Plasmoelectronics: Coupling Plasmonic Excitation with Electron Flow

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Plasmoelectronics: Coupling Plasmonic Excitation with Electron Flow Scott C. Warren,† David A. Walker,‡ and Bartosz A. Grzybowski*,†,‡ †

Department of Chemistry and ‡Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ABSTRACT: Explorations of the coupling of light and charge via localized surface plasmons have led to the discovery that plasmonic excitation can influence macroscopic flows of charge and, conversely, that charging events can change the plasmonic excitation. We discuss recent theory and experiments in the emerging field of plasmoelectronics, with particular emphasis on the application of these materials to challenges in nanotechnology, energy use, and sensing.

optical elements with exceptionally tunable absorption and scattering properties.40 Overall, the emerging field of plasmoelectronics provides a new means of influencing electrical conductivity and optical response by coupling localized electron oscillations with charge transport. The twofold objective of this Article is to expound basic physical principles underlying plasmoelectronic phenomena and then to illustrate how these phenomena have been, or could be, implemented in various types of plasmoelectronic materials or devices. The possibility of plasmoelectronic effects was hinted at in a series of experiments by Heath and co-workers in which the metal−insulator transition in a compressible two-dimensional (2-D) monolayer of ligand-stabilized metal nanoparticle films was probed.44−47 The films were assembled on a Langmuir− Blodgett trough and the optical response and electrical conductivity of the film was monitored as a function of pressure or, equivalently, the distance separating the metallic core of the nanoparticles. At small separations, the reflectivity increased and the electrical conductivity switched from activated to metallic transport. These changes reflected the increasing delocalization of electrons in these 2-D monolayers. Although conductivity was extensively studied, the possibility of influencing charge transport with optical stimuli was not explored.

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ocalized surface plasmons,1−3 that is, collective oscillations of electrons in metallic nanoparticles, have been successfully used in a broad range of biochemical and chemical sensors, detecting DNA hybridization,4,5 protein binding,6−8 or the presence of heavy metals.9,10 In addition, plasmonic particles have been used to measure distances as plasmonic rulers,11−13 enhance solar-to-electrical14,15 and solar-to-chemical16−18 energy conversion, and have been implemented as nanometer-scale lasers,19,20 antennas,21 or waveguides that transmit plasmonic signals.22,23 The vast majority of applications to date have relied on spectral shifts of the plasmon resonance due to the electrodynamic coupling24−26among adjacent particles or the environment.27 These kinds of effects are based on the oscillation of electrons within or near individual nanoparticles,28 and therefore, the plasmons, by themselves, do not give rise to or manipulate the flow of current on macroscopic scales. In this context, the use of plasmons to control the electronic flow through and electronic properties of bulk materials could provide a foundation for a new class of light-responsive elements with potential applications in sensing29 or optoelectronics.30 As we narrate here, prototypes of such “plasmoelectronic” materials have, indeed, been recently synthesized and, with appropriate tailoring of their nanoscale and molecular properties, have been shown to act as photoconductors,31−36 inverse photoconductors,31,37 and solid-state chemical sensors.31,38 One of the unique features of such materials is that instead of tailoring the optical excitation by traditional bandgap engineering,39 it is possible to control their optical response by simply adjusting the material’s properties, including the sizes, shapes, and environments of the constituent nanoparticles.2 In this way, modulation of currents passing through a macroscopically sized material derives from the plasmonic response of its discrete nanoscopic components. Interestingly, the phenomenon can also be “reversed” such that the plasmonic response can be modulated by an applied electrical bias causing the injection/ removal of electrons into/from the nanoparticle.40−42 In this modality, plasmoelectronic materials can act as precise sensors for electrochemical42 and photoelectrochemical reactions,43 and © 2012 American Chemical Society

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COUPLING PLASMONS TO ELECTRON FLOW The first fundamental examination of the effects of plasmonic excitation to increase or decrease a material’s electrical conductivity dates to 2009.31 In this paper, we studied the properties of thin (∼100−300 nm) films of metallic nanoparticles (Au or Ag, ∼5 nm in diameter) stabilized with selfassembled monolayers, SAMs,48 of various alkane thiols Special Issue: Colloidal Nanoplasmonics Received: January 26, 2012 Revised: March 1, 2012 Published: March 2, 2012 9093

