the number of layers: the coverage of the first and second layers is less than unity. After the second layer, the film thickness increases linearly with the number of layers, showing that the initial deposit does not correspond to unit coverage. This situation is quite different from the coverage of unity observed with a gold substrate when an alkanedithiol is used as a linker.17 In this case, the linker forms a packed selfassembled monolayer, rendering all the metal surface accessible for attachment of Au nanoparticles. MPS-derivatized surfaces are less well-ordered, and it is possible that polymeric compounds are attached to the surface, resulting in a smaller number of sites available for reaction with Au. The further derivatization of the Au clusters with a bifunctional thiol is very effective, and subsequent layers lead to an increase in the coverage per layer of Au attached to the surface. Eventually this process results in a saturation in the number of clusters that can be introduced on the surface in each layer. These growth characteristics are clearly illustrated in Figure 4 where a constant value for the increase of thickness of 5.8 nm/ layer can be observed after three layers of Au have been deposited. This most probably results in an open structure, as has been recently proposed by Musick et al.12 Potential Modulated Transmittance Spectroscopy (PMTS). Figure 5 shows the PMTS results for four layers of Au nanoparticles supported on ITO using 1,4benzenedimethanethiol as ligand. These results give a clear indication that the particles in the film are able to accumulate charge when the electrical potential is modulated and that, as a consequence, their optical properties are changed. In what follows, we shall give a simple analysis of these effects and point to the improvements in theory which are required for a better understanding of their properties. The dependence of the reflectance of a metal-aqueous electrolyte interface on the applied potential has been known for a long time (the so-called electroreflectance effect, ER).18,19 The origin of ER is the modulation of the (17) Brust, M.; Calvo, E. J.; Etchenique, R.; Gordillo, G. Chem. Commun. 1996, 1949. (18) McIntyre, J. D. E.; Aspens, D. E. Surf. Sci. 1971, 24, 417.
(4)
As is common in the analysis of ER and of size effects,5b,20,21 ˆ b is regarded as being independent of electron density because it involves only intraband electronic transitions and core polarizabilities, and the classical Drude free-electron model18,19 is used. The bound-electron contribution to the dielectric function is considered constant, and all changes are considered to be due to the free-electron contribution. The difference in the metal dielectric function between two states of charge 1 and 2 is given by
(1) - (2) )
-ωp2(2) + ωp2(1) ω(ω - iτ-1)
(5)
with ωp, the bulk plasma frequency, given by
ωp2 )
Ne2 m0
(6)
where ω is the frequency of the incident light, τ is the electron scattering time,19 N is the number density of electrons, e is the electronic charge, m is the effective mass of the electron, and 0 is the permittivity of free space. For a change in surface charge density, the new bulk plasmon frequency is given by
ωp2(2) )
(N(1) + ∆N)e2 m0
(7)
where N(1) and (N(1) + ∆N) are the number densities at the two surface charge densities and ∆N is the change in N. For a change in charge density at the metal surface of ∆qM ) qM(1) - qM(2),
(1) - (2) )
∆qMe δTFm0ω(ω - iτ-1)
(8)
and the charge-dependent reflectance changes can be calculated from eq 8 and the corresponding Fresnel coefficients.18 The potential dependence of the optical properties of nanocrystallites differs from that of the surface of bulk metal in three important respects. First, the geometry of small particles gives rise to Mie resonances due to coherent oscillation of the free electrons induced by the electromagnetic field.7,22 Second, the optical constants are size-dependent because of limitations of the electron mean free path by surface scattering.20-22 Finally, the simple linear geometry implied in the Thomas-Fermi screening length may not be valid. However, since the optical response of the particles is dominated by coherent (19) Kolb, D. In Spectroelectrochemistry; Gale, R. J., Ed.; Plenum Press: New York, 1998; Chapter 4. (20) Ho¨vel, H.; Fritz, S.; Hilger, A.; Kreibig, U.; Vollmer, M. Phys. Rev. B. 1993, 48, 18178. (21) Underwood, S.; Mulvaney, P. Langmuir 1994, 10, 3427. (22) Kreibig, U.; Fragstein, C. V. Z. Phys. 1969, 224, 307.
