We present a detailed description of the apparatus and techniques that we have utilized in our experimental study of individual plas on resonant nanoparticles,along with a brief description of some major results. The apparatus consists of a spectroscopic system combined with a modified darkfield microscope, which enables the user to sequentially select individual resonant nanostructures in the microscopic field of view for spectroscopic study. Plasmon resonant
nanostructures scatter light elastically,and typically have very large scattering cross-sections at their resonant optical
wavelengths. In general, spectra can be obtained with acquisition times between .1 to 30 seconds,and color images can be captured using consumer digital color cameras. Spheres,tetrahedrons,and pentagonal platelets were fabricated using colloidal chemistry techniques. To produce highly anisotropic structures such as nanorods and "barbells", templates were used. Many of these nanostructures have been individually spectroscopically characterized,and their spectra correlated with their shape and size as determined by transmission electron icroscope (TEM). The unique shape,size,
composition,and dielectric surroundings of the individual plasmon resonant nanostructures determine their plasmon resonant behavior. We will show how the composition of the substrate on which the particles are immobilized and the dielectric of the surrounding medium have a significant effect on the plasmon resonance of the individual particles.
The large scattering cross section of plasmon resonant gold and silver nanoparticles functionalized with the appropriate ligand allows for sensitive and specific detection of nucleic acids and proteins. By varying the size, shape, and material morphology populations with a specific peak plasmon resonance can be prepared. By varying the order and length of plasmon resonant bar segment in a composite nanowire one can obtain a large number of particle populations. Distinct populations can be used for labels for multiplexing or as a platform for biological assays. An larger number of color populations can be obtained with composite nanowires that are fabricated with various lengths of silver, gold, or nickel segments. The order and length of the different plasmon resonance rod segments can be used to uniquely identify a rod population allowing for a large degree of multiplexing within a single sample.
We report the photodeposition of polymeric layers of nanometer scale thickness onto two nanoparticle substrates. This was accomplished by ultraviolet irradiation of a solution of functionalized diacetylene monomers in which the nanoparticles were suspended. Following photodeposition, the coated nanoparticles were analyzed using transmission electron microscopy and UV-visible spectroscopy. Highly regular polydiacetylene films with thicknesses from 2.5 - 25 nm were produced. The thickness measurements were facilitated by the attachment of small gold nanoparticles onto the surface of silica nanoparticle substrates prior to photodeposition, to provide contrast in the final TEM image. Deposition onto gold nanoshells was also demonstrated. Photodeposition onto these particles resulted in more individual coated particles. Furthermore, short irradiation times (approximately 5 minutes) yielded coated particles without the extra oligomeric contaminants usually found. This substantiates the idea that photodeposition occurs preferentially on a substrate material. UV-visible spectroscopy of the deposited films indicate that approximately 40% less conjugation is present relative to macroscopic polydiacetylene thin films grown with the same approach. This process yields a unique `nanolaminate' coating which may be useful in the modification of the physical, chemical, or optical properties of nanoparticles.
We have used two-photon photoemission to probe the dynamics of the long-lived triplet excitons in pristine and photo- oxidized films of poly[2-methoxy,5-(2'-ethylhexoxy)-1,4- phenylenevinylene] (MEH-PPV). We use a rate equation model to explain the intensity dependence of the total photoelectron yield in terms of the relaxation dynamics of the triplet excitons. Observations of the pristine films indicate that triplet-triplet annihilation is an important deexcitation mechanism for photoexcited triplet excitons in MEH-PPV. Upon photo-oxidation, the measured triplet exciton energy is shifted downward and the triplet exciton-derived photoemission linewidth is dramatically narrowed. Both of these effects are attributed to modification of the physical and electronic structure of the films arising from the formation of carbonyl defects on the polymer chains. We also observe significant oxygen-induced lifetime quenching of the triplet exciton.
We have succeeded in adsorbing individual C60 molecules onto the tunneling region of an STM tip. The individual tip- adsorbed molecules are imaged by scanning the fullerene- adsorbed tip over a defect covered graphite surface. The nanometer-size defects serve as a surface tip array which 'inverse images' the molecules adsorbed to the tip when the surface is scanned. These tips were subsequently used to observe threefold symmetric electron scattering from point defects on a graphite surface, an effect that could not be observed using bare metal tips. Functionalizing an STM tip with an appropriate molecule adsorbate alters the density of states near the Fermi level of the tip and changes its imaging characteristics.
Conference Committee Involvement (1)
Plasmonics: Metallic Nanostructures and Their Optical Properties
3 August 2003 | San Diego, California, United States