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chosen to illustrate what perhaps is the most striking property of these materials, namely that when the thiols coating the NPs are uncharged, irradiation causes an increase in the current (Δj > 0, “normal” photoconductance) but when they are terminated in charged groups, the current decreases upon irradiation (Δj < 0, “inverse” photoconductance). It is worth emphasizing that while the sign of the photoresponse is solely due to the SAMs, it is the plasmonic properties of the metal cores that control the wavelength at which photoconductance effects are maximized, in particular, the magnitudes of |Δj| are the largest when the films are irradiated with the wavelength of light that corresponds to the particles’ surface plasmon resonance (SPR), |Δj|max ⇔ λSPR. In this way, the photoresponse of the material can be tuned flexibly by adjusting the material properties and/or sizes of the metallic cores. For instance, films comprising 5 nm Ag NPs exhibit maximal Ag_film photoconductance at irradiation wavelength close to λSPR = 470 nm, those made of 5 nm AuNPs show maximal values of Au_film |Δj| at λSPR = 580 nm irradiation, and those comprising mixtures of Ag and Au NPs have a broad range of photoconductance spanning two individual SPR bands.50 An interesting and practically important property of the films is that, to a good approximation, their overall properties reflect the additivity of the properties of the constituent pieces. One illustrative example is provided by the films comprising both NPs coated with uncharged/neutral thiols and NPs coated with charged ligands. The overall photocurrent changes for such a composite material can be expressed as a linear combination of the current changes of the two monocomponent films, Δjtotal = χneutralΔjneutral + χchargedΔjcharged, where χneutral and χcharged = 1 − χcharged are the fractions of NPs of each type in the composite film. Interestingly, since neutral and charged thiols have opposite effects on photoconducatnce (Δj > 0 and Δj < 0, respectively), the proportions of the two components can be chosen such that the film shows neither normal nor inverse photoconductance, that is, Δjtotal = 0. The above examples illustrate the possibilities of “engineering” photocurrent through the NP materials, but they do not explain the fundamental difference between the behaviors of NPs coated with neutral and charged SAMs. The difference arises from the structure and associated dynamics of the molecular bridges (i.e., SAMs) separating nearby nanoparticles.31 Figure 1e has a qualitative electronic structure diagram for two proximal NPs capped with uncharged ligands. The yellow bars represent nanoparticles’ metal cores characterized by Fermi energies, EF, which in the absence of irradiation define an approximate border between occupied and unoccupied states. Upon irradiation, electrons are promoted into excited metal states as much as ∼2 eV above51−56 EF, effectively lowering the height of the potential barrier due to nonpolar alkane thiols (∼3−5 eV above57,58 EF), and ultimately leading to an increased tunneling current between the NP cores such that Δj > 0. This scenario changes dramatically in the case of NPs capped with charged ligands (Figure 1f); here, one also needs to consider the interactions between the charged groups at the NPs’ surfaces and the charge carriers (i.e., flowing electrons). From solid-state physics, it is well-known that either positively or negatively charged groups can trap electrons in polaron-like states, whereby electron localization occurs through the reorganization of the nuclear environment of the ionic moieties in the presence of an electron51−53 (most probably, the polaronic state forms over a period of nanoseconds59 and consists of an electron with a first solvation

terminated in either neutral (e.g., CH3, OH, PhOH; Figure 1a) or charged (COO−, N(CH3)3+; Figure 1b) groups. The films

Figure 1. Coupling plasmonic excitation to electron flow. Upon plasmonic excitation, films comprising Au NPs coated with SAMs of either (a) neutral or (b) charged ligands can (c) increase (“normal” photoconductance) or (d) decrease (“inverse” photoconductance) their conductivity, respectively. These phenomena are quantified by the relative changes in current densities through the films, here defined as Δj ≡ (jirr − j0)/j0 in which the change in current is normalized with respect to the dark current, j0. (e) Qualitative electronic structure diagram of a molecular bridge between two proximal NPs coated with uncharged ligands; yellow bars represent the nanoparticles’ metallic cores. Excitation of the SPR effectively lowers the potential barrier due to the SAM molecules and thus increases current density. (f) For charged ligands, polaronic traps are formed that upon light irradiation capture charge carriers (green spheres in panel b) and ultimately reduce current density.

