Atomic Point Contact Raman Spectroscopy of a Si(111)-7 × 7 Surface

Cavity-Modified Chemiluminescent Reaction of Dioxetane

Chemiluminescence is a thermally activated chemical process that emits a photon of light by forming a fraction of products in the electronic excited state. A well-known example of this spectacular phenomenon is the emission of light in the firefly beetle, where the formation of a four-membered cyclic peroxide compound and subsequent dissociation produce a light-emitting product. The smallest cyclic peroxide, dioxetane, also exhibits chemiluminescence but with a low quantum yield as compared to that of firefly dioxetane. Employing the strong light-matter coupling has recently been found to be an alternative strategy to modify the chemical reactivity. In the presence of an optical cavity, the molecular degrees of freedom greatly mix with the cavity mode to form hybrid cavity-matter states called polaritons. These newly generated hybrid light-matter states manipulate the potential energy surfaces and significantly change the reaction dynamics. Here, we theoretically investigate the effects of a strong light-matter interaction on the chemiluminescent reaction of dioxetane using the extended Jaynes-Cummings model. The cavity couplings corresponding to the electronic and vibrational degrees of freedom have been included in the interaction Hamiltonian. We explore how the cavity alters the ground- and excited-state path energy barriers and reaction rates. Our results demonstrate that the formation of excited-state products in the dioxetane decomposition process can be either accelerated or suppressed, depending on the molecular orientation with respect to the cavity polarization.

Intermolecular and Electrode-Molecule Bonding in a Single Dimer Junction of Naphthalenethiol as Revealed by Surface-Enhanced Raman Scattering Combined with Transport Measurements

Electron transport through noncovalent interaction is of fundamental and practical importance in nanomaterials and nanodevices. Recent single-molecule studies employing single-molecule junctions have revealed unique electron transport properties through noncovalent interactions, especially those through a π-π interaction. However, the relationship between the junction structure and electron transport remains elusive due to the insufficient knowledge of geometric structures. In this article, we employ surface-enhanced Raman scattering (SERS) synchronized with current-voltage (I-V) measurements to characterize the junction structure, together with the transport properties, of a single dimer and monomer junction of naphthalenethiol, the former of which was formed by the intermolecular π-π interaction. The correlation analysis of the vibrational energy and electrical conductance enables identifying the intermolecular and molecule-electrode interactions in these molecular junctions and, consequently, addressing the transport properties exclusively associated with the π-π interaction. In addition, the analysis achieved discrimination of the interaction between the NT molecule and the Au electrode of the junction, i.e., Au-π interactions through-π coupling and though-space coupling. The power density spectra support the noncovalent character at the interfaces in the molecular junctions. These results demonstrate that the simultaneous SERS and I-V technique provides a unique means for the structural and electrical investigation of noncovalent interactions.

Imaging and controlling coherent phonon wave packets in single graphene nanoribbons

The motion of atoms is at the heart of any chemical or structural transformation in molecules and materials. Upon activation of this motion by an external source, several (usually many) vibrational modes can be coherently coupled, thus facilitating the chemical or structural phase transformation. These coherent dynamics occur on the ultrafast timescale, as revealed, e.g., by nonlocal ultrafast vibrational spectroscopic measurements in bulk molecular ensembles and solids. Tracking and controlling vibrational coherences locally at the atomic and molecular scales is, however, much more challenging and in fact has remained elusive so far. Here, we demonstrate that the vibrational coherences induced by broadband laser pulses on a single graphene nanoribbon (GNR) can be probed by femtosecond coherent anti-Stokes Raman spectroscopy (CARS) when performed in a scanning tunnelling microscope (STM). In addition to determining dephasing (~440 fs) and population decay times (~1.8 ps) of the generated phonon wave packets, we are able to track and control the corresponding quantum coherences, which we show to evolve on time scales as short as ~70 fs. We demonstrate that a two-dimensional frequency correlation spectrum unequivocally reveals the quantum couplings between different phonon modes in the GNR.

