Plasmons: Why Should We Care?

In the Classroom edited by

Advanced Chemistry Classroom and Laboratory

Joseph J. BelBruno Dartmouth College Hanover, NH 03755

Plasmons: Why Should We Care? Dean J. Campbell* Department of Chemistry and Biochemistry, Bradley University, Peoria, IL 61625; *[email protected] Younan Xia Department of Chemistry, University of Washington, Seattle, WA 98195

The physical phenomenon of plasmons has helped to enhance methods of chemical analysis in recent decades, even to the point where it is possible to detect a single molecule. Though many of these methods have not yet found their way into traditional chemistry classes and labs, it is likely that many of today’s chemistry and biochemistry majors will encounter plasmon-based techniques in their future. Some techniques, such as surface plasmon resonance, staining for microscopy, and colorimetry have already been commercialized. Other applications for plasmons are still being explored, for example, plasmon-based alternatives to traditional electronic circuits. This review will provide a brief overview of plasmons and the techniques that build upon them. Strictly speaking, plasmons are quantized waves in a collection of mobile electrons that are produced when large numbers of these electrons are disturbed from their equilibrium positions (1, 2). Electrons present in classic gaseous plasmas can support plasmons, hence the name of these waves. The collection of mobile electrons in metals is referred to as a quantum plasma (2). This is composed of delocalized electrons that bathe the metal nuclei and their localized core electrons, as described by the free-electron theory of metals (1). This description of metallic bonding has been used to explain many metallic properties. Metals that best exhibit this free-electron plasma behavior include the alkali metals, Mg, Al, and noble metals such as Cu, Ag, and Au (1). Plasmons can exist within the bulk of metals, and their existence was used to explain energy losses associated with electrons beamed into bulk metals (3). For a bulk metal of infinite size, the frequency of oscillation, ωp, of the plasmons can be described by the following equation (1): ωp = (Ne2兾ε0me)1兾2

• Propagating SPs, which occur on extended metal surfaces • Localized SPs, which occur in small volumes such as metal particles • Acoustic SPs, which are predicted to exist for some structures and metal surfaces and are the subject of continued study (3). These plasmons have not yet seen extensive use in chemistry and will not be discussed further in this article.

The combination of photons and SPs produce electromagnetic excitations on extended surfaces known as surface plasmon polaritons (SPPs) or propagating surface plasmons (PSPs) (1, 5). PSPs have lower frequencies and energies than bulk plasmons; metal surfaces in contact with a vacuum have PSPs with a theoretical frequency of ωp兾√2 (1, 3). Figure 1A shows a representation of PSPs. As the waves of electron density travel along the surface, alternating regions of posi-

(1)

where N is the number density of mobile electrons, ε0 is the dielectric constant of a vacuum, e is the charge of an electron, and me is the effective mass of an electron. Surface plasmons (SPs) are a type of plasmon associated with the surfaces of metals. They are significantly lower in frequency (and energy) than bulk plasmons and can interact, under certain conditions, with visible light in a phenomenon called surface plasmon resonance (SPR). SPs have been the most useful to chemists. For example, the electric fields of SPs amplify optical phenomena such as Raman scattering (1, 4), which will be discussed later in the article. There are

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at least three types of SPs:

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Figure 1. (A) Depiction of propagating surface plasmons (1). (B) Depiction of localized surface plasmons (1). Regions of greater electron density are represented by lighter shading. Electric fields are represented by arrows.

Vol. 84 No. 1 January 2007

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Journal of Chemical Education

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Figure 2. Extinction spectra of 10 nm, 50 nm, and 100 nm Au particles. The spectra are scaled to the same maximum absorbance. Note the red shift of the plasmons as particle size increases. The colloids were obtained from British Biocell International, Cardiff, Britain.

tive and negative charges are produced. The electric fields produced by these regions of differing charge decay exponentially away from the metal surface (5). A macroscopic physical analogy that has been used to describe surface plasmons is that of seaweed floating in water near a shoreline. The waves of water lapping along the shoreline represent the electric fields and the seaweed sloshed back and forth by these waves represent electrons (6). Though PSPs can be produced with light, simply shining light on a smooth metal surface in air is not sufficient because the momentum of the light does not match that of the SPs (3). Neither will the smooth metal surface emit light from plasmons.1 Various methods are employed to enable light to produce and to be produced by PSPs. Some methods involve passing the light through a medium with a higher refractive index than air, such as glass, before it comes near to the metal surface (3, 5). To better explain the role of refractive index, consider what happens when a beam of light passes from a medium with a higher index into a medium with a lower index. In many cases, part of the incident beam is reflected from the interface and part of the incident beam passes into the medium with the lower refractive index (and is refracted in the process). However, when the angle of the incident light exceeds a critical angle, no portion of the light leaves the medium with higher refractive index and is completely reflected. This is the condition of total internal reflectance (7). Many fiber optic cables utilize the same principle to transmit light down curved strands: the refractive index of the core of the fiber optic is higher than that of its cladding layer, and the condition of total internal reflectance keeps the light in the fiber (8). Under the conditions of total internal reflectance, light waves that strike the interface between two materials with differing refractive indices produce new light waves that propagate from the interface a very short distance into the medium with the lower refractive index. These evanescent waves decay exponentially as the distance from the interface increases. At a particular angle of incident light beyond the critical angle, evanescent waves can 92