were drop-cast from a nanoparticle suspension onto a glass or polymeric substrate, and electrodes were thermally evaporated on top of the film.49 In the absence of irradiation, both Au and Ag films exhibited Ohmic current density-applied field (j−E) characteristics in the field intensity range from 100 V/m to ca. 20 kV/m (at higher fields, irreversible changes in the NP films occurred). When irradiated with white light (at an intensity on the order of tens of μW/cm2), the conductance of the films changed. This effect is quantified in Figure 1c and d, which plots the changes in current density Δj ≡ (jirr − j0)/j0 standardized with respect to the dark current, j0. The specific plots shown were 9094

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shell of cations, analogous to similar systems60). For the NPs we used, the degree of energetic stabilization of these polaronic traps via nuclear reorganization was determined to be ∼1.0 eV. In the dark, the trapping states lie above the Fermi energy of the NPs, and act only as parts of the overall tunneling barrier. Upon irradiation, however, excitation at the plasmon resonance of the NPs promotes charge carriers to an injection energy that is ∼2 eV above their injection energy in the dark51−53 and now close to the energy of the trap states; as a consequence, the charged ligands become effective trap sites upon irradiation. The trapped electrons increase the space-charge in the NP film, the density of free carriers decreases, and so does the current through the material, all in all, Δj < 0. While the above description is quite intuitive, it should be emphasized that it is only a first-approximation of a rather more complex physics underlying photoconductance. The full model31 that reproduces the material’s kinetic response to light irradiation provides a more detailed description of NPs photoexcitation and charge-carrier trapping. In this model, it is assumed that the NP material has a density Nt of discrete “trap sites” of which nt0 are filled at thermal equilibrium and in the absence of light or applied bias. Physically, these traps correspond to sites on or within the NP cores that the injected or photoexcited charge carriers can occupy, either because the site is unpassivated by the SAM molecules or because the energy of the charge carrier is greater than the charging energy of the NP (the number of charges a metal NP can accommodate depends on its radius and capacitance).61,62 The initial equilibrium between the trapped and free charge carriers can be quantified by a mass-action relation with an apparent/effective equilibrium constant K: n0[Nt − n t0] =K n t0

reflect conservation laws for the free and trapped charge carriers, respectively, and account for the advective transport of the free carriers as well as the trapping kinetics, characterized by a rate constant, k, and equilibrium constant, K. The boundary conditions for these equations are specified by the applied voltage difference across the material, φ(0,t′) = 0 and φ(L,t′) = −V0, and the condition n(0,t′) → ∞ at the injection electrode implying an ideal Ohmic contact (such that the rate of transport is not injection-limited). With an additional but realistic assumption that the density of filled traps is small compared to the total number of traps, nt(x,t) ≪ Nt, eq 5 simplifies to ∂n t(x′, t ′) 1 = {n(x′, t ′) − θn t(x′, t ′)} ∂t ′ T

where θ = K/Nt = n0/nt0 is the ratio of free carriers to trapped carriers in the absence of light or applied bias and T = 1/kNt is a characteristic trapping time. When these equations are solved by numerical integration, the effects of light irradiation on the system’s response can be taken into account by considering the shifts in the trapping equilibria (eq 1). Specifically, light acts instantaneously to change the number of traps, Nt → N′t, and the magnitude of the change, ΔNt = N′t − Nt, depends on the intensity of the irradiation; importantly, the model predicts this dependence to be linear, as in fact observed in experiments. The difference between NPs capped with charged and uncharged thiols is in the sign of ΔNt. For uncharged thiols, ΔNt < 0, which reflects the light-induced trap filling within the NP cores. For the charged thiols, this effect is offset by an increase in the effective number of traps within the organic layer due to polaronic states; consequently, ΔNt > 0. In both cases, the model predicts that upon irradiation (or after removal of light) the conduction current relaxes to the new steady-state value in an exponential manner with characteristic time scale of ∼100 ms corresponding to the trapping time, and commensurate with the experimentally observed switching times, τS ≈ T = 1/kNt. In other words, the speed with which NP plasmonic materials respond to the light irradiation is dictated by the speed with which photoexcited electrons can fill up (or empty) the trapping states. Finally, the model predicts that switching times (1) scale with the thickness of the SAM, δ, and the particle radius R as τS ∝ 1 + ((3δ)/R) and (2) do not depend on the separation of the electrodes; both of these predictions are in excellent agreement with experiments. With the basic understanding of the physics underlying the plasmon-driven photoconductors, we turn our attention to possible applications of these materials; compared to traditional photoconductors, there are both limitations and advantages. The chief shortcoming of the NP based materials is their slow response times. Although, as discussed in the theory part above, these times decrease with decreasing SAM thickness and also with increasing particle size, and they are limited, at least for spherical particles, at ∼100 ms time scales characterizing the kinetics of charge trapping. Such values naturally preclude the uses of NP photoconductors in high-speed information processing such as fiber optic systems65 and certain types of integrated circuits.66 On the other hand, NP-based photoconductors offer some unique opportunities in conjunction with chemical reactions/processes taking place on NP surfaces. In this context, the key feature is that a complete switch between “normal” and “inverse” photoconductance can be achieved by charging/discharging the molecules on the NPs

