Ion-Selective, Atom-Resolved Imaging of a 2D Cu2N Insulator: Field and Current Driven Tip-Enhanced Raman Spectromicroscopy Using a Molecule-Terminated Tip.

Tip-enhanced Raman scattering (TERS) with a cobalt tetraphenylporphyrin (CoTPP)- terminated silver tip is used to obtain ion-selective, atomically resolved images of an insulating Cu2N monolayer grown on Cu(100). Ion selective images are obtained through vibrational frequency shift maps using CoTPP vibrations with oppositely signed Stark tuning rates (STR). The images allow a quantitative analysis of the electrostatic field of the ionic lattice using in situ calibrated STRs. Both intensity and Stark shift maps yield atomically resolved images in the tunneling regime of plasmons. We show that the CoTPP is bonded to the Ag tip through its central Co atom, whereby TERS taps into intramolecular currents and polarizations. The bias dependence of vibrational line intensities shows diode-like response with opposite polarity for current carrying modes of opposite polarization phase. The phase sensitive detection of vibrational lines and their voltage gating is explained in terms of distinct field- and phototunneling current-driven Raman, offering an alternate paradigm for the long-sought optoelectronic rectifier in molecular electronics.

Visualizing vibrational normal modes of a single molecule with atomically confined light.

The internal vibrations of molecules drive the structural transformations that underpin chemistry and cellular function. While vibrational frequencies are measured by spectroscopy, the normal modes of motion are inferred through theory because their visualization would require microscopy with ångström-scale spatial resolution-nearly three orders of magnitude smaller than the diffraction limit in optics1. Using a metallic tip to focus light and taking advantage of the surface-enhanced Raman effect2 to amplify the signal from individual molecules, tip-enhanced Raman spectromicroscopy (TER-SM)3,4 reaches the requisite sub-molecular spatial resolution5, confirming that light can be confined in picocavities6-10 and anticipating the direct visualization of molecular vibrations11-13. Here, by using TER-SM at the precisely controllable junction of a cryogenic ultrahigh-vacuum scanning tunnelling microscope14-16, we show that ångström-scale resolution is attained at subatomic separation between the tip atom and a molecule in the quantum tunnelling regime of plasmons6,8,9,17. We record vibrational spectra within a single molecule, obtain images of normal modes and atomically parse the intramolecular charges and currents driven by vibrations. Our analysis provides a paradigm for optics in the atomistic near-field.

Junction Plasmon Driven Population Inversion of Molecular Vibrations: A Picosecond Surface-Enhanced Raman Spectroscopy Study.

Molecular surface-enhanced Raman spectra recorded at single plasmonic nanojunctions using a 7 ps pulse train exhibit vibrational up-pumping and population inversion. The process is assigned to plasmon-driven, dark, impulsive electron-vibration (e-v) excitation. Both optical (Raman) pumping and hot-electron mediated excitation can be rejected by the characteristic spectra, which allow the simultaneous measurement of vibrational temperature of the molecules and electronic temperature of the metal. Vibrational populations are determined from anti-Stokes to Stokes intensity ratios, while the electron temperature is obtained from the anti-Stokes branch of the electronic Raman scattering continuum. Population inversion survives in high-frequency vibrations that effectively decouple from the metal.

Microscopy with a single-molecule scanning electrometer.

The vibrational spectrum of a single carbon monoxide molecule, adsorbed on the tip apex of a scanning tunneling microscope, is used to image electrostatic fields with submolecular spatial resolution. The method takes advantage of the vibrational Stark effect to image local electrostatic fields and the single-molecule sensitivity of tip-enhanced Raman scattering (TERS) to optically relay the signal. We apply the method to single metalloporphyrins adsorbed on Au(111) to image molecular charges, intramolecular polarization, local photoconductivity, atomically resolved hydrogen bonds, and surface electron density waves.

Hovering and Twirling of Tethered Molecules by Confinement between Surfaces.

Through STM images, we show that azobenzene-terminated alkanethiols hover and twirl when confined between the Ag tip and Au(111) substrate of an STM junction. In contrast with mechanisms of activation used to drive molecular rotors, twirling is induced by the effective elimination of lateral corrugation in the energy landscape when molecules hover by their van der Waals attraction to the approaching tip. While in the stationary state the benzenes of the head group lie flat with an inter-ring separation of 7.5 Å, they stand on-edge as the molecule twirls and their separation contracts to 5.2 Å, close to the value of the free molecule. The captured images of motion allow the characterization of physisorption potentials.