Inelastic Light Scattering in the Vicinity of a Single-Atom Quantum Point Contact in a Plasmonic Picocavity

Electromagnetic fields can be confined in the presence of metal nanostructures. Recently, subnanometer scale confinement has been demonstrated to occur at atomic protrusions on plasmonic nanostructures. Such an extreme field may dominate atomic-scale light-matter interactions in "picocavities". However, it remains to be elucidated how atomic-level structures and electron transport affect plasmonic properties of a picocavity. Here, using low-temperature optical scanning tunneling microscopy (STM), we investigate inelastic light scattering (ILS) in the vicinity of a single-atom quantum point contact (QPC). A vibration mode localized at the single Ag adatom on the Ag(111) surface is resolved in the ILS spectrum, resulting from tip-enhanced Raman scattering (TERS) by the atomically confined plasmonic field in the STM junction. Furthermore, we trace how TERS from the single adatom evolves as a function of the gap distance. The exceptional stability of the low-temperature STM allows to examine distinctly different electron transport regimes of the picocavity, namely, in the tunneling and QPC regimes. This measurement shows that the vibration mode localized at the adatom and its TERS intensity exhibits a sharp change upon the QPC formation, indicating that the atomic-level structure has a crucial impact on the plasmonic properties. To gain microscopic insights into picocavity optomechanics, we scrutinize the structure and plasmonic field in the STM junction using time-dependent density functional theory. The simulations reveal that atomic-scale structural relaxation at the single-atom QPC results in a discrete change of the plasmonic field strength, volume, and distribution as well as the vibration mode localized at the single atom. These findings give a qualitative explanation for the experimental observations. Furthermore, we demonstrate that strong ILS is a characteristic feature of QPC by continuously forming, breaking, and reforming the atomic contact and how the plasmonic resonance evolves throughout the nontunneling, tunneling, and QPC regimes.

Computational study on the catalytic control of endo/exo Diels-Alder reactions by cavity quantum vacuum fluctuations

Achieving control over chemical reaction's rate and stereoselectivity realizes one of the Holy Grails in chemistry that can revolutionize chemical and pharmaceutical industries. Strong light-matter interaction in optical or nanoplasmonic cavities might provide the knob to reach such control. In this work, we demonstrate the catalytic and selectivity control of an optical cavity for two selected Diels-Alder cycloaddition reactions using the quantum electrodynamics coupled cluster (QED-CC) method. Herein, we find that by changing the molecular orientation with respect to the polarization of the cavity mode the reactions can be significantly inhibited or selectively enhanced to produce major endo or exo products on demand. This work highlights the potential of utilizing quantum vacuum fluctuations of an optical cavity to modulate the rate of Diels-Alder cycloaddition reactions and to achieve stereoselectivity in a practical and non-intrusive way. We expect that the present findings will be applicable to a larger set of relevant reactions, including the click chemical reactions.

Development of low-temperature and ultrahigh-vacuum photoinduced force microscopy

In this paper, we develop optical and electronic systems for photoinduced force microscopy (PiFM) that can measure photoinduced forces under low temperature and ultrahigh vacuum (LT-UHV) without artifacts. For our LT-UHV PiFM, light is irradiated from the side on the tip-sample junction, which can be adjusted through the combination of an objective lens inside the vacuum chamber and a 90° mirror outside the vacuum chamber. We measured photoinduced forces due to the electric field enhancement between the tip and the Ag surface, and confirmed that photoinduced force mapping and measurement of photoinduced force curves were possible using the PiFM that we developed. The Ag surface was used to measure the photoinduced force with high sensitivity, and it is effective in enhancing the electric field using the plasmon gap mode between the metal tip and the metal surface. Additionally, we confirmed the necessity of Kelvin feedback during the measurement of photoinduced forces, to avoid artifacts due to electrostatic forces, by measuring photoinduced forces on organic thin films. The PiFM, operating under low temperature and ultrahigh vacuum developed here, is a promising tool to investigate the optical properties of various materials with very high spatial resolution.

Toward a New Era of SERS and TERS at the Nanometer Scale: From Fundamentals to Innovative Applications

Surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS) have opened a variety of exciting research fields. However, although a vast number of applications have been proposed since the two techniques were first reported, none has been applied to real practical use. This calls for an update in the recent fundamental and application studies of SERS and TERS. Thus, the goals and scope of this review are to report new directions and perspectives of SERS and TERS, mainly from the viewpoint of combining their mechanism and application studies. Regarding the recent progress in SERS and TERS, this review discusses four main topics: (1) nanometer to subnanometer plasmonic hotspots for SERS; (2) Ångström resolved TERS; (3) chemical mechanisms, i.e., charge-transfer mechanism of SERS and semiconductor-enhanced Raman scattering; and (4) the creation of a strong bridge between the mechanism studies and applications.

Multimodal (Non)Linear Optical Nanoimaging and Nanospectroscopy

This Perspective highlights recent advances in linear and nonlinear spectral nanoimaging. The described developments are motivated by the need to characterize molecular and material systems noninvasively with nanometer spatial and femtosecond temporal resolution. Indeed, the ability to image and chemically characterize heterogeneous interfaces with joint nano-femto resolution is a prerequisite to advancing our fundamental understanding of processes as diverse as heterogeneous catalysis, microbial communication, and energy flow in pristine/defect-containing low-dimensional quantum materials, to name a few. We describe pioneering work and recent demonstrations of (non)linear optical nanoimaging and nanospectroscopy, with an emphasis on high spatial resolution measurements conducted under ambient laboratory conditions.