(1)

The kinetic response of the material to an applied field is then governed by the transport equations63,64 (on the domain 0 ≤ x′ ≤ L between two Ohmic electrodes) that describe how the free carrier density, n, the trapped carrier density, nt, and the electric potential, φ, evolve in time upon application of an external field. Jc (x′, t ′) = −qμn(x′, t ′) ∂ 2φ(x′, t ′) ∂x′2

=

∂φ(x′, t ′) ∂x′

(2)

−q [n(x′, t ′) − n0 + n t(x′, t ′) − n t0] ε 0ε (3)

∂J (x′, t ′) ∂n (x′, t ′) ∂n(x′, t ′) =− c − t ∂t ′ ∂x′ ∂t ′

(6)

(4)

∂n t(x′, t ′) = k{n(x′, t ′)[Nt − n t(x′, t ′)] − Kn t(x′, t ′)} ∂t ′ (5)

In this system of coupled partial differential equations, eq 2 relates the conduction current to the local carrier density and the local field, with the latter defined in terms of an electric potential φ, and governed by the Poisson eq 3. The charge density (so-called space charge) is the difference between the total density of carriers at position x′ (both free and trapped, n(x′,t′) + nt(x′,t′) and that present in the same position in the absence of external bias, n0 + nt0. The remaining eqs 4 and 5 9095

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illumination, significantly higher light intensities were employed (up to 40 mW/cm2) and a factor-of-ten enhancement in photoconductance was reported. It therefore appears that the photoconductance and, possibly, inverse photoconductance scale linearly with light illumination intensity across 3 orders of magnitude in light illumination intensity. These photoconductance effects have been explored across a wide variety of device geometries. In a particularly interesting design, Selzer and colleagues developed “suspended-wire” molecular junctions in which a single ligand-coated gold or silver nanowire was deposited between two gold electrodes.35 Unlike the earlier work, which relied on localized surface plasmons, the large dimensions of the nanowires (about 10 μm long) led to the formation of surface plasmon polaritons that enhance the electric field intensity between the regions of the ligands. Here, enhancement photoconductance factors of up to 35 were obtained, and a linear scaling of the photoconductance enhancement with light intensity was observed. As in the earlier work, the enhancement could be attributed to photon-assisted tunneling.68,69 Other device geometries that have been explored include metal nanoparticles trapped in insulating nanowires70 in which photoconductance correlated with the plasmon resonance,36 and silver nanoparticles trapped in the pore space of anodic aluminum oxide.33,34

(see example in Figure 2). Since, in typical films we studied, there are only on the order of 1012 such groups on NP surfaces,

Figure 2. Switching between “normal” and “inverse” photoconductance using redox active ligands. Au NPs functionalized with tetrathiafulvalene (TTF) derivatives can exist in either (a) an uncharged or (b) a charged state, depending on the redox state of the molecule. (a) When the ligands stabilizing the Au NPs are uncharged, the films exhibit “normal” photoconductance, (b) but a two-electron oxidation of the TTF moiety alters the electronic state of the film such that an “inverse” photoconductance is observed. (c) Accordingly, the potential at which the NP films transition between “normal” and “inverse” photoconductance corresponds to the known oxidation potential for TTF → TTF2+ (∼2 V).