A theoretical simulation of the resonant Raman spectroscopy of the H2O⋯Cl2 and H2O⋯Br2 halogen-bonded complexes.

The resonant Raman spectra of the H2O⋯Cl2 and H2O⋯Br2 halogen-bonded complexes have been studied in the framework of a 2-dimensional model previously used in the simulation of their UV-visible absorption spectra using time-dependent techniques. In addition to the vibrational progression along the dihalogen mode, a progression is observed along the intermolecular mode and its combination with the intramolecular one. The relative intensity of the inter to intramolecular vibrational progressions is about 15% for H2O⋯Cl2 and 33% for H2O⋯Br2. These results make resonant Raman spectra a potential tool for detecting the presence of halogen bonded complexes in condensed phase media such as clathrates and ice.

Note: Automated electrochemical etching and polishing of silver scanning tunneling microscope tips.

Fabrication of sharp and smooth Ag tips is crucial in optical scanning probe microscope experiments. To ensure reproducible tip profiles, the polishing process is fully automated using a closed-loop laminar flow system to deliver the electrolytic solution to moving electrodes mounted on a motorized translational stage. The repetitive translational motion is controlled precisely on the µm scale with a stepper motor and screw-thread mechanism. The automated setup allows reproducible control over the tip profile and improves smoothness and sharpness of tips (radius 27 ± 18 nm), as measured by ultrafast field emission.

Quantum tomography of a molecular bond in ice.

We present the moving picture of a molecular bond, in phase-space, in real-time, at resolution limited by quantum uncertainty. The images are tomographically reconstructed Wigner distribution functions (WDF) obtained from four-wave mixing measurements on Br2-doped ice. The WDF completely characterizes the dissipative quantum evolution of the system, which despite coupling to the environment retains quantum coherence, as evidenced by its persistent negative Wigner hole. The spectral decomposition of the WDF allows a direct visualization of wavefunctions and spatiotemporal coherences of the system and the system-bath interaction. The measurements vividly illustrate nonclassical wave mechanics in a many-body system, in ordinary condensed matter.

Dynamics behind the long-lived coherences of I2 in solid Xe.

The absorption spectrum of I2 in solid Xe shows resolved zero-phonon lines and phonon side bands near the origin of the B←X transition (550-625 nm). The long-lived |B⟩⟨X| coherence in this energy range (T2 = 600 fs on average) emerges as vibrationally unrelaxed fluorescence in resonance Raman (RR) spectra. Upon excitation in the structureless continuum at 532 nm, the oscillatory RR progression exhibits electronic dephasing time of T2 = 150 fs. Two RR progressions with markedly different vibrational coherence on the X-state are observed. The main progression of sharp overtones (T2 > 21 ps) is assigned to molecules trapped in double-substitution sites. The minor progression, which shows dephasing times T2 = 6-0.6 ps for v = 1-8, is assigned to molecules in triple-substitution sites. The line progressions allow a detailed characterization of the solvated B- and X-state potentials. Time-resolved coherent anti-Stokes Raman scattering is used to probe selected vibrational coherences on the X-state. Assignments are obtained through molecular dynamics simulations, which reproduce the relative dephasing rates between the two sites, clarify the role of rotation-translation dynamics, and enable quantum dynamics simulations of the spectra by the potentials of mean force that accurately describe the molecule-surrounding interactions.

Nonlinear femtosecond laser induced scanning tunneling microscopy.

We demonstrate ultrafast laser driven nonlinear scanning tunneling microscopy (STM), under ambient conditions. The design is an adaptation of the recently introduced cross-polarized double beat method, whereby z-polarized phase modulated fields are tightly focused at a tunneling junction consisting of a sharp tungsten tip and an optically transparent gold film as substrate. We demonstrate the prerequisites for ultrafast time-resolved STM through an operative mechanism of nonlinear laser field-driven tunneling. The spatial resolution of the nonlinear laser driven STM is determined by the local field intensity. Resolution of 0.3 nm-10 nm is demonstrated for the intensity dependent, exponential tunneling range. The demonstration is carried out on a junction consisting of tungsten tip and gold substrate. Nano-structured gold is used for imaging purposes, to highlight junction plasmon controlled tunneling in the conductivity limit.

Raman scattering at plasmonic junctions shorted by conductive molecular bridges.

