Anionic Effects on Concentrated Aqueous Lithium Ion Dynamics.

The dynamics, orientational anisotropy, diffusivity, viscosity, and density were measured for concentrated lithium salt solutions, including lithium chloride (LiCl), lithium bromide (LiBr), lithium nitrite (LiNO2), and lithium nitrate (LiNO3), with methyl thiocyanate as an infrared vibrational probe molecule, using two-dimensional infrared spectroscopy (2D IR), nuclear magnetic resonance (NMR) spectroscopy, and viscometry. The 2D IR, NMR, and viscosity results show that LiNO2 exhibits longer correlation times, lower diffusivity, and nearly 4 times greater viscosity compared to those of the other lithium salt solutions of the same concentration, suggesting that nitrite anions may strongly facilitate structure formation via strengthening water-ion network interactions, directly impacting bulk solution properties at sufficiently high concentrations. Additionally, the LiNO2 and LiNO3 solutions show significantly weakened chemical interactions between the lithium cations and the methyl thiocyanate when compared with those of the lithium halide salts.

Comparable Enhancement of TERS Signals from WSe2 on Chromium and Gold.

Plasmonic tip-sample junctions, at which the incident and scattered optical fields are localized and optimally enhanced, are often exploited to achieve ultrasensitive and highly spatially localized tip-enhanced Raman scattering (TERS). Recent work has demonstrated that the sensitivity and spatial resolution that are required to probe single molecules are attainable in such platforms. In this work, we observe and rationalize comparable TERS from few-layer WSe2 single crystals exfoliated onto Au- and Cr-coated Si substrates, using a plasmonic TERS probe excited with a 638 nm laser. Our experimental observations are supported by finite-difference time-domain simulations that illustrate that the attainable field enhancement factors at the Au-Au and Au-Cr tip-sample junctions are comparable in magnitude. Through a combined experimental and theoretical analysis, we propose that besides Au/Ag, several metallic substrates may be used to record bright TERS spectral images.

Femtosecond photoemission electron microscopy of surface plasmon polariton beam steering via nanohole arrays.

Directional control over surface plasmon polariton (SPP) waves is a prerequisite for the development of miniaturized optical circuitry. Here, the efficacy of single and dual component SPP steering elements is explored through photoemission electron microscopy. Our imaging scheme relies on two-color photoemission and counter-propagating SPP generation, which collectively allow SPPs to be visualized in real space. Wave-vector difference mixing between the two-dimensional nanohole array and photon momenta enables SPP steering with directionality governed by the array lattice constant and input photon direction. In our dual component configuration, separate SPP generation and Bragg diffraction based steering optics are employed. We find that array Bragg planes principally influence the SPP angles through the array band structure, which allows us to visualize both positive and negative refractory waves.

A Closer Look at Corrugated Au Tips.

A series of optical and electron microscopies are utilized in concert to unravel the properties of corrugated metallic tips. While the overall microscopic shapes of the tips dictate their optical resonances and plasmonic field enhancement factors, nanometric structural details govern their tip-enhanced Raman (TER) spectra and images. Using 4-thiobenzonitrile (TBN) as a molecular reporter, spatially resolved TER spectra reveal that optical rectification and molecular charging are among the prominent observables in the tip-tip TER geometry. We show the spurious appearance of anions is driven by highly localized resonances that appear as a result of surface corrugation and their manifestation throughout the course of TER nanospectroscopy complicates spectral assignments. Overall, nanoscale spatial variations in the TERS spectra suggest that the tip-tip geometry sustains junction plasmons that appear very different from what may be expected from the hybridization of the bulk tip resonances.

Tip-Enhanced Raman Nanospectroscopy of Smooth Spherical Gold Nanoparticles.

We record nanoscale-resolved chemical images of thiobenzonitrile (TBN)-functionalized smooth gold nanospheres on silicon via tip-enhanced Raman (TER) nanospectroscopy. The recorded images trace the nascence of the familiar doughnut-shaped scattering profile of nanoparticles on silicon at its origin (the particle surface), which appears as a horseshoe-shaped scattering pattern under our experimental conditions. The local optical field maps are in agreement with their simulated finite-difference time-domain analogues. Analysis of the recorded spectra with the aid of ab-initio-molecular-dynamics-based Raman spectral simulations further suggests that optical rectification and molecular charging take place throughout the course of atomic-force-microscopy-based TER nanoscale chemical imaging.