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propagate PSPs along a thin layer of metal placed at the interface between the media with differing refractive indices (5, 6, 9). This SP resonance angle is susceptible to the refractive index of the media near the metal film, making it sensitive to chemical changes that involve changes in refractive index (or its square, the dielectric constant). Other methods to enable light to access a metal surface and produce PSPs involve roughening the surface, thereby changing its momentum. If light shines on a metal diffraction grating with parallel, linear features, it can be diffracted by that grating. At the same time, however, PSPs will be propagated along the surface of the grating in the direction perpendicular to the linear features (5, 10, 11). SPs can also be enhanced by randomly roughened metal surfaces. The rough surfaces can be considered as a superposition of many gratings (11). Localized surface plasmons (LSPs), are the collective electron oscillations in small volumes, Figure 1B. LSPs also have lower frequencies and energies than bulk plasmons; metal particles in contact with a vacuum have LSPs with a theoretical frequency of ωp兾√3 (1, 3). LSP resonances are produced in a somewhat different fashion from PSPs. An example of this interaction between light and the electrons of a metal particle is illustrated in Figure 1B (1, 12). For this phenomenon to occur, the particle must be much smaller than the wavelength of incident light. The electric field of the incident light can induce an electric dipole in the metal particle by displacing many of the delocalized electrons in one direction away from the rest of the metal particle and thus producing a net negative charge on one side. Since the rest of the metal particle is effectively a cationic lattice of nuclei and localized core electrons, the side opposite the negative charge has a net positive charge. LSPs have also been referred to as dipole plasmons, but the oscillating field of the incident light can induce quadrupole as well as dipole resonances, especially for particles greater than 30 nm in diameter (12, 13). If a particle with a dipole can be considered to have a positively charged pole and a negatively charged pole, then a particle with a quadrupole can be considered to have two positively charged poles and two negatively charged poles. The energy of light required to produce LSP resonance depends on a number of factors, including the size, shape, and composition of the particles, as well as the composition of the surrounding media. The interactions of light with spherical metal particles can be described by Mie theory, which is based upon an exact solution to Maxwell’s equations (1). This theory also predicts what fraction of light impinging upon colloidal metal particles will be absorbed and what fraction will be scattered. The sum of absorption and scattering is the extinction of light due to the particles. This is actually what is measured when one places a colloidal metal suspension into a UV–vis spectrometer. Figure 2 shows the extinction spectra for spherical Au colloids with differing diameters. Generally speaking, as the particle size increases, the plasmon resonant frequency decreases (shifts to longer wavelengths) (1). Small Au particles (with diameters less than 20 nm) exhibit extinction that is primarily due to absorption; larger particles tend to exhibit much stronger scattering (1). Though solid spherical Au and Ag particles are easy to synthesize, their plasmon energies do not cover much of the electromagnetic spectrum. Particles with other shapes are ca-


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+ + + + + +

− − − plasmon of sphere

− + − + − +

adsorbates

− − −

surface plasmons metal

+ − + − + − + + +

+ − + − + −

− − −

hybridized plasmons

θ

incident light

reflected light

plasmon of cavity glass prism

Figure 3. A hybridization model for the plasmon behavior of Au nanoshells (15). The plasmons of a spherical shell can be considered a hybribization of the plasmon from a solid sphere and the plasmon of a spherical void.