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the changing photocurrent characteristics can be used to monitor surface redox or ion-binding processes with picomolar sensitivity (one particularly intriguing opportunity is to use photoconductance changes to monitor the binding of toxic metal cations onto nanoparticles functionalized with SAMs presenting electrically neutral crown-ether receptors). We observe that, in these and other applications, it should be possible to amplify the changes in conductance by using nonspherical nanoparticles. In particular, polygonal nanoparticles appear to be interesting candidates, since upon plasmonic excitation they are known to give rise to electromagnetic hot-spots near the particle’s vertices;67 in these regions, it should be easier for the photoexcited electrons to fill the trap states on the particles surfaces31 and effectively increase photocurrent (Figure 3).

COUPLING ELECTRON FLOW TO PLASMONICS

Having discussed the ways in which plasmons can modulate current, we now consider the reciprocal problem of manipulating the plasmonic resonance by charging and decharging plasmonic nanostructures. This has recently captured significant interest because of the desire to rapidly and efficiently induce large optical changes in devices such as smart windows and displays.40 Moreover, such tunable materials have potential applications in signal transduction and processing, where electronically tunable plasmonic waveguides71 are emerging as a potential solution to the miniaturization of metallic interconnects in integrated circuits,66 to the design of signal encoders in fiber optic communications,72 and to the design of responsive materials for thermal imaging and spectroscopy.73 Before turning to specific examples, it is instructive to derive and examine the relationship between the plasmon resonance frequency and the carrier concentration. The model is most accurate for metals described by the Drude74,75 and Mie theories, that is, free electron metals, although it has frequently been used to rationalize the shifts in plasmon resonance frequency for metals that have optical contributions from bound electrons.76 From Mie theory, the extinction cross section of a nanoscopic spherical particle in an absorbing medium is

Figure 3. Can electromagnetic “hot-spots” amplify photoconductance? (a) The sharp vertices of polygonal platelike nanoparticles make them interesting candidates for plasmoelectronic materials, as the electromagnetic “hot-spots” (indicated in red) should facilitate the filling of the trap states. (b) One could expect that these effects will lead to higher photocurrents and shorter response times.

Cext =

Since the original publication in 2009, several other groups have investigated the plasmonic photoconductance effects. In one notable paper, even larger enhancements in photoconductance were observed.32 In that work, devices were built that contained films of uncharged ligand-stabilized gold nanoparticles sandwiched between two gold electrodes. Although the use of two gold electrodes limited direct

24πR3εm3/2 ε″ λ (ε′ + 2εm)2 + ε′′2

(7)

where R is the particle radius, εm is the dielectric function of the medium, λ is the wavelength of incident light, and ε′ and ε″ are the real and imaginary parts of the metal dielectric function. Following the Drude model, for frequencies ω substantially 9096

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also suggests that regulating electron concentration by varying the applied potential can lead to similar variations in the plasmon resonance frequency. Studies on the coupling of the electron plasma and electronic charge were first predicted and conducted in continuous metallic films. When the potential of a gold film electrode was cycled within a range that allowed capacitive currents but prevented Faradaic reactions, a change in the attenuated total reflection of as much as 7% was observed.78 An excellent correspondence with theory was obtained when a measurement of the capacitive charging and corresponding change in electron density of the gold in the near-surface layer79 (by about 1%) was used to calculate the change in absorption and reflection by the gold film (Figure 5).

higher than phonon frequencies, the real and imaginary parts of the dielectric functions can be written as27 ε′ = 1 −

ω p2 ω2

,

ε″ =

ω p 2γ ω3

(8)

where γ is the damping parameter and ωp, the plasma frequency, is given as77

ωp =

ne 2 mε0

(9)

in which n is the electron concentration, e is the electronic charge, m is the effective electron mass, and ε0 is the vacuum permittivity. The plasma resonance occurs at the frequency where the Fröhlich condition is met (ε′ = −2εm). Application of the Fröhlich condition to eq 7, followed by substitution of eqs 8 and 9 reveals that the plasmon resonance wavelength is related to the relative electron concentration by

λ f = λi

ni nf

(10)

This result describes the red-shift in plasmon resonance with decreasing electron concentration. This relationship can be used to justify the broad range of accessible plasmon resonance wavelengths by selecting materials with different electron concentrations; this is illustrated in Figure 4, which shows that plasmon resonances are tunable from the ultraviolet to infrared by varying electron density from 1023 to 1019 cm−3. This result

Figure 5. Coupling optical response and potential in continuous gold films. Observed (red) and calculated (blue) changes in attenuated total reflection caused by a 0.7 V anodic scan of a flat gold electrode. The peak at 2.5 eV corresponds to gold’s plasma resonance. Adapted from ref 78.

