Catherine J. Murphy University of South Carolina

Transcription

1 520 A ANALYTICAL CHEMISTRY / OCTOBER 1, 2002

2 ne of the most exciting areas of chemical research is inorganic nanomaterials metals, semiconductors, or insulators made with dimensions on a scale of nm. The Journal of the American Chemical Society, Nature, and Science frequently have a paper or two describing some of the latest advances in making and measuring properties of nanomaterials. The National Nanotechnology Initiative ( started by former President Clinton, promoted the formation of nanoscience and nanotechnology research groups across the country. Chemists, physicists, engineers, and biologists are collaborating to study these materials and to harness their properties for many different applications. Why are these materials so exciting? The size range of nanomaterials coincides with some fundamental length scales in physics. The mean free path of an electron in a metal at room temperature, for example, is ~ nm (1). The Bohr radius of photoexcited electron hole pairs in semiconductors is ~1 10 nm (2). The 1- to 100-nm dimension is the range over which molecules assemble into viruses; in solids, chemical bonds give way to electronic bands (Figure 1). There is still a great deal of fundamental physics and chemistry to discover in nanomaterials, but now applications are being explored. This article will give an overview of these exciting materials and describe their potential, especially in optical detection schemes. Catherine J. Murphy University of South Carolina What s a quantum dot? A quantum dot is a semiconductor particle that has all three dimensions confined to the 1- to 10-nm-length scale. The literature also refers to them as zero-dimensional (0-D) materials, semiconductor nanoparticles, or nanocrystallites. (Confinement in two dimensions produces 1-D quantum wires, and confinement in one dimension produces 2-D quantum wells.) To understand why a semiconductor particle of this size would be interesting from a quantum mechanical point of view, consider what happens in a material when an electron is promoted to the conduction band from the valence band (Figure 2). Left behind in the valence band is a hole, which can be thought of as a particle with its own charge (+1) and effective mass. The electron and hole are considered bound to each other via Coulombic attraction, and this quasiparticle is then known as an exciton. The exci- OCTOBER 1, 2002 / ANALYTICAL CHEMISTRY 521 A

3 ton can be considered a hydrogenlike system, and a Bohr approximation of the atom can be used to calculate the spatial separation of the electron hole pair of the exciton by r = h 2 /πm r e 2 (1) where r is the radius of the sphere (defined by the 3-D separation of the electron hole pair), is the dielectric constant of the semiconductor, m r is the reduced mass of the electron hole pair, h is Planck s constant, and e is the charge on the electron. For many semiconductors, the masses of the electron and hole have been determined by ion cyclotron resonance and are generally in the range m e (m e is the mass of the electron). For typical semiconductor dielectric constants, the calculation suggests that the electron hole pair spatial separation is ~1 10 nm for most semiconductors (2). The electronic structure of these quantum dots, then, becomes intermediate between localized bonds and delocalized bands (Figure 1). Because the physical dimensions of a quantum dot can be smaller than the exciton diameter, the quantum dot is a good example of the particle-in-a-box calculations of undergraduate physical chemistry. In those calculations, the energies of the particle in the box depend on the size of the box. In the quantum dot, the bandgap energy becomes size-dependent (2 7), which becomes obvious when simple absorption spectra of quantum dot solutions are taken (Figure 3). The bandgap energy of the semiconductor from such spectra is generally taken as the absorption energy onset. As the particle size decreases, the absorption onset shifts to higher energy (blue shifts), indicating an increase in bandgap energy (3 9). According to an early effective mass model calculation by Brus (8), estimating particle size from such data can be done as E g (quantum dot) = E g (bulk) + (h 2 /8R 2 )(1/m e + 1/m h ) 1.8e 2 /4π 0 R Energy p s sp 3 σ Conduction band Bandgap Valence band (2) Atomic orbitals Hybrid orbitals Molecular orbitals Density of states FIGURE 1. Silicon as a prototype semiconductor. σ Comparison of the electronic structure of the atomic orbitals in a silicon atom (left) to that of a silicon cluster molecule (middle) and to that of bulk silicon (right). Atomic orbitals in the atom give rise to bonding and antibonding molecular orbitals in the cluster molecule, which give rise to the filled valence band and (mostly) empty conduction band in the bulk semiconductor. (Adapted with permission from Ref. 2.) in which E g is the bandgap energy of a quantum dot or bulk solid, R is the quantum dot radius, m e is the effective mass of the electron in the solid, m h is the effective mass of the hole in the solid, is the dielectric constant of the solid, and 0 is the permittivity of a vacuum. The middle term on the right-hand side of the equation is a