Figure 4. Schematic of the surface plasmon resonance technique (4, 9). Light passing through a prism onto a metal film at the proper incident angle θ will amplify propagating surface plasmons in the metal film. The nature of the adsorbates helps determine the actual SPR angle.

pable of covering much wider energy ranges (14). Moving beyond spherical particles requires more advanced methodology, both synthetically and theoretically. Many of these plasmon resonant structures feature dimensions between 1 and 100 nm, making them “nanostructures” (1). Technology that deals with such nanostructures is called “nanotechnology”. Non-spherically symmetric particles can be synthesized by directionally dependent growth rate control with polymers or surfactants (15, 16). Another method for controlling particle shapes uses other particles or surfactant assemblies as templates (15, 16). Other template-directed patterning methods have been used to produce structures on surfaces that exhibit LSPs (17). Plasmons and their associated electric fields can be calculated for a variety of different particle shapes. The plasmonic behavior of these particles has been successfully described by the discrete dipole approximation (DDA) method, which subdivides a particle of interest into an array of polarizable cubes (1, 17). It seems that the most intense plasmon electric fields tend to be located near areas of the particles with the sharpest curvature (15). Particles of different shapes have different plasmon properties. For example, rod-shaped Ag or Au particles exhibit two plasmons: a longitudinal plasmon, corresponding to the long axis of the rod, and a transverse plasmon, corresponding to the short axis of the rod. Keeping the diameters of the rods constant while increasing their lengths result in the transverse plasmon remaining essentially constant in frequency (or energy) while the longitudinal plasmon decreases in frequency (1, 15). Hollow metal particles tend to have lower plasmon resonant frequencies than solid metal particles (18). The plasmon behavior of small hollow spherical metal particles, or shells, has recently been described by using a hybridization model that resembles, at first glance, the hybridization of molecular orbitals, Figure 3. In this model, the plasmons associated with a shell can be thought of as a combination of the plasmon from a solid sphere of the same diameter as the exterior of the shell and a plasmon from a spherical cavity in extended metal with the same diameter as the interior of the shell. The plasmons hybridize to produce a new higher energy plasmon and a new lower energy plasmon (the lower energy plasmon is more likely to interact with light). As the diameter of the solid sphere approaches that of

the spherical cavity (that is, as the thickness of the metal shell decreases) their plasmons interact more strongly and increase the energy separation of the hybridized plasmons and ultimately decrease the energy of the lower-energy hybridized plasmon. The net result is that the shells with thinner metal layers have plasmons that are shifted to lower frequencies (18). Aggregation of colloidal metal particles can also lower their plasmon frequencies (1, 19). Finally, the refractive index of the medium surrounding the metal particles can influence the frequency of their LSPs (1, 18, 20). Typically, a higher refractive index (or dielectric constant) of the medium produces a lower plasmon frequency. A macroscale physical analogy one might make to this phenomenon is using an oscillating weighted spring to represent the electric field of the LSP. A high dielectric medium can be represented by a viscous, higher-friction medium like oil. A spring in a vacuum will oscillate with a higher frequency than the spring in the oil.

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Applications Using PSPs One of the major applications of the phenomenon of PSP is surface plasmon resonance analysis. The specific geometry of this technique varies. The Kretchmann–Raether arrangement is described in Figure 4 (10). This involves shining a monochromatic light source through a prism onto a thin metal film deposited along one of its flat faces. A detector senses the light reflected from the film. As the angle between the incident light and the normal to the film is increased through the critical angle, the intensity of light reaching the detector increases. However, when the angle is increased even further through the SPR angle, the intensity of light reaching the detector decreases to a minimum. The specific SPR angle of the system varies as the refractive index of the medium adjacent to the metal film changes. Molecules can be attached to the metal film, and the changes in interactions of these molecules under different conditions can be monitored as changes to their collective refractive index. These SPR systems are commercially available and have been used to study the binding interactions of biomolecules (6, 9). These systems can be integrated with other analytical systems, for example, mass spectrometry may be performed on the species adsorbed to the metal film (9).