Intensity spikes in Raman scattering, accompanied by switching between line spectra and band spectra, can be assigned to shorting the junction plasmon through molecular conductive bridges. This is demonstrated through Raman trajectories recorded at a plasmonic junction formed by a gold AFM tip in contact with a silver surface coated either with biphenyl-4,4'-dithiol or biphenyl-4-thiol. The fluctuations are absent in the monothiol. In effect, the making and breaking of chemical bonds is tracked.

Laser-induced scanning tunneling microscopy: Linear excitation of the junction plasmon.

We introduce the cross-polarized double-beat method for localized excitation of the junction plasmon of a scanning tunneling microscope with femtosecond laser pulses. We use two pulse trains derived from a Ti:sapphire laser operating at a repetition frequency of f(s)=76 MHz, with a relative shift between their carrier frequencies ω(a)/2π=f(s)+f(b) controlled with an acousto-optic modulator. The trains are cross-polarized and collinearly focused on the junction, ensuring constant radiation flux. The anisotropic susceptibility of the junction plasmon mixes the fields, which modulate the tunneling current at f(b) (the difference between carrier beat and repetition frequency) at base-band frequencies that can be used for direct detection of the tunneling current. The interferometric cross-correlation of the pulses and the polarization dependence of the mixing identify the coupling to the radiation to be through the coherent z-displacement of the tip plasmon. Single Ag atoms are used to demonstrate microscopy under irradiation. In the linear coupling regime, the laser-induced displacement of the plasmon is operationally indistinguishable from the mechanical displacement of the junction gap.

Solidlike coherent vibronic dynamics in a room temperature liquid: Resonant Raman and absorption spectroscopy of liquid bromine.

UV-visible absorption and resonance Raman (RR) spectra of liquid bromine are presented and rigorously interpreted. The RR spectra, which show an anharmonic vibrational progression of up to 30 overtones, define the ground state potential in the range 2.05 Aor=25 to >or=2.4 ps between v=1 and v=25. Remarkably, the RR line shapes are skewed toward the red, indicating upchirp in frequencies that develop over a period of 400 fs. Evidently, the molecular vibrations adiabatically follow the solvent cage, which is impulsively driven into expansion during the approximately 20 fs evolution on the electronically excited state. Liquid bromine retains coherence in ordered sluggish local cages with quadrupolar interactions-dynamics akin to molecules isolated in structured cryogenic rare gas solids.

Graphitic electrical contacts to metallic single-walled carbon nanotubes using Pt electrodes.

We investigate electronic devices consisting of individual, metallic, single-walled carbon nanotubes contacted by Pt electrodes in a field effect transistor configuration, focusing on improvements to the metal-nanotube contact resistance as the devices are annealed in inert environments including ultrahigh vacuum. At moderate temperatures (T < 880 K), thermal processing results in high resistance contacts with thermally activated barriers. Higher temperatures (T > 880 K) achieve nearly transparent contacts. In the latter case, analytical surface measurements reveal the catalytic decomposition of hydrocarbons into graphene layers on the Pt surface, suggesting that improved electronic behavior is primarily due to the formation of an all-carbon nanotube-graphite interface rather than to the improvement of the nanotube-Pt one.

Spectroscopic signatures of halogens in clathrate hydrate cages. 2. Iodine.

UV-vis and Raman spectroscopy were used to study iodine molecules trapped in sII clathrate hydrate structures stabilized by THF, CH(2)Cl(2), or CHCl(3). The spectra show that the environment for iodine inside the water cage is significantly less perturbed than either in aqueous solution or in amorphous water-ice. The resonance Raman progression of I(2) in THF clathrate hydrate can be observed up to v = 6 when excited at 532 nm. The extracted vibrational frequency omega e = 214 +/- 1 cm(-1) is the same as that of the free molecule to within experimental error. At the same time, the UV-vis absorption spectrum of I(2) in the sII hydrate exhibits a relatively large, 1440 cm(-1), blue-shift. This is mainly ascribed to the differential solvation of the I(2) electronic states. We conclude that iodine in sII hydrate resides in a 5(12)6(4) cavity, in which the ground-state I(2) potential is not significantly perturbed by the hydrate lattice. In contrast, in water and in ice, the valence absorption band of I(2) is dramatically broadened and blue-shifted by 3000 cm(-1), and the resonance Raman scattering is effectively quenched. These observations are shown to be consistent with a strong interaction between water molecule and iodine through the lone pair of electrons on water as in the case of bromine in the same media. The results presented here, and the stability of other halogen hydrates, were used to test the predictions of simple models and force-field calculations of the host cage-guest association energy.