The influence of charge density on the properties of localized surface plasmons in metallic nanoparticles was first studied by Henglein and Mulvaney in 1991.80 By use of pulsed radiolysis, strong reductants or oxidants were created in aqueous solutions of 3 nm silver nanoparticles. It was hypothesized that these reductants or oxidants would act as electron or hole donors, effectively turning each nanoparticle into an isolated, nanoscopic electrode. Indeed, when reductants were generated in situ, a blue-shift in the Fermi level occurred; when oxidants were produced, a red-shift was observed. Under the most oxidizing conditions studied, where an amount of hydroxy radicals was produced equivalent to 36% of the silver atoms in the nanoparticles, there was a red-shift from 375 to 410 nm and significant damping of the plasmon resonance (see Figure 6a). Likewise, under reducing conditions, a blue-shift was observed. In 1997, Mulvaney and co-workers built on this initial work by examining the physical properties, including the plasmon resonance and electrophoretic mobility, of poly(acrylic acid)stabilized 10 nm silver nanoparticles.41 The nanoparticles were dispersed in aqueous solutions inside a spectroelectrochemical cell designed to simultaneously apply potentials and monitor optical transmission. Diffusion of the nanoparticles to an electrode led to electron transfer. As many as 1600 ± 300 electrons transferred to each nanoparticle during a single nanoparticle−electrode collision, and a corresponding decrease in the plasmon resonance wavelength of 13 nm was observed when a negative potential was applied (see Figures 6b and 7a). Following eq 10, a net change in electron concentration of 6% can be estimated, although a quantitative analysis was complicated by the change in polymer coverage with nanoparticle charge. Interestingly, the shift in the Fermi level

Figure 4. Coupling electron flow to plasmonic excitation. By either injecting or “withdrawing” electrons, one can tune the plasmon resonance of a material. The degree to which the resonance can be manipulated is strongly dependent upon the material’s carrier concentration. Materials that have a higher initial electron concentration (i.e., metals) are generally limited to smaller shifts in plasmon resonance, as the injection or removal of even large numbers of electrons results in a small relative change in the electron concentration (see eq 10 for justification). Conversely, materials with low carrier concentrations typically have plasmon resonances in the infrared, but much larger relative changes in electron concentration and, therefore, plasmon resonance are possible. This allows the plasmon resonance to be tuned from the infrared to the visible.40 9097

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surface chemical reactions, especially those involving electron transfer. With the advent of improved synthetic methods over the past decade, the above-described spectroelectrochemical methods began to be applied to other nanoparticle shapes and materials. Of considerable note was the discovery that significantly larger shifts in wavelength could be obtained by increasing the aspect ratio of nanorods (see photograph in Figure 8a).86 A shift of up

Figure 6. Shifts in plasmon resonance due to charging. (a) Blue-shift and enhancement of plasmon resonance of silver nanoparticles upon scanning the potential from +0.15 to −0.6 V vs Ag/AgCl, followed by the recovery of the original spectrum on scanning to +0.15 V vs Ag/ AgCl.41 (b) Red-shift and damping of plasmon resonance upon exposing silver nanoparticles to an oxidant that was generated radiolytically.80 The percentages reflect the amount of oxidant with respect to the number of silver atoms. Graphs adapted from refs 41 and 80.

Figure 8. Impact of nanoparticle shape on SPR maximum. (a) Photograph of vials containing nanorods before (left) and after (right) the addition of sodium borohydride, which induced a blue-shift in the plasmon resonance. Reprinted with permission from the American Chemical Society.86 (b) Calculated shifts in the longitudinal plasmon resonance for gold nanorods of two different aspect ratios (2.5:1 and 4.5:1) upon a change in electron density of 14%.87 Data used to generate the plots were taken from ref 87.

to 50 nm in SPR maximum was observed, which is consistent with models that account for both the shape and electron concentration in the nanoparticle. Specifically, the shift in the plasmon resonance was estimated as

Figure 7. Effect of applied potential on the surface plasmon resonance. (a) The SPR maximum of silver nanoparticles was varied by the application of a potential across a 2 V window, which induced a shift of ∼13 nm in the surface plasmon resonance. This shift was used to estimate the number of electrons transferred to each ∼10 nm nanoparticle.41 (b) Temporal response of an electrode comprising polymer-coated silver nanoparticles during an excursion to negative applied potentials.81 Data were taken from refs 41 and 81.