Ultrashort Pulse Excited Tip-Enhanced Raman Spectroscopy in Molecules

Vibrational fingerprints of molecules and low-dimension materials can be traced with subnanometer resolution by performing Tip-enhanced Raman spectroscopy (TERS) in a scanning tunneling microscope (STM). Strong atomic-scale localization of light in the plasmonic nanocavity of the STM enables high spatial resolution in STM-TERS; however, the temporal resolution is so far limited. Here, we demonstrate stable TERS measurements from subphthalocyanine (SubPc) molecules excited by ∼500 fs long laser pulses in a low-temperature (LT) ultrahigh-vacuum (UHV) STM. The intensity of the TERS signal excited with ultrashort pulses scales linearly with the increasing flux of the laser pulses and exponentially with the decreasing gap-size of the plasmonic nanocavity. Furthermore, we compare the characteristic features of TERS excited with ultrashort pulses and with a continuous-wave (CW) laser. Our work lays the foundation for future experiments of time-resolved femtosecond TERS for the investigation of molecular dynamics with utmost spatial, temporal, and energy resolutions simultaneously.

Wavefunction embedding for molecular polaritons

Polaritonic chemistry relies on the strong light-matter interaction phenomena for altering the chemical reaction rates inside optical cavities. To explain and to understand these processes, the development of reliable theoretical models is essential. While computationally efficient quantum electrodynamics self-consistent field (QED-SCF) methods, such as quantum electrodynamics density functional theory (QEDFT) needs accurate functionals, quantum electrodynamics coupled cluster (QED-CC) methods provide a systematic increase in accuracy but at much greater cost. To overcome this computational bottleneck, herein we introduce and develop the QED-CC-in-QED-SCF projection-based embedding method that inherits all the favorable properties from the two worlds, computational efficiency and accuracy. The performance of the embedding method is assessed by studying some prototypical but relevant reactions, such as methyl transfer reaction, proton transfer reaction, as well as protonation reaction in a complex environment. The results obtained with the new embedding method are in excellent agreement with more expensive QED-CC results. The analysis performed on these reactions indicate that the electron-photon correlation effects are local in nature and that only a small region should be treated at the QED-CC level for capturing important effects due to cavity. This work sets the stage for future developments of polaritonic quantum chemistry methods and it will serve as a guideline for development of other polaritonic embedding models.

Surface Resonant Raman Scattering from Cu(110)

We report the first evidence of Raman scattering from surface phonons of a pristine metal surface. Our study reveals a Raman-active surface vibrational resonance on Cu(110) with a surprisingly large scattering efficiency. With the incident photon energy close to the energy of the Cu(110) surface state electronic transition, the Raman scattering from the surface optical resonance can be significantly enhanced, while any contribution from bulk phonons is absent.

Cavity-Modified Unimolecular Dissociation Reactions via Intramolecular Vibrational Energy Redistribution

While the emerging field of vibrational polariton chemistry has the potential to overcome traditional limitations of synthetic chemistry, the underlying mechanism is not yet well understood. Here, we explore how the dynamics of unimolecular dissociation reactions that are rate-limited by intramolecular vibrational energy redistribution (IVR) can be modified inside an infrared optical cavity. We study a classical model of a bent triatomic molecule, where the two outer atoms are bound by anharmonic Morse potentials to the center atom coupled to a harmonic bending mode. We show that an optical cavity resonantly coupled to particular anharmonic vibrational modes can interfere with IVR and alter unimolecular dissociation reaction rates when the cavity mode acts as a reservoir for vibrational energy. These results lay the foundation for further theoretical work toward understanding the intriguing experimental results of vibrational polaritonic chemistry within the context of IVR.

Charge Transfer-Mediated Dramatic Enhancement of Raman Scattering upon Molecular Point Contact Formation

Charge-transfer enhancement of Raman scattering plays a crucial role in current-carrying molecular junctions. However, the microscopic mechanism of light scattering in such nonequilibrium systems is still imperfectly understood. Here, using low-temperature tip-enhanced Raman spectroscopy (TERS), we investigate how Raman scattering evolves as a function of the gap distance in the single C₆₀-molecule junction consisting of an Ag tip and various metal surfaces. Precise gap-distance control allows the examination of two distinct transport regimes, namely tunneling regime and molecular point contact (MPC). Simultaneous measurement of TERS and the electric current in scanning tunneling microscopy shows that the MPC formation results in dramatic Raman enhancement that enables one to observe the vibrations undetectable in the tunneling regime. This enhancement is found to commonly occur not only for coinage but also transition metal substrates. We suggest that the characteristic enhancement upon the MPC formation is rationalized by charge-transfer excitation.