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Another application of PSPs is based on a phenomenon called extraordinary transmission (21). Ordinarily one would expect a screen placed in front of a light beam to block a significant fraction of the light being transmitted through the screen. However, if a metal film containing a two dimensional array of holes of the proper size and periodicity is placed in the light beam, more light is transmitted through the screen at certain wavelengths than is expected. It is postulated that the light resonates with PSPs at the incident side of the metal film. The PSPs propagate to the other side of the film, where they produce light that transmits away from the other side. This phenomenon is being explored as a method for studying species adsorbed to the metal films, for example, the infrared absorbance of some alkanethiols on Cu are enhanced by a factor of 102 (22). The technique of scanning near-field optical microscopy (SNOM) can access PSPs on a metal surface. In this technique, light is shone down a glass fiber that has been reduced to a diameter less than the wavelength of light. When the tip of this fiber is brought very close to the metal surface, circular PSPs will radiate from the tip to the metal (5). This process, which has been explained by both evanescent waves from the high refractive index glass fiber and diffraction from the fiber tip itself (5), enables PSPs to be initiated at specific locations on a surface with great precision. Finally, PSPs are being explored as an alternative means of transmitting information (23, 24). Plasmons travel through metals with optical frequencies, about 105 times faster than the frequency of today’s microprocessors. At the same time, there is not as much need for light transmitting materials as would be necessary for entirely light-based (photonic) computers; many plasmonic components would use the same materials as electronic components. One challenge to overcome is that the distance plasmons can travel is currently less than that the dimensions of a computer chip (24). Applications Using LSPs One of the most recognizable results of LSP resonance is the dramatic absorption of light by some small metal particles. Solid Au and Ag particles can have extinction coefficients 106 times greater than molecular chromophores (1). The deep red and purple colors associated with colloidal Au have been known since antiquity, for example, in tinted glass such as the Lycurgus cup from the 4th century C.E. (25). “Purple of Cassius” was used as a coloring agent for glass and enamel and is thought to be a colloidal mixture of Au and hydrated SnO2 (26). Michael Faraday first reported the production of aqueous colloidal Au suspensions (using phosphorus as a reducing agent) in 1857. Some of these colorful suspensions are still preserved today at the Faraday Museum in London (1, 27). Au and Ag colloids can be produced and used conveniently in the student laboratory, and some of these educational preparations and applications have been published in this Journal and elsewhere (28–34). The high light-absorbing properties of Au colloids enable them to serve as structural and chemical labels for light microscopy (35). For example, Au colloids can be placed into biological tissue for use as optical contrast agents to enhance imaging of these tissues. The colloid particle surfaces can be derivatized with molecules such as antibodies to enhance their 94


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selective binding to tissue surfaces (16). Visibility of the bound Au colloids can be enhanced by depositing Ag onto the metal particles (36). Most colloidal particles of Au cannot be easily made to produce light like many colloidal semiconductor particles. Very small (< 5 nm) Au clusters can produce light by photoluminescence. Though the photoluminescence phenomenon itself is attributed to electronic transitions within the Au clusters, it is believed that plasmons associated with these very small particles amplify the intensity of the incoming light and therefore enhance their photoluminescence (37). Recently Au colloids have also been induced to produce light by a phenomenon called third harmonic generation (38). This may enable these Au particles to be used as light-producing labels in the future. Fluorescence of chemical species near a metal surface can also be enhanced by plasmons, but the challenge in using this method of enhancement is that if the species get too close (less than about 5 nm) to the metal the fluorescence is actually diminished (quenched) (11, 15). One application based on the intense light absorbance of Au colloids is photothermal therapy. Au colloids themselves are fairly chemically and biologically inert, but Au particles bound to living tissue can absorb light energy and convert it to thermal energy, destroying the tissue. Hollow Au particles can now be produced with plasmon resonances in the near infrared region of the electromagnetic spectrum. Since water and most biomolecules do not absorb in this region of the electromagnetic spectrum, it is possible to beam this energy some distance into tissue to heat the Au particles (15, 18). One way to illustrate this principle to a class would be to place Au colloid (e.g., by evaporating a drop of suspension) onto a polystyrene Petri dish and to place the dish near the Fresnel lens focal point of a high intensity overhead projector. We used a 7800 lumen projector with the mirror and the focusing lens removed. A ring stand and clamp were used to hold the assemblage about 40 cm above the glass top surface of the projector. (CAUTION: The focused light from an overhead projector can be dazzlingly bright and can heat some objects dramatically.) The transparent Petri dishes themselves are not melted by the focused light, but light-absorbing materials such as Au colloids, carbon nanotubes, and even dark permanent marker ink can absorb sufficient energy to damage the polystyrene dish at the location of these light absorbing materials (39). The surrounding transparent polystyrene is not affected. Some hollow colloidal metal cages synthesized by the Xia group absorb light energy so well that the light of a camera flash can melt the cages (40). These flash-induced shape changes have been verified by transmission electron microscopy (Figure 5) and scanning electron microscopy. These colloidal cages can also melt transparent polystyrene when irradiated by intense projector light. Au colloids and their LSPs can also be used as colorimetric chemical sensors. The pink test stripe of a positive pregnancy test can be composed of Au colloids coated with antibodies that have bound to hormone human chorionic gonadotropin (hCG) in urine. These complexes are then captured by more antibodies in the test band region (41). The pink control stripe can also be composed of colloidal Au. Au colloids coated with other biologically interacting molecules have shown aggregation-induced changes to their plasmon frequencies (1, 13, 35). For example, the surfaces of some