Δλ = −

⎛1 ⎞ Δn λ p ε∞ + εm⎜ − 1⎟ ⎝ ⎠ 2n L

(11)

where λp is the bulk plasma resonance and L is the shapedependent depolarization factor. For spheroids and similar shapes, L decreases toward zero as the aspect ratio increases; in terms of eq 11, the model predicts that higher aspect ratios lead to a larger change in wavelength for the same change in electron concentration. Both this relatively simple model86 as well as more rigorous theoretical treatments87 made predictions that were in excellent agreement with experiments86 (Figure 8b). One of the challenges in achieving even larger SPR shifts in metals is that for most metals, the high initial electron concentration ni (∼1023 cm−3) implies that large net flows of electrons are needed to induce large changes in the SPR maximum (see eq 10). These large net flows are limited by the constraints of material stability and the stability of the medium in which the nanoparticles are suspended. Therefore, materials with lower electron concentrations are extremely attractive because a similar net flow of electrons can lead to larger relative changes in electron concentration (and therefore plasmon resonance), as depicted in Figure 4. This opens up new possibilities for highly tunable plasmoelectronic devices in which the flow of small amounts of current induces a large plasmonic response. Quite recently, these concepts have been applied to indium tin oxide (ITO), which has an electron concentration that is 1 to 2 orders of magnitude below that of either gold or silver.40

also induced other changes in the physical properties of the nanoparticles, such as an enhanced electrophoretic mobility, which corresponded to a surface zeta potential shift from −30 mV in the initial particles to between −100 and −300 mV in the fully charged nanoparticles. These initial experiments showed the power of electrochemical methods to directly probe charge transfer to metal nanoparticles and to make direct correlations to changes in their spectroscopic signature. In subsequent work, silver nanoparticles were immobilized directly on a transparent electrode by using the layer-by-layer assembly of polyelectrolytes, one of which (poly(acrylic acid)) was electrostatically bound to silver nanoparticles.81 In this system, an excursion to negative potentials induced an 18 nm blue-shift in the plasmon resonance as well as an increase in extinction at the SPR maximum by up to 40% (Figure 7b). The large changes in the extinction coefficient with charging (also observed in Figure 6a upon oxidizing the particles) are consistent with earlier models.80,82,83 By exploiting the ability to modify the extinction spectrum of gold nanoparticles through charging, more recent experiments have explored the use of gold and silver nanoparticles as sensors of chemical reactions.84,85 Taken in aggregate, these experiments have shown how spectroscopic measurements are a powerful tool to monitor the kinetics of 9098

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of the ascorbic acid, the plasmon resonance gradually returned to its original position, owing to the injection of electrons into dissolved oxygen to form water (see Figure 9b). By relating the wavelength shift to the electron concentration, it was determined that, during the decomposition of ascorbic acid, 830,000 electrons were added to the nanoparticle over 3 min, that is, at a rate of 4600 electrons per second. Likewise, the oxidation consumed 65 oxygen molecules per second. These results have generated significant excitement because they provide the first direct measurement of a full reduction/ oxidation cycle on a single nanocrystal. The power of this technique to elucidate important electrochemical information, particularly as a function of particle size and faceting, highlights just one way in which plasmoelectronics has a rich future.

Ligand-stabilized ITO nanocrystals were synthesized in solution, deposited on quartz substrates, and the ligands were carefully removed by a combination of chemical and thermal treatments. The ITO was used as the working electrode in an electrochemical cell and the applied potential was changed by as much as 2.5 V. As more negative potentials were applied, a shift of over 1200 nm was obtained in the SPR maximum, consistent with a large relative increase in electron density (see Figure 9a). Indeed, calculations suggested that the electron