particle-in-a-box-like term for the exciton, and the third term represents the electron hole pair Coulombic attraction, which is mediated by the solid. Implicit in this equation is the assumption that the quantum dots are spherical and that the effective masses of the charge carriers and the dielectric constant of the solid are constant as a function of size. The Brus model maps E g and size well for larger quantum dots, but its predictions do not match experimental data well for very small particle sizes. Other calculations have been performed that better match experimentally determined bandgap energies and quantum dot sizes for smaller particle sizes (9). The above calculations usually are done for materials in the strong confinement limit, that is, the physical dimensions of the semiconductor nanoparticle are substantially smaller than the excitonic Bohr radius. It is also possible for a material to be in weak confinement, in which case the particle size is somewhat larger than the excitonic Bohr radius. Another predicted property of quantum dots is that the oscillator strength of the lowest-energy transition becomes size-dependent (2). In the weak confinement case, the oscillator strength of the excitonic transition in a quantum dot appears to be proportional to the dot volume (2). In the strong-confinement case, the oscillator strength is nearly size-independent (2, 8). How to make them Inorganic semiconductors include the Group 14 (old Group IV) elements silicon and germanium; compounds such as GaN, GaP, GaAs, InP, and InAs (collectively the III V materials); and ZnO, ZnS, ZnSe, CdS, CdSe, and CdTe (II VI materials). Periodic properties of the electronic properties of semiconductors are observable; as one goes down the periodic table, the bandgap energy of the solid decreases (10). Solid solutions of many of these semiconductors can be made, and the bandgap of the resulting solid solutions is intermediate between the two end-members; thus GaP has a room-temperature bandgap of 2.3 ev ( onset ~540 nm), GaAs has a room-temperature bandgap of 1.4 ev ( onset ~890 nm), and GaP x As 1 x has a bandgap energy that depends nearly linearly on x (10). Quantum dots are not yet commercially available, thus a great deal of work (literally hundreds of papers on CdS and CdSe alone) has gone into their synthesis and characterization. Quantum dots can be made as colloidal solutions or grown on solid substrates. In the colloidal approach, precursors of the material are reacted in the presence of a stabilizing agent that will restrict the growth of the particle and keep it within the quantum confinement limits estimated by Equation 1. Examples are abundant for the II VI quantum dots; aqueous Cd(II) salts can be mixed with anionic or Lewis basic polymers such as sodium polyphosphate or polyamines (11 13), and the subsequent addition of a sulfide source produces CdS nanoparticles that are in the 1- to 10-nm-size range. Size tuning is possible by controlling relative concentrations and rates of addition. 522 A ANALYTICAL CHEMISTRY / OCTOBER 1, 2002

4 The most popular route to synthesizing CdSe quantum dots is organometallic (14): Dimethylcadmium is reacted with a selenium reagent in the presence of a phosphine oxide surfactant at high temperature. Careful control of reaction conditions produces CdSe quantum dots that are quite homogeneous in size (<5% standard deviation in diameter for a given batch) and tunable in size from 2.0 to 8.0 nm. Other synthetic schemes rely on the confined space within a host to make quantum dot guests in situ; examples of such hosts are inverse micelles, lipid bilayers, porous glasses, zeolites, and even hollow biological macromolecules (15 19). In addition to these solution methods, there are many examples of supported quantum dots grown by vapor deposition reactions of precursors on solid substrates (20). The most popular choices of quantum dot materials for chemists are CdSe and CdS. Their bulk bandgaps are 1.7 ev for CdSe and 2.4 ev for CdS (corresponding to absorption onsets at ~720 nm and ~520 nm, respectively), which means that their absorption energies are tunable throughout the visible region. Thus, CdSe quantum dots have absorption spectra with absorption onsets at wavelengths corresponding to the red through the violet; the smaller the particle, the shorter the wavelength. Quantum dot size can be measured in several ways, including transmission electron microscopy (TEM), line-broadening of X-ray diffraction lines of quantum dot powders, and electronic absorption spectroscopy, as given by Equation 2 or related derivations. In the field s infancy, preparative procedures yielded nanomaterials that had standard deviations in particle diameters of ~50% or more; at present, state-of-the-art syntheses should produce nanoparticles with ~5% standard deviations in measured diameters (for which several hundred nanoparticles should be measured in TEM experiments). Knowledge of the surface chemistry of quantum dots is needed to understand their optical properties and to manipulate them chemically for a desired application. For sufficiently small quantum dots, which contain ~1000 atoms, 1/3 to 1/2 of the constituent atoms are on the surface. Some NMR experiments on quantum dots have been performed