Spectroscopic signatures of halogens in clathrate hydrate cages. 1. Bromine.

We report the first UV-vis spectroscopic study of bromine molecules confined in clathrate hydrate cages. Bromine in its natural hydrate occupies 51262 and 51263 lattice cavities. Bromine also can be encapsulated into the larger 51264 cages of a type II hydrate formed mainly from tetrahydrofuran or dichloromethane and water. The visible spectra of the enclathrated halogen molecule retain the spectral envelope of the gas-phase spectra while shifting to the blue. In contrast, spectra of bromine in liquid water or amorphous ice are broadened and significantly more blue-shifted. The absorption bands shift by about 360 cm-1 for bromine in large 51264 cages of type II clathrate, by about 900 cm-1 for bromine in a combination of 51262 and 51263 cages of pure bromine hydrate, and by more than 1700 cm-1 for bromine in liquid water or amorphous ice. The dramatic shift and broadening in water and ice is due to the strong interaction of the water lone-pair orbitals with the halogen sigma* orbital. In the clathrate hydrates, the oxygen lone-pair orbitals are all involved in the hydrogen-bonded water lattice and are thus unavailable to interact with the halogen guest molecule. The blue shifts observed in the clathrate hydrate cages are related to the spatial constraints on the halogen excited states by the cage walls.

Nonresonant ionization of oxygen molecules by femtosecond pulses: plasma dynamics studied by time-resolved terahertz spectroscopy.

We show that optical pump-terahertz probe spectroscopy is a direct experimental tool for exploring laser-induced ionization and plasma formation in gases. Plasma was produced in gaseous oxygen by focused amplified femtosecond pulses. The ionization mechanisms at 400- and 800-nm excitation wavelengths differ significantly being primarily of a multiphoton character in the former case and a strong-field process in the latter case. The generation of the plasma in the focal volume of the laser and its expansion on subnanosecond time scale is directly monitored through its density-dependent susceptibility. A Drude model used to evaluate the plasma densities and electron-scattering rates successfully captures the observations for a wide range of pump intensities. In addition, rotational fingerprints of molecular and ionic species were also observed in the spectra.

Vibrational dissipation and dephasing of I2 (v = 1-19) in solid Kr.

Time- and frequency-resolved coherent anti-Stokes Raman scattering is used to carry out systematic measurements of vibrational dephasing on I2 (v = 1-19) isolated in solid Kr, as a function of temperature, T = 7-45 K. The observed quantum beats, omega(v', v") allow an accurate reconstruction of the solvated molecular potential, which is well represented by the Morse form: omega(e) = 211.56 +/- 0.14, omega(e)chi(e) = 0.658 +/- 0.006. Near T = 7 K, the coherence decay rates gamma(v,0) become independent of temperature and show a linear v-dependence, indicative of dissipation, which must be accompanied by the simultaneous creation of at least four phonons. At higher temperatures, the T-dependence is exponential and the v-dependence is quadratic, characteristic of pure dephasing via pseudo-local phonons. A normal mode analysis suggests librations as the principle modes responsible for pure dephasing.

Onset of decoherence: six-wave mixing measurements of vibrational decoherence on the excited electronic state of I2 in solid argon.

Pump-probe, four-wave, and six-wave mixing measurements of I2 isolated in solid argon are used to provide a clear experimental measure for the onset of vibrational quantum decoherence on the excited electronic state. The electronically resonant, six-wave mixing measurements bypass the rapid electronic dephasing, and measure the quantum cross-correlation between two packets launched on the B-state. The vibrational quantum coherence survives one period of motion, 400 fs, during which approximately 2000 cm(-1) of energy is transferred to the lattice. The decoherence occurs during the second cycle of motion, while classically coherent motion measured via pump-probe spectroscopy using the same electronic resonances continues for approximately 15 periods. This is contrasted with vibrational dephasing on the ground electronic surface, which lasts for 10(2) periods, as measured through time-resolved coherent anti-Stokes Raman scattering. The measurements and observables are discussed through time-circuit diagrams, and a mechanistic description of decoherence is derived through semiclassical analysis and simulations that reproduce the experiments.