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electron motions as described by Mie’s theory, screening effects are not considered. In this approximation, the main effect of different states of change due to the applied interfacial potential will appear through the changes in the dielectric function of Au arising from the changes in the bulk plasma frequency given by eqs 5-7. The calculation of the charge-modulated transmittance is based on the Beer-Lambert law considering that the particles within the film can be treated as independent of each other. The transmittance at any state of charge is given by
Ti ) e-2.3dC(q)
(9)
where C is the molar concentration of gold atoms in the film and (q) is the molar absorption coefficient of the gold particles at a charge q in the particle. On the assumption that the polarizability of the particles can be regarded as being independent of their concentration, and considering the first term only in Mie’s series, i.e., for d , λ, is given by23,24
)
0.02455VMm3/2 ′′ λ (′ + 2 )2 + ′′2
Figure 6. Comparison between the experimental data presented in Figure 5 and calculated potential modulated transmittance spectra (from eq 11) for two different values of the effective electron mean free path.
(10)
m
where ′ and ′′ are the real and imaginary components of the dielectric function of gold given by ˆ Au ) ′ - ′′i; m is the dielectric function of the medium in which the particles are embedded (see later), VM is the molar volume (cm3 mol-1); when the wavelength of light in vacuum, λ, is given in cm, the units of are in M-1 cm-1. From eqs 9 and 10, the relative change in transmittance between potentials 2 and 1 is
∆T/T ) 1 - e-2.3dC[(2) - (1)] ≈ 2.3dC[(2) - (1)] (11) The dielectric function of gold as a function of charge can now be calculated from eqs 5-7. In order to take into account size effects in these calculations, the method discussed by Kreibig, Heath, and others was used.5b,21,22 The bound-electron contribution to Au is calculated by subtracting ˆ f - 1 from ˆ Au. ˆ f was calculated from Drude’s model for a free-electron gas:19
ˆ f ) 1 -
ωp2 ω(ω - iωτ-1)
(12)
For a particle of radius R, τ-1 is given by vf/Leff (vf ) electron velocity at the Fermi level and Leff ) effective electronic mean free path).21,22 A comparison of the experimental results with ∆T/T calculated from eq 11 for Leff ) 3 and 1.5 nm is shown in Figure 6. The effect of the changes in the charge density of the particles on their dielectric function was calculated from eq 5. Although there is a slight shift in the position of the maximum in transmittance, the agreement between experiment and calculations for Leff ) 1.5 nm is remarkable considering that the calculation ignores the effect of coherent oscillations involving more than one particle. Absorbance measurements for this type of nanostructure6 indicate that cooperative phenomena in which interparticle coupling takes place do not influence significantly the potential modulation of the transmittance. The reason (23) Mulvaney, P. Langmuir 1996, 12, 788. (24) Chumanov, G.; Sokolov, K.; Gregory, B. W.; Cotton, T. M. J. Phys. Chem. 1995, 99, 9466.
Figure 7. Potential modulated electroreflectance spectra of a pure gold electrode (1), a gold electrode derivatized with 1,4benzenedimethanethiol (2) and a gold electrode covered with two layers of ∼6-nm gold particles (9) in 0.1 M KClO4 at 0.1 V vs SCE. Modulation at 34 Hz with 140 mV peak-to-peak amplitude.
