The overarching goal of the Link lab is to understand the physical principles that govern the interaction of plasmonic nanoparticles with each other and their molecular environment and to determine the emerging collective optical properties that arise from novel composite nanomaterials. The research efforts can be broadly grouped into the following three themes:
1) Collective optical properties of plasmonic nanoparticle assemblies. Building new photonic materials and devices with nanoparticle building blocks is a central goal in nanoscience. Using metallic nanoparticles that support collective excitations of conduction band electrons – surface plasmons – requires a detailed understanding of how the optical properties of the individual nanoparticles change as they are assembled into complex structures, especially for chemically prepared nanoparticles because of their inherent size and shape polydispersity. To overcome this problem, the Link lab is applying and developing novel single particle spectroscopy techniques, which, when correlated with structural characterization of the same nanostructure and detailed electromagnetic modeling, have allowed us to characterize coupled plasmon modes in 1-dimensional nanoparticle chains, termed plasmonic polymers based on the strong dependence of their optical properties on the plasmonic repeat unit. Because of strong near-field coupling, these nanoparticle waveguides furthermore support subradiant plasmon modes that enable plasmon propagation distances comparable to nanowires with the additional benefit that no bending losses are introduced at sharp bends. Plasmonic waveguides with dimensions below the light diffraction limit are potential elements in opto-electronic circuits. Recent group efforts have focused on chiral assemblies of plasmonic nanoparticles for which single particle spectroscopy always measures only one enantiomer at a time even when ensemble preparation techniques yield racemic mixtures, and far-field coupling of aluminum nanorods to generate near-perfect red, green, and blue color pixels that can be electrically switched with liquid crystals for display and anti-counterfeiting applications.
2) Light harvesting and energy conversion with surface plasmons. Plasmons decay radiatively via scattering or non-radiatively generating an excited electron-hole pair. Unlike in semiconductors where a large bandgap enables long carrier lifetimes, excited electrons and holes in metals relax much more quickly due to the large density of states. While short hot carrier lifetimes appear counterproductive for photo-initiated chemical reactions on the surface of metal nanoparticles, many reactions have been demonstrated as the enormous absorption cross sections offset short carrier lifetimes. Much new physics and chemistry is however still to be learned about hot carrier generation and subsequent transfer to electron and hole acceptors, possibly revolutionizing how we currently think about photocatalysis. The Link lab has been contributing to this field in several different ways. We have shown how the homogeneous plasmon linewidth obtained from single particle measurements yields electron transfer times and efficiencies to a strong electron acceptor like graphene. We have developed single particle absorption spectroscopy based on photothermal imaging in order to determine the hot carrier generation efficiency free from background scattering for large nanostructures with bright and dark plasmon modes. We have discovered one-photon photoluminescence in gold nanorods, which presents a direct signature of the hot carrier distribution. Finally, we study the ultrafast relaxation dynamics of hot carriers directly through single particle ultrafast pump-probe spectroscopy. In collaboration with the Landes lab, all these studies can furthermore be carried out in an electrochemical cell applying potentials that tune the Fermi level and give mechanistic insight into photo-initiated, electro-catalytic redox reactions for energy harvesting and storage applications.
3) Interactions between nanoparticles and proteins. The fate of nanoparticles interacting with the human body is strongly influenced by a protein corona that forms when the nanoparticles come in contact with biological fluids. The type and number of proteins that absorb strongly depend on the particle size and surface chemistry and furthermore can change over the time period that the nanoparticles stay in the body. In situ, spectroscopic techniques, ideally at the single particle/molecule level, are therefore needed. The Link lab has developed correlation spectroscopy approaches utilizing the plasmon resonance to determine changes in the nanoparticle hydrodynamic radius due to protein binding, while our collaborators in the Landes lab are using super-resolution fluorescence imaging to identify binding of individual dye-labeled proteins. To date, we have focused on bovine serum albumin (BSA) as it is the most abundant protein in blood. We have found that on negatively charged nanoparticles BSA forms a monolayer, which protects the nanoparticles from aggregation even at high salt concentration. However, at low BSA concentrations and for positively charged nanoparticles we surprisingly observed that BSA unfolds and then triggers nanoparticle aggregations. The ultimate goal of these studies is to understand the competitive binding of all serum proteins in real time while nanoparticles interact with cells for the safe use of nanomaterials, and to potentially engineer protein coronas for enhanced medical applications.
Stephan Link is Professor of Chemistry and of Electrical and Computer Engineering at Rice University in Houston. He received his PhD in chemistry in 2000 from the Georgia Institute of Technology where he worked for Professor Mostafa A. El-Sayed. In 2006, he joined the Rice Chemistry Department after postdoctoral positions at Georgia Tech and the University of Texas at Austin, where he worked for Professor Paul F. Barbara. His main research interests include the optical properties of single and assembled metallic nanoparticles. Some of the honors he has received include an NSF CAREER award and a Norman Hackerman Award in Chemical Research.