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 at least three types of SPs: enhance methods of chemical analysis in recent decades, even •Propagating SPs, which occur on extended metal sur- to the point where it is possible to detect a single molecule. faces Though many of these methods have not yet found their way •Localized SPs, which occur in small volumes such as into traditional chemistry classes and labs, it is likely that metal particles many of today’s chemistry and biochemistry majors will en- counter plasmon-based techniques in their future. Some tech- •Acoustic SPs, which are predicted to exist for some niques, such as surface plasmon resonance, staining for structures and metal surfaces and are the subject of microscopy, and colorimetry have already been commercial- continued study (3). These plasmons have not yet seen ized. Other applications for plasmons are still being explored, extensive use in chemistry and will not be discussed for example, plasmon-based alternatives to traditional elec- further in this article. tronic circuits. This review will provide a brief overview of The combination of photons and SPs produce electro- plasmons and the techniques that build upon them. magnetic excitations on extended surfaces known as surface Strictly speaking, plasmons are quantized waves in a col- plasmon polaritons (SPPs) or propagating surface plasmons lection of mobile electrons that are produced when large num- (PSPs) (1, 5). PSPs have lower frequencies and energies than bers of these electrons are disturbed from their equilibrium bulk plasmons; metal surfaces in contact with a vacuum have ͞ positions (1, 2). Electrons present in classic gaseous plasmas PSPs with a theoretical frequency of ωp √2 (1, 3). Figure can support plasmons, hence the name of these waves. The 1A shows a representation of PSPs. As the waves of electron collection of mobile electrons in metals is referred to as a density travel along the surface, alternating regions of posi- quantum plasma (2). This is composed of delocalized elec- trons that bathe the metal nuclei and their localized core elec- trons, as described by the free-electron theory of metals (1). This description of metallic bonding has been used to ex- plain many metallic properties. Metals that best exhibit this free-electron plasma be- havior 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 en- ergy losses associated with electrons beamed into bulk metals (3). For a bulk metal of infinite size, the frequency of oscil- lation, ωp, of the plasmons can be described by the follow- ing equation (1): 2͞ 1͞2 ωp = (Ne ε0me) (1) where N is the number density of mobile electrons, ε0 is the dielectric constant of a vacuum, e is the charge of an elec- tron, 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 inter- act, under certain conditions, with visible light in a phenom- enon called surface plasmon resonance (SPR). SPs have been Figure 1. (A) Depiction of propagating surface plasmons (1). (B) the most useful to chemists. For example, the electric fields Depiction of localized surface plasmons (1). Regions of greater elec- of SPs amplify optical phenomena such as Raman scattering tron density are represented by lighter shading. Electric fields are (1, 4), which will be discussed later in the article. There are represented by arrows. www.JCE.DivCHED.org • Vol. 84 No. 1 January 2007 • Journal of Chemical Education 91 In the Classroom propagate PSPs along a thin layer of metal placed at the in- terface between the media with differing refractive indices (5, 6, 9). This SP resonance angle is susceptible to the re- fractive index of the media near the metal film, making it sensitive to chemical changes that involve changes in refrac- tive 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 diffrac- tion 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 elec- Figure 2. Extinction spectra of 10 nm, 50 nm, and 100 nm Au tron oscillations in small volumes, Figure 1B. LSPs also have particles. The spectra are scaled to the same maximum absorbance. lower frequencies and energies than bulk plasmons; metal Note the red shift of the plasmons as particle size increases. The particles in contact with a vacuum have LSPs with a theo- colloids were obtained from British Biocell International, Cardiff, ͞ retical frequency of ωp √3 (1, 3). LSP resonances are pro- Britain. duced in a somewhat different fashion from PSPs. An example of this interaction between light and the electrons of a metal tive and negative charges are produced. The electric fields pro- particle is illustrated in Figure 1B (1, 12). For this phenom- duced by these regions of differing charge decay exponen- enon to occur, the particle must be much smaller than the tially away from the metal surface (5). A macroscopic physical wavelength of incident light. The electric field of the inci- analogy that has been used to describe surface plasmons is dent light can induce an electric dipole in the metal particle that of seaweed floating in water near a shoreline. The waves by displacing many of the delocalized electrons in one direc- of water lapping along the shoreline represent the electric tion away from the rest of the metal particle and thus pro- fields and the seaweed sloshed back and forth by these waves ducing a net negative charge on one side. Since the rest of represent electrons (6). the metal particle is effectively a cationic lattice of nuclei and Though PSPs can be produced with light, simply shin- localized core electrons, the side opposite the negative charge ing light on a smooth metal surface in air is not sufficient has a net positive charge. LSPs have also been referred to as because the momentum of the light does not match that of dipole plasmons, but the oscillating field of the incident light the SPs (3). Neither will the smooth metal surface emit light can induce quadrupole as well as dipole resonances, especially from plasmons.1 Various methods are employed to enable for particles greater than 30 nm in diameter (12, 13). If a light to produce and to be produced by PSPs. Some meth- particle with a dipole can be considered to have a positively ods involve passing the light through a medium with a higher charged pole and a negatively charged pole, then a particle refractive index than air, such as glass, before it comes near with a quadrupole can be considered to have two positively to the metal surface (3, 5). To better explain the role of re- charged poles and two negatively charged poles. fractive index, consider what happens when a beam of light The energy of light required to produce LSP resonance passes from a medium with a higher index into a medium depends on a number of factors, including the size, shape, with a lower index. In many cases, part of the incident beam and composition of the particles, as well as the composition is reflected from the interface and part of the incident beam of the surrounding media. The interactions of light with passes into the medium with the lower refractive index (and spherical metal particles can be described by Mie theory, is refracted in the process). However, when the angle of the which is based upon an exact solution to Maxwell’s equa- incident light exceeds a critical angle, no portion of the light tions (1). This theory also predicts what fraction of light im- leaves the medium with higher refractive index and is com- pinging upon colloidal metal particles will be absorbed and pletely reflected. This is the condition of total internal re- what fraction will be scattered. The sum of absorption and flectance (7). Many fiber optic cables utilize the same scattering is the extinction of light due to the particles. This principle to transmit light down curved strands: the refrac- is actually what is measured when one places a colloidal metal tive index of the core of the fiber optic is higher than that of suspension into a UV–vis spectrometer. Figure 2 shows the its cladding layer, and the condition of total internal reflec- extinction spectra for spherical Au colloids with differing di- tance keeps the light in the fiber (8). Under the conditions ameters. Generally speaking, as the particle size increases, the of total internal reflectance, light waves that strike the inter- plasmon resonant frequency decreases (shifts to longer wave- face between two materials with differing refractive indices lengths) (1). Small Au particles (with diameters less than 20 produce new light waves that propagate from the interface a nm) exhibit extinction that is primarily due to absorption; very short distance into the medium with the lower refrac- larger particles tend to exhibit much stronger scattering (1).