to characterize the surface layer (21), but in general, workers generally assume that the stabilizing polymer or surfactant that they put in the reaction mixture is still on the surface. The competition of thiols (or thiolates, depending on ph) with sulfide for metal binding can be used to manipulate the surface chemistry of thiophilic materials, such as CdS, CdSe, ZnS, and ZnSe; the resulting nanoparticle s growth is restricted by the surface-bound thiolates (22, 23). If the solubility characteristics of the thiol are sufficiently different from the solvent (e.g., hydrophobic thiol for an aqueous preparation), precipitation and washings can be used to good effect for purification and, incidentally, to provide evidence that the thiol is, in fact, on the surface. Postsynthetic reaction of thiols with thiophilic quantum dots (again, generally the II VI materials) also modifies the surface, on average, in an understandable way. The kinetics and mechanisms of ligand chemistry on the surfaces of quantum dots are just beginning to be explored (24). Optical properties of quantum dots Quantum dots have bandgap energies, and hence onsets of light absorption, which vary as a function of size, as described Energy e h + E E g Conduction band Valence band Trap Luminescence Trap FIGURE 2. Energy-level diagram showing promotion of an electron from the valence band to the conduction band, leaving a hole behind. The electron must have a minimum E g bandgap energy to undergo this promotion. The created electron hole pair, an exciton, may recombine immediately to produce heat or light with an energy equal to E g, but it is more likely that trap states within the material trap either the electron or the hole. These trap states result from numerous factors, including structural defects, atomic vacancies, dangling bonds, and adsorbates at the interface. Radiative recombination of the trapped charge carriers then produces luminescence that is substantially redshifted from the absorbed light. earlier. Ideally, the corresponding light emission from quantum dots should follow this trend (Figure 3). However, the nature of the quantum dot surface is critical for photoluminescence experiments (Figure 4). The influence of the surface on photoluminescence can be understood in terms of the trap states described in Figure 2. These trap states are caused by defects, such as vacancies, local lattice mismatches, dangling bonds, or adsorbates at the surface. The excited electron or hole can be trapped by these local energy minima states and become less available for the radiative recombination of luminescence. For bulk semiconductors, surface passivation is a well-known phenomenon that decreases the possibility of charge carriers residing in traps. Silicon, for example, is passivated with a layer of silicon dioxide. For quantum dots, surface passivation has most frequently been achieved by overcoating the quantum dot with a higher-bandgap semiconductor quantum shell (25, 26). CdSe coated with ZnS is an example of a core-shell nanocomposite material (27). Thus, for well-passivated surfaces, the emission spectrum of the quantum dot does indeed blue-shift with a decrease in particle size (Figure 3) because of (essentially) band-edge to bandedge recombination of the electron hole pair. Many timeresolved spectroscopic experiments have been performed to estimate the recombination times for the electrons and holes from the band edges (picoseconds or faster) and to estimate the times for these charge carriers to be trapped by either defects within the particle or by adsorbates on the surface of the particle (nanoseconds and slower) (28 30). Evidence is accumulating that a dark excitonic state may exist in these materials, which renders band-edge recombination far less sensitive to surface states than Figure 2 s steady-state picture would seem to indicate (28). OCTOBER 1, 2002 / ANALYTICAL CHEMISTRY 523 A

5 (a) Absorbance (arbitrary units) Diameter Wavelength (nm) 90 Å 78 Å 69 Å 60 Å 55 Å 49 Å 44 Å 39 Å 37 Å 33 Å 30 Å 26 Å 22 Å 19 Å 17 Å Luminescence (arbitrary units) (b) Band edge Deep trap Radius (Å) FIGURE 3. Room-temperature spectra of CdSe quantum dots. (a) Absorption and photoluminescence spectra as a function of diameter. (b) Quantum yield of photoluminescence as a function of size. Squares represent deep-trap emission, and circles represent band-edge emission. (Adapted from Ref. 70.) 10 Quantum yield Quantum dots as fluorescent dyes If the surface of the quantum dot is sufficiently passivated, it can have quantum yields of light emission that approach those of organic dyes (~0.60) (25 27 ). The luminescence of the passivated material becomes relatively insensitive to its local environment, and quantum dots can therefore be used as large, inorganic analogues of fluorescent dyes. What are the advantages to the analytical chemist of using quantum dots instead of conventional fluorescent dyes? The emission spectra of quantum dots can be much narrower (fwhm ~30 nm) than organic dyes (25 27, 31 33). This property depends on the size monodispersity of the quantum dots; for a heterogeneous quantum dot sample with many different particle sizes, broad emission spectra will be obtained. In applications where multiplexing is important, and