Direct Visualization of Counter-Propagating Surface Plasmons in Real Space-Time.

We deploy two-dimensional nanohole arrays as resonant surface plasmon polariton (SPP) couplers that enable counter-propagation and excitation field interference-free imaging of SPP wave packets. We monitor the spatiotemporal evolution of the resulting SPPs using two-color photoemission electron microscopy. The measurements track the electric field envelope of the SPP in real space and time and enable direct characterization of their spatiotemporal properties in a regime where the SPP wave packet is the principal observable. We provide an analysis of the observables for both the co- and counter-propagating directions via SPP trajectories that are recorded in tandem. Our results highlight the advantages of isolating SPPs through counter-propagation, where excitation field-SPP interactions are suppressed.

Imaging the Optical Fields of Functionalized Silver Nanowires through Molecular TERS.

We image 4-mercaptobenzonitrile-functionalized silver nanowires (∼20 nm diameter) through tip-enhanced Raman scattering (TERS). The enhanced local optical field-molecular interactions that govern the recorded hyperspectral TERS images are dissected through hybrid finite-difference time-domain density functional theory simulations. Our forward simulations illustrate that the recorded spatiospectral profiles of the chemically functionalized nanowires may be reproduced by accounting for the interaction between orientationally averaged molecular polarizability derivative tensors and enhanced incident/scattered local fields polarized along the tip axis. In effect, we directly map the enhanced optical fields of the nanowire in real space through TERS. The simultaneously recorded atomic force microscopy (AFM) images allow a direct comparison between our attainable spatial resolution in topographic (13 nm) and TERS (5 nm) imaging measurements performed under ambient conditions. Overall, our described protocol enables local electric field imaging with few nm precision through molecular TERS, and it is therefore generally applicable to a variety of plasmonic nanostructures.

Surface Plasmon-Based Pulse Splitter and Polarization Multiplexer.

Surface plasmon polaritons (SPPs) launched from a protruded silver spherical cap structure using s-polarized femtosecond laser excitation are investigated using photoemission electron microscopy. The resulting SPP is comparable in intensity to SPPs launched with p-polarized excitation but propagates with a distinct spatial profile. The spatial and temporal properties of the nascent SPP are determined by splitting the femtosecond pulse into a spatially separated pump-probe pair of orthogonal polarizations. The s-polarized pump pulse initiates the SPP, which is then visualized by the photoelectron emission induced by a spatially and temporally separated p-polarized probe pulse. The s-polarization launched SPP displays a bifurcated spatial structure with an antisymmetric mirror plane and may be regarded as two spatially distinct, temporally phase-locked wave packets. Significantly, the wave packets are one-half period out of phase with each other governed by the phase of the driving laser field. Finite difference time domain calculations corroborate the experimental results. The resulting SPP can be utilized for either polarization multiplexing or as a pulse splitter in nanophotonic circuits.

Visualizing Electric Fields at Au(111) Step Edges via Tip-Enhanced Raman Scattering.

Tip-enhanced Raman scattering (TERS) can be used to image plasmon-enhanced local electric fields on the nanoscale. This is illustrated through ambient TERS measurements recorded using silver atomic force microscope tips coated with 4-mercaptobenzonitrile molecules and used to image step edges on an Au(111) surface. The observed two-dimensional TERS images uniquely map electric fields localized at Au(111) step edges following 671 nm excitation. We establish that our measurements are not only sensitive to spatial variations in the enhanced electric fields but also to their vector components. We also experimentally demonstrate that (i) few nanometer precision is attainable in TERS nanoscopy using corrugated tips with nominal radii on the order of 100-200 nm, and (ii) TERS signals do not necessarily exhibit the expected E4 dependence. Overall, we illustrate the concept of electric field imaging via TERS and establish the connections between our observations and conventional TERS chemical imaging measurements.

Surface Plasmon Coupling and Control Using Spherical Cap Structures.

