Emilie Ringe

SERS: Materials, applications, and the future

Surface enhanced Raman spectroscopy (SERS) is a powerful vibrational spectroscopy technique that allows for highly sensitive structural detection of low concentration analytes through the amplification of electromagnetic fields generated by the excitation of localized surface plasmons. SERS has progressed from studies of model systems on roughened electrodes to highly sophisticated studies, such as single molecule spectroscopy. We summarize the current state of knowledge concerning the mechanism of SERS and new substrate materials. We highlight recent applications of SERS including sensing, spectroelectrochemistry, single molecule SERS, and real-world applications. We also discuss contributions to the field from the Van Duyne group. This review concludes with a discussion of future directions for this field including biological probing with UV-SERS, tip-enhanced Raman spectroscopy, and ultrafast SERS.

Chemical Vapor Deposition Growth of Crystalline Monolayer MoSe2

Recently, two-dimensional layers of transition metal dichalcogenides, such as MoS2, WS2, MoSe2, and WSe2, have attracted much attention for their potential applications in electronic and optoelectronic devices. The selenide analogues of MoS2 and WS2 have smaller band gaps and higher electron mobilities, making them more appropriate for practical devices. However, reports on scalable growth of high quality transition metal diselenide layers and studies of their properties have been limited. Here, we demonstrate the chemical vapor deposition (CVD) growth of uniform MoSe2 monolayers under ambient pressure, resulting in large single crystalline islands. The photoluminescence intensity and peak position indicates a direct band gap of 1.5 eV for the MoSe2 monolayers. A back-gated field effect transistor based on MoSe2 monolayer shows n-type channel behavior with average mobility of 50 cm(2) V(-1) s(-1), a value much higher than the 4-20 cm(2) V(-1) s(-1) reported for vapor phase grown MoS2.

Single-Molecule Surface-Enhanced Raman Spectroscopy of Crystal Violet Isotopologues: Theory and Experiment

Single-molecule surface-enhanced Raman spectroscopy (SMSERS) of crystal violet (CV) has been reported since 1997, yet others have offered alternative explanations that do not necessarily imply SMSERS. Recently, the isotopologue approach, a statistically significant method to establish SMSERS, has been implemented for members of the rhodamine dye family. We provide the first demonstration of SMSERS of a triphenylmethane dye using the isotopologue approach. Two isotopologues of CV are employed to create chemically identical yet vibrationally distinct probe molecules. Experimental spectra were compared extensively with computational simulations to assign changes in mode frequencies upon deuteration. More than 90 silver nanoparticle clusters dosed with a 50:50 mixture of CV isotopologues were spectroscopically characterized, and the vibrational signature of only deuterated or undeuterated CV was observed 79 times, demonstrating that the isotopologue approach for proving SMSERS is applicable to both the CV and the rhodamine systems. The use of CV, a minimally fluorescent dye, allowed direct evaluation of enhancement factors (EF), which are reported herein. Through experiment and theory, we show that molecular electronic resonance Raman (RR) and surface-enhanced Raman effects combine synergistically in SMSERS. Excluding RR effects, the EF(SERS) is ∼10(9). Variations and relationships between substrate morphology and optical properties are further characterized by correlated SMSERS-localized surface plasmon resonance (LSPR)-high-resolution transmission electron microscopy (HRTEM) studies. We did not observe SMSERS from individual nanoparticles; further, SMSERS-supporting dimers are heterodimers of two disparately sized particles, with no subnanometer gaps. We present the largest collection to date of HRTEM images of SMSERS-supporting nanoparticle assemblies.

Correlated structure and optical property studies of plasmonic nanoparticles

This article provides a review of our recent studies of single metal nanoparticles and single nanoparticle clusters aimed at correlating the structural and plasmonic properties of the same entity. The correlation between the structure and the optical properties arising from the localized surface plasmon resonance (LSPR) on single nanoparticles from various samples is described. Nanoparticles of different materials (Ag and Au) and shapes (spheres, cubes, triangles) are considered. Experiments were carried out using transmission electron microscopy (TEM), dark-field spectroscopy, and surface-enhanced Raman spectroscopy (SERS). Results of those measurements were compared with electrodynamics calculations to provide insight into the interpretation and physical meaning of the experimental results. We examine correlated studies of triangular nanoparticle arrays to highlight the significance of single entity measurements over ensemble-averaged measurements. Furthermore, we show how an examination of statistics on large data sets helps draw quantitative structure―LSPR relationships. We also show that implementing SERS in correlated measurements improves the understanding of factors important in determining SERS enhancements. Finally, we extend the scope of correlated measurements to the tracking and controlled manipulation of single nanoparticles, thus paving the way for in vivo diagnostics using nanomaterials.

Plasmon Length: A Universal Parameter to Describe Size Effects in Gold Nanoparticles.

Localized surface plasmon resonances are central to many sensing and signal transmission applications. Tuning of the plasmon energy and line width through particle size and shape is critical to the design of such devices. To gain quantitative information on the size dependence of plasmonic properties, mainly due to retardation effects, we correlated optical spectra and structures for 500 individual gold particles of five different shapes. We show that the effects of size on the dipolar plasmon frequency and line width are shape-independent when size is described by the plasmon length, the length over which the oscillations take place. This result suggests that edge effects are rather unimportant for dipolar modes in a large size range between 50 and 350 nm. Therefore, in describing the size-dependent plasmonic properties of nanoparticles, one should focus on the distance along which the oscillation occurs rather than its intrinsic shape.