the overlap between different emission spectra needs to be minimized, narrow emission spectra are key. The maximum wavelength of emission is tunable by quantum dot size. For CdSe, well-passivated, monodisperse quantum dot solutions emit, depending on size, in the blue to the red (25 27, 31 33). The long wavelength limit corresponds to the bulk bandgap of the semiconductor (if the quantum dot is well passivated). Thus, once the synthesis conditions are worked out for one color of material, adjusting the conditions to tune the particle size will produce other colors of material. In addition, quantum dots appear to be less susceptible to photobleaching than organic dyes (33, 34). In one side-by-side comparison, rhodamine B underwent photobleaching in 10 min, compared with 4 h for CdSe quantum dots (33). For all of these reasons, there is great excitement about using quantum dots as fluorescent dyes. In 1998, two groups independently reported using quantum dots as specific fluorescent biological stains in cells (31, 32). In these experiments, both groups began with CdSe quantum dots of various sizes, which emitted at different wavelength maxima. Both groups passivated the surface and then covalently linked proteins to the nanoparticles via different surface chemistries. Incubating the protein-decorated quantum dots with different cell lines led to the specific fluorescent labeling of the cells, where the protein overcoat gave specificity and the quantum dot core gave color-coded luminescence (31, 32). Continuing work at this bio-nano interface is one of the exciting multidisciplinary frontiers of science. Quantum dots as fluorescent probes Even if quantum dots are not well passivated, they can be useful optical probes of local environments. For bulk semiconductors, it is well known that their photoluminescence is sensitive to the presence and nature of adsorbates (35 38). Again, the energies of the trap states become important when describing these effects. Cohen et al. have developed a theory and experiments that suggest that the lowest unoccupied molecular orbitals of adsorbates can interact with trap states within the bandgap of a bulk semiconductor in a donor acceptor adduct fashion. This interaction influences the intensity and lifetime of light emission from the semiconductor (35). Analogous effects are likely operative in quantum dots. Indeed, in Figure 4, the emission spectrum of unpassivated CdS was radically changed by an activation step by using additional Cd(II) in the basic aqueous solution and without a change in particle size. Presumably, trap states are either being filled or energetically moved closer to the band edges by this simple chemical treatment. This kind of behavior may form the basis for chemical sensing with quantum dots. So far, in the literature, there are some reports of chemical sensing of (for the most part) small molecules and ions with quantum dots via analyte-induced changes in photoluminescence (39 42). Particle size and surface treatment can yield a range of emission colors easily visible to the naked eye. In the opening art on p 521 A, the blue flask contains 2-nm CdS quantum dots stabilized by poly(amido)amine dendrimers in methanol; the red flask, 4.5-nm CdS dots stabilized by sodium polyphosphate in water; the green flask, 3-nm CdS dots stabilized by poly(amido)amine dendrimers in water; the orange flask, 4.5-nm CdS dots stabilized by sodium polyphosphate in water after activation with Cd(II) at ph 10. The flasks were irradiated with broadband UV light. Another set of experiments at the bio-nano interface uses CdS quantum dots to detect local, unusual DNA conformations. In these experiments, the quantum dots are not particularly well passivated, but their surfaces are modified with cationic or hydrogenbonding groups that are presumably attractive to the anionic DNA polymer. Photoluminescence titrations are then performed with short DNA oligomers that are either normal or unusual in terms of local structure and flexibility (43 46). The shape of the emission spectrum does not change, only its intensity. Equilibrium binding constants and adsorption on-rates can be estimated from the data. From these experiments, valuable biophysical insight has been gained into the way that different DNAs bind to generic 524 A ANALYTICAL CHEMISTRY / OCTOBER 1, 2002

6 protein-sized curved surfaces without the need for researchers to label the DNA or use radioactivity (43 46). Some of these unusual DNA structures are correlated with human disease (47), and thus the development of this optical detection scheme is of interest to the bioanalytical community. There is an open frontier here for the analytical chemist. Is it possible to make analyte-specific quantum dots (e.g., by covalently attaching specific receptor groups to their surfaces), which will give a measurable color change when the analyte is bound? A reversible surface passivation scheme needs to be developed for this sort of experiment to work. In addition to chemical effects that alter photoluminescence, electron injection into a quantum dot can alter its optical properties. Upon application of an electrochemical potential to quantum dots, new intraband transitions appear, excitonic transitions are bleached, and the