Propagating surface plasmons (PSPs) launched from a protruded silver spherical cap structure are investigated using photoemission electron microscopy (PEEM) and finite difference time domain (FDTD) calculations. Our combined experimental and theoretical findings reveal that PSP coupling efficiency is comparable to conventional etched-in plasmonic coupling structures. Additionally, plasmon propagation direction can be varied by linear rotation of the driving laser polarization. A simple geometric model is proposed in which the plasmon direction selectivity is proportional to the projection of the linear laser polarization on the surface normal. A application for the spherical cap coupler as a gate device is proposed. Overall, our results indicate that protruded cap structures hold great promise as elements in emerging surface plasmon applications.

Polarization-Directed Surface Plasmon Polariton Launching.

The relative intensities of propagating surface plasmons (PSPs) simultaneously launched from opposing edges of a symmetric trench structure etched into a silver thin film may be controllably varied by tuning the linear polarization of the driving field. This is demonstrated through transient multiphoton photoemission electron microscopy measurements performed using a pair of spatially separated phase-locked femtosecond pulses. Our measurements are rationalized using finite-difference time domain simulations, which reveal that the coupling efficiency into the PSP modes is inversely proportional to the magnitude of the localized surface plasmon fields excited at the trench edges. Our combined experimental and computational results allude to the interplay between localized and propagating surface plasmon modes in the trench; strong coupling to the localized modes at the edges correlates to weak coupling to the PSP modes. Polarization-directed PSP launching measurements reveal an optimal PSP contrast ratio of 4.2 using a 500 nm wide trench.

A nanosecond pulsed laser heating system for studying liquid and supercooled liquid films in ultrahigh vacuum.

A pulsed laser heating system has been developed that enables investigations of the dynamics and kinetics of nanoscale liquid films and liquid/solid interfaces on the nanosecond time scale in ultrahigh vacuum (UHV). Details of the design, implementation, and characterization of a nanosecond pulsed laser system for transiently heating nanoscale films are described. Nanosecond pulses from a Nd:YAG laser are used to rapidly heat thin films of adsorbed water or other volatile materials on a clean, well-characterized Pt(111) crystal in UHV. Heating rates of ∼10(10) K/s for temperature increases of ∼100-200 K are obtained. Subsequent rapid cooling (∼5 × 10(9) K/s) quenches the film, permitting in-situ, post-heating analysis using a variety of surface science techniques. Lateral variations in the laser pulse energy are ∼±2.7% leading to a temperature uncertainty of ∼±4.4 K for a temperature jump of 200 K. Initial experiments with the apparatus demonstrate that crystalline ice films initially held at 90 K can be rapidly transformed into liquid water films with T > 273 K. No discernable recrystallization occurs during the rapid cooling back to cryogenic temperatures. In contrast, amorphous solid water films heated below the melting point rapidly crystallize. The nanosecond pulsed laser heating system can prepare nanoscale liquid and supercooled liquid films that persist for nanoseconds per heat pulse in an UHV environment, enabling experimental studies of a wide range of phenomena in liquids and at liquid/solid interfaces.

Enhanced Raman scattering from aromatic dithiols electrosprayed into plasmonic nanojunctions.

We describe surface enhanced Raman spectroscopy (SERS) experiments in which molecular coverage is systematically varied from 3.8 × 10(5) to 3.8 × 10(2) to 0.38 molecules per µm(2) using electrospray deposition of ethanolic 4,4'-dimercaptostilbene (DMS) solutions. The plasmonic SERS substrate used herein consists of a well-characterized 2-dimensional (2D) array of silver nanospheres (see El-Khoury et al., J. Chem. Phys., 2014, 141, 214308), previously shown to feature uniform topography and plasmonic response, as well as intense SERS activity. When compared to their ensemble averaged analogues, the spatially and temporally averaged spectra of a single molecule exhibit several unique features including: (i) distinct relative intensities of the observable Raman-active vibrational states, (ii) more pronounced SERS backgrounds, and (iii) broader Raman lines indicative of faster vibrational dephasing. The first observation may be understood on the basis of an intuitive physical picture in which the removal of averaging over multiple molecules exposes the tensorial nature of Raman scattering. When an oriented single molecule gives rise to the recorded SERS spectra, the relative orientation of the molecule with respect to vector components of the local electric field determines the relative intensities of the observable vibrational states. Using a single molecule SERS framework, described herein, we derive a unique molecular orientation in which a single DMS molecule is isolated at a nanojunction formed between two silver nanospheres in the 2D array. The DMS molecule is found lying nearly flat with respect to the metal. The derived orientation of a single molecule at a plasmonic nanojunction is consistent with observations (ii) and (iii). In particular, a careful inspection of the temporal spectral variations along the recorded single molecule SERS time sequences reveals that the time-averaged SERS backgrounds arise from individual molecular events, marked by broadened SERS signatures. We assign the broadened spectra along the SERS time sequence--which sum up to a SERS background in the averaged spectra--to instances in which the π-framework of the DMS molecule is parallel to the metal at a classical plasmonic nanojunction. This also accounts for Raman line broadening as a result of fast vibrational dephasing, and driven by molecular reorientation at a plasmonic nanojunction. Furthermore, we report on the molecular orientation dependence of single molecule SERS enhancement factors. We find that in the case of a single DMS molecule isolated at a plasmonic nanojunction, molecular orientation may affect the derived single molecule SERS enhancement factor by up to 5 orders of magnitude. Taking both chemical effects as well as molecular orientation into account, we were able to estimate a single molecule enhancement factor of ∼10(10) in our measurements.

