The tip is key to the successful execution of tip-enhanced Raman scattering (TERS) measurements in the single molecule limit. We show that nanoscopically smooth silver tips, batch produced through ...field-directed sputter sharpening, reliably attain TERS with enhancement factors that reach 1013, as measured by the Raman spectra of single CO molecules attached to the tip apex. We validate the bare tips by demonstrating spectromicroscopy with submolecular spatial resolution and underscore that TERS is a near-field effect that does not obey simple selection rules. As a more gainful analytical approach, we introduce TERS-relayed molecular force microscopy using CO-terminated tips. By taking advantage of the large Stark tuning rate of the CO stretch, molecular structure and charges can be imaged with atomic resolution. As illustration, we image a single Ag atom adsorbed on Au(111) and show that the adatom carries +0.2e charge.
Atomically terminated and nanoscopically smooth silver tips effectively focus light on the angstrom scale, allowing tip-enhanced Raman spectromicroscopy (TER-sm) with single molecule sensitivity and ...submolecular spatial resolution. Through measurements carried out on cobalt-tetraphenylporphyrin (CoTPP) adsorbed on Au(111), we highlight peculiarities of vibrational spectromicroscopy with light confined on the angstrom scale. Field-gradient-driven spectra, orientational fingerprinting, and sculpting of local fields by atomic morphology of the junction are elucidated through measurements that range from 2D arrays at room temperature to single molecule manipulations at 5 K. Notably, vibrational Stark tuning within molecules, reflecting intramolecular charge distributions, becomes accessible when light is more localized than the interrogated normal modes. The Stark images of CoTPP reveal that it is saddled, and the distortion is accompanied by charge transfer to gold through the two anchoring pyrroles.
The structure and ultrafast photodynamics of ∼8 nm Au@Pt core–shell nanocrystals with ultrathin (<3 atomic layers) Pt–Au alloy shells are investigated to show that they meet the design principles for ...efficient bimetallic plasmonic photocatalysis. Photoelectron spectra recorded at two different photon energies are used to determine the radial concentration profile of the Pt–Au shell and the electron density near the Fermi energy, which play a key role in plasmon damping and electronic and thermal conductivity. Transient absorption measurements track the flow of energy from the plasmonic core to the electronic manifold of the Pt shell and back to the lattice of the core in the form of heat. We show that strong coupling to the high density of Pt(d) electrons at the Fermi level leads to accelerated dephasing of the Au plasmon on the femtosecond time scale, electron–electron energy transfer from Au(sp) core electrons to Pt(d) shell electrons on the sub-picosecond time scale, and enhanced thermal resistance on the 50 ps time scale. Electron–electron scattering efficiently funnels hot carriers into the ultrathin catalytically active shell at the nanocrystal surface, making them available to drive chemical reactions before losing energy to the lattice via electron–phonon scattering on the 2 ps time scale. The combination of strong broadband light absorption, enhanced electromagnetic fields at the catalytic metal sites, and efficient delivery of hot carriers to the catalyst surface makes core–shell nanocrystals with plasmonic metal cores and ultrathin catalytic metal shells promising nanostructures for the realization of high-efficiency plasmonic catalysts.
Tip-enhanced Raman spectromicroscopy (TERS) with CO-terminated plasmonic tips can probe angstrom-scale features of molecules on surfaces. The development of this technique requires understanding of ...how chemical environments affect the CO vibrational frequency and TERS intensity. At the scanning tunneling microscope junction of a CO-terminated Ag tip, we show that rather than the classical vibrational Stark effect, the large bias dependence of the CO frequency shift is due to ground-state charge transfer from the Ag tip into the CO π* orbital softening the C–O bond at more positive biases. The associated increase in Raman intensity is attributed to a bias-dependent chemical enhancement effect, where a positive bias tunes a charge-transfer excited state close to resonance with the Ag plasmon. This change in Raman intensity is contrary to what would be expected based on changes in the tilt angle of the CO molecule with bias, demonstrating that the Raman intensity is dominated by electronic rather than geometric effects.
Electroluminescence (EL) in scanning tunneling microscopy (STM), which enables spectroscopy with submolecular spatial resolution, is shown to be due to radiative ionization with vibronic shape ...resonances that carry Fano line profiles. Since Fano progressions retain phase information, the spectra can be transformed to the time domain to reconstruct the vibronic motion. In effect, measurements within a molecule are accessible with joint space–time resolution at the Å–fs limit. We demonstrate this through EL-STM on the Jahn–Teller-active Zn-etioporphyrin radical anion and visualize the orbiting motion of scattered electrons upon sudden reduction and oxidation. We discuss the elements that enable spectroscopy with submolecular spatial resolution through EL-STM and the closely related STM-Raman process.