photoluminescence is quenched (48, 49). These results suggest that quantum dots may also function as electrochromic sensors. Photonics Light-emitting diodes based on quantum dots (50) and quantum cascade lasers based on quantum wells (2-D semiconductor nanostructures) have been reported (51). In theory, quantum dot cascade lasers are achievable (52). In quantum cascade lasers, unlike in semiconductor diode lasers, the wavelength output is dependent upon quantum confinement effects. Stimulated emission and optical gain from CdSe quantum dots have been reported (53). In recent exciting work, semiconductor nanorods (cylinders, not spheres, on the nanometer scale) have exhibited polarized light emission and lasing (54, 55). These exciting results suggest another use of quantum dots: light sources. Quantum dots organized in three dimensions are also good candidates for photonic crystals, materials which are predicted to have controllable optical bandgaps in the same way semiconductors have electronic bandgaps (56). The field is in its infancy, and it will likely be some time before the wider chemical community embraces quantum dots commercially in these photonic applications. Nonetheless, it is tempting to speculate that integrated devices could be designed in which quantum dots function as both light sources and sensors (57). Quantum dots versus other nanoparticles A perusal of the past five years of Analytical Chemistry does not turn up a great many papers with the keyword nanoparticle, only ~30. Searching anywhere in the article yields about 100 papers in which the word nanoparticle is used. In only one of these papers do the researchers use quantum dots, conjugating them to antibodies for fluoroimmunoassays (58). Other papers use metallic nanoparticles (~10- to 100-nm diam) for detection of analytes via surface plasmon resonance (SPR) spectroscopy (59), surface-enhanced Raman spectroscopy (SERS) (60), optical microscopy (61), or electrochemical methods (62). Additional papers describe fluorescent nanoparticles that are, in fact, polymer nanoparticles that contain organic fluorescent dyes (63). Fundamental electrochemical studies of monolayer-protected gold nanoparticles are in the analytical literature, and there are reports of visible photoluminescence from such species (64, 65). Colorimetric detection of analytes using analyte-mediated aggregation of gold nanoparticles to produce a red-to-blue color change has been reported (66). If the surfaces of the gold nanoparticles are modified with DNA sequences, adding complementary DNA strands to the solution produces a visible color change with femtomolar detection limits (67, 68). Speculation There is a great deal of ongoing work to develop and improve the syntheses of quantum dots. Even so, the analytical community is beginning to explore these nanomaterials for optical sensors. As inorganic analogues of organic dyes, quantum dots show great promise. Quantum dot photoluminescence can respond via changes in intensity to the presence of analytes, affording another method of optical sensing; control of the surface chemistry remains a challenge. The optical signal most readily monitored for semiconductor quantum dots is photoluminescence. Metallic nanoparticles can exhibit analyte-induced optical changes due to SERS, SPR, aggregation effects (the latter resulting in visible color changes), and possibly photoluminescence. Yet supported semiconductor quantum dots may provide novel detection schemes, in addition to photoluminescence, for analytes. The bulk semiconductor silicon can be electrochemically etched to produce porous silicon, which is thought to contain silicon quantum dots sup- (a) (b) Photoluminescence (arbitrary units) Wavelength (nm) Cd(II) FIGURE 4. Activated quantum dots. (a) Photoluminescence spectra of 4.5-nm CdS quantum dots, M in cadmium, stabilized by sodium polyphosphate in water. Bottom purple spectrum is as prepared ; rest of the spectra are activated with increasing Cd(II) in basic solution. (b) The activation process does not change the particle size and works for many divalent metal ions, presumably due to surface passivation with a metal hydroxide overlayer. (Adapted from Ref. 11.) OCTOBER 1, 2002 / ANALYTICAL CHEMISTRY 525 A

7 ported on a silicon base (7, 39). Adsorption of analytes on derivatized thin films of porous silicon leads to changes in refractive index, with subsequent detection limits down to the femtomolar level (69). Further explorations of the potential of quantum dots in analytical chemistry are sure to be fruitful. I thank my co-workers, whose names are in the references, for their contributions. Our work has been supported by the National Science Foundation, the National Institutes of Health, the Research Corp., the Alfred P. Sloan Foundation, and the Dreyfus Foundation. Catherine J. Murphy is a professor at the University of South Carolina. Her research interests include the optical sensing properties of metallic and semiconductor nanomaterials and optical probes of DNA structure and dynamics. Address correspondence to her at the Department of Chemistry and Biochemistry, University of South Carolina, 631 Sumter St., Columbia, SC (murphy@mail.chem.sc.edu). 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