Ultrafast imaging of surface plasmons propagating on a gold surface.

We record time-resolved nonlinear photoemission electron microscopy (tr-PEEM) images of propagating surface plasmons (PSPs) launched from a lithographically patterned rectangular trench on a flat gold surface. Our tr-PEEM scheme involves a pair of identical, spatially separated, and interferometrically locked femtosecond laser pulses. Power-dependent PEEM images provide experimental evidence for a sequential coherent nonlinear photoemission process, in which one laser source launches a PSP through a linear interaction, and the second subsequently probes the PSP via two-photon photoemission. The recorded time-resolved movies of a PSP allow us to directly measure various properties of the surface-bound wave packet, including its carrier wavelength (783 nm) and group velocity (0.95c). In addition, tr-PEEM images reveal that the launched PSP may be detected at least 250 µm away from the coupling trench structure.

Tip-enhanced Raman nanographs: mapping topography and local electric fields.

We report tip-enhanced Raman imaging experiments in which information on sample topography and local electric fields is simultaneously obtained using an all-optical detection scheme. We demonstrate how a Raman-active 4,4'-dimercaptostilbene (DMS)-coated gold tip of an atomic force microscope can be used to simultaneously map the topography and image the electric fields localized at nanometric (20 and 5 nm wide) slits lithographically etched in silver, all using optical signals. Bimodal imaging is feasible by virtue of the frequency-resolved optical response of the functionalized metal probe. Namely, the probe position-dependent signals can be subdivided into two components. The first is a 500-2250 cm(-1) Raman-shifted signal, characteristic of the tip-bound DMS molecules. The molecules report on topography through the intensity contrast observed as the tip scans across the nanoscale features. The variation in molecular Raman activity arises from the absence/formation of a plasmonic junction between the scanning probe and patterned silver surface, which translates into dimmed/enhanced Raman signatures of DMS. Using these molecular signals, we demonstrate that sub-15 nm spatial resolution is attainable using a 30 nm DMS-coated gold tip. The second response consists of two correlated sub-500 cm(-1) signals arising from mirror-like reflections of (i) the incident laser field and (ii) the Raman scattered response of an underlying glass support (at 100-500 cm(-1)) off the gold tip. We show that both the reflected low-wavenumber signals trace the local electric fields in the vicinity of the nanometric slits.

Electric field enhancement in a self-assembled 2D array of silver nanospheres.

We investigate the plasmonic properties of a self-assembled 2D array of Ag nanospheres (average particle diameter/inter-particle separation distance of 9/3.7 nm). The structures of the individual particles and their assemblies are characterized using high-resolution transmission electron microscopy (HR-TEM). The plasmonic response of the nanoparticle network is probed using two-photon photoemission electron microscopy (TP-PEEM). HR-TEM and TP-PEEM statistics reveal the structure and plasmonic response of the network to be homogeneous on average. This translates into a relatively uniform surface-enhanced Raman scattering (SERS) response from biphenyl,4-4(')-dithiol (BPDT) molecules adsorbed onto different sites of the network. Reproducible, bright, and low-background SERS spectra are recorded and assigned on the basis of density functional theory calculations in which BPDT is chemisorbed onto the vertex of a finite tetrahedral Ag cluster consisting of 20 Ag atoms. A notable agreement between experiment and theory allows us to rigorously account for the observable vibrational states of BPDT in the ∼200-2200 cm(-1) region of the spectrum. Finite difference time domain simulations further reveal that physical enhancement factors on the order of 10(6) are attainable at the nanogaps formed between the silver nanospheres in the 2D array. Combined with modest chemical enhancement factors, this study paves the way for reproducible single molecule signals from an easily self-assembled SERS substrate.