While most active plasmonic efforts focus on responsive metamaterials to modulate optical response, we present a simple alternative based on applied orientation control that can likely be implemented ...for many passive plasmonic materials. Passive plasmonic motifs are simpler to prepare but cannot be altered postfabrication. We show that such systems can be easily manipulated through substrate orientation control to generate both active plasmonic and active chiral plasmonic responses. Using gold nanocrescents as our model platform, we demonstrate tuning of optical extinction from −21% to +36% at oblique incidence relative to normal incidence. Variation of substrate orientation in relation to incident polarization is also demonstrated to controllably switch chiroptical handedness (e.g., Δg = ± 0.55). These active plasmonic responses arise from the multipolar character of resonant modes. In particular, we correlate magnetoelectric and dipole–quadrupole polarizabilities with different light-matter orientation-dependence in both near- and far-field localized surface plasmon activity. Additionally, the attribution of far-field optical response to higher-order multipoles highlights the sensitivity offered by these orientation-dependent characterization techniques to probe the influence of localized electromagnetic field gradients on a plasmonic response. The sensitivity afforded by orientation-dependent optical characterization is further observed by the manifestation in both plasmon and chiral plasmon responses of unpredicted structural nanocrescent variance (e.g., left- and right-tip asymmetry) not physically resolved through topographical imaging.
We demonstrate a conductance switch controlled by the spin-vibronic density of an odd electron on a single molecule. The junction current is modulated by the spin-flip bistability of the electron. ...Functional images are provided as wiring diagrams for control of the switch’s frequency, amplitude, polarity, and duty-cycle. The principle of operation relies on the quantum mechanical phase associated with the adiabatic circulation of a spin-aligned electron around a conical intersection. The functional images quantify the governing vibronic Hamiltonian.
Vibrational energy transfer has been studied experimentally, both in the gas and solid phases, with the aim of obtaining a description of the equilibration pathways in multimode systems and obtaining ...specific state-to-state rate constants. The experimental results are compared with theoretical models in an attempt to understand the dynamics underlying the exchange of energy among the different degrees of freedom of two interacting systems. The experimental method used in these studies is that of laser induced fluorescence. It is shown that a detailed kinetic treatment is essential for extracting state-to-state rate constants from the experimental observables. Thus the general kinetic problem of vibrational equilibration of a multimode system is considered in some detail. Some mathematical and experimental techniques are introduced for the reduction of experimental data. It is shown that a careful study of the rare gas dependence of rise rates is a powerful tool for determining whether the steps leading to the population of a given state are resonant or nonresonant. Once a resonant step has been identified by this technique, the magnitude of the rate constant associated with that step can be measured. This is particularly useful since resonant rate constants are particularly difficult to obtain by direct methods. An analytical description of the time evolution of the population of a state is very important because of its predictive power. A convolution method is introduced which can be conveniently applied to obtain analytical expressions in multimode systems. The method is especially useful for manifolds with "stacked" configurations of states. The method is used to explicitly obtain solutions of a five level system and the generalization to an arbitrary number of levels is detailed. To verify the validity of these methods and the conclusions reached with their applications, extensive use of numerical modeling is made. A numerical solution is possible for any kinetic system. Thus the understanding of overall equilibration routes in a molecule can be tested by numerical modeling. These computational methods are succinctly described. Gas phase CH(,3)F studies served as a testing ground for the techniques introduced. As a result it was possible to unambiguously determine the pathways leading to the population of {(nu)(,1),(nu)(,4)} subsequent to exciting (nu)(,3). Additionally, it was possible to measure the rate constant for the first "ladder step" up the {(nu)(,2),(nu)(,5)} manifold and to establish upper and lower limits on many rate constants which had not been previously measured. Gas phase studies of CH(,2)D(,2) afforded a near complete map of energy transfer pathways amongst the states below 3000 cm('-1) and some specific rate constants. When compared with theoretical models, it could be concluded that V-V processes in CH(,2)D(,2) are dominated by hard-core collisional processes as opposed to long range energy transfer. SSH calculations proved to be quite successful in obtaining V-V rates. From the observed rare gas dependence of deactivation rates, it could be concluded that the rotational channel is important and that a simple V-T model is insufficient for explaining observed trends. Studies of vibrational energy transfer and deactivation of CH(,3)F in low temperature Kr and Xe matrices are presented. It is shown that CH(,3)F trapped in matrices as dilute as (TURN)10('4):1 belong to the fast migration regime and thus ordinary kinetic rate equations and concepts of vibrational temperatures are applicable. The temperature, concentration and power dependence of rise and fall rates can be interpreted according to a simple kinetic model when a single phonon assistance is included for both V-V and relaxation processes. Additionally, barriers to rotation are established from the high resolution FTIR spectra and a hole burning effect is observed at low temperatures.
PDF