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Au particles have been derivatized with strands of DNA. When these fixed strands match complementary strands of free-floating DNA, the strands all link together by hydrogen bonding and the colloidal particles aggregate together. The Au colloid plasmons shift in frequency during aggregation, and the suspension changes color, indicating a positive test for that particular sequence of free-floating DNA (42). Another biochemical detection method that is somewhat reminiscent of the SPR analysis method mentioned previously uses LSP-active features patterned onto an Au surface. Binding of molecules of interest to other molecules already attached to these Au features produces a change in the refractive index near these features, resulting in a change in their plasmon energy. This energy change can be monitored by UV–vis spectroscopy. This method has recently been shown to be capable of detecting amyloid-β derived diffusible ligands (ADDLs), which are believed to play a role in Alzheimer’s disease (43). Resonance of LSPs amplifies electric fields, E, near the particle surfaces. The |E|2 of the plasmon electric field can be 102 to 104 of the |E|2 of the incident light (17). The electric fields of plasmons can amplify the Raman signals of chemical species near the metal surface, thus enabling the technique of surface-enhanced Raman spectroscopy (SERS).2 Weaver and Norrod have already described SERS in this Journal and experiments suitable for the undergraduate laboratory are available (28, 33, 44). Briefly, Raman spectroscopy is a type of vibrational spectroscopy that is sensitive to polarizable bonds within molecules (typically different than bonds that are already polarized—the domain of infrared spectroscopy) (45). The electric field interacts with polarizable molecules to produce dipoles, described by the equation (28):

Figure 5. Flash melting of Au cages (35). (A) Transmission electron microscope (TEM) image of unmelted Au cages formed by reduction around polyhedral Ag templates (the Ag is oxidized away in the process). (B) TEM image of Au droplets formed by melting Au cages with a camera flash.

p = αE where α is polarizability of the molecule, E is the applied electric field, and p is the electric dipole induced in the molecule As the equation shows, p can be made larger by increasing either α or E, but plasmons tend to enhance E. As the intensity of the induced dipoles increases, the intensity of the Raman signals from those molecules increase. This surface enhancement of Raman signals was first observed from pyridine adsorbed onto a roughened Ag substrate (46). Reproducible control of the degree of enhancement has been problematic. It appears that many of the roughened substrates have “hot spots” with exceptionally good signal enhancement (18). Precise control of the nature of the surface roughness (e.g., by patterning techniques) allows for better control of the degree of Raman enhancement and of the wavelength required to access the surface plasmons (17). Recently, a patterned Ag substrate was used to detect calcium dipicinolate, a compound present in anthrax spores, by SERS (47). It is estimated that this method can detect as little as 2600 spores, which is less than the infectious dose of approximately 10,000 spores. The Raman signal of the adsorbed species can be dramatically enhanced with colloids as well. Enhancement of the electric fields of both the incident and Raman scattered light by 104 produces an overall theoretical enhancement of 108 (17). The regions where colloidal particles come into close proximity appear to be the location of intense plasmon electric fields (15). The fields that can arise between two parwww.JCE.DivCHED.org

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ticles that are very close (on the order of one nanometer) to one another have enabled the detection of SERS signals from single molecules located between these particles (48). Conclusion This brief survey has merely scratched the surface, so to speak, of many of the chemical applications of plasmons. Plasmon-enhanced applications have begun to make inroads into chemistry education and are well-suited for introduction in physical chemistry and instrumental analysis classes. Some methods of fabrication and analysis of plasmon-producing structures are sufficiently simple for use in labs in general, physical, and inorganic chemistry. Readers who are interested in knowing more about research and educational aspects of plasmons and plasmon-producing structures are encouraged to peruse the references provided, especially the May, 2005 issue of the MRS Bulletin (1,15–18, 23, 42). Acknowledgments DJC would like to acknowledge Bradley University for sabbatical support. DJC would like to thank Benjamin Wiley and Jingyi Chen for helpful discussions, supplying the Au cages, and assistance with transmission electron microscopy. YX has been supported in part by a grant from the NSF


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(DMR-0451788) and a fellowship from the David and Lucile Packard Foundation. YX is a Camille Dreyfus Teacher Scholar. Work was performed in part at the University of Washington Nanotech User Facility (NTUF), a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the NSF. Note 1. The blackbody glow of a light bulb filament is based on a completely different mechanism from plasmons. 2. The signal enhancement for SERS may also be due in part to at least one other non-plasmon mechanism, referred to as chemical enhancement. In this situation, some adsorbed molecules may interact with surface atoms, shifting or creating new electronic states in the visible light region in a manner similar to charge-transfer complexes (4, 11). These light-absorbing new species would have enhanced Raman signals due to a resonance Raman mechanism (4, 11).

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