Cavity-Modulated Proton Transfer Reactions

Proton transfer is ubiquitous in many fundamental chemical and biological processes, and the ability to modulate and control the proton transfer rate would have a major impact on numerous quantum technological advances. One possibility to modulate the reaction rate of proton transfer processes is given by exploiting the strong light-matter coupling of chemical systems inside optical or nanoplasmonic cavities. In this work, we investigate the proton transfer reactions in the prototype malonaldehyde and Z-3-amino-propenal (aminopropenal) molecules using different quantum electrodynamics methods, in particular, quantum electrodynamics coupled cluster theory and quantum electrodynamical density functional theory. Depending on the cavity mode polarization direction, we show that the optical cavity can increase the reaction energy barrier by 10-20% or decrease the reaction barrier by ∼5%. By using first-principles methods, this work establishes strong light-matter coupling as a viable and practical route to alter and catalyze proton transfer reactions.

Tip-Enhanced Raman Scattering on Both Sides of the Schrödinger Equation

Historically, molecular spectroscopists have focused their attention to the right-hand side of the Schrödinger equation. Our major goal had and still has to do with determining a (bio)molecular system's Hamiltonian operator. From a theoretical spectroscopist's perspective, this entails varying the parameters of a model Hamiltonian until the predicted observables agree with their experimental analogues. In this context, less emphasis has been put on the left-hand side of the equation, where the interplay between a system and its immediate local environment is described. The latter is particularly meaningful and informative in modern applications of optical microscopy and spectroscopy that take advantage of surface plasmons to enhance molecular scattering cross-sections and to increase the attainable spatial resolution that is classically limited by diffraction. Indeed, the manipulation of light near the apex of a metallic nanotip has enabled single molecule detection, identification, and imaging. The distinct advantages of the so-called tip-enhanced optical nanospectroscopy/nanoimaging approaches are self-evident: ultrahigh spatial resolution (nanometer or better) and ultimate sensitivity (down to yoctomolar) are both attainable, all while retaining the ability to chemically fingerprint one molecule at a time (e.g., through Raman scattering). An equally interesting aspect of the same approach stems from using the properties of a single molecule to characterize the local environment in which it resides. This concept of single molecule spectroscopy on the left-hand side of the Schrödinger equation is certainly not novel and has been discussed in pioneering single molecule studies that ultimately led to a Nobel prize in chemistry. That said, local environment mapping through ultrasensitive optical spectroscopy acquires a unique flavor when executed using tip-enhanced Raman scattering (TERS). This is the subject of this Account.In a series of recent reports, our group utilized TERS to characterize different properties of nanolocalized and enhanced optical fields. The platforms that were used to this end consist of chemically functionalized plasmonic nanostructures and nanoparticles imaged using visible-light-irradiated gold- or silver-coated probes of an atomic force microscope. Through a detailed analysis of the recorded spectral nanoimages, we found that molecular Raman spectra may be used to track the magnitudes, resonances, spatiotemporal gradients, and even vector components of optical fields with nanometer spatial resolution under ambient conditions. On the other side of the equation, understanding how spatially varying optical fields modulate molecular nano-Raman spectra is of utmost importance to emerging areas of nanophotonics. For instance, tracking plasmon-enhanced chemical transformations via TERS necessitates a deeper fundamental understanding of the optical signatures of molecular reorientation and multipolar Raman scattering, both of which may be driven by local optical field gradients that are operative in TERS. We illustrate these concepts and introduce the readers to the generally less appreciated and equally exciting world of TERS on the left-hand side of the Schrödinger equation.

Locating Single-Atom Optical Picocavities Using Wavelength-Multiplexed Raman Scattering

Transient atomic protrusions in plasmonic nanocavities confine optical fields to sub-1-nm³ picocavities, allowing the optical interrogation of single molecules at room temperature. While picocavity formation is linked to both the local chemical environment and optical irradiation, the role of light in localizing the picocavity formation is unclear. Here, we combine information from thousands of picocavity events and simultaneously compare the transient Raman scattering arising from two incident pump wavelengths. Full analysis of the data set suggests that light suppresses the local effective barrier height for adatom formation and that the initial barrier height is decreased by reduced atomic coordination numbers near facet edges. Modeling the system also resolves the frequency-dependent picocavity field enhancements supported by these atomic scale features.