for this is most likely related to the different phenomena that are being considered here. Potential modulation results in charge accumulation in the individual clusters, and only the relative change in the free-electron contribution is measured, whereas an absorbance experiment measures all of the optical contributions within the film. Reflectance Spectroscopy. Figure 7 illustrates a typical response of a gold electrode covered with two layers of Au nanoparticles attached to the surface with 1,9nonanedithiol as the linker. The same figure shows the absence of an ER signal when the gold electrode is terminated with the dithiol. This is expected because the interfacial capacitance is strongly decreased by the lower dielectric constant of the film which results in very small changes in the surface charge density as the potential is modulated. The observation of a strong ER signal in the presence of attached Au clusters is a clear indication that there is electronic communication between the bulk metal and the nanoparticles. Similar effects have been reported for these structures for electron-transfer reactions to aqueous redox couples.8 The wavelength dependence of the electroreflectance is similar to the results in transmittance, clearly indicating that the observed effects refer to the nanoparticles and not to the electrode support. In particular, the measured ER of bulk gold is very different from the results for the nanoparticles, not showing the characteristic peak in the
Electrochemical Charge Injection
reflectance spectrum observed for the latter in the plasmon resonance region. The ER spectrum for gold is qualitatively described by the Drude model (eqs 4-8) using the bulk dielectric function for the metal.19 The spectrum of the particles reflects the different nature of the interaction of light with the two surfaces. It can be concluded that coherent electron oscillations in the particles lead to a different influence of charge injection on the reflectance/ transmission characteristic compared with bulk metal as discussed above for the transmittance results. The dielectric function for the medium used in these calculations corresponded to a refractive index of 1.5. The effective medium that a particle will experience is, however, very complex. The optical structure that should be considered is glass (n ∼ 1.5), ITO (n ∼ 2), the organic ligand (n ∼ 1.5), and an aqueous solution (n ∼ 1.33). In addition, the particles themselves contribute to the effective medium. The correct approach to solve this very complex optical problem is through the complete solution of Maxwell’s equations. However, it is very surprising to find that the charge-modulated transmittance and reflectance of the nanostructured films appear to follow a very simple classical model. We inquire first into which part of the film the charge is being injected, i.e., where is the site of the potential drop. It has to be recognized that the results in Figure 7 appear to indicate that there is good electronic communication between the particles and the electrode surface. This point was further investigated by comparing an electron-transfer reaction for a thiol and a nanoparticleterminated surface for the aromatic linker employed; this comparison is shown in Figure 8. Although the 1,4-benzene dimethanethiol linker molecule self-assembled on gold inhibits electron transfer to the hexacyano Fe2+/Fe3+ couple, terminating the surface with gold nanoparticles restores facile electron transfer. The cyclic voltammogram shown in Figure 8 is indistinguishable from that for bulk gold. Similar results are obtained when an n-alkane thiol linker is used to make the nanostructured film.8 The restoration of electroactivity by changing the surface termination indicates that the potential drop occurs between the outermost particles and the solution, although these are not in direct contact with the electrode. In addition, from transmission spectroscopy,6 there is no evidence of sintering when an increasing number of layers is deposited. It is concluded that in these experiments most of the charge is injected to the outer layer of particles, and the effective medium that has to be considered is that experienced by the outer layer, the rest of the film acting mainly as a support through which electron transfer takes place. This is a different situation from that present when measuring the optical properties of the whole film, in which case the effective medium in the film determines the overall optical properties measured. In particular, as discussed by Heath et al.,5a for this case an effective medium approximation taking into account many-body coupling effects must be considered. Conclusions The optical properties of nanostructured films made by a self-assembly technique using dithiol linkers show that the particles retain their integrity in the film. The potential modulation of the transmittance and reflectance of nano-
Langmuir, Vol. 15, No. 3, 1999 871
a
b
Figure 8. Cyclic voltammogram of 0.1 mM K4Fe(CN)6 in 0.1 M aqueous K2SO4 at a gold disk electrode terminated (a) with 1,4-benzene dimethanethiol and (b) with ∼6 nm gold particles (4 layers). Sweep rate: 50 mV/s.
structured films has been demonstrated. Both transmission/reflectance and electron-transfer measurements show that the outer layer of particles is the site were charge localization occurs because the applied potential drop appears to occur entirely across this layer. These measurements were unable to reveal the Coulombic cascade effects observed for nanoparticles in solution4k,m because the particles employed in the present work are much larger than those used in the electrochemical experiments, and no size separation was attempted. Nevertheless, the change of the optical properties of the particles on changing their state of charge has been clearly demonstrated, and their behavior can be compared with that of individual quantum dots, capable of responding to electron injection. Acknowledgment. The financial support from the Wolfson Foundation, the EPSRC (Grant GR/J38185), and the EU Human Capital and Mobility Program (Contract No. ERB-CHBI-CT94-1118) is gratefully acknowledged. We are grateful to Dr. R. Greef (Southampton University, U.K.), and Professor R. Sambles (Exeter University, U.K.) for useful comments on ellipsometric calculations and optical properties of materials. We thank Mr. S. Maier for conducting some of the electron-transfer experiments. LA980930K