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OUTLOOK The electrodynamic coupling of photons and electrons via localized surface plasmons has been intensively studied for over 100 years. Nevertheless, an appreciation for the material design considerations needed to fully harness this coupling has only begun to emerge. As we have discussed, the recent discovery of photoconductivity and inverse photoconductivity in films of ligand-stabilized metal nanoparticles has allowed an unexpected parallel to be made between metal nanoparticles and semiconductors by demonstrating the possibility that macroscopic charge transport can be controlled by optical means. Likewise, investigations on the charging of nanoparticles have led to the surprising discovery that the plasmon resonance (and therefore the light absorption and scattering) is sensitive to charging events. These two sets of phenomena, which form the core of plasmoelectronic effects, suggest that a complete description of the electronic behavior and plasmonic response of a material requires knowledge of not only nanoparticle shape, size, and dielectric environment,2 but also of charge. Already, these plasmoelectronic effects are being used to solve important problems, such as the design of energy-efficient building materials,40 improved photocatalysts,43 and environmental sensors.31 The discovery of photoconductivity in metal nanoparticle arrays could serve as the basis for many types of sensing93 and energy conversion devices,94 including solar cells. These materials are enabling further scientific discovery by facilitating and improving understanding of redox reactions at the single-nanoparticle level.92 Even more significantly, the possibility of studying one-electron charge transfer events on individual nanoparticles has been proposed, which would enable access to the so-called “quantum catalysis” regime.42 Indeed, if such a regime could be accessed, the individual or multielectron transfer events that underlie the operation of fuel cells, solar cells, batteries, and capacitors could be directly probed, thereby providing fundamental mechanistic insight into the operation of these materials. These plasmoelectronic probes could also be used in wide variety of biological applications where the conformation and activity of biomolecules crucially depend on the redox state.95 From a more fundamental perspective, these plasmoelectronic materials provide new insights into the coupling of light with charge in complex heterogeneous materials. As evidenced by the studies presented herein, significant progress has been made in recent years in understanding the coupling of plasmons with net flows of charge carriers. In spite of the significant progress, however, we feel that the field of plasmoelectronics is yet in its infancy and that fundamental insights as well as a full exploration of potential applications are yet to be revealed.

Figure 9. Applications of plasmoelectronic effects in smart windows (a) and monitoring chemical reactions at the single-nanoparticle level (b). (a) Shifts in ITO plasmon resonance across a 1200 nm range, including a small change in optical density at visible wavelengths.40 (b) Measurement of SPR shift of a single gold decahedron (diameter ∼100 nm). For minutes 1−3, a blue-shift in the plasmon resonance was observed as ascorbic acid donated electrons to the nanoparticle. Upon exposure to oxygen for 60 min, a red-shift was observed in the plasmon resonance. This was attributed to electron transfer to oxygen. Plots adapted from refs 40 and 92.

concentration tripled from 5 × 1020 to 1.5 × 1021 cm−3. The authors proposed that their materials could be used to regulate the transmittance of smart windows because of the large dynamic change. These experiments hint that there are significant opportunitiesto design plasmoelectronic materials when particle shape, composition, environment, and carrier concentration are independently controlled. Plasmoelectronic effects have begun to appear in applications related to solar energy conversion as well. Kamat and coworkers reported core−shell Ag-TiO2 nanoparticles that, when illuminated with ultraviolet light, exhibit a 30 nm shift in plasmon resonance due to electron transfer from the TiO2 conduction band to Ag.88 These core−shell particles are of interest in solar89 and photoelectrochemical cells,90 where the plasmonic metal enables near-field light absorption, hot electron transfer, and far-field scattering path-length enhancements17 and the semiconductor provides a surface for dye adsorption.89 In these and future devices, a quantitative assessment of enhancement mechanisms will require the potential- and illumination-dependence of the plasmon resonance to be accounted for. Finally, we focus on one more emerging application of plasmoelectronic materials, namely, their utility in detecting chemical events at the single-nanoparticle level. Dark field optical microscopy has allowed the light scattered from individual plasmonic particles and aggregates of nanoparticles to be detected and related to their shape and orientation.91 Recently, dark field microscopy was performed on individual nanoparticles as they were charged and decharged by redox reactions92 whereby ascorbic acid decomposed in the presence of a gold nanocrystal over the course of 3 min to inject electrons and blue-shift the plasmon resonance. Upon removal 9099

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Non-Equilibrium Energy Research Center (NERC), which is an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award DE-SC0000989. D.A.W. gratefully acknowledges the support provided by the NSF MRSEC program (DMR-1121262) at Northwestern University and the Ryan Fellowship program.

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