Metal-insulator photocathode heterojunction for directed electron emission.

We use angle-resolved photoemission under ultraviolet laser excitation to demonstrate that the electron emission properties of Ag(001) can be markedly enhanced and redirected along the surface normal by the deposition of a few monolayers of epitaxial MgO. We observe new low-binding energy states with small spreads in their surface parallel momenta as a result of MgO/Ag(001) interface formation. Under 4.66 eV laser excitation, the quantum efficiency of MgO/Ag(001) is a factor of 7 greater than that of clean Ag(001), revealing the utility of such heterojunctions as advanced photocathodes.

Interferometric Plasmonic Lensing with Nanohole Arrays.

Nonlinear photoemission electron microscopy (PEEM) of nanohole arrays in gold films is used to map propagating surface plasmons (PSPs) launched from lithographically patterned structures. Strong near-field photoemission patterns are observed in the PEEM images, recorded following low angle of incidence irradiation of nanohole arrays with sub-15 fs laser pulses centered at 780 nm. The recorded photoemission patterns are attributed to constructive and destructive interference between PSPs launched from the individual nanoholes which comprise the array. By exploiting the wave nature of PSPs, we demonstrate how varying the array geometry (hole diameter, pitch, and number of rows/columns) ultimately yields intense localized photoemission. Through a combination of PEEM experiments and finite-difference time-domain simulations, we identify the optimal array geometry for efficient light coupling and interferometric plasmonic lensing. We show a preliminary application of inteferometric plasmonic lensing by enhancing the photoemission from the vertex of a gold triangle using a nanohole array.

Plasmon-induced optical field enhancement studied by correlated scanning and photoemission electron microscopy.

We use multi-photon photoemission electron microscopy (PEEM) to image the enhanced electric fields of silver nanoparticles supported on a silver thin film substrate. Electromagnetic field enhancement is measured by comparing the photoelectron yield of the nanoparticles with respect to the photoelectron yield of the surrounding silver thin film. We investigate the dependence of the photoelectron yield of the nanoparticle as a function of size and shape. Multi-photon PEEM results are presented for three average nanoparticle diameters: 34, 75, and 122 nm. The enhancement in photoelectron yield of single nanoparticles illuminated with femtosecond laser pulses (400 nm, ~3.1 eV) is found to be a factor of 10(2) to 10(3) times greater than that produced by the flat silver thin film. High-resolution, multi-photon PEEM images of single silver nanoparticles reveal that the greatest enhancement in photoelectron yield is localized at distinct regions near the surface of the nanoparticle whose magnitude and spatial extent is dependent on the incident electric field polarization. In conjunction with correlated scanning electron microscopy (SEM), nanoparticles that deviate from nominally spherical shapes are found to exhibit irregular spatial distributions in the multi-photon PEEM images that are correlated with the unique shape and topology of the nanoparticle.

Near-field focused photoemission from polystyrene microspheres studied with photoemission electron microscopy.

We use photoemission electron microscopy (PEEM) to image 3 µm diameter polystyrene spheres supported on a metal thin film illuminated by 400 nm (∼3.1 eV) and 800 nm (∼1.5 eV) femtosecond (fs) laser pulses. Intense photoemission is generated by microspheres even though polystyrene is an insulator and its ionization threshold is well above the photon energies employed. We observe intense photoemission from the far side (the side opposite the incident light) of the illuminated microsphere that is attributed to light focusing within the microsphere. For the case of p-polarized, 800 nm fs laser pulses, we observe photoemission exclusively from the far side of the microsphere and additionally resolve sub-50 nm hot spots in the supporting Pt∕Pd thin film that are located only within the focal region of the microsphere. We compare the PEEM images with finite difference time domain (FDTD) electrodynamic simulations to model our experimental results. The FDTD simulations predict light focusing in the microsphere and subsequent interaction with the supporting metal surface that is consistent with the experimental observations.