Wulff construction for alloy nanoparticles

The Wulff construction is an invaluable tool to understand and predict the shape of nanoparticles. We demonstrate here that this venerable model, which gives a size-independent thermodynamic shape, becomes size dependent in the nanoscale regime for an alloy and that the infinite reservoir approximation breaks down. The improvements in structure and energetic modeling have wide-ranging implications both in areas where energetics govern (e.g., nucleation and growth) and where the surface composition is important (e.g., heterogeneous catalysis).

Heterometallic antenna−reactor complexes for photocatalysis

Significance Plasmon-enhanced photocatalysis holds significant promise for controlling chemical reaction rates and outcomes. Unfortunately, traditional plasmonic metals have limited surface chemistry, while conventional catalysts are poor optical absorbers. By placing a catalytic reactor particle adjacent to a plasmonic antenna, the highly efficient and tunable light-harvesting capacities of plasmonic nanoparticles can be exploited to drastically increase absorption and hot-carrier generation in the reactor nanoparticles. We demonstrate this antenna−reactor concept by showing that plasmonic aluminum nanocrystal antennas decorated with small catalytic palladium reactor particles exhibit dramatically increased photocatalytic activity over their individual components. The modularity of this approach provides for independent control of chemical and light-harvesting properties and paves the way for the rational, predictive design of efficient plasmonic photocatalysts. Metallic nanoparticles with strong optically resonant properties behave as nanoscale optical antennas, and have recently shown extraordinary promise as light-driven catalysts. Traditionally, however, heterogeneous catalysis has relied upon weakly light-absorbing metals such as Pd, Pt, Ru, or Rh to lower the activation energy for chemical reactions. Here we show that coupling a plasmonic nanoantenna directly to catalytic nanoparticles enables the light-induced generation of hot carriers within the catalyst nanoparticles, transforming the entire complex into an efficient light-controlled reactive catalyst. In Pd-decorated Al nanocrystals, photocatalytic hydrogen desorption closely follows the antenna-induced local absorption cross-section of the Pd islands, and a supralinear power dependence strongly suggests that hot-carrier-induced desorption occurs at the Pd island surface. When acetylene is present along with hydrogen, the selectivity for photocatalytic ethylene production relative to ethane is strongly enhanced, approaching 40:1. These observations indicate that antenna−reactor complexes may greatly expand possibilities for developing designer photocatalytic substrates.

From tunable core-shell nanoparticles to plasmonic drawbridges: Active control of nanoparticle optical properties

Redox electrochemistry was used to reversibly tune the optical properties of plasmonic core-shell nanoparticles and dimers. The optical properties of metallic nanoparticles are highly sensitive to interparticle distance, giving rise to dramatic but frequently irreversible color changes. By electrochemical modification of individual nanoparticles and nanoparticle pairs, we induced equally dramatic, yet reversible, changes in their optical properties. We achieved plasmon tuning by oxidation-reduction chemistry of Ag-AgCl shells on the surfaces of both individual and strongly coupled Au nanoparticle pairs, resulting in extreme but reversible changes in scattering line shape. We demonstrated reversible formation of the charge transfer plasmon mode by switching between capacitive and conductive electronic coupling mechanisms. Dynamic single-particle spectroelectrochemistry also gave an insight into the reaction kinetics and evolution of the charge transfer plasmon mode in an electrochemically tunable structure. Our study represents a highly useful approach to the precise tuning of the morphology of narrow interparticle gaps and will be of value for controlling and activating a range of properties such as extreme plasmon modulation, nanoscopic plasmon switching, and subnanometer tunable gap applications.

Plasmonic Near-Electric Field Enhancement Effects in Ultrafast Photoelectron Emission: Correlated Spatial and Laser Polarization Microscopy Studies of Individual Ag Nanocubes

Electron emission from single, supported Ag nanocubes excited with ultrafast laser pulses (λ = 800 nm) is studied via spatial and polarization correlated (i) dark field scattering microscopy (DFM), (ii) scanning photoionization microscopy (SPIM), and (iii) high-resolution transmission electron microscopy (HRTEM). Laser-induced electron emission is found to peak for laser polarization aligned with cube diagonals, suggesting the critical influence of plasmonic near-field enhancement of the incident electric field on the overall electron yield. For laser pulses with photon energy below the metal work function, coherent multiphoton photoelectron emission (MPPE) is identified as the most probable mechanism responsible for electron emission from Ag nanocubes and likely metal nanoparticles/surfaces in general.

Correlating the structure and localized surface plasmon resonance of single silver right bipyramids

Localized surface plasmon resonances (LSPRs), collective electron oscillations in metal nanoparticles, are being heavily scrutinized for applications in prototype devices and circuits, as well as for chemical and biological sensing. Both the plasmon frequency and linewidth of a LSPR are critical factors for application optimization, for which their dependence on structural factors has been qualitatively unraveled over the past decade. However, quantitative knowledge based on systematic single particle studies has only recently become available for a few particle shapes. We show here that to understand the effect of structure (both size and shape) on plasmonic properties, one must take multiple parameters into account. We have successfully done so for a large data set on silver right bipyramids. By correlating plasmon energy and linewidth with edge length and corner rounding for individual bipyramids, we have found that the corner rounding has a significant effect on the plasmon energy for particles of the same size, and thus corner rounding must be taken into account to accurately describe the dependence of a LSPR on nanoparticle size. A detailed explanation of the phenomena responsible for the observed effects and their relationship to each other is presented.